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Phase transformations of siderite ore by the thermomagnetic analysis data
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
crossmark
Phase transformations of siderite ore by the thermomagnetic analysis data ⁎
V.P. Ponomar , N.O. Dudchenko, A.B. Brik M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine, Kyiv, Ukraine
A R T I C L E I N F O
A BS T RAC T
Keywords: Siderite ore Thermal decomposition Thermomagnetic analysis Curie temperature Phase transformations
Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650 °C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was ~320 °C. Saturation magnetization of obtained samples increases up to 20 Am2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite.
1. Introduction Siderite is a mineral of calcite group with chemical formula FeCO3. It is a widespread iron mineral which finds application in various industries (iron industry, mining, etc.). Natural samples of siderite often show a significant amount of substitution of iron in the lattice by magnesium, calcium, manganese [1]. The pure siderite is rarely found. Siderite is commonly found in hydrothermal veins. It may be also deposited by sedimentary processes. Siderite crystallizes in the trigonal crystal system, and is rhombohedral in shape, typically with curved and striated faces. The color of siderite ranges from yellow to dark brown or black, the latter being due to the presence of manganese. This is an antiferromagnetic mineral with Neel temperature of 38 K, which is paramagnetic at ambient temperature [2,3]. Moreover, metamagnetism was reported to be magnetic property of siderite, namely, antiferromagnetic lattice gradually transforming into a ferromagnetic lattice when exposed to strong magnetic fields of 12–14 T. Nowadays, the usage of weakly magnetic iron ores for iron production is of very importance, because the deposits of magnetite iron ores are becoming exhausted. The main problem that interferes the usage of weakly magnetic siderite ores for iron production is complexity of their beneficiation, i.e. separation of siderite from other admixtures in the ore. In this case, thermal decomposition of the mineral siderite is the topic of interest because of possibility of strongly magnetic phase formation without any reducing agents in contradistinction to goethite and hematite [4,5]. Thermal decomposition of siderite in different atmospheres (air, ⁎
oxygen, nitrogen, etc.) is widely used for different applications. For example, thermal decomposition characteristics of siderite are utilized in paleomagnetic studies [6]. Interest for such investigation is caused by transformation of siderite into magnetite and maghemite which can potentially carry a chemical remanent magnetization. As far as roasting of siderite yields to high pore volume and high surface area species, the end products also could be used to capture the sulfur dioxide (SO2) in a wide temperature range. The decomposition product of siderite is also used as a natural brown pigment in the paint industry [7]. Besides, magnetite is present in the ALH84001 Martian meteorite thought to be produced by thermal decomposition of siderite [8]. However, second possibility of magnetite presence in the ALH84001 is of biogenic origin [9,10]. The final products of thermal decomposition of mineral siderite are generally hematite in an oxidizing atmosphere [11,12], magnetite in a carbon dioxide atmosphere, and magnetite and wustite in an inert atmosphere or in vacuum [13,14]. Identification of decomposition products of natural siderite is difficult due to the significant amount of substitution of iron in the lattice by magnesium, calcium, manganese [15]. The aim of present work was to investigate the thermal decomposition of siderite ore of Bakal's deposits at low- and high-oxygen content by the method of thermomagnetic analysis. 2. Materials and methods The samples of Bakal siderite ore (the western slope of the Southern Urals) that was grinded up to 0.07 mm were investigated.
Correspondence to: 34 Palladina Prospect, Kyiv, 03680, Ukraine. E-mail address: [email protected] (V.P. Ponomar).
http://dx.doi.org/10.1016/j.jmmm.2016.09.124 Received 11 March 2016; Received in revised form 20 September 2016; Accepted 27 September 2016 Available online 28 September 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
V.P. Ponomar et al.
Phase transformations of siderite ore were studied by thermomagnetic analysis (up to 600 °C) at low-oxygen content (Fig. 3). There is no peak on heating curve, while magnetization of the sample increases considerably after cooling. So, we could conclude that magnetic phase formation occurs at temperatures above Curie temperature of obtained sample. Authors [6] report that formation of magnetic phase is already observed at the thermomagnetic curves at 400 °C with Curie temperature approximately 580 °C. We haven’t observed this in our experiments. Moreover, Curie temperature determined by differentiating the cooling curve is 508 °C. This value is slightly lower than Curie temperature of pure magnetite (580 °C). Measured Curie temperature is typical for series of solid solutions of magnetite – magnesioferrite (magnesiomagnetite) with ~9% MgO according to equation of Curie temperature dependence on magnesium content [18]:
Thermomagnetic analysis was performed with laboratory-built facility that allows automatic registration of sample magnetization as a function of temperature. Phase transformations of siderite ore was carried out in quartz mini-reactor, at low- and high oxygen content under heating up to 650 °C. The rate of sample heating/cooling was 65°/minute. Chemical composition of the samples was investigated using X-Ray Fluorescence analysis (XRF). X-Ray Fluorescence analysis of the samples was performed at Thermo ARL Optim’X spectrometer equipped with a Rh-anode X-ray tube of 50 W power, goniometer with three crystals (AX06, PET, LiF 200), and two detectors (FPC, SC). Preparation to XRF analysis included the samples milling to the powder and pellet pressing with boric acid. Measurements were done according to the requirements of OptiQuant software. Magnetic characteristics of initial and obtained samples were determined using magnetometer with Hall sensor. An external magnetic field of magnetometer varied in the range of 0 to ± 0.45 T. Mineral composition of siderite ore and products of its thermal decomposition were determined by the methods of X-Ray Diffraction (XRD) (X-Ray diffractometer DRON-4). The XRD phase diagnostics was performed using [16] by detected d-spacing. Thermogravimetric analysis of siderite ore was carried out in air at a heating rate of 10°/min in the 20−1000 °C range using a Derivatograph Q-1500D (Paulik, Paulik and Erdey, MOM, Budapest) with TG-DTA (differential thermal analysis).
