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Liquid-phase catalytic oxidation of smelting-gases containing SO2 in low concentration
Journal of Cleaner Production 6 (1998) 323–327
Liquid-phase catalytic oxidation of smelting-gases containing SO2 in low concentration Sun Pei-Shi *, Ning Ping, Song Wen-Biao Department of Environmental and Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan Province, PR China
Abstract In order to approach an applicable method of purifying smelting-gases containing SO2 in low concentration, the catalytic oxidation of SO2 in low concentration in smelting-gas, using Mn+2, Fe+2 and Zn+2 in liquid-phase, was performed separately in a foam absorbing column in a copper smelter. The absorption solution, containing metal ions in an optimal proportion according to an orthogonal test, demonstrates an improved purification capability for smelting-gas. When the concentration of H2SO4 in the solution rises to 20%, the removal efficiency of SO2 in smelting-gas can be still maintained above 85%. The method is demonstrably effective for the waste gas treatment at small and mid-scale smelters of non-ferrous metals. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Liquid-phase catalytic oxidation; Smelting-gases containing SO2 in low concentration; Purification of waste gases
1. Introduction Sulfur dioxide (SO2) is one of the air pollutants affecting the people all over the world. Recently, the discharge amount of SO2 in China has increased quickly. In 1994, it reached 18.25 million tons [1]. Because of a large quantity of SO2 exhaled into the atmosphere, the air quality has steadily deteriorated in China. In the nonferrous metals industry in China, due to the production techniques and the raw material, a great amount of smelting-gases containing SO2 is produced in the process of smelting operations. Although some of the smelting-gases containing more than 3% SO2 have been used in producing sulfuric acid, most of the smelting-gases containing SO2 in low concentrations have almost all been discharged directly into the atmosphere, because of large gas flows, low concentrations of SO2, the complex composition of the gases and the difficulty of using purifying treatment. At present, the amount of SO2 exhausted by the nonferrous metals industry in China has reached 600 thousand tons each year [2]. These SO2 discharges seriously pollute the atmosphere * Corresponding author. Tel./Fax: ⫹ 86 871 5318294.
0959-6526/98/$19.00 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 9 - 6 5 2 6 ( 9 8 ) 0 0 0 2 0 - 1
and the surroundings near some plants, and have caused such losses, for example, as decreasing the yield of crops, some crop death, and sometimes loss of vegetation cover in the leeward area near some plants. In recent years, a lot of work has been done in China on treatment of the smelting-gases containing SO2 in low concentrations, but some technical and economic problems remain to be solved. Generally speaking, the work on purifying the smelting-gases containing SO2 in low concentrations is still in the stage of research for suitable technologies for the situation of China [3]. Therefore, it is necessary to carry on researching for a new method that has both high technical reliability and economic benefits for purifying the smelting-gases containing SO2 in low concentrations in China. Based on the principle that SO2 can be catalytically oxidized to H2SO4 in solution by some metal ions, such as Mn+2, Fe+2, Zn+2, Cu+2, Se+3 and so on [4], and the experience of pilot tests in the lab [5], the removal of SO2 from the smelting-gases in a foam absorbing column in a copper smelter, by using an absorption solution containing Mn+2, Fe+2, Zn+2, has been studied. By using this method, the SO2 contained in the tail gas of a sulfuric acid production system in the copper smelter was removed and the solution containing H2SO4 was obtained.
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S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Table 1 Conditions of smelting-gas in the experiment Component
SO2
SO3
O2
N2
Temperature
Proportion (%)
0.1–0.2
0.018
7.8–17
86–91
22–30(°C)
2. Experimental setup
2SO2 ⫹ O2 ⫹ 2H2OMetal ions2H2SO4
The experiments were performed in the sulfuric acid production workshop in a copper smelter, located in Yunnan Province. The absorption solutions were made separately by Mn+2, Fe+2 and Zn+2 sulfate with water. The conditions of smelting-gas used in the experiment are shown in Table 1 and the experimental system is depicted schematically in Fig. 1. The absorbing column is made of a glass pipe with a diameter of 100 mm. There are three plates in the column. The diameter of the hole in the plate is 2 mm. The holes are arranged triangularly at intervals of 5 mm. In the experimental operation, the gas/liquid ratio is 1000:1, the foam height above each plate is 150–200 mm and the pressure drop is 80–136 mm H2O. The purpose of this experimental research is to investigate the capability of the absorption solution containing metal ions to purify SO2 in smelting-gas. So, the absorbing column was operated with a single plate in the experiments. The concentration of SO2 in gas and H2SO4 in liquid were analyzed by routine chemical methods.
