Sub-Product

In the chlor-alkali process, depleted brine is a brine solution with a lower salt concentration than the saturated brine (approximately 25 wt-% NaCl) fed to the electrolytic cell:

• Mercury cells are usually operated to maintain a 21–22 wt-% concentration of salt in the spent brine discharged from the cell.
• In the diaphragm cell, saturated brine is decomposed to approximately 50 % of its original concentration in a passage through the cell, compared to a 16 % decomposition of salt per passage through mercury cells.
• Brine depletion in membrane cells is two or three times greater than in mercury cells, which allows the brine system to be smaller, resulting in significantly lower recycling rates and less equipment needed compared to mercury cell plants of the same capacity^[1].

The table below gives the density (kg/L) and the corresponding concentration (% weight) of sodium chloride (NaCl) in water at different temperatures in degrees centigrade (°C)^[2].

References

1. Source: T. Brinkmann et al. Best Available Techniques (BAT) Reference Document for the Production of Chlor-alkali. Industrial Emissions Directive 2010/75/EU. EUR 26844. Luxembourg: Publications Office of the European Union; 2014. JRC91156.
2. handymath.com, The Complete Sodium Chloride Density-Concentration Table Calculator.