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Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis
Accepted Manuscript Title: Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis Authors: Yaxian Zhang, Qi Shi, Muxi Luo, Hongtao Wang, Xuejiao Qi, Chia-Hung Hou, Fengting Li, Zisheng Ai, Jose Tacares Araruna Junior PII: DOI: Reference:
S0304-3894(18)30932-4 https://doi.org/10.1016/j.jhazmat.2018.10.023 HAZMAT 19843
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-7-2018 13-9-2018 9-10-2018
Please cite this article as: Zhang Y, Shi Q, Luo M, Wang H, Xuejiao Q, Hou CHung, Fengting L, Ai Z, Araruna Junior JT, Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.10.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis
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Yaxian Zhang1,2, Qi Shi1,2, Muxi Luo1,2, Hongtao Wang1,2*, Xuejiao Qi1,2, Chia-Hung Hou3*, Fengting Li1,2, Zisheng Ai4, Jose Tacares Araruna Junior5 1. State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze River Water Environment, Ministry of Education, College of Environmental Science and Engineering, UNEP-TONGJI Institute of Environment for Sustainable Development, Tongji University, Siping Rd 1239, Shanghai 200092, P. R. China 2. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R. China 3. Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC 4. Department of Medical Statistics, School of Medicine, Tongji University, 1239 Siping Road, Yangpu District, Shanghai 200092, PR China 5. Department of Civil and Environmental Engineering, Pontifical Catholic University of Rio de Janeiro, Rio de Janeiro, Brazil * Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (C.-H. Hou). Tel: +86-21-65978598 (H. Wang), +886-2-33664400(C.-H. Hou). Fax: +86-21-65985059 (H. Wang),+886-2-23928830(C.-H. Hou).
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To be submitted to Journal of Hazardous Materials
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Graphical abstract
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Highlights
Combined washing and electrodialysis improved dealkalination of bauxite residue.
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Aeration enhanced the dealkalination efficiency by increasing ion transfer rate. Electrodialysis with aeration can efficiently recover NaOH and separate NaAl(OH)4. Al(OH)3 was the main component of membrane scaling. 2
Al(OH)3 resulted from Al(OH)4- and AlO2-.
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Abstract Bauxite residue, a major by-product of the alumina-producing Bayer
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process, is a serious environmental pollutant due to its high alkalinity.
Here, we reported an operation system designed in our laboratory that
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included washing and electrodialysis dealkalization systems with aeration
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pipes. Washing with aeration releases a substantial amount of free alkali
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and attached alkali into water and increases the dealkalization efficiency.
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The washing liquid was treated with five steps of batch-mode electrodialysis. The average removal of total dissolved solids (TDS) after
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the aeration and non-aeration electrodialysis processes were 61.30% and
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39.61%, respectively. The average removal of OH− under aeration conditions was 76.62%, a value that was greater than the value produced
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under non-aeration conditions (68.48%). This efficiency was also higher than that of some other reports (64.90-68.50%). Aeration decreased the
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energy consumption to a greater extent than the non-aeration condition. NaOH was recovered in terms of the concentration chamber, and the NaAl(OH)4 present in the dilution chamber was separated for the electrodialysis treatment. Membrane scaling was generated to a lesser amount under aeration conditions than that of non-aeration conditions, 3
which would improve the dealkalization efficiency. The high repeatability of the experiments was indicated by the intraclass correlation coefficient (P<0.05).
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Keywords: Bauxite residue; Aeration; Electrodialysis; Dealkalization;
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Membrane.
1. Introduction
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Bauxite residue, a by-product of alumina production through the
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Bayer process, exerts a substantial impact on environments by
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contaminating water and soil due to alkali liquid leakage, polluting the air
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with bauxite residue dust, and altering land occupation [1,2]. Bauxite
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residue is a type of solid waste with high alkalinity and a high heavy metal content [3], which is a potential threat to the surrounding soil and
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water ecosystems. The highly alkaline leachate pollute the ground water
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or surface water. The production of bauxite residue in China can reach up to 70 million tons per year, accounting for a relatively significant share (up to 30%) of the global production. The total weight of bauxite residue
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was approximately 0.51 billion tons by the end of 2016, but only 4% of these residues were comprehensively used as subgrade materials, adsorbents, and flocculants [4]. Bauxite residue has previously been treated by direct discharge into 4
seas [5], but this method has been prohibited in China since the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter began to be enforced in 1985. Since the late 1970s, bauxite residue has been treated by stacking and then burying [6].
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However, this method not only used nearly all land but also resulted in the easy percolation into the soil and groundwater, which then causes
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serious environmental pollution [7-9]. Moreover, when the volume of the bauxite residue dam increases to a certain extent, pits may collapse and
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severely endanger the safety of the people and property.
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Although the properties of bauxite residues from various sites in the
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world differ, the general characteristics of bauxite residue are its high
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alkalinity and abundant Al and Fe contents [10]. The high alkalinity
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makes bauxite residue difficult to recycle; thus, dealkalization methods, which are expected to broaden the recycling methods, decrease resource
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loss, and mitigate the risk of environmental pollution and dam collapse,
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should be studied. Currently available dealkalization methods include water leaching [11], calcification–carbonation [12,13], acid neutralization [14-16], mild hydro-chemical processes [16], the CO2 sequestration cycle
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[17-19], wet leaching [20], and bacterial amelioration [21,22], but nearly all of these methods exhibit several disadvantages that seriously affect their practical application. For example, water leaching effectively removes free alkali but not bound alkali [11]. In addition, this process 5
requires a long time and produces a large amount of alkali washing liquid [23]; thus, the approach should be combined with other technologies [24]. With the exception of the dealkalization methods described above, other developing methods, such as flocculation and membrane dealkalization
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method, exhibit satisfactory treatment efficiency in the bench scale but present poor biological stability or severe membrane fouling, thus
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rendering their practical application difficult [25,26].
In recent years, electrodialysis technology has garnered considerable
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attention in the field of alkali liquid recycling [27,28]. Hwang et al. [29]
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used electrodialysis with double polar films to treat high concentrations
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of NaOH and studied changes in membrane stability at different
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temperature. Kinčl Jan et al. [30] employed electrodialysis to dealkalize
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and concentrate the alkaline liquid of bauxite residue. Yan et al. [31] simulated the alkalinity and caustic ratio of bauxite residue using NaOH
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and NaAl(OH)4 to examine the efficiency of the electrodialysis
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dealkalization technology. This technology is efficient and requires a low energy consumption of only 12.43 kW·h/kg. Because bauxite residue exhibits poor fluidity and settles easily,
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which are unfavorable characteristics for electrodialysis technology, pretreatment of bauxite residue and a combination with other technologies are necessary. Therefore, we developed a process for the dealkalization of bauxite residue by employing washing in combination 6
with electrodialysis to investigate the actual dealkalization efficiency. The ion turbulence of the system is poor under non-aeration. Meanwhile, aeration can accelerate the mass-transfer rate of ions, decrease concentration polarization, and improve the conductivity of the
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electrodialysis system [32]. We also placed aeration pipes in both processes to increase the dealkalization efficiency. The major aims of this
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study are: (I) to explore the feasibility of the practical bauxite residue
dealkalization process based on washing and electrodialysis technologies,
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(II) to study the bauxite residue dealkalization efficiency of washing and
2.1 Materials
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2. Materials and Methods
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the changes in membrane properties.
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electrodialysis processes under aeration conditions, and (III) to analyze
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All reagents used in this study were of analytical grade and were
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purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
The bauxite residue and its alkali liquid used in this study were
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generated by the Bayer process, and the bauxite residue mainly contained SiO2 (20.76 wt%), Al2O3 (20.40 wt%), Na2O (17.33 wt%), CaO (17.56 wt%), and Fe2O3 (6.44 wt%), as mentioned in our previous publications [24,33]. These materials were collected from Shanxi Province, China. 7
(The pretreatments performed before the experiment are described in the Supplementary Materials.) A ruthenized titanium plate (1 mm, 9 cm × 20 cm) was provided by Baoji Baoye Titanium-Nickel Industry Co., Ltd. (Shanxi, China). The cation exchange membrane (CEM, JCM-II-07) and
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anion exchange membrane (AEM, JAM-II-07) were purchased from Beijing Jierui Environmental Science and Technology Co., Ltd. (Beijing,
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China) and displayed a range of properties, as shown in Table 1. Table 1 Properties of the CEM and AEM CEM
JCM-Ⅱ-07 −SO3H Homogeneous and acid-alkali resistance 0.16–0.23 33–40 1.3–4.0 95–99 ≥ 0.25 < 45 °C
Characteristic
JAM-Ⅱ-07 −NR3OH Homogeneous and acid-alkali resistance 0.16–0.23 24–28 3.0–6.5 90–95 ≥ 0.25 < 45 °C
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2.2 Experiments
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Thickness (mm) Water content (%) Membrane area resistance (Ω·cm2) Selective permeability (%) Bursting strength (MPa) Temperature range
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N
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Specifications Functional group
AEM
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Every experiment in our study was performed in triplicate. Figure 1
shows the complete process used in our study. The left panel shows the
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washing system, and the right panel shows the electrodialysis system.
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Figure 1 Schematic of the system combining washing with electrodialysis
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2.2.1 Washing system
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We designed a washing system, including a washing tower and a
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storage tower, as shown in Figure 1 (left panel). Bauxite residue and water entered the washing tower from the bottom at a ratio of 1:7, as
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determined in a preliminary experiment (shown in Figure B and C in the
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Supplementary Materials), because a considerable washing efficiency was observed, and the slurry-to-water ratio was similar to a study of
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dealkalization of bauxite residue with roasting and water leaching [11]. An aeration pipe was also utilized to increase the washing efficiency. Due
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to its slow-release property, bauxite residue was washed with 3 min of aeration and 30 min of settling for six cycles. After each washing step, the system remained static without aeration to allow the bauxite residue to precipitate, the washing alkali liquid to flow into storage tower, and the bauxite residue settling at the bottom to 9
be discharged by the sludge pipe. The first two volumes of alkali washing liquid were discharged into water storage chamber 1 and mixed; given the high alkalinity, the mixture entered the electrodialysis–dealkalization device immediately after filtration.
