Abstract:
A hybrid system for processing papermaking fibers includes a multistage array of forward cleaners coupled with a flotation cell which increases overall efficiency of the system. In a typical embodiment, a first rejects aqueous stream from a first stage bank of centrifugal cleaners is treated in a flotation cell before being fed to a second stage bank of centrifugal cleaners.

Description:
CLAIM FOR PRIORITY  
       [0001]    This non-provisional application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/180,348, of the same title, filed Feb. 4, 2000. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to papermaking fiber processing and more particularly to a method and apparatus useful for cleaning secondary pulp by way of a multistage forward cleaner system with an integrated flotation cell which cooperates with the forward cleaners to boost efficiency of the system.  
         BACKGROUND  
         [0003]    Processing of papermaking fibers to remove contaminants is well known in the art, including the use of forward cleaners and flotation cells. Such technology is used, for example, to treat secondary (recycle) fiber sources for re-use in paper products such as towel and tissue, paperboard, coated writing and printing papers and so forth. Following is a brief synopsis of some patents of general interest.  
           [0004]    According to U.S. Pat. No. 4,272,315 to Espenmiller waste paper containing materials, e.g., commercial “waste paper”, are treated for recovery of reusable paper therefrom by slushing in a pulper from which two fractions are continuously extracted—a first fraction through small holes, e.g. {fraction (3/16)} inch in diameter, and a second fraction through substantially larger holes, e.g., 1 inch in diameter. The second fraction is screened, preferably after a centrifugal cleaning operation, in a screen having small perforations sized to accept only substantially defibered paper, and the accepts flow is mixed directly with the first extracted fraction. The reject flow from this screen is conducted, with or without an intermediate deflaking operation, to a tailing screen from which the accepts are recycled to the pulper and the rejects are eliminated from the system. Advantages of this method and system include the continuous elimination of plastic and other floating trash from the pulper, a high degree of essentially complete defibering in the pulper, and minimal recycling of adequately defibered stock.  
           [0005]    U.S. Pat. No. 4,983,258 to Maxham discloses a process for the production of papermaking fiber or pulp from waste solids emanating from pulp and paper mills, particularly waste solids in process water streams containing fibrous solids that cannot be directly recycled by paper mill “saveall” devices, from pulp and paper mill process water streams conveyed by the sewerage system to wastewater treatment plant facilities, and from “sludge” emanating from the underflow of a primary clarifier or sedimentation basin at pulp and paper mill wastewater treatment facilities either before or after the “sludge” is thickened and dewatered. The said process comprises a defibering stage to release individual fibers from bundles, a screening stage to separate long fiber and debris from short fiber and clay, a centrifugal cleaning stage to separate debris from the long fiber, a bleaching stage to increase the brightness of the fiber, a dewatering stage to remove excess water from the pulp, a sedimentation stage to separate the short fiber-clay-debris from the defibering effluent which is substantially recycled, and a biological treatment process to remove dissolved organic materials from the excess water generated which can be either discharged from the process or recycled as process water.  
           [0006]    U.S. Pat. No. 5,240,621 to Elonen et al. discloses a method of separating an aqueous solids containing suspension which includes (a) subjecting a first solids containing suspension to centrifugal forces so as to separate the suspension into a first gas containing flow, a second gas-free flow and a third flow; (b) feeding the third flow into a flotation cell having a bottom; (c) introducing air at the bottom of the flotation cell into the third flow for separating from the third flow a fourth partial flow; (d) withdrawing the air containing third flow after the separation of the fourth partial flow from the flotation cell; and (e) subjecting the third flow to the centrifugal forces of step (a). An apparatus for the separation of gas and lightweight material from a gas and lightweight material containing aqueous solids suspension is also described and includes a centrifugal pump for separating the gas and lightweight material from the solids suspension with a suspension inlet and an outlet for the lightweight material; a flotation cell for separating the lightweight material from a solids suspension; and a circulation loop connecting the outlet of the centrifugal pump, the flotation cell and the suspension inlet of the pump.  
           [0007]    In U.S. Pat. No. 5,693,222 to Galvan et al. a dissolved gas flotation tank system is disclosed which is configured to provide educted gas or air into recirculated effluent fluid from the tank which includes a pump system which increases the dissolution rate of gas into the effluent fluid thereby eliminating the need for retention tanks and related equipment which adds to high equipment costs. The dissolved gas flotation tank system also provides a pre-contact chamber for assuring immediate and intimate contact between the suspended solids in an influent feed stream and the recirculated effluent fluid in which gas is dissolved, as well as flocculant when used, to produce a better agglomerate structure for improved flotation and separation. The dissolved gas flotation tank also provides an improved means of removing and processing float from the tank, and employs a dewatering system enhanced by the addition of chemicals or flocculants into the float removal system.  
