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Material World
Process

Electrorefining

Dissolving an impure metal anode and replating it pure — and collecting, in the sludge beneath, most of the world's tellurium, selenium and precious metals.

Smelted copper is around 99% pure, and 99% is not good enough to draw into wire: the last percent of impurity costs a disproportionate amount of conductivity. Electrorefining removes it by dissolving the impure metal as an anode and depositing it again on a cathode, at a voltage chosen so that copper moves and most impurities do not.

What does not move is the interesting part. Metals nobler than copper — gold, silver, platinum, palladium — refuse to dissolve and fall to the bottom of the cell, along with selenium and tellurium. The sludge that collects there is called anode slime, and it is the world's principal source of several elements that are never mined for their own sake.

Economic significance

Anode slime inverts the usual relationship between a metal's value and its production. Tellurium and selenium have no ore bodies worth mining; their supply is set by how much copper the world refines, and no increase in demand can call forth more of them directly.

For the refiner the slimes are often worth a substantial fraction of the plant's margin — the precious metals recovered from them can exceed the profit on the copper itself, which is why a refinery's economics are not the simple ones its name suggests.

Medium confidence Weak evidence

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.

takes as input

  • Copper — element · as impure smelted anodes, around 99% copper, which is not pure enough for wire

produces

  • Tellurium — element · from the anode slimes; tellurium has no ore body worth mining and no other significant source
  • Selenium — element · from the same slimes, and by the same accident of the refining cell
  • Gold — element · nobler than copper, so it never dissolves and falls to the bottom of the cell
  • Silver — element · recovered from anode slimes alongside the gold
  • Platinum — element · collected in the slimes, where the copper's own refining concentrates it
  • Bismuth — element · separated from the lead it accompanies during refining, which is where essentially all of it comes from
  • Palladium — element · from the anode slimes of copper and nickel refining, alongside platinum
  • Rhodium — element · from the same slimes and the same platinum-group concentrate, and rarer in them than palladium by an order of magnitude
  • Ruthenium — element · separated late in the platinum-group refining sequence, after the more valuable metals have been taken
  • Iridium — element · from platinum-group concentrate, and among the rarest elements in the crust that is produced at all
  • Osmium — element · the last of the platinum group to be separated, and produced in quantities measured in tonnes a year worldwide

succeeded

  • Smelting — process · smelting produces metal at about 99%; electrorefining takes it the rest of the way and harvests what is left behind

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Electrorefining back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is extracted from (the principal copper ore worldwide) → Chalcopyrite
  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (reduced with coke in a blast furnace) → Hematite
  • Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (the phosphate rock charged to the electric furnace, with coke to reduce it and silica to take up the calcium) → Apatite

These are the most distinct paths back. Electrorefining can be traced through others besides.

Downstream — what it becomes

  • Electrorefining → produces (recovered from anode slimes alongside the gold) → Silver → is a component of (a few per cent in the lead-free alloys, which measurably raised world silver demand when the transition happened) → Solder → is used in (every joint on it, and tin-silver-copper since RoHS pushed the lead out from 2006) → Printed circuit board → is used in (and it is the change that made electronics manufacturable: assembly stopped being a wiring job and became a printing one) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Electrorefining → produces (nobler than copper, so it never dissolves and falls to the bottom of the cell) → Gold → is used in (about thirty milligrams, on contacts and bond wires, because a gold-plated contact still works in ten years — a tonne of phones assays richer than a tonne of ore from any mine on earth) → Smartphone → is used in (and it is where four separate raw-material arguments meet — the 3TG conflict minerals, cobalt, the rare earths, and everything too dispersed to recover) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Electrorefining → produces (separated late in the platinum-group refining sequence, after the more valuable metals have been taken) → Ruthenium → is a component of (in the newest generations, to stop the heavy elements segregating into phases that embrittle the blade) → Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is associated with (the jet engine created the superalloy industry, because no existing material survived the turbine inlet — and turbine temperature has risen about 500 °C since, almost entirely on the blade) → The wartime materials programmes complete chain
  • Electrorefining → produces (collected in the slimes, where the copper's own refining concentrates it) → Platinum → is a source for (and of platinum mining) → Palladium → is used in (in petrol exhaust, and the largest single use of the metal in the world) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis
  • Electrorefining → produces (from the anode slimes of copper and nickel refining, alongside platinum) → Palladium → is used in (in petrol exhaust, and the largest single use of the metal in the world) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis
  • Electrorefining → produces (from the same slimes and the same platinum-group concentrate, and rarer in them than palladium by an order of magnitude) → Rhodium → is used in (a fraction of a gram, doing the nitrogen oxide reduction that neither of the others does well — and worth enough on its own to make converter theft organised crime) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis

These are the most distinct paths onward. Electrorefining ends up in others besides.