Millfeed Pelleting For Flour Mills

by Teresa Acklin
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   By David Sugden, a grain industry consultant.

   In the long history of flour milling, pelleting of millfeed is relatively new and not particularly widespread.

   The pelleting of millfeed has been adopted by flour millers following the example of the animal feed industry, which has used pelleting as one of its major feed manufacturing processes since well before World War II. And the animal feed industry is the major market for millfeed pelleted by flour millers.

   The distinction between pelleting millfeed and pelleting animal food is that the former applies solely to the pelleting of byproducts from flour mills, while animal food pelleting involves formulations that may include many raw materials, e.g. fish and bone meal, soybeans, maize, barley, molasses — the list is almost endless.

   The flour mill producing millfeed will include not only materials from the mill proper but also screenings from the wheat cleaning plant and certain suitable dockage from the elevator or silo. Millfeed can be defined as all materials from the flour mill that is safely edible but that is not made into flour.

   Whether to pellet millfeed or not is dictated by the market. The local animal feed compounders (feed millers) must be receptive.

   Advantages to the animal feed manufacturer of using pelleted millfeed include conservation and ease of storage. Pelleted millfeed typically has a specific gravity of 0.65 to 0.7, almost the same as wheat at about 0.75; whereas, the specific gravity of loose millfeed is around 0.35 to 0.4. So, the storage benefit is immediately apparent. Moreover, pellets ranging in diameter from 6 mm to 12 mm are stored in bins similar to those used for wheat because of their free flowing characteristics.

   The disadvantage to the feed miller is that normally he has to grind pelleted millfeed; hence, there is a power penalty that is not realized when using loose millfeed.

   Pelleting has two basic advantages from the flour miller's point of view. The first is that pelleted millfeed requires less storage capacity for the same tonnage. The second advantage is the ease of transporting pelleted millfeed to the feed miller. The latter factor is often enough to justify capital expenditure on the pelleting process. The trick is to ensure, from the flour miller's standpoint, that pellets are sold either on a delivered basis or with a premium ex mill. This is a discipline that must be adhered to on every trade; otherwise, the return on investment will be squandered.

   The disadvantages of pelleting for the flour miller include more maintenance requirements, another process to run, the cost of power and, of course, capital expenditure.

   Nevertheless, if the market is there, a return on investment can be justified in some cases. Circumstances that make or break the decision are local in character.

   Climate can be a problem. Hot, humid conditions are anathema to pellets, especially if transportation is necessary over long distances. This is because pellets may sweat, encouraging mold growth and bacteria spoilage. Cold and dry conditions, however, are ideal for pellets.

   The use of ships, barges and the like are excellent for loading, carrying and unloading pellets, because of the pellet's specific gravity. Also, pellets can be stored and conveyed in large grain elevators or flat grain stores.

   Let's turn to how pelleting of millfeed is accomplished. There are many variations in facilities and capacities. The plant diagrammed is rated at 7 tonnes per hour for pelleting and at 75 tonnes per hour for outloading. The plant can handle millfeed for a 700-tonne-per-day flour mill (producing 12,000 cwt of flour). It is designed to run 24 hours per day. If run for eight hours, it can serve a mill one-third of the size as that cited above — for example, to take account of special electric power tariffs.

   Into bin No. 1 (capacity of 30 tonnes, or four hours' run) is fed loose millfeed. The bin features level controls, preweighing and exhaust. This bin feeds the pellet press, which, in this case, has steam addition. The likely power requirement using steam would be about 130-150 kW. Using cold water, the power requirement increases to about 200 kW (about a 30% increase). After steam addition, the pellets are cooled in order to stabilize them for storage. They then are conveyed to the four outloading bins, Nos. 2, 3, 4 and 5. Each outloading bin has a capacity of 65 tonnes. The total capacity of 260 tonnes represents 37 hours' run on the press.

   The whole system is equipped with dust extraction fans and filters, including the cooler. The cooler usually uses ambient temperature air (beware in hot humid climates).

   The plant diagrammed utilizes a blowline to convey the pellets, which tends to cause a small amount of breakage. Mechanical chain or hand conveyors and elevators cause less breakage but take up more space.

   What if the pellet press has a major breakdown? The answer is a by- pass (see center of diagram). In this event, great care is needed to ensure that bins Nos. 2, 3, 4 and 5 can store and discharge loose millfeed. If they cannot, the answer is to place a truck under the outloading conveyor, open the discharger of the bin being fed and use it as a gravity spout without interrupting the flow.

   Pellet presses and coolers from reputable machinery manufacturers are inherently reliable and excellent. Features include fully remote controlled automation by programmable logic controllers (PLCs) or computer. Maintenance in the form of automatic lubrication is easy. Roll gap setting to ensure pellet hardness can be remotely accomplished and the changing of worn dies is greatly facilitated.

   One last point is moisture gain/loss in pelleted millfeed. As with loose millfeed, this can be controlled. Typically, it is neutral deliberately for stability reasons. A slight drying on the order of 0.5 to 1% is helpful in humid climates, the reverse is true for dry ones.

   Should you require further information, contact World Grain.

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