The advantages and disadvantages of roller mill, hammer mill, multicracker system as well as multi-stage grinding are introduced.Dry sieving determines particle size based on maximum width and maximum thickness; other characteristics of particles like volume or shape can be determined using laser diffraction and image analysis. Wet sieving can determine particle size of pellets.Particle size is reduced during the pelleting or expander process.Recalculated equivalent particle size may be superior in explaining pig growth performance compared to the use of geometric mean diameter.Breakage functions used to predict output particle size in roller mill grinding with wheat may be used in future to predict size reduction of feed ingredients in hammer milling.
Dry sieving determines particle size based on maximum width and maximum thickness; other characteristics of particles like volume or shape can be determined using laser diffraction and image analysis. Wet sieving can determine particle size of pellets.
Particle size of diets or ingredients plays an important role in pig growth and gut health. The way the size of particles is measured and expressed, however, is limited in explaining pig growth performance differences. This review explores new possibilities to determine, express and predict particle size. Different grinding methods, including the use of roller mills, hammer mills, multicracker and multi-stage grinding were reviewed. Roller milling tends to produce a more uniform particle size distribution (PSD) and consumes less energy, whilst hammer mills have a greater grinding capacity and a higher reduction ratio compared to roller mill. The multicracker system, a more recently developed technology, can be considered cost-effective and ensures grinding capacity. Since the effects of different grinding methods vary, multi-stage grinding, combining different grinding methods, might be a solution to obtain a defined PSD. Particle size determination techniques, including dry/wet sieving, laser diffraction, microscopy, and static/dynamic image analysis are described and compared. It is concluded that more characteristics of particles (e.g. shape, volume or surface area) should be investigated. Besides geometric mean diameter (GMD), particle size can also be expressed with parameters such as D50, D4,3 and span of PSD. Equivalent particle size (EPS) is introduced as a mean of describing the size of particles related to a functional trait of the particles. A meta-analysis was performed by collecting particle size and pig performance data from scientific studies examining the effect of recalculated EPS on pig performance (feed conversion ratio, FCR). Regression/linear modelling shows that recalculated EPS was not better than GMD in explaining pig performance differences due to the high variation among studies. Different expressions of PSD may result in different conclusions. An introduction of describing the breaking behavior of diet ingredients via mathematical models is provided. The development in breakage functions of wheat in roller milling in food preparations indicates that breakage functions are applicable in predicting the output PSD. Functions may also be extended to diet ingredients to be ground in animal feed manufacture. In feed manufacturing diagrams, particle size reduction for downstream processes (e.g. pelleting, extruding, expander processing) should be taken into account when the relationship between pig performance and particle size of diets is investigated. In conclusion, the determination, expression and prediction of particle size can be a new direction for controlling the grinding process in the feed mill to better explain its relationship with pig performance.
Reduction of feed particle size improves pigs performance.Pelleting induces intensive grinding of particles.On the other hand, fine particle size of the diet negatively affects health of GIT.Optimal particle size of pig diet should be in between 500 and 1600m.Particles smaller than 400m are considered as undesirable.
Pigs are monogastric animals with simple, single-chambered stomach and require easily digestible, high quality feed. One of the most important factors that determine feed utilization by pigs is the particle size distribution. The reduction of particle size of feed improves pigs performance due to increased specific surface of feed particles allowing better contact with digestive enzymes. In this respect, fine grinding could be recommended in production of pig feed. Additionally, in modern pig production dry feed is predominantly used in pelleted form, which is mainly due to improved (i.e. decreased) feed conversion ratio (FCR) of pigs fed pelleted feed, but also due to other advantages of pelleted over mash feed.
Size of feed particles is strongly reduced during pelleting process. Consequently, digestibility of nutrients in pig feed could be improved. On the other hand, presence of high quantities of fine particles in pig feed (both mash and pelleted) negatively affects the health of gastro-intestinal tract (GIT) leading to higher incidence of stomach ulceration and other negative alterations of gastric mucosa (keratization, erosions). Gastric ulcers are one of the most important causes of sudden death on farm that can result in large economic losses in pig production. Concerning that the animal therapy is expensive, labor-intensive, and mostly non-effective due to late recognition of ulceration, prophylactic recommendations are required. Thus, according to literature data, decreasing the quantity of fine particles in pig feed is strongly recommended.
