niobium and niobium compounds

2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Article No : a17_251

Niobium and Niobium Compounds

JOACHIM ECKERT, Hermann C. Starck Berlin, Werk Goslar, Goslar, Germany

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 133

2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . 133

2.1. Physical Properties . . . . . . . . . . . . . . . . . . 133

2.2. Chemical Properties . . . . . . . . . . . . . . . . . 134

3. Occurrence . . . . . . . . . . . . . . . . . . . . . . . . 134

4. Processing of Niobium Ores. . . . . . . . . . . . 135

4.1. Winning of Pyrochlore Concentrates . . . . . 135

4.2. Treatment of Pyrochlore Concentrates . . . 135

4.3. Production of Niobium Oxide from

Columbites and Tantalites . . . . . . . . . . . . . 135

4.3.1. Extraction Processes . . . . . . . . . . . . . . . . . . 135

4.3.2. Chlorination . . . . . . . . . . . . . . . . . . . . . . . . 136

5. Niobium Compounds . . . . . . . . . . . . . . . . . 138

5.1. Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

5.2. Halides. . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

5.3. Carbides and Hard Materials . . . . . . . . . . 139

6. Niobium Metal. . . . . . . . . . . . . . . . . . . . . . 141

6.1. Reduction of Niobium Pentoxide . . . . . . . . 141

6.2. Reduction of Halides . . . . . . . . . . . . . . . . . 141

6.3. Refining . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

6.4. Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

7. Ferroniobium. . . . . . . . . . . . . . . . . . . . . . . 143

7.1. Production . . . . . . . . . . . . . . . . . . . . . . . . . 143

7.2. Uses of Ferroniobium . . . . . . . . . . . . . . . . 144

8. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

9. Economic Aspects . . . . . . . . . . . . . . . . . . . 145

References . . . . . . . . . . . . . . . . . . . . . . . . . 145

1. Introduction

Niobium [7440-03-1], Nb, atomic number 41,Ar 92.91, is also known as columbium, Cb, inthe United States. There are many artificialradionuclides, but only one known natural non-radioactive isotope, 93Nb. The electronic con-figuration of the ground state is 4s2p6d3 5s2,which explains the existence of the oxidationstates 2 to 5.

Niobiumwas discovered byHATCHETT in 1801in the mineral columbite and was named colum-bium. In 1844 the name niobium was proposedby ROSE.

2. Properties

2.1. Physical Properties

Some important physical properties of niobiummetal are as follows [13]:

Density at 20 C 8.57 g/cm3

Crystal structure body-centered cubic

(a 3.31010 m)mp 2468 10 Cbp 4927 CLinear coefficient of thermal

expansion 6.892106 K1Specific heat 0.26 kJ kg1 K1

Latent heat of fusion 290 kJ/kg

Latent heat of vaporization 7490 kJ/kg

Thermal conductivity at 0 C 0.533 Wcm1 K1

Electrical resistivity at 0 C 15.22 mW cmTemperature coefficient

0 600 C 0.0396 K1

Electrochemical equivalent 0.19256 mg/C

Standard electrode potential

E Nb/Nb5 0.96 VMagnetic susceptibility

at 25 C 2.28106Superconductivity TC 9.13 K

Spectral emissivity at

6501010 m (at 2003 K) 2.28106Ionisation potential 6.67 eV

Work function 4.01 eV

DOI: 10.1002/14356007.a17_251.pub2

The mechanical properties, like those of mostrefractory metals, are influenced by the purityof the metal, the production method, and themechanical treatment. Even small amounts ofinterstitial impurities increase the hardness andstrength but reduce the ductility. Some importantmechanical properties of commercial niobiumare as follows [4]:

Annealed niobium

Ultimate tensile strength 195 MPa

Yield strength 105 MPa

Elongation 30% Reduction in area 80% Hardness 60 HV

Poissons ratio 0.38

Strain hardening exponent 0.24

Elastic modulus

tension 103 GPa

shear 17.5 GPa

Ductile brittle

transition temperature < 147 K

Recrystallization temperature 800 1000 CCold worked niobium

Ultimate tensile strength 585 MPa

Elongation 5%

Hardness 150 HV

2.2. Chemical Properties

Niobium is very resistant to most organic andinorganic acids, with the exception of HF, attemperatures up to 100 C [1]. Concentratedsulfuric acid above 150 Ccauses embrittlement.The resistance towards alkaline solutions islower. Because niobium has a marked tendencyto form oxides, hydrides, nitrides, and carbides,its use in air is limited to temperatures up to ca.200 C.

3. Occurrence

Niobium occupies the 33rd place in order ofnatural abundance, being present in the earths

crust at 24 mg/g. It is thus more common thancobalt, molybdenum, or tantalum.

The most important niobium mineral is pyr-ochlore, a compound with the general formula(Ca,Na)2mNb2O6 (O,OH,F)1n x H2O. Thelattice positions of Na and Ca can also beoccupied by Ba, Sr, rare earths, Th, and U. Thelatter two elements are responsible for theradioactivity of some pyrochlore concentrates.

