| 摘要 |
The sulfide ion concentration and sulfidity of kraft cooking liquor during kraft cooking of cellulose pulp is selectively increased. After treatment in a first treatment zone in which impregnation or kraft cooking takes place using a kraft cooking liquor having a first sulfide ion concentration and sulfidity, black liquor is extracted from the material, liquid is withdrawn from the material and dilution liquid is added to the withdrawn liquid and the withdrawn liquid with dilution liquid is reintroduced. In a second treatment zone after the first zone a second kraft cooking liquor is introduced having a second sulfide ion concentration and sulfidity greater than the first sulfide ion concentration and sulfidity (typically by about 20-50%), including by manipulating controlling the flow rate of extraction and the flow rates of withdrawal of liquid and addition of dilution liquid. The pressure of the continuous digester is controlled in a unique manner that avoids disruptions to the column of pulp continuously moving downwardly in the digester, anywhere in the digester, but particularly avoids non-uniform, unstable material in the countercurrent washing zone. The pressure is controlled by withdrawing liquid from, and introducing liquor into, the digester at at least one additional extraction-dilution loop aside from the main extraction of the digester and the wash dilution liquid introduction mechanism below the wash screens. Pressure can also be maintained (e.g. at about 130-170 psi) by also controlling the amount of wash dilution liquor, and by varying the extraction flow.
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