Cleaner alumina production from coal fly ash: Membrane electrolysis designed for sulfuric acid leachate

Journal of Cleaner Production 243 (2020) 118470

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Cleaner alumina production from coal fly ash: Membrane electrolysis designed for sulfuric acid leachate Yuan Shi a, Kai-xi Jiang a, b, Ting-an Zhang a, *, Guo-zhi Lv a a

Key Laboratory of Ecological Metallurgy of Multi-metal Intergrown Ores of Ministry of Education, Special Metallurgy and Process Engineering Institute, Northeastern University, Shenyang, 110819, China b China National Gold Group Co., Ltd, Beijing, 100000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2018 Received in revised form 19 August 2019 Accepted 17 September 2019 Available online 18 September 2019

The clean and efficient extraction of alumina from coal fly ash renders environmental and economic benefits. The current study mainly aimed to develop a nonhazardous approach to conduct electrolysis with two-membrane and three-chamber cell for alumina extraction from coal fly ash. The influences of the aluminum sulfate concentration, distance between electrodes, current density, temperature and electrolysis duration were systematically explored. The results revealed that Al(OH)3 was a main electrolytic product with a yield of 64.48% and the energy consumption was 4.88 kWh/kg Al(OH)3 after 20 h of electrolysis process. The concentrations of ions were determined by inductively coupled plasma atomic emission spectrometry, indicating that Al3þ in the middle chamber migrated to the cathodic chamber to form Al(OH)3 precipitate under the combined action of direct current and the exchange membranes. The electrolysis mechanism was analyzed by cyclic voltammetry, which showed that H2O gained electrons and reacted with Al3þ on the cathode to form Al(OH)3 and H2. On the anode side, H2SO4 and O2 were generated due to the oxidation of H2O. Al(OH)3 is an alumina resource, H2 is a clean energy, and H2SO4 is a common coal fly ash leaching agent and can be returned to the leaching process to promote zero pollution discharge. The solution in the middle chamber can be refreshed to attain continuous electrolysis. The proposed electrolysis process provides an efficient and environmentally friendly approach for the extraction of high-quality Al(OH)3 from coal fly ash. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: CT Lee Keywords: Coal fly ash Membrane Electrolysis Aluminum hydroxide Sulfuric acid

1. Introduction Coal fly ash (CFA) is a residue generated from the combustion of coal in power plants. More than 800 Mt of CFA is produced per annum worldwide (Belviso, 2017) and 580 Mt are generated annually in China (Yao et al., 2014). CFA is considered as one of the largest solid waste resources. The large amounts of CFA contribute to numerous environmental problems (Grethel et al., 2018) and need to be disposed of properly. Unfortunately, only 50% of CFA is utilized (J.W. Chen et al., 2018). A substantial amount of CFA is disposed of in landfills and impoundments (Almahayni and Vanhoudt, 2018), which causes adverse environmental and health hazards (Zhu et al., 2018). A significant amount of attention is being paid to the natural environment by global policy makers. The development of eco-

* Corresponding author. E-mail address: [email protected] (T.-a. Zhang). https://doi.org/10.1016/j.jclepro.2019.118470 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

friendly applications for CFA disposal has sparked worldwide interest. Several innovative applications, such as iron and aluminum oxide separation (Y.G. Chen et al., 2018), construction (Quesada et al., 2018) and building materials (Cheng et al., 2018), catalysis (Asl et al., 2018) and adsorbents for wastewater treatment (Wang et al., 2018), have emerged as potential candidates for CFA disposal. Among these applications, the recovery of alumina from CFA is being widely investigated because CFA is mainly composed of SiO2, Al2O3, FeO, MgO and CaO (Ma et al., 2018a,b) and the alumina component of high-alumina CFA exceeds 40%, even reaching 60%. 20 Mt high-alumina CFA is produced annually (Hua et al., 2018) in Inner Mongolia and Shanxi provinces, China (Liu et al., 2017), which makes it a potential substitute for bauxite. China's aluminum industries produce approximately half of the world's primary aluminum output. However, China's current domestic bauxite reserves will last for only 18 y (Paraskevas et al., 2016). Alumina extraction from high-alumina CFA can solve the problem of bauxite shortages, avoid excess land occupancy and reduce environmental pollution.