           [0008]    The disclosures of the foregoing patents are hereby incorporated for reference.  
           [0009]    While flotation and separation technologies are fairly advanced, there is an ongoing need to increase overall fiber-cleaning system performance and to reduce the amount of waste and capital investment in the plant.  
         SUMMARY OF INVENTION  
         [0010]    The present invention provides a hybrid system for processing papermaking fibers and includes a multistage array of forward cleaners coupled with a flotation cell which increases overall efficiency of the system. In a typical embodiment, a first rejects aqueous stream from a first stage bank of centrifugal cleaners is treated in a flotation cell before being fed to a second stage bank of centrifugal cleaners.  
           [0011]    One advantage of feeding the second accepts stream forward is that it does not have to be returned to the first bank of cleaners for re-cleaning. This reduces the size of the first bank of cleaners or allows an existing installation to operate at a lower consistency. (The cleaners operate more efficiently at a low consistency of 0.5% than at 0.8 or 1%). Another advantage is that the flotation cell operates at greater than 60% efficiency on removing hydrophobic contaminants from the first cleaner rejects, while another cleaner stage removes less than 50% of the hydrophobic contaminants. As a result a large quantity of hydrophobic contaminants are removed in the flotation stage, which makes the remaining cleaner stages work more efficiently with less good fiber loss.  
           [0012]    Investigation showed that the number of hydrophobic contaminants in the second cleaner accepts after the flotation stage was lower than the number of hydrophobic contaminants in the first cleaner accepts. Without the flotation stage the number of hydrophobic contaminants in the second accepts is much higher than the first accepts, so that the second accepts have to be returned to the first bank of cleaners for more cleaning.  
           [0013]    As will be appreciated form the discussion which follows, the size and cost of a flotation stage for treating secondary fiber can be reduced by up to 75% if it is installed in centrifugal cleaner system as compared to a full scale treatment of the stock by flotation. The centrifugal cleaner system modeling indicates a 34% reduction in ink speck area of total centrifugal cleaner system accepts by removing ink specks from the first stage rejects with 80% efficiency in a flotation stage and then feeding the flotation accepts forward after centrifugal cleaning of the second stage. (24% reduction if second stage rejects are treated in a similar manner). The ability to feed the centrifugal cleaner rejects forward (after the flotation stage and additional centrifugal cleaning in the next stage) reduces the stock consistency in the first stage, thereby improving the efficiency of the first stage. The capacity of the system is also increased by feeding the second stage centrifugal cleaner accepts forward. The other centrifugal cleaner stages can also be operated more efficiently since more than 50% of the ink in the first stage centrifugal cleaner rejects has been removed in the flotation stage. When the centrifugal cleaner accepts are thickened in a press, a large amount of ink ends up in the pressate. This ink can also be removed by using the ink-laden pressate as dilution water for the centrifugal cleaner rejects going to the flotation stage.  
           [0014]    A conventional centrifugal cleaner system (as shown in FIG. 1) normally consists of several stages, whereby the rejects of each centrifugal cleaner stage are diluted for cleaning in the next stage and the centrifugal cleaner accepts are fed backwards to the feed of the previous stage. The ink speck removal efficiency of the centrifugal cleaner is usually much less than 50% on toner inks in office waste paper. As a result the total centrifugal cleaner system ink speck removal efficiency can drop to 30% or less on a furnish containing a large proportion of office waste.  
           [0015]    By sending the first or second stage centrifugal cleaner rejects to a flotation stage (as shown in FIG. 2) it is possible to remove a much higher percentage of the ink specks in office waste. (It was possible to obtain 80% removal of ink specks during a pilot plant trial with a flotation cell operated on second stage centrifugal cleaner rejects.) If the accepts of the flotation cell are cleaned in the next centrifugal cleaner stage, the centrifugal cleaner accepts from that stage can then be fed forward to the thickener. Sending centrifugal cleaner accepts forward reduces the load and improves the efficiency of the previous centrifugal cleaner stage.  
