Production of Titanium Dioxide by Sulfuric Acid Method--Hydrolysis of Titanium Solution (III)

    Seven, various methods of hydrolysis and its pros and cons
The method of hydrolysis can be classified from the following three aspects.
1. Classification in the form of seed crystals can be divided into two types: the autogenic seed dilution method and the additional seed crystal method. In the former method, concentrated titanium solution is added to boiling water to be diluted to produce crystal nuclei, which are then hydrolyzed. The hydrolysis time of this method is long, but large equipment can be used, and the adverse effects of foreign impurities can be reduced to ensure the performance of the pigment titanium dioxide; the latter method is to add a pre-made seed crystal in the titanium liquid, and then Hydrolysis is carried out. This method is simple and easy to control, but the pigment properties of the precipitate are poor.
2. Classification by hydrolysis pressure, can be divided into two types: pressurization method and atmospheric pressure method. The pressurization method is to seal the entire hydrolysis tank and let it hydrolyze at a pressure greater than lapm1 (1 latm = 101,325 Pa). This method can control the higher hydrolysis temperature, so the hydrolysis rate is fast, the time is short, and the titanium formed by hydrolysis is formed. The acid particle size is fine, the product has good color-eliminating power, no dilution water is added for hydrolysis, and the concentration of waste acid is high, which is convenient for recycling. However, the obtained metatitanic acid particles are not uniform, the consistency is poor, the filtration and washing are difficult, the filtration cycle is long, the yield is not high, and the washing is not easy, and the whiteness of the product is poor. The equipment is more complicated, it is easy to damage the air leakage, and the maintenance is difficult. The hydrolysis pot cannot be made too large, generally only has a volume of 2-5 m ³ , which also affects the increase of the output. At the same time, the hydrolysis rate is not high. In the normal pressure method, the feed port is opened during thermal hydrolysis, and it is hydrolyzed at 1 atm. Atmospheric hydrolysis has the following four major advantages.
1 atmospheric pressure hydrolysis tank can be as large as tens of cubic meters, or even more than 100 cubic meters. The volume of imported technology in China is 120m ³ . One pot is equivalent to about 20 pots of pressurized hydrolysis; the titanium solution prepared by atmospheric pressure hydrolysis requires relatively high iron and titanium, and the titanium liquid is allowed to contain higher ferrous sulfate, which can accelerate freezing; the metatitanic acid particles formed by hydrolysis are uniform It is smooth and maintains a large porosity in the filter cake. The washing water easily passes the filter layer to carry away the impurities, which can accelerate the washing and calcining, thereby increasing the yield.
2 The metabetic acid particles produced by atmospheric pressure hydrolysis are uniform and smooth, have good consistency, narrow particle size distribution and strong anti-interference ability, which greatly improve the whiteness, color reduction and dispersibility of the product. Improve the quality of titanium dioxide.
3 atmospheric pressure hydrolyzed titanium liquid requires low F value, can save acid hydrolysis of sulfuric acid; titanium liquid iron-titanium ratio increased by 0.08, freezing temperature increased by about 10 °C, can shorten the freezing time, save energy, the frozen liquid can use water instead of salt water, It can reduce the cost of freezing; increase the amount of hydrolysis, which can save coal ; shorten the washing time, save water and electricity; accelerate calcination, save diesel (or gas); reduce equipment loss, speed up freezing, hydrolysis, washing, save power costs In particular, it can save expensive enamel pressure cooker; if it adopts self-produced seed crystal, it can shorten the hydrolysis time and washing time and increase the yield and quality. It can also improve the hydrolysis rate and recovery rate, reduce the seeding process and reduce the system. Seed crystals, equipment, caustic soda, steam for heating, and freezing liquid for cooling. In short, the production cost can be greatly reduced, and if the output and quality can be improved, the economic benefits will be significantly improved.
4The atmospheric pressure hydrolysis equipment is simple, no sealing, easy to operate and maintain, less work, can reduce labor intensity, and at the same time its safety is good, it can completely avoid accidents.