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Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

The major methods of alumina recovery from CFA can be classified into acid method (Bojinova and Teodosieva, 2016), alkali method (Li et al., 2014), and acid-alkali combination method (Han et al., 2018). The alkali method is an industrially mature technology in China, it raises some serious issues, such as high energy consumption, complexity and secondary pollution. Moreover, it requires a large amount of calcium oxide and sodium carbonate to be added to the CFA during the alkaline sintering process, which results in a large amount of residue (Jiang et al., 2015). The alkali method cannot handle the large amounts of waste CFA. The acid method can decrease the quantity of the residue. Thus, the acid method is also widely used to extract alumina from CFA (Yao et al., 2014). Sulfuric acid is one of most reactive acids for the alumina extraction from CFA (Liu et al., 2016). The sulfuric acid method involves mixing CFA with H2SO4 to obtain sulfuric acid leachate (Tanvar et al., 2018) and subsequently extracting alumina from the sulfuric acid leachate. The carbonation method is a common method for alumina recovery from sulfuric acid leachate of CFA. Briefly, the leachate is processed by NaOH solution to become NaAlO2 solution and then decomposed by carbonation to obtain Al(OH)3 during the carbonation method (Ding et al., 2017). This method involves a long process and a large amount of wastewater containing a large amount of Naþ produced. Naþ is among the most difficult compounds to remove from water (Toze, 2006). Elevated Naþ levels in wastewater usually have negative consequences for soil quality and crop growth (Wen et al., 2018). The pyrolysis method overcomes the drawbacks of the carbonation method. However, harmful sulfur trioxide is produced in this method because the Al2(SO4)3$18H2O crystalloids are decomposed during the pyrolysis process (Bai et al., 2011). Sulfur trioxide has environmental consequences since it can be released as an acid aerosol, which creates visible plumes and leads to acid rain (Ahn et al., 2011). All current methods cause secondary pollution to the environment. An alternative method for the clean extraction of alumina from CFA leachate is desired. This study presents a clean and efficient electrolysis process to extract Al(OH)3 from sulfuric acid leachate of CFA. Compared with current approaches, the proposed electrolysis method is less harmful and can be summarized by Eqs. (1)e(4): Anode: 2H2O ¼ 4Hþ þ O2 þ 4e

(1)

Cathode: 2H2O þ 2e ¼ H2 þ 2OH

(2)

3OH þ Al3þ ¼ Al(OH)3

(3)

2. Materials and methods 2.1. Chemicals and analytical methods The chemical composition of high-alumina CFA is shown as follows: SiO2, 45.19%; Al2O3, 47.84%; FeO, 1.24%; CaO, 1.52%; MgO, 0.16%; and loss of ignition, 1.55% (Hua et al., 2018). When CFA is mixed with H2SO4, all other components can react with H2SO4 except SiO2. The products are Al2(SO4)3, Fe2(SO4)3/FeSO4, MgSO4 and CaSO4. CaSO4 is insoluble (Seidel and Zimmels, 1998) and Mg2þ is several orders of magnitude less concentrated than Al3þ throughout the experiment. The sulfuric acid leachate of CFA is mainly composed of Al2(SO4)3, a small amount of Fe2(SO4)3/FeSO4 and a very small amount of MgSO4 and CaSO4. H2O obtain electrons on the cathode to generate OH, and the OH reacts with Al3þ, Mg2þ, Fe3þ/Fe2þ and Ca2þ to precipitate as Al(OH)3, Fe(OH)3/ Fe(OH)2, Mg(OH)2 and Ca(OH)2 during electolysis. Based on the solubility products (pKSP; AlðOHÞ3 ¼ 33.14, pKSP; FeðOHÞ2 ¼ 16.90, pKSP; FeðOHÞ3 ¼ 38.60, pKSP; MgðOHÞ2 ¼ 11.25 and pKSP; CaðOHÞ2 ¼ 5.3, at 25  C) (Lide, 2003), the order of precipitation formation is Fe(OH)3 > Al(OH)3 > Fe(OH)2 > Mg(OH)2 > Ca(OH)2. As highlighted earlier, the CFA composition can vary considerably between regions (Ma et al., 2018a,b). Considering all these factors, Al2(SO4)3 aqueous solution was used as a synthetic electrolyte in the electrolysis process. Al2(SO4)3·18H2O (99% pure) was purchased from Sinopharm Chemical Reagent Company Limited (Shenyang, China) and used without further purification. Deionized water was prepared by a laboratory-grade RO ultrapure water system, which was used to dissolve Al2(SO4)3$18H2O. The mineralogy phases of the precipitate in the cathodic chamber were identified by X-ray diffraction (XRD, Bruker D8 Advance, USA). Scanning electron microscopy (SEM, Hitachi SU8010, Japan) was used to observe the morphology of the precipitates. The particle size distribution of the precipitates was analyzed by laser particle size analyzer (Mastersizer 3000, UK). The concentrations of ions were observed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman Prodigy XP, USA). Cyclic voltammetry (CV) was conducted with an electrochemical workstation (Zahner PP211, Germany) to analyze the electrochemical reactions of the electrode. 2.2. Electrolytic cell construction

Overall reaction: 18H2O þ 4Al3þ ¼ 4Al(OH)3 þ 12Hþþ 6H2þ 3O2(4) Eqs. (1)e(4) demonstrate that harmful products are not generated during the electrolysis process. H2SO4 and O2 are produced in the anodic chamber, whereas Al(OH)3 and H2 are produced in the cathodic chamber. Al(OH)3 is produced as an alumina resource, the product H2 is considered as a source of new clean energy, and H2SO4 is a common CFA leaching agent and can be returned to the leaching process to promote zero pollution discharge. Hence, the two-membrane and three-chamber electrolysis approach is a clean and efficient process. The influences of various factors, such as the concentration of Al2(SO4)3 aqueous solution, the current density, the distance between the cathode and the anode, the temperature and the electrolysis time, on the electrolytic yield of Al(OH)3 from synthetic leachate of CFA were investigated and optimized. The results provide a theoretical basis for the development of a novel, clean and efficient approach to extract alumina from CFA.

