A method for separation of lignin from original black liquor (BL IN) having a first alkaline pH value, comprising the following stages in sequence: a first precipitation stage wherein an acidifier charge is added to the original black liquor in order to decrease the pH value of the original black liquor to a second pH level initiating precipitation of lignin whereby said second pH level is above pH 7 and below 11.5, followed by a first separation stage (FP 1) wherein the precipitated lignin is separated as a lignin cake (LIG 1) with a content of carbohydrates from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, lignin from the lignin cake with a content of carbohydrates is mixed in a subsequent stage with a second acidifier added to the lignin cake forming an acidic slurry establishing a pH value in the range 1 -3, establishing a reaction temperature in the range 100-120°C in the acidic slurry, maintaining the acidic slurry at the reaction temperature for a reaction time period during which at least 60% of the carbohydrates content is hydrolysed, said reaction time period is in the range 10-60 minutes and in inverse proportion to temperature, keeping the amount of lignin dissolved from the lignin cake below 15%, said reaction temperature and time period corresponding to a P-factor as calculated by the formula P-factor = e^(40.48-15106/(273.15+temp))*time/60, keeping the acidic slurry at the reaction temperature for a reaction time period corresponding to a P-factor above 1 and below 8, followed by a second separation stage in which the treated lignin content is separated from the acidic slurry and the carbohydrates dissolved in the acidic slurry forming a low carbohydrate lignin cake, characterized in that after the acidic slurry has been kept at the reaction temperature for a reaction time period during which at least 60% of the carbohydrates content is hydrolyzed, said acid slurry is subjected to cooling before subsequent separation of the treated lignin content. a first precipitation stage wherein an acidifier charge is added to the original black liquor in order to decrease the pH value of the original black liquor to a second pH level initiating precipitation of lignin whereby said second pH level is above pH 7 and below 11.5, followed by a first separation stage (FP 1) wherein the precipitated lignin is separated as a lignin cake (LIG 1) with a content of carbohydrates from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, lignin from the lignin cake with a content of carbohydrates is mixed in a subsequent stage with a second acidifier added to the lignin cake forming an acidic slurry establishing a pH value in the range 1 -3, establishing a reaction temperature in the range 100-120°C in the acidic slurry, maintaining the acidic slurry at the reaction temperature for a reaction time period during which at least 60% of the carbohydrates content is hydrolysed, said reaction time period is in the range 10-60 minutes and in inverse proportion to temperature, keeping the amount of lignin dissolved from the lignin cake below 15%, said reaction temperature and time period corresponding to a P-factor as calculated by the formula P-factor = e^(40.48-15106/(273.15+temp))*time/60, keeping the acidic slurry at the reaction temperature for a reaction time period corresponding to a P-factor above 1 and below 8, followed by a second separation stage in which the treated lignin content is separated from the acidic slurry and the carbohydrates dissolved in the acidic slurry forming a low carbohydrate lignin cake, characterized in that after the acidic slurry has been kept at the reaction temperature for a reaction time period during which at least 60% of the carbohydrates content is hydrolyzed, said acid slurry is subjected to cooling before subsequent separation of the treated lignin content.