Volume 6  Number 2                            Dennis R. Dinger                                1 December 2007

Updates

"... 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 E-zine

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

Mixing

Introduction

Several questions were recently submitted concerning mixing of slips, dry particles, colloids, etc.,  -- the whole gamit of possibilities.  These questions will be addressed in a general, broad way in this article.

Obviously, the purpose of mixing is to take two or more powders, pastes, slurries, slips, or liquids and combine them so the resulting powder, paste, slurry, slip, or liquid becomes a single, uniform, homogeneous mixture.  Lots of different types of commercial mixers are available to perform these tasks.  We will not get into specific brands or types of mixers, however, in these articles.  Each device, in the wide variety of mixers available, is designed primarily for a specific type of mixing process:  dry powders, suspensions, extrusion pastes, etc.  One should make sure the right mixer is used for the task at hand.

Definition of Sufficient Mixedness

Just how homogeneous is "homogeneous enough" for any given process?  The answer to this will be process-specific.  When one powder is white and another is black, it is relatively easy to see whether a mix is homogeneous.  But most ceramic powders are white or tan and it is not easy to look at a suspension or powder with the naked eye and decide whether the resulting mix is sufficiently homogeneous to send on to the next process.  In my opinion, when we err on this point, we tend to err on the side of less than adequate mixing.  Why?  If it looks mixed now, and we subject it to another 3 hours of mixing, after which it looks about the same as it does now, why spend the money to power the mixer for 3 more hours?  It costs less to perform less rather than more mixing, and this is sometimes used to determine the final amount of mixedness used in a process.  

Mixedness questions are process specific.  The level of mixedness required in a brick plant will be very different than the level of mixedness required in a catalytic convertor substrate plant.  The reason:  mixing variations in a brick body may make interesting variations in the appearances of the brick (architects frequently like this);  mixing variations in the very precise extrusion processes of catalytic convertor substrates, however, can cause flaws, holes, tears, and reject products.  Each process and each product will have its own definition of "sufficient mixedness".  

Mixing is considered a fundamental process and to my knowledge, no one pulls samples of powders or suspensions to analyze under a microscope to determine whether the particles are sufficiently homogeneously distributed.  This might be a good way to determine the ultimate level of mixedness, but I am not aware that this is happening.  In most cases, when it looks "homogeneous enough", that is sufficient.  Mixing then stops and the body is sent on to the next step in the process.

In many systems, rather than using an eyeball method, it may be advisable to define a more objective means of defining "sufficient mixedness."  Such a criterion should be defined at the particle or molecular level.  This type of criterion could require some type of microscopic analysis, or an electrical measurement, or some other simple test, appropriate to the particular body, that can be performed on a small sample to show the level of mixedness.

Achieving Sufficient Mixedness

          Mixers Destroy Agglomerates and/or Flocs of Particles 

When mixedness is defined on the particle level (which it appears it should be),  it is desirable to destroy all agglomerates and flocs so particles can be uniformly distributed and dispersed throughout the suspensions and/or dry powders.  Some operations, however, define sufficient mixedness to include the minimum required mixing energy to make a body appear uniform to the naked eye.  Once this is achieved, mixing energies are reduced to bare minimum amounts to prevent any further changes to the body.  When shear intensities are reduced, they appear to put an end to the destruction of agglomerates or flocs by the mixing impellor.  They actually reduce the destruction of agglomerates and flocs to very low levels, but even the slowest impellors can break agglomerates and flocs under the right conditions.  I personally have seen operations like this.  The problem they had when mixing was 'finished', was that even the storage tank impellors, which are supposed to prevent settling in holding tanks, have sufficient energy to cause agglomerates and flocs to be broken down.  It happens slowly --- verrrrrryyyyyyy sloooowwwwwwwlllllyy --- but it HAPPENS!  Mixers and mixing phenomena cause particles to be sheared off of agglomerates and flocs -- which means that even in holding tanks, particles will be freed from agglomerates and flocs to travel independently.  And when this continues to occur in suspensions, new surfaces continue to be exposed to interparticle fluids.  The problem is that required additive chemicals needed to adjust the characteristics of the newly liberated particles with their newly exposed surfaces are usually not present to do so.  When this happens, viscosities change slowly with time.  In the specific case I witnessed, suspension viscosities increased overnight -- every night -- in all storage tanks --- which required deflocculants to be added to the holding tanks every morning to adjust the viscosities back to the desired values.

