Ore-dependent cobalt release from Mn–Co laterites during reductive nitric acid leaching: matrix-assisted, poorly responsive and reductant-responsive behaviour

Ores of similar bulk composition can respond very differently to reductive leaching, which complicates Co recovery from Mn–Co laterites. Three goethite-dominated laterites (L1–L3) were leached in concentrated nitric acid at 110 °C with and without Fe 2+ or NO 2 − . Based on a series of characterisations, the ores followed three distinct mechanisms. L1 was matrix-assisted: its nanocrystalline goethite matrix (13–22 nm crystallites) dissolved almost completely, releasing ∼ 76 % of the Co without added reductant. L2 was poorly responsive: Co extraction rose only from ∼ 20 % to 65–70 % with either reductant, and its phyllomanganate reflections persisted. L3 was reductant-responsive: Co extraction rose from ∼ 40 % to 82–85 %, and its phyllomanganate reflections were strongly attenuated. This contrast arose although L2 and L3 had nearly identical surface areas, Fe extraction changed little on reductant addition, and Co K-edge HERFD-XAS showed the same Co(III)-bearing phyllomanganate environment in all three feeds. After leaching, 88–100 % of the residual Co remained in this environment, so incomplete recovery reflects surviving host domains rather than a new Co phase. Mn K-edge XANES showed that the two reductants act by different routes: a reduced Mn(II,III) component (24–36 %) accumulated in every NO 2 − -assisted residue, against 0–15 % in the Fe 2+ -assisted and reductant-free residues. Co recovery is therefore governed by the reactivity of the Fe matrix and the reductive lability of the Mn–Co host, not by bulk composition or surface area.