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

A novel route for the synthesis of complex metal sulfides using hydrothermal coupled dissolution−reprecipitation 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 (