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Effect of microwave irradiation and conventional calcification roasting with calcium hydroxide on the extraction of vanadium and chromium from high-chromium vanadium slag
Minerals Engineering 145 (2020) 106056
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Effect of microwave irradiation and conventional calcification roasting with calcium hydroxide on the extraction of vanadium and chromium from highchromium vanadium slag
T
Huiyang Gaoa, Tao Jianga,b, Mi Zhoua,b, Jing Wena, Xi Lia, Ying Wanga, Xiangxin Xuea,b a b
School of Metallurgy, Northeastern University, Shenyang 110819, Liaoning, China Liaoning Key Laboratory for Recycling Science of Metallurgical Resources, Shenyang 110819, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-chromium vanadium slag Microwave irradiation Calcification roasting Acid leaching
A novel high-efficiency method based on microwave irradiation roasting has been proposed to extract valuable metals from minerals. In this study, we employed microwave irradiation and conventional roasting for extracting vanadium and chromium from high-chromium vanadium slag (HCVS) with the additive calcium hydroxide. Differential thermal-thermogravimetry analysis (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy analysis (SEM-EDS), particle size distribution analysis and chemical analyses were used for the characterization and analysis to explain the mechanism of the roasting process. The results shown that roasting temperature, the m(CaO)/m(V2O5) ratio, and roasting time roasted by microwave irradiation and muffle furnace had significantly effect on extraction ratio of vanadium while the effect on the extraction ratio of chromium was relatively small. For the same roasting temperature, the degree of oxidation of HCVS roasted by microwave irradiation was higher than that roasted by muffle furnace. When the m(CaO)/m (V2O5) ratio is low, Mn4V2O9 and CaV2O6 are formed; with the m(CaO)/m(V2O5) ratio increased, Mn4V2O9 and CaV2O6 gradually converted to CaV2O7, and CaCrO3 is formed. Microwave irradiation decreased the particles size and shortened the roasting time. The optimal roasting conditions was determined to be: roasting temperature of 850 °C, m(CaO)/(V2O5) ratio of 0.95, and roasting time of 1.5 h. The maximum extraction ratio of vanadium and chromium roasted by microwave irradiation and muffle furnace were 98.29%, 4.32% and 85.36%, 2.48%, respectively.
1. Introduction Vanadium is an important metal that is widely used in iron and steel manufacturing as a non-ferrous metal and in chemical industries (Moskalyk and Alfantazi, 2003; Zhang et al., 2011; Zhao et al., 2014). In China, most vanadium is extracted from vanadium titanium magnetite ores in the Pan-Xi and Cheng-De areas, while the rest of the vanadium is extracted from stone coal, uranium ore, spent catalyst, petroleum residues, shale, etc. (Duan et al., 2006; Chen et al., 2013; Li et al., 2016). In recent years, with the ordinary vanadium titanium magnetite ores (Cr2O3 less than 0.05 wt%) in the iron and steel industry consumption, high-chromium vanadium titanium magnetite ores (Cr2O3 0.5–1.2 wt%) has attracted attention of researchers (Zhang et al., 2016). Vanadium slag is the direct raw material for vanadium extraction (Moskalyk and Alfantazi, 2003). High-chromium vanadium slag (HCVS)
is a special kind of vanadium slag which is produced from high-chromium vanadium titanium magnetite ores (Li et al., 2015). During smelting processes, high-chromium vanadium titanium magnetite concentrate is transported into a blast furnace and smelted to form vanadium and chromium-containing molten iron. Then through the blowing process in converter vanadium and chromium are separated from the molten iron and remain in the slag phase forming HCVS, in which content of Cr2O3 greater than 9 wt%. (Yu et al., 2016; Zhang et al., 2016). For considerations of resource utilization and environmental safety, both, vanadium and chromium, should be extracted from HCVS (Li et al., 2015). However, the study of HCVS is not deep enough. Sodium salt roasting-leaching process was a well-known method to recover vanadium from vanadium slag. This method involves the roasting of vanadium slag in air with sodium salts (NaCl, Na2CO3, and Na2SO4) to transform the spinel into sodium vanadate and then leached by water (Lozano and Godinez, 2003; Li, 2012). However, this method
E-mail addresses: [email protected] (H. Gao), [email protected] (T. Jiang), [email protected] (M. Zhou), [email protected] (J. Wen), [email protected] (X. Li), [email protected] (Y. Wang), [email protected] (X. Xue). https://doi.org/10.1016/j.mineng.2019.106056
0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
Minerals Engineering 145 (2020) 106056
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has the following drawbacks: (1) The roasting process has a low energy efficiency; (2) A large volume of harmful gases, such as Cl2, HCl, and SO2, are discharged during the roasting process which corrode equipment and severely contaminate the environment (Zheng et al., 2006); (3) The method is limited by the calcium content due to formation of water insoluble calcium vanadate or calcium-containing vanadium bronzes during the roasting process (Zhou et al., 2012); (4) Water-soluble sodium chromate in the tailings also causes a great environmental pollution. Thus, a new efficient and environmentally friendly method for extracting vanadium and chromium must be developed. The sub-molten salt method is discussed for extraction of vanadium and chromium from HCVS. However, this method consumes a large amount of NaOH, and the high alkaline conditions are very harsh for reactors, pipes, valves and flanges, leading to a high production costs (Chen et al., 2013). Calcification-roasting processing of vanadium slag was first proposed by the Russia Tula factory in the 1970s (Yang et al., 2014). Compared with sodium salt roasting process, calcificationroasting does have the benefits of low costs of additives, no emission of harmful gases, low environment risk due to water-insoluble calcium chromate in tailing. Hence, this method is an economical and promising approach. However, due to a lower recovery of vanadium than achieved with sodium salt roasting, so it was not applied commercially. Microwave irradiation extraction of minerals in metallurgy has shifted into the focus of attention in recent years (Al-Harahsheh and Kingman, 2004, Ouyang et al., 2008; Pickles, 2009a, 2009b). The major advantages of microwave irradiation used in the metallurgical field are the rapid heat transfer, volumetric and selective heating, higher thermal efficiency, pollution-free environment and allows a convenient automatization and process control (Chang et al., 2015). The application of microwave irradiation has been reported for the extraction of vanadium from stone coal (Zhang et al., 2011; Yuan et al., 2015). Microwave roasting has the potential for the reduction of the energy costs of comminution, enhancement of the mineral surface chemistry, and for the facilitation of new forms of metal extraction in a controlled environment (Kingman and Rowson, 1998). However, microwave irradiation used to extraction vanadium from HCVS have rarely been reported. Hence, compare to muffle furnace roasting, the application of microwave irradiation calcification roasting with calcium hydroxide and acid-leaching for HCVS for extraction vanadium and chromium was investigated in this work. We studied the influence of each processing parameter on microwave irradiation roasting and muffle furnace process; these parameters included roasting temperature, the mass ratio of calcium oxide and vanadium oxide (m (CaO)/m(V2O5)), and roasting time. The mechanisms involved in microwave irradiation roasting and muffle furnace roasting were also investigated.
Fig. 1. XRD pattern of HCVS.
Fig. 2. SEM image of HCVS.
and Table 2, respectively. According to the EDS analysis, the bright white area in Fig. 2 is metallic iron which is surround by FeO; the white area is the spinel phase; the gray area is the fayalite phase; and the black area is the augite phase. Ca(OH)2 was dried in an oven at 105 °C for 24 h before used. Deionized water was produced by Millipore Aquelix 5 (Merck Milipore, USA). All reagents used for the leaching process, i.e., hydrochloric acid, sulfuric acid, ammonium ferrous sulfate, urea, sodium nitrite, potassium permanganate and N-phenylanthranilic acid used in the chemical analysis, were of analysis grade.
2. Experiments 2.1. Materials
2.2. Experimental apparatus
HCVS used in this study was collected from the Jian long Iron and Steel Corporation, Ltd., Heilongjiang Province, China. The chemical analysis of the HCVS is shown in Table 1. Table 1 shows that Fe is the most abundant element in HCVS followed by Si, V, and Cr. The X-Ray diffraction (XRD) pattern of HCVS is shown in Fig. 1 and indicates that the main crystalline phases of the sample are spinel, fayalite, and augite. Results from scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy analysis (EDS) of HCVS are given in Fig. 2
Conventional roasting was conducted with a muffle furnace (XL100, Hebi City Billion Yan Instruments Co. LTD, China). The reaction chamber of the muffle furnace was 30 cm × 20 cm × 12 cm in volume. Microwave irradiation roasting experiments were carried with a microwave furnace (MobileLab Workstation, Tangshan Nano Source Microwave Thermal Instrument Manufacturing Co. LTD, China). The
Table 1 Chemical composition of HCVS (mass fraction, %). SiO2
MgO
Al2O3
CaO
Cr2O3
V2O5
TiO2
FeO
MFe
Na
Mn
P
20.45
1.45
3.45
1.25
11.28
12.10
6.66
30.78
4.31
0.68
6.57
0.05
2
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Table 2 EDS analysis of HCVS in Fig. 2 (mass fraction, %). Region
O
Mg
Na
Al
Si
K
Ca
Ti
V
Mn
Fe
Cr
1 2 3 4 5
– 19.96 26.47 38.48 17.91
– – 1.20 – 0.51
– – – 2.71 –
– – 1.39 6.74 1.83
– 1.37 16.42 29.21 –
– – 0.38 1.96 –
– 0.61 0.77 3.82 –
– – 1.92 3.01 6.48
0.43 0.72 0.75 0.67 18.40
– 0.83 9.13 2.20 5.01
99.23 76.51 41.57 11.20 32.23
0.34 – – – 17.62
test, the leach slurry was separated by vacuum filtration, and the residue was washed with deionized water. Then, the residue was dried at 105 °C for 5 h.
frequency of the microwave irradiation was 2.45 GHz. The reactor contained four 1-kW microwave generators that could be cooled with a blowing fan. A thermal insulation sleeve was used to put the microwave transparent crucible. The temperature of sample in the furnace was measured continuously using a stainless steel-sheathed, K-type thermocouple.
2.4. Characterization For chemical analysis, HCVS was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300DV, PerkinElmer, USA). The mineral compositions of the HCVS in this study were characterized by XRD (X’ Pert Pro MPD/PW3040, PANalytical B.V. Corporation, Netherlands) at a scanning rate of 8°/min using Cu Kα radiation with a 2θ angle ranging from 5° to 90°. Microstructure, surface morphology and elemental composition of HCVS and sintered HCVS were characterized by SEM-EDS (SSX-550, Shimadzu Corporation, Japan). The thermal behavior of HCVS, Ca (OH)2, and sintered HCVS was characterized by differential thermalthermogravimetric analysis (TG-DSC, STA449CD, NETZSCH, Germany) with a heating rate of 10 °C/min in atmosphere. The extraction ratio of chromium was characterized with an atomic absorption spectrophotometer (TAS-990, Beijing Purkinje General Instrument Co. LTD., China). The extraction ratio of vanadium was characterized by the ferrous ammonium sulfate titration method and was calculated with the following equation:
2.3. Experimental procedure In this paper, roasting parameters including heating mode, roasting temperature, m(CaO)/m(V2O5) ratio, and roasting time were studied. The experimental flow chart of the novel process based on microwave irradiation and muffle furnace roasting to extract vanadium and chromium is shown in Fig. 3. HCVS was mixed with Ca(OH)2 as an additive into column-shaped samples (diameter: 30 mm, height: 15 mm). The weight of each sample was about 30 g. The prepared five samples with a total mass of 150 g were placed in a 200-mL microwave transparent crucible and heated to holding temperatures in the range of 700–1000 °C in the microwave furnace. Samples prepared with the same weight were place in a corundum crucible and heated to the previously specified temperature in a muffle furnace. Microwave irradiation roasting could achieve the specified roasting temperature in 6 min and thus significantly reduce the roasting time. Further, different m(CaO)/m(V2O5) ratios ranging from 0.15 to 1.15 by adjusting the amount of Ca(OH)2 were considered in the roasting experiments. The microwave irradiation power was set to 2 kW and the roasting time was varied from 0.5 h to 2 h. After the roasting process, the sintered samples were taken out and cooled in air, then crushed and ground to particle sizes below 150 µm for further analysis. The leaching experiments were performed in a three-necked flask at atmospheric pressure. The extraction ratio of vanadium and chromium was characterized based on the acid leaching of 10.00 g sintered HCVS by a 20% H2SO4 solution with a solid-liquid ratio of 1:10 at 95 °C ( ± 0.5 °C) for 2 h and a stirring speed of 500 rpm. After each leaching
η=
m 0 ω0 − m1 ω1 × 100% m 0 ω0
(1)
where η represents the extraction ratio of vanadium, m0 is the mass of sintered sample, m1 is the mass of the tailing after leaching, ω0 is the mass fraction of the vanadium in the sintered sample and ω1 is the mass fraction of vanadium in the tailing. 3. Results and discussion 3.1. TG-DSC curves of HCVS and Ca(OH)2 The TG-DSC curves of HCVS is shown in Fig. 4. The roasting process of HCVS in Fig. 4 can be divided into three different stages: (1) From room temperature to 414.15 °C, HCVS is chemically stable; only
Fig. 3. Flow chart of the novel process based on microwave irradiation and muffle furnace roasting to extract vanadium and chromium.
Fig. 4. TG-DSC curves of HCVS. 3
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Fig. 5. TG-DSC curves of Ca(OH)2.
appearing at 449.89 °C was the decomposition of calcium hydroxide. The decomposition of calcium hydroxide can generate calcium oxide which reacts violently with vanadium oxide in the roasting process. Compared to the endothermic peak at 467.49 °C in Fig. 5, the endothermic peak at 449.98 °C was shifted towards lower temperatures due to the production of CaO which could react with SiO2 and accelerate the reaction. (3) From 460.15 °C to 695.16 °C, a dramatic weight gain of 0.73% and a slight endothermic peak at 628.34 °C could be observed. The endothermic peak at 628.34 °C was similar to that in Fig. 5. (4) The fourth stage proceeded at temperatures ranging from 695.16 °C to 1000 °C with a weight gain of 3.51%, which is in good agreement with Fig. 4. From 610.24 °C to 850.35 °C, a distinct exothermic peak occurred around the 762.04 °C. The exothermic peak was similar to that in the third stage in Fig. 4. In this case, the oxidation of ferrous ions was accompanied by the oxidation of trivalent vanadium and trivalent chromium. In this stage, vanadium oxide and chromium oxide reacted with calcium oxide and generated the corresponding calcium vanadate and calcium chromate. From 850.35 °C to 1000 °C, there was a slight exothermic peak, indicating the augite oxidation and the formation of CaSiO3.
moisture evaporates, resulting in a weight loss of 2.81%. (2) From 414.15 °C to 700.02 °C, the weight of HCVS increases by 1.46%, indicating that a partial oxidation has occurred. (3) When the temperature exceeds 700.02 °C, a dramatic weight gain of 3.94% and a broad exothermic peak occur simultaneously. The cause of the exothermic peak appearing at 754.98 °C is a partial sintering of the HCVS, which accords for the experimental phenomenon. According to the thermodynamic analysis (J. Zhang et al., 2015), the oxidation of fayalite and spinel are both exothermic. Hence, it is assumed that fayalite was oxidized in the second stage from 414.15 °C to 700.02 °C. Based on the SEM-EDS chemical composition analysis of the HCVS, ferrous ions were distributed in the fayalite, spinel, and augite phases. Thus, spinel oxidation and augite oxidation overlap in the temperature range from 700.02 °C to 1000 °C. Fig. 5 shows the TG-DSC curve of Ca(OH)2. When the temperature increased to 360.38 °C, the weight of Ca(OH)2 was reduced by 0.068%. A slight endothermic peak was observed at 100.02 °C, indicating the removal of water of crystallization from Ca(OH)2. From 360.38 °C to 478.00 °C, a dramatic endothermic peak and a weight loss of 21.54% occurred. The cause of this endothermic peak appearing at 467.49 °C was the decomposition of calcium hydroxide. From 607.32 °C to 693.54 °C, an endothermic peak and a weight loss of 1.67% occurred. The cause of the endothermic peak at 658.37 °C was the decomposition of calcium carbonate. The calcium carbonate was generated during the TG-DSC experiment. Compared to the theoretical weight loss rate (24.32%), the calcium hydroxide of weight loss rate was 23.27%. Hence, this result is in good agreement with the theoretical value within the experiment error. To explore the effect of Ca(OH)2 on TG-DSC behavior of HCVS, we conducted measurement of HCVS with Ca(OH)2. The TG-DSC curve of HCVS with the addition of Ca(OH)2 in the m(CaO)/m(V2O5) ratio of 0.95 is shown in Fig. 6. Compared with the curves of HCVS in Fig. 4, the roasting process of HCVS with addition of Ca(OH)2 in Fig. 6 can be divided into four different stages: (1) From room temperature to 383.89 °C, there was an endothermic peak around 129.27 °C and the weight of the HCVS decreased by 0.62%. The weight loss rate of HCVS could be attributed to the loss of crystallization. (2) From 383.89 °C to 460.15 °C, a dramatic endothermic peak occurred at 449.89 °C accompanied by a weight loss of 2.53%. The cause of the endothermic peak
3.2. Effect of roasting temperature on the extraction ratio of vanadium and chromium It was found that microwave irradiation had an effect on the extraction ratio of vanadium (G. Zhang et al., 2015). Notable, the crucial point of roasting by either microwave irradiation or muffle furnace was the extent of oxidative roasting of HCVS. This section of paper discusses the effect of roasting temperature on the extraction ratio of vanadium and chromium under the conditions where Ca(OH)2 was added with a m(CaO)/m(V2O5) ratio of 0.95 and a roasting time of 2 h. Fig. 7 shows that effect of roasting temperature on the extraction ratio of vanadium and chromium. Compared to muffle furnace, microwave irradiation improved the extraction ratio of vanadium. The maximum extraction ratio of vanadium increased by around 12.93% at 850 °C. The extraction ratio of vanadium roasted by microwave irradiation was higher than that of roasted by muffle furnace for temperature from 300 °C to 1000 °C, due to the fact that microwave irradiation roasting caused cracks in the mineral particles (Zhang et al., 4
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Fig. 6. TG-DSC curves of HCVS with addition Ca (OH)2 of the m(CaO)/m(V2O5) ratio of 0.95.
vanadates would be impeded. On the contrary, a further increase in the roasting temperature led to a decrease in the vanadium extraction; due to the fact that the oxidation and formation of calcium vanadates are exothermic and thus increasing roasting temperature will decrease the vanadium recovery. For all roasting temperatures, the extraction ratio of chromium was constant and remained lower than 8%. Taking this observation into account, the optimum roasting temperature was 850 °C. To investigate the evolution of the vanadium and chromium bearing phase during the roasting process, HCVS roasted by microwave irradiation and muffle furnace at different temperatures was investigated by XRD. Fig. 8 shows XRD patterns of HCVS with the addition Ca(OH)2 in a m(CaO)/m(V2O5) ratio of 0.95 roasting by muffle furnace at different temperatures for 2 h. The XRD measurements demonstrated that, at 500 °C, the fayalite phase was not completely oxidized and decomposed into Fe2O3 and SiO2. When roasting temperature increased to 700 °C, the fayalite phase was completely oxidized and decomposed. The decomposition of calcium hydroxide took place at temperatures below 500 °C, which agrees well with the TG-DSC result displayed in
Fig. 7. Effect of roasting temperature on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
2016). The extraction ratio of both, vanadium roasted by microwave irradiation and muffle furnace, initially increased with the roasting temperature but decreased for temperatures greater than 850 °C. Compared to the effects of the roasting temperature and heating mode on the extraction ratio of vanadium, the effects of roasting temperature and heating mode on extraction ratio of chromium were smaller and the extraction ratio of chromium was lower than 10%. The extraction ratio of chromium roasted by microwave irradiation increased from 4.75% to 7.35% and then slightly decreased to 3.62% for roasting temperatures from 300 °C to 600 °C and up to 1000 °C. Similarly, the extraction ratio of chromium roasted by muffle furnace increased from 3.67% to 6.51% and then slightly decreased to 2.16% for roasting temperatures from 300 °C to 600 °C and up to 1000 °C. As shown in Fig. 7, the roasting temperature is a key parameter for extraction ratio of vanadium. When the temperature was lower than 500 °C, the oxidation of fayalite and spinel was insufficient. According to the TG-DSC analyses in Fig. 5, the decomposition of calcium hydroxide was not complete and the reaction to generate the calcium
Fig. 8. XRD pattern of HCVS with addition Ca(OH)2 of the m(CaO)/m(V2O5) ratio of 0.95 roasting by muffle furnace at different temperatures for 2 h. 5
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Fig. 9. XRD pattern of HCVS with addition Ca(OH)2 of the m(CaO)/m(V2O5) ratio of 0.95 roasting by microwave irradiation at different temperatures for 2 h.
