Sodium
Hypochlorite
plant

The production of chlorine, caustic soda and hydrogen is obtained by electrolysis of a concentrated NaCl solution. Weak or depleted dechlorinated brine from the membrane cells is returned to saturation by dissolving additional NaCl salt in the brine saturation unit.
The basic reaction in the electrolysis process is as follows:

NaCl + H₂O → NaOH + ½ H₂ + ½ Cl₂

It takes place in the electrolyzer, which single cell comprises an anode compartment made from titanium, a membrane installed between anode and cathode compartments and a cathode compartment made from nickel.
The energy consumption in a membrane cell process is in the order of 2100÷2150 kWh/ton of NaOH.
A chilled water cooled heat exchanger is part of the reaction loop.

The plant includes several units:

BRINE SYSTEM

Brine dechlorination: The depleted brine coming from the electrolyzer must be dechlorinated to remove any chlorine trace before being delivered to the saturation unit.

Brine saturation: Additional raw salt is added to the depleted brine in the saturator together with process water. The optimum NaCl concentration range is 295÷305 gpl.

Brine primary purification: Chemicals are added to the brine stream to remove mainly Ca⁺² and Mg⁺² cations and SO₄^-2 anions. Precipitates are collected in a receiver while treated brine is sent to filtration.

Brine filtration: Treated brine is filtered to remove suspended solids as CaCO₃, Mg(OH)₂ and BaSO₄.

Brine super purification: After filtration, brine is sent to the brine super purification towers. It flows through a bed of ion-exchange resin in order to reach the purity level requested by the membrane electrolyzer process. Modern technology is based on two or three resin bed towers. Concentration of Ca⁺² and Mg⁺² is so reduced to less than 20 ppb.

BRINE SLUDGE TREATMENT

Sludge filtration: Brine sludge are collected and sent to a filter-press type filter. In this equipment sludge dewatering takes place. The recovered brine is recycled back to the brine system, the sludge cake is sent to disposal.

CHLORINE GAS TREATMENT

Cooling: Chlorine gas stream is cooled to condensate the maximum amount of water vapour leaving the anodic chamber with the gas phase. It is then sent to the hypochlorite synthesis section.

HYDROGEN GAS TREATMENT

Cooling and discharging: Hydrogen gas stream is cooled to remove the water content. It is vented to atmosphere through a flame arrestor.

CATHOLYTE HANDLING

Cooling and dilution: The 32% caustic soda stream leaving the cathodic chamber is cooled. A portion is recycled back to the electrolyzer while the other part is diluted with demineralized water down to 18% concentration required by the synthesis of sodium hypochlorite at 12.5%.

SODIUM HYPOCHLORITE PRODUCTION

Synthesis process: Sodium hypochlorite is produced by intimate contact between caustic soda and chlorine. The synthesis reaction takes place under continuous operation in a liquid jet ejector. Available concentration range is between 12.5 ÷ 15.5%. Final dilution with demineralized water allows the achievement of requested commercial concentration.