Cationic processing of mineral ores

Abstract: A method of wet processing mineral ore (e.g. kaolin crude) to recover negatively charged finely divided mineral, which method includes dispersing an aqueous pulp of the ore with wate-soluble organic cationic dispersant prior to said recovery - for example, a method for recovering a fine particle size fraction of a negatively charged mineral from an ore, such as a kaolin clay crude, which comprises forming an aqueous pulp of the ore, dispersing the pulp by adding thereto sufficient water-soluble organic cationic dispersant to a positive zeta potential to said particles, and preferably an acidic pH to said pulp removing coarse particles from said dispersed pulp and fractionating the dispersed pulp to separate a fine particle size fraction of mineral particles from coarser particles. Bleach may be added before or after fractionation. The fine fraction may be pulverised, calcined and repulverised to give low abrasion pigment.

Description: This invention relates to the art of upgrading mineral ores (e.g. kaolin clay crudes, aluminous ores, titania, calcium carbonate etc.) by processing which includes treating an aqueous suspension of the ore with dispersant before carrying out steps to produce refined mineral products. In particular, the invention relates to processing aqueous suspensions of ores to provide refined products using cationic dispersants, preferably at acidic pH, throughout the processing. The invention also concerns such cationic processing of kaolin crudes prior to calcination that leads to low abrasion calcined kaolin pigments. Minerals almost invariably occur in nature in ores which contain a variety of materials in addition to the particular mineral constituent that is to be marketed. The nondesired mineral matter may be, for example, an impurity or a particle size fraction of the desired mineral that is too coarse (or too fine) for an intended use. Especially where the desired mineral material is very finely divided, for example, a material having an appreciable content of particles finer than 2 micrometers, it is conventional to disperse the ore in water to form an aqueous pulp before attempting to upgrade (beneficiate) the ore. Dispersion (deflocculation) is practiced to fluidize mineral pulps and it enhances the separation of individual mineral particles from others by increasing the electrical charge on the individual particles. Anionic dispersants such as condensed phosphates and sodium silicate are frequently used to fluidize ore pulps containing negatively charged mineral particles at near neutral to mildly alkaline pH values (e.g., pH 6-9) without the need to consider the effect of using such dispersants on subsequent processing. An exception is the case of finely mineralized ores such as kaolin clay crudes in which chemical treatment after dispersion is frequently essential and the presence of the particular dispersant used to upgrade the ore may result in undesirable interaction between dispersant and subsequently or previously added reagents. See, for example, U.S. 3,594,203 (Sawyer et al). Naturally-occurring sources of kaolin, generally known as kaolin crudes, such as those mined in Georgia, U.S.A. and Cornwall in England, are processed to recover upgraded kaolin products. Many Georgia crudes contain well-crystallized finely divided particles of kaolin having a median particle size of about 1.5 micrometers in association with silica and silicate impurities as well as colored ferruginous and titaniferous impurities. Other commercially important Georgia crudes contain less well-crystallized kaolin which are finer in particle size, e.g., median size below 1 micrometer. The Cornwall crudes typically contain only about 10% to 15% kaolin which is coarser than the kaolin in the Georgia deposits. Vast tonnages of such kaolin crudes are mined for the ultimate recovery of purified grades of kaolin having a higher brightness and different particle size distribution than that of the kaolin in the crude. The processing, frequently referred to as "wet processing", almost invariably involves crushing the kaolin crude, pulping the crushed crude in water, removing coarse impurities (so-called grit), fractionating the degritted crude to recover one or more fractions of desired particle size distribution, bleaching to brighten one or more of the fractions of kaolin, filtration and washing. In some cases, additional upgrading to remove colored impurities by flotation, selective flocculation and/or magnetic purification is practiced and usually takes place before filtration. Such additional upgrading also takes place when the clay is in the form of a dispersed pulp. When kaolin crudes are mixed in water without addition of dispersant the resulting pulps are usually mildly acidic, typically having a pH of 4-6, and the kaolin particles are negatively charged. To the best of our knowledge, one or more anionic dispersants have invariably been added to a pulp of the crude to create near neutral or mildly alkaline systems before degritting and fractionation. When these steps are followed by bleaching with a reducing bleach, usually a hydrosulfite (dithionite) salt, the previously dispersed alkaline pulp of clay is first flocculated by adding an acid or alum to pH 3-5 because such bleaches are generally more effective at acidic pH values and flocculation aids subsequent filtration. However, the added acid or alum interacts with the dispersant to form salts which are frequently deleterious to performance properties of the kaolin product. After flocculation, the bleached clay, in the form of a filter cake, must usually be washed to reduce salt content. Filtration and washing add considerably to the cost of processing. Subsequent processing depends on the form in which the purified kaolin is to be supplied. So-called "predispersed grades" are usually manufactured by adding a dispersant to the previously flocculated clay to form a high solids slurry (e.g., 65-70%) which is spray dried to provide a dry product containing a dispersant. The production of slurried grades also entails the addition of dispersant(s). Dispersant(s) added to previously dispersed and flocculated clay are generally termed "secondary" dispersants to distinguish them from dispersants used to prepare pulps for degritting, fractionation, etc., which are generally referred to as "primary" dispersants. Acid grades of wet processed kaolins are produced without secondary dispersion of acid-flocculated filtered clay. The history of using alkali or negatively charged material as primary dispersants for negatively charged clay dates at least back to 1912 (U.S. Reissue Patent 14,583, Scherwin). This patent teaches using "electrolytes containing ions of an electric character which is the same as the character of the part of the composite mass which it is desired shall remain in suspension in sol-condition". This concept is pursued in Scherwin's subsequent patent (U.S. 1,233,713), which discloses fractional sedimentation of kaolin in a dispersed system. An advance is reflected in U.S. 1,324,958 (Feldenheimer) in which an acidic material is used to neutralize the alkaline deflocculating agent and to floc the clay in order to settle materials not ordinarily settled in a deflocculated condition. Subsequent patents disclose particle size fractionation of kaolin in dispersed state in centrifuges, usually followed by flocculation by acid addition, filtration and washing of the filter cake. A major advance in the development of the kaolin industry was reflected in the establishment of a 2 micrometers particle size cut-off point when fractionating kaolins to prepare paper coating clay and the use of centrifuges to carry out the desired fractionation of the crude clay. Crudes having a median particle size of 1.5 micrometers are conventionally classified in centrifuges to recover premier coating grades (at least 90% by weight finer than 2 micrometers) and coarse filler kaolin having a median size of 4-6 micrometers. A major present use of kaolin crudes is as a source of bleached minus 2 micrometers fractions of kaolin clay supplied principally for the paper coating industry. The growth of the kaolin industries in the U.S. and abroad has also generated many other innovations including the use of a variety of primary dispersants, all anionic, and including sodium silicates, silicate hydrosols, condensed phosphate salts, polyacrylate salts, and "auxiliary" dispersants such as sulfonates. Similarly, the art of producing carbonate pigments and fillers from naturally-occurring dolomitic and calcitic ores by processes utilizing wet milling and wet particle size classification has used anionic dispersants, exemplified by sodium polyacrylates, added, for example, during grinding, followed in some instances by flocculation. See, for example, U.S. 3,980,240 (Nott), U.S. 3,990,642 (Nott), and U.S. 4,165,840 (Lewis et al). The concept of using a negatively charged dispersant (sodium silicate, mixtures thereof with sodium carbonate, or hydrosols derived from sodium silicate) in the froth flotation of kaolin has been extended to other finely mineralized negatively charged mineral oxide and silicate ores, for example, talc ores, tin (cassiterite) ores, (U.S. 