In the process of catalytic oxidizing SO2 in the absorption solution, the Mn+2, Fe+2 and Zn+2 change their own valence in playing their catalytic function. In general, when the metal ions are oxidized from low valence to high valence, the O2 dissolved in the solution is used to oxidize SO2. And then, when the metal ions reduce from high valence to low valence, they oxidize the SO2 dissolved in the solution by themselves. The liquid-phase catalytic oxidation reaction in the process can be described as follows [6]: (Me stands for Mn, Fe and Zn)
3. Capability of mono-metal ion purifying SO2 in smelting-gas 3.1. Chemical process of liquid-phase catalytic oxidation of SO2 The general reaction in which SO2 is oxidized catalytically by Mn+2, Fe+2 and Zn+2 into H2SO4 in liquid phase can be described as follows:
Fig. 1. Experimental setup. 1Liquid container; 2Foam absorbing; 3 Plate with sieve meshes; 4High level liquid tank; 5Gas flow meter; 6 Gas muffer tank; 7Gas blower; 8Pipeline of smelting-gas; GSampling point for gas; LSampling point for liquid.
2Me+2 ⫹ SO2 ⫹ O2 ⫽ 2Me+3 ⫹ SO−2 4 + 2Me+3 ⫹ SO2 ⫹ 2H2O ⫽ 2Me+2 ⫹ SO−2 4 ⫹ 4H
Because the metal ions are oxidized and reduced automatically, the reaction process of liquid-phase catalytic oxidation of SO2 in the absorption solution can be carried out continuously. Based on the principle of chemical balance, while the process goes on, the H2SO4, as product, will hinder the catalytic oxidation of SO2 in the absorption solution. On the other hand, increasing the concentration of H2SO4 will improve the reused value of the absorption solution. Therefore, the effects of the concentration of metal ions and H2SO4 in the absorption solution on the purification of smelting-gas containing SO2 were investigated emphatically in this experimental research. 3.2. Purification capability of the solution with Mn+2 Fig. 2 indicates that the increase of the concentration of Mn+2 in the solution leads to the decrease of purification efficiency of SO2 at a different rate. When the concentration of Mn+2 is bigger than 0.36%(wt.), the purification efficiency of SO2 is almost of no change. The effect of the concentration of H2SO4 in the absorption solution on the capability of the solution with Mn+2 purifying SO2 is shown in Fig. 3. Using an absorption solution containing 0.18% Mn+2 to remove SO2 can produce an effective result. When the concentration of H2SO4 in the solution rises to 12%(wt.), the purification efficiency of SO2 can be kept at 72%, for a single plate in the absorbing column, and tends to rise with the increase of the concentration of H2SO4 in the solution. So, it is suitable to use the absorption solution containing 0.18% Mn+2 to remove SO2 from smelting-gases.
S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Fig. 2.
Purification efficiency versus conc. of Mn+2 in the solution.
Fig. 3. Purification efficiency versus conc. of H2SO4 in the solution with Mn+2.
325
Fig. 4. Purification efficiency versus conc. of Fe+2 in the solution.
Fig. 5. Purification efficiency versus conc. of H2SO4 in the solution with Fe+2.
3.3. Purification capability of the solution with Fe+2 The effects of the concentration of Fe+2 and H2SO4 in the absorption solution on the capability of the solution with Fe+2 purifying SO2 are shown in Fig. 4 and Fig. 5. With the increase of the concentration of Fe+2 in the solution, the purification efficiency of SO2 drops down (Fig. 4) and with the rise of the concentration of H2SO4 in the solution, the capability of the solution with 0.37% Fe+2 purifying SO2 is obviously higher than that of the solution with 1.11% Fe+2 (Fig. 5). Therefore, when the absorption solution containing Fe+2 is used to remove SO2 from smelting-gases, the suitable concentration of Fe+2 in the solution is 0.37%(wt.). 3.4. Purification capability of the solution with Zn+2 Fig. 6 shows that when the concentration of Zn+2 in the solution is close to 0.2%(wt.), a higher purification efficiency of SO2 can be obtained. Fig. 7 indicates that
Fig. 6.
