Apparatus for thickening pulp and paper stock

Abstract: Apparatus for thickening papermaking pulp in water has a pair of spaced-apart rolls (20, 22) and a woven wire belt (30) trained around these rolls. The pulp is supplied at the first roll (20) for centrifugal extraction and is carried by the belt to the second roll (22) for further water extraction and thickening, and is then discharged from the apparatus. In one embodiment, the first roll (20) is formed with an open surface, and the suspension is delivered to the belt at least partially by flowing through the first roll from a headbox (40) in the on-running wedge zone. The second roll (22) is porous and has auxiliary water-removing appliances associated therewith, including blow boxes for forcing air either inwardly or outwardly of the second roll.

Description: This application is directed to improvements in the apparatus forming the subject of our US-A-4,722,793 (Seifert) and corresponding EP-A-0251787. Our prior apparatus for thickening a water suspension of solid particles including pulp fibers basically comprises a frame, first and second rolls rotatably mounted in spaced relation in the frame, an endless belt in the form of a loop trained around both rolls and cooperating therewith to define a space bounded by the rolls and opposed runs of the belt, the belt having multiple openings whereby water from the suspension can flow through the belt whilst the particles in the suspension will be trapped by the belt, a headbox mounted to discharge the suspension to be thickened into a tapered entry zone or wedge zone defined by the first roll and the inner surface of the portion of the belt approaching the first roll, a motor for driving the belt around the rolls at a speed to develop centrifugal force to expel water from the suspension through the openings in the portion of the belt trained around the first roll, thereby to thicken the suspension carried on the inner surface of the belt, and means to collect and discharge thickened suspension from a zone bounded by the portion of the belt leaving the second roll. The first and second rolls of our prior apparatus are liquid-impervious, and are rotatable about substantially horizontal axes in the frame. It is an object of the present invention to provide an improved apparatus, and method, by which the capacity of our prior thickening apparatus may be increased without loss in effectiveness. Basically, this can be achieved if the roll to which the suspension or feed stock is initially delivered, and which may be identified as the first, primary or breast roll, is hollow, and includes a discontinuous outer surface through which the feed stock, including the particulate component, can enter, and be discharged from, a chamber within the roll. For example, the discontinuous surface of the roll may be provided by a cylindrical outer surface which has multiple openings therethrough. Such openings have a greater flow area than the maximum size of the particles in the feed stock, to provide for a free flow of the stock in both directions through the outer surface. More particularly, in accordance with one aspect of the present invention, there is provided apparatus for thickening a water suspension of solid particles including pulp fibers of the type disclosed in our EP-A-0251787, and as defined in claim 1. According to a further aspect of the present invention, there is provided a method of thickening a water suspension of solid particles including pulp fibers, as defined in claim 12. The chamber enclosed by the outer surface or shell of the first roll may be the entire interior of that roll. The feed stock is delivered to the infeed wedge zone defined by the belt or wire and the outer surface of the first roll in such a manner that a substantial volume of stock enters the chamber through the openings in the outer surface. Whatever stock enters the chamber in the roll is subsequently thrown by centrifugal force outwardly of the chamber and through the openings in the outer surface, against the portion of the wire wrapping the first roll. Much of the liquid component of that stock is expressed through the wire, leaving a layer or mat of thickened pulp on the inner surface of the wire loop. This layer is carried by the inside surface of the wire into wrapping relation with the second roll, where it is further dewatered, and is then collected and removed, for example as described in our first mentioned US-A and EP-A. A primary advantage of the apparatus and method embodying the invention is the delivery of a substantially higher volume of stock to the interior of the wire loop as it wraps the first roll. Increasing the volume of low consistency stock which is subjected to dewatering around this first roll correspondingly increases the amount of fiber deposited on the wire as the liquid component of the suspension is expressed through the wire by centrifugal force. At this point, reference is made to US-A-1,241,905 (Behr) which discloses an apparatus for separating liquids from solid material, which is somewhat similar in some respects to our previously mentioned prior apparatus. The apparatus described in the latter US-A incorporates a filter pulley or roll having a perforated rim. However, the perforations are filtering perforations which are too small to allow the solid material to pass into the interior of the pulley, and serve only as air passages to permit air under pressure to pass outwards from the interior of the pulley, the air being introduced between the pulley face and the belt to aid the expulsion of liquid outwards through the belt. This prior apparatus does not possess any of the aforesaid advantages of the apparatus embodying the present invention. In one embodiment of the present invention, the second roll may also have an "open" or liquid-permeable surface. For example, the roll could have a drilled shell which is covered by a wire mesh. The use of an open or liquid-permeable second roll obtains a higher discharge consistency of the thickened pulp. Higher consistency is achieved because the liquid which is squeezed from the fiber network as a result of the pressure induced by the wire tension can now travel either radially outwardly or radially inwardly, thereby reducing the required drainage pressure to achieve a certain consistency, or, at a given drainage pressure, increasing the water removal quantity and therefore increasing discharge consistency. The liquid which migrates inwardly is removed from the inner surface of the shell; otherwise, the liquid would leave the shell and rewet the pulp, immediately after the pulp leaves the squeezing zone. An internal doctor, or a wet suction box, as commonly used in paper machine suction presses, may be arranged on the inside surface