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Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating
PTEC-14897; No of Pages 9 Powder Technology xxx (2019) xxx
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Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating Kangqiang Li a, Jin Chen a,⁎,1, Jinhui Peng a,b, Roger Ruan d, C. Srinivasakannan e, Guo Chen a,b,c,⁎ a
Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming 650093, PR China Key Laboratory of Green-Chemistry Materials in University of Yunnan Province, Kunming Key Laboratory of Energy Materials Chemistry, Yunnan Minzu University, Kunming 650500, PR China Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, PR China d Center for Biorefining, Bioproducts and Biosystems Engineering Department, University of Minnesota, 1390 Eckles Ave., Saint Paul, MN 55108, USA e Chemical Engineering Department, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates b c
a r t i c l e
i n f o
Article history: Received 7 April 2019 Received in revised form 31 October 2019 Accepted 6 November 2019 Available online xxxx Keywords: Enhanced carbothermal reduction Low-grade pyrolusite Microwave heating Pilot-scale
a b s t r a c t Microwave heating through materials' dielectric loss endows energy saving and consumption reduction and production clean characteristics. Pilot-scale study was initiated to evaluate the enhanced effect of microwave heating on carbothermal reduction process of pyrolusite. Results indicated that carbothermal reduction process for lowgrade pyrolusite was divided into three stages identified by temperatures: b145 °C, 145 °C–400 °C, N400 °C. Meanwhile, ƞMn value of 95.38% can achieve at 650 °C for 60 min with 15% coal addition, with low Fe2+ content, indicating efficient pyrolusite reduction by microwave heating. Moreover, MnO2 peaks disappeared and MnO peaks were detected and product surface became loose and porous with numerous cracks and holes, meanwhile grain shape became more regular with a smaller particle size after further microwave treatment. The study highlights that the non-conventional technology by microwave heating to reduce low-grade pyrolusite is very promising and could be considered for full scale applications. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Manganese as important national strategic reserve, is predominantly (N90%) utilized as deoxidizers, desulfurizers, and alloying agents in the steelmaking process [1–3]; additionally, it finds far wider applications such as catalysts, electrode materials, magnetic materials, and building materials, etc. [4–7]. As the world's largest consumer of manganese resources and a major producer of manganese products, China reflects an increasingly huge demand for manganese ore reserves [8]. While chinese manganese ore resources are generally poor, fine, miscellaneous and difficult to utilize, and the grade and impurity content of manganese ore in different mining areas are quite different and complex; therefore, it is difficult for a single mineral processing process to be universal [9]. Meanwhile, the vigorous exploitation renders the sharp decrease of manganese resources and the synchronous decline of ore grade, further forcing industrial production of manganese series products being blocked [10]. Moreover, the low-grade manganese ore resources are abundant, accounting for about 93% [11]. Therefore, exploring an effective reduction method for low-grade pyrolusite will alleviate the severe contradiction between the current domestic ⁎ Corresponding author at: Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming 650093, PR China. E-mail addresses: [email protected] (J. Chen), [email protected] (G. Chen). 1 Co-Corresponding author.
demand and supply for manganese products, and also provide extremely important strategic significance for the sustainable development of manganese materials production. Among the reduction technologies reported for pyrolusite, pulverized coal is mostly widely chosen as a reducing agent in coal reduction roasting method [12], which is also named carbothermal reduction method, widely applied in the Mn and Fe reduction [13–18]. The method to treat high-grade manganese ore offers advantages such as high reduction rate, simple process and convenient operation, while the method is processing low-grade manganese ore, quartz (SiO2) component with high content is easily softened and sintered at temperatures higher than 600 °C; meanwhile, the reduction temperature by coal reduction roasting method with conventional heating is generally above 850 °C, which causes the simultaneous reduction of iron (Fe) in manganese ore, rendering manganese product in low purity [9,19]; additionally, the process is plagued with high-energy consumption, high temperature, high costs and environmental restrictions [9,20]. Therefore, it urgently demands development of new processes for lowgrade pyrolusite that are potentially environmentally benign and cost effective to produce high-purity manganese products. The traditional reverberatory furnace is plagued with low output, high energy consumption and serious pollution; meanwhile, the rotary kiln encounters troubles around easy “coupling” in the kiln, resulting in poor production continuity and high energy consumption, further rendering it inefficient utilization for low-grade pyrolusite [9,21].
https://doi.org/10.1016/j.powtec.2019.11.015 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Compared with conventional heating, microwave heating directly transfers the energy required for reactions to the reacting molecules or atoms through the dielectric loss inside the material [22–24]. According to the interaction between microwave and material, the microwave absorption by the material is realized through coupling microwave energy and magnetic field, followed by the dielectric loss of materials converts microwave energy into internal energy, usually as heat energy [25]. Under sufficient intensity of microwave energy density, the in-situ energy conversion method enables rapid accumulation of energy in the material micro-areas, and makes it preferentially heat the valuable minerals rather than the gangue in the ore by utilizing the difference of electromagnetic properties for minerals and gangue [26–28]. Furthermore, the difference results in uneven temperature distribution in the multiple and multi-phase complex ore system, and caused thermal stress between the useful mineral and the ore interface, further promoting the effective separation of valuable minerals and gangue in the inclusion minerals, increasing the effective reaction area with minerals and accelerating the interface chemical reaction and the diffusion rate in the reaction process [29,30]. Microwave heating as a novel green method, has been increasingly frequent in the comprehensive utilization of manganese resources in recent years [31–34]. The ability to rapidly heat dielectric materials is often used as a heat source and a traditional alternative to conductive heating. Meanwhile, the ability of a material to absorb microwaves primarily depends on its dielectric properties, which are affected by the frequency of the electric field, temperature, moisture content and chemical composition etc. He et al. [23] and Li et al. [24] investigated the dielectric properties of mixtures of low-grade pyrolusite with different reducing agents, and found the low-grade pyrolusite showed excellent microwave-absorbing properties, wherein the mixture of low-grade pyrolusite with 10% coal can be heated to from temperature to 800 °C within 200 s and the mixture of low-grade pyrolusite with 12% walnut shell can be heated to from temperature to 800 °C within 11.0 min, indicating microwave heating technology can be efficiently applied to the comprehensive utilization of low-grade pyrolusite resources. Actually, Chen et al. [35] applied microwave heating to investigate the thermal decomposition and dissociation behavior of manganese ore and reported that manganese ore could be rapidly heated to 1000 °C in 17 min by microwave heating, with an increase of manganese content from 30% to 40%. The increase of manganese content and shorter heating time indicated that microwave heating replacing traditional heating can obviously improve the decomposition efficiency of manganese ore with energy saving and consumption reduction. Earlier work have studied the small-scale laboratory experiments about the microwave carbothermal reduction and conventional carbothermal reduction of low-grade pyrolusite [23,36,37], and highlighted that the feasibility of microwave heating with efficient manganese recovery and complete elimination of Fe2+; while few detailed studies have reported the available applications regarding on microwave heating in larger experiments to the carbothermal reduction of lowgrade pyrolusite. Meanwhile, such information is urgently required if microwaves are to be used as energy source for industrial application systems. Therefore, in the present study, the technical viability of this novel enhanced-reduction process for low-grade pyrolusite by microwave heating at a pilot scale was systematically investigated; meanwhile, the thermodynamics characteristics and thermochemical characteristics were measured to explore the carbothermal reduction behavior of pyrolusite-coal mixtures; moreover, the quality indexes were analyzed to verify the technical viability of microwave enhanced-reduction method for low-grade pyrolusite at a pilot scale, including manganese reduction ratio and Fe2+ content; additionally, the particle size distributions, phase compositions and microstructures before and after microwave heating were characterized to evaluate effects of microwave heating on the carbothermal reduction of lowgrade pyrolusite.
