Titanium liquid hydrolysis method

There are many patents and reports on the hydrolysis method of titanium liquid, but only two methods are still widely used in various countries. One is the successful self-produced seed dilution hydrolysis process, also known as the Bruenfield method, by Joseph Blu-menfeld, France, in 1923; the other is Meklenberg. Developed in 1930, the process of adding seed crystals by alkali-neutralizing titanium liquid, also known as the McLenberg method or precipitation method. Although the operation details of these two methods have changed a lot in the past decades, the basic principles and basic operation methods are almost the same. The quality of the products obtained by the two methods is not significantly different. The two methods do not have the advantages of being superior to each other. In the operation, the concentration of the titanium solution is sensitive when the seed crystal method is hydrolyzed, and the seed crystal dilution method is adopted. The hydrolysis process is sensitive to the F value of the titanium liquid; the direct seed steam is generally used for the hydrolysis of the seed crystal dilution method, and most of the seed crystals are heated by indirect steam, and a few of them are directly steamed; the hydrolysis operation control of the seed crystal dilution method is complicated. Although the control of the seed crystal is relatively simple, it is necessary to add a set of equipment for preparing the seed crystal.

(1) Hydrolysis by autotrophic seed dilution

The method is to dilute the concentrated titanium liquid under strict strict conditions to form a batch of desirable crystal centers (nucleation or seed crystal) in the solution, and then continue to add the titanium liquid to be hydrolyzed, in its Heating and hydrolysis are carried out around the boiling point. In this way, a more concentrated solution of titanyl sulfate (TiO ₂ : 240 ~ 260g / L, effective acid is 480 ~ 520 / L) to ensure the number of autogenous seeds in the initial stage of hydrolysis, to prevent the titanium liquid during warm-up period Hydrolysis, the equipment is simple and economical in large-scale industrial production.

In this method, the titanium liquid and the water are pre-cooked in advance, according to a certain ratio of hot titanium liquid and hot water; the titanium liquid is added to the hot water at a certain rate for a prescribed time, and this step is the hydrolysis process. The core part is the only way to form a certain amount and quality of seed crystals, and then continue to add the remaining titanium solution for thermal hydrolysis while stirring and heating.

However, when the first batch of titanium solution is added to hot water, slight turbidity will occur. Whoever continues to add titanium solution, the mixture dissolves quickly and the gauge is relatively transparent. This dissolution process is only a surface spec. In fact, the colloidal precipitate is Dispersed into the titanium solution, acting as a seed crystal and a crystallization center, and in the subsequent hydrolysis process, hydrated titanium dioxide is deposited on these seed crystals or crystal centers. After about 20 minutes, the solution changed from black to olive green and then to steel gray. At this point, heating and stirring should be stopped immediately, so that the hydrated titanium dioxide particles grow in a milder and more uniform condition. After a few minutes, the color teeth turn from tan to milky white. During the period of stopping heating and stirring, the industry is called the "induction period" of hydrolysis. The filtration rate of the hydrolyzed product after stopping the heating and stopping the stirring is more than that of continuous stirring. The hydrolysis rate of the hydrolyzate is 50% faster; during this period, although it is not heated, the particles of the hydrolyzate are still growing, and the growth of the particles throughout the hydrolysis process is as follows:

When the solution turns white, the hydrolysis is actually 60%~70%, but the first few minutes of the addition of titanium to the hot water is very important. It basically determines the rate of the hydrolysis reaction and the quality of the hydrolysis product. Some processes specify the ion concentration after one minute (80~120g/L). The time of stopping heating and stirring is generally controlled at about 20~60min, and then reheating and stirring. The rate of hydrolysis is generally fast and slow, but the hydrolysis is close to the end point. When heating continues, no new precipitates will be precipitated. The hydrolysis reaction makes the hydrolysis more complete to increase the hydrolysis yield. Dilution generally uses hot water, which is a seemingly simple, actually technically strong work, can not wait for it, otherwise there will be a large number of fine particles, so that the hydrolyzed material is milky, the particle size distribution is worse, so that hydration Filtration and washing of titanium dioxide becomes very difficult. [next]

Some factories change the time of adding dilution water to the end of the hydrolysis induction period. When reheating and mixing, the dilution water is uniformly added to the hydrolyzed material throughout the boiling period. They think that this can maintain the acidity during the whole hydrolysis. Relatively constant, the hydrolyzed precipitated particles are relatively uniform.

