A cleaner electrolysis process to recover alumina from synthetic sulfuric acid leachate of coal fly ash

Hydrometallurgy 191 (2020) 105196

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A cleaner electrolysis process to recover alumina from synthetic sulfuric acid leachate of coal fly ash Yuan Shia, Kai-xi Jianga,b, Ting-an Zhanga,

T

⁎

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 LE I N FO

A B S T R A C T

Keywords: Coal fly ash Electrolysis Aluminum hydroxide Sulfuric acid

The realization of clean extraction of aluminum hydroxide from high-alumina coal fly ash would be beneficial to achieve better overall coal utilization, and renders environmental and economic benefits. The current study mainly aimed to develop a nonhazardous approach to conduct electrolysis for alumina extraction from sulfuric acid leachate of high-alumina coal fly ash. The XRD, SEM, LPSA, FT-IR and ICP-AES results revealed that aluminum hydroxide was a main electrolysis product, with current efficiency of 83.25% under the optimized conditions as t = 1 h, J = 600 A/m2. The median particles of aluminum hydroxide after 1-h and 6-h electrolysis were 35.3 and 40.1 μm, respectively. The electrolysis mechanism was analyzed by cyclic voltammetry, which showed that the H2O gained electrons on the cathode to form OH– and H2. The OH– reacted with Al3+ to form Al (OH)3. In addition, the effluent of electrolysis was sulfuric acid and can be reused as a leaching reagent to promote zero pollution discharge. Hence, the electrolysis process provided an environment-friendly and efficient approach for alumina extraction from high-alumina coal fly ash.

1. Introduction

meaningful integrated utilizations of HACFA appeared, including adsorbents for wastewater treatment (Lieberman et al., 2018), building materials (Cheng et al., 2018) and iron and aluminum oxide separation (Chen et al., 2018). Among these utilizations, alumina extraction from HACFA has been widely investigated (Wang et al., 2019). Following the recovery of alumina (Albanese et al., 2008), the HACFA residual can be utilized as building materials which would be economically beneficial and environmentally sustainable. The current methods of extraction of alumina from HACFA can be classified into acid process (Bojinova and Teodosieva, 2016), alkali process (Li et al., 2014), and acid-alkali combination process (Han et al., 2018). During these processes, the acid process featured with lower temperature (T < 350 °C) and amendable operations is a technical reliable and economically profitable to extract alumina from HACFA (Guo et al., 2013). Sulfuric acid as one of most reactive acid was widely used in the acid process (Li et al., 2017). The sulfuric acid process generally includes two steps. Firstly, HACFA mixed with sulfuric acid to get the sulfuric acid leachate of HACFA. Secondly, alumina was extracted from the leachate (Ding et al., 2017). Alumina extraction from the leachate has been widely studied, the current methods can be classified into pyrolysis method (Bai et al., 2011) and carbonation method (Ding et al., 2017). The pyrolysis method has a short process and easy to realize.

Waste management is among the environmental problems threatening sustainable development (Wang et al., 2018). Coal is the second largest-energy source and plays a vital role in development of the world's non-renewable primary energy (Oliveira et al., 2019). However, a large amount of solid waste is produced by coal combustion process, such as, high-alumina coal fly ash (HACFA). HACFA has been attracting much attention due to its high alumina content. The alumina component of HACFA exceeds 40 wt% even to 60 wt% (Asl et al., 2018) and is equivalent to mid- or low-grade bauxite ores (Yao et al., 2014). HACFA is mainly generated by coal-fired power plants in the northern part of Shanxi province and middle-western part of Inner Mongolia, China. Its production is approximately 30 Mt. per year (Zhang et al., 2018). The utilization rate of the total HACFA is only 20 wt% (Hu et al., 2018), indicating a great deal of HACFA is disposed of in landfills. Landfilled and impounded HACFA has amassed in tremendous quantities across the globe. There are adverse environmental (Sivalingam and Sen, 2019) and health effects (Almahayni and Vanhoudt, 2018) caused by the landfilled and impounded HACFA. Compared to disposal of HACFA in landfills, clean utilization of HACFA offers numerous advantages. Currently, instead of disposal of HACFA in landfills, many

⁎

Corresponding author. E-mail address: [email protected] (T.-a. Zhang).

https://doi.org/10.1016/j.hydromet.2019.105196 Received 7 July 2019; Received in revised form 14 September 2019; Accepted 2 November 2019 Available online 13 November 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.

