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Separation and recovery of arsenic from arsenic-bearing dust
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JECE 700 1–7 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
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Separation and recovery of arsenic from arsenic-bearing dust
2 Q1
Xueyi Guoa,b,* , Jing Shia , Yu Yia,b , Qinghua Tiana,b , Dong Lia,b
3 4
a b
School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China Cleaner Metallurgical Engineering Research Center, Nonferrous Metal Industry of China, Changsha, Hunan 410083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Accepted 27 June 2015
A hydrometallurgical process, including selective extraction, As-precipitation, the insoluble and sodium removal, acid dissolution and recovery of arsenic trioxide, was developed to treat arsenic-bearing dust. Arsenic was selective extracted by a mixed NaOH–Na2S leaching system with 2.5 mol/L NaOH and 0.2 mol/L Na2S. In this system, more than 90.0% As was extracted, while Sb and Pb were precipitated in residue as NaSb(OH)6 and PbS. Based on the solubility of As2O5, a method for precipitating arsenic in the form of sodium arsenate from alkaline leachate by oxidization-precipitation was established. Then, the sodium arsenate was converted to Ca–As compound by adding exceeding CaO into sodium arsenate solution. H3AsO4 solution, prepared by dissolving Ca5(AsO4)3OH in dilute H2SO4, was further reduced to HAsO2 by H2SO3 and then reduction solution was concentrated and crystalized as octahedral shaped As2O3. This process transformed the hazardous material to valuable material and realized the resource recycling of arsenic. ã 2015 Published by Elsevier Ltd.
Keywords: Arsenic-bearing dust Alkali leaching Sodium arsenate Arsenic trioxide Recovery of arsenic
5
Introduction
6
Arsenic, a toxic element aroused major public concern, is an unwanted hazardous waste generated from nonferrous pyrometallurgical industries in smelting slag, dust, anode slime and so on [1,2]. The arsenic-bearing dust is one of the most important secondary resources, which contains a large amount of valuable metals, such as lead, antimony and indium. Therefore, separating arsenic from arsenic-bearing dust to reclaim valuable metals is of great environmental and economic meanings. There are some studies on treating arsenic-bearing materials by pyrometallurgy or hydrometallurgy [3,4]. Pyrometallurgical processes usually result in secondary As pollution and have high energy consumption [5], while hydrometallurgical processes, including removal of arsenic by dilute H2SO4 [6], sodium-sulfide solution [7], NaOH solution, mixed solution of NaHS/NaOH [8] and so on, are more environmental friendly. Yu et al. [9] has studied the extraction of arsenic from arsenic-containing cobalt and nickel slag by alkaline leaching, which contains alkaline leaching with pressured oxidation, cooling crystallization, arsenate reduction by SO2 gas and arsenic trioxide precipitation. This process can realize the clean extraction of arsenic and preparation of As2O3, but has to bear high energy consumption with high temperature and pressure. What is more, many Na2SO4 generated in the reduction
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* Corresponding author. E-mail address: [email protected] (X. Guo).
solution need extra way to remove. Li et al. [10] reports a method to remove arsenic from secondary zinc oxide using a mixed NaOH–Na2S leach, then oxidation using hydrogen peroxide and precipitation with lime. This process transforms the hazardous wastes into values by selective extraction of arsenic, but the leaching efficiency of arsenic is low and arsenic is not recovered. Therefore, it is important to find a novel technique to recover Q2 arsenic from arsenic-bearing dust. In this paper, an effective and environmental friendly method, including separating arsenic from hazardous materials and recovery of arsenic (Fig. 1), is proposed to realize a comprehensive utilization of the arsenic-bearing dust. The influence of processing parameters on separating arsenic from hazardous materials have been investigated systematically in this study, such as leaching system, leaching time, alkali concentration, temperature and Na2S concentration. Furthermore, the further study respect to Asprecipitation, the insoluble and sodium removal, acid dissolution and preparation of As2O3 have been also studied so as to realize the recovery of As2O3 from alkaline leachate.
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Experimental
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Materials
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The arsenic-bearing dust samples obtained from blast furnace smelting of copper dross in lead smelter, Guangxi Province, China, were used to carry out the experimental work. Arsenic-bearing dust were crushed, ground, and screened using a 100 mesh sieve.
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http://dx.doi.org/10.1016/j.jece.2015.06.028 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: X. Guo, et al., Separation and recovery of arsenic from arsenic-bearing dust, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.028
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Fig. 1. Technical schematic of arsenic-bearing dust treatment.
Table 1 Chemical compositions of arsenic-bearing dust (wt%).
53 54 55 56 57
Element
As
Sb
Pb
Sn
Zn
Cu
Fe
S
In
Content
6.86
9.55
49.13
2.80
2.40
1.10
1.80
5.50
0.26
The chemical analysis is shown in Table 1 and the XRD presented in Fig. 2(a). The result shows that there are three main phases of Pb5(AsO4)3OH, PbS and Sb2O3. Reagents of NaOH, Na2S9H2O and H2O2 (30%) were of analytical grade, manufactured by Sinopharm Chemical Reagent Co., Ltd.
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Experimental procedures
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Batch alkaline leaching experiments were conducted in a 500 mL four-necked round-bottomed flask with ancillary system to control the temperature and the stirring speed. All the experiments were carried out with 60 g of arsenic-bearing dust in a certain volume of leaching solution. A typical experiment was carried out as: heating the solution to about 80 C, temperature at which the flask was opened to add the dust sample. The flask was then sealed and the system was further heated to the set temperature for the experiment. At the end of the experiment, the leachate filtered for collecting the leaching residues. The residue was washed, dried and analyzed to determine the leaching ratio of As and the leachate was analyzed for Pb and Sb. Then, the alkaline leachate was used to prepare As2O3 by processes of oxide-precipitation, the insoluble and sodium removal, H2SO4 dissolution and reduction-crystallization.
