Effect of Adding Phosphoric Acid to Titanium Dioxide
In the production of electrode grade titanium dioxide, due to the high phosphorus content of titanium dioxide, it is cold and brittle to the weld, so the phosphorus content of titanium dioxide is required not to exceed 0.05%. In this way, phosphoric acid cannot be added in production, and strict Control, especially high phosphorus content is not allowed in the ore source. However, in the production of anatase titanium dioxide pigments, not only is there no strict phosphorus requirement, but phosphoric acid or other salts must be added to improve the whiteness and weather resistance of titanium dioxide. However, too much phosphorus in titanium ore is not good, because phosphorus exists in the form of phosphoric acid or dihydrogen phosphate after acidolysis, and it is easy to combine with titanium to form the corresponding insoluble titanium salt. As the residue is removed, titanium recovery occurs. rate decreased.
Based on the fact that in the production of anatase titanium dioxide, the pH value of metatitanic acid is still between 2 and 3 after washing or bleaching, and ferrous sulfate has not yet reached its hydrolysis pH value of 6.5, and still exists as ferrous ions. Some manufacturers do not have a bleaching process. Part of the ferrous iron is oxidized by dissolved oxygen in water during washing to become ferric iron. When the ferric iron is washed to a pH value of 1.5, it will be hydrolyzed to form ferric hydroxide precipitate, which is mixed in partial titanium sour. If these divalent iron and iron hydroxide impurities are directly sent to calcination without treatment, reddish-brown iron oxide will be formed under calcination conditions, which will greatly affect the whiteness of titanium dioxide. Therefore, salt treatment must be carried out before calcination. Add appropriate amount of phosphoric acid or other salts in the salt treatment to make phosphoric acid react with ferric hydroxide to form light yellow stable iron phosphate; react phosphoric acid with divalent iron to form white or off-white Ferrous phosphate monobasic or dibasic salts, these divalent iron phosphates are also oxidized into light yellow ferric phosphate under calcination conditions. Since the light yellow iron phosphate is far lighter than the reddish-brown iron oxide, the effect on the whiteness of titanium dioxide is much smaller, so as to achieve the purpose of improving the whiteness of titanium dioxide.
