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September - October 2013

Sieving technology in feed pellet production

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The appropriate feed processing technology and a proper set of feed processing equipment will offer you

assured feed products. Thus feed milling technology plays quite a very important role in feed production. It ensures the benign and steady development of the feed industry. Technology also provides a technique guarantee for converting animal nutrition research into high quality feed products and makes a great contribu-tion to improving feeding efficiency and the value of feedstuffs. Feed pellets, as a kind of palatable and nutritious feed, enjoy wide popularity in feeding poultry, livestock and aquatic animals. At present, more than half of the world’s feedstuff is processed by pelleting technology.

Feed pellet processing technology

Feed pellets have been defined as ‘agglomerated feeds formed by extruding individual ingredients or mixtures by com-pacting and forcing through die openings by any mechanical process’. Basically, the purpose of pelletizing is to process finely divided, sometimes dusty, unpalatable and difficult-to-handle feed materials and, by way of heat, moisture and pressure, into larger particles. These larger particles are easier to

handle, more palatable and usually nutritious enough to ensure animal healthy growth. For the pelletizing process technology, it has the following steps.

Grain material selecting and sieving

Grain materials are sieved to remove the big and magnetic impurities. The siev-ing machine structure should be capable of avoiding material blocking and ensure a consistent smooth flow. The mass flow concept features once the feeder opens, the mass material will flow down without block-ing, packing and will be graded automatically.

Grain grinding Grinding or particle-size reduction is a

major step in pelleted feed manufactur-ing. Grain grinding generally improves feed digestibility, acceptability, mixing properties, pellet quality, and increases the bulk density of some ingredients. According to research, feed pellet quality depends on the following factors: 40 percent diet formulation, 20 per-cent particle size, 20 percent conditioning, 15 percent die specifications, and 5 percent cooling and drying.

Apart from diet formulation, particle size is the first factor that will dictate about 20 percent of pellet quality and also has a significant effect on the other factors, such as decreasing the par-ticle size of ingredients results in a greater surface area per unit volume for absorption of condensing steam and increases the surface area available for bonding. Moreover, penetration of heat and moisture to the core of a particle can be achieved in a shorter amount of time with small particles and a large surface area per unit of weight.

But this is not a one-size-fits-all situation. The ideal finished particle size varies by the

grain being processed, and depends on the species, the life stage of the animal. Bearing all these in mind, select the proper method of grain particle size reduction is a critical decision.

Hammer mills for grinding and sieving

In general, both roller mills and ham-mer mills have been applied to the task of particle size reduction or grinding in feed pelletizing technology. The hammer mill has been traditionally used to produce the finer materials commonly used for pelleting, and for many mash feed applications. A hammer mill is essentially a steel box surrounding a rotor. Swinging hammers attached to the rotor, flail out when the rotor spins. A screen covers the discharge opening to regulate the finished particle size.

In the hammer mill, the motor drives the rotor to rotate at a high speed through the belt, and on the rotor there are series of ham-mers. When the materials get into the work-ing area of ham-mers, the

Sieving technology in feed pellet productionby Amanda Zhou, Allance Machinery, China

Table 1: Example particle size distribution

US sieve Grams Percentage

12 0.03 0.02

16 1.64 1.20

20 27.21 19.88

25 43.29 31.64

30 40.33 29.47

40 17.09 12.49

Pan 7.25 5.30

Total 136.84 100

1/8’’ grind milo based ration

Grain&feed millinG technoloGy34 | September - october 2013

FEATURE

rotating hammers with high rotation speed strike them over and over until they are small enough to fit through a screen. A large amount of air is also moves through the hammer mill and it drags the material small enough to make it through screen holes and become the final products; the larger prod-ucts are brought back to the crushing area by the hammers for being re-crushed until they reach the required size.

As materials move through the grinding chamber, they tend to approach hammer tip speed. Since reduction only occurs when significant energy is transferred from the hammer to the particle (large differ-ence in velocities), less grinding takes place when the particles approach hammer tip

speed. Many manufacturers incorporate devices within their mills to interrupt this product flow, allowing impact and reduc-tion to continue. Tear circle hammer mills have a more positive, natural redirection of product at the inlet than ‘full circle’ design machines.

Factors determining ground particle size: sieves

Precision and consistency of the finished particle size has a strong effect on the digestibility of feed pellets. A finished particle size that is either too large or too small will have adverse effects on quality of pellets and animal performance.

The primary factor determining finished parti-

cle size is the measurement of the perforations in the screen covering the discharge area. The size of these perforations is equal to the maximum finished particle size of the material processed.

Large particles will cause cracks and fractures in pellets. It is recommended that the hammer mill screens for grinding do not exceed 1/8” diameter. Better results can be achieved from use of a #7 (7/64”) screen.

Smaller particles will increase horsepower efficiency by increasing the throughput of material through the die holes with less horsepower. The finer grind also extends the die life as it decreases the ‘grinding’ or milling of material on the solid surface between the holes on the die face.

