Process for pulping lignocellulosic material with an alkaline sulfide cooking liquor containing an accelerating additive and reducing assistant

A process for pulping lignocellulosic material comprising delignifying, at a temperature of from 140.degree. to 190.degree. C., a lignocellulosic material with an alkaline sulfide cooking liquor containing:

• (a) sodium sulfide;
• (b) sodium hydroxide;
• (c) a delignification-accelerating additive which consists of at least one cyclic organic compound selected from the group consisting of quinone compounds, hydroquinone compounds, 9,10-diketohydroanthracene compounds and 9,10-dihydroxyhydroanthracene compounds, and which is in an amount of from 0.01 to 5% based on the bone dry weight of said lignocellulosic material;
• and
• (d) a reducing assistant which consists of at least one compound selected from the group consisting of sodium sulfite and potassium sulfite, and which is in an amount of from 0.49 to 5.0% in terms of Na.sub.2 O, based on the bone dry weight of said lignocellulosic material, and separating the resultant delignified material from the delignifying mixture;
• the sodium sulfide and the sodium hydroxide being present in said alkaline sulfide cooking liquor in amounts to provide therein (i) an effective alkali content of from 8 to 40%;
• and (ii) a sulfidity of from 3 to 50%, both based on the bone dry weight of said lignocellulosic material.