Identically sized plastic two-membrane and three-chamber cells (15  10  10 cm, LWD), consisting of an anodic chamber (5  10  10 cm, LWD), a middle chamber (5  10  10 cm, LWD) and a cathodic chamber (5  10  10 cm, LWD), were used in this study. The cell was separated by a proton exchange membrane (PEM, Nafion 117, DuPont Co., USA) and an anion exchange membrane (AEM, 7001, ULTREX Co., USA). Prior to use, the PEM was soaked in 5 wt% hydrogen peroxide at 80  C for 1 h, followed by pretreatment with 0.5 M H2SO4 solution for 1 h (80  C) and deionized water for 0.5 h. The AEM was pretreated by soaking in 5 wt% sodium chloride aqueous solution for 12 h. The new cell was soaked in absolute ethyl alcohol for 2 h to remove organics, then, soaked in 5% hydrochloric acid for 12 h to remove alkali, lead and arsenic and then washed with deionized water five times, followed by washing with the Al2(SO4)3 electrolyte five times. Two electrodes, with dimensions of 2  0.1  8 cm (LWH), TaOx/ IrOx-coated titanium as the anode and a plate of titanium as the cathode, were connected to a DC power supply. The effective surface area of both electrodes was 6 cm2.

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

2.3. Experimental procedure All three chambers were inoculated with the same aqueous solution of Al2(SO4)3. The electrolytes were prepared with deionized water and kept at a constant temperature in a water bath. The electrolytic variables included the current density, concentration of the electrolyte, distance between the anode and cathode, temperature and electrolysis duration. The precipitates were separated from the aqueous solution by using a core funnel with a filter membrane. Table 1 presents the details of the five tests, which were designed to determine the optimal conditions for electrolysis by utilizing the connections, as shown in Fig. 1. Each test was conducted thrice and the results were averaged for data analysis. 3. Results and discussion 3.1. Electrolysis experiment 3.1.1. Influence of concentration on electrolysis (test 1) The electrolytic yield was noticeably influenced by the concentration of Al2(SO4)3 during 4 h of electrolysis (Fig. 2). When the Al2(SO4)3 concentration was equal to or higher than 0.1 M, lower concentrations of Al2(SO4)3 resulted in a higher yield of the products with a current density of 600 A/m2. When the Al2(SO4)3 concentration was less than or equal to 0.1 M, the electrolytic yield reached a peak level at 0.05 M and then decreased with a current density of 300 A/m2. 0.05 M of Al2(SO4)3 aqueous solution was chosen as the optimal concentration, as shown in Fig. 2. Electrolyzed Al2(SO4)3 aqueous solution has rarely been studied. Due to the development of the chlor-alkali industry, the electrolysis of NaCl solution is a relatively mature process and has been widely studied. Faverjon et al. (2006) explored the membrane electrolysis of NaCl solution and found that the current efficiency increased with increasing reactant concentration. The opposite conclusion was obtained in our experiment. This is because a higher concentration of Al2(SO4)3 aqueous solution result in a lower pH value. Low pH values have certain disadvantages in the formation of Al(OH)3. The Al(OH)3, as an amphoteric oxide, can be dissolved in low pH, resulting in a decrease in the electrolytic yield. Strong acidity means that a large amount of Hþ exists in aqueous solution; Hþ can replace H2O to obtain electrons on the cathode, decreasing the OH generated (Arslan et al., 2005). The reduced OH concentration directly leads to a decrease in electrolytic yield. Regarding the influence of low Al3þ concentrations on electrolytic yield, Llanos et al. (2019) proposed that working with a low concentration resulted in mass transfer limitations and subsequently contributed to slow reaction on the cathode and low electrolytic yield. Savari et al. (2008) found that a decrease in salt concentration caused a decrease in the driving force for the movement of ions during electrolysis. An excessively low concentration of Al3þ is also not beneficial to the electrolytic yield. 3.1.2. Influence of distance between electrodes (test 2) The influence of the distance between electrodes on the

