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Development of microwave-assisted sintering of Portland cement raw meal
Journal of Cleaner Production xxx (2016) 1e7
Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Development of microwave-assisted sintering of Portland cement raw meal Praphatsorn Kaewwichit a, Jaroon Junsomboon b, Parinya Chakartnarodom a, Chayanee Tippayasam a, Thapanee Srichumpong a, Parjaree Thavorniti c, Cristina Leonelli d, Duangrudee Chaysuwan a, * a
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand Physics and Engineering 2 Sub-division, Physics and Engineering Program, Department of Science Service, Ministry of Science and Technology, Bangkok, 10400, Thailand c National Metal and Materials Technology Center (MTEC), Pathumthani, 12120, Thailand d Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, 41125, Modena, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2016 Received in revised form 1 July 2016 Accepted 1 July 2016 Available online xxx
Typically, using a rotary furnace as a heat generator, a temperature of approximately 1450 C and a time of 60 min is needed to produce clinker requiring large amounts of energy. Recently, a method of sintering Portland cement by microwave furnace has been developed with the aim to reduce this high consumption of energy in the conventional cement production. In this work, cement raw meal was calcined by a microwave furnace operating at 2.45 GHz with 900 W at 1150 C at several periods of time but was not completely successful in terms of clinker formation. Therefore, an electric furnace was used at 1300 C and 1350 C for 30 min to further heat the material. Chemical compositions of the formed clinker, characterized by XRD, presented C3S, C2S, C3A and C4AF as the main constituents confirming a clinker similar to those of clinker produced by rotary kiln or conventional technique. Loss on ignition and insoluble residue of the resultant clinker were analyzed by chemical analysis and the results were found to pass ASTM C-114. It was found that the raw meal sintering process using a microwave furnace followed by transfer to an electric furnace could reduce not only the temperature by at least 100 C but also the processing time of the clinker. In addition, there is no grinding cost for clinker preparation in this process. This processing of clinker would decrease energy consumption and carbon dioxide emission to the atmosphere, a major cause of global warming. © 2016 Elsevier Ltd. All rights reserved.
Keywords: CO2 emission Cement raw meal Clinker Microwave furnace Electric furnace Sintering
1. Introduction Portland cement is currently widely used to build various constructions because of its high compressive strength. Cement production is a high-energy consumption process, especially in clinker production (Bye, 1999). Most energy is consumed in the burning process for clinkerization. It was found that the burning process requires approximately 3.2e6.3 GJ of energy per ton of clinker production (Ying et al., 2010) and the energy is generally associated with CO2 emission (Parrott, 2002). Behind power generation, the cement industry has always been among the largest CO2 emitting industries (Fairbairn et al., 2010) with its processing emitting
* Corresponding author. E-mail address: [email protected] (D. Chaysuwan).
around 900 kg of CO2 for every ton of cement production (Benhelal et al., 2013) and affecting the level of greenhouse gas emissions and contributing to the global warming issue. In the conventional process of cement manufacture high temperatures are maintained by rotary furnace around 1450 C for 60 min to obtain the clinker, however the clinker process, through microwave furnace, needs lower temperature (Fang et al., 1996). Microwave heating is a process of microwave-material interaction. The heating efficiency mainly depends on the dielectric properties of the material to be heated (Fang et al., 1996; Makul et al., 2014). Microwave heating is superior to conventional heating methods in terms of energy-saving, rapid heating rates and short processing times (Fang et al., 1996; Makul et al., 2014; Long et al., 2002). A 95% of energy saving can be achieved with the use of microwave energy over conventional heating techniques
http://dx.doi.org/10.1016/j.jclepro.2016.07.009 0959-6526/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
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(Quemeneur et al., 1983). Microwave processing of ceramic materials has been reported by several researchers (Fang et al., 1996; Makul et al., 2014; Long et al., 2002; Minay et al., 2004; Leonelli et al., 2006; Monaco et al., 2015). Calcium oxide (CaO) and magnesium oxide (MgO) are the typical clinker components, which account for 64e68% of the clinker weight. Normally, limestone used in cement production has 75e90% CaCO3 in raw meal. Most CO2 is produced in converting calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) into CaO and MgO when the temperature is above 900 C. At this stage, the CO2 leaves the system and the raw meal loses over one third of its original weight. When the temperature reaches 1300e1450 C, the reaction of clinkerization takes place, with parts of the materials becoming liquid, and finally, nodules known as clinker are formed (Tianming et al., 2015). The aim of this research was to utilize the microwave furnace operating at the frequency at 2.45 GHz at 900 W to reduce the sintering temperature by at least 100 C and reduce times in order to decrease energy consumption and produce less CO2 emission to the atmosphere. 