Abstract: A process for separating lignin and volatile reaction products from a black liquor derived from pulp production, lignin is demethylated in a heat treatment reactor and CH3+ ions separated out are reacted with reduced sulphur-containing components from cooking chemicals in the black liquor to form DMS a rise in pressure in the heat treatment reactor is controlled and DMS is withdrawn when a pressure of ?15 bar to ?40 bar, is reached in the heat treatment reactor the black liquor which is depleted in DMS is transferred to a first precipitation stage and CO2 and/or sulphur-containing reaction products are added to precipitate out a slurry containing demethylated lignin which is transferred into a second precipitation stage with at least one sulphur-containing acidifying agent, and the precipitated unrefined slurry containing demethylated lignin is discharged from the second precipitation stage; and supplied to a demethylated lignin separation step.