Treatment of effluents

In a process for the conversion of soluble thiosulphate to sulphates, said thiosulphates being present in an aqueous redox solution comprising at least one salt of an anthraquinone di-sulphonic acid and at least one water soluble vanadium compound, which process comprises introducing sulfuric acid, as such or as oleum, into a deep vessel containing a deep body of said thiosulphate-containing redox solution, the improvement wherein:

• the sulfuric acid introduced has a concentration of from 10-98% w/w H.sub.2 SO.sub.4 ;
• the amount of sulfuric acid introduced into said deep vessel is such that the gram molecular weight ratio of sulfuric acid to thiosulphate is from 1:2 to 1:3;
• introducing the sulfuric acid into the base of said vessel and into the bottom of the deep body of said redox solution therein;
• maintaining the contents of said vessel at a temperature of from 70.degree. C. up to the boiling point of the redox solution;
• maintaining said vessel at ambient pressure;
• and
• thereafter separating the sulphate formed, whereby the generation of SO.sub.2 is not more than 10 grams per mole of thiosulphate present in said aqueous redox solution, and whereby not more than 20% of said thiosulphate remains in said aqueous redox solution after not more than 31/2 hours.