This is by far the most general method of preparing elements. The elements commonly used as displacing agents are :-
The oxide or chloride is placed in a tube of hard glass, heated to 6oo° or 7000 in a tube-furnace, and a stream of dry hydrogen is passed through the tube. Water or hydrogen chloride is formed, and is carried on by the current of hydrogen, and the element i-s left. Indium, thallium, germanium, tin, lead, antimony, and bismuth are left in fused globules, solidifying to white lustrous metallic beads ; arsenic gasifies and condenses in the unheated part of the tube as a grey deposit; tellurium, which is also volatile, condenses as a lustrous metallic solid; while iron, cobalt, nickel, copper, and silver do not fuse at that temperature. The first three remain as grey powders, the copper as a red powder, and the silver in a white spongy condition. These metals can be fused by heating them in a crucible to a sufficiently high temperature ; it is well to use a " flux," or substance to make them flow, such as sodium carbonate or borax; the flux fuses, and dissolves any film of oxide off the surface of the metallic beads, and they then join up to form a single mass of molten metal.
The chlorides of beryllium, magnesium, calcium, strontium, barium, aluminium, scandium, yttrium, lanthanum, ytterbium, cerium, thorium, vanadium, niobium, and tantalum are all reduced when added to sodium kept melted in an iron crucible. For boron, silicon, and titanium the double fluoride is more convenient, for the chlorides are volatile liquids. The process for manufacturing magnesium, which is carried out on a large scale, may be more minutely described as an example. The double chloride of magnesium and potassium, MgCl2.KCl, carefully dried, is mixed with sodium in proportion to unite with the chlorine of the MgCl2, the sodium being in small lumps. The iron crucible containing the mixture is heated; a violent reaction takes place, and magnesium is liberated:
MgCl2. KC1 + 2Na = Mg + 2NaCl + KC1. As magnesium is volatile, and can be distilled, it is purified by this operation. The contents of the crucible are treated with water; the potassium and sodium chlorides dissolve, and the globules of magnesium are collected, dried, and placed in a crucible, through the bottom of which a tube is fixed reaching nearly to the lid, and projecting some distance below the bottom. This crucible is placed in a furnace, and on raising the temperature, the magnesium volatilises up, passes down the tube, and the vapour condenses in the cooler part of the tube which projects below the furnace. This particular method of distillation is called destillatio per descensum. The other elements mentioned are too little volatile to admit of purification by this means. In their case, the cooled mass is treated with alcohol in order to remove the excess of sodium, and then with water to dissolve the resulting salt; the element is left in the state of powder.
This process is sometimes used to prepare the element from its oxide. A mixture is made of magnesium filings with the oxide of the element, and it is heated in an iron crucible. The resulting mass is then treated with hydrochloric acid to remove the oxide of magnesium, which is thus converted into the soluble chloride. It is, of course, essential that the liberated element shall not be attacked by hydrochloric acid. The process works for the preparation of boron, silicon, and titanium ; also of rubidium and caesium, by distilling a mixture of their hydroxides with magnesium in an iron tube, and collecting the distilled metals under vaseline.
This process is of the most general application. If the element is volatile, it is distilled from an iron or fireclay retort; in this way sodium, potassium, rubidium, arsenic, zinc, and cadmium are prepared. If non-volatile at a red heat, a mixture of the oxide with charcoal is heated to bright redness in a clay crucible. On a manufacturing scale, coal or coke is substituted for the charcoal. The process is applicable to the production of indium, thallium, germanium, tin, lead, manganese, iron, cobalt, nickel, and copper. To exemplify this method, four instances will be described-the preparation of phosphorus, sodium, zinc, and iron.
The commonest natural compounds of phosphorus are phosphorite or calcium phosphate, Ca3(PO4)2, and gibbsite or aluminium phosphate, A1PO4. It is accordingly convenient and economical to prepare phosphorus from one of them. The process depends on the displacing action of carbon on the oxide at a high temperature. There are two methods of effecting this. The first is: the phosphorite is mixed with dilute sulphuric acid; the hydrogen of the sulphuric acid replaces the calcium of the calcium phosphate: Ca3(PO4)2 + 3H2SO4.Aq = 3CaSO4 +2H3PO4.Aq. Coke or charcoal is impregnated with the phosphoric acid and heated to redness, when the phosphoric acid loses water : H3PO4 = HPO3 + H2O. The mixture of metaphosphoric acid, HPOg, with carbon is charged into retorts of Stourbridge clay, the mouths of which are attached to a vertical copper tube, the lower end of which dips under water. On raising the retorts to a white heat, phosphorus distils over and condenses in the water. The final equation is : 4HPO3 + 12C = 2H2 + P4 + 12CO. By the second method, the calcium and aluminium phosphates are mixed with silica and carbon, and distilled from an electric furnace heated to whiteness by an arc in its interior.