Often an almost forgotten piece of machinery, the feed mill mixer has no adjustments that need to be made between batches and, so long as it remains operational, millers generally pay little or no attention to it apart from routine servicing, such as checking drive chain and gearbox oil levels.
Annual mixer uniformity tests are done to establish that the mixer does what it is supposed to do and that levels of homogeneous mixing are still being achieved despite natural and expected wear and tear on blades and casings.
However, you are only able to treat mixers in this manner when you have made the correct choice of mixer at the time you build your mill or when you remodel or upgrade your facilities. Naturally, feed mill mixers are quite large, irrespective of whether you choose to blend by weight on a batch basis or volumetrically on a continuous basis in single species, limited-diet-range mills.
In recent years, there have been more specialist mixing arrangements applied in mills where proteins are mixed separately from cereals, and minerals and vitamins are also mixed separately. This has been the case in post-grind mills, and it has been done in such a manner that the ingredients with different particle size characteristics can be routed to different hammermills ahead of the main batch mixer. This ensures optimum performance and power consumption through the hammermills, with computer control dictating screen sizes, beater speeds and mixing times.
Traditionally, horizontal mixers have always been the same shape, with flap tops allowing easy access to perform maintenance tasks on mixer blades and liquids injection systems. The more modern mixer has a much more efficient shape. Often referred to as a “tear drop,” and pioneered mainly by Buhler AG, the mixer has a tapered upper half which allows the full internal area to be utilized, thus saving considerable space within the mill building without detracting from maintenance access arrangements and capabilities.
SELECTING A MIXER
As with all the machines that you select for your feed mills, the type of mixer you install has a significant impact on the overall technical performance of the mill and influences efficiency tremendously. Obviously, size is extremely important, and you must give due consideration not only to your blending batch size but to the size of your pre-press bins and your finished product holding bins in order to optimize the use of your plant.
For example, it is not particularly wise to choose a mixer with a three-tonne capacity if you have a maximum batch size of two tonnes when fully utilizing batch weighers. With finished-product storage in 10-tonne units, a two-tonne mixer would be more appropriate. Mixers generally are most efficient when they are filled to capacity. A three-tonne mixer will not produce a homogenous mixed ration when it is only filled with two tonnes of ingredients. Trying to vary batch sizes is not advisable and can lead to variations throughout the mix in terms of analyses.
Mills nowadays are mostly operated on a batch basis. But where only a single species of animal is being fed, in a feed lot for example, continuous mixers are occasionally used and blending is done volumetrically. With volumetric blending, there are obviously some variations as ingredient densities vary,
but continuous mixing does enable the whole plant to operate efficiently with maximum outputs being achieved. In this instance, ribbon-bladed mixers are used to avoid any separation of ingredients within the mix when subsequent machines go down and materials must be held in the mixer longer than normal.
In almost all cases, it is important to minimize mixing times in order to optimize plant capacity. Most mills operate with mixing times of around four minutes, although this does vary. It is important to establish early on in the life of a mixer precisely how well it can mix. To do this, common practice is to simply take samples from the mixer at periodic intervals throughout an individual batch and analyze them for homogeneity. Invariably, a simple salt test will suffice to establish when complete mixing has been achieved.
In concentrate plants, it is common practice to employ a premix arrangement where vitamins and minerals being added at high levels to the main mix are mixed prior to incorporation in the main mix. In this case, a smaller mixer, appropriate to the size of the premix, will be used. Perhaps a half-tonne or one-tonne mixer will be used for this purpose. Again, this will likely be a ribbon-bladed mixer.
In the case of coarse rations, where some of the ingredients are friable, such as flaked maize, rolled oats or rolled barley, a slower, gentler type of mixer may be used.
When liquids such as fat are to be added at the main mixer, a sparge pipe is fitted which runs along the length of the mixer. This allows liquids to be incorporated into the side of the mixer where materials are on the upward sweep. This allows the fat to be added to the materials rather than being sprayed on the mixer side, where it will stick and cause problems. Obviously, timing the additions is important, and fat must be kept or preheated to the most appropriate temperature for rapid incorporation into the blend.
Vertical mixers are rarely supplied these days, although there are probably a considerable number still operating on European farms where farmers produce small quantities of feed using their own homegrown cereals and ingredients. Regulations concerning incorporation of feed additives have reduced the amount of on-farm mixing that takes place, but there are still a few installations operating. Often such mixers are used in equine facilities where breeders prefer to source their own, limited range of raw materials and
make up their own blends for horses at stud or racehorses in training. These vertical mixers, where bags can easily be tipped in at the same level at which the blend is bagged off after mixing, have been extremely popular in the past and continue to be used in certain areas of the world.
The use of load cells in most feed and flour mill mixing applications has enabled significant technical progress to be made. Today you see farm mixers mounted on trailers where silage and fodder crops are being incorporated with pelleted feed and where the mixer also doubles up as a batch weigher. This is also being done in feed mills where space is at a premium. However, while this saves on space, it significantly impacts productivity, since one batch must be completed before another one can start being assembled.
In the case of the regular blending system, it is common practice to have several batches being processed at the same time, especially where three-tier systems are adopted. For those not familiar with three-tier systems, this is where one batch is held in a top-reserve hopper over the mixer while another batch is being mixed and a third batch is held in a hopper below the mixer ready to feed the next available pelleting press. With computer controls and accurate load cell applications, three tier mixing systems can produce high throughputs in complex mill configurations. Good stock control is also maintained in such circumstances with continuous updating of inventory by virtue of load cell measurement data.
MIXER BLADE CONFIGURATION
The majority of mixers used in feed mills today are ribbon bladed. In other words, they consist of an inner and outer scroll, pushing materials in opposite directions. The outer scroll is usually quite close to the mixer wall and the inner scroll is of equal distance between the outer scroll and the main central shaft. Such scrolls are bolted to the main shaft so they can be easily changed when worn out. The blades are usually sectional for practical reasons of removal and replacement. Other blade configurations are in the form of a series of “Tshaped” arms projecting outward from the main shaft. The relative positions of these arms produce a homogeneous mix very rapidly, and it is this configuration which is most often adopted in fats coating applications.
One small but significant point to consider when designing new mills or upgrading an older facility is the total weight of an installation. When a three tier-system is fully loaded, there is considerable weight in motion. With the use of “bomb-type” doors under hoppers and mixers to assist with rapid transfer from one tier to another, the dynamic loads are also high. Allowance must be made for these factors and steelwork should be strong enough to support equipment at maximum load stress.
Bomb doors under hoppers and mixers must be closely monitored to ensure they open and close properly and effectively. There have been situations where this has not been the case and leakage
from one batch into another has gone undetected. Simple monitoring of load cell readings will indicate to the operators whether doors are correctly closed. Regular observation and maintenance is important. With simple slide arrangements, maintenance doesn’t have to be so regular, but transfer times from hoppers and mixers using slides are obviously somewhat slower and efficiency can be slightly impaired.
The final point to consider in selecting the correct mixer and installing it is to ensure that exhaust systems are adequate and appropriate. When three tonnes of material is dropped from a hopper into a mixer, there is significant displacement of air that needs to be recycled with the system. It is worth spending some time and money to get the calculations right to avoid serious consequences.
Good design and planning are always important, and a mixer only becomes a forgotten piece of equipment when care is taken with the original design and installation.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J B Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: firstname.lastname@example.org .