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The pelleting process
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The importance of continuing development
ince the late 1800s, animal feed manufacture has become a huge worldwide business and an essential link in the supply chain to sustain our ever expanding and protein-demanding world. Today in the UK alone we produce in the region of 13,3 mT/a of compounded feed (DEFRA October 2016) and 100,000 tonne capacity mills are average size plants. Our crops are more productive and we are producing fresh meat from ‘straw’ to shelf quicker than ever before. There have been significant advances in nutrition and animal processing over the last decades and ever more research is being carried out to push the boundaries of possibility for our all-consuming markets. The demands on feed processing and pelleting operations have subsequently meant these have had to evolve too. A broiler chicken that consumes 110g per day, a pig that consumes 3kg per day, or a cow that consumes over 4kg per day should all be fed the correct diet, each pellet containing the correct nutritional balance. The pellets should also be hard and importantly durable. Feed also needs to be made at high throughput rates e.g. 15 t/hr, which equates to 4kg/second of feed manufactured. Failing to do this, and adversely affecting the performance or welfare of your customers stock, can be very detrimental to a mill’s future business. What makes the pelleting process so challenging is that all of the above needs to be assured every production minute, hour and day of every year. But the following change all the time. • Formulation - a mill can manufacture 20 different formulations per day on the same line. • Raw material changes are usually market price governed. • Climate changes over 365 days, with temperatures variations of 30°C per season. • Operator ability: depending on education, training and the right experience.
Machine wear and tear
A pelleting operation needs to be able to respond to all these changes and still ensure quality consistency whilst operating economically in a competitive market place. In order to be able to deal with this the mill designer, through 66 | February 2017 - Milling and Grain
by Tom Connor, Condex (UK) to the mill operator, must consider the following parameters pre-pelleting: grist size, bound moisture content, mixing CV’s (5 percent being an industry standard) and liquid viscosity and distribution. In order to ensure a good quality pellet, all of these factors can influence your pellet mill capacity and pellet quality. Pelleting is a combination of mechanical forces and chemical changes in the product on a microscopic level that ensure all particles in each pellet adheres to its neighbour to provide better durability and hardness.
Grinding
Reducing particle size increases surface area and materials are easier to compact. Grinding efficiencies play a role in this aspect and a φ 3.2 mm screen tends to be the industry standard for particle size versus grinding cost. Grinding can result in as much as 1.5 percent of bound moisture being lost. By ensuring your grinder screen area doesn’t blind, you can keep this value lower. Inverters are now common practice on grinders and offer more control of the grinding process and produce a more consistent grist spectrum. Pre-sieving on a grinder operation can improve grist, grinder capacity and efficiency.
Viscosity
Viscosity and thorough mixing are also important, as any moisture introduced to the batch that isn’t bound moisture (naturally stored in the product) introduces possible slip issues further on in the system. The liquids added should be absorbed by the feed particles prior to pelleting, giving better pelletability characteristics. The lower the liquid viscosity (Viscosity is the natural resistance of a liquid to flow, for many liquids this is very much affected by temperature) the easier the liquid will be absorbed by the feed particles. This is very important for pellet mills as moisture creates the potential for roller slip. This is the point where the roller looses traction in the layer of material on the die track, resulting in the roll not rotating at the same speed as the die and the die not pelleting. Also, too much moisture acts as a lubricant and reduces the compression forces of the pelleting process as a result of reduced friction, resulting in a less durable/ hard pellet.
Conditioning
The conditioning of the feed prior to the pellet mill is probably one of the most important stages of the pelleting process. At