Abstract: A new type depressant was introduced in the flotation test of a certain fluorite ore containing carbonates and lots of sludge. Part of calcite and most of fluorite were separated from sludge by bulk flotation under conventional conditions through a rougher as first step, then the fluorite and calcite were separated by another rougher. A process of several steps in cleaning flotation was needed to obtain desirable fluorite concentrates, including the grinding once, roughing twice, clean-out five times and scavenging twice successively. The contents of CaF2 and CaCO3 in concentrate are 95.37% and 3.06%, respectively, with a recovery rate of CaF2 76.61% and the grade of CaF2 in tailing 12.36%. Theoretical study showed that Si(OH)4 and SiO2(OH)2^2- are the main components to depress calcite mineral in modified sodium silicate and the sequence of depressibility is SiO2(OH)2^2- > Si(OH)4 > HSiO3^-. The depressibility was mainly due to the different adsorbabilities of Si(OH)4 and SiO2(OH)2^2- for different minerals among which the highly adsorbable ones are easy to be depressed, while those minerals which cannot adsorb Si(OH)4 and SiO2(OH)2^2- firmly even are unable to adsorb them are not easily to be depressed or cannot be depressed at all.