It is precisely because the atmospheric pressure hydrolysis has more advantages than the hydrolysis method, so that in recent years, most manufacturers have eliminated the hydrolysis by pressurization and switched to the hydrolysis by atmospheric pressure.
3. Classification by heating, can be divided into direct steam heating method, indirect steam heating method and mixed heating method. The direct steam heating method is to directly transfer the steam of a certain pressure to the inside of the titanium liquid for heating and hydrolysis. The method has the advantages of simple equipment, high heat utilization rate, fast hydrolysis rate and high hydrolysis rate. However, the prepared titanium dioxide has poor pigment properties. This is because the steam condensate before the hydrolysis reaction reduces the concentration of the titanium liquid, and at the outlet of the steam heating tube, the titanium liquid generates an irregular crystallization center due to high temperature and strong impact, so that the performance of the product pigment is degraded. Therefore, it can only be used for the hydrolysis of non-pigmented titanium dioxide production, but it has been improved in recent years. It is also good for the hydrolysis of titanium liquid used in pigment-grade titanium dioxide. The indirect steam heating method uses a serpentine tube or a jacket to guide people. Steam heat transfer for hydrolysis, this method is better, heating is easy to control, so it is widely used in industrial production; mixed heating method is both direct steam heating method and indirect steam heating method, which are separately controlled according to the requirements of different stages of hydrolysis. . For example, some manufacturers have atmospheric pressure hydrolysis, and in the early stage, the titanium liquid is heated by indirect steam, and the latter is heated by direct steam heating. [next]
8. Quality requirements for hydrolyzed titanium liquid when producing anatase pigment titanium dioxide
The composition and quality of the hydrolyzed titanium solution have a great influence on the purity of the metatitanic acid, the structure of the microcrystals, the size of the colloidal particles, and the quality of the product. Therefore, the titanium liquid must meet the requirements of the specified index (see Table 1).

Table 1 Quality requirements for hydrolyzed titanium solution

    9. Effect of stability and clarity of titanium solution on hydrolysis and product quality
During the storage process, the titanium liquid tends to precipitate and gradually precipitate the colloid. In the poor stability of the titanium liquid, some colloidal substances have been produced before the hydrolysis, and these colloids are present in the titanium liquid. It becomes a bad crystallization center, which makes the obtained metatitanic acid particles uneven, and easily adsorbs more impurities, which not only prolongs the time of washing the metatitanic acid, but also easily sinters the titanium dioxide particles obtained during calcination, so that the final Titanium dioxide has a significant decrease in whiteness, achromatic power and dispersibility.
The clarity of the titanium solution is not good, indicating the presence of colloidal substances or solid suspended impurities in the titanium solution. The presence of these substances will be brought to the hydrolysis to become a poor crystalline center, which will eventually affect the whiteness, achromatic power and dispersibility of the product.
