The question at the heart of this article is, "How much energy should one expend in an attempt to fully deagglomerate laboratory samples prior to particle size analysis to find the 'ultimate' particle sizes in a distribution?"  In response to this question, the question I would then pose is, "Do you really want to know the 'ultimate' particle size of particles in your distribution?"  The answers and discussions applying to these questions are appropriate for most ceramic process engineers and anyone else who routinely performs particle size analysis.

Lab and Production Techniques Must Be Similar

As far back as I can remember, the late Jim Funk (who was Professor Emeritus of Ceramic Engineering at Alfred University) taught that lab and plant procedures MUST be identical for the two to correspond and for results to be meaningful.  I'm sure I told this story in the E-zine already but I'll repeat here:  One process engineer came to Jim and with excitement, said, "We use a kitchen blender to prepare ALL our lab samples!!"  Jim's response was, "Show me the kitchen blenders you use on the production bodies."  The point was that lab preparation techniques and plant preparation techniques MUST be totally similar for the lab procedures to produce results that are representative of plant procedures.

He and I worked with one or two companies who could develop bodies in the lab and duplicate them out on the plant floor, and vice versa.  How many of you can say that about your own lab and plant techniques?  It's more likely that you can make good products in the lab, but you struggle to duplicate those products out on the plant floor.  Or the opposite may be true -- you can make good product in large scale out on the plant floor, but you can't duplicate it in the lab.  Does that sound familiar?

The problem at work here is that the lab contains mixers, instruments, and other tools that are not completely similar to the corresponding mixers, instruments, and tools used in production.  Let me give you an example:  A commercial Hamilton Beach milkshake mixer running at medium or high speed with an approximately 1" diameter carbide blade produces high intensity dispersion (HID) conditions.  Milkshake mixers are easy to use and they make great mixers for use in ceramics labs.  Hang the mixing cup on the milkshake mixer automatically turns on the mixer.  Once the cup is in position, the operator can walk away while it's mixing.  You've all seen milkshake mixers in action, so you all know how they work.  (If you don't know how they work, you need to go to a gourmet ice cream store, order a milkshake, and watch them make it ... then enjoy!!)

OK -- now let's suppose you want to scale up the milkshake mixer results to production scale.  Down at school, we had a pilot scale HID mixer in the lab.  It worked well on a 5 gallon bucket of slip.  New ceramic engineering students used it to make slip in their first ceramic lab course.  The carbide mixing blade was approximately 4" in diameter, which produced an approximately 12" circumference on the blade.  Since HID conditions are defined as >5000ft/min tip speed, and 12" = 1', this blade had to run at about 5000 rpm to produce HID conditions.  The lab mixer really churned samples at 1000 rpm, but we would continue to raise the rpm and listen to, see, and feel the intensity increase until we reached ~5000 rpm.  It really ripped the slip apart at 5000 rpm -- sounded like the bucket would explode (which never happened), but that is the nature of HID.

I once toured another lab and saw a similarly sized HID mixer.  The engineers told me, "We tried HID, but it didn't work."  When I looked into their mixing chamber, the mixing blade and chamber capacity were similar in size to our lab HID mixer at school.  Then I asked how they run the mixer.  They told me that they routinely mix bodies at about 1000 rpm.  So they had an "HID mixer," but they ran it at intensities well below HID requirements, and they used those relatively mild, non-HID results to conclude that HID didn't work.  The samples they could make in a milkshake mixer couldn't be duplicated in a 5 gallon lab sample.  Why?  Mixing conditions were not similar.  For scale-up, mixing conditions MUST be similar.

The next question is: If HID conditions work in lab and pilot scale bodies, what should a company use out on the production floor to produce HID conditions?  The company we worked with in Colombia, South America, built production mixers (HID cisterns) into the floors of their plants.  These were octagonal cisterns, which were lined with heavy refractory brick to withstand the HID conditions.  Each held several cubic meters of slip.  Motor sizes were 100-150hp, and rpms were appropriate to produce 5000ft/min tip speeds with the carbide mixing blades used.