aMgO = 85 − 0, 149⋅Q where aMgO is content of MgO, and Q is Curie temperature. XRD pattern for obtained sample is shown at Fig. 4. In comparison with initial sample, the intensity of siderite peaks with d-spacings (Å): 2.776; 2.134; 1.951; 1.717; 1.508 decreases after the first cycle of thermomagnetic analysis. Characteristic peak of ankerite with d-spacing 2.894 Å is also observed. At the same time, characteristic peaks of magnesiomagnetite (with d-spacings (Å): 2.978; 2.539; 2.103; 1.717; 1.618; 1.487) and magnesiowustite (with dspacings (Å): 2.462; 2.134; 1.508) appeared. One could attribute peaks of magnesiowustite to mineral with the following composition: (MgO)0,432(FeO)0,568 (PDF 77-2368). At the study [13] it was also detected magnesiowustite as final product of thermal decomposition of natural siderite at high vacuum. Thus, we could propose the model of phase transformation of siderite: at the first stage siderite decomposes with formation of phase Mg1−xFexO (0 < x < 1) and carbon dioxide. At the second stage partial oxidation of Fe (II) occurs with formation of magnesiomagnetite Mg1−xFexO·Fe2O3. Further, thermomagnetic analyses of the sample were carried-out at low- and high-oxygen content (Fig. 5). The behavior of the samples depending on different oxygen content differs significantly. During the second cycle of heating/cooling at low-oxygen content the magnetization of the sample continues to increase. Thus, the formation of a magnetic phase continues due to the partial oxidation of magnesiowustite and decomposition of the residual siderite. Curie temperature has not changed significantly compared to the first cycle (Fig. 3). During the second cycle of heating/cooling at high-oxygen content the magnetization of the sample decreases. Curie temperature (328 °C) significantly decreases compared to the first cycle. One could observe two peaks on cooling curve and, therefore, we could conclude formation of two phases in the sample. Subsequent dynamics of change of Curie temperature and magnetization during all 20 cycles of heating/cooling are shown at Fig. 6. During initial 7 cycles of heating/cooling at low-oxygen content the magnetization of the sample increases (Fig. 6a) due to partial oxidation of Fe (II) in magnesiowustite and thermal decomposition of siderite. So, one could conclude, that magnetic phase formation occurs. Obviously, the ratio of Fe (II)+Mg (II)/Fe (III) after 7 cycles is about
3. Results and discussion Chemical composition of siderite ore determined by the method of XRF is shown in Table 1. It was shown by the XRD-data that the initial sample of siderite ore was composed by siderite (FeCO3) and ankerite (Ca(Fe, Mg)(CO3)2) (Fig. 1). Characteristic peaks (d-spacing) (Å) for siderite are shown in Table 2. However, siderite thought to be a complex of iron-magnesium carbonates solid solution series. Characteristic peaks: 2.889 Å and 2.539 Å are typical for both ankerite and dolomite. We attributed these peaks for the phase of ankerite according to the initial chemical composition of the sample. Shift of characteristic peaks of siderite compared with the standard from Powder Diffraction File (PDF) database, probably, could be interpreted by isomorphic substitutions of iron in the structure of siderite by magnesium. It was shown by the DTA method that by sample heating up to 1000 °C the weight loss was equivalent to 37% (Fig. 2). As the analyzed samples consist mainly of carbonate minerals, we could relate such weight change with the loss of CO2. The moderate weight loss (about 2%) in the range 25 – 400 °C is attributed to the residual water loss. The curve of differential thermogravimetric analysis (DTG) points up the most significant change of weight in the range from 460 to 670 °C due to decarbonization. It was observed that start decomposition temperature of investigated siderite (460 °C) is higher than that of pure siderite (350 °C), which is attributed to the presence of Mg impurities in siderite ore [7]. Obtained data correspond with literature data [7,17] for decomposition of natural siderite containing Mg impurities. It was shown that maximum peak at DTG-curve (640 °C) is shifted by almost 100 °C to the area of higher temperatures in comparison with literature data (550 °C) [17]. There is another small endothermic peak at a temperature ~750 °C which we refer to thermal decomposition of ankerite. It was shown on thermogravimetric (TG) curve that contribution of this peak to total weight loss is only 2%. Consequently, the remaining amount of weight loss (33%) can be attributed to siderite. We could suggest that the oxidation can also occur and is observed as exothermic effect on the DTA curve in the temperature range 650–900 °C. Initial sample of siderite ore does not demonstrate magnetic properties and its saturation magnetization was ~0.3 Am2/kg.
Table 1 Chemical composition of siderite ore.
374
Component
wt%
Fe2O3 MgO SiO2 Al2O3 CaO others
27.6 13.5 2.1 1.3 1 <1
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
V.P. Ponomar et al.
Fig. 1. XRD pattern of the initial sample of siderite ore. The numbers correspond to the phases: 1 – siderite; 2 – ankerite.
