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The application of variable atmosphere thermomagnetometry to the thermal decomposition of pyrite
Reactivity of Solids, 8 (1990) 159-168 Elsevier Science Publishers B.V., Amsterdam
159 - Printed
in The Netherlands
The application of variable atmosphere thermomagnetometry to the thermal decomposition of pyrite H.J. Hurst, J.H. Levy CSIRO Division of Fuel Technology, Lucas Heights Research Laboratories, Private Mail Bag 7, Menai, NS W 2234 (Australia)
and S.St.J. Wame Department of Geology, The University of Newcastle, Shortland, NS W 230% (Australia) (Received
9 May 1989; accepted
25 October
1989)
Abstract Thermomagnetometric studies of pyrrhotites, Fet, _x,S, formed during the decomposition of pyrite, were made in dry, oxygen-free nitrogen, 3% hydrogen in nitrogen, and 1.2% oxygen in nitrogen. Decomposition was demonstrated to be more complex than could be determined by thermogravimetric measurements, X-ray powder diffraction techniques, or chemical analysis of quenched pyrite samples. Thermomagnetometry was shown to be a sensitive and useful technique, enabling the detection of a number of magnetic phases within the decomposition products.
Introduction The thermal decomposition of pyrite, Fe&, in various atmospheres to give a series of pyrrhotites, Fe(,_,,S, has been extensively studied, not only on its own account [l-4], but also because of its importance as a source of sulfur in the gaseous products in the combustion and pyrolysis of coals [5] and oil shales [6]. The phase diagram of the FeS-FeS, system [7] is complex and shows that several pyrrhotite phases can coexist during pyrite decomposition. Magnetic susceptibility measurements of pyrrhotites Feel _X,S have usually been made on samples synthesized by heating the appropriate amounts of Fe and S, but a recent in situ magnetic susceptibility and mass change study [8] has been made on the conversion of pyrite in different atmospheres. In thermomagnetometry (TM), a mass change proportional to the magnetic susceptibility of a substance is measured as a function of temperature 0168-7336/90/$03.50
0 1990 - Elsevier Science Publishers
B.V.
160
whilst the substance is subjected to a controlled temperature programme 191. This technique is increasingly being used to detect the formation of magnetic phases and its applications to mineralogy, metallurgy, and geology have recently been comprehensively reviewed by Warne et al. [lo]. The present study, aimed at the detailed elucidation of the decomposition reaction products of pyrite under different flowing furnace atmosphere conditions, has employed the techniques of powder X-ray diffraction (XRD), thermogravimetry (TG), and thermomagnetometry (TM). Of these, it was demonstrated that TM yielded the most useful complimentary information and was the most sensitive of these techniques.
Experimental
Fragments were obtained from a monomineralic crystallized mass of pyrite, and stored in a nitrogen dry box. An estimated purity of > 99% confirmed by powder XRD which showed a low background pattern and could be assigned solely to the pyrite specimen 26-801 in the JCPDS Powder Diffraction File. Single particles of about 2 mg were preferred over powdered samples, to avoid loss of sample from the sample pan because of the magnetic attraction of some magnetic pyrrhotites formed during the TM experiments. The decomposition of pyrite was studied in furnace atmospheres of dry, oxygen-free nitrogen, 3% hydrogen in nitrogen, and 1.2% oxygen in nitrogen, at flow rates of 35 ml mm’. The nitrogen was reticulated from a bulk liquid nitrogen tank, the 3% HZ/N, was a commercially available mixture (non-explosive at this concentration), while the 1.2% 0,/N, mixture was obtained by metering dry air into the nitrogen supply. Non-absolute TM measurements were made using a Stanton-Redcroft TG760 thermobalance with a horseshoe magnet, of about 20 mT magnetic flux density, reproducibly positioned on top of the furnace, some 15 mm above the sample pan. The TM measurements consisted of three sequential TG experiments using convenient heating and cooling rates of 10 o C mini’. First, pyrite is heated, in the absence of the magnet, to a predetermined temperature to produce a residue with a known percentage pyrite mass loss. Depending on the temperature reached, the residue consists of either a mixture of pyrrhotites and unchanged pyrite, or pyrrhotites. The second experiment involves cooling the residue back to room temperature in the presence of the static magnetic field. Pyrite is weakly diamagnetic and little apparent mass change is observed in the presence of a magnetic field. Assuming that further decomposition of the pyrite has ceased, any mass changes are due to magnetic transitions in the pyrrhotite phases. For the third run, the residue is reheated without the magnetic field. The TG mass
161
loss showed that further decomposition only occurs at the upper limit of the heating range. Where no mass loss occurs, plots of the magnetic loss against temperature could be obtained by taking the difference between the last two measurements and expressing them as percentages of the initial pyrite mass. Since the chemical analysis and XRD measurements methods required larger amounts of material, pyrrhotite samples were prepared by decomposing pyrite particles of between 100 and 200 mg to a known percentage mass loss in the three atmospheres using a Cahn RH thermobalance. The pyrrhotites formed from incomplete conversion of the pyrite are less dense and form on the outside of the particles. The material could be removed by careful scraping, leaving any unreacted pyrite to be cleaned and weighed. Except in the case of the oxygen containing atmosphere, the composition of the pyrrhotites formed could then be determined by a combination of the TG mass loss and the mass of unreacted pyrite, or by the TG mass loss for complete pyrite conversion. The composition of the pyrrhotite residues could also be calculated from chemical analysis of the iron content. The XRD measurements on the powdered pyrrhotite samples were made using unfiltered Co-K,,,2 radiation on a Philips PW 1050/25 powder diffractometer.
