A reviewofthedepositsandbeneficiation of lower-gradechromite ·...

A review of the deposits and beneficiation oflower-grade chromite

by RALPH H. NAFZIGER*, Ph.D., M.A.S.M., M.S.M.A., M.S.R.S.

SYNOPSISThis paper reviews the ~ajor deposits of lower-grade chromite in the world, and a wide variety of methods used

for the recovery of ch.romlum f~om them. Ga~gue materials in more coarsely sized ores can be separated by gravity~ean~, whereas flotation techniques are required for finer-grained materials. However, unless there is considerableIron In the ore.s, p.hysical m~ans cannot !ncrease the er/Fe ratios. Chemical techniques such as hydrometallurgicalmethods, chlorination, r~astlng and lea:hlng, and s~elting are required for this purpose.

A greater und.erstandlng ?f the physical properties of chromites and the chemical reactions that take place in therecovery operat~ons IS required .befo~e. the recove~ies from lower-grade chromites can be maximized. More efficientrecovery. operatu:ms ca~ be reali~ed If It can be shown definitively that ther.eduction of chromites occurs via eithera.gas-solld or solid-solid mechanism, for example. The effects of specific impurities on the chromite reduction reac-tions need to be better understood, as do reagent and pH control in the flotation of chromite.

SAMEVATTINGHier.die ref~raat gee 'n oors!g °.or die ~el~ngrikste afsett;ings ~an laergraads.e chromiet in die wereld en 'n groot

~ers.keldenheld metodes wat vir die herwlnnlng van chromium Ult sulke afsettlngs gebruik word. AarsteenmateriaalIn die .growwer .ertse kan deur middel van swaartekrag geskei word, terwyl flottasietegnieke vir fynerkorrelrigem~terlaal gebrulk moet word. T~nsy daar .heelwat rster in die .ertse is, kan die Cr/Fe-verhouding egter nie deurfislese. metode~ verho.og word ~Ie. .Chemlese te~nleke soos hldrometallurgiese metodes, chlorering, roosteringen loglng, en ultsmeltlng moet vir die doel gebrulk word.

. 'n Beter b~gri~ van die fisies.e eiens~ka:ppe v~n chromiet en die chemiese reaksies wat in die herwinningsbewer-kings p!aasvlnd, IS nodlg om die he~wlnnlngs Ult laergr~a.dse chromiet te maksimeer. Doeltreffender herwinnings-bewerklngs sal byvoorbeeld moontllk wees as daar definltlef getoon kan word dat die reduksie van chromiet via 'nga.s-vastestc:>fof 'n ~astestof-vastestofmeganisme plaasvind. Die uitwerking van spesifieke onsuiwerhede op die chro-mletredukslereaksles asook reagens-en pH-beheer in die flottasie van chromiet moet ook beter begryp word. i

Introduction

Chromium is one of the most versatile and widelyused elements. Its use in the metallurgical, chemical, andrefractory industries is well known, and it is an essentialelement in the production of a wide variety of stainlesssteels, tool and alloy steels, nickel-chromium heatingelements, and plating metals. Its widespread use in themetallurgical industry is attributed to its capability ofenhancing properties such as resistance to corrosion oroxidation, creep and impact strengths, and hardenability.Chromium compounds are used extensively in the manu-facture of paint pigments and chemicals, as oxidizingagents in organic syntheses, as electrolytes in chromiumplating baths, and as agents for the tanning and dyeingof leather. In the form of chromite, it is used in theproduction of refractory bricks for the lining of high-temperature furnaces, and in the glassmaking and cementindustries. There are few, if any, materials that canreplace chromium economically and satisfactorily usingpresent technology. Owing to these factors, chromium isconsidered a strategic material, a point that has beenexphasized in a NMAB report!.

Before the early twentieth century, chromium fromMaryland, Pennsylvania, Virginia, Turkey, and the UralMountains in Russia was used to supply the limiteddemands of the chemical industry. From that time, thedemand increased substantially to satisfy the newlyemerging metallurgical and refractory industries.

The world reserves of chromium are concentrated inthe Eastern Hemisphere in over 20 countries (Fig. 1),

*Research Supervisor, Albany Research Centre, D.S. Depart-ment of the Interior, Bureau of Mines, P.O. Box 70, Albany,Oregon 97321, D.S.A.

~ 1982.

most of the producing countries containing limitedreserves. Although chromium occurs in a variety ofminerals, chromitc (FeCr20 4) is the sole commercialsource.

Chromites can be classified on the basis of their er/Feratio. The highest-grade chromites2,3 are those having aer/Fe ratio of more than 2,0 (usually approximately 2,8)and those containing a minimum of 46 to 48 per cent

Cr203' These metallurgical-grade chromites occur insignificant quantities in South Africa, Zimbabwe, andthe U.S.S.R. Chemical and refractory-grade chromitestypically have er/Fe ratios ranging from 1,4 to 2,0.Chemical-grade (high-iron) chromites contain largeamounts of iron, which often results in Cr/Fc ratios ofclose to 1,0 (the ratios are usually from 1,5 to 2,1),although the absolute amount of contained chromiumranges from 40 to 46 per cent Cr203' However, it shouldbe pointed out that South African high-carbon chro-mium is produced mainly from Transvaal chromite witha er/Fe ratio of approximately 1,5. Low-grade chromitesare those that have low er/Fe ratios and contain relative-ly small amounts of chromium. Refractory-grade chro-mites contain relatively large quantities of A12O3 (greaterthan 20 per cent) and have Cr203+ Al203levels of morethan 60 per cent. This paper concerns all classes of chro-mite except metallurgical grades.

The world-wide demand for primary chromium isexpected to increase at an annual growth rate of 3,4 percent from 1975 to 2000. The total demand for primarychromium in 2000 is forecast at 4800000 t (5300000sh. t)4. Although world reserves of high-grade ores canmeet this demand, these ores are often located at greatdistances from highly industrialized areas. Accordingly,demand for more readily available lower-grade chromiteores increases annually as the higher-grade ores become

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1982 205

206

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AUGUST 1982 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

iChemical grade Refractory grade

Country

I I

Total lower-Reserves Other Total Reserves Other Total grade ores

South Africa I 100 2200 3300 I 3300Zimbabwe 56 56 112

I

112U.S.S.R. 1,1 2,2 3,3 11 11 22 ! 25,3Finland 11 i',6 16,6 16,6Greenland 11 11 11,0Philippines 4,5 2,2 6,7 6,7Brazil i 3,9 2,2 6,1 0,2 0,1 0,3 6,4United States 5,6 5,6

I

0,1 0,1 ;5,7India 2,2 2,2 4,4 4,4Malagasy Republic 1,1 2,2 3,3 3,3Canada 2,8 2,8 2,8Cuba 0,3 1,1 1,4

I1,4

Greece 0,05 0,05 0,1 0,1I

Deposit Cr.O. Fe.O. FeO CaOI

MgOI

AI.O. SiO. Cr/Fe Footnote

Oregon coastal beach sands 9,40 * 14,1 8,10 3,50 14,4 34,8 0,59 t(Shepard deposit)

Stillwater Complex, Montana:Mouat mine 34,0 5,89 15,2 0,67 19,1 12,5 11,4 1,46 :j:Benbow mine 46,1 * 24,1 NA 5,43 23,8 1,02 1,68 t:j:Gish mine 46,0 3,87 18,4 0,04 11,2 19,4 0,28 1,85 t:j:§

Grant County, Oregon 37,8 0,8 14,6 0,3 15,2 27,0 1,9 2,17 tt::.(Chambers mine)

Seiad Creek area, California:Emma Belle" 20,0

*11,4 1,21 34,4 4,40 26,0 1,54

Seiad Creek" 18,3*

11,3 3,40 30,4 5,34 27,2 1,42

TABLE IWORLDWIDE CHROMITE ORES (IN MILLIO~S OF SHORT TONS)*

*Data from reference 4, except for Canada (from reference 5).

depleted, and economic and efficient methods for therecovery of chromium from lower-grade ores are receivingmore attention. With present technology, most of thelow-grade ores cannot compete with the higher grades.An estimate of the world's lower-grade chromite ores isgiven in Table I, which was derived from surface geologyand information on reserves, and indicates only ordersof magnitude for comparative purposes.

The objects of this paper are to review methods for therecovery of chromium from lower-grade chromite ores,and to assess means whereby the recovery of chromiumcan be maximized from these deposits. A survey of themajor lower-grade chromite deposits in the world anda review of the recovery methods used on them are in-eluded.

United States of America

Chromite Deposits

All the chromite deposits in the U.S.A. are relatively~mall and most are of lower grade. Being generallylocated in Alaska, Montana, Oregon, and California, afurther complication is their distance from consuming

plants. As most of the podiform deposits in Alaska andCalifornia contain metallurgical-grade chromite, theseare not discussed here.

The largest deposit in the United States lies in thecoastal beach sands of south-west Oregon. Althoughrelatively extensive, these deposits, which can be classi-fied as high-iron deposits, are very low-grade and wouldrequire extensive beneficiation (Table H). The beachsands contain quartz, olivine, pyroxene, ilmenite, fUtile,zircon, garnet, chromite, magnetite, hornblende, andepidote2.

The second most-extensive chromite deposit in theUnited States lies in the Stillwater Complex in south-central Montana. From the typical compositions of theconcentrates produced from this deposit (Table H), itcan be seen that they are either of chemical (high-iron)or refractory grade6,7.

A refractory-grade chromite deposit lies in east-central Oregon in Grant County, south-east of the townof John Day. The chromite occurs in belts of peridotiteand dunite, which have largely been altered to serpen-tine. A representative chemical analysis is given inTable H.

REPRESENTATIVE COMPOSITIONS OF CHROMITES IN THE U.S.A. IN MASS PERCENTAGES

TABLE II

NA = Not available.* Total iron as FeO.t Dr George H. Reynolds, personal communication, 1977.:j: Concentrate, ore contains more than 20 per cent Cr.O. by mass.§ 0,18 per cent MnO and 0,50 per cent TiO. by mass.t::. 0,3 per cent TiO. by mass.0 Avewage of 2 samples".

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METAllURGY AUGUST 1982 207

One of the few chromite deposits in California thatare not considered to be of metallurgical grade is thatin Siskiyou County, near Seiad Creek in the extremenorthern portion of the state. The ore is disseminated inserpentinized dunite. Associated minerals include olivinewith chlorite, limonite, quartz, feldspar, kammererite,serpentine, calcite, epidote, orthopyroxene, actinolite,mangetite, and fUtile. An examintttion of typical chemi-cal analyses of the ore from two prospects (Table H)shows that this ore can be classified as chemical grade(high-iron).

Minor low-grade, widely scattered deposits also existin Washington, Wyoming, North Carolina, Maryland,and Pennsylvania. Some of these were mined in the earlytwentieth century, but this is no longer economicallyfeasible.

Beneficiation

During World War H, the beach-sand deposits inOregon were gravity-concentrated by tabling or by theuse of Humphreys spirals. Some of these concentrateswere further beneficiated by magnetic separation afterdrying, but the recoveries of approximately 65 per centwere considered lowz. The concentrate contained from40,4 to 41,9 per cent CrZO3 by mass, and a er/Fe ratioaveraging 1,59 was obtained. Flotation techniques couldnot be recommended owing to the low recoveries (57 percent). The best flotation concentrate contained 36 percent CrZO3 by mass.

Considerable beneficiation work was conducted on thechromites from Montana over several decades. Earlywork by the Bureau of Mines focused on the roastingof the ore under strongly reducing conditions withcarbon at 1300°C to reduce the iron, which was solublein dilute HzSO4 while most of the chromium was inso-luble. In pilot-scale reduction experiments with coke,er/Fe ratios of 2,4 were obtained after leaching withacid 9. At lower temperatures, the er/Fe ratio was greaterthan 3, and a deep bed was preferred. An increase in thecarbon contents raised both the soluble iron and thesoluble chromium. The er/Fe ratio of the leached residueincreased to a maximum when increasing amounts ofcarbon were used, but then decreased as more carbonwas added 9.

Hunter and Paulson 10 conducted laboratory-scalereduction experiments with graphite on chromite fromthe Mouat Mine, starting the reduction at approximately1150°C. A reduction of nearly 50 per cent was achievedslightly above 1300°C, while approximately 80 per centwas obtained at 1450°C. The evaluation of several car-bonaceous reductants on the Mouat chromite showedthat coal char yielded the most reduction between1100 and 1300°C, whereas metallurgical coke was thepreferred reductant 7 at 1400 and 1500°C. The samerelationships applied with respect to the degree ofmetallization. The rate of reduction was greatest duringthe first 15 minutes, and higher temperatures increasedthe degree of reduction and metallization.

The Bureau of Mines conducted extensive smeltingtests on Mouat chromite. In one series of tests, thechromite was smelted with either a sub-bituminous coalor a char made from that coal, the carbon-to-chromium

208 AUGUST 1982 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

ratio of the charge being 1,02 for each testll. In anotherseries of tests, in which the carbon-to-chromium ratiowas varied, decreases in the ratio resulted in lower chro-mium and iron recoveries in the metal, lower yield, andlower er/Fe ratios in the metal. An acceptable ferro-chromium could be prepared having an average er/Feratio of 1,32, and higher er/Fe ratios were obtainedwhen the smelting operation was conducted on a silica(acid) hearthll.

Further smelting tests showed that Mouat chromitecan be used for the production of acceptable ferro-alloys,although the er/Fe ratio could not be improved. It wasfound that approximately 130 per cent of the stoichio-metric carbon requirement to reduce the chromium andiron appeared to be optimumlZ. T'his work also demon-strated that low-carbon ferrochromium-silicon can beprepared by a two-stage process that involves the addi-tion of silica after an initial direct smelting step, itsreduction to silicon with sufficient reductant, and meltingto produce low-carbon ferrochromium-silicon.

In another study, iron was reduced selectively fromchromite by carbothermic reduction in an electric-arcfurnace. The use of basic slags resulted in better chro-mium recovery in the slag, and higher er/Fe ratios wereobtained with acid slagsl3. Experiments were also con-ducted to remove much of the iron by the chlorinationof mixtures of carbon and chromite. Higher temperaturesproduced greater er/Fe ratios, and additions of CaF Zimproved the ratios. The upgraded slags were smeltedin an electric-arc furnace to produce high-carbon ferro-chromium containing up to 65 per cent Cr. Coke and hog-ged wood were used as reductants in these tests.

In a few smelting tests conducted on the chromiteores of central Oregon, low-carbon ferrochromium-silicon was produced in a two-step process as previouslydescribed. A recovery of up to 85 per cent Cr was realized.However, higher chromium recoveries were obtained bythe smelting of the ores than by gravity concentration ofthe ores and smelting of the concentrate14. Thesechromites were judged to be suitable for the productionof refractory brick with appropriate additions of MgO.

