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Review

A Mini-Review on Safe Treatment and Valorization of Salt Waste in Chemical Production Processes in China

by
Yang Lv
1,
Yi Wang
1,
Dapeng Zhang
1,
Chaoyue Wu
1,
Jun Zhang
1,
Zehua Zhao
1,*,
Mohammad Nabi
2,3,
Xuan Luo
2,3 and
Keke Xiao
2,3,4
1
Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment of the People’s Republic of China, Nanjing 201142, China
2
Environmental Science and Engineering Program, Guangdong Technion-Israel Institute of Technology, 241 Daxue Road, Shantou 515063, China
3
Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
4
Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion, Guangdong Technion-Israel Institute of Technology, Shantou 515063, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(11), 1620; https://doi.org/10.3390/w16111620
Submission received: 14 May 2024 / Revised: 29 May 2024 / Accepted: 30 May 2024 / Published: 5 June 2024
(This article belongs to the Special Issue Dissolved Organic Matter in Sludge)
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Abstract

:
In chemical production processes in China, a huge volume of salt waste is produced, contributing to environmental pollution due to the lack of proper treatment procedures to manage this waste effectively. With the current lack of proper treatment methods for salt waste disposal, landfill emerges as the predominant disposal approach adopted in China, exacerbating environmental concerns associated with the accumulation of such waste. In addition, this method fails to reuse and recycle salt waste. This review paper presents pertinent research on the safe disposal and utilization of salt waste in China. Well-known valorization methods, such as oxidation, thermal treatment, washing separation, precipitation, and evaporation crystallization, are comprehensively reviewed. The current technologies for recovering resources from salt waste and recommendations for its treatment and valorization are analyzed. This research work offers a reference for future resource utilization of industrial salt waste.

1. Introduction

Chemical industrial salt waste mainly originates from the production of refined petroleum products, coal-to-synthetic gas production, coal-to-liquid fuel production, the generation of chemical raw materials and chemical products, and the production of raw materials for chemical drugs [1]. It is produced from salt-containing waste liquids or solids by different chemical raw material reactions [2]. Additionally, salt waste is generated from the “zero discharge” treatment of wastewater and the neutralization of waste acid and alkali solutions [3]. The composition of salt waste varies significantly among industries, and it contains high concentrations of soluble salts, toxic and hard-to-degrade organic substances, and heavy metals [4]. Some salt wastes are contaminated with toxic and hazardous substances, posing environmental risks [4].
Chemical industrial salt wastes are known for the extensiveness of their production, their large volumes, diverse types, and complex components, despite being disposed of at a relatively low rate [5]. Currently, the main disposal methods for salt wastes involve refining them for use as industrial raw materials or additives. Some salt wastes with high impurity contents are buried, some are discharged into wastewater or the sea, and there are also instances of illegal dumping [6]. Due to low corporate enthusiasm for utilizing salt wastes, large amounts continue to be accumulated, posing significant environmental risks [7].
High-salt wastewater originating from chemical industrial processes requires suitable treatment to enable reuse, thereby reducing wastewater discharge [5]. The key technologies involved in this process include salt concentration and crystallization. Although thermal evaporation is a common method for concentration [8], its high investment and operational costs render it unsuitable for treating large-volume wastewater, presenting a major hurdle to its widespread adoption [9]. Furthermore, evaporative crystallization often yields soluble mixed salts that cannot be recycled, potentially leading to secondary pollution when they are disposed of in landfills. This environmental pressure, coupled with significant economic burdens on industrial enterprises, underscores the necessity of pretreating wastewater prior to crystallization to enhance salt content and reduce energy consumption and costs [10]. Ensuring the recycling of permeate water and producing solid salt products of sufficient purity and reuse value are crucial for both environmental sustainability and the economic viability of enterprises.
Research on salt waste management has become increasingly vital, with researchers making significant efforts to develop efficient treatment technologies. An obvious trend is evident from the data illustrated in Figure 1a showing a significant rise in publications from a mere 13 in 2005 to 68 in 2023. This rise stresses the growing recognition of salt waste as a critical research domain globally. Notably, China emerges as a prominent contributor to this field, as depicted in Figure 1b (https://www.engineeringvillage.com, data updated on 25 April 2024). This trend highlights the country’s commitment to addressing salt waste challenges and advancing sustainable solutions on a significant scale.
This study comprehensively reviews research related to the harmless treatment and valorization of chemical industrial salt wastes in China. It examines the current technologies for resource recovery from salt wastes and proposes suggestions for the treatment and valorization of salt wastes. The aim is to provide a reference for the future valorization of industrial salt wastes.

2. Technologies for the Treatment and Valorization of Chemical Industrial Salt Waste

In China, there are primarily two types of technologies for the treatment and valorization of salt waste: harmless treatment and valorization. Harmless treatment uses technologies like incineration, landfill, and sea disposal [7]. With these technologies, the toxic and hazardous components in chemical industrial salt waste can be reduced or eliminated. Valorization involves using integrated processes to treat salt waste, producing by-product salts or using processed salt as an industrial raw material and additive for comprehensive valorization [7].