Hydrometallurgy 191 (2020) 105196

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transform infrared spectrometer (FT-IR, Nicolet 6700, USA). The concentration of the ions was observed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman Prodigy XP, USA). The morphology of the electrolysis products was observed by a scanning electron microscopy (SEM, Hitachi SU8010, Japan). The cyclic voltammetry (CV) was conducted with an electrochemical workstation (Zahner PP211, Germany).

However, harmful sulfur oxides are produced due to the Al2(SO4)3·18H2O crystalloids are decomposed into γ-Al2O3 at 870 °C (Bai et al., 2011). Carbonation method overcomes this disadvantage, brief, the carbonation method is that the leachate is processed by NaOH solution and then decomposed by carbonation to obtain Al(OH)3 (Ding et al., 2017). However, the carbonation process has a long process and a large amount of wastewater with a large amount of Na+ is produced. High Na+ levels in wastewater usually have negative consequences for soil quality and crop growth (Wen et al., 2018). Thus, these current methods cause secondary pollution to the environment. Alumina extraction from HACFA have to evaluate from the environmental perspective and cleaner extraction of alumina from HACFA should be paid more attention. Compared with the current pyrolysis (Bai et al., 2011) and carbonation (Ding et al., 2017) methods, electrolysis method is considered to show the most promise to cleanly extract alumina from sulfuric acid leachate of HACFA. Theoretically, hydrogen, oxygen, sulfuric acid and aluminum hydroxide can be produced from sulfuric acid leachate of HACFA by electrolysis (18H2O + 4Al3+ = 4Al(OH)3 + 12H+ + 6H2 + 3O2). The hydrogen product emits only water without any carbon emissions during the burning process and has high energy density (140 MJ/kg) which is more than two times higher than typical solid fuels (50 MJ/kg) (Kumar and Himabindu, 2019). The wastewater (sulfuric acid solution) after electrolysis can be reused as a leaching reagent to realize non-wastewater generation. No member and addition reagent used in this one-chamber electrolysis make this process environmental and economic benefits. Moreover, electric energy from wind power and solar energy, which cannot be incorporated into the national grid (Shah and Ali, 2019), can be used to extract alumina from HACFA leachate. Hence, the proposed one-chamber electrolysis method is a promising technique of an efficient and potential zero-waste integrated process for the utilization of HACFA.

2.2. Electrolytic cell construction The identical sized plastic cells (15 × 10 × 10 cm, LWD) were used in this study. Iridium oxides and tantalum oxides are the most active and stable catalysts for the oxygen evolution reaction under acidic conditions. They are used as catalysts because iridium/tantalum in their oxides contain a large number of unsaturated ligands. Iridium oxides and tantalum oxides were widely used as catalysts for the oxygen evolution. Such as, Niyitanga and Jeong (2019) mentioned that iridium oxides were used as catalysts in the oxygen evolution reaction in acidic media of 0.5 M H2SO4 electrolyte. Li et al. (2006) found that Ti anodes coated with TaOx/IrOx had good electrocatalytic activity for oxygen evolution. Thus, the commercially TaOx/IrOx-coated titanium as an anode and a plate of titanium as a cathode were connected to a DC power supply. Both electrodes were with the dimensions 2 × 0.1 × 8 cm (LWH). In the case of the cell, the effective surface area of both electrodes was 6 cm2. 2.3. Experimental procedure The diagram of electrolysis is shown as Fig. 1. All electrolysis experiments were carried out with a single chamber. The electrolytic variables included the electrolysis duration and current density to determine the optimal conditions. The precipitate was separated from the Al2(SO4)3 aqueous solution by using a core funnel with a filter membrane. Table 2 presents the details of two tests of electrolysis. These two tests were designed to determine the optimal conditions for electrolysis. Each test was conducted thrice and results were averaged for data analysis.

