An Introduction to the Study of Metallurgy, etc

Introduction

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INTRODUCTION.

TO THE STUDY OF METALLURGY BY W. C. ROBERTS-AUSTEN, C.B., F.R.S., ASSOCIATE OF THE ROYAL SCHOOL OF MINES; CHEMIST AND ASSAYER OF THF. ROYAL MINT PROFESSOR OF METALLURGY IN THE ROYAL COLLEGE OF SCIENCE, WITH WHICH THE ROYAL SCHOOL OF MINES IS INCORPORATED. WITH NUMEROUS DIAGRAMS. LONDON: CHARLES GRIFFIN AND COMPANY, EXETER STREET, STRAND. 1891. [AU rights reserved.]

Preface

. The literature of metallurgy is rich, but those who are beginning to study it need guidance to a knowledge of the principles on which the art is rightly practised. It depends, as is well known, on the application of chemistry, physics, and mechanics ; but the methods of metallurgists vary greatly from those of chemists, who, however, frequently fail to apjjreeiate the nature of the difference. Ten years' experience has convinced me that it is more im portant at the outset for the student to know what was the scope of mind of the early practisers of metallurgy, and to see what kind of aid the art may be expected to receive in future from the sciences, than to acquire familiarity with complicated details of processes and appliances. In this little volume I have, therefore, devoted four chapters to these branches of the subject, embodying in them portions of lectures which I have delivered from time to time.

In all English works on metallurgy, the important metals are dealt with separately and in detail. In this, however, an attempt has been made to treat the subject as a whole, giving no minute descriptions of processes, but choosing typical appliances and indicating their use in connection with groups of metals. Such a method was adopted by the late M. Gruner, Professor of Metal lurgy at the LEcole des Mines, Paris, to whom I have reason to be grateful, for I have closely followed him in my class lectures. The student will, I trust, be led to study the works in which

PREFACE.

. the extraction of metals from their ores and fitting them for use are fully discussed. Such are the classical treatises of Percy, the monographs of Sir Lowthian Bell, and the manual of Phillips and Bauerman, from which volume a few illustrations have been borrowed. The literature of the subject has been enriched by Howe, of Boston, who has collected a store of facts in his elaborate and recently-published volume on steel. In the preparation of a portion of this little work, I have been aided by my colleague, Mr. Bennett Brough, whose help has been specially useful in passing the work through the press. I hope that the book will be found useful to my own students, for whose progress I feel sincere solicitude. Chilworth, Surrey, December 23, 1890. CONTENTS.

Chapter II Physical Properties Of Metals

PAGE Molecular structure . . 7 Density 8 Fracture 9 Malleability 10 Ductility 10 Tenacity 10 Toughness 10 Hardness 10 Brittleness 11 Elasticity, extensibility and strength of metals . . . . .11 Influence of foreign elements on the strength of metals ... 20 Colour 33 Fusibility 33 Welding 34 Properties common to fluids and solid metals 35 Occlusion of gases 45 Table of physical constants of metals 48

Chapter III. Alloys

Early investigations 51 Views as to the constitution of alloys X CONTENTS. PAGE Union of metals by fusion 55 compression ........ 5f> electro-deposition ....-•• 59 Liquation 59 Action of electric currents on molten alloys 65 Conduction of electricity by alloys at high temperature ... 67 Allotropic modifications of metals resulting from alloying ... 70 Influence of varying quantities of metals on each other ... 76 Colour of alloys 83 Alloys of industrial importance ........ 87

Chapter IV. The Thermal Treatment Op Metals

Annealing, hardening, and tempering . . . . . . - 91 History 93 Internal constitution of steel 95 Working of steel . 112 Classification of steel 116 Summary 11S

Chapter V. Fuel

Classification of fuel 120 Calorific power 122 Calorific intensity 123 Pyrometry 124 I. — Natural fuels 133 (1) Wood "... 133 (2) Peat I34 (3) Lignite i35 (4) Coal I36 (5) Anthracite ! (6) Liquid fuels • .139 (7) Natural gas .140 CONTENTS. XI PAGE II. —Prepared fuels 141 (1) Compressed fuels 141 (2) Dried fuels 141 (3) Carbonised fuels . . - 141 (a) Charcoal 141 (6) Coke 143 4) Gaseous fuel 153 Water Gas 157 Bibliography 158

