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Three-step vacuum separation for treating arsenic sulphide residue
Vacuum 111 (2015) 170e175
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Three-step vacuum separation for treating arsenic sulphide residue Haijin Hu a, b, Keqiang Qiu a, b, * a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China Key Laboratory of Resources Chemistry of Nonferrous Metals, Central South University, Ministry of Education of The People's Republic of China, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2014 Received in revised form 25 September 2014 Accepted 26 September 2014 Available online 22 October 2014
Up to now, most of arsenic sulphide residue can't be properly disposed. The traditional processes for treating arsenic sulphide residue have some widespread drawbacks such as the complex process, the high cost in operation, and the less than satisfactory removal effect and environmental protection. In view of this, a process of three-step vacuum separation was proposed for treating arsenic sulphide residue in this work. During vacuum separation, elemental sulphide and arsenic trioxide could be recovered effectively in primary distillation at temperature of 180 C, distillation time of 2.0 h, corresponding to the residual gas pressure of 15 Pa. In secondary distillation, arsenic sulfide was obtained under the condition of 450 C for 30 min, with a residual gas pressure of 15 Pa. When the temperature increased to 1000 C for 2 h, lead sulfide was evaporated out in third distillation. Through the three-step vacuum separation, calcium fluoride was left behind in the third distilland, in which the content of arsenic was 5.65 104 wt.%. Correspondingly, the removal rate of arsenic was almost 100%. The vacuum process is expected to be effectively employed for recycling valuable components from arsenic sulphide residue, and meanwhile eliminating its pollution. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Arsenic sulphide residue Three-step vacuum separation Recover
1. Introduction As a common element existing in ores, arsenic is widely distributed in multiple procedures of mineral processing processes, which result in the problem of residual arsenic in the intermediate and final products of the processes, and the trouble in recovery of arsenic. In the case of filling these compounds containing arsenic, a significant contamination problem is likely to appear, due to the acute toxicity of these substances. The landfills will release toxins into the groundwater. It is worth pointing out that the treating of the hazardous materials contained arsenic has serious occupational and environmental implications, especially when the treating process would not like to take the necessary precautions to protect the environment and employees' health [1]. As is known, the arsenic pollution has become a global problem, which is reported frequently in many countries such as America, Japan, India, Mexico and China [2]. In the pollution of environment report by WHO in 1968, arsenic was ranked first. Therefore, much more special attention has been drawn to arsenic problem. There are billions tons of waste acid liquid containing arsenic produced by the factories in where arsenic-bearing minerals act as * Corresponding author. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China. Tel.: þ86 731 88836994; fax: þ86 731 88879616. E-mail address: [email protected] (K. Qiu). http://dx.doi.org/10.1016/j.vacuum.2014.09.018 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
the raw materials all over the world annually. The existing methods for treating waste acid liquid containing arsenic fall into several categories: ion exchange, adsorption (activated alumina and activated carbon); ultrafiltration, reverse osmosis, and precipitation or adsorption by metals (predominately ferric chloride) followed by coagulation [1,3e5]. At present, one of the most widely used methods is vulcanizing agent leaching, in which arsenic from waste acid liquid reacts with the vulcanizing agent, such as sodium sulfide. This method is simple, low in cost and high in separation rate of arsenic (about 99.97%) [6]. However, significant quantities of arsenic sulfide byproduct, are produced through this process, which brings about a series of challenges for the disposal of them. Arsenic sulphide residue, which is a dreadful catastrophe to human health, may result in serious environmental pollution without reasonable disposal. On the other hand, as is well-known, arsenic sulfide has high application value in field of medicine [7e11] and the metallurgical industry [12]. For instance, arsenic sulfide has antitumor effects by the way of inhibiting the growth of K562-cells and SMMC-7721-cells. In addition to this, it is also a necessary raw material in the production of herbicides and wood preservative. Therefore, it is of great interest to turn the hazardous arsenic sulphide residue into good one. What's more, the recovery of valuable components from arsenic sulphide residue is one of way to enlarge utilization of arsenic and sulphide resources. So far, the publicly reported researches about treating arsenic sulfide residue mainly focus on curing, landfill, or recycling in the
H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
form of arsenic trioxide, arsenate, arsenic trichloride and so on [1,13]. One of the most attractive options is a Japanese technology, which is detailed as follows: arsenic sulfide is repulped and heated to 70 C in CuSO4 solution, under which arsenic sulfide reacts with the CuSO4 to form CuS. Meanwhile, the arsenic is dissolved as arsenious acid. Subsequently, As2O3 is precipitated from arsenious acid solution by cooling at 25 C [14]. Obviously, the separation of solideliquid is necessary for each steps, and the emission of arsenic trioxide in the process of crystallization should not be ignored either. As things now stand, the existing methods, including the Japanese technology, are less than satisfactory due to the drawbacks such as complex and time-consuming process flow, low recovery efficiency, high consumption in chemical reagents, high cost in operation. Above all, these processes are inevitably produce toxic gas and large quantities of waste acid liquid, which are potential to form secondary environmental pollution. Enlightened by above thought, a method of vacuum separation, which shows many advantages such as simple technological flow sheet, low consumption of raw material and energy, no secondary off-gas or wastewater and so on [15e17], is proposed for treating arsenic sulfide residue in this work, with the purpose of recovering valuable components without negative impact on the environment. During vacuum separation, a method of three-step distillation was employed. Through three-step distillation, elemental sulphide/ arsenic trioxide, arsenic sulfide, lead sulfide were evaporated into distillate in primary distillation, secondary distillation and third distillation, respectively. Yet the calcium fluoride is left behind as the third distilland. 2. Experimental section 2.1. Experimental materials The arsenic sulphide residue used in the experiments was acquired from lead plant in which arsenic was recycled from waste water through sodium sulfide leaching. A small amount of arsenic trioxide was formed, due to oxidation of the waste residue after pile-up for a period of time. A homogenized sample was characterized by titrimetric analysis. Correspondingly, the contents of As, S, Pb and Ca are shown in Table 1. 2.2. Apparatus The experiments were carried out in the self-made distillation apparatus, which was mainly comprised of quartz tube, quartz crucible, water condenser, resistance furnace, intelligent temperature controller and vacuum pump, as shown in Fig. 1. The quartz tube loaded with a quartz crucible, which is used for holding the experimental sample, was placed in the self-design resistance furnace. The water condenser was inserted into middle of quartz tube and exactly right above the quartz crucible. The furnace temperature was controlled by the temperature controller with a thermocouple connected. The outlet of the quartz tube was connected with a vacuum pump to maintain constant pressure of the whole distillation system.
