Nickel

Properties

Nickel is a silvery-white metal with a faint golden tint that can be polished to a high shine. Alongside iron, cobalt, and gadolinium, it is one of just four elements that are ferromagnetic near room temperature. Above 355 degrees Celsius, its Curie temperature, bulk nickel loses its magnetism. Its crystal structure is a face-centered cube with a lattice parameter of 0.352 nanometers, which gives each atom a radius of 0.124 nanometers.

This arrangement remains stable under pressures up to at least 70 gigapascals. The metal is hard, malleable, and ductile, and it conducts electricity and heat relatively well for a transition metal. An ideal crystal of nickel would have a compressive strength of 34 gigapascals, but in real bulk material, dislocations prevent this from being achieved; however, nickel nanoparticles have reached that value. Nickel has two possible electron configurations, [Ar] 3d8 4s2 and [Ar] 3d9 4s1, which are extremely close in energy, and experts disagree on which is lower.

Chemistry textbooks typically list it as [Ar] 4s2 3d8, following the Madelung rule that 4s fills before 3d. Yet the average energy of the [Ar] 3d9 4s1 states is actually lower than that of the [Ar] 3d8 4s2 states, so atomic research literature gives the ground state as [Ar] 3d9 4s1. Naturally occurring nickel consists of five stable isotopes, with nickel-58 being the most common at 68.077 percent abundance. Nickel-62 has the highest binding energy per nucleon of any known nuclide, at 8.7946 megaelectronvolts per nucleon, but in stars, intense photodisintegration breaks down nickel, making iron the far more abundant heavy element.

Production

Nickel is produced through extractive metallurgy, starting with conventional roasting and reduction of ore to create metal that is over 75% pure. For many stainless steel uses, this level of purity is sufficient without additional refining, depending on the impurities present. Historically, most sulfide ores are first concentrated using froth flotation, then processed with pyrometallurgical methods to create a matte for further treatment. Hydrometallurgical techniques are also employed.

The nickel matte then undergoes the Sherritt-Gordon process: hydrogen sulfide is added to remove copper, leaving a concentrate of cobalt and nickel. Solvent extraction separates these two metals, yielding nickel with a purity above 86%. Another common refining route involves leaching the matte into a nickel salt solution, followed by electrowinning, where nickel plates onto a cathode as electrolytic nickel.

The highest purity nickel, exceeding 99.99%, comes from the Mond process, patented by Ludwig Mond and used industrially since before the 20th century. In this method, nickel reacts with carbon monoxide at 40–80 °C, with a sulfur catalyst, to form nickel carbonyl. If needed, the nickel can be separated by distillation. The nickel carbonyl is then converted back to pure metal in one of two ways.

In the first, the gas passes through a large chamber at high temperature where tens of thousands of nickel pellets are constantly stirred; the carbonyl decomposes and deposits pure nickel onto the pellets. In the alternative, the carbonyl is decomposed in a smaller chamber at 230 °C, producing a fine nickel powder. The carbon monoxide byproduct is recirculated and reused. The resulting ultra-pure metal is called carbonyl nickel.

Applications

Nickel's primary role today is as an alloying metal, with the largest share—68%—going into stainless steel production. The remaining uses are spread across nonferrous alloys (10%), electroplating (9%), alloy steel (7%), foundries (3%), and a 4% category that includes batteries. You'll find nickel in a wide range of everyday and industrial items: stainless steel, alnico magnets, coins, rechargeable nickel-iron batteries, electric guitar strings, microphone capsules, plated plumbing fixtures, and specialty alloys like permalloy, elinvar, and invar.

It also adds a green tint to glass and serves as a protective plating. In nickel steels and cast irons, it boosts tensile strength, toughness, and elastic limit. Other alloys containing nickel include brasses, bronzes, and combinations with copper, chromium, aluminium, lead, cobalt, silver, and gold—examples being Inconel, Incoloy, Monel, and Nimonic. In Southeast Asia, it has a traditional role in making Kris blades.

Thanks to its corrosion resistance, nickel once stood in for decorative silver. It also acts as an excellent alloying agent for precious metals and is used in fire assays to collect platinum group elements, fully capturing all six from ores and partially collecting gold. High-volume mines, such as those in Norilsk, Russia, and the Sudbury Basin, Canada, may recover platinum and palladium as byproducts. Nickel foam or mesh appears in gas diffusion electrodes for alkaline fuel cells.

As a catalyst, nickel and its alloys—like Raney nickel, a finely divided nickel-aluminium mix—are common in hydrogenation reactions. Nickel is naturally magnetostrictive, meaning it contracts slightly (about 50 parts per million) in a magnetic field. Finally, it serves as a binder in cemented tungsten carbide, making up 6% to 12% of the weight, which adds magnetic properties and corrosion resistance, though the resulting hardness is lower than with a cobalt binder.

Toxicity

Nickel enters the body mainly through food and drink, since plants need it to grow. In the environment, typical levels are low: up to 20 nanograms per cubic meter in air, 100 milligrams per kilogram in soil, 10 milligrams per kilogram in plants, 10 micrograms per liter in fresh water, and 1 microgram per liter in seawater. Human activities can raise these levels. Nickel-plated faucets may leach into water and soil, mining and smelting can release it into wastewater, and cookware made from nickel-steel alloys or dishes with nickel pigments can transfer it into food.

Air pollution from nickel refining and burning fossil fuels also adds nickel. People can absorb it through tobacco smoke or skin contact with items like jewelry, shampoos, detergents, and coins. A less common exposure route is hemodialysis, where nickel ions from the equipment may enter the blood through binding with albumin.

For most people, the daily amount absorbed is not harmful. The kidneys remove most nickel, excreting it in urine, or it passes through the digestive tract without being taken up. Nickel does not build up in the body over time, but high doses or long-term inhalation can be toxic and may cause cancer, posing a risk for certain workers. Nickel compounds are considered human carcinogens, based on studies of refinery workers exposed to sulfidic ore who showed higher rates of respiratory cancer.

Animal tests with nickel subsulfide and nickel oxide support this. Evidence from both humans and animals indicates that eating nickel does not cause cancer; the risk is limited to lung tumors from breathing it in. Nickel metal itself is a suspected carcinogen, but workers mainly exposed to metallic nickel have not shown increased respiratory cancer, and rats inhaling nickel metal powder did not develop lung tumors. In rodent studies, eating water-soluble nickel salts led to higher death rates in newborn pups of pregnant animals, but whether this applies to humans is uncertain, since studies of female workers with high exposure found no developmental harm.

Did You Know?

• Unintentional use of nickel has been traced back as far as 3500 BCE in bronzes from what is now Syria, which contained as much as 2% nickel.
• In medieval Germany, miners called the ore Kupfernickel, blaming a mischievous sprite named Nickel for their inability to extract copper from it.