Patent Abstract:
A method for producing bleached wood pulp in which wood chips are digested in polysulfide liquor to produce brown stock pulp. The brown stock pulp is washed to produce washed brown stock wood pulp and weak black liquor and the washed wood pulp is then delignified in an oxygen delignification stage to produce oxygen delignified wood pulp. The delignified wood pulp is then ozone bleached in an ozone bleaching stage in which a waste stream principally containing ozone, carbon dioxide and oxygen is produced. The ozone-bleached pulp is introduced into an extractive oxidation stage which can include peroxide to further bleach the pulp and the product of the extractive oxidation stage is then either introduced into either a peroxide or chlorine dioxide bleaching stage. The waste stream is recovered and scrubbed with either white liquor, oxidized white liquor, or fully oxidized white liquor either in a separate scrubber or during oxidation reactions occurring in either polysulfide, white liquor or complete white liquor production stages. The scrubbing with white liquor or oxidized white liquor removes ozone and carbon dioxide so that the scrubbed stream can be utilized in the oxygen delignification stage. This eliminates the need for ozone destruct units. Moreover, the polysulfide liquor is utilized in the digestion of the wood pulp and the thiosulfate liquor is used in the oxygen delignification of the washed wood pulp. The fully oxidized white liquor can be utilized within the extractive oxidation stage and optionally can be used in a peroxide bleaching stage if present. The oxygen removed from the scrubbed stream can be balanced with oxygen demand of the foregoing stages.