Results and discussion Nitrogen atmosphere A recent TG study of the decomposition of pyrite in nitrogen [2] has shown that decomposition takes place by a two stage mass loss process. The major mass loss occurs during the conversion of pyrite to a pyrrhotite of composition Fe,.,, S by a chemically controlled three-dimensional process. The second, smaller mass loss, takes place after the total decomposition of the pyrite and is caused by a further loss of sulfur by a three-dimensional diffusion process to give a pyrrhotite with composition Fe,.,,S. Two pyrrhotite samples for chemical analysis and XRD were selected to correspond with partial and complete conversion of pyrite during the first stage mass loss. Good agreement was obtained between the TG and chemical analysis results for the pyrrhotite formed by complete conversion, Fe o,ssS. A similar value was obtained by the equivalent to the composition TG method for the pyrrhotite formed by partial conversion but the chemical analysis indicated a composition with a lower iron content, suggesting that some pyrite was removed when the pyrrhotite residue was scraped away from the quenched pyrite particle. The XRD measurements were inconclusive since pyrrhotites with the composition Fe,,_,,S where 0 < x < 0.12, all show four strong lines with very similar d values. However, they did confirm the presence of pyrite in
162
0 5 10
0
200
coo
600
0
TEMPERATURE
200
400
600
800
C
Fig. 1. Thermomagnetometry curves for pyrite residues formed by heating indicated temperatures. Arrows show the magnetic transition at 320°C.
in nitrogen
to the
the partially converted sample, and showed differences in the XRD profile of the d = 0.206 nm line for the two samples. No absolute matches were obtained between the XRD spectra and the pyrrhotites listed in the Powder Diffraction File, but partial correspondences were found with the monoclinic 24-79, and hexagonal 24-220 and 22-1120 pyrrhotites, all with the composition Fe,,,, S. This suggested that the two pyrrhotite samples were mixtures containing different proportions of these phases. Figuie 1 shows the magnetic mass losses for pyrite samples cooled from maximum temperatures ranging from 570 to 780°C at 30” C intervals. The five lowest temperatures correspond to the first stage process and all show a magnetic transition at 320 o C, with the room temperature magnetic mass loss reaching a maximum value for the samples cooled from 630 and 660 o C. The three highest temperatures correspond to the second stage process, and show the presence of a magnetic offset which does not undergo a magnetic transition on cooling. A previous magnetic susceptibility study of pyrite conversion [8] supplies a convenient overview of the properties of the pyrrhotite Feo_,,S system, and can be used with supporting evidence from the TG and XRD methods to identify the magnetic phases. It is likely that the magnetic phase observed in the first stage mass loss is the monoclinic pyrrhotite Fe,,,,S which
163
Fig. 2. Variation
of magnetic
phase with temperature
for pyrite residues
formed
in nitrogen.
