Volume 6  Number 4                            Dennis R. Dinger                                1 February 2008

Updates

Short Courses in June 2008????

It has been several years since I have offered short courses on ceramic processing topics.  Is there any interest among readers of this e-zine for me to offer 3- or 4-day Predictive Process Control courses, or a 1-day course on Rheology, or a 1-day course on Add-in Functions and Ceramic Calculations this summer (June, 2008????) here in Clemson, SC, or at some other location?  If interested, send an e-mail with feedback:  QuestionsandComments@DingerCeramics.com  .

The E-zine

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"... for Ceramists" Series Books

          Requests for Multiple Copies

I have had several recent inquiries about the purchase of multiple copies of these books.  Here are my two suggestions:  

          (1)  If you purchase downloadable versions, purchase the required number of copies (please be honest about the number) from the Books and Downloads page of this website.  Then download a single copy and distribute it (or print it and distribute it) to the people for whom you purchased the copies. ... or ... 

          (2) Purchase the required number of paperback copies from the Books and Downloads page of this websiteand distribute them to your people.  My books are priced $19.95, $24.95, and $29.95 with this in mind.  You won't find many other good ceramics books in this price range.  Most others start at $80 to $100 each and prices rise from there.  For example, our PPC book (when it was available) was $195 per copy.  (I had no input when that price was set.  During one phone conversation, after they made sure I was sitting down, they simply told me the price.)          

          Spanish Language Books

For those of you who speak Spanish as your primary language, a downloadable PDF version of Rheology for Ceramists in Spanish is currently in progress.  Reología para Ceramistas is currently being edited to be made available as soon as possible.  Best estimate at this time is that it will be available sometime in 2008 because the editing process is proceeding slowly.  The PDF file will be set up so it can be printed on your printer if you prefer a hard copy.  Depending on the reception this version receives, I will then consider translating the Particle Calculations book as well.  I will also then consider translating it into Portuguese.  Any thoughts, comments, and/or suggestions will be appreciated.

          English Language Books

The paperback version of Characterization Techniques for Ceramists is available on the Books and Downloads page at the web site!    Retail price is $29.95 plus shipping and handling. The book has 256 pages and it covers 34 different characterization techniques that are commonly used by ceramists.  Purchase a copy NOW!

The book sets on the web site have also been revised to include this new book.  A 3-book set of paperbacks, including one each of Particle Calculations for Ceramists, Rheology for Ceramists, and Characterization Techniques for Ceramists, is now available for $64.85 plus shipping and handling.  This is a $10 saving off the total retail price of the 3 paperback books.  A 3-book set of downloads is also available for $52.85.  This, too, represents a $10 saving off the total retail price of the 3 downloadable books.  

The E-Book version of Characterization Techniques for Ceramists is available for downloading at the Books and Downloads page of the website for $24.95.  The download is a 2.889 Mb self-extracting Zip® file for the Windows® environment which unzips to the 2.998 Mb book in PDF file format.  Those of you who order the downloadable book will want to know that the PDF book is formatted to print on 5.5" X 8.5" paper (i.e., 8.5" X 11" sheets cut in half.)

The other two books, Rheology for Ceramists and Particle Calculations for Ceramists, continue to be available for purchase as downloadable E-books and as paperback books at the Books and Downloads page of the web site.

The following articles were requested concerning mixing.  This article will address more of the questions submitted.

Three Major Phenomena Required for Excellent Mixing:  2 -- Impacts

Introduction

The second major phenomenon required for excellent mixing in suspensions is IMPACTS.  The only possibility for impacts in the mixing of simple fluids is between the mixing blade and the fluid and maybe also between the moving fluid and any stationary vanes that may be in the tank to prevent rotation of the whole fluid mass.  We do not usually regard collisions between fluid molecules, however, as "impacts."  Even the interactions between impellors and stationary vanes and the fluid mass are not regarded as "impacts."  Some molecules will remain stationary relative to impellor and vane surfaces, and other molecules will form smooth flow patterns around those boundary layer molecules.  

Impacts, therefore, are usually considered to be a phenomenon that is unique to the mixing of particulate suspensions.  Impact is the major phenomenon that is overlooked when designing suspension pumping, flow, and mixing systems.  Since impacts distinguish suspensions from simple fluids, they MUST be considered.  This article will address this phenomenon.

