Chloralkali process
Overview
The Chloralkali process is an industrial process for the electrolysis of sodium chloride solutions. It is primarily used to produce chlorine, hydrogen, and sodium hydroxide (caustic soda) at large scale. The process is named after the products – chlor for chlorine, alkali for the sodium hydroxide. These chemicals are fundamental building blocks in a vast range of industrial and consumer products.
History
The chloralkali process has been in use since the late 19th century. Before the advent of this process, chlorine and sodium hydroxide were produced using the Leblanc process, which was much less efficient and produced a significant amount of waste.
Process
The chloralkali process involves the electrolysis of a sodium chloride solution (brine). The overall reaction can be written as:
2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH
This reaction is carried out in a cell containing an anode and a cathode. The anode is often made of titanium coated with a thin layer of ruthenium and iridium, while the cathode is usually made of steel. The sodium chloride solution is introduced into the cell, and a direct current is passed through the solution.
At the anode, chloride ions are oxidized to chlorine gas:
2 Cl− → Cl2 + 2 e−
At the cathode, water is reduced to hydrogen gas and hydroxide ions:
2 H2O + 2 e− → H2 + 2 OH−
The sodium ions in the solution combine with the hydroxide ions to form sodium hydroxide.
Types of Cells
There are three main types of cells used in the chloralkali process: mercury cells, diaphragm cells, and membrane cells. Each type of cell has its own advantages and disadvantages, and the choice of cell type depends on a variety of factors, including the desired product purity and the specific requirements of the process.
Mercury Cells
Mercury cells were the first type of cell used in the chloralkali process. In a mercury cell, the cathode is a pool of mercury. The sodium formed at the cathode amalgamates with the mercury, and this sodium-mercury amalgam reacts with water to produce sodium hydroxide and hydrogen. Mercury cells produce a very pure sodium hydroxide, but they have been largely phased out due to environmental concerns about mercury emissions.
Diaphragm Cells
Diaphragm cells use a porous diaphragm, often made of asbestos, to separate the anode and cathode compartments. The sodium hydroxide produced in the cathode compartment is contaminated with sodium chloride and sodium chlorate, so it must be concentrated and purified before it can be used. Diaphragm cells are less energy-efficient than membrane cells, but they are more tolerant of impurities in the brine.
Membrane Cells
Membrane cells use a selective ion-exchange membrane to separate the anode and cathode compartments. The membrane allows sodium ions to pass through, but it prevents hydroxide ions and chloride ions from crossing. This results in a very pure sodium hydroxide, but the membrane is sensitive to impurities in the brine. Membrane cells are the most energy-efficient type of cell, and they are currently the most commonly used type of cell in the chloralkali process.
Applications
Chlorine and sodium hydroxide produced by the chloralkali process are used in a wide variety of applications. Chlorine is used in the production of polyvinyl chloride, disinfectants, and insecticides, among other things. Sodium hydroxide is used in the manufacture of paper, textiles, and detergents, as well as in water treatment.
Environmental Impact
The chloralkali process has a significant environmental impact. The production of chlorine gas is a major source of chlorinated organic pollutants, including dioxins. Mercury and asbestos used in some types of cells can also be a source of environmental contamination. In addition, the process is energy-intensive, contributing to carbon dioxide emissions.
