Patent ID: 6825379
Filing Date: 2004-11-30
Classification: B01D,C07C

Abstract:
A method for recovering and producing C4-C6 dicarboxylates from an alkaline waste solution generated in a caprolactam preparation process, comprising the steps of:(1) adding sulfuric acid to the alkaline waste solution generated from the caprolactam preparation process to adjust a pH value thereof to a pH of less than 7, separating the alkaline waste solution into an aqueous phase and an organic phase; adding nitric acid to the organic phase for oxidizing and converting valuable substances selected from adipic acid, 6-hydroxycaproic acid, saponified esters or ketones of cyclohexanol, and C4-C6-lactones, contained in the organic phase into dicarboxylic acids, thereby obtaining an oxidized reaction mixture containing dicarboxylic acids; (2) introducing the oxidized reaction mixture obtained from the step (1) into a two-stage concentration apparatus; in first-stage concentration, distilling out low boiling-point monocarboxylic acids and nitric acid; in second-stage concentration, the concentration is carried out at a temperature of 120Â° C. to 200Â° C. and an absolute pressure of 0.5 to 2.0 kg/cm2, so as to obtain crude concentrates mainly containing C4-C6 dicarboxylic acids; (3) adding C1-C4 alkyl alcohol to the crude concentrates mainly containing C4-C6 dicarboxylic acids obtained from the step (2), and proceeding two-stage esterification, whereby half-esterified intermediates are obtained in first-stage esterification, and crude dicarboxylates ore obtained in second-stage esterification; and (4) distilling the crude dicarboxylates obtained from the step (3), so as to get single-species dicarboxylates or a mixture of dicarboxylates.