- Email: [email protected]
Experimental investigation on the drying process of the sinter mixture
Powder Technology 218 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental investigation on the drying process of the sinter mixture Hui Dong a, FengRui Jia b,⁎, Yong Zhao a, Meng Wang a, JiuJu Cai a a b
SEPA Key Laboratory on Eco-industry, Northeastern University, Shenyang 110004, China College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 18 September 2011 Accepted 31 October 2011 Available online 16 December 2011 Keywords: Sintering Sinter mixture Drying process Waste heat Drying rate Moisture content
a b s t r a c t Achieving efficient recovery and utilization of waste heat during sintering is crucial for iron and steel plants. Previous studies on waste heat utilization have shown that steam and electricity are generated, after which the air and sinter mixture is preheated. However, few studies on the mechanisms of the drying process of the sinter mixture exist. This paper presents the results of an experimental investigation on the drying process that occurs during sintering. The results show that the drying process of the sinter mixture consists of three stages, namely, a short rising-rate stage, a long constant-rate stage, and a falling-rate stage. The moisture contents of specimens are 0.055–0.06 kg/kg in the first 5–7 min, and the critical moisture contents are approximately 0.023–0.03 kg/kg. The drying efficiency is mainly attributed to the temperature and the flow rate of the hot air, and the former one is more significant. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Achieving efficient recovery and utilization of waste heat during sintering–cooling process is crucial for iron and steel plants [1–3]. The energy consumption of the sintering process accounts for 15% of the total energy consumption, and the average energy consumption per ton of sinter in China is higher than those of developed countries (20%) [4, 5]. Low utilization and recovery rate of waste heat is one of the main reasons for high energy consumption during sintering. Therefore, the efficient recovery of waste heat resources is important to further reduce the energy consumption of the sintering process. These waste heat resources are mainly sensible heat, which is taken along by exhaust gas and products; waste heat accounts for 15%–20% of the heat input in the process, and sensible heat accounts for 40%–45% [6]. According to the second law of thermodynamics, if the temperature of the waste heat is relatively low (generally below 230 °C) and not high enough to be utilized in the waste heat boiler, it should be used to dry and preheat the sinter mixture which is applied to reduce moisture content in the material before ignition to prevent overmoisturising of the bottom layer. In this way, the low quality waste heat can be recycled efficiently, and energy consumption in the sintering process is reduced at the same time. In China, the primary attempt has been carried out in Anshan Iron and Steel Group Company (Ansteel) and a small number of enterprises [7]. As regards the sinter mixture, few studies on the mechanisms of drying process exist [8, 9], the drying process during sintering is an interdisciplinary subject,
⁎ Corresponding author: Tel./fax: + 86 2483686994. E-mail address: [email protected] (F.R. Jia). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.10.063
which involves the disciplines of metallurgy, chemical engineering and thermal energy engineering. The drying process is aim to remove the moisture from the sinter mixture as far as possible by heating, in this case, the dried sinter mixture could be beneficial to sintering. In the present study, a series of experiments (level 10 mg, level 10 g, and level 100 g) is carried out with thermogravimetric analysis, chamber dryer, and tunnel drier. Furthermore, several key parameters that influence the drying process, such as, critical moisture content, flow rate, and temperature of the hot air, are also investigated. In addition, critical moisture content stands for the start of fallingrate drying period. Promising results have been obtained. These can provide fundamental data for efficient recovery and utilization of waste heat in the sintering process in China. 2. Experiment platform and procedure Three different parts of platform and equipment were used in this study, namely, thermogravimetric analysis experiment, chamber dryer experiment, and tunnel drier experiment, with the weights of 10 mg, 10 g and 100 g, respectively. The thermogravimetric analysis and chamber furnace are used to retrieve basic knowledge on drying kinetics of sinter mixture, furthermore, the tunnel dryer was used to study effect of gas flow and temperature. The thermogravimetric analyzer STA409 was obtained from NETZSCH-Ger Inc., and the chamber dryer and the tunnel dryer were obtained from Changzhou Wanji and Suzhou Yihui respectively. Sinter mixture specimens were applied in Ansteel, and density of the specimens were measured using the Archimedes method. A result of 1865.30 kg/m3 was achieved, the composition of sinter mixture was shown in Table 1, and the initial moisture content in the mixture is 0.075 kg/kg.
