CA2240030A1 - Method for washing bleached wood pulp - Google Patents
Method for washing bleached wood pulp Download PDFInfo
- Publication number
- CA2240030A1 CA2240030A1 CA 2240030 CA2240030A CA2240030A1 CA 2240030 A1 CA2240030 A1 CA 2240030A1 CA 2240030 CA2240030 CA 2240030 CA 2240030 A CA2240030 A CA 2240030A CA 2240030 A1 CA2240030 A1 CA 2240030A1
- Authority
- CA
- Canada
- Prior art keywords
- stage
- pulp
- filtrate
- bleaching
- washer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Landscapes
- Paper (AREA)
Abstract
A process for washing bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence in which a reduced amount of bleaching chemicals and/or makeup water is used.
Description
D-20,596 M~T-HOi~ FOR WAS~rN-G B~EAC~E-~ WW~ FJL~
FIELD OF THE INVENTION
The present invention relates to a process for washing bleached wood pulp using a multi-stage 5 bleaching sequence. More particularly, the present invention is directed to a process for washing bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence in which a reduced amount of bleaching chemicals and/or makeup water is 10 used.
BACKGROUND OF THE INVENTION
After the pulping operation, bleaching processes remove ligneous and coloring material remaining in the pulp. It is important that bleaching be accomplished 15 with as little degradation of the pulp fibers as possible. Consequently, pulp bleaching has advanced to a high degree and incorporates a multi-stage sequence utilizing various bleaching chemicals and washing processes. Each stage in a bleaching sequence is 20 composed of a tower where bleaching chemicals react with the pulp and a washer where spent bleaching chemicals and undesirable constituents are removed.
Chemicals used to bleach pulp include, but are not limited to, elemental chlorine, chlorine dioxide, 25 oxygen, hypochlorites, chlorides, peroxides, chlorates, dichromates, and permanganates. For chemical pulp, i.e., those made by the kraft, sulfite or soda processes, traditional bleaching agents include chlorine and chlorine dioxide.
Conventionally, a bleachlng stage performed using chlorine is designated a C stage. A bleaching stage D-20, 596 performed using chlorlne dioxide is designated a D
stage. C and D stages have low pH values, around pH of about 1. 5 to pH of about 5 . 0~ due to the reaction of chlorine and chlorine dioxide with lignin and water.
5 Chlorine dioxide may be wholly or partially substituted for chlorine in the first stage of a bleach plant; such a stage is commonly denoted as CD. As used herein, the first stage of the bleach plant may be called a C stage when only chlorine is used, a D stage in which only 10 chlorine dioxide is applied or a CD stage in which chlorine dioxide is substituted in part for chlorine.
An alkaline extraction, called an E stage, is commonly placed between successive bleaching stages (i.e., C and/or D stages). If the extraction stage 15 incorporates oxygen, or oxygen and peroxide, in combination with the alkaline medium, the E stage is designated as EO or EOP~ respectively. E stages, used to extract dissolved lignin from the pulp, are called alkaline or extraction stages because caustic soda 20 (NaOH) is used in those stages to raise the pH of those stages anywhere from about pH 9.0 to about pH 12.5.
Typical five-stage bleaching sequences are designated CEDED, DEDED, CEOPDED~ DEOPDED~ etc. Typical four-stage bleaching sequences may be designated CEDD, 25 DEDD, CEOPDD~ DEOPDD~ etc. Typical three-stage bleaching sequences include DED, CED, and CDEopD~
Additionally, subscripts may be used to distinguish similar stages, for example, CE1D1E2D2 and D1ED2D3.
In addition to the order and type of bleaching 30 stages, there are various washing methods incorporated in a multi-stage bleach plant. The three principal mllltl-st2ge washing processes are direct countercurrent washing, split-flow countercurrent washing, and D-20,596 jump-stage countercurrent washing. The differences among the three processes are in the manner in which the wash water is applied to and the filtrates are recycled from the washing steps of each stage.
As used herein, "makeup water" refers to either fresh water, white water (water recycled from the paper machines), or cleaned condensates (condensate from a stripper or the first stages of evaporation). Makeup water represents additional water added to the 10 bleaching process and is used to wash the pulp. Makeup water may be applied to any bleaching or extraction stage, however, the majority of the makeup water typically is applied to the last extraction stage and the last bleaching stage. "Filtrate" refers to the 15 liquor (liquid) displaced from the pulp slurry as the pulp traverses a washer. "Dilution water" is filtrate or makeup water that is used to dilute the stock in the washer vat. "Wash water" is either makeup water or filtrate and is used to wash pulp. Filtrate has value 20 as wash water on another stage and typically is recycled. Filtrate may also be used as dilution water for the washer vat or the repulper dilution water. A
portion of the filtrate is discarded from each washing stage, wherein the majority of the filtrate discarded 25 is from the first bleaching stage (acid filtrate) and the first extraction stage (alkaline filtrate).
Dilution water typically is used to dilute thickened pulp from a consistency of 10-15% down to a consistency of approximately 1-3%. As used herein, "bleaching 30 chemicals" refers to all chemicals used within the bleach plant. Generally, "bleaching chemicals" refers tc the traditional bleaching aids of chlorine and chlorine dioxide but can also refer to caustic soda, D-20,596 hypochlorite, oxygen, peroxide, defoamers, drainage aids and the like.
Although a variety of methods for washing bleached wood pulp are available, the state of the art still 5 suffers from expensive bleach plant costs due to the use of bleaching and extraction stage chemicals, the high use of fresh water and poor runnability.
Runnability generally refers to how the bleach plant is running. Poor runnability typically results in 10 decreased production and/or excess downtime to fix problems or make adjustments.
A number of works have been reported in the prior art, none of which is satisfactory in reducing the plant cost associated with chemical usage, fresh water 15 usage and poor runnability.
U.S. Patent No. 3,698,995 to Rapson is directed to multi-stage bleaching using countercurrent washing with the object of recovering spent chemicals. Carbon dioxide is not used in the washing.
U.S. Patent No. 3,907,632 to Christiansen, et al.
is directed to bleaching of cellulosic pulp with gaseous chlorine. It is stated that the pure chlorine gas may be diluted with an inert gas such as air, carbon dioxide, or nitrogen to provide a gaseous medium 25 wherein the active contacting component consists essentially of gaseous chlorine. Countercurrent washing is not believed to be used.
U.S. Patent No. 5,139,613 to Lachapelle is directed to a process for acidifying chemical and 30 mechanical pulp using carbon dioxide at the end of the bleaching process. It is said that carbon dioxide can be used in place of sulfur dioxide to stabilize the bleached pulp and prevent deterioration of pulp D-20,596 strength. Carborl dioxiae is introduceu after ine las bleaching stage, and not used internally during a multi-stage bleaching process.
U.S. Patent No. 5,429,717 to Bokstrom is directed 5 to a method of washing alkaline, cellulosic pulp wherein the pH is lowered in one or more consecutive washing stages by the addition of carbon dioxide. The pH of the filtrate or wash water for the extraction is above 6.8.
OBJECTS OF THE INVENTION
It is a principal object of the present invention to reduce the use of bleaching chemicals in the D
stage of bleach plants using alkaline wash water, comprising filtrate recycled from the second extraction 15 stage, or the first extraction stage washer of a CE1D1E2D2 bleaching sequence.
It is another object of the present invention to reduce the use of bleaching chemicals in any bleaching stage in bleach plants using countercurrent washing 20 with at least two extraction stages and applying alkaline wash water, whether comprised of filtrate from a downstream extraction stage or not, on any extraction stage. Such bleaching sequences include, but are not limited to, DEDED, CEo~DED/ DEoDED/ etc. Note that the 25 importance lies in the method of washing and not on the actual bleaching chemicals.
It is another object of the present invention to reduce the amount of bleaching chemicals and makeup water applied to any stage of a bleaching sequence 30 having only one extraction stage and using a split-flow washing arrangement.
D-20,596 It is another object of the present invention to reduce the amount of bleaching chemicals and makeup water applied to any stage of a bleaching sequence having a plurality of extraction stages and using a 5 split-flow washing arrangement.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic of a five-stage, bleaching process using conventional countercurrent washing;
Figure 2 is a schematic of a five-stage, bleaching 10 process using jump-stage countercurrent washing; and Figure 3 is a schematic of a five-stage, bleaching process using split-flow countercurrent washing.
SUMMARY OF THE INVENTION
This invention provides a process for the 15 preparation of bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence having at least one extraction stage. This process comprises lowering the pH of an alkaline filtrate with an acidic medium, and using the filtrate 20 to wash an upstream pulp.
The pulp is from an upstream extraction stage or an upstream bleaching stage. The filtrate is recycled from a downstream extraction stage. Although a number of acidic media may be used, carbonic acid formed from 25 carbon dioxide is preferred. The pH of the alkaline filtrate is lowered to a range of from about pH 4.5 to about pH 6.5. Prior to acidification, the pH of the filtrate is from about pH 9.0 to about pH 12.5. The pH
of the pulp after extraction stage washing is from 30 about pH 6.5 to about pH 9Ø The countercurrent washing may include direct countercurrent washing, CA 02240030 l99X-06-09 D-20,596 jump-stage countercurrent washing or split-flow countercurrent washing.
In another embodiment, this invention is also directed to a process for the preparation of bleached 5 wood pulp utilizing countercurrent washing in a multi-stage sequence. The process comprises lowering the pH of a wash water to a range of from about pH 4.5 to about pH 6.5 with an acidic medium, and using the wash water to wash the bleached wood pulp in a washer.
The washer is either a bleaching stage washer or an extraction stage washer. The acidic medium is preferably carbonic acid formed from carbon dioxide.
The wash water is an alkaline wash water. The pH of the pulp after extraction stage washing is from about 15 pH 6.5 to about pH 9Ø
In yet another embodiment, this invention is also directed to a process for the preparation of bleached wood pulp in a multistage bleaching sequence having at least one extraction stage. This process comprises 20 iowering the pH of the pulp to a range of from about pH
6.5 to about pH 9.0 with an acidic medium.
The pulp is acidified in the washer vat or the repulper of the extraction stage. Preferably, carbonic acid formed from carbon dioxide is used.
In still yet another embodiment, the invention is directed to a process for the preparation of bleached wood pulp in a multi-stage bleaching sequence having at least one extraction stage, the process comprising lowering the pH of a filtrate to a range of from about 30 pH 4.5 to about pH 6.5 with an acidic medium, and using the filtrate as dilution water.
The filtrate is used as dilutiGn water Gn an extraction stage or an upstream bleaching stage.
D-20,596 Preferably, the acidic medium used is carbonic acid formed from carbon dioxide.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention has numerous 5 advantages over prior art washing processes. The process of the invention reduces fiber swelling and has greatly improved washing efficiency, requiring less makeup water in the overall process. The process results in reduced oxidized material in the pulp slurry 10 and can be used to provide a higher consistency pulp which is cleaner, as measured by dirt count, and wherein the overall process uses a lower amount of bleaching chemicals. In addition, the process of the present invention reduces foaming and scaling tendency 15 and reduces upsets associated when dissimilar filtrate mixes. Finally, the increased consistency and lower water usage translate into considerable energy savings because less wash water must be heated to obtain proper washing. As used herein, pulp consistency refers to 20 the concentration and its is defined as the weight in grams of oven dry fiber in 100 grams of pulp-water mixture. See, TAPPI test method 204.