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Aeration promotes the washing effect, based on a preliminary experiment in which the pH, TDS, and alkalinity were compared under
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aeration conditions and in a non-aerated system. Finally, we selected 15 L/min as the optimal aeration condition based on our results (shown in
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(right
panel)
shows
a
schematic
of
the
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Figure
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2.2.2 Electrodialysis dealkalization system
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Figure D in the Supplementary Materials).
electrodialysis-dealkalization system designed by our group. The whole
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system was divided into two floors: the upper floor contained the
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electrodialysis unit, and the lower floor contained the adjustment and water storage chamber unit. The CEM, AEM, and electrode plates
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(effective area: 180 cm2) were alternatively distributed in the device, thereby forming the anode chamber, concentration chamber, dilution
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chamber, and cathode chamber, respectively. Aeration pipes were placed in the concentration and dilution chambers. According to the preliminary experiments, by maintaining a certain conductivity, low energy consumption and low membrane scaling, the water quality of the effluent was a pH of 11.60~11.80 and a TDS concentration of 3.50-5.50 g/L. 10
Considering the bauxite residue-washing performance and water quality requirement of the outflow in this study, we collected the alkaline liquid from the first and second wash steps, which was disposed of by the electrodialysis system, in the washing system and recovered the alkaline
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liquid from the electrodialysis system using ion exchange. The washing liquid was treated with batch-mode electrodialysis in
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five cycles. Batch experiments were performed consecutively without substitution of membranes. The standard method “Determination of
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alkalinity, bicarbonate, and carbonate” (SL 83-1994) [34] was used to
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measure the alkalinity of bauxite residue and the washing and desalinated
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alkaline liquids. Moreover, the contents of metallic elements in the
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alkaline liquid were measured using an inductively coupled plasma
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optical emission spectrometer (ICP–OES, Agilent720ES, Agilent Technologies). The contents of salt anions in the alkaline liquid were
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measured using ion chromatography (ICS 1000+AS20).
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Different aeration intensities (0, 5, 15, and 30 L/min) were also selected for this process based on the results from the preliminary
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experiments (shown in Figure E in the Supplementary Materials). Energy consumption was calculated using the following equation: E=
t
U (t ) I
0
C0 Ct '
'
dt
(1)
,
where E is the energy consumption (kW·h/kg), U (t) is a function of the change in voltage with time, t is the time (s), I is the current (A), and 11
C0' and Ct' are the concentrations of TDS in the dilution chamber at times 0 and t, respectively. Current efficiency was calculated using the following equation: =
(C 0 C t ) V F
100%
(2)
,
Id t 0
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N
t
where ζ is the current efficiency, V is the volume of the alkaline
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liquid in the dilution chamber, F is Faraday’s constant, 96,485 C/mol, N is the unit number, N = 1, I represents current (A), t represents time (s), and
C0 and Ct are the molar concentrations of NaOH and NaAl(OH)4 in the
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dilution chamber at times 0 and t, respectively.
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2.2.3.1 Ion exchange capacity
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2.2.3 Characterization of fouling during electrodialysis
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The pretreatment of membranes in studies aiming to determine the ion exchange capacity is based on the Electrodialysis Technology
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Industry Standard (HY/T 034.1~034.5-1994). Specific experimental
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procedures are described in the Supplementary Materials. HCl acidification was used to remove the scale from the membrane after the
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electrodialysis process. The ion exchange capacity (IEC) was calculated using the following
equation. IE C
Cn V m (1 C w )
,
(3)
where IEC is the ion exchange capacity of the CEM and AEM 12
(mol/kg), Cn is the concentration of the titration reagent (mol/L), V is the volume of the titration reagent consumed (L), m is the weight of membrane (kg), and Cw is the water content of the membrane (%). 2.2.3.2 Morphological analysis was
analyzed
using
scanning
electron
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Morphology
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microscopy–energy dispersive X-ray spectroscopy (JSM-6701F, Japan). (The pretreatment is shown in the Supplementary Materials.)
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2.2.3.3 Analysis of the composition of membrane fouling
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X-ray diffraction (D8 Advance, BRUKER AXS GMBH) was
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utilized to measure the phase of membrane fouling. Elements and their
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Agilent Technologies).
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contents in the membrane were measured using ICP–OES (Agilent720ES,
SPSS 24 software was used to perform the statistical analyses in our
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study.
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3. Results and Discussion As shown in Table 2, the molar ratio of aluminate to sodium in the
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alkaline liquid was approximately 1:2. The primary soluble metal elements were Al, K, and Na, as shown in our previous publication [24]. The main components in the alkaline solution were NaCO3 and NaAl(OH)4, according the determination of alkalinity. For a convenient experimental analysis, the present study mainly considered the changes in 13
the concentrations of Al and Na in the alkaline liquid. This simplified analysis has been applied in many studies [16,25,31]. Table 2 Concentrations of Different Ions in the Alkaline Liquid K
Na
F-
SO42-
PO43-
NO3-
2434.12
668.73
4165.21
60.06
1.98
35.39
0.62
3.1 Washing process for dealkalization determined
the
washing
times,
water
consumption,
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We
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Concentration (mg/L)
Al
slurry-to-water ratio, and aeration intensity as parameters in six preliminary tests to evaluate the feasibility of the dealkalization of
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bauxite residue using the washing system.
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The pH and concentrations of TDS and soluble metal elements were
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determined together; therefore, pH was used as representative parameter to calculate the intraclass correlation coefficient (ICC). The results are
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presented in Table 3. The ICCs (P<0.05) of a single group and the whole
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group were all greater than 0.995, indicating that the results are highly reproducible. The results of the T test are shown in the Supplementary
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Materials (Table B).
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Table 3 Intraclass Correlation Coefficients for pH in the Washing Process Intraclass
95% Confidence Interval
F test using a truth value of 0
Correlation
Lower
Upper
Value
df 1
df 2
Group (Non-aeration)
0.996
0.984
0.999
730.589
5
12
0.000
Group (Aeration)
1.000
0.999
1.000
14971.558
5
12
0.000
0.999
0.997
1.000
2802.740
11
24
0.000
P
Group (Non-aeration or Aeration) 14
One-way random effect model with random personnel effect.
As shown in Figure 2, the pH and concentrations of TDS and soluble metal elements in the alkaline washing liquid of bauxite residue
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were considerably decreased as the washing times increased, regardless of the use of non-aeration or aeration conditions. In addition, free metal
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elements were rapidly removed by washing. This finding is consistent
with the results obtained from washing processes used in another study [35].
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As outlined in Figure 2 (A), the pH decreased after each wash.
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Notably, under aeration conditions, the washing liquids obtained after the
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first to third washes exhibited higher pH values than the liquids obtained under non-aeration conditions, namely, a greater amount of alkaline
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material entered the washing liquid and the reside was completely washed
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under aeration conditions because of a rapid ion mass-transfer rate [36]. Aeration substantially increases the mixing efficiency by increasing the
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liquid flow velocity and the contact areas [37]. The alkaline components in bauxite residue consist of free alkali, attached alkali and binding alkali.
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Washing transferred much of the free alkali and attached alkali from bauxite to water under aeration conditions. However, as the washing times increased, the pH did not exhibit apparent changes, due to the sustained release of sodalite and cancrinite from bauxite residue [38], which decreased the rate of alkali entering the water. 15
As illustrated in Figure 2 (B), a considerably greater amount of TDS was detected in the washing liquid under aeration conditions than under non-aeration conditions. TDS will enter the washing liquid after numerous or extensive washes. Aeration effectively induces the
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crystallization of some substances, resulting in an increase in the release of solids in the washing effluent [39]. Importantly, aeration enhanced the
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efficiency of dealkalization. The TDS concentration in combination of all
six washing liquid samples was 39.45 g under aeration conditions and
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only 2.18 g under non-aeration conditions. After two washes, the removal
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of TDS exceeded 95%, but the concentration remained high; therefore,
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we used electrodialysis to decrease the TDS concentration.
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Figure 2 pH (A), TDS concentration (B), and concentrations of soluble metal elements in the alkaline liquid (C and D) after different washes during washing under non-aeration and aeration conditions
As shown in Figures 2 (C) and 3 (D), considerable concentrations of soluble metal elements were detected in the bauxite residue slurry.
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Overall, the removal of soluble metal elements was considerably increased under aeration conditions than under non-aeration conditions.
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The final concentration of Na+ in the washing liquid was similar with the results of a leaching process for sodium from roasted bauxite residue
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cooled by water, indicating that the water cooling method was helpful for
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leaching sodium from a pretreated bauxite residue [40]. When the pH >
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11, Al(OH)3 started dissolving. The concentration of Al3+ increased. The
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concentrations of soluble metal elements might affect the concentration of
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TDS. Therefore, the changes in pH, TDS concentration and soluble metal concentrations exhibited similar trends in the Figure 2 (A) and (B).
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According to some studies [35,41],41] washing confers good
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efficiency on dealkalization processes, but aeration was not performed in these washing processes. Based on the results from the present study, the efficiency of dealkalization was considerably improved by a washing
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process incorporating aeration.
3.2 Electrodialysis process for dealkalization An increase in the current can accelerate the ion mobility rate, but energy consumption would increase as the current increased. Hence, in 17
this study, 2.7 A was selected as the suitable current, resulting in a current efficiency of 59.62% and an energy consumption of 12.97 kW·h/kg, which is slightly higher than the values of 56.90% and 12.26 kW·h/kg reported in another study [42]. The details of the method used to select
presented in the Supplementary Materials (Figure F).
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the current based on current efficiency and energy consumption are
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As outlined in Figure 3, compared to non-aeration conditions,
aeration decreased the energy consumed by the system, due to the
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reduction of concentration polarization. The average energy consumption
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values under non-aeration and aeration conditions were 21.49 and 11.15
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kW·h/kg, respectively. The value obtained under aeration conditions was
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lower than the value of 12.43 kW·h/kg reported by Yan [31].
Figure 3 Energy consumption of the electrodialysis system
3.2.1 Removal of OH− and TDS The dealkalization efficiency of five batch experiments performed 18
under aeration or non-aeration conditions is shown in Table 4. The results of the T test are shown in the Supplementary Materials (Table C). Table 4 Removal of OH− and TDS from the Dilution Chamber under Aeration and Non-Aeration Conditions Non-aeration
Batch
Aeration
Removal of OH− (%)
Removal of TDS (%)
TDS (g/L)
Removal of OH− (%)
Removal of TDS (%)
1
9.68
54.29
24.38
4.48
82.62
65.54
2
8.42
65.33
35.73
4.78
81.80
63.79
3
6.97
73.70
46.79
4.94
80.50
61.41
4
6.97
73.08
46.38
5.42
69.80
58.63
5
7.07
76.01
44.77
5.57
68.38
57.15
Mean
―
68.48
39.61
―
76.62
61.30
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N
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No.
