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DENSE MEDIUM SEPARATION OF SUGARCANE

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The Dominance of Billet Harvesters
Since 1995, there has been a major shift in the sugarcane industry away from whole-stalk harvesting. Today worldwide more than 80% of sugarcane is harvested as billets. A billet harvester cuts the cane into pieces ranging from 150 mm to 350 mm inches in length and discharges it into a trailer moving alongside.

The Problems Created by Billet Harvester
The billet harvester knocks the cane down and bites into the surface of the row, taking along with the cane an abundance of trash such as; tramp iron, sand, clay, clay balls, stones, bricks, leaves and tops. On average, a ton of cane contains 13% trash (8% inorganic and 5% organic). Under wet conditions, a ton of cane may contain up to 40% trash (30% inorganic and 10% organic).

Huge Losses of Recoverable Sucrose
Not only does this trash not contain sugar, but as it leaves the mill in the form of filter cake or bagasse, it carries away sugar at a rate of roughly 0.35 kg per percentage point of trash. Moreover, each percentage point of trash represents a loss in production of approximately 1.35 kgs of sugar.

Trash Cripples Profitability
In the case of 13% trash, the loss of sugar averages 4.4 kgs per ton of cane and production losses exceed 16%. In the case of 40% trash, the loss of sugar stands at 13.6 kgs per ton of cane, and production losses exceed 50%. More trash means more maintenance, more down-time, more flocculent, more lime, more natural gas, more unburned bagasse, more filter cake, more front-end loaders, more water treatment, more settling basins, more draglines to clean out settling basins, more haulage, more inversion, more molasses, less sugar and more cost. Every aspect of this complex sugar-making process is negatively impacted by the presence of trash. If growers and mills are intent on making money, they can no longer tolerate the presence of trash.

All mechanical harvesters whether of potatoes, beets, carrots, salsifies and peas, face the same problem of the intrusion of foreign matter that varies widely in terms of grain size and density. For over 40 years, the vegetable industry in Europe battled this problem by means of vibratory screens, rotary trommels, optical sorters, air separators and a variety of dynamic effect separators employing water. None of these technologies worked.

Failure of All Attempts to Date to Deal with Trash
Likewise, we may try to screen, rinse, wash or scrub the billet, but all of these steps fall far short of producing an acceptable product. The average washing efficiency for a single deck washing table is about 53%, for a dual deck washing table, it is about 59%, and for a cane washing drum and it is about 44%. In the wet season, these efficiencies will decline considerably. If a sugar mill wants to make money, produce a consistent product, eliminate the randomness associated with weather, and aggressively compete with the rest of the world, it must look for a technology that can make a perfect separation of trash from billets. Obviously the first question someone might ask: does the density of the billet make it difficult to separate from everything else?

The Specific Gravity of Sugarcane
The density of a sugarcane billet is no different from that of a sugar beet or potato, as is readily seen in the results of density tests conducted on January 17 and November 18 of 2003 at the USDA laboratory in Houma, Louisiana.
It terms of size and shape, the billet is no different from a large carrot or salsify. The salsify is a long root vegetable grown in a soil that is predominately clay, and since it is harvested in the middle of winter when it rains continuously, it generally comes from the field with over 60% clay. Nothing of this clay, not a single ounce, can ever be found in salsifies exiting a dense medium separator (DMS).

Since the size, shape and density of the billet appear quite normal; since metal, stone and clay all have densities at least twice that of the billet and since Newton’s laws continue to operate at sugar mills, nothing is unproven about dense medium separation technology. This technology has been around for over 20 years, and anyone trained in the art of dense medium separation would find it impossible to explain how it would not work in sugarcane.

What is dense medium separation?
The Dense Medium Separation of Cane
Here we find the dynamic of a quiescent bath where the density of water is changed by means of fine particles in suspension. At first glance, nothing could be simpler: one fraction floats, while the other fraction sinks. But at high tonnages even the best dense medium separators generate errors, and where should one go to find the suspension materials needed to change the density of water in separation of waste materials?