Chapter VI. Materials And Products Op Metallurgical Processes

Ores 159 Fluxes 159 Metallurgical agents . . . . ' 160 Selection of fluxes 161 Slags 161 Economic application of slags 163 Calculation of furnace charges 165 Classification of metallurgical operations 171 Roasting and calcination 173 Oxidising agents 174 Reducing agents 175 Chemical agents * 176

Chapter VII. Furnaces

Materials used in the construction of furnaces 178 Crucibles 180 Classification of furnaces 181 (1) Hearths 184 (2) Shaft furnaces 188 (3) Reverberatory furnaces 206 (4) Closed-vessel furnaces 217 The Bessemer converter CONTENTS.

Chapter VIII. Means Op Supplying Air To Furnaces

PAGE Means of producing draught ......•■ 226 Blowing engines ........... 230 Hot-blast stoves . . . ... . . . . . 231

Chapter IX. Typical Metallurgical Processes

Classification of processes ......... 238 Welsh method of copper smelting ....... 241 Smelting of gold and silver ores in Colorado 245 Freiberg process 248 Wet processes for treating argentiferous copper ores .... 264 Chlorination of gold ores 266 Wet process for treating nickel and cobalt ores .... 268 System of teaching metallurgy . -271

Chapter X. Economic Considerations

Object of the metallurgist 274 Capital and labour 274 Production of metals 282 Price of metals 283 Conclusion

INTRODUCTION.

TO THE STUDY OF METALLURGY.

Chapter I. The Relation Of Metallurgy To Chemistry

The distinguished metallurgist who may be said to have created the English literature of the subject concluded the introductory lecture he delivered* at the Royal School of Mines, by pointing out to the students who were then beginning their course, that "in proportion to the success with which the metallurgic art is prac tised in this country, will the interests of the whole population, directly or indirectly, in no inconsiderable degree, be promoted." During the period of nearly forty years since Dr. Percy began to teach, the conditions under which metallurgy is practised have changed considerably ; for the field of knowledge has so widely extended, the scale on which operations are conducted is now so great, and the mechanical appliances they involve are so varied and complicated, that while the interest of the subject is deepened, its difficulty is gravely increased.

Metallurgy involves not only a knowledge of " the art of extracting metals from their ores," but also of the means of " adapting them for use," which is effected by complicated mechanical operations, and, strange as it may seem, by varying the degree of purity, often to a very slight extent. It will thus be evident that the student must possess adequate knowledge of Inorganic Chemistry, Mechanics, and Physics. Besides this, he must acquire aptitude for dealing with economic details. * Records ofthe School of Mines, vol. i. pt. i. (1852), p. 127 a\ ■2 AX

INTRODUCTION.

TO METALLURGY. Historical Sketch. — In turning to the history of metallurgy, it is easy to be led away by the charm of the antiquarian store of riches into devoting too much time to literary research. It should be remembered, however, that much of what is both interesting and full of suggestion, even at the present day, is to be found buried in the treatises by the old writers whose work we inherit and continue. Primitive metallurgical processes are referred to in some of the oldest known historical records ; naturally, therefore, the scientific development of metallurgy must have been long preceded by its empirical practice as an art, an art for which a place has even been claimed among the rehgious systems of antiquity.* The earlier literature of the subject consists mainly of descriptions of processes ; but it is well known that chemistry was to a great extent built up on a metallurgical basis, and Black's singularly advanced definition of chemistry as the " effects produced by heat and mix ture "t might well be applied to metallurgy.

The library of Ley den contains a papyrus which has been described by M. Reuvens,J and which is considered by Kopp § to be the oldest known chemical manuscript, its date being possibly as early as a.d. 200. It treats generally of metallurgical matters, and the purification of gold and silver is frequently mentioned. Of all chemical phenomena, pro bably none have more contributed to advance chemistry as a science than those bearing upon the relations between oxygen and lead ; and the interest attaching to the mutual behaviour of these two elements is so great that the student will do well to consider the influence of one very ancient metallurgical process on the scientific views of the present day. When lead is melted with free access of air, a readily fusible substance forms on its surface.

