Extractive metallurgy of nickel is the set of operations that allow the manufacture of nickel metal from ore. It also concerns the recycling of metallurgical waste containing nickel (40% of nickel consumed in 2005 was recycled).
Nickel is extracted from two types of ores: laterites and sulfides. Although 70% of nickel reserves are lateritic ores, these only account for 40% of global production. Lateritic ores are primarily used for the production of ferronickel, while sulfide ores are generally used for the production of very pure nickel.
Whether lateritic or sulfide, nickel ores are mined when their nickel content exceeds 1.3%. This low content explains the complexity and diversity of processes, determined by the nature of the ore's gangue, as well as the desired quality of nickel at the end of extraction.
History
Jules Garnier traveled through New Caledonia from 1863 to 1866 and discovered an ore containing a maximum of 6 to 7% nickel, which was named garnierite in his honor. Nickel production began there in 1875. At that time, there was a heated debate between those who advocated a hydrometallurgical treatment and Garnier, who opted for a pyrometallurgical process.
During the construction of the Canadian Pacific Railway in 1883, nickel was discovered in the Sudbury Basin in Ontario. This discovery led to significant European immigration. The abundance of nickel in the region earned it the nickname "Nickel Capital."
Developed in Sudbury (Canada) in 1905, the production of nickel from sulfide ores, which are also rich in copper and cobalt, quickly surpassed production from laterites.
In 1921, nickel veins were discovered in Finland in the Petsamo region. Nickel exploitation began in 1935 by the Canadian company Inco.
In 1935, Stalin decided to create the city of Norilsk in Siberia for the exploitation of a nickel deposit by the company Norilsk Nickel and a forced labor camp known as the Norillag.
After the Winter War between Russia and Finland, in 1940, a German-Soviet consortium, including IG Farben and Krupp, shared the exploitation of nickel in the Petsamo region. After World War II, the Russian nickel industry was developed by the Soviet company Norilsk Nickel.
Primary ores
Laterites
Laterites are generally located in tropical regions. They are notably exploited in New Caledonia, Indonesia, the Philippines, and Cuba. This ore is mined when its nickel content exceeds 1.3%, and its cobalt content exceeds 0.1%.
Laterites are complex minerals resulting from the disintegration of oceanic peridotite floors when they emerge due to tectonic movements (as in New Caledonia). They are therefore surface deposits. The weathering of peridotite (a mixture of olivine and pyroxene) causes vertical segregation, from the soil surface to the bedrock:
limonite, rich in hydrated iron (goethite) and low in magnesia. It forms the iron cap of the deposit; smectites, such as nontronite, are found in some deposits (Western Australia), beneath the limonite; saprolites, such as garnierite. Unlike limonite, they are rich in magnesia and poorer in iron.
The configuration of the strata changes according to climate and soil age. Each stratum measures 2 to 5 meters thick. Rain and vegetation leach the surface limonite layer, removing magnesia and silica from the original peridotite rock, enriching it with iron, nickel, and cobalt. Dissolved nickel also tends to descend through percolation, enriching the deep saprolite layer. This leaching occurs over a period of 1 to 10 million years, and its progress differs depending on the deposit.
Sulfide ores Unlike laterites, the formation of sulfide ores is independent of climate. They are found in Canada and northern Siberia. Sulfide ore deposits originate from:
magma upwelling through the Earth's crust; metal concentration due to the presence of geothermal waters.
The most common mineral found in nickel sulfide deposits is pentlandite. The sulfur comes from the parent rock, and the molar ratio of nickel to iron ranges from 0.34 to 2.45, with an average of 1.15. This mineral is frequently accompanied by pyrrhotite and chalcopyrite, as well as more or less precious metals such as cobalt, silver, and platinum group metals. Deposits used for nickel extraction contain 1 to 3% nickel.
Except for pyrrhotite, where nickel substitutes for iron in varying proportions, sulfide nickel ore deposits contain very few minerals of nickel sulfide. Therefore, like lateritic deposits, the nature of their gangue has a significant influence on extraction processes. +Sulfide minerals that may contain nickelMineralChemical formulaChalcopyriteCuFeS2MagnetiteFe3O4CubaniteCuFe2S3Chromite(Mg,Fe)Cr2O4GalenaPbSSphaleriteZnSBorniteCu5FeS4Mackinawite(Fe,Ni,Co)SValleriiteCu3Fe4S7+Nickel sulfide and similar mineralsMineralChemical formulaTheoretical nickel content(% by weight)PentlanditeNi9Fe9S334.2MilleriteNiS64.7HeazlewooditeNi3S273.4PolydymiteNi3S457.9ViolariteNi2FeS438.9Siegenite(Co,Ni)3S428.9FletcheriteNi2CuS475.9NickelineNiAs43.9MaucheriteNi11As851.9RammelsbergiteNiAs235.4BreithauptiteNiSb32.5AnnabergiteNi3As2O8·8H2O34.2Pyrrhotite(Ni,Fe)7S81–5
Extraction
Mining extraction
Lateritic deposits It is important to distinguish between surface ores (limonite and smectite) and deep ores (saprolite) in the deposit. Indeed, the high iron content in limonite and smectite penalizes pyrometallurgical processes, whereas when dissolved in hot sulfuric acid, iron precipitates as hematite or jarosite. Moreover, the sulfuric acid consumption due to magnesia is economically acceptable, as it does not exceed 3% of the ore weight to be treated.
