Kaolinite impurities

China clay or kmlin, which is predominantly kaolinite, is particularly valuable because it is essentially free from iron impurities (and therefore colourless). World production in 1991 was 24.7M1 (USA 39%, UK 13%, Colombia, Korea and USSR 7% each). In the USA over half of this vast tonnage is used for paper filling or paper coating and only 130000 tonnes was used for china, crockery, and earthenware, which is now usually made from ball clay, a particularly fine-grained, highly plastic material which is predominantly kaolinite together with clay-mica and quartz. Some 800000 tonnes of ball clay is used annually in the USA for white ware, table ware, wall and floor tiles, sanitary ware, and electrical porcelain. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

Kaolin clays are naturally occuring sedimentary deposits composed largely of kaolinite mineral. Typical impurities in these deposits are iron oxides, titanifer-ous minerals, silica, feldspar, mica, sulfides and organic matter. The majority of kaolin clay produced in the world is used in the paper industry as coating and filler materials. This mineral also makes an excellent filler, carrier, opacifier and diluent in a variety of industrial products such as paints, plastics, cement, rubber, pharmaceuticals, etc. 

Froth flotation has proven to be an efficient method of removing titaniferous impurities (mainly iron-rich anatase) from kaolin clays. Fatty acid reagent, primarily tall oil, is used extensively in the reverse flotation of these impurities. This flotation collector typically requires divalent cations (usually Ca +) to activate the coloured impurities and enhance collector adsorption. This is not very selective since the tall oil can also absorb on the kaolinite particles. Alkyl hydroxamate collectors are relatively new in the kaolin industry but provide significant advantages. Hydroxamates do not require activators, substantially increase the removal of colored impurities and are very selective. 

From initial deposition and burial under overlying sedimentary materials through succeeding geological periods, coal beds are continually subject to the action of ground water. Thus, some coal beds have developed a system of essentially vertical fractures—thin cracks, often filled with coatings of pyrite. calcile. kaolinite and other minerals deposited from ground water. Impurities from these veins lower the quality of the coal. 

Fe203, Ti02, MgO, and CaO are nearly always present in kaolinite samples and K20 and Na20 are usually present. Most samples either have excess Si02 or A1203-Mineral impurities such as quartz, anatase, rutile, pyrite, limonite, feldspar, mica, montmorillonite, and various iron and titanium oxides are commonly present in addition to a number of other minerals. Si and Al, in the form of hydroxides, apparently can occur as coatings on the kaolinite layers. Although many of these impurities are usually identified, seldom is the analysis sufficiently quantitative to determine if all the deviation from the ideal composition is due to these impurities. 

Fire clays, ball clays, flint clays are kaolinite-rich clays, usually of the 6-axis disordered variety, which contain a relatively high impurity content. Illite, montmoril-lonite, diaspore, boehmite, quartz, and organic material are the minerals usually associated with these deposits. Few, if any, of the kaolinite minerals in these clays have been concentrated enough to afford meaningful chemical data. 

One of the major differences in the reported chemical composition of the kaolinite minerals is in the H20+and H20 values. In part, these variations may be real but many must be due to the presence of halloysite and other impurities, variation in grain size and surface area, and in the methods of dehydration. H20 increases linearly with increase in surface area and with decrease in grain size. 

The Si02 /Al2 03 ratios for the various kaolinite minerals suggest that most of the variations are a function of impurities. Ninety percent of the analyses show a ratio less than 2 1 suggesting that an excess of A1203 is much more common than an excess of... 

In carrier flotation, small-sized (several pm diameter) particles become attached to the surfaces of larger particles (perhaps 50 pm diameter, the carrier particles) [630]. The carrier particles attach to the air bubbles and the combined aggregates of small desired particles, carrier particles, and air bubbles float to form the froth. An example is the use of limestone particles as carriers in the flotation removal of fine iron and titanium oxide mineral impurities from kaolinite clays [630]. The use of a fatty acid collector makes the impurity oxide particles hydrophobic these then aggregate on the carrier particles. In a sense, the opposite of carrier flotation is slime coating, in which the flotation of coarse particles is decreased or prevented by coating their surfaces with fine hydrophilic particles (slimes). An example is the slime coating of fine fluorite particles onto galena particles [630],... 

Several studies of the effects of heating on pure clays have been reported in archaeological literature, but very little systematic work has been done on the effects of admixed mineral impurities upon the clays that constitute ceramic paste. The purpose of this chapter is to study the controlled firing of four measured mixtures of the clays, kaolinite and montmorillonite, with the common carbonates, calcite and dolomite. 

In industry, cordierite is usually obtained by calcination of the mixtures containing talc, kaolinite and silica at 1300-1450°C for 20-60 h. The product contains the impurity phases spinel, mullite, clinoenstatite, etc., that worsen the exploitation characteristics of cordierite. Since the mentioned minerals contain structural water, chemical interaction between them during mechanical activation can be considered from the viewpoint of soft mechanochemical synthesis. Mechanical activation of this mixture does simplifies the interaction between its components. It is sufficient to heat this mixture for 2 h at a temperature of 1260°C to obtain practically homogeneous cordierite without impurity phases (Fig. 7.2) [2-9]. 

The Chemical Dictionary defines porcelain as "ceramic wear made largely of baked clay (kaolin) coated or glazed with a fusible substance." Kaolin is defined as "(china clay white bole argilla porcelain clay white clay). A whiteburning clay, which, due to its great purity, has a high fusion point and is the most refractory of all clays." It gives the composition as "mainly kaolinite (40% alumina, 55% silica) plus impurities and water."... 

The conditions under which a high mullite content is created are obvious from the phase diagram. The first necessary condition is a suitable composition of the system. At low temperatures (up to 1300 °C), ihe system cannot be expected to approach the state of equilibrium. When formed from kaolinite, mullite in this case frequently keeps the pseudomorphous shapes of kaolinite. The typical mullitic forms, usually needle-shaped crystals, arise only at higher temperatures. The rate of formation as we]] as the final shape of its crysta]s may be considerab]y aflected by the impurities present or by the non-clay components of the raw materia] mixture. These components also take part in the formation of the melt which is then produced in higher amounts and at lower temperatures than would correspond to the pure system A1203 —Si02. 

The principal raw materials are refractory clays (fireclays) flint clays are refractory clays with a hard dense texture consolidated by geological pressure. The main clay mineral is usually kaolinite, and the content of impurities should be lower than 5-6%. 

Zeolites, particularly zeolite A, can be manufactured from kaolinitic clays, which as particularly found in Central Europe, Great Britain, Japan, China and USA. To transform kaolin into zeolite, it has to be thermally converted, e.g. by shock heating to > 550°C, to metakaolin. The metakaolin is then su.spended in sodium hydroxide solution and converted at 70 to 100°C into zeolite A. Some of the impurities contained in the natural raw material are retained in the final product. If amorphous silica is added, Si02-rich zeolites are produced. This process enables the transformation of preformed bodies into zeolite materials. 

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