This substance may be allowed to flow away, or, if the metal is contained in a suitable porous receptacle, the fusible oxide sinks into this containing vessel ; in either case the oxidation of the lead affords a means of separating it from precious or inoxidisable metals, if any were originally present in the lead. The above fact has been known from remote antiq uity, and the early Jewish writers allude to it as old and well-known. They clearly show, for instance, that lead can be removed from silver by being " consumed of the fire," while the silver is not affected. That the Greeks knew and practised this * Rossignol, Les Metaur de VAntiquite (1S63). t Lectures. By Joseph Black, M.D., vol. i. pp. 11, 12 (Edin. 1803). % Reuvens, Lettres a M. Let ronne (Leyden, 1S30), quoted by Prof. Fer guson in an address to the Glasgow Phil.

Soc. (1876), p. 19. § Beitrtit/c zur Geschichte der Chemie, 1869. THE RELATION OF METALLURGY TO CHEMISTRY. 3 method of cupellation is abundantly proved, if only by certain specimens of gold and silver which were discovered by Dr. Schlie mann on the site of ancient Troy. The Arabians investigated the subject, as is shown by the writings of Geber,* the greatest of the early chemists (he died in 777), who gives a remarkable account of cupellation ; he also describes the conversion of lead into a fine powder by calcination with much clearness, and he noticed the fact that after calcination the mass has " acquired a new weight in the operation." His subsequent observations on the reduction of altered metals from their "calxes" show that he knew the weight to be increased ; in any case it is interesting to remember that his work was, in a sense, quantitative.

He, moreover, was cognisant of the fact that two different substances may be pro duced by heating lead in air, and he assumed that " in the fire of calcination a fugitive and inflammable substance is abolished." The alchemists refer continually to the subject, and " deliver themselves," as Roger Bacon said, in his Speculum Alchimie, " in the enigmas and riddles with which they clouded and left shadowed to us the most noble science." Eck of Sulzbach showed (1489) that metals augment in weight by calcination, and that what we now call red oxide of mercury gives off " a spirit " when heated ; had he named and isolated the spirit, he would have discovered oxygen. The great metallurgist of the sixteenth century, Agricola,! points out that lead increases in weight when it is exposed to the action of moist air.

In the middle of the sixteenth century, the equally accomplished metallurgist, Biringuccio,J con temporary of Paracelsus and Agricola, seems to have been specially attracted by the phenomenon in question, and he remarks : — " If we had not lead we should work in vain for the precious metals, for without its aid we could not extract gold or silver from the stones containing them The alchemists also," he says, " make use of it in their operations, calcining it by itself or with other substances ; but," he goes on to observe, " the calcina tion, conducted in a reverberatory furnace is accompanied by a marvellous effect, the result of which should not be passed by in silence, for lead thus treated increases ten per cent, in weight, and considering that most things are consumed in the fire, it is remarkable that the weight of lead is increased and not diminished."

Although he subsequently gives evidence of much accurate know- * The works of Geber, translated by R. Russell (1686), pp. 74, 78, 220, 234- t De ortu et causis subterraneorum, p. 519. t Pirotechnia (Vinezia, 1540), translated into French by T. Vincent (Rouen, 1627), p. 41. ledge of practical metallurgy, his views as to this particular phenomenon were hardly in advance of Geber's ; but we may claim Biringuccio as an early metallurgist, who knew the facts and recognised that they were theoretically important. Cæsal pinus, in his work, De Metallicis, showed that the film which covers lead exposed to moist air and augments its weight, is due to an aeriform body. It was not till nearly a century later (1630) that a French chemist, Jean Rey,* stated that the increase in weight came from the air.

