Production of ferronickel from nickel laterites in a dc arc furnace

PRODUCTION OF FERRONICKEL FROM NICKEL LATERITESIN A DC-ARC FURNACE

H. Lagendijk and R.T. Jones

Pyrometallurgy Division, Mintek, Private Bag X3015, Randburg, 2125, South Africa

E-mail: [email protected] [email protected]

Nickel – Cobalt 9736th Annual Conference of Metallurgists, Sudbury, Canada, August 1997

ABSTRACT Laterites and other oxidized nickel ores constitute a very important part of world-wide nickel

reserves. In the conventional production of ferronickel from these ores, much fine material is produced whichcannot readily be accommodated directly in existing three-electrode or six-in-line AC furnaces. DC-arcfurnace technology allows ore particles less than 1 mm in size to be treated directly, thereby improving theoverall recovery of nickel without the need for expensive agglomeration techniques. Because of the highmoisture content of laterites, the ores should be dried and calcined before smelting. In order to decrease theenergy consumption further, the ores could also be pre-reduced. The CO-rich off-gas from the furnace couldbe used to supplement the energy requirements, and is also a good reducing agent. Because fine ore particlesare readily treated in a DC-arc furnace, units such as fluidized beds (which require materials of small particlesize) can be used for the pre-treatment stage of the process.

A process has been developed whereby nickel laterites of a wide compositional range can be smelted

in a DC-arc furnace, to produce ferronickel. The flexible operation of a DC-arc furnace (especially its lowerdependence on electrical properties of the slag, because of open-arc operation, in addition to the ability to runat an optimum slag temperature, due to the open-bath mode of operation) allowed for the successfultreatment of ores with a SiO2/MgO ratio between 1.2 and 3.0, as well as ores containing up to 30 per centby mass of iron (which tends to cause slag foaming in a conventional immersed-electrode furnace). A frozenlining can be maintained between the molten bath and the refractory lining, in order to minimize refractorywear (especially at high SiO2 contents). Results of furnace testwork at power levels up to 750 kW arepresented. Tests at the 120 kW furnace scale, together with some preheating in a 300 mm diameter bubblingfluidized bed, are also described.

INTRODUCTION

The name laterite (from the Latin, later, ‘a brick’) is used to describe the weathering product offerruginous rock exposed to strongly oxidizing and leaching conditions, usually in tropical and subtropicalregions. This porous, claylike rock comprises hydrated oxides of iron, aluminium, etc.

The ‘oxidized’ ores of nickel constitute by far the world's largest known reserves of this metal. The

ores include the true laterites (in which the nickel oxide is intimately associated with limonitic iron oxide) andthe silicate ores which often contain the mineral garnierite. These oxidized ores are found in regions of theworld where tropical weathering occurs, or where at least sub-tropical conditions have prevailed in past

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geological times. Mined laterite ore, being of a porous nature, can hold a large content of free moisture, commonly 25

to 30% H2O, although it can contain even 40% or more. In addition to this, combined water, which is notcompletely driven off until a temperature of 700 to 800°C is reached, can amount to up to 15% based on thedry ore weight. Because of the quantity of water present, and because water requires so much energy toevaporate and heat up, it is clear that some pretreatment of the ore (at least drying and calcining) is requiredbefore smelting.

Because of the friable nature of laterites, run-of-mine ore is normally screened as the first step of

ferronickel production. In the screening operation alone, up to 50% undersize particles may be screened outbecause of the difficulties in effecting a physical separation of wet laterite at a smaller size than 6 to 8 mm. This material would then require some form of agglomeration (e.g. sintering or pelletizing) before furtherprocessing. It is conceivable that the material could be dried before screening to say 1 or 2 mm prior tocalcining. However, fines and dust are generated in the drying process, and dust generation in the calciningstep is also increased. The large quantities of dust that are generated in the drying and calcining steps areunsuitable for smelting in a conventional electric-arc furnace. Once again, to utilize these dusts, they wouldneed to be agglomerated at considerable expense.

