Abstract:
An apparatus for improving the pulping characteristics of wood chips in which a pair of closely operating rolls are provided for supplying compressive force to chips passed therebetween, at least one roll having an aggressively contoured surface for causing chips to crack in the thickness dimension of the chip as compressive force is applied to the chip.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for treating wood chips, to enhance liquor penetration and subsequent pulping operations, and relates more particularly to destructuring apparatus in which chips are passed between closely operating rolls with compressive forces being exerted on the chips by the rolls. 
     BACKGROUND OF THE INVENTION 
     In a typical paper making process, logs are debarked and chipped, and individual cellulose fibers are then freed or liberated from the chip for subsequent treatment and ultimate paper web formation. A common way to liberate the cellulose fibers is by cooking the wood chips with chemicals at elevated temperatures and pressures in digesters to remove lignen from the chips, which holds the fibers together. For the subsequent paper making process, it is desirable that the delignified fibers obtained exhibit substantially similar characteristics. To minimize the production of undercooked or overcooked chips in the digester, it is necessary that the cooking liquor penetration into the chips is substantially similar for all chips, so that the effects of temperature, pressure, and time are similar for all chips. Therefore, it has been found desirable in the past to utilize chip screening apparatus which removes both undersized and oversized chips, so that the undersized can be treated separately and the oversized passed through chip size reducing apparatus prior to digesting. 
     A commonly used apparatus for reducing the size of oversized chips separated from a chip stream by screens is a chip slicer. The basic operation of a chip slicer includes a rotor operating within a drum, wherein the oversized chips are forced against knives and are thereby sliced to acceptable thickness. An example of a chip slicer can be found in U.S. Pat. No. 4,235,382 issued to William C. Smith for a &#34;Method and Apparatus for Rechipping Wood Chips&#34;. While chip slicers such as that taught in U.S. Pat. No. 4,235,382 work effectively to reduce the size of oversized chips, thereby substantially reducing the occurrence of undercooked chips in a digesting process, chip slicers which are not working within optimum design parameters, such as when knives are dull, or improper speed or loading occurs, tend to generate fines while reducing oversized chips. Thus, while minimizing the problem associated with oversized chips, chip slicers tend to increase the problem of undersized chips or fines. Therefore, it is desirable to develop an apparatus for treating oversized wood chips which does not compound the problems associated with fines or undersized chips. 
     Closely operating rolls have been utilized in the past for treating oversized chips by compression, and thereby affecting liquor penetration into the chips. For example, U.S. Pat. No. 4,050,980 issued Sept. 27, 1977 to Fred L. Schmidt and Frank J. Steffes for &#34;Selective Delamination of Wood Chips&#34;. This patent teaches screening a chip stream and passing the oversized chips through closely operating rolls for selective delamination by compression. 
     U.S Pat. 3,393,634 issued July 23, 1968 to John M. Blackford for a &#34;Method and Apparatus for Loosening Fibers of Wood Chips&#34;. This patent teaches closely operating rolls with an apparatus for directing chips edgewise into the crotch between the rolls, with the rolls compressing the chips transversely of their thickness to at least about one-fifth of their original thickness, but not more than about one-tenth of their original thickness. Thereafter, the chips are allowed to expand to their original shape, with the fibers therein having been loosened and the porosity of the chips having been increased. 
     In each of the two above-mentioned patents, the opposed, closely operating rolls, or delamination rolls compress the chips for loosening the fibers therein. The rolls are smooth, so that the only action on the chips is compressive, whereby the chip structure is not substantially changed other than for a loosening of the fibers. 
     A problem associated with the use of delamination rolls is that throughput is low. Chips tend to stay in the pocket above the rolls, and, particularly the larger chips which are most in need of delamination, tend to ride between the rolls in the upper portion of the roll couple, without being drawn through the rolls. 
