Aluminothermic reduction

The vanadium alloy is purified and consoHdated by one of two procedures, as shown in the flow diagram of the entire aluminothermic reduction process presented in Figure 1. In one procedure, the brittle alloy is cmshed and heated in a vacuum at 1790°C to sublime most of the aluminum, oxygen, and other impurities. The aluminum faciHtates removal of the oxygen, which is the feature that makes this process superior to the calcium process. Further purification and consoHdation of the metal is accompHshed by electron-beam melting of pressed compacts of the vanadium sponge. 

Aluminothermal Method. Calcium metal is produced by high temperature vacuum reduction of calcium oxide in the aluminothermal process. This process, in which aluminum [7429-90-5] metal serves as the reducing agent, was commercialized in the 1940s. The reactions, which are thermodynamically unfavorable at temperatures below 2000°C, have been summarized as ... 

Even though the standard free energy change associated with an aluminothermic reduction reaction... 

One of the important differences between calciothermic and aluminothermic reduction of oxides concerns the interaction between the reduced metal and the reductant. Calcium does not form stable solid solutions or alloys with the reduced metals calcium contamination in the metal is, therefore, relatively small. Aluminum, on the other hand, readily forms solid solutions with the reduced metals, and the product generally contains appreciable quantities of residual aluminum. This is not a serious problem because in many cases either a certain aluminum content is desired in the reduced metal or the residual aluminum can be effectively removed in post-reduction purification operations. The extent of the contamination of a reduced metal with the reductant can be related to factors such as the reaction temperature, the standard free energy change associated with the reaction, and the slag composition. Let the following generalized reaction be considered ... 

Aluminothermic reduction is one among the few pyrometallurgical processes where the actual process closely follows the route theoretically predicted from thermodynamic data. This characteristic, coupled with the simplicity associated with the whole process, makes it well suited for demonstration experiments. The reduction of magnetite by aluminum is a suitable example in this context. 

The reduction of manganese oxides by aluminum has been used for the production of manganese metal. The process is also a classic example to illustrate thermal energy management in a typical aluminothermic reduction process. 

The production of chromium metal by the aluminothermic reduction of chromium sesquioxide can be represented by the equation ... 

In the aluminothermic reduction of niobium oxides, the products must reach a temperature of at least about 2470 °C, and hence the heat required to raise niobium metal and alumina from room temperature to this temperature must be estimated. Using the values of the heat capacities and the heats of fusion for niobium and alumina, the following figures can be obtained ... 

The reactor used for the aluminothermic reduction of niobium pentoxide is shown schematically in Figure 4.17 (A). It is a steel pipe, lined on the inside with alumina and provided with a pipe cap. The charge, consisting of stoichiometric amounts of niobium pentoxide and aluminum powder, is blended and loaded in the lined pipe, and covered with alumina. The cap is closed and the reaction initiated by placing the loaded bomb in a gas-fired furnace, preheated to 800 °C, and by raising the temperature of the furnace to 1100 °C. 

Instead of a closed-bomb reactor, an open reactor (shown in Figure 4.17 B) has also been used for this aluminothermic reduction. The mild steel reactor is lined on the inside with calcined magnesia. An arrangement is made to initiate the reaction in the center of the... 

Figure 4.17 B Schematic of the reactor for open aluminothermic reduction of niobium pentoxide.

Vanadium forms numerous oxides, the most important of which are vanadium monoxide, vanadium sesquioxide, vanadium dioxide and vanadium pentoxide. In the earlier examples (e.g., oxides of chromium and of niobium) the enthalpy values for the aluminothermic reduction of each of the oxides was given for the purpose of illustration. Normally, the consideration can be restricted to only those oxides which are readily obtained and which can be handled freely without any special or cumbersome precautions. In the case of vanadium for example, it is sufficient to consider the reduction of the sesquioxide (V203) and the pentoxide (V2Os). The pertinent reactions are ... 

In practice, the production of vanadium by aluminothermic reduction is also governed by some other considerations. The reduction has to be carried out under an inert atmosphere (helium or argon) to avoid nitrogen pick-up from the air by vanadium metal. The composition of the oxide-aluminum charge has to be so chosen that the thermit (metal obtained by aluminothermic reduction) contains between 11 and 19% aluminum. This is necessary for the subsequent refining step in the vanadium metal production flowsheet. Pure vanadium pentoxide and pure aluminum are used as the starting materials, and the reduction is conducted in a closed steel bomb as shown in Figure 4.17 (C). 

Figure 4.17 C Schematic of the closed-bomb reactor for aluminothermic reduction of vanadium pentoxide.

Ferrovanadium stands as a major industrially used form of vanadium. Similarly, it is in the form of ferroniobium that the bulk of niobium is used industrially. Aluminothermic reduction is an elegant metallurgical process for the production of these ferroalloys in their practically carbon-free forms. 

Table 4.3 Details of ferroniobium production from pyrochlore by aluminothermic reduction.

Columbium (also known as niobium) and tantalum metals are produced from purified salts, which are prepared from ore concentrates and slags resulting from foreign tin production. The concentrates and slags are leached with hydrofluoric acid to dissolve the metal salts. Solvent extraction or ion exchange is used to purify the columbium and tantalum. The salts of these metals are then reduced by means of one of several techniques, including aluminothermic reduction, sodium reduction, carbon reduction, and electrolysis.19-21 Owing to the reactivity of these metals, special techniques are used to purify and work the metal produced. 

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