402 



NATURE 



[August 27, 1891 



the zinc is sufficient to exert this protecting action on that metal, 

 1 am satisfied that if our metallurgists are to advance their 

 industrial practice, they must, if I may use such an expression, 

 persistently think in calories, and not merely employ the ordinary 

 atomic "tools of thought." They will then be able to state 

 what reactions can, under given conditions, take place ; to indi- 

 cate those which will be completed ; and to avoid those that are 

 impracticable. 



In France, the country of so many great metallurgists, men 

 like Le Chatelier and Ditte are doing admirable service, by 

 bringing the results of the labours and teaching of St. Claire 

 Deville within the range of practical men. And if I do not 

 refer more specifically to their work it is for want of space and 

 not of appreciation, but a few simple cases of reversible actions 

 will perhaps rcake the subject clear. In the blast-furnace the 

 main reducing agent, carbonic oxide, is produced from the solid 

 fuel by the reaction COg -t- C = 2CO, a reaction which is 

 theoretically impossible because it is endothermic, and would be 

 attended by absorption of heat. But heat external to the system 

 intervenes, and acts either by depolymerizing the carbon into a 

 simpler form which can combine with oxygen of the CO2 with 

 evolution of heat, or by dissociating carbonic anhydride sets 

 oxygen free which combines with the carbon. Reduction of 

 oxide of iron in the blast-furnace is mainly effected by carbonic 

 oxide according to the well-known reaction 



FesOg -f 3CO = 2Fe -f 3CO2. 



But the gas issuing from a blast-furnace contains car- 

 bonic oxide, an important source of heat. The view 

 that this loss of carbonic oxide was due to the fact that 

 the contact of the ore and the reducing gas was not sufficiently 

 prolonged, led to a great increase in the height of blast-furnaces, 

 but without, ,as Griiner showed, diminishing the proportion 

 of carbonic oxide escaping from the throat. The reduction of 

 an iron ore by carbonic oxide only takes place within certain 

 well-defined limits, and a knowledge of the laws of chemical 

 equilibrium would have saved thousands and thousands of 

 pounds which have been wasted in building unduly high furnaces. 

 I would add that large sums have also been sacrificed in the vain 

 attempt to smelt oxide of zinc in the blastfurnace, for which 

 operation patents have frequently been souglit, in ignorance or 

 defiance of the readiness with which the inverse action occurs, 

 so that the reducing action of carbon on oxide of zinc may be 

 balanced by the re-oxidation of the reduced zinc by carbonic 

 anhydride, which is the product of the reduction. A further in- 

 stance may be borrowed from an electro-chemical process which 

 has been adopted for obtaining alloys of aluminium. As is well 

 known, all attempts to effect the direct reduction of alumina by 

 carbon have failed, because the reaction 



2(Al203) + 3C = 4Al + 3COo 



requires 783'2 calories, while only 291 calories would result 

 from the conversion of carbon into carbonic anhydride, there- 

 foie the reaction cannot be effected ; but in Cowles's process 

 aluminium is nevertheless liberated when alumina is mixed with 

 charcoal and strongly heated by the passage of an electric cur- 

 rent. This result is due, not to a simple reduction of alumina, 

 but to its dissociation at the high temperature produced by the 

 passage of a current of 1600 amperes between carbon poles, the 

 liberated aluminium being at once removed from the system by 

 metallic copper, which is simultaneously present and may not be 

 without action itself. 



An instance of the importance of these considerations is pre- 

 sented in the manufacture of steel by the basic process. Much 

 care is devoted to obtainirg conditions which will insure, not 

 only the elimination, but the order of the disappearance of the 

 impurities from the molten pig-iron. In the basic process as 

 conducted in the closed converter, the phosphorus does not dis- 

 appear until the carbon has left the fluid bath, whilst, when 

 the open-hearth furnace is used, the elimination of the 

 phosphorus may be effected befoie that of the carbon, 

 and it is asserted that, if the carbon goes before the 

 phosphorus is got rid of, a further addition of carbon is 

 necessary. A curious and subtle case of chemical equilibrium 

 is here presented. In the open-hearth furnace and Bessemer 

 converter respectively, the temperatures and pressures are 

 different, and the conditions as to the presentation of oxygen 

 to the fluid bath are not the same. The result is that the 

 relative rates of oxidation of the phosphorus and carbon are 

 different in the two cases, although in either case, with a given 



NO. II 39, VOL. 44] 



method of w 01 king, there must be a ratio between the phos- 

 phorus and carbon in which they disappear simultaneously. 

