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Batch cooking with black liquor pretreatment
| Details |
Inventors: Henricson, Kaj; Kettunen, Auvo; Ramark, Hannu;
Assignee: Andritz-Ahlstrom Oy (Helsinki, FI)
Primary Examiner: Silverman; Stanley S.
Assistant Examiner: Halpern; Mark
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
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). |
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DETAILED DESCRIPTION The present invention relates to a method and apparatus for cooking comminuted cellulosic fibrous material to produce cellulosic pulp.
Especially preferably, the invention relates to an alkaline batch cooking process, i.
e.
, to a so called Batch cooking process.
Alkaline cooking processes, in particular the sulphate or kraft process, have conventionally been performed in such a way that the wood matter, i.
e.
the chips, and the cooking liquor are added into a cooking vessel, the temperature of which vessel is then raised to the cooking temperature for a given period of time, whereby lignin is detached from the wood matter, the end result being chemical pulp and waste liquor.
In the sulphate process, white liquor, i.
e.
, a mixture of the active cooking chemicals sodium hydroxide (NaOH) and sodium sulfide (Na2S), is used as the cooking liquor.
The concentration of the active cooking chemicals is typically given in terms of "effective alkali" (EA) expressed as g/l as equivalent NaOH.
The waste liquor is typically referred to as "black liquor" which contains some amount of residual cooking chemicals, usually below 10 g/l EA as NaOH as well as dissolved wood material.
Modifications have been made to the conventional batch sulphate process, including the known SUPERBATCH and RDH batch cooking processes.
By reusing waste liquor, i.
e.
black liquor, generated in the cook a good heat economy and a low cooking Kappa number can be achieved with these processes.
However, mainly due to the reuse of the black liquor, the processes involve a great problem, namely inferior bleachability of pulp, that is, the pulp produced by these processes typically requires more bleaching chemical to obtain a comparable brightness compared to pulps produced from other methods.
In a cooking method according to our invention, it has been possible to eliminate the problem of bleachability by further development of the kind of process mentioned.
This has resulted above all in better bleachability, but in addition, the yield and relative strength of the pulp has also improved
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