Smelter Sulfuric Acid: Output, Capture and Where It Goes

A copper or zinc smelter that burns the sulfur out of a sulfide concentrate has to do something with that sulfur, and the usual answer is sulfuric acid. Southern Copper reported about 2.2 million tonnes of acid in 2025 from three sites: the Ilo smelter in Peru (976.5 thousand tonnes), La Caridad in Mexico (1,053.8 thousand) and a zinc refinery at San Luis Potosí (168.6 thousand). As a rule of thumb, Chile’s copper commission measured 3.14 tonnes of acid for every tonne of copper smelted in 2016. That acid mostly goes into leach pads, nickel plants and fertilizer plants, so a smelter outage in one country can raise the cost of mines in another.

This page follows the sulfur from the ore to the customer. It covers which minerals carry the most sulfur, how an acid plant captures sulfur dioxide (SO2) gas, how much acid a smelter makes, where the acid is used and what moves its price. It also has a calculator that turns your own concentrate tonnage into acid tonnes. Most of the world’s sulfur is recovered from oil and gas processing, as the U.S. Geological Survey (USGS) explains; this page deals only with the share that starts in a sulfide mineral. Tonnes are metric unless a long ton is named.

Where the sulfur in mining comes from

Metal ores are often sulfides, minerals in which the metal is bound to sulfur. When the ore is crushed and floated into a concentrate, the sulfur travels with the metal to the smelter. The chart below, drawn from Webmineral’s composition data, shows how much sulfur by weight sits in the common sulfide minerals. Pyrite, an iron sulfide with little commercial metal, carries the most; chalcopyrite, the main copper mineral, carries about a third.

Chart

Sulfur by weight in common sulfide minerals

Percent sulfur by weight, ideal composition (sphalerite includes some iron)

A real concentrate is a mix of minerals, so its sulfur content is lower than the pure-mineral figures. The U.S. Environmental Protection Agency (EPA) describes copper concentrates with about 25 percent sulfur and 25 percent iron, plus 10 to 15 percent water. Your own concentrate assay is the number to use.

The USGS notes that sulfur reserves in sulfide ores are large, and that supply is expected to be adequate. It also counts the metallurgical share separately in the United States: byproduct sulfuric acid came from five nonferrous-metal smelters in four states run by four companies, about 6% of U.S. sulfur production in all forms in 2025. Domestic byproduct acid met about 3% of U.S. consumption, elemental sulfur 63% and imports 34%. For China, the USGS table includes the sulfur in acid made from metallurgical gas and from pyrite in its national total of 19 million tons, which shows that smelter gas and pyrite are counted as sulfur sources there.

How a smelter turns sulfur dioxide gas into acid

Smelting and converting oxidize the sulfide in the concentrate. The metal goes into matte (a molten copper-iron sulfide), then blister copper (about 98 to 99 percent copper), and the sulfur leaves as SO2 in the furnace gas. Releasing that gas would be both a regulatory problem and a lost product, so most smelters pass it through an acid plant. EPA says control of SO2 at smelters is commonly done in a sulfuric acid plant, which needs a gas with the dust removed and at least 3 percent SO2 to run.

Infographic

From concentrate to acid in 5 steps

1. 1Smelt and convertFurnaces oxidize the sulfide, so the metal goes to matte and blister copper and the sulfur leaves as SO2 gas.
2. 2Clean the gasDust is removed and the gas is dried before it reaches the acid plant.
3. 3Check the strengthThe plant needs at least 3 percent SO2 to run, so weak gas is a problem.
4. 4Convert to SO3A catalyst adds oxygen to SO2 to make sulfur trioxide.
5. 5Absorb into acidSO3 is absorbed in strong acid, giving about 98.5% sulfuric acid to sell or use.

Chart

SO2 strength in smelter off-gas, by source

Typical volume percent SO2; an acid plant needs at least 3 percent

That 3 percent minimum is why the type of furnace matters. The chart shows EPA’s typical SO2 strength by source. A flash furnace gives a strong, steady stream, while an old reverberatory furnace gives a weak one that EPA says is usually released without SO2 reduction.