Two types of niobium ore deposits are known.In primary deposits, the pyrochlore is alwaysinterstratified in carbonatites. This is so in theCanadian deposits at Niobec and Oka, in whichcalciopyrochlore is interstratified in dolomite.The ore contains 0.5 0.7% niobium pentoxide.In the important secondary Brazilian deposits atAraxa and Catalao, the niobium content ofthe carbonate minerals has been considerablyenriched by weathering. Here, the ore is presentin combination with apatite, iron oxide, andbarite and contains about 3% Nb2O5, 46%Fe2O3, 17 18% BaO, and 1.5% P as apatite[5]. Table 1 shows the average Nb2O5 capacity,the production, and the reserves of variousdeposits [6]. In addition to the sources listed inTable 1, other, potential sources exist in Canada,Africa, Brazil, China, the United States, and theformer Soviet Union.

The most commercially important depositsare in Brazil, Canada, Nigeria, and Zaire. Worldreserves are estimated to be 4.1106 t Nb, ofwhich 78% are in Brazil. The Canadian reservesare estimated at 0.12106t.

The second most important niobium mineralis columbite, (Fe,Mn)(Nb,Ta)2O6, in whichniobium is nearly always present with tantalum.These ores are referred to as columbites if theNb2O5 content is greater than that of Ta2O5,otherwise as tantalites. Columbites and tantalitescontain at least 60% combined pentoxides.These minerals occur as primary deposits ingranites and pegmatites, or in alluvial secondary

Table 1. Niobium deposits [6]

Capacity, Reserves, Production,

Location 106 kg Nb2O5 Ore grade 106 t Nb2O5 10

6 kg Nb2O5

Araxa, Brazil 24.75 3.0 500 9.45St. Honore, Quebec 3.15 0.7 11 3.15

Catalao, Brazil 2.48 1.5 50 2.48

Nigeria 0.9

Thailand 0.45 0.9

Zaire pilot scale 2 3 extensive 0.23

134 Niobium and Niobium Compounds Vol. 24

deposits. Table 2 lists the chemical compositionsof various niobium and tantalum minerals [7].

Columbites are mined in Australia, Brazil,Nigeria,Malaysia, and Zaire. Since the discoveryof the enormous deposits of pyrochlore in Brazil,world production of columbite has decreasedconsiderably. This trend could change when theextraction of columbite as a byproduct of tinextraction is started with the Pitinga deposits inBrazil [8]. The discovery of large deposits ofpyrochlore in South Greenland (MotzfeldCenter) was reported in 1986.

A further source of niobium is provided bytantalum niobium slags from tin production,since columbites and tantalites are often associ-ated with cassiterite; niobium and tantalum con-centrate in the slags as oxides. The niobium andtantalum pentoxide content of various tin slags isgiven in Table 3 [9]. Niobium and tantalum arealso found in rare minerals such as stibiocolum-bites (Sb,Nb)TaO4, fergusonites (Re

3 )NbO4,and euxenite Y(Nb,Ti)2O6. The last two of these,however, are of no commercial significance.

4. Processing of Niobium Ores

4.1. Winning of PyrochloreConcentrates

Concentrates containing 50 60% Nb2O5 areobtained from pyrochlore-containing carbona-tites or weathered ores by conventional benefici-

ation processes such as crushing, grinding,magnetic separation, and flotation. Brazilianconcentrates (Araxa) are also treated chemicallyto remove lead, phosphorus, and sulfur. The rawconcentrate is roasted in the presence of calciumchloride and calcium oxide in a rotary furnace at800 900 C, and the product is leached withhydrochloric acid. In the case of Brazilian pyro-chlore, this procedure also leads to replacementof barium by calcium [10].

4.2. Treatment of PyrochloreConcentrates

Whereas pyrochlore from Araxa can be con-verted directly to ferroniobium, pyrochloreconcentrates from other sources must first bechemically pretreated before niobium can beobtained. Some well-known processes are treat-ment with concentrated sulfuric acid or withmolten soda. Reductive chlorination of pyro-chlore at ca. 1000 C produces volatile chlor-ides of niobium and other metals. Theseprocesses are, however, generally regarded asuneconomical. This is also true of the process inwhich digestion of pyrochlore with mixtures ofhydrofluoric and sulfuric acids is followed bysolvent extraction [11].

