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HomeIndustry InformationProduction of Titanium Dioxide by Sulfuric Acid Method--Blanching of Metatitanic Acid (I)

Production of Titanium Dioxide by Sulfuric Acid Method--Blanching of Metatitanic Acid (I)

2019-09-25

First, the reason for bleaching partial titanate role and production of rutile titanium dioxide pigment must be bleached
Traces of iron impurities in metatitanic acid after washing are in the form of solid ferric hydroxide, and the content of Fe 2 0 3 is more than 0.009%. For the production of titanium dioxide for coatings, it will seriously affect the whiteness of titanium dioxide. After being bleached, the Fe 2 O 3 content can be reduced to less than 0.003%, and the harmful impurities such as chromium , copper and vanadium adsorbed in the metatitanic acid can be removed, so that the prepared whiteness and gloss of the finished product are remarkable. improve.
Because trace iron in metatitanic acid is calcined at high temperature, red Fe 2 0 3 is formed , and Fe 2 0 3 has the same crystal structure as rutile titanium dioxide, and is often mixed with eutectic crystal lattice in the same crystal mixture. The lattice deformation is caused, so that the influence of rutile titanium dioxide on iron impurities is much more sensitive than the effect of anatase titanium dioxide on iron impurities. When the Fe 2 O 3 content is 0.003%, the rutile titanium dioxide has been rendered yellow; and the Fe 2 0 3 content is 0.009%, which begins to affect the whiteness of the anatase titanium dioxide. The influence of iron impurities is not only the red color of Fe 2 0 3 itself, but more importantly, Fe 2 0 3 enters the Ti0 2 lattice, causing lattice defects and forming a color activation point, thereby improving the light absorption energy of the product. It causes a decrease in whiteness and achromatic power, which affects the optical properties of titanium dioxide. Therefore, when producing rutile coating titanium dioxide, it must be bleached. By bleaching, the high-iron hydroxide is removed, and after calcination, Fe 2 0 3 having the same crystal structure as the rutile-type titanium dioxide is not produced, so that the whiteness and the achromatic power of the titanium white powder are not lowered.
2. Method for producing trace iron impurities in metatitanic acid after washing and removing method thereof
Traces of iron impurities in metatitanic acid after washing are produced in two ways.
1. Since the hydrolysis of ferrous sulfate is carried out under the condition of pH>6.5, the acidity in the early stage of the water washing process is large, and the direct hydrolysis of ferrous sulfate to form precipitates of ferrous hydroxide is impossible. However, as the washing continues, the trivalent titanium ions remaining in the metatitanic acid filter cake are quickly washed away, but the ferrous ion content in the filter cake is still relatively high, and the ferrous ions are gradually washed in the water. Oxygen or tap water disinfection chlorine gas is oxidized into high-iron ions. As the water washes, the Sulfuric Acid content in the filter cake is less and less, and the pH value is getting higher and higher. When the pH value is greater than 1.5, the high-iron ion is hydrolyzed. , the formation of high-pressure iron hydroxide precipitate. Its reaction formula is as follows:

2. In some factories, the tap water used for washing titanic acid is ferrous sulfate as a water purifying agent. First, the ferrous ions are oxidized into high-iron ions, and then the pH value is adjusted. The high-iron ions are hydrolyzed to form a large amount of flocculent high-iron precipitates and co-precipitate with suspended impurities such as sediment in the water to achieve the purpose of purifying water. In this way, although most of the high-iron has precipitated, a small amount of iron hydroxide particles can float in the water. During the washing process, the particles accumulate on the surface of the white metatitanic acid filter cake, and finally cover a layer of brownish yellow iron. .
Traces of iron impurities in metatitanic acid after washing are solid, high-iron, water-insoluble, and it is impossible to continue washing the water with water, which must be removed by bleaching. Bleaching is the first reaction of solid ferric hydroxide with sulfuric acid, conversion to soluble ferric sulfate, and then reduction to a low-cost ferrous sulfate with a chemically active metal or metal ion, which is finally removed by washing with water. The relevant reaction formula is as follows: [next]


Third, the reasons why bleaching cannot be carried out in copper equipment
Bleaching cannot be done in copper equipment. Because copper equipment will form a layer of basic copper carbonate on the surface of moist air, commonly known as patina. The patina is also dissolved in sulfuric acid during the acid-dissolving process of bleaching to form copper sulfate. During the reduction of bleaching, copper sulphate is reduced to copper powder. The solid copper powder entrained in metatitanic acid cannot be removed during the washing process. When calcined, the copper powder forms black copper oxide, which causes new pollution to the titanium dioxide and loses the bleaching effect. Its relevant reaction formula is as follows:

Fourth, the method of bleaching metatitanic acid
Partial titanate bleaching method can be divided into: ① zinc powder bleach; ② aluminum powder bleach; ③ bleaching trivalent titanium; ④ bleaching power (bleaching power allows Fe 2 0 3 content is less than 0.01%, and the consumption of H 2 S0 4 Low amount, no zinc powder and aluminum powder, and can be continuous and automated); 5 in the titanic acid slurry, first add ionic surfactant, then add metal powder as reducing agent for bleaching, this bleaching can make The contents of Fe, Cr, and V are not more than 0.002%. Aluminum powder bleaching and trivalent titanium bleaching are commonly used in the industry. Among them, trivalent titanium bleaching is better because the trivalent titanium solution bleaching has less sulfuric acid, less reducing agent, lower operating temperature, shorter bleaching time, no residue pollution, compared with zinc powder or aluminum powder bleaching. The bleaching effect is good and so on. However, the preparation of the trivalent titanium solution requires an increase in equipment and an increase in the amount of preparation work, and the preparation process is complicated, the energy consumption during preparation is high, the acid consumption is large, and the cost is high. In order to overcome these shortcomings, a factory has made improvements (the improvement method is detailed later).
Five, zinc powder bleaching steps and shortcomings
1. Acid-soluble in a enamel reaction pot with a stirred jacket heating, add water to wash the qualified meta-titanic acid and water, stir and adjust the slurry concentration between 180-220g / L (if used as rutile titanium dioxide) In order to promote the conversion of the crystal form and to make the product have better pigment properties, it is necessary to add 2.5%-5.0% of the calcined seed crystal. Because of the preparation of the calcined seed crystal, the metatitanic acid and the alkali solution are in the stainless steel reactor. Boiling for a long time, and later adding Hydrochloric Acid for acid dissolution, there will be metal ions such as iron and chromium. If you add calcined seeds before bleaching, you can bring the calcined seeds at the same time as the metatitanic acid bleaching. The high-priced metal ions and hydroxides are reduced to a low-cost state and then removed by washing with water). Then, a certain amount of industrial sulfuric acid is added to make the sulfuric acid concentration of the slurry between 65-80 g/L, and heated to 70-75 ° C by steam, so that the high-iron hydroxide is dissolved in sulfuric acid to form a high-iron sulfate solution. At the same time, a part of metatitanic acid is dissolved to form a titanyl sulfate solution. Its reaction formula is as follows: [next]

2. Reduction Add 0.5%-1.0% reducing agent to Ti0 2 . The reducing agent must be an industrial zinc powder with good activity. Some zinc powders that have been stored for a long time and agglomerated have surface passivation to reduce the activity, which will affect the bleaching effect and should not be used. Zinc powder is a solid powder. In order to uniformly disperse the zinc powder in the ferric acid slurry, the dry zinc powder should be firstly pulverized with water and then added in portions. Since the zinc powder can not only react with the high-sulfur sulfate in the slurry, but also reacts with the sulfuric acid in the slurry to release hydrogen, a large amount of foam is generated in the slurry, so that it is necessary to slowly add people to avoid the reaction being too intense and causing the slurry. Splash or pan. After the addition of the zinc powder, the heating is continued, and the temperature is maintained at 85-90 ° C for 1.5-2 h, so that all the high-sulfur sulfate is reduced to ferrous sulfate, and a small amount of titanium is also reduced to ferric iron, so that the slurry is light. purple. The sample was measured to determine the trivalent titanium content (calculated as Ti0 2 ) in the slurry from 0.3 to 0.5 g/L. After the heat preservation, the cooling water is changed to cool the material to below 45 °C. Its relevant reaction formula is as follows:

3. Washing The bleached metatitanic acid slurry is reduced and filtered by suction with a vacuum filter or a vacuum filter equipped with a vacuum vacuum. Wash the metatitanic acid with sand or deionized water (preferably with 0.1%-2.0% acidic water at 40-60 ° C), and further remove the ferrous sulfate to reduce the iron content until the meta-titanic acid A medium Fe 2 0 3 content of less than 0.003% is acceptable. Since the material itself has a low iron content, the washing time is short, generally not exceeding 10 hours. Some manufacturers take a long-term boiling method during bleaching, which is said to have a good effect on removing chromium and vanadium ions.
The disadvantages of zinc bleaching are as follows: In addition to the above-mentioned large amount of sulfuric acid, a large amount of reducing agent, high operating temperature, long bleaching time, and poor bleaching effect, residue contamination is also a disadvantage. Because the zinc bleaching process is a liquid-solid reaction, the reducing agent zinc powder permeates the inside of the titanic acid particles for a long time, and the effect is also poor. If the quality of the zinc powder is poor, too much is added or the control conditions are not proper, so that some of the zinc powder remains in the metatitanic acid, which often makes the finished paint have poor gloss. Therefore, the bleaching of metatitanic acid has been replaced by bleaching of aluminum powder and bleaching of trivalent titanium salt solution. Aluminum powder bleaching is commonly used in the introduction of technology. [next]
Sixth, the operation steps and requirements of aluminum powder bleaching
The principle and operation steps of aluminum powder bleaching are basically the same as zinc powder bleaching.
1. Acid-dissolved in a enamel reaction pot with a stirred jacket heating, add water-washed meta-titanic acid and water, stir and adjust the slurry concentration between 180-220g / L (if rutile titanium dioxide, this Adding a satin-sintered seed crystal), then adding a certain amount of industrial sulfuric acid (31% in terms of Ti0 2 ) with stirring, so that the sulfuric acid concentration of the slurry is between 65-80 g/L, and heated to 70-75 by steam. At ° C, iron hydroxide is dissolved in sulfuric acid to form a high-iron sulfate solution. At the same time, a part of metatitanic acid is dissolved to form titanium sulfate. Its reaction formula is as follows:

2. Reduction According to Ti0 2 , add 0.5%-1.0% aluminum powder. The aluminum powder is a solid powder. In order to uniformly disperse the aluminum powder in the metatitanic acid, the aluminum powder should be firstly pulverized with water and then added in portions. Since the aluminum powder can react with the high-iron iron in the slurry and the sulfuric acid of the slurry to release hydrogen, a large amount of bubbles are generated in the slurry, so that it is necessary to slowly add people to avoid the reaction being too intense and causing the slurry. Splash or pan. After the aluminum powder is added, the heating is continued, and the temperature is maintained at 80 to 90 ° C for 1.5 to 2 hours, so that all the high-sulfur sulfate is reduced to ferrous sulfate, and the titanium sulfate is reduced to ferric iron to make the slurry lavender. The sample was measured to determine the trivalent titanium content of 0. 3-0. 5g / L (in terms of Ti0 2 ), that is, the end of the reduction. After the end of the heat preservation, the cooling water is turned off to cool the material to below 45 °C. Its reaction formula is as follows: