Electrolysis

In industry, there are two methods for producing caustic soda: electrolysis and causticization. The electrolytic process is further divided into three types: ion-exchange membrane electrolysis, diaphragm electrolysis, and mercury electrolysis.
Electrolytic process: This method produces caustic soda via an electrochemical reaction using a sodium chloride solution as the feedstock, with the principal reaction as follows:
On the anode side　　　2Cl⁻-2e⁻→Cl₂↑
On the cathode side　　2H₂O+2e⁻→2OH⁻+H₂↑
Overall reaction　　　　2NaCl+2H₂O→2NaOH+H₂↑+Cl₂↑
Causticization process: This method produces caustic soda via a causticization reaction using sodium carbonate solution and lime as raw materials, with the principal reaction as follows:

Na₂CO₃+Ca(OH)₂→2NaOH+CaCO₃↓

The causticization process was once one of the primary methods for caustic soda production. However, due to its relatively low product purity and lower economic efficiency compared to the modern electrolytic process, it has been completely phased out. Therefore, the following sections will focus on the electrolytic process.

1. Ion-exchange membrane electrolysis
Ion-exchange membrane electrolysis is a process developed in the 1970s. In this method, a cation-exchange membrane is used to divide the electrolytic cell into an anode compartment and a cathode compartment. The cation-exchange membrane allows Na⁺ ions to pass through while preventing Cl⁻ and OH⁻ ions from passing, thereby separating the electrolytic products, as shown in the figure below:

Main Advantages of Ion-Exchange Membrane Electrolyzers:
A. Significantly reduced energy consumption: This is the most critical advantage of this technology. Its DC power consumption is approximately 1950 kWh/t, with the overall energy consumption being about 25% lower than that of the diaphragm  electrolysis and about 15% lower than that of the mercury electrolysis. This is primarily attributed to the low resistance and high current efficiency of the ion-exchange membrane.
B. High product purity: The ion-exchange membrane effectively prevents chloride ions (Cl⁻) from passing through, resulting in high-purity caustic soda with extremely low salt content, which can be directly used in high-end applications. Meanwhile, the purity of the by-product chlorine gas can reach 98.5% – 99%, and the hydrogen gas purity can reach 99%.
C. Environmentally friendly: This process completely eliminates the environmental issues associated with the mercury process (mercury pollution) and the diaphragm process (asbestos contamination).
D. Excellent overall economic benefits: 
This technology offers significant long-term operational advantages through its high efficiency and compact design, delivering overall economic performance that is markedly superior to that of the diaphragm and mercury processes. The overall operating costs are approximately 25% lower than those of the diaphragm process and 10% lower than those of the mercury process, while production costs also remain competitive.
E. High degree of automation and stable operation: The entire process flow is easily amenable to a high level of automation, ensuring stable operation and convenient management.
Main Disadvantages of Ion-Exchange Membrane Electrolyzers:
A. High initial investment and maintenance costs: 
The ion-exchange membrane, as the core component, is expensive and consumable in nature. During operation, it gradually ages and its performance deteriorates, requiring regular replacement, which leads to high maintenance costs. At the same time, in order to protect the membrane and ensure its long-term stable operation, a more complex brine purification system must be installed, which further increases the initial capital investment.
B. Extremely high operational requirements: The ion-exchange membrane imposes stringent quality requirements on the feed brine. The sum of calcium and magnesium ion content is less than 20ppb. Otherwise, membrane performance will rapidly deteriorate. Furthermore, any drastic fluctuations in pressure or temperature during operation can damage the membrane.
C.Potential full-line shutdown for maintenance: In some electrolyzer designs, if a single unit cell malfunctions, the entire system may need to be shut down for repair, which significantly impacts production and increases costs.
On the whole, the advantages of ion-exchange membrane electrolyzers far outweigh their disadvantages. By improving product quality, reducing energy consumption, and enabling cleaner production, this technology delivers substantial comprehensive benefits. The ion-exchange membrane process has become the inevitable choice for the global chlor-alkali industry.

2. Diaphragm Electrolysis
The diaphragm electrolysis process involves placing a diaphragm between the anode and the cathode to separate the anodic and cathodic products. The diaphragm is a porous permeable barrier that does not impede ion migration or current passage, allowing ions to flow toward the cathode at a certain rate, while preventing OH⁻ from diffusing to the anode, thereby keeping the anodic and cathodic products from mixing.