3

electrolytic yield and voltage is presented in Fig. 3. It can be clearly observed that the distance between the anode and the cathode did not influence the electrolytic yield at 4 h of electrolysis. A greater distance between electrodes resulted in a higher voltage, as shown in Fig. 3. The distance between electrodes has little effect on the electrolytic yield due to the following reasons: the electrolytic yield is correlated with the formation of Al(OH)3. The reaction between Al3þ and OH on the cathode determines the electrolytic yield. The concentration of Al3þ and OH near the cathode can obviously influence the electrolysis yield. Shiva and Himabindu (2019) proposed that acid electrolytes can provide OH due to the reduction of H2O. Lv et al. (2014) reported that the reaction was very slow when the initial reactant concentration was low during electrolysis. The electrolytic reaction is related to the concentration of the reactant. The distance between the cathode and anode has nothing to do with the reaction between Al3þ and OH on the cathode. Thus, the distance between electrodes has little effect on the electrolysis yield. The distance between the anode and the cathode conspicuously affected the voltage. This is because the further the distance between the electrodes can cause the higher the voltage, and the greater the voltage drop across the solution. As a result, a higher voltage is required to maintain a constant current density. 3.1.3. Influence of current density on electrolysis (test 3) Fig. 4 presents the effect of current density on the electrolysis of Al2(SO4)3 aqueous solution. The electrolytic yield gradually increased with current density during 4 h of electrolysis and reached a maximum value at 400 A/m2, followed by a gradual decrease. A higher current density results in higher electrolytic yields. If the current density is higher than the electrolytic cell capacity, the PEM is plugged and results in a reduced current density at the same voltage. With continued electrolysis, the adversely blocked PEM results in reduced conductivity. Faverjon et al. (2006) also observed that the current efficiency decreased with increasing current density and that the low current density (400 A/m2) rendered optimal performance. The current efficiency was low because the cationexchange membrane was not adapted to high current densities. The electrolyzed Al2(SO4)3 aqueous solution utilized with membranes yielded comparable results to Faverjon et al.‘s membrane electrolysis experiments. It appears that a high current density does not favor current efficiency. A current density of 300 A/m2 was chosen as the optimal value instead of 400 A/m2. 3.1.4. Influence of temperature on electrolysis (test 4) The relationship between the electrolytic yield and the temperature is shown in Fig. 5. The electrolytic yield was significantly influenced by temperature during 4 h of electrolysis. The maximum yield was attained at 30  C, and the yield decreased at higher temperatures. All these experiments were conducted in a water bath, except for electrolysis at room temperature (20 ± 2  C). The electrolytic yields remained almost the same in the temperature

Table 1 The designed tests to determine the optimal electrolysis conditions. Number of test

C (mol/L)

d (cm)

J (A/m2)

T ( C)

t (h)

Test Test Test Test Test

Cx CSelected CSelected CSelected CSelected

11 dx dSelected dSelected dSelected

300 300 Jx JSelected JSelected

20 ± 2 20 ± 2 20 ± 2 Tx TSelected

4 4 4 4 tx

1 2 3 4 5

in test 1 in test 1 in test 1 in test 1

in test 2 in test 2 in test 2

in test 3 in test 3

in test 4

Note: Cx are 0.033, 0.05, 0.067, 0.083, 0.1, 0.2, 0.3, 0.4, 0.5; dx are 9, 11, 13, 15; Jx are 100, 200, 300, 400, 500; Tx are 20, 30, 40, 50, 60; tx are 4, 8, 12, 16, 20.

4

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

Fig. 1. Process flow diagram of electrolysis of Al2(SO4)3 aqueous solution.

15

15

15

10

Electrolytic yield (%)

Electrolytic yield (%)

Electrolytic yield (%)

10

5

0

0.1

0.2 0.3 0.4 Concentration (mol/L)

0.5

5

10

5

0 0

0.04

0.06 0.08 Concentration (mol/L)

100

0.10

200 300 400 Current density (A/m2)

500

Fig. 4. Influence of current density on electrolysis of Al2(SO4)3 aqueous solution. Fig. 2. Influence of concentration on electrolysis of Al2(SO4)3 aqueous solution.

15 10

30

Voltage (V)

18

6

12

4

6

2

0

8

9

10

11 12 13 14 Electrode distance (cm)

15

Electrolytic yield (%)

8

24

0 16

Fig. 3. Influence of distance between electrodes on the electrolysis of Al2(SO4)3 aqueous solution.

Electrolytic yield (%)

Voltage Electrolytic yield

10

5

0

20

30

40 Temperature (

50

60

)

Fig. 5. Influence of temperature on electrolysis of Al2(SO4)3 aqueous solution.

