High-performance Al separation and Zn recovery from a simulated hazardous sludge

2.1 Pre-treatment of simulated Zn-bearing wastewater

The simulated electroplating wastewater with 290 mg/L Zn²⁺ was prepared with zinc chloride (analytically pure) and deionised water. Black sludge was generated with the addition of 600 mg/L PAC (poly aluminum chloride), and the mixture was centrifuged and dried for 5 h at 105 °C.

2.2 Hydrothermal precipitation of impure Al³⁺

Th sludge (20 g) was dissolved in a mixed acid containing 2.5 M nitric acid and 2 M sulphuric acid at 50 °C under stirring at 90 rpm for 2.5 h. A yellow solution with Zn²⁺ (5.7 g/L) and Al³⁺ (3.2 g/L) was generated. In the solution, Al³⁺ was efficiently separated as follows. The solution pH was adjusted to 0.21 by adding 8 M NaOH. Then, 30 mL of the solution was put into a 50 mL vessel and heated at 270 °C for 20 h, Afterward, the solution was cooled to room temperature. A white precipitate was produced at the bottom of the reactor, collected and vacuum dried at 55 °C for 24 h. To improve the Al³⁺ removal rate, 0.1 mL of ethylene glycol was added to the vessel before heating, and then treated following the above method. The dose of ethylene glycol was optimised in the range of 0.1–0.4 mL, and the reaction temperature and time were also investigated in the range of 150–270 °C and 0.08–20 h, respectively.

2.3 Recovery of zinc

After Al³⁺ removal, Zn²⁺ was residual in the solution and then precipitated by adjusting the solution to pH 7.5 with 4 M NaOH. A large amount of white Zn-bearing precipitate was generated, collected and vacuum dried.

3.1 Al³⁺ removal from Zn/Al bearing solution

The solution with 3.2 g/L Al³⁺ and 5.7 g/L Zn²⁺ was directly treated hydrothermally. Only 55% Al³⁺ was precipitated as irregular particles of AOSH (aluminium oxonium sulphate hydroxide) and boehmite, with Zn²⁺ loss of 0.32% (Fig. 1). The pH of the corresponding solution decreased from 0.21 to 0.09 due to the hydrolysis of Al³⁺ [Fig. 2(a)]. In the hydrothermal reaction, nitrate was pyrolysed into N₂ and NO₂ (Wang and Sun, 1999), and its concentration decreased from 59.6 g/L to 37.1 g/L, along with the small reduction of TOC (total organic carbon) from 21.6 mg/L to 10.2 mg/L [Fig. 2(b) and (c)]. However, the concentration of sulphate ions also decreased significantly from 1.67 g/L to 0.04 g/L [Fig. 2(d)], indicating that sulphates are involved in the production of AOSH with a chemical composition of (H₃O) Al₃(SO₄)₂ (OH)₆.

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Fig. 1

Al³⁺ and Zn²⁺ removal rates by varying the ethylene glycol dose from 0 mL to 0.1, 0.2 and 0.4 mL at 270 °C for 20 h.

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Fig. 2

Supernatant after Al³⁺ removal by different doses of ethylene glycol at 270 °C for 20 h. (a) pH, (b) nitrate concentration, (c) TOC and (d) sulphate concentration.

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Ethylene glycol was effective in promoting the removal of Al³⁺. When 0.1 mL of ethylene glycol was added, the removal rate of Al³⁺ increased rapidly to 91.3%, and Zn²⁺ loss was 0.09%. When the dose of ethylene glycol increased from 0.1 mL to 0.2 mL and 0.4 mL, the removal rate of Al³⁺ was further increased to 99.8% and then slightly dropped to 98%, during which Zn²⁺ loss increased slightly to 1.4% but increased significantly to 52.6%, respectively (Fig. 1). Accordingly, the pH of the solution increased from 0.72 to 1.83 and 2.74, but the nitrate concentration correspondingly decreased from 28.9 g/L to 8.34 g/L and 1.7 g/L. With the decrease in nitrate concentration, the consumed TOC correspondingly rose from 0.21 g/L to 0.41 g/L and 0.72 g/L. However, the concentration of residual sulphate increased slightly from 0.17 g/L to 0.18 g/L and then increased significantly to 1.47 g/L (Fig. 2), demonstrating that the removal of sulphate is detrimental at high pH.

