Volume 6  Number 3                            Dennis R. Dinger                                1 January 2008

Updates

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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:  1 -- Shear

Introduction

The question to be addressed in this series of articles concerned optimum conditions for mixing -- mixing of liquids, mixing of suspensions, mixing of colloidal particles, mixing of dry solids, etc.  Experience has shown that three major phenomena must be present individually and/or in concert with each other, as appropriate to the type of materials being mixed:  shear, impacts, and shear stresses.  We will consider these individually and in combination.

Shear is fundamental to achieving excellent mixtures of any kind.  It goes without saying that if shear is not present, most mixtures will not achieve any kind of homogeneity.  We will, nevertheless, consider shear as it applies to the different types of mixtures.

Mixtures of Fluids with Fluids

When fluids are moving, there is generally shear between their molecules.  Some molecules will be moving slower and some will be moving faster, and the zone between the two will experience shear.  Shear must be provided, however, for excellent mixing to occur.  It is simply not sufficient to expect the fluids to mix with one another merely by the fact that they have been combined in a container.  We will consider fluid mixtures in two parts:  gases with gases and liquids with liquids.

          Gaseous Mixtures

The greatest difference between gases and liquids is the velocity of motion of each of the molecules.  Gaseous molecules move at very high velocities and one could easily expect that merely by putting two gases into a single container -- they will mix.  This is a reasonable assumption, and if sufficient time is available and a totally homogeneous mixture is required, this would be a valid assumption.  

In production plants, however, we sometimes mix gases to achieve other results.  For example, many burners in industrial combustion systems are considered to be "nozzle mix" burners because the natural gas (or other fuel gas) is only mixed with the air in the nozzle in the burner.  Prior to that point, there are piping systems containing 100% air and 100% fuel gas.  Once mixed at the nozzle, however, we expect the fuel gases to burn.  This occurs when the fuel/air mixture is within the flammability range for the particular fuel gas.  

Industrial flames can be made long, slow, and lazy, or short and very energetic by means of the level of mixing that is produced by the burner.  A long, slow, lazy flame is technically called a "laminar diffusion flame" because great efforts are made to NOT provide shear at the nozzles (which would enhance mixing) and TO allow the air and fuel gas to mix as slowly as possible as they diffuse together into the flammability range of the fuel gas. 

There's a rule of thumb in the combustion industry which says "When it's mixed, it's burned."  With an ignition source provided, as soon as the fuel and air mix into the flammability range, they will burn.  Delay the mixing and the flame becomes long, slow, and lazy.  Provide as much shear as possible and the flame will be very short and intense.

Even in oil burners, the liquid oil does not burn -- the oil vapor burns.  In this case, time must be provided for the heat from the flame to vaporize the oil and only then when the vapor mixes with the air -- will it burn.  This requires a more complex burner system because the oil must be atomized and vaporized before it can burn.  Fortunately, the atomization process is a high shear process.  The higher the shear, the better the atomization, the smaller the oil droplets, the higher the surface area of the oil droplets, and the faster the vaporization process will be.  During all of this shear, the air will be mixed well with the droplets so as the droplets vaporize, the oil vapor can diffuse into the air and burn and it will do so quickly.  

The point to be emphasized here is that the level of shear needs to be controlled to control the speed of mixing and other shear related processes such as atomization.  Industrial burners produce flames in many shapes and sizes that are controlled by the shear intensity and the geometric arrangements of the gas and air jets and the shear forces produced.  

In an effort to achieve improved temperature uniformities within production kilns, shear on a slightly different scale must also be provided.  In this case, high velocity burners have been developed to stir the kiln atmosphere around and through the ware.  Before the advent of high velocity burners, firing times were extended to wait for the kiln atmosphere to achieve temperature uniformity -- a much slower process with low velocity burners.  In this case, however, high velocity burners not only have the requirements imposed to cause the fuel and air to mix and burn quickly, but they must also then produce high velocity flames to stir the kiln atmosphere to achieve uniformity -- both uniformity of composition and more importantly, uniformity of temperature.  The firing can proceed no faster than the temperature rise at the slowest (coolest) point in the kiln.