4.3. Production of Niobium Oxidefrom Columbites and Tantalites

4.3.1. Extraction Processes

Niobium and tantalum always occur together incolumbites and tantalites and must be separatednot only from the other elements present but alsofrom each other. The industrial separation oftantalum from niobium has long been carriedout by the Marignac process of fractionalcrystallization of potassium heptafluorotantalate

Table 2. Chemical composition of the principal niobium-bearing minerals, % [7]

Mineral Nb2O5 Ta2O5 TiO2 Fe MnO SnO2

Pyrochlore, NaCaNb2O6F 40 65 2 1 6 2

Columbite, (Fe,Mn)(Nb,Ta)2O6 40 75 1 40 0.5 3 10 20 2.6 2

Tantalo-columbite, (Fe,Mn)(Nb,Ta)2O6 25 60 20 50 0.5 3 10 20 2.6 2

Tantalite, (Fe,Mn)(Ta,Nb)2O6 2 40 42 84 0.5 3 10 20 2.6 2

Microlite, Ca2(Ta,Nb)2O6(OH,F) 60 70

Table 3.Niobium and tantalum pentoxide content of various tin slags,

% [9]

Country Nb2O5 Ta2O5

Malaysia 4 4

Nigeria 14 4

Portugal 7 7

Singapore 3 2

Thailand 8 12

Zaire 5 9

Vol. 24 Niobium and Niobium Compounds 135

and potassium heptafluoroniobate [12]. This ex-pensive process of precipitation and crystalliza-tion, which is also environmentally unaccept-able, has been abandoned, together with the longestablished Fansteel process [12], in favor ofprocesses based on solvent extraction.

Tantalite and columbite, either naturallyoccurring or synthetically produced as concen-trates from tin slags [13, 14], are digested withhydrofluoric and sulfuric acids at elevatedtemperature. The accompanying elements aredissolved along with the tantalum and niobium,which form the complex heptafluorides H2TaF7and H2NbOF5 or H2NbF7. After filtering offthe insoluble residue (fluorides of alkaline earthand rare earth metals), the aqueous solution ofTa Nb in hydrofluoric acid is extracted inseveral continuously operated mixer-settlerswith an organic solvent, e.g., methyl isobutylketone (MIBK) [1517]. The complex fluoridesof niobium and tantalum are extracted by theorganic phase, whereas most of the impuritiesand other elements, such as iron, manganese,titanium, etc., remain in the aqueous phase. Inpractice, Nb2O5 Ta2O5 concentrations of150 200 g/L in the organic phase are used.The organic phase is washed with 6 15 Nsulfuric acid and then reextracted with water ordilute sulfuric acid to obtain the niobium. Theaqueous phase takes up the complex fluoronio-bate and free hydrofluoric acid, while the com-plex fluorotantalate remains dissolved in theorganic phase. The aqueous niobium solution isreextracted with a small amount of MIBKto remove traces of tantalum. The resultingorganic phase is returned to the combinedtantalum niobium extraction stage. Gaseousor aqueous ammonia is added to the aqueousniobium solution to precipitate niobium oxidehydrate. Crystallization of K2NbF7 can onlybe achieved in strong hydrofluoric acid solution;therefore, it is only carried out on a small scalebecause of the high costs arising from theincreased consumption of hydrofluoric acid.

The tantalum is reextracted from the organicphase with water or dilute ammonia solution, andtantalum oxide hydrate is precipitated by ammo-nia, or potassium salts are added to produceK2TaF7, which is used in the production oftantalum metal.

The oxide hydrates are collected by filtration,dried, and calcined at up to 1100 C. Variation of

the conditions of precipitation, drying and calci-nation produces different particle sizes, givingoxides suitable for various applications. Depend-ing on the quality requirements, the calcination iscarried out in directly or indirectly heated cham-ber or rotary furnaces. The nature of the furnacelining has considerable influence on purity.

Sophisticated process control and optimiza-tion enable niobium and tantalum to be pro-duced with high yield (> 95%) and purity(> 99.9%).

A number of alternative extraction mediahave been reported in the literature, most ofwhich have never been used in industry, exceptfor tributyl phosphate (TBP) [18] and tri-n-octylphosphine oxide (TOPO) [19].

Figure 1 shows the flow diagram of an indus-trial installation for the processing of tantalum niobium raw materials [20].

4.3.2. Chlorination

The chlorination process is a modern alternativeto the extraction process. There are two versions:reductive chlorination of natural and syntheticraw materials, and chlorination of tantalum niobium ferroalloys.

In reductive chlorination, the ore or concen-trate is pelletized with coal/coke and pitch, dried,and reacted in a stream of chlorine at 900 C. Thenonvolatile alkaline earth metal chlorides remainbehind, while the readily volatilized tetrachlor-ides of silicon, tin, titanium, the pentachloridesNbCl5 and TaCl5, and WOCl4 are distilled offand fractionated. The waste gas, which containslarge amounts of phosgene and chlorine, must berigorously purified.

The chlorination of ferroalloys is much sim-pler andmore economical [21, 22]. Ferroniobiumor ferroniobium tantalum are produced by thealuminothermic or electrothermic process, sizereduced, and fed together with sodium chlorideinto a NaCl FeCl3melt. The chlorinating agentis NaFeCl4. Chlorine is passed into the melt,continuously regenerating NaFeCl4. The follow-ing overall reactions take place:

FeTaNbNaCl7 NaFeCl4!Ta=NbCl58 NaFeCl38 NaFeCl34 Cl2!8 NaFeCl4

The reaction temperature of 500 600 C ismuch lower than that required for reductive


chlorination. The volatile chlorides are evolvedfrom the molten salt bath. The boiling pointsof NbCl5, TaCl5, and WOCl4 lie between 228and 248 C, and these compounds must there-fore be separated by means of a distillationcolumn.