Methods for the utilization of chromite from SeiadCreek are described by Hunter and Sullivan8. Initially,an average of 86 per cent Cr was recovered in the sinkproduct (35 per cent CrZO3 by mass) from sink~floattests. Tabling resulted in concentrates containing atleast 49 per cent CrZO3 by mass, but the recoveries werelow (33 to 45 per cent). The recoveries were improved(to 73 per cent) when tabling and electrostatic separationof the table middlings were used. The chromite can beselectively concentrated by flotation in the presence ofslimes. Concentrates containing up to 45 per cent CrZO3by mass were recovered, the Cr recoveries ranging from83 to 91 per cent. Somewhat higher CrZO3 levels wereobtained with lower recoveries when a combination offlotation of the minus 200-mesh fraction and electro-static precipitation of the plus 200-mesh fraction wasused. Magnetic separation resulted in lower chromiumrecoveries and CrZO3 contents. The results of smeltingtests on the ore showed that a higher-carbon ferrochro-mium containing 47 to 51 per cent Cr could be prepared,with Cr recoveries as high as 86 per cent. Smelting of the

TABLE III

Deposit Fe.O.

ANALYSES OE' REPRESENTATIVE SOUTH AFRICAN CHIWMITES IN MASS PERCENTAGES

SiO. RefereneeFeO

Union CorporationtMoreesbergBushveld Complex, Steelpoort

Seam:Winter veld mine, TransvaalWinterveld mine, TransvaalWinterveld mine, Transvaal

Kroondal ore, TransvaalGrootboom ore

26'4

1

20,7

116,8

I

21.616,4

I25,0

**

7,928,3

*6,21*

NA = Not available.* Total iron computed as FeO.t Total iron computed as Fe.O..t TiO. = 0,5 per cent by mass.

sink-float concentrates with hogged-wood waste yieldeda high-carbon ferrochromium containing 52 to 58 percent Cl' with a Cl' recovery of 72 per cent. The energyrequirements were lower when the concentrates weresmelted than when the ore was smelted. Commercial-grade ferrochromium was produced by a combination ofconcentration and smelting8.

Republic of South Africa

Chromite Deposits

The large Bushveld Complex in South Africa is amajor world source of chromium, having estimated15reserves of up to 10 billion tons*. The Complex is locatedin the north-eastern portion of the country in theTransvaal, north of Pretoria. The chromite deposits inthe eastern portion of the Complex occur in a layeredsequence of mafic to ultramafie rocks, which containolivine, pyroxenes, and plagioclasel6, the layers being upto 18 m thick. Most of the Bushveld chromite is classi-fied as chemical-grade (high-iron), and representativeanalyses are presented in Table Ill. Thin seams of chro-mite also occur in the Merensky Reef17.

Beneficiation

In an investigation18 aimed to improve both the gradeand the er/Fe ratio of a chromite ore from Moreesberg(Table Ill), samples were crushed to minus 14 mesh andgravity-separated. One sample was separated into foursize fractions, portions of which were mixed with coal(in the proportions 4 to I) and reduced at 1200 and1300°C. The metallic phase was removed by 10 per centsulphuric acid. Increases in temperature and time, and/or decreases in particle size, resulted in increases in theer/Fe ratio but also decreases in chromium recovery.The chromium recoveries and er/Fe ratios were best atshort time intervals. The fine particle size necessary forliberation made removal ofthe metallic phase by gravityseparation unsuccessful. Corrosion tests to remove metal-lic iron with brine solutions, sea water, and ammoniumchloride solutions were equally ineffective. Leaching ofiron in a ferric salt solution was partially successful.

*This and subsequent references to tons in this paper are to

short tons.

I I'I

Cr.O. Cr/FeI

CaD 'Al.O. MgO MnO

~~I

I-=-~~I~30,5 1,30 NA 16,9 10,8

I

NA

42,9 1,.58'I

0,5'

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14,1 I 12,4

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43,6 1,51 0,2 15,4 8,644,3 1,59' 04 I 16,1 11,2 I NA35,3 1,41

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0,1842,4 1,49 0,09

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0,315,4 18

4,71,22,3

13,06,0

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51;)

-,.

Since chromite is weakly magnetic, it cannot be separatedfrom the metallic phase by magnetic separation 18.

Several investigations have been conducted to deter-mine the reducibility of South African chromites as afunction of temperature and composition3,5,lO,19-22.

When Hunter and PaulsonlO reduced beneficiatedTransvaal chromite with graphite in the temperaturerange 940 to 1600°C in a horizontal-tube furnace underargon, reduction began at 1150°C and reached over 80per cent above 1300°C. The chromite reduction wasaffected by temperature, ferric iron and alumina con-tents, and ferrous iron content, in that order.

Rankin 5,21investigated the reduction of beneficiatedKroondal ore thermogravimetrically (Table Ill) withgraphite under an argon or carbon monoxide atmosphere.With graphite and temperatures of up to 1200°C, hefound that iron formed first, followed by an impure

Cr203' At higher temperatures, the formation of iron wasfollowed by that of Fe3C and (Fe, Cr7C3), The initialmetallic iron was eventually converted to Fe3C, and thechromite spinel gradually transformed to MgA1204' Asimilar reaction sequence occurred when carbon mon-oxide was used as the reductant. These results suggestTransvaal chromite is reduced through carbon mon-oxide, an interpretation supported by microstructuralevidence that the initial reduction occurred at the edgesofthe chromite particles (topochemicaI5).

Dewar and See20 conducted reducing experiments onchromite from the Wintervcld mine (Table Ill) usingchars, coke, coal, and graphite as reducing agents in athermogravimetric furnace. In the Ih tests, 35 to 45 percent reduction occurred at 1500°C, whereas negligiblereduction occurred at 1350°C, the reduction mechanismsbeing independent of the type of reductant uscd. Therates of reduction increased with increased reactivitiesof the reducing agents towards carbon dioxide. In thelater stages of reduction, the reduction was influencedboth by the above factors and by the fixed-carboncontent of the reducing agent.

Another sample of chromite from thc Winterveldmine (Table Ill) was subjected to a series of reductiontests at 1200 to 1500°CI9,22. Observations showed atopochemical reaction, but other mechanisms mayoperate over such a wide temperature range. Up to1500°C, Urquhart believed that the reduction of this

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGYAUGUST 1982 209

Deposit FeO* Cr203 Cr/Fe MnOI

Reference

Great Dyke 19,1-23,0 51,7-53,9 2,06-2,38 NA 23Mount Claims 25,5 41,6 1,52 0,89 24

Transvaal chromitc ore occurred via a solid-state reac-tion with a solid reductant.

Cohen and Yalcin3 conducted experiments on a SouthAfrican chromite (Table Ill) using chromium as a redu-cing agent. The advantages claimed for this process arethat only the iron oxide is reduced, the reduction isexothermic, and the product is carbon-free and not other-wise contaminated by the redttctant. Chromite andchromium (17,5 per cent by mass) were briquetted andheated to temperatures between 700 and 1l00°C forIh. Total iron reduction was possible only at tempera-tures above 1050° C. It was found that the amount ofchromium reductant influences the time required to com-plete the reduction reaction, approximately 17,5 per centchromium being optimum; thorough mixing is necessaryto maintain the chromium losses at a minimum, the ironcan be leached subsequently with sulphuric acid, andthe resulting concentrates can be used for the productionof chromic oxide. Low-carbon ferrochromium can alsobe produced but, as a result of the advent of the argon-oxygen decarburization (AOD) process, this product isnot required.

Zimbabwe

The reserves of high-iron chromites in the Transvaaland Zimbabwe constitute probably the greatest resourceof chromium known in the world. In Zimbabwe, thechromites occur as seams in the Great Dyke, an extensivelayered intrusion extending NNE-SSW for almost thelength of the country and consisting of four ultrabasiccomplexes. Associated minerals include pyroxenes,olivines, and plagioclase23. Chromite occurs as the domi-nant phase in the chromite seams and as disseminatedcrystals in the olivine-bearing rocks. Intrusions ofchromiferous peridotite also occur in the Selukwe areain central Zimbabwe. Most of the mines in this areayield metallurgical-grade chromites, the average ana-lyses of the high-iron ore from Mount Claims and theGreat Dyke being given in Table IV.

Hunter and Paulsonl0 conducted laboratory-scalereduction experiments with graphite in a horizontal-tubefurnace under argon to determine the degree of reductionpossible as a function of temperature and chromitecomposition. They found that temperatures of at least1400°C were required to obtain a significant degree ofreduction, most of the reduction occurring between 1400and 1500°C. This chromite was more difficult to reducethan the other chromites tested (Stillwater, Transvaal,U.S.S.R.). The Zimbabwean chromite concentrate tested~ontained more ferric iron than the others.

U.S.S.R.

Chromite Deposits

Major chromite deposits in the Soviet Union occur inthe Ural and Caucasus Mountains, in central and easternSiberia, and in Kamchatka, Chukotka, and Sakhalin.All of the ores are contained in ultramafic massifs infolded rocks, the most important deposits being locatedin the Ural Mountains25. These ores are associated withperidotite rocks in which harzburgites predominate. Theprimary minerals include olivine, orthopyroxene, andaccessory chromium spine I (chromite). The chromites,which are low in iron but contain considerable amountsof A12O3' are classified as refractory-grade. Typicalaverage analyses are given in Table V. The chromites ofthe Urals are associated with serpentinized dunites, andferruginous chromium-picotites occur in troctolites(Table V). Metallurgical-grade chromite ores occur in thesouth-east but, being high-grade ores, are not consideredin this paper. Several other areas in the Soviet Unioncontain chromites with less alumina and/or higher ironlevels, and some can be considered to be chemical-gradechromites. Among these are deposits of the Shorzhamassif in Armenia, near Lake Sevan (in peridotites anddunites) and ores in the Saranovsk massif on the westernslopes of the Ural Mountains (in dunites).

Beneficiation

Sobieraj and Laskowski26 investigated the flotationresponses of a Russian chromite (Table V, last column)in 10-4 M solutions of sodium laurate and dodecylam-monium hydrochloride. The tests were conducted on aminus 0,2 plus 0,075 mm size fraction in a modifiedHallimond tube with conditioning and flotation times of5 min at an air flow of 51jh. Nearly 100 per cent recoverywas realized with sodium laurate in a pH range of 2,5 to8,5, and approximately 65 per cent recovery was obtainedat pH values of 10 to 12. With the dodecylammoniumhydrochloride, complete recovery was attained in a pHrange of 5 to 12. Comparisons with other chromitesindicated that the differences in flotation response weredue to variations in the composition of the gangueminerals. Ionic and molecular adsorption is believed tobe responsible for the flotation capabilities of sodiumlaurate, whereas an electrostatic interaction betweenmineral and collector is suggested from the work withdodecylammonium hydrochloride.

The presence of aluminum in refractory chromitesaffects the flotation of chromite depending on the pH.For pH ranges between 2,5 and 4,5, no decrease inrecovery was observed. However, for pH values between

ANALYSES OF REPRESENTATIVE CHROMITES IN ZIMBABWE IN MASS PERCENTAGES

TABLE IV

NA = Not available.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

* Total iron as FeO

210 AUGUST 1982

Chromites Chromium Chromites

I

Chromites Chromites Chromitesfrom picotites from from from of

harzburgites from dunites I Shorza Saranovsk unspecifiedtroctolites

I

massif massif origin

Cr,O, 38,9 35,0 37,5 I 47,5 40,6 41,8Fe,O, 2,5 5,2 4,9 5,2 7,5 4,4FeO 15,3 20,4 11,3 13,9 19,3 12,8Al,O, 27,1 25,4 27,5 13,2 16,3 23,8MgO 14,4 10,6 15,8 11,8 12,7 15,1CaO 0,0 0,0 0,0

I

0,6 0,1 0,8SiO, 1,2 2,2 1,5 2,2 2,4 1,2er/Fe 2,0 1,2 2,1 2,3 1,4 2,2Density, g/cm ND ND 4,15

I

ND ND ND

TABLE VAVERAGE ANALYSES OF U.S.S.R. CHROMIUM-BEARING ORES IN PERCENTAGES'5,"

ND = Not determined.

8 and 10, the AP+ ions interacted strongly with themineral surface. Depressed flotation occurred betweenpH 4,5 and 8, and activation between pH 10 and 12.This phenomenon is related to the region of stability ofaluminum hydroxides and to the resulting precipitationof aluminium species on the mineral surface. It wasconcluded that the flotation of chromite ores should beconducted under alkaline conditions with a possiblesecond upgrading in acid solutions.

India

Indian lower-grade chromite ores contain up to 40 percent Cr203 and considerable iron, with er/Fe ratiosranging from approximately 1,0 to 1,8. They are usuallyassociated with chlorite, serpentine, and magnesite ordolomite, and are typically hard and fine-grained27. Theores can be considered to be of chemical grade and arelocated in Karnataka (Mysore State).

The Indian chromites can be concentrated by conven-tional ore-dressing techniques owing to the difference inrelative density between the ore and the gangue materials.However, chemical means are necessary to increase theer/Fe ratio substantially. The optimum method of con-centration27 is determined by the particle size of the ores.

Several techniques to decrease the iron content of theconcentrates have been evaluated. One method involvedthe decomposition of the ore either with an alkali or withan acid. Sodium chromate or dichromate was producedin the alkali process, and these compounds were subse-quently reduced to chromium oxide28. The acid processinvolved pressure leaching with concentrated sulphuricacid, which yielded a basic chromium sulphate thatcould be decomposed into the oxide.

Another technique involved the reduction of the ironoxide in chromite ore with gaseous reducing agents suchas hydrogen, carbon monoxide, and coke-oven gas 9,29.Coke-oven gas was used on Indian ores to obtain a er/Feratio of 3 and a chromium recovery of 80 per cent. Theore was roasted for Ih at 1l00°C and then leached with10 per cent sulphuric acid for 1 h3°.

Solid-state carbon reduction to preferentially reducethe iron in the Indian chromites, followed by sulphuricacid leaching, was attempted on a laboratory scale27,31.The er/Fe ratio of the ores ranged from 1,01 to 1,33. Inone investigation, ore briquetted with a 1 per cent

dextrin binder and coke was heated to 1300°C, andthe reduced product was leached with 10 per cent boilingsulphuric acid for Ih to yield a product with a er/Feratio of 3. The use of a 5 per cent NaCI binder improvedthe solid-state reduction process when graphite, charcoal,and coke were employed. Selective reduction was con-ducted at 1150°C for 2,5 h and subsequently at 1350°Cfor 0,25 h. The same leaching solution was used to obtaina product with a er/Fe ratio ranging from 2,7 to 3.