2. Material and methods 2.1. Materials Manganese ore used in the pilot test was provided locally by Dameng Manganese Industry Group Co., Ltd. (Nanning City, Guangxi Province, P.R. China). The chemical compositions of raw ore and the microwave reduced samples were analyzed by Analysis and Testing Center of Daxin Branch of Dameng Manganese Industry Group Co., Ltd. (Chongzuo City, Guangxi Province, P.R. China), performed in accordance with the recommended methods of National Standard of the People's Republic of China (GB/T1506-2002), and the analytical results were illustrated in Table 1 and Fig. 1. It was characterized from Table 1 that the manganese ore was low-grade pyrolusite (TMn b 30%) of high iron type (Mn/Fe b 3) and high silicon type ((SiO2 + Al2O3) ≥20%), with minor oxides and elements such as CaO, MgO, S and P. Therefore, selective reduction of manganese and iron oxides will contribute to alleviate the pressure of subsequent impurity removal processes. After the bulk manganese ore was crushed, ground and screened, the ground pyrolusite were measured with a median particle diameter (D50) of 23.416 μm, as presented in Fig. 1(a). Moreover, it can be concluded From Fig. 1(b) that the main phases of raw pyrolusite contained SiO2, MnO2, and (Fe, Mn)2SiO4. Meanwhile, it was observed from Fig. 1(c) and (d) that there was no cracks or voids in the surface of the ore and existed irregular blocks with different shapes, indicating the carbothermal reduction of low-grade pyrolusite with those bad kinetics conditions would be hard to process. Pulverized coal was used as the reducing agent, with a median particle diameter (D 50 ) of 34.216 μm (as illustrated in Fig. 1(a)), also locally received from Dameng Manganese Industry Group Co., Ltd. (Nanning City, Guangxi Province, P.R. China). The proximate analysis of pulverized coal was performed in accordance with the recommended methods of National Standard of the People's Republic of China (GB/T28731-2012), provided by Kunming Metallurgical Research Institute (Kunming City, Yunnan Province, P.R. China). As presented in Table 2, the proximate analysis of pulverized coal included 3.01% of moisture, 8.43% of volatiles, 59.67% of fixed carbon and 28.89% of ash.
2.2. Characterization The particle size distributions of low-grade pyrolusite, the microwave reduced pyrolusite and pulverized coal were examined by a laser particle size analyzer (JL-1177, Chengdu Jingxin Powder Analyze Instruments Co. Ltd). The crystal structures of pyrolusite before and after microwave reduction were characterized by XRD analysis (X'Pert3 powder, Panaco, Netherlands), with the XRD patterns recorded at a scanning rate of 1.6°/min with 2-thera ranging from 10° to 90° using a diffractometer with CuKα radiation (λ = 1.540598 Å, 40 KV, 40 mA) and a PIxcel1D-medipix3 detector. The microstructure morphologies of raw pyrolusite and the microwave reduced samples were characterized by a scanning electron microscope (Phenom ProX, Phenom-World, Netherlands). The thermochemical characteristics of pyrolusite-coal mixtures were measured using a simultaneous thermal analyzer (STA 449F3, NETZSCH, Germany).
Table 1 Chemical composition of low-grade pyrolusite. Compositions Mass/W% Compositions Mass/W%
TMn 21.53 CaO 2.01
Mn4+ 20.00 MgO 1.65
Mn2+ 0.89 SiO2 37.99
Fe3+ 9.53 P 0.10
Al2O3 3.51 S 0.13
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Fig. 1. Characterization analysis for pyrolusite and pulverized coal, (a) particle size distributions; (b) XRD pattern of pyrolusite; (c) SEM pattern of pyrolusite, 3000×; (d) SEM, 6000×.
2.3. Instrumentation The pilot-scale reduction experiments were performed in a microwave high temperature experimental furnace, independently produced by Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology (Kunming city, P.R. China). The schematic diagram of the microwave furnace (MYWAVE) was shown in Fig. 2. On the whole perspective, the microwave furnace can be divided into three parts, including feed system, feeding system and discharge system, with feed motor, feeding motor and discharge motor to supply power to support separate operation. Wherein the feed system controls the feed quantity, the feeding system controls the residence time of the material in the furnace, and the discharge system can control the discharge quantity, with two thermocouples (K-type) to detect Table 2 Proximate analysis of pulverized coal (%). Moisture
Volatiles
Fixed carbon
Ash
3.01
8.43
59.67
28.89
the front temperature and the back temperature of the furnace body, respectively. The pilot test furnace can control continuous feed and discharge of the material, further to achieve the purpose of continuous production. In addition, from a partial perspective, the high temperature furnace mainly consisted of microwave reactor, spiral discharge pipe, motors, thermocouples, a computer control system, controller and a water circulation system. The microwave reactor was cooled by water circulation when working. The length of the cavity was 1288 mm, and the inner and outer diameters of the tube were 147 and 207 mm, respectively. The black spiral inside the dotted frame shown in Fig. 2 referred to the stirring paddle attached to the microwave furnace, therefore, the penetration depth should be in the entire cavity. The blade of the spiral feeder can't fully fit with the pipe wall; therefore, there existed a dead material layer between the blade and the pipe wall, with the minimum and maximum baking capacity at 7.865 kg and 78.783 kg, respectively. The temperature was measured by K-type thermocouple, with a maximum service temperature of 1300 °C. Continuous controllable microwave power was adjustable ranging from 0 to 22.5 kW with provided by 15 magnetrons at 2.45 GHz microwave frequency.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Fig. 2. Schematic diagram of microwave high temperature experimental furnace.
2.4. Procedure Before the pilot test, it is necessary to adjust and set the parameters of the three motors in the microwave furnace. Determining the relationships between the inlet and outlet frequency of the material and the speed of the motor in the cold state experiment, will contribute to avoid air burning or overload roasting, further to balance the feed and discharge of material. The forward and reverse rotation of the feeding motor determines the roasting time and roasting state of the material. Continuous forward rotation is not enough to stir the material sufficiently, further rendering that the material receive microwave radiation unevenly. While the combination of forward and reverse rotation can fully stir the material; therefore, the free mode of forward and reverse rotation combination is more suitable and chosen for carbothermal reduction of low-grade pyrolusite. The frequencies of the feed motor and the discharge motor were both set to 10 Hz, with the feeding motor set to 30 Hz in the forward rotation mode. The microwave source was open to heat the material when feed starts. When the material came out from the outlet, the forward mode of feeding motor was turned off, followed by the free mode was turned on, with the forward rotation time was set to 25 s, stay time of 5 s, and reverse rotation time of 25 s. The furnace temperature should be hold for 10 min for every 100 °C increase until reaching the target temperature. Finally, the feeding motor was set to turn forward rotation for 25 s, stay for 5 s and reverse rotation for 15 s. After continuous feed and discharge, the forward and reverse rotation time of the feeding motor can slightly adjust according to the color of the roasted sample observed. After the three motors parameters determined, the pyrolusite-coal mixtures with certain mixing proportions were introduced into the microwave furnace to be treated with specific reduction temperatures and holding times. Referred to earlier small-scale laboratory experiments parameters about the microwave carbothermal reduction of lowgrade pyrolusite, the manganese recovery can reach 96.65% at 550 °C for 50 min with 10% of coal addition and 400 W of microwave power [23] and achieve 97.2% at 800 °C for 40 min with 10% of coal addition and 380 W of microwave power [36], wherein the low-grade pyrolusite used as the raw material in the earlier work were the same batch of materials in the present work. Therefore, in the present pilot-scale experiments, 40.0 kg of low-grade pyrolusite powder was evenly mixed with 10% and 15% of coal addition, adding 4.0 kg and 6.0 kg of pulverized coal, respectively, and with the same particle size of 200 meshes for pyrolusite powder and pulverized coal. The microwave power of five sets