The chlorination of ferroalloys produces verypure niobium pentachloride and tantalum pen-tachloride in tonnage quantities. The NbCl5contains less than 30 mg/g Ta, and other metallicimpurities only amount to 1 2 mg/g. Niobiumpentachloride is an intermediate in the production

Figure 1. Flow diagram for the processing of tantalum niobium raw materials [20]


of oxides and organometallic compounds, and isthe starting material for production of niobiumpowder and for chemical vapor deposition (CVD)of niobium coatings.

5. Niobium Compounds

5.1. Oxides

Niobium pentoxide [1313-96-8], niobic acid,Nb2O5, mp 265

C, bp 1495 C, is a colorlesspowder that can only be dissolved by fusionwithacidic or alkaline fluxes such as NaOH orKHSO4, or in hydrofluoric acid. It is preparedby hydrolyzing solutions of alkali-metal nio-bates, niobium alkoxides (e.g., Nb(OC2H5 )5 ),or niobium pentachloride, or by precipitationfrom hydrofluoric acid solutions with alkali-metal hydroxides or ammonia. Dependingon the method used, the oxide hydrate formedis either a gel that is difficult to filter or isflocculent. The oxide hydrate is filtered,washed, and calcined at 800 1100 C. Thetemperature and treatment time determinewhich of the various crystalline modificationsis formed. Nearly all the phase changes areirreversible [2325].

Uses. Niobium pentoxide is used in metal-lurgy, for the production of hard materials, inoptics, and in electronics. Hydrated Nb2O5,which can be regarded as an isopolyacid, has ahigh surface acidity, and catalyzes the polymeri-zation of alkenes, e.g., propylene [26].

Various applications require different quali-ties of Nb2O5. For the electrothermic andmetallothermic manufacture of niobium metalor its alloys, technical quality niobium pentoxide(> 98 99%) is suitable. For higher qualityrequirements, chemically pure niobium pentox-ide (> 99.7%) is used.

In optics, niobium pentoxide is used as anadditive tomolten glass to prevent devitrificationand to control properties such as refractive indexand light absorption [27]. Optical grade niobiumpentoxide, with a purity of > 99.9%, must befree from colored impurities such as chromium,nickel, iron, manganese, etc.

Extremely stringent purity requirements applyto Nb2O5 used in the manufacture of LiNbO3[12031-63-9] or KNbO3 [12030-85-2] single

crystals (Nb2O5 ultra pure grade, > 99.995%).These compounds are used for electroacousticand electrooptical components such as modula-tors, frequencydoublers, andwavefilters [28, 29].

Ceramic grade niobium pentoxide, used formaking dielectric materials, must be manufac-tured with a special particle size distribution.This market sector will demand greatlyincreased quantities of niobium pentoxide if thedevelopment of the new class of ferroelectricperovskites (relaxors) is successful, e.g., Pb(Mg1/3 Nb2/3)O3 for the manufacture of ceramiccapacitors [3032].

Details of the various commercial qualities ofniobium pentoxide are given in Table 4 [20].

5.2. Halides

Niobium pentachloride [10026-12-7], NbCl5,Mr270.2,mp 209.5 C, bp 249 C, is now producedexclusively by chlorination of ferroniobium,niobium metal, or niobium scrap (see Section4.3.2.).

Niobium pentachloride forms strongly hygro-scopic yellow crystals that react with water toformNbOCl3 orNb2O5xH2O. The pentachlorideis very soluble in dry ethanol, tetrahydrofuran,and benzene. Alcoholic solutions of niobiumpentachloride are used in the production ofniobium alkoxides such as the pentaethoxide,Nb(OC2H5 )5, from which micronized niobiumpentoxide is produced [33]. Pure niobiumpentachloride is used for large scale productionof niobium pentoxide and niobium metal.

Niobium tetrachloride [13569-70-5], NbCl4,sublimes between 350 and 400 C. Niobiumtrichloride [13569-59-0], NbCl3, disproportion-ates between 900 and 1000 C. Niobiumtrichloride and niobium dichloride [13777-09-8], NbCl2, are formed by reduction of NbCl5with hydrogen, but are of no industrialimportance.

Niobium oxychloride [13597-20-1], NbOCl3,Mr 215.28, is a colorless, crystalline compoundthat sublimes at 400 C and partly decomposesinto niobium pentoxide and niobium pentachlo-ride on further heating.

Niobium pentafluoride [7783-68-8], NbF5,Mr 187.91, mp 72

C, bp 236 C, can be pre-pared by fluorination of niobium pentachlorideor niobiummetal with fluorine or anhydrous HF.


The reaction of Nb2O5 with aqueous HF yieldsfluoroniobic acids of various compositions (e.g.,H2NbF7 or H2NbOF5 ), depending on the acidconcentration. They are soluble in organic sol-vents and consequently play an important part inthe separation of tantalum from niobium bysolvent extraction.

5.3. Carbides and Hard Materials

The carbides, borides, silicides, and nitridesof niobium are metallic hard materials [34].Some of their physical properties are listed inTable 5. Only the carbides are commerciallyimportant.

