nickel processing
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- materials processing
- nickel
nickel processing, preparation of the metal for use in various products.
Although it is best known for its use in coinage, nickel (Ni) has become much more important for its many industrial applications, which owe their importance to a unique combination of properties. Nickel has a relatively high melting point of 1,453 °C (2,647 °F) and a face-centred cubic crystal structure, which gives the metal good ductility. Nickel alloys exhibit a high resistance to corrosion in a wide variety of media and have the ability to withstand a range of high and low temperatures. In stainless steels, nickel improves the stability of the protective oxide film that provides corrosion resistance. Its major contribution is in conjunction with chromium in austenitic stainless steels, in which nickel enables the austenitic structure to be retained at room temperature. Modern technology is heavily dependent on these materials, which form a vital part of the chemical, petrochemical, power, and related industries.
History
Nickel was used industrially as an alloying metal almost 2,000 years before it was isolated and recognized as a new element. As early as 200 bce the Chinese made substantial amounts of a white alloy from zinc and a copper-nickel ore found in Yunnan province. The alloy, known as pai-t’ung, was exported to the Middle East and even to Europe.
Later, miners in Saxony encountered what appeared to be a copper ore but found that processing it yielded only a useless slaglike material. They considered it bewitched and ascribed it to the devil, “Old Nick.” Thus, it became known as kupfernickel (Old Nick’s copper). It was from this ore, studied by Axel Fredrik Cronstedt, that nickel was isolated and recognized as a new element in 1751. In 1776 it was established that pai-t’ung, now called nickel-silver, was composed of copper, nickel, and zinc.
Demand for nickel-silver was stimulated in England about 1844 by the development of silver electroplating, for which it was found to be the most desirable base. The use of pure nickel as a corrosion-resistant electroplated coating developed a little later; both these uses are still important.
Small amounts of nickel were produced in Germany in the mid-19th century. More substantial amounts came from Norway, and a little came from a mine at Gap, Pennsylvania, in the United States. A new source, New Caledonia in the South Pacific, came into production about 1877 and dominated until the development of the copper-nickel ores of the Copper Cliff–Sudbury, Ontario, region in Canada, which after 1905 became the world’s largest source of nickel. By the late 1970s, production in Soviet Russia had exceeded that in Canada. By the early 21st century, China had become the world leader in nickel production, followed by Russia, Japan, Australia, and Canada.
Ores
Sulfides
Canadian ores are sulfides containing nickel, copper, and iron. The most important nickel mineral is pentlandite, (Ni, Fe)9S8, followed by pyrrhotite, usually ranging from FeS to Fe7S8, in which some of the iron may be replaced by nickel. Chalcopyrite, CuFeS2, is the dominant copper mineral in these ores, with small amounts of another copper mineral, cubanite, CuFe2S3. Some gold, silver, and the six platinum-group metals also are present, and their recovery is important. Cobalt, selenium, tellurium, and sulfur may be recovered from the ores as well.
Laterites
Other important classes of ore are the laterites, which are the result of long weathering of peridotite initially containing a small percentage of nickel. Weathering in subtropical climates removes a major portion of the host rock, but the contained nickel dissolves and percolates downward and may reach a concentration sufficiently high to make mining economical. Because of this method of formation, laterite deposits are found near the surface as a soft, frequently claylike material, with nickel concentrated in strata as a result of weathering. Garnierite, (NiMg)6Si4O10(OH)8, a nickel-magnesium silicate, is the richest in nickel, but nickeliferous limonite, (Fe, Ni)O(OH)·nH2O, constitutes a major portion of the laterites. The New Caledonian deposits are of the garnierite type, and numerous other laterite deposits are scattered around the world, presenting a wide range of mining, transport, and recovery problems. The nickel content of laterites varies widely: at Le Nickel in New Caledonia, for example, the ore delivered to the smelter in 1900 contained 9 percent nickel; currently it contains 1 to 3 percent.
Mining
With nickel found in two radically different types of ore, it is not surprising that the mining methods differ. Sulfide deposits are usually mined by underground techniques in a manner similar to copper, although some deposits have been mined using open pits in the early stages. The mining of laterites is basically an earth-moving operation, with large shovels, draglines, or front-end loaders extracting the nickel-rich strata and discarding large boulders and waste material. The ore is loaded into trucks at the face, as would be the case in an open pit, and hauled to the smelter.
