Analysis of factors affecting hydrolysis

(1) Properties and composition of raw material titanium liquid

In the production of pigment grade titanium white powder, titanium nature of the feedstock liquid and the composition and properties of the nature and composition of the final hydrolyzate finished pigment has a greater influence, and different hydrolysis process, requires different nature and composition of the titanium sulfate solution.

a. Titanium liquid stability, poor stability of titanium liquid, there are many irregular crystal centers that are invisible to the naked eye, and these irregular colloidal titanium dioxide (early hydrolysis) are easily precipitated during long time in the summer or in the "Huangmei" season. The number, composition, particle size, etc. of these poor crystalline centers are disordered, and more impurities are adsorbed. Their presence not only makes the hydrated titanium dioxide particles formed by hydrolysis uneven, prolongs the time of filtration and washing, but also calcination. When the desulfurization is difficult, the particles are easily sintered and hardened, and the optical properties and pigment properties of the calcined product are deteriorated, and the whiteness, the achromatic power, the dispersibility, the oil absorption and the like of the final product are inferior.

Due to the unstable crystalline titanium center poor stability in solution are anatase, upon hydrolysis tend to produce stable lattice of anatase type, so that the production of rutile titanium dioxide, rutile type conversion is low, Therefore, the stability of the titanium liquid for hydrolyzing the pigment-grade titanium dioxide is generally not lower than 500 mL.

Titanium liquid with poor stability can not only be used to produce pigment-grade titanium dioxide, but also can not be used to prepare seed crystals. For the titanium liquid with poor stability, try to increase its concentration and then use it. Because the concentration of titanium liquid is increased, the total ion concentration in the solution can be increased, the normal crystal center is increased, and the surface on the crystal center is free. If it can be enlarged, most of the products can be grown on the crystallization center when the titanium liquid is hydrolyzed, so as to improve the quality of the hydrolyzed product. In the case of titanium liquid which is not well stabilized, more attention should be paid to the neutralization temperature and the neutralization speed in the preparation of the seed crystal.

b. The clarity of the titanium liquid and the poor clarity of the titanium liquid indicate that the impurity content is high. These impurities include the colloidal titanium dioxide particles which are hydrolyzed at an early stage, and the filter aid (charcoal powder, diatomaceous earth, etc. which cannot be separated and cleaned when the filtration is controlled). ) and very fine particles of mud. These impurities contain many heavy metal impurity sulfates and sulfides harmful to titanium dioxide. Their presence not only prolongs the washing time, but also reduces the purity of titanium dioxide, which makes the whiteness of the product worse or even the phenomenon of mutual color change. When impurities phosphate product inhibition rutile conversion, thus producing rutile titanium dioxide, iron titanium ore phosphorus content should be <0.025%.

The degree of clarity is closely related to the stability. The titanium solution with poor stability contains colloidal titanium dioxide particles which are analyzed by early water, which increases the viscosity of the titanium solution, and the irregular colloidal titanium dioxide particles easily block the pores of the filter cloth. It is difficult to filter, and if the filtration pressure is increased, leakage and permeation are easily caused to lower the clarity of the titanium liquid. In addition, the surface of the unstable colloidal titanium dioxide particles which are analyzed by these early waters adsorbs many impurities, and these impurities are precipitated together and mixed into the metatitanic acid to reduce the purity and whiteness of the product.

Since these impurity particles are extremely fine and sometimes invisible to the naked eye, factories at home and abroad attach great importance to controlling the quality of the filtration products. Sometimes, it is necessary not only to increase the filtration step, but also to add a flocculant and then add a sedimentation and filtration process.

c. Total titanium concentration of titanium liquid

The total titanium concentration of titanium liquid not only affects the hydrolysis rate, but also affects the product quality. The total titanium concentration is further divided into the total titanium concentration of the titanium liquid, the total titanium concentration of the hydrolysis starting titanium liquid, and the total titanium concentration at the end of the hydrolysis. When discussing the total titanium concentration, the three different total titanium concentrations are first distinguished. The titanium liquid with high total titanium concentration generally has good stability, the stability of the titanium liquid is also good, the impurity content is low, and there is almost no bad crystal center, and the titanium liquid not only has good quality of the product, but also It is easy to store and does not precipitate colloidal titanium dioxide immediately upon dilution.

