Abstract:
In a kraft pulp mill, a process and apparatus for removing and washing dregs from green liquor so that the pH of the dregs is below &#34;hazardous waste&#34; levels. Specifically, green liquor clarifier underflow is filtered using a simple non-precoat cassette filter to form a first dregs cake. The dregs cake is diluted, preferably with water. The diluted dregs cake is then filtered in a precoat dregs filter to produce a dregs cake that is at least 50% solid and has a pH of about 12.0 or less.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and process for removing suspended dregs from green liquor in a kraft pulp mill and washing the same in preparation for disposal. This application is specifically directed to an apparatus and process that uses much less water than conventional processes. 
     BACKGROUND OF THE INVENTION 
     Wood pulp or cellulose is prepared from wood or other vegetable material by a process of chemical dissolving and softening. The wood or other vegetable material is subjected to a preliminary cleaning treatment, mechanically chopped up and then boiled in large tanks with hot solutions. The boiling process, known as digesting, is performed with a caustic liquid referred to as white liquor. Lignin, which acts to adhere cellulose fibers, is dissolved by this treatment and is, in part, chemically decomposed, leaving a soft pulp which consists primarily of cellulose. 
     The present invention is intended for use in kraft pulp mills. Kraft pulping is performed by cooking wood chips in a highly alkaline liquor that selectively dissolves lignin and releases cellulosic fibers from the wood matrix. The two principal chemicals in the liquor are sodium hydroxide and sodium sulphide. Sodium sulphide, also a strong alkali, readily hydrolyses in water, producing sodium hydroxide and sodium hydrosulphide. 
     At the beginning of the kraft process, white liquor is fed to the digester. This liquor contains a high amount of effective alkali which, as explained above, is used to digest the wood. At the terminus of the digester, spent liquor, known as black liquor, is removed from the digester. 
     The pulping chemicals are then recovered from the black liquor by a process that is referred to as the recovery process. To begin with, black liquor from the digester contains low levels of effective alkali. Black liquor also contains large amounts of organic compounds that are removed and burned in a recovery furnace. The resultant mass of inorganic residue, called smelt, is dissolved to form green liquor having a low concentration of effective alkali and a high concentration of sodium carbonate. The green liquor is led to a green liquor clarifier for the removal of solid particles called dregs. 
     The clarified green liquor is subsequently causticized in a conventional process. Note, for example the process described in U.S. Pat. No. 4,941,945 to Pettersson. In particular, white liquor is regenerated from the green liquor by causticizing the carbonate through addition of lime. After the recausticizing operation, a small residual amount of sodium carbonate is carried to the digester. The total amount of sodium hydroxide, sodium sulphide and sodium carbonate is called the total titratable alkali (TTA). 
     The chemistry of the causticizing reaction is simple and known to those skilled in the art. In particular, lime (CaO) reacts with sodium carbonate (Na 2  CO 3 ) in the green liquor to produce sodium hydroxide (NaOH) and calcium carbonate (CaCO 3 ), called lime mud. Because of its low solubility, calcium carbonate precipitates from solution. 
     Although the chemistry of the reaction is simple, the continuous nature of the process involves a number of unit operations--filtration, classification, mixing, sedimentation, calcining, and material handling. The entire process may be thought of a closed circuit operation involving solids as one cycle and the liquor as another. The two are brought together to furnish the cooking liquor and each subsequently is recycled and converted to that chemical form necessary to achieve the causticizing of the liquor. There are, however, some waste products. One such waste product is the dregs that are removed from the green liquor by the green liquor clarifier. 
     Since a kraft pulp mill produces large quantities of dregs, it is highly desirable to dispose of dregs in an inexpensive manner such as landfill. A problem arises, however, if the pH of the dregs exceeds the EPA standard for alkaline waste. Specifically, any solid waste material that is aqueous and has a pH above 12.5 is classified as a &#34;hazardous waste&#34; and is subject to extremely cumbersome regulations. Accordingly, the dregs must be washed to a pH below 12.5 before disposal in a landfill. 
     In addition, the material must be solid, so that it does not drip; a &#34;paint filter&#34; test is conventionally used--if it does not drip any water in five minutes, then it is considered solid. If foreign materials are added to make the dregs solid, then the dregs must be subjected to a compression test at 60 psi to see if it is &#34;solid&#34;. This application is specifically directed to an apparatus and process for treating dregs to achieve a solid mass having a pH below 12.5, ideally about 12.0 or less. 
     The treatment of dregs produced while recausticizing kraft green liquor is discussed in several U.S. patents. For example, U.S. Pat. No. 4,668,342 to Blackwell describes a process for recausticizing kraft green liquor in which the underflow from the green liquor clarifier is filtered in a rotary vacuum filter and washed with hot water. The dregs are then passed to waste. 
     U.S. Pat. No. 4,941,945 to Pettersson describes a method for clarifying green liquor in which the green liquor is clarified by filtering and the sludge separated by the filter is discarded for dumping. 
     U.S. Pat. No. 5,082,526 to Dorris discloses a process by which raw green liquor is passed through a buffer tank and into a clarifier to remove dregs. The dregs then pass to a green dregs filter. 
     Likewise, U.S. Pat. No. 5,145,556 to Westerberg et al. discloses a process in which dregs that settle in the green liquor clarifier are pumped to a dregs precoat filter for thickening and washing. The patent also suggests that mixing lime mud and grits with the unclarified green liquor enhances the settling of the dregs in the clarifier and the washing on the precoat filter. 
     U.S. Pat. No. 5,282,931 to Le Clerc et al. discloses a process by which dreg deposits are filtered in a precoat dregs filter before passing through the disposal line. 
