Green Processing and Synthesis 2020; 9: 734–743 Research Article Ran Zhao, ZiChen Tian, and Zengwu Zhao* Effect of calcination temperature on rare earth tailing catalysts for catalytic methane combustion + + https://doi.org/10.1515/gps-2020-0053 Ce3 and Ce4 oxidation states and the coexistence of Fe received June 27, 2020; accepted August 09, 2020 in the form of Fe2+ and Fe3+. Moreover, XPS revealed a higher surface O /O ratio. This study provides Abstract: Bayan Obo tailings are rich in rare earth ads latt evidence for the green reuse of Bayan Obo mine tailings elements (REEs), iron, and other catalytic active sub- in secondary resources. stances. In this study, mine tailings were calcined at different temperatures and tested for the catalytic Keywords: rare earth tailings, low-concentration combustion of low-concentration methane. Upon calci- methane, catalytic combustion, calcination nation at 600°C, high catalytic activity was revealed, with 50% CH4 conversion at 587°C (space velocity of 12,000 mL/g h). The physicochemical properties of cata- lysts were characterized using thermogravimetric ana- 1 Introduction lysis, X-ray diffraction, scanning electron microscopy, hydrogen temperature-programmed reduction (H2-TPR), Coal mine gas is an associated gas in coal seams. The and X-ray photoelectron spectroscopy (XPS). Compared main component of coal mine gas is methane (CH4), to the raw ore sample, the diffraction peak intensity of which is often released into the atmosphere during Fe2O3 increased post calcination, whereas that of CeCO3F mining through ventilation pipes. The greenhouse decreased. A porous structure appeared after the catalyst contribution of CH4 to global warming is the emission was calcined at 600°C. Additionally, Fe, Ce, Ti, and other of harmful NOx, CO, and other hydrocarbons. However, metal elements were more highly dispersed on the the synthesis of a reasonably designed catalyst with catalyst surface. H2-TPR results revealed a broadening good low-temperature ignition performance and high- of the reduction temperature range for the catalyst temperature thermal stability remains a challenge [6,7]. calcined at 600°C and an increase in the reduction peak. Although noble metal catalysts, particularly, palladium- XPS analysis indicated the presence of Ce in the form of based catalysts exhibit high activity in the catalytic, 21 times higher than the greenhouse effect of carbon dioxide (CO2)[1–5]. Therefore, effectively eliminating * Corresponding author: Zengwu Zhao, Key Laboratory of low-concentration CH is highly significant for environ- Integrated Exploitation of Baiyan Obo Multi-metal resources, Inner 4 Mongolia University of Science and Technology, Inner Mongolia mental protection. The catalytic combustion of CH4 has Autonomous Region, Baotou, 014010, China; Inner Mongolia many advantages over thermal combustion: reduction in Autonomous Region Rare Earth Secondary Resources Sustainable ignition temperature, complete combustion, and reduc- Utilization Engineering Technology Research Center, Inner Mongolia tion in combustion of CH4; their price and easy sintering University of Science and Technology, Baotou, 014010, China, at high temperatures limit their industrial application e-mail: [email protected], tel: +86-04-72-595-1507, [8–10]. Therefore, searching for a nonnoble metal fax: +86-04-72-595-1507 - Ran Zhao: School of Materials and Metallurgy, Inner Mongolia catalyst to completely oxidize methane at low tempera University of Science and Technology, Baotou, 014010, China; Key ture is important. Laboratory of Integrated Exploitation of Baiyan Obo Multi-metal The different metal components in a mixed metal Resources, Inner Mongolia University of Science and Technology, oxide can interact with each other, e.g., through electron Inner Mongolia Autonomous Region, Baotou, 014010, China coordination or for structural stability, which increases ZiChen Tian: Key Laboratory of Integrated Exploitation of Baiyan Obo Multi-metal Resources, Inner Mongolia University of Science their catalytic activity compared to the corresponding and Technology, Inner Mongolia Autonomous Region, Baotou, single-component catalyst. Currently, catalysts con- 014010, China taining Cu and Mn exhibit high catalytic activity in the Open Access. © 2020 Ran Zhao et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. Effect of calcination temperature on rare earth tailing catalysts for catalytic methane combustion 735 Table 1: Chemical composition of Bayan Obo mine tailings Components SiO2 Fe2O3 CaO TiO2 Na2O MnO2 MgO Al2O3 BaO F REO Others Amount (wt%) 11.9 27.7 27.2 1.00 1.28 1.96 3.31 1.26 2.70 8.92 5.88 6.89 combustion of CH4. Fe and Co catalysts modified by the The composition of the mine tailings is complex; they addition of rare earth elements (REEs) as additives have contain numerous metal elements, including RE oxides attracted research attention [11–13]. Zhang et al. pre- (REO)(5.88%) and Fe2O3 (27.7%). Table 3 displays the pared an Mn–Ni catalyst using a coprecipitation method major mineral species in the mine tailings, including iron and investigated its role in the catalytic combustion of ore, fluorite, bastnaesite, monazite, barite, and ankerite. low-concentration CH4 [14]. Li et al. showed that a The following steps were performed for catalyst Ce1−xFexO2−δ catalyst exhibits not only improved reduci- preparation: a certain amount of tailings containing bility but also increased lattice oxygen, thereby in- REEs were crushed, ground, sieved, and dried and were creasing the catalytic oxidation capacity of the catalyst then divided using 100–200, 200–300, 300–400, and [15]. Zhang et al. showed that the addition of CeO2, ZrO2, 400–500 meshes. The particle size with the best catalytic La2O3, and CeO2–ZrO2 as additives can significantly activity was calcined at 400°C, 500°C, 600°C, and 700°C improve the specific surface area of the catalyst, promote for 4 h, and the resulting samples were labeled as 2, 3, 4, the dispersion of each active component, and increase the and 5, respectively. The original REE-containing mine surface oxygen concentration; thus, the activity of the tailing sample of dolomite was labeled as 1. Cu–Mn–O/Al2O3/COR catalyst is significantly improved [16]. The Bayan Obo deposit in China is the largest REE resource worldwide [17]. The mine tailings in Bayan Obo are generated via separation and flotation. Till date, 160 2.2 Catalytic activity tests million tons of mine tailings have been stockpiled in the Bayan Obo tailings dams. Using or recycling these Figure 1 shows the setup for measuring the activity for resources is difficult owing to many complex factors, catalytic oxidation of methane. The catalysts’ activity such as low grade, fine grain size, and a complex was evaluated using a continuous-flow fixed-bed quartz mineral composition [18,19]. However, the mine tailings tube microreactor (diameter 10 mm). The reaction con- still contain REEs and transition metal elements, of ditions were as follows: 2% CH4, 18% O2, and N2 as the which REEs and iron oxides are a common raw material balance gas, ordinary atmospheric pressure, a volu- for catalyst preparation. Elements, such as Fe, Ce, and metric space velocity (SV) of 20,000 mL/g h, and 500 mg Mn, present in the mine trailing may display synergistic of catalyst. The reaction temperature was increased from effects with other components to increase the catalytic 300°C to 750°C at a heating rate of 5°C/min, and data activity in methane combustion. Therefore, this study were recorded at each temperature point that is uses mine tailings as a raw material to prepare a catalyst stabilized for 30 min. Finally, the methane content was for the catalytic combustion of low-concentration CH4. monitored online via gas chromatography (Agilent This reduces greenhouse gas emission and realizes the 8890B) using a thermal conductivity detector (TCD). reuse of mine tailing resources. The catalytic efficiency of methane was calculated as follows: ωω01− η = × 100% (1) 2 Experiment ω0 where η is the conversion rate of CH4, ω0 is the methane content before the reaction, and ω is the methane 2.1 Catalyst preparation 1 Table 2: Content of RE oxides in Bayan Obo mine tailings The raw materials used in this experiment were mine ( “ tailings containing REEs henceforth referred to as mine Components REO CeO2 Pr2O3 Nd2O3 La2O3 tailings”) from the Bayan Obo area [20]. Tables 1 and 2 Amount (wt%) 5.88 3.01 0.33 1.10 1.44 display the main elements present in the mine tailings. 736 Ran Zhao et al. Table 3: Content of major mineral species in Bayan Obo mine tailings Mineral species Hematite/Magnetite Bastnaesite Fluorite Ankerite Barite Monazite Apatite Amphibole Pyroxene Amount (wt%) 30.01 9.83 17.58 6.51 8.63 3.72 2.79 3.51 1.76 techniques were applied. The thermogravimetric (TG) analysis curve was measured using STA449C, NETZSCH (Germany). The online mass spectrometer (MS)(HPR20) used in this study was manufactured by HIDEN (UK). X-ray fluorescence (XRF) was obtained using a Rigaku ZSX primus (Japan). XRF samples were prepared on glass disk melts in lithium tetraborate using automatic fusion apparatus with a mass ratio of 1:15. The operating voltage and current were 60 kV and 60 mA, respectively. XRF spectrometry was performed under vacuum. X-ray Figure 1: Experimental setup for the catalytic oxidation of methane. diffraction (XRD) results were recorded on a Bruker D8 ( ) ( ) ( ) 1 N2/CH4 mixed gas cylinders; 2 O2 gas cylinders; 3 pressure Advance X-ray diffractometer, wherein the radiation ( ) - ( ) fl ( ) gauge; 4 pressure reducing valve; 5 mass ow meter; 6 source was Cu-Kα, the scanning angle range was reaction valve; (7) quartz tube; (8) thermocouple; (9) furnace wall; – (10) catalyst; (11) quartz cotton; (12) gas chromatograph; (13) 20° 80° with a scanning speed of 3°/min, and the exhaust gas. voltage and current were 40 kV and 40 mA, respectively.