Thermodynamic analysis for the separation of tungsten and aluminium in alkaline medium using solvent extraction

Abstract

The thermodynamic equilibrium diagrams for distribution of species at different pH values of tungsten and aluminium with the systems of W-H2O and Al-H2O at 298K were established, separately. The effects of the concentrations of W, Al and Na2CO3 in feed, and the equilibrium pH on the extractions of W and Al were investigated. Thermodynamic analysis results revealed that the tungsten and aluminium exist as WO42− and Al(OH)4−, respectively, in a solution containing 0.5mol/L [W]T and 0.1mol/L [Al]T in the pH range of 11.5–14.0. The decrease in the pH range to 4.0–11.5 resulted in the precipitation of aluminium as Al(OH)3. The extractant of quaternary ammonium salt has stronger affinity with WO42− than that with Al(OH)4−. The solvent extraction of tungsten in alkaline medium using the extractant with the HCO3− form would result in the decrease of the raffinate pH and the formation of Al(OH)3 precipitate at pH <11.5, causing the emulsion in the solvent extraction process. By controlling the raffinate pH higher than 11.5, the extractions of tungsten and aluminium were 99.2% and 1.2%, respectively after a 12-stage counter-current extraction, resulting in a separation factor βAlWO3>9800. This indicates that the efficient separation of tungsten and aluminium can be achieved in by direct solvent extraction in alkaline medium.