undergoes a phase transition at 320 o C on cooling with the onset of ferro- or ferri-magnetism [8]. Some idea of the rate of formation and decay of this phase may be seen in Fig. 2, which plots the magnetic mass loss observed at room temperature, normalized by dividing by the amount of decomposed pyrite, against prior treatment temperature. The curve shows a maximum rate at about 620 “C. The identity of the pyrrhotite formed in the second stage remains unknown. It shows none of the magnetic transitions [8] expected for pyrrhotites of the approximate composition Fe&S, and could be metastable. 3% Hydrogen
in nitrogen atmosphere
The TG results for the reaction of 2 mg pyrite single particles in a 3% H/N, atmosphere also show a two-stage mass loss process and suggest similar mechanisms to the decomposition in nitrogen. It is likely that the first stage also represents the conversion of pyrite to pyrrhotite, but in this case the composition of Fe ,,91S at the end of this stage probably represents an average composition of a mixture of pyrrhotite phases, since the surface material can also react with hydrogen to give hydrogen sulfide. The second stage represents the further loss of sulfur by diffusion and reaction to give a pyrrhotite with an average composition approaching FeS. The possibility also arises of further reduction to metallic iron, which forms the basis of the method for determining pyrite in coal [l&12], but only in the absence of siderite, FeCO,, which also provides a source of metallic iron [13]. Three larger pyrrhotite samples were prepared for chemical analysis and XRD with compositions typical of intermediate and total pyrite conversion during the first stage process followed by a sample from the partially-complete second stage, respectively. The TG and chemical analysis results for the first two samples were similar to those in the nitrogen atmosphere, giving a pyrrhotite composition F&s S. This is difficult to reconcile with the
I
i
L
0
200
COO
600 TEnPERRTURE
0
200
400
600
800
c
Fig. 3. Thermomagnetometry curves for pyrite residues formed by heating in 3% hydrogen nitrogen to the indicated temperatures. Arrows show the magnetic transitions occurring 220, 280 and 320’ C.
in at
observed TG results for the smaller particles where the first stage mass loss corresponds to a pyrrhotite with composition F,.,,S calculated for complete conversion. The third sample had the composition Fe,,,,S determined from TG mass loss. The XRD results for the first two samples showed differences in line profiles for the four strongest lines, and again gave correspondences with the patterns for the hexagonal and monoclinic pyrrhotites with composition Fe,,,,S. The XRD pattern for the third sample most closely matched the hexagonal pyrrhotite 20-535 in the Powder Diffraction File with the composition FeO.,,S. The TM results shown in Fig. 3 demonstrate that more pyrrhotite phases are formed in the decomposition of pyrite in the 3% Hz/N, atmosphere than are indicated from the TG results. The first four decomposition temperatures: 590, 620, 650, and 690” C, are in the region governed by the first stage process, and show magnetic transitions at about 220, 280 and 320 OC which are indicated on the figure. The phase with the transition at 320” C is most abundant during the range of pyrite decomposition of temperatures 590 to 650°C and as for pyrite in nitrogen, may again be associated with the pyrrhotite of composition Fe,.,,S. The phase with the transition at 280°C is present over the range 590 to 680” C, while the phase with the transition at 220 o C only occurs over the range 620 to 650 o C. The TM curves involving the transitions at 220 and 280” C are similar to those reported for the conversion of pyrite in hydrogen [8], where they are ascribed to mixed type pyrrhotites [14] with compositions in the range Fe 0.90_,,.92S.The magnetic transitions at 220 and 280°C are associated, so
165
that the failure to observe the former transition over the temperature range 590 to 680 o C is probably explained by the small amounts of these phases at the extremes of the range. No magnetic phases were observed for the fifth decomposition temperature at 710 o C, corresponding to the second stage process. The TG measurements show that insufficient mass loss occurs for the composition of the pyrrhotite formed to approach FeS, which has a magnetic transition at 140°C. 1.2 5%Oxygen in nitrogen atmosphere The combustion of pyrite is exothermic and is strongly dependent on particle size, heating rate, and atmospheric conditions so that a wide range of products can be formed [3-51. Rapid conversion to the final product hematite, Fe,O,, takes place on oxidation in air or pure oxygen, and it is necessary to use both lower heating rates and oxygen concentrations to reduce the reaction rate. Under these conditions magnetite, Fe,O,, can also form, and Nishihara and Kondo [15] have proposed the stepwise process FeS, + Fee, -xj S -+ Fe0 -+ Fe,O, + Fe,O,. Furthermore, if the oxygen contents are lowered to l-2%, the process may be stopped at the magnetite step. The TG results for the 2 mg particles used in the TM study and the 100 mg agglomerates used for chemical analysis and XRD both show a two-stage mass loss process, with the curve for the larger mass displaced 100°C to higher temperatures. The products of decomposition are governed by the experimental conditions, but the first stage probably involves the total conversion of the pyrite to a mixture of pyrrhotites and iron oxides, while the second stage corresponds to the conversion of the pyrrhotite to iron oxide. In a manner similar to the hydrogen in nitrogen case, three larger samples were produced for chemical analysis and XRD: two from partial and total completion of the first stage while the third sample represented partial completion of the second stage process. The residues from the first two samples visually resembled the products in the other atmospheres and showed no sign of an oxide layer. Their chemical analyses showed an iron content compatible with a pyrrhotite of composition Fe,,,& assuming no oxide formation. The third sample had a thin red-brown coating and gave a much higher percentage iron content than any of the other samples. The XRD powder patterns of the samples could again be matched with the hexagonal and monoclinic pyrrhotites of composition Fe,,,,S, but only the third sample showed additional lines corresponding to the hematite 13-534 in the Powder Diffraction File. These methods indicate that the first stage process for these massive particles involves conversion of the pyrite to pyrrhotite, although the presence of a thin outer oxide layer cannot be
s20
c
610
C
.I'
:-
C
670
C
1::
560
0
635
200
400
C
600
0
TEnPERRTURE
200
so0
600
600
c
Fig. 4. Thermomagnetometry curves for pyrite residues formed by heating in 1.2% oxygen in nitrogen to the indicated temperatures. The arrow shows the magnetic transition at 320 o C. The bars indicate the magnetic offsets.
discounted. The chemical analysis and XRD methods are of little help, since the former is based on the iron content, and the latter is known to be insensitive to small amounts of material in mixtures. The second stage then involves the conversion of pyrrhotite to hematite. No trace of the strongest XRD magnetite line was found for any of the samples, and this was probably due to the higher temperatures reached in their formation from the larger pyrite particles. The TM results for the compounds formed at various decomposition temperatures are shown in Fig 4, with the first four corresponding to the first stage process, and the last two to the second stage. The behaviour is different from the other atmospheres and shows a large magnetic offset in addition to a magnetic transition. This occurs at 320 o C and in the sample formed at the decomposition temperature of 550” C and may again be associated with the monoclinic pyrrhotite of composition Fe,,,,S. The offset reaches a maximum for the decomposition temperatures 550 and 580 o C and then decreases to a smaller value with increasing decomposition temperatures. This could be due to the presence of magnetite and its gradual conversion to hematite, and is supported by the reduction of the offset to smaller values above the Curie temperature of 585 o C for magnetite, and the similarity in behaviour to that described in a study of the magnetic properties of pyrite in anoxic atmospheres [3].
167
Conclusions
This work has demonstrated the sensitivity of thermomagnetometry in detecting the presence of additional phases that were not apparent by TG and XRD techniques. In fact, the results obtained from chemical analysis and XRD were of little help in interpreting the TM results, because the samples were prepared from larger pyrite particles having different thermal histories to those in the TM experiments. Thermomagnetometry has shown the presence of magnetic pyrrhotites Fe,,,,S during the simple decomposition of pyrite in nitrogen, and the presence of Fe,s,S and FeO.,_,,, S during the more complex reaction in 3% Hz/N, atmosphere. It has also shown the presence of Fe,,.,,S and magnetite during the slow reaction of pyrite in 1.2% 0,/N, atmosphere. The identification of these magnetic pyrrhotite phases arising from the thermal decomposition of pyrite in different atmospheres could have a bearing on industrial processes involving coal and oil shale. These include beneficiation to remove the original pyrite from coal or to reduce the sulfur content, which in turn reduces the environmental impact of combustion. Furthermore, the magnetic phases examined probably apply in the use of pyrite as a catalyst during coal hydroliquefaction and similar effects are also expected during oil shale retorting.
Acknowledgements We extend our thanks to Bill Stuart for providing helpful discussion. This work was partially funded by a CSIRO/University Collaborative Research Fund and a Collaborative Research Agreement with Esso Australia Ltd.
References
1 A.W. Coats and N.F.H. Bright, Can. J. Chem., 44 (1966) 1191. 2 I.C. Hoare, H.J. Hurst, W.I. Stuart and T.J. White, J. Chem. Sot., Faraday Trans. I, 84 (1988) 3071. 3 A.N. Thorpe, F.E. Sentfle, C.C. Alexander, F.T. Dulong, R.B. LaCount and S. Friedman, Fuel, 66 (1987) 147. 4 F.R.A. Jorgensen and F.J. Moyle, J. Therm. Anal., 25 (1982) 473. 5 A.N. Thorpe, F.E. Sentfle, CC. Alexander and F.T. Dulong, Fuel, 63 (1984) 662. 6 S.St.J. Wame and D.H. French, Thermochim. Acta, 79 (1984) 131. 7 J.C. Ward, Rev. Pure Appl. Chem., 20 (1970) 175. 8 M.S. Jagadeesh and M.S. Sehra, J. Phys. D. Appl. Phys., 14 (1981) 2153. 9 P.K. Gallagher and S.St.J. Warne, Thermochim. Acta, 43 (1981) 253.
168 10 S.St.J. Warne, H.J. Hurst and W.I. Stuart, (Invited review), Therm. Anal. Abs., 17 (1988) 1. 11 M. Hyman and M.W. Rowe, “New Approaches in Coal Chemistry”, ACS, 169, Washington, D.C., (1981) 389. 12 D.M. Aylmer and M.W. Rowe, Thermochim. Acta, 78 (1984) 81. 13 S.St.J. Wame, Thermochim. Acta, 87 (1985) 353. 14 K. Hayase, R. Otsuka and T. Mariko, Mineral J., 4 (1963) 41. 15 K. Nishihara and Y. Kondo, Mem. Fat. Eng. Kyoto Univ., 20 (1959) 285; 20 (1959) 307; 21 (1959) 214.
159 - Printed
in The Netherlands
The application of variable atmosphere thermomagnetometry to the thermal decomposition of pyrite H.J. Hurst, J.H. Levy CSIRO Division of Fuel Technology, Lucas Heights Research Laboratories, Private Mail Bag 7, Menai, NS W 2234 (Australia)
and S.St.J. Wame Department of Geology, The University of Newcastle, Shortland, NS W 230% (Australia) (Received
9 May 1989; accepted
25 October
1989)
Abstract Thermomagnetometric studies of pyrrhotites, Fet, _x,S, formed during the decomposition of pyrite, were made in dry, oxygen-free nitrogen, 3% hydrogen in nitrogen, and 1.2% oxygen in nitrogen. Decomposition was demonstrated to be more complex than could be determined by thermogravimetric measurements, X-ray powder diffraction techniques, or chemical analysis of quenched pyrite samples. Thermomagnetometry was shown to be a sensitive and useful technique, enabling the detection of a number of magnetic phases within the decomposition products.
Introduction The thermal decomposition of pyrite, Fe&, in various atmospheres to give a series of pyrrhotites, Fe(,_,,S, has been extensively studied, not only on its own account [l-4], but also because of its importance as a source of sulfur in the gaseous products in the combustion and pyrolysis of coals [5] and oil shales [6]. The phase diagram of the FeS-FeS, system [7] is complex and shows that several pyrrhotite phases can coexist during pyrite decomposition. Magnetic susceptibility measurements of pyrrhotites Feel _X,S have usually been made on samples synthesized by heating the appropriate amounts of Fe and S, but a recent in situ magnetic susceptibility and mass change study [8] has been made on the conversion of pyrite in different atmospheres. In thermomagnetometry (TM), a mass change proportional to the magnetic susceptibility of a substance is measured as a function of temperature 0168-7336/90/$03.50
0 1990 - Elsevier Science Publishers
B.V.
160
whilst the substance is subjected to a controlled temperature programme 191. This technique is increasingly being used to detect the formation of magnetic phases and its applications to mineralogy, metallurgy, and geology have recently been comprehensively reviewed by Warne et al. [lo]. The present study, aimed at the detailed elucidation of the decomposition reaction products of pyrite under different flowing furnace atmosphere conditions, has employed the techniques of powder X-ray diffraction (XRD), thermogravimetry (TG), and thermomagnetometry (TM). Of these, it was demonstrated that TM yielded the most useful complimentary information and was the most sensitive of these techniques.