In studying the effect of the total titanium concentration of the titanium solution on the hydrolysis operation, it is usually referred to as the total titanium concentration in the titanium liquid at the start of the hydrolysis. For the hydrolysis of the autogenous seed crystal dilution method, it refers to the total titanium concentration of the titanium liquid added to the hot water after preheating. Although the concentration of the titanium liquid selected by each factory is high or low, the amount of bottom water they match is sometimes different. The initial ion concentration of the final hydrolysis is not much different, that is to say, the amount of autogenous seed crystals is similar in the initial stage of hydrolysis; for the additional seed crystal hydrolysis method, it refers to the total titanium concentration in the titanium liquid when the seeding starts to be hydrolyzed. The total titanium concentration of the titanium liquid during hydrolysis is controlled mainly to control the initial velocity at the time of hydrolysis, since the concentration of free acid in the initial solution of hydrolysis is the lowest, and thus the hydrolysis rate is also the fastest, which is disadvantageous for controlling the particle size of the product. For the self-produced seed dilution process, since the titanium liquid and water are preheated by high temperature in advance, the hydrolysis reaction starts quickly after the contact between the two, so the total titanium concentration of the titanium liquid is generally required to be higher, and the addition of the hydrolysis process for the seed is generally used. The titanium liquid is generally not preheated, and the hydrolysis temperature is slower, so the total titanium concentration of the titanium liquid during hydrolysis can be lower. The concentration of total titanium diluted with water before the end of hydrolysis is mainly to increase the hydrolysis rate, because the concentration of free acid in the solution is higher and higher in the late stage of hydrolysis, the hydrolysis rate is slower and slower, and the concentration of free acid can be reduced by dilution to facilitate the hydrolysis reaction. complete.

Under normal circumstances, the total titanium concentration of the hydrolyzed titanium liquid is high, and the hydrated titanium dioxide particles formed by hydrolysis are fine, and the color reduction of the obtained product is high. FIG. 1 is 1% of the seed crystal addition, and the metatitanic acid after the regular pressure hydrolysis is calcined at 950 ° C. Tinting strength (achromatic power) of the anatase titanium dioxide. However, the total titanium concentration is high, and the hydrolysis rate of the titanium liquid is slow. On the contrary, the total titanium concentration is low, the hydrolysis rate is high, the hydrolysis rate is high, and the produced hydrated titanium dioxide particles are coarse. Although the water is washed well, the product has low color reduction and poor product quality.

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It can be seen from Fig. 1 that when the concentration of TiO ₂ exceeds 200 g/L, the effect of the increase of total titanium concentration on the tinting strength is not significant, so the initial concentration of total titanium in the hydrolysis by the external seeding method is generally controlled at 200 g/L. The higher the total titanium concentration at the start of hydrolysis by the autotrophic seed dilution method is mainly to control the speed of the initial stage of hydrolysis and prevent the titanium liquid from prematurely hydrolyzing during the preheating period. Figures 2 and 3 show the effect of total titanium concentration on hydrolysis rate and hydrolysis rate. In industrial production, the concentration of titanium solution used for seeding is generally 190~230g/L, the initial total titanium concentration of hydrolysis is about 200g/L, and the concentration of titanium solution used for hydrolysis by autotrophic seeding method is generally 220~260g/L.

d. The F value of the titanium liquid, and the F value of the titanium liquid is one of the most important factors affecting the hydrolysis process. The acidity coefficient of titanium liquid is different, and the degree of polymerization of hydrated titanium dioxide is different at the time of hydrolysis. Under the same concentration of titanium liquid, the F value is generally high, and the acidity in the solution is also high. From the chemical equilibrium of the following hydrolysis reaction, the hydrolysis reaction is affected. Inhibition, slowing down the hydrolysis rate, low hydrolysis rate