Extraction and refining
The extraction of nickel from ore follows much the same route as copper, and indeed, in a number of cases, similar processes and equipment are used. The major differences in equipment are the use of higher-temperature refractories and the increased cooling required to accommodate the higher operating temperatures in nickel production. The specific processes taken depend on whether the ore is a sulfide or a laterite. In the case of sulfides, the reaction of oxygen with iron and sulfur in the ore supplies a portion of the heat required for smelting. Oxide ores, on the other hand, do not produce the same reaction heats, making necessary the use of energy from other sources for smelting.
From sulfide ores
Sulfide ores are crushed and ground in order to liberate nickel minerals from the waste materials by selective flotation. In this process, the ore is mixed with special reagents and agitated by mechanical and pneumatic devices that produce air bubbles. As these rise through the mixture, the sulfide particles adhere to their surfaces and are collected as a concentrate containing 6 to 12 percent nickel. The waste material, or tailings, is frequently run through a second cleaning step before it is discarded. Because some nickel-bearing sulfides are magnetic, magnetic separators can be used in place of, or in conjunction with, flotation. In cases such as the Sudbury deposit, where the copper content of the ore is almost equal to that of nickel, the concentrate is subjected to a second selective flotation whereby the copper is floated to produce a low-nickel copper concentrate and a separate nickel concentrate, each to be processed in its respective smelting line.
Nickel concentrates may be leached with sulfuric acid or ammonia, or they may be dried and smelted in flash and bath processes, as is the case with copper. Nickel requires higher smelting temperatures (in the range of 1,350 °C [2,460 °F]) in order to produce an artificial nickel-iron sulfide known as matte, which contains 25 to 45 percent nickel. In the next step, iron in the matte is converted to an oxide, which combines with a silica flux to form a slag. This is done in a rotating converter of the type used in copper production. The slag is drawn off, leaving a matte of 70 to 75 percent nickel. Because the conversion of nickel sulfide directly to metal would require an extremely high temperature (in excess of 1,600 °C [2,910 °F]), the removal of sulfur at this stage of the converting process is controlled in order to produce the 70–75 percent nickel matte, which has a lower melting point. On the other hand, the relatively high ratio of sulfur (a major pollutant) to nickel in most nickel concentrates increases the burden of sulfur containment in the smelters.
Various processes are used to treat nickel matte. One process is the ammonia pressure leach, in which nickel is recovered from solution using hydrogen reduction, and the sulfur is recovered as ammonium sulfate for use as fertilizer. In another, the matte may be roasted to produce high-grade nickel oxides; these are subjected to a pressure leach, and the solution is electro- and carbonyl refined. In electrorefining, the nickel is deposited onto pure nickel cathodes from sulfate or chloride solutions. This is done in electrolytic cells equipped with diaphragm compartments to prevent the passage of impurities from anode to cathode. In carbonyl refining, carbon monoxide is passed through the matte, yielding nickel and iron carbonyls [Ni(CO)4 and Fe(CO)5]. Nickel carbonyl is a very toxic and volatile vapour that, after purification, is decomposed on pure nickel pellets to produce nickel shot. Copper, sulfur, and precious metals remain in the residue and are treated separately.
From laterite ores
Being free of sulfur, laterite nickel deposits do not cause a pollution problem as do the sulfide ores, but they do require substantial energy input, and their mining can have a detrimental effect on the environment (e.g., soil erosion). The range of process options is limited by the nature of the ore. Being oxides, laterites are not amenable to conventional concentration processes, so that large tonnages must be smelted. In addition, they contain large amounts of water (in the range of 35 to 40 percent) as moisture and chemically bound in the form of hydroxides. Drying of moisture and removal of the chemically bound water are therefore major operations. These are carried out in large rotary-kiln furnaces. Dryers 50 metres long and 5.5 metres in diameter are common, while reduction kilns 5 to 6 metres in diameter and more than 100 metres long are required to handle the large tonnages of ore and to provide the necessary retention time.
Next, it is necessary to reduce the oxide to nickel metal. Electric furnaces rated at 45 to 50 megavolt-amperes and operating at 1,360 ° to 1,610 °C (2,480 ° to 2,930 °F) are standard in modern laterite nickel smelters. The high magnesia content in most laterite ores and the liquidus temperature of the furnace products necessitate these higher smelting temperatures, which in turn make necessary an extensive system of cooling blocks within the refractory lining of the furnace. In some plants, sufficient sulfur is added to produce a furnace matte that can be further processed like matte from a sulfide smelter. However, the majority of laterite smelters produce a crude ferronickel, which, after refining to remove impurities such as silicon, carbon, and phosphorus, is marketed as an alloying agent in steel manufacture.
