Production of Titanium Dioxide by Sulfuric Acid Method--Hydrolysis of Titanium Solution (4)

    18. When using hydrolysis by hydrolysis, controlling the F value of titanium liquid can increase the product quality.
Some titanium dioxide plants, when washing with metatitanic acid, add a small amount of sulfuric acid in the later stage to keep the washing water at pH ≥1.5, which is good for preventing iron ion hydrolysis, improving water washing efficiency and improving product whiteness. It has been shown that the water is washed 5 times with the pouring method, and the dry-base titanic acid still contains Fe ₂ 0 ₃ 5%. When washed with water containing 1% sulfuric acid for 4 times, the Fe ₂ 0 ₃ content is reduced to 2%, and acidity is observed. Water can speed up washing. Controlling the F value of the titanium liquid to a large value will increase the acidity of the titanium liquid, and it will play the role of adding sulfuric acid without adding an acid, and achieve the same effect as an external acid, so that the water of the titanic acid is washed. The speed is increased, which increases the whiteness of the finished product.
If the F value of the titanium liquid is low, the stability of the titanium liquid is poor, and the colloidal impurities are many, not only the sedimentation is difficult, but some colloidal impurities have become the center of crystallization before hydrolysis, and these irregular crystallization centers play an adverse role in hydrolysis. The obtained metatitanic acid particles are not uniform, and it is easy to adsorb a large amount of impurities, and not only the water washing time of the metatitanic acid is prolonged, but also the particles are easily sintered during the satin burning, so that the whiteness, the color reduction power and the dispersion property of the final product are obtained. decline. The F value is controlled too much, and the stability of the titanium liquid is improved, so that the early hydrolysis phenomenon is not easy to occur, the colloid is less in the titanium liquid, and the irregular crystal center is formed at the time of hydrolysis, and the uneven titanic acid particles are produced. Less, less impurities are adsorbed, and even due to the F value, the acidity is large, and some non-titanium impurities in the metatitanic acid can be dissolved, and the particle size of the metatitanic acid particles is not fine. This not only makes water washing easy, but also does not cause particle sintering during calcination. The performance of the prepared titanium dioxide is better. This will be beneficial to the improvement of the quality of titanium dioxide. The relevant data in Table 2 can also show that the F value of the titanium liquid is controlled to be too large, the effective acid of the titanium liquid is high, and the decoloring power of the product with high free acid is greatly improved.

    19. Controlling the large F value of titanium solution during pressure hydrolysis can improve the recovery rate
Since the F value of the hydrolysis method can be too large, the concentrated acid can be returned and utilized. Since the concentrated acid is used for acid hydrolysis and leaching, the concentrated acid can replace a part of sulfuric acid, so that less acid can be added during acid hydrolysis, thereby saving sulfuric acid. The concentrated acid also contains 3%-4% of unhydrolyzed titanium, and all the concentrated acid is returned. This 3%-4% of the titanium is transferred to the next period of titanium to make the next batch of titanium. The total TiO ₂ concentration of the liquid is increased so that the recovery rate is over 80%. [next]
Twenty, the difference between the atmospheric pressure method hydrolysis and the pressurized hydrolysis of titanium liquid to control the F value
The F value of the hydrolyzed titanium solution controlled by atmospheric pressure is completely different from the pressurization method. Generally, the F value control of the hydrolyzed titanium solution can be relaxed to 2.2, and the control of the F value of the titanium solution at normal pressure is more than 1.95, and the hydrolysis is carried out. Unfortunately, the resulting titanium dioxide has a color reduction of less than 100. Between the 1.75-1. 95, the F value of the titanium liquid must be controlled. Generally, the concentration of the titanium solution in the atmospheric pressure method is 220-230 g/L, and the F value can be controlled between 1.85-1.95, and the concentration of the titanium solution in the atmospheric hydrolysis of the autogenous seed crystal is 250-260 g. / L, then its F value should be controlled between 1. 75-1. The control of the F value is not only related to the concentration, but also to the iron-titanium ratio and the trivalent titanium content (see Table 1). In order to control these indicators of titanium liquid, in order to ensure the quality of hydrolysis, modern large titanium dioxide factory, before the hydrolysis, added a titanium liquid preparation process, the titanium liquid is formulated to meet the requirements of various indicators before being used for hydrolysis. Since the F value of the hydrolysis control by the atmospheric pressure method is low, that is, the acidity is small, the acid for acid hydrolysis is less, and the waste acid should not be added too much at the lower limit, acid hydrolysis and leaching. In this way, the concentrated acid can not be used up, only a part.
