Method for producing wet-process phosphoric acid and by-producing alpha-hemihydrate gypsum and high-purity and high-whiteness alpha-hemihydrate gypsum

A method for producing wet-process phosphoric acid and at the same time obtaining an alpha-hemihydrate gypsum I as well as an alpha-hemihydrate gypsum II as by-products, comprising the following steps:

• (1) mixing phosphoric acid and phosphate rock powder, performing an extraction reaction under stirring;
• after completion of the extraction reaction, adding sulfuric acid solution, continuing the reaction under stirring to obtain an extraction slurry;
• separating solid and liquid of the extraction slurry to obtain a phosphoric acid extraction solution A and a slag slurry B;
• wherein the amount of the sulfuric acid solution added is controlled, so that 10% to 50% of calcium ions are transformed into dihydrate gypsum and the other 50% to 90% of calcium ions exist in a form of calcium dihydrogen phosphate;
• (2) mixing the phosphoric acid extraction solution A obtained in step (1) and the sulfuric acid solution, performing a decalcification reaction;
• after completion of the reaction, separating solid and liquid to obtain a solid C and a filtrate D;
• (3) performing a crystal transformation reaction with the solid C obtained in step (2), part of the filtrate D obtained in step (2), a sulfuric acid solution and a crystal transformation agent;
• after completion of the reaction, separating solid and liquid to obtain the alpha-hemihydrate gypsum II as well as a filtrate F;
• (4) performing crystal transformation reaction with the slag slurry B obtained in step (1), part of the filtrate D obtained in step (2), a sulfuric acid solution and a crystal transformation agent;
• after completion of the reaction, separating solid and liquid to obtain the alpha-hemihydrate gypsum I and a filtrate J,
• wherein a purity of the alpha-hemihydrate gypsum II is higher than that of the alpha-hemihydrate gypsum I.