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Recovery of sulfuric acid from a stone coal acid leaching solution by diffusion dialysis
Hydrometallurgy 173 (2017) 9–14
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Recovery of sulfuric acid from a stone coal acid leaching solution by diffusion dialysis
MARK
Kui Wanga,c,d, Yimin Zhanga,b,c,d,⁎, Jing Huanga,c,d, Tao Liua,c,d, Jingpeng Wanga,c,d a
School of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China c Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Wuhan 430081, China d Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan 430081, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Diffusion dialysis Sulfuric acid recovery Anion exchange membrane Stone coal Acid leaching solution
Diffusion dialysis was employed to recover sulfuric acid from a stone coal acid leaching solution. The dialysis coefficients of H+, V, Al, Fe, Mg, K, F, P, and S ions in a stone coal acid leaching solution for an anion exchange membrane were determined. The effects of the flow rate, flow rate ratio, and water osmosis rate as well as ion rejection on sulfuric acid recovery were investigated. The results demonstrated that the DF120-III anion exchange membrane showed a good separation performance for separating sulfuric acid from the stone coal acid leaching solution. In the diffusion dialysis process, the water osmosis rate, sulfuric acid recovery, ion rejection, water equilibrium and processing ability were taken into consideration. Controlling water osmosis was more important than obtaining a higher acid recovery. Under the optimum operating conditions of a feed flow rate of 12 mL/min and flow rate ratio of water to feed of 1–1.1, sulfuric acid recovery reached 71.12%, the water osmosis rate was controlled at approximately 14.95%, and vanadium rejection was approximately 95.50%. The rejection of impurity ions, such as Al, Fe, Mg, K, F, P, and S was approximately 99.04%, 97.37%, 98.01%, 85.12%, 98.33%, 91.16%, and 69.96%, respectively. The high rejections of F in the form of complexes and P in the form of incompletely dissociated acid were disadvantageous to the recovery of sulfuric acid. The recovered acid was able to be reused in the acid leaching process by the addition of fresh acid.
1. Introduction In China, in addition to vanadium‑titanium magnetite, stone coal is an important vanadium-bearing resource due to its vast reserves and wide distribution (Zhang et al., 2011). Currently, acid leaching of stone coal with H2SO4 is the most common method in vanadium metallurgical plants because of its high vanadium recovery and low pollution (Zhou et al., 2009; Ma et al., 2015; Li et al., 2009; Li et al., 2015). However, a quantity of residual free sulfuric acid is present in acid leaching solution after leaching with H2SO4, and the acidity of the acid leaching solution is relatively high, making it unsuitable for the subsequent extraction of vanadium. Currently, a conventional and popular method to treat the acid leaching solution is neutralization with ammonia water or lime. However, such a neutralization process will generate a large quantity of highly concentrated ammonia‑nitrogen wastewater or vanadium-containing gypsum, which cause disposal problems and serious environmental pollution. Moreover, valuable resources, such as sulfuric acid, bases and a portion of vanadium, are wasted (Wei et al., 2010; Li et al., 2012; Yang et al., 2016a).
⁎
Obviously, a new method is needed to both improve the economics of the process as well as avoid the vanadium loss and environmental pollution caused by the neutralization process. Diffusion dialysis with an anion exchange membrane seems to be a desirable method due to its low energy consumption, low installation and operating costs and lack of pollution (Luo et al., 2011a). It has been widely exploited to recover acid from metal treatment wastes, such as waste streams generated in steel, metal-refining, and electroplating industries (Oh et al., 2000; Xu and Yang, 2001, 2003; Xu et al., 2009a, 2009b). However, as opposed to recovering acid from general waste acid systems, the stone coal acid leaching solution is a mineral leaching solutions that is used with the ultimate goal of separating valuable metal resources from it rather than recovering acid from it and discarding it; the residual solution will be used in the subsequent separation and enrichment process of vanadium and the recovered acid should be able to be reused in the ore acid leaching process. In this way, a closed-circuit is created, and the economic and environmental effects are remarkable. Studies have been performed on the recovery of sulfuric acid from a stone coal acid leaching solution by diffusion dialysis. Wei et al. (2010)
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.hydromet.2017.07.005 Received 13 March 2017; Received in revised form 16 July 2017; Accepted 24 July 2017 Available online 25 July 2017 0304-386X/ © 2017 Published by Elsevier B.V.
Hydrometallurgy 173 (2017) 9–14
K. Wang et al.
Peristaltic pump P
Diffusate cell
Dialysate cell
investigated the effects of the sulfuric acid, FeSO4 and VOSO4 concentrations; flow rate; and flow rate ratio on the recovery of H2SO4. Acid recovery exceeded 80%. The V and Fe ion rejection levels were within 93–95% and 92–94%, respectively. Li et al. (2012) investigated the effects of the flow rate, flow rate ratio, as well as V, Al and Fe ion concentrations on the recovery of H2SO4 and metal ion rejection. Over an 84% H2SO4 recovery efficiency was achieved. The V, Al and Fe ion rejection values reached 93%, 92% and 85%, respectively. However, these authors did not conduct detailed research on the behavior of vanadium and the main impurity ions (Al, Fe, Mg, K, F, P, S), the volumetric expansion of the acid leaching solution because of water osmosis and the effects on sulfuric acid recovery. However, these factors will directly affect the subsequent separation and enrichment of vanadium as well as the utilization of the recovered acid. Therefore, in this study, the dialysis coefficients of H+,V, Al, Fe, Mg, Na, K, F, P, and S ions in a stone coal acid leaching solution for the anion exchange membrane were determined in a cycling type dynamic diffusion dialysis experiment to investigate the separation performance of the anion exchange membrane. Furthermore, one-pass type dynamic diffusion dialysis experiments were conducted to investigate the factors that impacted the recovery of sulfuric acid, water osmosis and ion rejection.
P
Peristaltic pump
Membrane
Fig. 1. The schematic of the cycling type dynamic diffusion dialysis system.
late stage of the experiment, only the first six sets of data were analyzed. The schematic of the cycling type dynamic diffusion dialysis system is shown in Fig. 1.
2.2.2. One-pass type dynamic diffusion dialysis experiments One-pass type dynamic diffusion dialysis experiments were conducted using a HKY-001 diffusion dialyzer supplied by Shandong Tianwei Membrane Technology Co., Ltd. (Shandong, China). The diffusion dialyzer was separated by 19 sheets of an anion exchange membrane (DF120-III, Shandong, China) into dialysate cells and diffusate cells through which the feed and running water, respectively, passed in a countercurrent fashion. The effective area of each membrane for mass transfer was 0.08 m2 (0.2 × 0.4), and the total effective membrane was 1.52 m2 (19 × 0.2 × 0.4). During the tests, the diffusion dialyzer was initially fed with feed (0.7 L) and running water (0.7 L) and maintained for 2 h to reach equilibrium. Then, the test began and proceeded until a dynamic equilibrium state was reached; then, samples were taken. The schematic of the one-pass type dynamic diffusion dialysis experimental set-up is shown in Fig. 2. The DF120-III anion exchange membrane was produced from engineering polymer poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by bromination and amination. The functional group attached on the anion exchange membrane was a quaternary ammonium group, and the physical aperture was approximately 5 nm. The membrane's lifetime is up to 5 years (Wei et al., 2010). Its characteristics are shown in Table 2 (Xu et al., 2009a; Li et al., 2012). The membranes used in all experiments were from the same production batch. All experiments were conducted at an ambient temperature of approximately 25 °C.
2. Experimental 2.1. Materials The solution used for cycling type dynamic diffusion dialysis and one-pass type dynamic diffusion dialysis was the actual acid leaching solution generated from the stone coal direct acid leaching process. Before the experiments, the acid leaching solution was filtered through a 0.45 μm pore size filter membrane to remove suspended solids and prevent blockage of the membrane. The concentrations of various ions in the solution are listed in Table 1. 2.2. Diffusion dialysis experiments 2.2.1. Cycling type dynamic diffusion dialysis experiment The cycling type dynamic diffusion experiment was carried out using a custom-built mini diffusion dialyzer. The diffusion dialyzer was separated by 20 sheets of an anion exchange membrane (DF120-III, Shandong, China) into dialysate cells and diffusate cells through which the feed and de-ionized water, respectively, passed in a countercurrent fashion. The effective area of each membrane for mass transfer was 0.0112 m2 (0.56 × 0.1 × 0.2), and the total effective membrane area was 0.224 m2. Feed (1.5 L) and de-ionized water (1.5 L) were cycled, respectively, in dialysate cells and diffusate cells by a peristaltic pump with identical flow rates of 75 mL/min. The whole experiment ran continuously for 6 h. Small amounts of samples were taken from the dialysate and the diffusate to analyze the H+ concentration and other ion concentrations every 30 min. Due to the severe water osmosis at the
Water
Peristaltic pump
P
Acid leaching solution Exhaust vent
P
Peristaltic pump
Table 1 The chemical composition of the acid leaching solution used in the experiment. Element +
H V Al Fe Mg K F P S
Concentration (mg/L) 1750 2101.81 17,560 5828 4503 6779 11,244.6 1463 61,670
Diffusion dialyzer
Residual solution
Recovered acid
Fig. 2. The schematic of the one-pass type dynamic diffusion dialysis experimental set-up.
