Method for extracting tungsten from scheelite

A method for extracting tungsten from scheelite, the method comprising the steps of:

• 1) adding a mixed acid comprising sulfuric acid and phosphoric acid into a decomposition reactor, wherein the sulfuric acid is present in the mixed acid at a concentration of 150-500 g/L and the phosphoric acid is present in the mixed acid at a concentration corresponding to 15-35 wt. % of P2O5;
• 2) heating the mixed acid to a temperature of 70-100° C.;
• adding scheelite to the decomposition reactor and controlling an amount of the mixed acid present in the decomposition reactor at 3-8 L per kg of scheelite;
• allowing components in the decomposition reactor to react for 1-6 hr, and filtering to obtain a filtrate;
• 3) supplementing the filtrate with sulfuric acid;
• 4) crystallizing the filtrate of step 3) to obtain phosphotungstic acid crystals and a mother liquor;
• 5) dissolving the phosphotungstic acid crystals in water to obtain a phosphotungstic acid solution;
• 6) transforming the phosphotungstic acid solution to an ammonium tungstate solution for preparing ammonium paratungstate (APT);
• and
• 7) supplementing the mother liquor of step 4) with phosphoric acid and water to obtain a diluted mother liquor comprising sulfuric acid and phosphoric acid at the same concentrations as those in step 1), and using the diluted mother liquor as the mixed acid in step 1) for leaching scheelite.