of magnetrons (a total of 15 magnetrons) was set to 700 W, with a total power of 10.5 kW. Meanwhile, the reduction temperature was set to separate 450 °C, 500 °C, 550 °C, 600 °C, and 650 °C, the holding time ranged from 20 min to 80 min, with 20 min as a variable node. Upon reaching the set reduction temperature and holding time, the reduced samples were discharged by the discharge system of the microwave furnace, to be naturally cooled to room temperature for subsequent characterization analysis. 3. Results and discussion 3.1. Thermodynamic analysis The relationships between equilibrium constants and temperature for various reduction reactions were calculated and calibrated by thermodynamic software, including FactSage and HSC, and thermodynamics graphs for reduction reactions of pyrolusite-coal mixture were plotted in Fig. 3. The organic components of coal are composed of aromatic rings with high substituents and relatively weak aliphatic bridge bonds and ether bonds, in which the number of functional groups is related to the degree of coalification [38]. Carbothermal reduction reactions of low-grade pyrolusite is known to be the solid–solid direct reductions (C–Ferromanganese oxide) directly reacting with C, and gas–solid reductions (CO–Ferromanganese oxide) indirectly reacting with CO simultaneously [39,40]. From the proximate analysis of pulverized coal, it can clearly conclude that there were about 8.43% of volatiles in pulverized coal which contributed to the gas–solid reductions; meanwhile, fixed carbon (C) contributed to the solid-solid reductions, with a content of about 59.67%; hence, it can be speculated that pulverized coal has good reduction effects for pyrolusite, with high content of reducing ingredients. Moreover, considering the pyrolusite-coal mixtures were prepared by manual mixing during the pilot test, rendering the contact between pyrolusite powder and pulverized coal insufficient, accompanying with the particles filled with residual air. Therefore, during the process of high temperature carbothermal reduction, CO was firstly produced by the reaction of pulverized coal with air, then the CO produced reacted with metal oxides to produce CO2; followed by the CO2 produced reacted with carbon particles for carbon gasification reaction and Boundouard reaction, referring to the reactions of C + H2O(g) = CO(g) + H2(g) and C + CO2(g) = 2CO(g), respectively [36]. The CO generated by the two reactions, promoted the
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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3.2. Thermo-gravimetric analysis Thermogravimetric analysis was conducted to evaluate the weight loss and thermochemical behavior of pyrolusite-coal mixtures. Mixtures at two different additions of coal were studied in the temperature regime from 30 °C up to 800 °C at a heating rate of 20 °C/min with Argon (Ar) as a shielding gas. The TG/DTG/DSC curves were illustrated in Fig. 4. The thermodynamic behavior of pyrolusite-coal mixtures (Fig. 3) and thermogravimetric analysis results (Fig. 4) were combined to analyze thermochemical behavior of the mixtures. It was observed that TG (Fig. 4(a)), DTG (Fig. 4(b)) curves of two mixtures with different coal additions showed the same trend, with two endothermic peaks and one exothermic peak at temperatures ranging from 270 °C–560 °C, and the reduction process of low-grade pyrolusite by coal can be divided into three stages: the first stage (b145 °C), corresponded to two temperatures at 145.6 °C and 145.1 °C (marked Fig. 4(b)), wherein the masses of two mixtures with 10% and 15% coal addition decreased by 4.34% and 0.45%, respectively, which the decrease of weight was attributed to the release of volatiles in the coal and the evaporation of free water and bound water of the pyrolusite and coal. The second stage (145 °C–400 °C), corresponded to two temperatures at 387.1 °C and 399.3 °C (marked Fig. 4(b)), wherein the masses of two mixtures with 10% and 15% coal addition decreased by 2.51% and 1.46%, respectively, which the decrease at this stage was mainly ascribed to the endothermic reduction reactions of 2MnO2 + C = Mn2O3 + CO(g) and 3Mn2O3 + C = 2Mn3O4 + CO(g), with the endothermic peaks appeared at 273.1 °C and 272.4 °C, which the peak temperatures were lower than the conversion temperature of Mn3O4 → MnO with temperatures higher than 288.9 °C (Fig. 3), indicating there just occurred the conversion of MnO2 → Mn2O3 → Mn3O4 in the second stage, mainly occurring
Fig. 3. Thermodynamics graph for carbothermal reduction reactions of low-grade pyrolusite, (a) direct reduction reactions; (b) indirect reduction reactions.
carbothermal reduction of low-grade pyrolusite in such a cycle [22]. Meanwhile, observed from Fig. 3, according to the principle of stepby-step conversion, the carbothermal reduction mechanism for lowgrade pyrolusite by coal could be understood to have undergone these phase changes: MnO2 → Mn2O3 → Mn3O4 → MnO and Fe2O3 → Fe3O4 → FeO [38–40], referring that the metal oxides in pyrolusite are gradually reduced from the high valence state to the lower valence state. From Fig. 3(a), it can be concluded that during the direct reduction process, MnO2 → Mn2O3 → Mn3O4 could be achieved at room temperature, while conversion of Mn3O4 → MnO could be achieved only at temperatures higher than 288.9 °C, and Fe3O4 → FeO reaction needs temperatures N719.1 °C. Additionally, concluded from Fig. 3(b), MnO2 → Mn2O3 → Mn3O4 → MnO and Fe2O3 → Fe3O4 can both occur at temperatures above 46 °C, while Fe3O4 → FeO reaction needs temperatures N685.1 °C. In summary, whether the direct reductions or indirect reductions, the driving force for MnO2 and Fe2O3 reduction increases significantly with temperature. Based on the above analysis (Fig. 3), the selective reduction temperature between MnO2 and Fe2O3 in pyrolusite was found with to be required b685.1 °C, referring to that the reduction of MnxOy to MnO and the reduction of Fe2O3 to Fe3O4 instead of Fe2O3 to FeO; therefore, in the pilot test, the carbothermal reduction temperatures of low-grade pyrolusite were set from 450 °C to 650 °C. Moreover, it can be concluded from Fig. 3 that using carbothermal reduction to converting MnO2 and Fe2O3 in low-grade pyrolusite to MnO and Fe3O4, respectively, was thermodynamically feasible at the chosen temperatures.
Fig. 4. TG-DTG-DSC curves of pyrolusite-coal mixtures, (a) TG curves; (b) DTG curves; (c) DSC curves.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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the reduction of MnO2 to Mn2O3, and some Mn2O3 was reduced to Mn3O4, without the conversion of Mn3O4 → MnO; meanwhile, it can be observed from Fig. 4(c) that both the first stage and the second stage were endothermic, verifying the rationality of analysis for the first stage and the second stage. Third stage (N400 °C), mainly occurred the conversion of Mn2O3 to Mn3O4, and some Mn3O4 was reduced to MnO, and part of the unreduced MnO2 continued to be reduced to Mn2O3. As temperature increases, the reaction degree of Mn3O4 + C = 3MnO + CO(g) is more complete, indicating a higher manganese reduction ratio for low-grade pyrolusite.
size of the sample decreased to a certain extent, which was attributed to the unique selective heating and volume heating characteristics of microwave heating. The excellent heating characteristics of microwave heating can open the mineral inclusions, further to make the particle size smaller; meanwhile, the smaller particle size is beneficial to the subsequent leaching process of roasted products. Therefore, it can conclude that increasing carbon content, reduction temperature and holding time are beneficial to the decrease of particle size and regular growth of grains. 3.4. Quality indexes analysis
3.3. Particle size analysis Effects of microwave heating on particle size compositions of lowgrade pyrolusite samples treated with different reduction temperatures and holding times were investigated, and the results were illustrated in Fig. 5. The median particle diameter (D50) of raw pyrolusite was 23.416 μm, concluded from Fig. 1(a). After treated by microwave heating at 500 °C for 40 min, it can be observed from Fig. 5 that the median particle diameter (D50) of the pyrolusite-coal mixture with 10% of coal addition decreased to 16.921 μm, with a volume average particle size (D [3,4]) of 19.264 μm and a surface area average particle size (D [2,3]) of 2.247 m2/cm3. Additionally, for the mixture with 15% of coal addition and reduced at 600 °C for 60 min, the D50, D [3,4], D [2,3] value of the sample changed to 15.11 μm, 17.775 μm, 2.710 m 2 /cm 3 , respectively. For laser particle size analyzer, the more similar to spherical particles, the more accurate the measurement results will be. Wherein the D-value marked in Fig. 5 was the difference value of D [3,4] and D [2,3]. The D-value is smaller, indicating that the shape of the sample particles is more regular, with a more concentrated particle size distribution. Therefore, it was observed from Fig. 5 that the D-value for the pyrolusite-coal mixture with 10% of coal addition treated at 500 °C for 40 min was 17.017 μm, much bigger than that of the mixtures with 15% of coal addition reduced at 600 °C for 60 min, with a D-value of 15.065 μm. Meanwhile, the D50 of the sample roasted at 500 °C for 40 min was also bigger than that of the sample roasted at 600 °C for 60 min, with 16.921 μm and 15.11 μm, respectively. Hence, it can be concluded from Fig. 5 that compared with the raw pyrolusite and the sample roasted at 500 °C for 40 min, the sample roasted at 600 °C for 60 min had a much more regular shape and a more concentrated particle size distribution. After microwave reduction, the particle
Fig. 5. Particle size analysis for pyrolusite-coal mixtures, (a) with 10% coal; (b) with 15% coal.