     A typical structure for a chip destructuring apparatus is disclosed in an article entitled &#34;A Machine For Destructuring Wood Chips by Rolling&#34; by John A. Oldham in the July 1983 issue of APPITA, Volume 37, Number 1. In the last paragraph of the first column on Page 65, the destructuring machine is described as having &#34;smooth, chrome surfaced, very rigid rollers&#34;. The aforedescribed problem of passing larger chips through the nip is discussed in the first paragraph on Page 66. The larger chips &#34;often would not enter between the smooth rollers; the surface of the rollers slipped over the chips&#34;. It is then described that the chips remaining above the rolls obstructed feeding of succeeding chips causing clotting or bridging. In the third paragraph on Page 66, a solution is discussed wherein small grooves, only one millimeter deep were cut parallel to the roll axis at approximately 10 millimeter spacings. Harsher roll surfaces are not deemed appropriate, since an unacceptable amount of fiber damage would be created. General roughening of the roll surface is also described as being likely to improve feed reliability. 
     An analysis of the effects of chip destructuring or delamination was presented at the 1984 TAPPI Pulping Conference by D. Lachenal, P. Monzie, and C. deChoudens in a paper entitled &#34;Chip Destructuring Improves Kraft Pulping&#34;, TAPPI November 1984, Book 1, Pages 13-17. In the apparatus used for the pulping trials discussed in the article, again the rollers were smooth, and the chips were compressed. 
     Destructuring or delamination as known previously has not been accepted as a standard process in pulping operations, largely, it is believed, due to the low capacities of delamination devices and inconsistent results and subsequent effects on digesting operations. 
     It is therefore one of the principal objects of the present invention to provide an apparatus for treating oversized chips in a manner to reduce the necessary cooking time therefore, to achieve in the treated oversize chips delignification levels similar to that for smaller chips during identical delignification processes, with resultant pulps having similar characteristics and properties. 
     It is another object of the present invention to provide an apparatus for treating oversized chips quickly and efficiently with rapid throughput, while minimizing plugging or blinding of the apparatus. 
     It is yet another object of the present invention to provide a wood chip treating apparatus which cracks or fractures oversized chips without generating additional fines or pin chips, and which is simple in operation, requiring minimal adjustment for optimal operation. 
     A still further object of the present invention is to provide an apparatus for treating wood chips to increase the rate of liquor impregnation particularly of large chips and for providing an apparatus to destructure wood chips which is not dependent on a particular chip orientation between the closely operating rolls. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved in the present invention by providing closely operating, oppositely rotating rolls having highly aggressive surfaces. In a preferred design, the rolls have matrices of pyramid shaped projections machined into their surfaces. In a preferred embodiment, the peaks of the pyramids are spaced one-half inch apart, and the depth of the machining from the peak to the base of an individual pyramid is approximately one-quarter inch. In operation, the peaks of the rolls may be placed in peak-to-peak orientation or in peak-to-valley orientation. In use, the chips are fractured along the direction of fiber orientation, and with the present apparatus, the chips will crack there along regardless of how the chip enters the nip between the rolls. 
     The present invention differs from conventional thinking for destructuring or delamination devices, in that a highly aggressive surface is used, not merely to compress the chips, but to actually break or fracture the chip, generally through the thickness dimension of the chip previously such chip cracking has been believed undesirable. 
     Additional objects and advantages of the present invention will become apparent from the following detailed description and the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional, end view of a wood chip cracking apparatus embodying the present invention. 
     FIG. 2 is a vertical cross-sectional view of the wood chip cracking apparatus shown in FIG. 1, taken generally along line II--II of FIG. 1. 
     FIG. 3 is a perspective view of a portion of the roll surface for one of the rolls of a wood chip cracking apparatus embodying the present invention. 
     FIG. 4 is a fragmentary end view of one of the roll couples in a wood chip cracking apparatus embodying the present invention, showing one manner of adjacent roll orientation. 
     FIG. 5 is a fragmentary end view similar to that of FIG. 4, but showing another manner of roll orientation. 
     FIG. 6 is yet another fragmentary end view similar to that of FIGS. 4 and 5, but showing yet another manner of roll orientation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now more specifically to the drawing, and to FIG. 1 in particular, numeral 10 designates a wood chip cracking apparatus embodying the present invention. The apparatus 10 receives wood chips from a distributing device 12 which supplies an even flow of wood chips generally indicated by numeral 14 to top and bottom roll couples 16 and 18. The roll couples 16 and 18 are disposed in a housing 20 having a top opening 22 through which the wood chips 14 enter, and a bottom opening 24 through which the treated wood chips flow from the apparatus. The incoming flow of chips 14 is directed by baffles 26 and 28 to the upper roll couple 16, and the chips passing through the upper roll couple are directed by baffles 30 and 32 to the bottom roll couple 18. A suitable conveying apparatus, not shown, carries the treated chips from the apparatus 10 to subsequent process steps. 