As shown in the figures, the present invention is directed to a washing sequence, preferably a 25 multi-stage bleaching sequence using countercurrent washing, whereby alkaline wash water or dilution water is applied to an extraction stage washer or bleaching stage washer. The pH of alkaline dilution water or wash water is reduced with an effective amount of 30 acidic medium. While various acids, including mineral acids and sulfur dioxide, couid be used to reduce pH, carbon dioxide is highly preferred. Carbon dioxide is CA 02240030 l998-06-09 D-20, 596 less corrosive and poses less of an environmental threat, as compared to mineral acids and sulfur dioxide. Generally, the pH of the alkaline wash water or dilution water is reduced to as low as practical to 5 a pH of from about pH 4. 5 to about pH 6 . 5, preferably about pH 5.5. By reducing the pH of the wash water or dilution water, pulp pH typically is reduced to between about pH 6 . 5 and about pH 9.0 for alkaline stage washers and to between about pH 5 . 5 and about pH 8.0 10 for acid stage washers. Reducing the pH to this level requires from about 5 to 20 lbs of CO2 per ton of pulp processed, depending upon the initial pH of the wash water or dilution water. If filtrate from an extraction stage serves as the wash water or dllution 15 water, then typically the initial pH of the filtrate ranges from about pH 9.0 to about pH 12 . 5 . Fresh water, however, may have a pH in the range of from about pH 14.0, preferably about pH 12.0, to about pH
7Ø
For the purposes of this invention, the focus of this application is on the washing steps of each stage, and more particularly, the process of washing in each stage. The three washing methods are detailed below.
The descriptions below only detail the major water 25 flows to the washers.
Fig. 1 provides a schematic drawing for direct countercurrent washing. In direct countercurrent washing, filtrate from the washer of a downstream stage is applied as wash water and dilution water to the 30 washer of the adjacent upstream stage. Usually, direct countercurrent washing is avoided since mixing of caustic and acidic filtrates promotes foaming, scaling, and excessive heat generation. In addition, if the CA 02240030 l998-06-09 D-20,596 _ 1 n _ wasn water 'fbreaks through" the pulp mat on the washer, acid and alkaline filtrates will mix and disrupt (or upset) the pH balance necessary for efficient operation of the bleach plant. To alleviate these problems, 5 large amounts of fresh water are added to each stage, and excess filtrate is purged from the system. The main advantage of this method of washing is that pulp is prepared for the next stage by using filtrate from the next stage. For example, the pulp exiting the E1 10 stage is washed with Dl filtrate immediately before it enters the D1 stage. In this way, the pulp pH is brought closer to the desired reaction pH in the next stage, and the amount of bleaching chemicals required in downstream stages is reduced.
In a typical direct countercurrent washing sequence as shown in Fig. 1, fresh water stream 1 washes pulp 54 on the D2 washer, resulting in filtrate 6 and washed pulp 55. Filtrate stream 6 is split into wash water 7 and acid filtrate 8. Wash water 7 and 20 fresh water stream 2 wash pulp 53 on the E2 alkaline-stage washer, resulting in filtrate 9 and washed pulp 54. Filtrate stream 9 is split into wash water 10 and alkaline filtrate 11. Wash water 10 and fresh water stream 3 wash pulp 52 on the D1 acid-stage 25 washer, resulting in filtrate 12 and washed pulp 53.
Filtrate 12 is divided into two streams 13 and 14.
Fresh water stream 4 and filtrate stream 13 are used as wash water on the E1 stage washer to wash pulp 51, resulting in washed pulp 52 and filtrate 15. Filtrate 30 stream 15 is split into wash water 16 and alkaline filtrate 19. Wash water 16 and fresh water stream 5 wash pulp 50 on the CD stage washer, resulting in filtrate 17 and washed pulp 51. Alkaline filtrates 19 D-20,596 and 11 are discarded, and acid filtrates 8, 14 and 17 are also discarded.
Fig. 2 provides a schematic drawing for jump-stage countercurrent washing. Jump-stage washing alleviates 5 some of the problems associated with direct countercurrent washing. In jump-stage countercurrent washing, filtrate from a downstream extraction stage is used to wash pulp entering an upstream extraction stage, and filtrate from later bleaching stages is used 10 to wash pulp entering earlier bleaching stages. For a typical five-stage bleach plant (CE1D1E2D2), pulp is introduced in the first stage, C, and traverses in series through the remaining stages: El, D1, E2, and D2.
The bulk of the makeup water is applied to the last two 15 stages, the E2 and D2 stages. This makeup water displaces the filtrate within the pulp slurry as the pulp traverses the washer. As mentioned previously, these filtrates cannot be mixed with filtrate from dissimilar stages because the large differences in pH
20 would cause considerable heat generation, foaming and scaling. In addition, filtrate from a D2 stage (or E2 stage) is not applied to the immediate prior stage (E2 stage (or D1 stage), respectively) for similar reasons.
Hence, filtrates are jumped and are only applied on 25 like stages. That is, the E2 filtrate ~umps over the D1 washer and is applied as wash water on the E1 washer; likewise, the D2 filtrate jumps over the E2 washer and is applied as wash water on the Dl washer.
The D1 filtrate is used as wash water on the C washer.
30 Finally, the C and E1 stage filtrates are discarded.
This process's main advantages over direct countercurrent washing are that filtrates of dissimilar pH do not contact one another and less makeup water is D-20,596 required to ensure ~leach plant performance. The method described above is the current state of the art.
In a typical jump stage countercurrent washing sequence as shown in Fig. 2, white water stream 201 5 washes pulp 254 on the D2 acid-stage washer, producing filtrate 202 and washed pulp 255. Filtrate 202 from this washing step is split into streams 203 and 204.
Filtrate stream 203 jumps over the E2 alkaline-stage and is applied as wash water on the D1 acid-stage 10 washer. Together with fresh water stream 205, this filtrate washes pulp 252 and produces washed pulp 253 and filtrate 206. Filtrate 206 is split into wash water stream 207 and filtrate stream 208. Wash water stream 207, together with hot condensate 209, washes 15 pulp 250 to produce washed pulp 251 and filtrate 210.
Filtrates 204, 208, and 210 combine to form acid filtrate 211, which is discarded. For the alkaline washing sequence, hot condensate 220 washes pulp 253, resulting in filtrate 221 and washed pulp 254.
20 Filtrate 221 splits into filtrate stream 223 and wash water stream 222. Wash water stream 222 jumps over the D1 stage and is applied to the E1 alkaline-stage washer. Together with white water stream 224, wash water 222 washes the pulp on the E1 stage washer, 25 resulting in washed pulp 252 and filtrate 225.
Filtrates 223 and 225 combine to form alkaline filtrate 226 for disposal.
Fig. 3 is a schematic drawing for split-flow countercurrent washing. Split-flow washing combines 30 the characteristics of direct countercurrent washing and jump-stage countercurrent washing. Split-flow washing involves washing the pulp wherein a first set of showers applies wash water that is similar to the D-20,596 stage the pulp is ieaving. Lne second set o~ showers on the washer then washes the pulp with filtrate similar to the stage the pulp is entering. In a five-stage bleach plant using a split-flow arrangement, 5 wash water is applied in a similar fashion as that in a conventional bleach plant using jump-stage washing, with two differences. First, the wash water that is jumped is applied to the pulp using the first set of showers, as the pulp is lifted out of the vat and 10 applied to the washer. Secondly, a portion of the filtrate from the downstream washer is applied to the back end of the washer, on the second set of showers, to prepare the pulp for the next stage. For example, on the E1 washer, the first wash water applied to the 15 pulp is filtrate from the E2 washer; after the E2 filtrate is applied, low pH filtrate from the D1 washer is applied.
This arrangement has several advantages over the previously described washing methods. First, it uses 20 up to 80% less water than the jump-stage washing method. Secondly, as in the direct countercurrent method, the pulp is prepared for the next stage because filtrate of similar pH to the next stage is applied to the pulp immediately prior to its application on the 25 next stage. Nevertheless, this method still has some of the disadvantages associated with direct countercurrent washing, in that mixing of dissimilar filtrates promotes scaling, foaming, and heat generation.
In a typical split-flow countercurrent washing sequence as represented in Fig. 3, fresh water stream 304 is applied to the D2 acld-stage washer using the second set of showers. White water stream 301 is split CA 02240030 l998-06-09 D-20,596 into two streams 3û2 and 303. White water stream 302 is applled to the D2 washer using the first set of showers. Filtrate 305 from this stage is split into streams 306 and 307. Filtrate 306 is applied to the E2 5 alkaline stage washer using the second set of showers, and white water stream 303 iS applied to the E2 washer using the first set of showers. These wash waters wash pulp 353, resulting in filtrate 3 U 8 and washed pulp 354. Filtrate 308 is split into streams 309 and 31û.
10 Filtrate 309, applied to the D1 washer using the second set of showers, and filtrate 307, applied using the first set of showers, wash pulp 352 to produce washed pulp 353 and filtrate 311. Filtrate 311 is split into streams 312 and 313. Filtrate 312 is applied to the 15 E1 alkaline-stage washer using the second set of showers, and filtrate stream 310 iS applied to the E
washer using the first set of showers. These wash waters wash pulp 351, resulting in filtrate 314 and washed pulp 352. Filtrate 314 is split into streams 20 315 and 316. Filtrate 316 is discarded as alkaline filtrate. Filtrate 315, applied to the Dc washer using the second set of showers, and filtrate 313, applied using the first set of showers, wash pulp 350 to produce washed pulp 351 and filtrate 317. Filtrate 25 stream 317 is split into recycle stream 319 and alkaline filtrate stream 318. Alkaline filtrate stream 318 is discarded whereas recycle stream 319 dilutes incoming pulp 350.
In the case of three- or four-stage bleaching 30 sequences, makeup water may be applied to the E stage washer. Generally, in this bleach plant scheme, pulp travels though a ~ stage, Eop stage, and one or two more D stages in order. Each stage includes a tower D-20,596 and washer. In this arrangement, makeup water is applied to the last D stage, the first D stage, and the Eop stage. None of the alkaline filtrate is recycled.
Of the acid filtrates, the D3 filtrate (if present) is 5 recycled to the D2 washer, the D2 filtrate is recycled to the Eop and the Dl washers (as in a split-flow arrangement), and the Dl and Eop filtrates are discarded.
In the embodiments in which the pH of the wash 10 water is reduced, the reduced pH wash water is applied to the pulp through the shower head of an upstream extraction stage or bleaching stage. In the embodiments in which the pH of the dilution water is reduced, this dilution water is applied to the pulp in 15 the washer vat or repulper. For an alkaline extraction stage, the dilution water may be applied to the pulp through the vat to obtain maximum benefits. If the washer is an acidic stage washer, the dilution water should be applied to the repulper to prepare the pulp 20 for the next stage.
Each of the washing method embodiments described herein enhances bleach plant operation or, in the specific embodiment in which alkaline filtrates are acidified and applied as wash water on upstream 25 washers, replaces makeup water on previous stages. In general, the process of the present invention at least 1) transitions the pulp from stage to stage, 2) enhances washing efficiency, 3) reduces makeup water requirements and 4) reduces bleach plant operating 30 costs. Reducing the pH of the pulp prevents fiber swelling and enhances washing. As a result, more organic and inorganic chemicals are washed from the pulp slurry, yielding a brighter pulp that is easier to CA 02240030 l998-06-09 3-20, 596 bleach in subsequent stages. ~n addition, by controlling pH, both scaling and pitch deposition will be reduced. Also, because washing is more efficient, consistency is improved, wash water volume is reduced, 5 energy consumption is lowered and final brightness is increased.
In the embodiment in which the dilution water is added to the vat of an alkaline washer, the dilution water lowers the pH by combining with the high pH
10 liquor already present. In the embodiment in which the dilution water is added to the repulper of an acidic washer, the pH will be increased by diluting the thickened pulp with the dilution water. A repulper is a device that catches the pulp as it comes off the 5 washer; it breaks up the pulp mat, and sometimes dilution water is added to the repulper before the pulp is sent to the next stage. In the embodiments in which the pH of the wash water is reduced, the reduced pH
wash water displaces filtrate within the pulp.