2. Experimental 2.1. Materials preparation and sintering At first, the raw meal of commercial type I Portland cement was intended to use for clinkerization by only microwave technique with a microwave furnace (CEM, MAS7000, USA, 900 W, max temp. 1200 C). This furnace has a limitation of a maximum temperature of 1150 C which was used for the microwave sintering. So, in addition an extra electric furnace (Nabertherm, HT18, Germany, 1200 W, max temp. 1800 C) was utilized, for final sintering of the raw meal (at 1300 C and 1350 C). The specimens were prepared by slightly compressing the raw meal powder (20 g) into alumina boats (of volume 25 ml), and then heating in a microwave furnace at 1150 C for soaking times of 1, 3, 5, 7, 9, 11 and 20 min, respectively. Consequently, all specimens were brought to continue sintering in an electric furnace at either 1300 C or 1350 C for 30 min and the resultant clinkers obtained. 2.2. XRD characterization The mineralogical compositions of the raw meal, commercial clinker and resultant clinkers were characterized. They were investigated by x-ray diffraction (XRD) (X'Pert, Philips, Netherland) in order to determine the optimum sintering conditions. The diffraction patterns of resultant clinkers were recorded by using Cu Ka radiation and generator setting was 40 kV excitation potential
with a current of 35 mA. The programs typically used were scanned from 10 to 60 2q with step size 0.02 2q. 2.3. SEM analysis The microstructures of resultant clinkers were determined by a scanning electron microscope (SEM) (XL 30, Philips) on the powders glued to Al stub with an accelerating voltage of 15 kV. 2.4. Chemical analysis The main compounds of clinker such as SiO2, Fe2O3, Al2O3, CaO and MgO, loss on ignition (LOI), insoluble residue (IR) of the commercial and resultant clinkers were characterized by wet chemical analysis according to ASTM C114-04 (ASTM, 2004). 2.5. Compressive strength The compressive strength of the mortar from commercial and the experimental clinkers was determined by a universal testing machine (UTM) (Shimadzu, Japan). The mortars of both commercial clinker and experimental clinkers were prepared with a water: clinker: sand ratio of 0.485: 1: 2.75. The pastes were cast and compacted by tamping for two layers in cube molds (2.5 2.5 2.5 cm3). Mortar cubes were demolded after being at room temperature for 24 h. Consequently, they were cured in a plastic box under water for 7 days and then tested for compressive strength according to ASTM C 109-02 (ASTM, 2002). 3. Results and discussion 3.1. XRD analysis The XRD patterns in Fig. 1 show sharp peaks for the raw meal and commercial clinker corresponding to CaCO3 (01-072-1650), SiO2 (00-046-1045), NaAlSiO4 (01-076-1733), C3S (Ca3SiO5: 00042-0551), C2S (Ca2SiO4: 00-049-1672), C4AF (CaAl2Fe4O10: 00021-0830), C3A (Ca3Al2O6: 00-038-1429) and CaO (00-002-1088). XRD patterns of the resultant clinker prepared in the microwave furnace for different soaking times follower by an electric furnace at 1300 C are shown in Fig. 2. The peaks of the main compounds of clinker such as C3S, C2S, C4AF, C3A and, especially, free lime (CaO) (37.4 and 53.8 2q) were found. Their effect on the mortar strength is discussed in Sections 2.5 and 3.4. With the aim to enhance the reactivity of the free lime, higher temperature treatments at 1350 C with an electric furnace were performed. Microwave furnace sintered samples followed by electric furnace sintering at
Fig. 1. XRD patterns of (a) raw meal and (b) commercial clinker. C Ca(CO3) - SiO2
NaAlSiO4 A C3S
: C2 S
C4AF
C3A
; CaO.
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7
Fig. 2. XRD patterns of resultant clinkers from microwave furnace for varied soaking times followed by heating in an electric furnace at1300 C for 30 min A C3S : C2S C3A ; Free CaO.
1350 C diffraction patterns (Fig. 3) indicated a more complete reaction, shown by the disappearance of the main free lime peak (53.8 2q), i.e., the complete conversion of CaO to C3S, C2S and C4AF occurred. It was shown that mineralogical composition of resultant clinker was similar to that of commercial Portland cement and the XRD results indicate the main compounds of cement (C3S, C2S, C3A, C4AF) were certainly formed already at the lower temperature of 1300 C. Thus, the resultant clinker could be processed at lower temperature with respect to that of the conventional production by approximately 100 C. The conventional preparation of the main compounds of cement requires a high sintering temperature and long holding time due to the low diffusion rates for solid-state
3
C4AF
reaction. Due to the efficient conversion of electromagnetic energy into heat, the microwave assisted sintering technique lowered the temperature for clinker formation and reduced the holding time leading to a reduction of CO2 emission (Haoxuan et al., 1999; Makul et al., 2014). 3.2. Microstructures of clinkers SEM micrographs of the resultant clinkers show different types of structure as similar as commercial clinker in Fig. 4. The raw meal sintered by microwave for 1, 9, 11 and 20 min and transferred to an electric furnace at 1300 C for 30 min (Fig. 5(a), (b), (c) and (d)) showed that hexagonal particles of C3S and spherical particles of
Fig. 3. XRD patterns of resultant clinkers from microwave furnace for varied soaking time followed by heating in an electric furnace at 1350 C for 30 min A C3S : C2S C3A ; Free CaO.
C4AF
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the dielectric properties of the materials to be heated (Gupta and Eugene, 2007). The oxide was in the raw meal to enhance the microwave process and attributed to the reduction of sintering temperature as well as shorter sintering times. 3.3.1. Loss on ignition (LOI) Weight percentage of LOI determined at 1000 C of the commercial and resultant clinkers was characterized by the chemical analysis. It was found that results of weight loss of our clinkers during sintering were higher than that of the commercial one. However, it passes the ASTM C-150 (less than 3%) (ASTM, 2015). At higher temperature (1350 C) in the electric furnace, the LOI was lower due to the more complete sintering which also supported the XRD and SEM results, as reported in Table 2.
Fig. 4. Microstructures of commercial clinker.