Fig. 3. The thermomagnetic curve for siderite ore. The arrow points Curie temperature.
Table 2 Characteristic peaks (d-spacing) (Å) for investigated sample and standard d-spacing for siderite (PDF 83-1764). Registered d-spacing for investigated sample
Standard d-spacing for siderite (PDF 83-1764)
3.569 2.76 2.332 2.12 1.953 1.786 1.719 1.498 1.415 1.386
3.5924 2.7926 2.3458 2.1330 1.9641 1.7962 1.7305 1.5059 1.4261 1.3812 Fig. 4. XRD pattern of the obtained sample after the first cycle of thermomagnetic analysis. The numbers correspond to the phases: 1 – siderite; 2 – ankerite; 3 – magnesiowustite; 4 – magnesiomagnetite.
analysis curves one could detect gradual decrease of 440 °C peak and increase of 330 °C peak from 7 to 17 cycle (Fig. 7). This Curie temperature (330 °C) is close to that of magnesioferrite (310 °C). These data explain the unusual shape of thermomagnetic curve for second cycle of heating/cooling at high-oxygen content (Fig. 5b). Obviously, Curie temperature shift is caused by change of ratio Mg (II)/Fe (II) which determines transition of magnesiomagnetite into magnesioferrite. Contradictory literature data about magnesioferrite Curie temperature values could be interpreted by the presence of Fe (II) in the crystal lattice. Thus, one could attribute this mineral to magnesiomagnetite. At high oxygen content Curie temperature decreases already after the second cycle of heating/cooling and does not change during the further cycles (Fig. 6b). At high oxygen content, the oxidation is so rapid, that only final phase could be detected. Thermomagnetic curves after 20 cycles of heating/cooling are shown at Fig. 8. It was shown that shape of curves after 20 cycle of heating/cooling change significantly in comparison with the second cycle. But it was no change observed between heating/cooling curves at 20 cycles at lowand high-oxygen content. Therefore, one could conclude that the final product of thermal decomposition of siderite ore at low- and highoxygen content is the same. The Curie temperature of obtained samples is about 318 °C in both cases that is close to Curie temperature of magnesioferrite (310 °C) [18]. Magnetization of the samples is not very different. Saturation magnetization, determined for obtained sample was ~20 Am2/kg (magnetization curve is shown at Fig. 9), that is close to saturation magnetization of pure magnesioferrite (27 Am2/kg) [18].
Fig. 2. Differential thermal analysis of siderite ore.
½. Namely, formation of stoichiometric spinel occurs. Subsequent decrease of magnetization happens due to further oxidation of Fe (II) to Fe (III) in magnesiomagnetite. Oxidation of Fe (II) to Fe (III) at high oxygen content happens during the initial three cycles of heating/cooling. No changes of magnetization (Fig. 6a) were observed during further cycles of heating/cooling. Curie temperature gradually decreases within initial 11 cycles of heating/cooling at low-oxygen content from 508 to 440 °C (Fig. 6b). Thus, we could assume that the ratio Mg (II)/Fe (II) increases due to oxidation of Fe (II). Further, the sharp decrease of Curie temperature from 440 to 330 °C is observed. At differential thermomagnetic
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Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
V.P. Ponomar et al.
Fig. 5. The second cycle of sample heating/cooling at low- (a) and high-oxygen content (b). The arrows point Curie temperatures.
Fig. 6. The changes of magnetization (a) and Curie temperature (b) during 20 cycles of heating/cooling of sample (1 – at low-oxygen content: 2 – at high-oxygen content).
Authors [15] also studied thermal decomposition of natural manganese-bearing siderite in air and show that at the first stages of siderite decomposition the hematite appears. The final product was spinel structure (manganese spinel) with unit cell parameters intermediate between those of magnetite and maghemite. It was shown that the presence of Mn in the crystal lattice lowers the Curie temperature and explains the stability of the spinel phase. Results of thermal decomposition of natural siderite in air atmosphere were published in [6] and it was shown that at 490–510 °C the main phases were siderite and magnetite, the phases of maghemite with admixtures of hematite were observed at the temperature of 580 and at 690 °C the main phase was hematite. No phase of wustite was observed. In our case, hematite was not registered, but the intermediate phase was magnesiowustite. We also could conclude that magnesium stabilizes spinel phase. Calculated unit cell parameter of final spinel phase was a=8.3966 which is close to unit cell parameter of magnesioferrite (a=8.397, PDF 17-0465). Fig. 7. Differential thermomagnetic analysis of sample at low-oxygen content (cooling curves). The number indicates cooling cycle.
4. Conclusions
The sample was a soft magnetic material due to the very low remanent magnetization and coercivity. The result of XRD-measurements of obtained sample after 20th cycles of heating/cooling is shown at Fig. 10. In comparison with the XRD pattern, obtained after the first cycle of thermomagnetic analysis (Fig. 4), peaks of siderite and magnesiowustite were not observed. Ankerite was not decomposed at given experimental conditions. The d-spacing for characteristic peaks of obtained and standard samples (Å) are shown at Table 3.
