Production of Titanium Dioxide by Sulfuric Acid Method--Calcium Sulfate Crystallization and Crystal Separation

    First, the relationship between the solubility of ferrous sulfate and temperature
The solubility of ferrous sulfate decreases greatly with decreasing temperature (see Table 1).

Table 1 Solubility of ferrous sulfate in titanium at different temperatures

Temperature / °C Solubility ( FeSO 4 ) /(g/L) Temperature / °C Solubility ( FeSO 4 ) /(g/L)
30 240 5 95
20 190 0 79
15 130 -2 59
10 117 -6 38

    Second, the crystallization of ferrous sulfate
After the ilmenite powder and the sulfuric acid are acidified, the titanium liquid obtained by leaching has a relatively high iron content, but at this time, the temperature of the titanium liquid is relatively high, so the solubility of the ferrous sulfate is relatively large, and the actual The amount of dissolution has not reached its solubility and this solution is not saturated. If the temperature of the titanium liquid drops, the solubility of ferrous sulfate also decreases, and when the solubility drops to the same amount as the actual amount of dissolution, the solution becomes a saturated solution. If the temperature is continued to decrease, the solubility continues to decrease, and its solubility is less than the actual dissolved amount of ferrous sulfate, which becomes a supersaturated solution. The supersaturated solution is unstable, and as long as there is a trace amount of solid or the surface of the vessel is rough, a crystal center is formed, and the portion of the ferrous sulfate that exceeds saturation is crystallized.
Third, the iron-titanium ratio and its control
The ratio of the total iron content in the titanium liquid to the total Ti0 2 content is referred to as the iron-to-titanium ratio. Its formula is as follows:

The ratio of iron to titanium has a certain influence on the particle size and structure of the metatitanic acid of the hydrolyzate. Therefore, in the production of titanium dioxide, especially in the production of titanium dioxide powder, the iron-titanium ratio must be controlled within a certain range. [next]
Fourth, the reasons for the removal of ferrous sulfate
The titanium ore is subjected to acid hydrolysis, followed by water leaching and reduction with iron filings or iron powder, and the obtained titanium liquid contains a large amount of ferrous sulfate. The purpose of freeze-crystallization of the titanium liquid is mainly to cause ferrous sulfate to be crystallized, and then to filter out most of the ferrous sulfate from the titanium liquid. The purpose of removing ferrous sulfate from titanium liquid is mainly the following aspects.
1 is advantageous for water washing of metatitanic acid obtained after post-hydrolysis. When the amount of iron in the titanium liquid is reduced, the washing time of the metatitanic acid can be shortened, thereby improving the washing efficiency and increasing the yield.
2 The ratio of iron to titanium can be adjusted so that the ratio of ferrous sulfate to Ti0 2 meets the requirements of the hydrolysis process.
3 Removal of ferrous sulfate can take away a large amount of crystal water at the same time, so that the total titanium concentration of the titanium liquid is increased from 120-140 g/L to 150-180 g/L, thereby reducing the burden of concentration in the later stage.
4 Recycled ferrous sulfate is a chemical raw material that allows the material to be fully utilized and sold, and the resulting income can offset the cost of the main product.
5 After the acid hydrolysis, the titanium solution obtained by leaching and reduction has a higher concentration of ferrous sulfate. When stored and transported, it is slightly cooled, that is, ferrous sulfate crystals are precipitated, which may affect the utilization of the storage tank or block the pump body and the pipeline. . Therefore, it is necessary to remove ferrous sulfate in time so that subsequent storage and infusion will not cause trouble.
5. The nature of ferrous sulfate and the reasons for its whitening and long-term yellowing
The ferrous sulfate crystallized from the aqueous solution or the dilute sulfuric acid solution is a light green crystal having a shape, and contains 7 mol of crystal water per 1 mol of ferrous sulfate, and the molecular formula is FeSO 4 ·7H 2 O, which is commonly called green 矾. . Its crystal form is monoclinic, melting point 64 ° C, relative density is 1. 899; at 90 ° C will lose 1 mol of crystal water, become hexahydrate FeS0 4 · 6H 2 O; at 300 ° C and then remove 5 mol of water It becomes a monohydrate FeS0 4 ·H 2 O; gradually loses the last 1 mol of crystal water above 300 ° C; completely removes crystal water above 700 ° C, and decomposes into sulfur dioxide, sulfur trioxide and iron oxide red. The ferrous sulfate of monohydrate and anhydrate is a white powder, which changes to green again upon contact with water. When exposed to air, the surface layer is oxidized by oxygen in the air to high-sulfur sulfate or basic ferric sulfate under the action of free acid. Due to the weak acidity, the high-sulfur sulfate is easily hydrolyzed to form brown ferrous hydroxide. Its oxidative hydrolysis reaction formula is as follows:

