Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor

Definitions

• Pulping is the changing of wood chips or other wood particulate matter to fibrous form. Chemical pulping requires cooking -of the chips in " solution with a chemical, and includes partial removal of the coloring matter such as lignin associated with the wood. Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.
• Consistency is the amount of pulp fiber in a slurry, expressed as a percentage of the total weight of the oven dry fiber and the solvent, usually water. It is sometimes called pulp concentration.
• the consistency of the pulp will depend upon the type of dewatering equipment used. The following definitions are based on those found in Rydholm Pulping Processes, Interscience Publishers, 1965, pages 862-863 and TAPPI Monograph No. 27, The Bleaching of Pulp, Rapson editor, The Technical Association of Pulp and Paper Industry, 1963, pages 186-187.
• Tank 22 may be a diffusion washer instead of a storage tank. Diffusion washers are described in Rydholm Pulping Processes, Interscience Publishers, 1965, pages 725-730 and illustrated in Figure 10.14 on page 728. The diffusion washer would be followed by a storage tank.
• the material passing through the blow line is a slurry which contains the remaining lignin and carbohydrates, "the spent digestion chemicals, and the fibers formed from the chips as they are blown from the digester.
• the chips will be formed into fibers when the pressure on the chips is partially released, usually at the outlet of digester Ik.
• the slurry will still be under some pressure to move it through the blow line. If the digester is continuous, then additional fiberizing may be done by a refiner, or refiners, in the blow line.
• the refiners will fiberize the large particles that have not been reduced to fibers earlier in the process.
• two refiners - 18 and 19 - are shown.
• the first refiner 18 does course refining and the second refiner 19 does fine refining.
• the refiners are optional. They are usually encountered in a linerboard mill.
• the digester in a bleached pulp mill normally would not have refiners in the blow line. Neither would they be used with a batch digester.
• the fibers and liquor are carried by pump 23 through line 2k to the washers and screens.
• the system will be described by following the pulp through the system, and then following the wash water through the system.
• the pulp mat 33 is removed from the face of the drum 32 by a doctor blade, carrier wires or strings between the drum and the mat, rolls or any other standard manner and carried to the vat 50 of the second brownstock washer 51. Again, the fibers are picked up on vacuum drum 52.
• the pulp mat 53 is washed with still weaker filtrate, removed from the vacuum drum 52 and carried to the vat 70 of brownstock washer 71.
• the operation of this washer is the same as the others, the vacuum drum being 72 and the mat 73.
• the mat 73 is carried to the vat 90 of the last brownstock washer 91. Again, the operation of this washer is the same as the others, the vacuum drum being 92 and the mat 93.
• the pulp mat 93 is carried to storage tank 110 with the aid of thick stock pump 96.
• the pulp is diluted and then carried through line 111 by pump 112 to screens ID in which the larger fiber bundles and knots are removed.
• the bundles and knots 114 are carried to further treatment by suitable transpor ⁇ tation means.
• the pulp 115 is carried from the screens 113 to the vat 120 of decker 121 in which additional water is removed.
• the operation of the decker is similar to that of the washers. Washing showers may or may not be used in the decker.
• the vacuum drum is 122 and the pulp mat is 123.
• the pulp 123 is carried by thick stock pump 126 to a high-density storage tank
• the liquor or filtrate from the vat 120 and the mat 123 flows through piping which extends radially from the vacuum chambers at the surface of the vacuum drum 122 to a pipe in the central shaft of the rotating drum. This liquor or filtrate passes through the central pipe and an external line 128 to a filtrate storage tank or seal tank 129.
• the tank 129 is called both a storage tank and a seal tank because it acts both to store the filtrate for further use and to seal the vacuum drum 122 from the outside
• the filtrate from tank 129 may be handled in several ways. Several of the uses may occur simultaneously. Although the following description is specific to the effluent from tank 129, it is also illustrative of how the effluent from any of the washers in brownstock washing system 28 would be handled.
• the filtrate from tank 129 is reused to reduce the consistency of the pulp slurry either entering the decker 121, entering the screens 113 or leaving storage tank 110.
• Line 130 carries the filtrate to lines 131, 133 and 135.
• Line Dl and pump D2 carry the filtrate back to screened pulp 115 to reduce the consistency of the pulp slurry entering vat 120 to around 1-1/2%.
• Line 133 and pump 134 carry the filtrate back to line 111 to reduce the consistency of the pulp slurry entering the screens ID to from 0.2 to 2%.
• Line 135 and pump D6 carry the filtrate back to storage tank 110 to reduce the consistency of the pulp slurry leaving the tank to around 5%.
• the filtrate not reused for dilution may be taken to an effluent treatment system by line 130 and effluent line 29.
• This treatment may include combining the effluent with the effluent in line 16, or carrying the effluent directly to the cooking liquor recovery system. It should be understood that in a batch digester system the digester effluent is recovered completely from the brownstock washing system while in a continuous digestion system only a portion of the digester effluent would be recovered from the brownstock washers.
• the filtrate from tank 129 may be used as wash water in the brownstock washing system 28 in a counterflow washing system. In this system, the filtrate flow is counter to the flow of pulp.
• the line 137 and pump 138 carry the filtrate back to brownstock washer 91 for use as wash water.
• the filtrate is sprayed on the pulp mat by washer heads 95 and displaces the liquor within the mat.
• This filtrate may also be sprayed on the carrier wires, strings or rolls after the pulp mat is separated from them to remove any pulp fibers that cling to the wires, strings or rolls if water instead of air is used for this operation. This is done by cleanup washer 94. Additional water may be required to supplement the filtrate. This is provided through process water line 97.
• the flow of filtrate through brownstock washer 91 is the same as the flow through decker 121.
