WO1996009433A1

WO1996009433A1 - Paper pulp washing method

Info

Publication number: WO1996009433A1
Application number: PCT/US1995/011635
Authority: WO
Inventors: Terry L. Bliss
Original assignee: Black Clawson Co
Current assignee: Black Clawson Co
Priority date: 1994-09-20
Filing date: 1995-09-14
Publication date: 1996-03-28
Anticipated expiration: 1997-03-20

Abstract

The method and apparatus for washing papermaker's stock comprised of waste paper products having suspended, undesirable solids and fines includes the pre-thickening the stock on a mat-forming, low turbulence thickener (20) in which very little of the fines is lost during thickening, re-dilution the pulp from the pre-thickener and applying the re-diluted stock to the inlet of the headbox of a high speed, high turbulence belt-type washer (10) to wash out the undesirable solids while retaining a substantial portion of the fines content in the washed stock. The belt-type washer has a belt speed up to 6,000 feet per minute or more. The clear water effluent from the pre-thickener is used for re-dilution, and a portion of the white water effluent from the belt-type washer may be recirculated directly to the washer inlet or a clarified portion thereof may be recirculated, or both.

Description

PAPER PULP WASHING METHOD

This application pertains to a system and method for washing paper making stock containing, or consisting entirely of, recycled paper products employing a combination of mat-forming thickeners or filters to increase stock consistency, with low loss of fines in combination with a high-turbulence, belt-type pulp washer.

Certain belt-type washers have been found useful and effective for washing and de-inking papermaking stocks which contain waste paper products. For the purpose of this disclosure, these washers may be referred to as high turbulence washers.

One example of a high turbulence washer is the single pass machine sold under the trade name "Vario Split", described in Biondetti, U. S. Patent No. 4,501,040 issued February 26, 1985, and in an article by Lang, "Improved Quality in Waste Type in Paper Stock Reclaimed via Twin-Wire Concept", Paper Trade Journal October 30, 1982. According to Lang, the best ash removal efficiency is always obtained at low basis weights, low stock inlet consistencies, and high speeds. Inlet consistencies of 1.5%, preferably less, are described. A headbox delivers the paper stock to be washed or thickened between a porous belt and the closed outer surface of a roll. The term "belt" or "porous belt" is used herein as a generic term to cover various kinds of porous moving supports used or which may be used for high turbulence thickeners, including woven and non-woven porous fabrics, plastic and metal belts, and the like. The belt is maintained under tension and the stock is squeezed between the belt and the roll surface. At the off-running side of the belt, the stock follows the smoother roll surface and is doctored off for removal. The excess water, ash, ink and other fine particles are pressed out of the space between the belt and the roll and form the washer effluent.

Another high turbulence washer is the "DNT" washer of The Black Clawson Company, described in U. S. Patent 4,722,793, which patent is incorporated herein by reference. The DNT washer employs a two-step washing and thickening process, in which the pulp is passed between the belt and a first roll, usually called a breast roll, where it is partially dewatered, and is then carried by the belt to a second roll, usually called a couch roll. The stock is further dewatered as it passes between the belt and the couch roll. The dewatered or thickened stock, sometimes referred to as "accepts", is then doctored off the couch roll at the off-running side and removed for further processing, and the "white water" effluent is gathered from a catch basin and drained from the machine.

High turbulence washers remove ink, minerals, and short fibers from the waste paper pulp more efficiently than do slow speed mat-forming washers such as disc filters and deckers, or mildly turbulent washers such as thickeners or screw presses. However, the same characteristics which make the high turbulence washers quite efficient, also results in a washing out and potential loss of short paper fibers, called "fines".

"Fines" can be defined as fibers having a length of less than about 200 microns.

The term "suspended solids" includes not only clays, ash and ink, but also includes papermaking fibers, and fiber fines, as defined above. A large portion of the suspended solids, which include its ash and ink components as well as other items of trash, commonly have a size in the 3-4 micron range up to about 30-40 microns. While "fines" may be undesirable for certain grades of paper, an excess loss of fines in certain cases is considered to be an economic loss, where the fines contribute to bonding and opacity, such as in certain grades of fine papers and in newsprint. For example, while a de-inked pulp which has the minerals and most of the fines washed out may be desirable for tissue, the wash out of excessive fines is an economical loss to a paper mill in which fines are not only tolerated, but are desired, to enhance the quality of the finished product.