The most difficult part of the hydrolysis of the seed crystal dilution method is how to control the amount of titanium addition and the rate of addition, especially the amount and rate of the first 4 minutes. Because the titanium liquid is added too fast, because it is too late to produce enough colloidal titanium dioxide (seed), the hydrolysis rate is low, and the addition speed is too slow, or the temperature is too high, and may even appear in the large amount before the total titanium liquid is added. Irregular precipitation, not only poor hydrolysis rate, but also poor filtration performance. On this point, modern metrology and automatic control technology can basically solve this problem; the second is how to observe the discoloration to determine the time to stop heating and stirring. The general operation is mainly based on the naked eye observation, supplemented by time and temperature, but this is large In industrial production, there are many artificial subjectivity. It is proposed in U.S. Patent No. 1,335, 537 and U.S. Patent No. 3,706, 829 to determine the critical point of stopping the heating and stirring time by measuring the reflectance of the hydrolyzed titanium liquid. They use a color master (Colormaster) and use a green filter to continuously detect on the mirror of the hydrolysis tank wall. When the discoloration of the hydrolyzed titanium liquid reaches a certain critical value, it is found that the reflectivity curve has a turning point. Stop heating and stop stirring for the most ideal time. However, due to the pollution of the sight glass and the influence of the color of the titanium liquid, the application in actual production is not much, and it is mainly determined by mastering the temperature, time and visual observation. Table 1 is an example of the Brumentfield process hydrolysate data.

Table 1 Composition of titanium liquid in the initial stage of dilution

U.S. Patent Nos. 3,706,829 and 4,014,977 have a detailed introduction to the hydrolysis of authigenic seed crystals. In the above patent, the concentration of TiO ₂ in the titanium solution is required to be 230 to 260 g/L, and the F value is 1.75 to 1.85, and it is considered to be the best at 1.8. Before the start of hydrolysis, the titanium liquid and water are required to be preheated to 88~98 °C, preferably 96 °C, and the ratio of titanium liquid to water is 3.5~4.5:1, preferably 4:1.

In the hydrolysis operation, the hot water is first placed in the hydrolysis tank, and then the hot titanium liquid is added to the hydrolysis tank at a certain rate within 16~20 minutes, and heated by direct steam under stirring. After about 20 minutes, the titanium liquid boils (about 108 °C). ), called the first boiling point. However, when the critical point of hydrolysis is reached, that is, when the titanium liquid is just whitened from black (the hydrolysis rate is about 15% to 25%), immediately stop heating and stirring for about 45 minutes, then start stirring and steam to continue heating. To boil again (about 112 ° C or so), the second boiling point, and then keep boiling for 3h, then add hot water to dilute to TiO ₂ concentration of 155 ~ 175g / L, and then stir for 10 ~ 30min to cool the discharge.

After the discharge, the hydrolysis tank must be cleaned without leaving any residue of hydrolysis, otherwise these products will play the role of a bad crystallization center in the next hydrolysis. When hydrolyzing with a large-scale hydrolysis tank (100 m ³ or more), steam may be continuously stirred only because the growth of the hydrated titanium oxide particles precipitated during the stirring is increased, the viscosity of the solution is also sharply increased, and the start of the motor is required to start the stirring again. The power is too large. Titanium liquid should maintain a certain rate during hydrolysis, and increase the direct steam inlet amount under normal pressure, which will not only cause the reaction temperature to rise quickly, but will also waste Yanqi. A large amount of direct steam will cause intense stirring and will destroy. The flocculation of the hydrated titanium dioxide particles is such that as long as the micro-boiling is maintained, the large-scale hydrolysis tank is adjusted by the micro-pressure gauge to adjust the amount of steam to be added for the purpose of observation control. [next]

