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Effects of coal rank, Fe3O4 amounts and activation temperature on the preparation and characteristics of magnetic activated carbon
M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0872–0876
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Effects of coal rank, Fe3O4 amounts and activation temperature on the preparation and characteristics of magnetic activated carbon YANG Mingshun, XIE Qiang*, ZHANG Jun, LIU Juan, WANG Yan, ZHANG Xianglan, ZHANG Qingwu School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, China Abstract: Coal-based Magnetic Activated Carbons (CMAC’s) were prepared from three representative coal samples of various ranks: Baorigele lignite from Inner Mongolia; Datong bitumite from Shanxi province; and Taixi anthracite from Ningxia Hui Autonomous Region. Fe3O4 was used as a magnetic additive. A nitrogen-adsorption analyzer was used to determine the specific surface area and pore structure of the resulting activated carbons. The adsorption capacity was assessed by the adsorption of iodine and methylene blue. X-ray diffraction was used to measure the evolution behavior of Fe3O4 during the preparation process. Magnetic properties were characterized with a vibrating-sample magnetometer. The effect of the activation temperature on the performance of CMAC’s was also studied. The results show that, compared to Baorigele lignite and Taixi anthracite, the Datong bitumite is more appropriate for the preparation of CMAC’s with a high specific surface area, an advanced pore structure and suitable magnetic properties. Fe3O4 can effectively enhance the magnetic properties and control the pore structure by increasing the ratio of mesopores. An addition of 6.0% Fe3O4 and an activation temperature of 880°C produced a CMAC having a specific surface area, an iodine adsorption, a methylene blue adsorption and a specific saturation magnetization of 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/g and 4.623 emu/g, respectively. The coal used to prepare this specimen was Datong bitumite. Keywords: magnetic activated carbon; coal rank; Fe3O4; activation temperature
1
Introduction
Tons of Spent Activated Carbon (SAC) are derived from a variety of applications, including electrode materials, gas adsorption and water purification, gold recovery from cyanide solutions and catalyst carrier[1-9]. These SACs challenge general separation methods and cause economic losses. Coal-based Magnetic Activated Carbon (CMAC) manifests its efficiency and economy in the recovery of spent activated carbons by the magnetic separation approach. Many researchers have prepared CMAC’s using various methods[6,10-11]. However, there is still room for improving the adsorption capacity and magnetic performance of current CMAC’s. Also, the effects of coal rank, magnetic additives and activation temperature on the performance of CMAC’s remain uncertain. In this study, CMAC’s were prepared from samples Received 11 March 2010; accepted 22 May 2010 *Corresponding author. Tel: 86 10 62331014 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60298-2
of lignite (from Baorigele, Inner Mongolia), bitumite (from Datong, Shanxi province) and anthracite (from Taixi, Ningxia Hui Autonomous Region) along with various amounts of Fe3O4. Efforts were made to determine the effects of coal rank, magnetic additives and activation temperature on CMAC’s performance as characterized by pore structure, specific area and magnetic properties. Of special importance in this study is the idea that it could facilitate the effective recovery of spent activated carbons.
2
Experimental
2.1 Preparation of magnetic activated carbons Table 1 shows the proximate analysis of the three coal samples, labeled C1, C2 and C3 for the lignite, bitumite and anthracite, respectively. Table 1
Proximate analysis of raw coals
(%)
Coal sample
Mar
Ad
Vdaf
C1
27.26
7.82
48.78
51.22
C2
4.03
3.82
30.23
69.77
C3
0.54
2.98
9.03
90.97
FCdaf
Effects of coal rank, Fe3O4 amounts and activation …
2.2
Characterization of the magnetic activated carbons
The iodine and methylene blue adsorption of the CMAC’s were measured referring to China Standards GB7702.7-2008 and GB7702.6-2008. Nitrogen-adsorption isotherms were obtained with a nitrogen-adsorption analyzer (model NOVA-1200, Quanta Chrome, USA). The specific surface area was then estimated from the BET (Brunauer, Emmett, Teller) equation and the total pore volume was calculated assuming a relative pressure P/P0=0.995. The micropore volume was determined by the t-plot method, the mesopore volume and the pore size distribution by the BJH (Barrett, Joyner, Halender) method[12]. The crystal structure of the Fe3O4 at each period —precursor, carbonized and activated—was monitored by X-Ray Diffraction (XRD, model D-MAXRB, Rigaku, Japan) to investigate its conversion and evolution behavior. The parameters of the XRD were Cu, Ka and l=0.154 nm. A Vibrating Sample Magnetometer (VSM, model 7307, Lakeshore, USA) was employed to characterize magnetic properties that included coercivity, magnetization, residual magnetism and specific magnetic susceptibility.