C2S (Bye, 1999) were dispersed throughout the samples. However, in Fig. 6, the denser microstructures of clinker (especially those of 9, 11 and 20 min in microwave) treated at 1350 C were observed supporting the XRD results. 3.3. Chemical analysis of clinker The chemical composition of both commercial and resultant clinkers is shown in Table 1. It is seen that the major compositions of cement is rich in CaO. For the resultant clinkers, the sum of the major oxide was higher than the minimum according to ASTM C150 (ASTM, 2015). The presence in the clinker composition of oxides, especially, Fe2O3 (Fang et al., 1996) is a key composition as the absorber of microwave energy. The heating efficiency depends on
3.3.2. Insoluble residue (IR) The weight percentage of IR of the resultant clinkers was higher than that of the commercial clinker and was lower for the clinker that was sintered at 1350 C (Table 3). It was less than 0.75% for all clinkers and passes ASTM C-150 (ASTM, 2015). Insoluble residue is a measure of the adulteration of cement, largely coming from impurities and affects the properties of cement, especially its compressive strength. That's why the results of compressive strength (in Section 3.4) of resultant clinkers sintered in a microwave furnace and an electric furnace at 1350 C was higher than those at 1300 C. 3.4. Compressive strength The compressive strength results at 7 days for the commercial and resultant clinkers with different soaking times in the microwave furnace followed by an electric furnace 1300 C are presented in Fig. 7(a). When the soaking time was increased from 5 to 40 min,
Fig. 5. Microstructures of resultant clinkers heated by a microwave furnace at 1150 C for different soaking times (a) 1 min (b) 9 min (c) 11 min and (d) 20 min, then transferred to an electric furnace at 1300 C for 30 min (3000x).
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
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Fig. 6. Microstructures of resultant clinkers heated by a microwave furnace at 1150 C for different soaking times, (a) 1 min (b) 9 min (c) 11 min and (d) 20 min, then transferred to an electric furnace at 1350 C for 30 min (3000x).
Table 1 Weight percentage chemical composition of the commercial and resultant clinkers sintered by a microwave furnace at 1150 C for different soaking times (1 min, 9 min, 11 min and 20 min), then transferred to an electric furnace at either 1300 C or 1350 C for 30 min. Chemical composition
Resultant clinkers (wt%) 1300 C
Commercial clinker (wt%)
20.00 3.77 5.75 0.85 66.84
SiO2 Fe2O3 Al2O3 MgO CaO
9
11
20
1
9
11
20
18.32 3.29 5.74 0.97 62.50
19.11 3.48 6.36 0.90 65.25
19.32 3.29 6.64 0.96 65.44
19.82 3.56 6.14 0.93 64.65
18.44 3.46 5.98 0.63 62.89
20.24 3.42 6.71 0.46 65.92
19.22 3.38 6.65 0.24 65.88
19.21 3.45 6.33 0.36 65.68
Table 2 Weight percentage of LOI of commercial and resultant clinkers with different soaking times (min) in a microwave furnace and then transferred to an electric furnace at either 1300 C or 1350 C for 30 min. Commercial clinker (wt%)
Resultant clinkers (wt%) 1300 C
2.56
1350 C
1
9
11
20
1
9
11
20
2.87
2.86
2.85
2.86
2.73
2.63
2.62
2.66
Table 3 Weight percentage of IR of commercial clinker for resultant clinkers with different soaking times (min) in a microwave furnace at 1150 C followed by an electric furnace at either 1300 C or 1350 C for 30 min. Commercial clinker (wt%)
Resultant clinker (wt%) 1300 C
0.14
Resultant clinkers (wt%) 1350 C
1
1350 C
1
9
11
20
1
9
11
20
0.27
0.25
0.25
0.27
0.23
0.22
0.22
0.24
the strength development of resultant cements increased significantly. Fig. 7(b) shows the compressive strength of specimens with different soaking times in the microwave furnace followed by an electric furnace at 1350 C. It shows higher compressive strengths than those treated at 1300 C. Moreover, at 9 min in the microwave furnace, the compressive strength records the highest value, similar to commercial clinker, but at 11, 20 and 40 min the compressive strength is decreasing. So compressive strength results confirmed what was found in the mineralogical analyses on the development of the specific phases of the clinker developed in the microwave þ electrical furnace treatments. 3.5. Energy consumption The net heat balance for the manufacture of Portland cement has been calculated theoretically as 4 106 kJ/ton (Engin and Ari, 2005). The clinkerization of the raw meal during pyro-processing is considered one of the most energy intensive steps, accounting for approximately more than 60% of the total energy required (Chen
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7
18.85
20
18 16 14 12 9.07
10 8
5.42
6
6.16
6.18
9.64
7.07
4 2 0
Compressive Strength (MPa)
Compressive Strength (MPa)
20
18.85
18.67 17.69
16.85
18 16
13.69
14
12.74
12
9.69
10 8 6 4 2 0
C
5
7
9
11
20
C
40
5
(a)
7
9
11
20
40
(b)
Fig. 7. The 7 day compressive strength results of the commercial clinker and resultant clinker mortars for commercial clinker (C) and for the resultant clinkers soaked from 5 min to 40 min in microwave furnace followed by an electric furnace at (a) 1300 C and (b) 1350 C.
Table 4 Energy consumption of cement production (per ton) by conventional process compared with new clinker formation method.
Conventional process New method
Microwave furnace
Electric furnace
Rotary kiln
Total
e 1.14 106 kJ
e 1.44 106 kJ
4 106 kJ e
4 106 kJ 2.58 106 kJ
et al., 2010; Behdad et al., 2015). Any reduction in residence time within the sintering furnace and the required sintering temperature would realize significant process benefits in terms of reduced production cost, less energy and lower carbon dioxide emissions. However, the energy of clinker from sintering by microwave þ electric furnaces (new method) is 2.58 106 kJ/ton (Table 4). The energy consumption by this new hybrid method in which cement is sintered by microwave furnace (at 1150 C for 9 min) and electric furnace (at 1350 C for 30 min) provides lower temperature and shorter residence time than that of the conventional process. Hence, the energy consumption is reduced. Furthermore, there is no grinding cost needed for clinker preparation in our process.
