Sulphuric acid: Production route
One process, four sources of SO2, very different economics
What this tab covers How it is made: the main production routes or, for product families, the main product types and technologies, compared on share of supply, inputs per tonne, plant scale, yield, co-products and unit cost.All commercial acid is made by the contact process: SO2 is oxidised to SO3 over a vanadium catalyst and absorbed in strong acid. What differs is where the SO2 comes from. Sulphur burning gives the cleanest gas and the most energy. Metallurgical (smelter) acid is a by-product of controlling SO2 at copper, zinc and nickel smelters, so its volume follows metal output, not acid demand. Pyrite roasting survives mainly in China. Spent acid regeneration and wet gas processes recycle refinery acid and H2S.
Routes compared
| Route | Share of supply | Inputs per tonne | Typical scale | Yield | Co-products | Unit cost |
|---|---|---|---|---|---|---|
| Sulphur burning, double contact double absorption | about 60-65% of world output (indicative) | 0.33 t sulphur per t H2SO4 (stoichiometric 0.327); small amounts of catalyst and water | single trains of 1,000-5,000 t/day; OCP Jorf Lasfar runs many such lines (indicative) | about 99.7-99.9% SO2 conversion (indicative) | high-pressure steam about 1.2-1.4 t per t acid, turned into power or process heat (indicative) | feedstock about $60/t acid at $180/t sulphur (US 2025 average) and about $245/t at $740/t (QatarEnergy May 2026); conversion about $15-25/t (indicative) |
| Metallurgical (smelter) acid | about 25-30% of world output (indicative); about 6% of US sulphur output in all forms (USGS 2025) | about 1 t acid per t of copper concentrate smelted (indicative); no purchased sulphur | LS MnM's Onsan smelter has 1.925 Mt/yr of acid capacity | up to 99.9% sulphur capture in modern smelters (indicative) | acid is itself the by-product; impurities such as mercury and arsenic must be removed | low marginal cost; producers will sell at near zero or negative netbacks rather than stop the smelter (indicative) |
| Pyrite roasting | about 5-10% of world output, nearly all in China (indicative) | about 0.7-0.8 t pyrite concentrate per t acid (indicative) | typical plants 100-400 kt/yr (indicative) | lower than sulphur burning (indicative) | iron oxide cinder that can feed steel or cement | competitive when imported sulphur is expensive (indicative) |
| Spent acid regeneration and wet gas sulphuric acid (WSA) | small; 2.5-5 Mt a year of spent acid reclaimed in the US (USGS) | spent acid plus fuel gas | typically 100-1,500 t/day (indicative) | high recovery of sulphur values | steam (WSA) | a service fee to the refiner rather than a commodity price (indicative) |
How each route works
Sulphur burning, double contact double absorptionMolten sulphur is sprayed into a furnace and burnt with dried air to about 10-12% SO2. The gas passes through catalyst beds with intermediate absorption to reach high conversion. Waste heat boilers raise steam, and many plants generate power for neighbouring phosphoric acid units.
Lowest cost and energy positive, but fully exposed to sulphur price spikes.
USGS MCS 2026; CRU BCInsight (May 2026); stoichiometry Metallurgical (smelter) acidSO2 in copper, zinc, nickel and lead smelter off-gas is cleaned of dust and metals and fed to a contact plant. The smelter must sell or neutralise the acid whatever the price, so smelter acid is often sold cheaply in weak markets and becomes scarce when smelters cut output.
Cheap and low carbon, but volume follows metal smelting, and quality can be lower.
CRU BCInsight (2024, LS MnM figure); USGS MCS 2026 Pyrite roastingIron pyrite (FeS2) is roasted in fluidised beds to give SO2 and iron oxide cinder. The gas is dirtier and needs more cleaning. The route is largely limited to China, where it uses domestic pyrite and reduces reliance on imported sulphur.
Uses domestic ore, but has more waste and higher emissions of arsenic and dust.
Indicative; USGS MCS 2026 notes Chinese pyrite acid in sulphur totals Spent acid regeneration and wet gas sulphuric acid (WSA)Refinery alkylation acid and other spent acid is decomposed in a furnace to SO2 and remade into acid. WSA plants turn H2S or SO2-rich gas directly into concentrated acid without first making sulphur. Both are recycling routes tied to refineries and chemical plants.
Closes the loop for refiners; capacity is local.
USGS MCS 2026; indicative Unit economics are indicative ranges from the sources named; costs move with feedstock and energy prices, plant size and region.