Kaolin Wet-Processing

Prepared by Eng. Atef Helal / May 4, 2012

Sources :Mainly SME / www.smenet.org / The Georgia Kaolins Geology and Utilization – Copyright © 2002 & HAYDN H. MURRAY; Industrial Applicaions of Kaolin;  Georgia Kaolin Company,Elizabeth, New Gersy & Hayden H. Murray 2007 / Applied Clay Mineralogy (Occurrences,Processing and Application)

-------------

Kaolinite is the proper name of the mineral about which this article is written, but in industrial terminology the mineral is known as kaolin, so all references will be to kaolin.

The theoretical chemical composition of kaolin and a chemical analysis of a crude kaolin from Dry Branch, Georgia, shown below, indicate the relative purity of many of the Cretaecous kaolins in Georgia and South Carolina.

Kaolin has many industrial uses. It is soft, has low viscosity at high solids content in many systems, is readily wet and dispersed in water and some organic systems, and can be produced with a controlled particle size distribution Some of the important physical constants of kaolin are: specific gravity, 2.60; index of refraction, 1.56; hardness (Mobs scale), 2; fusion temperature, 1850o C; dry brightness, 78-92 percent,( measured at 458 mµ on a General Electric recording speetrophotometer, against MgO Reference).

As mentioned previously many kaolins have a low viscosity at relatively high solids content. For example, many kaolins can be dispersed readily in water at 70 percent solids by weight and the resultant slurry pours like fresh milk. This is an attractive property for the paper coating industry.

 

The ranges of particle size distribution in which kaolins are produced commercially are shown in Fig. 1. Several end uses of kaolin depend upon the particle size distribution. In many applications a coarse-particle kaolin may not work whereas a fine-particle kaolin will, and vice versa. Some of the more important industrial uses will be described in this paper, but it must be realized that these descriptions, of necessity, are brief and incomplete. More than 2,000,000 tons of kaolin is used annually in the United States (de Polo, 1960, p.207).

 

Dry Processing

Two basically different processes are used to refine kaolins and remove the major impurities. The simplest process is called air flotation or the dry process. The properties of the finished product depend to a large extent on those properties inherent in the crude kaolin. In the dry process operation, a deposit must be chosen with desirable properties of color and relatively low content of grit ( > 44 µ). The crude kaolin is transported to the mill where the large chunks are reduced to about egg size by roll crushers. The crushed kaolin is fed into rotary driers and then into airfloating equipment. The latter usually consists of a pulverizing unit and an air separator. The fine particles are transported to collecting chambers and the coarse particles are fed back into the pulverizer. Dry processing yields a product of relatively low cost.

Air- float Kaolin are limited in their ablility to improve clay brightness or viscosity; they typically shred,dry, and mill the crude clay while removing coarse grit impurities on a moving column of air. Today air-float kaolin products make up only about 2% of the total U.S. kaolin production value.

 

 

Wet Processing

 

The second process used to produce kaolins is much more complex and is called the wet process. The kaolin is dispersed in water after it is mined. The first step after dispersion is the removal of the coarse grit ( > 44 µ) by settling procedures and vibrating screens. The resultant degritted slurry is fed into centrifuges to separate the kaolin into fine, intermediate, and coarse particle size fractions. These fractions can be chemically bleached to remove some coloration caused by iron impurities. The kaolin is then dewatered through a filtration process, dried in either rotary, apron or spray driers, and prepared for shipment. This process is used to produce highly refined kaolins having controlled properties.

 

Water-wash processing of the crude clay, yields a product of considerably higher value, and accounts for most of the kaolin produced in the Georgia district. One of the main objectives of the water-wash process is to totally or substantially remove all pigmentary impurity minerals that discolor the crude. Reductive leaching and magnetic seperation are used to remove iron oxide mineral. Iron-stained titanium dioxides are removed by magnetc seperation, selective flocculation, and froth flotation. Organics are typically oxidized using ozone, which breaks down the discolored organic molecules and makes pigmentary iron oxides available for removal be reductive leaching. Complete removal of these impurities is virtually impossible, largely because of the limitations imposed by process cost.

 A typical wet-processed kaolin shown above undergoes many of the following steps:

§  Blunging (high-energy mixing with water) to a dispersed 30-40% solids liquid slurry.

§  Degritting to – 220 mesh (- 75 µm) to remove coarse sand and mica.

§  Blending to achieve optimum mix of particle size, and to equalize variation of other quality parameters.

§  Centrifugation to product particle size ( 50% - 100%  < 2 µm).

