Abstract: In order to recover about 35% feldspar and 35% quartz from a molybdenum tailings in Inner Mongolia, the flotation separation of quartz and feldspar by sodium oleate in the presence of magnesium ions was studied. Through the optimization of flotation conditions, at pH=11.0, Mg²⁺ ion dosage of 300 g/t, and sodium oleate dosage of 900 g/t, the quartz products with yield of 27.28%, SiO₂ content of 94.67% and feldspar products with yield of 36.47% and K₂O+Na₂O content of 11.13% were obtained. Zeta potential test and chemical analysis of magnesium ion solution showed that when the pH value of pulp was 11.0, the concentration of Mg(OH)⁺ was relatively high, and the precipitation of Mg(OH)₂ was just started to form. At this time, the activated anion collector had the most obvious effect of capturing quartz, that is, the yield of flotation concentratrate was high, and the recovery rate of feldspar flotation in the flotation concentrate was relatively low. When the pH value of the pulp is greater than 11.0, the formation of Mg(OH)₂ precipitation on the mineral surface may be an important reason for the loss of selectivity of the agent, leading to the large amount of feldspar beginning to float.