CA1076322A - Gas cleansing means - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Method and apparatus for treating gases by passing said gases horizontally through a plurality of spray zones wherein the gases are contacted in each spray zone with scrubbing medium in a plurality of vertically disposed bays; utilizing a scrubbing medium of varied concentration or chemical reactivity between the adjacent spray zones and bays;
combining and reamalgamating the spray medium and re-spraying said amalgamated medium in one or more spray zones and bays.

Description

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In the art of gas cleansing it is well known to provide wet scrubbing apparatus or wet electrostatic precipi-tating apparatus for the purpose of purging a pollutant laden ~as stream of entrained gaseous or particulate pollutants before discharging such a s~ream to the atmosphere. For example, users of large quantities of coal have commonly employed well known wet scrubbing apparatus adapted to chemically purge coal combus-tion gases of entrained gaseous sulfur dioxide and additionally to physically arrest particulate impurities such as fly ash.
Likewise, wet electrostatic precipitating apparatus has been employed in the art to purge such pollutants as fly ash or liquid droplets from a polluted gas stream.
Although heretofore known wet scrubbing and wet pre-cipitating devices generally have served the purposes intended, they have nonetheless been subject to certain undesirable defic-iencies. For example, although known wet electrostatic precipi-tators have been successfully employed to purge particulate impurities, they generally have not been adaptable for the re-moval of gaseous pollutants from gas streams; thus the utility of such precipitators have been limited. Additionally, known precipitators have often been susceptible to overloading by large quantities of particulate matter. By way of further example, it is noted that some prior wet scrubbers have required very large and powerful fans to force polluted gases at relatively high velocity through relatively restricted flow passages with-in the scrubber. Not only is the use of such fans unnecessari.ly wasteful of energy, but the resultant high velocity gas flow through the scrubber has in some cases complicated gas cleansing operations.

3~2 The cited deficiencies of prior gas cleansing apparatus are nullified by the present invention which provides a combined and substantially unitary wet scrubbiny and wet electrostatic precipitating apparatus having substantially open and unobstructed gas flow passages for the passing of a pollutant laden gas stream therethrough at a relatively low velocity. The present invention additionally includes means for contacting such a gas stream with sprays of liquid or slurried scrubbing medium in a plurality of spray zones spaced along the gas flow path within the scrubbing chamber, means to vary scrubbing medium concen-ration or chemical reactivity between adjacent spray zones, and means to rejuvenate the scrubbing medium by reamalgamation and respraying thereof with one or more spray zones.
In one particular aspect the present invention provides a pollution control system for treating gas containing oxides of sulfur comprising: wet scrubbing means adapted to discharge a scrubbing liquor for removing oxides of sulfur in the form of wet precipitates from gas passing therethrough; a wet demister means located downstream of said scrubbing means for arresting droplets of scrubbing liquor entrained in gas which passed through said wet scrubbing means and adapted to have a continuous flow of washing liquor therein; first conveying rneans for conveying at least a portion of the liquors exiting from said scrubber means and said demister to a common conditioning means;
said conditioning means being operable to separate such

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liquors into a first zone of liquor and a second zone of concentrated slurry; and a second conveying means for conveying at least a portion of said liq~or in said first zone to said demister.
In a further particular aspect the present invention provides a method of removing air pollutants from a stream of gas comprising the steps of: scrubbing said stream of gas with a scrubbing liquor containing reactants which absorb at least a portion of said pollutants from said gas;
subsequent to said scrubbing, passing said stream of gas through a wet electrostatic precipitator to demist said stream of gas and remove at least a poriion of the residue pollutants therein; simultaneously with said first mentioned passing, passing a washing liquor over the collecting surfaces of said electrostatic precipitator;
subsequent to said scrubbing, collecting -2a bm:-~

