Patent Application
20090000751
This invention relates to the bleaching process with at least one extraction stage. More particularly, the invention relates to decreasing bleaching chemical (ClO2 and NaOH) charge and steam required in the bleaching extraction stage.
The recycling of some of the extraction effluent during chlorination pulp bleaching is a common practice. See for example U.S. Pat. No. 5,352,332.
One aspect of this invention relates to an improved pulp bleaching process having at least one Do delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stage, the process comprises treating the delignified pulp Do pulp from the Do stage in a MDE stage with a filtrate from the at least one extraction stage after the Do stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with interstage washing, wherein:
Do is a delignification stage in which pulp is treated with an agent comprising chlorine dioxide preferably no or substantially no elemental chlorine dioxide (ECF);
E0 is an extraction stage in which a pulp is extracted with base in the presence of oxygen;
E is an extraction stage in which a pulp is extracted with a base;
Eop is an extraction stage in which pulp is treated with a composition comprising oxygen, base, and peroxide;
Ep is an extraction stage in which pulp is treated with a composition comprising base and peroxide;
and
MDE is a treatment stage in which delignified pulp is treated with a filtrate from the at least one extraction stage. The MDE can be accomplished in the bottom dilution zone of the Do tower (for a down flow Do tower) or a pipe to provide contact and mixing of extraction filtrate and Do pulp slurry.
Another aspect of the invention relates to an improved pulp bleaching process having at least one D0 delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stages which is followed by at least one D1 bleaching stage, the process comprises treating the delignified pulp from the D0 stage in a MDE stage with a filtrate from the at least one extraction stage wherein D0, Eo, Eop, Ep, and E are as defined above and D1 is a bleaching stage in which pulp is bleached with a bleaching agent comprising chorine dioxide.
The process of the present invention provides one or more advantages over prior processes for brightening bleached pulps. For example, advantages of some of the embodiments of the process of this invention include 1) a reduction in the amount of base used in the at least one bleaching stage, 2) reducing the steam/energy consumption, 3) ClO2 reduction in the D1 stage, 4) lower at least one extraction stage pulp kappa number and less carryover in the at least one extraction stage, 5) reduce Eop filtrate discharge at least one extraction stage, 6) reduction in filtrate AOX and color reduction or 7) a combination of two or more of the aforementioned advantages. Some embodiments of this invention may exhibit one of the aforementioned advantages while other preferred embodiments may exhibit two or more of the foregoing advantages in any combination.
A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawing in which:
While this invention is susceptible of embodiment in many different forms, there is shown and described in drawing, figures, and examples and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.
One aspect of this invention relates to an improved pulp bleaching process having at least one D0 delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stage. The process comprises treating the delignified pulp D0 pulp from the D0 in a MDE stage with a filtrate from the at least one extraction stage after the D0 stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with an interstage washing, wherein D0, Eo, E, Eop, Ep, and MDE are as defined above.
MDE is a mixing stage in which the pulp is mixed with a filtrate. The method and apparatus of the present invention can be varied widely, for example, the mixing stage may be conducted in a separate tower at a point between the chlorination washing step and the step of mixing the pulp with an alkaline extracting solution. Alternatively, the MDE can also be accomplished in the dilution zone of the bottom of the D0 tower (for a down flow D0 tower) or a pipe to provide contact and mixing of extraction filtrate and D0 pulp slurry.
The pH of the MDE stage can be varied widely provided the pH range is greater than 2.5. Any pH within this range can be used. For example, the pH can be as high as about 4.5 and as low as about 2.5 to about 3.5. In the preferred embodiments of the invention, the pH is greater than or equal to 3. In the more preferred embodiments of the invention, the pH is from about 3.5 to about 6.0 and in the most preferred embodiments of the invention, the pH is from about 3.5 to about 4.5.
The consistency (CSC) of the pulp in the MDE stage may vary widely and any consistency that provides the desired increase in pulp brightness may be used. The pulp may be bleached under low consistency conditions (i.e. from about 3 to about 5 based on the total weight of the mixture of pulp and bleaching chemicals), medium consistency conditions (i.e. from about 8 to about 15 based on the total weight of the mixture of pulp and bleaching chemicals) or high consistency conditions (i.e. from about 25 to about 30 based on the total weight of the mixture of pulp and bleaching chemicals). The consistency is preferably from about 5 to 30, more preferably from about 10 to 20, and most preferably from about 10 to about 15.
The MDE stage retention times will vary widely and retention times used in the conventional bleaching stages may be used. Usually, retention times will be at least about 2 minutes. Retention times are preferably from about 2 min. to about 120 min., and are more preferably from about 5 minutes to about 60 min. and most preferably from about 5 min. to about 15 min.
Similarly, The MDE stage treatment temperatures employed in the critical treatment stage may vary widely and temperatures employed in the conventional bleaching stages may be used. For example, useful temperatures can be as low as about 50° C. or lower and as high as about 80° C. or higher. In the process of this invention, the treatment temperature is usually from about 50° C. to about 85° C., preferably from about 60° C. to about 80° C., more preferably from about 65° C. to about 75° C. and most preferably from about 65° C. to about 70° C.
After MDE stage, the pulp is preferably washed or is washed at some subsequent point in the process prior to the Ep or Eop stage. Pulp washing after the MDE stage removes the acidic species (NaHCO3 and dissolved CO2) to minimize its caustic demand in the Eop stage.
After the washing stage, the pulp is subjected to extraction in an E, E0, Eop or Ep stage. The Eop stage refers to alkaline extraction of pulp with oxygen (O) and hydrogen peroxide (P), and Ep stage refers to alkaline extraction of pulp with hydrogen peroxide (P). Conventional processes and apparatus can be used in the Ep or Eop stage. See for example “Pulp Bleaching Principles and Practice of Pulp Bleaching” Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and references cited therein.
An advantage of a preferred embodiment of this invention is the reduction of extraction chemicals such as NaOH in the Eop and/or Ep stages as compared to the same or substantially the same bleaching processes which do not include the MDE stage. For example, the reduction in the amount of NaOH is typically at least about 10%, preferably at least about 20%, more preferably from about 15% to about 30% and most preferably from about 20% to about 25% less than the amount of NaOH used in the same or substantially the same bleaching processes which do not include the MDE stage.
The amount of extraction agent used (e.g. sodium hydroxide, magnesium hydroxide, potassium hydroxide, etc.) used in the practice of the process of this invention can vary widely and any amount sufficient to provide the desired lignin extraction efficiency and the desired degree of brightness can be used. The amount of caustic used is usually at least about 0.5% based on the dry weight of the pulp. Preferably the amount of bleaching agent is from about 1% to about 8%, more preferably from about 1.5% to about 3% and most preferably about 1-2% on the aforementioned basis.
Another advantage of a preferred embodiment of this invention is the reduction of AOX resulting from the Eop and/or Ep stages as compared to the same or substantially the same bleaching processes which do not include the MDE stage. For example, the reduction in the amount of AOX is typically at least about 20%, preferably at least about 30%, more preferably from about 10% to about 50% and most preferably from about 20% to about 30% less than the amount of AOX produced in the same or substantially the same bleaching processes which do not include the MDE stage to obtain the same or substantially level of pulp brightness in the Eop and/or Ep stages.
The bleaching process of this invention may include other bleaching stages as for example bleaching with Cl2, peroxy acids, chlorine dioxide, ozone and the like, and extraction stages such as extraction with oxygen, ozone, borohydride, chlorine dioxide and the like in the presence other bases such as Mg(OH)2. Illustrative of such bleaching processes are OD0/C MDEEop, D0 MDEEopD, D0 MDE EOPDD, D0 MDE EopED, D0 MDE EDEpEopD, ZE MDE (Eop)D, Z MDE (Eop)D, D0 MDE EpZ(Eop), D0 MDE EpZD(ZD), D0 MDE (Eop)D(ZD), D0 MDE (Eop)PP, D0 MDE (Eop)DZ, D0 MDE EopD1, ODo MDE EopD1, Do MDE EopD1D2, ODo MDE EopD1D2, Do MDE EopD1EpD2, ODo MDE EopD1EpD2, D MDE EopD1P and the like in which D is as described above and Z is ozone, E is extraction in the presence of base, O is oxygen, P is peroxide, D/C is a mixture of chlorine dioxide and elemental chlorine and two or more symbols in parenthesis indicate an absence of an intermediate washing stage. The Eop, Ep, and Do are defined above herein. The processes and apparatus used in the D, Z, E, 0, P, D/C are conventional and there are well known in art. See for example, “Pulp Bleaching Principles and Practice of Pulp Bleaching” Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and references cited therein.
In the preferred embodiment of the invention, the process comprises at least one D1 bleaching stage following the extraction stage. Another advantage of a preferred embodiment of this invention is the reduction of bleaching chemicals such as ClO2, H2SO4, or combination thereof in the D1 stage as compared to the same or substantially the same bleaching processes which do not include the MDE stage. For example, the reduction in the amount of ClO2 is typically at least about 5%, preferably at least about 10%, more preferably from about 5% to about 20% and most preferably from about 10% to about 20% less than the amount of ClO2 used in the same or substantially the same bleaching processes which do not include the MDE stage to obtain the same or substantially level of pulp brightness in the Eop and or Ep stages.
The plant source of pulp for use in this invention is not critical and may be any fibrous plant which can be subjected to chemical pulp bleaching. Examples of such fibrous plants are trees, including hardwood fibrous trees such as aspen, eucalyptus, maple, birch, walnut, acacia and softwood fibrous trees such as spruce, pine, cedar, including mixtures thereof. In certain embodiments, at least a portion of the pulp fibers may be provided from non-woody herbaceous plants including, but not limited to, kenaf, hemp, jute, flax, sisal, or abaca although legal restrictions and other considerations may make the utilization of hemp and other fiber sources impractical or impossible. The source of pulp for use in the practice of this invention is preferably hardwood and softwood fibrous trees, more preferably Eucalyptus, Spruce and Aspen and is most preferably Aspen and Spruce.
The pulp used in the process of this invention can be obtained by subjecting the fibrous plant to any chemical pulping process. Following the wood digestion process, pulp is separated from the spent pulping liquor. The spent pulping liquor is then recovered and regenerated for recycling. The pulp is then bleached and purified in a bleach plant operation.
The pulp of this invention can also be used in the manufacture of paper and packaging products such as printing, writing, publication and cover papers and paperboard products. Illustrative of these products and processes for their manufacture are those described in U.S. Pat. Nos. 5,902,454 and 6,464,832.
For example, in the paper or paperboard making process, the bleached pulp of this invention or pulp mixtures comprising the bleached pulp of this invention is formulated into an aqueous paper making stock furnish which also comprises one of more additives which impart or enhance specific sheet properties or which control other process parameters. Illustrative of such additives is alum which is used to control pH, fix additives onto pulp fibers and improve retention of the pulp fibers on the paper making machine. Other aluminum based chemicals which may be added to furnish are sodium aluminate, poly aluminum silicate sulfate and poly aluminum chloride. Other wet end chemicals which may be included in the paper making stock furnish for conventional purposes are acid and bases, sizing agents, dry-strength resins, wet strength resins, fillers, coloring materials, retention aids, fiber flocculants, defoamers, drainage aids, optical brighteners, pitch control chemicals, slimicides, biocides, specialty chemicals such as corrosion inhibitors, flame proofing and anti-tarnish chemicals, and the like.
The aqueous paper making stock furnish comprising the bleached pulp and the aluminum based compounds is deposited onto the forming wire of a conventional paper making machine to form a wet deposited web of paper or paperboard and the wet deposited web of paper or paperboard is dried to form a dried web of paper or paperboard. Paper making machines and the use of same to make paper are well known in the art and will not be described in any great detail. See for example, Pulp and Paper Chemistry and Handbook for Pulp & Paper Technologies, supra. By way of example, the aqueous paper making stock furnish containing pulp, aluminum based and other optional additives and usually having a consistency of from about 0.3% to about 1% is deposited from the head box of a suitable paper making machine as for example a twin or single wire Fourdrinier machine. The deposited paper making stock furnish is dewatered by vacuum in the forming section. The dewatered furnish is conveyed from the forming section to the press section on specially-constructed felts through a series of roll press nips which removes water and consolidates the wet web of paper and thereafter to the dryer section where the wet web of paper is dried to form the dried web of paper of this invention. After drying, the dried web of paper may be optionally subjected to several dry end operations such as and various surface treatments such as coating, and sizing and calendering.
