A PROCESS AND APPARATUS FOR CONTINUOUSLY TREATING PULP WITH A TREATING LIQUID

Abstract: In continuously treating pulp in an apparatus with a treating liquid wherein a web of cellulosic material is moved enclosed between two belts running together over rotatable rolls in a treating section while supplying liquid to the web and displacing it therefrom in at least two consecutive treating stages, it is proposed according to the invention that the belts (3, 4) and the web (14) enclosed therebetween, are moved in an upwardly running winding path (9, 10), for instance, and that in a first treating stage within this path treating liquid is supplied to the web (14) on a first surface thereof and in a second treating stage on a second surface thereof, and that in the first treating stage liquid is displaced from said second surface and in the second treating stage liquid is displaced from said first surface by compression of the web (14) between the belts (3, 4) during their movement around a first and a second guide roll (5), respectively, so that the liquid is displaced through the web (14) in direction from the envelope surface of the guide roll which in that case is solid, the web (14) acquiring a relatively high dryness content by each liquid displacing compression. The displacement of liquid at a given guide roll occurs in a flow direction through the web (14) that is opposite to the flow direction of the liquid at the immediately following or immediately preceding guide roll (5).

Description: A process and apparatus for continuously treating pulp with a ttrr*e__attii_ng_ lliiαqiuiiirdl The present invention relates to a process for continuously treating pulp with a treating liquid, in which a web of cellulosic material is moved enclosed between two belts running together over rotatable rolls in a treating section, the treating liquid being supplied to the web and displaced therefrom in at least two con¬ secutive treating stages. The invention also relates to an appa¬ ratus for carrying out this process. The term "treating" applies to washing, bleaching or some other chemical treatment of pulp. In the manufacture of pulp wood chips or other disintegrated cel- lulosic materials are digested with a pulping liquor containing chemicals which dissolve a part of the soluble substance. The ma¬ terial is then defibrated mechanically or by decreasing the pres¬ sure, depending on the apparatus used for said dissolving. Before the fibres, which have been exposed in this way, can be used, the chemicals and the substance dissolved must be separated from the fibres, and this is accomplished by pulp washing. At the beginning this washing was performed in the same apparatus as was used for the digestion. Later on a technique was developed in which the pulp was diluted with liquor, deposited on rotating filters and washed by means of a displacement process once a web had formed. The web was then dewatered, dissolved and passed to the next fil¬ ter. The washing liquid flowed in counter-current. The recovery of chemicals required improved washing technique and the liquor therefore had to be evaporated before combustion. It thus became increasingly important to recover the chemicals with a minimum of dilution since each time water was added it must be removed by evaporation. Increased costs for energy and chemicals enhance the need for recovery and for less dilution. When digestion is carried out continuously, washing may be per- formed in the lower part of the digester. This washing is extreme¬ ly efficient due to the high temperature and the compressed fibre bed. Washing is usually required even after the digester wash, for environmental and economic reasons. Such washing is performed on filters, for instance, or similar equipment, the web being formed from a diluted suspension produced by recirculation of liquor. The dilution itself has negative effects on the washing result. More¬ over, filters are open units and they have the problems associated therewith. A washing technique has therefore been developed using what are known as continuous diffuser washers and pressure dif us¬ ers, where the pulp is enclosed during the washing process. In some cases presses, both screw presses and roll presses, are also used in conjunction with the washing process. Some roll presses are also equipped with displacement zones, the actual dis- placement being completed in the pressure nip. These are known as washing presses. This type of washing apparatus, which has a rotatable drum with a perforated envelope surface is no doubt suitable at the end of the washing process or when a high dryness content is desired for some other reason, such as prior to an oxygen gas stage. Examples of such washing apparatuses are de¬ scribed in SE 318182, 318183, 349340, 413685 and 434859. A special apparatus for displacing one liquid with another liquid is described in SE 157240. The displacing liquids are transported between different displacement zones by means of gravity, the dis- placement zones being at different levels in that a transport path for the web runs in steps. The web passes a number of liquid pools defined by rolls around which a wire passes. Liquid is continuous¬ ly forced through the web in the same direction and no compression of the web to achieve high dryness content is described prior to a further displacement