November 6, 1919] 



NATURE 



245 



than otherwise would be justified in making their 

 own steel. The rapid growth of the electric 

 furnace is shown by the fact that the total number 

 in operation in March, 1910, was 114, and in 

 January, 1917, 471. 



As an alternative to flotation concentration and 

 smelting operations, which, as mentioned, are par- 

 ticularly suitable for sulphide ores, the achieve- 

 ments of hydrometallurgy have also to be con- 

 sidered. The cyanide process, which has long 

 been established as the most suitable method of 

 extracting gold and silver from low-grade ores, 

 depends on the fact that the dilute solutions em- 

 ployed exercise a selective solv.ent action on the 

 precious metals, and is the best-known instance 

 of the application of leaching on a large scale. 



Recently the extraction and refining of copper 

 by hydrometallurgical and electrolytic methods 

 direct from the ore has become a commercial 

 process. Many of the low-grade ores of copper, 

 particularly the vast porphyry deposits in the 

 highly mineralised mountainous country in the 

 south-west of U.S.A. and Mexico, are oxidised 

 and not amenable to flotation concentration. For 

 their beneficiation leaching is the most suitable 

 method. A famous example of this kind is the 

 treatment of the ore obtained from the mine of 

 the Chile Exploration Co. situated at Chuqui- 

 camata, which, as regards tonnage and contents 

 of valuable metal, is one of the greatest known 

 copper deposits in the world. The high point of 

 the mine lies at an altitude of 9890 ft., while 

 the extraction plant is situated at 9023 ft. on a 

 plateau of the Andes, 160 miles north-east of 

 Antofagasta. Here a plant of 10,000 tons daily 

 capacity has been designed and erected in a 

 desert 5000 miles away from the base of supplies. 

 The ore, which carries about 2 per cent, of copper, 

 is an oxysulphate known as "brochantite." 



The process chosen utilises its sulphuric acid 

 ion for the solution of the copper, and allows a 

 percentage discard of the liquid after each opera- 

 tion, thus avoiding its fouling by continual use. 

 The ore is crushed to about half-an-inch mesh 

 and leached with sulphuric acid. The greater part 

 of the chlorine is eliminated in tube mills by treat- 

 ment with metallic copper. The remaining copper 

 is precipitated from solution by electrolysis and 

 the cathodes are melted and cast into commercial 

 wire bars. In this way a high-grade commercial 

 metal is produced direct from the ore in three 

 operations, in only the last of which is a furnace 

 treatment necessary. Against this the ordinary 

 concentrating, smelting, and refining operations 

 involve no fewer than seven stages. The output of 

 refined copper from this plant is at the rate of 

 200 tons per day. 



The refining of metals by electrolysis, of which 

 the previous process is an illustration, is one of 

 the most important features of the industry of 

 to-day. It derives its importance from two con- 

 siderations which are inter-related : first, that it 

 permits the production of the commercial metal 

 in a highly purified form ; secondly, arising out 

 of this, it allows the complete recovery of the 

 NO. 2610, VOL. 104] 



precious metal values from base metal ores, which 

 thus increases their commercial value. To-day, 

 iron, copper, zinc, lead, aluminium, sodium, 

 magnesium, nickel, gold, and silver are obtained 

 in a marketable form by such methods. To take 

 one instance only, upwards of 90 per cent, of 

 the world's annual production of copper, which 

 in 1913 was about a million tons, was refined by 

 electrolysis within 20 miles of New York City. 

 The cathode copper thus produced did not contain 

 more than 2-3 parts of impurities in 10,000. As 

 a by-product of this refining, there was obtained 

 nearly 20 per cent, of the world's entire output 

 of silver, a substantial amount of gold, small 

 amounts of platinum and palladium, together with 

 notable quantities of nickel, selenium, and tel- 

 lurium. 



Viewing the industry to-day, it is manifest that 

 there is a notable trend towards the substitution 

 of furnace- or pyro-metallurgy by hydro- and 

 electro-metallurgy. Even where furnace operations 

 still hold the field, attempts are being continually 

 made towards the substitution of fuel heat by 

 electric heat. Iron is being produced commerci- 

 ally in Sweden, Canada, and the U.S.A. by elec- 

 tric smelting and refining. There is a clear trend 

 of a similar character in the metallurgy of certain 

 ores of copper which are not amenable to direct 

 flotation or leaching. Metals such as aluminium, 

 sodium, and magnesium are produced direct by 

 electrolysis. The great importance of this ten- 

 dency, as already suggested, is that it permits 

 of a more complete beneficiation of any given 

 ore, and, indeed, brings a far wider range of raw 

 materials within the scope of economic exploita- 

 tion than otherwise would be the case. How 

 clearly this is so may be seen from the following 

 instances :■ — • 



Previous to the introduction of the cyanide 

 process, it did not pay to extract gold from any 

 sulphide ore unless it contained at least 0-5 oz. 

 of this metal. By means of this process, however, 

 the limit of such payable ores has been brought 

 down to about i dwt. per ton, and in the case of 

 clean gravel containing native gold to as low as 

 3 grains — i.e. 0006 of an oz. of gold per ton. 

 Similarly in the case of the sulphide ores of silver, 

 the previous limit of 20 oz. has been lowered to 

 about 2 oz. per ton by the same process. As 

 regards copper, the economic percentage was 

 about 5 per cent, down to the year 1890. By the 

 introduction of leaching processes this figure was 

 quickly reduced to about 2 per cent. Progress 

 has been continuous in lowering the limit, and 

 to-day tailings from concentrating tables con- 

 taining only 0-5 per cent, of copper are being 

 treated for extraction at Cananea (Mexico) and 

 Anaconda (U.S.A.). In regard to such materials 

 it is more than possible that the flotation process 

 will lower the limit still further. 



Limits of space prevent any reference to the 

 trend of current practice in the mechanical and 

 thermal treatment of metals and alloys and the 

 ever-deepening influence of metallography on this 

 great branch of the metallurgical industry. 