2. Materials and methods 2.1. Chemicals and analytical methods The chemical composition of HACFA and HACFA leachate varies considerably between regions and is shown in Table 1. The chemical composition of HACFA leachate was calculated by Luo et al.'s conclusion. Luo et al. (2013) reported that the maximum leaching efficiencies were 71.8% for aluminum, 87.2% for calcium and 95.9% for iron of HACFA. The impurities are several orders of magnitude less concentrated than Al3+ and vary considerably in the leachate of HACFA (Table 1). To eliminate the interference of impurities to have a better understanding of the mechanism, Al2(SO4)3 aqueous solution was used as a synthetic electrolyte in the electrolysis process. The aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O, 99% pure) was purchased from Sinopharm Chemical Reagent Company Limited (Shenyang, China) and used without further purification. A laboratory grade RO-ultrapure water system was used to prepare deionized water, and the deionized water was used to dissolve Al2(SO4)3·18H2O. The particle size distribution of the electrolysis product was analyzed by Laser particle size analyzer (LPSA, Mastersizer 3000, UK). The mineralogy phases of the electrolysis products were identified by X-ray diffraction (XRD, Bruker D8 advance, USA). The Fourier transform infrared spectra of the electrolysis product was obtained with a Fourier

2.4. Calculation of the current efficiency, energy consumption and electrolysis yield The current efficiency, energy consumption and electrolysis yield of aluminum hydroxide were calculated according to Eqs. (1)–(3):

nNF It

(1)

W = UIt

(2)

η=

E=

mb − ma × 100% mb

(3)

where η is current efficiency, %; n represents amount of substance, mol; I corresponds to current value, A; t denotes reaction time, s; N is number of electrons transferred; F refers to Faraday constant, 96,480C/ mol; W represents energy consumption, Wh; U is average voltage, V; m corresponds to quality, g; E denotes the electrolysis yield, %. In addition, mb and ma are the mass of aluminum present in Al2(SO4)3 aqueous solution before and after electrolysis, g.

Table 1 The mainly composition of HACFA (Hu et al., 2018) and sulfuric acid leachate of HACFA (wt%). Component

SiO2

Al2O3/Al3+

Fe2O3/Fe3+

CaO/Ca2+

MgO/Mg2+

HACFA HACFA leachate

39.17–50.10 –

37.85–50.14 87.03–90.89

1.47–2.76 3.50–8.49

1.61–3.07 4.45–5.50

0.17–0.37 <1

2

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

(a) 28

d (cm)

T(°C)

t (h)

1 2

0.05 0.05

1 1

20 ± 2 20 ± 2

tx t Selected

J (A/m2)

in test 1

1500 Jx

Note: tx are 1, 2, 3, 4, 5, 6. Jx are 600, 900, 1200, 1500, 3000, 4500. 0.05 M Al2(SO4)3 aqueous solution was used in this study.

3. Results and discussion

100

21

80 60

14

40 20

7

3.1. Electrolysis experiments

1

3.1.1. Influence of time on electrolysis (test 1) Influence of electrolysis time on electrolysis yield and energy consumption is presented in Fig. 2a. The electrolysis yield increased intensely within 5-h and then slowly increased with electrolysis time. Moreover, the energy consumption increased with the electrolysis duration, as shown in Fig. 2a. The slow increase of electrolysis yield after 5 h can be explained as follows: water molecules get electrons to become OH– on the cathode, and the Al3+ near the cathode can react with the OH– to generate aluminum hydroxide. This results in the decrease of Al3+ concentration. However, Savari et al. (2008) proposed the decrease in salt concentration caused a decrease in the driving force (concentration gradient) for the movement of ions during electrolysis. Thus, the decrease of Al3+ concentration near cathode caused a slow increase of electrolysis yield. In addition, the water molecules were oxidized to form sulfuric acid on the anode, and this resulted in the decrease of pH values of the Al2(SO4)3 aqueous solution. The low pH value was disadvantage to the formation of Al(OH)3 because the Al(OH)3 can be dissolved in acid solution. To obtain highly efficient electrolysis, electrolysis process should be completed within the initial 5 h.

2

3 4 Time (h)

5

6

0

(b) 20

Electrolysis yield (%)

Electrolysis yield Current efficiency

80

15

60

40

10

Current efficiency (%)

C (M)

Electrolysis yield (%)

Number of tests

120 Energy consumption Electrolysis yield

Energy consumption (Wh)

Table 2 The designed tests to determine the optimal electrolysis conditions.

20 5

1000

2000 3000 Current densithy (A/m2)

4000

Fig. 2. Influence of (a) time and (b) current density on electrolysis of Al2(SO4)3 aqueous solution.

3.1.2. Influence of current density on electrolysis (test 2) The current efficiency and electrolysis yield for the production of aluminum hydroxide are evaluated. The influence of current density on electrolysis yield and current efficiency of 1-h electrolysis is presented in Fig. 2b. The electrolysis yields sharply increased with current density when the current density was < 1200 A/m2 and then slowly increased when the current density was > 1200 A/m2. Moreover, the current efficiency decreased with current density (Fig. 2b). The change of current efficiency can be explained by two factors. First, working with a high concentration ensured full availability of the raw material at the anode surface, avoiding mass transfer limitations (Llanos et al., 2019). The concentration of Al3+ was relatively high at the early stage of the electrolysis. Thus, the current efficiency was high

at the beginning of the electrolysis. Second, Azizi et al. (2011) proved that a high concentration of available hydroxyl radicals can promote the production of product. In this work, the reduction of water molecules produced a high concentration of available hydroxyl radicals at high current density. However, the current efficiency did not increase with the current density may due to the limitations of the mass transfer of Al3+. This suppose can be supported by Govindan et al.'s (2015) opinion that increment of current density had a substantial effect on reducing the concentration of reactant. Thus, high current density can cause a rapidly decrease of Al3+ concentration in the aqueous solution and further lead to slower mass transfer. The electrolysis yields slowly increase with current density when 3

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(a)

40.76 and 53.29, indicating the product is aluminum hydroxide after 5h and 6-h electrolysis. Moreover, in order to have a better understand of the product after 1-h electrolysis. The FT-IR spectrums of products were obtained after 1h electrolysis at different current densities (Fig. 3b). The peak at 3355 cm−1 in the FT-IR spectrum was attributed to the hydroxyl stretch of the gibbsite phase. The peaks at 1077 cm−1, 977 cm−1 and 518 cm−1 were also associated with the gibbsite phase, whereas the peak at 1636 cm−1 corresponded to the bending vibration of H2O (Fig. 3b). These results provided evidence for the formation of aluminum hydroxide after 1-h electrolysis with different current densities. The results of XRD patterns and FT-IR spectrums indicated that the precipitate was mainly composed of aluminum hydroxide by electrolysis process. The aluminum hydroxide characteristic peaks appeared in the XRD patterns at 5-h and 6-h instead of 1-h electrolysis. This can be ascribed to the OH– enriched on the cathode (Arslan et al., 2005) met and reacted with Al3+ to produce aluminum hydroxide. As time went on, the aluminum hydroxide grain grew up near the cathode and the crystallinity of the aluminum hydroxide increased. Thus, the sharp aluminum hydroxide characteristic peaks appeared in the XRD patterns after 5-h and 6-h electrolysis. It is interesting to note that even after washing, vacuum filtration, and drying, the Al(OH)3 precipitate charged. Further analysis of the precipitate indicated the presence of SO42− of about 10 wt% after washing three times (100 ml of deionized water per time). This suggested that a large of SO42− were trapped during the precipitation. The trapped SO42− may result in the electrolytic product charged. This characteristic of aluminum hydroxide may be used to absorb ions (SO42−). Al(OH)3 has been reported to be a gelatinous precipitate (Peng et al., 2001), and it has been found to serve as an efficient adsorbent (Asl et al., 2019). It has been reported to precipitate with different crystallinity. Such as Chen et al. (2018) obtained high crystallinity of aluminum hydroxide from HACFA leachate by adding the NaOH solution and then continuous CO2 to yield high crystallinity Al(OH)3. Wei et al. (2018) obtained the γ-Al2O3 from HACFA leachate by heating to obtain Al2(SO4)3·18H2O crystalloids and further calcining to get γAl2O3. Yan et al. (2018) synthesized γ-Al2O3 from HACFA with simultaneous on-site utilization of CO2. Compared with current methods to prepare alumina from HACFA leachate, electrolysis method presents its advantage. The electrolysis process can prepare aluminum hydroxide products with different crystallinity at short process. Zhang and Jia (2016) suggested that the adsorption of fluoride on the amorphous aluminum hydroxide was a feasible, spontaneous and exothermic process. Therefore, the aluminum hydroxide prepared by electrolysis may be a promising candidate as a water adsorbent.

