Industrially, NaOH, Cl2 and H2 are produced by electrolytically saturating NaCl solution, and a series of chemical products, called chlor-alkali industry, are produced from them. The chlor-alkali industry is one of the most basic chemical industries. Its products are widely used in the light industry, textile industry, metallurgical industry, petrochemical industry and public utilities, in addition to the chemical industry itself.
The chlor-alkali industry is widely used, and titanium anodes and titanium cathodes are also required for electrolytically electrolyzing saturated NaCl solutions.
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There are mainly three kinds of process diaphragm methods, double electrolytic cell method, mercury electrolytic cell method.
Anode reaction: 2Cl–2e-=Cl2↑ (oxidation reaction)
H+ readily acquires electrons than Na+, and thus H+ continuously recovers electrons from the cathode to hydrogen atoms, and combines them into hydrogen molecules to be emitted from the cathode.
Cathodic reaction: 2H++2e-=H2↑ (reduction reaction)
In the above reaction, H+ is generated by the ionization of water. Since H+ continuously generates electrons on the cathode to generate H2, which destroys the ionization balance of nearby water, the water molecules continue to ionize H+ and OH-, and H+ is continuously obtained. The electrons become H2, and as a result, the concentration of OH- in the cathode region solution relatively increases, causing the phenolphthalein test solution to turn red. Therefore, the total reaction of electrolytically saturated brine can be expressed as:
This reaction principle is industrially used to produce caustic soda, chlorine and hydrogen.
In the above experiment of electrolytically saturated brine, a chemical reaction can occur between the electrolysis products, such as NaOH solution and Cl2 can react to form NaClO, H2 and Cl2, and the mixture can explode in case of fire. In industrial production, mixing of these products is avoided, and the reaction is often carried out in a special electrolytic cell. Production principle and process Industrial production of caustic soda, chlorine and hydrogen from electrolytic salt solution is one of the important basic chemical industries. China’s chlor-alkali industry mainly uses two production processes.
Diaphragm electrolysis separates the anode and cathode regions with a porous membrane, avoiding mixing of the bipolar products. The principle of diaphragm electrolysis is shown in Figure 2. Saturated brine is added from the anode region, and the alkali and undecomposed brine formed in the cathode region continuously flow out. By properly adjusting the brine flow rate, the liquid level in the anode region can be higher than the liquid level in the cathode region, thereby generating a certain static pressure difference, so that the anolyte flows through the diaphragm to the cathode chamber, and the flow direction is opposite to the anodic anode region of the cathode region. And the opposite direction of diffusion, thereby greatly reducing the amount of OH entering the anode region, inhibiting the occurrence of oxygen evolution reaction and other side reactions, and improving the anode efficiency to over 90%. However, the concentration of lye can be increased to 100-140 g/L due to the decrease of OH loss in the cathode region.
2. Double electrolysis cell method (Diaphragm cell)
This method is similar to the membrane method, but the reaction is divided into two electrolytic cells for reaction. This further avoids the reaction of chlorine and sodium hydroxide. The chlorine thus synthesized contains a small amount of oxygen.
3. Mercury Cell Process (Castner–Kellner process)
The sodium hydroxide obtained in the above electrolytic cell contains a trace amount of sodium chloride, and the mercury electrolytic cell method can synthesize sodium hydroxide completely free of sodium chloride. As shown, the mercury electrolytic cell method separates the two electrolytes from mercury. The anode was placed in a saturated saline solution and the cathode was placed in a sodium hydroxide solution. When energized, chloride ions are anodized:
Chemical production based on chlor-alkali industry
NaOH, Cl2 and H2 are important chemical production raw materials, which can be further processed into various chemical products and widely used in various industries. Therefore, the chlor-alkali industry and related products involve almost all areas of the national economy and people’s lives.
The catholyte flowing out of the electrolytic cell contains 30% NaOH, which is called liquid alkali, and the liquid alkali is evaporated and crystallized to obtain a solid base. Another product of the cathode zone, wet hydrogen, is cooled, washed, compressed and sent to a hydrogen storage tank. The liquid chlorine in the anode region is obtained by cooling, drying, purifying and compressing the wet chlorine gas.
2NaOH + CO2 = Na2CO3 (soda) + H2O
NaOH+CO2=NaHCO3 (sodium bicarbonate)
With the increasing awareness of environmental protection, more and more attention has been paid to the pollution caused by the chemical production process based on the chlor-alkali industry and the environmental impact of its products. For example, it has been identified that certain organochlorine solvents are carcinogenic and that HCFCs can destroy the ozone layer and so on, so measures to produce certain organochlorine products have ceased. While giving full play to the role of chlor-alkali industry and chlor-alkali-based chemical production in the development of the national economy, we should minimize its adverse impact on the environment.