Nickel starts as a mineral locked inside rock, and turning it into usable metal requires a chain of increasingly aggressive chemistry: crushing, concentrating, smelting or acid leaching, and finally refining. The exact route depends heavily on whether the ore is a sulfide or a laterite, two fundamentally different geological deposit types that demand different technologies. The process has evolved over more than a century, and today the rising demand for battery-grade nickel is reshaping how producers think about every step from mine to finished product.
Two Kinds of Ore, Two Different Starting Points
Nearly all commercially mined nickel comes from one of two deposit types: sulfide ores and laterite ores. Sulfide deposits form deep underground in igneous rock, where nickel bonds with sulfur in minerals like pentlandite. Laterite deposits form near the surface in tropical regions, where millions of years of weathering concentrate nickel in iron-rich, clay-like soils. Despite the fact that laterite ores hold the majority of the world’s known nickel resources, the bulk of historic production has come from sulfides, largely because sulfide ores are cheaper and simpler to process.1Ore Geology Reviews. Global trends and environmental issues in nickel mining: Sulfides versus laterites That balance is shifting, though. As the richest sulfide deposits are depleted and demand grows, the industry has invested heavily in laterite processing technology.
The distinction matters because it determines everything that follows. Sulfide ores go through a concentration-then-smelting pathway. Laterite ores can go through either a pyrometallurgical route (smelting in a furnace) or a hydrometallurgical route (dissolving the metal out with acid). Each path has different energy costs, different chemical challenges, and different environmental footprints.
Processing Sulfide Ores
When sulfide ore arrives at a processing plant, it first gets crushed and ground into a fine powder. The goal is to physically liberate the nickel-bearing mineral grains from the surrounding waste rock, called gangue. Once the ore is ground fine enough, it enters a process called froth flotation. In flotation, the ground ore is mixed with water and chemical reagents in large tanks. Air is blown through the mixture, creating a froth. The reagents make the nickel-bearing particles hydrophobic, so they attach to air bubbles and float to the surface, where they are skimmed off as a concentrate. The waste rock, now called tailings, sinks and is discarded. This step typically boosts the nickel content from a few percent in the raw ore to somewhere around ten to twenty percent in the concentrate.
The concentrate then goes to a smelter. Flash smelting is one of the dominant technologies for nickel sulfide concentrates. In a flash smelting furnace, dried concentrate is injected into a stream of oxygen-enriched air at the top of a reaction shaft. The sulfide minerals ignite almost instantly in midair, generating enough heat from their own combustion to melt. The molten material separates by density at the bottom: a heavier sulfide layer called matte, which contains the nickel, settles below a lighter slag layer of iron silicates and other waste minerals.2Metallurgical and Materials Transactions B. Reaction Sequences in Flash Smelting and Converting Furnaces: An In-depth View The slag is tapped off and discarded. The matte moves on to a converting step, where more air is blown through the molten material to oxidize remaining iron and sulfur, leaving a high-grade nickel matte.
Flash smelting is efficient because the fuel is essentially the sulfur already present in the ore. The process also captures sulfur dioxide in the off-gas, which gets converted to sulfuric acid rather than escaping into the atmosphere. That sulfuric acid, incidentally, often gets sold as a byproduct or used in other metal extraction processes.
The Laterite Challenge
Laterite ores present a different problem. They contain almost no sulfur, so flotation does not work. The nickel is distributed throughout iron oxide and silicate minerals at relatively low concentrations, often less than two percent. You cannot simply float the nickel away from the waste because it is chemically bound into the mineral structure, not sitting in separate grains. This is the core reason laterite processing lagged behind sulfide processing for so long: the easy separation techniques do not apply.
Two main routes have developed to handle laterites, and which one a producer chooses depends partly on the ore’s composition and partly on what end product is needed.
The Rotary Kiln-Electric Furnace Route
The primary pyrometallurgical route for laterite nickel ores is the Rotary Kiln-Electric Furnace process, commonly known as RKEF. The ore is first dried and then fed into a long rotating kiln, where it is heated to several hundred degrees. Inside the kiln, the ore undergoes calcination, which drives off moisture and chemically bound water, and partial reduction, which begins converting nickel oxides toward a metallic state.3Alexandria Engineering Journal. Improving the rotary kiln-electric furnace process for ferronickel production: Data analytics-based assessment of dust insufflation into the rotary kiln flame The hot, partially reduced material then moves into an electric arc furnace, where temperatures climb high enough to fully melt it. The nickel and iron reduce together and settle as a molten alloy called ferronickel, while the remaining silicate waste floats on top as slag.