In addition to the retention of fines, many mills would like to be able to adjust the washing process to allow for a range of washing efficiencies, to obtain a higher yield when less washing is required, or accept a lower yield when more washing is required.

As a practical matter, the only methods which have been available for adjusting such washing efficiencies, and the resulting yields, have included selection of the type of primary washing equipment, the adjustment of the clarifier effectiveness, and the adjustment of the amount of wash water which is allowed to bypass the clarifier and be recirculated.

Recirculation of fines around the clarifier back to the inlet headbox of the washer, for the purpose of controlling the rate of loss of fines, can be self- defeating. The recirculation loop can quickly become loaded with fines which are then repeatedly washed out, thereby lowering the capacity of the system. Also, operating the clarifier at reduced levels of effectiveness, such as with reduced levels of flocculating additives and chemicals, may selectively increase the amount of fines being removed at the expense of removing ink and other contaminants, with the obvious result of a preferential removal of the very component which is most desired to be recirculated.

A need therefore exists for a system and method by which the washing of papermaking stock, containing fines, ink, and ash may be controlled, as desired, to remove the maximum quantity of ink while retaining selected fractions of fines and ash, and to do so at a capacity which exceeds that now handled by a high turbulence washer apparatus operating under conventional consistencies and speeds.

The method and system of this invention is, in part, based upon the synergistic employment, in tandem or sequence, of a low turbulence mat-forming thickener for supplying a pre-thickened stock to a high turbulence washer, as previously described. It is important to the practice of this invention that the stock pre-thickener causes only a moderate, or preferably low, loss of suspended solids, so that at least most of the desirable fines are retained in the pre-thickened stock output.

Equipment which meets the requirements of pre- thickening with low loss of suspended solids and fines within a given range of consistencies include vacuum disc filters and drum-type thickeners. Appropriate drum thickeners include deckers, and vacuum and pressure drum washers.

When low turbulence mat-forming equipment is used to pre-thicken the paper making stock in question, the consistency may be increased from the conventional stock supply input value of about 0.5% to 1.5% to an output value in the range of 4% to 15%, and this can be accomplished with little loss of suspended solids. The particular outlet consistency of the pre-thickener will depend on the actual inlet consistency, the nature of the thickening apparatus and its speed of operation. Those solids which are lost are generally in the 2-30 micron range as distinguished from the "fines" range or size, as previously defined, due in part to the self-filtering formation of a pulp mat which forms on the filtering or separating surfaces.

Effluent taken from this first thickening step may be advantageously used to re-dilute the thickened pulp from its high outlet consistency down to an intermediate but still high consistency in excess of 1.5% and as high as 3% to 6%, for application to the inlet headbox of a high turbulence washing device. Also, for best results, the high turbulence washer should have a belt speed in excess of 3,000 ft./min. and as high as 6,000 ft./min. or more. The washing of pre-thickened stock in such a high turbulence device, at high belt speed, has been shown to provide a substantial increase in yield while maintaining about the same final pulp brightness as obtained in conventional practice. The increase in yield primarily consists of retained fines which would otherwise be lost when operating at conventional inlet consistencies of 1.5% or less. The amount of fines in the retained solids may advantageously be controlled by controlling the amount of washer effluent which is permitted to bypass the clarifier and return to the process. In another aspect of the invention, the operator is given the choice of adjusting the overall yield and washing effectiveness over a wide range, simply by adjusting the set point of a consistency controller or valve positioned in an effluent inflow line between the low turbulence pre-thickener and the high turbulence washer. Such an adjustment of consistency may, for example, be made in response to a change in the incoming raw material composition, to provide a different grade of final pulp, or to minimize the cost of obtaining the required final brightness by balancing washing effectiveness and yield against subsequent bleaching costs.

