Caustic soda is usually obtained from the electrolysis of sodium chloride brine, by the chlor-alkali process (*). So, as a first step, purified concentrated brine should be obtained from sea water. Many references to the related processes are found in the chemical engineering literature; e.g.: Vollrath Hopp, Ingo Hennig, "Handbook of Applied Chemistry", MacGraw-Hill, 1983.  

(*) https://en.wikipedia.org/wiki/Chloralkali_process

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