The produced hydrated titanium dioxide particles are finer and the washing speed is slow; on the contrary, the titanium liquid with low F value has low acidity in the solution, high hydrolysis rate, high hydrolysis rate, coarse hydrated titanium dioxide particles formed by hydrolysis, and low F value. The titanium liquid generally has poor stability, and it is not only possible that some unfavorable crystallization centers are present when it is not hydrolyzed, resulting in uneven particle size of the hydrolyzed product and poor performance of the product pigment. In addition, due to the low acidity in the solution, it is possible that the sulfate of some metal impurity ions will be precipitated together with the metatitanic acid at a higher pH, so the titanium liquid with too low F value is not suitable for the production of pigment grade. Titanium dioxide.

The F value of titanium liquid in industrial production is generally between 1.6 and 2.1, otherwise the quality of the hydrolyzed product is not good. Many data reports that the F value of 1.8 is the most ideal value. However, the F value of the titanium liquid obtained in industrial production generally fluctuates between 1.7 and 1.9, and sometimes the amplitude is higher, which indicates that the hydrolysis results of different batches of titanium liquid are different. According to the requirements of USP 4014977, the F value of the titanium liquid during hydrolysis should be controlled at about 1.8 (1.75~1.85), especially when the autogenous seed crystal dilution method is hydrolyzed, the first part of the titanium liquid used as the seed crystal must be added. Make it happen. In order to control the F value within this range before hydrolysis, the alkali solution (ferrous hydroxide, sodium carbonate, ammonia, sodium hydroxide, etc.) may be used to locally neutralize the titanium solution, so that the F value of the titanium solution is guaranteed to be about 1.8. The product thus hydrolyzed has a good particle size and achromatic power.

However, different hydrolysis processes have different F values. Even in the same hydrolysis method, the total titanium concentration, F value, and iron-titanium ratio can be adjusted accordingly. For example, the same is the seed crystal dilution method. The hydrolysis process uses a titanium solution with a total titanium concentration of 250-260 g/L, and its F value is generally controlled between 1.75 and 1.85; while the titanium solution with a total titanium concentration of 220-230 g/L is used, its F value is Can be controlled between 1.85 ~ 1.95. However, the adjustment of this index is relatively fixed. In production, it cannot be considered that the F value and the total titanium height can be used in this proportion. Otherwise, the product quality is not only unstable, but also causes confusion to the operator, resulting in disorder. operating. Therefore, in the modern large-scale titanium dioxide factory, most of the titanium before the hydrolysis is provided with a titanium liquid mixing process to ensure that the total titanium concentration, F value, iron-titanium ratio, and trivalent titanium concentration in the titanium liquid before hydrolysis are controlled within a certain limit, Ensure the stability and improvement of the quality of the hydrolysate.

e. Titanium-to-titanium ratio (Fe/TiO ₂ ) The ratio of iron to titanium is the ratio of the concentration between iron ions and titanium dioxide in the titanium solution. The ferrous sulfate in the titanium liquid not only increases the relative density and viscosity of the solution, but also increases The total ion concentration in the solution, so it can also play a role in regulating the hydrolysis rate and controlling the particle size, because during the hydrolysis process, the concentration of titanium dioxide in the liquid phase is continuously reduced, and the ferrous sulfate is not hydrolyzed. The mother liquor still maintains a certain ion concentration and acts similarly to increasing the concentration of titanium dioxide. In general, the titanium-titanium solution with high iron-titanium ratio has a slow hydrolysis rate, and the produced hydrated titanium dioxide particles are finer and the washing time is longer; the titanium-titanium solution with lower iron-titanium ratio has a faster hydrolysis rate, and the produced hydrated titanium dioxide particles are coarser. The washing time is short, but the iron-titanium ratio is too low, and not only the hydrolyzed product particles are too coarse, but the pigment performance is not good, and the burden of the freezing crystallization process is increased. In winter, the iron-titanium ratio is too high, and the ferrous sulfate dissolved in the mother liquor precipitates ferrous sulfate crystals as the temperature is lowered, making the water washing operation more difficult. Therefore, the titanium liquid for hydrolysis should control a certain range of iron-titanium ratio, and the value thereof should be comprehensively considered according to the difference of the hydrolysis process and the total titanium content and F value in the titanium liquid. Table 1 shows the relationship between the ferrous sulfate content in the titanium solution and the product's color reduction.

Table 1 Relationship between ferrous sulfate, free acid and achromatic power

TiO 2 content / g / L	207.7	207.7	207.7
H 2 SO 4 content / g / L	19.6	127	127
FeSO 4 content / g / L	0	0	167
Achromatic power (Reynolds number)	200	1200	1670

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f. The content of trivalent titanium in titanium liquid, the concentration of trivalent titanium in titanium liquid is both a technical indicator and an economic indicator. From a technical point of view, the presence of trivalent titanium ions in the titanium solution prevents the ferrous ions in the titanium liquid from being oxidized to ferric ions because the oxidation potential of the trivalent titanium is higher than that of the ferrous iron. When the titanium liquid is oxidized by oxygen in the air during storage and hydrolysis, the first oxidation reaction occurs with trivalent titanium, the trivalent titanium is oxidized, and the ferrous iron begins to oxidize. Because ferric iron is easily hydrolyzed to form iron hydroxide during hydrolysis, the adsorption is precipitated on the surface of metatitanic acid, and the solubility of ferric ion in water is very low, which cannot be removed by subsequent washing operation, and eventually will be polluted. Products that reduce the whiteness of the product. Therefore, from a technical point of view, the presence of a certain amount of trivalent titanium ions in titanium liquid (1~3g/L in terms of TiO ₂ ) is good for product quality, but the trivalent titanium content is too high, which has an inhibitory effect on hydrolysis. The example shows that when the trivalent titanium content is >7 g/L, the hydrolysis rate is slowed down.

Trivalent titanium should be hydrolyzed at pH>3, while the acidity in the solution is high when the titanium solution is hydrolyzed, and the trivalent titanium does not undergo hydrolysis at the same time. The trivalent titanium remaining in the mother liquor is filtered and washed with water and filtrate. Discharge together to reduce the rate of hydrolysis and cause unnecessary waste. The trivalent titanium remaining in the mother liquor and in the wastewater will gradually oxidize or decompose in the waste acid concentration and wastewater treatment, which brings certain difficulties and losses to the purification operation of these departments.

(2) The influence of the number of seeds

During the hydrolysis process, the seed crystal can accelerate the hydrolysis rate, shorten the hydrolysis cycle, increase the hydrolysis rate, control the size of the hydrated titanium dioxide primary particles, directly affect the product's color reduction, hiding power and other pigment properties.

Since the seed crystal is added to the titanium liquid in the form of a regular crystal center, it is used to properly induce the hydrolysis of the titanium liquid, so the quality (activity) of the seed crystal and the amount of addition (how much of the crystal center) hydrolyze the titanium liquid. The process has a big impact.

The amount of seed crystal added directly determines the number of crystal centers in the solution. Under normal circumstances, the amount of seed crystal is increased, the hydrolysis rate is increased, and the hydrolysis rate is also high (Fig. 4). For example, 1% of the seed crystal is added to the concentrated titanium solution, and the hydrolysis rate can reach 90% or more by atmospheric pressure hydrolysis for 3 hours. Under the same conditions, the hydrolysis rate is only 30%~40% without seeding, but when the seeding amount exceeds 2%, the hydrolysis rate is not obvious.

However, the amount of seed crystal added is too small (less than 0.6% in terms of TiO ₂ ). Because the number of crystal centers is too small, some irregular crystal centers are formed when the titanium liquid is heated and hydrolyzed, resulting in uneven particle size. When the product's achromatic power (tinting power) is lowered, a milky acid-like slurry will appear in a serious case, which makes filtration and washing difficult. As can be seen from Fig. 5, the seeding amount exceeds 2%, and the tinting strength is There is a downward trend.

