Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With washing
Reexamination Certificate
2000-07-21
2001-10-16
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With washing
C162S049000, C008S156000
Reexamination Certificate
active
06302996
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of washing a fiber pulp mixture, in which method a fiber pulp mixture which is substantially constant in consistency is fed between two filter wires running in the same direction, at a substantially constant input flow, the wires being run into a closed dewatering space which converges in their direction of travel, whereby the filter wires support themselves on to surfaces equipped with dewatering holes, and water is removed from the fiber pulp mixture through the wires and the surfaces of the dewatering space, after which the washed fiber pulp mixture exits, supported by the wires, the dewatering space through a slice at its output end.
The invention also relates to an arrangement for washing a fiber pulp mixture, which arrangement comprises two water permeable filter wires arranged to run in the same direction, the wires being run into a closed dewatering space which converges in their direction of travel, in which dewatering space the filter wires are arranged to support themselves against surfaces equipped with dewatering holes, and means for feeding the fiber pulp mixture between the wires, whereby the water in the fiber pulp mixture is arranged to drain through the wires and the surfaces of the dewatering space and whereby the washed fiber pulp is arranged to exit, supported by the wires, the dewatering space through a slice at its output end.
BACKGROUND OF THE INVENTION
Fiber pulp, such as waste fiber, contains not only fiber-like raw materials, but also filling agents, such as ash and also fines originating from the fiber-like material. To be able to reuse such a fiber pulp in paper-making, it needs to be washed in a de-inking plant. There, a certain part of the fine fraction of the fiber pulp is removed by means of water, and the aim is to leave only useful fibers in the pulp. Thus, ash and other particle-like materials, for instance, are washed away by means of filtrate waters. Further, when washing fiber pulp, water is removed from it with the aim of affecting the consistency of the washed pulp. The washed accept is led on in the process to be used as raw material for newsprint and soft tissue, for instance.
There are several different apparatuses on the market for the above-mentioned pulp washing. The known apparatuses remove fines relatively well. The problem is, however, that fines loss cannot be adjusted during operation in the present apparatuses, and such an adjustment always requires a shutdown. When process conditions change, the apparatus is stopped and washing is changed by changing the wires, for instance. A further problem may be that when attempting maximum ash removal, fines loss becomes too high, which is uneconomical with respect to efficient use of the pulp. Changes in the process impair the running of the washers, producing a varying washing result which then causes problems in the phases after the washers.
The object of this invention is to provide a method and an arrangement which prevent the problems occurring in prior art solutions.
SUMMARY OF THE INVENTION
The method of the invention is characterized in that the thickness, i.e. pulp weight per surface area, of the fiber pulp layer being fed between the wires is adjusted during operation, whereby the ash and fines content of the washed fiber pulp can be adjusted to desired values.
Further, the arrangement of the invention is characterized in that the arrangement comprises means for adjusting during operation the layer thickness of the fiber pulp mixture being fed between the wires, to adjust the ash and fines content of the washed fiber pulp to desired values.
The essential idea of the invention is that the fiber pulp is fed between two wires which are arranged to run towards a slice at the output end of a convergent dewatering space. In the dewatering space, liquid in the pulp drains through the wires and further through the holed surfaces of the dewatering space. Fiber pulp is fed into the apparatus substantially at a constant flow. According to the idea of the invention, the grammage of the fiber pulp being fed between the wires, i.e. the thickness of the pulp layer running between the wires during washing, is adjusted. Ash removal ability is essentially dependent on the thickness of the fiber web being washed. Further, the essential idea of a preferred embodiment of the invention is that the grammage of the fiber pulp being fed between the wires is adjusted by adjusting the height of the slice, i.e. the quantity of the fiber pulp being fed between the wires. The idea of a second preferred embodiment of the invention is that the speed of the wires, i.e. the running speed of the washer, is adjusted. Another essential idea is that the layer thickness of the pulp being fed in is adjusted on the basis of the ash and fines content measured from the washed fiber pulp, the fiber mixture fed between the wires and/or the removed water.
The invention provides the advantage that this way it is possible to adjust the ash and fines content of the washed fiber material to be suitable for each use. Large quantities of waste fiber are used in newsprint and soft tissue, for instance, and the requirements for their ash and fines content are different. Soft tissue requires maximum ash removal, whereas newsprint does not require complete ash removal, but a certain level with which the desired strength and other technical properties can be achieved. With the solution of the invention, it is possible to exactly adjust the ash content. It is then possible to manufacture pulp having different fines contents simply and quickly in the same apparatus without complex changing of settings and wires. In pulp of this kind, the relative proportion of ash and fines in the washed fiber pulp, and correspondingly the proportion of other fibers, is as desired. Thus, the same kind of apparatus can be used in various factories, as the paper and process engineering properties of the washed pulp can always be tuned as required by each factory. Further, other processes of the factory can be kept as they are or the processes can be adjusted quite freely independent of the washing. In addition, the solution of the invention is more environment-friendly than before, because ash and other fines are not unnecessarily washed from the pulp. This means that smaller quantities of fines find their way outside the process with filtrate waters. Because the washing is, due to the adjustment, done at exactly the right efficiency, the power consumption of washing is smaller than before. This matter is naturally significant for both the environment and the price of the final product. Further, the process becomes more stable and easier to run due to measurements and the adjustments made on the basis thereof, whereby a fiber pulp having a more even quality exits the washer for the following phases of production. This way, process variations occurring during the phases prior to washing and sudden peaks caused by process disturbances can also be evened out. Owing to the adjustment according to the invention, the washing can be adjusted without needing to change the flow or the consistency of the pulp being fed into the washer. Thus, the washing does not require changes in the prior process phases and expensive additional equipment.
BRIEF DESCRIPTION OF THE FIGURE
The invention is described in greater detail in the attached drawing which shows a schematic sectional side view of the principle of a washing apparatus of the invention.
REFERENCES:
patent: 4501040 (1985-02-01), Biondetti et al.
patent: 5196090 (1993-03-01), Corbellini et al.
patent: 5257438 (1993-11-01), Fass et al.
patent: 0 162 179 A1 (1985-11-01), None
patent: 0 488 058 A1 (1992-06-01), None
patent: WO 96/09433 A1 (1996-03-01), None
Tappi, Test Method T211 om-93, “Ash in Wood, Pulp, Paper, and Paperboard: Combustion at 525 Degrees C” (abstract only from www.tappi.org).*
Tappi, Test Method T261 cm-00, “Fines Fraction by Weight of Paper Stock by Wet Screening” (abstract only from www.tappi.org).*
Tappi, Test Method T413 om-93, “Ash
Alston & Bird LLP
Chin Peter
Hug Eric
Metso Paper Inc.
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