Module 3 · Smelting, refining and the narrow link

Smelting: from concentrate to blister

Level 2 · Working knowledge 10 min read 4 questions

Concentrate is melted with oxygen. Iron and sulfur burn off, the sulfur is captured as acid rather than released, and what is left is about 98% copper.

Molten copper being cast from a furnace — Iron prefers oxygen, copper prefers sulfur, and the sulfur reaction supplies much of the heat.Alfred T. Palmer · Public domain · Wikimedia Commons

Concentrate arriving at a smelter is roughly a quarter to a third copper. The rest is mostly iron and sulfur, bound up with the copper in chalcopyrite, plus whatever gangue survived flotation. Smelting removes the iron and the sulfur by exploiting a convenient piece of chemistry: at high temperature, in the presence of oxygen, iron prefers to become an oxide and copper prefers to stay a sulfide.

The three operations inside a smelter

Smelting properConcentrate is fed into a furnace with oxygen-enriched air. Sulfur burns, releasing heat - the reaction is strongly exothermic, so a well-run flash smelter needs little added fuel. Iron oxidises and combines with added silica to form a slag that floats.Matte, plus slag

ConvertingThe matte - a copper-iron sulfide melt - is blown with air or oxygen in two stages: first the remaining iron is oxidised into slag, then the sulfur is blown out of the copper sulfide itself.Blister copper, about 98% Cu

Fire refining and anode castingResidual oxygen and sulfur are adjusted, then the copper is cast into thick plates shaped for the next step.Anodes, about 99% Cu

The name blister comes from the appearance: sulfur dioxide escaping as the metal solidifies leaves the surface blistered. It is copper, and it is nowhere near good enough to use as a conductor.

The sulfur is the point

Each tonne of copper smelted liberates a substantial mass of sulfur. Released, it would be sulfur dioxide, and the environmental history of copper smelting is largely the history of that fact. Modern smelters capture the gas and convert it to sulfuric acid, which is sold. The copper to the grid chain page states the modern practice directly: the sulfur is captured as sulfuric acid rather than released.

NoteThis makes a copper smelter an acid plant with a metal by-product, from a tonnage point of view. And because acid is expensive to ship and to store, a smelter needs a buyer within economic reach - often the leach operations from lesson 6. Where there is no such buyer, acid disposal becomes a cost that constrains where smelters can be built.

What a smelter charges, and why it is a market

A smelter does not usually buy copper. It buys concentrate and charges for the service of turning it into metal, under terms that are among the most closely watched numbers in the industry.

Treatment charge (TC): A fee per dry tonne of concentrate treated.
Refining charge (RC): A fee per pound of payable copper refined.
Payable metal: The share of contained copper the miner is actually paid for. The smelter keeps a small deduction, because some copper is unavoidably lost in slag.
Penalties: Deductions for deleterious elements - arsenic especially, also bismuth, antimony, fluorine, mercury - which complicate smelting and restrict where the concentrate can go at all.
Credits: Payment for gold and silver in the concentrate, recovered downstream.

Read the numbers correctlyTC/RCs are a price for smelting capacity, and they move inversely to how tight that capacity is. When concentrate is plentiful relative to smelters, smelters can charge more and TC/RCs rise. When concentrate is scarce, smelters compete for it and TC/RCs fall, sometimes to levels at which smelting is loss-making. A falling TC is bullish for the mine and bearish for the smelter, and headlines routinely get the direction backwards.

Arsenic, and why some concentrate has nowhere to go

Arsenic deserves its own paragraph because it is an increasingly binding constraint. As the clean upper parts of deposits are mined out, more concentrate carries arsenic, and smelters have limits - regulatory and technical - on how much they can accept. Such material is blended down into cleaner concentrate, routed to the minority of smelters equipped for it, or treated in a dedicated roasting step first. A high-arsenic concentrate can be perfectly good copper and still have almost no buyers, which is a very clear illustration of this site's recurring theme: the constraint is somebody else's plant, not the resource.

What to carry forward

Smelting exploits iron's preference for oxygen over copper's, and the sulfur reaction supplies much of the heat.
Modern smelters capture sulfur as sulfuric acid, which links them to leach operations and fertiliser.
TC/RCs price smelting capacity and move inversely to concentrate scarcity.

Terms introduced here

Matte: The copper-iron sulfide melt produced by smelting, before converting.
Blister copper: Converter product, about 98% copper.
TC/RC: Treatment and refining charges: what a smelter is paid to turn concentrate into metal.
Payable metal: The share of contained metal a miner is actually paid for.

Go deeper

Guixi Smelteron this site
Atlantic Copper Smelter, Huelvaon this site
Smelting and refiningon this site
Sulfuron this site

Check yourself

1. Smelting copper concentrate requires relatively little added fuel because:

2. Concentrate becomes scarce relative to world smelting capacity. Treatment and refining charges will most likely:

3. A concentrate with high arsenic content is difficult to sell because:

4. Blister copper is: