A comparison of liquid and supercritical carbon dioxide as an extraction solvent for plating bath treatment

Because of increasing federal regulations, alternative metal recovery methods for spent electroplating baths are needed. Supercritical extraction of waste metals using CO2 is potential candidate for this process. However, because of the high-cost of supercritical fluid processing equipment, an extraction system modeled on the use of liquid CO2 rather than supercritical CO2 may be more economical. For this reason, the use of liquid CO2 contained beta-diketones as an extraction solvent for the removal of Ni2+ and Zn2+ from an electroplating solution was investigated and compared to results obtained with supercritical CO2. Acetylacetone (ACAC) and its fluorinated analogues were used as the ligands for the extraction of Ni2+ and Zn2+ from the electroplating bath. While the fluorinated ACAC showed a potential for a higher extraction efficiency, over 70% of both Ni2+ and Zn2+ was extracted using both liquid and supercritical CO2 using a 100-fold molar excess of ACAC. With isopropyl alcohol as a modifier, extraction efficiency was increased to over 80% for both metals using both phases of CO2 as the carrier fluid.