Purification efficiency versus conc. of Zn+2 in the solution.
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S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Fig. 7. Purification efficiency versus conc. of H2SO4 in the solution with Zn+2.
although the capability of the absorption solution containing 0.2% Zn+2 to purify SO2 drops down slowly with the increase of the concentration of H2SO4 in the solution, its purification efficiency of SO2 is obviously higher than that of the solution containing 0.4% Zn+2 in the range of 2–12%(wt.) H2SO4 in the solution. For this reason, when the absorption solution containing Zn+2 is used to remove SO2 from smelting-gases, a matter worthy of note is that the concentration of Zn+2 in the solution is no bigger than 0.2%.
4. Capability of the solution with mixed metal ions purifying SO2 Generally speaking, it is difficult to get a satisfactory result to remove SO2 from smelting-gases by absorption solutions containing mono-metal ions. Because of this, an orthogonal test was performed to find out the optimal proportion of the metal ions used, when the absorption solution containing mixed metal ions was used in purifying the smelting-gases. The results of the orthogonal test are shown as follows: 1. the order of the capability of catalytic oxidizing SO2 for the metal ions used is: Mn+2 > Fe+2 > Zn+2. 2. the optimal proportion of the metal ions in the absorption solution is: Mn+2:Fe+2:Zn+2 ⫽ 1:1:2. When the absorption solution containing mixed metal ions in the optimal proportion was used to remove SO2 from the smelting-gas, an improved purification result was achieved as shown in Fig. 8. When the concentration of H2SO4 in the solution rises to 20%, the purification efficiency of SO2, for a single plate in the absorbing column, can still be maintained above 85%. Higher concentrations of H2SO4 contained in the solution will be favorably purified by reuse of the used absorption solution.
Fig. 8. Purification efficiency versus conc. of H2SO4 in the solution with mixed metal ions.
5. Utilization of the used absorption solution After the absorption solution is used in purifying the smelting-gases containing SO2 in low concentration, it generally contains 10–20%(wt.) H2SO4. The effluent solution can be put into the production system of sulfuric acid, using the smelting-gas containing SO2, as the supplementary water in some mid-scale smelters. Mixed with sulfuric acid, the effluent solution can be used as the steeping solution in the section of hydrometallurgical production in some small and mid-scale smelters of non-ferrous metals.
6. Conclusion Using the method of liquid-phase catalytic oxidizing SO2 to purify the smelting-gases containing SO2 in low concentration is technologically feasible and is quite appropriate for the treatment of waste gas containing SO2 at small and mid-scale smelters of non-ferrous metals. By recovering and reutilizing the SO2, this method will produce obvious environmental and economic benefits.
Acknowledgements The author would like to express his thanks to Professor Deng Yanjuan, at the Department of Foreign Language in Kunming University of Science and Technology, for her valuable help in preparing and writing this paper.
S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
References [1] EPA of China. 1994 Environmental Situation of China, Environmental Newspaper of China, June 3, 1995. [2] EPA of China. Waste Gas Treatment of Nonferrous Metals Industry, Publishing House of Environmental Sciences of China, Beijing, China, 1993:1–16. [3] Dehong Y. Treatment and utilization of smelting-gases containing SO2. Journal of Nonferrous Metals (Smelting Part) 1995;1:42–4.
327
[4] Nanjing Chemical Industrial Company. Desulfurization of Waste Gases Containing SO2 in Low Concentration, Publishing House of Science and Technology in Shanghai, Shanghai, China, 1981:237–45. [5] Pei-Shi S, Wen-Biao S. Study on purifying smelting-gases containing SO2 in low concentration by liquid-phase catalytic oxidation method. Journal of Environmental Protection of Nonferrous Metals Industry 1988;1:6–11. [6] Hong, Z. Air Pollution Control in Chemical Plants, Publishing House of Chemical Industry, Beijing, China, 1987:147–59.
