Highly pure para-toluene-sulphonic acid mono:hydrate prepn. from sulphuric acid contg. para-toluene-sulphonic acid by mixing with water, cooling, filtration and crystallisation steps

Residual H2SO4, contg. 60% H2SO4 and 27% p-toluenesulphonic acid, is stirred with water, in water:acid wt. ratio 0.1-0.5 (0.25-0.35). The reaction mixt., at 60-85 deg.C, is cooled to 0-30 (10 deg.C for 15 mins. After filtration and centrifuging, over 93%pure p-toluenesulphonic acid monohydrate crystals are recovered
and also a filtrate contg. 55% H2SO4 and 7.5% toluenesulphonic acid. The filtrate can be used as such; conc. to 75% H2SO4 by mixing with conc. H2SO4 or oleum; or worked up either for complete conversion to p-toluenesulphonic acid or for the recovery of H2SO4 as Na- or Ba sulphates.
The p-toluenesulphonic acid crystals obtd. in the 1st crystallization step can be purified by redissolving
in water, at wt. ratio water:crystals 0.1-0.3 (0.14) at 60-100 (90) deg.C. The soln. obtd. is filtered and then cooled at 0-30 (10) deg.C to recover 98% p-toluenesulphonic acid mono-hydrate, which is sepd. by filtration and dried, esp. in vacuo, at 50-100 (80) deg.C. The filtrate obtd., contg. 65% p-toluenesulphonic
acid and 4.5% H2SO4 can be recirculated to the dilution crystallization process of p toluene-sulphonic
acid; or used as final prod. or purified to reduce the content of H2SO4 to less than 1%. The plant used is claimed.