(1)

Intensity

(2)

(3)

Al(OH)3

20

40

60

80

2-theta (°)

10 97 77 7

600 A/m2 900 A/m2 1200 A/m2 1500 A/m2 3000 A/m2 4500 A/m2

3600

51 8

Transmittance (%)

33 55

16 36

(b)

2700 1800 Wavenumber (cm-1)

900

Fig. 3. (a) The XRD patterns of Al(OH)3 product after (1) 1-h (2) 5-h and (3) 6-h electrolysis and (b) infrared spectrum of aluminum hydroxide obtained after 1h electrolysis at different current densities.

the current density is > 1200 A/m2. On one hand, the increase of current density leads to the increase of the redox reaction of water molecules on the electrodes. Arslan et al. (2005) proved that water molecules were reduced to produce hydroxyl radicals. Thus, a large amount of hydroxyl radicals enriches on the cathode. However, a large amount of hydroxyl radical product enriched on the cathode hinders the further reduction reaction of water molecules. This results in the slow generation of OH– and subsequent slow generation of Al(OH)3. On the other hand, more generated OH– lead to more consumption of Al3+ near the cathode. However, Llanos et al. (2019) proposed that a low concentration resulted in mass transfer limitations. Thus, the consumed Al3+ cannot be replenished in time. Resulting the slow increment of electrolysis yield when the current density is > 1200 A/m2.

3.2.2. Particle size distribution and SEM characterization The distribution curves of products by electrolysis are shown in Fig. 4a. Clearly, a peak was achieved in the particle size distribution of aluminum hydroxide products after electrolysis. The median sizes of the particles after 1-h and 6-h electrolysis were 35.3 μm and 40.1 μm, respectively. The particle size of aluminum hydroxide slowly increased with electrolysis time and directly determined its application. Different particle sizes of aluminum hydroxide powders can be used to meet different requirements of preparing smelter grade alumina (Hind et al., 1999), adsorbent materials (Asl et al., 2019), ultrasonically assisted material (Mushtaq et al., 2019), etc. For instance, Hind et al. (1999) produced the smelter grade alumina (−45 μm ≤ 18%) with Bayer process. Panda et al. (2006) synthesized nanoparticles boehmite (~35–75 nm) as water adsorbents. Al-Hamadani et al. (2017) reported alumina component can enhance the sonochemical degradation. Therefore, the particle size distribution is very important performance indices of the aluminum hydroxide. The aluminum hydroxide prepared by electrolysis has potential broad application prospects. The SEM images of the products by electrolysis are shown in Fig. 4. As shown in

3.2. Characterization of electrolysis products 3.2.1. XRD and FT-IR characterization The XRD patterns of electrolysis products after 1-h, 5-h and 6-h of electrolysis are presented in Fig. 3a. From the Fig. 3a, it is seen that there are no peaks appeared after 1-h electrolysis. However, the sharp and well-defined peaks were observed at 2θ = 18.93, 20.42, 27.97, 4