The problem attracted much attention in England, and it is not a little interesting that among the very first experiments recorded by our own Royal Society, is a metallurgical series relating to the weight of lead increased in the fire on the " copels " at the assay oflice in the Tower, the account being brought in by Lord Brouncker in February, 1661.T Sub sequently, in 1669, John Mayo showed that the increase in weight of calcined metals was due to a " spiritus," or distinct constituent of ordinary air. J Nevertheless, Boyle heated lead in a small retort,§ and attributed the increase in weight, as Lemery also did,|| to his having " arrested and weighed igneous corpuscles." One of the most curious passages known is in the HipjMcrates Chemicus of Otto Tachen, a German who lived at Venice, and published his book there in 1666.

He describes how lead, when burnt to minium, increases in weight. This increase he ascribes to a substance of acid character in the wood used for burning, and then, by a very ingenious course of argument, based on the saponifying powers of litharge, makes out that lead is of the nature of, or contains an alkali which combines with, the " occult acid of the fat." This is a curious anticipation of a very modern classification, which brings lead into relationship with the alkalies and alkaline earths, as well as of Chevreul's investigations on saponification. Cæsalpinus had previously called lead " a soap " which in cupellation washes gold and silver. It is hardly necessary to point out how important this calcination of lead was considered by those who defended the Phlogistic theory in regard to chemical change, the theory propounded by the metallurgist Becker, which, for more than a century, exerted so profound an influence on scientific thought.

His views were first embodied in the Physica Subterranea (1669) and in the Alphabet hum Minerale (1682). * Essais de Jean Re;/ (reprinted in Paris, 1777), p. 64. t MS. register book of the Royal Society. t Tractatus quinque Medico-R/iysici, p. 25 et seq. (Oxonii, 1674). § Collected works, vol. iii. (1744), p. 347. || Pours de Chymie (1675), 2nd English edition (1686). p. 107. THE RELATION OF METALLURGY TO CHEMISTRY. 5 According to his still more famous pupil Stahl, the litharge produced by the prolonged calcination of lead in air, is lead de prived of its phlogiston ; but he and his followers were indifferent to the fact that when lead is burnt the weight of the resulting mass is greater than that of the original metal, and were content to insist that the burnt lead had lost its inflammable principle — that is, Phlogiston.

Tillet, assayer of the Paris Mint, made some quantitative ex periments which led up in a singular way to the work of Lavoisier, who, as is well known, overthrew the old phlogistic theory by showing that a chemical combination takes place, resulting in an augmentation of weight which represents the exact weight of the gaseous body added. At the same time it should be remembered that the phlogistic chemists made a great step in advance, as was admitted by J. R. Mayer* in his memoir on the mechanical theory of heat ; and Odling, discussing the experiments on the oxidation of lead, has pointed out t that an error has arisen in consequence of the same word being used in a different sense at different periods of time ; chemists, in fact, now substitute the words potential energy for phlogiston, or, as Dr. Crum Brown well observed, | we recognise "that no compound contains the sub stances from which it was produced, but that it contains them minus something.

We know now what this something is, and can give it the more appropriate name of potential energy ; but there can be no doubt that this is what the chemists of the seventeenth century meant when they spoke of phlogiston." It will thus be evident that the main aim of chemical investigation down to the end of the last century was the explanation of calcination, combustion, or oxidation, and that lead was especially useful in solving the problem. It might, perhaps, be added that the absorption of oxygen by molten litharge furnished Ste. Claire Deville,§ a physicist and metallurgist, with an important step in the argument as to dissociation, thus connecting the history of the metal, lead, with the great advance on the borderland of chemistry and physics which has been made in modern times. The above remarks will be sufficient to show that conclusions of the utmost importance in the history of chemical theory were based on the very ancient metallurgical process of cupellation of lead, a process which affords an appropriate illustration, because, * Bemerlcungen -iiber die Krclfte der itnbelebten Natur, Liebig's Ann., vol. xiii.