of the shell of the second roll to remove the water from the roll inner surface, collect it, and discharge it from the roll interior to the outside. Water discharge could be effected through a hollow shaft, through openings in the end of the second roll, or back through the open surface of this second roll. The latter discharge method could be accomplished after the point at which the pulp leaves the roll and before the tapered entry zone or wedge zone, where new wet pulp contacts the second roll. A suitable trough can catch such liquid. The radially inward removal of water from the pulp layer though the second roll openings has advantage where the pulp mat or layer between the wire or belt and the roll surface has a substantial drainage resistance. The drainage resistance may vary with the thickness of the mat and with the characteristic of the fibers and the pores therein. By providing a second path for water removal, the maximum distance which the water has to travel is halved, and the amount of water which has to travel in either direction is also halved, providing the possibility of a four-fold increase in water removal efficiency. In centrifugal water separation apparatus, the inflow of water against centrifugal force is enhanced where a minimum of water is permitted to accumulate at the radial inside surface, or where the water is removed from the inside surface about as soon as it arrives through the roll openings. The supplemental water removal of the suction or vacuum box is enhanced where an upstream dam or diverter to the box scrapes away the water layer before it enters the box. On the other hand, where the drainage resistance of the pulp layer is low, either because of openness and/or thinness of the layer, water removal may be enhanced by applying a stream of air against the inside of the roll and through the roll openings, to entrain water for removal outwardly through the pulp layer. Liquid migration can be enhanced by blowing air or other gas radially inwardly through the wire, the pulp layer, and the open roll shell, such as by an external air blow box mounted in close proximity to the wire belt and fed by air under pressure. Some of the air can escape through a gap between the blow box and the belt, and purge water out of the mesh cavities of the wire into the filtrate space. Air can be used to assist centrifugal force to move liquid radially outwardly by means of an interior blow box and an exterior suction box adjacent the surface of the second roll. The cavities in the mesh-type wire or belt, at the end of the travel of the wire around the second roll, may be partially or completely filled with liquid, since the surface tension retards the centrifugal escape of this water. Also, a thin layer of water remains on the outside surface of the belt or wire for the same reason. When the thickened pulp is separating from the wire, for eventual discharge, some or all of the liquid may be drawn out of the wire and follow the path of the pulp, virtually inseparable from it. In order to prevent such rewetting, a jet or a stream of air is directed at the outer surface of the wire, just before the pulp separates from the wire, across the width of the machine. The air tends to blow the liquid out of the wire cavities into the filtrate space. When the point of separation of wire and pulp is reached, the wire cavities are now largely filled with air and rewetting is minimized. When an open surface roll is used as the second roll, it may be advantageous to blow the water out of the wire prior to the position where the inner suction box or the outer blow box are arranged. Also, a nip roll may be applied to the thickening apparatus embodying this invention. A nip roll may have a smooth cylindrical surface, or have a more open surface with circumferential, axial, or spiral, reverse, blind or through going holes, or it could be a wire mesh covered roll. The nip roll should be located in a zone at the second roll where the centrifugal water throw off is largely completed. If an air jet device is used to blow water out of the wire, the nip roll is located before such a device. If the nip roll cooperates with an open second roll, it would be located at a point prior to drawing liquid radially inward. The nip roll acts to compress the fiber mat more than that of wire tension alone, so that more water is squeezed out of the mat. After the squeezing zone, the pulp may follow either the wire or the roll surface, and it must be removed from either of these surfaces and put into the discharge conveyor. The surface of the open second roll may not lend itself to doctoring, and it is necessary to lift the pulp mat off the roll surface. An internal blow box, or a brush roll may be mounted inside the roll, whose bristles penetrate through the openings and push the pulp away from the roll outer surface. An external stationary brush or rotating brush roll could be used. In cases where the pulp follows the wire, an air blow box, doctor, or similar means may be used to remove the pulp from the wire. In some instances the openings in the second roll may be large, and include such a high percentage of the total surface area, that a mechanical squeezing effect is not achieved, and the second roll dewaters only by centrifugal action, similarly to the open first roll embodying this invention. Such a design is suitable for pulps or suspended materials which dewater very easily, or in applications where high discharge consistency as achieved by additional squeezing action is not required. In this mode, the pulp travels, supported by the wire, around the second roll until the wire leaves this roll. The pulp must now be removed from the wire by appropriate means and guided into the discharge conveyor for discharge from the machine. Either or both of the rolls may have an open surface provided by construction of the roll consisting of a shaft which carries a number of disks of a diameter equal to the roll diameter. As applied to the second roll, at a minimum, two disks are arranged with one at each end of the roll, and the wire is supported and guided by these disks. As in other designs, centrifugal action of liquid and pulp, and the mass of the wire itself, create tension which keeps the wire adequately tensioned, even in those areas where the wire is not supported by a roll surface. According to another embodiment of the invention, as described in detail hereinafter, the wire-supporting rolls may be mounted in spaced relation with their axes at different horizontal levels, and particularly with their axes in the same vertical plane so that one roll is spaced directly above the other, thereby reducing to a minimum the floor space required by the apparatus as a whole. The roll axes may be tilted out of the