Large quantities of laterite fines and dust have been stockpiled over the years. These can be used

profitably as a feedstock for the production of ferronickel. The chemical composition of laterite usually varies considerably, and this adds to the difficulty in

processing this material. Normal electric furnaces, operating with a layer (or partial layer) of feed material ontop of the melt, are reportedly difficult to operate with an SiO2/MgO ratio greater than 2.0, or an iron contentof more than 20% by mass, as these conditions can cause operational instability, mainly due to a tendency toslag foaming. These compositional problems can be overcome, to some extent, by blending different ores.

DC laterite process DC-arc furnace technology can be used to provide an economical process for the smelting of nickel-

containing laterite ores and dust, to produce a crude ferronickel product that could be refined to saleableferronickel by conventional techniques.

Mintek has been working on the production of unrefined ferronickel from nickel-containing laterite inDC-arc furnaces since 1993. In this process, lateritic material is fed, together with a carbonaceous reducingagent, to the central region of the molten bath of a cylindrical DC-arc furnace. This feed material is preferablypre-treated, i.e. hot (calcined), and optionally pre-reduced.

DESCRIPTION OF A DC-ARC FURNACE

The development of DC-arc furnace technology at Mintek has been described in detail previously(1). Mintek’s 500 kW pilot plant, being a typical DC-arc furnace with ancillaries, was utilized for some of thelarger-scale testwork described in this paper. The DC-arc furnace (Figure 1) comprises a water-cooledrefractory-lined cylindrical shell, a conical roof, a graphite electrode, and an anode configuration comprisingvarious pins protruding through the hearth refractory.



The furnace has a single electrode (cathode) situated centrally in the roof and positioned above the

molten bath. The electrode may be hollow, and the feed materials may be introduced through the hollowcentre of the electrode. The molten bath forms part of the electrical circuit (anode). The return electrode, oranode, consists of multiple steel rods built into the hearth refractories and connected at their lower end to asteel plate which, via radially extending arms, is linked to the furnace shell, and further to the anode cable.

The water-cooled roof is lined with an alumina refractory, and contains the central entry port for the

graphite electrode and three equi-spaced feed ports. The outer sidewalls of the furnace are spray-cooled withwater to protect the refractories, and to promote (together with a tight control of the power and feed rate ofmaterial to the furnace) the formation of a slag freeze-lining within the vessel.

The furnace is fed more or less continuously, and is tapped periodically (or even continuously, if so

desired). The furnace is operated at a slightly higher pressure than that of the surrounding atmosphere, inorder to substantially exclude the ingress of air into the furnace.

Figure 1 - Schematic diagram of a DC-arc furnace

The solid-feed system for the furnace comprises a batching plant and a final controlled feedingsystem. The batching plant consists of feed hoppers mounted on load cells, vibratory feeders positionedunder the hoppers, an enclosed belt conveyor, a bucket elevator, and a pneumatically activated flap valve todirect the feed to one of two final feed hoppers. The final feeding system is made up of separate centre andside feeding arrangements. The centre feeder comprises a screw feeder discharging into a telescopic pipeattached to the hollow graphite electrode. The side feeders are vibratory, and discharge into feed chutesleading to the feed ports.

The gas-cleaning system consists of a water-cooled off-gas pipe, a refractory-lined combustion

chamber, water-cooled ducting, a forced-draft gas cooler, a reverse-pulse bag filter, a fan, and a stack. Thecondensed fume and dust, which accumulates in the lower conical section of the bag plant, is discharged via arotary valve into a collecting drum. This dust would, of course, be recycled back to the furnace in an



industrial situation.

PRE-TREATMENT Energy may be recovered from the process by using the thermal calorific value of the off-gases from

the furnace to assist in the energy requirements for drying or calcining of the laterite feed. The CO-rich off-gasmay also be used for pre-reduction purposes.

It is advantageous to pre-reduce the laterite prior to its introduction into the furnace. In such a case,

the laterite ore is dried first, followed by dry milling, and calcining at 700 to 900°C in a fluidized bed. Thecalcined laterite is then pre-reduced in a fluidized reduction reactor, using a solid carbonaceous or gaseousreductant, at 800 to 850°C, prior to feeding to the furnace bath. As the DC-arc furnace can smelt finematerials, a fluid bed reactor can be linked to the furnace.

