Recovery of chlorine from dilute hydrochloric acid by electrolysis using a chlorine resistant anion exchange membrane

Abstract

A new chlorine resistant anion exchange membrane enables innovative possibilities for hydrochloric acid electrolysis for recovery of chlorine. This is of interest for hydrochloric acid that is neutralized in the chemical industry because purity and concentration are not sufficiently high for recycling. In the common electrolysis process hydrochloric acid is fed into the anode compartment and needs a satisfactory HCl concentration for supplying the anode with chloride ions. Using an anion exchange membrane as a cell separator the feed flows into the cathode chamber where a low HCl concentration is acceptable because Cl− ions at the anode can be supplied by addition of a salt which is not consumed. Experimental data of the membrane and the process are presented: membrane permselectivity improved up to above 97% using CaCl2 as added salt, chlorine current efficiency up to 98% and oxygen content as low as 0.5 vol%, cell voltage at 4 kA m−2 2.3 V, equivalent to 1740 kWh per t produced chlorine, even at low HCl concentrations. Thus, the power consumption is comparable with the common process. A problem of the new process is the high water transport through the membrane. Therefore, experiments for two process alternatives were carried out. Disadvantages of water transport can be avoided by using a high concentrated CaCl2 solution as anolyte and catholyte and as absorption medium for diluted HCl gas streams. Additionally, a cell design was investigated where the anode is directly connected to the membrane in an empty (gas filled) anode compartment.