Electro-reductive leaching of 18.1% Mn residues (tailings) generated after enrichment of raw manganese oxide ore deposits from the Chiatura region in Georgia was conducted in a non-diaphragm electrolyzer using an Fe3+/Fe2+ redox system as a mediator in a suspension with a limited concentration of Fe3+ (up to 3.5 g∙L−1).
For regeneration of the Fe2+ reductant agent, the enhanced surface of an AISI 304 stainless steel mesh-cathode was chosen, which unlike the graphite felt electrodes (GFE-1) did not become clogged with solid silicates and sand particles. The apparatus, which was designed to ensure electro-leaching efficiency, consisted of a non-diaphragm electrolyzer with an intermediate tank, a peristaltic pump, and a compressor. The optimal conditions for the electro-reductive leaching process using a mesh cathode were as follows: i) a relatively low concentration of acid (16–20 g∙L−1 H2SO4 + 2.0 –2.5 g∙L−1 Fe3+; pH of 1), a liquid/solid ratio in the suspension of 0.08 kg/L, ii) taking into account the overall surface of the cathode, the current density ic=0.6A/dm2; A∙dm−2, and iii) intensive turbulence (boiling) of the suspension. Under these conditions, the Mn extraction rate was 96.0–98.1% and the current efficiency was 84.2–86.6%, while only 20–25% of the iron from raw material was transferred to the solution and its content did not exceed the limited concentration of 3.5 g∙L−1 Fe3+.
Large-scale laboratory testing of electro-reductive leaching showed that the total specific electricity consumption for raw material processing was 3760.8 kWh∙t−1, and for electro-reductive leaching, the value was 586.8 kWh∙t−1. After electro-leaching, filtration, and drying, light brown solid stuff was obtained, which could be used for cement production as an additive.
Key words: Low-grade manganese oxide raw materials; electro-reductive leaching; redox system Fe3+/Fe2+; leaching efficiency, current efficiency
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