Table 4. Commercially available niobium oxides [20]

Technically Chemically Grade HPO

Specification pure grade pure grade Ceramic grade (high purity optical) Grade UP (ultra pure)

Composition

Nb2O5 min. 99% min. 99.7% min. 99.8% min. 99.95% min. 99.997%

Ta max. 1600 ppm max. 800 ppm max. 500 ppm max. 100 ppm max. 8 ppm

Al max. 2 ppm max. 0.5 ppm

B max. 0.2 ppm

Bi max. 0.5 ppm

Ca max. 700 ppm max. 200 ppm max. 10 ppm max. 1 ppm

Cl max. 10 ppm

Co max. 2 ppm max. 0.2 ppm

Cr max. 2 ppm max. 0.2 ppm

Cu max. 2 ppm max. 0.2 ppm

F max. 1 ppm

Fe max. 1000 ppm max. 200 ppm max. 100 ppm max. 10 ppm max. 1 ppm

K max. 1 ppm

Mg max. 0.5 ppm

Mn max. 2 ppm max. 0.2 ppm

Mo max. 2 ppm

Na max. 400 ppm max. 200 ppm max. 1 ppm

Ni max. 100 ppm max. 2 ppm max. 0.2 ppm

Pb max. 0.5 ppm

Rare earths max. 1 ppm

S max. 30 ppm max. 1 ppm

Si max. 1000 ppm max. 250 ppm max. 50 ppm max. 50 ppm max. 1 ppm

Sn max. 0.5 ppm

Ti max. 300 ppm max. 100 ppm max. 100 ppm max. 1 ppm

V max. 2 ppm max. 0.5 ppm

W max. 50 ppm max. 1 ppm

Zr max. 50 ppm max. 1 ppm

Alkali max. 200 ppm

Loss on ignition max. 1% max. 0.5% max. 0.4% max. 0.2% max. 0.005%

Average particle 1 10 mm 0.5 10 mm 0.7 0.8 mm max. 5 mm 1 3 mmsize (FSSS)

Tap density 0.8 1 g/cm3 0. 8 1.25 g/cm3

Apparent density 0.5 0.7 g/cm3

(Scott)

Microtac analysis 90% < 1.2 mm50% 0.7 0.8 mm10% < 0.5 mm

Grain size 1. < 600 mm < 255 mmHPO 600

2. < 400 mmHPO 400

3. < 150 mmHPO 150

Crystal structure orthorhombic monoclinic monoclinic (H-phase) (T-phase) (H-phase) orthorhombic (T-phase)


Niobium Carbides (! Carbides). Theniobium carbon phase diagram [35] indicatesthe existence of a face-centered cubic compound,NbC, which melts without decomposition at3600 C with a broad region of homogeneity,and of Nb2C, which has a peritectic meltingpoint. Only NbC has practical applications. Inaddition to its original use as a grain growthinhibitor in tungsten carbide cobalt hardmaterials, it is now usedmainly as solid solutionswith titanium carbide, tantalum carbide, andtungsten carbide for cutting tools. The preciseeffect of niobium on the composition, micro-structure, and properties of these hard materialsis not yet fully understood [36]. In practice, upto 50% of the more expensive TaC can bereplaced by NbC without appreciably affectingthe hardness or fracture strength [37].

Production, like that of TaC, is by carburizingthe oxide, hydride, or metal at 1500 C in acarbon tube furnace, vacuum furnace, or in thepresence of a molten metallic menstruum such asiron or aluminum. Some of the physical proper-ties of niobium carbides are given in Table 5.

Niobium Borides. The niobium boronphase diagram [38] shows the existence ofNb3B2, NbB, Nb3B4, and NbB2. However, thesehigh-melting, very hard materials, have noindustrial uses. Table 5 lists their physicalproperties.

Niobium Silicides, Nb4Si, Nb5Si3, andNbSi2, are produced by the silicothermic reduc-tion of Nb2O5 or from the elements by sintering,pressure sintering, or fusion in an electric arcfurnace. Some physical data are given in Table 5.These compounds are not used industrially.

Niobium Nitrides. Various nitride phases,with regions of homogeneity varying in distinct-ness, are obtained by heating niobium metal ormixtures of niobiumoxide and carbon in a streamof ammonia or nitrogen [39]. Niobium metal canbe produced by thermal decomposition ofNbN inhigh vacuum. The pure nitrides have not as yetfound any industrial application. Table 5 listssome of the physical properties of niobiumnitrides.

