Optimize the way of hydrated titanium dioxide washing operation

Hydrated titanium dioxide is filtered and washed with water operation, although it is a simple physical filtering and washing process, but the metatitanic pigment grade titanium dioxide in claim wash low impurity content and excellent in whiteness of titanium dioxide Fe ₂ O ₃ content sometimes required to be less than 30X10 ^-6 , which not only requires sufficient washing time, but also to grasp several aspects that affect the quality of washing.

(1) Particle size and particle size distribution of hydrated titanium dioxide

The particle size and particle size distribution of hydrated titanium dioxide are the main internal factors affecting the washing speed and washing quality, and it is also one of the main reasons for the excessive washing time of most titanium dioxide factories in China.

The hydrated titanium dioxide particles are uniform in size and fast in water washing. Under the same conditions at the same time, the material has the lowest impurity content after water washing. The particles of hydrated titanium dioxide are too fine, not only easy to pass through the filter to cause material loss, but also easy to block the filter cloth, so that the filter cake is too solid, the water flow rate is slow, the washing time is long, and the washing quality is poor. If the hydrated titanium dioxide flocculated particles are relatively coarse, although the water flow rate is fast when washing, the particles inside the floc are not easily washed, so the uniformity and size of the hydrated titanium dioxide particles during hydrolysis are very important, which not only affects the washing operation, It also affects the quality of the final product. When the Blumenfield method is hydrolyzed, the purpose of stopping the heating and stopping the stirring at an appropriate time is to allow the hydrolyzed hydrated titanium dioxide microcrystals to slowly grow to an optimum particle size and particle size distribution under a better condition.

The kind of meta-titanic acid slurry with uneven particle size, too much colloidal substance and milk appearance, it is difficult to suck the film during filtration, the filter cake is not thick, the surface material flows down, the washing time is long, and the filter wear is lost. The surface of the filter cake is uneven or even cracked. The more the material is washed, the less the iron content can not be lowered. The sulfur is not easily removed during calcination, it is easy to be sintered, and the product quality is inferior.

(2) Vacuum degree and location of washing equipment

The degree of vacuum and the reasonable arrangement of the washing equipment are one of the important external factors that affect the washing speed and the quality of washing.

The degree of vacuum is the main external driving force when filtering and washing water of hydrated titanium dioxide, that is, the pressure difference between the two sides of the filter cake. The pressure difference between the two sides of the vacuum is large, the speed of filtration and washing is fast; the degree of vacuum is small on both sides. The pressure difference is small, the washing speed is slow, and even the falling phenomenon (the filter cake is detached from the vacuum leaf filter), but the vacuum is too high, not only the filter cake is prone to cracks, but also the water flow is short-circuited, and because the pressure difference is too large, The filter cake plate actually reduces the water flow speed, and does not reach the speed. At the same time, the vacuum of the general mechanical vacuum pump is smaller, the smaller the vacuum is, the larger the vacuum is, so maintaining a proper vacuum and the thickness of the filter cake is very important for the normal operation of the water washing. The experienced operator slowly opens the vacuum valve when starting the suction washing to prevent the vacuum from being too high, and the titanate particles quickly block the pores of the filter cloth, or the filter cake is too tight and difficult to wash, and then over time. The gradual shift gradually increases the degree of vacuum. Generally, the water washing operation requires that the thickness of the filter cake of metatitanic acid is controlled to be 30 to 40 mm, and the degree of vacuum is -0.04 to -0.065 MPa.

The relative position (dislocation) of the vacuum system (including the automatic drain tank) to the vacuum vane filter is important and is a prerequisite for affecting the washing speed. Almost all titanium dioxide factories in China (except for imported equipment), vacuum pumps, vacuum drain tanks and water wash tanks (vacuum leaf filter machines) are all arranged in the same plane, generally ±0.00 ground, usually automatic pouring tank and pouring The filtrate inlet pipe above the tank is higher than the outlet of the leaf filter. This difference in position will cause useless work on the vacuum, which actually reduces the effective vacuum. The washing equipment (washing tank and vacuum leaf filter) of foreign titanium dioxide factories are generally arranged on the third floor (10m or more), while the vacuum pump and drain tank are arranged on the ground of ±0.00, and this 10m high drain pipe is used. Like the atmospheric leg of a pneumatic condenser, the filtrate is quickly and continuously extracted under the action of vacuum, siphon and gravity, and does not destroy the stability of the vacuum. The vacuum only needs to remove a small amount of non-condensable gas entrained. And water vapor, so the vacuum energy consumption is small, and the washing speed is fast.