The above variant of the process has even greater advantages in cases where the SiO2/MgO ratio is

what would normally be regarded as excessively high and the overall reduction requirement is conducted inthe pre-reduction stage. No SiO2 would be reduced in the pre-reduction step, and accordingly very littlesilicon appears in the molten metal produced in the furnace. Also, by selective pre-reduction of NiO andFe2O3 in the pre-reduction step, the ratio of Ni to Fe in the metal can be controlled to obtain a high-grade

ferronickel. A lower-powered furnace can be used, as the melting requires less energy than is the case for thesmelting reactions (especially if some SiO2 were also to be reduced to some significant extent in the smeltingprocess).

FURNACE OPERATION The feed rate of materials and the energy input into the furnace are adjusted to achieve and maintain

desired bath and tapping temperatures of the slag and metal. Cooling of the furnace walls assists in theformation of a slag freeze lining, which is particularly required in the case where the SiO2/MgO ratio is greaterthan 1.5, to protect the refractories from excessive wear.

The carbonaceous reducing agent is added in such quantities that the oxygen in the off-gases is

substantially in the form of carbon monoxide, and the nickel content of the slag is below 0.15%. Thetemperature of the furnace is controlled to between 1500 and 1700°C, depending on slag composition.

EXPERIMENTAL RESULTSExample 1 - 50 kW tests

At this preliminary small-scale stage of experimentation, smelting tests were carried out on:a) laterite ore fines and dusts (even 100% less than 100 µm),b) partially calcined laterite ores, andc) nickel-containing slags.

Tests were conducted in a 50 kW furnace with an outside diameter of 600 mm, and a refractory lining

thickness of 114 mm. The refractory material had a 96% MgO content. The hearth was lined with a



chrome-magnesite rammable material to a thickness of 310 mm, and a number of mild steel rods were usedto make the DC (anode) electrical connection from the molten bath through the hearth refractory to the anodecable. The molten bath in the furnace was heated to the desired operating temperature, with an initial metalcharge.

The feed materials consisted of calcined laterite dust (< 100 µm) from an industrial rotary kiln calciner

(for two tests), and laterite ore fines (< 6 mm) dried at 250°C (also for two tests). Charcoal (< 4 mm) wasused as the reductant in all these tests. The compositions of the feed materials, and masses of the feedmaterials and products, are shown in Tables I and II respectively. The feed materials were passed through afeed port in the furnace roof into a reaction zone, and the liquid products were tapped intermittently from thefurnace. Some additional tests were carried out using a somewhat smaller furnace shell, in order to increasethe number of samples obtainable at this scale of operation. This smaller furnace was equipped with water-cooling of the sidewalls. Results of the smelting tests, showing metal and slag compositions, are presented inTables III and IV respectively.

Table I - Composition of feed materials for 50 kW tests, mass %

Component Laterite fines

( dried at 250oC)

