To obtain high whiteness, good performance pigment titanium white powder, titanium dioxide must minimize the impurities, these impurities affect not only the whiteness of titanium dioxide into the crystal lattice during calcination also produces photosensitive phenomenon, extended washing time and rely The purpose of thorough purification is not achieved, so when producing pigment grade titanium dioxide or high-purity titanium dioxide, it is necessary to bleach the metatitanic acid after washing.
Bleaching is actually a reduction process using a reducing agent to metatitanic acid ferric ion metal hydroxide and impurities all reduced to low state, so that they can be re-dissolved in water, is removed by washing with water.
The bleaching operation is carried out in an acidic medium because the hydroxide precipitate is first decomposed before reduction to form a soluble state and then subjected to reduction bleaching. For example, iron hydroxide in metatitanic acid is first reacted with sulfuric acid to form high-sulfur sulfate. Participate in the reduction reaction.
2Fe(OH) 3 +3H 2 SO 4 →Fe 2 (SO 4 ) 3 +6H 2 O
Fe(OH) 2 +H 2 SO 4 →FeSO 4 +2H 2 O
Industrial bleaching methods for metatitanic acid are as follows.
(1) Zinc powder bleaching
This is the earliest method for the bleaching of metatitanic acid. Zinc reacts with sulfuric acid to form hydrogen. The new ecological hydrogen is a strong reducing agent; the high-iron sulfate is reduced to ferrous sulfate, but when the reducing agent is excessive, some of the meta-titanic acid is also reduced. It is trivalent titanium and exhibits lavender. Its chemical reaction formula is as follows:
Zn+H 2 SO 4 →ZnSO 4 +2[H]
Fe 2 (SO 4 ) 3 +2[H]→2FeSO 4 +H 2 SO 4
H 2 TiO 3 +2H 2 SO 4 →Ti(SO 4 ) 2 +3H 2 O
2Ti(SO 4 ) 2 +2[H]→Ti 2 (SO 4 ) 3 +H 2 SO 4
Zinc powder bleaching is generally carried out in an acid-resistant container of an enamel reaction tank or a steel-lined porcelain plate. First, the metatitanic acid is pumped into the bleaching tank, a certain amount of process water is added, and the concentration of the TiO 2 is adjusted to 200-220 g/L, and then Add sulfuric acid (preferably sulfuric acid for battery with low impurity content) under stirring to make the concentration of sulfuric acid in the slurry reach 60~80g/L, then heat it by steam (jacket heating, coil heating, direct steam heating) , if the copper heating coil, it is possible that the divalent copper is reduced to copper, copper oxide upon calcination and black product contamination reduction in the bleaching process, the reaction is as follows:
When the temperature is heated to 60~70 °C, Zn 2 0.5%~1% (mass) of zinc powder is added. The zinc powder can be added into the slurry by water and added in fractions. If the addition speed is too fast, the zinc powder reacts too hard with sulfuric acid. The generated hydrogen generates a large amount of foam, which not only wastes the reducing agent, but also causes a pan accident. After continuing to raise the temperature to 90 ° C, the temperature is kept for 2 h, and after cooling, the material is floated and washed with water. The bleached titanic acid slurry should contain trivalent titanium (calculated as TiO 2 ) of 0.3 to 0.5 g/L. qualified.
In the production of rutile titanium dioxide, it is necessary to add a calcined seed crystal (secondary seed crystal), preferably before the bleaching, because in the preparation of the calcined seed crystal, the metatitanic acid and the base will be boiled in the stainless steel reactor for a long time. Metal ions such as iron and chromium are introduced, and if added before bleaching, the high-valent metal ions and hydroxides carried by the calcined seeds can be reduced to a low-cost state while being bleached, and then removed by washing with water.
Some factories take a long-term boiling method during bleaching, which is said to have a good effect on removing chromium and vanadium ions. It is reported that in order to remove residual copper, nitric acid or hydrogen peroxide can be added after iron washing. The copper is converted into soluble divalent copper for removal by washing with water; in order to remove calcium, magnesium and silicate ions, it can be treated with a basic iron compound after removing iron, and the pH is brought to 5-8, and then washed with water to remove.