A further advantage of the system and method of this invention resides in the fact that in addition to yield improvements, increasing the feed consistency to a high turbulence washer has the effect of increasing the through-put capacity of the system. Likewise, the capacity of the high turbulence washer is significantly increased by increasing its feed consistency and by increasing its belt speed. A much smaller clarifier is then needed to handle the overflow of white water effluent from the high turbulence washer, and less sludge thickening capacity is required. These provide savings in the process equipment costs which offset at least in part the added costs of the pre-thickener.

For the same net output from the systems, the raw material costs may be lowered by using the process of this invention, and the overall operating costs of the method may be reduced, when it is considered that operation of the system results in lower sludge- thickening and disposal costs. Overall, the operating costs for the systems according to the method of this invention are lower for the cost of sludge thickening and disposal than are conventional washing and thickening systems.

In a broader aspect of the invention, a pre- thickener which, in its operation, is characterized by a very low loss of fines, is used for the purpose of supplying a pre-thickened papermaking stock to a second washer, characterized as a high turbulence washer, which, when operated using stock of conventional consistencies, would be accompanied by a loss of fines in the white water and therefore a reduction in overall yield. The loss of fines is offset by operating such a high turbulence washer at uncommonly high inlet consistencies, usually accompanied by operating the washer at uncommonly higher speeds, with the end result that the washed and thickened stock retains much of its fines content at a brightness which compares favorably with that which is obtained in conventional practice.

The method practiced by the invention for washing ash and other small suspended solids from paper maker stock, in which the stock has a large proportion of waste paper products including a fines content, is characterized by the steps of applying the stock to be washed to a low turbulence thickener for extracting a portion of the water content to form a pre-thickened pulp having a consistency in the range of about 4% to 20%, diluting the pre-thickened pulp with water to a consistency which exceeds 1.5%, applying the re-diluted pulp to an inlet of a high turbulence-type pulp washer which uses a closed loop belt and which has a belt speed greater than 1200 ft./min., and then removing the thickened pulp from the high turbulence-type pulp washer with a major portion of the fines retained in the thickened pulp.

In a further aspect of the invention, an apparatus or system for washing ash, ink, and other suspended solids from paper making stock which is, in part, made up of waste paper products and which includes ink, ash, and fines while still retaining a major proportion of the fines content in the washing process, the system including both a low turbulence paper stock thickener and a high turbulence belt-type washer, characterized by the thickening or pre-thickening of the stock in the stock thickener to a range of about 40% to 20%, the system and apparatus include means for applying a portion of the effluent from the thickener for re- diluting the pulp to a lower consistency which exceeds

1.5% and preferably is in a range of about 3% to 6% which is then applied to the high turbulence belt-type washer.

Fig. 1 is a process diagram showing typical conventional practice in the employment of a high turbulence washer;

Fig. 2 is a process diagram of a two-stage thickener/washer in accordance with this invention; and

Fig. 3 is a plot of stock brightness versus yield for three different feed consistencies and at five different speeds of a high turbulence stock washer according to the data of Tables 1, 2 and 3 herein.

Referring, for background, to the process diagram of Fig. 1, a high speed, high turbulence type of washer 10, previously described, is fed a slurry of paper pulp and water. The white water effluent from the washer is directed to a clarifier 12. A portion may be shunted around the clarifier by a bypass line 13. The overflow from the clarifier 12 and the water from the bypass line 13 are recirculated into the process, while the underflow is directed to a sludge press 15 for disposal, typically in a landfill. The pulp accepts output 16 from the high speed, high turbulence washer is washed pulp at a consistency of about 8% to 14% and is ready for further processing or use.

It is helpful here to consider a typical operating example in reference to the system which is illustrated in Fig. 1. In this example, a washer output 16 is assumed to be 400 dry tons a day at 12.9% solids. When de-inking a difficult waste product such as offset printed newspaper (ONP) 70%, magazines (OMG) 30%, typically having a high ash content of about 10% as well as a high fines content, this typical washing setup could result in as much as a 33% loss of suspended solids in the form of ink, ash and fines, in order to provide an acceptable brightness at the accepts.

To make up for the loss, the washer inlet feed to the headbox on line 17 would have to be 597 dry tons a day at 1.0% consistency. In this example, the washer effluent on lines 18 to the clarifier 12, is 197 dry tons a day at 0.35% solids.