3,915,391, Mercade), fluorspar ores (U.S. 3,893,915, Mercade) and scheelite (tungsten) ores, (U.S. 3,915,391, Mercade). Cationic surfactants have been disclosed as a means to disperse various clays in organic liquids. See, for example, U.S. 2,797,196, Dunn et al. In accordance with U.S. 3,804,656, Kaliski, et al, a negatively charged filler or pigment such as kaolin is dispersed with a combination of nonionic and cationic surface active agents at a strongly alkaline pH. The cationic material is not added as a primary dispersant nor is it present during wet processing. U.S. 4,738,726, Pratt et al, discloses high bulking pigments obtained by adding a cationic polyelectrolyte to previously degritted and fractionated kaolin clay to partially flocculate the clay. Anionic primary and secondary dispersants are used and the cationic material is used to partially flocculate, not to disperse the clay. U.S. 4,210,488 (Reus et al) discloses addition of a cationic polymer (polypiperidine halide) "absorbed" on a fine particle carrier such as kaolin to improve the dry strength of paper or to improve the effect of optical brighteners. The absorbed polymer is said to create a positive charge on the kaolin particles. The cationic material is not added during wet processing of the carrier. Numerous publications make reference to the addition of cationic polyelectrolytes to paper coating composition containing mineral pigments such as clay or calcium carbonate to improve the coating structure by imparting a bulked structure. See, for example, Coco, C.E., Soy Protein Latex Interpolymers - Properties and Function, Preprints TAPPI Coating Conference 1987. EPA 281,134 (Weige) relates to cationic pigment dispersion, especially for producing paper coating compositions, containing a pigment component, a cationised polymer, which encloses the pigment particles as a protective colloid, and optionally a cationic polymer and/or ammonium compound as dispersant for the coated pigment particles. The cationic polymer PVA increases the zeta potential of the dispersion to the isoelectric point or into the cationic region. The pigment can be ground or precipitated calcium carbonate, (calcined) kaolin, TiO 2 , ZnO, satin white, aluminum hydrosilicates or their mixtures. In the manufacture of paper and paper board, it is well-known to incorporate quantities of inorganic fillers into the fibrous web in order to improve product quality. It is also well known to coat paper and paper board with inorganic pigments to improve opacity, print quality, and frequently sheet brightness. Titanium dioxide is widely used to improve brightness and opacity of coated and filled sheets but it is an expensive pigment. In recent years, considerable efforts have been made to develop satisfactory replacements for titanium dioxide. Substantially anhydrous kaolin clays prepared by partially or fully calcining a fine particle size fraction of crude kaolin clay is now a replacement pigment of choice - see, for example, U.S. 3,586,523, Fanselow et al, to which attention is directed for detail. Calcined kaolin clay opacifying pigments, such as the products supplied under the registered trademarks ANSILEX and ANSILEX 93 by Engelhard Corporation are exemplary. These products are substantially anhydrous white pigments and are widely used as fillers in paper sheets and paper board, as a coating pigment for paper, and as a pigment in paints and other filled systems. They consist of aggregates of clay particles, and exhibit exceptionally high light scattering and opacifying characteristics when incorporated as a filler into paper. The particle size of these pigments is typically at least 65 percent by weight finer than 2 micrometers equivalent spherical diameter (esd), and at least 50 percent by weight finer than 1 micrometer. Especially when used to coat or fill paper products, low abrasion is essential. The calcined kaolin pigments presently used by the paper industry exhibit low abrasion values, generally less than 50 mg., and usually below 30 mg., as measured by the Valley test method. Calcined kaolin clay pigment includes kaolin clays which have been dehydroxylated by heating to temperatures over 400°C. The term "calcined" thereby embraces fully calcined kaolins-which usually have been heated above the 980°C exotherm, as well as so-called "metakaolin," which results from heating to lower temperatures, below the exotherm. Reference is made to the following which relate to fine particle size, low abrasion, opacifying calcined kaolin pigments, Proctor, U.S. 3,014,836; Fanselow et al., U.S. Pat. No. 3,586,523; Morris, U.S. Pat. No. 3,519,453; Podschus, U.S. Pat. Nos. 3,021,195 and 3,309,214, and British Pat. No. 1,181,491 and U.S. Pat. No. 4,381,948, McConnell et al. Naturally-occurring sources of kaolin, generally known as kaolin crudes, such as those mined in Georgia, U.S.A. and Cornwall in England, are processed to recover upgraded kaolin products. Many Georgia crudes contain well-crystallized finely divided particles of kaolin having a median particle size of about 1.5 micrometers in association with silica and silicate impurities as well as colored ferruginous and titaniferous impurities. Other commercially important Georgia crudes contain less well-crystallized kaolin which are finer in particle size, e.g., median size below 1 micrometer. The latter find widespread use in calcined form in applications such as paper coating and filling in which high opacification and low abrasivity, frequently also high brightness, are demanded. Vast tonnages of such kaolin crudes are mined for the ultimate recovery of purified grades of kaolin having a higher brightness and different particle size distribution than that of the kaolin in the crude. The processing, frequently referred to as "wet processing", almost invariably involves crushing the kaolin crude, pulping the crushed crude in water, removing coarse impurities (so-called grit), fractionating the degritted crude to recover one or more fractions of desired particle size distribution, optionally bleaching to brighten one or more of the fractions of kaolin, filtration and washing. In some cases, additional upgrading to remove colored impurities by flotation, selective flocculation and/or magnetic purification is practiced and usually takes place before filtration and drying. The manufacture of low abrasion calcined kaolin opacifying pigment has heretofore invariably utilized anionic wet processing of carefully selected crudes by processes including degritting fractionation, recovery of the fine fraction (see, for example, Fanselow et al and McConnell patents, supra). In some cases, flotation, magnetic purification, bleaching or combinations of such steps is practiced. The recovered fine fraction of a selected crude, typically one composed of poorly crystallized kaolin, is recovered, pulverized, calcined and repulverized. Other low abrasion calcined pigments utilize as calciner feed material generally known as mechanically delaminated clay, a material recovered by mechanical delamination of coarse particle size fractions of well-crystallized kaolin recovered during wet processing - see, for example, Morris et al, supra. U.S. 4,078,947, Bundy et al, relates to a process for producing a calcined clay product having finer particles and producing higher void volume in coatings wherein a Georgia kaolin is blunged with an anionic dispersant (sodium polymetaphosphate), fractionated and the fine fraction is treated with citric acid and polyamine fatty acid amine or a quaternary ammonium salt, flocculated, filtered, dried, pulverized, calcined and repulverized. To the best of our knowledge, the prior art does not disclose or suggest the use of cationics as primary dispersants for kaolin or other ores, much less for wet processed kaolin intended to be utilized as calciner feed to produce calcined kaolin pigments. The present invention provides a method of wet processing crude kaolin ore to recover a fine particle size fraction of kaolin which comprises forming an aqueous pulp of the crude kaolin ore, dispersing the crude kaolin in the aqueous pulp as particles, removing coarse particles from said dispersed pulp and then fractionating the dispersed pulp to separate and recover a fine particle size fraction of kaolin, and optionally adding bleach reagent to said dispersed pulp to bleach kaolin particles, characterised in that said dispersing is effected by adding to the aqueous pulp sufficient water-soluble organic cationic polyelectrolyte dispersant to impart a positive zeta potential to said particles and form a cationically dispersed acidic pulp Such upgraded kaolin can be calcined to give low abrasion opacifying pigment. Such pigments can be composed of calcined kaolin particles having a coarser particle size than is obtained using conventional anionic prior art primary dispersants. Unexpectedly, coarsening of the particles prior to and subsequent to calcination results in pigments having remarkably low abrasion. The prior art (see Proctor and McConnell et al, supra) suggest that finer particle size calciner feed results in reduced abrasion. This is contrary to our findings regarding fineness as affected by changing from anionic to cationic primary dispersion. Another