Preferably, the pH of the pulp is reduced to less than about pH 9.0, thereby converting alkaline earth metal salts of calcium or barium to soluble bicarbonates and aiding their removal from the pulp slurry. With this reduction in calcium and barium 25 ions, reduced scale formation is realized. Also, free calcium and barium ions are known to increase the consumption of downstream bleaching chemicals. By removing these ions, less bleaching chemicals are required in downstream bleaching stages. In addition 30 to lowering calcium and barium ion levels, lowering the pH of the pulp on the washer improves pulp drainage.
Consequently, more dirt, COD, and spent chemicals are washed from the pulp, producing a cleaner, brighter D-20,596 puip t~lat is easier to DieaCn in subsequenl bleacn1ng stages. Finally, the pulp pH is modified so as to transition pulp between stages as is accomplished less efficiently in state of the art split-flow washing 5 methods. That is, when the pulp is acidified between an alkaline stage and a subsequent acidic stage, the pH
is increased and less caustic soda is required in the alkaline stage to raise the pH to the proper operating conditions at from about pH 9.0 to about pH 12.5.
10 Alternatively, pulp pH can be reduced by lowering the wash water pH so as to transition the pulp from an alkaline stage to a subsequent acidic stage.
Therefore, less bleaching chemicals are required in the acidic stage to lower the pH to the proper operating 15 conditions, that is from about pH 1. 5 to about pH 5Ø
The pH of alkaline filtrates may be lowered so that the filtrates can be applied on previous washers in a split-flow fashion. In this embodiment, filtrate from a downstream extraction stage is acidified with 20 CO2, and the resultant filtrate is applied on an upstream bleaching stage. The resultant filtrate replaces makeup water that would normally be used on the bleaching stage.
The embodiments of the present invention in which 25 wash water is acidified and then applied to the washers in split-flow fashion provide a similar process as that for jump-stage washing with some additional benefits.
Namely, split-flow washing using the process of the present invention reduces the operational problems 30 associated with break through on all washers and reduces the problem of foaming. These benefits are realized because filtrates of vastly different pHs are no longer being applied on a single washer. Instead of - D-20,596 , ~ _ using both iow pn filtrates (from acid stages) and high pH filtrates (from alkaline stages) together on a single washer, near neutral pH filtrates (from the alkaline stages, modified with CO2) and low pH
5 filtrates (from acid stages) are used.
Break through occurs when wash water penetrates the pulp on the washer and mixes directly with the extracted filtrate. As the filtrates mix, the pH
balance necessary for proper mill operation is 10 adversely affected, causes upsets in bleach plant operation, and impairs pulp quality. By using the process of the present invention, if break through does occur and the filtrates mix, the chances of foaming, pitch deposition, scaling, and heat generation are 15 significantly reduced.
Foaming tendency is greatly reduced. Mixing of unlike filtrates causes foaming. By using the process of the present invention, differences in pH are minimized and less foam is created. Thus, less 20 defoamers are required by the process.
The changes in pH can cause pitch to be deposited onto the washer and can cause considerable heat generation. Pitch is defined as a resinous substance, composed of resin and fatty acids, found in wood pulp 25 that is soluble in neutral organic solvents. By using the process of the present invention, these operational problems can be avoided or minimized. Moreover, the pulp quality may be improved by lowering the dirt count of the pulp. "Dirt" is defined according to Tappi test 30 method T 213, which is incorporated herein by reference.
By using the process of the present invention, scaling tendency is reduced. This reduction is D-20,596 accompl1shed by controlling wash water pH such that scale does not form. As a result, the present bleach stock washing process reduces the formation of scale deposits on extraction stage washes in mills having 5 elevated calcium or barium levels in their washing system. Those skilled in the art will realize that scaling is a combination of many factors, including, but not limited to, water hardness, pH, ion concentration, and temperature.
Finally, with proper control of wash water, the total amount of makeup water can be reduced. This reduction is beneficial because more filtrate from the extraction stage can be recycled to the acid-stage washers. Consequently, less acid-stage makeup water is 15 required, and the total makeup water requirements are reduced.
Although various parameters may be used to evaluate the efficiency of the extraction stage washers, the parameter chosen for the purpose of this 20 application is the removal efficiency of chemical oxygen demand (COD). This calculation is performed by measuring the COD of the filtrate entering the washer with the pulp minus the COD of the filtrate exiting the washer with the pulp, divided by the COD of the 25 filtrate entering the washer with the pulp, in kg/tonne pulp, or similar units. The lower the COD of the filtrate leaving with the pulp (throughout the course of the bleaching sequence), the lower will be the bleaching chemicals required in subsequent stage(s).
30 The efficiency of the extraction stage and its associated washer is increased by decreasing the pH of the wash water or the pH of the pulp mat on the washer.
For purposes of this invention, it is expected that the D-20,596 amount of CûD reauction correlates tO an equal amount of reduction in the required use of bleaching chemicals. For example, one kg reduction in CûD
provides one kg reduction in chlorine dioxide addition.
Other parameters may also be used to measure efficiency of the improved washing mechanism. These parameters include measuring the Kappa number (K#) or the brightness of the pulp exiting each stage or measuring COD carryover to each stage. By using the 10 process of the present invention, brightness in each stage will increase slightly, Kappa numbers will decrease, and COD carryover will drop. Water consumption can be measured by monitoring either the addition of makeup water to the system or measuring the 15 dilution or thickening factor of each washer. In addition, CûD displacement ratio can be used to demonstrate washing efficiency. The scaling tendency of each washer can be estimated by monitoring the pH of the wash water of each stage and calculating the 20 Langelier and Ryznar indexes for the washer. Foaming tendency and bleach plant upsets are measured discreetly; that is, either the vat foams and interferes with operation or it does not, or either the mixing of filtrates affects operation of the washer or 25 it does not. It should be noted that other methods for evaluating the efficiency of the extraction and bleaching stage and their associated washers, and of reduction in scaling and foaming, are known to the person skilled in the art.
The bleach stock washing process of the present invention is applicable to both elemental chlorine bleaching technology, elemental chlorlne free (ECF?
technology, and totally chlorine free technology (TCF).
D-20,596 - 2' -~e ~ ea~ s~v_,~ v~s~ y ,vlv~e~ vL ~lle ~le~ell~
invention may also be used with bleaching agents incorporated within the extraction stage such as hydrogen peroxide and oxygen. It is the purpose of the 5 present invention to focus on the method of washing the pulp rather than the method of bleaching the pulp.
The following examples further illustrate various features of the invention, but are intended to in no way limit the scope of the invention as set forth in 10 the appended claims.
Example 1: Conventional Five-Stage Bleach Plant with Jump-Stage Washing This example uses a bleach plant similar to the one represented in Figure 2. The bleach plant 15 processes 1000 air dried tons per day (ADTPD) of softwood kraft pulp. Incoming kappa # is between 20 and 26, averaging 24. 3. The bleach plant uses Ingersoll-Rand rotary vacuum drum washers 4.2 meters in diameter and 9.0 meters in length. Each washer has 20 five rows, or bars, of showers. Up to two different shower waters can be applied to each washer, depending on the valve configuration of the shower bars.
Chlorine and chlorine dioxide are applied in the first stage, the CD stage, to delignify the pulp and 25 separate the lignin from the cellulose. In the first stage, the treatment consistency is between 2 and 4%, averaging 3.0%. Chlorine is applied between 0. 5 and 4.0% on pulp, averaging 1. 3%~ and chlorine dioxide is applied between 0. 5 and 4% on pulp, averaging 1.9%.
30 Bleaching is performed at an average pH of 2, temperature of about 35 to 40~C~ and atmospheric pressure for 30~45 minutes. The pulp is washed on the CD stage washer before entering the E1 stage.
CA 02240030 l99X-06-09 D-20,596 Brightness altel this staye ls meas-ured at between 3U . U
and 57.2 ISO, averaging 53.9 ISO.
In the E1 stage 0.5-4.û% on pulp of caustic sodâ
(average 3.0%) is applied to attain a pulp pH of about 5 10.6. The consistency of the pulp is between 10-15%
and is treated at a temperature of 60-80~C (average 76~C) for 6û minutes. Under these conditions, lignin is soluble and is easily washed out of the pulp in the E1 washer. At this point, brightness is measured at lû 62.4 to 69.1 ISO, averaging 67.4 ISO.
Following the E1 stage, the pulp is treated with chlorine dioxide in the D1 stage to further remove the lignin from the pulp. The chlorine dioxide stage is performed at medium consistency (10-15%). Chlorine 15 dioxide is applied between 0.1-3.0% on pulp (average 0.49%) and the final pH of the pulp is between 3 and 4, averaging 3.6. The treatment is carried out at approximately 70~C for 30 minutes at atmospheric pressure. The pulp is again washed before entering the 2û E2 stage.
The conditions during the E2 stage are similar to that of the E1 stage. The objective here is to dissolve the lignin fragments separated during D1 stage bleaching so they can be removed during washing on the 25 E2 stage washer. Caustic chemical charge is maintained at 0.5% on pulp to yield the desired pH of 10.6.
Finally, the pulp is brightened in the final D2 stage with chlorine dioxide. The purpose here is to increase the brightness of the pulp as most of the 30 lignin was removed in the previous stages. The pulp is treated with approximately 0.68% chlorine dioxide on pulp at a pH of between 3.3 and 4.7, averaging 4.4.
The pulp is washed thoroughly after the final D2 stage D-20, 596 to al~ain a final brlghtness of between 88. 2 and 90.3 ISO, averaging 88.7 ISO.
When operating in jump-stage countercurrent washing mode, the bleach plant uses makeup water 5 composed of 9.0 m3/tonne of white water, 9 m3/tonne of recycled condensates and 2 m3/tonne of fresh water.
Starting on the last washer, 7.0 m3/tonne of white water is used as wash water on the D2 washer displacing 7.0 m3/tonne of filtrate. 5 m3/tonne of this filtrate 10 combines with 2 m3/tonne of white water. This filtrate/white water mixture is fed as wash water to the D1 washer, producing 7 m3/tonne of D1 filtrate. 2 m3/tonne of hot condensate and 5 m3/tonne of Dl filtrate are used to wash the pulp on the CD stage washer. The 15 filtrate from the CD/ D1, and D2 stages are combined and discarded as acid filtrate.
For the alkaline washing stages, 7 m3/tonne of hot condensate is used as wash water on the E2 stage.
5 m3/tonne of the resultant filtrate is used in 20 conjunction with 2 m3/tonne of fresh water to wash the pulp on the El stage. The 7 m3/tonne of E1 filtrate combines with the residual 2 m3/tonne of E2 filtrate to produce 9 m3/tonne of alkaline filtrate for disposal.
For the experimental portion, 10 kg of CO2 per 25 tonne of pulp is sparged into the E2 filtrate using a 20 micron sparger inserted after the E2 filtrate combines with the fresh makeup water. The combined wash water pH~ and thus the pH of the wash water applied to the El washer, is reduced to 6 . 5 . Control 30 is accomplished using a feed-back control mechanism incorporating a pH meter inserted directly before the shower water discharge on the El washer. As a result of the experiment, the chlorine dioxide charge to the D-20,596 D1 ~uwer was red-uce~ i9~ to û.~û~ on pulp. lt was also noted that the consistency of the pulp on the discharge of the E1 washer increased 1% from an average of 11.3%
to an average of 12.3%. This result was taken as an 5 indication of improved drainage on the washer. As a result, the amount of fresh water applied to the E1 washer was reduced by appromixately 200 gallons per ton of pulp (0.83 m3/tonne) without adversely affecting the process. The decreased hot water requirement of the 10 bleach plant also reduced the overall heating requirements by approximately 90 MM BTU/day. Finally, the COD displacement ratio was increased from 0.66 to û.81.
Example 2: Conventional Five-Stage Bleach Plant Using 15 Split-Flow Washing A conventional five-stage bleach plant incorporating split-flow washing as represented by Figure 3, is specifically designed to demonstrate how the process of the present invention transitions the 2û pulp from stage to stage and reduces the amount of bleaching chemicals applied in each stage. The same bleach plant as described in Example 1 was used to carry out the experiment except that the washing was modified from jump-stage countercurrent washing to 25 split-flow washing.
In modifying the bleach plant for split-flow jump-stage washing, 9 m3/tonne of white water and 4 m3/tonne of fresh water are used as makeup water.
Excess water enters the system at the following points:
30 l.û m3/tonne enters with the caustic solution applied to the E1 stage, 2.5 m3/tonne enters with the incoming CA 02240030 l998-06-09 D-20, 596 2_ --pulp, a"d 1.5 r,~~,Gnlle en.els wi.h ~ chi~ e dioxide solution in the first CD stage.
On the D2 stage, 5 m3/tonne of white water is applied to the first set of showers and 4 m3/tonne of 5 fresh water is applied to the second set. This wash water displaces 9 m3/tonne of filtrate. 5 m3/tonne of the filtrate is applied as wash water using the second set of showers on the E2 washer, and 4 m3/tonne of filtrate is applied as wash water using the first set 10 of showers of the D1 washer.
On the E2 washer, 4 m3/tonne of white water is applied using the first set of showers. This wash water, along with the wash water from the D2 filtrate, displaces 9 m3/tonne of filtrate from the pulp on the 15 E2 washer. The filtrate is divided. 5 m3/tonne of the filtrate is applied on the D1 washer using the second set of showers, and 4 m3/tonne of the filtrate is applied on the E1 washer using the first set of showers.
The filtrates from the E2 and D2 washers were applied to the D1 washer to displace 9 m3/tonne of filtrate. This filtrate is also divided. 5 m3/tonne of the filtrate is applied on the E1 washer using the second set of showers, and 4 m3/tonne of the filtrate 25 iS applied on the CD washer using the first set of showers.
The filtrates from the D1 and E2 washers were applied to the E1 washer to displace 9 m3/tonne of filtrate. In addition, the 1 m3/tonne of water 30 entering with the caustic solution is also removed for a total of 10 m3/tonne of filtrate. Five m3/tonne is discarded. The remaining 5 m3/tonne is applied to the CD stage using the second set of showers.
D-20,596 2~ -~n the CD slage, the flltrates from the E1 and D
stages wash the pulp and displace 10.5 m3/tonne of filtrate. 4.5 m3/tonne of the filtrate is recycled as dilution water, and 6 m3/tonne is discarded.
In this example, E2 filtrate pH is reduced to 6.5 in a similar manner and with similar chemicals as that in Example 1. In this situation, filtrate from the E2 washer is applied as wash water to two different washers: the E1 washer using the first set of shower 10 bars and the D1 washer using the second set of shower bars. To allow for independent pH control of each wash water stream, two control schemes and two 20 micron injectors are required. The injectors are installed after the E2 filtrate tank but before the shower water 15 is discharged onto the washers. 10 kg of CO2 per ton of pulp is injected at each of the spargers serving the E1 and D1 washers. In addition to these injection points, filtrate from the E1 washer is treated and applied to the CD stage washer; a third injector and 20 control scheme are established for this filtrate. Only 7 kg of CO2 per ton of pulp is required at this location to lower the pH to 6.5.
After acidification with CO2, filtrates from the E1 and E2 washers are applied as wash water on the CD
25 and D1 washers, respectively, using the second set of shower bars of each washer. These filtrates serve to transition the pulp from the low pH CD and D1 washers, respectively, to the high pH E1 and E2 towers, respectively. Likewise, the pH of the E2 filtrate 30 applied as wash water to the E1 washer is lowered to a pH of from about 11.5 to about 6.5; this arrangement lowers the pulp pH and transitions the pulp from the high pH E1 washer to the low pH D1 tower.
D-20,596 As a result of the experiment, chemical charges were reduced significantly and discharge consistencies were increased slightly. The results of the trial are summarized in the table below (see Table I). Although 5 break through was never noted, it can be seen from the table that the differences in the pHs of the various streams have been reduced, thus minimizing any adverse affects that break through might cause. For example, without CO2, the wash waters on the E1 stage washer 10 have pHs of 10.1 and 3.1. These wash waters wash high pH pulp to generate a pulp of pH 9.1 and filtrate of pH
10.5. When using CO2, the wash water pHs have been altered to 6.5 and 5.9. The exiting pulp now has a pH
of 7.1 and the filtrate pH is 9.4. Another consequence 15 of the pH shift in the system is that the extent of pitch deposition, foaming, and scaling are minimized.
(Pitch deposition and scaling were not measured during the trial, however, defoamer usage was monitored and showed a prominent decrease). Lastly, final brightness 20 was not affected by the trial.
D-20,596 T~bl~ t~ ûllV~ ullai r i v ~ ag~ d~n ~lân Using Split-Flow Washing Without CO2 With CO2 Chemical Charqes Caustic, E, 3.0% 2.6%
CIO2, D, 0.49% 0.44%
Caustic,E, 0.5% 0.48%
C1O2, D2 0.68% 0.59%
Defoamer 0.74 kg/t 0.23 kg/t 10 Stock pH (exitinq washer) Dc 5.8 6.2 E, 9.1 7.1 D, 6.2 6.4 E2 7.5 7.4 Filtrate pH (before CO2l Dc 2.9 3.8 E, 10.5 9.4 D, 3.1 5.9 E2 10.1 10.1 D2 4.1 4.1 Discharqe ConsistencY
Dc 12.0% 12.6%
E, 11.3% 12.0%
D, 12.2% 12.9%
Final Briqhtness 88.7 ISO 88.9 ISO
Example 3: Modern Four-Stage Bleach Plant Using Split-Flow Washing A D1EopD2D3 bleach plant processing kraft pulp is 30 modified by treating the Eop filtrate with CO2. The purpose of this experiment WâS to demonstrate the effectiveness of the present invention in reducing the amount of makeup water required by the bleaching process. Operating conditions for this bleach plant 35 follow that of the traditional bleach plant as described in Example 1 except in the manner the filtrates are recycled and the absence of the second E
D-20,596 s~dge. Elnal p-uip brigntness was 86 l~O; final viscosity was 764 dm3/kg.
Prior to this process, the bleach plant added 10 m3/tonne of makeup water to the D3 stage washer and 5 5 m3/tonne of makeup water to each of the Eop and D1 stage washers. The makeup water applied to the D3 stage essentially displaces 10 m3/tonne of filtrate which is recycled to the D2 washer. This filtrate displaces another 10 m3/tonne of filtrate, half of which jumps to 10 the D1 stage washer and is applied using the first showers of the D1 stage washer and half of which is recycled in split-flow fashion to the Eop washer using the latter shower bars of the Eop washer. The makeup water and D2 stage filtrate displace 10 m3/tonne of 15 alkaline filtrate from the Eop washer which is discarded; the makeup water and D2 stage filtrate also displace 10 m3/tonne of acid filtrate from the D1 washer which is also discarded. In total, 20 m3/tonne of makeup water is required for the process, and 20 20 m3/tonne of filtrates must be treated for disposal.
By using the process of the present invention, 5 m3/tonne of the alkaline filtrate from the Eop washer is removed as a side stream. This filtrate is treated with 8 kg of CO2/tonne of pulp by injecting CO2 into 25 the filtrate using a 20 micron sparger. pH is controlled at approximately 6.5. This acidified filtrate is then applied as wash water on the D1 stage in place of the 5 m3/tonne of makeup water that would normally be added. As a result, only 15 m3/tonne of 30 makeup water is required, and only 15 m3/tonne of filtrates must be disposed (10 m3/tonne of acid filtrate and 5 m3/tonne of alkaline filtrate). This reduction represents a 25% savings in water usage over D-20,596 the s~dte of the art. Eirlai brignlness ana viscosity were not affected by the experiment.
Example 4: Addition of Acidic Medium into the Extraction Vat by Acidification of the Vat Dilution 5 Line This example demonstrates an alternative method for delivering the carbon dioxide into the pulp. Since filtrate from a downstream extraction stage is used as dilution water in the vat of an upstream extraction 10 stage washer, this filtrate modifies the pH of the filtrate in the upstream extraction stage washer. The bleach plant described in Example 2 serves as the basis.
12 kg of CO2 per tonne of pulp is injected into 15 the filtrate from the downstream extraction stage E2 to lower the pH of this stream from 11.0 to 6Ø This filtrate is then used to dilute the stock in the E~
vat. As a result, results similar to those from Example 1 are realized.
FIELD OF THE INVENTION
The present invention relates to a process for washing bleached wood pulp using a multi-stage 5 bleaching sequence. More particularly, the present invention is directed to a process for washing bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence in which a reduced amount of bleaching chemicals and/or makeup water is 10 used.
BACKGROUND OF THE INVENTION
After the pulping operation, bleaching processes remove ligneous and coloring material remaining in the pulp. It is important that bleaching be accomplished 15 with as little degradation of the pulp fibers as possible. Consequently, pulp bleaching has advanced to a high degree and incorporates a multi-stage sequence utilizing various bleaching chemicals and washing processes. Each stage in a bleaching sequence is 20 composed of a tower where bleaching chemicals react with the pulp and a washer where spent bleaching chemicals and undesirable constituents are removed.
Chemicals used to bleach pulp include, but are not limited to, elemental chlorine, chlorine dioxide, 25 oxygen, hypochlorites, chlorides, peroxides, chlorates, dichromates, and permanganates. For chemical pulp, i.e., those made by the kraft, sulfite or soda processes, traditional bleaching agents include chlorine and chlorine dioxide.
Conventionally, a bleachlng stage performed using chlorine is designated a C stage. A bleaching stage D-20, 596 performed using chlorlne dioxide is designated a D
stage. C and D stages have low pH values, around pH of about 1. 5 to pH of about 5 . 0~ due to the reaction of chlorine and chlorine dioxide with lignin and water.
5 Chlorine dioxide may be wholly or partially substituted for chlorine in the first stage of a bleach plant; such a stage is commonly denoted as CD. As used herein, the first stage of the bleach plant may be called a C stage when only chlorine is used, a D stage in which only 10 chlorine dioxide is applied or a CD stage in which chlorine dioxide is substituted in part for chlorine.
An alkaline extraction, called an E stage, is commonly placed between successive bleaching stages (i.e., C and/or D stages). If the extraction stage 15 incorporates oxygen, or oxygen and peroxide, in combination with the alkaline medium, the E stage is designated as EO or EOP~ respectively. E stages, used to extract dissolved lignin from the pulp, are called alkaline or extraction stages because caustic soda 20 (NaOH) is used in those stages to raise the pH of those stages anywhere from about pH 9.0 to about pH 12.5.
Typical five-stage bleaching sequences are designated CEDED, DEDED, CEOPDED~ DEOPDED~ etc. Typical four-stage bleaching sequences may be designated CEDD, 25 DEDD, CEOPDD~ DEOPDD~ etc. Typical three-stage bleaching sequences include DED, CED, and CDEopD~
Additionally, subscripts may be used to distinguish similar stages, for example, CE1D1E2D2 and D1ED2D3.
In addition to the order and type of bleaching 30 stages, there are various washing methods incorporated in a multi-stage bleach plant. The three principal mllltl-st2ge washing processes are direct countercurrent washing, split-flow countercurrent washing, and D-20,596 jump-stage countercurrent washing. The differences among the three processes are in the manner in which the wash water is applied to and the filtrates are recycled from the washing steps of each stage.
As used herein, "makeup water" refers to either fresh water, white water (water recycled from the paper machines), or cleaned condensates (condensate from a stripper or the first stages of evaporation). Makeup water represents additional water added to the 10 bleaching process and is used to wash the pulp. Makeup water may be applied to any bleaching or extraction stage, however, the majority of the makeup water typically is applied to the last extraction stage and the last bleaching stage. "Filtrate" refers to the 15 liquor (liquid) displaced from the pulp slurry as the pulp traverses a washer. "Dilution water" is filtrate or makeup water that is used to dilute the stock in the washer vat. "Wash water" is either makeup water or filtrate and is used to wash pulp. Filtrate has value 20 as wash water on another stage and typically is recycled. Filtrate may also be used as dilution water for the washer vat or the repulper dilution water. A
portion of the filtrate is discarded from each washing stage, wherein the majority of the filtrate discarded 25 is from the first bleaching stage (acid filtrate) and the first extraction stage (alkaline filtrate).
Dilution water typically is used to dilute thickened pulp from a consistency of 10-15% down to a consistency of approximately 1-3%. As used herein, "bleaching 30 chemicals" refers to all chemicals used within the bleach plant. Generally, "bleaching chemicals" refers tc the traditional bleaching aids of chlorine and chlorine dioxide but can also refer to caustic soda, D-20,596 hypochlorite, oxygen, peroxide, defoamers, drainage aids and the like.
Although a variety of methods for washing bleached wood pulp are available, the state of the art still 5 suffers from expensive bleach plant costs due to the use of bleaching and extraction stage chemicals, the high use of fresh water and poor runnability.
Runnability generally refers to how the bleach plant is running. Poor runnability typically results in 10 decreased production and/or excess downtime to fix problems or make adjustments.
A number of works have been reported in the prior art, none of which is satisfactory in reducing the plant cost associated with chemical usage, fresh water 15 usage and poor runnability.
U.S. Patent No. 3,698,995 to Rapson is directed to multi-stage bleaching using countercurrent washing with the object of recovering spent chemicals. Carbon dioxide is not used in the washing.
U.S. Patent No. 3,907,632 to Christiansen, et al.
is directed to bleaching of cellulosic pulp with gaseous chlorine. It is stated that the pure chlorine gas may be diluted with an inert gas such as air, carbon dioxide, or nitrogen to provide a gaseous medium 25 wherein the active contacting component consists essentially of gaseous chlorine. Countercurrent washing is not believed to be used.
U.S. Patent No. 5,139,613 to Lachapelle is directed to a process for acidifying chemical and 30 mechanical pulp using carbon dioxide at the end of the bleaching process. It is said that carbon dioxide can be used in place of sulfur dioxide to stabilize the bleached pulp and prevent deterioration of pulp D-20,596 strength. Carborl dioxiae is introduceu after ine las bleaching stage, and not used internally during a multi-stage bleaching process.
U.S. Patent No. 5,429,717 to Bokstrom is directed 5 to a method of washing alkaline, cellulosic pulp wherein the pH is lowered in one or more consecutive washing stages by the addition of carbon dioxide. The pH of the filtrate or wash water for the extraction is above 6.8.
OBJECTS OF THE INVENTION
It is a principal object of the present invention to reduce the use of bleaching chemicals in the D
stage of bleach plants using alkaline wash water, comprising filtrate recycled from the second extraction 15 stage, or the first extraction stage washer of a CE1D1E2D2 bleaching sequence.
It is another object of the present invention to reduce the use of bleaching chemicals in any bleaching stage in bleach plants using countercurrent washing 20 with at least two extraction stages and applying alkaline wash water, whether comprised of filtrate from a downstream extraction stage or not, on any extraction stage. Such bleaching sequences include, but are not limited to, DEDED, CEo~DED/ DEoDED/ etc. Note that the 25 importance lies in the method of washing and not on the actual bleaching chemicals.
It is another object of the present invention to reduce the amount of bleaching chemicals and makeup water applied to any stage of a bleaching sequence 30 having only one extraction stage and using a split-flow washing arrangement.
D-20,596 It is another object of the present invention to reduce the amount of bleaching chemicals and makeup water applied to any stage of a bleaching sequence having a plurality of extraction stages and using a 5 split-flow washing arrangement.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic of a five-stage, bleaching process using conventional countercurrent washing;
Figure 2 is a schematic of a five-stage, bleaching 10 process using jump-stage countercurrent washing; and Figure 3 is a schematic of a five-stage, bleaching process using split-flow countercurrent washing.
SUMMARY OF THE INVENTION
This invention provides a process for the 15 preparation of bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence having at least one extraction stage. This process comprises lowering the pH of an alkaline filtrate with an acidic medium, and using the filtrate 20 to wash an upstream pulp.
The pulp is from an upstream extraction stage or an upstream bleaching stage. The filtrate is recycled from a downstream extraction stage. Although a number of acidic media may be used, carbonic acid formed from 25 carbon dioxide is preferred. The pH of the alkaline filtrate is lowered to a range of from about pH 4.5 to about pH 6.5. Prior to acidification, the pH of the filtrate is from about pH 9.0 to about pH 12.5. The pH
of the pulp after extraction stage washing is from 30 about pH 6.5 to about pH 9Ø The countercurrent washing may include direct countercurrent washing, CA 02240030 l99X-06-09 D-20,596 jump-stage countercurrent washing or split-flow countercurrent washing.
In another embodiment, this invention is also directed to a process for the preparation of bleached 5 wood pulp utilizing countercurrent washing in a multi-stage sequence. The process comprises lowering the pH of a wash water to a range of from about pH 4.5 to about pH 6.5 with an acidic medium, and using the wash water to wash the bleached wood pulp in a washer.
The washer is either a bleaching stage washer or an extraction stage washer. The acidic medium is preferably carbonic acid formed from carbon dioxide.
The wash water is an alkaline wash water. The pH of the pulp after extraction stage washing is from about 15 pH 6.5 to about pH 9Ø
In yet another embodiment, this invention is also directed to a process for the preparation of bleached wood pulp in a multistage bleaching sequence having at least one extraction stage. This process comprises 20 iowering the pH of the pulp to a range of from about pH
6.5 to about pH 9.0 with an acidic medium.
The pulp is acidified in the washer vat or the repulper of the extraction stage. Preferably, carbonic acid formed from carbon dioxide is used.
In still yet another embodiment, the invention is directed to a process for the preparation of bleached wood pulp in a multi-stage bleaching sequence having at least one extraction stage, the process comprising lowering the pH of a filtrate to a range of from about 30 pH 4.5 to about pH 6.5 with an acidic medium, and using the filtrate as dilution water.
The filtrate is used as dilutiGn water Gn an extraction stage or an upstream bleaching stage.
D-20,596 Preferably, the acidic medium used is carbonic acid formed from carbon dioxide.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention has numerous 5 advantages over prior art washing processes. The process of the invention reduces fiber swelling and has greatly improved washing efficiency, requiring less makeup water in the overall process. The process results in reduced oxidized material in the pulp slurry 10 and can be used to provide a higher consistency pulp which is cleaner, as measured by dirt count, and wherein the overall process uses a lower amount of bleaching chemicals. In addition, the process of the present invention reduces foaming and scaling tendency 15 and reduces upsets associated when dissimilar filtrate mixes. Finally, the increased consistency and lower water usage translate into considerable energy savings because less wash water must be heated to obtain proper washing. As used herein, pulp consistency refers to 20 the concentration and its is defined as the weight in grams of oven dry fiber in 100 grams of pulp-water mixture. See, TAPPI test method 204.
As shown in the figures, the present invention is directed to a washing sequence, preferably a 25 multi-stage bleaching sequence using countercurrent washing, whereby alkaline wash water or dilution water is applied to an extraction stage washer or bleaching stage washer. The pH of alkaline dilution water or wash water is reduced with an effective amount of 30 acidic medium. While various acids, including mineral acids and sulfur dioxide, couid be used to reduce pH, carbon dioxide is highly preferred. Carbon dioxide is CA 02240030 l998-06-09 D-20, 596 less corrosive and poses less of an environmental threat, as compared to mineral acids and sulfur dioxide. Generally, the pH of the alkaline wash water or dilution water is reduced to as low as practical to 5 a pH of from about pH 4. 5 to about pH 6 . 5, preferably about pH 5.5. By reducing the pH of the wash water or dilution water, pulp pH typically is reduced to between about pH 6 . 5 and about pH 9.0 for alkaline stage washers and to between about pH 5 . 5 and about pH 8.0 10 for acid stage washers. Reducing the pH to this level requires from about 5 to 20 lbs of CO2 per ton of pulp processed, depending upon the initial pH of the wash water or dilution water. If filtrate from an extraction stage serves as the wash water or dllution 15 water, then typically the initial pH of the filtrate ranges from about pH 9.0 to about pH 12 . 5 . Fresh water, however, may have a pH in the range of from about pH 14.0, preferably about pH 12.0, to about pH
7Ø
For the purposes of this invention, the focus of this application is on the washing steps of each stage, and more particularly, the process of washing in each stage. The three washing methods are detailed below.
The descriptions below only detail the major water 25 flows to the washers.
Fig. 1 provides a schematic drawing for direct countercurrent washing. In direct countercurrent washing, filtrate from the washer of a downstream stage is applied as wash water and dilution water to the 30 washer of the adjacent upstream stage. Usually, direct countercurrent washing is avoided since mixing of caustic and acidic filtrates promotes foaming, scaling, and excessive heat generation. In addition, if the CA 02240030 l998-06-09 D-20,596 _ 1 n _ wasn water 'fbreaks through" the pulp mat on the washer, acid and alkaline filtrates will mix and disrupt (or upset) the pH balance necessary for efficient operation of the bleach plant. To alleviate these problems, 5 large amounts of fresh water are added to each stage, and excess filtrate is purged from the system. The main advantage of this method of washing is that pulp is prepared for the next stage by using filtrate from the next stage. For example, the pulp exiting the E1 10 stage is washed with Dl filtrate immediately before it enters the D1 stage. In this way, the pulp pH is brought closer to the desired reaction pH in the next stage, and the amount of bleaching chemicals required in downstream stages is reduced.
In a typical direct countercurrent washing sequence as shown in Fig. 1, fresh water stream 1 washes pulp 54 on the D2 washer, resulting in filtrate 6 and washed pulp 55. Filtrate stream 6 is split into wash water 7 and acid filtrate 8. Wash water 7 and 20 fresh water stream 2 wash pulp 53 on the E2 alkaline-stage washer, resulting in filtrate 9 and washed pulp 54. Filtrate stream 9 is split into wash water 10 and alkaline filtrate 11. Wash water 10 and fresh water stream 3 wash pulp 52 on the D1 acid-stage 25 washer, resulting in filtrate 12 and washed pulp 53.
Filtrate 12 is divided into two streams 13 and 14.
Fresh water stream 4 and filtrate stream 13 are used as wash water on the E1 stage washer to wash pulp 51, resulting in washed pulp 52 and filtrate 15. Filtrate 30 stream 15 is split into wash water 16 and alkaline filtrate 19. Wash water 16 and fresh water stream 5 wash pulp 50 on the CD stage washer, resulting in filtrate 17 and washed pulp 51. Alkaline filtrates 19 D-20,596 and 11 are discarded, and acid filtrates 8, 14 and 17 are also discarded.
Fig. 2 provides a schematic drawing for jump-stage countercurrent washing. Jump-stage washing alleviates 5 some of the problems associated with direct countercurrent washing. In jump-stage countercurrent washing, filtrate from a downstream extraction stage is used to wash pulp entering an upstream extraction stage, and filtrate from later bleaching stages is used 10 to wash pulp entering earlier bleaching stages. For a typical five-stage bleach plant (CE1D1E2D2), pulp is introduced in the first stage, C, and traverses in series through the remaining stages: El, D1, E2, and D2.
