acid and or sulphuric acid and or sulphurous acid both obtained from gaseous so 2 in turn obtained from rich gases and or by collecting recycled h 2 s gas from the suspending step b and converting it into said so 2 or sulphuric acid obtained by electrolysis of sodium sulphate said sodium sulphate is preferably emanating from esp dust electrostatic precipitator dust also known as esp catch or esp ash the recovery boiler in a pulp mill produces significant amount of esp dust which contains mainly sodium sulphate this esp dust may to a large extent be recycled to the recovery boiler some amounts may further be removed to adjust imbalances in s or na in the recovery cycle said sodium sulphate may as set out above be converted into sodium hydroxide and sulphuric acid by electrolysis see e g u s pat no 4 561 945 the sodium hydroxide can further be used in different positions in the pulp mill such as the bleach plant or in the recovery area this approach eliminates the need for purchased sulphuric acid the need to purge excess s by purging esp dust and thus the intake of sodium make up can be eliminated according to.