CBOT sampling protocols

Best Practices in Wood Waste Recycling

Vibrating and Gyratory Screens

Material: Wood Waste

Issue: The processed wood waste discharged from a size-reduction machine varies widely in sizes. Depending on the method of size-reduction (e.g., hog, shredder, chipper), the general geometric form of the processed wood may range from a coarse fragment or shred of wood to a sharp-edged chip. Most manufacturing feedstock specifications call for a certain size range of shreds and chips; thus, processors must remove the unacceptable sizes from the fiber to satisfy these size requirements. Fragments or chips that are larger than the allowable size ranges are referred to as overs. Fragments or chips that are smaller than the allowable size ranges are referred to as fines. The wood fiber that meets the specified size range is referred to as accepts. A manufacturing feedstock specification limits the amount of overs and fines that can accompany the accepts in a given shipment of wood fiber, usually by establishing an allowable weight percentage.

Although careful adjustments to the size-reduction equipment can maximize the production of accepts from the wood waste, the production of overs and fines is an inevitable side effect of any size-reduction process. (Some size-reduction equipment has a tendency to produce more overs and fines than others).

Best Practice: This Best Practice recommends using vibrating or gyratory screens to separate overs and fines from acceptable processed wood fiber.

Vibrating or Gyratory Screens could be square- or rectangular-shaped steel mesh with carefully sized openings that segregate material according to size. Screens may be either dedicated to size segregation or designed like a conveyor to transport the material as well. Screens will have one or more layers that start the segregation process by removing the over-sized material first and then graduating down to the smallest desired size. Through a vibrating or gyrating movement of the equipment, the relatively small particles fall through the screen while the relatively large particles remain on top of the screen. Wood waste that cannot fall through the screen vibrates to the sides or apron of the screen where it exits through an out-feed chute to a dedicated conveyor.

Implementation: Install vibrating or gyratory screens downstream from the size-reduction equipment at a wood-waste processing facility to remove excess overs and fines from the end product. Following is a description of the selection considerations for vibrating and gyratory screens:

Selection Considerations. Primary issues to consider in selecting vibrating or gyratory screens include the following: operational issues (e.g., effectiveness, compatibility with other process equipment, capability to handle targeted production throughput), capital cost, maintenance issues, and safety issues.

Operational: Through work with an equipment vendor, a processor can size vibrating or gyratory screens to handle the types and volumes of wood waste processed at a given facility. Vibrating or gyratory screens effectively segregate by size and are standard equipment in the industry.

Normally, a vibrating screen is defined as screening device that incorporates a straight-line vibrating action. Viewed from the side, one would see a back-and-forth motion following a straight-line path from about 30 to 45 degrees from the horizontal direction of material flow. Screening occurs as each wood chip instantaneously encounters an opening in the screening surface. If the chip is positioned correctly and clears the opening, it passes through the screen. If it is slightly larger it will pass over to encounter another opening

or pass all the way across as an over. Vibrating conveyors do not ensure that the chip will change its orientation as it moves down its deck. This factor is important, because a straight-line conveying motion typically causes wood particles to convey in a direction parallel to their longer dimension. This unit is excellent for removing larger waste from sawdust and similar small materials.

A gyratory screen, on the other hand, is usually installed on a decline and provides an elliptical motion in a primarily horizontal plane. The effect of gravity causes the conveying force that carries the material across the screen. The gyratory motion is very efficient for the accurate sizing of wood chips because it causes each chip to constantly reorient itself while moving across the screen surface. The chip will tend to pass through the screen if any of its dimensions qualifies for the correct size. This unit is widely used in sizing chips for pulp production. The elliptical motion also spreads material across the entire surface to maximize throughput and effectiveness.

Maintenance: Vibrating and gyratory screens require good support structures and foundations because of the dynamic action of the machines. If properly installed, these screens will not require excessive maintenance. They do require regular maintenance attention to ensure that fasteners remain tight and support structures are sound so that the units will not self-destruct if left unattended for an extended time. The screening of wet material can create maintenance and performance problems when holes become constricted or plugged.

Safety: The general operation of these screens creates no unique safety issues so long as they are properly installed, maintained, and monitored. The screens must have adequate clearance for operation so as not to create potential pinch points. The gyratory screens can be noisy because of the movement of the chips, whereas vibrating screens are relatively noise-free when properly designed and installed. Processors can enclose the screens to control any dust problems.

Benefits: Using vibrating or gyratory screens downstream from size-reduction equipment is an essential processing step in the production of wood fiber feedstocks that meets the size-distribution specifications of wood-waste end users.

Application Site: This Best Practice applies to wood-waste processing facilities.

Contact: For more information about this Best Practice, contact CWC (206) 443-7746, e-mail info@cwc.org.

References:

1. BM&M Partnership; Vancouver, BC, Canada.

2. General Kinematics Corp.; equipment brochure.

3. Lyman, Mark. West Salem Machinery; Salem, OR.

4. Musschoot, Albert, Raymond Sherman, and William Guptail. “Vibrating Process Equipment for Resource Recovery.” American Society of Mechanical Engineers; Proceedings from National Waste Processing Conference. 1992

5. Sherman, Raymond. General Kinematics Corp.; Barrington, IL.

6. West Salem Machinery; equipment brochure.

(See Appendix for an Equipment Manufacturers List)

Issue Date / Update: March 1997