Preparation of Niobium Metal

Niobium is a very important metal in both ferrous and nonferrous metallurgy. As an additive to alloys or when alloyed with other metals niobium imparts high mechanical strength, high electrical conductivity, and ductility to alloys. It enhances corrosion resistance of most alloys. The metal and several of its alloys exhibit superconductivity.

Nobium is used as an additive in the manufacture of most high strength, low alloy carbon steels and microalloyed steels that are used in the construction of oil and gas pipelines, bridges, buildings, concrete bars, and automobiles. Niobium can be isolated from tantalum and prepared in several ways from different niobium-containing materials.

Niobium-tantalum concentrates

(i) The primary source is the columbotantalite ore, (Fe,Mn)(Ta,Nb)₂O₆, obtained after concentration of the raw ore by means of common ore-beneficiation processes (e.g., froth flotation, gravity separation).

(ii) A Nb-Ta source of secondary importance consists in treating tin-slag resulting from the smelting of niobium-tantalum-bearing cassiterite. Actually, the tin slag usually contains between 1.5 and 10 wt.% Ta₂O₅ and 1 to 3 wt.% Nb₂O₅. Tin slags are then leached by either an acid or alkaline solution, providing an insoluble Nb-Ta-bearing residue, principally made of niobium and tantalum pentoxides.

(iii) A third recent source of niobium and tantalum comes from recycling of hardmetal that contains tantalum and niobium carbides. After recycling, the process provides a Nb- Ta-bearing oxide residue.

Processing

These three Nb-Ta sources can be processed to produce niobium-tantalum concentrates.  Two niobium chemicals are industrially used as feedstock to prepare the niobium metal: niobium heptafluorotantalate (K₂NbF₇) and niobium pentoxide (Nb₂O₅).
Niobium heptafluorotantalate is obtained by adding potassium fluoride, KF, to the purified stripping solution in order to precipitate the insoluble crystals. The settled crystals of K₂NbF₇ are easily removed from the solution by centrifugation and filtration.

Once separated, the crystals are dried. Niobium pentoxide is prepared by precipitation of niobium hydroxide, Nb₂O₅.xH₂O, by adding ammonia gas, NH3, to the stripping solution containing niobium. The settled precipitate is then filtrated, washed with deionized water, dried, and calcinated, giving off water, to obtain the anhydrous niobium pentoxide.

Preparation of niobium metal

Niobium metal is further obtained by either thermal reduction of niobium pentoxidepentoxide (Nb₂O₅) or the thermal reduction of niobium heptafluorotantalate (K₂NbF₇). There are two processes for producing niobium metal by thermal reduction of niobium pentoxide. Carbothermic reduction consists in reducing Nb₂O₅ by carbon. The reaction is carried out in two steps: the first step involves preparation of niobium carbide by vacuothermal reduction of Nb₂O₅ with carbon at 1650°C under a pressure of 133 mPa.

The carbide is then mixed with additional niobium pentoxide and reacted at 2020°C under a diminished pressure of 1.33 mPa. After the reaction is complete, the reacted mass is cooled in a vacuum. Metallothermic reduction uses aluminum powder as a reducing agent and calcia as flux. The reduction takes place in a magnesia refractory brick-lined reactor, and after the reaction is complete, the reacted mass is allowed to cool and the slag separated from the metal. In the metallothermic reduction of niobium heptafluorotantalate, sodium metal is used as reducing agent.

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