Effect of hydrogen on nickel extraction from low-grade ultramafic nickel sulfide concentrate

Low-grade ultramafic nickel sulfide ores represent an important but underutilized resource for nickel production. However, their high MgO content poses significant challenges to conventional smelting processes, limiting their economic and environmental viability. The authors proposed a solid-state nickel extraction method, employing metallic iron as a nickel extractant under inert or H2–Ar atmospheres, to avoid high-temperature smelting and enable direct extraction of nickel as ferronickel. Although hydrogen was present in the atmosphere, the authors observed the loss of iron into iron-magnesium oxides and silicates. The aim of the current work is to understand the mechanism of iron oxidation and the effect of atmosphere on it. It was found that hydrogen helps create a low oxygen potential environment around the sample (logPO2 = −22 to −17 at 750–920 °C), but oxygen-buffering minerals in the ultramafic concentrate create local oxidizing conditions, leading to the oxidation of iron. Kinetic analysis reveals that hydrogen reduction of iron oxide in the complex compounds with MgO and sulfides is hindered by high activation energy barriers (Ea = 240 kJ/mol). These findings highlight critical factors affecting the efficiency of hydrogen-based reductio in complex mineral matrices.