Solvent extraction and electrowinning
Dissolving metal out of low-grade ore, moving it into a clean solution with an organic reagent, and plating it out with electricity — the route that made waste rock into copper ore.
SX-EW wins metal without ever melting anything. Dilute acid is trickled through a heap of crushed ore for months and comes out carrying copper; an organic reagent then picks the copper out of that dirty solution and hands it to a clean one; and an electric current plates it onto a cathode at a purity high enough to sell directly.
What makes it important is what it can be pointed at. Smelting needs a concentrate, and a concentrate needs flotation, and flotation needs sulfide minerals. SX-EW works on oxidised ore that flotation cannot treat and on grades far too low to justify moving through a mill — including, routinely, the waste dumps of mines that closed decades ago.
Processing
Leaching is deliberately slow. Ore is stacked on a lined pad, acid is dripped over the top, and the solution that drains out at the bottom is collected — a cycle measured in months, with bacteria doing much of the work on sulfide ores by oxidising them into a form the acid can attack.
The solution that results is far too dilute and too impure to plate from. Solvent extraction fixes both problems at once: an organic reagent designed to bind copper and nothing else is mixed with it, takes the copper into the organic phase, and is then stripped with strong acid into a small volume of clean electrolyte. The metal has been concentrated perhaps forty-fold and separated from the iron that came with it.
Electrowinning is the same chemistry as electrorefining with a different anode. There is no impure metal to dissolve — the copper comes from the solution — so the anode is inert and oxygen is released at it instead.
Uranium follows the same sequence and stops earlier. The product is ammonium diuranate, dried to the yellowcake that is traded as uranium, and the metal itself is several conversion steps further on.
Economic significance
Roughly a fifth of the world's copper is now produced this way, and the share is not really the point: SX-EW changed what counts as ore. A deposit too low-grade to mill in 1970 is a leach pad today, and the same is true of the tailings the mill produced.
Its environmental account is genuinely mixed rather than rhetorically so. There is no smelter, so no sulfur dioxide and far less energy per tonne. There is instead an acidic solution circulating through a heap for years, on a liner that has to hold, above groundwater that is affected if it does not.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
produces
- Copper — element · roughly a fifth of world production, plated straight from solution at cathode purity without ever being melted
- Cobalt — element · from the copper belt of central Africa, where cobalt follows copper into the leach solution and is separated from it downstream
- Nickel — element · from laterite ores, which have no sulfide to float and cannot be smelted economically — leaching is what made them a resource
- Zinc — element · where an oxidised or low-grade ore does not justify a roaster, though most zinc still comes from the roasting route
- Uranium — element · as yellowcake rather than metal: leaching and solvent extraction end at a uranium oxide concentrate, and the metal is several conversion steps further on
- Gallium — element · recovered from the Bayer process liquor, which carries the gallium that came in with the bauxite
- Scandium — element · from the residues of titanium and uranium processing, in quantities small enough that supply is measured in tonnes a year
- Germanium — element · leached from zinc smelting residues and coal fly ash and separated by solvent extraction — dispersed everywhere at parts per million and concentrated nowhere
- Indium — element · from the residues of zinc refining, which is the only source there is; indium ore does not exist in any workable sense
- Thorium — element · leached from monazite processing residues, and as an oxide rather than a metal — thorium is a by-product nobody currently wants, which is the difficulty rare earth operations keep running into
- Tantalum — element · separated from niobium by solvent extraction, which is the hard step; the metal itself is reduced from the resulting salt afterwards
takes as input
- Chalcopyrite — mineral · in the low-grade and oxidised ores that flotation cannot economically treat, with bacteria oxidising the sulfide into a form the acid can attack
- Uraninite — mineral · leached with acid or alkali depending on the host rock, which is what decides the flowsheet of a uranium mill
- Malachite — mineral · a carbonate dissolves straight into acid, which is why an oxidised orebody goes to a leach pad and the sulfide beneath it goes to a smelter
succeeded
- Froth flotation — process · the alternative to the smelter for what flotation cannot concentrate — a parallel route rather than a later stage
Sources
- Material WorldOur own writing