• the liquor, either from the mat or the vat, is carried through internal piping to line 98 and through line 98 to filtrate storage tank or seal tank 99. Again, the filtrate from the seal tank 99 may be handled in a number of ways. Line 100 would carry it to effluent line 29.
• Line 101 and pump 102 would carry the filtrate to pulp 73 to reduce the consistency of the pulp slurry to 1 -1/2 to 3-1/2% as it enters vat 90.
• Line 103 and pump 104 would carry the filtrate " to brownstock washer 71 to be used as wash water.
• the process in brownstock washers 71, 51 and 31 are, for the most part, identical to the process in brownstock washer 91 so the parts are similarly numbered.
• the washer heads are 75, 55 and 35 respectively.
• the cleanup washers are 74, 54 and 34 respectively.
• the filtrate lines are 78, 58 and 38 and the filtrate storage or seal tanks are 73, 59 and 39.
• the filtrate lines from the seal tanks to effluent line 29 are 80, 60 and 40.
• the lines and pumps carrying the filtrate to the pulp to reduce the consistency of the slurry entering a vat are 81 and 82, 61 and 62, and 41 and 42, respectively.
• the counterflow wash water lines and pumps are 83 and 84, and 63 and 64.
• line 43 and pump 44 carry the filtrate into storage tank 22 to reduce the consistency of the pulp slurry in the bottom of the tank to 2 to 3-1/2% before it exits the tank.
• the washed pulp which has passed through the brownstock washing system 28, the screens 113 and decker 121 remains in storage tank 140 until it is carried into the bleaching system.
• the first stage is chlorine and subsequent stages use chlorine dioxide, hydrogen peroxide, or a hypochlorite. These stages are interspersed with alkali extraction stages.
• the slurry in line 158 is combined with additional water or filtrate to reduce the consistency to about 1 to 1-1/2%.
• This dilute slurry flows into vat 160 of washer 161.
• a vacuum drum washer or filter is shown. The operation of this washer is the same as that of the brownstock washers.
• the vacuum drum 162 revolves through the vat. The vacuum pulls the fibers in the slurry onto the outer filter .surface of the drum and holds them against the surface, forming a mat, while pulling the liquid or filtrate through the filter cloth to the interior piping of the vacuum system in the drum to be discharged as effluent.
• the revolving drum 162 carries the fiber mat from the vat past a bank of washer heads which spray water or weak filtrate onto the mat to displace reaction products and unreacted chlorine entrained in the mat.
• the pulp mat 163 is removed from the face of drum 162. The means of removal is the same as in the brownstock washers - a doctor blade, carrier wires or strings between the drum and the mat, rolls or in any other standard manner.
• the pulp mat 163 is moved to mixer 166. This movement usually is by gravity fall through a chute from the washer to the mixer. Prior to leaving washer 161, the pulp mat 163 is impregnated with the caustic or alkali extraction solution from line 167. A sodium hydroxide solution is usually used.
• the pulp is then moved to steam mixer 186 of the chlorine dioxide stage. This transfer may again be by gravity drop through a chute.
• the mat 183 Prior to leaving washer 181, the mat 183 is treated with a slight amount of alkali from line 187.
• a sodium hydroxide solution is usually used. It is added to the mat at a point on the drum which will allow the solution to stay in the mat and not pass into the filtrate.
• the purpose of this treatment is not further extraction but adjustment of the pH of the pulp prior to being treated with chlorine dioxide.
• the pH of the pulp should be in the range of 5 to 7, preferably 6, for optimum brightness when bleaching with chlorine dioxide.
• the alkali may be added in the steam mixer 186 instead of the washer 181.
• the pulp 183 is mixed with steam from line
• the pulp will have a consistency of approximately 1% less than from the washer when it leaves a steam mixer.
• the slurry is carried from dilution zone 234 through line 235 by pump 236. During its travel through line 235, the pulp is again treated with additional sulfur dioxide or alkali from line 238 to remove any free chlorine dioxide and is further diluted so that the slurry is at a consistency of about 1 to 1-1/2% when it reaches vat 240 of washer 241. It is picked up by vacuum drum 242, washed and discharged from the bleaching system as bleached pulp 243.
• the passage of liquid through the washer is the same as in the brownstock washers. Wash water is sprayed onto the mat by the washer heads. This water displaces the entrained liquid within the pulp mat on the drum. The displaced liquid is carried through piping internally of the rotating vacuum drum to a pipe in the central shaft of the drum. Here, it is combined with the liquor being pulled into the drum from the washer vat. This combined liquor passes outwardly through a central pipe in the drum and an external line to a seal or storage tank which maintains the vacuum in the drum by providing a seal between the vacuum inside the drum and the ambient pressure externally of the drum.
• the filtrate is also supplied to dilute the slurry in "the dilution zone of the tower in the stage.
• the filtrate from seal tank 273 is carried into the dilution zone 174 by line 281 and pump 282.
• line 301 and pump 302 carry the filtrate from the seal tank 293 into the dilution zone 194.
• line 321 and pump 322 carry the filtrate from the seal tank 313 into dilution zone 214
• line 341 and pump 342 carry the effluent from the seal tank 333 into dilution zone 234.
• Methods of measuring the practicality and efficiency of a* pulping or a bleaching process are the pulp yield, the physical properties of the pulp, the degree of pulp delignif ication, the pulp brightness, and the cost of obtaining the pulp.
• pulp bright ⁇ ness There are also a number of methods of measuring pulp bright ⁇ ness. It usually is a measure of reflectivity and its value is expressed as a percent of some scale. A standard method is GE brightness which is expressed as a percentage of a maximum GE brightness as determined by TAPPI Standard Method TPD-103.