Fig. 10. XRD pattern of HCVS with the addition of Ca(OH)2 at different m (CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h.
Fig. 5. When roasting temperature increased to 700 °C, the spinel phase was oxidized and reacted with the calcium oxide. Peaks of Ca2V2O7 gained intensity above 700 °C, which is in accordance with the steep increase in the extraction ratio of vanadium in the temperature range from 500 °C to 700 °C, as shown in Fig. 7. It is noteworthy that, above 700 °C, the intermediate product Fe3O4 was converted completely to Fe2O3 by reacting with O2. As roasting temperature increased, peaks of Cr2O3 gained intensity above 700 °C. When roasting temperature was higher than 800 °C, FeCr2O4 was converted into Fe2O3 and Cr2O3. Fig. 9 shows the XRD patterns of HCVS with the addition Ca(OH)2 in a m(CaO)/m(V2O5) ratio of 0.95 roasting by microwave irradiation at different temperatures for 2 h. Fig. 9 shows that roasting temperature for the complete fayalite oxidation was 500 °C while the roasting temperature roasting by muffle furnace is 700 °C. The decomposition of calcium hydroxide took place at temperatures below 500 °C, which agrees well with the results displayed in Fig. 8. When roasting temperature increased to 700 °C, the spinel phase was oxidized and reacted with calcium oxide. As roasting temperature increased further, peaks of Ca2V2O7 increased in intensity, which is in accordance with the results shown in Fig. 8. Along with the increase in the peak intensity of Fe2O3, Fe3O4 was completely oxidized above 900 °C. As roasting temperature increased, peaks of Cr2O3 and CaSiO3 increased in intensity. It is noteworthy that, above 500 °C, FeCr2O4 was converted into Fe2O3 and Cr2O3. During the roasting process, the fayalite and diopside phases were oxidized and generated a large amount of SiO2, which then was converted to calcium silicate above 500 °C. With the same roasting temperatures, we can conclude that the degree of oxidation of HCVS roasted by microwave irradiation is larger than that roasted by muffle furnace which is in agreement with the results shown in Fig. 7.
CaO + Cr2O3 = CaCr2O4
Fig. 10 shows the XRD of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasted by microwave irradiation at 850 °C for 2 h. CaV2O6 and Mn4V2O9 were the major vanadates observed for m(CaO)/m(V2O5) ratio of 0.15 and 0.30, while Ca2V2O7 and Ca3V2O8 were not observed due to the low content of Ca(OH)2. Furthermore, Mn4V2O9 could also be leached by sulfuric acid to obtain (VO2)SO4. As Ca(OH)2 was added up to a ratio of 0.60, Ca2V2O7, CaCrO3, and CaSiO3 were generated; additionally, CaV2O6 and Mn4V2O9 were observed. The intensity of diffraction peaks of CaSiO3 increased with increasing m(CaO)/m(V2O5) ratio. The XRD results in Fig. 10 reveal that the ratio of 0.75 was the same as 0.60. When more Ca(OH)2 was added up to a m(CaO)/m(V2O5) ratio of 0.95, the diffraction peaks of Mn4V2O9 disappeared and the diffraction peaks of CaV2O6 and Ca2V2O7 could be observed. Along with the increase of m (CaO)/m(V2O5) ratio to 1.15, the observed calcium vanadate was Ca2V2O7. The diffraction peak intensity of CaSiO3 became gradually stronger with an increase in m(CaO)/m(V2O5) ratio from 0.60 to 1.15. In order to study the effect of m(CaO)/m(V2O5) ratio of on extraction ratio of vanadium and chromium, SEM with EDS was employed, the images of which shown in Fig. 11. In this study, the roasting process of HCVS was performed with Ca(OH)2 as additive at different m(CaO)/ m(V2O5) ratios and roasted by microwave irradiation at 850 °C for 2 h. The results of relevant EDS analysis of different regions in Fig. 11 are listed in Table 3. The elements Fe, O, and Si were distributed homogenously, while V, Ti, Al, K, Mg, Na, and Cr were inhomogenously distributed. According to Table 3, the points (c)-1 and (d)-4 both corresponded to Ca2V2O7. The point (d)-1 was composed of Ca2V2O7 and CaSiO3. With an increased m(CaO)/m(V2O5) ratio, the Ca, V, and Mn were mainly distributed in the bright white region where the content of V, Ca, and Mn was increased, which is accordance with the calculation shown in Fig. 11. Furthermore, the particle size increased significantly as m(CaO)/m(V2O5) ratio increased. Fig. 12 shows the combined effect of m(CaO)/m(V2O5) ratio on the extraction ratio of vanadium and chromium. In this section, the roasting process of HCVS was performed under the condition of adding Ca(OH)2 as additive at different m(CaO)/m(V2O5) ratios roasted by microwave irradiation and muffle furnace at 850 °C for 2 h. It is obvious that, for m (CaO)/m(V2O5) ratio from 0.15 to 1.15, the extraction ratio of vanadium roasted by microwave irradiation is higher than of that of roasted by muffle furnace. The maximum extraction ratio of vanadium roasted by microwave irradiation was 97.53% for the m(CaO)/m(V2O5) ratio of 0.95, while the extraction ratio of vanadium roasted by muffle furnace was 87.52%. According to the XRD analysis, when HCVS was roasted
3.3. Effect of m(CaO)/m(V2O5) ratio on the extraction ratio of vanadium and chromium The m(CaO)/m(V2O5) ratio is a further important parameter for the extraction ratio of vanadium and chromium. Yang et al. (2014) investigated calcification roasting vanadium slag with a high CaO content. CaO was used as additives during the calcification roasting process to produce calcium vanadates and calcium chromate, including CaV2O6, Ca2V2O7, Ca3V2O8 and CaCr2O4. The reactions are expressed as follows: CaO + V2O5 = CaV2O6
(2)
2CaO + V2O5 = Ca2V2O7
(3)
3CaO + V2O5 = Ca3V2O8
(4)
(5)
6
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Fig. 11. SEM images of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h: (a) 0.15; (b) 0.30; (c) 0.60; (d) 1.15.
3.4. Effect of roasting time on the extraction ratio of vanadium and chromium
with the addition of Ca(OH)2 at a mass ratio of 0.95, the present calcium vanadate was CaV2O7. Our results reveal that the calcium vanadate CaV2O7 can be easily dissolved in sulfuric acid and thus can improve the extraction ratio of vanadium. When more Ca(OH)2 was added to the HCVS, a large amount of SiO2 was converted to CaSiO3 which decreased the extraction ratio of vanadium during the acid leaching process. With m(CaO)/m(V2O5) ratio increasing from 0.15 to 1.15, the extraction ratio of chromium roasted by microwave irradiation and muffle furnace increased slowly from 3.29% to 4.53% and from 1.22% to 2.62%, respectively. In order to obtain a high extraction ratio of vanadium with less required Ca(OH)2 during roasting processing, 0.95 is believed to be the optimal m(CaO)/m(V2O5) ratio. With this ratio, vanadium and chromium extractions are at a maximum with 97.53% and 4.32%, respectively.
