As the critical link between its upstream customer (the producer) and its downstream customer (the textile mill), the cotton ginning plant shoulders a great responsibility to do everything it can to maintain the cotton’s quality properties. Since 2010-11, world cotton production is substantially higher, while the ginning infrastructure remains relatively static (if not reduced in some areas). Existing ginneries will be expected to process even greater throughputs without abnormally extending the length of the processing season.
While this may seem a daunting challenge to some, it does not have to be. The fiber quality properties required by textile mills around the world (reduced short fiber content, less neps, higher uniformity, etc.) are in no way new to the industry. The basic concepts for ginnery design and operation have been developed over decades of ginning research and empirical experience. This information, combined with an industry mindset geared toward continual improvement, will result in a product that will lend itself to a multitude of end uses, through the wide array of spinning technologies found around the globe. This article will discuss many of these concepts, along with some of the latest issues facing the ginning industry, and the responses of the industry to those issues.
Lint contamination continues to be one of the industry’s most formidable challenges. Regardless of the harvesting method, non-cotton content within the incoming seed cotton—be it paper, plastic, wood, or metallic in composition—poses problems in many forms. Of course, the worst issue is for any contamination to find its way through the ginning process all the way to the final bale. When this happens and the bale enters the textile mill processes, contamination can result in major downtime, and fibrous types of contamination can actually reach the final finished yarn—and even into the fabric. This is disastrous, because the marketability of the end product is highly compromised, and therefore, the mill’s profitability is adversely affected. Improved bale identification technology frequently allows the mill to trace a defective bale back to its source, and this can have negative financial repercussions throughout the stream of commerce.
Even if the contamination does not reach the finished bale, non-cotton content entering the ginnery can cause a myriad of problems. Large pieces of contamination (e.g., wood stumps, wrenches, wire, etc.) will, in most cases, cause machinery damage and downtime. Since the ginnery is a seasonal business, its ability to maintain consistent throughput with little or no unscheduled downtime is tantamount to its financial success.
Because a large share of the responsibility for contamination prevention rests with the producer, the ginnery must nevertheless be diligent about reducing contamination in any way it can. Some of these techniques include inspection of the incoming seed cotton as much as possible, in whatever form it reaches the ginnery (sack, wagon, truck or module) to identify any signs of possible contamination. Many of the lightweight plastic shopping bags and snack food wrappers that blow into the fields are brightly colored, which aids in their identification. Worn tarps that cover trailers or modules and the twine used to secure them should be scrutinized to see when they are becoming worn and frazzled. These types of contamination can often make their way through the ginning process and into the finished bale.
Hand tools used in maintenance of the ginnery and fasteners (nuts, screws, bolts, etc.) must be closely monitored to ensure that they do not fall into machinery or ductwork, because they usually cause significant and expensive damage to the gin machinery. Finally, proper and timely machinery maintenance must reduce the potential for actual pieces of the machinery (rubber flashing, spikes from cleaning cylinders, internal fasteners, etc.) to become loose and make their way into downstream machinery or, worse, the finished bale.
Drying and Precleaning Systems
Although there are many areas of the world that still harvest cotton by hand, mechanical harvesting—either spindle picking or stripper harvesting—has been the industry standard in many regions for years. Additionally, the past decade has seen a significant increase in mechanical harvesting in countries like Brazil and China. Machine-harvested seed cotton contains significantly more moisture and trash, which must be removed prior to the ginning process.
In order to accomplish this, properly designed seed cotton conditioning (drying) and precleaning systems must be designed to facilitate effective trash removal, while preserving the fiber quality. Any ginnery drying and precleaning system should be designed with flexibility in mind—to allow for only as much drying and machining of the cotton as is required to maximize value and minimize damage. Heaters should be designed for worst-case moisture conditions and feature modulating capability to throttle back on fuel usage when applicable.
The drying system itself should be engineered to provide for a high ratio of heated-air-to-seed cotton, whether it is a single-stage or multiple-stage system. Higher volumes of air allow for more heat-carrying capacity in the air stream at lower temperatures.
The importance of adequate seed cotton precleaning was documented by research performed at the USDA-ARS Lubbock Ginning Lab in the early 1990s. Pepper trash (very small pieces of leaf and sticks), a major problem for textile mill processing, had long been assumed to be created in saw-type lint cleaners at the ginnery.
The only place in the ginnery in which pepper trash is increased is in the saw gin itself, and it is the result of larger leaf (which is not removed in the upstream precleaning) being ground into smaller particles in the saw gin stand. Hand-harvested seed cotton typically requires minimal seed cotton conditioning and precleaning (normally only one stage).
However, the increased moisture and foreign matter content in machine-harvested seed cotton mandates that two stages of precleaning be used for successful ginning. Precleaning machinery should be specified to adhere to the accepted capacity range of two- to two-and a-half 500 lb. (228 kg) bales per hour per foot of machine width (2-2.5 bales/hour/ft width), or six-and-a-half to eight bales per hour per meter of machine width (6.5-8 bales/hour/m width). When the seed cotton is in optimum condition, capacities exceeding these recommendations can be attained, but not usually without some adverse effects on fiber quality. Finally, machinery should be arranged with bypass capability wherever possible to allow only as much machining of the cotton as is necessary.
Ginning and Lint Cleaning Systems
Saw-type ginning continues to be the predominant method of lint/seed separation around the world. Continued product development has resulted in individual saw-type gin stand capacities reaching anywhere from 15 to 25 bales per hour. Within the industry, there has been arisen a misconception that high-capacity ginning results in lower-quality fiber.