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Table 2 The characteristics of the DF120-III anion exchange membrane.
Table 3 Dialysis coefficients of different ions and the corresponding separation factors.
Item
Specification
Ion
Dialysis coefficient Ui (m/h)
Separation factor UH+/Ui
Water content (%) Thickness (mm) Transference number of ions in membrane Membrane area resistance (Ω cm2) Ion exchange capacity (mmol/g) Burst strength (MPa) Selective penetration (%) Potential of membrane (mV)
42.34 0.20–0.23 0.98 3.4 (16 °C) or 2.7 (25 °C) 1.7–1.9 > 0.9 96.2 14.9 (14 °C)
H+ V Al Fe Mg K F P S
2.230 × 10− 3 6.864 × 10− 5 5.525 × 10− 5 4.654 × 10− 5 4.621 × 10− 5 1.510 × 10− 4 1.038 × 10− 5 1.092 × 10− 4 5.809 × 10− 4
− 32.49 40.36 47.92 48.26 14.77 214.84 20.42 3.84
2.3. Analyses and calculations 3. Results and discussion The concentration of sulfuric acid was determined by titration using 0.1 mol/L Na2CO3 with methyl orange as an indicator. The vanadium concentration in the aqueous phase was determined by ferrous ammonium sulfate titration. The concentration of fluoride was measured with a fluoride ion selective electrode (GB/T 7484-1987, 1987). The concentration of the other elements in the acid solution was analyzed by using ICP-AES. Without accounting for the volume changes, according to Fick's law, the dialysis coefficient of substance i can be calculated by the following equation (Xu et al., 2009a; Tang et al., 2006):
V1V2 ΔCi0 ln (V1 + V2)St ΔCit
Ui =
3.1. Cycling type dynamic diffusion dialysis In the diffusion dialysis process, the anion exchange membrane is the key component. The anion exchange membrane has to have high proton permeability but strongly reject salts (Xu and Yang, 2004). The dialysis coefficient is an important parameter in the diffusion dialysis process, as it is related to the membrane, species and concentrations of the diffusion materials, temperature and solution system. It represents the transport characteristics, including the diffusion coefficients of acid and metal salts through the membrane as well as the sorption equilibrium (Oh et al., 2000). The dialysis coefficients of different ions for the membrane and the corresponding separation factors that were calculated according to the results of the experiment are presented in Table 3. As Table 3 shows, the dialysis coefficient of the H+ ion is the largest of all of the investigated ions, and the separation factor between H+ and S is the smallest. The dialysis coefficient of monovalent K+ ion is larger than that of the other metal cations. Separation factors are observed over the range of 14.77 to 48.26 for metal ions. The separation factor between H+ and F is as high as 214.84. Although positively charged, H+ ions show higher competition in diffusion than metal ions because of their smaller size, lower valence state and higher mobility and diffuse along with the SO42 − to meet the requirement of solution electrical neutrality (Luo et al., 2011b). A larger separation factor represents more efficient separation of an acid and metal salt (Oh et al., 2000). Based on these results, it is observed that the DF120-III anion exchange membrane has high proton permeability and strongly rejects salts; the membrane also functioned efficiently regarding the separation of sulfuric acid from the stone coal acid leaching solution.
(1)
where Ui is the dialysis coefficient of substance i (m/h); V1 and V2 are the volumes of the dialysate and the diffusate (m3), respectively; ΔCi0 and ΔCit are the concentration gradient of substance i between the dialysate and the diffusate at time 0 and t, respectively (mol/L); S is the effective membrane area (m2); and t is the time (h). Then, Eq. (1) can be transformed to Eq. (2):
ln ΔCit = ln ΔCi0 − Ui S
V1 + V2 t V1V2
(2)
According to Eq. (2), lnΔCit is linear with respect to t, and the dialysis coefficient Ui can be calculated using the method of linear regression. The separation factor was determined to be the ratio of the dialysis coefficient for H+ to that for substance i in this study (HG/T 5112-2016, 2017). The total acid recovery ratio R was calculated by the following equation:
R=
Q r × CH r × 100% H Q r × CH r + Q d × Cd
(3) 3.2. One-pass type dynamic diffusion dialysis
where Qr is the flow rate of the recovered acid (mL/min), Qd is the flow rate of the dialysate (mL/min), and CrH and CdH are the H+ concentrations in the recovered acid and dialysate, respectively (mol/L). The ion rejection ratio was calculated as follows:
η=
Qd × CId × 100% Q r × CIr + Q d × CId
3.2.1. Effect of the flow rate on the diffusion dialysis performance To investigate the effect of the flow rate on the diffusion dialysis process, several tests were conducted. During the tests, the flow rate ratio of water to feed was maintained at 1.0. Fig. 3 shows the variation of the sulfuric acid recovery and H+ concentration in the recovered acid and dialysate as the flow rate increases. We observed that the sulfuric acid recovery increased with the increase of the flow rate and reached a maximum at 8 mL/min; the sulfuric acid recovery was 73.50%. Over this flow rate, the sulfuric acid recovery began to decrease. The H+ concentration in the recovered acid decreased from 1.80 mol/L to 1.24 mol/L. The H+ concentration in the dialysate decreased first and then increased; it reached a minimum at 8 mL/min, and the H+ concentration in the dialysate was 0.37 mol/L. When the flow rate increased, the thicknesses of the two concentration boundary layers over the membrane decreased and the mass transfer resistance weakened, which was beneficial to the diffusion of H+ ions. Since the surface area of the membrane is constant, too high of a flow rate could shorten the residence time for the feed passing through the
(4)
where CrI and CdI are the concentrations of I ions in the recovered acid and dialysate, respectively (mol/L). The water osmosis rate was used to quantify the extent of water osmosis and label the extent of the volumetric expansion of the dialysate and was calculated as follows:
Q δ = ⎛ d − 1⎞ × 100% ⎠ ⎝ Qf ⎜
⎟
(5)
where Qf and Qd are the flow rates of the feed and the dialysate, respectively (mL/min). The residence time was defined as the ratio of the volume of the dialysate chamber to the flow rate of the feed. 11
Hydrometallurgy 173 (2017) 9–14
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2.1
75
reduced the volume of the recovered acid, which was apparently detrimental to the process. Therefore, controlling water osmosis is more important than obtaining a higher acid recovery. Under the low flow rate conditions, a large amount of water transported from the diffusate side diluted the dialysate, the solution concentration difference on both sides of the membrane subsequently decreased. According to the principle of diffusion dialysis, the driving force for the diffusion of H+ ions decreased and sulfuric acid recovery decreased (as shown in Fig. 3). Although sulfuric acid recovery reached its highest value at 8 mL/min (as shown in Fig. 3), water osmosis still occurred, and the water osmosis rate was 24.23%. Under the high flow rate conditions, the water osmosis rate decreased, but acid recovery decreased as well. The flow rate also reflected the processing ability of the dialyzer per unit time. Therefore, a suitable feed flow rate was needed to control the relatively low water osmosis rate, achieve relatively high sulfuric acid recovery and guarantee a relatively high processing ability. Taking all of the above factors into consideration, 12 mL/min was chosen as the optimum flow rate, and the water osmosis rate was 14.95% under this condition. Ion rejection, especially V rejection, is another important concern and is related to the subsequent extraction of vanadium and whether the recovered acid can be reused in the acid leaching process. If the concentration of impurity ions in the recovered acid is high, a decrease in the extraction performance will occur due to the accumulation of impurity ions in the acid leaching solution. The effect of the flow rate on ion rejection is shown in Fig. 5. From this figure, we observe that all of the ion rejection values increased as the flow rate increased. Among them, V rejection increased from 92.62% to 96.39%. Al, Mg, Fe ions rejections were all > 96%. K and P rejections obviously increased. F rejection was > 98%. S rejection was the smallest. With an increasing flow rate, the residence time became shorter for the feed passing through the dialysate unit. The effect of ion rejection was enhanced since there was not sufficient time for the mass transfer through the membrane. K rejection was remarkably lower than that of other metal ion rejection. The hydrated radii and charge valences for the ions K+, VO2 +, Al3 +, Fe3 +, Fe2 +, and Mg2 + are collected in Table 4 (Nightingale, 1959). In comparison with K+, the VO2 +, Al3 +, Fe3 +, Fe2 + and Mg2 + ions were more effectively rejected by the positively charged fixed groups on the membrane since their higher charges make them suffer greater electrostatic repulsion. In addition to valence, the hydrated radii of their complexes and themselves were greater than the hydrated radius of K+, which also increased their resistance, affecting their ability to penetrate the membrane. Hence, K+ ions transported through the membrane into the diffusate more easily under the same
H2SO4 recovery H in diffusate + H in dialysate
1.7
69
1.5
67
1.3
1.1
65 3
5
7
9
11
13
15
17
0.45
0.40
+
71
0.50
H in dialyste (mol/L)
1.9
+
73
H in diffusate (mol/L)
H2SO4 recovery (%)
+
0.35
19
Flow rate (mL/min) Fig. 3. Effect of the flow rate on sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate.
