1. Overview of the core production process of the chlor-alkali industry
2. Principles and equipment of the ion membrane electrolysis process
3. History and limitations of the diaphragm method and mercury method
4. By-product treatment and resource recycling
5. Process optimization and energy-saving technology progress
6. Environmental challenges and clean production technology
1. Overview of Core Production Processes
Chlor-alkali plants produce caustic soda (NaOH), chlorine (Cl₂), and hydrogen (H₂) through the electrolysis of sodium chloride (NaCl) solution, a cornerstone of the basic chemical industry. Over 90% of global chlor-alkali capacity employs the ion-exchange membrane process, with the remaining using the phased-out diaphragm and mercury cell methods.
2. Principles and Equipment of the Ion-Exchange Membrane Process
Core Mechanism
The perfluorinated ion-exchange membranes, featuring a backbone of fluorocarbon chains with sulfonic acid functional groups, exhibit superior resistance to corrosion and chemical degradation, maintaining stable performance even in highly acidic (anode) and alkaline (cathode) environments. To further optimize membrane efficiency, the process incorporates advanced brine pretreatment systems, such as dual-stage filtration and ion chromatography, which reduce trace impurities like iron and silica to sub-ppb levels, thereby preventing membrane fouling and extending operational life by 20–30%. Additionally, the integrated design of the electrolysis system allows for precise regulation of the anode-cathode gap to less than 2 mm, minimizing ohmic resistance and further lowering energy consumption by an additional 5–8% compared to conventional designs. Finally, the process enables continuous production of high-purity caustic soda with a consistent sodium chloride content below 50 ppm, eliminating the need for downstream desalination steps and making it ideal for demanding applications in pharmaceuticals, electronics, and food processing industries.
Key Equipment
Electrolyzers: Classified into bipolar and monopolar types. Bipolar electrolyzers operate in series with high voltage but occupy less space, while monopolar ones run in parallel with high current requiring independent rectifiers. Modern "zero-gap" designs reduce electrode spacing to <1 mm for further energy savings.
Brine Purification Systems: Membrane-based sulfate removal (e.g., Ruipu Brine Refining System) and chelating resin adsorption reduce Ca²⁺ and Mg²⁺ to <1 ppm, extending membrane lifespan.
Chlorine and Hydrogen Treatment Units: Chlorine is cooled (12–15°C) and dried with 98% H₂SO₄ before compression for PVC production; hydrogen is cooled, compressed, and used for hydrochloric acid synthesis or as fuel.
3. Historical Context and Limitations of Diaphragm and Mercury Processes
The process principle and historical application of the diaphragm method
The diaphragm electrolyzer uses a porous asbestos diaphragm as a physical barrier between the anode and cathode chambers. The core principle is to use the pore size selectivity of the diaphragm (about 10~20 microns) to allow the electrolyte (NaCl solution) to pass through, while preventing the generated Cl₂ and H₂ gases from mixing. At the anode, Cl⁻ loses electrons to generate Cl₂ (2Cl⁻ - 2e⁻ → Cl₂↑); at the cathode, H₂O gains electrons to generate H₂ and OH⁻ (2H₂O + 2e⁻ → H₂↑ + 2OH⁻), and OH⁻ combines with Na⁺ to form NaOH. Because the asbestos diaphragm cannot completely block the reverse migration of Na⁺, the NaOH solution produced at the cathode contains about 1% NaCl, with a concentration of only 10~12%, and needs to be concentrated to more than 30% by evaporation to meet industrial needs. This process was widely used in the mid-to-late 20th century. China once relied on this technology to solve the problem of shortage of basic chemical raw materials, but with the improvement of environmental awareness, its inherent defects were gradually exposed.
Fatal defects and elimination process of the diaphragm method
The three core disadvantages of the diaphragm method eventually led to its comprehensive replacement:
High energy consumption and low efficiency: Due to the high resistance of the asbestos diaphragm, the cell voltage is as high as 3.5~4.5V, and the power consumption per ton of alkali is 3000~3500 kWh, which is 40~70% higher than the ion membrane method. It is only suitable for areas with low electricity prices;
Insufficient product purity: The dilute alkali solution containing NaCl needs additional evaporation and desalination, which increases the process cost and cannot meet the demand for high-purity NaOH in high-end fields (such as alumina dissolution);
Asbestos pollution crisis: Asbestos fibers are easily released into the air and wastewater during the production process. Long-term exposure leads to diseases such as lung cancer. The International Agency for Research on Cancer (IARC) listed it as a Class I carcinogen as early as 1987. In 2011, China revised the "Guidelines for Industrial Structure Adjustment", which clearly stated that all diaphragm caustic soda plants would be eliminated by 2015, with a total of more than 5 million tons/year of production capacity shut down.
