Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1996-09-11
1998-08-04
Popovics, Robert
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 3011, 162 37, 162 38, 162 43, 162 44, 162 45, D21C 1100
Patent
active
057888132
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
This application is a U.S. national phase of PCT/Fl95/00125, International Publication No. WO 95/25198.
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method of and an apparatus for treating green liquor formed in a sulphate pulp mill, especially to a method of and an apparatus for enhancing the clarifying of green liquor.
An essential part of the process of producing kraft pulp is chemical recovery. The chemical recovery process includes production of white liquor by causticizing, whereby lime milk and green liquor are allowed to react so as to produce lime mud and white liquor. Green liquor is produced by directing a chemical melt containing sodium chemicals from the bottom of the furnace of a recovery boiler into a separate dissolving tank, in which the melt is dissolved into weak white liquor. The most important chemicals contained by the green liquor are sodium carbonate and sodium sulphide. Green liquor also contains insoluble compounds, such as oxides of metals, silicates, soot and other impurities. The insoluble impurities, dregs, are a light, fluffy material that must be removed from the green liquor. Otherwise these compounds will be concentrated in the chemical recovery loop which is detrimental to the production of white liquor by causticizing.
Usually the dregs are separated from the green liquor by settling in a clarifier, which is a vertical cylinder with a spherical bottom. In the middle of the clarifier is a smaller cylinder, into which the green liquor is introduced and in which the flow is damped. The dregs settle on the bottom of the clarifier and are scraped from there into a pocket at the bottom of the clarifier and then introduced to a dregs filter, in which the dregs are washed with water for recovering alkali. The alkaline washing liquid is used in the melt dissolver, while the clarified green liquor is directed to causticizing.
Usually the clarifier functions both as a storage tank and a dregs separator. The bottom functions as a clarifier and the rest as a storage tank. Clarified liquor rises from the clarifier portion to the storage portion, from which clarified liquor is discharged as needed through a liquor discharge duct. Because of the fineness of the dregs to be separated the separation process must be highly efficient. As the size of the mills increases and environmental regulations tighten, separation by clarifying conventionally leads to large equipment, with the diameters of clarifiers exceeding 30 meters. Additionally, a clarifying process is always sensitive to disturbances, such as fluctuations in pulp production process, whereby even a correctly dimensioned equipment yields green liquor with too much impurities.
So, in order to succeed, clarifying requires stability in both the quality and quantity of the introduced green liquor. An increase in the quantity of treated liquor would thereby necessitate an additional clarifier. An alternative is to treat part of the green liquor by filtering it with a filter parallel with the clarifier. Nevertheless, in this case directing the dregs from both the clarifier and the additional filter to the dregs filter, in which the separated dregs are washed, could result in overloading the dregs filter of the mill.
The purpose of the present invention is to provide a method capable of increasing the capacity and/or operation capability of an existing green liquor clarifier without sacrificing the quality of the clarified green liquor. An enhanced operation capability can be useful, should the rate of settling of the dregs decrease for some reason. Further, another purpose of the invention is to improve the operation of the dregs filter regardless of any increase in the quantity of green liquor.
Characterizing features of the method according to the invention for achieving the above-mentioned purposes are that a clarifier, from which clarified green liquor is discharged from the top and dregs-containing green liquor from the bottom; and in which it is filtered for s
REFERENCES:
patent: 4388197 (1983-06-01), Lumikko
patent: 4668342 (1987-05-01), Blackwell
patent: 4929355 (1990-05-01), Ragnegard et al.
patent: 5145556 (1992-09-01), Westerberg et al.
Engdahl Holger
Pokkinen Martti
Tormikoski Pekka
Ahlstrom Machinery Oy
Popovics Robert
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