Production of pure tungsten oxide from scheelite concentrates

Abstract

Scheelite concentrates, obtained from Uludag, Bursa (Turkey), containing roughly 50% W, are treated by the chelate‐added acidic leaching route. Dissolved tungsten ions are then selectively precipitated by the addition of ‘ammonia ion‐containing’ reagents – either organic or inorganic – and thus are purified from other ions present in the solution. Selective precipitation of pure (> 99%) tungstate salt was realized from the polytungstate solution at 25°C by adding a ‘chelate‐breaking agent’ at a NH4+/WO3 weight ratio of ≥ 1/4. The formula of the complex salt is indicated by (NH4)3PW12O40·9H2O. The optimum precipitation conditions are investigated with the aim of making pure tungsten oxide via a thermal breakdown process. Tri‐ammonium‐dodeka‐tungstate‐phosphate salt can be easily converted to nanometer‐sized WO3 particles by a simple calcination operation carried out above 650°C. Weight loss ratios of the samples vary between 8.7 and 9.6% at the end of this 2‐h thermal breakdown process. Tungsten trioxide obtained can be suitably employed in tungsten‐related powder metallurgy applications with its extremely low impurity content of 0.04% Mo, 0.15% SiO2, 0.06% Fe and 0.03% Al.