Key Takeaways & Executive Findings
- •• • Coal chemical waste salt comprises >90% NaCl and Na2SO4, with organic pollutants (e.g., phenols, PAHs) adsorbed on crystal surfaces and inorganic impurities (heavy metals, hardness ions) posing significant environmental risks; TDS of concentrated brine can exceed 35,000 mg/L. • • Treatment follows a 'remove impurities first, then separate salts' route: organic removal via pyrolysis, advanced oxidation, or biochemical methods; inorganic deep removal via chemical precipitation-adsorption-membrane coupling; salt separation via evaporative crystallization (solubility differences) or membrane methods like electrodialysis. • • Ion-exchange membrane caustic soda production is viable for NaCl reuse, but strict control of Fe3+ and TOC is critical; for Na2SO4, the 'short-process ammonium-alkali' method is mature with engineering demonstrations, while bipolar membrane electrodialysis shows green potential but faces economic and stability challenges. • • The current standard system lacks effective integration between waste salt products, pollution control, and resource utilization; a holistic framework from pollution control to product standards to resource use is needed to drive clean, industrial, and value-added development.
Abstract
Coal chemical waste salt, a solid residue from evaporative crystallization of high-salinity wastewater, poses significant environmental risks and challenges for resource utilization due to its complex composition. This study systematically analyzes its composition and environmental hazards, highlighting its typical "mixed salt" nature and the potential threats of organic pollutants and heavy metals to soil, water, and ecosystems. It reviews mainstream treatment pathways, including organic degradation, inorganic impurity removal, and salt separation, with a focus on the resource utilization of sodium chloride and sodium sulfate and their industrial prospects. The current pollution control technical specifications and product quality standards are examined, comparing the scope and technical points of relevant standards such as the "Technical Specification for Pollution Control of Chemical Waste Salt." Finally, countermeasures are proposed to address challenges including difficult treatment of mixed salts, insufficient resource utilization incentives, and incomplete standard systems, emphasizing technological innovation, policy guidance, and standard improvement.
1. Introduction
Coal chemical industry, a cornerstone of China's energy sector, has pursued 'near-zero discharge' strategies to maximize water reuse (≥95%) and recover salts via evaporative crystallization. However, this process generates substantial solid waste salt—over 5 million tonnes annually as of 2019, accounting for >20% of industrial waste salt—creating a new environmental bottleneck. The waste salt, primarily NaCl and Na2SO4, is contaminated with organics (e.g., phenols, PAHs) and heavy metals (e.g., Cr(VI), Pb2+, Cd2+), posing risks of groundwater contamination and soil degradation if mismanaged. Existing treatment costs (1,000–1,600 CNY/t) far exceed the market price of raw salt (~440 CNY/t), undermining economic incentives for resource recovery.
This study addresses the critical gap between waste salt generation and sustainable management by systematically analyzing composition, treatment technologies, and resource utilization pathways. It evaluates the efficacy of organic degradation, impurity removal, and salt separation methods, and assesses the feasibility of converting NaCl and Na2SO4 into valuable products such as caustic soda and ammonium alkali. By examining the current regulatory framework and standards, the study identifies systemic deficiencies and proposes integrated solutions to foster a circular economy for coal chemical waste salt, aligning with national green development goals.
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YANG Jun, HE Yi, XU Jie, LIU Yanping, LIU Guoliang, LIU Haibing (2026). Analysis of Treatment and Resource Utilization of Coal Chemical Waste Salt: Current Status and Prospects. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202507081
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Frequently Asked Questions
What are the primary technical bottlenecks in removing organic contaminants from coal chemical waste salt, and how do different methods compare in efficiency and cost?
Organic contaminants, including phenols and PAHs, adsorb onto salt crystals, complicating purification. Thermal pyrolysis can achieve >95% TOC removal but requires high energy input (typically >500°C). Advanced oxidation processes (AOPs) like Fenton or ozone can degrade organics at ambient conditions but may leave residual oxidants. Biochemical methods are cost-effective but slow and sensitive to salt concentrations. The choice depends on the organic load and desired purity; for instance, TOC levels range from 56 to 3,000 mg/L, necessitating tailored approaches.
How does the 'short-process ammonium-alkali' method for Na2SO4 utilization achieve economic viability compared to traditional routes, and what are its scale-up challenges?
The short-process ammonium-alkali method converts Na2SO4 to Na2CO3 and (NH4)2SO4 via reaction with NH3 and CO2, offering a closed-loop process with high atom economy. It has been demonstrated at engineering scale, achieving Na2SO4 conversion rates >90% and producing saleable products. However, challenges include managing heat integration, preventing scaling from impurities like Ca2+ and Mg2+, and ensuring consistent product quality. Economic viability depends on local market prices for Na2CO3 and (NH4)2SO4, but the process can be profitable when waste salt disposal costs are avoided.
What specific impurity limits must be met for NaCl to be used in ion-exchange membrane caustic soda production, and how are these achieved?
For membrane cell operation, NaCl brine must have low concentrations of hardness ions (Ca2+, Mg2+ < 0.1 mg/L), heavy metals (Fe3+ < 0.1 mg/L), and TOC (< 10 mg/L) to prevent membrane fouling and poisoning. Achieving these limits requires a combination of chemical precipitation (e.g., NaOH-Na2CO3 softening), adsorption (e.g., activated carbon or ion exchange), and membrane filtration (e.g., ultrafiltration). Advanced oxidation may be needed to degrade organic impurities that can deactivate the membrane.
How does the current standard system in China fail to support waste salt resource utilization, and what specific improvements are recommended?
The existing standards, such as the 'Technical Specification for Pollution Control of Chemical Waste Salt' (under development), focus on pollution control but lack clear criteria for product quality and end-use applications. This creates a regulatory gap: waste salt treated to meet pollution control limits may not satisfy product specifications for industries like chlor-alkali or soda ash. Recommendations include establishing tiered standards that define acceptable impurity levels for different applications, harmonizing testing methods, and creating a certification system to track waste salt from generation to resource utilization, thereby ensuring environmental safety and market confidence.
What are the main economic barriers to large-scale resource utilization of coal chemical waste salt, and how can policy interventions mitigate them?
The primary barrier is the cost disparity: treatment costs (1,000–1,600 CNY/t) exceed the market price of virgin salt (~440 CNY/t), making recycling unprofitable. Additionally, regional imbalances—waste salt concentrated in the Yellow River basin while downstream users are elsewhere—increase logistics costs. Policy interventions could include subsidies for waste salt treatment, tax incentives for using recycled salt, and mandates for government procurement of products made from recycled salt. Establishing a national trading platform for waste salt by-products could also improve market liquidity and reduce transaction costs.
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