3656
Ind. Eng. Chem. Res. 1997, 36, 3656-3661
POZONE Technology to Bleach Pulp Hai Wang,† Yao Shi,‡ Loc Le, Shu-Mei Wang,† Julie Wei,§ and Shih-Ger Chang* Environmental Energy Technology Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720
The POZONE process, a chemical means of ozone production, has been used to bleach wood pulp. The brightness, Kappa number, and viscosity of wood pulp subjected to POZONE treatment have been determined. Brightness increases of up to 44 points and Kappa number decreases of as much as 22 points have been achieved. Promise for effective industrial application has been demonstrated. Introduction Currently, there has been a move in the pulp and paper industry to reduce or eliminate chlorine-based bleaching due to environmental concerns. The discharge of hazardous chlorinated organic compounds into streams and rivers from kraft mill bleach plants has led to government legislation mandating that such effluents be eliminated through the phasing out of chlorine-based bleaching. In the place of chlorine, oxygen-based chemicals, including hydrogen peroxide, molecular oxygen, and ozone, have become alternative choices. Ozone treatment, in particular, has the potential to replace chlorine chemical usage, promising to be an effective method for both delignification (prebleaching) and brightening (Gangolli, 1982; Allison, 1985). Despite the advantages, a number of limitations have prevented ozone from completely replacing chlorine in industrial application. Although successful in delignifying pulp, ozone also attacks cellulose, resulting in the degradation of pulp strength (reduction of viscosity). Further research needs to be conducted to determine how to improve ozone selectivity. Ozone’s short halflife has also reduced its efficiency, potentially raising costs. Because on-site ozone generation is required for ozone bleaching, installation of expensive new equipment is needed. Finally, conventional electricity-based ozone production methods are expensive and energy inefficient. About 90% of electric energy used in conventional ozone production is wasted as heat (Allison, 1991). An efficient and cost-effective means of ozone production would make ozone a more viable replacement for chlorine. The newly developed POZONE process (Chang et al., 1994) could offer a more cost-effective method for ozone production. Developed as a means of removing NOx and SO2 from flue gas (Chang and Liu, 1990; Liu et al., 1991; Chang and Lee, 1992), the application of POZONE technology has been extended to bleach and delignify wood pulp. The POZONE process involves the use of the reaction of yellow phosphorus (P4) with oxygen molecules in moist air to produce oxygen atoms (O) and ozone (O3). In addition to O and O3, several reactive phosphorus oxide radicals, such as PO and PO2, are generated during the course of the reaction; these radicals are chemically active and could also contribute to the destruction of lignin, leading to pulp bleaching. * Author to whom correspondence should be addressed. † On leave from the Research Center for Eco-Environmental Sciences, Academia Sinica, Beijing, 100085 China. ‡ On leave from the Chemical Engineering Department, Zhejiang University, Hangzhou, 310027 China. § On leave from the Engineering Technology Department, California State Polytechnic University, Pomona, CA 91768. S0888-5885(96)00621-5 CCC: $14.00
The chemical reactions of the POZONE process is summarized as follows:
P4 + 20O2 f P4O10 + 10O3
(1)
P4O10 + 6H2O f 4H3PO4
(2)