           [0016]    The present invention is particularly useful in connection with removing stickies from the recycle fiber product stream; likewise, it is believed pitch removal is enhanced. Stickies are generally a diverse mixture of polymeric organic materials which can stick on wires, felts or other parts of paper machines, or show on the sheet as “dirt spots”. The sources of stickies may be pressure-sensitive adhesives, hot melts, waxes, latexes, binders for coatings, wet strength resins, or any of a multitude of additives that might be contained in recycled paper. The term “pitch” normally refers to deposits composed of organic compounds which are derived form natural wood extractives, their salts, coating binders, sizing agents, and defoaming chemicals existing in the pulp. Although there are some discrete characteristics, there are common characteristics between stickies and pitch, such as hydrophobicity, low surface energy, deformability, tackiness, and the potential to cause problems with deposition, quality, and efficiency in the process. Indeed, it is possible with the present invention to reduce stickies by 50%, 80% or even more by employing a flotation cell in a multistage forward cleaner system as hereinafter described in detail.  
           [0017]    The rejects from the flotation stage are so full of ink and ash that they can be rejected without any further treatment.  
           [0018]    There is provided in one aspect of the present invention, a method of processing papermaking fibers with a multistage array of forward cleaners including a plurality of centrifugal cleaners configured to generate accepts streams and rejects streams which concentrate heavy waste, the method including (a) feeding a first aqueous feed stream including papermaking fibers to a first stage bank of centrifugal cleaners of the multistage array; (b) generating a first accepts aqueous stream and a first rejects aqueous stream in the first stage bank of centrifugal cleaners, the first aqueous rejects stream being enriched in heavy waste with respect to said first aqueous feed stream; (c) supplying the first rejects aqueous stream to a flotation stage; (d) treating the first rejects aqueous stream in the flotation stage to remove hydrophobic waste from the first aqueous rejects stream and produce an intermediate aqueous purified feed stream; and (e) feeding the aqueous purified intermediate feed stream to a second stage bank of centrifugal cleaners of the multistage array, the second centrifugal cleaner being configured to generate a second accepts aqueous stream, wherein the second rejects aqueous stream is enriched in heavy waste with respect to said aqueous purified intermediate feed stream. The method may further include feeding the first accepts aqueous stream and said second accepts aqueous stream to another cleaning device or a thickening device. Suitable additional cleaning devices include screening devices, reverse cleaners and the like. In a preferred embodiment, the first aqueous feed stream comprises a preliminary accepts stream generated by way of a preliminary bank of centrifugal cleaners dividing a preliminary feed stream into a preliminary accepts stream and a preliminary rejects stream. A preferred method may include feeding the preliminary rejects stream to the flotation stage and treating the preliminary rejects stream along with the first rejects aqueous stream to remove hydrophobic waste therefrom whereby the aqueous purified intermediate stream includes treated components from both the preliminary rejects stream and the first rejects aqueous stream.  
           [0019]    In other preferred embodiments, the process may include feeding the second rejects aqueous stream to a third centrifugal cleaner operative to generate a third accepts aqueous stream and a third rejects aqueous stream.  
           [0020]    Preferably, the multistage array of forward cleaners comprises at least 3 banks of centrifugal cleaners, and still more preferably, the multistage array of forward cleaners comprises at least 5 banks of centrifugal cleaners. The first aqueous feed stream generally has a consistency of from about 0.3% to about 0.9%, whereas the first aqueous stream more typically has a consistency of from about 0.4% to about 0.7%. The hydrophobic waste removed from the first aqueous stream by the flotation stage often includes an ink and stickies composition, toner ink compositions being typical in office waste and stickies compositions frequently being obtained from pressure sensitive adhesives in office waste.  
           [0021]    In another aspect of the invention there is provided a hybrid apparatus for processing papermaking fibers with a multistage array of forward cleaners including (a) a first bank of centrifugal cleaners configured to generate a first accepts stream and a first rejects stream upon operating on a first aqueous feed stream, the first rejects stream being enriched with respect to heavy hydrophobic contaminants with respect to the first aqueous feed stream; (b) a flotation cell connected to the first bank of centrifugal cleaners so as to receive the first rejects stream and adapted to remove hydrophobic contaminants such as ink, stickies and the like from the first rejects stream, the flotation cell being constructed and arranged so as to generate a flotation rejects stream and a flotation accepts stream which is purified with respect to hydrophobic contaminants in said first rejects stream; and (c) a second bank of centrifugal cleaners coupled to the flotation cell so as to receive the flotation accepts stream as a second feed stream, the second bank of centrifugal cleaners being likewise configured to generate an accepts stream hereinafter referred to as a second accepts stream and a second rejects stream respectively. In a preferred embodiment, a preliminary bank of centrifugal cleaners is provided upstream of the first bank of centrifugal cleaners and coupled thereto whereby the accepts stream of the preliminary bank of centrifugal cleaners is fed to the first bank of centrifugal cleaners. The banks of centrifugal cleaners are typically hydrocyclone type cleaners.  
