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Titanium filtration
Titanium, also known as filtration control liquid was filtered, and titanium sulfate was precipitated and separated after ferrous, entering some hydrolysis should be very clean, free of any insoluble impurities. However, during the coarse filtration process of separating ferrous sulfate, a small amount of extremely fine suspended impurities are still filtered through the titanium liquid, and the temperature and viscosity of the titanium liquid after freezing and crystallization are further reduced, and some of the naked eyes are further reduced. The fine colloidal impurities that are not visible can be precipitated and must be further filtered and separated before use. Since these colloidal particles charged fine large specific surface area, the surface will not only absorption of harmful heavy metal ions affect product appearance and chemical purity whiteness, and these foreign particles during formation of undesirable hydrolysis center of the crystal, the influence hydrolyzate The particle structure causes lattice defects to cause impurity ions to be mixed into the crystal lattice, resulting in a decrease in optical properties and pigment properties of the titanium dioxide. This is the last purification process from black to white in the production of titanium dioxide in sulfuric acid. It should be especially careful, otherwise it will cause irreparable consequences.
Control filtration in industrial production usually uses pressure filtration, and most use plate and frame filter presses. In pressure filtration, the filtration rate is proportional to the filtration area and the pressure on the filtration area, and inversely proportional to the viscosity of the filtrate and the thickness of the filter cake. Generally, the temperature rises and the viscosity decreases to increase the filtration rate.
Since the molten iron is a thin adhesive solution, the colloidal impurity particles are very fine, and even fine ferrous sulfate crystals are further precipitated at a low temperature, and the titanium liquid is unstable at a high temperature, and it is impossible to filter. When the temperature is raised too high (generally no more than 40 ° C). If the filtration pressure is increased, not only will fine particles be squeezed out of the gap of the filter cloth, but also the filtration speed will be slowed down due to the clogging of the filter cloth until the filtration process is terminated. In this case, a filter aid can be formed on the filter medium by adding a filter aid, which can increase the porosity, reduce the compression ratio of the filter cake, prevent the plugging of the pores, improve the filtration efficiency, and increase the filtration. Liquidity. The choice of filter aid should meet the following principles.
a. The filter aid should not decompose in the solution and does not react with the filtered solution;
b. The particles of the filter aid must be easily dispersed in the solution, have good suspension property, and the formed filter layer is loose, porous, and has good adsorption performance;
c. The particle size of the filter aid is uniform, and within a certain range of particle size distribution, the particles are relatively hard, incompressible, and not broken during pressure filtration;
d. The price is low and the source is abundant. The filter aid after use can be discarded together with the filter cake.
The filter aids commonly used in the filtration of titanium liquid are: diatomaceous earth , charcoal powder, wood powder, white clay, rice husk ash, pulp and the like.
Diatomaceous earth is a sediment of ancient single-cell diatom microbial residues. Its main components are SiO 2 , A1 2 O 3 , Fe 2 O 3 , CaO, MgO, etc., of which SiO 2 content is 80%-85%, relative Density 2.1~2.5, good acid resistance, various colors are earthy opaque, white quality is best, diatomaceous earth has natural and processed products. The diatomaceous earth for titanium liquid filtration is usually after pickling and drying, and the processed diatomaceous earth has a high content of SiO2 (SiO 2 88%, A1 2 O 3 2.0%, Fe 2 O 3 1.3). %), the particle size is about 150 mesh (the remaining 7% of the sieve), the dosage is about 0.5kg/m2, and the thickness of the filter layer is about lmm.
Charcoal powder is generally prepared by burning wood into charcoal and then pulverizing it. Charcoal powder is suitable for filtration due to its spherical shape and large porosity. It is light in weight and easy to disperse. Its surface porosity is very favorable for adsorbing colloidal particles; its ash content should be <10%, fineness is 80~120 mesh, no Containing MnO and Fe 2 O 3 to avoid reducing the content of trivalent titanium in the titanium liquid, the dosage is generally 1kg/m 2 , and the thickness of the filter layer is 1~3mm.
