Addressing the Challenges of Chlor-alkali Process for Brines, Fine Caustics Production

Brine (salt water), the feedstock of the chlor-alkali industry, is used in the production of chlorine, caustic soda (sodium hydroxide), soda ash (sodium carbonate), hydrogen (hydrochloric acid) and a wide range of other products that can be used as raw material in other industries and downstream processes.

Chlorine is used to produce polymers, which are then used in the manufacture of plastics, synthetic fibers and synthetic rubber. Caustic soda is used in the production of polyester, other synthetic fibers, plastics, soaps and detergents. Soda ash is used in the manufacture of glass, pulp and paper, and rayon. Hydrogen is used in the production of hydrochloric acid, ammonia and hydrogen peroxide.

A critical parameter in the chlor-alkali process is determining the purity of the feed brine. In the membrane cell production process of caustic soda and trace-level alkaline earth metals (such as calcium and magnesium) are analyzed using the Thermo Scientific™ Dionex™ ICS-6000 Standard Bore and Microbore HPIC™ Systems and Thermo Scientific™ Dionex™ IonPac™ CS16 Column for Cation analysis (Ca and Mg). To ensure production efficiencies, the sum of calcium and magnesium must be less than 20 ppb in 30% sodium chloride (NaCI).

The produced gases (Cl₂ and H₂) have their own unique challenges. Cl₂ and H₂ can contain moisture after the production process and lead to corrosion issues during storage and transportation. For the determination of organic acids and inorganic anions, the Thermo Scientific™ Dionex™ IonPac AS28-Fast-4µm column provides fast, accurate and repeatable results.

Monitoring the formation of contaminants and byproducts during salt electrolysis enables purity objectives to be achieved in an efficient and safe manner. The formation of nitrogen trichloride during the production of high-grade chlorine is a key example. When its concentrations exceed 20,000 ppm, the compound becomes explosive.

Quality assurance testing of the final caustic or chlorine product ensures trace contaminants are within specification for key applications or as feedstock in follow-on production processes. Trace levels of chloride and chlorate in sodium hydroxide used in plating baths can cause expensive inconsistencies in the production of electronic and semiconductor devices. Similarly, pure-grade chlorine, used as feedstock for the production of plastic products such as urethanes, epoxy and glycol, is essential to avoid potential corrosion issues.