Kaolins

Kaolin is a hydrated aluminium silicate of variable composition, derived from clay minerals. The main constituent is kaolinite, with the formula Al2O3.2SiO2.2H2O, usually accompanied by a variable amount of feldspar, quartz, mica or similar minerals. The structure consists of hexagonal sheets, with an aspect ratio of 10. Purification is required before use. 

Kaolin is a low cost, naturally occurring, white or pale coloured mineral, very widely used in the paper industry but also to a lesser extent in plastics for cable insulation, wire coating, footwear, flooring, pipes, window frames, garden furniture, agricultural film, medical devices and automotive parts. It is mined in North America, Brazil, the CIS, Europe and Australia. 

It has good dielectric properties, electrical resistivity and chemical resistance, and is particularly favoured in polymers for electrical cable insulation. Grades for wire and cable are normally first calcined, i.e., strongly heated to remove most of the surface hydroxyl groups. Between 450 and 700 °C kaolin is converted to metakaolin, and at temperatures around 1000 °C it imdergoes further changes to form defect spinel. 

Kaolin s physical properties (Table 4.4) depend on the calcining temperature as well as on contamination by minerals. Calcination increases the oil absorption and improves the optical brightness, opacity and dielectric properties, together with the water absorption and hardness, but decreases the reinforcing capacity. 

Kaolin was produced 150 million years ago. Its main content is kaolinite, occurring with other silicates such as mica, feldspar, and quartz or metallic oxides such as hematite and rutile. In form it consists of thin pseudo-hexagonal lamellar particles. When heated to above 500°C, kaolinite loses its water of crystallization and changes to metakaolinite, which is stable up to 960°C. 

New kaolins give excellent tensile and tear strength, and abrasion resistance in general purpose compounds where colour is not a critical factor. They also offer the processability and particle shape advantages of kaolin, and can be used as a partial replacement for carbon black, where they give good cost effectiveness. 

Note that the PhEur 2005 contains a monograph on heavy kaolin (kaolinum ponderosum). The BP 2004 in addition to the monograph for heavy kaolin also contains monographs for light kaolin (natural) and light kaolin. 

Argilla bolus alba China clay E559 kaolinite Lion porcelain clay Sim 90 weisserton white bole. 

The USP 28 describes kaolin as a native hydrated aluminum silicate, powdered and freed from gritty particles by elutriation. The BP 2004 similarly describes light kaolin but additionally states that it contains a suitable dispersing agent. Light kaolin (natural) BP contains no dispersing agent. Heavy kaolin is described in the BP 2004 and PhEur 2005 as a purified, natural hydrated aluminum silicate of variable composition. The JP 2001 describes kaolin as a native hydrous aluminum silicate. 

Adsorbent suspending agent tablet and capsule diluent. 

Kaolin is a naturally occurring mineral used in oral and topical pharmaceutical formulations. 

The silicates, which are important as ceramic materials, are feldspar, kaolin, mica, and mullite. About 70% of feldspar produced goes into the making of glass. Kaolin is used in fine china, paper, and rubber. The use of mica is in terms of hundreds of thousands of tons per year. The use of mullite is again in hundreds of thousands of tons for refractory blocks. 

Feldspar constitutes about 60% of the earth s crust. They are present in many sedimentary deposits and are found in igneous and metamorphic rocks. Sedimentary deposits form when small particles become cemented together. Feldspar is a source for alumina. It is also used in whiteware bodies as flux. 

Kaolin is a clay mineral. Clays are the primary ingredients in the making of traditional ceramic products. They are the layer silicates with a grain size of less than 2 p. Mechanical and chemical weathering of feldspars in igneous 

Refractive Index 1.55-1.57/calcined 1.62 Specific Gravity 2.58 / partially calcined 2.50 / fully ealcined 2.63 Mohs Hardness 2 / partially calcined 4-6 / fully ealeined 6-8 

Commercial grades of kaolin are composed primarily of the mineral kaolinite, a sheet sihcate, and may contain greater or lesser quantities of related sheet silicates (mica, illite, chlorite, smectite) and quartz. An individual kaolinite particle has the shape of an hexagonal plate. In nature these plates occur in stacks or books that exhibit varying degrees of staeking regularity. Kaolin is hydrophilic (readily water dispersible) for nonaqueous apphcations matrix compatibility can be improved by siuface treatment. 

Descriptive classifications of kaolin products are many and in eertain cases specific to particular markets. The most common designations are as follows. 

Primary (residual) kaolin - In reference to geologic origin, the clay occurs in 

Sedimentary (secondary) kaolin - In reference to geologie origin, the elay has been eroded and transported fix)m its site of formation and deposited at a distant location. The world s major kaolin belt, the 250 miles between Aiken, SC and Eufala, AL, consists of sedimentary kaolin. 

---

Acid refractory materials include fireclays, flint clays, china clays (kaolins), silica, flint, chalcedony, ganister and titanium dioxide. 

Inequality Re > H corresponds to the other case, when only a part of a penetrant is extracted by a developer and can form crack s indication. Such a situation can take place when one use kaolin powder as the developer. We measured experimentally the values Rj for some kaolin powders. For the developer s layer of kaolin powder, applied on tested surface. Re = 8 - 20 pm depending on powder s quality. 