dialysate unit. Thus, there was not sufficient time for the H+ ions to permeate through the membrane, and this resulted in a decrease of sulfuric acid recovery and the H+ concentration in the recovered acid and an increase of the H+ concentration in the dialysate. The volume of the dialysate expanded and the volume of the diffusate reduced during the diffusion dialysis process. There was competition between water osmosis from the diffusate to the dialysate due to the osmosis pressure and water transport from the dialysate to the diffusate due to the diffusion of hydrated ions (acid or salt) (Xu and Yang, 2004). Fig. 4 shows the variation of the water osmosis rate and ratio of the vanadium concentration in the dialysate and feed with the increase of flow rate. The water osmosis rate decreased as the flow rate increased and reached a stable value of approximately 10% over a flow rate of 14 mL/min. YC increased from 68.20% to 93.49%. Obviously, water osmosis was reduced by increasing the flow rate. Water osmosis was related to the feed concentration (osmosis pressure difference) and membrane hydrophilicity; a large concentration difference or a membrane with a higher water content permitted a larger water osmosis flux (Xu and Yang, 2004). In diffusion dialysis of the stone coal acid leaching solution, water osmosis is a key factor that affects the operation. In contrast to recovering most of the acid from waste and discarding the waste, the dialysate in this experiment was used in the subsequent separation and enrichment process of vanadium. The recovered acid was returned to the acid leaching process. The stone coal acid leaching solution had a high salt content, which contributed to a high osmotic pressure. Water osmosis not only decreased the vanadium concentration but also expanded the volume of the dialysate and
100 40
95
35
90
Ion rejection (%)
90 30 85
25 80
20
YC (%)
Water osmosis rate (%)
95
Water osmosis rate YC
75
15
85
V Fe K P
80 75
Al Mg F S
70
10
70
5
65 3
5
7
9
11
13
15
17
65
19
5
Flow rate (mL/min)
7
9
11
13
15
Flow rate (mL/min)
Fig. 4. Effect of the flow rate on the water osmosis rate and ratio of the vanadium concentration in the dialysate and feed (YC).
Fig. 5. Effect of the flow rate on ion rejection.
12
17
19
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100
Table 4 The hydrated radii and charge valences for K+, VO2 +, Al3 +, Fe3 +, Fe2 +, and Mg2 +. Ion
Ion hydrated radius (nm)
Valence
K+ VO2 + Al3 + Fe3 + Fe2 + Mg2 +
0.331 – 0.475 0.457 0.428 0.428
1 2 3 3 2 2
90
Ion rejection (%)
1.8
85
95
0.7
H2SO4 recovery
1.4
65 1.2
0.5
0.4
60
55 0.7
0.9
1.1
1.0 1.5
1.3
H in dialysate (mol/L)
70
0.6
0.7
1.0
1.1
1.2
1.3
1.4
1.5
Table 5 The chemical composition of the residual solution and recovered acid.
0.3
120
Element
Residual solution (mg/L)
Recovered acid (mg/L)
H+ V Al Fe Mg K F P S
480 1832.15 15,810 5203 4017 5688 9519.55 1198 37,920
1520 128.25 211.9 194.2 112.8 1372 119.09 160.3 22,470
110
solutions composed of 330 g/L HNO3 and HF 3 g/L (Xu and Yang, 2003), the recovery ratio of HF reached 90%.
10 90
YV (%)
Water osmosis rate (%)
0.9
Fig. 8. Effect of the flow rate ratio on ion rejection.
100
3.2.2. Effect of the flow rate ratio (water/feed) on the diffusion dialysis performance To investigate the effect of the flow rate ratio on the diffusion dialysis process, several tests were conducted. During the tests, the flow rate of the feed was maintained at 12 mL/min. Fig. 6 shows the variation of sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate as the flow rate ratio increases. With the increase of the flow rate ratio, the sulfuric acid recovery increased from 60.63% to 80.64% and the H+ concentration in the recovered acid decreased from 1.70 mol/L to 1.17 mol/L. The H+ concentration in the dialysate decreased from 0.62 mol/L to 0.315 mol/ L. An increase in the flow rate ratio means that the contact time for a given volume of feed is elongated. Moreover, the H2SO4 concentration in the recovered acid decreased and the difference in concentration over the membrane increased. The driving force for the diffusion of H+ ions increased, and therefore, more H+ ions permeated the membrane into the recovered acid. Fig. 7 shows the variation of the water osmosis rate and ratio of the volume of the dialysate and feed (YV) with the increased flow rate ratio. As Fig. 7 shows, the water osmosis rate remained approximately unchanged. YV showed a close to linear increase. When the flow rate ratio was 1.1, the water osmosis rate was 13.58% and YV was 95.83%. A higher flow rate ratio prompts the recovery of sulfuric acid (as shown in Fig. 6), but also resulted in a conspicuous increase in the volume of the recovered acid and a significant decrease of the H+ concentration in the recovered acid. Yamashita et al. (1996) utilized diffusion dialysis to separate sulfuric acid from the process solution of a zinc leaching
80
5
70
0 0.7
0.8
Flow rate ratio (water/feed)
130
15
70
60
Fig. 6. Effect of the flow rate ratio on sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate.
Water osmosis rate YV
75
Al Mg F S
65
Flow rate ratio (water/feed)
20
V Fe K P
80
+
+
H2SO4 recovery (%)
1.6
75
H in diffusate (mol/L)
+
H in diffusate + H in dialysate
80
85
0.9
1.1
1.3
60 1.5
Flow rate ratio (water/feed) Fig. 7. Effect of the flow rate ratio on the water osmosis rate and ratio of the volume of the dialysate and feed (YV).
external operating conditions. Theoretically speaking, F− ions could readily permeate through the anion exchange membrane due to their negative charge, but this was not the case probably because F− ions formed complexes with Al3 + ions that were present in the acid leaching solution (Wang et al., 2015). F− ions and Al3 + ions can form a series of AlFx(3 − x)+ complexes, depending on the relative content of the two ions. In this solution, AlF2 +, AlF2+ may be the main complex forms, which contributed to the high rejection of F ions due to electrostatic repulsion. However, blockage of a large amount of F ions on the dialysate side was disadvantageous to the separation of H+. HF is a useful component to destroy the crystal structure of vanadium-containing minerals and strengthen the leaching process (Wang et al., 2015). If F ions can pass through the anion exchange membrane in the form of anions, more H+ ions will permeate through the membrane to meet the requirements of solution electrical neutrality due to the principle of diffusion dialysis. In the system containing titanium spent leaching 13
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Under the optimum operating conditions, the sulfuric acid recovery reached 71.12%, water osmosis rate could be controlled at approximately 14.95%, and vanadium rejection was approximately 95.50%. The values of the rejection of impurity ions, such as Al, Fe, Mg, K, F, P, and S, were approximately 99.04%, 97.37%, 98.01%, 85.12%, 98.33%, 91.16%, and 69.96%, respectively. (3) In the diffusion dialysis process, controlling water osmosis was more important than obtaining a higher acid recovery; water osmosis could be reduced by increasing the flow rate. (4) The high rejections of F in the form of complexes and P in the form of incompletely dissociated acid were disadvantageous to the recovery of sulfuric acid.
