Module 3 · Smelting, refining and the narrow link

Electrorefining, and what falls to the bottom of the tank

Level 2 · Working knowledge 9 min read 4 questions

Two weeks in a tank takes copper from 98% to 99.99%. The impurities that drop out are a significant share of the world's supply of several other elements.

Blister copper is 98% copper, and the last 2% is what stops it being usable. Small quantities of the wrong elements - particularly oxygen, sulfur, bismuth and antimony - degrade conductivity and mechanical behaviour disproportionately. Removing them is done electrochemically.

How it works

Cast anodes of impure copper are hung in a tank of acidified copper sulfate solution, alternating with thin starter sheets or permanent blanks that will become the cathodes. A current is passed. Copper dissolves from the anode, travels through the solution as ions, and plates onto the cathode. The trick is selectivity: elements more reactive than copper dissolve but do not plate back, and elements less reactive than copper do not dissolve at all. They simply fall off the anode as it erodes and settle at the bottom of the tank.

Anode slimes

The sludge at the bottom of the tank is called anode slime, and it is one of the most valuable materials in the non-ferrous industry. It contains the elements too noble to dissolve: gold, silver, selenium, tellurium and the platinum-group metals. It is collected periodically and processed in a dedicated precious metals plant.

Why the cathode has to be this pure

Copper's selling point is conductivity, and conductivity is extraordinarily sensitive to dissolved impurities. Elements that stay in solid solution in the copper lattice scatter electrons; a fraction of a per cent of the wrong element costs a measurable percentage of conductivity. Oxygen is a special case: it is deliberately controlled rather than eliminated, and for applications where hydrogen embrittlement matters an oxygen-free grade is produced instead. The difference between 99.9% and 99.99% copper is not fastidiousness; it is the difference between a conductor that meets specification and one that does not.

Finished cathode is then registered against an exchange specification - LME Grade A is the usual reference - which is what allows a plate produced in Chile to settle a contract written in London and be delivered in Rotterdam. Standardisation at this step is what makes copper a traded commodity rather than a negotiated product.