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Review

Treatment and Recycling of Tungsten Beneficiation Wastewater: A Review

by
Wenxia Zhu
1,
Jianhua Kang
1,2,*,
Danxian Zhang
1,
Wei Sun
1,
Zhiyong Gao
1,
Haisheng Han
1 and
Runqing Liu
1
1
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
2
State Key Laboratory of Mineral Processing, BGRIMM Technology Group, Beijing 102600, China
*
Author to whom correspondence should be addressed.
Separations 2024, 11(10), 298; https://doi.org/10.3390/separations11100298
Submission received: 22 August 2024 / Revised: 13 October 2024 / Accepted: 14 October 2024 / Published: 16 October 2024
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Abstract

:
The large amount of wastewater containing various pollutants generated during the tungsten beneficiation process has become a bottleneck for the sustainable development of tungsten mining enterprises. Typical pollutants mainly include suspended solids (SSs), silicate ions, metal ions, and residual organic reagents. The direct discharge of untreated tungsten beneficiation wastewater can cause serious harm to the ecological environment, while recycling can significantly affect flotation indicators. In this paper, the sources and characteristics of typical pollutants were analyzed, and various purification techniques were outlined, including coagulation, adsorption, chemical precipitation, oxidation, and biological treatment methods. Among these techniques, coagulation is particularly effective for the removal of SSs, while adsorption and chemical precipitation are recommended for the removal of soluble ions. For residual organic reagents, oxidation methods have demonstrated high treatment efficiencies. The mainstream methods for wastewater recycling were summarized, including centralized recycling, as well as internal recycling at certain stages. For tungsten beneficiation such a complex process, where the quality of wastewater varies greatly between different stages, it is suitable to recycle the wastewater after appropriate treatment at a specific stage. Furthermore, this study provided a perspective on the future directions of tungsten beneficiation wastewater treatment, serving as a reference for related research and industrial practices.

Graphical Abstract

1. Introduction

Non-ferrous metals are indispensable raw materials for modern industry and strategic support for the national economy [1]. Beneficiation, as an essential part of the development and utilization of non-ferrous metals, is the key to ensuring the quality of non-ferrous metal products [2,3,4]. In the process of non-ferrous metal beneficiation, a large amount of wastewater is generated, and the wastewater usually contains a variety of pollutants that cannot be directly reused in production or discharged into the environment [5,6]. In addition, with the continuous improvement of industrialization, the innovation of new technologies, and the high attention to environmental protection from all walks of life, the environmental pollution caused by the wastewater generated in the process of non-ferrous metal beneficiation has received widespread attention.
Tungsten is a typical non-ferrous metal with important economic value and strategic significance. Wolframite ((Fe, Mn)WO4) and scheelite (CaWO4) are the main tungsten resources with industrial value [7]. Tungsten deposits are diverse and mainly include skarn, quartz vein, and porphyry deposits [8]. The associated gangue minerals of tungsten ores vary across different deposit types. In skarn deposits, the main gangue minerals are garnet (Ca3Al2(SiO4)3) and diopside (CaMg(SiO3)2) [9]. Quartz vein deposits commonly include quartz (SiO2), fluorite (CaF2), pyrite (FeS2), and arsenopyrite (FeAsS) as gangue minerals [10]. Feldspar (KAlSi3O8), biotite (K(Mg,Fe)3(AlSi3O10)(OH)2), and calcite (CaCO3) often coexist with tungsten minerals in porphyry deposits [11]. According to the difference in mineralogical properties between tungsten minerals and gangue minerals, the beneficiation methods of tungsten ore mainly include flotation, gravity, and magnetic separation. Most of the tungsten beneficiation plants in China focus on flotation, supplemented by gravity and magnetic separation, and it takes 4~7 t of water to float 1 t of tungsten raw ore. Tungsten beneficiation wastewater generally contains a high concentration of suspended solids (SSs), soluble ions, and organics, especially in the flotation wastewater of low-grade complex tungsten ores. If this wastewater is directly discharged, it will cause great harm to the ecological environment and threaten people’s health.
The treatment methods for tungsten beneficiation wastewater mainly include standard discharge and recycling. The standard discharge of wastewater requires that all pollutants meet the discharge standards, which often rely on deep purification technologies. Recycling refers to the removal of specific pollutants from wastewater and then returning the treated wastewater to the appropriate beneficiation process without affecting production indicators. Currently, the literature related to tungsten beneficiation wastewater primarily focuses on individual treatment technologies, lacking comprehensive summaries and analyses. Hence, the aim of this study is to analyze the sources, occurrence state, and transformation forms of pollutants in tungsten beneficiation wastewater, review the purification technologies of typical pollutants, summarize the mainstream methods of wastewater recycling, and look forward to the future development direction of tungsten beneficiation wastewater treatment.