Phosphoric acid, a valuable byproduct, can be collected for sale or use. This nonelectrical method of ozone generation has been estimated to reduce costs by more than 30% when compared to conventional electrical generation processes. Since the molecular weight of P4 is 124 and that of O3 is 48, the overall reaction shows that 1 lb of phosphorus could generate 3.87 lb of ozone at 100% efficiency. Assuming an additional 50% of phosphorus is needed to account for process inefficiencies and losses (Lee, 1994), 1 lb of phosphorus could generate 2.58 lb of ozone (3.87/1.5 ) 2.58 lb). With the current cost of P4 at $0.98/lb, the reagent cost of phosphorus ozone is $0.38/lb. Ozone produced by the conventional corona discharge method costs from $0.55 to $0.75/lb, depending on the cost of energy. Thus the POZONE process may offer large potential savings to the pulp and paper industry. This paper presents the findings of POZONE pulp bleaching on high- and low-consistency pulp in three different bleaching reactors. Optimal bleaching conditions were determined, identifying the effects of acidification, temperature, oxygen, air flow, and P4 concentration. The success of POZONE bleaching was evaluated on the basis of increase in brightness, reduction of Kappa number, and maintenance of viscosity. The objectives were to maximize brightness and decrease the Kappa number within a short treatment period, while retaining maximum pulp viscosity. The experimental results indicate that the POZONE process can be cost-effective for pulp bleaching and delignification, and it shows potential for industrial application. Experimental Section Materials. P4 and O2 (99.6%) were obtained from the Monsanto Chemical Co. and the Matheson Co., respectively. Deionized H2O was used throughout the experiments. Softwood pulp was supplied by the GeorgiaPacific Co., International Paper, Louisiana-Pacific (Samoa Pulp Mill), Potlatch, and Pulp and Talbot Companies. A hardwood pulp sample was supplied by Westvaco Co. Apparatus and Procedure. Two kinds of POZONE bleaching reactors were used to treat high-consistency pulp: the basket column reactor with the nozzle above water (Figure 1), and the basket column reactor with the nozzle under water (Figure 2). The reactor is a glass © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3657
Figure 3. Schematic diagram for a low-consistency bleach reactor. (1) Water bath; (2) glass beaker; (3) stirring motor; (4) stirring blade; (5) air distribution tube; (6) pulp in water.
Figure 1. Schematic diagram of a basket column reactor system with the nozzle above water. (1) Sintered glass; (2) thermometer; (3) transformer; (4) first basket and pulp; (5) second basket and pulp; (6) third basket and pulp; (7) fourth basket and pulp; (8) fifth basket and pulp; (9) sixth basket and pulp; (10) heating tape; (11) spray nozzle; (12) centrugal pump; (13) glass column.
Figure 2. Schematic diagram of a basket column reactor system with an underwater spray nozzle. (1) Sintered glass; (2) thermometer; (3) transformer; (4) first basket and pulp; (5) second basket and pulp; (6) third basket and pulp; (7) fourth basket and pulp; (8) fifth basket and pulp; (9) sixth basket and pulp; (10) heating tape; (11) spray nozzle; (12) centrugal pump; (13) glass column.
column with a dimension of 70 mm diameter and 800 mm height. A sintered glass is located at the bottom of the reactor. Once 700 mL of water, poured into the bottom of the glass column, was heated to 60 °C by the heating tape, 0.7-7 g (0.10-1.0 wt %) of yellow phosphorus was added. Air or oxygen bubbled upward through the sintered glass while the liquid was circulated by a centrifugal pump through a spray nozzle located either above (Figure 1) or below (Figure 2) the water surface. The air flow rate was 800 mL/min. The liquid flow rate was 7000 mL/min. Yellow phosphorus exists in the liquid state at 60 °C and becomes emulsified into the solution by bubbling air and recirculating
liquid. Phosphorus spraying downward reacted with O2 flowing upward to produce ozone as well as PO and PO2. The reaction time ranged from 15 to 60 min. The ozone produced and then bleached and delignified pulp suspended above. This bleaching took place at 1 atm and about 60 °C. The baskets were approximately 45 mm in height and 65 mm in diameter and were stacked directly above each other. Three stainless steel rods attached to the baskets held them in place. The pump hose that led water to the nozzle ran through a hole in the bottom of the baskets. The pump was temporarily switched off in order to remove each of the baskets from the column to fill with about 1 g of fluffed pulp (by hand). The untreated pulp had been pressed to a 50% consistency and separated into 1 cm pieces. After the pulp baskets were installed into the reactor, the pump was switched backed on. A schematic diagram of the reactor employed to bleach low-consistency pulp is shown in Figure 3. A 1000 mL glass beaker, containing a 2% pulp solution, was immersed in a water bath at 60 °C. Then a motorized stirring blade suspended above the water bath mixed the solution while air bubbled through. The addition of P4 initiated the reaction, which produced ozone to bleach the pulp fiber. An apparatus based on TAPPI standard methods was set up to characterize the following pulp properties. (1) Brightness. An optical reflectance meter was set up to measure pulp brightness. Brightness was determined to be the percent reflection of 457 nm light off a pulp disk sample, with a paper disk coated with magnesium oxide being standard 100% reflectance (ANSI/TAPPI, 1987). (2) Kappa Number. The Kappa number measures the amount of lignin and the bleachability of pulp. Kappa number is the number of cubic centimeters of 0.1 N potassium permanganate solution consumed by 1 g of moisture-free pulp under the conditions specified in the standard. The results are corrected to 50% consumption of the permanganate added (ANSI/TAPPI, 1978). (3) Pulp Viscosity. A 0.5% cellulose solution with 0.5 M cuprioethylenediamine as a solvent was measured with a Cannon-Fenske capillary viscometer. The viscosity (V) was calculated by the formula V ) Ctd, where C is the viscometer constant, t is the average efflux time, and d is the density of the pulp solution (ANSI/TAPPI, 1982).
3658 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. Brightness, Kappa Number, and Viscosity of Pulp (0.5 wt % P4, 60 °C, Underwater Nozzle, Air Flow ) 800 mL/min, 0.5 h Reaction Time) brightness pulp source Samoa Mill International Paper Westvaco Potlatch Pulp and Talbot
O2 air O2 air O2 air O2 air O2 air
Kappa number
viscosity (cP)
initial
final
change
initial
final
change
initial
final
change
38.0 38.0 31.0 31.0 41.0 41.0 43.0 43.0 34.0 34.0
58.7 52 42.0 41.0 67.6 59.0 68.5 58.0 49.7 36.0
20.7 14.0 11.0 10.0 26.6 18.0 25.5 15.0 15.7 2.0
24.84 24.84 18.95 18.95 11.80 11.80 12.55 12.55 37.77 37.77
7.46 8.97 12.95 14.26 6.34 7.85 7.41 10.00 15.71 20.02
17.38 15.87 6.00 4.69 5.46 3.95 5.14 2.55 22.06 17.75
14.34 14.34 17.38 17.38 21.28 21.28 16.02 16.02 18.68 18.68
5.87 7.79 9.44 11.04 9.01 11.33 6.43 9.14 6.70 11.56
8.46 6.55 7.94 6.31 12.27 9.95 9.58 6.88 11.98 7.12
Results and Discussion High-Consistency Pulp. Unbleached pulp samples from different sources were bleached by the POZONE system at varying experimental conditions. Two types of basket column reactors were designed to determine the effectiveness of pulp bleaching as a function of spray conditions. In the first system (Figure 1), the nozzle was installed to spray the liquid in the airspace from above the aqueous emulsion of yellow phosphorus, while in the second system (Figure 2), the nozzle was placed in the water to better agitate the liquid. Parametric studies were done for high-consistency POZONE bleaching to determine optimum bleaching conditions. Parameters included pulp acidification, temperature, P4 concentration, air flow rate, oxygen concentration, and bleaching time. Corresponding Changes in Brightness, Kappa Number, and Viscosity. The improved brightness on the changes of Kappa number and viscosity was studied for high-consistency pulp bleached by the POZONE method. Pulp was bleached using the basket column reactor with the nozzle beneath the water surface (Figure 2). The reaction was carried out using 0.5 wt % of P4 with air/O2 flow ) 800 mL/min at 60 °C for 30 min. After bleaching, the pulp from the six baskets was mixed together in this set of experiments (Table 1). Brightness, Kappa number, and viscosity were then determined. Pulp samples from the International Paper Co., Potlatch Co., Pulp and Talbot, Samoa Mill of Louisiana-Pacific, and Westvaco Co. were used. Table 1 shows a general decrease in Kappa number and viscosity with a corresponding increase in brightness. For example, the Samoa Mill softwood kraft pulp sample had a 20.7 point brightness increase with a Kappa number decrease of 17.38 after 30 min of treatment with oxygen. Viscosity, however, decreased by 8.46 cP. The Westvaco hardwood kraft pulp sample had a brightness increase of 26.6 points when bleached using oxygen. Its corresponding Kappa number decreased by 5.46 points from 11.80 to 6.34. Viscosity dropped 12.27 cP from 21.28 to 9.01 cP. The International Paper sample showed an 11 point brightness increase with a 6.00 and 7.94 cP decrease in Kappa number and viscosity, respectively. The Potlatch sample had a 25.5 brightness increase, while Kappa number fell 5.14 and viscosity decreased by 9.58. Lastly, the Pulp and Talbot sample showed a 15.7 brightness increase, 22.06 Kappa number decrease, and 11.98 cP viscosity decrease. The differences, in Kappa number and viscosity decrease and brightness increase, among the softwood pulps can be attributed to the difference in pretreatment of pulps prior to POZONE bleaching. On the other hand, the difference between hardwood
and softwood pulps may be attributed to any of a large number of different physical and chemical properties between the two, such as fiber morphology and proportions of the cellulose, hemicellulose, and lignin components. Blank experiments by bubbling air or oxygen through water without phosphorus, under otherwise same conditions, were performed. The results did not indicate any measurable improvement in pulp brightness. Although high-consistency POZONE bleaching can greatly reduce Kappa number, the above results also show that it weakens pulp fibers. These results are consistent with other studies which have shown that ozone weakens fibers because of unselective attack on cellulose (Allison, 1991; Lindholm, 1987). Viscosity loss, per unit Kappa number reduction, however, was less pronounced among pulp with high initial Kappa numbers. Of all the samples tested, the Pulp and Talbot pulp had the highest lignin content with an initial Kappa number of 37.7. Treatment using air resulted in a viscosity decrease of 7.12 cP, a smaller loss compared to the other pulps. Kappa number was reduced by 17.75 points from 37.7 to 20.02, while brightness increased by 2.0 points. The Samoa Mill pulp exhibited the same characteristics as the Pulp and Talbot sample when bleached in air. It had the second highest initial Kappa number (24.84). While brightness increased 14.0 points and Kappa number decreased 15.87 points, viscosity decreased by 6.55 cP, lower than the other samples. In contrast, the Westvaco sample, which had the lowest initial Kappa number, showed the greatest viscosity loss under the same conditions (bleaching with air) with a 9.95 cP drop. These results indicate that high-consistency POZONE treatment is more selective with high-lignin-content pulp, where ozone and/or phosphorus oxide react(s) more readily with lignin than cellulose. With industrial application, POZONE bleaching could possibly be used to delignify high-lignin pulp before bleaching to limit viscosity loss. Above Water Nozzle Spray Versus Underwater Nozzle Spray. In the basket column reactor, the nozzle sprays and recirculates the water/phosphorus solution in the glass column. The recirculation prevents coagulation and precipitation of P4. This spray emulsifies the liquid phosphorus to ensure better contact with the oxygen/air bubbled through the sintered-glass bottom. We studied nozzle placement to determine maximum bleaching results. Bleaching was carried out using two types of basket column reactorssthe above water nozzle spray reactor (Figure 1) and underwater spray reactor (Figure 2). Bleaching was carried out using 1 % (wt) P4 with air flow ) 800 mL/min at 60 °C for 1 h.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3659 Table 2. Effect of Nozzle Location: Brightness (%) for POZONE Bleaching at Conditions of 1 wt % P4, 60 °C, Air Flow ) 800 mL/min, 1 h Reaction Timea nozzle
initial
1st basket
2nd basket
3rd basket
4th basket
5th basket
6th basket
avg
change
above water underwater
38 38
59 75
67 72
66 66
60 65
59 61
55 54
61 66
23 28
a
Pulp from Samoa paper mill of Louisiana-Pacific Corp.