171
Fig. 1. Distillation apparatus.
was placed at the bottom of the quartz tube and then heated to preset temperature for a given period of time after the whole system was evacuated to a certain pressure. During the experiment, the volatile components were evaporated out of the arsenic sulphide residue into gas phase, and then condensed in the surface of the water condenser as distillate. Meanwhile, the other components were left in the crucible as distilland. After the experiment, the distilland left in the quartz crucible was taken out of the furnace, weighed. The distillate attached to the condenser was also cleaned out, collected and sampled. Subsequently, the residual distilland was evaporated again at a higher temperature. As can been seen from Fig. 2, three different kinds of distillate could be obtained after three-step distillation, which elemental sulphide/arsenic trioxide, arsenic sulfide and lead sulfide, respectively. Correspondingly, the calcium fluoride was left in the crucible as third distilland. The removal percent (R) of arsenic were calculated by the following formula:
R¼
m1 w1 m4 w4 100% m1 w1
Where m1 and m4 are the mass of the arsenic sulphide residue and the third distilland (g), respectively; w1 and w4 are the content of arsenic (wt.%) of the arsenic sulphide residue and the third distilland respectively. 3. Principle analysis It should be noted that, the difference in saturated vapor pressure of different components in arsenic sulphide residue at the same temperature is the basic principle for separating them from each other under vacuum. At the same temperature, the greater the saturation vapor pressure of substance, the easier it evaporates.
2.3. Processes The processes of the experiments were shown in Fig. 2. The quartz crucible loaded with about 4 g of arsenic sulphide residue Table 1 Chemical composition of arsenic sulphide residue. Element
As
S
Pb
Ca
Content (wt.%)
34.69
36.82
1.15
9.29
(1)
Fig. 2. Flowsheet for treating arsenic sulphide residue.
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H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
According to the saturated vapor pressure parameters, which are quoted from the related literature [18,19], the calculation formulas of elemental sulphide, arsenic trioxide, arsenic sulfide, lead sulfide and calcium fluoride are as follows:
logðp=PaÞ ¼ 4830T
1
5 log T þ 26 388 718 K
(2)
logðp=PaÞ ¼ 5282T 1 þ 12:31 373 573 K
(3)
logðp=PaÞ ¼ 4307T 1 þ 9:373 729 959 K
(4)
logðp=PaÞ ¼ 13300T 1 0:81 log T 0:00043T þ 16:97 298 1200 K
(5)
logðp=PaÞ ¼ 23600T 1 4:525 log T þ 29:62 298 1424 K
(6)
According to the calculation formulas above, the saturated vapor pressure of the substance in different temperatures can be calculated. The results are shown in Fig. 3. As can been seen from Fig. 3, the saturated vapor pressures of substance is proportional to the temperature. More important, however, it is the fact that the saturation vapor pressure of calcium fluoride is much lower when compared with other four substances at the same temperature. Hence, it is easy to separate elemental sulphide, arsenic trioxide, arsenic sulfide and lead sulfide from the calcium fluoride at the appropriate temperature via vacuum distillation. At the same time, another significant fact is that there are remarkable differences in saturated vapor pressure between elemental sulphide/arsenic trioxide, arsenic sulfide and lead sulfide, which indicate the possibility for separating them from each other at different distillation temperature. However, it is difficult to isolate elemental sulphide and arsenic trioxide separately, due to the small difference in saturated vapor pressure between them, as shown in the figure above.
