METALURGIADE

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HISTORY AND USE.• Bentonite mining began on the Taylor ranch near Rock River, WY, in

1888. This bentonite reportedly was shipped crude to Philadelphiawhere is sold for $25 a ton and was used in making cosmetic.

• The value of bentonite as foundry sand bond was recognized in the1920s, and the iron and steel foundries have since been majorconsumers of bentonite.

• Bentonite was first used as drilling mud in the late 1920s or early1930s, and bentonite is still one of the most efficient materials fordrilling mud where the rocks penetrated contain only freshwater.

HISTORY AND USE.• In addition to the major uses, bentonite is used in many miscellaneos

products, and hundreds of parents for specialty uses have been issuedor applications filed. The specialty uses include:– Filtering agents (one of which is a high value product for clarifying

wine, and another is a less costly one for treating waste water),– Water impedance (preventing seepage loss from reservoirs,

irrigation ditches, and waste disposal ponds, and seepage throughbasement walls, tunnel walls, and other structures)

– Ingredients in cosmetics,– Animal feed,– Pharmaceuticals,– Colloidal filler for certain types of paints,– An additive to ceramic raw materials to increase plasticity,– Fire retarding materials,– And many other purposes.– Another use for bentonite is as a coating on some types of computer

paper and on no-carbon required copy paper.

PRODUCT SPECIFICATIONS.• Product specifications have been tentatively standardized for

bentonite and/or fuller’s earth sold for drilling mud, foundry sandbond absorbent granules, and oil bleaching and will be outlinedbriefly. Specifications for most of the many other products made fromthese clays are prepared to meet the requirements of the individualcustomer, and they commonly vary with each.

– Drilling mud.• Suspension Properties.• Wet Screen Analysis• Moisture.

– Foundry Sand Bond.– Iron Ore Pelletizing– Absorbent Granules– Bleaching Oils

PRODUCT SPECIFICATIONS.• DRILLING MUD.• The most critical specifications for bentonite for drilling mud are:

– The suspension properties,– Wet screen analysis,– And moisture as shipped.

• Suspension Properties: The test for suspension properties involvesthe preparation of a suspension consisting of 22.5 g of bentonite in 350cc of distilled water. The suspension is aged and the viscositydetermined and yield point calculated from dial readings at 300 and 600rpm with a direct indicating viscometer, according to procedure andequipment requirements outlined in American Petroleum Institutespecification API STD 13A

PRODUCT SPECIFICATIONS.• DRILLING MUD.• Another important suspension requirement is the filtrate test, which is a

measure of the volume of water lost from the prepared suspensionwhen tested in a pressurized filter press according to procedure outlinedin American Petroleum Institute specification API RP 13.

• Wet Screen Analysis: Wet-screen analysis (grit test) is a measure ofthe material in bentonite mud that is coarser than 200-mesh USSeries sieve.. The specified test is made by mixing 10 g bentonite in350 cc water containing 0.2 g dispersing agent, aging, stirring, andwashing through a sieve with a specified spray system. The residue onthe sieve is then dried, weighed, and the percentage of the originalbentonite is determined. Bentonite to fulfill the specifications fordrilling mud must have no more than 2.5% residue (grit) andpalygorskite no more than 8% residue.

• Moisture: The maximum moisture content of bentonite when shippedfrom the plant where it is processed is 10% and that for palygorskite is16%.

PRODUCT SPECIFICATIONS.• FOUNDRY SAND BOND.• Tentative specifications for Western bentonite for bonding foundry sand

are outlined in Steel Founders Society of America.

• This specification requires the following characteristic and properties forWestern bentonite, as tested by methods described in the specification:1) Water content shall not exceed 12% or be less than 6%,2) pH value shall be equal to or greater than 8.2,3) Calcium oxide content shall not exceed 0.70%,4) And the liquid limit shall not be less than 600 or greater than 850.

• Each foundry has its own green, dry, and hot strengthspecifications for bentonite, which vary with the type of metal, size ofcasting, and foundry production procedures.

PRODUCT SPECIFICATIONS.• IRON ORE PELLETIZING.• Specifications for bentonite used for pelletizing taconite-type iron ore

have not been standardized, and several tests are used.

• As green pellets must be capable of withstanding handling,compaction, and drying and the dry pellets must be even stronger,mixtures of bentonite, iron ore, and water are commonly tested for wet-drop stregth, wet compression strength, plastic deformation, anddry compression stregth.

PRODUCT SPECIFICATIONS.• ABSORBENT GRANULES.• The size and absorbent characteristics required of granules as outlined

in the federal specification are listed in Table 1.

