Carbothermal reduction of low-grade pyrolusite by microwave heating

Abstract

Pyrolusite was carbothermally reduced using coal by microwave heating, and the crystal structures and microstructures of the samples were characterized after microwave heating using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). When the reductants proportion was of 10 percent, the reduction temperature was of 800 °C and the holding time was of 40 minutes, the reduction ratio of MnO₂ to MnO was 97.2%, and Fe₂O₃ was almost completely transformed to Fe₃O₄ and there was no Fe(II) produced. Moreover, it was found that the low-grade pyrolusite was carbothermally reduced using microwave heating with lower temperature and shorter processing time. These results show that microwave heating can be applied effectively and efficiently to the carbothermal reduction processes of low-grade pyrolusite.

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1. Introduction

With the extensive exploitation and consumption of high grade manganese ore (total Mn >35%),¹ considerable attention is focused on the development and utilization of the low grade (total Mn <35%) manganese ore, which has a low business value.^2,3

Pyrolusite is usually formed by MnO₂ and other oxides such as SiO₂ and Fe₂O₃.^4,5 It is granular, fibrous or columnar-like, which are good properties for absorbing microwave energy.⁶ Pyrolusite is also a significant strategic resource and a key material for the production of MnO₂ and Mn. MnO₂ has many polymorphic forms such as α-, β-, γ- and ε- type.⁷ It is widely used in the fields of catalysis, ion sieve, electrode materials of Li/MnO₂ and semiconductors.^8–14 Manganese is essential to iron and steel production because of its sulfur-fixing, deoxidizing, and alloying properties. Manganese from these ores can be extracted selectively using hydrometallurgical techniques.

One thing is to pre-reduce pyrolusite at high temperature (1000–1350 °C).^15–18 The reductants are used as natural gas and methane gas, which contains H₂, CO, CH₄, and carbonaceous materials like coal, wood-charcoal and graphite.^19–27 Firstly, MnO₂ in the pyrolusite is reduced to MnO, and then leached using a hot acid solution, and Mn²⁺ is obtained from the leaching solution. The leaching solution containing Mn²⁺ and Fe²⁺ needs to be purified and electrolyzed, and then the high quality electrolytic MnO₂ is acquired. Although this method is well established, it is time-consuming and requires a large amount of energy (2–4 h).²⁸ Secondly, the current research hotspot is the direct reductive leaching method in a water solution. The reductants are oxalic acid,²⁹ pyrite,³⁰ aqueous sulfur dioxide,³¹ iron powder,³² iron(II) sulfate,³³ hydrogen peroxide,³⁴ organic biomass reductants and bio-battery.^35–37 To leach pyrolusite, pyrolusite and another mineral are mixed in an acid solution, by which the leaching solution containing Mn²⁺ and Fe²⁺ is obtained. The advantage of this method is the short process path, low energy-consumption and short processing cycle, but purification of the leaching solution is very difficult because of its complex chemical constituents.

Microwave heating has many characteristics, such as selective, uniform and fast heating, no pollution, low equipment cost, very fast reaction speed and high product yields.³⁸ It can successfully avoid the disadvantages of traditional heating methods, such as large temperature gradient, long processing period, low heating efficiency, high energy consumption and high pollution industries.^39,40

In this study, we try to make use of the advantage of microwave heating to solve the current problems (high temperature, high energy consumption and high reductants ratio) in the pre-reduction process of pyrolusite. The influence of reaction temperature and holding time on the reduction of pyrolusite were systematically investigated, and the aim is to determine a low temperature and a short period process for the reduction of pyrolusite.

2. Experimental

2.1. Materials

The chemical composition of the low-grade pyrolusite is presented in Table 1, and the compositions of the coal are shown in Table 2. The particle size distribution of the pyrolusite and coal used are presented in Fig. 1, and all these materials were milled by a ball grinding mill.

