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Best Practices in Wood Waste Recycling Click here for printable PDF version Chip Washing for Removal of Dirt and Grit Material: Wood Waste
Issue: Dirt and grit are common contaminants in recovered
wood waste that resist removal by
mechanical screening. The persistence of dirt and grit in wood-waste
derived feedstocks tends to cause
significant problems for the high
value end use markets.
For example, the presence of
dirt and grit in composite panel board
feedstocks causes excessive equipment
wear in the manufacturing plant as
well as for the end users of panel
board products.
For the manufacturing plant,
dirt and grit causes refiner plates
in the size reduction equipment to
wear down at a faster rate than normal.
For the end-user, dirt and
grit embedded in the panel board causes
saw blades to dull more rapidly.
Dirt and grit can also reduce
the brightness of a panel board which
may be a problem for end users who
overlay the board with thin melamines
or other coatings.
Best Practice: This best practice recommends the use of chip washing equipment to remove persistent dirt and grit contaminants from recovered wood waste feedstocks for users of exceptionally clean supplies of wood waste. Mechanical screening for dirt and grit removal is adequate.
Chip Washer. A chip washer operates by mixing wood waste feedstocks with water. The mixing process needs to be fairly dynamic to remove dirt and grit. The energetic mixing can be accomplished through an enclosed ‘waterfall’, turbulent pumping, or other mechanisms. Once the dirt and grit is washed free from the surface of the wood waste material, the wood and water slurry is ready to be dewatered by straining the slurry over a relatively fine screen. To reduce costs and avoid excessive waste water disposal, it is advantageous to continuously cycle the cleaning water through the system. A calm holding tank will prepare the water for reuse by allowing dirt, grit, and other particulates to settle out of the dirty water. The settled material can then be readily removed from the system.
In order for a chip washer to work most effectively, the fines must be removed from the feedstock prior to washing. The presence of fines in the material flow has a tendency to hamper dewatering. To aid dewatering, the ‘acceptable fines’ fraction should be screened out prior to washing and remixed after dewatering.
The process of washing wood waste feedstocks has the side effect of reintroducing moisture into the material. Wood waste feedstocks tend to have very low moisture content and rehydration may improve the quality of refined manufacturing furnish. Or, it may increase the drying requirements. The desired moisture content will be determined by the requirements (specification) of the manufacturer.
Implementation: The overriding consideration with respect to installing a chip washing system is cost. Chip washing systems tend to be very expensive. As a result, they are usually installed at the manufacturing plant as opposed to the processing facility. At the manufacturing plant, processing volumes are large and the operators can take advantage of economies of scale.
The
material handling and storage systems
downstream from the chip washer need
to be designed to accommodate wet
feedstock because the material will
continue to dewater somewhat after
it leaves the washing system.
Significant maintenance and
safety issues may arise if the storage
and material handling systems are
not compatible with a wet feedstock.
Benefits: Chip washing equipment results in the production
of a cleaner wood waste derived feedstock
that will not cause equipment wear
and other problems for the manufacturer
and end user. In addition, because the wood waste material
is cleaner, manufacturers are able
to use a greater percentage of the
recovered wood waste in their process
relative to their use of conventional
wood fiber.
Application Site: Processing facility.
Contact: For more information about this Best Practice, contact CWC (206) 443‑7746, e-mail info@cwc.org. References:
1.
Smith, Dave.
CE/Western Engineering. Albany, OR. 2. International Resources Unlimited, Inc.; Eugene, OR.
Issue Date /
Update: March 1997 |