2.1. Harmless Treatment

Incineration is a feasible technology used for disposing of industrial salt waste. In real applications, it has been found that at lower incineration temperatures, it is not effective in removing organic substances from salt waste. Higher temperatures are beneficial for decomposing organic substances in salt waste. When the temperature exceeds the melting point of the salt, the molten salt waste can cause equipment corrosion, thereby increasing operational costs [11].
Currently, landfill is the primary method of disposal for industrial mixed salt waste in China. According to the “Hazardous Waste Landfill Pollution Control Standard” (GB 18598-2019, https://www.mee.gov.cn) from 1 June 2020, hazardous waste with a water-soluble salt content greater than 10% must not be disposed of in flexible landfills and can only be buried in rigid landfills. Most of the industrial salt wastes, due to their high solubility, require disposal in rigid landfills, which does not fundamentally eliminate the pollution characteristics and environmental risks of salt waste, and the treatment cost is high. Currently, most landfills in China are flexible, and the disposal of salt waste is restricted, leading to limited options for many enterprises.
In other countries, the main disposal method for final salt waste is sea discharge. There had been less research on the valorization of salt waste until the United States officially launched a salt waste treatment facility in 2020 [12]. The advantages and limitations of different salt waste treatment methods are presented in Figure 2. Regarding incineration technology, it reduces high volumes of salt waste, and destroys hazardous components. But it emits pollutants like dioxins and heavy metals, requires high energy inputs, and it is costly to implement and maintain. Landfill technology is a relatively inexpensive method, provides controlled waste containment, and facilitates land post closure. However, it requires significant land areas, causes groundwater contamination, and contributes to harmful gas emissions and environmental degradation. The technology of sea disposal is a potentially cost-effective solution, as it offers a vast disposal space and the dilution effect of the sea mitigates localized impacts.
Developed countries generally use membrane technology to directly treat high-salinity wastewater to achieve zero-liquid-discharge requirements [13]. The disposal of salt waste into the sea is only applicable in coastal regions of China, where saline wastewater is discharged directly into nearby marine environments, and salt waste is transported to international waters for deep-sea disposal. However, the brine from the harmless treatment of chemical industrial salt waste discharged into the sea should meet the requirements of national policies and standards related to marine ecological environment and wastewater discharge and undergo risk assessment. Compared with membrane technology and incineration, landfill and sea disposal are relatively cheaper. For membrane technology, expensive membranes and relative modules are needed. For incineration, extra heat and transportation are needed. Although landfill and sea disposal are cheaper, these two techniques fail to achieve resource valorization. Landfill causes pollution of soil and ground water, while sea disposal causes pollution of the sea environment.

2.2. Valorization

According to the “Environmental Management Guide for Hazardous Waste: Chemical Industrial Salt Waste”, chemical industrial salt waste that has undergone harmless treatment can be refined and separated to produce industrial by-product salts, such as industrial NaCl, Na2SO4, PO43−, KCl, CaCl2, NH4Cl, and (NH4)2SO4. This not only facilitates the recycling and valorization of raw materials like sodium and potassium, enhancing economic benefits, but also addresses issues such as land occupation due to stacking and landfilling, thereby avoiding environmental pollution. Valorization also facilitates resource recovery from salt waste, as shown in Table 1. Common valorization technologies include thermal treatment, washing separation, double decomposition, and oxidation methods. Currently, thermal treatment technology achieves the highest industrial application degree. As shown in Table 1, currently, expanding chlor-alkali projects using industrial salt waste as a raw material is not restricted by production capacity policies. However, as chlor-alkali projects are high-energy-consuming endeavors, while they facilitate the environmentally friendly disposal of industrial salt waste, they are subject to constraints such as energy assessments and carbon emission audits. This imposes significant limitations and restrictions on the expansion of chlor-alkali projects using industrial salt waste.

2.2.1. Thermal Treatment

Thermal treatment has been widely used to process chemical industrial salt wastes containing a high amount of organic impurities. The primary methods employed are pyrolysis and high-temperature melting, which vaporize and decompose organic impurities into volatile gases under elevated temperatures. The removal rate of organic substances can reach 82.93–99.48% [24]. The treated salt waste can be crystallized to produce industrial by-product salts that meet industrial standards [25,26], as summarized in Table 2. Among the by-product salts, potassium salts can be used as fertilizers, and sodium salts can be used as additives in the construction industry [27] or as solvents in the metallurgy industry [28], turning waste into wealth while creating economic benefits. However, due to management regulations, this technique fails to be widely promoted on a large scale.
The optimal conditions for thermal treatment of salt waste depend on the nature of the organic impurities present [29]. The efficiency of organic removal during thermal treatment of salt waste determines the difficulty of subsequent processing steps and the organic treatment load, which in turn affect the investment and operational costs of salt waste valorization [30]. Pyrolysis technology generally can only remove some of the organic materials from salt waste. However, for other types of salt wastes, supplementary combined units are still required for further treatment. The use of high-temperature melting technology for treating salt waste is limited because it lacks corresponding standards for valorization, and the treated salt waste can only be disposed of as general solid waste through landfilling.
Additionally, furnace corrosion and flue gas treatment are two major issues with high-temperature treatment of salt waste. Salt waste mainly consists of NaCl and Na2SO4. Many researchers have shown that chlor-alkali metal compounds, such as NaCl and KCl, are the primary causes of furnace corrosion, with Cl being the most critical factor in corrosion. To prevent salt clumping while improving thermal efficiency, there is also the valorization of new thermal fluidization technology to remove organic substances from the surface of salt waste. It has been found that when the fluidization temperature is 400 °C and the fluidization time is 10 min, the removal rate of organic substances from salt waste can reach 99.5%, resulting in NaCl crystalline salt with a purity of 99.1% [31].