X. Effect of the concentration of titanium solution on hydrolysis and product quality
The concentration of titanium liquid is low (such as Ti0 ₂ concentration < 150g / L), and the metatitanic acid obtained by hydrolysis is converted into coarsely dispersed titanium dioxide after calcination, and the pigment performance is extremely poor. The reason is that the concentration of the non-pigmented titanium liquid is low, and the produced product cannot be made of pigment titanium dioxide. As can be seen from Fig. 1, as the concentration of Ti0 ₂ is increased, the decoloring power is also improved. However, increasing the concentration of Ti0 ₂ slows down the rate of hydrolysis and prolongs the time to start hydrolysis (see Figure 2), and the rate of hydrolysis decreases (see Figure 3). When the Ti0 ₂ concentration is >200g/L, the effect of the increase of the concentration on the color reduction of the product is not obvious, and the washing rate of the metatitanic acid is also slowed down. Therefore, when selecting the concentration of titanium liquid, it is necessary to meet the quality requirements of the product, and also to consider the improvement of the hydrolysis rate and the economic rationality of the concentration of titanium liquid. Generally, the production of non-pigment titanium white powder, due to the requirement of non-pigmenting properties of the product, is mainly carried out by hydrolysis with a low concentration titanium solution to obtain a higher hydrolysis rate and easy to wash the metatitanic acid. In the production of finely divided titanium dioxide for pigments, it is necessary to increase the concentration of the titanium liquid so that the product produced meets the requirements for pigment properties. [next]

The rate of hydrolysis and the pigment properties of the final product, in addition to the initial concentration and F value of the titanium solution, are also affected by the final acidity at the end of the hydrolysis. For pressurized hydrolysis, the hydrolysis rate is significantly reduced when the concentration of titanium solution is higher than 230 g/L due to the absence of water dilution at the later stage of hydrolysis. For the atmospheric pressure hydrolysis diluted with water at the later stage of hydrolysis, the concentration of titanium solution is proportional to the hydrolysis rate. Influential, but not as significant as the F value. [next]
+1. Effect of iron-titanium ratio on hydrolysis and product quality
The iron-titanium ratio for controlling titanium liquid has the following three aspects.
1. Ensure that the hydrolyzate does not precipitate ferrous sulfate crystal during cooling and storage. Because the concentration of free acid in the hydrolyzed mother liquor is high, the solubility of ferrous sulfate will decrease, so that crystals are easily precipitated, and once precipitated crystals are sandwiched with metatitanic acid. Medium will greatly extend the washing time, which is especially important in winter.
2. The concentration of ferrous sulfate is high, and the viscosity and density of the mother liquor are increased, so that the washing is slowed down.
3. The iron-titanium ratio is stable, so that the total ion concentration of hydrolysis is stable, which has certain significance for maintaining the stability of hydrolysis rate and the particle size of metatitanic acid.
When hydrolyzed, the concentration of ferrous sulfate in the titanium solution has a certain influence on the hydrolysis reaction. The presence of ferrous sulfate increases the relative density and viscosity of the solution and increases the total ion concentration. When the hydrolysis, the Ti0 ₂ content in the solution will gradually decrease and its concentration will continue to decrease, while the ferrous sulfate will not hydrolyze. It remains in the mother liquor, and the mother liquor continues to maintain a certain ion concentration, which is similar to maintaining A certain concentration of Ti0 _{2 is} present. Therefore, increasing the concentration of ferrous sulfate, the rate of thermal hydrolysis will be slower, the rate of hydrolysis will decrease, and the particle size of the obtained metatitanic acid particles will be finer. However, there are reports in foreign countries that high FeS0 ₄ content in titanium liquid will increase the color reduction of the product (see Table 2). However, an excessively low iron-titanium ratio, while facilitating the washing of metatitanic acid, increases the burden of freezing of the titanium liquid and makes the particle size of the obtained metatitanic acid particles coarse. Therefore, the ratio between the concentration of ferrous sulfate ions in the titanium solution and the concentration of Ti0 ₂ , that is, the ratio of iron to iron (Fe/Ti0 ₂ ) must be kept within an appropriate range.
12. Effect of the F value of titanium solution on hydrolysis and product quality t
The reaction formula of the thermal hydrolysis reaction of titanium liquid is as follows:

In the same concentration of titanium solution, the larger the F value, the greater the acidity. It can be seen from the above hydrolysis reaction formula that the higher the acidity, the higher the concentration of the product sulfuric acid, and the reaction is reversible, so that not only the hydrolysis reaction is suppressed, but also the hydrolysis rate is slowed, and the produced metatitanic acid is dissolved to make the particle size. It is thinned to lower the hydrolysis rate.
The titanium oxysulfate in the titanium solution is mostly in the form of a short-chain unbranched linear polymer (activated titanium). If the acidity is high, a small portion is still in a molecular state where hydrolysis does not occur. As the hydrolysis progresses, the acidity of the mother liquor becomes higher and higher, and the titanium of this part of the molecular solution does not transform into active titanium, which is the essence of the high F value affecting the hydrolysis rate.
