Determination of gold in geological samples by x-ray fluorescence spectrometry after extraction with tributyl phosphate

Abstract

The behaviour of gold and elements impeding its x-ray fluorescence spectrometric (XRF) determination, namely zinc, lead and arsenic, was studied during their extraction from hydrochloric acid, nitric acid, and aqua regia solutions using tributyl phosphate as a solid extractant [SE(TBP)]. Extraction of gold from pulps after aqua regia leaching was found to be the most favourable approach for the quantitative and selective recovery of gold. The gold distribution ratio, D Au, is ca. 10 4 ml g −1. For extraction from hydrochloric acid solutions the D Au value also exceeds 10 4 in the whole range of gold concentrations studied (10 −8−10 −4 M), but it decreases substantially with increasing extraction temperature, from 5 × 10 5 ml g −1 at 20°C to 9 × 10 3 ml g −1 at 70°C. An anomalously high distribution ratio of lead, D Pb ≈ 10 3 ml g −1, was observed during extraction from hydrochloric solutions in the presence of chlorine. This could be explained by the formation of the chloro complexes of lead(IV). An XRF method for the determination of gold in natural samples was developed, which includes back-extraction of gold from SE(TBP) using a hot 0.025 M thiourea solution, providing a thin sample layer for secondary XRF. For 25 g of sample material the limit of determination is 10 ng g −1 (10 −6%). The accuracy of the technique was checked using different reference materials. The results agreed within 10%.