character of the calcined kaolin pigments is the improved printability of paper sheets coated with our pigments. The coarser particle size of pigments made by the invention obtained by cationic dispersion is in contrast to the finer particle size achieved by Bundy et al (supra) as result of anionic dispersion followed by flocculation with a cationic material. Pursuant to one aspect of the invention, crude kaolin clay is subjected to processing including degritting, fractionation and bleaching (e.g. with a hydrosulfite salt), all of these steps being carried out while a pulp of the clay is cationically dispersed at an acidic pH which, as mentioned, is desirable for effective bleaching. Most preferably, anionic dispersants or added basic materials are not present during any of these steps. The processing takes advantage of the fact that the cationically dispersed slurry of degritted fractionated kaolin is already acidic when the clay is in a condition suitable for bleaching. In conventional kaolin processing, the anionically dispersed clay is flocculated with acid before bleaching and the added acid interacts with the primary anionic dispersant(s) to form salts. Since, in the present invention, the kaolin is already at an acid pH, no separate acid addition is required. Thus, the process of the present invention would be expected to result in a bleached kaolin slurry with a lower quantity of deleterious soluble salts. Further, in practice of our invention the clay is dispersed and therefore bleaching can be carried out at solids higher than those that can be effectively employed when bleaching flocculated slurries. The pulp of fractionated (or fractionated and bleached) clay may then be spray dried to produce a dry kaolin product which, when added to water, results a cationically dispersed clay product. A small amount of additional cationic dispersant may be required before spray drying. When carried out at high solids, such processing avoids the need to employ filtration and washing steps which add significantly to the cost of the conventional wet processing of kaolin crudes. In another embodiment, slurries of the cationically dispersed acidic slip of bleached fine clay, wet processed in accordance with this invention at lower solids, is flocculated by adding an anionic material which neutralizes the cationic dispersant. The flocced clay system is filtered to produce a filter cake which is washed and then fluidized by adding a secondary dispersant which may be either cationic (to produce an acidic cationically dispersed high solids slurry of beneficiated clay intended for shipment in that form) or anionic (to produce an anionically dispersed high solids slurry of anionically dispersed clay intended for shipment in that form.) Accompanying Figure 1 illustrates embodiments for upgrading kaolin crude, all utilizing a cationic material for primary dispersion. One embodiment results in cationically predispersed dry clay (A); another results in a high solids slurry of cationically dispersed clay (B); a third results in a high solids slurry of anionically predispersed dry clay (C); and a fourth results in a high solids slurry of anionically dispersed clay (D). The term "predispersed" is used herein in the conventional sense, i.e., a dry product which when mixed with water results in a slurry in which the kaolin particles are in a dispersed condition without further addition of dispersant. Accompanying Figure 2 illustrates embodiments for upgrading appropriate kaolin crude to produce calcined pigments, all utilizing a cationic material for primary dispersion. One results in a cationically predispersed dry intermediate kaolin (INTERMEDIATE A) and another results in a anionically predispersed intermediate kaolin (INTERMEDIATE B). The term "predispersed" is used herein in the conventional sense, i.e., a dry product which when mixed with water results in a slurry in which the kaolin particles are in a dispersed condition without further addition of dispersant. After a double pulverization step, the hydrous predispersed kaolin products are calcined to produce anhydrous opacifying kaolin pigments. The calcined pigments illustrated are: cationically predispersed hydrous pigment after calcining at 1680°F for 40 minutes (PRODUCT A); cationically predispersed hydrous pigment after calcining at 1980°F for 40 minutes (PRODUCT B); anionically predispersed hydrous pigment after calcining at 1680°F for 40 minutes (PRODUCT C); and anionically predispersed hydrous pigment after calcining at 1980°F for 40 minutes (PRODUCT D). A pulverization step incorporated after the calcining step is required for all products. This invention is described in connection with its utility in upgrading kaolin crudes. Figure 1 illustrates the processing steps used to produce various cationically processed kaolin pigments that result in cationic as well as anionic pigments. The process of the present invention is conveniently carried out by adding a dispersant effective amount of one or more cationic materials to the required amount of water for the desired solids in a vessel equipped with a stirrer. Once the cationic dispersant has dissolved, the crushed kaolin crude is added slowly with sufficient agitation to give a well dispersed fluid suspension. If necessary, the slurry may be passed through a sieve or other conventional degritting apparatus such as a "sandbox" to remove undispersed aggregates or coarse impurities. A crude kaolin cationically dispersed and degritted at a high solids level of 55% to 65% may be further upgraded at high solids by subsequent centrifugation steps to classify the kaolin into more desirable particle size fractions. Since the cationically dispersed kaolin slurries are at an acidic pH ranging from 3.0 to 5.0, reducing bleaches such as sodium dithionite may be added directly, using the same quantities used in conventional anionic processing systems which normally range from 4 lb. to 15 lb./ton active bleach (based on the weight of the dried kaolin in the slurry.) Bleaching improves the color properties of kaolin pigments. The kaolin slurry is then spray dried directly after bleaching thereby eliminating a costly filtration step necessary in low solids processing. The spray dried material is a cationically predispersed hydrous dry kaolin product (PRODUCT A in Figure 1, INTERMEDIATE A in Figure 2). When Product A is added to a bleached slurry of cationically processed clay to increase the solids content, the result is a high solids (above 50%) cationically dispersed hydrous kaolin slurry product with a pH ranging from 3.0 to 5.0 (PRODUCT B in Figure 1). As shown in Figure 1, pigments similar to those processed at high solids to produce A and B, may also be produced by a low solids process, ranging from 20% to 30% slurry solids; however, a slurry filtration step is necessary to increase the solids content prior to spray drying or to produce a high solids cationically dispersed hydrous kaolin slurry. In Figure 2, a product similar INTERMEDIATE A processed at high solids may be produced by a low solids process, ranging from 20% to 30% slurry solids; a slurry filtration step is again necessary to increase the solids content prior to spray drying. To carry out this embodiment of the invention, an appropriate anionic dispersing agent such as sodium silicate, sodium polyacrylate, sodium condensed phosphate or mixture thereof is added to the low solids cationically dispersed slurry in amount sufficient to effect charge reversal of the kaolin to negative and to flocculate the slurry which is filtered, washed, redispersed with a cationic dispersant and spray dried. The spray dried product is pulverized, calcined and repulverized. Thus, cationics are used as both primary and secondary dispersants in practice of this embodiment of the invention. Anionic pigments can be produced from cationically processed kaolin by adding anionic dispersing agents after the bleaching step. To prepare such pigments, appropriate anionic dispersing agents such as alkali silicates (sodium silicate), sodium polyacrylates, tetrasodium pyrophosphate, etc. may be added to the cationic slurries individually or in various combinations in amounts adequate to render a charge reversal of the pigment to negative. The addition of anionic materials to effect charge reversal may be carried out using either low or high solids slurries. When added to high solids slurry a costly filtration step is eliminated and the slurry is sent directly to the spray drier which produces a dry anionically predispersed hydrous kaolin product (PRODUCT C in Figure 1, INTERMEDIATE B in Figure 2). In low solids processing, sufficient anionic dispersant is added to flocculate the slurry which is then filtered with subsequent water washing of the filtercake. The filtercake is redispersed with a secondary dispersant (anionic) to produce a high solids anionically dispersed slurry of hydrous kaolin (PRODUCT D in Figure 1). Calcination of pulverized intermediate products is carried out to produce metakaolin or fully calcined pigments using known pulverizing and calcination equipment such as rotary calciners or Nichols-Herreshoff furnaces. As in conventional processing to produce calcined kaolin pigments, the calciner discharge is