TDS (g/L)
Test
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As shown in Table 4, the average removal of OH− under aeration was 76.62%, while under non-aeration it was 68.48%. A study on
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electrodialysis with membrane stack configuration optimization reported
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the removal of OH− as 64.90-68.50% [31]. The result was 63.80% in Yan et al. [25], which treated alkaline solution through diffusion dialysis and
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electrodialysis. The average removal of TDS under aeration was 61.30%, while under non-aeration it was 39.61%. Aeration could enhance the ion
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transfer and decrease concentration polarization. In addition, aeration could prevent ion channels from blocking, improving the removal efficiency. The removal trend of OH− and TDS increased under non-aeration conditions, but decreased under aeration conditions. A potential 19
explanation is that the concentration of alkaline liquid was considerably higher in the concentration chamber in the batch experiments performed under aeration conditions (Figure 2), resulting in a large concentration gradient difference between the two sides of the membrane and
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concentration and dilution chambers, as well as a strong self-diffusion effect [28]. Therefore, the removal of OH− and TDS decreased as the
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number of batch experiments performed under aeration conditions
increased. Antagonism occurs between electrodialysis and the diffusion
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effect, possibly due to a slow anion migration rate [36]. However, under
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non-aeration conditions, substantial water dissociation occurred on both
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sides of the membrane, and ion movement was negligible.
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In addition, the TDS concentration in the outflow under non-aeration
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conditions did not satisfy the requirement (TDS = 3.50–5.50 g/L), but stable effluent water quality might be achieved under aeration conditions.
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3.2.3 Dealkalization under aeration conditions
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Considering the satisfying behaviors of the system under aeration
conditions, the dealkalization efficiency under aeration conditions was
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further studied. This experiment aims not only to recover the alkali but also to separate NaAl(OH)4 from the alkaline liquid for further treatment.
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Figure 4 Concentrations of Na and Al in the concentration and dilution chambers in consecutive batch experiments
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As outlined in Figure 4, the concentrations of Na and Al in the
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concentration chamber increased as the number of batch experiments
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increased. The concentration of Na in the concentration chamber showed
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a larger increase than the Al concentration. This difference may be attributed to the more rapid migration rate of Na+ than Al(OH)4− through
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the ion exchange membrane [28]. Al(OH)4− and OH− passed through the
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AEM and entered the concentration chamber by the driving force of the electric field; Na+ entered the concentration chamber through the CEM from the anode chamber. Therefore, the electrodialysis process attained
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the goal of recovering the alkali and separate NaAl(OH)4 from the alkaline liquid. The amounts of Na and Al removed from the dilution chamber in each batch experiment were approximately equal to the amounts 21
recovered from the concentration chamber, indicating that the material flow was substantially balanced in the system (Figure 4). The rate of increase in the Na and Al concentrations in the concentration chamber gradually decreased due to the diffusion and dialysis of the ion exchange
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membrane used for electrodialysis, thus restraining the migration and concentration of desorbed ions.
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3.3 Evaluation of the fouling of ion exchange membranes
Ion exchange membranes play an important role in determining the
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separation performance of electrodialysis. The formation of scale on the
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membrane surface is a direct cause of membrane contamination [43,44].
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Therefore, the ion exchange membranes used in the present study were further evaluated and treated to improve the properties of the membranes.
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3.3.1 Scaling of membranes
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The surface morphology of the CEM and AEM before and after HCl treatment under aeration and non-aeration conditions is shown in the
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Supplementary Materials (Figure G).As shown in Figure G (left panel), the surface morphology of CEM did not change considerably under
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different conditions, indicating that the surface structure of the CEM remained intact. The surface structure and active functional groups in the CEM were not markedly damaged after HCl acidification. The acidification treatment effectively removed the scale attached to the membrane and purified the cations into hydrogen. In addition, the 22
proportion of aluminum and sodium in the CEM was consistent with the results of ICP (Figure G). High electro-convection at the CEM caused an increase in mass transfer, which facilitated the removal of the scale from the surface [43].
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However, compared with the initial AEM, the surface morphology of the AEM did not change considerably under aeration conditions. Under
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non-aeration conditions, a considerable amount of damage was observed on the AEM surface (Figure G (right panel)), and scale was attached to
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the surface of the membrane. The aluminum fractions in AEM under
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aeration and non-aeration conditions were 6.90% and 9.39%, respectively,
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indicating that the acidification treatment did not completely remove the
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scale from the membrane. In one study [45], scale formation was evident
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on the dilute side of the AEM, and the water splitting rate was higher at
side.
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the CEM than that at the AEM, thereby preventing scaling on the dilute
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Membrane scaling is one of the main factors contributing to the decreases in the water content and ion exchange capacity of the CEM and
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AEM and subsequent decrease in the dealkalization efficiency [44,45]. In the present study, the membrane scale produced during
electrodialysis was collected. According to the XRD results (Figure 5 (A)), the phase composition of the membrane scale was aluminum hydroxide (α-Al(OH)3), consistent with the analytical results from the 23
electrodialysis-concentrated alkali recovery [31]. This finding is also consistent with findings from another study [42]. As shown in Figure 5 (B), the content of Al in the CEM was low under every condition, with a markedly lower content measured under conditions
than
under
non-aeration
conditions.
Under
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aeration
non-aeration conditions, concentration polarization around the membrane
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was evident, and AlO2− and Al(OH)4− were converted into an Al(OH)3 precipitate to form membrane scaling, which is attached to the membrane
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surface and ion channels. This finding also indirectly decreased the
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membrane moisture content and ion exchange capacity.
Figure 5 XRD of membrane scale (A) and contents of Al and Na in the membrane (B)
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3.3.2 Ion exchange capacity of experimental membranes before and after the acid treatment
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Figure 6 Ion exchange capacity (A: CEM and B: AEM) of the experimental membranes and HCl-pretreated membranes
As shown in Figures 6 (A) and (B), a slight change in ion exchange
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capacity was observed in CEM and AEM under aeration conditions than
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in the initial membrane, decreasing 0.03 mol/kg (1.8%) and 0.93 mol/kg
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(7.5%), respectively. However, the ion exchange capacity decreased under non-aeration conditions to a greater extent than under aeration
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conditions, 0.47 mol/kg (28.8%, CEM) and 4.29 mol/kg (34.5%, AEM),
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respectively. Aeration could increase the ion migration rate, and few ion channels were occupied, keeping the option performance of membranes
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and enhancing ion exchange membrane flux. Acidification also increased the ion exchange capacity of the CEM and AEM because the acidification
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of HCl, CEM, and AEM were purified by hydrogen and chlorine, and membrane scales of CEM and AEM were removed to a certain extent [43]. Thus, the number of active sites in the ion exchange membrane increased, and the ion exchange capacities of the CEM and AEM were increased. The result is similar to the findings reported by Yan [31]. 25
3.4 Mechanism of the electrodialysis process According to the results of our experiments, a summary of the
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mechanism is shown in Figure 7.
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(1) Force of the electrical field
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Figure 7 Mechanism of the electrodialysis process
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Due to the selective permeability of ion exchange membranes, ions
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are transferred through the anion or cation exchange membrane following exposure to a direct current Therefore, desalination and concentration
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might be achieved.
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H+, Na+, Al3+, OH−, Al(OH)4− and AlO2− moved in response to the force of the electrical field. H+ and Na+ in the anode chamber passed through the CEM to enter the concentration chamber. Meanwhile, OH−
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and Al(OH)4− in the dilution chamber passed through the AEM to enter the concentration chamber. As a result, NaOH was recovered in the concentration chamber. Na+ and Al3+ in the dilution chamber passed through the CEM to enter the cathode chamber; therefore, NaOH and 26
NaAl(OH)4 in the dilution chamber were separated into the concentration chamber and the cathode chamber, respectively. (2) Concentration diffusion Concentration diffusion in the concentration chamber was another
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driving force that affected ion migration, particularly Al(OH)4−, Na+, and OH−. Concentration diffusion resulted in a direction of ion migration
concentration diffusion was inhibited by aeration.
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(3) Generation of membrane scaling
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opposite the force of the electrical field. Therefore, the effect of
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Al(OH)4− was ionized to produce Al(OH)3 (Eq. (4)). Al(OH)4−
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reacted with H+ to generate Al(OH)3 (Eq. (5)), which is the main
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component of membrane scaling. Al(OH)3 reacted with H+ to produce Al3+
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(Eq. (6)). Al(OH)3 reacted with OH− to produce AlO2− (Eq. (7)). AlO2− was hydrolyzed to generate Al(OH)3 (Eq. (8)), and AlO2− also reacted
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with Al3+ to form Al(OH)3 (Eq. (9)).
A l (O H )
4
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A l (O H )
A l (O H )
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A l (O H )
A l (O H )
A lO 3
Al
2
3H
OH
3
3
H
4
2H O 2
3 A lO
2
2
(5)
H O 2
3
(6)
3
A l 3H O 2
(7)
A lO 2 H O 2
2
A l (O H )
6H O
(4)
-
OH
A l (O H )
3
3
(8)
OH
4 A l (O H )
(9) 3
In the concentration chamber, the high concentration of OH− may 27
degrade Al(OH)3 and inhibit the hydrolysis of AlO2− and ionization of Al(OH)4−. Therefore, less membrane scaling was generated on the CEM. However, in the dilution chamber, the concentration of OH-
Al(OH)3 attached to the membrane to form a scale.
In this study, the
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4 Conclusions
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decreased as the number of batch experiments increased, and much more
combination of aeration
washing and
electrodialysis was found to be effective for dealkalizing bauxite residue.
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Aeration promoted dealkalization during the washing process because
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aeration increased the mixing efficiency by increasing the areas of contact
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between the alkaline liquid and water. The electrodialysis process performed under aeration conditions improved the removal of OH− and
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TDS and efficiently recovered NaOH and separated NaAl(OH)4 from the
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alkaline liquid. In addition, aeration decreased the energy consumption requirements of the electrodialysis system by reducing membrane fouling,
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which was 11.15 kW·h/kg and was less than 21.49 kW·h/kg of the energy consumed under non-aeration conditions. The ion exchange capacity of
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the CEM decreased by 7.5% after the experiment was performed under aeration conditions, while the capacity obtained under non-aeration conditions was 34.5%. For the AEM, the ion exchange capacity decreased by 1.8% and 28.8% under aeration and non-aeration conditions, respectively. Al(OH)3 was the main component of membrane scaling. An 28
acid treatment restored the performance of the membrane by removing scaling.