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Fig. 4. (a) Particle size distributions and (b and c) SEM images of Al(OH)3 product after 1-h and 6-h of electrolysis.

thermodynamically stable region of soluble ions as Al3+ and AlO2− is < 4.3 and > 11.1 of pH values. The E-pH diagrams indicate that extraction of alumina from the leachate by electrolysis is hard in the view of thermodynamics. For example, the pH value of the 0.05 M Al2(SO4)3 aqueous solution is about 2.88. The pH value of Al2(SO4)3 aqueous solution decreases with the increases of Al2(SO4)3 concentration. Thus, studying the change of pH values is beneficial to understand the generation of aluminum hydroxide by electrolysis method. Furthermore, as the electrolysis went on, the pH decreased (far from the electrodes) with electrolysis time (Fig. 5b). The pH values gradually decreased with electrolysis time. Finally, the pH values were from 2.88 to 1.71 after 6-h electrolysis (Fig. 5b). The redox reaction happened on the electrode resulted in the H+ and OH– generated on the anode and cathode, respectively (Arslan et al., 2005). The OH– reduced due to OH– reacted with Al3+ on the cathode. Thus, the H+ enriched in the aqueous solution and the pH values gradually decreased with electrolysis time. Further indicating that it is difficult to prepare aluminum hydroxide product from Al2(SO4)3 aqueous solution by electrolysis due to the aluminum hydroxide can be dissolved in the acidic solution. Influence of electrolysis time on temperature is presented in Fig. 5b. It can be obviously seen that the temperature increased with the electrolysis time within 6-h electrolysis. Roh (2014) proposed that the internal energy fluxes were obtained as functions of temperature. Indicating part of the electrolytic energy became internal energy of the aqueous solution during electrolysis. After a serious of electrolysis experiments, an interesting phenomenon was discovered by observing the change of pH values near the cathode during the electrolysis process. The pH values around the cathode changed quickly and were significantly higher than that of other areas of the chamber, as shown in Fig. 5c. The pH values varied from 9.5 to 12.7 (Fig. 5c). The optimal pH values for the formation of aluminum hydroxide were from 4.3 to 11.1 (Fig. 5a). However, the pH values (far from the electrodes) were < 2.88 (Fig. 5b). Comparing the pH values far from the electrodes and very near the Ti electrode, the aluminum hydroxide precipitate was easily formed around the Ti electrode during electrolysis. This can be ascribed to the OH– enriched near the cathode when electrolysis (Arslan et al., 2005). When the aluminum hydroxide products produced near the cathode, the suitable pH values made the small aluminum hydroxide particles grew up and this observation was a good explanation for the SEM images mentioned above.

Fig. 4b, the products are consisted of many small spherical structures with smooth surface after 1-h electrolysis. Fig. 4c indicates that most of the aluminum hydroxide products after 6-h electrolysis has a sheet structure. Furthermore, the small spheroidal particles grew up with electrolysis time. As a result, the size of aluminum hydroxide products at 6-h electrolysis became bigger than that of 1-h electrolysis and its morphologies were from small spherical particle to sheet structure. This has a good agreement with the XRD patterns conclusion mentioned before. Different morphology of Al(OH)3 crystals have been prepared from HACFA by different acid. Such as Chen et al. (2018) prepared the morphology of the Al(OH)3 crystals as plate-like structure with smooth surface using hydrochloric acid method to extract aluminum hydroxide from HACFA. Shayanfar et al. (2018) prepared ovoid-shaped aluminum hydroxide using carbon dioxide gas in aluminate solution obtained from sintered nepheline syenite. Compared with the morphology of the Al(OH)3 prepared by current methods, both spherical particles and sheet structure of aluminum hydroxide can be prepared by electrolysis method. Asl et al. (2018) reported that smaller particle size created higher surface area, hence higher adsorption capacity. The aluminum hydroxide prepared by electrolysis has a potential large specific surface area and provides sufficient sites for ions adsorption. Thus, it may be a promising candidate as a water adsorbent and this agrees with the conclusion mentioned before in the XRD analysis.