Thermal decomposition of Bakal siderite ore in low-oxygen condition leads to formation of magnetic phase with Curie temperature ~500 °C, which is typical for solid solution series of magnetitemagnesioferrite with ~9% MgO. Formation of magnetic phase occurs above the temperature of 500 °C. Herewith, phases of magnesiowustite and magnesiomagnetite were detected using XRD. The obtained sample was investigated by thermomagnetic analysis at low- and high-oxygen content. At low-oxygen content magnetization of the sample increases during initial cycles of heating/cooling due to 376
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
V.P. Ponomar et al.
Fig. 8. The thermomagnetic curves of sample (20th cycles of heating/cooling) at low- (a) and high-oxygen content (b).
partial oxidation of Fe (II) in magnesiowustite and siderite. During further cycles of heating/cooling magnetization of the sample decreases due to the oxidation of Fe (II) in magnesiomagnetite. Curie temperature gradually decreases from 500 to 440 °C during initial cycles of heating/cooling due to oxidation of Fe (II) in spinel structure and increase of Mg (II)/Fe (II) ratio. Further, heating/cooling leads to disappearing of phases with Curie temperature ~440 °C and formation of the phase with Curie temperature ~330 °C. We could attribute this change to transition of magnesiomagnetite into magnesioferrite. At high-oxygen content the decrease of Curie temperature and magnetization occurs rapidly. Saturation magnetization of obtained samples was ~20 Am2/kg, which is close to saturation magnetization of pure magnesioferrite. XRD pattern of final samples shows the presence of spinel phases, which we could attribute to magnesioferrite phase. We could conclude that magnesium stabilize spinel phase, even at high-oxygen content and no hematite phase was detected.
Fig. 9. Magnetization curve for obtained sample.
References [1] Q.J. Fisher, R. Raiswell, J.D. Marshall, Siderite concretions from nonmarine shales (Westphalian A) of the Pennines, England: controls on their growth and composition, J. Sediment. Res. 68 (5) (1998) 1034–11045. [2] T. Frederichs, T. von Dobeneck, U. Bleil, M.J. Dekkers, Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties, Phys. Chem. Earth 28 (16–19) (2003) 669–679. [3] I.S. Jacobs, Metamagnetism of siderite (FeCO3), J. Appl. Phys. 34 (1963) 1106–1107. [4] Y. Wu, Y. Li, X. Yang, P. Zhang, Z. Bao, The reduction mechanism of biomass roasting of goethite ores, Adv. Mater. Res. 560–561 (2012) 441–446. [5] S. Ellid, Y.S. Murayed, M.S. Zoto, S. Musiс, S. Popoviс, Chemical reduction of hematite with starch, J. Radioanal. Nucl. Chem. 258 (2) (2003) 299–305. [6] Y. Pan, R. Zhu, S.K. Banerjee, J. Gill, Q. Williams, Rock magnetic properties related to thermal treatment of siderite: behavior and interpretation, J. Geophys. Res. 105 (2000) 783–794. [7] A.P. Dhupe, A.N. Gokarn, Studies in the thermal decomposition of natural siderites in the presence of air, Int. J. Miner. Process. 28 (1990) 209–220. [8] T.M. McCollom, Formation of meteorite hydrocarbons from thermal decomposition of siderite (FeCO3), Geochim. Cosmochim. Acta 67 (2003) 311–317. [9] K.L. Thomas-Keprta, D.A. Bazylinski, J.L. Kirschvink, S.J. Clemett, D.S. McKay, S.J. Wentworth, H. Vali Jr, E.K. Gibson, C.S. Romanek, Elongated prismatic magnetite crystals in ALH84001 carbonate globules: potential martian magnetofossils, Geochim. Cosmochim. Acta 64 (2000) 4049–4081. [10] D.S. McKay, E.K. Gibson Jr., K.L. Thomas-Keprta, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F Chillier, C.R. Maechling, R.N. Zare, Search for past life on Mars: Possible relic biogenic activity in martian meteonte ALH84001, Science, 273, 1996, pp. 924–930. [11] P.K. Gallagher, S.St.J. Warne, Thermomagnetometry and thermal decomposition of siderite, Thermochim. Acta 43 (3) (1981) 253–267. [12] S. Shaoxian, J. Feifei, P. Changsheng, Study on Decomposition of Goethite/siderite in Thermal Modification Through XRD, SEM and TGA Measurements, Surf. Rev. Lett. 21 (1) (2014) 1450019-1–1450019-6. [13] F.J. Gotor, M. Macias, A. Ortega, J.M. Criado, Comparative study of the kinetics of the thermal decomposition of synthetic and natural siderite samples, Phys. Chem.
Fig. 10. XRD pattern of the obtained sample after the 20th cycle of thermomagnetic analysis. The numbers correspond to the phases: 1 – ankerite; 2 – magnesioferrite. Table 3 Characteristic peaks (d-spacing) (Å) for obtained sample (magnesioferrite) and standard d-spacing for synthetic magnesioferrite (PDF 17-0465). Registered d-spacing for obtained sample (magnesioferrite)
Standard d-spacing for synthetic magnesioferrite (PDF 17-0465)
2.969 2.532 2.099 1.711 1.616 1.484
2.969 2.532 2.099 1.713 1.616 1.485
377
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
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X-ray Diffraction Data, Philadelphia, 1946–1969. [17] D. Alkaç, Ü. Atalay, Kinetics of thermal decomposition of Hekimhan–Deveci siderite ore samples, Int. J. Miner. Process. 87 (34–) (2008) 120–128. [18] G.P. Kudryavtseva, Ferrimagnetism of Natural Oxides (in Russian), Nedra, Moscow, 1988.