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Acknowledgments The research was partially supported by the Science and Technology
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Commission of Shanghai Municipality (No. 15230724300), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRE16007), and Fundamental Research Funds for the Central
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Universities (No. 0400219312).
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References
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[1] Y. Hua, K.V. Heal, and W. Friesl-Hanl, The use of red mud as an immobiliser for metal/metalloid-contaminated soil: A review. J. Hazard. Mater. 325 (2017) 17-30.
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[2] I. Smičiklas, S. Smiljanić, A. Perić-Grujić, M. Šljivić-Ivanović, M. Mitrić, and D. Antonović, Effect of acid treatment on red mud properties with implications on Ni(II) sorption and stability. Chem. Eng. J. 242 (2014) 27-35.
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[3] Ying Z, Wang J, Luan Z, et al. Removal of phosphate from aqueous solution by red mud using a factorial design[J]. Journal of Hazardous Materials, 165(2009):1193-1199.
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[4] Z. Wen, S. Ma, S. Zheng, Y. Zhang, and Y. Liang, Assessment of environmental risk for
red mud storage facility in China: a case study in Shandong Province. Environ. Sci. Pollut. R. 23 (2016) 11193-11208. [5] G. Power, M. Gräfe, and C. Klauber, Bauxite residue issues: I. Current management,
A
disposal and storage practices. Hydrometallurgy 108 (2011) 33-45. [6] K. Evans, The History, Challenges, and New Developments in the Management and Use
of Bauxite Residue. Journal of Sustainable Metallurgy 2 (2016) 316-331. [7] W.M. Mayes, I.T. Burke, H.I. Gomes, Á.D. Anton, M. Molnár, V. Feigl, and É. Ujaczki, Advances in Understanding Environmental Risks of Red Mud After the Ajka Spill, Hungary. Journal of Sustainable Metallurgy 2 (2016) 332-343. 29
[8] BEH Jones, and RJ Haynes, Bauxite Processing Residue: A Critical Review of Its Formation, Properties, Storage, and Revegetation. Critical Review of Environmental Science & Technology. 41 (2011) 271-315. [9] S. Samal, A.K. Ray, and A. Bandopadhyay, Proposal for resources, utilization and processes of red mud in India — A review. Int. J. Miner. Process. 118 (2013) 43-55. [10] S. Smiljanić, I. Smičiklas, A. Perić-Grujić, M. Šljivić, B. Đukić, and B. Lončar, Study of
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factors affecting Ni2+ immobilization efficiency by temperature activated red mud. Chem. Eng. J. 168 (2011) 610-619.
[11] X. Zhu, W. Li, and X. Guan, An active dealkalization of red mud with roasting and
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water leaching. J. Hazard. Mater. 286 (2015) 85-91.
[12] R. Li, T. Zhang, Y. Liu, G. Lv, and L. Xie, Calcification–carbonation method for red mud processing. J. Hazard. Mater. 316 (2016) 94-101.
[13] S.J. Palmer, M.K. Smith, and R.L. Frost, Effect of High Concentrations of Calcium
N
Residues. Ind. Eng. Chem. Res. 50 (2011) 1853-1859.
U
Hydroxide in Neutralized Synthetic Supernatant Liquor − Implications for Alumina Refinery
[14] M. Johnston, M.W. Clark, P. McMahon, and N. Ward, Alkalinity conversion of bauxite
A
refinery residues by neutralization. J. Hazard. Mater. 182 (2010) 710-715.
M
[15] S. Khaitan, D. A. Dzombak and G. V. Lowry, Chemistry of the Acid Neutralization Capacity of Bauxite Residue. Environmental Engineering Science. 26 (2009) 873-881.
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[16] L. Zhong, Y. Zhang, and Y. Zhang, Extraction of alumina and sodium oxide from red mud by a mild hydro-chemical process. J. Hazard. Mater. 172 (2009) 1629-1634. [17] R.C. Sahu, R.K. Patel, and B.C. Ray, Neutralization of red mud using CO 2 sequestration
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cycle. J. Hazard. Mater. 179 (2010) 28-34. [18] Khaitan S, Dzombak D A, Lowry G V, et al. Mechanisms of neutralization of bauxite
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residue by carbon dioxide.[J]. Journal of Environmental Engineering. 135(2009):433-438. [19] P. Renforth, W.M. Mayes, A.P. Jarvis, I.T. Burke, D.A.C. Manning, and K. Gruiz,
Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill:
A
The effects of gypsum dosing. Sci. Total Environ. 421-422 (2012) 253-259. [20] Y. Zhang, Y. Zhang, S. Zheng, H. Du, S. Wang, and Y. Peng, Alumina recovery from
spent Bayer liquor by methanol. Inorganic Chemicals Industry. 20 (2010) 165-168. [21] M.K. Hamdy, and F.S. Williams, Bacterial amelioration of bauxite residue waste of industrial alumina plants. J Ind Microbiol Biotechnol 27 (2001) 228-233. [22] F. Zhu, S. Xue, W. Hartley, L. Huang, C. Wu, and X. Li, Novel predictors of soil genesis following natural weathering processes of bauxite residues. Environ. Sci. Pollut. R. 23 30
(2016) 2856-2863. [23] C. Löser, A. Zehnsdorf, P. Hoffmann, and H. Seidel, Remediation of heavy metal polluted sediment by suspension and solid-bed leaching: Estimate of metal removal efficiency. Chemosphere 66 (2007) 1699-1705. [24] M. Luo, X. Qi, Y. Zhang, Y. Ren, J. Tong, Z. Chen, Y. Hou, N. Yeerkebai, H. Wang, S. Feng, and F. Li, Study on dealkalization and settling performance of red mud. Environ. Sci. Pollut.
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R. 24 (2017) 1794-1802. [25] H. Yan, S. Xue, C. Wu, Y. Wu, and T. Xu, Separation of NaOH and NaAl(OH)4 in alumina alkaline solution through diffusion dialysis and electrodialysis. J. Membrane Sci. 469
SC R
(2014) 436-446.
[26] X. Zhang, C. Li, X. Wang, Y. Wang, and T. Xu, Recovery of hydrochloric acid from simulated chemosynthesis aluminum foils wastewater: An integration of diffusion dialysis and conventional electrodialysis. J. Membrane Sci. 409-410 (2012) 257-263.
U
[27] T. Xu, and C. Huang, Electrodialysis‐ based separation technologies: A critical review.
N
54 (2008) 3147-3159.
[28] Y. Tanaka, Ion-Exchange Membrane Electrodialysis of Saline Water and Its Numerical
A
Analysis. Ind. Eng. Chem. Res. 50 (2011) 10765-10777.
M
[29] U. Hwang, and J. Choi, Changes in the electrochemical characteristics of a bipolar membrane immersed in high concentration of alkaline solutions. Sep. Purif. Technol. 48 (2006)
ED
16-23.
[30] J. Kinčl, V. Kysela, F. Toman, and J. Thomas, Application of electrodialysis for mine water treatment. Freiberg Online Geoscience. 122 (2015) 39-42.
PT
[31] H. Yan, C. Wu, and Y. Wu, Separation of alumina alkaline solution by electrodialysis: Membrane stack configuration optimization and repeated batch experiments. Sep. Purif. Technol.
CC E
139 (2015) 78-87.
[32] Y. TANAKA, Limiting current density of an ion-exchange membrane and of an
electrodialyzer. J. Membrane Sci. 266 (2005) 6-17. [33] X. Qi, H. Wang, C. Huang, L. Zhang, J. Zhang, B. Xu, F. Li, and J.T.A. Junior, Analysis
A
of bauxite residue components responsible for copper removal and related reaction products. Chemosphere (2018). [34] Determination of alkalinity (total alkalinity, bicarbonate and carbonate) (acid titration), SL 83-1994, http://www.bzwxw.com/html/15/5558.html ,2018 (accessed 1 January 2018). [35] G. Zhang, S. Li, X. Zhang, and Z. Zhang, Study on the effect of fresh water washing on dealkalization of red mud from Bayer process. Journal of Qingdao Technological University, 33 31
(2012) 59-62. [36] Xu Zhenliang, Membrane Water Treatment Technology. Chemical Industry Press. Beijing, 2001. [37] Ueda T, Hata K, Kikuoka Y, et al. Effects of aeration on suction pressure in a submerged membrane bioreactor[J]. Water Research, 31(1997):489-494. [38] Snars, K.;Gilkes, R., Evaluation of bauxite residues (red muds) of different origins for
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environmental applications. Applied Clay Science, 46 (2009): 13-20. [39] Suzuki K, Tanaka Y, Osada T, et al. Removal of phosphate, magnesium and calcium
from swine wastewater through crystallization enhanced by aeration[J]. Water Research,
SC R
36(2002):2991-2998.
[40] Z. Liu, H. Li, M. Huang, D. Jia, and N. Zhang, Effects of cooling method on removal of sodium from active roasting red mud based on water leaching. Hydrometallurgy 167 (2017) 92-100.
U
[41] L. Zhang, G. Wang, and L. Duan, Preliminary dealkalization of red mud by washing
N
process. Inoganic Chemical Industry. (2011) 57-58.
[42] T. Wang, and W. Yang, Factors affecting the current and the voltage efficiencies of the
A
synthesis of quaternary ammonium hydroxides by electrolysis–electrodialysis. Chem. Eng. J. 81
M
(2001) 161-169.
[43] W. Garcia-Vasquez, L. Dammak, C. Larchet, V. Nikonenko, and D. Grande, Effects of
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acid–base cleaning procedure on structure and properties of anion-exchange membranes used in electrodialysis. J. Membrane Sci. 507 (2016) 12-23. [44] S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, T. Maruyama, and H. Matsuyama,
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Improvement of the antifouling potential of an anion exchange membrane by surface modification with a polyelectrolyte for an electrodialysis process. J. Membrane Sci. 417-418 (2012) 137-143.
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[45] J. Park, H. Lee, and S. Moon, Determination of an optimum frequency of square wave power for fouling mitigation in desalting electrodialysis in the presence of humate. Sep. Purif.
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Technol. 30 (2003) 101-112.