Particle size distribution of the pigs feed has a strong influence on presence of pathogen bacteria in GIT of pigs. Feeding pigs with coarse mash feed decreases pH value of stomach content compared to pigs fed finely ground mash feed and compared to pigs fed pelleted feed. This can be explained by slower passage rate, increased dry matter, and more dense consistency of stomach content in pigs fed coarse mash diets. Consequently, feed acidification in stomach is better, number of lactic acid bacteria and concentration of organic acids is higher, and pH of stomach content is lower. These conditions create additional barrier against pathogen bacteria.
According to available data, optimal particle size of diets for pigs is in the range between 500 and 1600m, while particles smaller than 400m are considered as undesirable with high ulcerogenic capacity. Optimal particle size could be designed in the grinding process, and it was shown that the most convenient grinding method is to combine roller mill and hammer mill. Concerning that nowadays pigs are mainly fed pelleted feed, and that pelleting causes strong additional grinding of feed particles, particle size distribution (PSD) obtained within the grinding process would be dramatically changed during pelleting. The possibilities to decrease the intensity of grinding of particles during pelleting, by variation of parameters of pelleting process, are very limited. Modified extrusion process (i.e. processing using expander) followed by shaping element, is suggested in the literature as an alternative for pelleting in order to obtain agglomerated pig feed with preserved PSD, but this process is not extensively studied so far.
Hammer mill machine is essential part of feed processing and an effective way to improve the digestibility of feed for livestock and poultry. Its purpose and function are to improve the utilization value of materials and improve the processing quality of feed products. Grinding process is an important procedure to improve feed quality during feed processing and is also one of the necessary means to make rational usage of feed. The size of the crushed particle should be determined according to the types of raw materials, the type of feeding animals, growth stage, and technological requirements. Too coarse or too fine particle size have disadvantages and cannot reach the best production status.View all feed hammer mills
3) The tip linear velocity of hammer: in a certain range, the greater the tip linear velocity of hammer, the higher the productivity of grinder and the lower power consumption of unit product. But too faster speed will increase no-load power, vibration and noise, decrease the grinding efficiency.
4) Screen size and opening rate: the larger the sieve pore, the greater the output of crushed materials, and the coarser the material pellets; With the same sieve pore, the larger the opening rate, the greater the output of crushed material, and more uniform the material particles. But too large opening rate will reduce the strength of the screen and shorten the service life.
6) Clearance between hammer and screen: The clearance is the distance between the top of hammer and the inner surface of screen when rotor is running. It is an important factor affecting the crushing efficiency. Too large or too small clearance will increase power consumption, reduce grinding efficiency, and make the particle size of crushed material uneven.
7) The structure of grinding chamber and the installation of toothed plate: all of them can effectively destroy the material circulation layer, increase the number of effective hits to the material, and thus improve the grinding efficiency.
8) Discharging way: discharging device should be able to discharge and convey the crushed materials in time. At present, there are three kinds of discharging ways: self-weight dropping, pneumatic conveying and mechanical conveying plus wind suction. Mechanical conveying plus wind suction has best effect, which is the main discharging way used by large grinder at present.
Some raw materials, such as corn, wheat, rice, soybean etc., have hard and dense skin. If they are directly used to feed livestock and poultry, the digestive juice is difficult to immerse the inside. After being crushed, the cereal skin is torn and the internal nutrients are exposed, to ensure that the material has a larger surface area, the digestive juice is easier to immerse the material, and improve the digestibility of animal feed.
Studies have shown that the digestibility of grain with diameter of 0.85 ~ 0.95mm is increased by 3% to 5% compared with diameter of 1.7 to 1.8mm. At the same time, the material after being crushed improves the palatability of livestock and poultry, and reduces the consumption of feed energy, thus reduces the utilization rate of feed. Certainly, too fine material does not mean better effect. The size of the general particle from 0.5 to 2.0mm is suitable, which varies according to different varieties of livestock and poultry, the feeding stage and raw materials. Because too fine feed will make the palatability of livestock and poultry worse, the material will be stuck in the mouth, difficult to swallow. At the same time, too fine material easily causes stomach ulcers in livestock and poultry. Moreover, it will increase the power consumption of feed processing, reduce the output, increase the wear of grinder and its parts like hammer, screen and so on, and reduce the service life.
