IMPROVED PULP BLEACHING APPARATUS AND METHOD

Abstract: A bleaching process and apparatus for producing high consistency pulp mats with relatively uniform chemical and temperature distribution in the pulp mat employs a washing press (100) in which the mat is formed in a first chamber (I) while a chemical solution which may first be heated or cooled is added to a second chamber (II). Liquor extraction begins in the first chamber (I) and continues through the second chamber (II) to the nip of the rolls (62). Displacement washing of liquor from the pulp mat begins at the juncture of the first and second chambers and continues until the pulp mat enters the nip. By this process, the adjustment of temperature and chemical content is uniformly accomplished with a minimum loss of energy or materials.

Description: IMPROVED PULP BLEACHINGAPPARATUSAND METHOD This invention relates to the making of pulp for paper products and, more particularly to an improved method and apparatus for adding chemicals and/or adjusting temperature in a bleaching operation. BACKGROUND OF THE INVENTION Chemical removal of lignin from wood pulp is a well known process which is constantly being studied and incrementally improved. For high quality paper, it is important that delignification (or bleaching) be as complete as practicably attainable. One of the earliest methods for removal of residual lignin remaining in the pulp after it has passed through a digesting stage involves the use of chlorine as the bleaching chemical for the first bleaching stages. Apart from the corrosive tendency of chlorine, it is difficult to remove from the pulp liquor and may contribute to harmful emissions from the mill. Accordingly, a great deal of effort has been concentrated on attempting to replace chlorine based bleaching reagents using a variety of alternative processes, including oxygen bleaching, which produce more benign byproducts. Oxygen bleaching is performed on pulp of high and medium consistency. In addition, if chlorine is totally eliminated, oxygen bleaching may be followed by an ozone bleaching process. For ozone to be most effective and to minimize pulp degradation, it is best to achieve maximum bleaching before the ozone stage by high consistency oxygen delignification, for example. It is very difficult to add or extract chemicals and/or heat in a high consistency (27%-45%) pulp. Sodium hydroxide, or some other alkaline compound, is required by the oxygen delignification process to adjust the pH of the pulp. Uniform distribution of temperature and of the alkali added is especially difficult to achieve in high consistency pulp. Lowering the consistency of the pulp, to obtain a more uniform distribution of the chemical, wastes expensive chemicals because dewatering of the pulp removes dissolved chemicals as well and transfers a portion upstream in the process along with extracted liquor. After oxygen delignification, the pulp may be exposed to an ozone bleaching step. Ozone bleaching has markedly different requirements from those of the oxygen bleaching process. For example, oxygen bleaching is most efficient at high temperatures (approximately 190 degrees F) and high pH (8-9), while ozone bleaching is most efficient at low pH and a generally low temperature (120 degrees to 140 degrees F). Thus, the pulp must be acidified and cooled, with the same concerns for economy and efficiency, after exiting the oxygen delignification stage. The foregoing illustrates limitations known to exist in present day bleaching methods and apparatus. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features and advantages more fully described hereinafter. SUMMARY OF THE INVENTION In one aspect of the invention, this is accomplished by a method for uniformly adjusting chemical content and/or temperature of a wood pulp slurry in a pulp bleaching system by introducing pulp into a liquid filled wash press which has provisions for displacement washing and for extraction of liquid from the pulp. Liquid is extracted from the pulp to form a pulp mat in a first chamber, and the pulp mat is conveyed into a second chamber. A heated or cooled chemical solution is introduced into the second chamber in an amount and at a concentration and temperature sufficient to perform displacement washing and to bring the pulp mat to a predetermined temperature and chemical content. Liquid is displaced from the pulp mat by extracting liquid introduced with the pulp while simultaneously replacing the liquid with liquid from the second chamber to an extent required to yield pulp at a desired temperature and a desired chemical content, after which, liquid extraction is continued in order to bring pulp consistency to a desired level for discharge from the washing press. The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing a washing press used to add chemicals, to increase consistency of the pulp, and/or to adjust temperature of the pulp at the end of a bleaching stage; Fig. 2 is another schematic diagram of the washing press as applied in another bleaching stage in a pulp making process. DETAILED DESCRIPTION Fig. 1 shows a washing press in which the chemical content and temperature of a pulp slurry is uniformly adjusted while the consistency of