horizontal, to any common plane, including the vertical. According to another embodiment, which is also described in detail hereinafter, a highly porous endless belt may be wrapped around the second roll between the surface of the second roll and the wire belt, and around a guide roll of small diameter spaced from the second roll within the space between the belt runs and between the rolls to receive pulp from the surface of the second roll for delivery. In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Fig. 1 is a somewhat diagrammatic side view, partly in section, of thickening apparatus in accordance with one embodiment of the invention, wherein the first, primary or breast roll, to which the feed stock is delivered, incorporates a perforate outer shell of honeycomb construction; Fig. 2 is an enlarged fragment of Fig. 1 illustrating the course of the feed stock as it is delivered to the primary roll; Fig. 3 is a somewhat diagrammatic sectional view, similar to Fig. 2, illustrating another construction of perforate-surfaced roll in the position of the primary roll in Fig. 1; Fig. 4 is a somewhat diagrammatic sectional view through the second roll in which the roll is formed with an open surface, and in which auxiliary dewatering devices are employed for removing liquid from the interior of the second roll; Fig. 5 is a sectional view similar to Fig. 4 and showing the employment of blow boxes to assist in liquid and pulp removal; Fig. 6 is also a view of the second roll employing auxiliary mechanical apparatus in the form of a brush for removal of the stock fibers or mat from the roll surface; Fig. 7 is a view similar to Fig. 6 illustrating the use of a couch roll for removing the thickened mat from the surface of the second roll; Fig. 8 is a view similar to Fig. 7 illustrating the use of an endless belt of string or open mesh for removing the thickened mat from the surface of the second roll; Fig. 9 is a fragmentary view illustrating the use of a belt of string or open mesh to assist in removing the thickened mat from the surface of the primary or breast roll and laying it down on the wire for travel to the second roll; Fig. 10 is a view similar to Fig. 4, employing a solid or impervious second roll, combined with air impingement devices for directing air jets laterally onto the wire belt surface to remove entrained liquid therefrom; Fig. 11 is a view similar to Fig. 10, illustrating the employment of an auxiliary nip roll for liquid removal; Fig. 12 is a diagrammatic perspective view of another embodiment of the apparatus in which the wire belt is guided over two or more shaft-mounted disks, to provide a maximum degree of openness; Fig. 13 is a diagrammatic view illustrating a form of the invention wherein the axes of the two rolls lie in a plane inclined to the horizontal, with the breast roll being at a higher level than the second roll; Fig. 14 is a view similar to Fig. 13 illustrating an alternative arrangement and incorporating a flooded nip type of headbox; Fig 15 is a fragmentary view similar to Fig. 14 and illustrating the use of a nozzle-type headbox; Fig. 16 is a view similar to Fig. 13 illustrating a form of the invention wherein the breast roll is at a level lower than the second roll; Fig. 17 is a view similar to Fig. 16 and showing an alternate arrangement; Fig. 18 is a diagrammatic view illustrating an embodiment of the invention wherein the two wire-supporting rolls are in vertically superimposed and spaced relation; Fig. 19 is a diagrammatic view illustrating another embodiment wherein the axes of the two rolls are tilted in the cross-machine direction; Fig. 20 is a diagrammatic view in side elevation, partly broken away, illustrating an embodiment wherein the two rolls are mounted with their axes vertical; and Fig. 21 is a plan view of the apparatus shown in Fig. 19 with the top wall removed. Fig. 1 illustrates the structure of the thickening apparatus which is basically of the type disclosed in the above-mentioned US-A-4,722,793. It includes a frame comprising two columns 10 on each side connected by side beams 11 and suitable cross braces 13. The two rolls 20 and 22 form respectively the first and second rolls which are major operating elements of this apparatus and are mounted adjacent opposite ends of the frame. The roll 22 is shown as mounted by pillow block bearings 23 on the tops of columns 10. This roll 22 is driven, through the belt drive indicated generally at 24, by a motor 25 mounted on the cross brace 13 between the columns. A highly porous endless belt 30 of woven foraminous "wire" is trained around the rolls 20 and 22 and defines therewith a space in which the other operating parts of the apparatus are located. Preferably the belt or wire 30 will consist of any plastic material currently used for paper machine wires, e.g. polyester. Also, since the product of the machine is pulp, rather than a sheet on which wire marking may be undesirable, the belt or wire 30 may be of the pin seam type wherein the ends of the wire belt include overlapping loops which are releasably fastened together by a metal "pin" inserted through these loops. Use of pin seam wires is particularly desirable because it makes possible the changing of wires without the cantilevering of the rolls which is required if the wire is an endless loop without a seam. Roll 20 has an adjustable mounting on the frame which includes means for tensioning the wire. Each of the journals of the roll 20 are mounted by a pillow block 31 on a base 33 which is in turn mounted for sliding movement on the side beam 11. Means such as a pair of jack screws connected between bases 33 and the adjacent columns 10 cause and control this movement to effect corresponding control of the tension in wire 30. The roll 20 functions similarly to the breast roll of a Fourdrinier paper machine in combination with a headbox 40 mounted on the same base members 33 as the roll 20 so that it maintains a fixed spacing with respect to roll 20. This headbox is shown as including lower and upper walls 41 and 42 which define a spout through which a relative flat jet 44 of feed stock is discharged into the tapered entry zone or wedge zone 45 defined by the upper run of wire 30 approaching roll 20 and the adjacent surface of the roll itself. The feed stock to be thickened is fed to the headbox by any convenient feed line (not shown) from the usual stock supply pump (not shown). At the other end of the space defined by the upper and lower runs of the wire or belt 30 and the rolls 20 and 22 is a trough 50 having a screw conveyor 51 mounted in the bottom thereof for receiving thickened pulp from the surface of roll 22 in the wedge zone 52 defined by the roll surface and the upper run of wire 30 leaving this roll. A doctor blade 55 bridges