Process for the delignification of cellulose fiber raw materials using alcohol and alkali

A process for the delignification of fibrous material comprising:

• heating a mixture of alcohol and water,
• contacting the fibrous material with impregnation liquor at a temperature in the range of 100.degree.-160.degree. C. in an impregnation stage comprising the addition of said heated mixture of alcohol and water,
• impregnating the fibrous material in the impregnation stage until a phase equilibrium is attained between the impregnation liquor and the liquid contained in the fibrous material,
• heating an aqueous mixture of alkali and alcohol,
• contacting the fibrous material with delignifying liquor in a single delignification stage comprising the addition of said heated aqueous mixture of alkali and alcohol,
• wherein the concentration of alcohol in the delignification stage is lower than the concentration of alcohol in the impregnation stage, and
• the pH in the delignification stage is in the range pH 9-12;
• decreasing uniformly the concentration of alcohol contained in the fibrous material during the delignification stage, and
• steadily increasing the concentration of the alkali in the fibrous material during the initial delignification stage, and then decreasing the concentration of alkali until a phase equilibrium is again reached between the liquid in the fibrous material and the delignification liquor.