2
H. Dong et al. / Powder Technology 218 (2012) 1–4
Table 1 Sinter mixture composition. TFe/%
FeO/%
CaO/%
MgO/%
SiO2/%
C/%
R = CaO/SiO2
57.4
8–10
9.34
2.2
4.67
0.1–0.2
2
The thermogravimetric analysis experiment was performed on STA409 using a heating rate was 10 °C/min. Specimens were dried at temperatures ranging from 30 to 300 °C for a holding time of 40 min, in addition, due to the limitation of the capacity in the STA409, sinter mixtures in smaller size were picked.. These were used to analyze moisture content and drying rate during heat-up [10–12]. For the chamber dryer experiment, specimens were dried at temperatures of 60, 80, 100, 150, 200, and 250 °C with the same holding time of 40 min for all experiments. The specimens, which were weighed out using the same quality, were divided into 40 weighing bottles. Subsequently, 2 specimens were taken out every 2 min and were immediately weighed again. The specimens were used to calculate the moisture content, the advantage of using specimens from “interrupted” drying runs is that the weight of these specimens can be measured and used to study moisture content during heat-up. The effects of cooling on the other bottles inside the dryer are considered insignificant. Meanwhile, hot air temperature had significant impact on moisture content of the sinter mixture. In addition, every working condition was repeated at least twice in order to prevent segregation of the experimental results, and then, all the specimens in the bottles were kept in the chamber dryer for over 24 h, so the temperature in bottles was equilibrated with chamber temperature, furthermore, all the bottles were put away from the furnace door, in order to avoid the effect of opening of furnace's door. The pressure ratings used in the tunnel drier experiments were 0.32, 0.41, and 0.54 MPa, corresponding to flow rates of 63, 90 and 119 m 3/h, respectively. Specimens were dried at temperatures ranging from 50 to 90 °C for a holding time of 40 min. These were placed in an oblong-shaped gauze and dried by the hot air from one end of the drying chamber. The schematic of the tunnel drier is shown in Fig. 1, the captions details as follows: 1, centrifugal fan; 2, orifice-plate flow-meter; 3, 15-temperature demonstrator; 4, 17-weight sensor demonstrator; 5, drying materials; 6, electric heater; 7, dry-bulb; 8, 14-wetbulb; 9, box-dryer; 10, waste valve;11, waste cyclic valve; 12, fresh air
Fig. 2. Thermal analysis curves (TG, DTG, DSC).
inlet valve; 13, electric heating control instrument; 16, differential pressure transmitter and demonstrator of orifice-plate flow-meter. 3. Results and discussions 3.1. Thermogravimetric analysis experiment In the thermogravimetric analysis experiment, the thermal analysis curves were shown in Fig. 2.As functions of drying time defined as TG, DTG and DSC, these represent the interplay between moisture content and drying time, drying rate and drying time and heat discharge and drying time, respectively. Fig. 2 allows the study of change as a function of drying time during the heat-up from 30 °C–300 °C. (1) Based on the TG curve, the free moisture content was approximately 0.064 kg/kg, and the initial moisture content average of the sinter mixture was 0.08 kg/kg. Thus, the equilibrium moisture content in sinter mixture was approximately 0.016 kg/kg. The weight loss of the sinter mixture was more obvious in the first 5 min. The mass fraction of the sinter mixture was 94.2% when the time reached the fifth minute, indicating the removal of approximately 90% of the free moisture content during the first 5 min. (2) At point A (peak of the DSC curve) or at point B (trough of the DTG curve), the water content of the material approximated the critical moisture content, i.e., the critical moisture content of the sinter
Fig. 1. The schematic of tunnel drier.