repulverized. Presently preferred primary cationic dispersants (or secondary cationic dispersants, when used) are water soluble cationic polyelectrolytes - see, for example, U.S. 4,174,279. Cationic polyelectrolytes are characterized by a high density of positive charge. Positive charge density is calculated by dividing the total number of positive charges per molecule by the molecular weight. Generally the high charge density of polyelectrolytes exceeds 1X10 -3 and such materials do not contain negative groups such as carboxyl or carbonyl groups. In addition to the alkyl diallyl quaternary ammonium salts, other quaternary ammonium cationic polyelectrolytes are obtained by copolymerizing aliphatic secondary amines with epichlorohydrin - See U.S. Pat. No. 4,174,279. Still other water-soluble cationic polyelectrolytes are poly(quaternary ammonium) polyester salts that contain quaternary nitrogen in a polymeric backbone and are chain extended by the groups. They are prepared from water-soluble poly(quaternary ammonium salts) containing pendant hydroxyl groups and bifunctionally reactive chain extending agents; such polyelectrolytes are prepared by treating an N, N, N (1) , N (1) tetraalkylhydroxyalkylenediamine and an organic dihalide such as a dihydroalkane or a dihaloether with an epoxy haloalkane. Such polyelectrolytes and their use in flocculating clay are disclosed in U.S. Pat. No. 3,663,461. Other water soluble cationic polyelectrolytes are polyamines. Polyamines are usually supplied commercially under trade designations; chemical structure and molecular weight are not provided by the suppliers. The aforementioned cationic dispersants are known when used at appropriate dosages to partially flocculate negatively charged clays - see, for example, U.S. 4,738,726 (Pratt et. al.), and references cited therein. It should be noted that as incremental dosages of such cationic materials are added to anionically charged particles, the initial effect is that of flocculation. As dosages increase beyond the levels at which flocculation occurs, dispersion (deflocculation) occurs and the charge on the particles becomes positive. Charge may be determined by the use of the Lazer Zee Meter, Model 501, manufactured by PEN KEM, Inc; ether zeta potential measuring devices can be used. Cationic dispersants used in practice of this invention also include low molecular weight polyamines (e.g., ethylene diamine or hexamethylene diamine), long carbon chain amines or quaternary ammonium salts (e.g., "dimethylditallow" ammonium chloride). Foaming may be a problem with surface active quaternary ammonium compounds such as dimethylditallow ammonium chloride. The amount of cationic dispersant required depends on the nature of the cationic dispersant as well as the nature of the surface of the mineral particles. A lower molecular weight diallyl polymer salt is less effective in conferring a cationic charge than is the same polymer of higher molecular weight. Quaternary ammonium polymers of high charge density are more effective than those of lower charge density. Higher surface area, fine particle minerals require more dispersant than do coarser particles. The magnitude of the anionic charge before treatment with the cationic dispersant also affects the amount required. A mineral carrying a high anionic charge will require a greater amount of cationic dispersant than will a mineral which initially has a lower anionic charge. A dimethyl diallyl quaternary ammonium chloride polymer commercially available under the trademark designation Polymer 261LV from the Calgon Corporation has been found to be particularly useful in the practice of the present invention. The polymer is supplied as an aqueous solution containing approximately 42% active polymer; the supplier estimates the molecular weight of the reagent to be between 50,000 and 250,000. Generally, 0.2 to 0.75 percent active Calgon 261LV is required to disperse crude kaolins in aqueous suspensions in the range of 45% to 60% solids. The particle size distribution and surface area of the kaolin dictate the amount of polymer required to impart cationic dispersion in the slurries. cationically dispersed kaolin slurries are acidic and pH ranges between 3.0 and 5.0 as measured on a pH meter. Kaolins that have been previously treated with anionic agents require even higher amounts of Calgon 261LV to disperse them cationically. The following nonlimiting Examples are given to illustrate the best modes presently contemplated for practicing this invention. All particle sizes in these Examples in the micrometer size range were determined by a conventional sedimentation technique using the Sedigraph® 5000 analyzer. All pH values were determined by an Orion Research pH meter (model 701A). All charge values were obtained by use of the Lazer Zee Meter (supra). When sodium silicate was used in illustrative Examples, N®Brand sodium silicate was employed. N®Brand sodium silicate is a solution containing about 8.9% wt. Na 2 O, about 28.7% wt. SiO 2 and the balance water. Example 1 This example illustrates the use of cationic dispersion in the wet processing of a Georgia kaolin crude (Washington County, Georgia) of the type conventionally processed using only anionic dispersants to produce a No. 1 grade of paper coating kaolin. The crude kaolin used was a well crystallized kaolin. Typical crude of this type contains about 1.76% wt. titania and about 0.34% wt. iron (as Fe 2 O 3 ) as impurities. Median particle size of the degritted kaolin is about 1.6 micrometers (equivalent spherical diameter); about 92% by weight of the particles is <10 micrometers, 80% <5 micrometers, and 56% <2 micrometers. The crushed crude kaolin was received at 78% solids (22% volatiles). Deionized water (1540 g) was added to a beaker. Using moderate speed stirring provided by a drill press mixer equipped with a 2" uplift stainless steel propeller, 5128 g of the crude kaolin was gradually added to the water with alternate drop-by-drop additions of Calgon 261LV polymer (42% active) until the crude appeared dispersed when examined visually. The slurry was stirred for an additional 90 minutes after combining all of the ingredients. The total amount of cationic polymer added was 0.175% (100% active weight basis) based on the dried weight of the kaolin. Solids content in the dispersed slurry was 60% and the pH was 3.6. The 60% solids slurry was then diluted to 42% solids by adding 2858 g of deionized water while the slurry was stirred. The slurry pH after dilution was 3.6. The diluted acidic cationically dispersed slurry was degritted by passing it over a vibrating 325 mesh sieve (U.S.). The >325 mesh portion that collected on top of the screen was set aside and the <325 mesh portion passing through the screen was collected in a beaker. Solids content in the <325 mesh slurry measured after mixing with a spatula was found to be 34.9%, resulting in a 82% yield. The lower than normal yield obtained for the <325 mesh portion may have been caused by an inadequate amount of Calgon 261LV to disperse the crude kaolin sufficiently prior to degritting. A +47mv zeta potential value was obtained using the Lazer Zee Meter after a portion of the <325 mesh fraction was diluted with supernatant extracted from a separate portion of the same fraction. The <325 mesh slurry was then fractionated to obtain the <2 micrometers particles by using a conventional gravitational sedimentation method. The slurry was divided equally into two-gallon plastic beakers. The slurry height in each of the beakers was adjusted to 22 cm by adding more deionized water followed by stirring on the drill press. Solids content in each slurry was 17.7%. After permitting the slurries to stand undisturbed for 16 hours, no hard sediment had formed in either beaker, indicating that the slurries were not adequately dispersed. To improve dispersion, 0.05% Calgon 261LV polymer (based on the weight of the dried kaolin) was added to one of the slurries during stirring on the drill press mixer. After permitting the slurry to stand undisturbed for 6 hours, a hard sediment had formed indicating adequate dispersion. The same amount of cationic polymer was then added to the other slurry which resulted in a total of 0.225% (active) polymer addition based on the weight of the dried kaolin in each slurry. Both of these further dispersed slurries were recombined, stirred manually, and redivided into three separate containers. The slurry height in each of the containers was 8 cm. All three slurries were permitted to stand undisturbed for 6 hours before gently pouring out the supernatants. The three supernatants were retained together in a single plastic bucket and the remaining hard sediments at the bottom of each container were diluted by adding approximately 1 liter of deionized water to each one. The sediments were redispersed during stirring on the drill press mixer. The three slurries formed from the sediments were recombined and mixed. A second gravitational sedimentation was carried out to remove additional <2 micrometers material from the sediments formed by the previous fractionation. The slurry (from sediments) was diluted with enough deionized water to give 8 cm slurry heights after dividing into the same three containers