4. Results and discussion 4.1. Factors on primary distillation In the process of primary distillation, the dependence of removal efficiency of elemental sulphide and arsenic trioxide on experimental conditions including distillation temperature and time was investigated. 4.1.1. Effect of distillation temperature The experiment was performed at temperature of 180 C for 1.0 h, with the residual gas pressure of 15 Pa. After the primary distillation, 0.23 g of pale yellow distillate was obtained. Fig. 4 shows the XRD pattern of the corresponding distillate. According to Fig. 4, the primary distillate is a mixture of elemental sulphide and arsenic trioxide based on the standard Inorganic Crystal Structure Database (ICSD) card. When the distillation temperature increased to 200 C, the primary distillate turned to reddish orange, which could be due to the slight evaporation of arsenic sulfide at 200 C, according to saturated vapor pressure curve shown in Fig. 3. In order to verify this assumption, 3.62 g of arsenic sulfide distillate collected from the process of the secondary distillation was heated at 200 C for 1.0 h under 15 Pa. The experimental result revealed that 0.04 g of arsenic sulfide was evaporated out. For this reason, the temperature of 180 C is suitable for separating elemental sulphide and arsenic trioxide from arsenic sulphide residue independently, in order to avoid the evaporation of arsenic sulfide. 4.1.2. Effect of distillation time Investigating of effect of distillation time on primary distillation was carried out in time ranging from 0.5 to 2.5 h at 180 C and 15 Pa. The results showed that 0.15, 0.23, 0.27, 0.29 and 0.29 g of primary distillate could be obtained, corresponding to the distillation time of 0.5, 1.0, 1.5, 2.0, 2.5 h, respectively. When the distillation time was more than 2.0 h, the weight of primary distillate remained constant as the vacuum treatment time increased. Therefore, it is of no consideration for the extension of distillation time after 2.0 h. 4.2. Factors on secondary distillation After the primary distillation, the main component of primary distilland was arsenic sulfide. In this series of experiments, the
Fig. 3. Relationship between the saturated vapor pressure and temperature for various substances.
Fig. 4. The power X-ray diffraction pattern of the primary distillate.
H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
173
factors influencing secondary distillation were studied under different experimental conditions including distillation temperature, distillation time and residual gas pressure. 4.2.1. Effect of distillation temperature The experiments as to the effect of distillation temperature were conducted in the temperature ranging from 300 C to 500 C, maintaining distillation time at 1 h, and the residual gas pressure of 15 Pa. The relationship between distillation temperature and evaporation percent (E1) of primary distilland is shown in Fig. 5a. The evaporation percent (E1) of primary distilland was figured up by the following equation:
E1 ¼
m2 m3 100% m2
(7)
Where m2 is the mass of primary distilland (g); m3 is the mass of the secondary distilland (g). As can be seen from Fig. 5a, the evaporation percent (E1) of primary distilland increased obviously with the increase of distillation temperature. When the temperature changed from 300 C to 450 C, the evaporation percent (E1) of primary distilland increased from 30.98% to 80.16%. This can be explained by the fact that the saturation vapor pressure of arsenic sulfide increases with the rise of the temperature. However, the evaporation percent almost remained constant, when the temperature was higher than 450 C. Consequently, it is ineffectual and energy-consuming for a further increase of distillation temperature after 450 C. 4.2.2. Effect of distillation time This effect was examined by running experiments in distillation time ranging from 0.25 h to 1.5 h, when the distillation temperature was 450 C and the residual gas pressure was 15 Pa. Fig. 5b summarizes the relationship between evaporation percent and distillation time. As clearly seen in Fig. 5b, the evaporation percent (E1) of primary distilland hovered around 80.25% while the distillation time
Fig. 6. The Energy-dispersive X-ray spectroscopy pattern of secondary distillate.
increased from 0.25 h to 1.5 h. According to the formula (4), the saturated vapor pressure of arsenic sulfide is approximately 3000 Pa at 450 C, which means arsenic sulfide is highly volatile under the experimental condition. Moreover, in a vacuum, the evaporation resistance is greatly reduced due to the rarefied air. Accordingly, the evaporation rate of arsenic sulfide is greatly improved compared with atmospheric pressure. Therefore, arsenic sulfide can almost be evaporated out of the primary distilland completely in a relatively short period of time under vacuum, which can be a reasonable explanation for the experimental results.
4.2.3. Effect of residual gas pressure Experiments were conducted at various residual gas pressure, ranging from 15 to 135 Pa at temperature of 450 C, distillation time of 0.5 h. Fig. 5c shows the experimental results.
Fig. 5. Effects of (a) temperature, (b) distillation time, (c) residual gas pressure on evaporation percent of primary distilland.
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H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
result is shown in Fig. 6. As can be seen from Fig. 6, there is almost no impurity in the secondary distillate, which can be a satisfactory target product in the process of vacuum separation. 4.3. Factors on third distillation The XRD pattern of the secondary distilland is shown in Fig. 7a. As can be seen from Fig. 7a, the secondary distilland is a mixture of lead sulfide and calcium fluoride. In order to separate lead sulfide and calcium fluoride, the secondary distilland was treated again under vacuum. The factors influencing the evaporation percent of the secondary distilland were studied under different experimental conditions including distillation temperature and time. 4.3.1. Effect of distillation temperature The experiments were conducted in the temperature ranging from 800 C to 1100 C, maintaining distillation time at 2 h, corresponding to the residual gas pressure of 15 Pa. The relationship between distillation temperature and evaporation percent (E2) is shown in Fig. 8a. The evaporation percent (E2) of secondary distilland can be calculated by the following formula:
E2 ¼
Fig. 7. The power X-ray diffraction pattern of (a) secondary distilland, (b) third distilland.
