The quality of kaolin is mainly reflected in three aspects of whiteness, purity and fineness. The higher the whiteness, the higher the purity, and the finer the particle size, the better the quality and the higher the price on the market. There are many factors that affect the quality of kaolin, mainly iron and titanium. According to the different properties of kaolin, its processing technology mainly includes kaolin beneficiation and purification, superfine grinding and surface modification.
Gravity separation is mainly used in the beneficiation and purification of sandy and soft kaolin. The process is to add kaolin, water and dispersant to the ramming machine to mix a certain concentration of ore slurry, and pass the high-speed rotating impeller in the ramming machine The kaolin is separated from other minerals and organic matter such as quartz and feldspar, and the mashed ore slurry is then passed through centrifugal classification equipment such as spiral classifiers and hydrocyclones to remove the fine sand in the kaolin for further purification. The purified product can be used as a kaolin product after it meets the use requirements, and the quality of kaolin can also be improved by further removing iron and whitening.
The magnetic minerals present in kaolin mainly include:
- Fine-grained iron-containing titanium oxides, such as magnetite, ilmenite, etc.
- The iron in the crystal lattice or the iron in the form of lattice substitution In the weakly magnetic minerals, such as biotite and so on.
There are two main methods for the separation of weak magnetic minerals: high-gradient magnetic separation and magnetic roasting. High-gradient magnetic separation is commonly used in industry. The difference between high-gradient magnetic separation and conventional magnetic separation is that it can generate a magnetic field strength of the order of 107 Gauss/cm, which can efficiently capture fine-grained weakly magnetic mineral particles present in kaolin.
As the demand for kaolin continues to increase and mining, high-quality kaolin resources are becoming less and less, most of which are low-grade kaolin that is difficult to develop and utilize. The ore contains high impurity content and is finely embedded, and high-gradient magnetic separation cannot be done. In order to effectively remove iron, it is the development trend of high-gradient magnetic separation technology to achieve the application of superconducting magnetic separation in industrial production by increasing the magnetic field strength in the future.
The flotation of kaolin is mainly to add flotation agent to make the fine-particle iron-titanium impurity minerals in the kaolin enter the foam and be scraped out to achieve the purpose of removing iron and whitening. Due to the natural flotability of iron sulfide minerals, its flotation separation can be achieved by adding sulfide mineral flotation collectors for froth flotation.
A clay development company uses carrier-free flotation technology to remove iron from kaolin by flotation. During the flotation process, Ca2+ is added to activate the flotation process, and at the same time, high-strength scrubbing of the kaolin slurry is carried out to remove surface contaminants. A good flotation effect has been achieved.
Selective flocculation is based on the difference in properties between minerals, by adding flocculants to selectively aggregate some fine-grained particles of the slurry to form flocs with a larger particle size. The principle of selective flocculation of kaolin for iron removal is mainly to flocculate and settle the fine-grained kaolin by adding a kaolin flocculant, while the iron-titanium impurity minerals in kaolin remain in the upper suspension, and then magnetic separation or flotation Iron and titanium impurity minerals are removed to achieve the purpose of removing iron and whitening.
The chemical method of whitening is mainly through the addition of agents to react with the impurity components in the kaolin, and then water washing to remove. Commonly used methods include acid solution, oxidative bleaching, reductive bleaching, and oxidation-reductive bleaching.
The principle of the acid solution method is to add an acid to the kaolin to react with the insoluble impurities, and then wash it with water to remove it. The commonly used acids are mainly hydrochloric acid, sulfuric acid, and oxalic acid.
The principle of reductive bleaching is to add a reducing agent in an acidic environment with a pH of 2 to 5 to convert the insoluble Fe3+ in the kaolin into soluble Fe2+, and then wash the soluble Fe2+ to remove it. The commonly used reducing agent is sodium dithionite (Sodium sulfide), sodium thiosulfate, etc.
The principle of oxidative bleaching is similar to reductive bleaching, mainly by adding strong oxidants (such as hydrogen peroxide, potassium permanganate, chlorine, etc.) to kaolin to oxidize insoluble impurities (such as FeS2 and organic matter), and then rinse with water to remove .
In many cases, ore contains insoluble trivalent iron and divalent iron at the same time. Single reduction bleaching or oxidation bleaching cannot effectively remove iron. At this time, the method of oxidation-reduction combined bleaching can be used to remove iron. The principle is to first add an oxidant to oxidize the insoluble FeS2 to Fe3+, and then add a reducing agent to convert Fe3+ into Fe2+ that can be removed by washing with water. Compared with single reduction bleaching or oxidative bleaching, this method has significantly improved iron removal effect, but its process flow is more complicated.
Chlorination roasting is a commonly used kaolin whitening process. Its main principle is to add a solid chlorinating agent or chlorine to react with the iron and titanium impurities in the kaolin in a reducing environment to generate chlorides and discharge them. Thereby improving the whiteness. Coal-based kaolin ore itself contains a certain amount of carbon, which is beneficial to the chlorination reaction. In addition, coal-based kaolin calcining and decarbonization is one of the necessary processing processes, combining decarbonization with chlorination and removal of iron and titanium. Therefore, the chlorination roasting method is widely used in the purification of coal series kaolin.
The particle size of kaolin is the key to measure the quality of its products. There are certain requirements for the particle size of kaolin in the fields of papermaking, coatings, and rubber and plastics, so it is necessary to superfine the kaolin.
Because kaolin has a lamellar structure, the ultrafine crushing of kaolin is also called peeling. It is mainly through physical or chemical action that the laminated kaolin is ground and peeled into a single layer of flakes, and its particle size is reduced without destroying its lattice structure. Commonly used equipment is stirring mill and ball mill.
Nano-kaolin has many unique properties of nanoparticles, such as surface effects, macro-quantum tunneling effects, etc., which make it widely used in various fields and can greatly improve the physical and chemical properties of the material. With the development of science and technology, the nanoization of kaolin will also become the main research direction of kaolin deep processing.
Kaolin surface modification is to use physical or chemical means to treat the surface of kaolin, so that the whiteness, brightness, chemical activity and other physical and chemical properties of kaolin are changed. The surface modification methods of kaolin mainly include coupling agent treatment, adsorption, surface coating, etc. The most widely used is coupling agent modification. The principle of coupling agent modification is to change the physical and chemical properties of the surface through the interaction of the coupling agent and the active groups on the surface of kaolin.