Dadape, V. V.

Kulkarni, M. P.

English

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IDdlaD JourDol of ChemistryVol. 15A. April 1977. pp. 188-190Dissociation of Manganous Sulphide at High Temperatures &the Study of Its Reaction with Carbon*:'11:. P. KULKARNI & V. V. DADAPEtNational Chemical Laboratory, Poona 411008Received 31 January 1976; accepted 14 October 1976The dissociation of manganous sulphide has been studied between 1746° and 1850'K employingtranspiration technique. Since the dissociation is quite small even on prolonged heating (3 hr),longer time, it could not be of much use to calculate the thermodynamic parameters. The re-action of MnS with carbon has been carried out in the temperature between range 1657° and 1823°K.The heat of reaction is found to be 158·5±2·9 kcal rnole? at 1740oK. Employing the Cp valuesavailable in the literature, the value for the heat of reaction (AHr) at 298°K is found to be 168·4±2·9kcal rrio'le", Using this value, the heat of forrnation AHf298 for MnS has been calculated by secondlaw method and found to be AHf298= -46·6±2·9 kcal rnole? which compares well with that (AHf298= -5·5±7·6 kcal mote'<) obtained from Third Law. These values compare favourably with thatreported in the literature (-49 kcal mole'<),REFRACTORY sulphides offer a fertile field forfuture development, despite their susceptibi-lity to oxidation at elevated temperatures.Manganous sulphide! can be considered to be onthe border line between refractory sulphides andnon-refractory sulphides, since its melting point ishigh (1893°K) and is fairly volatile in vacuum at1648'K. In connection with the studies on MnS-Csyste:n. Wiedemeir and Schafer- have emphasizedthe importance of vaporization of MnS..'\. number of w)[ke,s2.3 have studied the dissocia-tion pressures of M'lS(C). employing effusion methodand mass spectrometric study.Wnile studying the sublimation, Gilles'' observedthe formation of small quantities of MnST and MnS;and ~lnS; and reported the dissociation energy ofMnS(g) as 71 ± 4 kcal, mole=. Wiedemeir andSC;lafe,4 employed the Knudsen effusion method tovaporize MnS in molybdenum crucibles. In orderto study reaction (1)MnS(c)+C(c) = Mn(g)+CS(g) ... (1)information about reaction (2)MnS(c) = Mn(g) +iS2(g) I...(2)was necessary. They further showed that reactions(1) and (2) were indeed p.edominant. Reactions (1)and (2) have nJW been investigated using transpira-tion technique.Materials and MethodsManganous sulphide was prepared by passingcarb m disulphide vapours over manganese sulphatemonohydrate (AR, Sarabhai Merck) kept in a silicaboat in a tube furnace at 1023°K for ,,:4 hr.Nitrogen gas was bubbled through liquid carbondisulphide kept at 298°K to carry CS2 vapours toreact with the sulphate. The manganous sulphide*~CL Communication No. 1989.t Present address: Development Manager, MIs DharamsiMorarji Chemicals (Pvt) Ltd, Ambarnath P.O., Dist. Thana.288formed was cooled in nitrogen atmosphere. Thesample was ground and purified by heating at 673°Kunder hydrogen atmosphere, thus removing theexcess sulphur. The sulphide was subjected tochemicals and X-ray analyses. The chemical ana-lysis is in good agreement with the correspondingtheoretically calculated values. The 'd' spacingscompared well with those of MnS reported earliers.Purified argon gas was used as a carrier gas.Impurities like oxygen, nitrogen, carbon dioxideand moisture were removed by methods describedby Ra07. In addition light lanthanon sesquisul-phidess (i.e. Sm2S3 and Nd2S2) I kept at about773°K were used to remove traces of oxygen(partial pressure of oxygen in argon gas was 10-18atrn.).The furnace was constructed" by winding aplatinum rhodium (13%) wire (diam. 1·5 mm) on arecrystallized alumina tube (length 55 cm, intodiam. 