Method for recycling a polyamide under acidic conditions

A method of recovery of the monomers from a polyamide (PA) of the AABB type comprising the following steps:

• a) a product (P) comprising the polyamide (PA) is optionally comminuted;
• b) an aqueous medium comprising water, the product (P) and an acid (Ac) having at least one pKa lower than 0 and selected in the group consisting of the nitrogen-free inorganic acids, methanesulfonic acid and combinations of two or more of these acids, is heated to induce the dissolution and the hydrolysis of the polyamide (PA);
• c) a stream (S) comprising the diacid (A) and the diamine (B) in the form of a salt with the acid (Ac), recovered at the end of step b) is further processed in order to either separate and recover the diacid (A) and the diamine (B) (embodiment E1) or collect the salt formed by the diacid (A) and the diamine (B) (embodiment E2);
• wherein step b) is performed with the following conditions:
• (i) the initial proportions in the aqueous medium are the following:
• proportion of polyamide (PA):
• at least 15.0 wt. %;
• proportion of water:
• between 20.0 and 70.0 wt. %;
• the remainder as acid (Ac) with the condition that the proportion of acid (Ac) is at least 2.0 wt. %,
• these proportions being based on the total weight of polyamide (PA), water and acid (Ac);
• the molar ratio (H/N) representing the amount of H from the acid (Ac) over the amount of N from the amide bonds of the polyamide (PA) between 1.1 and 7.0;
• (ii) the temperature is between 80° C. and 150° C.;
• wherein after step b), the method optionally comprises a total or partial removal of the unreacted acid (Ac) from the reaction mixture;
• wherein:
• according to embodiment (E1), step c) comprises a separation substep c1), wherein the diacid (A) and the salt of the diamine (B) are separated and a neutralisation substep c2), wherein the salt of diamine (B) separated from the diacid (A) is neutralised with at least one inorganic base of formula XOH, X being Li, Na, K or a combination of two or more of these cations to release the diamine (B);
• according to embodiment (E2), step c) comprises a neutralisation substep c2) in which the salt of diamine (B) is neutralised with an inorganic base of formula XOH to release the diamine (B);
• and wherein the α ratio is less than or equal to 7.0, this ratio being defined as the molar ratio=[salt of the acid (Ac) with XOH released by the method]/[amide bonds of the polyamide (PA)].