Kaolin

KAOLIN

Synonyms

China clay.

Source

Kaolin is a purified native hydrated aluminium silicate free from gritty particles. It is obtained by powdering the native kaolin, elutriating and collecting the fraction, which complies with the requirements of particle size. The native clay is derived from decomposition of the feldspar (potassium aluminosilicate) or granite rock and contains silica (47%), alumina (40%), and water (13%).

History

The word kaolin was derived from the Chinese word ‘Kauling’, meaning high ridge. Kaolin was first mined in Colonial days in Georgia and then shipped to England. Georgia was the source of clay for famous Wedgwood Pottery and this resulted in the end of mining in Georgia for over a century. By 1876, mining here was resumed and today it continues as the major mineral production of the state by producing 72% of the total kaolin.

Collection and Preparation

Kaolin is mined from the surface layer of stones, clay and sand which are in depth up to 100 feet. The average thickness of clay varies from 12 to 15 feet. Kaolin is removed by firing a high-pressure water jet at the quarry face. The clay is then sifted and refined to remove impurities before finally being dried to reduce its moisture content. The impurities like sand are removed by washing where the impurities settle down and the slurry is then pumped into a long channel of drags. The coarse particles in the slurry settle down, whereas the lighter ones move slowly along with water and flow into a settling pits, where the clay is deposited.

Description

Kaolin is white soft plastic clay composed of well-ordered kaolinite with low iron content. In many parts of the world, it is coloured pink-orange-red by iron oxide, giving it a distinct rust hue. Lighter concentrations yield white, yellow or light orange colours also. It is made up of a loose aggregation of randomly oriented stacks of kaolinite flakes, smaller packets and sheaves and individual flakes.

The median particle size of kaolin is 0.78 μ, l.02 μ, 1.1 μ, 1.2 μ and 3.8 μ. It has loose bulk density of approximately 25 lbs/cubic feet and packed bulk density of 46 lbs./cu ft, the hardness factor is 6.0, specific gravity: 2.6 g/cc, pH: 6.0, surface area: 10–29. Odourless when dry but has clay like odour when wet. Kaolin when treated with concentrated HCl, decomposes partially but on heating it with conc. sulphuric acid, it is converted into insoluble silica and aluminium sulphates.

There are two types of kaolin: coarse (heavy) and colloidal (light). The coarser kaolin when treated with water forms a plastic and slightly sticky mass while colloidal kaolin with water forms sticky, stiff mass and if suspended in water forms a turbid solution or slurry. The standard grades of kaolin available are: calcined, Sanitary ware grade, tableware grade, and porcelain grade.

Chemical Constituents

Chemically kaolin is anhydrous aluminium silicate with a chemical formula: Al₂O₃2SiO₂2H₂O or H₄Al₂Si₂O₉. The percentage composition are as follows: silicon dioxide (wt %): 56.91, iron oxide: 0.93, titanium dioxide: 0.54, aluminium oxide: 39.68, calcium oxide: 0.16, magnesium oxide: 0.16, sodium oxide: 0.60, potassium oxide: 0.60, and water: 12.6. Natural kaolinite usually contains small amounts of uranium and thorium, octahedral sheet of alumina octahedral.

Identification

Heat kaolin on charcoal black with cobalt nitrate, it forms blue mass due to alumina.

Uses

It is used as an adsorbent by oral administration, in the treatment of enteritis, dysentery and in alkaloidal and food poisoning. It is also applied externally as a dusting powder and also as clarifying agent during the filtration. Mostly, light kaolin with a particle size less than 10 μ is used in pharmaceutical preparations. Heavy kaolin with particle size up to 60 μ is only used in the preparation of kaolin poultice.

It is used as filler in paper, rubber, ceramics, cement, and fertilizer industries. It is used in anticaking preparations, cosmetics, insecticides, paints, and as source of alumina.