residue in place of the limestone neutralization process. Because the gypsum produced from the limestone neutralization process contains low contents gallium and indium, it is understood that their recovery from gypsum is impractical. Comparing the high value of gallium and indium with the lower cost of dealing with the diffused solution in this situation, the flow rate ratio can achieve 5 times or even higher in order to separate more sulfuric acid from the process solution of the zinc leaching residue, though this would generate a large quantity of diffused solution. Correspondingly, the concentration of sulfuric acid in the dialyzed solution can be reduced to 0.0125 mol/L or even lower from approximately 0.5 mol/L. In practical applications of this study, the recovered acid was returned to the acid leaching process for reuse. To maintain the equilibrium of water throughout the process, the volume of the recovered acid must be less than the volume of the feed pumped into the dialyzer. Otherwise, the extra recovered acid cannot be consumed completely and still needs to be neutralized. However, compared with the precious metals gallium and indium, the price of vanadium is much lower. Ultimately, the cost is increased. Moreover, a low concentration of recovered acid is unfavorable for utilization in the acid leaching process because more acid is needed to reach a certain concentration. Therefore, taking the above factors into consideration, the optimum flow rate ratio should be controlled in the range of 1 to 1.1. The effect of the flow rate ratio on ion rejection is shown in Fig. 8. According to this Figure, all ion rejection showed a small decrease as the flow rate ratio increased. The increase of the flow rate ratio resulted in an increase of the concentration difference over the membrane which increased the driving force for the diffusion of various ions. The higher P rejection meant that most P in the form of H3PO4, H2PO4−, and so on (Yang et al., 2016b) remained in feed side, resulting in that this part of H+ ions cannot be recovered. However, more S in the form of HSO4− and SO42 − (Wei et al., 2010) permeating through the membrane promoted the diffusion of H+ ions to meet the requirements of solution electrical neutrality due to the principle of diffusion dialysis. This difference is attributed to the fact that the permeability of the membrane to H3PO4 is lower than that to H2SO4 which is mainly related to the sorption properties of membrane to them and the diffusivity of them in membrane as well as the difference of their initial concentrations (Luo et al., 2011b). In the flow rate and flow rate ratio range selected in this paper, V rejection was over 93%, while Al, Fe, Mg and K rejections were over 99%, 97%, 98% and 85%, respectively. This also confirmed the results of the cycling type dynamic diffusion dialysis experiments on the DF120-III membrane separation performance. Diffusion dialysis was performed at the optimum conditions. Table 5 lists the chemical composition of the residual solution and recovered acid. Compared with the initial feed, the acidity of the residual solution was reduced and the vanadium concentration in the residual solution decreased by approximately 13%. However, the impurity ion concentration in the residual solution was still high, so further purification enrichment is needed. Apart from S, the contents of the impurity ions in the recovered acid were relatively low. The recovered acid can be recycled to the acid leaching process by the addition of fresh acid.
Acknowledgements This study was financially supported by a Project of the National Natural Science Foundation of China (No. 51404174 & No. 51474162) and the Key Science and Technology Support Program (No. 2015BAB18B01) from the Ministry of Science and Technology of China. References GB/T 7484-1987, 1987. Water Quality Determination of Fluoride ion Selective Electrode Method. China. (in Chinese). HG/T 5112-2016, 2017. Diffusion Dialysis Anion Exchange Membrane. China. (in Chinese). Li, M.T., Wei, C., Fan, G., Li, C.X., Deng, Z.G., Li, X.B., 2009. Extraction of vanadium from black shale using pressure acid leaching. Hydrometallurgy 98, 308–313. Li, W., Zhang, Y.M., Huang, J., Zhu, X.B., Wang, Y., 2012. Separation and recovery of sulfuric acid from acidic vanadium leaching solution by diffusion dialysis. Sep. Purif. Technol. 96, 44–49. Li, X.B., Deng, Z.G., Wei, C., Li, C.X., Li, M.T., Fan, G., Huang, H., 2015. Solvent extraction of vanadium from a stone coal acidic leach solution using D2EHPA/TBP: continuous testing. Hydrometallurgy 154, 40–46. Luo, J.Y., Wu, C.M., Wu, Y.H., Xu, T.W., 2011a. Diffusion dialysis processes of inorganic acids and their salts: the permeability of different acidic anions. Sep. Purif. Technol. 78, 97–102. Luo, J.Y., Wu, C.M., Xu, T.W., Wu, Y.H., 2011b. Diffusion dialysis-concept, principle and applications. J. Membr. Sci. 366, 1–16. Ma, Y.Q., Wang, X.W., Wang, M.Y., Jiang, C.J., Xiang, X.Y., Zhang, X.L., 2015. Separation of V(IV) and Fe(III) from the acid leach solution of stone coal by D2EHPA/TBP. Hydrometallurgy 153, 38–45. Nightingale, E.R., 1959. Phenomenological theory of ion solvation. effective radii of hydrated ions. J. Phys. Chem. 63 (9), 1381–1387. Oh, S.J., Moon, S.H., Davis, T., 2000. Effects of metal ions on diffusion dialysis of inorganic acids. J. Membr. Sci. 169, 95–105. Tang, J.J., Zhou, K.G., Zhang, Q.X., 2006. Sulfuric acid recovery from rare earth sulphate solutions by diffusion dialysis. Trans. Nonferrous Met. Soc. China 16, 951–955. Wang, F., Zhang, Y.M., Liu, T., Huang, J., Zhao, J., Zhang, G.B., Liu, J., 2015. A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal. Int. J. Miner. Process. 145, 87–93. Wei, C., Li, X.B., Deng, Z.G., Fan, G., Li, M.T., Li, C.X., 2010. Recovery of H2SO4 from an acid leach solution by diffusion dialysis. J. Hazard. Mater. 176, 226–230. Xu, T.W., Yang, W.H., 2001. Sulfuric acid recovery from titanium white (pigment) waste liquor using diffusion dialysis with a new series of anion exchange membranes-static runs. J. Membr. Sci. 183, 193–200. Xu, T.W., Yang, W.H., 2003. Industrial recovery of mixed acid (HF +HNO3) from the titanium spent leaching solutions by diffusion dialysis with a new serious of anion exchange membranes. J. Membr. Sci. 220, 89–95. Xu, T.W., Yang, W.H., 2004. Tuning the diffusion dialysis performance by surface crosslinking of PPO anion exchange membranes―simultaneous recovery of sulfuric acid and nickel from electrolysis spent liquor of relatively low acid concentration. J. Hazard. Mater. B109, 157–164. Xu, J., Fu, D., Lu, S.G., 2009a. The recovery of sulfuric acid from the waste anodic aluminum oxidation solution by diffusion dialysis. Sep. Purif. Technol. 69, 168–173. Xu, J., Lu, S.G., Fu, D., 2009b. Recovery of hydrochloric acid from the waste acid solution by diffusion dialysis. J. Hazard. Mater. 165, 832–837. Yamashita, S., Ishikawa, K., Goto, S., HATA, K., 1996. Basic study on removal of sulfuric acid from process solution of zinc leaching residue by diffusion-dialysis. Metall. Rev. MMIJ 13 (1), 97–112 (Mining and Metallurgical Institute of Japan). Yang, X., Zhang, Y.M., Bao, S.X., 2016a. Separation and recovery of sulfuric acid from acidic vanadium leaching solution of stone coal via solvent extraction. J. Environ. Chem. Eng. 4, 1399–1405. Yang, X., Zhang, Y.M., Bao, S.X., Shen, C., 2016b. Separation and recovery of vanadium from a sulfuric-acid leaching solution of stone coal by solvent extraction using trialkylamine. Sep. Purif. Technol. 164, 49–55. Zhang, Y.M., Bao, S.X., Liu, T., Chen, T.J., Huang, J., 2011. The technology of extracting vanadium from stone coal in China: history, current status and future prospects. Hydrometallurgy 109, 116–124. Zhou, X.Y., Li, C.L., Li, J., Liu, H.Z., Wu, S.Y., 2009. Leaching of vanadium from carbonaceous shale. Hydrometallurgy 99, 97–99.
