Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Patent
1995-11-06
1997-09-30
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
162 55, 162243, D21C 324, D21C 900
Patent
active
056722451
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a process for continuous cooking of chemical pulp, and relates in particular to a process for dealing with knots in conjunction with the continuous cooking of chemical pulp.
PRIOR ART AND PROBLEMS
When cooking pulp, the majority of the raw wood which is cooked will have been delignified to a sufficient extent for defibration to be achieved. However, a relatively small proportion of the raw wood consists of knots which differ in character from the rest of the raw wood insofar as they are more difficult to cook, i.e., they require a longer dwell time in order for sufficient delignification to be achieved. This problem is usually solved by separating these knots off, after cooking, and returning them to the digester for repeated treatment. In this respect, the knots are normally separated off by screening and are then conveyed to a washing stage and, directly thereafter, to a knot bin. The knots are then fed/blown from the knot bin into the chip bin and thereby commence their second cooking run. The disadvantage of such a process is that a relatively large amount of energy is required to transport the knots right up to the top of the relatively high chip bin. Moreover, fairly expensive equipment, such as a sluice feeder, etc., is needed for transporting the knots. In addition, it is of course a disadvantage that the chip bin has to be constructed in such a way as to take into account the extra load which is exerted by the equipment for transporting and, if appropriate, for dewatering.
SOLUTION AND ADVANTAGES
According to the present invention the knots are conveyed from the knot dewatering device to a tank, from which they are transported to the chute circulation without first being fed down through the chip bin or the steaming vessel. The knots are preferably fed to the chip chute in the form of a relatively low consistency suspension, for example approximately 5%, with the aid of a centrifugal pump. By proceeding according to the invention, most of the abovementioned disadvantages are eliminated or at least minimized.
According to a further aspect of the invention, use is made of a black liquor and/or filtrate from one of the subsequent pulp washing stages, preferably following the screening, in order to dilute the knots/suspension to the desired strength.
BRIEF DESCRIPTION OF THE FIGURES
The invention will be described in greater detail hereinbelow with reference to the attached figures, in which:
FIG. 1 shows a preferred embodiment of the invention in conjunction with a one-vessel digester, and
FIG. 2 shows an alternative embodiment of the invention in conjunction with a two-vessel system, i.e., with a separate impregnation vessel.
FIG. 1 thus shows the "front part" of a fibre line. Chips are fed into a chip bin, preferably a HULA BINX.TM. (trademark of Kvaerner Pulping Technologies AB). From the chip bin 1 the chips are fed via a low pressure feeder into a steaming vessel 2, and from the end of the steaming vessel the chips are fed down into a chip chute 3. The pressure in the steaming vessel is approximately 1.5 bar. The chip chute 3 is mounted on top of a high pressure feeder 4, the purpose of which is to pass the chips into the digester 5 which is at a considerably higher pressure (at least 8 bar). The chips are thus fed with the aid of the high pressure feeder 4 to the top of the digester 5, where some of the transport liquor is separated off and re-circulated to the high pressure feeder. By means of holding a suitable temperature/temperatures and adding suitable chemicals, as is generally known to the person skilled in the art, the chips are cooked in the desired manner in the digester 5, by means of which a desired delignification is achieved. The ITCX.TM. method developed by Kamyr is preferably used, in which essentially the same temperature level is maintained in all the cooking zones so that the pulp can be cooked to very low kappa numbers, while retaining good strength properties. The cooked pulp, which has now been defibred, is now con
REFERENCES:
patent: 3393121 (1968-07-01), Lea et al.
patent: 3886035 (1975-05-01), Laakso
patent: 4002528 (1977-01-01), Laakso
patent: 5302247 (1994-04-01), Richardsen et al.
patent: 5401361 (1995-03-01), Prough et al.
Andtbacka Elisabeth
Andtbacka Stig
Bergqvist Anders
Czaja Donald E.
Kananen Ronald P.
Kvaerner Pulping Technologies Aktiebolag
Nguyen Dean T.
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