We investigated the influence of a salt solution on the flotation behaviour of synthetic malachite using a sodium oleate as a collector. We found that the floatability of oleate ions-adsorbed synthetic malachite monotonically increases with an increase in the overall ionic strength (IS) range (1-1000 mM) of the monovalent cation (Na+), while the floatability sharply increases only up to a level of 30 mM of the divalent cation (Ca2+), followed by decreased floatability with further increases in the IS. For a monovalent salt, Na+ ions effectively screen a negatively charged bubble surface, reducing the energy barrier for attachment and thus increasing floatability. Moreover, electrophoretic mobility measurements revealed that oleate ions-adsorbed synthetic malachite is negatively charged in a NaCl solution while it is positively charged in a CaCl2 solution over the entire IS, indicating the specific adsorption of Ca2+ ions onto the surface of oleate ions-adsorbed synthetic malachite in a CaCl2 solution. This surface charge reversal in the CaCl2 solution suggests that electrostatic interaction between positively charged oleate ions-adsorbed synthetic malachite and a negatively charged bubble is attractive, yielding high flotation efficiency. The increased floatability of both the entire IS of NaCl and a low IS of CaCl2 can be well explained by the extended DLVO theory. At a high IS (100-1000 mM) of CaCl2, however, the decreased floatability did not follow the extended DLVO prediction, implying that additional non-DLVO type interactions are likely to be involved. Direct contact angle measurements and an inductively coupled plasma (ICP) analysis with the electrophoretic mobility measurements revealed that this unusual flotation behaviour is likely due to the hydration layer by the hydrated Ca2+ adsorption onto oleate ions-adsorbed synthetic malachite. (C) 2015 Elsevier B.V. All rights reserved.