Niobium Hydrides. On heating niobium ina pure hydrogen atmosphere to 350 500 C, Ta

ble5.Physicalproperties

ofborides,carbides,nitrides,andsilicides

ofniobium

[34]

Nb3B2

NbB

Nb3B4

NbB2

Nb2C

NbC

Nb2N

NbN

Nb4Si

Nb5Si 3

NbSi 2

CASregistry

number

[12045-55-5]

[12045-19-1]

[12045-89-5]

[12007-29-3]

[12011-99-3]

[12069-94-2]

[12033-43-1]

[24621-21-4]

[12164-59-9]

[12060-34-3]

[12034-80-9]

Crystalstructure

tetragonal,

ortho-

ortho-

hexagonal,

hexagonal

face-centered

hexagonal,

face-centered

hexagonal,

a-tetragonal

hexagonal,

U3Si 2type

rhombic,

rhombic,

AlB

2type

cubic,

W2Ctype

cubic,

ZrSi 4type

b-tetragonal

CrSi 2type

CrB

type

Ta 3B4type

NaC

ltype

NaC

ltype

Density,g/cm

38.0

6.6

7.83

7.78

8.33

8.2

7.74

6.26

5.45

Hardness*

2060

2200

2290

2600

2123

2400

2123

8**

550

600

700

mp, C

1860

2280

2700

3000

3100

3600

1950

2480

1950

(decomp.)

(decomp.)

(decomp.)

(decomp.)

Electrical

resistivity,

mWcm

64.5

12/34

35

60

10

40

50.4

Superconductivity

transition

temperature,K

8.25

1.27

1.27

9.18

69.5

15.2

1.27

1.2

*Vickershardnessat0.5

Nloading.

**Mohshardness


hydrogen (ca. 44 atom %) is absorbed, causingexpansion of the lattice and embrittlement. Apartfrom the NbH0.9 [13981-86-7] thus formed, thehydride Nb4H3 [56941-03-8] [40, 41] and theunstable dihydride NbH2 [13981-96-9] [42] arealso known.

The reversible uptake and release of hydro-gen by NbH is used in industry as a meansof producing niobium powder from compactniobium, e.g., scrap or ingots. The metal is firsthydrogenated, then ground, and finally dehy-drogenated in a vacuum furnace or under aninert gas.

6. Niobium Metal

The industrial production of niobium is usuallycarried out by the reduction of the pentoxide orhalides. The crude metal is then refinedelectrothermally.

6.1. Reduction of Niobium Pentoxide

Niobium pentoxide is reacted with carbon,aluminum, or silicon at high temperature. In thecarbothermic process, niobium pentoxide ismixed with carbon black, and the mixture ispelletized and reduced in a vacuum furnace ina two-stage process at 1950 C:

Nb2O57 C!2 NbC5 CO

Nb2O57 C!2 NbC5 CO

The reaction takes place with the formation ofNbC, Nb2C, NbO2 and NbO [43, 44]. A crudeproduct containing oxygen and carbon is pro-duced, which must be refined in further high-temperature processes.

The following one-stage carbonitrothermicreduction of niobium pentoxide has been suc-cessfully carried out on the pilot-plant scale. Apelletized mixture of niobium pentoxide andcarbon black is reacted in a stream of ammoniain an induction furnace at 1570 20 C to formniobium nitride, NbN, which is then thermallydecomposed in high vacuum at 2000 2100 C. However, the process does not yetappear to have been used on an industrial scale[45, 46].

Over 90% of niobium metal is produced byaluminothermic reduction:

5 NbCNb2O5!7 Nb5 CO

Niobium metal produced by this process isdesignated ATR niobium. In general, an excessof aluminum is used, producing a niobium aluminum alloy which is melted in a vacuum,electric arc, or electron beam furnace to producelow-oxygen, carbon-free niobium [47, 48]. Theamount of excess aluminum determines the yieldof niobium and its oxygen content [49].

The silicothermic reduction of niobiumpentoxide can lead to silicide formation, andthe thermodynamics of this reaction are lessfavorable. This process is therefore not inregular use.

6.2. Reduction of Halides

The reduction of niobium pentachloride can beachieved with hydrogen at 600 650 C [50].Alternatively, niobium pentachloride vapor isreduced with hydrogen in a fluidized bed furnaceat 750 1050 C [51] or in a hydrogen plasmaabove 2000 C [52]. Metallothermic reductionwith sodium or magnesium has also beenreported [53]. The company TOHO TitaniumCorporation has commissioned a plant with acapacity of 30 t/a niobium in which niobiumpentachloride is reduced by magnesium [54].A reduction process with zinc using the auxiliarymetal bath technique is also known [55]. Incontrast to the production of tantalum, the reduc-tion of the fluoroniobates K2NbF7 and K2NbOF5has not become industrially significant.

Very pure niobium can be obtained by elec-trowinning fromoxygen-freemolten salt systemswith double fluorides or chlorides as the source ofmetal [56]. These molten salts are very corrosiveand the current efficiencies are low. For thesereasons, molten salt electrowinning of metallicniobium is not carried out commercially.

6.3. Refining

Crude niobium must be refined in order toremove impurities introduced either from theraw materials or during the treatment stages.


Purification can be achieved by high-temper-ature treatment because the melting point ofniobium (2497 C) is so high that most otherelements can be removed by vaporization. Nio-bium is extremely reactive towards all but theinert gases [57], and melting must thereforealways be carried out in a high or ultra-highvacuum or under a pure inert gas to minimizethe concentration of harmful interstitial foreignatoms.

