Abstract:
Collection of a post-chlorination washer filtrate and recycle thereof to acidify a wood pulp stream entering the initial chlorine dioxide bleaching stage is provided in order to reduce the formation of barium scale.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the bleaching of wood pulp and a method of reducing the amount of barium scale that is formed on equipment used in the bleaching process. 
     BACKGROUND OF THE INVENTION 
     For many years chlorine was used predominantly as the primary bleaching agent for wood pulp. When chlorine was used as the bleaching agent in the first bleaching stage, the consistency of this stage was preferably in the range of about 2.0-5.0% solids. Since the process stream coming to the first bleaching stage typically had a consistency much higher than desired for the first bleaching stage, it was often diluted with filtrate from a chlorination washer for the first bleaching stage. This recycle of the washer filtrate reduced water usage and reduced energy costs. There was also an additional benefit in that with the filtrate recycle, the pulp delignification was improved slightly. 
     As the industry moved away from the use of chlorine and substituted chlorine dioxide, it became necessary to increase the retention time of the pulp in the reaction tower because of the reduced reaction rate of chlorine dioxide compared to reactions involving chlorine. A typical way of increasing the retention time has been to increase the consistency of the pulp stream to about 10 to 12% solids. As a result of increasing the consistency of the pulp stream in chlorine dioxide bleaching processes, it became unnecessary to recycle chlorination washer filtrate for dilution. 
     Barium is present in the pulp stream and originates from the wood chips that serve as the raw material for the wood pulp. 
     The bleaching ability of chlorine dioxide depends in part on pH. The bleaching efect of chlorine dioxide is maximized at a pH of about 2.3 to 3.0. Since the pH of the pulp stream into the bleaching stage is typically more basic than 2.3 to 3.0, acid is typically added to the pulp stream. The most common acid has been sulfuric acid because of its relatively low cost and availability. While the addition of sulfuric acid effectively reduces the pH, it also increases the number of sulfate ions which are available to react with barium and precipitate as barium sulfate, forming unwanted scale on equipment. When scale forms on equipment, such as the chlorination washer, the ability of the equipment to remove unwanted materials, reaction products and unreacted chlorine dioxide is reduced. One of the side effects of reducing the ability of a washer to remove unwanted materials is that less calcium is removed in the washer, and thus passes downstream to extraction stages where the calcium is available to react with carbonate ions and precipitate out as calcium carbonate and form additional unwanted scale. 
     Hydrochloric acid is another acid that has been used in a manner similar to sulfuric acid to acidify the pulp stream; however, when hydrochloric acid finds its way into filtrates, the utility of such filtrates is reduced. Often such hydrochloric acid containing filtrates require expensive treatment in order to render the filtrate stream disposable. 
     In certain bleaching processes, pH of the pulp stream has been controlled by applying excess amounts of chlorine dioxide to the pulp. Apparently, such larger doses of chlorine dioxide are effective to sufficiently acidify the pulp such that the addition of sulfuric acid is not needed. In those chlorine dioxide bleaching processes that do not or cannot apply excess amounts of chlorine dioxide, and thus employ sulfuric acid as a pH adjustment chemical, the formation of barium scale continues to be problematic. 
     In view of the costs involved in removing barium scale and calcium scale from pulp bleaching process equipment, the need exists for a solution to reduce or avoid the formation of barium sulfate which can lead to scale formation on process equipment. Such an improvement would reduce the cost of pulp bleaching processes by reducing down time and costs associated with removal of both barium scale and calcium scale from equipment. 
     As an aid in understanding the prior art as it relates to a typical pulp mill, and more specifically, the bleaching process within a pulp mill, a typical pulp mill operation is described below. 
     FIGS. 1A-1C illustrate a typical pulp mill. In the mill, means of transporting chips or pulp from one operation to another will depend upon the consistency of the pulp and the location of the equipment. The transportation may be accomplished by a conveyor or a chute if the consistency is too high for the pulp or chips to be pumped. If the pulp or chips can be pumped, a pipe can be used to transport the material. 
