Kaolin is widely used in ceramics and refractory materials, followed by industrial raw materials such as paper, paint, paint, building materials and rubber. In recent years, with the rapid development of China's paper industry and paint and paint industry, the demand for high-quality kaolin products in the industry has increased year by year, and the market gap is large. However, there are fewer and higher high-quality kaolin resources in nature, which can not meet the market requirements. Washed kaolin is an ideal raw material for paper coating in China. It has strong hiding power, high whiteness and uniform particle size distribution. Features. It is easier to improve the printability, ink absorption and finish of the paper during the paper coating process. Calcined washed kaolin as one of the means to prepare high-quality kaolin, has a good development and application prospects.
In industrial production, kaolin mineral processing can be divided into dry and wet methods. For soft kaolin and sandy kaolin, the wet beneficiation process is generally used. The main process of kaolin wet process is: coarse crushing of kaolin ore → pulping and purification → classification → mechanical superfine pulverization → centrifugal dehydration → drying → grinding.
In order to disperse the kaolin well, it may be stirred while feeding, and a small amount of a dispersing agent may be added if necessary. Common dispersants are sodium carbonate, sodium silicate, sodium polyacrylate, and trimer phosphate sodium hexametaphosphate. Based on the theory of friction in the pulp, the mechanism of particle dispersion, and the influence factors of viscosity, Zhang Yan used the Ener viscometer to test the influence of various dispersants on the viscosity of the slurry. It was determined that the method of effectively reducing the viscosity of Maoming washed kaolin was to adjust. The pH of the pulp and the addition of reasonable dispersants DC and STPP.
In this paper, the processing methods (calcination method), beneficiation classification technology, iron removal technology, chemical modification and existing problems of washed kaolin are summarized, and the future is prospected.
Calcination method
It can dehydrate and devolatize kaolin, and can also change the crystal structure of kaolin to obtain special surface properties. Its main purpose is to improve the whiteness, purity, crystal structure, surface physicochemical properties and processing properties of kaolin. Wait.
Chen Qiang carried out a calcination experiment to improve the whiteness of washed kaolin. The whiteness of the washed kaolin after calcination can reach more than 90%. Because of the high iron content before calcination, the magnetic separation operation is added to the beneficiation process and calcination, the whiteness can reach 94.5, which provides a basis for the continuous development and deep processing of washed kaolin.
Liu Qinqi et al. investigated the effects of calcined kaolin on the mechanical properties of rubber under different temperature conditions. The calcined product changed from crystalline to amorphous amorphous. The properties of calcined washed kaolin products varied with calcination temperature, oil absorption and volume resistivity. It works best at 700 ° C and has good mechanical properties in styrene butadiene rubber.
On the basis of the traditional processing technology of washed kaolin, Zeng Wein can carry out deep processing of kaolin resources through appropriate temperature calcination, high-efficiency depolymerization, dispersion and modification to prepare active kaolin with pozzolanic properties, which can be used as high-end concrete additive and cable filler. In the field of water-based paints, it has the characteristics of short setting time, high early strength, compensation shrinkage, no corrosion to steel bars and high electrical resistivity. It increases the added value of kaolin resources and has huge market potential.
Li Sanhua has repeatedly tested that the calcination temperature is controlled at 960-980 °C. The calcined product has good oil absorption value and large specific surface area, and has good hiding rate and opacity. At the same time, the whiteness of the product can reach 91.8%-93. %.
Li Weiwei has a calcination temperature of 900 ° C and a holding time of 1 h. When 2% NaCl is added as a whitening agent, the maximum whiteness of the product reaches 92.84%.
He Lixi analyzed the effects of calcination temperature and constant temperature on its crystalline structure, and investigated the variation of whiteness and oil absorption of calcined products during calcination. The highest whiteness was 86.27% at 750 °C, and the highest oil absorption was 83.8 mL/100 g of washed kaolin. The whiteness increased first, and the increase trend became smaller after 50 minutes. There was no significant change in oil absorption and extended holding time.
Centrifugal classification
Generally, the horizontal snail centrifugal grading technology is adopted to make the product fineness reach -2 μm and account for more than 90%, and the classification efficiency is above 80%. Li Qicheng used a centrifugal classifier to carry out super-segmentation test on kaolin. Through a centrifugal classification, the content of -2μm in the graded overflow can reach 85%-90%, which meets the particle size requirements of the blade coating.
Tang Hongwei determined that the optimal dispersion conditions for the precise classification of weathered kaolin are: solid content 5%, hexa-polar concentration 0.2%, DC concentration 0.1%, pH neutral, good dispersion, better can achieve different Free sedimentation and separation of large and small particles.
Yi Longsheng used a certain Australian sandy kaolin as raw material. After centrifugal sand removal, the centrifugal yield reached 58.5%, the total yield reached 28.1%, the SiO2 content of the product decreased from 52.4% to 47.2%, and the Al2O3 content increased from 36.5%. 38.0%. The chlorination of the concentrate was carried out to remove iron and titanium , so that the content of Fe2O3 and TiO2 decreased from 1.26% and 1.03% to 0.35% and 0.46%, respectively. The SiO2 content of the calcined product was 52.9%, the Al2O3 content was 45.8%, and the whiteness of calcination was from 86.3. % increased to 93.6%, meeting the requirements of the national standard calcined kaolin ZT-(D)1 for papermaking.