2.2.2. Oxidation Method

The oxidation method is a treatment process that uses oxidants to oxidize organic impurities in salt waste, thereby purifying inorganic salts. Lin et al. [32] used the molten salt oxidation method to treat salt waste arising from triazinone production. Their study showed that high-salt organic waste can be efficiently oxidized in a molten salt bath. In addition, when the temperature was raised from 600 °C to 750 °C, the oxidation efficiency increased from 91.1% to 98.3%.
A certain dye manufacturer involved in diazotization reactions in the production of reactive dyes found that the salt waste residue contained organic substances and chromophoric groups. The manufacturer used multiple oxidation units to break down the chromophoric groups and then applied a combined process to remove the organic substances, resulting in brine that meets the reuse requirements for ion-exchange membrane caustic soda brine [11].
Li et al. [33] adopted a combination of air stripping and diatomaceous earth filtration for pretreatment, along with the Fenton oxidation method, to treat high-salt wastewater from epoxy resin production. With wastewater treatment agent costs controlled at 150 CNY/ton, they ensured an effluent total organic carbon removal rate of about 98%, making the treated water suitable for use as a raw material in the production of chlorine gas and caustic soda.

2.2.3. Metathesis

Wang [34] introduced H2SO4 to react with salt wastes from the pesticide and pharmaceutical industries. The reaction of H2SO4 with NaCl under low-temperature heating produces NaHSO4 and HCl. The NaHSO4 solution, after pH adjustment and evaporative crystallization, becomes Na2SO4, which can be used to manufacture industrial anhydrous Na2SO4 (also known as anhydrous Glauber’s salt). Meanwhile, the HCl can be absorbed by water to form industrial by-product hydrochloric acid.
Zheng et al. [35] utilized NaCl salt waste to react with concentrated phosphoric acid, producing sodium dihydrogen phosphate. This process also yields industrial concentrated hydrochloric acid as a by-product. Not only does this approach resolve the pollution problem associated with salt-waste NaCl, it also converts concentrated phosphoric acid into the more valuable sodium dihydrogen phosphate, while simultaneously producing another type of industrial concentrated acid.
Li [36] used industrial NaCl salt waste as a raw material, utilizing it to react with ammonium bicarbonate through metathesis to produce Na2CO3 (with a purity exceeding 98%) and generating NH4Cl as a by-product. This process achieves the valorization of NaCl salt waste.
Han [37] utilized coal chemical high-salinity wastewater containing large amounts of NaCl and Na2SO4 to produce soda ash (sodium bicarbonate), achieving the successful production of ultralight pure soda ash with a purity of 96–99%. The co-produced mixed ammonium salts can be used as fertilizer.
Chen et al. [38] used the high-salinity wastewater produced during the methanol and light hydrocarbon production process at a coal chemical plant as a raw material. They concentrated the wastewater until Na2SO4 precipitated out at saturation. Then, they applied metathesis to produce K2SO4 as a by-product, meeting the standards for superior-grade K2SO4 for agricultural use (GB/T 20406 2017, https://www.mee.gov.cn).

2.2.4. Washing Separation Method

Starting from the resource recovery of phosphorus-containing salt wastes generated as by-products in the pharmaceutical industry, a method was developed involving the utilization of a wash eluent (methanol–ethanol) mixed with activated carbon. This method aimed to remove organic impurities from the phosphorus-containing salt waste produced during the production of pharmaceutical intermediates at a factory in Jiangsu. The technique of recrystallization and separation was used to separate the mixed salts, recovering usable phosphates and purifying the salt waste. The obtained sodium phosphate had a purity of over 98%, and all parameters met the requirements of the “Industrial Na3PO4” standard (HG/T 2517-2009, https://www.mee.gov.cn) [39].
Hydrazine hydrate is an important raw material in chemical production, primarily produced industrially by the urea method. This method generates a large amount of alkaline salt waste containing NaCl and Na2CO3. Yao [40] employed pure water to wash the hydrazine hydrate salt waste, maintaining a solid-to-liquid ratio of 3:7 and a reaction time of 1 h. After filtration, the purity of NaCl in the salt residue reached 85%. Na2CO3 in the filtrate was precipitated out by freeze crystallization. Ammonium bicarbonate was then added to the salt residue to produce pure soda ash, achieving valorization.
Ning and Gong [41] chose xylene as the solvent for washing and recovering salt waste residue from the etherification of furan phenol. Using a double-cone rotary vacuum dryer under conditions of steam pressure between 0.20 and 0.25 MPa and a vacuum of −0.090 MPa, the washed and filtered salt residue was dried, successfully recovering most of the monoethers and organic solvents from the salt waste residue.