2. Sources and Transformation of Typical Pollutants in Tungsten Beneficiation Wastewater

2.1. Sources and Characteristics of Typical Pollutants

Tungsten ores are usually associated with valuable minerals [12,13], and many tungsten beneficiation plants recover tungsten resources while comprehensively recycling these metal or non-metal resources with economic value [14]. Therefore, the flotation process of tungsten typically includes the sequential selection of sulfide minerals, tungsten minerals, and fluorite [15]. As shown in Figure 1, the tungsten beneficiation process is relatively complex. In order to improve the separation efficiency of the target minerals, a large number of collectors, frothers, and regulators are usually added during the flotation process, making the composition of the generated wastewater complex and difficult to treat. Fatty acid (200–500 g/t), hydroxamic acid (100–300 g/t), and sodium silicate (500–1000 g/t) are commonly used reagents in tungsten flotation [16]. For tungsten-associated polymetallic sulfide ore flotation, it is also necessary to add sulfide mineral flotation reagents, such as xanthate (50–200 g/t) and Aerofloat (100–300 g/t) [17,18]. The addition of various flotation reagents will aggravate the pollution of beneficiation wastewater.
The high content of SSs is the most intuitive characteristic of tungsten beneficiation wastewater. The fine mineral particles generated during the crushing and grinding before flotation are the main source of SSs. Meanwhile, the sodium silicate added in the flotation process has a dispersion effect, which makes the fine mineral particles repel each other and form a stable dispersion system, resulting in SSs being difficult to settle [19]. The concentration of SSs in tungsten beneficiation wastewater typically ranges from 1000 to 5000 mg/L [20]. The presence of a large amount of SSs in the slurry has a significant impact on the flotation efficiency. SSs typically have a large specific surface area and surface energy, which can adsorb onto the surface of minerals, impeding the interaction between minerals and flotation reagents, thus affecting the flotation effect [21]. Additionally, the presence of colloidal substances increases the viscosity of the slurry, making gangue minerals easily entrained into the concentrate by froth entrainment, and thereby reducing the concentrate grade [22]. Li et al. [23] found that the mechanical entrainment of sericite has a significant impact on the selective flotation of microcrystalline graphite.
The soluble ions in tungsten beneficiation wastewater mainly include silicate ions and metal ions, which primarily originate from the addition of inorganic flotation reagents and the dissolution of minerals. During the beneficiation process, the minerals undergo dissolution, releasing various metal ions such as Ca2+, Fe2+/Fe3+, Mn2+, and other trace elements into the wastewater. Heavy metal ions, in particular, are toxic and non-biodegradable, and the direct discharge of wastewater containing heavy metal ions poses a serious threat to the ecological environment, biodiversity, and human health [24]. If wastewater containing a large amount of silicate ions and metal ions is reused in the flotation process, it will cause significant interference in the flotation of the target minerals. Soluble ions in the slurry not only consume flotation reagents but can also adsorb onto the mineral surfaces, altering the interaction between minerals and reagents, and thereby affecting flotation performance [25,26].
Most of the organics in tungsten beneficiation wastewater come from underutilized organic reagents, which are also the primary factors leading to excessive chemical oxygen demand (COD) in wastewater [27]. The concentration of COD in the wastewater is typically between 200–500 mg/L [28]. Sodium oleate, benzohydroxamic acid, and terpineol are commonly used in tungsten flotation processes [29]. Recycling wastewater containing an appropriate amount of residual reagents can reduce the corresponding reagents’ demand. However, if the concentration of residual reagents in the reused wastewater is too high, non-target minerals can also be captured, thereby reducing the selectivity of the flotation process [30].