           [0022]    Unless otherwise indicated, terminology appearing herein is given its ordinary meaning; %, percent or the like refers, for example, to weight percent and “consistency” refers to weight percent fiber or solids as that term is used in papermaking. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0023]    The invention is described in detail below with reference to numerous examples and the appended Figures wherein like numbers designate similar parts throughout and wherein:  
         [0024]    [0024]FIG. 1 is a schematic of a conventional multistage forward centrifugal cleaner system wherein each bank of cleaners are designated by a conical element;  
         [0025]    [0025]FIG. 2 is a schematic diagram of a hybrid multistage forward cleaner/flotation apparatus and process of the present invention, wherein a flotation stage is provided to treat the second stage rejects stream;  
         [0026]    [0026]FIG. 3 is a schematic diagram of a hybrid multistage forward cleaner/flotation apparatus and process of the present invention wherein a flotation stage is provided to treat the first stage rejects stream;  
         [0027]    [0027]FIG. 4 is a schematic diagram of a hybrid multistage forward cleaner/flotation apparatus and process of the present invention wherein a flotation stage is provided to treat the first stage rejects and third stage accepts; and  
         [0028]    [0028]FIG. 5 is a schematic diagram illustrating an apparatus and process of the present invention wherein the hybrid system has dual forward cleaner banks in series and the rejects stream from both of the forward cleaner banks are provided to a flotation cell. 
     
    
     DETAILED DESCRIPTION  
       [0029]    The invention is described in detail below for purposes of illustration and exemplification only. Such explanation of particular embodiments in no way limits the scope of the invention which is defined in the appended claims. Referring to FIG. 1, there is shown a conventional forward cleaner system  10  of the type employed at a paper mill, for instance, as part of the cleaning process for processing secondary pulp into paper products. System  10  has five stages  12 ,  14 ,  16 ,  18  and  20  of banks of centrifugal cleaners interconnected in the manner shown. Such connections may include suitable piping, mixing tanks, holding vessels and the like (not shown) as may be convenient for operating the system. Pulp is fed at low consistency to the system at  22  to the first bank of cleaners  12  through inlet  24  and centrifugally treated in the first stage by a bank of hydrocyclones, for example, such that the accepts are fed forward at  26  to a thickener (or another cleaning device) at  28  whereas the rejects, concentrating the heavy, hydrophobic waste in the system are fed to second stage  14  at  28  for further treatment in a second stage made up of a second bank of centrifugal cleaners  14 . Diluent water is added to the rejects stream from the first stage as indicated at  30  in an amount suitable for the particular system or operating conditions. Stream  28  (first stage rejects) is thus fed to the second stage cleaners whereupon bank  14  of cleaners generates an accepts stream  32  and a rejects stream  34 . Stream  32  is a recycled to the feed  22  and makes up a portion of the material fed to the first stage bank of cleaners  12 . The first bank of cleaners may be made up of 50 or more hydrocyclones depending on capacity and performance desired. Subsequent stages will each contain fewer cleaners than the previous stage depending upon the amount of rejects, until the final stage contains less than 10 cleaners.  
         [0030]    Stream  34  is again enriched with respect to heavy components (with respect to stream  32 ) and is fed to the third stage  16  bank of cleaners for further processing. Diluent water may again be added at  36  if so desired to stream  34 . Stage  16  generates another accepts stream  38  which is fed back to the second stage (stream  28 ) and another rejects stream  40  enriched in heavy hydrophobic components.  
         [0031]    In like fashion, stream  40  is fed to the fourth stage  18  bank of cleaners at  42  where diluent water may again be added. The fourth stage generates another accepts stream  44  and another rejects stream  46 . These streams have the rejects/accepts characteristics noted above.  
         [0032]    Stream  46  is fed to yet another stage  20  of forward cleaners at  48  wherein stream  46  is divided into an accepts stream  50  and a rejects stream  52  as indicated on the diagram. Accepts stream  50  is recycled to the fourth stage as shown and rejects stream  52  is discarded or further processed if so desired. There is thus described a conventional forward cleaner system utilizing centrifugal cleaners in cascaded/refluxing fashion to concentrate the waste material and purify the pulp which is fed forward at a papermaking process to a thickening device or a cleaning device such as screens or a reverse cleaner.  