used for the first gravitational sedimentation. The three slurries were permitted to stand undisturbed for 6 hours before gently pouring out the supernatants. The three supernatants were retained in a separate container and the remaining sediments formed in the three containers were transferred to a single 2 gallon plastic beaker. A third and final gravitational sedimentation was carried out to recover more <2 micrometers material from the sediments formed by the second fractionation. The previously combined sediments were diluted with deionized water until the slurry was 21 cm high in one beaker and after stirring on the drill press mixer the slurry pH was 3.6. After permitting the slurry to stand undisturbed for 16 hours, the supernatant was gently poured out and collected in a separate beaker and the remaining sediment was set aside. All supernatants retained from the three gravitational sedimentations were mixed together in a single container resulting in a 5.38% solids slurry. The yield of <2 micrometers kaolin was determined to be 1556 g or 47.5% based on <325 mesh fraction. The <2 micrometers fine fraction kaolin was bleached with 8 lb. of sodium dithionite reducing bleach per ton of dry kaolin. This was performed by sifting 6.225 g of the bleach powder into the slurry during low speed stirring on the drill press mixer. After 5 minutes of mixing the slurry appeared visually brighter. After static aging the slurry for 18 hours its pH was 3.2 and specific conductivity was 0.586 micromhos. A +17 mv zeta potential value was obtained using the Lazer Zee meter after diluting a portion of the bleached slurry with clear supernatant extracted by centrifuging another portion of the main bleached slurry. A preliminary filtration test was performed by extracting three 100 g samples from the main bleached slurry after aging and mixing. The first sample was filtered "as is" on a Buchner funnel under vacuum using Whatman #5 filter paper. Aside from a few cloudy drops of filtrate at the beginning of the filtration the filtrate was clear. Sodium hydroxide solution (2% active) was added to the second sample to increase pH to 7.0 and after permitting the sample to static age for a few minutes clear supernatant formed at the top portion of the slurry indicating good flocculation of the kaolin particles. Sodium polyacrylate solution (2%) was added to the third slurry to increase pH to 7.0 and after static aging the sample for several minutes a cloudy supernatant formed at the top indicating poorly flocculated kaolin. The main portion of bleached slurry was divided into two separate portions for filtration. One portion weighing 5700 g was filtered "as is" on a 24 cm Buchner funnel with vacuum (Whatman #5 paper), and a 1000 ml deionized water rinse followed. The pH of the second portion of bleached slurry was adjusted to 7.0 with sodium hydroxide solution (10% active) with mixing. After 1 hour static aging the clear supernatant that formed at the top of the slurry was gently poured off and discarded and a zero mv zeta potential value was obtained from the flocculated kaolin portion. The flocculated slurry was then divided into three equal portions for filtration on three 24 cm Buchner funnels. Each filtercake was rinsed with 1000 ml of deionized water. Example 2 This example illustrates the use of cationic dispersion in the wet processing of a 50%:50% blend of two Georgia kaolin crudes from mines commonly referred to as Dixie and Califf. The crude blend contains a large number of finely divided kaolin particles and is of the type useful in producing low abrasion calcined kaolin opacifying pigment. A control sample to illustrate conventional anionic wet processing was also prepared. The kaolins were received as crushed crudes. The Dixie sample was at 80.2% solids (19.8% volatiles); and the Califf sample was at 81.6% solids (18.4% volatiles). Following is a description of the wet processing of the crude blend using cationic primary dispersion. Deionized water (4930 g) was weighed into a two gallon beaker. Using low speed stirring (300 r.p.m.) provided by a drill press mixer equipped with a 2" uplift stainless steel propeller, 0.60% active Calgon 261LV (44.5% active aqueous solution) based on the dried weight of the crude kaolin was added to the deionized water. Slowly and simultaneously 2500 g of each crude calculated on a dry weight basis (3117 g Dixie and 3064 g Califf) was added to the diluted polymer solution during continuous stirring at a moderate speed. To further disperse the slurry, an additional 0.15% active Calgon 261LV (44.5%) based on the dried weight of the kaolin was added to the slurry and stirring continued for one hour at an increased speed of 1000 r.p.m. The dispersion of the slurry was checked by examining the sediment that formed after permitting the slurry to settle in the beaker overnight. The supernatant portion was poured out and retained and the sediment that had formed was hard and evenly stratified, indicating adequate dispersion in the slurry for degritting and fractionation. The supernatant portion was recombined-with the sediment and stirring at 1000 r.p.m. on the drill press followed. The slurry pH was 4.0. Degritting the acidic cationically dispersed kaolin slurry was carried out by passing it over a 100 mesh (U.S.) sieve. The >100 mesh residue was set aside and the <100 mesh slurry that passed through the sieve was collected and immediately passed over a 325 mesh sieve (U.S.). The >325 mesh residue that collected on top of this sieve was combined with the previously collected >100 mesh residue. This portion was set aside. Yield of the <325 mesh portion was found to be 67% and the slurry solids was 25.2%. A +50 mv zeta potential value on the <325 mesh fraction was obtained with the aid of the Lazer Zee Meter (model 501 Pen Kem Inc.) after diluting a portion of the slurry with deionized water. During moderate stirring on the drill press, the 25.2% solids slurry was diluted to 15% solids with an addition of 8770 g of deionized water and the slurry was dispersed further by adding 0.05% active Calgon 261LV based on the dried weight of the kaolin. The <325 mesh slurry was then fractionated to extract the <1 micrometer particles (equivalent spherical diameter) by using a conventional gravitational sedimentation method. The slurry was divided equally into two five gallon plastic buckets (having straight walls), and the resulting slurry height in each of the buckets was 19 cm. After permitting the slurries to static settle for 53 hours, the supernatant portions were gently poured out and retained. The hard sediments that had formed on the bottom of the buckets were diluted with deionized water, mixed on the drill press, combined and remixed vigorously on the drill press. The slurry formed from the sediments was at 17.9% solids. A second gravitational sedimentation was carried out to recover additional <1 micrometer material from the sediments formed by the previous fractionation. The slurry (from sediments) was diluted to 15% solids by adding deionized water during stirring on the drill press. The height of the slurry in the bucket was 21 cm. After permitting the slurry to settle undisturbed for 40 hours, the top 13 cm of supernatant was gently siphoned off and collected in a separate beaker. The supernatants obtained from the first and second gravitational sedimentations were combined and stirred. Yield of the <1 micrometer fraction was found to be 64.0% and the slurry solids measured 7.6%. The <1 micrometer fraction was upgraded further by passing the slurry through a high intensity magnetic separator Model # WHIMS 3X4L (Carpco Inc.) containing a steel wool matrix. Slurry was passed through the magnet at 100 ml per minute flow rate. The product collected was visually brighter and had a 6.1% solids content and a pH of 4.0. To improve the brightness of the <1 micrometer fraction of acidic cationically dispersed magnetically purified kaolin slurry, it was bleached with 6 lb. of sodium dithionite reducing bleach based on one ton of dried kaolin. This was performed by sifting bleach powder into the slurry during manual stirring followed by 5 more minutes of stirring. The slurry was then permitted to static age overnight in a covered plastic bucket. The final step in the cationic wet processing of this sample was to remove excess liquid. Since the solids content in the slurry was only 6.2% and filtration by vacuum in Buchner funnels would consume too much time, the slurry solids was increased to 30.5% before filtration by centrifugation. This was done by centrifuging several portions of the low solids slurry in a SORVALL® SS-3 centrifuge at 9500 r.p.m. until a clear supernatant was obtained. The clear supernatants were poured off and discarded and the sediments formed during centrifugation were mixed into the bleached slurry. The 30.5% solid slurry was then filtered under vacuum on several Buchner funnels by placing 655 g of slurry in each funnel (200 g dry kaolin). Each filter cake was rinsed twice with 200 ml of deionized water. The rinsed filter cakes were then removed from the funnels and dried at 180°F for 4 hours. The dried kaolin was then pulverized twice in a MIKRO-SAMPLMILL using the 0.039" round hole screen. The particle