In another patent, it is considered that the F value of the titanium liquid provided in the actual production fluctuates widely, generally between 1.7 and 1.9, and sometimes higher. Since the hydrolysis result is different due to the change of the F value, the patent seeks to The titanium liquid to be hydrolyzed is divided into two parts. The F value of the titanium liquid (3% to 12% of the total titanium liquid) added before the first lmin must be accurately controlled between 1.75 and 1.85, and then the total titanium concentration is the same, but The second titanium liquid having a high F value (up to 2.1 to 2.4) can be added to the hydrolysis tank for hydrolysis. The preheating temperature, boiling time, time to stop stirring and heating, time of boiling and dilution of the remaining titanium liquid and water are substantially the same as those described above. The decolorization power of the hydrolyzate of this method is relatively high, and generally is about 100 (1750~1850) higher than the Reynolds number of the above hydrolysis method. Another advantage of this method is that the particle size of the hydrolyzed hydrated titanium dioxide can be controlled by adjusting the amount of the first low F-value titanium solution. Generally, the amount of the multi-particles is small, and the amount of the particles is relatively small. Big.

Compared with the external seed hydrolysis method, the autogenous seed dilution method has the advantages of simple operation, less equipment, and no need for special seed crystal manufacturing equipment, which can save raw materials and energy costs when preparing seed crystals. When hydrolyzed by the seed crystal dilution method, the condensed water produced by the direct steam can alleviate the effect of the effective acid in the hydrolysis process on the hydrolysis process, and also the effect of adding the dilution water in the late stage of hydrolysis, and adopt direct steam heating. There is no heating coil in the hydrolysis tank, which is convenient for cleaning the hydrolysis tank to avoid the influence of coil corrosion on product quality. However, it is difficult to control the hydrolysis of the autogenous seed crystal dilution method, the initial addition of the titanium liquid to the formation of the seed crystal and the color change of the titanium liquid. In order to overcome this shortcoming, a hydrolysis method has been developed in recent years. Before the hydrolysis of the titanium liquid, the seed crystal is added, and the rest of the water and the titanium liquid are preheated, stopped stirring, steam stopped, boiled and kept for a long time, and the autogenous seed crystal dilution method hydrolyzes the sample, which actually combines the two methods. Advantages, simple operation control, and good product quality, which is an additional set of seed preparation equipment and seed preparation process.

(2) Additional seed hydrolysis method (McLenberg method)

The operation process of the additional seed hydrolysis method is relatively simple, and the key to the process is the method of preparing the seed crystal and the quality of the seed crystal. The operation process of the hydrolysis by the self-produced seed dilution method has not changed much in the past ten years, and the preparation method of the seed crystal hydrolysis method has changed a lot, and the hydrolysis is sensitive to the concentration change of the titanium liquid.

The first thing to introduce is the preparation of seed crystals and seed crystals. The so-called seed crystal is a colloidal titanium hydroxide solution prepared by incomplete neutralization of a titanyl sulfate solution, which acts as a crystallization center of hydrated titanium dioxide during hydrolysis, which not only accelerates the hydrolysis reaction, but also shortens the hydrolysis cycle, and It has a great influence on the particle size, particle size distribution and final product quality of the hydrolyzed precipitated product. In industrial production, in order to obtain hydrated titanium dioxide with uniform particle size and certain composition, it is necessary to add titanium (pre-seed method) or culture (self-crystal seeding method) before the thermal hydrolysis. A good crystallization center that must be composed in order to properly guide the progress of thermal hydrolysis. If the quality of the seed crystal is poor or there is no seed crystal at all, the hydrolysis operation is not normal, and the obtained product is often a suspension of fine and uneven "milk" of the particles. The hydrolyzed product does not precipitate, is difficult to be filtered and washed, and The produced titanium dioxide pigment has poor performance.

During the hydrolysis process, the titanium oxysulfate is hydrolyzed under the induction of heating and seed crystal, and the formed hydrated titanium dioxide precipitates on the surface of the seed crystal as long as there is a sufficient amount of seed crystal in the titanium liquid, and the heating rate and stirring speed are high. When diluted properly, the resulting hydrated titanium dioxide precipitates on these crystallization centers, and no new crystallization center occurs, so that not only the hydrolysis can be carried out more completely, the hydrated titanium dioxide has a relatively uniform particle size, and the pigment performance is superior. The titanium dioxide is also easier to filter and wash, and the filtration loss is less.

The quality (activity) of the seed crystal directly affects the hydrolysis rate. The amount of the seed crystal directly affects the primary particle size of the hydrated titanium dioxide, and the uniformity of the seed crystal itself directly affects the particle distribution of the hydrated titanium dioxide.