Fig. 13 shows the effect of roasting time on the extraction ratio of vanadium and chromium under the conditions of a roasting temperature 850 °C and with the addition Ca(OH)2 of m(CaO)/m(V2O5) ratio of 0.95. Fig. 13 reveals that roasting time had as large effect on the extraction ratio of vanadium. When roasting time was short, oxidation and calcification of vanadium and chromium were insufficient. Conversely, a long roasting time reduced the production efficiency. Compared with the roasted by muffle furnace, Fig. 13 also shows that roasted by microwave irradiation was beneficial for improving the extraction ratio of vanadium; in detail, the maximum extraction ratio of vanadium was increased by 10.22%. For the same roasting time, the oxidation degree of HCVS roasted by microwave irradiation was larger than for that roasting by muffle furnace; an observation which is in
Table 3 EDS analysis of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h (mass fraction, %). Region
O
Al
Si
K
Ti
Ca
V
Cr
Mn
Fe
Na
Mg
(a)-1 (a)-2 (a)-3 (b)-1 (b)-2 (b)-3 (c)-1 (c)-2 (c)-3 (c)-4 (d)-1 (d)-2 (d)-3 (d)-4
37.50 9.26 42.52 44.98 43.96 24.40 46.87 9.86 38.77 22.88 42.21 44.58 31.45 44.30
0.84 1.03 3.21 2.05 5.41 0.83 0.24 0.78 0.61 0.64 1.02 11.02 1.19 0.16
3.61 2.22 19.26 12.86 31.55 – 1.61 0.60 4.90 1.69 5.82 27.24 5.40 1.22
0.17 – 0.48 0.30 2.31 – – – – – 0.14 0.50 0.15 –
13.91 4.75 2.01 2.29 1.06 1.21 0.71 3.43 6.54 3.74 2.63 0.35 2.76 0.57
1.29 0.87 1.74 2.37 0.89 1.56 20.91 3.44 8.03 6.19 19.80 5.38 10.10 21.72
4.01 4.01 2.63 6.82 1.39 5.80 23.47 6.13 3.86 6.62 13.65 1.40 8.60 25.97
9.92 4.13 1.47 5.03 0.25 22.56 0.34 33.51 2.70 10.60 1.86 0.63 11.18 0.69
3.71 3.77 4.20 4.50 4.34 16.04 2.38 24.70 4.55 7.00 3.48 0.60 6.91 2.04
25.05 69.55 25.51 18.80 6.18 26.25 2.71 16.71 29.01 39.74 8.30 3.28 20.58 3.01
– 0.25 1.31 – 2.22 – 0.49 0.24 – 0.36 0.69 5.02 0.92 0.33
– 0.16 0.67 – 0.45 1.35 0.28 0.60 1.02 0.54 0.40 – 0.75 –
7
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bed roasting, microwave irradiation is a high-efficiency method which requires less time and thus decreases the cost of production. As an effective and environmentally friendly technique, the analysis of the interaction mechanism of microwaves with HCVS is attracting increasing attention. 3.5. Analysis of the mechanisms in HCVS roasted by microwave irradiation and muffle furnace Understanding the roasting process is the key to extract vanadium and chromium efficiently from HCVS. In order to understand the effect of roasted by microwave irradiation and muffle furnace on the oxidation behavior, the apparent morphology and mechanism of the roasting process should be studied. We can calculate that the high efficiency of the microwave irradiation can be explained by the heating mechanism. Fig. 14 shows a schematic representation of the mechanisms of HCVS roasted by microwave irradiation and muffle furnace. Microwave is energy that is converted into heat energy depending upon the dielectric and magnetic properties of the heated media. Microwave irradiation causes heating from the interior (Zhang et al., 2016). The surface temperature of a sample is lower than its inside temperature and the heating rate is very high. Due to different dielectric constant and thermal capacities, different minerals phases have different heat propagation speeds and temperature, thereby giving rise to large temperature gradients which can induce cracks at the mineral phase boundaries (Jiang et al., 2016). Hence, microwave irradiation results in tiny cracks on the surface of the HCVS samples and reduces the strength of HCVS. So, the particle size of HCVS roasted by microwave irradiation reduces and the oxidation of HCVS roasted by microwave irradiation is full. HCVS roasted by muffle furnace is heated from the surface to the interior and the heating rate is low. The small temperature gradient cannot induce cracks. This is the reason why the degree of oxidation of HCVS roasted by microwave irradiation is higher than that of HCVS roasted by muffle furnace. To better interpret this mechanism, particle size distribution analysis of HCVS roasted by microwave irradiation and muffle furnace under the optimal roasting condition is shown in Fig. 15. The particle size distribution of the HCVS roasted by muffle furnace decreased and the D(50) decreased to 12.36 μm. However, the particle size distribution of the HCVS roasted by microwave irradiation is noticeably smaller and the D(50) decreased to 11.73 μm. The smaller size of HCVS can increase the contract area for gas-solid reactions. In addition, the results of Yuan et al. showed that microwave irradiation roasted stone coal contains more cracks and that the coal particles are more porous compared to the conventionally roasted samples (Yuan et al., 2015). Microwave irradiation causes a larger change of the mineral structure at lower temperatures and for shorter roasting time than conventional roasting methods. With the increasing roasting temperatures, HCVS will be sintered and cracks will be reduced or even disappear completely. The extraction ratio of vanadium and chromium initially increased and then decreased for higher temperature.
Fig. 12. Effect of m(CaO)/m(V2O5) ratio on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
Fig. 13. Effect of roasting time on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
agreement with the results shown in Figs. 8 and 9. The extraction ratio of vanadium roasted by microwave irradiation increased from 75.94% to 98.47% and then remained constant for roasting times from 0.5 h to 2.5 h. Likewise, the extraction ratio of vanadium roasted by muffle furnace increased from 65.83% to 88.25% for roasting time from 0.5 h to 2.5 h. As the roasting time increased, the extraction ratio of chromium increased slowly. The extraction ratio of chromium roasted by microwave irradiation increased from 3.92% to 4.39% and was higher than the extraction ratio of chromium roasted by muffle furnace. In order to obtain a high extraction ratio of vanadium, a roasting time of 1.5 h for microwave irradiation is considered as optimal roasting time. Li et al. have reported that 87.9% of vanadium can be extracted from vanadium-chromium slag during the stage of fractional sodium roasting-water leaching, while only 6.3% Cr can be extracted within 2 h (Li et al., 2015). Liu et al. (2013) developed a new NaOH-NaNO3 melt to treat vanadium-chromium slags. Under the optimum conditions, the extraction ratio of vanadium can reach 93.7% in 6 h. Zeng, Wang et al. have developed a roasting technology (static roasting in a muffle oven and fluidized bed roasting) for extracting vanadium from stone coal and the available maximum vanadium extraction ratio reached 91% (Zeng et al., 2015). Compared to the method of stepwise roasting with sodium salts, sub-molten salt, static roasting in a muffle furnace, and fluidized
4. Conclusions A novel effective method based on roasted by microwave irradiation with the help of a muffle furnace to extract vanadium and chromium from HCVS has been proved to be feasible. For the same roasting temperature and roasting time, the oxidation degree of HCVS roasted by microwave irradiation was higher than that of HCVS roasted by muffle furnace. The optimal temperature for extraction ratio of vanadium roasted by microwave irradiation and muffle furnace were 850 °C; the extraction ratios of vanadium and chromium were 98.29%, 4.32% and 85.36%, 2.48%, respectively. During the calcification roasting process, when the m(CaO)/m (V2O5) ratio is low, Mn4V2O9 and CaV2O6 are formed; with the m (CaO)/m(V2O5) ratio increased, Mn4V2O9, CaV2O6 gradually converted 8
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Fig. 14. Schematic representation of the mechanisms of HCVS roasting by microwave irradiation and muffle furnace.
while muffle furnace roasting does not have this effect. The effect of roasting time on the extraction ratio of vanadium roasted by microwave irradiation and muffle furnace followed the same trend. The optimal roasting time for roasting by microwave irradiation and muffle furnace was 1.5 h in both case. Acknowledgement This research was financially supported by the Programs of the National Natural Science Foundation of China (Nos. 51604065 and 51574082), the National Basic Research Program of China (973 Program) (No. 2013CB632603), the Fundamental Funds for the Central Universities (Nos. 150203003, 150202001). References Al-Harahsheh, M., Kingman, S.W., 2004. Microwave-assisted leaching—a review. Hydrometallurgy 73 (3–4), 189–203. Chang, Jun, Zhang, Libo, Yang, Changjiang, et al., 2015. Kinetics of microwave roasting of zinc slag oxidation dust with concentrated sulfuric acid and water leaching. Chem. Eng. Process. 97, 75–83. Chen, Desheng, Zhao, Longsheng, Liu, Yahui, et al., 2013. A novel process for recovery of iron, titanium, and vanadium from titanomagnetite concentrates: NaOH molten salt roasting and water leaching processes. J. Hazard. Mater. 244–245, 588–595. Duan, Lian, Tian, Qinghua, Guo, Xueyi, 2006. Review on production and utilization of vanadium resources in China. Hunan Nonferrous Metals 22 (6), 17–20. Jiang, Tao, Zhang, Qiaoyi, Liu, Yajing, et al., 2016. Influence of microwave irradiation on boron concentrate activation with an emphasis on surface properties. Appl. Surf. Sci. 385, 88–98.
Fig. 15. Particle distribution of HCVS roasting by microwave irradiation and muffle furnace with the m(CaO)/m(V2O5) ratios of 0.95 at 850 °C in 2 h.
to CaV2O7 and CaCrO3 is formed. The maximum extraction ratio of vanadium roasted by either, microwave irradiation or muffle furnace, was realized for m(CaO)/m(V2O5) ratio of 0.95. Microwave irradiation could achieve the specified roasting temperature within 6 min and thus significantly reduce the roasting time. Microwave irradiation is beneficial to reduce the particle size of HCVS 9
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133, 105–111. Yu, Kaiping, Chen, Bo, Zhang, Hongling, et al., 2016. An efficient method of chromium extraction from chromium-containing slag with a high silicon content. Hydrometallurgy 162, 86–93. Yuan, Yizhoug, Zhang, Yimin, Liu, Tao, et al., 2015. Comparison of the mechanisms of microwave roasting and conventional roasting and of their effects on vanadium extraction from stone coal. Int. J. Miner., Metall. Mater. 22 (5), 476–482. Xi, Zeng, Wang, Fang, Zhang, Huifeng, et al., 2015. Extraction of vanadium from stone coal by roasting in a fluidized bed reactor. Fuel 142, 180–188. Zhang, Yimin, Bao, Shenxu, Liu, Tao, et al., 2011. The technology of extracting vanadium from stone coal in China: history, current status and future prospects. Hydrometallurgy 109 (1–2), 116–124. Zhang, Xuefei, Liu, Fengguo, Xue, Xiangxin, et al., 2016. Effects of microwave and conventional blank roasting on oxidation behavior, microstructure and surface morphology of vanadium slag with high chromium content. J. Alloys Comp. 686, 356–365. Zhang, Guoquan, Zhang, Tingan, Lü, Zhiguo, et al., 2015. Effects of microwave roasting on the kinetics of extracting vanadium from vanadium slag. Jom 68 (2), 577–584. Zhang, Juhua, Zhang, Wei, Li, Zhang, et al., 2015. Mechanism of vanadium slag roasting with calcium oxide. Int. J. Miner. Process. 138, 20–29. Zhao, Longsheng, Wang, Lina, Qi, Tao, et al., 2014. A novel method to extract iron, titanium, vanadium, and chromium from high-chromium vanadium-bearing titanomagnetite concentrates. Hydrometallurgy 149, 106–109. Zheng, S., Zhang, Y., Li, Z., et al., 2006. Green metallurgical processing of chromite. Hydrometallurgy 82 (3–4), 157–163. Zhou, Lanhua, Wang, Jun, Gou, Shuyun, et al., 2012. Development of utilization of vanadic titanomagnetite. Appl. Mech. Mater. 184–185, 949–953.
