Desenvolvimento de procedimento para análise química de alumina combinando fusão e espectrometria de fluorescência de raios x

Abstract

Alumina, or aluminum oxide, is a refractory material with excellent mechanical properties and is the primary raw material for the production of metallic aluminum (Smelter Grade Alumina—SGA). To ensure the purity of alumina, monitoring the levels of contaminants such as CaO, Fe2O3, SiO2 and Na2O is essential and is carried out by determining its chemical composition, typically using X-ray fluorescence spectrometry (XRF). At Companhia Brasileira de Alumínio (CBA), the traditional method for preparing alumina samples involves pulverization followed by pressing, with subsequent analysis by XRF. Although well-established, this method has disadvantages, such as sample heterogeneity, matrix effects, contamination, and ergonomic risks to analysts. This professional master’s project focused on developing a method for preparing alumina samples via fusion, followed by XRF measurements. Fusion offers advantages such as more homogeneous samples, reduced matrix effects, greater safety for analysts, and shorter preparation times. However, it presents challenges, such as sample dilution in the flux, which can reduce the sensitivity of certain elements and affect the accuracy of the results. Sample preparation is crucial for ensuring the accuracy of analytical results, and both pulverization followed by pressing and fusion are relevant strategies for preparing samples for XRF. Choosing the most appropriate method is essential for optimizing resources, ensuring analyst safety, and guaranteeing the reliability of results, in accordance with safe practices. For the fusion method, we used a 2-g sample of aluminum oxide and 10 g of flux containing 49.5% m/m lithium tetraborate, 49.5% m/m lithium metaborate, and 1% m/m lithium bromide. The mixture was transferred to a 95%-5% Pt-Au crucible and subjected to a heating program with a maximum temperature of 1050 °C and a total duration of 13.5 min. After fusion, the solution is poured into a mold to form the glass pellet. The entire procedure is performed mechanically using a fusion apparatus (Katanax, Model K2 Prime). The finished glass pellet is analyzed by WDXRF. Instrumental measurements using the glass pellet were conducted according to the strategy adopted for the pressed pellet. The concentrations determined for the main contaminants i.e., CaO, Fe₂O₃, Na₂O, and SiO₂ fall within the process specification range for quality control. The analytical procedure developed was evaluated using both qualitative and quantitative approaches. The qualitative approach proved suitable for rapid process control. On the other hand, the quantitative approach was effective, and its accuracy was demonstrated by comparing the results obtained by fusion and by grinding-pressing. Validation parameters were also investigated. The linearity of the analytical curves for CaO, Fe₂O₃, Na₂O, and SiO₂ was adequate according to the statistical parameters evaluated, such as homoscedasticity, residuals, and correlation and determination coefficients. The detection limits (LDs) and quantification limits (LQs) were determined, and low sensitivity was observed for Na₂O. Precision was evaluated using the repeatability parameter in the CRM samples. In this study, the concentration range of the fusion procedure was evaluated from 100 to 1000 mg/kg applying tablet dilution tests, by optimizing the masses of the flux and the sample. Trueness was determined using two certified reference materials.