However, when the ginnery and all its sub-systems have been properly designed, operated, and maintained, this has been shown not to be the case. Increased ginning capacity without compromised quality will continue to be one of the major goals of the industry, since it will result in lower per-bale ginning costs and more cotton being processed in a given amount of time.
Roller ginning, while making up a much smaller percentage of the ginning market, has seen advancements in the last few years, primarily due to groundbreaking work performed at the USDA-ARS Mesilla Park Ginning Lab in New Mexico. Their high-speed roller gin, now commercially available and in operation at numerous locations in the United States, shows great promise to revolutionize not only the processing of extra-long staple (Pima) cotton, but premium Upland (fuzzy seed) cultivars as well, producing finished fiber that is one or two staple lengths longer, has higher uniformity, and with fewer neps and reduced short-fiber content (all traits that are highly desired by the textile industry).
Ginning at the proper moisture content (typically between 5 percent and 7 percent) and maintaining the gin stand’s manufacturer-recommended settings are the best way to ensure peak performance. Monitoring of residual lint content on the seed (keeping it typically at 10 percent or below by weight) is an excellent measure of effective seed cleaning, and when this goes above that figure, it is time to perform maintenance on the gin stand.
Lint cleaners in modern ginneries fall into two basic categories: flow-through, air-type centrifugal lint cleaners and saw-type lint cleaners. Because they have no moving parts, air-type lint cleaners cause no fiber damage, and they should be designed into any modern ginnery as the first stage of lint cleaning.
Controlled-batt saw-type lint cleaners were introduced in cotton gins throughout the United States in the late 1940s, in order to reduce the unprecedented amounts of trash which resulted when mechanical harvesting replaced hand harvesting.
Numerous tests run on saw-type lint cleaners have quantified the level of fiber breakage that takes place in these machines. Further, these studies show that almost all of the fiber breakage takes place in saw-type lint cleaners where the sharp saw teeth plow through the firmly held batt of fibers being fed to the saws.
Based on this information, many ginneries over the last decade have opted to install an alternative saw-type lint cleaner, which is does not utilize the conventional lint cleaner feed works to meter the lint onto the saw. This non-controlled-batt saw-type lint cleaner applies individual tufts of fiber directly to the saw (through the use of a high-speed applicator brush cylinder and stationary perforated panels for air/dust removal), rather than agglomerating the fiber into a batt on a low-speed revolving condenser drum.
Studies in both single-stage and two-stage (tandem) installations of these new lint cleaners have documented substantially better fiber properties (brighter color, better uniformity, and reduced dust, short fiber content, and neps) than conventional controlled-batt, saw-type line cleaners, with no reduction in visual quality grade.
Ginneries equipped with tandem saw-type lint cleaners should bypass the second lint cleaner whenever possible, using it only when the trash content of the lint warrants doing so. Also, lint cleaners should be strictly maintained and adjusted according to the manufacturer’s specifications, because those that get out of adjustment or that have badly worn parts (saw cylinders, doffing brushes, grid bars, etc.) not only can result in fiber damage, but they can also be prone to expensive downtime, damage, and even fires. If the speeds of specific rotating components are not correct for the given ginning rate, the lint cleaner’s performance can be severely compromised.
There are two major areas in which moisture may be introduced to the cotton. The first is in the seed cotton just prior to ginning, and the second is in the lint cotton prior to the baling press. In many regions of the world, high ambient temperatures and low relative humidity during the harvesting and ginning season can produce seed cotton with moisture content levels in the 3 percent to 4 percent range. Since some warm air in the drying system is critical to condition the seed cotton to release its trash, this moisture content can be even lower when the cotton enters the hoppers above the extractor feeder.
Thus, moisture restoration hoppers, which induce moist air into the seed cotton, are recommended in arid regions to raise the seed cotton moisture level to the recommended ginning range of 5 percent to 7 percent). Doing so helps to preserve fiber strength and reduces fiber breakage during the ginning process.
Restoring moisture to the lint cotton prior to the press typically allows the press to handle higher capacities while expending less energy in the tramping and final pressing of the bale. Uniform lint moisture restoration–to no more than 7.5% at any point in the bale–can reduce the amount of hydraulic pressure required to compress the finished bale from 30 percent to 40 percent, which means less wear on both the tramping and compression systems of the press.
It is important to note that USDA-ARS tests have shown that lint moisture restoration at levels above 7.5 percent resulted in fiber degradation and reduced performance at the textile mill. Therefore, it is imperative that moisture levels be monitored on a regular basis.
Maintenance and Safety
In addition to the adherence to recommended ginning practices, two essential components to successful ginnery operation are a well-planned program of in-season and off-season maintenance and a comprehensive safety program.
In order to maintain consistent capacity and daily/weekly throughput, the ginnery must operate at its peak efficiency whenever it is not down for scheduled maintenance. This can only be accomplished by keeping all the machinery properly adjusted, in good repair, and operated per the manufacturer’s recommendations. A detailed log of all downtime–including cause, duration, and corrective action–can provide a blueprint for off-season expenditure, whether it is a repair or a replacement of a specific machine or system within the ginnery.
Gin safety can never be emphasized enough. The ginnery personnel are your most valuable asset, and their performance can have more effect on whether the ginnery operates well or poorly. Proper employee training in the purpose and operation of the machinery and safe work practices can result in substantial improvement in ginnery performance, reduced injuries and downtime, and an overall better work environment.
Any type of machinery is dangerous, and gin machinery is no exception. Proper guarding of all drives and maintenance of all safety devices can significantly reduce the chances of injury. Finally, good housekeeping within the ginnery and around the gin yard is both a sound maintenance and safety practice that can help uncover potential problems, eliminate hazards, and promote pride within the workforce.