It can be seen from Fig. 5c, the decrease of residual gas pressure had no significant improvement on the evaporation percent (E1) of primary distilland. The value of E1 increased slightly from 79.15% to 80.16%, with the residual gas pressure ranging from 135 Pa to 15 Pa. This can be explained by that the evaporation rate of arsenic sulfide almost reaches its maximum under the experimental conditions we employed. However, the process of vacuum separation should be performed in lower residual gas pressure in order to prevent the formation of oxides. It is worth noting that the arsenic sulfide obtained from secondary distillation is difficult to be characterized by XRD, due to its amorphous structure. In view of this, Energy-dispersive X-ray spectroscopy (EDS) is used to characterize the arsenic sulfide. The
m3 m4 100% m3
(8)
Where m3 is the mass of the secondary distilland (g); m4 is the mass of the third distilland (g). As can be seen from Fig. 8a, the evaporation percent (E2) of the secondary distilland increased from 17.81% to 36.73% between the temperatures of 800 C and 1050 C. This can be explained by the fact that the vapor pressure of lead sulfide increases as the temperature increases. However, the increase in the evaporation percent begins to weaken, as the temperature is higher than 1050 C. 4.3.2. Effect of distillation time The experiments were conducted at various distillation time, ranging from 1.0 h to 3.0 h at temperature of 1000 C with the residual gas pressure of 15 Pa. Fig. 8b shows the experimental results. As shown in Fig. 8b, the evaporation percent (E2) of the secondary distilland increased from 31.29% to 33.25% with the rise of distillation time. However, the evaporation percent almost remained constant as the distillation time was more than 2 h. Therefore, after the vacuum treatment time of 2 h, prolonging of the time will result in more energy consumption, which comes to naught.
Fig. 8. Effects of (a) distillation temperature, (b) distillation time on evaporation percent of secondary distilland.
H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
The XRD pattern of the third distilland is shown in Fig. 7b. As can be seen, the main component of the third distilland is calcium fluoride. The content of arsenic in the third distilland was 5.65 104 wt.% (characterized by ICP), which indicated that a arsenic removal ratio of almost 100% could be achieved through three-step vacuum distillation of arsenic sulphide residue. 5. Conclusions In this work, a novel process of three-step vacuum distillation was developed for treating arsenic sulphide residue. The experimental results are summarized as follows: (i) Elemental sulphide, arsenic trioxide, arsenic sulfide and lead sulfide could be separated respectively from the arsenic sulphide residue by three-step vacuum distillation under different temperature. (ii) Through three-step vacuum distillation, 0.29 g of the mixture of elemental sulphide and arsenic trioxide, 3.01 g of arsenic sulfide, 0.23 g of lead sulfide could be obtained as distillate, while 0.47 g of CaF2 was left behind as the third distilland. The content of arsenic in the third distilland was 5.65 104 wt.%, which indicated that a arsenic removal percent (R) of arsenic sulphide residue almost 100% could be achieved. References [1] Leist M, Casey RJ, Caridi D. The management of arsenic wastes:problems and prospects. J Hazard Mater 2000;76(1):125e38. [2] Ravenscroft P, Brammer H, Richards K. Arsenic pollution: a global synthesis. New York: Wiley; 2009.
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[3] Mohan D, Pittman CU. Arsenic removal from water/wastewater using adsorbents-a critical review. J Hazard Mater 2007;142(1):1e53. [4] Mondal P, Majumder CB, Mohanty B. Treatment of arsenic contaminated water in a batch reactor by using Ralstonia eutropha MTCC 2487 and granular activated carbon. J Hazard Mater 2008;153(1):588e99. [5] Ladeira ACQ, Ciminelli VST. Adsorption and desorption of arsenic on an oxisol and its constituents. Water Res 2004;38(8):2087e94. [6] Monhe AJ, Swash PM. Removing and stabilizing arsenic from copper refining circuits by hydrothermal processing. JOM 1999;51(9):30e3. [7] Zhu J, Gu C, Chen F, Teng Y, Han J, OuYang R. Study on the mechanism of apoptosis-related gene PNAS-2 in APL cells treated by arsenic sulfide. Acta Univ Med Second Shanghai 2005;25(5):443e6 [in Chinese]. [8] Wang L, Zhou G, Liu P, Song J, Liang Y, Yang X, et al. Dissection of mechanisms of Chinese medicinal formula Realgar-Indigo naturalis as an effective treatment for promyelocytic leukemia. Proc Natl Acad Sci U S A 2008;105(12): 4826e31. [9] Q. Zhang, Mao J, Liu P, Huang Q, Liu J, Xie Y, et al. A systems biology understanding of the synergistic effects of arsenic sulfide and imatinib in BCR/ABLassociated leukemia. Natl Acad Sci U S A 2009;106(9):3378e83. [10] Wang Z. Arsenic compounds as anticancer agents. Cancer Chemother Pharmacol 2001;48(1):S72e6. [11] Anderson NP. Bowen's precancerous dermatosis and multiple benign superficial epithelioma: evidence of arsenic as an etiologic agent. Arch Derm Syph 1932;26(6):1052e64. [12] Riveros PA, Dutrizac JE, Spencer P. Arsenic disposal practices in the metallurgical industry. Can Metall Quart 2001;40(4):395e420. [13] Qiu Y, Lu B, Chen B, Zhong Y, Fu W, Yang. Commercial scale test of antipollution control technique for slag of arsenic and soda. J Cent South Univ 2005;36(2):234e7. [14] Armando V. Arsenic management in the metallurgical industry. 