March 22, 1917] 



NATURE 



67 



is, however, the prime factor, and the reason the 

 sulphide particles float is because of the air-film 

 attached to them and not because of oil buoyancy. 

 Accordingly, though a small amount of oil is 

 generally used, flotation may be, and in particular 

 cases is, achieved without oil. Nor is it necessary 

 to bring the mixture gently on to the water sur- 

 face; the sulphide particles, if introduced below 

 the surface, will attach themselves to air-bubbles 

 introduced at the same time, and rise. This 

 result is probably more readily completed if 

 enough oil is present to cover the sulphides with 

 the filmiest covering of oil, though an excess of 

 oil would agglomerate such particles and cause 

 them to sink. Be that as it may, it is considered 

 that the particular function of the oil is to lower 

 the surface tension of the water and so permit the 

 mineral-laden bubbles to form a froth which is the 

 stronger both because of the presence of the oil 

 and because of the strengthening effect of the 

 mineral particles themselves ; in this latter con- 

 nection the retention of the globular form by 

 drops of water thrown on to a dusty floor is 

 interesting. When thus assembled into a froth 

 the collection of separated sulphides is easy. 



Two main theories have been put forward to 

 explain flotation. The first, and probably the most 

 applicable, is that of interfacial tensions between 

 the different phases, sulphide particle, air-bubble, 

 water, and sometimes oil droplet. This was sug- 

 gested by the difficulty of wetting sulphides. It 

 is conceivable that the sulphide surface has a 

 potentiality to oxidise or otherwise change its 

 chemical state, and for that reason to stick to an 

 air-bubble when brought in contact with one ; 

 whereas the oxides, carbonates, and silicates of 

 the worthless material, having nothing to get from 

 the air, are inert. Flotation may then for con- 

 venience be said to depend upon the surface 

 energy of mineral particles, just as magnetic 

 separation is dependent upon their magnetism. 



The second theory, and one of great assistance 

 as a working hypothesis, is based on the fact that 

 mineral particles in water, by reason of the film 

 around them, are electrically charged, the sulphide 

 particles positively and the non-metalliferous 

 particles and air-bubbles negatively. Under these 

 conditions the attachment of the sulphide particles 

 to the air-bubbles is readUy understood; air being 

 a non-conductor, discharge would not come with 

 contact. 



In addition to oil, sulphuric acid is also generally 

 used. Its effect is to increase the wetting powers 

 of the water, so that less of the waste is 

 entrained with the sulphides and the concen- 

 trate consequently cleaner. Whether the view 

 be taken that the acid achieves this eff"ect 

 simply by cleaning the surfaces, or by acting as 

 an electrolyte, largely depends upon what theory 

 is being applied. If carbonates be present in the 

 ore, an additional effect of the acid is that the 

 generation of carbonic acid gas may render the 

 special introduction of air unnecessary, since ex- 

 perience has shown that bubbles of this gas may 

 take the place of air. 



XO. 2473, VOL. 99] 



Whatever the true theory, it is impossible to 

 question the fact of the great importance of flotation 

 concentration. Its success upon the zinc middling 

 product at Broken Hill was immediate. At first, 

 in 1900, used only for treating a sandy material, 

 with the elimination of imperfections and the intro- 

 duction of improvements it eventually became ap- 

 plied to the slime, the position now being that the 

 whole range of zinc products on that field is treated 

 by flotation, and zinc ore to the extent of about 

 500,000 tons per year is being recovered. 



Such a success could not t)e without influence 

 upon the recovery of fine material elsewhere, and 

 at this time the large disseminated copper deposits 

 of America were becoming big producers of 

 copper. With these low-grade deposits ordinary 

 gravity-concentration was yielding at most, even 

 with an extensive plant, a 70 per cent, recovery of 

 the contained copper, the larger part of the loss 

 being in the very fine material. Upon this material 

 flotation tests showed a much better recovery, and 

 many plants have now been provided with a flota- 

 tion equipment to treat this fine material, bringing 

 the total recovery of the copper up to about 85 per 

 cent. One large mine, having a capacity of 

 several thousand tons a day, has indeed gone to 

 the extent of making flotation the prime concen- 

 tration process employed, in spite of the fact that 

 a gravity-concentration plant had been designed 

 and was about to be put into execution. 



Lead ores in their turn have had this process 

 applied to their finer material, to the much- 

 improved recovery of the lead contents ; while 

 simple zinc ores have similarly benefited. Flotation 

 has also in some cases been applied to the bene- 

 ficiation of the fine sulphides of silver and the 

 tellurides of gold, encroaching in these cases upon 

 a field long the monopoly of the cyanide process ; 

 while among the ores of the minor metals, molyb- 

 denite, the sulphide of molybdenum, except for 

 what can be done by hand-picking, is entirely 

 recovered this way. 



Finally, it may be said that though Australia 

 led the way, there is scarcely a metalliferous dis- 

 trict in the world where flotation has not become a 

 factor of the greatest interest, while its advent 

 has been to the base metals the same beneficent 

 revolution that the cyanide process was to the 

 precious metals. 



It is a pleasure to know that this process, like 

 the cyanide process, was largely the discovery of 

 British experimentalists, and in connection with it 

 the names of Elmore, Sulman, and others will 

 become historical. The only regret is that it 

 should have been the subject of so much litigation 

 and the source of so much animosity. 



MAJOR SYDNEY D. ROWLAND. 



WE regret to announce the death on March 6, 

 in France, from cerebro-spinal fever, of 

 Major Sydney Donville Rowland, R.A.M.C., 

 M.R.C.S. 



Sydney Rowland was born in 1872 and edu- 

 cated at Berkhamsted School, whence he pro- 



