Solvent extraction in one sitting

Solvent extraction is a metal shuttle run between two liquids that do not mix. An organic phase carrying a tailored extractant pulls the target metal out of the aqueous leach solution, then a second contact with a small, strong aqueous strips the metal back off the organic into a clean, concentrated solution. Held in one sitting, SX is two reversible reactions run in opposite directions on the same organic, and everything else — the O/A ratio, the staged mixer-settlers, the McCabe-Thiele staircase — is detail hung on that frame.

Extract, then strip

In extraction, the aqueous pregnant solution is mixed with an immiscible organic — an extractant reagent dissolved in a kerosene-like diluent — that selectively bonds the target metal and carries it from the aqueous into the organic. The phases disengage in a settler; the now-loaded organic separates from the raffinate, the depleted aqueous that leaves carrying the impurities the extractant did not want. In stripping, that loaded organic is contacted with a different, small-volume aqueous — strong sulfuric acid, in copper SX — that reverses the reaction, driving the metal back into a concentrated strip liquor and regenerating the barren organic to recirculate. Selectivity goes in at extraction; concentration comes out at strip.

The O/A ratio

The organic-to-aqueous flow ratio, O/A, is the dial that turns SX into both a purifier and a concentrator. Run a high aqueous flow against a smaller organic flow on extraction, then strip that organic into a still smaller aqueous, and the metal is upgraded in tenor across the loop while the impurities stay behind in the raffinate. The O/A ratio sets where the operating line sits against the equilibrium, and therefore how fully each stage loads or strips — it is the first number an SX circuit is read and tuned on.

The McCabe-Thiele staircase

How many stages a duty needs is read off a McCabe-Thiele construction, and a crosser from distillation will recognise it on sight. The equilibrium between the phases is a distribution isotherm — metal in organic against metal in aqueous; the operating line is fixed by the O/A ratio; and the number of counter-current stages is the staircase stepped between the isotherm and the operating line. It is a conceptual construction here, an intuition rather than a plotted figure: each step is one mixer-settler, and a duty that needs a tall staircase needs many stages. Extraction and strip each have their own staircase.

Where SX rules

Solvent extraction is the workhorse of copper SX-EW: a dilute, impure heap-leach solution is upgraded and purified into a clean copper electrolyte that electrowinning can plate as cathode, the combination that made low-grade copper oxide ores economic. It is central to nickel and cobalt separation, where extractants split metals with very close chemistry, and it is the backbone of rare-earth separation, where dozens — sometimes hundreds — of mixer-settler stages tease apart lanthanides almost identical in behaviour. Wherever a target must be both purified from close neighbours and concentrated from a large flow, SX is the first mechanism considered.