The microwave reduced samples treated with different reduction temperatures and holding times were taken sampling every other 20 min after continuous feeding and discharging, and the manganese reduction ratio and Fe2+ content were detected to evaluate the reduction effects of microwave heating on low-grade pyrolusite reduction, wherein the analytical methods for manganese reduction ratio and Fe2+ content were provided in Supplementary material, and the analytical results were plotted in Fig. 6. Reduction effects of minerals by carbothermal reduction are generally measured by the reduction ratio. From Fig. 6(a), it can be seen that after stable and continuous feeding and discharging, the manganese reduction ratios of low-grade pyrolusite were mostly higher than 90%, significantly increased with carbon addition increasing. The manganese monoxide (MnO) powder can be introduced to the
Fig. 6. Quality indexes analysis for pyrolusite-coal mixtures, (a) manganese reduction rate; (b) Fe2+ content.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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electrolysis workshop with a reduction rate N88% in the factory; therefore, it can be concluded form Fig. 6(a) that the microwave reduced samples in the pilot test all met the quality requirements of enterprises. More importantly, the reduced sample with a low Fe2+ content will contribute to alleviate the pressure of subsequent electrolysis and impurity removal processes. It can be observed from Fig. 6(b) that the average Fe2+ content of the sample with 10% of coal addition roasted at 500 °C was only 1.85%; meanwhile, for the sample with coal addition of 15% roasted at 600 °C, the average Fe2+ content was just 1.675%, much lower than that by conventional heating, which was attributed to that microwave heating can decrease reduction temperature [22,36]. The lower Fe2+ content indicates that only a small number of ions will enter the solution during acid leaching, which greatly decreases the burden for iron removal during the subsequent purification process. In addition, He et al. [23] and Ye et al. [36] have reported the small-scale laboratory experiments about microwave carbothermal reduction of low-grade pyrolusite, and results indicated the manganese recovery can reach 96.65% at 550 °C for 50 min with 10% of coal addition and achieve 97.2% at 800 °C for 40 min with 10% of coal addition, respectively. Therefore, combined with the small-scale laboratory findings [23,36], the information for pilot-scale study presented in Fig. 6 can provide a sound basis for the full-scale applications of microwave heating to the carbothermal reduction of low-grade pyrolusite, furthermore demanding further studies to explore the economic feasibility for commercial adoption.
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3.5. Phase structure characterization The crystal structures and microstructure morphology of microwave reduced samples treated with different reduction temperatures and holding times were characterized by XRD and SEM, and the results were displayed in Fig. 7. Wherein Fig. 7(a)–(c), illustrate the XRD and SEM patterns of the sample with coal addition of 10% roasted at 500 °C for 40 min, with a ƞMn value of 91.36% (Fig. 6(a)). Fig. 7(d)–(f), present the XRD and SEM patterns of the sample with coal addition of 15% roasted at 600 °C for 60 min, with a ƞMn value of 95.38% (Fig. 6(a)). For the crystal structures of pyrolusite, it can be concluded from Fig. 7(a) that the main phase of the sample contained SiO2 (JCPDS: 461045), MnO (JCPDS: 78-0424), Mn2SiO4 (JCPDS: 74-0716) and Mn3O4 (JCPDS: 80-0382). Compared with Fig. 7(a) and (d), for the sample with coal addition of 15% roasted at 600 °C for 60 min, it was observed that the Mn3O4 phase disappeared. The disappearance of Mn3O4 phase was attributed to that the increase of roasting temperature and coal addition accelerated the carbothermal reduction of low-grade pyrolusite, rendering the process of MnO2 converted to MnO more complete. Moreover, compared to Fig. 1(b), it was clearly observed from Fig. 7 (a) and (d) that traces of Mn2SiO4 were detected, which was ascribed to the decomposition of (Mn, Fe)2SiO4; meanwhile, the peaks of MnO2 were disappeared, indicating that MnO2 had almost been reduced to lower-valence manganese oxide. Thus, it can be summarized from Fig. 7 that MnO2 was transformed into MnO successfully and completely, without Fe2+ detected, which the patterns results were consistent with the quality indexes analysis results (Fig. 6); moreover,
Fig. 7. XRD and SEM patterns of pyrolusite-coal mixtures, (a) XRD for pyrolusite with 10% coal; (b) SEM, 2000×; (c) SEM, 25000×; (d) XRD for pyrolusite with 15% coal; (e) SEM, 10000×; (f) SEM, 70000×.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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the findings from Fig. 7 also indicated that the pilot study on microwave-enhanced carbothermal reduction of low-grade pyrolusite was feasible and successful. For the microstructure morphology of pyrolusite, referring to Fig. 1(c) and (d), it can be seen from Fig. 7(b) and (c) that the tighter surface morphology completely disappeared, and was replaced by numerous irregular cracks, holes, and pits. The great change of surface morphology was attributed to the selective heating characteristic of microwave heating. Based on the excellent microwaveabsorbing properties of MnO2 phase, MnO2 was quickly heated to a higher temperature, while gangue components such as SiO2 were difficult to heat, forming a large temperature gradient and causing thermal stress at the interface, further to open the inclusions and produce structural cracks [23,24]. Compared with Fig. 7(b) and (c), it was observed from Fig. 7(e) and (f) that the molten granule were further grown, then interconnected and stacked. The grain shape became more regular and the particle size was smaller, corresponding to the particle size analysis results (Fig. 5). Moreover, after further microwave treatment, it was observed from Fig. 7 that the surface of sample became loose and porous, which was attributed to the release of gas and the volatiles in the pyrolusite-coal mixtures during carbothermal reduction process. 4. Conclusions In the pilot-scale study, applying microwave heating to enhance carbothermal reduction of low-grade pyrolusite was attempted. Results indicated that carbothermal reduction process of lowgrade pyrolusite was divided into three stages: b145 °C, 145 °C– 400 °C, N400 °C. The pyrolusite-coal mixture with 15% coal was completely reduced to MnO powder at 600 °C for 60 min, with a ƞMn value of 95.38%, corresponding to the disappearance of MnO2 peaks and the appearance of MnO peaks. Moreover, the product surface became loose and porous with numerous cracks and holes, and the grain shape became more regular with a smaller particle size after further microwave treatment, which was attributed to the unique selective heating characteristics of microwave heating. Meanwhile, the release of gas and volatiles by carbothermal reduction reactions also contributed to form the porous microstructure. The information presented in this work should provide a sound basis for the full scale applications of microwave heating to the carbothermal reduction of low-grade pyrolusite, furthermore demanding further studies to explore the economic feasibility for commercial adoption. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial supports from the National Natural Science Foundation of China (No: U1802255), the Key Projects in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (No. 2015BAB17B00), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), and Innovative Research Team (in Science and Technology) in University of Yunnan Province were sincerely acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2019.11.015.