Full Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of producing bleached wood pulp in which wood chips are digested and are then subjected to subsequent bleaching stages that are conducted in the presence of a sodium hydroxide. More particularly the present invention relates to such a method that includes an ozone bleaching stage in which a waste stream produced from an ozone bleaching stage is scrubbed to produce an oxygen containing stream useful in an oxygen delignification stage of the pulping process. Even more particularly, the present invention relates to such a method in which the waste stream is scrubbed by white liquor either in an external stage or during production of polysulfide liquor and then is separately reacted with white liquor to produce oxidized white liquor containing an appreciable amount of thiosulfate species of sulfur (thiosulfate liquor), and fully oxidized white liquor, containing almost no thiosulfate sulfur, to serve as sodium hydroxide in bleaching stages. 
     In the formation of bleached wood pulp, wood chips are digested in the presence of white liquor, which contains sodium sulfide and sodium hydroxide for such digestion, to produce brownstock pulp and weak black liquor. It is known that pulping with polysulfide liquor has advantages over conventional white liquor cooking in the wood chip digestion stage. The brownstock pulp is then washed and weak black liquor is extracted for reprocessing. The pulp is then subjected to oxygen delignification. The oxygen delignification is conducted in the presence of thiosulfate liquor, oxygen and steam. After the oxygen delignification, the wood pulp is sequentially subjected to an ozone bleaching stage, an extractive oxidation stage, which may be conducted in the presence of peroxide, and a final peroxide or chlorine dioxide bleaching stage. The extractive oxidation stage is conducted in the presence of thiosulfate liquor. Fully oxidized white liquor is a sodium hydroxide source for peroxide based bleaching stages and has advantages in such bleaching stages over thiosulfate liquor. 
     The ozone feed to the ozone bleaching stage is made in an ozone generator from air or more preferably oxygen. The end result is a mixture of ozone and oxygen containing about 5% ozone if air is used and anywhere from 10 to 14% ozone if the ozone is generated from oxygen. Not all of the feed to the ozone bleaching stage is consumed and as a result, a waste stream is produced that contains ozone, oxygen, carbon dioxide and water. This waste stream is further processed by an ozone destruct unit and a carbon dioxide scrubber to produce oxygen that can be used in an oxygen deliqnification stage. Ozone is destroyed so that the stream may be recycled to the ozone generator after CO 2  removal and drying. Also, some of the waste stream may be vented atmosphere and ozone must be destroyed for industrial hygienic reasons. Carbon dioxide must be removed, otherwise it would consume sodium hydroxide inside the oxygen delignification stage, limiting the extent of lignin removal. 
     As will be discussed, the present invention provides a method of producing bleached wood pulp in which a waste stream produced from an ozone bleaching stage is scrubbed and then used as a source of oxygen for oxygen delignification. Expensive ozone destruct units are not used and in fact oxygen requirements can be balanced with oxygen recovery from the waste stream. The implication of this is that an oxygen recycle involved in the utilization of the ozone destruct unit can be eliminated together with its attendant capital and power consumption. Additionally, there is no need to further purify the waste stream to remove carbon dioxide and water. Moreover, the present invention advantageously utilizes polysulfide liquor in the wood chip digestion stage, oxidized white liquor in oxygen delignification and extractive oxidation stages, and fully oxidized white liquor in the peroxide bleaching stage. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of producing bleached wood pulp. In accordance with the method, wood chips are digested in a digestion stage to produce brownstock pulp and weak black liquor. The brownstock pulp is washed and the weak black liquor is extracted. The brownstock pulp after having been washed is introduced into sequential bleaching stages, including oxygen delignification and ozone bleaching stages, to produce a bleached wood pulp product. The oxygen delignification stage utilizes an oxygen containing stream and the ozone bleaching stage utilizes an ozone/oxygen containing stream. The ozone bleaching stage produces a waste stream principally containing water vapor, carbon dioxide, ozone, and oxygen. The waste stream is recovered and scrubbed with an aqueous, sodium sulfide and sodium hydroxide containing solution to remove ozone and carbon dioxide from the waste stream and thereby form a scrubbed stream. The oxygen containing stream, used in the oxygen delignification stage, is formed from at least part of the scrubbed stream. 
     Residual ozone is consumed from the waste stream by oxidizing sodium sulfide to an oxygenated sulfur species such as sulfite, thiosulfate or sulfate. Sodium hydroxide reacts with carbon dioxide to form sodium carbonate. In this manner, the waste stream becomes a scrubbed stream to eliminate the need for an ozone destruct unit. Additionally, since carbon dioxide has been removed, it will not neutralize the alkalinity required in the oxygen deliqnification process. Furthermore, in another aspect of the present invention, that will be discussed hereinafter, the oxygen recovered from the waste stream can be balanced with oxygen usage by utilizing the waste stream as an oxidant in a polysulfide production stage. Such usage will scrub the waste stream and will produce polysulfide that can be advantageously used in the wood chip digestion stage. Furthermore, the resultant scrubbed stream can also be used in oxidizing and fully oxidizing the white liquor in oxidized white liquor and fully oxidized white liquor stages. The fully oxidized white liquor can also advantageously be used in a peroxide based bleaching stage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic representation of a method of producing bleached wood pulp in accordance with the present invention; 
     FIG. 2 is a schematic view of an alternative embodiment of a method of producing bleached wood pulp in accordance with the present invention; and 
     FIG. 3 is a schematic of a reactor used in producing fully oxidized white liquor. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, a process flow sheet of a method for producing bleached wood pulp is illustrated. Wood chips  10  and a polysulfide liquor stream  12  enter a digestion stage  14 , which can be provided by a known wood pulp digester, to produce brownstock pulp and weak black liquor. The brownstock pulp is introduced into a washing stage  16 , which can be a rotary washer, along with oxygen stage filtrate. The brownstock pulp is washed with the water and the weak black liquor is extracted as a weak black liquor stream  17 . Although not illustrated but as would be known to those skilled in the art that digestion and washing stages  14  and  16  could be integrated and generally could also include a knotting stage separating the digestion and washing stages  14  and  16  and a screening stage following washing stage  16 . 
     Weak black liquor stream can be processed in a manner well known in the art to produce white liquor. This is accomplished by introducing the weak black liquor into multiple effect evaporators and a recovery boiler to convert the weak black liquor to smelt. The smelt is then dissolved in a dissolving tank to produce green liquor. The green liquor is then causticized in a causticizing tank by the addition of lime from a lime kiln and is then subjected to a clarifying stage to produce the white liquor. All of these stages, which are known in the art, are designated by white liquor regeneration stage  18 . White liquor from white liquor regeneration stage  18  can be held for use within a holding tank  19 . It is to be noted that although not illustrated, washers would be placed between each of the stages with countercurrent flow of washer filtrate from washer to washer and then back to washing stage  16 . This is the manner in which oxygen stage filtrate is obtained for washing stage  16 . At the same time, the weak black liquor is being reprocessed to form white liquor which is in turn used throughout the bleaching process. It is to be noted that a mass balance can be maintained throughout the mill without the resort found in the prior art of adding sodium hydroxide. When sodium hydroxide is added, it can potentially build and thus be discharged from the mill. 
     The brownstock pulp is then introduced into an oxygen delignification stage  20  along with steam and a thiosulfate liquor stream  22  and a first oxygen containing stream  24 . The oxygen delignification stage  20  would be provided by a reactor, known in the art. The delignified wood pulp is then introduced into an ozone bleaching stage  26  along with an ozone/oxygen containing stream  28  to produce an ozone-bleached pulp and a waste stream  29 . The ozone-bleached pulp and a second oxygen-containing stream  28  is then introduced into a known extractive oxidation stage  30  along with a first fully oxidized white liquor stream  32  and a second oxygen containing stream  34 . The extractive oxidation stage is provided to remove soluble alkaline reaction products produced in ozone bleaching stage  26 . In the illustrated process, extractive oxidation stage  30  utilizes peroxide (the stream containing peroxide is not illustrated). It is to be noted that in some extractive oxidation processes peroxide is not utilized. In this regard, first fully oxidized white liquor stream  32  is optional because it is only required when peroxide is present within extractive oxidation stage  30 . When peroxide is not present, thiosulfate liquor may be substituted for fully oxidized white liquor. The ozone bleached pulp produced in extractive oxidation stage  30  is then introduced into a known peroxide bleaching stage  36  along with a second fully white oxidized liquor stream  38  to produce a bleached wood pulp product  39 . Alternatively, a chlorine dioxide bleaching stage could be used in place of the peroxide bleaching stage. In such case, second fully oxidized white liquor stream  38  would not be used. 
     Waste stream  29  from the ozone bleaching stage  26  is then introduced along with a subsidiary stream  40  composed of white liquor into a polysulfide reaction stage  42 . Polysulfide reaction stage  42  can be a stirred tank, a pipeline reactor or a device using counter-current contact devices such as structured packing. In any of these reactors, the white liquor serves to strip the carbon dioxide from the waste stream while the white liquor is oxidized by the oxygen contained within waste stream  29  to produce the polysulfide liquor. The sulfide reactions remove ozone. Thus, waste stream  29  is introduced into polysulfide reaction stage  42  as a third oxygen containing stream which becomes scrubbed with respect to carbon dioxide and ozone to become scrubbed stream  44 . 
     Scrubbed stream  44  is then compressed by a compressor  45  to an elevated pressure at which oxygen delignification stage  20 , white liquor and complete white liquor oxidizing stages, designated by reference numbers  46  and  48 , operate. These foregoing stages operate at an elevated pressure as compared with the remainder of the apparatus illustrated in FIG.  1 . After compression, scrubbed stream  44  is subdivided into first and second oxygen containing streams  24  and  34  and a forth and a fifth oxygen containing streams  50  and  52  which are then introduced into white liquor and complete white liquor oxidizing stages  46  and  48 , respectively, along with two other subsidiary streams  54  and  56  containing white liquor. Thiosulfate liquor is produced in white liquor oxidizing stage  46  and fully oxidized white liquor is produced in complete white liquor oxidizing stage  48  which in turn respectively serve as makeup for thiosulfate liquor stream  22  and first and second fully oxidized white liquor streams  32  and  38 . 
     As possible alternative embodiments, either thiosulfate liquor, white liquor, or fully oxidized white liquor could be used as an alkaline, aqueous solution to scrub carbon dioxide from waste stream  29 . In such alternative embodiments, waste stream  29  could be used as either the forth or fifth oxygen containing streams  50  and  52  to produce a scrubbed stream emanating from either white liquor and complete white liquor oxidizing stages  46  and  48 . Thereafter, such scrubbed stream would be subdivided into first and second oxygen containing streams  24  and  34 , a third oxygen containing stream to be introduced into polysulfide reaction stage  42  and either the remaining forth or fifth oxygen containing streams  50  and  52  which was not formed by waste stream  29 . As could be appreciated, in any of the foregoing embodiments in which waste stream  29  is used to directly form either fourth or fifth oxygen containing streams  50  and  52 , waste stream  29  must be compressed to the elevated operating pressure of white liquor and complete white liquor oxidizing stages  46  and  48 . For that matter, in any possible embodiment of the present invention, waste stream  29  could be compressed in lieu of compressing the scrubbed stream. 
     The oxygen requirements of a method in accordance with the present invention, such as outlined above, will depend upon whether the final bleaching stage is a peroxide bleaching stage or a chlorine dioxide bleaching stage. Chlorine dioxide bleaching is an acidic process that does not consume oxidized white liquor or oxygen and as such will not consume oxygen. Additionally, the amount of polysulfide produced will also effect oxygen consumption. On the supply side, the amount of oxygen produced will depend on the ozone requirements in the ozone bleaching stage. The greater the requirement for ozone, the greater will be the oxygen production. The following is a calculated chart of oxygen production versus usage is a process conducted in accordance with the present invention as set forth in FIG.  1 . In the first column, the term, “W % O 3 ” means the percentage by weight ozone in the ozone/oxygen containing stream produced by the ozone generator and used in ozone bleaching stage  26 . The term “O 3  charge on pulp” is the ozone requirement for the particular pulp being bleached. The next column, headed, “O 2  produced from O 3  gen” is the oxygen content in the ozone/oxygen containing stream. Under the grouping “oxygen Usage in Mill, the “%PS as S” is the percentage poly sulfide charge on the pulp expressed as sulfur. “PS-OZE op -P” indicates the use of a peroxide bleaching stage with an extractive oxidation stage using peroxide. “PS-OZE op -D” indicates a chlorine dioxide bleaching stage. For comparison purposes, the oxygen usage of a prior art pulp bleaching process that does not use polysulfide is labeled, “No PS”. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 OXYGEN REQUIREMENTS FOR 1000 MFPD O.D. PULP 
               
             
          
           
               
                   
                   
                 O 2   
                   
               
               
                   
                   
                 pro- 
               
               
                   
                   
                 duced 
               
               
                   
                 O 3   
                 from 
                 Oxygen Usage in Mill 
               
             
          
           
               
                 Wt. 
                 charge 
                 O 3   
                 2% PS as S 
                 1% PS as S 
               
             
          
           
               
                 % 
                 on pulp 
                 gen 
                 PS- 
                 PS- 
                 PS- 
                 PS- 
               
               
                 O 3   
                 (% wt.) 
                 mtpd 
                 OZE OP -P 
                 OZE OP D 
                 OZE OP -P 
                 OZE OP D 
               
               
                   
               
               
                 10 
                 0.8 
                 72 
                 72 
                 60 
                 64 
                 52 
               
               
                 10 
                 1.0 
                 90 
                 72 
                 60 
                 64 
                 52 
               
               
                 12 
                 0.8 
                 59 
                 72 
                 60 
                 64 
                 52 
               
               
                 12 
                 1.0 
                 73 
                 72 
                 60 
                 64 
                 52 
               
               
                 14 
                 0.8 
                 49 
                 72 
                 60 
                 64 
                 52 
               
               
                 14 
                 1.0 
                 61 
                 72 
                 60 
                 64 
                 52 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 OXYGEN REQUIREMENTS FOR 1000 MTPD 0.D. PULP 
               
               
                 Oxygen Usage in Mill 
               
             
          
           
               
                   
                 O 3  charge 
                 O 2  produced 
                   
               
               
                   
                 on pulp 
                 from O 3  gen 
                 No PS 
               
             
          
           
               
                 Wt. % O 3   
                 (% wt.) 
                 mtpd 
                 PS-OZE OP -P 
                 PS-OZE OP D 
               
               
                   
               
               
                 10 
                 0.8 
                 72 
                 57 
                 45 
               
               
                 10 
                 1.0 
                 90 
                 57 
                 45 
               
               
                 12 
                 0.8 
                 59 
                 57 
                 45 
               
               
                 12 
                 1.0 
                 73 
                 57 
                 45 
               
               
                 14 
                 0.8 
                 49 
                 57 
                 45 
               
               
                 14 
                 1.0 
                 61 
                 57 
                 45 
               
               
                   
               
             
          
         
       
     