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S0304-3894(18)30932-4 https://doi.org/10.1016/j.jhazmat.2018.10.023 HAZMAT 19843
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-7-2018 13-9-2018 9-10-2018
Please cite this article as: Zhang Y, Shi Q, Luo M, Wang H, Xuejiao Q, Hou CHung, Fengting L, Ai Z, Araruna Junior JT, Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.10.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis
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Yaxian Zhang1,2, Qi Shi1,2, Muxi Luo1,2, Hongtao Wang1,2*, Xuejiao Qi1,2, Chia-Hung Hou3*, Fengting Li1,2, Zisheng Ai4, Jose Tacares Araruna Junior5 1. State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze River Water Environment, Ministry of Education, College of Environmental Science and Engineering, UNEP-TONGJI Institute of Environment for Sustainable Development, Tongji University, Siping Rd 1239, Shanghai 200092, P. R. China 2. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R. China 3. Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC 4. Department of Medical Statistics, School of Medicine, Tongji University, 1239 Siping Road, Yangpu District, Shanghai 200092, PR China 5. Department of Civil and Environmental Engineering, Pontifical Catholic University of Rio de Janeiro, Rio de Janeiro, Brazil * Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (C.-H. Hou). Tel: +86-21-65978598 (H. Wang), +886-2-33664400(C.-H. Hou). Fax: +86-21-65985059 (H. Wang),+886-2-23928830(C.-H. Hou).
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To be submitted to Journal of Hazardous Materials
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Graphical abstract
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Highlights
Combined washing and electrodialysis improved dealkalination of bauxite residue.
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Aeration enhanced the dealkalination efficiency by increasing ion transfer rate. Electrodialysis with aeration can efficiently recover NaOH and separate NaAl(OH)4. Al(OH)3 was the main component of membrane scaling. 2
Al(OH)3 resulted from Al(OH)4- and AlO2-.
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Abstract Bauxite residue, a major by-product of the alumina-producing Bayer
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process, is a serious environmental pollutant due to its high alkalinity.
Here, we reported an operation system designed in our laboratory that
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included washing and electrodialysis dealkalization systems with aeration
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pipes. Washing with aeration releases a substantial amount of free alkali
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and attached alkali into water and increases the dealkalization efficiency.
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The washing liquid was treated with five steps of batch-mode electrodialysis. The average removal of total dissolved solids (TDS) after
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the aeration and non-aeration electrodialysis processes were 61.30% and
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39.61%, respectively. The average removal of OH− under aeration conditions was 76.62%, a value that was greater than the value produced
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under non-aeration conditions (68.48%). This efficiency was also higher than that of some other reports (64.90-68.50%). Aeration decreased the
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energy consumption to a greater extent than the non-aeration condition. NaOH was recovered in terms of the concentration chamber, and the NaAl(OH)4 present in the dilution chamber was separated for the electrodialysis treatment. Membrane scaling was generated to a lesser amount under aeration conditions than that of non-aeration conditions, 3
which would improve the dealkalization efficiency. The high repeatability of the experiments was indicated by the intraclass correlation coefficient (P<0.05).
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Keywords: Bauxite residue; Aeration; Electrodialysis; Dealkalization;
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Membrane.
1. Introduction
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Bauxite residue, a by-product of alumina production through the
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Bayer process, exerts a substantial impact on environments by
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contaminating water and soil due to alkali liquid leakage, polluting the air
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with bauxite residue dust, and altering land occupation [1,2]. Bauxite
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residue is a type of solid waste with high alkalinity and a high heavy metal content [3], which is a potential threat to the surrounding soil and
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water ecosystems. The highly alkaline leachate pollute the ground water
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or surface water. The production of bauxite residue in China can reach up to 70 million tons per year, accounting for a relatively significant share (up to 30%) of the global production. The total weight of bauxite residue
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was approximately 0.51 billion tons by the end of 2016, but only 4% of these residues were comprehensively used as subgrade materials, adsorbents, and flocculants [4]. Bauxite residue has previously been treated by direct discharge into 4
seas [5], but this method has been prohibited in China since the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter began to be enforced in 1985. Since the late 1970s, bauxite residue has been treated by stacking and then burying [6].
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However, this method not only used nearly all land but also resulted in the easy percolation into the soil and groundwater, which then causes
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serious environmental pollution [7-9]. Moreover, when the volume of the bauxite residue dam increases to a certain extent, pits may collapse and
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severely endanger the safety of the people and property.
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Although the properties of bauxite residues from various sites in the
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world differ, the general characteristics of bauxite residue are its high
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alkalinity and abundant Al and Fe contents [10]. The high alkalinity
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makes bauxite residue difficult to recycle; thus, dealkalization methods, which are expected to broaden the recycling methods, decrease resource
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loss, and mitigate the risk of environmental pollution and dam collapse,
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should be studied. Currently available dealkalization methods include water leaching [11], calcification–carbonation [12,13], acid neutralization [14-16], mild hydro-chemical processes [16], the CO2 sequestration cycle
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[17-19], wet leaching [20], and bacterial amelioration [21,22], but nearly all of these methods exhibit several disadvantages that seriously affect their practical application. For example, water leaching effectively removes free alkali but not bound alkali [11]. In addition, this process 5
requires a long time and produces a large amount of alkali washing liquid [23]; thus, the approach should be combined with other technologies [24]. With the exception of the dealkalization methods described above, other developing methods, such as flocculation and membrane dealkalization
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method, exhibit satisfactory treatment efficiency in the bench scale but present poor biological stability or severe membrane fouling, thus
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rendering their practical application difficult [25,26].
In recent years, electrodialysis technology has garnered considerable
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attention in the field of alkali liquid recycling [27,28]. Hwang et al. [29]
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used electrodialysis with double polar films to treat high concentrations
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of NaOH and studied changes in membrane stability at different
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temperature. Kinčl Jan et al. [30] employed electrodialysis to dealkalize
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and concentrate the alkaline liquid of bauxite residue. Yan et al. [31] simulated the alkalinity and caustic ratio of bauxite residue using NaOH
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and NaAl(OH)4 to examine the efficiency of the electrodialysis
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dealkalization technology. This technology is efficient and requires a low energy consumption of only 12.43 kW·h/kg. Because bauxite residue exhibits poor fluidity and settles easily,
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which are unfavorable characteristics for electrodialysis technology, pretreatment of bauxite residue and a combination with other technologies are necessary. Therefore, we developed a process for the dealkalization of bauxite residue by employing washing in combination 6
with electrodialysis to investigate the actual dealkalization efficiency. The ion turbulence of the system is poor under non-aeration. Meanwhile, aeration can accelerate the mass-transfer rate of ions, decrease concentration polarization, and improve the conductivity of the
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electrodialysis system [32]. We also placed aeration pipes in both processes to increase the dealkalization efficiency. The major aims of this
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study are: (I) to explore the feasibility of the practical bauxite residue
dealkalization process based on washing and electrodialysis technologies,
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(II) to study the bauxite residue dealkalization efficiency of washing and
2.1 Materials
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2. Materials and Methods
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the changes in membrane properties.
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electrodialysis processes under aeration conditions, and (III) to analyze
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All reagents used in this study were of analytical grade and were
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purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
The bauxite residue and its alkali liquid used in this study were
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generated by the Bayer process, and the bauxite residue mainly contained SiO2 (20.76 wt%), Al2O3 (20.40 wt%), Na2O (17.33 wt%), CaO (17.56 wt%), and Fe2O3 (6.44 wt%), as mentioned in our previous publications [24,33]. These materials were collected from Shanxi Province, China. 7
(The pretreatments performed before the experiment are described in the Supplementary Materials.) A ruthenized titanium plate (1 mm, 9 cm × 20 cm) was provided by Baoji Baoye Titanium-Nickel Industry Co., Ltd. (Shanxi, China). The cation exchange membrane (CEM, JCM-II-07) and
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anion exchange membrane (AEM, JAM-II-07) were purchased from Beijing Jierui Environmental Science and Technology Co., Ltd. (Beijing,
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China) and displayed a range of properties, as shown in Table 1. Table 1 Properties of the CEM and AEM CEM
JCM-Ⅱ-07 −SO3H Homogeneous and acid-alkali resistance 0.16–0.23 33–40 1.3–4.0 95–99 ≥ 0.25 < 45 °C
Characteristic
JAM-Ⅱ-07 −NR3OH Homogeneous and acid-alkali resistance 0.16–0.23 24–28 3.0–6.5 90–95 ≥ 0.25 < 45 °C
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2.2 Experiments
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Thickness (mm) Water content (%) Membrane area resistance (Ω·cm2) Selective permeability (%) Bursting strength (MPa) Temperature range
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Specifications Functional group
AEM
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Every experiment in our study was performed in triplicate. Figure 1
shows the complete process used in our study. The left panel shows the
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washing system, and the right panel shows the electrodialysis system.
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Figure 1 Schematic of the system combining washing with electrodialysis
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2.2.1 Washing system
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We designed a washing system, including a washing tower and a
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storage tower, as shown in Figure 1 (left panel). Bauxite residue and water entered the washing tower from the bottom at a ratio of 1:7, as
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determined in a preliminary experiment (shown in Figure B and C in the
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Supplementary Materials), because a considerable washing efficiency was observed, and the slurry-to-water ratio was similar to a study of
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dealkalization of bauxite residue with roasting and water leaching [11]. An aeration pipe was also utilized to increase the washing efficiency. Due
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to its slow-release property, bauxite residue was washed with 3 min of aeration and 30 min of settling for six cycles. After each washing step, the system remained static without aeration to allow the bauxite residue to precipitate, the washing alkali liquid to flow into storage tower, and the bauxite residue settling at the bottom to 9
be discharged by the sludge pipe. The first two volumes of alkali washing liquid were discharged into water storage chamber 1 and mixed; given the high alkalinity, the mixture entered the electrodialysis–dealkalization device immediately after filtration.