One more obvious example illustrates strong influence of particle s sedimentation upon the sensitivity threshold. Assume that we have to ensure the detection of the cracks with the depth 10 > 2 mm in the case when the same product family indicated above is applied and h = 20 pm. The calculation using formula (1) shows that in the absence of sedimentation only the cracks with the width H > 2 pm could be detected. But when the effect of sedimentation results in the reduction of the value of developer layer thickness from h = 20 pm to h = 8 pm, then the cracks of substantially smaller width H > 0,17 pm can be revealed at the same length lo = 2 mm. Therefore we can state that due to the sedimentation of developer s particles the sensitivity threshold has changed being 12 times smaller. Similar results were obtained using formula (2) for larger particles of the developers such as kaolin powder. 

Very finely divided minerals may be difficult to purify by flotation since the particles may a ere to larger, undesired minerals—or vice versa, the fines may be an impurity to be removed. The latter is the case with Ii02 (anatase) impurity in kaolin clay [87]. In carrier flotation, a coarser, separable mineral is added that will selectively pick up the fines [88,89]. The added mineral may be in the form of a floe (ferric hydroxide), and the process is called adsorbing colloid flotation [90]. The fines may be aggregated to reduce their loss, as in the addition of oil to agglomerate coal fines [91]. 

Aluminium is not found free but its compounds are so widespread that it is the most abundant metal in the earth s crust. Aluminosilicates such as clay, kaolin (or china clay), mica and feldspar are well known and widely distributed. The oxide. AI2O3. occurs (anhydrous) as corundum and emery, and (hydrated) as bauxite. Cryolite. Na,AlF. (sodium hexafluoroaluminate). is found extensively in Greenland. 

Fig. 16. Two-hquid flotation flow sheet (39). The original ROM is kaolin (white clay) that contains 11% impurity in the form of mica, anatase, and siUca. Treatment produces high purity kaolin and a Ti02-rich fraction. A, Kaolin stockpile D, dispersant (sodium siUcate plus alkah) W, water K, kerosene C, collector (sodium oleate) RK, recycled kerosene S, screen M, inline mixer SPR, separator CFG, centrifuge P, product and T, to waste.

C or higher for the kaolin-based products to 1425°C and above for the zirconium-containing materials. At temperatures above 1000°C these ceramic fibers tend to devitrify and partially crystallize. Specially prepared ceramic fibers are used to protect space vehicles on re-entry and can withstand temperatures above 1250°C (see Ablative materials Refractory fibers). 

A process has been developed by J. M. Huber Co. to treat kaolin clay pigments using a hydrothermal process (see Clays) (25). The products, called synthetic alkah metal aluminosihcates (SAMS), have superior pigmentary quahties for paper (qv) coating. 

Flake Mica. Flake mica is mined from weathered and hard rock pegmatites, granodiorite, and schist and gneiss by conventional open-pit methods. In soft, residual material, dozers, shovels, scrapers, and front-end loaders are used to mine the ore. Often kaolin, quartz, and feldspar are recovered along with the mica (see also Clays Silicon compounds). 

Sodium sihcate (41°Bh, 1 3.22 ratio Na20 Si02) is added in the milling operation to disperse the slime, mosdy kaolin. Dispersion also aids the grinding process. The rod mill serves to grind the ore to 0.833 mm (—20 mesh) or to the point where mica, quart2, feldspar, and iron minerals are Hberated. Cyclones, or rake, hydrauhc, or other types of classifiers, are used after grinding to produce coarse and fine mica fractions that are treated separately. 

The slime, consisting of kaolin, fine quart2, and feldspar, is sometimes used as is after being dewatered. This material may be used in the manufacture of light-colored brick or may be further processed to produce a high grade ceramic kaolin used in the manufacture of dinnerware, electrical porcelain, or sanitary-ware (see Ceramics). Floes of kaolin may be sold in bulk from the drier or pulveri2ed and sold in a powdered form. 

The main by-products of mica processiag plants are kaolin, quart2, and feldspar. Some plants produce all of these products for sale. 

Glass-grade siUca can be produced from most mica operations with additional beneficiation of the quart2. Ceramic-grade kaolin can be produced from some mica flotation plants by selective mining and additional processiag of the clay slime removed prior to mica flotation (see Clays). 

Cl y Conversion. The starting material for this process is kaolin, which usually must be dehydroxylated to y /i7-kaolin by air calcination. At 500—600°C, yW /t -kaohn forms, followed by a mulliti2ed kaolin at 1000—1050°C. 

The 2eohtes are prepared as essentially bindedess preformed particles. The kaolin is shaped in the desired form of the finished product and is converted in situ in the pellet by treatment with suitable alkaU hydroxide solutions. Preformed pellets of 2eohte A are prepared by this method. These pellets may be converted by ion exchange to other forms such as molecular sieve Type 5A (1). ZeoHtes of higher Si02/Al202 ratios, eg, 2eohte Y, can be obtained by the same method, when sodium metasiUcate is incorporated in the preshaped pellets, or when acid-leached metakaolin is used. 

---