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Characterization
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The phase of the solid sample was detected by X-ray diffraction (XRD) (The emission target of XRD was CuKa, the emission power was 40 kV 250 MA, the step width was 0.01, the scanning rate was 8 /min and 2u was 1080 , D/max-rA, Rigaku Corporation of Japan). The concentration of As(III) in liquid was detected by the substoichiometric oxidation of As(III) to As(V) with potassium bromate. The total As content was detected by an atomic fluorescence spectrometry (AFS-2202E, Haiguang Corp., Beijing) coupled with a hydride generator. The other components were detected by inductively coupled plasma optical emission spectrometer (ICP-OES, Intrepid II XSP, Thermo Elemental Corporation, America).
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Results and discussion
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Separate arsenic from hazardous materials
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Effect of leaching system on the leaching of As, Sb and Pb The effect of the NaOH–Na2S and NaOH leaching system on the leaching of As, Sb and Pb are shown in Fig. 3(a). As is seen, higher leaching efficiency of As is observed in the mixed NaOH–Na2S leaching system compared to that of NaOH alone leaching system. It can be explained that arsenic is extracted as Na3AsO4 in NaOH leaching solution but as Na3AsO4 and Na3AsS4 in mixed NaOH– Na2S leaching solution, where the solubility of Na3AsO4 is lower than that of Na3AsS4 in concentrated NaOH solution [9,11]. The main reactions of this process are shown as follows:
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Pb5(AsO4)3OH + 9NaOH = 3Na3AsO4 + 5 petabits(OH)2
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(1)
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Fig. 2. XRD patterns of solid phases: (a) arsenic-bearing dust; (b) alkaline leaching residue; (c) sodium arsenate crystals; (d) insoluble substance; (e) Ca–As compounds; (f) acid leaching residue. 100
Na3AsO4 + 4Na2S + 4H2O = Na3AsS4 + 8NaOH
(2)
101
Sb2O3 + 6NaOH = 2Na3SbO3 + 3H2O
(3)
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Sb2O3 + 6Na2S + 3H2O = 2Na3SbS3 + 6NaOH
(4)
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Pb(OH)2 + 2NaOH = Na2PbO2 + 2H2O
(5)
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Na2PbO2 + Na2S + 2H2O = PbS + 4NaOH
(6)
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On the other hand, a large amount of lead would be extracted in NaOH alone leaching system, which is not benefit to separate As from other valuable metals. According to Reaction (6), the dissolved Pb2+ can be precipitated by Na2S as PbS (Ksp = 1 1028) to reduce the loss of lead [10]. As seen in the Fig. 3(a), the leaching
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of Sb is about 10.0% in both leaching systems when reaction time less than 2.0 h. Giving consideration on the leaching of As, Sb and Pb, the mixed NaOH–Na2S leaching system and time of 2.0 h are determined as the optimal conditions.
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Effect of alkali concentration on the leaching of As and the loss of Sb and Pb As seen from Fig. 3(b), the leaching of As increases from 46.21% to 89.26% when alkali concentration increases from 0.5 to 3.0 mol/L, then decreases with the further increasing of alkali concentration. In general, the solubility of Na3AsO4 decreases rapidly with Na2O concentration increases from 5.0% to 27.0% [9]. Thus, the leaching of arsenic decreases as NaOH concentration more than 4.0 mol/L (Na2O 12.4 wt%). However, Sb and Pb concentration in the leachate increase rapidly with NaOH concentration increasing, that because NaOH can promote Reactions (3) and (5) [12]. Giving consideration on the leaching of As and the loss of Sb and Pb, the optimal NaOH concentration is determined as 2.5 mol/L.
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Fig. 3. (a) Effect of leaching system on the leaching of As, Sb and Pb. Leaching conditions: 1: mixed Na2S–NaOH leaching system with Na2S of 0.2 mol/L and NaOH of 2.5 mol/L and L/S of 5.0 at 80.0 C for 2.0 h; 2: NaOH alone leaching system with NaOH of 2.5 mol/L and L/S of 5.0 at 80.0 C for 2.0 h; (b) effect of alkali concentration on the leaching of As and the loss of Sb and Pb. Leaching conditions: 80 C, 2.0 h, S/L ratio = 1/5, 60 g of dust, 0.2 mol/L Na2S; (c): effect of reaction temperature on the leaching of As and the loss of Sb and Pb. Leaching conditions: 2.5 mol/L NaOH, 2.0 h, S/L ratio = 1/5, 60 g of dust, 0.2 mol/L Na2S.
Effect of temperature on the leaching of As and the loss of Sb and Pb The effect of temperature on the leaching of As and the loss of Sb and Pb are shown in Fig. 3(c). It indicates that the leaching of As and the loss of Sb and Pb increase with the elevation of temperature. At 30 C, the leaching of As is 58.58%, the concentration of Sb and Pb in leachate are 0.40 and 0.03 g/L, respectively, in comparison to those of 92.93%, 0.92 and 0.89 g/L at 95 C, correspondingly. Considering on the cost and the leaching of As and the loss of Sb and Pb, 90 C is determined as the optimal temperature.
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Effect of Na2S on the leaching of As and the loss of Sb and Pb The effect of Na2S content on the leaching of As and the loss of Sb and Pb are shown in Table 2. It is found that the leaching of As and the concentration of Sb increase from 86.48% to 94.31% and 0.46 to 1.37 g/L respectively, while the concentration of Pb decreases from 1.75 to 0.06 g/L, when the Na2S concentration increases from 0.1 to 0.3 mol/L. Considering on the leaching of As and the loss of Sb and Pb, the optimum concentration of Na2S is determined as 0.2 mol/L.
