Directional transformation of pyrite in sodium aluminate solution for desulfurization and the reduction of red mud based on electrochemical mechanism

The directional transformation of pyrite in the concentrated sodium aluminate solution at high temperatures will benefit the sulfur removal and the reduction of red mud, leading to green utilization of bauxite containing sulfur. After calculation of the activity coefficient through the Bromley model, $\phi$-$\rho \left(N{a}_{2}O\right)$ diagrams of Fe-S-H₂O were provided at 298 K and 533 K, respectively. Regulating the potential, caustic alkali concentration, and temperature allowed pyrite to directionally transform into S₂O₃²⁻ and Fe₃O₄. Increasing caustic alkali concentration enlarged the stable region of S₂O₃²⁻ and Fe₃O₄, while reducing the stable region of FeS and FeS₂. The electrochemical reaction of pyrite depended on the sulfur-bearing anions with different potentials in the sodium aluminate solution. Owing to S² preferentially adsorbed on the pyrite and readily changing into S⁰, the thin dense S⁰ layer enriched on the pyrite surface, inhibiting the reaction of pyrite. S₂O₃²⁻ contributed to the formation of an iron hydroxide layer on the pyrite surface at room temperature, the thick layer of iron hydroxide and iron-deficient sulfide (Fe_1-xS₂) on pyrite then remarkably restrained the reaction of pyrite. In addition, the negative potential from S²⁻ and S₂O₃²⁻ and high temperatures promoted the formation of magnetite. Inhibiting pyrite reaction, transformation of S₂O₃²⁻, and formation of magnetite in the digestion of pyrite were all achieved by adding 1 g·L⁻¹ NaNO₃, whereas more NaNO₃ notably promoted pyrite reaction and formation of hematite at 260 ℃. The mechanism of the directional transformation of pyrite was elucidated, while a green novel process to efficiently utilize bauxite containing sulfur for synchronous desilication and sulfur removal, together with reduction of the red mud through iron recovery was provided. The results provide an insight into electrochemical reaction mechanism of pyrite in Bayer liquor, and novel approach to sulfur removal in desilication and the reduction of red mud after iron recovery.