Material Insights · Understanding the Material World

Phosphoric acid and Sulphuric acid

Wet-process phosphoric acid is made by dissolving phosphate rock in sulphuric acid, so large phosphoric acid plants usually run their own sulphuric acid plant beside them. That makes sulphur a direct cost of phosphate fertiliser: the 2026 sulphur spike lifted India's phosphoric acid contract by $340 a tonne of P2O5 in one quarter.

Phosphoric acid (chemical) and Sulphuric acid (chemical): 4 relationships.

How they are linked

1. Acid dissolves phosphate rock

Physical flow · Strong · Sulphuric acid to phosphoric acid

Sulphuric acid attacks the calcium phosphate in rock, releasing phosphoric acid and leaving the calcium as gypsum.

  • about 2.3 t sulphuric acid per tonne of P2O5, in theory (about 0.75 t of sulphur)
  • 1.36 to 1.49 t phosphogypsum per tonne of rock digested

Because every calcium atom in the rock must be tied up as gypsum, acid use and gypsum waste both rise in step with output. The same reaction also releases fluorine as HF, which links the industry to fluorine recovery.

Sources: MDPI Sustainability

2. Acid plant on site

Joint production · Strong · Sulphuric acid plant feeds phosphoric acid plant

Phosphoric acid plants need large, steady volumes of acid, so complexes usually make their own sulphuric acid on site, from sulphur or from smelter gas.

  • 1.2 Mt/yr sulphuric acid capacity, Sterlite Copper, Thoothukudi
  • 230 kt/yr P2O5 capacity of its in-house phosphoric acid plant
  • 2 Mt and 0.6 Mt sulphuric and phosphoric acid capacity, ICS Senegal

At Thoothukudi most smelter acid went to the in-house phosphoric acid plant, turning a copper by-product into fertiliser feed, but the complex has been shut since May 2018 and the Supreme Court refused to let it reopen in February 2024. At ICS in Senegal, belt conveyors bring rock concentrate from the flotation plant to the acid site, so mine, acid and phosphoric plants form one chain.

Sources: Sterlite Copper (Vedanta); ICS; ThePrint

3. Price link: main use

Cost and price · Strong · Both ways

Phosphate fertiliser is a major use of sulphuric acid, so fertiliser cycles drive both, and acid cost feeds into phosphoric acid prices.

  • 0.18 monthly change correlation
  • 0.81 12-month change correlation
  • 0.84 fitted elasticity

Monthly moves correlate at only 0.18, with a two-month lag, but yearly moves at 0.81, because quarterly contracts such as India's phosphoric acid price reset slowly after sulphur and acid costs move.

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

4. Sulphur shock into phosphoric acid

Cost and price · Strong · Sulphur price to phosphoric acid price

Producers that make their acid by burning sulphur feel a sulphur price spike in their costs, and contract prices reset within a quarter.

  • $340/t P2O5 rise in India's Q3 2026 phosphoric acid contract, to $1,700
  • 49% rise in dry bulk sulphur prices to India over Q2 2026
  • about 7% elemental sulphur share of phosphoric acid operating cost, normal markets

OCP reported sulphur roughly tripled in the first half of 2026 while phosphate fertiliser prices rose only about 20%, and its EBITDA fell about 28.5%. A small cost share in normal times becomes a large margin hit when sulphur triples.

Sources: Argus Media; Fertilizer Daily; Thunder Said Energy

For investors

Key learnings

  1. Sulphuric acid is the reagent that turns rock into phosphoric acid, so sulphur is a direct cost of DAP and MAP.
  2. Sulphur is only about 7% of phosphoric acid operating cost in normal markets, but in the first half of 2026 it roughly tripled while phosphate prices rose about 20%, and OCP's EBITDA fell about 28.5%.
  3. Producers with smelter acid or their own sulphur supply are best placed to absorb a sulphur shock.

What to track

  • Sulphur price, Middle East FOB and CFR India: Leading signal for phosphoric acid contracts (Weekly · Argus, CRU)
  • India quarterly phosphoric acid contract: Cost of P2O5 for Indian DAP makers (Quarterly · Argus)
  • Phosphate fertiliser price against sulphur cost: Margin squeeze on phosphate producers (Monthly · World Bank Pink Sheet (DAP), Argus or CRU (sulphur))

For policymakers

Key learnings

  1. India buys phosphoric acid on quarterly contracts, so a sulphur shock reaches domestic DAP costs within one quarter.
  2. Using smelter acid locally, as Thoothukudi did before its 2018 closure, links copper and zinc smelting policy to fertiliser supply.
  3. Phosphogypsum, 1.36 to 1.49 t per tonne of rock, needs managed storage or reuse wherever acid output grows.

What to track

  • India phosphoric acid and sulphur imports: Exposure to external shocks (Monthly · DGCI&S)
  • Domestic sulphuric acid output from refineries and smelters: Local reagent supply (Annual · DCPC, company reports)

India

India settled its Q3 2026 phosphoric acid contract at $1,700 a tonne of P2O5, up $340 on sulphur costs; Sterlite Copper's Thoothukudi complex, built to use most of its smelter acid in-house, has been shut since 2018.

Sources

  1. Phosphogypsum management and reuse (Sustainability 17(19) 8828), MDPI Sustainability (2025)
  2. Phosphoric acid and sulphuric acid plants, Sterlite Copper (Vedanta)
  3. Acid site, Darou (Industries Chimiques du Senegal), ICS
  4. India 3Q phosacid settles at $1,700/t P2O5 cfr, Argus Media (2026)
  5. OCP H1 2026: sulfur costs triple as EBITDA falls 28.5%, Fertilizer Daily (2026)
  6. Price correlation, Material Insights, Material Insights (World Bank, FRED, IMF data)
  7. Phosphoric acid production costs, Thunder Said Energy
  8. SC rejects Vedanta's plea against closure of copper plant in TN, says public health of utmost concern, ThePrint (2024)