Remember Rubik’s Cubes?  Mechanical 3-D puzzles invented in 1974, the toys were multi-colored mosaic of squares.  “All mixed-up” can mean a person is confused or perplexed, and that certainly applies when trying to solve this puzzle.  The goal of the Rubik’s Cube is to start with a randomly mixed-up mosaic and work it until all the sides show uniformly colored faces.

Your goal in feed mixing is just the opposite.  You start with distinctly separated ingredients and work to combine them so each one is completely dispersed and intermingled among all the others.  Getting “all mixed-up” can be a good thing in the feed business.

Uniform, properly mixed feeds provide the most accurate nutrition for animals and help ensure every customer gets what they pay for.  Feed uniformity, sometimes called “mixing efficiency” is important to satisfy customers who judge their suppliers on quality and consistency.

New world technology meets old-style craftsmanship

ASP’s Tech Reps help customers apply and fine-tune various aspects of feed manufacturing technology.  Think of us as your “Quality Cohorts.”  Helping you test and validate mixer uniformity accomplishes several goals, among them:

• Optimize your mix time.  This requires testing feed that has been mixed for different lengths of time, using a tracer or marker to assess internal variation within each mix.

• Formally validate and document your process controls.  This provides proof that your equipment is capable of producing uniform, properly blended feed complying with FDA regulations in 21 CFR 225.130 (for non-licensed mills) or 21 CFR 225.30 (for FDA licensed plants).

Newly-engineered mixer designs are tested to validate their uniformity before production.  Mark Hayes of Hayes and Stolz Industrial Manufacturing Company describes their process: A prototype mixer is filled with ground corn to its rated capacity.  One percent salt is hand- added in the corner of the mixer and the mixing is timed.  Technicians stop the mix every 30 seconds and pull 10 samples from set locations.  Salt migration throughout the mix is tracked to

validate mixing patterns, and the mixers must meet a final uniformity standard before the design is certified.  For example, single-shaft ribbon mixers must achieve 10% or less variation within two minutes.  Hayes’ standard for twin-shaft mixers shortens to 1 minute.

Not every mixer that comes off the production line is tested, so it is important that a new mixer be validated after installation.  Operating mixers should also be checked following repair and validated at least quarterly.  Calling us at 936-560-0003 is the first step.

Preparing for the test

Select the feed mix you plan to use for the validations.  It should be a formula with a representative particle size and ingredient makeup.  Depending on your objectives, you may need to produce several batches of this formula to complete the test.

Coordinate with ASP to determine the most appropriate marker for your test.  Salt is commonly used, but has one significant drawback; other ingredients containing chloride will interfere with the accuracy of the test.   Discuss marker options with us if the feed contains potassium chloride (Dyna-K), calcium chloride, lysine monohydrochloride etc.  In these cases we may consider alternate markers such as dye-impregnated ferrous or vegetable-based particles or selected micronutrients.

Plan to add the marker at a concentration that is similar to your hand-adds, ordinarily 1-5 pounds per ton but usually not more than 10 lb per ton.

Prepare to take large samples of the finished feed, from 2-5 lb each.  Larger sample sizes reduce sampling error, and ASP can provide large sample bags for your test.  Pre-labeling the bags helps the test stay organized and flowing smoothly.  Label the sample bags with the length of the batch mix time, sample number within the batch and marker concentration used.

Mixing options for test batches

Depending on your mixer setup and goals, you may be mixing either one batch or several batches.  For every batch in the test, fill the mixer to capacity.  Never conduct a test with less than half the mixer filled and never over-fill the mixer.  You should be able to see several inches of ribbon or paddles extending above the feed ingredients when properly filled.

Weigh and add the correct amount of the marker you have chosen for the batch size, and continue the test using one of the two strategy options below. 

Option 1- Testing a single batch with multiple mix times

You can use a single batch of feed to test multiple mix times if you have safe access to the entire top of the mixer.  After adding all ingredients including tracer, mix the feed for the shortest time you feel is reasonable, for example 3 minutes.  Stop the mixer, lock it out and tag it for safety.  Probe 10 different locations from the top of the mix, collecting 2-5 pounds per sample.  Bag the samples and number them 1-10.  The sample numbering should be organized in a set pattern, with each number consistently corresponding to a specific location in the mixer (see figure 1).  The sample number may help you find the location of any un-mixed or “dead” zones after the samples are analyzed.

Restart the mixer and run it for another increment of time, for example add 3 more minutes for a total of 6 minutes, then repeat the sampling.  Continue adding mix time to the same batch until you reach your longest reasonable duration.  In our example you may have accumulated samples at 3, 6, 9 and 12 minutes, with 10 numbered samples for each of the 4 mixer times.

Option 2- Testing multiple mix times in different batches

You may not have full access to the top of a mix, or you may prefer not to test a single batch.  If this is the case you can test several batches, using a different mix time for each batch.  Your plan is to take multiple samples from each batch as the feed stream exits the mixer, or as close to the mixer as safe and practical.  Stay safe; do not expose yourself or others to operating equipment.

Fully load the mixer and add the selected marker.  Run the mixer for the longest mix time you feel is practical, in our example 12 minutes.  Stop the mixer and let it discharge, taking 10 samples each weighing 2-5 lb, evenly distributed from the beginning to the end.   Label each sample with the length of the mix time used and the order the samples were taken in. 

Reload the mixer with the same amount of the same formula, including the marker.  Run the mixer for the next shorter mix time, in our example 9 minutes.  Stop the mixer and repeat the sampling process during discharge.  Continue to shorten the mixing times until the shortest practical time is reached.

Analyzing samples and results

Send the labeled samples to ASP, making sure each sample’s label contains its batch mix time, sample number within the batch and concentration of the marker added.  ASP will test the samples, calculate the uniformity and report the “coefficient of variation” or “CV” for each batch or mix time.  The total CV indicates the percentage of variation that exists and contains all the variation from mixing, sampling and analytical processes.  A CV of 10% or less in a batch of feed is considered acceptable, indicating an adequately uniform mix.  Under certain circumstances, such as feeding animals with small intakes, lower CV recommendations have been made.  Selecting the shortest mix time that produces an acceptably low CV of 10% or less is generally most efficient.

When CV is above 10%, it can be caused by a combination of factors.  The most common factor is mixing equipment that is worn or out of adjustment.  Figure 2 compares the CV at different mixing times for a new and worn ribbon mixer.  Note that the new ribbon achieved a CV of 10% or less after 4 minutes, whereas the worn ribbon could not reach an acceptably low variation after 10 minutes.

In addition to sampling and lab variation, other sources of variation abound.  Among them are particle shape, static charge, ingredient sequencing, mixer design, over-filling or under-filling, slow RPM, fat build-up and micronutrient sifting.  Some suggest that “over-mixing” can occur if the feed is mixed too long.  Potential “un-mixing” does not happen in the mixer, but can result when fine micro-ingredients sift out of feed, separating from the larger particles.  Sifting can happen in handling systems, on trucks or even within bags.  This can be addressed by managing ingredients and particle sizes.

Maintaining uniform feed mixtures improves animal nutrition and ensures each of your customers gets what they paid for. Mixer validations are essential to help you produce the quality and consistency your customers want and FDA requires. Calling your ASP Technical Representatives at 936-560-0003 is the first step. We will be delighted to help you define your mixer’s optimum mix time and provide ongoing documentation to monitor your equipment’s performance through timely mixer validation tests.