4. Conclusions In this study, diffusion dialysis was employed to recover sulfuric acid from a stone coal acid leaching solution. Based on the results obtained from the current work, the following conclusions were drawn: (1) The DF120-III anion exchange membrane showed good performance in separating sulfuric acid from a stone coal acid leaching solution. (2) Comprehensively considering the water osmosis rate, sulfuric acid recovery, ion rejection, water equilibrium and processing ability, the optimum operating conditions for this process included a flow rate of 12 mL/min and flow rate ratio of water to feed of 1–1.1. 14
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Recovery of sulfuric acid from a stone coal acid leaching solution by diffusion dialysis
MARK
Kui Wanga,c,d, Yimin Zhanga,b,c,d,⁎, Jing Huanga,c,d, Tao Liua,c,d, Jingpeng Wanga,c,d a
School of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China c Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Wuhan 430081, China d Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan 430081, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Diffusion dialysis Sulfuric acid recovery Anion exchange membrane Stone coal Acid leaching solution
Diffusion dialysis was employed to recover sulfuric acid from a stone coal acid leaching solution. The dialysis coefficients of H+, V, Al, Fe, Mg, K, F, P, and S ions in a stone coal acid leaching solution for an anion exchange membrane were determined. The effects of the flow rate, flow rate ratio, and water osmosis rate as well as ion rejection on sulfuric acid recovery were investigated. The results demonstrated that the DF120-III anion exchange membrane showed a good separation performance for separating sulfuric acid from the stone coal acid leaching solution. In the diffusion dialysis process, the water osmosis rate, sulfuric acid recovery, ion rejection, water equilibrium and processing ability were taken into consideration. Controlling water osmosis was more important than obtaining a higher acid recovery. Under the optimum operating conditions of a feed flow rate of 12 mL/min and flow rate ratio of water to feed of 1–1.1, sulfuric acid recovery reached 71.12%, the water osmosis rate was controlled at approximately 14.95%, and vanadium rejection was approximately 95.50%. The rejection of impurity ions, such as Al, Fe, Mg, K, F, P, and S was approximately 99.04%, 97.37%, 98.01%, 85.12%, 98.33%, 91.16%, and 69.96%, respectively. The high rejections of F in the form of complexes and P in the form of incompletely dissociated acid were disadvantageous to the recovery of sulfuric acid. The recovered acid was able to be reused in the acid leaching process by the addition of fresh acid.
1. Introduction In China, in addition to vanadium‑titanium magnetite, stone coal is an important vanadium-bearing resource due to its vast reserves and wide distribution (Zhang et al., 2011). Currently, acid leaching of stone coal with H2SO4 is the most common method in vanadium metallurgical plants because of its high vanadium recovery and low pollution (Zhou et al., 2009; Ma et al., 2015; Li et al., 2009; Li et al., 2015). However, a quantity of residual free sulfuric acid is present in acid leaching solution after leaching with H2SO4, and the acidity of the acid leaching solution is relatively high, making it unsuitable for the subsequent extraction of vanadium. Currently, a conventional and popular method to treat the acid leaching solution is neutralization with ammonia water or lime. However, such a neutralization process will generate a large quantity of highly concentrated ammonia‑nitrogen wastewater or vanadium-containing gypsum, which cause disposal problems and serious environmental pollution. Moreover, valuable resources, such as sulfuric acid, bases and a portion of vanadium, are wasted (Wei et al., 2010; Li et al., 2012; Yang et al., 2016a).
⁎
Obviously, a new method is needed to both improve the economics of the process as well as avoid the vanadium loss and environmental pollution caused by the neutralization process. Diffusion dialysis with an anion exchange membrane seems to be a desirable method due to its low energy consumption, low installation and operating costs and lack of pollution (Luo et al., 2011a). It has been widely exploited to recover acid from metal treatment wastes, such as waste streams generated in steel, metal-refining, and electroplating industries (Oh et al., 2000; Xu and Yang, 2001, 2003; Xu et al., 2009a, 2009b). However, as opposed to recovering acid from general waste acid systems, the stone coal acid leaching solution is a mineral leaching solutions that is used with the ultimate goal of separating valuable metal resources from it rather than recovering acid from it and discarding it; the residual solution will be used in the subsequent separation and enrichment process of vanadium and the recovered acid should be able to be reused in the ore acid leaching process. In this way, a closed-circuit is created, and the economic and environmental effects are remarkable. Studies have been performed on the recovery of sulfuric acid from a stone coal acid leaching solution by diffusion dialysis. Wei et al. (2010)
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.hydromet.2017.07.005 Received 13 March 2017; Received in revised form 16 July 2017; Accepted 24 July 2017 Available online 25 July 2017 0304-386X/ © 2017 Published by Elsevier B.V.
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Peristaltic pump P
Diffusate cell
Dialysate cell
investigated the effects of the sulfuric acid, FeSO4 and VOSO4 concentrations; flow rate; and flow rate ratio on the recovery of H2SO4. Acid recovery exceeded 80%. The V and Fe ion rejection levels were within 93–95% and 92–94%, respectively. Li et al. (2012) investigated the effects of the flow rate, flow rate ratio, as well as V, Al and Fe ion concentrations on the recovery of H2SO4 and metal ion rejection. Over an 84% H2SO4 recovery efficiency was achieved. The V, Al and Fe ion rejection values reached 93%, 92% and 85%, respectively. However, these authors did not conduct detailed research on the behavior of vanadium and the main impurity ions (Al, Fe, Mg, K, F, P, S), the volumetric expansion of the acid leaching solution because of water osmosis and the effects on sulfuric acid recovery. However, these factors will directly affect the subsequent separation and enrichment of vanadium as well as the utilization of the recovered acid. Therefore, in this study, the dialysis coefficients of H+,V, Al, Fe, Mg, Na, K, F, P, and S ions in a stone coal acid leaching solution for the anion exchange membrane were determined in a cycling type dynamic diffusion dialysis experiment to investigate the separation performance of the anion exchange membrane. Furthermore, one-pass type dynamic diffusion dialysis experiments were conducted to investigate the factors that impacted the recovery of sulfuric acid, water osmosis and ion rejection.
P
Peristaltic pump
Membrane
Fig. 1. The schematic of the cycling type dynamic diffusion dialysis system.
late stage of the experiment, only the first six sets of data were analyzed. The schematic of the cycling type dynamic diffusion dialysis system is shown in Fig. 1.
2.2.2. One-pass type dynamic diffusion dialysis experiments One-pass type dynamic diffusion dialysis experiments were conducted using a HKY-001 diffusion dialyzer supplied by Shandong Tianwei Membrane Technology Co., Ltd. (Shandong, China). The diffusion dialyzer was separated by 19 sheets of an anion exchange membrane (DF120-III, Shandong, China) into dialysate cells and diffusate cells through which the feed and running water, respectively, passed in a countercurrent fashion. The effective area of each membrane for mass transfer was 0.08 m2 (0.2 × 0.4), and the total effective membrane was 1.52 m2 (19 × 0.2 × 0.4). During the tests, the diffusion dialyzer was initially fed with feed (0.7 L) and running water (0.7 L) and maintained for 2 h to reach equilibrium. Then, the test began and proceeded until a dynamic equilibrium state was reached; then, samples were taken. The schematic of the one-pass type dynamic diffusion dialysis experimental set-up is shown in Fig. 2. The DF120-III anion exchange membrane was produced from engineering polymer poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by bromination and amination. The functional group attached on the anion exchange membrane was a quaternary ammonium group, and the physical aperture was approximately 5 nm. The membrane's lifetime is up to 5 years (Wei et al., 2010). Its characteristics are shown in Table 2 (Xu et al., 2009a; Li et al., 2012). The membranes used in all experiments were from the same production batch. All experiments were conducted at an ambient temperature of approximately 25 °C.
2. Experimental 2.1. Materials The solution used for cycling type dynamic diffusion dialysis and one-pass type dynamic diffusion dialysis was the actual acid leaching solution generated from the stone coal direct acid leaching process. Before the experiments, the acid leaching solution was filtered through a 0.45 μm pore size filter membrane to remove suspended solids and prevent blockage of the membrane. The concentrations of various ions in the solution are listed in Table 1. 2.2. Diffusion dialysis experiments 2.2.1. Cycling type dynamic diffusion dialysis experiment The cycling type dynamic diffusion experiment was carried out using a custom-built mini diffusion dialyzer. The diffusion dialyzer was separated by 20 sheets of an anion exchange membrane (DF120-III, Shandong, China) into dialysate cells and diffusate cells through which the feed and de-ionized water, respectively, passed in a countercurrent fashion. The effective area of each membrane for mass transfer was 0.0112 m2 (0.56 × 0.1 × 0.2), and the total effective membrane area was 0.224 m2. Feed (1.5 L) and de-ionized water (1.5 L) were cycled, respectively, in dialysate cells and diffusate cells by a peristaltic pump with identical flow rates of 75 mL/min. The whole experiment ran continuously for 6 h. Small amounts of samples were taken from the dialysate and the diffusate to analyze the H+ concentration and other ion concentrations every 30 min. Due to the severe water osmosis at the
Water
Peristaltic pump
P
Acid leaching solution Exhaust vent
P
Peristaltic pump
Table 1 The chemical composition of the acid leaching solution used in the experiment. Element +
H V Al Fe Mg K F P S
Concentration (mg/L) 1750 2101.81 17,560 5828 4503 6779 11,244.6 1463 61,670
Diffusion dialyzer
Residual solution
Recovered acid
Fig. 2. The schematic of the one-pass type dynamic diffusion dialysis experimental set-up.
