Method for preparing dicarboxylic acid by two-step oxidation of cycloalkane

The invention discloses a method for preparing dicarboxylic acid by two-step oxidation of cycloalkane. The method comprises the following steps of: continuously feeding the raw material cycloalkane and a catalyst into an oxidation reaction kettle for catalytic oxidation, separating an unreacted part of cycloalkane and a solvent in oxidation liquid through flash evaporation and distillation processes, dehydrating the separated cycloalkane and the solvent, and returning the separated cycloalkane and the solvent to the oxidation reaction kettle; cooling, crystallizing and filtering the residual oxidation liquid to obtain crude dicarboxylic acid and a filtrate, and returning the filtrate to the oxidation reaction kettle; and carrying out second-step oxidation on the washed crude dicarboxylic acid by using nitric acid under the action of a catalyst, filtering the oxidation liquid, carrying out cooling, crystallizing and centrifugal separating, dissolving filter residues, removing cobalt ions, performing decoloring