2nd ed. Ottwa: National Library of Canada; 2000. [15] Lin D, Qiu K. Removing arsenic from anode slime by vacuum dynamic evaporation and vacuum dynamic flash reduction. Vacuum 2012;86(8):1155e60. [16] Lin D, Qiu K. Removal of arsenic and antimony from anode slime by vacuum dynamic flash reduction. Environ Sci Technol 2011;45(8):3361e6. [17] Dai Y, Yang B. Vacuum metallurgy of nonferrous materials. Beijing: Metallurgical Industry Press; 2000 [in Chinese]. [18] Kubaschewski O, Alcock CB. Metallurgical thermochemistry. 5th ed. Oxford: Pergamon Press; 1979. [19] Speight JG. Lange's chemistry handbook. 16th ed. New York: McGraw-Hill; 2005.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Three-step vacuum separation for treating arsenic sulphide residue Haijin Hu a, b, Keqiang Qiu a, b, * a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China Key Laboratory of Resources Chemistry of Nonferrous Metals, Central South University, Ministry of Education of The People's Republic of China, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2014 Received in revised form 25 September 2014 Accepted 26 September 2014 Available online 22 October 2014
Up to now, most of arsenic sulphide residue can't be properly disposed. The traditional processes for treating arsenic sulphide residue have some widespread drawbacks such as the complex process, the high cost in operation, and the less than satisfactory removal effect and environmental protection. In view of this, a process of three-step vacuum separation was proposed for treating arsenic sulphide residue in this work. During vacuum separation, elemental sulphide and arsenic trioxide could be recovered effectively in primary distillation at temperature of 180 C, distillation time of 2.0 h, corresponding to the residual gas pressure of 15 Pa. In secondary distillation, arsenic sulfide was obtained under the condition of 450 C for 30 min, with a residual gas pressure of 15 Pa. When the temperature increased to 1000 C for 2 h, lead sulfide was evaporated out in third distillation. Through the three-step vacuum separation, calcium fluoride was left behind in the third distilland, in which the content of arsenic was 5.65 104 wt.%. Correspondingly, the removal rate of arsenic was almost 100%. The vacuum process is expected to be effectively employed for recycling valuable components from arsenic sulphide residue, and meanwhile eliminating its pollution. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Arsenic sulphide residue Three-step vacuum separation Recover
1. Introduction As a common element existing in ores, arsenic is widely distributed in multiple procedures of mineral processing processes, which result in the problem of residual arsenic in the intermediate and final products of the processes, and the trouble in recovery of arsenic. In the case of filling these compounds containing arsenic, a significant contamination problem is likely to appear, due to the acute toxicity of these substances. The landfills will release toxins into the groundwater. It is worth pointing out that the treating of the hazardous materials contained arsenic has serious occupational and environmental implications, especially when the treating process would not like to take the necessary precautions to protect the environment and employees' health [1]. As is known, the arsenic pollution has become a global problem, which is reported frequently in many countries such as America, Japan, India, Mexico and China [2]. In the pollution of environment report by WHO in 1968, arsenic was ranked first. Therefore, much more special attention has been drawn to arsenic problem. There are billions tons of waste acid liquid containing arsenic produced by the factories in where arsenic-bearing minerals act as * Corresponding author. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China. Tel.: þ86 731 88836994; fax: þ86 731 88879616. E-mail address: [email protected] (K. Qiu). http://dx.doi.org/10.1016/j.vacuum.2014.09.018 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
the raw materials all over the world annually. The existing methods for treating waste acid liquid containing arsenic fall into several categories: ion exchange, adsorption (activated alumina and activated carbon); ultrafiltration, reverse osmosis, and precipitation or adsorption by metals (predominately ferric chloride) followed by coagulation [1,3e5]. At present, one of the most widely used methods is vulcanizing agent leaching, in which arsenic from waste acid liquid reacts with the vulcanizing agent, such as sodium sulfide. This method is simple, low in cost and high in separation rate of arsenic (about 99.97%) [6]. However, significant quantities of arsenic sulfide byproduct, are produced through this process, which brings about a series of challenges for the disposal of them. Arsenic sulphide residue, which is a dreadful catastrophe to human health, may result in serious environmental pollution without reasonable disposal. On the other hand, as is well-known, arsenic sulfide has high application value in field of medicine [7e11] and the metallurgical industry [12]. For instance, arsenic sulfide has antitumor effects by the way of inhibiting the growth of K562-cells and SMMC-7721-cells. In addition to this, it is also a necessary raw material in the production of herbicides and wood preservative. Therefore, it is of great interest to turn the hazardous arsenic sulphide residue into good one. What's more, the recovery of valuable components from arsenic sulphide residue is one of way to enlarge utilization of arsenic and sulphide resources. So far, the publicly reported researches about treating arsenic sulfide residue mainly focus on curing, landfill, or recycling in the