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Optimum operating conditions Based on the above results, the optimal leaching conditions are determined: a mixed NaOH–Na2S leaching system with 2.5 mol/L NaOH and 0.2 mol/L Na2S, L/S of 5.0, temperature of 90 C and time of 2.0 h. Under these conditions, more than 90.0% of As is extracted, and leachate contains 13.30 g/L As, 1.64 g/L Sb, 0.83 g/L Pb and 2.0 mol/L alkali. Fig. 2(b) shows the XRD pattern of the leach residue. Comparing to the XRD pattern of arsenic-bearing dust shown in Fig. 2(a), there is a new characteristic peak of NaSb(OH)6 observed but the characteristic peak of Pb5(AsO4)3OH is disappeared. The chemical analysis shows that the leach residue contain 0.79% As, 10.77% Sb and 55.7% Pb, which can be delivered to pyrometallurgical process to recovery valuable metals.
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Recovery of arsenic
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As-precipitation The chemical analysis of alkaline leachate indicates that the contents of As(III) and As(V) are 5.96 and 7.34 g/L, respectively. The solubility of As2O5 changes significantly with temperature in highalkaline solution, and its solubility is small at low temperature [9], and As (V) would be precipitated as sodium arsenate when the As2O5 concentration higher than its solubility in Na2O–As2O5–H2O system. Therefore, As in alkaline leachate can be effectively precipitated as sodium arsenate crystals by adding H2O2. The reactions of this process are shown as follows:
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NaAsO2 + H2O2 + 2NaOH = Na3AsO4 + 2H2O
(7)
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Na3AsS4 + 4H2O2 = Na3AsO4 + 4S + 4H2O
(8)
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Na3SbS3 + 4H2O2 = NaSb(OH)6 + 3S + 2NaOH
(9)
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(10)
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3S + 6NaOH = 2Na2S + Na2SO3 + 3H2O
Table 2 Experiments condition and results of effect of the leaching of arsenic by Na2S. No.
1 2 3
CNaOH (mol/L)
2.5 2.5 2.5
CNa2 S (mol/L)
0.1 0.2 0.3
Temperature ( C)
90 90 90
Time (h)
2 2 2
Leaching ratio of As (%)
86.48 93.17 94.31
Concentration in leaching solution (g/L) Sb
Pb
0.46 0.64 1.37
1.75 0.83 0.006
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Fig. 4. Effect of molar ratio (H2O2/As) on As precipitation. Conditions: 25 C, 2.0 h.
Table 3 Chemical composition of sodium arsenate crystals (wt%). Elements
Na
As
Sb
Sn
Pb
Zn
Content
18.7
17.8
2.27
0.14
0.027
0.0085
5
molar ratio of 4.0, initial pH of 11.0, temperature of 85.0 C and stirring time of 3.0 h. Under these conditions, the precipitation efficiency of arsenic reaches 99.99%. The solution after precipitation was return to dissolve sodium arsenate. From composition analysis of precipitation residue as shown Table 4, the Ca–As compound contains 32.39% Ca and 25.27% As. However, the content of Ca and As of pure Ca5(AsO4)3OH is 30.60% and 35.44%, respectively, that is due to the excess of CaO is deposited as Ca (OH)2. Fig. 2(e) shows the XRD pattern of precipitation residue, which indicates the reactions of this process shown as follows:
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CaO + H2O = Ca(OH)2
(11)
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3N3AsO4 + 5Ca(OH)2 = Ca5(AsO4)3OH + 9NaOH
(12)
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A surplus amount of CaO is thus likely to enhance the elimination of As [13].
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Acid dissolution The H3AsO4 solution is prepared by dissolving Ca–As compounds in dilute H2SO4 solution. The main reactions of this process are shown as follows [13,14]:
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H2SO4 + Ca(OH)2 = CaSO42H2O
4rG
u
298 = 208.965 kJ
Ca5(AsO4)3OH + 5H2SO4 = 5CaSO4 + 3H3AsO4 + H2O 4rGu298 = 800.40 kJ Table 4 Components content in Ca–As compounds residue (wt%).
174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196
Elements
As
Ca
Na
Content
25.27
32.39
0.04
The effect of H2O2 amount on the precipitation efficiency of As is presented in Fig. 4. As seen, the precipitation efficiency of As increases rapidly with the addition of H2O2. The precipitation efficiency of As reaches 91.3% and a large amount of crystals is observed when 0.8 times stoichiometric of H2O2 is added. Fig. 2(c) and Table 3 show the XRD and chemical compositions of sodium arsenate, respectively, from which the main phase of the sample is Na3AsO410H2O, and the content of arsenic is 17.8%. Solution after precipitation contains alkali of 80 g/L, which can be returned to alkaline leaching process. When the solution after precipitation was recycled to treat a second batch of arsenic-bearing dust, very similar extraction results were obtained. The extraction of As is 90.15%, while the concentration of Sb and Pb in leachate remain low (1.53 and 0.67 g/L, respectively). Therefore, recycling of the solution after precipitation is feasible. Insoluble and sodium removal This process of dissolving sodium arsenate crystals into deionized water is aimed to remove the insoluble from the crystals and prepare sodium arsenate solution. It is found that the insoluble is NaSb(OH)6 from the XRD analysis as shown in Fig. 2(d). And As concentration in sodium arsenate solution is 21.08 g/L. Then, CaO was added into sodium arsenate solution to remove sodium ions from As-compound under the conditions as: Ca/As
(13)
199 200 201 202 203 204 205 206
210
212 213 214 215
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(14)
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Table 5 shows the effect of H2SO4 concentration on dissolution of As. It reveals that the dissolution of As increases from 92.08% to 99.06%, and As content of gypsum decreases from 1.57% to 0.19%, when H2SO4 concentration increases from 100 to 120 g/L. With H2SO4 concentration increases to 200.0 g/L continuously, the dissolution of As remains almost unchanged. Due to pentavalent arsenic is a mediate strong acid (pK1 = 2.2) [15], addition sulfuric acid is beneficial for the extraction of arsenic from Ca–As compounds [16]. The dissolution of As is more than 99.0% and the As content in acid leaching residue is less than 0.5% with acid solution of 120.0 g/L H2SO4 at S/L ratio of 8.0 and 85 C for 1.5 h. Fig. 2(f) shows the XRD pattern of acid leaching residue. It is found that the main components of leach residues are CaSO4 and CaSO40.15H2O, which can be used to produce cement.