§  Delimination to reduce particle size, increase aspect ratio (ratio of diameter to thickness), and improve brightness.

§  Brightness improvement by one or more methods : reductive leaching, ozone oxidation, magnetic seperation, froth flotation, or selective flocculation, foe example.

§  Filtration to ~ 55% solids, spray drying or evaporation, repulpingto 70% solids slurry or selling as dry-pigment product.

§  Optional calcination to a higher brightness and more opaque pigment product.

 

Blunging and Dispensing

The process of converting run-of-mine crude kaolin to stable suspended slurry is accoplished by mixing the kaolin with water and small percentage of dispersant. Sodium hexametaphosphate (NaPO3)6, Sodium metasilicate (Na2SiO3 known as water glass or liquid glass and produced according to the reaction: Na2CO3 + SiO2 → Na2SiO3 + CO2), or organic colloid polymors are the most commonly used dispersants. The clay may be mixed either in mobile blungers at the mine or in fixed blungers at the processing plant. If the clay is blunged in the mine, it is transported by slurry pipeline. The blunger is a high speed, high horsepower mixer which breaks up the kaolin lumps into discrete individual particles.

A dispersant is necessary in order to keep the discrete particles seperated from each other because otherwise the particles would flocculate. Fig 2 is a diagram showing flocced particles and dispersed particles. Fig. 3  is a diagram showing the charges on the crystals of kaolinite. Because of the positive and negative charges, the kaolinite particles are attracted and form large aggregates or flocs. The addition of soluble dispersant which ionizes to produce cations that are attracted to the negative charges on the clay particles so that each kaolinite plate or stack has a similar charge and thus they repel each other.The amount os dispersant added is quite small, of the order 4 – 12 Ib/ton of kaolin, which is 0.2- 0.6 based on the dry weight of the kaolin.

 

 

Degritting (Grit is defined as particles coarser than 325 mesh or 44 µm)

Coarse impurities (such as quartz, sand, muscovite mica, and heavy minerals) are removed from the dispersed kaolin slurry during the degritting step. One common method for removing grit is to pass the slurry through drag boxes,  which are known as sandboxes. A residence time of about 30 min is adequate to allow the coarse grit particles to settle to the bottom of the drag box. These coarse settled impurities are then removed by drag slats and disposed of in waste impoundments. Mica, which is flake shaped, does not settle as rapidly as the quartz and heavy minerals so the slurry goes from the drag box to a vibratory screen which removes the coarse mica and other floating debris that may be present. Hydrocyclones are sometimes used instead of drag boxes, particularly if the grit percentage is higher than about 15%. Hydroseparators are also used to remove grit.

Crude Clay Blending

Typically, kaolin from several mines are blended in mine or in processing plant terminal tanks to achieve the necessary quality. This step when carried out in the mine by pumping the kaolin slurry from previous step  to a large  holding tanks, which when filled and checked for quality, is then pumped through a pipeline to terminal tanks at the processing plant. The mine holding tanks are also used to blend kaolins in order to meet viscosity and brightness specifications. The longest pipeline in Georgia is about 35 miles (56 km) in length and the longest in Brazil is about 100 miles (160 km) in length. Further blending, if necessary to meet quality specifications, can be accomplished in the terminal tanks at the processing plant.

The next step in the wet process (Fig. 4) is to fractionate the kaolin into coarse and fine fractions. This is accomplished by continuous bowltype centrifuges, hydroseparators, or hydrocyclones. After fractionation to a particular particle size, the fine fraction and the coarse fraction of the kaolin are pumped to holding tanks. The coarse fraction may be delaminated (which will be described later) or is filtered and dried to produce filler clays. The fine fraction can then be passed through a high intensity magnetic separator which removes discrete iron and titanium minerals. Other processes used to remove the iron containing titanium minerals, usually anatase, are selective flocculation and flotation.

These processes will be described later in this chapter. The fine fraction slurry can go through one of the above processes before going to the floc and leach step or it can go directly to floc and leach depending on the brightness of the grade to be produced. The floc and leach step is to acidify and floc the slurry at a pH between 2.5 and 3, which solubilizes some of the iron compounds which stain the kaolin. Alum is sometimes used in combination with sulfuric acid to give a tighter floc. At essentially the same time, a strong reducing agent, sodium hydrosulfite, is added to the slurry to reduce ferric iron to ferrous iron, which then combines with the sulfate radical to form a soluble iron sulfate, FeSO4. The iron sulfate is removed in the filtration step, which is the next step in the process. Quality control determines the quantity of acid, alum, and hydrosulfate that is needed to give the best brightness result.