High velocity burners produce lots of shear which produces lots of mixing which produces much more uniform temperatures within all regions of the kilns.  One simply cannot throw the fuel and air into the kiln in any manner and expect the flames to produce uniform temperatures.  The nature of the shear and the velocity of fuel, air, and combustion gases provided by the burners controls flame types and shapes, and the velocity and nature of the shear provided by the high velocity flames produces mixing within the kiln.  

During firing with high velocity burners, one must also consider the arrangement of the ware within the kiln.  Tightly stacked ware will reduce the capability of the hot gases to uniformly penetrate within the ware stacks and distribute evenly throughout the kiln.  In such cases, hot and cold spots can be present in different zones in the kiln -- simply because the ware was stacked poorly.  

Not only must shear be present for mixing to occur -- it must be CONTROLLED.  The author once worked on a boiler burner project in which one large flame provided the heat to a boiler.  When the fuel and air were thrown into the burner in the normal way, the oil vaporized reasonably quickly and it burned as soon as it was mixed into its flammability range with the air.  This boiler chamber was about 10' diameter and 40' long (inside dimensions).  In this normal configuration, the flames filled the whole chamber and the mixing produced lots of smoke (incomplete combustion) at a air/fuel ratio of about 20% excess air.  When the exact same amounts of fuel and air were put through this burner in a very intense manner, the flame shortened to about a 10' diameter ball and the rest of the chamber was clear to see through (that is, no smoke was produced).  This second flame could be run without smoke down to very low levels (~1%) of excess air.  The difference?   In the second, more intense case, the high shear conditions were forced to occur within the burner.  Intense shear was produced in the burner under the control of the combustion engineers who designed the burner.  The first, less intense flame, however, more closely followed the rule -- throw it into the combustion chamber and allow it to vaporize, mix, and burn whenever and wherever it has time to vaporize, mix, and burn.

If you want excellent mixing to occur, you must FORCE shear to occur when and where you want it.  If you don't FORCE it, the alternative is to provide lots of time and/or volume for the mixing to occur.  In this case, the time it takes and the mixing will expand to fill the available volume.  In my experience, excellent mixing will occur when you FORCE it to occur here and now.  If you don't force it, the mixing will never be quite as good as you want.

Sure -- when dealing with gases, diffusion occurs quickly (compared to diffusion in liquids), but when other processes are relying on the mixing achieved, as in the case of combustion systems, a well-mixed system of air and a small percentage of unburnt fuel (which usually presents itself as smoke) is not the desirable goal.  Either YOU control the mixing, force shear to occur, and mix the gases when and where YOU want them to mix, or you settle for less desirable levels of mixing and associated less desirable consequences.

More energy is usually required to produce better shear, so it will usually cost more to mix well and under control.  Remember -- the amount of energy used during mixing is not inversely proportional to efficiency.  Some would like to define the most efficient mixers as the ones that use the least amount of energy.  The problem with this definition of efficiency is that the homogeneities of the resulting mixtures are never very good.  The least amount of energy used in a mixer usually produces the poorest mixture.  Efficiency of mixing must be based on the level of homogeneity achieved -- not on the amount of energy used in the process.  Then, when two different mixing configurations can produce the desired level of homogeneity, the one that uses the least energy can be declared the 'most efficient'.  Apart from achieving the desired level of homogeneity, efficiency cannot even be discussed.

          Liquid Mixtures

Since each molecule in a liquid is free to move around, diffusion will usually produce homogeneity of mixing EVENTUALLY.  But again, as with gases, if one wants a homogeneous mixture, one must usually expend the energy required to perform the mixing here and now.  Waiting on the system to equilibrate will frequently take much longer than desired (or than expected.)  This is especially important when a liquid with a large molecule is to be mixed with a liquid with small molecules.  The imposition of appropriate amounts of shear will cause all molecules to move relative to one another and allow homogeneity to be achieved more quickly. 

