Kaolin often contains harmful impurities like iron, organic matter, and dark substances. These harmful impurities will cause kaolin to show different colors and affect its whiteness. Therefore, we must remove impurities from kaolin before use. The common kaolin purification methods include the following items.

Kaolin gravity separation

The gravity separation and purification process mainly uses the density and particle size differences between kaolin and gangue minerals to remove light organic matter and high-density impurities containing elements such as iron, titanium, and manganese to purify kaolin.
Centrifugal concentrators are usually used to remove high-density impurities. The hydrocyclone group can also complete the washing and screening operations in selecting kaolin. It can not only achieve the purpose of washing and classifying but also remove some impurities, which have good application value.

Kaolin magnetic separation

The magnetic separation process removes weak magnetic dyeing impurities such as hematite, siderite, pyrite, and rutile from kaolin. For highly magnetic minerals such as magnetite and ilmenite or iron filings mixed during processing, it is more effective to use magnetic separation to separate kaolin. There are two main methods for weakly magnetic minerals: roast them first to turn them into strongly magnetic iron oxide, then perform magnetic separation; the other is to use high-gradient strong magnetic separation to do the magnetic separation. The magnetic separation method effectively solves the problem of developing and utilizing low-grade kaolin with no commercial mining value due to high iron ore content. The superconducting magnetic separator has high field strength, energy saving, high productivity, etc., and can be directly processed in Kaolin clay, which contains more impurities. However, obtaining high-grade kaolin products using only a single magnetic separation method is difficult. Other processes, such as chemical treatment, are also required to further reduce the iron content in kaolin products.

Kaolin flotation

The flotation purification process can effectively remove iron, titanium, and carbon impurities in kaolin, enabling the recovery and reuse of low-grade kaolin resources such as coal-based kaolin. Kaolin particles are finer and more difficult to float than gangue minerals. Therefore, the kaolin flotation purification process mainly uses reverse flotation to achieve better impurity removal effects, such as reverse flotation for carbon removal, desulfurization, and iron removal. Kerosene is used as a collector, pine alcohol oil is used as a foaming agent, and water glass is used as an inhibitor. The flotation purification process is mostly used to treat raw kaolin ore with more impurities and lower whiteness to utilize low-grade kaolin resources comprehensively.

Kaolin chemical leaching

Certain impurities in kaolin are selectively dissolved through leaching agents such as sulfuric acid, hydrochloric acid, and nitric acid to remove impurities. This method can remove hematite, limonite, siderite, etc. from low-grade kaolin.

Kaolin chemical bleaching

Impurities in kaolin clay are oxidized into soluble substances through bleaching and then washed and removed to improve the whiteness of kaolin clay products. However, the cost of chemical bleaching is relatively high and is usually used for kaolin concentrates that require further purification after impurity removal.

Kaolin roasting and purification

By utilizing the difference in chemical composition and reactivity between impurities and kaolin, magnetization roasting, high-temperature roasting, or chlorination roasting is performed to remove iron-containing impurities, carbon-containing impurities, sulfides, and other impurities in kaolin. This method can improve roasted products’ chemical reaction activity, improve kaolin’s whiteness, and obtain high-grade kaolin products. However, the disadvantage of roasting and purification is that it consumes a lot of energy and quickly causes environmental pollution.