BISSETT: THE REMOVAL OF SULPHUR FROM SILVER. 2829CCLXV.-The Removal of Sulphur from Silver.By CRELLYN COLGRAVE BISSETT.THE fact that sulphur is found a t times in silver, especially in thecase of metal recovered from materials containing thiocyanates, hasbeen mentioned by hhe aathor in a previous paper (this vol., p.1223). When recovering metal from such residues the danger canbe removed to a large extent by a preliminary digestion of thematerial with warm ccacentrated hydrochloric acid. The lasttraces of sulphur can be removed by the addition of a little potass-ium nitrate to the sodium carbonate used in the recovery.The removal of sulphur from metallic silver by fusion withpotassium nitrate is very tedious, as the oxidation proceeds veryslowly. It appeared possible that the sulphur could be removedmore readily by the addition of the necessary quantity of a secondmetal.The results obtained by adding copper and iron respec-tively have been studied to some extent. The effect produced byblowing air through the molten metal has also been determined.VOL. cv. 8 2830 BISSETT: THZ REMOVAL OF SULPHUR FROM SILVER.The results obtained by the metallic additions have a furtherinterest, since they show the nature of the chemical equilibriumin these cases at moderately high temperatures.I. Znfiuence of Copper on Silver containin,g Sulphur.The freezing-point diagram for the mixture silver sulphide-cuprous sulphide has been determined by Friedrich (Metallurgie,1907, 4, 671), who found that mixtures of these substances forman unbroken series of solid solutions, the freezing-point curve fallingto a minimum a t 70 per cent.of silver sulphide, the freezing pointof this mixture being 67F.Cuprous sulphide is the only sulphide of copper stable a t hightemperatures.The results given below tend to show that the equilibriumbetween the eulphides is not affected by the presence of excessof either or both of the metals.The effect produced by adding copper to silver containing 13.5per cent. of silver sulphide was first studied. It was found that theaddition of small amounts of copper caused the first freezing pointof thei alloy to be lowered, whilst a t the1 same time the temperatureof the halt in the cooling curve due to the' materials being incom-pletely niiscible in the liquid state was raised.This change con-tinued until 2 per cent. of copper had been added; the mixturethen had only one main freezing point. An examination of thevertical section of the ingot a t this stage showed that the mixturehad separated into two ldyers in the liquid state. A further addi-tion caused the freezing point t o rise to a maximum, with approxi-mately 3.5 per cent. of copper, and then to fall gradually. Whenabout 9 per cent. of copper had been added, the cooling curve ofthe mixture showed a second break after the first freezing point,due to the separation of the copper-silver eutectic. The eutecticarrest, when it first appeared, occurred a t a temperature consider-ably below the average value.The mixture gave only one freezingpoint wheil approximately 31 per cent. of copper had been added.It was evident after the first few determinations that copper wasof little use for removing sulphur from silver.The effect of small additions of copper can only be explained onthe principlo that part alloys with the silver, whilst the remainderreplaces the silver in a portion of the silver sulphide, formingcuprous sulphide which dissolves in the excess of silver sulphidepresent in the mixture. The initial rise in the temperature ofincomplete miscibility is apparently due t o the influence of thecopper in lessening the solubility of the sulphide. This is to bBISSETT: THE REMOVAL OF SULPHUR FROM SILVER. 2831expected since cuprous sulphide is insoluble in copper.The lowtemperature of the eutectic halt in its early stages may be due t othe presence of sulphur in solution.Microscopic evidence supports the above interpretation, since theamount of sulphide present remains fairly constant until 2 percent. of copper has been added. A further addition of copperlessens the area of sulphide slightly, whilst the addition of morethan 3.5 per cent. of copper rapidly removes practically the wholeof the sulphide from the silver.I n order to follow the changes a little more completely the effectof heating practically pure silver sulphide with varying percentagesof copper was examined to some extent. It was found that thebehaviour was very similar to that described above. The mixtureseparated into two layers in the liquid &ate, when about 3 per cent.of copper had been added.The freezing point of the layer rich insulphide was determined in a number of cases. As was to beexpected, since cuprous sulphide and silver sulphide forni a con-tinuous series of solid solutions when melted together, these freezingpoints were very poorly defined. The temperature of the freezingpoint was lowered rapidly by the addition of copper.On examining these parts of the alloys under the microscope, itwas found that tho metallic portion of the mixtures had a veryconsiderable solubility, especially in the case of mixtures containinga moderately high percentage of copper.EXPERIMENTAL,The determinations were carried out in a manner analogous tothat described in the previous paper (Zoc. cit.).It was necessaryto stir t'he various mixtures very thoroughly in order t o ensureequilibrium.The following tables give information obtained from tlie coolingcurves :TABLE r.Ttzpuences of Copper o n Silver containing 13.5 per cent. ofSilver Sulphide.Alloy number.A.C. 1718192021222324Percentage of First freezing Second freezingcopper. point. point.0 920" 903"0.5 917 90s1.0 916 9101.5 915 9112.0 9122.8 9173.5 917 -4.3 9106.2 903--8 . Y 2832 BISSETT: THE REMOVAL OF SULPHUR FROM SILVER.Alloy number.A.C. 2526272829303132 . 333435TABLE I (continued).Percentage of First freezingcopper. point.6.2 gooo7.3 8929.0 87311.9 86315.6 84019.8 81227.1 78230.0 77337.5 79645.6 83754.5 867Second freezingpoint.