• The specifications require that absorbent granules consist of a uniformmixture of minerals of the silicate type. They also must be clean,uniform, and free of lumps or foreing matter, and no more than 10% ofthem can pass through an 80 – mesh sieve in the attritionresistance test. The attrition resistance test is made by shaking withsteel balls on a screen, according to a specified procedure.

PRODUCT SPECIFICATIONS.

PRODUCT SPECIFICATIONS.• BLEACHING OILS.• Test methods for evaluating bentonite and other types of fuller´s earths

for bleaching soybean and cottonseed oils are outlined in the AmericanOil Chemists Society AOCS Ofificial Method Cc 8ª-52 (1958). Thesespecifications contain instructions on bench-type tests, including stirringtime, heating rates and temperatures; approved equipment; quantitiesof raw oil and clay required; methods of color determinations; and soforth. The color determination of the bleached oils and their comparisonwith the oils bleached by the official earth in the same test are the basisfor purchase specifications.

PRODUCT SPECIFICATIONS.

GEOLOGY.• MINERALOGY.• Smectite: The major mineral constituent of bentonite, and the mineral

that gives bentonite its unique physicochemical properties, is smectite.• Smectite is the general term given to a group of clay minerals that

includes beidellite, montmorillonite, nontronite, saponite, andhectorite (Table 2).

• Each of these minerals shares a similar structure but each ischemically distinct. Nontronite, for instance, is an iron´-rich smectiteand hectorite is a lithium-rich smectite.

• Structure: The most common smectite group mineral found ineconomic bentonite deposits is montmrillonite. The calcium andsodium varieties, named based on the predominant exchangeablecation, are most abundant. Hectorite is also a major constituent ofimportant commercial deposits.

• The smectite minerals are divided into two structural groupsdepending on whether all three octahedral sites are occupied(trioctahedral) or two out of three are occupied (dioctahedral).

GEOLOGY.

GEOLOGY.

GEOLOGY.• MINERALOGY.• The theorical structural formula for the dioctahedral smectite minerals

montmorillonite, beidellite, and nontronite is

• [AlxFeyMgz)2.00(Si4.00-(u+v)FevAlu)O10(OH)2]M +u+v+z

• where M+ represents the exchangeable cations.

• The trioctahedral smectites, hectorite and saponite, have the followingtheoretical formula:

• [Mg3.00-ZLiZ)(Si4.00-uAlu)O10(OH2]M+z + u

GEOLOGY.• MINERALOGY.• Other Minerals: Other minerals commonly found in bentonites fall into

three categories:1) Those that are volcanic in origin,2) Those that form in situ as secondary minerals in response to

diagenesis and weathering, and3) Those that are detrital contaminants.

• Tipical volcanic minerals include feldspars, biotite, quartz,cristobalite, apatite, zircon, magnetite, amphiboles, and sometimesunaltered volcanic glass.

• Minerals of low temperature origin include various forms of poorlyordered opaline silica, zeolites, calcite, selenite, other clayminerals including kaolinite and illite, and various iron sulfateminerals. Detrital minerals are the least abundant and include all ofthe major rock forming minerals found in most sedimentaryenvironments. These minerals rarely constitute more than 10% of acommercially viable deposit.

OCCURRENCE.• Bentonite is typically found as beds in marine and nonmarine strata

ranging in age from Jurassic to Pleistocene. These beds which tend tobe parallel to the overlying and underlying strata, range in thicknessfrom several centimeters to tens of meters and can begeographically very extensive.

• Bedded bentonite can also occur as small lens shaped bodies with alimited lateral extent.

• A less common occurrence of bentonite is as irregular shaped bodiesthat grade into unaltered host rock..

• Bentonite ranges in color from white to yellow to olive green tobrown to blue. It has a characteristic soapy texture and a waxyappearance. Weathered sodium bentonite has a distinctive “popcorn”texture.

• Calcium bentonite weathers to an “aligator skin” texture.

ORIGIN.• Bentonite most commonly forms as a result of the in situ alteration of

volcanic ash or tuff. Another less common origin is as thehydrothermal alteration of volcanic rocks.

• The alteration of ash to smectite is a hydration reaction that requiresmagnesium and produces excess silica as well as alkali ions. Becauseof these requirements bentonite is most commonly found in marinestrata where the ash apparently altered in contact with seawater. Insome instances it also formed in contact with alkaline lake water andgroundwater.

• The evidence for bentonite having altered from volcanic ash is:1. Mineralogy--- bentonite beds often contain euhedral minerals

characteristic of volcaniclastic rocks,2. Chemistry--- the major element and the rare earth element chemistry

of the smectite is consistent with a volcanic precursor,3. Texture--- relict glass shards pseudomorphically replaced by smectite

have been observed both in thin section and under the scanningelectron microscope.

• The most compelling evidence for a volcanic parent, however, is thepresence of unaltered glass shards in some deposits.