     Finally, U.S. Pat. No. 4,322,266 to Nelson discloses a process by which some dregs are pumped back to the recovery furnace. 
     None of these patents seem to acknowledge, much less address, the need to reduce the pH of dregs. One conventional system for reducing the pH of dregs involves washing dregs using a sedimentation dregs washer located upstream of a precoat dregs filter. In one known embodiment, about 20% of the available wash water is used to wash the dregs. This recovers approximately 80% of the soda in the dregs underflow. The dregs washer underflow is sent to a standard precoat dregs filter where it is dewatered to about 50% solids, and further washed with 1.65 displacements. This lowers the soda in the dregs cake to 0.14%, and the pH to an acceptable 11.93. 
     The trouble with the conventional dregs washing system is that it uses too much water and sends too much soda to the weak wash system. The dregs washer overflow and the dregs filter filtrate must go to the weak wash system. This can be viewed as diverting wash water from the mud, or adding soda to the mud washing system, but the result is the same--mud washing suffers. Other problems are also expected. 
     Likewise, washing the dregs to the required pH on a precoat dregs filter is not practical. The number of wash displacements is so great that the washing efficiency cannot be maintained. Moreover, it is doubtful that the required amount of water would flow through the cake, even at half the filter loading. 
     Thus, there is a need for an alternative process for washing dregs. 
     Finally, in describing the kraft pulp mill process, the terms overflow and underflow are commonly used. These are terms of art. In general, the term &#34;underflow&#34; is used to refer to the material, typically solid concentrate, that is removed from the flow or retained by the filter or clarifier. The term &#34;overflow&#34; refers to the filtrate or the material that passes through the filter or clarifier. 
     SUMMARY OF THE INVENTION 
     The present invention proposes an alternative process by which dregs are filtered, rather than washed, before passing to the precoat dregs filter. Assuming the existing green liquor clarifier is of adequate size and performance, a simple filter is used to dewater the underflow, from 5-7% suspended solids to 50% suspended solids. Since the material has passed through the green liquid clarifier, the filtrate (green liquor) can be returned to storage, rather than the weak wash. 
     The process of the present invention includes the steps of: clarifying the green liquor in a green liquor clarifier to separate the green liquor into an overflow comprising clarified green liquor and an underflow comprising dregs and green liquor; filtering the underflow to form a first dregs cake that is at least 50% solid; diluting the dregs cake, preferably by water flushing, to less than 20% solid; filtering the diluted dregs cake to form a second dregs cake that is at least 50% solid. 
     The apparatus of the present invention includes a green liquor clarifier for separating green liquor into an overflow containing clarified green liquor and an underflow containing dregs and green liquor; a first filter for filtering green liquor from the underflow so as to form a dregs cake; means for diluting the dregs cake; and a second filter for filtering the diluted dregs cake. 
     Through the use of a simple filter, it is possible to filter the dregs to form a dregs cake that is about 50% solid. In this way, about 92% of the soda is removed in this initial filtering step and returned to the green liquor system. The resultant dregs cake will contain much less soda than the underflow of a conventional dregs washer. By back-flushing the dregs cake with water to a dilution of 15%, then filtering and washing on the dregs filter, the soda content is further reduced and the pH of the dregs cake is reduced to an acceptable level. 
     The process of the present invention requires only a small fraction of the amount of water required by conventional dregs washing. According to one proposed application of this process, the dregs filter filtrate is the only water sent back to the weak wash system. As a result, less than 2% of the water needed with conventional technology is used in this embodiment of the present invention. 
     Thus, the present invention differs from known kraft mill processes and provides an alternative to dregs washing. According to this alternative, a filter is used in place of the dregs washer. Thus, two stage filtration is used instead of a simple dregs filter or a sedimentation dregs washer followed by filtration. 
     A precoat filter is often used as the dregs filter in a conventional kraft pulp mill having one filter with or without dregs washing. In the two stage filtration process of the present invention, two precoat filters in series could be used. 
     The inventor has found, however, that there are disadvantages associated with the use of two precoat filters in series. To begin with, two filters means two precoats using two to three times the lime mud used by a single standard precoat filter. Another problem is oxidation: the precoat vacuum filter will suck a lot of air through the cake, oxidizing an appreciable amount of the sodium sulphide to sodium thiosulfate which is highly corrosive. 
     Accordingly, in accordance with the best mode currently contemplated by the present invention, the initial filtration is performed by a cassette type pressure filter such as that sold under the trademark CAUSTEC™ by Kvaerner Pulping Technologies AB of Karlstad, Sweden. The CAUSTEC™ casette filter is an enclosed vessel containing a series of perforated tubes covered with a filter media. The CAUSTEC™ cassette filter does not require a precoat and uses the pressure of the liquid feed pump to provide the driving force for filtration, thus, there is no vacuum pump and no air infiltration. Moreover, the system uses much less power. At the same time, a greater differential power is developed, so a drier cake is formed. This filter can recover at least 93% of the soda in the green liquor clarifier underflow. Since liquor is simply filtered out of the dregs, there is no dilution, and the filtrate can be returned to green liquor storage without any problems. 
     Naturally, other suitable filters could be used. In that regard, it should be appreciated that one of the advantages of the present invention is that a variety of conventional filters can be used to perform the novel step of filtering the dregs before the dregs are sent to the precoat dregs filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed descriptions when considered in connection with the accompanying drawings, in which: 
     FIG. 1 depicts a conventional kraft mill recausticizing system; 
     FIG. 2 represents a portion of the kraft mill recausticizing system employing an embodiment of the present invention, the green liquor filter; and 
     FIG. 3 depicts a preferred embodiment of the present invention in which a sump tank is used in conjunction with the dregs filter of the present invention in a kraft mill recausticizing system. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, in a conventional kraft liquor cycle, the digestion of wood chips takes place in one or more digesters employing an aqueous mixture of sodium hydroxide and sodium sulfide called white liquor. After the cooking operation, the waste liquor, sometimes referred to as black liquor, is separated from the pulp fibers, concentrated and burnt in a recovery furnace to form a smelt. The smelt, which consists mainly of sodium carbonate and sodium sulfide, is then dissolved in an aqueous solution, usually referred to as a weak wash, to form the green liquor. 
