Seed Cleaning Method – Pre-Conditioning and Pre-Cleaning
Pre- conditioning refers to operations such as shelling, debearding, etc, that prepare seed lots for basic seed cleaning, and also to the removal of particles such as pieces of trash, stones, clods etc, larger in size than desirable crop seed, from threshed seed lots. Some pre-cleaners, in addition to removing larger sized particles, also remove particles that are lighter in weight and smaller in size than the crop seed. The necessity of both of these operations, other than shelling, is associated with advanced mechanised agriculture, no pre-cleaning is usually required on hand harvested and winnowed seed lots.
Pre-conditioning and Pre-cleaning Equipment and their Use:
The most common equipment used in these operations are scalers, debearders, huller-scarifier, buckhorn machine and maize Sheeler.
Scalper /Rough Cleaner:
Scalpers are simple devices intended to remove only large trash. Such units basically consist of a vibrating or rotating screen or sieve. The screen perforations are larger enough to allow the rough seed to pass through readily while the larger inert material is ‘scalped off’ and removed from the seed lot. The scalpers manufactured for pre- cleaning may consist of several screens or reels, with one or more controlled air separations. The single sieve pre-cleaners are called scalpers and the multiple sieve units are referred to as rough cleaners. The rough cleaners are essentially the simple air screen seed cleaners that make possible a separation of light chaff and dust with a controlled air current; a separation of large trash over a large hole screen; and a separation of small foreign material through a small hole screen. Most scalpers are arranged to make the air separation before the seeds reach the screens.
After scalping / rough cleaning many kinds of seeds can be cleaned without any further pre-processing. Seeds of some crops, however, may require hulling, scarification, etc, after scalping.
Huller and scarifies usually abrade the seeds between two rubber-faced surface, or impel seeds against roughened surfaces, such as sandpaper. In a huller –scarifier, the seeds fall from the feed hopper on to a rotating distributing disc, where they are thrown against the hulling and scarifying surface by centrifugal force either once or twice, depending upon the machine. At this point the seed are hulled and or scarified. After this operation the seeds are moved into a suction chamber where the suction removes the light, fine dust, and the seed discharge at the bottom of the chamber. The severity of abrasion or impact must be controlled accurately to prevent damage. Hulling (removal of an outer coat or husk) and scarification ( Scratching of the seed coat) can be done separately or jointly with a huller scarifier.
The debearding machines have a horizontal beater with arms rotating inside a steel drum. The arms are pitched to move the seeds through the drum. Stationary posts, adjustable for clearances, with arms, protrude inward from the drum.
These machines, rub the seeds against the arms and against each other. The time, the seeds remain in the machine, is varied by regulating a discharge gate. The degree of action is determined by the processing time, beater clearance and beater speed.
The pebble mill is used for removing cob-webby hairs from blue grass and similar seeds. It has a drum rotating about a shaft, in seeded off-centre at opposite ends. The mill is loaded with seeds and smooth half-inch pebbles and turned at a slow speed until the rubbing action of the pebbles roll the fuzz from the seeds into small round balls. The mixture of pebbles , seeds and matted fuzz is then run over a scalper to remove the pebbles.
The maize sheller varies in size from small hand-powered Sheller to large motor- driven sheelers with capacities up to ten tonnes per hour.
Small hand-power shellers consist of a crank a small feed inlet , a heavy cast iron fly whell and burns that remove the maize seed from the ear, seeds drop out to the bottom and into a container, and the cobs are discharged out from the rear of the Sheller. These types of shellers are useful for small lots of breeder’s seed of inbred lines.
At processing plants, power sellers are installed to give high capacity shelling. The power Sheller has four main parts, namely, inlet hopper, rotating beating cylinder, concave and fan. The inlet hopper is kept sufficient large to feed several cobs at a time. The rate of feeding could be adjusted through the sliding gate inside the inlet hopper. The rotating beating cylinder has spirally arranged sheelings lugs. On one cylinder a drive pulley is fixed. Drive pulley can be driven by an oil engine, tractor belt pulley, or an electric motor. Most indigenous power shellers require a 7.5 h.p motor. The lower concave is made of a perforated steel sheet to allow the seed to pass through , but to retain and divert cobs towards the vibrating screen so as discharge them outside. Shelled seed coming out of the lower concave is passed through an air blast to remove small cob piece and dust.
For obtaining satisfactory shelling, the cylinder shaft speed should be in the ranger of 450 to 500 RPM.