The rice husk ash is composed of porous silicic acid material, and the SiO 2 content is more than 90%, which has a good adsorption effect.
Both wood flour and pulp are cellulosic, not only porous, but also negatively charged when in contact with a fast-flowing titanium liquid phase, so that positively charged colloidal particles in solution can be trapped, often in combination with diatomaceous earth. The disadvantages are more expensive.
For the filtration operation of titanium liquid, the filter aid is generally pumped into the filter with water (or small water, light waste acid), so that the filter aid forms a uniform filter layer on the filter cloth until After the circulating liquid is clarified, the titanium liquid to be filtered is pumped into the filter for circulation filtration, and after checking the clarity of the filtrate to meet the standard, the circulation is stopped for continuous filtration. When the filtration pressure is getting higher and the filtrate flow rate is getting less and less, it indicates that the filter cloth hole has been blocked. At this time, the filtration should be stopped, the filter cloth should be disassembled, and the above steps should be used again. Do not use the filtration pressure to force the filtration. In order to avoid the fineness of the particles, the clarity is unqualified. In winter, due to the high viscosity of titanium liquid, when the room temperature is lower than the freezing crystallization temperature, there will be fine industrial iron crystallizing to affect the normal filtration operation. At this time, the titanium liquid can be heated to 30~40 °C with hot water to increase the filtration rate. .
For some products, products with low impurity content and high whiteness requirements, such as food, medicine, reagents, and high-whiteness, high-brightness pigment-grade titanium dioxide, can not meet the needs of the above filtration operations, and sometimes It is necessary to further remove some traces of soluble impurities to avoid hydrolysis of these impurities together with the titanium liquid during hydrolysis to precipitate and adsorb on the surface of the meta-titanic acid particles to be mixed into the product.
The soluble impurities in the titanium liquid are generally classified into ferrous sulfate dissolved in the titanium liquid, because the ferrous sulfate does not precipitate in the acidic solution, and is still present in the mother liquid after hydrolysis, and is easily removed by water washing; product quality is more harmful metal ions such as Cu, Pb, Co, Cr, Mn , etc., by the addition of copper sulfate, sodium sulfide, copper sulfide so that the impurity generation, lead sulfide, cobalt sulfide, chromium sulfide, or manganese Metal sulfides are precipitated and these sulfides are insoluble in acidic media and can be removed by filtration. Since the amount of these harmful impurities is very small (0.004%~0:01%), even if it is completely converted into sulfide, because the amount is small and the particles are fine, it is still possible to pass through the filtration when filtering. The role of copper sulfate is as these small The carriers of the colloidal particles allow them to co-precipitate.
Some factories in foreign countries have high requirements for the purification of titanium liquid. Titanium liquid sometimes undergoes three filtrations, namely: filtration of precipitated titanium liquid before crystallization (hot filtration), crystallization after plate frame filtration (control filtration), concentration after è¶The hot tube filter is filtered once more (precision filtration), and the titanium dioxide produced by the thus filtered titanium liquid has a good whiteness.
After checking the quality of the filtered titanium liquid, there are two kinds of quantitative analysis and qualitative analysis. The quantitative method is to determine the residual amount of suspended solid matter in the filtered titanium liquid, generally <0.03~0.04g/L; qualitative method is to filter some titanium liquid in the Buchner funnel, then wash the filter paper with a small amount of water, drain After checking the appearance of the second filter paper, it is required to be white and free of foreign matter and without any traces of spots. After controlled filtration of titanium liquid; industrially known as "clear titanium liquid", the quality index of clear titanium liquid according to the requirements of different products and different hydrolysis processes, generally controlled as follows:
Titanium dioxide containing disc / g / L 135 ~ 175 iron to titanium ratio 0.18 ~ 0.37
F value 1.8~2.1 Stability/mL ≥400
Trivalent titanium content / g / L 1 ~ 5