Laterite dust Charcoal

NiO 1.96 2.45 -Fe2O3 39.1 36.8 -MgO 12.2 17.5 0.2SiO2 30.7 34.4 4.3

Al2O3 5.60 3.83 0.8CaO 0.50 0.25 0.4MnO 0.69 0.58 -Cr2O3 2.51 1.15 -

Fixed carbon - 0.79 64.0Moisture 9.17 2.0 5.6Volatiles - - 23.3

Total 102.43 99.75 98.6

Table II - Masses of feed materials and products of the 50 kW tests

Feed materials, kg Products, kgTest Series Batch Mild

SteelLaterite

finesDust Char-

coalSlag Ferro-

nickelSlag

A Start12345

1.5 5.97.07.07.07.0

0.510.520.440.440.37

0.220.121.86

4.39.58.316.6

B 123456

7.07.07.07.010.07.0

0.600.520.440.440.630.37

5.14.28.627.5

C Start 6.0 1

23

10.010.010.0

0.630.630.63

0.08

21.4Digout 7.00 8.6D Start

1

4.5 +1.5 Ni

0.11

4.9

0.20

2.3



23456

5.05.05.0

0.110.120.700.700.84

4.05.0

0.78

0.278.00

2.52.13.32.64.9

Table III - Metal compositions from 50 kW tests, mass %

Test series Batch Ni Fe Si Cr P S CA 3

45

0.060.2114.70

99.199.684.3

0.150.110.05

0.080.050.06

0.023

0.10

0.05C 3 7.82 90.5 0.18 0.04 0.10Furnacecontents

8.43 90.9 <0.02 0.05 0.02

D 56

23.3016.60

72.883.1

0.75<0.05

0.200.40

Table IV - Slag compositions from 50 kW tests, mass %

Test Batch NiO FeO SiO2 Cr2O3 MgO CaO MnO Al2O3A 2

345

0.110.060.040.19

43.142.841.940.2

20.321.423.217.7

5.454.984.886.23

27.627.027.030.9

0.21<0.02<0.02<0.02

0.520.530.530.47

4.673.994.054.72

B 3456

1.371.351.041.18

36.737.938.537.3

25.424.926.828.0

4.744.693.704.12

26.524.824.325.3

0.400.460.490.40

0.570.600.630.59

5.895.806.145.55

C 3 0.15 29.9 29.3 4.09 28.3 0.39 0.65 4.73D 1

23456

0.240.880.010.030.710.19

32.333.133.423.026.016.5

25.024.826.534.134.336.7

3.734.213.133.832.853.55

29.329.828.328.427.432.9

0.550.550.640.700.760.70

0.960.720.741.121.000.95

7.106.436.938.518.168.83

From the above results, it can be seen that even extremely fine (< 100 µm) nickel-containing laterite

can be utilized effectively in a DC-arc furnace. Nickel levels in the slag could be lowered below 0.1%, whenlaterite fines and dusts were smelted, as well as when nickel-containing slags were processed.

Relatively high power fluxes at this small scale, and high slag temperatures around 1700°C, caused

substantial erosion of the MgO-based sidewall refractory material, as is evident from the differences betweenthe SiO2/MgO ratios of the feed materials and that of the tapped slags, as summarized in Table V.

Table V - Summary of 50 kW test: Power, power flux per unit hearth area, tapping temperatures, as well as theSiO2/MgO ratios of the feed materials and that of the slags

Test No. Furnace ID,

mAveragePower,

kW

Water-cooling(Y/N)

Powerflux,

kW/m2

SiO2/MgOratio in the

feed material

SiO2/MgORatio in the

Slag

Temperature,°C

A 0.372 47.5 N 437 1.96 0.90 1710B 0.372 46.4 N 427 2.52 1.08 1693



C 0.372 47.6 N 438 1.96 1.04 1676D (Heats 1-3)

0.200 29.1 Y 926 1.02 0.87 -

D (Heats 4-6)

0.200 27.9 Y 888 2.52 1.18 -

Example 2 - 120 kW test

A 120 kW DC-arc furnace was used for the processing of laterite calcine blends, with the objectivebeing the study of the smelting behaviour of materials with SiO2/MgO ratios ranging from 1.2 to 3.0. The

smelting of laterite types having a range of Fe contents from 15 to 20% was tested at the same time. A partof the test was devoted to the smelting of lateritic material that was preheated in a bubbling fluidized-bedreactor linked to the DC-arc furnace.

High-quality magnesite refractories were used for the sidewalls and hearth. The internal diameter of

the furnace was 760 mm. The outer shell was spray water-cooled to protect the sidewall refractories. Twotapholes were used; the lower taphole for the metal product, and the upper for slag drainage. The two-taphole system ensured efficient slag-metal separation.

The smelting campaign was conducted on a continuous basis over a period of seven days, and about

7.2 tons of material was smelted. Prior to the smelting testwork, the laterite ores were calcined to reduce theloss on ignition (LOI) value from 11.5% (dry basis) to a residual LOI of 6.5%.