After the floc and leach process, the flocced slurry is pumped to filters to remove water and the soluble iron sulfate. Usually water spray bars are used to wash the filter cake to remove more of the iron sulfate. Commonly, the percent solids after the floc and leach is around 25%. Large rotary vacuum filters or plate and frame pressure filters are used to dewater the kaolin, raising the percent solids to 60–65%. After filtration, the filter cake is redispersed and pumped to a spray drier where it is dried for bulk or bag shipments or the percent solids is increased to 70% by adding dry spray dried clay or by large evaporators which is the slurry solids necessary for most tank car or tank truck shipments. The filter cake can be extruded and dried to make what is termed an acid kaolin product.

 

 

The coarse fraction from the centrifuges is used either to make coarse filler clays or as feed to produce delaminated kaolins (Fig. 5). The coarse thick vermicular stacks and books of kaolin are pumped to delaminators which shears the plates making up the stack or book into large diameter thin plates (Kraft et al., 1972). These large diameter thin plates have what is termed a high aspect ratio which is a ratio of the diameter to the thickness of the plate. The stacks and books have a prominent cleavage, which is parallel to the (001) basal plane. The coarse particles are cleaved by placing them in a baffled vessel filled with media in which impellers strongly agitate the slurry. The spherical media which can be used is wellrounded sand, alumina proppants, and/or glass, plastic, zirconia, or alumina beads. This vigorous agitation of the media and the coarse kaolin cause the kaolin to shear upon collision between the media beads to produce a coarse delaminated plate with a high aspect ratio .

 

The magnetic separation process involves the use of powerful magnets with field strengths ranging from 2 to 6 T (Tesla is the international unit of magnetic flux intensity and equals to 10,000 gauss - cgs system of measure) . The range from 2 to 6T is achieved by using liquid helium cooled superconducting coils which results in considerable savings in electric power. The kaolin slurry is pumped through a highly compressed fine stainless steel wool matrix, which when energized, separates the magnetic minerals and allows the non-magnetic kaolinite to pass through the matrix. The magnetic field is periodically switched off so that the accumulated magnetic particles can be rinsed with water, thus cleaning the steel wool matrix. Fig. 6 is a diagrammatic representation of a 2 T magnet. The magnetic minerals that are removed are dominantly hematite and yellowish iron enriched anatase along with some ilmenite, magnetite, and biotite. The magnetic separation process was described by Iannicelli (1976) who was one of the first to advocate the use of magnetic separation in order to brighten kaolin clays. The development of high intensity wet magnetic separation for use in the kaolin industry has resulted in a huge increase in kaolin reserves which can be used commercially.

 

 

The froth flotation process used to remove dark iron stained anatase which discolored the kaolin was initially developed by Greene and Duke (1962). They used a calcium carbonate carrier which was termed a ‘‘piggy back’’ process. Since then, the flotation process has been improved so that now it has evolved into a standard method in processing Georgia kaolins to make high brightness products of 90% or higher. The dark iron stained anatase is selectively coated with a reagent which causes it to adhere to air bubbles sprayed into the slurry. The air bubble froth which contains the stained anatase rises to the surface of the float cell and is skimmed off and discarded. Denver-type conditioners and float cells are the most commonly used equipment. Recently, vertical column flotation cells have been used which improves the separation of fine particles and also increases product recovery. Most of the Georgia kaolins contain up to 2.5% TiO2 and by using the flotation process, the percentage can be reduced to as low as 0.3.

 

Selective flocculation is another process that can be used to reduce the TiO2 percentage. The process was introduced in the late 1960s by Bundy and Berberich (1969) to produce high brightness products of 90% or higher. Since its initial development, the selective flocculation process has been continually improved and is now a process which is used extensively to produce high brightness products (Shi, 1986, 1996; Pruett, 2000). This process is the reverse of flotation in that the dark iron stained anatase is selectively flocculated so that it settles in a hydroseparator while the kaolin remains suspended in a dispersed condition. The flocculated anatase is discarded into waste impoundments.