Carrying the process further, without clarifier bypass 13, 177 dry tons per day at 2.0% solids would typically pass to the sludge press 15. This represents the loss of fines plus ash and other contaminants. 20 dry tons a day would be recirculated into the process at 0.04% consistency from the clarifier in its effluent. A 33% solids loss is considered to be unacceptable by many mills. To compensate for the loss, a clarifier bypass circuit 13 is opened by which a major portion of the material which otherwise would be sent to the clarifier is bypassed back to the washer feed line 17. This bypass can reduce the solids loss to a more acceptable level, but does so at the expense of de- optimizing the system, including a possible loss of brightness.

Also, the bypass 13 encourages a massive build¬ up of inks, fines, and ash within the recirculation loop and tends to apply an unacceptably high level of washed solids for recirculation. It has the effect of unbalancing consistencies, and in fact increases consistencies both to the clarifier and to the washer feed, so that these consistencies tend to move out of the optimum working range. In addition, the mere quantity of solids which become entrained within the recirculation loop is a burden on the pumping and piping system and when it becomes a common practice, it results in a requirement for larger capacity pumps and larger piping. However, clarifier bypass is a common technique in an attempt to hold down and control the total loss of solids when washing used paper pulp in a high turbulence belt- type washer/thickener 10.

The loss of solids and particularly the loss of fines can be reduced, according to this invention, and controlled, by first subjecting the stock to pre- thickening in a non-turbulent, mat-forming washer or thickener, usually followed by intermediate dilution and then followed by application, in a pre-thickened state, to a high turbulence type of washer of the kind previously described. The inventor has discovered that a belt-type, high speed, high turbulence washer/thickener, when operated at high speed and at high inlet consistencies, has the ability of providing the required increase in brightness accompanied by a substantial retention of the fines which would otherwise be lost when operating the same machine at lower inlet consistencies and at lower speeds. This fact is demonstrated by the chart of Fig. 3 on which is plotted thirty test runs of such a high speed, high turbulence belt-type washer/thickener of the "DNT" type constructed generally in accordance with the disclosure of U. S. patent No. 4,722,793, previously mentioned.

Table No. 1 herein shows the test conditions in which a prepared paper stock consisting of 70% old newsprint (ONP) and 30% old magazines (OMG) was applied at just over 3% consistency to the headbox of such a washer operating at speeds ranging from 2,000 feet per minute to 6,000 feet per minute. The brightness was measured according to the G.E. method, also known as the TAPPI Official Method T452om-92, and the beginning brightness was 47%.

Table No. 2 represents a series of runs in which the same stock was applied at a 2% feed consistency, all other conditions remaining the same. However, it may be noted that the G.E. brightness of the more diluted stock prior to washing, was slightly reduced to 46.1%.

Table No. 3 represents a third series of tests in which the same stock material, now diluted to just under 1% consistency, was applied to the same machine under the same conditions of belt speed ranging from 2,000 to 6,000 feet per minute, although at this further reduced consistency, the G.E. brightness of the original stock had reduced slightly to 45.7%.