3.3. Mechanism of electrolysis 3.3.1. The pH changes of electrolysis Ions phase stability has a significant effect on alumina extraction via electrolysis. The stability of Al-contained ion is determined by potential, pH, temperature and pressure, etc. Predominance area phase diagram is a phase diagram for chemical reaction system, and that represents a comprehensive thermodynamic analysis for abstract chemical reaction thermodynamic equilibrium relationship. E-pH diagrams at 25 °C elevated temperature have been calculated using the equilibrium data (Zhang et al., 2015). As Al-H2O, all lines represent reaction conditions in the E-pH diagrams, and the substances are stable in Al2(SO4)3 aqueous solution within the area surrounded graph line. Fig. 5a shows the E-pH diagrams of Al-H2O systems at 25 °C. It is shown that the behavior of alumina in the solution is complex, with different forms at different pH values. With the increase of pH values, the Al3+ tend to transfer into Al(OH)3 and AlO2−. Meanwhile, the 5

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(a) 0.006

(a)

1

1.6

0.004

AlO-2

Current (A)

E (volts)

0.8 Al2O3(H2O)(s)

Al3+

O (1 2 atm)

0.0

-0.8 AlH3(s)

4

8

12

2 3

-0.004 -0.8

pH (b) 42

3.2 pH

-0.6

-0.4

-0.2 0.0 0.2 Potential (V)

0.4

0.6

0.8

(b)

Temperature

36

2.8

30 2.4

pH

Temperature (°C)

0.000

-0.002

H2 (1atm )

0

0.002

24 2.0 18 0

1

2

3 Time (h)

4

5

6

1.6

(c) 12

(c) pH

8

4

0

0

7

14 21 Time (s)

28

35

Fig. 5. (a) E-pH diagrams for Al-H2O at 25 °C and pH variation (b) far from the electrodes and (c) very near the Ti electrode in Al2(SO4)3 aqueous solution.

Fig. 6. Working principles of (a) CV curve, (b) electrolysis process and (c) Ti electrode in Al2(SO4)3 aqueous solution.

3.3.2. The CV analyzes of electrolysis A curve between voltage and current of this work is shown in Fig. 6a. A three-electrode system was used to characterize the mechanism. A Ti electrode was used as the working electrode, a Hg/ Hg2SO4 electrode was performed as a reference electrode and a Pt electrode was served as a counter electrode in 0.05 M Al2(SO4)3 aqueous solution. During the initial negative and positive scanned, peak 1, 2 and 3 corresponded to the oxide adsorption, oxide desorption and hydrogen desorption, respectively (Fig. 6a). Electron transfer and its consequences can be studied by the CV (David and Gosser, 1994). The CV was utilized to study the electrolysis mechanism of Al2(SO4)3 aqueous solution in this study. A curve was produced to show the relationship between voltage and electricity. The peak potential and current for an irreversible couple can be given as Eqs. (4), (5):

EP = E 0 −

RT ⎡ k0 αn Fv 1/2 0.78 − In 1/2 + In ⎛ a ⎞ ⎤ ⎢ αna F ⎣ D ⎝ RT ⎠ ⎥ ⎦

ip = (2.99 × 105) n (αna )1/2ACD1/2v1/2

(4) (5)

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). The CV curve in this work indicated the detail of electrode reactions during electrolysis and this process can be expressed as Eqs. (6), (7). Meanwhile, the electrode reactions are that the water molecules get/ lost electrons on the electrodes. The reaction between OH– and Al3+ is a chemical reaction happened in the aqueous solution (near the cathode). This confirmation of the electrode mechanism further support to the aforementioned aluminum hydroxide formation. Fig. 6b presents the reactions conducted in the electrolysis of 6

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Declaration of Competing Interest