size distribution of the cationically processed hydrous kaolin product was: 92% <2 micrometers, 80% <1 micrometer, 53% <0.5 micrometer, 0% <0.2 micrometer and median size was 0.48 micrometer. Block brightness of the pigment when measured on the Elrepho Reflectance Meter (Carl Zeiss Corp.) was 87.9%. Preparation of an anionically processed kaolin pigment was carried out for purposes of comparison following the same wet processing steps used to produce the cationic pigment. The processing steps were slurry makedown with a primary dispersant, degritting, fractionating, high intensity magnetic separation, bleaching, filtration, and washing. In the initial step of a slurry makedown with a primary dispersant, N®Brand sodium silicate (37.7% active aqueous solution) was added to the deionized water in the amount of 0.20% sodium silicate (dry basis) based on the dried weight of the crude kaolin blend (2500 g of Dixie and 2500 g of Califf). The crude kaolin was added to the diluted sodium silicate solution until the slurry-solids was 45%. The slurry pH was 8.5. The yield of <325 mesh kaolin after degritting she slurry was 65% and the slurry was 31.1% solids (pH 8.5). Two gravitational sedimentations to obtain the <1 micrometer size kaolin particles were carried out after diluting the 31.1% solids degritted portion to 15% solids with deionized water. After the initial fractionation the sediment was diluted to 15% solids again, however, 0.05% sodium silicate based on the dried weight of the kaolin was added to the slurry to improve dispersion. The slurry pH was 8.4 and the kaolin was sedimented again to recover more of the <1 micrometer particles. The yield of the <1 micrometer fraction was 69.0% based on the <325 mesh portion and the slurry contained 9.0% solids. High intensity magnetic separation followed producing a brighter slurry. The slurry solids were 7.3%. The magneted slurry was further upgraded by dithionite bleaching. Since the slurry pH was 7.9 and dithionite bleaching is most effective in acidic systems, the slurry pH was adjusted to 3.0 by adding sufficient sulfuric acid (10% active solution) prior to the addition of bleach. Bleaching with sodium dithionite in an amount equivalent to 6 lb. per ton of dried kaolin was carried out by adding the bleach to the slurry while stirring. The pH of the bleached slurry after static aging overnight was 3.5. To remove the liquid phase from the slurry, it was filtered under vacuum on several Buchner funnels (2700 g per funnel) and the filter cakes were rinsed twice with 200 g of deionized water (2:1 rinse). The filter cakes were removed from the funnels and dried at 180°F for 4 hours. Pulverization of the dried filter cakes followed by passing them twice through the MIKRO-SAMPLMILL. The particle size distribution of the anionically processed hydrous kaolin product was: 98% <2 micrometers, 96% <1 micrometer, 82% <0.5 micrometer, 36% <0.2 micrometer, and the median size was 0.25 micrometer. Block brightness of the pigment when measured on the Elrepho Reflectance Meter was 87.2%. Example 3 This example illustrates the use of a cationic primary dispersant in the wet processing of a Georgia kaolin crude from a mine commonly referred to as Dixie. A control sample to illustrate conventional anionic wet processing was also prepared. As in Example 2, the crude used contains a large number of finely divided kaolin particles and is of the type useful in producing low abrasion calcined kaolin opacifying pigments. The Dixie crude was received as a crushed crude containing 80.2% solid materials (19.8% volatiles). The use of a primary cationic dispersant in wet processing is described first. The crude kaolin was made down in a slurry form at 45% solids as follows. Deionized water (4876 g) was weighed into a two gallon plastic beaker. Using low speed stirring (300 r.p.m.) provided by a drill press mixer equipped with a 2" uplift stainless steel propeller, 0.53% active Calgon 261 LV (44.5% solids) based on the weight of the dried kaolin was added to deionized water. Slowly and continuously 6235 g of Dixie crude (5000 g dry) was added to the diluted polymer solution during continuous stirring at a moderate speed (600 r.p.m.). After all of the crude was added, the stirring speed was increased to and maintained at 1000 r.p.m. for one hour. The dispersion of the slurry was checked by examining the sediment that formed after permitting the slurry to settle in the beaker overnight. The supernatant was poured out and retained and the sediment that had formed was hard and uniformly stratified, indicating adequate dispersion of the slurry for degritting and fractionating. The supernatant was recombined with the sediment and stirring at 1000 r.p.m. followed. The slurry was pH was 4.0. Degritting the acidic cationically dispersed kaolin slurry was carried out by first passing it over a 100 mesh (U.S.) sieve. The >100 mesh residue was set aside and the <100 mesh slurry that passed through the sieve was collected and immediately passed over a 325 mesh (U.S.) sieve. Deionized water was used to rinse both of the sieves after which the two residues were recombined and set aside. Yield of the <325 mesh portion was found to be 86.9% and the slurry solids was 26.4%. The degritted crude was then prepared for fractionating by a conventionally used gravitational sedimentation method to obtain only the finely divided particles measuring <1 micrometer (equivalent spherical diameter) in size. During moderate speed stirring on the drill press, the 26.4% solids cationically dispersed slurry was diluted to 15% solids with deionized water. The slurry was then divided equally into two straight walled five gallon plastic buckets which gave a 21 cm slurry height in each. After static settling the slurries for 53 hours, the top 17.5 cm of supernatant was siphoned off and retained. The hard sediments that had formed at the bottom of each bucket were diluted with deionized water to 15% solids, redispersed on the drill press mixer, recombined and then remixed at 1000 r.p.m. by the drill press. The height of the slurry in the bucket was 20 cm and pH was 4.0. To recover more fine material, the slurry was permitted to settle for 40 hours and the top 13 cm of supernatant was siphoned off and combined with the supernatant retained from-the previous sedimentation. Yield of <1 micrometer material was found to be 63.8%. The final step of cationically wet processing this sample was to remove the excess liquid. Since the volume of the slurry was large and the solids content low and filtration by vacuum in Buchner funnels would take a long time, the slurry solids was increased by centrifuging several portions of the slurry in the SORVALL centrifuge at 9500 r.p.m. until clear supernatants were obtained. The clear supernatants were poured out and discarded and the sediments formed during centrifugation were mixed into the original slurry, thereby increasing its solids content. This higher solids slurry was then filtered under vacuum on several Buchner funnels, in an amount equal to 200 g of dried kaolin. The filter cakes were removed from the funnels and dried at 180°F for 4 hours and double pulverization in a MIKRO-SAMPLMILL using the 0.039" round hole screen followed. The particle size distribution of the cationically processed hydrous kaolin product was: 95% <2 micrometers, 81% <1 micrometer, 50% <0.5 micrometer, 0% <0.2 micrometer, and the median size was 0.50 micrometer. Block brightness of the pigment was 85.4%. For purposes of comparison, an anionically processed pigment was prepared by following the same wet processing steps used to produce the previously made cationic pigment, but using an anionic primary dispersant. Processing steps included slurry makedown with a primary dispersant, degritting, fractionating, and filtering. Processing sequence, equipment, addition rates, and mixing speeds to process the anionically dispersed system were the same as those used for the previously processed cationically dispersed system. In the initial step of slurry makedown with a primary dispersant, sodium silicate (37.7% active aqueous solution) was added to the deionized water in the amount of 0.20% based on the dried weight of the crude kaolin (5000 g). The crude kaolin was added to the diluted sodium silicate solution which resulted in 45% solids slurry and the slurry pH was 8.6. After degritting, the <325 mesh yield was 83.2% and the slurry contained 33.3% solids. Two gravitational sedimentations to obtain the <1 micrometer size kaolin particles were carried out by diluting the degritted slurry to 15% solids with deionized water. After the initial fractionation the sediment was diluted to 15% solids and the slurry was settled again. Yield of the combined fine fractions was 64.1% and the pH of the slurry was 8.3. The next step in processing was to remove the excess liquid by filtration. To speed filtration, the fine fraction slurry was flocculated by adding sulfuric acid (10% active solution) in an amount sufficient to reduce the pH to 3.5. After permitting the flocculated slurry to stand overnight, the kaolin flocs settled and the clear supernatant was poured out and discarded. The concentrated slurry was then filtered on several Buchner funnels (200 g dry kaolin