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said scrubbln~ liquor and classifying said collected scrubhing liquor into a ligllter portion and a heavier portlon; recir-culating said lighter portion from said scrubbLng; collecting said washing liquor passing over said collecting surfaces and classifying said heavier portlon and said collected washing liquor into a reusable portion and a non-reusable portion;
and combining at least a part of said reusable portion with said scrubbing liquor.
Other objects and advantages of the present invention are more fully specified in the following description and illustrations in which:
Fig. 1 illustrates in schematic a gas cleansing apparatus constructed in accordance with the principles of the present invention;
Fig. 2 is a partly cut away trimetric projection of the scrubber portion of the present invention;
Fig. 3 is an enlarged fragmentary portion of Fig. 1 illustrating in schematic the wet scrubber portion of the present invention; and Fig. 4 illustrates in schematic the wet scrubber portion of Fig. 3 substantially as seen from line 4--4 of Fig. 3.
There is generally indicated as 10 in Fig. 1 a greatly simplified gas cleansing apparatus constructed in accordance with the principles of the present invention and including a wet scrubbing means 16 and a demister, such as the wet electrostatic precipitating means 18, both of which operate according to well known Eundament~l principles. Those versed in the art will ~ 2b-7!63~2 recognize that the apparatus 10 may in practice be adapted to purge various gaseous, particulate or liquid pollutants, or combinations thereof from a gas stream passed therethrough.
Ho~ever, for purposes of illustrating one embodiment of the present invention the recitation hereinbelow is directed to a simplified ~as cleansing apparatus which is adapted to purge ~oth gaseous and particulate pollutants from waste gases such as flue gas of fossil fuel combustion. Of course, it is to be understood from the outset that such simplification is not intended to unduly limit the scope of the invention described.
During combustion of fossil fuels such as coal or oil and particularly during combustion of fossil fuels having a relatively high sulfur content, oxidation of sulfur produces quantities of gaseous sulfur oxidesj notably sulfur dioxide, a noxious and objectionable air pollutant the purging of which from flue gases is highly desirable. Fossil fuel combustion may addi-tionally produce other undesirable air pollutants, for example the fly ash generated by burning powdered coal or droplets of sulfur dioxide in aqueous solution suspended in the flue gas stream. Preferably these and all other objectionable pollutants are to be substantially arrested prior to discharging the gas stream to the atmosphere~
Control of such undesirable emissions may be effected by means of the present invention. Accordingly, in the apparatus 10 the scrubber 16 and precipitator 18 are housed within a gas cleansing module 12 having a gas inlet portion 4 and a gas out-let portion 6. Inlet portion 4 communicates with any convention-al source of pollutant laden gas such as the flue of a powdered coal burner (not shown), and outlet portion 6 communicates with 3Q the atmosphere via a stack 8 whereby in practice a continuous stream of flue gas bearing such pollutant components as men -tioned hereinabove is directed via inlet 4 into module 12 , ~763ZZ

through scrubber 16 and precipitatox 18 within the module 1~, and thence to the atmosphere via outlet 6 and stack 8. Within scrubber 16 the flue gas stream is subjected to the wet scrub-bing action of a chemical scrubbing medium for chemical absorp-tion of gaseous sulfur dioxide and physical arrest of particulate contaminants such as fly ash. A suitable scrubbing medium may comprise a slurry of metal carbonate, o~ide or hydroxide, for example a lime slurry.
In practice the scrubbing slurry is continuously recirculated within a scrubbing loop generally indicated at 20 which comprises~` a hold and classification tank 26 containing the bulk of the scrubbing slurry; a pump 32 which is operable to impel a flow of slurry from tank 26 via a conduit 30 into portion 16; means within portion 16 to be described hereinbelow which are adapted to disperse a flow of slurry within the portion 16; and a conduit 28 communicating between portion 16 and tank 26 to provide a slurry return flow path therebe-tween.
The scrubbing slurry dispersed within portion 16 con-tacts the flue gas stream passing therethrough to absorb there-from a major portion of the entrained gaseous sulfur dioxideand to arrest particulates such as fly ash as indicated herein-above for subsequent transport thereof via conduit 28 into the tank 26. As is well known the absorbed sulfur dioxide com-bines with dissolved lime in the slurry to form calcium com-pounds, predominately calcium sulfite and calcium sulfate, which under opportune conditions will precipitate out of solution to form suspended sulfite and sulfate particles. Thus within the tank 26 there is provided an agitator 14 which is adapted to classify contained particulates such as sulfite and sulfate ~ILGi763Z2 particles or fly ash into a lower solids concentrated region of tank 26. A pump 34 is pro~ided to selectively impel a flow of such solids concentrated slurry via a conduit 36 into a known solids processing apparatus shown as a thickener 50.
Subsequent to treatment within scrubber 16 the effluent ~as stream is direc-ted into precipitator 18 wherein remaining particulate contaminants such as residual fly ash and spurious scrubbing slurry droplets entrained in the gas stream are arrested by electrostatic precipitation prior to release of the gas stream ~o the atmosphere. The precipitator 18 may be of any well known type such as a hollow pipe or flat plate design wherein particulate contaminants are attracted to collector surfaces by electrostatic forces in the known manner, and are subsequently flushed from collector surfaces b~ bathing of such surfaces with a suitable wash liquor such as water which is con-tinuously recirculated within a wash loop generally indicated at 40.
The loop 40 includes: a hold tank 38 wherein is con-tained the bulk of the precipitator wash liquor; a pump 42 adapted to impel a flow of wash liquor from tank 38 via a conduit 44 into the precipitator 18; spray heads 46 shown as being loca-ted upwardly adjacent the precipitator 18 which communicate with conduit 44 and are adapted thereby to disperse sprays of wash liquor upon electrodes and collector surfaces of precipitator 18 to flush collected particulates therefrom; a sump portion 48 of module 12 located subjacent precipitator 18 and adapted thereby to collect therein the used wash liquor which falls from bathed surfaces of precipitator 18; and a conduit 52 which pro-vides a flow path for used liquor from sump 48 to thickener 50.

