Sulphuric acid

Sulphuric acid (H₂SO₄) ranks as the highest-tonnage chemical manufactured globally. Its production hinges on the catalytic oxidation of sulphur dioxide (SO₂) to sulphur trioxide (SO₃), followed by absorption of the latter into concentrated sulphuric acid — oleum — rather than water, as the latter approach generates an acid mist that is troublesome to condense. The contact process, whether single or double absorption, prevails industrially; the double contact/double absorption (DC/DA) variant delivers superior yields and lower emissions. Sources of SO₂ are manifold: combustion of elemental sulphur, roasting of pyrite, smelter gases from non-ferrous metals, or regeneration of spent acids^[1][2].

Manufacture and raw material sources

A typical production unit comprises sulphur storage and melting, filtration, combustion with dried air supply, heat recovery in waste-heat boilers, the catalytic converter, intermediate and final absorption towers, and the exhaust stack^[1]. In Europe, EU-25 production capacity stood at roughly 22 Mt/year around 2004, spread across 95 plants of widely varying scale: 32 % of units produced under 250 t/d, 29 % between 250 and 500 t/d, 27 % between 500 and 1,000 t/d, and 12 % exceeded 1,000 t/d^[3]. European producers listed by the ESA (European Sulphuric Acid Association) operate sites with diverse profiles, from sulphur refining to smelter gas valorisation^[4][5][6].

In the United States, recovered elemental sulphur and by-product sulphuric acid came in 2025 from 86 operations across 26 states. Elemental sulphur output reached 7.6 Mt, with Louisiana and Texas accounting for 54 %; petroleum refineries, natural gas processing plants, and coke ovens form the main sources. By-product sulphuric acid, originating from five non-ferrous metal smelters in four states, represents roughly 6 % of total sulphur production across all forms^[7]. The average price of elemental sulphur, FOB mine or plant, rose from $58.90/t in 2024 to $180/t in 2025. Total shipments of sulphur in all forms were valued at approximately $1.4 billion in 2025, some $1 billion more than in 2024, this increase stemming chiefly from the price surge^[7].

Agronomic uses and place in the fertiliser chain

Sulphuric acid intervenes at several stages of the fertiliser chain. It is chiefly employed to attack phosphate rock to produce single superphosphate (SSP) through the reaction of tricalcium phosphate with the acid, or triple superphosphate (TSP) via phosphoric acid as an intermediate^[8]. It also enters into the manufacture of ammonium sulphate by neutralising ammonia: two molecules of NH₃ react with one molecule of sulphuric acid to yield ammonium sulphate, a fertiliser traditionally used for simultaneous supply of ammoniacal nitrogen and sulphate sulphur^[9]. Process flow diagrams of the industry show sulphuric acid at the heart of phosphate and nitrogen fertiliser production routes^[10][11].

Market and trade

Apparent US consumption of sulphur in all forms amounted to 9.1 Mt in 2025^[7]. Domestic sulphur consumption broke down as 63 % domestic elemental sulphur, 3 % by-product sulphuric acid, and 34 % imports of elemental sulphur or sulphuric acid^[7]. Approximately 90 % of sulphur consumed is in the form of sulphuric acid^[7]. US sulphuric acid imports came predominantly from Canada (54 %), Mexico (22 %), and Spain (7 %) over the 2021-2024 period^[7]. In the international phosphate market, sulphuric acid used in the wet acid route most commonly derives from elemental sulphur combustion, but may also be "spent acid" from non-ferrous metal smelting, or regenerated acid. Only mercury and possibly lead may constitute significant impurities, particularly with spent acid (0.1-1 ppm mercury versus under 0.01 ppm for acid from elemental sulphur)^[12].