Acetylacetone as a ligand in gas-phase extraction of gold and iron

Gas-phase extractions using acetylacetone as a ligand are reported to be a promising alternative method – particularly in gold extraction as an alternative to cyanide leaching. Extraction work has focussed on iron, aluminium, vanadium, and other transition metals. It has most recently targeted gold extraction. While previous extraction work focussed on metal recovery, the fundamental mechanism of extraction is not understood. This dissertation set out to improve the fundamental understanding of gas-phase extractions with acetylacetone as a ligand. An ab initio study of potential gold complexes with density functional theory (DFT) suggested that Au(III) acetylacetonate or [Au(acac)2] + would be the most stable complex out of the following complexes considered: Au(I) acetylacetonate or Au(acac); Au(I) thioacetyl acetonate or Au(Sacac); Au(I) β-ketiminato substituted variant of acetylacetone or Au(Nacac); Au(III) acetylacetonate or [Au(acac)2] +; Au(III) acetylacetonate or Au(acac)3; Au(III) thioacetyl acetonate or Au(Sacac)3; Au(III) βketiminato substituted variant of acetylacetone or Au(Nacac)3. DFT work showed good agreement with previous DFT work on Au β-ketonato complexes. However, the work did not agree with empirical evidence. A goldacetylacetonate complex synthesis from AuCl3 did not yield any goldacetylacetonate. Gas-phase extraction work at 170-190 °C for 30-180 min, on 99.99 % gold beads ranging from approximately 1.0-1.7 g did not yield any detectable gold extraction, measured on ICP-MS, despite the gold samples being several orders of magnitude larger than that of previous gold extraction work with acetylacetone. The extraction work did reveal that acetylacetone may decompose during the process at 165-167 °C – supported by thermodynamic equilibrium modelling and the presence of free carbon detected on the samples in Raman spectra. Further attempts to replicate some of the earliest previous acetylacetone extraction work on iron followed a similar outcome to gold. No iron extraction could be detected with ICP-MS on extractions from hematite at 220 °C for 200-210 min in both air and nitrogen atmosphere. Pyrite extraction attempts revealed that there is a surface reaction between pyrite and acetylacetone; however, no extraction was measured by mass difference or in concentrations of the condensate measured with ICP-MS. A full investigation and discussion of previous acetylacetone extraction work found that cumulative summation for plotting extraction curves imposed a linear bias on the curve. Measurement of changes in baseline metal content of acetylacetone condensate could have produced the impression of metal extraction as starting baseline metal content in the acetylacetone was not considered in previous work.
A dissertation submitted in partial fulfilment of the requirements for the degree Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2022