Material Insights · Understanding the Material World

Sulphuric acid and Zinc

Most zinc is made by roasting sulphide concentrate, and the sulphur dioxide given off is turned into sulphuric acid, so every zinc smelter is also an acid plant. Acid is a by-product that must be sold or disposed of whatever its price: when acid is dear it props up smelter margins, and when it is cheap it becomes a cost.

Sulphuric acid (chemical) and Zinc (commodity): 4 relationships.

How they are linked

1. Acid from roasting

Joint production · Strong · Zinc smelting yields sulphuric acid

Zinc concentrate is mostly zinc sulphide, and roasting it to zinc oxide for leaching drives off sulphur dioxide, which is cleaned and converted to acid by the contact process.

  • about 89% of world zinc made by roast-leach-electrowin, 2003
  • 30 to 32% sulphur in typical zinc concentrate
  • about 1.5 t sulphuric acid per tonne of zinc from the sulphur in zinc sulphide alone, by stoichiometry

Roasting runs in fluid bed roasters at about 900 to 1,000 C, and the gas carries around 10% sulphur dioxide, rich enough for an acid plant. Because concentrate is about a third sulphur, acid output is large relative to zinc output and rises in proportion to smelter throughput. Smelters cannot cut acid production without cutting zinc.

Sources: AusIMM

2. Acid credit in smelter margins

Cost and price · Asymmetric · Acid prices move zinc smelter profits

Smelters earn treatment charges from miners plus credits for by-products such as acid; when treatment charges are low or negative, acid income can decide whether a smelter makes money.

  • -500 to -1,000 yuan per tonne, Chinese domestic zinc treatment charges, July 2026
  • RMB 1,500 to 1,900 per tonne, Chinese acid prices supporting smelters, July 2026

In 2026, with treatment charges below zero, high acid prices kept Chinese smelters profitable. In 2023 falling acid prices cut smelter profits, though output stayed high, which shows that acid swings change margins more than zinc supply. The effect runs one way: acid prices matter to smelters, but they barely move the zinc price.

Sources: Shanghai Metals Market (SMM); Shanghai Metals Market (SMM)

3. Acid markets shape process choice

Joint production · Medium · Acid outlets decide where roast-leach plants work

A roast-leach smelter needs a buyer for its acid, so the route is preferred where acid markets are close, and inland or remote smelters face freight or neutralisation costs.

  • about 6% of US sulphur production recovered as smelter acid, 2025

Smelter acid flows to fertiliser plants and to copper leaching, and in 2025 China, Peru and Japan were the main suppliers of acid to Chile's copper mines. Where outlets are weak, smelters may have to sell acid at very low prices or neutralise it, which turns a credit into a cost.

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

4. Price link: by-product

Cost and price · Weak · Both ways

Acid credits help smelter margins but are too small a share of revenue to move zinc prices, which follow galvanised steel demand and mine supply.

  • 0.04 monthly change correlation
  • 0.15 12-month change correlation
  • 0.36 fitted elasticity

The near-zero correlation fits the by-product logic: acid supply from smelters rises with zinc output, but acid prices are set by sulphur and fertiliser demand. The site's acid series is a US index.

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

For investors

Key learnings

  1. Zinc smelter earnings depend on treatment charges plus by-product credits, so acid prices matter most when treatment charges are low or negative.
  2. Acid output is tied to zinc throughput, so smelters cannot respond to acid prices; a weak acid market becomes a disposal cost.
  3. Acid price spikes driven by sulphur shortages are a windfall for zinc and copper smelters but a cost for fertiliser and leaching buyers.

What to track

  • Zinc concentrate treatment charges: Low charges make acid and other credits decisive for smelter margins (Monthly and annual benchmark · SMM, industry reports)
  • Sulphuric acid price in China and Chile: Indicates the value of smelter acid credits (Weekly · SMM, S&P Global Platts)
  • Middle East sulphur price: Sets the competing cost of sulphur-burnt acid (Weekly · industry price reports)

For policymakers

Key learnings

  1. Zinc smelters are a domestic source of sulphuric acid, so expanding smelting adds acid supply that can reduce sulphur imports for fertiliser.
  2. Acid from smelters must find a nearby user, which argues for siting fertiliser or chemical plants near smelters or securing rail logistics.
  3. Smelter acid output is fixed by metal output, so policies for acid security should not assume smelters can raise acid supply on demand.

What to track

  • Domestic smelter acid output and sales: Shows how much fertiliser acid demand domestic metal smelting covers (Quarterly · company filings)
  • Sulphuric acid imports: Rising imports point to a gap that smelter acid is not filling (Monthly · DGCIS)

India

Hindustan Zinc's smelters at Chanderiya, Dariba and Debari roast zinc concentrate and send the gases to acid plants, giving India a domestic acid source tied to its zinc output.

Sources

  1. Price correlation, Material Insights, Material Insights (World Bank, FRED, IMF data)
  2. Extractive metallurgy of zinc (Spectrum Series), AusIMM
  3. High sulfuric acid prices support zinc smelters amid tight domestic concentrate market, Shanghai Metals Market (SMM) (2026)
  4. Chinese zinc smelters maintained high output despite lower profits due to falling prices of sulphuric acid, Shanghai Metals Market (SMM) (2023)
  5. Mineral Commodity Summaries 2026: Sulfur, USGS (2026)
  6. No quick sulfuric acid fix for Chilean copper sector: analysts, S&P Global Commodity Insights (2026)
  7. Dariba Smelting Complex, Hindustan Zinc