10
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Table 2 The characteristics of the DF120-III anion exchange membrane.
Table 3 Dialysis coefficients of different ions and the corresponding separation factors.
Item
Specification
Ion
Dialysis coefficient Ui (m/h)
Separation factor UH+/Ui
Water content (%) Thickness (mm) Transference number of ions in membrane Membrane area resistance (Ω cm2) Ion exchange capacity (mmol/g) Burst strength (MPa) Selective penetration (%) Potential of membrane (mV)
42.34 0.20–0.23 0.98 3.4 (16 °C) or 2.7 (25 °C) 1.7–1.9 > 0.9 96.2 14.9 (14 °C)
H+ V Al Fe Mg K F P S
2.230 × 10− 3 6.864 × 10− 5 5.525 × 10− 5 4.654 × 10− 5 4.621 × 10− 5 1.510 × 10− 4 1.038 × 10− 5 1.092 × 10− 4 5.809 × 10− 4
− 32.49 40.36 47.92 48.26 14.77 214.84 20.42 3.84
2.3. Analyses and calculations 3. Results and discussion The concentration of sulfuric acid was determined by titration using 0.1 mol/L Na2CO3 with methyl orange as an indicator. The vanadium concentration in the aqueous phase was determined by ferrous ammonium sulfate titration. The concentration of fluoride was measured with a fluoride ion selective electrode (GB/T 7484-1987, 1987). The concentration of the other elements in the acid solution was analyzed by using ICP-AES. Without accounting for the volume changes, according to Fick's law, the dialysis coefficient of substance i can be calculated by the following equation (Xu et al., 2009a; Tang et al., 2006):
V1V2 ΔCi0 ln (V1 + V2)St ΔCit
Ui =
3.1. Cycling type dynamic diffusion dialysis In the diffusion dialysis process, the anion exchange membrane is the key component. The anion exchange membrane has to have high proton permeability but strongly reject salts (Xu and Yang, 2004). The dialysis coefficient is an important parameter in the diffusion dialysis process, as it is related to the membrane, species and concentrations of the diffusion materials, temperature and solution system. It represents the transport characteristics, including the diffusion coefficients of acid and metal salts through the membrane as well as the sorption equilibrium (Oh et al., 2000). The dialysis coefficients of different ions for the membrane and the corresponding separation factors that were calculated according to the results of the experiment are presented in Table 3. As Table 3 shows, the dialysis coefficient of the H+ ion is the largest of all of the investigated ions, and the separation factor between H+ and S is the smallest. The dialysis coefficient of monovalent K+ ion is larger than that of the other metal cations. Separation factors are observed over the range of 14.77 to 48.26 for metal ions. The separation factor between H+ and F is as high as 214.84. Although positively charged, H+ ions show higher competition in diffusion than metal ions because of their smaller size, lower valence state and higher mobility and diffuse along with the SO42 − to meet the requirement of solution electrical neutrality (Luo et al., 2011b). A larger separation factor represents more efficient separation of an acid and metal salt (Oh et al., 2000). Based on these results, it is observed that the DF120-III anion exchange membrane has high proton permeability and strongly rejects salts; the membrane also functioned efficiently regarding the separation of sulfuric acid from the stone coal acid leaching solution.
(1)
where Ui is the dialysis coefficient of substance i (m/h); V1 and V2 are the volumes of the dialysate and the diffusate (m3), respectively; ΔCi0 and ΔCit are the concentration gradient of substance i between the dialysate and the diffusate at time 0 and t, respectively (mol/L); S is the effective membrane area (m2); and t is the time (h). Then, Eq. (1) can be transformed to Eq. (2):
ln ΔCit = ln ΔCi0 − Ui S
V1 + V2 t V1V2
(2)
According to Eq. (2), lnΔCit is linear with respect to t, and the dialysis coefficient Ui can be calculated using the method of linear regression. The separation factor was determined to be the ratio of the dialysis coefficient for H+ to that for substance i in this study (HG/T 5112-2016, 2017). The total acid recovery ratio R was calculated by the following equation:
R=
Q r × CH r × 100% H Q r × CH r + Q d × Cd
(3) 3.2. One-pass type dynamic diffusion dialysis
where Qr is the flow rate of the recovered acid (mL/min), Qd is the flow rate of the dialysate (mL/min), and CrH and CdH are the H+ concentrations in the recovered acid and dialysate, respectively (mol/L). The ion rejection ratio was calculated as follows:
η=
Qd × CId × 100% Q r × CIr + Q d × CId
3.2.1. Effect of the flow rate on the diffusion dialysis performance To investigate the effect of the flow rate on the diffusion dialysis process, several tests were conducted. During the tests, the flow rate ratio of water to feed was maintained at 1.0. Fig. 3 shows the variation of the sulfuric acid recovery and H+ concentration in the recovered acid and dialysate as the flow rate increases. We observed that the sulfuric acid recovery increased with the increase of the flow rate and reached a maximum at 8 mL/min; the sulfuric acid recovery was 73.50%. Over this flow rate, the sulfuric acid recovery began to decrease. The H+ concentration in the recovered acid decreased from 1.80 mol/L to 1.24 mol/L. The H+ concentration in the dialysate decreased first and then increased; it reached a minimum at 8 mL/min, and the H+ concentration in the dialysate was 0.37 mol/L. When the flow rate increased, the thicknesses of the two concentration boundary layers over the membrane decreased and the mass transfer resistance weakened, which was beneficial to the diffusion of H+ ions. Since the surface area of the membrane is constant, too high of a flow rate could shorten the residence time for the feed passing through the
(4)
where CrI and CdI are the concentrations of I ions in the recovered acid and dialysate, respectively (mol/L). The water osmosis rate was used to quantify the extent of water osmosis and label the extent of the volumetric expansion of the dialysate and was calculated as follows:
Q δ = ⎛ d − 1⎞ × 100% ⎠ ⎝ Qf ⎜
⎟
(5)
where Qf and Qd are the flow rates of the feed and the dialysate, respectively (mL/min). The residence time was defined as the ratio of the volume of the dialysate chamber to the flow rate of the feed. 11
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2.1
75
reduced the volume of the recovered acid, which was apparently detrimental to the process. Therefore, controlling water osmosis is more important than obtaining a higher acid recovery. Under the low flow rate conditions, a large amount of water transported from the diffusate side diluted the dialysate, the solution concentration difference on both sides of the membrane subsequently decreased. According to the principle of diffusion dialysis, the driving force for the diffusion of H+ ions decreased and sulfuric acid recovery decreased (as shown in Fig. 3). Although sulfuric acid recovery reached its highest value at 8 mL/min (as shown in Fig. 3), water osmosis still occurred, and the water osmosis rate was 24.23%. Under the high flow rate conditions, the water osmosis rate decreased, but acid recovery decreased as well. The flow rate also reflected the processing ability of the dialyzer per unit time. Therefore, a suitable feed flow rate was needed to control the relatively low water osmosis rate, achieve relatively high sulfuric acid recovery and guarantee a relatively high processing ability. Taking all of the above factors into consideration, 12 mL/min was chosen as the optimum flow rate, and the water osmosis rate was 14.95% under this condition. Ion rejection, especially V rejection, is another important concern and is related to the subsequent extraction of vanadium and whether the recovered acid can be reused in the acid leaching process. If the concentration of impurity ions in the recovered acid is high, a decrease in the extraction performance will occur due to the accumulation of impurity ions in the acid leaching solution. The effect of the flow rate on ion rejection is shown in Fig. 5. From this figure, we observe that all of the ion rejection values increased as the flow rate increased. Among them, V rejection increased from 92.62% to 96.39%. Al, Mg, Fe ions rejections were all > 96%. K and P rejections obviously increased. F rejection was > 98%. S rejection was the smallest. With an increasing flow rate, the residence time became shorter for the feed passing through the dialysate unit. The effect of ion rejection was enhanced since there was not sufficient time for the mass transfer through the membrane. K rejection was remarkably lower than that of other metal ion rejection. The hydrated radii and charge valences for the ions K+, VO2 +, Al3 +, Fe3 +, Fe2 +, and Mg2 + are collected in Table 4 (Nightingale, 1959). In comparison with K+, the VO2 +, Al3 +, Fe3 +, Fe2 + and Mg2 + ions were more effectively rejected by the positively charged fixed groups on the membrane since their higher charges make them suffer greater electrostatic repulsion. In addition to valence, the hydrated radii of their complexes and themselves were greater than the hydrated radius of K+, which also increased their resistance, affecting their ability to penetrate the membrane. Hence, K+ ions transported through the membrane into the diffusate more easily under the same
H2SO4 recovery H in diffusate + H in dialysate
1.7
69
1.5
67
1.3
1.1
65 3
5
7
9
11
13
15
17
0.45
0.40
+
71
0.50
H in dialyste (mol/L)
1.9
+
73
H in diffusate (mol/L)
H2SO4 recovery (%)
+
0.35
19
Flow rate (mL/min) Fig. 3. Effect of the flow rate on sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate.