the space between the bare surface of roll 22 and the upstream wall 56 of trough 50 to transfer the thickened pulp from the surface of roll 22 to trough 50, and the screw 51 forces the accumulated pulp to a chute (not shown) at the back of the machine which leads to the next station in the system. Details of the structure and mode of operation of the preferred form of doctor blade 55 are disclosed in the commonly owned Bliss et al, U. S. patent application Serial No. 229,673, filed August 8, 1988 and corresponding EP-A-89307966.5, filed 4th August 1989. In accordance with one embodiment of the present invention, and as shown in Fig. 2, the roll 20, which is termed the "first" or the "primary" roll because it initially receives the feed stock, includes a cylindrical outer shell 60 having openings 61 therethrough which provide the roll with a discontinuous outer surface. The openings are of sufficiently large size to assure full flow therethrough of particles of the maximum size which may be suspended in the feed stock. For example, the shell 60 may be a perforated cylinder constructed as shown in our US-A-4,106,980 (Seifert). Alternatively, and as represented in Fig. 2, the shell 60 may be of a honeycomb structure such as is conventionally used in the manufacture of cylinder molds for the paper industry. Such honeycomb shells may be made of sheet metal strips of a width ranging from 1.27 to 2.54 cm (0.5 to 1.0 inch) which are arranged and configured to define interfitting rows of hexagonal openings 61 with each side wall 62 thereof in the order of 1.27 cm (0.5 inch) long. Where such a honeycomb shell would serve as a support for a wire covering when used in a cylinder mold, however, it is left bare for the purposes of the invention to form the open faced surface which is wrapped by the wire or belt 30. The requirement that the openings 61 be barge enough to pass the largest solid particles in the stock, which may include relatively large pieces of reject material such as paper clips, is also desirable for another reason. This is that openings of such size cause the roll 20 to have an outer surface less smooth than the wire 30, and because a pulp or paper web has a natural tendency to transfer to the smoother surface, the layer of pulp accumulated between the wire and roll 20 will tend to deposit itself on the bottom run of the wire without requiring doctoring. This result could be further assured by the provision of the doctor in the vicinity of the five o'clock position on roll 20 as shown in our previously mentioned US-A-4,722,793, but generally no such doctor would be needed. The internal construction of the roll 20 is not important to the invention, so long as the interior of the roll is not partitioned in such manner as to prevent free circulation of stock into and out of the roll through the shell openings 61. The ends of the roll are preferably closed by end heads so that fluid communication with its interior can be only through the openings 61. The roll 20 may be made with both ends closed, so that the interior of the roll acts as a chamber enclosed by the shell 60 but providing for the in and out flow of feed stock through the openings 61. In operation, and as illustrated in Fig. 2, the flat jet 44 of feed stock from the headbox outlet is directed into the wedge zone 45 and preferably at the apex of the zone where the wire and roll shell meet. Some of this stock impinges on the wire or belt, and is immediately partially dewatered because the force of the jet cause liquid to travel through the wire. Some stock is received between the shell 60 and the wire, but since the shell is open faced, a substantial volume of stock enters the interior of the roll through the openings 61. With the roll 20 rotating at a relatively high surface speed, such as 914.4-1219.2 metres per minute (3000-4000 feet per minute), whatever stock enters the openings 61 within the shell 60 will immediately be subjected to correspondingly high centrifugal force, since the walls 62 defining the openings 61 are traveling at essentially the same speed as the wire 30, This not only causes the stock in the openings 61 to travel at the same speed, but centrifugal force urges it back out and toward the wire covering the outer ends of those openings. Similarly, stock which passes entirely through the shell 60 into the interior of the roll is thrown by centrifugal force back into and through the shell openings 61 along the side of roll 20 opposite the headbox 40. Since only the wire 30 then prevents free centrifugal discharge of the stock from the roll, the continuous application of centrifugal force causes the liquid component of the stock to be expressed through the belt while the pulp materials suspended therein is held by the wire and forms a layer on its inner surface. The liquid expressed in this manner through the wire, both in wrapping the first roll 20 and in subsequently wrapping the second roll 22, is collected in a trough 65 (Fig. 1) which extends under both of rolls 20 and 22 and is provided with a drain outlet 66. A hood 67 is mounted above the apparatus as a whole, and it fits into the top of the trough 65 so that any liquid hitting the inner surface of this hood will drip therefrom into the trough 65. As is explained in our previously mentioned US-A-4,722,793, the primary action of the wire 30 is to serve as a filter medium that holds the fiber and other solid constituents of the stock on its inner surface against the action of centrifugal force, which is the major factor causing dewatering of the retained pulp. In order to provide for maximum capacity for retaining pulp, the wire need not be maintained under more than enough tension to transmit the drive from the roll 22 to the roll 20, particularly since the pressure between the wire and rolls is augmented by the layer of pulp which accumulates therebetween. As noted in the above patent, the tension may be in the range of 0.18 to 7.14 kg per linear cm (1 to 40 pounds per linear inch). The initial dewatering takes place through the portions of the wire which successively wrap the roll 20. Generally, if the feed stock has a consistency of 0.5%, the pulp layer on the bottom run of the wire will be dewatered to a consistency in the range of 6 to 8%. Dewatering to higher consistencies is generally more difficult since the liquid in pulp of that consistency is no longer free. Nevertheless, under the operating conditions described above, a consistency in the range of 12 to 14% for the pulp removed from the roll 22 can be readily obtained. Whatever liquid enters the interior of the roll 20, which can be only through the perforations 61, leaves the roll through some of the same perforations. While it might be possible to flood the apparatus by supplying the