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Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating Kangqiang Li a, Jin Chen a,⁎,1, Jinhui Peng a,b, Roger Ruan d, C. Srinivasakannan e, Guo Chen a,b,c,⁎ a
Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming 650093, PR China Key Laboratory of Green-Chemistry Materials in University of Yunnan Province, Kunming Key Laboratory of Energy Materials Chemistry, Yunnan Minzu University, Kunming 650500, PR China Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, PR China d Center for Biorefining, Bioproducts and Biosystems Engineering Department, University of Minnesota, 1390 Eckles Ave., Saint Paul, MN 55108, USA e Chemical Engineering Department, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates b c
a r t i c l e
i n f o
Article history: Received 7 April 2019 Received in revised form 31 October 2019 Accepted 6 November 2019 Available online xxxx Keywords: Enhanced carbothermal reduction Low-grade pyrolusite Microwave heating Pilot-scale
a b s t r a c t Microwave heating through materials' dielectric loss endows energy saving and consumption reduction and production clean characteristics. Pilot-scale study was initiated to evaluate the enhanced effect of microwave heating on carbothermal reduction process of pyrolusite. Results indicated that carbothermal reduction process for lowgrade pyrolusite was divided into three stages identified by temperatures: b145 °C, 145 °C–400 °C, N400 °C. Meanwhile, ƞMn value of 95.38% can achieve at 650 °C for 60 min with 15% coal addition, with low Fe2+ content, indicating efficient pyrolusite reduction by microwave heating. Moreover, MnO2 peaks disappeared and MnO peaks were detected and product surface became loose and porous with numerous cracks and holes, meanwhile grain shape became more regular with a smaller particle size after further microwave treatment. The study highlights that the non-conventional technology by microwave heating to reduce low-grade pyrolusite is very promising and could be considered for full scale applications. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Manganese as important national strategic reserve, is predominantly (N90%) utilized as deoxidizers, desulfurizers, and alloying agents in the steelmaking process [1–3]; additionally, it finds far wider applications such as catalysts, electrode materials, magnetic materials, and building materials, etc. [4–7]. As the world's largest consumer of manganese resources and a major producer of manganese products, China reflects an increasingly huge demand for manganese ore reserves [8]. While chinese manganese ore resources are generally poor, fine, miscellaneous and difficult to utilize, and the grade and impurity content of manganese ore in different mining areas are quite different and complex; therefore, it is difficult for a single mineral processing process to be universal [9]. Meanwhile, the vigorous exploitation renders the sharp decrease of manganese resources and the synchronous decline of ore grade, further forcing industrial production of manganese series products being blocked [10]. Moreover, the low-grade manganese ore resources are abundant, accounting for about 93% [11]. Therefore, exploring an effective reduction method for low-grade pyrolusite will alleviate the severe contradiction between the current domestic ⁎ Corresponding author at: Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming 650093, PR China. E-mail addresses: [email protected] (J. Chen), [email protected] (G. Chen). 1 Co-Corresponding author.
demand and supply for manganese products, and also provide extremely important strategic significance for the sustainable development of manganese materials production. Among the reduction technologies reported for pyrolusite, pulverized coal is mostly widely chosen as a reducing agent in coal reduction roasting method [12], which is also named carbothermal reduction method, widely applied in the Mn and Fe reduction [13–18]. The method to treat high-grade manganese ore offers advantages such as high reduction rate, simple process and convenient operation, while the method is processing low-grade manganese ore, quartz (SiO2) component with high content is easily softened and sintered at temperatures higher than 600 °C; meanwhile, the reduction temperature by coal reduction roasting method with conventional heating is generally above 850 °C, which causes the simultaneous reduction of iron (Fe) in manganese ore, rendering manganese product in low purity [9,19]; additionally, the process is plagued with high-energy consumption, high temperature, high costs and environmental restrictions [9,20]. Therefore, it urgently demands development of new processes for lowgrade pyrolusite that are potentially environmentally benign and cost effective to produce high-purity manganese products. The traditional reverberatory furnace is plagued with low output, high energy consumption and serious pollution; meanwhile, the rotary kiln encounters troubles around easy “coupling” in the kiln, resulting in poor production continuity and high energy consumption, further rendering it inefficient utilization for low-grade pyrolusite [9,21].
https://doi.org/10.1016/j.powtec.2019.11.015 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Compared with conventional heating, microwave heating directly transfers the energy required for reactions to the reacting molecules or atoms through the dielectric loss inside the material [22–24]. According to the interaction between microwave and material, the microwave absorption by the material is realized through coupling microwave energy and magnetic field, followed by the dielectric loss of materials converts microwave energy into internal energy, usually as heat energy [25]. Under sufficient intensity of microwave energy density, the in-situ energy conversion method enables rapid accumulation of energy in the material micro-areas, and makes it preferentially heat the valuable minerals rather than the gangue in the ore by utilizing the difference of electromagnetic properties for minerals and gangue [26–28]. Furthermore, the difference results in uneven temperature distribution in the multiple and multi-phase complex ore system, and caused thermal stress between the useful mineral and the ore interface, further promoting the effective separation of valuable minerals and gangue in the inclusion minerals, increasing the effective reaction area with minerals and accelerating the interface chemical reaction and the diffusion rate in the reaction process [29,30]. Microwave heating as a novel green method, has been increasingly frequent in the comprehensive utilization of manganese resources in recent years [31–34]. The ability to rapidly heat dielectric materials is often used as a heat source and a traditional alternative to conductive heating. Meanwhile, the ability of a material to absorb microwaves primarily depends on its dielectric properties, which are affected by the frequency of the electric field, temperature, moisture content and chemical composition etc. He et al. [23] and Li et al. [24] investigated the dielectric properties of mixtures of low-grade pyrolusite with different reducing agents, and found the low-grade pyrolusite showed excellent microwave-absorbing properties, wherein the mixture of low-grade pyrolusite with 10% coal can be heated to from temperature to 800 °C within 200 s and the mixture of low-grade pyrolusite with 12% walnut shell can be heated to from temperature to 800 °C within 11.0 min, indicating microwave heating technology can be efficiently applied to the comprehensive utilization of low-grade pyrolusite resources. Actually, Chen et al. [35] applied microwave heating to investigate the thermal decomposition and dissociation behavior of manganese ore and reported that manganese ore could be rapidly heated to 1000 °C in 17 min by microwave heating, with an increase of manganese content from 30% to 40%. The increase of manganese content and shorter heating time indicated that microwave heating replacing traditional heating can obviously improve the decomposition efficiency of manganese ore with energy saving and consumption reduction. Earlier work have studied the small-scale laboratory experiments about the microwave carbothermal reduction and conventional carbothermal reduction of low-grade pyrolusite [23,36,37], and highlighted that the feasibility of microwave heating with efficient manganese recovery and complete elimination of Fe2+; while few detailed studies have reported the available applications regarding on microwave heating in larger experiments to the carbothermal reduction of lowgrade pyrolusite. Meanwhile, such information is urgently required if microwaves are to be used as energy source for industrial application systems. Therefore, in the present study, the technical viability of this novel enhanced-reduction process for low-grade pyrolusite by microwave heating at a pilot scale was systematically investigated; meanwhile, the thermodynamics characteristics and thermochemical characteristics were measured to explore the carbothermal reduction behavior of pyrolusite-coal mixtures; moreover, the quality indexes were analyzed to verify the technical viability of microwave enhanced-reduction method for low-grade pyrolusite at a pilot scale, including manganese reduction ratio and Fe2+ content; additionally, the particle size distributions, phase compositions and microstructures before and after microwave heating were characterized to evaluate effects of microwave heating on the carbothermal reduction of lowgrade pyrolusite.