H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
form of arsenic trioxide, arsenate, arsenic trichloride and so on [1,13]. One of the most attractive options is a Japanese technology, which is detailed as follows: arsenic sulfide is repulped and heated to 70 C in CuSO4 solution, under which arsenic sulfide reacts with the CuSO4 to form CuS. Meanwhile, the arsenic is dissolved as arsenious acid. Subsequently, As2O3 is precipitated from arsenious acid solution by cooling at 25 C [14]. Obviously, the separation of solideliquid is necessary for each steps, and the emission of arsenic trioxide in the process of crystallization should not be ignored either. As things now stand, the existing methods, including the Japanese technology, are less than satisfactory due to the drawbacks such as complex and time-consuming process flow, low recovery efficiency, high consumption in chemical reagents, high cost in operation. Above all, these processes are inevitably produce toxic gas and large quantities of waste acid liquid, which are potential to form secondary environmental pollution. Enlightened by above thought, a method of vacuum separation, which shows many advantages such as simple technological flow sheet, low consumption of raw material and energy, no secondary off-gas or wastewater and so on [15e17], is proposed for treating arsenic sulfide residue in this work, with the purpose of recovering valuable components without negative impact on the environment. During vacuum separation, a method of three-step distillation was employed. Through three-step distillation, elemental sulphide/ arsenic trioxide, arsenic sulfide, lead sulfide were evaporated into distillate in primary distillation, secondary distillation and third distillation, respectively. Yet the calcium fluoride is left behind as the third distilland. 2. Experimental section 2.1. Experimental materials The arsenic sulphide residue used in the experiments was acquired from lead plant in which arsenic was recycled from waste water through sodium sulfide leaching. A small amount of arsenic trioxide was formed, due to oxidation of the waste residue after pile-up for a period of time. A homogenized sample was characterized by titrimetric analysis. Correspondingly, the contents of As, S, Pb and Ca are shown in Table 1. 2.2. Apparatus The experiments were carried out in the self-made distillation apparatus, which was mainly comprised of quartz tube, quartz crucible, water condenser, resistance furnace, intelligent temperature controller and vacuum pump, as shown in Fig. 1. The quartz tube loaded with a quartz crucible, which is used for holding the experimental sample, was placed in the self-design resistance furnace. The water condenser was inserted into middle of quartz tube and exactly right above the quartz crucible. The furnace temperature was controlled by the temperature controller with a thermocouple connected. The outlet of the quartz tube was connected with a vacuum pump to maintain constant pressure of the whole distillation system.
171
Fig. 1. Distillation apparatus.
was placed at the bottom of the quartz tube and then heated to preset temperature for a given period of time after the whole system was evacuated to a certain pressure. During the experiment, the volatile components were evaporated out of the arsenic sulphide residue into gas phase, and then condensed in the surface of the water condenser as distillate. Meanwhile, the other components were left in the crucible as distilland. After the experiment, the distilland left in the quartz crucible was taken out of the furnace, weighed. The distillate attached to the condenser was also cleaned out, collected and sampled. Subsequently, the residual distilland was evaporated again at a higher temperature. As can been seen from Fig. 2, three different kinds of distillate could be obtained after three-step distillation, which elemental sulphide/arsenic trioxide, arsenic sulfide and lead sulfide, respectively. Correspondingly, the calcium fluoride was left in the crucible as third distilland. The removal percent (R) of arsenic were calculated by the following formula:
R¼
m1 w1 m4 w4 100% m1 w1
Where m1 and m4 are the mass of the arsenic sulphide residue and the third distilland (g), respectively; w1 and w4 are the content of arsenic (wt.%) of the arsenic sulphide residue and the third distilland respectively. 3. Principle analysis It should be noted that, the difference in saturated vapor pressure of different components in arsenic sulphide residue at the same temperature is the basic principle for separating them from each other under vacuum. At the same temperature, the greater the saturation vapor pressure of substance, the easier it evaporates.
2.3. Processes The processes of the experiments were shown in Fig. 2. The quartz crucible loaded with about 4 g of arsenic sulphide residue Table 1 Chemical composition of arsenic sulphide residue. Element
As
S
Pb
Ca
Content (wt.%)
34.69
36.82
1.15
9.29
(1)
Fig. 2. Flowsheet for treating arsenic sulphide residue.