each). Each filtercake was rinsed twice with 200 ml of deionized water after which they were dried at 180°F for 4 hours and pulverized with two passes through the MIKRO-SAMPLMILL using the 0.039" round hole screen. The particle size distribution of the anionically processed hydrous kaolin product was: 98% <2 micrometers, 93% <1 micrometer, 75% <0.5 micrometer, 33% <0.2 micrometer, and the median size was 0.28 micrometer. Block brightness was 85.7%. Example 4 Tests were carried out to compare the effect of cationic and anionic wet processing on the properties of calcined kaolin opacifying pigment obtained by calcining kaolin products obtained in Examples 2 and 3. The objective was to determine whether cationic processing would influence significant properties of the calcined pigment such as brightness, opacification, abrasion and printing properties. Abrasiveness of pigment slurries was measured using the "Einlehner" AT-1000 tester. This test measures weight loss of bronze fourdrinier wire when subjected to the rubbing action of a plastic abrader immersed in the pigment slurry. Results are expressed as mg of weight lost per 100,000 revolutions of the abrader. Complete details of the test are given by Neubold, Sennett and Morris, "Abrasiveness of pigments and extenders", Tappi Journal, Dec. 1982, p. 90. The "needle" abrasion test is designed to measure the abrasivity of pigment coated or filled paper. In this test, the weight loss of a brass needle is measured after a known number of penetrations of a brass needle through the paper. A complete description of the method is given by Kurrle, "Correlation of IPC needle penetration test with guillotine trimmer knife life", Tappi Journal, July 1980, p. 100. Samples of wet processed, dried and pulverized fine fractions of the kaolin from Example 2 (Dixie/Califf crude) and Example 3 (Dixie crude) were used in the tests. Portions of the dried and double pulverized fine fractions were calcined in laboratory muffle furnaces preheated to 1680°F or 1980°F. The 100% Dixie fractions (anionic and cationic processed) of Example 3 were treated at 1680°F for 40 minutes in an attempt to yield 90% block brightness products and the 50% Dixie/50% Califf magnetically treated and bleached fractions (anionic and cationic processed) were calcined at 1980°F for 40 minutes to yield 93% block brightness products. All of the calcined products were then pulverized in the MIKRO-SAMPLMILL, 1 pass through the 0.039" round hold screen. All four calcined, repulverized products were initially made down at low shear into 50% solids slurries with deionized water and 0.10% polyacrylate (C211) dispersant, followed by high shear mixing on the Waring Blendor® mixer for 1 minute. Additional dispersant was added to each slurry until a minimum Brookfield viscosity was obtained. Each optimally dispersed slurried sample was then incorporated into a rotogravure paper coating. The coating formulations prepared at 57% solids contained: 85 parts Nuclay® hydrous kaolin, 15 parts calcined clay, 7 parts Penford Gum® 280 starch, 4 parts Dow 620A latex, and 0.5 parts Nopcote® C-104 calcium stearate emulsion (all parts on dry basis). All coatings were finally adjusted to pH 8.0 with dilute ammonium hydroxide. Coating weights of 4.4, 5.5, and 6.7 pounds coating per 3300 square feet of paper for each coating was applied to the wire side of a light weight (25#/ream) publication base stock paper with the aid of the Dow blade coater. Paper optical and print properties were determined using conventional techniques on sheets supercalendered 2 nips at 260 pli and 140°F. TABLE I lists physical properties of the processed 100% Dixie crudes from Example 3. Particle size distribution of the cationic calciner feed shows no particles with diameters less than 0.2 micrometer while the anionic calciner feed has a considerable amount of these particles. Calcination of the two feeds at 1680°F for 40 minutes resulted in block brightness increases for the cationic material from 85.4% to 89.8%, a 4.4 point improvement, and for the anionic material from 85.7% to 88.5%, a 2.8 point improvement. Also shown in TABLE I is that high shear mixed and optimally dispersed slurries prepared at 50% solids content for both systems had equivalent slurry pH and Brookfield viscosity. Hercules viscosity of the cationically calcined clay product was improved. Films laid on black glass plates from 30% solids slurries (diluted 50% solids slurries) showed a 64% gloss for the cationically processed calcined material and a 70% gloss for the anionically processed calcined material. Light scattering coefficients of both systems were virtually the same. An Einlehner abrasion of 6 mg loss/100,000 revolutions for the cationically processed calcined products was 33% lower than the 9 mg loss/100,000 revolutions value obtained for the anionically processed calcined product. Surprisingly, particle size distribution for the cationically processed calcined product was considerably coarser than the anionically processed product in spite of the fact that it was less abrasive. TABLE II lists physical properties of the processed 50%/50% blend of Dixie/Califf crudes (Example 2). Particle size distribution of the cationic calciner feed show no particles with diameters less than 0.2 micrometers while the anionic calciner feed has a considerable amount of these particles. Block brightness was marginally better for the cationic calciner feed (87.9%) than the anionic calciner feed (87.2%). Calcining both materials increased their block brightness values to 93.0%. High sheared optimally dispersed 50% solids content slurries for both systems had equal slurry pH and Brookfield viscosity. Hercules viscosity was better for the cationically processed calcined product. Films laid on black glass plates from 30% solids slurries (diluted 50% solid slurries) had 57% gloss for the cationically processed calcined material and 65% gloss for the anionically processed calcined material. Light scattering coefficients of both systems were the same. Einlehner abrasion of 20 mg value obtained for the cationically processed calcined product was 30% lower than the 29 mg loss/100,000 revolutions value obtained for the anionically processed calcined product. However, needle abrasion was higher. Particle size distribution for the cationically processed calcined product was considerably coarser than the anionically processed calcined product. TABLE III lists optical properties obtained from supercalendered sheets coated at 4.4, 5.5, and 6.7 lb/3300 ft 2 with 15 parts of each of the four calcined pigments and 85 parts Nuclay starch/latex coating formulations on a 25#/ream base stock. No significant differences in sheet gloss, brightness and opacity at equivalent coating weights were observed. TABLE IV lists printing properties for the same supercalendered coated sheets. The test procedures used are known in the industry and are described or cited in U.S. 4,738,726, Pratt, et al supra. All four calcined products exhibit equal IGT dry pick strength at equal coating weights. However, Heliotest data showed cationically processed calcined products had significantly better rotogravure printability than anionically processed calcined products. Chemical compositions determined on the calcined products are listed in TABLE V. The cationically processed materials had marginally lower amounts of titanium dioxide and calcium oxide than the anionic materials which may be due to the differences in particle size distribution between the two. In conclusion, overall evaluation of all systems show greater improvements in rotogravure printability and Einlehner abrasion for kaolins processed cationically before calcination than ones processed anionically. While this invention has been described with particular emphasis on its application to upgrading kaolin ores, it will be understood that principles of the invention can be applied to upgrading other ores such as other silicate or aluminous ores and carbonate ores. When processing carbonate ores such as calcitic or dolomitic ores the pH during primary dispersion should be sufficiently high to avoid decomposition of the ore. Physical Properties of 100% Dixie Fine Fraction Kaolins Processed Anionically and Cationically. DIXIE PROPERTIES ANIONIC PROCESS CATIONIC PROCESS HYDROUS KAOLIN Particle size distribution, e.s.d. wt. % less than 2 micrometers 98 95 wt. % less than 1 micrometer 93 81 wt. % less than 0.5 micrometer 75 50 wt. % less than 0.2 micrometer 33 0 median (micrometers) 0.28 0.50 Block Brightness, % 85.7 85.4 CALCINED KAOLIN Temp./Time 1680°F/40 min. 1680°F/40 min. Block Brightness, % 88.5 89.8 50% Solids highly sheared slurries % C211 (polyacrylate) dispersant 0.200 0.175 slurry pH 6.1 6.0 Brookfield viscosity cps @ 10 rpm 40 36 cps @ 20 rpm 38 36 cps @ 100 rpm 58 58 Hercules viscosity rpm/dyne-cm X10 5 360/16 580/16 30% Solids slurries black glass evaluation 75° gloss (%) 70 64 S457 m 2 /kg 218 227 S577 m 2 /kg 132 142 Einlehner Abrasion (mg/100,000 revolutions) 9 6 Particle size distribution, e.s.d. % less than 2 micrometers 98 92 % less than 1 micrometer 84 71 % less than 0.5 micrometer 24 19 % less than 0.2 micrometer 0 0 median (micrometer) 0.64 0.74 Physical Properties of 50% Dixie/50% Califf Fine Fraction Kaolins Processed Anionically and Cationically. 