Novel route to synthesize complex metal sulfides: Hydrothermal coupled dissolution-reprecipitation replacement reactions

Abstract

A novel route for the synthesis of complex metal sulfides using hydrothermal coupled dissolutionreprecipitation reactions is reported. Two thiospinels, (Ni,Fe)₃S₄ (violarite) and Co₃S₄ (linnaeite), were synthesized using (Fe,Ni)₉S₈ (pentlandite) and Co₉S₈ (cobaltpentlandite) as precursors. The Fe/Ni ratio of (Ni,Fe)₃S₄ can be adjusted by varying the reaction conditions, for example, temperature (125–145 °C), pH (2.90, 3.90, 5.00), and precursor stoichiometry ((FexNi1-x)₉S₈, x = 0.4, 0.5, 0.55, 0.6). Pure (Ni,Fe)₃S₄ can be synthesized by utilizing a flow-through hydrothermal cell rather than a static hydrothermal cell as the fluid flow improves mass transfer and flushes away the Fe₂O₃ byproduct from the reaction front. Synthesis times range from 10 to 20 days, compared to the traditional dry synthesis route for (Ni,Fe)₃S₄ that requires 3 months annealing to obtain a product of only 72 ± 3 wt % purity. This synthesis route is ideal for preparing compounds with low thermal stabilities (<500 °C).