3 3.1
Results and discussion Effect of coal rank
The effect of coal rank on carbonizing behavior can be determined from the d(002) peaks in the XRD patterns of the chars. A sharp and narrow peak indicates a higher degree of graphitization and vice versa[13]. As shown in Fig. 1, the d(002) peaks become sharper and narrower as the coal rank increases. It is
safe to conclude that sample Char-C3-6 graphitizes more easily than the other two. The values of d(002) shown in Table 2 further demonstrate this.
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The three coal samples were ground into fine particles smaller than 0.15 mm in diameter: 90% (by weight) of every sample passed a 200 mesh (0.076 mm) screen. Fe3O4 was used as the magnetic additive. A powdered coal sample was mixed well with a weighed portion of Fe3O4 that varied from 0 to 8% in 2% increments. Coal tar and deionized water were also used in the formulation. The mixture was then compressed into a cylinder. These cylinders were air dried and then carbonized in a tube furnace at a heating rate of 5°C/min from room temperature to 600 °C and then maintained for 45 minutes. The resulting chars were then activated at a temperature from 850 to 940 °C in 30 °C increments. In this way a series of activated carbons was obtained. The resulting products, the so-called CMAC’s, are distinguished this way: the sequence (CMAC-Coal sample) and the amount of Fe3O4. For example, CMAC-C1-6 represents the CMAC prepared by using lignite, C1, with a Fe3O4 amount of 6% (by weight).
873
2θ
Fig. 1 XRD patterns of the resulting chars Table 2
Value of d(002) of the resulting chars
Sample
2ș (°)a
d(002) (nm)b
Char-C1-6
21.9
0.4058
Char-C2-6
24.2
0.3667
Char-C3-6
25.3
0.3517
Note: a. Read at the peaks of d(002); b. Calculated by the following formula: d(002)=l/2sinș.
The effect of coal rank on the pore structure of the CMAC’s is shown in Table 3. The SBET of the CMAC’s falls in the order: CMAC-C2-6>CMACC3-6>CMAC-C1-6. Sample CMAC-C2-6, derived from bitumite, manifests advanced pore structure with a specific surface area, SBET, of 1152.0 m2/g due to its favorable index of burn-off (58.32%)[14]. However, samples CMAC-C1-6 and CMAC-C3-6 have lower SBET and an unsuitable mesopore ratio. As shown in Table 1, when compared with the other two samples the bitumite sample (C2) has a reasonable ratio of volatile to fixed carbon capable of facilitating the formation and enlargement of pores[13], especially mesopores that constitute 40.50% of the total. As shown in Fig. 2, the diameter of the mesopores is mainly in the range of 3.0~5.0 nm. In contrast, the lignite (C1) sample shows a higher ratio of mesopores. The anthracite CMAC (C3) contains micropores.
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YANG Mingshun et al
Fig. 2 Pore distribution of the CMAC’s
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Mining Science and Technology
Table 3 Sample
Burn-off (%)
SBET (m2/g)
Vmicr
Vmeso
Mesopore percentage (%)
Average pore size (nm) 3.597
CMAC-C1-6
21.63
485.5
0.4365
0.1525
0.2840
65.06
CMAC-C2-6
58.32
1152.0
0.6758
0.4021
0.2737
40.50
2.347
CMAC-C3-6
54.17
993.5
0.5086
0.4344
14.59
2.048
$GVRUEHGDPRXQW PJJ
Iodine and Methylene Blue (MB) were used as adsorbates to evaluate adsorption capacity. The relative amounts of iodine and methylene blue adsorbed indicate the relative ratio of micropores and mesopores, respectively[12]. As shown in Fig. 3, the CMAC-C2-6 prepared from bitumite has a higher iodine and methylene adsorption compared to the other two samples, which is consistent with the results shown in Table 3.
