Sodium Hydroxide Production from Sodium Chloride (Membrane Process)

The process under analysis comprises three major sections: (1) Brine purification; (2) Electrolysis; and (3) Products Recovery.

Brine purification. Initially, recycled depleted brine is mixed with water and resaturated with fresh NaCl salt (sodium chloride). Heavy metal ions (e.g. Ca2+ and Mg2+) present in the brine would be harmful to the membranes. So the brine is treated with precipitants, in such a way the heavy metals precipitate forming a sludge, which is removed by settling in a clarifier. Subsequently, the clarified brine is filtered and then purified by ion exchange resins to remove residual hardness and achieve acceptable levels.

Electrolysis. The ultrapure brine and electricity are the main inputs to the electrolysis area. The brine is fed into the anolyte compartments of the cells, separated from the catholyte by cation-exchange membranes. Chlorine gas is generated at the anodes and sodium ions migrate through the membranes into the catholytes. The depleted brine from the anode compartments is dechlorinated downstream and then returned to brine saturation. On the catholyte side, water is electrolyzed generating hydrogen gas and hydroxyl ions. The membranes prevent the migration of such hydroxyl ions into the anolytes, in such a way that hydroxyl ions combine with the sodium ions forming caustic soda. The addition of demineralized water keeps the catholytes concentration at the desired level.

Products recovery. Hydrogen from electrolysis is compressed and directed to consumers. The caustic soda solution is concentrated to a saturated 50% NaOH solution – the commercialized form. The chlorine gas produced is sent to a drying system, consisting of drying towers where concentrated sulfuric acid circulates as dehydrating agent. The dry chlorine gas is compressed and then liquefied before being sent to storage.