     Top roll couple 16 includes rolls 40 and 42 closely spaced and oppositely driven, so that in the upper pocket between the rolls, the surfaces are running toward a narrow region formed by the closely spaced rolls 40 and 42, as indicated by the arrows 44 and 46. 
     The bottom roll couple 18 includes rolls 50 and 52 closely spaced and oppositely driven, so that in the upper pocket between the rolls, the surfaces are running toward a narrow region formed by the closely spaced rolls 50 and 52, as indicated by the arrows 54 and 56. 
     Each of the rolls 40, 42, 50, and 52 is suitably journalled in bearings generally indicated at numeral 60 in housing 20, and a drive mechanism 62 is provided for turning the rolls. The drive mechanism 62 may include a motor 64, or other source of power, and a drive train 66. The drive train 66 may drive each of the rolls; however, it has been found that in some applications of the present invention, it is necessary to drive only one roll of each roll couple. The mating roll in each roll couple opposite the driven roll can merely idle, and, in this manner, the energy requirements for operating the machine are reduced, in that when chips are not flowing to the apparatus, only one roll of each couple is being driven. As chips enter the apparatus and wedge between the driven and non-driven rolls, the non-driven roll will rotate, aiding in the cracking operation and in the passing through of wood chips. 
     The distributing device 12 includes a housing 70 having an opening 72 for receiving chips from a chip supply apparatus not shown, a distributing screw 74 for evening the flow of chips along the distributing device, and a distributing grid 76 through which chips pass from the distributing device 12 to the first roll couple 16. The distributing screw 74 is driven at 78 by a suitable source of power and is journalled in bearings 80 in the housing 70. 
     It should be understood by those skilled in the art that the arrangement shown in FIGS. 1 and 2 for the wood chip cracking apparatus of the present invention is merely one example of a suitable arrangement. In some installations, it may be desirable to use only one roll couple or to use more than two roll couples, and the apparatus for supplying chips to the roll couple or couples may be of types other than the distributing device 12 described above. 
     The surfaces of the rolls used in the present invention differ from that of rolls used for delaminating chips previously, in that the roll surfaces of the present invention are aggressively contoured. In the embodiment shown in FIG. 3, the roll surface comprises a matrix of pyramid shaped projections 100 which are formed by machining into the roll surface circumferential v-shaped valleys 102 and axial v-shaped valleys 104 in the roll at right angles. By machining such intersecting valleys, four-sided pyramids are formed extending radially outward on the roll surface. Each of the projections 100 has a peak 106 formed by the remaining material from the outer portions of the machined roll surface, and a base 108 defined by the depth of the intersecting valleys 102 and 104 in the machined material zone. Normally both rolls of the roll couples have similar surface configuration; however, it may be desirable to have one roll of each roll couple be smooth or otherwise have a more aggressively or less aggressively contoured surface than that of the other roll in the roll couple. 
     In one structure found to work advantageously, the roll surface was formed wherein the peaks 106 were spaced one-half inch apart, and each peak comprised a flattened surface approximately one-sixteenth inch square. The depth of each pyramid, from peak 106 to base 108 was six millimeters. 
     In the use and operation of an apparatus for destructuring wood chips as depicted in the aforedescribed drawings, chips are supplied to the distributing device 12, and from the distributing device 12 are supplied evenly along the axial extent of the first roll couple 16. The chips entering the distributing device 12 can be from a previous screening step, and comprise only the oversize chips separated at a previous screening step, or the entire chip flow to a pulping operation can be processed through the apparatus of the present invention. In yet other applications, it may be desirable to separate from the total chip stream only the under size chips, and then process both oversize and acceptable size chips through the present apparatus. 