(3) Temperature and acidity during washing

Washing mainly metatitanic acid and washed free acid hydrolysis and solubility of the iron-based metal sulfates, temperature and pH directly affect the iron salt (ferrous sulfate) sulfate and other metal ions in water to generate hydroxide The pH value at the time of precipitation, therefore, controlling the temperature and acidity at the time of washing is one of the main measures for speeding up the washing speed and improving the quality of washing.

Taking ferrous sulfate as an example, its solubility in water increases with temperature, the solubility at 20 ° C is about 22g / L, the solubility at 60 ° C is the largest, up to 50g / L, while the temperature is high, water The viscosity is lower, so increasing the water temperature can significantly accelerate the washing speed and reduce the iron content in the metatitanic acid. Especially in the winter, if the washing water is not heated, the ferrous sulfate dissolved in the mother liquor of the meta-titanic acid is likely to crystallize. To make the washing operation more difficult. However, vacuum filter machines are mostly made of plastic materials (hard PVC, PP, ABS, etc.). It is not suitable for long-term operation above 60 °C. Therefore, the optimum temperature for water washing is controlled at 40~50 °C, generally not more than 60 °C. At the same time, it is necessary to choose the material for manufacturing the vacuum leaf filter. The filter of the hollow type filter machine made of two pieces of PVC board will be deformed even after long-term work at 40~50 °C. The hot water required for water washing can directly use the cooling water in the titanic acid cooling tank or the cooling water and condensed water in the titanium liquid concentrator. [next]

In the initial stage of water washing of metatitanic acid, the acidity in the material is high, the concentration gradient is large, and the washing efficiency is high. As the washing time is prolonged, the acidity is gradually reduced. When the acidity reaches a certain pH value, the metal impurity ions of iron and the like are sulfuric acid. The salt will hydrolyze to form a water-insoluble hydroxide precipitate that cannot be removed by water washing, which will ultimately affect the purity and whiteness of the finished product. For example, high iron ions in metatitanic acid will be hydrolyzed to form water-insoluble ferric hydroxide Fe(OH) ₃ or insoluble in water-soluble basic ferric sulfate Fe(OH)SO ₄ at pH >2.5. When the pH exceeds 4, the ferrous sulfate in the metatitanic acid may also be hydrolyzed to form a precipitate of ferrous hydroxide Fe(OH) ₂ . Table 1 shows the pH at the time of precipitation of a part of the hydroxide.

Table 1 pH of the metal hydroxide at the beginning of precipitation

It can be seen from Table 2 that increasing the acidity during washing can suppress the hydrolysis of impurity ions such as iron salts. If a small amount of sulfuric acid can be added to the washing water to maintain the pH at 1-2, the impurity ions such as iron salts remain soluble. It is easily removed by washing in the state. In his book "The Discovery of Titanium and Its Chemical Technology", Barksdahle introduced that metatitanic acid can be washed almost at 50-60 ° C in a 0.5% to 5% sulfuric acid or hydrochloric acid solution. Metatitanic acid. See also the data on the small test results in the laboratory (Table 2), washing 5 times in cold water, Fe ₂ O ₃ content per gram of meta-titanic acid is 5%; if washed 4 times with cold water, plus 1 time The 1% cold sulfuric acid solution is washed once, and the Fe ₂ O ₃ content can be reduced to 2%; if it is washed 4 times with cold water and then washed once with a 1% sulfuric acid solution at 50 ° C, the Fe ₂ O ₃ content is only 0.05%. French patent FR1422120 describes that adding trivalent titanium to the wash water can also achieve better iron removal effect.