Sixth, the method of crystallization, the principle of evaporation crystallization and its adaptation range
Crystallization can be divided into three types: evaporation crystallization, freeze crystallization, and vacuum crystallization. Evaporation crystallization is a process in which the solvent in the solution is continuously evaporated by heating, the solution is continuously concentrated, and finally supersaturated to precipitate crystals. This method is suitable for certain salts, such as table salt, which have little change in solubility at different temperatures, and is not suitable for the crystallization of green mites. The latter two methods are applicable to green crystallization. [next]
Seven, the principle and method of frozen crystallization
The principle of freeze crystallization is to reduce the temperature of the solution by heat exchange, and finally to achieve supersaturation to precipitate crystals. Commonly used methods are natural cooling crystallization and mechanical freezing crystallization.
1. Natural cooling crystallization This method of crystallization is relatively primitive and simple, that is, the titanium liquid is stored, and the temperature difference between the temperature of the titanium liquid and the room temperature is used to naturally cool the ferrous sulfate. Since the temperature is slow and it is still crystallized, a large crystal is obtained.
The advantage of natural cooling crystallization is that the structure of the device is simple, generally ceramic cylinders, plastic tanks, etc. can be used, and no energy is consumed at the same time. The disadvantage is that a large amount of cooling equipment is required, and the floor space is large; the final cooling temperature is determined by the temperature, cannot be adjusted, the crystallization process is slow, the summer temperature is high, the crystallization is slower, and the obtained crystals are less; the iron-titanium ratio is difficult to control; the crystal recovery rate is Low; high operating intensity, not suitable for industrial production.
2. Mechanical Freeze Crystallization Mechanical freeze crystallization is the use of a low temperature chilled liquid to exchange heat with a titanium liquid through a heat exchanger (freezer coil) to rapidly cool the titanium liquid. Therefore, a coil and a stirrer with a rotational speed of 50-70 r/min are installed in the freezing tank. The specific operation is as follows: Firstly, the titanium liquid is injected into the freezing crystallization tank. In order to save energy, the liquid surface should be completely flooded, and the agitator should be driven to send the chlorination obtained by the ammonia compressor (refrigerator) to the coil. Low-temperature chilled brine of calcium or sodium chloride, tap water can be used for atmospheric hydrolysis. The brine temperature at the beginning should not be too low, so as to quickly crystallize a thick crystal hard shell outside the coil and affect the heat exchange. The brine temperature is then gradually lowered as the temperature of the titanium liquid decreases until the desired final freezing temperature is reached. When the pigment titanium dioxide is produced by the pressurization method, the ratio of the Fe content to the TiO2 content is required to be 0.20-0.25, and the freezing end temperature is controlled at 4-5 ° C; the atmospheric pressure method requires the iron-titanium ratio to be 0.28-0.33, and For the production of enamel or electrode with titanium dioxide, there is no strict requirement for iron-titanium ratio, and the freezing temperature can be controlled at 10-15 °C.
The advantage of this method is that it solves various shortcomings of the natural cooling crystallization method. However, the disadvantage is that the equipment is more complicated and consumes energy. Most of the smaller titanium dioxide plants in China use this crystallization method.
In order to save energy, some manufacturers have made the following improvements to freeze crystallization.
1 Tap the tap water in the freezer coil, freeze it to a certain temperature, and switch to low temperature chilled brine.
2 Install a double-row freezer coil in the frozen crystallization tank, respectively pass the tap water and the frozen brine, first pass the tap water, then pass the frozen brine.
3 Let the titanium liquid pass through a three-stage stepped freezing crystallization tank and freeze it in three steps. The first tank has the highest position, is cooled by tap water, and then flows into the second tank. The cold titanium liquid which has been frozen and filtered out of the green pot is used, and finally the third tank which is frozen by the frozen brine is used.
