Zinc is recovered from purified leach liquor (Zn2+ in dilute sulfuric acid, with residual Fe3+ and Mn2+ impurities) using D2EHPA (di-(2-ethylhexyl) phosphoric acid) dissolved in kerosene. D2EHPA extracts divalent and trivalent cations by cation exchange, swapping metal ions for the acidic hydrogen on the extractant and releasing protons back to the raffinate:
Zn2+(aq) + 2(HR)2(org) ⇌ ZnR2·2HR(org) + 2H+(aq)
Fe3+(aq) + 3(HR)2(org) ⇌ FeR3·3HR(org) + 3H+(aq)
Because Fe(III) releases three protons per extraction event against zinc's two, its distribution coefficient falls off faster as acidity rises — the real basis for Zn/Fe selectivity in this circuit. This simulator solves both equilibria live from the sliders:
D_Zn = Kex,Zn · [D2EHPA]^2 / (1 + [H+])^2
D_Fe = Kex,Fe · [D2EHPA]^3 / (1 + [H+])^3
Each distribution coefficient is then carried through a real two-stage countercurrent mixer-settler cascade (E1/E2) at the organic:aqueous phase ratio set by the flow slider: per-stage efficiency η = D·(O/A)/(D·(O/A)+1), overall transfer over two stages = 1-(1-η)². Loaded-organic zinc concentration follows an actual mass balance, [Zn]org = [Zn]feed·η_overall/(O/A), so a lean organic flow concentrates the same recovered zinc into less volume and can push the extractant past its own D2EHPA capacity.
- D2EHPA concentration — raises extraction of both Zn and Fe/Mn together (it appears in both distribution coefficients), so on its own it does not improve selectivity, only throughput.
- Feed acidity — the real selectivity lever: because Fe(III) extraction is cubic in acid suppression against zinc's square law, raising acid rejects iron and manganese noticeably faster than it costs zinc recovery, which is exactly how a real Zn-D2EHPA plant buys separation factor β = D_Zn/D_Fe.
- O/A phase ratio — a process (not chemical) variable: it trades recovery speed against loaded-organic concentration and overload risk, same as the underlying equilibrium chemistry demands.
In real plants, most residual Fe(III) is precipitated upstream (as jarosite or goethite) before the SX circuit, precisely because it co-extracts by this same mechanism and often more readily than zinc — the acidity slider here shows why the feed's residual iron still matters even after that step. Loaded organic then leaves for the strip stage (S1), where spent electrolyte returning from zinc electrowinning — already carrying ~150–180 g/L H2SO4 — reverses the exchange and pushes Zn2+ back into the aqueous phase as zinc-rich electrolyte, closing the loop this diagram animates.