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Review

Properties, Treatment and Resource Utilization of Bauxite Tailings: A Review

by
Yuansheng Peng
1,2,
Zhongping Chen
1,2,
Xiaohui Sun
1,2,*,
Yuefu Zhou
1,2 and
Xiaoduo Ou
3
1
College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2
Shenzhen Key Laboratory of Green, Efficient and Intelligent Construction of Underground Metro Station Shenzhen, Shenzhen 518048, China
3
College of Civil Engineering and Architecture, Guangxi University, Nanning 530004, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(16), 6948; https://doi.org/10.3390/su16166948
Submission received: 25 June 2024 / Revised: 7 August 2024 / Accepted: 8 August 2024 / Published: 14 August 2024
(This article belongs to the Section Waste and Recycling)
Browse Figures
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Abstract

:
A substantial amount of bauxite tailings (BTs) at abandoned mine sites have been stored in waste reservoirs for long periods, leading to significant land occupation and environmental degradation. Although many studies of the resource utilization of BTs were conducted to address this challenge, there is still a lack of efforts to systematically review the state of the art in BTs. In the present paper, a systematic literature review was carried out to summarize and analyze the properties, treatment, and resource utilization of BTs. Physical characteristics and the mineral and chemical composition of BTs are introduced. The efficacy of physical, chemical, and microbial treatment methods for BTs in terms of dehydration are outlined, and their respective benefits and limitations are discussed. Moreover, the extraction process of valuable elements (e.g., Si, Al, Fe, Li, Na, Nd, etc.) from BTs is examined, and the diverse applications of BTs in adsorption materials, ceramic materials, cementitious materials, lightweight aggregates, foamed mixture lightweight soil, among others, are studied. Finally, an efficient and smart treatment strategy for BTs was proposed. The findings of the present review provide a scientific basis and reference for future research focusing on the treatment and resource utilization of BTs.

1. Introduction

In recent decades, there has been a growing emphasis on sustainable waste management and recycling practices, emerging as a significant focal point of concern [1]. According to data from the International Aluminium Institute, global alumina (Al2O3) production is estimated to be 142 million tons in 2023. The primary contributors to this production were China, Australia, Brazil, India, and Russia. After grinding the raw bauxite ore, particles larger than 1 mm are selected for alumina extraction. Currently, the primary method employed to convert medium- and low-grade bauxite into alumina is the flotation Bayer method [2,3]. For every 1 tonne of alumina, 1 to 1.5 tonnes of high alkaline red mud is produced. However, particles smaller than 1 mm formed a suspended slurry of bauxite tailings (BTs) with water. Then, BT slurry is pumped and discharged into waste reservoirs when concentration is reached approximately 24 wt% to 32 wt%. Due to the long pumping distance and large pipeline span, preventive measures are required to avoid blockages in the BT pumping process. For example, it is necessary to pump the freshly reclaimed BT slurry with a high water content of up to 300 wt%, resulting in a fluid-like surface and minimal structural strength. In addition, unlike red mud, BTs have a large water content, high alumina content, and are neutral. The generation process of BTs is illustrated in Figure 1. Approximately 0.2 tons of discarded BTs are generated for every 1 ton of bauxite raw ore processed through flotation [4], resulting in an annual volume of BTs reaching hundreds of millions of cubic meters. The high water content, fine particle size, and low strength characteristics of BTs pose significant challenges in the processing of BTs. As depicted in Figure 1, the long-term storage of substantial quantities of BTs that occurs in the waste reservoirs is one of the three key controlling projects of the mine. As of 2022, China had accumulated more than 70 billion tons of waste rocks, including a huge amount of tailings, distributed in nearly 10,000 waste reservoirs. For instance, there are presently 14 large tailing reservoirs in Guangxi, China. The maximum accumulation depth of these tailing reservoirs was more than 70 m, with a total storage capacity ranging from 1145 × 104 m3 to 6181 × 104 m3, and the largest storage area covers an area of 495 × 104 m2 [5]. BTs in waste reservoirs not only consume land resources but also pose a risk of man-made debris flow. Frequent leakages of BTs, as illustrated in Figure 1, have severely contaminated the surrounding environment and jeopardized the safety of residents and their properties. China, as the largest global recycled Al production country in the world [6], is facing a serious environmental challenge due to BTs. Hence, the effective utilization of BTs as a valuable resource becomes crucial in the pursuit of green industry development and the global objective of carbon emissions reduction.
The main chemical components of BTs are A12O3, SiO2, and Fe2O3, with respective contents of 57.1 wt%, 22.8 wt%, and 11.4 wt% [7]. BTs have a fine particle size, a substantial specific surface area, pronounced functional group activity, as well as notable adsorption and ion exchange properties [8]. The chemical and mineral compositions of BTs make their surface highly reactive in alkaline environments. Specifically, BTs can undergo volcanic ash reactions with calcium ions. Accordingly, this promotes their value as building materials for enhancing early strength. However, newly reclaimed BTs are characterized by a water content of up to 300% and a flowing surface. To address the elevated moisture content, consolidation and solidification technologies are the prevailing approach for BTs treatment. They include methods such as mud-water separation, step-by-step sand stacking, quicklime solidification, and co-solidification of quicklime and microorganisms [9,10,11,12]. However, in practical engineering, the extensive volume and vast area are occupied by tailing reservoirs (as depicted in Figure 1). It is difficult to consolidate and solidify BTs directly to attain usable strength, while time and economic costs are high as well. Hence, the direct application of consolidation and solidification technologies in the treatment of sites having BTs present challenges and difficulties.
Due to the considerable content of highly viscous kaolinite and various mineral components, BTs have prominent plasticity and properties of non-toxicity and harmlessness [8]. Consequently, the synthesis of various valuable engineering materials from BTs served as an effective means for their recycling. Notably, BTs can be utilized for the production of zeolites [13,14,15,16], adsorption materials [4,17,18], ceramics [19,20,21], cementitious materials [22,23,24], and lightweight aggregates [1,25,26], in addition to other applications. Moreover, Peng et al. [5] successfully developed foamed and mixture lightweight soil by utilizing BTs. Their study revealed that incorporating BTs at a content range of 10% to 40% resulted in unconfined compressive strength at 7 days, reaching 73% to 83% of those at 28 days, indicating notable early strength characteristics. Such findings presented effective avenues for the resource utilization of BTs. Additionally, the life cycle assessment (LCA) model [27,28] can be used to evaluate resource utilization methods for BTs, more accurately reflecting the life cycle of the resource utilization process and thereby reducing their environmental impact. Despite extensive study in this field, there remains a lack of systematic efforts to review the treatment and resource utilization for BTs. This gap promotes us to conduct our present research.
With the objective of advancing the green and efficient resource utilization of BTs, this study presented a comprehensive analysis of the particle size distribution, physical properties, and chemical composition of BTs. It also summarized the consolidation and solidification technologies applied to BTs. The application and effect of BTs in metal extraction, adsorption materials, ceramic materials, cementitious materials, and lightweight aggregate are studied. Furthermore, the mechanism of performance improvement is analysed. Based on existing treatment and resource utilization methods, our present study would propose future strategies for the treatment and utilization of BTs. The review findings provided valuable insights and approaches for achieving environmentally friendly, low-carbon, and highly efficient resource utilization of large quantities of BTs. These efforts hold immense significance in mitigating challenges associated with land resource scarcity, environmental pollution, and the growth of sustainable industries.