The paper manufactured in accordance with this invention can be used for conventional purposes. For example, the paper is useful as printing paper, publication paper, newsprint and the like.
The present invention is described in more detail by referring to the following examples and comparative examples which are intended to more practically illustrate the invention and not to be a limitation thereon.
The extraction stage filtrate and the D stage delignified pulp 12 are preferably mixed at a pH greater 2.5 and more preferably at pH 3.5 to 6.0. The consistency of the pulp and extraction filtrate mixture can be varied widely, but the consistency of the pulp is preferably from about 10 to 20.
The retention time (t) can be varied widely, but the retention time can be from about 5 to 60 minutes and most preferably the retention time (t) is from about 5 to 15 min. The treatment temperature (T) can be also varied widely, but preferably the treatment temperature is from 60° C. to 80° C. and most preferably is from 65° C. to 70° C.
After the extraction filtrate and the D0 pulp are mixed for a desired period of time, the pulp is preferably washed prior to transfer to the extraction stage to remove acidic species such as NaHCO3, and dissolved CO2) to minimize caustic demand in the extraction stage. For example, as depicted in
The pulp can be processed from system and used for conventional purposes or the pulp can be subjected to one or more additional acidic and/or alkaline bleaching stages either before or after the first acidic bleaching, alkaline bleaching stage and/or second acidic bleaching stage. As for example, further pulp bleaching with one or more bleaching agents selected from the group consisting of peroxide, chlorine dioxide and ozone. Such additional bleaching stages may be without subsequent washing or may be followed by subsequent wash stage or stage(s). The washed pulp exits the bleaching sequence via line 46 for conventional use as for example in a paper making process.
Unbleached pulps and Eop filtrates were supplied by Mill A. The unbleached hardwood pulp had 24.6% brightness and 11.7 P#. The unbleached softwood pulp had 24.6% brightness and 13.4 P#. The pHs of the Eop filtrates were unusually higher at 10.4 and 11 for hardwood and softwood bleach lines, respectively. The 200 OD gram of the Do pulp along with Do filtrate at 3.5% consistency was treated with 2 liter of Eop filtrate, assuming the availability of 2640 gallon Eop filtrate per ton of pulp for recycling.
All bleaching except for the Eop stage was conducted in sealed plastic bags. All pulp samples were preheated to the bleaching temperature, and all the chemicals were added sequentially and mixed thoroughly with the pulp before addition of another chemical. The chemical addition sequence in the D stages are deionized water, caustic (for pH control), and ClO2.
After completing the D1 bleaching stage, the pulp was squeezed to collect filtrate for pH, residual, and COD measurement. The pulp was repulped at 1% consistency with deionized water and dewatered on a Buchner funnel and repeat a couple of time to simulate a pulp washing stage in mills. The washed pulp was analyzed for brightness, viscosity, and pulp dirt. All the filtrate and pulp analysis was done with the standard published procedures understood by all the people working in the field.
The Eop stage was simulated in a pressurized reactor where O2 pressure can be applied to simulate the mill practice. The Quantum reactor widely known to all in the field was used for lab simulation of the Eop stage.
Initial experiments were conducted at three different temperatures for 30 min of treatment time. The effect of Eop filtrate preextraction of Do pulp on Do brightness is shown in Table 1. The 3-4% brightness increase was observed, depending upon the treatment temperature. The pretreated Do pulp pHs were at 3.9 compared with the control Do pulp pH of 2.82.
The Do pulps were then subject to normal Eop treatment at the conditions simulating those in Mill A operation. The results are shown in Table 2. All the Eop pHs were lower than normal mill Eop pH target because of insufficient caustic charge and more H2O2 usage in the lab than in mill practice. The caustic charges for preextracted Do pulps were 4 lb/t less than that of the control by design. The Eop end pHs for extracted Do pulps were slightly higher than that of the control Eop because of the removal of the neutralized acidic chloride and organic acids in the Eop stage. The Eop treatment efficiency is higher for the preextracted Do pulp than the control Do pulp, manifested by a 10% lower Eop P#. For the unknown reasons, the Eop brightness of preextracted pulps was lower than that of the control.
Lower Eop pulp P# should lead to some ClO2 reduction in the D1 stage as the ClO2 bleachability in the D1 stage is dependent upon the Eop pulp P#. Lab bleaching at the same 0.6% ClO2 charge shows 0.4% higher D1 brightness for Eop filtrate preextracted pulps than the control at the unoptimized D1 pHs (Table 2). Based on the 10% lower Eop pulp P#, greater benefits in terms of D1 stage ClO2 savings or D1 brightness increase are expected for Eop filtrate preextracted pulps than the control. Perhaps, there is some lignin reprecipitation occurring when bringing the Eop filtrate down to the acidic pH.
The effect of treatment time was subsequently investigated to determine if the treatment can be accomplished in the existing pipe from the top or bottom of the Do tower to Do washer vat without additional retention tower. The results are shown in Table 3. Treatment time at 30 and 2 minutes produced similar results except for a slightly higher Eop P# at 2 minute treatment time than at 30 minutes, consistent to the expected less efficient lignin preextraction by Eop filtrate at shorter time. Among the three-tier caustic demands, neutralization of chloride and organic acids in the Do pulp and filtrate are inorganic reactions and should be largely completed within a minute or so available in the pipe. Therefore, the premise of using Eop filtrate for neutralizing the Do pulp and filtrate for Eop caustic reduction and delignification efficiency enhancement for D1 ClO2 reduction is achievable at the short time available with the existing equipment.
Similar results are achieved in preextraction of softwood Do pulp and filtrate with softwood Eop filtrate (Table 4). The results for softwood pulp in Table 4 validate those achieved for HW. Therefore, the strategy is potentially feasible for both species to achieve 4 lb/t caustic savings in the Eop stage and some ClO2 reduction in the D1 stage due to lower Eop pulp P#.
The results in Table 5 show that reduced treatment time (from 30 mins versus 2 mins) and the amount of Eop filtrate (from 2 liter versus 1 liter) in the Eop filtrate preextraction of Do pulp and filtrate still allow about the same (4 lb/t) caustic usage savings in the Eop stage with enhanced Eop extraction efficiency. The Eop filtrate preextracted Do pulp had higher Eop brightness and lower P# than the control. The lower Eop P# of the Eop filtrate preextracted pulp is translated to better D1 bleachability measured by higher D1 brightness. The Eop filtrate preextraction of Do pulp and filtrate had very limited effect on pulp viscosity and dirt removal (Table 5).
Approximately 5 grams of pulp is rolled or pressed on a disc and is permitted to completely dry. The brightness is measured on both sides of the brightness pad, at least four readings per side and then the average is calculated. These readings are performed on a GE brightness meter which reads a directional brightness or on an ISO brightness meter which reads a diffused brightness. Both instruments are made by Technidyne Corp.
Reverted brightness, a standard lab test for pulp brightness stability, was conducted by placing the pulp brightness pad (after brightness reading) in an oven at 105° C. for 60 min. After that, the brightness pad is read for brightness as reverted brightness.
The viscosity is a measurement used to compare a relative strength property of the pulp.
This property is used to determine the percentage of hardwood/softwood for making different grades of paper. A Cannon-Fenske (200) viscometer tube, calibrated for 25° C., is used for testing bleached pulps. The sample size is 0.2000 grams, using 20 ml, 1.0 molar CED and 20 ml DI water mixed thoroughly to break down the pulp fiber.
The Permanganate Number indicates the amount of lignin that is in the pulp. (The Kappa number is generally used only on the brownstock, while the value for the Permanganate Number is comparative to the bleached pulp.) The procedure for determining the Permanganate Number is:
Pulp dirt count is done by a visual count of all the dirt spots on the brightness pad and is the size weighted sum of the total dirt spots according to a Tappi temperature rate.
Various modifications and variations may be devised given the above-described embodiments of the invention. It is intended that all embodiments and modifications and variations thereof be included within the scope of the invention as it is defined in the following claims.