When mixing is actually complete and a system is truly homogeneous, further changes with time will not occur.  If suspensions and bodies change their properties with time, something is changing inside those bodies --- and they are becoming more homogeneous.  If a suspension is truly homogeneous and a particle is kicked free from a floc, it will eventually be pulled back into the growing floc structure without change to the fundamental nature of the structure and without change to fundamental structure properties.  When changes occur to bodies that have been defined as "sufficiently homogeneous", the changes demonstrate that the bodies were not actually homogeneous --- and further changes will continue to move the body towards true homogeneity.

          Using Any and All Devices (Other Than Mixers) to Perform Mixing

Many companies utilize whatever mixers or other devices are on hand to perform their mixing requirements.  One of the most common examples of this is when ball mills are used to perform mixing, in lieu of using an actual mixer.  Sometimes a mixer is available, but its use would force an extra step to be added to the process.  In many cases, the addition of an extra step to a process is undesirable.  Sometimes, the required mixer is not available, and the engineers do not want to purchase the appropriate device to perform the mixing.  

When devices other than mixers are used to perform mixing operations, just remember that they may perform some mixing phenomena, but they are not usually efficient mixers.  Ball mills are designed to break particles and perform comminution.  They are not primarily designed as mixers -- although they do actually perform some mixing.  They are not highly efficient mixers;  they are not high shear devices; and the question that must be asked is this:  Is the level of mixedness following a ball milling operation sufficient for the next step in this process?  

Another example of this is when companies use their main blunger tank to mix suspensions, but they rely on holding tanks and their low-speed impellors to mix additive chemicals into the body.  Holding tank impellors, when designed properly, prevent settling in the suspension by providing some upward flow near the circumference of the tank to balance downward flow and settling of large particles near the more quiescent central axis of the tank.  Holding tank impellors are NOT designed to perform mixing as the main blunger tank impellors are designed to do.  So to use a holding tank impellor to fine-tune slip viscosities is to use a holding tank impellor in a way it was not designed to perform.  Sure, it will perform some mixing over long periods, but it is a slow process that will not sufficiently disperse additive chemicals and the dispersion that it can perform will occur very slowly.  

Dispersants want to be dispersed, but in concentrated forms, dispersants can cause local flocculation -- IF they are not dispersed sufficiently and quickly.  Most engineers and technicians will not take the time to dribble additives slowly into holding tanks.  They are more likely to pour (glug, glug, glug) a liter of additive into a holding tank and rely on the impellor to mix it.  But in some cases, each glug can cause local flocculation to form a rock and then the rock will settle to the bottom of the tank before any action at all by the impellor.  In such cases, the proper amounts of additive may have been added to the tank, but the body will certainly not contain the concentration of additive the process engineers think.

The best place to add additive chemicals to a suspension is just prior to passing suspension through a mixer or stirred ball mill in a recirculation loop off to the side of a holding or blunging tank.  Some operations use continuous side loops to subject bodies in holding and blunging tanks to high intensity dispersion and/or to stirred ball milling.  When such loops are used, additives should be injected near the intake to these devices in the recirculation loop.  In this way, the additive chemicals will be distributed first as suspension passes through the HID or stirred ball mill, and the second as the larger volumes of products from the mixing or ball milling operations are then mixed back into the bulk suspension in the holding tank by its slow moving impellor.

Other Considerations

          Solids Content

The higher the solids content of a body, the more difficult it will be to mix and the more powerful the mixing motor needs to be.  If one measures the energy or torque required to mix a system of particles, dry powders require less power from mixing impellors than when a little water or other fluid is added to form a high solids paste.  As more and more fluid is added, to a system of dry powder, the viscosity will increase and torque requirements on the mixing impellor will increase -- through maxima -- before viscosities and torque requirements begin to decrease.  Eventually when sufficient fluids have been added to suspend all particles at reasonable distances from one another, viscosities will decrease and torque requirements will decrease as well.  These decreases will be back to and below the original values required to mix the original dry powders. 

Each different process will have its own optimum solids content.  Generally speaking, the higher the solids content, the more difficult it will be to perform adequate mixing.  Experience shows that for low solids suspensions, high shear rates are required to perform adequate mixing operations.  As solids contents increase, lower and lower shear rates but higher and higher shear stresses are required.  For extrusion bodies, really high shear stresses that operate at relatively low shear rates will perform excellent mixing.  The easiest way to describe this is to consider an orange juicer onto which you are twisting 1/2 of an orange at low shear rate (low rotational velocity) but at a high shear stress.  In other words, pushing the orange with lots of force while twisting it very slowly on the juicer is equivalent to what you need to produce in a high solids, extrusion body mixer.  A slow moving, powerful rotor that forces the extrusion body under great pressure against a stator will perform excellent mixing on extrusion bodies.  