2.2. Occurrence State and Transformation Forms of Typical Pollutants

The occurrence state and transformation forms of pollutants have a significant impact on the effectiveness of wastewater treatment. A thorough understanding of the occurrence state and transformation forms of typical pollutants in tungsten beneficiation wastewater is of great significance in selecting appropriate treatment methods and enhancing treatment efficiency.
The SSs in tungsten beneficiation wastewater are mainly composed of fine mineral particles, colloidal particles, and organic debris, which have a certain dispersion stability. Coagulation can disrupt the stability of the dispersed system primarily by reducing the electrostatic repulsion between particles, bridging particles with flocculants to form flocs, and coalescing hydrophobic particles using oily hydrocarbons to create hydrophobic aggregates [31,32].
Soluble ions in tungsten beneficiation wastewater mostly exist in a free or colloidal state and can be converted into a stable state through coagulation, adsorption, and precipitation. The main anion in the tungsten beneficiation wastewater is silicate ion, with its concentration depending on the amount of sodium silicate used, typically ranging from 500 to 2000 mg/L [33]. The hydrolysis reactions of silicate ions are described in Equations (1)–(3) [34,35]. The distributions of silicate ions with pH are shown in Figure 2. Most of the silicon existed in the form of Si(OH)4 at pH < 9.46, while in the pH range of 9.46~12.56, the main species of silicon is SiO(OH)3−. The forms of SiO2(OH)22− is dominant at pH > 12.56.
S i O 2 ( S , a m o r p h o u s ) + 2 H 2 O ⇌ S i ( O H ) 4 ( a q )   K s p = 10 − 2.7
S i O 2 ( O H ) 2 2 − + H + ⇌ S i O ( O H ) 3 −       K 1 = 10 12.56
S i O ( O H ) 3 − + H + ⇌ S i ( O H ) 4        K 2 = 10 9.46
Metal ions in tungsten beneficiation wastewater mainly include Pb2+ and Zn2+. The concentration of Pb2+ is influenced by the amount of Pb(NO3)2 used, typically between 10 and 20 mg/L. Zn2+ mainly originates from sphalerite, with concentrations generally ranging from 5 to 10 mg/L [36]. At different pH values, the metal ions in the water will hydrolyze to form various hydroxyl complexes. As shown in Figure 3a, at pH < 7.7, lead in water is mainly in the form of Pb2+. In the pH range of 7.7~9.4, Pb(OH)+ is the dominant species. As the pH increases, most of the Pb(OH)+ is replaced by Pb(OH)2(aq) and Pb(OH)3−. Pb(OH)2(aq) is the most prominent species in the pH range of 9.4~11, while Pb(OH)3− is mainly present at pH > 11. The species distribution of zinc with different pHs is shown in Figure 3b, from which it can be seen that Zn2+ is predominantly present in the water when the pH < 8, whereas the main species is Zn(OH)2 in the pH range of 8~11. The forms of Zn(OH)3– and Zn(OH)4– are the dominant forms in the pH range of 11–12.5 and pH > 12.5, respectively.
Most of the organics in tungsten beneficiation wastewater exist in the form of ions, molecules, or micelles. Under aeration and microbial action, some organics can be naturally degraded [37,38], while the remaining non-biodegradable fraction can be treated with strong oxidants such as hydrogen peroxide, sodium hypochlorite, and sodium chlorate. These oxidants can degrade organics into harmless products such as carbon dioxide, water, or non-toxic, easily decomposable small organic molecules [39,40,41].

3. Purification Technologies of Tungsten Beneficiation Wastewater

The commonly used wastewater treatment technologies include coagulation [42], adsorption [43], precipitation [44], oxidation [45], biodegradation [46], and membrane separation [47,48]. Selecting the appropriate purification techniques based on the characteristics of pollutants, emission standards, or the quality requirements of recycled water is the key to the efficient treatment of wastewater.