stage. The stepped arrangement of such an in¬ stallation makes it relatively long and it thus requires consider¬ able space. The specification states that the direction of treat¬ ment can be changed when the web is between two conveyors so that washing and displacement can be done in the direction of the one or the other conveyor. Below the conveyor is a liquid receptacle from which liquid is pressed up through the web after passage of a liquid pool supplying the web with l quid from above. The arrange- ment proposed for this purpose extends horizontally and is there¬ fore rather long. Compression of the web each time liquid is sup¬ plied cannot be performed in such an apparatus. Through the Finnish patent application 834486 it is known to wash pulp fibres in a longitudinal tank through which a wire runs, the tank being divided into a plurality of chambers filled with stock disposed above the wire and a corresponding number of suction chambers disposed below the wire. As the wire advances, water is sucked through it so that a fibre layer is formed on the wire and this fibre layer is washed in accordance with the counter-current principle on its way to a picking-up roll where it is compressed between the envelope surface of said roll and the wire to remove water, before reaching a screw conveyor. Washing is carried out in the same direction in all the washing stages and no compression of the web supported by the wire and entirely surrounded by liquid is possible between each washing stage. A new type of washing apparatus has latterly come into use, em¬ ploying a single wire, which is therefore generally known as a fourdrinier wire washer. In such a fourdrinier wire washer a com¬ paratively thin web is produced with good formation. The web is then washed in counter-current, liquid being added and withdrawn along the path of movement of the wire. The equipment is relative¬ ly simple, easily controlled and is extremely efficient. The main drawbacks with this technique are that, due to friction between the wire and the suction boxes required for removing the water, the wire is subjected to considerable wear and its service life is therefore extremely short. The high temperature, which is desir¬ able, also means that the material used for wires is not very suitable and experiments are in progress to replace it by other material. Washers of this fourdrinier wire type are described in US 4.014.736, 4.046.621, 4.154.644 (corresponding to Swedish pat¬ ent application 7811285-1), FI 25895 and Tappi Journal 66 (7), pages 43-45. Most of these known washers are provided with a hood. The big advantage of the fourdrinier wire washing technique is primarily that extremely good washing results can be achieved with minimum dilution. This is of great importance nowadays in view of high energy costs, and the significance increases with increasing energy costs. The object of the invention is to provide a novel process for con¬ tinuously treating a web with treating liquid, which process fur¬ ther increases the possibility of efficient treating using a mini¬ mum of treating liquid which are achieved with the known fourdrin- ier wire technique, and at the same time eliminate the drawback of friction between wire and suction boxes. The process for continuously treating pulp with a treating liquid according to the invention is substantially characterized in that the belts and the web enclosed therebetween, are moved in a down- wardly or upwardly running winding path and that in a first treating stage within said path treating liquid is supplied to the web on a first surface thereof and in a second treating stage on a second surface thereof opposite said first surface, and that in the first treating stage liquid is displaced from said second sur- face and in the second treating stage liquid is displaced from said first surface by compression of the web between the belts during their movement around a first and a second guide roll, re¬ spectively, so that the liquid is displaced through the web either from the envelope surface of the guide roll which in that case is solid, or towards the envelope surface of the guide roll which in that case is provided with means permitting the liquid to be drained at the envelope surface, the web acquiring a relatively high dryness content by each liquid displacing compression, the displacement of liquid at a given guide roll occuring in a flow direction through the web that is opposite to the flow direction of the liquid at the immediately following or immediately preced- ing guide roll . An apparatus for carrying out this process is substantially char¬ acterized in that the rolls in the form of guide rolls are mounted at different levels within the treating section in such a way that the belts describe a winding path, that the distribution means are mounted in a first treating zone to supply treating liquid to the web on a first surface thereof and in a second treating zone on a second surface thereof opposite said first surface, and that the belts and guide rolls are mounted to cooperate with each other to effect compression of the web enclosed between the belts and dis- placement of liquid out from the web. A preferred embodiment of the apparatus according to the invention comprises a former located upstream of the treating section and comprising a headbox arranged