     As indicated by the charts, oxygen usage can be balanced. Also, under certain circumstances, the combination of ozone output and ozone charge required will not produce enough oxygen to sustain a process in accordance with the present invention. For instance, where the weight percent ozone in the ozone/oxygen containing stream  12  and the required ozone charge on the pulp is 0.8, then the 59 kg of oxygen per metric ton per day of oven dried pulp would only be sufficient to sustain a process in accordance with the present invention in which a chlorine dioxide bleaching stage were used and with a polysulfide stage that produced 1% sulfur in the polysulfide. 
     With reference to FIG. 2, waste stream  29  can be scrubbed within a scrubbing stage  58  by a partial stream  60  formed of thiosulfate liquor produced within white liquor oxidizing stage  46  to form a scrubbed stream  44   b  which is then introduced into polysulfide reaction stage  42  as the third oxygen containing stream. The excess of scrubbed stream  44   b  not used within polysulfide reaction stage  42  is then subdivided into first and second oxygen containing streams  24  and  34  and forth and fifth oxygen containing streams  50  and  52 . The thiosulfate liquor after having served its scrubbing function is returned as a recycled thiosulfate stream  61  which is added to the white liquor and used in forming subsidiary streams  54  and  56 . 
     Alternatively, partial stream  60  could be formed of fully oxidized white liquor from complete white liquor oxidizing stage  48 , white liquor, or polysulfide liquor from polysulfide reaction stage  42 . If fully oxidized white liquor is used, only carbon dioxide will be removed. No ozone destruct tubes place. As a result the residual ozone would eventually be consumed. This would not be preferred because the ozone would adversely effect conventional equipment and fittings. The resultant scrubbed stream could then again be introduced into polysulfide reaction stage  42  with the excess being subdivided into first and second oxygen containing streams  24  and  34 , the third oxygen containing stream, and forth and fifth oxygen containing streams  50  and  52 . A further alternate is that scrubbed stream  44   b  could be compressed and introduced into either white liquor or complete white liquor oxidizing stage  46  or  48  and then, the excess subdivided into first and second oxygen containing streams  24  and  34 , the third oxygen containing stream, and either the forth or fifth oxygen containing stream  50  and  52 . As is apparent from the above discussion, in the embodiment of FIG. 2, the scrubbed stream is being used to form all oxygen containing streams. 
     With reference to FIG. 3, a preferred fully oxidized white liquor reactor  62  is illustrated. Reactor  62  consists of a liquid/vapor contacting column  64  of approximately 9.84 meters in height by about 0.9 meters in diameter. Column  64  is provided with an a white liquor inlet  66  and an oxygen inlet  68  to top and bottom regions  70  and  72  of column  64 , respectively. An oxygen stream is introduced into the column through inlet  66  and a white liquor stream is introduced into the column through inlet  68 . 
     The white liquor and oxygen are brought into intimate contact by contacting elements which are preferably formed by beds of structured packing designated by reference numeral  74 . As would be known by those skilled in the art, liquid distributors would be located between pairs of beds. The white liquor is introduced into structured packing  74  by a liquid distributor  76  and the oxygen rises through the open area of structured packing  74 . Structured packing is efficient and has a very low pressure drop. This allows the recycling of the gas stream with a blower or an eductor. It is to be noted that to preclude clogging of the packing by particulates, the packing type and crimp angle are important. In this regard, structured packing  74  can have a packing density of between about 500 m 2 /m 3  and is preferably Koch Type  1 X or  1 Y which can be obtained from Koch Engineering Company, Inc. of Wichita, Kansas. Random packing and trays could also be used with less effectiveness. 
     Column  64  should be operated at a pressure of no less than 9.2 atmospheres absolute. The oxygen should have a purity as high as is economical with 90% and above being preferred. The reaction should proceed at a total pressure of no less than about 9.2 atmospheres absolute and more preferably at least about 11.2 atmospheres absolute. Additionally, the reaction between the oxygen and the sodium sulfide should occur at a minimum temperature of about 110° C. A minimum reaction temperature of about 120° C. is more preferred and reaction temperatures at or above 150° C. are particularly preferred. A particularly preferred temperature and pressure is about 200° C. and about 18 atmospheres absolute. 
     The reaction of oxygen and sodium sulfide is an exothermic reaction. However, to start the reaction heat must be added to the white liquor to raise it to the requisite reaction temperature. To this end, a heat exchanger  78  can be provided before inlet  66  in which the incoming white liquor is heated by indirect heat exchange with steam. After the reaction progresses, heat exchanger  78  can be shut down. 
     The oxidized white liquor collects as a column bottom  80  within bottom region  72  of column  64 . A product stream  82  of the oxidized white liquor is removed from bottom region  70  of column  64  and divided into first and second fully oxidized white liquor streams  32  and  38 . At the same time, an oxygen containing tower overhead collects within top region  70  of column  64 . 
     Tower overhead stream is circulated by an eductor  82  having a low pressure inlet  84 , a high pressure outlet  86 , and a high pressure inlet  88 . A stream of in-process white liquor is pumped by a pump  90  through eductor  82 . Low pressure inlet  84  of eductor  82  draws the tower overhead stream from top region  70  of column  64 . The pumped oxidized white liquor is introduced into a high pressure inlet  88  of eductor  82  and a combined stream of tower overhead and oxidized white liquor is discharged from high pressure outlet  86  of eductor  82 . High pressure outlet  86  is connected by a conduit  92  to bottom region  70  of column  64  in order to circulate the oxygen-containing column overhead back into bottom region  70 . 
     Stripped gas impurities and reaction products which may serve to dilute the tower overhead stream and thereby lower oxygen partial pressure can collect at the top of column  64 . In order for such gas impurities and reaction products to not affect the reaction, they can be periodically or continually vented through the use of a small vent  94  provided for such purpose. 
     The following are examples of the method of the present invention as carried out in FIGS. 1 and 2. 
     EXAMPLE 1 
     The following is an example of a practice of the invention in accordance with the embodiment illustrated in FIG.  1 . For purposes of the examples set forth herein it is assumed that the white liquor has the following composition: 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Unoxidized White Liquor (UWL) Composition 
               
             
          
           
               
                   
                 g/L as salt 
                 g/L as sulfur 
               
               
                   
                   
               
             
          
           
               
                   
                 Na 2 S 
                 40 
                 16.4 
               
               
                   
                 NaOH 
                 100 
                 — 
               
               
                   
                 Na 2 CO 3   
                 33.7 
                 — 
               
               
                   
                 Na 2 S 2 O 3   
                 1.3 
                 0.5 
               
               
                   
                 Na 2 S x   
                 0 
                 — 
               
               
                   
                 Na 2 SO 4   
                 1.0 
                 0.2 
               
               
                   
                 Total 
                 176 
                 17.1 
               
               
                   
                   
               
             
          
         
       
     
     In the following discussion, the term “kg/mtpd pulp” means kilograms per metric ton per day of oven dried wood pulp being processed. In this Example 1, about 333 kg/mtpd pulp of white liquor is introduced into polysulfide reaction stage  42 . Additionally, 813 kg/mtpd pulp of white liquor is divided so that subsidiary stream  54  flows at about 250 kg/mtpd pulp and subsidiary stream  56  flows at approximately 563 kg/mtpd pulp to supply white liquor and complete white liquor oxidizing stages  46  and  48 . Polysulfide reaction stage  42  in this example operates at approximately 80° C. and at 1 atm and produces 20 kg/mtpd pulp of polysulfide expressed as sulfur. 
     The typical composition of the polysulfide liquor, expressed in grams/liter salt or grams/liter sulfur is as follows: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Grams/Liter as 
                   
               
               
                   
                 Sulfur 
                 Grams/Liter as Salt 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Na 2 S x   
                 5.0 
                 — 
               
               
                   
                 NaOH 
                 — 
                 100 
               
               
                   
                 Na 2 CO 3   
                 — 
                 33.7 
               
               
                   
                 Na 2 SO 4   
                 1.0 
                 — 
               
               
                   
                 Na 2 S 2 O 3   
                 2.0 
                 — 
               
               
                   
                   
               
             
          
         
       
     
     The production of thiosulfate liquor and fully oxidized white liquor of partial and complete white liquor oxidizing stages  46  and  48  are roughly equal to the flow rates of white liquor entering these stages. The composition of the thiosulfate liquor and the fully oxidized white liquor is as follows when expressed in g/L as salt. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Thiosulfate 
                 Fully Oxidized 
               
               
                   
                 Liquor 
                 White Liquor 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Na 2 S 
                 0.0 
                 0 
               
               
                   
                 NaOH 
                 100 
                 85 
               
               
                   
                 Na 2 CO 3   
                 33.7 
                 33.7 
               
               
                   
                 Na 2 SO 4   
                 — 
                 73 
               
               
                   
                 Na 2 S 2 O 3   
                 — 
                 &lt;1.0 
               
               
                   
                   
               
             
          
         
       
     
     All of the thiosulfate liquor is utilized in oxygen delignification stage  20  while about 188 kg/mtpd pulp of the fully oxidized white liquor is used in extractive oxidation stage  30  with peroxide and about 375 kg/mtpd pulp of fully oxidized white liquor is used in a final peroxide bleaching stage  36 . An ozone generator (not illustrated) is required to produce a mixture of about 10 kg/mtpd pulp of ozone and 73 kg/mtpd pulp of oxygen. In ozone bleaching stage  26 , roughly 0.2% of the ozone is lost and waste stream  29  has the following approximate composition: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 WL for Ozone Stage off-gas Cleaning 
               
               
                 Typical composition on a weight percent basis 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Oxygen 
                 83% 
               
               
                   
                 O 3   
                  0.2% 
               
               
                   
                 CO 
                 30-40 ppm 
               
               
                   
                 CO 2   
                  8-9% 
               
               
                   
                 H 2 O 
                 satd at 40° C. 
               
               
                   
                 Organics 
                 &lt;10 ppm 
               
               
                   
                   
               
             
          
         
       
     
     All of waste stream is introduced into polysulfide reaction stage  42  which in turn uses about 14.9 kg/mtpd pulp of oxygen. Scrubbed stream  44  contains approximately 58 kg/mtpd pulp of oxygen, approximately 30 ppm carbon monoxide and water saturated at 80° C. Scrubbed stream  44  is compressed in compressor  45  to between about 100 and 150 psig and approximately 25 kg/mtpd pulp of oxygen is introduced into oxygen delignification stage  20 , about 5 kg/mtpd pulp of oxygen is introduced into the extractive oxidative stage  30 . Approximately 4.9 kg/mtpd pulp of oxygen is introduced into the partial white liquor oxidation stage  46  and about 22.2 kg/mtpd pulp of oxygen is introduced into the complete white liquor oxidizing stage  48 . The result of this is about 72 kg/mtpd pulp of oxygen is consumed and about 1 kg/mtpd pulp of oxygen is lost or vented from the process. 
     As can be seen from this example, a major advantage of the present invention is that most of the oxygen can be recycled back into the pulp bleaching apparatus and process if the waste stream  29  is first introduced into polysulfide reaction stage  42 . Polysulfide reaction stage  42  will scrub carbon dioxide from waste stream  30  while consuming some of the oxygen. This will produce a lesser volume to be compressed by compressor  45  which is an advantage to be realized in lower power consumption. 
     EXAMPLE 2 
     Example 1 has particular application to white liquor that does not have too high a sulfidity. When sulfidity is high, the carbonic acid formed in the polysulfide reactor due to the presence of carbon dioxide will tend to neutralize the alkalinity of the polysulfide. In such case, the waste stream is scrubbed by a scrubber as illustrated in FIG.  2 . In this example the flow rates of the various sodium hydroxide streams and oxygen containing streams will be the same as in the previous example. The main difference is that more white liquor will be needed to scrub waste stream  29 . In this regard, 1,266 kg/mtpd pulp of white liquor is consumed in this example as compared with 1146 kg/mtpd pulp of white liquor in Example  1 . The incoming white liquor is distributed so that about 933 kg/mtpd pulp of white liquor is used in white liquor oxidation (white liquor and complete white liquor oxidizing stages  46  and  48 ) and again, about 333 kg/mtpd pulp of white liquor is utilized in polysulfide reaction stage  42 . Approximately 370 kg/mtpd pulp of white liquor is introduced into partial white liquor oxidizing stage  46  and about 563 kg/mtpd pulp of white liquor is introduced again into complete white liquor oxidizing stage  48 . About 126 mtpd pulp of white liquor is used in forming scrubbing stream  60 . 
     While the invention has been illustrated with reference to a preferred embodiment, it will be understood by those skilled in the art that numerous additions, modifications, and omission may be made without departing from the spirit and scope of the present invention.

Technology Classification (CPC): 3