 

Calcination

 

Another special process used to produce value-added products is calcination, which was introduced in the early 1950s. The kaolinite is processed to remove impurities and a fine particle size gray kaolin is a preferred feed (Fanselow and Jacobs, 1971). The fine gray kaolin is spray dried, pulverized, and then fed to either rotary or large hearth calciners and heated to as high as 1300oC. The highest temperature of 1300oC is used to produce granules for use in making refractory shapes and bricks. Most of the pigment grade of calcined kaolin is heated to a temperature between 1000 and 1050oC. Shown in the follwing reaction, the temperature at which the kaolin is dehydroxylated to form metakaolin which is then transformed into mullite  The metakaolin is an amorphous mixture of alumina and silica that is used in several applications. The phase change at 980oC transforms the amorphous metakaolin into mullite (Al2SiO5). This causes a significant increase in brightness and opacity which is discussed later.

 

 

 

The hardness of the calcined kaolin is about 6.5 on the Mohs scale, which is considerably harder than the 1.5–2 hardness of hydrous kaolin. An 85% brightness feed to the calciner will produce a product with a brightness of 91–93%.

 

Special processes are used to modify the surface properties of kaolinite in order to improve the functionality and dispersion of the product (Grim, 1962; Nahin, 1966; Libby et al., 1967; Bundy et al., 1983; Iannicelli, 1991). The hydrophilic surface of kaolinite can be chemically treated to make them hydrophobic or organophilic. These surface modified kaolins can then be used as a functional pigment and/or extender in systems where the natural hydrophilic kaolin cannot be used. The uses of these surface modified kaolins are discussed in the following items.

 

 

Main Industrial Uses

 

Paper

 

The largest single user of kaolin is the paper industry, which used approximately1,200,000 tons in 1958 (de Polo, 1960, p. 207). Because kaolin is used, paper products print better and are made whiter and smoother. Kaolin used as a filler in the interstices of the sheet adds ink receptivity and opacity to the paper sheet. Kaolin used to coat the surface of the paper sheet makes possible sharp photographic illustrations and bright printed colors. Kaolin constitutes nearly one-third the weight of today's slick sheet magazines. The significant properties of kaolin of greatest value to the paper industry are whiteness, low viscosity, non-abrasiveness, controlled particle sizes, and flat hexagonal plates. Opacity is an extremely important property to the paper industry and Fig. 7 shows the relationship between the particle size of the kaolin and opacity. Brightness, gloss, and viscosity properties also depend on particle size (Lyons, 1958, p. 81).

 

 

 

Flow properties or rheology of kaolin clays, especially the kaolin coating clays used in the paper industry, are very important because of their influence on coat weight, smoothness, texture, and other properties. Figure 8 shows plots of viscosity vs. shear rate for Newtonian, thixotropie, and dilatant fluids. These three types of viscous flow are of primary interest to paper coaters. Particle size distribution, particle shape, electrokinetic effects between particles, presence of impurities and degree of flocculation and dispersion all affect rheology. Rheology of kaolin slurries has been the subject of several papers and patents in recent years (Albert, 1951, p. 456; Millman and Whitley, 1959; Murray, 1961).

 

 

If one were going to design in a laboratory a coating pigment for the paper industry, it would be white, disperse readily in water, and have low viscosity, be soft, have a fine particle size, and be a thin plate-shaped particle. Nature produced a mineral which has essentially all the above properties, and that mineral is kaolin. It can be readily seen why kaolin is an ingredient essential to the paper industry.

 

Rubber

 

Kaolin is used as a filler in many rubber goods. It adds strength, abrasion resistance, and rigidity to both natural and synthetic rubber products. In general, most rubber products extrude more easily after kaolin filler is added. The major reason that kaolin is used in rubber compounds is its whiteness and low cost. Although kaolin costs less than most other rubber pigments, it has excellent functional properties.

 

Ceramics

 

Kaolin is used in ceramic whiteware products, insulators, and refractories(Smoot, this Volume). In whitewares, kaolin aids accurate control of molding properties, and adds dry and fired strength, dimensional stability, and a smooth surface finish to the ware. The excellent dielectric properties and chemical inertness of kaolin make it well suited for porcelain electrical insulators. In refractory applications, the dimensional stability, high fusion point, and low water content, along with high green strength, make kaolin an important constituent.

 

Paint

 

Kaolin is used in paint because it is chemically inert and insoluble in the paint system, has a high covering power, gives the paint desirable flow properties, and is low in cost.

 

Plastics

 

The addition of kaolin to thermosetting and thermoplastic mixes gives smoother surfaces, a more attractive finish, good dimensional stability, and high resistance to chemical attack. In addition, the flat hexagonal kaolin plates hide the reinforcing fibers and give the mix flowability to simplify tile molding of complex shapes.