2.2.5. Precipitation Method

The precipitation method involves dissolving salt waste in water and adding specific chemical reagents to eliminate particular characteristic pollutants from the salt waste. This method provides a stable effect in treating salt waste but can also produce secondary pollution [42]. The precipitation method is only suitable for processing salt wastes with a single and stable composition. In addition, the precipitated waste residue also requires secondary treatment.
During the chloride process for producing titanium dioxide, a large amount of titanium white salt waste is generated, primarily composed of CaCl2, MgCl2, MnCl2, and NaCl. Yang et al. [43] used titanium white salt waste as a raw material and employed the precipitation and calcination method, using sodium bicarbonate as a carbonizing agent to prepare manganese carbonate. Manganese carbonate can be used to synthesize manganese oxide and other manganese salts.
A pharmaceutical intermediate salt waste rich in phosphates was processed using the coprecipitation method by converting it into hydroxyapatite. While recycling the phosphates in the salt waste, the remaining NaCl solution was also recovered through evaporation crystallization and high-temperature thermal treatment. This not only alleviates the disposal pressure of phosphate-containing salt waste but also reduces the production cost of hydroxyapatite, aligning with the environmental principles of the “green economy” [44].

2.2.6. Evaporation Crystallization Method

The key to the valorization of salt waste lies in “detoxification”, typically achieved by removing organic and inorganic impurities from salt waste followed by evaporation crystallization to produce product salt. The typical process flow for the valorization of salt waste includes “pretreatment → organic removal → dissolution filtration → inorganic impurity removal → recrystallization separation → product salt drying → final product sale” [45].
Hao et al. [46] used activated carbon for the decolorizing pretreatment of high-salinity wastewater, reducing its chemical oxygen demand (COD) and removing insoluble impurities, calcium and magnesium ions, silicates, and other soluble substances. Subsequently, electrodialysis and mechanical vapor recompression were used to concentrate the high-salinity wastewater until the Na2SO4 and NaCl content approached saturation. Finally, Na2SO4 was obtained through cooling crystallization, and after washing and drying, anhydrous Na2SO4 products were produced, meeting national standard requirements.
Coal chemical high-salinity water is treated with a dual-membrane method to reduce impurities in the crystalline salt. This is followed by evaporation crystallization and salt separation processes to separate the industrial wastewater into anhydrous Na2SO4 and NaCl products for the valorization of crystalline salt. The anhydrous Na2SO4 in industrial wastewater meets the Class I top-grade standard of “Industrial Anhydrous Na2SO4” (GB/T 6009-2014, https://www.mee.gov.cn), and the NaCl meets the refined industrial salt first-grade standard of “Industrial Salt” (GB/T 5462-2003, https://www.mee.gov.cn). The remaining less than 10% of mixed salts are treated as hazardous waste, significantly reducing the cost and pressure of mixed salt disposal [47].
Although crystallization separation technology can separate mixed salts into industrial Na2SO4 and NaCl, the utilization of produced Na2SO4 and NaCl poses significant challenges: Firstly, the industrial salt market in China, including industrial Na2SO4 and NaCl, is already severely over-capacitated, with an overall capacity valorization rate of only 50%, making it difficult for salt products processed from industrial waste to enter the market. Secondly, due to the varying compositions of industrial salt waste from different enterprises, the small amount of impurities in industrially separated salt could severely impact the existing salt-using production units, and impurity metal ions could cause irreversible damage to the ion-exchange membranes of chlor-alkali plants.
In the fine chemical industry, the long production process, complex technologies, and low product yield result in a large amount of raw and auxiliary material intermediates entering the original liquor. If only evaporation crystallization methods are used to treat the original source liquor, the toxic and harmful substances contained in it will enter the evaporated salt waste, posing significant environmental risks if not treated properly. Besides evaporation crystallization, other processes are often combined for deeper treatment of salt waste. For example, a pesticide company in Shandong produces high-salinity wastewater during the condensation and chlorination steps of pesticide production, generating NaCl through evaporation crystallization, which contains organic impurities such as dicamba, dichlorophenol, and ortho-cresol. Because the organic compounds in the high-salinity wastewater are not effectively treated, the quality of the obtained NaCl is severely affected. To address the organic content in the salt residue, a combined process of resin adsorption coupled with ultraviolet photocatalysis is used for deep treatment of the salt waste, meeting the requirements for ion-exchange membrane caustic soda, truly turning waste into treasure and providing a new approach for the valorization disposal of pesticide industry salt waste [48].
In the production process of 2,4-dichlorophenoxyacetic acid, a pesticide company generated wastewater containing NaCl. However, the direct recovery of NaCl via evaporation and crystallization resulted in an odor due to organic substances such as 2,4-dichlorophenoxyacetic acid and phenol. In the engineering process, a special resin with good adsorption properties for phenolic substances was used. After pretreatment with the special resin, photocatalytic wet oxidation technology was applied to improve the water quality, allowing the refined and purified NaCl salt waste to meet the reuse requirements for ion-exchange membrane caustic soda, achieving valorization [49].
In the production of polyphenylene sulfide (PPS), the main components of the generated salt waste are NaCl and Na3PO4. Through a process of synthesis → washing → inorganic membrane filtration → evaporation drying → salt dissolving → filtration → activated carbon purification → evaporation (crystallization) → centrifugal separation → transport use, the inorganic salts can be separated using organic solvent extraction, and the inorganic salts containing lithium can be separated using precipitation [50]. Then, the remaining NaCl and Na3PO4 are processed for salt separation and can be used in the production of chlor-alkali ion-exchange membrane electrolysis, fully realizing the resource reuse of PPS industrial salt waste [51].

