Cementation: metal displacement

Cementation is purification by displacement: add a more active metal to the solution, and it pushes a dissolved, more noble metal out as solid metal, going into solution itself in the trade. The added metal corrodes; the value plates onto it. It is one of the oldest hydrometallurgical recoveries and still a standard, and it rewards being read as the electrochemistry it is rather than memorised as a recipe.

A displacement driven by potential

Every metal-and-ion couple sits somewhere on the electrochemical series, ranked by its standard electrode potential. A metal will reduce — plate out — the ions of any metal more noble than itself, oxidising and dissolving in exchange. Put zinc into a gold solution and the gold deposits while the zinc dissolves; put iron into a copper solution and copper deposits while the iron dissolves. The two half-reactions run on the same metal surface, a short-circuited galvanic cell that needs no external current — the potential difference between the two couples is the entire driving force. The wider that gap, the more completely the value is displaced, which is why an active metal like zinc or iron drives a noble one like gold or copper essentially to completion.

The classic duties

Two duties define cementation in practice. In the Merrill-Crowe process, fine zinc dust is added to a clarified, de-aerated gold-and-silver solution — the polishing front end of the previous module exists precisely to prepare that solution — and the precious metals cement onto the zinc as a precipitate that is filtered and refined. In copper launders and cones, scrap or sponge iron contacts a dilute copper leach solution and copper cements onto it as impure "cement copper", historically the recovery of choice for weak heap and dump liquors. Both are the same mechanism: a sacrificial active metal trading itself for the value in solution.

Where cementation fits

Cementation earns its place by being near-complete on dilute solutions and by delivering a metal product directly, without the electrowinning or the elution that other routes need. Its costs are the sacrificial metal consumed and a cement product that usually needs downstream refining, and a sensitivity to conditions — dissolved oxygen will redissolve a cemented precious metal, which is why de-aeration matters. It competes with adsorption, solvent extraction and electrowinning for the recovery step, and it is the defining mechanism of the Merrill-Crowe gold route and the zinc-cementation purification of zinc-circuit leach liquor.