H. Dong et al. / Powder Technology 218 (2012) 1–4
3
Fig. 3. Drying curve at different holding temperatures.
mixture was approximately 0.048 kg/kg–0.053 kg/kg (average of 0.05 kg/kg). 3.2. Chamber dryer experiment The mechanisms of the drying process were theorized based on the specimens of 10 mg during heat-up. However, the experiment was unable to completely reflect the actual conditions. In the chamber dryer experiment, by plotting the moisture content as a function of drying time from heating at different holding temperatures, we found that the moisture content of the sinter mixture decreased drastically as heating increased (Fig. 3). (1) Under the experimental conditions, the drying rate was slower when the hot air temperature was below 100 °C, specifically below 80 °C. Taking the first 10 min of the drying process for example, moisture contents of the sinter mixtures were recorded at 0.0758 kg/kg, 0.0744 kg/kg, and 0.067 kg/kg when the corresponding temperatures were 60, 80 and 100 °C, respectively. The corresponding decreases were 0.0012 kg/kg, 0.0026 kg/kg, and 0.01 kg/kg, respectively. (2) The drying rate changed with time within a narrow range when the hot air temperature was below 100 °C. When the hot air temperature reached 150 °C in the first 4 min, the drying rate became slower, changed with time within a narrow range, and varied within the wide range. When the drying
Fig. 4. Drying curve (Constant flow rate and different temperatures).
Fig. 5. Drying curve characteristics (Constant flow rate and different temperatures).
time was more than 20 min, the rate change with time slowed down. Therefore, the drying rate was larger in the interval from the fourth to the twentieth minute when the hot air temperature was higher than 150 °C. (3) Comparing the drying curves of various temperature conditions helped arrive at 150 °C as the established demarcation point. The hot air temperature had a greater impact on the drying rate when it was below 150 °C, although it had less impact when it was higher than 150 °C. This situation became more obvious when temperature was higher than 150 °C. Based on (2) and (3), hot air temperature should be at least higher than 150 °C, in order to ensure the fine drying effect under the experimental conditions. 3.3. Tunnel dryer experiment With regards the performance of the tunnel drier, both the hot air flow rate and temperature were regulated. There were three working conditions for both flow rate and temperature: 63, 90, and 119 m 3/h at 50, 70 and 90 °C, respectively. When the temperature effect was considered, the flow rate (60 m 3/h) became constant (Figs. 4 and 5). When the
Fig. 6. Drying curve (Constant temperature and different flow rates).
4
H. Dong et al. / Powder Technology 218 (2012) 1–4
under the condition of improving air flow from 60 m3/h to 90 m3/h was better than the air flow from 90 m3/h to 119 m3/h. Thus, an appropriate range should be established to enforce the dying effect. (2) The effect of the hot air flow on the drying process occurred mainly at a constant rate period. The effect was weaker at the rising-rate period and almost negligible at the falling-rate period. (3) The drying rate was greater, and the corresponding drying time was contained within the first 5–7 min when the moisture content of the sintering materials ranged from 0.055 to 0.06 kg water/kg material based on the experimental conditions. (4) The critical moisture content of the sinter mixture was approximately 0.023 kg/kg–0.03 kg/kg, and the moisture content of the rising and falling rate period was approximately 7.0% under the experimental condition.