to supply a jet of fibre suspension onto one of said belts before the belts converge. The invention will be described further in the following with ref¬ erence to the accompanying drawings. Figure 1 shows schematically an apparatus according to a first em¬ bodiment of the invention. Figure 2 shows schematically an apparatus according to a second embodiment of the invention. The apparatus shown in Figure 1 comprises a forming section 1 and a treating section 2. Two belts 3, 4 run in endless loops through these sections over a plurality of rolls comprising guide rolls 5 and tension rolls 6, each loop describing an outer loop part 7, 8 and an inner loop part 9, 10 in relation to the forming section 1 and the treating section 2. The two outer loop parts 7, 8 are spaced from each other, one on each side of the forming section 1 and the treating section 2. The inner loop parts 9, 10 of the belts are arranged to run together into contact with each other before the treating section 2 and run away from each other at the end of the treating section 2. The belts 3, 4 are pervious to liq¬ uid. Preferably the belts consist of wires of the same type as used for forming paper webs or some other wires having similar properties. It is understood that the belts are advanced at the same speed. In the embodiment shown the forming section 1 is of fourdrinier wire type and includes a headbox 11 from which a fibre suspension 12 of cellulosic material in suspension liquid is caused to flow out onto one of the belts, i.e. the lower belt 3, so that a con¬ tinuous web 14 of predetermined thickness is formed thereon in the forming zone 13. Immediately before the forming zone 13 the direc¬ tion of travel of the belt 3 is deflected by one of said rolls in the form of a breast roll 15. Drainage means in the form of one or more suction boxes 16 are mounted in the forming zone 13 below said belt 3. The suction boxes operate under sufficient vacuum to draw the suspension liquid out of the web through the belt 3. The term "vacuum" means as used herein a subatmospheric pressure. In the embodiment shown, the treating section 2 includes four rotatable guide rolls 5a, 5b, 5c, 5d for the two belts 3, 4. The guide rolls are journalled at different levels and are alternately displaced in relation to each other in such a way that the con¬ verging belts describe a winding path with their inner loop parts 9, 10. In the embodiment shown the guide rolls are mounted in two vertical rows disposed at a predetermined distance from each other. Before each of the three guide rolls 5a, 5b, 5c in the treating section 2, seen in the direction of travel of the belts, are dis¬ tribution means 17 for the treating liquid to be supplied trans- versely across the advancing web. Each distribution means is spaced from the relevant guide roll 5a, 5b, 5c and is followed by drainage means below the belts, in the form of one or more suction boxes 18, operating under sufficient vacuum to draw liquid out of the web through the lowermost belt 3 or 4. Between two consecutive distribution means 17, each mounted immediately after a preceding guide roll 5d, 5a, 5b, 5c, a treating zone is formed for a treating stage starting with the supply of treating liquid to the web at a position where the belts are moving tangent ally from an immediately preceding guide roll and close to this, and terminates with liquid being displaced out from the web at the immediately following guide roll 5a, 5b and 5c, respectively, by the web being compressed between the belts as it passes the guide roll. The flat, substantially horizontal part of a treating zone can thus be defined as a fourdrinier wire part. As will be understood the one belt 3 and the other belt 4 act alternately as supporting means and covering means for the enclosed web. In the embodiment shown the treating section is provided downstream with a supplementary treating zone comprising an equivalent distribution means 20 for fresh treating liquid and a drainage means in the form of one or more suction boxes 21. The envelope surfaces, i.e. the cylindric wall, of guide rolls 5a, 5b, 5c, 5d are solid and the liquid is therefore displaced through the web in a direction away from the envelope surfaces. The liquid from this displacement at the guide rolls is collected in a receptacle 19 below and carried to a dis¬ tribution means in a treating zone located upstream, while the liquid from the uppermost suction box 21 and the two suction boxes 18 immediately subsequent thereto is carried to respective distri¬ bution means 17 in a treating zone located upstream. The treatment is thus carried out in accordance with the counter-current princi¬ ple, fresh treating liquid being supplied in the last treating stage, which does not therefore include compression around a guide roll. Since the web and belts advance upwards the treating liquid can be transported by means of gravity between the separate treating stages. The finished web 14 is collected in a trough 22 and disintegrated by a screw 23 or the like. The web is compressed already at the first guide roll 5d in the treating section to give it a relatively high dryness content be¬ fore the first complete treating stage with treating liquid and compression is started. The liquid from this initial compression is collected in a receptacle 24 and thereafter in a tank 25, to¬ gether with liquid drained from the forming zone 13 and from the first treating zone. Alternatively, treating liquid from the first treating zone may be supplied to the web at a place before the < first guide roll 5d as soon as the web has been formed. A complete treating stage is thus obtained which is substantially similar to the three following treating stages. Figure 2 shows another embodiment of an apparatus according to the present invention. The same reference numbers are used for similar elements and parts in the two cases. The apparatus thus comprises a forming section 1 and a treating section 2. Two belts 3, 4 run through these sections in loops in the same way as in the embodi- ent described above, except that they run in the other direction so that the inner loop parts 9, 10 of the belts thus passing from the top downwardly in a winding path. The forming section 1 with its headbox 11 is thus located above the treating section 2. Distribution means in the form of liquid boxes 30 are mounted below the belts in each treating zone immedi¬ ately before a guide roll 5e, 5f, 5g, 5h, 5i, said rolls having solid envelope surfaces. Treating liquid is pumped into the liquid boxes 30 and is thus pressed up through the web. In the embodiment shown in Figure 2 each treating zone is also provided with a dis¬ tribution means 31 for treating liquid, located immediately after each guide roll and followed by suction boxes 32 to drain the web in the same flow direction as at the guide roll immediately pre- ceding. Liquid treatment is in accordance with the counter-current principle in this case also, the liquid being transported up to a treating zone at a higher level by means of a pump 33. Alternatively, the supplementary treating means in the form of the distribution means 31 and suction boxes 32 may be omitted and the two vertical rows of guide rolls can therefore be mounted much closer to each other, e.g. immediately adjacent or even in a com- mon vertical line. Instead of supplying treating liquid via liquid boxes 30, spray nozzles may be mounted below the belts before each guide roll, to spray treating liquid in the nip between the guide roll and belts. In a further alternative embodiment of the apparatus according to Figure 2, not however shown, treating liquid is supplied by dis¬ tribution means from above, as shown in Figure 1. The envelope surfaces of the guide rolls are in this case provided with means enabling liquid to be drained off at the envelope surface. The en- velope surface may, for instance, be perforated or provided with channels so that the liquid is displaced through the web towards the envelope surface of the roll as the web is compressed between the belts. Each guide roll is then suitably arranged to cooperate with an endless, impermeable pressure strip travelling at the same speed as the belts and arranged to press sealingly against the belts within their contact circle sectors with the guide roll to prevent liquid from flowing out radially. The pressure strip is preferably common to all guide rolls mounted in a vertical row. The apparatus according to the invention, with its arrangement of treating zones disposed immediately above or below each other as described may be defined, for its specific application for washing cellulosic material, for instance, a story washer. The desired ef¬ ficient treatment is obtained with a minimum of treating liquid since the liquid can be displaced at each compression in an ex- tre ely effective manner so that the web achieves a very high dryness content after each such compression while at the same time the direction is changed for each new or immediately subsequent treating stage. The dewatering of the web occurs thus in the first hand during its passage over the guide rolls by the pressure ef¬ fected by the belt tension, and each compression results in con¬ siderably higher dryness content than are obtained in a fourdrini- er wire type washer of conventional design. The compression de¬ sired can be determined by selection of roll diameter and is con¬ trolled during operation by selection of belt tension. The altera¬ tion in the direction of displacement, which is thus also preceded by a corresponding change in the direction of supply of the treating liquid prevents the web from always being pressed into one and the same face of the belt. Felting of the web into the belt, which is a drawback if the treating direction is the same through several treating stages as in fourdrinier wire washers of conventional design, is thus prevented. The most concentrated side of the web after each compression is also the side which will ab¬ sorb the fresh supply of treating liquid. The effect of the en¬ hanced dryness content is that the result of washing will be es¬ sentially improved even at greatly reduced dilution factors. Each guide roll may constitute a drive point for the belts, thus minimizing the forces on the belts. Since most of the dewatering occurs at the guide rolls, there is less need for suction boxes, which in turn reduces the space requirement in horizontal direc¬ tion. However, the greatest saving in floor space is achieved thanks to the treating zones being arranged immediately over or under each other in stories as shown. The whole apparatus can thus easily be built in to be surrounded by a hermetically closed hood.