Kingman, S.W., Rowson, N.A., 1998. Microwave treatment of minerals—a review. Miner. Eng. 11 (11), 1081–1087. Li, Hongyi, Fang, Haixing, Wang, Kang, et al., 2015. Asynchronous extraction of vanadium and chromium from vanadium slag by stepwise sodium roasting–water leaching. Hydrometallurgy 156, 124–135. Li, Hongyi, Wang, Kang, Hua, Weihao, et al., 2016. Selective leaching of vanadium in calcification-roasted vanadium slag by ammonium carbonate. Hydrometallurgy 160, 18–25. Li, Xinsheng, 2012. Extraction of vanadium from high calcium vanadium slag using direct roasting and soda leaching. Liu, Biao, Du, Hao, Wang, Shaona, et al., 2013. A novel method to extract vanadium and chromium from vanadium slag using molten NaOH-NaNO3 binary system. AIChE J. 59 (2), 541–552. Lozano, L.J., Godinez, C., 2003. Comparative study of solvent extraction of vanadium from sulphate solutions by primene 81R and alamine 336. Miner. Eng. 16 (3), 291–294. Moskalyk, R.R., Alfantazi, A.M., 2003. Processing of vanadium: a review. Miner. Eng. 16 (9), 793–805. Ouyang, Guoqiang, Zhang, Xiaoyun, Tian, Xueda, et al., 2008. Effect of microwave roasting on vanadium extraction from stone coal. Chinese J. Nonferrous Metals 18 (4), 750–754. Pickles, C.A., 2009a. Microwaves in extractive metallurgy: Part 1 – Review of fundamentals. Miner. Eng. 22 (13), 1102–1111. Pickles, C.A., 2009b. Microwaves in extractive metallurgy: Part 2 – A review of applications. Miner. Eng. 22 (13), 1112–1118. Yang, Zhao, Li, Hongyi, Yin, Xuchen, et al., 2014. Leaching kinetics of calcification roasted vanadium slag with high CaO content by sulfuric acid. Int. J. Miner. Process.
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Effect of microwave irradiation and conventional calcification roasting with calcium hydroxide on the extraction of vanadium and chromium from highchromium vanadium slag
T
Huiyang Gaoa, Tao Jianga,b, Mi Zhoua,b, Jing Wena, Xi Lia, Ying Wanga, Xiangxin Xuea,b a b
School of Metallurgy, Northeastern University, Shenyang 110819, Liaoning, China Liaoning Key Laboratory for Recycling Science of Metallurgical Resources, Shenyang 110819, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-chromium vanadium slag Microwave irradiation Calcification roasting Acid leaching
A novel high-efficiency method based on microwave irradiation roasting has been proposed to extract valuable metals from minerals. In this study, we employed microwave irradiation and conventional roasting for extracting vanadium and chromium from high-chromium vanadium slag (HCVS) with the additive calcium hydroxide. Differential thermal-thermogravimetry analysis (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy analysis (SEM-EDS), particle size distribution analysis and chemical analyses were used for the characterization and analysis to explain the mechanism of the roasting process. The results shown that roasting temperature, the m(CaO)/m(V2O5) ratio, and roasting time roasted by microwave irradiation and muffle furnace had significantly effect on extraction ratio of vanadium while the effect on the extraction ratio of chromium was relatively small. For the same roasting temperature, the degree of oxidation of HCVS roasted by microwave irradiation was higher than that roasted by muffle furnace. When the m(CaO)/m (V2O5) ratio is low, Mn4V2O9 and CaV2O6 are formed; with the m(CaO)/m(V2O5) ratio increased, Mn4V2O9 and CaV2O6 gradually converted to CaV2O7, and CaCrO3 is formed. Microwave irradiation decreased the particles size and shortened the roasting time. The optimal roasting conditions was determined to be: roasting temperature of 850 °C, m(CaO)/(V2O5) ratio of 0.95, and roasting time of 1.5 h. The maximum extraction ratio of vanadium and chromium roasted by microwave irradiation and muffle furnace were 98.29%, 4.32% and 85.36%, 2.48%, respectively.
1. Introduction Vanadium is an important metal that is widely used in iron and steel manufacturing as a non-ferrous metal and in chemical industries (Moskalyk and Alfantazi, 2003; Zhang et al., 2011; Zhao et al., 2014). In China, most vanadium is extracted from vanadium titanium magnetite ores in the Pan-Xi and Cheng-De areas, while the rest of the vanadium is extracted from stone coal, uranium ore, spent catalyst, petroleum residues, shale, etc. (Duan et al., 2006; Chen et al., 2013; Li et al., 2016). In recent years, with the ordinary vanadium titanium magnetite ores (Cr2O3 less than 0.05 wt%) in the iron and steel industry consumption, high-chromium vanadium titanium magnetite ores (Cr2O3 0.5–1.2 wt%) has attracted attention of researchers (Zhang et al., 2016). Vanadium slag is the direct raw material for vanadium extraction (Moskalyk and Alfantazi, 2003). High-chromium vanadium slag (HCVS)
is a special kind of vanadium slag which is produced from high-chromium vanadium titanium magnetite ores (Li et al., 2015). During smelting processes, high-chromium vanadium titanium magnetite concentrate is transported into a blast furnace and smelted to form vanadium and chromium-containing molten iron. Then through the blowing process in converter vanadium and chromium are separated from the molten iron and remain in the slag phase forming HCVS, in which content of Cr2O3 greater than 9 wt%. (Yu et al., 2016; Zhang et al., 2016). For considerations of resource utilization and environmental safety, both, vanadium and chromium, should be extracted from HCVS (Li et al., 2015). However, the study of HCVS is not deep enough. Sodium salt roasting-leaching process was a well-known method to recover vanadium from vanadium slag. This method involves the roasting of vanadium slag in air with sodium salts (NaCl, Na2CO3, and Na2SO4) to transform the spinel into sodium vanadate and then leached by water (Lozano and Godinez, 2003; Li, 2012). However, this method
E-mail addresses: [email protected] (H. Gao), [email protected] (T. Jiang), [email protected] (M. Zhou), [email protected] (J. Wen), [email protected] (X. Li), [email protected] (Y. Wang), [email protected] (X. Xue). https://doi.org/10.1016/j.mineng.2019.106056
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has the following drawbacks: (1) The roasting process has a low energy efficiency; (2) A large volume of harmful gases, such as Cl2, HCl, and SO2, are discharged during the roasting process which corrode equipment and severely contaminate the environment (Zheng et al., 2006); (3) The method is limited by the calcium content due to formation of water insoluble calcium vanadate or calcium-containing vanadium bronzes during the roasting process (Zhou et al., 2012); (4) Water-soluble sodium chromate in the tailings also causes a great environmental pollution. Thus, a new efficient and environmentally friendly method for extracting vanadium and chromium must be developed. The sub-molten salt method is discussed for extraction of vanadium and chromium from HCVS. However, this method consumes a large amount of NaOH, and the high alkaline conditions are very harsh for reactors, pipes, valves and flanges, leading to a high production costs (Chen et al., 2013). Calcification-roasting processing of vanadium slag was first proposed by the Russia Tula factory in the 1970s (Yang et al., 2014). Compared with sodium salt roasting process, calcificationroasting does have the benefits of low costs of additives, no emission of harmful gases, low environment risk due to water-insoluble calcium chromate in tailing. Hence, this method is an economical and promising approach. However, due to a lower recovery of vanadium than achieved with sodium salt roasting, so it was not applied commercially. Microwave irradiation extraction of minerals in metallurgy has shifted into the focus of attention in recent years (Al-Harahsheh and Kingman, 2004, Ouyang et al., 2008; Pickles, 2009a, 2009b). The major advantages of microwave irradiation used in the metallurgical field are the rapid heat transfer, volumetric and selective heating, higher thermal efficiency, pollution-free environment and allows a convenient automatization and process control (Chang et al., 2015). The application of microwave irradiation has been reported for the extraction of vanadium from stone coal (Zhang et al., 2011; Yuan et al., 2015). Microwave roasting has the potential for the reduction of the energy costs of comminution, enhancement of the mineral surface chemistry, and for the facilitation of new forms of metal extraction in a controlled environment (Kingman and Rowson, 1998). However, microwave irradiation used to extraction vanadium from HCVS have rarely been reported. Hence, compare to muffle furnace roasting, the application of microwave irradiation calcification roasting with calcium hydroxide and acid-leaching for HCVS for extraction vanadium and chromium was investigated in this work. We studied the influence of each processing parameter on microwave irradiation roasting and muffle furnace process; these parameters included roasting temperature, the mass ratio of calcium oxide and vanadium oxide (m (CaO)/m(V2O5)), and roasting time. The mechanisms involved in microwave irradiation roasting and muffle furnace roasting were also investigated.
Fig. 1. XRD pattern of HCVS.
Fig. 2. SEM image of HCVS.
and Table 2, respectively. According to the EDS analysis, the bright white area in Fig. 2 is metallic iron which is surround by FeO; the white area is the spinel phase; the gray area is the fayalite phase; and the black area is the augite phase. Ca(OH)2 was dried in an oven at 105 °C for 24 h before used. Deionized water was produced by Millipore Aquelix 5 (Merck Milipore, USA). All reagents used for the leaching process, i.e., hydrochloric acid, sulfuric acid, ammonium ferrous sulfate, urea, sodium nitrite, potassium permanganate and N-phenylanthranilic acid used in the chemical analysis, were of analysis grade.