In a two-stage reduction process using a 25 k V . Aelectric furnace, 80 per cent of the stoichiometric carbonrequirement was smelted with Indian chromite orehaving a er/Fe ratio of 1,0. A chromium-rich slag with aer/Fe ratio of 2,7 was obtained when a slag basicity of0,9 was used, although this variable had a minor in-fluence on the chromium recovery27. In the second stage,the slag was smelted to produce a high-carbon ferrochro-mium. The economics of the process are dependent onthe cost of electric power.

Athwale and Altekar32 investigated the chlorinationof chromite with hydrochloric acid using a fluidized-bedtechnique. Improvements in the er/Fe ratio were notedbut the economics on a large scale are unknown.

CanadaChromite Deposits

Major chromite deposits in Canada are located inBritish Columbia, south-eastern Manitoba, Ontario,Quebec, and Newfoundland. The lower-grade ores con-tain a relatively large amount of iron, and thereforcpossess a low er/Fe ratio and are classified as chemical-grade. One of the largest single chromite deposits in NorthAmerica is located in south-eastern Manitoba (BirdRiver). The ores include epidote and serpentine, inaddition to an impure chromite (Fe, Mg)O (Cl', AI, Fe)203'An average grade of approximately 1l,5 Cr203 (7,8 percent chromium) has been estimated33. Concentratescontaining approximately 44 per cent Cr203 should bepossible, based on mineralogical examinations, andshould contain approximately 21 per cent iron, yieldinga er/Fe ratio of 1,5.Beneficiation

A variety of physical methods, such as sink-float,jigging, tabling, magnetic separation, and flotation, havebeen employed on Bird River ores. The resulting Cr203


contents ranged from 36 to 42 per cent, with iron con-tents ranging from 19 to 24 per cent. This yields Or/Feratios ranging from 1,1 to 1,5, which is insufficient formost metallurgical applications.

Several pyrometallurgical and hydrometallurgicalmethods have been devised to remove iron and ganguematerials from Bird River chromite ores in order toincrease the Or/Fe ratio. An oxidation and reductiontechnique was developed to prepare a high-grade chro-mium oxide that would be added to the concentrate toachieve metallurgical-grade specifications34. Soda ashwas added to the chromite to produce sodium chromate,followed by aqueous dissolution of the chromate. Fil-tration removed the iron-bearing residue, and a high-grade chromium oxide was produced by evaporation,crystallization, and reduction of the chromate (NaZOrO4)with coke. The chromium recovery was approximately90 per cent of the chromium in the concentrate. Whenthe high-grade chromium oxide was mixed with thelower-grade chromite concentrate in a 2:1 ratio, theresulting product assayed 57 per cent OrZO3 and 13 percent Fe, yielding a Or/Fe ratio of 3 :1.

Smelting of the chromite concentrate with lime, silica,and coke in an electric-arc furnace produced iron anda chromium-containing slag34. The chromic oxide wasrecovered by water classification from the disintegratedslag. Although the Or/Fe ratio was approximately 20,the overall chromium recoveries ranged from 30 to 77per cent.

Leaching of the finely ground chromite concentratewith a 30 per cent sulphuric acid solution at 200 to 250°0and elevated pressure, followed by cooling, filtration,waHhing, and calcining at 1000°0, resulted34 in a productcontaining 43,6 per cent OrZO3 with a Or/Fe ratio of3,17. The chromium recovered was about 96 per centof the chromium in the concentrate.

Other methods34 included leaching of the chromite withHulphuric acid and chromic acid at 140 to 160°0 to pro-duce OrO3 after oxidation and evaporation in a diaphragmcell. This procedure yielded a product with 90 per cent

OrO3' and the chromium recovery was approximately75 per cent of the chromium in the ore. Bird Riverchromite concentrate can be roasted in a kiln at 1200°0or higher with carbon, or at 1000 to 1l0000 with reducinggascs (e.g. hydrogen, methane, natural gas), followed byatmospheric leaching of the calcine in 10 per cent hot:mlphuric acid. The iron was dissolved selectively, andany Or/Fe ratio can be obtained as a function of theamount of iron dissolved. A series of tests yielded aconcentrate with 43,2 per cent OrZO3 and a Or/Fe ratioof 2,82. The recovery was approximately 90 per cent ofthe chromium in the original concentrate.

More-recent investigations have emphasized the directproduction of chromium metal, carbides, and sodium'chromates. The Ontario Research Foundation reducedBird River chromite with carbon in the solid state35between 1025 and 1425°0 to produce chromium carbide.The carbide (Or703) is recovered as a magnetic fractionand contained 40 per cent chromium, 35 per cent iron,and 5,2 per cent carbon. High-grade chromium carbidescan also be prepared from sodium chromate and carbonat temperatures from 1000 to 1270°0 in a nitrogen

atmosphere36. The carbide can be used to producechromium metal or can be employed as an alloying addi-tion. Ferrochromium can be prepared direct37 from BirdRiver chromite concentrate by reduction of chromite~carbon pellets in a methane-hydrogen mixture at 800 to1050°0. Solid-state reduction of chromium carbides withchromium oxide at temperatures ranging from 1300to 1750°0 or the rcduction of sodium dichromate withcarbon can produce sponge chromium meta}36. High-purity sodium chromate can be prepared from an acidi-fied solution that causcs precipitation of alumina andsilica35,36.

Canadian chromite-ore fines having a Or/Fe ratio of1,7 with 45,3 per cent OrZO3 and 23,2 per cent FeO weremelted in an extended arc flash reactor (EAFR), whichemploys an indirect arc to melt metal and slag by radia-tion from a soft diffuse plasma that is derived from gasintroduced through axial holes in graphite electrodes38.The advantages of this system over conventional sub-merged-arc processes are that low electrical resistivity ofthe slag is not a factor for efficient processing, there is nosharp temperature gradient in the bath, fluid slags aremore easily obtained, the composition and size of thecharge are less critical, a higher-quality product is ob-tained, andslag~metal reactions are enhanced. Ohromiteore fines were mixed with appropriate reductants andflux additives and heated to approximately 800°0 in apreheater, the iron oxide in the charge being reduced ina reactor column at temperatures of 1600 to 2000°0.Alternatively, the ore can be fed through the preheaterwithout a carbon addition. Anthracite in the hearthproduced the required reducing gases. Ferrochromiummetal containing an average of 56 per cent chromium,28 per cent iron, and 7,7 per cent carbon was obtained.This molten metal could be charged direct to an AODunit or vacuum furnace for decarburization, and appro-priate scrap and alloy additions made to produce stain-less steel. However, on a laboratory scale, the energyconsumption was three times higher per unit mass thanthat realized in conventional operations.

Cuba

Cuban chromite ores contain considerable AIZO;j(25 per cent) and are therefore classified as refractory-grade materials. A typical analysiH is given in Table VI.The ores occur in peridotites and are associated withdunites, troctolites, and harzburgites in the Majari-Baracoa region.

Sobieraj and Laskowskiz6 conducted flotation testson a Cuban chromite using procedures and equipmentsimilar to those previously described for the beneficiationof Russian chromites. The results were similar to those

TABLE VI

ANALYSIS OF A OUBAN OHROMITE IN MASS PEROENTAGES"----- --

.---

. . . . . . . . . . . . . . . .Cr.O.Fe.O.FeOCaDMgOAI.O.SiO. .Cr/Fe

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . .. . . . . . . .

. . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .


Sample 3

Cr.O. 40,2Fe.O. *FeO 18,7er/Fe 1,9CaD 4,3MgO 8,0AI.O. , 18,2SiO. 7,2 10,4 9,5Reference 38 39 40

TABLE VIITYPICAL ANALYSIS OF RUN-OF-MINE CHROMITE ORE FROM TROODOS,

CYPRUS, IN MASS PERCENTAGES

Cr.O. ................Fe.O. . . . . . . . . . . . . . . . .FeO ................er/Fe ................CaD ................MgO

""""""AI.O. ............SiO. . . . . . . . . . . . . . . . . .TiO. . . . . . . . . . . . .MnO . . . . . . . . . . . .

31,714,513,11,074,5

11,29,8

14,30,30,2

1

obtained with the Russian material. Maximum chromiumrecovery occurred in a slightly narrower pH range (3 to 8)with sodium laurate, and at a slightly higher pH (nearly6) with dodecylammonium hydrochloride than thatachieved with the Russian chromite. The behaviour ofboth these refractory chromites was generally similar,although the Russian chromite exhibited better flotationproperties at pH values ranging from 10,5 to 12.

Cyprus

Podiform chromite deposits are located in dunites inthe Troodos area of the island of Cyprus. These orescontain massive and disseminated chromites39. A typicalanalysis of run-of-mine ore is given in Table VII. TheTroodos chromites can be classified as high-iron chromiteores.

Preliminary tests on this ore were designed to recovera maximum amount of massive ore in the feed whilesimultaneously minimizing the size reduction39. Therecovery of the massive chromite was accomplished bytwo heavy-medium drums, and one heavy-mediumcyclone separator was used to recover chromite grainsdown to 1 mm. Liberation, followed by grinding, jigging,and screening, was used, and the fine fraction was takenfor final treatment. The fine fraction was recovered ondiagonal shaking tables. This procedure provided anoverall Cr203 recovery of 93 per cent and a recovery ofmassive coarse chromite of more than 80 per cent. Thecoarse, 'intermediate', and fine concentrates contained48 to 49 per cent Cr2O3 by mass, giving a Cr/Fe ratio of2,75. Although this is a satisfactory concentrate, it wasnot accomplished without some disadvantages. Theseincluded the use oflow-capacity and bulk shaking tables,some costly handpicking, and a relatively slow feed rateowing to the size of the ore.

Egypt

Chromite DepositsBarramyia chromite ore is located in the Eastern

Desert, approximately 130 km west of Marsa Alam. Itis a high-iron podiform deposit, and typical chemicalanalyses of the ore are presented in Table VIII (samples1 and 2). Chromite [(Mg, Fe)O (Cr, AI, FehO3] consti-tutes 60 to 70 per cent of the ore, with serpentine com-prising much of the remainder (20 to 30 per cent)40,41.Minor amounts of magnetite and chromium-bearingsilicates, such as kammererite and uvaronite, are alsopresent. The analyses of another Egyptian chromite orewith similar mineralogy are also given in Table VIII(sample 3). This ore also contains small amounts of

chromium silicates such as kammererite and uvaronite42.

BeneficiationBarramyia chromite was concentrated on a shaking

table to separate the lighter silicate minerals from thechromite minerals. The results for sample 1 (Table VIII)showed that higher feed rates and water flowratesdecreased the recovery and quality of the chromite.Optimum motor speeds of 370 r/min and a deck slopeof 4 degrees from the horizontal yielded the greatestrecovery of chromites41. Cleaning was conducted on thetable concentrate and middling, which resulted in con-centrates containing 51,2 and 43,8 per cent Cr203'respectively. These concentrates are suitable for theproduction of ferrochromium and chemical applications,respectively. Similar tabling operations for sample 2(Table VIII) yielded a concentrate containing 36,0 per centCr203, which is suitable for refractory applications.Experiments using a cross-belt high-intensity separatorwere conducted in an attempt to remove most of theiron and gangue materials in the concentrates. Theoptimum main- belt speed was found to be approximately6 m/min at a current of 1 A. These variables gave aCr203 content of 54,6 per cent, with a recovery of 69 percent and a er/Fe ratio of 3,4. Smaller particle sizesincreased the Cr203 content of the concentrate. Inchlorination experiments to increase the er/Fe ratio,various amounts of carbon were added to reduce the ironand chromium oxides and chlorine gas was allowed toreact with the ores40. Preliminary tests indicated thatfine-grained ore chlorinated most satisfactorily, and thata gas flowrate of 3 l/g per hour was optimum. Fourtimes the stoichiometric carbon requirement was foundto provide the maximum degree of chlorination. Highertemperatures (up to 800°C) and times of reaction (up to5 h) increased the degree of chlorination.

Laboratory-scale beneficiation work on the secondEgyptian chromite was directed towards depression ofthe serpentine and trapping of magnetite in a flotationcell. Deslimed feed was added to the magnetoflotationapparatus (a flotation cell positioned between the polesof a C-shaped magnet). Selective flotation of chromiteusing dilute hydrofluoric acid to obtain a pH of 5,5,Na2SiFo as a serpentine depressant, oleic acid in ker~seneas a collector, and pine oil as a frother resulted m aconcentrate with a er/Fe ratio of 3,2 and a Cr203

TABLE VIIIANALYSES OF EGYPTIAN CHROMITE ORES, IN MASS PERCENTAGES

Barramyia Ore

Sample 1 Sample 2

38,25,69,11,94,1

21,018,5

35,55,3

21,31,35,8

10,59,0

* Total iron given as FeO


recovery of approximately 57 per cent. A er/Fe ratio ofonly 2,5 was obtained without use of the magnetic field42.However, high magnetic field intensities are detrimentalfor grade and recovery.

Finland

The chromites of the Kemi district in Finland compriseeight ore bodies in association with a basic-ultra basicplutonic intrusion. Chromium in the ores typically rangesfrom 16,8 to 20,8 per cent, and iron ranges from 1l,1 to12,8 per ceut. In geueral, the er/Fe ratio varies from1,52 to 1,65, making these chemical or refractory-gradechromites. Mineralogically, the chromites are associatedwith serpentines, tremolite, or talc and carbonates.

Lukkarinen4a summarized the results of an extensiveseries of tests to establish optimum conditions for theconcentrating of Finnish ores. Mixed results were ob-tained during flotation experiments owing to the altera-tion of gangue minerals, which produces similar surfacecharacteristics for different minerals. Rapid changes inore grade and magnetic layers on fractured chromitesurfaces made flotation control difficult, but gravity sepa-ration was effective in concentrating the chromite andmagnetite, most of which is altered. Low-intensity mag-netic separation caused the magnetite layer on chromiteparticles to be drawn into the concentrate, thus decrea-sing the chromite recovery. Fine-grained magnetiteattached to silicate minerals reported to the concentratein high-intensity magnetic separation, but the chromiterecovery was improved by completion of the magneticseparation with gravity methods. The resulting middlingcould be ground to remove silicates from the chromite.It was found that each ore body requires its own concen-trating procedures for maximum recovery.

