Breaking the Bottleneck of Industrial Waste Salt Resource Utilization: From "Waste" to "Wealth"
- Gu Zhouying
- Jul 4
- 6 min read
Introduction
In the field of environmental protection, Zero Liquid Discharge (ZLD) technology has become a symbol of innovation and strength, drawing widespread attention. However, beneath its seemingly perfect surface lies a challenging issue: what to do with the waste salt generated from evaporation and crystallization processes. Whether it’s the complex mixed salts from traditional processes or separated salts like sodium sulfate and sodium chloride, they all face the same dilemma: hard to sell, difficult to store, and costly to treat. These waste salts are a double-edged sword — leaving them untreated risks severe environmental pollution, yet disposing of them via landfilling contradicts the very essence of zero discharge.
Today, a silent revolution is emerging — one focused on the resource recovery of waste salts. The goal is to convert salts extracted from wastewater into valuable chemicals like acids and bases, thereby realizing a truly circular economy.
Article contents:
1. Status Quo and Challenges: The "Dead End" of Mixed Salt Disposal
Currently, nearly 80% of ZLD projects still rely on evaporation crystallization to produce mixed salts. However, due to the complexity of their composition — often containing heavy metals and organics — these salts are classified as hazardous waste. Disposal by landfill can cost as much as 3,000–4,000 RMB per ton (approximately USD 400–$550 per ton). Even with advanced salt-separation technologies such as nanofiltration or thermal crystallization, the separated sodium sulfate and sodium chloride often fail to meet industrial-grade purity standards.
Key Problems:
Purity vs. Marketability: Sodium sulfate often has a purity below 92%, while sodium chloride lags below 95%, with insufficient whiteness, making them unsuitable for use in major industries like chemicals and construction.
Lack of Economies of Scale: Most ZLD systems produce only 1–5 tons of salt per day, which is too little to form stable supply chains, making downstream customers reluctant to purchase.
2. The ZLD Paradox: Industrial Waste Salt as a Liability
Although evaporation crystallization helps achieve zero discharge, the resulting mixed salts become a heavy burden for industries. Disposal costs are high, and improper handling could result in serious environmental penalties.
While salt-separation techniques improve purity, they still fall short due to:
Limited wastewater volumes which hinder large-scale production.
Residue impurities in crystallized salts prevent compliance with industry standards.
Low-value market saturation, where industrial salt prices are driven down, making impure salts commercially unviable.
As a result, these salts are often either stockpiled or sent to landfills, creating a persistent barrier to achieving full ZLD goals.
3. New Direction: From Passive Separation to Active Resource Recovery
Researchers are now pursuing a transformative approach — one that bypasses crystallization and extracts high-value chemicals directly from concentrated brine. This approach, known as in-situ salt resource recovery, is gaining traction with three main technologies leading the way:
Acid-Base Co-production via Bipolar Membrane Electrodialysis (BMED)
This method splits salt solutions (e.g., NaCl, Na₂SO₄) into hydrochloric acid, sodium hydroxide, or sulfuric acid using bipolar membranes. Requirements include:
Salt concentration of 10–15%
Impurity levels (Ca²⁺, Mg²⁺) below 1 ppm
Key Figures:
Energy consumption: 120–180 kWh per ton of water
Investment cost: 3–5 million RMB per ton/day capacity
Benefits: The acids and bases produced can be reused in the same treatment plant (e.g., for pH adjustment or RO cleaning), creating a closed loop. Example: A coal chemical plant generated 12,000 tons of HCl and 8,000 tons of NaOH annually using BMED, saving over 10 million RMB/year in chemical purchases.
On-site Sodium Hypochlorite Generation
For low-concentration NaCl brine (3%–5%), sodium hypochlorite (0.5%–0.8%) can be produced directly through electrolysis.
Salt consumption is low: ~4.5 kg salt per 1 kg NaOCl
Ideal for internal use (e.g., membrane cleaning, disinfection)
Limitation: Sodium hypochlorite degrades quickly and must be used immediately — not suitable for commercial distribution.