4. Conclusions Fig. 7. Drying curve characteristics (Constant temperature and different flow rates).
flow rate effect was considered, the temperature (70 °C) became constant (Figs. 6 and 7). 3.3.1. Part A: influence of hot air temperature on the drying process We observed a sharp decrease in the moisture content of the sinter mixture during heat-up using different hot air temperatures but with a constant flow rate (Fig. 4). The moisture contents of specimens were obtained by measuring the weights of the weighting bottle containing the specimens. Achieving the final moisture content of specimens took approximately 15 min at 90 °C, 20 min at 70 °C, and 40 min at 50 °C under continuous heat-up. Drying time was significantly shortened as hot air temperature increases. Figs. 5 and 7 exhibit three stages: the initial stage of short risingrate; the second but extremely long constant-rate stage, during which a large fraction of desiccation is accomplished; and the third stage during which desiccation rate decreases as the material approaches the absolute dried specimens. The results listed below were obtained. (1) The drying rate was greater, and the corresponding drying time was contained within the first 5 min when the moisture content of the sintering materials ranged from 0.055 to 0.06 kg/kg. This result is different from the outcome of the chamber dryer experiment. In this experiment, the heat and mass transfer between the gas and solid occurred quickly because of the full contact. (2) The effect of heat and mass transfer enforcement under the condition of temperature increase from 50 °C to 70 °C was better than that of the increase from 70 °C to 90 °C. Therefore, when the hot air temperature was lower, increasing air temperature became one of the effective means of enforcing drying. (3) Under the experimental condition, the critical moisture content of the sinter mixture was approximately 0.023 kg/kg–0.03 kg/kg. The moisture content of the rising-rate and falling-rate period ranged approximately 0.065 kg/kg–0.07 kg/kg. 3.3.2. Part B: influence of hot air flow rate on the drying process In Fig. 6, approximately 25 min at 119 m 3/h, 30 min at 90 m 3/h, and 36 min at 63 m 3/h during continuous heat-up were required to achieve the final moisture contents of the specimens. Likewise, drying time shortened as hot air flow rate increased, but the effect was not as significant compared with the increase in hot air temperature. (1) In the range of experiment adjustment, the greater the air flow rate, the higher the drying rate. The effect of drying enforcement
In order to understand the mechanisms of the drying process of the sinter mixture during sintering, experimental studies have been carried out. Results show that over 90% of dehydration can be achieved during heat-up. Initially, the drying rate of sinter mixture increased drastically and subsequently slowed down. During heat-up, the drying of the sinter mixture changed from a short rising-rate stage to a long constant-rate stage. Finally, drying changed to a falling-rate stage. Based on the experimental conditions, the moisture content of the sinter mixture ranged from 0.055 to 0.06 kg/kg, and the critical moisture content ranged from 0.023 kg/kg–0.03 kg/kg. The results also demonstrate that the hot air temperature and flow rate are the main factors that affect the drying rate. Moreover, significant drying process occurs when the heat-up temperature increases considerably along with an increase in flow rate. These results suggest that the appropriate scopes of both hot air temperature and flow rate should be established. Acknowledgments The authors wish to thank Ansteel, China for supplying the sinter mixture. The research was supported by the National High Technology Research and Development Program of China (no. 2009AA05Z215) and the Natural Science Foundation of Liaoning Province (no. 20102069). References [1] J.J. Cai, J.J. Wang, et al., Recovery of residual-heat integrated steelworks, Iron and Steel 6 (2007) 61–67. [2] R.Y. Yin, Development and evaluation on Chinese steel industry, Acta Metallurgica Sinica 38 (6) (2002) 561–567. [3] W.X. Wang, Analysis on energy-saving potential of iron and steel industry, Energy For Metallurgical Industry 21 (3) (2002) 5–23. [4] The European Commission, Best available techniques reference document on the production of iron and steel. 2001. [5] W.E. Brunbauer, E. Zwittag, et al., New process of sinter off gas in Voestalpine Stahl sinter plant, Metallurgical Plant and Technology International 26 (4) (2007) 21–27. [6] M. Nobuhiro, M. Toshio, P. Hadi, A. Tomohiro, Feasibility study for recovering waste heat in the steelmaking industry using a chemical recuperator, ISIJ International 44 (2) (2004) 257–262. [7] X.C. Yang, J.J. Li, D.Q. Guo, The existing conditions of sintering heat overseas, Sintering and Pelletizing 21 (5) (1996) 39–40. [8] H.S. Lou, The current situations of energy consumption and development trend and countermeasure of domestic and international steel and iron industry, Energy for Metallurgical Industry 26 (2) (2007) 7–11. [9] T.S. Zhang, Hot air sintering process in number 1 sintering machine of Anshan Iron and Steel Group Company, Sintering and Pelletizing 21 (4) (1994) 41–52. [10] T. Hatakeyama, FX Quinn, Thermal analysis fundamentals and applications to polymer science, Wiley Publisher, 1999. [11] T. Hatakeyama, H. Hatakeyama, Thermal Properties of Green Polymers and Biocomposites, Kluwer Academic Publisher, Dordecht, 2004. [12] Z.H. Liu, G.H. Xu, H.L. Zhang, Thermal Analysis Instruments, Beijing Chemical Industry Press, 2006, pp. 177–220.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental investigation on the drying process of the sinter mixture Hui Dong a, FengRui Jia b,⁎, Yong Zhao a, Meng Wang a, JiuJu Cai a a b
