niobium processing
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What is niobium?
Niobium (Nb) is a chemical element and refractory metal that is used in alloys, tools and dies, and superconductive magnets. Niobium is associated with tantalum in ores and in properties.
Niobium production is complex, especially separating it from tantalum. Solvent extraction separates niobium and tantalum. Niobium is precipitated and roasted to niobium pentoxide, then reduced to powder through metallothermic and hydriding processes. The powder is consolidated and purified by electron-beam melting or vacuum sintering.
Niobium enhances steel properties, leading to high-strength, low-alloy (HSLA) steels. It is added to stainless steel to prevent chromium carbide formation, improving creep resistance and toughness. Niobium alloys are also used in high-pressure sodium lamps and nuclear reactors due to their chemical resistance, ductility, and strength at high temperatures.
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Where is niobium found in nature?
Niobium is found in nature primarily as an oxide, often alongside tantalum, with which it shares a strong geochemical relationship. The main niobium-bearing minerals are pyrochlore and columbite. Pyrochlore is commonly located in carbonatites and pegmatites associated with alkalic rocks, often alongside minerals containing zirconium, titanium, thorium, uranium, and rare earths. Columbite, on the other hand, is typically found in intrusive pegmatite and biotite, as well as in alkalic granites.
Significant pyrochlore mines operate in the Brazilian states of Minas Gerais and Goi\u00e1s, and in Saint Honor\u00e9, Quebec, Canada. Substantial columbite deposits are located in Nigeria and Congo (Kinshasa), with columbite concentrates also being extracted as by-products of tin mining in Nigeria.
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How is niobium extracted from its ores?
Niobium extraction from ores involves several steps. Niobium mainly occurs as an oxide, often found with tantalum in pyrochlore and columbite minerals.
To extract it, the ore is crushed, ground, and decomposed using hydrofluoric acid. Then, a liquid-liquid extraction process separates tantalum and niobium. Niobium remains in the aqueous residue and is precipitated as a fluoride by adding ammonium hydroxide. The resulting filter cake is then dried and calcined at high temperatures to produce niobium pentoxide. This oxide is reduced to impure metallic globules, which can be further purified through electron-beam melting or a hydriding/dehydriding process to obtain niobium powder.
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What steps are involved in purifying niobium after it is mined?
After niobium is mined, the following steps are involved in purifying it:
- The ore is crushed, ground, and decomposed in hydrofluoric acid.
- The niobium is then separated from tantalum through a liquid-liquid extraction process. Niobium remains in the aqueous residue, or raffinate.
- The niobium is precipitated from the solution as a fluoride by adding ammonium hydroxide.
- The filter cake is then dried and roasted, or calcined, at 900–1,000 °C (1,650–1,800 °F) to produce niobium pentoxide.
- This oxide may be reduced aluminothermically to produce niobium reguli (impure metallic globules), as in the production of ferroniobium.
- The reguli may be further purified by electron-beam melting into ingots, or put through a hydriding and dehydriding process to produce niobium powder.
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What products use refined niobium?
Refined niobium is used in a variety of products across several industries.
- Aerospace: Niobium-based alloys are used in aerospace applications due to their workability. Niobium alloys like WC-103, WC-129Y, and Cb-752 are used in rocket nozzles and reentry guides because of their strength at high temperatures.
- Steel Production: Niobium is a key additive in high-strength, low-alloy (HSLA) steels, enhancing their strength and toughness. It is also added to stainless steel to prevent detrimental effects and improve creep resistance.
- Lighting: Niobium and zirconium alloys are used in high-pressure sodium lamps for their resistance to sodium vapor and similar expansion coefficients with alumina tubes.
- Nuclear Industry: Niobium, niobium-1-percent-zirconium, and zirconium-2.5-percent-niobium are used in nuclear reactors. Its compatibility with uranium, corrosion resistance, and low thermal-neutron cross-section make it suitable for cladding nuclear reactor cores.
- Electronics: Niobium is used in cryogenic electronic devices for its low power consumption. Niobium alloys, such as niobium-titanium, niobium-zirconium, and niobium-tin, are used in superconductivity applications like machinery, levitated trains, high-energy physics, and magnetic resonance imaging. Niobium-tin (Nb3Sn) is a superconductor below 18.45 Kelvins (K), and niobium metal itself, below 9.15 K.
- Tools and Dies: Cemented carbides containing niobium are used as hot-pressing dies and cutting tools, increasing their hardness, shock resistance, and erosion resistance.
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Why is controlling impurities important in niobium processing?
Controlling impurities is crucial in niobium processing because, during vacuum sintering, the evaporation of impurities leads to an improvement of its mechanical properties. Niobium is susceptible to oxidation above approximately 400 °C (750 °F), so eliminating interstitial impurities such as carbon and oxygen helps avoid intergranular corrosion. Also, when niobium exceeds its solubility limit in the steel matrix, it precipitates as a carbide or nitride, acting as a grain refiner and further improving the toughness and strength of the steel.
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niobium processing, preparation of niobium ore for use in various products.
Niobium (Nb) has a body-centred cubic (bcc) crystal structure and a melting point of 2,468 °C (4,474 °F). Of the refractory metals, it has the lowest density and best workability; for this reason, niobium-based alloys are often used in aerospace applications. Because of its strengthening effect at elevated temperatures, its principal commercial use is as an additive in steels and superalloys. Niobium-titanium and niobium-tin alloys are used as superconducting materials.
