Manufacture of hydrated titanium dioxide---hydrolysis of titanium liquid

Hydrated titanium dioxide manufacturing, typically includes titanium dioxide hydrate hydrolysis and purification of liquid titanium. Hydrolysis of titanium can be used as part of the hydrolysis of salts. The most common understanding of the hydrolysis reaction of a salt is to treat it as a reverse reaction of the neutralization reaction, namely:

However, due to the different ions of the constituent salts, the salt and the water will produce a weak electrolyte surface to change the pH of the solution. For example, when the weak acid strong alkali salt sodium acetate is dissolved in water, the hydrolysis reaction causes the solution to be alkaline:

CH ₃ COONa+H ₂ O===CH ₃ COOH+NaOH(Na ⁺ , OH ^- )

The strong acid weak base salt, such as ammonium chloride dissolved in water, the hydrolysis reaction occurs to make the solution acidic:

NH ₄ Cl+H ₂ O===NH ₄ OH+HCl(H ⁺ , Cl ^- )

It can be seen from the above salt hydrolysis reaction that the pH value is the main factor affecting the hydrolysis of the salt. After the salt hydrolysis reaches equilibrium, the change of the concentration of H ⁺ and OH ^{- in the} solution can destroy the equilibrium of the hydrolysis reaction and make the reaction to the left. Or proceed to the right. For strong acid and weak base salt, the hydrolysis rate is higher in the case of alkaline solution. If the acidity of the solution is increased at this time, the hydrolysis reaction cannot be carried out; also for the strong base weak acid salt, the hydrolysis rate is in the case where the solution is acidic. Will be high.

Table 1 pH of partial metal hydroxide precipitation

Table 1 is a partial metal hydroxide precipitate when the pH value, due to the different salts have different pH value of the hydrolysis, the titanium sulphate solution is dissolved in the other soluble metal impurities, such as iron (Fe ^2+, Fe ^{3 +} ), aluminum (A1 ³⁺ ), manganese (Mn ²⁺ ), copper (Cu ²⁺ ), lead (pb ²⁺ ), nickel (Ni ²⁺ ), chromium (Cr ³⁺ ), cobalt (Co ^{2+ )} ), cerium (Ce ⁴⁺ ), etc., their pH values ​​are higher when hydrolyzed, such as the pH value of 4.5 to 7 when Fe ²⁺ starts to hydrolyze and precipitate, and the pH value of 8.6 to 10.8 when Mn ^{2+ is} hydrolyzed and precipitated only when hydrolysis is achieved. At pH, their hydroxide precipitates are produced. In the production process of titanium dioxide by sulfuric acid method, the hydrolysis of titanium liquid is carried out under the condition of high acidity, and free acid is continuously generated during the hydrolysis process, so the above metal impurity ions are inhibited by acidity in the solution without occurrence. Hydrolyzed and precipitated, so that not only the hydrated titanium dioxide can be precipitated when the titanium liquid is hydrolyzed, but other impurity ions remain in the mother liquid, so that the precipitated hydrated titanium dioxide can be well separated from the soluble impurities in the titanium liquid.

In addition to the pH value, the factors affecting the hydrolysis of the salt are of course the temperature and concentration of the solution. The acid and alkali neutralization reaction is an exothermic reaction, and the hydrolysis of the reverse reaction salt is definitely an endothermic reaction, which increases the reaction temperature during hydrolysis, and the reaction can move in the direction of hydrolysis, and the hydrolysis will be more complete. The degree of hydrolysis of the salt is inversely proportional to the square root of the salt concentration. Therefore, lowering the salt concentration is beneficial to the ionization of the salt, and can increase its hydrolysis rate and degree of hydrolysis.

However, the hydrolysis of titanium liquid has the sameities and differences in the hydrolysis of general salts. The most obvious difference is that the hydrolysis of the donor liquid does not have a fixed pH value, as long as it is heated or diluted, no additional The reactants can resolve the precipitation of hydrated titanium dioxide by water, and even in the case of high acidity (H ₂ SO ₄ 400-500 g/L), precipitation of hydrated titanium dioxide can be produced after prolonged boiling. Due to the nature of hydrolysis of titanium liquid due to heating and dilution, the process of leaching, filtration washing, concentration, etc. in the preparation of titanium liquid not only does not have too high operating temperature, but also avoids dilution with water as much as possible to avoid titanium. Early hydrolysis of the liquid. [next]

Hydrolysis of titanium solution with water at room temperature produces a titanic acid or orthotitanic acid (H ₄ TiO ₄ ) such as colloidal hydroxide precipitation. This composition can also be written as Ti(OH) ₄ , which is equivalent to the dihydrate of titanium dioxide. ——TiO ₂ ·2H ₂ O, which is soluble in organic acid, dilute mineral acid and titanium solution. The solution has obvious colloidal characteristics. If the solution is aged or heated, the colloidal characteristics will be lost, and the precipitate a titanium The acid is also converted to beta titanate (meta-titanate), which is only soluble in hot concentrated sulfuric acid. Its chemical reaction formula is as follows:

