A Perspective On Mixing And Mix Uniformity: Part 4 - Problem Ingredients, Discharge And Cleanout Of

Problem Ingredients

The physical properties of ingredients can affect mixing. If all of the physical properties are relatively the same, then mixing becomes fairly simple. As the physical characteristics of ingredients begin to vary widely, blending and segregation problems are compounded. Some of these factors are: 1. Particle size
2. Particle shape
3. Density, or specific weight
4. Hygroscopicity
5. Static charge
6. Adhesiveness

Of those listed, particle size, shape, and density are the most important. Large and small particles do not mix well. They will mix better if there are particles of varying size in between. High density particles, such as minerals, tend to segregate and go to the bottom of the bin. An illustration would be the blending of cracked corn and fine mixing salt. The corn may have a particle size of 1200-1500 microns (m) and a density of 1.35 g/cc while the salt might have a particle size in the range of 200 m with a density of 2.6 g/cc. While it is not altogether impossible to obtain and maintain a satisfactory blend of these two ingredients, it is difficult.

Hygroscopicity refers to the ability to attract or take-up water. A very hygroscopic material, such as urea, may absorb enough water from the atmosphere to cause caking. Some ingredients, particularly vitamins and drugs, may possess a large static charge making them tend to stick to bin walls and sides of mixers. As the number of ingredients and variations increases, a greater appreciation of the complexity of the mixing operation is formed.

One of the places we can control ingredient variation is ahead of the mixing operation. Are the grinders, either roller mill or hammermill, producing the desired particle sizes? Is the particle size from the grinders ever measured? We may also need to measure the particle size of ingredients that are normally utilized as received. In some cases they may be contributing to poor mixing.

Discharge And Cleanout Of Mixers

There are obvious advantages to the drop-bottom type horizontal mixer in terms of discharge and cleanout. Discharge is nearly instantaneous with the lighter material being discharged within two seconds of the heavier material. There is essentially no carryover from one batch to the next, especially if equipped with air sweep systems.

Liquid Addition

The incorporation of fats, oils, molasses, and other liquids into the vertical mixer is being done today. Many fats and oils are hand-dumped along with the feed ingredients into the dump-in hopper at the base of these mixers. A better way to introduce fats and oils would be through a spray bar installed in the top of the mixer and the liquid metered thereto. The mixes incorporating liquids should be mixed longer than the dry feeds to ensure uniformity and to break up fat or molasses balls that form. To facilitate better mixing and breaking up, a high speed paddle type mixer can be installed in one of the discharge parts and the feed re-elevated to the top of the mixer.

The incorporation of molasses into a vertical mixer is not recommended above 3% if only one mixer is used for all feeds. Even this percentage will require more cleanup maintenance. This requires the installation of access panels in the tub and into the elevating tube to permit cleaning. Molasses builds up on the interior of the tub as well as on the mixing flights and will seriously reduce mixer efficiency. If not kept clean, rejections by the state feed inspection authorities and customer complaints will occur. Teflon lining of the tub and stainless steel screws will reduce maintenance when molasses is being added to the feed mix.

When higher percentages of molasses (3-8%) are going to be mixed continuously in a mixer, then two mixers should be used; one for feed requiring molasses addition and the other for dry feeds.

Liquid addition is best accomplished at the custom mix plant by working on the batch principle. Metering devices can be preset for automatically delivering exact quantities to the mix. The mixers should then operate long enough to properly incorporate or blend the total batch. Attempts to use a continuous, auxiliary mixer for molasses while discharging from the batch mixer can result in headaches. This procedure must be planned very carefully for good results and conditions must be ideal. Getting those conditions ideal is difficult and, for a quantity of molasses (ordered by a customer), the feed man may experience variations in the percentages of molasses applied to the feed as well as a shortage of molasses at the beginning and at the end of the run. If his customers are in the habit of specifying a certain amount, then the feed man should be aware of the continuous system or he may lose customers or give molasses away, thus cutting into his operating profits.

Liquids can be handled equally well in drop-bottom or nondrop-bottom horizontal mixers. About 10% of added liquids are normally considered in the top range of practical use. More than this amount begins to add problems in other areas such as your materials handling equipment. Adding liquids at the batch mixer is a common practice in many feed manufacturing operations. Adding liquids at this point can add to the contamination problem whether they be a drop-bottom or standard discharge type mixer. The addition of liquids (other than molasses) slightly increases horsepower requirements. Regulatory requirements may create a new concept in importance of where and when to add liquids. Where conflicting drug additives follow each other in mixing sequences, contamination is a major problem.