     FIG. 1 shows schematically a recausticizing plant wherein the smelt and water are added to a smelt dissolver tank 101. The raw green liquor is then passed to a green liquor clarifier 102 where the dregs, comprising mainly carbonaceous particles and metallic compounds insoluble in the green liquor are removed. The dregs are conveyed to dregs storage 103 and then to a dregs filter 104. The clarified green liquor is mixed with lime in a combination slaker 105 to convert the sodium carbonate to sodium hydroxide in accordance with the formula: 
     
         CaO+Na.sub.2 CO.sub.3 +H.sub.2 O␣2NaOH+CaCO.sub.3 
    
     (The lime is added by means of a number of components comprising a hot lime conveyor 106, a lime bucket elevator 107, a combination lime bin 108, and a lime screen feeder 109.) During this reaction the lime and calcium carbonate are insoluble and are, therefore, present in the liquid as suspended solids. Several other impurities from the lime or from the dregs are also insoluble and become part of the suspended solids. The mixture of these insoluble compounds forms the lime mud. 
     To complete the reaction, the slurry, which comprises sodium hydroxide and calcium carbonate particles, is passed through a series of agitated vessels called causticizers 110. The slurry is allowed to react at temperatures between about 90° and 105° C. for a period varying from 60 to 180 minutes. The reacted mixture is then passed to a white liquor clarifier 111 to separate the lime mud from the white liquor. The white liquor then goes to the white liquor storage 112 for use in the digestion of wood chips and contains mostly an aqueous solution of sodium hydroxide and sodium sulfide. 
     The thickened lime mud is washed in a lime mud washer 113, passes through a lime mud storage tank 114 and a lime mud filter 115 where it is dewatered and then calcined in a lime kiln 116 or in a fluidized bed calciner 117 to yield reburned lime which is reused for causticizing green liquor in the slaker. 
     In the conventional process shown in FIG. 1, the underflow of the green liquor clarifier is passed directly to a dregs filter 104. The dregs filter is typically a precoat filter. The dregs cake formed on the precoat filter is discarded, while the overflow is directed to a weak wash storage tank 118. This process does not reduce the pH of the dregs to an acceptable level, however. Thus, in accordance with the process of the present invention, the underflow from the green liquor clarifier is subjected to a preliminary filtering step to remove green liquor from the underflow so that the dregs constitute about 50% suspended solids. In this way, a significant percentage of the soda is removed from the underflow before the precoat dregs filter step. 
     FIG. 2 depicts one embodiment of the apparatus of the present invention in which the kraft mill recausticizing system employs a green liquor filter. More specifically, the raw green liquor passes through a conduit 1 to a green liquor clarifier 2. The overflow from the green liquor clarifier flows on through additional conduit 3 for the green liquor circuit. The underflow from the green liquor clarifier passes on to a green liquor filter 4, which filters the underflow. The filtrate from the green liquor filter is returned to the conduit 3 constituting the green liquor circuit. The resultant cake from this filter is washed with water and passes on to a precoat filter 5. 
     This precoat filter operates in a conventional fashion, in which there is an initial cake formation and the resultant overflow from formation of the cake is passed through conduit 6 to the white liquor. The cake is subsequently washed and the washing is also passed through to the white liquor conduit 6. By virtue of this process, the pH of the dregs cake is reduced to an acceptable level. The cake resulting from the wash is then discarded as waste 7. 
     FIG. 3 depicts a preferred embodiment of the present invention in which the underflow of the green liquor clarifier 2 is directed to a sump tank 8, the contents of which are pumped by pump 9, either periodically or continuously, to the green liquor filter 4 (alternatively referred to as a dregs filter). The filtrate is then directed to the green liquor circuit 3, while the underflow is directed to the precoat filter 5 (or second dregs filter) indicated in FIG. 2. 
     Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are provided for illustration of the invention and are not intended to be limiting thereof. 
     In the description of the invention, all percentages are given in term of a ratio of weight to weight. 
     A production rate of 1680 BDTPD (bone dry tons per day) kraft linerboard on two lines and 450 BDST/day (bone dry short tons) semichemical corrugating medium is assumed, using a modified green liquor cooking in which the green liquor/white liquor ratio is higher than the normal 1.2. 
     
         ______________________________________White liquor flow  734 GPMGreen liquor to semichem               50 GPMGreen liquor flow  924 GPMGreen liquor to slaker              874 GPM (depends on WLC U-              flow consistency)TTA (total titratable alkali)              7.800 lb./cu. ft.AA (active alkali) 6.668 lb./cu. ft.EA (effective alkali)              5.835 lb./cu. ft.Sulfidity          25%- of AACausticity         81.5%Lime availability  85%Scrubber recycle   0% (precipitator, dry return)Dregs              2774 ppm or 17.0 lb./BDT pulpWLC (white liquor clarifier) underflow              42% ss (suspended solids)LMW (lime mud washer) underflow              44% ssLM (lime mud) filter feed              34% ssLM filter cake     75% TS (total solids)Wash displacements 0.9 (30 GPM) (gallons per              minute)EQUIPMENT SIZEGreen liquor clarifier              70 ft. dia. × 40 ft. highDregs filter        6 ft. dia × 8 ft. face______________________________________ 
    
     The process of the present invention requires two stage filtration of the underflow of the green liquor clarifier. This could be done using two precoat filters in series, but this means two precoats, using two to three times the lime mud. In such an example, the estimated total lime mud required is 40 TPD or 8% of the process flow. In contrast, a standard precoat filter would use about 15 TPD or 3% of the process flow. 