Impacts Transfer Momentum

          Momentum Transfer

One of our computer models which attempted to calculate and predict rheological properties in suspensions used the "billiard ball approach."  At the time, we were heavily criticized for this approach by the chemical engineering community.  The majority of their computer models did not allow particles to collide.  There was no code in their models to handle particle collisions, so their models simulated suspensions at relatively low solids contents under low shear conditions.  Their models predicted ONLY shear-thinning (pseudoplastic) behaviors, but NOT dilatancy.

Our model allowed the particles (the billiard balls) to collide both with each other and with the walls of the chamber.  In fact, we essentially ignored the the carrier fluid.  Actually, we allowed the fluid viscosity to be a drag (a hindrance) to the movement of each of the particles -- but that was it.  The simulation chamber had a top moving wall and a bottom stationary wall.  Transfer of energy within the system was limited to momentum transfer between the top moving wall and the particles, between particles and other particles, and between particles and the bottom stationary wall.  Different levels of friction were simulated on the surfaces of the walls and on the surfaces of the particles (from max-friction conditions to non-friction conditions.)  

In this model, because we had ignored particle surface forces that extended out into the interparticle fluids due to adhered additives and electrostatic surface charging, we saw NO shear-thinning behavior.  The model predicted ONLY dilatancy.  

We also learned that some particles move independently of the carrier fluids.  This needs some explanation:  As mentioned above, a thin layer of fluid molecules is always supposed to remain stationary against a solid surface during flow of simple fluids.  Even high speed airplanes experience this phenomenon.  The nose of the plane and the wing surfaces, etc., have a thin, stationary boundary layer of fluid that travels with the plane.  Impacts and abrasion are not considered because they do not occur in simple fluid systems.  

But pilots, passengers, and management do worry about their airplanes flying through flocks of birds!  Why?  Because impacts from birds can damage external surfaces and internal engine surfaces.  I also recollect that when Mt. St. Helens erupted, pilots kept their airplanes well away from the down stream dust cloud to prevent their windshields and other outside surfaces from being "sand-blasted" by high speed particle impacts.

A thin layer of particles in suspensions does not remain stationary with solid container, pipe wall, or impellor surfaces (as is the case in simple fluid systems.)  Impacts -- collisions -- occur;  momentum is transferred;  energy is transferred;  and abrasion occurs.   In our computer model, regardless of the friction conditions simulated, the layers of particles close to the walls ALWAYS moved at different speeds than the walls.   

The late Jim Funk, who was a Professor of Ceramic Engineering at Alfred University, taught for years that if fluid boundary layer theory was true and applicable in ceramic extrusion systems (extrusion bodies are high solids particulate suspensions), the inside surfaces of extrusion dies would RUST.  But he also noted that everyone who has ever extruded high solids particulate suspensions knows that the dies are POLISHED -- not rusted.  In some extreme cases, extrusion dies are CHEWED OUT and DESTROYED by the flow of particles.  (We had one engineer tell us that he had tried to extruded a high solids suspension of beach sand through a 1" thick hardened steel pelletizing die and within an hour, the center of the die was destroyed by the severe abrasive conditions.)

In suspensions, particles collide with other particles and with the container walls -- transferring energy and momentum in the process.  They do not flow smoothly as do fluid molecules.  They do not flow at the same rates as do fluid molecules.  And they do not even flow in the same directions as do fluid molecules.  (Consider bug splats on your windshields [bugs :: particles], with no similar marks from the air through which the cars move.)

          Momentum Transfer Is Desirable

In ceramic suspensions, momentum transfer is a GOOD thing!  We want it to happen!  If we want the particles to be uniformly distributed throughout the suspension volume, we need to force the particles to bump and collide with one another and push each other around.  Only through such a process can we expect the particles to distribute themselves uniformly throughout a suspension.  

Impacts happen!  We want ceramic design engineers to realize this, admit that impacts are a fact of life, and use this information to design and control better mixing and process systems.  