Inside the plant, the gas is dried and passed over a catalyst that adds oxygen to make sulfur trioxide (SO3), which is absorbed in strong acid to make more acid. EPA’s AP-42 section 12.3 (text dated 1986, reformatted 1995) reports that single-contact plants convert 92.5 to 98 percent of the SO2 in the plant gas, while double-contact plants collect 98 to more than 99 percent and emit about 500 parts per million (ppm) of SO2. Southern Copper describes the Ilo smelter gas stream, at 11.34% SO2, as being split between two double-absorption, double-contact plants; the company states that the off-gas treatment recovers over 92% of the sulfur arriving in the concentrate and produces 98.5% acid.

Those two numbers measure different things. The plant conversion rate says how much of the SO2 that enters the acid plant is converted. The whole-smelter capture rate says how much of the sulfur in the concentrate ends up as acid, after fugitive gas and gas that bypasses the plant. When you read a capture figure, check which one it is.

The weight arithmetic is simple. One tonne of sulfur (atomic mass 32.06) makes 98.08 / 32.06 = 3.06 tonnes of 100% sulfuric acid, so a plant that sells 98.5% acid ships about 3.11 tonnes of product per tonne of sulfur captured. EPA also notes that some U.S. smelters have absorbed SO2 in a dimethylaniline solution to make liquid SO2, an alternative product to acid. Plain SO2 dissolved in water gives sulfurous acid, a weak acid that is not what smelters sell as acid.

▶ Explains the seven environmental impacts of mining in plain words, including air emissions such as sulfur dioxide. · 2:31

How much acid does a copper smelter make per tonne of copper?

Chile’s copper commission, Cochilco, divides the acid made at copper smelter-refinery complexes by the copper smelted (4.687 million tonnes of acid in 2016, not the national total of 5.708 million, which includes sulfur burners). Over 2007 to 2016 the ratio ran between 3.04 and 3.44 tonnes of acid per tonne of copper and stood at 3.14 in 2016. Cochilco attributes the changes to the capture rate and the composition of the sulfide ore being smelted. The ratio also tracks the sulfur in the concentrate: a high-pyrite feed gives more acid per tonne of copper than a clean chalcocite feed.

Southern Copper’s 2025 numbers fit that range. The Ilo smelter made 305.2 thousand tonnes of copper anode and 976.5 thousand tonnes of acid, or about 3.2 tonnes of acid per tonne of anode. Acid fell 17.6% from 2024 while anode output fell 15.1%, so the two moved together, as the ratio would suggest. The table gives the company’s three acid sites.

Source: Southern Copper Corporation, Form 10-K for 2025. SX-EW means solvent extraction and electrowinning, the leach-based route that makes copper cathode directly from oxide ore and leach solutions. The company says it sells the balance to mining and fertilizer companies, mainly in Mexico, Peru, the United States and Chile.

The calculator below turns your own concentrate tonnage and sulfur grade into an acid estimate. It uses the 3.06 weight factor from above and lets you set the capture rate, so you can test the 92% Ilo case against a better or worse plant.

Is smelter acid a profit or a by-product?

For a copper smelter the acid is a by-product that has to go somewhere. Southern Copper’s by-product credits show how big it is next to other products. In 2025 the company booked US$261.4 million of sulfuric acid credits, about US$0.13 per pound of copper, against US$1,405.1 million for molybdenum and US$880.4 million for silver. The acid credit was US$262.4 million in 2024 and US$318.4 million in 2023, so it has been a steady, mid-sized line rather than a swing factor. As of 31 December 2025 the company also had contracts to deliver an estimated 353,000 tonnes of acid in 2026, subject to annual volume negotiations.

Chart

By-product credits at Southern Copper, 2025

US$ million; credits against operating cash cost

Because the acid cannot be switched off, a smelter needs a buyer within reach. A smelter near leach operations or a port has more buyers than one far inland. Southern Copper’s Ilo smelter has a marine trestle, started in 2010, that lets acid be loaded directly onto ships without hauling it through the city of Ilo.

