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Waste Generation
Wastes of greatest concern from bleaching are chloroform and dioxins. Elemental chlorine and sodium hypochlorite are the two bleaching chemicals most associated with chloroform and dioxin formation. For this reason, most mills have eliminated sodium hypochlorite and replaced chlorine to some degree. Of the six virgin bleached Kraft pulp mills in Georgia, three have replaced elemental chlorine with chlorine dioxide. The other three have substituted varying amounts of chlorine dioxide.
Bleaching, as previously discussed, is a multistage process. Historically, bleaching was considered a separate process from digestion and began with a chlorine stage. Currently, with elemental chlorine use in decline, it is more difficult to define where digestion or delignification ends and bleaching begins. Some of the pollution prevention options discussed below perhaps are closer to delignification than bleaching in historical terms. In either case, the basic purpose of bleaching is to further selectively delignify pulp.
Pollution Prevention Options
Georgia pulp mills have proven that 100% chlorine dioxide replacement for elemental chlorine is technically and economically feasible. Mills nationally have reported pulp quality improvements by switching to chlorine dioxide. Some mills using less than 100% chlorine dioxide report that increases in substitution rate are not economically feasible. The lack of economic feasibility seems to be related to equipment costs as opposed to chlorine dioxide cost. Replacement or upgrade of washers, bleach tower materials, and chlorine dioxide generators were all mentioned as needed prior to increasing substitution rates at these mills. Full substitution can also cause reduced pulp yield and reduced brightness according to some published data.
Chlorine dioxide costs more than elemental chlorine. Even with chlorine dioxides greater reactivity, chlorine dioxide can cost more than eight times as much as elemental chlorine according to EPA documents. It is estimated by the EPA that the total cost of generating one pound of chlorine dioxide is approximately $0.50. The cost of an equivalent amount of chlorine is $0.06. In a 1,000 ton per day mill operating at 11% substitution, an increase to 50% substitution would cost $5 million initially with an increase in operating costs of $1.9 million annually or $5.40 per ton. Increasing to 100% substitution would cost $15.9 million initially with an increase in operating cost of $7.1 million annually or $20.30 per ton. Considering that pulp prices can approach $1,000 per ton, 100% substitution represents an increase in operating cost of about 2%. These estimates are based on general averages and not one particular mill.
Reducing dependence on chlorine bleach can best be achieved prior to bleaching. Bleaching is basically an extended chemical delignification process. Chlorine is used to selectively delignify pulp without causing the damage to cellulose that traditional pulp digestion causes. The key to reducing chlorine use is to find methods which delignify further prior to the bleaching stage.
Uniform, thin chips achieve a greater degree of delignification in standard or modified cooking processes by allowing the chip charge to reach a predictable amount of digestion using a specific chemical and thermal mix. Various methods are available and include improved screening and rechipping of oversized chips. This is a lower cost option for improved delignification with or without bleaching.
A number of methods exist for improving delignification without extensive cellulose degradation. At least four mills in Georgia use extended delignification techniques. These techniques are primarily applicable to continuous digesters. New digesters can cost upwards of $15 million. Some Kamyr digesters, which are the most common type, can be refitted at costs reported in the $1 million range. Batch digesters can also be refitted in some cases. Reduced operating cost by refitting results in payback periods of approximately 18 months according to EPA documents. The cost savings are reported to be in steam and reduction in bleach chemical usage.
Oxygen delignification requires the addition of a reaction tower between the brownstock washers and bleach plant. Oxygen and sodium hydroxide is added to brownstock. Reduction of bleaching chemistry by 50% can be achieved in the bleaching process if preceded by oxygen delignification. Washing follows oxygen delignification; effluent can be recovered or discharged. At least two Georgia mills use oxygen delignification. Cost of building a new oxygen delignification tower can be in the $10 to $30 million range, but may be offset by reducing bleaching needs. Operating costs have been estimated to decrease by up to $3.70 per ton for softwood and to increase by up to $6.65 per ton for hardwood based on studies conducted in Sweden and published by the EPA.
Ozone delignification is not used by any Georgia mills; only one full-scale mill operates within the U.S. Ozone delignification is similar to oxygen delignification. The Union Camp mill in Franklin, Virginia, began operation in 1992 with an OZEoD extended delignification and bleaching process at 1,000 tons per day of bleached kraft production. The overall installation cost was $113 million. Operating costs of the OZEoD delignification and bleaching process are reported to be about 50% less when compared to the more conventional CEDED or DEDED process. D represents chlorine dioxide, E represents extraction, O represents oxygen, and Z represents ozone. There is one pilot mill in Georgia capable of ozone delignification and bleaching. The mill is operated by the University of Georgia for the use of the school and industrial customers for research.
Chlorine and chlorine dioxide are the only bleaching chemicals used in Georgia by virgin Kraft pulp mills. Oxygen and peroxide are used to augment sodium hydroxide extraction. Oxygen and peroxide act to delignify in the extraction stage essentially replacing chlorine in later stages. Sodium hydrosulfite is used by one mill in a bleaching operation producing newsprint. No Georgia mills use ozone in bleaching, although ozone bleaching is used in a pilot mill in South Carolina and a full-scale mill in Virginia.
Enzymes are used by wood eating insects and bacteria to break lignin bonds or to delignify pulp. Xylanase is an enzyme family related to the chemicals secreted by such bacteria; it appears to be useful in pulp bleaching, extended delignification, and deinking. Mills nationally have used this chemical in trial runs; results indicate that up to 50% reductions in chlorine requirements can be achieved without damage to the cellulose. Application of xylanase appears to be simple. Following brownstock washing, xylanase is applied to the pulp as it enters the high density storage tank. Xylanase works in the high density storage tank and requires between 30 and 180 minutes of reaction time. A washing stage follows the xylanase treatment. No Georgia mills report using xylanase, but it appears to be a low cost method of achieving significant results. Other enzymes such as mannanase are being investigated.
Anthraquinone
Anthraquinone is a pulping reaction catalyst which was found to increase the speed of pulping, increase yield, and reduce pulping chemical usage by up to 10%. Anthraquinone is one of the few new technological discoveries in the Kraft pulping field. Due to cost, anthraquinone has not been used extensively in the U.S. except in testing. Approximately 100 mills worldwide use anthraquinone. Although developed as a pulping pollution prevention technology, it may have greater use and be more cost effective if used as an aid to extended delignification. The cost of anthraquinone is expected to drop in the near future due to patent expiration. Renewed interest may develop as a result. Anthraquinone has been estimated to increase pulping costs by approximately $5.00 per ton, but with increased yields of 0.75% and reduced recovery boiler loading. An increase yield of 0.75% represents an increase in production of 7.5 tons per day in a 1,000 tons per day mill.
Polysulfide
Polysulfide is another catalyst mentioned in trade magazines applicable in pulping. Yield increase of 1.5% are possible with improvements in strength. It is possible to use both anthraquinone and polysulfide together. TAPPI publications report that yield increases of approximately 2.5% are possible.
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