feed stock at too great a volumetric feed rate, balancing of the supply rate with the capability of the apparatus to effect adequate dewatering would be no more than a simple matter of observation and control of the feed rate. Fig. 3 is a view similar to Fig. 2 showing a different construction of the primary or first roll 70 which can be used in place of the roll 20. This includes a cylindrical shell 71 having a discontinuous outer surface defined by multiple relatively flat bars 72 supported at their opposite ends by heads 75 which are in turn mounted for rotation in the same manner as the roll 20. The bars 72 are arranged in circumferentially spaced relation to provide slot-like perforations 77 therebetween, and they may also be supported at positions intermediate their ends by suitable disk members, not shown. When the roll 70 is utilized in the position of roll 20 in Fig. 1, the operation is essentially as described in connection with Figs. 1-2, with the slots 77 functioning like the openings 61, and with the flat bars 72 acting as paddles which force stock in the slots 77 to travel at the same speed as the wire and to be discharged outwardly from the shell. The apparatus embodying the invention may also employ an open faced or liquid permeable roll for the second roll 22, and one preferred arrangement is shown by the roll 100 in Fig. 4. The roll 100 is hollow and has an outer shell 102 which could be drilled, for example, and covered by a wire mesh 103. For maximum water removal, ultilizing the squeezing pressure or force of the wire in tension, the openings 105 therein should be so small as to pass liquid but not pulp. The use of the perforate roll 100 possesses the advantage that higher consistency is achieved since the water, expressed from the pulp or stock layer 106, now has a second path for removal. As previously mentioned the drainage pressure may be reduced, by providing a radially inward path in the zone or region of compression of the layer 106. In the system as shown in Fig. 4, it is important to remove the free water which migrates inwardly to the inner surface of the shell, to hold the hydraulic pressure at a minimum, and to prevent rewetting of the pulp layer 106 when it leaves the pressure zone. For this purpose, an internal doctor in the form of a suction box 110 is arranged on the inside surface of the shell, preferably at a location therein more than half way around the pressure or squeeze zone but before the wedge zone 52 at the discharge or the off-running side of the wire 30. A wiper or doctor blade 111 at the upstream or leading side of the box 110 removes the film 112 of water from the inside surface of the shell 102, and discharges it into a removal trough 113. The water discharge may conveniently be routed through a hollow roll shaft (not shown). Fig. 4 also illustrates a preferred arrangement for the removal of whatever portion of the fiber layer 106 may follow either the outer surface of the roll 100 or the inside surface of the wire, in the form of air pressure doctors or blow boxes. A blow box 115 positioned above the wire and wedge zone 52 will force pulp material entrained within the wire into the conveyor trough 50, and if required, a second blow box 116 may be positioned on the inside surface of the shell 102 at the wedge zone 52 to direct entrained stock material from the roll shell into the trough 50 and screw 51. The arrangement of the open second roll 100 in Fig. 5 illustrates the advantageous use of an air jet, or air pressure, to assist in water removal, in combination with the open shell 102. An air blow box 120 is mounted so as to direct a stream of air onto the outer surface of the wire, preferably at a location more than half way around the squeezing or pressure zone. The side walls 121 and 122 of the box 120 terminate in spaced relation to the outer surface of the wire, and define air outlet passages or slots 124 which enable some of the air to exit therethrough at high velocity tangent to the outer surface of the wire. These air streams, directed onto and closely adjacent the wire, entrain residual water out of the wire openings or cavities, and tend to purge these cavities of water by overcoming the surface tension tending to hold the water in place. The remaining air tends to assist in water removal by pushing water through the pulp layer 106 into the shell interior, for collection by a trough 125. In some instances, the surface of the second roll 100 may not lend itself to conventional doctoring, such as where the surface includes or is formed as a wire mesh 103. An internal blow box 116 can be used, but alone may be not sufficient. In such cases, as shown in Fig. 6, a brush 130 can be mounted on the inside of the shell 102 and arranged so that the bristles penetrate through the shell openings 105. Fig. 6 also illustrates the use of an external rotating brush 131 in the off-running wedge zone 52, with its bristles in contact with the shell outer surface, to sweep the fiber particles into the conveyor screw 51. In some instances, which could depend on the thickness and permeability of the stock layer 106, and the tension pressure applied by the wire, it may be found advantageous to direct purging air in a generally radially outward direction, in the direction of the already present centrifugal force, so as to assist in water removal. Such an internal blow box 132 is also shown in Fig. 6. As in the case of the vacuum or suction box 110 in Fig. 4, the leading edge of the box may have a wiper or doctor 111, for water film removal into a trough 113, as previously described. Fig. 7 illustrates another arrangement for removing the layer of thickened pulp from the second roll 22 which is particularly advantageous when the roll 22 has a discontinuous surface of any of the types discussed above. In Fig. 7, the roll 135 is of the type often employed as a couch roll with a thickener of the decker type and therefore has an outer surface, such as a rubber covering, which is smoother than the surface of the roll 22. The positioning of the two rolls should be such that the roll 135 is in gentle pressure engagement with the mat of pulp on the surface of roll 22 so that this mat will transfer to the smoother surface of roll 135, from which it is removed by a doctor 136 into the trough 50. In Fig. 8, an additional highly porous endless belt 140 composed, for example, of multiple parallel strings or a wide mesh woven fabric, wraps the roll 22 and a guide roller 141 of small diameter positioned above the trough 50. With this arrangement, the mat of pulp is compressed onto the belt 140 as it travels around the roll 22 and thus follows belt 140 away from roll 22 until it is removed by gravity into the trough 50. A doctor (not shown) may be provided, positioned