Liquid-phase catalytic oxidation of smelting-gases containing SO2 in low concentration Sun Pei-Shi *, Ning Ping, Song Wen-Biao Department of Environmental and Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan Province, PR China
Abstract In order to approach an applicable method of purifying smelting-gases containing SO2 in low concentration, the catalytic oxidation of SO2 in low concentration in smelting-gas, using Mn+2, Fe+2 and Zn+2 in liquid-phase, was performed separately in a foam absorbing column in a copper smelter. The absorption solution, containing metal ions in an optimal proportion according to an orthogonal test, demonstrates an improved purification capability for smelting-gas. When the concentration of H2SO4 in the solution rises to 20%, the removal efficiency of SO2 in smelting-gas can be still maintained above 85%. The method is demonstrably effective for the waste gas treatment at small and mid-scale smelters of non-ferrous metals. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Liquid-phase catalytic oxidation; Smelting-gases containing SO2 in low concentration; Purification of waste gases
1. Introduction Sulfur dioxide (SO2) is one of the air pollutants affecting the people all over the world. Recently, the discharge amount of SO2 in China has increased quickly. In 1994, it reached 18.25 million tons [1]. Because of a large quantity of SO2 exhaled into the atmosphere, the air quality has steadily deteriorated in China. In the nonferrous metals industry in China, due to the production techniques and the raw material, a great amount of smelting-gases containing SO2 is produced in the process of smelting operations. Although some of the smelting-gases containing more than 3% SO2 have been used in producing sulfuric acid, most of the smelting-gases containing SO2 in low concentrations have almost all been discharged directly into the atmosphere, because of large gas flows, low concentrations of SO2, the complex composition of the gases and the difficulty of using purifying treatment. At present, the amount of SO2 exhausted by the nonferrous metals industry in China has reached 600 thousand tons each year [2]. These SO2 discharges seriously pollute the atmosphere * Corresponding author. Tel./Fax: ⫹ 86 871 5318294.
0959-6526/98/$19.00 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 9 - 6 5 2 6 ( 9 8 ) 0 0 0 2 0 - 1
and the surroundings near some plants, and have caused such losses, for example, as decreasing the yield of crops, some crop death, and sometimes loss of vegetation cover in the leeward area near some plants. In recent years, a lot of work has been done in China on treatment of the smelting-gases containing SO2 in low concentrations, but some technical and economic problems remain to be solved. Generally speaking, the work on purifying the smelting-gases containing SO2 in low concentrations is still in the stage of research for suitable technologies for the situation of China [3]. Therefore, it is necessary to carry on researching for a new method that has both high technical reliability and economic benefits for purifying the smelting-gases containing SO2 in low concentrations in China. Based on the principle that SO2 can be catalytically oxidized to H2SO4 in solution by some metal ions, such as Mn+2, Fe+2, Zn+2, Cu+2, Se+3 and so on [4], and the experience of pilot tests in the lab [5], the removal of SO2 from the smelting-gases in a foam absorbing column in a copper smelter, by using an absorption solution containing Mn+2, Fe+2, Zn+2, has been studied. By using this method, the SO2 contained in the tail gas of a sulfuric acid production system in the copper smelter was removed and the solution containing H2SO4 was obtained.
324
S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Table 1 Conditions of smelting-gas in the experiment Component
SO2
SO3
O2
N2
Temperature
Proportion (%)
0.1–0.2
0.018
7.8–17
86–91
22–30(°C)
2. Experimental setup
2SO2 ⫹ O2 ⫹ 2H2OMetal ions2H2SO4
The experiments were performed in the sulfuric acid production workshop in a copper smelter, located in Yunnan Province. The absorption solutions were made separately by Mn+2, Fe+2 and Zn+2 sulfate with water. The conditions of smelting-gas used in the experiment are shown in Table 1 and the experimental system is depicted schematically in Fig. 1. The absorbing column is made of a glass pipe with a diameter of 100 mm. There are three plates in the column. The diameter of the hole in the plate is 2 mm. The holes are arranged triangularly at intervals of 5 mm. In the experimental operation, the gas/liquid ratio is 1000:1, the foam height above each plate is 150–200 mm and the pressure drop is 80–136 mm H2O. The purpose of this experimental research is to investigate the capability of the absorption solution containing metal ions to purify SO2 in smelting-gas. So, the absorbing column was operated with a single plate in the experiments. The concentration of SO2 in gas and H2SO4 in liquid were analyzed by routine chemical methods.
