Hydrometallurgical recovery and leaching studies for selected valuable metals from fly ash samples by ultrasound-assisted extraction followed by ICP-OES determination

Abstract

Au, Co, Cu, Cr, Ir, Ni, Pd, Pt, V and Zn are metals used widely as raw materials in industry. They can all be considered as valuable metals. Out of these metals Co, Cr, Ir, Pd and Pt were recognized as critical raw materials in the ad-hoe working group of the European Commission's report in 2014. While the primary sources of these metals are drying up it seems to be reasonable to investigate new methods to recover the metals from secondary sources e.g., from industrial fly ash. In this work, new methods for the recovery of all metals (Au, Co, Cu, Cr, Ir, Ni, Pd, Pt, V, Zn) and determination of the noble metals (Au, lr, Pd, Pt) from fly ash samples were developed. The samples were collected from Finnish power plants using a mixture of biomass and peat as a fuel. An ultrasound-assisted digestion was utilized as a sample pre-treatment method and the concentrations of elements were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). The measurement conditions were optimized by applying a standard reference material, a recovery test and synthetic samples. The determination of the noble metals investigated was found to be affected by spectral interferences due to the emission of matrix elements Cu, Fe, Mn, Si, Ti and W. The interferences were corrected successfully for lines Pd 340.458 and Ir 224.268 nm by the multiple linear regression (MLR) technique. An ultrasound-assisted leaching procedure with oxalic acid as a leaching agent was developed for the recovery of elements. A leaching rate from 70% to 90% was obtained for the elements investigated. The parameters of extraction were optimized for a sample weight of 10g. Liquid-liquid extraction and sulfide precipitation methods in the recovery of the noble metals from oxalic acid leachate were investigated. The former method was found to be more specific for the noble metals examined and thus preferred to the latter method.