The invention relates to a method and an apparatus for washing a crude lignin slurry (5) formed from plant based raw material, wherein the method comprises separating a soluble carbohydrate containing fraction (10) from the crude lignin slurry (5) by using displacement washing in at least one solid-liquid separation stage (6) so that the crude lignin slurry is prepressed, washed and pressed, and recovering a solid fraction (11) and the soluble carbohydrate containing fraction (10). Further, the invention relates to the soluble carbohydrate containing fraction and the solid fraction, and their uses.