Kaolin Processing Technology

Kaolin processing technology encompasses three core stages: purification and whitening, ultrafine processing, and modification. Depending on the properties of the raw ore and product requirements, either dry or wet processing methods are employed, as detailed below:

I. Purification and Whitening Process

Purification and whitening aim to remove impurities from kaolin, improving its purity and whiteness. The main methods include physical methods, chemical methods, and high-temperature calcination:

Physical Purification Methods

Hand Separation: Separates minerals based on differences in particle size or density, removing impurities such as feldspar and quartz, improving the purity and calcined whiteness of kaolin. Suitable for small-scale enterprises.

Water Separation: Uses water as a medium to separate minerals from impurities. Removes larger particles of minerals such as quartz, feldspar, and mica, or iron-titanium minerals. This method is simple, easy to implement, and low-cost, suitable for high-quality deposits.

Flotation: Separates useful minerals from ore based on differences in the physicochemical properties of mineral particle surfaces. Various flotation techniques exist, such as carrier flotation, selective coagulation, and two-liquid flotation, and it is an effective means of pretreatment for ceramic raw materials.

Magnetic Separation: Utilizes differences in mineral magnetic properties, separating them under the influence of magnetic force and other forces. Magnetic separation can effectively remove iron impurities from kaolin, improving its purity. For weakly magnetic minerals, high-gradient magnetic separation can be used to remove iron.

Chemical purification and bleaching: Primarily used to remove iron-bearing minerals from kaolin. Main methods include acid leaching, chlorination, alkali treatment, reduction, oxidation, and combined redox methods.

Reduction bleaching for iron removal: Insoluble ferric iron (such as limonite) is reduced to soluble ferrous iron using a reducing agent. After filtration and washing, the ferrous iron is removed with the filtrate.

Acid leaching for iron removal: Kaolin is treated with an acid solution, converting insoluble iron-bearing minerals into soluble salts. Further washing and other processes whiten the kaolin.

Oxidative bleaching for iron removal: A strong oxidizing agent is used as the bleaching agent in the kaolin slurry system. Pyrite and other minerals in the kaolin are oxidized to soluble Fe²⁺, while organic matter is oxidized into easily washable colorless oxides.

High-temperature calcination is the best method for removing carbon and whitening kaolin, and it is also an essential processing method for kaolin from coal-bearing strata. Calcination is typically carried out at 650–1050℃ to completely alter the phase composition of the kaolin, while some impurities volatilize, thus improving whiteness and insulation. Sometimes, high-temperature treatment methods such as chlorination, roasting,g and oxidative roasting can also be used to remove iron and titanium elements from the kaolin.

II. Ultrafine processing technology

Ultrafine processing aims to refine kaolin into ultrafine powder to meet the needs of specific industrial sectors. The main methods include:

Mechanical pulverization: Utilizing the layered structure of minerals, external force is used to break the bonds between layers, achieving ultrafine grinding. However, this method is energy-intensive.

Classification: Based on Stokes' rule, kaolin is settled in a liquid to obtain ultrafine kaolin. However, this method is costly and has a low yield.

Intercalation-exfoliation method: Intercalation causes expansion between mineral layers in kaolin, greatly weakening the bonding forces. After removing the intercalating material, the originally stacked flaky kaolinite naturally decomposes into smaller flakes, achieving natural exfoliation. This method is currently the main direction of research on ultrafine kaolin.

Chemical synthesis: General, usually using rock minerals or aluminosilicate gel as raw materials, ultrafine synthetic kaolinite is obtained through a hydrothermal method. It has high purity, good suspension stability, good light scattering properties, and excellent other properties.

III. Modification Process

Modification aims to alter the physicochemical properties of the kaolin surface and improve its application performance. The main methods include:

Acid-base modification: Primarily for calcined kaolin, this involves calcining and activating kaolin at a specific temperature based on the different chemical environments of Al and Si during phase transitions. This allows Al and Si to react with acids or alkalis, thereby changing their acidity, pore size, and specific surface area.

Surface modification: This involves treating the surface of kaolin powder using physical, chemical, or mechanical methods to change its physicochemical properties. Surface modification can effectively improve the whiteness, brightness, surface chemical activity, and polymer compatibility of kaolin.

Intercalation modification: This refers to the direct or indirect insertion of organic molecules into the interlayers of kaolinite without destroying its original layered structure.

IV. Process Flow

Industrially, kaolin is commonly processed using two methods: dry and wet processes.

Dry Process: This generally includes crushing, drying (usually in a rotary dryer), fine grinding, and air flotation. This process removes most of the sand and gravel, making it suitable for ores with high whiteness, low sand and gravel content, and suitable particle size distribution. Dry processing has low production costs and is generally suitable for dry areas. The product is typically used as a low-cost filler in industries such as rubber, plastics, and papermaking.

Wet Process: This includes several stages such as slurry dispersion, classification, impurity separation, and product treatment. The general flow is: Raw ore → Crushing → Slurrying → Sand removal → Hydrocyclone classification → Flaking → Centrifuge classification → Magnetic separation (or bleaching) → Concentration → Filtration → Drying → Packaging.

V. Special Processes (Coal-Assembled Kaolin)

Coal-assembled (hard) kaolin is a unique kaolin resource in my country. Currently, production primarily employs either a pre-ultrafine grinding followed by calcination or a pre-calcination followed by ultrafine grinding process:

Ultrafine grinding followed by calcination process: Raw ore → Crushing → Pulverizing → Slurrying → Wet ultrafine grinding or flake removal → Drying → Calcination → Deagglomeration → Grading → Packaging.

Calcination followed by ultrafine grinding process: Raw ore → Crushing → Pulverizing → Calcination → Wet ultrafine grinding → Drying → Packaging.