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Aeration promotes the washing effect, based on a preliminary experiment in which the pH, TDS, and alkalinity were compared under
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aeration conditions and in a non-aerated system. Finally, we selected 15 L/min as the optimal aeration condition based on our results (shown in
1
(right
panel)
shows
a
schematic
of
the
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Figure
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2.2.2 Electrodialysis dealkalization system
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Figure D in the Supplementary Materials).
electrodialysis-dealkalization system designed by our group. The whole
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system was divided into two floors: the upper floor contained the
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electrodialysis unit, and the lower floor contained the adjustment and water storage chamber unit. The CEM, AEM, and electrode plates
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(effective area: 180 cm2) were alternatively distributed in the device, thereby forming the anode chamber, concentration chamber, dilution
A
chamber, and cathode chamber, respectively. Aeration pipes were placed in the concentration and dilution chambers. According to the preliminary experiments, by maintaining a certain conductivity, low energy consumption and low membrane scaling, the water quality of the effluent was a pH of 11.60~11.80 and a TDS concentration of 3.50-5.50 g/L. 10
Considering the bauxite residue-washing performance and water quality requirement of the outflow in this study, we collected the alkaline liquid from the first and second wash steps, which was disposed of by the electrodialysis system, in the washing system and recovered the alkaline
IP T
liquid from the electrodialysis system using ion exchange. The washing liquid was treated with batch-mode electrodialysis in
SC R
five cycles. Batch experiments were performed consecutively without substitution of membranes. The standard method “Determination of
U
alkalinity, bicarbonate, and carbonate” (SL 83-1994) [34] was used to
N
measure the alkalinity of bauxite residue and the washing and desalinated
A
alkaline liquids. Moreover, the contents of metallic elements in the
M
alkaline liquid were measured using an inductively coupled plasma
ED
optical emission spectrometer (ICP–OES, Agilent720ES, Agilent Technologies). The contents of salt anions in the alkaline liquid were
PT
measured using ion chromatography (ICS 1000+AS20).
CC E
Different aeration intensities (0, 5, 15, and 30 L/min) were also selected for this process based on the results from the preliminary
A
experiments (shown in Figure E in the Supplementary Materials). Energy consumption was calculated using the following equation: E=
t
U (t ) I
0
C0 Ct '
'
dt
(1)
,
where E is the energy consumption (kW·h/kg), U (t) is a function of the change in voltage with time, t is the time (s), I is the current (A), and 11
C0' and Ct' are the concentrations of TDS in the dilution chamber at times 0 and t, respectively. Current efficiency was calculated using the following equation: =
(C 0 C t ) V F
100%
(2)
,
Id t 0
IP T
N
t
where ζ is the current efficiency, V is the volume of the alkaline
SC R
liquid in the dilution chamber, F is Faraday’s constant, 96,485 C/mol, N is the unit number, N = 1, I represents current (A), t represents time (s), and
C0 and Ct are the molar concentrations of NaOH and NaAl(OH)4 in the
N
U
dilution chamber at times 0 and t, respectively.
M
2.2.3.1 Ion exchange capacity
A
2.2.3 Characterization of fouling during electrodialysis
ED
The pretreatment of membranes in studies aiming to determine the ion exchange capacity is based on the Electrodialysis Technology
PT
Industry Standard (HY/T 034.1~034.5-1994). Specific experimental
CC E
procedures are described in the Supplementary Materials. HCl acidification was used to remove the scale from the membrane after the
A
electrodialysis process. The ion exchange capacity (IEC) was calculated using the following
equation. IE C
Cn V m (1 C w )
,
(3)
where IEC is the ion exchange capacity of the CEM and AEM 12
(mol/kg), Cn is the concentration of the titration reagent (mol/L), V is the volume of the titration reagent consumed (L), m is the weight of membrane (kg), and Cw is the water content of the membrane (%). 2.2.3.2 Morphological analysis was
analyzed
using
scanning
electron
IP T
Morphology
SC R
microscopy–energy dispersive X-ray spectroscopy (JSM-6701F, Japan). (The pretreatment is shown in the Supplementary Materials.)
U
2.2.3.3 Analysis of the composition of membrane fouling
N
X-ray diffraction (D8 Advance, BRUKER AXS GMBH) was
A
utilized to measure the phase of membrane fouling. Elements and their
ED
Agilent Technologies).
M
contents in the membrane were measured using ICP–OES (Agilent720ES,
SPSS 24 software was used to perform the statistical analyses in our
PT
study.
CC E
3. Results and Discussion As shown in Table 2, the molar ratio of aluminate to sodium in the
A
alkaline liquid was approximately 1:2. The primary soluble metal elements were Al, K, and Na, as shown in our previous publication [24]. The main components in the alkaline solution were NaCO3 and NaAl(OH)4, according the determination of alkalinity. For a convenient experimental analysis, the present study mainly considered the changes in 13
the concentrations of Al and Na in the alkaline liquid. This simplified analysis has been applied in many studies [16,25,31]. Table 2 Concentrations of Different Ions in the Alkaline Liquid K
Na
F-
SO42-
PO43-
NO3-
2434.12
668.73
4165.21
60.06
1.98
35.39
0.62
3.1 Washing process for dealkalization determined
the
washing
times,
water
consumption,
SC R
We
IP T
Concentration (mg/L)
Al
slurry-to-water ratio, and aeration intensity as parameters in six preliminary tests to evaluate the feasibility of the dealkalization of
N
U
bauxite residue using the washing system.
A
The pH and concentrations of TDS and soluble metal elements were
M
determined together; therefore, pH was used as representative parameter to calculate the intraclass correlation coefficient (ICC). The results are
ED
presented in Table 3. The ICCs (P<0.05) of a single group and the whole
PT
group were all greater than 0.995, indicating that the results are highly reproducible. The results of the T test are shown in the Supplementary
CC E
Materials (Table B).
A
Table 3 Intraclass Correlation Coefficients for pH in the Washing Process Intraclass
95% Confidence Interval
F test using a truth value of 0
Correlation
Lower
Upper
Value
df 1
df 2
Group (Non-aeration)
0.996
0.984
0.999
730.589
5
12
0.000
Group (Aeration)
1.000
0.999
1.000
14971.558
5
12
0.000
0.999
0.997
1.000
2802.740
11
24
0.000
P
Group (Non-aeration or Aeration) 14
One-way random effect model with random personnel effect.
As shown in Figure 2, the pH and concentrations of TDS and soluble metal elements in the alkaline washing liquid of bauxite residue
IP T
were considerably decreased as the washing times increased, regardless of the use of non-aeration or aeration conditions. In addition, free metal
SC R
elements were rapidly removed by washing. This finding is consistent
with the results obtained from washing processes used in another study [35].
N
U
As outlined in Figure 2 (A), the pH decreased after each wash.
A
Notably, under aeration conditions, the washing liquids obtained after the
M
first to third washes exhibited higher pH values than the liquids obtained under non-aeration conditions, namely, a greater amount of alkaline
ED
material entered the washing liquid and the reside was completely washed
PT
under aeration conditions because of a rapid ion mass-transfer rate [36]. Aeration substantially increases the mixing efficiency by increasing the
CC E
liquid flow velocity and the contact areas [37]. The alkaline components in bauxite residue consist of free alkali, attached alkali and binding alkali.
A
Washing transferred much of the free alkali and attached alkali from bauxite to water under aeration conditions. However, as the washing times increased, the pH did not exhibit apparent changes, due to the sustained release of sodalite and cancrinite from bauxite residue [38], which decreased the rate of alkali entering the water. 15
As illustrated in Figure 2 (B), a considerably greater amount of TDS was detected in the washing liquid under aeration conditions than under non-aeration conditions. TDS will enter the washing liquid after numerous or extensive washes. Aeration effectively induces the
IP T
crystallization of some substances, resulting in an increase in the release of solids in the washing effluent [39]. Importantly, aeration enhanced the
SC R
efficiency of dealkalization. The TDS concentration in combination of all
six washing liquid samples was 39.45 g under aeration conditions and
U
only 2.18 g under non-aeration conditions. After two washes, the removal
N
of TDS exceeded 95%, but the concentration remained high; therefore,
A
CC E
PT
ED
M
A
we used electrodialysis to decrease the TDS concentration.
16
Figure 2 pH (A), TDS concentration (B), and concentrations of soluble metal elements in the alkaline liquid (C and D) after different washes during washing under non-aeration and aeration conditions
As shown in Figures 2 (C) and 3 (D), considerable concentrations of soluble metal elements were detected in the bauxite residue slurry.
IP T
Overall, the removal of soluble metal elements was considerably increased under aeration conditions than under non-aeration conditions.
SC R
The final concentration of Na+ in the washing liquid was similar with the results of a leaching process for sodium from roasted bauxite residue
U
cooled by water, indicating that the water cooling method was helpful for
N
leaching sodium from a pretreated bauxite residue [40]. When the pH >
A
11, Al(OH)3 started dissolving. The concentration of Al3+ increased. The
M
concentrations of soluble metal elements might affect the concentration of
ED
TDS. Therefore, the changes in pH, TDS concentration and soluble metal concentrations exhibited similar trends in the Figure 2 (A) and (B).
PT
According to some studies [35,41],41] washing confers good
CC E
efficiency on dealkalization processes, but aeration was not performed in these washing processes. Based on the results from the present study, the efficiency of dealkalization was considerably improved by a washing
A
process incorporating aeration.
3.2 Electrodialysis process for dealkalization An increase in the current can accelerate the ion mobility rate, but energy consumption would increase as the current increased. Hence, in 17
this study, 2.7 A was selected as the suitable current, resulting in a current efficiency of 59.62% and an energy consumption of 12.97 kW·h/kg, which is slightly higher than the values of 56.90% and 12.26 kW·h/kg reported in another study [42]. The details of the method used to select
presented in the Supplementary Materials (Figure F).
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the current based on current efficiency and energy consumption are
SC R
As outlined in Figure 3, compared to non-aeration conditions,
aeration decreased the energy consumed by the system, due to the
U
reduction of concentration polarization. The average energy consumption
N
values under non-aeration and aeration conditions were 21.49 and 11.15
A
kW·h/kg, respectively. The value obtained under aeration conditions was
A
CC E
PT
ED
M
lower than the value of 12.43 kW·h/kg reported by Yan [31].
Figure 3 Energy consumption of the electrodialysis system
3.2.1 Removal of OH− and TDS The dealkalization efficiency of five batch experiments performed 18
under aeration or non-aeration conditions is shown in Table 4. The results of the T test are shown in the Supplementary Materials (Table C). Table 4 Removal of OH− and TDS from the Dilution Chamber under Aeration and Non-Aeration Conditions Non-aeration
Batch
Aeration
Removal of OH− (%)
Removal of TDS (%)
TDS (g/L)
Removal of OH− (%)
Removal of TDS (%)
1
9.68
54.29
24.38
4.48
82.62
65.54
2
8.42
65.33
35.73
4.78
81.80
63.79
3
6.97
73.70
46.79
4.94
80.50
61.41
4
6.97
73.08
46.38
5.42
69.80
58.63
5
7.07
76.01
44.77
5.57
68.38
57.15
Mean
―
68.48
39.61
―
76.62
61.30
SC R
U
N
IP T
No.
TDS (g/L)
Test
M
A
As shown in Table 4, the average removal of OH− under aeration was 76.62%, while under non-aeration it was 68.48%. A study on
ED
electrodialysis with membrane stack configuration optimization reported
PT
the removal of OH− as 64.90-68.50% [31]. The result was 63.80% in Yan et al. [25], which treated alkaline solution through diffusion dialysis and
CC E
electrodialysis. The average removal of TDS under aeration was 61.30%, while under non-aeration it was 39.61%. Aeration could enhance the ion
A
transfer and decrease concentration polarization. In addition, aeration could prevent ion channels from blocking, improving the removal efficiency. The removal trend of OH− and TDS increased under non-aeration conditions, but decreased under aeration conditions. A potential 19
explanation is that the concentration of alkaline liquid was considerably higher in the concentration chamber in the batch experiments performed under aeration conditions (Figure 2), resulting in a large concentration gradient difference between the two sides of the membrane and
IP T
concentration and dilution chambers, as well as a strong self-diffusion effect [28]. Therefore, the removal of OH− and TDS decreased as the
SC R
number of batch experiments performed under aeration conditions
increased. Antagonism occurs between electrodialysis and the diffusion
U
effect, possibly due to a slow anion migration rate [36]. However, under
N
non-aeration conditions, substantial water dissociation occurred on both
A
sides of the membrane, and ion movement was negligible.
M
In addition, the TDS concentration in the outflow under non-aeration
ED
conditions did not satisfy the requirement (TDS = 3.50–5.50 g/L), but stable effluent water quality might be achieved under aeration conditions.
PT
3.2.3 Dealkalization under aeration conditions
CC E
Considering the satisfying behaviors of the system under aeration
conditions, the dealkalization efficiency under aeration conditions was
A
further studied. This experiment aims not only to recover the alkali but also to separate NaAl(OH)4 from the alkaline liquid for further treatment.
20
IP T SC R
U
Figure 4 Concentrations of Na and Al in the concentration and dilution chambers in consecutive batch experiments
N
As outlined in Figure 4, the concentrations of Na and Al in the
A
concentration chamber increased as the number of batch experiments
M
increased. The concentration of Na in the concentration chamber showed
ED
a larger increase than the Al concentration. This difference may be attributed to the more rapid migration rate of Na+ than Al(OH)4− through
PT
the ion exchange membrane [28]. Al(OH)4− and OH− passed through the
CC E
AEM and entered the concentration chamber by the driving force of the electric field; Na+ entered the concentration chamber through the CEM from the anode chamber. Therefore, the electrodialysis process attained
A
the goal of recovering the alkali and separate NaAl(OH)4 from the alkaline liquid. The amounts of Na and Al removed from the dilution chamber in each batch experiment were approximately equal to the amounts 21
recovered from the concentration chamber, indicating that the material flow was substantially balanced in the system (Figure 4). The rate of increase in the Na and Al concentrations in the concentration chamber gradually decreased due to the diffusion and dialysis of the ion exchange
IP T
membrane used for electrodialysis, thus restraining the migration and concentration of desorbed ions.
SC R
3.3 Evaluation of the fouling of ion exchange membranes
Ion exchange membranes play an important role in determining the
U
separation performance of electrodialysis. The formation of scale on the
N
membrane surface is a direct cause of membrane contamination [43,44].
M
A
Therefore, the ion exchange membranes used in the present study were further evaluated and treated to improve the properties of the membranes.
ED
3.3.1 Scaling of membranes
PT
The surface morphology of the CEM and AEM before and after HCl treatment under aeration and non-aeration conditions is shown in the
CC E
Supplementary Materials (Figure G).As shown in Figure G (left panel), the surface morphology of CEM did not change considerably under
A
different conditions, indicating that the surface structure of the CEM remained intact. The surface structure and active functional groups in the CEM were not markedly damaged after HCl acidification. The acidification treatment effectively removed the scale attached to the membrane and purified the cations into hydrogen. In addition, the 22
proportion of aluminum and sodium in the CEM was consistent with the results of ICP (Figure G). High electro-convection at the CEM caused an increase in mass transfer, which facilitated the removal of the scale from the surface [43].
IP T
However, compared with the initial AEM, the surface morphology of the AEM did not change considerably under aeration conditions. Under
SC R
non-aeration conditions, a considerable amount of damage was observed on the AEM surface (Figure G (right panel)), and scale was attached to
U
the surface of the membrane. The aluminum fractions in AEM under
N
aeration and non-aeration conditions were 6.90% and 9.39%, respectively,
A
indicating that the acidification treatment did not completely remove the
M
scale from the membrane. In one study [45], scale formation was evident
ED
on the dilute side of the AEM, and the water splitting rate was higher at
side.
PT
the CEM than that at the AEM, thereby preventing scaling on the dilute
CC E
Membrane scaling is one of the main factors contributing to the decreases in the water content and ion exchange capacity of the CEM and
A
AEM and subsequent decrease in the dealkalization efficiency [44,45]. In the present study, the membrane scale produced during
electrodialysis was collected. According to the XRD results (Figure 5 (A)), the phase composition of the membrane scale was aluminum hydroxide (α-Al(OH)3), consistent with the analytical results from the 23
electrodialysis-concentrated alkali recovery [31]. This finding is also consistent with findings from another study [42]. As shown in Figure 5 (B), the content of Al in the CEM was low under every condition, with a markedly lower content measured under conditions
than
under
non-aeration
conditions.
Under
IP T
aeration
non-aeration conditions, concentration polarization around the membrane
SC R
was evident, and AlO2− and Al(OH)4− were converted into an Al(OH)3 precipitate to form membrane scaling, which is attached to the membrane
U
surface and ion channels. This finding also indirectly decreased the
CC E
PT
ED
M
A
N
membrane moisture content and ion exchange capacity.
Figure 5 XRD of membrane scale (A) and contents of Al and Na in the membrane (B)
A
3.3.2 Ion exchange capacity of experimental membranes before and after the acid treatment
24
IP T
SC R
Figure 6 Ion exchange capacity (A: CEM and B: AEM) of the experimental membranes and HCl-pretreated membranes
As shown in Figures 6 (A) and (B), a slight change in ion exchange
U
capacity was observed in CEM and AEM under aeration conditions than
A
N
in the initial membrane, decreasing 0.03 mol/kg (1.8%) and 0.93 mol/kg
M
(7.5%), respectively. However, the ion exchange capacity decreased under non-aeration conditions to a greater extent than under aeration
ED
conditions, 0.47 mol/kg (28.8%, CEM) and 4.29 mol/kg (34.5%, AEM),
PT
respectively. Aeration could increase the ion migration rate, and few ion channels were occupied, keeping the option performance of membranes
CC E
and enhancing ion exchange membrane flux. Acidification also increased the ion exchange capacity of the CEM and AEM because the acidification
A
of HCl, CEM, and AEM were purified by hydrogen and chlorine, and membrane scales of CEM and AEM were removed to a certain extent [43]. Thus, the number of active sites in the ion exchange membrane increased, and the ion exchange capacities of the CEM and AEM were increased. The result is similar to the findings reported by Yan [31]. 25
3.4 Mechanism of the electrodialysis process According to the results of our experiments, a summary of the
U
SC R
IP T
mechanism is shown in Figure 7.
A
(1) Force of the electrical field
N
Figure 7 Mechanism of the electrodialysis process
M
Due to the selective permeability of ion exchange membranes, ions
ED
are transferred through the anion or cation exchange membrane following exposure to a direct current Therefore, desalination and concentration
PT
might be achieved.
CC E
H+, Na+, Al3+, OH−, Al(OH)4− and AlO2− moved in response to the force of the electrical field. H+ and Na+ in the anode chamber passed through the CEM to enter the concentration chamber. Meanwhile, OH−
A
and Al(OH)4− in the dilution chamber passed through the AEM to enter the concentration chamber. As a result, NaOH was recovered in the concentration chamber. Na+ and Al3+ in the dilution chamber passed through the CEM to enter the cathode chamber; therefore, NaOH and 26
NaAl(OH)4 in the dilution chamber were separated into the concentration chamber and the cathode chamber, respectively. (2) Concentration diffusion Concentration diffusion in the concentration chamber was another
IP T
driving force that affected ion migration, particularly Al(OH)4−, Na+, and OH−. Concentration diffusion resulted in a direction of ion migration
concentration diffusion was inhibited by aeration.
U
(3) Generation of membrane scaling
SC R
opposite the force of the electrical field. Therefore, the effect of
N
Al(OH)4− was ionized to produce Al(OH)3 (Eq. (4)). Al(OH)4−
A
reacted with H+ to generate Al(OH)3 (Eq. (5)), which is the main
M
component of membrane scaling. Al(OH)3 reacted with H+ to produce Al3+
ED
(Eq. (6)). Al(OH)3 reacted with OH− to produce AlO2− (Eq. (7)). AlO2− was hydrolyzed to generate Al(OH)3 (Eq. (8)), and AlO2− also reacted
PT
with Al3+ to form Al(OH)3 (Eq. (9)).
A l (O H )
4
CC E
A l (O H )
A l (O H )
A
A l (O H )
A l (O H )
A lO 3
Al
2
3H
OH
3
3
H
4
2H O 2
3 A lO
2
2
(5)
H O 2
3
(6)
3
A l 3H O 2
(7)
A lO 2 H O 2
2
A l (O H )
6H O
(4)
-
OH
A l (O H )
3
3
(8)
OH
4 A l (O H )
(9) 3
In the concentration chamber, the high concentration of OH− may 27
degrade Al(OH)3 and inhibit the hydrolysis of AlO2− and ionization of Al(OH)4−. Therefore, less membrane scaling was generated on the CEM. However, in the dilution chamber, the concentration of OH-
Al(OH)3 attached to the membrane to form a scale.
In this study, the
SC R
4 Conclusions
IP T
decreased as the number of batch experiments increased, and much more
combination of aeration
washing and
electrodialysis was found to be effective for dealkalizing bauxite residue.
N
U
Aeration promoted dealkalization during the washing process because
A
aeration increased the mixing efficiency by increasing the areas of contact
M
between the alkaline liquid and water. The electrodialysis process performed under aeration conditions improved the removal of OH− and
ED
TDS and efficiently recovered NaOH and separated NaAl(OH)4 from the
PT
alkaline liquid. In addition, aeration decreased the energy consumption requirements of the electrodialysis system by reducing membrane fouling,
CC E
which was 11.15 kW·h/kg and was less than 21.49 kW·h/kg of the energy consumed under non-aeration conditions. The ion exchange capacity of
A
the CEM decreased by 7.5% after the experiment was performed under aeration conditions, while the capacity obtained under non-aeration conditions was 34.5%. For the AEM, the ion exchange capacity decreased by 1.8% and 28.8% under aeration and non-aeration conditions, respectively. Al(OH)3 was the main component of membrane scaling. An 28
acid treatment restored the performance of the membrane by removing scaling.