The size of the hydrated titanium dioxide primary particles after hydrolysis can be adjusted by the amount of seed crystal addition, which is one of the main means for controlling the particle size of the product in industrial production and adjusting the product variety. When discussing the effect of seed addition on the size of the hydrolyzed particles, it is necessary to distinguish between the primary particles and the flocculated particles. From the crystallographic principle, the amount of seed crystals is increased, and the number of crystal centers is also large. The primary particles of hydrated titanium dioxide are finer. Because the surface free energy of the fine particles is large, it is easy to flocculate into large particles and settle down. It is also easy to sinter together when calcined. Under normal circumstances, the amount of seed crystals is increased, the primary particles generated during hydrolysis are fine, the product has high decoloring power, and has a blue bottom phase. At the same time, the secondary flocculated particles are large, and the water is washed well, and the particles are easily sintered during calcination. Harder. [next]

(3) Influence of hydrolysis operating conditions

In the case where the composition of the titanium liquid, the quality of the seed crystal and the amount of addition have been determined, the most important thing in the hydrolysis operation is to control its hydrolysis rate and hydrolysis rate, and the main factors affecting the hydrolysis rate and hydrolysis rate are the hydrolysis temperature and the hydrolysis time. , followed by the stirring speed and heating method.

a. The hydrolysis temperature also affects not only the rate of hydrolysis but also the particle size of the hydrolyzed product (hydrated titanium dioxide). Since the hydrolysis reaction of the titanium liquid is an endothermic reaction, increasing the hydrolysis temperature can accelerate the hydrolysis rate, and the temperature increase can also accelerate the growth rate of the hydrated titanium dioxide particles.

Under normal conditions, the titanium solution begins to hydrolyze at 90 ° C, and accelerates significantly at 100 ° C. It is significantly accelerated at the boiling temperature. The boiling temperature is related to the concentration of the titanium liquid, and the concentration is high and the boiling point is also high. Low temperature, long hydrolysis time, and long-term hydrolysis at low temperatures produce very fine hydrated titanium dioxide particles, and the pigment properties of the product are poor. However, if the temperature is too high, not only steam is wasted, but intense boiling will destroy the flocculation of the primary particles of hydrated titanium dioxide to the secondary particles, which will deteriorate the filtration performance of the product. If the temperature is too high, the water in the solution will evaporate too quickly, affecting hydrolysis. The concentration of the material, while the temperature is too high, the hydrolysis rate is too fast, and the size of the hydrated titanium dioxide particles is not uniform. Generally, the hydrolysis temperature is only required to be maintained near the boiling point, and the micro-boiling state can be maintained.

According to the data, under the conditions of atmospheric hydrolysis, at a temperature slightly lower than the boiling point (about 100 °C), the product has higher color reduction, but the hydrolysis time is long, it is difficult to filter and it is not economical, and it is hydrolyzed at the boiling point (112). ~114 ° C), although the product colorlessness is not lower than the boiling point, but the hydrolysis rate is fast, the filtration performance is good, so atmospheric hydrolysis is generally hydrolyzed near the boiling temperature, as the color reduction is lower, other Ways to solve it.

In the case of pressurized hydrolysis, as the pressure increases, the hydrolysis temperature also increases. At an internal pressure of 0.2 MPa, the boiling point can be as high as 136 to 140 ° C. At such high temperatures, the hydrolysis rate is fast and the particles are also relatively high. Fine, in order to increase the particle size of the hydrolyzed product hydrated titanium dioxide to a good degree of filtration, the pressure hydrolysis operation, after the internal pressure reaches 0.2 MPa, it is necessary to keep pressure for a period of time to vent, cool, discharge, and thus can also make the hydrolysis rate Further improve.

b. The length of hydrolysis time of hydrolysis not only affects the degree of hydrolysis, but also has a significant effect on the size and uniformity of hydrated titanium dioxide particles.