4. Conclusion Conventional cement production consumes large amounts of energy and is one of main sources of CO2 emissions. A worldwide increase in construction using concrete and therefore an increasing need for cement production leading to an increase consumption energy also aggravates the problem. The benefit of a reduction in the temperature for clinker formation and/or a reduction in sintering time using a microwave treatment is therefore worthy of consideration. The results of this research indicated that the new hybrid method, the cement raw meal sintering by a microwave furnace prior to an electric furnace, was able to reduce the temperature by up to 100 C and reduce sintering time. XRD showed that after microwave and electric furnace sintering CaO transformed to the main cement compounds C2S, C3S, C3A and C4AF. For the SEM results, the microstructure of resultant clinkers were similar shape to conventionally formed cement compounds. The resultant clinkers, also, were found to pass ASTM C-150 for chemical composition, LOI and IR. The mortar compressive strength of the resultant clinkers sintered in a microwave furnace at 1150 C for 9 min and then in the electric furnace at 1350 C for 30 min presented the highest strengths and was similar to that of commercial clinker. The energy
consumption of the conventional process is higher than that of new method. The new hybrid process would lead to decrease the energy consumption and carbon dioxide emission. Acknowledgement The authors wish to thank the Siam Cement Group (SCG) for providing raw meal and clinker and Department of Science Service for the supports on analytical instruments and work places. References ASTM C 10902, (2002), Standard test method for compressive strength of hydraulic cement mortars. ASTM C 11404, (2004), Standard test methods for chemical analysis of hydraulic cement. ASTM C 15015, (2015), Standard specification for Portland cement. Behdad, A., Akbarian, B., Narges, B.A., Kakaee, A.H., Shabani, B., 2015. Energy consumption assessment in a cement production plant. Sustain. Energy Technol. Assess. 10, 84e89. Benhelal, E., Zahedi, G., Shamsaei, E., Bahadori, A., 2013. Global strategies and potentials to curb CO2 emissions in cement industry. J. Clean. Prod. 51, 142e161. Bye, G.C., 1999. Portland Cement, second ed. Thomas Telford, London. Chen, C., Habert, G., Bouzidi, Y., Jullien, A., 2010. Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. J. Clean. Prod. 18, 478e485. Engin, T., Ari, V., 2005. Energy auditing and recovery for dry type cement rotary kiln systemseea case study. Energy Convers. Manage 4, 551e562. Fang, Y., Della, M.R., Thiago, P.P., Romildo, D., Filho, T., Silvoso, M.M., 1996. Microwave clinkering of ordinary and colored Portland cements. Cem. Concr. Res. 26, 41e47. Fairbairn, E.M.R., Americano, B.B., Cordeiro, G.C., Paula, T.P., Filho, R.D.T., Silvoso, M.M., 2010. Cement replacement by sugar cane bagasse ash: CO2 emissions reduction and potential for carbon credits. J. Environ. Manag. 9, 1864e1871. Gupta, M., Eugene, W.W.L., 2007. Microwaves and Metals. Wiley, Singapore. Haoxuan, L., Agrawal, D.K., Cheng, J., Silsbee, M.R., 1999. Formation and hydration of C3S prepared by microwave and conventional. Cem. Concr. Res. 29, 1611e1617. Leonelli, C., Siligardi, C., Veronesi, P., Corradi, A., 2006. Application of Microwave to Glaze and Ceramic Industry in Advances in Microwave and Radio Frequency Processing - Report from the 8th International Conference on Microwave and High Frequency Heating, pp. 703e709. Long, S., Yan, C., Dong, J., 2002. Microwave-promoted burning of Portland cement clinker. Cem. Concr. Res. 32, 17e21. Makul, N., Rattanadecho, P., Agrawal, D.K., 2014. Applications of microwave energy
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7 in cement and concrete. Renew. Sust. Energy Rev. 37, 715e733. Minay, E.J., Boccaccini, A.R., Veronesi, P., Cannillo, V., Leonelli, C., 2004. Processing of novel glass matrix composites by microwave heating. J. Mater. Process. Technol. 155e156 (1e3), 1749e1755. Monaco, C., Prete, F., Leonelli, C., Esposito, L., Tucci, A., 2015. Microstructural study of microwave sintered zirconia for dental applications. Ceram. Int. 41 (1), 1255e2126. Parrott, L., 2002. Cement Concrete and Sustainability, a Report on the Progress of the UK Cement and Concrete Industry towards Sustainability. British Cement Association, London.
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Quemeneur, L., Choisnet, J., Raveau, B., Thiebaut, J., Roussy, G., 1983. Microwave clinkering with a grooved resonant applicator. J. Am. Ceram. Soc. 66, 855e859. Tianming, G., Shen, L., Shen, M., Chen, F., Liu, L., Gao, L., 2015. Analysis on difference of carbon dioxide emission from cement production and their major determinant. J. Clean. Prod. 103, 160e170. Ying, L.C., Chang, J.E., Shin, P.H., Ko, M.S., Chang, Y.K., Chiang, L.C., 2010. Reusing pretreated desulfurization slag to improve clinkerization and clinker grindability for energy conservation in cement manufacture. J. Environ. Manage. 9, 1892e1897.