Al2(SO4)3 aqueous solution. Many negative ions, positive ions and neutral particles exist in the Al2(SO4)3 aqueous solution. For instance, a large amount of SO42−, H2O, Al3+ and a small amount of H+ and OH– exist in the Al2(SO4)3 aqueous solution. The oxidation order of ions is H+ (H2O) > Al3+ according to the Nernst equation. Thus, the water molecules get electrons on the cathode to generate H2 and OH−. At the same time, the Al3+ reacted with the OH– near the cathode to form Al (OH)3 precipitate. The reducibility order of ions is OH– (H2O) > SO42−. Thus, the water molecules lost electrons on the anode to generate H+ and O2. The Fig. 6b illustrates electrode process and chemical process. These processes can be expressed as Eqs. (6)–(9):

Cathode:2H2 O + 2e− = H2 + 2OH−, E = −0.059 × pH V (vs. SHE) Anode:2H2 O =

4H+

+ O2 +

4e−,

None. Acknowledgements 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).

(6)

References

E = 1.229–0.059 × pH V (vs. SHE) (7)

Overall electrochemical reaction:2H2 O = 2H2 + O2

(8)

Chemical reaction:3OH− + Al3 + = Al(OH)3

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The chemical composition of HACFA is mainly SiO2, Al2O3, Fe2O3, CaO, MgO (Hu et al., 2018). When HACFA is mixed with H2SO4, all other components can react with H2SO4 except SiO2. Thus, Al2(SO4)3, Fe2(SO4)3, MgSO4 and CaSO4 are produced. CaSO4 is insoluble (Seidel and Zimmels, 1998) and Mg2+ is several orders of magnitude less concentrated than Al3+ in the sulfuric acid leachate of HACFA. The leachate is mainly composed of Al3+, a small amount of Fe3+ and a very small amount of Mg2+ and Ca2+. During electolysis, OH– is genereated by the reduction of H2O. Al3+, Fe3+, Mg2+ and Ca2+ react with the generated OH– to precipitate as Al(OH)3, Fe(OH)3, Mg(OH)2 and Ca (OH)2, respectively. Based on the solubility products (pKSP, Ca(OH)2 = 5.3, pKSP, Mg(OH)2 = 11.25, pKSP, Al(OH)3 = 33.14 and pKSP, Fe(OH)3 = 38.60, at 25 °C) (Lide, 2003), the order of precipitation formation is Fe (OH)3 > Al(OH)3 > Mg(OH)2 > Ca(OH)2. Fig. 6c shows the working principles that water molecule is reduced to become aluminum hydroxide on the Ti cathode. Arslan et al. (2005) proposed that hydroxyl radicals were adsorbed on the anode surface during the electrolysis. This work agrees with Arslan's opinion that the water molecules were attracted by the electrons on the cathode and tended to become hydroxyl radicals. The negatively charged particles attract positive charge (Al3+) (Huang et al., 2012). The aluminum ions are attracted by the negatively charged oxygen atoms and promote the break of HeO bonds in water molecules. Once the HeO bonds in water molecules break, the Al(OH)3 precipitate and H2 are produced. 4. Conclusions The results of electrolysis provided a theoretical basis for the development of an efficient and potential zero-waste integrated for the utilization of HACFA. (1) The as-prepared Al(OH)3 was produced by electrolysis with current efficiency of 83.25% under the optimized conditions as t = 1 h, J = 600 A/m2. The median particles of the as-prepared aluminum hydroxide after 1-h and 6-h electrolysis were 35.3 and 40.1 μm, respectively. (2) The electrochemical evaluation of electrolytic reaction on the cathode demonstrated that the H2O gained electrons to become OH– and H2. At the time, OH– reacted with the Al3+ on the cathode to form Al(OH)3 precipitate. Moreover, the water molecule lost its electrons to form sulfuric acid and O2 on the anode. (3) The pH values near the cathode were obviously different from that of other zones in case of power up. The pH values near the cathode ranged from 9.5 to 12.7 and reached the optimal values for the precipitation of Al(OH)3. Thus, the Al(OH)3 precipitate was easily generated near the cathode without membrane and additives.

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