     According to the best mode of the present invention, a CAUSTEC™ pressure filter is used for this application. Adapted as a green liquor filter, the CAUSTEC™ cassette filter requires no precoat and can recover at least 93% of the soda in the green liquor clarifier underflow. Since the liquor is simply filtered out of the dregs, there is no dilution, and the filtrate can be returned to green liquor storage without any problems. 
     The CAUSTEC™ cassette filter is an enclosed vessel, which uses the pressure of the liquor feed pump to provide the driving force for filtration. Thus, there is no vacuum pump and no resultant air infiltration, and the system uses much less power. At the same time a greater differential pressure is developed, so a drier cake is formed. 
     The soda content of the dregs in the green liquor clarifier underflow is about 44%. The soda content in the green liquor filter cake at 50% ss is about 9.4%. Specifically, 1912 lb./hr. of soda is recovered. Out of 2068 lb./hr.in the underflow, only 156 lb./hr. of soda is left in the cake. 50% solids in the filter cake has been assumed for the calculations, but 70% solids or more can be achieved in a preferred embodiment employing green liquor filtration. 
     The cake is washed after cake formation. Calculations show that a very reasonable amount of wash, between approximately 1 and 2 displacements, should be sufficient to wash the cake down to a pH of 12. 
     The dregs cake is sluiced off the CAUSTEC™ cassette filter by backwashing with water. This water also serves to wash out a great deal of soda when the dregs are filtered on a precoat filter. In this example sufficient water is used to form a 10% suspended solids slurry to feed the precoat filter. This is thin enough to form a good cake and do a good washing job. Consideration of the mechanism of precoat filtration shows that adding more water at this point does not affect the precoat filter sizing, which is based on solids, not liquid flow. The dregs filter filtrate flow is only 39 GPM and can be easily absorbed in the weak-wash system. The weak-wash flow is calculated at 914 GPM, so the dregs filtrate is about 4% of the total weak wash flow. 
     In alternate embodiments of the present invention, filters other than a CAUSTEC™ cassette filter may be employed. Such filters can be conventional filter devices employed in the industry which can effectively separate the underflow from the green liquor clarifier. 
     Tables 1 and 2 further demonstrate the advantages offered by the present invention relative to the prior art. Table 1 represents a flowsheet summary of a conventional kraft mill recausticizing system. Table 2 depicts a flowsheet summary of a kraft mill recausticizing system embodying the present invention. 
     
                                           TABLE 1__________________________________________________________________________                     05/18/93                           &#34;WLC2DW&#34;  WATER                                          SOLIDSEXAMPLE FLOWSHEET SUMMARY   CFD   CFH   GPM LB/HR                                          LB/HR STPD                                                    %__________________________________________________________________________                                                    Na2O1A    RAW GREEN LIQUOR FLOW 90° C.                     148538                           6189  772 372481                                          753   9.01B    G L CLARIFIER OVERFLOW 90° C.                     144776                           6032  752 363492                                          44.2  0.51C    G L CLARIFIER UNDERFLOW                     3265  136   17  7741 708   8.5 44.2%2A    WATER TO DREGS WASHER                     16352 681   85  415902B    DREGS WASHER FEED   20324 847   106 50580                                          704   8.4 44.3%2C    DREGS WASHER OVERFLOW                     16565 690   86.1                                     41590                                          4.3   0.12D    DREGS WASHER UNDERFLOW                     3761  157   19.5                                     8989 704   8.4 21.5%3A    DREGS FILTER LIME MUD PRECOAT                     568   23.6  2.9 1054 1056  12.73B    TOTAL TO DREGS FILTER                     4328  180   22.5                                     10043                                          1760  21.1                                                    10.8%5A    FIRST DREGS FILTER CAKE 70° C.                     1037  43.2  5.4 1756 1760  21.1                                                    0.44%4     DREGS FILTER WASH WATER 70° C.                     1139  47.5  5.9 28985B    WASHED DREGS FILTER CAKE 70° C.                     1037  43.2  5.4 1756 1760  21.1                                                    0.14%6     DREGS FILTER FILTRATE 70° C.                     3983  166   20.7                                     10131                                          0.0   0.07     TOTAL DREGS SYSTEM OVERFLOW                     20336 847   105.6                                     51721                                          4.3   0.18     WATER TO SLAKE LIME (95° C.)                     2788  116.2 14.5                                     69779     LIME USE, BONE DRY  6179  257   32.1     25549 306.6 MUD GENERATED, BONE DRY                     6179  257   32.1     42592 511.110    GRIT LOSS (WET)     182   7.6   0.9 275  511   6.111    GRIT WASH WATER     108   4.5   0.6 27512    FLOW THROUGH CAUSTICIZERS 95° C.                     148601                           6192  772 356515                                          42126 505.513    WHITE LIQUOR FLOW AT 95° C.                     124579                           5191  647 311702                                          37.3  0.4514    W L C UNDERFLOW AT 95° C.                     24017 1001  125 44813                                          42088 505.1                                                    12.2% W L C UNDERFLOW AT 70° C.                     23726 989   123 44813                                          42088 505.115    SCRUBBER RECYCLE 70° C.                     81438 3393  423 204455                                          7242  86.926    FILTRATES           66366 2765  345 168791                                          8.5   0.17     DREGS SYSTEM OVERFLOW                     20336 847   106 51721                                          4.3   0.116    L M W FEED 70° C.                     175520                           7313  912 428191                                          49382 592.6                                                    14.1%17    L M W OVERFLOW (WEAK WASH) 70° C.                     146457                           6102  761 372481                                          38.1  0.4618    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     29063 1211  151 55710                                          49344 592.1                                                    2.94% DILUTION WATER-DF PRECOAT                     758   32    3.9 1928 DILUTION WATER-LMF FEED                     34651 1444  180 8813019    TOTAL DILUTION WATER                     35409 1475  184 9005820    LIME MUD TO DREGS FILTER                     1380  57    7.2 3120 1056  12.721    FILTER FEED SLURRY  63091 