Miner. 27 (2000) 495–503. [14] P.K. Gallagher, S.St.J. Warne, Application of thermomagnetometry to the study of siderite, Mater. Res. Bull. 16 (2) (1981) 141. [15] A. Isambert, J.-P. Valet, A. Gloter, F. Guyot, Stable Mn-magnetite derived from Mnsiderite by heating in air, J. Geophys. Res. 108 (B6) (2003) 2283. [16] ASTM, Diffraction Data Cards and Alphabetical and Grouped Numerical Index of
378
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
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Phase transformations of siderite ore by the thermomagnetic analysis data ⁎
V.P. Ponomar , N.O. Dudchenko, A.B. Brik M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine, Kyiv, Ukraine
A R T I C L E I N F O
A BS T RAC T
Keywords: Siderite ore Thermal decomposition Thermomagnetic analysis Curie temperature Phase transformations
Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650 °C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was ~320 °C. Saturation magnetization of obtained samples increases up to 20 Am2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite.
1. Introduction Siderite is a mineral of calcite group with chemical formula FeCO3. It is a widespread iron mineral which finds application in various industries (iron industry, mining, etc.). Natural samples of siderite often show a significant amount of substitution of iron in the lattice by magnesium, calcium, manganese [1]. The pure siderite is rarely found. Siderite is commonly found in hydrothermal veins. It may be also deposited by sedimentary processes. Siderite crystallizes in the trigonal crystal system, and is rhombohedral in shape, typically with curved and striated faces. The color of siderite ranges from yellow to dark brown or black, the latter being due to the presence of manganese. This is an antiferromagnetic mineral with Neel temperature of 38 K, which is paramagnetic at ambient temperature [2,3]. Moreover, metamagnetism was reported to be magnetic property of siderite, namely, antiferromagnetic lattice gradually transforming into a ferromagnetic lattice when exposed to strong magnetic fields of 12–14 T. Nowadays, the usage of weakly magnetic iron ores for iron production is of very importance, because the deposits of magnetite iron ores are becoming exhausted. The main problem that interferes the usage of weakly magnetic siderite ores for iron production is complexity of their beneficiation, i.e. separation of siderite from other admixtures in the ore. In this case, thermal decomposition of the mineral siderite is the topic of interest because of possibility of strongly magnetic phase formation without any reducing agents in contradistinction to goethite and hematite [4,5]. Thermal decomposition of siderite in different atmospheres (air, ⁎
oxygen, nitrogen, etc.) is widely used for different applications. For example, thermal decomposition characteristics of siderite are utilized in paleomagnetic studies [6]. Interest for such investigation is caused by transformation of siderite into magnetite and maghemite which can potentially carry a chemical remanent magnetization. As far as roasting of siderite yields to high pore volume and high surface area species, the end products also could be used to capture the sulfur dioxide (SO2) in a wide temperature range. The decomposition product of siderite is also used as a natural brown pigment in the paint industry [7]. Besides, magnetite is present in the ALH84001 Martian meteorite thought to be produced by thermal decomposition of siderite [8]. However, second possibility of magnetite presence in the ALH84001 is of biogenic origin [9,10]. The final products of thermal decomposition of mineral siderite are generally hematite in an oxidizing atmosphere [11,12], magnetite in a carbon dioxide atmosphere, and magnetite and wustite in an inert atmosphere or in vacuum [13,14]. Identification of decomposition products of natural siderite is difficult due to the significant amount of substitution of iron in the lattice by magnesium, calcium, manganese [15]. The aim of present work was to investigate the thermal decomposition of siderite ore of Bakal's deposits at low- and high-oxygen content by the method of thermomagnetic analysis. 2. Materials and methods The samples of Bakal siderite ore (the western slope of the Southern Urals) that was grinded up to 0.07 mm were investigated.
Correspondence to: 34 Palladina Prospect, Kyiv, 03680, Ukraine. E-mail address: [email protected] (V.P. Ponomar).
http://dx.doi.org/10.1016/j.jmmm.2016.09.124 Received 11 March 2016; Received in revised form 20 September 2016; Accepted 27 September 2016 Available online 28 September 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 423 (2017) 373–378
V.P. Ponomar et al.
Phase transformations of siderite ore were studied by thermomagnetic analysis (up to 600 °C) at low-oxygen content (Fig. 3). There is no peak on heating curve, while magnetization of the sample increases considerably after cooling. So, we could conclude that magnetic phase formation occurs at temperatures above Curie temperature of obtained sample. Authors [6] report that formation of magnetic phase is already observed at the thermomagnetic curves at 400 °C with Curie temperature approximately 580 °C. We haven’t observed this in our experiments. Moreover, Curie temperature determined by differentiating the cooling curve is 508 °C. This value is slightly lower than Curie temperature of pure magnetite (580 °C). Measured Curie temperature is typical for series of solid solutions of magnetite – magnesioferrite (magnesiomagnetite) with ~9% MgO according to equation of Curie temperature dependence on magnesium content [18]:
Thermomagnetic analysis was performed with laboratory-built facility that allows automatic registration of sample magnetization as a function of temperature. Phase transformations of siderite ore was carried out in quartz mini-reactor, at low- and high oxygen content under heating up to 650 °C. The rate of sample heating/cooling was 65°/minute. Chemical composition of the samples was investigated using X-Ray Fluorescence analysis (XRF). X-Ray Fluorescence analysis of the samples was performed at Thermo ARL Optim’X spectrometer equipped with a Rh-anode X-ray tube of 50 W power, goniometer with three crystals (AX06, PET, LiF 200), and two detectors (FPC, SC). Preparation to XRF analysis included the samples milling to the powder and pellet pressing with boric acid. Measurements were done according to the requirements of OptiQuant software. Magnetic characteristics of initial and obtained samples were determined using magnetometer with Hall sensor. An external magnetic field of magnetometer varied in the range of 0 to ± 0.45 T. Mineral composition of siderite ore and products of its thermal decomposition were determined by the methods of X-Ray Diffraction (XRD) (X-Ray diffractometer DRON-4). The XRD phase diagnostics was performed using [16] by detected d-spacing. Thermogravimetric analysis of siderite ore was carried out in air at a heating rate of 10°/min in the 20−1000 °C range using a Derivatograph Q-1500D (Paulik, Paulik and Erdey, MOM, Budapest) with TG-DTA (differential thermal analysis).