The ideal particle size range of 650 to

Table 2: Tip speed-feet/minute. Rotor diameter*RPM

Diameter 1200RPM 1800RPM 3600RPM

19’’ NA 7536 17898

22’’ NA 10362 20724

28’’ NA 13194

38’’ 11938 17670

42’’ 13194 19782

44’’ 13823 20724

54’’ 16964 25434

Grain&feed millinG technoloGy September - october 2013 | 35

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750 microns can easily and consistently be achieved through proper screen selection. In addition, screens can easily be changed to accommodate the processing goals of a variety of grains using the same hammer mill.

Machinery sifting is the main screen technology applied in animal feed machinery at present. In the present market, punching, woven screen and bar grizzly are widely used as screen plates. The key part of screening equipment which affects sieving efficiency is screen surface.

Factors determining ground particle size: tip speed

Tip speed, in addition to screen size, has a significant influence on ground and sifted particle size. High tip speeds (>18,000 Ft/min) will always grind finer and produce more fines than lower tip speeds. Low tip speeds (<13,000 Ft/min), on the other hand, produce a coarser granulation with fewer fines. As a rule, smaller screen hole sizes should be used with higher tip speeds, and larger screen hole sizes with lower tip speeds. Tip speed is sim-ply a factor of mill diameter and motor RPM and is not easily changed on direct coupled machines. There are a few V-belt drive ham-mer mills on the market today.

To produce a uniform granulation with few fines on materials like corn, wheat, grain sorghum, pelleted ingredients and solvent extracted meals, an intermediate tip speed is normally desirable. Hammer mills with a tip speed of 13,000-18,000 Ft/Min will produce a high quality finished product with excellent capacity and efficiency. 38" diameter mills with 1800 RPM motors (17,800 Ft/Min) and 44" mills with 1200 or 1500 RPM motors (13,500 or 17,250 Ft/Min) are both used extensively in the processing of all kinds of feed ingredients.

For finely ground products and tough to

grind materials like soybean hulls, mill feed, and mixtures with animal protein products, a higher tip speed is needed because more energy is required to grind these kinds of materials. Normal tip speeds for fine grinding and fibrous materials are obtained on 42" and 44" mills operating at 1800 RPM (19,500 and 20,000 Ft/Min), or 28" mills operating at 3000 RPM and 54" mills operating at 1500 RPM (21,000 Ft/Min). Recent developments in hammer mill grinding have included the use of 54’’ diameter mills operating at 1800 RPM. This very high tip speed (>25,000 Ft/Min) is particularly well suited to fine grinding at high capacities and high efficiency. Because a larger screen (hole) sizes can be used while maintaining the fineness of the grind, operat-ing costs are reduced as well.

MixingMixing or blending can be either a batch

process or a continuous process. Batch mixing can be done on an open flat surface with shovels or in containers shaped such as cylinders, half-cylinders, cones or twin-cones with fixed baffles or moving augers, spirals, or paddles. Continuous mixing proportions by weight or volume is a technique best suited for formula feeds with few ingredients and minimal changes. Different types of mix-ers, such as horizontal or vertical as well as special mixers like liquid mixers are used to achieve mixing.

Pelleting Mechanically, the pelleting process

involves forcing soft feed through holes in a metal ring die or flat die. The holes may be round or square, tapered or non-tapered. This is done either by using a locally fabricat-ed pelleting machine, which is operated by diesel engine, electricity powered machine or manually, or by using automatic highly sophisticated machines with hot air or water

conditioning. The machine consists of cylin-drical dies of different diameters to pelletize varying sizes of feed, which depends on the age, size and spices of animals involved. Most feeds produced are compressed pellets. Pelleted feeds have numerous advantages which include less feed wastage, uniform feed intake, and destruction of growth inhibi-tors.

The ring die feed pellet mill is equipped with one or more layer conditioner to added in the form of steam which softens the feed and partially gelatinizes the starch content of the ingredients, so as to process firmer (and for aquatic feed more stable) pellets. Generally speaking, the starch in grain mate-rials can be ripened about 60-80% by hot air in conditioner so that high starch or molas-ses content pelleted feed can be processed.

CoolingPellets from dry pellet machines may

exit at up to 88°C and 17-18% moisture. The temperature must be quickly reduced to ambient or less and the moisture level to 10-12% or less for proper storage and handling. Pellets must therefore be cooled and dried. Moist pellets, if they are going to be converted to dry pellets, also need drying although their temperature is not normally much elevated during manufacture.

Sifting Pellet sifting is the last process in pellet

manufacturing and this equipment should be located just above the bagging or final discharge bin. A rotary sieve, as the specially designed sieving device, is used to grade and screen feed pellets having undergone the secondary crushing in the large or medium-sized animal feed manufacturing plant. The undesired feed pellets can be removed from cooled pellets to get uniform finished pellets. In order to get perfect product, more and

more feed pellet plants choose proper sifters in their feed pelletizing process.

Conclusion Feed pellet production needs

not only a complete proper set of equipment but high efficient and cost-effective processing solutions. A finer grinded material or mixture will pro-duce a better quality pelleted feed at a lower cost both in terms of energy and maintenance. The fineness of the grind must be matched to both the particle size and capacity requirements of the entire feed manufacturing process.

More InforMatIon:Email: service@pellet-machine.netWebsite: www.pellet-machine.net

Grain&feed millinG technoloGy36 | September - october 2013

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