Process for the preparation of nitro-T-acid (8-nitro-naphthalene-1,3,6-trisulphonic acid)

In the process of the preparation of 8-nitro-naphthalene-1,3,6-trisulfonic acid by nitration of naphthalene-1,3,6-trisulfonic acid or mixture containing the latter with nitric acid in sulfuric acid the improvement comprising

• (a) carrying out the nitration in continuous operation in a plurality of reactors of which the first reactor is a loop reactor;
• (b) placing initially into the first reactor a nitration mixture in which naphthalene-1,3,6-trisulfonic acid has been nitrated to the extent of at least 80%;
• (c) adding to this nitration mixture simultaneously while thoroughly mixing the reaction mixture and maintaining the reaction temperature in the range from 30.degree. to 60.degree. C. (i) naphthalene-1,3,6-trisulfonic acid or a mixture containing the latter dissolved in sulfuric acid having a temperature between 100.degree. C. and 200.degree. C. and (ii) 1.0 to 1.4 mols of nitric acid per mol of naphthalene sulfonic acid in such a manner that a sulfuric acid concentration in the range of 86 to 94% by weight relative to the sum of water and sulfuric acid present is maintained during the nitration;
• (d) carrying out the nitration in the loop reactor to the extent of at least 80%;
• (e) completing the nitration reaction by transferring the nitration mixture into a series of one or two stirred reactor vessels until reaction is complete;
• and
• (f) then working up the completed reaction mixture in known manner.