The processing technology of kaolin ore depends on the nature of the ore and the end use of the product. There are two kinds of dry process and wet process used in industrial production. Usually, hard kaolin is produced by dry process, and soft kaolin is produced by wet process.
Dry beneficiation process
The dry process is a simple and economical process. After the mined raw ore is crushed to 25.4mm by a hammer crusher, it is fed into a cage crusher to reduce the particle size to 6.35mm. The hot air in the cage crusher reduces the moisture of the kaolin from 20% About 10%. The crushed ore is further refined by a blown Raymond mill equipped with a centrifugal separator and cyclone dust collector. This process can remove most of the sand and gravel. The products are usually used as low-cost fillers in the rubber, plastics and paper industries. When used in the paper industry, this product can be used as a filler where the ash content of the filler layer is less than 10% or 12%, and the brightness of the product is not required at this time.
When the dry method requires high whiteness of the product, the product produced by the Raymond mill must be subjected to dry iron removal. The advantage of the dry process is that it can save the process of product dehydration and dry operation, reduce the loss of ash, the process is short, the production cost is low, and it is suitable for arid and water-scarce areas. But to get high-purity high-quality kaolin, we must rely on wet process.
Wet beneficiation process
The wet process includes three stages: ore preparation, mineral processing and product processing. The preparation stage includes batching, crushing and tamping operations. Tamping is to mix the raw kaolin ore with water and dispersant in the tamping machine to make pulp. The tamping operation can disperse the raw ore, prepare the kaolin ore pulp of appropriate fineness for the sorting operation, and remove the large-grained sandstone. The beneficiation stage may include hydraulic grading, flotation, selective flocculation, magnetic separation, chemical treatment (bleaching) and other operations to remove different impurities. The prepared pulp is de-sanded by rake washing box, float tank classifier or cyclone, and then it is divided into continuous centrifuge, hydrocyclone, hydraulic separator or vibrating fine screen (325 mesh) Two grades of thickness. The fine-grained grade of the classifier is sent to HGMS (high gradient magnetic separator) to remove iron and titanium impurities. The product is leached with iron oxide after being stirred and scrubbed and peeled off. The clay with high brightness and good coating performance can be removed without magnetic separation and Stripped and sent directly to the leaching operation. After leaching, alum is added to the slurry to make the clay minerals coagulate and facilitate dehydration. The bleached clay is dehydrated with a high-speed centrifuge, rotary vacuum filter or filter press. Filter or filter press dehydration. The filter cake is redispersed into 55% to 65% solid slurry, and then spray dried to make a loose dry product. Part of the dry product is mixed into dispersed pulp to make 70% solids and shipped to the paper mill by boat.
The final product without selection has lower brightness, and only high-brightness clay products can be obtained by configuring magnetic separation, foam flotation or selective flocculation in the process. However, these independent operations all have their own advantages and disadvantages, so the industry usually adopts a combined process of two to three of these processes for the comprehensive utilization of clay resources.
High gradient magnetic separation
Dyeing impurities in kaolin (such as hematite, etc.) have weak magnetic properties, so they can be removed by a high gradient magnetic separator. The US uses PEM-84 wet high-gradient magnetic separator to reduce the Fe2O3 in the kaolin ore from 0.9% to 0.6% and Ti2O3 from 1.8% to 2.0% to 0.8%. This kind of high-gradient magnetic separator uses stainless steel wool as the medium. When the field strength is 1.5 ～ 2.0T, the power consumption is 270 ～ 500kw. In China, wet methods have been carried out on Hunan Huling, Leiyang, Poluo and Hengling clays [3-6], all of which have achieved good test results, especially the removal of iron and titanium in kaolin by vibrating high gradient magnetic separation A very good test index. From the comparison test results of China's CLY500 vibrating high-gradient magnetic separator for Leiyang Kaolin in Hunan and the high-gradient magnetic separator of PEM-84 in the United States, China's high-gradient magnetic The machine selection performance is better than the United States.
Because some iron impurities in some kaolin mines exist in the form of silicate, the magnetic property is very weak, while titanium exists in the form of rutile, the magnetic separation method is difficult to work, so the process is usually equipped with flotation, selective flocculation and other operations To improve product quality. In recent years, superconducting magnetic separators have been successfully used in kaolin separation. Not only does energy consumption decrease, but the field strength can be greatly improved, the quality of kaolin concentrate is also higher. Eriez superconducting magnetic separator has the characteristics of rapid magnetization, can reach the highest design field strength (5T) within 60s, and the demagnetization time is short, which greatly shortens the time required to flush magnetic impurities from the magnet during the load cycle. Its energy consumption is low, about 80% less than the conventional magnetic separator, and the processing capacity is large, which can reach more than 100th. The United Kingdom has tested a reciprocating helical tube superconducting magnetic system. Its design is similar to a conventional pot-shaped magnetic filter. The difference is that it still keeps the superconducting magnet in the excitation state during the working cycle without the need for switch control. Continuous operation. The 3048mm, superconducting high gradient magnetic separator designed by Humboldt Company of Germany has a simple structure, low operation and maintenance costs, and good stability.