2. Experiments 2.1. Materials
2.2. Experimental apparatus
HCVS used in this study was collected from the Jian long Iron and Steel Corporation, Ltd., Heilongjiang Province, China. The chemical analysis of the HCVS is shown in Table 1. Table 1 shows that Fe is the most abundant element in HCVS followed by Si, V, and Cr. The X-Ray diffraction (XRD) pattern of HCVS is shown in Fig. 1 and indicates that the main crystalline phases of the sample are spinel, fayalite, and augite. Results from scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy analysis (EDS) of HCVS are given in Fig. 2
Conventional roasting was conducted with a muffle furnace (XL100, Hebi City Billion Yan Instruments Co. LTD, China). The reaction chamber of the muffle furnace was 30 cm × 20 cm × 12 cm in volume. Microwave irradiation roasting experiments were carried with a microwave furnace (MobileLab Workstation, Tangshan Nano Source Microwave Thermal Instrument Manufacturing Co. LTD, China). The
Table 1 Chemical composition of HCVS (mass fraction, %). SiO2
MgO
Al2O3
CaO
Cr2O3
V2O5
TiO2
FeO
MFe
Na
Mn
P
20.45
1.45
3.45
1.25
11.28
12.10
6.66
30.78
4.31
0.68
6.57
0.05
2
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Table 2 EDS analysis of HCVS in Fig. 2 (mass fraction, %). Region
O
Mg
Na
Al
Si
K
Ca
Ti
V
Mn
Fe
Cr
1 2 3 4 5
– 19.96 26.47 38.48 17.91
– – 1.20 – 0.51
– – – 2.71 –
– – 1.39 6.74 1.83
– 1.37 16.42 29.21 –
– – 0.38 1.96 –
– 0.61 0.77 3.82 –
– – 1.92 3.01 6.48
0.43 0.72 0.75 0.67 18.40
– 0.83 9.13 2.20 5.01
99.23 76.51 41.57 11.20 32.23
0.34 – – – 17.62
test, the leach slurry was separated by vacuum filtration, and the residue was washed with deionized water. Then, the residue was dried at 105 °C for 5 h.
frequency of the microwave irradiation was 2.45 GHz. The reactor contained four 1-kW microwave generators that could be cooled with a blowing fan. A thermal insulation sleeve was used to put the microwave transparent crucible. The temperature of sample in the furnace was measured continuously using a stainless steel-sheathed, K-type thermocouple.
2.4. Characterization For chemical analysis, HCVS was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300DV, PerkinElmer, USA). The mineral compositions of the HCVS in this study were characterized by XRD (X’ Pert Pro MPD/PW3040, PANalytical B.V. Corporation, Netherlands) at a scanning rate of 8°/min using Cu Kα radiation with a 2θ angle ranging from 5° to 90°. Microstructure, surface morphology and elemental composition of HCVS and sintered HCVS were characterized by SEM-EDS (SSX-550, Shimadzu Corporation, Japan). The thermal behavior of HCVS, Ca (OH)2, and sintered HCVS was characterized by differential thermalthermogravimetric analysis (TG-DSC, STA449CD, NETZSCH, Germany) with a heating rate of 10 °C/min in atmosphere. The extraction ratio of chromium was characterized with an atomic absorption spectrophotometer (TAS-990, Beijing Purkinje General Instrument Co. LTD., China). The extraction ratio of vanadium was characterized by the ferrous ammonium sulfate titration method and was calculated with the following equation:
2.3. Experimental procedure In this paper, roasting parameters including heating mode, roasting temperature, m(CaO)/m(V2O5) ratio, and roasting time were studied. The experimental flow chart of the novel process based on microwave irradiation and muffle furnace roasting to extract vanadium and chromium is shown in Fig. 3. HCVS was mixed with Ca(OH)2 as an additive into column-shaped samples (diameter: 30 mm, height: 15 mm). The weight of each sample was about 30 g. The prepared five samples with a total mass of 150 g were placed in a 200-mL microwave transparent crucible and heated to holding temperatures in the range of 700–1000 °C in the microwave furnace. Samples prepared with the same weight were place in a corundum crucible and heated to the previously specified temperature in a muffle furnace. Microwave irradiation roasting could achieve the specified roasting temperature in 6 min and thus significantly reduce the roasting time. Further, different m(CaO)/m(V2O5) ratios ranging from 0.15 to 1.15 by adjusting the amount of Ca(OH)2 were considered in the roasting experiments. The microwave irradiation power was set to 2 kW and the roasting time was varied from 0.5 h to 2 h. After the roasting process, the sintered samples were taken out and cooled in air, then crushed and ground to particle sizes below 150 µm for further analysis. The leaching experiments were performed in a three-necked flask at atmospheric pressure. The extraction ratio of vanadium and chromium was characterized based on the acid leaching of 10.00 g sintered HCVS by a 20% H2SO4 solution with a solid-liquid ratio of 1:10 at 95 °C ( ± 0.5 °C) for 2 h and a stirring speed of 500 rpm. After each leaching
η=
m 0 ω0 − m1 ω1 × 100% m 0 ω0
(1)
where η represents the extraction ratio of vanadium, m0 is the mass of sintered sample, m1 is the mass of the tailing after leaching, ω0 is the mass fraction of the vanadium in the sintered sample and ω1 is the mass fraction of vanadium in the tailing. 3. Results and discussion 3.1. TG-DSC curves of HCVS and Ca(OH)2 The TG-DSC curves of HCVS is shown in Fig. 4. The roasting process of HCVS in Fig. 4 can be divided into three different stages: (1) From room temperature to 414.15 °C, HCVS is chemically stable; only
Fig. 3. Flow chart of the novel process based on microwave irradiation and muffle furnace roasting to extract vanadium and chromium.
Fig. 4. TG-DSC curves of HCVS. 3
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Fig. 5. TG-DSC curves of Ca(OH)2.
appearing at 449.89 °C was the decomposition of calcium hydroxide. The decomposition of calcium hydroxide can generate calcium oxide which reacts violently with vanadium oxide in the roasting process. Compared to the endothermic peak at 467.49 °C in Fig. 5, the endothermic peak at 449.98 °C was shifted towards lower temperatures due to the production of CaO which could react with SiO2 and accelerate the reaction. (3) From 460.15 °C to 695.16 °C, a dramatic weight gain of 0.73% and a slight endothermic peak at 628.34 °C could be observed. The endothermic peak at 628.34 °C was similar to that in Fig. 5. (4) The fourth stage proceeded at temperatures ranging from 695.16 °C to 1000 °C with a weight gain of 3.51%, which is in good agreement with Fig. 4. From 610.24 °C to 850.35 °C, a distinct exothermic peak occurred around the 762.04 °C. The exothermic peak was similar to that in the third stage in Fig. 4. In this case, the oxidation of ferrous ions was accompanied by the oxidation of trivalent vanadium and trivalent chromium. In this stage, vanadium oxide and chromium oxide reacted with calcium oxide and generated the corresponding calcium vanadate and calcium chromate. From 850.35 °C to 1000 °C, there was a slight exothermic peak, indicating the augite oxidation and the formation of CaSiO3.
moisture evaporates, resulting in a weight loss of 2.81%. (2) From 414.15 °C to 700.02 °C, the weight of HCVS increases by 1.46%, indicating that a partial oxidation has occurred. (3) When the temperature exceeds 700.02 °C, a dramatic weight gain of 3.94% and a broad exothermic peak occur simultaneously. The cause of the exothermic peak appearing at 754.98 °C is a partial sintering of the HCVS, which accords for the experimental phenomenon. According to the thermodynamic analysis (J. Zhang et al., 2015), the oxidation of fayalite and spinel are both exothermic. Hence, it is assumed that fayalite was oxidized in the second stage from 414.15 °C to 700.02 °C. Based on the SEM-EDS chemical composition analysis of the HCVS, ferrous ions were distributed in the fayalite, spinel, and augite phases. Thus, spinel oxidation and augite oxidation overlap in the temperature range from 700.02 °C to 1000 °C. Fig. 5 shows the TG-DSC curve of Ca(OH)2. When the temperature increased to 360.38 °C, the weight of Ca(OH)2 was reduced by 0.068%. A slight endothermic peak was observed at 100.02 °C, indicating the removal of water of crystallization from Ca(OH)2. From 360.38 °C to 478.00 °C, a dramatic endothermic peak and a weight loss of 21.54% occurred. The cause of this endothermic peak appearing at 467.49 °C was the decomposition of calcium hydroxide. From 607.32 °C to 693.54 °C, an endothermic peak and a weight loss of 1.67% occurred. The cause of the endothermic peak at 658.37 °C was the decomposition of calcium carbonate. The calcium carbonate was generated during the TG-DSC experiment. Compared to the theoretical weight loss rate (24.32%), the calcium hydroxide of weight loss rate was 23.27%. Hence, this result is in good agreement with the theoretical value within the experiment error. To explore the effect of Ca(OH)2 on TG-DSC behavior of HCVS, we conducted measurement of HCVS with Ca(OH)2. The TG-DSC curve of HCVS with the addition of Ca(OH)2 in the m(CaO)/m(V2O5) ratio of 0.95 is shown in Fig. 6. Compared with the curves of HCVS in Fig. 4, the roasting process of HCVS with addition of Ca(OH)2 in Fig. 6 can be divided into four different stages: (1) From room temperature to 383.89 °C, there was an endothermic peak around 129.27 °C and the weight of the HCVS decreased by 0.62%. The weight loss rate of HCVS could be attributed to the loss of crystallization. (2) From 383.89 °C to 460.15 °C, a dramatic endothermic peak occurred at 449.89 °C accompanied by a weight loss of 2.53%. The cause of the endothermic peak
3.2. Effect of roasting temperature on the extraction ratio of vanadium and chromium It was found that microwave irradiation had an effect on the extraction ratio of vanadium (G. Zhang et al., 2015). Notable, the crucial point of roasting by either microwave irradiation or muffle furnace was the extent of oxidative roasting of HCVS. This section of paper discusses the effect of roasting temperature on the extraction ratio of vanadium and chromium under the conditions where Ca(OH)2 was added with a m(CaO)/m(V2O5) ratio of 0.95 and a roasting time of 2 h. Fig. 7 shows that effect of roasting temperature on the extraction ratio of vanadium and chromium. Compared to muffle furnace, microwave irradiation improved the extraction ratio of vanadium. The maximum extraction ratio of vanadium increased by around 12.93% at 850 °C. The extraction ratio of vanadium roasted by microwave irradiation was higher than that of roasted by muffle furnace for temperature from 300 °C to 1000 °C, due to the fact that microwave irradiation roasting caused cracks in the mineral particles (Zhang et al., 4
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Fig. 6. TG-DSC curves of HCVS with addition Ca (OH)2 of the m(CaO)/m(V2O5) ratio of 0.95.