Table 1 Compositions of the pyrolusite (Total manganese 27.51 percentages, mass%)

MnO2	Mn3O4	MnO	Fe2O3	Fe3O4	SiO2	Al2O3	K2O	CaO	BaO
41.00	1.67	0.51	11.97	1.15	36.73	3.66	0.86	0.82	0.38

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P2O5	MgO	TiO2	SO3	Co2O3	NiO	ZnO	SrO	CuO	Y2O3
0.38	0.36	0.16	0.11	0.06	0.05	0.05	0.04	0.02	0.01

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Table 2 Compositions of the coal (mass%)

Fixed carbon	Volatile organic matter	H2O	SiO2	Al2O3	SO3	Fe2O3	TiO2
67.58	10.89	4.67	7.19	3.55	2.55	1.83	0.76

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CaO	K2O	MgO	MnO	ZrO2	P2O5	Cr2O3	SrO
0.63	0.19	0.08	0.02	0.02	0.02	0.01	0.01

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Fig. 1 Particle size distributions of pyrolusite and coal.

2.2. Instruments

A self-made microwave tube furnace, which utilizes a single-mode continuous controllable power was utilized for all experiments and are shown in Fig. 2. The microwave frequency was 2.45 GHz, whereas the output power was controlled within the maximum of 3000 W. The activation temperature was controlled by varying the input microwave power and measured by a nickel chrome–nickel silicon armor type thermocouple, which was in contact with the materials. The dimensions of the thermocouple were 1000 mm in length, 3 mm in diameter with the temperature range of 0–1250 °C, and a measurement precision up to ±0.5 °C.

Fig. 2 Diagram of microwave tube furnace.

2.3. Methods

The pyrolusite powder and the coal were thoroughly mixed in an agate mortar and placed into the corundum crucible, which has a temperature test hole at the waist, and the mixed material surface was covered by a layer of coal powder (about 1 g), and the reductants proportion was 10%. The mixed materials were placed inside a microwave heating reactor. The schematic of the microwave reactor is shown Fig. 2. The cavity of the microwave heating reactor was filled with nitrogen by a gas cylinder for 10 min. Then, the carbothermal reduction experiments were started according to the process.

Reduction ratios of Mn and the valence state of iron were chosen as independent research factors. At the end of the experiment, the reduction ratio of Mn was calculated based on the following equation:

where M₁ is the mass of Mn, M₂ is the mass of Mn(II), and the determination of degree of reduction using ferrous ammonium sulfate as redox indicators.

2.4. Characterization

After microwave heating, the phase transitions of the pyrolusite were identified using XRD technology (D/Max 2200, Rigaku, Japan). XRD patterns were recorded using Rigaku diffractometer with CuKα radiation and a Ni filter operated at the voltage of 35 kV, anode current of 20 mA and a scanning rate of 0.25° min⁻¹, respectively. The microstructure morphology of the pyrolusite and the microwave heat treated samples were investigated by scanning electron microscopy (SEM). The SEM instrument (XL30ESEM-TMP, Philips, Holland) was operated at 20 kV in a low vacuum while the energy dispersion scanner spectrometer (EDX, USA) attached to the SEM was used for semi-quantitative chemical analysis.

3. Results and discussion

Fig. 3 shows the thermodynamics graph of the directly reduced manganese and iron oxide. The main chemical reductions can be calculated by the following equations:

2MnO₂ + C = Mn₂O₃ + CO(g) (2)

3Mn₂O₃ + C = 2Mn₃O₄ + CO(g) (3)

Mn₃O₄ + C = 3MnO + CO(g) (4)

MnO + C = Mn + CO(g) (5)

2MnO₂ + C = Mn₂O₃ + CO(g) (6)

3Fe₂O₃ + C = 2Fe₃O₄ + CO(g) (7)

Fe₃O₄ + C = 3FeO + CO(g) (8)

FeO + C = Fe + CO(g) (9)

Fig. 3 Thermodynamics graph of direct reducing manganese and iron oxide.