In comparison to the classical sintering andelectric arc melting processes, electron beammelting (EBM) has considerable technical ad-vantages for the production of pure metal.

The vaporization of the impurities can becontrolled in each melting cycle by[58, 59]:

1. Optimizing the melting rate,2. Maintaining the bath in a liquid state for a

long period, and3. Controlled superheating of the molten metal.

Sufficient refinement cannot be achievedwithone melting cycle only, so that in practice thesolid ingot produced must be remelted. Themelting rate (kg/h Nb) for the first melt (mainrefining step) depends on the raw material and islower for ATR niobium because of the highcontent of aluminum and NbO that must beevaporated compared with compacted granularniobium. Further development of the plasmamelting technique would enable a simple plasmafurnace to be used for the first melting operation[60, 61]. In the second and third melting cycles,the melting rate can be increased by a factor of3 4, depending on the specification of thematerial.

Optimization of the equipment used in theelectron beam melting process, leading to reduc-tion in residual gas pressure and leakage rate, hasenabled standard quality niobium to be producedfromATRniobium on a large scale with only twomelting cycles [62].

The electron beam melting and remeltingtechnique enables niobium metal with less than50 mg/g interstitial impurities to be produced forhigh-frequency superconductors. Figure 2 showsthe concentration of interstitial impurities (O, N,and C) as a function of the number of meltingcycles [63].

Ultra-high purity niobium can be obtainedfrom metal melted by the EBM method by a

series of additional sophisticated process steps,including electrorefining and electron beam zonemelting, followed by high-temperature treatmentin high vacuum. By these methods, niobiummetal can be prepared with metallic impuritiesin the parts per billion range and with interstitialimpurities < 1 ppm [64].

6.4. Uses

Niobium has good resistance towards corrosivechemicals [4, 65] even at high temperatures, andis therefore used in the construction of chemicalequipment, though it is not quite so resistant astantalum. The process of oxide dispersion hard-ening of niobium with titanium dioxide gives amaterial which can be used both for chemicalequipment and for medical implants subjected tohigh mechanical loading [66]. High-niobiumalloys such as KBI 40/41 [67] can also be usedunder these conditions, for which formerly themore expensive metal tantalum had to be used.

Because niobium has a low neutron capturecross-section and is unusually resistant towardscorrosion by liquid sodium, it is used in the purestate or as the alloy NbZr1 in the nuclear industryfor the production of fuel element cans. Thisalloy is also used for the sealing caps of sodiumvapor lamps.

Figure 2. Contents of interstitial impurities (O, N, and C) ofan electron beam melted niobium ingot as a function of thenumber of melting cycles [63]


World consumption of niobium for the pro-duction of superconducting materials, such asNb3Sn and NbTi is estimated to be 60 70 t/a.

The addition of niobium to titanium aluminum alloys imparts the ductility neededto fabricate this material for the aircraft andspace industry [68].

Other newly developed alloys C 103(NbHf10Ti1) and Nb 752 (NbW10Zr25) com-bined with a special coating technique enableniobium to be used in the manufacture ofnozzles and combustion chambers for rocketpropulsion [69].

7. Ferroniobium

Approximately 85 90% of the total niobiumproduction is used in the steel industry in theform of iron niobium alloy (ferroniobium)containing 40 70% niobium. Depending onthe application, the alloy can also containsmall amounts of Ta (ferroniobium tanta-lum), e.g., FeNb65Ta2, which contains 65%Nb and 2% Ta. Other alloy specifications aregiven in [70].

7.1. Production

Ferroniobium is usually produced by alumino-thermic reduction of niobium oxide ores, withthe addition of iron oxides if the niobium oreused contains insufficient iron. The startingmaterials are mainly columbites and pyrochloreconcentrates.

The enthalpy of the reaction between Nb2O5and Al is276.1 kJ/mol Al, which is lower thanthe threshold value for self-sustaining alumi-nothermic reactions. The mixture must thereforeeither be preheated or mixed with oxygenreleas-ing compounds such as BaO2, CaO2, KClO4,KClO3 or NaNO3 .

Concentrates with lower percentages of niobi-um (ca. 40%) can also be treated by the alumi-nothermic process in an electric arc furnace.Also,a two-stage electroaluminothermic process forthe production of ferroniobium from columbitehas been developed [71].

The method of operation is to charge themixture of niobium concentrate with the addi-tives to refractory lined reaction vessels. Either

the wholemixture is reacted, or a small amount isset aside, ignited with a special exothermic mix-ture, and added to the bulk mixture. The moltenreaction product is allowed to solidify in thefurnace, and the block of metal separates fromthe slag. After cooling, it is broken into pieces ofthe required size.