When the amount of seed crystals is large, the hydrated titanium oxide particles formed by hydrolysis are fine and have a large specific surface area, so that the surface free energy is also large, and it is easy to aggregate into large particles of metatitanic acid; when the amount of seed crystals is small, the result is opposite, which may affect Filtration and washing speed. If the number of seed crystals is too small and there is insufficient crystallization center in the hydrolysis, the titanium oxysulfate will generate a new crystallization center under heating and dilution, and the structure, composition and quantity of such uncontrolled crystallization centers vary greatly. It will cause the hydrolyzate particles to be uneven, which makes it difficult to wash the water. It is one of the process design methods for titanium dioxide products to adjust and control the size of hydrated titanium dioxide primary particles by using the amount of seed crystals.

Many compounds can be used as seed crystals, but colloidal titanium hydroxide solution neutralized with alkali is the most effective seed crystal for thermal hydrolysis of titanium liquid. There are many patents related to seed crystal preparation, but generally they are all suspended. Colloidal titanate, then acid soluble in dilute sulfuric acid or hydrochloric acid to form a positively charged titanium dioxide colloidal solution, the difference is the type of alkali (sodium hydroxide, sodium carbonate, ammonia, etc.), preparation method, The seed crystal concentration, the F value, the addition amount and the manner of addition are different. It has been proved that the seed crystal of good quality has a very significant effect even if the amount of the seed crystal is small.

Alkali-neutralized seed crystals (usually anatase-type seed crystals) are the most common seed crystals used in the addition of seed crystal hydrolysis. They are usually prepared by taking out a portion of the titanium liquid to be hydrolyzed and maintaining a certain temperature with a dilute alkali under stirring. The liquid is neutralized to a certain pH value, and the ortho-titanic acid precipitate formed by the neutralization reaction is dispersed into the solution by mechanical stirring, and heated and heated (acid-soluble) together with the free acid remaining in the solution to form a charged micro Crystallized rubber particles. The main chemical reactions that occur when the titanium solution is neutralized with a base to prepare seed crystals are as follows:

H ₂ SO ₄ +2NaOH→Na ₂ SO ₄ +2H ₂ O

The titanyl sulfate in the titanium solution reacts with a base to form an orthotitanic acid precipitate.

TiOSO ₄ +2NaOH + H ₂ O→Ti(OH) ₄ ↓Na ₂ SO ₄

The trivalent titanium in the titanium solution is hydrolyzed to form a blue titan hydroxide precipitate under the low acidity after neutralization with a base, so the colloidal solution of the seed crystal is generally blue, and the trivalent titanium content in the titanium liquid The higher the color, the deeper the color is even blue and purple.

Ti ₂ (SO ₄ ) ₃ +6H ₂ O→2Ti(OH) ₃ ↓+3H ₂ SO ₄

The neutralized orthotitanic acid is chemically reacted in a heated ripening acid to form a certain charge of TiO ²⁺ and SO ₄ ^2- , which adsorb on the surface of the hydrated titanium dioxide to make it positively charged and become insoluble. Dilute colloidal solution (seed).

Ti(OH) ₄ +H ₂ SO ₄ →TiOSO ₄ +3H ₂ O

TiOSO ₄ ===TiO ²⁺ +SO ₄ ^2-

Ti(OH) ₃ in the solution does not participate in the acid-soluble reaction during the ripening acid dissolution process because the acidity of the solution is not high. The colloidal seed solution after heating and aging should be cooled to room temperature immediately after use if it is not used immediately. [next]

When the seed crystal is hydrolyzed, the titanium liquid generally does not need to be preheated in advance, but the time of the seed crystal addition and the temperature at the time of the addition are specified, and the temperature of the titanium liquid to be hydrolyzed is usually heated to the vicinity of the acid temperature of the seed crystal, or slightly higher than the acid. It is better to add seed crystals at the solution temperature. The addition amount of the seed crystals is 0.6%~2% according to the TiO ₂ according to the product variety and the hydrolysis process. There are rarely more than 5% of the examples, and the unqualified crystal seeds are rather discarded. Can not be used.