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Recovery of As2O3 As seen in Table 6, there is 28.16 g/L As(V) in H3AsO4 solution. Arsenate acid is a strong oxidant and easy to be reduced to HAsO2 by H2SO3. The reactions of reduction process are indicated as follows [17]:
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H2SO3 + H3AsO4 = HAsO2 + H2SO4 + H2O, = 77.56 kJ/mol
4rGu298
H2SO3 + H3AsO4 = AsO+ + HSO4 + 2H2O, = 86.85 kJ/mol
4rGu298
219 220 221 222 223 224 225 226 227 228 229 230 231
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(15)
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(16)
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The reduction of As(V) was carried out under temperature of 30 C, H2SO3 addition of 1.5 times of theory amount and 2.5 h. After reduction, the reduced solution was concentrated to 90 g/L, and then, the concentrated solution was cooled to 25 C and
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Table 5 Effect of sulfuric acid concentration on arsenic leaching ratio and arsenic content of gypsum. Factors Sulfuric acid concentration (under S/L of 8, reaction temperature 85 C for 1.5 h)
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100 g/L 120 g/L 160 g/L 200 g/L
Arsenic leaching ratio (%)
Arsenic content of gypsum (%)
92.08 99.06 98.73 98.96
1.57 0.19 0.42 0.39
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Table 6 The valence states’ change of As in solution during the recovery of As2O3. Name
H3AsO4 solution (g L1)
Reduced and concentrated solution (g L1)
Crystallization solution (g L1)
As(V) As(III) H2SO4
28.16 0 –
0.12 90.00 –
0.16 10.00 632.85
Table 7 As2O3 solubility in water at varied temperature. T ( C)
0
15
25
39.8
48.2
62
75
98.5
g per 100 g H2O As(III) concentration (g L1)
1.21 9.16
1.66 12.57
2.05 15.53
2.93 22.19
3.43 25.98
4.45 33.70
5.62 42.56
8.18 61.95
Table 8 Component of As2O3 product (wt%). Elements
As2O3
Ca
Cu
Sb
Fe
Pb
Bi
Content
99.67
0.002
0
0
0
0
0
Fig. 5. SEM (a) and XRD (b) pattern of As2O3 product.
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crystallized product was obtained after filtration. In order to improve the purity of the As2O3 crystal, recrystallization was carried out. The valence states’ change of As in solution during recovery of As2O3 process is listed in Table 6. It shows that As(III) concentration in crystallization solution is only 10.0 g/L, which is less than its solubility in water shown in Table 7 [18]. This is because that the solubility of As2O3 decreases to a rock bottom gradually when H2SO4 concentration increases from 0 to 850 g/L [19]. It also can be calculated that the reduction efficiency and the crystallization efficiency of arsenic as high as 99.6% and 88.9%, respectively. The compositions, XRD and SEM pattern of the product are shown in Table 8 and Fig. 5, which show that the recrystallized product is octahedral shaped As2O3, and its purity is 99.67%.
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Conclusions
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A research on the recovery of As2O3 from arsenic-bearing dust generated from pyrometallurgical process of jamesonite mine were performed with the hydrometallurgical processes. For this aim, selectively extraction of arsenic, As-precipitation, the insoluble and sodium removal, acid dissolution and recovery of As2O3 were carried out in laboratory scale.
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(1) The selective As extraction enhanced obviously when leaching with mixed Na2S and NaOH system compared to that with NaOH alone system. >90% As was extracted from arsenic-containing dust with 2.5 mol/L NaOH and 0.2 mol/L Na2S at 90 C for 2.0 h using L/S ratio of 5, and leachate contains 13.30 g/L As, 1.64 g/L Sb, 0.83 g/L Pb and 2 mol/L alkali. The leach residue contained 0.79% As, 10.77% Sb and 55.7% Pb. (2) 91.3% As was precipitated as the form of Na3AsO410H2O by using hydrogen peroxide under condition of molar ratio (H2O2/ As) of 0.8. By dissolving sodium arsenate into deionized water and Ca–As precipitation, the insoluble NaSb(OH)6 and sodium ions in the As-compounds were removed, Then, H3AsO4 solution was prepared by dissolving Ca–As compounds into dilute H2SO4 solution. Further, octahedral shaped As2O3 was prepared from H3AsO4 solution by H2SO3 reduction.
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Acknowledgement
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This work was financially supported by the science and technology Research Foundation of Guangxi Province, China (No. 2012AA04022).
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[10] Y.H. Li, Z.H. Liu, Q.H. Li, et al., Removal of arsenic from arsenate complex contained in secondary zinc oxide, Hydrometallurgy 109 (3–4) (2011) 237– 244, doi:http://dx.doi.org/10.1016/j.hydromet.2011.07.007. [11] W. Tongamp, Y. Takasaki, A. Shibayama, Precipitation of arsenic as Na 3AsS4 from Cu3AsS4–NaHS–NaOH leach solutions, Hydrometallurgy 105 (2011) 42– 46. [12] X. Zhang, Z.H. Liu, Y.H. Li, et al., Oxygen pressure leaching of arsenic and antimony bearing flue dust in NaOH solution, J. Cent. South Univ. Sci. Technol. 45 (2014) 1390–1397 (in Chinese). [13] T. Fujita, R. Taguchi, E. Shibata, et al., Preparation of an As (V) solution for scorodite synthesis and a proposal for an integrated as fixation process in a Zn refinery, Hydrometallurgy 96 (4) (2009) 300–312, doi:http://dx.doi.org/ 10.1016/j.hydromet.2008.11.008. [14] X.W. Yang, Thermodynamic Data Calculation of High Temperature Aqueous Manual, Metallurgical Industry Press, Beijing, 1983 (in Chinese). [15] T. Suzuki, M. Moribe, Y. Okabe, et al., A mechanistic study of arsenate removal from artificially contaminated clay soils by electrokinetic remediation, J. Hazard. Mater. 254–255 (2013) 310–317, doi:http://dx.doi.org/10.1016/j. jhazmat.2013.04.013. 23643955. [16] T. Nishimura, R.G. Robins, A re-evaluation of the solubility and stability regions of calcium arsenites and calcium arsenates in aqueous solution at 25 C, Miner. Process. Extract. Metall. Rev. 18 (3–4) (1998) 283–308, doi:http://dx.doi.org/ 10.1080/08827509808914159. [17] Y. Peng, Y. Zheng, W. Zhou, et al., Separation and recovery of Cu and As during purification of copper electrolyte, Trans. Nonferrous Met. Soc. China 22 (9) (2012) 2268–2273, doi:http://dx.doi.org/10.1016/S1003-6326(11)61459-3. [18] Y.J. Liang, Y.C. Che, Handbook of Thermodynamic Data for Inorganic Substances, Dong bei University Press, Shenyang, 1993 (in Chinese). [19] F. Dalewski, Removing arsenic from copper smelter gases, JOM 51 (9) (1999) 24–26, doi:http://dx.doi.org/10.1007/s11837-999-0153-0.