Loop 40 additionally includes a pump 54 ~hich is adapted to impel a flow of relatively clear and solids-free recovered liquid from thickener 50 via a conduit 56 into ~ank 3~ for recirculation thereo~ as wash liquor thr~ugh precipitator 18 in the manner hereinabove described.
Inasmuch as the structure and operation o~ ~he thick-ener 50 is conventional in every respect, detailed description thereof is omitted herefrom. Suffice it to note in this regard that within thickener 50 the spent slurry and liquor delivered thereinto via respective conduits 36 and 52 are classified in the conventional manner into a waste solids concentrated lower xegion from which solids may be drawn as indicated at 58 for further processing or disposal thereof, and a relatively solids-fr~e upper region wherein is contained a volume of relatively clear liquor which is suitable for recycling into tank 38 via the conduit 56 as hereinabove described or in a similar manner into the tank 26 via a conduit 62 and a pump 60. Thus the thickener 50 functions as a common solids processing unit for spent media from both the scrubber loop 20 and the precipitator wash loop 40, and further functions as a source of recyclable li~uid for both of the loops 20 and 40, thereby significantly increasing system efficiency by elimina~ing excess or redundant solids processing capacity from the apparatus 10.
It will be evident to those versed in the art that apparatus 10 must of necessity additionally include a suitable source of makeup ~ater such as indicated at 64 to replace system losses due to leakage, evaporation and spills, and a suitable source of reagent such as indicated at 66 to inject fresh lime as necessary for rejuvenation of spent scrubbing slurry.

