Volume 6  Number 5                            Dennis R. Dinger                                1 March 2008

Updates

The E-zine

If this is the first issue of the Ceramic Processing E-zine that you've seen, you can add your name to the mailing list by clicking HERE.  All back issues can be accessed from the Publications page at the web site.  For those of you whose e-mail programs don't properly show the figures in these E-zines, go to the Publications page of the web site using your web browser to open any and all issues.  All figures should open properly when issues are accessed from the web site.  Questions, suggestions, and/or requests for topics to be covered in future issues of this e-zine can be sent to QuestionsandComments@DingerCeramics.com   .

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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:  3 -- Shear Stresses

Introduction

The third phenomenon that should be present for excellent mixing is a sufficient amount of shear stress.  Since one is trying to force particles to mix with one another, shear must occur between individual particles.  Sometimes this occurs at relatively low viscosities at high shear rates.  Sometimes this can be forced to occur at relatively high solids contents, when high shear rates are difficult (i.e., almost impossible) to achieve.  Then, instead of high shear, one must use high forces and high shear stresses to drive the mixing.  

Attempting High Shear Flow in Extrusion Bodies

The most common example of high shear flow in an extrusion body occurs in the die of the extruder.  When dealing with extrusion bodies (high viscosity pastes), high shear rates are impossible due to dilatancy.  Attempting to use a high shear rate anyway (--- even after being warned) on high solids content bodies will cause all of the particles to lock up.  Dies are frequently and quickly ruined.  If the die is made from a particularly hard, strong material, a blockage can form which allows no flow to occur -- that is, the body will clog the die and flow will be blocked.  When this happens in an extruder, WATCH OUT!!! because the pressure will be building as long as the extruder continues to run --- and eventually, something will BREAK!!!.  

If a high solids extrusion body is forced to flow down a length of straight pipe at too high a velocity, a plug flow condition can occur in which the particles are all physically locked together and the whole structure will slide along the pipe.  It will move along the pipe as fast as possible, depending on the pressure available at the entrance to the pipe -- but it will SLIDE --- dragging particles along the walls and producing severe scraping and abrasion.  In many cases the pipe will become permanently blocked.  If the plug actually moves, the severe abrasion will occur.  Any downstream bend or fitting will then provide the shape necessary for the plug to permanently lodge in the pipe.  At this point, throw away the blocked pipe and start again at lower shear rates.

High Solids Bodies Require Counter-Intuitive Thinking

Attempting to do much of anything with high solids bodies is counter-intuitive.  One would think that shear rates and shear stresses must be increased so particles and bodies flow at high velocities.  Mixing of high solids bodies produces the same kind of thinking.  I watched one day as a high solids refractory body was being spun in a mixer for 30 minutes.  The operator thought it was being 'mixed' for 30 minutes, but no mixing whatsoever was occurring.  It was such a high solids, dilatant body that it could hardly ooze at all.  It oozed a grand total of about a centimeter downhill as it was being spun in the bottom of a sloped mixer.  By the time the mixture had oozed the 1 cm downhill, the mixer had moved the lowest part of the sample to the top side of the mixer where it could ooze back to its original starting point.  This 'oozing' was the sum total of 'mixing' that occurred for 30' in this mixer.  The mixer had blades in it, but the blades only ever just dented the surface of the sample.  The body was too dilatant for the blades to cause any actual mixing.  I suggested changes, but to no avail.  They would have needed to slow the rotation speed of the mixer severely so the blades could actually cut and mix the body.  They told me it was a 'single speed' mixer.  

High Solids, Low Shear Rate, High Shear Stress Mixing

There is actually a way to force flow in high solids bodies.  One must be able to provide sufficient shear stress to force the body to move, and then one must force it to move slowly through some kind of restriction at a SLOW flow rate.  High stresses are required to force flow at such slow rates, but it can be done.  Dilatant bodies exhibit extremely high viscosities and particle/particle structures when sheared at high shear rates, but the level of particle/particle interactions can actually be reduced by shearing at low shear rates.

One cannot use a standard mixer that relies on the body to flow by itself, with the aid of mixing blades, or under the influence of gravity.  One must force such bodies to flow --- and the easiest way to do this is to push them through a pelletizing die or other restriction.

          Using the Extruder as a Mixer

We have found that the best mixer for a high solids extrusion body is frequently the extruder itself.  An excellent example of high solids, low shear rate, high shear stress mixing would be to extrude a high solids body through a pelletizing die at a low overall flow rate.  This produces a relatively low shear rate in all locations except within the die holes (where shear rates are higher --- not extremely high --- just higher).  Lots of force should be available from the extruder, but the desired flow rate must remain low.  This is not to suggest that any mixing must necessarily occur between the auger and the extrusion barrel.  This procedure applies to both auger and piston extruders.  The trick (if it can be called that) is to pass the extruded material through the same extruder and die several times.  