Twenty years ago a unique bi-directional dense medium drum was invented that has proved to be the most accurate separator ever employed in the separation of a variety of root vegetables such as carrots, potatoes, salsifies and beets. Not only does this separator remove both high-and low-density trash, but it is also able to distinguish good vegetables from bad vegetables, vegetables that at times might differ in density by only a few points to the third decimal place.

Suspension Fines from Harvester Dirt
In this revolutionary sorting process, the suspension fines needed to change the density of water are obtained nowhere else but from the dirt that mechanical harvesters extract along with the vegetables. Two stages of classifying cyclones isolate ultra-fine sand from the scrub and rinse water of the pre-processing and cleaning line. In using these fines, one avoids the dreadful health issues associated with the use of clay or sand contaminated with dioxins or heavy metals, and one avoids as well the thorny environmental and maintenance issues associated with the use of salt and the disposal of brine.

Eventually sixteen bi-directional dense medium separators were sold in Belgium and France. These separators have been in continuous operation in Europe, some for almost 20 years. It is truly remarkable that no one to this day has ever been able to establish a single separation error in the finished product.

Putting Billets in Water
Perhaps the biggest objection to the dense medium separation of sugarcane billets concerns the loss of sugar associated with putting them in water. The washing of whole stalk cane had been in practice in Louisiana for about 50 years. But since the harvesting of a whole stalk involves but one or two cuts, the loss of sugar here does not exceed about .5 kg of sugar per ton of cane.
There are four factors that determine the loss of sugar with respect to billeted cane:

  1. the number of harvester cuts (billet length),
  2. the precision of the harvester cut (clean or frayed),
  3. the forces applied to the billet (agitation/tumbling),
  4. the amount of time the billet is subjected to these forces.

The Number of Harvester Cuts
In general, it is fairly safe to say that if all other factors are the same, twice the number of harvester cuts will yield twice the loss of sugar. In the study by Birkett and Stein, we see that short billets of an average length of 135 mm lose twice as much sugar as long billets of an average length of 245 mm.

The Precision of the Harvester Cut
If the billet harvester cutting blades are not sharp, they will damage and fray the ends of the billet. The same study by Birkett and Stein shows that sugar losses increase almost threefold if the billet is damaged. Therefore it is very important to change the cutter blades on the combine harvester at the frequency recommended by the harvester manufacturer.

The Force Applied to the Billet
In vegetable separation, the handling of the carrot or salsify within the dense medium separation process is a very important issue. If, for example, a carrot or salsify is broken, it has little or no value. Several studies conducted in Europe have shown that the carrot remains undamaged throughout the entire DMS process.

Likewise, if a billet is agitated or tumbled, either before or during the cleaning process, it will lose sugar and drop in value. That is why the scrubbing of the billet, as in a typical wash drum, is far from ideal. It would take at least 5 minutes of violent scrubbing to break down the large clay balls that often accompany the billets. But a retention time of five minutes would require a scrub drum of an enormous length, and during this time, far too much sugar would be lost.

Time of Agitation
The study by Birkett and Stein shows that if billets are placed in a cane preparation index apparatus and rotated at 19 RPM, a lot of sugar is released by this tumbling action. The longer the billet is tumbled, the greater the loss of sugar. In the first 30 seconds, we see a loss of about .5 kg of sugar per ton of cane, while after 15 minutes, this loss increases to almost 3.6 kgs of sugar per ton of cane.

Water Alone Has Little Impact
It would appear, therefore, that the simple act of putting the billet in water releases very little sugar from within the billet. To determine the density of the billet the USDA lab in Houma weighed billets underwater. The billets were left in a tranquil bath of water for several hours at a time, and throughout the entire day, the water used in this test was never changed. At the end of the day, the amount of sugar in this water was indistinguishable from the amount of sugar present in the original tap water used for this density test. What can we learn from all of this?

