Tungsten recovery from alkaline leach solutions as synthetic scheelite

The recovery of tungsten from alkaline leach solutions has been studied examining the effect of temperature, pH, Ca/WO3 molar ratio and nature of the precipitated calcium tungstate. The precipitation kinetics of calcium tungstate, upon the addition of aqueous sodium tungstate to calcium solutions, was followed by potentiometric measurements using a calcium ion-selective electrode. Two models, a crystal growth model and a second-order reaction opposed by zero-order reaction, have been used to test the experimental data. Both models show that the apparent activation energy of CaWO4 precipitation falls in the range 58 to 67 kJ mol(-1). The kinetic data shows that the maximum recovery of precipitated calcium tungstate occurs at pH >= 8.5 with a 10% excess of CaCl2 at 50 degrees C over a period of 20 min using sodium tungstate solutions of 100 g L-1 and 150 g L-1 WO3. (C) 2006 Elsevier B.V. All rights reserved.