~7~3Z2 Regarding the structural and operational arrangement of the module 12, it will be seen by reference to Figs. 1 and 2 that the module 12 comprises a closed, generally rectangular vessel 68 which has the respective inlet and outlet portions 4 and 6 communicating with the inner confines thereof adjacent respective longitudinally opposed vessel end walls 70 and 72.
The inlet 4 includes an .inlet flue 22 shown as a rectangular form which communicates with the inner confines of vessel 68 over substantially the entire area of the respective end wall 70 (Fig. 2) and which further communicates with the dirty gas source (not shown) via a duct 7~. In an entirely similar manner the outlet 6 communicates with the inner confines of vessel 68 via an outlet flue 24 adjacent the end wall 72 and further communi-cates with stack 8 via a duct 76 whereby there is established a substantially hori~ontal gas flow path within vessel 68 ex-tending longitudinally intermediate the inlet 4 and the outlet 6 and over substantially the entire cross sectional area of the vessel 68.
Within vessel 68 the scrubber 16 occupies an upstream portion of the gas flow path, being disposed adjacent inlet 4, and precipitator 18 occupies a down stream portion of the gas flow path, being disposed longitudinally intermediate scrubber 16 and outlet 6. Both scrubber 16 and precipitator 18 occupy substantially the entire cross sectional area o~ the vessel 68 whereby each is effective to treat gas flowing in any and all cross sectional portions of the gas flow path.
Precipitator 1~ may be of any conven-tional type such as the types hereinabove suggested, the particular design there-of being limited only by applicable operating requirements and by the requirement that the precipitator 18 be adapted to pro-vide substan-tially unobstructed gas flow intermediate scrubber 16 and outlet 6 throughout the entire cross sectional area of `. C~763i2~
various ways, for example by employing a flat plate precipita-tor 18 having a plurality of banks 41 (Fig~ 1) of laterally spaced vertically extending collector plates which are oriented such that the flat collecting surfaces of each plate are aligned to the direction of gas flow so as to face laterally opposed sides of vessel 68 whereby ~ree flow of gas intermediate lateral-ly adjacent plates is possible. Those versed in the art will recognize that various other precipitator designs are equally well suited for providing the required unobstructed gas flow therethrough.
As shown in Figs. 2 through 4 the scrubber 16 com-prises a plurality of vertically stacked, generally horizontal-ly extending chambers 77 through 80 numbered from top to bottom as viewed in Figs. 2 and 3. The chambers 77 through 80 are bounded laterally by respective opposed side walls of vessel 68 and vertically by a respective plurality of flat, rigid plates 81 through 84 which are vertically spaced such that each plate 81 throu~h 84 underlies one of respective chambers 77 through 80.
Each plate 81-84 extends longitudinally to the full length of the scrubber 16 and transversely intermediate opposed side walls of the vessel 68, sloping transversely from an upper plate por-tion 86 (Fig. 4) located adjacent one vessel side wall to a lower plate portion 88 located adjacent the opposite vessel side wall at an ele~ation below that of the respective portion 86. The plates 81-84 are rigidly affixed in the positions described by any suitable means such as welding to the vessel 68 by side walls.
Each of the plates 81-84 has rigidly affixed adjacent the upper surface thereof and extending vertically upwardly therefrom a plurality of transversely extending, longitudinally spaced partitions or dams 85 which subdivide the respective plate 81-84 into a plurality of longitudinally adjacent bays ~7~322 indicated in Figs 2 and 3 from left to right by capital letters A through D. For simplified description hereinbelow, specifc bays will be designated by the number of the subject plate and the letter of the subject bay. For example the lower right bay in Fig. 3 will be designated bay 84D. Additionally, the desig-nation for each bay shall be understood to include the overlying portion of the respective chamber 77-B0.
The dams 85 are advantageously small in vertical extent relative to the vertical extent of chambers 77-80 to provide within the chambers 77-80 a substantially unobstructed horizontal gas flow path therethrough. Thus it will be seen by reerence to description presented hereinabove that within all portions of the module 1~ including inlet flue 22, scrubber 16, precipitator 18 and outlet flue 24, the cross sectional area of the gas flow path is relatively large and unobstructed whereby it is feasible to direct an unobstructed flow of gas therethrough for cleansing thereof at very low flow velocities of, for example 5 to 15 feet per second through scrubber 15 and 5 to 25 feet per second through precipitator 18, using only the natural pressure of the gas source. In this manner the need for large fans or like devices to impel the gas stream through the cleansing apparatus 10 is obviated. Additionally, lower flow velocities permit more thorough cleansing of the gas stream and greater control of the cleansing process.
~ he respective upper and lower side portions 88 and 86 of plates 81-84 define respective upper and lower end portions of the bays 81A-81D whereby there is formed adjacent the lower end portion of each bay 81A-84D a sump portion 90 (Fig. 4) which is adapted to collect and contain therewithin scrubblng medium which has been dispersed within the overlying portion of respec~
tive chamber 77-80 in the following manner.