ORIGIN.

DISTRIBUTION OF DEPOSITS.• North America

– Western Bentonite.– Southern Bentonite.– Fuller’s Earth– Hectorite.

• Other Deposits.– Canada.– Mexico: Bentonite and fuller’s earth deposits are distributed

throughout central Mexico where volcanoclastic rocks arepervasive. The majority of these bentonites contain calcium as thedominant exchangeable cation and many of them occur inmarine rocks of Tertiary age.

– Both calcium and sodium bentonites are mined in Mexico whichproduced 136 kt of bentonite in 1986. The largest calciumbentonite deposits are located in Tlaxcala and Morelos inHidalgo.

– The principal sodium bentonite producing region is in Durango.

DISTRIBUTION OF DEPOSITS.• Other Deposits.

– The sodium bentonite is marketed to the foundry industry and is alsoused in detergents, pencil manufacture, and cosmetics.

– Acid activated and sodium exchanged calcium bentonites are alsoproduced in plants in Durango and Puebla.

• South America.– Brazil– Peru– Chile.

DISTRIBUTION OF DEPOSITS.• Europe.

– United Kingdom– Germany– Sweden– France– Greece– Italy– Spain– Cyprus– Turkey.

• Africa– Morocco– South Africa

DISTRIBUTION OF DEPOSITS.• Australia.• Asia

– India– Pakistan– Japan

EVALUATION OF DEPOSITS.EXPLORATION.• Exploration for bentonite and fuller`s earth deposits consists of several

steps.– The first step in exploration is to identify areas that are likely to

contain bentonite deposits.– Once target areas have been identified, a field reconnaisance is

usually conducted to establish whether or not clay deposits arepresent. If clay is found then the next step is to map the extent ofthe deposits. In other areas drilling must be conducted toaccurately map the deposits. Usually both surface mapping anddrilling are utilized to determine the extent of available reserves.

– After deposits has been located and mapped other factors must beconsidered before capital is invested in evaluation and extensivetesting of the deposit. Factor such as amount of overburden,distance to markets, available transportation, and amount ofreserves must be considered.

EVALUATION OF DEPOSITS.MINERALOGICAL INVESTIGATIONS.• The mineralogy, and the chemistry of a clay are important properties

that must be considered when evaluating the economic use of adeposit. A detailed understanding of the particle size and shape of theclay fraction can also be essential to making the appropriatedetermination for the end-use of a clay.

• Simple physical testing is no longer sufficient for a number or reasons.1. One reason is that these clays are being used in an increasing

number of specialty products that require very stringentspecifications, many of which are related to chemistry, particlesize, and shape.

2. Another is that reserves of the high quality clays, for which thestandard testing procedures and specifications were written, arebecoming depleted and different quality clays are being mined.These clays often must be beneficiated to meet the traditionalspecifications and this requires a detailed understanding ofmineralogy and chemistry.

EVALUATION OF DEPOSITS.MINERALOGICAL INVESTIGATIONS.

3. A third reason for characterizing mineralogy and chemistry isthat there is an increasing interesting in the research anddevelopment of these clays for use in more highly technicalindustrial applications such as catalysis, non –linear optics,advanced ceramics, and polymer composites.

• Analytical methods for studying clay minerals include X-raydifraction, scanning electron microscopy, transmission electronmicroscopy, infrared spectroscopy, differential thermal analysis,and optical microscopy. These methods are used to study thestructure and chemistry of raw clays and clay based products. Othermethods used to investigate clays include various methods fordetermining structural chemistry, surface chemistry, surface area, andparticle size distribution to name a few.

EVALUATION OF DEPOSITS.APPRAISAL OF FIELD AND LABORATORY RESULTS.• Testing:• Mineralogy.• Appraisal of Field and Laboratory Results.

EVALUATION OF DEPOSITS.PREPARATION FOR MARKETS.• Mining Methods: Bulldozers and scrapers or pans are most

commonly used in removing overburden. In gthe typical pit, theoverburden is removed in panels, the clay mined by loading trucks withdragline or endloaders, and the overburden from an adjacent panelshifted to the mined out area. Thicknesses of overburden removed varyconsiderably. The overburden stripped from fuller`s earth depositsranges from a few meters to more than 23 m.

• The rocks overlying most bentonite deposits are soft and can beloaded directly by self-propeller or pushed scrapers, but in places theymust be loosened by bulldozers and rippers before loading.

• Most bentonite produced in other countries is also strip mined, butunderground methods are used in a few places.

• Because of variable physical properties, most bentonite deposits areselectively mined. Bentonite from a single pit or bed may by separatedinto as many as three stockpiles at the plant, each with differentphysical properties. Various grades of bentonite suitable for differentuses are then prepared from the separate piles and by blending clayfrom more than one pile. Blending bentonite as it is dumped onstockpiles, using earth-moving to obtain a unifom clay, is also commonpractice.

