Migration and Conversion of Al Element in the Hydrometallurgical Preparation of Al₂O₃ from Secondary Aluminium Dross

Abstract

The amount of secondary aluminium dross in China exceeds one million tons annually, posing environmental and disposal challenges. This study explores acid leaching as an alternative to conventional alkali methods for recovering Al from secondary aluminium dross to produce Al₂O₃. Research has focused on optimizing leaching conditions. Under optimized H₂SO₄ leaching conditions, an Al³⁺ leaching ratio of 86.5% is achieved. By maintaining a pH below 9 during hydrolytic precipitation and multiple washes, the leaching efficiency of Al from Al(OH)₃ reached 95.97%. The original dross, which is primarily composed of Al, Al₂O₃, and AlN, undergoes a transformation where AlN becomes Al(OH)₃ during washing. Thermal decomposition then yields Al₂O₃. The overall recovery of Al reaches 83.11%.

1. Introduction

In 2024, China concurrently generated more than 43,396 thousand metric tons of aluminium (Total) from primary aluminium dross (AD) and approximately 500 thousand tons of secondary aluminium dross (SAD) as consequential byproducts [1]. SAD is derived from the remelting of AD or from the recycling of waste aluminium materials, which contains approximately 5–10 wt% Al, 20–60 wt% Al₂O₃, 10–30 wt% AlN, 5–15 wt% salts and various other oxides, rendering it a valuable resource with prospects for comprehensive recycling [2,3,4].

The key to the successful hydrometallurgical processing of SAD hinges upon the selection of an appropriate leaching medium and the precise control of leaching parameters, with a primary focus on the recovery of Al₂O₃ from residual aluminium elements [5,6]. Wet metallurgical methods for SAD can be broadly classified into alkali leaching and acid leaching processes, which are largely dictated by the choice of leaching medium [7]. Alkali leaching, for example, efficiently recovers most of the Al, typically by employing a NaOH solution as the leaching agent. During alkali leaching, Al and its oxides react with NaOH, yielding a leachate rich in sodium aluminate [8]. By adjusting the pH and introducing seed crystals for Al(OH)₃ precipitation, Al₂O₃ products can be obtained through subsequent calcination [9]. Conversely, acid leaching employs acidic leaching agents to dissolve Al from SAD. After filtration, which effectively separates the insoluble components, leachate enriched with Al₂(SO₄)₃ is produced [3]. By controlling the pH, Al(OH)₃ can be precipitated from the leachate, subsequently undergoing calcination to yield Al₂O₃ products [10]. Compared with alkaline leaching, acid leaching is preferred based on the following considerations: First, secondary aluminium dross contains a large number of alkali-sensitive components such as AlN, which would release ammonia gas during alkaline leaching, posing potential safety hazards. Second, acid leaching can more effectively dissolve target components such as Al₂O₃ and Al(OH)₃, thereby improving the aluminium recovery efficiency. We chose H₂SO₄ because it is commonly used in industry, cost-effective, easy to handle, and does not introduce interfering ions that may form precipitates with aluminium salts [3].

Owing to the complex composition of SAD, which includes soluble salts and metal oxides, a preliminary water-washing step is deemed essential prior to wet metallurgy [11,12]. Water washing facilitates the separation of soluble substances from the insoluble constituents within SAD, permitting the recovery of salts from the soluble fraction. In contrast, the insoluble fraction primarily comprises Al₂O₃ and minor oxides [13,14]. The use of pretreatment methods such as crushing and sieving can significantly increase the removal of fluoride and aluminium nitride (AlN) from SAD [15,16,17]. Consequently, pretreatment measures are deemed indispensable before wet metallurgy can be applied. It is worth mentioning that after washing, trace amounts of AlN and F may still remain in SAD. During alkali leaching, these react vigorously with NaOH, potentially releasing hazardous gases like NH₃ and causing exothermic reactions, which pose safety risks and complicate gas treatment [3]. Besides, the downstream precipitation of Al(OH)₃ from sodium aluminate requires precise pH control and seed crystals to promote nucleation, which adds complexity to the process. Conversely, H₂SO₄ allows for more straightforward precipitation via pH control [18].

In the preparation of alumina via acid leaching and hydrolytic precipitation, acid leaching emerges as a critical component. Different acids were evaluated, including H₂SO₄, HCl, HNO₃, and H₃PO₄, and the H₂SO₄ reaction achieved all Al leaching with the best technical–economic analysis [18]. Comprehending the complex behaviour of soluble Al components and leaching conditions in SAD is essential for achieving a high leaching ratio [19]. Additionally, during ambient temperature precipitation, the pH value plays a pivotal role in determining the purity of Al₂O₃ [20]. Throughout the wet metallurgical process, the utilization ratio of Al serves as a critical indicator for evaluating the process’s efficiency, thereby underscoring the importance of investigating Al element recovery from SAD within the context of solid waste wet metallurgy [21,22].

In summary, existing studies have not yet systematically elucidated the migration and transformation behaviour of aluminium during its green recovery from secondary aluminium dross, particularly lacking in high-efficiency processes based on sulfuric acid leaching coupled with selective precipitation. This study employs an acid-leaching-hydrolytic precipitation process to recover Al from SAD for Al₂O₃ production, with a primary emphasis on elucidating the transformation processes during acid-leaching and hydrolytic precipitation. Furthermore, research has revealed the migration and transformation patterns of Al during acid leaching and hydrolytic precipitation.

2. Materials and Methods

The SAD used in this study was supplied by Delta Aluminium Industry Co., Ltd. (Zhaoqing, China). We procured 99.9% pure sulfuric acid (H₂SO₄) from Shanghai Titan Chem Co., Ltd., Shanghai, China, and 99.9% pure sodium hydroxide (NaOH) from Adamas Reagent Co., Ltd., Shanghai, China.

Figure 1 illustrates the characteristics of the SAD material. In Figure 1a, a macroscopic image showcases SAD in a finely powdered form. Simultaneously, Figure 1b,c, SEM-EDS images reveal the combination of AlN, Al₂O₃, and MgAl₂O₄ contributing to the formation of larger particles. Figure 1d presents the particle size distribution of the original SAD, indicating a non-uniform dispersion of particles with sizes ranging from 3 to 155 μm. Figure 1e displays the X-ray diffraction (XRD) pattern of SAD, highlighting its complex composition containing aluminium in the forms of Al, AlN, Al₂O₃, and MgAl₂O₄ [22].

As depicted in Figure 2, the experimental process was designed to recover Al element from SAD for Al₂O₃ production, involving a sequence of steps, including water washing, acid leaching, hydrolytic precipitation, and calcination.

The purpose of the water washing step is to remove soluble salts (such as NaCl and KCl) and partially water-soluble metal salts in order to prevent interference with aluminium dissolution during subsequent acid leaching. To ensure thorough dissolution of substances in SAD that could react with H₂SO₄, we employed an excess of 20% H₂SO₄ to react with 20 g of SAD while maintaining continuous stirring at 200 r/min. Theoretically, 50 g of 98% concentrated H₂SO₄ was required to completely dissolve substances in 20 g of SAD that reacted with H₂SO₄ on the basis of chemical equation calculations. We calculated the minimum solid-liquid ratio as M_s:M_a:M_w (where M_s represents the SAD mass, M_a represents the H₂SO₄ mass, and M_w represents the pure water mass). However, the experiments utilized a 20% excess of H₂SO₄. To assess the progress of the reaction, we periodically filtered the SAD acid leaching system and measured the remaining filter cake mass for further analysis.

The influence of various parameters, including the solid-liquid ratio (1:2.5:10–1:5:10), leaching temperature (50–90 °C), and leaching time (0–4 h), on the Al³⁺ leaching ratio was investigated. X-ray fluorescence (XRF) was employed to measure the Al³⁺ content in the filter cake during the acid-leaching experiments. The Al³⁺ leaching ratio was calculated via the Al³⁺ content in the filter dross and the Al³⁺ content in the SAD.

Sample compositions were analyzed using X-ray fluorescence (XRF; ED 2000 X-ray, Oxford Instruments, Abingdon, Oxfordshire, UK). Phase compositions were determined using X-ray diffraction (XRD; D8 Advance A25, Bruker AXS GmbH, Karlsruhe, Germany), with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA, and diffraction angles (2θ) were scanned from 10° to 90° with a step size of 0.02° and a scan speed of 1°/min. The micromorphology of the samples was examined using scanning electron microscopy (SEM; Nano SEM-450, FEI USA Inc, Hillsboro, OR, USA), and the chemical composition of the sample particles was analyzed using energy-dispersive X-ray spectroscopy (EDS).

3. Results and Discussion

3.1. Impact of Water Washing on SAD

Prior to stirring and washing, the SAD underwent ball milling and sieving processes to achieve particle sizes smaller than 20 μm. It should be noted that AlN is formed during the high-temperature smelting of aluminium in the presence of nitrogen gas, which is used as a protective atmosphere. Due to its low density, AlN becomes incorporated into the dross after smelting. It remains stable under anhydrous conditions. During water washing, the primary reaction involved the hydrolysis of AlN [23].

$$AlN+3H_{2}O\to Al{(OH)}_3+N{H}_3\uparrow$$

(1)

Figure 3a shows that the enthalpy change (ΔH) for the hydrolysis of AlN is negative (ΔH < 0) within the temperature range of 0–100 °C, indicating the exothermic nature of the reaction. The Gibbs free energy change (ΔG) for AlN hydrolysis is also negative (ΔG < 0), suggesting that this reaction can occur spontaneously at room temperature. Furthermore, as the temperature increases, the ΔG for AlN becomes more negative, indicating that higher temperatures promote forward reactions [24].

Crucial factors influencing the hydrolysis of AlN include hydrolysis temperature and hydrolysis time [25]. Considering that low temperatures have a minimal impact on AlN hydrolysis and that excessively high temperatures increase water washing costs, a temperature range of 50–90 °C was chosen. Figure 3b reveals that as the hydrolysis temperature and duration increase, the hydrolysis ratio of AlN gradually increases. However, complete AlN hydrolysis does not occur rapidly. During the initial 0–0.5 h of hydrolysis, vigorous reactions produce a substantial amount of gas (NH₃) and result in approximately 50% AlN leaching. With increasing temperature, the ratio of improvement in leaching efficiency decreases. After 2–4 h of water washing, the leaching ratio of AlN remains relatively low and decreases further after 4 h. Hence, 4 h is determined as the optimal duration for large-scale AlN leaching from SAD [20].

For process practicality, the effective decomposition of AlN could realize the maximum recovery of aluminium in SAD [26]. Considering all the factors, the optimal conditions for AlN hydrolysis in SAD were established: a hydrolysis temperature of 80 °C and a hydrolysis duration of 4 h. The water-washed SAD sample underwent subsequent drying treatment and chemical composition analysis, as detailed in

Table 1. The chemical composition of SAD before and after washing.

Component/%	Al2O3	MgO	SiO2	Fe2O3	Cl	CuO	CaO	Other
Before washed	72.94	7.36	4.96	3.74	3.45	1.95	1.67	2.93
After washed	74.02	6.98	5.63	5.22	0.04	2.67	1.66	3.78
1 Leaching percentage/%	−1.48	5.16	−13.51	−39.57	98.84	−36.92	0.60	−29.01

¹ “−”is “negative leaching rate”.

.

From Table 1, we know that the leaching rate of Cl is the highest, reaching 98.84%, indicating that soluble salts were effectively removed during the water-washing stage. In contrast, the contents of Al₂O₃, SiO₂, Fe₂O₃, CuO, and “Other” increased. This “negative leaching rate” suggests that these components were not washed out, but rather, their relative proportions increased due to the overall mass reduction caused by the dissolution of salts and AlN. The contents of MgO and CaO showed a slight decrease, indicating that some soluble or hydrolyzable compounds were removed. F was not detected, likely because of its low content.

3.2. Acid Leaching Mechanism

As outlined in Table 1, the products derived from water washing and drying of SAD predominantly comprise Al₂O₃ and other insoluble oxides. The resulting chemical reactions during acid leaching are detailed below [27]:

$$A{l}_2{O}_3+3H_{2}S{O}_4\to A{l}_2{(S{O}_4)}_3+3H_{2}O$$

(2)

$$2Al+3H_{2}S{O}_4\to A{l}_2{(S{O}_4)}_3+3H_2\uparrow$$

(3)

$$MgO·A{l}_2{O}_3+4H_{2}S{O}_4\to MgS{O}_4+A{l}_2{(S{O}_4)}_3+4H_{2}O$$

(4)

$$F{e}_2{O}_3+3H_{2}S{O}_4\to F{e}_2{(S{O}_4)}_3+3H_{2}O$$

(5)

$$CuO+H_{2}S{O}_4\to CuS{O}_4+H_{2}O$$

(6)

$$MgO+H_{2}S{O}_4\to MgS{O}_4+H_{2}O$$

(7)

$$CaO+H_{2}O\to Ca{(OH)}_2$$

(8)

$$Ca{(OH)}_2+H_{2}S{O}_4\to CaS{O}_4+2H_{2}O$$

(9)

Figure 4a visually represents the interaction between H₂SO₄ and SAD. Figure 4b–d depicts the leaching ratios of Al³⁺ at various solid-liquid ratios, acid leaching temperatures, and acid leaching durations, respectively.

In Figure 4a, the solid–liquid ratio ratio of M_s:M_a:M_w = 1:5:10, the acid leaching temperature was 100 °C, and the acid leaching time was 5 h. It is evident that during the initial 0–70 min, there is a vigorous interaction between SAD and H₂SO₄, accompanied by the emission of a substantial volume of white gas and a pungent odour. However, after 70 min, the reaction ratio gradually decreased.

Figure 4b shows that as the H₂SO₄ content in the material mixture increases, the leaching ratio of Al³⁺ in SAD continues to rise. This increase is particularly pronounced up to a solid-liquid ratio of M_s:M_a:M_w = 1:4:10, beyond which additional H₂SO₄ content has a diminished effect and may introduce impurities. Hence, the optimal solid-liquid ratio ratio is determined to be M_s:M_a:M_w = 1:4:10.

Figure 4c illustrates that increasing the acid leaching temperature led to a higher leaching ratio of Al³⁺. However, beyond 80 °C, the incremental effect becomes negligible. The maximum Al³⁺ leaching ratio is observed at 100 °C. Thermodynamically, higher temperatures hinder these reactions, but they increase the dissolution ratio of Al³⁺. Consequently, the suitable acid leaching temperature is 80 °C.

Figure 4d shows that as the acid leaching duration increases, the leaching ratio of Al³⁺ continues to increase, although it decreases. Beyond 3 h, no significant increase was observed due to the encapsulation of residual Al³⁺ by other soluble acids. Hence, the optimal acid leaching duration is 3 h.

The optimal acid leaching conditions are as follows: a solid-liquid ratio of M_s:M_a:M_w = 1:4:10, acid leaching temperature of 80 °C, and acid leaching duration of 3 h. Under these conditions, acid leaching of water-washed SAD yields an Al³⁺ leaching ratio of 86.5%.

In previously reported studies on acid leaching, the optimal acid-base conditions were: temperature of 90 °C, H₂SO₄ concentration of 15 wt%, reaction time of 5 h, and a solid-liquid ratio of 1:12 [28]. Compared with our acid leaching conditions, this method requires a longer leaching time and consumes more H₂SO₄ under the same SAD treatment conditions. Moreover, under stoichiometric acid conditions, approximately 85% of alumina in SAD can be leached [29], whereas our leaching rate reached 86.5%, which is higher than that reported in previous studies.

Figure 5 shows the acid-leaching principle of aluminium dross, which aims to separate soluble and insoluble substances in SAD and eliminate SiO₂. The acid-leaching process unfolds in three stages:

First Stage: Metal oxides in secondary aluminium dross (SAD), such as Al₂O₃, MgO, CuO, CaO, and Fe₂O₃, actively react with H₂SO₄, while SiO₂ remains unreactive and settles at the bottom. Continuous stirring is necessary to keep the unreacted metal oxides suspended and prevent them from being trapped beneath the nonreactive substances. As the reaction progresses, the consumption of H⁺ ions and the formation of water reduce the concentration of H⁺ in the solution. This leads to a slower reaction rate and a gradual drop in temperature. Second Stage: In this phase, Al₂O₃ particles that were previously covered by soluble metal oxides begin to dissolve as the surrounding layers are removed. Third Stage: After the main reactions are complete, the remaining suspended solids, including SiO₂ and partially dissolved or encapsulated Al₂O₃ particles, begin to settle out, forming the final leaching residue [30]. Besides, a high acid concentration inhibited the ionization of H⁺, and a high acid concentration increased the concentration of metal ions, which competed with Al³⁺ to react with SO₄²⁻. A high acid concentration increased the leaching speed of alumina, and the concentration of Al³⁺ reached a high value rapidly. So, the reaction with a low acid concentration is controlled by the reaction process, and the reaction with a high acid concentration is controlled by the diffusion process. The reaction rate is much higher than the diffusion rate when the acid concentration is high. Thus, the leaching rate decreases remarkably as the acid concentration increases to a high level [28].

3.3. Hydrolytic Precipitation

As elucidated in the chemical reactions detailed during the acid-leaching process, it is evident that the primary substance in the filtrate obtained under optimal acid-leaching conditions is Al₂(SO₄)₃. However, other sulfate salts remain present in this chemical reaction. To further increase the material purity and prepare a high-purity Al(OH)₃ precursor, the introduction of a NaOH solution becomes imperative [31]. The potential chemical reactions involved in this process are as follows:

$$A{l}_2{(S{O}_4)}_3+6NaOH\to 3N{a}_{2}S{O}_4+2Al{(OH)}_3\downarrow$$

(10)

$$MgS{O}_4+2NaOH\to N{a}_{2}S{O}_4+Mg{(OH)}_2\downarrow$$

(11)

$$F{e}_2{(S{O}_4)}_3+6NaOH\to 3N{a}_{2}S{O}_4+2Fe{(OH)}_3\downarrow$$

(12)

$$CaS{O}_4+2NaOH\to N{a}_{2}S{O}_4+Ca{(OH)}_2\downarrow$$

(13)

$$CuS{O}_4+2NaOH\to N{a}_{2}S{O}_4+Cu{(OH)}_2\downarrow$$

(14)

Upon the addition of a 5 mol/L NaOH solution to the acid-leaching filtrate and the subsequent adjustment of pH to values of 7, 8, 9, 10, 11, and 12 while maintaining continuous stirring, an Al(OH)₃ precursor forms along with other precipitates. Figure 6 visually represents the evolving colour of the precursor.

Notably, the physical properties of the materials in this context are as follows: Al(OH)₃, Mg(OH)₂, and Ca(OH)₂ all exhibit a white appearance, Cu(OH)₂ appears blue, and Fe(OH)₃ takes on a reddish-brown hue. With increasing pH, the colour of the precursor gradually intensifies from white to reddish-brown. Notably, the colour transformation initiates at pH 9, where the precipitation of Fe(OH)₃ and Cu(OH)₂ begins within the precursor, leading to an increased impurity content. Thus, maintaining the pH value below 9 during alkaline conversion is imperative to ensure that the resulting Al(OH)₃ has the highest purity [30].

Table 2. XRF elemental analysis of the precursors at different pH conditions.

Element	pH Value
	7	8	9	10	11	12	Washed *
O	48.20	47.50	47.30	47.20	47.50	47.50	58.2
Na	19.80	21.30	23.40	23.70	24.50	24.90	0.09
Al	13.20	12.60	11.50	10.20	9.60	4.30	37.77
S	18.40	18.20	16.70	16.80	17.30	17.10	0.26
Mg	0.26	0.37	0.38	0.78	1.02	1.98	2.55
Ca	0.14	0.26	0.33	0.65	0.66	0.71	1.13
Cu	0.03	0.02	0.04	0.11	0.14	0.19	——
Fe	0.22	0.47	0.69	0.75	0.75	0.86	——
Other	0.23	0.51	1.44	1.11	1.77	0.23	0.36

* Washed sample: acid leaching and alkali conversion were carried out, followed by multiple rounds of water washing.

presents the elemental analysis data of the Al(OH)₃ precursor under various pH conditions. Following filtration and drying of the precursor, X-ray fluorescence (XRF) analysis revealed that the primary elements within the precursor were O, Na, Al, and S. As the pH increased, the contents of O, Na, S, and other impurity elements gradually increased, whereas the Al content decreased. This phenomenon is attributed to the reaction of Al(OH)₃ with excess NaOH to form NaAlO₂, which is soluble in water, consequently increasing the concentration of insoluble substances [32]. This chemical reaction can be represented as follows:

$$Al{(OH)}_3+NaOH\to NaAl{O}_2+2H_{2}O$$

(15)

Importantly, the Al(OH)₃ sample contains a notable amount of Na₂SO₄. After multiple washes with deionized water, the final composition of the material achieves an aluminium and oxygen content of 95.97%, less than 99% extraction of alumina in a two-step process [30], with impurities primarily comprising MgO and CaO.

At elevated temperatures, Al(OH)₃ experiences thermal decomposition, resulting in the formation of Al₂O₃, and the corresponding reaction equation is presented as follows [33]:

$$2Al{(OH)}_3\to A{l}_2{O}_3+3H_{2}O\uparrow$$

(16)

Al₂O₃ manifests as a white crystalline powder, and researchers have substantiated its existence in various crystal structures, such as α, γ, and θ [10]. The specific structure of nano-Al₂O₃ depends on the preparation method and processing conditions, with the calcination temperature and duration playing pivotal roles. The calcination temperature influences the transition temperature, whereas the calcination time impacts the degree of transformation. Notably, once the calcination time exceeds 3 h, prolonging the heating duration ceases to significantly affect the degree of transformation [34].

3.4. Migration and Transformation of Al Element

Given the pivotal role of the Al element in the wet process treatment of SAD, it is imperative to elucidate the migration and transformation patterns of the Al element within SAD.

As illustrated in Figure 7, during the initial SAD stage, Al is present predominantly in the forms of elemental Al, Al₂O₃ (including MgAl₂O₄), and AlN. To facilitate the assessment of Al utilization, the Al content within 20 g of the original SAD is considered the functional unit. Following the washing process, AlN reacts with H₂O, resulting in the formation of Al(OH)₃. During the drying stage, Al(OH)₃ subsequently undergoes thermal decomposition, resulting in the formation of Al₂O₃. After this washing and drying sequence, 99.86% of the Al remains intact.

Upon subjecting the material to H₂SO₄ acid leaching, Al and Al₂O₃ transform into Al³⁺, with 86.38% of the initial Al remaining preserved. Subsequent hydrolytic precipitation leads to the interaction of Al³⁺ with OH⁻ from NaOH, regenerating Al(OH)₃, with 83.11% of the Al remaining. Finally, during the calcination process, all the Al(OH)₃ compounds thermally decompose into Al₂O₃. Notably, phase transformations occur sequentially, giving rise to γ-Al₂O₃, θ-Al₂O₃, and α-Al₂O₃ within the temperature range of 800–1300 °C [7].

Consequently, the final utilization ratio of Al in the SAD stands at 83.11%. The relatively low utilization ratio of Al is attributed to the limited leaching efficiency, which is intricately linked to factors such as SAD particle size, leaching conditions, and the presence of Al₂O₃ particles that are insoluble in H₂SO₄ and encase Al. Enhancing the leaching ratio is imperative for improving the overall utilization of Al.

4. Conclusions

In this study, an optimized hydrometallurgical process was developed to recover Al elements from secondary SAD and produce high-purity Al₂O₃. The process involved sequential water washing, acid leaching, hydrolytic precipitation, and calcination, enabling efficient aluminium extraction.

Under optimized acid leaching conditions (solid-liquid ratio M_s:M_a:M_w = 1:4:10, leaching temperature of 80 °C, and duration of 3 h), an Al³⁺ leaching efficiency of 86.5% was achieved. The hydrolytic precipitation stage demonstrated that maintaining a pH below 9 effectively minimized impurity incorporation, yielding a high-purity Al(OH)₃ precursor. Subsequent calcination led to the transformation of Al(OH)₃ into Al₂O₃.

Overall, the total recovery efficiency of the Al element reached 83.11%, highlighting the effectiveness of this process in converting SAD into valuable Al₂O₃ while minimizing waste. This research provides valuable insights into the migration and transformation of Al elements during hydrometallurgical processing, offering a sustainable and efficient pathway for the resource utilization of aluminium dross. Future work should focus on further optimizing leaching efficiency and exploring industrial-scale applications of this method.