     One significant advantage of the present invention is that the highly aggressive surface on the rolls significantly minimizes, virtually eliminating the heretofore recognized problem of chips not being pulled between the rolls, but instead, particularly with overlarge chips, riding above the rolls, with rolls sliding there along. Thus, a high volume of chips can be passed through the present apparatus, making it possible to process the entire chip flow in the pulp mill, potentially even eliminating the need for screening out oversized chips. If acceptable and oversized chips all can be passed through the apparatus, it is unnecessary to separate the overlarge for separate treatment. The small and acceptable chips, through proper roll spacing, will pass through the device substantially untreated, while only the oversize will be cracked. However, after treatment, the acceptable and treated oversize chips will respond similarly to pulping. 
     From the distributing device 12, the chips enter the region above the roll couple. The rolls may be separately driven, and positions controlled such that they are aligned in a peak to valley orientation such as shown in FIG. 4. Alternatively, in some processes and for some types of wood chips, it is desirable to control the roll&#39;s orientation in a peak-to-peak orientation as shown in FIG. 5. In yet other processes wherein a substantial compression in addition to cracking is desired, or wherein the acceptable chip thickness is quite thin, a closely intermeshed peak-to-valley relationship, as shown in FIG. 6, may be desirable. In yet other operations, particularly when the power input to the apparatus is to be minimized as much as possible, only one roll of each roll couple is driven, and the other merely idles. As chips approach the rolls and are pinched therebetween, the idle roll is driven by the driven roll through the driving connection formed by the wood chips compressed therebetween. 
     As chips are passed between the roll couples, regardless of the chip orientation, the chips tend to crack or split parallel to the fiber orientation in the chip. This is true whether the chip passes between the rolls lengthwise or endwise. 
     When the peak-to-valley orientation, as shown in FIGS. 4 or 6, is used, together with pyramid-shaped projections spaced one-half inch from each other, and being approximately six millimeters high, the cracks created in the chips occur approximately every one-fourth inch. This spacing of the cracks formed generally corresponds to the typically acceptable chip thickness in pulping operations. By cracking the chips, openings are created in the larger surfaces of the chips to aid liquor penetration. In addition to any fiber loosening which may result from compression, liquor penetration into the chip is aided by the actual physical openings created by the cracks. Displacement of the material near the crack is generally greater for thicker chips than for thinner chips, and thus, the opening for liquor penetration is less obstructed for thicker chips than thinner chips, thereby equalizing liquor penetration rates in the thicker and thinner chips. Because the rolls are spaced apart, the core of the chip is not displaced, and even with very thick chips, although surface displacement near the cracks may be significant and the general shape of the chip may be slightly changed, the integrity of the chip is not compromised, and the chip remains whole without the generation of pins, fines, or broken chips. 
     When a plurality of vertically arranged roll couples are used, as shown in FIGS. 1 and 2, it may be advantageous to provide progressively decreasing roll spacing on the lower roll couples. In this way, the largely oversized will be compressed and/or fractured by the upper rolls, with the acceptable and minimally oversized passing therethrough. Subsequent roll couples will further process the greatly oversized and process the minimally oversized. 
     Laboratory pulping studies have been conducted on chips processed through a single roll couple of the present invention wherein the projections of the adjacent rolls were intermeshed, as shown in FIG. 6. As a control, one sample was not treated, and other samples were sliced by conventional chip thickness slicing techniques. 
     Several different samples were treated in a wood chip cracking apparatus of the present invention. Several samples were treated in what is termed a &#34;mild treatment&#34; and others were treated in a &#34;harsh treatment&#34;. In the mild treatment, the spacing between the projections in the region where projections from each roll are at their closest was six millimeters. In the harsh treatment, the spacing at the closest point of spacing between projections on separate rolls was three millimeters. Table 1 hereinafter summarizes the characteristics of the various samples on which pulping studies were conducted. 
     
                       TABLE 1______________________________________(Sample Characteristics)Sample       Species      Treatment______________________________________1            Pine         Not Treated2            Pine         Mild3            Pine         Harsh4            Pine         Sliced5            Pine/Fir     Sliced6            Pine/Fir     Harsh______________________________________ 
    
     The samples were fractionated in a Rader Companies CC2000 Chip Classifier. Samples were divided into fines, which would pass through a 3 millimeter round hole; pins which were between 0 and 2 millimeters thick; accepts, which were between 2 and 8 millimeters thick; total over thick greater than 8 millimeter; and highly over thick greater than 14 millimeter. Table 2 summarizes the thickness characteristics of each sample. 