2.2.7. Other Valorization Technologies

Some enterprises utilize ion-exchange membrane caustic soda units to resourcefully use chlorinated salt waste that has been treated through valorization processes. The salt waste can meet the entry requirements for ion-exchange membrane caustic soda, including acceptable levels of total organic carbon (TOC), inorganic ammonium, and total ammonium [52].
Compared to existing salt waste treatment technologies, there is a novel technique called Hazardous Industrial Salt Waste Innocuous Disposal Integration (HDI). This technology completely transforms industrial salt waste by converting large amounts of soluble salts, such as NaCl and Na2SO4, into glass-phase products. The Nenglu Industrial Park in Ordos used high-multiplication particle circulation and rapid heat dispersion technology at temperatures around 1200 °C with the synergistic action of high-reactivity salt waste transformation agents. The soluble salts and heavy metal components in the salt waste react efficiently with reactive silicon and aluminum species in the system. This reaction produces glass-phase and mineral crystal products with stable lattice network structures. With efficient and harmless transformation through HDI technology, the total dissolved solids (TDS) components in industrial salt waste are converted into environmentally benign inert glassy mineralized products, such as silico-aluminate feldspars, albite, and blue albite, with over 90% glass-phase contents. These can be used as proppants in oil and gas fracking and as building materials [53].
In Quzhou City, through the continuous advancement of the “Zero Waste City” construction, salt waste valorization industrial chains have been established in industries such as fluorosilicate, chlor-alkali, and cement production. Successful models include salt waste to ion-exchange membrane caustic soda, water washing + cement kiln co-disposal, incineration + by-product acid extraction + waste heat valorization, and one material, one unit for recyclable landfill. These models provide new demonstrations for the full-process valorization of high-salt waste in Zhejiang Province [54].
A Chinese company, namely, Shaoxing Yuexin Environmental Protection Technology Co., Ltd. (Shaoxing, China), used an internal-heat dual-stage structured pyrolysis furnace to effectively remove organic pollutants from salt waste materials. High-salt solutions were treated in an integrated treatment unit to remove non-target impurities, and special nanofiltration equipment was used for the separation of target salts, achieving the mono-salination of mixed salts, reducing salt wastewater discharge, and realizing water conservation and waste valorization. The evaporative system’s condensate water was recycled back to the front-end process as water for dissolving solid salt waste, avoiding the discharge of saline wastewater. A comprehensive collection of miscellaneous-use rinse water, emergency discharges, and high-pressure spray were used for atomization and injection into the rotary kiln, avoiding wastewater discharge and serving a temperature regulation function. The main technological route in the valorization of salt waste includes dissolution, flocculation precipitation, immersed ultrafiltration, low-temperature catalytic oxidation, special nanofiltration, evaporative crystallization, and source-liquor triple-effect evaporative crystallization. Sludge was passed through a buffer tank into a plate-and-frame filter press, and the resulting cake and mixed salts were handled by a third-party unit with relevant qualifications. This process proved to be suitable for treating high-salt mixed salt solutions or solid mixed salts. Traditional processes often treat the product of crystallization evaporation as solid waste, generating a large amount of solid waste and relatively low-concentration saline wastewater (evaporative condensate). This process, through the optimization of the technological route and the introduction of new equipment, addresses the problem of large-scale discharge of crystallized salt solid waste (by separating the salts using special nanofiltration to produce relatively single-variety industrial-grade crystallized salts). The evaporative system’s condensate water reuse for solution preparation solves the issue in traditional processes of relatively low-concentration saline condensate water still needing to be separated by a membrane system before it can be reused [55,56].
After processing, different types of salt wastes are utilized for resource recovery purposes, as outlined in Table 3. For pyrolysis, the process is conducted in a controlled oxygen environment and at temperatures below the melting point of the salt waste. By heating the salt waste, some organic materials in the salt waste are vaporized into gas and are moved into subsequent treatment units, while other organic materials are converted into ash. This process effectively treats industrial salt waste and reduces the content of toxic and hazardous substances in the salt waste [57,58]. Practice has shown that variations in pyrolysis temperature and other process parameters significantly affect the final treatment outcome. High-temperature melting refers to treating salt waste at higher temperatures, typically requiring calcination temperatures between 800 °C and 1200 °C. Since the calcination temperature is often higher than the melting point of the salt waste, the salt is in a molten state during treatment, allowing for the thorough removal of organic materials from the salt waste. Although high-temperature melting thoroughly removes organic materials from salt waste, impurities such as oxidized carbon (C), sulfur (S), and phosphorus (P) may still be present, requiring further treatment to meet reuse standards. Additionally, the high temperature of the molten salt can cause salt components to volatilize under ventilated conditions, which then crystallize in subsequent cooling units, potentially clogging pipes. The refining process of the salt after melting treatment still requires the involvement of evaporation units, resulting in high energy consumption throughout this part of the process. For high-temperature fluidized beds, large contact areas and uniform mixing are needed. This process prevents the clumping of salt waste and enhances thermal efficiency. The fluidization condition within the bed layer must be monitored in real-time during production, and appropriate measures should be taken to promptly intervene. The oxidation method uses oxidizers to oxidize organic impurities in salt waste, thereby purifying the inorganic salts. Advanced oxidation is characterized by the production of hydroxyl radicals (•OH) with strong oxidative capabilities, transforming large and difficult-to-degrade organic molecules into less toxic or non-toxic smaller molecules. This method is a green, pollution-free, and efficient water treatment technology. The effectiveness of the oxidation method is closely related to the properties of the organic materials present. The types of organic compounds in salt waste can vary significantly between different products and production processes, making it difficult to determine operational parameters, such as the amount of oxidizer needed and the treatment time. This variability restricts the application of the oxidation method. The washing and separation method refers to the method of purifying salt by dissolving the salt to be purified in water or organic solvents, allowing some organic substances and heavy metal ions in the salt to remain in the solution, thereby achieving the purpose of purifying the salt waste. The solution produced by this method is difficult to handle, and, often, a large amount of water or solvent is used during the process to ensure the effectiveness of salt washing, which can lead to secondary pollution. In practice, it has been found that the quality of salt waste fluctuates significantly, making it difficult to accurately calculate the required amount of water or solvent, resulting in resource wastage. Some impurities may be trapped within the salt particles and cannot be thoroughly cleaned. The precipitation method involves dissolving the salt waste in water and adding chemical reagents to remove certain characteristic pollutants from the salt waste. While this method offers relatively stable results in treating salt waste, it may also lead to secondary pollution. Removing organic substances is challenging, and the process is lengthy. Evaporation crystallization is a method where a mixture of salts containing one or more impurities are dissolved in water. Then, by adjusting the temperature or evaporating water, one component reaches a saturated state and crystallizes, thereby achieving the separation and purification of salts. High energy consumption and the need for further treatment of the source liquor after system processing are required. It is often combined with other processes for further treatment of salt waste. Currently, the primary valorization direction for industrial salt waste, after purification, is its utilization as a raw material for the production of caustic soda and soda ash, with caustic soda accounting for approximately 60% of the total capacity.