Fig. 3
No.6
Pore structure of the CMAC’s
Pore volume (m3/g) Vtotal
Vol.20
0.0743
separate routes of micropore formation and enlargement that depends on the degree of catalysis. When the content of Fe3O4 is less than 6.0% there are fewer active catalyzing sites and the distribution of Fe3O4 is unsymmetrical. Then a portion of the micropores enlarge into mesopores and macropores, so the iodine adsorption decreases but the methylene blue adsorption increases. The adsorption capacity deteriorates at higher Fe3O4 amount (8.0%), because the catalysis becomes extensive and the speed of enlargement exceeds the regeneration rate of the micropores. Besides, the steric hindrance of excess Fe3O4 on the pore volume cannot be ignored. Table 4
Adsorption capacity with various amounts of Fe3O4
Adsorption
Adsorption capacity of the CMAC’s
3.2 Effect of the amount of added Fe3O4 A previous investigation had suggested that Fe3O4 was preferable to other iron containing magnetic additives including Fe, Fe2O3 and Fe2(C2O4)3[15]. Consequently, of the special interests is to further study the effect of Fe3O4 amount on the performances of the resulting CMAC’s. Compared with the other two coal samples, the anthracite coal from Taixi was selected as the precursor for its ultra low ash content and trace iron element with the aid to elucidate the effect of Fe3O4 on the preparation and characteristics of CMAC’s. The adsorption capacities of CMAC’s prepared from the C3 sample using various Fe3O4 amounts are listed in Table 4. The optimum adsorption occurs at 6.0% Fe3O4 amount. The adsorbed amounts of iodine and methylene blue have increased by 17.1% and a remarkable 352.7%, respectively, compared to CMAC-C3-0. However, higher or lower Fe3O4 content decreases adsorption capacity, in agreement with several previous reports[15-18]. Metallic Fe, formed by Fe3O4 reduction during the activating period as shown in Fig. 4b, can catalyze the surface reaction between carbon and steam. This would cause micropores to be enlarged into mesopores or macropores. The latter could function as a bridge to allow more steam into the interior of the specimen thereby facilitating the formation of more micropores. As a result, the pore distribution is controlled and regulated along
Fe3O4 (%) 0
2.0
4.0
6.0
8.0
Iodine (mg/g)
725.6
704.6
655.8
849.8
564.6
MB (mg/g)
23.9
48.0
129.5
108.2
53.9
Fig. 4 shows clearly the crystal structure and conversion of Fe3O4 for each period of the CMAC-C3-6 transformation. As indicated in Fig. 4a, the sample with 6.0% Fe3O4, CMAC-C3-6, may be distinguished from the sample without Fe3O4, CMAC-C3-0, by noting the peaks from Fe3O4 and FeO. Fe3O4 still exists in the CMAC after activation so it is obvious that Fe3O4 will cause the magnetism of the CMAC (FeO is non-magnetic). However, as shown clearly in Fig. 4b, the amount of Fe3O4 tends to decrease as the reaction proceeds. At the same time FeO appears as a new phase, in a rapidly growing amount, during the final activating period. During carbonization only a small portion of Fe3O4 is converted to FeO (this occurs at 600°C). While as the temperature rises and the steam diffuses during the activating period, the reactions become increasingly vigorous, leading to a more reducing environment with reaction products including hydrogen and carbon monoxide, which could definitely convert Fe3O4 into FeO, Fe2O3 and Fe as shown in Fig. 4b[16-19]. Metallic Fe, as explained above, can provide catalyst sites that generate proper pore distributions. It also helps during magnetic separation due to its magnetic property. The magnetic properties of the CMAC’s were examined because the conversion of Fe3O4 to FeO, shown in Fig. 4b, was of concern. The hysteresis loops of the CMAC’s are shown in Fig. 5 and the magnetic properties are listed in Table 5. The specific saturation magnetization of CMAC-C1-6, the sample with 6% Fe3O4 addition, is 13.2 times higher than that
YANG Mingshun et al
Effects of coal rank, Fe3O4 amounts and activation …
of simple separation tests using a magnet, shown in Fig. 6, demonstrate that it is relatively easy to separate CMAC-C3-6 due to its high magnetic susceptibility of 3.073×10–6 m3/kg.