Table 1

Belt Consist¬ Flow Rates Ash Solids Brightness

Speed ency t Gal./Min. Removal Loss TAPPI

Run fpm Feed Feed T.S. W.W. Percent Percent Percent

1 2.000 3.20 770 331 439 66.2 11.8 49.4

2 2.000 3.20 580 213 367 55.8 12.8 49.6

3 3.000 3.20 985 339 646 62.1 15.0 50.5

4 3.000 3.20 690 189 501 60.4 17.2 51.7

5 4.000 3.20 1.100 325 775 63.5 17.4 51.4

6 4.000 3.20 775 189 586 72.4 19.6 52.8

7 5.000 3.20 1.290 292 998 71.8 19.8 52.3

8 5.000 3.20 925 193 732 78.4 22.8 55.0

9 6.000 3.20 1.350 275 1.075 78.7 21.2 55.4

10 6.000 3.20 950 187 763 84.0 23.6 55.8

Table 1 Data: Stock - 70* ONP. 30X OMG C.S. Freeness 174 Ash 10*

G.E. Brightness 47.OX T.S. - Thickened Stock Output W.W. - Effluent Table No 2

Belt Consist¬ Flow Rates Ash Solids Bπgl

Speed ency * Gal /Mm Removal Loss Perci

Run fpm Feed Feed T S w w Percent Percent

11 2.000 201 830 199 631 646 193 504

12 2.000 201 580 103 477 691 201 509

13 3000 2.01 1.045 183 862 730 226 524

14 3.000 201 730 117 613 724 255 53.7

15 4.000 201 1.320 221 1.099 73.3 240 539

16 4.000 2.01 920 131 789 801 277 54.2

17 5.000 2.01 1.385 185 1.200 810 284 550

18 5.000 2.01 970 127 843 83.0 285 555

19 6.000 2.01 1.500 193 1.307 820 286 554

20 6.000 2.01 1.080 129 951 797 289 55.3

Table No 2 Data Stock - 70* ONP. 30* OMG C S Freeness 149 Ash 1000X

G E Brightness 46.1* T S - Thickened Stock Output W W - Effluent

Table No 3

Belt Consist¬ Flow Rates Ash Solids Brightness

Speed ency * Gal /Mm Removal Loss Percent

Run fpm Feed Feed T s W W Percent Percent

21 2000 095 1.140 85 1.055 817 302 536

22 2.000 095 800 64 736 872 320 555

23 3.000 095 1.660 188 1.472 839 27 1 53.9

24 3000 095 1.160 85 1.075 889 342 567

25 4.000 0.95 1.815 126 1.689 896 31.3 590

26 4.000 095 1.270 78 1.192 921 385 578

27 5000 095 1.815 94 1.721 936 389 582

28 5.000 095 1.270 69 1.201 938 388 589

29 6000 095 1.540 76 1.464 937 380 583

30 6.000 095 1.110 55 1.055 944 390 588

Table No 3 Data

Stock - 70* ONP 30* OMG

C S Freeness 128

Ash 106*

G E Brightness 457*

T S - Thickened Stock Output

W W - Effluent

Tables 1, 2 and 3 further respectively show the percentage of solids loss. Solids loss is the complement of yield, and is translated to percent yield in Fig. 3. Therefore, on the chart of Fig. 3, there has been plotted markers identifying each of the three sets of runs corresponding to Tables 1, 2 and 3. The runs of Table No. 1 are represented in Fig. 3 by square markers. The runs of Table No. 2 are represented by triangular markers, while the runs described in Table No. 3 are represented by dots. These three sets of runs are plotted on the chart of Fig. 3 in which the percent of brightness is represented by the ordinate and the percentage of yield (the complement of solids loss) is represented by the abscissa.

It may be noted that the plots of Table 3 representing a conventional 1.0% consistency feed, provided the highest gain of brightness, but were accompanied by the lowest yield, whereas all of the points represented by the data of Table No. 1 at the unusually high 3% feed consistency showed much higher yields by as much as 15% to 25%, and in most cases, had only a tolerable loss of brightness averaging about 5-8 points less than the maximum brightness achieved with the data of Table No. 3. Table No. 2 data at 2% feed consistency shows points which are scattered between those of Table No. 1 and Table No. 3, but with higher brightness versus yield results when compared to the Table No. 1 data at 3% consistency.

The results of this investigation demonstrate that acceptably good brightness increases with very substantial gains in yield may be achieved by operating a high turbulence type of washer/thickener at very high feed consistencies, preferably combined with high belt speeds. The use of higher speeds substantially offsets the loss of brightness points. Implementing this finding, the process and apparatus of the invention, as illustrated in the systems flow diagram of Fig. 2, employs a pre-thickener apparatus in the form of a non- turbulent, ma -forming thickener, to take a stock at a 1% consistency or less, such as at 0.5%, and dewater the same to a higher outlet consistency, on line 22.