Since the addition of the seed crystal hydrolysis method requires strict concentration of the titanium liquid during the hydrolysis, the concentration of the titanium liquid to be hydrolyzed is generally maintained at about 200 g/L after the seed crystal is added, so that the product quality and the hydrolysis rate of the hydrolysis are relatively good. In order to avoid the water brought in by the direct steam to make the concentration of the titanium liquid thin, most of the additional seed crystals are heated by indirect steam.

The discoloration of titanium liquid by the addition of seed crystal hydrolysis method is not so obvious that it is hydrolyzed by the seed crystal dilution method. It mainly controls the temperature of the titanium liquid to be boiled after the seed crystal is added. It should not be too long, generally not more than 1 h, so the design of the hydrolysis tank coil is usually designed. Take full account of this feature when heating the area. It is the same as the hydrolysis of the authigenic seed crystal dilution method. At the end of the hydrolysis, it is also diluted with hot water to increase the hydrolysis rate.

Since the addition of the seed crystal hydrolysis method involves the dispersion of the seed crystal into the titanium liquid as much as possible, and most of them are heated by inquiring; therefore, the stirring speed of the seeding method is faster than that of the autogenous seed crystal hydrolysis method. some. After the end of the hydrolysis to cool the discharge, the hydrolysis tank should also be rinsed with clean water, and the unloaded metatitanic acid should not remain in the tank.

The quality of the hydrolyzed product is detected after the end of the hydrolysis. From the viewpoint of preparation of the hydrated titanium dioxide particles, the particle size and particle size distribution of the hydrated titanium oxide should be determined by electron microscopic observation and particle size analyzer, but these instruments are not only expensive but also expensive. The test time is long, and the following methods are generally used in industrial production to identify the quality of the hydrolysis operation:

a. The hydrolysis rate, that is, the ratio of the titanium dioxide in the liquid phase (titanium liquid) before the hydrolysis to the solid phase titanium dioxide, expressed as a percentage, which is an indicator for checking the degree of completion of hydrolysis, has important economic value, and the general hydrolysis rate should not be low. At 95%. When tests are water slurry of metatitanic acid solution before the titanium solution and hydrolysis of the 1, wherein the content of TiO ₂ were determined or directly measured after hydrolysis mother liquor (spent acid hydrolysis) in TiO ₂ content.

b. Particle sedimentation rate This is a method for indirectly observing the size of hydrated titanium dioxide particles after hydrolysis. Usually, 100 mL of the hydrolyzed slurry is taken, diluted in a 1000 mL measuring cylinder to 1000 mL with water, shaken and allowed to settle for 0.5 h. The scale at which the solid-liquid boundary is measured can be calculated in milliliters (or millimeters).

c. Filtration speed This method can not only indirectly estimate the particle size and particle uniformity of hydrated titanium dioxide after hydrolysis, but also can directly grasp the difficulty of the subsequent washing operation by the filtration speed, usually after taking a certain amount of hydrolyzed slurry. Filter in the funnel and measure the time after draining, in seconds.

The above three methods are suitable for various hydrolysis processes.

(3) Preparation method of seed crystal

a. Preparation of anatase seed crystal

Take a certain amount of titanium liquid (control the unconcentrated titanium liquid after filtration), the content of titanium dioxide is about 130~170g/L, the content of trivalent titanium is about 2~5g/L, and the dilute lye of concentration of 100g/L left stone is used. (NaOH), while maintaining the neutralization temperature not exceeding 45 ° C, well, and with good agitation, slowly add to the diluted titanium solution for neutralization, the speed of lye addition is fast and slow, at the end of the neutralization The pH value is 2~3, the acidity coefficient is 0.26~0.30, and the TiO ₂ concentration is 50~60g/L. Samples should be sampled and pre-analyzed near the end point. If the above indicators are not met, the ratio of water, titanium solution and alkali can be adjusted, and then used. The indirect steam was heated to 60 ° C in 10 min, and the solution was aged for 30 min to form a blue colloidal colloidal seed solution, which was then quenched to room temperature for use.