3. Discussion and Outlook

Currently, the treatment and valorization of chemical industrial salt waste remain significant challenges in the solid waste management sector, both domestically and internationally. Given the production and disposal status of salt waste, there is a vast market potential for the reuse of salt waste in China. Most domestic enterprises adopt high-temperature pyrolysis for salt waste treatment to meet landfill standards before disposal; however, this does not achieve valorization of the salt waste, and research on related valorization technologies is still in its infancy.
The composition of industrial salt waste varies across different industries, and the chemical salt waste valorization industry lacks sufficiently advanced technologies. There is an urgent need to explore advanced and practical technologies and methods for valorization. Using either solid or liquid industrial salt waste can promote the transition from waste generation to separation, purification, and product transformation, achieving resource recycling and fostering a sustainable cycle. This can actively promote the development of a green circular economy and create significant environmental benefits for society. During the initial technology development and usage phases, enterprises face substantial technological and economic risks, necessitating government and industry association support through environmental tax exemption and considerations of energy and carbon emission benchmarks. Additionally, the current cooperation mode for the valorization of industrial salt waste is relatively limited, with production and usage companies needing to operate on a one-to-one basis, and the transfer and disposal control is very strict. There is a need for further optimization by the industry and relevant departments to form new guidelines and broaden usage channels. Encouragement of research, development, and demonstrational promotion of valorization technologies for difficult-to-dispose-of hazardous wastes like salt waste and the advancement of comprehensive valorization projects for salt waste are essential.
On the other hand, the valorization of enterprise salt waste still lacks comprehensive legal and regulatory guidance. Currently, there are no unified standards specifying to what extent salt waste can be reused, making implementation challenging for enterprises. China lacks product standards for salt waste-derived product salts; relying solely on regulation is insufficient to solve the disposal challenges of salt waste. There is a need to accelerate the establishment of technical specifications and product standards for salt waste disposal.

4. Conclusions

The valorization of salt waste faces challenges due to the lack of unified policy documents and standard specifications, rendering industrialization a challenging endeavor. Among the available methods, the washing separation method typically serves only as a pretreatment technique. Both the metathesis and oxidation methods exhibit narrower applicability and higher costs compared to the thermal treatment method. Considering all factors, feasible recommendations for the valorization of industrial salt waste are as follows:
  • For salt wastes with low and uniform organic content: salt washing + impurity removal (removing impurities and separating salts) + crystallization to produce salt that meets national product standards.
  • For salt wastes with high organic contents and large-scale and mixed salts: stable high-temperature oxidation + impurity removal (removing impurities and separating salts) + crystallization to produce salt that meets national product standards.