• a special oxygen reactor design is shown in Jamieson U.S. Patent No. 3,754,417, issued August 28, 1973.
• the reactor has a series of trays and the pulp slurry cascades from one tray to another.
• the oxygen or air is above the tray, and the slurry on the tray is agitated. Kirk et al., "Low Consistency Oxygen Deiignification in a
• the oxygen reactor used in the commercial version of this sytem is described in Verreyne, et al. U.S. Patent No. 3,660,225 granted May 2, 1972.
• the reactor is complex, having individual trays for each layer of pulp.
• the pulp consistency is between 16 and 67%.
• An example gives the height of pulp in the reaction vessel as 15 meters, the reaction time as 30 minutes, and the pressure as 150 psig.
• the reactor is a large, costly pressure vessel.
• the Billeruds system is described in U.S. Patent No. 4,004,967, issued January 25, 1977. This also is a high-consistency, high-pressure system.
• Hirtie 23 (2), 58-62 (1974) describes AODED, COD, CODED and OCDED sequences.
• the swept area should be in the range of 10,000 to 1,000,000 square meters per metric ton of oven dry pulp. They determined that within this range there was a range of 25,000 to 150,000 square meters per metric ton of oven dry pulp which had certain character ⁇ istics that were better: less power was required or the kinetics of the reaction were substantially better. The optimum swept area is around 65,400 square meters per metric ton of oven dry pulp.
• the rotors in the mixer preferably have leading and trailing edges, each with a radius of curvature of 0 ' 5 to 15 mm, and an elliptically generated cross section.
• the oxygen is introduced into the mixing zone through the stators.
• the inventors decided to investigate both the need for costly expenditures and for lengthy times in which to do oxygen bleaching. They decided to add oxygen to an existing system and determine the results.
• the oxygen may be added into an extraction stage, between washers, between a washer and a subsequent storage tank or in the blow line of a continuous digester, after the pulp is washed in the digester. Alkali, steam and oxygen are added to the blow line and the oxygen treated with the pulp.
• the blow line may carry the pulp to either a storage tank, a diffusion washer or other processing. These are not a necessary part of the oxygen treatment.
• O-X-O and O-O-X-O in which X is chlorine, chlorine dioxide, a combination of chlorine and chlorine dioxide, a hypochlorite, a peroxide or ozone.
• X is chlorine, chlorine dioxide, a combination of chlorine and chlorine dioxide, a hypochlorite, a peroxide or ozone.
• the sequence may be followed by a D step.
• the mixer was originally designed to overcome a problem in the oxygenation of pulp, it is also useful for noncondensable gases such as ozone, air, chlorine, chlorine dioxide, sulfur dioxide, ammonia, nitrogen, carbon dioxide, hydrogen chloride, nitric oxide and nitrogen peroxide. These gases may also be described as unsaturated in that they will not condense into liquid but will be superheated even after contact with the pulp.
• the mixer may also be used to mix highly superheated steam with the pulp.
• Figure 1 (A-C) is a diagram of a prior art pulping and bleaching process.
• Figure 2 is a diagram of the present oxygen system used in the blow line in conjunction with a refiner..
• OMPI Figure 3 is an oxygen diffuser used with a refiner to add the oxygen to the refiner.
• Figure 7 is a diagram of a prior art oxygen bleaching system.
• Figure & is a diagram of the present oxygen bleaching system.
• Figure 9 is a diagram of the present oxygen system in an extraction stage.
• Figure 10 is a diagram of the present oxygen system between washers.
• Figure 11 is a diagram of the present oxygen system between a washer and storage.
• Figure 15 is a diagram of a pulping and bleaching process using the present systems.
• Figure 16 is an isometric view of a mixer that may be used in the present invention.
• Figure 17 is a side plan view of the mixer shown in Figure 16.
• Figure 18 is a cross section of the mixer taken along line 18-18 of Figure 17.
• Figure 19 is a cross section of the mixer taken along line 19-19 of Figure 18.
• Figure 20 is a plan view of a rotor.
• Figure 21 is a cross section of the rotor taken along line 21-21 of Figure 20.
• Figure 22 is a plan view, partially in cross section, of a modified rotor.
• Figure 23 is a cross section of the modified rotor taken along line 23-23 of Figure 22.
• OMPI Figure 24 is a plan view, partially in cross section, of a stator which may be used with the mixer.
• Figure 25 is a side plan view, partially in cross section, of a modified stator taken along a line corresponding to line 25-25 of Figure 24.
• Figure 26 is a cross section of the stator taken along line 26-26 of Figure 24.
• Figure 27 is a cross section of a valve taken along line 27-27 of Figure 25.
• Figure 28 is an isometric view of a modified mixer.
• Figure 29 is a side plan view of the mixer of Figure 28.
• Figure 30 is a cross section of the mixer taken along line 30-30 of Figure 29.
• Figure 31 is a cross section of the mixer taken along line 31-31 of Figure 30.
• Figure 32 is a cross section of a rotor used in the reactor of
• Figure 33 is a cross section of the rotor taken along line 33-33 of Figure 32.
• Figure 34 is a graph comparing two mixers.
• Figure 35 is a cross section of a modi ied mixer.
• Figure 36 is a cross section of the modified mixer taken along line 36-36 of Figure 35.
• Figure 37 is an enlarged cross section of the interior of the mixer shown in Figure 35. DESCRIPTION OF THE PREFERRED EMBODIMENT
• Figures 2-5 show our invention applied in the blow line at the refiner.
• Figure 2 is a diagram of the process and Figures 3-5 are proposed changes to the refiner to add the oxygen to the pulp slurry nearer to the actual refining mechanism.
