Liquid purification or separation – Processes – Separating
Patent
1991-05-30
1992-09-22
Dawson, Robert A.
Liquid purification or separation
Processes
Separating
210167, 210188, 210253, 2102571, 210258, 210332, 21033301, 210340, 210346, 210411, 210486, 210771, 210772, 210797, B01D 2962
Patent
active
051494493
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method for filtering a suspension, e.g. filtering for separating liquor and lime sludge in a causticising process, and a device for performing the method.
For performing the filtration a stationary filter is used being arranged in a pressure vessel, and after each filtration a re-conditioning of the filter is made including back-washing the filter before start of the next filtration. The filtering takes place through at least one substantially vertically arranged filter element covered with filter cloth surrounding at least one filter channel through which flows filtrate obtained during filtration, for instance white liquor or green liquor, while filtered material, for instance lime sludge, is deposited on the filter cloth. The pressure vessel is provided with at least inlet means for the suspension to be filtered, outlet means for filtrate obtained during filtration, outlet means for material obtained during filtration, and inlet means for media for re-conditioning the filter.
According to prior art utilizing a stationary filter, separation of white liquor and lime sludge in a causticising process is performed in a so called tube filter, i.e. depending perforated tubes covered with filter cloth which have their filter outlets directed upwards. This tube filter is divided in separate parts having separate white liquor outlets from each part. While parts of the tube filter are back-washed, which takes place with liquid already filtered, i.e. white liquor, filtering takes place in the other parts. This involves, a.o., a bad ratio of utilization of the filter. Further, during the entire process a continuous discharge of material removed by the back-washing, i.e., lime sludge, takes place from the pressure vessel. This brings about a.o. the drawback, that the lime sludge gets a great content of unfiltered liquid including a large content of chemicals.
The object of the present invention is to provide a new and efficient method and an improved device for filtering a suspension. This has been achieved by the method stated in claims 1-18 and the device stated in claims 19-32.
The drawbacks mentioned above have widely been eliminated by the method according to the present invention. By discharging filtrate from the bottom of the filter it is assured that the filter elements are emptied of filtrate when back-washing takes place. Further, since back-washing according to the present invention is made with another medium than filtrate, preferably water, it is achieved that material removed from the filter cloth is not mixed with a washing medium having a large content of chemicals, as is the case when filtrate is used as back-wash liquid. At the same time a dilution of the removed filter cake is obtained already in the filtering vessel which in turn results in that the mixture of washing liquid and removed filter cake self-drains out through an outlet provided for this purpose without any need for a scraper mechanism or other emptying means. In the drying step, preferably taking place before the back-wash, a gas medium is preferably used which is circulating at least between the pressure vessel and a separator for separating the gas medium from the filtrate, but preferably also between a tank for collecting filter cake and washing medium. By circulating the gas medium, mainly air and water vapour, is achieved that the oxygen contents of the gas medium is so low that oxidation of the filtrate is avoided. In all, in improved filtering result having high capacity and efficiency is obtained with a method according to the invention as compared to the prior art.
A method according to the invention enables in a simple manner a continuous filtering and re-conditioning cycle in a unit including at least two pressure vessels. According to the present invention, while the filtering process according to the above proceeds in one pressure vessel the re-conditioning process as stated above is performed in the other pressure vessel. The invention enables in an advantageous manner common sources for f
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Oswaldsson Rolf
Strid Kent
Dawson Robert A.
Kim Sun Uk
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