Third, the effect of temperature, concentration and acidity on salt hydrolysis
The relationship between salt hydrolysis is as follows:
1. Temperature Salt hydrolysis is the reverse reaction of acid-base neutralization reactions. If the neutralization reaction is an exothermic reaction, then the hydrolysis reaction must be an endothermic reaction. Therefore, increasing the temperature facilitates the reaction to proceed in the direction of the positive reaction and promotes the hydrolysis of the salt. For example, when washing oil stains, the hot soda ash water decontamination effect is better because heating promotes the hydrolysis of soda ash, increases the OH - concentration, and enhances the decontamination ability.
2. Concentration Increasing the water (that is, reducing the concentration of the salt contained in the solution) is beneficial to the ionization of the salt, facilitating the reaction to the positive reaction direction, and also promoting the hydrolysis of the salt. For example, when the stability of the titanium liquid is measured, a hydrolysis reaction occurs to a certain extent when water is added to a certain extent to form white orthotitanic acid.
3. Acidity For a strong base weak acid salt, the solution is alkaline. If the acidity is increased (or the alkalinity is lowered), the alkalinity of the solution is neutralized, and the reaction proceeds to the positive reaction direction, which is also beneficial for hydrolysis. If the alkalinity is increased (or the acidity is lowered), the concentration of the product (base) is increased, and the reaction proceeds in the reverse reaction direction, which inhibits hydrolysis. For the hydrolysis of strong acid and weak base salts, the solution is acidic. If the alkalinity (or the acidity is lowered), the acidity of the solution is neutralized, and the reaction proceeds to the positive reaction direction, which is favorable for hydrolysis; for example, increasing the acidity ( Or lowering the alkalinity), the concentration of the product (acid) is increased, and the reaction proceeds in the reverse reaction direction, which inhibits hydrolysis. For example, iron (Fe 3+ , Fe 2+ ), aluminum (A1 3+ ), lead (Pb 2+ ), manganese (Mn 2+ ), copper (Cu 2+ ), nickel (Ni 2+ ), vanadium ( salts thereof Vo 2+) and the like and metal sulfate salts belonging WEAK, such salts are not hydrolyzed at high acidity. These salts are present in the titanium liquid, but since the acidity of the titanium liquid is high, the metal ions contained in the titanium liquid during hydrolysis are inhibited by the acidity and are not hydrolyzed. Hydrolysis begins to form the corresponding hydroxide precipitate only when the acidity is reduced to a certain value. In the case of water washing with metatitanic acid, the acidity is continuously lowered as the water washing progresses. When it is lowered to a certain acidity, some impurity metal ions are hydrolyzed to precipitate and exist in the metatitanic acid, which contaminates the metatitanic acid. For example, when the acidity is lowered to pH ≥1.5, Fe 3+ is hydrolyzed to form iron hydroxide precipitate, and the reaction formula is as follows:
When the acidity continues to decrease, and the pH drops to ≥6.5, Fe 2+ is also hydrolyzed to form ferrous hydroxide precipitate. The reaction formula is as follows:
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Fourth, the commonality and characteristics of titanium liquid hydrolysis
Hydrolysis of titanium liquid has the commonality of general salt hydrolysis. For example, it belongs to strong acid weak base salt. When the pH value is 0.5, it starts to hydrolyze. The hydrolyzed solution is an acidic solution. It has colloidal properties and has its own hydrolysis. characteristic.
1. When the titanium liquid is diluted to a certain level at normal temperature, hydrolysis occurs to form a white floc ortho-titanate precipitate. Its reaction formula is as follows:
2. In the case of not diluting, it is kept in a boiling state, and even if the acidity is large, even if a new acid is produced by hydrolysis, hydrolysis can occur to form a white metatitanic acid precipitate. Its reaction formula is as follows:
The molecular formula of the produced hydrated titanium dioxide, that is, metatitanic acid, can be written as Ti0 2 ·H 2 0 or H 2 TiO 3 or Ti0(OH) 2 or Ti0 2 ·xH 2 O·yS0 3 .
Since the produced metatitanic acid is colloidal, the colloidal particles have a small surface area and adsorb a lot of impurities and sulfuric acid, so the metatitanic acid must be washed with water.
Fifth, the process of thermal hydrolysis of titanium liquid
The thermal hydrolysis of titanium liquid follows the following three processes.
1. Precipitating a tiny crystal center from a completely clarified titanium liquid - crystal nucleus Generally, a self-generated seed crystal of a dilution method is used to prepare a crystal nucleus or a crystal nucleus is first added and a titanium liquid is added as a crystal center. The unstable titanium liquid is used to contain some colloidal impurities before hydrolysis. These colloidal impurities are also nucleation or crystallization centers. These nucleuses do not meet the requirements and often have adverse effects. Therefore, these must be carried out before hydrolysis. The nucleus (ie, colloidal impurities) is removed.
2. Growth of crystal nuclei and formation of precipitates When the nucleation is formed, if the hydrolysis proceeds, the solidification of titanium occurs on the surface of the crystal nucleus, causing the crystal nuclei to grow gradually. When it reaches a considerable size, it becomes a precipitate. And precipitated.
3. The precipitation and the composition of the meta-titanic acid particles change. As the hydrolysis continues, the precipitate grows to a certain size, and then the solid phase particles are precipitated from the solution and become a suspension. At this time, the composition of the solution changes continuously. The content of Ti0 2 is continuously decreasing, while the concentration of free acid is continuously increasing. Further, the precipitated ferric acid particles are partially dissolved to reprecipitate the precipitate of the new composition. This process continues until the hydrolysis forms a dynamic equilibrium, leaving only a very small amount of titanium and concentrated sulfuric acid in the solution, at which point the composition of the precipitate is finally fixed.
6. Changes in trivalent titanium during the thermal hydrolysis of titanium liquid
During the thermal hydrolysis of titanium liquid, trivalent titanium does not undergo hydrolysis reaction and does not precipitate and remains in the mother liquor because the hydrolysis pH of trivalent titanium ions is close to 3, and the titanium liquid greatly exceeds this. acidity, even when the pH reaches 3, which is hydrolyzed to generate metatitanic acid is not generated instead Ti (OH) 3, the Ti (OH) 3 does not have colloidal properties. However, due to the agitation and boiling of the thermal hydrolysis process, a portion of the trivalent titanium is oxidized to tetravalent titanium by air and still hydrolyzes. Its reaction formula is as follows:
