Comprehensive Extraction of Molybdenum from an Off-Grade Concentrate for the Production of Molybdenum Trioxide

Nitric–sulfuric acid leaching of substandard molybdenite concentrate from the Aktogay deposit (Kazakhstan) transfers only part of the molybdenum into solution: under the baseline process conditions, approximately 30% of the sulfide is oxidized, while roughly 70% of the molybdenum remains in the leach cake as unoxidized molybdenite. Molybdenum is thus distributed between two technologically dissimilar streams—the solution and the solid cake—and complete recovery requires processing of both. In the present work, both branches are combined into a single integrated scheme applied to a common feedstock. In the first (sorption) branch, molybdenum is concentrated from the solution. Among four anionites tested (Purolite A-100, Amberlite IRA-95, AV-17-8, and Lewatit M-800), Amberlite IRA-95 exhibited the best performance (sorption 84.21%, desorption with ammonia solution 82.15%, and selectivity coefficients Mo/Cu = 304.7 and Mo/Fe = 192.2). To remove iron, copper, and silicon impurities prior to concentration, a two-stage scheme is proposed incorporating preliminary impurity sorption on KU2-8 cationite (separation coefficients Mo/Cu = 5.7, Mo/Fe = 3.58, Mo/SiO2 = 2.17). In the second (roasting) branch, the cake is processed: the cake, granulated with molasses, is subjected to oxidative roasting at 600 °C for 150 min under an oxygen flow (2 dm3/min) until complete desulfurization is achieved, after which the calcine is leached with ammonia. Both streams converge into a single product: the ammoniacal desorbate from the sorption branch and the alkaline solution from the calcine are acidified with nitric acid to pH 2–3, ammonium paramolybdate is precipitated by evaporation, and the precipitate is calcined to molybdenum trioxide. The resulting MoO3, according to XRD data, is represented predominantly by orthorhombic α-MoO3 (molybdite). Overall, the proposed two-branch scheme closes the molybdenum material balance for the substandard concentrate from both sides and outlines a technically straightforward pathway for bringing stockpiled technogenic raw material into processing.