Producing Chlorine

Salt (sodium chloride) obtained from dried-up prehistoric seas is dissolved in water to form a solution called brine. This solution is placed in a cell and a current of electricity is passed through it. Chlorine gas bubbles off in one part of the cell and sodium metal is produced in the other. The sodium reacts with water to form caustic soda (sodium hydroxide) and hydrogen gas. Both of which have important commercial uses.

Generation of chlorine gas involves the liquid metal mercury (compounds of which are toxic) and can cause disastrous effects on the environment. One such example was in 1950s in Japan at Minamata Bay where mercury escaped and contaminated fish and entered the food chain which led to the death of many local people. Industrial plants therefore take the utmost care in preventing mercury from escaping however there is always some mercury lost which is why a new method for making chlorine is now used. This new method involves an asbestos diaphragm in a cell which is porous and allows an electric current to flow as well as resisting corrosion from chlorine and caustic soda. This method is safer as no asbestos is lost and when the diaphragm is replaced it can be disposed of safely and easily. However, one downfall of this method is that it forms a more dilute solution of caustic soda so requires steam heating to remove excess water and make it more concentrated before it can be sold on for commercial use.

The manufacture of PVC accounts for 30% of the chlorine produced industrially. The presence of chlorine makes PVC compatible with a wide range of other materials making PVC very versatile. Also, chlorine makes PVC flame retardant and allows PVC to be distinguished when sorting plastics for recycling. However chlorine itself is highly corrosive and is a lethal gas. It is dangerous to handle and people have died in industrial incidents involving chlorine. Rigorous safety measures are therefore taken where chlorine is concerned including in the transportation of this chemical.