EVALUATION OF DEPOSITS.PREPARATION FOR MARKETS.• Beneficiation and Processing techniques: Bentonite is processed by

simple milling techniques that involve removal of water and in someinstances, other volatile matter, and grinding to suitable sizes. (Fig.9).The high swelling bentonite, when delivered to the plant, containsapproximately 30% moisture; the free water in the calcium-typebentonite is about 25%. Some fuller`s earth contains as much as50% volatile matter and 10% undesirable impurities.

• The processed bentonite ordinarily contains only 7 or 8% water, butbecause it is hygroscopic, it may contain considerably more when used.Moisture content of the fuller`s earth is approximately the same as thatof bentonite. Most absorbent granules, however, are heated to atemperature high enough to drive off interlayer and zeolitic water in theminerals present. Any moisture in the granules has been absorbed afterprocessing.

• In most plants, the raw clay is passed through some sort of a clay“slicer” to break up the large chunks before drying. The temperature indryers varies with the intended use of the clay. The desirableproperties of bentonite is reduced greatly if the clay is heated toomuch.

EVALUATION OF DEPOSITS.PREPARATION FOR MARKETS.• Beneficiation and Processing techniques:• The temperature in dryers processing bentonite is likely to be

approximately 800ºC at the inlet, 100 to 200ºC at he outlet, and 400 to500ºC in main drying zone. The bentonite itself is kept at temperaturesof less than 150 ºC.

• The dried clay is ground and sized in several ways. In some plants,rods in rotary dryers do much of the grinding, but most of thepowdered product is ground with roll and hammer mills or otherpulverizer, and screened. Most bentonite is ground to approximately90% finer than 200 mesh.

• Granular grades of bentonite are also sold, and the wide use ofabsorbent granule-type fuller`s earth requires the preparation of acoarse product. The preparation of coarse absorbent granules is, ineffect, a beneficiation process, because much of the fine and mediumgrained sand impurities in the fuller`s earth are in the size fractiondiscarded.

EVALUATION OF DEPOSITS.PREPARATION FOR MARKETS.• Beneficiation and Processing techniques:• Hectorite and some Southern bentonite, in addition to being processed

like other bentonite, are beneficiated by a hydroclassificationprocess. In this process, the clay is dispersed in water and pumpedinto a multistage concentrating circuit where nonclay and otherundesirable materials are separated and removed. The slurry is thenpassed through drum or spray-type dryers, and the concentrated ispulverized. A high value material is prepared in one plant by centrifuginga dilute slurry.

EVALUATION OF DEPOSITS.PREPARATION FOR MARKETS.

EVALUATION OF DEPOSITS.SYNTHETIC AND ORGANIC-CLAD “BENTONITE”.• Synthetic hectorite is made in the United Kingdom. This material is

purer than naturally occurring varieties, as it contains no fluoride. Itpresumably is suitable for some of the same uses that syntheticbentonite was developed for in the Unites States.

• For several decades an organic treated bentonite has been on themarket under the registered trade name Bentone. This product isprocessed in such a way that the original inorganic exchangeable ion onthe bentonite is replaced by an alkyl amine organic cation. This reactionproduces a hydrophobic clay because the inorganic ions that can behydrated are removed, and a large part of the clay mineral surfaceformely capable of absorbing water is coated by hydrocarbon chains.Organic-clad bentonites are used in making paint, to gel organicliquids, to produce greases having superior properties for adherenceto metal, ability to repel water, and resistance to high temperatures, andin other products.

EVALUATION OF DEPOSITS.FUTURE CONSIDERATIONS AND TRENDS.• The growth rate in fuller`s earth requirements will probably exceed

the rate forecast for bentonite.• Both bentonite and fuller`s earth are used in so many different products

and in so many ways that there are many possible substitutes forspecific uses and both have lost markets to other materials in the past.

• Bentonite is in some danger of a market loss which may be caused by abreakthrough in the proportion of bentonite added in taconite bonding issmall, it is still a contaminant, and some type of bonding material thatwill burn off completely or even add heat in the steel furnace willcontinue to be sought.

• The manufacture of synthetic hectorite and other montmorillonite-likemineral products formerly in the United States and now in the UnitedKingdom marks a technological breakthrough. World-wide markets fororganic-clad bentonite and washed or otherwise pufified white bentonitefor specialty products are also likely to increase with generaleconomic growth and the development of new uses and products.

• Rising cost of transportation and fuel are two of the majorpressures on the bentonite and fuller`s earth industries that are likely tohave a significant influence on future trends.