Sodium silicate is used as a dispersant in the selective flocculation of iron ores with starch as the polymeric floccullant. The mechanism of its dispersive effect on quartz has been reported to be related to its ability to remove coagulants, such as calcium and magnesium ions, from the process water. The interaction between iron oxides and calcium and sodium silicate, which has not appeared in literature, is also an important factor that would contribute to the success of the selective flocculation process. To elucidate the nature of this interaction at pH 11, where selective flocculation of iron oxides is commonly carried out, the effect of calcium and sodium silicate on the flocculation-selective flocculation behavior of hematite was investigated. Abstraction of calcium and adsorption of starch by hematite at different concentrations of sodium silicate and calcium were determined. It was found that calcium silicate precipitates as well as sodium silicate prevented starch adsorption and, therefore, sodium silicate could only be used effectively below 10 mg/L calcium and 1000 mg/L SiO2 concentrations while dispersing a hematite-quartz mixture.