Synergistic extraction of molybdenum and tungsten from low-grade tungsten-molybdenum ores with poor flotation separability using a sodium hypochlorite-trisodium phosphate-sodium hydroxide system

The mineral composition of a low-grade tungsten-molybdenum ore (LGTMO) in Henan, China is very complex with a fine distribution size. Molybdenite, scheelite, powellite (CaMo_xW_1-xO₄) in the ore cannot be separated by flotation. In this study, tungsten and molybdenum were extracted from LGTMO by hydrometallurgy with using a mixture of sodium hypochlorite (NaClO), trisodium phosphate (Na₃PO₄) and sodium hydroxide (NaOH). Through single-factor experiments, the highest molybdenum and tungsten leaching efficiencies of 91.14 % ± 1.30 % and 83.12 % ± 1.03 % were attained under the following conditions: a milling time of 20 min, a stirring speed of 350 r/min, a NaClO concentration of 0.072 mol/L, a Na₃PO₄ concentration of 0.15 mol/L, a NaOH concentration of 0.070 mol/L, a liquid–solid ratio of 3:1 at 80 °C for 240 min. Subsequently, the concentrations of NaOH, NaClO and Na₃PO₄ were optimized by the response surface methodology. The model predicted that the concentrations of NaOH, NaClO and Na₃PO₄ under the optimal experimental conditions were 0.070, 0.079 and 0.173 mol/L and the corresponding leaching efficiencies of Mo and WO₃ were 94.84 % and 86.74 %, respectively. Finally, SEM-EDS and XPS analyses were conducted to characterize the leaching residue. The formation of hydroxyapatite is the main reason for the incomplete leaching of tungsten and molybdenum. It covers the surface of the ore and thus limits the leaching of tungsten and molybdenum. This study provided an alternative for the efficient extraction of tungsten and molybdenum from LGTMO at lower reagent costs and temperatures.