This company could develop bodies in the lab and then put them into production because they knew exactly how the various lab, pilot, and production equipment scaled.  They could duplicate conditions at any scale, so their lab samples and their production bodies were produced under similar conditions.  This is the type of correspondence everyone wants between lab, pilot, and production conditions.

Does Similarity Continue to Lab Samples?  Analysis Techniques MUST Show
           the True Characteristics of the Samples

          From the Point of View of Particle Size Analyzer Companies

This is where the question becomes muddy.  We'll start from the point of view of the particle size analyzer companies.  They are in business to measure particle sizes.  Their whole raison d'etre is to accurately measure the sizes of individual particles.  They have learned from experience that many particulate systems are agglomerated or flocculated.  As a result, they offer accessory devices (mixers, ultrasonic fingers, etc.) that can be routinely used in the preparation of samples prior to particle size analysis.  This is all good and proper from their point of view.  These accessory devices maximize probabilities that all particles will report as individuals in all size analysis samples.

          From the Point of View of the Production Company

The original question dealt with the ability to find the 'ultimate' particle size.  Knowing that their particles were agglomerated, the questioner wanted to know how much energy to put into the deagglomeration process so the 'ultimate' particle size could be measured.  This is where my question applies:  Do you really want to see 'ultimate' particle sizes?  If you have agglomerates or strong flocs, do you want to break them up during particle size analysis preparation procedures, or do you want to see what's actually present in the body?

          What Do You Really Want to Learn?

Following the recommendations of particle size analyzer companies for sample preparation techniques, the attempt will usually be made to completely deagglomerate samples so all particles report individually.  This is especially the case when someone phones an analyzer company rep and says, "My system contains many agglomerates.  How best do we deagglomerate so we can see the individual particles?"  In response to such questions, you may learn about HID, or high energy ultrasonic fingers (which are used for cell disruption in biological labs), or other methods to deagglomerate.  Instrument manufacturers' representatives don't necessarily know why you want to deagglomerate, but if you ask "How?", they should be able to answer your questions and point you to better methods to deagglomerate --- again, of course, if that's really what you want to do.

The question I'm asking here is:  Do you really want to deagglomerate to get at the 'ultimate' particle size? 

The answer to this goes back to the similarity condition discussed above.  In this case, 'similarity' refers not just to the similarity between samples (be they lab, pilot, or production samples) but also to similarity between these three categories of samples and the corresponding particle size analysis samples.  They all (i.e., all four of them -- lab, pilot, production, and analysis samples) need to be similar.

If you go to great trouble to produce similarity between lab, pilot, and production procedures, but then you totally change the nature of the samples before you send them to the particle size analyzer, the results once again will be meaningless.

An Example of This Problem

Consider calcined alumina powders.  The least expensive way to buy these powders is as relatively large agglomerates.  When you buy powders with average particle sizes of ~4 microns, they come as 100-200 micron diameter agglomerates.  Extra milling (available at extra cost) must be used to produce (i.e., to release them from the agglomerates) the individual deagglomerated particles of the ~4 micron size.  Either the supplier performs the extra milling (and the cost per ton rises) or the customer must mill the agglomerates (and the production cost rises).

          Do You Want All Individual Particles?

These powders (and many others) are chemically prepared.  In this example, the precipitates are then also calcined.  Chemical precipitation frequently produces very narrow distributions of 'ultimate' particle sizes, and calcination turns them into relatively hard agglomerates.  If all individual particles are approximately the same size, you probably won't want all individual particles.  Very narrow particle size distributions produce very poor, frequently dilatant, rheological suspension properties.  From the point of view of processing, very narrow particle size distributions are not desirable.  So it may be desirable to have a whole range of sizes of agglomerates -- ranging from the individual particle sizes up to a large fraction of the whole original agglomerate size.

