A process combination of ion exchange and electrodialysis for the recovery and purification of hydroxy acids from secondary sources

Abstract

Ion exchange and electrodialysis processes for the production of hydroxy acids (HAs) from alkaline soda and kraft black liquors (BLs) were investigated. Ion exchange with a type-2 strong anion exchange resin in sulfate form was used to separate thiosulfate from alkaline kraft-BL-based mixture of sodium (Na+) salts of HAs. Efficient regeneration of the spent resin to sulfate form was achieved only through hydroxide form. Ion exchange with strong cation exchange (SAC) resins was successfully used to convert the Na+ salts of HAs in soda-BL-based solution and thiosulfate-free kraft-BL-based solution into free acids with complete removal of Na+ ions from the solution. The spent resin was efficiently regenerated with 1–2 mol/L sulfuric acid (H2SO4). Electrodialysis with bipolar membranes (EDBM) was used for the generation of H2SO4 and sodium hydroxide (NaOH) from the spent SAC resin regenerants (sodium sulfate–sulfuric acid mixtures). Overall, 1 mol/L H2SO4 with 95% purity and 90% yield could be obtained, but excess water flux through the membranes limited the maximum obtainable H2SO4 concentration to 1.17 mol/L. Simultaneously, 0.79 mol/L NaOH was produced with 93% purity. EDBM-based H2SO4 was successfully used for the regeneration of the SAC resin. With EDBM, a closed loop can be built for the regeneration of SAC resin with the simultaneous generation of NaOH. The results of this study demonstrate that a process combination of ion exchange and electrodialysis decreases the overall chemical consumption related to HA production and enables efficient chemical recycling.