dialysate unit. Thus, there was not sufficient time for the H+ ions to permeate through the membrane, and this resulted in a decrease of sulfuric acid recovery and the H+ concentration in the recovered acid and an increase of the H+ concentration in the dialysate. The volume of the dialysate expanded and the volume of the diffusate reduced during the diffusion dialysis process. There was competition between water osmosis from the diffusate to the dialysate due to the osmosis pressure and water transport from the dialysate to the diffusate due to the diffusion of hydrated ions (acid or salt) (Xu and Yang, 2004). Fig. 4 shows the variation of the water osmosis rate and ratio of the vanadium concentration in the dialysate and feed with the increase of flow rate. The water osmosis rate decreased as the flow rate increased and reached a stable value of approximately 10% over a flow rate of 14 mL/min. YC increased from 68.20% to 93.49%. Obviously, water osmosis was reduced by increasing the flow rate. Water osmosis was related to the feed concentration (osmosis pressure difference) and membrane hydrophilicity; a large concentration difference or a membrane with a higher water content permitted a larger water osmosis flux (Xu and Yang, 2004). In diffusion dialysis of the stone coal acid leaching solution, water osmosis is a key factor that affects the operation. In contrast to recovering most of the acid from waste and discarding the waste, the dialysate in this experiment was used in the subsequent separation and enrichment process of vanadium. The recovered acid was returned to the acid leaching process. The stone coal acid leaching solution had a high salt content, which contributed to a high osmotic pressure. Water osmosis not only decreased the vanadium concentration but also expanded the volume of the dialysate and
100 40
95
35
90
Ion rejection (%)
90 30 85
25 80
20
YC (%)
Water osmosis rate (%)
95
Water osmosis rate YC
75
15
85
V Fe K P
80 75
Al Mg F S
70
10
70
5
65 3
5
7
9
11
13
15
17
65
19
5
Flow rate (mL/min)
7
9
11
13
15
Flow rate (mL/min)
Fig. 4. Effect of the flow rate on the water osmosis rate and ratio of the vanadium concentration in the dialysate and feed (YC).
Fig. 5. Effect of the flow rate on ion rejection.
12
17
19
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100
Table 4 The hydrated radii and charge valences for K+, VO2 +, Al3 +, Fe3 +, Fe2 +, and Mg2 +. Ion
Ion hydrated radius (nm)
Valence
K+ VO2 + Al3 + Fe3 + Fe2 + Mg2 +
0.331 – 0.475 0.457 0.428 0.428
1 2 3 3 2 2
90
Ion rejection (%)
1.8
85
95
0.7
H2SO4 recovery
1.4
65 1.2
0.5
0.4
60
55 0.7
0.9
1.1
1.0 1.5
1.3
H in dialysate (mol/L)
70
0.6
0.7
1.0
1.1
1.2
1.3
1.4
1.5
Table 5 The chemical composition of the residual solution and recovered acid.
0.3
120
Element
Residual solution (mg/L)
Recovered acid (mg/L)
H+ V Al Fe Mg K F P S
480 1832.15 15,810 5203 4017 5688 9519.55 1198 37,920
1520 128.25 211.9 194.2 112.8 1372 119.09 160.3 22,470
110
solutions composed of 330 g/L HNO3 and HF 3 g/L (Xu and Yang, 2003), the recovery ratio of HF reached 90%.
10 90
YV (%)
Water osmosis rate (%)
0.9
Fig. 8. Effect of the flow rate ratio on ion rejection.
100
3.2.2. Effect of the flow rate ratio (water/feed) on the diffusion dialysis performance To investigate the effect of the flow rate ratio on the diffusion dialysis process, several tests were conducted. During the tests, the flow rate of the feed was maintained at 12 mL/min. Fig. 6 shows the variation of sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate as the flow rate ratio increases. With the increase of the flow rate ratio, the sulfuric acid recovery increased from 60.63% to 80.64% and the H+ concentration in the recovered acid decreased from 1.70 mol/L to 1.17 mol/L. The H+ concentration in the dialysate decreased from 0.62 mol/L to 0.315 mol/ L. An increase in the flow rate ratio means that the contact time for a given volume of feed is elongated. Moreover, the H2SO4 concentration in the recovered acid decreased and the difference in concentration over the membrane increased. The driving force for the diffusion of H+ ions increased, and therefore, more H+ ions permeated the membrane into the recovered acid. Fig. 7 shows the variation of the water osmosis rate and ratio of the volume of the dialysate and feed (YV) with the increased flow rate ratio. As Fig. 7 shows, the water osmosis rate remained approximately unchanged. YV showed a close to linear increase. When the flow rate ratio was 1.1, the water osmosis rate was 13.58% and YV was 95.83%. A higher flow rate ratio prompts the recovery of sulfuric acid (as shown in Fig. 6), but also resulted in a conspicuous increase in the volume of the recovered acid and a significant decrease of the H+ concentration in the recovered acid. Yamashita et al. (1996) utilized diffusion dialysis to separate sulfuric acid from the process solution of a zinc leaching
80
5
70
0 0.7
0.8
Flow rate ratio (water/feed)
130
15
70
60
Fig. 6. Effect of the flow rate ratio on sulfuric acid recovery and the H+ concentration in the recovered acid and dialysate.
Water osmosis rate YV
75
Al Mg F S
65
Flow rate ratio (water/feed)
20
V Fe K P
80
+
+
H2SO4 recovery (%)
1.6
75
H in diffusate (mol/L)
+
H in diffusate + H in dialysate
80
85
0.9
1.1
1.3
60 1.5
Flow rate ratio (water/feed) Fig. 7. Effect of the flow rate ratio on the water osmosis rate and ratio of the volume of the dialysate and feed (YV).
external operating conditions. Theoretically speaking, F− ions could readily permeate through the anion exchange membrane due to their negative charge, but this was not the case probably because F− ions formed complexes with Al3 + ions that were present in the acid leaching solution (Wang et al., 2015). F− ions and Al3 + ions can form a series of AlFx(3 − x)+ complexes, depending on the relative content of the two ions. In this solution, AlF2 +, AlF2+ may be the main complex forms, which contributed to the high rejection of F ions due to electrostatic repulsion. However, blockage of a large amount of F ions on the dialysate side was disadvantageous to the separation of H+. HF is a useful component to destroy the crystal structure of vanadium-containing minerals and strengthen the leaching process (Wang et al., 2015). If F ions can pass through the anion exchange membrane in the form of anions, more H+ ions will permeate through the membrane to meet the requirements of solution electrical neutrality due to the principle of diffusion dialysis. In the system containing titanium spent leaching 13
Hydrometallurgy 173 (2017) 9–14
K. Wang et al.
Under the optimum operating conditions, the sulfuric acid recovery reached 71.12%, water osmosis rate could be controlled at approximately 14.95%, and vanadium rejection was approximately 95.50%. The values of the rejection of impurity ions, such as Al, Fe, Mg, K, F, P, and S, were approximately 99.04%, 97.37%, 98.01%, 85.12%, 98.33%, 91.16%, and 69.96%, respectively. (3) In the diffusion dialysis process, controlling water osmosis was more important than obtaining a higher acid recovery; water osmosis could be reduced by increasing the flow rate. (4) The high rejections of F in the form of complexes and P in the form of incompletely dissociated acid were disadvantageous to the recovery of sulfuric acid.
