Chloralkali process
A chloralkali process always implies the electrolysis of common salt or sodium chloride. Depending on the method several products beside hydrogen can be produced. If the products are separated chlorine and sodium hydroxide are the products while by mixing sodium hypochlorite or sodium chlorate are produced depending on the temperature. Higher temperatures are needed for the production of sodium chlorate instead of sodium hypochlorite. Industrial scale production began in 1892.
Using calcium chloride and potassium chloride the products contain calcium or potassium instead of sodium.
The process has a high energy consumption, for example over 4 billion kWh per year in West Germany 1985, and produces equal (molar) amounts of chlorine and sodium hydroxide, which makes it necessary to find a use for the product with less request, usually the chlorine.
Procedures
There are two production methods in use. While the mercury cell method produces chlorine free sodium hydroxide the use of several tonnes of mercury leads to environmental problems, although in normal production cycle the emission of mercury is low. The membrane and diaphragm method use no mercury, but the sodium hydroxide contains chlorine which has to be removed.
Membrane cell
The most common chloralkali process involves the electrolysis of aqueous sodium chloride (a brine) in a membrane cell.
Saturated brine is passed into the first chamber of the cell where the chloride ions are oxidised at the anode to chlorine:
- 2Cl– → Cl
2 + 2e–
At the cathode hydrogen in the water is reduced to hydrogen gas, releasing hydroxide ions into the solution:
- 2H
2O + 2e– → H2 + 2OH–
The non-permeable ion exchange membrane at the center of the cell allows the sodium ions (Na+) to pass to the second chamber where they react with the hydroxide ions to produce caustic soda (NaOH). The overall reaction for the electrolysis of brine is thus:
- 2NaCl + 2H
2O → Cl
2 + H
2 + 2NaOH
A membrane cell is used to prevent the reaction between the chlorine and hydroxide ions. If this reaction were to occur the chlorine would be disproportionated to form chlorine and hypochlorite ions:
- Cl
2 + 2OH– → Cl– + ClO– + H
2O
At higher temperatures, 333K, chlorate can be formed:
- 3Cl
2 + 6OH– → 5Cl– + ClO
3– + H
2O
Because of the corrosive nature of the chlorine produced, the anode has to be made from a non-reactive metal such as titanium, whereas the cathode can be made from steel.
In the membrane cell, the anode and cathode are separated by an ion-permeable membrane. Saturated brine is fed to the compartment with the anode (the anolyte). A DC current is passed through the cell and the NaCl splits into its constituent components. The membrane passes Na+ ions to the cathode compartment (catholyte), where it forms sodium hydroxide in solution. The chloride ions are oxidised to chlorine gas at the anode, which is collected, purified and stored. Hydrogen gas and Hydroxide ions are formed at the cathode.
Diaphragm cell
In the diaphragm cell process, there are two compartments separated by a permeable diaphragm, often made of asbestos fibers. Brine is introduced into the anode compartment and flows into the cathode compartment. Similarly to the Membrane Cell, chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. The diaphragm prevents the reaction of the caustic soda with the chlorine. A diluted caustic brine leaves the cell. The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of caustic soda. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.
Mercury cell
In the mercury-cell process, also known as the Castner-Kellner process, a saturated brine solution floats on top of the cathode which is a thin layer of mercury. Chlorine is produced at the anode, and sodium is produced at the cathode where it forms a sodium-mercury amalgam with the mercury. The amalgam is continuously drawn out of the cell and reacted with water which decomposes the amalgam into sodium hydroxide and mercury. The mercury is recycled into the electrolytic cell. Mercury cells are being phased out due to concerns about mercury poisoning from mercury cell pollution such as occurred in Ontario Minamata disease.
Operator associations
The interests of chloralkali process plant operators are represented at regional, national and international levels by associations such as Euro Chlor and The World Chlorine Council.