Philippines

Refractory chromites are mined from the Masinlocmine in Zambales Province in northern Luzon44, whichis one of the two largest producing mines in the Philip-pines. (The other produces metallurgical-grade chromite.)The chromites are located in a belt of peridotitic rockadjacent to a north-northeast- trending belt of olivine-rich gabbroic rock, which has been serpentinized to someextent. The analysis, in mass (percentages), of the oreis as follows44:

Cr20a Fe20a FeO er/Fe Al2Oa MgO+CaO SiO236,5 4,75 10,8 2T 31,0 17,2 0,85

Australia

A chemical-grade high-iron chromite from Australiacontaining enstatite (MgSiOa), pyrope (MgaAI2SiaO12)'and hohmannite [Fe2(OH)2(SO 4)2 7H2O] was the subject

TABLE IXANALYSIS OF AUSTRALIAN CHROMITE IN MASS PERCENTAGES'S

Cr.O. ........Total Fe (as FeO) ..........CaO . . . . . . . . . . . .MgO ,...........AI.O. ,

SiO2 . . . . . . . . . . . . . . . . .er/Fe ,.....

49,424,7

0,36,59,59,21,8

of hydrochlorination tests by Maude and Sale45. Achemical analysis of this ore is given in Table IX.

Experiments on the reduction of carbon at relativelylow temperatures (approximately 700 to 1O60°C) yieldedonly a 17 per cent reduction after 16 hours when 32,2 percent carbon by mass was added45. At 1O60°C, considerablechlorination of the chromite occurred when 49 per centcalcium chloride dihydrate (by mass) was added. At thattemperature, nearly all the iron and 22 per cent of thechromium were removed from the ore. Higher iron andchromium yields than those obtained in experimentswhere no carbon had been added were found to bepossiblea2. Increases in the er/Fe ratio can be obtainedby chlorination without carbon, bu~ carbon is requiredif both the chromium and the iron are to be chlorinated.The decomposition of calcium chloride dihydrate togenerate HCI gas resulted in lower chromium yields thanthose achieved with pure anhydrous HCI, owing to theformation of ft-calcium chromium oxide from the CaOdecomposed from the calcium chloride dihydrate.

Other Countries

Other smaller, lower-grade chromite deposits occur inGreenland, Colombia, Brazil, the Malagasy Republic,and Japan.

,I

Recovery of Chromium

Both surface and underground mining techniques areused to obtain raw chromite ore. Most of the podiformor lens-like deposits are surface mined, whereas thestratiform (layered) deposits such as those in SouthAfrica and Zimbabwe are usually mined by undergroundmethods. Specific mining techniques vary widely depen-ding on the locality46.

Usually, chromite is cleaned by hand-sorting, washing,and screening, or by gravity concentration. Concentratesare prepared from fines or crushed lower-grade ore usingjigs, tables, spirals, or magnetic separators. Gravity-separation methods predominate over flotation tech-niques.

The successful beneficiation of lower-grade chromiteores depends on the ore mineralogy, the size requirementsof the concentrates, and the chemical composition of theore. Gangue materials can be separated by physicalmethods such as gravity concentration (mainly tablingand jigging). Low-intensity magnetic separation can beused to reject the magnetite, but physical methods cannotbe used to increase the Cr /Fe ratio or to separate the chro-mites that are present in fine intergrowths with othermaterials. Upgrading to yield higher er/Fe ratios requirespyrometallurgical and/or hydrometallurgical treatment.

Pyrometallurgical techniques are used to producechromium alloys or additives for the metallurgicalindustry. Chromite ore, fluxes, and reducing agents areusually smelted in ~ctric-arc furnaces, adjustmentsbeing made to the composition of the charge and theprocess variables to produce the desired ferrochromiumproducts.

Either electrochemical or pyrometallurgical processesare used to produce chromium metal. In the electro-chemical process, a solution of ferrochromium is used toproduce a purified chromium-alum solution, which is

AUGUST 1982 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY214

Cr.O. Concentrate Cr.O. inreporting to Cr/Fe concentrate

concentrate % % (by mass)

! 22 2,35 51,1: 64 2,30 51,0

58 1,96 52,934 2,24 49,745 2,33 49,076 2,25 50,684 1,49 40,789 1,22 40,081 1,21 37,869 1,32 38,569 1,40 40,182 2,58 48,074 1,69 33,868 1,50 37,6

electrolysed to chromium metal. The pyrometallurgicalprocess involves the aluminothermic reduction of pureCr20a.

A pyrometallurgical-hydrometallurgical process, whichincludes the roasting of chromite with sodium carbonateand lime, followed by water leaching to remove sodiumdichromate and sodium sulphate, is often used. The leachliquor is purified, the sodium sulphate is removed, andthe purified sodium dichromate solution is crystallized,the product being used either directly or to produce avariety of chromium chemicals.

Conventional crushing equipment is used to size-reducechromite for refractory applications. Screening tovarious particle ranges follows, and specific quantitiesof these ranges are blended with magnesite and pressedinto suitable refractory shapes.Gravity Concentration

Crushing and grinding of chromite ores often result inthe production of excessive amounts of fines owing to thebrittle, fractured nature of the ore. These fines can berecovered only with difficulty by gravity processes.Massive and disseminated ores require the use of tables,jigs, and spirals for effective concentration.

The D.S. Bureau of Mines has for many years conduc-ted extensive tabling tests on lower-grade chromite ores.The early work involved crushingcand grinding of theores to minus 28 mesh, hydraulic sizing of the ore, andtabling47,48. The results for ores from California, Mon-tana, and Oregon are summarized in Table X. Magneticseparation of the gravity concentrates from the table feedwas not satisfactory and failed to increase the Cr/Feratio. Chromium was lost primarily in a slime fractionthat could not be beneficiated by either gravity or flota-tion methods. For the Montana ores, the degree of chro-mium recovery in the concentrate from the table feedwas often inversely proportional to the grade.

Further research on Seiad Creek chromites showed thata considerable amount of chromium was present in themiddling fractions after tabling8. Although the chromitewas essentially liberated from the gangue materials,

significant amounts of either concentrates or tailingscould not be prepared by repeated tabling. The chro-mium recoveries from tabling methods alone wererelatively low (Table X).

The chromium recoveries for Stillwater chromite werehigher from tabling methods (Table X), but significantamounts of chromium remained in the middling frac-tion49. Tabling with middling re treatment representedone of the two most satisfactory concentrating schemes.

Tabling was found to be effective on other ores con-taining serpentine gangue materials owing to the differen-ces in size between the chromite and the serpentine par-ticles41. Complete slime separation was also accomplished.Tabling is often regarded as an effective way of removinggangue materials.

The D.S. Bureau of Mines conducted jigging experi-ments on several samples of domestic chromites. Thejigging of a plus 20-mesh fraction of a California orecontaining 18,1 per cent Cr20a by mass resulted47 in arecovery of 77 per cent of the chromite in the productwith 43,5 per cent Cr20a, 8,9 per cent SiO2, and a Cr/Feratio of 2,38. However, the liberation at that size wasnot sufficiently satisfactory to produce a low-silicaproduct. Jigging could be an appropriate coarse bene-ficiation technique for other ores, such as those from theStillwater Complex, in combination with sink-floatmethods.

Panning was used to separate metaIIics from partiallyreduced chromite ores that had been ground to minus270 mesh18. Poor results were obtained, possibly dueeither to the lack of separation of the metaIIics or to theextremely fine grain size.

Heavy-liquid separations on chromite ore from SeiadLake sized to plus 100 mesh showed that consider-able coarse gangue can be rejected at a solution densityof 3,32 or higher without excessive chromium losseswhen thallous formate solutions are used as the separa-tion media8. Typical sink products contained 35,0 percent Cr20a by mass and 86 per cent of the chromium fedto the unit.

TABLE XRESULTS FROM THE TABLING OF CHROMITE ORES

-~----

Ore Ore sizemesh

I Cr.O. inore %

(by mass)

Seiad Creek, California -20-20-28

-48 to -200-48 to -200

-65-28-28-35-35-48

-20 to -100-28 to -100-48 to -100

. NANA

9,120,018,3NA9,9522,813,58,047,7528,824,520,9

. . . !

Stillwater Complex, Montana.

John Day, Oregon. . . . .

NA = Not available.* Wet table.t Air table.

Reference

47*47t46

55

47*4846464848464646


Magnetic SeparationMagnetic separation was not as satisfactory as other

methods such as gravity separation and flotation for therecovery of chromium from California chromites8. Thechromium recoveries did not exceed 62 per cent, and themagnetic concentrates graded from 41 to 42 per centCr203 by mass. The limited chromite recovery may bedue to the presence of a magnetite layer on the chromiteparticles that draws the chromite into the concentratorof a low-intensity separator. However, controlled mag-netic separation can be used on table or flotation concen-trates to reject the magnetite and improve the er/Feratio. Magnetic separation has also been used to eliminatemost of the iron and gangue materials in table concen-trates41. Slower main-belt speeds, magnetic fieldstrengths of 1 A, and finer particle sizes improved theer/Fe ratios. Recoveries of up to 80 per cent chromium,er/Fe ratios of 3,7, and concentrates containing up to68 per cent Cr203 by mass were realized.Electrostatic Separation

Electrostatic separation at 15000 to 20000 V produceda high-grade chromite concentrate8 containing 54 percent Cr203 by mass with a er/Fe ratio of 2,42. However,the chromium recovery was only 58 per cent owing to thefine material that was not treated and that was maskedby adhering dust, so precluding selective separation.Combinations of tabling and electrostatic separation, orelectrostatic separation and flotation, failed to improvethe chromium recoveries.

Comparative electrostatic-separation experiments onsized and unsized material from Montana indicate thatsizing can improve the chromium recovery49. High-tension electrostatic separation of a tabling concentrateresulted in recoveries of 86 per cent and a CraO3 contentof 42 per cent by mass. Ores that contain considerableserpentine do not respond well to electrostatic separationbecause serpentine can respond as a conductor. Someimprovement resulted when electrostatic separation wasconducted on tabling concentrates.Flotation

Although gravity methods are well known and widelyused for the concentration of chromites, such techniquesfail to recover the size fractions below approximately100 /Lm. Recovery from the fine size fractions is particu-larly important in finely disseminated ores. Flotationoffers an alternative concentration process for the sepa-ration of the fine materials and the avoidance of chro-mium losses. In general, earlier results of flotationof lower-grade chromite ores were inferior tothose obtained by tabling. Flotation required fine grin-ding, which necessitated greater slime loss and carefulreagent control. Slimes were depressed by a variety ofmaterials, and the use of a fluoride ion with an anionic(fatty acid) collector for the selective flotation of chro-mite ores is effective for sands, residues from gravityplants, and other deslimed ores 5°. However, fine-graineddisseminated ores do not respond well to this technique.It was found that additional dispersing depressants wererequired for the gangue materials. Some ions such asiron and lead activated the chromite for flotation.Chromite could not be floated below pH 6, the prevention offlotation of the gangue materials being the major problem.

Tabling of flotation concentrates improved the results, buta recovery of only 55 per cent in a product containing 37per cent Cr203 by mass with a er/Fe ratio of 0,89 wasobtained from one Montana ore47. With an ore from theStillwater Complex, the recoveries were improved to89 per cent, which was lower than those obtained bytabling treatments alone. The concentrates obtainedusing the most efficient size ranges contained up to 40per cent Cr203 by mass and had a er/Fe ratio of 1,23.

Further studies by the U.S. Bureau of Mines showedthat lower-grade chromites could be selectively concen-trated by flotation in the presence of slimes. Precon-ditioning with fuel oil permitted flotation of the chromitefrom the flocculated siliceous-gangue slimes. Recoveriesof up to 91 per cent were realized with products contain-ing up to 45 per cent Cr203 by mass8. The er/Fe ratiosranged from 2,17 to 2,46. Oleic acid was used for flota-tion. Sodium fluoride and sulphuric acid yielded betterresults than dilute hydrofluoric acid when these reagentswere added to free the 'floes' from the siliceous gangue.

Flotation results have shown that the compositionof the chromite phase in the ore affects the grade of theconcentrate more than does the grade or the compositionof the ore. This was particularly evident in work conduc-ted on chromite ores in the north-western UnitedStates51 in which cheaper collectors than oleic acid wereinvestigated. Taggart and Arbiter52 showed that fuel oilalone does not collect in many cases, and that fatty acidalone results in non-selective collection. The use of bothreagents allows a neutral oil film to support a weaklyanchored fatty acid. The recoveries ranged from 78 to91 per cent, with concentrates grading from 36 to 43 percent Cr203 by mass. For the concentrates derived fromnorth-western ores, the er/Fe ratios averaged 2,41. Thistechnique was also used on Montana ores, which yieldedconcentrates ranging from 24 to 38 per cent CraO3 bymass, with recoveries from 65 to 89 per cent.

One problem in the development of flotation techni-ques for chromite ores is the variability of the oresthemselves. A process developed for one ore does notusually yield optimum results for another ore of differentorigin. It is thought that differences in the compositionof the gangue materials, as well as in the composition ofthe chromium spinels, are responsible for these varia-tions in flotation behaviour. Fundamental research isrequired to alleviate this problem. Investigations involv-ing the flotation of a pure chromite and a pure serpen-tine using sodium oleate as the collector showed 53 thatthe chromite can be collected at collector concentrationsof between 0,3 and 1000 mg/I. Effective flotation occurswhen the collector concentration is between 10 and 100mg/l at pH values between 3 and 11. Sodium oleatefloats serpentine poorly, but appears to be an effectiveseparator for chromite and serpentine. Metal ions suchas Mg2+, Ca2+, and Fe3+ depress chromite and do notinfluence serpentine, but the depression of chromite byMg2+ depends on the anions in the solution. The preci-pitation or chelation of such potentially harmful ions isrequired for selective flotation of chromite. Othercollectors such as alizarin red S or hexametaphosphatein acid pulps, or sodium silicate or carbonymethyl cellu-lose in alkaline media, offer no selectivity42. The best

"

iI

,1

1'(

~


depressant for serpentines in an acid circuit is N a2Si]'6at pH 5,5 with oleic acid as collector. The grade of chro-mite can be improved by the use of a magnetic field, buthigh field intensities are detrimental to grade andrecovery.

Among the various ions that can be present in the pulp,A}3+ ions exert the most influence on the flotation beha-

viour of refractory-grade chromites. These ions causedepression in the pH range 4,5 to 8 and activationbetween pH 10 and 12. The properties of the mineralsapparently do not affect depression, whereas activationis influenced by the surface characteristics of theminera}26.

Amines have been used as collectors in the flotation ofchromites containing olivine and serpentine 54,55. Itwas found that serpentine could be floated in the pHrange 3 to 12, olivine in neutral solutions, and chromitein acid and alkaline solutions. Amines with 8 to 14carbon atoms were the optimum collectors. Improvedflotation of chromite in the second acid step resultedfrom extensive conditioning of the pulp in an alkalineenvironment. Morawietz56 found amines with 12 to 18carbon atoms to be effective collectors for a pure chromite.Regulators including HCl, NaOH, H2SO

4' and H3PO 4could be used in different pH ranges. There is evidenceof an electrostatic interaction between collector ions andmineral surfaces. ]'or some chromite ores, sodiumfluorosilicate was found to be an effective regulator57.Abid058 showed that hydrofluoric acid also has goodselectivity in the amine flotation of chromite, in contrastto sodium fluorosilicate. Again, the degree to which agiven fluoride ion influences the floatability of chromitedepends on the source and composition of the mineralphase.