If the titanium liquid is directly heated and kept boiling, a white precipitate of metatitanic acid (H ₂ TiO ₃ ) can be generated by hydrolysis, and the composition of the hydrate can also be written as TiO(OH) ₂ , which is close to the monohydrate of titanium dioxide ( TiO ₂ ·H ₂ O), which is the only method in the industry for the preparation of metatitanic acid by hydrolysis of titanium liquid. The chemical reaction formula is as follows:

X-ray diffraction analysis of the above hydrolyzed product showed that orthotitanic acid is an amorphous compound, and metatitanic acid has a less pronounced crystal structure, which is identical to the crystal structure of anatase titanium dioxide. Since boiling and dilution can promote the hydrolysis of titanium liquid, it is often used in industrial production. Usually, the product of the hydrolysis of titanium sulfate is anatase type titanium dioxide, and the titanium halide or nitrate is hydrolyzed to obtain rutile type titanium dioxide.

The hydrated titanium dioxide can be obtained from the titanium salt, and the alkali precipitation method can also be used. This is an early method for preparing titanium hydroxide from the titanium liquid, and the chemical reaction formula is as follows:

TiOSO ₄ +Na ₂ CO ₃ +2H ₂ O→Ti(OH) ₄ +Na ₂ SO ₄ +CO ₂

TiOSO ₄ +Na ₂ CO ₃ +H ₂ O→TiO(OH) ₂ +Na ₂ SO ₄ +CO ₂

There are many reports on the reaction mechanism of titanium hydrothermal hydrolysis, but the representative ones are H ⁺ transfer and colloidal coagulation during hydrolysis. It is generally considered that the gelation process is mainly in the titanium liquid with a low F value and a high total titanium content, and the reaction between ions is mainly in the titanium liquid with a high F value and a low total titanium.

Since titanium is located in Group IVB of the periodic table, its ionic radius of the positive tetravalent ion is very small, so tetravalent titanium is difficult to exist in simple ionic form in aqueous solution, but forms a complex with water to hydrate the complex ion. The form exists, usually a 6-coordinated hydration complex ion [Ti(H ₂ O) ₆ ] ⁴⁺ . The first step in hydrolysis is to remove 1 H ⁺ from a water molecule, thus forming a A complex ion composed of 5 water molecules and 1 OH ^- reduces the charge of titanium. OH ^- plays the role of "bridge".

As the acidity in the solution gradually increases, a more stable "oxygen bridge" is formed due to the continued transfer of H ⁺ on the "hydroxy bridge" complex of titanium. This transfer of H ⁺ forms a multinuclear complex as the hydrolysis process continues.

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The sequentially formed multinuclear complex is in the form of a lock or a network of colloidal particles, and finally aggregates into large particles, which can be precipitated when the agglomerated particles reach about 10 μm. Some scholars believe that this multi-nuclear complex with oxygen as a chain bridge actually has the following long chain structure in solution.

As the thermal hydrolysis progresses, the chain length becomes longer and longer, and under the action of heating and stirring, they are entangled with each other to cause aggregation and precipitation. This agglomeration can be carried out even at a higher acidity, and the above-mentioned coagulation and precipitation processes are continuously repeated to continue the hydrolysis reaction until most of the titanium ions are hydrolyzed to form a hydrated titanium oxide colloid which is separated and separated from the mother liquor.

Baricsdahle of the United States discusses this in his book "The Discovery of Titanium and Its Chemistry and Technology": The final precipitated product of hydrolysis is a white floc cluster of 10~200μm size, which is directly determined by its size. The filtration and washing speed of the hydrolysate metatitanic acid; it is independent of the performance, it is formed by agglomerating particles of 0.6~0.7μm, and the primary aggregated particles are the base particles that determine the performance of the pigment. It consists of about 1000 microcrystals of 60-75mμm, and each microcrystal of 60-75mμm is a network structure composed of 10 atoms arranged linearly.

The views of the former Soviet Union, Bielki and Lisgen, in their book "Pigment Chemistry and Technology" are similar to those of the American Barxtel. They believe that a metastable anatase is formed before the hydrolysis begins. Microcrystals, 3~10nm in diameter, 20~30 such microcrystals are aligned and arranged into colloidal particles, which determine the size of the titanium dioxide particles. The colloidal particles are in a sheet-like structure with a length of 45-90 nm and a thickness. About 0.25 nm. As the hydrolysis proceeds, the colloidal particles accelerate to agglomerate into primary aggregated particles of 0.55 to 0.75 μm, which determine the basic properties of the pigment particles, and have a specific surface area of ​​about 60 to 70 m ² /g, so that a large amount of water and sulfate ions can be adsorbed. The orthotitanic acid adsorbs about 0.3 mole of sulfate per mole of titanium, and the metatitanic acid adsorbs about 0.1 mole of sulfate (SO ₄ ^2- ) per mole of titanium, so the composition of metatitanic acid is approximately 10H ₂ TiO ₃ ·SO ₃ .