3.1. Coagulation of SSs

Coagulation is one of the most widely used techniques for removing SSs from wastewater, and coagulants play a crucial role in the coagulation of wastewater. Relevant studies indicate that adding coagulants to wastewater can alter the physical state of pollutants, destabilize charged particles, promote aggregation, and facilitate precipitation [49]. According to the different components and molecular weights, coagulants can be divided into inorganic low-molecular coagulants (lime, ferric chloride (FeCl3), aluminum sulfate (Al2(SO4)3)) [50], inorganic polymer coagulants (polyferric sulfate (PFS), polymerized aluminum chloride (PAC), polyaluminum silicate (PASiC)) [51], and organic polymeric coagulants (starch, gelatin, chitosan, polyacrylamide (PAM)) [52,53].
Tungsten ore is brittle and mostly occurs in fine-grained disseminated form, leading to the generation of a large amount of fine mud during the beneficiation process. In tungsten beneficiation wastewater, the residual sodium silicate combines with the fine mud to form colloids with a negative charge, inducing SSs to remain in a stable dispersed state. In response to the characteristics of SSs in tungsten beneficiation wastewater, Zhai et al. [54] used CaCl2 as a coagulant to treat it, and the removal rate of SSs could reach more than 99% when the dosage of CaCl2 was 2 g/L. The study indicated that the mechanism of using CaCl2 to remove SSs was primarily based on Ca2+ first neutralizing the anions in the solution through electrostatic adsorption, thereby disrupting the solution’s stable state. Moreover, the differences in the molecular electrostatic potentials (MESPs) between Ca2+ and anions such as CO32− and SiO32− significantly influence the strength of electrostatic interactions, with a higher difference in MESPs corresponding to stronger interactions [55,56]. Subsequently, Ca2+ reacted with CO32− and SiO32− to form precipitates, which then naturally settle under gravity. This treatment process has been applied at Hengyang Yuanjing Tungsten Co., Ltd. (Hengyang, Hunan, China), and has yielded positive results. Ahmad et al. [57] evaluated the performance of chitosan, alum, and PAC as coagulants for the removal of SSs in wastewater. Compared with alum and PAC, chitosan has the advantages of faster settling velocity and lower dosage. As a natural, safe, non-toxic, and biodegradable material, chitosan has no side effects during its application. The chemical formula of chitosan is (C6H11NO4)n, where n represents the degree of polymerization, and its molecular structure is derived from chitin through deacetylation, resulting in a high density of active amino groups. In acidic conditions, these amino groups (–NH2) are protonated to form –NH3+ groups, making chitosan a positively charged polyelectrolyte. The protonated amine groups interact with negatively charged particles and colloids in wastewater, facilitating charge neutralization, electrostatic attraction, and bridging flocculation. Furthermore, the high charge density and multiple binding sites of chitosan promote the adsorption of suspended solids and other contaminants through hydrogen bonding and van der Waals interactions. These characteristics enable chitosan to achieve high removal efficiency at a lower dosage compared to conventional coagulants like alum and PAC. However, the limited raw material supply and relatively complex preparation process hinder the large-scale production of chitosan [58].
From the perspective of performance and cost, environmentally friendly coagulants with simple production processes, economic feasibility, and high removal efficiency are still a research hotspot in the treatment of SSs in wastewater [59].

