Caustic soda is obtained from the electrolysis of salt. A concentrated solution of purified salt (NaCl) in demineralized water (i.e. brine) is decomposed in an electrolytic cell by the passage of an electrical current (DC). In the electrolytic cell, the sodium chloride solution is decomposed to chlorine at the anode and to a sodium hydroxide solution and hydrogen at the cathode.
AkzoNobel Industrial Chemicals utilizes two processes to produce caustic soda lye. These processes are:
In the membrane process the membrane divides the electrolysis cell in the two parts: the anode and cathode compartment. The anode and the cathode are separated by an ion-exchange membrane. Only sodium ions and a little water pass through the membrane. The impoverished brine is de-chlorinated and re-saturated with solid salt. After purification the brine is recirculated over the cell. The caustic soda lye leaves the cell with a 32% concentration, and can be concentrated to 50% in a later stage in the process.
In the amalgam process, sodium forms an amalgam with mercury at the cathode. The amalgam reacts with the demi-water in a separate reactor called the decomposer where hydrogen gas and caustic soda lye at 50% are produced. The impoverished brine is de-chlorinated and re-saturated with solid salt. After purification the brine is recirculated over the cell.