Equipment for Cleaning Seeds

LEONARD M. KLEIN, JAMES HENDERSON, AND ABRAHAM D. STOESZ.

SEED as it comes from a field is never pure. Mixed with it are seeds of weeds and other plants. They have to be separated after harvest so as to get pure, live crop seeds for replanting.

Seed from each crop is basically different in physical makeup from the others and can be identified easily. The differences in size, shape, weight, surface area, specific gravity, color, electrical properties, texture, stickiness, and pubescence can be measured or sensed by mechanical devices, called separators, which cull unwanted seeds from wanted ones on the basis of one or more of those physical differences.

Seed separators also remove dirt, leaves, stems, and chaff. Cleaning reduces the bulk to be handled and stored and removes moist material that may cause heating in storage. All seed crops, whether of grasses, legumes, grains, vegetables, flowers, fibers, trees, and shrubs, require some cleaning.

In the usual sequence through a processing plant, seeds first go through a precleaning operation. It may include scalping, the removal of material coarse enough to be easily separated by screens; hulling, a completion of the field threshing operation; and scarifying, the scratching of hard-coated seed so that moisture can enter it when it is planted. The processing of seeds of native grasses and other seeds that have awns and appendages usually requires additional precleaning to remove awns and beards.

The seed is then processed on an air-screen cleaner, in which the bulk of the foreign material is removed by screens and air.

The final separation is made on one or more finishing machines, which generally separate only one type of contaminating seed from the desired clean product.

Specific gravity separators will divide seeds according to their weight and size.

Indent disk and cylinder separators will remove long seeds from short ones. Pneumatic and aspirator separators will separate seeds that present a different resistance to airflow.

Velvet roll separators remove smooth seeds from rough seeds.

Spiral, inclined draper, timothy bumper mill, vibrator, and horizontal disk separators divide seeds according to their shape.

Electronic separators sense a difference in the electrical properties of seeds. Magnetic separators and the buck-horn machine separate rough or sticky-surfaced seeds from smooth seeds.

Color separators divide the light-colored seed from the dark ones.

SEEDS FROM a thresher or combine brought to a cleaning plant may contain a great deal of trash, green leaves, green weed seeds, and insects. Because of the moisture in the leaves and weed seeds, the seeds cannot be safely stored, or efficiently handled, or accurately cleaned until most of the foreign material has been removed. Many seed-cleaning plants therefore separate the foreign material immediately with a machine called a scalper.

Scalpers are of many types. One consists of a reel of perforated metal screen, which is inclined slightly and turns on a central shaft. Seeds fed into the higher end tumble inside the reel until they drop through the perforations, but longer and larger trash continues through the reel and is discharged separately. Another type of scalper makes the same type of separation with a single, flat, perforated screen, which is mechanically shaken. The two types are simple devices intended to remove only large trash. Because some seeds are mixed with small weed seeds as well as large trash, many seedsmen prefer a more complete precleaner to a simple scalper. It is a simple air-screen seed cleaner that makes 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.

Scalping, or rough cleaning, has certain advantages: The mechanical handling is facilitated. Subsequent seed flow is more even. The time required for artificial or natural drying is reduced. Succeeding machines can have a higher capacity.

After scalping, many kinds of seeds can be cleaned without any further preprocessing, but others may require hulling or scarifying.

Hulling is the removal of an outer coat or husk.

Scarification is scratching the seed-coat.

The hulls of some seeds are impermeable to water, and the seeds will not germinate promptly unless the outer coat or husk is removed before the seed is planted. Some legume seeds are hard and must be hulled or scarified if they are to absorb water and sprout promptly and evenly.

Hullers and scarifiers usually abrade the seeds between two rubber-faced surfaces or impel seeds against roughened surfaces, such as sandpaper. The severity of the abrasion or impact must be controlled accurately to prevent damage.

Seeds of a high moisture content are harder to hull or scarify than seeds with less moisture. Because a huller or scarifier adjusted for moist seeds may damage dry seeds, the moisture content usually is determined before hulling or scarifying and the necessary adjustments are made.