Experimental
Fragments were obtained from a monomineralic crystallized mass of pyrite, and stored in a nitrogen dry box. An estimated purity of > 99% confirmed by powder XRD which showed a low background pattern and could be assigned solely to the pyrite specimen 26-801 in the JCPDS Powder Diffraction File. Single particles of about 2 mg were preferred over powdered samples, to avoid loss of sample from the sample pan because of the magnetic attraction of some magnetic pyrrhotites formed during the TM experiments. The decomposition of pyrite was studied in furnace atmospheres of dry, oxygen-free nitrogen, 3% hydrogen in nitrogen, and 1.2% oxygen in nitrogen, at flow rates of 35 ml mm’. The nitrogen was reticulated from a bulk liquid nitrogen tank, the 3% HZ/N, was a commercially available mixture (non-explosive at this concentration), while the 1.2% 0,/N, mixture was obtained by metering dry air into the nitrogen supply. Non-absolute TM measurements were made using a Stanton-Redcroft TG760 thermobalance with a horseshoe magnet, of about 20 mT magnetic flux density, reproducibly positioned on top of the furnace, some 15 mm above the sample pan. The TM measurements consisted of three sequential TG experiments using convenient heating and cooling rates of 10 o C mini’. First, pyrite is heated, in the absence of the magnet, to a predetermined temperature to produce a residue with a known percentage pyrite mass loss. Depending on the temperature reached, the residue consists of either a mixture of pyrrhotites and unchanged pyrite, or pyrrhotites. The second experiment involves cooling the residue back to room temperature in the presence of the static magnetic field. Pyrite is weakly diamagnetic and little apparent mass change is observed in the presence of a magnetic field. Assuming that further decomposition of the pyrite has ceased, any mass changes are due to magnetic transitions in the pyrrhotite phases. For the third run, the residue is reheated without the magnetic field. The TG mass
161
loss showed that further decomposition only occurs at the upper limit of the heating range. Where no mass loss occurs, plots of the magnetic loss against temperature could be obtained by taking the difference between the last two measurements and expressing them as percentages of the initial pyrite mass. Since the chemical analysis and XRD measurements methods required larger amounts of material, pyrrhotite samples were prepared by decomposing pyrite particles of between 100 and 200 mg to a known percentage mass loss in the three atmospheres using a Cahn RH thermobalance. The pyrrhotites formed from incomplete conversion of the pyrite are less dense and form on the outside of the particles. The material could be removed by careful scraping, leaving any unreacted pyrite to be cleaned and weighed. Except in the case of the oxygen containing atmosphere, the composition of the pyrrhotites formed could then be determined by a combination of the TG mass loss and the mass of unreacted pyrite, or by the TG mass loss for complete pyrite conversion. The composition of the pyrrhotite residues could also be calculated from chemical analysis of the iron content. The XRD measurements on the powdered pyrrhotite samples were made using unfiltered Co-K,,,2 radiation on a Philips PW 1050/25 powder diffractometer.
Results and discussion Nitrogen atmosphere A recent TG study of the decomposition of pyrite in nitrogen [2] has shown that decomposition takes place by a two stage mass loss process. The major mass loss occurs during the conversion of pyrite to a pyrrhotite of composition Fe,.,, S by a chemically controlled three-dimensional process. The second, smaller mass loss, takes place after the total decomposition of the pyrite and is caused by a further loss of sulfur by a three-dimensional diffusion process to give a pyrrhotite with composition Fe,.,,S. Two pyrrhotite samples for chemical analysis and XRD were selected to correspond with partial and complete conversion of pyrite during the first stage mass loss. Good agreement was obtained between the TG and chemical analysis results for the pyrrhotite formed by complete conversion, Fe o,ssS. A similar value was obtained by the equivalent to the composition TG method for the pyrrhotite formed by partial conversion but the chemical analysis indicated a composition with a lower iron content, suggesting that some pyrite was removed when the pyrrhotite residue was scraped away from the quenched pyrite particle. The XRD measurements were inconclusive since pyrrhotites with the composition Fe,,_,,S where 0 < x < 0.12, all show four strong lines with very similar d values. However, they did confirm the presence of pyrite in
162
0 5 10
0
200
coo
600
0
TEMPERATURE
200
400
600
800
C
Fig. 1. Thermomagnetometry curves for pyrite residues formed by heating indicated temperatures. Arrows show the magnetic transition at 320°C.
in nitrogen
to the
the partially converted sample, and showed differences in the XRD profile of the d = 0.206 nm line for the two samples. No absolute matches were obtained between the XRD spectra and the pyrrhotites listed in the Powder Diffraction File, but partial correspondences were found with the monoclinic 24-79, and hexagonal 24-220 and 22-1120 pyrrhotites, all with the composition Fe,,,, S. This suggested that the two pyrrhotite samples were mixtures containing different proportions of these phases. Figuie 1 shows the magnetic mass losses for pyrite samples cooled from maximum temperatures ranging from 570 to 780°C at 30” C intervals. The five lowest temperatures correspond to the first stage process and all show a magnetic transition at 320 o C, with the room temperature magnetic mass loss reaching a maximum value for the samples cooled from 630 and 660 o C. The three highest temperatures correspond to the second stage process, and show the presence of a magnetic offset which does not undergo a magnetic transition on cooling. A previous magnetic susceptibility study of pyrite conversion [8] supplies a convenient overview of the properties of the pyrrhotite Feo_,,S system, and can be used with supporting evidence from the TG and XRD methods to identify the magnetic phases. It is likely that the magnetic phase observed in the first stage mass loss is the monoclinic pyrrhotite Fe,,,,S which
163
Fig. 2. Variation
of magnetic
phase with temperature
for pyrite residues
formed
in nitrogen.
