Method for dealkalizing Bayer process red mud by using CO2 and waste acid in combined way

The invention discloses a method for dealkalizing Bayer process red mud by using CO2 and waste acid in a combined way. The method comprises the following steps: smashing the Bayer process red mud, and mixing the smashed Bayer process red mud with water in the weight ratio of 1:(3-10); grinding the red mud till the granularity is 160-200 meshes to form a solid-liquid mixture, and introducing CO2 into the Bayer process red mud for performing an acidification reaction at the temperature of 60-80 DEG C for 0.5-2 hours, wherein the CO2 is introduced at the flow of 0.2-1L/min; adding 20 percent by mass waste hydrochloric acid into the solid-liquid mixture obtained in the first step for reacting at the temperature of 60-80 DEG C for 0.5-2 hours to obtain a solid-liquid mixture, wherein the mass ratio of the Bayer process red mud to the added waste hydrochloric acid is (2-4):1; and performing suction filtration separation on the obtained solid-liquid mixture, and separating treated residues to recover aluminum and iron. Through adoption of the method for treating the Bayer process red mud, a solution is provided for the application of the red mud to building materials.