Smelter acid or burnt-sulfur acid: two routes to the same product

Sulfuric acid can be made from smelter gas or by burning elemental sulfur in a furnace and feeding the SO2 into the same type of contact plant. The difference is what drives output. A smelter makes acid because it is processing concentrate, whatever the acid market looks like; Cochilco’s reports call this smelter route “obligatory” production and count burner output separately. A sulfur burner’s output depends on the price of sulfur against the price of acid.

The two routes dominate in different places. In Indonesia, CRU says burnt-sulfur acid is the main source because the nickel industry built sulfur-burning capacity alongside its plants. In the United States, the USGS reports that domestic elemental sulfur met 63% of consumption and byproduct acid about 3%. In Chile, smelters and some mine-side sulfur burners share the market. For a miner choosing where to site a leach operation, the practical question is which route will supply the acid in the region and what each does when prices swing.

Where sulfuric acid goes: five uses compared

Smelter acid is one of several sources for each use below. The comparison shows scale and what is at stake for each buyer, using the latest figure each source gives.

Copper leaching and SX-EW in Chile

Chile makes less acid than it uses. Cochilco’s balance, reported by Diario Financiero, shows 4.8 million tonnes of national production and 3.4 million tonnes of imports in 2023, for total consumption of 8.29 million tonnes. Cochilco expects production to stay near 5.7 million tonnes from 2029 to 2033, so the gap widens as consumption stays high. The commission publishes an annual acid market report with data and projections for smelters and leach operations.

The acid is used to dissolve copper from oxide ore on a leach pad. Acid use per tonne of cathode has risen as the ore gets poorer: Cochilco found it went from 3.22 tonnes of acid per tonne of SX-EW cathode in 2007 to 4.29 in 2016, while the average oxide copper grade fell from 0.717% to 0.500%. A simple comparison of the two ratios shows why a deficit builds: a tonne of smelted copper yields about 3 tonnes of acid, while a tonne of leached cathode uses more than 4.

In April 2026, Reuters reported that China’s acid exports to Chile fell to zero in March, from 151,268 tonnes in March 2025. HSBC data cited in the story put China at 37% of Chile’s acid imports, and Morgan Stanley estimated that 1.1 million tonnes of Chile’s leached copper could be at risk from a Chinese export ban. Rio Tinto’s chief commercial officer said Chile is the country most exposed to acid imports because it has the most leached copper.

Mines respond by contracting and by changing the plan. Capstone Copper said in its results of 30 July 2026 that it expected to use about 243,000 tonnes of acid over the rest of 2026, with 80% fixed-price at an average of US$190 per tonne CFR Chile (cost and freight, delivered to a Chilean port) and the rest on variable pricing. Its Mantoverde mine cut heap throughput of high-carbonate ore, which uses more acid per tonne, and prioritized sulfide production to avoid spot purchases. A US$45 million pyrite augmentation project there aims to cut heap acid needs by about 20%, or roughly 90,000 tonnes a year, by recovering pyrite in the concentrator.

Copperbelt leaching in Zambia and the DRC

The Copperbelt leaches copper and cobalt from oxide ores, and acid is a main input. Zambian smelters make about 2 million tonnes of acid a year, mostly as a byproduct used by local mines, with the surplus going to the Democratic Republic of Congo (DRC). Zambia banned acid exports in September 2025 and moved to a permit policy in March 2026. On 14 May 2026 its trade minister said Chambishi Copper Smelter and Mopani Copper Mines could resume shipments in limited quantities after local stocks recovered. DRC miners had cut usage and considered output cuts. Argus reported that in the second quarter of 2026 US Gulf sulfur reached unusual buyers, including east African copper-belt consumers, as Middle East exports fell.