As is clear from Fig. 4 and the hydrolysis reaction formula, the lower the F value of the titanium liquid, the smaller the acidity, and the more favorable the hydrolysis reaction proceeds to the positive reaction direction, the more difficult it is to suppress the progress of hydrolysis, and the earlier the reaction starts. However, the F value of the titanium liquid is too low, its stability is greatly reduced, and there is a danger of early hydrolysis. When the titanium liquid is hydrolyzed, not only the size of the metatitanic acid particles is coarse and irregular, but also the pigment properties are improved. Poor and uncontrollable, so it must be avoided. The rate of hydrolysis is gradually slowed as the value of F increases. When F>2.1, the hydrolysis start time is prolonged and the hydrolysis rate is also lowered. [next]

When F=1.6, the color reduction is the largest. When F=1.4-1.8, the oil absorption is the best. When F<1.3, it will be hydrolyzed without adding seed crystals. The “milk” accident often occurs. For the production of non-pigment titanium dioxide, the F value is generally controlled between 1. 7-l. 9, low F value is beneficial to reduce the consumption of sulfuric acid, increase the hydrolysis rate and accelerate the washing of meta-titanic acid; With the production of titanium dioxide, the F value should be correspondingly increased. Generally, the F value of the hydrolysis control is between 1.8 and 2.1; the atmospheric pressure method controls the F value between 1.75 and 1.95.
Thirteen, the effect of stirring and heating methods on hydrolysis
Good agitation can improve the heat transfer effect and make the particles of metatitanic acid evenly dispersed. If the stirring is too fast, the particle size of the metatitanic acid particles becomes fine, and the particle flocculation is affected, which affects the filtration and washing speed. Generally, it is heated by direct steam at a speed of 10~30r/min. Some manufacturers use variable speed motors, which are faster during seeding and slower during hydrolysis.
Direct steam heating, simple equipment, fast heating rate, easy control, less steam consumption, saving energy. However, the condensed water in the steam will reduce the concentration of the titanium liquid during the hydrolysis, and it is disadvantageous to the additional seed hydrolysis method which requires strict hydrolysis initial concentration, and at the outlet of the steam heating tube, the titanium alloy will be caused by the high temperature and strong gas impact. acid irregular particles; slow indirect steam heating, more than the amount of steam, the heat utilization is low, high energy consumption, high manufacturing and maintenance costs of the coil, the coil copper corrosion may contaminate the product, but it does not dilute the concentration of the solution of titanium . Therefore, by the addition of seed crystal hydrolysis, most manufacturers use indirect heating; mixed heating is added to the seed crystal, first indirectly heated to the critical point of hydrolysis, and then changed to direct steam heating. The same high quality hydrolysate can be obtained by all three heating methods.
14. If the ilmenite contains high ferric iron and concentrated waste acid, all of them are returned for use, and the control of F value
If the ilmenite contains high ferric iron, the control of the F value should be relaxed to 1.8-2.2 because the ferric iron has a high content of ferric iron, and the heat released during acid hydrolysis is large, and at the same time, in order to increase the yield, shorten the ripening and cooling. At the time, the operating temperature is high, and if there is no high F value, it is difficult to inhibit the early hydrolysis of the titanium liquid. In addition, in order to improve the recovery rate, the unhydrolyzed titanium in the concentrated waste acid is completely recovered, and the concentrated waste acid is all returned for acid hydrolysis and leaching. Since the free acid of the waste acid is relatively large, the Ti0 ₂ content is relatively small. The F value of the spent acid reaches about 30, so that the high-value waste acid is added to the leached titanium solution, and the F value of the titanium liquid is definitely too large. The practice has proved that the use of pressurized hydrolysis, the F value is controlled between 1. 8-2. 2, will not affect production.