to assure removal of all of the mat from belt 140, and it may also be desirable to provide a blow box 115 above the belt as described in connection with Fig. 6. Fig. 9 illustrates the use of a similar highly porous belt 145 in conjunction with the roll 20 for the purpose of assuring that all of the pulp mat thickened in passing around the roll 20 will be removed therefrom for travel on the inside of the wire 30 to the second roll. As shown, the belt 145 wraps the roll 20 and a guide roller 146 of relatively small diameter so that as the belt 145 takes a sharp turn around roller 146, any thickened pulp which has not already dropped off should be dislodged and thrown forward and then down onto the inner surface of the wire 30. A belt 140 or 145 may be used with either permeable or impervious second rolls. Fig. 10 illustrates the use of an air jet doctor 150 in a number of alternate positions for the removal of water from the belt or wire openings or surface cavities. The nozzle 151 of the doctor 150 directs its stream of air against the outer surface of the belt at a location adjacent the off-running side but before the end of the pressing zone. The optimum location of the nozzle 151 may depend on the geometry of the belt cavities and may require consideration of the velocity vector of the belt at the region of impingement. The air stream from nozzle 151 overcomes the surface tension which otherwise retards the escape of the water. The removal of such crevice water reduces the rewetting which would otherwise occur when the pulp layer enters the wedge zone 52. The water removed by the air jet doctor 150 enters the filtration space under the hood 67 and is drained by the trough 65 and outlet 66, as shown in Fig. 1. This air jet arrangement for the removal of water trapped in the belt may be used with either permeable or impervious second rolls. A further preferred arrangement for enhancing water removal is provided by a nip roll 155, as shown in Fig. 11. The nip roll 155, usually of a smaller diameter than that of the roll 100 or 22, applies a pressing force at the outer surface of the belt at the pressing, compression or squeeze zone. It is preferably located more than half way around the pressing zone, at a point where water removal is already partially accomplished at the second roll. The nip roll 155 may have a smoother surface, or may have a more open surface with either axial or spiral grooves, for example, or with blind or through openings. It could also be a wire mesh covered roll. Where an air jet doctor 150 is used, the nip roll 155 should be located before the air jet. If the nip roll is used in combination with a suction box 110 or a blow box 120, it would precede such device. The nip roll 155 compresses the pulp or fiber mat or layer 106 to a greater extent than is possible by belt tension alone. In some instances, the openings in the second roll can be so large that a mechanical squeezing effect is not achieved, and the dewatering takes place principally through the action of centrifugal force. Such an arrangement may be preferred where the stock has suspended material which dewaters easily, or where higher consistency by squeezing is not required, so that the pulp layer is primarily or entirely supported by the belt 30. Such an arrangement is illustrated by the open second roll 200 in Fig. 12, which includes a support shaft 202 having a limited number of identical disks 205 of the same diameter mounted thereon. At a minimum, two of the disks 205 are provided, one at each end of the roll 200, and in effect define the roll. The belt 30 is supported and guided by the disks, and the pulp is applied to and carried by the inside surface of the belt, as previously described. As with the other embodiments, the belt itself and the action of the liquid and the pulp create tension to keep the belt tight, even in those regions where the belt is not supported by a roll, and the afore-described headbox applicators and dewatering devices may be used. It should be understood that, in each of the embodiments described above, means should be provided for guiding the belt to travel in a path perpendicular to the axes of the two rolls on which it is supported, and preferred means for effecting such guiding are disclosed in our U.S. patent application Serial No. 229,393 (Marsh), filed August 8, 1988 and corresponding EP-A-0341913, filed 5 May 1989. Such means comprise a guide strip secured along one or both edges of the inner surface of the belt loop, and complementary grooves in the surfaces of the rolls, or a pair of guide strips at opposite edges of the belt which overhang the ends of the rolls wrapped by the belt as is shown diagrammatically at 206 in conjunction with the disks 205 at each end of the second roll 200 in Fig. 12. Figs. 13 to 18 show a variety of embodiments of the invention wherein the axes of the rolls 20 and 22 are not in the same horizontal plane. More specifically, the first or breast roll 20 in Fig. 13 is at a higher elevation than the second roll 22 such that their axes define a plane extending at approximately 45° to the horizontal. In Fig. 13, the two rolls and the belt travel in counterclockwise directions, and the headbox 40 is positioned to discharge the suspension to be thickened into the wedge zone 45 on the upper side of roll 20. The thickened stock is removed from roll 22 in the same manner as shown in Fig. 1, by means of a doctor 55 which directs the mat or pulp into the trough 50. In Fig. 14, the arrangement of the rolls is the same as in Fig. 13, but the rolls and belt are shown as traveling clockwise, and an open-sided headbox 210 is provided which is in effect an open-sided trough in that its side adjacent the belt is formed by the belt itself. With this feed arrangement, the belt and a lower portion of the roll 20 are flooded with suspension to be thickened, so that appreciable thickening takes place by drainage of liquid through the belt before the thickened suspension reaches the space between the belt and roll 20. The thickened pulp is removed from the surface of the roll 22 by a doctor 212 of curved configuration so that it directs the mat of pulp into a trough 50 as illustrated in Fig. 13. Fig. 15 illustrates an alternative to Fig. 14 wherein the belt arrangement is the same as in Fig. 14, but the headbox 215 is similar in structure and mode of operation to the headbox identified as 40 in Fig. 1 of US-A-4,722,793, namely a pressure headbox is provided having one side 42 shorter than its other side 41 so that it forces stock against the belt as well as into the wedge zone 45. The arrangement for removing the thickened pulp would be the same in the form shown in Fig. 15 as in Fig. 14. Figs. 16 and 17 show alternative arrangements similar to those in