iL~7~i3;Z~, Within each of bays 81A, B, C through 84A, B, C
there is disposed one or more spray heads 92 ~or dispersing therewithin scrubbing slurry for the purpose of purging pollut-ants from the flue gases being passed horizontally through chambers 77-80. The scrubbing slurry is initially supplied to heads 92 of bays 81C and 82C (Fig. 3) vla the conduit 30 from tank 36 as hereinabove described for controlled dispersion there-of within bays 81C and 82C whereby the dispersed slurry contacts the flue ~as passing therethrou~h to purge 1y ash and gaseous sulfur dioxide therefrom. The dispersed slurry thence falls onto the respective portions of plates 81 and 82 underlyin~ ba~rs 81C
and 82C and collects within respective sump portions 90 thereo~.
Outlet connections 94 and 96 in the respective sumps 90 of bays 81C and 82C receive the collected slurry and by way of respective connecting conduits 98 and 100 and other spray heads 92,supply the slurry from sumps 90 of bays 81C and 82C to respective bays 83C and 84C for dispersion therewithin by means of gravity flow~
Sumps 90 of bays 83C and 84C collect slurry in a like manner as described hereinabove and other outlets 99 and 97 respectively deliver the collected slurry via a conduit 95 to a pump93 which is shown in Fi~. 3 as being located in an operating space 39 subjacent plate 84 and suitably isolated from the gas flow channels as by extensions 84' and 84''of plate 84. The pump 93 directs the flow of slurry thus received to spray heads 92 in bays 81B and 82B for dispersion therewithin and for subse-quent collection and redispersion in bays 83B and 84Bas herein-above described for the bays C. In an entirely similar manner yet another pump 91 directs a flow of slurry Erom bays 83B and 84B to bays 81A and 82A for dispersion therewithin, and Eor collection and redispersion within bays 83A and 84A in sub-stantially the manner as hereinabove described. The sumps 90 ~763Z2 of bays 83A and 84A are connected by means of respective outlets 89 and 87 to conduit 28 for return of used slurry to the tank 26.
Although no spray heads or outlet connections are shown in the bays 81D-84D, it is to be noted that such may be provided therein to disperse scrubbing slurry as hereinabove descirbed for the bays A, B and C. However, as shown, the bays 81D-84D provide a gas flow path portion upstream of precipitator 18 wherein such contaminants as heavy spurious slurry droplets entrained in the flowing gas or heavy wetted fly ash particles may settle out of the flowing gases prior to electrostatic pre-cipitating operations. Such natural settling of heavy particu-l~tes is considered desirable to the extent that it reduces required precipitator capacity.
It is to be noted that by virtue of the scrubber structure described hereinabove fresh slurry from tank 26 is provided for the sprays 92 of bays C inasmuch as these bays represent the farthest downstream scrubbing chamber of scrubber 16 where the lowest sulfur dioxide concentration is encountered.
The freshest and most concentrated slurry is thus required to purge as much as possible of this residual sulfur dioxide.
Accordingly, the slurry used for scrubbing in bays B is slightly less reactive than fresh slurry, it having previously absorbed residual sulfur dioxide during scrubbing in bays C. Likewise, the slurry used in bays A is somewhat less reactive than fresh slurry, it having previously absorbed sulfur dioxide during scrubbing in bays B and C. Therefore, it will be seen that by virtue of the scrubber arrangement defined hereinabove means are provided to vary slurry reactivity between adjacent bays A, 30 B and C to improve scrubber efficiency.

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Moreover, it is to be noted that in each set oE bays A, B and C the collectingsumps 90 and respective cooperable out~
lets and spray heads provide means for collecting, mixing and redispersing slurry whereby the scrubbing slurry is rendered more reactive than slurry not so collected and redlspersed. Such collecting and redispersing of slurry minimizes the use of ineffective slurry droplets whose surface portions have absorbed their full complement of sulfur dioxide during initial stages of scrubbing. In order to advantageously implement this feature of the present invention the chambers 77 through 80 are advan-tageously limited to a relatively small vertical height of, for example 5 eet to limit the total time that free falling slurry droplets are able to contact the gas stream. Thus r the slurry droplets remain effective for sulfur dioxide absorption through-out a relatively short scrubbing interval after which the drop-lets are collected, reamalgamated and redispersed to create fresh droplets with surface portions of renewed reactivity.
An alternative means of collecting and redispersing slurry is illustrated in Fig. 2 wherein there are shown fragmen-tary portions of a plurality of horizontally extending, vertical-ly spaced expanses of screen mesh 43 which replace all plates 81-84 and some of the sprays 92 by capturing slurry droplets falling thereupon and releasing individual reamalgamated drop-lets to fall to a lower screen 43 or into a sump (not shown) as gravity overcomes cohesion between the accumulating slurry droplets and the screen 43. According to this structure, the size of slurry droplets generated could be readily controlled by the mesh size of the screen 43. It will be noted that substi-tution of screens 43 for plates 81-84 preserves the unobstructed horizontal gas flow capability through chambers 77 80.

