Introduction to kaolin iron removal process

1 Water separation method

The first step of the water washing process for the production of kaolin coatings and fillers is to simply make a slurry of crude clay. In order to separate kaolinite from mineral impurities such as quartz and mica and grade it into three grades of fine, medium and coarse, the slurry must be composed of individual mineral particles separated from each other and suspended in water.

Kaolin particles present opposite charges on the edges and surfaces of the mixer, and they attract each other to form flocs. Dispersants such as sodium polyphosphate are added to separate particles in the flocs. The clay slurry is sent from the mixer to a settling box and screen using a pump to remove grit coarser than 44 microns. After the grit is removed, the desired kaolin is obtained.

2 Magnetic separation method

The process of magnetic separation is based on the difference between the magnetic susceptibility of different types of minerals. Colored impurities in kaolin, such as anatase, rutile, hematite, mica and pyrite are free magnetic, and the magnetic susceptibility is usually as low as 10-6 cm-3. When placed in a magnetic field, they become magnetized, thereby assisting or opposing the applied magnetic field. High gradient magnetic separation has achieved considerable success in the beneficiation of industrial minerals, and kaolin samples that do not respond to conventional bleaching techniques have proven particularly successful in magnetic separation.

3 Flotation

Flotation has been applied to the treatment of kaolin from primary and secondary deposits. During the flotation process, kaolinite and mica particles are separated, and the resulting purified products are several suitable industrial grade raw materials. The selective flotation separation of kaolinite and feldspar is usually carried out in a water slurry with controlled pH. The flotation process of separating kaolin and feldspar at natural pH, using water-soluble salts of trivalent metal ions as activators and depressants, ultimately obtains qualified concentrates.

4 Reduction method

Trivalent iron (Fe3+) is only soluble at an acidic pH of 3 or less. Ferrous iron (Fe2+) is soluble in a wide range of pH values, but at neutral or above pH, Fe2+ is only stable under reducing conditions.

Removal of Fe3+ impurities from industrial kaolin is usually achieved by combining physical techniques (magnetic separation, selective flocculation) with chemical treatment under acidic or reducing conditions.

Thiourea dioxide ((NH2)2CO2, TD) is a strong reducing agent that has long been widely used in leather processing, textile printing and dyeing, papermaking, bleaching and other fields [58]. Compared with other reducing agents such as borohydride and hydrosulfite, TD has the advantages of strong reducing ability, environmental friendliness, low decomposition rate, safety and low mass production cost. Insoluble Fe3+ in kaolin can be reduced to soluble Fe2+ by TD. Subsequently, after filtering and washing processes, the whiteness of kaolin can be increased.

5 Oxidation method 

Oxidation treatment includes the use of ozone, hydrogen peroxide, potassium permanganate and sodium hypochlorite to remove the adsorbed carbon layer to improve whiteness. The kaolin that appears deeper under the thicker cover is gray, and the iron in the kaolin is in a reduced state. Strong oxidants such as ozone or sodium hypochlorite are used to oxidize the insoluble FeS2 in pyrite into soluble Fe2+, which is then washed with water to remove Fe2+ from the system.