Sodium Hydroxide from Sodium Chloride (HCl as Co-Product)

The process under analysis comprises four major sections: (1) Brine purification; (2) Electrolysis; (3) Sodium Hydroxide recovery; and (4) Hydrochloric acid production.

Brine purification. Initially, recycled depleted brine is resaturated with fresh sodium chloride. The brine is treated with precipitants, in such a way the heavy metals (harmful to the membranes) precipitate forming a sludge, which is removed by settling in a clarifier. Subsequently, the clarified solution is filtered and then purified by ion exchange resins to remove residual hardness until acceptable levels are achieved.

Electrolysis. 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.

Sodium Hydroxide recovery. The Caustic Soda solution is concentrated to a saturated 50% NaOH solution, the commercialized form, before being sent to storage. The evaporation of the Caustic Soda solution is carried out in a triple effect evaporator, having nickel as the material of construction.

Hydrochloric acid production. Chlorine and hydrogen are reacted to produce hydrogen chloride. A slight excess of hydrogen is used to assure a chlorine-free product (an additional amount of hydrogen is purchased from an external supplier). The reaction takes place in a combustion chamber. The hydrogen chloride formed is fed to a falling film absorber where it is absorbed in a weak acid stream to produce 35 wt% hydrochloric acid. The gas from the absorber is sent to a tails tower, where the remaining hydrogen chloride is absorbed in demineralized water, forming a weak acid stream, which is recycled to the absorber.