Table 2 Effect of temperature and acidity on water washing

(4) Washing time and trivalent titanium content

With the extension of the washing time, the acidity decreases, washing efficiency is lowered, tests have demonstrated a leaf filter of 0.54m ² surface of the vacuum, at a degree of vacuum of 8.67x10 ⁴ Pa, cake thickness of 25 ~ 30mm, by After 0.5h of water washing, the iron content in the filter cake can be reduced by 80%. After 1h, the falling speed is very slow, and the washing curve becomes flat. It takes a long time to further reduce the iron content, which means that with the prolonged washing time, As the concentration gradient of the impurity between the filter cake and the filtrate is gradually reduced, the ion concentration is continuously lowered, the colloidal property of the metatitanic acid tends to be conspicuous, and the water washing efficiency is lowered. Secondly, in addition to the decrease in acidity, impurities such as iron in metatitanic acid may be hydrolyzed into hydroxide precipitates. Oxygen in water may also oxidize ferrous ions into high-iron ions and cannot be removed by water washing because of the residual three in metatitanic acid. The valence titanium ion is also oxidized by oxygen in water to tetravalent titanium, which does not inhibit oxidation. In addition to controlling the titanium liquid before hydrolysis, it must contain a certain amount of trivalent titanium, and sometimes it is necessary to wash it in water. Trivalent titanium ions are added to maintain the entire water washing process in a reduced state. The above problems can be solved by bleaching operations in industrial production.

In addition, the practice proves that if the water quality is not good for a long time, the water washing will not only reduce the iron content, but also the iron, calcium, magnesium , aluminum , silicon and other impurity ions in the water, due to the filtration and adsorption of the metatitanic acid filter cake. The deposition is concentrated on the surface of the filter cake, and the yellowing of the surface of the filter cake can also be observed by the naked eye, which not only fails to purify the water, but also increases the content of impurities. [next]

(5) Cleaning of filter cloth and filter cloth

In the case where the properties of the metatitanic acid and the structure and vacuum of the vacuum filter are relatively fixed, the selection of the filter cloth becomes an important factor.

The metatitanic acid is acidic, especially in the initial stage of filtration washing. Therefore, the selection of the filter cloth is first to be acid resistant. Synthetic fibers such as polyester (polyester), polypropylene (polypropylene) and caprolactam (nylon) are commonly used as filter cloth. Among them, polyester and polypropylene fibers are more. In addition, the tightness and gas permeability of the filter cloth directly affect the washing speed. Polyester long fibers (such as Shanghai's 240) are directly woven from filaments. The fabric is tight, the pores are small, and the filtration is small, but it is easy to block, filtering and washing time. Longer; short-fiber polyester filter cloth (such as Shanghai-made 758) is soft, breathable, easy to filter and wash, but more filter, easy to fall off, if the above two filter cloth, use the filter layer, The above disadvantages can be overcome. Recently, SCAPA Filter Equipment Company of the United Kingdom has developed a composite filter cloth: a polyester gauze with a coarse mesh structure on the bottom, a short fiber filter layer similar to felt in the middle, and a resin film on the outer layer (polytetrafluoroethylene). Ethylene), it is said that the filter cloth has good filtration performance, neither filtering nor blocking, acid, alkali and high temperature resistance. It is very convenient to remove the filter cake. The characteristics of commonly used synthetic fibers are shown in Table 3. Table 4 is a filter cloth commonly used in vacuum leaf filters.

Table 3 Performance of various synthetic fibers

Table 4 Filter cloth for vacuum leaf filter recommended by SCAPA

Regardless of the type of material used in industrial production, the filter cloth should be washed after a period of use, and the material in the pores and gaps of the filter cloth should be cleaned. In the past, many titanium dioxide factories in China like to soak them with alkali (NaOH, Na ₂ CO ₃ ) and then wash them. In fact, the alkali resistance of polyester is not good, it looks very soft after washing, and the strength of filter cloth is greatly reduced after alkali soaking. The porosity becomes larger and the penetration is serious. The correct way is to soak it with 2% HF for 1~2h, then wash it after soaking in severe condition. [next]

(6) filter layer

The filtration rate of the metatitanic acid filter washing is proportional to the degree of vacuum and inversely proportional to the filtration resistance. When the degree of vacuum is constant, the filtration resistance is smaller, and the filtration rate is faster. The method for reducing the filtration resistance is as follows.