the slurry is increased. This press and the method of its operation promote uniform mixing of the chemical added with the pulp slurry, uniformity of pulp temperature, and minimum waste of the chemical. Washing press 100 contains two identical rolls 62, mounted on shafts 62b, all within a tank 64. Tank 64 has an arcuately shaped nip wedge 63 to decrease volume of chamber II of tank 64 and to train pulp into the nip between rolls 62. The press further comprises two baffles 66, substantially separating chambers, I and II. Streams 10, 15, and 17 combine in mix chest 60 to form stream 70 and to enter tank 64 in the chamber I between rolls 62 and baffles 66. Streams 14 are also shown as entering tank 64 in the chambers II between baffles 66 and tank 64. In operation, the streams 10 enter mix chest 60 where the consistency is reduced by the addition of liquor from streams 15 and 17. Stream 15 contains a part of the effluent stream 11 from rolls 62. Stream 14 is a solution of chemical 16, like sodium hydroxide, mixed with wash liquor from stream 19. A portion of wash liquor 19 is preferably mixed with chemical 16 to produce a chemical solution stream 14 which is then supplied to tank 64. The chemical material which is supplied to chamber II of stream 14 is of a sufficient strength and amount to provide the output pulp 13 from washing press 100 with a sufficient amount of chemical to satisfy downstream bleaching operations. For the particular example of alkali addition, the downstream operation would be an oxygen stage. It is well known that, for oxygen to react with lignin, typically 1-4% by weight of caustic soda (sodium hydroxide) has to be added to the pulp. In operation, an approximately 4% consistency pulp stream is provided from chest 60 as feed stock to both sides of the tank 64 along with the chemical solution stream 14. Consistencies are used for illustrative purposes to indicate the desirability, at various process stages, of lesser or greater consistencies. Rolls 62 are counter- rotating and will produce an output pulp mat 13 having a consistency of approximately 28%, or greater - a high consistency pulp mat. The functions within tank 64 take place under differential pressure with pressurized feed stock, stream 70 from chest 60 and stream 14 injected into tank 64, for liquid extraction and displacement washing of the feed pulp slurry. Liquid extraction is driven by pressurized slurry in chambers I and II losing liquor to the relatively low pressure within perforated rolls 62. Properly applied, pressure or vacuum will drive the operation with equal effectiveness. Bleaching systems of the prior art, have introduced the required chemicals, e.g., alkali materials for a subsequent oxygen delignification stage, either into mix chest 60, or into the high consistency output stream 13 of the extraction press. While addition of chemical to a mix chest before the extraction press promotes good mixing of the chemical with the pulp with relatively low power cost (because only a 4% consistency solution resides within the mix chest 60), it leads to an excessive loss of chemical; because some of the chemical is lost upstream with extracted liquor. The recycle line 21 will contain about 50% of the lost chemical. If chemicals are added to the high consistency pulp output stream, there are no losses of chemical, but it is very difficult to obtain a uniform chemical distribution in the pulp. This results in non- uniform bleaching results. In short, supplying the required chemical to the low consistency pulp at the inlet of the extraction press, or supplying the chemical to the output stream thereof, has two different but undesirable consequences. This invention follows neither prior art approach but instead concentrates on where and how the pulp and chemicals are combined for the best results. As applied in this invention, baffles 66 are mounted on pivot points 66a and have mechanical positioning devices 66b which establish a "no load" baffle position. In operation of wash press 100, a pulp mat forms on rolls 62 by liquor extraction through the porous roll surfaces. Compaction baffles 66 gently iron the pulp mat but, more importantly, separate mat formation chamber I from chemical and temperature adjusting solution contained in chamber II. The space between the periphery of roll 62 and baffle 66 defines chamber I, into which is directed a preferably pressurized stream 70 of pulp slurry. Chamber II is defined between baffle 66, tank 64, and the arcuately shaped nip wedge 63. A chemical containing input stream 14, pressurized as is stream 70, is introduced into chamber II and comes in contact with the pulp mat as the mat emerges from Chamber I (behind baffles 66). The chemical containing solution of Chamber II begins to displace liquor from the pulp mat, after the pulp mat passes the end of baffles 66, and continues to do so until the pulp mat enters the nip between rolls 62 and above nip wedge 63. The pulp slurry stream 70 is directed into the first chamber I which, at its throat, would typically have a width of, approximately 20 to 80 millimeters, while the distance between baffles 66 and rollers 62, at the juncture of chambers I and II, would be on the order of 6 millimeters. Thus