As a separate condition, about 600 kg of the feed materials were preheated (prior to smelting) to

about 600°C in a fluidized bed, using liquefied petroleum gas (LPG). A schematic layout of the fluidized bedis shown in Figure 2. An enlargement of diameter in the upper portion of the reactor allowed solids to dropout of the gas, in order to decrease the carryover of fines in the off-gas.

Figure 2 - Schematic diagram of the 300 mm ID bubbling fluid bed reactor, which was linked to the 120 kW DC-arc



furnace

For all tests in which laterite was fed directly to the furnace, the laterite was screened to a particle

size of less than 8 mm. In the case of heating the feed materials to 600°C in the fluidized bed, screening wascarried out to a size range of less than 2 mm.

Table VI summarizes, on a test condition basis, the masses and critical chemical analyses of the feed

materials and the products.

Table VI - Feed material and product masses, with chemical compositions for the 120 kW test

TeststageNo.

Lateritemass,

kg

Coalmass,

kg

Ni inlaterite ,

%

Fe inlaterite ,

%

LateriteSiO 2/MgO

ratio

Slagmass,

kg

Ni inslag,

%

Fe inslag,

%

SlagSiO 2/MgO

ratio

Metalmass,

kg

Ni inmetal,

%

1 1685 142 1.57 17.3 1.53 1456 0.17 14.5 1.45 114 6.32 1750 128 1.54 14.9 1.23 1240 0.04 10.3 1.19 126 16.12 FBR 919 58 1.36 14.9 1.23 727 0.30 15.3 1.29 50 16.83 1300 80 1.57 19.4 1.52 1067 0.15 18.8 1.41 36 16.84 900 54 1.49 15.0 1.73 841 0.12 14.9 1.60 67 22.15 600 36 1.22 20.3 3.03 340 0.12 14.1 2.18 34 20.9Dig-out

- - - - - 161 0.34 12.0 1.67 122 20.3

Slag temperatures varied between 1600 and 1700oC, depending on the chemical composition.

During this test, conducted at power fluxes between 280 and 300 kW/m2 of hearth area, a substantialimprovement in the match between slag and feed SiO2/MgO ratios was obtained, as opposed to Example 1,where the power fluxes had been substantially higher. However, there was some erosion of the refractories,especially at high SiO2/MgO ratios.

Example 3 - 500 kW test : laterite calcine smelting

The aim of this test was to gather further information, in addition to that obtained from the testdescribed in Example 2, on the smelting of laterite calcine in a DC-arc furnace. Further data was requiredwith respect to reductant addition, energy consumption, pre-baked electrode consumption, refractoryperformance, dust carry-over, and arc length–voltage characteristics.

To meet these objectives, a five-day smelting campaign was conducted on Mintek’s 500 kW DC-arcfurnace, during which the following conditions were investigated: 1. Coarse calcine (2 to 10 mm) with a SiO2/MgO ratio of 1.29, and 16% iron content2a. Fine calcine (< 2 mm) with a SiO2/MgO ratio of 1.78, and 23% iron content

2b. Fine calcine (< 2 mm) with a SiO2/MgO ratio of 1.88, and 26% iron content.

Additional heats were completed, to determine arc length–voltage characteristics, the effect of ahigher power level (750 kW) on furnace performance, and the feasibility of feeding through the bore of thegraphite electrode.



The screening and blending of the laterite calcine (< 0.05% LOI) was done at Mintek. In order toobtain the desired chemical compositions for the various test conditions, additions of hematite, silica sand, andcalcined magnesite were made.

In total 30.8 tons of material was processed, with the overall mass balance given in Table VII.

Table VII - Overall mass balance for 500kW test on laterite calcine smelting

Warm-up metal Feed material, t Products, t Calcine Coal Slag Metal Dust

0.50 28.89 1.44 26.81 1.03 0.55

The dust losses to the furnace off-gas system amounted to about 2%, for both the fine and coarse

materials.

Averaged operating details for the campaign are summarized in Table VIII.

