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.
The Ganlin process, reported successful on Burma zinc-lead middlings, consists in feeding the dry pulverized ore into a molten bath of Zn and NaCl in equal parts. Zinc replaces lead and silver, which are dissolved as chlorides, ZnS being precipitated. When this reaction is complete the lead and silver are precipitated as metals by granulated spelter added to the amount of 35 per cent. of their weight, the dissolved spelter forming ZnCl2. The silver-bearing lead is tapped off, the residue granulated, the salts leached with water, and the zinc-bearing gangue freed from lead shot by tabling, leaving a zinc ore free from lead.
Electrolytic refining has been one of the greatest advances in the industry; it makes possible the preparation of pure lead from any source and the recovery of numerous by-product metals.