2629  328 142647                                          48288 579.5                                                    3.00%22    CAKE WASH WATER     15778 657   82  4012923    FILTER CAKE         13315 555   69  16052                                          48279 579.4                                                    0.086%24    LMF FILTRATE 70° C.                     65554 2731  341 166725                                          8.5   0.1025    DREGS FILT PRECOAT FILTRATE                     812   34    4.2 206626    TOTAL FILTRATES     66366 2765  345 168791                                          8.5   0.1027    SMELT                                    715   8.628    SMELT &amp; WEAK WASH (RAW GREEN LIQUOR)__________________________________________________________________________                     SLURRY                          SLURRY                               LB/HR   BREAKDOWN OF CHEM-LB/HREXAMPLE FLOWSHEET SUMMARY   SS   SP GR                               CHEM                                   TTA NaOH                                           Na2S                                               SO4 Na2CO3__________________________________________________________________________1A    RAW GREEN LIQUOR FLOW 90° C.                     1669 1.17 77809                                   47859                                       4520                                           17088                                               3554                                                   526471B    G L CLARIFIER OVERFLOW 90° C.                     100  1.17 78430                                   47251                                       4520                                           16325                                               4940                                                   526441C    G L CLARIFIER UNDERFLOW                     7.00%                          1.19 1670                                   1006                                       96  348 105 11212A    WATER TO DREGS WASHER    0.98 625 -605    -762                                               13862B    DREGS WASHER FEED   1.3% 1.00 1670                                   1006                                       96  348 105 11212C    DREGS WASHER OVERFLOW                     100  1.00 1305                                   786 75  272 82  8762D    DREGS WASHER UNDERFLOW                     7.00%                          1.03 365.5                                   220.2                                       21.1                                           76.1                                               23.0                                                   245.43A    DREGS FILTER LIME MUD PRECOAT                     38.6%                          1.853B    TOTAL TO DREGS FILTER                     13.1%                          1.20 365.5                                   220.2                                       21.1                                           76.1                                               23.0                                                   245.45A    FIRST DREGS FILTER CAKE 70° C.                     36.5%                          1.79 12.4                                   7.5 0.72                                           2.59                                               0.78                                                   8.354     DREGS FILTER WASH WATER 70° C.                          0.98 4.0 2.4 0.23                                           0.82                                               0.25                                                   2.665B    WASHED DREGS FILTER CAKE 70° C.                     50.0%                          1.31 361.6                                   217.8                                       20.8                                           75.3                                               22.8                                                   242.76     DREGS FILTER FILTRATE 70° C.                          1.01 361.6                                   217.8                                       20.8                                           75.3                                               22.8                                                   242.77     TOTAL DREGS SYSTEM OVERFLOW                          0.98 1666.4                                   1003.9                                       96.0                                           346.9                                               105.0                                                   1118.58     WATER TO SLAKE LIME (95° C.)                          0.96 -10035                                   0   30882                                           CAUSTICIZING                                                   -409189     LIME USE, BONE DRY  100% 1.59 625 -605    -762                                               1386                                                   OXIDATION MUD GENERATED, BONE DRY                     100% 2.6510    GRIT LOSS (WET)     65.0%                          1.66 -9411                                   -605                                       30882                                           -762                                               1386                                                   -4091811    GRIT WASH WATER          0.9812    FLOW THROUGH CAUSTICIZERS 95° C.                     9.0% 1.21 69019                                   46647                                       35402                                           15564                                               6326                                                   1172713    WHITE LIQUOR FLOW AT 95° C.                          1.15 60889                                   40255                                       30952                                           12942                                               6743                                                   1025314    W L C UNDERFLOW AT 95° C.                     44.0%                          1.53 8754                                   5787                                       4450                                           1861                                               969 1474 W L C UNDERFLOW AT 70° C.                     44.0%                          1.55 624 -605    -761                                               138615    SCRUBBER RECYCLE 70° C.                     3.3% 1.0526    FILTRATES                0.99 2205                                   1294                                       924 341 414 5277     DREGS SYSTEM OVERFLOW         1666                                   1004                                       96  347 105 111916    L M W FEED 70° C.                     10.3%                          1.05 12625                                   8086                                       5469                                           2548                                               1488                                                   311917    L M W OVERFLOW (WEAK WASH) 70° C.                     99   0.98 10547                                   6571                                       4520                                           1967                                               1483                                                   