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Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Development of microwave-assisted sintering of Portland cement raw meal Praphatsorn Kaewwichit a, Jaroon Junsomboon b, Parinya Chakartnarodom a, Chayanee Tippayasam a, Thapanee Srichumpong a, Parjaree Thavorniti c, Cristina Leonelli d, Duangrudee Chaysuwan a, * a
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand Physics and Engineering 2 Sub-division, Physics and Engineering Program, Department of Science Service, Ministry of Science and Technology, Bangkok, 10400, Thailand c National Metal and Materials Technology Center (MTEC), Pathumthani, 12120, Thailand d Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, 41125, Modena, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2016 Received in revised form 1 July 2016 Accepted 1 July 2016 Available online xxx
Typically, using a rotary furnace as a heat generator, a temperature of approximately 1450 C and a time of 60 min is needed to produce clinker requiring large amounts of energy. Recently, a method of sintering Portland cement by microwave furnace has been developed with the aim to reduce this high consumption of energy in the conventional cement production. In this work, cement raw meal was calcined by a microwave furnace operating at 2.45 GHz with 900 W at 1150 C at several periods of time but was not completely successful in terms of clinker formation. Therefore, an electric furnace was used at 1300 C and 1350 C for 30 min to further heat the material. Chemical compositions of the formed clinker, characterized by XRD, presented C3S, C2S, C3A and C4AF as the main constituents confirming a clinker similar to those of clinker produced by rotary kiln or conventional technique. Loss on ignition and insoluble residue of the resultant clinker were analyzed by chemical analysis and the results were found to pass ASTM C-114. It was found that the raw meal sintering process using a microwave furnace followed by transfer to an electric furnace could reduce not only the temperature by at least 100 C but also the processing time of the clinker. In addition, there is no grinding cost for clinker preparation in this process. This processing of clinker would decrease energy consumption and carbon dioxide emission to the atmosphere, a major cause of global warming. © 2016 Elsevier Ltd. All rights reserved.
Keywords: CO2 emission Cement raw meal Clinker Microwave furnace Electric furnace Sintering
1. Introduction Portland cement is currently widely used to build various constructions because of its high compressive strength. Cement production is a high-energy consumption process, especially in clinker production (Bye, 1999). Most energy is consumed in the burning process for clinkerization. It was found that the burning process requires approximately 3.2e6.3 GJ of energy per ton of clinker production (Ying et al., 2010) and the energy is generally associated with CO2 emission (Parrott, 2002). Behind power generation, the cement industry has always been among the largest CO2 emitting industries (Fairbairn et al., 2010) with its processing emitting
* Corresponding author. E-mail address: [email protected] (D. Chaysuwan).
around 900 kg of CO2 for every ton of cement production (Benhelal et al., 2013) and affecting the level of greenhouse gas emissions and contributing to the global warming issue. In the conventional process of cement manufacture high temperatures are maintained by rotary furnace around 1450 C for 60 min to obtain the clinker, however the clinker process, through microwave furnace, needs lower temperature (Fang et al., 1996). Microwave heating is a process of microwave-material interaction. The heating efficiency mainly depends on the dielectric properties of the material to be heated (Fang et al., 1996; Makul et al., 2014). Microwave heating is superior to conventional heating methods in terms of energy-saving, rapid heating rates and short processing times (Fang et al., 1996; Makul et al., 2014; Long et al., 2002). A 95% of energy saving can be achieved with the use of microwave energy over conventional heating techniques
http://dx.doi.org/10.1016/j.jclepro.2016.07.009 0959-6526/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7
(Quemeneur et al., 1983). Microwave processing of ceramic materials has been reported by several researchers (Fang et al., 1996; Makul et al., 2014; Long et al., 2002; Minay et al., 2004; Leonelli et al., 2006; Monaco et al., 2015). Calcium oxide (CaO) and magnesium oxide (MgO) are the typical clinker components, which account for 64e68% of the clinker weight. Normally, limestone used in cement production has 75e90% CaCO3 in raw meal. Most CO2 is produced in converting calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) into CaO and MgO when the temperature is above 900 C. At this stage, the CO2 leaves the system and the raw meal loses over one third of its original weight. When the temperature reaches 1300e1450 C, the reaction of clinkerization takes place, with parts of the materials becoming liquid, and finally, nodules known as clinker are formed (Tianming et al., 2015). The aim of this research was to utilize the microwave furnace operating at the frequency at 2.45 GHz at 900 W to reduce the sintering temperature by at least 100 C and reduce times in order to decrease energy consumption and produce less CO2 emission to the atmosphere. 