3.2. Adsorption and Chemical Precipitation of Soluble Ions

Tungsten beneficiation wastewater contains a wide variety of soluble ions, especially silicate ions and metal ions, which need to be removed by appropriate methods. There are several removal methods available, including adsorption [60], neutralization [61], chemical precipitation [62], electrocoagulation [63], membrane [64], and crystallization [65]. Among them, the commonly used and cost-effective ones are adsorption and chemical precipitation.
In recent years, industrial waste residues such as steel slag, fly ash, activated carbon, and red mud have been widely studied as adsorbents for treating wastewater. Wang et al. [66] found that steel slag could synergistically remove collectors, Cu(II) and Pb(II), from flotation wastewater, and the metal ions had a promoting effect on the adsorption. When the dosage of steel slag was 12.0 g/L, the concentrations of Cu(II) and Pb(II) decreased from 4.0 mg/L to below 10.0 μg/L. The primary adsorption mechanism involves both chemical precipitation and complexation reactions. Steel slag, primarily composed of CaO and MgO (total content of 42.26%), can release Ca2+ and OH− ions into the solution, significantly increasing the pH and promoting the formation of metal hydroxides like Cu(OH)2 and Pb(OH)2 on its surface. Chen et al. [67] used modified fly ash to remove heavy metal ions from wastewater. The study indicated that the removal efficiency of Pb2+, Cd2+, and AsO33− by modified fly ash in actual wastewater reached 85.31%, 95.98%, and 47.57%, respectively. Electrostatic interactions and chelation reactions are pivotal mechanisms in this adsorption process. The negatively charged sites on the surface of modified fly ash can attract positively charged heavy metal ions, facilitating adsorption through electrostatic attractions. This interaction is particularly significant for cations like Pb2+ and Cd2+. Additionally, chelation reactions occur when functional groups present in the modified fly ash, such as carboxyl and amino groups, forming coordinate bonds with heavy metal ions. This process enhances the stability of the adsorbed species and increases the overall removal efficiency. The application of industrial waste residues as adsorbents in wastewater treatment not only provides an economically feasible way to remove pollutants from wastewater, but also improves the utilization of waste resources.
Chemical precipitation is another effective method for removing soluble ions from wastewater. The fundamental principle involves adding a precipitant to the wastewater, which reacts with the soluble ions to form insoluble compounds, thereby purifying the wastewater [68,69]. For wastewater containing a large number of silicate ions, Ca2+ is often employed as a precipitant, reacting with silicate ions to form insoluble CaSiO3, as shown in the reaction (4) [70].
C a 2 + + H 3 S i O 4 − + O H − → C a S i O 3 + 2 H 2 O
Chemical precipitation for removing metal ions from wastewater can be categorized into hydroxide precipitation, sulfide precipitation, and chelating agent precipitation depending on the precipitates. Hydroxide precipitation is a method that employs pH adjustment to convert metal ions into hydroxide or salts, which then can be removed by physical means [71]. Commonly used hydroxide precipitants in industrial treatment include CaCO3, Ca(OH)2, and CaO. It is worth noting that using the hydroxide precipitation to remove metal ions does not imply that higher pH adjustments are always better. Some metal hydroxides are amphoteric compounds and can dissolve under highly alkaline conditions. If the pH value is too low, metal ions may not completely precipitate, while excessively high pH values can lead to redissolution, increasing the concentration of metal ions in the wastewater [72].
Compared to the hydroxide precipitation, the products of sulfide precipitation have lower solubility, leading to higher removal efficiency of metal ions. Additionally, this method allows for selective precipitation of metal ions over a wide pH range [73]. Alvarez et al. [74] used biologically produced H2S to treat heavy metal-contaminated wastewater, achieving a copper removal rate of approximately 100%, a zinc removal rate of over 94%, and a lead removal rate of over 92%. However, sulfide precipitants will generate H2S fumes when exposed to acids, causing secondary pollution [75].
In view of the limitations of hydroxide and sulfide precipitation, chelating agent precipitation has gained increasing attention in recent years [76,77]. Fu et al. [78] employed a chelating reagent with two chelating groups, N,N-bis-(dithiocarboxy) piperazine (BDP) to remove Cu(II) from wastewater. The results indicated that BDP could effectively form a highly insoluble linear chelating polymer with Cu(II) through bridging, thus reducing the concentration of Cu(II) from 50.00 mg/L to below 0.5 mg/L. Ayalew et al. [79] compared the removal performance of four types of oligomeric ethylenediamine dithiocarbamate (OEDTC) on Cu, Ni, and Zn in acidic wastewater. The results showed that the OEDTC with longer carbon and amine chains exhibited higher removal ability for metal ions. The removal preference of the four OEDTC for Cu was significantly higher than Ni and Zn from simulated wastewater. The primary functional groups involved in metal ion interactions are the dithiocarbamate (-CSSNa) and amine (-NH) groups, which contain S and N atoms with lone pair electrons that readily coordinate with metal ions, forming stable metal complexes. The molecular electrostatic potential (MESP) analysis indicated that the high electron density around these functional groups promotes the formation of coordinate-covalent bonds, particularly with Cu, which acts as a soft acid.