Figs. 13 and 14 except that the positions of the rolls 20 and 22 are reversed, with the breast roll 20 in the lower position. In Fig. 16, the breast roll 20, second roll 22 and belt 30 travel counterclockwise, and the headbox 40 is positioned to discharge the suspension to be thickened into the wedge zone 45 as in Fig. 1. Similarly, the thickened pulp is removed from the roll 22 by the doctor 55 into the trough 50. A practical advantage of the arrangement of Fig. 16 over that in Fig. 1 is the reduction in floor space required, resulting from tilting of the apparatus as a whole. In Fig. 17, the arrangement of the rolls is the same as in Fig. 16, but they and the belt travel clockwise. The headbox 215 may be of the nozzle type shown at 40 in Figs. 1 and 13, but is shown as of the same construction as in Fig. 15 so that it discharges the suspension directly against the belt as well as into the wedge zone 45 on the downward side of the roll 20. The second roll 22 in Fig. 17 should preferably have surface characteristics such that the thickened mat of pulp will remain thereon rather than transferring to the belt in the wedge zone 52, and it would be desirable to provide a slide 216 in the position shown, to convey the thickened pulp by gravity into the trough 50. Any thickened material which is not thrown off from roll 22 by centrifugal force may be removed and delivered to the trough 50 by a doctor 55. Note also that it is immaterial if some of the thickened pulp remains on the belt, since it will simply be mixed with fresh suspension in the wedge zone 45 and recirculated to roll 22. Fig. 18 shows an embodiment of the invention which has the practical advantage of requiring minimum floor space, because the rolls 20 and 22 are mounted with their axes in the same vertical plane. In its other respects, the vertical orientation of the roll axes in Fig. 18 is comparable to the inclined orientation in Fig. 17, and the operations of these two forms of the invention are therefore closely comparable, whether the headbox 215 is of the same type as in Fig. 17, as shown, or of the nozzle type shown in other views. In either case, initial dewatering is essentially downward and is therefore easily handled by a "save-all" (not shown) below the machine which will also receive the liquid collected by a hood enclosing the sides and top of the machine. While in all of the embodiments shown in Figs. 1-18 the rolls 20 and 22 are shown with their axes horizontal, the invention is not limited to such an arrangement. On the contrary, the axes of these rolls may be at any angle to the horizontal, and there are practical advantages in having the two axes substantially parallel but at an angle to the horizontal in the cross machine direction. For example, if they are tilted to 45°, as illustrated in Fig. 19, the trough into which the pulp is discharged from the second roll will be similarly tilted, so that pulp will be sluiced out from the interior of the belt loop. In a further such modification, Figs. 20 and 21 show a specific embodiment of the invention wherein the rolls 220 and 222 are mounted in laterally spaced relation with their axes vertical, which is an arrangement that is both advantageous and is made practical by reason of the high belt speeds at which the apparatus embodying the invention is capable of operating. More specifically, with the headbox 40 positioned to discharge the feed suspension into the wedge zone 45 as a jet flow which extends vertically, the suspension is trapped between the belt 230 and roll 220 before it has an opportunity for downward flow under the influence of gravity. Then, when the thickened pulp emerges on the outrunning side of roll 220, the speed of the belt 230 and the centrifugal force effective on the pulp mat formed during travel around roll 220 will hold that mat firmly on the belt as it travels to roll 222, during which travel it will be subjected to further dewatering by the combined centrifugal and catenary forces. As this thickened mat then travels around roll 222, it is again subjected to further dewatering by centrifugal force, so that it will emerge on the outrunning side of roll 222 at a consistency comparable to that of the paper web at the couch roll of a Fourdrinier paper machine. If the roll 222 is smooth and liquid-impervious, the mat will follow that roll surface away from the belt, and it can then be readily removed from the roll by any suitable doctor blade 255 that deflects it towards a suitably curved vertical wall 256 by which it is intercepted and guided downwardly to a pipe or chute 257 which leads to the next station in the system. This arrangement for removing the thickened pulp from inside the loop of belt 230 is simple and effective, and it eliminates the need for a conveyor such as is shown in the other views. Similarly, the removal of the liquid expressed through the belt is simple and easy. A bottom wall 265 underlies all of the apparatus except for openings therethrough for the pulp outlet pipe 257 and the liquid outlet pipe 266, and the apparatus is also enclosed by side walls 267, and a top wall 268 which is removable to facilitate changing the belt 230. All of the liquid expressed through the belt 230 will therefore either fall first directly to bottom wall 265 or will bounce off the side walls 267 and thence to the bottom wall. The apparatus shown in Figs. 20 and 21 thus offers the same advantages as the forms of the invention shown in other views, and in addition it has the practical advantage that the belt can be changed from above without requiring cantilevering of rolls 220 and 222. It is also to be understood that any of the various roll constructions and combinations described above can be used as the rolls 220 and 222. All of the forms of the invention shown in Figs. 13 to 21 include open-faced first or breast rolls as described above in connection with Figs. 1-3, and can advantageously include open-faced second rolls as previously described and illustrated, although the second rolls may be liquid-impervious. The choice of particular rolls for a given installation may be made in accordance with the teaching of the foregoing description as applied by persons skilled in the art. In every case, suitable provision is needed for guiding the belt, as explained above in connection with Fig. 12. Since the thickening apparatus embodying the invention was developed for use in the treatment of papermaking pulp or fiber suspensions, it has been described with specific reference thereto. It is to be understood, however, that the apparatus could be used to thicken other dilute liquid suspensions of solid particles, such for example as sludges and the like, and use of the term "pulp" herein is to be understood as applying to any such suspensions.

