Formic acid regeneration by electromembrane processes

Waters containing low amounts of sodium formate and sodium hydroxide were processed in order to regenerate formic acid. The treatment was performed in three steps: wastewaters neutralization, sodium formate concentration by conventional electrodialysis (ED), and sodium formate splitting into formic acid and sodium hydroxide by bipolar membrane electrodialysis (BMED). A coupling of these processes was performed. ED was carried out with a current efficiency of 90% and sodium formate concentration up to 2 mol dm⁻³. BMED was performed in a three-compartment cell configuration. Formic acid solution up to 30% was obtained with current efficiency of 80% under a current density of 500 A m⁻². Diffusion of molecular formic acid explains the current efficiency loss. The current efficiency varies with acid concentration and current density. Diffusion is more important through the anion-exchange membrane than through the bipolar membrane (2.5-fold).

Depleted salt produced in BMED was recycled to the neutralisation step.