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Flotation. The principles of flotation do not appear to be entirely understood from the physical or physicochemical point of view, as there is considerable divergence of opinion on the question whether flotation phenomena are the result of purely "surface tension" forces or whether electrical forces play an important part. The opera- tions involved are, however, sufficiently clearly denned, and consist essentially of three steps which may overlap or merge into one an- other. The first of these may be described as the "oiling" process. This consists in adding to the slip or mixture of finely ground ore and water some " oiling " reagent. This may be either an essential oil or one of a great range of chemical substances, generally organic in character. Only a very small amount of such a reagent is used, since the addition of larger quantities produces entirely different effects due to the formation of oil-films of appreciable thickness. The minute amount of oiling reagent, on the other hand, appears to produce some change on the surfaces of certain minerals having a metallic or semi-metallic character zinc-blende being a typical example which makes them less readily " wetted " by the liquor of the slip, or, put in another way, increases the angle of contact between these surfaces and the liquid. The gangue of the ore is less affected or not all affected by the oiling reagent, but in many cases it is necessary artificially to increase the difference between mineral and gangue by the addition of some " gangue modifying reagent" which renders the gangue more readily wetted by the aqueous liquid of the slip. These " gangue modifiers " are as a rule alkalies or mineral acids.

The third step in the process consists in adding to the slip some substance which causes the ready production of a stiff and lasting froth when air is introduced, either by agitation or by blowing or drawing it through the liquid in a finely divided state. Very Ire- quently the " oiling reagent " also serves as the froth-producer. When the slip thus prepared is treated so as to produce a froth, the oiled mineral particles adhere to the air-bubbles very firmly, the mineral-air surfaces evidently being the seat of less potential energy than the mineral-water surfaces, and consequently the buoyancy of the combined bubble with its mineral burden causes it to float to the surface where it accumulates as a very stiff mineralized froth which can be mechanically separated in various ways. Actually, a further action appears to take place, which is related to the floccula- tion of the mineral particles in contact with air-bubbles. This results in the attachment to each bubble of much larger and heavier agglom- erations of mineral particles, thus greatly increasing the efficiency of the whole process.

The flotation process has found its largest application in the con- centration of various types of sulphide ores and, on this ground alone, has attained very great industrial and technical importance. Its application to other minerals has, however, been actively pursued, and in a great many cases with considerable success. Minerals having a more or less metallic character are particularly suited for flotation, so that native metals constitute an obviously promising application. It is quite possible that ultimately the treatment of finely divided gold-bearing ores may revert to flotation, but as yet the older cyanid- mg processes in their modern forms continue to hold their own. Application to minerals of the oxide and carbonate type is less sim- ple, and here it seems to be necessary, in many cases, to submit the ore to a previous " activating " treatment. Preliminary roasting in a reducing atmosphere in the hope of producing a more or less metallic surface coating upon the ore particles has been tried, while " sul- phidizing " by exposing the finely ground ore to the action of hydro- gen (or ammonium-sulphide) has also been tried, in some cases with success. Another problem, which offers the promise of most impor- tant results when a full solution has been found, is that of " differen- tial flotation " for the purpose of separating different metalliferous minerals in a complex ore, such as the separation of zinc-blende or sphalerite from galena. In a certain number of cases, oiling reagents have been found which act differentially upon different flotable minerals, but every particular case still requires extensive research which does not always lead to a satisfactory industrial solution. Recently, an attempt has been made to recover coal from the waste heaps of collieries by means of flotation plant; technically entire success has been attained, since coal can be readily floated. Whether commercial success can be attained is another question, since the cost of fine grinding is a serious matter where a mineral of relatively low value is to be recovered. There is the further doubt whether coal which has been exposed to the weather, resulting in a dulling of its surface, can be made to float.

Smelting. Beyond its immensely important direct effects in rendering possible the economic concentration of a number of ores and in cheapening the concentration of others, the development of flotation has also profoundly affected smelting practice, since the product of flotation, being very finely divided and wet, differs widely from the drier product of gravity concentration. The concomitant progress of the reverberatory furnace, to which reference has already been made, has been accompanied by another development which, although to a large extent independent, has also received indirect stimulus from flotation. This is the use of powdered coal as fuel, a development which is still in active progress and shows promise of very wide extension. Extremely finely divided coal is blown or otherwise forced into the actual combustion-space of the furnace, where it burns with a flame very similar to that of a jet of gas. The

