ASSIGNMENT OF SEED PROCESSING ON GRAVITY SEPARATOR AND COLOR SORTER
The first commercial color sorter was manufactured in 1931 by ESM (Electric Sorting Machine) in Lowell, Michigan, to sort Michigan Pea Beans (now called Navy beans). Since that time, sorters have been adapted to sort/separate many different products, including seed, fresh and cooked food products, snack items, minerals, plastics, etc. Basically, any dry particulate solid that requires visual inspection and can be accelerated to a common speed and singulated (i. e. , placed in a row so they can be viewed one by one), is a good candidate for color sorting.
Many large crop seed such as peas and beans differ in color between varieties, and can be separated efficiently by the color sorter. Traditionally, small seed have not been sorted due to low capacity, but today’s multiple-channel machines with advanced electronics make small-seed sorting amiable option. Color variation may also occur due to immaturity or disease, which makes it possible to remove affected seed. Some diseased bean seed carry pathogens, which fluoresce under ultraviolet light; with a UV light source in the color sorter, diseased seed can be removed.
Rice processing plants also use color sorters as a final cleaner for polished rice, to remove red rice, stones and other off-color particles. Many food processing plants also use color sorters. Place in Conditioning Color sorters are used in various locations in a seed conditioning facility, but are generally placed after the standard mechanical cleaning/sizing processes, and are used as a final inspection before bagging. Each application is different and technology continues to evolve, so there may be applications where the sorter can replace standard equipment, but for the most part, they complement standard cleaning machines.
In corn and soybean conditioning, the sorter is usually placed before the gravity, as this provides the largest seed savings and highest-quality final seed lot. There are occasions where the sorter can be installed to sort the middling culls from the gravity, which increases yield of good seed and eliminates the need to re-run the middlings. Regardless of its position in the cleaning sequence, the sorter enables the plant to run other equipment more efficiently, increase final seed quality, and generally increase out-turn of good crop seed.
Sometimes, the polisher is installed just before the color sorter, to polish the seed and remove dust and other minor surface discolorations which could interfere with the color separation. Operating Process The color sorter accentuates the color (tone or shade) difference between acceptable seed and unwanted seed/particles, and then automatically removes unwanted seed from a continuously-flowing stream of the un-separated seed lot. The electronic color sorter views each seed individually, on one or two sides, by using various sensing devices.
Each seed is compared with a pre-set selected background or color range, and is discharged from the machine according to its color. If it is of the desired color, the seed is discharged out the good seed spout. If its color or shade or spots on the seed falls within the reject range, a blast of compressed air deflects the seed and sends it into the reject discharge spout. By utilizing a spectrophotometer, a specific light wavelength or wavelengths can be selected, which will create the largest differential between good and unwanted seed.
After this wavelength is determined, an optical filter which narrows the transmitted light down to narrow bandwidths is placed in front of the sorter’s camera lens, which is focused on a pre-selected background. The ideal background makes the good seed appear neutral or invisible, while any seed of unacceptable color will appear lighter or darker. It is possible that one wavelength (mono-chromatic) or two (bi-chromatic), or possibly three (trichromatic) different wavelengths can accentuate the difference.
Mono-chromatic is the simplest method, and works for the majority of seed separations. If there is little or no color difference between the acceptable and unwanted seed, the spectro-photometer may determine that an infrared filter is required. This requires that the sorter be equipped with sensors that respond in the IR range and with halogen lighting to illuminate in the IR range. A monochromatic or color-only sorter can use fluorescent lighting, and this lighting can be generated in specific wavelengths to enhance the specific wavelength filters.
Once the required configuration for the viewer is determined-proper sensor, lighting, background and filtration-the un-separated seed lot must be presented to the viewer in a single file or monolayer of seed, with all seed traveling at exactly the same speed. This is usually accomplished by using vibratory feeder to meter seed onto an inclined chute or trough-shaped conveyer belt that is traveling at a carefully controlled speed. The chute can be flat or segmented into channels, and is often Teflon-impregnated to provide good wear characteristics and low friction.