aMgO = 85 − 0, 149⋅Q where aMgO is content of MgO, and Q is Curie temperature. XRD pattern for obtained sample is shown at Fig. 4. In comparison with initial sample, the intensity of siderite peaks with d-spacings (Å): 2.776; 2.134; 1.951; 1.717; 1.508 decreases after the first cycle of thermomagnetic analysis. Characteristic peak of ankerite with d-spacing 2.894 Å is also observed. At the same time, characteristic peaks of magnesiomagnetite (with d-spacings (Å): 2.978; 2.539; 2.103; 1.717; 1.618; 1.487) and magnesiowustite (with dspacings (Å): 2.462; 2.134; 1.508) appeared. One could attribute peaks of magnesiowustite to mineral with the following composition: (MgO)0,432(FeO)0,568 (PDF 77-2368). At the study [13] it was also detected magnesiowustite as final product of thermal decomposition of natural siderite at high vacuum. Thus, we could propose the model of phase transformation of siderite: at the first stage siderite decomposes with formation of phase Mg1−xFexO (0 < x < 1) and carbon dioxide. At the second stage partial oxidation of Fe (II) occurs with formation of magnesiomagnetite Mg1−xFexO·Fe2O3. Further, thermomagnetic analyses of the sample were carried-out at low- and high-oxygen content (Fig. 5). The behavior of the samples depending on different oxygen content differs significantly. During the second cycle of heating/cooling at low-oxygen content the magnetization of the sample continues to increase. Thus, the formation of a magnetic phase continues due to the partial oxidation of magnesiowustite and decomposition of the residual siderite. Curie temperature has not changed significantly compared to the first cycle (Fig. 3). During the second cycle of heating/cooling at high-oxygen content the magnetization of the sample decreases. Curie temperature (328 °C) significantly decreases compared to the first cycle. One could observe two peaks on cooling curve and, therefore, we could conclude formation of two phases in the sample. Subsequent dynamics of change of Curie temperature and magnetization during all 20 cycles of heating/cooling are shown at Fig. 6. During initial 7 cycles of heating/cooling at low-oxygen content the magnetization of the sample increases (Fig. 6a) due to partial oxidation of Fe (II) in magnesiowustite and thermal decomposition of siderite. So, one could conclude, that magnetic phase formation occurs. Obviously, the ratio of Fe (II)+Mg (II)/Fe (III) after 7 cycles is about
3. Results and discussion Chemical composition of siderite ore determined by the method of XRF is shown in Table 1. It was shown by the XRD-data that the initial sample of siderite ore was composed by siderite (FeCO3) and ankerite (Ca(Fe, Mg)(CO3)2) (Fig. 1). Characteristic peaks (d-spacing) (Å) for siderite are shown in Table 2. However, siderite thought to be a complex of iron-magnesium carbonates solid solution series. Characteristic peaks: 2.889 Å and 2.539 Å are typical for both ankerite and dolomite. We attributed these peaks for the phase of ankerite according to the initial chemical composition of the sample. Shift of characteristic peaks of siderite compared with the standard from Powder Diffraction File (PDF) database, probably, could be interpreted by isomorphic substitutions of iron in the structure of siderite by magnesium. It was shown by the DTA method that by sample heating up to 1000 °C the weight loss was equivalent to 37% (Fig. 2). As the analyzed samples consist mainly of carbonate minerals, we could relate such weight change with the loss of CO2. The moderate weight loss (about 2%) in the range 25 – 400 °C is attributed to the residual water loss. The curve of differential thermogravimetric analysis (DTG) points up the most significant change of weight in the range from 460 to 670 °C due to decarbonization. It was observed that start decomposition temperature of investigated siderite (460 °C) is higher than that of pure siderite (350 °C), which is attributed to the presence of Mg impurities in siderite ore [7]. Obtained data correspond with literature data [7,17] for decomposition of natural siderite containing Mg impurities. It was shown that maximum peak at DTG-curve (640 °C) is shifted by almost 100 °C to the area of higher temperatures in comparison with literature data (550 °C) [17]. There is another small endothermic peak at a temperature ~750 °C which we refer to thermal decomposition of ankerite. It was shown on thermogravimetric (TG) curve that contribution of this peak to total weight loss is only 2%. Consequently, the remaining amount of weight loss (33%) can be attributed to siderite. We could suggest that the oxidation can also occur and is observed as exothermic effect on the DTA curve in the temperature range 650–900 °C. Initial sample of siderite ore does not demonstrate magnetic properties and its saturation magnetization was ~0.3 Am2/kg.