History
Niobium was discovered in 1801 by an English chemist, Charles Hatchett. Since Hatchett’s mineral sample came from New England, he named it columbium (Cb), after Columbia, another name for America. In 1844 Heinrich Rose, a German chemist, announced his discovery of an element that he named niobium, after Niobe, the mythical daughter of Tantalus (who in turn gave his name to tantalum, with which niobium is often associated in minerals). Niobium was later proved to be the same element as columbium, and niobium was accepted as the official name by the International Union of Pure and Applied Chemistry in 1950.
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In 1905 W. von Bolton, a German chemist, succeeded in producing niobium in a pure, ductile state. Niobium was first added to tool steel around 1925 and was first used to stabilize austenitic stainless steel in 1933. Interest in adding niobium to high-strength low-alloy (HSLA) steel can be traced to the work in 1939 of F.M. Becket and R. Franks, who demonstrated that niobium strengthening reduced reliance on conventional hardeners such as carbon, manganese, chromium, and molybdenum, thereby improving weldability. In 1958 Norman F. Tisdale of Molybdenum Corporation of America added 0.01–0.034 percent niobium to carbon steel as a grain refiner for improving toughness. The development of niobium-based alloys for aerospace applications began in the late 1950s.
Ores
Niobium occurs mostly as an oxide and has a strong geochemical coherence with tantalum. Major minerals of niobium are pyrochlore [(Na, Ca)2Nb2O6F] and columbite [(Fe, Mn)(Nb, Ta)2O6], consisting of niobate, tantalate, iron, and manganese. Pyrochlore occurs usually in carbonatites and in pegmatite derived from alkalic rocks, commonly in association with zirconium, titanium, thorium, uranium, and rare-earth minerals. Columbite is normally found in intrusive pegmatite and biotite and in alkalic granites. However, since most such deposits are small and erratically distributed, they are usually mined as a by-product of other metals.
There are large pyrochlore mines in the Brazilian states of Minas Gerais and Goiás and at Saint Honoré, Quebec, Canada. Large columbite deposits are found in Nigeria and Congo (Kinshasa); also, columbite concentrates are obtained as by-products of tin mining in Nigeria.
Mining and concentrating
Because of the altered and decomposed nature of overburden and ore materials, the Brazilian deposits are mined by the open-pit method. The ore is generally divided into blocks and processed by ripping, bulldozing, loading, and transporting. Mining in Quebec follows underground methods.
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Concentration of ore is accomplished by crushing and grinding, magnetic separation to remove magnetite, and then desliming and flotation separation.
Extraction and refining
Ferroniobium
Pyrochlore concentrates are commonly reduced to ferroniobium through an aluminothermic process. In this process, the concentrate is mixed with hematite (an iron ore), aluminum powder, and small quantities of fluorspar and lime fluxes in a rotary mixer and then unloaded into steel containers lined with magnesite refractory bricks. Here the charge is placed in circular concave pits made of a mixture of lime, fluorspar, and silica sand, and reduction is initiated by the ignition of a mixture of aluminum powder and sodium chlorate or barium peroxide. The exothermic reaction lasts about 15 to 30 minutes, and the temperature reaches about 2,400 °C (4,350 °F). Most of the gangue impurities from the concentrate, including all the thorium and uranium oxides, enter the molten slag. When the reaction is finished, the slag is tapped off and the vessel is lifted, leaving the metal to solidify in the sand. The ferroniobium alloy is then crushed to particle sizes of 10 millimetres (about three-eighths of an inch) for marketing. The content of this alloy is 62–69 percent niobium, 29–30 percent iron, 2 percent silicon, and 1–3 percent aluminum.
Niobium powder
Columbite concentrates and tin slags with a high tantalum content (greater than 10 percent) are directly dressed in most cases by a wet chemical process. Low-tantalum tin slags, on the other hand, are first melted in an electric-arc furnace with the addition of a flux material, and the tantalum-niobium content is collected as a ferroalloy. The tantalum-rich ore or ferroalloy is then crushed, ground, and decomposed in hydrofluoric acid. This is followed by a liquid-liquid extraction process, in which the two metals are dissolved in a slightly acidic aqueous feed solution into which an organic solvent, usually methyl isobutyl ketone, is mixed. The tantalum is extracted as a fluoride in the organic solution, while niobium remains in the aqueous residue, or raffinate. The niobium is then precipitated from solution as a fluoride by the addition of ammonium hydroxide, and the filter cake is dried and roasted, or calcined, at 900–1,000 °C (1,650–1,800 °F) to obtain niobium pentoxide. This oxide may be reduced aluminothermically to produce niobium reguli (impure metallic globules), as in the production of ferroniobium. The reguli may be further purified by electron-beam melting into ingots, or they may be put through a hydriding and dehydriding process to produce niobium powder.
In the hydriding process, the impure niobium is crushed into chunks and placed in a furnace, which is evacuated and heated to 800–950 °C (1,450–1,750 °F). Hydrogen is then fed to the furnace and passed over the charge for two to four hours. After hydriding, the niobium is crushed and pulverized to fine powder, which is then reheated and dehydrided in a vacuum to produce niobium powder. The powder can be pressed with a mechanical or isostatic press into “green” (that is, unfired) compacts with a density of 60–65 percent of the theoretical maximum and then sintered. Sintering is carried out in a vacuum at 2,100–2,300 °C (3,800–4,150 °F), either by direct-resistance heating or by indirect heating. When direct-resistance is applied, electrical contact is made via water-cooled copper clamps with brazed tungsten facings. The temperature is increased in stages to permit the evaporation of impurities and to prevent the sudden release of gas. During vacuum sintering, a purification of the metal takes place, leading to an improvement of its mechanical properties.