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

range of 20  Ce30  C. In the 21st century mankind must cope with one of the biggest challenges: mitigating greenhouse gas emissions to limit climate change while supplying increasing energy demand to favor economic growth. To reduce energy consumption and protect the environment (Oliviera et al., 2017), we used room temperature without additional heating. Wang et al. (2010) reported that the temperature influenced the speed of the ions and the rate of the electrochemical reaction on the surface of the electrode. The influence of temperature on the electrolytic yield can be explained as follows: the strong diffusion of OH and Al3þ at high temperature results in the low concentration of OH and Al3þ near the cathode, which does not favor the generation of Al(OH)3 (Llanos et al., 2019). When the temperature is equal to or less than 30  C, the speed of Al3þ diffusion is beneficial to the formation of Al(OH)3 because the diffusion of Al3þ can replace the Al3þ consumed by reaction with OH. Thus, the optimal temperatures in this work ranged from 20  C to 30  C. 3.1.5. Influence of time on electrolysis (test 5) As the electrolysis time increased, the electrolytic yield and energy consumption increased rapidly, as shown in Fig. 6. A large amount of Al3þ was consumed in the electrolysis process, which caused a decrease in the concentration of Al3þ. The low concentration of Al3þ resulted in a slow increase in electrolytic yield, which implies that electrolysis becomes inefficient. To obtain efficient electrolysis, the electrolysis process should be completed within the initial 20 h. Hence, the solution in the middle chamber should be refreshed every 20 h to obtain a continuous and efficient electrolysis process. The change in electrolytic yield with time can be explained as follows: the reaction between OH and Al3þ results in a decrease in Al3þ concentration. Savari et al. (2008) proposed that a decrease in concentration caused a decrease in ion movement during electrolysis. The slow movement of Al3þ, resulting from the consumption of Al3þ near the cathode, was not replenished in time. The decrease in Al3þ concentration near the cathode caused a slow increase in the electrolysis yield. The energy consumptions were 4.8 (Qu, 1998) and 12.01 kWh/kg Al2O3 (Le, 2004) of primary aluminum production from bauxite by Bayer and sintering processes. Ding et al. (2012) proposed that the total energy consumption of recycled aluminum was 4.86% of the energy consumption of primary aluminum production from bauxite. Qiu (2010) reported that the energy consumption was 13.24 kWh/kg Al from Al2O3 by high temperature electrolysis

90 Energy consumption Electrolytic yield

60

60 45

30

30

0

15 0

Electrolytic yield (%)

Energy consumption (Wh)

75

-30 4

8

12 Time (h)

16

20

Fig. 6. Influence of time on electrolysis of Al2(SO4)3 aqueous solution.

5

method. The main energy consumption of proposed electrolysis method was 7.46 kWh/kg Al2O3 (4.88 kWh/kg Al(OH)3) in the treatment of CFA leachate. The energy costs calculated from the theoretical perspective would amount for 4009 and 464 USA$/t Al(OH)3 for CFA leachate by carbon (Ding et al., 2017) and pyrolysis methods (Bai et al., 2011). At the electricity price of 0.021 USA$/ kWh (Antofagasta, Chile). The energy cost would be 102.48 USA$/t Al(OH)3 for CFA leachate by proposed electrolysis method. Thus, the proposed electrolysis method exhibits superior performance in the treatment of CFA leachate. Validity was determined after electrolysis in three replicate experiments performed with the same membrane electrolysis cell, including same electrodes and membranes. The low values of standard deviation (S.D.) were obtained from different conditions (Figs. 2e6). Reliability was studied by the test-retest reliability. All indicators loaded significantly (r > 0.6) on test-retest reliability (Table 2). In sum, r and S.D. results provided sufficient evidence for demonstrating the high validity and reliability of this electrolysis method. 3.2. Characterization of electrolysis products The XRD patterns of the electrolysis products obtained under different conditions are presented in Fig. 7 and Fig. 8. The sharp and well-defined peaks, observed at 2q ¼ 18.93, 20.42, 27.97, 40.76 and 53.29, correspond to Al(OH)3, which is consistent with Eric et al.‘s results. Eric et al. (2013) discovered that alumina obtained in ammonium media (neutral pH) crystallizes mainly in the form of boehmite (AlOOH), whereas a mixture of boehmite, bayerite and nordstrandite (Al(OH)3) was obtained in all other environments. The intensity of the Al(OH)3 characteristic peaks at different current densities remained almost the same, which implies that current density has little influence on crystallization (Fig. 7). The intensity of the Al(OH)3 characteristic peaks at 30  C was higher than those at other temperatures (Fig. 8), which can be ascribed to the disadvantageous nature of OH enrichment in the double layer at high temperature (Arslan et al., 2005). The pH changed too quickly to be accurately recorded and the pH value around the electrode was significantly higher than that in the other areas of the chamber. Al(OH)3 precipitates easily formed around the electrode, but OH quickly diffused to other regions at high temperatures. High-temperature electrolysis required more energy and the electric current density increased due to the relaxed activation barriers at the electrolyte surfaces (Shin et al., 2007). The proposed electrolysis process was carried out at room temperature without using a water bath, making it an energy-efficient and environmentfriendly process. The relaxation of the activation barriers at the electrolyte surface increased with the reaction rate which occurred at the PEM. Reaction at the PEM resulted in a clogged membrane, which is not desirable for continuous reaction. Herein, high temperature is not conducive to the formation of Al(OH)3 crystals in Table 2 Calculation of the reliability of proposed electrolysis method. Variables

