Liquid purification or separation – With gas separator
Patent
1995-06-01
1997-04-15
Popovics, Robert J.
Liquid purification or separation
With gas separator
210194, 210197, 2103232, 210332, 210409, 2104161, 210456, B01D 2900
Patent
active
056205961
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method of filtering suspension and an apparatus for realizing said method. The present invention may be applied for filtering green liquor containing finely divided matter, generated, for example, in the chemical circulation of a pulp mill.
A significant subprocess in the manufacture of sulphate cellulose is the recovery of cooking chemicals. Part of said regeneration is formed by the manufacture of white liquor by causticizing, whereby lime milk and green liquor are allowed to react in order to form lime sludge and white liquor. The green liquor is generated when a chemical melt containing sodium chemicals is led from the bottom of the furnace of a soda recovery boiler to a separate dissolving vessel, in which the melt is dissolved in a dilute liquor. The most significant sodium chemicals of the green liquor are sodium carbonate and sodium sulphide. The green liquor also contains insoluble compounds, such as metal oxides, silicates, soot and other impurities. Said dregs containing impurities must be discharged from chemical circulation, since the dregs otherwise concentrates in the chemical circulation and disturbs the manufacture of white liquor in causticizing. The cleaning of green liquor is generally performed in clarifiers. When the mills become larger and the environmental regulations more stringent cleaning by sedimentation principle results in large apparatuses, the diameters of which exceed 30 meters. Moreover, clarifiers are always liable to disturbances, whereby too impure green liquor is obtained even from the correctly dimensioned apparatuses.
An alternative method is to clean green liquor by filtering. Since the dregs mainly consist of finely divided matter, the filterability of which is poor and which rapidly clog the filtering surface, said method :results in a filtering apparatus, which has a large surface area and is thus expensive. A cake containing fine particles must thus be removed from the surface often, whereby it, of course, remains thin. When such a cake is washed off, a very dilute sludge is obtained, which must be further concentrated in several process stages.
In order to improve the filterability, for example, lime sludge obtained from the causticizing process is used as filtration aid, by means of which a filtering layer having good filtering capacity when filtering finely divided green liquor is formed on the surface of the filter element. Said method results, however, in additive costs and increased amount of waste being transported to a landfill site.
The filterability of a suspension, which is difficult to filter, has been suggested to be improved also in such a way that the formation of a filter cake on the filtering surfaces is prevented by removing separated solids by causing a strong shear force on the suspension close to the filtering surface, which mixes solids back to the suspension to be filtered. The problem with this cross-flow filtration has been large liquid volumes, which must be circulated in order to generate sufficient velocity and turbulence to remove the separated solids from the filtering area.
The purpose of the present invention is to provide a filtering method and apparatus, by means of which said disadvantages may be eliminated. The present invention provides thus a simpler and more economic method and apparatus for filtering liquid-solids suspensions, especially those which have so far been difficult to filter. Although the background of the problem has been discussed above in view of the green liquor, it is apparent that it is only an example and that the invention may be applied to be used also for filtering other liquid-solids suspensions, such as kaolin sludge, clarified white liquor, soda liquor, bleaching effluent of cellulose and white water.
In the present filtering method the suspension to be filtered is brought into contact with the filtering surface of the filter element, whereby due to the pressure difference across the filtering surface the filtrate flows thro
REFERENCES:
patent: 3696933 (1972-10-01), Pall et al.
patent: 3835999 (1974-09-01), Moore
patent: 5008009 (1991-04-01), Ciaffoni
Engdahl et al, "Clarification of Green Liquor by a Falling Film . . . ", Paper and Timber vol. 76/No. 5/1994, pp. 326-329, May 24, 1994.
Engdahl Holger
Tormikoski Pekka
Ahlstrom Machinery Oy
Popovics Robert J.
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