Determination of SiO2 in lime mud by gravimetry

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 tre ated 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 dryne ss. 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 t reatments 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 Sta ndards and Technology Standard Reference Materials, SRM, (1c, 694 and 88b). The method was found to be accurate and precise in a 95% confidence interv al, rapid, and highly repeatable. The repeatability standard deviation for SRMs was calculated to be 0.49%.