2. Material and methods 2.1. Materials Manganese ore used in the pilot test was provided locally by Dameng Manganese Industry Group Co., Ltd. (Nanning City, Guangxi Province, P.R. China). The chemical compositions of raw ore and the microwave reduced samples were analyzed by Analysis and Testing Center of Daxin Branch of Dameng Manganese Industry Group Co., Ltd. (Chongzuo City, Guangxi Province, P.R. China), performed in accordance with the recommended methods of National Standard of the People's Republic of China (GB/T1506-2002), and the analytical results were illustrated in Table 1 and Fig. 1. It was characterized from Table 1 that the manganese ore was low-grade pyrolusite (TMn b 30%) of high iron type (Mn/Fe b 3) and high silicon type ((SiO2 + Al2O3) ≥20%), with minor oxides and elements such as CaO, MgO, S and P. Therefore, selective reduction of manganese and iron oxides will contribute to alleviate the pressure of subsequent impurity removal processes. After the bulk manganese ore was crushed, ground and screened, the ground pyrolusite were measured with a median particle diameter (D50) of 23.416 μm, as presented in Fig. 1(a). Moreover, it can be concluded From Fig. 1(b) that the main phases of raw pyrolusite contained SiO2, MnO2, and (Fe, Mn)2SiO4. Meanwhile, it was observed from Fig. 1(c) and (d) that there was no cracks or voids in the surface of the ore and existed irregular blocks with different shapes, indicating the carbothermal reduction of low-grade pyrolusite with those bad kinetics conditions would be hard to process. Pulverized coal was used as the reducing agent, with a median particle diameter (D 50 ) of 34.216 μm (as illustrated in Fig. 1(a)), also locally received from Dameng Manganese Industry Group Co., Ltd. (Nanning City, Guangxi Province, P.R. China). The proximate analysis of pulverized coal was performed in accordance with the recommended methods of National Standard of the People's Republic of China (GB/T28731-2012), provided by Kunming Metallurgical Research Institute (Kunming City, Yunnan Province, P.R. China). As presented in Table 2, the proximate analysis of pulverized coal included 3.01% of moisture, 8.43% of volatiles, 59.67% of fixed carbon and 28.89% of ash.
2.2. Characterization The particle size distributions of low-grade pyrolusite, the microwave reduced pyrolusite and pulverized coal were examined by a laser particle size analyzer (JL-1177, Chengdu Jingxin Powder Analyze Instruments Co. Ltd). The crystal structures of pyrolusite before and after microwave reduction were characterized by XRD analysis (X'Pert3 powder, Panaco, Netherlands), with the XRD patterns recorded at a scanning rate of 1.6°/min with 2-thera ranging from 10° to 90° using a diffractometer with CuKα radiation (λ = 1.540598 Å, 40 KV, 40 mA) and a PIxcel1D-medipix3 detector. The microstructure morphologies of raw pyrolusite and the microwave reduced samples were characterized by a scanning electron microscope (Phenom ProX, Phenom-World, Netherlands). The thermochemical characteristics of pyrolusite-coal mixtures were measured using a simultaneous thermal analyzer (STA 449F3, NETZSCH, Germany).
Table 1 Chemical composition of low-grade pyrolusite. Compositions Mass/W% Compositions Mass/W%
TMn 21.53 CaO 2.01
Mn4+ 20.00 MgO 1.65
Mn2+ 0.89 SiO2 37.99
Fe3+ 9.53 P 0.10
Al2O3 3.51 S 0.13
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Fig. 1. Characterization analysis for pyrolusite and pulverized coal, (a) particle size distributions; (b) XRD pattern of pyrolusite; (c) SEM pattern of pyrolusite, 3000×; (d) SEM, 6000×.
2.3. Instrumentation The pilot-scale reduction experiments were performed in a microwave high temperature experimental furnace, independently produced by Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology (Kunming city, P.R. China). The schematic diagram of the microwave furnace (MYWAVE) was shown in Fig. 2. On the whole perspective, the microwave furnace can be divided into three parts, including feed system, feeding system and discharge system, with feed motor, feeding motor and discharge motor to supply power to support separate operation. Wherein the feed system controls the feed quantity, the feeding system controls the residence time of the material in the furnace, and the discharge system can control the discharge quantity, with two thermocouples (K-type) to detect Table 2 Proximate analysis of pulverized coal (%). Moisture
Volatiles
Fixed carbon
Ash
3.01
8.43
59.67
28.89
the front temperature and the back temperature of the furnace body, respectively. The pilot test furnace can control continuous feed and discharge of the material, further to achieve the purpose of continuous production. In addition, from a partial perspective, the high temperature furnace mainly consisted of microwave reactor, spiral discharge pipe, motors, thermocouples, a computer control system, controller and a water circulation system. The microwave reactor was cooled by water circulation when working. The length of the cavity was 1288 mm, and the inner and outer diameters of the tube were 147 and 207 mm, respectively. The black spiral inside the dotted frame shown in Fig. 2 referred to the stirring paddle attached to the microwave furnace, therefore, the penetration depth should be in the entire cavity. The blade of the spiral feeder can't fully fit with the pipe wall; therefore, there existed a dead material layer between the blade and the pipe wall, with the minimum and maximum baking capacity at 7.865 kg and 78.783 kg, respectively. The temperature was measured by K-type thermocouple, with a maximum service temperature of 1300 °C. Continuous controllable microwave power was adjustable ranging from 0 to 22.5 kW with provided by 15 magnetrons at 2.45 GHz microwave frequency.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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Fig. 2. Schematic diagram of microwave high temperature experimental furnace.
2.4. Procedure Before the pilot test, it is necessary to adjust and set the parameters of the three motors in the microwave furnace. Determining the relationships between the inlet and outlet frequency of the material and the speed of the motor in the cold state experiment, will contribute to avoid air burning or overload roasting, further to balance the feed and discharge of material. The forward and reverse rotation of the feeding motor determines the roasting time and roasting state of the material. Continuous forward rotation is not enough to stir the material sufficiently, further rendering that the material receive microwave radiation unevenly. While the combination of forward and reverse rotation can fully stir the material; therefore, the free mode of forward and reverse rotation combination is more suitable and chosen for carbothermal reduction of low-grade pyrolusite. The frequencies of the feed motor and the discharge motor were both set to 10 Hz, with the feeding motor set to 30 Hz in the forward rotation mode. The microwave source was open to heat the material when feed starts. When the material came out from the outlet, the forward mode of feeding motor was turned off, followed by the free mode was turned on, with the forward rotation time was set to 25 s, stay time of 5 s, and reverse rotation time of 25 s. The furnace temperature should be hold for 10 min for every 100 °C increase until reaching the target temperature. Finally, the feeding motor was set to turn forward rotation for 25 s, stay for 5 s and reverse rotation for 15 s. After continuous feed and discharge, the forward and reverse rotation time of the feeding motor can slightly adjust according to the color of the roasted sample observed. After the three motors parameters determined, the pyrolusite-coal mixtures with certain mixing proportions were introduced into the microwave furnace to be treated with specific reduction temperatures and holding times. Referred to earlier small-scale laboratory experiments parameters about the microwave carbothermal reduction of lowgrade pyrolusite, the manganese recovery can reach 96.65% at 550 °C for 50 min with 10% of coal addition and 400 W of microwave power [23] and achieve 97.2% at 800 °C for 40 min with 10% of coal addition and 380 W of microwave power [36], wherein the low-grade pyrolusite used as the raw material in the earlier work were the same batch of materials in the present work. Therefore, in the present pilot-scale experiments, 40.0 kg of low-grade pyrolusite powder was evenly mixed with 10% and 15% of coal addition, adding 4.0 kg and 6.0 kg of pulverized coal, respectively, and with the same particle size of 200 meshes for pyrolusite powder and pulverized coal. The microwave power of five sets
