Lime neutralization is widely used to precipitate heavy metals including copper and nickel from wastewater. Limestone (calcium carbonate: CaCO3) is too stable to be used directly for this purpose. Grinding of CaCO3 in the solutions of copper and nickel sulfate was conducted to raise its reactivity. During the mechanochemical activation, CaCO3 reacted with copper sulfate to form insoluble copper compounds as basic carbonate and basic sulfate. No reaction between CaCO3 and nickel sulfate occurred even with such activation. This new phenomenon allowed an easy separation of copper from nickel. Several parameters including grinding rotational speed, Cu to Ni ratio (Cu/Ni), solid to liquid ratio and Ca in CaCO3 to Cu ratio (Ca/Cu) were investigated to examine the effects on separation efficiency and to find out the conditions at which more than 99% of the copper (Cu(II)) ions were precipitated with more than 99% of the nickel (Ni(II)) ions remaining in the aqueous solution. Changes in phase composition by XRD analysis, solution pH and particle size of solid sample were investigated to understand the probable mechanism involved in the separation process, together with the comparisons in solubility of copper and nickel carbonates to that of CaCO3. (C) 2016 Elsevier B.V. All rights reserved.