Quantitative in-situ analysis of impurity elements in primary aluminum processing using laser-induced breakdown spectroscopy

Abstract

We report the application of laser-induced breakdown spectroscopy (LIBS) for on-line chemical analysis in a primary aluminum smelter. Measurements were performed for fourteen trace elements (Fe,Si,Cu,Ni,Ti,Cr,Mn,Sn,V,Ga,Zn,Sb,Mg,Na) and correlated with laboratory measurements on corresponding solid samples. Real-time quantification of trace elements down to ppm levels directly in the molten aluminum has been demonstrated for a number of elements (Cu,Cr,Mn,Sn). For elements Fe and Si, typically present in elevated concentrations in primary aluminum (approximately 1000 ppm and 300 ppm, respectively, in the present work) the average difference between concentration measurement carried out on the melt and laboratory results was found to be within 2.5% and 5% of the measured concentration, respectively. The observed differences can be partly related to uncertainties in the reference laboratory measurements, originating from the process of casting of solid samples from the melt. For elements with the highest vapor pressure, comparison with laboratory results on manually collected samples is shown to be less adequate. Nevertheless, we show that LIBS can provide reliable real-time measurements of the relative concentration of volatile elements, e.g., down to ppm levels in the case of Na. We conclude that for many technically important trace elements in primary aluminum, automated LIBS analysis on the molten metal is fully competitive with off-line laboratory analysis of solid process samples in terms of accuracy and precision, in addition to improving worker safety and providing significantly faster measurement results.