The term “wood flour” is somewhat imprecise, with a rather ambiguous meaning, though it generally describes wood particles with a texture, particle size and appearance similar to that of cereal flours. Generally, wood flour refers to wood particles that can pass through screens with 20 mesh (850 micron) openings. It’s made by pulverizing dried wood, often from scraps of wood. Flour-like wood particles work as a natural substitute for various synthetic fillers, and wood flour is used for many other applications.
Made typically from pine or maple, wood flour can also be made from other types of wood. Though it looks and feels like sawdust, its particles are much finer. The processing of wood flour is largely based upon the reuse and recycling of wood scrap by lumber mills, paper mills, furniture factories, and other facilities that produce scrap wood, all of whom can benefit from what would otherwise be considered waste material.
Uses for Wood Flour
The commercial use of wood flour goes back more than a century. Since 1906, wood flour has been used to produce absorbents, explosives, glues, soil amendments, and other products. An early commercial use was in a composite known as Bakelite, a wood flour and phenol-formaldehyde composite. The 1916 Rolls Royce utilized a composite containing wood flour for its gear shift knob, one of the first commercial products made from a plastic-wood composite.
Though once a common filler for thermoset plastics, wood flour’s use in this manner has since declined. However, in recent decades, wood flour has seen widespread use in thermoplastic composites. In the mid-1990s, the worldwide market only produced about 50,000 tons of wood-plastic composites, rising to 600,000 tons by 2003. As of 2015, the US market alone for wood-plastic composites reached 1.3 million tons. The most rapid growth has been in exterior building products, including decking, door profiles, railings, and window frames.
Applications for wood flour include:
- Absorbent for cleaning chemical, petroleum, or other industrial spills
- Animal feed
- Bioplastics for flooring, composite decking, and other products
- Decorative baseboards and moldings
- Electronic housings and fuses
- Epoxy or wood filler, when mixed with resin
- Filtering water or other liquids
- Indoor and outdoor construction
- Interior and exterior automotive components
- Partitioning walls and façade paneling
- Soil remediation
As it’s unstable at higher temperatures, wood flour tends to be used only in plastics processed below approximately 392°F (200°C), with polyethylene largely utilized as the matrix in these composites. Forestry waste can’t be utilized to make wood flour, as scrap wood must be free of bark, dirt, and other foreign substances. For this reason, scrap wood for flour is almost exclusively sourced from waste wood generated by wood processors.
Though wood-plastic composites made in the United States usually use maple, oak, or pine, many other tree species can be used. Most often, the type of wood used depends on the availability in the region, though color differences and familiarity with specific types of woods’ properties can also affect selection. Certain types of wood may have unwanted properties, as with red oak, due to a lack of certain compounds, for which composites made from it may stain when the wood repeatedly gets wet.
Making Wood Flour from Wood Scrap
The process by which wood flour is made from scrap wood is relatively simple. However, if it contains too much moisture, the wood flour won’t hold up well, so it needs to be thoroughly dried. Larger lumber mills might have a kiln powered by electricity, solar energy, or other means to dry their leftover pieces of wood. Flour can also be made from wood scrap that can sit and dry naturally. Though pine and maple chips are the most commonly used, and some wood flours contain both, they can instead be made from oak, hickory, or other types of wood.
Most scrap that a mill generates is dimensional cut-offs, which pass through a shredder before grinding in a hammermill.
Once the scrap dries, it goes through the following processes to become wood flour:
- Wood scrap enters the grinding chamber of the mill through a transition tray or chute.
- A high-speed rotor with attached hammers breaks the wood scraps into smaller particles within the grinding chamber.
- As the hammers rotate and strike the material, fragments of wood scrap move across a screen.
- Once small enough, the wood particles pass through the screen and drop into a discharge chute, where the finished wood flour falls into a collection chamber.
The most used mills for wood flour production are hammer mills, with flat-edged hammers being the most efficient for pulverizing scrap wood. Flour can also be produced by first going through a mill with knife-edged hammers. Flour from wood scrap often has a similar consistency to that of most wheat flours. Typically, its coloration is a medium to light brownish color, though this depends on the wood used to produce it. Though it may look like sawdust, wood flour shouldn’t feel grainy or rough.
Hammer Mills for Wood Flour Processing
Hammer mills are the most effective equipment for both the breaking down into wood scrap and grinding of this scrap into wood flour. Reliable and versatile, they can be configured to process most types of scrap wood into flour, which is why they’re often used for recycling efforts. The particle size for wood flours can additionally be tweaked by changing hammer type, rotor speed, and size of screen openings. Prater’s Full-Screen Hammer Mill and Mega Hammer Mill offer an effective and efficient way to turn wood scrap into wood flour.
Prater Full-Screen Hammer Mill
Capable of handling high capacities and producing uniform distributions of particle size, Prater’s robust and durable Full-Screen Hammer Mill can handle oddly shaped and sized scrap wood.
This hammer mill’s features include the following:
- Ability to configure hammer sizes, patterns, and screen openings.
- Boosted efficiency via use of the entirety of the screen.
- Easy access for replacement or removal of screens.
- Large access doors make maintenance and cleaning simple.
- Maximum power efficiency by balancing horsepower with screen area.
- Rigid-frame mounting allows uniform clearance between the screen and hammers.
- Screen changing requires no tools.
- Symmetrical, reversible, and precision-balanced rotors.
With a full-screen design, this hammer mill’s durability is augmented by sturdy pillow-block bearings, while optional armored or stainless steel construction extends its lifespan.
Prater Mega Hammer Mill
Prater’s Mega Hammer Mill offers minimal thermal build-up with a uniform grind. With a distinctive cantilevered rotor for grinding, the Mega Mill provides quiet and smooth operation that allows for constant operation. This Prater hammer mill requires less airflow and functions more efficiently than standard hammer mills. It’s built to operate with minimal power, requires little maintenance, and decreases downtime.
This hammer mill’s features include the following:
- Cantilevered design of the rotor, along with hinged doors, makes maintenance and cleaning simpler.
- Dust-tight design makes it perfect for use with pneumatic conveying systems.
- Horsepower to screen ratio is maximized to ensure uniform grinding and optimal capacity without much thermal buildup.
- Interrupter plates that intensify grinding action are placed to allow easy removal of screens.
- Numerous shapes and styles of hammers available.
- Precision-built bearing assembly and rotor assembly that are balanced for smoother operation.
- Robust construction makes it last longer.
- Screens and rotor remove easily.
The Prater’s Mega Mill requirements allow operators to regulate particle size and capacity simply by changing the number and size of screen holes, hammer tip speed, number of hammers, the thickness of hammerheads, and clearance between screens and hammers.