Another anatase type seed crystal is prepared by using a concentrated titanium solution before hydrolysis, diluting to 30 g/L with demineralized water, neutralizing with a concentration of 100 g/L of alkali solution (NaOH) under stirring, and controlling the whole process. and time within 30min, and slow down after holding and the temperature 60 ~ 65 ℃, and the final pH is 3.3 ~ 3.7, TiO ₂ concentration of 18 ~ 22g / L, and then rapidly cooled to 30 deg.] C or less for use. The method is characterized in that the temperature at the time of neutralization is in the acid-soluble temperature range, the appearance of the seed crystal is turbid, and the concentration of TiO ₂ is low. According to the general principle of colloidal chemistry, when the electrolyte concentration is lowered, the stability of the colloidal solution is increased, and the seed activity is improved due to the decrease in acidity. However, too much seed crystal concentration will increase too much, which will reduce the concentration of titanium solution at the beginning of hydrolysis. Therefore, this method is mainly suitable for the application of additional seed hydrolysis method with high concentration of titanium solution (215~230g/L) before hydrolysis. Otherwise, the initial concentration of hydrolysis of 200 g/L cannot be guaranteed. [next]

Recently, many factories have used a kind of alkali-neutralized anatase type seed crystal called "fast seed crystal". The preparation method is that the purified titanium liquid is quickly added to the lye of 80-100 g/L at a time under stirring, and the temperature during the neutralization period is below 50 ° C, and the acidity coefficient at the neutralization end point is 0.42 to 0.50. Then, the acid solution was heated at a rate of 2 ° C / min, but it was found that the ortho-titanate particles had been clearly peptized and clarified, kept for 5 min, then increased to 70 ° C at a rate of 2 ° C / min, and quenched after 15 min of heat preservation. spare. The method is characterized by the acidity coefficient of the neutralization end point. It is not measured by the test paper or the acidity meter during the neutralization period, but the amount of the titanium liquid and the alkali liquid is added in advance, so it is relatively simple and can be operated. A portion of the desalinated water is reserved for addition to the seed crystal during quenching, which can be used for dilution water and to help cool down. Calculated as follows:

Where v _titanium - the volume of titanium liquid required to prepare the amount of seed crystals as needed, L;

v _crystal - the amount of seed crystals to be formulated, L;

c _Titanium - TiO2 concentration of titanium solution for preparing seed crystals, g/L;

c _crystal - seed crystal concentration (according to 30g / L);

v _alkali - the volume of the lye used, L;

c _{titanic acid} - an effective acid content of a titanium solution for preparing a seed crystal, g / L;

c _{crystal acid} - effective acid content of the seed crystal (g / L, in 0.48);

c _alkali - the dissolved alkali concentration has been dissolved, g / L;

v' _water - the amount of water required to dilute the lye to 100 g / L, L;

v _water - the amount of cooling water required to be added after quenching in acid solution, L;

0.815 - acid-base neutralization constant (N _NaOH / N _H2SO4 = 40 / 49);

0.9 - empirical constant.

This seed crystal can be added when the titanium liquid is boiled.

Preparation b. below. rutile seed

Parallel seed crystal: This co-current seed crystal has been widely used in the production of rutile titanium dioxide in China. It is prepared by neutralizing the calculated clear titanium solution and dilute alkali solution (Na ₂ CO ₃ , NaOH) by a cocurrent method, maintaining the pH of 3.8 to 4.5 throughout the neutralization period, and then removing the orthotitanic acid by water washing. The sulfate ion in the medium is heated and acidified with hydrochloric acid to convert the orthotitanic acid into a sol. When the solution becomes transparent and the turbidity is opalescent, the heating is stopped and quenched and used. The activity of this kind of seed crystal is relatively high, and it is relatively stable, but the pH control during neutralization is very strict. It is time-consuming and troublesome to wash with orthotitanic acid. If the sulfate is not cleaned, the mixed crystal seed crystal is obtained. use.

Titanium tetrachloride seed crystal: Titanium tetrachloride seed crystal has been used in foreign countries in the past. The seed crystal has high activity. The preparation method is to dissolve titanium tetrachloride in water to prepare 475g/L tetrachloride. Titanium solution, sodium hydroxide is prepared into 67g/L alkali solution, and the calculated titanium tetrachloride solution and alkali solution are placed in a seed preparation tank (enamel tank), and partially neutralized under stirring, and retained. A part of excess hydrochloric acid is used for acid dissolution, the neutralization temperature is kept below 10 °C, the neutralization end point is controlled by an acidity coefficient of 0.7-0.8 (HCl/TiO ₂ ), and then heated to 80 ° C for 30 minutes for acid-solution ripening, followed by quenching. Stand by to room temperature. This method is very troublesome because of the storage, transportation and dilution of titanium tetrachloride. Once a large amount of hydrogen chloride gas is spilled to pollute the environment, it is rarely used in China.