SEPA Key Laboratory on Eco-industry, Northeastern University, Shenyang 110004, China College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 18 September 2011 Accepted 31 October 2011 Available online 16 December 2011 Keywords: Sintering Sinter mixture Drying process Waste heat Drying rate Moisture content
a b s t r a c t Achieving efficient recovery and utilization of waste heat during sintering is crucial for iron and steel plants. Previous studies on waste heat utilization have shown that steam and electricity are generated, after which the air and sinter mixture is preheated. However, few studies on the mechanisms of the drying process of the sinter mixture exist. This paper presents the results of an experimental investigation on the drying process that occurs during sintering. The results show that the drying process of the sinter mixture consists of three stages, namely, a short rising-rate stage, a long constant-rate stage, and a falling-rate stage. The moisture contents of specimens are 0.055–0.06 kg/kg in the first 5–7 min, and the critical moisture contents are approximately 0.023–0.03 kg/kg. The drying efficiency is mainly attributed to the temperature and the flow rate of the hot air, and the former one is more significant. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Achieving efficient recovery and utilization of waste heat during sintering–cooling process is crucial for iron and steel plants [1–3]. The energy consumption of the sintering process accounts for 15% of the total energy consumption, and the average energy consumption per ton of sinter in China is higher than those of developed countries (20%) [4, 5]. Low utilization and recovery rate of waste heat is one of the main reasons for high energy consumption during sintering. Therefore, the efficient recovery of waste heat resources is important to further reduce the energy consumption of the sintering process. These waste heat resources are mainly sensible heat, which is taken along by exhaust gas and products; waste heat accounts for 15%–20% of the heat input in the process, and sensible heat accounts for 40%–45% [6]. According to the second law of thermodynamics, if the temperature of the waste heat is relatively low (generally below 230 °C) and not high enough to be utilized in the waste heat boiler, it should be used to dry and preheat the sinter mixture which is applied to reduce moisture content in the material before ignition to prevent overmoisturising of the bottom layer. In this way, the low quality waste heat can be recycled efficiently, and energy consumption in the sintering process is reduced at the same time. In China, the primary attempt has been carried out in Anshan Iron and Steel Group Company (Ansteel) and a small number of enterprises [7]. As regards the sinter mixture, few studies on the mechanisms of drying process exist [8, 9], the drying process during sintering is an interdisciplinary subject,
⁎ Corresponding author: Tel./fax: + 86 2483686994. E-mail address: [email protected] (F.R. Jia). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.10.063
which involves the disciplines of metallurgy, chemical engineering and thermal energy engineering. The drying process is aim to remove the moisture from the sinter mixture as far as possible by heating, in this case, the dried sinter mixture could be beneficial to sintering. In the present study, a series of experiments (level 10 mg, level 10 g, and level 100 g) is carried out with thermogravimetric analysis, chamber dryer, and tunnel drier. Furthermore, several key parameters that influence the drying process, such as, critical moisture content, flow rate, and temperature of the hot air, are also investigated. In addition, critical moisture content stands for the start of fallingrate drying period. Promising results have been obtained. These can provide fundamental data for efficient recovery and utilization of waste heat in the sintering process in China. 2. Experiment platform and procedure Three different parts of platform and equipment were used in this study, namely, thermogravimetric analysis experiment, chamber dryer experiment, and tunnel drier experiment, with the weights of 10 mg, 10 g and 100 g, respectively. The thermogravimetric analysis and chamber furnace are used to retrieve basic knowledge on drying kinetics of sinter mixture, furthermore, the tunnel dryer was used to study effect of gas flow and temperature. The thermogravimetric analyzer STA409 was obtained from NETZSCH-Ger Inc., and the chamber dryer and the tunnel dryer were obtained from Changzhou Wanji and Suzhou Yihui respectively. Sinter mixture specimens were applied in Ansteel, and density of the specimens were measured using the Archimedes method. A result of 1865.30 kg/m3 was achieved, the composition of sinter mixture was shown in Table 1, and the initial moisture content in the mixture is 0.075 kg/kg.