It can be seen in Fig. 3 that the reactions of MnO₂, Mn₂O₃, Mn₃O₄ (eqn (2) and (3)) can be performed at room temperature and the starting reduction temperatures of the reactions, 289.8–719.74 °C, were used further in the research in order to transform Mn_xO_y to MnO.

Mainly carbothermal reductions of the pyrolusite are shown in the following equation, which can be observed from the thermodynamic graph of the manganese and iron oxide by indirect reduction, as shown in Fig. 4.

MnO₂ + CO(g) = MnO + CO₂(g) (10)

3Mn₂O₃ + CO(g) = 2Mn₃O₄ + CO₂(g) (11)

Mn₃O₄ + CO(g) = 3MnO + CO₂(g) (12)

MnO + CO(g) = Mn + CO₂(g) (13)

3Fe₂O₃ + CO(g) = 2Fe₃O + CO₂(g) (14)

Fe₃O₄ + CO(g) = 3FeO + CO₂(g) (15)

FeO + CO(g) = Fe + CO₂(g) (16)

2C + O₂(g) = 2CO(g) (17)

C + CO₂(g) = 2CO(g) (18)

Fig. 4 Thermodynamics graph of reductions of manganese and iron oxide.

Quantitative evaluations of the studied reactions from the perspective of thermodynamics were characterized. It can be seen from Fig. 4 that the reactions of MnO₂, Mn₂O₃, Mn₃O₄, MnO (eqn (10), (11) and (12)) can be achieved at room temperature. The reactions of Fe₂O₃, FeO can be performed at room temperature under standard conditions, and the temperature should be controlled at 575 °C. According to the thermodynamic analysis of Fig. 4, Mn_xO_y can be reduced into MnO and Fe_xO_y is reduced into FeO at room temperature.

The microwave heating curves of the raw material, with a coal proportion of 10%, total material of 50 g and microwave power of 200 W, are characterized and the results are illustrated in Fig. 5. It can be seen from Fig. 5 that the mixed materials could be heated to 800 °C from room temperature in 6 min under the low microwave power density (4 W g⁻¹) with the highest heating rate at 372 °C min⁻¹ and the average heating rate at 165.2 °C min⁻¹. It can be concluded that the mixed pyrolusite powder, which contains coal, has the good characteristic of microwave absorption.

Fig. 5 Microwave heating curve of the raw material with 10% coal.

Fig. 6 shows the relationship between the reduction ratio of Mn (η_Mn) and the holding time (T), when the reductant proportion of pyrolusite is 10%, and the holding time is 40 min. It can be seen from Fig. 6 that the reduction ratio of pyrolusite increases gradually from 16.56% to 97.2% with increase in microwave heating temperature from 400 °C to 800 °C. It can be concluded that with the temperature increasing, the ΔG^Θ value of the reaction MnO₂ → Mn₂O₃ → Mn₃O₄ → MnO is obviously decreasing (Fig. 3 and 4), which indicates that the thermodynamic condition becomes better; moreover, while the temperature is high, the dynamic condition is also markedly improved.

Fig. 6 Influence of reduction temperature on the reduction ratio of pyrolusite.

The relationship between the reduction ratio of Mn (η_Mn) and the holding time (T) of the pyrolusite under microwave heating are obtained, and the results are illustrated in Fig. 7. It can be seen from Fig. 7 that the reduction ratio of Mn increases with the extension of the microwave holding time. After holding for 40 minutes, the reduction ratio increases slowly because of the amount of un-reduced MnO₂ and the remaining reductant are reduced.

Fig. 7 Influence of holding time on the reduction ratio of pyrolusite.