• the system shown in Figure 2 should be contrasted to the blow line oxygen system described in the Kamyr patent and article noted above. It should be remembered that the Kamyr system required a special upflow- downflow tower after the refiner in order to provide a retention time of at least 20 minutes after treating the pulp with oxygen. In contrast to this, our system merely requires adding the oxygen, alkali and heat prior to a refiner in a standard continuous digester
• O ⁇ ylPI blow line refining system In a two-refiner system, the second refiner is preferable because there are more individual fibers in this stage. This is the system shown in Figure 2.
• the reference numerals are identical to those found in Figure 1, 10' being the incoming chips, 11' being the process water, 12' being steam, 13' being the pulping chemicals, and 14* being the continuous digester.
• the chips 10 * may be treated prior to entering the digester 14' by presteaming or impregation with digestion chemicals or any other type of treatment.
• any type of pulping process may be used, and the pulping conditions for a particular process will depend upon the species of wood chip and the product desired. The pulping conditions and the amounts of chemical are well known.
• the digester 14' should be continuous because a major portion of the deiignification products should be removed prior to the oxygen treat- ment. Otherwise too much of the oxygen will be used in reaction with the deiignification products and not with the pulp fibers.-
• the washing stage of the continuous digester provides this washing.
• Reference numerals 15' and 16' refer respectively to t ⁇ e wash water entering and the effluent leaving the washing stage of the continuous digester.
• reference numerals 17*, 20', and 21' are the three sections of the blow line
• 18' and 19' are the two refiners
• 22' is the storage tank, or a diffusion washer and tank
• 23' and 24' are the pump and line carrying pulp from the tank 22' to further processing.
• the purpose of the present invention is to treat the washed pulp with oxygen with as little ch.ange to the equipment as possible.
• Sodium hydroxide and. steam are added to the pulp slurry in line 20' between refiners 18' and 19'.
• Sodium hydroxide which both adjusts the pH of the pulp and buffers the oxygen reaction, is added through line 25.
• Other suitable alkalies, such as white liquor, may also be used.
• Steam is added through line 26, The steam raises the temperature of the pulp to a temperature appropriate for the oxygen treatment.
• Oxygen is added to the pulp through line 27. In a two-refiner system, the addition of the chemicals and steam prior to the second refiner is preferable because there are more individual fibers in the second refining stage.
• Line 360' carries process water to lines 11' and 15'.
• Line 362' carries sodium hydroxide to line 25.
• Line 364' carries steam to lines 12' and 26.
• Line 366 carries oxygen to line 27.
• the alkali is used both as a digestion chemical and for the oxygen treatment, as in the soda process in which sodium hydroxide is used for both digestion and oxygen treatment, or the kraft process in which white liquor is used for both digestion and oxygen treatment.
• line 362' would also supply line 13'.
• the amount of oxygen used will depend upon the yield and K or Kappa number of the pulp to be treated, and the desired result of the treatment. Between 5 to 50 kilograms of oxygen per metric ton of oven-dry unbleached wood pulp is required for the oxygen treatment.
• high Kappa number pulp of the type usually used for linerboard the purpose of the oxygen treatment is to improve certain properties of the product.
• the blow line and brownstock Kappa number for this pulp is usually around 80 to 120. This allows the mill either to increase certain property values of the product at the same pulp yield or to maintain the property value while increasing the yield.
• high Kappa number pulp will either increase the ring crush of a liner prepared from the pulp or maintain the ring crush at the same value and increase the yield. Ring crush is determined by TAPPI Standard T 818 OC-76.
• the pH for any oxygen treatment in any environment should be between 8 and 14. In this environment, the amount of alkali, expressed as sodium hydroxide, required to obtain this pH is between 0.25 to 8% of the oven-dry weight of the unbleached wood pulp.
• the temperature for any oxygen treatment in any environment is usually between around 65° C to around 121°aC The usual oxygen stage temperatures are around 82°C to around 99°C. However, the actual temperature in any oxygen stage will depend
• OMPI upon the ability to heat the pulp, so it may vary from around 65°C to around 121°C depending on the location of the oxygen stage * in the system.
• the pulp from the digester may be at the temperature required for the oxygen reaction, li not, the pulp would be heated to either adjust it to or maintain it at the temperature required for the oxygen reaction during the mixing step.
• A is in the blow line 17' after the continuous digester 14'; B is in the blow line 21' after refiner 19'; and C is in the outlet from storage tank 22'. These are the three points at which samples were taken and tested in a mill trial of this system.
• the system was first tested for a period of approximately 6 hours to determine the amount of residual lignin in the unbleached pulp at point A and point C to determine if there were any bleaching e fects in a standard system.
• Samples were taken at points A and C at the time intervals indicated in Table I, and the Kappa number of each of the samples was determined.
• the accuracy of the measurements of these discrete samples was checked during the test by taking a number of samples, averaging the Kappa numbers of these samples and comparing this average to the Kappa number of the discrete sample.
• the A Avg and C Avg Kappa numbers in Table I, and the discrete sample Kappa numbers are within experimental accuracy.
• the various Kappa numbers for the samples of unbleached pulp are shown in Table I. TABLE I
• the oxygen flow varied from a low of 6.7 kilograms per oven-dry metric ton of unbleached pulp to a high of 40 kilograms per oven-dry metric ton of unbleached pulp.
• the average oxygen rate was 15 kilograms per oven-dry metric ton of unbleached pulp.
• Sodium hydroxide was added at a rate of 64 kilograms per oven-dry metric ton of unbleached pulp, and the steam was added at a rate of 612 kilograms per oven-dry metric ton of unbleached pulp.
• the pulp entered the second refiner 19' at a temperature of 70-97°C, a pressure of 621 kPa gage and a pH of 12.5. Pulp samples were again taken at points A and C at the time intervals indicated in Table II and the Kappa numbers of the pulp sample
• OMPI measured. This was to determine whether the oxygen bleached the pulp.
• Kappa numbers are given in columns A, and C, in Table II.
• the Kappa number at point C was, on the average, 7.5 points less than the Kappa number at point A during the oxygen treatment, showing that bleaching occurred.
• the first Kappa number at B, 28.5 is the same as the corresponding number at C j
• the second Kappa number B, 31.4 is almost the same as the corres ⁇ ponding number at C 2 , 31.5. Consequently, these tests have shown that bleaching occurs in the refiner 19' between the oxygen addition and point B.
• the brightness of samples taken at points A and C was also checked.
• the average brightness of A 2 pulp samples " was 18.9 and of C 2 pulp samples was 22.3.
• the shives, unbroken fiber bundles, were measured at points A and C.
• the average shive content was 2.2 percent of the oven-dry weight of the pulp at point A, and 0.67 percent of the oven-dry weight of the pulp at point C.
• the filtrate solids in the blow line material were 3.6 percent of the oven-dry weight of the pulp.
• the physical properties of the pulp were also tested. These properties were freeness, burst, tear, fold, breaking length, density and viscosity.
• the results of these tests are given in Table in. Three sets of data are given. The first set is for an average of all the bleached pulps tested. The second set is for a specific sample of pulp. The third set is for a control and is an average of tests of unbleached pulps produced on the apparatus before and after the bleaching trial.
• Breaking Length Initial 2700 m 2700 m 2700 m Breaking Length (3 550 CSF 7700 m 7300 8000 m Breaking Length (3 400 CSF 8700 m 7800 m 8500 m Breaking Length (3 250 CSF 9000 m 8200 m 8900 m
• the treated pulp had a Kappa number of about 65. It was compared to a kraft pulp having a 58 Kappa number. The tests were at 675 Canadian Standard Freeness. The oxygen treated pulp had a ring crush 15% greater than the kraft pulp and a burst 2% greater than the kraft pulp.
• the oxygen application may be from 12 to 50 kilograms per metric ton of oven- dry pulp.
• the alkali addition, expressed as sodium hydroxide, would normally be from 3.6 to 4.9% and the temperature would normally be from 82 to 95°C.
• a slight amount of protector might be used. This would not exceed 0.5% based on the weight of the oven-dry pulp.
• the final product would have a Kappa ranging from 65 to 69; a ring crush, compared to a kraft pulp, of from 3% less when yield is increased to 28% more if better properties are desired; and a burst, compared to kraft pulp, of the same number if yield is increased to 6%. greater if better properties are desired.
• Much of the treatment would occur in the mixer and a majority in the mixer and through to the back pressure valve or top of the pipe in the mixer outlet line.
• Figures 3, 4 and 5 show a unit that would provide better distribution of oxygen.
• Figure 3 shows the distribution unit by itself and
• Figures 4 and 5 show cross sections of refiners which include the unit.
• the unit 370 consists of an inlet sleeve 371 which fits into the refiner casing inlet and is ixed in place by bolts which extend through holes 372 in flange 373.
• Each diffuser has an inlet section 375 and an outlet section 376.
• the inlet section 375 extends radially along flange 373 and the outlet section 376 extends longitudinally of the sleeve 371.
• the tube may have any cross section.
• the inlet section 375 may be along the interior or exterior face of flange 373, be fitted in recesses in the interior or exterior face of flange 373, or be within and formed by the walls of the flange. In the latter design,
• an outlet tube would extend radially from the flange 373 as shown in Figure 2.
• the outlet section 376 could be affixed to the inner or outer wall of the sleeve 371, fitted into recesses in the inner or outer walls of the sleeve or be within and formed by the walls of the sleeve. They would be formed within and by the walls by casting when the sleeve and flange are formed or by drilling through the wall.
• the preferred form is shown in Figure 3.
• the inlet section 375 is formed in the flange and the outlet section 376 affixed to the inner wall of the sleeve.
• the diffuser will have one or more outlets for oxygen. Six diffusers should adequately disperse the oxygen in the pulp.
• the refiner 380 has an inlet 381, a screw conveyer section 382, a refiner section 383, and an outlet 384.
• the refiner shaft 385 is within the casing. Attached to the shaft are screw conveyer 386 and the revolving refiner member 387.
• the revolving refiner plate 388 is attached to member 387. Attached to the refiner casing 389 are the fixed refiner member 390 and the fixed refiner plate 391 which is aligned with revolving plate 388.
• the shaft 385, conveyer 386, revolving refiner member 387 and plate 388 are rotated by a suitable motor 392.
• the unit 370 would be part of the wear plate for the conveyer.
• the diffusers 374 would either be recessed in the sleeve 371 or formed in the sleeve as described above. This would allow the oxygen to be admitted after the conveyer section. Oxygen is fed to the diffusers through the oxygen manifold 394. The oxygen " enters the diffusers 374 through the manifold 394 and is added to the pulp after the conveyer 386. The oxygenated pulp leaves the refiner through the outlet 384.
• the blow line 20' is attached to inlet 381.
• Figure 5 shows the use of unit 370 in a refiner that does not have a conveyer section.
• the reference numerals are the same as in Figure 3.
• any type of refiner there is relative rotative movement between two opposed surfaces which are spaced to allow passage of material between them.
• Disc refiners are normally used because of the ability to change the clearance and pressure on the plates, depending on the furnish to the refiner and the end product desired. There are other types of refiners that may be used.
• the rotating disc In the usual double-disc refiner, the rotating disc has
• OMPI WI? ° refiner plates on both faces which act against opposing fixed plates.
• Another type of double-disc refiner has both refiner plates mounted on discs which rotate in opposite directions to provide both a rolling and an abrading action.
• the discs are mounted on separate shafts which may be concentric.
• a conical refiner may also be used.
• Mixer 116 may be a refiner such as the refiner shown in Figures 4 or 5.
• the refiner when stopped, may be used as a mixing device.
• the clearance between discs has been tested at around 13 mm and can be up to 75 mm. Consequently, the clearance may be from a few millimeters to around 75 millimeters. This narrow passage causes the pulp slurry and oxygen to mix.
• Another suitable mixer would be one that is relatively small and intensively mixes the pulp and gas. Several suitable mixers are described later in this application.
• a back pressure on the pulp in the mixer or refiner may be provided by an upflow line after the mixer which creates a hydrostatic head at the mixer.
• a pressure valve is preferred.
• the valve may be combined with the upflow line.
• the valve may be placed in the blow line 21' downstream of the refiner 19* or line 21' * downstream of the mixer 116.
• the valve may be either right after the mixer or refiner or at the top of the line before the outlet.
• the maximum pressure in the mixer would normally not exceed 830 kPa gage, and at the top of the pipe would normally not exceed 345 kPa gage.
• Figures 7 and 8 compare the size and complexity of a prior art oxygen bleaching system of the type shown in Verreyne et al, U.S. Patent No. 3,660,225 with the present system. Both drawings are to the same scale. Both units would handle the same amount of pulp in an oven-dry weight basis.
• pulp 400 from mill 401 is carried by pump 402 to a storage tank 403.
• storage tank 403 the pulp is mixed with an alkali solution 404 from filtrate storage tank 405.
• a protector would be added to the pulp at this time also.
• the treated pulp mixture 406 is moved by pump 407 to a dewatering press 408 which removes enough water from the pulp to raise the consistency of the pulp slurry to around 20-30%.
• This material is then carried by pump 409 to the top of the oxygen reactor.
• the pump 409 is a series of screw conveyers, the only way to pressurize pulp of this consistency.
• At the top of the reactor 410 is a fluffer 411 which spreads the pulp uniformly over the top tray 412 of the reactor.
• the pulp passes down through the other trays 413-416 and is treated with oxygen during its passage through the trays. From the bottom of the trays the bleached pulp 417 is carried to storage tank 418.
• Figure 9 shows the oxygen mixer in a standard caustic extraction stage of a bleaching system. It shows that a simple change can turn a caustic extraction stage into an oxygen treatment stage. To allow com ⁇ parison of this extraction stage with the same one in Figure 1, the same reference numerals have been used.
• the operation of the various pieces of equipment - the washers 201' and 221', the steam mixer 206', the extraction tower 213* and the seal tanks 293' and 313' - are the same as in the prior art extraction stage in Figure 1.
• the flows of pulp and wash water through the system are also the same as in Figure 1.
• the pulp 195' enters washer 201' where it is washed, dewatered and treated with alkali, usually sodium hydroxide.
• alkali usually sodium hydroxide.
• the consistency of the pulp leaving the washer is usually in the range of 8 to 15%.
• the exiting pulp 203' then Is mixed with the alkali and steam in steam mixer 206'. Pulp consistency is reduced about 1% in the steam mixer. From the steam mixer the pulp goes to extraction tower 213' where it remains for the usual period of time. It is diluted and carried to washer 221', where it is washed and dewatered.
• washer 221' may be a diffusion washer, it is shown and described as a vacuum or pressure drum washer.
• washer 221' the water is either fresh process water through line 310', counterflow filtrate through line 343' or a combination of these, and in washer 201' the wash water is either fresh process water through line 290', or counterflow filtrate through line 323', or a combination of these.
• the mixing produces an intimate contact between the gas and the slurry, and appears to divide the gas into mostly small bubbles. There may be some larger bubbles and gas pockets, however. The presence of some large bubbles and gas pockets up to the size of the pipe through which the pulp slurry was passing have been observed. These have not affected the quality of the pulp or the treatment of the pulp.
• a pressure valve is preferred.
• the valve may be combined with the upflow line.
• the valve may be placed in the line 209'B downstream of the mixer 211.
• the valve may be either right after the mixer or at the top of the line before the outlet. The maximum pressure in the mixer would normally not exceed
• the decker 121' has been converted to a washer by the addition of washer heads 125, a process water line 127 and a clean-up washer 124.
• the system has been further modified into an oxygen system by the addition of an alkali line 425, a steam mixer 426, a steam line 427, an oxygen mixer 428 and an oxygen line 429. These are placed between
• the time in this mixer, as in the oxygen mixer, is less than 1 minute, and normally would be only a few seconds. Pulp traveling at 18.3 meters per second would pass through an 2.4 or 3 meter long reactor in an exceedingly short time. The chlorine would be treated at the temperature of the pulp off the washer, 54 to 60°C, rather than the cooler chlorination temperature.
• bracket 437 There is one other change indicated by bracket 437. This is the addition of E and D stages at the end of process. Again, the process conditions for this last extraction stage are the same as those for the other extraction stages and for this last chlorine dioxide stage are the same as those for the other chlorine dioxide stages. It should also be realized that the only additional equipment required for these two stages are the two additional washers. The extraction equipment that was eliminated at 433' can be used in this extraction stage and the chlorine dioxide equipment eliminated at 434* can be used in this chlorine dioxide stage. In an actual modification, this equipment would be left in place and repiped.
