How Is Nickel Extracted From Its Ores?

Nickel extraction is not one fixed sequence. The route depends mainly on whether the ore is a sulfide or a laterite (an oxide-rich, weathered ore). Processing often produces an intermediate such as a nickel-rich matte, ferronickel, or a nickel compound rather than pure nickel metal straight from the ore.

The two main nickel ore types

Important sulfide minerals include pentlandite, often found with iron sulfides and copper-bearing minerals. Laterite deposits form when rocks weather in warm, wet climates. Their nickel-bearing minerals occur in zones that can include limonite, rich in iron oxides, and saprolite, which contains magnesium silicates.

Because the minerals and impurities differ, sulfide ores are commonly concentrated and smelted, while laterites are treated by either hydrometallurgy (leaching metals into solution) or pyrometallurgy (high-temperature processing).

Sulfide ore: concentration and smelting

First, the ore is crushed and ground to free the valuable mineral grains. Froth flotation separates many nickel sulfide minerals from waste rock: chemicals make selected mineral grains attach to air bubbles, which carry them into a froth. The resulting concentrate contains a higher proportion of nickel than the original ore, but it still contains other metals and minerals.

The concentrate is smelted with flux and oxygen. Iron and other impurities are separated into a slag, while a molten matte—a mixture rich in metal sulfides, commonly including nickel and copper sulfides—forms beneath it. Converting the matte with oxygen removes more iron and sulfur. Sulfur dioxide in the furnace gases can be captured and processed rather than simply released.

The matte is then refined. Depending on its composition and the desired product, refiners may leach it and recover nickel from solution, or use electrorefining. These stages separate nickel from copper, cobalt, and other metals. The final product can be high-purity nickel metal, but some operations instead make an intermediate for another refinery.

Laterite ore: leaching or smelting

Laterite ores are commonly treated by one of two routes:

• High-pressure acid leaching (HPAL): A limonite-rich ore is treated with sulfuric acid at elevated temperature and pressure. Nickel and often cobalt dissolve into the solution. Impurities are removed, and nickel can be recovered in a mixed hydroxide or sulfide product, or further purified and recovered as metal.
• Reductive smelting: A saprolite-rich ore is dried and smelted with a reducing agent. The process produces ferronickel—an iron-nickel alloy—or nickel pig iron. These products are used mainly as feedstock for stainless-steel production; they are not the same as pure nickel.

The choice depends on ore composition, energy and reagent costs, available equipment, and the product a plant is designed to make. A deposit may also contain different ore zones that need different treatment.

Worked example: estimating nickel in an ore shipment

Suppose a shipment contains 100 tonnes of ore with a nickel grade of 1.5% by mass. The contained nickel is:

$$100\ \text{tonnes}\times 0.015=1.5\ \text{tonnes of nickel}$$

If a hypothetical processing route recovers 85% of that nickel, the recovered amount is:

$$1.5\ \text{tonnes}\times 0.85=1.275\ \text{tonnes of nickel}$$

This is a simplified mass-balance example, not a claim that every plant achieves an 85% recovery. It also gives the mass of nickel recovered, not necessarily the mass of the saleable product: ferronickel, for example, contains iron as well as nickel.

How nickel metal may be purified

One specialized refining method is the Mond process, also called carbonyl refining. Nickel reacts with carbon monoxide to form volatile nickel tetracarbonyl, which can be separated from some impurities and decomposed to deposit nickel:

$$\mathrm{Ni + 4CO \rightleftharpoons Ni(CO)_4}$$ $$\mathrm{Ni(CO)_4 \rightarrow Ni + 4CO}$$

The reactions are carried out under different conditions: carbonyl forms at a relatively lower temperature and decomposes at a higher temperature. Nickel tetracarbonyl is extremely toxic, so this is a specialized industrial process requiring strict containment. It is not the standard route for every nickel ore or refinery.

Common mistakes

• Treating all nickel ores alike: Sulfide and laterite ores usually need different preparation and extraction routes.
• Calling every nickel product “pure nickel”: Matte, ferronickel, nickel pig iron, and mixed hydroxide are intermediate or alloy products with different compositions.
• Assuming flotation extracts metal: Flotation concentrates mineral grains; smelting or leaching and subsequent refining are still needed.
• Confusing grade with recovery: Grade is the nickel proportion in the ore. Recovery is the fraction of its contained nickel that reaches a specified product.
• Leaving out by-products and emissions: Iron-rich slag, sulfur-bearing gases, and other residues need appropriate handling as part of the process.