21. Effect of seed crystal activity and amount on hydrolysis and product quality
The seed crystal is induced by its regular crystallization center to induce hydrolysis. Therefore, the activity and quantity of the seed crystal have a great influence on the rate of thermal hydrolysis, the hydrolysis rate, the recovery rate, the size of the metatitanic acid particles, the average particle size of the finished product, and the achromatic power.
The activity of the seed crystal is determined by the preparation conditions of the seed crystal. The seed crystal has good activity, high hydrolysis rate, uniform metatitanic acid particles, and high color reduction of the finished product.
As can be seen from Fig. 6, the amount of seed added is increased and the hydrolysis rate is increased. However, when the amount of seed added is more than 2%, the increase in hydrolysis rate is not obvious.
As can be seen from Fig. 7, the amount of seed crystal added is as follows. The color reduction is best at 0.6%-2%. When the seed crystal is less than 0.6%, since the crystal center is insufficient, some irregular crystal centers formed by itself are formed, and thus the achromatic power is drastically lowered. When the amount of seed added is more than 2%, the achromatic power also slowly decreases.
As can be seen from Fig. 8, the amount of seed added is increased, and the average grain size of the finished product is increased. Since the amount of the crystallization center is increased, the particle size of the meta-titanic acid virgin granules becomes finer, and aggregates into larger condensate particles. It is easy to sinter during calcination. When the seed crystal is greater than 2%, the product particle size is significantly increased.

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    Twenty-two, the effect of the concentration of trivalent titanium in titanium solution on hydrolysis
Since the oxidation potential of the trivalent titanium is larger than the oxidation potential of the ferrous iron, both the presence of trivalent titanium and the presence of divalent iron exist in the titanium solution. Therefore, if there is a possibility of oxidation, the trivalent titanium is first oxidized, and it is the turn that the divalent iron is oxidized. If the ferrous iron is oxidized to ferric iron, the ferric iron is easily hydrolyzed to form reddish-brown iron hydroxide mixed in the metatitanic acid, so that the finally produced titanium white powder is not pure white. Therefore, a certain amount of trivalent titanium exists in the titanium liquid to prevent oxidation of the ferrous iron. However, trivalent titanium is easily oxidized to tetravalent titanium. Titanium can be oxidized when placed, transported, and hydrolyzed. Therefore, the titanium liquid must be kept in the mother liquor after hydrolysis to still contain a certain amount of trivalent titanium to prevent the oxidation of the divalent iron into ferric iron. 5克以上。 Preferably, the trivalent titanium is preferably about 0. 5g / L. It is not good to have too much trivalent titanium because it has an inhibitory effect on the thermal hydrolysis of the titanium liquid, and the ferric iron does not undergo thermal hydrolysis reaction, and will remain in the mother liquor to lower the hydrolysis rate and recovery rate. However, in the case of pressurized hydrolysis, the higher the trivalent titanium is not affected, because it remains in the mother liquor, and the mother liquor is recycled, and the titanium can not run.
Twenty-three, the effect of hydrolysis temperature on hydrolysis and product quality
During the hydrolysis process, the temperature has a large influence on the rate of hydrolysis and the particle size of metatitanic acid.
