Why Should grain be cleaned before storage?
With modern high quality harvesters, grain is often received fairly well cleaned if the settings were correct and if capacity was not pushed to the limit and if grain was dry and if there were not too many dirty patches and if no heat damage took place and if conveyors did not damage the kernels etc.
Lots of If's. So iIf in doubt therefore, it is recommended that fines and broken kernels in the case of maize especially, at least be removed prior to storage.
Pre-Cleaning grain prior to intake into a silo system is one of the most important ways in which to add value to the crop and to assist in the process to secure a long life for the grain during storage.
Keep in mind, the grain quality will start to deteriorate immediately once it has been harvested. The deterioration process must be delayed as far as possible to ensure the end user still has a quality product to work with.
Pockets of foreign material in silos, especially fines, which tend to group together during conveyance, can cause hot spots and will negatively affect aeration. Those pockets would have a higher density than the rest of the grain and during aeration, air would by-pass those dense pocket, causing it to retain more moisture, as moisture is captured there and not allowed to move away and out of the grain with the air that is forced through the grain.
The dockage furthermore takes up valuable storage space - even pu to 5% or more may be taken up with a low value product that can furthermore contain poisonous seeds that can be spread throughout the silo and can be cause for downgrading the product.
If a large parcel of grain is rejected by the buyer or the central co-op, the cost of cleaning and resubmitting the grain to the negatove bueyr, can be well in excess of the cost of a good cleaner.
Even if grain is only cleaned before selling, the owner has control over the dockage and can use it as feed for livestock or sell the dockage while under his control but have no claim over the dockage once sold. He may just face deductions in the selling price.
Precleaning and final cleaning:
Pre-cleaning refers to grain that has undergone cleaning where either over or under or overs and unders have been removed from the grain to an acceptable level. This may be up to 97 or 98 or 99%, depending on sales terms. We would recommend pre-cleaning grain to as high a level as possible and usually to around 98 or 99%.
All cleaners are more effective at lower capacities, so if a pre-cleaner benchmarked at 40 tons per hour top capacity is used at full capacity, it would inevitably have a lower cleaning efficiency than if utilised at 30 tons per hour. The operator has control over this and should find an optimal level at which to operate the system.
The ABC Hansen range of pre-cleaners include the Egret Rotary cleaner with optional aspiration system for capacities up to 40 tons per hour, the Weaver range of gravity cleaners up to 200 tons per hour and the oscillating Heron range with each stage at 80 tons per hour and a maximum ot three stages for a total maximum capacity at 240 tons per hour.
Final cleaning refers to the process grain undergoes before it enters a processing facility such as a mill. Capacities are usually lower and is geared towards the capacity of the processing facility ie. a 5 ton per hour maize mill would usually work well with a 10 ton per hour final cleaning system but does not require a 40 tons per hour final cleaning system which would simply not be cost effective.
Final cleaning usually also includes an aspiration system whereby dust not removed through grading alone, as well as light particles, the same size as the grain kernel and therefore not removed through grading, is included.
ABC Hansen's final cleaners have capacities between 2-3 tons per hour on the Quail eccentric cleaner and from 5 tons to 15 tons per hour on the Francolin range.
What cleaner should be selected?Other than own storage, the cleaning of grain represents the most logical step for the Grainman to add value to his product.
Annual seasonal changes require different approaches to cleaning. In some years heavy rains may have cause a lot of rotten and undersized kernels in the crop.. This causes large tonnages to be downgraded and after a thorough cleaning by the buyers, the grain is again blended into first grade. The mills are also battling to deal with grain received at farm gate that has not been thoroughly pre-cleaned since it places too much stress on their own final cleaners causing bottlenecks and poorly cleaned grain to enter the mill which can be disastrous.
Cleaning normally includes the following elements:
- Grading for size: This is usually the least expensive equipment and can consist of the following main types of cleaners:
- Rotary drum cleaners: It usually has relative large capacity between 20 and 60 tons per hour and should isolate over- as well as undersized particles.
- Vibration cleaners: Usually smaller capacity final cleaners (5-15 tons per hour) but with a high quality end product.
- Eccentric cleaners: Smaller (2-8 ton per hour).
- Oscillating cleaners with large capacities of 80 - 240 tons per hour per cleaner.
- Gravity cleaners: Only isolate throughs and can not isolate overs as well. Large capacities of more than 200 tons per hour are achievable. (Do not confuse this with gravity cleaners that separate particles on the basis of specific weight)
- Aspiration: This refers to a suction process with a fan, cyclone, ducting and rotaty valve and is used in combination with graders for size as the light foreign particles of the same size as the kernel is isolated. This equipment is likewise not excessively priced.
- De-stoners and grading according to the kernel’s specific weight. Stones that passed through a size grader can be isolated here while rotten kernels not light enough to be aspirated off can also be isolated. Wheat with falling number problems can, on good separators also be isolated. This is usually more expensive equipment.
- Colour sorters can isolate speckled kernels or deviations based on colour. These machines are normally used in high value items such as seed material, coffee, beans, nuts etc for human consumption and is also used in packing plants and food processing plants. While the cost of these sorters have come down recently, entry level machines would start at around R500,000 at 2-3 tons per hour. Lazer sorters form the new generation.
- Other special cleaning equipment includes trieurs and other indented cylinders grading by length of kernel for instance as well as magnets (often used in conjunction with basic graders also), centrifugal cleaners etc.
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