Feed Milling Processes (MIXING)

The objective of feed mixing is to start with a certain assortment of ingredients called a "formula", totalling some definite weight. This is processed so that each small unit of the whole, either a mouthful or a day's feeding, is the same proportion as the original formula. Mixing is recognized as an empirical unit operation, which means that it is more of an art than a science and must be learned by experience.
Feed mixing may include all possible combinations of solids and liquids. Within each ingredient are differences in physical properties. For solids there are differences in particle size, shape, density, electostatic charge, coefficient of friction as represented by the angle of repose, elasticity or resilience and, of course, colour, odour, and taste. For liquids there are differences in viscosity and density.
The term "mixed" can mean either blended, implying uniformity, or made up of dissimilar parts, implying scattering. As applied to formula feeds, the objective of mixing combines each of these definitions; i.e., the scattering of dissimilar parts into a blend. However, it is improbable that uniformity is attained with particles within a, sample arranged in some order of position or concentration. That is only a quality control; goal. It has been suggested that a proper title for a discussion of mixing should be "mixing and unmixing", for during the operation there is a constant tendency of particles which have been mixed to become separated. Three mechanisms are involved in the mixing process:
(a) the transfer of groups of adjacent particles from one location in the mass to another,
(b) diffusion distribution of particles over a freshly developed surface,
(c) shear slipping of particles between others in the mass.
In theory, the position of particles within a container is determined by chance, and the effects of chance accumulate until they outweigh the direct effects of mixing action. In the mixing of liquids, chance movement of components creates order or uniformity. With dry solids, chance distribution creates disorder. When disorder is at a more or less stable maximum, it may be called "random". Many factors in dry solids cause particles to avoid a chance or random arrangement. In fact, the result of mixing feed ingredients may be a definite pattern of particle segregation or non-random arrangement.
Particle segregation is due to differences in the physical properties of ingredients and the shape and surface characteristics of the mixer. Particle size may be the most important factor in causing segregation. An improvement in mixing which approaches random distribution of solids by decreasing particle size can be measured quantitatively by statistical methods. In general, the smaller and the more uniformally sized the ingredients are prepared, the more nearly they will approach random distribution during mixing.
In many formulae, a decrease in particle size is necessary to attain a sufficient number of particles of an essential additive (vitamin, mineral, medication) for dispersion in each daily feed unit. This may require the particle size to be the diameter of dust, 10 to 50 microns. Certain ingredients are unstable in finely divided form and likely to acquire an electrostatic charge. Concentration of particles on a charged surface, roughness of the mixed and stickiness of oily and wet ingredients are factors in causing segregation when very small particles are mixed and when these are much smaller than the bulk of other ingredients.
Mixing may be either a batch or a continuous process. Batch mixing can be done on an open flat surface with shovels or in containers shaped 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.

Horizontal Mixers

.1 Continuous ribbon mixers
The continuous or "twin-spiral" mixer consists of a horizontal, stationary, half-cylinder with revolving helical ribbons placed on a central shaft so as to move materials from one end to the other as the shaft and ribbon rotate inside (Figure 2). Capacity can be from a few litres to several cubic metres. The speed of shaft rotation will vary inversely as the circumference of the outer ribbon; usually optimum between 75-100 metres per minute. Since material travel is from one end to the other, either end may be used for discharge. These mixers may be inverted for cleaning.
.2 Non-continuous ribbon mixers
Non-continuous or interrupted ribbons are similar to the continuous ribbon mixers except that short sections called "paddles" or "ploughs" are spaced in a spiral round the mixer shaft. Action is different from that of continuous ribbon mixers, and may be more satisfactory for mixing liquids with dry solids. These mixers are made in a wide variety of sizes with travel of the outer diameter of paddles from 100 to 120 metres per minute.

Vertical Mixers

Vertical mixers may consist of a cylinder, cone, or hopper-shaped container, with a single or double screw (auger) located vertically through the centre (Figure 3). The screw operates at speeds of 100 to 200 rpm and vertically conveys incoming materials from the bottom (generally the intake) end, like a screw conveyor, to the top where they are scattered and fall by gravity. This sequence is repeated several times until a blend is attained (usually from 10 to 12 minutes). These mixers may also be loaded from the top. Results show that vertical mixers are not efficient for uniform mixing of solids and liquids or for materials of quite different particle size or density. This unit is difficult to clean and there may be inter-batch contamination.

Other Types of Mixers

A third type of mixer is the horizontal revolving drum. This can be a straight-sided cylinder or a cylinder tapered at each end. The sides may be smooth or fixed with baffles or shelves to pick up and drop ingredients. Smooth, dry materials of uniform physical properties are blended best in this type of mixer.
A modification of this type is the turbine mixer which is a fixed cylinder with revolving shaft to which are fixed paddles, ploughs, scrapers, or shelves designed to re-pile materials continually. This mixer is often used as a cooker to dry fish wastes and to blend various types of fish meal into a standardized product. They are also particularly efficient for mixing heavy ingredients and for adding liquids to mixtures which would clump or cake in another type of mixer. Some particle size reduction (grinding) may occur on soft materials, such as rice bran and alfalfa leaves. A complete mixing can usually be attained in 3 to 6 minutes unless longer time is necessary to eliminate lumps caused by added liquids. Mixer shaft rotation is regulated to provide some centrifugal action, but this must not be excessive.
The "Nauta" mixer originated in Holland and is constructed in the form of an inverted cone with a mixing screw inside rotating around the inside wall. The mixer is made in a variety of sizes from laboratory models, for premixing chemical and vitamin additives, to very large production sizes. It is excellent for premixing trace elements and works very well for adding moderate amounts of liquids into dry ingredients.
Another type of mixer called the "entoleter" consists of a high-speed rotating disc which throws the ingredient charge with considerable force against the walls of a chamber. This mixer functions well to smooth out clumps or balls of compacted ingredients and will cause eggs of grain weevils to become inactive. However, since it may shatter vitamin A beadlets encapsulated in gelatine, it is not recommended for all mixtures.

Liquid Mixers

Oils and water-miscible oil preparations are often added to dry ingredients as a source of energy or as a specific nutrient. Although the oil-soluble vitamins. A, D, E, and K, are available in dry carrier concentrates, they may be obtained in pure form and premixed by the feed manufacturer. Liquids containing nutrients can be mixed faster and with more uniformity than the same nutrient in dry concentrate condition. Therefore, a liquid blender may be needed in the feed plant.
Liquid blenders usually consist of a horizontal tub or cylinder with a number of wires or paddles equally spaced around a shaft which revolves inside. Sometimes the shaft is hollow and liquids are forced through holes in the paddles in a spray effect. Some models have a shaft speed of 400 to 600 rpm while others rotate at 1 200 rpm. Ingredients such as condensed fish or fermentation solubles, molasses, or fish oils are often premixed in a bowl type variable speed mixer, blending the liquid with dry ingredients.

Mixing Operation and Evaluation

Accurate mixing requires the addition of ingredients in a tested sequence from batch to batch. The usual practice is to add large-volume ingredients first, then those of smaller amount. Unless already premixed, liquids should be added after all dry ingredients have been mixed. Total mixing time is critical and is influenced by the composition of the formula. All mixers should be calibrated by laboratory recovery of known additives (physically or chemically) so that under and over mixing does not occur. Uniformly sized salt, graphite, or iron particles coated with water soluble dyes are often used as "tracers". Each mixer should be calibrated for its mixing time and capacity by volume for best results.