         [0033]    In accordance with the present invention, a flotation stage is advantageously integrated into a multistage forward cleaner system to remove hydrophobic material and increase the cleaning efficiency. Flotation utilizes the phenomenon that the minerals which are present in the ground ore can partially be wetted, i.e., they are hydrophilic, while other parts of the minerals are hydrophobic. Hydrophobic particles have a clear affinity to air. Accordingly, finely distributed air is introduced into the solid-water-mixture so that the air will attach to the hydrophobic particles causing them to rise to the surface of the mixture or suspension. The hydrophobic particles, such as valuable minerals or the above-mentioned contaminants present in repulped stock suspensions, collect as froth at the surface of the suspension and are skimmed off with a suitable means such as a paddle or weir. The hydrophilic particles of the ore or stock suspension remain in the flotation vat. It is also possible to separate two or more useful minerals selectively by the flotation method, for example, in the separation of sulfidic lead/zinc ores. For controlling the surface properties of the minerals small amounts of additives of chemical agents are introduced such as, for example, foaming agents which will help to stabilize the air bubbles, so-called collecting agents which actually cause the hydrophobic effect and prepare the mineral particles for attachment to the air bubbles, and floating agents which temporarily impart hydrophilic properties to the hydrophobic minerals and later return the hydrophobic properties for selective flotation, as mentioned above. The latter are generally inorganic compounds, mostly salts, while the collectors are mostly synthetic organic compounds, and the foaming agents are oily or soapy chemicals such as fatty acid soap.  
         [0034]    The apparatus of the present invention may utilize a variety of readily available components. The centrifugal cleaners, for example, are available from Ahlstrom (Noormarkku, Finland) or Celleco (Model 270 series) (Lawrenceville, Ga., USA) and are arranged in banks as shown in FIGS.  2 - 5 . The flotation stage, which may be multiple cells, are likewise readily available from Comer SpA (Vicenza, Italy). Comer Cybercel® models FCB1, FCB3 and FCB4 are suitable as discussed further herein.  
         [0035]    There is illustrated in FIG. 2 an apparatus  100  and method in accordance with the present invention. Apparatus  100  operates similarly to apparatus  10  in FIG. 1. Like ports are given like numbers for purposes of brevity and only differences noted from the discussion above. The system  100  of FIG. 2 operates as described in connection with system  10  of FIG. 1 and is so numbered in the drawing except that system  100  has a flotation stage  75  for treating the rejects stream  34  of second stage cleaner  14 . Diluent water may be added at  36  as before, and hereafter, stream  34  is treated in the flotation stage to remove hydrophobic material. The accepts from the flotation stage, that is purified as shown by removing hydrophobic waste from stream  34 , is then fed in stream  34 ′ to third stage cleaner  16 . Instead of refluxing the accepts from the third stage back to the second stage, the accepts material is fed forward in a product stream  26 ′ for downstream processing. The hydrophobic rejects ( 31 ′) from flotation stage ( 75 ) are removed from system  100 .  
         [0036]    In FIG. 3 there is illustrated another apparatus  200  and method of the present invention. Here again similar functioning parts are numbered as in FIGS. 1 and 2, the discussion of which is incorporated by reference here. Apparatus  200  of FIG. 3 differs from apparatus  10  of FIG. 1 in that a flotation stage  75  is added to treat the first stage rejects stream  28  to remove hydrophilic waste to produce an intermediate purified stream  28 ′ which is fed to the second stage bank of cleaners  14 . Bank  14  generates a purified accepts stream  32 ′ which is fed forward to the thickening or other device  28  along with stream  26 . The hydrophobic rejects ( 21 ′) from flotation stage ( 75 ) are removed from system  200 .  
         [0037]    In FIGS. 4 and 5 there are illustrated alternate embodiments of the present invention. Like components are numbered as in FIGS.  1 - 3  above, the discussion of which is incorporated by reference. In the apparatus  300  of FIG. 4, there is provided a flotation cell  75  which treats rejects stream  28  from the first centrifugal cleaning stage along with accepts stream  38 ′ from the third centrifugal cleaning stage. Stream  38 ′ is combined with rejects stream  28  and fed to the flotation stage where hydrophobic material is removed and an intermediate purified stream  28 ′ is produced. Stream  28 ′ is fed to the second stage  14  of centrifugal cleaners. The accepts stream from stage  14  is fed forward as stream  32 ″ and combined with stream  26  in thickening device  28 . The hydrophobic rejects ( 21 ′) from flotation stage ( 75 ) are removed from system  300 .  