Sommerlatte59 mentions a flotation plant having acapacity of 10 000 tons of chromite concentrate per monththat yielded a recovery of only 55 per cent and a concen-trate with 54 per cent Cr203 by mass. Tall oil, sulphatedkerosene, fuel oil, and sulphuric acid were used 59 toreach a pulp pH of 6. Organic regulators and selectiveflocculants facilitated the flotation without desliming.

The flotation of chromite ores should be conducted inalkaline conditions with a possible second upgrading inacid solutions, owing to a high consumption of acid.H ydrometallurgy

For the production of ferrochromium, the Cr/]'e ratioof the starting material should be as high as possible.Conventional methods of beneficiation such as gravityconcentration and flotation can increase the metal con-tent of chromite ores by removing gangue materials, butthe Cr/]'e ratio is difficult to change since it depends onthe spinel composition. To increase the er/Fe ratio, ironmust be removed by structural dissociation of the spine!.This can be accomplished in two ways: by selectivereduction of the iron oxides with carbon, gases, or chlo-rine at temperatures below 1350°C, followed by leachingof the metallic iron with dilute sulphuric acid; or byselective chlorination of the iron oxides using a reducingagent with chlorine or hydrochloric acid. Gaseous reduc-tion with or without a prior oxidative roasting step hasalso been evaluated, and solvent extraction techniqueshave been employed for the recovery of chromic oxide.

Chemical UpgradingEarly D.S. and British patents described the chlorina-

tion of chromium-bearing materials. In one60, iron asferric chloride is separated by the use of a gas containingCO, S2Cl2, and Cl2 at temperatures ranging from 600 to900°C according to the reaction

2FeO.Cr203 + 3 Cl2 = 2 FeCla + 2 Cr203 .(1)In other patents61, hydrochloric acid with excess hydro-gen is used to upgrade chromites according to thereactionFeO.Cr203 + 2 HCl = FeCl2 + H2O + Cr203 . .(2)Khundkar and Talukdar62 used CCl4 to chlorinate chro-mite ores at temperatures between 700 and 800°C accor-ding to the reaction4 FeO.Cr203+3 CC14=4 FeC13+4 Cr203+3 CO+0,5 (3)Using this method, they chlorinated 80 per cent of theiron and 30 per cent of the chromium in 2 h, and increasedthe er/Fe ratio more than threefold. Subsequent separa-tion of the chlorides is required.

Reducing agents such as carbon increase the degree ofchlorination of chromites, depending on the amount ofstoichiometric carbon used, and the presence of carbonmay cause incipient reduction of the oxides. Generally,lower oxides are chlorinated more readily. Iron oxidesin the ore chlorinate more readily than chromium oxides,and from four to five times the stoichiometric carbonrequirements are necessary to completely chlorinate thechromites4°. The use of this amount of carbon requiresa temperature of approximately 800°C for maximumchlorination. Periods from I to 5 h exert a minimal in-fluence on the degree of chlorination. The overall chlori-nation reaction can be expressed asFeO. Cr203+4 C+3 CI2=FeCI2+2 CrC12+4 CO (4)Carbides and/or COCl2 can also be formed during thechlorination process. The phosgene can form at lowertemperatures and decompose into CO and Cl2 at 800°Cor above, which may result in less chlorination than hasbeen observed at 900 to 1O00°C. The formation of car-bides would also result in more difficult chlorinationconditions.

Pokorny63 has shown that complete chlorination of theiron and chromium constituents in chromites can beaccomplished with HCl or Cl2 at 800°C if the chromiteis briquetted with a mixture of coke, pitch, and tar.Maier64 used a stepwise chlorination process withbriquettes of ore and carbon reductant. At 700°C, CrCl3formed, which made the briquettes very dense, and at800°C, some CrCl3 began to volatilize and CrCl2 appeared.Chlorination proceeded rapidly at temperatures between900 and 1O00°C. Chlorination of chromite concentratesby a fluosolids-reductive chlorination technique usingcoke, CO, and fluorspar increased the er/Fe ratios by upto ten times32. In general, the iron and chromium oxidesare chlorinated in proportion to their molar ratio.Chlorination of iron oxide is favoured over chromiumoxide especially when Al2O3 is present, but also in thepresence of Mg065. Maude and Sale45 showed that hydro-chlorination of chromite ore with carbon can be achievedat temperatures approximating 1O00°C. The HCl gaswas generated by the thermal decomposition of calciumchloride dehydrate. This reaction produced CaO, whichreacted with the Cr203 in the chromite to produce


f3-calcium chromium oxide. The chromium yields weretherefore lower than those obtained with pure anhydrousHCI.Pyrometallurgical- H ydrometallurgical Treatment

Chromite ores have been roasted with Na2C03 to formwater-soluble sodium chromates66,67. The impuritiescan be precipitated by adjustment of the pH, andNa2CrO 4 can be recovered66 at pH 6, or Na2Cr2O7 at pH3. The chromates can be reacted with carbon to formCr203 for further reduction, and Na2C03 can be recoveredfor recycling.

Chromitc ores can be processed with either an alkalior an acid treatment. The alkali process consists of theproduction of sodium chromate or dichromate and thesubsequent reduction to Cr203 for blending withuntreated ores28. In the acid process, pressure leachingof chromite ores with H2SO4 yields a basic chromiumsulphate that can be decomposed to Cr203' Chromiumalso can be obtained by H2SO4 leaching in the presenceof chromic oxide and subsequent electrolytic oxidationof the chromium sulphate to chromic acid, which can beseparated by fractional crystallization. The leaching oflower-grade chromite ores produces chromic acid, whichcan be electrolysed to produce electrolytic chromium.

Early work by the V.S. Bureau of Mines focused on theselective reduction of iron in chromite ores with carbon,which subsequently was removed by acid leaching,leaving a chromium-containing residue 9. Laboratory-scale investigations showed that finely divided chromite(minus 28 mesh), carbon, and a minimum temperatureof 1200°C are required. The use of excess carbon over thestoichiometric amounts required to reduce all the iron isnot recommended, although excess carbon increases theoxidation resistance of the products68,69. Pilot-scalework in an oil-fired kiln designed to minimize trouble-some temperature gradients and premature reductantoxidation produced a calcine at 1395°C, which could beleached at 85 to 90°C with 8 per cent H2SO4 to yield aresidue with a er/Fe ratio higher than 3.

Harris7° continued this approach and showed that anoff-grade chromite could be reduced by the use of excessreductant, with the selective leaching action dependenton the formation of carbide. At lower temperatures suchas 1150°C, Cr3C2 is formed, together with iron or ironcarbide. Acid leaching dissolves the iron and iron car-bide, and the Cr3C2 and unreduced chromite remain inthe residue. At higher temperatures (1350°C), Cr7C3 isformed. This carbide is acid-soluble and causes chro-mium losses in the residue. Various leaching agents weretested; the results showed ferric sulphate to be an effec-tive substitute, with the advantages of closed-cycleleaching and a minimal disposal problem. However, thebuild-up of aluminium and magnesium salts in the elec-trolyte might pose problems. Fluxes added prior to roast-ing, such as borates or carbonates, increased the er/Feratios by increasing the rate and extent of reduction, butthe chromium losses were excessive. Fine grinding andpelletization were beneficial by decreasing the dust losses,easing the density, and improving the handlingcharacteristics. A treatment involving sulphating roast-ing and leaching with ammonium sulphate was not con-sidered economic owing to excessive losses of ammonia

II

and the required crystallization steps for separation.McRae conducted experiments in which coal was used

as the reductant at 1200 and 1300°C to reduce a chro-mite18 and the metallic phase was leached with 10 percent H2SO4' The chromium recovery decreased as theCr/Fe ratio increased in the residue. Smaller particlesizes yielded higher er/Fe ratios. Other leaching techni-ques such as rusting and the use of a ferric salt solutionwere attempted. Rusting with water at pH 4, brine, andseawater was unsuccessful, possibly owing to the effectsof the chromium content of the metallic phase. Theferric salt solution separated some of the iron, but furtherresearch is required.

Various gases have been used to selectively reducechromites followed by acid leaching 70. Carbon mon-oxide gas reduced the chromites at temperatures rangingfrom 1130 to 1250°C, yielding a maximum er/Fe ratio of3,38. A considerable amount of chromium as well as ironwas reduced. However, the percentage of gas used in thereaction is Iow. When methane gas was used as a reduc-tant at 1190°C, a maximum Cr/Fe ratio of approximately2,5 was obtained. Longer roasting resulted in greaterchromium and iron losses. The use of gaseous mixtures ofS02 and air in partial equilibrium with S03 yielded onlya superficial sulphation of the chromite. The best resultswere obtained at 470°C. StilI less suIphation occurred attemperatures above 860°C.

Another process that has been investigated is roastingfollowed by gaseous reduction. Experiments usingoxygen and wood charcoal to form CO were unsatisfac-tory 71. However, the optimum conditions were found toinclude pre-oxidation with oxygen at 1200°C, followed byreduction roasting with town gas at 1175°C. Both theCr/Fe ratio and the chromium recovery were excellent(6,4:1 and 90,9 per cent respectively). The pre-oxidationstep precludes a maximum particle-size limitation. Thepre-oxidation roast oxidizes the FeO to a hematite-typephase, which breaks the iron-chromium oxide bonds.Reduction then forms carbides throughout the grainsrather than on the surface only. Evidently, the hematite-type phase enhances the permeability to the reducinggas and the iron oxide is reduced to iron carbide. Theenriched chromium-containing phase is less susceptibleto reduction, and the chromium recovery is enhanced.

In most of these investigations, the ultimate goal wasto prepare suitable material for the production of ferro-chromium. In some applications in the chemical industry,however, chromic oxide is the desired product. Oneprocess involves the fusion of finely ground chromite orewith sodium carbonate at 900°C for 16 hours 72. Theresulting sodium chromate was leached in boiling wateror in a molten mixture of NaCI and Na20. 2 B2O3' and

an effective separation of chromium was achieved fromiron and vanadium by use of the latter method. The saltphase, which contained most of the chromium, wasmelted under reducing conditions and reacted with boricoxide and sodium carbonate. Chromic oxide formed as afine solid dispersion throughout the oxide phase. Thistreatment further decreased the iron. The borate glasscontaining the chromic oxide product was crushed, boiledin water, and filtered, the boric oxide being recycled.When borate glass from the first step was crushed, boiled,

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and filtered, the residue was Cr203 that could be com-bined.Electrornetallurgy

In the 1940s, the Burcau of Mines developed a newcommercial method for the production of elcctrolyticchromium from lower-grade chromites. This techniquehas been the basis for nearly all the electrolytic chromiummetal produced in the United States4. Small lots ofroom-temperature ductile chromium used for metalhu'-gical and medical research were produced by anothertechnique developed by the Bureau of Mines. Ductilewire for permanently irradiated implants in the treat-ment of cancer was produced by this method. Chromiummetal produced electrolytically in fused salt baths hasbeen used for basic research applications4.Pyrornetallurgy

In 1979, 61 per cent of the chromite consumed in theU.S.A. was used in the metallurgical industry for theproduction of ferro-alloys that are used primarily in themanufacture of stainless steels 73. Chromium alloys oradditi,'es are produced by pyrometallurgical techniques,which makes pyrometallurgical processing one of themost important methods for the recovery of chromiumvalues. Traditionally, chromite ore, fluxes, and reducingagents are smeltcd in three-phase submerged-arc furnaces.Chromite fincs should be agglomerated prior to beingcharged into the furnace. More recently, various methodsfor prereduction of the chromites prior to melting haveattracted considerable attention.

Pelletizing and briquetting are the two major techni-ques for the agglomeration of finely divided chromiteores, which represent the majority of the ores. Lankesand Boehm 74have described a method for the productionof pellets used as charge material for the travellinggratc-rotary kiln LEPOL process. Bentonitc (1 per cent)is used as a binder for the pelletization, which is conduc-ted on a disc pelletizer. During the operation of theLEPOL process, the pellets are strengthened in the rotarykiln at temperatures ranging from 1300 to 1500°C.Excellent compressive strengths were obtained, and thepellets were judged to be completely satisfactory forsmelting purposes.

The Japanese developed a similar process 75 using anannular vertical kiln for the firing of pellets containing2,5 to 3 per cent bentonite by mass at 1150°C. Theadvantages claimed are that less area is required than fora rotary kiln, the thermal efficiency is better, and noseparatc preheating equipment is required. Pelletizingoffers the following advantages over the use of lumpyores: variations in the composition of the charge can behandled wcll, and the permeability and reactivity arebetter than those of lumpy ores.

South African investigators showed that additions ofhydraulic bindcrs of up to 10 per cent by mass for thepelletizing of Transvaal chromite fines resulted in pelletswith very Iow strengths 76. A bentonite binder (up to 2,5per cent by mass) was preferred. The addition of a redu-cing agent in the form of char did not significantly alterthe pelletizing characteristics of the ore. The firingtemperature should be below the softening point of thechromite pellets.

Because pelletization requires very fine grinding and

subsequent induration at high temperatures, briquettingthat involves a reductant and a binder, such as calciumlignosulphate, molasses and lime, or mixtures of thesematerials, has been investigated. Two South Africancompanies now prefer briquetting to pelletization 77.

Several techniques have been proposed for the reduc-tion of chromites using gaseous reductants includingcarbon monoxide and mixtures of methane and hydrogen.

