Abstract: A method for washing sulphate cellulose pulp in the washing department of a sulphate cellulose mill, in which method a first pulp flow (1) is conducted to a first washer (101), where a washing liquid (12) is combined with the first pulp flow (1). A second pulp flow (2) is conducted from the first washer (101) to a second washer (102), where a washing liquid (3) is combined with the second pulp flow (2). The washed pulp flow (5) is proceeded from the second washer (102) in the process and the filtrate (4) displaced by the washing liquid (3) is recycled to the process so that the filtrate (4) displaced by the washing liquid (3) in the second washer (102) is divided into a first part flow (6) and a second part flow (7) and the pH of the second part flow (7) is dropped by combining an acidifier (8) with the second part flow (7).

Abstract: The present invention relates to a method of processing white liquor (3) obtained from the causticization step of a pulp mill, in which method infeed white liquor (3) principally contains sodium hydroxide (6) and sodium sulfide (7). The method according to the invention is characterized in that the sodium hydroxide (6) contained in the white liquor (3) is separated from the white liquor (3) either entirely or partly by means of a diffusion dialysis membrane process (4). The invention is further characterized in that the sodium hydroxide (6) separated from the white liquor (3) is advantageously entirely returned back to the chemical circulation of the pulp mill.