Eight, the operation of vacuum crystallization and its advantages and disadvantages
In the production process of large scale titanium dioxide, vacuum crystallization is often used to remove ferrous sulfate. The method is to clarify the titanium liquid pump with a stirring and good sealing performance in the bottom of the crystal tank, the volume of the titanium liquid is between 35-45 m 3 . A vacuum ring vacuum pump is used to evacuate the crystallization tank, causing the temperature of the titanium liquid to drop. When the solid nucleus begins to appear in the titanium liquid (ie, the temperature is 36-37 ° C), the steam ejector is turned on to further enhance the vacuum in the crystallization tank, and at the same time increase the flow rate of the cooling water to cool the secondary steam and the jet steam to Condensed water, non-condensable gas is discharged by a vacuum pump. In the first crystallization stage, only the water ring pump is opened, generally 40-60 min. In the second crystallization stage, the ejector is added, and it usually takes 2 h to reach the required crystallization temperature (15-20 ° C). Then slowly close the steam ejector and adjust the water ring pump valve to the venting position to return the crystallization tank to normal pressure. The suspension was discharged from the bottom valve of the crystallization tank, and separated by a centrifuge to remove ferrous sulfate heptahydrate. Wherein the vapor pressure P ≥0.7MPa for vapor injector, the injector steam flow is typically 1. 5-1.8 tons / h, circulating cooling water flow rate of 60-250m 3 / h. [next]
The process of vacuum crystallization is the same as the process of mechanical freeze crystallization, in which the titanium liquid is frozen until the ferrous sulfate is supersaturated to precipitate crystals. However, there is no heat exchange process in vacuum crystallization, and the heat of the titanium liquid is removed by evaporation of a small portion of the solution. This is because under reduced pressure (vacuum), the boiling point of the titanium liquid is lowered to form partial boiling, and the solvent is evaporated. Due to the latent heat of vaporization, a large amount of heat needs to be absorbed, and the titanium liquid is rapidly frozen to crystallize the green cerium crystal.
Compared to mechanical freeze crystallization, vacuum crystallization has the following advantages:
1 evaporation and cooling simultaneously, the crystallization efficiency is high;
2 equipment selection is simple, less maintenance, long life;
3 The accessory equipment is simple and the production capacity is large;
4 solution adiabatic evaporative cooling, does not require the heat transfer area required for heat exchange, equipment investment costs are low;
5 low comprehensive energy consumption and low production costs;
6 The equipment has a small footprint and the overall average cost is low;
7 After the crystallization, the concentration of the titanium liquid is high, and the temperature at the end of the crystallization is high, which can alleviate the workload after concentration.
Disadvantages:
1 operation control is more complicated;
2 consumes more steam and cooling water;
3 The obtained green germanium crystal has a small particle size, and this factor should be taken into consideration when separating the centrifuge.
9. Reasons for using calcium chloride or sodium chloride as a freezing medium
The function of the freezing medium is to continuously transfer the "cold" of the freezing mechanism to the titanium liquid in the crystallization tank. Since water forms ice at 0 ° C, it cannot be recycled, while the solution of calcium chloride and sodium chloride has a lower freezing point. The applied concentration should be determined by the temperature of the desired brine. The lower the temperature of the brine, the thicker the brine. For example, if the minimum temperature of the brine is required to be -10 ° C, the concentration of the calcium chloride solution should be not less than 14.7%; if sodium chloride is used, the concentration should be not less than 14%.
X. Separation of ferrous sulfate crystals and washing after separation
The ferrous sulfate crystals are separated from the titanium liquid by filtration. Since the ferrous sulfate crystal has some residual titanium liquid between the voids, if it is not washed and recovered, the titanium liquid loss affects the recovery rate and reduces the quality of the ferrous sulfate, so it should be washed with water to recover.
