The Willamette Valley Company, also known as Wilvaco, is a renowned multinational specialist supplying products globally for the panel industry.

Among the products it offers are raw material for the adhesives used in plywood manufacture. One of these is bark from the western red alder. Turned to a fine powder, then mixed with other powdered wood material, this is shipped to customers as an extender and filler in plywood adhesives.

Uniformity in this blend of powders is important. So too is throughput. Previously Willamette used a ribbon blender to achieve as even a mix of the different powders as possible. The powders are held in a U-shaped trough where an inner and an outer set of helical ribbon-shaped blades agitate them.

The outer set moves them in one direction, the counter-rotating inner set move them in the other direction, and the hoped-for result is a thorough mixing.

There was, though, a problem. ‘The ribbon blender did not mix materials thoroughly enough,” says Don Coleman, plant manager for Willamette Valley. “The material, which customers mix in vats in their own formulations, could clog filters and shut down lines if not properly blended. Our glue customers are very particular when it comes to their ingredients, so consistent quality is important. We wanted pure blending in high volume.” Furthermore, the ribbon blender was slow, and its output was being outpaced by demand.

The company has therefore recently replaced its ribbon blender with a rotary batch mixer. The motion is different: Instead of agitating the material, it tumbles it.

A cone-shaped drum rotates on trunnion rings; lifters inside it constantly raise the powders and gently fold them back towards the centre. The action is not unlike that of the more familiar concrete mixer, though more sophisticated. The mixing action is in fact four way: folding, tumbling, cutting and turning.

The machine is a Munson model 700-TH- 140-MS. “Portions of the batch are constantly contacting the sides of the cone,” says manufacturer Munson, based in New York State, “and are gently folded back towards the horizontal axis. This gives a diagonal as well as a vertical tumbling motion. In addition, lifters inside the drum continuously cut out portions of the batch and fold them back into the main body. These mixing actions are in addition to the constant tumbling action induced by the drum's rotation.”

Several advantages follow. Not least is a rapid and uniform distribution of particles. The machine rotates constantly. It does not stop during charging or discharge as the material enters or leaves it; this ensures uniform distribution of batch ingredients during the loading and discharge cycles, as well as during the mixing cycle, and prevents powders from separating into regions of different composition. The blend at the output is therefore much more homogenous.

Nor does it press the powders together in the way that a ribbon blender can – so it does not produce regions of higher densities. Its mixing action is more gentle than that of a ribbon mixer; materials that in optimum condition are light and fluffy remain so during mixing and upon discharge, with no densification.

Similarly, powders do not break down under the mixing action but remain in their original particle sizes. “The gentle mixing action enhances product quality,” says Munson.

Less poetically, but more perhaps importantly, Willamette Valley is blending wood powder having particle sizes as small as 74 microns – finer than baking flour – and achieving it without densification, Mr Coleman says, and without stratification.

The constant rotation also means energy consumption remains stable and relatively low, especially when making multiple batches of one blend. The “soft start” of the motor and slow mixing speed further increase energy saving, adds Mr Coleman.

There is an output gain through the machine’s continuous rotation and therefore an efficiency gain: since there is never a need to stop the mixer for loading or discharging it boosts output in high volume processes. Mixers that stop and start become "nothing more than stationary hoppers" during loading and discharge, says Munson.

The machines have applications well outside the forest industry. Mr Colman learned about the rotary batch mixer from a colleague who is plant manager at Idaho Milling and Grain, a Wilvaco company in Idaho that blends wheat flour for glue extenders.

Mr Coleman wanted to replace his ribbon blender with a faster, higher throughput machine. His colleague, citing Idaho Milling’s good experience, recommended the rotary batch mixer. Given the mixer's high output, uniform blend quality and short cycle times, Willamette Valley made the investment. It has speeded up output, and improved quality of product, as desired.

The Willamette operation runs as follows. Tree bark – or 'hog fuel'as the industry calls it – is delivered to the Willamette Valley plant, where operators run it through a 3600rpm grinder for cleaning and size reduction. The moisture content of the bark is 45 to 60%, so the powder is transported to a dryer which reduces moisture to 6 or 7%.

The bark powder is loaded into a distribution bin and conveyed to one of three PLCcontrolled hoppers supplying the mixer.

Each hopper has capacity of 1,362kg and is mounted on load cells. A rotary air lock at each hopper outlet discharges material into the mixer’s inlet chute. An operator selects a recipe on the computer and pushes a 'go' button, causing material to discharge from the feed hoppers. A PLC receives weight loss information from load cells and automatically closes the rotary valves once an accurate batch weight has been discharged.

As the drum rotates, the proprietary mixing blades, or flights, lift, cut, fold and tumble the material, achieving complete blend uniformity. At Willamette the process takes about four minutes. The gravity driven process produces a rapid, thorough blend while imparting minimal energy and intensity to the product. This maximises equipment life without sacrificing blend quality.

The machine is self-emptying. When a pneumatically actuated plug gate valve is opened the internal flights also serve to elevate the material for discharge through it. This leaves no residues other than dust. “Some fine dust may be left on the flights” says Mr Coleman, “but this can be vacuumed away when the machine shuts down for material changeover.”

The continued rotation during the emptying process prevents stratification of ingredients from start of discharge until it is completely empty.

From the mixer discharge a pneumatic conveyor transports the blended material 4.3m vertically to an overhead weigh hopper equipped with load cells and a rotary valve that allows filling of 23kg paper sacks or 1362kg bulk bags under PLC control.

Mr Coleman says 21,792kg of material can be blended in eight hours. “We could do more, but we continually double check our products to ensure consistent quality.” Examples of the products blended at Willamette include Modal SPR-Dark and Modal SPR-Lite, both bulking agents for plywood glue mixes.

The design is totally dust-tight, so is suitable for enclosed, automated systems where dust-tight connections link it to other equipment.

Munson claims that its rotary batch mixer offers 100% uniform particle distribution, the fastest cycle times, gentle product handling, and the lowest energy consumption per kilo of blended product per hour, as well as complete discharge with no segregation of blended products. They also claim renown as makers of the most robust mixer available. They say that Munson mixers that were built in the 1930s are still in 24/7 operation today.

The Munson blender performs uniformly and speedily, with a throughput of 2,721kg per four-minute batch. Capacity of the previous ribbon blender was one-third of that amount.