Leaching kinetics of scheelite with nitric acid and phosphoric acid

• The leaching kinetics of scheelite with H3PO4 in HNO3 solutions was studied.
• The controlling kinetic model of the reaction process was established.
• HNO3/H3PO4 utilized as leachant media eliminate the impact of solid film of H2WO4.
• The reaction rate was increased compared to the traditional HNO3 leaching process.
• Scheelite could be completely decomposed in this leaching system.

The extraction of tungsten from scheelite concentrate using phosphoric acid as chelating agent in nitric acid solutions was investigated. The effects of the leaching parameters, such as stirring speed, temperature, concentrations of HNO3 and H3PO4 as well as particle size were examined in order to model the kinetics of scheelite dissolution. It was determined that the leaching rate was practically independent of stirring speed and H3PO4 concentration at an investigated range, while increased with increasing temperature and HNO3 concentration. The leaching data agreed quite well with the shrinking core model, with surface chemical reaction as the rate-controlling step. The apparent activation energy was calculated as 52.91 kJ/mol, and the reaction order with respect to HNO3 concentrations was determined to be 0.23. The leaching process using the shrinking sphere model was established as:1−1−x1/3=3.2×102CHNO30.23r0−1exp−52910RT.