Segmented channels provide the highest accuracy, as the seed are forced to travel in a path directly in front of the camera and air ejection nozzle. However, a non-channelized feed system will provide higher capacities. As the unseparated seed lot travels down the chute or belt, it is accelerated and cingulated so that as it enters the viewing area, all seed are traveling at the same speed and in a common trajectory in the center of the viewer. Once in the viewer, each seed can be viewed from either one or two sides. The moving mono-layer of seed is adjusted so that it travels very close to the air ejection nozzles, but is not touching them.
This position increases the accuracy of defective-seed removal after the defective seed is identified . When the seed are viewed in midair in its flight path through the viewer, any seed that is lighter or darker than the pre-established set point is identified byte viewing/ comparison system, and is then rejected by a controlled blast of compressed air. The air ejectors are programmed to wait a specific time (delay) and to stay open a selected time (dwell) to insure that each unwanted seed is removed with minimal loss of good seed. Fractions Separated The color sorter normally discharges only two separated fractions from each channel: 1.
Seed (the good seed) whose color is within the pre-set acceptable color range. 2. Seed (unacceptable rejects) whose color is not within the acceptable range. Using filters of specific wavelength allows the sorter to accentuate the difference between good and unacceptable seed. Generally speaking, the good seed is accepted and the undesirable rejected, regardless of the amount of reject material in the incoming seed lot. To achieve ultimate purity, especially with incoming defective seed levels above 10%, it may be necessary to color-sort the seed lot for several consecutive times, to be certain of removing all undesirable seed.
Some modern machines have a number of separating channels which operate independently. Also, machines can be used in series to make more precise separations. For example, as the air-jet reject-seed ejector blows out unwanted seed, a few good seed will also be rejected. This seed can be salvaged, by utilizing a portion of the machine to re-sort the rejected seed to Seed Conditioning: Technology reclaim any good seed which are in the reject fraction. When using the re-sort option, the first pass is sorted aggressively to insure maximum seed purity by removing all rejects.
This, however, means that the reject fraction will contain a higher percentage of good seed. To salvage these good seed, this rejected-seed stream is then re-sorted at a lower feed rate and colorsensitivity to create two fractions: (1) A very concentrated final reject stream and (2) An accept stream of good seed that contains only a few unwanted seed. This re-sort good seed fraction is sent back to the initial incoming seed lot to be re-sorted and save the good seed. Structure and Components While construction and configuration vary among models, the operating system f the electronic color sorter is basically composed of the following components: 1. Feed hopper. 2. Chute or belt seed metering/feeding system. 3. Viewing area, of electronic “eyes” to view seed. 4. Accepted seed area. 5. Rejected seed area, and ejection system for unacceptable seed. 6. Discharge spouts. Electronic circuitry and control/adjustment systems are employed in these operations. Adjustments General adjustments of an electronic color sorter include pre-selection of accept-reject color limits, lighting, cameras, and circuitry systems.
Follow the recommendations of the manufacturer is setting up and maintaining adjustments and selectivity. Modern color sorters are designed for minimal operator attention once set for a specific separation product. Initial setup adjustment: Make sure selection items-filters, background, lighting, program-for the specific crop seed and separation are installed and activated. Set feed rate to achieve desired separation and capacity. Check to be sure that the background is balanced so there is the greatest possible differential between acceptable and unacceptable seed. Set sensitivity to remove unwanted seed with minimal good seed loss.
After initial adjustments are made: – The operator should periodically check for cleanliness of machine and viewing area, to be sure it is free of dust, etc. Sensitivity should be re-adjusted when there are major changes in the seed lot or incoming level of undesirable seed. The higher the sensitivity setting, the more subtle the defects that will be removed. Special Separations Modern machines have been developed to make special separations, including: 1. Fluorescence under infra-red light: by using infra-red lights and special components, materials which fluoresce under IR (infra-red) light can be identified and separated.
