Sodium Carbonate-Roasting-Aqueous-Leaching Method to Process Flot-Grade Scheelite-Sulfide Tungsten Ore Concentrates

Abstract

Tungsten, the strongest natural metal on earth, has two main economic minerals: Wolframite (Fe,Mn)WO₄ and scheelite (CaWO₄). Approximately two-thirds of the world’s tungsten reserves consist of scheelite deposits. Scheelite high grade ore concentrate is expected to contain more than 50% W. However, low-grade (flot grade) scheelite concentrates with W-content as low as 11% are also reported. During the scheelite ore concentration process at the mine site, two grades of ore concentrates are prepared. These are known as high grade ore concentrate containing 55–60% tungsten and flot grade ore concentrate containing 12–36% tungsten. Most APT (Ammonium Para Tungstate) producing plants prefer to process only high-grade ore concentrates. Scheelite in flot grade ore concentrates is diluted with the presence of non-tungsten minerals such as phlogopite: KMg₃(Si₃Al)O₁₀(OH)₂; talc: Mg₃Si₄O₁₀(OH)₂; calcite: CaCO₃; sulfide phases such as chalcopyrite: CuFeS₂; and pyrrhotite: Fe_1–xS; and calcium hydroxy apatite: Ca₅(PO₄)₃(OH). These concentrates also contain flotation-chemicals, especially fatty acids which are not acceptable to the APT plants due to their suspected interference in the LIX (liquid ion exchange) step of the APT process. In fact, the conventional LIX process is not suitable to process flot grade scheelite ore concentrate without removing flotation chemicals and sulfide phases by heat treatment, an additional step that needs to be performed either at a third-party site, at the APT plant site or the mine site. In this paper, we propose an economic method that uses high and/or low temperature roasting of scheelite-sulfide flot grade ore concentrate in sodium carbonate. Tungsten from roasted scheelite flot concentrate is extracted in water, leaving a residue that could be disposed of or used in other applications. Sodium carbonate roasting experiments were conducted with 100 g of scheelite-sulfide flot grade ore concentrate in a furnace at the temperature 650–900 °C for 1–15 h. No oxidant, such as NaNO₃, was used. The method is optimized for the amount of sodium carbonate, roasting temperature, and time as well as corrosion of the calciner tube metal. The amount of sodium carbonate used was 30–45% less than the amount of ore concentrates used in the roasting. Aqueous leaching of roasted concentrate resulted in concentrated filtrates containing 78 to120 g per liter tungsten, which can be economically processed for APT production (using LIX). The method aligns with circular hydrometallurgy for processing flot grade scheelite-sulfide ore concentrate, as it has one fewer step and uses cheaper Na₂CO₃ in place of NaOH.