Improvements in Recycling Wood and Wood-Fiber Products

by Ted Laufenberg, Richard Horn, and Ted Wegner, Forest Products Laboratory, Forest Service, USDA, Madison, WI, and Stan Bean, Forest Products and Harvesting Research, Forest Service, USDA, Washington, DC (retired).

Since the turn of this century, the United States has been dubbed by some "the throw-away society," and it generates approximately 50 percent of the world's solid and industrial waste. Almost half the municipal solid waste (MSW) that goes into landfills consists of paper and wood fiber (table 1). Recovery and reuse of these materials would offer a significant opportunity for saving landfill space as well as for reducing our impact on the environment.

The pulp and paper industry has expanded recycling of some grades of paper, including corrugated containers and cartonboard, newspapers, and white office paper. In addition to this effort, alternative uses of recovered fibers and wood from MSW need to be developed. Wood fibers can be recovered from industrial operations or the MSW stream to produce useful, ecologically sound commodities.

This chapter highlights research on recycling that is being conducted by USDA's Forest Products Laboratory. These research activities represent a step toward achieving the technology needed to expand recycling opportunities in this country.

Improved Technology for Recycling Paper

An intensive research and development effort by the USDA Forest Service promises to reduce disposal of wood fiber wastes by an additional 30 million tons per year by the year 2000. Research at the Forest Products Laboratory (FPL) in Madison, WI, is focused on (1) removing contaminants from and deinking recycled papers, (2) improving fiber bonding in recycled papers, (3) developing new bleaching technologies for recycled fibers, and (4) transforming the structure of recycled fibers.

Removing Contaminants. Nonfiber components added to paper and paperboard products complicate the recycling of wastepaper. Before wastepaper can be reused as a fiber source, many of these components, such as adhesives, inks, dyes, metal foils, and plastics, have to be removed.

Many contaminants can be removed on the basis of size and density by cleaning and screening. However, some contaminants are similar in size and density to the fibers and are much more difficult to remove. Nonwettable synthetic adhesives, called stickies, fall into this category and area major obstacle to wastepaper recycling. Current techniques for controlling stickies include furnish selection, improved pulping and deflaking, well-designed screening and cleaning systems, dispersion, and use of stabilizing additives.

Recent advances in stickie control have focused on pulper design. New pulpers are equipped with mechanical devices that remove stickies at the beginning of the stock preparation system. Drum pulpers and low-speed, high-torque agitators can gently fiberize the wastepaper without reducing the size of the stickies, resulting in highly consistent pulping. Low-density stickies are removed by through-flow cleaners, an energy-efficient process that reduces fiber loss. This type of cleaner design has been critical in removing contaminants from corrugated containers.

Deinking. Deinking poses a different set of problems. When ink is fused to paper, as in laser printing and the photocopy process, it cannot be dispersed by conventional methods for removing contaminants. Detaching nondispersible inks from fibers requires intensive mechanical, chemical, and thermal action. The detached ink is then removed by screens, cleaners, and the flotation process.

Recent deinking technology has resulted in brighter and cleaner paper through high-consistency dispersion, together with bleaching or use of agglomerating chemicals. Flotation units have been improved to generate a wider range of air bubbles, which permit removal of a wider range of ink particle sizes. Pressurized refiners have been adapted for dispersing noncontact inks. Dispersion has been especially useful in recycling high-quality office wastepaper into printing- and writing-grade papers. The process disperses both contaminant particles and stickies, facilitating their subsequent removal.

Dave Bromett, a chemical engineer with the Forest Service Forest Products Laboratory in Madison, WI, begins the process of separating oversized particles, lumps, and other contaminants, called "stickies," by passing the liquid paper pulp over a vibrating flat screen to produce usable fiber.

Bob Nichols/USDA 92BW0734-15

An important development in deinking technology is the multidisc refiner, which packs a large amount of refining surface in a single refiner housing. This technology reduces refining intensities drastically compared to conventional double-disc refiners and permits treatment of delicate pulps. Multidisc refiners are especially well suited for treating deinked newsprint. Increases in strength of 15 to 25 percent are possible without generating the short, unusable fibers (fines) that can clog papermaking systems.

Improved washing units have resulted in more efficient treatment of deinked stock and other recycled wastepaper pulp. Advanced diffusion washing technology, displacement wash presses, vacuum deckers, and other stock washing equipment are being used throughout the recycling industry. These new systems typically cost less than their older counterparts, an important feature for an industry with high capital costs.

Finally, researchers are using computer-aided visual analysis of paper samples to determine the number and size of ink specks.

Improved Fiber Bonding. In the final stage of papermaking, the sheet of paper is dried at a relatively high temperature. The combination of dehydration and elevated temperatures hardens the surface of the wood fibers and stiffens their internal structure. These effects must be reversed if the fibers are to be recycled into paper. The surface hardening limits interfiber bonding and the internal stiffening reduces fiber conformability, which is needed to consolidate a paper web. Variations on traditional refining and stock preparation can reverse internal stiffening for some applications. However, the surface hardening effect is not as readily reversed.