     Chips  10 , process water  11 , steam  12  and pulping chemicals  13  are placed in digester  14 . Wood chips  10  may optionally be treated prior to entering digester  14  by conventional means such as by pre-steaming the chips in a steaming vessel or impregnating the chips with digestion chemicals in an impregnation vessel. Chemicals  13  that are contacted with the chips will depend on the process being used, e.g., sulfate, sulfite, or soda, and whether or not digester  14  is operated in a batch or continuous mode. In FIGS. 1A-1C, a continuous digester is illustrated. Chips are cooked under appropriate conditions within digester  14 . The cooking conditions will depend upon the species of chip and the type of pulp being used and are well known. 
     The products of the digestion process are delignified or partially delignified wood chips, spent pulping chemicals, and lignin and carbohydrate products which have been removed from the wood chips in the digestion process. Treatment of the chips after cooking, will depend in part on the type of digester being used. A major portion of the spent pulping chemicals and lignin products are removed from the chips prior to further processing by washing. In a continuous digester as illustrated, the chips are washed in a washing section of the digester. This is indicated by process water  15  entering the washing stage of digester  14  and an effluent stream  16  leaving the washing stage of digester  14 . Effluent  16  will consist of the lignin and carbohydrates which have been removed from the chips during the digestion process and spent pulping chemicals. This effluent is carried to a treating facility for processing. If the pulp is Kraft or sulfate, such treating facility would include a recovery system wherein the liquor is burned to recover the pulping chemicals for reuse. Such treatment would not occur in a batch digester where all the washing would occur in the following brown stock washing system. 
     Following this washing treatment, the chips are passed from digester  14  through a blow line to storage or blow tank  22 . It is customary in pulp mills to have storage tanks between separate processes so that the entire mill will not shut down if one section of the mill is taken off line. Storage tank  22  is located between the digester stage and the subsequent washing or bleaching stages. 
     Material passing through the blow line comprises a slurry that contains the remaining lignin and carbohydrates, spent digestion chemicals, and fibers formed from the chips as they are blown from the digester. The chips are formed into fibers when the pressure on the chips is partially released, usually at the outlet of digester  14 . The slurry will still be under some pressure to move it through the blow line. If digester  14  is a continuous digester, additional fiberizing may be accomplished by a refiner, or refiners, in the blow line. Such refiners fiberize large particles that have not been reduced to fibers earlier in the process. In FIG. 1A, two refiners,  18  and  19 , are illustrated; first refiner  18  does coarse refining and second refiner  19  does fine refining. It should be understood that such refiners are optional and often are encountered in a liner board mill; however, in a bleached pulp mill refiners would normally not be included in a blow line. In addition, such refiners would not be employed if digester  14  was operated in a batch mode. 
     The blow line is shown in three sections, section  17  between digester  14  and refiner  18 ; section  20  between refiners  18  and  19 ; and section  21  between refiner  19  and storage tank  22 . 
     From storage tank  22 , fibers and liquor are carried by pumps  23  through line  24  to washing and screening equipment. The system will be described by first following the pulp through the system and then following the wash water through the system. 
     The pulp slurry is first carried to brown stock washers  28 , where preferably substantially all of the remaining lignin and spent digestion chemicals are removed from the fibers. In FIG. 1A, four washers are illustrated. Typically, this is the number of washers that would be used for the batch digester. The washing section of a continuous digester may replace the first two brown stock washers. Each of the washers is usually a vacuum or pressure drum washer or vacuum or pressure drum filter and the operation of each is the same and well known. The operation of a vacuum or pressure drum washer will be described. Some of the washers may, however, be diffusion washers in which case, the pulp slurry would not be diluted prior to entering the washer. 
     Pulp slurry from line  24  enters vat  30  of washer  31 . Vacuum drum  32  revolves through the vat, and the vacuum pulls the fibers in the slurry onto the outer surface of the filter drum and holds the fibers, in that form, against the surface. Liquor or filtrate are pulled through the filter cloth to the interior piping of the vacuum drum to be discharged as effluent. The revolving drum carries the fiber mat from the vat past a bank of washer heads that spray a weak filtrate onto the mat to displace the liquor from the mat. The vacuum also pulls this displaced liquor into the interior piping of the drum. The consistency of the mat leaving a washer, either the brown stock washers described here or the bleach washers described later, will usually be between about 8 to 15 wt. %. 
     Pulp mat  33  is removed from the face of 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 vat  50  of second brown stock washer  51 . Again, the fibers are picked up on vacuum drum  52  where the mat is washed with still weaker filtrate, removed from vacuum drum  52  and carried to vat  70  of brown stock washer  71 . Operation of this washer is the same as the others. In brown stock washer  71 , vacuum drum is identified by reference numeral  72  and the mat is identified by reference numeral  73 . Mat  73  is carried from brown stock washer  71  to vat  90  of the last brown stock washer  91 . Again, the operation of this washer is the same as the others with the vacuum drum being identified as reference numeral  92  and the mat as reference numeral  93 . 
     From the brown stock washers, the pulp mat is carried to storage tank  110  with the aid of stock pump  96 . In the lower section of tank  110 , the pulp is diluted and then carried through line  111  by pump  112  to screens  113  in FIG.  1 B. In screens  113 , the larger fiber bundles and knots are separated. The bundles and knots  114  are delivered to further treatment sites. 
     Pulp  115  is carried from screens  113  to decker  121  where additional water is removed. The operation of the decker is similar to that of the washers and is well understood. Washing showers may or may not be used in the decker. The decker includes vacuum drum  122  and pulp mat  123 . The pulp mat  123  is pumped by stock pump  126  to a high density storage tank  140  where the pulp is stored until it is bleached. 
     The liquor or filtrate from decker  121  may be handled in several ways, which may or may not occur simultaneously. While 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 the brown stock washing system  28  can be handled. 
     First, filtrate from tank  129  can be reused to reduce the consistency of pulp slurry either entering decker  121 , entering screens  113 , or leaving storage tank  110 . Line  130  carries filtrate to lines  131 ,  133  and  135 . Line  131  and pump  132  carry filtrate back to screened pulp  115  to reduce the consistency of the pulp slurry entering decker  121 , preferably to around 1.5 wt. %. Line  133  and pump  134  carry filtrate back to line  111  to reduce the consistency of the pulp slurry entering screens  113  to about 0.2 to 2 wt. %. Line  135  and pump  136  carry filtrate back to storage tank  110  to reduce the consistency of the pulp slurry leaving the tank to about 5 wt. %. 
     Second, 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 a cooking liquor recovery system. It should be understood that in a batch digester system, the digester effluent is recovered completely from the brown stock washing system while in a continuous digestion system only a portion of the digester effluent is recovered from the brown stock washers. 
     All of the remaining filtrate would be handled as effluent if counterflow washing, described below, is not employed. Some of the filtrate may be handled as effluent even if counterflow washing is employed. 
     Third, filtrate from tank  129  may be used as wash water in the brown stock washing system  28  in a counterflow washing system. In this configuration, filtrate flows counter to the flow of pulp. Line  137  and pump  138  carry filtrate back to brown stock 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 therefrom to remove any pulp fibers that cling to the wires, strings or rolls. Additional water may be required to supplement the filtrate and may be provided through process water line  97 . 
     The flow of filtrate through brown stock 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 . The filtrate from the seal tank may be handled in a number of ways. For example, line  100  can carry it to effluent line  29 . Line  101  and pump  102  can carry the filtrate to pulp  73  to reduce the consistency of the pulp slurry to about 1½ to 3½ wt. % as it enters vat  90 . Line  103  and pump  104  can carry the filtrate to brown stock washer  71  to be used as wash water. 