-752O764771768772771771773772TABLE 11.Influence of Copper on Silver Sulphide containing 1 per cent.of Free Silver.Percentage of First freezingAlloy number.copper. point.- 0 802'A.C. 36 0.6 85937 1.2 88339 2-5 89840 3.2 90041 4.0 90343 7.5 91844 10.8 92045 14.1 92 146 16.9 92147 19.3 92149 23.2 91060 28.4 89261 33.3 84752 39.9 7 8253 47.2 803Second freezingpoint.798'794777766756-720674-761770774774The second ,series of points for alloys A.C. 36-A.C. 44 are inevery case the freezing points of the layer rich in sulphur. It wasfound impossible to determine similar freezing points in mixturescontaining a higher percentage of copper with any degree of accu-racy.So far as could be determined, the freezing point appeared t orise steadily as the percentage oi' copper was increased.The compositions of the inixtures tabulated above were deter-mined from the weights of material used, since in the majority ofcases there wese two liquid layers, and it was found difficult todetermine exactly the relative amounts of these.The curves obtained by plotting the above results are shown inFig, 1BISSETT: THE REMOVAL OF SULPHUR FROM SILVER. 2833II. Influence of Iron, on Silver containing Sulphur.The relation between siIver sulphide and ferrous sulphide hasbeen determined by Schoen (Uetallurgie, 1911, 8, 737), who foundthat mixtures of these substances form a simple eutectif erous series,the eutectic containing 11 per cent.of ferrous sulphide, with amelting point of 6 1 5 O .It appeared highly probable that iron would be a suitable metal930"91 0"890"870"850"8 30"810"790"770"750"730"710"690"670-__. 13.5 per cent. solution.99'0 ,, - - - - - - - - 9 ,Percentage of copper.to add in order to remove sulphur, since silver and iron areimmiscible eves a t 1600O (Petrenko, Zeitsch. unorg. Chem., 1907,53, 212), and also ferrous sulphide is stable in comparison with themajority of other sulphides.I n the present investigation the effect produced by adding vary-ing percentages of iron to molten silver containing 11.6 per cent.of silver sulphide has been studied2834 BISSETT: THE REMOVAL OF SULPHUR FROM SILVER.Three mixtures only were examined, since these gave sufficientThe following table' gives the result of the thermal investigation :information.TABLE In.Alloy number.Percentage of First freezing Second freezing - iron. point. point.A.F. 1 0 927' 903"A.F. 2 1.2 95 1 904A.F. 3 5.8 960 noneA.F. 4 10 960 noneAlloys after A.F. 1 were found to contain two layers in the liquidstate. The lower layer was rich in silver, whilst the upper layerwas rich in sulphur. Tho freezing points given refer in all cases tothe layer rich in silver. The upper layer in A.F. 4 appeared tofreeze a t a temperature above 1000°.Alloy A.F. 2 contains sufficient iron to saturate half the sulphurpresent. It will be seen that whilst the first freezing point israised to 951°, the second freezing point is practically constant.It seems probable, thesefore, that the iron combines with as muchas possible of the sulphur present, and a t the same time thesulphide formed is insoluble in silver.Microscopic evidencesupports this view, since the sulphide remaining in solution in thesilver in this alloy behaves in a manner similar to that of silversulphide, towards etching media. The freezing point, 951°, corre-sponds with that of an alloy containing 4.5 per cent. of silversulphide. Apparently , therefore, the ferrous sulphide formed dis-solves a considerable portion of the silver sulphide left in themetal. This is made the more probable by the fact that a polishedsection of the upper layer of the ingot shows traces of a eutecticstructure.Alloy A.F.3 contains sufficient iron to saturate the whole of thesulphur present. Apparently this is what does occur, since thefreezing point of the mixture rises t o that of pure silver, and thereis no second freezing point. It was found, however, on examiningthe layer rich in sulphur under the microscope that silver sulphidewas present. On adding a considerable excess of iron above that'required to saturate the sulphur, the silver present in the upperlayer was displaced by iron, the upper layer then consisting ofpractically pure ferrous sulphide.Iron, theref ore, when added to molten silver containing silversulphide appears to remove the sulphur from solutionBISSETT: THE REMOVAL OF SULPHUR FROM SILVER.2835111. Effect Produced by Blowing Air through Molten Silvercontaining Sulphur.I n the experiments being described dry air was blown throughthe molten metal a t about 1000°, the rate being approximately thesame in every case.The alloy contained 16 per cent. of silver sulphide, and 30 gramsof alloy were used in each experiment. Table I V gives the resultsobtained.TABLE IV.Experiment Time of blowing air.number. Minutes. Freezing point.1 0 907'960"950"940'930"92G"910"900-27162944 -FIG. 2.91392 193394394595910 20 30 40 50Time of blowing air: in minutes.I n experiment 7 the material from experiment 6 was re-melted.Charcoal was added, and the mixture was well stirred.The curve obtained by plotting the results is shown in Fig. 2.It will be seen that the removal of sulphur is somewhat slow underthe conditions of the experiments. The rate a t which air wasblown through the material was of necessity slow, owing to theviolent spitting caused by a rapid stream of gas.Charcoal was added in experiment 7 to remove the oxygenpresent in solution in the silver, in order to determine whether theproduct a t that stage was pure silver. The freezing point obtaine2836 MARTIN: RESEARCHES ON SILICON COMPOUNDS. PART VI.is slightly lower hhan that of pure silver. Microscopic examinationBhowed that no sulphide was present. The low freezing pointobtained was due probably t o the small weight of material used.I wish to thank Mr. C. T. Heycock for suggesting the w.ork t ome, and the Research Fund Committee of the Chemical Societyf o r a grant towards the cost of apparatus.METALLURGICAL DEPARTMENT,CHEMICALABORATORIES,CAMBRIDGE