The idea that one would try to minimize impacts because they required more powerful drive motors is to produce suspensions that have not been properly mixed.  I have seen this.  According to some, who equated power indirectly with efficiency, the less power required to mix a suspension, the more efficiently that product is mixed.  This is not true, but some apparently think so.  The characteristic forgotten in this case is the actual state of mixedness of the suspension.  Such a 'high efficiency mixing system' (as this), however, uses little power and produces suspensions that are NOT well-mixed.  According to this definition of efficiency, 'less efficient operating conditions' (those that use more power during mixing) used more energy and were undesirable, even though they produced more well-mixed suspensions.  Notice:  the state of mixedness is not even part of this definition of efficiency.  Their design choice was the system that used the least power -- and it was defined as the most efficient system.  I repeat -- I have seen this in industry.

I hope you all can understand that the least power used does not define the most efficient system.  If less power defines more efficiency, the most efficient mixer would be one that doesn't use any power at all.  Stop the mixing before it starts.  Simply dumping fluid and powder into a mixing tank and declaring it to be the 'most efficient mix' is absurd. No mixing occurs, power draw is zero, and this is the most efficient operating conditions???!!! -- ABSURD!!

The mixing of suspensions requires lots of energy, lots of IMPACTS, and lots of collisions between particles, impellors, and container walls.  With particulate suspensions, these phenomena are PRESENT and DESIRABLE!!    

A proper definition of efficiency requires one to achieve a certain level of MIXEDNESS.  When that is achieved by more than one set of conditions, then one can select the more efficient system by comparing the power requirements.  

Impacts Help Deagglomeration

Other structures present in suspensions but not in simple fluids are flocs, gel structures, and agglomerates.  The distinction between flocs and agglomerates lies in the strength of the bonds:  flocs are weakly bonded, whereas many agglomerates have much stronger bonds.  Flocs are usually contain particles that are electrostatically bonded although they can also be tied together with a variety of binding agents.  Agglomerates usually have strong chemical bonds tieing the constituent particles together.  Consider, for example, calcined powders which can contain agglomerates with partially sintered necks between particles.  These are much stronger and more stable in suspensions than flocs and gel structures.

Particle/particle and particle/wall impacts during mixing helps with all of these structures.  In fact, experience has shown that slightly higher solids, more crowded systems will mix better than less crowded, low solids suspensions.  Spray dry suspensions, which are frequently fairly low solids, will frequently benefit from mixing at higher solids contents followed by dilution to appropriate spray drying levels.  

Small particles tend to flow with carrier fluids, compared with larger particles which (as the proverbial 500lb gorillas) do their own things.  If solids contents are too low, particles can flow with the carrier fluids around other particles and impellor surfaces and avoid the impacts.  This usually hurts the mixing process.  By raising solids contents slightly, particles become more and more crowded, and they impact one another, impellors, and walls with much greater frequencies.  The surface areas of impellors and walls represent much smaller areas than the surface areas of the powders themselves.  So it is much more likely that particle/particle impacts will do more of the work of mixing and deagglomeration than will impacts with impellor and wall surfaces.  To be sure, the energy for this whole process comes from the drive motor through the impellor, but the majority of work of mixing comes from the momentum transferred to the particles which produces a vast number of particle/particle impacts.  To raise solids contents to enhance this phenomenon is to improve mixing and deagglomeration processes.

Some agglomerates are too strong to be fully deagglomerated during mixing.  It may then be necessary to use traditional ball milling or stirred ball milling to achieve the necessary deagglomeration.  Even in such systems, slightly more crowded systems (slightly higher solids contents) work better than lower solids systems.  Even when milling, the goal is to break agglomerates and particles -- which is enhanced when particles are close enough to be crowded.  When suspensions are crowded, particles cannot escape impending impacts and collisions.  In low solids systems, if the particles can squirt out of the way of an impending impact or collision -- they will do so.  In high solids, crowded systems, they cannot.

Impacts Cause Severe Abrasion

Simple fluid systems hardly ever (never???) present an abrasion problem.  Suspensions present severe abrasion problems all the time.  Impellor blades and other sharp corners will be quickly rounded off, and obstructions to flow will be eaten away.  Count on it!!

We once tried to test an in-line viscometer and an in-line high shear rotor/stator mixer in a high solids slurry line.  The heads of both instruments were chewed off and gone within the first hour of testing.  Even at low speeds, such as during extrusion, the high power/slow speed environment produces enough force to chew out extrusion dies when particularly strong, angular particles are present.  Just remember -- the polishing process uses a variety of sizes of powder grits to abrade and smooth surfaces.  Any suspension system will produce similar polishing/grinding phenomena during flow.  You may be attempting to mix a suspension, but the particles will be abrading any and all surfaces with which they come in contact.  