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1
Separation and recovery of arsenic from arsenic-bearing dust
2 Q1
Xueyi Guoa,b,* , Jing Shia , Yu Yia,b , Qinghua Tiana,b , Dong Lia,b
3 4
a b
School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China Cleaner Metallurgical Engineering Research Center, Nonferrous Metal Industry of China, Changsha, Hunan 410083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Accepted 27 June 2015
A hydrometallurgical process, including selective extraction, As-precipitation, the insoluble and sodium removal, acid dissolution and recovery of arsenic trioxide, was developed to treat arsenic-bearing dust. Arsenic was selective extracted by a mixed NaOH–Na2S leaching system with 2.5 mol/L NaOH and 0.2 mol/L Na2S. In this system, more than 90.0% As was extracted, while Sb and Pb were precipitated in residue as NaSb(OH)6 and PbS. Based on the solubility of As2O5, a method for precipitating arsenic in the form of sodium arsenate from alkaline leachate by oxidization-precipitation was established. Then, the sodium arsenate was converted to Ca–As compound by adding exceeding CaO into sodium arsenate solution. H3AsO4 solution, prepared by dissolving Ca5(AsO4)3OH in dilute H2SO4, was further reduced to HAsO2 by H2SO3 and then reduction solution was concentrated and crystalized as octahedral shaped As2O3. This process transformed the hazardous material to valuable material and realized the resource recycling of arsenic. ã 2015 Published by Elsevier Ltd.
Keywords: Arsenic-bearing dust Alkali leaching Sodium arsenate Arsenic trioxide Recovery of arsenic
5
Introduction
6
Arsenic, a toxic element aroused major public concern, is an unwanted hazardous waste generated from nonferrous pyrometallurgical industries in smelting slag, dust, anode slime and so on [1,2]. The arsenic-bearing dust is one of the most important secondary resources, which contains a large amount of valuable metals, such as lead, antimony and indium. Therefore, separating arsenic from arsenic-bearing dust to reclaim valuable metals is of great environmental and economic meanings. There are some studies on treating arsenic-bearing materials by pyrometallurgy or hydrometallurgy [3,4]. Pyrometallurgical processes usually result in secondary As pollution and have high energy consumption [5], while hydrometallurgical processes, including removal of arsenic by dilute H2SO4 [6], sodium-sulfide solution [7], NaOH solution, mixed solution of NaHS/NaOH [8] and so on, are more environmental friendly. Yu et al. [9] has studied the extraction of arsenic from arsenic-containing cobalt and nickel slag by alkaline leaching, which contains alkaline leaching with pressured oxidation, cooling crystallization, arsenate reduction by SO2 gas and arsenic trioxide precipitation. This process can realize the clean extraction of arsenic and preparation of As2O3, but has to bear high energy consumption with high temperature and pressure. What is more, many Na2SO4 generated in the reduction
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
* Corresponding author. E-mail address: [email protected] (X. Guo).
solution need extra way to remove. Li et al. [10] reports a method to remove arsenic from secondary zinc oxide using a mixed NaOH–Na2S leach, then oxidation using hydrogen peroxide and precipitation with lime. This process transforms the hazardous wastes into values by selective extraction of arsenic, but the leaching efficiency of arsenic is low and arsenic is not recovered. Therefore, it is important to find a novel technique to recover Q2 arsenic from arsenic-bearing dust. In this paper, an effective and environmental friendly method, including separating arsenic from hazardous materials and recovery of arsenic (Fig. 1), is proposed to realize a comprehensive utilization of the arsenic-bearing dust. The influence of processing parameters on separating arsenic from hazardous materials have been investigated systematically in this study, such as leaching system, leaching time, alkali concentration, temperature and Na2S concentration. Furthermore, the further study respect to Asprecipitation, the insoluble and sodium removal, acid dissolution and preparation of As2O3 have been also studied so as to realize the recovery of As2O3 from alkaline leachate.
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Experimental
47
Materials
48
The arsenic-bearing dust samples obtained from blast furnace smelting of copper dross in lead smelter, Guangxi Province, China, were used to carry out the experimental work. Arsenic-bearing dust were crushed, ground, and screened using a 100 mesh sieve.
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http://dx.doi.org/10.1016/j.jece.2015.06.028 2213-3437/ ã 2015 Published by Elsevier Ltd.
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Fig. 1. Technical schematic of arsenic-bearing dust treatment.
Table 1 Chemical compositions of arsenic-bearing dust (wt%).
53 54 55 56 57
Element
As
Sb
Pb
Sn
Zn
Cu
Fe
S
In
Content
6.86
9.55
49.13
2.80
2.40
1.10
1.80
5.50
0.26
The chemical analysis is shown in Table 1 and the XRD presented in Fig. 2(a). The result shows that there are three main phases of Pb5(AsO4)3OH, PbS and Sb2O3. Reagents of NaOH, Na2S9H2O and H2O2 (30%) were of analytical grade, manufactured by Sinopharm Chemical Reagent Co., Ltd.