2. Research Methods

The literature search for this review was conducted using several prominent academic databases, including Web of Science, Google Scholar, and CNKI (China National Knowledge Infrastructure). Specific keywords such as “bauxite tailings”, “bauxite residue”, “bauxite”, and “slurry treatment” were utilized to ensure the inclusion of a wide range of studies relevant to the review’s focus.
The search targeted articles published between 2014 and 2024 to capture the latest advancements and developments in the field. Additionally, key studies from 2007 to 2013 were included to provide a foundational context and historical perspective on earlier research. The selected literature covers various aspects of BTs, including their physicochemical properties, consolidation, and solidification treatments, and resource utilization approaches.
The article selection process involved an initial screening of titles and abstracts to exclude studies that did not meet the review’s relevance criteria. Following this, a detailed full-text review was conducted for the shortlisted articles to ensure their inclusion based on comprehensive criteria. Priority was given to studies that offered in-depth insights into the characterization, innovative treatment methods, and practical applications of BTs in resource utilization. Data processing and visualization were conducted using Origin 2016 and Microsoft Office PowerPoint 2023, which facilitated precise data analysis and effective graphical presentation, ensuring a clear communication of complex information.

3. Results

3.1. Properties of BTs

3.1.1. Physical Characteristics

The discharge of BTs occurred during the grinding and washing of raw bauxite ore, resulting in potential soil structure damage. Throughout the process of pumping and warehousing, the properties of BTs exhibited spatiotemporal variability influenced by time and location. What’s more, the composition of BTs particles stored at different locations within the tailing reservoirs demonstrated variation. Coarse-grained silty sand settled near the discharge outlet due to gravitation, while fine-grained clay and silty sand gradually deposited towards the central and surrounding areas of the depression through hydraulic sorting. Figure 2 shows the particle size distribution curves of BTs. Jiang et al. [29] indicated that near the discharge outlet, the majority of particles comprised silty sand, while the remaining particles consisted of sand with fine-grained soil. The central region of the waste reservoirs predominantly consisted of clay and clayey sand, where clay is approximately half of the total mass, while silty soil and clayey sand accounted for the remaining portion. It can also be seen from Figure 2 that BTs used in most studies [25,30,31] were classified as clay according to GB/T 50145-2007. The median particle size (D50) obtained from the laser particle size analyzer was within 0.52 μm [25]. The proportion of BTs with a particle size smaller than 69.5 μm was close to 90% [31]. The frequency of powder separation had a great influence on particle size distribution. Zhou et al. [30] showed that with the frequency increased, the particle number less than 6.0 μm increased significantly, as shown in Figure 2.
BTs samples from Guangxi, China, were analyzed using a Hitachi S-3400N scanning electron microscope (SEM), and the SEM microstructure is presented in Figure 3. It is evident from the figure that the dissolution of alumina resulted in an irregular and rough surface morphology of BTs particles, leading to a significantly large specific surface area and heightened adsorption activity. This was consistent with the conclusion drawn by Anawati et al. [32], which indicated that the surface area of BTs was 21.98 m2 /g, and a large surface area of BTs was available for the digestion reaction. Moreover, in the presence of an alkaline environment created by cement, the contact surface between BTs and OH- ions was larger, which made BTs more susceptible to be activated and exhibit notable chemical activity.
Through the analysis of water content in the BTs samples collected from different regions of China Aluminum Guangxi Company’s No. 1 and No. 2 tailing reservoirs, the available data indicated that samples near the ore discharge port of the tailing reservoirs showed relatively low water content, with a range from 8.23 wt% to 74.78 wt% and an average of 35.73 wt%. This is because of the coarse particles of BTs in this area, which were mainly in a plastic state or a soft plastic state. In the peripheral zone of the tailing reservoirs, BTs were covered with shallow water or no water. In this area, BTs displayed a soft plastic or flow plastic state, characterized by significantly variable water content with depth. The water content ranged from 13.25 wt% to 176.82 wt%, with an average of 66.08 wt%. Moving towards the central area of the tailing reservoirs, the accumulation of BTs became more pronounced, with a notable presence of accumulated water. In this region, BTs exhibited a flow plastic state consisting of fine particles and exceptionally high water content. The water content ranged from 81.52 wt% to 239.65 wt%, with an average of 140.50 wt%. These observations revealed that BTs in the central area of the reservoirs possess extremely high water content, posing significant challenges for on-site treatment.

3.1.2. Mineral and Chemical Composition

The main mineral compositions of BTs are listed in Table 1. It can be seen that diaspore (AlO(OH)), kaolinite (Al4(Si4O10)(OH)8), gibbsite (Al(OH)3), muscovite (KAl2(AlSi3O10)(OH)2), illite (K2O·3Al2O3·6SiO2·2H2O), anatase (TiO2), lepidolite (K(Li,Al)3(Si,Al)4O10(F,OH)2), goethite (FeO(OH)), hematite (Fe2O3), quartz (SiO2) and montmorillonite ((Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·nH2O) were determined as the main mineral compositions of BTs. Most of the literature stated that diaspore and kaolinite were the predominant mineral compositions of BTs. The kaolinite content was as high as 54 wt% [22]. However, there is also evidence that the mineral component with the highest content in BTs was illite (36.2 wt%), followed by diaspore (21 wt%) and kaolinite (14.5 wt%) [25]. Based on the mineral composition, the applications possible for BTs were [33]: (a) pozzolan; (b) inert material (aggregate or filler); (c) precursor; or (d) plasticity-reducing waste. The predominant form of BTs was kaolin-layered silicate minerals, with characteristics of fine particle size, a substantial specific surface area, and increased functional group activity [8]. These BTs displayed strong adsorption and ion exchange properties akin to those observed in bentonite.
Table 2 presents the chemical composition of BTs. It is evident that the primary elements in BTs are Al, Si, and O. The main chemical compositions of China’s BTs are Al2O3 and SiO2, with the highest recorded contents of 60.9 wt% and 57.6 wt%, respectively. The Al/Si ratio ranged from 0.75 to 5.82. The presence of these oxides sets BTs distinct from typical soft soil, rendering them highly reactive in alkaline environments, akin to volcanic ash effects. Moreover, besides the substantial amounts of Al2O3 and SiO2, BTs from Brazil and South Korea had significant concentrations of Fe2O3, with a maximum content of 46.6 wt%. Additionally, the BTs sample contained minor quantities of K2O, TiO2, CaO, MgO, SO3−, and Na2O. Notably, trace levels of Li2O (0.21–0.59 wt%) were also present in the BTs sample, as reported by Zhang et al. [38] and Han et al. [45]. The presence of lithium in BTs, an emerging energy material, held considerable application value. Overall, some elements present in BTs showcased notable economic significance, while others represented impurities that required elimination and extraction of valuable elements during practical applications.

3.2. Treatment of BTs

3.2.1. Consolidation of BTs

Consolidation technology is the predominant method employed for the treatment of BTs with high water content, as demonstrated in the model test shown in Figure 4. The newly reclaimed BTs exhibited a water content of up to 300 wt% and displayed a flowing surface. To lower the water content of BTs, mud-water separation technology was utilized. Post-separation, the sediment has a water content of about 100 wt% [57]. However, in a natural drainage state, the fine particles would seep along with water, forming localized regions of extremely low permeability with varying thicknesses near the drainage system. These dense areas ultimately impeded effective drainage, leading to the water content of BTs ranging from 60 wt% to 75 wt% at different positions after the completion of self-weight drainage. To further enhance the dewatering process of BTs, applying a controlled vacuum load with three drainage paths reduced the water content from 281.9 wt% to 53.6 wt% within 60 h [57]. Moreover, a step-by-step stacking test of BTs with sand and a model test involving quicklime curing were recommended [11]. The results revealed that following the addition of quicklime, the average water content of BTs in the sand stacking model decreased by 19.9 wt%.

