1. A process for the oxidation of p-xylene to terephthalic acid comprising the following steps:a) oxidizing p-xylene using at least one oxidizing agent in the presence of at least one metal salt as a catalyst and at least one promoter in at least one aliphatic carboxylic acid as a fluid medium to obtain a reaction mixture comprising terephthalic acid, oxidation intermediates of terephthalic acid, trimellitic acid, at least one metal salt, the promoter, the aliphatic carboxylic acid and water; wherein the oxidation intermediates of terephthalic acid comprise at least one compound selected from the group consisting of p-tolualdehyde, p-toluic acid and 4-carboxy benzaldehyde;
b) separating a partial amount of the terephthalic acid from the reaction mixture by crystallization and filtration to obtain crude terephthalic acid and a mother liquor; wherein the mother liquor comprises trimellitic acid, the promoter, at least one metal salt, water and the aliphatic carboxylic acid;
c) dividing the mother liquor into a recycle stream and a purge stream; wherein the ratio of the amount of the recycle stream and the purge stream is in the range from 19:1 to 4:1; and recycling the recycle stream to step (a);
d) extracting p-xylene soluble chemicals from the purge stream by adding p-xylene to the purge stream and mixing at a predetermined temperature to obtain a biphasic mixture, which after separation, provides an organic stream and an aqueous stream; wherein the organic stream comprises p-xylene, the aliphatic carboxylic acid and the oxidation intermediates of terephthalic acid, and the aqueous stream comprises the aliphatic carboxylic acid, trimellitic acid, the promoter and at least one metal salt;
e) recycling the organic stream comprising p-xylene, the aliphatic carboxylic acid and the oxidation intermediates of terephthalic acid to step (a); and
f) recovering at least one metal salt from the aqueous stream and recycling the recovered metal salt to step (a);
wherein, the concentration of trimellitic acid in the reaction mixture is below 2000 ppm.