Sodium hydroxide digestion of scheelite by reactive extrusion

According to the thermodynamic analysis, scheelite can be decomposed as long as the NaOH concentration is high enough, even at low temperatures. As for the approaches of increasing NaOH concentration, it is more economical to reduce the water dosage at fixed NaOH dosage than to increase the NaOH dosage at a fixed ratio of liquid to solid. But the viscosity of slurry will be increased with the increase of the solid to liquid ratios, which will make the mass transfer more difficult if the conventional stirring equipment is still employed. Twin screw extruder is a kind of equipment which is suitable to work with highly viscous macromolecular materials. Good mass transfer can be easily obtained in this process. Therefore, it is considerable to apply this equipment to treat the highly viscous scheelite slurry. Based on these, a novel technology is proposed to digest scheelite with sodium hydroxide. Under the conditions of temperature 120 °C, screw rotating speed 160 rpm, reactive time 3.5 h and the NaOH dosage 2.2 times to stoichiometry, the content of WO3 in the residue and the WO3 extraction are 1.54% and 99.18%, respectively. The results indicate that scheelite can be decomposed with sodium hydroxide by reactive extrusion at a lower temperature.

Research highlights
► A novel technology of sodium hydroxide digestion of scheelite by reactive extrusion is investigated.
► Alkali dosage is 2.2 times to stoichiometry, temperature at 120 °C, screw rotating speed at 160 rpm and reaction time for 3.5 h.
► Under the conditions, the content of WO3 in the residue and WO3 extraction are 1.54% and 99.18%, respectively.