3.2.2. Biochemical Solidification of BTs

Microbially induced calcite precipitation (MICP) is a novel and ecologically sustainable technique that enhances soil strength [58]. In order to achieve enhanced solidification and higher strength of BTs, the combined solidification technology utilizing quicklime and MICP has shown notable effectiveness. Comparative analysis with quicklime treatment alone revealed that the compression coefficient decreased by up to 63.2%, when the moisture content of tail clay, treated with both quicklime and microbial technology, was 60 wt% [59]. It was because the internal structure of BTs after biochemical treatment was denser, as shown in Figure 5e,f. However, it was observed that the compression coefficient av1–2 of tail clay with a moisture content of 60 wt%, following biochemical solidification, was comparatively lower than that of 80 wt% moisture content [59], as depicted in Figure 5a,b. With an increase in the quantity of quicklime, the compression coefficient av1–2 and cumulative pore volume of w = 60 wt% tail clay, after biochemical solidification, initially declined and subsequently exhibited an increase. Conversely, the percentage of small pore volume experienced an initial rise followed by a decline. This phenomenon can be attributed to the lime solidification process, which expands soil pores and provides a conducive environment for microbial activity. The ensuing microbial mineralization contributed to pore filling and soil particle binding, ultimately resulting in a denser soil structure through biochemical processes. However, excessive water content or high quicklime content led to intense hydration reactions, resulting in the release of heat and high temperatures reaching up to 50.5 °C, accompanied by an increase in OH− concentration. The elevated temperature and alkaline environment significantly inhibited microbial activity, thereby reducing the effectiveness of biochemical solidification. As depicted in Figure 5c,d, the cumulative pore volume at w = 60 wt% was found to be smaller than that at w = 80 wt%. However, the volume percentage of small pores (10 nm ≤ d < 100 nm) was observed to be 26.7% higher than that at w = 80% [59]. The effects varied for different particle sizes of BTs. The addition of urease-producing bacteria and gel resulted in a reduction of pore areas by 11.7% and 6.2% for BTs silty sand and clay samples, respectively [12]. It can be attributed to the utilization of calcium carbonate, a reaction product, for pore filling in BTs silty sand particles, leading to a gradual decrease in pore size. In the case of detrital BTs clay, the primary mechanism involved bonding, resulting in the gradual disappearance of small cracks. The reaction mechanism is shown in Equations (1)–(3) [12]. Furthermore, the alkali-activated binary system comprising sugarcane bagasse and carbide lime was used as a green stabilization technology for BTs. This approach extended the curing time of the alkali-activated gel from 7 to 28 days, resulting in an average increase of 65% in its unconfined compressive strength. Notably, it exhibits sustained reactivity throughout the curing period [53,60].
Ca2+ + Cell → Cell-Ca2+
NH2-CO-NH2 + 2H2O → 2NH4+ + CO32−
CO32− + Cell-Ca2+ → Cell-CaCO3 ↓

3.3. The Resource Utilization of BTs

3.3.1. Extraction of Metals

BTs exhibited high concentrations of Si, Al, and Fe, along with trace amounts of K, Ti, Ca, and Mg, indicating their significant economic potential. Efficient extraction of these valuable components through suitable processes is crucial for promoting the resource utilization of BTs. Clay minerals constitute the primary component of BTs. The acid methods [61,62] and the alkali methods [63,64,65] were often used for pre-roasting treatments. However, the roasting process consumes much energy, and, therefore, it is not economic. To achieve sustainable and energy-efficient utilization of BTs, a direct leaching process with a mixed acid for Al and Li extraction was employed, eliminating the need for roasting and thereby reducing energy consumption. This method represented a low-energy treatment approach [38]. Research indicated that optimal leaching rates of Al and Li reached 88.64% and 96.35%, respectively, under conditions of 60% acid concentration, a liquid-solid ratio of 4 mL/g, a reaction temperature of 100 °C, and a reaction time of 3 h, as shown in Figure 6a. The acid-baking water-leaching method can recover valuable elements from BTs [32]; the maximum leaching rate of each element at 30 min leaching time is shown in Figure 6a. It indicated that the leaching rates of scandium (Sc), samarium (Sm), cerium (Ce), neodymium (Nd), hafnium (Hf), iron (Fe), aluminum (Al), titanium (Ti), calcium (Ca), and sodium (Na) from BTs fall in the range 59–98%.
The leaching rate of elements was closely related to leaching time, the particle size of BTs, and reaction temperature, as shown in Figure 6b–d. It can be seen from Figure 6b that with the increase in reaction time, the leaching rates of Sc, Li, and Al gradually increased and tended to be stable, but the stability time of each element was different [32,38]. The baking temperature of BTs had a great influence on the leaching rate and stability time. With the increase of baking temperature, the leaching rate of Sc increased, and the time of leaching rate reaching stability is longer [32]. It can be seen from Figure 6c that the extraction efficiency of Al and Li decreased with the increase particle size of BTs [38]. The reduction of particle size was beneficial to leach Al and Li because with the gradual decrease of particle size from 150 µm to 44 µm, the mineral dissociation rate increased, and the exposed area of each particle increased. The increase of the contact area between particles and acid solution can improve the reaction efficiency. However, when the particle size continued to decrease to a certain size, the acid solution consumed on the particle surface could not be replenished rapidly, resulting in a decrease in the reaction rate and a reduction in the leaching rate of Al and Li. Therefore, the results showed that 74 µm of BTs was the best extraction size for Al and Li. Figure 6d shows that as reaction temperature increased, the leaching rates of Al and Li were greatly increased, and there was still a trend of continuous increase [38]. Therefore, 100 °C was selected as the best temperature for Al and Li extraction.
Based on the findings given in Section 3.1, it was evident that BTs exhibited a remarkably high content of Al and Si elements, indicating substantial potential for their application in the production of Al-Si alloy materials. Primary Al-Si alloys can be synthesized by carbothermal reduction experiments on BTs [51]. The significant influence of pressure and temperature on the carbon thermal reduction process indicates that the low pressure is not suitable for achieving the desired Al-Si alloy compositions. Moreover, the heating temperature in the range of 1600~2000 °C led to the persistent presence of silicon carbide, which is easy to form and hard to decompose, impeding the formation of primary Al-Si alloys. Notably, Fe2O3 played a vital role in the carbothermal reduction of Al2O3 and SiO2, which enhanced the reduction rate of Al2O3 and promoted the decomposition of carbides such as Al4C3 and SiC at elevated temperatures to facilitate the progress of the reduction reaction. Based on these observations and conclusions, it is suggested that optimal conditions for obtaining primary Al-Si alloys involved atmospheric pressure of 0.1 MPa, a heating temperature of 1900 °C, a smoke content equivalent to 95% (mass fraction) of the theoretical smoke content, and a sintering time of 1 h.

3.3.2. Preparation of Adsorption Materials

Due to the high concentration of aluminosilicate in BTs, the presence of Si and Al elements presented valuable opportunities for the utilization of BTs in the preparation of adsorption materials, such as zeolite and CaO-based adsorbents. These materials were commonly employed for the adsorption of heavy metal ions in soil and wastewater, as well as CO2 in the atmosphere.
The Si and Al elements present in BTs played a crucial role in the synthesis of zeolite. Among the numerous zeolite types available, BTs were primarily utilized in the production of zeolite 4A, zeolite X, and zeolite Y. Scanning Electron Microscopy (SEM) images of the synthesized zeolites are depicted in Figure 7, illustrating the distinct morphological characteristics. Zeolite 4A exhibited a tetrahedral or chamfered cubic shape, whereas zeolite X and zeolite Y possessed octahedral structures. Before zeolite synthesis, it is essential to remove impurities such as Fe2O3, TiO2, and K2O from BTs, as they can interfere with the synthesis process and impact the adsorption capacity of the resulting zeolite. Various methods can be employed to achieve impurity removal, including direct acid leaching and active calcination acid leaching, effectively eliminating impurities such as K2O and Fe2O3 [37], as shown in Equations (4) and (5). Wet chemical techniques can also be utilized to remove Fe2O3 impurities, involving an initial reaction with HCl followed by alkali melting [13]. Mechanical crushing and ball milling of BTs can enhance the reaction rate. The alkali melting method [13,34,66] was commonly employed for the extraction of useful Al2O3 and SiO2 (see Equations (6) and (7) [13]) while facilitating the removal of K2O and TiO2 impurities through solid-liquid separation. Following impurity removal, the remaining Al2O3, SiO2, and Na2O can be utilized to prepare pure zeolite. The hydrothermal method was a widely adopted approach for zeolite synthesis from BTs, as shown in Figure 8.
K2O + 2HCl = 2KCl + H2O
Fe2O3 + 6HCl = 2FeCl3 + 3H2O
Al2O3(s) + 2NaOH(s) → Na2Al2O4(s) + H2O(g)
SiO2(s) + 2NaOH(s) → Na2SiO3(s) + H2O(g)
Zeolites derived from BTs exhibited promising capabilities for the adsorption of various heavy metal ions, including Cr(III), Cu(II), Pb(II), and CO2. The maximum removal efficiency is shown in Figure 9. Notably, zeolite 4A synthesized from BTs demonstrated high efficiency in removing Cr(III), surpassing the performance of chemical precipitation methods [66]. Under conditions of an initial pH value of 4 and an initial Cr(III) concentration of 5 mg/L, zeolite 4A achieved a maximum removal rate of 85.1 mg/g. In addition, the maximum removal efficiency of Cr(III) reached 96.8%. Similarly, the synthesis of zeolite 4A from BTs resulted in a remarkable removal rate of 96.2% for Cu(II) within just 30 min when the initial pH value was 5.0 [14]. After three cycles of adsorption and desorption, the removal efficiency of Cu(II) was as high as 78.9%. Zeolite X, characterized by its distinctive crystal structure, exhibited exceptional adsorption capacity and selective adsorption performance. Zeolite X with high stability can be successfully synthesized by hydrothermal methods [15,34]. At the temperature of 273 K and 298 K, the maximum adsorption capacity for CO2 reached 7.3 mmol/g and 6.4 mmol/g, respectively, surpassing that of zeolite X derived from feldspar by a factor of 1.64 and bentonite zeolite by a factor of 1.3. This value was comparable to the maximum adsorption capacity of zeolite X synthesized from kaolinite. Even after 11 adsorption and desorption cycles, the CO2 adsorption capacity remains at 6.3 mmol/g, decreasing by only 1.6% from the initial cycle. Zeolite Y was known for its exceptional thermal and chemical stability. It was synthesized from BTs and demonstrated remarkable efficacy in removing Pb (II) from wastewater [16]. The adsorption capacity for Pb (II) reached an impressive 443.87 mg/g, accompanied by a maximum removal rate of 99.95%. Notably, even after undergoing five cycles of adsorption and desorption, the removal rate of Pb (II) by zeolite Y remained at a high level of 99.2%.
In addition to zeolite, BTs have been utilized for the preparation of CaO-based adsorbents, which exhibited notable performance in CO2 adsorption and desorption. This was attributed to the utilization of the abundant Al2O3 content in BTs, as the incorporation of Al2O3 played a pivotal role in enhancing the durability of CaO-based adsorbents for CO2 absorption, as reported by Luo et al. [67] and Wu et al. [68]. The reaction between Al2O3 and CaO led to the formation of the Ca12Al14O33 phase, which provided a stable structural framework for the CaO-based adsorbents and effectively inhibited the occurrence of multiple cyclic sintering. The effect of different BTs/CaO ratios on carbonation conversions during the calcination/carbonation cycles is shown in Figure 10. The carbonation conversion of CaO in sorbents and the absorption capacity of CaO-based sorbents were determined as indicated in Equation (8) [4]:
X = [(m − m0)/(m0 × γ)] × MCaO/MCO2
Here, X represents the carbonation conversion of CaO in the sample, m denotes the sample mass during the reaction, m0 is the sample mass after the initial calcination, γ indicates the CaO content in the CaO-based sorbents, and MCaO and MCO2 refer to the molar masses of CaO and CO2, respectively.
It can be clearly seen from Figure 10a that the CaO-based adsorbent with higher BT content had better stability but lower conversion. However, Figure 10b showed a different trend; the CaO-based adsorbent with different BT content showed an increase in carbonation conversion through several initial cycles and then stayed almost constant in the subsequent cycles. In addition, there were different conclusions on the optimal content of BTs. The synthesis of a stable CaO-based adsorbent can be achieved using CaCO3 and BTs [52]. The addition of 5% wt% BTs resulted in excellent cycling stability for the CaO-based adsorbents, even after calcination at 900 °C for 3 h and undergoing 50 carbonization/calcination cycles, achieving a conversion rate of 42% after 50 cycles. The synthesis of CaO-based adsorbents for CO2 absorption can be accomplished through a dry mixing method, utilizing lime mud as the calcium source and BTs as an additive [17]. The experimental results exhibited a low conversion rate of 27.95% after 30 cycles of carbonization/calcination, despite the use of a CaO-based adsorbent doped with 15 wt% BTs. However, the adsorbent demonstrated notable cycling absorption stability. By employing BTs and eggshells as raw materials, low-cost porous CaO-based adsorbents can be developed using the solid-state method [4]. The incorporation of 10 wt% BTs in the CaO-based adsorbent exhibited favorable stability in CO2 absorption throughout multiple adsorption and desorption cycles, with a conversion rate of 55% after 40 cycles. It should be noted that an appropriate amount of the BT content contributed to the reduction in crystal particle size, thereby facilitating enhanced CO2 absorption capabilities.
Additionally, BTs modified with FeCl3·6H2O proved to be directly applicable as an adsorbent, achieving an impressive 99.3% removal rate for Cr(VI) [35]. Ye et al. [41] observed similarly high removal rates of 99%, 99%, and 90% for Cr(VI), As(V), and F(I), respectively. BTs can serve as a cost-effective precursor for synthesizing pyroaurite-like (Mg-Fe-Al-NO3) layered double hydroxide (LDH) nanosorbents [39]. It was used to remove acid red B dye from aqueous solutions. The nanostructured pyroaurite demonstrated exceptional adsorption capacity, with a specific surface area of 81 m2/g and a rapid adsorption rate within the initial 20 min. Notably, as the temperature increased from 35 °C to 55 °C, the dye removal rate exhibited a corresponding increase from 89% to 93%. Despite the cost-effectiveness of the physical and chemical methods mentioned, they pose the risk of secondary pollution during the treatment process. Specific types of BTs contain various fungal species that exhibit resistance to multiple metals and serve as effective adsorbents [69]. Notably, the BI-II fungal strains exhibited remarkable resistance to Cr(VI) (1500 μg mL−1), Cu(II) (600 μg mL−1), Pb(II) (500 μg mL−1), and Zn(II) (500–1500 μg mL−1), highlighting their significant potential for metal resistance. The excellent metal-tolerant isolate was characterized and identified as Aspergillus tubingensis AF3 through 18S rRNA sequencing method. It was submitted to GenBank and received an accession number (MN901243). Standard growth conditions resulted in absorption efficiencies below 70% for Cr(VI) and 46.3% for Cu(II) by A. tubingensis AF3. However, under optimized conditions, the absorption efficiency for Cr(VI) reached an impressive 74.48%, demonstrating the high capacity of A. tubingensis AF3 biomass to absorb elevated concentrations of Cr(VI).
After undergoing crushing, grinding, and roasting, BTs were subjected to hydrochloric acid dissolution, resulting in the dissolution of Al2O3 and Fe2O3 present in BTs. Subsequently, a polymerization process was employed to synthesize a polymeric aluminum ferrc chloride (PAFC) flocculant for efficient turbidity removal in wastewater treatment, demonstrating an impressive turbidity removal rate of up to 90% [40]. Comparative studies with traditional Polymeric aluminum chloride (PAC) revealed that PAFC derived from BTs exhibited superior performance in terms of turbidity removal efficiency. The utilization of BTs as a precursor also extended to their co-pyrolysis with microalgae (chlorella vulgaris, CV) for the production of metal-biochar catalysts [56]. Notably, BTs covered the surface of the biochar, increasing BT content and leading to enhanced coverage (as illustrated in Figure 11), thus promoting the development of distinct mesopores and macropores within the metal-biochar. As a result, BTs served as a stable support for the deposition of solid carbon. Yoon et al. [56] also indicated that catalysts prepared with equal mass ratios of CV and BTs exhibited the highest degradation efficiency for methyl orange (MO), achieving a remarkable degradation rate of 75.4% at a pH of 3 during a 60-min reaction period.

3.3.3. Preparation of Ceramic Materials

As industrial solid waste comprising clay minerals, BTs have good plasticity and can replace clay in ceramic manufacturing processes. Incorporating BTs can significantly lower the sintering temperature of ceramics, typically from 800 °C to 1190 °C [3,19,20,70]. Ren et al. [19] attributed the low-temperature sintering to the formation of a flux from potassium mica decomposition (yielding K2O) and partial Fe2O3 decomposition (generating FeO), as well as a low melting point eutectic within the sintered body (composed of Al2O3 and SiO2). This eutectic facilitated the dissolution of crystals such as quartz, corundum (Al2O3), hematite, and mullite (3Al2O3·2SiO2), fostering the formation of a liquid phase at lower temperatures. The presence of this low-temperature liquid phase accelerated the sintering kinetics.
The ceramic materials derived from BTs showed high flexural strength, as illustrated in Figure 12. The flexural strength increased with higher sintering temperatures [3,20] due to the formation of a beneficial liquid phase that promotes densification [19]. However, exceeding a critical temperature threshold may decrease the strength [19]. At 1130 °C, the ceramics achieved a peak flexural strength of 197.41 MPa. The main minerals present in the sintered ceramics comprised pyroxene [(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6] or anorthite (CaAl2Si2O8), exhibiting excellent mechanical strength [20]. Furthermore, higher Al2O3 content increased the proportion of corundum crystals, enhancing the flexural strength [19].
Furthermore, at sintering temperatures between 930 °C and 1000 °C, the ceramic materials displayed Vicker’s hardness values ranging from 5.86 GPa to 6.63 GPa [3]. The change in Vickers hardness was correlated with the volume friction and microstructure of the crystal phase. Within this temperature range, the variations in bulk density and water absorption of the substrate were minimal, ranging from 2.13 g/cm3 to 2.14 g/cm3 and 8.81% to 8.24%, respectively. Despite higher water absorption in the substrate layer, the double-layer bricks performed similarly to porcelain bricks. Sintering at 1000 °C for 30 min produced a glass ceramic disk with a water absorption of 0.19% and a bulk density of 2.16 g/cm3 [70]. As the sintering temperature increased from 1050 °C to 1150 °C, the water absorption of the ceramics steadily decreased [19]. In contrast, Li et al. [20] created porous permeable ceramics (PPC) by granulating composite ceramic (CC) particles with steel slag as the core and BTs as the outer layer. The findings revealed that as the sintering temperature rose, the micropores gradually vanished, and the threshold diameter of the micropores expanded from 45 μm at 1180 °C to 70 μm at 1190 °C. This increase in the threshold diameter led to a reduction in pore quantity. The PPC sintered at 1180 °C had a porosity of 27.5%, a medium pore size of 92.7 μm, a bending strength of 10.92 MPa, and a water permeability of 0.039 cm/s. Moreover, it guarantees excellent leaching of harmful elements, such as Mn, Cr, V, Pb, and others.

3.3.4. Preparation of Cementitious Materials

Due to their volcanic ash properties, BTs can be used as a supplementary material in cement production to reduce energy use and carbon emissions. Using BTs in clinker synthesis cuts carbon dioxide emissions by 32% compared to traditional methods [22]. Untreated BTs have a stable crystal structure, so thermal activation is used to make them more reactive [71]. Thermal activation induced modifications in the mineral phase of BTs. At 500 °C, the hydroxy group of diaspore and kaolinite in BTs began to be removed in large quantities and converted into corundum and metakaolinite (Al2Si2O7), respectively. These hydroxyl groups are completely removed at 600 °C and 700 °C respectively [23]. However, higher temperatures may cause the recrystallization of metakaolinite into less reactive minerals like spinel [(Mg,Fe)Al2O4)] [72]. Ye et al. [36] observed that at 800 °C, diaspore and kaolinite transformed into corundum and metakaolinite through dehydration, while other minerals like anatase and muscovite remained unchanged. The smaller particle sizes of corundum and metakaolinite compared to diaspore and kaolinite result in a reduction in the particle size of the thermally activated BTs. Between 400 °C and 700 °C, BTs displayed a gradual decrease in particle size with increasing activation temperature. However, when the activation temperature exceeded 700 °C, particle size tended to increase due to sintering-induced agglomeration [23]. Additionally, sintering led to reduced exposure of active sites, consequently lowering the reactivity of BTs.
When activated BTs are used in mortar preparation, both the compressive strength and activity index of the mortar increased with the fineness of BTs [23]. This is because finer BTs have a larger surface area, which exposes more active sites and promotes the formation of hydration products. Moreover, the unreacted fine BTs effectively filled pores in the mortar, creating a denser structural arrangement. At an activation temperature of 700 °C, both coarse and fine BTs showed their best performance, with activity indices of 55.6% and 69.6%, respectively [23]. Fine BTs were more reactive than coarse BTs at the same temperature, leading to greater consumption of calcium hydroxide and the formation of C-S-H gel and ettringite. Consequently, at 700 °C, mortar samples with fine BTs achieved the highest 28d unconfined compressive strength of 36.8 MPa, while those with coarse BTs reached 29.5 MPa, as shown in Figure 13. Fine BTs are thus effective additives for Portland cement production, meeting the Chinese national standard GB/T 2847-2005, which requires an activity index over 65%.
The BT content also significantly affected the hydration process of cementitious materials, as illustrated in Figure 14. Over 72 h, the hydration heat profile of the samples progresses through five stages: dissolution, induction, acceleration, deceleration, and diffusion. As the BT content increased, the degree of hydration in the BTs-lime-gypsum system decreased, leading to lower calcium hydroxide levels in the binder compared to pure cement, especially when the BT content exceeded 30% [30]. Higher BT content promotes the formation of more hydration products, contributing to a denser structure that slows water and ion migration within the cementitious system. However, as the hydration reaction progressed, more Ca(OH)2 was consumed, reducing its content during the diffusion stage. Increased BT content enhanced the formation of ettringite and C(A)-S-H gel due to the pozzolanic reaction with calcium hydroxide, consuming Ca2+ and OH− and forming these hydration products [50].
The compressive strength of cement paste made with BTs, combined with clinker and gypsum in ratios of 90:10, 85:15, and 75:25, was higher than that of standard Portland cement [22]. Among these ratios, the paste with 15% gypsum had the highest strength after 28 days, reaching 51 MPa. However, a paste with a 25% gypsum content experienced a decline in strength to 33 MPa over the same period, as shown in Figure 15a. Thermal treatment also had a significant impact on geopolymers made from BTs and blast furnace slag, as illustrated in Figure 15b. At 1000 °C, the gel in the geopolymer changed from an amorphous state to crystalline phases, such as calcium aluminum oxide and anorthite. The difference in thermal expansion between the shrinking gel and the expanding quartz sand led to a notable drop in the mortar’s compressive strength, reducing it by about 40.0 MPa. However, at temperatures above 1200 °C, the strength improved significantly by around 100% due to densification, self-healing through viscous sintering, and the formation of anorthite and other structural components.

3.3.5. Preparation of Lightweight Aggregate

In recent years, BTs have been widely used as a primary raw material for producing lightweight aggregate (LWA) [1,25,26]. Extensive research has underscored the considerable advantages offered by LWA derived from BTs. The fine LWA, possessing a high water absorption capacity (20%), is generated from BTs, silica fume, and papermaking sludge. It exhibited a well-connected fine pore structure in the range of 200 nm to 2000 nm and absorbed over 95% moisture at 97% relative humidity (RH) [25]. Compared to high-strength mortars with the same water content, mortars with fine LWA showed an 88% reduction in self-shrinkage and a 2.5% strength increase. Additives like CaF2 and CaCO3 further enhanced pore size, especially at higher concentrations [1]. High water-absorbing LWA significantly reduces mortar self-shrinkage while mitigating compressive strength loss, as shown in Figure 16. The stacking density of silica fume deposits from BTs and silica fume ranged from 931 kg/m3 to 1100 kg/m3, with a water absorption rate of 1.2% to 16.6%, meeting the standards. LWA with over 20% silica fume content and sintering temperatures above 1100 °C is suitable for lightweight concrete and insulation wall panels. Conversely, LWA with less than 20% silica fume content and sintering temperatures below 1100 °C is ideal for internal curing. Effective gas release control via a two-step firing method enabled LWA expansion [26].
Using BTs as a fine aggregate to replace natural sand in 3D-printed mortar formulations proved effective [31]. Incorporating BTs reduced fluidity, slump, and setting time while delaying cement hydration. This addition also significantly increased the static yield stress, dynamic yield stress, plastic viscosity, and thixotropy of the mortar. As BTs content increased, both macroscopic and structural deformation rates of the 3D printed components gradually decreased. Compressive and flexural strengths initially increased but later declined (Figure 17). The optimal BTs to natural sand ratio was found to be 35%, offering favorable workability and high mechanical strength for 3D-printed mortar.

3.3.6. Other Materials

A foamed mixture of lightweight soil is a widely used geotechnical material in projects like soft foundation treatment, road widening, and subgrade backfilling [73]. Ordinary Portland cement, commonly used in its preparation, significantly contributes to carbon emissions (0.8–0.95t CO2 per 1t of cement). This has led researchers to seek alternatives, such as alkali-activated industrial solid waste, to partially or fully replace cement. A foamed mixture of lightweight soil with BTs (FMLSB) has shown promising performance [5]. At a density of approximately 500 kg/m3, the FMLSB exhibited unconfined compressive strengths between 0.31 MPa and 0.71 MPa. Adding 10–40% BTs yielded 7-day unconfined compressive strengths, reaching 73–83% of the 28-day strength. These mixtures also demonstrated water absorption rates of 25.7–43.1% and water stability coefficients above 0.7, indicating good early strength and durability. Increased area porosity led to an exponential decrease in unconfined compressive strength (Figure 18a). Analysis of the chemical composition of FMLSB revealed the presence of calcium silicate hydrate (C-S-H), calcium hydroxide crystals (C-H), hydrated calcium aluminate (C-A-H), hydrated calcium aluminosilicate (C-A-S-H), and a small amount of ettringite (AFt), as demonstrated in Figure 18b. BTs were roasted at 600 °C for 2 h, then mixed with phosphoric acid and subjected to two heat treatments, and used to synthesize aluminum tri-polyphosphate, an anticorrosion pigment [42]. This pigment exhibited enhanced pH (7.2), whiteness (85.4%), and an average particle size of 4.8 μm, surpassing industry standards when compared to the unmodified form.

4. Discussion

4.1. Challenges and Strategies for Efficient Dehydration and Solidification of BTs

The high moisture content of BTs posed treatment challenges. Consolidation and solidification technology was well-established and effective in reducing the water content of BTs and enhancing their strength. However, in practical engineering applications, the large volume and open nature of the BTs storage areas hindered the direct consolidation and solidification to achieve the required strength, resulting in high time and economic costs. Hence, it was imperative to integrate existing methods and explore novel technologies to conduct systematic experimental research in stages and blocks. The ultimate goal is efficient dehydration and solidification of BTs in waste reservoirs to meet reclamation and reuse criteria. For example, combining solar evaporation and surface drainage can first remove surface water, followed by accelerated drainage consolidation, chemical solidification, and microbial solidification to treat specific sections of the waste reservoirs.

4.2. Multifaceted Utilization and Resource Management of BTs

The multifaceted utility of BTs stemmed from their rich mineral and chemical composition. BTs are high in Si, Al, Fe, and trace amounts of K, Ti, Ca, and Mg, making them economically valuable. Following metal extraction, significant residual material remains. Therefore, it is crucial to repurpose this residual material to enable its large-scale utilization and maximize the overall resource efficiency of BTs. Notably, BTs’ abundance in Al2O3, SiO2, and Fe2O3 enhances the adsorption efficiency of adsorption materials for heavy metal ions and CO2 and substantially improves the flexural strength of ceramic materials. The siliceous aluminate in BTs imparts pozzolanic characteristics activated by calcination or an alkaline environment. In cementitious materials, BTs exhibit impressive compressive strength, surpassing Portland cement, thus reducing carbon emissions and energy consumption. Consequently, their use as a cement substitute contributed to reduced carbon emissions and energy consumption. Furthermore, BTs’ fine particle size, large specific surface area, and high functional group activity make them ideal for producing fine lightweight aggregates. These aggregates have high water absorption rates, aiding internal curing and reducing shrinkage in concrete. Using BTs as a fine aggregate decreases mortar fluidity, slump, and setting time while improving static and dynamic yield stress, plastic viscosity, and thixotropy.
Despite their immense application potential, the absence of a systematic BTs management platform hinders their utilization. Moreover, the preparation process for most materials necessitated high-temperature calcination and intricate procedures. There was a problem of separation between drying and curing, which was time-consuming and labor-intensive and also caused certain energy consumption and carbon emissions. Consequently, large quantities of BTs remain stored in tailing reservoirs with low utilization rates. To optimize BTs’ utilization, initial chemical synthesis and extraction should be followed by resource utilization of the remaining residues. The synergistic utilization of multiple solid wastes represents a future development direction. For example, BTs can be combined with fly ash, waste-to-energy ash, or industrial sludge to produce construction materials. This approach aims to achieve comprehensive resource utilization and facilitate the large-scale application of all BTs components.
The proposed processing and resource utilization strategies are shown in Figure 19. First, a database should be established to document the physicochemical properties of BTs in various locations within waste reservoirs. A comprehensive information management platform should be developed by the government and relevant enterprises to store this data and monitor the real-time storage volume of BTs. This platform would facilitate the utilization and preparation of BTs into products aligned with actual engineering needs. Second, an integrated treatment process and equipment for BTs should be developed that incorporates flocculation, filtration, and solidification techniques. Sun et al. [74,75] proposed a geopolymeric Flocculation-Filtration-Solidification method, which can utilize waste shield slurry with high efficiency. It provides a good idea for the treatment of BT slurry. The BTs treated using this technology can be applied in three aspects (Figure 19): (1) flow casting type packing, (2) mud cake utilization, and (3) building materials application. By combining these treatment methods with the management platform, efficient resource utilization of the BTs can be achieved. Developing an integrated treatment process will significantly reduce the time, cost, and resources required for the treatment and utilization of BTs. To achieve efficient BT utilization, it is crucial to align their utilization with specific project needs and to develop environmentally-friendly and low-carbon resource utilization methods based on existing research.

5. Conclusions

In the context of global carbon emissions, the low utilization rate of BTs has attracted significant scholarly attention, leading to extensive research on promoting their resource utilization. This review examined the properties, treatment methods, extraction processes of valuable elements, and resource utilization of BTs, highlighting their benefits and limitations.
The main chemical components of BTs are Al2O3, SiO2, and Fe2O3, with an Al/Si ratio between 0.75 and 5.82. BTs have fine particle size, large specific surface area, and high functional group activity. In alkaline environments, BT activity was easily stimulated, resulting in a volcanic ash effect. At the same time, BTs had strong adsorption and ion exchange properties.
Technologies like rapid drainage consolidation, chemical solidification, and microbial solidification offer promising solutions for high-water-content BTs. However, due to the large storage volume and wide area of waste reservoirs, single technologies are time-consuming and costly. Thus, combining various existing methods or developing new technologies to conduct experimental research in stages and blocks is necessary, ultimately achieving dehydration and solidification of BTs in waste reservoirs for reclamation and reuse.
BTs’ diverse mineral and chemical composition underscores their potential for valuable element extraction, including significant concentrations of Si, Al, and Fe and trace amounts of K, Ti, Ca, and Mg. These elements enhance BTs’ economic value and industrial utility.
BTs’ physical characteristics and chemical composition make them valuable for the production of adsorption materials, cementitious materials, ceramic materials, and lightweight aggregates, offering a promising solution for large-scale utilization. However, challenges like high energy consumption and carbon emissions from high-temperature calcination, along with complex preparation processes, remain.
For the time being, the storage capacity of BTs remains substantial, and with the continuous growth of the aluminum industry, the future production of billions of BTs is expected. Extensive research has confirmed the immense value and potential applications of BTs. However, further advancements are required in the realm of green utilization technology for BTs. On the one hand, the resource utilization of BTs should be directed towards in-situ, green, economically viable, and highly efficient utilization. On the other hand, effective management and deployment of BT resources based on actual requirements is crucial to achieve optimal resource utilization of BTs.

Author Contributions

Conceptualization, Y.P. and X.S.; methodology, Y.P.; software, Y.P.; Drawing, Y.P.; writing—original draft preparation, Y.P.; Supervision, Z.C., X.S. and X.O.; writing—review and editing, Z.C., X.S., Y.Z. and X.O.; funding acquisition, Y.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 52208353).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Production process of BTs and consequent environmental problems.
Figure 1. Production process of BTs and consequent environmental problems.
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Figure 2. Cumulative size curves of the investigated BTs. (Data are from Refs. [25,29,30,31]).
Figure 2. Cumulative size curves of the investigated BTs. (Data are from Refs. [25,29,30,31]).
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Figure 3. Back-scattered electron (BSE) images obtained from the SEM study.
Figure 3. Back-scattered electron (BSE) images obtained from the SEM study.
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Figure 4. Suggested model test for the treatment of BTs.
Figure 4. Suggested model test for the treatment of BTs.
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Figure 5. Effects of biochemical treatment and quicklime treatment: (a,b) represent av-P curves for BTs; (c,d) represent pore volume percentage under different treatment; (e,f) represent BSE images to show microstructure of quicklime treatment and biochemical treatment, respectively. (Data are from Ou et al. [59]).
Figure 5. Effects of biochemical treatment and quicklime treatment: (a,b) represent av-P curves for BTs; (c,d) represent pore volume percentage under different treatment; (e,f) represent BSE images to show microstructure of quicklime treatment and biochemical treatment, respectively. (Data are from Ou et al. [59]).
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Figure 6. Recovery of elements of BTs: (a) maximum recovery; (b–d) represent the effect of leaching duration, particle sizes, and temperature on the recovery, respectively. (Data are from Refs. [32,38]).
Figure 6. Recovery of elements of BTs: (a) maximum recovery; (b–d) represent the effect of leaching duration, particle sizes, and temperature on the recovery, respectively. (Data are from Refs. [32,38]).
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Figure 7. BSE images of the zeolite samples: (a,b) represent the zeolite 4A samples synthesized by two methods; (c,d) represent the zeolite X and zeolite Y samples, respectively [13,14,15,16].
Figure 7. BSE images of the zeolite samples: (a,b) represent the zeolite 4A samples synthesized by two methods; (c,d) represent the zeolite X and zeolite Y samples, respectively [13,14,15,16].
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Figure 8. Synthesis of zeolite from BTs.
Figure 8. Synthesis of zeolite from BTs.
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Figure 9. Maximum removal efficiency: (a) heavy metal ions; (b) CO2. (Data are from Refs. [14,15,16,34,66]).
Figure 9. Maximum removal efficiency: (a) heavy metal ions; (b) CO2. (Data are from Refs. [14,15,16,34,66]).
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Figure 10. Carbonation conversion of CaO-based adsorbent prepared with different BTs/CaO ratios: (a) Refs. [17,52]; (b) Ref. [4].
Figure 10. Carbonation conversion of CaO-based adsorbent prepared with different BTs/CaO ratios: (a) Refs. [17,52]; (b) Ref. [4].
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Figure 11. FE-SEM/EDX of metal-biochar [56]: (a) CV; (b) C1B0.5; (c) C1B1; and (d) C1B2. Note: C1B0.5, C1B1, and C1B2 represent the mass ratios of CV to BTs of 1:0.5, 1:1 and 1:2, respectively.
Figure 11. FE-SEM/EDX of metal-biochar [56]: (a) CV; (b) C1B0.5; (c) C1B1; and (d) C1B2. Note: C1B0.5, C1B1, and C1B2 represent the mass ratios of CV to BTs of 1:0.5, 1:1 and 1:2, respectively.
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Figure 12. The flexural strength of ceramics varies with temperature. (Data are from Refs. [19,20,70]).
Figure 12. The flexural strength of ceramics varies with temperature. (Data are from Refs. [19,20,70]).
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Figure 13. Compressive strength of mortar including coarse BTs and fine BTs at different activation temperatures (Data are from Zhou et al. [23]).
Figure 13. Compressive strength of mortar including coarse BTs and fine BTs at different activation temperatures (Data are from Zhou et al. [23]).
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Figure 14. Hydration heat evolution rate (a) and cumulative hydration heat curves (b) of paste samples for 72 h (Data are from Zhou et al. [30]). Note: T0 is the pure cement; T80-20 signifies that the separation frequency of BTs is 80 Hz and the BTs content is 20% by weight; others are similar.
Figure 14. Hydration heat evolution rate (a) and cumulative hydration heat curves (b) of paste samples for 72 h (Data are from Zhou et al. [30]). Note: T0 is the pure cement; T80-20 signifies that the separation frequency of BTs is 80 Hz and the BTs content is 20% by weight; others are similar.
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Figure 15. Effect of clinker to gypsum ratio (a) and elevated temperature (b) on the compressive strength of mortars. (Data are from Refs. [22,36]).
Figure 15. Effect of clinker to gypsum ratio (a) and elevated temperature (b) on the compressive strength of mortars. (Data are from Refs. [22,36]).
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Figure 16. Autogenous shrinkage of mortars (Data are from Yang et al. [1]). Note: REF represents the mortar without LWA; M0, MF, and MC represent the mortars mixed with LWA, LWA + 10 wt% CaF2, and LWA + 20 wt% CaCO3, respectively.
Figure 16. Autogenous shrinkage of mortars (Data are from Yang et al. [1]). Note: REF represents the mortar without LWA; M0, MF, and MC represent the mortars mixed with LWA, LWA + 10 wt% CaF2, and LWA + 20 wt% CaCO3, respectively.
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Figure 17. Mechanical properties of 3D printing mortar at different curing ages (a) Compressive strength; (b) Flexural strength. (Date are from Zhou et al. [31]).
Figure 17. Mechanical properties of 3D printing mortar at different curing ages (a) Compressive strength; (b) Flexural strength. (Date are from Zhou et al. [31]).
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Figure 18. Characteristics of FMLSB: (a) relationship between unconfined compressive strength and area porosity; (b) XRD analysis. Note: S = C-S-H, C = C-H, E = AFt, H = C-A-S-H, A = C-A-H, B = C2S or C3S, G = CaSO4·2H2O, D = SiO2, O = CaCO3; B0, B10, B20, B30, and B40 represent the replacement ratio of BTs for cement at 0%, 10%, 20%, 30%, and 40% by weight, respectively. (Date are from Peng et al. [5]).
Figure 18. Characteristics of FMLSB: (a) relationship between unconfined compressive strength and area porosity; (b) XRD analysis. Note: S = C-S-H, C = C-H, E = AFt, H = C-A-S-H, A = C-A-H, B = C2S or C3S, G = CaSO4·2H2O, D = SiO2, O = CaCO3; B0, B10, B20, B30, and B40 represent the replacement ratio of BTs for cement at 0%, 10%, 20%, 30%, and 40% by weight, respectively. (Date are from Peng et al. [5]).
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Figure 19. Treatment and resource utilization strategy of BTs.
Figure 19. Treatment and resource utilization strategy of BTs.
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Table 1. Main mineral compositions of BTs.
Table 1. Main mineral compositions of BTs.
BTs NumberMineral Composition or MineralogyReference
1Diaspore, hematite, anatase, kaolinite and illite[34]
2Diaspore, kaolinite, illite, anatase, hematite and quartz[35]
3Kaolinite, diaspore, muscovite, anatase, corundum[36]
4Diaspore, illite, kaolin, illite, hematite, anatase and quartz[37]
5Kaolinite, diaspore, boehmite, anatase and illite[38]
6Diaspore, kaolinite, with a small amount of hematite,
anatase, and clay minerals		[13]
7Kaolinite, hematite, gibbsite, anatase and quartz[39]
8Illite, diaspore, kaolinite, hematite and montmorillonite[25]
9Diaspore, hematite, illite and quartz[19]
10Kaolinite and gibbsite[33]
11Diaspore, kaolinite, with a small amount of hematite, anatase[40]
12Diaspore, illite, kaolinite, and hematite with minor montmorillonite.[1]
13Diaspore, kaolinite, illite, anatase, hematite and quartz[41]
14Diaspore, kaolinite, illite, anatase, hematite and quartz[42]
15Kaolinite, gibbsite, goethite, hematite, and anatase[22]
16Hematite, quartz, gibbsite, and anatase[43]
17Kaolinite, muscovite, quartz, illite, vaterite, and albite[44]
Table 2. Chemical composition of the BTs (wt%).
Table 2. Chemical composition of the BTs (wt%).
BTs NumberSiO2Al2O3Fe2O3K2OTiO2CaOMgOSO3−Na2OCountry of
Production		Reference
130.5140.7110.242.45/0.90.94/0.67China[46]
23440.561.834.343.310.30.37/0.069China[45]
335.0144.5711.183.773.030.830.450.770.11China[47]
437.2239.4610.614.14/2.670.73//China[48]
528.3238.114.93/1.70.32///China[5]
638.7441.569.683.612.580.621.360.82/China[31,49]
736.7943.519.583.412.650.751.680.86/China[23]
825.4837.127.53/2.970.72///China[4]
937.5544.790.50.322.4/0.16//China[38]
1021.4547.8713.12.252.610.41//0.23China[13]
1136.6840.5410.283.473.212.980.531.73/China[50]
1237.22–37.2336.17–39.4610.61–13.663.28–4.143.63–3.742.67–3.150.61–0.730.72–1.35/China[30]
1334.5444.6810.644.86/0.550.46/0.49China[25]
1425.2737.1910.663.063.896.050.15/0.99China[3]
1522.4549.8713.12.252.610.41//0.23China[40]
1626.3343.810.520.143.082.80.42//China[51]
1728.8939.527.314.713.120.610.460.130.82China[41]
1841.4435.28.383.171.630.52///China[19]
1919.9754.737.692.172.51///0.03China[15]
2028.939.57.34.73.10.60.50.10.8China[35]
2119.857.64.840.692.181.081.6/0.82China[42]
2221.4547.8713.12.252.610.41//0.23China[37]
2360.922.757.9341.191.780.77/0.17China[20]
2425.4837.127.53/2.970.72///China[17]
2532.2437.398.67/2.313.150.85//China[36]
2625.4837.127.53/2.970.72///China[52]
2724.9831.2623.46/4.03////Brazil[39]
2818.221.630.4//1.47<0.1/10.9Brazil[53]
2931.5333.8528.54.15///0.56/Brazil[33]
3025.16–25.3541.12–41.2812.4–12.670.022.04–2.270.020.12/0.03Brazil[22]
3117.853.58.30.22.60.70.8/0.1Russia[54]
3218.838.920.4/1.5////Guinea[55]
3322.857.111.40.532.31.221.4/0.24Ghana[7]
3414.11846.6/9.32.6//7.9Korea[56]
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Peng, Y.; Chen, Z.; Sun, X.; Zhou, Y.; Ou, X. Properties, Treatment and Resource Utilization of Bauxite Tailings: A Review. Sustainability 2024, 16, 6948. https://doi.org/10.3390/su16166948

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Peng Y, Chen Z, Sun X, Zhou Y, Ou X. Properties, Treatment and Resource Utilization of Bauxite Tailings: A Review. Sustainability. 2024; 16(16):6948. https://doi.org/10.3390/su16166948

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Peng, Yuansheng, Zhongping Chen, Xiaohui Sun, Yuefu Zhou, and Xiaoduo Ou. 2024. "Properties, Treatment and Resource Utilization of Bauxite Tailings: A Review" Sustainability 16, no. 16: 6948. https://doi.org/10.3390/su16166948

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