In the absence of ethylene glycol, Al³⁺ was precipitated in the form of irregular particles of AOSH and boehmite [Figs. 3(a) and 4(a)]. With the addition of ethylene glycol, Al³⁺ was removed as regular hexahedron block with a few irregular particles, in agreement with the crystal growth of AOSH [Figs. 3(b), 3(c) and 4(b), 4(c)]. When the added ethylene glycol was 0.4 mL, the regular hexahedron block disappeared, and new octahedral particles were observed. These particles were gahnite [Figs. 3(d), 4(d) and 4(e)], in accordance with 53% Zn²⁺ loss in the hydrothermal system [Fig. 1].

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Fig. 3

XRD curves of the solid precipitate at different doses of ethylene glycol (mL) at 270 °C for 20 h: (a) 0, (b) 0.1, (c), 0.2 and (d) 0.4.

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Fig. 4

SEM images of the solid precipitate at different doses of ethylene glycol (mL) at 270 °C for 20 h: (a) 0, (b) 0.1, (c) 0.2 and (d and e) 0.4.

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3.2 Effect of hydrothermal temperature

The hydrothermal reaction temperature is very important for the hydrolysis and precipitation of Al³⁺ and was optimised in the range of 150–270 °C at the optimal ethylene glycol dose of 0.2 mL. The Al³⁺ removal rate was only 24.2% at 150 °C and gradually increased to nearly 100% when the temperature was increased to 270 °C (Fig. 5). The solution pH was 0.21 but rose to 0.31 at 150 °C and further to 1.83 at 270 °C [Fig. 6(a)]. With the variation in the solution pH, the nitrate concentration decreased from 40.17 g/L to 34.11, 22.7, 17.2 and 8.34 g/L [Fig. 6(b)], in accordance with the consumption of TOC [Fig. 6(c)]. The residual sulphate showed a similar decreasing trend to nitrate, and its concentration was 0.47 g/L at 150 °C, which was reduced to 0.18 g/L at 270 °C [Fig. 6(d)], indicating that the generation of AOSH was accelerated at high temperature.

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Fig. 5

Al³⁺ and Zn²⁺ removal rates at 150 °C, 180 °C, 210 °C, 240 °C and 270 °C for 20 h after the addition of 0.2 mL of ethylene glycol.

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Fig. 6

(a) Solution pH, (b) nitrate concentration, (c) TOC, and (d) sulphate concentration when the temperature changed from 150 °C to 180 °C, 210 °C, 240 °C and 270 °C for 20 h at ethylene glycol dose of 0.2 mL.

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The Al-containing precipitates were also analysed, in which irregular lumps of AOSH were observed at 150 °C, and these lumps were converted to regular hexahedron with side length of 3 μm at 180 °C. This result demonstrated the crystal growth of AOSH at high temperature [Figs. 7(a), 7(b), 8(a) and 8(b)]. As the temperature was increased to 210 °C, 240 °C and 270 °C, the side lengths of the regular hexahedron AOSH further grew to 4 μm, along with the formation of a small portion of boehmite [Figs. 7(c), 7(d), 8(c) and 8(d)]. Such boehmite was generated from the direct hydrolysis of Al³⁺ without the involvement of sulphate, and the related mechanism will be discussed in detail in Section 3.5.

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Fig. 7

XRD curves at ethylene glycol (0.2 mL) and reaction temperature of (a) 150 °C, (b) 180 °C, (c) 210 °C and (d) 240 °C for 20 h.

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Fig. 8

SEM images at ethylene glycol (0.2 mL) and reaction temperature of (a) 150 °C, (b) 180 °C, (c) 210 °C and (d) 240 °C for 20 h.

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3.3 Effect of hydrothermal time

The hydrothermal reaction time was also investigated, and the results are shown in Fig. 9. As the reaction time was increased from 0.08 h to 5, 10, 15 and 20 h, the removal rate of Al³⁺ gradually increased from 37.1% to 74.1%, 85.6%, 95.6% and 99.8%, respectively. These results show that the removal of Al³⁺ was related to the reaction time. The loss of Zn²⁺ increased slightly from 0.7% at 0.08 h to 1.4% at 20 h, which indicated the low hydrolysis of Zn²⁺ in a hydrothermal system. With the extension of the reaction time, the pH of the solution also rose from 0.21 to 0.3 at 0.08 h and to 1.83 at 20 h [Fig. 10(a)]. Accordingly, the corresponding nitrate and sulphate concentrations dropped from 59.6 g/L and 1.67 g/L to 37.6 g/L and 0.45 g/L at 0.08 h and finally to 8.34 g/L and 0.18 g/L at 20 h [Fig. 10(b) and (c)]. As the concentration of nitrate decreased, the TOC consumed gradually increased from 0.11 g/L at 0.08 h to 0.41 g/L at 20 h [Fig. 10(d)].

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Fig. 9

Al³⁺ and Zn²⁺ removal rates at ethylene glycol (0.2 mL) and reaction temperature at 270 °C for times of 0.08, 5, 10 and 15 h.

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Fig. 10

(a) Solution pH, (b) nitrate concentration, (c) sulphate concentration, and (d) TOC at reaction temperature 270 °C for times of 0.08, 5, 10 and 15 h and ethylene glycol dose of 0.2 mL.

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When the reaction time was 0.08 h, only 37.1% Al³⁺ was eliminated as AOSH aggregates, along with small boehmite chips [Figs. 11(a) and 12(a)]. As the reaction time was extended to 5 h, the AOSH in the shape of a regular hexahedron appeared [Figs. 11(b) and 12(b)]. After 10 h, such AOSH aggregates disappeared, but abundant well-formed AOSH in the shape of a regular hexahedron were generated [Figs. 11(c) and 12(c)]. The AOSH in the shape of a regular hexahedron became rich with extension of time from 10 h to 15 h and 20 h, although boehmite particles were also generated [Figs. 3(c), 4(c), 11(d) and 12(d)].

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Fig. 11

XRD curves with ethylene glycol dose of 0.2 mL at reaction temperature 270 °C and reaction times (h) of (a) 0.08, (b) 5, (c) 10 and (c) 15.

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Fig. 12

SEM images at ethylene glycol dose of 0.2 mL at reaction temperature 270 °C and reaction times (h) of (a) 0.08, (b) 5, (c) 10 and (c) 15.

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3.4 Zn recycling from the residual solution

Nearly 100% Al³⁺ in the leaching solution was removed by hydrothermal treatment at 270 °C for 20 h with the addition of 0.2 mL of ethylene glycol, whilst 6.6 mg/L Al³⁺ and 5.62 g/L Zn²⁺ remained in the supernatant. The remaining supernatant was collected, and its pH level was adjusted to pH 7.5 to precipitate the residual Zn²⁺ as white simonkolleite chips with 63.1 wt% ZnO and less than 0.8% impure Al [Fig. 13]. This result demonstrated that the recovered simonkolleite was highly purified. Such simonkolleite is a marketable chemical raw material to produce rubber and anticorrosion materials in the rubber and metal product processing technology (Martinez et al., 1994, Chen et al., 2011, Tanaka et al., 2011).

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Fig. 13

(a) XRD curves and (b) picture and (c) SEM images of the Zn precipitate.

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3.5 Mechanism of Al/Zn separation

After the dissolution of the sludge, impurity Al³⁺ was rich in the solution. After adjusting the pH of the solution to pH 4.1, nearly 96.6% Al³⁺ was removed, whilst 29.1% Zn²⁺ was also eliminated because of the coprecipitation of Al³⁺ and Zn²⁺ in the solution (Fig. S1).

When the solution was hydrothermally treated at 270 °C, Al³⁺ was spontaneously hydrolysed and reacted with sulphate to generate AOSH via Eq. (1). In parallel, the hydrolysis of Al³⁺ as AlOOH occurred in the absence of sulphate, and the produced AlOOH was weakly crystallised and further converted to boehmite via Eq. (2). With the hydrolysis of Al³⁺, H⁺ was produced, resulting in the decrease in the pH of the solution from 0.21 to 0.09. With the accumulation of H⁺, the hydrolysis of Al³⁺ reached equilibrium, and only 55% Al³⁺ was removed. During the hydrolysis of Al³⁺, less than 0.3% Zn²⁺ was removed, suggesting a very slow hydrolysis of Zn²⁺. The hydrolysis products of Al³⁺, i.e. AOSH, AlOOH and boehmite, had numerous surface hydroxyl groups for the coordination of Zn²⁺. However, H⁺ was abundant in the solution and competed with Zn²⁺ to complex with the surface hydroxyl group of the Al-bearing products. Thus, the adsorbed Zn²⁺ was replaced by H⁺ and then released to the solution.

Ethylene glycol plays an important role in the reaction. When added to the reaction system, ethylene glycol reacted with nitrate via Eq. (3) to generate N₂, CO₂ and H₂O, in which a high amount of H⁺ was consumed. Thus, the pH of the solution rose, and the hydrolysis of Al³⁺ was accelerated. In the hydrothermal system, Al³⁺ was hydrolysed to produce two products, namely, AOSH and AlOOH, in which only sulphate was involved in the generation of AOSH. With the increasing pH, the direct hydrolysis of Al³⁺ to AlOOH was also accelerated. The total amount of Al³⁺ in the formation of AOSH decreased, resulting in the residual sulphate at high pH. For instance, by adding 0.4 mL of ethylene glycol, the residual sulphate was 1.47 g/L with the solution pH increased to 2.74 [Fig. 2(a) and (d)]. The optimal dosage for Al³⁺ removal was 0.2 mL of ethylene glycol, in which nearly100% Al³⁺ and 1.4% Zn²⁺ were removed. The removal rate of Zn²⁺ increased slightly, because the hydrolysis of Zn²⁺ was enhanced at high pH. When the dose of ethylene glycol was 0.4 mL, the pH of the solution rose to 2.74, during which the Al³⁺ removal rate was almost unchanged, but the removal rate of Zn²⁺ increased to 52.6%, in accordance with the hydrolysis of Zn²⁺ as Zn(OH)₂ via Eq. (4). In the hydrothermal system, the formed AlOOH and Zn(OH)₂ were small flocs, which were polymerised and coprecipitated to form Al/Zn-bearing polymers and further recrystallised in the form of octahedral sphalerite [Eq. (5)].

Hydrothermal temperature is important for Al³⁺ hydrolysis. The Gibbs free energy of hydrolysis of Al³⁺ as AOSH (Eq. (1)) was −3162.2 kJ mol⁻¹ (25 °C) (Rudolph and Mason, 2001, Majzlan et al., 2004), demonstrating that Al³⁺ hydrolysis was endothermic. Accordingly, the Al³⁺ hydrolysis accelerated with the increase of temperature in hydrothermal system, resulting in the Al³⁺ removal efficiency at a high level at 270 °C. The redox reaction of nitrate to ethylene glycol was both endothermic (Su et al., 2020) and accelerated with the increase in temperature. Correspondingly, more H⁺ was consumed in the redox reaction at high temperature, which was profitable for the hydrolysis and precipitation of Al³⁺.

The hydrolysis of Al³⁺ was very rapid at reaction time of 0.08 h, where 37.1% Al³⁺ was removed to generate AOSH aggregates, along with a small amount of boehmite. Then, the AOSH crystals grew to well-crystallised regular hexahedron block with the side length of 3 μm, as the reaction time was extended to 15 h. The residual Al³⁺ concentration was reduced to 428 mg/L at 15 h and further to 6.6 mg/L at 20 h. Given the decrease in Al³⁺ concentration, the hydrolysis of Al³⁺ became slow, and additional time was needed to remove Al³⁺. The hydrolysis of Al³⁺ was also affiliated with other two parameters, e.g., pH value and Al³⁺ concentration, besides hydrolysis temperature. In the hydrothermal reaction, the solution pH steadily increased due to the H⁺ consumption in the redox reaction between nitrate and ethylene glycol. This behavior was profitable for Al³⁺ hydrolysis. As the Al³⁺ hydrolysis continued, the concentration of residual Al³⁺ in the solution gradually reduced, and thereby the hydrolysis rate constant (R) steadily became small. For instance, the R value was $4.08\times {10}^9$ at the Al³⁺ concentration of $4.004\times {10}^{-4}$ mol/l (Holmes et al., 1968), but changed to $3.89\times {10}^9$ when the Al³⁺ concentration was $1.502\times {10}^{-4}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{l}$ . Thus, the lower the Al³⁺ residual was, the slower the Al hydrolysis became, which consumed more time to reach a promising equilibrium of Al hydrolysis. Accordingly, Al was residual at a low level after treatment for 20 h.

After the hydrothermal treatment, Zn²⁺ was residual at 5.66 g/L in the supernatant and then precipitated as simonkolleite by directly adjusting the solution to pH to 7.5. Such precipitation route of Zn²⁺ recycling is simple, cheap and easy to perform at mass production.

3.6 Potential application in recycling Zn-bearing sludge

Zn-bearing sludge is common in electroplating sludge, flue ash and zinc smelting industry waste from electroplating factories (Cao et al., 2017), steel factories (Yuhua, 2010) and smelt industries (Yan, 2010). Therefore, the method in Section 3.5 can be applied in the recycling of such waste. The obtained Al-bearing by-product and simonkolleite are valuable and marketable chemical APIs (active pharmaceutical ingredients). The former is a common raw material for producing aluminium alloy (Kaufman and Rooy, 2007), accessories (Calignano et al., 2013) and appliances (Ja-Liang et al., 1997). The later contained 63.1% ZnO, close to that of zinc fertiliser raw materials, zinc oxide raw material for rubber and zinc oxide for pigments. In addition, simonkolleite can be directly used as a substitute in the zinc fertiliser processing, rubber and pigment processing industries (Wu et al., 1985, Sahoo et al., 2007, Osmond and Gillian, 2012). Hence, with the proposed method, Al and Zn were successfully recovered from the Zn-bearing sludge without generating any secondary solid waste.