Kaolin selection and purification test uses kaolin, which is the original ore purified soil of China Kaolin Company. The process route is: the three-stage cyclone for crushing and pulping of raw ore is selected, filtered, dried and pulverized. Sample A is the original ore of Yangdong Mine and Taoyuan Mine, and the mineral crystal structure is mainly tubular; sample B is the original ore of Yangxi Mine, and the mineral crystal structure is mainly flaky. Result of sample composition analysis.
Experimental materials and equipment Kaolin samples: A and B samples. Instruments and equipment: sealed reactor, made of stainless steel; extruder, made of stainless steel; constant temperature bath, produced by Shanghai Instrument Factory; muffle furnace, produced by China Experimental Electric Furnace Factory; precision temperature controller, produced by Shanghai Medical Instrument Factory.
Sample preparation Weigh 20g kaolin sample and a certain amount of NaAlO2 and water glass, mix thoroughly, add about 10ml of water, mix evenly, then extrude the strip with an extruder, then dry the sample at 100°C for 24h, and then put it in the muffle The furnace is roasted at 500 for 810h. The roasted sample is put into a stainless steel sealed reactor, and then 200ml of 20% NaOH is added. After the reactor is sealed, it is put into an oven and crystallized under pressure at 120 for about 10h. Take out the reactor, pour out the excess lye, remove the crystallized sample, and then wash it to neutrality, and then activate the obtained sample at 100 for 4 hours to obtain a desiccant sample.
This process adopts the natural kaolin molding-roasting into granular shape, and crystallizing in NaOH solution. Different from the traditional process, it avoids raw material drying, grinding, sieving and other processes, and the raw material particles after roasting have considerable strength, which is sufficient to meet the requirements of the handling process. After crystallization, the process of washing and drying 4A molecular sieve particles is much simpler than powder, but the crystallization conditions are different. The raw material of kaolin we use is a porous amorphous substance, which is a mixture of Al2O3 and SiO2, which is more active. When it comes into contact with the NaOH solution, the latter penetrates the entire particle interior through its pores. First, the alkali dissolves the Al2O3 and SiO2 on the inner wall of the hole, 14Page2
forms a kind of silica-alumina gel. Between the gel and the solid phase, the solubility is balanced. When heating. The solid phase solubility increases, increases the concentration of silicate, aluminate, and aluminosilicate ions in the liquid phase, and strengthens the polycondensation between the ions. When it reaches a certain level, crystallization begins. Since the solubility of 4A molecular sieve crystals is lower than that of the gel phase, the balance is broken, and the dissolution and crystallization will continue until the raw materials are completely dissolved. Since the crystallization is carried out inside the entire raw material particles, the crystallized 4A molecular sieve crystal grains are crowded in a small space, forming a structure in which the crystal grains are inlaid with each other. Therefore, the higher the degree of crystallinity, the higher the strength of the particles, and the degree of crystallinity and strength are reflected in the water absorption.
Performance tests include static water absorption rate test, phase test (XRD), crush strength test, etc. XRD phase analysis of the sample: The test spectrum shows that, in the case of uncompressed crystallization, although the sample contains additives, its crystal phase has no change relative to the kaolin raw material. The XRD spectrum of the sample containing additives and treated with 20% alkali under pressure crystallization conditions can find that a new crystal phase (relative to the kaolin raw material) has been produced, and there are characteristic peaks that are different from other silicon-aluminum compounds ( d=47), which coincides with the characteristic peak of 4A molecular sieve (d=47). The 8 strong lines are basically in line with it. It can be considered that this desiccant sample has formed a 4A molecular sieve crystal phase, and its molecular formula is found to be: Na12Si12O48XH2O. The pressure crystallized sample containing additives treated with 75% alkali has a better crystal phase. The raw materials of kaolin have not changed, so it can be considered that the alkali concentration is a very important parameter in the crystallization process of kaolin forming 4A molecular sieve, and there is no condition for crystallization if the concentration is lower than a certain concentration. Although the sample without additives was subjected to compression crystallization and 20% alkali treatment, the characteristic peak of 4A molecular sieve did not appear in the phase diagram. However, the phase diagram of the kaolin raw material crystal phase has changed a lot, and its structure is Na4Al3Si3O12. Both the experiment and the above analysis show that the 4A molecular sieve structure can be formed only under the condition of adding additives and homogenizing under pressure.
Results and discussion The water absorption of the blank sample: A (powder), 12%; B (powder), 163%; A (bar), 72%; B (bar), 9%. It can be seen that A , B sample has a certain water adsorption capacity when it is in powder form; but after forming, its water absorption rate drops sharply. Therefore, samples A and B as desiccants for automobile brake systems are far from reaching the required requirements in terms of adsorption capacity and strength, and they must be studied to improve their performance.
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