3.3. Oxidation and Biological Treatment of Organics

3.3.1. Advanced Oxidation Treatment of Organics

With the continuous depletion of high-quality tungsten resources, flotation of low-grade complex tungsten ores has become mainstream. Its process is becoming increasingly intricate, and the types and quantities of flotation reagents used are also increasing. The residual flotation reagents, especially organic ones, are some of the most difficult causes of environmental pollution caused by tungsten beneficiation wastewater. For the treatment of organic wastewater, oxidation methods exhibit excellent performance, especially advanced oxidation processes (AOPs) [80,81,82].
Fenton oxidation is one of the most mature and popularly used AOPs for treating organic wastewater. It utilizes the reaction of Fe2+ and H2O2 to generate highly reactive hydroxyl radicals (·OH) which can non-selectively degrade the majority of organic pollutants [83]. The Fenton process involves several chain chemical reactions, the cores of which are widely recognized, as shown in Equations (5) and (6), and the reaction mechanism is shown in Figure 4.
F e 2 + + H 2 O 2 + H + → F e 3 + + H 2 O + ⋅ O H
F e 3 + + H 2 O 2 → F e 2 + + H 2 O ⋅ + H +
Meng et al. [85] compared the performance of coagulation–flocculation, adsorption, and Fenton processes for the removal of COD from wastewater and found that the Fenton process was the best in terms of treatment efficiency and economic benefits. Currently, the Fenton process for treating COD in tungsten beneficiation wastewater has been successfully applied at Shizhuyuan Mine, significantly improving wastewater treatment efficiency. Guo et al. [86] employed the Fenton process to treat wastewater with high toxicity and high concentrations of COD, as well as a low BOD5/COD ratio (<0.1). Under the optimal experimental conditions, COD removal was 85.29%, TOC removal was 75.23%, color removal was 99.99%, and the biodegradability of the wastewater was also improved. Although Fenton oxidation has shown good performance in the treatment of organic wastewater, it is still hampered by some drawbacks in practical applications. In the Fenton process, the regeneration of Fe2+ (Equation (6)) is very slow, which greatly limits the cycling of Fe3+ and Fe2+ and leads to the accumulation of Fe3+ in solution [87]. When the pH of the solution gradually increases, Fe3+ forms iron sludge precipitation, which is difficult to separate and can cause secondary pollution to the environment [88]. Therefore, how to reduce the generation of sludge will be a key focus of future research on the Fenton process.
Persulfate oxidation is a newly developed AOP in recent years, which is used to convert organic pollutants into CO2, H2O, inorganic salt, and other small molecular substances by generating reactive oxygen species (ROS) through a variety of activation methods [89]. External energy sources like ultraviolet [90], thermal activation [91], electrochemistry [92], ultrasound [93], and microwave [94] can effectively activate persulfate. The non-homogeneous activation method by introducing an external catalyst can activate persulfate without external energy, which has the advantages of convenient operation, low cost, mild reaction conditions, and high generation rate. In the practical application of persulfate oxidation for treating beneficiation wastewater, some transition metal ions in the wastewater can play a catalytic role and promote the degradation of pollutants [95,96]. Compared with the traditional AOPs, persulfate oxidation has the advantages of strong oxidizing capacity, stability, and a wide pH adaptation range, which has a broad application prospect in the field of water treatment.

3.3.2. Biological Treatment of Organics

Biological treatment is a method that utilizes the metabolic activities of microorganisms to remove pollutants from the environment [97]. Compared with chemical and physical treatment methods, biological treatment is more environmentally friendly and cost-effective [98]. In recent years, it has gained increasing attention in the field of environmental remediation.
The activated sludge process is the most widely used biological treatment method, which can effectively remove organics and nutrients from wastewater [99]. The process can be divided into three parts as shown in Figure 5. Firstly, wastewater is subjected to coagulation and sedimentation in the primary sedimentation tank to reduce solid pollutants. Subsequently, the effluent from the first step enters the aeration tank for biochemical reactions, which forms the core of the entire treatment process. Activated sludge possesses a substantial specific surface area and is populated with a multitude of microorganisms on its surface. At the initial stage of contact between activated sludge and wastewater, part of the organic pollutants can be removed by coagulation and adsorption [100]. Organics adsorbed on the surface of the microorganisms will be taken into the microorganisms successively after several hours of aeration. Under the catalytic action of various intracellular enzymes, the microorganisms carry out metabolic reactions to remove organics from wastewater [101]. The effluent from the aeration tank enters the secondary sedimentation tank for solid–liquid separation. The liquid portion is discharged after meeting the required standards, while a portion of the solids is returned as recycled sludge to the aeration tank to maintain a certain range of microbial concentration, and the excess is discharged from the system.
Biological purification technology is a promising wastewater treatment method, which has the characteristics of environmental friendliness, low energy consumption, and good selectivity. However, this method has certain limitations. One of the primary challenges is maintaining an optimal microbial population. The efficacy of biological treatment is highly dependent on the microbial communities present, and fluctuations in microbial concentration or changes in environmental conditions such as pH, temperature, and nutrient availability can significantly affect treatment performance. Additionally, sludge disposal is another issue that needs to be carefully managed. The accumulation of excess sludge is inevitable in biological treatment processes, and improper disposal can lead to secondary pollution. Effective strategies such as sludge minimization techniques or sludge digestion and stabilization are required to address this challenge.

4. Recycling Methods of Tungsten Beneficiation Wastewater

With the increasing prominence of water scarcity and water pollution, the recycling of wastewater is receiving more attention [102,103]. Recycling of beneficiation wastewater can not only effectively reduce the amount of fresh feed water and wastewater discharge, but also make full use of the residual useful reagents to reduce the demand for additives. This way of treating wastewater aligns well with the concept of sustainable development in green mining [104]. The recycling of beneficiation wastewater typically involves centralized recycling and internal recycling at certain stages [105].