Some kinds of seeds that maintain viability for long periods after being hulled and scarified can be processed immediately after harvest and stored until the following season. Others that lose viability quickly can be hulled and scarified shortly before planting time. Hulling and scarification may be performed separately or jointly, depending on the presence of unhulled or hard seed or both.

Some seeds that may require hulling are bermudagrass, bahiagrass, buffalo-grass, and Korean, Kobe, common, and bicolor lespedeza.

Some seeds that may require scarification are wild winter peas, hairy indigo, alfalfa, crotalaria, subclover, and suckling clover.

Some seeds that may require both hulling and scarification are sweet-clover, sericea lespedeza, crownvetch, black medic, and sourclover.

Many native grasses, small grains, and other plants produce seed units that have awns, beards, hairs, glumes, and other appendages, which make them difficult to handle in processing and planting operations because they tend to interlock and cause undesirable clustering. They can be removed by a precleaning treatment, which improves their flow properties, cleaning characteristics, and quality of the seeds.

Especially troublesome awned grasses are species of Stipa, or the needle-grasses, and species of Elymus, or wildrye. In other grasses, like the bluestems and gramas, hairy appendages make the seeds fluffy and bulky.

After scalping to remove excess straw and trash, several mechanical actions may be employed to remove awns and appendages. These include a highspeed thresher, the hammermill, a debearding machine, and a tumbling pebble mill. All employ a vigorous abrading action and must be operated carefully to insure little damage to seed and maximum removal of awns.

High-speed threshing or rethreshing of the seed to remove awns without too much seed damage is done by running the once-threshed seed material through the thresher or combine a second time at a high cylinder speed.

A concave setting of minimum clearance and a reduced airblast gives best results. Complete removal of awns by threshing is controlled by closing the seed pan screen so that only deawned seeds drop through into the seed auger. The tailings screen must be opened enough to allow the awned seeds to drop into the return auger and be rerun.

THE COMMONEST type of machine used successfully for deawning and debearding is the farm hammermill. The results depend on cylinder speed, size of the screen openings, rate of feed, and condition of the crop. The speed of the hammermill best suited for pretreating is about 50 percent of that used in normal grinding operations 600 to 1,400 revolutions a minute. Long, thin seeds need a slower cylinder speed to avoid loss through breakage. Excessive hammer speed will mutilate, crack, or groat the seeds. If the speed is too slow, awns will not be removed.

Openings in the screen must be slightly larger than the deawned seeds. Oversized screen openings will pass a high percentage of seeds with awns. If the openings are too small, the seeds will be damaged and capacity reduced. Those with slots instead of round openings will handle slender seeds with less breakage.

The mill should be fed to full capacity so the hammers will rub and roll the sufficiently trimmed seeds through the openings. When feeding is reduced, the cushioning effect is also reduced and more seeds will be damaged. Seed moisture should be held within close tolerances. Awns on moist seeds often are limber and will not break off easily; cracking damage results in lowered germination if the seeds are too dry.

The following steps should be followed in adjusting the hammermill:

First, choose a screen with openings slightly larger than the deawned seeds.

Second, start the cylinder at slow speed fill the mill and keep it full.

Third, examine the seeds after a short trial run. If the appendage removal is incomplete and no damaged seeds are found, advance the cylinder speed about 100 revolutions per minute. Be careful to avoid cracking or otherwise damaging the seeds.

Fourth, repeat the third step until the most trimmed seed is obtained with the least breakage. Small seeds that come through untrimmed should be rerun through a screen with smaller openings.

Seeds of the following species have been successfully processed in a hammermill: Native grasses like blue-bunch wheatgrass, blue wildrye, Canada wildrye, Siberian wildrye, the gramas, the bluestems, the needle-grasses, and tame species like tall oat-grass, bulbous barley, squirreltail, alfilaria, and virgins-bower.

Debearding machines, used to precondition grasses, have a larger capacity, are simpler to operate, and damage the seeds less than hammer-mills.

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 clearance with the arms, protrude inward from the drum.