IP T
Acknowledgments The research was partially supported by the Science and Technology
SC R
Commission of Shanghai Municipality (No. 15230724300), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRE16007), and Fundamental Research Funds for the Central
N
U
Universities (No. 0400219312).
A
References
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[1] Y. Hua, K.V. Heal, and W. Friesl-Hanl, The use of red mud as an immobiliser for metal/metalloid-contaminated soil: A review. J. Hazard. Mater. 325 (2017) 17-30.
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[2] I. Smičiklas, S. Smiljanić, A. Perić-Grujić, M. Šljivić-Ivanović, M. Mitrić, and D. Antonović, Effect of acid treatment on red mud properties with implications on Ni(II) sorption and stability. Chem. Eng. J. 242 (2014) 27-35.
PT
[3] Ying Z, Wang J, Luan Z, et al. Removal of phosphate from aqueous solution by red mud using a factorial design[J]. Journal of Hazardous Materials, 165(2009):1193-1199.
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[4] Z. Wen, S. Ma, S. Zheng, Y. Zhang, and Y. Liang, Assessment of environmental risk for
red mud storage facility in China: a case study in Shandong Province. Environ. Sci. Pollut. R. 23 (2016) 11193-11208. [5] G. Power, M. Gräfe, and C. Klauber, Bauxite residue issues: I. Current management,
A
disposal and storage practices. Hydrometallurgy 108 (2011) 33-45. [6] K. Evans, The History, Challenges, and New Developments in the Management and Use
of Bauxite Residue. Journal of Sustainable Metallurgy 2 (2016) 316-331. [7] W.M. Mayes, I.T. Burke, H.I. Gomes, Á.D. Anton, M. Molnár, V. Feigl, and É. Ujaczki, Advances in Understanding Environmental Risks of Red Mud After the Ajka Spill, Hungary. Journal of Sustainable Metallurgy 2 (2016) 332-343. 29
[8] BEH Jones, and RJ Haynes, Bauxite Processing Residue: A Critical Review of Its Formation, Properties, Storage, and Revegetation. Critical Review of Environmental Science & Technology. 41 (2011) 271-315. [9] S. Samal, A.K. Ray, and A. Bandopadhyay, Proposal for resources, utilization and processes of red mud in India — A review. Int. J. Miner. Process. 118 (2013) 43-55. [10] S. Smiljanić, I. Smičiklas, A. Perić-Grujić, M. Šljivić, B. Đukić, and B. Lončar, Study of
IP T
factors affecting Ni2+ immobilization efficiency by temperature activated red mud. Chem. Eng. J. 168 (2011) 610-619.
[11] X. Zhu, W. Li, and X. Guan, An active dealkalization of red mud with roasting and
SC R
water leaching. J. Hazard. Mater. 286 (2015) 85-91.
[12] R. Li, T. Zhang, Y. Liu, G. Lv, and L. Xie, Calcification–carbonation method for red mud processing. J. Hazard. Mater. 316 (2016) 94-101.
[13] S.J. Palmer, M.K. Smith, and R.L. Frost, Effect of High Concentrations of Calcium
N
Residues. Ind. Eng. Chem. Res. 50 (2011) 1853-1859.
U
Hydroxide in Neutralized Synthetic Supernatant Liquor − Implications for Alumina Refinery
[14] M. Johnston, M.W. Clark, P. McMahon, and N. Ward, Alkalinity conversion of bauxite
A
refinery residues by neutralization. J. Hazard. Mater. 182 (2010) 710-715.
M
[15] S. Khaitan, D. A. Dzombak and G. V. Lowry, Chemistry of the Acid Neutralization Capacity of Bauxite Residue. Environmental Engineering Science. 26 (2009) 873-881.
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[16] L. Zhong, Y. Zhang, and Y. Zhang, Extraction of alumina and sodium oxide from red mud by a mild hydro-chemical process. J. Hazard. Mater. 172 (2009) 1629-1634. [17] R.C. Sahu, R.K. Patel, and B.C. Ray, Neutralization of red mud using CO 2 sequestration
PT
cycle. J. Hazard. Mater. 179 (2010) 28-34. [18] Khaitan S, Dzombak D A, Lowry G V, et al. Mechanisms of neutralization of bauxite
CC E
residue by carbon dioxide.[J]. Journal of Environmental Engineering. 135(2009):433-438. [19] P. Renforth, W.M. Mayes, A.P. Jarvis, I.T. Burke, D.A.C. Manning, and K. Gruiz,
Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill:
A
The effects of gypsum dosing. Sci. Total Environ. 421-422 (2012) 253-259. [20] Y. Zhang, Y. Zhang, S. Zheng, H. Du, S. Wang, and Y. Peng, Alumina recovery from
spent Bayer liquor by methanol. Inorganic Chemicals Industry. 20 (2010) 165-168. [21] M.K. Hamdy, and F.S. Williams, Bacterial amelioration of bauxite residue waste of industrial alumina plants. J Ind Microbiol Biotechnol 27 (2001) 228-233. [22] F. Zhu, S. Xue, W. Hartley, L. Huang, C. Wu, and X. Li, Novel predictors of soil genesis following natural weathering processes of bauxite residues. Environ. Sci. Pollut. R. 23 30
(2016) 2856-2863. [23] C. Löser, A. Zehnsdorf, P. Hoffmann, and H. Seidel, Remediation of heavy metal polluted sediment by suspension and solid-bed leaching: Estimate of metal removal efficiency. Chemosphere 66 (2007) 1699-1705. [24] M. Luo, X. Qi, Y. Zhang, Y. Ren, J. Tong, Z. Chen, Y. Hou, N. Yeerkebai, H. Wang, S. Feng, and F. Li, Study on dealkalization and settling performance of red mud. Environ. Sci. Pollut.
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R. 24 (2017) 1794-1802. [25] H. Yan, S. Xue, C. Wu, Y. Wu, and T. Xu, Separation of NaOH and NaAl(OH)4 in alumina alkaline solution through diffusion dialysis and electrodialysis. J. Membrane Sci. 469
SC R
(2014) 436-446.
[26] X. Zhang, C. Li, X. Wang, Y. Wang, and T. Xu, Recovery of hydrochloric acid from simulated chemosynthesis aluminum foils wastewater: An integration of diffusion dialysis and conventional electrodialysis. J. Membrane Sci. 409-410 (2012) 257-263.
U
[27] T. Xu, and C. Huang, Electrodialysis‐ based separation technologies: A critical review.
N
54 (2008) 3147-3159.
[28] Y. Tanaka, Ion-Exchange Membrane Electrodialysis of Saline Water and Its Numerical
A
Analysis. Ind. Eng. Chem. Res. 50 (2011) 10765-10777.
M
[29] U. Hwang, and J. Choi, Changes in the electrochemical characteristics of a bipolar membrane immersed in high concentration of alkaline solutions. Sep. Purif. Technol. 48 (2006)
ED
16-23.
[30] J. Kinčl, V. Kysela, F. Toman, and J. Thomas, Application of electrodialysis for mine water treatment. Freiberg Online Geoscience. 122 (2015) 39-42.
PT
[31] H. Yan, C. Wu, and Y. Wu, Separation of alumina alkaline solution by electrodialysis: Membrane stack configuration optimization and repeated batch experiments. Sep. Purif. Technol.
CC E
139 (2015) 78-87.
[32] Y. TANAKA, Limiting current density of an ion-exchange membrane and of an
electrodialyzer. J. Membrane Sci. 266 (2005) 6-17. [33] X. Qi, H. Wang, C. Huang, L. Zhang, J. Zhang, B. Xu, F. Li, and J.T.A. Junior, Analysis
A
of bauxite residue components responsible for copper removal and related reaction products. Chemosphere (2018). [34] Determination of alkalinity (total alkalinity, bicarbonate and carbonate) (acid titration), SL 83-1994, http://www.bzwxw.com/html/15/5558.html ,2018 (accessed 1 January 2018). [35] G. Zhang, S. Li, X. Zhang, and Z. Zhang, Study on the effect of fresh water washing on dealkalization of red mud from Bayer process. Journal of Qingdao Technological University, 33 31
(2012) 59-62. [36] Xu Zhenliang, Membrane Water Treatment Technology. Chemical Industry Press. Beijing, 2001. [37] Ueda T, Hata K, Kikuoka Y, et al. Effects of aeration on suction pressure in a submerged membrane bioreactor[J]. Water Research, 31(1997):489-494. [38] Snars, K.;Gilkes, R., Evaluation of bauxite residues (red muds) of different origins for
IP T
environmental applications. Applied Clay Science, 46 (2009): 13-20. [39] Suzuki K, Tanaka Y, Osada T, et al. Removal of phosphate, magnesium and calcium
from swine wastewater through crystallization enhanced by aeration[J]. Water Research,
SC R
36(2002):2991-2998.
[40] Z. Liu, H. Li, M. Huang, D. Jia, and N. Zhang, Effects of cooling method on removal of sodium from active roasting red mud based on water leaching. Hydrometallurgy 167 (2017) 92-100.
U
[41] L. Zhang, G. Wang, and L. Duan, Preliminary dealkalization of red mud by washing
N
process. Inoganic Chemical Industry. (2011) 57-58.
[42] T. Wang, and W. Yang, Factors affecting the current and the voltage efficiencies of the
A
synthesis of quaternary ammonium hydroxides by electrolysis–electrodialysis. Chem. Eng. J. 81
M
(2001) 161-169.
[43] W. Garcia-Vasquez, L. Dammak, C. Larchet, V. Nikonenko, and D. Grande, Effects of
ED
acid–base cleaning procedure on structure and properties of anion-exchange membranes used in electrodialysis. J. Membrane Sci. 507 (2016) 12-23. [44] S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, T. Maruyama, and H. Matsuyama,
PT
Improvement of the antifouling potential of an anion exchange membrane by surface modification with a polyelectrolyte for an electrodialysis process. J. Membrane Sci. 417-418 (2012) 137-143.
CC E
[45] J. Park, H. Lee, and S. Moon, Determination of an optimum frequency of square wave power for fouling mitigation in desalting electrodialysis in the presence of humate. Sep. Purif.
A
Technol. 30 (2003) 101-112.
32