3·5 cm) (Morgan Refractories, UK). Thefurnace cone was enclosed in a rectangular boxmade of light refractory bricks. The box washeated by connecting the heating elements to themains through a voltage stabilizer. An aluminatube (length 75 ern, into diam. 2·5 cm) both ends ofwhich were connected to ground silica joints with arefractory cement, was used as the reaction tube.A molybdenum boat contained in an alumina boatwas employed.Temperatures of the hot zone, in which thesample was located, were measured by means of(a) Pt-PtJRh-13% thermocouple and (b) an opticalpyrometer (disppearing filament type, Polish make.type EP-5). Thermocouple as well as pyrometerwere calibrated by standard methods as describedby Bockris-''Procedure - Pure solid MnS was weighed in amolybdenum boat contained in an aluminaboat placed in a glass capsule which was tightlyclosed. The operation of transforming MnS to theKULKARNI & DAD APE : DISSOCIATION OF MANGANOUS SULPHIDEboat was carried out in a dry box in an inert atmo-sphere.Having attained the required temperature, thereaction tube was flushed with purified argon gas.After adjusting the argon gas rate to a desiredvalue, the molybdenum boat containing the sulphurfree MnS placed in an alumina boat (in order toavoid the reaction between molybdenum and mullite)was introduced from the down stream end and keptin the calder zone of the furnace for about 30 min.The tube was then mived SJ that the boat couldbe located in the hot zone. The experiment wasconducted for a definite time (3 hr) after which theheating was stopped and the tube moved so thatthe boat got into the colder part of the furnace.The boat was allowed to cool to room temperaturein argon atmosphere. The sintered mass of thesulphide was weighed after cooling. The experi-mental data are presented in Table 1.In all these experiments it was observed thatthere was no change in colour of the green MnS.X-ray powder patterns of the mass after experi-mental run revealed only the presence of MnS.Reaction between MnS and carbon - The mate-rials and apparatus are similar as described above.A weighed quantity of green purified MnS wasmixed with excess graphite powder and placed in agraphite boat contained in an outer alumina boat.The operations were conducted in a dry box in aninert atmosphere. The rest of the experimentaldetails are the same as given above. The sinteredmass containing MnS and graphite was removedand analysed for manganese by bismuthate methodand sulphur content by wet process. From thequantities of Mn and S the loss in weight of MnSwas found out.ResultsThe sublimation of MnS is too small at the tem-peratures listed in Table 1, even if the experimentTABLE 1 - DISSOCIATIO:>r OF Mn SNo. Temp. OK Rate of flow Loss of MnSof argon mg(litrejhr)1 1746 4·40 1·52 1774 4·35 2·03 1798 4·00 2·54 1821 3·80 4·55 1850 4·13 6·5was carried out for a long time (3 hr). The weightloss data could not be of much value as it couldwell be due to experimental errors involved in thetranspiration technique. When these studies werecarried out using a graphite boat the loss in weightof the sulphide was substantial indicating a reactionbetween the boat material and the sulphide. Itwas, therefore, decided to study the reaction of MnSwith carbon at high temperatures. The results ofthe study are given in Table 2.Treatment of the data - In the study of the re-action of MnS with carbon over the temperaturerange 1657-1822°K the experimental weight losswas used to calculate the vapour pressure of Mn(g)/CS(g) employing Eq. (3)P = n RT, (3)V,. ...where p = pressure in atmospheren = number of moles of MnSR = gas constantVr = volume of the argon in litres at tem-perature r,OK.The equilibrium constant Kp can be calculatedfrom Eq. (4)Kp = P of Mn(g) xp of CS(g) ... (4)The free energies and heats of reaction at varioustemperatures were calculated employing Eq. (5)aGo = RT In k ... (5)The free energy functions for CS(g) were takenfrom JANAF tables and12 for Mn and graphite weretaken from Stull and Sinke-" and free energy func-tions for MnS(c) were calculated from heat contentsand entropies tabulated by Kelleyl' and by Kinglli.The free energies and calculated Third Law heatsfor the reaction have been summarized in Table 2.Employing the Vant Hoff's equation, the heat ofreaction (a !I,) has been determined from the plotof log K vs liT. The slope of the line obtained bythe method of least squares gave a value for theheat of reaction AH1740 = 158·5 ± 2·9 kcal mole'<.This value represents the mean over the temperaturerange studied and does not include the Cp values.The value for AH'28S could be calculated applyingKirchoff's law (Eq. 6).298AH'28S = AH1740 - J ACp d T ... (6)1740where ACp is change in heat capacities of productsTABLE 2 .