Material Insights · Understanding the Material World

Phosphate rock and Sulphuric acid

Sulphuric acid is the reagent that unlocks phosphate rock: it dissolves the rock into phosphoric acid and leaves four to five tonnes of gypsum per tonne of P2O5. Phosphate fertiliser is the largest use of sulphur, so sulphur and acid price spikes go straight into phosphate costs.

Phosphate rock (commodity) and Sulphuric acid (chemical): 4 relationships.

How they are linked

1. Acid to digest rock

Physical flow · Strong · Sulphuric acid to phosphate rock processing

The calcium phosphate in rock reacts with concentrated sulphuric acid, three molecules of acid for each molecule of tricalcium phosphate, to give phosphoric acid and calcium sulphate; sulphuric acid is the standard reagent of the wet process.

  • more than half of world sulphur supply used for phosphate fertiliser, its largest use
  • about 90% of US sulphur consumed as sulphuric acid, 2025

Because the reaction consumes acid in proportion to the calcium in the rock, rock with more carbonate or other calcium minerals needs more acid per tonne of P2O5. Phosphate complexes often make their own acid by burning sulphur, so the input actually bought is often sulphur rather than acid.

Sources: EFMA, now Fertilizers Europe (copy hosted by Universidad Politécnica de Madrid); USGS; Kelewell

2. Phosphogypsum waste

Joint production · Strong · Acid digestion leaves gypsum

The sulphate from the acid ends up as calcium sulphate, filtered off as phosphogypsum, which carries impurities from the rock and is mostly stacked rather than reused.

  • 4 to 5 t phosphogypsum per tonne of P2O5
  • about 5 t gypsum per tonne of P2O5 in the dihydrate process

The sulphate from the acid used in digestion ends up in the gypsum, so acid use and waste volume rise together. Disposal is regulated, and stacks at older sites are a lasting liability. Some is used in cement, plaster and soil conditioning where impurity limits allow.

Sources: EFMA, now Fertilizers Europe (copy hosted by Universidad Politécnica de Madrid); ISPRA (Italian Institute for Environmental Protection and Research); Polish Journal of Chemical Technology

3. Price link: processing input

Cost and price · Medium · Both ways

Acid is a major cost of turning rock into fertiliser, so higher sulphur and acid prices squeeze processors and tend to lift phosphate prices over a year.

  • 0.16 monthly change correlation
  • 0.57 12-month change correlation
  • USD 46 and 180 average US sulphur unit value per tonne, 2024 and 2025

The site's acid series is a US index, which moves with phosphate demand over a year, in the same month with no lag. The US sulphur unit value rose nearly fourfold in 2025, and in 2026 the blockade of the Strait of Hormuz pushed sulphur and acid prices to record highs in some markets, raising the cost of every tonne of acid used to digest rock. Integrated producers with their own sulphur or long contracts felt it less than buyers on the spot market.

Sources: Material Insights (World Bank, FRED, IMF data); USGS; CRU

4. Gulf sulphur supply

Shared risk · Strong · Gulf sulphur feeds acid for phosphate plants worldwide

Almost half of seaborne sulphur comes from the Middle East, much of it recovered at Gulf refineries and gas plants and shipped through the Strait of Hormuz, so a disruption there cuts the acid supply of phosphate producers that lack their own sulphur.

  • almost half of global seaborne sulphur from the Middle East, 2026
  • 16.6 of 84 Mt sulphur output of Saudi Arabia, the UAE and Qatar, 2025, about 20% of world output

Phosphate producers without refinery or gas sulphur of their own buy it from the Gulf, so a Gulf outage hits rock processing in several countries at once while rock supply itself is unaffected. In 2026 a naval blockade of the Strait of Hormuz sharply cut shipping, and CRU reported record sulphur and acid prices in some markets and continued rises in phosphoric acid prices outside the US.

Sources: USGS; CRU; S&P Global Commodity Insights

For investors

Key learnings

  1. For phosphate processors without their own sulphur, sulphur is the most volatile cost; a sulphur spike can turn acid and DAP margins negative even when rock is cheap.
  2. Gypsum is a cost that rises with output, so stack capacity and disposal rules can constrain expansions as much as rock or acid supply.
  3. Gulf supply risk is shared across phosphate producers, so events there move the whole sector rather than one company.

What to track

  • Middle East sulphur price (FOB Qatar or Saudi): Leads acid and phosphoric acid costs by weeks (Weekly · industry price reports (Argus, CRU, Platts))
  • Sulphur price against DAP price: A rising ratio signals squeezed non-integrated DAP and acid margins (Monthly · World Bank Pink Sheet, industry reports)
  • Shipping through the Strait of Hormuz: Disruption cuts sulphur supply for acid plants (Daily · shipping trackers, news)

For policymakers

Key learnings

  1. India's phosphate fertiliser output depends on imported sulphur or acid as well as imported rock, so security needs cover all three inputs.
  2. Sulphur recovered at Indian refineries is a domestic source of acid, and routing it to fertiliser plants reduces import exposure.
  3. Phosphogypsum rules decide whether expanding domestic acid production is environmentally viable, and reuse standards can turn waste into a resource.

What to track

  • India sulphur and sulphuric acid imports: Rising imports increase exposure to Gulf supply (Monthly · DGCIS, port data)
  • Phosphogypsum generation and reuse: Shows whether waste stocks are growing faster than reuse (Annual · CPCB, state pollution control boards)

India

India produced about 3.7 Mt of sulphur in 2025, largely from refineries, but its phosphate makers also import sulphur and acid. With rock mostly imported too, Gulf sulphur disruptions and sulphur price spikes reach India's DAP costs and subsidy bill.

Sources

  1. Price correlation, Material Insights, Material Insights (World Bank, FRED, IMF data)
  2. Best Available Techniques booklet: production of phosphoric acid, EFMA, now Fertilizers Europe (copy hosted by Universidad Politécnica de Madrid) (2000)
  3. Phosphogypsum generation in phosphoric acid production, ISPRA (Italian Institute for Environmental Protection and Research)
  4. Phosphogypsum from wet process phosphoric acid (Polish Journal of Chemical Technology), Polish Journal of Chemical Technology (2012)
  5. Mineral Commodity Summaries 2026: Sulfur, USGS (2026)
  6. Surging sulfur prices: who is choking the global phosphate fertilizer industry?, Kelewell (2025)
  7. Middle East conflict pushes sulphur and acid prices to new heights, CRU (2026)
  8. No quick sulfuric acid fix for Chilean copper sector: analysts, S&P Global Commodity Insights (2026)