Direct determination of zirconium and silicon in zircon by flame atomic absorption spectrometry using two rapid decomposition methods

Abstract

Analysis of zircon for its zirconia and silica content is important as part of quality control. Due to its exceptional thermal and chemical stability, decomposition is difficult under ordinary conditions. Two relatively rapid and accurate decomposition methods using alkali fusion and hydrofluoric acid have been described. In the present work, zircon has been fused with molten sodium hydroxide in 1 : 6 mole ratio in an open vessel at 550 °C to give acid soluble sodium zirconate and water soluble sodium ortho silicate. The acid soluble zirconate and water soluble silicates were separated and analysed by flame atomic absorption spectrometry (FAAS) for zirconia and silica content. The effects of zircon to alkali ratio and decomposition temperature on degree of decomposition have been studied. The acid decomposition method was developed using hydrofluoric (HF) acid at elevated temperatures in closed microwave (MW) digestion system. The MW digested samples containing water soluble fluoride compounds of Zr and Si were directly analysed by FAAS. Several temperature profiles of MW system were studied for acid strengths of 40% and 48% and optimised conditions were achieved to investigate the effect of strength on the extent of decomposition and decomposition temperature. Complete digestion was observed at 225 °C (48%) and 250 °C (40%). MW acid digestion decomposes samples with a very short heating time of 20 min with small amount of reagents (∼10 mL) as compared to conventional methods. For increasing absorption for Zr in FAAS, all solutions were analysed in presence of 0.5 M HF + Al salt. The proposed MW digestion method using HF has also been applied successfully for analyzing zirconium carbide and is well suited for other refractory zirconium compounds.