(2) Aluminum powder bleaching
Bleaching and bleaching powder zinc aluminum principles, and chemical reaction during operation is completely the same, but the impurity content is less than aluminum zinc, some aluminum electrode potential between the iron and the reduction electrode ratio of zinc to iron The potential difference is larger, so it is easier to react. The acidity of the solution (the amount of acid used) and the amount of aluminum powder can be lower than that of zinc powder. Many factories have changed the zinc powder to aluminum powder. [next]
(3) Trivalent titanium bleaching
In the initial stage of zinc powder bleaching and aluminum powder bleaching reaction, zinc powder reacts with aluminum powder and sulfuric acid to form a new ecological hydrogen, which is a solid-liquid phase reaction. When the new ecological hydrogen is reduced with high-valent iron plasma in solution, it is a gas-liquid phase reaction. From the chemical reaction point of view, these two types of reactions are not easy to carry out completely, so the amount of reducing agent is more than the theoretical addition, but after adding more often there are residual unreacted zinc powder or aluminum powder Mixed into metatitanic acid, the quality of the product will be affected after calcination. Trivalent titanium bleaching is a liquid-liquid phase reaction, and the reaction can be carried out relatively completely. There is no zinc powder or aluminum powder mixed into the product, and the amount and acid addition amount and reaction temperature can be lower than that of zinc powder or aluminum powder. Its chemical reaction formula is as follows:
Fe 2 (SO 4 ) 3 +Ti 2 (SO 4 ) 3 →2Ti(SO 4 ) 2 +2FeSO 4
The preparation and use of the trivalent titanium solution is as follows: the titanic acid after the water washing with a concentration of about 180 g/L is heated in an acid-resistant enamel reaction tank at a ratio of H 2 SO 4 :TiO 2 =5:1. Acid soluble. When the acid is dissolved, it is stirred under stirring and heated to boiling. The boiling point of the evaporated slurry gradually increases with boiling. When the temperature reaches 120-150 ° C, the meta-titanic acid begins to dissolve, and the stirring is continued until the solution turns into brownish brown. After the titanium sulphate solution, the heating and cooling are stopped and diluted with water to make the concentration of TiO 2 be about 50-70 g/L.
When the temperature drops to 75~80 °C, add the aluminum powder which has been adjusted into slurry beforehand to carry out the reduction operation. The addition of the aluminum powder slurry should be very careful and slow, preferably added in fractions of 20~30min, otherwise it will be intense due to the reaction. In the event of a pan accident, the reduction reaction is as follows:
3H 2 SO 4 +2A1→A1 2 (SO 4 ) 3 +6[H]
Ti(SO 4 ) 2 +TiOSO 4 +2[H] →Ti 2 (SO 4 ) 3 +2H 2 O
The amount of aluminum powder for reduction was 1.5 times of the theoretical amount. After the addition of the aluminum powder, the temperature was further increased to 90 ° C, and the temperature was kept for 1 hour. After cooling, the concentration of trivalent titanium in the solution was analyzed to calculate the reduction rate. The reduction rate must not be less than 90%.
The prepared trivalent titanium solution should be used after filtration to prevent unreacted aluminum powder and unacidified metatitanic acid from being carried into the trivalent titanium solution. The storage time of the solution should not be too long, and it is best to use it within 48 hours, because trivalent titanium is a strong reducing agent, and it will be oxidized by oxygen in the air during storage to reduce the reduction effect.
Although trivalent titanium bleaching has many advantages over zinc powder or aluminum powder bleaching, the preparation process is complicated, the energy consumption during preparation is high, the acid consumption is large, and the cost is high.
(4) rinsing
The rinsing operation is the same as the water washing before bleaching, except that the water quality during rinsing is very strict. It is best to use ion-exchanged water to prevent the impurities in the water from contaminating the product again. The content of Fe 2 O 3 in the meta-titanic acid after rinsing is controlled at 30X10. About -6 , titanium dioxide with excellent whiteness can be obtained.
Sometimes the content of trivalent titanium cannot be measured in the metatitanic acid after bleaching. The main reason may be that the content of ferric iron in the metatitanic acid is too high, and the amount of the zinc powder, the aluminum powder or the trivalent titanium solution added is insufficient. Reduce them to divalent iron, or zinc powder, aluminum powder storage time is too long or the moisture surface is passivated. In addition, when the trivalent titanium solution is used, the storage time of the trivalent titanium solution is too long, the concentration of the trivalent titanium becomes low, and the amount added is not enough according to the original calculation, or the amount of sulfuric acid added is too small, the bleaching temperature is too low, and the time is fast. Re-oxidation of the trivalent titanium in the agitated slurry may result in the inability to detect trivalent titanium ions.
However, the content of trivalent titanium in the metatitanic acid after bleaching should not be too high or too low. If the trivalent titanium is still contained at the end of the rinsing, the product will be grayed out when calcined; if trivalent If the titanium content is too low, there is no guarantee that trivalent titanium will inhibit the reoxidation of ferrous iron to ferric iron during rinsing. If there is no trivalent titanium, there is a certain amount of ferric iron in the metatitanic acid. Reduced to divalent iron.
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