• the vat is 480, the washer 481, the drum 482, the exiting pulp 483, the cleanup washer 484, the incoming process water 510, the washer heads 511, the filtrate line 512, the seal tank 513, the effluent line 514, the dilution lines 515, 517 and 521 ' and their pumps 516, 518 and 522, and the counterflow wash line 523 and its pump 524.
• the pulp 93"'" from brownstock washers 28'" is carried by thick stock pump 96"" to high-density storage 110"".
• the pulp slurry is moved through line 111"" by pump 112"" to a mix tank 116 in which it is mixed with water to reduce its consistency.
• a pump 117 carries the pulp slurry through line 118 to screens - 113'".
• the pulp slurry 115'" enters decker 121'" where it is dewatered.
• the filtrate goes through filtrate line 128'" into seal tank 129'", while the pulp 123'" is moved by thick stock pump 126'" to high-density storage 140'".
• the pulp 163"' exits the washer, goes to steam mixer 166'" and is mixed with sodium hydroxide and steam and carried though line 169' by a thick stock pump 170'" to extraction tower 173'".
• the alkali in this extraction stage may be added at the washer or the steam mixer.
• the extracted pulp is moved through line 175' by pump 176'" to washer 181"'.
• the filtrate from this washing step leaves through line 272'" into seal tank 273'".
• the pulp 183"' passes to steam mixer 186'" and again
• the slurry is carried through line 215"" by pump 216"" to washer 221"" and washed.
• the filtrate leaves the washer through line 312"" to seal tank 313"", while the pulp 223"" is mixed with steam and possibly sodium hydroxide in steam mixer 226'".
• the pulp is carried through line 229'" by thick stock pump 230'" to the chlorine dioxide mixer 231"' and the chlorine dioxide tower 233'".
• the pulp slurry is then carried through line 235"' by pump 236"' to washer 241'" where it is again washed.
• the filtrate passes through line 332'" to seal tank 333'".
• the pulp 243"' is carried by thick stock pump 450' through line 455* to storage tank 527. From the storage tank, the material is carried by pump 528 through line 529 to any additional processing. This should be contrasted with the oxygen system shown in
• FIG. 15 The eight storage tanks of the prior system become four storage tanks in the present system. This number could be reduced to three because the one chlorine dioxide tower in the system shown in Figure 15 can also be eliminated. Its purpose is as a storage tank within the system. It need not be used as a chlorine dioxide tower.
• the pulp slurry from the high-density storage is carried through line 141"" by pump 142"" to washers 161"" and 181"".
• the filtrate from these two washers passes through lines 252"" and 272"" to seal tanks 253"" and 273”" respectively.
• the pulp 183"" from washer 181"" is carried by thick stock pump 190"" to chlorine mixer 438". From the mixer, the slurry passes through line 195"" to washer 201"'".
• the filtrate from this washer passes through line 292'"" to seal tank 293"'".
• the pulp 203"'" is carried to steam mixer 206'"", mixed with sodium hydroxide and steam and then carried through line 209'"" by thick stock pulp 210'"" to oxygen mixer 211'". From the oxygen mixer, the pulp passes through line 215'"" to washer 221'"". The filtrate from this washer passes through line 312'"" to seal tank 313"'".
• the sodium hydroxide, or other alkali may be added either at a washer or at a steam mixer.
• the washer after the mixer may be a diffusion washer.
• sequence X may be chlorine, chlorine dioxide, a combination of chlorine or chlorine dioxide -
• OMPI C j - j , D or a mixture of chlorine and chlorine dioxide, hypochlorites, peroxides or ozone.
• the mixers to be described may be used to mix these.
• the pulp may be treated with ozone by the treatment described in United States Patent Application serial number 836,449 filed September 26, 1977 or United States Patent Application serial number 2,491 dated January 11, 1979.
• the amount of oxygen and the chemical used will depend, of course, on the K number of the unbleached pulp, the desired brightness and the number of bleach stages.
• an OOCOD sequence might use 14 to 20 kilograms of oxygen and 22 to 28 kilograms of sodium hydroxide per metric ton of oven-dry pulp in the first stage; 11 to 17 kilograms of oxygen and 17 to 22 kilograms of sodium hydroxide per metric ton of oven- dry pulp in the second stage; around 56 kilograms of chlorine per metric ton of oven-dry pulp in the third stage; 8 to 11 kilograms of oxygen per metric ton of oven-dry pulp in the fourth stage; and 14 to 16 kilograms of chlorine dioxide per metric ton of oven-dry pulp in the last stage.
• the temperature of the pulp would not be changed from the temperature of the washer for the chlorine treatment.
• a shaft 560 extends longitudinally of the mixer and is supported on bearings 561 and 562 and is rotated by rotational means 563.
• a chain belt drive is shown, but any other type of rotational means may be used.
• Rotors 570 are attached to the shaft 560.
• a typical rotor construction is shown in Figures 20-21.
• the rotor 570 has a body 571 which is tapered outwardly from the shaft and has an elliptically generated cross section. The preferred cross section is an ellipse.
• the major axis of the rotor is aligned with the direction of rotation of the rotor.
• Each of its leading and trailing edges 572 and 573 has a radius of the curvature in the range of 0.5 to 15 mm. The radii are usually the same, though they need not
• OMPI be. If different, then the leading edge would have a greater radius than the trailing edge.
• a modification is shown in Figures 22-23.
• a groove 574 is formed in the trailing edge 573' of the rotor.
• the groove is about 0.1 mm across.
• the groove may be coated with a hydrophobic material.
• the rotors are usually arranged in rings on the central shaft.
• the number of rotors in a ring will depend upon the circumference of the central shaft and the size of the rotor base. A greater number of rotors would req ⁇ ire a longer and stiffer shaft. Fewer rotors would require longer rotors. Consequently, space for the mixer would determine the actual rotor configuration. Normally, there are a total of 4 to 400 rotors, and from 2 to 20 rotors in a ring.
