Before being used in feed formulas, the dry ingredients need to be of similar particle size and distribution to blend and stay properly distributed in the formulation when mixed or further processed such as pelleting.
Some ingredients normally are received in ready-to-use form and particle size. This includes minerals, soybean meal, cottonseed meal and others. Other ingredients need additional preparation before they are ready for use. This is generally processing to obtain the necessary particle size for blending and mixing. This may include one or more of the following processes: cracking, grinding, steam rolling or steam flaking. In this article, we will look at the equipment used for this processing.
The most used ingredients in full feed formulas are grains. They often make up 65% to 70% of the formula. Corn is by far used the most, but other grains such as wheat, sorghum, barley and others are used or substituted for corn based on cost and/or availability of the grains. These grains are purchased and arrive in their natural kernel size and need to be ground to smaller particles to be used in most formulas. Other ingredients such as beet pulp pellets or cottonseed pellets, which are residues left from the extraction of oil from cottonseed and starches from sugar production from the sugar beets, are used and must be ground to use.
Particle size reduction is normally accomplished by grinding the whole kernels or pellets using hammermills or roller mills. Each of these pieces of equipment has its own grinding characteristics. Hammermill grinding to a specified mean particle size produces a wide range of small to large size particles. Roller mills grinding to the same mean particle size have a much smaller range of particle sizes and fewer small and large particles. All particle sizing equipment must have magnets in the product stream before entering the sizing equipment. Foreign materials, such as stones, etc., should be screened out before the product enters the sizing equipment.
The hammer mill consists of hammers mounted on a spinning shaft. Whole grain or product is introduced into the hammer chamber. These hammers are surrounded by a perforated screen through which the ground product exits the grinding chamber. A hammermill breaks up the product into smaller particles through the impact of hammers shattering the product until the size is small enough to exit through the perforated screen surrounding the grinding chamber. Each time a particle is hit, it shatters into more fine particles. Hammermills are used when grinding grain or other products into a particle size that can be combined with other ingredients and further processed into pelleted products. They also can be used to size grain for mash (meal) products where the presence of fine particles is not a problem. Ground particle sizes must be similar to the particle size of other ingredients to ensure a uniform product is added at mixing and remains that way after mixing.
The hammermill system’s capacity may be increased 15% or more by adding an air assist system as shown in Figure 1 (above). When grinding through 5/32 (10/64) diameter or smaller screen perforation size, the particle size range will be reduced as the air helps pull the product through the screen.
The air assist system also reduces the amount of heat generated in the grinding process. The amount of air needed in the system is 1.25 to 1.50 CFM (cubic feet per minute) per square inch of screen area in the mill. The ground product and air are discharged into a large plenum where the air velocity drops to a point that the ground product falls out of the air stream and into a gathering screw conveyor at the bottom of the plenum. The air then passes through a baghouse filter and is discharged back into the atmosphere.
A roller mill uses a pair of rolls mounted horizontally through which the grain or product is passed. The mill may have one pair of rolls to as many as four pairs of rolls. The surface of each roll has teeth and grooves known as corrugations to do the grinding. Roller mills are used where particle size control is needed. Each set of rolls may be designed to crack, crimp, flake or grind products.
Fibrous grains and products do not process well in roller mills.
A set of rolls may be designed so that both rolls turn the same speed, which is a ratio of 1:1 of the roll speeds. These rolls may be used to crush, crimp, roll or flake grains. Other sets of rolls have one roll turning faster than the other. As an example, the fast roll may turn at 150% of the slow speed roll. In this case, the speed differential between the fast roll and the slow roll is 1.5:1.
A single pair of rolls are used in cracking, crimping, rolling and flaking operations. For cracking, course particle sizing, the roll surfaces are built with coarse corrugations such as four corrugations per inch of surface. The cracking rolls may have a speed differential of 1:1, which makes the rolls do a crushing action. If a speed differential is used on the rolls, a cutting action occurs.
If the grain is just to be cracked for use in scratch feed for poultry or for direct feed into a roughage mix for cattle, it is normally passed through a single set of rolls. The particle size varies between 1,500 and 1,800 microns with large particles and few fines. The roll surfaces are corrugated with coarse corrugations and both rolls turn the same speed.
Crimping occurs when the grain is softened by adding steam before passing through rolls having coarse corrugations. Both rolls turn the same speed (1:1). The amount of gap between the roll surfaces determines the degree of crimping that occurs. Crimping is used on grains such as wheat, oats and barley to open the kernel shell and expose the endosperm in the grain. For flakes, the roll surfaces have shallow corrugations and the rolls have a large diameter (24 to 36 inches). The grain is injected with steam and held for an hour, passing through a steam chest above the flaking rolls. The gap between the rolls is very small to squeeze the grain into flakes. The degree of flaking (flattening) is determined by the pressure put on the grain as it passes through the rolls. In cattle feed yards using flaked grains, a good flake will weigh about 26 to 27 pounds per bushel. That converts to about 20 pounds per cubic foot. These flakes are wet and either dried for future use or put into live bottom bins for short-time storage. Wet flakes need to be used within a few hours. Flaking helps with gelatinization and the feed is easier to digest.
When grinding for a given particle size range with roller mills, more than one set of rolls are used. This is done by stacking sets of rolls in 2-high or 3-high configurations. Starting with the top set of rolls, the corrugations become smaller in each succeeding roll set. Using a setup like this, the quantity of fine particles produced can be limited to less than that of a hammermill-ground grain. Pelleted feed has few fines, but if a non-pelleted meal is made to feed swine, fines must be limited as they can cause ulcers and respiratory problems.
Selection of particle sizing equipment must be based on the type of products to be made. Capacity should be a minimum of 10% more than the amount in a formula and the facility production rate.