Author Contributions

Conceptualization, and original draft preparation, Y.L.; investigation, Y.W.; resources, D.Z.; data curation, C.W.; literature collection, J.Z., funding acquisition, Z.Z.; revision and comments, M.N., revision and comments, X.L.; revision and comments, K.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Jiangsu Province Ecological Environment Research Project (2022016) and the Key Research and Development Program of Jiangxi (no. 20203BBG72W012). Support for this work received from the Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion (MATEC2022KF011), the National Natural Science Foundation of China (52170133), and Start Funding of the Guangdong Technion Israel Institute of Technology (ST2300014) is acknowledged. And The APC was funded by the Jiangsu Province Ecological Environment Research Project (2022016) and the Key Research and Development Program of Jiangxi (no. 20203BBG72W012).

Data Availability Statement

Data will be available once requested.

Acknowledgments

This study was funded by the Jiangsu Province Ecological Environment Research Project (2022016) and the Key Research and Development Program of Jiangxi (no. 20203BBG72W012). Support for this work received from the Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion (MATEC2022KF011), the National Natural Science Foundation of China (52170133), and Start Funding of the Guangdong Technion Israel Institute of Technology (ST2300014) is acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Water 16 01620 g001
Figure 1. Status of research on salt waste: (a) number of publications in the last twenty years and (b) number of publications from the top twenty countries (https://www.engineeringvillage.com, data updated on 25 April 2024).
Figure 1. Status of research on salt waste: (a) number of publications in the last twenty years and (b) number of publications from the top twenty countries (https://www.engineeringvillage.com, data updated on 25 April 2024).
Water 16 01620 g001
Water 16 01620 g002
Figure 2. Advantages and limitations of different salt waste treatment methods.
Figure 2. Advantages and limitations of different salt waste treatment methods.
Water 16 01620 g002
Table 1. Different valorization purposes of salt waste.
Table 1. Different valorization purposes of salt waste.
Type of Salt WasteValorization ApplicationMain Conclusions
NaClUsed in the chlor-alkali industry as a raw material [14] or as a deicing agent (must meet the GB/T 23851 standard (https://www.mee.gov.cn), with very strict requirements for heavy metals in the product [15]) or as a raw material for bipolar membranes to produce acids and bases [16] and as a coal additive [17]Currently, expanding chlor-alkali projects using industrial salt waste as a raw material is not restricted by production capacity policies. However, as chlor-alkali projects are high-energy-consuming endeavors, while they facilitate the environmentally friendly disposal of industrial salt waste, they are subject to constraints such as energy assessments and carbon emission audits. This imposes significant limitations and restrictions on the expansion of chlor-alkali projects using industrial salt waste
Na2SO4Used as soda ash [18] or as a raw material for bipolar membrane production for acid–base production [19]/
A mixture of NaCl and Na2SO4Used to produce soda ash or reused after fractional crystallization [20,21]/
CaCl2Used as a raw material for gypsum [22] or as an additive [23]/
FeCl3Used as a water purifier/
Table 2. Standards for industrial by-product salt produced in China.
Table 2. Standards for industrial by-product salt produced in China.
Standard NameImplementation Date (Day Month Year)Main Contents
Standard for glyphosate by-product industrial salts
(HG/T 5531.1-2019, https://www.mee.gov.cn)		1 January 2020Specifies that by-product NaCl must have a purity of more than 94%, with total phosphorus less than 0.15% and total organic carbon less than 0.03%
Coal chemical by-product industrial sodium sulfate (T/CCT 001-2019, https://www.mee.gov.cn)1 January 2020After refining high-salt wastewater, the by-product industrial NaCl must have a purity of more than 96% and grade A Na2SO4 must have a purity of more than 97%
Coal chemical by-product industrial sodium sulfate (T/CCT 002-2019, https://www.mee.gov.cn)1 January 2020
Industrial salts (GB/T 5462-2015, https://www.mee.gov.cn)1 May 2016Sets physicochemical benchmarks for refined industrial salts, including dry and wet salts, as well as sun-dried salts
Industrial anhydrous sodium sulfate (GB/T 6009-2014, https://www.mee.gov.cn)1 December 2014Specifies product grades and other requirements
Management of hazardous solid waste in heat-treatment salt baths (GB/T 27945.1-201, https://www.mee.gov.cn)1 October 2012Standardizes methods for the harmless treatment of residues such as barium salts, nitrate salts, and cyanide salts
Iron and steel enterprises use mixed wastewater by-product industrial salt (T/CISA 225-2022, https://www.mee.gov.cn)1 August 2022Establishes physicochemical standards for industrial salts derived as by-products from mixed wastewater of steel companies
Industrial salt as a by-product of coking wastewater (T/CISA 227-2022, https://www.mee.gov.cn)1 August 2022Sets physicochemical benchmarks for industrial salts obtained as by-products from coking wastewater
Technical specification for pollution control and treatment of high-salt wastewater in textile printing and dyeing industry (DB 37/T 3536-2019, https://www.mee.gov.cn)2 May 2019Outlines regulations for the solidification and recycling use of salts
Table 3. Salt waste treatment methods.
Table 3. Salt waste treatment methods.
Treatment MethodPrincipleApplicable ScopeAdvantages and Limitations