(1842). p. 233. t Proc. Roy. Inst., vol. vi. (1871), p. 323. t Edin. Roy. Soc. Proc, vol. v. (1866), p. 328. § Ann. de Chim. et de Phys. [3], vol. xlvi. p. 182. in the gradual development of the knowledge derived in the first instance from the metallurgy of lead, there is much that is typical of the mutual relation of theory and practice that still prevails. Now, as in the past, in the study of metallurgy, a prominent position must be given to the production of high temperatures, as it will be obvious that metallurgists have principally to consider the reactions of the elements when under the influence of heat. In the first half of the present century, temperatures higher than the melting point of zinc had not been determined with any degree of certainty; but, in 1856, Henri Ste. Claire-Deville pointed out that chemistry at high temperature, that is to say, up to the bluewhite heat at which platinum volatilises and silica fuses, remained to be studied, as under such conditions ordinary chemical reactions may be modified or even reversed.

Since then, in conjunction with Troost, he has given certain fixed points, such, for instance, as the boiling-points of cadmium and zinc ; and his researches on dissociation have entirely modified the views generally entertained in regard to the theory of com bustion. Indeed, so much is due to this illustrious teacher, that the best homage that can be offered him will be to work in the directions he has indicated. The essential difference in the properties of metals produced by a small difference of composition introduces one very distinctive feature of metallurgy — the enormous influence exerted on a large mass of metal by a " trace " of another metal or metalloid, that is, by a quantity so small that it appeal's to be out of all propor tion to the mass in which it is distributed ; and it may safely be asserted, that in no other branch of applied science has the operator to deal with quantities that are at once so vast and so minute.

It may be that the trace is alone of value, as, for instance, the few grains of gold that can be profitably extracted from a ton of material, wliich, though containing only one part of gold in five millions by volume, is thereby entitled to be regarded as an auriferous deposit that can be profitably worked ; or it may be that the presence of a minute percentage of a metalloid is prejudicial and must be extracted, in order that the physical properties of the remaining mass of metal may not be such as to render it use less. Due prominence is given to such facts in the following pages.

Chapter II. Physical Properties Of Metals

Molecular Structure. — The physical aspects of metals are so pronounced as to render it difficult to abandon the old view that metals are sharply defined from other elements, and form a class by themselves. The term metal is in fact somewhat arbitrary. Zinc and bismuth, when they were first discovered, were considered to be semi-metals, and it was not until mercury was frozen by Braune in 1759 that it was recognised as a metal. Like all other elements, metals are composed of atoms grouped in molecules, and any force that alters the relations of the atoms in the molecules modifies the physical properties of the metals. Indeed, it would be easy to show that the physical constants of each metal vary with its degree of purity. The molecular grouping of metals is doubtless very varied, and little definite is known re garding the structural stability of most of them ; but it may be assumed that it is not very great, as some metals split up into single atoms when they are volatilised, and most of them unite readily with chlorine and with oxygen.* It is probable that in many pure metals, such as gold, silver, copper and iron, the indi vidual molecules are of simple atomic constitution, and that these fundamental molecules bear a uniform relationship to one another.

Consequently, any mass, of which the fundamental molecules are the constituent particles, may practically be regarded as a single molecule. Two fundamental molecules must, however, be held to be capable of uniting to form complexes that have less power of cohering, and any circumstance tending to bring about the for mation of such complexes would also tend to make the material less tough. This may account for the extraordinary alteration in the properties of many metals produced by very small quantities of incompatible foreign matters.! It will be shown in a future chapter that the effect produced by a small quantity of an im- * Lothar Meyer, Modern Theories of Chemistry, English translation, 1888, p. 568. t H. E. Armstrong, Min. Proc. Inst. C.E., vol. xciii. (1S88), p. 112. purity added to a mass of metal bears a close relation to the atomic volume of the added element.

Density. — The density of a metal is dependent on the intimacy of the contact between the molecules. It is dependent, there fore, on the crystalline structure, and is influenced by the tem perature of casting, by the rate of cooling, by the mechanical treatment, and by the purity of the metal. With the exception of bismuth, all metals are lighter when molten than when in the solid state. In the case of cast iron, which passes through a pasty state on solidification, the density is less in that state than in the fluid or solid. The density of a metal is augmented by wire-drawing, hammering, and any other physical method of treatment in which a compressing stress is employed. Mere traction, however, may diminish the density by tending to de velop cavities in the metal. Pressure on all sides of a piece of metal increases its density.

