Pulp & Paper
The Pulp & Paper Industry
Until recently, the use of enzymes in the pulp and paper industry was not considered technically or financially viable. Quite simply, suitable enzymes were not readily available, except for the limited use of enzymes to modify starch for paper coatings. However, new enzymes offer significant benefits for the industry, particularly from an environmental point of view. Three examples of applications made possible by new enzyme developments are presented here: enzymatic deinking of waste paper, bleach boosting and pitch control.
How pulp is made
Before explaining how enzymes can be beneficial in the manufacture of pulp and paper, here is a brief introduction for those unfamiliar with the processes involved:
The raw material is wood, which consists of three major natural polymers – cellulose, hemicellulose and lignin. There is also a minor fraction of extractives. Wood fibres contain cellulose and hemicellulose. Lignin can be thought of as the glue holding the wood fibres together. The extractives, also known as sap, pitch or resin, act as a tree’s defence mechanism against microbial attack.
In the pulping process, the goal is to form a suspension of wood fibres – the pulp.
Two different types of pulping process are used. Mechanical pulping is an attrition process in which the fibres are separated mechanically with the input of large amounts of energy. Mechanical pulps are often called highyield pulps since all the wood components are conserved in the pulp, including the lignin. They are less expensive to produce than chemical pulps, but they have the disadvantage that they become darker when exposed to sunlight. They are used mainly in the manufacture of newsprint and magazine paper.
In chemical pulping, wood chips are cooked in chemicals until the lignin dissolves, releasing the wood fibres. The dominant chemical pulping process is the kraft process, which gives a dark brown pulp due to the residual lignin. This residual lignin must undergo some type of bleaching process to yield a bright, white wood pulp before it can be used for paper manufacture. In one end-use, it will be converted into fine paper grades.
Enzymatic deinking of waste paper
Deinking of waste paper is an area with large potential for enzymes. This technology appears to be well-suited for mixed office waste (MOW). The current deinking methods mostly involve deinking in an alkaline environment. Moving to a neutral deinking system which can employ neutral/alkaline enzyme classes requires some change in the chemistry of the system, but can result in improvements in both the process and the final product. This can include improved pulp cleanliness, improved operation of the grey-water loops, less deposit potential and a brighter final pulp.
Bleach boosting of kraft pulps
Chlorine and derivatives of chlorine have been the cheapest and most versatile bleaching agents available for the bleaching of chemical pulps. This class of compounds has the disadvantage of forming chlorinated organic substances (some of which are toxic) during bleaching. The pulp and paper industry is under growing pressure from authorities, consumers and environmental groups to reduce the use of chlorinebased bleaching chemicals and the discharge of chlorinated organic compounds.
By treating the kraft pulp enzymatically prior to bleaching, it is possible to obtain a very selective partial hydrolysis of the hemicellulose which has precipitated onto the fibres during the kraft cooking process. The enzyme has two indirect effects – firstly, it is possible to wash out more lignin from the pulp, and, secondly, the pulp becomes more susceptible to the bleaching chemicals. The technique is called ‘bleach boosting’ and gives a significant reduction in the need for chemicals in the subsequent bleaching stage.
Enzymatic pitch control
Pitch and deposit problems are common in paper mills. In some cases, the cause of these problems may be the extractives which are present in the mechanical pulps. Pitch agglomerates form on the processing equipment such as the chests, felts and rollers. These agglomerates can cause holes in the paper so it has to be recycled or downgraded in quality. In the worst cases, the paper web can break, causing costly paper machine downtime.
A commercial lipase has been developed for use in mill operations. This enzyme has proved its ability to reduce pitch deposits significantly on rollers and other equipment. It breaks down triglycerides in the wood resin in the pulp in much the same way as fungal and bacterial growth reduces the pitch content of the wood during conventional seasoning. However, unlike seasoning, where the wood is stored for a long time, the enzyme acts immediately and does not reduce brightness or yield.
Starch modification for paper coating applications
In the manufacture of coated papers, a starch-based coating formulation is used to coat the surface of the paper. The coating provides improved gloss, smoothness and printing properties compared to the uncoated product. Raw starch is unsuitable for this application, since the flow properties would be unsuitable. In one case, chemically modified starch with a much lower solution viscosity is used. As an economical alternative to modifying the starch with aggressive oxidizing agents, the starch can be treated with enzymes (alpha-amylases) to obtain the same viscosity reduction.
Modified starch is available from starch producers or can be produced on site at the paper mill using a batch or continuous process.
There are interesting possibilities for future applications of enzymes in the pulp and paper industry. One possibility is the selective action of an endo-cellulase which can improve individual fibre characteristics, for example, in producing a softer tissue product. Furthermore, other types of carbohydrate are reported to reduce the amount of energy required for pulp refining, or in reducing contrary components like vessel segments, which can cause printing problems with the final paper.
Further improvements are expected in bleach boosting enzymes, which today are capable only of replacing part of the bleaching agents currently used for chemical pulps with either oxygen or hydrogen peroxide. Researchers around the world are looking for more efficient enzyme systems. Novozymes researchers expect a new group of enzymes, oxidoreductases, to be a future candidate for more environment-friendly pulp bleaching processes.
The standard Product Range for the Pulp & Paper industry looks as follows. Most products are available in liquid as well as solid form, and in different concentrations. Please contact your local sales office for further details as well as with inquiries about special products not listed here.
Please note that all products listed are not necessarily available in all countries. Contact your local sales office for details.
Aquazym®, BAN (Bacterial Amylase Novo), Fungamyl®
Amylases for low-temperature modification of starch.
A cellulase preparation used for deinking of Mixed Office Waste.
A xylanase preparation for reducing the need of bleaching chemicals when bleaching kraft pulp.
Resinase® A 2X
A preparation used to eliminate pitch/resin-related problems.