Basic investigations have used chromium sesquioxidcwith mixtures of methane and hydrogen, and have shownthat methane is generally an inert component withrespect to the reduction reaction, carbides are formedwhen excess carbon is available, hydrogen exerts a cata-lytic effect on the solid-state reduction reaction with COdecreasing this effect, and reduction reactions occur at100 to 150°C lower in hydrogen than when CO or aninert atmospherc is used 78.Additional work on chromitesusing mixtures of methane and hydrogen showed thatchromite reduction can begin at 800°C whcn methaneproduced by the hydrogenation of carbon is used37. Onlythe iron and chromium oxides are reduced, the othcroxide impurities remaining inert. The presence of theseimpurities decreases the reduction rate. Excess carbonresults in exothermic hydrogenation reactions, whichprovide some of the heat required for single-step reduc-tion. Further investigations5,21 using CO demonstratedthat iron and Cr2C3 are formed up to 1200°C. At highertemperatures, iron forms first, followed by (Cr,Fe)7C3,and the original iron is then transform cd to Fe3C,

Considerable work has been conducted on the solid-state reduction of chromites with carbon. Basic investi-gations79,80 involving the delincation of the kincticsand mechanisms of chromite reduction with solid carbon(graphite) showcd that the initial reaction at 1000 to1050° C resulted in reduction of Fc3+ to Fe2+. From1050 to 1200°C, iron is totally reduced to the metal anda carbide. This is true for chromium if no impurities arepresent. Otherwise, chromium is totally reduccd to acarbide at 1250°C. Huntcr and Paulson 10investigated theease of reduction of four chromites with graphite as afunction of composition in the temperature range 600to 1400°C. Higher tcmperatures yielded a greater degreeof reduction, the degree of reduction being inverselyproportional to the fcrric iron content and directlyproportional to the alumina and ferrous iron levels.Barcza81 further demonstrated that reduction (as repre-sented by mass losses) was proportional to the amountand mineralogical composition of the contained gangue.The mechanism was believed to be a three-dimensionaldiffusion of the reactants through the reaction product.Rankin 5,21 showed that the solid-state reduction ofchromites with graphite yielded phases similar to thoseobtained when chromites were reduced with CO. Nafzi-gel' et al. 7 investigated the solid-state reduction of twochromites with various reductants, and found that theuse of more-reactive reductants such as coal char resultedin a greater degree of reduction. The ease of reductionfor the two chromites was proportional to the amountof contained iron.

The data obtained forof chromites formed thement by the Japanese


the solid-state reductionbasis for the develop-of reduction processes

AUGUST 1982 219

involving shafts or grates and rotary kilns on a pilotscale in the late 1960s. Dried pellets prepared from low-grade chromite fines, a reductant (such as anthracite orcoke), a binder, and a flux were reduced in a rotary kiln,one-half of the kiln being regarded as a preheating area.Approximately 30 per cent of the carbon in the pelletswas lost by combustion. During roasting at 1300 to1435°C, an outer shell of oxides was formed on the pelletsfrom the flux materials in the zone between the pre-heating and reduction areas in the kiln. The exothermicoxidation increased the pellet temperature to a point~mfficiently high for the subsequent reduction reactionto occur (greater than 1275° C). The temperature must becarefully controlled to avoid softening of the pellets andyet be high enough to effect sufficient metallization. Areduction of 60 per cent of the iron and chromium oxideswas achieved in the kiln82. A plant was constructed inSouth Africa to process 120 kt of ferrochromium peryear, at least 25 per cent of the production being expor-ted to Japan83. Further reduction investigations84 inSouth Africa were conducted using char in pellets con-taining a high-iron chromite at temperatures rangingfrom 1l00 to 1400°C. At the higher temperatures, reduc-tion proceeded faster, approximately 35 per cent of theiron being metallized before any significant amount ofchromium was metallized. A maximum of 75 per centreduction was achieved by the addition of 100 per centof the stoichiometric carbon requirement to completelyreduce all the iron and chromium oxides. Recent pilot-scale investigations have shown that off-grade chromitescan be metallized up to 90 per cent in 2 to 3 hours,beginning at approximately 1150°C and ending atapproximately 1350°C. The chromites were pelletizedwith 125 to 150 per cent of the stoichiometric carbonrequirement (as coal char) to reduce all the iron and chro-mium oxides. These pellets were charged into a batchrotary kiln, together with a continuous addition of cokebreeze during the test. The prereduced pellets wereremoved from the kiln under an inert atmosphere, anda layer of coal char was placed on top of the hot pelletsto prevent reoxidation85.

The D.S. Bureau of Mines has conducted extensivesmelting studies on unreduced lower-grade chromitesover the past several decades. Wessel and Rasmussenllprepared ferrochromium with an average Cr/Fe ratio of1,32 by smelting high-iron chromite with a sub-bitumi-nous coal and coal char in an open-top electric-arcfurnace. The average chromium and iron recoverieswere 97 and 94 per cent, respectively. Walsted12 notedthat the er/Fe ratio was not improved, but that suitableferro-alloys could be smelted from these chromites. Theselective reduction of iron from other off-grade chromitesresulted in Cr/Fe ratios greater than 4 in the metal, withCr recoveries of 89 per cent in a chromium-bearing slagl3.Additional research on one chromite ore showed thathigh-carbon ferrochromium containing approximately50 per cent Cr by mass with Cr recoveries of up to 86 percent could be prepared8. However, the consumption ofelectrical energy was high. The consumption of energywas less when the ore was beneficiated. Recent investiga-tions85 have shown that submerged-arc conditions arepreferable to open-bath operations with respect to

chromium recovery, mass of metal, and reductionefficiency. Coal char (100 to 120 per cent of the stoichio-metric carbon requirement for complete reduction) wasgenerally the reductant of choice. Initial slag basicities(defined as (CaO + MgO)jSiO2 on a mass basis) between0,6 and 0,9 and a C/Cr mass ratio between 0,6 and 1,0were recommended. Satisfactory ferrochromium for usein the AOD manufacture of stainless steel can be pro-duced from lower-grade chromites under submerged-arcconditions provided that the operating variables arecontrolled within relatively narrow limits.

Other investigators have smelted chromite concentrates(with Cr/Fe ratios of 1,6 to 1,7) with a limited amountof coke and slagging constituents under submerged-arcconditions. In a two-stage operation, an iron alloy and aslag containing 27 per cent Cr203 by mass were obtainedinitially. In the second stage, the chromium-rich slagwas smelted with appropriate slagging components anda reductant to produce high-carbon ferrochromium con-taining 63 per cent Cr and a slag with only 1,8 per centCr203 by mass. The slag-metal separation was difficultowing to the refractory nature of the first-stage slag 70.A similar scheme has been reported by Mousoulos86, inwhich chromium contained in pig iron (equal to or lessthan 2 per cent Cr by mass) smelted from laterites isremoved during steelmaking, and the ironmaking slag,which contains most of the chromium, is processedfurther. Seetharaman and Abraham27 report that 80 percent of the stoichiometric carbon requirement forcomplete reduction and a slag basicity of 0,9 werepreferred for the first-stage smelting of a low-grade chro-mite ore with a Cr/Fe ratio of 1. The activity of Cr203 inthe slag should be as low as possible, whereas the activityof FeO should be high for optimum separation. A high-carbon ferrochromium was produced during the second-stage smelting operation. High carbon contents87 arefavoured by agglomeration of the ore with finely dividedcarbon, an MgOIAI2O3 ratio in the slag greater than 1,the presence of CaO, and a C/Cr ratio greater than 1. Ahigher carbon content than normal can be expected inthe high-carbon ferrochromium produced from lower-grade ores. Although alternative smelting schemes invol-ving a silicon reductant have been proposed, it has beendemonstrated that the power requirements are higherbecause silicochromium must be produced first and thelime-chromite slag melted88.

In summary, extensive testing has shown that low-grade chromites can be smelted to produce acceptablehigh-carbon ferrochromium. The use of wood wastesprovides distinct advantages in a typical two-stagesmelting operation for the upgrading of lower-gradechromites.

Considerable recent interest is evident in the proces-sing of chromites for the production of high-carbonferrochromium suitable for the production of stainlesssteel. Prereduction of the chromite prior to melting offerssome advantages over direct smelting. Otani and Ichika-wa82 emphasized that, when hot prereduced pellets arecontinuously charged into an electric-arc furnace, theenergy savings can be as high as 500 kW, h per ton ofalloy produced, compared with operations in whichcharging techniques are used. In addition, the coke

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requirements are halved, the total energy consumptionis less, and the chromium recoveries are higher. Furtheradvantages include higher productivity and a smoother,more controllable melting operation in the furnace85.Prereduction also possesses some inherent disadvantagessuch as the necessity for fine grinding and for very closetemperature control to prevent excessive sintering,sensitivity to the carbon balance, and possible increasedsulphur content in the metal 76, 85.

Special AlloysIn addition to high-carbon ferrochromium, which

accounts for the major metallurgical end-use of chro-mium, other metals and alloys can be produced. Theseinclude ferrochromium-silicide, chromium, and low-carbon ferrochromium. A single-stage process for theproduction of ferrochromium-silicide in two carbon-lined submerged-arc furnaces is described by 01ark89,who used lumpy chromium ore, quartz, and coke as thecharge. The alloy contained 39 per cent chromium and4-3 per cent silicon by mass, and a chromium recovery of90 to 93 per cent was realized. However, such alloyscan be produced only from higher-grade chromite ores.

Oohen and Yalcin3 describe a process, in which chro-mite is reduced by chromiun, that they claim to be moreselective in reducing iron oxides; it is exothermic, thusproviding added heat, and very Iow carbon levels can beachieved in the alloys. A minimum temperature of1050°0 was required to achieve total reduction of theiron. Improvements in the Or/Fe ratio were realized, butthe use of chromium as a reducing agent is not attractivecommercially.

A number of other processes have been used to produceIow-carbon ferrochromium. However, with the adventof the AOD process for the production of stainless steelin 1967, the demand for special alloys including low-carbon ferrochromium diminished from 45 per cent of thetotal consumption in 1970 to 12 per cent in 1978. Inaddition, stainless steel can be made in an L.D. converteror in an electric furnace, and the carbon can be removedin a vacuum degassing vessel while oxygen is blown intothe vessel. For these reasons, techniques for the produc-tion of Iow-carbon ferrochromium are not treated inthis paper inasmuch as they have become of academicinterest only.Special Melting Techniques

The energy crisis prompted several investigators toseek alternative methods for the production of ferro-chromium. One of these is plasma technology, which hasthe following advantages: it requires lower specific-energy owing to the higher operating temperatures,which decrease the flux burden and increase the recoveryof heat from off-gases; no agglomeration or preparation ofthe charge is required; and water-cooled electrodes canbe used, instead of the more expensive graphite elec-trodes. Fey and Harvey 90 proposed a process in whichlower-grade chromite ore is charged into a prereductionvessel where some of the iron oxide is selectively reducedand the iron removed. The upgraded chromite is thencarbothermically reduced by a pyrolysed gaseous hydro-carbon such as natural gas at over 1700°0. At Bethle-hem Steel, a reactor has been developed that employs afilm rather than discrete particles entrained in the plasma

gas 76. This process increases the residence time andprovides intimate contact between the oxide and thecarbon reductant. Hamblyn 91 compares the energyrequirements for the production of ferrochromium usingplasma technology with those in submerged electric-arcfurnaces. The production of ferrochromium in a plasmafurnace required less energy (between 13 and 18 per cent)owing to the lower reaction energies, heating of rawmaterials (e.g. fluxes), and higher heat recovery in off-gases during plasma melting. Preheating of the chromitecould recover between 10 and 14 per cent of the theoreti-cal input energy. Moore et al. 92describe their equipmentand results using a laboratory-scale sustained shock-wave plasma reactor to successfully reduce a Iow-gradedomestic chromite in flight. Metallizations of 46 and 7,3per cent for iron and chromium, respectively, wereachieved by the use ofJignite char as the reductant.

Pickles et al.38 developed an extended-arc flash reactorto prepare very high-purity ferrochromium, using coke,coal, coke breeze, or graphite as reductants. The smallscale of their work resulted in a very high consumptionof energy. The temperatures in the plasma zone exceeded2000°0. The advantages ofthis process are that no agglo-meration is necessary, any grade of ore can be used, thereductant can be of any size and resistivity, the electricalefficiency is greater than for conventional smelting in asubmerged electric-arc furnace, the process is simple andnot capital-intensive, and the produced ferrochromiumis pure.

Discussion and Conclusions

Comparison of Recovery TechniquesAlthough this paper has considered a wide variety of

lower-grade chromite ores and concentrates, certaingeneralizations can be made with regard to maximizingthe chromium recovery from these materials. Successfulrecovery depends on the chemical composition, mineral-ogy, and particle size of the material. Physical methodscan be successful in removing some or all of the ganguematerials. For example, gravity-concentration methodscan be used effectively if the density difference betweenthe ore and the gangue is sufficiently large (for example0,8 g/cm3). However, chemical means (hydrometallurgi-cal and/or pyrometallurgical) must be used to improvethe Or/]'e ratio in chromites. The techniques used oftenpresent a compromise between maximum Or203 contentof the product, chromite recovery, Or/Fe ratio, andeconomlCS.

Most lower-grade chromite ores respond to some typeof gravity separation for the removal of gangue materialsprovided that the chromite is not finely intergrown withsuch materials. Finer grinding of a high-iron ore does notnecessarily yield concentrates containing more Or203 orhigher Or/Fe ratios. Ohromite fines are not generallyamenable to gravity methods of separation owing toexcessive chromium losses. High-iron ores containingless Cr2O3 can provide concentrates by gravity methodssuch as tabling that have equal or greater Or/Fe ratiosand Or203 contents than concentrates from ores withhigher Or203 levels. Ohromite ores having significantsize differentials between the chromite and the ganguematerials can be tabled effectively. For more coarsely


sized ores, gravity methods generally provide superiorresults over flotation techniques in the beneficiation ofmost off-grade chromite ores.

Magnetite in the ores can be separated by magneticmeans provided that the magnetite particles have notadhered to the surface of the chromite grains. Relativelylow belt speeds and low magnetic fields are recommended.Electrostatic-separation techniques can be used in com-bination with other beneficiation techniques if the gan-gue contains little or no serpentine.

For fine-grained materials (less than 100 /Lm), flotationis an accepted technique for the beneficiation of chro-mites. Careful control of reagents and pH is required formaximum recovery. The composition of the chromitephase in the ore affects the selection of reagents and thegrade of the concentrate more than it affects the bulkcomposition of the ore. For example, the presence ofAP+ profoundly affects the flotation behaviour of refrac-tory-grade chromites and requires careful pH control. Avariety of flotation reagents have been developed forchromites to depress slimes, to act as dispersants, and toselectively float the chromites. The wide variability ofthe ores precludes the recommendation of optimumreagents. Economics may also play an important part.

In some cases, it should be noted that a specific bene-ficiation technique (such as tabling) will increase theer/Fe ratio, but simultaneously decrease the Cr recoveryand/or the Cr203 content of the concentrate. Evidently,when some of the iron is removed, some chromium isinterlocked with it and a certain amount is lost. Moreselective separation techniques are required.

A summary of the effect of some beneficiation techni-ques on selected ores is presented in Table XI, togetherwith partial analyses of the ores. The recommendedmethod of concentration cannot be correlated well withthe ore composition. Detailed mineralogical studies arenot available.Chernical Upgrading

As has been demonstrated, ore-dressing methods can beused for the removal of gangue materials for upgradingpurposes. However, physical beneficiation of chromiteores cannot increase the er/Fe ratio from that present inthe ore unless the gangue materials contain considerableiron. As no ore-dressing process can change the er/Feratio of the chromite spinel in the ore, some form ofpyrometallurgical and/or hydrometallurgical processinvolving chemical attack is required. Chemical benefi-ciation methods have not been enthusiastically receivedowing to economic considerations.

