Liquid purification or separation – Processes – Separating
Patent
1996-05-20
1999-05-04
Popovics, Robert J.
Liquid purification or separation
Processes
Separating
210777, 210780, 210331, 210332, 210391, 210396, 210408, B01D 3702, B01D 3306
Patent
active
059001585
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for thickening lime mud by means of a disc filter. The invention also relates to a method of cleaning the filter sectors of a disc filter. The method and apparatus according to the invention are especially suitable for continuously renewing the precoat layer and/or washing the wire surfaces.
Prior to the lime reburning kiln, the lime mud having a dry solids content of about 25% is thickened usually to a dry solids content of about 70-85% by means of a drum vacuum filter. A so-called precoat filtering layer is formed on the surface of the filter drum by lime mud, the thickness of which is 10-20 mm. The lime mud thickened on the precoat is removed therefrom by means of a scraper, subsequent to which the lime mud falls on a conveyor belt, which then feeds the lime mud to a lime reburning kiln.
The surface layer of the precoat gets clogged while the filtering proceeds and must be removed from time to time. The technique normally used is to move the automatic scraper gradually inwards following a predetermined program, thus removing the clogged surface layer. After the scraper has moved in several steps closer to the drum the rest of the precoat is removed and a new precoat is formed.
All the movements of the scraper described above and the removal of the whole precoat from the surface of the drum cause disturbances in the operation of the lime mud reburning kiln. Every time the clogged surface layer of the precoat has been removed by means of the scraper the filtering is more efficient and the lime mud volume is larger for a moment Further, also the dry solids content of the lime mud is higher after the precoat has been scraped which results in and calls for changes in the operation of the lime mud reburning kiln. When the whole precoat is renewed at the same time, the supply of the mud into the kiln is stopped altogether for a moment, which naturally causes a break in the operation of the kiln. Typically, the precoat is renewed once a shift, or correspondingly three times in 24 hours. In the long run, the wire is also so badly clogged that it must be washed either with acid or with a high-pressure washer. This operation also causes a long disturbance or interrupts the production of the lime mud reburning kiln.
The arrangement disclosed in U.S. Pat. No. 5,149,448 reduces the disadvantages and problems of the known techniques described above by removing a strip at a time from the filtering layer by means of a liquid jet reciprocating continuously in the longitudinal direction of the drum. This has been effected by installing one nozzle or several nozzles under the scraper, continuously reciprocating in the longitudinal direction of the drum. The high-pressure nozzles, for example two in number, have been provided under the scraper so that the purpose of one of the nozzles is to break the precoat layer and to drop it to the filter vat after which the second nozzle is used to wash the filter cloth clean. The pressure of the water sprayed from the washing nozzles is approx. 50-100 bar which gives a very good cleaning result. The nozzle disperses the jets so that the precoat is removed and the wire washed from a strip of approx. 10-50 mm in width at a time.
According to the arrangement of the above US patent, the washing nozzles have been installed in a rack designed for this purpose for moving the nozzles sideways. The nozzles are moved continuously to and fro in the longitudinal direction of the drum thus continuously renewing the precoat and cleaning the filter cloth. The advantages provided by this kind of continuous high-pressure water scraping are: mud reburning kiln is continuously uniform which allows smooth running of the kiln; a strip at a time; thus disturbances in the operation of the kiln due to a change of the precoat are avoided; wire.
According to the invention, the lime mud filtering by means of a disc filter is effected almost correspondingly: The lime mud is pumped via a feed duct or ducts into a dividing duct, w
REFERENCES:
patent: 2839194 (1958-06-01), Lopker et al.
patent: 3521751 (1970-07-01), Holthuis
patent: 4943372 (1990-07-01), Kohonen
patent: 5149448 (1992-09-01), Mattelmaki
patent: 5759397 (1998-06-01), Larsson et al.
Ruokolainen Pekka
Titoff Juha
A. Ahlstrom Corporation
Popovics Robert J.
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