The thermal hydrolysis of titanium fluid is an endothermic reaction, and increasing the temperature accelerates the hydrolysis rate. Titanium solution is hydrolyzed at a lower temperature, and it is more difficult to precipitate metatitanic acid. At 90 ° C, the hydrolysis reaction began to proceed weakly, and the reaction accelerated significantly at 100 ° C, but it took a long time to complete. Only at boiling temperatures, the rate of hydrolysis can meet the needs of industrial production, and operation control is also the easiest.
If the temperature is too high, the following drawbacks will occur: 1 waste steam; 2 intense boiling will destroy the flocculation of the meta-titanic acid primary particles to the secondary particles, making filtration difficult; 3 water evaporation is fast, affecting the concentration of titanium liquid; 4 hydrolysis rate Too fast, the size of the metatitanic acid particles is not uniform. It is generally required to carry out the hydrolysis in a slightly boiling state. In order to maintain micro-boiling, micro-pressure is often used for observation.
After prolonged hydrolysis at a low temperature, the obtained metatitanic acid particles are extremely fine, so that the finished product obtained after the burning of the horn is horny and the pigment performance is poor. Therefore, in order to avoid the production of such metatitanic acid in industrial production, it is required to shorten the time from 80 ° C to boiling. [next]
Some foreign researchers believe that the quality of metatitanic acid produced by thermal hydrolysis at 100 ° C is superior. If it is hydrolyzed at boiling temperature, the resulting metatitanic acid particles are coarser, which makes the achromatic power slightly lower (see above). 9), but it is difficult to filter and wash the metatitanic acid formed by hydrolysis at 100 °C. Therefore, industrial production still uses boiling temperature for hydrolysis. Although the performance of pigments such as decolorizing power is less, it can be compensated by adjusting other conditions. At present, many manufacturers use a micro-boiling state for hydrolysis.
Twenty-four, the length of hydrolysis time on the hydrolysis and product quality
The length of hydrolysis can determine the completeness of the hydrolysis process. The long hydrolysis time can increase the hydrolysis rate, but has a significant adverse effect on the size and uniformity of the metatitanic acid particles. It can be seen from Fig. 10 that the hydrolysis time has an effect on the hydrolysis rate and the achromatic power. At the beginning of the induction period, the hydrolysis is relatively rapid, but after 3 hours, it gradually becomes equilibrium, and then the time is extended, and the hydrolysis rate is not obvious. With the prolongation of hydrolysis time, due to the coarsening of the particle size of metatitanic acid, the color reduction power decreased after 4 hours, and the atmospheric pressure hydrolysis time (meaning boiling after whitening) was 2-4 h. Preferably, the pressure is hydrolyzed to maintain a pressure of 19.6*10 ⁴ Pa for 15-30 min.

    25. Operation of pressurized hydrolysis with additional seed crystals and indirect steam heating
This method is suitable for the preparation of pigment titanium dioxide which is usually fine and dispersible. The operation process is as follows: firstly, by adding 85% of the volume of the pot, the titanium liquid is added to the pressurized hydrolysis pot by metering, the stirrer is started, and the steam is heated to 60-80 ° C by a steam serpentine tube or a steam jacket, and then pressed. Ti0 ₂ adds 1% of the seed crystal (also added at room temperature). Close the feed port and seal it to prevent air leakage and continue heating. The pressure of the steam should reach (49-58.8)*10 ⁴ Pa. When the titanium liquid in the pot heats up to boiling, the secondary steam generated will cause the internal pressure to rise rapidly. . It is required to raise the feed port from the addition of seed crystals to 19.6*10 ⁴ Pa within 30-40 min, hold pressure for 15-30 min, and complete the hydrolysis. Then slowly open the vent valve, let the pot slowly depressurize, and finally discharge. [next]
Twenty-six, the operation of external pressure hydrolysis of seed crystals and autogenous seeds
The concentrated titanium liquid is added to an open atmospheric pressure hydrolysis pot lining two layers of acid-resistant ceramic tiles with a steel shell, and the stirring is started. If the seed crystal is hydrolyzed, it is heated by indirect steam (also used for direct steam heating). When the temperature rises to the temperature at which the seed acid is dissolved, the metered seed crystal is added; if the seed crystal is hydrolyzed, the body titanium to be hydrolyzed to 90-100 ° C is added immediately after the seed crystal is whitened. In the liquid. As for the control of the hydrolysis process, the two methods are similar, the production conditions of each plant are different, and the hydrolysis methods and control indicators are also different.