The bulk of the makeup water is applied to the last two 15 stages, the E2 and D2 stages. This makeup water displaces the filtrate within the pulp slurry as the pulp traverses the washer. As mentioned previously, these filtrates cannot be mixed with filtrate from dissimilar stages because the large differences in pH
20 would cause considerable heat generation, foaming and scaling. In addition, filtrate from a D2 stage (or E2 stage) is not applied to the immediate prior stage (E2 stage (or D1 stage), respectively) for similar reasons.
Hence, filtrates are jumped and are only applied on 25 like stages. That is, the E2 filtrate ~umps over the D1 washer and is applied as wash water on the E1 washer; likewise, the D2 filtrate jumps over the E2 washer and is applied as wash water on the Dl washer.
The D1 filtrate is used as wash water on the C washer.
30 Finally, the C and E1 stage filtrates are discarded.
This process's main advantages over direct countercurrent washing are that filtrates of dissimilar pH do not contact one another and less makeup water is D-20,596 required to ensure ~leach plant performance. The method described above is the current state of the art.
In a typical jump stage countercurrent washing sequence as shown in Fig. 2, white water stream 201 5 washes pulp 254 on the D2 acid-stage washer, producing filtrate 202 and washed pulp 255. Filtrate 202 from this washing step is split into streams 203 and 204.
Filtrate stream 203 jumps over the E2 alkaline-stage and is applied as wash water on the D1 acid-stage 10 washer. Together with fresh water stream 205, this filtrate washes pulp 252 and produces washed pulp 253 and filtrate 206. Filtrate 206 is split into wash water stream 207 and filtrate stream 208. Wash water stream 207, together with hot condensate 209, washes 15 pulp 250 to produce washed pulp 251 and filtrate 210.
Filtrates 204, 208, and 210 combine to form acid filtrate 211, which is discarded. For the alkaline washing sequence, hot condensate 220 washes pulp 253, resulting in filtrate 221 and washed pulp 254.
20 Filtrate 221 splits into filtrate stream 223 and wash water stream 222. Wash water stream 222 jumps over the D1 stage and is applied to the E1 alkaline-stage washer. Together with white water stream 224, wash water 222 washes the pulp on the E1 stage washer, 25 resulting in washed pulp 252 and filtrate 225.
Filtrates 223 and 225 combine to form alkaline filtrate 226 for disposal.
Fig. 3 is a schematic drawing for split-flow countercurrent washing. Split-flow washing combines 30 the characteristics of direct countercurrent washing and jump-stage countercurrent washing. Split-flow washing involves washing the pulp wherein a first set of showers applies wash water that is similar to the D-20,596 stage the pulp is ieaving. Lne second set o~ showers on the washer then washes the pulp with filtrate similar to the stage the pulp is entering. In a five-stage bleach plant using a split-flow arrangement, 5 wash water is applied in a similar fashion as that in a conventional bleach plant using jump-stage washing, with two differences. First, the wash water that is jumped is applied to the pulp using the first set of showers, as the pulp is lifted out of the vat and 10 applied to the washer. Secondly, a portion of the filtrate from the downstream washer is applied to the back end of the washer, on the second set of showers, to prepare the pulp for the next stage. For example, on the E1 washer, the first wash water applied to the 15 pulp is filtrate from the E2 washer; after the E2 filtrate is applied, low pH filtrate from the D1 washer is applied.
This arrangement has several advantages over the previously described washing methods. First, it uses 20 up to 80% less water than the jump-stage washing method. Secondly, as in the direct countercurrent method, the pulp is prepared for the next stage because filtrate of similar pH to the next stage is applied to the pulp immediately prior to its application on the 25 next stage. Nevertheless, this method still has some of the disadvantages associated with direct countercurrent washing, in that mixing of dissimilar filtrates promotes scaling, foaming, and heat generation.
In a typical split-flow countercurrent washing sequence as represented in Fig. 3, fresh water stream 304 is applied to the D2 acld-stage washer using the second set of showers. White water stream 301 is split CA 02240030 l998-06-09 D-20,596 into two streams 3û2 and 303. White water stream 302 is applled to the D2 washer using the first set of showers. Filtrate 305 from this stage is split into streams 306 and 307. Filtrate 306 is applied to the E2 5 alkaline stage washer using the second set of showers, and white water stream 303 iS applied to the E2 washer using the first set of showers. These wash waters wash pulp 353, resulting in filtrate 3 U 8 and washed pulp 354. Filtrate 308 is split into streams 309 and 31û.
10 Filtrate 309, applied to the D1 washer using the second set of showers, and filtrate 307, applied using the first set of showers, wash pulp 352 to produce washed pulp 353 and filtrate 311. Filtrate 311 is split into streams 312 and 313. Filtrate 312 is applied to the 15 E1 alkaline-stage washer using the second set of showers, and filtrate stream 310 iS applied to the E
washer using the first set of showers. These wash waters wash pulp 351, resulting in filtrate 314 and washed pulp 352. Filtrate 314 is split into streams 20 315 and 316. Filtrate 316 is discarded as alkaline filtrate. Filtrate 315, applied to the Dc washer using the second set of showers, and filtrate 313, applied using the first set of showers, wash pulp 350 to produce washed pulp 351 and filtrate 317. Filtrate 25 stream 317 is split into recycle stream 319 and alkaline filtrate stream 318. Alkaline filtrate stream 318 is discarded whereas recycle stream 319 dilutes incoming pulp 350.
In the case of three- or four-stage bleaching 30 sequences, makeup water may be applied to the E stage washer. Generally, in this bleach plant scheme, pulp travels though a ~ stage, Eop stage, and one or two more D stages in order. Each stage includes a tower D-20,596 and washer. In this arrangement, makeup water is applied to the last D stage, the first D stage, and the Eop stage. None of the alkaline filtrate is recycled.
Of the acid filtrates, the D3 filtrate (if present) is 5 recycled to the D2 washer, the D2 filtrate is recycled to the Eop and the Dl washers (as in a split-flow arrangement), and the Dl and Eop filtrates are discarded.
In the embodiments in which the pH of the wash 10 water is reduced, the reduced pH wash water is applied to the pulp through the shower head of an upstream extraction stage or bleaching stage. In the embodiments in which the pH of the dilution water is reduced, this dilution water is applied to the pulp in 15 the washer vat or repulper. For an alkaline extraction stage, the dilution water may be applied to the pulp through the vat to obtain maximum benefits. If the washer is an acidic stage washer, the dilution water should be applied to the repulper to prepare the pulp 20 for the next stage.
Each of the washing method embodiments described herein enhances bleach plant operation or, in the specific embodiment in which alkaline filtrates are acidified and applied as wash water on upstream 25 washers, replaces makeup water on previous stages. In general, the process of the present invention at least 1) transitions the pulp from stage to stage, 2) enhances washing efficiency, 3) reduces makeup water requirements and 4) reduces bleach plant operating 30 costs. Reducing the pH of the pulp prevents fiber swelling and enhances washing. As a result, more organic and inorganic chemicals are washed from the pulp slurry, yielding a brighter pulp that is easier to CA 02240030 l998-06-09 3-20, 596 bleach in subsequent stages. ~n addition, by controlling pH, both scaling and pitch deposition will be reduced. Also, because washing is more efficient, consistency is improved, wash water volume is reduced, 5 energy consumption is lowered and final brightness is increased.
In the embodiment in which the dilution water is added to the vat of an alkaline washer, the dilution water lowers the pH by combining with the high pH
10 liquor already present. In the embodiment in which the dilution water is added to the repulper of an acidic washer, the pH will be increased by diluting the thickened pulp with the dilution water. A repulper is a device that catches the pulp as it comes off the 5 washer; it breaks up the pulp mat, and sometimes dilution water is added to the repulper before the pulp is sent to the next stage. In the embodiments in which the pH of the wash water is reduced, the reduced pH
wash water displaces filtrate within the pulp.
Preferably, the pH of the pulp is reduced to less than about pH 9.0, thereby converting alkaline earth metal salts of calcium or barium to soluble bicarbonates and aiding their removal from the pulp slurry. With this reduction in calcium and barium 25 ions, reduced scale formation is realized. Also, free calcium and barium ions are known to increase the consumption of downstream bleaching chemicals. By removing these ions, less bleaching chemicals are required in downstream bleaching stages. In addition 30 to lowering calcium and barium ion levels, lowering the pH of the pulp on the washer improves pulp drainage.
Consequently, more dirt, COD, and spent chemicals are washed from the pulp, producing a cleaner, brighter D-20,596 puip t~lat is easier to DieaCn in subsequenl bleacn1ng stages. Finally, the pulp pH is modified so as to transition pulp between stages as is accomplished less efficiently in state of the art split-flow washing 5 methods. That is, when the pulp is acidified between an alkaline stage and a subsequent acidic stage, the pH
is increased and less caustic soda is required in the alkaline stage to raise the pH to the proper operating conditions at from about pH 9.0 to about pH 12.5.
10 Alternatively, pulp pH can be reduced by lowering the wash water pH so as to transition the pulp from an alkaline stage to a subsequent acidic stage.
Therefore, less bleaching chemicals are required in the acidic stage to lower the pH to the proper operating 15 conditions, that is from about pH 1. 5 to about pH 5Ø
The pH of alkaline filtrates may be lowered so that the filtrates can be applied on previous washers in a split-flow fashion. In this embodiment, filtrate from a downstream extraction stage is acidified with 20 CO2, and the resultant filtrate is applied on an upstream bleaching stage. The resultant filtrate replaces makeup water that would normally be used on the bleaching stage.
The embodiments of the present invention in which 25 wash water is acidified and then applied to the washers in split-flow fashion provide a similar process as that for jump-stage washing with some additional benefits.
Namely, split-flow washing using the process of the present invention reduces the operational problems 30 associated with break through on all washers and reduces the problem of foaming. These benefits are realized because filtrates of vastly different pHs are no longer being applied on a single washer. Instead of - D-20,596 , ~ _ using both iow pn filtrates (from acid stages) and high pH filtrates (from alkaline stages) together on a single washer, near neutral pH filtrates (from the alkaline stages, modified with CO2) and low pH
5 filtrates (from acid stages) are used.
Break through occurs when wash water penetrates the pulp on the washer and mixes directly with the extracted filtrate. As the filtrates mix, the pH
balance necessary for proper mill operation is 10 adversely affected, causes upsets in bleach plant operation, and impairs pulp quality. By using the process of the present invention, if break through does occur and the filtrates mix, the chances of foaming, pitch deposition, scaling, and heat generation are 15 significantly reduced.
Foaming tendency is greatly reduced. Mixing of unlike filtrates causes foaming. By using the process of the present invention, differences in pH are minimized and less foam is created. Thus, less 20 defoamers are required by the process.
The changes in pH can cause pitch to be deposited onto the washer and can cause considerable heat generation. Pitch is defined as a resinous substance, composed of resin and fatty acids, found in wood pulp 25 that is soluble in neutral organic solvents. By using the process of the present invention, these operational problems can be avoided or minimized. Moreover, the pulp quality may be improved by lowering the dirt count of the pulp. "Dirt" is defined according to Tappi test 30 method T 213, which is incorporated herein by reference.
By using the process of the present invention, scaling tendency is reduced. This reduction is D-20,596 accompl1shed by controlling wash water pH such that scale does not form. As a result, the present bleach stock washing process reduces the formation of scale deposits on extraction stage washes in mills having 5 elevated calcium or barium levels in their washing system. Those skilled in the art will realize that scaling is a combination of many factors, including, but not limited to, water hardness, pH, ion concentration, and temperature.