For example, some bean seed affected by specific diseases will fluoresce, and the sorter can identify and separate them. 2. Shape: new developments in circuitry and systems begin to show promise in being able to allow the sorter to distinguish between different shapes. Thus, only desired shapes will be accepted; undesirable shapes will be rejected. 3. Near Infrared: use of infrared filters allows the machine to see defects that may not be visible to the human eye. Decay, rocks, glass, stones, etc. , that may be the same color as the good seed can be detected and removed from the good seed. 4.
Translucence: while most sorts (separations) rely on reflected light, some. Gravity Separator Separation Principle Undesirable seed and contaminants are often so similar to good seed in size, shape and surface texture that they cannot be separated by the air- screen cleaner, magnetic, roll mill, or width/thickness separator. These materials, however, may differ from good seed in density—unit weight or specific gravity. The gravity separator separates seed and/or particles by differences in their specific weight (specific gravity, density, or relative weight which affect terminal velocity).
Seed of the same size and general shape can often be separated because they differ in specific gravity. This difference is very useful in removing light immature seed or heavy sand and rocks to improve both the purity and germination of crop seed. Uses The gravity was not designed originally for seed; it was first used in mining to separate ores, as an improved sluice trough technique as is used in gold mining. Various models of gravity separators are also used for many other weight separations such as processing food beans and coffee beans, removing stones from coal, etc.
Seed of different specific gravity can be separated with a specific gravity separator, usually called gravity table or just gravity. Seed which are similar in size and shape may be separated if they differ in specific gravity—density, unit weight, or relative weight—and are lighter or heavier than the good seed. This can separate light seed to improve seed germination, and to remove lighter undesirable particles and heavier sand/rocks to improve purity. Common gravity separations include: 1. Other crop or weed seed of the same size and shape may be lighter or heavier than good crop seed. 2.
Insect-damaged seed usually are the same size as undamaged seed, but are lighter because of damage. 3. Deteriorated, moldy, or rotten seed are usually similar in size, but have lower specific gravity. 4. Many crops often have empty, blind or sterile seed coats or hulls which look like good seed, but are lighter. 5. Seed lots of some crops often contain mud balls, soil particles, gravel or sand which are close to the crop seed in size and shape, so basic cleaning cannot remove them. However, they have higher specific gravity than good seed and the gravity can separate them. Place in Conditioning
The gravity is a very sensitive separator which makes a specific separation by differences in seed density. For an effective gravity separation, the seed mass flowing across the deck must first be precisely stratified. It can be successfully used only after the seed have been closely sized on the air- screen cleaner and other sizes such as length separators, so that the seed/ particles to be gravity-separated are all of nearly the same size, and differ only in density. This is possible only after seed have been closely size-separated on air-screen cleaner and other dimensional separators to eliminate size differences.
Therefore, the gravity is usually one of the last machines in the processing sequence. However, in cleaning alfalfa or clover seed, the gravity is usually used before the roll mill or magnetic separator, in order to remove sand or rocks which could damage the rolls of the roll mill, or to reduce the amount of material which must be removed by the magnetic separator. In some special operations, gravity is used before other upgrading or separating machines, as before the roll mill and magnetic separator in cleaning alfalfa and clover seed.
Here, the gravity removes sand to reduce wear of the velvet-covered rolls, and to reduce the amount these machines must separate so as to increase total processing output. Only machines which do not use dimensional characteristics—roll mill, magnetic separator, etc. —follow the gravity, as their separation would not affect the gravity separation, but gravity separation first would improve the separation made by the roll mill or magnetic separator. Gravity Separator Action on a Seed Mass A seed’s size, surface texture and specific gravity influence seed terminal velocity and thus control stratification of the seed mass and the separation ade by gravity. Overall seed size also affects its total weight and terminal velocity; to some extent, shape and surface texture also affect a seed’s resistance to the flowing air stream, and have some influence on its terminal velocity. The gravity separator will react to seed size and weight differences in this manner 1. Seed of the same size but different in specific gravity are separated by differences in their specific gravity. 2. Seed of the same specific gravity but different in size are separated by differences in their size. 3.
