Sulfur in HPAL Nickel Processing

What is HPAL and which nickel ore does it treat?

HPAL is a hydrometallurgical process that dissolves nickel and cobalt from laterite ore with sulfuric acid at high temperature and pressure. Total Materia describes the process as particularly suited to limonite, a laterite ore high in iron and low in magnesium and silica.

In the iron-rich limonite layer, nickel sits inside the mineral goethite (USGS). The USGS Mineral Commodity Summaries 2026 put about 54% of world nickel resources in laterite deposits. In Indonesia, the energy and mineral resources ministry (ESDM) stated in July 2020 that low-grade limonite would go through the hydrometallurgical route.

Leaching runs in titanium-lined autoclaves at about 240–270 °C and 3.8–5.4 MPa, with a residence time of 30–90 minutes (Total Materia). The leach reactions absorb heat, so the ore slurry is first preheated with steam injection (Sherritt patent US12286686B2).

Why does HPAL need sulfuric acid?

HPAL needs sulfuric acid because the nickel in limonite is locked inside iron minerals. A 2003 SGS technical paper explains that nearly all of the iron must react with acid for nickel extraction to reach 90% or more.

If that iron stayed dissolved, stoichiometric acid demand would be about 700–800 kg of H₂SO₄ per tonne of ore or more. At HPAL temperatures, dissolved iron hydrolyses to hematite (Fe₂O₃) and releases acid again. Laboratory tests on one limonite by Abbasi Gharaei and colleagues (2024) found that this keeps net acid use in HPAL below that of atmospheric leaching.

Some acid is still consumed for good, mainly by the magnesium and aluminium in the ore (SGS). The solution leaving the autoclave also carries about 30–60 g/L of free acid (Total Materia). SGS names sulfuric acid and the energy to heat pulp above 240 °C as the two main operating costs of HPAL.

How much sulfuric acid does HPAL use per tonne of ore?

Total Materia puts typical HPAL acid consumption at 200–520 kg of H₂SO₄ per tonne of dry ore. The figure varies from plant to plant because it depends on ore composition, especially magnesium and aluminium content.

The table holds historical data up to 2003, none of it from Indonesian plants. It shows that some early plants made acid from sulfur while others bought it in.

A Sherritt patent granted in April 2025 notes that HPAL economics are closely tied to sulfur cost and to acid demand per tonne of nickel.

How does an acid plant turn sulfur into sulfuric acid?

A sulfur-burning acid plant uses the contact process: sulfur is burned to SO₂, SO₂ is oxidised to SO₃, and SO₃ is absorbed to form H₂SO₄. In theory, 1 tonne of sulfur yields about 3 tonnes of sulfuric acid.

This sequence follows US EPA AP-42 section 8.10. Argus (October 2024) gives the 1:3 ratio for sulfur burners. SGS (2003) reached the same ratio in a test that oxidised sulfur inside limonite pulp in an autoclave: adding 171 kg of sulfur per tonne of ore equals 513 kg of acid at full conversion. Actual output in an acid plant depends on its efficiency.

The industry also counts sulfur per tonne of nickel. FINI, quoted by CNBC Indonesia (April 2026), estimated 10–12 tonnes of sulfur per tonne of nickel in MHP, while Mysteel data published by Petromindo (March 2026) cited 8–10 tonnes per tonne of MHP. The basis differs between sources (tonnes of MHP or tonnes of contained nickel), so these figures work only as ranges.

1. Solid sulfur is melted and filtered to remove ash, then the molten sulfur is sprayed into a furnace.
2. The sulfur burns in air to form SO₂. A waste heat boiler cools the hot gas and raises steam.
3. SO₂ reacts with oxygen to form SO₃ over a vanadium pentoxide catalyst in the converter. This releases a lot of heat, and the converter exit gas is cooled again to raise more steam.
4. SO₃ is absorbed in sulfuric acid of about 98% strength to form H₂SO₄.

Why does steam from the acid plant matter in HPAL?

