Nickel and copper are separated using their differences in physical density, chemical reactivity, and electrochemical behavior. Because the two metals often occur together in nature and in manufactured alloys, several distinct methods have been developed depending on whether you’re starting with raw ore, a smelted intermediate, or scrap metal. The main approaches fall into five categories: flotation, smelting, chemical leaching, electrorefining, and the Mond carbonyl process.
Flotation: Separating the Minerals Before Smelting
When nickel and copper come out of the ground, they’re locked in sulfide minerals. The nickel sits primarily in pentlandite, while the copper is concentrated in chalcopyrite. Froth flotation exploits differences in how these mineral surfaces interact with water and air bubbles to physically sort them.
In practice, the crushed ore is mixed with water and chemical reagents, then air is pumped through the slurry. Certain minerals attach to the rising bubbles and float to the surface, while others sink. To separate copper-bearing minerals from nickel-bearing ones, operators depress the nickel mineral (keep it from floating) while allowing the copper mineral to rise. This is done commercially by adding lime to raise the pH, by using a combination of sulfur dioxide and diethylenetriamine, or by adding sodium cyanide. Each of these reagents selectively coats the pentlandite surface so it stays in the slurry while chalcopyrite floats off for collection.
Smelting Into Matte
After flotation concentrates are produced, they’re typically smelted at 1,200 to 1,300°C to create what’s called a nickel-copper matte. At these temperatures, nickel, copper, and cobalt bond strongly with sulfur and collect in a dense molten layer, while iron bonds preferentially with oxygen and silica to form a lighter slag that floats on top. The two layers are separated by density, much like oil and water.
The resulting matte contains a complex mix of metal sulfides and alloys: pentlandite, bornite, chalcocite, copper-nickel alloy, iron-nickel alloy, and pyrrhotite, along with small amounts of precious metals. To pull the nickel and copper apart from this matte, processors turn to either pyrometallurgical or hydrometallurgical methods.
Pyrometallurgical Roasting
The matte can be roasted under controlled conditions to convert the metal sulfides into oxides or soluble salts that are easier to separate. The main variations include calcification roasting (with calcium compounds), chlorination roasting (with chlorine-bearing reagents), sulfation roasting (converting metals to sulfates), and oxidation roasting. Each approach changes the chemistry of one metal faster or more completely than the other, creating a mixture where selective dissolution or physical separation becomes possible.
Chemical Leaching
Hydrometallurgy dissolves the metals into solution, then separates them using their different chemical behaviors. One of the most established approaches is ammonia pressure leaching, developed at the Sherritt Gordon operation in Canada. In this process, the matte or concentrate is placed in a sealed vessel (autoclave) at around 100 psig total pressure, with a partial oxygen pressure of about 10 psi. The temperature is held between 160°F and 190°F, and the solution contains 50 to 60 grams per liter of free ammonia. Under these conditions, nickel, copper, and cobalt dissolve as ammonia complexes while iron is rejected into the solid residue.
Once both metals are in solution, the next challenge is pulling them apart. Solvent extraction is the most common route. A chelating reagent dissolved in kerosene is mixed with the metal-bearing solution. Different reagents grab copper and nickel at different rates and under different conditions. One well-studied system uses a reagent called LIX 984N diluted in kerosene: it extracts both copper and nickel from ammonia-based solutions, but the two metals can then be stripped selectively using acid at different strengths. Copper comes out first under acidic conditions, and nickel is recovered afterward from a basic (alkaline) solution.
Electrorefining and Electrowinning
Electricity provides one of the cleanest separations of nickel and copper, because the two metals deposit onto a cathode at very different voltages. Copper requires roughly 0.6 volts less energy to plate out of solution than nickel does. This voltage gap means you can set up an electrolytic cell that deposits copper first, leaving nickel behind in the liquid.
In industrial practice, the copper is deposited from an acidic solution at a pH of about 2.0 to 2.2 and a temperature of 40°C. Once the copper is removed, the remaining nickel-rich solution is made alkaline (typically to a pH above 10.5) by adding ammonia, and a second electrolysis step plates out the nickel. This two-stage approach achieves current efficiencies around 95%, meaning almost all the electrical energy goes toward depositing metal rather than being wasted on side reactions. Running the nickel step in alkaline conditions is key: it suppresses hydrogen gas formation at the cathode, which would otherwise waste energy and reduce purity.
The Mond Carbonyl Process
The Mond process, discovered by Ludwig Mond in 1890, is uniquely selective for nickel. It works by passing carbon monoxide gas over a mixture of metals at 50 to 60°C. At this temperature, nickel reacts with carbon monoxide to form nickel carbonyl, a volatile gas. Copper does not form a stable carbonyl under these conditions, so it stays behind as a solid.
The nickel carbonyl gas is then piped to a separate chamber and heated to 200 to 280°C, where it decomposes back into pure solid nickel and carbon monoxide. The carbon monoxide is recycled. This produces extremely high-purity nickel in a single pass, and the copper-rich residue left behind can be processed separately.
There is a serious safety consideration with this method. Nickel carbonyl is extraordinarily toxic and a recognized occupational carcinogen. The workplace exposure limit set by both NIOSH and OSHA is just 0.001 parts per million, and concentrations above 2 ppm are classified as immediately dangerous to life and health. Exposure to just 3 ppm for 30 minutes can be lethal. Industrial facilities using this process require rigorous gas containment, continuous air monitoring, and specialized respiratory protection.
Separating Nickel From Copper in Scrap and Alloys
Recyclers face a different version of this problem when processing scrap alloys like Monel (about 67% nickel, 30% copper) or cupronickel (used in coins and marine hardware), as well as metal-coated plastics from electronics. The metals in these materials are already alloyed at the atomic level, so physical methods like flotation won’t work. Instead, the alloy is dissolved in acid or ammonia-based solutions, and the metals are separated from the liquid.
The main separation techniques for recycled material include chemical precipitation, ion exchange resins, adsorption with activated carbon, and solvent extraction. One effective approach for electronics waste uses a pair of chemical extractants (LIX 84-I mixed with TBP) in an ammonia leach solution. The two extractants interact with each other in a way that suppresses nickel extraction while allowing copper to be pulled out selectively. In testing with real waste solutions containing about 8 grams per liter of copper and 2.5 grams per liter of nickel, this method achieved complete copper extraction with zero nickel contamination across four counter-current stages. The copper was then stripped from the organic phase using sulfuric acid, producing a concentrated copper solution of about 21 grams per liter. Alkaline leaching routes for this type of recycling also carry a lower carbon footprint than acid-based alternatives.