Solvometallurgical Process for the Recovery of Tungsten from Scheelite

Abstract

The industrial state-of-the-art processes to extract tungsten from scheelite (CaWO4) require high pressures and temperatures. These flowsheets consume a large excess of chemicals (which are very hard to recycle) and generate up to 25 tons of high-salinity wastewater per ton of ammonium paratungstate product. In this work, a more sustainable conceptual flowsheet for the recovery of tungsten from high-grade (55.0 wt % W) and medium-grade (3.3 wt % W) scheelite ores was developed at the lab scale. Leaching of CaWO4 was tested in solutions of 37.0 wt % aqueous HCl in organic solvents (ethylene glycol, poly(ethylene glycol) 200, acetonitrile) and ionic liquids (Aliquat 336). The tungstate (HxWyOzn–) generated by the reaction between CaWO4 and HCl was only solubilized in the ethylene glycol system with an appropriate amount of 37.0 wt % aqueous HCl, whereas in all other solvents, it either precipitated or CaWO4 did not dissolve. After the dissolution of tungsten, nonaqueous solvent extraction was used to separate tungsten from calcium, by means of a solvent consisting of 20 vol % Aliquat 336 in the aliphatic diluent GS190 with 10 vol % 1-decanol as a modifier. Scrubbing with water removed the co-extracted iron. Finally, tungsten was recovered as ammonium tungstate (NH4)2WO4, the precursor of ammonium paratungstate, by stripping in a mixture of aqueous ammonia and ammonium chloride. Paratungstate is the most common intermediate for the production of tungsten oxide or tungsten metal. One drawback that needs to be adequately addressed, prior to further upscaling of this conceptual flowsheet, is the potential formation of trace levels of 2-chloroethanol in the leaching stage. It is hypothesized that this problem can be circumvented by further optimizing process conditions to enhance the mass transfer and reduce the reaction time, such as better mixing of the solid and the lixiviant.