2
H. Dong et al. / Powder Technology 218 (2012) 1–4
Table 1 Sinter mixture composition. TFe/%
FeO/%
CaO/%
MgO/%
SiO2/%
C/%
R = CaO/SiO2
57.4
8–10
9.34
2.2
4.67
0.1–0.2
2
The thermogravimetric analysis experiment was performed on STA409 using a heating rate was 10 °C/min. Specimens were dried at temperatures ranging from 30 to 300 °C for a holding time of 40 min, in addition, due to the limitation of the capacity in the STA409, sinter mixtures in smaller size were picked.. These were used to analyze moisture content and drying rate during heat-up [10–12]. For the chamber dryer experiment, specimens were dried at temperatures of 60, 80, 100, 150, 200, and 250 °C with the same holding time of 40 min for all experiments. The specimens, which were weighed out using the same quality, were divided into 40 weighing bottles. Subsequently, 2 specimens were taken out every 2 min and were immediately weighed again. The specimens were used to calculate the moisture content, the advantage of using specimens from “interrupted” drying runs is that the weight of these specimens can be measured and used to study moisture content during heat-up. The effects of cooling on the other bottles inside the dryer are considered insignificant. Meanwhile, hot air temperature had significant impact on moisture content of the sinter mixture. In addition, every working condition was repeated at least twice in order to prevent segregation of the experimental results, and then, all the specimens in the bottles were kept in the chamber dryer for over 24 h, so the temperature in bottles was equilibrated with chamber temperature, furthermore, all the bottles were put away from the furnace door, in order to avoid the effect of opening of furnace's door. The pressure ratings used in the tunnel drier experiments were 0.32, 0.41, and 0.54 MPa, corresponding to flow rates of 63, 90 and 119 m 3/h, respectively. Specimens were dried at temperatures ranging from 50 to 90 °C for a holding time of 40 min. These were placed in an oblong-shaped gauze and dried by the hot air from one end of the drying chamber. The schematic of the tunnel drier is shown in Fig. 1, the captions details as follows: 1, centrifugal fan; 2, orifice-plate flow-meter; 3, 15-temperature demonstrator; 4, 17-weight sensor demonstrator; 5, drying materials; 6, electric heater; 7, dry-bulb; 8, 14-wetbulb; 9, box-dryer; 10, waste valve;11, waste cyclic valve; 12, fresh air
Fig. 2. Thermal analysis curves (TG, DTG, DSC).
inlet valve; 13, electric heating control instrument; 16, differential pressure transmitter and demonstrator of orifice-plate flow-meter. 3. Results and discussions 3.1. Thermogravimetric analysis experiment In the thermogravimetric analysis experiment, the thermal analysis curves were shown in Fig. 2.As functions of drying time defined as TG, DTG and DSC, these represent the interplay between moisture content and drying time, drying rate and drying time and heat discharge and drying time, respectively. Fig. 2 allows the study of change as a function of drying time during the heat-up from 30 °C–300 °C. (1) Based on the TG curve, the free moisture content was approximately 0.064 kg/kg, and the initial moisture content average of the sinter mixture was 0.08 kg/kg. Thus, the equilibrium moisture content in sinter mixture was approximately 0.016 kg/kg. The weight loss of the sinter mixture was more obvious in the first 5 min. The mass fraction of the sinter mixture was 94.2% when the time reached the fifth minute, indicating the removal of approximately 90% of the free moisture content during the first 5 min. (2) At point A (peak of the DSC curve) or at point B (trough of the DTG curve), the water content of the material approximated the critical moisture content, i.e., the critical moisture content of the sinter
Fig. 1. The schematic of tunnel drier.