C1 (mol/L) C2 (mol/L) d (cm) J (A/m2) T ( C) T (h)

r I

II

III

Ⅳ

Ⅴ

0.96 0.98 0.96 e 0.98 0.99

0.98 0.89 0.99 0.66 0.99 0.97

0.86 0.91 0.97 0.99 0.98 0.99

0.86 0.69 0.99 0.97 0.97 0.98

0.63 0.91 e 0.89 0.99 0.99

Note: C1 (I, II, III, Ⅳ, Ⅴ) are 0.033, 0.05, 0.067, 0.083, 0.1; C2 (I, II, III, Ⅳ, Ⅴ) are 0.1, 0.2, 0.3, 0.4, 0.5; d (I, II, III, Ⅳ) are 9, 11, 13, 15; J (I, II, III, Ⅳ, Ⅴ) are 100, 200, 300, 400, 500; T (I, II, III, Ⅳ, Ⅴ) are 20, 30, 40, 50, 60; t (I, II, III, Ⅳ, Ⅴ) are 4, 8, 12, 16, 20.

(1,6,-1) A

(3,3,0) A

(2,0,2) A

(1,3,-1) A

A--Al(OH)3

Intensity(CPS)

(1,1,0) A

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

(0,0,1) A (1,1,0) A

6

@500

@400

@300

20

40

60

80

2-theta(°)

(1,6,-1) A

(2,0,2) A

(1,3,-1) A

(1,1,0) A

(0,0,1) A (1,1,0) A

Fig. 7. The XRD patterns of Al(OH)3 at different current densities.

A--Al(OH)3 @60

Intensity(CPS)

@50 @40

shape factor (in general k ¼ 1), l denotes the wavelength, q represents the angle of incidence and b refers to the full width at half maximum (FWHM) of the Bragg peak, which is corrected by using the corresponding peak in micron-sized powder (Kurian and Kunjachan, 2014). The XRD patterns of Al(OH)3 obtained under optimal conditions at various times from 4 to 20 h were recorded and the calculation of mean particle size is shown in Table 3. The results revealed that the products of various shapes had a mean particle size of 30.29 nm. Crystallinity increased asymptotically with the increase of time. The electrolytic reactions occurred on the electrode at the beginning of electrolysis. The small grained Al(OH)3 products dropped to the bottom of the cathode chamber due to the force of the constantly rising H2 being produced. The particle sizes of products were observed at optimal conditions of 20 h (Fig. 9). Small sheets of Al(OH)3 products were evidenced by the spherical structure in the SEM images (Fig. 9). With increased electrolysis time, particles accumulated on the electrode and H2 was not able to push the particles towards the bottom of the cathode chamber. Different average diameters of electrolysis product are observed in Fig. 9. A more precise particle-size distribution was measured by the laser diffraction method (Fig. 10). The distribution curves are plotted on a logarithmic scale of particle diameter, as shown in Fig. 10. The distribution curves exhibit that a uniform particle-size distribution was achieved. The median size of the sorted particles varied from 0.87 to 21.20 mm, as shown in Fig. 10. The log-normal distribution is often used as an excellent approximation for particles. In terms of particle size (D), the lognormal distribution pLN (D) can be given as Eq. (6):

pLN ðDÞ ¼

ðln DεÞ2 1 pffiffiffiffiffiffiffiffiffiffiffiffie 2s2 D 2ps2

@30 ε ¼ lnðMÞ; s ¼

@20

20

40

2-theta(°)

60

80

Fig. 8. The XRD patterns of Al(OH)3 at different temperatures.

membrane electrolysis. Extracting alumina from CFA has been widely studied. Yao et al. (2014) prepared a-alumina from CFA by calcination. Zhang et al. (2016) synthesized g-Al2O3 with a high specific area from CFA via the coprecipitation method. Compared with current methods for extracting alumina from CFA, the electrolysis method has advantages: different crystallinities of the Al(OH)3 product can be produced by controlling the conditions in the electrolysis method. A defect structure in crystals causes lattice strain, which is a quantitative measure of dislocations and crystal defects because nanoparticles reduce their strain by lattice expansion. Strain due to lattice deformation induces peak broadening in XRD patterns. Williamson-Hall analysis is an accurate method to calculate crystalline size. From Debye-Scherrer's formula, the full width at half maximum can be given as Eq. (5):

d¼

kl b cos q

(5)

where d refers to the average crystallite size, k corresponds to the

1 sþ ln 1:2816

(6) pffiffiffiffiffiffiffiffiffiffiffiffiffi s2 þ 4 2

(7)

