Abstract: The invention is directed to a method for producing a purified lignin salt from alkaline spent liquor from the chemical cooking of lignocellulosic raw materials. The method comprises the steps of decreasing the spent liquor pH to form a lignin precipitate suspension; filtering the lignin precipitate to form a lignin filter cake and a filtrate; mixing the lignin filter cake into a dilution liquid to form an alkalilignin agglomerate colloid; subjecting the alkalilignin agglomerate colloid to large-pore membrane filtration; and recovering a purified lignin salt concentrate.