Over 95% of the capacity to produce chlorine and essentially 100% of the capacity to produce caustic soda (sodium hydroxide, NaOH) are based on the electrolysis of brine. In this process a sodium chloride (NaCl, salt) solution (brine) is electrolytically decomposed to elemental chlorine (in the anode compartment), and sodium hydroxide solution and elemental hydrogen (in the cathode compartment). A chemical facility which co-produces caustic soda and chlorine is typically referred to as a chlor-alkali (C/A) facility.
The overall reaction for the electrolytic production of caustic soda and chlorine is:
The electrolytic route to making caustic soda, by using sodium chloride as a raw material, is a co-product process, i.e., for each 1.1 pounds of sodium hydroxide produced, one pound of chlorine is also produced. This ratio is referred to as an electrochemical unit (ECU).
Dow utilizes two electrolytic processes to produce caustic soda solution and chlorine. These processes are:
- Diaphragm cell
- Membrane cell
Using diaphragm cell technology, chlorine, caustic soda and hydrogen are produced simultaneously. Saturated brine enters the anode compartment of the cell, where chlorine gas is liberated. The function of the diaphragm is to separate the brine from the caustic solution (called cell effluent) at the cathode side, which is also where hydrogen gas is released.
Membrane cell technology is a relatively recent development. It differs from diaphragm cell technology in that the solutions surrounding each electrode are separated by a membrane rather than a diaphragm. The membrane is very selective and primarily allows the migration of sodium ions from the anode chamber to the cathode chamber. Saturated brine enters the anode compartment of the cell where chlorine gas is liberated. Since only sodium ions can pass through the membrane to the cathode (brine cannot pass through the membrane), the caustic soda (cell effluent) contains substantially less sodium chloride. No salt removal capabilities are required as in the diaphragm cell process.