The particle size of the material affects the mixing uniformity of feed. According to the test, it is found that the smaller the average particle size of mixed material is, the more uniform particle size is, the slower mixing speed is, the higher mixing uniformity can be achieved, and not easy to produce the classification phenomenon during the subsequent transportation and storage. On the contrary, when the material with different particle diameters are mixed, the uniformity of mixing can be poor. Therefore, in terms of mixing process, the crushing particle size of various raw materials is required to be smaller and similar, so as to achieve good mixing effect and ensure product quality.
The particle size of materials has a great effect on the pelleting process and feed quality. The same materials with different particle diameter and size produce feed with different quality. At the same time, the fine material can reduce the wear of the die and produce feed with smooth appearance, compact structure, low powder rate and without crack. After testing, the maximum particle diameter of materials to be pelleted is not larger than half of ring dies aperture, and the proportion of particles above 1mm is not more than 20%. Then the ideal particles can be produced and the negative effects of too fine materials can be avoided effectively.
This wasnt always the case. In the early days of compound feed milling, when raw materials were homegrown and power sources were either wind or water, the effort needed to grind or flake cereals into a form where animal uptake was optimized dictated that roller milling was a more economic and popular means of size reduction.
Thus, roller milling was the traditional method of preparing cereals and fodder for on-farm consumption by livestock. Today, millers have the option of using either method, or both, and there are many factors that impact their choice.
First, and in some peoples eyes the most important consideration, is power consumption per tonne of grinded product. In this case, I am referring to general processing of cereals and proteins, and my comments do not relate to the specialist grinding of micro-ingredients and high-fat raw materials, which both need careful and specific attention when being ground.
In recent years, hammermill diameters have gradually increased, which obviously allows for greater peripheral beater tip speed at lower revolutions. This has meant the impact effect on cereals at the outer extremities of the grinding chamber is increasingly severe. Consequently, power consumption levels in such hammermills have been reduced to a minimum. This is partly due to a combination of increased screen-hole diameter that complements the increased peripheral beater tip speed by accelerating the impact of individual particles between beater and screen.
This increased diameter of hammermill grinding chambers has led to the adoption of machines with greater throughput capacity, and there has been a progressive shift toward the adoption of post-grinding techniques in most mills built today. In this case, post grinding refers to the positioning of the hammermill after the blending stage as opposed to pre-grinding positioning, when hammermills are placed at the early stage in the mill flow, before ingredients are combined together.
There are distinct advantages to adopting the post-grind position for hammermills. Building layout is simplified, overall bulk ingredient storage capacity is reduced, and capital costs are thus minimized in new installations. There are some disadvantages, however, as millers are aware, particularly those who have been called out in the early hours of the morning when a hammermill has broken down and there has been no reserve of ground product for manufacture through the pellet mills while the hammermill is being repaired.
Essentially, hammermills rely on the impact of screens and beaters on the product being ground to reduce it to the desired granularity for incorporation into a balanced ration. Roller mills simply roll or crush product between two revolving cylinders. This latter process has the distinct advantage of requiring considerably less power, although it is not possible to achieve the fineness of final grind through a roller mill that can be achieved through a hammermill. In a hammermill, the screen-hole diameter controls the maximum finished particle size of any ground product. When using roller mills, there is no screen being used, and unless the product is sifted and the coarse fraction reprocessed, the resultant particle size is purely reliant on the millers skill in setting the roller mill effectively.
Roller mills, particularly single pass installations, require more care and attention than hammermills in order to achieve a consistent and accurate grind. Ensuring the feed is spread thinly across the face of the roller mills can present some problems as mechanical feed gates can easily become obstructed, impairing the smooth and regular flow of product into the nip of the roller mill. Variability of raw material also needs regular adjustments as opposed to the all-encompassing grinding nature of the hammermill.
The available capacity is also a major consideration when using roller mills as there is a need for machines of considerable size or number to achieve the similar capacity as that of hammermills in the same circumstances. There are other general considerations that may affect capacity such as the cleanliness of the grain and the presence of foreign objects that may restrict flow through the roller mill feed mechanisms.