The ferrous sulfate crystal itself is soluble in water, so when it is rinsed with water, some crystals are inevitably re-dissolved. In order to minimize this re-dissolution and to rinse the crystal as much as possible, the flushing can be carried out twice. The first flushing uses the second flush of water from the previous batch of crystals, and the washing solution can not affect the titanium liquid index. In the case of human-filtered titanium liquid, or for leaching of acid-decomposed solid phase; the second flush with tap water, the washing liquid is retained for the first flush of the next batch of crystals, the tap water used most Use ice water to reduce redeposition of crystals. [next]
XI. Construction of vacuum suction filter tank and suction filtration operation
The vacuum suction filter tank is a rectangular groove, and the groove body and the groove bottom are made of a hard polyvinyl chloride plate, and are reinforced by angle steel. The bottom is in the shape of a bottomed cylinder with a false bottom in the middle of the groove. The false bottom is a porous plastic plate. The plate is covered with a filter cloth, generally acid-resistant felt or polyester flannel. A porous, slightly thin laminated plastic panel is placed over the filter cloth. During operation, a vacuum is applied to the lower half of the tank, the slurry is placed in the upper half, the titanium liquid permeates the filter cloth into the lower portion, and the ferrous sulfate crystal remains on the filter cloth. After the suction-filtered titanium liquid is sucked dry, the water of the second batch of the green sputum is washed with a high-pressure water gun, and the mixture is washed with tap water and then rinsed with tap water. After suction filtration, after drying, the ferrous sulfate crystals in the tank were manually removed. This filtering method is simple in equipment, easy to manufacture, easy to operate, and has little investment and is practical. However, its labor intensity is relatively large. Most of the smaller titanium dioxide plants currently use this filtration method.
Twelve, the structure of the centrifuge and its filtration principle
The centrifuge is a filter that separates solid and liquid by utilizing the centrifugal force generated when rotating at a high speed.
A titanium liquid containing green enamel crystals is added to the drum at full speed. The drum is covered with an acid-resistant filter cloth bag, and when the feeding amount reaches the loading limit, the feeding is stopped immediately. When the drum rotates at a high speed, the liquid liquid passes through the filter cloth under the action of the centrifugal force, and the drum is discharged to form a filtrate, which flows out through the lower outlet. The green enamel crystals are left in the drum. After drying, rinse with a small amount of water, then dry and then stop, manually unloading from the top.
The centrifuge has a high filtration speed and high production efficiency, but still needs manual discharge, and still has a certain labor intensity. Due to the large size of the green enamel crystal particles, it can also be filtered by a horizontal automatic discharge centrifuge. The program control, automatic feeding, rinsing, drying and unloading can greatly reduce the labor intensity.
A factory introduced a 2050mm*1000mm horizontal centrifuge with a rotating drum. The single processing capacity can meet the production requirements of 10,000 tons/year titanium dioxide. In addition, a factory introduced a disc filter to separate green mites.
Thirteen, the quality index control of the freeze crystallization process 1 brine temperature high temperature frozen brine 10-20 ° C, low temperature frozen brine <-10 ° C.
2 frozen titanium liquid temperature pressure hydrolysis method pigment grade 4-5 ° C, kept for 20 min; atmospheric pressure hydrolysis method pigment grade and non-pigment grade 10-15 ° C until the standard is reached.
2%。 The ferrous sulfate content of FeS0 4 · 7H 2 0 content ≥ 90%; residual titanium content ≤ 0.2%.
4 The quality index of titanium liquid after freezing is shown in Table 2.

Table 2 Quality indicators of titanium liquid after freezing

Indicator name Pigment level Non-pigment level
Pressure hydrolysis method Atmospheric hydrolysis
Total TiO 2 content / ( g / L ) 150~180 150~180 120~130
F value 1.8~2.1 1.75~1.9 1.7~1.9
Trivalent titanium content / ( g / L ) 2.0~5.0 1.0~3.0 1.5~3
stability ≥ 350 ≥ 300 ≥ 300
Iron-titanium ratio 0.2~0.25 0.28~0.33 -

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