H. Dong et al. / Powder Technology 218 (2012) 1–4
3
Fig. 3. Drying curve at different holding temperatures.
mixture was approximately 0.048 kg/kg–0.053 kg/kg (average of 0.05 kg/kg). 3.2. Chamber dryer experiment The mechanisms of the drying process were theorized based on the specimens of 10 mg during heat-up. However, the experiment was unable to completely reflect the actual conditions. In the chamber dryer experiment, by plotting the moisture content as a function of drying time from heating at different holding temperatures, we found that the moisture content of the sinter mixture decreased drastically as heating increased (Fig. 3). (1) Under the experimental conditions, the drying rate was slower when the hot air temperature was below 100 °C, specifically below 80 °C. Taking the first 10 min of the drying process for example, moisture contents of the sinter mixtures were recorded at 0.0758 kg/kg, 0.0744 kg/kg, and 0.067 kg/kg when the corresponding temperatures were 60, 80 and 100 °C, respectively. The corresponding decreases were 0.0012 kg/kg, 0.0026 kg/kg, and 0.01 kg/kg, respectively. (2) The drying rate changed with time within a narrow range when the hot air temperature was below 100 °C. When the hot air temperature reached 150 °C in the first 4 min, the drying rate became slower, changed with time within a narrow range, and varied within the wide range. When the drying
Fig. 4. Drying curve (Constant flow rate and different temperatures).
Fig. 5. Drying curve characteristics (Constant flow rate and different temperatures).
time was more than 20 min, the rate change with time slowed down. Therefore, the drying rate was larger in the interval from the fourth to the twentieth minute when the hot air temperature was higher than 150 °C. (3) Comparing the drying curves of various temperature conditions helped arrive at 150 °C as the established demarcation point. The hot air temperature had a greater impact on the drying rate when it was below 150 °C, although it had less impact when it was higher than 150 °C. This situation became more obvious when temperature was higher than 150 °C. Based on (2) and (3), hot air temperature should be at least higher than 150 °C, in order to ensure the fine drying effect under the experimental conditions. 3.3. Tunnel dryer experiment With regards the performance of the tunnel drier, both the hot air flow rate and temperature were regulated. There were three working conditions for both flow rate and temperature: 63, 90, and 119 m 3/h at 50, 70 and 90 °C, respectively. When the temperature effect was considered, the flow rate (60 m 3/h) became constant (Figs. 4 and 5). When the
Fig. 6. Drying curve (Constant temperature and different flow rates).
4
H. Dong et al. / Powder Technology 218 (2012) 1–4
under the condition of improving air flow from 60 m3/h to 90 m3/h was better than the air flow from 90 m3/h to 119 m3/h. Thus, an appropriate range should be established to enforce the dying effect. (2) The effect of the hot air flow on the drying process occurred mainly at a constant rate period. The effect was weaker at the rising-rate period and almost negligible at the falling-rate period. (3) The drying rate was greater, and the corresponding drying time was contained within the first 5–7 min when the moisture content of the sintering materials ranged from 0.055 to 0.06 kg water/kg material based on the experimental conditions. (4) The critical moisture content of the sinter mixture was approximately 0.023 kg/kg–0.03 kg/kg, and the moisture content of the rising and falling rate period was approximately 7.0% under the experimental condition.