Regardless of the hydrolysis mechanism, the hydrolysis process is always completed in the following three stages.

(1) Formation of the crystallization center (phase of formation of crystal nuclei), which is the smallest particle that can be detected, which cannot be broken and can only be dissolved, and its size depends mainly on the concentration of the seed crystal;

(2) The growth of crystal nucleus and the stage where hydrated titanium dioxide begins to precipitate, the nucleus grows to form a primary aggregate, and the size of the aggregate depends on the hydrolysis condition, which directly affects the performance of the pigment and can be broken by chemical and mechanical forces;

(3) The stage of agglomeration and sedimentation of hydrated titanium dioxide and the composition of the precipitate. At this time, the size of the agglomerated particles affects the filtration and washing performance of metatitanic acid, and has little effect on the properties of the pigment.

The first stage is the nucleation stage. The hydrolysis begins by first depositing a very fine crystallization center called nucleus from the clarified titanium solution. The number, nature, structure and composition of the nucleus are the properties of the final hydrolyzate. And the composition laid the foundation.

In a metastable titanium solution with poor stability, a part of such a very fine colloidal nucleus is formed during storage before hydrolysis, except that the amount thereof is insufficient to induce a hydrolysis reaction in the short term as a crystallization center, but these The presence of a poor crystallization center can seriously affect the performance of the hydrolyzed product, which is also detrimental to the quality of the finished titanium dioxide. Therefore, when the "fresh" titanium liquid is hydrolyzed together with the "old" titanium liquid with poor stability, the composition of the precipitate is different, so the bad crystal nucleus (including the residue after the hydrolysis) is removed before hydrolysis. product). In order to properly guide the hydrolysis process, a certain number of crystal nucleuses having a certain composition structure must be provided in the solution as a crystallization center. These controlled crystallization centers can be produced by heating in a certain program (self-produced seeds), or artificially preparing a batch of crystal nucleus and placing them in the titanium solution to be hydrolyzed (plus Seed crystals. Since the number and composition of this part of the crystal nucleus are often not fixed, the titanium liquid having the same chemical composition is sometimes hydrolyzed under exactly the same conditions, and different hydrolyzates are obtained due to the difference in seed crystals. If hydrolysis is the core of titanium dioxide production, the formation of crystal nuclei is the most important part of the hydrolysis process.

In the second stage, that is, the growth stage of the particles, titanium is gradually precipitated into hydrated titanium dioxide particles in the form of hydrated titanium dioxide, but it is not enough to be precipitated. This stage is just found to be discolored when hydrolyzed. At this stage, the chemical composition of the solution does not change. The composition of this material is constant over a wide range of concentrations of TiO ₂ and H ₂ SO ₄ , but this nucleus is hydrolyzed by the use of additional seed crystals. The stage of growth is not obvious when the seed crystals are hydrolyzed.

In the third stage, the hydrated titanium dioxide particles gradually aggregate and grow up, and the size and dispersion degree of these aggregated particles have a great influence on the subsequent washing operation. In this stage, due to the precipitation of solid metatitanic acid particles from the solution, the equilibrium of hydrolysis in the original solution is broken, the hydrolysis is carried out at a large speed, and the titanium dioxide component in the liquid phase is continuously converted to solid phase titanium. The precipitation of acid, the concentration of titanium dioxide in the solution is continuously decreased, and the concentration of free acid is sharply increased. During this period, local dissolution of precipitated particles occurs simultaneously and a new precipitation process is re-precipitated until the hydrolysis process is completed.

The surface of the precipitated metatitanic acid adsorbs the mother liquor, that is, the free acid contained in the mother liquor, the ferrous sulfate, and other sulfates from which the metal impurities are separated. Fortunately, the thermal hydrolysis of the titanium liquid can be carried out at a higher acidity. These impurity ions do not precipitate under such high acidity and can be removed by washing in the future. However, when the ferric ion is hydrolyzed, the acidity is high (pH 2~3), and it is likely to precipitate together with the metatitanic acid during the hydrolysis. Therefore, the titanium liquid for hydrolysis in industrial production must contain a certain amount of trivalent. Titanium, the mother liquor after hydrolysis should also contain trace amounts of trivalent titanium, and the trivalent titanium content is generally controlled at 0.1~0.5g/L.

Hydrolysis is an extremely important process in the production of titanium dioxide. In the production of pigment-grade titanium dioxide, controlling the particle size of the precipitate and making the precipitated particles uniform is the key to the hydrolysis process. Because the operating conditions during hydrolysis basically determine the size of titanium dioxide microcrystals, hydrated titanium dioxide colloidal particles and the composition of metatitanic acid, although the salt hydrolysis reaction is reversible, the growth of particles in hydrolysis and release is irreversible, operation Improper rework can be remedy, ultimately affecting the operation of the post-process and the quality of the finished titanium dioxide.