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Experimental procedures
59
Batch alkaline leaching experiments were conducted in a 500 mL four-necked round-bottomed flask with ancillary system to control the temperature and the stirring speed. All the experiments were carried out with 60 g of arsenic-bearing dust in a certain volume of leaching solution. A typical experiment was carried out as: heating the solution to about 80 C, temperature at which the flask was opened to add the dust sample. The flask was then sealed and the system was further heated to the set temperature for the experiment. At the end of the experiment, the leachate filtered for collecting the leaching residues. The residue was washed, dried and analyzed to determine the leaching ratio of As and the leachate was analyzed for Pb and Sb. Then, the alkaline leachate was used to prepare As2O3 by processes of oxide-precipitation, the insoluble and sodium removal, H2SO4 dissolution and reduction-crystallization.
60 61 62 63 64 65 66 67 68 69 70 71 72 73
Characterization
74
The phase of the solid sample was detected by X-ray diffraction (XRD) (The emission target of XRD was CuKa, the emission power was 40 kV 250 MA, the step width was 0.01, the scanning rate was 8 /min and 2u was 1080 , D/max-rA, Rigaku Corporation of Japan). The concentration of As(III) in liquid was detected by the substoichiometric oxidation of As(III) to As(V) with potassium bromate. The total As content was detected by an atomic fluorescence spectrometry (AFS-2202E, Haiguang Corp., Beijing) coupled with a hydride generator. The other components were detected by inductively coupled plasma optical emission spectrometer (ICP-OES, Intrepid II XSP, Thermo Elemental Corporation, America).
75
Results and discussion
87
Separate arsenic from hazardous materials
88
Effect of leaching system on the leaching of As, Sb and Pb The effect of the NaOH–Na2S and NaOH leaching system on the leaching of As, Sb and Pb are shown in Fig. 3(a). As is seen, higher leaching efficiency of As is observed in the mixed NaOH–Na2S leaching system compared to that of NaOH alone leaching system. It can be explained that arsenic is extracted as Na3AsO4 in NaOH leaching solution but as Na3AsO4 and Na3AsS4 in mixed NaOH– Na2S leaching solution, where the solubility of Na3AsO4 is lower than that of Na3AsS4 in concentrated NaOH solution [9,11]. The main reactions of this process are shown as follows:
89
Pb5(AsO4)3OH + 9NaOH = 3Na3AsO4 + 5 petabits(OH)2
99
(1)
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3
Fig. 2. XRD patterns of solid phases: (a) arsenic-bearing dust; (b) alkaline leaching residue; (c) sodium arsenate crystals; (d) insoluble substance; (e) Ca–As compounds; (f) acid leaching residue. 100
Na3AsO4 + 4Na2S + 4H2O = Na3AsS4 + 8NaOH
(2)
101
Sb2O3 + 6NaOH = 2Na3SbO3 + 3H2O
(3)
102
Sb2O3 + 6Na2S + 3H2O = 2Na3SbS3 + 6NaOH
(4)
103
Pb(OH)2 + 2NaOH = Na2PbO2 + 2H2O
(5)
104
Na2PbO2 + Na2S + 2H2O = PbS + 4NaOH
(6)
105
On the other hand, a large amount of lead would be extracted in NaOH alone leaching system, which is not benefit to separate As from other valuable metals. According to Reaction (6), the dissolved Pb2+ can be precipitated by Na2S as PbS (Ksp = 1 1028) to reduce the loss of lead [10]. As seen in the Fig. 3(a), the leaching
106 107 108 109
of Sb is about 10.0% in both leaching systems when reaction time less than 2.0 h. Giving consideration on the leaching of As, Sb and Pb, the mixed NaOH–Na2S leaching system and time of 2.0 h are determined as the optimal conditions.
110
Effect of alkali concentration on the leaching of As and the loss of Sb and Pb As seen from Fig. 3(b), the leaching of As increases from 46.21% to 89.26% when alkali concentration increases from 0.5 to 3.0 mol/L, then decreases with the further increasing of alkali concentration. In general, the solubility of Na3AsO4 decreases rapidly with Na2O concentration increases from 5.0% to 27.0% [9]. Thus, the leaching of arsenic decreases as NaOH concentration more than 4.0 mol/L (Na2O 12.4 wt%). However, Sb and Pb concentration in the leachate increase rapidly with NaOH concentration increasing, that because NaOH can promote Reactions (3) and (5) [12]. Giving consideration on the leaching of As and the loss of Sb and Pb, the optimal NaOH concentration is determined as 2.5 mol/L.
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Fig. 3. (a) Effect of leaching system on the leaching of As, Sb and Pb. Leaching conditions: 1: mixed Na2S–NaOH leaching system with Na2S of 0.2 mol/L and NaOH of 2.5 mol/L and L/S of 5.0 at 80.0 C for 2.0 h; 2: NaOH alone leaching system with NaOH of 2.5 mol/L and L/S of 5.0 at 80.0 C for 2.0 h; (b) effect of alkali concentration on the leaching of As and the loss of Sb and Pb. Leaching conditions: 80 C, 2.0 h, S/L ratio = 1/5, 60 g of dust, 0.2 mol/L Na2S; (c): effect of reaction temperature on the leaching of As and the loss of Sb and Pb. Leaching conditions: 2.5 mol/L NaOH, 2.0 h, S/L ratio = 1/5, 60 g of dust, 0.2 mol/L Na2S.
Effect of temperature on the leaching of As and the loss of Sb and Pb The effect of temperature on the leaching of As and the loss of Sb and Pb are shown in Fig. 3(c). It indicates that the leaching of As and the loss of Sb and Pb increase with the elevation of temperature. At 30 C, the leaching of As is 58.58%, the concentration of Sb and Pb in leachate are 0.40 and 0.03 g/L, respectively, in comparison to those of 92.93%, 0.92 and 0.89 g/L at 95 C, correspondingly. Considering on the cost and the leaching of As and the loss of Sb and Pb, 90 C is determined as the optimal temperature.