- TRANSPIRATiON DATA FOR THE REACTION MnS(e) + C(e) = Mn(g) + CS(g)Temp. Rate of Loss of pMn( x 10') Kp(xl07) -Log Kp !IF Sfef H·fef !lH298OK argon MnS(e) atm. kcal kcal kcal kcal mole'?(litre/hr) mg mole= mole"! mole'?1657 3·60 6·0 1-561 0·2436 7·6133 57·70 68'64 113-73 171·431734 2·70 10·5 3·643 1'327 6·8771 54·57 68·47 118·72 173·291752 2·88 15·0 4·879 2·380 6·6234 53·10 68·34 119·73 173·231773 2·52 16·5 6·133 3·761 6·4247 52·12 68'40 121'27 173·391793 2·52 21·0 7·811 6·101 6'2146 50·98 58·54 122·89 173·871808 2·94 24·0 7·649 5·850 6·2328 51'57 68·27 123·43 175·001810 2·04 33·0 1·515 2·295 5·5392 46·71 68·27 123'56 170·271822 3·00 42·0 1'311 1·718 5·7650 48·09 68·33 124·51 172-61,~9INDIAN J. CHEM., VOL. 15A, APRIL 1977and reactants. Thus the value for AH,298 is 168·4± 2·9 kcal mole-I. The corresponding Third Lawvalue is 172·9 + 2·6 kcal mole-I.Heat of formation of MnS - The heat of formationAHf298 of MnS(c) has been calculated from theheat of reaction at 298°K.AH, = AH, Mn(g)+AHfCS(g)-AHf MnS(c)-AH, C(c) ... (7)Hence AHf298= (-168·4±2·9)+66·73+(55·0±5·0)= -46·6 ± 2·9 kcal mole'< and the correspondingThird Law value is -51·1 + 7·6 kcal mole=.The values for heat of formation of Mn(g) andCS(g) were taken from Stull and Sinke-" andJANAp2 tables respectively.Entropy of the reaction - The entropy of thereaction was calculated by the least squaresmethod and was found to be 60·2 ± 1·6 e.u. at1740oK.AcknowledgementThis work was carried out under the PL-480scheme sponsored by the National Bureau of Stan-dards, Washington DC, USA.References1. CAMPBELL, I. E., High temperature technology (JohnWiley, New York), 1956, 187.2902. WIEDEMEIR, H. & SCHAFER, H., Z. anorg. allg, Chem.,326 (1964), 235.3. WIEDEMEIR, H. & GILLES, P. W., J. chem. Phys., 41(1965), 2765.4. WIEDEMEIR, H. & SCHAFER, H., Z. anorg. allg. Chem.,326 (1964), 230.5. HILL, S. D., CLElLAND, C. A., ADAMS, ARNE LANDSBERG& BLOCK, F. E., J. chem. Engng Data, 14 (1969), 84.6. Standard X-ray diffraction powder pattern, N.B.S. CircularNo. 539, Vol. IV (United States Dept. of Commerce),1955, 11.7. BHOGESWARARAO, D. & DADAPE, V. V., J. phys. Chem.,70 (1966), 1349.8. ATCHAYYA,M. & DAVAPE, V. V., J. Less common Metals,13 (1967). 559.9. THOMAS, J. RHODES, Industrial instruments for measure-ment and control (McGraw-Hill, New York), 1941.257, 329.10. BOCKRIS, J. O'M., \VHITE, J. L. & MACKENZIE, J. D.,Physicochemical measurements and high temperatures(Butterworths, London), 1959, 22; 28.11. VOGEL, A. I., A text book of quantitative inorganic analysis(The English Language Book Society and Longman),1961, 299, 466.12. STULL, D. R. et al., JANAF thermochemical tables,PB 168 370 (Dow Chemical Co., Midland, Michigan),Aug. 1965.13. STULL, D. R. & SINKE, G. C,; Thermodynamic propertiesof the elements (American Chemical Society, WashingtonDC), 1956, 127.14. KELLEY, K. K. & KING, E. G., Contribution to the dataon theoretical metallurgy, Bulletin 584 (US Bureau ofMines, Washington), 1960, 119.15. KELLEY, K. K. & KI:-IG, E. G., Contributions to the dataon theoretical metallurgy, Bulletin 592 (US Dept. ofInterior, Bureau of Mines), 1961. 64.

Dissociation of Manganous Sulphide at High Temperatures & the Study of Its Reaction with Carbon

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Dissociation of Manganous Sulphide at High Temperatures & the Study of Its Reaction with Carbon

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