Mercury electrolysis process: mercury toxicity hidden dangers behind high purity
Technical characteristics and historical value of the mercury method
The mercury method was once a "high-end process" for producing high-purity caustic soda due to the unique properties of the mercury cathode. Its principle is to use mercury as a mobile cathode. During the electrolysis process, Na⁺ and mercury form sodium amalgam (Na-Hg alloy), and then the sodium amalgam reacts with water to generate 50% high-concentration NaOH (Na-Hg + H₂O → NaOH + H₂↑ + Hg), which can be used directly without evaporation and concentration. The significant advantage of this process is that the output NaOH is extremely pure (NaCl content <0.001%), which is particularly suitable for industries such as pharmaceuticals and chemical fibers that have strict requirements on alkali purity. In the middle of the 20th century, this process was widely adopted in Europe, America, Japan and other countries. The Japanese chlor-alkali industry once relied on the mercury method to occupy 40% of the global high-end caustic soda market.
Mercury pollution disaster and global ban process
The fatal flaw of the mercury method is the irreversible pollution of mercury:
Mercury vapor volatilization: Mercury escapes in the form of vapor during electrolysis, and the mercury concentration in the working environment often exceeds the standard by dozens of times, resulting in frequent mercury poisoning incidents among workers (such as the Minamata disease incident in Japan in 1956, which was caused by mercury pollution);
Wastewater discharge hazards: About 10-20 grams of mercury is lost for every ton of NaOH produced, which is converted into methylmercury after entering the water body, and enriched through the food chain to harm the ecosystem;
Difficulty in recycling: Although mercury can be recovered by distillation, long-term operation still leads to excessive mercury content in the soil, and the cost of remediation is high. With the entry into force of the Minamata Convention (2013), more than 90% of countries in the world have pledged to phase out the mercury method by 2020. As the world's largest chlor-alkali producer, China completely banned the mercury process in 2017, completely cutting off the "mercury-caustic soda" pollution chain and promoting the industry's transformation to a single process of ion membrane. Today, only a few countries such as India and Pakistan still retain less than 5% of mercury production capacity and face severe international environmental pressure.
4. By-Product Management and Resource Recycling
High-Value Utilization of Chlorine
Basic Chemicals: Used in PVC production (30–40% of chlorine demand) and propylene oxide synthesis.
High-End Applications: Electronic-grade chlorine (≥99.999% purity) for semiconductor etching commands 5–8 times the price of industrial-grade chlorine.
Emergency Treatment: Accidental Cl₂ is absorbed in a two-stage NaOH scrubber (15–20% concentration), ensuring emissions <1 mg/m³.
Hydrogen Recovery and Utilization
Hydrochloric Acid Synthesis: Reacted with Cl₂ to produce HCl for pickling and pharmaceuticals.
Green Energy: Purified hydrogen fuels fuel cells or ammonia synthesis, with one plant reducing carbon footprint by 60% through hydrogen integration.
Safety Control: Hydrogen pipelines incorporate flame arresters and pressure relief devices, with real-time H₂/Cl₂ purity monitoring to prevent explosions.
5. Process Optimization and Energy-Saving Technologies
Oxygen Cathode Technology
Principle: Replacing hydrogen evolution with oxygen reduction lowers cell voltage by 0.8–1.0 V, reducing energy consumption to <1500 kWh/ton NaOH while co-producing hydrogen peroxide (H₂O₂).
Application: Beijing University of Chemical Technology's 50,000-ton/year plant achieved 30% power savings.
High-Current-Density Electrolyzers
Advancement: Increasing current density from 4 kA/m² to 6 kA/m² boosts capacity by 30%, commercialized by Asahi Kasei (Japan) and ThyssenKrupp (Germany).
Digital Transformation
Intelligent Control Systems: AI algorithms optimize current efficiency to >96% and predict membrane lifespan with <5% error, reducing costs by ¥80/ton at one plant.
AI-Powered Inspection: Hangzhou-based chemical plants use AI-equipped robots to inspect chlorine facilities, achieving 99.99% accuracy in detecting Teflon tube blockages.
6. Environmental Challenges and Clean Production Technologies
Wastewater Treatment
Dechlorination: Vacuum dechlorination (residual Cl₂ <50 ppm) and ion exchange recover NaCl with >95% reuse.
Zero Liquid Discharge (ZLD): Multi-effect evaporation (MVR) crystallizes industrial salt, implemented in Xinjiang and Shandong.
Exhaust Gas Treatment
Sulfuric Acid Mist Control: Electrostatic precipitators (>99% efficiency) and wet scrubbing meet GB 16297-2025 emission standards.
Mercury Pollution Prevention: Low-mercury catalysts are promoted, with Yunnan Salt and Haohua Yuhang receiving state funding for mercury-free catalyst R&D.
Solid Waste Management
Membrane Recycling: Closed-loop recovery of precious metals (titanium, ruthenium) achieves >98% efficiency.
Salt Sludge Utilization: Used in construction materials or landfill covers, with 100% comprehensive utilization of carbide slag.