Fifteen, in the production of pressurized hydrolyzed titanium dioxide, control whether the F value of titanium liquid is too large will bring disadvantages
Conventionally, a large F value will bring six major drawbacks: 1 increase in acid consumption; 2 delay in the start of hydrolysis, so that the entire hydrolysis time is prolonged; 3 hydrolysis rate decreases, from the chemical equilibrium of the hydrolysis reaction, TISO0 ₄ + 2H ₂ 0 ← →H ₂ Ti0 ₃ +H ₂ S0 ₄ , the F value of the titanium liquid is large, indicating that the acidity is increased, and the hydrolysis reaction is inhibited; 4 because the acidity is large, the metatitanic acid particles are difficult to grow and the particle size is fine, and it is difficult to wash; The fine particle size of the metatitanic acid particles causes a lot of water washing and filtering, and the recovery rate is lowered; 6 the meta-titanic acid particles are too fine, and the sintering is easy to be sintered, which affects the quality of the product. [next]
However, the F value is raised to 2.2, and the above-mentioned drawbacks are not present, and there is no adverse effect on the production. The high F value is increased by the waste acid, and the acid consumption is not increased; the hydrolysis rate is averaged at 96.2% per month. , the sedimentation rate can be controlled between 435-455, the particle size of the metatitanic acid particles is not fine, so the water washing is not difficult, the filtration is not much, and the sintering phenomenon has not appeared. In fact, the F value is too high to 2.2, the titanium liquid is still easily hydrolyzed titanyl sulfate, and has not reached the hardly hydrolyzed titanium sulfate above 2.45. The titanium liquid has the acidity of the free acid, and can inhibit the titanium liquid at normal temperature. Early hydrolysis stabilizes the titanium liquid, but does not inhibit the thermal hydrolysis of the titanium liquid in the boiling state, nor is it sufficient to prevent the growth of the metatitanic acid particles.
Sixteen, pressurized hydrolysis, if there is a high settlement rate, the size of the titanic acid particles is fine
If the hydrolysis has a high sedimentation rate and the particle size of the metatitanic acid particles is fine, it will cause easy penetrating and difficult washing of the metatitanic acid, and the following measures can be taken to remedy:
1 after the hydrolysis, add a little aminomethyl modified polyacrylamide;
2 add a little more seed crystals during hydrolysis;
3 Add more sodium hydroxide to the seed crystal, and properly reduce the effective acid of the seed crystal;
4 The concentration of hydrolyzed titanium solution is low;
5 reducing the iron-to-titanium ratio of the titanium liquid;
6 Prolong the hydrolysis time;
7 Increase the pressure and increase the hydrolysis temperature appropriately.
All of the above measures are beneficial to the coarsening of the particle size of the metatitanic acid particles and the decrease of the sedimentation rate, thereby remedying the problem of fine particle size of the hydrolyzed metatitanic acid particles, filtration washing, easy filtration and difficult washing. Of course, if the F value is too high and adversely affects, the lye may be appropriately neutralized to neutralize excess acid, thereby lowering the F value.
17. When pressurized hydrolysis is used, controlling the F value of the titanium liquid to increase the output can increase the yield.
The sedimentation quality of the titanium liquid often deteriorates as the F value decreases. Because the F value decreases, the stability of the titanium liquid also decreases, and the decrease in stability indicates an increase in colloidal impurities, which increases the difficulty of sedimentation and affects the increase in yield. The F value of the titanium liquid is too large, indicating that the free acid of the titanium liquid is increased, so that the stability of the titanium liquid is improved, which is advantageous for inhibiting the early hydrolysis of the titanium liquid. It can suppress the early hydrolysis of the titanium liquid, and the colloidal impurities of the titanium liquid are small, and it is easy to settle and remove the slag. The sedimentation curves of different F-value titanium liquids are shown in Fig. 5. As can be seen from the figure, the F value increases, the sedimentation effect becomes better, the sedimentation and slag removal is easy, and the pressure filtration is easy, and the prepared titanate is also washed. It is easy to make the whole production smooth from start to finish, which ensures the increase of titanium dioxide production.