Consumption of kaolin is concentrated in the paper, rubber, ceramics, refractories, glass fibre and cement industries.  Smaller, but still important consuming industries are paint, plastics, adhesives and sealants and catalyst supports.  Outside of these industries, kaolin is used in small quantities by a number of industries.  Roskill estimates consumption of kaolin outside of the industries mentioned above was around 720,000t in 2002.

 

1.1Kaolin applications in chemicals

 

Kaolin is used in the production of aluminium sulphate, also known as alum, and was formerly used in the production of aluminium metal.  The use of kaolin as a source of alumina for chemical manufacture declined following rapid growth of the aluminium metal market led to the widespread availability of alumina trihydrate (ATH).  Alumina trihydrate is both purer than kaolin and contains a higher proportion of alumina so is a more efficient feedstock.

 

1.1.1Aluminium sulphate

 

Aluminium sulphate occurs as hydrated or anhydrous varieties.  Hydrated aluminium sulphate, or alum (Al2[SO4]3.18H2O), is marketed in industrial, low iron and iron free forms.  Aluminium sulphate is unusual in that it can be produced from bauxite, natural clays, alumina trihydrate and by-product or rejected alumina raw materials.  

 

Industrial grades are normally produced from chemical grade bauxite, which is digested in sulphuric acid at 110oC for up to twenty hours.  The purer low iron and iron free grades are produced in a similar fashion to industrial grades, except that alumina trihydrate and sulphuric acid are normally used.  Iron free aluminium sulphate, the purest grade material, is mainly used in high quality paper production to clarify the process water, control the pH of the pulp slurries, set dyes and in paper sizing.  To do this and produce a stain free product, the aluminium sulphate must contain less than 0.005% Fe2O3.

  

1.1.1.1Use of aluminium sulphate in water treatment

 

The most common use for aluminium sulphate is in water treatment with smaller quantities used in paper manufacture and other minor end uses.  US production of aluminium sulphate largely depends on demand for coagulants in the water treatment market.  In the USA, aluminium sulphate use in water treatment has come under pressure from a number of factors:

 

·Concerns have been raised over the disposal of sludge resulting from the use of aluminium sulphate made from bauxite

·Technological developments have allowed the recovery and reuse of aluminium sulphate

·Competition with other aluminium salts such as polyaluminium chloride (PAC) and other materials including ferric chloride and organic polymer flocculants

·Concerns over a possible connection between aluminium in the water supply and Alzheimer’s disease

 

1.1.1.2Use of aluminium sulphate in paper manufacture

 

In paper manufacture, aluminium sulphate is used in conjunction with rosin as a sizing agent to curb the spread of ink and water into the paper and improve its printing quality.  Rosins are also used, with or without aluminium sulphate, as internal sizing agents in acid and neutral paper manufacture and as surface sizing agents in acid, neutral and alkaline paper manufacture.  Increasing filler content and the rapidly increasing use of computers and copiers are driving consumption of mineral fillers in the world paper sizing market.

 

The use of aluminium sulphate as a sizing agent in the USA has fallen because of the switch to the use of neutral sizing agents, mainly alkylketenedimer and alkenyl succinic anhydride.  This followed the switch from acid to alkaline paper manufacture.  When a pulp mill switches to alkaline sizing its consumption of aluminium sulphate falls by between 85% and 90%.  This change has now largely been completed so the rapid fall in aluminium sulphate use in US paper manufacture has ceased.  

  

1.1.2Aluminium chloride

 

Aluminium chloride is marketed as anhydrous AlCl3 or Al2Cl6, aluminium chloride hexahydrate AlCl3.6H2O and as a 27.82%, 1.28g/cm3 (32o Baume) aqueous solution.  Commercial production is undertaken by reacting chlorine with either molten aluminium or alumina in an aluminous material, such as clay or more usually bauxite, in the presence of a reductant.  Carbon, carbon monoxide, phosgene or tetrachloromethane are used as reductants but in the case of the last two substances, the gaseous reductant supplies both the carbon reductant and part of the chlorine for the halogenation of the alumina.