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of CMAC-C1-0, the sample with no Fe3O4 addition. Once susceptibility exceeds 1.26×10–7~7.5×10–6 m3/kg, the substance can be separated magnetically by a magnetic field of 800~1600 kA/m[20]. The results
875
2θ
2θ
(a) X-ray diffraction of the CMAC’s and Fe3O4
(b) Conversion of Fe3O4 during each period: CMAC-C3-6
Fig. 4 XRD patterns
(b)
(a)
(a)
(b)
(c)
Fig. 6 Magnetic separation tests: (a) CMAC-C3-6, (b) CMAC-C3-0 and (c) magnet Table 5
Magnetic properties of CMAC-C3-6 and CMAC-C3-0
Specific Residual saturation magnetism magnetization (emu/g) (emu/g)
Magnetic Coercivity susceptibility (G) (10–7 m3/kg)
CMAC-C3-0
0.1831
0.0374
247.86
4.03
CMAC-C3-6
2.4158
0.2364
271.01
30.73
Sample
Fig. 7
$FWLYDWLRQWHPSHUDWXUH
Iodine and methylene blue adsorption: activation at various temperatures
0%DGVRUSWLRQ PJJ
Fig. 5 Magnetization hysteresis loop: CMAC-C3-6 and CMAC-C3-0
The effect of the activation temperature on the adsorption capacity of CMAC’s prepared from coal C3 (the Taixi anthracite) was investigated by programming the temperature from 850 to 940°C in 30 °C increments at a constant steam flow rate of 0.77 mL/(h·g char)[13]. The results are shown in Fig. 7. Initially, as the temperature rises from 850 to 880 °C, both the iodine and methylene blue adsorptions increase slightly. Then adsorption decreases markedly as the temperature is further increased from 880 to 940 °C. Since the reaction between carbon and steam is endothermic higher temperatures theoretically shift the reaction toward product, leading to advanced pore structure and a suitable ratio of micropores and mesopores. The iodine and methylene blue adsorption reaches 849.7 and 108.2 mg/g respectively as the temperature increases. However, the intensity of the burn-off becomes uncontrollable as the temperature rises further. As reported in previous investigations[14,21], once burn-off reaches 75% macropores become the predominant pore structure of the resulting activated carbon. Then the iodine and methylene blue adsorptions drop significantly as a consequence. ,RGLQHDGVRUSWLRQ PJJ
0RPHQWPDVV HPXJ
3.3 Effect of activation temperature
876
4
Mining Science and Technology
Conclusions
1) Compared to Baorigele lignite and Taixi anthracite the Datong bitumite was more favorable for preparation of CMAC’s having high specific area and advanced pore structure, especially mesopores. 2) Fe3O4 both enhances the magnetic properties of the activated carbon and controls pore distribution due to the catalytic function of metallic Fe, which is the product of Fe3O4 reduction during the activating period. 3) Added Fe3O4 is mostly converted into FeO during activation, while residual Fe3O4 and any Fe formed are sufficient to create magnetic properties appropriate for magnetic separation. 4) The specific surface area, the iodine and methylene blue adsorption and the specific saturation magnetization of CMAC prepared from Datong bitumite with added (6.0%) Fe3O4 and an activation temperature of 880 °C were 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/g, 4.623 emu/g, respectively.
Acknowledgements The research is supported by the National Natural Science Foundation of China (No.20776150) and the National High Technology Research and Development Program of China (No.2008AA05Z308). Prof. H. K. Lin from the University of Alaska is honored for his tolerance in reviewing the paper.
References [1] Li L T, Xie Q, Hao L N, Zheng Y F, Zhou T Q, Xing W
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W, Wang Y Y. Preparation and characterization of metal containing activated carbon for electrode. Journal of China University of Mining & Technology, 2008, 37(2): 225-230. (In Chinese) Zhang C X, Duan Y L, Xing B L, Qiao W M, Ling L C. Influence of nitrogen hetero-substitution on the electrochemical performance of coal-based activated carbons measured in non-aqueous electrolyte. Mining Science and Technology, 2009, 19(3): 295-299. Zhang S Q, Ma R, Zhu W K, Fan Y J, Zhang B T, Yu X D. Research on activated carbon for separating CO2 by adsorption. Journal of China University of Mining & Technology, 2004, 33(6): 683-686. (In Chinese) Nakagawa K, Mukai S R, Suzuki T, Tamon H. Gas adsorption on activated carbons from PET mixtures with a metal salt. Carbon, 2003, 41(4): 823-831. Xie Q, Hu W C, Zhang Y Z, Xu H Q. Consideration and proposal for the development of activated carbon industry in our country. Coal Processing & Comprehensive Utilization, 2003(5): 36-39. (In Chinese) Kahani S A, Hamadanian M, Vandadi O. Deposition of magnetite nanoparticles in activated carbons and prepa-
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ration of magnetic activated carbon. Nanotechnology and Its Applications, 2009, 929: 183-188. Wang C L, Huang H Q, Wei S Q, Cheng X Z, Jiang W B, Zhao Y. Feasible study on gold extraction by magnetic carbon-in-pulp process. Gold, 1995, 16(6): 27-31. (In Chinese) Tseng H H, Wey M Y. Study of SO2 adsorption and thermal regeneration over activated carbon-supported copper oxide catalysts. Carbon, 2004, 42(11): 2269-2278. Torimoto T, Okwawa Y, Norihiko, Yoneyama H. Effect of activated carbon content in TiO2-loaded activated carbon on photodegradation behaviors of dichloromethane. Journal of Photochemistry and Photobiology A: Chemistry, 1997, 103(1/2): 153-157. Demircan Z, Tekol E, Tanyolac D, Ozdural A R. Para-magnetic polyvinylbutyral particles containing activated carbon as a new adsorbent. Chem