Process for substantially removing unwanted metal ions, in particular vanadium ions, when concentrating dilute iron(II) sulphate-containing sulphuric acid solutions

The sulphuric acid solutions are boiled down by evaporating water and by separating off iron(II) sulphate to a content of 60 to 70% by weight of H2SO4. According to the invention, as the sulphuric acid solution is boiled down, the use of trivalent titanium ensures that the content of trivalent iron in the sulphuric acid solution does not exceed 0.1 g/l. The trivalent titanium, if it is not already present in sufficient quantity in the solution, can be added from outside or can be formed from quadrivalent titanium already present in the solution by the addition of a reducing agent. Prior to the final concentration step, pre-concentrating and cooling of the pre-concentrated sulphuric acid solution can be employed to deposit and to separate part of the iron as iron(II) sulphate heptahydrate.