Manufacture of Sodium hydroxide by Nelson's process
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A solution of sodium chloride in water contains Na+, Cl-, H+, and OH- ions because of the dissociation of sodium chloride and water:
During electrolysis, Na+ and H+ ions move toward the cathode, but H+ ions are discharged more easily than Na+ ions at the cathode, so the hydrogen gas is liberated. Similarly, Cl- ions are oxidized at the anode and liberated as chlorine, so the solution becomes richer in Na+ and OH- ions.
Process: A perforated U-shaped steel tube is suspended in a rectangular steel tank to act as a cathode. The steel tube is lined with asbestos inside to act as a diaphragm and prevent the reaction between the products formed during electrolysis. A saturated solution of sodium chloride in water (brine) is placed in the cell and a carbon is dipped in the solution to act as an anode.
The solution becomes richer in sodium hydroxide, which collects in the catch basin at the bottom of the tank. Chlorine gas is evacuated through an outlet at the top. Steam is passed into the tank through an inlet to keep the electrolyte warm and keep the perforations clear. Sodium hydroxide solution is then evaporated to form liquid solution. It is solidified in the form of pellets or flakes which are 98% sodium hydroxide. The pellets or flakes may be further purified by dissolution in alcohol. The impurities like NaCl and Na2SO4 are not soluble and are removed by filtration. The filtrate is evaporated to remove alcohol. The purification yields lye that ranges from 99-100% pure sodium hydroxide; however, prolonged contact with air allows the formation of sodium carbonate and decreases the purity of the sodium hydroxide.[1]
[edit] References
- Safety Precautions for Lye (Sodium Hydroxide) (HTML). Certified Lye.
