USES OF LEAD
Lead is used in the form of the metal, of alloys with other metals, and of various chemical compounds. As metal its chief uses are as pipe for water and corrosive solutions; for protective covering of electrical cables; as sheet lead for lining chambers for the manufacture of sulphuric acid and vats for chemical manufacturing processes. In smelting, lead is used as a collector of other metals, particularly of gold and silver, from which it is later separated, now most generally by the use of zinc by the Parkes process of desilverization.
Lead alloys readily with nearly all other metals in all proportions. Its alloys of industrial importance comprise type metal, bearing or babbitt metals, shot, solders, casting metals, some brasses, and the fusible alloys used for the protection of electrical apparatus and in automatic sprinklers for the protection of buildings against fire. Type metal, originally composed of 83 per cent. lead and 17 per cent. antimony, now often contains bismuth and sometimes a little copper and iron. An alloy of 9 parts lead, 2 antimony and 2 bismuth is used for stereotype plates. Less than 2 per cent. of arsenic is added to lead used to make shot to increase the hardness and sphericity of the product. Antimony also imparts the hardness essential to shrapnel, etc. Bearing metals comprise alloys of lead and antimony or these with copper, tin, and zinc. Antimony imparts to lead the property of expansion on solidification, essential to type metal and casting materials generally. Lead makes a brass that is soft and machines easily. Solder is commonly an alloy of lead and tin. The melting point varies with the proportions of these constituents and others, sometimes added for special purposes. The cheapest solder in general use is 30 per cent. tin and 70 per cent. lead. Solders seldom contain more than 50 per cent. tin. The addition of bismuth, cadmium, or mercury lowers the melting below the boiling point of water. Fuses can thus be obtained which interrupt electric circuits at any desired temperature.
The largest uses of lead compounds are as pigments. White lead or basic carbonate, 2(PbCO3)Pb(OH)2, is the most extensively consumed, being used alone or mixed with zinc oxide and barytes. Red lead (Pb3O4) is used for painting structural steel, as a pipe-joint cement, and in the manufacture of glass. Litharge, another oxide, is used in assaying as a flux, in rubber manufacture, and in making glass. The acetate, carbonate and other chemical compounds are used in medicine.
The relative amounts of lead consumed in the various uses in 1913 were: In pigment, comprising white lead, red lead, litharge, and orange mineral, 38.0 per cent.; in alloys such as type metal, bearing metals, and solders, 29.7 per cent.; in pipe, 15.2 per cent.; in shot, 10.4 per cent.; and in sheets, 6.7 per cent.
CHANGES IN PRACTICE
The most revolutionary advance in ore dressing of recent years has been the development of oil-flotation, electromagnetic, and electrostatic processes for the concentration of lead-zinc ores. These processes permit the elimination of the objectionable zinc content of many ores and render it an additional credit of great importance in the exploitation of low-grade complex ores.
The Murex process applied to the treatment of complex ores consists of coating the metallic sulphide minerals with oil and particles of magnetite and pyrite roasted to magnetic sulphide, then separating them from the gangue by an electromagnetic machine. The Lyster preferential flotation of galena depends on the presence of various salts in the water used. In the Broken Hill mines these salts are present in the mine waters. After removal of galena the blende may be preferentially floated by the Bradford copper salt or Bradford hyposulphite or sulphurous acid processes.
In the reduction of lead ores, there are improvements constantly being made. These are chiefly in the mechanical appliances, such as mechanical ore hearths and continuous roasting machinery, such as the Dwight-Lloyd, and in details of furnace construction and the handling of materials, rather than in processes or recognized principles. Various new processes have been proposed, most of which are intended to make available the ores now of too low grade, or the complex ores of lead and zinc whose separation is difficult or commercially impracticable. These processes are now the subject of experiment, with some indications that successful applications may be found. One process involves the volatilization by chloridizing roast of sulphide ores, the precipitation of the lead chloride fume by Cottrell electric precipitation, and smelting the fume with lime. At least 50 per cent. of the chlorine is recovered as calcium chloride, which can be substituted for salt in the further operations. With oxidized ores it is proposed to dissolve the lead by means of brine acidified with sulphuric acid and to precipitate metal sponge by electrolysis.
Gillies’s process consists in roasting the complex sulphides to a low sulphur content, mixing with carbonaceous matter, distilling in excess of air, and volatilizing the lead, zinc, bismuth, cadmium, arsenic, etc., as oxides and sulphates, the lead in form of sulphate, the zinc chiefly as oxide. The fume is caught and digested with a solution of ZnSO4 and free H2SO4 from the electrolytic vats; PbSO4 remains, which can be used as pigment or smelted. The ZnSO4 solution is electrolyzed, in the presence of gum arabic in the electrolyte, rendering the zinc deposit more dense.