Battery cobalt recovery: leach, neutralize, D2EHPA Mn SX, Cyanex 272 Co SX, electrowinning

Spent NMC111 black mass is leached in sulfuric acid with the acid consumption tracked (lithium as Li2SO4, each transition metal as its sulfate), so the pregnant leach solution carries the residual free acid a real leach leaves. A neutralizer then titrates that liquor with 20 wt% caustic to pH 4.0 on the aqueous speciation: the caustic dose is solved, not assumed, and the heat of neutralization (55.8 kJ per mole of acid neutralized) warms the liquor; the caustic header is the supply, and what the titration does not take is returned. Two solvent-extraction circuits on the pH-isotherm chemistry follow, the common NMC arrangement: D2EHPA at pH 4.0 takes manganese into the organic while cobalt and nickel stay behind, and Cyanex 272 at pH 5.0 then takes cobalt and rejects nickel and lithium to the raffinate. The Cyanex 272 isotherms are one published measurement (Liu et al., Metals 11 (2021) 401: 0.4 M in ShellSol D70, 40 C, sulfate; pH50 cobalt 4.15, nickel 6.06), and the 1.9 pH-unit gap between them is why the cobalt organic carries about 0.6% of the nickel, against about 3% with D2EHPA at the same cobalt recovery. The loaded cobalt organic is stripped into a dilute sulfuric strip liquor, and that advance electrolyte goes to cobalt electrowinning: Faraday's law at a 90% current efficiency, 90% of the cobalt plated as cathode cobalt, the anode's oxygen and the 10% of the charge that evolves hydrogen reported as cell gas, and one mole of sulfuric acid regenerated per mole of cobalt plated. Honesty notes: (1) the leach yield and sulfate stoichiometry are screening inputs, and the hydrogen-peroxide reductant a real NMC leach needs to bring cobalt from Co(III) to Co(II) is not modelled; (2) the D2EHPA pH50 values used for manganese are representative literature-order values, not one cited plant isotherm; the Cyanex 272 table carries only zinc, cobalt and nickel (the metals the cited study measured), so the trace of manganese that passes the D2EHPA circuit is shown in the nickel raffinate although Cyanex 272 would extract it with the cobalt: on the Cytec brochure's single-metal sulfate isotherms (0.6 M, 50 C), which do carry manganese, cobalt and nickel together, manganese's pH50 sits 0.34 below cobalt's and the whole trace follows the cobalt into the organic. That basis is offered as an alternative extractant setting but not used here, because it is measured one metal at a time and so rejects nickel some 80 times better than the mixed-liquor Liu measurement (0.008% against 0.6%); the trace is about 3e-8 mol/s either way. The isotherms have no loading term, so they overstate the cobalt/nickel separation factor by up to about 3x at high cobalt loading, and there are no scrub stages; (3) the extraction stages hold pH as an operator setpoint, so the acid D2EHPA releases on loading and the acid stripping consumes are not balanced (a real circuit saponifies the organic with more caustic); (4) the strip liquor, the regenerated organic and the spent electrolyte are not recycled (open loop), where a tankhouse returns the spent electrolyte to the strip; (5) the cell voltage of 4.0 V and the 333 K bath are illustrative inputs to replace with the tankhouse's own measurements; and (6) the manganese organic, the nickel-lithium raffinate and the leach residue leave as products, their own recovery circuits not drawn.