50% DIXIE : 50% CALIFF PROPERTIES ANIONIC PROCESS CATIONIC PROCESS HYDROUS KAOLIN Particle size distribution, e.s.d. wt. % less than 2 micrometers 98 92 wt. % less than 1 micrometer 96 80 wt. % less than 0.5 micrometer 82 53 wt. % less than 0.2 micrometer 36 0 median (micrometers) 0.25 0.48 % Block Brightness 87.2 87.9 CALCINED KAOLIN Temp./Time 1980°F/40 min. 1980°F/40 min. % Block Brightness 93.0 93.0 50% Solids highly sheared slurries % C211 disp. on clay 0.150 0.125 slurry pH 5.7 5.6 Brookfield viscosity cps @ 10 rpm 44 40 cps @ 20 rpm 40 38 cps @ 100 rpm 70 69 Hercules viscosity rpm/dyne-cm X10 5 515/16 1035/16 30% Solids slurries black glass evaluation 75° gloss (%) 65 57 S457 m 2 /kg 266 261 S577 m 2 /kg 169 171 Einlehner Abrasion (mg/100,000 revolutions) 29 20 Needle Abrasion (mg) 370 480 Particle size distribution % less than 2 micrometers 95 86 % less than 1 micrometer 76 65 % less than 0.5 micrometer 16 9 % less than 0.2 micrometer 0 0 median (micrometers) 0.72 0.80 Optical Properties of Anionically and Cationically Processed Calcined Kaolins Coated on 25 lb. Lightweight Publication Base Stock Paper OPTICAL PROPERTIES 100% DIXIE CALCINED AT 1680°F 50% DIXIE/50% CALIFF CALCINED AT 1980°F ANIONIC PROCESS CATIONIC PROCESS ANIONIC PROCESS CATIONIC PROCESS coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) 4.3 5.5 6.7 4.4 5.5 6.7 4.4 5.4 6.7 4.4 5.6 6.7 75° GLOSS (%) 35.3 43.5 52.4 36.8 44.9 53.8 37.9 45.6 54.0 37.8 45.1 52.5 BRIGHTNESS (%) 75.7 76.6 77.4 75.7 76.5 77.4 76.1 76.9 77.6 75.9 76.9 77.5 OPACITY (%) 85.4 86.8 88.2 85.4 86.6 87.8 85.6 86.7 88.1 85.1 86.4 88.0 Printing Properties of Anionically and Cationically Processed Calcined Kaolins Coated on 25 lb. Lightweight Publication Base Stock Paper PRINTING PROPERTIES 100% DIXIE CALCINED AT 1680°F 50% DIXIE/50% CALIFF CALCINED AT 1980°F ANIONIC PROCESS CATIONIC PROCESS ANIONIC PROCESS CATIONIC PROCESS coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) coating weight (#/3300 sq. ft.) 4.3 5.5 6.7 4.4 5.5 6.7 4.4 5.4 6.7 4.4 5.6 6.7 IGT DRY PICK (vvp) 3.0 3.4 3.8 3.2 3.6 4.0 3.2 3.4 4.0 3.0 3.6 4.2 HELIOTEST (cm to 20th missing dot) 2.6 3.6 5.5 3.6 4.4 6.7 3.4 4.3 7.4 4.2 4.8 8.8 Chemical Composition of Cationically and Anionically Processed Calcined Products COMPONENT* 100% DIXIE 1680°F/40 min. 50% DIXIE/50% CALIFF 1980°F/40 min. ANIONIC PROCESS (wt.%) CATIONIC PROCESS (wt.%) ANIONIC PROCESS (wt.%) CATIONIC PROCESS (wt.%) SiO 2 52.9 53.5 53.3 53.1 Al 2 O 3 44.5 44.8 44.7 44.9 Fe 2 O 3 0.96 0.96 0.90 0.88 TiO 2 1.50 1.41 1.38 1.27 CaO 0.14 0.02 0.09 0.01 K 2 O 0.20 0.12 0.11 0.11 Na 2 O 0.16 0.17 0.18 0.18 MgO 0.07 0.06 0.09 0.09 P 2 O 5 0.04 0.05 0.04 0.03

Claims: A method of wet processing crude kaolin ore to recover a fine particle size fraction of kaolin which comprises forming an aqueous pulp of the crude kaolin ore, dispersing the crude kaolin in the aqueous pulp as particles, removing coarse particles from said dispersed pulp and then fractionating the dispersed pulp to separate and recover a fine particle size fraction of kaolin, and optionally adding bleach reagent to said dispersed pulp to bleach kaolin particles, characterised in that said dispersing is effected by adding to the aqueous pulp sufficient water-soluble organic cationic polyelectrolyte dispersant to impart a positive zeta potential to said particles and form a cationically dispersed acidic pulp A method according to claim 1 which includes pulverising said crude kaolin ore before pulping the pulverised crude in water. A method according to claim 2 wherein said pulp is fractionated and the fine particle size fraction is bleached at high solids and then spray dried without prior filtration to provide a cationically dispersed grade of kaolin clay. A method according to claim 2 wherein said bleach comprises dithionite salt. A method according to claim 4 wherein said fine particle size fraction is bleached with dithionite salt in a pulp at low solids, said pulp is flocculated before or after bleaching by adding anionic electrolyte, and the flocculated pulp is filtered to produce a filter cake which is redispersed by adding anionic or cationic electrolyte to form a high solids slurry. A method according to claim 5 wherein said anionic electrolyte is selected from polyacrylates, condensed phosphate salts, and alkali metal silicates. A method according to claim 2 wherein said fine particle size fraction is bleached in a pulp at low solids and said pulp is flocculated by adding anionic electrolyte, and the flocculated pulp is filtered to produce a filtercake which is redispersed by adding cationic dispersant to form a high solids slurry. A method according to claim 1 which comprises blunging said crude kaolin ore in water in the presence of said cationic polyelectrolyte dispersant to produce said acidic dispersed pulp, removing grit from said pulp and then fractionating the degritted pulp to recover a fine particle size fraction having a greater content of particles finer than 2 micrometers than the degritted kaolin, bleaching said fine particle size fraction kaolin with dithionite bleach reagent, and recovering bleached cationically dispersed fine particle size kaolin, said degritting, fractionation and bleaching all taking place while said kaolin is cationically dispersed in a pulp having an acidic pH. A method according to claim 8 in which said crude kaolin is degritted, fractionated and bleached at a kaolin solids content of (a) at least 40% or (b) at least 50%, and the pulp of bleached kaolin is spray dried without filtration. A method according to claim 8 or 9 wherein said pulp of bleached kaolin is diluted with water containing anionic material in an amount sufficient to flocculate the pulp which is filtered to produce a filter cake, and said filter cake is redispersed to produce a dispersed slurry of fine particle size bleached kaolin. A method according to claim 10 wherein said anionic material is selected from polyacrylates, condensed phosphate salts and alkali metal silicates, and/or said filter cake is redispersed by adding polyacrylate salt or cationic dispersant. A method according to claim 1 which comprises forming the crude kaolin ore into an aqueous pulp containing water and said cationic polyelectrolyte dispersant in said amount sufficient to disperse the crude kaolin particles, removing grit from said cationically dispersed pulp, centrifugin or otherwise fractionating the resulting degritted cationically dispersed pulp to separate at least one fine particle size fraction, drying said fine particle size fraction of kaolin, and pulverising, calcining and repulverising the fine particle size fraction to form a low abrasion calcined kaolin pigment, bleach being added prior to fractionation. A method according to claim 12 wherein said cationically dispersed fine particle size fraction is bleached at high solids without adding acid and then spray dried without filtration prior to pulverisation. A method according to claim 12 wherein said pulp of cationically dispersed fine particle size fraction is flocculated by adding anionic dispersant, the resulting flocculated slurry is filtered and washed, the resulting filter cake is dispersed by adding anionic or cationic dispersant and the dispersed filter cake is spray dried prior to pulverisation. A method according to claim 1 which comprises blunging said crude kaolin ore in water in the presence of said cationic polyelectrolyte dispersant to produce said acidic dispersed clay pulp, the kaolin in said crude being at least 80% by weight finer than 2 micrometers, removing grit from said pulp and then fractionating the degritted pulp to recover a fine particle size fraction having a greater content of particles finer than 2 micrometers than the degritted kaolin, drying said fine particle size fraction, and then pulverising, calcining and repulverising said calcined kaolin to form a low abrasion calcined kaolin pigment. A method according to claim 15 in which said crude ore is degritted, fractionated and bleached while the kaolin is cationically dispersed and the pulp of bleached kaolin is spray dried without filtration before pulverisation and calcination. A method according to any of claims 1, 2, 15 and 16 wherein bleach reagent is added to the dispersed pulp before or after fractionation. A method according to any of claims 1 to 4, 8, 9, 12, 13, 15, 16 and 17 wherein anionic dispersant is absent throughout. A method according to any preceding claim wherein said cationic polyelectrolyte dispersant has a molecular weight of 1x10 4 to 1x10 6 . A method according to claim 19 wherein said cationic dispersant is selected from quaternary ammonium polymer (including diallyl ammonium polymer) salts and/or has a molecular weight between 50,000 and 250,000. A method according to any preceding claim wherein the pH of said cationically dispersed pulp is (a) from 3 to 5 or (b) about 4. A method according to any preceding claim wherein sufficient of said cationic dispersant is added to impart to said pulp a pH below 5.

Inventors: BROWN STEVEN L, SENNETT PAUL, BROWN, STEVEN L., SENNETT, PAUL
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1990
Main classification: B03B1
IPC codes: B03B1, C09C1