2. Experimental 2.1. Materials preparation and sintering At first, the raw meal of commercial type I Portland cement was intended to use for clinkerization by only microwave technique with a microwave furnace (CEM, MAS7000, USA, 900 W, max temp. 1200 C). This furnace has a limitation of a maximum temperature of 1150 C which was used for the microwave sintering. So, in addition an extra electric furnace (Nabertherm, HT18, Germany, 1200 W, max temp. 1800 C) was utilized, for final sintering of the raw meal (at 1300 C and 1350 C). The specimens were prepared by slightly compressing the raw meal powder (20 g) into alumina boats (of volume 25 ml), and then heating in a microwave furnace at 1150 C for soaking times of 1, 3, 5, 7, 9, 11 and 20 min, respectively. Consequently, all specimens were brought to continue sintering in an electric furnace at either 1300 C or 1350 C for 30 min and the resultant clinkers obtained. 2.2. XRD characterization The mineralogical compositions of the raw meal, commercial clinker and resultant clinkers were characterized. They were investigated by x-ray diffraction (XRD) (X'Pert, Philips, Netherland) in order to determine the optimum sintering conditions. The diffraction patterns of resultant clinkers were recorded by using Cu Ka radiation and generator setting was 40 kV excitation potential
with a current of 35 mA. The programs typically used were scanned from 10 to 60 2q with step size 0.02 2q. 2.3. SEM analysis The microstructures of resultant clinkers were determined by a scanning electron microscope (SEM) (XL 30, Philips) on the powders glued to Al stub with an accelerating voltage of 15 kV. 2.4. Chemical analysis The main compounds of clinker such as SiO2, Fe2O3, Al2O3, CaO and MgO, loss on ignition (LOI), insoluble residue (IR) of the commercial and resultant clinkers were characterized by wet chemical analysis according to ASTM C114-04 (ASTM, 2004). 2.5. Compressive strength The compressive strength of the mortar from commercial and the experimental clinkers was determined by a universal testing machine (UTM) (Shimadzu, Japan). The mortars of both commercial clinker and experimental clinkers were prepared with a water: clinker: sand ratio of 0.485: 1: 2.75. The pastes were cast and compacted by tamping for two layers in cube molds (2.5 2.5 2.5 cm3). Mortar cubes were demolded after being at room temperature for 24 h. Consequently, they were cured in a plastic box under water for 7 days and then tested for compressive strength according to ASTM C 109-02 (ASTM, 2002). 3. Results and discussion 3.1. XRD analysis The XRD patterns in Fig. 1 show sharp peaks for the raw meal and commercial clinker corresponding to CaCO3 (01-072-1650), SiO2 (00-046-1045), NaAlSiO4 (01-076-1733), C3S (Ca3SiO5: 00042-0551), C2S (Ca2SiO4: 00-049-1672), C4AF (CaAl2Fe4O10: 00021-0830), C3A (Ca3Al2O6: 00-038-1429) and CaO (00-002-1088). XRD patterns of the resultant clinker prepared in the microwave furnace for different soaking times follower by an electric furnace at 1300 C are shown in Fig. 2. The peaks of the main compounds of clinker such as C3S, C2S, C4AF, C3A and, especially, free lime (CaO) (37.4 and 53.8 2q) were found. Their effect on the mortar strength is discussed in Sections 2.5 and 3.4. With the aim to enhance the reactivity of the free lime, higher temperature treatments at 1350 C with an electric furnace were performed. Microwave furnace sintered samples followed by electric furnace sintering at
Fig. 1. XRD patterns of (a) raw meal and (b) commercial clinker. C Ca(CO3) - SiO2
NaAlSiO4 A C3S
: C2 S
C4AF
C3A
; CaO.
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7
Fig. 2. XRD patterns of resultant clinkers from microwave furnace for varied soaking times followed by heating in an electric furnace at1300 C for 30 min A C3S : C2S C3A ; Free CaO.
1350 C diffraction patterns (Fig. 3) indicated a more complete reaction, shown by the disappearance of the main free lime peak (53.8 2q), i.e., the complete conversion of CaO to C3S, C2S and C4AF occurred. It was shown that mineralogical composition of resultant clinker was similar to that of commercial Portland cement and the XRD results indicate the main compounds of cement (C3S, C2S, C3A, C4AF) were certainly formed already at the lower temperature of 1300 C. Thus, the resultant clinker could be processed at lower temperature with respect to that of the conventional production by approximately 100 C. The conventional preparation of the main compounds of cement requires a high sintering temperature and long holding time due to the low diffusion rates for solid-state
3
C4AF
reaction. Due to the efficient conversion of electromagnetic energy into heat, the microwave assisted sintering technique lowered the temperature for clinker formation and reduced the holding time leading to a reduction of CO2 emission (Haoxuan et al., 1999; Makul et al., 2014). 3.2. Microstructures of clinkers SEM micrographs of the resultant clinkers show different types of structure as similar as commercial clinker in Fig. 4. The raw meal sintered by microwave for 1, 9, 11 and 20 min and transferred to an electric furnace at 1300 C for 30 min (Fig. 5(a), (b), (c) and (d)) showed that hexagonal particles of C3S and spherical particles of
Fig. 3. XRD patterns of resultant clinkers from microwave furnace for varied soaking time followed by heating in an electric furnace at 1350 C for 30 min A C3S : C2S C3A ; Free CaO.
C4AF
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the dielectric properties of the materials to be heated (Gupta and Eugene, 2007). The oxide was in the raw meal to enhance the microwave process and attributed to the reduction of sintering temperature as well as shorter sintering times. 3.3.1. Loss on ignition (LOI) Weight percentage of LOI determined at 1000 C of the commercial and resultant clinkers was characterized by the chemical analysis. It was found that results of weight loss of our clinkers during sintering were higher than that of the commercial one. However, it passes the ASTM C-150 (less than 3%) (ASTM, 2015). At higher temperature (1350 C) in the electric furnace, the LOI was lower due to the more complete sintering which also supported the XRD and SEM results, as reported in Table 2.
Fig. 4. Microstructures of commercial clinker.
