Hammer & pellet mills
September 01, 1999
by Teresa Acklin
By David Sugden
To the uninitiated, the features, capabilities and uses of hammer and pellet mills are almost entirely opposite.
The definition of a mill, in most dictionaries, is a machine that grinds material into smaller particle size. A hammer mill falls into this category. A pellet mill, on the other hand, agglomerates or amalgamates a series of fine particles into a (larger) pellet. This machine is also known as a pellet press, for that is precisely its function — to press smaller particles together to form a larger one.
The hammer mill is used in a number of industries such as animal feed, brewing and cereal milling. The latter includes flour milling, where it is used to either reduce bran to a finer particle size or screenings/dockage. The typical reason for doing this is to grind the parent material into a state suitable for addition to straight grade millfeed.
Although its use in a flour mill is not widespread, a hammer mill can be found in place of a roller mill passage at the head end of the reduction system. The reason for the unpopularity of the hammer mill in this role is the high degree of severity leading to an inseparable high mineral content of ground product. Fixed pin flake disrupters are less harsh. So, the use of a hammer mill is a reduction in particle size by means of impact by force due to the action of grinding against a screen until small enough to pass through.
Few adjustments are needed in a hammer mill besides speed and screen size. The effect on the particle size range that can be manipulated is not all that great compared to a roller mill.
Energy consumption in a hammer mill is high — up to twice that of a roller mill at a given capacity. However, the capital cost, when comparing like for like, is lower.
Noise is a problem, as is the risk of dust explosion. Dust explosion protection can, and should be, further enhanced by the efficient adjustment of destoners in the wheat cleaning section plus suitable explosion vents and even inert gas suppressors. Consequently, for these reasons, hammer mills are very often encased in a separate room.
In many configurations, the machine is designed to rotate in either direction by means of a suitable control gear, which ensures even wear. With some models it is possible to change the screen while the motor is running. With dust explosion sparks as well as damage, a magnet preceding the main machine is good practice.
This machine type will only process dry as opposed to wet matter. It requires a relatively coarse feedstock and is inefficient with fine.
Maintenance costs are fairly high due to the wear factor on the hammers (rotor) and on the screen (stator). The hammers are called such because a single pin attaches them to the rotating disc, providing a hammer-like action. Other similar mills have fixed beaters instead of hammers, but have lower capacities.
Heat generation can be a problem. Excess heat can reduce capacity by up to 50% at higher than 45oC (112oF). Moisture loss in a single pass is high — up to 1% or more. Increased moisture content of the feed stock means decreased capacity. The more sophisticated units benefit from air assistance, allowing both cooler ground material as well as greater capacity by keeping the screen clear and clean.
Rotor speeds are generally between 1,500 rpm and 3,000 rpm. Common screen sizes vary between one and four millimeters; the smaller the screen, the lower the capacity and higher the power consumption.
A 3mm screen will provide an "open" area of about 40%. Discharge of ground product can be directed to a bin underneath, to a hopper with a screw discharge or by pneumatic conveying. Controls include a compensating link between the full load amps current of the motor drive and the hammer mill feed mechanism to ensure efficient processing.
Machine sizes vary from the laboratory scale up to well over 100 kilowatt motors handling 15 tonnes per hour of grain. Bran, being both light in bulk density and fibrous, will need a higher energy per tonne rating.
The character and capability of the hammer mill clearly has its place in flour milling but to a limited degree. The hammer mill is widely used in animal feed production and is increasing used in brewing by reducing malted barley in certain processes in place of roller milling.
The pellet mill, or press, has different characteristics and purposes but processing animal feed is its most common industrial use.
The pellet mill is capable, when employing different die sizes, of making pellets between 6 millimeters to 12mm in diameter with lengths of up to two and a half to three times the diameter, as well as much larger cubes or pencils. All kinds of raw materials are processed on pellet mills, including meals, cereals and legumes. The pellet mill is also found in the pharmaceutical industry.
Increasing the size of millfeed, for example, enhances bulk density. Loose millfeed may have a bulk density of 0.35 tonnes per cubic meter. This increases to around 0.7 tonnes after pelleting, which is almost as heavy as wheat and therefore capable of being stored in bulk in bins designed for grain. Pellets are conveyed on equipment suitable for wheat, such as chain and band conveyors and bucket elevators.
The machine can increase, by added value, reclaimed fine material. When preconditioners are used in advance, it is possible to incorporate additives such as fats, molasses, hardeners and steam, preferably dry steam.
Preconditioning usually means the addition of steam in flour milling. This has three advantages: greater capacity, increased digestibility and sterilization, and increased moisture.
Greater capacity results by the cooking or gelatinization of starch, aiding agglomeration of particles. There is always starch in millfeed from even the best flour mills.
The problem with increased moisture is that poorly adjusted equipment can lead to mold development. Further, millfeed pelleting may not be appropriate in hot, humid climates even where a market is available. In any event, the press exists to bind dissimilar materials. Oil from wheat germ is particularly useful.
Other characteristics of the pellet mill include the potential to compress and extrude fine materials from less than 100 microns to greater than 18mm. Raw material particle size can be from less than 50 microns to more than 1,000 microns.
Noise is a factor to a limited extent, but certainly nothing like that of a hammer mill. Additionally, the likelihood of a dust explosion when using a pellet mill is comparatively low.
Capital, energy and maintenance costs are not cheap. Claw back can be gained by selling on a delivered, rather than ex, mill basis solely because of increased bulk density. Pellet mills operate by feeding raw material into the pellet mill from the top to the center from a suitable raw material mechanism or preconditioner. The pellet die rotates, which is secured by bolts and is driven from a main drive shaft. Two adjustable rollers also rotate and compress the feedstock against the pellet die. Pellets emerge from the outside and are broken off by simple static knives. Discharge of pellets by gravity takes place at the outlet.
All pellet mills operate in the same way and many are automatically lubricated. These machines need readjustment of rollers every few hours, depending on the application. In case of the presence of foreign matter, a shear pin protects the unit. Both dies and rollers can be quickly changed.
Energy consumption for millfeed is on the order of 15 to 20 kilowats per tonne processed. As with the hammer mill, most pellet mills are equipped with automatic constant load controls.
While just about all applications need a following cooler (usually by ambient air) and sieve to recycle fines, it is possible to pellet millfeed with water as opposed to steam preconditioning. Steam preconditioning decreases capacity about 30%. Conversely, the main drive motor would need to be increased to achieve a similar capacity.
The local market will indicate to the flour miller whether pelleting is a consideration.
David Sugden, independent consultant to the grain industries, may be reached at the Coach House, Killigrews, Margaretting, Ingatestone, Essex CM4 0EZ, U.K. 44-1245-352048, or fax: 44-1245-251162.