Process for the manufacture of pulp using sodium carbonate and oxygen

A process for the manufacture of pulp in a closed system, which comprises

• 1. mixing a fibrous raw material with an alkaline aqueous solution of sodium carbonate or a mixture of a major proportion of sodium carbonate and a minor proportion of sodium bicarbonate as the only alkaline ingredients, which alkaline aqueous solution is completely free of sulfite and sulfide compounds and sulfur-containing ions resulting from dissolution of said compounds, the amount of sodium carbonate or mixture of sodium carbonate and sodium bicarbonate being 5 - 20% by weight, as sodium oxide, based on the weight of the raw material,
• 2. pre-cooking the fibrous raw material in the alkaline aqueous solution at a temperature of 90.degree. to 190.degree. C,
• 3. defibrating the pre-cooked material by mechanical means,
• 4. removing a major proportion of waste liquor, formed during the pre-cooking step, from the defibrated material,
• 5. pulping the defibrated material in an alkaline aqueous solution of sodium carbonate or a mixture of a major proportion of sodium carbonate and a minor proportion of sodium bicarbonate as the only alkaline ingredients, which alkaline aqueous solution is completely free of sulfite and sulfide compounds and sulfur-containing ions resulting from dissolution of said compounds, the amount of sodium carbonate or mixture of sodium carbonate and sodium bicarbonate being 5 - 20% by weight, as sodium oxide, based on the weight of the raw material, in a pressurized pulping vessel at a temperature of 90.degree. to 170.degree. C in the presence of an oxygen-containing gas,
• 6. discharging waste liquor, formed during the pulping step, from the pulping vessel,
• 7. subjecting the waste liquor discharged from at least one of the pre-cooking and pulping steps to wet combustion, in which the waste liquor is treated at a temperature exceeding 230.degree. C in the presence of an oxygen-containing gas, to recover an alkaline aqueous substance containing sodium carbonate or a mixture of a major proportion of sodium carbonate and a minor proportion of sodium bicarbonate as the only alkaline ingredients and
• 8. recycling the recovered alkaline aqueous substance, without any chemical treatment thereof, to at least one of the pre-cooking and pulping steps.