Method for preparing alum from lepidolite in tantalum and niobium tailings

The invention provides a method for preparing alum from lepidolite in tantalum and niobium tailings. The method comprises the process steps: adding dilute sulfuric acid solution to raw material lepidolite to carry out acid leaching to remove fluoride; separating; and deslagging. The method is characterized in that in respect of the mixed solution subjected to acid leaching, fluoride removal, separation and deslagging, under certain temperature and pressure, mixed solution of potassium forms mixed solution of aluminum potassium sulfate, and then the concentration of Al in the mixed solution of aluminum potassium sulfate is controlled to be in a saturated (or supersaturated) state so as to separate out and precipitate alum.