Case of Japan's JFE Steel: 30-year application history of magnesium hydroxide desulfurization in sintering flue gas

I. Background and challenges of technical application

As a pioneer in global steel environmental protection technology, Japan's JFE Steel (formerly NKK) has faced severe challenges in sintering flue gas treatment since the 1990s. The flue gas generated by the sintering process has the characteristics of large fluctuations in SO₂ concentration (300-1000mg/m³), high dust content (＞200mg/m³), and strong Cl⁻ corrosiveness (＞500mg/m³). In the early 1990s, Keihin Steel used ammonia-ammonium sulfate desulfurization. Although the desulfurization efficiency was stable at more than 95%24, the equipment corrosion problem was prominent - 316L stainless steel pipelines had pitting perforation after 3 years of operation, and the annual maintenance cost exceeded 8 million yen.

In 1995, JFE began to explore magnesium hydroxide wet desulfurization technology. Compared with the traditional limestone method, the reaction activity of magnesium hydroxide is increased by 40%, and the generated magnesium sulfate crystal particles are finer (average particle size <50μm), which greatly reduces the risk of pipeline wear. However, in the initial application, slurry pipelines still face three threats:

Chemical corrosion: The pH value of the slurry fluctuates between 5.5-6.5, and the Cl⁻ concentration is as high as 20,000ppm, resulting in an average annual corrosion rate of 1.2mm for the carbon steel matrix;

Crystallization blockage: Magnesium sulfate is deposited in the low flow rate area (<1m/s), and the pipe diameter is reduced by 30% within 6 months;

Cavitation damage: When the outlet flow rate of the circulating pump is >5m/s, the rubber lining will peel off78.

2. Material iteration and anti-corrosion technology evolution

Phase 1 (1995-2005): Rubber-lined carbon steel pipe dominant period

JFE initially adopted Q235 carbon steel + butyl rubber lining (thickness 5mm) solution, which was put into use in the No. 3 sintering machine of Chiba Plant:

Advantages: Resistant to Cl⁻ corrosion (＞50000ppm), excellent wear resistance (Rockwell hardness ≥85), and the overall cost is only 1/3 of stainless steel;

Disadvantages: The rubber layer ages faster at temperatures ＞120℃, and the average annual breakage rate at the flange joint is 15%. In the 2001 renovation, honeycomb fiberglass reinforced flanges were introduced to reduce the breakage rate to 5%9.

The second stage (2006-2015): FRP pipes and super duplex steel in parallel

For high dust content flue gas sections (such as the absorption tower entrance), JFE uses epoxy vinyl ester FRP pipes:

Fluid optimization: The inner wall roughness is 0.01mm, which reduces the pressure drop by 25% compared with the rubber lined pipe. The measured flow rate of the No. 3 unit of Kimitsu Plant has increased to 3.5m/s;

Anti-blocking design: The pipe diameter adopts a gradient change from coarse to fine (DN400→DN300), combined with a 0.8% slope design, which reduces the amount of crystal deposition by 60%10.

In high-pressure areas such as the oxidation fan outlet, super duplex steel 2507 (PREN value>40) began to ｒｅｐｌａｃｅ traditional materials. The transformation data of the No. 2 unit of the Oita Plant showed that the corrosion rate of the duplex steel pipeline in a Cl⁻ concentration of 30000ppm was <0.05mm/a7 within a 10-year maintenance-free period.

The third stage (2016 to present): Intelligent monitoring and material composite

In 2018, the Fushan plant introduced silicon carbide coated fiberglass pipes:

Improved wear resistance: surface hardness reaches 2200HV, which is 3 times longer than the life of traditional materials;

Intelligent operation and maintenance: embedded fiber optic sensors monitor wall thickness in real time, and AI models warn high-risk pipe sections 14 days in advance, shortening maintenance response time by 70%11.

III. Process package innovation and energy efficiency breakthrough

JFE's magnesium hydroxide desulfurization system integrates four core modules:

Gradient oxidation tower design:

The first tower strengthens absorption (pH 6.0-6.5) and the second tower deeply oxidizes (aeration volume 0.8m³/min·m³ slurry), which increases the oxidation rate of magnesium sulfite from 75% to 98%. The actual measurement of the Nagoya plant shows that the purity of magnesium sulfate reaches 99.5%, which can be directly sold as agricultural fertilizer46.

Thermal energy synergy network:

Using sintering waste heat (140-160℃) to drive the triple-effect evaporator, the steam consumption per ton of magnesium sulfate crystallization is reduced from 1.2 tons to 0.6 tons, and the annual energy saving benefits exceed 200 million yen5.

Anti-blocking spray system:

The swirl atomizing nozzle (atomizing particle size ≤80μm) and the 120° coverage angle design increase the contact area between droplets and flue gas by 40%, and the system pressure loss is controlled within 800Pa9.

Digital twin operation and maintenance:

Based on CFD simulation of the flow field distribution in the tower, the spray layer opening is dynamically adjusted. After the application of Keihin Plant, the fluctuation range of desulfurization efficiency has narrowed from ±15% to ±3%211.

4. Economic Benefits and Industry Impact

In the past 30 years, JFE has transformed 17 sintering machine desulfurization systems, 9 of which use magnesium hydroxide technology:

Operation data: Under the condition of inlet SO₂ concentration of 800mg/m³, the outlet is stable <50mg/m³, and the consumption of magnesium hydroxide is 0.85kg/kg SO₂;

By-product income: the annual output of magnesium sulfate exceeds 300,000 tons, and the product with purity above 99.2% accounts for 80%, and the sales volume in 2024 will exceed 12 billion yen;

Carbon asset value-added: the carbon footprint of the process package is 55% lower than that of the limestone method, and the annual CCER income is about 1.5 billion yen46.

This technology has been exported to South Korea's POSCO, China's Baosteel and other companies. In 2023, a steel plant in Tangshan introduced JFE's fourth-generation technology, achieving a desulfurization efficiency of 98% in an environment with a Cl⁻ concentration of 50,000ppm, and the equipment investment recovery period was shortened to 2.8 years10.

V. Future Technology Direction

Self-repairing materials: Microencapsulated rubber lining can automatically repair scratches, and tests show that it can extend the life of the pipeline by 40%;

Hydrogen energy drive: electrolysis of magnesium sulfate solution to produce green hydrogen (62m³ H₂ per ton of byproduct), building a desulfurization-energy storage integrated network;

Biological anti-scaling: polysaccharide scale inhibitors are extracted from deep-sea microorganisms, and the equipment cleaning cycle is extended to 18 months in the pilot test of the Fushan plant711.

When the first generation of rubber-lined pipelines were tested in the acid mist of the sintering machine in the Chiba plant, and when smart sensors began to capture nano-level corrosion precursors, JFE's 30 years of exploration has gone beyond simple environmental governance and evolved into a symphony of materials science, fluid mechanics and digital technology. From Keihin to Fukuyama, the material upgrade of each section of the pipeline is a precise response to the corrosion mechanism, and every gram of regenerated magnesium sulfate is writing a new paradigm of circular economy. Under the wave of carbon neutrality, this continuously iterative technology philosophy may be the key to breaking through the traditional manufacturing industry.