The two parameters ε and s (Eq. (7)) can be easily evaluated from the median M ¼ D50 and the span S ¼ (D90 - D10)/D50, where Dx refers to the diameter below which x % of the particle diameters are found. Based on the extended Bruggeman formula, the effective permeability of magneto-dielectric composites with log-normal particle-size distributions can be predicted from the median and the span of the distributions (Li et al., 2018). Several non-electrochemical studies have reported the synthesis of nano-crystalline boehmite. Chen et al. (2008) utilized a hydrothermal method to prepare boehmite of various shapes and sizes, varying from 20 to 150 nm. Panda et al. (2006) synthesized boehmite nanoparticles (~35e75 nm) by a hydrothermal process and utilized them as water adsorbents. Eric et al. (2013) proposed an exchange reaction between hydroxide and certain anions (Cl and SO2 4 ), which resulted in absorption. The titanium electrode adsorbed sulfur-containing species, which was similar to the behavior of platinum, as reported by Grgur and Mijin (2014). The Al(OH)3 on the surface of the titanium electrode contained a large 2 amount of SO2 4 . The SO4 content in the products of this work was approximately 9.78 wt% after washing five times with 100 mL of Table 3 Crystallinity and mean crystallite size of Al(OH)3 under optimal conditions at various times. Time (h)

4

8

12

16

20

24

Crystallinity (%) Mean crystalline size (nm)

5.53 30.29

6.13

7.30

8.35

9.18

12.22

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

7

Fig. 9. SEM images of Al(OH)3 under optimal conditions after 20 h of electrolysis.

5

Percentage (%)

4 3 2 1  þ Fig. 11. Schematic illustration of the migration of Al3þ, SO2 4 , OH and H3O in.

0 0.1

1

10 Diameter ( m)

100

1000

Fig. 10. The particle size distribution of as-prepared Al(OH)3 powder.

deionized water per wash. This suggests that the exchange reaction between hydroxide and SO2 4 occurred but not significantly. It may be concluded that the interaction between the Al(OH)3 and SO2 4 is mainly physical absorption. The SEM image in Fig. 9 shows that Al(OH)3 was formed by the agglomeration of small balls and had a large specific surface area, which provided sufficient sites for ion adsorption. Herein, the adsorbed ions can be removed from the asprepared Al(OH)3 surface, which makes it reusable. These properties of the as-prepared Al(OH)3 are superior to those of Al(OH)3 prepared by Chen et al. (2008), Panda et al. (2006) and Eric et al. (2013). The as-prepared Al(OH)3 is a promising candidate for water adsorption. 3.3. Mechanism of electrolysis  þ Fig. 11 presents the migration of Al3þ, SO2 4 , OH and H3O in the electrolysis cell. The Al3þ and H3Oþ in the anodic chamber did not migrate to the other chambers, whereas the Al3þ and H3Oþ moved from the middle chamber to the cathodic chamber through the PEM. The Al3þ in the cathodic chamber reacted with H2O and electrons on the cathode to form Al(OH)3 precipitates and H2. The  SO2 4 and OH from the cathodic chamber did not migrate to other  chambers, whereas the SO2 4 and OH from the middle chamber diffused to the anodic chamber through the AEM. H2SO4 and O2 were generated in the anodic chamber due to the oxidation of H2O. Fig. 12 illustrates an electrochemical process and a chemical

process. During the chemical process, OH and Al3þ react to form Al(OH)3. The electrochemical process includes oxidation and reduction reactions. During the oxidation reaction, the oxygen evolution reaction of H2O happened on the TaOx/IrOx-coated titanium electrode. Iridium oxides and tantalum oxides are the most active and stable catalysts for the oxygen evolution reaction under acidic conditions. The iridium oxides and tantalum oxides are used as catalysts because iridium/tantalum in their oxides contain a large number of unsaturated ligands. Thus, the possibility of forming the highest-valence iridium oxide and tantalum oxide is reduced (Grgur and Mijin, 2014). The process by which H2O are catalyzed by iridium oxides and tantalum oxides in this work may be expressed in three steps: IrOx/TaOx þ H2O ¼ IrOx/TaOx-(OH*) þ Hþ þ e (Fig. 12 (a)). IrOx/ TaOx-(OH*) ¼ IrOx/TaOx-O þ Hþ þ e (Fig. 12 (b)). 2IrOx/TaOxO ¼ 2IrOx/TaOx þ O2 (Fig. 12 (c)). In the total oxidation reaction, H2O lose electrons at the anode, and a large number of Hþ ions appear in the aqueous solution of the anode chamber (Fig. 11). This process can be expressed as Eq. (8): 2H2O ¼ 4Hþ þ O2 þ 4e, E ¼ 1.229e0.059  pH V (vs. SHE)

(8)

During the reduction reaction, H2O are attracted by electrons and then the HeO bonds in a H2O break, producing Hþ and OH (Fig. 12 (d)). Two Hþ gain two electrons on the cathode to form H2 (Fig. 12 (e)). In the total reduction reaction, H2O gain electrons at the cathode and are converted into H2 and OH (Fig. 12), which can be expressed as Eq. (9): 2H2O þ 2e ¼ H2 þ 2OH, E ¼ - 0.059  pH V (vs. SHE)

(9)

In the total electrochemical process, H2O are oxidized to

8

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

the electrolysis chambers.