Eng Comm, 2003, 190(5/8): 831-842. Liu Z C, Ling L C, Qiao W M, Lu C X, Wu D, Liu L. Effects of various metals and their loading methods on the mesopore formation in pitch-based spherical activated carbon. Carbon, 1999, 37(4): 1333-1335. Gong G Z, Xie Q, Zheng Y F, Ye S F, Chen Y F. Regulation of pore size distribution in coal-based activated carbon. New Carbon Materials, 2009, 24(2): 141-146. Xie Q, Bian B X. Principles of Control over Coal Carbonization & Its Application in Preparation of Activated Carbon. Xuzhou: China University of Mining & Technology Press, 2002. (In Chinese) Xie Q, Zhang X L, Li L T, Jin L. Porosity adjustment of activated carbon: theory, approaches and practice. New Carbon Materials, 2005, 20(2): 183-187. (In Chinese) Xing W W, Zhou T Q, Zhang J, Li L T, Xie Q. Preparation of magnetic coal-based activated carbon. Journal of University of Science and Technology Beijing, 2009, 31(1): 83-87. (In Chinese) Yang J B, Kang F Y, Huang Z H. Influence of transition metals on the pore structure and adsorption properties of spherical activated carbon. Journal of Tsinghua University, 2002, 42(5): 688-691. (In Chinese) Zhang X L, Xu D P, Chen Q R. Preparation of activated carbon with mesopore by catalyzed method. Carbon, 2001(2): 22-25. (In Chinese) Yang J B, Lin L C, Lv C X, Liu L. Influences of ferrocene on the activation characteristics and pore size distribution of phenolic resin-based spherical activated carbon. Ion Exchange and Adsorption, 2000, 16(2): 155-161. (In Chinese) Yang J B, Kang F Y, Lin L C, Liu L, Huang Z H. An investigation on mesopore evolution mechanism of activated carbon spheres by examining the change of iron state. Ion Exchange and Adsorption, 2001, 17(2): 104-109. (In Chinese) Dai H X. Mineral Process and Magnetic Measurement Technology. Beijing: Chemical Industry Press, 2008. (In Chinese) Hao L N, Xie Q, Li L T, Duan M H, Yan W, Xing W W, Wang Y Y, Zhou T Q. Catalytical preparation of mesoporous coal-based activated carbon by nitrate copper and nitrate manganese. Carbon Techniques, 2008, 27(4): 27-29. (In Chinese)
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Effects of coal rank, Fe3O4 amounts and activation temperature on the preparation and characteristics of magnetic activated carbon YANG Mingshun, XIE Qiang*, ZHANG Jun, LIU Juan, WANG Yan, ZHANG Xianglan, ZHANG Qingwu School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, China Abstract: Coal-based Magnetic Activated Carbons (CMAC’s) were prepared from three representative coal samples of various ranks: Baorigele lignite from Inner Mongolia; Datong bitumite from Shanxi province; and Taixi anthracite from Ningxia Hui Autonomous Region. Fe3O4 was used as a magnetic additive. A nitrogen-adsorption analyzer was used to determine the specific surface area and pore structure of the resulting activated carbons. The adsorption capacity was assessed by the adsorption of iodine and methylene blue. X-ray diffraction was used to measure the evolution behavior of Fe3O4 during the preparation process. Magnetic properties were characterized with a vibrating-sample magnetometer. The effect of the activation temperature on the performance of CMAC’s was also studied. The results show that, compared to Baorigele lignite and Taixi anthracite, the Datong bitumite is more appropriate for the preparation of CMAC’s with a high specific surface area, an advanced pore structure and suitable magnetic properties. Fe3O4 can effectively enhance the magnetic properties and control the pore structure by increasing the ratio of mesopores. An addition of 6.0% Fe3O4 and an activation temperature of 880°C produced a CMAC having a specific surface area, an iodine adsorption, a methylene blue adsorption and a specific saturation magnetization of 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/g and 4.623 emu/g, respectively. The coal used to prepare this specimen was Datong bitumite. Keywords: magnetic activated carbon; coal rank; Fe3O4; activation temperature
1
Introduction
Tons of Spent Activated Carbon (SAC) are derived from a variety of applications, including electrode materials, gas adsorption and water purification, gold recovery from cyanide solutions and catalyst carrier[1-9]. These SACs challenge general separation methods and cause economic losses. Coal-based Magnetic Activated Carbon (CMAC) manifests its efficiency and economy in the recovery of spent activated carbons by the magnetic separation approach. Many researchers have prepared CMAC’s using various methods[6,10-11]. However, there is still room for improving the adsorption capacity and magnetic performance of current CMAC’s. Also, the effects of coal rank, magnetic additives and activation temperature on the performance of CMAC’s remain uncertain. In this study, CMAC’s were prepared from samples Received 11 March 2010; accepted 22 May 2010 *Corresponding author. Tel: 86 10 62331014 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60298-2
of lignite (from Baorigele, Inner Mongolia), bitumite (from Datong, Shanxi province) and anthracite (from Taixi, Ningxia Hui Autonomous Region) along with various amounts of Fe3O4. Efforts were made to determine the effects of coal rank, magnetic additives and activation temperature on CMAC’s performance as characterized by pore structure, specific area and magnetic properties. Of special importance in this study is the idea that it could facilitate the effective recovery of spent activated carbons.