         [0038]    Apparatus  400  of FIG. 5 resembles apparatus  200  of FIG. 3 except that there is provided a preliminary stage  12 ′ of centrifugal cleaners, the accepts stream  26 ″ of which is utilized as the feed to stage  12 . Rejects stream  28 ″ of stage  12 ′ is combined with rejects stream  28  of stage  12  and fed to flotation stage  75 . Accepts stream  32 ′ of the second stage cleaners is fed forward with accepts stream  26  of stage  12 . The hydrophobic rejects ( 21 ′) from flotation stage ( 75 ) are removed from system  400 .  
       EXAMPLES  
       [0039]    Pilot plant trials showed that flotation cells such as the Comer Cybercel ® can successfully deink secondary centrifugal cleaner rejects, with better results obtained if the consistency is kept close to 0.6%. Consistency refers to weight percent fiber or associated solids such as ash unless the context indicates otherwise. Results on 42% office waste (Grade A) and 100% office waste (Grade B) are shown in Table 1.  
                                     TABLE 1                           Pilot Plant Results for Brightness Gain, Dirt + Ash Removal Efficiency       on Grades A and B at Halsey and Results Used in Simulation Models                Grade:   A   B   Model                       Consistency:   0.69%   0.90%   0.62%           Brightness Gain:   18.5%    5.3%           Dirt Removal:   77-89%   65-87%     80%           Ash removal:     63%     64%     64%                      
 
         [0040]    A simulation model was used to calculate the impact of a Comer Cybercel® flotation cell to deink forward cleaner rejects on solids loss, ash removal and on removal efficiency of mid-dirt (&gt;150 microns) from a 1 st  washer to the deinked pulp (while running grade B at 336 tpd at the 1 st  washer):  
                                     TABLE 2                           Impact of Flotation Cell on Solids Loss, Ash Loss, and Mid-dirt       Removal Efficiency       (according to the Simulation Model for 6 different configurations on       Grade B)            Example       Solids loss   Ash loss   Mid-dirt Eff.               1   No Flotation cell   8.9 tpd   0.8 tpd   96.1%       2   Flotation cell on 2 nd     2.7 tpd   0.9 tpd   97.0%           stage Rejects       3   Flotation cell on   6.7 tpd   1.9 tpd   97.4%           1 st  stage Rejects       4   As 3 with 50% eff. in   6.7 tpd   1.9 tpd   97.7%           1 st  stage       5   Flotation cell on 1 st     8.9 tpd   1.9 tpd   97.7%           stage Rejects +           3 rd  stage accepts, 44%           eff. in 1 st  stage       6   Flotation cell on two   11.8 tpd    2.8 tpd   98.5%           1 st  stages                  
 
         [0041]    The following indicators were used to evaluate the performance of the pilot plant:  
         [0042]    feed consistency.  
         [0043]    brightness gain of handsheets from accepts compared to feed.  
         [0044]    Dirt removal efficiency of small dirt (&lt;150 microns), mid-dirt (&gt;150 microns) and large dirt (&gt;200 microns).  
         [0045]    Ash removal efficiency.  
         [0046]    The results in Table 3 below for examples 7-14 (duplicate runs) show that even at 0.90% feed consistency it was possible to obtain 5.3% points brightness gain, 73% mid-dirt removal efficiency and 64% ash removal on Grade B. Operating the flotation cell at 0.69% consistency on Grade A, it was possible to obtain 8.1% points brightness gain, 79% mid-dirt removal efficiency and 63% ash removal.  
                                                                                                                         TABLE 3                           Comer Pilot Plant Results on 2 nd  stage Cleaner Rejects                Feed   Brightness   Dirt + Ash Removal %                Example   Anal.   Cons. %   Ash %   Gain   Small   Mid   Large   Ash   Comments                    Grade B                                            7   1   0.86       3.3   88   71   64           2       4.4%   5.8   87   74   65   59   Accepts = 90% &gt; 200 m.        8   1   0.88       5.4   87   74   67           2       3.9%   4.6   86   69   57   52   Accepts = 99% &gt; 200 m.        9   1   0.88       6.3   88   78   74           2       5.9%   5.0   87   73   66   68       10   1   0.98       5.9   89   74   61                   3.8%   5.7   86   69   63   77           Average   0.90   4.5%   5.3   87   73   65   64       Grade A       11   1   0.53       7.3   .   .   .           2       15.9%   9.4   92   78   72       Accepts = 95% &gt; 200 m.       12   1   0.83       4.2   88   70   60   70           2       17.8%   8.2   87   70   64       Accepts = 90% &gt; 200 m.       13   1   0.70       8.6   89   88   92   53           2       16.5%   8.0   89   80   80       Accepts = 74% &gt; 200 m.       14   1   —       8.7   91   85   87   67           2       23.8%   10.4   89   85   85           Average   0.69   18.5%   8.1   89   79   77   63                  
 
         [0047]    The effect of incorporating a flotation stage in accordance with the present invention into a multistage forward cleaner system was evaluated with a computer model with respect to the systems illustrated in FIGS.  1 - 5 . Results are summarized in the tables below. DIP refers to deinked pulp and DRE refers to dirt removal efficiency.  