4. Conclusions Fig. 7. Drying curve characteristics (Constant temperature and different flow rates).
flow rate effect was considered, the temperature (70 °C) became constant (Figs. 6 and 7). 3.3.1. Part A: influence of hot air temperature on the drying process We observed a sharp decrease in the moisture content of the sinter mixture during heat-up using different hot air temperatures but with a constant flow rate (Fig. 4). The moisture contents of specimens were obtained by measuring the weights of the weighting bottle containing the specimens. Achieving the final moisture content of specimens took approximately 15 min at 90 °C, 20 min at 70 °C, and 40 min at 50 °C under continuous heat-up. Drying time was significantly shortened as hot air temperature increases. Figs. 5 and 7 exhibit three stages: the initial stage of short risingrate; the second but extremely long constant-rate stage, during which a large fraction of desiccation is accomplished; and the third stage during which desiccation rate decreases as the material approaches the absolute dried specimens. The results listed below were obtained. (1) The drying rate was greater, and the corresponding drying time was contained within the first 5 min when the moisture content of the sintering materials ranged from 0.055 to 0.06 kg/kg. This result is different from the outcome of the chamber dryer experiment. In this experiment, the heat and mass transfer between the gas and solid occurred quickly because of the full contact. (2) The effect of heat and mass transfer enforcement under the condition of temperature increase from 50 °C to 70 °C was better than that of the increase from 70 °C to 90 °C. Therefore, when the hot air temperature was lower, increasing air temperature became one of the effective means of enforcing drying. (3) Under the experimental condition, the critical moisture content of the sinter mixture was approximately 0.023 kg/kg–0.03 kg/kg. The moisture content of the rising-rate and falling-rate period ranged approximately 0.065 kg/kg–0.07 kg/kg. 3.3.2. Part B: influence of hot air flow rate on the drying process In Fig. 6, approximately 25 min at 119 m 3/h, 30 min at 90 m 3/h, and 36 min at 63 m 3/h during continuous heat-up were required to achieve the final moisture contents of the specimens. Likewise, drying time shortened as hot air flow rate increased, but the effect was not as significant compared with the increase in hot air temperature. (1) In the range of experiment adjustment, the greater the air flow rate, the higher the drying rate. The effect of drying enforcement
In order to understand the mechanisms of the drying process of the sinter mixture during sintering, experimental studies have been carried out. Results show that over 90% of dehydration can be achieved during heat-up. Initially, the drying rate of sinter mixture increased drastically and subsequently slowed down. During heat-up, the drying of the sinter mixture changed from a short rising-rate stage to a long constant-rate stage. Finally, drying changed to a falling-rate stage. Based on the experimental conditions, the moisture content of the sinter mixture ranged from 0.055 to 0.06 kg/kg, and the critical moisture content ranged from 0.023 kg/kg–0.03 kg/kg. The results also demonstrate that the hot air temperature and flow rate are the main factors that affect the drying rate. Moreover, significant drying process occurs when the heat-up temperature increases considerably along with an increase in flow rate. These results suggest that the appropriate scopes of both hot air temperature and flow rate should be established. Acknowledgments The authors wish to thank Ansteel, China for supplying the sinter mixture. The research was supported by the National High Technology Research and Development Program of China (no. 2009AA05Z215) and the Natural Science Foundation of Liaoning Province (no. 20102069). References [1] J.J. Cai, J.J. Wang, et al., Recovery of residual-heat integrated steelworks, Iron and Steel 6 (2007) 61–67. [2] R.Y. Yin, Development and evaluation on Chinese steel industry, Acta Metallurgica Sinica 38 (6) (2002) 561–567. [3] W.X. Wang, Analysis on energy-saving potential of iron and steel industry, Energy For Metallurgical Industry 21 (3) (2002) 5–23. [4] The European Commission, Best available techniques reference document on the production of iron and steel. 2001. [5] W.E. Brunbauer, E. Zwittag, et al., New process of sinter off gas in Voestalpine Stahl sinter plant, Metallurgical Plant and Technology International 26 (4) (2007) 21–27. [6] M. Nobuhiro, M. Toshio, P. Hadi, A. Tomohiro, Feasibility study for recovering waste heat in the steelmaking industry using a chemical recuperator, ISIJ International 44 (2) (2004) 257–262. [7] X.C. Yang, J.J. Li, D.Q. Guo, The existing conditions of sintering heat overseas, Sintering and Pelletizing 21 (5) (1996) 39–40. [8] H.S. Lou, The current situations of energy consumption and development trend and countermeasure of domestic and international steel and iron industry, Energy for Metallurgical Industry 26 (2) (2007) 7–11. [9] T.S. Zhang, Hot air sintering process in number 1 sintering machine of Anshan Iron and Steel Group Company, Sintering and Pelletizing 21 (4) (1994) 41–52. [10] T. Hatakeyama, FX Quinn, Thermal analysis fundamentals and applications to polymer science, Wiley Publisher, 1999. [11] T. Hatakeyama, H. Hatakeyama, Thermal Properties of Green Polymers and Biocomposites, Kluwer Academic Publisher, Dordecht, 2004. [12] Z.H. Liu, G.H. Xu, H.L. Zhang, Thermal Analysis Instruments, Beijing Chemical Industry Press, 2006, pp. 177–220.