Depending on the initial stock consistency, the type and speed of the thickener, the outlet therefrom should fall within the range of about 4% to 20%, with preferred ranges between about 8% and 12%. Suitable non-turbulent mat-forming thickeners for this purpose include gravity deckers, vacuum drums, and disc-type filters. Such suitable disc filters are sold under the trade name "Center Disc" of Cellico, Inc. of Lawrenceville, Georgia, or "Polydisc" of Beloit Corporation, Pittsfield, Massachusetts. Further mat- forming thickeners which have low loss of fines are sold under the names "Chemi-Washer" and "Flex-D" by The Black Clawson Company of Middletown, Ohio. Deckers, drum filters and disc filters are all characterized by the forming of a mat of pulp on a belt surface, providing some washing action which removes a certain portion of the ink and ash, but which retain a large percentage of the fines in the fiber mat, for delivery to the washer 10.

The pre-thickener effluent, on line 22, is a relatively clear water and is, in fact, much clearer than the white water effluent from the washer 10 and/or the overflow clarified effluent from the clarifier 12. It is at least partially reused in the process for re-dilution, and is adjustably or variably applied through a controllable valve 23 to the output line 21 for partial re-dilution of the pre-thickened stock to an input range, on the washer input line 17 to what is normally- considered to be an extremely high consistency input for such a washer, in the range of about 3% to 6%.

Fig. 2 shows the same clarifier 12 and sludge press 15 which had been illustrated in the system diagram of Fig. 1, except since the percentage of yield is improved, smaller and less expensive components may be used at these positions than have heretofore been possible. The clarified or partially clarified overflow effluent from the clarifier on line 16 is also reused in this process, and the underflow clarifier sludge is delivered to the press 15.

The loadings on the recirculation paths between the clarifier and the inlet 17 of the washer 10 of fines and recirculated solids, are substantially lessened due again to the increased yield translating into a decreased percentage of lost solids. The efficacy of the use of a mat-forming thickener for the purpose of pre-thickening the stock input to the preferred high speed, high turbulence, belt- type washer 10 may be demonstrated by considering the example previously given with respect to Fig. l. When using the apparatus of Fig. 3, 400 dry tons a day at 12% solids output, on output line 16, may be accomplished by using only 524 dry tons per day, 1.0% consistency, on the feed line as compared to the 597 tons per day required in the example for Fig. 1. This assumes, for the purpose of illustration, that the loss of solids is not 33%, but 23.7%. This closely parallels the results shown for test run No. 10 (Table 1) in which there was an acceptable 23.6% loss of solids, accompanied by almost nine points of brightness increase.

Under these conditions, 119 dry tons per day of solids are applied to the clarifier 12. Assuming the bypass line 13 to be closed, 113 dry tons per day are sent by the clarifier to the sludge press 15 and represents the loss of solids to the system.

Rather than 20 tons a day being recirculated by the clarifier and/or through the bypass 13, only six tons per day of stock at a consistency of 0.08% are recirculated as clarified white water from the clarifier for reuse in the process.

In addition to allowing an operation at or near the same final brightness, but at substantially higher yield, the invention allows a system operator the choice of adjusting system yield and washing effectiveness over a wide range. This may be accomplished by adjusting the set point of a consistency controller 23 which controls a consistency transmitter 24 in the line between the pre- thickener 20 and the washer 10. The controller 23 and transmitter 24 in turn regulate the valve position of a pre-thickener effluent dilution valve 25 or a high turbulence dilution valve 26, or both, to control or vary the amount of pre-thickener and/or high turbulence washer effluent applied to line 22 between the pre-thickener 20 and the washer 10. The construction of consistency controllers and transmitters, as defined above, are well known in the stock preparation industry. Consistency adjustments could be made in response to a change in incoming raw material composition, such as for the purpose of producing a different final grade of pulp, or for the purpose of minimizing the costs of obtaining a final required brightness and yield loss against known bleaching costs. For these reasons, the apparatus and method of this invention provide an operator with a wider range of capabilities.

The invention has further versatility in having provision by which the feed to the high turbulence waster 10 may be preferentially diluted either with partially clarified cloudy white water directly from the washer itself. This selection may be provided through bypass line 13 or line 13a by which the unclarified effluent may be selectively redirected to the inlet of the washer 10. The latter arrangement may be preferred, in some instances, in order to keep the fines content and the flow volume to the clarifier low. Also, such an arrangement has the tendency of retaining the fines content within the high turbulence washer loop and keeping this loop both small and tight.