Claims: Apparatus for thickening a water suspension of solid particles including pulp fibers, including: a frame (13), first and second rolls (20,22) rotatably mounted in spaced relation in the frame, an endless belt (30) in the form of a loop trained around both rolls and cooperating therewith to define a space bounded by the rolls and opposed runs of the belt, the belt having multiple openings whereby water from the suspension can flow through the belt whilst the particles in the suspension will be trapped by the belt, a headbox (40) mounted to discharge the suspension to be thickened into a tapered entry zone (45) defined by the first roll (20) and the inner surface of the portion of the belt approaching the first roll, a motor (25) for driving the belt around the rolls at a speed to develop centrifugal force to expel water from the suspension through the openings in the portion of the belt trained around the first roll, thereby to thicken the suspension carried on the inner surface of the belt, and means (50,51) to collect and discharge thickened suspension from a zone bounded by the portion of the belt leaving the second roll (22), characterised in that the first roll (20) has a discontinuous cylindrical outer surface enclosing a hollow interior, the outer surface having multiple openings (61) for conducting particles in the suspension therethrough to and from the roll interior, and the headbox (40) is arranged to cause some of the suspension to enter the first roll interior through the openings (61) in the portion of the roll defining the entry zone (45) for subsequent discharge therefrom through said openings (61) in response to centrifugal force. The apparatus of claim 1, in which the second roll (100) is hollow and includes a cylindrical shell (102) formed with openings (105) therethrough sized to permit the passage of water, but to prevent the passage of particles from the thickened suspension (106), into the interior of the second roll, and in which the belt (30) is tensioned against the outer surface of the second roll to provide for further water removal from the thickened suspension about the portion of the second roll wrapped by the belt. The apparatus of claim 2, including a suction box (110) positioned adjacent the inside surface of the shell (102) where the second roll is wrapped by the belt, for drawing water from the particles of the thickened suspension into the interior of the second roll through the openings (105). The apparatus of claim 3, including a doctor blade (111) upstream of the box (110) for removing water from the inside surface of the shell of the second roll. The apparatus of claim 2, including an air pressure box (132) positioned adjacent the inside surface of the shell (102) to direct air outwardly through the openings (105). The apparatus of any of claims 2 to 5, including a nip roll (155) mounted in pressure engagement with an outer surface of the belt where it wraps the second roll, for squeezing the layer of thickened suspension in the region between the belt and the outer surface of the shell (102). The apparatus of any of claims 2 to 6, including an air jet device (150) positioned adjacent an outer surface of said belt where it wraps the second roll, to direct a stream of air against the outer surface of the belt for the removal of water from belt cavities. The apparatus of claim 7, in which the air jet device (150) is arranged to direct an air stream at an acute angle to the outer surface of the off-running portion of the belt in relation to the second roll. The apparatus of any of claims 2 to 8, in which the second roll (200) has a support shaft (202) and at least a pair of disks (205) mounted in spaced relation on the shaft, each of the disks having the same diameter, the belt (30) being trained over the outer surfaces of the disks and spanning the open space between the disks. The apparatus of claim 1, in which the second roll (22) has an impervious surface. The apparatus of any preceding claim, in which at least one of the rolls (20, 22) has an additional highly porous endless belt (140, 145) wrapped therearound between the surface of the roll and the belt (30), the additional belt (140, 145) also being wrapped around a guide roller (141) of smaller diameter than the roll, which guide roller is spaced from said roll within the space between the rolls, the additional belt (140, 145) being operable to receive and separate the thickened suspension from the surface of the roll. The apparatus of any preceding claim, in which the axes of the first and second rolls are disposed in a common plane, and in which said plane is other than horizontal. The method of thickening a water suspension of solid particles including pulp fibers, in which a porous, endless wire or belt (30) in the form of a loop is wrapped around first and second rolls (20, 22) rotatably mounted in a space bounded by the belt loop, leaving portions of the roll surfaces exposed within the belt loop, the method including driving the rolls and belt to cause the belt to travel around the rolls (20, 22), directing a flow of the suspension from a headbox (40) into a tapered entry zone (45) defined by the first roll (20) and the inside surface of the approaching portion of the belt (30), to trap and form into a mat the particles in the suspension on the inside surface of the belt (30), dewatering the mat by centrifugal force created by the travel of the belt about the portion of the first roll outer surface wrapped by the belt, collecting the mat of thickened suspension from the belt (30), and removing the collected mat from the space within the belt loop; characterised by: the first roll (20) having a discontinuous cylindrical outer surface enclosing a hollow interior, the outer surface being provided with multiple openings (61) therethrough opening into the roll interior, the openings (61) being sufficiently greater in flow area than the effective maximum predetermined particle size of the suspension to provide for free flow of the suspension through said outer surface both into and out of the roll interior, and the method including directing the flow from the headbox (40) so that at least some of the suspension, including the particles in the suspension, enters into the roll interior through the openings (61) in the portion of the first roll outer surface not wrapped by the belt (30), and driving the roll at a speed sufficiently high to develop centrifugal force causing substantially all of the suspension within the roll interior to be discharged therefrom through the openings (61) in the portion of the first roll outer surface which is wrapped by the belt (30), while also causing the water component of the suspension to be expressed through that portion of the belt wrapping the roll surface while the particles in the suspension are formed into a mat. The method of claim 13, in which the second roll (100) is hollow and has a cylindrical surface formed by a wire mesh (103) or otherwise formed with openings (105) which can pass water therethrough, but are sufficiently small to prevent the passage of particles from the mat (106) into the interior of the second roll, the method including pressing the mat with the belt (30) against the second roll surface for further water removal therefrom around the portion of the second roll surface wrapped by the belt. The method of claim 14, which includes generating an air pressure differential between the interior of the second roll (100) and the exterior of the belt to cause air flow through the openings (105) in the wrapped portion of the second roll surface to entrain water therewith from the mat (106). The method of claim 14 or 15, which includes squeezing water from the mat (106) between the belt (30) and the wrapped portion of the second roll surface by means of a nip roll (155) mounted in pressure engagement with the outer surface of the belt where it is wrapped around said portion of the second roll surface. The method of any of claims 14 to 16, which includes positioning an air jet (150) to direct a stream of air against the outer surface of the belt (30) at a region thereof which is wrapped about said portion of the second roll surface, for the removal of water from cavities in the belt. The method of claim 13, in which the second roll (22) has an impervious surface, the method including pressing said mat with said belt (30) against the second roll surface for further water removal therefrom around the portion of the second roll surface wrapped by the belt.

Inventors: CHUPKA DAVID E, SEIFERT PETER, CHUPKA, DAVID E., SEIFERT, PETER
Applicants: BLACK CLAWSON CO, THE BLACK CLAWSON COMPANY
Application year: 1989
Main classification: D21C9
IPC codes: D21C9, D21F1, B01D33, D21F9