Jiang Lei, coal-based primaries kaolin ore, after crushing, grinding, flaking, use of hydrocyclones dressing experiment, the whiteness of the final product obtained is greater than 88%, the main impurities and the products of iron and titanium content were The decline.
Iron removal technology
Whiteness is the main parameter affecting the performance of kaolin. The higher the whiteness, the higher the purity and the higher the utilization value. Natural kaolin often reduces whiteness due to impurities. The substances affecting mineral whiteness in kaolin are mainly organic matter, iron, titanium-bearing ore and dark minerals. Among them, iron is the main substance affecting the whiteness of kaolin. The iron-containing mineral will become Fe2O3 when it is calcined at high temperature, causing the raw material to be yellow or brick red. These colored impurities are usually weakly magnetic and can be removed by strong magnetic separation.
Wang Hao through high gradient magnetic separation, the Fe203 content in the concentrate decreased from 1.33% to 1.04%, the iron removal rate was 21.80%, and the K2O content decreased from 1.68% to 1.27%, removing a large amount of impurity minerals such as mica in kaolin; The content of Fe203 in the concentrate after reduction bleaching decreased from 1.33% to 1.07%, the iron removal rate was 19.55%, and the content of iron impurities was reduced. The content of Fe203 in the high gradient magnetic separation-reduction bleaching process was reduced from 1.33% to 0.95. %, the iron removal rate is 28.57%, the K20 content is reduced by 1.29% from 1.68%, the calcination whiteness is increased from 76.53% to 84.00%, and the product meets the secondary standard (TC-2) for the ceramic industry.
Chemical modification
Song Haifeng hydrophilically modified the washed kaolin and prepared a hydrophilic modified kaolin/waterborne polyurethane composite by mechanical blending using a titanate coupling agent. When the mass fraction of modified kaolin is 2.0%, the tensile strength of the composite is 24.4 MPa, which is 53.1% higher than that of polyurethane, and the elongation at break is 492%, an increase of 9.6%, which enhances the toughening of the composite.
Sun Tao et al. found that kaolin had the highest oil absorption value when calcined at 750 °C, which was 80.272g/100g. At this time, calcined kaolin had the largest specific surface area, the pore size distribution was concentrated in micropores and mesopores, and the average pore diameter was the smallest. Gan Xuefeng et al. obtained a kaolin fine powder with a whiteness of 92.89% by a 10% pulp concentration, 4% insurance powder, a water bath temperature of 85 ° C and a NaCl content of 1%.
Luo Ronghuang takes the washed kaolin from some place in Fujian as the main raw material, and through the processes of mineral activation, acid decomposition and separation, the preparation of white carbon black co-production aluminum sulfate, the products reach the national standard and have good economic benefits. HongliangXu prepared kaolinite nanotubes with a length of 300nm-2000nm and internal and external diameters of 15-55 and 40-80 by methanol intercalation and solvothermal method.
Liu Xuening et al. prepared polypropylene/intercalated PP/kaolin nanocomposites by blending different modified kaolin with polypropylene. Yan Tao et al. used acid modification to change the adsorption activity site of kaolin, and improved its adsorption and purification performance of phosphorus. The aluminum in kaolinite was activated by calcination to further improve its adsorption and purification performance of phosphorus.
SoniaZulfiqar uses dimethyl sulfoxide to increase the interlayer spacing of kaolin from 0.712 nm to 1.12 nm, which enhances the adsorption performance of kaolin. Kaolinite organic intercalation compound will become a new functionalized nanocomposite with broad application prospects. Dong Wenhui and other methods using reduction and complexation significantly improved the whiteness of kaolin, effectively reduced the iron content of kaolin, and made the final product whiteness reach 86%, meeting the application requirements of rubber fillers and plastic fillers. Yang Mingan et al. activated the kaolin and added polyaluminum chloride and polyacrylamide to increase the whiteness from 72% to 82% without decomposing flocculation and calcination.
The single beneficiation and purification method has limited purification effect on kaolin. In the industry, a combination of various processes is generally used to select and purify kaolin. Zhang Lingyan] et al. used a sandy kaolin in a certain area of ​​Guangdong as the research object. After “magnetic separation---reduction bleachingâ€, the whiteness of kaolin increased from 56.04% to 66.61%, and the whiteness after calcination at 1200°C reached 86.10%. SamyaEl improve optical properties of paper through the copper plate and the acid leaching of the calcination method, stretching and tensile strength increase of 17.5% and 19.7%, respectively.
Problem
At present, the washed kaolin in the domestic market contains more adsorbed water, structural water, organic matter and other colored impurity elements, so that it has defects such as low whiteness, low void ratio, poor ink absorption, low hiding power, and high viscosity. There are few varieties and specifications, and the products lack market competitiveness. When kaolin is used as a high value-added paper-making filler, paper coating, etc., the whiteness of the product is very high, and the whiteness is generally required to be greater than 90%. Especially when used for paper coating, the product is required to have a relatively low viscosity.
Outlook
The application of kaolin in various industries is very extensive. With the increase of demand, the supply and demand of high-quality kaolin resources are gradually tense. However, ordinary kaolin cannot meet the industrial demand. Therefore, the deep processing of low-quality kaolin has become a hot issue of comprehensive utilization of kaolin resources. At present, in some application fields, the “Double 90†standard can no longer meet the application needs. The particle size and whiteness of kaolin need to be further improved, which will lay a solid foundation for the deep processing and utilization of kaolin, and rationally develop and utilize kaolin. Resources will also generate huge economic and social benefits.
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