          Do You Want the Agglomerates?

In this example, if body specifications call for average particle sizes of 4 microns, the agglomerates may not be desirable.  Normal processing will break up some (many?) agglomerates.  The types of processes used will control the extent of breakage of agglomerates during processing.  Blungers won't do much deagglomeration.  Extruders and muller-type mixers can cause agglomerates to be broken.  Ball milling will deagglomerate reasonably well -- the duration of ball milling will control the final size distribution ranges.  Which of these devices are used in your process?  How much effort is intentionally applied to deagglomeration?   Or are you simply processing the body and achieving an unknown level of  deagglomeration?

Deagglomeration and Particle Size Analysis

Here, we finally arrive at the real issue behind the original question.  If you have agglomerates in your production body, or in your pilot body, or in your lab samples, do you really want to destroy them after samples have been taken, but prior to performing particle size analysis?  I think not.

Excellent rheology can be achieved if agglomerates provide a wide range of particle sizes in a production body.  Excellent rheology can be achieved in the absence of agglomerates, when the whole particle size distribution of 'ultimate' particles covers a wide range of particle sizes.  Isn't the whole purpose of particle size analysis to show the particle size distribution extant in the production body?  Yes, it is! 

What particle sizes or agglomerate sizes are actually in your production body?  This is what you want to learn from particle size analysis.

The Answer to the Original Question

So part one of the answer to the original question is:  You want to provide enough energy so all 'particles' (be they individual 'ultimate' particles, or be they agglomerates of a variety of sizes) report individually during particle size analysis.  This means that sufficient energy must be provided to homogeneously mix the dilution water throughout suspensions so all 'particles' are well-separated from one another and homogeneously distributed throughout suspensions sent to particle size analyzers.  This means that sufficient energy must be provided to free all 'particles' from one another before they are sent to dry particle size analysis techniques. 

Part two of the answer is:  You don't want to provide too much energy to cause 'particles' in lab, pilot, or production samples to change significantly before particle size analysis.

When you perform particle size analysis, you want the results to show the sizes of particles present in lab, pilot, or production samples.  This requires that you toe a fine line.  This is a delicate balancing act to perform.  You must apply enough energy to disperse weak flocs so individual particles report individually, but you don't want to apply too much energy to deagglomerate particles that are not free in the body to report individually.

What is your goal?

There's no easy specific answer that applies to every case.  I could say that you should all apply a 300W ultrasonic finger for 30 minutes to all suspensions prior to particle size analysis, but that would be incorrect.  Each body and each production process is unique.  Each requires a unique answer.  This means each production engineer must decide for him- or her-self how much energy is enough, and how much energy is too much.

If it's necessary that you populate bodies with particles of 'ultimate' size, that needs to be done during body preparation processes.  If the lab sample shows that you need to fully deagglomerate and populate the body with particles of 'ultimate' size, then pilot-scale and production-scale processes must be similar to lab processes to produce similar particle size distributions.  If the HID milkshake mixer in the lab produces excellent body properties by eliminating all agglomerates, then similar HID processes must be applied to pilot-scale and production-scale processes.  If HID isn't used in the lab, it shouldn't be used in pilot or production processes.  The different scale processes MUST be similar.  If lab samples are only mixed with a spatula, then maybe the corresponding production bodies should only be mixed with a boat oar.  If that's deemed to be insufficient mixing intensity, then a more intense mixer should be found for use in the lab that will duplicate the mixing intensity in the plant.

Having gone to the trouble to make all of your processes similar, don't change the particle size distribution of samples before particle size analysis simply because you are following a 'traditional' method of sample preparation, or one recommended by the analyzer company, or a 'new' method which will better allow you to see 'ultimate' particle sizes.  Decide what you need to see;  use appropriate processing techniques to achieve that end;  and then use sufficient energy to disperse particle size analysis samples to show the sizes of the actual particles present during production.

Miscellany

Suggested topics .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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