Finally, with proper control of wash water, the total amount of makeup water can be reduced. This reduction is beneficial because more filtrate from the extraction stage can be recycled to the acid-stage washers. Consequently, less acid-stage makeup water is 15 required, and the total makeup water requirements are reduced.
Although various parameters may be used to evaluate the efficiency of the extraction stage washers, the parameter chosen for the purpose of this 20 application is the removal efficiency of chemical oxygen demand (COD). This calculation is performed by measuring the COD of the filtrate entering the washer with the pulp minus the COD of the filtrate exiting the washer with the pulp, divided by the COD of the 25 filtrate entering the washer with the pulp, in kg/tonne pulp, or similar units. The lower the COD of the filtrate leaving with the pulp (throughout the course of the bleaching sequence), the lower will be the bleaching chemicals required in subsequent stage(s).
30 The efficiency of the extraction stage and its associated washer is increased by decreasing the pH of the wash water or the pH of the pulp mat on the washer.
For purposes of this invention, it is expected that the D-20,596 amount of CûD reauction correlates tO an equal amount of reduction in the required use of bleaching chemicals. For example, one kg reduction in CûD
provides one kg reduction in chlorine dioxide addition.
Other parameters may also be used to measure efficiency of the improved washing mechanism. These parameters include measuring the Kappa number (K#) or the brightness of the pulp exiting each stage or measuring COD carryover to each stage. By using the 10 process of the present invention, brightness in each stage will increase slightly, Kappa numbers will decrease, and COD carryover will drop. Water consumption can be measured by monitoring either the addition of makeup water to the system or measuring the 15 dilution or thickening factor of each washer. In addition, CûD displacement ratio can be used to demonstrate washing efficiency. The scaling tendency of each washer can be estimated by monitoring the pH of the wash water of each stage and calculating the 20 Langelier and Ryznar indexes for the washer. Foaming tendency and bleach plant upsets are measured discreetly; that is, either the vat foams and interferes with operation or it does not, or either the mixing of filtrates affects operation of the washer or 25 it does not. It should be noted that other methods for evaluating the efficiency of the extraction and bleaching stage and their associated washers, and of reduction in scaling and foaming, are known to the person skilled in the art.
The bleach stock washing process of the present invention is applicable to both elemental chlorine bleaching technology, elemental chlorlne free (ECF?
technology, and totally chlorine free technology (TCF).
D-20,596 - 2' -~e ~ ea~ s~v_,~ v~s~ y ,vlv~e~ vL ~lle ~le~ell~
invention may also be used with bleaching agents incorporated within the extraction stage such as hydrogen peroxide and oxygen. It is the purpose of the 5 present invention to focus on the method of washing the pulp rather than the method of bleaching the pulp.
The following examples further illustrate various features of the invention, but are intended to in no way limit the scope of the invention as set forth in 10 the appended claims.
Example 1: Conventional Five-Stage Bleach Plant with Jump-Stage Washing This example uses a bleach plant similar to the one represented in Figure 2. The bleach plant 15 processes 1000 air dried tons per day (ADTPD) of softwood kraft pulp. Incoming kappa # is between 20 and 26, averaging 24. 3. The bleach plant uses Ingersoll-Rand rotary vacuum drum washers 4.2 meters in diameter and 9.0 meters in length. Each washer has 20 five rows, or bars, of showers. Up to two different shower waters can be applied to each washer, depending on the valve configuration of the shower bars.
Chlorine and chlorine dioxide are applied in the first stage, the CD stage, to delignify the pulp and 25 separate the lignin from the cellulose. In the first stage, the treatment consistency is between 2 and 4%, averaging 3.0%. Chlorine is applied between 0. 5 and 4.0% on pulp, averaging 1. 3%~ and chlorine dioxide is applied between 0. 5 and 4% on pulp, averaging 1.9%.
30 Bleaching is performed at an average pH of 2, temperature of about 35 to 40~C~ and atmospheric pressure for 30~45 minutes. The pulp is washed on the CD stage washer before entering the E1 stage.
CA 02240030 l99X-06-09 D-20,596 Brightness altel this staye ls meas-ured at between 3U . U
and 57.2 ISO, averaging 53.9 ISO.
In the E1 stage 0.5-4.û% on pulp of caustic sodâ
(average 3.0%) is applied to attain a pulp pH of about 5 10.6. The consistency of the pulp is between 10-15%
and is treated at a temperature of 60-80~C (average 76~C) for 6û minutes. Under these conditions, lignin is soluble and is easily washed out of the pulp in the E1 washer. At this point, brightness is measured at lû 62.4 to 69.1 ISO, averaging 67.4 ISO.
Following the E1 stage, the pulp is treated with chlorine dioxide in the D1 stage to further remove the lignin from the pulp. The chlorine dioxide stage is performed at medium consistency (10-15%). Chlorine 15 dioxide is applied between 0.1-3.0% on pulp (average 0.49%) and the final pH of the pulp is between 3 and 4, averaging 3.6. The treatment is carried out at approximately 70~C for 30 minutes at atmospheric pressure. The pulp is again washed before entering the 2û E2 stage.
The conditions during the E2 stage are similar to that of the E1 stage. The objective here is to dissolve the lignin fragments separated during D1 stage bleaching so they can be removed during washing on the 25 E2 stage washer. Caustic chemical charge is maintained at 0.5% on pulp to yield the desired pH of 10.6.
Finally, the pulp is brightened in the final D2 stage with chlorine dioxide. The purpose here is to increase the brightness of the pulp as most of the 30 lignin was removed in the previous stages. The pulp is treated with approximately 0.68% chlorine dioxide on pulp at a pH of between 3.3 and 4.7, averaging 4.4.
The pulp is washed thoroughly after the final D2 stage D-20, 596 to al~ain a final brlghtness of between 88. 2 and 90.3 ISO, averaging 88.7 ISO.
When operating in jump-stage countercurrent washing mode, the bleach plant uses makeup water 5 composed of 9.0 m3/tonne of white water, 9 m3/tonne of recycled condensates and 2 m3/tonne of fresh water.
Starting on the last washer, 7.0 m3/tonne of white water is used as wash water on the D2 washer displacing 7.0 m3/tonne of filtrate. 5 m3/tonne of this filtrate 10 combines with 2 m3/tonne of white water. This filtrate/white water mixture is fed as wash water to the D1 washer, producing 7 m3/tonne of D1 filtrate. 2 m3/tonne of hot condensate and 5 m3/tonne of Dl filtrate are used to wash the pulp on the CD stage washer. The 15 filtrate from the CD/ D1, and D2 stages are combined and discarded as acid filtrate.
For the alkaline washing stages, 7 m3/tonne of hot condensate is used as wash water on the E2 stage.
5 m3/tonne of the resultant filtrate is used in 20 conjunction with 2 m3/tonne of fresh water to wash the pulp on the El stage. The 7 m3/tonne of E1 filtrate combines with the residual 2 m3/tonne of E2 filtrate to produce 9 m3/tonne of alkaline filtrate for disposal.
For the experimental portion, 10 kg of CO2 per 25 tonne of pulp is sparged into the E2 filtrate using a 20 micron sparger inserted after the E2 filtrate combines with the fresh makeup water. The combined wash water pH~ and thus the pH of the wash water applied to the El washer, is reduced to 6 . 5 . Control 30 is accomplished using a feed-back control mechanism incorporating a pH meter inserted directly before the shower water discharge on the El washer. As a result of the experiment, the chlorine dioxide charge to the D-20,596 D1 ~uwer was red-uce~ i9~ to û.~û~ on pulp. lt was also noted that the consistency of the pulp on the discharge of the E1 washer increased 1% from an average of 11.3%
to an average of 12.3%. This result was taken as an 5 indication of improved drainage on the washer. As a result, the amount of fresh water applied to the E1 washer was reduced by appromixately 200 gallons per ton of pulp (0.83 m3/tonne) without adversely affecting the process. The decreased hot water requirement of the 10 bleach plant also reduced the overall heating requirements by approximately 90 MM BTU/day. Finally, the COD displacement ratio was increased from 0.66 to û.81.
Example 2: Conventional Five-Stage Bleach Plant Using 15 Split-Flow Washing A conventional five-stage bleach plant incorporating split-flow washing as represented by Figure 3, is specifically designed to demonstrate how the process of the present invention transitions the 2û pulp from stage to stage and reduces the amount of bleaching chemicals applied in each stage. The same bleach plant as described in Example 1 was used to carry out the experiment except that the washing was modified from jump-stage countercurrent washing to 25 split-flow washing.
In modifying the bleach plant for split-flow jump-stage washing, 9 m3/tonne of white water and 4 m3/tonne of fresh water are used as makeup water.
Excess water enters the system at the following points:
30 l.û m3/tonne enters with the caustic solution applied to the E1 stage, 2.5 m3/tonne enters with the incoming CA 02240030 l998-06-09 D-20, 596 2_ --pulp, a"d 1.5 r,~~,Gnlle en.els wi.h ~ chi~ e dioxide solution in the first CD stage.
On the D2 stage, 5 m3/tonne of white water is applied to the first set of showers and 4 m3/tonne of 5 fresh water is applied to the second set. This wash water displaces 9 m3/tonne of filtrate. 5 m3/tonne of the filtrate is applied as wash water using the second set of showers on the E2 washer, and 4 m3/tonne of filtrate is applied as wash water using the first set 10 of showers of the D1 washer.
On the E2 washer, 4 m3/tonne of white water is applied using the first set of showers. This wash water, along with the wash water from the D2 filtrate, displaces 9 m3/tonne of filtrate from the pulp on the 15 E2 washer. The filtrate is divided. 5 m3/tonne of the filtrate is applied on the D1 washer using the second set of showers, and 4 m3/tonne of the filtrate is applied on the E1 washer using the first set of showers.
The filtrates from the E2 and D2 washers were applied to the D1 washer to displace 9 m3/tonne of filtrate. This filtrate is also divided. 5 m3/tonne of the filtrate is applied on the E1 washer using the second set of showers, and 4 m3/tonne of the filtrate 25 iS applied on the CD washer using the first set of showers.
The filtrates from the D1 and E2 washers were applied to the E1 washer to displace 9 m3/tonne of filtrate. In addition, the 1 m3/tonne of water 30 entering with the caustic solution is also removed for a total of 10 m3/tonne of filtrate. Five m3/tonne is discarded. The remaining 5 m3/tonne is applied to the CD stage using the second set of showers.
D-20,596 2~ -~n the CD slage, the flltrates from the E1 and D
stages wash the pulp and displace 10.5 m3/tonne of filtrate. 4.5 m3/tonne of the filtrate is recycled as dilution water, and 6 m3/tonne is discarded.
In this example, E2 filtrate pH is reduced to 6.5 in a similar manner and with similar chemicals as that in Example 1. In this situation, filtrate from the E2 washer is applied as wash water to two different washers: the E1 washer using the first set of shower 10 bars and the D1 washer using the second set of shower bars. To allow for independent pH control of each wash water stream, two control schemes and two 20 micron injectors are required. The injectors are installed after the E2 filtrate tank but before the shower water 15 is discharged onto the washers. 10 kg of CO2 per ton of pulp is injected at each of the spargers serving the E1 and D1 washers. In addition to these injection points, filtrate from the E1 washer is treated and applied to the CD stage washer; a third injector and 20 control scheme are established for this filtrate. Only 7 kg of CO2 per ton of pulp is required at this location to lower the pH to 6.5.
After acidification with CO2, filtrates from the E1 and E2 washers are applied as wash water on the CD
25 and D1 washers, respectively, using the second set of shower bars of each washer. These filtrates serve to transition the pulp from the low pH CD and D1 washers, respectively, to the high pH E1 and E2 towers, respectively. Likewise, the pH of the E2 filtrate 30 applied as wash water to the E1 washer is lowered to a pH of from about 11.5 to about 6.5; this arrangement lowers the pulp pH and transitions the pulp from the high pH E1 washer to the low pH D1 tower.
D-20,596 As a result of the experiment, chemical charges were reduced significantly and discharge consistencies were increased slightly. The results of the trial are summarized in the table below (see Table I). Although 5 break through was never noted, it can be seen from the table that the differences in the pHs of the various streams have been reduced, thus minimizing any adverse affects that break through might cause. For example, without CO2, the wash waters on the E1 stage washer 10 have pHs of 10.1 and 3.1. These wash waters wash high pH pulp to generate a pulp of pH 9.1 and filtrate of pH
10.5. When using CO2, the wash water pHs have been altered to 6.5 and 5.9. The exiting pulp now has a pH
of 7.1 and the filtrate pH is 9.4. Another consequence 15 of the pH shift in the system is that the extent of pitch deposition, foaming, and scaling are minimized.
(Pitch deposition and scaling were not measured during the trial, however, defoamer usage was monitored and showed a prominent decrease). Lastly, final brightness 20 was not affected by the trial.
D-20,596 T~bl~ t~ ûllV~ ullai r i v ~ ag~ d~n ~lân Using Split-Flow Washing Without CO2 With CO2 Chemical Charqes Caustic, E, 3.0% 2.6%
CIO2, D, 0.49% 0.44%
Caustic,E, 0.5% 0.48%
C1O2, D2 0.68% 0.59%
Defoamer 0.74 kg/t 0.23 kg/t 10 Stock pH (exitinq washer) Dc 5.8 6.2 E, 9.1 7.1 D, 6.2 6.4 E2 7.5 7.4 Filtrate pH (before CO2l Dc 2.9 3.8 E, 10.5 9.4 D, 3.1 5.9 E2 10.1 10.1 D2 4.1 4.1 Discharqe ConsistencY
Dc 12.0% 12.6%
E, 11.3% 12.0%
D, 12.2% 12.9%
Final Briqhtness 88.7 ISO 88.9 ISO
Example 3: Modern Four-Stage Bleach Plant Using Split-Flow Washing A D1EopD2D3 bleach plant processing kraft pulp is 30 modified by treating the Eop filtrate with CO2. The purpose of this experiment WâS to demonstrate the effectiveness of the present invention in reducing the amount of makeup water required by the bleaching process. Operating conditions for this bleach plant 35 follow that of the traditional bleach plant as described in Example 1 except in the manner the filtrates are recycled and the absence of the second E
D-20,596 s~dge. Elnal p-uip brigntness was 86 l~O; final viscosity was 764 dm3/kg.
Prior to this process, the bleach plant added 10 m3/tonne of makeup water to the D3 stage washer and 5 5 m3/tonne of makeup water to each of the Eop and D1 stage washers. The makeup water applied to the D3 stage essentially displaces 10 m3/tonne of filtrate which is recycled to the D2 washer. This filtrate displaces another 10 m3/tonne of filtrate, half of which jumps to 10 the D1 stage washer and is applied using the first showers of the D1 stage washer and half of which is recycled in split-flow fashion to the Eop washer using the latter shower bars of the Eop washer. The makeup water and D2 stage filtrate displace 10 m3/tonne of 15 alkaline filtrate from the Eop washer which is discarded; the makeup water and D2 stage filtrate also displace 10 m3/tonne of acid filtrate from the D1 washer which is also discarded. In total, 20 m3/tonne of makeup water is required for the process, and 20 20 m3/tonne of filtrates must be treated for disposal.
By using the process of the present invention, 5 m3/tonne of the alkaline filtrate from the Eop washer is removed as a side stream. This filtrate is treated with 8 kg of CO2/tonne of pulp by injecting CO2 into 25 the filtrate using a 20 micron sparger. pH is controlled at approximately 6.5. This acidified filtrate is then applied as wash water on the D1 stage in place of the 5 m3/tonne of makeup water that would normally be added. As a result, only 15 m3/tonne of 30 makeup water is required, and only 15 m3/tonne of filtrates must be disposed (10 m3/tonne of acid filtrate and 5 m3/tonne of alkaline filtrate). This reduction represents a 25% savings in water usage over D-20,596 the s~dte of the art. Eirlai brignlness ana viscosity were not affected by the experiment.
Example 4: Addition of Acidic Medium into the Extraction Vat by Acidification of the Vat Dilution 5 Line This example demonstrates an alternative method for delivering the carbon dioxide into the pulp. Since filtrate from a downstream extraction stage is used as dilution water in the vat of an upstream extraction 10 stage washer, this filtrate modifies the pH of the filtrate in the upstream extraction stage washer. The bleach plant described in Example 2 serves as the basis.
12 kg of CO2 per tonne of pulp is injected into 15 the filtrate from the downstream extraction stage E2 to lower the pH of this stream from 11.0 to 6Ø This filtrate is then used to dilute the stock in the E~
vat. As a result, results similar to those from Example 1 are realized.
Claims (10)
1. A process for the preparation of bleached wood pulp utilizing countercurrent washing in a multi-stage bleaching sequence having at least one extraction stage, the process comprising lowering the pH of an alkaline filtrate to a range of from 4.5 to 6.5 with an acidic medium, and using said filtrate to wash an upstream pulp.
2. The process of claim 1 wherein said upstream pulp is from an upstream extraction stage or bleaching stage.
3. The process of claim 1 wherein said filtrate is recycled from a downstream extraction stage.
4. The process of claim 1 further wherein said acidic medium comprises carbonic acid formed from carbon dioxide.
5. A process for the preparation of bleached wood pulp utilizing countercurrent washing in a multi-stage sequence, the process comprising lowering the pH of a wash water to a range of from 4.5 to 6.5 with an acidic medium, and using said wash water to wash pulp on a washer.
6. The process of claim 5 wherein said washer is a bleaching washer or an extraction washer.
7. A process for the preparation of bleached wood pulp in a multi-stage bleaching sequence having at least one extraction stage, the process comprising lowering the pH of said pulp to a range of from 6.5 to 9.0 with an acidic medium.
8. A process for the preparation of bleached wood pulp in a multistage bleaching sequence having at least one extraction stage, the process comprising lowering the pH of a filtrate to a range of from 4.5 to 6.5 with an acidic medium, and using said filtrate as dilution water.
9. The process of claim 8 wherein said filtrate is used as dilution water on an extraction stage or an upstream bleaching stage.
10. A process for the preparation of bleached wood pulp in a multistage bleaching sequence having two extraction stages, E1 and E2, and three bleaching stages, C D, D1 and D2, each stage having a respective washing step, the process comprising bleaching a digested pulp with at least a portion of a bleaching chemical in the C D stage followed by a first washing step to form a C D pulp and a C D filtrate; extracting said C D pulp in the E1 stage followed by a second washing step to form an E1 pulp and an E1 filtrate;
bleaching said E1 pulp with at least a portion of said bleaching chemical in a D1 stage followed by a third washing step to form a D1 pulp and a D1 filtrate;
extracting said D1 pulp in the E2 stage followed by a fourth washing step to form an E2 pulp and an E2 filtrate; washing said E2 pulp with at least a portion of said bleaching chemical in a D2 stage followed by a fifth washing step to form said bleached wood pulp and a D2 filtrate; lowering the pH of the E2 filtrate to a pH of from about pH 6.5 to about pH 4.5 with an effective amount of carbon dioxide; providing at least one stream of white water to said fourth and fifth washing steps; and providing at least one stream of fresh water to said fifth washing step, wherein at least one portion of said D2 filtrate is passed to said fourth and third washing steps; at least one portion of said E2 filtrate is passed to said third and second washing steps; at least one portion of said D1 filtrate is passed to said second and first washing steps; at least one portion of said E1 filtrate is passed to waste and to said first washing step; and at least one portion of said CD filtrate is passed to waste and to said digested pulp.
bleaching said E1 pulp with at least a portion of said bleaching chemical in a D1 stage followed by a third washing step to form a D1 pulp and a D1 filtrate;
extracting said D1 pulp in the E2 stage followed by a fourth washing step to form an E2 pulp and an E2 filtrate; washing said E2 pulp with at least a portion of said bleaching chemical in a D2 stage followed by a fifth washing step to form said bleached wood pulp and a D2 filtrate; lowering the pH of the E2 filtrate to a pH of from about pH 6.5 to about pH 4.5 with an effective amount of carbon dioxide; providing at least one stream of white water to said fourth and fifth washing steps; and providing at least one stream of fresh water to said fifth washing step, wherein at least one portion of said D2 filtrate is passed to said fourth and third washing steps; at least one portion of said E2 filtrate is passed to said third and second washing steps; at least one portion of said D1 filtrate is passed to said second and first washing steps; at least one portion of said E1 filtrate is passed to waste and to said first washing step; and at least one portion of said CD filtrate is passed to waste and to said digested pulp.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US87224997A | 1997-06-10 | 1997-06-10 | |
| US08/872,249 | 1997-06-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2240030A1 true CA2240030A1 (en) | 1998-12-10 |
Family
ID=25359169
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2240030 Abandoned CA2240030A1 (en) | 1997-06-10 | 1998-06-09 | Method for washing bleached wood pulp |
Country Status (2)
| Country | Link |
|---|---|
| BR (1) | BR9801812A (en) |
| CA (1) | CA2240030A1 (en) |
-
1998
- 1998-06-09 BR BR9801812-4A patent/BR9801812A/en not_active Application Discontinuation
- 1998-06-09 CA CA 2240030 patent/CA2240030A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| BR9801812A (en) | 1999-09-28 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1259456A (en) | Enhanced oxidative extraction | |
| JP2825346B2 (en) | Environmentally improved bleaching method for lignocellulosic materials | |
| US6569284B1 (en) | Elemental-chlorine-free bleaching process having an initial Eo or Eop stage | |
| JP5191536B2 (en) | Methods for cleaning pulp in chemical pulp mills. | |
| EP1026312A1 (en) | Method and apparatus for pulp yield enhancement | |
| CA2111519C (en) | Oxygen/ozone/peracetic acid delignification and bleaching of cellulosic pulps | |
| US5302244A (en) | Oxygen delignification of waste cellulosic paper products | |
| FI105213B (en) | Method for production of bleached pulp from lignocellulose material | |
| US5409570A (en) | Process for ozone bleaching of oxygen delignified pulp while conveying the pulp through a reaction zone | |
| US20150240423A1 (en) | Effect of low dose xylanase on pulp in prebleach treatment process | |
| US4537656A (en) | Method for delignifying or bleaching cellulose pulp wherein chlorine is added to recycle liquor to regenerate chlorine dioxide | |
| US5728264A (en) | Avoidance of salt scaling by acidic pulp washing process | |
| WO2005024126A1 (en) | Chlorine dioxide delignification of medium consistency pulp | |
| FI128111B (en) | A method and a system for treating liquid flows at a chemical pulp mill | |
| KR100538083B1 (en) | Oxygen delignification of lignocellulosic material | |
| CA2240030A1 (en) | Method for washing bleached wood pulp | |
| WO1995006772A1 (en) | Improved method for bleaching lignocellulosic pulp | |
| US5126009A (en) | Process for decreasing the charge of chemical required in a chlorine bleaching extraction stage | |
| US8524038B2 (en) | Bleaching process of chemical pulp | |
| WO2002014600A1 (en) | Two-stage chlorine bleaching process with filtrate recirculation | |
| EP0905307A1 (en) | Method for applying ozone in ECF bleaching | |
| WO1995006773A1 (en) | Medium consistency ozone brightening of high consistency ozone bleached pulp | |
| FI122773B (en) | Preparation of pulp | |
| Hart | Pulp bleaching | |
| Sudharsan et al. | Stretching Captive Pulp Production-A Case Study |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Dead |