The lower the solids content of a suspension, the higher the mixing shear rate must be.  At extremely low solids contents, it is not possible to reach the high shear rates necessary to achieve excellent mixing.  The solution to this is to mix at higher solids contents and then, finally, as the last process step, dilute to the desired solids content.  Spray dryer bodies which are low solids suspensions would benefit from such procedures. 

          % Composition

Another consideration deals with the percentage compositions of mixtures.  A homogeneous mixture of two powders (~50/50) is easier to achieve than when the process requires less than 1% of a powder to be mixed into 99⁺% of another.  Sometimes, one must also consider the particle size of the additive powder.  The smaller the additive percentage, the finer the powder needs to be.  (... but that is another story.)  The reason for this is that a 50/50 mixture will appear reasonably homogeneous when the mixture is anywhere between 48/52 to 52/48.  A <1/>99 mixture will not be well mixed when some regions of the mixture contain 1 or 2 % of the additive and other regions have none.  

          Dry Added to Wet

Another point is that one should always add dry to wet.  In a blunger, the water (or fluid) should go into the tank first and the dry powders should then be added to the water.  The simple explanation for this is to consider a kitchen mixer and the making of a cake.  In most cake recipes, the flour and other powders are added into the mixing bowl first, and the fluids are added second.  This procedure requires constant squeegeeing of the sides of the mixing bowl to scrape the high solids mixtures off the bowl and put them into the center where the mixing blades can operate on them.  This is okay in a small kitchen mixer, but it is unacceptable in a production blunger tank.  Adding powders to fluids eliminates this type of problem.

          Sufficient Shear

Colloidal particles and other fine particles tend to agglomerate and otherwise tightly associate with one another.  They will continue to travel as small agglomerates and the agglomerates will behave as larger particles --- until --- they see sufficient shear to knock them apart.  In my opinion, the biggest two mistakes made regarding mixing are that (1) insufficient shear rates are used because less power is required at lower shear rates, and (2) mixing is stopped as soon as the bodies look sufficiently homogeneous.  When two tan ball clay slurries are mixed, they will appear to be homogeneously mixed almost as soon as the two slurries have been completely poured into the mixing tank.  It is not possible to visually tell whether mixing is complete in most systems.  For this reason, I believe most process engineers stop mixing processes prematurely.

          Vortex Cannot Reach Down to the Impellor

Regarding bubbles in a suspension (which is of concern to some processes), the simple guide is to insure that the vortex around the impellor shaft in a blunger does not reach down to the level of the impellor blade.  When this happens, the impellor becomes a giant air pump which forces all sorts of bubbles out into the suspension.  This is not the goal of mixing.  If the vortex reaches so far down the shaft that you hear a giant sucking sound, the tank needs to be filled to a higher level.  The other alternative is to use a lower rotational velocity on the shaft.  Simply increase the size of the batch to insure that the vortex only appears as a small indentation in the surface of the suspension.  

This could also happen when the impellor blade is improperly located too high in the tank.  Most impellors should be within about a radius of the bottom of the tank.  If they are higher than this, they are more prone to suck and pump air, and they are less prone to properly mix the suspension in the bottom of the tank below the level of the impellor. 

Mixing Dry Particles

There are lots of types of mixers for mixing dry particles.  It is remarkable to me that many of these actually work -- but that too is another story.  The biggest problem with the mixing of dry particles deals with maintaining the level of mixedness achieved in the mixer after the powders have been removed from the mixer.  Dry particles will segregate by size and density during vibration -- and practically any motion or movement or stirring action of the 'mixed' powders will cause them to segregate.

The easiest way to demonstrate this to yourself is to put a tablespoon full of dry Folger's instant coffee crystals into a coffee cup, and add an equivalent amount of non-dairy coffee creamer to the cup as well.  Folger's contains many large flakes of dark brown coffee crystals.  Non-dairy creamer is fine white powder.  Stir the two powders until you think they are sufficiently well mixed.  When satisfied that you have a good mixture, remove the spoon, and tap the cup on the table a few times.  Make sure you are watching the powder mixture when you do this step.  You will see that it unmixes relatively easily.  

Summary

In this article, some of the simple considerations regarding mixing were considered.  The most important considerations were:

     1 -- One should determine an objective criterion for defining the appropriate level of mixedness that does not require a simple eyeball inspection.
     2 -- Low solids mixing should be performed at high shear rates while high solids mixing can be performed at lower shear rates as long as high shear stresses are used.  
     3 -- Mixing should utilize sufficient shear to break up agglomerates and flocs and to homogeneously distribute individual particles throughout the body.
     4 -- Just because devices (e.g., ball mills) produce some mixing action does not mean they should be used as "process mixers."
     5 -- Dry powders are relatively easy to mix, but dry powders will easily segregate by size and density as they are moved and/or vibrated.