Seed which differ in both specific gravity and size cannot be separated. Gravity Separation Process If seed which differ in specific gravity (relative weight per unit of volume) are placed on a substrate of intermediate density, seed of higher specific gravity will fall down through the substrate, while seed of lower specific gravity will be buoyed up by the pressure of the substrate. This is basically the separation made by the gravity separator, using air in an upward-flowing column as a separation substrate. To make a separation on the gravity separator, the seed material is: 1.
First, stratified into vertical layers of seed of different densities, with the heavy seed layer lying on the deck surface with layers of successively lighter seed above. 2. Second, the vertically-stratified layers are separated so they flow to different discharge spouts. Fractions Separated The gravity separator does not separate seed into a distinct heavy fraction and a distinct light fraction. Instead, it performs a grading operation which gradually changes from the lightest seed at the low side of the deck, through successively heavier fractions to the heaviest seed at the highest side of the deck.
At the discharge end of the deck, seed flow off the deck onto the “discharge apron”, a flat sloped surface which guides seed to the discharge spouts. On the discharge apron, “discharge fingers” can be adjusted to separate the gradually changing seed mass into the desired fractions, and send the desired part of the discharging gradient of seed/particles to the desired discharge spout. Discharged fractions— starting at the high side of the deck and moving downhill—normally include: 1. The heaviest fraction, at the high side of the deck. In some seed crops, this consists entirely of heavy good seed and is combined with fraction no. (below). In other seed crops, this fraction consists of a mixture of the heaviest seed and sand or mud balls. This fraction is kept separate, and discarded if good seed content is low and no stoner is available; or is sent to a stoner to salvage the good seed in it. 2. The second heaviest fraction normally composed entirely of the good crop seed. 3. Next, a middling’s fraction which includes both light good seed and undesirable light particles. In higher-capacity operations, the middling’s fraction can be re-cleaned to save the good seed it contains. . At the lowest side of the deck, the lightest fraction composed of undesirable light materials which are usually discarded. Gravity Separator Structure and Components The gravity separator consists essentially of (1) a stationary frame which must be bolted to a solid foundation and within this (2) an adjustable frame which can permit the deck surface to tilt. One or more (3) fans with air intake controls blow air into the (4) air chest, where pressure is built up and the air currents are diffused into a uniform air flow through the (5) deck.
The deck is an open-frame structure covered with an open-mesh material. It is vibrated by an (6) eccentric drive and controlled by a (7) variable speed assembly. A (8) feed hopper meters the desired rate of feed onto the feed end of the deck. The (9) discharge spouts carry the separated seed fractions to the desired place. Two or three (10) separating fingers or dividers on the (11) discharge apron of the discharge end give flexibility in controlling exactly which seed go into each discharge spout. Mechanically, gravity is simple. It consists of: 1.
Base and frame: the base of the stationary frame is bolted to a solid foundation. Within it, adjustable frame allows the deck surface to be tilted. 2. Fans: One or more fans, with air intake controls, blow air into the air chest. 3. Plenum chamber (air chest): beneath the deck; inside it, air pressure is built up to create air flow up through the deck separating surface. 4. Deck: the flat porous surface where seed are separated as they flow across it. It has an open frame covered with an open-mesh material. 5. Feed hopper: meters seed onto the deck at the proper rate to maintain the separation. . Drive system: an eccentric drive system powers the fans and oscillates the deck, controlled by a variable speed assembly. 7. Discharge system: spouts separated fractions (light undesirable fraction, middling (intermediate fraction), good seed, and heavy waste fraction) into different places. Adjustments The gravity separator is a versatile machine; it can accomplish a wide range of separations primarily because it has five different adjustments which allow the operator to control the separating action precisely.