172
H. Hu, K. Qiu / Vacuum 111 (2015) 170e175
According to the saturated vapor pressure parameters, which are quoted from the related literature [18,19], the calculation formulas of elemental sulphide, arsenic trioxide, arsenic sulfide, lead sulfide and calcium fluoride are as follows:
logðp=PaÞ ¼ 4830T
1
5 log T þ 26 388 718 K
(2)
logðp=PaÞ ¼ 5282T 1 þ 12:31 373 573 K
(3)
logðp=PaÞ ¼ 4307T 1 þ 9:373 729 959 K
(4)
logðp=PaÞ ¼ 13300T 1 0:81 log T 0:00043T þ 16:97 298 1200 K
(5)
logðp=PaÞ ¼ 23600T 1 4:525 log T þ 29:62 298 1424 K
(6)
According to the calculation formulas above, the saturated vapor pressure of the substance in different temperatures can be calculated. The results are shown in Fig. 3. As can been seen from Fig. 3, the saturated vapor pressures of substance is proportional to the temperature. More important, however, it is the fact that the saturation vapor pressure of calcium fluoride is much lower when compared with other four substances at the same temperature. Hence, it is easy to separate elemental sulphide, arsenic trioxide, arsenic sulfide and lead sulfide from the calcium fluoride at the appropriate temperature via vacuum distillation. At the same time, another significant fact is that there are remarkable differences in saturated vapor pressure between elemental sulphide/arsenic trioxide, arsenic sulfide and lead sulfide, which indicate the possibility for separating them from each other at different distillation temperature. However, it is difficult to isolate elemental sulphide and arsenic trioxide separately, due to the small difference in saturated vapor pressure between them, as shown in the figure above.
4. Results and discussion 4.1. Factors on primary distillation In the process of primary distillation, the dependence of removal efficiency of elemental sulphide and arsenic trioxide on experimental conditions including distillation temperature and time was investigated. 4.1.1. Effect of distillation temperature The experiment was performed at temperature of 180 C for 1.0 h, with the residual gas pressure of 15 Pa. After the primary distillation, 0.23 g of pale yellow distillate was obtained. Fig. 4 shows the XRD pattern of the corresponding distillate. According to Fig. 4, the primary distillate is a mixture of elemental sulphide and arsenic trioxide based on the standard Inorganic Crystal Structure Database (ICSD) card. When the distillation temperature increased to 200 C, the primary distillate turned to reddish orange, which could be due to the slight evaporation of arsenic sulfide at 200 C, according to saturated vapor pressure curve shown in Fig. 3. In order to verify this assumption, 3.62 g of arsenic sulfide distillate collected from the process of the secondary distillation was heated at 200 C for 1.0 h under 15 Pa. The experimental result revealed that 0.04 g of arsenic sulfide was evaporated out. For this reason, the temperature of 180 C is suitable for separating elemental sulphide and arsenic trioxide from arsenic sulphide residue independently, in order to avoid the evaporation of arsenic sulfide. 4.1.2. Effect of distillation time Investigating of effect of distillation time on primary distillation was carried out in time ranging from 0.5 to 2.5 h at 180 C and 15 Pa. The results showed that 0.15, 0.23, 0.27, 0.29 and 0.29 g of primary distillate could be obtained, corresponding to the distillation time of 0.5, 1.0, 1.5, 2.0, 2.5 h, respectively. When the distillation time was more than 2.0 h, the weight of primary distillate remained constant as the vacuum treatment time increased. Therefore, it is of no consideration for the extension of distillation time after 2.0 h. 4.2. Factors on secondary distillation After the primary distillation, the main component of primary distilland was arsenic sulfide. In this series of experiments, the
Fig. 3. Relationship between the saturated vapor pressure and temperature for various substances.
Fig. 4. The power X-ray diffraction pattern of the primary distillate.
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factors influencing secondary distillation were studied under different experimental conditions including distillation temperature, distillation time and residual gas pressure. 4.2.1. Effect of distillation temperature The experiments as to the effect of distillation temperature were conducted in the temperature ranging from 300 C to 500 C, maintaining distillation time at 1 h, and the residual gas pressure of 15 Pa. The relationship between distillation temperature and evaporation percent (E1) of primary distilland is shown in Fig. 5a. The evaporation percent (E1) of primary distilland was figured up by the following equation:
E1 ¼
m2 m3 100% m2
(7)
Where m2 is the mass of primary distilland (g); m3 is the mass of the secondary distilland (g). As can be seen from Fig. 5a, the evaporation percent (E1) of primary distilland increased obviously with the increase of distillation temperature. When the temperature changed from 300 C to 450 C, the evaporation percent (E1) of primary distilland increased from 30.98% to 80.16%. This can be explained by the fact that the saturation vapor pressure of arsenic sulfide increases with the rise of the temperature. However, the evaporation percent almost remained constant, when the temperature was higher than 450 C. Consequently, it is ineffectual and energy-consuming for a further increase of distillation temperature after 450 C. 4.2.2. Effect of distillation time This effect was examined by running experiments in distillation time ranging from 0.25 h to 1.5 h, when the distillation temperature was 450 C and the residual gas pressure was 15 Pa. Fig. 5b summarizes the relationship between evaporation percent and distillation time. As clearly seen in Fig. 5b, the evaporation percent (E1) of primary distilland hovered around 80.25% while the distillation time
Fig. 6. The Energy-dispersive X-ray spectroscopy pattern of secondary distillate.
increased from 0.25 h to 1.5 h. According to the formula (4), the saturated vapor pressure of arsenic sulfide is approximately 3000 Pa at 450 C, which means arsenic sulfide is highly volatile under the experimental condition. Moreover, in a vacuum, the evaporation resistance is greatly reduced due to the rarefied air. Accordingly, the evaporation rate of arsenic sulfide is greatly improved compared with atmospheric pressure. Therefore, arsenic sulfide can almost be evaporated out of the primary distilland completely in a relatively short period of time under vacuum, which can be a reasonable explanation for the experimental results.