Accordingly, extensive smelting tests have shown thatit is possible to produce a satisfactory high-carbon ferro-chromium containing higher-than-normal carbon levelsfrom lower-grade chromites. Hogged fuel such as woodwaste and sawdust can be used effectively.

Originally, it was thought that high-carbon ferro-chromium should contain 66 to 72 per cent chromium bymass, which required a er/Fe ratio in the ore of approxi-mately 3. With the advent of new and improved proces-ses for the production of stainless steel, lower grades offerrochromium can be used. Such lower grades (50 to 55per cent chromium by mass) can be smelted directlyfrom lower-grade chromites. Therefore, there is less

apparent need for chemical upgrading of the ore nowthan there was previously. However, chemical upgradingmay be required to offset transportation costs to themarket from remote chromite deposits.

In the past, high-carbon ferrochromium was unsuitablefor the production of high-chromium steels such asaustenitic stainless steels, which represent a large per-centage of the market for ferrochromium. The carbidestended to precipitate at grain boundaries at certaintemperatures, leading to selective corrosion at thosesites and subsequent embrittlement of the alloy. How-ever, with the development of the AOD process, in whichmolten steel is blown with a mixture of argon andoxygen to remove carbon and silicon, the argon decreasesthe partial pressure of carbon monoxide and alters theCr~C equilibrium, resulting in stainless steels havingcarbon contents as low as 0,01 per cent by mass. Anotherdevelopment involves the removal of carbon by vacuumdegassing while oxygen is simultaneously blown in theladle. Therefore, there is essentially no demand forexpensive low-carbon ferrochromium for metallurgicalapplications.

Techniques involving metallic chromium as a reduc-tant do not appear to be a practical means for theproduction of ferrochromium.Pyrornetallurgy

The reduction of lower-grade chromites with carbonis a complex multistep process, primarily owing to cationsubstitution and defects in the spine I structure. Expcri.mental results 7 have indicated that the iron in the chro-mites reduces first at lower temperatures according tothe generalized reactions(Fe, Mg)O' (Cr, Al)203+C-+[Fe] + MgO' (Cr, Al)203+CO

. . . . . . . . (5)

As the iron in the chromites is reduced, reduction of thechromium in the chromites begins at the higher tempera-tures according to the generalized reaction(Fe, Mg)O' (Cr, Al)203+C -+ [Cr] + (Cr, Fe)7C3+

MgO.AI2O3 + COOverall, the reaction can be written(Fe, Mg)O'(Cr, AlhO3+C -+ [Fe, Cr]+(Cr, Fe)7C3+

MgO'AI2O3 + CO ,.. (7)

If carbon monoxide is not an effective reducing agent forchromites37 , 81, its removal from the system should not

affect the rate of reduction. On the other hand, if ]'e3Cis formed as a reaction product, as suggested by ther-modynamics alone, this species can function as aneffective reductant by permitting the transport of car-bon through the carbide to the chromite. Such an inter-mediate carbide is short-lived and rarely appear;;; as areaction product 7. The presence of carbides has a signifi-cant effect on the reduction of oxides containing chro-mIUm.

The question of whether chromite is reduced by asolid-solid reaction or by a gas-solid reaction has receivedconsiderable attention. Barcza showed that a solid-solidreaction was responsible and that carbon was the diffu-sing specIes. It is important to understand the reductionreactions more thoroughly so that high-carbon ferro-chromium can be produced more efficiently 93.

Reduction reactions for the production of high-carbonferrochromium can be plotted on a Pourbaix diagram,

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which relates temperature and oxygen pressures. Athigher temperatures, lower-carbon chromium carbidesare more stable than those containing higher concen-trations of crabon87. Carbidization reactions occur atlower temperatures under reduced total pressures. Atlow temperatures and high oxygen pressures, the stablecondensed phases are Cr203 and carbon. As the tem-perature increases or the oxygen pressure decreases,Cr3C2 and Cr203 become the stable condensed phases 94.Pourbaix diagrams are useful in determining the mini-mum temperature necessary to obtain a metal or specificcarbide by carbothermic reduction at a specified oxygenpressure. For example, chromium can be obtained at aminimum temperature of 1255°C at 10-4 atm. COpressure or 1O40°C at 10-6 atm. CO pressure.

Of equal importance to the efficient reduction of chro-mites by carbon for the production of high-carbonferrochromium is an understanding of the kinetics of therelevant reduction reactions. Early experiments under1 atmosphere pressure conducted by Gel'd and Esin 95

on Cr203 and chromite ores with graphite showed thatthe rate of reduction of the ore was significantly slowerthan that of the Cr203' Impurities retarding the diffusionof the reactive species in the ore were thought to beresponsible. As expected, the reaction rates also dependinversely on the square root of the particle diameterof the ore and reductant. At total pressures of less than1 atm., topochemical mechanisms were believed to de-scribe the reactions in which reduction begins over theentire particle surface simultaneously. Others have foundthat Cr203 in the presence of iron was reduced by aprocess that was diffusion-controlled at carbon contentsof less than 1 per cent by mass, and was controlled bychemical reaction rates at carbon contents above 2 percent by mass. The overall reduction rate of iron andchromium oxides by carbon increases with temperature.However, at constant temperatures, the reaction ratedecelerates owing to a lower overall contact surface fordiffusion of the reducing agent.

UrquharP9 found that the presence of quartz did notaffect the rate of reduction of iron from chromite ore,but increased the rate of reduction of chromium. Solid-state diffusion of the reacting species through a productlayer most nearly described the experimental results.This is considered to be a topochemical reaction. Evident-ly, the diffusion of iron and chromium ions in thechromite spinel is the rate-limiting step.

Nafziger et al.7 found that the reduction may benucleation-controlled, although topochemical controlmay predominate under mildly reducing conditions.Diffusion control was suggested as the rate-determiningstep under mildly reducing conditions.

It is evident from this review that much more researchis required for a full understanding of the behaviourof chromites in conventional beneficiation and smeltingtechniques. This includes a greater understanding ofthe physical properties, as well as of the reactions appli-cable to flotation, roasting, solid-state reduction, andsmelting. Thermochemical and kinetic investigationsare required, and emerging techniques such as prereduc-tion and plasma-arc melting should be applied. Theeffective recovery of chromium also is dependent on a

Tmore thorough knowledge of the spinel structure and thedegree to which cations substitute into this structure.Detailed X-ray crystallographic studies are thereforerequired. A greater understanding of the fundamentalphysical, chemical, and metallurgical reactions in theextraction of chromium will permit a better recoveryfrom lower-grade chromites, including very low-gradematerials such as those found on the south-westerncoast of Oregon in the U.S.A.

~

II

\

References1. Contingency plans for chromium utilization. Report of the

Committee on Contingency Plans for Chromium Utilization,National Materials Advisory Board, National Academy ofSciences, Washington, D.C., Publication NMAB-335, 1978.347 pp.

2. HUNDHAUSEN, R. J. Chromiferous sand deposits in the CoosBay area, Coos County, Oregon. U.S. Bureau of Mines,Report of Investigations 4001, 1947. 18 pp.

3. COHEN, E., and YALCIN, T. The chromothermic reduction ofchromite. INF ACON 74 (First International Congress on Ferro-alloys). Helen Glen, editor. Johannesburg, South AfricanInstitute of Mining and Metallurgy, 1975. pp. 167 - 174.

4. MORNING, L. Chromium. U.S. Bureau of Mines, MineralCommodity Profile, No. 1, 1977. 14 pp.

5. RANKIN, W. J. Solid-state reduction by graphite and carbonmonoxide of chromite from the Bushveld Complex. Randburg,National Institute for Metallurgy, Report no. 1957. 1978.15 pp.

6. KOSTER, J. Studies on the treatment of domestic chromeores. U.S. Bureau of Mines, Report of Investigations 3322,October 1936. 37 pp.

7. NAFZIGER, R. H., TRESS, J. E. and PAIGE, J. 1. Carbother-mic reduction of domestic chromites. Aletall. Trans., SectionB, vol. lOB, no. 1. Mar. 1979. pp. 5 - 14.

8. HUNTER, W. L., and SULLIVAN,G. V. Utilization studies onchromite from Seiad Creek, Calif. U.S. Bureau of Mines,Report of Investigations 5576, 19('0. 37 pp.

9. LLOYD, R. R., GARST, O. C., RAWLES, W. T., SCHLOCKER,J.,DOWDING, E. P., MAHAN, W. M., and FucHsMAN, C. H.Beneficiation of Montana chromite concentrates by roastingand leaching. U.S. Bureau of Mines, Report of Investigations3834, Feb. 1946. 37 pp.

10. HUNTER, W. L., and PAULSON, D. L. Carbon reduction ofchromite. U.S. Bureau of Mines, Report of Investigations6755. 1966. 20 pp.

11. WESSEL, F. W., and RASMUSSEN,R. T. C. Ferrochromiumfrom low grade chromite ores and concentrates. ./. lVIetals,vol. 2, no. 8. Aug. 1950. pp. 984 - 988.

12. WALSTED, J. P. Electric smelting of low-grade chromiteconcentrates. U.S. Bureau of Mines, Report of Investigations5268, Oct. 1956.29 pp.

13. HUNTER, W. L., and BANNING, L. H. Pyrometallurgioalbeneficiation of offgrade chromite and production of ferro-chromium. U.S. Bureau of Mines, Report of Invest'igations6010, 1962. 16 pp.

14. H1JNDHAUSEN, R. J., BANNING, L. H., HARRIS H. M.,and KELLY, H. J. Exploration and utilization studies, JohnDay chromites, Oregon. U.S. Bureau of Mines, Report ofInvestigations 5238, Jul. 1956. 67 pp.

15. HUNTER, D. R. Some enigmas of the Bushveld Complex.Econ. Geol., vol. 71, no. 1. Jan.-Feb. 1976. pp. 229 - 248.

16. CAMERON,E. N., and DESBOROUGH,G. A. Occurrence andcharacteristics of chromite deposits - eastern BushveldComplex. Monograph 4, Economic Geology. H. D. B. Wilson,editor. 1969. pp. 23 - 40.

17. COUSINS, C. A. The Merensky Reef of the Bushveld IgneousComplex. Monograph 4, Economic Geology. H. D. B. Wilson,editor. 1969. pp. 239 - 251.

18. MCRAE,L. B. The upgrading of chromite ore from Morees-burg. Randburg, National Institute for Metallurgy, Reportno. 1513. 1973. 9 pp.

19. URQUHART,R. C. Reactions occurring during the smeltingof high-carbon ferrochromium from Transvaal chromite ores.Randburg, National Institute for Metallurgy, Report no. 1482.1973. 31 pp.

20. DEWAR, K., and SEE, J. B. The influence of carbonaceousreducing agents on the rate of reduction of representativemanganese and chromium ores. Randburg, National Institutefor Metallurgy, Report no. 1968. 1978.25 pp.

21. RANKIN, W. J. Reduction of ehromite by graphite and car-

r

..

I

II

I

224 AUGUST 1982 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND MET ALLUGRY

t

iI

1I

bon monoxide. Trans. Instn lV1in. JYIetall., Section C, vol. 88.Jun. 1979. pp. Cl07-C1l2.

22. URQUHART, R. C., JOCHENS, P. R., and HOWAT, D. D. Alaboratory investigation of the smelting mechanisms asso-ciated with the production of high-carbon ferrochromium.INF ACON 74 (First International Congress on Ji'erro-alloys).Helen Glen, editor, Johannesburg, South African Instituteof Mining and Metallurgy, 1975. pp. 195 - 205.

23. BICHAN, R. Origin ofchromite seams in the Hartley Complexof the Great Dyke, Rhodesia. Monograph 4, EconomicGeology. H. D. B. Wilson, editor. 1969. pp. 95 - 113.

24. COTTERILL,P. The chromite deposits of Selukwe, Rhodesia.Monograph 4, Economic Geology. H. D. B. Wilson, editor.1969. pp. 154 - 186.

25. SMIRNOV,V. I., editor. Ore deposits of the U.S.S.R. Belmont(California), Pitman Publishing Co., 1977. vol. 1, Depositsof chromium, pp. 179 - 236.

26. SOBIERAJ, S. and LASKOWSKI,J. Flotation of chromiteTrans. Instn Min. Metall. Section C, vol. 82, no. 805. Dec.1973. pp. C207 - C213.

27. SEETHARAMAN,S. and ABRAHAM,K. P. Upgradation of Iowgrade chromite ores of Mysore State. J. lnstn Engrs, Min.Metall. Div., vo!. 54, pt MM1. Nov. 1973, pp. 23 - 27.

28. DOWNES, K. W. and MORGAN,D. W. Utilization of low-grade domestic chromite. Ottawa, Department of Mineraland Technical Surveys, Canadian Branch of Mines, Memo-randum Series no. 116, Oct. 1951. 54 pp.

29. BATTY, J. V., MITCHELL, T. F., HAVENS, R., and WELLS,R. R. Beneficiation of chromite ores from western UnitedStates. U.S. Bureau of Mines, Report of Investigations 4079,1947.26 pp.

30. MISRA, R. N., SEN, M. C., and BHATNAGAR,P. P. Trans.Indian Inst. Metals, vo!. 12. 1959. p. 221.

31. MISRA, R. N., SEN, M. C. and BHATNAGAR,P. P. Trans.Indian Inst. Metals, vo!. 15. 1962. p. 227.

32. ATHWALE, A. S., and ALTEKAR, V. A. Chemical beneficia-tion of chromite by selective chlorination in fluidized bed.Trans Indian Inst. .Metals, vo!. 22, no. 2. Jun. 1969. pp. 29-37.

33. RAICEVIC. D. Methods for chromium recovery from ManitobaBird River chromite deposits. Canad. Min. J., vol. 98,no.l1.1977. pp. 66 - 68.

34. DOWNES, K. W., and MoRGAN, D. W. The utilization ofIow-grade domestic chromite. Ottawa, Canada Departmentof Mines and Technical Surveys, Mines Branch, ResearchReport no. MD79, 15th Oct., 1950. 54 pp.

35. BRANDSTATTER,H. G. Production and recovery of metalliccarbides from ores and concentrates. U.S. Patent 3,999,981.28th Dec. 1976.5 pp.

36. SMELLIE, A. M., and BRANDSTATTER,H. G. Production ofmetal carbides. U.S. Patent 4,000,247. Feb. 22nd, 1977. 7 pp.

37. QAYYUM,M. A., and REEVE, D. E. Reduction of chromitesto sponge ferro-chromium in methane-hydrogen mixtures.Canad. Metall. Q., vo!. 15, no. 3. Ju!.-Sep. 1976. pp. 193 - 200.