In the process of catalytic oxidizing SO2 in the absorption solution, the Mn+2, Fe+2 and Zn+2 change their own valence in playing their catalytic function. In general, when the metal ions are oxidized from low valence to high valence, the O2 dissolved in the solution is used to oxidize SO2. And then, when the metal ions reduce from high valence to low valence, they oxidize the SO2 dissolved in the solution by themselves. The liquid-phase catalytic oxidation reaction in the process can be described as follows [6]: (Me stands for Mn, Fe and Zn)
3. Capability of mono-metal ion purifying SO2 in smelting-gas 3.1. Chemical process of liquid-phase catalytic oxidation of SO2 The general reaction in which SO2 is oxidized catalytically by Mn+2, Fe+2 and Zn+2 into H2SO4 in liquid phase can be described as follows:
Fig. 1. Experimental setup. 1Liquid container; 2Foam absorbing; 3 Plate with sieve meshes; 4High level liquid tank; 5Gas flow meter; 6 Gas muffer tank; 7Gas blower; 8Pipeline of smelting-gas; GSampling point for gas; LSampling point for liquid.
2Me+2 ⫹ SO2 ⫹ O2 ⫽ 2Me+3 ⫹ SO−2 4 + 2Me+3 ⫹ SO2 ⫹ 2H2O ⫽ 2Me+2 ⫹ SO−2 4 ⫹ 4H
Because the metal ions are oxidized and reduced automatically, the reaction process of liquid-phase catalytic oxidation of SO2 in the absorption solution can be carried out continuously. Based on the principle of chemical balance, while the process goes on, the H2SO4, as product, will hinder the catalytic oxidation of SO2 in the absorption solution. On the other hand, increasing the concentration of H2SO4 will improve the reused value of the absorption solution. Therefore, the effects of the concentration of metal ions and H2SO4 in the absorption solution on the purification of smelting-gas containing SO2 were investigated emphatically in this experimental research. 3.2. Purification capability of the solution with Mn+2 Fig. 2 indicates that the increase of the concentration of Mn+2 in the solution leads to the decrease of purification efficiency of SO2 at a different rate. When the concentration of Mn+2 is bigger than 0.36%(wt.), the purification efficiency of SO2 is almost of no change. The effect of the concentration of H2SO4 in the absorption solution on the capability of the solution with Mn+2 purifying SO2 is shown in Fig. 3. Using an absorption solution containing 0.18% Mn+2 to remove SO2 can produce an effective result. When the concentration of H2SO4 in the solution rises to 12%(wt.), the purification efficiency of SO2 can be kept at 72%, for a single plate in the absorbing column, and tends to rise with the increase of the concentration of H2SO4 in the solution. So, it is suitable to use the absorption solution containing 0.18% Mn+2 to remove SO2 from smelting-gases.
S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Fig. 2.
Purification efficiency versus conc. of Mn+2 in the solution.
Fig. 3. Purification efficiency versus conc. of H2SO4 in the solution with Mn+2.
325
Fig. 4. Purification efficiency versus conc. of Fe+2 in the solution.
Fig. 5. Purification efficiency versus conc. of H2SO4 in the solution with Fe+2.
3.3. Purification capability of the solution with Fe+2 The effects of the concentration of Fe+2 and H2SO4 in the absorption solution on the capability of the solution with Fe+2 purifying SO2 are shown in Fig. 4 and Fig. 5. With the increase of the concentration of Fe+2 in the solution, the purification efficiency of SO2 drops down (Fig. 4) and with the rise of the concentration of H2SO4 in the solution, the capability of the solution with 0.37% Fe+2 purifying SO2 is obviously higher than that of the solution with 1.11% Fe+2 (Fig. 5). Therefore, when the absorption solution containing Fe+2 is used to remove SO2 from smelting-gases, the suitable concentration of Fe+2 in the solution is 0.37%(wt.). 3.4. Purification capability of the solution with Zn+2 Fig. 6 shows that when the concentration of Zn+2 in the solution is close to 0.2%(wt.), a higher purification efficiency of SO2 can be obtained. Fig. 7 indicates that
Fig. 6.
