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PRE - CLEANING
The first step in pelleting is conveying of ingredients from bulk silos or godown to the batching bins. It is essential to remove oversize foreign materials and iron trash etc. which otherwise can affect functioning of slide gates, gravity /screw feeder in batching or down stream.
Pre-cleaning machines generally include rotary sieve, jute remover, magnetic catchers etc. In automatic plants, it is desired that system is designed to give continuous efficient cleaning at maximum flow of ingredients.
After batching, next step is to reduce the material to the required degree of fineness to have maximum surface area for exposure to heat and moisture to accomplish gelatinisation in conditioners.
In pelleting process grinding and pelleting are two major energy consuming operations. If we can perform fine grinding with minimum of energy, it will not only cut grinding cost but also save on energy required in pelleting.
In modern plants, automatic weighing of different materials as per formulation to make one batch is called batching or proportioning. In this any no. of silos of suitable capacity as per total capacity of plant, are incorporated and are having gravity or screw feeders for controlled discharging of material into weigh bin under neath it. Generally two set of bin or silos are installed - one for major ingredients, second for minor ingredients. The discharge of both weighing bin come into common surge for further processing. Bin vibrator or shaking devices are installed on some or all of bins to make the material flow incase of jamming of bin.
The weighing is done through an electronic controller working on PLC or SCADA based, which can be operated through a computer. The computer control and record all the detail of a batch- over or under weighing of ingredients from set point, reporting on each batch, shift, day ,weak, month basis. The generally desired features of a batching system are:
It should give accurate weighing within specified time to meet output of plant.
Though it should be highly reliable and work automatically, it should have manual control also in case of auto failure.
Efficient mixing of micro ingredients in whole batch is significant in making quality feed. The designed features of a good mixing system are:
It should perform mixing to achieve minimum coefficient of variation in minimum time. Mixer discharging mechanism should aim at no leakage of gates with minimum residue in drum to avoid carryover to next batch.
Conditioning is done to add heat and moisture in mixed feed to achieve gelatinization and making the product more pliable for pelleting. An ideal conditioning system should meet the following:
It should raise the temperature and moisture of product by direct or indirect heating.
It should give desired retention time in conditioners.
After conditioning the product is passed through a die in pellet mill with the help of rollers and powdery material is converted into pellets. The quality and output of pellet mill. is dependent on so many parameters, like:
Quality of grinding, conditioning.
Formulation of feed: - (percentage of oil, protein, fiber etc.)
Die configuration: - Deciding working on product in pelleting.
Hot pellets coming out of pellet mill are required to be cooled immediately to remove moisture and heat. Generally counter flow cooler are used for the purpose. The desired feature of an efficient cooler are:
It should be capable to remove the unwanted heat and moisture from pellets to improve its shelf life.
The cooling should be uniform from all the sides.
Product coming out from pellet cooler. These are required to be screened to remove fines and overs. Single deck screeners are used to remove fines only while double deck screeners are used to remove both fine and overs.. The unbroken or the overs are returned and fines return to pelleting line for repelleting.
A screener should have following features:
It should remove the desired percentage of fines from product. Screens should not clog frequently.
Changing of screens of desired mesh should be easy and quickly.