Most mixers were designed to mix liquids.  Many mixers that are used to mix suspensions were not designed for mixing liquids with powders, but for mixing liquids with other liquids.  Good propellor blade impellors and other impellor shapes do excellent mixing of liquids.  

When mixing two or more liquids, one must usually pay attention to the order of mixing.  The higher viscosity fluid should be added to and mixed into the lower viscosity fluid(s).  This will eliminate the necessity to produce shear which scrubs the walls of the mixing container.  For example, consider mixing water and molasses.  If the molasses is poured into the mixing bowl and the water is added to it, the bowl will be coated with the molasses and this will be relatively difficult to remove unless the mixer utilizes a scraper of some sort.  On the other hand, if the bowl contains the water and the molasses is added to it, the water will more easily leave the bowl surface to be mixed with the higher viscosity fluid mixture.  Since most large production systems do not use scrapers, one must consider the order of mixing.  Just as one would add powder to liquid when mixing a suspension, one should add high viscosity to low viscosity when mixing fluids.

Once again, excellent results can be achieved mixing liquids with liquids in most mixers.  How long should one mix?  This question should be answered based on a good definition and a measure of the level of homogeneity of the mixture -- not on amount of energy used.  I am not suggesting the definition of a good measure to determine homogeneity is an easy requirement to achieve, but it must be done.  As one scales up a process, the duration in each larger batch should be sufficient to produce a similar homogeneity to that achieved in the smaller batches.  The time requirements will usually be different as scale-up occurs.  If the lab batch was mixed for 15 minutes, it is highly unlikely that the production batch will also require only 15 minutes.  (It can be done, but it is unlikely.)  Determine a method to guage homogeneity of mixture, and use that definition to define durations of mixing for each different size batch.

As for gases, shear must be provided when and where it is needed to achieve homogeneity.  The production of shear usually requires energy, so do NOT define efficiencies solely based on the amount of energy used.  If the smallest amount of energy utilized defined the most efficient mixer, the fluids would always be best mixed (least energy used = 0 energy) before they ever reached the mixer.  

Also -- the label on a mixer does not mean that it always runs consistent with its label.   What I mean by this is the following:  I toured a lab once in which they had a 'high intensity' mixer which they used routinely.  They didn't like the results 'high intensity mixing' produced, however.  When I asked a few more questions, it turned out that they were not running the high intensity mixer under high intensity conditions.  'High intensity' conditions would have required them to use their mixer at about 5000 rpm.  They admitted only to using it at about 1000 rpm.  So they had a lousy opinion of 'high intensity mixing' without ever having actually achieved 'high intensity' conditions.  

We had a similarly sized laboratory mixer at that time.  We needed ~5000 rpm to achieve high intensity conditions.  At 1000 rpm, the mixer sounded like it was doing a tremendous job of mixing.  At 5000 rpm, it sounded like it was ripping the mixture apart.  But 1000 rpm was insufficient to achieve high intensity conditions.  So in the case of that lab, they were turned off to 'high intensity' mixing because they were using a 'high intensity' mixer -- even though they only ever achieved low intensity conditions in it.  

One must also pay attention to whether or not air is being pumped into the mixture when using mixers.  If one hears that 'giant sucking sound' that Ross Perot always talked about (regarding NAFTA), but it is coming from the mixer, it is highly likely you are pumping air into the mix.  If you look at the surface of the liquid, if the vortex exposes most of the impellor shaft, you will hear the sucking sound whenever the impellor is exposed as well.  This means that there is not enough batch in the tank for the impellor/rpm being used.  Add more batch to raise the level in the tank and the sound will cease.  The vortex should be visible in most mixers, but it should not do more than make a slight indentation in the surface of the fluid.  If it is pulled down the shaft, add more fluid.

The other way to eliminate the sucking sound is to lower impellor rpm, but that is not the way to solve this mixing problem.  That will reduce intensities and it will decrease the efficiency of the mixer.  The way to solve this problem is to increase the fluids being mixed to appropriate levels.