257818    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     46.0%                          1.42 2216                                   1381                                       950 413 312 541 DILUTION WATER-DF PRECOAT                          0.98 138 -134    -168                                               306 DILUTION WATER-LMF FEED  0.9819    TOTAL DILUTION WATER     0.9820    LIME MUD TO DREGS FILTER                     25.0%                          1.18 47  3021    FILTER FEED SLURRY  25.0%                          1.18 2216                                   1381                                       950 413 312 54122    CAKE WASH WATER          0.98 51  -50     -63 11423    FILTER CAKE         75.0%                          1.86 62  36  26  10  12  1524    LMF FILTRATE 70° C.                     51   0.9825    DREGS FILT PRECOAT FILTRATE                          0.9826    TOTAL FILTRATES     50   0.99 2205                                   1294                                       924 341 414 52727    SMELT                         67261                                   41287                                       0   15121                                               2071                                                   5006928    SMELT &amp; WEAK WASH             77809                                   47859                                       4520                                           17088                                               3554                                                   52647 (RAW GREEN LIQUOR)__________________________________________________________________________                     BREAKDOWN OF TTA-LB/HREXAMPLE FLOWSHEET SUMMARY   NaOH  Na2S  SO4  Na2CO3 TOT S R.E.__________________________________________________________________________1A    RAW GREEN LIQUOR FLOW 90° C.                     3502  13570 1551 30786  15121 89.74%1B    G L CLARIFIER OVERFLOW 90° C.                     3502  12965 2156 30785  15120 85.74%1C    G L CLARIFIER UNDERFLOW                     75    276   46   656    322   85.74%2A    WATER TO DREGS WASHER     -605  605  CLC RE LOSS =                                                   -4.00%2B    DREGS WASHER FEED   75    276   46   656    322   85.74%2C    DREGS WASHER OVERFLOW                     58    216   36   512    252   85.74%2D    DREGS WASHER UNDERFLOW                     16.3  60.4  10.0 143.5  70.5  85.74%3A    DREGS FILTER LIME MUD PRECOAT3B    TOTAL TO DREGS FILTER                     16.3  60.4  10.0 143.5  70.5  85.74%5A    FIRST DREGS FILTER CAKE 70° C.                     0.56  2.06  0.34 4.88   2.40  85.74%4     DREGS FILTER WASH WATER 70° C.                     0.18  0.65  0.11 1.56   0.76  85.74%5B    WASHED DREGS FILTER CAKE 70° C.                     16.1  59.8  9.9  141.9  69.7  85.74%6     DREGS FILTER FILTRATE 70° C.                     16.1  59.8  9.9  141.9  69.7  85.74%7     TOTAL DREGS SYSTEM OVERFLOW                     74.4  275.5 45.8 654.1  321.3 85.74%8     WATER TO SLAKE LIME (95° C.)                     23928            -239289     LIME USE, BONE DRY        -605  605  S + C RE LOSS                                                   -4.00% MUD GENERATED, BONE DRY10    GRIT LOSS (WET)     23928 -605  605  -23928 011    GRIT WASH WATER12    FLOW THROUGH CAUSTICIZERS 95° C.                     27429 12360 2760 6857   15120 81.74%13    WHITE LIQUOR FLOW AT 95° C.                     23981 10278 2942 5995   13220 77.75%14    W L C UNDERFLOW AT 95° C.                     3448  1478  423  862    1901  77.75% W L C UNDERFLOW AT 70° C.                           -605  605  WLC RE LOSS =                                                   -4.00%15    SCRUBBER RECYCLE 70° C.26    FILTRATES           716   271   181  308    451   60.00%7     DREGS SYSTEM OVERFLOW                     74    275   46   654    321   85.74%16    L M W FEED 70° C.                     4238  2024  649  1824   2673  75.71%17    L M W OVERFLOW (WEAK WASH) 70° C.                     3502  1562  647  1507   2209  70.71%18    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     736   328   136  317    464   70.71% DILUTION WATER-DF PRECOAT -134  134  LMW RE LOSS =                                                   -5.00% DILUTION WATER-LMF FEED19    TOTAL DILUTION WATER20    LIME MUD TO DREGS FILTER21    FILTER FEED SLURRY  736   328   136  317    464   70.71%22    CAKE WASH WATER           -50   50   LMF RE LOSS =                                                   -10.71%23    FILTER CAKE         20    8     5    9      13    60.00%24    LMF FILTRATE 70° C.25    DREGS FILT PRECOAT FILTRATE26    TOTAL FILTRATES     716   271   181  308    451   60.00%27    SMELT               0     12008 904  29279  12912 93.00%28    SMELT &amp; WEAK WASH   3502  13570 1551 30786  15121 89.74% (RAW GREEN LIQUOR)__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________                     05/18/93                           &#34;WLCGLF2&#34; WATER                                          SOLIDSEXAMPLE FLOWSHEET SUMMARY   CFD   CFH    GPM                                     LB/HR                                          LB/HR STPD                                                    %__________________________________________________________________________                                                    Na2O1     RAW GREEN LIQUOR FLOW 90° C.                     145041                           6043   753                                     363703                                          750   9.02A    G L CLARIFIER UNDERFLOW                     3254  136    17 7715 706   7.5 44.2%2B    G L FILTER CAKE     258   11     1.3                                     194  706   8.5 3.5%2C    G L FILTER FILTRATE 90° C.                     2996  125    15.6                                     7522 0.23  0.003A    DILUTION WATER      1496  62     8  38053B    GLF UNDERFLOW SLURRY-, % ss                     1773  74     9  3998 706   8.5 3.5%3C    DREGS FILTER LIME MUD PRECOAT                     569   23.7   3.0                                     1056 1058  12.74     DREGS FILTER WASH WATER 70° C.                     1141  47.6   5.9                                     29035     DREGS FILTER CAKE 70° C.                     1038  43.2   5.4                                     1759 1764  21.2                                                    0.16%6     DREGS FILTER FILTRATE 70° C.                     2437  102    12.7                                     61987A    G L CLARIFIER OVERFLOW 90° C.                     141787                           5908   737                                     355987.3                                          43.9  0.57B    CLARIFIED GREEN LIQUOR FLOW 90° C.                     144783                           6033   751                                     363509                                          44.2  0.58     WATER TO SLAKE LIME (95° C.)                     2788  116.2  14.5                                     69779     LIME USE, BONE DRY  6179  257    32.1    25549 306.6 MUD GENERATED, BONE DRY                     6179  257    32.1    42593 511.110    GRIT LOSS (WET)     182   7.6    0.9                                     275  511   6.111    GRIT WASH WATER     108   4.5    0.6                                     27512    FLOW THROUGH CAUSTICIZERS 