 The industrial bearing of the question is very remarkable. In 

 the basic Bessemer process the tendency of the phosphorus to 

 linger in the bath renders an "after-blow " necessary ; it may be 

 only of a few seconds' duration, but much iron is nevertheless 

 burnt and wasted, and Mr. Gilchrist tells. me that, if this after- 

 blow could be avoided, a saving of some 6 per cent, of the yield 

 of steel would be effected annually, the value of which, at the 

 present rate of output and price of steel, is no less than a quarter 

 of a million sterling. 



The volatilization of sulphur in the converter while it is re- 

 tained by the steel in the open-hearth furnace, and the increase 

 in the percentage of manganese which leaves the slag and 

 returns to the bath of metal in the converter at the end of the 

 "blow," will probably be traced to the disturbance of equili- 

 brium which attends very slight variations in the conditions, 

 especially as regards temperature and pressure, under which the 

 operations are conducted. 



In the blast-furnace the reducing action must be greatly de- 

 pendent on the rate at which alkaline cyanides are formed, and 

 Hempel has recently shown, by the aid of well-devised experi- 

 ments, that the quantity of cyanides which may be obtained at 

 a high temperature from ca.rbon, nitrogen, and alkaline oxides, 

 increases as the pressure becomes greater. 



Metallurgical chemistry is, in fact, a special branch of chemical 

 science v\hich does nfit come within the ordinary sphere of 

 the academic teaching of chemistry. It is often urged that 

 metallurgical practice depends upon the application of chemical 

 principles which are well taught in every large centre of instruc- 

 tion in this country, but a long series of chemical reactions exist 

 which are of vital importance to the metallurgist, though they 

 are not set forth in any British manual of chemistry, nor are 

 dealt with in courses of purely chemical lectures. I feel bound 

 to insist upon this point, because, as Examiner in Metallurgy for 

 the Science and Art Deportment, I find that purely analytical 

 and laboratory methods are so often given in the belief that they 

 are applicable to processes conducted on a large scale, and at 

 high temperatures. 



We are told that technical instruction should be kept apart 

 from scientific education, which consists in preparing the student 

 to apply the results of past experience in dealing with entirely 

 new sets of conditions, but it can be shown that there is a whole 

 .side of metallurgical teaching which is truly educational, and 

 leads students to acquire the habit of scientific thought as surely 



I as the investigation of any other branch of knowledge. 



I It is, in fact, hardly possible in a course of theoretical 

 chemistry to devote much attention to specific cases of industrial 

 practice in which reactions are inconiplete, because they are 

 limited by the presence of bodies that cannot be directly 

 eliminated from the chemical system. Take, for instance, the 

 long series of reactions studied by Plattner, who- published the 

 results of his investigations in his celebrated treatise, "Die 

 Metallurgische Rostprozesse," Freiberg, 1856, whose work I 



I have chosen as a starting-point on account of our presence in 

 South Wales near the great copper-smelting district of Swansea, 

 A complex sulphide, of which copper is the main metallic con- 

 stituent, contains some fifty ounces of silver to the ton. The 

 problem may be supposed for the present to be limited to the 

 extraction of the precious metal from the mass in which it is 

 hidden, and the student deriving his knowledge from an ex- 

 cellent modern chemical treatise would find the case thus 

 stated : — 



"Ziervogel's process depends upon the fact that when argenti- 

 ferous copper pyrites is roasted, the copper and iron sulphides 

 are converted into insoluble oxides, whilst the silver is converted 

 into a soluble sulphate, which is dissolved out by lixiviating the 

 roasted ore with hot water, the silver being readily precipitated 

 from this solution in the metallic state." 



It is certain that if an observant, chemically- trained student 

 visited a silver extraction works, and possessed sufficient ana- 

 lytical skill to enable him to secure evidence as to the changes 

 that occur, he would find a set of facts which his training had 

 not enabled him to predict, and he would establish the existence 

 of a set of reactions to the nature of which his chemical reading 

 had hardly given him a clue. The process to be considered is 

 a simple one, but it is typical, and applies to a large proportion 

 of the 7,000,000 ounces of silver annually obtained in the world 

 from cupriferous compounds. He would be confronted with a 

 ton or more of finely- divided material spread iu a thin layer over 