Conversely, the magnesia content of saprolite (20%) makes its hydrometallurgical treatment too costly due to sulfuric acid consumption. But the low iron content (15%) allows obtaining a rich ferronickel containing 20 to 30% nickel. Some cobalt is present in this ferroalloy, but in too small a quantity to influence steel customers.
Ore concentration by hydrometallurgy Laterites, which typically contain between 1.3 and 2.5% nickel, are crushed, screened, and, if the process is wet, hydrocycloned to remove as much waste as possible. This typically doubles the nickel content. The finer the grinding (down to 75 μm), the more effective the enrichment. All laterite enrichment methods are based on the principle that rich minerals are softer than minerals not yet transformed into laterites (olivine) or transformed into hard and nickel-poor products (quartz). A condition is therefore to carry out "gentle" grinding that does not affect the hardest minerals.
Limonite is leached in a sulfuric acid solution at 250 °C and 40 bar.
Sulfide ores are finely ground (particle size less than 50 μm), typically in two stages: a gyratory crusher followed by a ball mill working on a 50% wet product. A flotation stage allows separating:
the sterile gangue (less than 0.3% nickel, rich in pyrrhotite) the nickel concentrate (10 to 20% nickel, mainly pentlandite) the copper concentrate.
Flotation is froth flotation with air, which consists of isolating naturally or artificially hydrophilic components (often pyrrhotite and gangue) from hydrophobic components (often pentlandite). Hydrophobic components attach to bubbles and rise in the bath, while hydrophilic ones remain in the aqueous solution. The copper and nickel concentrates are then treated by pyrometallurgy.
Fusion and pyrometallurgical refining
Saprolite Saprolite, which contains little iron, is the only laterite suitable for fusion in an electric arc furnace. It can be used to make either ferronickel or a matte of nickel, which will be refined by pyrometallurgy.
The production of ferronickel exploits two chemical characteristics of iron and nickel:
iron and nickel oxides are easily reduced by carbon at 800 °C through a direct reduction reaction. Alumina and magnesia are not reducible at these temperatures, while chromium(III) oxide (Cr2O3) and silica are to a small extent, which explains the presence of a few percent chromium and silicon in ferronickel; iron(II) oxide (or wüstite), which is the stable iron oxide at 800 °C, has a reducibility close to that of nickel(II) oxide: it is therefore not possible to reduce one without the other. Since iron does not add value to ferronickel, ores are selected to minimize its presence.
The concentrated saprolite ore intended for ferronickel production, which contains 35% water, is first dried in a first rotary drum (4 m in diameter and 30 m long), then calcined and pre-reduced in a large direct reduction drum (5 m in diameter and 100 m, or even 185 m, long) From the direct reduction drum emerges a powder, or dry calcine, containing 1.5 to 3% nickel at 900 °C, partially reduced This calcine can be obtained using other furnaces, but at the beginning of the 21st century, "static furnace processes have been tried, with limited success. Most ferronickel producers invest in rotary furnaces. In 2011, only one producer installs static shaft furnaces".
The calcine is then melted in electric furnaces 15 to 20 m in diameter, producing 100 to 200 tonnes of ferronickel per day. This melting stage allows the separation, by decantation, of oxides (which form slag) from molten iron and nickel. Melting, which occurs at 1500 °C, is very energy-intensive. Furthermore, the reduction of iron and nickel oxides by carbon, provided by the graphite electrodes or contained in the powder, is highly endothermic: furnaces typically consume 500 kWh per tonne of charged powder. However, in the molten metal, no reduction reaction affects the other oxides, which form the slag. The metal separates easily because, in addition to its much lower density, it is an ionic compound, not miscible with the molten metal. At this stage, raw or unrefined ferronickel is recovered from the electric furnace.
Since the electric furnace reduces iron and nickel, it cannot remove sulfur and phosphorus, undesirable elements present in ferronickel at 0.4% and 0.06%, respectively. As in steel production, these elements are typically removed by oxidation. Moreover, the elimination of their oxides requires a basic medium, rich in lime and poor in silica, to form CaS and (CaO)4P2O5, which then float in lime-rich slag. Desulfurization and dephosphorization are therefore carried out in a separate ladle metallurgy workshop, where much less powerful electric ladle furnaces ensure the stirring of the molten metal.
It is also possible to add sulfur in the direct reduction rotary kiln. This results in a sulfide matte (63% iron, 28% nickel, 10% sulfur) at the exit of the electric furnace, which is then refined in a . Blowing, limited to the first stage of the Manhès-David process, removes all iron from the matte. The "white matte" that comes out (78% nickel and 20% sulfur) is solidified, crushed, and roasted twice (an oxidizing roast to remove sulfur, then a reducing roast to remove oxygen introduced by the first roast) to produce pure nickel. It is also possible to purify this white matte by electrolysis, or to make nickel salts through hydrometallurgical processes. Resulfurization of nickel concerns approximately 10% of lateritic ore treatment, or less than 4% of nickel production from ores.
Limonite and smectite Limonite and smectite are too rich in iron to be economically treated by pyrometallurgical processes. However, their low magnesia content facilitates their treatment by hydrometallurgy. Indeed, it is important to minimize sulfuric acid consumption, which combines with magnesia, a base.
The concentrated ores from limonite and smectite are therefore leached with sulfuric acid. The reaction takes place in an autoclave 4.5 m in diameter and 30 m long, capable of treating 2,000 tonnes of concentrates per day. This process, called "HPAL process" for High-pressure Acid-Leaching process, is conducted at 250 °C and 40 bar. It removes more than 95% of nickel and cobalt and consists of: