Process for the pulping of lignocellulose materials with alkali or alkaline earth metal hydroxide or salt and a solvent

A process for the pulping of a raw lignocellulosic material which consists essentially of the steps of

• impregnating said raw lignocellulosic material in finely divided form at atmospheric pressure with an impregnating solution whose constituents consist essentially of water, 2 to 20% by weight of alkali metal hydroxide, alkali metal salt, alkaline earth metal hydroxide or alkaline earth metal salt and 2 to 20% by weight of a solvent having a boiling point above 150.degree. C. selected from the group consisting of diols, alkanolamines and sulfoxides, said impregnating being effected at a temperature of 80.degree. to 100.degree. C. using a quantity of impregnating solution such the amount of said alkali metal hydroxide or salt or alkaline earth metal hydroxide or salt retained by said lignocellulosic material is 11 to 29% of the dry weight of said material, and the amount of said solvent retained is sufficient to lower the melting point of said alkali metal hydroxide, alkali metal salt, alkaline earth metal hydroxide or alkaline earth metal salt in the said impregnated mass to between 150.degree. and 200.degree. C.,
• removing unabsorbed impregnating solution from the impregnated mass and adjusting the weight of the retained impregnating solution so that the ratio of the weight of said impregnated mass to the dry weight of said lignocellulosic material is not more than about 2.6:1,
• cooking a material consisting of the resulting adjusted impregnated solid mass at a temperature from 150.degree. and 200.degree. C. for 15 to 60 minutes in an exclusively externally heated closed reactor without exerting any mechanical disintegration action on the mass, and
• pressing the cooked product so as to extract therefrom the solubilized lignin in the form of a black liquor containing about 50% dry material.