chamber I exhibits a continuously decreasing width as the pulp mat advances to the end of baffles 66. The chemical containing stream 14 is introduced into chamber II, which is separated from chamber I by the compaction baffles 66. The stream 14 enters chamber II, joins with the pulp mat as it emerges from chamber I, and displaces liquor from the pulp mat until the mat enters the nip between rolls 62 just above nip wedge 63. After the respective rolls 62 have advanced sufficiently, the mat is discharged in stream 13 at high consistency (approximately 28%), at the required temperature, and having the required chemical content uniformly distributed therein. Note that drainage of the chemical solution through the perforated periphery of the rolls 62 is minimized, if not totally avoided, by exposing the mat and the chemical to one another only after the mat has already been formed by extraction of a substantial fraction of the pulp liquor and compacted by baffles 66. Thus, there is negligible dilution of the chemical solution, during displacement of pulp liquor by the chemical solution, due to the less-than-one ratio of the volume of chemical solution added to the liquor remaining in the mat as it leaves the press. Also, there is some resistance to penetration of chemical solution through the compacted mat. Slurry stream 70 is fed into washing press 100 at a pressure between 3 psi and 15 psi. This pressure is generally uniform throughout the press, except in the washing chamber II, where it is usually higher, and continually drives the liquid extraction due to the pressure differential through the surfaces of rolls 62. As the pulp layers formed on rolls 62 approach the nip of rolls 62, the mechanical pressure on the pulp mat increases significantly due to the convergence of nip wedge 63 and rolls 62 and because the rolls 62 are urged against each other at very high pressure. The liquor forced by hydrostatic pressure through the perforations into the interior of the rolls is drained away to seal tank 11. The loss of chemical material in the present process and apparatus is significantly reduced because only a small part of the original pulp liquor has to be displaced by the chemical solution. The balance of the liquor has been removed prior to contact of the pulp mat with the chemical solution. By this process, most of the liquor is removed from the pulp during formation of the pulp mat in chamber I and is not available in chamber II for intermixing with the chemical solution and for dilution thereof. The amount of chemical introduced into chamber II of wash press 100 is generally around 2.5% sodium hydroxide if the high consistency pulp discharge stream 13 is to be introduced into an oxygen reactor. In Fig. 2, the 28% consistency slurry from output stream 13 of the washing press enters an oxygen reactor 82. The pulp slurry effluent 86 from oxygen reactor 82, together with liquors flowing from lines 96 and 95, all of them feeding into mix chest 60 and from there into inlet box 88, create a 4% consistency pulp slurry which has to be acidified and cooled in preparation for ozone reactor 114. Again, a washing press is shown at 90, and it will drain liquor through line 92 into seal tank 6. Ozone bleaching requires a lower temperature for reaction than does oxygen bleaching and requires an acidic pulp, as opposed to an alkali pulp. Thus, acid (from a source not shown) is conveyed via conduit 140 to blend with wash liquor from conduit 120, which may be fresh water or liquor from a succeeding stage and which is passed through a heat exchanger 98 for cooling as required. The combined flow from conduits 120 and 140 is provided, via conduit 102, to a washing press 90, identical in operation and structure to that which has been already described. This time washing press 90 produces a very high consistency pulp slurry at approximately 35-45% consistency. Also the chemical solution, in this case acidic, as before is provided in one chamber, while the slurry is provided in another chamber. After a mat of pulp has been formed on the rolls in the first chamber, the mat is exposed to the chemical solution in the second chamber where the chemical solution displaces the liquor from the mat to adjust the pH and the temperature mat. After passing through the nip of the rolls the output slurry is conveyed to an ozone reactor 114. It should be noted that the wash liquor enters the washing press 90 in such a manner that only that portion which is to be combined with the acid, passes through heat exchanger 98, while the majority of the flow bypasses the heat exchanger as well as the acidification step and goes directly to mix chest 60. A comparison of the apparatus and techniques used to produce a relatively high consistency (28%) pulp mat with a relatively uniform alkali distribution throughout the pulp, at a relatively high temperature for the oxygen bleaching process, with the apparatus and technique used for producing a high consistency (approximately 35%) pulp mat with a relatively uniform acid distribution at relatively lower temperatures for the ozone bleaching process, shows that all objectives are achieved and the same economies are obtained in both examples by the same methods and apparatus.