of magnetrons (a total of 15 magnetrons) was set to 700 W, with a total power of 10.5 kW. Meanwhile, the reduction temperature was set to separate 450 °C, 500 °C, 550 °C, 600 °C, and 650 °C, the holding time ranged from 20 min to 80 min, with 20 min as a variable node. Upon reaching the set reduction temperature and holding time, the reduced samples were discharged by the discharge system of the microwave furnace, to be naturally cooled to room temperature for subsequent characterization analysis. 3. Results and discussion 3.1. Thermodynamic analysis The relationships between equilibrium constants and temperature for various reduction reactions were calculated and calibrated by thermodynamic software, including FactSage and HSC, and thermodynamics graphs for reduction reactions of pyrolusite-coal mixture were plotted in Fig. 3. The organic components of coal are composed of aromatic rings with high substituents and relatively weak aliphatic bridge bonds and ether bonds, in which the number of functional groups is related to the degree of coalification [38]. Carbothermal reduction reactions of low-grade pyrolusite is known to be the solid–solid direct reductions (C–Ferromanganese oxide) directly reacting with C, and gas–solid reductions (CO–Ferromanganese oxide) indirectly reacting with CO simultaneously [39,40]. From the proximate analysis of pulverized coal, it can clearly conclude that there were about 8.43% of volatiles in pulverized coal which contributed to the gas–solid reductions; meanwhile, fixed carbon (C) contributed to the solid-solid reductions, with a content of about 59.67%; hence, it can be speculated that pulverized coal has good reduction effects for pyrolusite, with high content of reducing ingredients. Moreover, considering the pyrolusite-coal mixtures were prepared by manual mixing during the pilot test, rendering the contact between pyrolusite powder and pulverized coal insufficient, accompanying with the particles filled with residual air. Therefore, during the process of high temperature carbothermal reduction, CO was firstly produced by the reaction of pulverized coal with air, then the CO produced reacted with metal oxides to produce CO2; followed by the CO2 produced reacted with carbon particles for carbon gasification reaction and Boundouard reaction, referring to the reactions of C + H2O(g) = CO(g) + H2(g) and C + CO2(g) = 2CO(g), respectively [36]. The CO generated by the two reactions, promoted the
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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3.2. Thermo-gravimetric analysis Thermogravimetric analysis was conducted to evaluate the weight loss and thermochemical behavior of pyrolusite-coal mixtures. Mixtures at two different additions of coal were studied in the temperature regime from 30 °C up to 800 °C at a heating rate of 20 °C/min with Argon (Ar) as a shielding gas. The TG/DTG/DSC curves were illustrated in Fig. 4. The thermodynamic behavior of pyrolusite-coal mixtures (Fig. 3) and thermogravimetric analysis results (Fig. 4) were combined to analyze thermochemical behavior of the mixtures. It was observed that TG (Fig. 4(a)), DTG (Fig. 4(b)) curves of two mixtures with different coal additions showed the same trend, with two endothermic peaks and one exothermic peak at temperatures ranging from 270 °C–560 °C, and the reduction process of low-grade pyrolusite by coal can be divided into three stages: the first stage (b145 °C), corresponded to two temperatures at 145.6 °C and 145.1 °C (marked Fig. 4(b)), wherein the masses of two mixtures with 10% and 15% coal addition decreased by 4.34% and 0.45%, respectively, which the decrease of weight was attributed to the release of volatiles in the coal and the evaporation of free water and bound water of the pyrolusite and coal. The second stage (145 °C–400 °C), corresponded to two temperatures at 387.1 °C and 399.3 °C (marked Fig. 4(b)), wherein the masses of two mixtures with 10% and 15% coal addition decreased by 2.51% and 1.46%, respectively, which the decrease at this stage was mainly ascribed to the endothermic reduction reactions of 2MnO2 + C = Mn2O3 + CO(g) and 3Mn2O3 + C = 2Mn3O4 + CO(g), with the endothermic peaks appeared at 273.1 °C and 272.4 °C, which the peak temperatures were lower than the conversion temperature of Mn3O4 → MnO with temperatures higher than 288.9 °C (Fig. 3), indicating there just occurred the conversion of MnO2 → Mn2O3 → Mn3O4 in the second stage, mainly occurring
Fig. 3. Thermodynamics graph for carbothermal reduction reactions of low-grade pyrolusite, (a) direct reduction reactions; (b) indirect reduction reactions.
carbothermal reduction of low-grade pyrolusite in such a cycle [22]. Meanwhile, observed from Fig. 3, according to the principle of stepby-step conversion, the carbothermal reduction mechanism for lowgrade pyrolusite by coal could be understood to have undergone these phase changes: MnO2 → Mn2O3 → Mn3O4 → MnO and Fe2O3 → Fe3O4 → FeO [38–40], referring that the metal oxides in pyrolusite are gradually reduced from the high valence state to the lower valence state. From Fig. 3(a), it can be concluded that during the direct reduction process, MnO2 → Mn2O3 → Mn3O4 could be achieved at room temperature, while conversion of Mn3O4 → MnO could be achieved only at temperatures higher than 288.9 °C, and Fe3O4 → FeO reaction needs temperatures N719.1 °C. Additionally, concluded from Fig. 3(b), MnO2 → Mn2O3 → Mn3O4 → MnO and Fe2O3 → Fe3O4 can both occur at temperatures above 46 °C, while Fe3O4 → FeO reaction needs temperatures N685.1 °C. In summary, whether the direct reductions or indirect reductions, the driving force for MnO2 and Fe2O3 reduction increases significantly with temperature. Based on the above analysis (Fig. 3), the selective reduction temperature between MnO2 and Fe2O3 in pyrolusite was found with to be required b685.1 °C, referring to that the reduction of MnxOy to MnO and the reduction of Fe2O3 to Fe3O4 instead of Fe2O3 to FeO; therefore, in the pilot test, the carbothermal reduction temperatures of low-grade pyrolusite were set from 450 °C to 650 °C. Moreover, it can be concluded from Fig. 3 that using carbothermal reduction to converting MnO2 and Fe2O3 in low-grade pyrolusite to MnO and Fe3O4, respectively, was thermodynamically feasible at the chosen temperatures.
Fig. 4. TG-DTG-DSC curves of pyrolusite-coal mixtures, (a) TG curves; (b) DTG curves; (c) DSC curves.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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the reduction of MnO2 to Mn2O3, and some Mn2O3 was reduced to Mn3O4, without the conversion of Mn3O4 → MnO; meanwhile, it can be observed from Fig. 4(c) that both the first stage and the second stage were endothermic, verifying the rationality of analysis for the first stage and the second stage. Third stage (N400 °C), mainly occurred the conversion of Mn2O3 to Mn3O4, and some Mn3O4 was reduced to MnO, and part of the unreduced MnO2 continued to be reduced to Mn2O3. As temperature increases, the reaction degree of Mn3O4 + C = 3MnO + CO(g) is more complete, indicating a higher manganese reduction ratio for low-grade pyrolusite.
size of the sample decreased to a certain extent, which was attributed to the unique selective heating and volume heating characteristics of microwave heating. The excellent heating characteristics of microwave heating can open the mineral inclusions, further to make the particle size smaller; meanwhile, the smaller particle size is beneficial to the subsequent leaching process of roasted products. Therefore, it can conclude that increasing carbon content, reduction temperature and holding time are beneficial to the decrease of particle size and regular growth of grains. 3.4. Quality indexes analysis
3.3. Particle size analysis Effects of microwave heating on particle size compositions of lowgrade pyrolusite samples treated with different reduction temperatures and holding times were investigated, and the results were illustrated in Fig. 5. The median particle diameter (D50) of raw pyrolusite was 23.416 μm, concluded from Fig. 1(a). After treated by microwave heating at 500 °C for 40 min, it can be observed from Fig. 5 that the median particle diameter (D50) of the pyrolusite-coal mixture with 10% of coal addition decreased to 16.921 μm, with a volume average particle size (D [3,4]) of 19.264 μm and a surface area average particle size (D [2,3]) of 2.247 m2/cm3. Additionally, for the mixture with 15% of coal addition and reduced at 600 °C for 60 min, the D50, D [3,4], D [2,3] value of the sample changed to 15.11 μm, 17.775 μm, 2.710 m 2 /cm 3 , respectively. For laser particle size analyzer, the more similar to spherical particles, the more accurate the measurement results will be. Wherein the D-value marked in Fig. 5 was the difference value of D [3,4] and D [2,3]. The D-value is smaller, indicating that the shape of the sample particles is more regular, with a more concentrated particle size distribution. Therefore, it was observed from Fig. 5 that the D-value for the pyrolusite-coal mixture with 10% of coal addition treated at 500 °C for 40 min was 17.017 μm, much bigger than that of the mixtures with 15% of coal addition reduced at 600 °C for 60 min, with a D-value of 15.065 μm. Meanwhile, the D50 of the sample roasted at 500 °C for 40 min was also bigger than that of the sample roasted at 600 °C for 60 min, with 16.921 μm and 15.11 μm, respectively. Hence, it can be concluded from Fig. 5 that compared with the raw pyrolusite and the sample roasted at 500 °C for 40 min, the sample roasted at 600 °C for 60 min had a much more regular shape and a more concentrated particle size distribution. After microwave reduction, the particle
Fig. 5. Particle size analysis for pyrolusite-coal mixtures, (a) with 10% coal; (b) with 15% coal.