Calcined seed crystal: calcined seed crystal, also known as secondary seed crystal, is a kind of rutile seed crystal commonly used at present. The first two kinds of rutile crystal seeds belong to hydrolyzed seeds (added during hydrolysis). In the production of modern rutile titanium dioxide, it is not necessary to add rutile-type hydrolyzed seeds during hydrolysis, but ordinary anatase-type hydrolyzed seeds are used. The calcined seed crystals are then added during bleaching or salt treatment and are therefore also referred to as secondary seed crystals. [next]

The calcined seed crystal is prepared by boiling the alkaline solution of metatitanic acid and alkali (NaOH) which have passed the bleaching water washing at a high temperature, and the ratio of the two is NaOH:H ₂ TiO ₃ =2.3:1, the slurry of metatitanic acid The concentration is ≥300g/L, the concentration of alkali solution is ≥42%, the alkali solution temperature is 110~115°C, and the boiling point is kept for 4h, so that metatitanic acid forms sodium metatitadate. The reaction formula is as follows:

H ₂ TiO ₃ +2NaOH→Na ₂ TiO ₃ +2H ₂ O

The reactants are placed in a water washing tank at 60 ° C under the cooling of jacket cooling water to wash off the free base and sulfate. The removal of sodium ions and sulfate can increase the activity of the seed crystals. Will hydrolyze to form orthotitanic acid.

Then, it was neutralized with hydrochloric acid to pH 3.5, and all of Na ₂ TiO _{3 was} precipitated to form H ₄ TiO ₄ .

Then, the precipitate was washed twice with water, washed with chlorine and then acid-dissolved, and the acid solution temperature was 110 ° C. After boiling for 2 hours, it was quenched to 40 ° C for use. The obtained calcined seed crystal concentration is 60~70g/L, the rutile type conversion rate is 98%~100%, and the electron microscope photo seed crystal is in a well-dispersed willow-shaped particle, and the calcined seed crystal is generally added in an amount of 2.5%~5%. .

The calcined seed crystal can also be prepared by using titanium tetrachloride, which is lower in temperature than the calcined seed crystal made of metatitanic acid during calcination, but the operation is complicated, and the transportation and transportation of titanium tetrachloride is difficult. Titanic acid is a raw material, and it can be directly prepared by using semi-finished products in production.

(4) Precautions when preparing seed crystals

a. neutralization operation

The neutralization reaction of titanyl sulfate and sodium hydroxide is an exothermic reaction. Therefore, it is necessary to control the rate and time of alkali addition, to avoid excessive temperature during neutralization, and local thermal reaction of sulfuric acid and orthotitanic acid to form metatitanic acid. The activity of the seed crystal, the reaction formula is as follows:

However, the neutralization temperature should not be too low (less than 30 ° C), too low will affect the dispersion of the alkali to make the reaction uneven, and even localized to neutralize and reduce product quality. Because of the excessive neutralization, the iron in the titanium solution will precipitate iron hydroxide and contaminate the product. The reaction formula is as follows:

FeSO ₄ +2NaOH→Fe(OH) ₂ ↓+Na ₂ SO ₄
Therefore, the pH at the neutralization end point generally does not exceed 4, and if the degree of neutralization is insufficient, the number of crystal nuclei in the produced seed crystal is reduced, and the hydrolysis rate is lowered due to insufficient number of crystal centers at the time of hydrolysis, and the hydrolyzed particles are fine and difficult. Washed.

The pH at the neutralization end directly affects the temperature and time of acid dissolution. When the pH is low, the precipitation of orthotitanic acid is completed in an acidic medium, and some TiO ₂ ²⁺ is adsorbed on the surface of the precipitate in a free state. It has a positive charge, and it consumes less hydrochloric acid when it is acid-soluble, and the seed crystal has good stability. If the pH is too high during neutralization, the precipitate is completed under neutral conditions (pH=7), the particles are not charged, and the TiO ₂ ²⁺ ions will combine with OH ^- ions to prevent the absence of TiO ₂ ²⁺ in the solution. It has low stability and poor activity, and consumes more hydrochloric acid when it is dissolved.