Table 1 Chemical composition of siderite ore.
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Component
wt%
Fe2O3 MgO SiO2 Al2O3 CaO others
27.6 13.5 2.1 1.3 1 <1
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Fig. 1. XRD pattern of the initial sample of siderite ore. The numbers correspond to the phases: 1 – siderite; 2 – ankerite.
Fig. 3. The thermomagnetic curve for siderite ore. The arrow points Curie temperature.
Table 2 Characteristic peaks (d-spacing) (Å) for investigated sample and standard d-spacing for siderite (PDF 83-1764). Registered d-spacing for investigated sample
Standard d-spacing for siderite (PDF 83-1764)
3.569 2.76 2.332 2.12 1.953 1.786 1.719 1.498 1.415 1.386
3.5924 2.7926 2.3458 2.1330 1.9641 1.7962 1.7305 1.5059 1.4261 1.3812 Fig. 4. XRD pattern of the obtained sample after the first cycle of thermomagnetic analysis. The numbers correspond to the phases: 1 – siderite; 2 – ankerite; 3 – magnesiowustite; 4 – magnesiomagnetite.
analysis curves one could detect gradual decrease of 440 °C peak and increase of 330 °C peak from 7 to 17 cycle (Fig. 7). This Curie temperature (330 °C) is close to that of magnesioferrite (310 °C). These data explain the unusual shape of thermomagnetic curve for second cycle of heating/cooling at high-oxygen content (Fig. 5b). Obviously, Curie temperature shift is caused by change of ratio Mg (II)/Fe (II) which determines transition of magnesiomagnetite into magnesioferrite. Contradictory literature data about magnesioferrite Curie temperature values could be interpreted by the presence of Fe (II) in the crystal lattice. Thus, one could attribute this mineral to magnesiomagnetite. At high oxygen content Curie temperature decreases already after the second cycle of heating/cooling and does not change during the further cycles (Fig. 6b). At high oxygen content, the oxidation is so rapid, that only final phase could be detected. Thermomagnetic curves after 20 cycles of heating/cooling are shown at Fig. 8. It was shown that shape of curves after 20 cycle of heating/cooling change significantly in comparison with the second cycle. But it was no change observed between heating/cooling curves at 20 cycles at lowand high-oxygen content. Therefore, one could conclude that the final product of thermal decomposition of siderite ore at low- and highoxygen content is the same. The Curie temperature of obtained samples is about 318 °C in both cases that is close to Curie temperature of magnesioferrite (310 °C) [18]. Magnetization of the samples is not very different. Saturation magnetization, determined for obtained sample was ~20 Am2/kg (magnetization curve is shown at Fig. 9), that is close to saturation magnetization of pure magnesioferrite (27 Am2/kg) [18].
Fig. 2. Differential thermal analysis of siderite ore.
½. Namely, formation of stoichiometric spinel occurs. Subsequent decrease of magnetization happens due to further oxidation of Fe (II) to Fe (III) in magnesiomagnetite. Oxidation of Fe (II) to Fe (III) at high oxygen content happens during the initial three cycles of heating/cooling. No changes of magnetization (Fig. 6a) were observed during further cycles of heating/cooling. Curie temperature gradually decreases within initial 11 cycles of heating/cooling at low-oxygen content from 508 to 440 °C (Fig. 6b). Thus, we could assume that the ratio Mg (II)/Fe (II) increases due to oxidation of Fe (II). Further, the sharp decrease of Curie temperature from 440 to 330 °C is observed. At differential thermomagnetic
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Fig. 5. The second cycle of sample heating/cooling at low- (a) and high-oxygen content (b). The arrows point Curie temperatures.
Fig. 6. The changes of magnetization (a) and Curie temperature (b) during 20 cycles of heating/cooling of sample (1 – at low-oxygen content: 2 – at high-oxygen content).
Authors [15] also studied thermal decomposition of natural manganese-bearing siderite in air and show that at the first stages of siderite decomposition the hematite appears. The final product was spinel structure (manganese spinel) with unit cell parameters intermediate between those of magnetite and maghemite. It was shown that the presence of Mn in the crystal lattice lowers the Curie temperature and explains the stability of the spinel phase. Results of thermal decomposition of natural siderite in air atmosphere were published in [6] and it was shown that at 490–510 °C the main phases were siderite and magnetite, the phases of maghemite with admixtures of hematite were observed at the temperature of 580 and at 690 °C the main phase was hematite. No phase of wustite was observed. In our case, hematite was not registered, but the intermediate phase was magnesiowustite. We also could conclude that magnesium stabilizes spinel phase. Calculated unit cell parameter of final spinel phase was a=8.3966 which is close to unit cell parameter of magnesioferrite (a=8.397, PDF 17-0465). Fig. 7. Differential thermomagnetic analysis of sample at low-oxygen content (cooling curves). The number indicates cooling cycle.
4. Conclusions
The sample was a soft magnetic material due to the very low remanent magnetization and coercivity. The result of XRD-measurements of obtained sample after 20th cycles of heating/cooling is shown at Fig. 10. In comparison with the XRD pattern, obtained after the first cycle of thermomagnetic analysis (Fig. 4), peaks of siderite and magnesiowustite were not observed. Ankerite was not decomposed at given experimental conditions. The d-spacing for characteristic peaks of obtained and standard samples (Å) are shown at Table 3.