     The process and brown stock washers  71 ,  51  and  31  are for the most part, identical to the process in brown stock washer  91  so the parts are similarly numbered. Washer heads are identified, respectively, as  75 ,  55  and  35 . The clean-up washers are identified by reference numerals  74 ,  54  and  34 , respectively. Filtrate lines are identified by reference numerals  78 ,  58  and  38 , respectively, and the filtrate storage or seal tanks are identified by reference numerals  79 ,  59  and  39 , respectively. The filtrate lines from the seal tanks to effluent  129  are identified by reference numerals  80 ,  60  and  40 , respectively. 
     The consistency of the slurry entering any of the vats  70 ,  50  or  30  is preferably about 1.5 to about 3.5 wt. %. 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 , respectively. 
     In brown stock washer  31 , 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 about 2 to about 3½ wt. % before it exits the tank. 
     In each of the brown stock washers, there is a possibility that additional process water may be needed to supplement the filtrate being used as wash water. Lines  77 ,  57  and  37  are for this purpose and provide all the wash water to the individual washers if the counterflow system described above is not used and parallel flow washing is used instead. 
     The washed pulp which has passed through the brown stock washing system  28 , the screens  113  and decker  121  remains in storage tank  140  until it is carried into the bleaching system. 
     The bleaching process illustrated in FIGS. 1B and 1C will also be described by following the pulp stream through the bleaching system from washed pulp to bleached pulp and then by following the wash water from its entry into the process through to bleach plant effluent. The particular bleaching sequence illustrated is DED, where “D” represents a chlorine dioxide stage and “E” represents an alkali extraction stage. The process conditions are taken from the Tappi Monograph, No. 27 , The Bleaching of Pulp , Rapson, Editor, The Technical Association of Pulp and Paper Industry, 1963, pages 186-187. It should be understood that there are many other bleaching sequences which are described in standard text. 
     Prior to decker  121 , pulp mat  123  can be treated with a slight amount of alkali from line  187 . Generally, a sodium hydroxide solution is used and is added to the mat at a point on the drum that will allow the solution to stay in the mat and not pass into the filtrate. The purpose of this treatment is to adjust the pH of a pulp prior to chlorine dioxide treatment. The pH of the pulp should be in the range of about 5 to about 7, preferably about 6, for optimum brightness when bleaching with chlorine dioxide. Alternatively, the alkali solution may be added in the storage tank  140  instead of decker  121 . Alternatively, if the pH of the pulp mat is elevated, an acid, such as sulfuric acid can be added to lower the pH. Such acid can be added at the same locations that the alkali solution described above can be added. 
     From tank  140 , the washed pulp is delivered to mixer  191  where it is combined with chlorine dioxide from line  192  prior to entering chlorine dioxide tower  193 . This tower can be an upflow or upflow/downflow tower with the pulp remaining in the tower long enough to allow the chlorine dioxide to react with the pulp. Generally, the reaction is about complete after one hour, but can continue for up to about four hours. Prior to leaving the tower, the slurry is diluted, e.g., to a consistency of about 5 wt. %, in dilution zone  194 . If necessary, based on the metallurgy of the equipment, the slurry may be treated with a small amount of sulfur dioxide or alkali from line  197 , which reacts with any excess chlorine dioxide to reduce the amount of free chlorine dioxide leaving the bleaching tower  193 . The diluted slurry is then carried by line  195  and pump  196  to vat  200  of washer  201 . During its passage through line  195 , the slurry is again diluted, e.g., to a consistency of about 1 to about 1.5 wt. %, when it reaches vat  200 , and is again treated with additional sulfur dioxide from line  198 . The pulp is picked up on vacuum drum  202 , and any reaction products and unreacted bleaching chemicals are washed from it prior to being removed as pulp mat  203 . 
     It should be understood that while the initial chlorine dioxide, stage is described above in relation to an extraction tower separate from washer  201 , a diffuser can be employed wherein the reaction between the pulp and chlorine dioxide occurs in the same vessel in which washing of the pulp occurs. Diffusers are available commercially from numerous sources such as Ahlstrom and Kvaerner and their operation is well understood. 
     The pulp is then moved to steam mixer  206  of the extraction stage, usually by gravity drop through a chute. Sodium hydroxide from line  207  is added on washer  201  or at mixer  206 . In mixer  206 , the treated pulp mat  203  is mixed with steam from line  208 . This slurry is then carried through line  209  by pump  210  to extraction tower  213 . The conditions in this extraction stage serve to remove lignin and unwanted chemicals. This tower may be a downflow or an upflow tower. 
     After the appropriate dwell time, the pulp enters dilution zone  214 , and its consistency is reduced, e.g., to approximately 5 wt. %. The pulp is then carried through line  215  by pump  216  to vat  220  of washer  221 . Washer  221  is also shown and described as a vacuum or pressure dome washer but it may be a diffusion washer. Again, the bleached pulp is diluted, e.g., to a consistency of about 1 to 1.5 wt. % before entering the vat. The slurry is picked up by vacuum drum  222  and washed and discharged as pulp mat  223 . If necessary, the pH of the pulp may be adjusted by treating the mat with sodium hydroxide from line  227 . This may occur on the drum  222  or in the steam mixer  226 . 
     The pulp then enters the last chlorine dioxide stage. The conditions and flow in this stage are the same as in the first chlorine dioxide stage. The pulp is dropped into or carried to steam mixer  226 , and mixed with steam from line  228 . The slurry is carried through line  229  by pump  230  to mixer  231 , mixed with chlorine dioxide from line  232  and carried into the chlorine dioxide tower  233 , shown as an upflow/downflow tower, where it remains from about 1 to 4 hours. The pulp then enters dilution zone  234  where its consistency is reduced, e.g., to about 5 wt. %. It is also treated with a small amount of sulfur dioxide from line  237  to remove any excess chlorine dioxide. 
     The slurry is then 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 go further diluted, e.g., to a consistency of about 1 to about 1.5 wt. %, 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 washers  201 ,  221  and  241  is the same as in the brown stock 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 the central pipe in the drum and an external line to a seal or storage tank that 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 after it leaves the seal or storage tanks can take one of two routes. First, the filtrate has been used to dilute the slurry within the washing stage or tower. For example, filtrate from seal tank  293  can be carried in line  297  by pump  298  into line  195  where the filtrate dilutes the slurry going to vat  200 . Line  315  and pump  316  and line  317  and pump  318  carry filtrate from seal tank  313  of a second extraction stage washer  221  into line  215  to dilute the slurry going to vat  220 ; and line  335  and pump  336 , and line  337  and pump  338 , carry filtrate from seal tank  333  of the second chlorine dioxide washer  241  into line  235  to dilute the slurry going to vat  240 . In the first chlorine dioxide stage, line  301  and pump  302  carry filtrate from seal tank  293  into the dilution zone  194 . In the extraction stage, line  321  and pump  322  carry filtrate from seal tank  313  into dilution zone  214 , and in the second chlorine dioxide stage, line  341  and pump  342  carry the effluent from the seal tank  333  into dilution zone  234 . 
     Second, the filtrate not reused for dilution is discharged as effluent or to further processing as by line  294  from tank  293 , line  314  from tank  313 , and line  334  from tank  333 . 
     The chemical, water and steam supplies to the system illustrated in FIGS. 1A,  1 B and  1 C are conventional. Process water is carried through line  360  to the various lines supplying water to the process, line  351  to digester lines  11  and  15 , lines  37 ,  57 ,  77  and  97  to the brown stock washers  28 , and lines  290 ,  310  and  330  to the bleach system washers. Alkali line  362  supplies dilute alkali to lines  187 ,  207  and  227 . Chlorine dioxide line  363  provides a chlorine dioxide solution to lines  192  and  232 . Steam is supplied through line  364  to steam lines  12 ,  209 , and  229 . Sulfur dioxide is supplied to lines  198 ,  237  and  238  from line  365 . 
     SUMMARY OF THE INVENTION 
     The present invention relates to applicant&#39;s solution of the problem relating to the formation of barium scale in pulp bleaching processes, particular those that employ chlorine dioxide as a bleaching chemical. A bleaching process to which the present invention pertains includes a primary process stream comprising wood pulp that is washed prior to contacting the wood pulp with chlorine dioxide. In accordance with the present invention, the washed wood pulp after being contacted with the chlorine dioxide is washed in a post-chlorination washing stage to remove reaction products and unreacted chlorine dioxide. The method of the present invention includes the step of collecting filtrate from the post-chlorination washing stage and combining a portion of the collected filtrate with the primary process stream, preferably in or subsequent to a pre-bleach wash stage and prior to the introduction of the chlorine dioxide, in order to decrease the pH of the primary process stream. By introducing the collected filtrate from the post-chlorination washing stage into the primary process stream in accordance with the present invention, the need for acidifying chemicals, such as sulfuric acid, is reduced and preferably eliminated. 
     The benefits of reducing barium scale formation: in accordance with the present invention include: (1) reducing process down time and the costs associated with cleaning equipment to remove barium scale; (2) reducing the amount of barium scale that forms on the equipment so that the adverse impacts of such barium scale on the efficiency of the washing equipment is reduced, and (3) reduced chemical requirements. The present invention should find widespread application in many pulp bleaching processes that employ chlorine dioxide as a primary bleaching chemical. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of the present invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIGS. 1A,  1 B and  1 C provide a schematic flow diagram for a typical wood pulp bleaching process that employs chlorine dioxide as a primary bleaching chemical and a DED system; 
     FIGS. 2A,  2 B and  2 C illustrate a schematic flow diagram for a wood pulp bleaching process using chlorine dioxide as a primary bleaching chemical and incorporating the method of the present invention for reducing the formation of barium scale; and 
     FIG. 3 illustrates a schematic flow diagram of a specific portion of the system of FIGS. 2A-2C that embodies the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 2A,  2 B and  2 C, one example of a wood pulp bleaching process that employs chlorine dioxide as a primary bleaching chemical to which the method of the present invention can be applied is substantially identical to the bleaching process described above in the background of the invention with reference to FIGS. 1A,  1 B and  1 C. It should be understood that the description of the present invention in the context of the specific configuration for a bleaching process is for illustrative purposes only and is not intended-to limit the scope of the present invention. The method of reducing barium scaling in accordance with the present invention is equally applicable to other pulp bleaching processes that employ chlorine dioxide as a bleaching chemical and suffer from the drawback of the formation of barium scale on equipment. 
     The present invention is best described with reference to FIGS. 2B and 2C. For the purposes of clarity, the same numbering convention has been adopted for FIGS. 2A,  2 B and  2 C for those features that are common between FIGS. 2A-2C and FIGS. 1A-1C. 
     As noted above in the background of the invention, brown stock solution in line  115  is delivered to decker  121  for removal of water and additionally washing to remove lignin and spent digestion chemicals. As an alternative to decker  121 , a pre-bleach washer or press, such as a twin roll press can be utilized. It should be understood that the present invention is not limited to the particular type of equipment used to remove water and provide additional washing of the brown stock solution prior to its contacting with chlorine dioxide. For purposes of clarity, the method of the present invention will be described below in the context of the use of a decker as a type of equipment used as a pre-bleach washer. 
     The washed pulp leaving decker  121  is delivered to storage tank  140 . From tank  140 , the washed pulp is delivered to chlorine dioxide tower  193 , where bleaching occurs under conventional conditions. After the bleaching process is considered complete, the pulp is delivered to post-bleach washer  201  where reaction products and unreacted chlorine dioxide are removed. As an alternative to the reaction tower  193  and washer  201  described above in the background of the invention, it is preferred in the context of the present invention that a diffusion tower be employed; however, other wash devices would be suitable, such as a drum or press. Such a diffuser tower combines the unit operations of the bleaching step and washing step into a single piece of equipment. In the following discussion, descriptions relating to washer  201  are equally applicable to the washing portion of a diffusion tower. 