2
Experimental
2.1 Preparation of magnetic activated carbons Table 1 shows the proximate analysis of the three coal samples, labeled C1, C2 and C3 for the lignite, bitumite and anthracite, respectively. Table 1
Proximate analysis of raw coals
(%)
Coal sample
Mar
Ad
Vdaf
C1
27.26
7.82
48.78
51.22
C2
4.03
3.82
30.23
69.77
C3
0.54
2.98
9.03
90.97
FCdaf
Effects of coal rank, Fe3O4 amounts and activation …
2.2
Characterization of the magnetic activated carbons
The iodine and methylene blue adsorption of the CMAC’s were measured referring to China Standards GB7702.7-2008 and GB7702.6-2008. Nitrogen-adsorption isotherms were obtained with a nitrogen-adsorption analyzer (model NOVA-1200, Quanta Chrome, USA). The specific surface area was then estimated from the BET (Brunauer, Emmett, Teller) equation and the total pore volume was calculated assuming a relative pressure P/P0=0.995. The micropore volume was determined by the t-plot method, the mesopore volume and the pore size distribution by the BJH (Barrett, Joyner, Halender) method[12]. The crystal structure of the Fe3O4 at each period —precursor, carbonized and activated—was monitored by X-Ray Diffraction (XRD, model D-MAXRB, Rigaku, Japan) to investigate its conversion and evolution behavior. The parameters of the XRD were Cu, Ka and l=0.154 nm. A Vibrating Sample Magnetometer (VSM, model 7307, Lakeshore, USA) was employed to characterize magnetic properties that included coercivity, magnetization, residual magnetism and specific magnetic susceptibility.
3 3.1
Results and discussion Effect of coal rank
The effect of coal rank on carbonizing behavior can be determined from the d(002) peaks in the XRD patterns of the chars. A sharp and narrow peak indicates a higher degree of graphitization and vice versa[13]. As shown in Fig. 1, the d(002) peaks become sharper and narrower as the coal rank increases. It is
safe to conclude that sample Char-C3-6 graphitizes more easily than the other two. The values of d(002) shown in Table 2 further demonstrate this.
,QWHQVLW\
The three coal samples were ground into fine particles smaller than 0.15 mm in diameter: 90% (by weight) of every sample passed a 200 mesh (0.076 mm) screen. Fe3O4 was used as the magnetic additive. A powdered coal sample was mixed well with a weighed portion of Fe3O4 that varied from 0 to 8% in 2% increments. Coal tar and deionized water were also used in the formulation. The mixture was then compressed into a cylinder. These cylinders were air dried and then carbonized in a tube furnace at a heating rate of 5°C/min from room temperature to 600 °C and then maintained for 45 minutes. The resulting chars were then activated at a temperature from 850 to 940 °C in 30 °C increments. In this way a series of activated carbons was obtained. The resulting products, the so-called CMAC’s, are distinguished this way: the sequence (CMAC-Coal sample) and the amount of Fe3O4. For example, CMAC-C1-6 represents the CMAC prepared by using lignite, C1, with a Fe3O4 amount of 6% (by weight).
873
2θ
Fig. 1 XRD patterns of the resulting chars Table 2
Value of d(002) of the resulting chars
Sample
2ș (°)a
d(002) (nm)b
Char-C1-6
21.9
0.4058
Char-C2-6
24.2
0.3667
Char-C3-6
25.3
0.3517
Note: a. Read at the peaks of d(002); b. Calculated by the following formula: d(002)=l/2sinș.