We once tried to extrude a high solids body consisting of extremely dilatant powder.  The first pass produced a pagoda shape (rather than a smooth cylindrical column) with extreme feathering or cracking at the surfaces of the extrusions.  After several passes through the same extruder and the same pelletizing die, the extrusions were well mixed with smooth shiny surfaces and cylindrical shapes that matched the die holes.  They looked great!  But they required several passes to achieve this finished state.  The mixing step prior to extrusion was insufficient to mix the particles and binders well enough to require only one pass through the extruder.  We did the best we could with the mixing step, but then we used the extruder itself to finish the mixing.  This was a piston extruder -- so no mixing occurred within the piston chamber.

Obviously, several passes through an extruder is not desirable from a production point of view.  But several passes through the extruder as a mixing step, followed by the actual production pass is a valid, desirable, and successful way to produce near perfect extrusions.

          A Simple Design for A High Shear Stress Mixer

We always talked about producing a 'Juicer' which would be a mixer that resembled an orange juice squeezer.  The central, rotating truncated cone (the part that resembles the orange juice squeezer) would be silicon carbide with grooves in it to grip the body.  The outer, stationary stator would be a matching ring of silicon carbide also with grooves to grip the body.  The gap between the two would be variable so we could alter the shear rate between the rotor and stator.  We would simply use this in the die position of an extruder and force the body through the narrow gap at using the high shear stresses produced by an extruder.

This is a low speed, high shear stress operation.  The goal is to allow particles, interparticle fluids, and additives to distribute themselves within the body as it passes through the mixing location.  The high shear stresses will allow the particles to slide upon one another and it will allow additives (and particularly binding additives) to be uniformly distributed as the body passes through this location.  We might need to pass the body through this mixing head several times, but that requirement would be a function of the homogeneity of the feed material achieved on the first pass.  It would not be necessary for all particles to be in equilibrium positions when they first passed the mixing head, but within the volume of the mixing head, one should have a homogeneous representation of all particle sizes, fluids, and chemicals.  If done properly, one pass through this type of mixing head could achieve the desired homogeneous results.    

Other Benefits of High Solids-High Shear Mixing

          Deagglomeration

Some feed powders, such as calcined aluminas, are agglomerated.  Slow, high stress mixing can be used to produce stresses sufficient to break agglomerates.  In fact, this should be one of the main goals of such a mixing operation.  Sometimes, it is not possible to use low solids and high shear rates to achieve deagglomeration.  Crushing and/or milling may then be required for the deagglomeration to occur.  But when a high shear stress mixer can be used to mix high solids bodies, the high stress conditions should be sufficient to break agglomerates and free all particles to travel as individuals.  A single 200 micron diameter agglomerate consisting of a bazillion 4 micron diameter particles is not the same as the bazillion 4 micron diameter particles each flowing freely and independently of one another.  If the high stress mixing step can break such agglomerates and free all constituent particles -- SUCCESS!!

          Particle Breakage

Sometimes, particles are large and can be broken relatively easily.  A high stress mixing step can also possibly cause particle breakage to occur.  If the particles are going to break anyway during processing, it is beneficial for them to break during a milling step prior to mixing, or during this type of high stress mixing.  When breakage occurs, the high stress mixing environment can then help to move all newly broken particles into homogeneous positions within the body.  This will not usually happen in low solids/high shear rate systems without also having dilatancy problems -- and when dilatancy occurs, sufficiently high shear stresses are usually not possible to deal with the problems. 

          Minimization of Dilatant Effects

The counter-intuitive part of dilatancy is that better mixing and better flow occurs when shear rates are low.  If dilatancy is the rise of viscosities as shear rates increase, then the reduction in shear rates will reduce both viscosities AND dilatancy problems.  Dilatancy in high solids bodies is also a problem, but when the mixing step has been designed to produce sufficiently high shear stresses at sufficiently low shear rates, the mechanism producing the high shear stresses and the mixing head should be sufficient and strong enough to overpower the dilatancy problem.  Experience has shown that when dilatancy can be overpowered, particle size distributions of constituent particles improve, and both viscous and rheological properties improve (i.e., viscosities decrease and rheologies move away from dilatancy towards shear-thinning behaviors).  

Particle/particle contacts cause dilatancy, but particle/particle contacts also enhance mixing and comminution, so the presence of particle/particle contacts in this type of a system is beneficial.  When a device is designed to impart high shear stresses on a body at low shear rates, mixing is enhanced because the system is designed not only to handle particles in close proximity but to FORCE them to mix and distribute homogeneously.

Summary

High shear stress/low shear rate mixing and processing is necessarily a slow operation.  High solids bodies must be mixed using both high shear stresses and low shear rates.  This is at the opposite extreme from many production goals which are to move bodies faster and faster through the various processing steps.  Dilatancy and particle/particle interactions REQUIRE both low flow rates and high stresses.  High stress, low shear rate environments take advantage of the fact that we know that dilatancy will cause all sorts of problems in high shear systems.  So, we move the mixing process to the other extreme -- low shear rates and high shear stresses -- to minimize problems and to allow successful mixing and forming.