In the aluminothermic process operated atAraxa, Fe Nb blocks of metal up to 11 t inweight are produced. The yield of niobiummetalis 96 97%. The typical composition of a reac-tion charge is as follows:

Pyrochlore concentrate 18 000kg (60% Nb2O5 )

Iron oxide 4000 kg (68% Fe)

Aluminum powder 6000 kg

Fluorspar 750 kg

Lime 500 kg

The reaction gives ca. 11 000 kg of ferroniobiumof composition:

Nb 66.0%

Fe 30.5%

Si 1.5%

Al 0.5%

Ti 0.1%

P 0.1%

S 0.04%

C 0.08%

Pb 0.02%

and ca. 20 000 kg of slag containing:

Al2O3 48.0%

CaO 25.0%

TiO2 4.0%

BaO 2.0%

Re2O3 4.0%

Nb2O5 trace

ThO2 2.0%

U3O8 0.05%

Special niobium alloys, e.g., nickel niobium,cobalt niobium, aluminum niobium andchromium niobium, are also manufacturedby the aluminothermic process. For thesealloys, which have various niobium contents[72], niobium oxide is the only raw materialused.


7.2. Uses of Ferroniobium

Niobium has very marked carbide and nitrideforming properties and is therefore mainly usedin the production of steels and gray iron [73].The addition of niobium prevents intercrystal-line corrosion in stainless austenitic chromi-um nickel steels and improves corrosion re-sistance, weldability, ductility, and toughnessin ferritic chromium steels [74]. Ferroniobiumis also used as a trace additive in large quanti-ties for the production of high-strength low-alloy (HSLA) steels used in automobile bodymanufacture, structural steels, concrete rein-forcing steel, and pipelines. Superalloys con-taining up to 5% niobium are used for gasturbine components. These materials havehigh hot strength [75, 76], and mostly have anickel or copper base. The niobium is added inthe form of nickel niobium or cobalt nio-bium.

8. Analysis

Extensive literature on the analytical determina-tion of niobium is available in handbooks andmonographs [7780].

The determination of niobium in raw materi-als is carried out by X-ray fluorescence [8183].For this purpose, test pieces in tablet form areproduced by reacting the niobium-containingmaterial with borate to produce a melt, or bycompressing it with binders such as wax or boricacid.

For ferroniobium, X-ray fluorescence anal-ysis of HF solutions [84] or borate tablets isused. For umpire assay, it is usual to separatethe niobium from the other material using ion-exchange resins with HF solutions, followed bygravimetric determination [85]. Small niobiumcontents in steels are determined photometri-cally [86].

Metallic impurities in niobium pentoxideor niobium metal are determined by atomicabsorption spectrometry (AAS) or atomicemission spectrometry (ICP-OES, DCP-OES)in HF solution [87]. Emission spectrum anal-ysis in a d.c. current plasma arc is also used.The nonmetals oxygen, nitrogen, hydrogen,

carbon, and sulfur are determined by extrac-tion at high temperature either with a carriergas or under vacuum, or by combustion anal-ysis in a stream of oxygen [88]. The hydride-forming elements arsenic, antimony, bismuth,selenium, and tellurium can be converted totheir hydrides and detected with high sensitiv-ity by the AAS method. The anions Cl andF are separated by distillation and determinedphotometrically, by ion-selective electrodes, orby ion chromatography.

In all the above-mentioned methods, the usuallimits of detection are from the low mg/gregion down to the ng/g region in routine qualitytesting. For the analysis of ng/g trace impuritiesit is necessary to separate and concentrate thematerial [89, 90]. The final determination iscarried out using AAS with a graphite furnace,ICP-OES, DCP-OES, or voltammetry. Recently,ICP mass spectrometry has become a powerfuladditional method for many metallic impurities.Niobium metal is analyzed simply and quicklyfor metallic and nonmetallic impurities downto the lower ng/g region by glow dischargemass spectrometry (GDMS) [91]. The additionaltechniques of neutron activation analysis [92, 93]or proton activation analysis [94] are used forverification.

Figure 3. World niobium consumption compared to worldcrude steel production from 1972 1988 [95]


9. Economic Aspects

The world consumption of niobium has, since1980, reached 16 20106 kg Nb2O5. Theannual rate of increase is ca. 2%. More than90% of total niobium production goes to thesteel industry in the form of ferroniobium orniobium alloys, and therefore the demand forniobium is determined by the world market foriron and steel [95]. Figure 3 shows the clearrelationship between steel production andniobium demand. Between 1984 and 1988,technical quality niobium pentoxide was mar-keted in the price range 14.3 15.2 $/kg. Theprice of standard grade ferroniobium was14.5 $/kg Nb. In mid 1987, the price of niobi-um concentrate (> 65% Nb2O5 Ta2O5,10 : 1) was 5.5 6.4 $/kg Nb2O5 CIF Europe.Canadian pyrochlore concentrate was quoted at5.9 $/kg Nb2O5 FOB.

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Further Reading

A. Agulyansky: The chemistry of tantalum and niobium

fluoride compounds, 1st ed., Elsevier, Amsterdam 2004.

J. E. Schlewitz: Niobium and Niobium Compounds, Kirk

Othmer Encyclopedia of Chemical Technology, 5th

edition, vol. 17, p. 132157, JohnWiley& Sons, Hoboken,

NJ, 2006, online: DOI: 10.1002/0471238961.

1409150219030812.a01.pub2.


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