Since each adjustment affects the action of the others, all adjustments must be coordinated and blended together to produce a sharp separation. On some machines, separate adjustments allow controlling the variable factors; on some machines, all controls are located in a control panel. Feed Rate Rate of feed is an important adjustment on the gravity separator. A constant and uniform feed rate is necessary to maintain a uniform bed of seed on the deck. Enough seed must be fed onto the deck so that a bed of seed thick enough to stratify into different layers will cover all parts of the deck at all times.
The thickness of the seed bed should be just enough to allow the most effective stratification and separation. This can only be determined by observing the separation obtained at the discharge spouts. Variations in feed rate will change the bed of seed and cause the points of discharge of different seed fractions to move up or down along the discharge end. A clean separation is impossible when the seed bed surges because of variable feed. The gravity separator should have a feed bin large enough to ensure a uniform flow of seed.
A bin-level sensing device should be installed in the lower part of the bin to signal the operator or stop the gravity when seed level in the bin is low. This prevents undesirable light seed from falling into the clean seed spout when the feed stops and the light seed shift uphill on the empty deck. Rate of feed must be coordinated and balanced with other adjustments. When seed are fed onto the deck faster than the actions created by other adjustments can handle, seed are not stratified and appear to lie dead on the deck. A feed rate too low will not cover the deck properly.
Seed should be fed onto the deck at a rate that can be fluidized and separated. If feed rate is changed, other adjustments must be changed to match the new feed rate. Changes in feed rate change the seed mass, and cause different seed fractions to discharge higher or lower along the discharge end. Rate of feed must be coordinated and balanced with other adjustments. If more seed are fed onto the deck than actions created by other adjustments can handle, seed are not stratified and appear to lie dead on the deck. A feed rate too low does not cover the deck properly.
Feed seed onto the deck at a rate which can be fluidized and separated. If feed rate is changed, other adjustments must be changed to match the new feed rate. Good separation is impossible if the seed mass varies due to variable feed. Constant, uniform feed rate must maintain a constantly uniform seed mass, thick enough to stratify into different layers, and cover the entire deck at all times. Proper seed mass thickness allows the most effective stratification and separation. The gravity must have an overhead bin large enough to ensure uniform seed flow.
A bin-level sensor should signal the operator or stop the gravity when seed level in the bin is low. This prevents undesirable light seed from falling into the clean seed spout if the feed stops and light seed shift uphill on the empty deck. Air The basic control is precisely adjusting the velocity or pressure of air flowing up through the deck. Properly-adjusted air fluidizes and stratifies the seed mass so it flows freely, with heavy seed lying on the deck and lighter seed lifted up and floating on the air. Excessive air forces heavy seed up into the layers of light seed, and destroys the stratification.
This is characterized by “boiling” or bubbling in the seed bed, and the discharge of heavy seed with the light seed. Insufficient air fails to lift light seed above the deck surface, and thus fails to stratify the seed. This causes the seed to lie “dead” on the deck and light seed to discharge with the heavy seed. End Slope End slope, or slope of the deck from the feed hopper to the discharge end, controls the speed at which seed move across the deck and thus the length of time they are on the deck and exposed to its separating action. The longer the seed remain on the deck the sharper the separation.
When differences between the seed to be separated are slight, the deck should have a relatively flat end slope to hold the seed on the deck longer. Crop seed and contaminants that differ greatly in specific gravity will stratify and separate quickly, so end slope can be increased to move seed off the deck rapidly. This increases capacity. Side Slope Side slope is the tilt or inclination of the deck from the low side to the high side. Side slope creates an inclined surface over which the stratified seed bed must flow to reach the discharge end.
This allows the light seed layers floating on a cushion of air to float downhill to the low side of the deck, while deck oscillation moves heavy seed uphill to the high side of the deck. Deck Oscillation Speed The pitching motion of deck oscillation causes heavy seed to move toward the high side as they flow across the deck. Increased deck speed moves heavy seed uphill faster, so they discharge further up the deck. Decreased deck speed causes heavy seed to discharge lower at the discharge end, since they do not move as far uphill.