CLARIFYING AND/OR FILTERING OF GREEN LIQUOR AND AN APPARATUS THEREFORE

Abstract: The present invention relates to a method of and an apparatus for treating the green liquor formed in a sulphate pulp mill, especially to a method of enhancing the clarification of the green filter. According to the invention, at least a portion of the green liquor stream of the mill is directed into a clarifier. Clarified green liquor is discharged from the clarifier while dregs-containing green liquor is discharged from the bottom of the clarifier. The dregs-containing green liquor is directed into a filter, in which it is filtered for forming a filtrate and a dregs. Dregs separated from the green liquor can be washed already in this step, thus lightening the load on the actual dregs filter. Filtrate, i.e. filtered green liquor, is directed for further treatment along with clarified green liquor. The connection according to the invention enables a portion of the stream of green liquor to be directed past the clarifier directly to the filter along with dregs-containing green liquor.

Description: Clarifying and/or filtering of green liquor and an apparatus therefore. 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. These 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) at least a portion of the green liquor stream of the mill is directed to a clarifier, from which clarified green liquor is discharged from the top and dregs-containing green liquor from the bottom; and b) dregs-containing green liquor from step a) is introduced into a filter, in which it is filtered for separating filtrate from dregs. Characterizing features of the apparatus according to the invention are that it comprises - a clarifier and a filter connected in series for introducing dregs-containing green liquor from the clarifier to the filter, - means for introducing green liquor into the filter, - means for discharging clarified green liquor from the clarifier and a duct for discharging filtered green liquor from the filter and - means for removing green liquor dregs from the filter. The connection according to the invention enables a portion of the green liquor stream to be introduced past the clarifier directly to the filter along with the dregs-containing green liquor. The method and apparatus according to the invention enhance the clarification of green liquor by connecting a filter in series with the clarifier, the filter further treating the dregs-containing green liquor from the clarifier. The dregs separated from the green liquor can, at this stage, be also washed in this filter for recovering alkali. According to present practice the dregs are washed at the dregs filter. Thus, the present invention greatly improves and enhances the operation of the solids filter, as the alkali content of the dregs to be filtered there is greatly reduced. Also, the filter produces a much more concentrated dregs than the clarifier. Usually, the solids concentration of the dregs from a clarifier is about 2-3%, whereas the connection according to the present invention can increase the concentration to about 5-15%. The method according to the invention is not as prone to various disturbances as clarification alone. Should the clarification be for some reason disturbed, the volume of clarified green liquor from the clarifier can be reduced to include only the clearest green liquor. The volume of the stream of dregs-containing green liquor to the filter is then accordingly increased. Thus, both the clarifier and the filter produce green liquor with a low enough dregs content for further use. The filter connected in series with the clarifier can be any filter capable of treating dregs-containing green liquor. Such filters include, e.g., drum, disk and tube filters with a filtering layer formed by lime mud. Corresponding vacuum filters can also be used. A preferred filter for this purpose is described in international patent application PCT/FI94/00485. In the following, the invention is described in more detail by way of reference to the accompanying drawing, which is a schematic illustration of an apparatus for carrying out the method according to the invention. The green liquor to be treated is introduced into the clarifier 10 via ducts 12 and 13. Clarified green liquor with low enough dregs content is discharged from the clarifier via conduit 14, while dregs-containing green liquor is directed from the bottom of the clarifier 10 to the filter 18 via duct 16. Because the dregs conduit 16 of many an old clarifier is underrated for the volume of green liquor to be treated by the filter after an increase in the stream of liquor, a portion of the green liquor is directed past the clarifier to the filter 18 via duct 15. In this embodiment the filter is an apparatus as described in the international patent application PCT/FI94/00485, the apparatus comprising a plurality of filter elements having filter surfaces. The green liquor to be treated is directed via ducts 16 and 17 to a liquid distribution tray 22 located above the filter elements, in which the green liquor is divided to flow down on the filter surface of the elements. The film of liquid, flowing downwards, is filtered through the filter surfaces of the elements so that the filtrate flows through the surface while the solids remain on the surface. The filtrate, i.e. filtered green liquor, is discharged from the filter via duct 24 into tank 26, where gas 28 is separated from the filtrate. The gas can be recycled to the filter via a compressor 30 located along duct 32. The filtered green liquor is directed away from the tank 26 along duct 34 and directed together with clarified green liquor via duct 36 to further treatment at the causticizing plant. The green liquor from the filter can also be separately treated, i.e. it is not taken to the causticizing plant for production of white liquor. Filtered green liquor is usually cleaner than clarified green liquor, so it can be utilized in, e.g., the bleaching process. Sodium carbonate is separated from the filtered green liquor and then further causticized into sodium hydroxide to be used in bleaching. Separation of sodium carbonate produces high-sulphidity green liquor to be preferably used in the initial phase of pulp cooking. Unfiltered dregs-containing green liquor is collected on the bottom of the filter, from where it is removed via duct 37 and redirected via sludge tank 38 (or directly from the filter) to the filtration surface via ducts 40 and 17. Dregs slurry is removed from the process via duct 42, tank 44 and duct 46 to the dregs filter 48. The load on the actual dregs filter 48 can be lightened by the method according to the invention, as the invention provides for washing the dregs already in the filter 18. In this case dregs are intermittently filtered in the filter 18. Instead of removing dregs slurry from the tank 38 during a filtration cycle, it is recycled and concentrated. At the end of the filtration cycle, the feed of green liquor to the filter is replaced by washing water from duct 50 and filtration is continued. Washing water, while mixing with the dregs, removes alkali therefrom. Subsequent to the washing, dregs are removed to the solids filter 48. Thus treated, dregs are cleaner and thicker than dregs that have been conventionally directed from a clarifier directly to the dregs filter. The efficiency of dregs washing can be improved by allowing it to form a cake on the filtration surface. The filter 18 can also be a pressure or vacuum filter known per se, with either a drum or a disk as the filter element. The present invention is highly preferable in cases where the existing clarifier is either overloaded or its capacity must be increased. The advantages provided by the invention are listed in the following: - the clarifier can be operated at its rated capacity, whereby it produces clean liquor; - the space required by the new piece of machinery (the filter) is small; - all the dregs of the mill are treated in the filter, whereby the concentration of the dregs is higher when it is directed for further treatment; - all the dregs can be pre-washed; - the actual dregs washing apparatus (dregs filter) is capable of treating more dregs than when collecting dregs from the bottom of the clarifier; - when increasing the production of the mill, the capacity of existing machinery is often sufficient for final washing of dregs, even though the amount of dregs increases.

