Process for the continuous preparation of 3,3'-dichloro-benzidine dihydrochloride

A continuous process for the preparation of 3,3'-dichlorobenzidine dihydrochloride from 2,2'-dichlorohydrazobenzene by treatment with aqueous sulfuric acid, which comprises:

• in a reaction zone from which a product of a reaction can be conveyed, continuously reacting the 2,2'-dichlorohydrozobenzene, dissolved in a water-immiscible aromatic solvent, at a temperature within the range of about 20.degree. to about 50.degree. C. in the presence of an alkali metal salt of a C.sub.8- C.sub.23 alkyl polyglycol ether sulfate, with such an amount of from about 50 to about 80% strength aqueous sulfuric acid that a conveyable suspension containing 3,3'-dichlorobenzidine sulfate is formed in said reaction zone,
• conveying said conveyable suspension from the reaction zone,
• continuously diluting the suspension conveyed from the reaction zone with water,
• heating the thus-diluted suspension to a temperature in the range of from about 90.degree. to about 95.degree. C. until a solution comprising ahot, sulfuric acid aqueous phase is obtained, the aromatic solvent being separable from said solution,
• separating off the aromatic solvent from the hot, sulfuric acid phase, and
• precipitating the 3,3'-dichlorobenzidine dihydrochloride by adding hydrochloric acid to the thus-separated sulfuric acid phase, and filtering off said 3,3'-dichlorobenzidine dihydrochloride.