Effect of sodium silicate on the reverse anionic flotation of a siliceous–phosphorus iron ore

• Sodium silicate is more effective than other depressants on iron oxides depression.
• Sodium silicate dosage, modulus and conditioning time affect iron ore flotation.
• Addition of hydrous calcium chloride enhances the silica separation from iron ore.
• An iron concentrate with Fe of 67%, SiO2 of 1.5% and P of 0.034%, is obtained.

In the flotation system of Chador Malu iron processing plant (Iran), the apatite is floated with fatty acid based collectors. Sodium silicate is also used as iron oxides depressant. While the phosphorous content of the iron concentrate is reduced at an acceptable level (0.047% P), the silica content (3.5% SiO2) is still higher than the required level.This study aimed to find an effective reagent combination to reduce the silica and phosphorous contents of iron concentrate. Batch flotation tests were carried out to investigate the influence of sodium silicate dosage, modulus and addition of Ca++ ions, on the separation selectivity of phosphorous and siliceous gangue minerals from iron oxides. The feed sample, with 57.20% Fe, 6.42% SiO2 and 0.748% P was mainly composed of haematite and magnetite and the main gangue minerals were apatite and quartz.The results of flotation tests showed that sodium silicate performed better as depressant of iron oxide, but had little effect on silica flotation in the apatite–iron oxide–silica flotation system. It was also found that by using sodium silicate of modulus of 2.5 at a concentration of 800 g/t, pH value of 10, and conditioning time of 10 min, an effective separation of apatite from iron oxides could be achieved, while the silica removal (<35%) was still poor. Addition of hydrous calcium chloride (CaCl2·2H2O), as a source for Ca++ ions, to the sodium silicate solution enhanced the separation selectivity of quartz and apatite from the iron oxides. Applying a silicate–CaCl2·2H2O mixture with Me/Si ratio of 0.4, resulted in a concentration with acceptable grades of Fe (67%), SiO2 (1.5%), and P (0.034%). In this case, 93.6% of Fe was recovered while 75% of SiO2 and 93.9% of P were also removed.