Mineral Processing and Purification of Kaolin

Hand Sorting Method

The hand sorting method leverages the size or density differences between mineral particles to separate them. Primarily used to remove impurities such as feldspar and quartz, it enhances the purity and fired whiteness of kaolin. This method is suitable for small-scale enterprises.

Hydraulic Washing Method

The hydraulic washing method uses water as a medium to separate minerals from impurities. It can effectively remove larger particles of minerals like quartz, feldspar, and mica, as well as iron and titanium minerals. This method is simple and cost-effective, ideal for high-quality ore deposits.

Flotation Method

Flotation is a technique that separates valuable minerals from ores based on the physical and chemical properties of their surfaces. Innovations in flotation technology include carrier flotation, selective flocculation, and dual-liquid flotation, which are effective in pre-treating ceramic raw materials. For example, adsorption flotation involves adding limestone powder to finely ground kaolin slurry, where the limestone acts as an adsorbent for Fe2O3. Dual-liquid flotation adjusts the pH of the kaolin slurry, adds collectors and organic solvents, mixes thoroughly, then allows settling and separation.

Magnetic Separation Method

Magnetic separation exploits the magnetic differences between various minerals or materials for separation. For coal-associated kaolin with complex iron compositions, different high-gradient magnetic separation techniques can be applied, such as pulsating high-gradient magnetic separation, dry high-gradient magnetic separation, vibrating high-gradient magnetic separation, combined selective flocculation and high-gradient magnetic separation, and magnetic agglomeration.

Chemical Purification

Chemical purification utilizes the acid-soluble or alkali-soluble properties of certain impurities present in minerals by dissolving them using acids or alkalis.

Reductive Bleaching Iron Removal Method

This method uses a reductant to convert insoluble trivalent iron (such as limonite) into soluble divalent iron, which is then removed through filtration and washing.

Characteristics: Requires strict reaction conditions, releases irritating odors, and may leave residues that affect downstream applications.

Acid Leaching Bleaching Iron Removal Method

In this method, kaolin is treated with an acid solution to transform iron-containing insoluble minerals into soluble salts, followed by washing to achieve whitening of the kaolin.

Characteristics: Less effective against sulfide iron minerals and titanium iron minerals; requires higher temperatures for the slurry, increasing equipment corrosion resistance requirements and production costs; may leave residues affecting downstream applications.

Oxidative Bleaching Iron Removal Method

Using strong oxidants as bleaching agents, this method oxidizes minerals like pyrite in the kaolin slurry to produce soluble Fe2+. It can also oxidize dark-colored organic matter into colorless compounds for bleaching.

Characteristics: Primarily effective for pyritic kaolin; has limited application for kaolin ores containing other coloring metal substances.

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Microbial Bleaching Method

The microbial bleaching method includes microbial leaching and microbial flotation technologies. It removes pyrite and other sulfide minerals from kaolin using microorganisms like Thiobacillus ferrooxidans and iron-reducing bacteria. This method is low-cost, energy-efficient, and environmentally friendly, does not alter the properties of kaolin, and is currently in the experimental stage.

Modification of Kaolin

Intercalation Modification

Intercalation modification overcomes interlayer hydrogen bonding and inserts materials into the interlayer space, thereby increasing the interlayer spacing without destroying the lamellar structure of kaolin. Techniques include liquid intercalation, evaporation solution intercalation, mechanochemical intercalation, microwave radiation intercalation, and ultrasonic intercalation.

Calcination Modification

Calcination modification is a common processing method for kaolin, involving thermal treatment by physical means. The calcination temperature varies depending on the intended use, with high-temperature calcination (450–925°C) being optimal for carbon removal and whitening. Calcined kaolin is used in ceramics, fillers, adsorbents, catalysts, and other applications.

Chemical Modification

Chemical modification includes acid and alkaline treatments, which improve the surface adsorption and reactivity of kaolin powders. Research on acid and alkali modifications is still in its early stages.

Polyhydroxy Iron Modification

This method involves mixing polyhydroxy iron solution with crushed and sieved kaolin, followed by modification under heating and stirring. The adsorption capacity of kaolin increases with rising temperature after modification with polyhydroxy iron.

Coupling Agent Modification

Coupling agent modification creates a layer of organic coupling compounds on the surface of kaolin particles through chemical means, altering the surface characteristics of kaolin. Common coupling agents include silane and titanate coupling agents, which enhance compatibility between kaolin and organics.

Kaolin Delamination

Kaolin delamination reduces particle size by breaking down the layers, increasing the surface area. Mechanical and chemical delamination techniques produce ultra-thin sheets with low abrasion and high whiteness, suitable for papermaking, cosmetics, pharmaceuticals, and other industries.

Blending Technology for Kaolin

Stability of product specifications is a key indicator of kaolin quality. Due to natural factors, there are significant variations in quality among different mines and strata within the same mine. Downstream industries demand consistent quality, making blending technology essential for kaolin producers.

Summary

To align with industry trends and leverage the unique characteristics of China’s kaolin resources, leading kaolin companies are continuously researching and applying advanced technologies such as purification, delamination, and modification. These efforts aim to improve the whiteness, fineness, and plasticity of kaolin products, enhancing their quality consistency. Future developments include improvements in calcination processes, ultrafine flotation technologies, whitening and viscosity reduction techniques, utilization of kaolin tailings, nanotechnology, and antibacterial materials.

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References:

- Liao Zihui et al. Distribution, Types, Applications, and Prospects of Kaolin Deposits in China

- Zeng Hongjiu et al. Current Status of Iron Removal Technologies for Coal-Associated Kaolin

- Prospectus of Longyan Kaolin Co., Ltd. (Translated)

(Edited and compiled by Black Gold, China Powder Network)