Mixtures of Fluid with Particles (Suspensions)

Most mixers have been designed to mix liquids.  Some have been designed to mix high solids content suspensions.  In most lower solids content suspensions (lots of fluid with minor amounts of powders), mixers that have been designed for liquids are routinely used.  Be aware that most engineers do not deal with suspensions very often.  Most engineering college curricula require an introduction to simple fluids by all undergraduates, but only in advanced (senior and graduate level) classes do they begin to touch on suspensions.  Ceramic engineers deal with suspensions from their first lab class onward, but they are the exception.  Even though most mixers are designed for liquids -- not for suspensions -- suspensions are usually treated as if they are equivalent to high viscosity simple fluids (which they are not) -- and are therefore covered by the various mixer designs.

Most mixer motors are sized based on fluid viscosity.  Suspensions have viscosities, so they are covered by these motor sizing rules.  But whenever powders are mixed with liquids, powders collide with impellor blades and with each other (this will be covered in the next article), and this takes extra energy.  (We don't talk about liquid molecules colliding with each other.  They do, but they are much less energetic collisions than when solid particles collide with impellors and with each other.)  So the rules for sizing mixer motors are usually inadequate for use with mixers to be used with suspensions. 

The first major concern, therefore, when sizing mixers for use with suspensions is that by simply following the viscosity recommendations, the motors will frequently be under-powered for the task.  Mixing liquids with liquids simply never requires as much energy as mixing powders with liquids.  

Another consideration in this vein is that when powders collide with each other, dilatancy can occur (and that takes more energy.)  Dilatancy usually occurs during the early stages of mixing.  When mixing suspensions, you actually want collisions to occur -- the collisions help the mixing process -- but when dilatancy occurs, more energy is required than usual.  Motor requirements must be increased to handle the dilatancy.  

Remember -- dilatancy is not viscosity.  We have charts in our green PPC textbook where viscosities of suspensions are increasing while measured power draws on the motors are decreasing.  This cannot happen when mixing simple liquids.  There are no other phenomena that occur in simple liquids that can account for such behavior.  But this happens commonly when mixing suspensions.

The actual phenomenon occuring is that particles are colliding with particles (the dilatancy) while the viscosity of the system is at some value.  As the particles (or agglomerated particles) are broken down due to the collisions, fewer particles collide (the dilatant effect is reduced) but the viscosity of the suspension increases due to the new free particles released into the suspension.  The net effect is that as the dilatancy and intensity of particle collisions decreases with time, the viscosity of the suspension increases, and the power draw on the motor decreases.  Deflocculants can then be used to decrease the viscosity of the suspension.  

The literature therefore suggests that as viscosity in simple fluids increases, motor horsepower should increase.  In most mixing literature, motors are sized based on viscosity alone (I have not seen a chart that takes suspensions and collisions into account.  I apologize in advance for the exceptions that are out there with which I am not familiar.)  Good mixer salesmen know that suspensions present unique problems for their mixers, and they can make excellent recommendations to handle the differences.  

When using a mixer that has been designed for fluids to mix suspensions, one should usually order a larger motor.  If the heaters are sized correctly, a small motor on a new mixer won't be ruined when used to mix a suspension.  But if the motor continues to kick out, that will require the ordering of a larger motor.

The second major concern is that of abrasion.  Fluids may corrode impellors, tank parts, and pumping systems, but fluid molecules do not abrade those same parts.  The particles in suspensions abrade anything and everything with which they come in contact.  For this reason, the types of mixers that have the impellor bearings below the surface of the suspension are highly susceptible to abrasion damage.  Any mixer designed like a typical kitchen blender (with motor below the tank and impellor in the bottom of the tank) falls into this category.  They, too, are designed for use with liquids and they frequently will not stand up to the abrasive environment of suspensions.  Even in a mixer like this that was designed to be used with suspensions, it took less than one hour of use after a rebuild by the manufacturer to ruin the bearings.  Mixers like the standard milkshake mixer with motor above and impellor at the bottom of a long shaft are best used for suspensions.