The invention, in its broader sense, may be considered as a method of thickening and washing by using a thickener which is characterized by retention of fines and solids and then washing the pre-thickened pulp in a washer which has a high capacity for removing the solids, and operating such washer in such a manner that a substantial portion of the fines are retained.

For most efficient operation, that is, the highest gain and brightness accompanied by the lowest loss of solids, the high turbulence, high speed belt-type thickener, such as the DNT washer previously described, should be operated at or near its high speed limit. For the apparatus upon which the test results of Table No. l were obtained, this limit was somewhat in excess of 6,000 feet per minute. However, the invention should not be construed as being limited to this speed, as the concepts herein described and the advantages which have been obtained, could be further improved by operating the thickener 10 at speeds in excess of 6,000 feet per minute. Lower speeds, of course, may be used where less brightness gain may be tolerated, with an accompanying higher yield.

Claims

- - CLAIMS - -

1. The method of washing ash and other small suspended solids from a suspension of papermaker's stock and waste in which the stock contains a substantial proportion of waste paper products having a fines content, comprising the steps of: applying the stock to a low turbulence thickener and extracting therefrom a portion of the water content to form a pre-thickened pulp having a consistency in the range of about 4% to 20%, diluting the pre-thickened pulp to a consistency in excess of 1.5%, applying said re-diluted pulp to the inlet of a high turbulence belt type pulp washer having a belt speed greater than 1200 ft./min., and removing the thickened pulp from the high turbulence pulp washer with a substantial major portion of the fines retained in the thickened pulp.

2. The method of claim 1 in which said pre- thickened pulp is diluted to a consistency in the range of about 3% to about 6%.

3. The method of claim 1 in which said belt speed is at least about 3000 ft./min.

4. The method of claim 1 further comprising the steps of applying effluent from the high speed, high turbulence belt-type washer to a clarifier, and directing at least a portion of the overflow from said clarifier at a position prior to the inlet of said mat-forming thickener.

5. The method of claim 4 further comprising the step of regulating the consistency of said re-diluted stock, applying effluent from said low turbulence thickener to said input to said headbox, and regulating the rate of flow of said effluent from the mat-forming, low turbulence thickener.

6. The method of claim 1 in which said high turbulence belt-type washer is operated at about 6,000 feet per minute.

7. A system for washing ash, ink, and other small suspended solids from papermaker's stock in which at least a portion of the stock is made up of waste paper products which include ink, ash, and fines, while retaining a large proportion of the fines content of the stock, comprising: a low turbulence paper stock thickener, means applying said stock to said thickener forming a thickened stock having a consistency in the range of about 4% to 20% and forming an effluent, means applying at least a portion of said effluent to said thickened stock to dilute said thickened stock to a range of about 3% to 6% consistency, a high turbulence bel -type washer having a stock inlet, and means applying said diluted pre-thickened stock to said stock inlet for washing therein, whereby a large percentage of the fines are retained within the washed stock.

8. Apparatus for washing ash, ink and other small suspended solids from papermaker's stock formed at least in part of waste paper pulp having a fines content while retaining a large proportion of said fines content, comprising: stock thickener apparatus in the form of mat- forming thickener, means applying said stock to said stock thickener and for extracting therefrom a portion of the water content to form a pulp mat at a consistency of about between 4% and 20%, means applying a portion of said water content to said pulp mat for re-diluting the pulp of said mat to a consistency greater than 1.5% and up to about 6%, stock washer apparatus in the form of a high turbulence, high speed belt-type washer having a stock inlet, a thickened stock outlet and an effluent outlet, means applying said re-diluted pulp from said stock thickener to said inlet of said stock washer, a stock clarifier connected to receive washer effluent from said stock washer and providing an output of clarified process water, means connecting said clarified process water to provide dilution water to said thickener apparatus, and means for removing washed and thickened pulp from said washer apparatus for further processing.

9. The apparatus of claim 8 further comprising a valve for controlling the amount of clarified process water added as dilution to control the consistency of said re-diluted pulp.

10. The apparatus of claim 9 further comprising means for bypassing at least a portion of said washer effluent to said washer stock inlet.