Table VIII - Averaged operating details for 500kW tests on laterite calcine smelting

ConditionProcess parameter 1 2a 2b 750 kW Centre feedMass fed, t 10.66 7.84 3.93 2.42 1.05Feed rate, kg/h 327 405 403 792 353Carbon addition, % 2.0 2.6 2.4 2.0 2.1Slag temperature, °C 1682 1603 1565 1538 1486Metal temperature, °C 1465 1520 1477 - -Power input, kW 500 470 405 720 370Measured rate of energy loss, kW 170 154 122 180 170Voltage, V 170 178 178 250 200Current, kA 2.92 2.06 2.28 2.89 1.85Resistance, mW 58 67 78 87 108

Power flux, kW/m2 hearth area 377 362 305 543 -

Energy consumption, kWh/kg feed 1.4 1.1 1.0 - -Thermal efficiency, % 54 61 64 - -

- Not measured/calculated

The overall electrode consumption was 3.1 kg/MWh or 5 kg/t calcine, and the average voltage dropin the arc, when side feeding was employed, was determined as 7 V/cm.

Table IX provides information on the most important chemical analyses of the composite calcine andsmelter products, as well as the nickel recoveries, which have been calculated on the assumption that dustcarry-over could be recycled and does not constitute a loss of nickel.

Table IX - Chemical analysis and nickel recoveries from 500 kW test on laterite calcine smelting

Condition Composite calcine analyses Slag analyses Alloynickelcontent

Nickelrecovery,

% Ni,

mass%Fe,

mass%SiO2/MgO

ratioNi,

mass%Fe,

mass%SiO2/MgO

ratioNi,

mass%



1 1.48 16.1 1.29 0.20 16.0 1.19 43.0 862a 1.23 23.2 1.78 0.10 20.8 1.55 26.9 952b 1.14 25.7 1.88 0.16 23.3 1.76 24.8 89

The nickel content of the composite calcine was diluted under condition 2, owing to the raw-materialadditions made to attain the desired chemical compositions. The selective reduction ratio was high, 7.4 and6.1 for conditions 1 and 2 respectively, where this ratio is defined as the nickel recovery to metal divided bythe iron recovery to metal.

Example 4 - 500kW test: Rotary kiln calciner dust smelting

The structure of this test campaign was very similar to that described in Example 3, but rotary kiln

calciner dust was used as feed material. The dust was dry, with 6.5% LOI. The following conditions wereinvestigated:1. Dust only, with a SiO2/MgO ratio of 1.71 and 26% iron content. Under this condition, additional heats

were completed to determine arc length-voltage characteristics, and the feasibility of feeding through thecentre of the hollow electrode.

2. Dust together with hematite and silica sand additions to increase the SiO2/MgO ratio to 2.20 and the ironcontent to 32%.

The overall mass balance for the test is given in Table X.

Table X - Overall mass balance for 500 kW test on rotary kiln calciner dust smelting

Feed material, t Products, tWarm-up metal Dust Charcoal Slag Metal Dust

0.47 25.80 0.89 21.38 1.30 1.22

About 4.5 per cent of the ultra-fine dust feed material (with a d50 of approximately 35 µm) carried

over to the off-gas system.

Table XI presents averaged operating parameters for the test campaign.

Table XI - Averaged operating details for the 500 kW test on rotary kiln calciner dust smelting

Condition Process parameter 1a 1b Centre feed 2a 2bMass fed, t 4.30 11.32 1.44 5.11 1.61Feed rate, kg/h 265 289 307 337 196Carbon addition, % 2.9 3.4 3.4 3.8 3.8Slag temperature, °C 1554 1576 1533 1526 1522Metal temperature, °C - 1540 - 1475 1500Power input, kW 373 374 364 372 267Measured rate of energy loss, kW 130 110 110 105 115Voltage, V 170 170 180 170 170

Current, kA 2.19 2.20 2.02 2.19 1.57



Resistance, mW 78 77 89 78 108

Power flux, kW/m2 hearth area 281 281 274 280 200

Energy consumption, kWh/kg feed 1.40 1.29 1.21 1.10 1.37Thermal efficiency, % 61 66 66 67 55

- Not measured

The results were very similar to those obtained in Example 3. The electrode consumption was

3.4 kg/MWh (corresponding to 5 kg/t dust), and the mean voltage drop in the arc, during around-the-electrode feeding, was 6.0 V/cm. The electrical resistivity of the slag was calculated to be approximately0.78 Wcm.

