Cooking process of lignocellulose material

In a continuous cooking process making use of a digester, which includes therein, from a top toward a bottom of the digester, a top zone, an upper cooking zone, a lower cooking zone and a cooking/washing zone and also includes strainers provided at the bottom of the respective zones and wherein a cooking black liquor extracted from at least one of the strainers is discharged to outside a digestion system, a process for cooking a lignocellulose characterized by comprising:

• feeding, upstream of the top of the digester, the following first cooking liquor;
• feeding the following second cooking liquor to the upper cooking zone;
• and
• feeding the following third cooking liquor to the cooking/washing zone.
• First cooking liquor:
• an alkaline cooking liquor that is made of polysulfide, and sodium hydroxide and sodium sulfide or sodium carbonate and sodium sulfide as main components, contains polysulfide sulfur at a sulfur concentration of 3˜20 g/L and contains not less than 99 mass % of a sulfur component and 80˜95 mass % of effective alkali relative to total sulfur component of cooking activity and contains 80-95 mass % of effective alkali relative to total alkali, respectively, contained in a total amount of alkali cooking liquors to be introduced into the digestion system.
• Second cooking liquor:
• an alkaline cooking liquor made mainly of sodium hydroxide.
• Third cooking liquor:
• an alkaline cooking liquor similar to the second cooking liquor.