38. PICKLES, C. A., WANG, S. S., McLEAN, A., ALCOCK,C. B.,and SEGSWORTH,R. S. A new route to stainless steel by thereduction of chrornite ore fines in an extended Arc FlashReactor. Trans. Iron Steel Inst. Japan, vo!. 18, no. 6. 1978.pp. 369 - 382.

39. MOUSOULOS,L., and PAPADOPOLOUS,M. Z. Gravity concen-tration of Troodos chromites, Cyprus. Trans. Instn Min.Metall., Section C, vo!. 85. Jun. 1976. pp. C73 - C77.

40. HUSSEIN, M. K., and EL-BARAWI, K. Study of thechlorina-tion and beneficiation of Egyptian ehromite ores. Trans.Instn Min. Metall., Section C, vo!. 80, no. 772. Mar. 1971.pp. C7 - Cll.

41. YOUSEF, A. A., BOULOS, T. R., KOLTA, G. A., and ABDEL,A. A. L. Concentration of Iow-grade chromite ores for metal-lurgical and chemical purposes. J. Mines, Metals, Fuels,vo!. 18, no. 1. Jan. 1970. pp. 12 - 18.

42. YOUSEF, A. A., BOULOS, T. R., and ARAFA, M. A. Magneticflotation beneficiation of chromite ore. Canad. Metall. Q., vo!.10, no. 4. 1971. pp. 323 - 326.

43. LUKKARINEN, T. The Kemi chromite . review of concentra-ting characteristics and concentration tests. Acta Polytech-nica Scandivanica, Chemistry including Metallurgy Series,no. 136. 1977. 40 pp.

44. BRYNER, L. Ore deposits of the Philippines-an introductionto their geology. Econ. Geol. vo!. 64, no. 6. 1969. pp. 644 . 666.

45. MAUDE, C. R., and SALE, F. R. Hydrochlorination ofchromite with in-situ generation of HCl. Trans. Instn Min.Metall. Section C, vo!. 86. Jun. 1977. pp. C82 . C87.

46. MORNING, L. Chromium. Mineral facts and problems. U.S.Bureau of Mines, Bulletin 667, 1975. pp. 241 - 252.

47. BATTY, J. V., MITCHELL, T. F., HAVENS, R., andWELLS, R. R. Beneficiation of chromite ores from westernUnited States. U.S. Bureau of Mines, Report of Investigations

4079, Jun. 1947.26 pp.48. ENGEL, A. L., SHEDD, E. S., and MORRICE, E. Concentration

tests of California chromite ores. U.S. Bureau of Mines,Report of Investigations 5172, Jan. 1956. 10 pp.

49. SULLIVAN,G. V., and WORKENTINE, G. F. Beneficiatinglow-grade chromites from the Still water Complex, Montana.U.S. Bureau of Mines, Report of Investigations 6448, 1964. 29pp

50. HAVENS, R. Froth flotation of chromite with fluoride. U.S..Patent 2,412,217. 1946. 7 pp.

51. SULLIVAN,G. V., and STICKNEY, W. A. Flotation of PacificNorthwest chromite ores. U.S. Bureau of Mines, Report ofInvestigations 5646, 1960. 13 pp.

52. TAGGART,A. F., and ARBITER, N. Collector coatings in soapflotation. Trans. AIME, vo!. 153. 1943. pp. 500 - 507.

53. SAGHEER, M. Flotation characteristics of chromite andserpentine. Trans. AIME, vo!. 235, no. 1. 1966. pp. 60 - 67.

54. SCHRANZ,H., and GOKSALTIK,S. Aufbereitungsversuche mitVerwachsenen Turkischen Chromerzen (Investigations withvarious Turkish chromium ores). Bergbauwissenschaften, vo!.2. 1955. pp. 103 - 107.

55. GOKSALTIK,S. Die Selecktive Flotation der Kefdag-Chro-merze (Selective flotation of Kefdag chromium ores). Berg-bauwissenschaften, vo!. 3. 1956. pp. 97 - 107.

56. MORAWIETZ,H. J. Ein Beitrag zur Losung des Problems derChromit-Flotation (A contribution to the solution of theproblems of the flotation of chromites). Zeitschriftfur Erzberg-bau Metallhuttenwesen, vo!. 12. 1959. pp. 309 - 321,388 - 393.

57. MALYGIN,B. V.,KUROCHKIN, M. G.,andPOTAPIENKO, V. E.On the effect of water hardness on flotation behavior ofchromite and serpentine. Obgashch. Polez. Iskop, no. 6. 1970.pp. 71 - 75.

58. ABIDO, A. M. Fluoride activation in the flotation of chromite.J. Appl. Chem., vo!. 21. 1971. pp. 19 - 21.

59. SOMMERLATTE,H. Zur Flotation der Chromerzen (On theflotation of chromium ores). Zietschrift fur Erzbergbau Metall-huttenwesen, vo!. 15. 1962. pp. 602 - 604.

60. GAILY, A. Removal of iron from material containing oxides ofboth chromium and iron, such as chromite ores. U.S. Patent2,277,220. 1942.

61. DYSON, W. H., and ATCHINSON,L. British Patents 176,201and 176,729. 28th Oct., 1920.

62. KHUNDKAR, M. H., and TALUKDAR,M. 1. Chlorination ofchromite with CCI. and C.Cl.. Chemistry and Industry,30th Mar. 1963. pp. 530 - 531.

63. POKORNY, E. A. Studies in the chlorination of some complexores-wolframite, vanadinite, and chromite. Extraction andRefining of the Rare Metals: A Symposium. London, Institut-tion of Mining and Metallurgy, 1957. pp. 34 - 48.

64. MAIER, C. G. Sponge chromium. U.S. Bureau of Mines,Bulletin 436, 1942. 109 pp.

65. HUSSEIN, M. K., WINTERHAGER, H., KAMMEL, R., andEL-BARAWI, K. Chlorination behaviour of the main oxidecomponents of chromite ores. Trans. Instn Min. Metall.,Section C, vo!. 83. 1974. pp. C154 - C160.

66. SMELLIE, A. M., and BRANDsTATTER,H. G. Metal carbidesand metals from ores and alloys. Canadian Patent 1,053,910,8th May, 1979. 24 pp.

67. SUGITA, J. Some extractive properties of chromite oreroasted with soda ash. J. Metall. Inst. Japan, vo!. 87, no. 996.Mar. 1971. pp. 173 -178.

68. BOERICKE, F. S. Selective reduction of iron in chromite bymethane-hydrogen and similar gas mixtures. U.S. Bureau ofMines, Report of Investigations 3847, 1946. 14 pp.

69. BOERICKE, F. S., and GANGER, W. M. Effect of. variables inchemical beneficiation of chromite ores. U.S. Bureau of Mines,Report of Investigations 3817,1945.26 pp.

70. HARRIS, D. L. Chemical upgrading of Stillwater chromite.Trans. AIME, vo!. 229, no. 9. 1964. pp. 267 - 281.

71. ANON. Pyrometallurgical beneficiation of chromite. Min.Mag. (Lond.), vo!. 112, no. 2. 1965. pp. 130 - 132.

72. WILLIAMS, D. E., and ROGERS, P. S. Novel high-temperatureroute for production of pure chromic oxide from chromite.Trans. Instn Min. Metall., Section C,vo!.87.1978. pp. C243.C248.

73. THOMSON,A. G. Chromite. Mining annual review. London,Mining Journal, Jun. 1980. pp. 86 - 88.

74. LANKES, K, and BoEHM, W. Experience and operationalresults with a chromium-ore pelletizing plant based on theLEPOL process. INFACON 74 (First International Con-ference on Ferro-alloys). Helen Glen, editor. Johannesburg,South Mrican Institute of Mining and Metallurgy, 1975.pp. 39 - 46.

75. NAGASAWA,S., AOKI, N., and IwABAom, H. Operation ofchrome ore pelletizing plant for large electric furnaces.Proceedings of the 33rd Electric Furnace Conference. New York,AIME, 1976. pp. 83 - 90.


76. SEE, J. B. Development in chromium -1979. J. Metals,vo!. 32. no. 4. 1980. pp. 52 - 55.

77. MORNING, J. L. Chromium. Minerals yearbook 1977. U.S.Bureau of Mines, vo!. 1. pp. 257 - 267.

78. READ, P. J., REEVE, D. A., WALSH, J. H., and REHDER,J. E. Reduction of chromites in methane-hydrogenmixtures - chromium sesquioxide. Canad. Metall. Q., vo!. 3,no. 4. 1974. pp. 587 - 595.

79. LISNYAK, S. S., BELIKOV, A. M., and MoRosov, A. N. Kine-tics and mechanism of chromite reduction by means ofsolid carbon. Sbornik Nauch. Teknn. Tr. Naugh-Issled Inst.Met. Chelyabinsk Sovnarehosa, no. 4, 1961, pp. 3 - II (EnglishTranslation no. 78, National Institute for Metallurgy).

80. MoRozov, A. N., LISNYAK, S. S., and BELIKOV, A. M.Changes in the composition and structure of chrome oresduring heating and reduction. Stal in Eng., no. 2. 1963. pp.ll9 - 122.

81. BARCZA,N. A. Studies of incipient fusion in the systemchromite-MgO-AI.Oa-SiO.-C. National Institute for Metal-lurgy, Report no. 1365, 1972. 14 pp.

82. OTANI, Y., and ICHIKAWA, K. Manufacture and use of pre.reduced chromium-ore pellets. INFACON 74 (First Interna-tional Congress on Ferro-alloys). Helen Glen, editor. Johannes-burg, South African Institute of Mining and Metallurgy,1975. pp. 31 - 37.

83. ICHIKAWA, K. High-carbon ferrochrome route slashes poweruse. Chem. Enyng, vo!. 81, no. 7. 1974. pp. 36 - 37.

84. McRAE, L. B., and SELMER-OLSEN, S. S. Investigation intothe pelletizing and prereduction of Transvaal chromites.Proceedinys of the 2nd International Symposium on Agglomera-tion, Atlanta, Georgia, March 1977. NewYork,AIME, 1977. pp.356 - 377.

85. NAFZIGER, R. H., SANKER, P. E., TRESS, J. E., andMcCuNE, R. A. Prereduction and melting of domesticchromites. Proceedings of the 38th Electric Conference, vo!. 38,

Computer applications

The Institution of Mining and Metallurgy will holdthe Eighteenth International Symposium on ComputerApplications in the Mineral Industries (APCOM) from26th to 30th March, 1984, at the Royal School of Mines,Imperial College, London.

The Symposium, in line with previous conferences, willcover a wide range of computer applications in themining industry, emphasizing techniques ofinterest to thepractising engineer. The special theme of the meeting -

the first to be held in the United Kingdom - will be thecomparison of actual operating experience with originalmodels, forecasts, and feasibility estimates.

Since the APCOM series of conferences was begun in1961 many examples must have accumulated of techni-ques having been put into practice and plans having beenimplemented. It is therefore timely to review the succes-ses and failures of computer applications since computersbecame available as a management tool. Case studies ofthis general theme under any of the subject headingsgiven below are particularly welcome.PapersThe Organizing Committee intends to invite a number ofauthors to present papers, but will welcome additionalsubmissions on the following principal topics:Exploration

Modelling mineral deposition processesRegional comparisons for target selectionComputer-aided selection of target areasExperience with computer analysis of geophysical andgeochemical dataGeostatistics and ore-reserve estimationTechniques of estimation

1981. pp. 27 - 45.86. MoussouLos, L. Extraction of nickel, cobalt, iron, and chro-

mium from laterites - LM and MLAR processes. Paper No.13, Advances in Extractive Metallurgy Symposium, London,England, April 17-20, 1967.

87. DoWNING, J. H. Smelting chrome ore. Geochim. Cosmoehim.Acta, vo!. 39, no. 6-7. 1975. pp. 853 - 856.

88. BLELOCH, W. Reduction of iron-rich chromite fines withferrosilicon and aluminoferrosilicon. Miner. Sei. Engng, vo!.2, no. 1. 1970. pp. 3 - 22.

89. Clark, P. W. The production of ferrosilicon-chromium by thesingle-stage process. INF ACON 74 (First InternationalCongress on Ferro-alloys). Helen Glen, editor. Johannesburg,South African Institute of Mining and Metallurgy, 197.5.pp. 275 - 280.

90. FEY, lVI.G., and HARVEY, F. J. Plasma heating devices in theelectric energy economy. Metals Engng Q., vo!. 16, no. 21976. pp. 27 - 30.

91. HAMBLYN, S. M. L. A review of applications of plasmatechnology with particular reference to ferro-alloy production.National Institute for Metallurgy, Report no. 1895, 1977.34 pp.

92. MooRE, J. J., REm, K. J., and TYLKo, J. K. In-flightplasma reduction of domestic chromite. J. Metals, vo!. 33,no. 8. Aug. 1981. pp. 43 - 49.

93. URQUHART,R. The production of high-carbon ferrochromiumin a submerged-arc furnace. Miner. Sci. Enyny, vo!. 4, no. 4.Oct. 1972. pp. 48 - 65.

94. WORRELL, W. L. A thermodynamic analysis of the Cr-C-O,Mo-C-O, and W-C-O systems. Trans. AIME, vo!. 233, no. 6.Jun. 1965. pp. lln - ll77.

95. GEL'D, P. V., and ESIN, O. A. Kinetics of reduction ofchromium oxide. Zhurnal Prikladnoi Khimii, vo!. 23, no. 12.1950. pp. 1271-1276. (Henry Brutcher Translation No. 2707, IIpp.)

)

Comparison of grade and tonnage estimates withproductionComputer graphic applications

Financial evaluation and planningTechniques for making early estimates of capital andoperating costsComparison of feasibility estimates with final costsand performance

Mine design and operationsSystem modelling and evaluationRock mechanics in design applicationsMaterials-handling studiesInteractive graphics

JJ1ineral processing and metals extractionModelling applications for plant management, designand controlComparison of designed and actual performance

Market analysis and predictionSimulation of commodity marketsSupply, demand and price studiesComparison of predictions with actual outturns

International climate for the mineral industriesAbstracts (250 to 300 words) should be submitted to

the Secretary, The Institution of Mining and Metallurgy,44 Portland Place, London WIN 4BR, England, before1st January, 1983. Completed manuscripts of approvedpapers will be required in September 1983, and a pre-printed volume will be sent to registrants in advance ofthe conference. A range of micros and/or terminals willbe available as an extension of visual aids. Intendingauthors should indicate whether they wish to make useof this facility.


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