Acidic leaching of scheelite concentrate and production of hetero-poly-tungstate salt

The aim of this study is to investigate the utility of adding a chelating agent during hydrochloric acid leaching of low-grade scheelite concentrates obtained from Uludag, Bursa Turkey. Examined parameters included leaching conditions, selective precipitation of tungsten salt from the acidic leach solution in which tungsten exists as hetero-poly-tungstate, optimum precipitation conditions, and the possibility of obtaining pure and nano-sized tungsten salt. The incorporation of a chelating agent in the leaching process reduces the number of steps compared to classical methods and facilitates the production of pure tungsten oxide without generating any spent liquor. The optimum leaching conditions identified are 80°C, 2 M HCl, a solid-liquid ratio of 1:10, a stirring speed of 900 min⁻¹, and a weight ratio of tungsten to phosphate of 7:1. Under these conditions, a tungsten leaching efficiency of 98% was achieved. Additionally, it was discovered that a pure tungstate salt (over 99%) could be selectively precipitated within minutes from the poly-tungstate solution at 25°C by adding a 'chelate breaking agent' at a weight ratio of ammonium to tungsten of greater than 1:4.