Thermal decomposition of Bakal siderite ore in low-oxygen condition leads to formation of magnetic phase with Curie temperature ~500 °C, which is typical for solid solution series of magnetitemagnesioferrite with ~9% MgO. Formation of magnetic phase occurs above the temperature of 500 °C. Herewith, phases of magnesiowustite and magnesiomagnetite were detected using XRD. The obtained sample was investigated by thermomagnetic analysis at low- and high-oxygen content. At low-oxygen content magnetization of the sample increases during initial cycles of heating/cooling due to 376
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Fig. 8. The thermomagnetic curves of sample (20th cycles of heating/cooling) at low- (a) and high-oxygen content (b).
partial oxidation of Fe (II) in magnesiowustite and siderite. During further cycles of heating/cooling magnetization of the sample decreases due to the oxidation of Fe (II) in magnesiomagnetite. Curie temperature gradually decreases from 500 to 440 °C during initial cycles of heating/cooling due to oxidation of Fe (II) in spinel structure and increase of Mg (II)/Fe (II) ratio. Further, heating/cooling leads to disappearing of phases with Curie temperature ~440 °C and formation of the phase with Curie temperature ~330 °C. We could attribute this change to transition of magnesiomagnetite into magnesioferrite. At high-oxygen content the decrease of Curie temperature and magnetization occurs rapidly. Saturation magnetization of obtained samples was ~20 Am2/kg, which is close to saturation magnetization of pure magnesioferrite. XRD pattern of final samples shows the presence of spinel phases, which we could attribute to magnesioferrite phase. We could conclude that magnesium stabilize spinel phase, even at high-oxygen content and no hematite phase was detected.
Fig. 9. Magnetization curve for obtained sample.
References [1] Q.J. Fisher, R. Raiswell, J.D. Marshall, Siderite concretions from nonmarine shales (Westphalian A) of the Pennines, England: controls on their growth and composition, J. Sediment. Res. 68 (5) (1998) 1034–11045. [2] T. Frederichs, T. von Dobeneck, U. Bleil, M.J. Dekkers, Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties, Phys. Chem. Earth 28 (16–19) (2003) 669–679. [3] I.S. Jacobs, Metamagnetism of siderite (FeCO3), J. Appl. Phys. 34 (1963) 1106–1107. [4] Y. Wu, Y. Li, X. Yang, P. Zhang, Z. Bao, The reduction mechanism of biomass roasting of goethite ores, Adv. Mater. Res. 560–561 (2012) 441–446. [5] S. Ellid, Y.S. Murayed, M.S. Zoto, S. Musiс, S. Popoviс, Chemical reduction of hematite with starch, J. Radioanal. Nucl. Chem. 258 (2) (2003) 299–305. [6] Y. Pan, R. Zhu, S.K. Banerjee, J. Gill, Q. Williams, Rock magnetic properties related to thermal treatment of siderite: behavior and interpretation, J. Geophys. Res. 105 (2000) 783–794. [7] A.P. Dhupe, A.N. Gokarn, Studies in the thermal decomposition of natural siderites in the presence of air, Int. J. Miner. Process. 28 (1990) 209–220. [8] T.M. McCollom, Formation of meteorite hydrocarbons from thermal decomposition of siderite (FeCO3), Geochim. Cosmochim. Acta 67 (2003) 311–317. [9] K.L. Thomas-Keprta, D.A. Bazylinski, J.L. Kirschvink, S.J. Clemett, D.S. McKay, S.J. Wentworth, H. Vali Jr, E.K. Gibson, C.S. Romanek, Elongated prismatic magnetite crystals in ALH84001 carbonate globules: potential martian magnetofossils, Geochim. Cosmochim. Acta 64 (2000) 4049–4081. [10] D.S. McKay, E.K. Gibson Jr., K.L. Thomas-Keprta, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F Chillier, C.R. Maechling, R.N. Zare, Search for past life on Mars: Possible relic biogenic activity in martian meteonte ALH84001, Science, 273, 1996, pp. 924–930. [11] P.K. Gallagher, S.St.J. Warne, Thermomagnetometry and thermal decomposition of siderite, Thermochim. Acta 43 (3) (1981) 253–267. [12] S. Shaoxian, J. Feifei, P. Changsheng, Study on Decomposition of Goethite/siderite in Thermal Modification Through XRD, SEM and TGA Measurements, Surf. Rev. Lett. 21 (1) (2014) 1450019-1–1450019-6. [13] F.J. Gotor, M. Macias, A. Ortega, J.M. Criado, Comparative study of the kinetics of the thermal decomposition of synthetic and natural siderite samples, Phys. Chem.
Fig. 10. XRD pattern of the obtained sample after the 20th cycle of thermomagnetic analysis. The numbers correspond to the phases: 1 – ankerite; 2 – magnesioferrite. Table 3 Characteristic peaks (d-spacing) (Å) for obtained sample (magnesioferrite) and standard d-spacing for synthetic magnesioferrite (PDF 17-0465). Registered d-spacing for obtained sample (magnesioferrite)
Standard d-spacing for synthetic magnesioferrite (PDF 17-0465)
2.969 2.532 2.099 1.711 1.616 1.484
2.969 2.532 2.099 1.713 1.616 1.485
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