Many ceramic powders are stronger than steel in piping systems.  So even in steel piping systems, it is necessary to assume not only that impacts are occurring, but that with the impacts, abrasion will also be occurring.

Impacts Cause Dilatancy

I believe it would be unusual (impossible???) to see true dilatancy in a simple fluid system.  For dilatancy to occur, powders that can collide with one another are required.  Molecules in simple fluids don't do this.  But as particles collide with one another, and the intensities of the collisions increase, lots of energy can be lost due to collisions and dilatancy.  Our computer model, described briefly above, demonstrated this:  put particles into a system and dilatancy ispossible.  Take away the possibility for collisions (remove the particles, lower the solids contents, or maintain really low shear rates) and dilatancy disappears.  The rest of the computer models of that day, which did not allow particles to collide, NEVER predicted dilatancy.  Our model, which primarily focussed on the particle/particle collisions, predicted ONLY dilatancy.

Impacts Increase Mixer Power Requirements

I have mentioned this in an earlier article, but it deserves repeating.  According to the 'big boys', mixing motor sizes are theoretically only dependent on the viscosity of the fluid to be mixed.  But we have seen that as suspensions are mixed, power draw can decrease while mixture viscosities actually increase.  This is inconsistent with the theory that viscosity alone determines motor sizes.  

What appears to be happening is this:  suspended particles and agglomerates are being broken during mixing.  When the size distribution of the free particles and agglomerates improves (and particles pack better with one another), the number of impacts and collisions will decrease, which in turn causes the impellor's power draw to decrease.  As energy required to overcome particle/particle collisions decreases, power draw decreases.  At the same time as this drop in power occurs, the freshly exposed surfaces of the new particles will search for any chemical additives in the interparticle fluid to maintain their original state of deflocculation.  If none can be found, or none are available, the average viscosity of the suspension will begin to increase.  

In suspensions, decreasing numbers of particle/particle collisions causes the power draw to drop.  Increasing viscosity causes the power draw to rise.  The net effect we have seen in production high intensity dispersion (HID) mixers for whiteware bodies is that the measured power draw decreased slowly as the measured viscosity increased.  Additions of chemical additives were used to bring the viscosity back to desired levels.  

The importance of this phenomenon to note is that collisions DO occur WHENEVER suspensions are mixed and that the collisions have a major effect on the power requirements for the particular mixer/suspension combination.  Suspension systems with broad particle size distributions may exhibit minimal effects on power draw as mixing proceeds.  Suspension systems with narrow particle size distributions, or with lots of agglomerated fine particles, however, may show major effects on power draw from collisions, impacts, and breakage events among the powders.  When this happens, one must consider the fact that motor horsepowers for suspension mixing must be increased well beyond the values predicted for simple fluids of the same viscosity.

Another way to say this is that when suspensions are mixed, rheological properties must also be taken into account -- not just viscosities.  To give you a ball park number, my recollection says that a 6 cubic meter HID mixer required a 150 hp motor.    

Summary

Over the years, I have seen quite a few engineering designs for pumping, mixing, extrusion systems, etc., that were excellent designs for simple fluids, but were lousy designs for suspensions.  Many engineers have never had to use suspensions, so they have no points of reference to the typical ceramic system.  As a result, they will ask for the expected viscosity of the fluid and make their design accordingly.  The problem is not ONLY that particles are present in suspensions, but that particles interact with each other, with the impellor, and with the walls to cause phenomena that are NOT present in simple fluid systems.  

If one needs to design a system for a high viscosity molasses, one does not need to take impacts and collisions, particle/particle momentum transfer, deagglomeration effects, abrasion, or dilatancy into account.  If it is a mixing system or a pumping system that is being designed, the mixer and pump sizes can usually be easily predicted from the molasses' viscosity.

If on the other hand, one needs to design a system for a high viscosity ceramic suspension, one MUST take impacts and collisions, particle/particle momentum transfer, deagglomeration effects, abrasion, dilatancy, and additive chemistries into account.  The sizing of mixers and pumps and the whole process design is much more complicated as a result -- and none of the phenomena are easily predicted from a single suspension viscosity alone.