Leaching of synthetic scheelite by nitric acid without the formation of tungstic acid

The reaction kinetics for the leaching of synthetic scheelite with nitric acid was studied to establish the effect of pH and temperature in the formation of tungstic acid. The yield of the leaching reaction in the temperature range of 30-90 degreesC and at pH between 1.5 and 3.0 is analyzed in terms of prevailing tungsten species present in the leaches. The results show that tungstic acid can be avoided during the digestion of synthetic scheelite in the acidic medium by monitoring the pH and controlling the temperature in the domain of metatungstates. In this case, the experimental results match those of the shrinking-core model and show that the solids leaching is under both chemical and mass-transfer control.