Important chemical analyses of the composite dust, and slag and metal products, are presented inTable XII. Also given is the recovery of nickel, once again based on the assumption that the only loss ofnickel is to the slag phase.

Table XII - Chemical analyses of dust feed and products, together with the nickel recoveries obtained during the 500 kWtest on rotary kiln calciner dust smelting.

Condition Composite calcine analyses Slag analyses Alloynickelcontent

Nickelrecovery,

% Ni,

mass%Fe,

mass%SiO2/MgO

ratioNi,

mass%

Fe,mass%

SiO2/MgO Ni,mass%

1a 1.63 25.7 1.71 0.22 27.8 1.61 32.2 891b 1.63 25.7 1.71 0.09 24.8 1.61 25.1 952a 1.22 32.0 2.19 0.11 32.9 1.88 24.8 922b 1.22 32.0 2.19 0.11 32.9 2.20 24.8 92

The selective reduction ratio (recovery of nickel to metal divided by recovery of iron to metal) was

7.7 when calculated over the whole test campaign, which illustrates the selective reducing capability of theDC-arc furnace while maintaining a high recovery of nickel. The crude ferronickel also contained on average0.045% Si, 0.01% C, 0.3% S, and 0.055% P, therefore necessitating further removal of sulphur andphosphorus to meet the ISO specifications of 0.03% S maximum and 0.03% P maximum.

DC-ARC FURNACE PROCESS ADVANTAGES The superior performance of a DC-arc furnace, compared with an AC three-electrode furnace or a

six-in-line furnace operated with immersed electrodes, for this type of process, is due to a number of factors. These have been presented previously (1-3). The most important advantage of the process is the fact that awide range of laterite ores (in terms of particle size and chemical composition) can be processed.

Feeding of fine material Problems have previously been noted with regard to the feeding of laterite ore fines to a conventional

shielded-arc AC furnace, leading to decreased power and production levels (4). On the other hand, the DC-arc furnace has the industrially-proven ability to process fine ores very successfully (1). Even dusts can be



utilized directly in the production of ferronickel, due to the highly stable nature of the DC arc.

Wide compositional range for feed In the DC-arc furnace, the bulk of the electrical resistance is located in the arc, and the smelting

process is conducted with an open bath. The furnace operates under open-arc conditions, with the electrodepositioned above the bath, so the resistivity of the slag has little influence on the supply of energy to thefurnace bath. Therefore, the bath temperature can be controlled to minimize the tendency to slag foaming. Effective energy supply depends less on slag composition, allowing the slag chemistry to be optimized for thebest recovery and minimum flux addition (instead of for the required electrical characteristics). Furthermore,the relatively high iron oxide content of some laterites, would (if they were not blended with lower iron-containing laterites) result in high iron-containing slags with high electrical conductivity, which do not permiteffective energy generation in the melt when using a slag-resistance furnace.

Effective control An additional advantage of the open-arc, open-bath smelting mode of operation is the effective

control of the reductant addition, as there is no direct contact between the graphite electrode and the melt.It is possible (and desirable) to maintain a layer of frozen material in contact with the sidewalls, in

order to protect the refractory lining of the furnace.

CONCLUSIONS · DC-arc furnace technology has been successfully applied to the production of ferronickel from laterite

ores and dusts. · Pilot-plant testwork at Mintek at power levels of up to 750 kW has demonstrated that laterites of wide

compositional range (with respect to iron content and SiO2/MgO ratio) can be smelted effectively toproduce crude ferronickel at high nickel recoveries.

· The smelting of dusts (100% < 35 mm) recovered from rotary kilns used to calcine nickel laterite ores has

also been demonstrated successfully at this scale.

ACKNOWLEDGMENTS

This paper is published by permission of Mintek.

REFERENCES

1. R.T. Jones, N.A. Barcza, and T.R. Curr, “Plasma Developments in Africa”, Second International Plasma

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Production of ferronickel from nickel laterites in a dc arc furnace

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