 1.2Kaolin applications in zeolites

 

Kaolin is the starting point for the manufacture of synthetic zeolites by the clay conversion process.  In this process, the kaolin is formed into binderless pellets that are then calcined at between 500oC and 600oC to form metakaolin.  The pellets are then treated with an alkai hydroxide solution to form pelletised zeolite A.  This process requires the use of high purity kaolinite as a feedstock.  Only Mizusawa Industrial Chemical of Japan is reported to have used this process in the recent past and it is unclear if it still does so.  All other producers use the hydrogel process that uses sodium aluminate.

  

1.3Kaolin applications in agriculture

 

Small amounts of kaolin and other materials are used in agricultural applications.  The main use is as a carrier and diluent in animal feeds, fertilizers, pesticides and related products.  The specifications for minerals used in agriculture are not as stringent as they are for more sophisticated applications.  For this reason, selection of the mineral tends to be highly dependent on price.

 

1.4Kaolin applications in pesticides and other biocides

 

Mineral fillers are used as carriers and diluents for pesticides and other biocides.  Before application, biocides are first mixed with a carrier to make a concentrate that is easy to store and transport.  The carrier must be highly absorbent, dispersible in liquids, so that it can be sprayed, and chemically inert with respect to the active ingredients of the biocide.  A small particle size and low abrasiveness are also required to avoid clogging and excess wear to nozzles and other application equipment.

 

The particle size of a carrier is typically between 0.5 microns and 4 microns.  The concentration of active ingredients in a formulation may be as low as 0.1% in some cases but it is usually between 5% and 10% by weight.  The choice of which carrier to use depends on technical factors, such as particle size, and cost.

 

The most commonly used minerals used as carriers and diluents are diatomite, fuller’s earth, kaolin, talc, gypsum, lime, perlite and pyrophyllite.  Minerals with a platey structure are very suitable because they aid the retention of the formulation on the plant.  Minerals that are highly absorbent, such as bentonite and fuller’s earth, are more commonly used in biocides.

 

1.6Kaolin applications in fertilizers

 

A wide variety of minerals are used in fertilizers in the same way as they are used in pesticides and biocides.  The most commonly used include diatomite, fuller’s earth, talc, gypsum, lime, perlite, pyrophyllite, sepiolite and attapulgite.  Of these minerals, diatomite is the most chemically inert so is possibly the most widely used.

 

Kaolin can be used in the production of ammonium nitrate and urea fertilizers, as these materials are hydroscopic and also highly soluble.  This means that the materials will stick together if there is only a very small amount of water present.  To prevent this, anti-caking agents, such as kaolin, are added in amounts of between two and three percent by weight.

 

The amount of kaolin used in fertilizers worldwide is probably in the region of a few tens of thousand tonnes per year.  US consumption of kaolin in these applications was reported as 3,580t in 2001.

 

1.7Kaolin applications in pharmaceuticals and cosmetics

 

Kaolin has been used in medical and cosmetic applications for many years in both oral and topical formulations.  

 

1.7.1Pharmaceuticals

 

In the pharmaceutical sector, kaolin is used as a diluent and filler in some drugs, in poultices and in a mixture with morphine used to treat gastro-intestinal conditions.  The absorbent properties of kaolin can reduce the rate at which a drug is released within the body and even the actual amount absorbed by the body.

 

1.7.2Cosmetics

 

The main use for kaolin in cosmetics is in face powder where it is added to absorb skin secretions.  Face powders are a blend of tinted and perfumed powder used to enhance skin appearance.  A number of materials, including titanium dioxide, zinc oxide and magnesium oxide are used for their covering properties.  Talc, stearates and starch assist in spreading the product and also impart a smooth feeling to the surface of the skin.  Fillers used include chalk, precipitated calcium carbonate (PCC), magnesium carbonate, starch and kaolin that absorb skin secretions.  The metallic soaps and talc enhance the adherence of the powder while chalk and PCC acts as whiteners.