Purification efficiency versus conc. of Zn+2 in the solution.
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S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
Fig. 7. Purification efficiency versus conc. of H2SO4 in the solution with Zn+2.
although the capability of the absorption solution containing 0.2% Zn+2 to purify SO2 drops down slowly with the increase of the concentration of H2SO4 in the solution, its purification efficiency of SO2 is obviously higher than that of the solution containing 0.4% Zn+2 in the range of 2–12%(wt.) H2SO4 in the solution. For this reason, when the absorption solution containing Zn+2 is used to remove SO2 from smelting-gases, a matter worthy of note is that the concentration of Zn+2 in the solution is no bigger than 0.2%.
4. Capability of the solution with mixed metal ions purifying SO2 Generally speaking, it is difficult to get a satisfactory result to remove SO2 from smelting-gases by absorption solutions containing mono-metal ions. Because of this, an orthogonal test was performed to find out the optimal proportion of the metal ions used, when the absorption solution containing mixed metal ions was used in purifying the smelting-gases. The results of the orthogonal test are shown as follows: 1. the order of the capability of catalytic oxidizing SO2 for the metal ions used is: Mn+2 > Fe+2 > Zn+2. 2. the optimal proportion of the metal ions in the absorption solution is: Mn+2:Fe+2:Zn+2 ⫽ 1:1:2. When the absorption solution containing mixed metal ions in the optimal proportion was used to remove SO2 from the smelting-gas, an improved purification result was achieved as shown in Fig. 8. When the concentration of H2SO4 in the solution rises to 20%, the purification efficiency of SO2, for a single plate in the absorbing column, can still be maintained above 85%. Higher concentrations of H2SO4 contained in the solution will be favorably purified by reuse of the used absorption solution.
Fig. 8. Purification efficiency versus conc. of H2SO4 in the solution with mixed metal ions.
5. Utilization of the used absorption solution After the absorption solution is used in purifying the smelting-gases containing SO2 in low concentration, it generally contains 10–20%(wt.) H2SO4. The effluent solution can be put into the production system of sulfuric acid, using the smelting-gas containing SO2, as the supplementary water in some mid-scale smelters. Mixed with sulfuric acid, the effluent solution can be used as the steeping solution in the section of hydrometallurgical production in some small and mid-scale smelters of non-ferrous metals.
6. Conclusion Using the method of liquid-phase catalytic oxidizing SO2 to purify the smelting-gases containing SO2 in low concentration is technologically feasible and is quite appropriate for the treatment of waste gas containing SO2 at small and mid-scale smelters of non-ferrous metals. By recovering and reutilizing the SO2, this method will produce obvious environmental and economic benefits.
Acknowledgements The author would like to express his thanks to Professor Deng Yanjuan, at the Department of Foreign Language in Kunming University of Science and Technology, for her valuable help in preparing and writing this paper.
S. Pei-Shi et al. / Journal of Cleaner Production 6 (1998) 323–327
References [1] EPA of China. 1994 Environmental Situation of China, Environmental Newspaper of China, June 3, 1995. [2] EPA of China. Waste Gas Treatment of Nonferrous Metals Industry, Publishing House of Environmental Sciences of China, Beijing, China, 1993:1–16. [3] Dehong Y. Treatment and utilization of smelting-gases containing SO2. Journal of Nonferrous Metals (Smelting Part) 1995;1:42–4.
327
[4] Nanjing Chemical Industrial Company. Desulfurization of Waste Gases Containing SO2 in Low Concentration, Publishing House of Science and Technology in Shanghai, Shanghai, China, 1981:237–45. [5] Pei-Shi S, Wen-Biao S. Study on purifying smelting-gases containing SO2 in low concentration by liquid-phase catalytic oxidation method. Journal of Environmental Protection of Nonferrous Metals Industry 1988;1:6–11. [6] Hong, Z. Air Pollution Control in Chemical Plants, Publishing House of Chemical Industry, Beijing, China, 1987:147–59.