Advantages of Diaphragm Electrolyzers:
A. Relatively low requirements for brine purity: The purity requirements for the feed brine in diaphragm electrolyzers are less stringent compared to those of the ion-exchange membrane process.
B. Relatively mature technology and extensive operational experience: As a traditional process that has been applied for a long time, it has accumulated a wealth of operational and management expertise.
C. Relatively low capital investment: Compared with the ion-exchange membrane process, the cost of diaphragm electrolyzer is lower.
Disadvantages of Diaphragm Electrolyzers:
A. Poor product quality and high salt content: This is the most prominent drawback of the diaphragm process. The caustic liquor produced contains 2% to 3% sodium chloride (NaCl), which cannot meet the requirements of industries such as rayon and synthetic fibers that demand high-purity caustic soda.
B. High energy consumption and poor economic efficiency: The overall energy consumption (including electricity and steam) of the diaphragm process is the highest among the chlor-alkali processes. Its DC power consumption per ton of caustic soda typically exceeds 2,400 kWh, which is considerably higher than that of the ion-exchange membrane process.
C. Short service life of the core diaphragm component, affecting operation cycles: The asbestos diaphragm, as the core component, typically has a service life of only about 4 months. This results in short operating cycles of the electrolyzer, requiring frequent shutdowns for diaphragm replacement, which increases maintenance costs and worker labor intensity.
D. Safety risk: Damage or poor performance of the diaphragm may lead to mixing of hydrogen and chlorine gas. When the hydrogen content in the gas mixture exceeds 2%, it poses a serious safety risk.
E. Environmental pollution: The traditional asbestos diaphragm material itself poses environmental pollution and health hazards to workers.
Based on the above comparison of advantages and disadvantages, the diaphragm electrolysis process has gradually withdrawn from the historical stage and has been replaced worldwide by the ion-exchange membrane electrolysis process.
3. Mercury Electrolysis
The mercury electrolysis process was first industrialized in 1897 and once held a significant position in the global chlor-alkali industry.
The core of the mercury electrolysis process lies in the use of liquid mercury as the cathode. During electrolysis, sodium ions (Na⁺) in the brine gain electrons at the mercury cathode surface to form sodium amalgam (an alloy of sodium and mercury).
The mercury electrolytic cell consists of three components: the electrolyzer, the decomposer, and the mercury pump, forming two circulation loops for mercury and brine. In the electrolyzer, brine generates chlorine gas, while sodium ions form sodium amalgam with mercury and are carried into the decomposer along with the mercury. In the decomposer, metallic sodium in the sodium amalgam reacts with water to produce high-purity caustic soda and hydrogen gas. The mercury is then recovered and recycled. The principal reactions are as follows:
Reaction in the electrolyzer:

2Cl⁻-2e⁻→Cl₂↑
Na⁺+e⁻+Hg→Na(Hg)

Reaction in the decomposer:

2Na(Hg)+2H₂O→2NaOH+H₂↑+2Hg

Overall reaction:

2NaCl+2H₂O→2NaOH+H₂↑+Cl₂↑

Core Advantage: High-Purity Products
A. Extremely high-quality caustic soda product: This is the most prominent advantage of the mercury process. Since caustic soda is not directly generated in the electrolytic cell, the product contains virtually no sodium chloride (NaCl) and is of extremely high purity, with NaCl content below 50 ppm. This ultra-high-purity caustic soda is particularly suitable for industries such as rayon manufacturing that require exceptionally high caustic purity.
B. High current efficiency: The mercury process exhibits relatively high current efficiency.
C. Mature technology: As one of the earliest chlor-alkali technologies to be industrialized, it has accumulated extensive operational experience.
Fatal Drawback: Mercury Pollution and Environmental Costs.
A. Severe environmental pollution: This is the fundamental reason for the phase-out of the mercury process. Mercury is a highly toxic substance that readily volatilizes and leaks during the process. Even under normal operation, hundreds of kilograms of mercury are released into the environment each year. The resulting mercury-containing wastewater, exhaust gases, and solid waste pose long-term and severe ecological and health risks.
B. High energy consumption: The overall energy consumption of the mercury process is relatively high.
C. Safety risks: Improper operation may lead to "cell explosion" accidents.
Based on the above advantages and disadvantages, and with the growing awareness of environmental protection, the severe mercury pollution problem has led to its gradual phase-out. China has gradually replaced the mercury process with the ion-exchange membrane process since the 1980s，while the European Union officially banned the use of mercury in the chlor-alkali industry effective December 11, 2017.
There are no new chlor-alkali plants worldwide that have adopted this scheme.