The effect of coal rank on the pore structure of the CMAC’s is shown in Table 3. The SBET of the CMAC’s falls in the order: CMAC-C2-6>CMACC3-6>CMAC-C1-6. Sample CMAC-C2-6, derived from bitumite, manifests advanced pore structure with a specific surface area, SBET, of 1152.0 m2/g due to its favorable index of burn-off (58.32%)[14]. However, samples CMAC-C1-6 and CMAC-C3-6 have lower SBET and an unsuitable mesopore ratio. As shown in Table 1, when compared with the other two samples the bitumite sample (C2) has a reasonable ratio of volatile to fixed carbon capable of facilitating the formation and enlargement of pores[13], especially mesopores that constitute 40.50% of the total. As shown in Fig. 2, the diameter of the mesopores is mainly in the range of 3.0~5.0 nm. In contrast, the lignite (C1) sample shows a higher ratio of mesopores. The anthracite CMAC (C3) contains micropores.
G9 P/ QPÂJ
YANG Mingshun et al
Fig. 2 Pore distribution of the CMAC’s
874
Mining Science and Technology
Table 3 Sample
Burn-off (%)
SBET (m2/g)
Vmicr
Vmeso
Mesopore percentage (%)
Average pore size (nm) 3.597
CMAC-C1-6
21.63
485.5
0.4365
0.1525
0.2840
65.06
CMAC-C2-6
58.32
1152.0
0.6758
0.4021
0.2737
40.50
2.347
CMAC-C3-6
54.17
993.5
0.5086
0.4344
14.59
2.048
$GVRUEHGDPRXQW PJJ
Iodine and Methylene Blue (MB) were used as adsorbates to evaluate adsorption capacity. The relative amounts of iodine and methylene blue adsorbed indicate the relative ratio of micropores and mesopores, respectively[12]. As shown in Fig. 3, the CMAC-C2-6 prepared from bitumite has a higher iodine and methylene adsorption compared to the other two samples, which is consistent with the results shown in Table 3.
Fig. 3
No.6
Pore structure of the CMAC’s
Pore volume (m3/g) Vtotal
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separate routes of micropore formation and enlargement that depends on the degree of catalysis. When the content of Fe3O4 is less than 6.0% there are fewer active catalyzing sites and the distribution of Fe3O4 is unsymmetrical. Then a portion of the micropores enlarge into mesopores and macropores, so the iodine adsorption decreases but the methylene blue adsorption increases. The adsorption capacity deteriorates at higher Fe3O4 amount (8.0%), because the catalysis becomes extensive and the speed of enlargement exceeds the regeneration rate of the micropores. Besides, the steric hindrance of excess Fe3O4 on the pore volume cannot be ignored. Table 4
Adsorption capacity with various amounts of Fe3O4
Adsorption
Adsorption capacity of the CMAC’s
3.2 Effect of the amount of added Fe3O4 A previous investigation had suggested that Fe3O4 was preferable to other iron containing magnetic additives including Fe, Fe2O3 and Fe2(C2O4)3[15]. Consequently, of the special interests is to further study the effect of Fe3O4 amount on the performances of the resulting CMAC’s. Compared with the other two coal samples, the anthracite coal from Taixi was selected as the precursor for its ultra low ash content and trace iron element with the aid to elucidate the effect of Fe3O4 on the preparation and characteristics of CMAC’s. The adsorption capacities of CMAC’s prepared from the C3 sample using various Fe3O4 amounts are listed in Table 4. The optimum adsorption occurs at 6.0% Fe3O4 amount. The adsorbed amounts of iodine and methylene blue have increased by 17.1% and a remarkable 352.7%, respectively, compared to CMAC-C3-0. However, higher or lower Fe3O4 content decreases adsorption capacity, in agreement with several previous reports[15-18]. Metallic Fe, formed by Fe3O4 reduction during the activating period as shown in Fig. 4b, can catalyze the surface reaction between carbon and steam. This would cause micropores to be enlarged into mesopores or macropores. The latter could function as a bridge to allow more steam into the interior of the specimen thereby facilitating the formation of more micropores. As a result, the pore distribution is controlled and regulated along
Fe3O4 (%) 0
2.0
4.0
6.0
8.0
Iodine (mg/g)
725.6
704.6
655.8
849.8
564.6
MB (mg/g)
23.9
48.0
129.5
108.2
53.9
Fig. 4 shows clearly the crystal structure and conversion of Fe3O4 for each period of the CMAC-C3-6 transformation. As indicated in Fig. 4a, the sample with 6.0% Fe3O4, CMAC-C3-6, may be distinguished from the sample without Fe3O4, CMAC-C3-0, by noting the peaks from Fe3O4 and FeO. Fe3O4 still exists in the CMAC after activation so it is obvious that Fe3O4 will cause the magnetism of the CMAC (FeO is non-magnetic). However, as shown clearly in Fig. 4b, the amount of Fe3O4 tends to decrease as the reaction proceeds. At the same time FeO appears as a new phase, in a rapidly growing amount, during the final activating period. During carbonization only a small portion of Fe3O4 is converted to FeO (this occurs at 600°C). While as the temperature rises and the steam diffuses during the activating period, the reactions become increasingly vigorous, leading to a more reducing environment with reaction products including hydrogen and carbon monoxide, which could definitely convert Fe3O4 into FeO, Fe2O3 and Fe as shown in Fig. 4b[16-19]. Metallic Fe, as explained above, can provide catalyst sites that generate proper pore distributions. It also helps during magnetic separation due to its magnetic property. The magnetic properties of the CMAC’s were examined because the conversion of Fe3O4 to FeO, shown in Fig. 4b, was of concern. The hysteresis loops of the CMAC’s are shown in Fig. 5 and the magnetic properties are listed in Table 5. The specific saturation magnetization of CMAC-C1-6, the sample with 6% Fe3O4 addition, is 13.2 times higher than that
YANG Mingshun et al
Effects of coal rank, Fe3O4 amounts and activation …
of simple separation tests using a magnet, shown in Fig. 6, demonstrate that it is relatively easy to separate CMAC-C3-6 due to its high magnetic susceptibility of 3.073×10–6 m3/kg.