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Effect of Na2S on the leaching of As and the loss of Sb and Pb The effect of Na2S content on the leaching of As and the loss of Sb and Pb are shown in Table 2. It is found that the leaching of As and the concentration of Sb increase from 86.48% to 94.31% and 0.46 to 1.37 g/L respectively, while the concentration of Pb decreases from 1.75 to 0.06 g/L, when the Na2S concentration increases from 0.1 to 0.3 mol/L. Considering on the leaching of As and the loss of Sb and Pb, the optimum concentration of Na2S is determined as 0.2 mol/L.
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Optimum operating conditions Based on the above results, the optimal leaching conditions are determined: a mixed NaOH–Na2S leaching system with 2.5 mol/L NaOH and 0.2 mol/L Na2S, L/S of 5.0, temperature of 90 C and time of 2.0 h. Under these conditions, more than 90.0% of As is extracted, and leachate contains 13.30 g/L As, 1.64 g/L Sb, 0.83 g/L Pb and 2.0 mol/L alkali. Fig. 2(b) shows the XRD pattern of the leach residue. Comparing to the XRD pattern of arsenic-bearing dust shown in Fig. 2(a), there is a new characteristic peak of NaSb(OH)6 observed but the characteristic peak of Pb5(AsO4)3OH is disappeared. The chemical analysis shows that the leach residue contain 0.79% As, 10.77% Sb and 55.7% Pb, which can be delivered to pyrometallurgical process to recovery valuable metals.
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Recovery of arsenic
159
As-precipitation The chemical analysis of alkaline leachate indicates that the contents of As(III) and As(V) are 5.96 and 7.34 g/L, respectively. The solubility of As2O5 changes significantly with temperature in highalkaline solution, and its solubility is small at low temperature [9], and As (V) would be precipitated as sodium arsenate when the As2O5 concentration higher than its solubility in Na2O–As2O5–H2O system. Therefore, As in alkaline leachate can be effectively precipitated as sodium arsenate crystals by adding H2O2. The reactions of this process are shown as follows:
160
NaAsO2 + H2O2 + 2NaOH = Na3AsO4 + 2H2O
(7)
170
Na3AsS4 + 4H2O2 = Na3AsO4 + 4S + 4H2O
(8)
171
Na3SbS3 + 4H2O2 = NaSb(OH)6 + 3S + 2NaOH
(9)
172
(10)
173
3S + 6NaOH = 2Na2S + Na2SO3 + 3H2O
Table 2 Experiments condition and results of effect of the leaching of arsenic by Na2S. No.
1 2 3
CNaOH (mol/L)
2.5 2.5 2.5
CNa2 S (mol/L)
0.1 0.2 0.3
Temperature ( C)
90 90 90
Time (h)
2 2 2
Leaching ratio of As (%)
86.48 93.17 94.31
Concentration in leaching solution (g/L) Sb
Pb
0.46 0.64 1.37
1.75 0.83 0.006
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Fig. 4. Effect of molar ratio (H2O2/As) on As precipitation. Conditions: 25 C, 2.0 h.
Table 3 Chemical composition of sodium arsenate crystals (wt%). Elements
Na
As
Sb
Sn
Pb
Zn
Content
18.7
17.8
2.27
0.14
0.027
0.0085
5
molar ratio of 4.0, initial pH of 11.0, temperature of 85.0 C and stirring time of 3.0 h. Under these conditions, the precipitation efficiency of arsenic reaches 99.99%. The solution after precipitation was return to dissolve sodium arsenate. From composition analysis of precipitation residue as shown Table 4, the Ca–As compound contains 32.39% Ca and 25.27% As. However, the content of Ca and As of pure Ca5(AsO4)3OH is 30.60% and 35.44%, respectively, that is due to the excess of CaO is deposited as Ca (OH)2. Fig. 2(e) shows the XRD pattern of precipitation residue, which indicates the reactions of this process shown as follows:
197
CaO + H2O = Ca(OH)2
(11)
207
3N3AsO4 + 5Ca(OH)2 = Ca5(AsO4)3OH + 9NaOH
(12)
208
A surplus amount of CaO is thus likely to enhance the elimination of As [13].
209
Acid dissolution The H3AsO4 solution is prepared by dissolving Ca–As compounds in dilute H2SO4 solution. The main reactions of this process are shown as follows [13,14]:
211
H2SO4 + Ca(OH)2 = CaSO42H2O
4rG
u
298 = 208.965 kJ
Ca5(AsO4)3OH + 5H2SO4 = 5CaSO4 + 3H3AsO4 + H2O 4rGu298 = 800.40 kJ Table 4 Components content in Ca–As compounds residue (wt%).
