Cobalt/nickel solvent extraction (D2EHPA)

A laterite leach liquor (Co/Ni/Mg in dilute sulfate solution) meets a D2EHPA-in-kerosene organic phase across a 20-stage countercurrent extraction circuit run at pH 5.2 — the real industrial operating window (e.g. Bulong, Murrin Murrin, Western Australia) that exploits the ~1 pH-unit gap between Co and Ni's D2EHPA extraction isotherms: Co extracts into the organic while Ni (and Mg gangue) are rejected to the raffinate. The loaded organic then meets fresh dilute-acid strip liquor across a 10-stage strip circuit at pH 1.0 (well below Co's isotherm), reversing the equilibrium to recover a concentrated cobalt strip liquor and regenerate barren organic. Real pH-isotherm chemistry throughout (thermo/organophosphorus_extraction.py), not a fitted shortcut K_D. Honesty note: the regenerated organic and strip liquor aren't recycled back upstream (an open-loop v1, same posture as this session's other new cycle examples) — a real plant recycles both; strip_stage's distribution_coefficients is the numeric inverse of the same isotherm (1/D_Co at pH 1.0 = 1e8, i.e. strongly favors the aqueous phase) since extraction_column.py's ph= mode always assumes the aqueous-feed/organic-solvent extraction direction, not reverse stripping.