                                                                                                                                                                                         TABLE 4                           System of FIG. 1 - Conventional Multi-Stage Cleaner System       SUMMARY                Flow   Cons.       Ash   Ash   Dirt &gt; 150   Dirt &gt; 160           gpm   %   STPD   %   STPD   ppm/1.2 g   m 2 /day                        Washer       Thick Stock    540   10.37   335.7   2.53   8 5    720   3310               DWw   4272    0.03    7.7   7   0.5    1504   158       Gyro       Accept   4812    1.19   343.4   2.63   9.0    738   3468       Gyro       Accept   4812    1.19   343.4   2.49   8 55   738   3468       Dil. Water           4741    0.03    8.5   7.00   0 60   1504   176           Total in       9553       351.9       9.15       3644       1 st  Stage Cleaner       Accept   9492    0.60   343 0   2.43   8 34    596   2798           Total out   Accept   9492       343.0       8 34   596   2798           Diff.   In-out    60        8.9       0.8        846       5 th  Stage Cleaner       Rejects    60    2 46    8.9   9.04   0.80   6957   847           Total   Rejects    60        8.9       0.8        847                Cleaner to Press DRE:   30.0% DRE            Dil. Water   Out   9334    0 03    16.8                       Press   Out     158.5   35.1    326.2   1.9   6.2    417   1863                Press to DIP DRE:   93.3% DRE            DIP   28                PROCESS   Washer - DIP   96.1% DRE                  
 
         [0048]    [0048]                                                                                                                                                                                         TABLE 5                           System of FIG. 2 - Multi-Stage Cleaner System       with Flotation Cell on 2 nd  Stage Rejects       SUMMARY                Flow   Cons.       Ash   Ash   Dirt &gt; 150   Dirt &gt; 160           gpm   %   STPD   %   STPD   ppm/1.2 g   m 2 /day                        Washer       Thick Stock   540   10.37   335 7    2.53   8.5    720   3310               DWw   4272   0.03    7 7   0 7   0.1    150.4    16       Gyro       Accept   4812   1.19   343.4    2.49   8 5    708   3326       Gyro       Accept   4812   1.19   343 4    2.49   8.55   708   3327       Dil. Water           5666   0.03    10.2    0.70   0.07   150    21           Total in       10478       353.5       8.62       3348       1 st  Stage Cleaner       Accept   9492   0.57   327.0    2.25   7.34   461   2063       3 rd  Stage Cleaner       Accept   927   0.43    23.8    1 39   0 33   373    121           Total out   Accept   10419   0.56   350.8       7.68   455   2185           Diff.   In-out   58        2.7       0.9        1164       Comer       Rejects   42   0.93    2.3   34.77   0.81   32762   1050       5 th  Stage Cleaner       Rejects   16   0.36    0.3   32 88   0.11   23680    113           Total   Rejects   58        2.7       0.9        1163                Cleaner to Press DRE:   30.0% DRE            Dil. Water   Out   10261   0.03    18.5                       Press   Out   158.5   35.1   332.4   1.9   6.3    318   1449                Press to DIP DRE:   93.3% DRE            DIP   21.3                PROCESS   Washer - DIP   97.0% DRE                    
         [0049]    [0049]                                                                                                                                                                                         TABLE 6                           System of FIG. 3 - Multi-Stage Cleaner System       with Flotation Cell on 1 st  Stage Rejects       SUMMARY                Flow   Cons.       Ash   Ash   Dirt &gt; 150   Dirt &gt; 150           gpm   %   STPD   %   STPD   ppm/1.2 g   m 2 /day                        Washer       Thick Stock   540   10.37   335.7   2.53   8.5    720   3310               DWw   4272    0 03    7 7   0.7   0.1    150.4   16       Gyro       Accept   4812    1.19   343.4   2.49   8.5    708   3326       Gyro       Accept   4812    1 19   343.4   2.49   8.55   708   3327       Dil. Water           7449    0 03    13.4   0.70   0.09   150   28           Total in       12261       356.8       8.64       3355       1 st  Stage Cleaner       Accept   9492    0.50   282.9   2.13   6.04   443   1715       2 nd  Stage Cleaner       Accept   2679    0.42    67.1   1.12   0.75   191   175           Total out   Accept   12171    0.48   350.1       6.79   394   1890           Diff.   In-out   90        6.7       1.85       1465       Comer       Rejects   74    1.45    6 4   25.91   1 66   15279   1337       5 th  Stage Cleaner       Rejects   16    0.28    0.3   69.31   0.19   34056   128           Total   Rejects   90        6.7       1.85       1465                Cleaner to Press DRE:   30.0% DRE            Dil. Water   Out   12012    0.03    21.6                       Press   Out   158.5   35.1    328.5   1.9   6.2    276   1241                Press to DIP DRE:   93.3% DRE            DIP   18.5                PROCESS   Washer - DIP   97.4% DRE                    