4.2.3. Effect of residual gas pressure Experiments were conducted at various residual gas pressure, ranging from 15 to 135 Pa at temperature of 450 C, distillation time of 0.5 h. Fig. 5c shows the experimental results.
Fig. 5. Effects of (a) temperature, (b) distillation time, (c) residual gas pressure on evaporation percent of primary distilland.
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result is shown in Fig. 6. As can be seen from Fig. 6, there is almost no impurity in the secondary distillate, which can be a satisfactory target product in the process of vacuum separation. 4.3. Factors on third distillation The XRD pattern of the secondary distilland is shown in Fig. 7a. As can be seen from Fig. 7a, the secondary distilland is a mixture of lead sulfide and calcium fluoride. In order to separate lead sulfide and calcium fluoride, the secondary distilland was treated again under vacuum. The factors influencing the evaporation percent of the secondary distilland were studied under different experimental conditions including distillation temperature and time. 4.3.1. Effect of distillation temperature The experiments were conducted in the temperature ranging from 800 C to 1100 C, maintaining distillation time at 2 h, corresponding to the residual gas pressure of 15 Pa. The relationship between distillation temperature and evaporation percent (E2) is shown in Fig. 8a. The evaporation percent (E2) of secondary distilland can be calculated by the following formula:
E2 ¼
Fig. 7. The power X-ray diffraction pattern of (a) secondary distilland, (b) third distilland.
It can be seen from Fig. 5c, the decrease of residual gas pressure had no significant improvement on the evaporation percent (E1) of primary distilland. The value of E1 increased slightly from 79.15% to 80.16%, with the residual gas pressure ranging from 135 Pa to 15 Pa. This can be explained by that the evaporation rate of arsenic sulfide almost reaches its maximum under the experimental conditions we employed. However, the process of vacuum separation should be performed in lower residual gas pressure in order to prevent the formation of oxides. It is worth noting that the arsenic sulfide obtained from secondary distillation is difficult to be characterized by XRD, due to its amorphous structure. In view of this, Energy-dispersive X-ray spectroscopy (EDS) is used to characterize the arsenic sulfide. The
m3 m4 100% m3
(8)
Where m3 is the mass of the secondary distilland (g); m4 is the mass of the third distilland (g). As can be seen from Fig. 8a, the evaporation percent (E2) of the secondary distilland increased from 17.81% to 36.73% between the temperatures of 800 C and 1050 C. This can be explained by the fact that the vapor pressure of lead sulfide increases as the temperature increases. However, the increase in the evaporation percent begins to weaken, as the temperature is higher than 1050 C. 4.3.2. Effect of distillation time The experiments were conducted at various distillation time, ranging from 1.0 h to 3.0 h at temperature of 1000 C with the residual gas pressure of 15 Pa. Fig. 8b shows the experimental results. As shown in Fig. 8b, the evaporation percent (E2) of the secondary distilland increased from 31.29% to 33.25% with the rise of distillation time. However, the evaporation percent almost remained constant as the distillation time was more than 2 h. Therefore, after the vacuum treatment time of 2 h, prolonging of the time will result in more energy consumption, which comes to naught.
Fig. 8. Effects of (a) distillation temperature, (b) distillation time on evaporation percent of secondary distilland.
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The XRD pattern of the third distilland is shown in Fig. 7b. As can be seen, the main component of the third distilland is calcium fluoride. The content of arsenic in the third distilland was 5.65 104 wt.% (characterized by ICP), which indicated that a arsenic removal ratio of almost 100% could be achieved through three-step vacuum distillation of arsenic sulphide residue. 5. Conclusions In this work, a novel process of three-step vacuum distillation was developed for treating arsenic sulphide residue. The experimental results are summarized as follows: (i) Elemental sulphide, arsenic trioxide, arsenic sulfide and lead sulfide could be separated respectively from the arsenic sulphide residue by three-step vacuum distillation under different temperature. (ii) Through three-step vacuum distillation, 0.29 g of the mixture of elemental sulphide and arsenic trioxide, 3.01 g of arsenic sulfide, 0.23 g of lead sulfide could be obtained as distillate, while 0.47 g of CaF2 was left behind as the third distilland. The content of arsenic in the third distilland was 5.65 104 wt.%, which indicated that a arsenic removal percent (R) of arsenic sulphide residue almost 100% could be achieved. References [1] Leist M, Casey RJ, Caridi D. The management of arsenic wastes:problems and prospects. J Hazard Mater 2000;76(1):125e38. [2] Ravenscroft P, Brammer H, Richards K. Arsenic pollution: a global synthesis. New York: Wiley; 2009.
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