95° C.                     148608                           6192   772                                     356533                                          42126 505.513    WHITE LIQUOR FLOW AT 95° C.                     124579                           5191   647                                     311702                                          37.3  0.4514    W L C UNDERFLOW AT 95° C.                     24024 1001   125                                     44831                                          42088 505.1                                                    12.2% W L C UNDERFLOW AT 70° C.                     23732 989    123                                     44831                                          42088 505.115    SCRUBBER RECYCLE 70° C.                     79566 3315   413                                     199698                                          7234  86.826    LMF FILTRATES       66424 2768   345                                     168938                                          8.5   0.16     DREGS FILTER FILTRATE 70° C.                     2437  102    13 619816    L M W FEED 70° C.                     172159                           7173   894                                     419663                                          49331 592.0                                                    12.5%17    L M W OVERFLOW (WEAK WASH) 70° C.                     143005                           5959   743                                     363703                                          37.3  0.4518    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     29154 1215   151                                     55961                                          49294 591.5                                                    2.59% DILUTION WATER-DF PRECOAT                     760   32     3.9                                     1932 DILUTION WATER-LMF FEED                     34616 1442   180                                     8804119    TOTAL DILUTION WATER                     35375 1474   184                                     8997320    LIME MUD TO DREGS FILTER                     1386  58     7.2                                     3133 1058  12.721    FILTER FEED SLURRY  63144 2631   328                                     142800                                          48235 578.8                                                    2.65%22    CAKE WASH WATER     15766 657    82 4010023    FILTER CAKE         13303 554    69 16040                                          48227 578.7                                                    0.075%24    LMF FILTRATE 70° C.                     65607 2734   341                                     166860                                          8.5   0.1025    DREGS FILT PRECOAT FILTRATE                     817   34     4.2                                     207826    TOTAL FILTRATES     66424 2768   345                                     168938                                          8.5   0.1027    SMELT                                    713   8.628    SMELT &amp; WEAK WASH (RAW GREEN LIQUOR)__________________________________________________________________________                     SLURRY                          SLURRY                               LB/HR   BREAKDOWN OF CHEM-LB/HREXAMPLE FLOWSHEET SUMMARY   SS   SP GR                               CHEM                                   TTA NaOH                                           Na2S                                               SO4 Na2CO3__________________________________________________________________________1     RAW GREEN LIQUOR FLOW 90° C.                     1696 1.17 77721                                   47874                                       4428                                           17086                                               3410                                                   527982A    G L CLARIFIER UNDERFLOW                     7.00%                          1.19 1662                                   1003                                       94  347 102 11212B    G L FILTER CAKE     75.00%                          1.40 42  25  2   9   3   282C    G L FILTER FILTRATE 90° C.                     26   1.17 1620                                   978 92  338 99  10923A    DILUTION WATER           0.98 612 -602    -766                                               13783B    GLF UNDERFLOW SLURRY-, % ss                     14.9%                          1.03 41.7                                   25.2                                       2.4 8.7 2.5 28.13C    DREGS FILTER LIME MUD PRECOAT                     38.8%                          1.844     DREGS FILTER WASH WATER 70° C.                          0.985     DREGS FILTER CAKE 70° C.                     50.0%                          1.31 4.6 2.8 0.3 1.0 0.3 3.16     DREGS FILTER FILTRATE 70° C.                          0.98 37.1                                   22.4                                       2.1 7.7 2.3 25.07A    G L CLARIFIER OVERFLOW 90° C.                          1.17 76683                                   46270                                       4336                                           15981                                               4688                                                   516777B    CLARIFIED GREEN LIQUOR FLOW 90° C.                     100  1.17 78304                                   47248                                       4427                                           16319                                               4787                                                   527698     WATER TO SLAKE LIME (95° C.)                          0.96 -10066                                   0   30976                                           CAUSTICIZING                                                   -410429     LIME USE, BONE DRY  100% 1.59 622 -602    -758                                               1380                                                   OXIDATION MUD GENERATED, BONE DRY                     100% 2.6510    GRIT LOSS (WET)     65.0%                          1.66 -9444                                   -602                                       30976                                           -758                                               1380                                                   -4104211    GRIT WASH WATER          0.9812    FLOW THROUGH CAUSTICIZERS 95° C.                     9.0% 1.21 68859                                   46646                                       35404                                           15561                                               6167                                                   1172713    WHITE LIQUOR FLOW AT 95° C.                          1.15 60744                                   40255                                       30952                                           12942                                               6598                                                   1025314    W L C UNDERFLOW AT 95° C.                     44.0%                          1.53 8737                                   5790                                       4452                                           1861                                               949 1475 W L C UNDERFLOW AT 70° C.                     