Water does not diffuse sugar out of a billet. Since the cell membrane of cane is resistant to osmotic effects, the cell wall must be ruptured and force must be applied to the wall of the billet. Once the billet is cut, every effort should be made to minimize all aspects of its handling. From field to factory, it should be loaded and unloaded but one time. It should never be dumped on a concrete slab or handled by a wheel loader. In the cleaning process, it should not be scrubbed, agitated or tumbled in any way, and it should not remain in a separator for more than about 15 seconds.

An Estimation of Sugar Losses
If we do all of the above, then we have every reason to expect a sugar loss of about 1.35 to 1.8 kgs of sugar per ton of cane. Keep in mind that a 1.35 kgs loss of sugar per ton of cane is made up for by removing only 4% trash per ton of cane. Under dry conditions, a mill would accept a loss of 1.35 kg to avoid a loss of 4.4 kgs, and certainly under wet conditions, a mill would accept a loss of 1.35 kgs to avoid a loss of 13.5 kgs of sugar per ton of cane.

Some people have put forward the concept that dense medium separator would involve a loss of sugar of about 9 kgs of sugar per ton of cane. But if the average loss of sugar in a wash drum is only 2.8 kgs of sugar per ton of cane, and if our separator affects only a fraction of the tumbling action of a wash drum, how does one justify such a claim?

The Vegetable Industry in Europe
The sugarcane industry has a lot to learn from the vegetable industry in Europe. If it handles billets with the same respect that the vegetable industry in Europe handles carrots, and if it separates billets with the same technology that Europe employs to separate carrots, then most of the problems created by the introduction of billet harvester are easily solved.

How to Handle a Billet
Putting a billet in water will not cause sugar to come out of a billet. Putting a billet in water simply shows us how much we have mishandled it prior to putting it in water. The gentle action of a dense medium separator can never be a major factor in sugar loss. Perhaps we need to adjust the length of the billet, change cutter blades more often, reduce the number of times we load and unload the billet, eliminate the use of wheel loaders, reduce the distance we allow a billet to fall, and so forth. It has taken eight years for the billet harvester to dominate the sugarcane industry but sugarcane industry does not have the luxury of waiting another eight years to address the problems and inefficiencies created by the billet harvester.

The Air Separation of Leaves and Tops
The leaves and tops of the sugarcane plant contain no sugar, and those that accompany the billet into the mill can be easily removed by means of an air separator. Since it is very important in air separation to minimize particle interference, the billets should be spread out over a broad two-dimensional plane, and the blower should be situated in-line with the flow of billets. The leaves and tops isolated by the air separator are ideal for composting.

The Elimination of Trash
Hopefully it is clear that this dual strategy of air separation and dense medium separation can eliminate virtually all organic and inorganic trash. What mill in has ever considered the possibility of processing billets without extraneous material? It is hard to imagine what this should mean. For the first time ever, a mill will be able to operate under constraints that are truly internal.

A mill would possess a level of control in daily operations that it has never before experienced. A lot more sugar would be recovered. Production would stabilize at a high rate. Operating and maintenance cost would decrease dramatically. Rain would have minimal impact on profitability.

The Sugarcane Industry Must Compete
Mills without effective control over trash such as; tramp iron, sand, clay, clay balls, stones, bricks, leaves and tops will never compete with the rest of the world if, during wet weather, it faces losses in production of over 50% and increases in operating and maintenance costs of over 50%. Sugar mills must adapt to compete in an ever aggressive marketplace, and no single technology offers more hope of doing so than the DMS technology briefly outlined in this paper.

The elimination of trash will greatly simplify every aspect of this complex sugar-making process. It will offer growers and factories a myriad of new options and new economic opportunities, and, at the same time, it has the power to rescue and stabilize one of world’s oldest and most important industries.

 

 

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