• the rotors rotate transversely of the direction of pulp movement through the mixer, describing a helical path through the pulp.
• the speed of rotation of the rotors would be determined by the motor, and the drive ratio between the motor and the central shaft.
• the diameter of the central shaft 560 is at least one half of the internal diameter of the mixer, forming an annular space 568 through which the slurry passes.
• the enlarged shaft requires scraper bars 564 and 565 on shaft ends 566 and 567. There normally would be four bars on each end. The bars remove fibers that tend to build up between the shaft and the mixer head plate. This prevents binding of the shaft in the mixer.
• the stators are shown in Figures 24-26.
• the stators add oxygen to the pulp in the mixing zone and also act as friction devices to reduce or stop the rotation of the pulp with the rotors so that there is relative rotative movement between the rotors and the pulp.
• Each stator 580 has a body 581, a central passage 582 and a base plate 583.
• the stators extend through apertures 556 in body 551.
• the stator is attached to the body 551 by a friction fit using a Van Stone flange 584. This allows the stator to be rotated if it is desired to change the oxygen placement.
• the base plate 583' is attached directly to the body 551 either by bolts or studs.
• the oxygen enters the mixer through check valves 590.
• the stators are round and tapered and the face having the check valves is flattened. The check valves face across a transverse plane of the mixer and in the direction of rotation of the
• the purpose of the check valve 590 is to prevent the pulp fibers from entering the passage 582.
• a typical check valve is shown in Figure 27.
• the valve 590 consists of a valve body 591 which is threaded into stator body 581.
• the valve body has a valve seat 592.
• the valve itself consists of a bolt 593 and nut 594 which are biased into a closed position by spring 595.
• the stators may also be arranged in rings. There being one ring of stators for each one or two rings of rotors. The number of stators in a
• OMPI ring will depend upon the size of the mixer. Usually, there are 4 stators in a ring, but this can normally vary from 2 to 8.
• Both the rotors and the stators should extend across the annular space.
• a normal clearance between the rotor and the inner wall of the mixer, or the stator and the outer wall of the central shaft is about 13 mm. This ensures that all of the pulp is contacted by the oxygen and there is no short circuiting of the pulp through the mixer without contact with oxygen.
• the rotors and stators should be between the inlet and outlet to ensure that all the pulp would pass through the swept area, and would be contacted with oxygen.
• Figures 28-33 disclose a modification to the basic mixer.
• Oxygen is carried to the rotors through pipe 600 and passage 601 which extends centrally of shaft 560'.
• Radial passages 602 carry the oxygen to the outer annular manifold 603.
• the oxygen passes from the manifold to the pulp through central passage 604 of rotor body 605 and through -check valve 590". These valves are the same as valve 590.
• the rotor is shown as round and tapered, but its shape may be different.
• the rotor may be round or square and nontapered such as those normally found in steam mixers.
• the round rotors would have radii of curvature exceeding 30 mm.
• Tapered rotors 606 having a rectangular cross section may also be used.
• Figure 34 compares the operation of a modified mixer similar to that shown in Figures 28-33 with the operation of the mixer of Figures
• the diameter of the shaft was 0.38 m and the swept area was 14,100 square meters per metric ton of oven-dry pulp.
• the mixer of Figures 16-27 had the same internal diameter but had a central shaft that was 0.508 m in diameter.
• the rotors were elliptical and linealy tapered.
• the major axis of the rotor extended in the direction of rotation of the rotor.
• the leading and trailing edges of the rotor had radii of curvature of 3.8 mm.
• the rotors were 19 cm long and extended to within about 13 mm of the reactor wall, and the stators extended to within about 13 mm of the central shaft.
• the speed of rotation of the rotors was 435 RPM.
• the swept area of the reactor was 72,200 square meters per metric ton of oven-dry pulp. Oxygen was admitted through the stators.
• Figure 34 compares the extracted K number of the pulp with the additional K number drop after passing through the mixer, and shows that the mixer achieved a greater K number drop than the modified mixer. It was also found that the mixer needed only half the amount of oxygen as in the modified mixer to obtain the same amount of deiignification; that is, with the other operating conditions remaining the same, to achieve the same K number drop, 11 kilograms of oxygen per metric ton of oven-dry pulp were required in the modified mixer, but only 5 kilograms of oxygen per metric ton of oven-dry pulp were required in the mixer. It was also found that the mixer could mix greater amounts of oxygen with the pulp than the modi ied mixer. Between 1-1/2 to 2 times as much oxygen could be mixed with the pulp with the mixer than with the modified mixer. For example, the modified mixer could mix a maximum of 15.1-20.2 kilograms of oxygen with a metric ton of oven-dry pulp. The mixer could mix 30.2-35.3 kilograms of oxygen with a metric ton of oven-dry pulp.
• the optimum swept area is achieved by reducing the number of rotors in the mixer from 224 to 203.
• the outer radius -of the rotors 575 is greater than the inner radius of the dams 612 so that the rotors extend beyond the inner wall 608 of the dam into the trapped gas between the dams.
• This construction allows the rotor to extend into a gas pocket and for the gas to flow down the trailing edge of the rotor as it passes through the pulp slurry.
• the rotors and stators may be flat with rounded leading and trailing edges. Again, the radius of curvature of the leading and trailing edges would be in the range of 0.5 to 15 mm, and the radii need not be the same.
• the rotors and stators may be as narrow as 6.35 mm in width.
• This design could also include the groove in the trailing edge of the rotor which may be covered with a hydrophobic coating.

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• 1981
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