However, there are some circumstances when roller mills have the edge and it is not completely desirable to reduce ingredients down to a very fine particle size. Ruminant animals prefer to consume flaked cereals, as do horses and outdoor pigs. In such instances, the roller mill comes very much to the fore, particularly where coarse or open rations are being produced and fed. In the case of beef lots, where the finished feed is not required to be pelleted for purposes of cost-effective transportation, the roller mill can be used quite effectively and can be a key part of reducing power consumption at the mill.
One advantage of using flaked cereals is that the ability to incorporate liquid ingredients into a ration is enhanced. The greater surface area presented by a flake allows for greater absorption of liquids. At the very least, it allows for coating of a greater surface area if absorption is not fully achievable with such ingredients as molasses and some fats and oils. In the brewing industry, a standard grist is required that has been proven to allow optimum application and absorption of enzymes into the mash stage of the process. This can only be achieved by the use of roller mills, often triple roller mills where product is ground twice to achieve the desired grist spectrum.
It should be stressed that the roller mill, when equipped with fluted or corrugated roll chills, can achieve a relatively fine grind, particularly when moisture content of cereals is optimized. The use of differential roll drive arrangements, which create a sheer effect between the chills, not only allows for a finer particle size output, but the sheering effect the roller mill has upon starch granules in cereals is advantageous to the nutritionist when compiling rations. This is especially true for young stock, such as baby piglets and veal calves, where the digestive tract is undeveloped and its sensitivity needs to be respected and treated kindly in early stage diets. The use of HTD belt drives to achieve differential roll speeds of up to 2.5:1 is now well proven, and as a result of such engineering technology there is little need for lubrication of the modern roller mill.
One of the biggest disadvantages of using roller mills is that when the roll chills become worn, replacing them with new chills and subsequently recorrugating the old chills is a major endeavor in terms of time and expense. The good news, however, is there are no screens that can burst or become damaged.
Another positive aspect of using roller mills is that they require little or no air flow to operate effectively due to the fact that, with the rollers being mounted horizontally, product passes through by gravity. Hammermills require a steady and balanced airflow in order to operate efficiently and to keep screens clear and unimpeded. The cost of moving that air, the capital cost of filters and fans, and the space requirement must all be borne in mind when drawing comparisons between grinding techniques.
Recent hammermill designs have been quite innovative, and we have seen the combination of roller mill and hammermill technologies begin to emerge. By using a roller mill, or adopting roller grinding principles as part of the feed mechanism on entry to the hammermill, the raw material is partially ground at that point, which then allows the hammers and screens in the grinding chamber of the hammermill to be fully effective, with often excellent grinding efficiency results.
Not only is a finer grind achievable with far less power consumption, but the control the miller has on the resultant particle size of the grinded ingredient is enhanced tremendously. By partial preparation of the product between the rollers in transit to the hammermill grinding chamber in such an arrangement, the best of both worlds is achieved.
As power consumption becomes increasingly important, you will likely see greater use of roller milling technology as part of overall grinding techniques. Rolls of up to eight inches in diameter are being adopted as feed mechanisms with differential drives and variable gap settings. Compared to conventional, straight forward hammermilling, these new hybrid arrangements can reduce power consumption by around 15%, which cannot be ignored in these stringent times.
The key to successful size reduction, however, is diligence and, as with all aspects of mill management, attention to detail is paramount. The daily walk around the mill, keenly observing minor daily changes in operations, will always prove to be the best defense against rising costs.
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: [email protected]?.
Hammer mill is the most widely used grinding mill and among the oldest. Hammer mills consist of a series of hammers (usually four or more) hinged on a central shaft and enclosed within a rigid metal case. It produces size reduction by impact.
The materials to be milled are struck by these rectangular pieces of hardened steel (ganged hammer) which rotates at high speed inside the chamber. These radically swinging hammers (from the rotating central shaft) move at a high angular velocity causing brittle fracture of the feed material.
The material is crushed or shattered by the repeated hammer impacts, collisions with the walls of the grinding chamber as well as particle-on-particles impacts. A screen is fitted at the bottom of the mill, which retains coarse materials while allowing the properly sized materials to pass as finished products.
The above subtype is based on the direction of the rotor (clockwise direction, anticlockwise directions or in both directions). Their working and grinding actions remain similar despite the fact that their construction differs in many respects.