Effects of Low Concentration Levels of Calcium and Magnesium in the Feed Brine on the Performance of a Membrane Chlor‐Alkali Cell

The influence of low concentration levels of calcium and magnesium ions present in the cell feed on the performance of a laboratory membrane‐type chlor‐alkali electrolysis cell was determined experimentally. The resultant effects appeared mainly as a decrease in current efficiency and an increase in cell potential caused by calcium precipitation primarily in the carboxylate layer (on the cathode side of the Nafion® 90209 2 membrane) and magnesium precipitation in the sulfonate layer (on the anode side of the membrane), respectively. Identification and location of the precipitates were obtained by evaluation of the degraded membranes by scanning electron microscopy (SEM) and electron probe x‐ray microanalysis (EPMA) after experimental determination of cell potential and current efficiency for various feed brine purities. Physical disruption of the polymer morphology of the carboxylate layer was determined as a possible reason for the decreased caustic current efficiency with calcium accumulation. For the first time, membranes taken from commercial electrolyzers after 2.5 years of service were also evaluated using SEM and EPMA and their performance compared to the laboratory cell results. Evaluation of the commercially used membranes confirmed the validity of the laboratory membrane tests for bilayer films of sulfonate/carboxylate‐type combinations.