         [0050]    [0050]                                                                                                                                                                                         TABLE 7                           System of FIG. 4 - Multi-Stage Cleaner System       with Flotation on 1 st  St. Rejects + 3 rd  St. Accepts       SUMMARY                                    Dirt &gt; 150               Flow   Cons.       Ash   Ash   ppm/1.2g   Dirt &gt; 150           gpm   %   STPD   %   STPD   Double-dirt   m 2 /day                        Washer       Thick Stock   546   10.37    339.5   2.51   8.52   1489   6921               DWw   4266   0.015    3.8   0.7   0.0    300   16       Gyro       Accept   4812   1.19    343.4   2.49   8.55   1476   6937       Gyro       Accept   4812   1.19    343.4   2.49   8.55   1476   6937       Dil. Water           7543   0 015    6 8   0.70   0.05   300   28           Total in       12355       350.1       8.60       6985       1 st  Stage Cleaner       Accept   10100   0 46    279.2   2.15   6.01   816   3118       2 nd  Stage Cleaner       Accept   2104   0.50     62.9   1.16   0.73   346   298           Total out   Accept   12204   0 47    342.2   1.97   6.74   729   3416           Diff.   In-out   151        8.0       1.9        3549       Comer       Rejects   143   0 91     7.8   23.75   1.85   31464   3347       5 th  Stage Cleaner       Rejects   8   0 41     0.2   7.68   0.02   72988   202           Total   Rejects   151        8.0       1.9        3549                Cleaner to Press DRE:   30.0% DRE            Dil. Water   Out   12045   0.015    10.8                       Press   Out   158.5   35.1     331.3   1.9   6 3    511   2316                                   Double-dirt                Press to DIP DRE:   93.3% DRE            DIP   34               Double-dirt            PROCESS   Washer - DIP   97.7% DRE                            
         [0051]    [0051]                                                                                                                                                                                         TABLE 8                           System of FIG. 5 - Multi-Stage Cleaner System       with Flotation Cell on both 1 st  Stage Rejects.       SUMMARY                                    Dirt &gt; 150               Flow   Cons.       Ash   Ash   ppm/1.2 g   Dirt &gt; 150           gpm   %   STPD   %   STPD   double-dirt   m 2 /day                        Washer       Thick Stock   546   10.37    339.5    2.51   8.5    1489   6920               DWw   4266   0.015   3.8   0.7   0.0    300   16       Gyro       Accept   4812   1.19    343.3    2 49   8.5    1476   6935       Gyro       Accept   4812   1.19    343.4    2.49   8.55   1476   6937       Dil. Water           7431   0.015   6.7    0.70   0 05   300   27           Total in       12243       350.0       8 60       6964       1 st  Stage       Accept   8417   0 44    223.0    1.89   4.21   523   1596       Cleaner 2       2 nd  Stage Cleaner       Accept   3619   0.53    115.3    1.36   1.56   388   612           Total out   Accept   12036   0.47    338.3       5.77   477   2208                   12036   0.55    400.0           Diff.   In-out   208       11.8       2.8        4756       Comer       Rejects   192   0.99    11.4   24.65   2.81   28167   4389       5 th  Stage Cleaner       Rejects   16   0.39    0.4    8.54   0.03   71490   367           Total   Rejects   208       11.8       2.8        4756                Cleaner to Press DRE:   30.0% DRE            Dil. Water   Out   11856   0.015   10.7    0 70   0.1                Press   Out   180.0   35.16    327.6    1 74   5.7    334   1497                       379.5           double-dirt                Press to DIP DRE:   93.3% DRE            DIP   22               double-dirt            PROCESS   Washer - DIP   98.5% DRE