The crystal structures of the raw materials after microwave heating are characterized by XRD, and the results are illustrated in Fig. 8. It can be seen that MnO, Fe₃O₄, MnSiO₄ and SiO₂ are the major phase compositions in the microwave treated samples. In addition, a minor amount of MnSiO₄ is also present. The XRD results of the microwave reduced product show that the XRD patterns of the reference MnO and all peaks match the standard spectra of MnO (JCPDS card no. 89-4835). The strongest and second preferential orientation of (2 0 0) and (1 1 1) planes of the reduced MnO are observed at 2θ = 40.577° and 2θ = 34.950°, respectively. It can be seen from Fig. 8 that the total Mn of raw materials completely transforms to MnO, and Fe₂O₃ is reduced to Fe₃O₄. Moreover, it can be seen from Fig. 6 and 7 that Fe₂O₃ transforms to Fe₃O₄ almost completely at 800 °C (eqn (8) and (15)), and there was no Fe(II) produced.

Fig. 8 XRD of the product prepared after microwave heating at 800 °C for 40 min.

The results also indicate that the low-grade pyrolusite was carbothermally reduced using microwave heating with lower temperature and shorter processing time. The major advantages of using microwave heating for industrial processing are rapid heat transfer, volumetric and selective heating, compactness of equipment, speed of switching on and off and a pollution-free environment.

The pyrolusite before and after microwave reduction at the microwave heating temperature of 800 °C and the holding time of 40 min are characterized by SEM and EDAX techniques, and the result as shown in Fig. 9 and 10, respectively. Compared with the untreated raw materials (Fig. 9(a)), from the SEM in Fig. 9(b), the results indicate that the size distribution of the microwave treated samples is wide (0.3–20 μm), and the product seems to be hard agglomerates, which may have been formed by the sintering of small particles. EDAX analyses of microwave treated pyrolusite are carried out to estimate the elemental composition of microwave heating prepared MnO, and the results are shown in Fig. 10. It was observed from Fig. 10(a) and (b) that the microwave treated product consists of Mn and Fe, and minor amounts of Si, Au, Ca and Al.

Fig. 9 SEM of the raw materials before and after microwave irradiation at 800 °C for 40 min. (a) Raw materials; (b) microwave treated samples.

Fig. 10 EDS of the product prepared after microwave heating at 800 °C for 40 min. (a) SEM of EDAX analysis; (b) EDAX analysis results of the red area.

4. Conclusion

The pyrolusite was microwave pre-reduced using coal at varying heating times. The optimum conditions for experimental parameters of microwave pre-reduced pyrolusite were obtained with the proportion of coal of 10%, process temperature of 800 °C, and holding time of 40 min. Under these optimum conditions, the reduction ratio of MnO₂ to MnO in the pyrolusite was 97.2%. Fe₂O₃ transforms to Fe₃O₄ completely and there was no Fe(II) produced from the microwave heating process. The reduction product was formed by sintering of small particles. Compared with the traditional high temperature (1000–1350 °C) method of pre-reduced pyrolusite, the microwave heating technique had the characteristics such as short time, low temperature, low consumption and high quality. Based on the above mentioned results, this method can be applied effectively and efficiently to the carbothermal reduction processes of low-grade pyrolusite.

Acknowledgements

Financial supports from the Key Projects in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (no. 2015BAB17B00), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20125314120014), the Applied Foundation Fund of Yunnan Province of China (no. 2012FD015), the Yunnan Provincial Science, and Technology Innovation Talents scheme – Technological Leading Talent (no. 2013HA002) and the award for new academic graduate student of Yunnan province are acknowledged.