4.1. Centralized Recycling

The centralized recycling of beneficiation wastewater is a common practice widely applied in mineral processing plants. This approach offers significant advantages in terms of safety, reliability, stability, and economic feasibility [106]. At present, many mineral processing plants in China concentrate the beneficiation wastewater into tailings dams, utilizing the comprehensive effects of natural sedimentation, photodegradation, oxidation, adsorption, and biodegradation of tailings dams to reduce pollutants in overflow water. If the pollutants in the overflow water have a less negative impact on the flotation effect, they can be directly reused. However, the composition of tungsten beneficiation wastewater is typically complex, with a high content of SSs, metal ions, and organics. Relying solely on the natural purification of the tailings dam is insufficient to meet the standards for direct reuse in the flotation process. Therefore, the treatment of centralized tungsten beneficiation wastewater often requires a combination of external purification technologies.
Wu et al. [107] employed natural marmatite (NM) as a photocatalyst for the centralized treatment of beneficiation wastewater. The results showed that when the dosage of NM was 4 g/L and the initial pH value was 4, the removal rate of TOC in the wastewater was 74.25% after 120 min of visible light irradiation. The treated wastewater can be reused in the flotation process without affecting the product index. Meanwhile, the freshwater consumption of the flotation system decreased 50.98 m3/h to 13.79 m3/h. Kang et al. [108] proposed a centralized treatment of beneficiation wastewater from a molybdenite and scheelite processing plant in Luanchuan by using hydrometallurgical waste acid as a precipitating agent. The treatment process is shown in Figure 6, where it can be seen that after adding the waste acid, the cleaning tailings of scheelite are pumped into the buffer pool and combined with the other tailings streams. The addition of waste acid leads to the reduction of pH and the dissolution of calcite in the tailings. The generated calcium ions then react with silicate to form insoluble calcium silicate precipitates. These precipitates not only facilitate the removal of silicate but also promote the adsorption and sedimentation of suspended solids. Then these tailings are pumped into the tailings dam after adding 10~15 t/d of lime to the buffer pool. Tailings dam can serve as a natural carrier, in which fine-grained scheelite flotation tailings are combined with coarse fractions. After a period of sedimentation, the overflow water is recycled for the flotation process. This process has been applied in the beneficiation plant that provides wastewater samples, achieving a reduction in industrial waste discharge while promoting the sustainable development of resources and the environment.

4.2. Internal Recycling at Certain Stages

In recent years, with the rapid advancement of industrialization and urbanization, the demand for mineral resources has been continuously increasing, promoting the production of beneficiation plants, while the generation of beneficiation wastewater has also been continuously increasing. However, the carrying capacity of tailings dams is limited, making it impossible to treat a large amount of beneficiation wastewater. If the tailings dam is far from the plant, the cost of backwater transportation is high [109]. Moreover, a single treatment method usually fails to achieve the removal of multiple contaminants, especially in the flotation of polymetallic ores, where the quality of the wastewater produced by each process varies greatly. Therefore, many researchers focus on recycling the wastewater generated in different stages of the flotation process into corresponding circuits after treatment [110].
Lin et al. [104] carried out internal recycling of process water according to the process shown in Figure 7. The specific recycling process involved diverting the tailings water back to the conditioning and rougher flotation stages, where it was mixed with the raw ore pulp. The recycled water was used in lieu of a certain proportion of freshwater to prepare the slurry for flotation. During the industrial tests, the flotation reagent regimes were dynamically adjusted to counteract potential adverse effects due to residual reagents in the reclaimed water. The results showed that there was no significant difference in the quality of the concentrate obtained with and without processed water recycling. In addition, the amount of reagents required for the flotation circuit was reduced by more than 10% after using recycled water, and among them, the dosage of sodium sulfide was significantly reduced by 18.56%. Most importantly, the internal recycling of process water reduced freshwater consumption by 34.62%. Shen et al. [111] concluded in the study of polymetallic beneficiation wastewater treatment that copper-molybdenum mixed flotation and copper-molybdenum separation wastewater can be directly reused in the corresponding system after natural clarification, while tungsten system wastewater needs to undergo flocculation precipitation and deep oxidation before being returned to the tungsten or copper-molybdenum systems for reuse. According to the above treatment method, the flotation indexes of the polymetallic ore were still stable after 10 consecutive closed-circuit flotation.
The purification technology and recycling method of beneficiation wastewater should be selected according to the nature of wastewater and specific conditions, taking into account the economy and environmental protection. For tungsten beneficiation such a complex process, the quality of wastewater from different processes varies greatly, the process wastewater after appropriate treatment is reused at a certain stage of the way and has a stronger adaptability.