Ferronickel is a useful product in its own right. Stainless steel producers, who account for a large share of global nickel consumption, can feed ferronickel directly into their steelmaking furnaces. But ferronickel is not pure nickel. It typically contains a substantial fraction of iron, which means it is not suitable for applications that need high-purity nickel, such as battery cathodes or specialty alloys. The RKEF route is also energy-intensive, consuming roughly twice the energy and producing roughly twice the carbon emissions of the main hydrometallurgical alternative.4China Geology. Nickel extraction from nickel laterites: Processes, resources, environment and cost
High-Pressure Acid Leaching
The hydrometallurgical alternative for laterites is high-pressure acid leaching, or HPAL. Instead of melting the ore, HPAL dissolves the nickel out using sulfuric acid at high temperature and pressure in large, titanium-lined vessels called autoclaves. The acid attacks the ore minerals, pulling nickel and cobalt into solution while leaving most of the iron behind as a residue. Research has shown that at higher temperatures, the primary ore minerals like goethite and gibbsite transform into different phases such as hematite, which changes the composition of what is left behind after leaching.5Hydrometallurgy. High-pressure acid leaching of laterite ores: Effect of acid and solid content on Ni and Co yield under non-isothermal and isothermal conditions
After leaching, the nickel-rich solution goes through a series of purification steps. Impurities like iron, aluminum, and chromium are removed by raising the pH to precipitate them out. The nickel and cobalt are then precipitated as a mixed hydroxide or mixed sulfide intermediate product. This intermediate can be shipped to a refinery for further processing into pure metal or into chemical compounds like nickel sulfate for batteries.
HPAL plants are notoriously difficult to build and operate. The combination of high acidity, high temperature, and high pressure is brutal on equipment, and early HPAL projects in the 1990s and 2000s struggled with cost overruns and reliability problems. The technology has matured since then, and newer plants in Indonesia and other tropical laterite regions have achieved more consistent results. But it remains one of the trickier operations in the metals industry.
Refining to High-Purity Nickel
Whether the intermediate product came from a sulfide smelter or a laterite leach plant, reaching the finish line of high-purity nickel metal requires a refining step. The two most established refining technologies are electrorefining and the carbonyl process.
In electrorefining, slabs of impure nickel are hung as anodes in tanks filled with a nickel-containing electrolyte solution. When electric current passes through, nickel dissolves off the anodes and plates out as pure metal on the cathodes. Impurities either stay in solution, fall to the bottom as anode slime, or are removed by chemical treatment of the electrolyte. The result is nickel cathode with a purity above 99.9 percent, suitable for the most demanding applications.
The carbonyl process takes a completely different approach. Impure nickel reacts with carbon monoxide gas at moderate temperatures to form nickel carbonyl, a volatile compound. Because nickel carbonyl is a gas under these conditions, it separates from solid impurities by simply floating away. The gas is then heated to a higher temperature, causing it to decompose and deposit pure nickel as pellets or powder. The carbonyl process can achieve very high purities and produces nickel in a convenient pellet form. It was pioneered over a century ago and is still used at some facilities today, though it requires careful handling because nickel carbonyl is extremely toxic.
Making Battery-Grade Nickel Sulfate
The explosive growth of lithium-ion batteries has created a new and specific demand: not just pure nickel metal, but nickel sulfate of particular chemical specifications. Battery cathode materials like NMC (nickel-manganese-cobalt) and NCA (nickel-cobalt-aluminum) need nickel delivered as a sulfate salt dissolved in solution, not as solid metal. This has added a new branch to the nickel production tree.
One route dissolves nickel matte, the intermediate product from sulfide smelting, in dilute sulfuric acid. Research on this process has found that dissolution works best at higher temperatures and longer reaction times. At around 95°C and three hours of processing, with manganese oxide added as a catalyst, conversion rates reach about 55 to 60 percent.6IOP Conference Series: Materials Science and Engineering. Preliminary Research of Synthesizing Battery Grade Nickel Sulphate from Dissolution of Nickel Matte in Dilute Sulphuric Acid The resulting nickel sulfate solution then undergoes purification to remove trace metals like copper, cobalt, and iron down to parts-per-million levels, because battery cathode chemistry is sensitive to contamination.
Other routes to battery-grade nickel sulfate start from the mixed hydroxide precipitate produced by HPAL, or even from refined nickel metal that is re-dissolved. The variety of pathways reflects a broader reality: the battery industry’s specifications are demanding enough that producers often need additional processing steps beyond what traditional nickel refining provides. Getting from “pure nickel” to “battery-grade nickel sulfate” is its own engineering challenge.
Environmental Costs Along the Way
Every step in nickel production generates waste and consumes energy, but the environmental burdens are not distributed evenly across the different routes. The RKEF process for laterite ores is particularly carbon-intensive because it requires sustained high temperatures in both the rotary kiln and the electric furnace. As noted earlier, its energy consumption and carbon emissions are roughly double those of the HPAL route for comparable nickel output.4China Geology. Nickel extraction from nickel laterites: Processes, resources, environment and cost
HPAL has its own problems. The process generates massive volumes of acidic, iron-rich residue that presents serious environmental and waste management challenges.7Results in Engineering. Sustainable valorization of lateritic nickel tailings: Valuable metals recovery and gas-purifying materials fabrication Storing this residue safely over the long term, often in tailings dams, carries the risk of leaks or failures that could contaminate waterways and soil. Some operations have used deep-sea tailings disposal, which pipes the waste to the ocean floor, a practice that remains controversial.
