Contemporary grain and feed handling plants are forced to satisfy increasingly strict requirements; African Farming looks at ways to ensure contamination-free conveyance
The most difficult and potentially dangerous phase in feed grain and materials handling, in relation to cross-contamination, occurs upstream during conveying to the mixer and formulator, silo and bagging line. The economic consequences occur downstream at the point of marketing and the short easy transfer of feed from trough to livestock mouth.
Contemporary grain and feed handling plants are forced to satisfy increasingly strict, stringent and enforceable requirements for health, hygiene and quality assurance. Net result is many long established plants forced to upgrade the design, manufacture and operation of plant and equipment.
Managers are essentially faced with four choices:
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To restrict product formulation and manufacturing flexibility,
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Carry out extensive remodelling,
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Change over to a single-species feed production operation
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Or to construct brand-new custom designed facilities.
The in-plant conveyor is the most common vector of cross-contamination resulting in rancidity, fungal mould growth, possibly with accompanying mycotoxin contamination, and insect and mite pest infestations. The design and development of conveyor systems to minimise grain and materials carry-over, especially when handling products for many different species, is the over-riding challenge. A well-designed multi-discharge conveyor significantly decreases the risk of cross-contamination when delivering feed grain and finished feed materials through a single conveyor discharging at several locations.
Drag conveyor design and development
Managers require residue-free handling, complete discharge from multiple points, dust tight construction and low life-time maintenance all at an economic cost. Many are going for contemporary multi-discharge ‘drag’ or drag-flight conveyors. So what range of features and capabilities should they be looking for?
The downward force of gravity is essentially free energy and labour to the feed plant manager and that is why the round bottom cross-section design is perfect for clean drag conveyors. With no corners or ledges, the round or ‘U-trough’ allows all free-flowing material including grain to constantly move to the bottom centre of the trough for discharge without residue.
Excessively sticky materials like inadequately processed soya meals can defy gravity but they will have already caused problems, such as bridging and caking, further back upstream in the silos, so the operator should be forewarned and forearmed. Contamination hazards created by sticky feed materials are even worse with alternative conveyor designs.
Problems for clean transfer of feed grain and materials do occur with round-bottom cross section conveyors but for a completely different reason, which is difficulty in incorporating clean discharge slide gates in a round trough with multiple discharge points. This led to alternative designs such as rectangular-bottom or angle-bottom cross-sections with alternative slide gate designs.
Aiming for residue-free
Flat-plate slide gates – the very first intermediate discharge gates on drag conveyors - were unable to achieve contamination-free transfer of product due to the cavity created between the trough bottom and the slide gate. Improvements came with a curved slide gate fitted in at the bottom of the ‘U-trough’. This development significantly reduced the cavity between the slide gate and trough bottom, but not enough to prevent all cross-contamination.
Material transferred over closed slide gates still remains trapped in the small cavity between the trough bottom and the curved slide plate, and when that slide gate discharges again trapped material falls into the receiving bin to contaminate the new product falling on top. The trough-bottom slide gate design may also require the operator to ‘flush’ or decant old product from downstream storage and conveying equipment to reduce the risk of cross-contamination.
Other designers have tried to solve contamination problems in conventional multi-discharge drag conveyors by altering the profile of the trough to accommodate a conventional slide gate. Such troughs possess a flat bottom and angular bottom sides that are compatible with incorporation and operation of flush-mounted slide gates.
When the slide is open there is an unobstructed discharge opening for the flat bottom section of the conveyor. On discharge, almost all the product falls through the outlet. However, the sides of the trough which are used to carry the paddle and chain system over the discharge opening offer opportunities for carry-over and cross-contamination.
Ideally, the angle of the trough sides and the velocity of the conveyor should be sufficient to make sure all product discharges, but this cannot account for any trapped between the paddle edge and the trough side angles. Another area of concern is the cross-sectional corners because they cannot create a perfect fit between the paddle and the trough. That said neither a rectangular-bottom or angle-bottom trough can achieve the well-established clean-out capacity of a round-bottom trough.
Clean clear-through discharge
The round-bottom or U-trough cross-sectional profile provides the cleanest possible method for transferring grain and other free flowing materials because it presents no corners or angles for the accumulation of residues. But the conveyor is still required to discharge product at designated discharge points on a single line conveyor while maintaining the integrity of the round trough shape.
Solution to this potential problem is a ‘drop gate’ discharge device in which the entire round-bottom trough section provides the discharge opening. Unlike the standard U-trough or angle-trough slide gates, the drop gate requires no paddle-carrying bridge or other paddle transfer device as part of the trough bottom. With the drop gate, there is 100% discharge of product without the paddle touching the trough bottom or sides as it passes over the discharge opening. The carrying system for the paddles and chain is housed in the upper chamber of the trough, away from the product.
The drop gate conveyor design additionally includes a drag paddle with a higher profile which raises the drag chain away from any contact with the materials being conveyed. This design feature is highly significant because the chain is invariably the next most important source of cross-contamination in drag conveyors.
Whenever the chain touches the product there is clear potential for product carry-over and cross-contamination because the chain is able to ferry product across the discharge opening. Flexing of the chain where the links join can grind product into fines, which is another reason why drag conveyors should always be designed to keep the chain away from product.
Consequences of failure to keep clean
Feed grain materials becoming trapped in cavities, spaces and gaps is one thing, but the length of time they remain is another. Rancidity is the most immediate result of trapped grains and materials and especially for those with relatively high oil contents. Growth of fungal moulds takes longer but the consequences are dire especially if the fungal pathogens are mycotoxin synthesisers. Mycotoxin contamination over above legally accepted levels will invariably render the commodity unmarketable.
If mycotoxin contaminated batches reach the animal trough then serious damage including livestock fatality may occur with contamination freely passed down the food chain into dairy products like milk. The most obvious problem of cross contamination, either on the conveyor itself or in bagged and stored commodities further downstream, is insect and mite infestation including beetles, weevils and moths.
Conveyors are not the only vector for cross-contamination and infestation in the feed plant. However, most of the plant’s materials move through them so improvements in clean conveying will immediately yield enhanced quality assurance.
Dr Terry Mabbett
