Abstract: In order to find out the reason for high content of magnesium in copper-nickel concentrate from Jinchuan No.2 mine area，the process mineralogy of copper-nickel concentrate from the on-site process had been researched.The result showed that：the magnesium-containing minerals in the mixed concentrate was mainly serpentine，secondly pyroxene，olivine，chlorite，talc and so on；The particle size of the main copper and nickel minerals in mixed concentrate was fine，but the liberation degree was low，only about 60%，objectively.Strengthening the recovery of copper and nickel minerals created the conditions for easy-floating magnesium-containing gangue minerals and it's intergrowth with copper and nickel minerals into the mixed concentrate；The magnesium-containing gangue minerals in mixed concentrate mostly exist in monomeric form，and there was a certain amount of intergrowth with magnetite，which was the main reason for the mixed concentrate with high magnesium content；The floating order of main magnesium-containing gangue minerals from high to low was：talc> magnesite>pyroxene>serpentine>chlorite>olivine.Therefore，it was important to improve the liberation degree of copper and nickel minerals for reducing the MgO content in copper-nickel mixed concentrate，but it's the key to be targeted to inhibit the magnesium-containing gangue minerals monomer and its intergrowth with magnetite.

Key words: Magnesium-containing gangue, Mixed concentrate, Liberation degree, Floatability, Monomer, Intergrowth, Approache