a. Full flocculation Because the particles of metatitanic acid are very fine, the fine particles quickly block the pores of the filter cloth when filtered, and the filtration resistance is increased, and the filtration speed is lowered. The method of full flocculation is to add a flocculant (usually polyacrylamide) to the metatitanic acid cooling tank after hydrolysis and cooling, so that the titanic acid particles flocculate to increase, increase the gap between the particles, and reduce the filtration resistance, but this The method sometimes makes it difficult to clean the particles inside the floc; the surface of the filter cloth is sticky for a long time.

b. Local flocculation, local flocculation is to first take out a part of the metatitanic acid suspension (may be the metatitanic acid to be washed, the washed metatitanic acid, or the recovery of metatitanic acid), adding flocculant (usually polyacrylamide) A flocated metatitanic acid suspension is formed. Before the metatitanic acid is washed with water, a layer of the above-mentioned flocculated metatitanic acid is sucked on the blades of the vacuum leaf filter, and then the filtered meta-titanic acid is washed for washing, and the method can also be reduced. The effect of filtering resistance, but the surface of the filter cloth used for a long time is also sticky, and it takes up a washing pool.

c. filter aid material layer, drainage layer may be selected pulp, wood flour, rice hull ash, etc., but as well the pulp, the pulp less impurities, a low ash content, when using cardboard (tissue plate, copper paperboard) It is crushed, heated and beaten, and is mixed to a certain concentration. Before washing with metatitanic acid filter, a layer of pulp is sucked as a filter layer, and then the material to be filtered is sucked for washing. The effect of this method is more obvious than the above two methods, and there is no phenomenon that the particles in the middle of the titanate floc are not cleaned, the filter cloth is not sticky, the filter cake is convenient to discharge, and the pulp is in the subsequent calcination. Burned, used in foreign countries, but in addition to occupying a washing pool, it is necessary to add a set of equipment for preparing pulp, and the price of pulp is more expensive.

The use of the filter layer not only can reduce the filtration resistance, accelerate the filtration rate, but also greatly reduce the penetrating phenomenon of the meta-titanate fine particles during the filtration washing, although the operation is troublesome and the cost is increased a little, but the filtration washing speed is increased and the titanium is reduced. The comprehensive economic benefits of the acid permeation filtration loss and the burden of the metatitanic acid recovery process are obvious.

(7) Water quality

Water quality has a great impact on the quality of titanium dioxide products, especially in water washing and subsequent processes (including seed preparation). Because there are many impurities in ordinary tap water, metal impurity ions such as iron in water will be concentrated on the surface of metatitanic acid during long-time washing, while calcium, magnesium, silicon and the like in water will form insoluble precipitate with sulfate. It not only affects the purity and whiteness of the product, but also affects the dispersion properties of titanium dioxide. Some factories have water source for groundwater, high hardness, and more attention to impurities. Generally, tap water can be washed with tap water before washing, and ion exchange water after washing with water. At least the tap water after purification (sand filter or activated carbon filter) must be used. Try to avoid the pollution caused by water quality.

In addition, the thickness of the filter cake should be moderately controlled; the slurry in the tank should be kept uniform up and down before the suction of the suction sheet to avoid thick and thinner under the filter cake; the drying time after the suction sheet is not too long, and cracks are generated to prevent water from flowing. Short circuit; the water in the water washing pool is best to go in and out, and should be replaced regularly, and the residual material at the bottom of the pool should be removed in time; the water in the washing pool should always keep the filter cake below the liquid level during the operation, to prevent the exposed piece, otherwise Not only can it be cleaned, the ferrous iron in the filter cake in contact with the air will oxidize to ferric iron, which is an effective operation experience in actual production.

The above washing operations are all carried out under vacuum. The pressurized water washing is described in U.S. Patent No. 3,981,978, which is a pressure filtration, washing and testing at a pressure of 1 to 16 x ^{10 5} Pa in a membrane filter. Prove that a 2.5m ² membrane filter has an ability to process anatase titanium dioxide of 10~15kg/(m ² ·h), and treat rutile titanium dioxide to 8kg/(m ² ·h), 2 times of water washing. The time is only 78min, the filter cake has a solid content of 50%, and the Fe content is 25X10 ^-6 .