Sulfide smelting generates sulfur dioxide, which historically was a major air pollutant around nickel smelters. Modern smelters capture the vast majority of this sulfur dioxide and convert it to sulfuric acid, but older or less regulated operations can still be significant polluters. Mining itself, whether open-pit for laterites or underground for sulfides, transforms landscapes, disrupts ecosystems, and consumes large quantities of water.
Recycling as a Second Source
Recycling is increasingly important to the nickel supply chain, especially as millions of electric vehicle batteries begin reaching end of life. If spent nickel-bearing batteries could be fully recycled, modeling suggests that around 44.5 million tonnes of nickel could eventually be recovered, accounting for roughly a third of total demand for manufacturing new nickel-bearing batteries.8Resources, Conservation and Recycling. The role of nickel recycling from nickel-bearing batteries on alleviating demand-supply gap in China’s industry of new energy vehicles
The recycling process typically begins by shredding spent batteries into a mixture called “black mass,” which contains nickel, cobalt, lithium, manganese, and other materials. The black mass is then leached with acid to dissolve the metals, producing a solution from which individual metals can be recovered. One approach uses electrowinning, where electric current deposits nickel from solution onto a cathode. Research into this process has shown that keeping the nickel concentration in the electrolyte bath stable, rather than letting it decline as nickel plates out, significantly improves the quality and efficiency of the deposit. With bath stabilization and appropriate additives, current efficiencies of 78 to 93 percent and deposit purities above 90 percent have been achieved.9PubMed Central. Electrowinning of Nickel from Lithium-Ion Batteries
Recycled nickel still makes up a modest share of total supply, but that share is expected to grow substantially in the coming decades as the first major waves of electric vehicle batteries reach retirement. The economic case improves as primary ore grades decline and as regulators push for circular material flows.
Growing Nickel With Plants
One of the more surprising developments in nickel extraction is phytomining, a process in which living plants pull nickel from the soil and accumulate it in their tissues. Certain plant species, called hyperaccumulators, can absorb nickel at concentrations hundreds or thousands of times higher than normal plants. After harvesting, the plant biomass is dried and burned, and nickel is recovered from the ash.
This is not purely theoretical. After more than three decades of research, phytomining has reached the stage of commercial-scale implementation for nickel.10PubMed Central. Harnessing hyperaccumulator plants to recover technology-critical metals: where are we at? The approach is particularly interesting for substrates that are too low-grade for conventional mining but still contain enough nickel to feed a hyperaccumulator. Researchers have demonstrated that the hyperaccumulator species Odontarrhena chalcidica can be grown on artificial soils made from industrial galvanic sludge, a nickel-rich waste product, achieving nickel concentrations in shoot tissue above 1,000 mg per kilogram and in some cases exceeding 26,000 mg per kilogram.11PubMed. Nickel phytomining from industrial wastes: Growing nickel hyperaccumulator plants on galvanic sludges Those yields are comparable to what the same plants achieve on natural nickel-rich soils.
Phytomining will not replace conventional mining anytime soon. The volumes are small, and the process is slow, constrained by growing seasons and plant biology. But it offers a genuinely green way to recover nickel from marginal sources, old mine tailings, contaminated industrial sites, and naturally nickel-rich soils that are uneconomic for conventional extraction. It also rehabilitates degraded land in the process, since the plants stabilize soil and reduce erosion while they work.
Why the Route Matters More Than Ever
For most of the twentieth century, the question of how nickel was made had a fairly stable answer: mine sulfide ore, concentrate it by flotation, smelt it, refine it. The product was nickel metal, and the main customer was the stainless steel industry. That picture has become considerably more complicated. The shift toward laterite ores has brought pyrometallurgical and hydrometallurgical processes into competition. The battery boom has created demand for nickel in chemical form rather than as metal, pulling the supply chain in new directions. Recycling is becoming a meaningful input rather than an afterthought.
The choice of production route now carries implications well beyond the factory floor. Battery manufacturers and automakers increasingly scrutinize the carbon footprint of the nickel in their supply chains, which puts pressure on producers to favor lower-emission pathways like HPAL over the more carbon-heavy RKEF process. Indonesia, which has become the world’s dominant nickel producer in recent years, has built out enormous RKEF and HPAL capacity to process its vast laterite reserves, and the environmental management of that rapid expansion is a live and contested issue. The nickel that ends up in your phone battery, your kitchen sink, or the landing gear of an aircraft all traces back to one of these pathways, and the economics, chemistry, and politics of each are in flux.