THE ALUMINIUM INDUSTRY In general, raw aluminium is converted into commodities by methods analogous to those used for other metals. The few radical differences are alone discussed here. Aluminium is melted in re verberatory furnaces or crucibles heated by oil, coke, gas or electricity, as circumstances and the scale of operations dictate. The pure metal requires no flux but a little zinc chloride is used with zinc alloys.
Ingots for rolling, extrusion, etc., are cast into iron moulds. Industrial castings are, however, prepared by three methods: large parts or those required in small numbers, in sand (sand castings) ; smaller parts required in numbers, by hand into per manent cast iron moulds (chill castings) ; whilst latterly consider able progress has been made in forcing the molten metal into closed moulds under pressure (die castings). Die castings are accurate to - —in. so that no subsequent machining is required, but very large numbers are required to justify the costly dies.
Rolling into sheets, foil, rods and sections, beating to leaf, extruding and drawing tubes and wire, spinning, pressing, riveting, machining, etc., follow customary practice, only minor modifica tions being demanded by the physical properties of aluminium or its alloys.
Annealing of work-hardened material is effected rapidly at C. to 400° C., a temperature recognizable by the charring of a soft wood splinter drawn across the metal. Scaling does not take place so that no protection from the flame or furnace gases is necessary. Pickling or frosting is effected by immersion in a hot io% caustic soda solution followed by dipping in dilute nitric acid. The alkali lightly attacks the surface of the metal giving a smooth, white finish. Forging is confined to certain alloys, nota bly duralumin (q.v.). Contrary to general opinion, soldered joints are easily made and are as strong, when first made, as those of other metals. They fail, however, without exception after a few weeks or even hours exposure to mild corrosives, because during soldering the solder dissolves some aluminium, forming alloys, generally of tin or Zinc. These corrode so readily that they freqeuntly disintegrate in damp air; but where complete pro tection can be afforded, as by painting, immersion in oil, etc., soldering is sometimes useful.
Welding, unlike soldering, demands no second metal and aluminium is successfully welded on a very large scale by sev eral methods. At about 5 75 ° C. aluminium is very soft and two pieces hammered or pressed together at this stage unite as steel unites, giving very durable welds (hammer welding). The process is, however, difficult and expensive except on bars and wires and is not applicable to alloys. Using fluxes, principally mixtures of alkaline chlorides and fluorides, which dissolve the oxide film, the edges of two pieces of aluminium and many aluminium alloys may be molten by the oxyacetylene flame and caused to flow together, whilst additional metal may be .applied by means of an aluminium welding rod (autogenous welding). This method is widely used for making articles of sheet metal too large to be pressed or spun, such as petrol tanks, vats, stills, etc. Broken castings are also repaired by autogenous welding, sometimes without the use of flux.
Electrical welding is mainly confined to the butt-welding of rods and wires, the ends to be joined being pressed together while partially fused by the passage of a current. Spot-welding is also practised for special purposes.
Near its melting point aluminium loses all cohesion and the grains may be separated. Alternatively, the molten metal is run into specially constructed disintegrators, the product after solidi fication being classified by sieving. This type of powder is in Europe known as granulated aluminium, a name reserved in the United States for molten metal which has been passed through sieves into water. Aluminium bronze powder is produced from pure aluminium sheet scrap by progressive stamping in cast iron mortars with mechanically operated stamps. The finest particles are separated from the coarse by air classifiers and are then polished by rotating brushes in a sheet iron drum.
Approximately 2 tons of alumina, ton of carbon and 2 5,000 kilowatt hours are required to make a ton of aluminium. As about two tons of bauxite and several tons of chemicals and fuel are required for every ton of alumina, large quantities of ma terials have to be assembled. In most cases the production of the metal has become centred near the cheap sources of electricity, namely, hydro-electric power stations. As a notable exception, in northern Germany, steam stations fed by cheap and abundant lignite, provide the necessary motive power. Table No. i gives a picture both of the distribution of the producing countries and the movements of the metal when produced. From this it is seen that production is confined to 9 countries and that some 4o of the world's output is derived from the United States which, nevertheless, import in addition some r o–i 5 % of what the world manufactures. From a comparison of the three columns in any year the amount converted into commodities in any coun try may also be gauged. Thus Norway, one of the largest produc ers, has but a poorly developed fabricating industry, whereas India and Japan, where no aluminium is produced, absorb large quantities for conversion into utensils and other manufactured products.
Finally, we may note that of the 7 50,000 tons of bauxite used for conversion into aluminium in 1925, the bulk was produced in 6 countries, viz., France, the United States, British Guiana, Italy, Dutch Guiana and Yugoslavia and none in Great Britain or Ireland.
The following table gives the world prices for ingot metal for the years 1924-27 inclusive:—