The displacement of a metal from its solid compounds by another metal through thermal methods was possible only after the discovery and isolation of the alkali metals by Humphry Davy in 1808. The technique was further developed after the discovery and isolation of aluminum by Hans Christian Ørsted in 1826 and Friedrich Wöhler from 1827-45. The fact that it was difficult to liberate the alkali metals and aluminum from their “earths” suggested these metals could themselves be used to liberate other difficult-to-isolate metals. A similar reaction takes place in aqueous solutions and is known as cementation – an ancient process used by alchemists.
The first industrial application of metallothermic reactions was in 1854 by Henri Sainte-Claire Deville, who produced metallic aluminum by reduction of the molten double chloride, AlCl3·NaCl, with sodium. The endothermic reaction: AlCl3·NaCl + 3Na → Al + 4NaCl was conducted in a reverberatory furnace.
When Charles Martin Hall in the U.S. and Paul Louis Toussaint Héroult in France simultaneously and independently invented the electrolytic process for the production of aluminum in 1888, their process immediately displaced Sainte-Claire Deville’s process.
The idea of displacement of a metal from its solid compounds by another metal with thermal methods was first applied by French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808, and independently by Davy who was the first to isolate boron by reacting boric acid with potassium. In 1811, Gay-Lussac and Thénard passed gaseous silicon tetrafluoride over heated potassium; although they did not identify the brown mass they obtained, it was believed to be impure, amorphous silicon. Silicon tetrafluoride had already been prepared by Swedish chemist Carl Wilhelm Scheele (1742-1786), who heated a mixture of sand and fluorspar with concentrated sulphuric acid in 1771. It was another Swedish chemist, Jöns Jakob Berzelius, however, who further developed the technique and was the first to prepare other metals using the method. He prepared zirconium in 1824, titanium in 1825, thorium in 1828, and vanadium in 1831. Other chemists followed suit.
Metallothermic reactions are concerned with the preparation of metals and alloys by reduction of their oxides or halides with a metal. These reactions can be expressed in general by the equation: AX + B → A + BX, where X is oxygen, chlorine, or fluorine, and A and B are two metals. It is characterized by the fact that the reducing metal is converted to either a solid or a liquid product, and not to a gas as in other reduction processes, e.g., by carbon or hydrogen where CO + CO2 and H2O are formed, respectively. In fact, this method is used when reduction by carbon and hydrogen or by electrowinning from aqueous solutions is not possible.
The most commonly used reducing metals are aluminum, calcium, ferrosilicon, magnesium, and sodium. The choice of a reducing metal is based on many factors. In general, the metal must have a strong affinity for the compound to be reduced, it should be cheap, have a high boiling point, low vapour pressure, produce a slag that can be easily melted or leached away, not form intermetallic compounds with the metal produced, and be easy to handle. Metallothermic reactions may be exothermic or endothermic. In the exothermic system, the reaction takes place in such a way that when the mixture is ignited at one place, the reaction continues spontaneously, usually with the formation of a fluid slag, and the reduced metal is obtained as a compact uniform regulus.