WO1990014463A1

WO1990014463A1 - Method for reducing chlorinated compounds produced during chlorination stage bleaching of pulp

Info

Publication number: WO1990014463A1
Application number: PCT/US1990/002612
Authority: WO
Inventors: Ronnie G. Hise
Original assignee: Westvaco Corporation
Priority date: 1989-05-15
Filing date: 1990-05-14
Publication date: 1990-11-29
Also published as: CA2033077A1; SE9100023L; CA2033077C; JPH03505241A; SE9100023D0; FI98838C; FI910178A0; FR2650605A1; FR2650605B1; FI98838B

Abstract

A chlorination stage bleaching method is disclosed which comprises suppressing formation of polychlorinated dioxins and furans without resulting in cellulose degradation and poor pulp viscosity by controlling the molecular chlorine:hypochlorous acid ratio by adding the chlorine to the pulp in multiple charges and by adding an alkaline material to the pulp during the chlorination stage such that chlorination stage pH is controlled to a value from above about 1.8 to below about 3.5.

Description

METHOD FOR REDUCING CHLORINATED COMPOUNDS PRODUCED DURING CHLORINATION STAGE BLEACHING OF PULP

This invention relates to a method for reducing polychlorinated dioxin and furan formation in chlorination stage bleaching typically as part of the process of chemical delignification and purification of fibers from wood chips. More particularly, this invention relates to bleaching pulp with sequential chlorine addition and at a controlled pH in the chlorination stage which results in diminished production of chlorinated dioxins and furans during the chlorination and alkaline extraction stages without sacrificing delignification of the pulp or pulp viscosity in the absence of cellulose protectors or other additives. Recent concern has been expressed about the environmental effects of chlorinated compounds formed by bleaching chemical pulp. Although investigations are incomplete and debate continues as to the risk to the environment represented by these compounds, special attentio has been given chlorinated dibenzo-p-dioxins and dibenzofurans.

Practical means for preventing or reducing formation of these and related compounds are being sought in laboratory and mill studies. Certain proposed approaches, such as oxygen delignification or high substi¬ tution of chlorine dioxide for chlorine in the chlorina¬ tion stage, involve great expense, both in terms of capital equipment and processing costs. In addition to cost, significant time is required to implement these options. Some inexpensive, shorter term solution is desired Investigation of the causes of the formation of chlorinated compounds during chlorination bleaching have led researchers to propose approaches to the reduction of TCDD/F levels focused primarily on modification of the chlorination stage of bleaching. Swedish researchers, for example, have claimed that TCDD/F levels are exponent¬ ially related to the "chlorine multiple" or "Kappa factor" — actually saying that the critical factor is the amount of chlorine applied to a certain amount of lignin ("The Influence of Lignin Content and Bleaching Chemicals on the Formation of Chlorinated Dioxins, Dibenzofurans and Phenolics" by Axegard et al and "Influence of Oxygen Pre- treatment and Chlorine Ratio on the Formation of PCDDs and PCDFs in Pulp Bleaching" by Swanson et al, presented at the Dioxin '88 Conference in Umea, Sweden). On the other hand, copending U.S. patent application Serial No. 262,534 reports the discovery that the amount of chlorine actually may remain at conventional levels as long as the concentration of chlorine does not exceed a level at which unwanted chlorinated dioxins and furnas are produced at any time during the chlorination bleaching stage, as a method for controlling formation of these undesirable compounds. Control of the chlorine concentra¬ tion is taught to be achieved by a split addition of chlorine chemicals.

Washing the pump after chlorination with excess water lowered the levels of 2378-TCDD and 2378-TCDF in the subsequent E-stage by only 5-10%. This suggests that the potential is low for removing chlorinated dioxins and furans from bleached pulp by improved bleach plant washing.

Copending U.S. patent application Serial No. 07/338,320, however, discloses that washing unbleached softwood pulp with aqueous alcohol decreases the amount of chlorinated dioxins and furans formed during subsequent chlorination and alkaline extraction stages by 80%, or higher, as compared to a control representing the prior art method which omits such pre-chlorination washing step. Of ourse, washing pulp with organic solvents may be too expensive at present for commercial feasibility.

Due to the equilibrium relationship between molecular chlorine (Cl2) and hypochlorous acid (HOC1) in water as shown in Figure 1, it is postulated that a shift in this equilibrium to decrease the concentration of molecular chlorine by raising the pH in the chlorination stage from the typical chlorination stage final pH 1.5 - 1.8 to above 1.8 reduces the formation of polychlorinated dioxins (PCDDs) and furans (PCDFs) . Chlorine (CI2) , available at lower pH value, both oxidizes and partici¬ pates in electrophilic substitution reactions with lignin forming chlorinated organic compounds and generally does not degrade cellulose. The resultant cellulose degrada- tion and unacceptable pulp viscosity would seem to dictate against an upward adjustment of pH in the chlorina¬ tion stage as a means of reducing PCDD and PCDF formation during pulp bleaching. On the other hand, since hypo¬ chlorous acid is a stronger oxidant and acts to oxidize and degrade cellulose, as well as lignin, high pH bleach¬ ing can result in lower pulp strength, as evidenced by low pulp viscosities.

Copending U.S. patent application Serial No. 07/351,826 teaches suppressing formation of polychlori- nated dioxins and furans by adding pH adjusting base incrementally during the chlorination stage and control¬ ling the final chlorination stage pH to a value greater than 2.2 without increasing the degree of cellulose degradation and reducing pulp viscosities. Even higher pH chlorination stage bleaching is taught by Canadian Patent No. 1,149,111 wherein the patentees seek to optimize the economics of substituting chlorine for chlorine dioxide without significantly increasing (rather than seeking to decrease) the amount of "toxic decomposition products" resulting from the use of chlorine. The patentees teach a minimum chlorination stage pH of 3.5, a minimum chlorine dioxide fraction of 30%, and chlorine addition is taught in all cases to follow an initial chlorine dioxide addition. Figure 1 depicts the equilibrium relationship of the molecular chlorine concentration and hypochlorous acid formation at rising pH values. As pH increases, the relative concentration of chlorine decreases and the concentration of hypochlorous acid increases.

Figures 2 and 3 depict the effects of process changes on the formation of 2378 TCDF in chlorination stage bleaching of softwood pulp at various chlorinated pulp brightnesses. Figure 2 compares a single chlorine charge process with multiple chlorine charge systems. Figure 3 compares multiple chlorine charge systems with varying chlorine dioxide substitution and with or without pH adjustment by alkali addition.

It has been discovered that the greatest reduction in polychlorinated dioxin and furan formation during chlorination stage bleaching is achieved by lowering the molecular chlorine concentration both by introducing the chlorine to the pulp in at least two increments and by adding alkaline material to the pulp. Moreover, this suppresses formation of PCDD/Fs without resulting in cellulose degradation and poor pulp viscosity. The viscosity is protected by raising the pH gradually and by controlling the pH in a range that avoids overproduction of hypochlorous acid. For example, one embodiment may involve adding the alkaline material (used to adjust the pH upward) in several small doses to control the final chlorination pH to a value above about 1.8 to below about 3.5. The ratio of molecular chlorine to hypochlorous acid is less than 20:80 at all times according to calculations that assume equilibrium conditions at a measured pH value. Figure 1 shows this equilibrium between chlorine and hypochlorous acid at various pH values.

Four ingredients are necessary to make paper: (1) raw materials — such as wood from the forest;

(2) energy — from coal, oil, gas or wood by-products, e.g., bark or wastes from the paper making process itself;

(3) water, much of which is used as a conveyer belt to transport material along the process and is recirculated, but some of which is discharged to the atmosphere as vapor from driers or as liquid after purification in a waste treatment plant; and (4) skilled operators and management. Not all pulps are produced chemically. "Ground- wood" or "mechanical pulp," as the names imply, is produced by grinding a log of wood against abrasive stone surfaces or between rotating steel discs with cutting bars against their faces to yield fibers and fragmented fiber bundles. Such pulp is used in newsprint and similar paper where high opacity and good printability are desirable properties but where mechanical strength is not a prime requirement.

Chemical pulping begins with cutting the wood into chips. The chips are screened, rejects being both oversize slivers or undersize fines, and are taken to the top of a "digester" or a high pressure cooking vessel. Chemicals are added, and the reaction is allowed some time under a prescribed program of temperatures for the lignin of the wood and some hemicelluloses to be dissolved and extracted from the chips. Then the cooked material is discharged discontinuously in a batch process or continuously into a blow-tank where steam and other volatiles are flashed off. The cooking liquor — which is now a "black liquor" because of the dissolved lignin — is passed on to a chemical recovery cycle.

The pulp is washed with water to remove black liquor on, for example, a series of wire covered rotating drums. The washed brownstock is screened, diluted, and may be passed on to arrays of centrifugal cyclonic cleaners to separate large and heavy "dirt" — e.g., silica or metal particles — before bleaching. Since the screening operation and cyclone cleaners are only efficient with dilute suspensions, while bleaching requires higher consistencies for economical reasons, the stock is "thickened" by extracting some of its water, using wire covered, perforated drums on which the stock is made to form a mat. The thick brownstock is next subjected to a series of bleaching operations. These can vary widely both in the types of chemicals used and their sequences. In a typical system, called CEDED (Chlorine- Extraction-chlorine IDioxide-Extraction-chlorine Dioxide) , the pulp is first delignified with elemental chlorine, then extracted with sodium hydroxide and finally bleached with two chlorine dioxide stages separated by an alkaline extraction. As is well known in the art, this first chlorination stage of bleaching often involves various combinations of chlorine and chlorine dioxide. Chlorine dioxide attacks lignin specifically to a far greater extent than it attacks cellulose — unlike chlorine which is a more indiscriminate oxidant — chlorine dioxide, however, is more expensive. Thus, it is preferably used for the final steps. After the final bleaching, the bright stock is washed to leave the pulp mill and enter the paper mill.

In the chlorination stage,chlorine and chlorine dioxide are functionally similar but not identical.

Various methods to optimize delignification and minimize viscosity loss are discussed in the literature. Mixtures of chlorine dioxide and chlorine are often superior to either chemical alone. Also, the effects of these chemicals can be varied by altering the sequence of their addition to the pulp. Many variations exist, but a common approach is to add chlorine dioxide first followed by chlorine after a short retention time to allow reaction of the chlorine dioxide. This method of sequencing the two chemicals is often claimed to give best delignification with the least cost. It is particularly applicable when relatively large portions of chlorine dioxide are used, i.e., 50% substitution of chlorine by chlorine dioxide on an active chemical basis. When smaller amounts of chlorine dioxide substitutions are used, the sequence of addition is reversed to provide best retention of pulp viscosity. If such amounts of chlorine must be used, compromise must be reached between the adverse effects of a high chlorine concentration resulting in TCDD/F forma¬ tion and excessive hypochlorous acid formation by bleach- ing at high pH values and causing cellulose degradation. As demonstrated in the following examples, a reduction in TCDD/F production of at least 50% is achieved without a reduction in total chlorine applied by controlling the molecular chlorine concentration. The invention described herein applies to all chlorination bleaching systems in which the applied chlorine level is sufficient to generate concentrations of molecular chlorine in the reaction system that result in formation of undesired levels of TCDD/F. The objective of the present invention is to permit enough molecular chlorine to be available for reaction with the lignin, but avoid an excess of molecular chlorine to limit TCDD/F formation. Limiting the forma¬ tion of hypochlorous acid by controlling the pH minimizes cellulose degradation and viscosity loss. This compromise is achieved by the combination of incremental addition of chlorine and addition to the pulp of a base compound to regulate the pH to from above about 1.8 to below about 3.5, which results in a molecular chlorine:hypochlorous acid ratio of less than 20:80 (Fig. 1). It follows from the chlorine-hypochlorous acid relationship (CI2 + H2O -^*==^HOC1 + H++C1") that the chlorine concentration can be lowered by (a) adding H2O, (b) decreasing [H⁺] (increasing the pH) , or (c) decreasing [Cl~] . Further, the equilibrium constant, K, described in the equation: [HOCll [H⁺] [Cl~]

[C1₂1 [H₂0] is known to increase slightly with increasing temperature. At 25°C, K = 4.3 x 10^~4, whereas at 45°C, K = 6.1 x 10~⁴. This equilibrium expression can be used to calculate the values which establish the curve of Fig. 1. This provides another opportunity to reduce the chlorine concentration, but the magnitude of the effect is small. The invention method is employed in processes of bleaching of pulp which employ a chlorination stage. In one embodiment, the invention method is employed in the CEDED system as earlier described. As one skilled in the art appreciates, there are many pulp bleaching sequences, and no limitation of the invention is intended by the following examples.

EXAMPLE I To demonstrate the improvements of sequential chlorine addition, softwood pulp (Kappa number 30.2) was bleached in the laboratory with chlorine and 10% chlorine dioxide substitution. The total active chlorine applied in all cases was calculated by multiplying 0.24 times the unbleached pulp Kappa number ("Kappa factor") . Total active chlorine is determined by multiplying the percent of chlorine dioxide by 2.63 and adding the percent of chlorine. (All percentages are by weight on oven dried pulp.)

Chlorinations were performed on 55 grams (oven dried basis) of pulp at 50°C in a two liter capacity glass reaction kettle, with constant stirring. Chlorine gas was dissolved in cold distilled water. The CI2 concentration was determined by titrating a sample of the solution just before addition to the pulp slurry. This C10₂ solution was added 15 seconds after the intial charge of Cl₂. The consistency after addition of all reagents was 2.5%.

After one hour, the slurry was thickened to near 15% consistency in a Buchner funnel using a 300 mesh stainless steel wire (both prewashed with absolute ethanol) Fines were retained with the pulp by pouring the initial filtrate back through the pulp pad.

Extractions were conducted in heat-sealable bags. Target extraction final pH was at least 11.0. Pulp consistency was 12%, reaction time one hour, at a tempera- ture of 10°- C . The extracted pulp was diluted to 1.5% consistency with distilled water and thickened to near 15% consistency. A sample of each chlorinated pulp was analyzed for TCDF. The results are reported in Table I.

TABLE I

EFFECT OF CHLORINE CONCENTRATION ON FORMATION OF

2378-TCDF IN THE C-STΛGE SOFTWOOD PULP

Kappa 30, Kappa Factor 0.24

2378- CE

Method of Kappa TCDF, ¹ CE K Vi se , C-Stage

Addition Factor DDt No. CD Final PH

One

Charge 0.24 175 4.5 22.1 1.8

Two

Charges 0.24 79 4.5 17.1 1.6

Three

Charges 0.24 27 4.7 18.2 1.7

(1 ) Studies have indicated that 2378-TCDF can be used as an i ndicator of the presence of 2378-TCDD, the corresponding chlorinated dioxin isomer, at l ower level s.

In one case , all the chlorine was added to the stirred pulp slurry at once , in about 5-10 seconds . Other experiments were conducted in which the same total amount of chlorine was divided into two or three separate charges , and added sequentially to maintain lower concentrations of chlorine in the slurry . The amount of 2378-TCDF was lowered by about 50% when the chlorine was added in two equal charges , spaced twenty minutes apart . The 2378-TCDF level was lowered by about 85% when the chlorine was added in three (unequal) charges . The second charge was added after ten minutes , with the final amount added after 30 minutes total reaction time . In all cases , all C10₂ was added 15 seconds after the first Cl charge . By adding the chlorine sequentially , there was never as high a concentration of Cl₂ as when it was added all at once . Thus , it is demonstrated that if the concentration of Cl₂ is kept low , formation of TCDD/Fs can be minimized. EXAMPLE II Mill studies were conducted to confirm the benefits of reduced chlorinated dioxin and furan formation on a commercial scale. Bleach plant samples of both softwood and hardwood pulps were taken from runs conducted using single chlorine addition and multiple (3) chlorine additions and were analyzed for 2,3,7,8-TCDD/F. Finally, runs were made, sampled, and analyzed which combined multiple chlorine addition with addition of alkali to maintain the pH to from above about 1.8 to below about 3.5, preferably 2.0 - 2.4. This last technique, as herein claimed, most consistently produced acceptable pulp viscosities with minimum 2,3,7,8-TCDD/F formation, as shown in Tables II and III.

TABLE II

2,3,7,8-TCDD/F ANALYSIS

BLEACH PLANT SAMPLES OF HARDWOOD PULP

(Temperature = 50'C ± 2'; Consistency * 2-3%)

Chlorination Stage Final Bleach Stage

2,3,7,8- 2,3,7,8- 2,3,7,8- 2,3,7,8-

Bleaching Run TCDD TCDF TCDD TCDF

Conditions No. (DDt)* (DDt)* (DDt) (DDt)

1 ND(2.0) 6.4 2.9 12

2 3.3 14 2.4 11

3 7.3 41 8.7 42

Single 4 - - 6.2 49 cι₂ 5 - - 5.9 19

Addition 6 5.3 82 1.3 21

7 3.3 20 2.4 21

8 - - 8.3 37

9 - - 2.6 5.7

10 - - 6.3 30

11 _ _ 1.2 10.1

12 - - D(0.8) 8.8

Multiple 13 - - 0.6 3.0

Cl₂ 14 - - ND(0.3) 3.9

Addition 15 - - ND(0.9) ND(0.5)

16 - - ND(0.9) 8.5

17 ND(2.3) ND(1.5)

18 - - ND(2.3) ND(1.2)

Multiple 19 - - ND(O.l) 0.6 cι₂ 20 - - ND(0.2) 1.0

Addition 21 - - ND(0.05) 0.45

& Alkali 22 - - ND(O.l) 0.51

Addition 23 - - ND(0.4) 0.4

24 - - ND(0.6) 0.86

25 - - ND(0.4) 0.34

26 ** ND(0.7) 0.51

*Where no data is reported, none was generated. TABLE III

2,3 ,7,8-TCDD/F ANALYSIS

BLEACH PLANT SAMPLES OF SOFTWOOD PULP

(Temperature - 50^*C ± 2* ; Consi stency » 3-4.5%)

Chlorinatlion Staαe Final Bleach Staαe

2,3,7,8- 2,3,7,8- 2,3,7,8- 2,3,7,8-

Bleaching Run TCDD TCDF TCDD TCDF

Conditions No. (DDt) (DDt) (DDt) (DDt)

1 31 215 19 87

2 - - 13 105

Single 3 ND(22*) 188 ND(3.8) 53 cι₂ 4 40 293 4.1 17

Addition 5 - - 4.6 20

6 - - 5.4 14

Multiple 7 ND(0.8) 5.3 ci, 8 - - 3.8 12.4

Addition 9 - - ND(1.8) 7.1

Multiple 10 ND(1.2) ND(0.7)

Cl₂ 11 - - ND(0.08) 0.6

Addition 12 - - 0.9 5.5

& Alkali 13 - - ND(0.3) 0.53

Addition

^♦Estimated Maximum Potential Concentration

Therefore , minimizing the 2 , 3 , 7 , 8-TCDD/F formation while maintaining good pulp viscosities during chlorination stage bleaching is optimally achieved by pH adjustment with alkali addition , combined with sequential addition of chlorine on pulp.

EXAMPLE III

Mill trials were conducted to determine the benefits of splitting chlorine charges and adding sodium hydroxide for pH control towards lowering the formation of TCDD/Fs during bleaching of softwood pulp. The results are graphically depicted in Figures 2 and 3. In Figure 2, data are presented from a series of mill tirals on softwood. In the curve marked "One Charge" brightness levels were varied by changing the level of tot active chlorine (Cl₂ + C10 ) in the C-stage. The level of C10₂ substitution (for Cl₂) was varied from 9% to 27% in these short-term trials. One of the benefits of higher substitution is that a higher C-stage pH results.

In the family of curves in Figure 2 marked "Three Charges", the level of C10₂ substitution was either 10% or 25%, and caustic was used for pH control in two cases, as is shown in Figure 3.

The individual contributions of some of the proce changes can be estimated from Figures 2 and 3. Comparing one charge (with 9-27% C10₂ substitution) with three charges (10-25% C10₂ substitution) at 28% GE brightness, a reduction in 2378-TCDF of 82-92% would be predicted as noted in Table IV. Using a base for pH control would further impact the TCDF levels, resulting in a predicted 99%+ reduction. Given a system with three mixers, these data would predict an 83-96% reduction in 2378-TCDF by adding an alkali for pH control.

TABLE IV

ESTIMATED CONTRIBUTION OF PROCESS CHANGES TO LOWER LEVELS OF 2378-TCDF IN C-STAGE SOFTWOOD PULP

Percent Reduction

2378-TCDF Compared Compared

(ppt) at Compared to to

28% GE¹ to One 25% C10₂, 10% C10₂,

Process Charge No NaOtf No NaOH

One Charge 118

Three Charges 25% C10₂, No NaOH 21.7 82 10% C10-, No NaOH 9.4 92 10% C10₂, NaOH added 1.6 99 93 83 25% C10₂, NaOH added 0.8 99+ 96 92

Interpolated from Figure 2. Based on the percent reduction in 2378-TCDF formation by the combination of multiple charges of chlorine addition with a 25% chlorine dioxide fraction and alkali addition versus the reduction achieved with multiple chlorine charges with a 10% chlorine dioxide fraction and alkali addition, further reductions are insignificant with up to below about a 30% chlorine dioxide fraction in the chlorine solution. Also, in view of the reductions achieved with less than a 30% chlorine dioxide fraction, further substitution of chlorine with relatively expensive chlorine dioxide is not economically justified. There¬ fore, the present invention relates to chlorine additions with up to below about a 30% fraction of chlorine dioxide. While the invention has been described and illustrated herein by reference to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials, combinations of materials, and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art.

Claims

1. A method of reducing polychlorinated dioxin and furan formation during chlorination stage bleaching of brownstock pulp comprising controlling molecular chlorine: hypochlorous acid ratio to less than about 20:80 by adding a chlorine generating compound to the pulp in multiple charges and by adding alkaline material to the pulp.

2. The method of claim 1 wherein the chlorine generating compound is a member of the group consisting of chlorine, chlorine dioxide, hypochlorous acid, hypochlorites, chlorine monoxide, monochlorine monoxide, and combinations thereof.

3. The method of claim 1 wherein the alkaline material is selected from the group consisting of sodium hydroxide, calcium oxide, magnesium oxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, calcium carbonate, and ammonia.

4. The method of claim 1 wherein the alkaline material is introduced to the pulp in multiple charges.

5. The method of claim 1 wherein the alkaline material is added between two or more of the multiple chlorine charges.

6. The method of claim 1 wherein the alkaline material is added prior to adding chlorine to the pulp.

7. The method of claim 1 wherein the chlorina¬ tion stage is the initial chlorination stage of a sequential pulp bleaching-extraction process. lo

8. The method of claim 7 wherein the chlorina¬ tion stage is the first stage of a CEDED pulp bleaching system.

9. The method of claim 2 wherein the chlorine generating compound is the combination of fractions of chlorine and chlorine dioxide up to a maximum chlorine dioxide fraction of up to below about 30%.

10. The method of claim 9 wherein the chlorine dioxide fraction is 25%.

11. The method of claim 9 wherein the chlorine dioxide fraction is 10%.