Kaolin comprehensive utilization technology (4)

8 , ultrafine grinding

In order to meet the high fineness requirements of kaolin in industries such as paper, plastics and rubber products, it is necessary to increase the fineness of kaolin to improve the quality of the product. It is difficult to achieve this by conventional methods. In recent years, foreign ultra-fine processing technology research has made great progress, such as the use of supersonic airflow pulverization and other methods to improve the fineness of kaolin, thus opening up new ways for the production of more paint-grade and high-grade filler-grade products. The resource utilization rate has been expanded and better economic benefits have been obtained. The ultrafine grinding process mainly includes grinding and stripping method, high pressure extrusion method and air flow pulverization method.

( 1 ) Kaolin peeling

Coarse kaolin is often made up of many monoliths. The stripping process divides the massive aggregates (>2 μm ) into monolithic or reduced superimposed layers from the lamellar kaolin crystal by peeling and delamination . Main points stripping apparatus used in a scale mill, the grinding media have ceramic beads, glass beads, synthetic corundum beads, nylon, polyethylene beads. The relative density of the beads is about 2.45 and the diameter is 2 ~ 3mm . The purpose of the stripping is achieved by a mixture of the agitating slurry and the fine grinding medium. After the kaolin is stripped, the crystal structure is generally not destroyed, the new surface is not polluted, and the colored impurities in the kaolin are released by dissociation and removed by sedimentation or centrifugation. Therefore, while the fineness is greatly improved, the whiteness and gloss are also improved. Used in the paper industry to greatly improve the gloss and opacity of paper.

( 2 ) High pressure extrusion method

The high pressure extrusion method is a high pressure homogenizer made by high shear force plus cavitation principle in a high pressure extrusion device, with pump high pressure ( up to 5.88 × 10 ⁷ Pa) , high pressure slurry 950m The /s line speed is frictionally extruded from the narrow slit and is sprayed at high speed onto the impeller in the normal pressure zone. When the material leaves the gap, the pressure is reduced. This sudden decrease in pressure produces a cavitation effect, like popcorn. This powerful impact combined with cavitation and frictional shear forces causes the crystal plane of the kaolin to be stripped along the weakly bonded hydrogen bond layer.

Kaolin with a particle size range of this treatment process is 2 - 20 μ m. The test proved that after the slurry with a high pressure homogenizer treatment time is less than 18% 2 μ m size fraction may be increased to 37% from the original. The Longyan kaolin, natural ore high whiteness (75--80), kaolinite content of 20--30% kaolin after the selection of a sheet, to a particle size 2 - 5 μ m, 5 - 10 μ m is Mainly, the coating grade product standard can not be reached. After the high pressure extrusion method, the coating grade product with less than 2 μ m particles and more than 80% can be obtained , and the maximum economic benefit is obtained.

( 3 ) Airflow pulverization

The essence of the jet pulverization method is to use the fluid energy to cause the powder to be subjected to a large shear collision, friction, and the like. When the force is greater than the breaking stress of the particle itself, the particle is pulverized. The method is to continuously feed the powder 750m In the supersonic airflow of /s or higher, the powder particles collide with each other to achieve grinding, and at the same time, the crushed particles are classified along with the jetted vortex flow in a special classification chamber provided in the pulverizer, and then by centrifugation, The coarse particles in the grading swirl flow outward, and are recycled back to the supersonic nozzle through the loop tube. The particles ejected from the nozzle at high speed collide with the coarse particles swirled in the grinding chamber, only the pulverized, Fine particles smaller than a certain size are discharged and collected in a trap. After jet milling, calcined kaolin particle size of more than 90% were 5 μ m or less, shows that using this method can receive good results.

9 , calcination processing

Calcination is a special processing method to improve the performance of kaolin. The use of calcined kaolin in the paper coating industry can increase the scattering power and hiding rate and increase the ink sucking speed. Used for cable packing to increase resistivity during synthesis 4A In the industry of zeolite , aluminum chloride and cryolite, calcination can increase the chemical activity of kaolin. High temperature calcination can increase the whiteness, the price may be partially replace expensive titanium white powder. Calcination produces mullite. For coal-based calcined Kaolinite is essential to the process, due to calcination can remove carbonaceous, improve the whiteness.

Kaolinite undergoes different phase transitions as the temperature increases during calcination. The reaction formula of the calcination phase transition process is as follows:

Al ₂ O ₃ · 2SiO ₂ · 2H ₂ O ^{550 ~ 700} Al ₂ O ₃ · 2SiO ₂ (meta-tilcite ) + 2H ₂ O

2(Al ₂ O ₃ · 2SiO ₂ ) ⁹²⁵ 2Al ₂ O ₃ · 3SiO ₂ ( silicon spinel ) + SiO ₂

2Al ₂ O ₃ · 3SiO _{2 Â}  ^{1100 　　　} 2(Al ₂ O ₃ · SiO ₂ ) ( like mullite ) + SiO ₂

3(Al ₂ O ₃ · SiO _{2 Â}  ) ^{1400 °C 　　　} 3Al ₂ O ₃ · 2SiO ₂ ( mullite ) + SiO ₂

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