3~76322 By virtue of the invention described hereinabove there is provided a gas cleansing apparatus including sub-stantially unitary wet scrubbing and wet electrostatic precipi-tating means wherein such wet scrubbing means includes means or treating a gas flow in a plurality of scrubber portions with a liquid or slurried scrubbing medium of variable reactivity or reagent concentration. The scrubber further includes means or collecting, mixing or reamalgamating, and redispersing scrubbing slurry within the scrubber. The present invention further includes scrubber and precipitator structures which per-mit substantially unobstructed flow of gas therethrough whereby ;~
the need for forced gas flo~ as by fans or like devices is obviated. The present invention still further includes waste processing means adapted to receive spent scrubbing slurr~
and used precipitator wash liquor for processing of waste solids therefrom and for recycling of recovered liquid to the scrubber and precipitator.
Notwithstanding reference hereinabove to a particularembodiment of the present invention, it is to be understood ~o that this invention may be practiced in various alternative embodiments and with numerous modifications thereto without departing from the broad spirit and scope of the invention.
~or example: gas flow through the scrubber and precipitator may be directed at an angle to the horizontal, including vertical gas flow; the relationship between the direction of slurry flow and gas flow within scrubbing chambers may be intersecting, opposing or concurrent flows; the thickener 50 may alternati~ely be any other suitable solid processing apparatus such as, for example, a centrifugal separator; the particular configurations of scrubber 16 and precipitator 18 may be varied within a wide design latitude subject only to the functional characteristics described hereinabove; the entire apparatus 10 may be refined 1~'763Z~

and specialzied by inclusion therein of cooperable ancillary sub-systems; and the like. These and other embodiments and modifications having been envisioned and anticipated it is requested that this invention be interpreted broadly and limited only by the scope of the claims appended hereto.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pollution control system for treating gas containing oxides of sulfur comprising: wet scrubbing means adapted to discharge a scrubbing liquor for removing oxides of sulfur in the form of wet precipitates from gas passing therethrough; a wet demister means located downstream of said scrubbing means for arresting droplets of scrubbing liquor entrained in gas which passed through said wet scrubbing means and adapted to have a continuous flow of washing liquor therein; first conveying means for conveying at least a portion of the liquors exiting from said scrubber means and said demister to a common conditioning means;
said conditioning means being operable to separate such liquors into a first zone of liquor and a second zone of concentrated slurry; and a second conveying means for conveying at least a portion of said liquor in said first zone to said demister.

2. A pollution control system as specified in claim 1 including third conveying means for conveying a portion of said liquor in said first zone to said scrubbing means to be effective as at least a portion of said scrubbing liquor.

3. A pollution control system as specified in claim 1 therein said wet demister means includes a wet electrostatic precipitator.

4. A pollution control system as specified in claim 1 wherein said conditioning means includes a settling tank,

5. A method of removing air pollutants from a stream of gas comprising the steps of: scrubbing said stream of gas with a scrubbing liquor containing reactants which absorb at least a portion of said pollutants from said gas;
subsequent to said scrubbing, passing said stream of gas through a wet electrostatic precipitator to demist said stream of gas and remove at least a portion of the residue pollutants therein; simultaneously with said first mentioned passing, passing a washing liquor over the collecting surfaces of said electrostatic precipitator;
subsequent to said scrubbing, collecting said scrubbing liquor and classifying said collected scrubbing liquor into a lighter portion and a heavier portion, recirculating said lighter portion from said scrubbing; collecting said washing liquor passing over said collecting surfaces and classifying said heavier portion and said collected washing liquor into a reusable portion and a non-reusable portion;
and combining at least a part of said reusable portion with said scrubbing liquor,

6. The method as specified in claim 5 including the further step of recirculating another part of said reusable portion as at least a portion of said washing liquor.