Working up spent sulphuric acid simultaneously with sulphate salts

Spent sulphuric acid, either alongside or mixed with contaminated metal salts such as iron sulphate heptahydrate, is regenerated to fresh acid by countercurrent contact with gases from cleavage of the sulphate salts, such gases containing SO3 which effects build up of the acid concentration. The built up acid is evaporated in another stage to obtain pure acid. The metal sulphate solids are subjected to high temperature cleavage, generating SO3 which effects the previous concentration. Cleavage also produces some SO2 which is converted to SO3 by wet catalysis.