(1) Promotion of HI decomposition by membrane separation of H₂ in an HI decomposer, (2) change from abandonment of HI and I₂ condensation heat to separate H₂ (left figure) to condensation heat recovery to the HI distillation column by feeding gas-phase HI and I₂ (right figure) and (3) heat recovery from the Bunsen reaction to the depressurized flash H₂SO₄ concentration. The excess amount is reduced by (1) and waste heat is recovered by (2) and (3).