In order to avoid the too long titration of the metatitanic acid particles formed by hydrolysis at low temperature for a long time, no matter which hydrolysis process is used, it is required to be as short as possible from 80 ° C to boiling, or 80 ° C to the time when the titanium liquid just changes color. In the process of stopping steam and stopping stirring when the titanium liquid is just discolored, the time during which the temperature is raised to the second boiling point by reheating should not be too long, so as to ensure that the particles are relatively uniform. However, the hydrolysis rate should not be too fast. If the hydrated titanium dioxide is too fast, it will not be uniformly and neatly precipitated on the crystal nucleus. It may be a disorderly accumulation, and even a new crystallization center will appear. Some data show that the hydrolysis rate is best controlled. 0.4%/min or so (calculated from the seed crystal).

Generally, in the initial stage of hydrolysis, since the free acid in the solution is less, the acidity is low, and the hydrolysis speed is fast. At the same time, since the concentration of titanium dioxide in the titanium liquid is high, the hydrolyzed product hydrated titanium dioxide particles are fine, and the hydrolysis is prolonged as the hydrolysis time is prolonged. The speed is gradually slow, and the particles of metatitanic acid (hydrated titanium dioxide) gradually become coarser. Therefore, the hydrolysis time is too long, which not only helps to increase the hydrolysis rate, but also reduces the performance of the pigment of the product, so that the color reduction of the product becomes low. Therefore, sometimes the one-sided pursuit of hydrolysis rate in production is at the cost of quality reduction. Figure 6 is the effect of hydrolysis time on product quality and hydrolysis rate.

It is more reasonable to control the hydrolysis time, which is determined by experiment. After the 1960s, a large number of patents and research reports related to the rate of hydrolysis and its control methods. Although the curves of hydrolysis temperature rise and temperature increase rate in these papers are not the same, the hydrolysis time is about 4h (from boiling to hydrolysis end). The average hydrolysis rate is also relatively uniform at about 0.4%/min.

c. Stirring during hydrolysis, the stirring speed not only affects product quality, hydrolysis time, but even determines the success or failure of the hydrolysis reaction. From the crystallographic point of view, the particle growth state is better without stirring, but the hydrolysis of the titanium liquid is heated, and heat exchange cannot be performed without stirring, and good agitation not only improves the heat transfer effect, but also The particles of metatitanic acid are dispersed relatively uniformly. If the stirring speed is too fast, not only the crystal particles are made fine, but also the particles of the metatitanic acid are flocculated, which affects the speed of filtration and water washing.

Generally, it is heated by direct steam. The stirring speed can be slower, usually 10~30r/min. With indirect steam heating, the stirring speed can be faster, generally 30~60 rpm. The use of an additional seed hydrolysis method allows for a uniform dispersion of the seed crystal into the titanium liquid, a faster stirring speed or a variable speed motor, which is faster during seeding and slows down during hydrolysis.

d. Heating method at the time of hydrolysis The heating methods at the time of hydrolysis include direct steam heating, indirect steam heating, and mixed heating. Direct steam heating, simple equipment, fast heating rate, good heating rate control, less steam consumption, energy saving, but the condensed water in steam will reduce the concentration of titanium liquid during hydrolysis, and the addition of crystals with strict initial concentration of hydrolysis The hydrolysis method is disadvantageous, and at the outlet of the steam heating tube, the crystal particles of the hydrated titanium oxide are irregular due to high temperature and strong gas impact.

Indirect heating, warming slowly, more than the amount of steam, the heat utilization is low, high energy consumption, producing not only the heating coil, high maintenance costs, long-term use of the copper coil corrosion may contaminate the product, but not dilute hydrolysis of titanium The concentration of the liquid, therefore, the addition of seeding method mostly uses indirect heating.

The mixed heating method is to add the titanium liquid after the seeding, and indirectly heat it to the hydrolysis critical point (boiling, or just discoloration, or 90 to 96 hours) in the hydrolysis tank, and then use direct steam heating.

The three kinds of heating methods can obtain the same high-quality hydrolyzed product as long as the heating time, the hydrolysis rate, the concentration of the titanium liquid during hydrolysis, and the F value can be controlled.