References

1. R. N. Sahoo, P. K. Naik and S. C. Das, Leaching of manganese from low-grade manganese ore using oxalic acid as reductant in sulphuric acid solution, Hydrometallurgy, 2001, 62(3), 157–163.
2. A. A. Ismail, E. A. Ali, I. A. Ibrahim and M. S. Ahmed, A comparative study on acid leaching of low grade manganese ore using some industrial wastes as reductants, Can. J. Chem. Eng., 2004, 82(6), 1296–1300.
3. Y. Zhang, Z. You, G. Li and T. Jiang, Manganese extraction by sulfur-based reduction roasting–acid leaching from low-grade manganese oxide ores, Hydrometallurgy, 2013, 133, 126–132.
4. K. P. Staudhammer and L. E. Murr, Characterization of natural pyrolusite by electron microscopy, Contrib. Mineral. Petrol., 1974, 45(3), 251–256.
5. W. Zhang and C. Y. Cheng, Manganese metallurgy review. Part I: leaching of ores/secondary materials and recovery of electrolytic/chemical manganese dioxide, Hydrometallurgy, 2007, 89(3), 137–159.
6. J. N. Das, The dielectric and piezoelectric behaviour of pyrolusite (polycrystalline ore of MnO₂), Zeitschrift für Physik, 1959, 155(4), 465–471.
7. W. Xun and L. Y. Dong, Selected-control hydrothermal synthesis of alpha- and beta-MnO₂ single crystal nanowires, J. Am. Chem. Soc., 2002, 124(12), 2880–2881.
8. T. Yamashita and A. Vannice, NO decomposition over Mn₂O₃ and Mn₃O₄, J. Catal., 1996, 163(1), 158–168.
9. T. S. Park, S. K. Jeong, S. H. Hong and S. C. Hong, Selective catalytic reduction of nitrogen oxides with NH₃ over natural manganese ore at low temperature, Ind. Eng. Chem. Res., 2001, 40(21), 4491–4495.
10. A. R. Armstrong and P. G. Bruce, Synthesis of layered LiMnO₂ as an electrode for rechargeable lithium batteries, Nature, 1996, 381(6582), 499–500.
11. M. M. Thackeray, Manganese oxides for lithium batteries, Prog. Solid State Chem., 1997, 25(1), 1–71.
12. N. J. Welham, Activation of the carbothermic reduction of manganese ore, Int. J. Miner. Process., 2002, 67(1), 187–198.
13. B. Ammundsen and J. Paulsen, Novel lithium-ion cathode materials based on layered manganese oxides, Adv. Mater., 2001, 13(12–13), 943–956.
14. J. N. Das, Study of the semi-conducting properties of pyrolusite, Zeitschrift für Physik, 1958, 151(3), 345–350.
15. O. Ostrovski, S. E. Olsen, M. Tangstad and M. Yastreboff, Kinetic modelling of MnO reduction from manganese ore, Can. Metall. Q., 2002, 41(3), 309–318.
16. W. J. Rankin and J. R. Wynnyckyj, Kinetics of reduction of MnO in powder mixtures with carbon, Metall. Mater. Trans. B, 1997, 28(2), 307–319.
17. G. Akdogan and R. H. Eric, Kinetics of the solid-state carbothermic reduction of wessel manganese ores, Metall. Mater. Trans. B, 1995, 26(1), 13–24.
18. W. J. Rankin and J. S. J. Van. Deventer, The kinetics of the reduction of manganous oxide by graphite, J. South. Afr. Inst. Min. Metall., 1980, 80(7), 239–247.
19. Y. Gao, M. Olivas-Martinez, H. Y. Sohn, H. G. Kim and C. W. Kim, Upgrading of low-grade manganese ore by selective reduction of iron oxide and magnetic separation, Metall. Mater. Trans. B, 2012, 43(6), 1465–1475.
20. Z. Cheng, G. Zhu and Y. Zhao, Study in reduction-roast leaching manganese from low-grade manganese dioxide ores using cornstalk as reductant, Hydrometallurgy, 2009, 96(1), 176–179.
21. R. Kononov, O. Ostrovski and S. Ganguly, Carbothermal reduction of manganese oxide in different gas atmospheres, Metall. Mater. Trans. B, 2008, 39(5), 662–668.
