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• The decrease of pulp viscosity enhanced the copper recovery and grade. • Entrainment still played a role as the salt concentration increased further. • The change of flotation solution chemistry affected the non-selective aggregation. • The underpinning mechanism was the ion exchange reaction. This study revealed the underpinning mechanisms in chalcopyrite flotation with high sodium bentonite content when using saline water containing divalent cations, Ca2+ and Mg2+. It was found that at a divalent cation concentration of 0.1 mol/L, the decrease of pulp viscosity significantly enhanced the copper recovery and grade. Entrainment still played a role as the salt concentration increased further. When pulp viscosity and entrainment were excluded, the non-selective aggregation of chalcopyrite and bentonite was also found to affect the copper grade. The change of flotation solution chemistry further influenced the electrostatic interaction between chalcopyrite and bentonite. The underpinning mechanism was the ion exchange reaction between sodium bentonite and saline water through which altered the elemental distribution and inner structure of bentonite. The findings in this work suggest that when using saline water containing Ca2+ and Mg2+ in flotation, the ion-exchange interaction makes the control of divalent cations concentration in industry flotation more complex, particularly for sodium bentonite-ore. However, within an appropriate range of divalent cations concentration, both copper grade and copper recovery can be enhanced, instead of removing Ca2+ and Mg2+. [ABSTRACT FROM AUTHOR]