C2S (Bye, 1999) were dispersed throughout the samples. However, in Fig. 6, the denser microstructures of clinker (especially those of 9, 11 and 20 min in microwave) treated at 1350 C were observed supporting the XRD results. 3.3. Chemical analysis of clinker The chemical composition of both commercial and resultant clinkers is shown in Table 1. It is seen that the major compositions of cement is rich in CaO. For the resultant clinkers, the sum of the major oxide was higher than the minimum according to ASTM C150 (ASTM, 2015). The presence in the clinker composition of oxides, especially, Fe2O3 (Fang et al., 1996) is a key composition as the absorber of microwave energy. The heating efficiency depends on
3.3.2. Insoluble residue (IR) The weight percentage of IR of the resultant clinkers was higher than that of the commercial clinker and was lower for the clinker that was sintered at 1350 C (Table 3). It was less than 0.75% for all clinkers and passes ASTM C-150 (ASTM, 2015). Insoluble residue is a measure of the adulteration of cement, largely coming from impurities and affects the properties of cement, especially its compressive strength. That's why the results of compressive strength (in Section 3.4) of resultant clinkers sintered in a microwave furnace and an electric furnace at 1350 C was higher than those at 1300 C. 3.4. Compressive strength The compressive strength results at 7 days for the commercial and resultant clinkers with different soaking times in the microwave furnace followed by an electric furnace 1300 C are presented in Fig. 7(a). When the soaking time was increased from 5 to 40 min,
Fig. 5. Microstructures of resultant clinkers heated by a microwave furnace at 1150 C for different soaking times (a) 1 min (b) 9 min (c) 11 min and (d) 20 min, then transferred to an electric furnace at 1300 C for 30 min (3000x).
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Fig. 6. Microstructures of resultant clinkers heated by a microwave furnace at 1150 C for different soaking times, (a) 1 min (b) 9 min (c) 11 min and (d) 20 min, then transferred to an electric furnace at 1350 C for 30 min (3000x).
Table 1 Weight percentage chemical composition of the commercial and resultant clinkers sintered by a microwave furnace at 1150 C for different soaking times (1 min, 9 min, 11 min and 20 min), then transferred to an electric furnace at either 1300 C or 1350 C for 30 min. Chemical composition
Resultant clinkers (wt%) 1300 C
Commercial clinker (wt%)
20.00 3.77 5.75 0.85 66.84
SiO2 Fe2O3 Al2O3 MgO CaO
9
11
20
1
9
11
20
18.32 3.29 5.74 0.97 62.50
19.11 3.48 6.36 0.90 65.25
19.32 3.29 6.64 0.96 65.44
19.82 3.56 6.14 0.93 64.65
18.44 3.46 5.98 0.63 62.89
20.24 3.42 6.71 0.46 65.92
19.22 3.38 6.65 0.24 65.88
19.21 3.45 6.33 0.36 65.68
Table 2 Weight percentage of LOI of commercial and resultant clinkers with different soaking times (min) in a microwave furnace and then transferred to an electric furnace at either 1300 C or 1350 C for 30 min. Commercial clinker (wt%)
Resultant clinkers (wt%) 1300 C
2.56
1350 C
1
9
11
20
1
9
11
20
2.87
2.86
2.85
2.86
2.73
2.63
2.62
2.66
Table 3 Weight percentage of IR of commercial clinker for resultant clinkers with different soaking times (min) in a microwave furnace at 1150 C followed by an electric furnace at either 1300 C or 1350 C for 30 min. Commercial clinker (wt%)
Resultant clinker (wt%) 1300 C
0.14
Resultant clinkers (wt%) 1350 C
1
1350 C
1
9
11
20
1
9
11
20
0.27
0.25
0.25
0.27
0.23
0.22
0.22
0.24
the strength development of resultant cements increased significantly. Fig. 7(b) shows the compressive strength of specimens with different soaking times in the microwave furnace followed by an electric furnace at 1350 C. It shows higher compressive strengths than those treated at 1300 C. Moreover, at 9 min in the microwave furnace, the compressive strength records the highest value, similar to commercial clinker, but at 11, 20 and 40 min the compressive strength is decreasing. So compressive strength results confirmed what was found in the mineralogical analyses on the development of the specific phases of the clinker developed in the microwave þ electrical furnace treatments. 3.5. Energy consumption The net heat balance for the manufacture of Portland cement has been calculated theoretically as 4 106 kJ/ton (Engin and Ari, 2005). The clinkerization of the raw meal during pyro-processing is considered one of the most energy intensive steps, accounting for approximately more than 60% of the total energy required (Chen
Please cite this article in press as: Kaewwichit, P., et al., Development of microwave-assisted sintering of Portland cement raw meal, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.07.009
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7
18.85
20
18 16 14 12 9.07
10 8
5.42
6
6.16
6.18
9.64
7.07
4 2 0
Compressive Strength (MPa)
Compressive Strength (MPa)
20
18.85
18.67 17.69
16.85
18 16
13.69
14
12.74
12
9.69
10 8 6 4 2 0
C
5
7
9
11
20
C
40
5
(a)
7
9
11
20
40
(b)
Fig. 7. The 7 day compressive strength results of the commercial clinker and resultant clinker mortars for commercial clinker (C) and for the resultant clinkers soaked from 5 min to 40 min in microwave furnace followed by an electric furnace at (a) 1300 C and (b) 1350 C.
Table 4 Energy consumption of cement production (per ton) by conventional process compared with new clinker formation method.
Conventional process New method
Microwave furnace
Electric furnace
Rotary kiln
Total
e 1.14 106 kJ
e 1.44 106 kJ
4 106 kJ e
4 106 kJ 2.58 106 kJ
et al., 2010; Behdad et al., 2015). Any reduction in residence time within the sintering furnace and the required sintering temperature would realize significant process benefits in terms of reduced production cost, less energy and lower carbon dioxide emissions. However, the energy of clinker from sintering by microwave þ electric furnaces (new method) is 2.58 106 kJ/ton (Table 4). The energy consumption by this new hybrid method in which cement is sintered by microwave furnace (at 1150 C for 9 min) and electric furnace (at 1350 C for 30 min) provides lower temperature and shorter residence time than that of the conventional process. Hence, the energy consumption is reduced. Furthermore, there is no grinding cost needed for clinker preparation in our process.