undergoes a phase transition at 320 o C on cooling with the onset of ferro- or ferri-magnetism [8]. Some idea of the rate of formation and decay of this phase may be seen in Fig. 2, which plots the magnetic mass loss observed at room temperature, normalized by dividing by the amount of decomposed pyrite, against prior treatment temperature. The curve shows a maximum rate at about 620 “C. The identity of the pyrrhotite formed in the second stage remains unknown. It shows none of the magnetic transitions [8] expected for pyrrhotites of the approximate composition Fe&S, and could be metastable. 3% Hydrogen
in nitrogen atmosphere
The TG results for the reaction of 2 mg pyrite single particles in a 3% H/N, atmosphere also show a two-stage mass loss process and suggest similar mechanisms to the decomposition in nitrogen. It is likely that the first stage also represents the conversion of pyrite to pyrrhotite, but in this case the composition of Fe ,,91S at the end of this stage probably represents an average composition of a mixture of pyrrhotite phases, since the surface material can also react with hydrogen to give hydrogen sulfide. The second stage represents the further loss of sulfur by diffusion and reaction to give a pyrrhotite with an average composition approaching FeS. The possibility also arises of further reduction to metallic iron, which forms the basis of the method for determining pyrite in coal [l&12], but only in the absence of siderite, FeCO,, which also provides a source of metallic iron [13]. Three larger pyrrhotite samples were prepared for chemical analysis and XRD with compositions typical of intermediate and total pyrite conversion during the first stage process followed by a sample from the partially-complete second stage, respectively. The TG and chemical analysis results for the first two samples were similar to those in the nitrogen atmosphere, giving a pyrrhotite composition F&s S. This is difficult to reconcile with the
I
i
L
0
200
COO
600 TEnPERRTURE
0
200
400
600
800
c
Fig. 3. Thermomagnetometry curves for pyrite residues formed by heating in 3% hydrogen nitrogen to the indicated temperatures. Arrows show the magnetic transitions occurring 220, 280 and 320’ C.
in at
observed TG results for the smaller particles where the first stage mass loss corresponds to a pyrrhotite with composition F,.,,S calculated for complete conversion. The third sample had the composition Fe,,,,S determined from TG mass loss. The XRD results for the first two samples showed differences in line profiles for the four strongest lines, and again gave correspondences with the patterns for the hexagonal and monoclinic pyrrhotites with composition Fe,,,,S. The XRD pattern for the third sample most closely matched the hexagonal pyrrhotite 20-535 in the Powder Diffraction File with the composition FeO.,,S. The TM results shown in Fig. 3 demonstrate that more pyrrhotite phases are formed in the decomposition of pyrite in the 3% Hz/N, atmosphere than are indicated from the TG results. The first four decomposition temperatures: 590, 620, 650, and 690” C, are in the region governed by the first stage process, and show magnetic transitions at about 220, 280 and 320 OC which are indicated on the figure. The phase with the transition at 320” C is most abundant during the range of pyrite decomposition of temperatures 590 to 650°C and as for pyrite in nitrogen, may again be associated with the pyrrhotite of composition Fe,.,,S. The phase with the transition at 280°C is present over the range 590 to 680” C, while the phase with the transition at 220 o C only occurs over the range 620 to 650 o C. The TM curves involving the transitions at 220 and 280” C are similar to those reported for the conversion of pyrite in hydrogen [8], where they are ascribed to mixed type pyrrhotites [14] with compositions in the range Fe 0.90_,,.92S.The magnetic transitions at 220 and 280°C are associated, so
165
that the failure to observe the former transition over the temperature range 590 to 680 o C is probably explained by the small amounts of these phases at the extremes of the range. No magnetic phases were observed for the fifth decomposition temperature at 710 o C, corresponding to the second stage process. The TG measurements show that insufficient mass loss occurs for the composition of the pyrrhotite formed to approach FeS, which has a magnetic transition at 140°C. 1.2 5%Oxygen in nitrogen atmosphere The combustion of pyrite is exothermic and is strongly dependent on particle size, heating rate, and atmospheric conditions so that a wide range of products can be formed [3-51. Rapid conversion to the final product hematite, Fe,O,, takes place on oxidation in air or pure oxygen, and it is necessary to use both lower heating rates and oxygen concentrations to reduce the reaction rate. Under these conditions magnetite, Fe,O,, can also form, and Nishihara and Kondo [15] have proposed the stepwise process FeS, + Fee, -xj S -+ Fe0 -+ Fe,O, + Fe,O,. Furthermore, if the oxygen contents are lowered