66 



NATURE 



[March 22, 1917 



able " Trilobite " larva? of what is presumed to be 

 a very larg^e, unidentified Malacoderm. The im- 

 portance and overwhelming number of ants in the 

 tropics, with a detailed account of their complex 

 relations with plants, occupy another chapter. In 

 the author's view the supposed benefits obtained 

 bv so-called myrmecophilous plants have been 

 exaggerated, and the wonderful development of 

 plant forms in this relation should be regarded 

 more as protective devices than for the purpose of 

 attraction. A whole chapter is devoted to mimicry, 

 of which theory the author was a strong supporter, 

 as indeed might be expected in one who had much 

 experience of the tropics. In this connection a 

 valuable list is given of the Bornean Longicorns 

 mimicking Hymenoptera and other Coleoptera. 



The later chapters are devoted to an attractive 

 account of expeditions into the interior, the fauna 

 of the seashore, and some notes on the manners 

 and customs of the natives, while a number of 

 explanatory notes by Prof. Poulton and others 

 form an appendix. 



The book is well printed and has a useful index, 

 while the excellent illustrations are mainly from 

 the author's own photographs. Though much has 

 been done by its distinguished editor to combine 

 it into a more or less connected whole, it is greatly 

 to be regretted that its author's untimely death 

 necessarily renders it less complete than we could 

 have w-ished. It will, nevertheless, be found most 

 fascinating reading by all lovers of Nature. 



THE FLOTATION METHOD OF ORE 

 CONCENTRATION. 



THE recovery of metalliferous minerals from 

 crude ores in the condition of cleanliness 

 necessary for metallurgical operations is accom- 

 plished by the removal of the non-metalliferous 

 material, leaving the valuable minerals in a con- 

 centrated condition. In this removal advantage is 

 chiefly taken of the higher specific gravity of the 

 metalliferous material, which permits its separa- 

 tion either when falling vertically in still water 

 or when travelling horizontally in moving water. 

 This is known as water- or gravity-concentration ; 

 it depends upon the differential movement of heavy 

 and light minerals in water. 



Other properties are also at times made use of in 

 effecting this removal, such, for example, as 

 magnetism in magnetic separatioji, and electric 

 conductivity in electrostatic separation. These 

 secondary methods have, however, found their 

 application almost exclusively in the separation 

 of the individual minerals of a complex concen- 

 trate already recovered by water concentration, 

 and the fact remains that, outside exceptional 

 cases, gravity-concentration has been practically 

 the one means of removing the waste material 

 from crude ore. 



This means has well-defined and inexorable 

 limitations. When the ore has to be crushed fine 

 in order to release the individual grains, some of 

 it unavoidably becomes rendered so impalpably 

 fine that all advantage of gravity becomes lost in 

 the greater factor of the water's resistance, and 

 NO. 2473, VOL. 99] 



no differential movement is any longer possible. 

 Gravity-concentration also demands for its success 

 that there shall be a suflScient difference in gravity 

 between the mineral to be recovered and that to 

 be removed. In cases where that difference does 

 not exist, and such are continually occurring, it 

 fails. 



•These limitations have until comparatively 

 recently been accepted as in the nature of things. 

 j If not in the mechanical preparation of ores, there 

 was in the hydro-metallurgical processes of 

 recovery the compensating factor of remarkable 

 progress. The advent of cyanidation in 1889 had 

 gradually effected a revolution in the recovery of 

 gold and silver. In this process no removal of 

 the worthless material was necessary, since the 

 active solutions themselves made the necessary 

 discrimination, attacking only the valuable 

 mineral and leaving the mechanical preparation 

 responsible simply for crushing the crude ore to 

 the necessary fineness. 



In the case of the base metals the position was 

 not so satisfactory. The greatly increased con- 

 sumption of these metals necessitated attention to 

 the more complex and poorer deposits. Among 

 them, opportunely, was the immense Broken Hill 

 deposit containing argentiferous lead and zinc- 

 ores in a heavy matrix. By gravity-concentration 

 it was possible to market only about 60 per cent. 

 of the lead, and less of the silver, leaving the bulk 

 of the zinc associated with the heavy waste as a 

 middle product, while the finest and lightest por- 

 tion of the crushed material w^as often allowed to 

 flow away. 



The quantities concerned were, however, so 

 enormous that every process possessing any pos- 

 sible chance was tried, with in almost every case 

 little success. Among them, however, was one 

 which took advantage of the property possessed 

 by metallic sulphides when in a fine condition 

 to float, and therefore known as the flotation 

 process. When, for instance, a mixture of sucb 

 sulphides and waste in a fine condition is gently 

 brought on to the surface of moving water, it will' 

 be found that the waste particles will break 

 through that surface and sink, whereas the sul- 

 phides will float away. Further, this differential 

 iDehaviour will be the more pronounced if in one 

 way or another the sulphide particles can be oiled 

 or greased. This, providentially enough, can 

 readily be done because of a second property 

 possessed by sulphide particles, that of adsorbing 

 oil when agitated with a small proportion of oil in 

 an aqueous mixture, a property which the particles 

 of waste do not possess. This effect of an oiled 

 or greased surface will be appreciated from the 

 comparative ease with which a small needle 

 which has been passed through the fingers can 

 be made to float, whereas it would be very difficult 

 indeed to get the same needle to float after it had 

 been cleaned in the flame or by alcohol. 



In this oiled condition the floating powers of 

 sulphides are so reinforced that what otherwise 

 might be an inadequate separation is then gener- 

 ally highly satisfactory. The air, and not the oily 