vanadates would be impeded. On the contrary, a further increase in the roasting temperature led to a decrease in the vanadium extraction; due to the fact that the oxidation and formation of calcium vanadates are exothermic and thus increasing roasting temperature will decrease the vanadium recovery. For all roasting temperatures, the extraction ratio of chromium was constant and remained lower than 8%. Taking this observation into account, the optimum roasting temperature was 850 °C. To investigate the evolution of the vanadium and chromium bearing phase during the roasting process, HCVS roasted by microwave irradiation and muffle furnace at different temperatures was investigated by XRD. Fig. 8 shows XRD patterns of HCVS with the addition Ca(OH)2 in a m(CaO)/m(V2O5) ratio of 0.95 roasting by muffle furnace at different temperatures for 2 h. The XRD measurements demonstrated that, at 500 °C, the fayalite phase was not completely oxidized and decomposed into Fe2O3 and SiO2. When roasting temperature increased to 700 °C, the fayalite phase was completely oxidized and decomposed. The decomposition of calcium hydroxide took place at temperatures below 500 °C, which agrees well with the TG-DSC result displayed in
Fig. 7. Effect of roasting temperature on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
2016). The extraction ratio of both, vanadium roasted by microwave irradiation and muffle furnace, initially increased with the roasting temperature but decreased for temperatures greater than 850 °C. Compared to the effects of the roasting temperature and heating mode on the extraction ratio of vanadium, the effects of roasting temperature and heating mode on extraction ratio of chromium were smaller and the extraction ratio of chromium was lower than 10%. The extraction ratio of chromium roasted by microwave irradiation increased from 4.75% to 7.35% and then slightly decreased to 3.62% for roasting temperatures from 300 °C to 600 °C and up to 1000 °C. Similarly, the extraction ratio of chromium roasted by muffle furnace increased from 3.67% to 6.51% and then slightly decreased to 2.16% for roasting temperatures from 300 °C to 600 °C and up to 1000 °C. As shown in Fig. 7, the roasting temperature is a key parameter for extraction ratio of vanadium. When the temperature was lower than 500 °C, the oxidation of fayalite and spinel was insufficient. According to the TG-DSC analyses in Fig. 5, the decomposition of calcium hydroxide was not complete and the reaction to generate the calcium
Fig. 8. XRD pattern of HCVS with addition Ca(OH)2 of the m(CaO)/m(V2O5) ratio of 0.95 roasting by muffle furnace at different temperatures for 2 h. 5
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Fig. 9. XRD pattern of HCVS with addition Ca(OH)2 of the m(CaO)/m(V2O5) ratio of 0.95 roasting by microwave irradiation at different temperatures for 2 h.
Fig. 10. XRD pattern of HCVS with the addition of Ca(OH)2 at different m (CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h.
Fig. 5. When roasting temperature increased to 700 °C, the spinel phase was oxidized and reacted with the calcium oxide. Peaks of Ca2V2O7 gained intensity above 700 °C, which is in accordance with the steep increase in the extraction ratio of vanadium in the temperature range from 500 °C to 700 °C, as shown in Fig. 7. It is noteworthy that, above 700 °C, the intermediate product Fe3O4 was converted completely to Fe2O3 by reacting with O2. As roasting temperature increased, peaks of Cr2O3 gained intensity above 700 °C. When roasting temperature was higher than 800 °C, FeCr2O4 was converted into Fe2O3 and Cr2O3. Fig. 9 shows the XRD patterns of HCVS with the addition Ca(OH)2 in a m(CaO)/m(V2O5) ratio of 0.95 roasting by microwave irradiation at different temperatures for 2 h. Fig. 9 shows that roasting temperature for the complete fayalite oxidation was 500 °C while the roasting temperature roasting by muffle furnace is 700 °C. The decomposition of calcium hydroxide took place at temperatures below 500 °C, which agrees well with the results displayed in Fig. 8. When roasting temperature increased to 700 °C, the spinel phase was oxidized and reacted with calcium oxide. As roasting temperature increased further, peaks of Ca2V2O7 increased in intensity, which is in accordance with the results shown in Fig. 8. Along with the increase in the peak intensity of Fe2O3, Fe3O4 was completely oxidized above 900 °C. As roasting temperature increased, peaks of Cr2O3 and CaSiO3 increased in intensity. It is noteworthy that, above 500 °C, FeCr2O4 was converted into Fe2O3 and Cr2O3. During the roasting process, the fayalite and diopside phases were oxidized and generated a large amount of SiO2, which then was converted to calcium silicate above 500 °C. With the same roasting temperatures, we can conclude that the degree of oxidation of HCVS roasted by microwave irradiation is larger than that roasted by muffle furnace which is in agreement with the results shown in Fig. 7.
CaO + Cr2O3 = CaCr2O4
Fig. 10 shows the XRD of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasted by microwave irradiation at 850 °C for 2 h. CaV2O6 and Mn4V2O9 were the major vanadates observed for m(CaO)/m(V2O5) ratio of 0.15 and 0.30, while Ca2V2O7 and Ca3V2O8 were not observed due to the low content of Ca(OH)2. Furthermore, Mn4V2O9 could also be leached by sulfuric acid to obtain (VO2)SO4. As Ca(OH)2 was added up to a ratio of 0.60, Ca2V2O7, CaCrO3, and CaSiO3 were generated; additionally, CaV2O6 and Mn4V2O9 were observed. The intensity of diffraction peaks of CaSiO3 increased with increasing m(CaO)/m(V2O5) ratio. The XRD results in Fig. 10 reveal that the ratio of 0.75 was the same as 0.60. When more Ca(OH)2 was added up to a m(CaO)/m(V2O5) ratio of 0.95, the diffraction peaks of Mn4V2O9 disappeared and the diffraction peaks of CaV2O6 and Ca2V2O7 could be observed. Along with the increase of m (CaO)/m(V2O5) ratio to 1.15, the observed calcium vanadate was Ca2V2O7. The diffraction peak intensity of CaSiO3 became gradually stronger with an increase in m(CaO)/m(V2O5) ratio from 0.60 to 1.15. In order to study the effect of m(CaO)/m(V2O5) ratio of on extraction ratio of vanadium and chromium, SEM with EDS was employed, the images of which shown in Fig. 11. In this study, the roasting process of HCVS was performed with Ca(OH)2 as additive at different m(CaO)/ m(V2O5) ratios and roasted by microwave irradiation at 850 °C for 2 h. The results of relevant EDS analysis of different regions in Fig. 11 are listed in Table 3. The elements Fe, O, and Si were distributed homogenously, while V, Ti, Al, K, Mg, Na, and Cr were inhomogenously distributed. According to Table 3, the points (c)-1 and (d)-4 both corresponded to Ca2V2O7. The point (d)-1 was composed of Ca2V2O7 and CaSiO3. With an increased m(CaO)/m(V2O5) ratio, the Ca, V, and Mn were mainly distributed in the bright white region where the content of V, Ca, and Mn was increased, which is accordance with the calculation shown in Fig. 11. Furthermore, the particle size increased significantly as m(CaO)/m(V2O5) ratio increased. Fig. 12 shows the combined effect of m(CaO)/m(V2O5) ratio on the extraction ratio of vanadium and chromium. In this section, the roasting process of HCVS was performed under the condition of adding Ca(OH)2 as additive at different m(CaO)/m(V2O5) ratios roasted by microwave irradiation and muffle furnace at 850 °C for 2 h. It is obvious that, for m (CaO)/m(V2O5) ratio from 0.15 to 1.15, the extraction ratio of vanadium roasted by microwave irradiation is higher than of that of roasted by muffle furnace. The maximum extraction ratio of vanadium roasted by microwave irradiation was 97.53% for the m(CaO)/m(V2O5) ratio of 0.95, while the extraction ratio of vanadium roasted by muffle furnace was 87.52%. According to the XRD analysis, when HCVS was roasted
3.3. Effect of m(CaO)/m(V2O5) ratio on the extraction ratio of vanadium and chromium The m(CaO)/m(V2O5) ratio is a further important parameter for the extraction ratio of vanadium and chromium. Yang et al. (2014) investigated calcification roasting vanadium slag with a high CaO content. CaO was used as additives during the calcification roasting process to produce calcium vanadates and calcium chromate, including CaV2O6, Ca2V2O7, Ca3V2O8 and CaCr2O4. The reactions are expressed as follows: CaO + V2O5 = CaV2O6
(2)
2CaO + V2O5 = Ca2V2O7
(3)
3CaO + V2O5 = Ca3V2O8
(4)
(5)
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Fig. 11. SEM images of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h: (a) 0.15; (b) 0.30; (c) 0.60; (d) 1.15.
3.4. Effect of roasting time on the extraction ratio of vanadium and chromium
with the addition of Ca(OH)2 at a mass ratio of 0.95, the present calcium vanadate was CaV2O7. Our results reveal that the calcium vanadate CaV2O7 can be easily dissolved in sulfuric acid and thus can improve the extraction ratio of vanadium. When more Ca(OH)2 was added to the HCVS, a large amount of SiO2 was converted to CaSiO3 which decreased the extraction ratio of vanadium during the acid leaching process. With m(CaO)/m(V2O5) ratio increasing from 0.15 to 1.15, the extraction ratio of chromium roasted by microwave irradiation and muffle furnace increased slowly from 3.29% to 4.53% and from 1.22% to 2.62%, respectively. In order to obtain a high extraction ratio of vanadium with less required Ca(OH)2 during roasting processing, 0.95 is believed to be the optimal m(CaO)/m(V2O5) ratio. With this ratio, vanadium and chromium extractions are at a maximum with 97.53% and 4.32%, respectively.
