Separation of Zinc(II) from Hydrochloric Acid Solutions in a Double Lewis Cell

Abstract

A double Lewis cell (bulk liquid membrane) was used to study the extraction of zinc(II) from model 10% HCl and industrial spent pickling liquor containing iron(II) and iron(III). Water was used for stripping. Tributyl phosphate (TBP), Cyanex® 923, HOE® F2562, and AMBERLITE® LA‐2 were used as carriers. The extraction–stripping process from model HCl solution was successfully carried out when undiluted or highly concentrated TBP was used as the membrane phase. CYANEX® 923 and AMBERLITE® LA‐2 were too strong as carriers to achieve successful stripping with water. TBP did not act as a phase‐transfer catalyst when added to the membrane phase containing CYANEX® 923. The process needed additionally 3 M NaOH for stripping. An interfacial gel layer was formed, especially on the strip side, when HOE® F2562 was used as the extractant. It seemed important to select the appropriate carriers for the membrane processes, because the interface could be easily blocked if trace impurities were present in industrial reagents. When TBP was used to recover zinc(II) from actual spent pickling solution, an undesired transfer of iron(III), first to the membrane and then to the strip phase, was observed. The dilution of TBP with kerosene decreased the co‐transfer of hydrochloric acid but did not improve the selectivity of zinc(II) transfer with respect to iron. The reduction of iron(III) to iron(II) and/or extraction of iron(III) with a small volume of TBP, carried out just before the process, enabled the selective extraction of zinc(II). The organic phase loaded with iron(III) needed quick stripping to avoid both the hydrolysis of TBP and then the undesired transfer of iron(II) with partial esters. The selective recovery of zinc(II) from industrial spent pickling solution is possible, but special care must be taken to avoid any oxidation during the process.