D-Electrolysis of NaCl
(click on the titles to see a sketch of each process)
a-Membrane electrolysis cell
An ion exchange membrane separates anode and cathode. Chlorine forms at the anode, but only sodium migrates through to cathode compartment. The membrane is not permeable to liquid/gas flow and only permits migration of cations. There are no known environmental problems associated with the use of the ion exchange membrane. It is also effective at keeping the products separate, leading to high quality products. Membrane cells consume the least energy per ton of chlorine produced. Click on the following link to see the electrochemical reactions taking place in such cells.
b-Diaphragm electrolysis cell
The diaphragm cell process uses an asbestos(nowadays replaced by a polymer) diaphragm deposited on an iron grid to prevent the chlorine and sodium hydroxide, which are formed at the anode and cathode respectively, from re-mixing. Anodes are titanium coated, and cathodes are made from steel. Hydrogen is liberated in the cathodic compartment with the formation of sodium hydroxide, which is present as a 10 to 12% solution and therefore requires further concentration to become marketable.
The energy consumption of diaphragm cells is more important than that of the membrane cell. Click on the following link to see the electrochemical reactions taking place in such cells.
c-Mercury electrolysis cell
The cell has titanium anodes located above a mercury cathode which flows along the bottom of the cell. Under the action of a direct electrical current on an aqueous solution of NaCl, Cl2 is liberated at the anode and the cation (Na+) dissolves into the mercury cathode to form an amalgam. This amalgam is passed into a separate reactor, where it reacts with water to produce hydrogen and a 50% NaOH solution, so regenerating the mercury which is returned to the electrolytic cell. The depleted brine is regenerated and recirculated. Mercury cells consume the largest quantities of energy.
In all three cells the operating temperatures are around 85ºC. Click on the following link to see the electrochemical reactions taking place in mercury cells.