     From washer  201 , as described above in the background of the invention, the washed pulp proceeds on to the extraction phase and then a subsequent bleaching stage for processing under known conditions. 
     FIG. 3 represents a portion of the pulp bleaching process illustrated in FIGS. 2B and 2C, but is simplified in order to exemplify the method of the present invention. FIG. 3 carries the same numbering convention as FIGS. 2B and 2C. Referring to FIGS. 2B,  2 C and  3 , in accordance with the present invention, filtrate from washer  201  is collected in line  292  and delivered via line  294  and  303  to decker  121 , where it is combined with brown stock solution preferably prior to or simultaneously with the washing that occurs in decker  121 . Filtrate from washer  201  is preferably introduced to the brown stock solution prior to or simultaneously with the washing that occurs in decker  121 , so that the filtrate is available to be carried through the bleaching process with the pulp. In accordance with the present invention, the pH of the filtrate is lower than the pH of the primary pulp stream to which the filtrate is added. By applying the filtrate to the pulp prior to the first bleaching stage, the pH of the primary pulp stream can be reduced, preferably without the need for an acidifying agent, such as sulfuric acid. Introduction of the filtrate to the wood pulp at this location is also preferred because applicants have observed that a major amount of barium scale forms on the initial wash stage after the initial chlorine dioxide bleaching stage. Therefore, in order to reduce the amount of barium scale that occurs at this juncture, it is necessary to eliminate the presence of the sulfate ions prior to the initial wash stage. 
     The pH of the pulp stream  115  can range from about 7 to about 11 depending on the effectiveness of the brown stock washing. Accordingly, in order for filtrate in line  303  to be able to reduce the pH of stream  115 , the pH of the filtrate should be lower than the pH of pulp stream  115 , i.e., &lt;7. Preferably, the pH of the filtrate in line  303  is about 2.0 to 5.0 and most preferably from about 2.5 to 3.0 in order to maximize the bleaching effect in tower  193 . In addition to the magnitude of the difference between the pH of the process stream  115  and the pH of the filtrate in line  303 , the amount of filtrate that is introduced into process stream  115  will effect the degree of reduction in the pH thereof While the present invention is not limited to any particular amount of filtrate in line  303  that is added to wood pulp stream  115 , as a practical matter, the amount that can be introduced will be limited by the volumemetric capacity of washer  121  or other downstream processing equipment. 
     Ultimately, the amount of filtrate from line  303  introduced into line  115  is selected so that the pH of the wood pulp after addition of the filtrate is adjusted to fall within the range of about 3 to about 5, preferably about 4 to about 5. These particular pH ranges are preferred in part because of the conditions needed to provide the desired amount of bleaching in bleach tower  193 . Other criteria to be considered in determining the pH of the wood pulp entering bleach tower  193  are well understood. 
     The filtrate can be applied to the wood pulp in a number of different manners, including applying it as a wash stream in decker  121 . When the filtrate is applied to the pulp as a wash stream in decker  121 , it is preferred that the filtrate be applied as high up on the drum of the washer as possible in order to minimize any break-through in the pulp sheet to the washer wire. 
     Referring to FIG. 3, it should be understood that in certain wood pulp bleaching processes, decker  121  may be replaced by a press as a means to dewater the brown stock solution. In those types of processes, the filtrate in line  303  can be applied after the press and prior to the first bleaching stage. 
     While the present invention has been described above in the context of a preferred embodiment, it should be understood that the method of reducing barium scale in accordance with the present invention is applicable to numerous configurations of chlorine dioxide bleaching of wood pulp. For example, the particular unit operation into which the recycled filtrate is introduced will vary from process to process. It will be appreciated that various changes can be made from the preferred embodiment illustrated and described above without departing from the spirit and scope of the present invention.