to l-2%, the process may be stopped at the magnetite step. The TG results for the 2 mg particles used in the TM study and the 100 mg agglomerates used for chemical analysis and XRD both show a two-stage mass loss process, with the curve for the larger mass displaced 100°C to higher temperatures. The products of decomposition are governed by the experimental conditions, but the first stage probably involves the total conversion of the pyrite to a mixture of pyrrhotites and iron oxides, while the second stage corresponds to the conversion of the pyrrhotite to iron oxide. In a manner similar to the hydrogen in nitrogen case, three larger samples were produced for chemical analysis and XRD: two from partial and total completion of the first stage while the third sample represented partial completion of the second stage process. The residues from the first two samples visually resembled the products in the other atmospheres and showed no sign of an oxide layer. Their chemical analyses showed an iron content compatible with a pyrrhotite of composition Fe,,,& assuming no oxide formation. The third sample had a thin red-brown coating and gave a much higher percentage iron content than any of the other samples. The XRD powder patterns of the samples could again be matched with the hexagonal and monoclinic pyrrhotites of composition Fe,,,,S, but only the third sample showed additional lines corresponding to the hematite 13-534 in the Powder Diffraction File. These methods indicate that the first stage process for these massive particles involves conversion of the pyrite to pyrrhotite, although the presence of a thin outer oxide layer cannot be
s20
c
610
C
.I'
:-
C
670
C
1::
560
0
635
200
400
C
600
0
TEnPERRTURE
200
so0
600
600
c
Fig. 4. Thermomagnetometry curves for pyrite residues formed by heating in 1.2% oxygen in nitrogen to the indicated temperatures. The arrow shows the magnetic transition at 320 o C. The bars indicate the magnetic offsets.
discounted. The chemical analysis and XRD methods are of little help, since the former is based on the iron content, and the latter is known to be insensitive to small amounts of material in mixtures. The second stage then involves the conversion of pyrrhotite to hematite. No trace of the strongest XRD magnetite line was found for any of the samples, and this was probably due to the higher temperatures reached in their formation from the larger pyrite particles. The TM results for the compounds formed at various decomposition temperatures are shown in Fig 4, with the first four corresponding to the first stage process, and the last two to the second stage. The behaviour is different from the other atmospheres and shows a large magnetic offset in addition to a magnetic transition. This occurs at 320 o C and in the sample formed at the decomposition temperature of 550” C and may again be associated with the monoclinic pyrrhotite of composition Fe,,,,S. The offset reaches a maximum for the decomposition temperatures 550 and 580 o C and then decreases to a smaller value with increasing decomposition temperatures. This could be due to the presence of magnetite and its gradual conversion to hematite, and is supported by the reduction of the offset to smaller values above the Curie temperature of 585 o C for magnetite, and the similarity in behaviour to that described in a study of the magnetic properties of pyrite in anoxic atmospheres [3].
167
Conclusions
This work has demonstrated the sensitivity of thermomagnetometry in detecting the presence of additional phases that were not apparent by TG and XRD techniques. In fact, the results obtained from chemical analysis and XRD were of little help in interpreting the TM results, because the samples were prepared from larger pyrite particles having different thermal histories to those in the TM experiments. Thermomagnetometry has shown the presence of magnetic pyrrhotites Fe,,,,S during the simple decomposition of pyrite in nitrogen, and the presence of Fe,s,S and FeO.,_,,, S during the more complex reaction in 3% Hz/N, atmosphere. It has also shown the presence of Fe,,.,,S and magnetite during the slow reaction of pyrite in 1.2% 0,/N, atmosphere. The identification of these magnetic pyrrhotite phases arising from the thermal decomposition of pyrite in different atmospheres could have a bearing on industrial processes involving coal and oil shale. These include beneficiation to remove the original pyrite from coal or to reduce the sulfur content, which in turn reduces the environmental impact of combustion. Furthermore, the magnetic phases examined probably apply in the use of pyrite as a catalyst during coal hydroliquefaction and similar effects are also expected during oil shale retorting.
Acknowledgements We extend our thanks to Bill Stuart for providing helpful discussion. This work was partially funded by a CSIRO/University Collaborative Research Fund and a Collaborative Research Agreement with Esso Australia Ltd.
References
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