Claims: WE CLAIM: 1. A method of treating green liquor of a sulphate pulp mill, especially a method of enhancing the clarification of green liquor, characterized in that a) at least a portion (13) of the green liquor stream (12) of the mill is directed to a clarifier (10), from which clarified green liquor (14) is discharged from the top and dregs-containing green liquor (16) from the bottom; and b) dregs-containing green liquor (16) from step a) is introduced into a filter (18), in which it is filtered for separating filtrate (24) from dregs (37). 2. A method according to claim 1, characterized in that a portion (15) of the green liquor (12) is directed past the clarifier (10) directly to step b) along with dregs- containing green liquor (16). 3. A method according to claim 1 or 2, characterized in that filtrate (34) , i.e. filtered green liquor, from step b) and clarified green liquor (14) from step a) are directed for further treatment (36). 4. A method according to claim 1, 2 or 3, characterized in that step b) and washing the dregs resulting from the step are effected in the same filter (18). 5. A method according to claim 4, characterized in that the washed dregs are treated in a dregs filter (48). 6. An apparatus for treating green liquor, especially an apparatus for enhancing the clarification of green liquor, characterized in that it comprises - a clarifier (10) and a filter (18) connected in series for conducting dregs-containing green liquor from the clarifier into the filter, - means (13) for directing green liquor into a clarifier (10), - means (14) for discharging clarified green liquor from the clarifier and means (24) for removing filtered green liquor from the filter, and - means (37) for discharging dregs from the filter (18). 7. An apparatus according to claim 6, characterized in that it additionally comprises means (15) for directing a portion of the green liquor directly to the filter (18).

Inventors: ENGDAHL HOLGER, POKKINEN MARTTI, TOERMIKOSKI PEKKA, ENGDAHL, HOLGER, POKKINEN, MARTTI, TOERMIKOSKI, PEKKA
Applicants: AHLSTROEM OY, ENGDAHL HOLGER, POKKINEN MARTTI, TOERMIKOSKI PEKKA, A. AHLSTROM CORPORATION, ENGDAHL, HOLGER, POKKINEN, MARTTI, TOERMIKOSKI, PEKKA
Application year: 1995
Main classification: D21C11
IPC codes: D21C11