In most cases, the method of adding alkali to the titanium solution is used for neutralization, because when the titanium liquid is used as a dispersed phase, iron in the titanium liquid can be prevented from entering the crystal lattice. The stirring speed during neutralization is also very important, generally controlled at about 60r/min. Too intense agitation will reduce the stability of the seed crystal. [next]

b. Acid solution operation

The ortho-titanate precipitate formed by neutralization is an amorphous dispersion, and only an acid solution can produce anatase or rutile seed crystals. The unacidified orthotitanic acid has a tendency to be dissolved by the free acid in the titanium liquid, and the storage time is too long to change the structure thereof, and the colloidal particles are microcrystallized by heating the acid to form a colloid which is insoluble in dilute acid. The granules cannot be separated by ordinary filtration methods. The colloidal solution has a slight opalescence without precipitation, and the end point of the acid dissolution can be judged from the appearance of the opalescence during the operation.

The temperature of acid dissolution has a certain dependence on the time of acid dissolution. Generally, the temperature is high and the time is short; while the temperature is low, the time is long. Since the thermal stability of the seed crystal is worse than that of the titanium liquid, the temperature at the time of acid dissolution cannot be high, and the time cannot be long. Too high a temperature and an excessively long time cause hydrolysis of the seed crystal to lower the activity.

Quenching after acid dissolution is important because the colloidal titanium dioxide content at this temperature is the highest and extremely unstable, and it must be cooled quickly to ensure its activity and stability.

When a strong acid (HCl, HNO ₃ , HF, etc.) of a monovalent anion is used for acid dissolution, the hydrated titanium oxide adsorbs a monovalent anion, and since the radius of the monovalent anion is small, it does not hinder the anatase type crystallite to the rutile crystal form. Conversion, so the rutile seed crystals are produced. The divalent or higher anions (SO ₄ ^2- , PO ₄ ^3-, etc.) hinder the conversion of anatase to rutile. In the preparation of anatase seed, the acid used in acid dissolution is an effective acid in titanium. (H ₂ SO ₄ ), so the seed crystal thus obtained is anatase type.

C. Concentration of finished seed crystals

Many research data have shown that the content of colloidal titanium dioxide in the seed crystal is the main indicator of the activity of the seed crystal, but the higher the content of colloidal titanium dioxide in the seed crystal, the more unstable it is. The test proves that when the concentration of the titanium dioxide in the seed crystal is 84.8g/L, the storage is carried out. After 72 hours, turbidity occurred. When the seed crystal concentration was diluted 1 times (42.4 g/L), turbidity occurred after 148 hours. However, if the concentration of the seed crystal is too low, the concentration of the hydrolyzed titanium liquid will be diluted. Generally, the TiO ₂ concentration of the rutile type seed crystal is relatively high, so that the particles of the hydrated titanium oxide during the hydrolysis can be finer, which is advantageous for the color reduction power and hiding power of the product. The anatase type crystal seed does not have to increase its concentration because of its fine crystal center. The reason why the parallel flow crystal has good stability is because its orthotitanic acid is washed before acid dissolution, and the purity of orthotitanic acid is relatively high, so the stability is good.

The quality of the seed crystal depends mainly on the activity, stability and uniformity of the seed crystal. There is no good analytical method at present. Potentiometric titration is used to determine the amount of polymerized ions and the degree of polymerization to indicate the level of active titanium dioxide in the solution. It is believed that the crystal center of the seed crystal is hydrolyzed by hydroxyl bridge (OH ^- ) and oxygen bridge (O ^{2 ). -} ) to induce hydrolysis, so the ratio of the hydroxyl group or the ratio of the hydroxy group and the oxo group [(OH ^- ) / (TiO ₂ ²⁺ )] can be used to indicate the activity of the seed crystal, but above The method has not been formally adopted in industrial production. The only way to ensure the quality of the seed crystals is to carefully and meticulously operate according to the process requirements.

d. Storage of seed crystals

The stability of a seed crystal (hydrolyzed seed crystal) is not very good, generally only can be stored for 24h, so the factory operation is now available, preferably 1 time, not allowed to use the remaining part and the next A batch of seed crystals are mixed together, and the secondary seed crystal (calcined seed crystal) has good stability and can be stored for a long time without deterioration.