5. Conclusions

The pollutants in tungsten beneficiation wastewater are complex and diverse, making them difficult to treat. This study analyzed the sources, characteristics, and forms of typical pollutants in tungsten beneficiation wastewater, reviewed the corresponding purification technologies, and summarized methods for wastewater recycling. The main conclusions are as follows:
(1) The typical pollutants in tungsten beneficiation wastewater mainly include SSs, silicate ions, metal ions, and organics, which can significantly affect the standard discharge and recycling of wastewater. In response to the characteristics of these pollutants, commonly used purification technologies include coagulation, adsorption, chemical precipitation, oxidation, and biological methods. In the practical treatment of tungsten beneficiation wastewater, it is often necessary to combine multiple techniques to achieve the treatment goals.
(2) The recycling of treated beneficiation wastewater not only effectively reduces the freshwater input and wastewater discharge, but also makes full use of the residual reagents to reduce the demand for additives. However, the quality of wastewater varies greatly between different stages, and it is suitable to recycle wastewater after appropriate treatment at specific stages.
(3) With the continuous depletion of high-quality tungsten resources, the beneficiation process for low-grade complex tungsten ores has become increasingly intricate, resulting in greater challenges in wastewater treatment. In light of these circumstances, developing advanced wastewater purification technologies and exploring efficient methods for wastewater recycling to promote zero wastewater discharge is an important direction in future research of tungsten beneficiation wastewater treatment.

Author Contributions

W.Z.: Investigation, Visualization, Data Curation, Writing—Original Draft. J.K.: Conceptualization, Methodology, Writing—Review & Editing. D.Z.: Investigation, Validation. W.S.: Supervision, Funding acquisition. Z.G.: Supervision, Project administration. H.H.: Resources, Validation. R.L.: Conceptualization, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Sustainable Development Demonstration zone project (2022sfq34), Natural Science Foundation of Hunan Province (2022JJ40598), Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2023-17), Natural Science Foundation of Changsha (kq2202095), and Open Foundation of State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control (HB202107, HB202207).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

Author Jianhua Kang was employed by the company BGRIMM Technology Group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Common flow chart of tungsten beneficiation.
Figure 1. Common flow chart of tungsten beneficiation.
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Figure 2. Species distribution of silicate ions in solutions as a function of pH.
Figure 2. Species distribution of silicate ions in solutions as a function of pH.
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Figure 3. Species distribution of lead (CPb = 1 × 10−4 mol/L) (a) and zinc (CZn = 1 × 10−4 mol/L) (b) in solution as a function of pH.
Figure 3. Species distribution of lead (CPb = 1 × 10−4 mol/L) (a) and zinc (CZn = 1 × 10−4 mol/L) (b) in solution as a function of pH.
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Figure 4. Reaction mechanism for the Fenton process [84].
Figure 4. Reaction mechanism for the Fenton process [84].
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Figure 5. Schematic diagram of activated sludge process for wastewater treatment.
Figure 5. Schematic diagram of activated sludge process for wastewater treatment.
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Figure 6. Schematic diagram of centralized treatment and recycling of wastewater from molybdenite and scheelite enrichment.
Figure 6. Schematic diagram of centralized treatment and recycling of wastewater from molybdenite and scheelite enrichment.
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Figure 7. Principled process of wastewater recycling [104].
Figure 7. Principled process of wastewater recycling [104].
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MDPI and ACS Style

Zhu, W.; Kang, J.; Zhang, D.; Sun, W.; Gao, Z.; Han, H.; Liu, R. Treatment and Recycling of Tungsten Beneficiation Wastewater: A Review. Separations 2024, 11, 298. https://doi.org/10.3390/separations11100298

AMA Style

Zhu W, Kang J, Zhang D, Sun W, Gao Z, Han H, Liu R. Treatment and Recycling of Tungsten Beneficiation Wastewater: A Review. Separations. 2024; 11(10):298. https://doi.org/10.3390/separations11100298

Chicago/Turabian Style

Zhu, Wenxia, Jianhua Kang, Danxian Zhang, Wei Sun, Zhiyong Gao, Haisheng Han, and Runqing Liu. 2024. "Treatment and Recycling of Tungsten Beneficiation Wastewater: A Review" Separations 11, no. 10: 298. https://doi.org/10.3390/separations11100298

APA Style

Zhu, W., Kang, J., Zhang, D., Sun, W., Gao, Z., Han, H., & Liu, R. (2024). Treatment and Recycling of Tungsten Beneficiation Wastewater: A Review. Separations, 11(10), 298. https://doi.org/10.3390/separations11100298

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