Claims: O 86 03795 PCT/SEKVW2- 11 1. A process for continuously treating pulp with a treating liquid, in which a web of cellulosic material is moved enclosed between two belts running together over rotatable rolls in a treating section, the treating liquid being supplied to the web and displaced therefrom in at least two consecutive treating stages, characterized in that the belts (3, 4) and the web (14) enclosed therebetween, are moved in a downwardly or upwardly run¬ ning winding path (9, 10) and that in a first treating stage with¬ in said path treating liquid is supplied to the web (14) on a 0 first surface thereof and in a second treating stage on a second surface thereof opposite said first surface, and that in the first treating stage liquid is displaced from said second surface and in the second treating stage liquid is displaced from said first sur¬ face by compression of the web (14) between the belts (3, 4) dur- 15 ing their movement around a first and a second guide roll (5), re¬ spectively, so that the liquid is displaced through the web (14) either from the envelope surface of the guide roll which in that case is solid, or towards the envelope surface of the guide roll which in that case is provided with means permitting the liquid to 20 be drained at the envelope surface, the web (14) acquiring a rela¬ tively high dryness content by each liquid displacing compression, the displacement of liquid at a given guide roll occuring in a flow direction through the web (14) that is opposite to the flow direction of the liquid at the immediately following or immediate- 25 ly preceding guide roll (5). 2. A process according to claim 1, characterized in that said web (14) is formed in a forming section (1) preceding the treating section (2) and comprising a headbox (11) arranged to supply a jet of fibre suspension (12) onto one (3) of said belts before the 30 belts (3, 4) converge. 3. A process according to claim 1 or 2, characterized in that the direction of movement of the belts (3, 4) after a guide roll (5) is opposite to the direction of movement before said roll, said directions of movement being substantially horizontal. 4. A process according to claim 1, 2 or 3, characterized in that the web (14) is subjected to initial drainage via a draining means (18) prior to each guide roll (5) within the treating sec¬ tion. 5. A process according to any of the preceding claims, charac¬ terized in that the treatment is carried out w th treating liquid conveyed in counter-current to the direction of movement of the belts. 6. A process according to any of the preceding claims, charac¬ terized in that the compression of the web (14) between the belts (3, 4) during passage of the guide rolls (5) is determined from the start by selection of the roll diameter and is controlled dur¬ ing operation by selection of belt tension. 7. An apparatus for carrying out the process according to any of claims 1 to 6 for continuously treating pulp with treating liq¬ uid, said apparatus comprising a treating section through which two belts are mounted to run together over rotatable rolls with a web enclosed therebetween, distribution means being mounted for the supply of treating liquid to the web and the treating section including at least two consecutive treating zones, characterized in that the rolls in the form of guide rolls (5) are mounted at different levels within the treating section in such a way that the belts (3, 4) describe a winding path, that the distribution means (17) are mounted in a first treating zone to supply treating liquid to the web (14) on a first surface thereof and in a second treating zone on a second surface thereof opposite said first sur- face, and that the belts (3, 4) and guide rolls (5) are mounted to cooperate with each other to effect compression of the web (14) enclosed between the belts (3, 4) and displacement of liquid out from the web. 8. An apparatus according to claim 7, characterized in that it comprises a forming section (1) located upstream of the treating section and comprising a headbox (11) mounted to supply a jet of fibre suspension (12) onto one (3) of said belts before the belts (3, 4) converge. 9. An apparatus according to claim 7 or 8, characterized in that the guide rolls (5) within the treating section are displaced alternately in relation to each other and located at different levels in such a way that the belts (3, 4) run between two guide rolls (5) in substantially horizontal direction. 10. An apparatus according to claim 9, characterized in that draining means (18) are mounted below the belts (3, 4) in connec- tion to their regions where they run in a substantially horizontal direction between two guide rolls (5). 11. An apparatus according to any of claims 7 to 9, character¬ ized in that the belts (3, 4) are arranged to be moved in a down¬ wardly running winding path and that the distribution means con- sist of liquid boxes (30) arranged below and transversely across the belts (3, 4) immediately before each guide roll (5e-5i) within the treating section (2), the surfaces of the guide rolls being solid. 12. An apparatus according to claim 11, characterized in that draining means (32) are mounted below the belts (3, 4) in connec¬ tion to their regions where they run in a substantially horizontal direction between two guide rolls (5), and that distribution means (31) for the treating liquid are arranged immediately after a guide roll (5) so as to form supplementary treating zones together with said draining means (32) between each two treating zones which comprise guide rolls. 13. An apparatus according to any of claims 7 to 11, character¬ ized in that the number of treating zones which comprise guide rolls is between three and five.

Inventors: INGEMARSSON GOESTA INGEMAR, TELL LARS ERIK KARL AXEL, INGEMARSSON, GOESTA, INGEMAR, TELL, LARS, ERIK, KARL, AXEL
Applicants: KMW AB, KMW AKTIEBOLAG
Application year: 1985
Main classification: D21C9
IPC codes: D21C9