Determination of SiO2 in lime mud by gravimetry

Abstract

A simple and rapid method for the determination of SiO₂ in lime mud used in current sulfate pulping practice is reported. The lime mud samples are treated three times with 2.5 mL of 6 mol/L hydrochloric acid. The samples are nearly evaporated to dryness two times and the third time to complete dryness. To determine the exact SiO₂ content of the residue, it is treated with an excess of concentrated hydrofluoric acid and a little hydrochloric acid, and the silica is expelled as volatile silicon tetrafluoride. The sample treatments take about 1.5 h and the heating periods take a total of about 3.5 h. The SiO₂ results were evaluated by analyzing National Institute of Standards and Technology Standard Reference Materials, SRM, (1c, 694 and 88b). The method was found to be accurate and precise in a 95% confidence interval, rapid, and highly repeatable. The repeatability standard deviation for SRMs was calculated to be 0.49%.