residue in place of the limestone neutralization process. Because the gypsum produced from the limestone neutralization process contains low contents gallium and indium, it is understood that their recovery from gypsum is impractical. Comparing the high value of gallium and indium with the lower cost of dealing with the diffused solution in this situation, the flow rate ratio can achieve 5 times or even higher in order to separate more sulfuric acid from the process solution of the zinc leaching residue, though this would generate a large quantity of diffused solution. Correspondingly, the concentration of sulfuric acid in the dialyzed solution can be reduced to 0.0125 mol/L or even lower from approximately 0.5 mol/L. In practical applications of this study, the recovered acid was returned to the acid leaching process for reuse. To maintain the equilibrium of water throughout the process, the volume of the recovered acid must be less than the volume of the feed pumped into the dialyzer. Otherwise, the extra recovered acid cannot be consumed completely and still needs to be neutralized. However, compared with the precious metals gallium and indium, the price of vanadium is much lower. Ultimately, the cost is increased. Moreover, a low concentration of recovered acid is unfavorable for utilization in the acid leaching process because more acid is needed to reach a certain concentration. Therefore, taking the above factors into consideration, the optimum flow rate ratio should be controlled in the range of 1 to 1.1. The effect of the flow rate ratio on ion rejection is shown in Fig. 8. According to this Figure, all ion rejection showed a small decrease as the flow rate ratio increased. The increase of the flow rate ratio resulted in an increase of the concentration difference over the membrane which increased the driving force for the diffusion of various ions. The higher P rejection meant that most P in the form of H3PO4, H2PO4−, and so on (Yang et al., 2016b) remained in feed side, resulting in that this part of H+ ions cannot be recovered. However, more S in the form of HSO4− and SO42 − (Wei et al., 2010) permeating through the membrane promoted the diffusion of H+ ions to meet the requirements of solution electrical neutrality due to the principle of diffusion dialysis. This difference is attributed to the fact that the permeability of the membrane to H3PO4 is lower than that to H2SO4 which is mainly related to the sorption properties of membrane to them and the diffusivity of them in membrane as well as the difference of their initial concentrations (Luo et al., 2011b). In the flow rate and flow rate ratio range selected in this paper, V rejection was over 93%, while Al, Fe, Mg and K rejections were over 99%, 97%, 98% and 85%, respectively. This also confirmed the results of the cycling type dynamic diffusion dialysis experiments on the DF120-III membrane separation performance. Diffusion dialysis was performed at the optimum conditions. Table 5 lists the chemical composition of the residual solution and recovered acid. Compared with the initial feed, the acidity of the residual solution was reduced and the vanadium concentration in the residual solution decreased by approximately 13%. However, the impurity ion concentration in the residual solution was still high, so further purification enrichment is needed. Apart from S, the contents of the impurity ions in the recovered acid were relatively low. The recovered acid can be recycled to the acid leaching process by the addition of fresh acid.
Acknowledgements This study was financially supported by a Project of the National Natural Science Foundation of China (No. 51404174 & No. 51474162) and the Key Science and Technology Support Program (No. 2015BAB18B01) from the Ministry of Science and Technology of China. References GB/T 7484-1987, 1987. Water Quality Determination of Fluoride ion Selective Electrode Method. China. (in Chinese). HG/T 5112-2016, 2017. Diffusion Dialysis Anion Exchange Membrane. China. (in Chinese). Li, M.T., Wei, C., Fan, G., Li, C.X., Deng, Z.G., Li, X.B., 2009. Extraction of vanadium from black shale using pressure acid leaching. Hydrometallurgy 98, 308–313. Li, W., Zhang, Y.M., Huang, J., Zhu, X.B., Wang, Y., 2012. Separation and recovery of sulfuric acid from acidic vanadium leaching solution by diffusion dialysis. Sep. Purif. Technol. 96, 44–49. Li, X.B., Deng, Z.G., Wei, C., Li, C.X., Li, M.T., Fan, G., Huang, H., 2015. Solvent extraction of vanadium from a stone coal acidic leach solution using D2EHPA/TBP: continuous testing. Hydrometallurgy 154, 40–46. Luo, J.Y., Wu, C.M., Wu, Y.H., Xu, T.W., 2011a. Diffusion dialysis processes of inorganic acids and their salts: the permeability of different acidic anions. Sep. Purif. Technol. 78, 97–102. Luo, J.Y., Wu, C.M., Xu, T.W., Wu, Y.H., 2011b. Diffusion dialysis-concept, principle and applications. J. Membr. Sci. 366, 1–16. Ma, Y.Q., Wang, X.W., Wang, M.Y., Jiang, C.J., Xiang, X.Y., Zhang, X.L., 2015. Separation of V(IV) and Fe(III) from the acid leach solution of stone coal by D2EHPA/TBP. Hydrometallurgy 153, 38–45. Nightingale, E.R., 1959. Phenomenological theory of ion solvation. effective radii of hydrated ions. J. Phys. Chem. 63 (9), 1381–1387. Oh, S.J., Moon, S.H., Davis, T., 2000. Effects of metal ions on diffusion dialysis of inorganic acids. J. Membr. Sci. 169, 95–105. Tang, J.J., Zhou, K.G., Zhang, Q.X., 2006. Sulfuric acid recovery from rare earth sulphate solutions by diffusion dialysis. Trans. Nonferrous Met. Soc. China 16, 951–955. Wang, F., Zhang, Y.M., Liu, T., Huang, J., Zhao, J., Zhang, G.B., Liu, J., 2015. A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal. Int. J. Miner. Process. 145, 87–93. Wei, C., Li, X.B., Deng, Z.G., Fan, G., Li, M.T., Li, C.X., 2010. Recovery of H2SO4 from an acid leach solution by diffusion dialysis. J. Hazard. Mater. 176, 226–230. Xu, T.W., Yang, W.H., 2001. Sulfuric acid recovery from titanium white (pigment) waste liquor using diffusion dialysis with a new series of anion exchange membranes-static runs. J. Membr. Sci. 183, 193–200. Xu, T.W., Yang, W.H., 2003. Industrial recovery of mixed acid (HF +HNO3) from the titanium spent leaching solutions by diffusion dialysis with a new serious of anion exchange membranes. J. Membr. Sci. 220, 89–95. Xu, T.W., Yang, W.H., 2004. Tuning the diffusion dialysis performance by surface crosslinking of PPO anion exchange membranes―simultaneous recovery of sulfuric acid and nickel from electrolysis spent liquor of relatively low acid concentration. J. Hazard. Mater. B109, 157–164. Xu, J., Fu, D., Lu, S.G., 2009a. The recovery of sulfuric acid from the waste anodic aluminum oxidation solution by diffusion dialysis. Sep. Purif. Technol. 69, 168–173. Xu, J., Lu, S.G., Fu, D., 2009b. Recovery of hydrochloric acid from the waste acid solution by diffusion dialysis. J. Hazard. Mater. 165, 832–837. Yamashita, S., Ishikawa, K., Goto, S., HATA, K., 1996. Basic study on removal of sulfuric acid from process solution of zinc leaching residue by diffusion-dialysis. Metall. Rev. MMIJ 13 (1), 97–112 (Mining and Metallurgical Institute of Japan). Yang, X., Zhang, Y.M., Bao, S.X., 2016a. Separation and recovery of sulfuric acid from acidic vanadium leaching solution of stone coal via solvent extraction. J. Environ. Chem. Eng. 4, 1399–1405. Yang, X., Zhang, Y.M., Bao, S.X., Shen, C., 2016b. Separation and recovery of vanadium from a sulfuric-acid leaching solution of stone coal by solvent extraction using trialkylamine. Sep. Purif. Technol. 164, 49–55. Zhang, Y.M., Bao, S.X., Liu, T., Chen, T.J., Huang, J., 2011. The technology of extracting vanadium from stone coal in China: history, current status and future prospects. Hydrometallurgy 109, 116–124. Zhou, X.Y., Li, C.L., Li, J., Liu, H.Z., Wu, S.Y., 2009. Leaching of vanadium from carbonaceous shale. Hydrometallurgy 99, 97–99.
4. Conclusions In this study, diffusion dialysis was employed to recover sulfuric acid from a stone coal acid leaching solution. Based on the results obtained from the current work, the following conclusions were drawn: (1) The DF120-III anion exchange membrane showed good performance in separating sulfuric acid from a stone coal acid leaching solution. (2) Comprehensively considering the water osmosis rate, sulfuric acid recovery, ion rejection, water equilibrium and processing ability, the optimum operating conditions for this process included a flow rate of 12 mL/min and flow rate ratio of water to feed of 1–1.1. 14