Nickel HPAL plants in Indonesia

High-pressure acid leach (HPAL) plants dissolve nickel and cobalt from laterite ore in hot acid. CRU reports that nickel-related acid demand in Indonesia reached 16 million tonnes a year in 2025, up from 3.5 million in 2021. Most of the acid there is made by burning imported sulfur, not captured from smelters, but imports of finished acid grew too, and CRU’s CFR Indonesia price rose from US$150 to 185 per tonne in the first quarter of 2026 to US$380 to 410 by late May. The Cobalt Institute’s market report says imported sulfur costs rose 165% in 2025, from US$182 to US$483 per tonne, and that each tonne of nickel needs about 11.4 tonnes of sulfur; it adds that 75% of Indonesia’s sulfur comes from the Middle East.

Phosphate fertilizer

The Fertilizer Institute (TFI) says about 90 percent of the world’s sulfur is converted into sulfuric acid, and 60 to 75 percent of that acid is used to make phosphate fertilizer. It takes 4 tonnes of sulfur for every 10 tonnes of finished MAP or DAP. The USGS expects new sulfur demand from phosphate fertilizer projects mostly in Africa and west Asia. Fertilizer plants and leach pads both buy sulfur or acid, so a high sulfur price reaches both.

Chile’s acid gap: what two Cochilco reports show

Chile is the best-documented acid market, because Cochilco surveys every producer and consumer each year. Two of its reports, seven years apart, show how the balance moved. The 2016 survey counted 7.494 million tonnes of consumption and 5.708 million tonnes of production, a gap of about 1.8 million tonnes. By 2023 consumption was 8.29 million tonnes and production 4.8 million.

Source: Cochilco, acid market report published in 2017 (2016 data); Diario Financiero report on Cochilco’s 2024 report (2023 data).

The reports also show how fast a forecast can turn. In the 2017 report, Cochilco’s maximum-balance case had Chile moving from deficit to surplus in 2022, with a surplus of 33.4% of 2016 production by 2026. The later report, published in 2024, instead sees a deficit through 2033, because high copper prices extended the life of leach operations. The 2017 report also records that acid made in sulfur burners at copper mines fell 18% in 2016, and that the closure of the Haldeman sulfur burner cut production in the Tarapacá region by 96.3%. A mine plan that assumes cheap local acid should be tested against both outcomes.

Sulfur and acid prices: how the swings reach the mine gate

Acid prices follow sulfur, and sulfur has moved sharply. The USGS reports an average U.S. unit value of US$180 per metric ton of elemental sulfur in 2025 (estimated), up from US$46.42 in 2024 and US$58.90 in 2023. Its Tampa, Florida contract series started 2025 at US$116 per long ton, rose to US$310 by early October 2025 and the USGS said fourth-quarter 2025 prices were the highest since the second quarter of 2022. Argus then reported that the third-quarter 2026 Tampa contract settled at a record US$705 per long ton delivered on 13 July 2026, up US$50 from US$655 in the prior quarter. A long ton is 2,240 pounds, or 1.016 tonnes.

Chart

Tampa contract price for sulfur

US$ per long ton at the dates shown; no value is plotted for the first quarter of 2026

Dated spot checks show prices easing from the peak. FertilizerField reported the Kuwait Petroleum Corporation October 2026 price at US$820 per tonne FOB (the price at the port of loading, before sea freight), down US$45 from September. Trading Economics showed its sulfur benchmark price at 6,669 yuan per tonne on 15 October 2026, down 6.94% on the day and 13.43% over the month, after an all-time high of 11,084 yuan in June 2026.

For a mine, the sensitivity can be small or large. Capstone estimates that each US$25 per tonne change in acid costs about US$1 million in consolidated C1 cash cost (the cash cost per payable pound of copper) over the rest of 2026, less than US$0.01 per payable pound. For its Mantoverde pyrite project it estimated yearly acid savings of US$18 million to US$40 million at acid prices of US$200 to US$450 per tonne. A leach-heavy operation with no contract cover feels much more.

What to check on a sulfide project before counting on acid

Consultants and licence holders weighing a sulfide or mixed oxide-sulfide project can use this short list to test the acid side of the plan.

