The effect of calcium ions, sodium silicate and surfactant on charge and wettability of magnetite

[Display omitted] ► Calcium ions can impair dispersing performance of sodium silicate in flotation. ► Adsorption of silicate on calcium modified magnetite increased with increasing pH. ► Subsequent surfactant adsorption in the pH range 7.5–9.5 was the highest at pH 8.5. ► In the presence of calcium, sodium silicate did not reduce surfactant adsorption. ► Adsorbed surfactant made magnetite hydrophobic which is unwanted in agglomeration. Anionic carboxylate surfactants and sodium silicate are used in the reverse flotation of iron ore to separate magnetite from apatite. In this work, consecutive adsorption of sodium silicate and an anionic surfactant on synthetic magnetite modified with calcium ions was studied in the pH range 7.5–9.5 using in situ ATR-FTIR spectroscopy. The effect of these chemicals on the zeta-potential and wetting properties of magnetite was also investigated. While adsorption of silicate increased with increasing pH, subsequent surfactant adsorption went through a maximum at pH 8.5. Surfactant adsorption in the presence of calcium ions was not affected by the amount of silicate adsorbed on magnetite. Calcium ions were found to render the magnetite surface positive in the pH range 3–10 and could reduce the dispersing effect of silicate in flotation of apatite from magnetite. While treatment with calcium chloride and sodium silicate made magnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the water contact angle on the magnetite surface from about 10° to 40–50°. Although the latter values are not high enough to make magnetite float, the hydrophobic areas on the magnetite surface could result in the incorporation of air bubbles inside the iron ore pellets produced by wet agglomeration, lowering the pellet strength.