174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196
Elements
As
Ca
Na
Content
25.27
32.39
0.04
The effect of H2O2 amount on the precipitation efficiency of As is presented in Fig. 4. As seen, the precipitation efficiency of As increases rapidly with the addition of H2O2. The precipitation efficiency of As reaches 91.3% and a large amount of crystals is observed when 0.8 times stoichiometric of H2O2 is added. Fig. 2(c) and Table 3 show the XRD and chemical compositions of sodium arsenate, respectively, from which the main phase of the sample is Na3AsO410H2O, and the content of arsenic is 17.8%. Solution after precipitation contains alkali of 80 g/L, which can be returned to alkaline leaching process. When the solution after precipitation was recycled to treat a second batch of arsenic-bearing dust, very similar extraction results were obtained. The extraction of As is 90.15%, while the concentration of Sb and Pb in leachate remain low (1.53 and 0.67 g/L, respectively). Therefore, recycling of the solution after precipitation is feasible. Insoluble and sodium removal This process of dissolving sodium arsenate crystals into deionized water is aimed to remove the insoluble from the crystals and prepare sodium arsenate solution. It is found that the insoluble is NaSb(OH)6 from the XRD analysis as shown in Fig. 2(d). And As concentration in sodium arsenate solution is 21.08 g/L. Then, CaO was added into sodium arsenate solution to remove sodium ions from As-compound under the conditions as: Ca/As
(13)
199 200 201 202 203 204 205 206
210
212 213 214 215
216
(14)
217
Table 5 shows the effect of H2SO4 concentration on dissolution of As. It reveals that the dissolution of As increases from 92.08% to 99.06%, and As content of gypsum decreases from 1.57% to 0.19%, when H2SO4 concentration increases from 100 to 120 g/L. With H2SO4 concentration increases to 200.0 g/L continuously, the dissolution of As remains almost unchanged. Due to pentavalent arsenic is a mediate strong acid (pK1 = 2.2) [15], addition sulfuric acid is beneficial for the extraction of arsenic from Ca–As compounds [16]. The dissolution of As is more than 99.0% and the As content in acid leaching residue is less than 0.5% with acid solution of 120.0 g/L H2SO4 at S/L ratio of 8.0 and 85 C for 1.5 h. Fig. 2(f) shows the XRD pattern of acid leaching residue. It is found that the main components of leach residues are CaSO4 and CaSO40.15H2O, which can be used to produce cement.
218
Recovery of As2O3 As seen in Table 6, there is 28.16 g/L As(V) in H3AsO4 solution. Arsenate acid is a strong oxidant and easy to be reduced to HAsO2 by H2SO3. The reactions of reduction process are indicated as follows [17]:
232
H2SO3 + H3AsO4 = HAsO2 + H2SO4 + H2O, = 77.56 kJ/mol
4rGu298
H2SO3 + H3AsO4 = AsO+ + HSO4 + 2H2O, = 86.85 kJ/mol
4rGu298
219 220 221 222 223 224 225 226 227 228 229 230 231
233 234 235 236 237
(15)
238
239
(16)
240
The reduction of As(V) was carried out under temperature of 30 C, H2SO3 addition of 1.5 times of theory amount and 2.5 h. After reduction, the reduced solution was concentrated to 90 g/L, and then, the concentrated solution was cooled to 25 C and
241
Table 5 Effect of sulfuric acid concentration on arsenic leaching ratio and arsenic content of gypsum. Factors Sulfuric acid concentration (under S/L of 8, reaction temperature 85 C for 1.5 h)
198
100 g/L 120 g/L 160 g/L 200 g/L
Arsenic leaching ratio (%)
Arsenic content of gypsum (%)
92.08 99.06 98.73 98.96
1.57 0.19 0.42 0.39
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Table 6 The valence states’ change of As in solution during the recovery of As2O3. Name
H3AsO4 solution (g L1)
Reduced and concentrated solution (g L1)
Crystallization solution (g L1)
As(V) As(III) H2SO4
28.16 0 –
0.12 90.00 –
0.16 10.00 632.85
Table 7 As2O3 solubility in water at varied temperature. T ( C)
0
15
25
39.8
48.2
62
75
98.5
g per 100 g H2O As(III) concentration (g L1)
1.21 9.16
1.66 12.57
2.05 15.53
2.93 22.19
3.43 25.98
4.45 33.70
5.62 42.56
8.18 61.95
Table 8 Component of As2O3 product (wt%). Elements
As2O3
Ca
Cu
Sb
Fe
Pb
Bi
Content
99.67
0.002
0
0
0
0
0
Fig. 5. SEM (a) and XRD (b) pattern of As2O3 product.
245
258
crystallized product was obtained after filtration. In order to improve the purity of the As2O3 crystal, recrystallization was carried out. The valence states’ change of As in solution during recovery of As2O3 process is listed in Table 6. It shows that As(III) concentration in crystallization solution is only 10.0 g/L, which is less than its solubility in water shown in Table 7 [18]. This is because that the solubility of As2O3 decreases to a rock bottom gradually when H2SO4 concentration increases from 0 to 850 g/L [19]. It also can be calculated that the reduction efficiency and the crystallization efficiency of arsenic as high as 99.6% and 88.9%, respectively. The compositions, XRD and SEM pattern of the product are shown in Table 8 and Fig. 5, which show that the recrystallized product is octahedral shaped As2O3, and its purity is 99.67%.
259
Conclusions
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A research on the recovery of As2O3 from arsenic-bearing dust generated from pyrometallurgical process of jamesonite mine were performed with the hydrometallurgical processes. For this aim, selectively extraction of arsenic, As-precipitation, the insoluble and sodium removal, acid dissolution and recovery of As2O3 were carried out in laboratory scale.
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(1) The selective As extraction enhanced obviously when leaching with mixed Na2S and NaOH system compared to that with NaOH alone system. >90% As was extracted from arsenic-containing dust with 2.5 mol/L NaOH and 0.2 mol/L Na2S at 90 C for 2.0 h using L/S ratio of 5, and leachate contains 13.30 g/L As, 1.64 g/L Sb, 0.83 g/L Pb and 2 mol/L alkali. The leach residue contained 0.79% As, 10.77% Sb and 55.7% Pb. (2) 91.3% As was precipitated as the form of Na3AsO410H2O by using hydrogen peroxide under condition of molar ratio (H2O2/ As) of 0.8. By dissolving sodium arsenate into deionized water and Ca–As precipitation, the insoluble NaSb(OH)6 and sodium ions in the As-compounds were removed, Then, H3AsO4 solution was prepared by dissolving Ca–As compounds into dilute H2SO4 solution. Further, octahedral shaped As2O3 was prepared from H3AsO4 solution by H2SO3 reduction.
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Acknowledgement
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This work was financially supported by the science and technology Research Foundation of Guangxi Province, China (No. 2012AA04022).
Please cite this article in press as: X. Guo, et al., Separation and recovery of arsenic from arsenic-bearing dust, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.028
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