44.0%                          1.55 622 -602    -758                                               138015    SCRUBBER RECYCLE 70° C.                     3.4% 1.0426    LMF FILTRATES            0.99 1894                                   1134                                       924 298 361 3116     DREGS FILTER FILTRATE 70° C.                          0.98 37  22  2   8   2   2516    L M W FEED 70° C.                     10.5%                          1.05 10668                                   6946                                       5378                                           2167                                               1312                                                   181117    L M W OVERFLOW (WEAK WASH) 70° C.                     100  0.98 8881                                   5625                                       4428                                           1665                                               1297                                                   149118    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     46.0%                          1.41 1906                                   1207                                       950 357 278 320 DILUTION WATER-DF PRECOAT                          0.98 118 -115    -144                                               263 DILUTION WATER-LMF FEED  0.9819    TOTAL DILUTION WATER     0.9820    LIME MUD TO DREGS FILTER                     25.0%                          1.17 41  2621    FILTER FEED SLURRY  25.0%                          1.17 1906                                   1207                                       950 357 278 32022    CAKE WASH WATER          0.98 42  -41     -51 9323    FILTER CAKE         75.0%                          1.86 53  32  26  8   10  924    LMF FILTRATE 70° C.                     51   0.9825    DREGS FILT PRECOAT FILTRATE                          0.9826    TOTAL FILTRATES     50   0.99 1894                                   1134                                       924 298 361 31127    SMELT                         68841                                   42250                                       0   15421                                               2113                                                   5130728    SMELT &amp; WEAK WASH             77721                                   47874                                       4428                                           17086                                               3410                                                   52798 (RAW GREEN LIQUOR)__________________________________________________________________________                     BREAKDOWN OF TTA-LB/HREXAMPLE FLOWSHEET SUMMARY   NaOH  Na2S  SO4  Na2CO3 TOT S R.E.__________________________________________________________________________1     RAW GREEN LIQUOR FLOW 90° C.                     3431  13569 1488 30874  15057 90.12%2A    G L CLARIFIER UNDERFLOW                     73    275   44   655    320   86.12%2B    G L FILTER CAKE     1.8   6.9   1.1  16.4   8.0   86.12%2C    G L FILTER FILTRATE 90° C.                     71    268   43   639    312   86.12%3A    DILUTION WATER            -602  602  CLC RE LOSS =                                                   -4.00%3B    GLF UNDERFLOW SLURRY-, % ss                     1.8   6.9   1.1  16.4   8.0   86.12%3C    DREGS FILTER LIME MUD PRECOAT4     DREGS FILTER WASH WATER 70° C.5     DREGS FILTER CAKE 70° C.                     0.2   0.8   0.1  1.8    0.9   86.12%6     DREGS FILTER FILTRATE 70° C.                     1.6   6.1   1.0  14.6   7.1   86.12%7A    G L CLARIFIER OVERFLOW 90° C.                     3359  12692 2046 30219  14737 86.12%7B    CLARIFIED GREEN LIQUOR FLOW 90° C.                     3430  12960 2089 30858  15049 86.12%8     WATER TO SLAKE LIME (95° C.)                     24000            -240009     LIME USE, BONE DRY        -602  602  S + C RE LOSS                                                   -4.00% MUD GENERATED, BONE DRY10    GRIT LOSS (WET)     24000 -602  602  -24000 011    GRIT WASH WATER12    FLOW THROUGH CAUSTICIZERS 95° C.                     27431 12358 2691 6858   15049 82.12%13    WHITE LIQUOR FLOW AT 95° C.                     23981 10278 2879 5995   13157 78.12%14    W L C UNDERFLOW AT 95° C.                     3449  1478  414  862    1892  78.12% W L C UNDERFLOW AT 70° C.                           -602  602  WLC RE LOSS =                                                   -4.00%15    SCRUBBER RECYCLE 70° C.26    LMF FILTRATES       716   236   158  182    394   60.00%6     DREGS FILTER FILTRATE 70° C.                     2     6     1    15     7     86.12%16    L M W FEED 70° C.                     4167  1721  573  1059   2294  75.03%17    L M W OVERFLOW (WEAK WASH) 70° C.                     3431  1322  566  872    1888  70.03%18    L M W U&#39;FLOW TO MUD STORAGE 70° C.                     736   284   121  187    405   70.03% DILUTION WATER-DF PRECOAT -115  115  LMW RE LOSS =                                                   -5.00% DILUTION WATER-LMF FEED19    TOTAL DILUTION WATER20    LIME MUD TO DREGS FILTER21    FILTER FEED SLURRY  736   284   121  187    405   70.03%22    CAKE WASH WATER           -41   41   LMF RE LOSS =                                                   -10.03%23    FILTER CAKE         20    7     4    5      11    60.00%24    LMF FILTRATE 70° C.25    DREGS FILT PRECOAT FILTRATE26    TOTAL FILTRATES     716   236   158  182    394   60.00%27    SMELT               0     12247 922  30003  13169 93.00%28    SMELT &amp; WEAK WASH   3431  13569 1488 30874  15057 90.12% (RAW GREEN LIQUOR)__________________________________________________________________________ 
    
     Additionally, Charts 1 and 2 demonstrate the superior characteristics of dregs obtained with the present invention relative to dregs produced by the conventional processes. In Chart 1, the Untreated column designates the pH of the untreated underflow of the green liquor clarifier, the Precoat Only column designates the pH of dregs resulting from precoat dregs filter treatment only, the CAUSTEC Only column designates the pH of dregs resulting from CAUSTEC™ treatment only, and the CAUSTEC+Precoat column designates the pH of dregs resulting from CAUSTEC™ and precoat dregs filter treatment dregs. As shown, it is only with the process of the present invention that an acceptable pH level is reached. In Chart 2, comparative data is presented regarding % Na 2  O (a measure of the amount of alkali) in the dregs resulting from the processes described above. As above, the Untreated column designates the amount of alkali in the untreated underflow of the green liquor clarifier, the Precoat Only column designates the amount of alkali in dregs resulting from precoat dregs filter treatment only, the CAUSETEC Only column designates the amount of alkali in dregs resulting from CAUSTEC™ treatment only, and the CAUSTEC+Precoat column designates the amount of alkali in dregs resulting from CAUSTEC™ and precoat dregs filter treatment dregs. Again, the dregs produced by the present invention utilizing a CAUSTEC cassette dregs filter in combination with a dregs precoat filter contains much less alkali then the other processes. 
     Modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than is specifically described herein.