Production of pure tungsten oxide from scheelite concentrates

Scheelite concentrates, obtained from Uludag, Bursa (Turkey), containing roughly 50% W, are treated by the chelate-added acidic leaching route. Dissolved tungsten runs 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.