244 



NATURE 



[November 6, igig 



being specifically heavier, tended to sink, while 

 the lighter gangue floated, never, at its best, gave 

 an extraction of more than 82 per cent. In the 

 "flotation" process of to-day the sulphide ore 

 particles are made to float on a froth produced by 

 the agitation of the pulp with the addition of a 

 small amount of oil and acid, while the gangue, 

 although specifically lighter, sinks. The flotative 

 agent is air, the froth being stabilised by the 

 particular oil mixture used. Surface tension and 

 not gravity is the principle utilised in the separa- 

 tion. The method has been principally applied to 

 the concentration of copper sulphide and mixed 

 lead and zinc sulphide ores. Its largest applica- 

 tion has been in copper reduction work in 

 America, where many millions of tons of ore are 

 being treated to-day. At Anaconda the total 

 recoveries in the concentration process have been 

 raised from 76 per cent, to as much as about 

 95 per cent. There can scarcely be any doubt 

 that flotation has a great future as a concentrator 

 of metal values. At the present, however, it is 

 limited to sulphide materials. For that reason it 

 has had no effect on the metallurgy of iron, where 

 the mineral is either an oxide or a carbonate ; but 

 it seems likely to have a very wide application to 

 the principal economic minerals of copper, lead, 

 zinc, gold, and silver, especially when the two 

 latter contain base metal values. 



Thus far the concentration has been mechanical 

 — 1.<'. there has been no change in the chemical 

 composition of the mineral itself. In the next 

 stage, in the great majority of cases, "smelting " 

 or "reduction" begins, which has for its object 

 the conversion of the ore into a metal, usually 

 unrefined. Hitherto the shaft or blast furnace 

 has held its own, with coke as the fuel. This 

 furnace has had its principal recent development 

 along the lines of better charge distribution, and, 

 in the iron industry, more efficient hot blast stoves 

 and more economical power plants. Fifteen years 

 ago many furnacemen were satisfied with almost 

 any distribution of charge they happened to get 

 from the apparatus installed, but this most im- 

 portant operation cannot be ignored without caus- 

 ing low output and high coke consumption. Dis- 

 tributors are now in use which give the charge 

 a columnar structure with alternate columns of 

 coarse and fine ore, instead of uniform layers 

 produced by most systems of filling. Highly bene- 

 ficial results are claimed for this improvement. 



Hitherto the reverberatory furnace has been 

 markedly inferior to the shaft furnace from the 

 point of view of thermal efficiency. For this 

 reason only the ore which was in too fine a state 

 of division for treatment in a shaft furnace was 

 smelted in a reverberatory. The very extensive 

 application of gravity and flotation concentration, 

 particularly to copper ores, with the fine grinding 

 which these processes involve, has, however, 

 necessitated the smelting concentration of such 

 materials in these furnaces. Until comparatively 

 recently the best practice in the reverberatory 

 furnace concentration of copper ores was about 

 48 tons of charge per ton of coal, against about 

 NO. 2610, VOL. 104] 



8 tons of charge per ton of coke in the blast 

 furnace. Here, too, a remarkable improvement 

 during the last five years has taken place. This 

 is due, in the main, to three factors : — ■ 



(i) To the increased efficiency of burning coal 

 in the form of dust as compared with burning it 

 on a grate. 



(2) To maintaining a very large mass of the 

 charge piled along each side of the furnace, which 

 increases the speed of heat absorption. 



(3) To the augmented size of. the hearth, which 

 has now reached a length of about 143 ft. and a 

 width of 30 ft. 



For successful practice, the coal, before pul- 

 verising, must be dried to a maximum of i per 

 cent, of moisture. It must be finely pulverised, 

 since the increased surface thus obtained has a 

 direct bearing on the efficiency. Upwards of 

 80 per cent, should be capable of passing through 

 a 200-mesh sieve. The delivery of coal and air 

 must be controlled so that the proper proportion 

 between them is maintained, and the coal itself 

 must contain enough volatile combustible matter 

 to give the required combustion. Only a few years 

 ago it used to be reckoned that to smelt from 

 230 to 270 tons of charge per 24 hours was good 

 work. The current practice to-day in the new 

 furnaces is to smelt between 600 and 700 tons, 

 and the ratio of charge to fuel has been brought 

 up to about 7:1, which raises the reverberatory 

 furnace almost to a level with the blast furnace 

 from the thermal efficiency point of view. Certain 

 of these large reverberatories are fired with oil, 

 and very satisfactory results, as regards both 

 economy in fuel consumption and weight of 

 charge smelted, have been obtained. 



Passing next to the refining of the metal, it is 

 here that "electric heat " is tending in some cases 

 to supplement, in others to supplant, fuel heat. 

 An instance is furnished by the refining of steel 

 on a large scale in the so-called triplex process, 

 in the second stage of which fuel heat, in the form 

 of producer gas, is used (the charge being worked 

 first in a converter and afterwards in an open- 

 hearth furnace), while in the third stage an elec- 

 tric resistance furnace is utilised which permits 

 the refining of the steel to a considerably further 

 degree. It is widely held that the quality of steel 

 which can be produced in this way is superior to 

 that obtained in the open-hearth furnace. This 

 is due to the fact that, owing to the high tem- 

 perature employed, more refractorv basic fluxes 

 can be used which permit of a greater removal 

 of sulphur and phosphorus, with a consequent 

 improvement in the properties of the refined steel. 

 Moreover, in the electric furnace the charge is 

 decidedly less contaminated with gases. For 

 high-grade materials, such as high-speed cutting 

 tools, where quality is of paramount importance, 

 the electric furnace seems to have a field all its 

 own, and, owing to the fact that, under special 

 conditions, current for it can be bought from 

 public service companies during " off peak " 

 periods, its installation cost is not necessarily 

 high. This permits of its use in plants smaller 