The density of standard gold, for example, by compression between dies is increased by 0.9, and cast discs of platinum, having a density of 21.21, may have the density increased to 21.46 by striking; whilst annealing such struck discs will again diminish their density. This shows that the compression is not permanent, and is solely due to the closing of pores. W. Spring* has even shown by careful experiments on lead, tin, bismuth, antimony, cadmium, aluminium, and zinc, that a pressure of 20,000 atmospheres continued for many days is insufficient to effect the obliteration of all the pores. A metal can only be really compressed if the result of the applica tion of pressure is to cause it to pass to an allotropic state, that is denser than that which it originally possessed. The specific gravities of the various metals are given in the table on p. 9.

Lithium is the lightest metal, and iridium the heaviest, the specific gravity of the former being 0.6, and that of the latter 22.38. So early as 1845, Joules recognised the importance of determining the specific gravity of melted metals, seeing that " this condition would completely obviate the influence of cohesion, or that of any particular molecular arrangement." His method, which was essentially that afterwards adopted by Mallet J and by the author,§ may be described as follows : — It consists in filling with molten metal a vessel, the capacity of which may be calculated for the particular temperature at which the molten metal is introduced. The weight of the metal when cold, divided by the * Bull. Soc. Chim. Paris, vol. xl. (1883), p. 515. t Collected Papers. Published by the Physical Society, vol. ii. p. 136.

X Pro::. Roy. Soc., vol. xxii. (1873), p. 366 ; and vol. xxiii. (1874I, p. 209. § Ibid., vol. xxiii. (1875), p. 4S1. weight of water which the expanded vessel is capable of holding, gives the fluid density of the molten metal. Subsequently, work ing in conjunction with T. Wrightson, the author * determined, by the aid of an instrument called the oncosimeter, devised by the former, the fluid density of several metals. The results which have as yet been obtained may be briefly summarised in the fol lowing table : — The question of the fluid densities has also been investigated by Nies and Winkelmann,f who have adopted another method, determining the liquid density by observing the weights of blocks which just sink and just swim. With regard to bismuth, C. Liidekingsf finds that this metal, like water, attains a maximum density just before becoming solid, the expansion at the moment of solidification being about 3 per cent, of the volume.

Fracture. — The appearance of the fractured surface of a metal depends partly on the nature of the metal and partly on the manner in which solidification occurred. Sudden cooling to a great extent prevents the formation of crystals, whilst slow cooling facilitates their development. Long-continued hammering, frequent vibrations, and intense cold will produce the latter result. Any condition that affects either the cohesion or the crystalline structure of a metal affects its fracture. Thus, lead broken when hot has a columnar structure ; when broken cold, this structure is not exhibited. * Phil. Mag., vol. xi. (1881), p. 295; vol. xiii. (1882), p. 360. t Sitzungsber. der Acad. der Wissen. zu Munchen, 1881, p. 63. % Ann. Rhys. Chem., vol. xxxiv. (1888), p. 21. Fluid Density. Specific Gravity of Solid.

By By Mallet's Oneosiilethod. ! meter. Percentage us Change in Volume from Cold Solid to Liquid. Bismuth Copper Lead . Tin Zinc .Silver . Iron (No. 4 foundry pig) . Iron, cast (mixed brands) . 9.82 8.80 II.40 7-5° 7.20 10.57 10.039 ; 10-055 8.217 IO.650 IO.370 6.974 7.025 6.550 6.480 9.460 9-510 Decrease of vol. 2.30 Increase of vol. 7.10 9-93 6.76 „ „ 11.10 „ „ 11.20 6.95 6.880 „ „ 1.02 7-i7 6.650 ordinary mineralogical terms regarding colour and fracture are used in relation to metals. Practice, however, can alone enable the student to accurately describe these appearances. Malleability. — This is the property of permanently extending in all directions, without rupture, by pressure produced by slow stress or by impact. As a rule, crystalline metals are not malleable, and any circumstance that tends to produce crystallisation must affect the malleability.