COTTON GINNING

Process Description
Figure 9.7-1 is a flow diagram of a typical cotton-ginning process. Each of the five ginning steps and associated equipment is described below.

Unloading System -
Module trucks and trailers transport cotton from the field to the gin. A pneumatic system removes the cotton from the trailers, and either a pneumatic system or a module feeder removes the cotton from modules. A combination conveyer and pneumatic system conveys the cotton to a separator and feed control unit. Prior to this first separator point, some gins use a stone and green boll trap for preliminary trash removal. The screen assembly in the separator allows air to escape but collects the cotton and allows it to fall into the feed control unit. The conveying air flows from the separator to a cyclone system, where it is cleaned and discharged to the atmosphere.

Seed Cotton Cleaning System -
Cotton is subjected to three basic conditioning processes--drying, cleaning, and
extracting- before it is processed for separation of lint and seed. To ensure adequate conditioning, cotton gins typically use two conditioning systems (drying, cleaning, and extracting) in series.

Seed cotton dryers are designed to reduce lint cotton moisture content to 5 to 8 percent to facilitate cleaning and fiber/seed separation. A high-pressure fan conveys seed cotton through the drying system to the first seed cotton cleaner, which loosens the cotton and removes fine particles of foreign matter (e. g., leaf trash, sand, and dirt). In the second cleaner, large pieces (e. g., sticks, stems, and burs) are removed from the cotton by a different process, referred to as "extracting". Different types of extractors may be used, including bur machines, stick machines, stick and bur machines, stick and green leaf extractors, and extractor/feeders. These machines remove burs, sticks, stems, and large leaves, pneumatically conveying them to the trash storage area. The cotton is pneumatically conveyed to the next processing step. Typically, all conveying air is cleaned by a cyclone before being released to the atmosphere.

Overflow System -
After cleaning, the cotton enters a screw conveyor distributor, which apportions the cotton to the extractor/feeders at a controlled rate. The extractor/feeders drop the cotton into the gin stands at the recommended processing rates. If the flow of cotton exceeds the limit of the extractor/feeder systems, the excess cotton flows into the overflow hopper. A pneumatic system (overflow separator) then returns this cotton back to the screw conveyor distributor, as required. Typically, the air from this system is routed through a cyclone and cleaned before being exhausted to the atmosphere.

Ginning and Lint Handling System -
Cotton enters the gin stand through a "huller front", which performs some cleaning. Saws grasp the locks of cotton and draw them through a widely spaced set of "huller ribs" that strip off hulls and sticks. (New gin stands do not have huller ribs.) The cotton locks are then drawn into the roll box, where fibers are separated from the seeds. After all the fibers are removed, the seeds slide down the face of the ginning ribs and fall to the bottom of the gin stand for subsequent removal to storage. Cotton lint is removed from the saws by a rotating brush, or a blast of air, and is conveyed pneumatically to the lint cleaning system for final cleaning and combing. The lint cotton is removed from the conveying airstream by a condenser that forms the lint into a batt. The lint batt is fed into the first lint cleaner, where saws comb the lint cotton again and remove part of the remaining leaf particles, grass, and motes. Most condensers are covered with fine mesh wire or fine perforated metal, which acts to filter short lint fibers and some dust from the conveying air.

Battery Condenser And Baling System -
Lint cotton is pneumatically transported from the lint cleaning system to a battery condenser, which is a drum covered with fine mesh screen or fine perforated metal that separates the lint cotton from the conveying air. The lint cotton is formed into batts and fed into a baling press, which compresses the cotton into uniform bales.

Most gins use a double-press box for packaging the cotton into bales. The lint drops into one press box and fills it while a bale is being pressed and strapped in the other box. Approximately 480 lb (217 kilograms [kg]) of cotton is pressed into a bale before it is wrapped with a cover and strapped. Modern gins are presently equipped with higher-tonnage bale presses that produce the more compact universal density cotton bales. In 1995, 96 percent of the U.S. crop was pressed into universal density bales at the gins. The finished cotton bale is transported to the textile mill for processing into yarn. Motes are sometimes cleaned and baled also.