Claims: What is claimed is: 1. A method for uniformly adjusting chemical content and/or temperature of a wood pulp slurry in a pulp bleaching system, comprising: introducing the pulp into a liquid filled washing press, said washing press having means for displacement washing and for extraction of liquid from said pulp; extracting liquid in a first chamber of said washing press to form a pulp mat; conveying said pulp mat into a second chamber; introducing into the second chamber a chemical solution at a concentration and temperature sufficient to perform displacement washing and to achieve a desired chemical content of said pulp mat and/or to cool said pulp mat to a predetermined temperature; displacing liquid from said pulp mat by simultaneously extracting liquid introduced with said pulp and replacing said liquid with liquid from said second chamber to an extent required to yield pulp at a desired temperature and a desired chemical content; and further extracting liquid from said pulp to increase pulp consistency to a desired level for discharge from said press. 2. The method of claim 1, further including the step of discharging pulp which is impregnated with the chemical solution and which has a consistency greater than 25%. 3. The method of claim 1, wherein the bleaching system incorporates oxygen bleaching, and the chemical solution is an alkali. 4. The method of claim 1, wherein the bleaching system incorporates ozone bleaching, and the chemical solution is an acid. 5. The method of claim 1, wherein both the pulp and the chemical solution are pressurized prior to their introduction into the liquid filled washing press. 6. The method of claim 1, further comprising the step of: adding a second bleaching stage to the bleaching system, and wherein the chemical solution comprises an alkali for one of the stages and an acid for the other stage. 7. The method of claim 1, wherein the liquid from said second chamber raises the temperature of said pulp. 8. The method of claim 1, wherein the liquid from said second chamber lowers the temperature of said pulp. 9. The method of claim 1, wherein the amount of liquid from said second chamber added to said pulp mat is less than 80% of the total liquid remaining in the pulp stream leaving the washing press. 10. The method of claim 1, wherein the amount of liquid from said second chamber added to said pulp mat is between 45% and 75% of the total hquid remaining in the pulp stream leaving the washing press. 11. In a pulp bleaching system, the improvement comprising: means for introducing a lignin containing pulp into a liquid filled washing press, the washing press including a first chamber and a second chamber, with the pulp being introduced into the first chamber of the washing press; means for creating a pulp mat in the first chamber while liquid is simultaneously extracted from the pulp; means for introducing into the second chamber of the treatment zone a chemical solution at a temperature and in an amount sufficient to achieve a desired chemical content of said pulp and to bring said pulp to a predetermined temperature by displacing liquid from the pulp layer with the chemical solution from the second chamber wherein liquid extraction continues and a chemical content and temperature of the pulp layer are adjusted. 12. The improvement of claim 11, further including means for discharging pulp which is impregnated with the chemical solution and which has a consistency greater than 25%. 13. The improvement of claim 11, wherein the bleaching system incorporates oxygen bleaching, and the chemical solution is an alkali. 14. The improvement of claim 11, wherein the bleaching system incorporates ozone bleaching and the chemical solution is an acid. 15. The improvement of claim 11, wherein both the lignin containing pulp and the chemical solution are pressurized prior to their introduction into the liquid filled washing press. 16. The improvement of claim 11, further comprising: a second bleaching stage, wherein the chemical solution comprises a cold alkali for one of the stages and a hot acid for the other stage. 17. The improvement of claim 11, wherein the first and second chambers are located within a washing press having a roll, the roll being located at least partially within a tank, and the tank having an arcuately shaped bottom. 18. The improvement of claim 17, wherein the first chamber is located between a periphery of the roll and a baffle. 19. The improvement of claim 17, wherein the second chamber is located between the baffle and the arcuately shaped bottom of the tank. 20. The improvement of claim 17, wherein the second chamber also extends beyond the baffles between the periphery of the roll and the arcuately shaped bottom of the tank.

Inventors: LUTHI OSCAR, LUTHI, OSCAR
Applicants: INGERSOLL RAND CO, INGERSOLL-RAND COMPANY
Application year: 1993
Main classification: D21C9
IPC codes: D21C9