Mechanochemically extracting tungsten through caustic processing of scheelite by controlling calcium dissolution

• Extract W from scheelite by milling with NaOH and avoiding calcium dissolution.
• Washing the ground product in soda solution gave 96% W recovery, with NaOH formed.
• Cogrinding of CaWO4, NaOH and Al(OH)3 can form Na2WO4 and katoite (Ca3Al2(OH)12).
• Wash the mixture of Na2WO4 and katoite in water, with 98% W and no NaOH recovered.

A novel process to extract tungsten from scheelite (CaWO4) was developed by dry grinding with sodium hydroxide (NaOH) and controlling the calcium dissolution based on two methods. One was to wash the ground product composed of sodium tungstate (Na2WO4) and calcium hydroxide (Ca(OH)2) in the solution of sodium salts instead of water. For example, W recovery was increased from 30% in water washing to 96% in a 0.1 M soda (Na2CO3) solution due to the formation of calcium carbonate (CaCO3). Another was to add Al(OH)3 further to the cogrinding of CaWO4 and NaOH to produce Na2WO4 and katoite (Ca3Al2(OH)12). Over 98% W recovery was simply obtained by only water washing the product due to the existence of insoluble Ca3Al2(OH)12. Grinding time and Al(OH)3 dosage were found to be key factors in affecting W recovery.