Table 3. Effect of Acidification: Brightness (%) for POZONE Bleaching at Conditions of 1 wt % P4, 70 °C, Underwater Spray, Air Flow ) 800 mL/min, 1 h Reaction Timea treatment
initial
1st basket
2nd basket
3rd basket
4th basket
5th basket
6th basket
avg
change
untreated acidified
38 40
60 69
60 67
59 67
59 64
59 61
59 53
59 64
21 24
a
Pulp from Samoa paper mill of Louisiana-Pacific Corp.
Results for the Samoa Mill (Louisiana-Pacific) sample shown in Table 2 indicate that bleaching is more effective when the nozzle spray is placed underwater. The average brightness increase of the underwater nozzle spray (28 points) was 5 points greater than that of the above water spray (23 points). The underwater spray is more effective for two reasons. First, in underwater spray (Figure 2), better liquid agitation is obtained, so P4 can disperse more easily and be emulsified into the solution. Better contact between phosphorus globules and oxygen can allow more P4 to vaporize for reaction with the O2. The increased P4 vapor concentration would lead to more efficient ozone production. Second, ozone flowing up the column to the pulp is not inhibited by the downward spray of solution from an above water nozzle. Because of ozone’s short half-life, increased travel time between the reaction source and pulp would allow for greater decomposition of ozone. Another advantage of an underwater nozzle is that pulp consistency would not be significantly affected. With the above water spray, the pulp in the first basket became very wet because of the spray. Consistency was lowered when the pulp soaked up moisture. Ozone cannot destroy lignin as rapidly at lower pulp consistencies. Therefore, with the above water nozzle, the first basket had a brightness 8 points lower than that of the second basket. The first basket should have had a higher brightness, however, because it was closer to the source of ozone. The pulp brightness should directly correlate to the distance from the water (ozone source), with the first (lowest) basket being the brightest and the sixth (highest) basket being the least bright. This correlation holds true with underwater nozzle spray conditions. Pulp Acidification. Studies were conducted to evaluate how pulp acidification affected POZONE bleaching. The underwater spray reactor was used to bleach pulp using 1% (wt) P4 with air flow ) 800 mL/min at 70 °C for 1 h. Pulp was acidified with sulfuric acid to pH 2. Results for the Samoa Mill sample (Table 3) show that acidification increases pulp brightness. When untreated pulp was acidified, the initial brightness increased by 2%. After ozone bleaching, brightness of acidified pulp increased by 24% while untreated pulp increased by 21%. It is well-documented that the ozone decomposition is enhanced at a higher pH, leading to poorer delignification results (Lachenal and Bokstrom, 1986; Liebergott and Van Lierop, 1978; Allison, 1985). Considerable amounts of heavy-metal ions can be removed from pulp during an acid prewash. This reduces the ozone decomposition initiated by heavymetal ions, which allows a larger proportion of the
Figure 4. Effect of temperature (1 wt % P4, air flow ) 800 mL/ min, underwater spray,1 h). Pulp: Samoa Mill (Louisiana-Pacific Corp.), initial brightness ) 38%.
charged ozone to react with lignin. A second reason may be that acidification is simply an additional washing stage. Temperature. Tests were conducted to determine the optimal aqueous phosphorus emulsion temperature for POZONE bleaching. Bleaching of the Samoa Mill pulp was carried out in an underwater spray reactor using 1% (wt) P4 at air flow ) 800 mL/min for 1 h. The initial temperature of the POZONE reaction is 50 °C. Temperatures studied ranged from 50 to 80 °C in 10 °C increments. Brightness increase was used as a measure of effectiveness. Brightness values for the first basket at 50, 60, 70, and 80 °C were 61, 75, 60, and 63, respectively. Results presented in Figure 4 indicate that the optimum bleaching temperature is 60 °C. Effectiveness decreased at temperatures above 60 °C. Although an increase in the aqueous phosphorus emulsion temperature would increase the P4 vapor pressure and increase ozone generation, high temperatures cause the ozone half-life to decrease. P4 Concentration. Studies were conducted to determine the best P4 concentration for POZONE bleaching. Samoa Mill pulp was bleached in an underwater spray reactor at 70 °C for 1 h. Air flow was 800 mL/ min. Studies were conducted for three concentrations: 0.1% (of water weight), 0.5%, and 1%. Brightness values for the first basket were 50, 65, and 60 for P4
3660 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 5. Effect of P4 concentration (air flow ) 800 mL/min, 70 °C, underwater spray, 1 h). Pulp: Samoa Mill (Louisiana-Pacific Corp.), initial brightness ) 38%.
concentrations of 0.1%, 0.5%, and 1%, respectively. Results presented in Figure 5 show that bleaching is most effective at a phosphorus concentration of 0.5 % (wt). At 0.1%, the P4 concentration may not have been high enough to produce sufficient amounts of ozone for effective bleaching. When the P4 concentration was greater than 0.5%, the reaction was less efficient because P4 globules may have coagulated too quickly, forming larger globules which decrease the surface area. P4 vapor pressure subsequently would be lowered, resulting in decreased production of O3. Air Flow Rate. Effects of the flow rate of air bubbled into the solution were studied. The bleaching of the Samoa Mill pulp was carried out in an underwater spray reactor using 1% (wt) P4 at 60 °C for 1 h. The brightness values of pulp in the first basket was 68, 75, and 62 for flow rates of 500, 800, and 1100 mL/min, respectively. Results (Figure 6) indicate that POZONE bleaching is most effective at an air flow rate of 800 mL/min. When the air flow rate was greater than 800 mL/min, the pulp brightness decreased. Greater air flow produced lower ozone concentration and lower contact time between pulp and ozone. However, when the air flow rate was too low (500 mL/min or less), the exceeded retention time may have destroyed the ozone due to its short half-life. Oxygen Concentration. Ozone production directly corresponds to the oxygen concentration in the reaction chamber. Bubbling oxygen (99.6%) into the reaction chamber would produce higher ozone concentrations than bubbling air. Studies were conducted to determine the effect on brightness. Pulp samples from the International Paper Co., Potlatch Co., and Pulp and Talbot Co. were bleached in an underwater spray reactor using 1% (wt) P4 with air flow ) 800 mL/min at 60 °C for 1 h. Higher oxygen concentrations resulted in greater brightness (Table 4). The International Paper, Potlatch, and Pulp and Talbot samples were 13, 3, and 5 points brighter, respectively, when oxygen was used. The advantage of using oxygen instead of air is that the bleaching time required to achieve equivalent brightness is less. In an additional bleaching test of
Figure 6. Effect of air flow rate (1 wt % P4, 60 °C, underwater spray, 1 h). Pulp: Samoa Mill (Louisiana-Pacific Corp.), initial brightness ) 38%.
Westvaco pulp (using 1 % (wt) P4 concentration, with air flow ) 800 mL/min at 60 °C, bleaching with only three baskets), pulp bleached for 15 min using oxygen achieved a brightness increase of 24 points while pulp bleached for 30 min using air achieved a brightness increase of 28 points. Bleaching with O2 achieved brightness nearly equivalent to that with air in only half the reaction time. There are, however, also disadvantages to using oxygen. Parametric studies on viscosity presented earlier indicated that oxygen bleaching caused greater fiber degradation than bleaching with air. To limit cellulose degradation, the bleaching time with oxygen should be reduced. Bleaching with oxygen is also more expensive than bleaching with air because additional equipment is needed. Further study is needed to determine whether using oxygen or air is more costefficient. Low-Consistency Pulp Bleaching. For comparison, we also studied the effectiveness of low-consistency POZONE bleaching. A. Samples in various stages of the conventional bleaching method were obtained from the GeorgiaPacific Co. These pulp samples had been pretreated with one or more of the following successive steps: chlorine, caustic step (NaOH) + oxygen, hypochlorite, and chlorine dioxide. Low-consistency POZONE bleaching was applied to all of these pulp samples. An initial unbleached, with no pretreatment, sample of 45% brightness showed a 7% increase to 52% after low-consistency POZONE bleaching, whereas an increase of 16% for the sample pretreated with the chlorine step (from 59% to 75%) was achieved. We obtained a 6% increase for the sample after the caustic stage (from 72% to 78%) and 5% for the sample after the hypochlorite pretreatment (from 78% to 83%). Low-consistency POZONE bleaching was most effective after the initial chlorine bleaching stage, in which a 16% increase was achieved. B. An unbleached pulp sample from the LouisianaPacific Co. was also bleached using the low-consistency method. This sample showed an improvement in brightness from an initial 38% to 47% after POZONE treatment.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3661 Table 4. Effect of Oxygen Concentration: Brightness (%) for POZONE Bleaching at Conditions of 1 wt % P4, 60 °C, Underwater Spray, Air Flow ) 800 mL/min, 1 h Reaction Time pulp source
gas (mL/min)
initial
1st basket
2nd basket
3rd basket
4th basket
5th basket
6th basket
avg
change
International Paper
air oxygen air oxygen air oxygen
31 31 43 43 34 34
56 69 78 82 49 63
52 65 77 83 52 62
50 64 75 80 56 60
47 60 74 74 55 55
46 58 73 74 52 52
45 54 72 74 48 49
49 62 75 78 52 57
18 31 32 35 18 23
Potlatch Pulp and Talbot
Our results indicate that low-consistency POZONE bleaching can increase pulp brightness by 5-9% for most pulps. The most effective was bleaching after the initial chlorine stage only. Since our goal was to replace the chlorine step with low-consistency POZONE bleaching, this part of our experiment showed limited success. We also encountered a number of drawbacks. Because P4 was not finely dispersed and it adhered to the pulp fibers as globules, sufficient amounts of air or oxygen bubbling time (1-2.5 h) were required to ensure that all P4 could completely react and to prevent phosphorus residue from remaining in the pulp. Therefore, a highsheer mixer to facilitate mixing will improve lowconsistency pulp bleaching (Blomberg and Wartiovaara, 1986). Quantitative ozone formation and consumption in the POZONE process are difficult to determine because of the interference of ozone measurements from the presence of phosphorus oxide intermediates (PO, PO2, etc.). These intermediates are produced during the chain reactions of phosphorus with oxygen. Because the phosphorus oxide intermediates are expected to react with lignin and cellulose, determining the amount of pulp bleached per mole of phosphorus consumed is more meaningful. This determination should be done in a pilot plant test, where the bleaching is performed in a more effective manner. The effectiveness of bleaching depends on parameters such as fluffiness of the pulp, mixing of pulp with phosphorus or ozone, and reaction temperature due to ozone lifetime dependency. Previously, the POZONE technology was applied to induce the oxidation of nitric oxide to nitrogen dioxide in a spray absorber (Chang and Lee, 1992), which rendered a P/NO molar ratio of 0.6:1simplying that 1 mol of NO was oxidized by ozone and/or phosphorus oxide radicals for every 0.6 mol of phosphorus consumed. This is equivalent to 1.67 mole of NO oxidized (or 1.67 mol of effective ozone produced) for every mol of yellow phosphorus consumed. Conclusion The experiments show that the brightness levels attained through high-consistency POZONE pulp bleaching hold promise for industrial application. It is possible that high-consistency POZONE bleaching can achieve brightness comparable to several steps of chlorine bleaching in only one chemical bleaching sequence. Optimum POZONE bleaching conditions for highconsistency pulp have been determined to be at the following conditions: 700 mL of H2O, 0.5% (wt) P4, 60 °C, underwater nozzle spray, and air flow rate of 800 mL/min. Further studies need to be done to determine the cost-efficiency of using air or oxygen.
between the POZONE and Corona discharge method. We thank the following pulp and paper companies for providing pulp samples, information, and assistance on this project: Georgia-Pacific Co., International Paper Co., Longview Fibre Co., Louisiana-Pacific Co., Potlatch Co., Pulp and Talbot Co., Samoa Pulp Mill of LouisianaPacific Co., Westvaco Co., and Weyerhaeuser Corp. This work was supported by the Assistant Secretary for Fossil Energy, U.S. Department of Energy, under Contract DE-AC03-76SF00098 through the Federal Energy Technology Center, Pittsburgh, PA. Literature Cited Allison, R. W. Effects of Temperature and Chemical Pre-Treament on Pulp Bleaching with Ozone. Proceedings of the 1985 International Pulp Bleaching Conference, Quebec City, 1985; pp 4754. Allison, R. W. Potential of Ozone in Kraft Pulp Bleaching. Appita 1991, 44 (6), 405-409. ANSI/TAPPI. Brightness of Pulp, Paper and Paperboard (Directional Reflectance at 457 nm), 1987, T 452 om-87. ANSI/TAPPI. KAPPA Number of Pulp, 1978, T 236 os-76. ANSI/TAPPI. Viscosity of Pulp (Capillary Viscometer Method), 1982, T 230 om-82. Blomberg, L.; Wartiovaara, I. Ozone Instead of Chloride in the First Bleaching Stage. Proceedings of the 12th EUCEPA Conference, Florence, 1986; Vol. 5, 1-12. Chang, S. G.; Liu, D. K. Removal of Nitrogen and Sulphur Oxides from Waste Gas Using a Phosphorus/Alkali Emulsion. Nature 1990, 343 (6254), 151-153. Chang, S. G.; Lee, G. C. LBL PhosSNOX Process for Combined Removal of SO2 and NOx From Flue Gas. Environ. Prog. 1992, 11 (1), 66-73. Chang, S. G.; Hu, K.; Wang, Y. The Use of Yellow Phosphorus to Destroy Toxic Organic Compounds. J. Environ. Sci. 1994, 6 (1), 1-12. Gangolli, J. The Use of Ozone in the Pulp and Paper Industry. Pap. Technol. Ind. 1982, June, 152-158. Lachenal, D.; Bokstrom, M. Improvement of Ozone Prebleaching of Kraft Pulps. J. Pulp Pap. Sci. 1986, 12 (2), 50-53. Lee, G. C. Phosphorus Ozone for Wood Pulp Bleaching, Bechtel Corporation Report, Bechtel Corp.: 1994. Liebergott, N.; Van Lierop, B. The Use of Ozone in Bleaching and Brightening Wood Pulps. Proceedings of 1978 Oxygen, Ozone and Peroxide Pulping and Bleaching Seminar, Raleigh, NC, 1978; pp 90-106. Lindholm, C.-A. Effect of Pulp Consistency and pH in Ozone Bleaching, Pap. Puu 1987, 69 (3), 211-218. Liu, D. K.; Shen, D. X.; Chang, S. G. Removal of NOx and SO2 from Flue Gas Using Aqueous Emulsions of Yellow Phosphorus and Alkali. Environ. Sci. Technol. 1991, 25 (1), 55-60.
Received for review October 4, 1996 Revised manuscript received May 20, 1997 Accepted May 27, 1997X IE960621U
Acknowledgment We acknowledge that Mr. George Lee of the Bechtel Corp. made the cost comparison of ozone production
X Abstract published in Advance ACS Abstracts, August 1, 1997.