4. Conclusion Conventional cement production consumes large amounts of energy and is one of main sources of CO2 emissions. A worldwide increase in construction using concrete and therefore an increasing need for cement production leading to an increase consumption energy also aggravates the problem. The benefit of a reduction in the temperature for clinker formation and/or a reduction in sintering time using a microwave treatment is therefore worthy of consideration. The results of this research indicated that the new hybrid method, the cement raw meal sintering by a microwave furnace prior to an electric furnace, was able to reduce the temperature by up to 100 C and reduce sintering time. XRD showed that after microwave and electric furnace sintering CaO transformed to the main cement compounds C2S, C3S, C3A and C4AF. For the SEM results, the microstructure of resultant clinkers were similar shape to conventionally formed cement compounds. The resultant clinkers, also, were found to pass ASTM C-150 for chemical composition, LOI and IR. The mortar compressive strength of the resultant clinkers sintered in a microwave furnace at 1150 C for 9 min and then in the electric furnace at 1350 C for 30 min presented the highest strengths and was similar to that of commercial clinker. The energy
consumption of the conventional process is higher than that of new method. The new hybrid process would lead to decrease the energy consumption and carbon dioxide emission. Acknowledgement The authors wish to thank the Siam Cement Group (SCG) for providing raw meal and clinker and Department of Science Service for the supports on analytical instruments and work places. References ASTM C 10902, (2002), Standard test method for compressive strength of hydraulic cement mortars. ASTM C 11404, (2004), Standard test methods for chemical analysis of hydraulic cement. ASTM C 15015, (2015), Standard specification for Portland cement. Behdad, A., Akbarian, B., Narges, B.A., Kakaee, A.H., Shabani, B., 2015. Energy consumption assessment in a cement production plant. Sustain. Energy Technol. Assess. 10, 84e89. Benhelal, E., Zahedi, G., Shamsaei, E., Bahadori, A., 2013. Global strategies and potentials to curb CO2 emissions in cement industry. J. Clean. Prod. 51, 142e161. Bye, G.C., 1999. Portland Cement, second ed. Thomas Telford, London. Chen, C., Habert, G., Bouzidi, Y., Jullien, A., 2010. Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. J. Clean. Prod. 18, 478e485. Engin, T., Ari, V., 2005. Energy auditing and recovery for dry type cement rotary kiln systemseea case study. Energy Convers. Manage 4, 551e562. Fang, Y., Della, M.R., Thiago, P.P., Romildo, D., Filho, T., Silvoso, M.M., 1996. Microwave clinkering of ordinary and colored Portland cements. Cem. Concr. Res. 26, 41e47. Fairbairn, E.M.R., Americano, B.B., Cordeiro, G.C., Paula, T.P., Filho, R.D.T., Silvoso, M.M., 2010. Cement replacement by sugar cane bagasse ash: CO2 emissions reduction and potential for carbon credits. J. Environ. Manag. 9, 1864e1871. Gupta, M., Eugene, W.W.L., 2007. Microwaves and Metals. Wiley, Singapore. Haoxuan, L., Agrawal, D.K., Cheng, J., Silsbee, M.R., 1999. Formation and hydration of C3S prepared by microwave and conventional. Cem. Concr. Res. 29, 1611e1617. Leonelli, C., Siligardi, C., Veronesi, P., Corradi, A., 2006. Application of Microwave to Glaze and Ceramic Industry in Advances in Microwave and Radio Frequency Processing - Report from the 8th International Conference on Microwave and High Frequency Heating, pp. 703e709. Long, S., Yan, C., Dong, J., 2002. Microwave-promoted burning of Portland cement clinker. Cem. Concr. Res. 32, 17e21. Makul, N., Rattanadecho, P., Agrawal, D.K., 2014. Applications of microwave energy
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P. Kaewwichit et al. / Journal of Cleaner Production xxx (2016) 1e7 in cement and concrete. Renew. Sust. Energy Rev. 37, 715e733. Minay, E.J., Boccaccini, A.R., Veronesi, P., Cannillo, V., Leonelli, C., 2004. Processing of novel glass matrix composites by microwave heating. J. Mater. Process. Technol. 155e156 (1e3), 1749e1755. Monaco, C., Prete, F., Leonelli, C., Esposito, L., Tucci, A., 2015. Microstructural study of microwave sintered zirconia for dental applications. Ceram. Int. 41 (1), 1255e2126. Parrott, L., 2002. Cement Concrete and Sustainability, a Report on the Progress of the UK Cement and Concrete Industry towards Sustainability. British Cement Association, London.
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Quemeneur, L., Choisnet, J., Raveau, B., Thiebaut, J., Roussy, G., 1983. Microwave clinkering with a grooved resonant applicator. J. Am. Ceram. Soc. 66, 855e859. Tianming, G., Shen, L., Shen, M., Chen, F., Liu, L., Gao, L., 2015. Analysis on difference of carbon dioxide emission from cement production and their major determinant. J. Clean. Prod. 103, 160e170. Ying, L.C., Chang, J.E., Shin, P.H., Ko, M.S., Chang, Y.K., Chiang, L.C., 2010. Reusing pretreated desulfurization slag to improve clinkerization and clinker grindability for energy conservation in cement manufacture. J. Environ. Manage. 9, 1892e1897.
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