Making sulfuric acid on site does not necessarily protect a copper mine from soaring sulfur prices. The protection depends on what feeds the acid plant.
The Financial Times reported on October 13 that sulfur prices had approached $1,000 per tonne this year, compared with typical levels of $100–200. That raises a specific question for copper producers: how much purchased sulfur does their processing route require?
In acid-leaching operations, sulfuric acid dissolves copper from suitable ore before the metal is recovered from solution. An interruption in acid supply can therefore affect production even when the mine has ore available and customers willing to buy its copper. Freeport’s documentation describes this process at its Sierrita operation.
But the acid itself can come from different sources.
Freeport provides a useful example within one company. Its Miami smelter produces sulfuric acid as a by-product of processing copper concentrates and supplies it to the group’s North American leaching operations. At Safford, a sulfur-burning plant produces acid for copper extraction. Both arrangements provide acid within the business, but they have different feedstock dependencies.
An on-site acid plant can still leave a mine exposed to purchased sulfur. Acid recovered through smelting draws on sulfur already contained in the concentrate. Burning purchased sulfur retains a direct link to that commodity’s price and availability.
Our reading is that this distinction could produce very different changes in cash costs across copper operations facing the same metal price. Contracts, transport costs and acid consumption per tonne of recovered copper would determine how much of the shock reaches each operation.
The practical question goes beyond whether a producer makes its own acid: what must it buy to keep making it? That is where a sulfur shortage can enter the copper supply chain.
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