Method for producing diamine and dicarboxylic acid, method for recycling polyamide, and polyamide

A method for producing a diamine and a dicarboxylic acid, the method comprising:

• pre-treatment by subjecting a polyamide-containing waste material to one or more selected from the group consisting of pulverization, washing, and foreign substance segregation, to obtain a crude polyamide,
• depolymerization by adding the crude polyamide to an acid-containing aqueous solution in a tightly closed reactor, raising a temperature to a target temperature of 90° C. or more and 160° C. or less, and then performing heating at or around the target temperature, to depolymerize 80% or more of an amide group in the entire polyamide contained in the crude polyamide to decompose the polyamide into a diamine and a dicarboxylic acid,
• separation by removing a component other than the diamine and a derivative of the diamine as well as the dicarboxylic acid and a derivative of the dicarboxylic acid, from a reaction liquid obtained in the depolymerization, to obtain the diamine and the derivative of the diamine as well as the dicarboxylic acid and the derivative of the dicarboxylic acid, and
• purification by isolating and purifying each of the diamine and the dicarboxylic acid, from the diamine and the derivative of the diamine as well as the dicarboxylic acid and the derivative of the dicarboxylic acid obtained in the separation.