Kaolin is a type of clay or clay rock primarily composed of the kaolin mineral group. Kaolin has a wide range of applications, including the ceramics industry, papermaking, refractory materials, cement industry, rubber industry, and more. Natural kaolin often contains impurities such as iron and organic matter. Removing these impurities to enhance purity and whiteness is the ultimate goal of kaolin purification. Here, we introduce four methods for kaolin purification and three commonly used technological routes.
Gravity Separation Method
The principle of the gravity separation method is to utilize the difference in density between kaolin and gangue minerals to remove gangue minerals and thereby enhance the whiteness and purity of kaolin. Using this method, it is possible to remove high-density impurities like organic matter, quartz, feldspar, and iron-titanium-manganese elements from kaolin.
Gravity separation methods often employ centrifugal beneficiation machines to remove high-density impurities. Additionally, hydrocyclones can be used for washing and screening during the separation process.
Magnetic Separation Method
The iron content in raw kaolin typically ranges from 0.5% to 3%. The principle of magnetic separation is to exploit the difference in magnetic properties between kaolin and iron impurities to remove iron. Common iron impurities in raw kaolin include magnetite, ilmenite, siderite, pyrite, and other colored impurities.
It's important to note that strongly magnetic impurities like magnetite and ilmenite can be directly removed using magnetic separation. However, weakly magnetic impurities may require pre-roasting or high-gradient strong magnetic field magnetic separation for effective removal.
Flotation Method
The flotation method relies on the difference in floatability between kaolin and gangue minerals to remove impurities. Flotation is mainly used to treat raw kaolin with numerous impurities and low whiteness. It is employed to remove iron, titanium, and carbon impurities, allowing for the comprehensive utilization of low-grade kaolin resources.
Kaolin is a typical clay mineral, and between its particles, there are often embedded impurity minerals like iron and titanium. Therefore, the original ore needs to be ground to a certain fineness, and the particle size often belongs to the mud-grade particles mentioned in the flotation process. Common flotation methods for kaolin include ultrafine particle flotation, double-liquid layer flotation, and selective flocculation flotation.
Chemical Treatment
Chemical Leaching: Specific leaching agents such as sulfuric acid, hydrochloric acid, and nitric acid can selectively dissolve certain impurities in kaolin, removing them. This method can be used to remove impurities like hematite, goethite, siderite, etc., from low-grade kaolin.
Chemical Bleaching: Chemical bleaching involves oxidizing impurities in kaolin into soluble forms, which are then washed away to enhance the whiteness of the kaolin product. However, chemical bleaching is relatively costly and is typically used for refined kaolin concentrates after impurity removal.
Calcination Purification: Calcination involves utilizing the differences in chemical composition and reactivity between impurities and kaolin. Processes like magnetic roasting, high-temperature calcination, or chlorination can be applied to remove iron impurities, carbon impurities, sulfides, and other impurities. This method can significantly enhance the whiteness of the calcined product but is associated with higher energy consumption and the potential for environmental pollution.
Natural kaolin ore is often categorized into hard kaolin, soft kaolin, and sandy kaolin, based on differences in origin, quality, plasticity, and sand content. Here are the technological routes for these types:
Hard Kaolin
Hard kaolin, also known as kaolin rock, typically has a hardness rating of 3-4 and only becomes plastic after fine grinding.
Technological Route for Purification of Hard Kaolin: Raw Ore → Crushing → Pulping → Cyclone Classification → Centrifugal Classification → Delamination → Bleaching → Filtration → Drying → Product.
Soft Kaolin
Soft kaolin refers to the clayey form of kaolin with relatively soft quality and moderate plasticity, generally containing less than 50% sand.
Technological Route for Purification of Soft Kaolin: Raw Ore → Crushing → Pulping → Cyclone Classification → Bleaching → Filtration → Drying → Product.
Sandy Kaolin
Sandy kaolin is characterized by its loose and sandy texture, with relatively weak plasticity, but becomes more plastic after sand removal. Sandy kaolin typically contains more than 50% sand.
Technological Route for Purification of Sandy Kaolin: Raw Ore → Pulping → Spiral Classifier for Sand Removal → Sedimentation Tank and Hydrocyclone Classification → Centrifugal Classification → Filtration → Drying → Product.
Properties and compositions of different kaolin ores vary, and it is essential to conduct ore dressing experiments and formulate suitable purification processes based on the test reports to obtain ideal kaolin products. The above purification methods and technological routes are provided for reference.
