DISSOLUTION CHARACTERISTICS OF SCHEELITE IN HCL-C2H5OH-H2O AND HCL-C2H5OH SOLUTIONS

The reaction of synthetic scheelite with hydrochloric acid, HCI-C2H5OH and HCl-C2H5OH-H2O (with varying proportions of H2O) solutions was studied at 70-degrees-C between HCI concentrations of 4 M and 5 M. Although the reaction product was solid tungstic acid when hydrochloric acid was used as a leachant, this behaviour was observed to change with a decrease in the amount of water in the reaction media. Complete dissolution was achieved with HCI-C2H5OH-H2O (25% H2O, v/v) and HCI-C2H5OH solutions. The same dissolution behaviour, with the formation of negligible or minor amounts of solid tungstic acid was observed in the leaching of Swedish scheelite (71.4% WO3) and upgraded Turkish scheelite (65.18% WO3) concentrates. Leach solutions were observed to be stable when hot and at room temperature in the case of HCI-C2H5OH leaching and only stable when hot in the case of HCl-C2H5OH-H2O leaching. Selective precipitation of tungsten from solution was possible by dilution with water at a ratio of 1:1. The reaction rate was controlled chemically rather than by diffusion in HCI-alcohol solutions. This great change in the dissolution behaviour was attributed to the prevention of hydrolysis, due to the replacement of water in the media by ethyl alcohol, which is a better solvator for chloride ions and has the proper-ty of forming hydrogen bonds.