Making sulfuric acid from sulfur releases heat that can be recovered as steam, and HPAL needs steam to heat its slurry. Many HPAL plants therefore rely on their sulfuric acid plant for both acid and high-pressure steam (Sherritt patent).

SGS (2003) calculated that producing 1 tonne of H₂SO₄ releases about 5.4 GJ of heat in theory. A conventional acid plant recovers about 70% of that heat in usable form, and a high-efficiency plant can recover 90% or more.

In an HPAL plant, flash steam from the autoclave discharge is topped up with fresh steam from the acid plant or a power plant, and most of it heats slurry. The same patent notes that acid plants started and stopped often have shorter lives. Since the acid plant supplies both acid and steam, a steady sulfur feed matters for both.

What is MHP and how does it relate to HPAL?

MHP (mixed hydroxide precipitate) is the nickel-cobalt intermediate product of HPAL. Nickel and cobalt are precipitated from the leach solution by adding a base such as MgO, NaOH or Ca(OH)₂ (Sherritt patent).

The alternative product is MSP (mixed sulphide precipitate), precipitated with H₂S. ESDM described MHP in July 2020 as a starting material for battery production. It still needs further processing before it becomes battery material for electric vehicles.

Why is sulfur supply a planning issue for HPAL in Indonesia?

According to CRU (May 2026), nickel-related acid demand in Indonesia has risen fast, and Indonesia's overall acid requirements now exceed local supply. Acid from sulfur burning is the main source, and most of that sulfur is imported.

Indonesia's first HPAL autoclave began trial production in late April 2021 on Obi Island, North Maluku (International Mining, April 2021). CRU (May 2026) estimates nickel-related acid demand in Indonesia at 16 million tonnes a year in 2025, up from 3.5 million tonnes in 2021.

Mysteel data published by Petromindo (March 2026) put Indonesia's 2025 sulfur imports at about 5.35 million tonnes, up 48% year on year. Perhapi, quoted by Kontan (April 2026), noted that supply generally comes from Gulf states through the Strait of Hormuz.

The 2026 supply constraints show up in forecasts. BMI, in a Bloomberg Technoz report republished by IMA (July 2026), expected 2026 HPAL output growth to fall well below its five-year average because Persian Gulf sulfur supply was constrained. The USGS (February 2026) also expects rising HPAL nickel output to lift sulfur demand.

What do procurement teams plan for when buying HPAL sulfur?

Procurement teams buying sulfur for HPAL acid plants usually plan around continuous volume, loading schedules, purity and impurities, and documents.

1. Volume and continuity. Base demand on your own ore tests and acid plant efficiency, not on generic figures.
2. Schedule and reserves. Kadin (April 2026) recommended medium- to long-term contracts and buffer reserves.
3. Supply sources. Perhapi (April 2026) recommended diversifying so that supply does not depend on one region.
4. Purity and impurities. The US EPA notes that sulfur with hydrocarbon impurities produces more water vapour and acid mist in an acid plant. Ask the seller for the full specification, and test a sample where possible.
5. Form and carriage schedule. The IMSBC Code separates lump sulfur from formed sulfur such as granules or prills, and each has its own carriage schedule.
6. Documents and trade terms. Agree who does what. Under FOB, for example, the buyer nominates the vessel, arranges insurance and handles import permits and duties.

What does this mean for FOB sulfur buyers?

Under FOB, the seller delivers the goods on board the vessel nominated by the buyer at the loading port, and the buyer arranges shipping to Indonesia. PT Samudra Rayya Nusantara is a trading company that sells lump sulfur from Oman with 99.54% purity FOB Sohar Port. The sulfur comes in jumbo bags, 500–20,000 tonnes per transaction. Sohar is on the Gulf of Oman coast, outside the Strait of Hormuz.

Goods stay in the country of origin. The company holds no stock in Indonesia and does not ship. Further specifications come with the offer. Samples and meetings are available to buyers with a clear purchase plan.

Nickel smelter requirements are covered on the sulfur for nickel smelters page, and product details are on the lump sulfur page.