WORKING TOWARD A WHITER SHADE OF PALE
May 1, 2000 By Pulp & Paper Canada
A quick survey of the program for the upcoming 2000 International Pulp Bleaching Conference in Halifax shows a heavy focus on the subject of oxygen delignification. Six oral presentations and seven po…
A quick survey of the program for the upcoming 2000 International Pulp Bleaching Conference in Halifax shows a heavy focus on the subject of oxygen delignification. Six oral presentations and seven poster presentations are scheduled on this topic, more than any other category, and several of the posters in other categories also deal with removing lignin before bleaching.
Delignification is so important to bleaching because of the potential economic and environmental gains. The economic advantages are the reduction of chemicals, and the increase in pulp yield, which can translate into reduced wood usage. Environmentally the process reduces the lignin entering the bleach plant and therefore reduces chemical oxygen demand (COD) and absorbable organic halides (AOX) discharged in effluent.
Lignin content is the cause of low brightness in kraft pulps. Once the bulk of the lignin is removed as economically as possible, selective but more expensive chemicals such as chlorine dioxide (ClO2) can be used to remove the small amount of remaining lignin.
“In the last decade,” says Arthur Ragauskas, Institute of Paper Science and Technology (IPST), “delignification research was primarily focused on environmental issues. As these issues have been addressed, studies are now directed at improving manufacturing technologies to reduce the cost of pulp delignification. Interest in reducing operating costs is popular but much greater interest is directed at improving the capital effectiveness of pulp bleaching operations. This can be accomplished in a variety of ways: reduced or simplified delignification/bleaching stages, and improved pulp yields via improved delignification selectivity.”
The goal is to improve oxygen delignification. However, if you remove too much lignin using a chemical that can degrade cellulose, pulp strength will decrease. How do we increase the selectivity of oxygen delignification? How do we take the lignin out of the pulp without damaging the cellulose which will be seen as a decrease in yield or a pulp with lower strength?
To date the average amount of delignification in an oxygen stage about 40 to 45%. Some processes can achieve 50 to 60%, says Richard Berry, Pulp and Paper Research Institute of Canada (Paprican). The goal of research, Berry said, is to ensure the full potential of oxygen delignification by application of the proper equipment, configuration and chemistry.
Getting a boost
Research trends show investigators trying to improve the selectivity by going to two-stage delignification and using pre-treatments, treatments between the stages or post treatments.
Pre-treatments and inter-stage treatments increase delignification up to 65% with improved selectivity, says Adriaan van Heiningen, University of Maine.
Research with peracids such as peracetic acid and peroxymonosulphuric acid show promise, say Gerhard Arnold, Degussa-Hls. The DegOX process has been specifically developed to use peroxymonosulphuric acid (Caro’s acid) as pre- and inter-stage treatments to enhance the subsequent oxygen delignification stage. Under these conditions applying Caro’s acid the active species present will be peroxymonosulphate (PMS). Caro’s acid is an equilibrium product obtained by reacting hydrogen peroxide with sulphuric acid. Commercially available Caro’s acid generation systems have been developed by Degussa-Hls and used in other industries for years. These generators can produce large quantities of Caro’s acid at very high conversion yields.
Current work at Paprican looks at the addition of peroxymonosulphate (PMS) to the oxygen stage. “This oxidant boosts delignification without affecting pulp strength when used in alkaline conditions,” said Jean Bouchard, one of the Paprican scientists working on this project. PMS, said Bouchard, is compatible with the recovery system because the end product is sodium sulphate.
Peroxymonosulphate is a strong and selective oxidant, says Bouchard. Within two minutes the reaction is completed. When combined with oxygen in a single stage it can boost delignification from 40% to 80% and significantly reduce the chlorine dioxide needed and the levels of COD and AOX in secondary treatment systems.
“Caro’s acid may help to improve selectivity of mini-oxygen and Eop stages,” Arnold said. “There is potential,” but he said that there is not enough return to justify the capital for this stage, he said. (His company Degussa-Hls, has also experimented with peroxymonosulphate in extraction stages and high density applications).
One problem is that this product is not commercially available in alkaline form. “Caro’s acid comes in three species: a very strong acid, a strong acid and an alkaline stream. Work needs to be done on how to generate it in industrial quantities.” Paprican has found a way to use it in alkaline form, and the research would benefit greatly if the chemical compound could be produced with high yield.
Some Paprican and UBC work looked at the feasibility of generating alkaline peroxymonosulphate, but economically it has not yet proven to be feasible. One challenge is to develop a way to deal with the sulphur imbalance in the effluent. The existing recovery strategies can be used up to a point, but limitations exist.
IPST has also sought ways to extend the limits of oxygen delignification, targeting the limiting factors that Arnold mentioned above. “We have been very active… developing new enzymatic delignification technologies that operate under milder operating conditions that need less capital equipment (with respect to chemical technologies) and provide pulp with better physical properties,” reported Ragauskas.
Continuing interest remains for enzyme use in the industry. “We and other research groups are aggressively studying this issue. At this time few enzymatic [technologies] have been able to demonstrate substantial cost benefits. The most beneficial commercial systems are the xylanase ‘pre-bleaching’ treatments that have been extensively studies and have seen some commercial applications.
Nonetheless, several other different types of enzymatic systems in the lab continue to demonstrate promising results which should lead to significantly improved delignification under milder conditions, requiring less costly capital equipment, and provide improved delignification selectivity.”
Forty per cent of Canadian mills are using delignification systems compared to about 15% 10 years ago. The high capital expenditure and lingering fears that it decreases the integrity of the pulp have limited the spread of its use across Canada. There is also the question of recovery. “You always open a can of worms if the mill’s recovery system can’t handle it,” says Arnold.
Mini-delignification systems are used in some mills, and have proven to be a cost-effective compromise to the dilemma.
The Harmac pulp operations of Pope and Talbot in Nanaimo, BC, has two mini-systems. The first was started in 1993 and the second in 1996. Doug McKenzie, process engineering superintendent, said decreasing bleaching costs was the impetus for the second system. The environmental advantages for the mill, were considered a given.
The first system, added to be able to manufacture a low AOX effluent product required for the European market, was put in for about $1.5 million, compared to a full system which McKenzie reported would have cost about $25 million. In terms of delignification, the mini system works at 30 to 35% delignification versus 40 to 45% for a full system, McKenzie reported. “Even though we don’t get quite as good delignification, the capital cost is less than one tenth. [This system] allows us to make product for the European market.”
The mini-systems are installed between the brownstock washers and the brownstock high-density storage tanks. “We used the high density storage tanks as blow tanks, then made use of existing bleach plant washers to remove solids. The mini-systems were built in house, with the actual research done by MB Research (the mill was at the time a MacMillan Bloedel property.) The process
engineering was also done in house, while NLK handled the mechanical design.
“The process is embarrassingly simple,” said McKenzie. An MC pump to an in-line steam mixer and an in-line O2 mixer. The retention tube is eight feet in diameter and 60 to 70 feet in height. The tube itself was built without mechanical scrapers, and uses duplex stainless steel, a first in oxygen reactor design. “Primarily 316 or 317 stainless steel was used previously.”
The second tube is a larger system and cost about $2.5 million to put in. The mill had to insulate the high-density storage tank to deal with hot alkaline conditions.
After oxygen delignification the pulp is sent through a conventional bleaching cycle of chlorine dioxide, and a Papricycle stage, followed by EopDED in the ‘B’ bleach plant. “We have a 100% substitution,” said McKenzie. The ‘C’ bleach plant has a similar sequence but with no Papricycle stage.
Bleach costs are reduced and AOX emissions at the mill average 0.3 to 0.4 kg of AOX per tonne of pulp.
The mill has decreased bleaching costs by about 12%, while maintaining the yield and pulp strength. “We do add magnesium sulphate as a viscosity protector to maintain pulp properties at the discharge of the preceding brownstock washers.” Brightness has remained constant.
The installation was relatively easy: all of the equipment was built off-line and tied in during regularly scheduled downtime. “Both systems started up trouble-free. We were already familiar with the equipment,” said McKenzie, as they were already running the MC pumps elsewhere in the plant. “There was no training curve.”
The mini-system was a cost-effective environmentally sound solution for Harmac, and an indicator of the pressures and challenges researchers face. The challenge is finding the science to increase the whiteness of the final product, all the while considering its effect on the environment, and while keeping costs within reach for mills. Research will continue to look at selectivity of oxygen delignification as a way to improve the capital efficiency of pulp bleaching operations.
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