Dissolution behavior of calcium tungstate in oxalic acid solutions

Scheelite mineral (CaWO4) is the main raw material used in tungsten production. In this study the dissolution of synthetically prepared CaWO4 in oxalic acid (H2C2O4) solutions was studied. The effect of following parameters on the dissolution of reaction was investigated: stirring speed, temperature, H2C2O4 concentration and particle size. The amounts of tungsten leached were followed by using Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) instrument. X-ray Powder Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analytical techniques were used to characterize the leach residues. The results obtained show that the dissolution reaction takes place in two steps: first, the intermediate calcium aqua oxalato tungstate (Ca[WO3(C2O4)H2O]) chelate compound was obtained. Second, this compound reacted with H2C2O4 to form water soluble hydrogen aqua oxalato tungstate (H2[WO3(C2O4)H2O]) and solid CaC2O4H2O. CaC2O4H2O formed covered the surfaces of unreacted CaWO4 particles as a protective layer and the dissolution reaction was interrupted after a certain conversion at high temperatures and H2C2O4 concentrations.