August 27, 1891] 



NA TURE 



403 



the bed of a reverberatory furnace. Suppose the material is 

 what is known as a complex regulus, as imported into Swansea 

 or produced at Freiberg, to which are added rich native sul- 

 phides. The mixture then consists of sulphides mainly of iron 

 and copper, with some sulphide of lead, and contains fifty or 

 sixty ounces of silver to the ton, and a few grains of gold. It may 

 also contain small quantities of arsenic and antimony as arsenides, 

 antimonides, and sulpho-salts, usually with copper as a base. 



The temperature of the furnace in which the operation is to 

 be performed is gradually raised, the atmosphere being an oxi- 

 dizing one. The first effect of the elevation of the tempera- 

 ture is to distil off sulphur, reducing the sulphides to a lower 

 stage of sulphurization. This sulphur burns in the furnace 

 atmosphere to sulphurous anhydride (SOo), and, coming in con- 

 tact with the material undergoing oxidation, is converted into 

 sulphuric anhydride (SO3). It should be noted that the 

 material of the brickwork does not intervene in the re- 

 actions, except by its presence as a hot porous mass, but its 

 influence is, nevertheless, considerable. The roasting of these 

 sulphides presents a good case for the study of chemical equili- 

 brium. As soon as the sulphurous anhydride reaches a certain 

 tension, the oxidation of the sulphide is arrested, even though 

 an excess of oxygen be present, and the oxidation is not resumed 

 until the action of the draught changes the conditions of the 

 atmosphere of the furnace, when the lower sulphides remaining 

 are slowly oxidized, the copper sulphide being converted into 

 copper sulphate mainly by the intervention of the sulphuric 

 anhydride formed as indicated. Probably by far the greater 

 part of the iron sulphide only becomes sulphate for a very brief 

 period, being decomposed into the oxides of iron, mainly ferric 

 oxide, the sulphur passing off. Any silver sulphide that is 

 present would have been converted into metallic silver at the 

 outset were it not for the simultaneous presence of other sul- 

 phides, notably those of copper and of iron, which enables the 

 silver sulphide to become converted into .sulphate. The lead 

 sulphide is also converted into sulphate at this low temperature. 

 The heat is now raised still further with a view to split up the 

 sulphate of copper, the decomposition of which leaves oxide of 

 copper. If, as in this case, the bases are weak, the sulphuric 

 anhydride escapes mainly as such ; but when the sulphates of 

 stronger bases are decomposed, the sulphuric anhydride is ro a 

 great extent decomposed into a n.ixture of sulphurous anhydride 

 and oxygen. The sulphuric anhydride, resulting from the de- 

 composition of this copper sulphate, converts the silver into 

 sulphate, and maintains it as such, just as, in turn, at a lower 

 temperature, the copper itself had been maintained in the form 

 of sulphate by the .sulphuric anhydride eliminated from the iron 



which is afforded by external heat. The facts appear to be as 

 follows : sulphide of antimony, when heated, dissociates, and 

 the tension of the sulphur vapour would produce a state of 

 equilibrium if the sulphur thus liberated were not seized by the 

 hydrogen and removed from the system. The equilibrium is 

 thus destroyed, and fresh sulphide is dissociated ; the general 

 result being that the equilibrium of the system is continually 

 restored and destroyed until the sulphide is decomposed. The 

 antimony combines with oxygen, and escapes as volatile oxide, 

 as does also the arsenic, a portion of which is volatilized as 

 sulphide. 



The main object of the process which has been considered is 

 the formation of soluble sulphate of silver. If arsenic and anti- 

 mony have not been eliminated, their presence at the end of the 

 operation would be specially inconvenient, as they give rise to 

 the formation of arseniate and antimoniate of silver, insoluble 

 in water, which may necessitate the treatment of the residues 

 by an entirely different process from that which has hitherto 

 been considered. 



It will have been evident that effecting this series of changes 

 demands the exercise of the utmost skill, care, and patience. 

 The operations beginning at a dull red heat, or a temperature of 

 some 500°, are completed at 700°, within a range, that is, of 200°. 

