Process for the conversion of lignocellulosic material to cellulose pulp by alkaline preoxidation followed by alkaline oxygen-free digestion both in the presence of a redox additive

A process for the essentially sulphur-free delignification of particulate lignocellulosic material that does not require oxygen during the delignification stage, with a short digestion time at high temperature, which comprises:

• (1) subjecting the lignocellulosic material to a preoxidation using an alkaline liquor at a temperature below 140.degree. C. in the presence of at least one redox additive that is converted into a reduced form during reaction with the lignocellulosic material;
• (2) withdrawing the reduced form of the redox additive with alkaline liquor and oxidizing the reduced form by oxygen gas in the absence of the lignocellulosic material at a rate sufficient to maintain the oxidized form of the redox additive in a major proportion and the reduced form in a minor proportion throughout the preoxidation;
• (3) continuing the preoxidation so that reducing sugar end groups in the lignocellulosic material are oxidized to aldonic acid end groups;
• and
• (4) then converting the lignocellulosic material to chemical cellulose pulp by delignification using strong alkali in the presence of at least one redox additive at a temperature within the range from about 160.degree. to 200.degree. C. without any addition of oxygen-containing gas.