22. F. W. Y. Momade and Z. G. Momade, Reductive leaching of manganese oxide ore in aqueous methanol–sulphuric acid medium, Hydrometallurgy, 1999, 51(1), 103–113.
23. X. Tian, X. Wen, C. Yang, Y. Liang, Z. Pi and Y. Wang, Reductive leaching of manganese from low-grade manganese dioxide ores using corncob as reductant in sulfuric acid solution, Hydrometallurgy, 2010, 100(3), 157–160.
24. K. L. Berg and S. E. Olsen, Kinetics of manganese ore reduction by carbon monoxide, Metall. Mater. Trans. B, 2000, 31(3), 477–490.
25. H. Hancock, Use of coal and lignite to dissolve manganese dioxide in acidic solutions, Chem. Ind., 1986, 569–571.
26. J. Chen, P. F. Tian, X. A. Song, N. Li and J. X. Zhou, Microstructure of solid phase reduction on manganese oxide ore fines containing coal by microwave heating, J. Iron Steel Res. Int., 2010, 17(3), 13–20.
27. D. Moradkhani, M. Malekzadeh and E. Ahmadi, Nanostructured MnO₂ synthesized via methane gas reduction of manganese ore and hydrothermal precipitation methods, Trans. Nonferrous Met. Soc. China, 2013, 23(1), 134–139.
28. R. K. Jana and B. D. Pandey, Ammoniacal leaching of roast reduced deep-sea manganese nodules, Hydrometallurgy, 1999, 53(1), 45–56.
29. R. N. Sahoo, P. K. Naik and S. C. Das, Leaching of manganese from low-grade manganese ore using oxalic acid as reductant in sulphuric acid solution, Hydrometallurgy, 2001, 62(3), 157–163.
30. A. G. Kholmogorov, A. M. Zhyzhaev, U. S. Kononov, G. A. Moiseeva and G. L. Pashkov, The production of manganese dioxide from manganese ores of some deposits of the Siberian region of Russia, Hydrometallurgy, 2000, 56(1), 1–11.
31. R. Raghavan and R. N. Upadhyay, Innovative hydrometallurgical processing technique for industrial zinc and manganese process residues, Hydrometallurgy, 1999, 51(2), 207–226.
32. M. S. Bafghi, A. Zakeri, Z. Ghasemi and M. Adeli, Reductive dissolution of manganese ore in sulfuric acid in the presence of iron metal, Hydrometallurgy, 2008, 90(2), 207–212.
33. S. C. Das, P. K. Sahoo and P. K. Rao, Extraction of manganese from low-grade manganese ores by FeSO₄ leaching, Hydrometallurgy, 1982, 8(1), 35–47.
34. T. Jiang, Y. Yang, Z. Huang, B. Zhang and G. Qiu, Leaching kinetics of pyrolusite from manganese–silver ores in the presence of hydrogen peroxide, Hydrometallurgy, 2004, 72(1), 129–138.
35. F. Veglio and L. Toro, Fractional factorial experiments in the development of manganese dioxide leaching by sucrose in sulphuric acid solutions, Hydrometallurgy, 1994, 36(2), 215–230.
36. D. Hariprasad, B. Dash, M. K. Ghosh and S. Anand, Leaching of manganese ores using sawdust as a reductant, Miner. Eng., 2007, 20(14), 1293–1295.
37. M. Trifoni, L. Toro and F. Veglio, Reductive leaching of manganiferous ores by glucose and H₂SO₄: effect of alcohols, Hydrometallurgy, 2001, 59(1), 1–14.
38. Q. X. Ye, H. B. Zhu, J. H. Peng, C. S. Kannan, J. Chen, L. Q. Dai and P. Liu, Preparation of reduced iron powders from mill scale with microwave heating: optimization using response surface methodology, Metall. Mater. Trans. B, 2013, 44(6), 1478–1485.
39. J. Vereš, Š. Jakabský and M. Lovás, Zinc recovery from iron and steel making wastes by conventional and microwave assisted leaching, Acta Montan. Slovaca, 2011, 16, 185–191.
40. J. Vereš, M. Lovás, Š. Jakabský, V. Šepelák and S. Hredzák, Characterization of blast furnace sludge and removal of zinc by microwave assisted extraction, Hydrometallurgy, 2012, 129, 67–73.

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