obvious advantages of such a system are that the loss of heat involved in the gasification of coal in producers is saved, together with the labour and the technical difficulties involved, while most of the ad- vantages of gas-firing can also be secured by burning powdered coal. As against this must be set the cost of grinding the coal sufficiently fine and of injecting it into the furnace, .while disadvantages, also attach to the fact that the ash of the coal is introduced into the fur- nace and thence into flues, regenerators, etc. The question of the manner in which the coal is to be powdered and conveyed into the furnace receives rather different treatment in different forms of the process, but it is coming to be recognized that extremely fine grinding is advantageous and in one process the coal is made into an " emul- sion " of coal-dust and air which is said to flow and to be capable of being pumped like a dense liquid. The coal particles appear to become coated with a closely adherent (probably " absorbed ") layer of air and as soon as the temperature becomes high enough for igni- tion to occur, very rapid combustion takes place. The use of coal- air mixtures or " emulsions," however, implies the introduction into the furnace of a relatively large amount of cold air and this materi- ally affects the question of regeneration or recuperation of the heat of the waste gases of the furnace. This is also affected by the pres- ence of the fine ash-dust which tends to clog or even to flux the tubes or chequer-work. In some furnaces, where an extremely high temperature is not required, the problem can be solved by dispens- ing with regeneration or recuperation entirely and utilizing the heat of the waste gases for raising steam, etc. Another point to be borne in mind is that a suspension of coal-dust in air may be a powerful explosive and must be treated with the care due to such substances.

In the methods for the extraction of non-ferrous metals, during the period under review, there has also been an important development in an entirely different direction. This is the very great advance in hydro-metallurgical processes, such as leaching and direct electro- lytic treatment of ores. The elimination of the German zinc smelters during the war, so far as supplies of zinc ores from the British Empire and the Allied countries were concerned, undoubtedly supplied a stimulus to this development, which has been particularly marked in connexion with the extraction of zinc from its ores. But the treat- ment of many other ores has also come within reach of " wet way methods, and these undoubtedly promise to play an increasingly prominent part in metallurgical extraction in the near future.

Organization. Among the factors of metallurgical progress, men- tion must be made of certain institutions and organizations which now play an important and growing part, particularly in regard to non-ferrous metals. The Imperial Mineral Resources Bureau, con- stituted in London towards the end of the war, and working by means of a large number of committees, assisted by all the leading metal- lurgists, serves to compile and publish a large amount of informa- tion, mainly in regard to the mineral resources of the British Empire, but dealing also in rather less detail with the resources of other countries. The continued progress and growth of the Institute of Metals has been a very marked feature of metallurgical activity; this body has now attained a membership of over 1,300, and issues two annual volumes of its " Journal," which constitute sources of stan- dard reference and information. In America, an Institute of Metals has been formed on slightly different lines, as part of the Institution of Mining and Metallurgical Engineers. The British Engineering Standards Assn., formerly the Engineering Standards Committee, has also come to exert a powerful influence on the metallurgy of those metals which form the primary materials of engineering. The issue of standard specifications for a large number of non-ferrous metals has been undertaken, mainly for air-craft purposes, while recently the formation of a new Sectional Committee on Non-Ferrous Metals promises a further series of more general specifications. Further, in connexion with the British Government Department of Scientific and Industrial Research, a Non-Ferrous Metals Research Assn. has been formed and has begun to initiate important researches, particularly in connexion with copper.

Copper. The effect of flotation, powdered-coal firing, and the advance of hydro-metallurgical methods on the metallurgy of copper, have already been mentioned. In regard to the metal itself, there is an increasing tendency for the exclusive employment of electrolytic copper. In part, this arises from the increased supplies of this quality of metal, but more from the increasing demand for high purity. " Best selected " and " tough " (arsenical) copper are still in demand for some purposes, particularly for locomotive fireboxes and stays ; but there is considerable doubt as to the value of the arsenic, partic- ularly as more complete de-oxidization of the copper is now practi- cable. For this purpose, particularly in connexion with the produc- tion of sound castings in high-conductivity copper, various agents have been advocated and employed. Crystalline Boron, and a sub- stance known as " Boron sub-oxide," have been advocated and some success has been attained by their use. With regard to finished cop- per, much attention has been paid to certain anomalies which occur during severe cold-working, such as wire-drawing, while the effect of heating the metal in a reducing atmosphere has also been further studied. A particularly interesting case of failure in copper when heated in a bath of fused sodium chloride has been studied by the Bureau of Standards, at Washington. This material became brittle and broke with a typical inter-crystalline fracture. It was shown that this arose from an electrolytic effect produced by the contact of the