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,QWHQVLW\
of CMAC-C1-0, the sample with no Fe3O4 addition. Once susceptibility exceeds 1.26×10–7~7.5×10–6 m3/kg, the substance can be separated magnetically by a magnetic field of 800~1600 kA/m[20]. The results
875
2θ
2θ
(a) X-ray diffraction of the CMAC’s and Fe3O4
(b) Conversion of Fe3O4 during each period: CMAC-C3-6
Fig. 4 XRD patterns
(b)
(a)
(a)
(b)
(c)
Fig. 6 Magnetic separation tests: (a) CMAC-C3-6, (b) CMAC-C3-0 and (c) magnet Table 5
Magnetic properties of CMAC-C3-6 and CMAC-C3-0
Specific Residual saturation magnetism magnetization (emu/g) (emu/g)
Magnetic Coercivity susceptibility (G) (10–7 m3/kg)
CMAC-C3-0
0.1831
0.0374
247.86
4.03
CMAC-C3-6
2.4158
0.2364
271.01
30.73
Sample
Fig. 7
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Iodine and methylene blue adsorption: activation at various temperatures
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Fig. 5 Magnetization hysteresis loop: CMAC-C3-6 and CMAC-C3-0
The effect of the activation temperature on the adsorption capacity of CMAC’s prepared from coal C3 (the Taixi anthracite) was investigated by programming the temperature from 850 to 940°C in 30 °C increments at a constant steam flow rate of 0.77 mL/(h·g char)[13]. The results are shown in Fig. 7. Initially, as the temperature rises from 850 to 880 °C, both the iodine and methylene blue adsorptions increase slightly. Then adsorption decreases markedly as the temperature is further increased from 880 to 940 °C. Since the reaction between carbon and steam is endothermic higher temperatures theoretically shift the reaction toward product, leading to advanced pore structure and a suitable ratio of micropores and mesopores. The iodine and methylene blue adsorption reaches 849.7 and 108.2 mg/g respectively as the temperature increases. However, the intensity of the burn-off becomes uncontrollable as the temperature rises further. As reported in previous investigations[14,21], once burn-off reaches 75% macropores become the predominant pore structure of the resulting activated carbon. Then the iodine and methylene blue adsorptions drop significantly as a consequence. ,RGLQHDGVRUSWLRQ PJJ
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3.3 Effect of activation temperature
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Mining Science and Technology
Conclusions
1) Compared to Baorigele lignite and Taixi anthracite the Datong bitumite was more favorable for preparation of CMAC’s having high specific area and advanced pore structure, especially mesopores. 2) Fe3O4 both enhances the magnetic properties of the activated carbon and controls pore distribution due to the catalytic function of metallic Fe, which is the product of Fe3O4 reduction during the activating period. 3) Added Fe3O4 is mostly converted into FeO during activation, while residual Fe3O4 and any Fe formed are sufficient to create magnetic properties appropriate for magnetic separation. 4) The specific surface area, the iodine and methylene blue adsorption and the specific saturation magnetization of CMAC prepared from Datong bitumite with added (6.0%) Fe3O4 and an activation temperature of 880 °C were 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/g, 4.623 emu/g, respectively.
Acknowledgements The research is supported by the National Natural Science Foundation of China (No.20776150) and the National High Technology Research and Development Program of China (No.2008AA05Z308). Prof. H. K. Lin from the University of Alaska is honored for his tolerance in reviewing the paper.
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