Fig. 13 shows the effect of roasting time on the extraction ratio of vanadium and chromium under the conditions of a roasting temperature 850 °C and with the addition Ca(OH)2 of m(CaO)/m(V2O5) ratio of 0.95. Fig. 13 reveals that roasting time had as large effect on the extraction ratio of vanadium. When roasting time was short, oxidation and calcification of vanadium and chromium were insufficient. Conversely, a long roasting time reduced the production efficiency. Compared with the roasted by muffle furnace, Fig. 13 also shows that roasted by microwave irradiation was beneficial for improving the extraction ratio of vanadium; in detail, the maximum extraction ratio of vanadium was increased by 10.22%. For the same roasting time, the oxidation degree of HCVS roasted by microwave irradiation was larger than for that roasting by muffle furnace; an observation which is in
Table 3 EDS analysis of HCVS with the addition of Ca(OH)2 at different m(CaO)/m(V2O5) ratios roasting by microwave irradiation at 850 °C for 2 h (mass fraction, %). Region
O
Al
Si
K
Ti
Ca
V
Cr
Mn
Fe
Na
Mg
(a)-1 (a)-2 (a)-3 (b)-1 (b)-2 (b)-3 (c)-1 (c)-2 (c)-3 (c)-4 (d)-1 (d)-2 (d)-3 (d)-4
37.50 9.26 42.52 44.98 43.96 24.40 46.87 9.86 38.77 22.88 42.21 44.58 31.45 44.30
0.84 1.03 3.21 2.05 5.41 0.83 0.24 0.78 0.61 0.64 1.02 11.02 1.19 0.16
3.61 2.22 19.26 12.86 31.55 – 1.61 0.60 4.90 1.69 5.82 27.24 5.40 1.22
0.17 – 0.48 0.30 2.31 – – – – – 0.14 0.50 0.15 –
13.91 4.75 2.01 2.29 1.06 1.21 0.71 3.43 6.54 3.74 2.63 0.35 2.76 0.57
1.29 0.87 1.74 2.37 0.89 1.56 20.91 3.44 8.03 6.19 19.80 5.38 10.10 21.72
4.01 4.01 2.63 6.82 1.39 5.80 23.47 6.13 3.86 6.62 13.65 1.40 8.60 25.97
9.92 4.13 1.47 5.03 0.25 22.56 0.34 33.51 2.70 10.60 1.86 0.63 11.18 0.69
3.71 3.77 4.20 4.50 4.34 16.04 2.38 24.70 4.55 7.00 3.48 0.60 6.91 2.04
25.05 69.55 25.51 18.80 6.18 26.25 2.71 16.71 29.01 39.74 8.30 3.28 20.58 3.01
– 0.25 1.31 – 2.22 – 0.49 0.24 – 0.36 0.69 5.02 0.92 0.33
– 0.16 0.67 – 0.45 1.35 0.28 0.60 1.02 0.54 0.40 – 0.75 –
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bed roasting, microwave irradiation is a high-efficiency method which requires less time and thus decreases the cost of production. As an effective and environmentally friendly technique, the analysis of the interaction mechanism of microwaves with HCVS is attracting increasing attention. 3.5. Analysis of the mechanisms in HCVS roasted by microwave irradiation and muffle furnace Understanding the roasting process is the key to extract vanadium and chromium efficiently from HCVS. In order to understand the effect of roasted by microwave irradiation and muffle furnace on the oxidation behavior, the apparent morphology and mechanism of the roasting process should be studied. We can calculate that the high efficiency of the microwave irradiation can be explained by the heating mechanism. Fig. 14 shows a schematic representation of the mechanisms of HCVS roasted by microwave irradiation and muffle furnace. Microwave is energy that is converted into heat energy depending upon the dielectric and magnetic properties of the heated media. Microwave irradiation causes heating from the interior (Zhang et al., 2016). The surface temperature of a sample is lower than its inside temperature and the heating rate is very high. Due to different dielectric constant and thermal capacities, different minerals phases have different heat propagation speeds and temperature, thereby giving rise to large temperature gradients which can induce cracks at the mineral phase boundaries (Jiang et al., 2016). Hence, microwave irradiation results in tiny cracks on the surface of the HCVS samples and reduces the strength of HCVS. So, the particle size of HCVS roasted by microwave irradiation reduces and the oxidation of HCVS roasted by microwave irradiation is full. HCVS roasted by muffle furnace is heated from the surface to the interior and the heating rate is low. The small temperature gradient cannot induce cracks. This is the reason why the degree of oxidation of HCVS roasted by microwave irradiation is higher than that of HCVS roasted by muffle furnace. To better interpret this mechanism, particle size distribution analysis of HCVS roasted by microwave irradiation and muffle furnace under the optimal roasting condition is shown in Fig. 15. The particle size distribution of the HCVS roasted by muffle furnace decreased and the D(50) decreased to 12.36 μm. However, the particle size distribution of the HCVS roasted by microwave irradiation is noticeably smaller and the D(50) decreased to 11.73 μm. The smaller size of HCVS can increase the contract area for gas-solid reactions. In addition, the results of Yuan et al. showed that microwave irradiation roasted stone coal contains more cracks and that the coal particles are more porous compared to the conventionally roasted samples (Yuan et al., 2015). Microwave irradiation causes a larger change of the mineral structure at lower temperatures and for shorter roasting time than conventional roasting methods. With the increasing roasting temperatures, HCVS will be sintered and cracks will be reduced or even disappear completely. The extraction ratio of vanadium and chromium initially increased and then decreased for higher temperature.
Fig. 12. Effect of m(CaO)/m(V2O5) ratio on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
Fig. 13. Effect of roasting time on extraction ratio of vanadium and chromium (H2SO4 concentration: 20%; S/L ratio: 1:10 g/ml; leaching temperature: 95 °C; leaching time: 2 h; stirring speed: 500 rpm).
agreement with the results shown in Figs. 8 and 9. The extraction ratio of vanadium roasted by microwave irradiation increased from 75.94% to 98.47% and then remained constant for roasting times from 0.5 h to 2.5 h. Likewise, the extraction ratio of vanadium roasted by muffle furnace increased from 65.83% to 88.25% for roasting time from 0.5 h to 2.5 h. As the roasting time increased, the extraction ratio of chromium increased slowly. The extraction ratio of chromium roasted by microwave irradiation increased from 3.92% to 4.39% and was higher than the extraction ratio of chromium roasted by muffle furnace. In order to obtain a high extraction ratio of vanadium, a roasting time of 1.5 h for microwave irradiation is considered as optimal roasting time. Li et al. have reported that 87.9% of vanadium can be extracted from vanadium-chromium slag during the stage of fractional sodium roasting-water leaching, while only 6.3% Cr can be extracted within 2 h (Li et al., 2015). Liu et al. (2013) developed a new NaOH-NaNO3 melt to treat vanadium-chromium slags. Under the optimum conditions, the extraction ratio of vanadium can reach 93.7% in 6 h. Zeng, Wang et al. have developed a roasting technology (static roasting in a muffle oven and fluidized bed roasting) for extracting vanadium from stone coal and the available maximum vanadium extraction ratio reached 91% (Zeng et al., 2015). Compared to the method of stepwise roasting with sodium salts, sub-molten salt, static roasting in a muffle furnace, and fluidized
4. Conclusions A novel effective method based on roasted by microwave irradiation with the help of a muffle furnace to extract vanadium and chromium from HCVS has been proved to be feasible. For the same roasting temperature and roasting time, the oxidation degree of HCVS roasted by microwave irradiation was higher than that of HCVS roasted by muffle furnace. The optimal temperature for extraction ratio of vanadium roasted by microwave irradiation and muffle furnace were 850 °C; the extraction ratios of vanadium and chromium were 98.29%, 4.32% and 85.36%, 2.48%, respectively. During the calcification roasting process, when the m(CaO)/m (V2O5) ratio is low, Mn4V2O9 and CaV2O6 are formed; with the m (CaO)/m(V2O5) ratio increased, Mn4V2O9, CaV2O6 gradually converted 8
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Fig. 14. Schematic representation of the mechanisms of HCVS roasting by microwave irradiation and muffle furnace.
while muffle furnace roasting does not have this effect. The effect of roasting time on the extraction ratio of vanadium roasted by microwave irradiation and muffle furnace followed the same trend. The optimal roasting time for roasting by microwave irradiation and muffle furnace was 1.5 h in both case. Acknowledgement This research was financially supported by the Programs of the National Natural Science Foundation of China (Nos. 51604065 and 51574082), the National Basic Research Program of China (973 Program) (No. 2013CB632603), the Fundamental Funds for the Central Universities (Nos. 150203003, 150202001). References Al-Harahsheh, M., Kingman, S.W., 2004. Microwave-assisted leaching—a review. Hydrometallurgy 73 (3–4), 189–203. Chang, Jun, Zhang, Libo, Yang, Changjiang, et al., 2015. Kinetics of microwave roasting of zinc slag oxidation dust with concentrated sulfuric acid and water leaching. Chem. Eng. Process. 97, 75–83. Chen, Desheng, Zhao, Longsheng, Liu, Yahui, et al., 2013. A novel process for recovery of iron, titanium, and vanadium from titanomagnetite concentrates: NaOH molten salt roasting and water leaching processes. J. Hazard. Mater. 244–245, 588–595. Duan, Lian, Tian, Qinghua, Guo, Xueyi, 2006. Review on production and utilization of vanadium resources in China. Hunan Nonferrous Metals 22 (6), 17–20. Jiang, Tao, Zhang, Qiaoyi, Liu, Yajing, et al., 2016. Influence of microwave irradiation on boron concentrate activation with an emphasis on surface properties. Appl. Surf. Sci. 385, 88–98.
Fig. 15. Particle distribution of HCVS roasting by microwave irradiation and muffle furnace with the m(CaO)/m(V2O5) ratios of 0.95 at 850 °C in 2 h.
to CaV2O7 and CaCrO3 is formed. The maximum extraction ratio of vanadium roasted by either, microwave irradiation or muffle furnace, was realized for m(CaO)/m(V2O5) ratio of 0.95. Microwave irradiation could achieve the specified roasting temperature within 6 min and thus significantly reduce the roasting time. Microwave irradiation is beneficial to reduce the particle size of HCVS 9
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