Sodium Hydroxide Production from Lime and Sodium Carbonate

The process under analysis comprises three major sections: (1) Causticization; (2) Sodium Hydroxide Purification; and (3) Calcium Carbonate Separation.

Causticization. Initially, soda ash is conveyed to a dissolution tank, also fed with a wash liquor recovered from a thickening step downstream. Here a strong soda solution is prepared, containing 18-20 wt% of sodium carbonate. In a rotary lime slaker, solid lime is slaked to a dry hydrate using wash liquor recovered from a thickening step downstream. The milk of lime is fed to a mechanical classifier for the removal of unburnt stone, overburnt lime and grit. The purified lime suspension produced is directed to the Causticizer, where a large amount of purified lime causticizes the sodium carbonate solution under agitation, yielding Sodium Hydroxide and calcium carbonate. The lye suspension, containing the sodium hydroxide produced, overflows to the 1st thickener, where the calcium carbonate settles and is separated as a sludge, generating a clear lye containing most of the Sodium Hydroxide.

Sodium Hydroxide Purification. The clear lye is concentrated in an evaporator, yielding a 50 wt% caustic soda solution. The suspension is then allowed to settle, recovering the soda solution.

Calcium Carbonate Separation. The cake from the 1st thickener is fed to two other thickeners, along with wash water, for further separation of remaining caustic soda from the sludge. The precipitated calcium carbonate obtained is partly dried, deagglomerated to the desired particle size and sold as by-product. The remainder is sent to a kiln, in which calcium carbonate is converted back to calcium oxide and carbon dioxide is generated. The calcium oxide obtained is reused in the process.