Enhanced flow injection leaching of rocks by focused microwave heating with in-line monitoring of released elements by inductively coupled plasma mass spectrometry

Abstract

A focused microwave digestion system was used to heat a mini-column of sample of crushed rock (hematite) during its successive leaching by repeated 250-μL injections of water, HNO 3 1%, 10% and 30% (v/v). The mini-column was connected to the nebulizer of an inductively coupled plasma mass spectrometry instrument, which allowed a continuous monitoring of the progressive release of elements by a given leaching reagent. Quantitation of the accessible fraction of Mg, V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Sb and Pb was done by calibration using 250-μL injections of standard solutions prepared in the leaching reagent matrices. Total digestion of the sample residue was also done to verify mass balance. With the exception of Mg, V and Co, where the same total amount was released with or without microwave heating, an increased release resulted from focused microwave heating, by up to an order of magnitude. Furthermore, mass balance was verified for more elements using microwave heating, presumably because of a lower relative proportion of spectroscopic interference as a result of an increased release of analytes. Using microwave energy in general resulted in the dissolution of additional phases, as evidenced by significantly different 208Pb/ 206Pb ratios as well as the increased release of elements with milder reagents. In fact, in the case of Pb, leaching with 30% HNO 3 was no longer necessary as all the Pb was released in the first three leaching reagents. Microwave heating could therefore be used advantageously in on-line leaching for exploration geochemistry and environmental monitoring.