 

Other Applications

 

Kaolin has many other industrial applications, some of which are listed here :

Ink, cement, detergents, adhesives, fertilizers, porcelain enamels, insecticides, plaster, paste, medicines, filter aids, roofing granules, food additives, cosmetics, sizing, foundries, catalyst preparations, chemicals, bleaching, crayons, linoleum, adsorbents, pencils, floor tiles, and textiles.

 

Special Applications Through Chemical Modifications

 

Kaolin is hydrophilic and can be dispersed in water and in some other systems. Because of the nature of the chemistry of its surface, kaolin can be chemically modified so that it will become hydrophobic or organophilic, or both. Generally, an ionic or a polar non-ionic surfactant is used as the surfacetreating agent.

Eleetrophoretic studies have shown that kaolin has an overall negative charge. The exchange capacity of kaolin results from broken bonds and isomorphous substitution of A1 for Si in the tetrahedral sheet in the structure (Grim, 1953, p. 132). The exchange sites are the locations on the surface where polar molecules can be adsorbed and oriented.

The choice of surface treatment or chemical modification depends upon the polarity, structure, and composition of the organic system into which the kaolin is to be utilized and the physical and chemical properties desired in the end product.

Wetting, dispersion, flow properties, and general physical-chemical behavior are most important in the organic medium into which the kaolin is to be utilized. Thorough wetting of kaolin by the vehicle is essential in order to derive maximum utility and functionality. Wetting breaks down the attractive forces between kaolin particles and facilitates the coating of each particle with the wetting medium. Even though wetting is complete, dispersion will not necessarily be the ultimate, because the attractive forces between kaolin particles may still be effective across interfaces causing loose agglomeration of the particles.

Flow properties are closely related to wetting and dispersion. Interparticle attraction produced by unlike surface charges causes the formation of an internal structure which inhibits flow and gives rise to thixotropy (Michaels, 1958, p. 26). Uniform wetting and good dispersion tend to give flow properties which approach Newtonian and dilatant systems.

Improvements in functionality of kaolin fillers after surface modifications have been noted in the ink, paint, rubber, and plastics industries (Albert, 1960a, 1960b; Bundy, 1961; Felletsehin, 1961; yon Volkenburgh, 1959: Werner, Marra, and Gilman, 1960; Wilcox, 1961a, 1961b). Dispersion is essential for smooth films with high finish and for improved color tone and texture. Decrease in viscosity by virtue of improved dispersion enables higher pigment loadings as well as easier workability.

0rganophilic kaolins are being used in the rubber industry where it has been found that the modified kaolin is easier to incorporate in the polymer system, higher pigment loadings are obtained, and faster cure rates, higher modulus, and higher tensile strength can be achieved. In paints, plastics, and inks, organophilie kaolins are being used because they disperse and wet out rapidly, have better suspension properties, and give superior water resistance and reduced viscosities. In polyester resins, for example, some organophilie kaolins give viscosities tenfold less than viscosities obtained for unmodified kaolins of exactly the same particle size distribution and tested under similar conditions.

The future industrial use for surface-modified kaolins looks very good indeed. Additional research will find new and better surfaetants for modifying the kaolin surface and new end uses will be forthcoming. Industrial applications for surface-modified kaolins will increase very rapidly in the next few years.

 

 

Calcined Kaolins

 

Another area that deserves mention is the large industrial use of calcined kaolin. When kaolin is heated to approximately 1050oC it is converted to mullite, eristobalite, and/or a silica alumina spinel (Brindley and Nakahira, 1959, p. 312). This calcined product is whiter and more abrasive than the original kaolin, and the surface chemistry and physical properties are completely changed. The largest utilization of calcined kaolinsis in paint, rubber, and plastics.

 

SUMMARY

 

Kaolin is a unique industrial mineral that has properties that make it a functional ingredient in many industrial applications. The largest tonnage of kaolin is used by the paper industry. Surface-modified kaolins have a rosy industrial future. Research efforts in this area are disclosing new and fascinating uses. Calcined kaolins also are in demand particularly for paints, rubber, and plastics.

---------------------------

 

End Note : Delamination is the grinding or cleaving of kaolinite stacks into finer platy particles. Delamination technology allowed producers to use coarse kaolin stacks that had mostly been used as paper filler. The US Bureau of Mines in the late 1950s developed attrition grinding of clay slurries with sand or other media to delaminate and separate fine kaolinite platy particles. Delaminated products show value in paper coating because they have good sheet coverage and printability.

 

---------------------------

 

 

______________

 

Index