     
                       TABLE 2______________________________________(Thickness Classification in Percentage)Sample  14 mm     8 mm   2-8 mm  0-2 mm                                  Fines______________________________________1       46.2      82.4   17.5    0     02       16.0      50.0   33.0    0.7   0.33       8.8       53.6   44.8    0.8   0.84       0         4.5    91.5    3.1   0.95       0.4       7.1    84.8    5.4   2.76       29.2      84.8   15.2    0     0______________________________________ 
    
     In all of the samples except those in which the overthick chips were sliced, fifty percent or more of the chips in each sample were greater than the maximum established acceptable thickness of 8 millimeters. Several samples included high percentages of overly thick chips greater than 14 millimeters. 
     The samples were cooked in a laboratory batch digester using kraft digesting processes. Several samples were cooked in separate batches under two separate cooking conditions. One batch was cooked using a 15%/85% blend of chips from samples 3 and 4. The pulping conditions used for each batch and the chip sample type are described below in Table 3. 
     
                       TABLE 3______________________________________(Pulping Conditions)            Eff Alkali % Yield Max Pressure            %/Resid.   Total/Rej./                                 KappaSample Min./P.S.I.            Wood/(g/e) Screened  Number______________________________________1     50/105     15.8/14.3   52.5/16.5/36.0                                 48.42     50/105     15.8/14.3  46.3/0.8/45.5                                 44.72     70/112     16.1/13.8  44.1/0.4/43.7                                 30.14     70/112     16.1/13.6  44.9/0.9/44.0                                 32.83     50/112     16.2/13.9  45.3/0.5/44.8                                 40.63     60/105     15.8/13.7  47.0/0.7/46.3                                 44.64     60/105     15.8/13.7  49.2/2.7/46.9                                 48.33/4   50/112     16.4/14.3  45.8/1.6/44.2                                 38.05     50/112     15.9/12.6  46.3/4.5/41.8                                 46.86     50/112     15.9/12.6  49.2/5.0/44.2                                 45.2______________________________________ 
    
     Pulp strength properties were calculated after refining the cooked pulps at 3000 revolutions, Table 4 shows these results. 
     
                       TABLE 4______________________________________(Unbleached Strength Properties)                    Break  Freeness          Length                          %Sample (CSF)    Porosity (Km)  Stretch                                Tear Mullen______________________________________1      600      606      7.7   3.7   246  1382      600      655      7.7   3.9   195  1202      534      312      7.9   3.8   200  1214      543      262      7.9   3.8   230  1343      540      264      7.8   3.5   187  1213      540      264      7.8   3.5   187  1204      570      307      7.6   3.1   217  1353/4    572      336      7.9   3.8   238  1345      543      141      9.8   3.8   189  1616      581      192      9.1   3.8   172  148______________________________________ 
    
     As seen in Table 4, the break length and stretch were substantially unaffected by the current chip cracking process of the present invention. Both sliced and cracked chips yielded similar strength characteristics. Tear, strength, and mullen, were, however, lower for the cracked chips. The decreased tear was realized at the entire freeness range examined, with the lowest tear from the harshly treated chips. However, when mixed with sliced chips, the resultant tear from pulps combining samples 3 and 4 was higher than that for the sliced chips (sample 4). Hence, mixtures of cracked chips with regular chips for pulping should be acceptable. 
     In terms of yield, pulps from chips treated by an apparatus according to the present invention contained minimal reject levels and substantially less rejects than pulp from the sliced chips. The overall yield out of the digester was, however, somewhat lower for the chips processed according to the present invention; however, this is believed to be less significant when the percent yield of acceptable fibers is compared. 
     It can be seen that the present invention provides a means for treating oversize chips which yields acceptable, usable pulp having characteristics similar to pulps obtained from acceptable size chips. At the same time, the apparatus of the present invention substantially reduces fines generation and reject fibers when compared to chips processed by conventional slicing techniques or pulps obtained from untreated chips. The simplicity of operation of the present invention makes it advantageous over chip slicers which require more frequent adjustment for proper operation. 
     While an apparatus for destructuring wood chips has been shown and described in detail herein, various changes may be made without departing from the scope of the present invention.