High-salt wastewater synergistic treatment: Circular economy case of magnesium hydroxide desulfurization and brine magnesium extraction
1. Dual challenges and synergistic opportunities of high-salt wastewater
The treatment of high-salt wastewater in the industrial field has long faced two major problems: sulfate enrichment generated by the desulfurization system and resource waste in the brine magnesium extraction process. Taking a coastal chemical park as an example, its flue gas desulfurization system discharges more than 500,000 tons of magnesium sulfate-containing wastewater annually, while the adjacent salt lake magnesium extraction project has a magnesium recovery rate of less than 60% due to brine calcium ion pollution. The traditional "headache treatment" governance model not only pushes up the treatment cost, but also causes a break in the circulation of resources such as magnesium sulfate and magnesium oxide.
The synergistic technology of magnesium hydroxide desulfurization and brine magnesium extraction is breaking this deadlock. By directional delivery of magnesium sulfate in desulfurization wastewater to the brine magnesium extraction link, it not only solves the problem of sulfate enrichment, but also provides low-cost raw materials for magnesium extraction. The practice of a park in Shandong shows that this model reduces the cost of desulfurization wastewater treatment by 42%, and increases the brine magnesium yield to 92%, realizing the industrial closed loop of "waste into raw materials".
II. The core logic of technology coupling
1. Resource utilization path of magnesium sulfate
The magnesium sulfate wastewater (concentration can reach 15%-20%) produced by traditional wet desulfurization can be converted into industrial-grade magnesium sulfate crystals after multi-stage membrane concentration + crystallization purification. A power plant in Liaoning uses the "nanofiltration salt separation-evaporation crystallization" process to increase the purity of magnesium sulfate to 99.3%, which is directly used as a magnesium supplement for magnesium extraction from brine.
2. Process innovation of magnesium extraction from brine
The extraction of magnesium from salt lake brine is often interfered by calcium ions, and a large amount of soda ash is required to remove calcium. After the magnesium sulfate in the desulfurization wastewater is introduced, the calcium impurities can be removed and magnesium ions can be enriched simultaneously through the calcium-magnesium replacement reaction (Ca²⁺ + MgSO₄ → CaSO₄↓ + Mg²⁺). A salt lake project in Qinghai uses this technology, and the soda ash consumption per ton of magnesium product is reduced by 1.2 tons, and the gypsum by-product can also be used in building materials production.
3. Internal circulation of magnesium hydroxide
The magnesium hydroxide slurry (concentration 8%-12%) produced as a byproduct of magnesium extraction from brine can be reused in the flue gas desulfurization system after active modification + particle size control. The measured data of a steel enterprise in Hebei Province showed that the desulfurization efficiency of modified magnesium hydroxide was 18% higher than that of traditional limestone, and the amount of sludge produced was reduced by 55%.
III. Engineering practice and benefit verification
Case 1: Binhai Chemical Park circulation network
System architecture: desulfurization wastewater → DTRO membrane concentration → magnesium sulfate crystallization → salt lake magnesium extraction → magnesium hydroxide reuse → desulfurization tower
Operation data:
Annual treatment of 300,000 tons of high-salt wastewater and recovery of 42,000 tons of magnesium sulfate
Brine magnesium extraction energy consumption reduced from 3.2 tons of standard coal/ton to 1.8 tons
The substitution rate of magnesium hydroxide reuse exceeded 75%, saving 26 million yuan in procurement costs annually
By-product value-added: gypsum is used to produce lightweight partition boards, Annual revenue of 8 million yuan
Case 2: Inland coal-fired power-salt chemical co-production project
Develop a "zero liquid discharge" synergistic solution for the water-scarce Northwest region:
Anhydrous transformation: Desulfurization wastewater is converted into magnesium sulfate powder through a spray drying tower and transported to a salt chemical plant 200 kilometers away
Energy synergy: Use the waste heat of the power plant to drive the brine evaporation system, reducing the overall energy consumption by 34%
Smart management and control: Build an Internet of Things platform to match supply and demand in real time, and increase the inventory turnover rate of magnesium sulfate by 3 times
IV. Economic model and competitive barriers
1. Cost reconstruction effect
Marginal cost advantage: The cost of obtaining magnesium sulfate from desulfurization wastewater is only 1/3 of the market price
Policy dividend: Under the carbon trading mechanism, each ton of coordinated processing can generate 0.8 tons of CO₂ equivalent carbon sink income
Industry chain extension: A Qinghai company developed flame-retardant magnesium hydroxide through by-products, and its gross profit margin increased to 45%
2. Technology moat construction
Crystallization control patent: Directed cultivation of flaky magnesium sulfate crystals (particle size 50-100μm) to improve the reaction activity of brine
Anti-fouling and clogging materials: Silicon carbide-polyethersulfone composite coating pipeline, resistant to Cl⁻ concentration＞50,000mg/L environment
Intelligent early warning system: Magnesium ion concentration prediction model based on digital twin, process fluctuation rate＜±2%
V. Future evolution direction
1. Hydrogen energy coupling development
Use green electricity to electrolyze magnesium sulfate solution and simultaneously produce hydrogen and magnesium oxide:
Each ton of magnesium sulfate can produce 62m³ of hydrogen with a purity of 99.97%
Magnesium oxide is used for CO₂ mineralization and storage, realizing the trigeneration of "desulfurization-hydrogen production-carbon sink"
2. Marine economic extension
Transplant technology to seawater desalination and brine treatment:
A pilot project in Bohai has achieved an 85% magnesium resource recovery rate in brine
Byproduct magnesium hydroxide is used for coastal saline-alkali land improvement, increasing rice production by 28%
3. Global resource network
Build a "Belt and Road" high-salt wastewater collaborative treatment alliance:
Middle East oil and gas field desulfurization system and Dead Sea magnesium extraction project docking
Southeast Asian palm oil mill wastewater and Australian salt lake form a transnational circulation chain
When the flue gas desulfurization tower is connected to the salt lake magnesium extraction tank through a pipeline, and when every gram of magnesium sulfate finds its destination, this industrial metabolic revolution has gone beyond the scope of environmental governance and evolved into a strategic fulcrum for reshaping the global resource pattern. From the recycling network of the Bohai Bay to the zero-emission salt lakes of the Qaidam Basin, from the electrolyzers of green hydrogen plants to the drip irrigation pipes of desert agriculture, the synergistic story of magnesium hydroxide and brine is writing the most concrete footnote of the circular economy - here, pollution and resources are no longer two sides of the same coin, but a symbiotic entity that circulates sustainably in the industrial chain.