The microwave reduced samples treated with different reduction temperatures and holding times were taken sampling every other 20 min after continuous feeding and discharging, and the manganese reduction ratio and Fe2+ content were detected to evaluate the reduction effects of microwave heating on low-grade pyrolusite reduction, wherein the analytical methods for manganese reduction ratio and Fe2+ content were provided in Supplementary material, and the analytical results were plotted in Fig. 6. Reduction effects of minerals by carbothermal reduction are generally measured by the reduction ratio. From Fig. 6(a), it can be seen that after stable and continuous feeding and discharging, the manganese reduction ratios of low-grade pyrolusite were mostly higher than 90%, significantly increased with carbon addition increasing. The manganese monoxide (MnO) powder can be introduced to the
Fig. 6. Quality indexes analysis for pyrolusite-coal mixtures, (a) manganese reduction rate; (b) Fe2+ content.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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electrolysis workshop with a reduction rate N88% in the factory; therefore, it can be concluded form Fig. 6(a) that the microwave reduced samples in the pilot test all met the quality requirements of enterprises. More importantly, the reduced sample with a low Fe2+ content will contribute to alleviate the pressure of subsequent electrolysis and impurity removal processes. It can be observed from Fig. 6(b) that the average Fe2+ content of the sample with 10% of coal addition roasted at 500 °C was only 1.85%; meanwhile, for the sample with coal addition of 15% roasted at 600 °C, the average Fe2+ content was just 1.675%, much lower than that by conventional heating, which was attributed to that microwave heating can decrease reduction temperature [22,36]. The lower Fe2+ content indicates that only a small number of ions will enter the solution during acid leaching, which greatly decreases the burden for iron removal during the subsequent purification process. In addition, He et al. [23] and Ye et al. [36] have reported the small-scale laboratory experiments about microwave carbothermal reduction of low-grade pyrolusite, and results indicated the manganese recovery can reach 96.65% at 550 °C for 50 min with 10% of coal addition and achieve 97.2% at 800 °C for 40 min with 10% of coal addition, respectively. Therefore, combined with the small-scale laboratory findings [23,36], the information for pilot-scale study presented in Fig. 6 can provide a sound basis for the full-scale applications of microwave heating to the carbothermal reduction of low-grade pyrolusite, furthermore demanding further studies to explore the economic feasibility for commercial adoption.
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3.5. Phase structure characterization The crystal structures and microstructure morphology of microwave reduced samples treated with different reduction temperatures and holding times were characterized by XRD and SEM, and the results were displayed in Fig. 7. Wherein Fig. 7(a)–(c), illustrate the XRD and SEM patterns of the sample with coal addition of 10% roasted at 500 °C for 40 min, with a ƞMn value of 91.36% (Fig. 6(a)). Fig. 7(d)–(f), present the XRD and SEM patterns of the sample with coal addition of 15% roasted at 600 °C for 60 min, with a ƞMn value of 95.38% (Fig. 6(a)). For the crystal structures of pyrolusite, it can be concluded from Fig. 7(a) that the main phase of the sample contained SiO2 (JCPDS: 461045), MnO (JCPDS: 78-0424), Mn2SiO4 (JCPDS: 74-0716) and Mn3O4 (JCPDS: 80-0382). Compared with Fig. 7(a) and (d), for the sample with coal addition of 15% roasted at 600 °C for 60 min, it was observed that the Mn3O4 phase disappeared. The disappearance of Mn3O4 phase was attributed to that the increase of roasting temperature and coal addition accelerated the carbothermal reduction of low-grade pyrolusite, rendering the process of MnO2 converted to MnO more complete. Moreover, compared to Fig. 1(b), it was clearly observed from Fig. 7 (a) and (d) that traces of Mn2SiO4 were detected, which was ascribed to the decomposition of (Mn, Fe)2SiO4; meanwhile, the peaks of MnO2 were disappeared, indicating that MnO2 had almost been reduced to lower-valence manganese oxide. Thus, it can be summarized from Fig. 7 that MnO2 was transformed into MnO successfully and completely, without Fe2+ detected, which the patterns results were consistent with the quality indexes analysis results (Fig. 6); moreover,
Fig. 7. XRD and SEM patterns of pyrolusite-coal mixtures, (a) XRD for pyrolusite with 10% coal; (b) SEM, 2000×; (c) SEM, 25000×; (d) XRD for pyrolusite with 15% coal; (e) SEM, 10000×; (f) SEM, 70000×.
Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015
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the findings from Fig. 7 also indicated that the pilot study on microwave-enhanced carbothermal reduction of low-grade pyrolusite was feasible and successful. For the microstructure morphology of pyrolusite, referring to Fig. 1(c) and (d), it can be seen from Fig. 7(b) and (c) that the tighter surface morphology completely disappeared, and was replaced by numerous irregular cracks, holes, and pits. The great change of surface morphology was attributed to the selective heating characteristic of microwave heating. Based on the excellent microwaveabsorbing properties of MnO2 phase, MnO2 was quickly heated to a higher temperature, while gangue components such as SiO2 were difficult to heat, forming a large temperature gradient and causing thermal stress at the interface, further to open the inclusions and produce structural cracks [23,24]. Compared with Fig. 7(b) and (c), it was observed from Fig. 7(e) and (f) that the molten granule were further grown, then interconnected and stacked. The grain shape became more regular and the particle size was smaller, corresponding to the particle size analysis results (Fig. 5). Moreover, after further microwave treatment, it was observed from Fig. 7 that the surface of sample became loose and porous, which was attributed to the release of gas and the volatiles in the pyrolusite-coal mixtures during carbothermal reduction process. 4. Conclusions In the pilot-scale study, applying microwave heating to enhance carbothermal reduction of low-grade pyrolusite was attempted. Results indicated that carbothermal reduction process of lowgrade pyrolusite was divided into three stages: b145 °C, 145 °C– 400 °C, N400 °C. The pyrolusite-coal mixture with 15% coal was completely reduced to MnO powder at 600 °C for 60 min, with a ƞMn value of 95.38%, corresponding to the disappearance of MnO2 peaks and the appearance of MnO peaks. Moreover, the product surface became loose and porous with numerous cracks and holes, and the grain shape became more regular with a smaller particle size after further microwave treatment, which was attributed to the unique selective heating characteristics of microwave heating. Meanwhile, the release of gas and volatiles by carbothermal reduction reactions also contributed to form the porous microstructure. The information presented in this work should provide a sound basis for the full scale applications of microwave heating to the carbothermal reduction of low-grade pyrolusite, furthermore demanding further studies to explore the economic feasibility for commercial adoption. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial supports from the National Natural Science Foundation of China (No: U1802255), the Key Projects in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (No. 2015BAB17B00), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), and Innovative Research Team (in Science and Technology) in University of Yunnan Province were sincerely acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2019.11.015.
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Please cite this article as: K. Li, J. Chen, J. Peng, et al., Pilot-scale study on enhanced carbothermal reduction of low-grade pyrolusite using microwave heating, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.015