Liquid purification or separation – Flow – fluid pressure or material level – responsive
Patent
1994-10-14
1996-05-21
Drodge, Joseph W.
Liquid purification or separation
Flow, fluid pressure or material level, responsive
210110, 2103232, B01D 2960, B01D 2988
Patent
active
055186092
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device for cleaning caustic liquor mixtures containing lime, comprising a housing, a substantially horizontal wall, which is arranged in the housing such that a closed chamber is formed in the housing below the horizontal wall, and a number of hollow filter elements, which are attached to the horizontal wall and extend from the latter downwardly in the chamber. The interior of each filter element communicates with the chamber via a great number of filtrate holes in the filter element. Each filter element is provided with a filter material, which covers the filtrate holes. In addition, the device comprises an inlet member for supplying the caustic liquor mixture to be cleaned to the chamber, means for discharging cleaned caustic liquor mixture from the filter elements, and means for providing a pressure difference between the chamber and the interior of the filter elements, such that during operation a clean fraction of the caustic liquor mixture is pressed through the filter material and the filtrate holes into the filter elements.
When producing sulphate pulp from raw material of wood, wood chips are cooked with white liquor containing caustic soda and sodium sulphide. During the cooking there is created black liquor containing dissolved wood substance. The black liquor is treated in a process for producing clean white liquor, which is reused for the cooking of wood chips. Intermediate products of caustic liquor mixtures are obtained during this process, such as uncleaned green liquor containing burnt wood substance, sodium sulphide and soda, uncleaned white liquor containing caustic soda, sodium sulphide and lime (calcium carbonate), and diluted lime mud containing lime and weak caustic liquor (low concentration of caustic soda and sodium sulphide).
The device of the kind described above is usually used for cleaning of said uncleaned white liquor, the white liquor being separated from lime, and for cleaning (washing) of said diluted lime mud, the lime being separated from weak liquor. During the cleaning of white liquor or lime mud, lime is deposited on the filter material, liquid impervious layers of lime being formed relatively rapidly on the filter material. At intervals of a few minutes the filter elements therefore are regularly backflushed with filtrate so that the layers of lime are released from the filter elements and sink towards the bottom of the chamber.
The device may also be used for cleaning of uncleaned green liquor obtained during said process, the green liquor being cleaned from burnt wood substance. However, lime has to be added to the uncleaned green liquor before the latter is supplied to the chamber in order to prevent an immediate clogging of the filter material with green liquor sludge. Thus, also in this case the filter elements have to be backflushed regularly to release the formed liquid impervious layers of lime.
A problem of this device is that liquid impervious permanent layers of lime often are formed on at least parts of the filter elements in spite of the regular backflushing of the filter elements. Such permanent layers of lime deteriorate the capacity of the device. Therefore the device has to be taken out of operation from time to time for manual cleaning of the filter elements.
The object of the present invention is to provide an improved device of the kind discussed above, so that the formation of permanent layers of lime on the filter elements is counteracted, whereby there is no need for taking the device out of operation as often as previously for the manual cleaning of the filter elements.
This object is obtained by means of a device of the kind described initially, which primarily is characterized in that the filtrate holes of the filter elements are arranged below a horizontal plane, which is situated below and at a distance from the horizontal wall such that during operation gas is enclosed between the wall and the caustic liquor mixture in the chamber. In consequence, during operation of the device the caustic liquor mixture
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Caustec
Drodge Joseph W.
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