Liquid-liquid synthesis of oximes from carbonyl compounds: Formation at neutral conditions and degradation at acidic hydrometallurgical process conditions

Abstract

BACKGROUND In order to elucidate kinetics in the synthesis of oxime liquid–liquid extraction reagents, used as ligands in hydrometallurgical copper production, the oximation reactions of salicylaldehyde and 2′‐hydroxyacetophenone with hydroxylamine were studied in a two‐phase system. Two experimental methods were applied: CSTR reactor under vigorous mixing and batch reactor under normal agitation. The reverse reaction in acidic conditions was also examined, because it results in hydroxyoxime reagent losses in copper liquid–liquid extraction process. RESULTS The overall oximation reaction was of a second‐order, first‐order with respect to each reactant. The estimated activation energy for oximation of salicylaldehyde was 34.2 kJ mol−1, and interfacial reaction rate constant at 40°C 4.81 × 10−3 L mol−1 s−1. The model used, including the empirical description for interfacial reactant concentrations, described the salicylaldehyde oximation reaction well over a wide concentration range. In the batch reactor more than 0.99 conversion for salicylaldehyde was achieved in 15 min at 50°C, but for 2′‐hydroxyacetophenone the maximum reaction conversion was only 0.17 at 70°C in 60 min. Both, aldoxime and ketoxime, were hydrolyzed into their corresponding carbonyl compounds in the presence of 180 g L−1 sulfuric acid. The ketoxime was completely converted into a ketone, but with the aldoxime 80% reagent degradation was observed at equilibrium. CONCLUSION A liquid–liquid system in neutral conditions is applicable for the oximation of aldehydes generated by degradation of hydroxyoxime solvent extraction reagents. For ketones the reaction is much slower. The degradation reaction was shown to be significant in conditions similar to the copper liquid–liquid extraction stripping stage.