US 6,350,348 B1Granted

Batch cooking with black liquor pretreatment

AssigneeAndritz-Ahlstrom Inc.

InventorsKaj Henricson, Auvo Kettunen, Hannu Ramark

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Abstract

A method of cooking pulp in an alkaline batch process (using one or more batch digesters) improves pulp bleachability. The wood material is supplied to the digester; the cellulosic material is pre-treated in the digester with a first liquor, the first liquor is displaced by a second liquor, the displaced first liquor is led to a further treatment, the cellulose material is treated with the second liquor, and the cooked pulp is removed from the digester. The second liquor is displaced as two consecutive parts, i.e. as strong black liquor and weak black liquor, and after cooling in a heat exchanger the strong black liquor thus obtained is fed into the digester as the first liquor in a subsequent batch (in the same, or a different, batch digester).

Claims28

Dependent claims

A method of cooking comminuted cellulosic fibrous material to produce cellulose pulp in at least one batch digester comprising substantially sequentially:

(a) introducing the material to the digester;
(b) introducing a first liquor having a first effective alkali concentration to the digester;
(c) treating the material in the digester with the first liquor;
(d) displacing first liquor from the digester with a second liquor having a second effective alkali concentration, to produce a third liquor, displaced from the digester, having a third effective alkali concentration lower than the first effective alkali concentration;
(e) treating the material in the digester with the second liquor;
(f) displacing the second liquor from the digester with a fourth liquor having a fourth effective alkali concentration to produce a fifth liquor, displaced from the digester, having a fifth effective alkali concentration, lower than the second effective alkali concentration and between 20-40 g/l, expressed as NaOH;
and
(g) removing the treated material from the digester;
and wherein the fifth liquor comprises at least some of the liquor to be used as the first liquor in (b) for a subsequent batch, and repeating (a)-(g) for the subsequent batch in the same or a different digester.

6 more elements

Dependencies are inferred from the claim text.