The purpose of the flotation operation is to float titanium impurities from kaolin. Because the impurity particles are very fine, the carrier flotation process is usually used. The carrier mineral can be calcite or silica sand (-325 mesh). The amount of carrier mineral is generally 10% to 20% of the weight of kaolin. A part of the carrier can be reused after being recycled. The agents used in the flotation process include the dispersant sodium silicate, the pH adjuster amine hydroxide and the caustic soda collector, tall oil, fatty acids and calcium petroleum sulfonate. However, there are many shortcomings in flotation. The hydrophobization of the carrier requires a large amount of reagents. The flotation process can only be effective at a low slurry concentration, thereby increasing dewatering costs. The loaded body must be removed from the clay product as much as possible. And from the foam The product is recycled for recycling. The chemicals and carrier minerals remaining in the clay are harmful to the final product. Cundy and Yong et al. Studied a flotation process that does not require a carrier, and floats anatase directly from kaolin, which is characterized by the presence of dispersants (such as sodium silicate) and pH adjusters (commonly used amine hydroxides). Under conditions, scrub with high slurry concentration (40% to 60% solids) to remove surface contaminants. At the same time, scrubbing also dissociates anatase and hematite from kaolin minerals, and then captures a small amount of activator and fatty acids The slurry is added into the slurry together, and the anatase covered by the collector forms selective agglomeration under high shear stirring conditions, thereby significantly increasing the particle size. The slurry after high shear mixing and slurry mixing is diluted to 15% ~ 20% solids are used for flotation, and alum in kaolin can also be removed by flotation.
At pH 8 ~ 11, adding Ca2 +, Mg2 + and other alkaline earth metal ions to kaolin slurry can observe the selective aggregation of iron-titanium impurities, and then use weak anion polyelectrolyte for selective flocculation. The process requires the slurry concentration to be less than 20%, so there must be a lot of water to be removed in subsequent operations, and the residual flocculant also affects the quality of the final product.
Selective flocculation of kaolin with polymer flocculant, kaolin particles flocculate and settle to the bottom, iron and titanium impurities are reddish brown in the upper suspension due to the fine particles, and the upper suspension can be removed by removing Most of the iron and titanium impurities are removed, and then processed by other operations (such as magnetic separation) to obtain high-quality kaolin. Suzhou Kaolin Company has achieved good indicators by adopting a new selective flocculation process. The use of selective flocculation plus high-gradient magnetic separation for kaolin treatment also achieved satisfactory indicators.
The leaching is carried out in the presence of a reducing agent (NaS2O4) in a weakly acidic solution (pH 3 ~ 4), which can keep the dissolved iron in the Fe2 + state, avoid the formation of Fe (OH) 3, and separate it from kaolin by washing with water. In order to remove dark-colored organic matter, strong oxidants (hydrogen peroxide, sodium hypochlorite, etc.) can be used for bleaching. The Suzhou Kaolin Company's selection plant has obtained high-quality kaolin products by oxidative bleaching. According to reports, the treatment of kaolin with microorganisms can significantly improve the quality of the product.
The selected clay is stored in the slurry tank for 6-8 hours, the pH is adjusted to 3-4, close to the zero electric point of the clay, so the clay particles are easy to agglomerate. The addition of alum to the ore pulp helps the agglomeration of clay particles and promotes dehydration. Cylinder filter is a commonly used dewatering device, it can increase the concentration of ore pulp to 55% ~ 60%. One of the important functions of the filtering operation is to remove the chemical agents in the clay. To strengthen this operation, water spray is often used. Spray drying has become a very effective process in the clay industry, but it is expensive. In recent years, a new type of filtration process using the electrophoretic properties of charged particles in an electric field has emerged. Kaolin particles are negatively charged at pH 3 and surrounded by oppositely charged ion mist to form an electric double layer. In the electric field, the clay particles move to the anode, and the counter ions in the ion mist move to the cathode. When the particles reach the anode, they are used to protect the filter cake formed on the anode film of the electrode. The anode filter cake is further dehydrated by electroosmosis, and excess water is pumped out through the negatively charged filter cake capillary according to the principle of electroosmosis.
The dewatering agent is used to agglomerate the kaolin particles into large particles, which can accelerate the precipitation speed of the particles, facilitate dehydration, and reduce the loss of fine particles of kaolin. Therefore, the development of new high-efficiency dehydrating agents for kaolin is also one of its research directions.