When the seed crystal is added, it is heated for about 20 minutes, the molten iron appears slightly boiling, and the solution changes from dark blue to dark gray. If the F value and the trivalent titanium content are high, the discoloration time may be prolonged. This obvious turning point of color change is called the critical point. This period of time is called the induction period of hydrolysis in the industry. At the end of the induction period, ie when the hydrolysis reaches a critical point, stirring and heating are stopped. At this time, the metatitanic acid particles are still growing, but the growth is relatively mild and uniform. This can significantly change the filtration and water wash properties of metatitanic acid, increasing the filtration and washing speed by 50%.
After about 30 minutes of rest, it was re- stirred and heated until it remained in a slightly boiling state after boiling. In order to control the steam to be added, the large-scale hydrolysis pot should not be too large, and is often adjusted by a micro-pressure gauge. In order to carry out the hydrolysis as far as possible at a fixed rate, the heating is sometimes carried out at a temperature lower than the boiling point and sometimes at a boiling point, mainly depending on the speed of the hydrolysis. When the hydrolysis rate is fast, the temperature is lowered, and when the hydrolysis rate is slow, the temperature is raised. When the reaction is in a boiling state and the hydrolysis rate is still not too high, the hydrolysis is mainly diluted by adding water to increase the hydrolysis rate. Since water is a reactant of the hydrolysis reaction, the addition of water (reactant) facilitates the progress of the hydrolysis reaction. Dilution with water to reduce the concentration of free acid is also beneficial to the hydrolysis reaction. Generally, 20 hours after boiling, the dilution water is added at a rate of about 15 L/min. In addition to adjusting the hydrolysis rate, it is more important to add the dilution water to a TiO ₂ content of 160 g/L to obtain a higher hydrolysis rate. When the water is added and the micro-boiling state is maintained until the sedimentation rate and the hydrolysis rate are satisfied, the hydrolysis reaction is completed. It usually takes about 4 hours from boiling to the end.
After the discharge, the hydrolysis pot must be washed with water to prevent the residue from becoming a poor crystallization center in the next hydrolysis.
The modern improved hydrolyzed titanium concentration can be controlled at ≤210g/L, and the iron-titanium ratio can be appropriately increased, which can reduce the burden of concentration and shorten the washing time; modern heating uses direct steam heating and maintains micro-boiling state hydrolysis. Before the boiling of the titanium liquid, it is directly heated by a large flow of steam. After the titanium liquid is boiled, it is directly heated by a small flow of steam to maintain a slightly boiling state. The hydrolysis pot has an open and sealed seal. The cover of the sealed hydrolysis pot prevents liquid leakage by liquid sealing technology, and the entire hydrolysis process does not require the addition of dilution water.
Twenty-seven, hydrolysis rate and its determination and calculation
The hydrolysis rate is a value reflecting the degree of completion of hydrolysis. That is, the liquid phase Ti0 _{2 is} converted into a solid phase Ti0 ₂ percentage. The level of hydrolysis indicates the conversion of Ti0 ₂ into solid phase Ti0 ₂ in the titanium solution.
Take a concentrated titanium solution before hydrolysis and a sample of the mother liquor after hydrolysis, and determine the total Ti0 ₂ and total ferrous content according to the method of measuring total titanium and total ferrous iron in titanium solution, and then calculate according to the following formula Hydrolysis rate:

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