Efficient extraction of tungsten, calcium, and phosphorus from low-grade scheelite concentrate

Abstract

• Scheelite and apatite in low-grade scheelite were digested completely by HNO 3 . • Tungsten was leached in the form of soluble phosphotungstic acid. • Calcium was leached and recovered in the form of gypsum whiskers. • The excess phosphorus can be recovered in the form of nitrophosphate fertilizer . In the traditional process for tungsten recovery from scheelite concentrate, phosphorus is regarded as a harmful impurity that should be strictly controlled. Moreover, the calcium that entered the resideue as Ca(OH) 2 or CaCO 3 was discarded or landfilled. To date, the residues from the traditional process have been listed as solid hazardous waste in China. A novel method was investigated to realize the comprehensive recovery of tungsten, phosphorus, and calcium. In the leaching process, 96% of scheelite (CaWO 4 ) and 99% of apatite (Ca 5 (PO 4 ) 3 F) were digested under the conditions of 3 M HNO 3 , at a liquid/solid ratio of 10:1, a leaching temperature of 90 °C, and leaching time of 2 h. After extracting tungsten from the leaching solution by solvent extraction, about 70% of Ca 2+ in the raffinate was recovered as calcium sulfate whiskers by adding H 2 SO 4 . Subsequently, the solution from the calcium removal process was reused to leach scheelite after adding the consumed HNO 3 . When the P 2 O 5 concentration in the circulating leaching solution was over 30%, it was discharged and neutralized by NH 3 ·H 2 O, and then used to produce a nitrophosphate fertilizer by spray granulation and drying processes.