(57) In the production of paper pulp by the sulfite pulping process, the amount of sulfite
cooking chemical consumed per ton of pulp produced is significantly lowered, a much
lower K-No. can be achieved without screen plugging, the wash circulation temperature
is lowered, and the digester (19) runs better, in the practice of the invention. According
to the invention, the wood chips entrained in sulfite cooking liquor are fed into
the top of a digester, and flow co-currently to a central portion (21) of the digester,
at which portion waste liquor is extracted and passed to flash tanks (25). An effective
amount of the sulfite cooking liquor (preferably about 5-20%) utilized for producing
the pulp is introduced into the vessel in the wash circulation (36), adjacent the
bottom of the digester, to flow countercurrently to the material moving downwardly
in the vessel. The produced sulfite pulp is withdrawn (at 33) from the bottom of the
digester. The invention is primarily applicable to acid or bisulfite pulping, but
also can be utilized with alkaline or neutral sulfite pulping. In some circumstances
the entire cooking flow can be countercurrent (e.g. 5% up to close to 100% of the
sulfite cooking liquor flows countercurrently to the material).
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