Thermal treatmentPyrolysisGenerally, the process is conducted in a controlled oxygen environment and at temperatures below the melting point of the salt waste. By heating the salt waste, some organic materials in the salt waste vaporize into gas and move into subsequent treatment units, while other organic materials are converted into ashSingle-stage carbonization process: suitable for treating industrial salt wastes with relatively simple structures. Multi-stage carbonization process: suitable for salt wastes containing long carbon chains and heterocyclic organic compounds [56]Using this process effectively treats industrial salt waste and reduces the content of toxic and hazardous substances in the salt waste [57,58]. Practice has shown that variations in pyrolysis temperature and other process parameters significantly affect the final treatment outcome
High-temperature meltingThis refers to treating salt waste at higher temperatures, typically requiring calcination temperatures between 800 °C and 1200 °C. Since the calcination temperature is often higher than the melting point of the salt waste, the salt is in a molten state during treatment, allowing for the thorough removal of organic materials from the salt wasteSuitable for treating salt wastes with high organic contents and complex functional groupsAlthough high-temperature melting thoroughly removes organic materials from salt waste, impurities such as oxidized carbon (C), sulfur (S), and phosphorus (P) may still be present, requiring further treatment to meet reuse standards. Additionally, the high temperature of the molten salt can cause salt components to volatilize under ventilated conditions, which then crystallize in subsequent cooling units, potentially clogging pipes. The refining process of the salt after melting treatment still requires the involvement of evaporation units, resulting in high energy consumption throughout this part of the process
High-temperature fluidized bed Salt waste containing organic materialsA large contact area and uniform mixing prevent the clumping of salt waste and enhance thermal efficiency. The fluidization condition within the bed layer must be monitored in real time during production, and appropriate measures should be taken to promptly intervene
Oxidation method (such as advanced oxidation, catalytic wet oxidation, hydrothermal oxidation techniques)The oxidation method uses oxidizers to oxidize organic impurities in salt waste, thereby purifying the inorganic salts. Advanced oxidation is characterized by the production of hydroxyl radicals (•OH) with strong oxidative capabilities, transforming large, difficult-to-degrade organic molecules into less toxic or non-toxic smaller molecules. This method is a green, pollution-free, and efficient water treatment technologyUsed for salt wastes with few organic impurities that are easily oxidized, this method does not produce secondary pollution and does not introduce new impurities after the reactionThe effectiveness of the oxidation method is closely related to the properties of the organic materials present. The types of organic compounds in salt waste can vary significantly between different products and production processes, making it difficult to determine operational parameters such as the amount of oxidizer needed and the treatment time. This variability restricts the application of the oxidation method
Washing and separation methodA method of purifying salt by dissolving the salt to be purified in water or organic solvents, allowing some organic substances and heavy metal ions in the salt to remain in the solution, thereby achieving the purpose of purifying the salt wasteIt generally applies only to salt waste with single components and few impuritiesThe solution produced by this method is difficult to handle, and, often, a large amount of water or solvent is used during the process to ensure the effectiveness of salt washing, which can lead to secondary pollution. In practice, it has been found that the quality of salt waste fluctuates significantly, making it difficult to accurately calculate the required amount of water or solvent, resulting in resource wastage. Some impurities may be trapped within the salt particles and cannot be thoroughly cleaned
Precipitation methodThe precipitation method involves dissolving salt waste in water and adding chemical reagents to remove certain characteristic pollutants from the salt waste. While this method offers relatively stable results in treating salt waste, it may also lead to secondary pollutionThe precipitation method is only suitable for treating salt waste with a relatively stable and uniform composition. Additionally, the waste residue after precipitation also requires secondary treatmentRemoving organic substances is challenging, and the process is lengthy
Evaporation crystallizationEvaporation crystallization is a method where a mixture of salts containing one or more impurities are dissolved in water. Then, by adjusting the temperature or evaporating water, one component reaches a saturated state and crystallizes, thereby achieving the separation and purification of saltsIt is suitable for high-salinity wastewater containing components with different solubilitiesHigh energy consumption and the need for further treatment of the source liquor after system processing are required. It is often combined with other processes for further treatment of salt waste
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Lv, Y.; Wang, Y.; Zhang, D.; Wu, C.; Zhang, J.; Zhao, Z.; Nabi, M.; Luo, X.; Xiao, K. A Mini-Review on Safe Treatment and Valorization of Salt Waste in Chemical Production Processes in China. Water 2024, 16, 1620. https://doi.org/10.3390/w16111620

AMA Style

Lv Y, Wang Y, Zhang D, Wu C, Zhang J, Zhao Z, Nabi M, Luo X, Xiao K. A Mini-Review on Safe Treatment and Valorization of Salt Waste in Chemical Production Processes in China. Water. 2024; 16(11):1620. https://doi.org/10.3390/w16111620

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Lv, Yang, Yi Wang, Dapeng Zhang, Chaoyue Wu, Jun Zhang, Zehua Zhao, Mohammad Nabi, Xuan Luo, and Keke Xiao. 2024. "A Mini-Review on Safe Treatment and Valorization of Salt Waste in Chemical Production Processes in China" Water 16, no. 11: 1620. https://doi.org/10.3390/w16111620

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