Co-Production of Ammonium Sulfate and Soda Ash (Modified Solvay Process)
This involves reacting NaCl and Na₂SO₄ with CO₂ and ammonia to produce sodium carbonate (soda ash) and ammonium sulfate. Conditions:
Requires access to high-concentration CO₂ (15%–35%)
Proximity to industrial kilns and ammonia sources
Economic Advantage:
Sodium carbonate market price: ~3,500 RMB/ton
Ammonium sulfate: ~1,000 RMB/ton
3–5x value increase compared to raw salts
Example: In Inner Mongolia, one park converts 15,000 tons of high-salinity brine into industrial soda ash, reducing CO₂ emissions by 8,000 tons annually.
4. Game-Changer: Regenerating Acids and Bases from Waste Salt
A key breakthrough is the ability to regenerate HCl and NaOH from waste salts, closing the resource loop and minimizing solid waste. Two main pathways are being pursued:
Salt Separation + Bipolar Electrodialysis: Use nanofiltration/electrodialysis to separate salts, then apply BMED to convert into acids and bases.
Thermochemical Conversion: High-temperature reactions convert Na₂SO₄ into sulfuric acid and sodium carbonate, and NaCl into HCl and NaOH.
Advantages:
High-value outputs with stable market demand
Acids/bases reused on-site, reducing purchasing costs
Eliminates hazardous solid waste, aligning with circular economy goals
5. Technical Challenges: Bridging Ideal and Reality
Despite promising potential, several challenges remain:
High energy consumption: Especially with BMED. Environmental benefits decline if the energy source isn’t renewable.
Membrane fouling: Organics and heavy metals in wastewater can degrade membranes and reduce product purity.
System complexity: Coordinated operation of separation, electrolysis, and purification requires high stability.
Capital investment: High upfront costs limit adoption among small- and medium-sized enterprises.
6. Future Outlook: Driving Progress with Policy and Innovation
Progress is underway, supported by favorable policies and emerging technologies. China's 14th Five-Year Plan for Green Industry Development encourages wastewater and waste salt resource utilization. Several provinces are piloting “point-to-point” hazardous waste use policies.
On the technology front:
Domestic bipolar membrane costs are declining
Renewable energy + electrolysis models are being promoted
Environmental and chemical enterprises are joining forces to develop integrated solutions
Case Study: In one pilot project, 2,000 tons of waste salt are converted annually into 1,500 tons of HCl and 800 tons of NaOH, reused directly in production, saving over 3 million RMB/year.
Conclusion
The true goal of Zero Liquid Discharge should not be waste salt accumulation, but efficient resource recovery. From landfill burdens to unsellable separated salts, and now toward acid and base regeneration, every step forward addresses one fundamental question: Can environmental protection and economic benefit go hand in hand?
The answer may lie in one word — circulation. When every gram of salt in wastewater becomes part of a renewed industrial cycle, ZLD will have fulfilled its ultimate promise.
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FAQs
What is industrial waste salt, and why is it a problem?
Industrial waste salt is the byproduct generated from processes like evaporation and crystallization in Zero Liquid Discharge (ZLD) systems. It often contains impurities such as heavy metals and organic compounds, making it hazardous and difficult to reuse or dispose of. High treatment costs and limited market value turn it into a major environmental and operational burden for industries.
Why can’t traditional salt separation technologies fully solve the waste salt issue?
Traditional separation methods, such as nanofiltration or thermal crystallization, can extract sodium sulfate and sodium chloride, but the resulting purity levels typically fall below industrial-grade standards. In addition, the low volume of salt produced daily (1–5 tons) limits economic scalability, making it difficult to form stable supply chains or attract buyers.
What are the leading technologies for waste salt resource recovery?
Three main technologies are currently at the forefront of waste salt resource recovery:
Bipolar Membrane Electrodialysis (BMED) – converts salts into hydrochloric acid and sodium hydroxide.
Sodium Hypochlorite Generation – produces disinfectant from low-concentration saltwater for on-site use.
Solvay-based Co-production – regenerates soda ash and ammonium sulfate using CO₂ and ammonia.
How does acid-base regeneration from waste salt benefit industries?
Acid-base regeneration transforms waste salts into valuable chemicals like HCl and NaOH, which can be reused in the same facility for pH control, cleaning, or pretreatment. This reduces external chemical procurement costs, eliminates hazardous solid waste, and supports a circular economy model, all while maintaining environmental compliance.
What are the main challenges in implementing salt resource recovery technologies?
The main barriers include high energy consumption (especially for BMED), membrane fouling from wastewater impurities, system complexity requiring precise operation, and significant upfront investment. However, falling membrane costs and supportive government policies are helping drive adoption across the industry.
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