1. Concentrate sulfur grade. Get the assay by mineral. A pyrite-rich concentrate gives more acid per tonne of metal and may carry penalties at the smelter.
2. Smelter route. Ask which smelter would treat the concentrate, its capture rate and whether it sells or neutralizes acid.
3. Acid buyer within reach. Count the haul to the nearest leach pad, port or fertilizer plant, and the number of buyers.
4. Own acid demand. If the project will leach oxide ore, estimate acid use per tonne of cathode and compare it with local supply, using ranges like Cochilco’s 3 to 4+ tonnes per tonne.
5. Acid price exposure. Test your cost model at a low and a high price, such as the US$190 contract price and the US$475 spot price Capstone quoted for Chile in July 2026.
6. Gas strength and permits. Check that the process can give the SO2 strength an acid plant needs, and what limits the regulator sets on emissions.

Where satellite images fit

Acid supply starts with the geology: whether a district holds copper, zinc or nickel sulfides, and how much pyrite comes with them. Farmonaut screens an area from satellite images, using a KML, KMZ or polygon, to flag zones that look like base-metal targets, including copper, zinc and nickel, before field crews go in. See satellite-based mineral detection for the method. Results are exploration targets, not resource estimates.

Frequently asked questions

How much sulfuric acid does a copper smelter make?

About 3 tonnes of acid per tonne of copper smelted. Cochilco measured 3.14 in Chile in 2016, in a 3.04 to 3.44 range for 2007 to 2016. Southern Copper’s Ilo smelter made 976.5 thousand tonnes of acid and 305.2 thousand tonnes of anode in 2025, about 3.2 to 1.

What happens to the sulfur dioxide from smelting?

Most of it is cleaned, dried and converted to sulfur trioxide over a catalyst, then absorbed to make sulfuric acid. EPA says this is the common way to control SO2 at copper smelters, provided the gas holds at least 3 percent SO2.

How much SO2 does an acid plant capture?

EPA gives 92.5 to 98 percent conversion for single-contact plants and 98 to more than 99 percent for double-contact plants. Whole-smelter capture is lower than plant conversion. Southern Copper states that Ilo recovers over 92% of incoming sulfur.

Is sulfur dioxide solution the same as sulfuric acid?

No. SO2 dissolved in water makes sulfurous acid, which is weak and unstable. A smelter acid plant adds oxygen to make SO3 and then sulfuric acid, a strong acid sold at about 98% strength. Some smelters have also made liquid SO2 as a separate product, according to EPA.

Which minerals carry the most sulfur?

By weight, pyrite is highest at 53.45% sulfur, then molybdenite at 40.06%, chalcopyrite at 34.94%, sphalerite at 33.06%, chalcocite at 20.15% and galena at 13.40% (Webmineral).

Where does sulfuric acid go?

To copper and cobalt leaching, nickel HPAL plants and phosphate fertilizer plants, plus other chemical uses. The share depends on the region: Chile and the Copperbelt use it mainly for leaching.

Why does Chile import acid?

Leaching uses more acid than its smelters make. Cochilco’s balance shows 8.29 million tonnes consumed in 2023, against 4.8 million tonnes made at home and 3.4 million tonnes imported.

How much of the world’s sulfur comes from mining and smelting?

The USGS does not give a world split. In the United States, byproduct acid from five smelters was about 6% of sulfur production in 2025, with refineries and gas plants supplying most of the rest.

What is the price of sulfur and acid?

There is no single price. Tampa contract sulfur settled at US$705 per long ton for the third quarter of 2026 (Argus, 13 July 2026), and Kuwait set US$820 per tonne FOB for October 2026. Acid is priced by region, such as CFR Chile or CFR Indonesia, and the figures above are dated.

Can satellites help find sulfide deposits?

Satellite data can screen large areas for alteration and structures that often go with base-metal sulfide deposits, so a field team knows where to sample first. It does not replace drilling or an assay.