 Judicious stirring has been necessary to prevent the formation 

 of crusts of sulphates, which would impede the reactions, and, 

 as has been shown, an undue elevation of temperature within a 

 very limited range would, at any stage, have been fatal to the 

 success of the operation. It is difficult to appreciate too highly 

 the delicacy of sight and touch which enables an operator to 

 judge by the aid of rough tests, but mainly from the tint of the 

 streak revealed when the mass is rabbled, whether any particu- 

 lar stage has or has not been reached, and it will be obvious 

 that the requisite skill is acquired solely by observation and ex- 

 periment. The technical instructor may impart information as 

 to the routine to be followed, and the appearances to be ob- 

 served, but scientific knowledge of a high order can alone enable 

 the operator to contend with the disturbing influences introduced 

 by the presence of unexpected elements or by untoward varia- 

 tions in temperature. In the training of a metallurgist it is im- 

 possible to separate education from instruction, and the above 

 description of a very ordinary operation will show the intimate 

 relations between science and practice which are characteristic 

 of metallurgical operations. Practice is dependent on science 

 for its advancemeuient, but scientific workers too often hesitate 

 to attack metallurgical problems, and to devote the resources of 

 modern investigation to their solution, because they are not 

 aware of the great interest of the physical and chemical prob- 



sulphide. When only a little of the copper sulphate remains | lems which are connected with many very simple metallurgical 



undecomposed, the silver sulphate begins to split up, and the 

 furnace charge must therefore be immediately withdrawn, or the 

 whole of the silver sulphate would be converted into metallic 

 silver, partly by the direct action of heat alone and partly by 

 reactions such as those shown in the following equations : — 

 Ag.,S04 -f 4Fe,0., = 2Ag -f dYc^O^ -f SO^, 

 AgoSOj + CuoO = 2Ag 4- CUSO4 -f- CuO. 

 If the charge were not withdrawn, the silver would thus be 

 effectually removed from the solvent action of water, and the : 

 smeller's efforts would have failed entirely. The charge still 

 contains lead sulphate, which cannot be completely decomposed | 

 at any temperature attainable in the roasting furnace, except in i 

 the presence of silica, and it is well to leave it where it is if the i 

 residue has subsequently to be smelted with a view to the j 

 extraction of the gold. The elimination of arsenic and antimony 

 gives rise to problems of much interest, and again confronts the 



processes, especially with those that are conducted at high 

 temperatures. 



Proceeding yet one step further, suppose that the copper- 

 smelter takes possession of the residual mass, consisting mainly 

 of oxide of copper, he would smelt it with fresh sulphide ores, 

 and obtain, as a slag from the earthy matters of the ore, a 

 ferrous silicate containing some small proportion of copper. The 

 displacement of the copper from this silicate may be effected by 

 fusing it with sulphide of iron, a fusible sulphide of iron and 

 copper being formed, which readily separates from the slag. 

 By this reaction some twenty thousand tons of copper are 

 added to the world's annual production. Proceeding yet a 

 step further, suppose the smelter to have reduced his copper to 

 the metallic state. If arsenic had been originally present in the 

 ore, and had not been eliminated entirely in the roasting, extra- 

 ordinary difficulties will be met with in the later stages of the 

 process, in extracting small quantities of arsenic which resist the 



smelter with a case of chemical equilibrium. For the sake of f^elter's efforts. Copper, moreover, containing arsenic cann..t 



brevity it will be well for the present to limit the consideration 

 to the removal of antimony, which may be supposed to be pre- 

 sent as sulphide. Some sulphide of antimony is distilled off, 

 but this is not its only mode of escape. An attempt to remove 

 antimony by rapid oxidation would be attended with the danger 

 of converting it into insoluble antimoniates of the metals present 

 in the charge. In the early stages of the roasting it is therefore 

 necessary to employ a very low temperature, and the presence 

 of steam is found to be useful as a source of hydrogen, which 

 removes sulphur as hydrogen sulphide, the gas being freely 

 evolved. The reaction 



SbjSs 4- 3H2 rr 3H2S + 2Sb 

 between hydrogen and sulphide of antimony is, however, endo- 

 thermic, and could not, therefore, take place without the aid 



NO. II 39, VOL. 44] 



be "overpoled," as the presence of arsenic hinders the reducing 

 action of gases on the copper. The amount of arsenic which 

 the copper-smelter has to remove may vary from mere traces up 

 to I per cent., and if the copper is destined for the use of the 

 electrical engineer, he will insist on its being as pure as pos- 

 sible, for the presence of a trace of arsenic would materially 

 increase the electrical resistance of the copper, and would be 

 fatal to its use in submarine telegraphy. If, on the other hand, 

 the copper is intended for the maker of locomotive fire-boxes, 

 he will encourage the retention of small quantities of arsenic, as 

 it is found to actually increase the endurance of the copper, and 

 the smelter will in such a case have no inducement to employ 

 the basic furnace lining which Mr. Gilchrist has offered him, nor 

 will he care to use the special methods for the removal of arsenic 



