Iron oxide in kaolin clay has a detrimental effect on its color, reducing its brightness and fire resistance, which will significantly decrease its commercial price. Even a small amount of iron oxide, hydroxide, and hydrated oxide, such as 0.4%, is enough to color the clay sediment reddish to yellowish. These iron oxides and hydroxides can be hematite (red), magnetite (reddish-brown), goethite (brownish-yellow), limonite (orange), and hydrated iron oxide (reddish-brown), among others. Therefore, the first beneficiation step to give these raw materials commercial value is effectively removing iron oxide from kaolin clay.
water washing method
The water-washing process for producing kaolin coating and filler involves several steps. Firstly, the crude clay is made into a slurry. To separate kaolin from mineral impurities such as quartz and mica and classify it into three grades (fine, medium, and coarse), the slurry must consist of individual mineral particles separated and suspended in water. The kaolin particles have opposite charges on their edges and surfaces in the mixer, causing them to attract each other and form flocs. Dispersants such as sodium polyphosphate are added to separate the particles in the flocs. The clay slurry is pumped from the mixer to settling tanks and screens to remove sand and gravel larger than 44 micrometers. After the sand particles are removed, the desired kaolin is obtained.
Magnetic separation
The magnetic separation process is based on the difference in magnetic susceptibility between different types of minerals to separate them. Colored impurities in kaolin, such as rutile, hematite, magnetite, mica, and pyrite, are naturally magnetic. High-intensity magnetic separation has achieved significant success in the beneficiation of industrial minerals.
Flotation
The flotation method has been applied to process kaolin from primary and secondary deposits. During the flotation process, kaolinite and mica particles are separated, resulting in purified materials suitable for industrial use. The selective flotation separation of kaolinite and feldspar is usually conducted in a water slurry with controlled acidity or alkalinity.
Reduction method
Trivalent iron is only soluble in acidic conditions with a pH of 3 or lower. Ferrous iron is soluble within a wider range of acidity, but under neutral or higher pH conditions, Fe2+ is only stable under reducing conditions. In the presence of oxygen, Fe2+ is rapidly oxidized to the trivalent form, producing solid precipitates containing Fe3+. Removing Fe3+ impurities from industrial kaolin clay is typically achieved through physical techniques (magnetic separation, selective flocculation) and chemical treatment under acidic or reducing conditions.
Sodium bisulfite, also known as sodium metabisulfite or sodium pyrosulfite, has effectively reduced and leached iron from kaolin clay and is currently being used in the kaolin industry. However, this method must be carried out under strongly acidic conditions (pH < 3), resulting in high operating costs and environmental impacts. Furthermore, the chemical properties of sodium bisulfite are unstable, requiring special and expensive storage and transportation arrangements.
Sulfur dioxide urea is a strong reducing agent widely used in leather processing, textile printing and dyeing, papermaking, and bleaching. Compared to other reducing agents such as borohydride and insurance powder, sulfur dioxide urea has a strong reducing ability, environmental friendliness, low decomposition rate, safety, and low production cost in bulk. Insoluble Fe3+ in kaolin can be reduced to soluble Fe2+ by sulfur dioxide urea. Subsequently, through the filtration and washing process, the whiteness of kaolin can be increased. Sulfur dioxide urea is very stable at room temperature and neutral conditions, and its strong reducing ability can only be obtained under strong alkaline conditions (pH>10) or heating (T>70°C), which leads to higher operating costs and operational difficulties.
Oxidation method
Oxidation treatment involves using ozone, hydrogen peroxide, potassium permanganate, and sodium hypochlorite to remove adsorbed carbon layers and improve whiteness. In areas beneath thicker cover layers, the kaolin clay appears gray, and the iron in the clay is reduced.
Using strong oxidizing agents such as ozone or sodium hypochlorite, insoluble FeS2 in pyrite is oxidized to soluble Fe2+, which is then removed from the system through water washing.
Acid leaching
Sidhu et al. used hydrochloric and perchloric acid leaching to treat iron oxides and hydroxides. However, the industrial removal of iron oxide from high-purity clay or sand mines using sulfuric acid and other inorganic acids has significant limitations, as the residual acid after treatment may contaminate the raw materials used in ceramic production.
Compared to other organic acids, oxalic acid is the most promising due to its acidity, good chelating properties, and high reducing power. Using oxalic acid, dissolved iron can be precipitated from the leaching solution as oxalate iron, which can be further processed through calcination to form pure hematite. Oxalic acid can be obtained cheaply from other industrial processes, and any remaining oxalate salts in the treated material will decompose into carbon dioxide during the firing stage of ceramic manufacturing.
High temperature calcination method
Kaolin changes structure and phase during high-temperature calcination, which can be divided into two processes: removal of structural water and phase transformation. Calcination is the process used to produce high-grade kaolin products. Two different grades of calcined kaolin are produced based on the processing temperature. Calcination at temperatures between 650-700°C removes structural hydroxyl groups and evaporates water vapor, resulting in increased elasticity and opacity of kaolin, which are ideal properties for paper coating applications. Additionally, heating kaolin at 1000-1050°C can increase its grinding performance and achieve 92-95% whiteness.
Chlorination calcination method
Jackson studied the chlorination of kaolin minerals to remove impurities, mainly iron and titanium, to achieve mineral bleaching. The chlorination method removes iron and titanium from clay minerals, especially kaolin. The process involves high temperatures (700℃-1000℃), at which point kaolinite undergoes dehydroxylation and transforms into metakaolinite. At even higher temperatures, spinel and mullite phases are formed. These transformations increase the kaolin particles’ hydrophobicity, hardness, and particle size through sintering. The treated minerals can be used in various industries, such as paper, PVC, rubber, plastics, adhesives, polishing, and toothpaste. The higher hydrophobicity of these minerals makes them more compatible with organic systems.