Fig. 12. Working principles of the electrolysis process using Al2(SO4)3 aqueous solution.

generate Hþ and O2 at the anode, whereas H2O are reduced to form H2 and OH at the cathode. We focused on attaining Al(OH)3 and H2SO4, which implies that the exchange of Hþ and Al3þ remained the focal point of our study. The Al3þ ions from the anodic chamber did not stop moving under the action of direct current until the Al3þ reached the AEM during electrolysis (Fig. 11). The Al3þ from the anode chamber were not exchanged with the Al3þ in other chambers and remained constant. The direct current drove the Al3þ from the middle chamber to the cathodic chamber under the action of direct current. When OH met and reacted with Al3þ, Al(OH)3 occurred according to Eq. (3). As the reaction between Al3þ and OH continued to form Al(OH)3 in the cathodic chamber, the concentration of Al3þ in the cathodic chamber decreased, and the Al3þ from the middle chamber diffused to the cathodic chamber through the PEM under

direct current and differential concentration (Fig. 13). The variation in Hþ concentration remained the same as Al3þ with some minor differences. pH in the anodic chamber became lower (Fig. 14), which allowed the formation of H2SO4. A two-electrode system was used to characterize the mechanism. A Ti electrode was used as the working electrode, and a Pt electrode was served as a counter electrode in 0.05 M Al2(SO4)3 aqueous solution, as shown in Fig. 15. During the initial negative and positive scans, the absence of H2 adsorption and H2 desorption, indicated that Al3þ altered the reaction of H2O reduction. In Fig. 15, peaks 1 and 2 corresponded to oxide adsorption and oxide desorption. CV was utilized in this work to study the detailed electrolysis mechanism. CV is the primary method used in the study of electron transfer and its consequences. David and Gosser (1994) proposed

0.05 a b c

n (Al3+) (mol)

0.04

0.03

0.02

0.01 0

4

8

12

16

20

Time (h) Fig. 13. The change of concentration of Al

3þ

in (a) the cathodic chamber (b) the middle chamber and (c) the anodic chamber.

Y. Shi et al. / Journal of Cleaner Production 243 (2020) 118470

oxide formation occurred on the anodic sweep and the reduction of oxide gave a cathodic peak on the reverse scan. When a compound was added to H2SO4 solution, the CV results can change, and the compound's oxidation displayed one oxidation peak. The CV results in this work agreed with the previous authors' opinion. Arslan et al. (2005) proposed that the adsorption of OH on the anode surface occurred as the first step. They agreed well with the mechanism of this work mentioned above in Fig. 12.

a b c

4

9

pH

3

4. Conclusions

2

1 0

4

8 12 Time (h)

16

20

Fig. 14. The pH variation in (a) the cathodic chamber, (b) the middle chamber and (c) the anodic chamber.

1

300

Acknowledgements

200

Current ( A)

The proposed membrane electrolysis method is a clean and efficient alternative for alumina extraction from sulfuric acid leachate of CFA. The as-prepared Al(OH)3 exhibited a uniform particle size distribution (0.87e21.20 mm) and a mean crystalline size of 30.29 nm. Electrolytic yield reached 64.48% and energy consumption was 4.88 kWh/kg Al(OH)3 after 20 h of electrolysis process under the optimal conditions: 0.05 mol/L, 11 cm, 300 A/m2 and 20e30  C. Al3þ from the middle chamber migrated to the cathodic chamber and reacted with the OH generated by water reduction to form Al(OH)3 and H2 in the cathodic chamber. H2O lost electrons to form O2 and contributed to a decrease in pH in the anodic chamber. The impurity ions (Ca2þ, Fe2þ, Fe3þ and Mg2þ) in the leachate might influence the electrolysis process. This factor has great potential for further research.

This study was supported by the National Natural Science Foundation of China (U1710257). Fundamental Research Funds for the Central Universities of China (Nos.N140203005, N140204015), Science and Technology Research Projects of Liaoning Education Department (No. L2014096), State Key Laboratory of Pressure Hydrometallurgical Technology of Associated Nonferrous Metal Resources (YY2016006).

100 2

0

-100

References

-2

-1

0 Potential (V)

1

2

Fig. 15. CV curve in 0.05 M Al2(SO4)3 aqueous solution using Ti-cathode as a working electrode.

that molecules were activated by electron transfer and subsequent chemical reactions can be probed with this method. It produces a curve that shows the relationship between voltage and electricity, and is shown as Eqs. (10), (11):

"  #  RT k0 ana Fv 1=2 0:78  In 1=2 þ In EP ¼ E  RT ana F D 0

  ip ¼ 2:99  105 nðana Þ1=2 ACD1=2 v1=2

(10)

(11)

where n refers to the number of electrons, A represents the electrode area (cm2), C corresponds to the concentration (mol/cm3), D denotes the diffusion coefficient (cm2/s) and v refers to the scan rate (V/s). Grgur and Mijin (2014) used CV to study the electrochemical behavior of electrodeposited palladium in 1 M NaOH solution. The three anodic peaks corresponded to H2 desorption, OH adsorption and Pd oxide formation. Nady et al. (2017) studied CV in 0.5 M H2SO4 solution using a platinum sheet and found that monolayer

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