Utilization of byproducts of magnesium hydroxide desulfurization: industrial practice of magnesium sulfate crystallization process

In the field of high-sulfur flue gas treatment such as thermal power and steel, magnesium hydroxide desulfurization technology has been rapidly popularized due to its high efficiency and low cost, but the treatment of byproduct magnesium sulfate has once become a key bottleneck restricting its development. With the industrial breakthrough of the crystallization process of magnesium sulfate heptahydrate (MgSO₄·7H₂O), this "environmental burden" is being transformed into a "circular economy gold mine" with an annual output value of over 10 billion. This article will deeply analyze the industrialization path from desulfurization waste liquid to high-purity magnesium sulfate, and reveal the innovative logic behind the technology upgrade.

1. From waste liquid to product: industrial process of magnesium sulfate crystallization

1. Pretreatment stage: accurate removal of impurities

After the desulfurization slurry is centrifuged, the waste liquid contains unreacted magnesium hydroxide, smoke particles and chloride ions (Cl⁻) in addition to magnesium sulfate. Industrial treatment uses three-stage purification:

First-stage impurity removal: Add polyacrylamide (PAM) flocculant to precipitate suspended matter, and reduce turbidity from 200NTU to below 10NTU;

Second-stage dechlorination: Use nanofiltration membrane (molecular weight cutoff 200Da) to separate Cl⁻, and reduce chloride ion content from 5000mg/L to 100mg/L;

Third-stage refining: Ion exchange resin adsorbs heavy metal ions, and the content of lead and cadmium is less than 0.1ppm.

Case data: Through the above process, the purity of magnesium sulfate solution at Baosteel Zhanjiang Base has increased from 75% to 99.2%, reaching the industrial standard (GB/T 26568-2011).

2. Evaporation crystallization: equipment selection and process control

The core equipment adopts DTB type (guide tube baffle) crystallizer, and the crystal size is controlled by precise temperature control:

Vacuum evaporation: Under -90kPa conditions, the boiling point of the solution drops to 60℃, and the energy consumption is reduced by 40% compared with normal pressure evaporation;

Gradient cooling: In the initial stage, the temperature is cooled to 45℃ at a rate of 2℃/h to promote the formation of crystal nuclei; in the later stage, it is slowly cooled to 30℃ at a rate of 0.5℃/h to promote crystal growth;

Particle size control: By adjusting the stirring speed (30-50rpm) and the amount of seed crystals added (5%-10%), uniform crystals of 0.3-1.2mm are produced.

Operation parameters: The annual production line of 50,000 tons of magnesium sulfate in Huaneng Yuhuan Power Plant shows that the steam consumption per ton of product has dropped from 6 tons to 3.8 tons, and the power consumption has dropped from 120kW·h to 75kW·h.

II. Three major technical challenges in the process of industrialization

1. Anti-scaling: the self-cleaning revolution of evaporators

The inner wall of traditional evaporators is prone to magnesium sulfate scaling, which requires shutdown and cleaning every 3 months. The latest solutions include:

Ultrasonic anti-scaling: Install a 40kHz ultrasonic generator on the outer wall of the heat exchange tube to reduce the scaling rate by 80%;

Flexible scraping wall: Use a rotating scraper made of polytetrafluoroethylene to remove tube wall deposits in real time;

Case results: After Shanghai Electric applied it in the Zhejiang Energy Group project, the continuous operation cycle was extended from 90 days to 300 days.

2. Crystal quality improvement: from industrial grade to food grade

In order to open up the high-end market (such as medicine and food additives), three key technologies need to be broken through:

Mother liquor reuse: adopt multi-effect evaporation (MVR) system, the mother liquor reuse rate reaches 95%, avoiding impurity enrichment;

Dynamic washing: introduce 75℃ hot water into the centrifuge for countercurrent washing, and the Cl⁻ residual amount is reduced from 0.1% to 0.01%;

Data verification: The food-grade magnesium sulfate produced by Shandong Haihua Group has passed USP/EP certification, and the unit price has increased from 3,400 yuan/ton to 8,500 yuan/ton.

3. Intelligent control system

Online monitoring: Raman spectrometer is used to analyze the supersaturation of the solution in real time with an accuracy of ±0.5%;

AI optimization: Based on historical data, the neural network model is trained to automatically adjust the evaporation temperature, feed rate and other parameters;

Industry 4.0 practice: A digital twin system was introduced in a project of Guodian Group, and the energy consumption of the crystallization process was reduced by another 18%.

3. Economic Benefits and Market Application Spectrum

1. Cost-benefit model

Take a 600MW unit that processes 100,000 tons of SO₂ per year as an example:

Project Value Remarks

Magnesium sulfate output 150,000 tons/year Calculated as Mg/S=1.5

Construction investment 120 million yuan Including evaporation crystallization system, purification equipment, etc.

Operation cost 980 yuan/ton Including energy consumption, labor, maintenance

Average sales price 3,400 yuan/ton (industrial grade) Average food grade price 6,500 yuan/ton

Annual income 510 million yuan 150,000 tons × 3,400 yuan

Investment payback period 2.3 years Including 1 year of construction period

2. Diversified application scenarios

Agriculture: As a direct supply planting base for magnesium fertilizer, applying 20kg per mu can increase production by 10%-15%;

Building materials: Preparation of magnesium sulfate cement, with a 3-hour compressive strength of 30MPa, for prefabricated buildings;

Energy storage: Development of magnesium sulfate heptahydrate phase change material, with a heat storage density of 180kJ/kg, for valley electricity heat storage;

Medicine: Purified to 99.9% for the production of laxatives and anticonvulsants, with a gross profit margin of over 60%.

IV. Industry trends and policy dividends

1. Technology iteration direction

Zero emission process: Electrolysis of crystallization mother liquor to regenerate magnesium hydroxide desulfurizer to form a closed loop (pilot project recovery rate of 92%);

Low-carbon preparation: ｒｅｐｌａｃｅ coal-fired boilers with photovoltaic steam, and the carbon footprint of a single ton of product is reduced from 1.2kg CO₂ to 0.3kg.

2. Policy-driven

Financial subsidies: included in the "National Advanced Environmental Protection Technology Catalogue", equipment investment deduction of 15% corporate income tax;

Standard upgrade: the "Flue Gas Desulfurization Byproduct Magnesium Sulfate" group standard to be implemented in 2024 will clarify industrial/food grade technical indicators;

Export opportunities: The EU has included magnesium sulfate in the white list of organic fertilizer raw materials, exempting it from double anti-dumping tariffs.

From "pollution control cost" to "circular gold mine", the industrial breakthrough of magnesium sulfate crystallization process marks that magnesium hydroxide desulfurization technology has truly completed the closed loop of environmental protection and economy. With the maturity of technologies such as food-grade purification and energy storage modification, this track is bursting with amazing commercial potential-according to the Gaogong Industry Research Institute, the global desulfurization magnesium sulfate market size will exceed 22 billion yuan in 2025, and China's production capacity will account for more than 70%. In this industrial revolution of turning waste into treasure, those companies that first mastered the tricks of crystallization technology are seizing the high ground of value in the wave of green transformation.