The third major concern is the subject of this article:  the production of shear.  Turbulence is the result of lots of shear, so one frequently might strive to achieve lots of turbulence.  The problem is that turbulence theory and Reynold's number calculations are based on fluids and gases.  It is easy to calculate the velocities that will produce turbulence in suspensions, but the calculations are not always accurate for such conditions.  We talk about achieving turbulent flow in slurry pipelines which will prevent particles from settling during transport through those lines, but it is not clear that turbulence is actually achieved.  It has been suggested that in many high solids suspensions, turbulence never occurs due to the presence of the particles.  The fact that the particles are present prevents the eddies and currents that define turbulence.  

High levels of shear are always necessary for mixing suspensions.  We can define those levels consistent with the definitions in the textbooks.  Whether or not they actually produce turbulent flow is the question.  If we can achieve the high levels of shear required, it really doesn't matter whether turbulence does actually occur.  

A fourth major concern is the definition of homogeneity.  I made some pan cookies the other day during Christmas vacation.  The recipe said to mix flour, sugar, and egg whites to a rather coarse consistency.  I have seen similar consistencies when mixing high solids castable bodies in Hobart-type mixers.  Is this type of mixture sufficiently homogeneous for all processes?  On a fine particle scale --  no.   For some bodies --   maybe yes.  The typical Hobart-type mixer is not a truly high shear mixer.  It is mostly a relatively low shear mixer that produces a coarse homogeneity, but not a homogeneity on a particle by particle basis.  Is that level of homogeneity sufficient for any particular process?  It depends on the process.  

On one project, we tried to extrude a very high surface area alumina powder (>100 m²/gm).  First, we mixed the body with appropriate fluids and binders in a Hobart-type mixer.  Then we tried to extrude it through a simple extruder.  The extrusions came out of the circular die looking like a Japanese pagoda.  The feathering at the edges of the extruded cylinders was terrible.  After passing the mixture repeatedly through the same extruder, after the 5^th or higher numbered pass, the surfaces of the extruded cylinders began to become smooth without tearing.  Eventually, we obtained very excellent extruded rods.  This suggested that mixing was occurring during the extrusion process that was not achieved in first mixer.  Could we have achieved better mixing during the first mixing step?  Sure!  But not with that same mixer.  We did the best we could with that type of mixer, but the homogeneity after the mixing step was insufficient for a good extrusion on the first pass through the extruder.  

The point of this concern is that just because the system looks homogeneous doesn't mean that it is homogeneous.  I have seen production plants in which the clay body suspension certainly looked homogeneous after only minimal mixing.  It was a tan suspension, approximately the consistency of a chocolate milkshake.  But it wasn't truly homogeneous because its viscosity continued to change over the next several days in a mildly agitated holding tank.  Just like the mixed alumina extrusion body, the clay body wasn't homogeneous after the initial mixing step.  In both cases, insufficient shear and insufficient mixing duration were used.  In both cases, the initial mixing step was probably incapable of producing a more homogeneous body.  Both mixers needed to be much more intense to produce higher levels of homogeneity.

The most important thing to remember when mixing suspensions is that they are a different breed of animal to deal with than when mixing fluids with fluids.  Choose production equipment appropriately to produce the high shear conditions needed in suspensions to allow them to be mixed homogeneously.  Remember that when insufficient shear is used, true homogeneity on the particle scale may not be achievable.  High shear and sufficient mixing duration are required to produce homogeneity in suspensions.  

A final concern when mixing suspensions is segregation of particles after having achieved homogeneity.  Unlike fluids in which fluid molecules can and will diffuse toward homogeneity, large particles in suspensions cannot diffuse, but they will settle when given the chance.  A low solids content suspension flowing in a pipe will usually allow the particles to settle when flow stops.  If one had to pump molasses through a horizontal 100' pipe, nothing would happen if the pump was started and stopped.  Molasses won't settle.  Because molasses has a high viscosity, the pipe can be relatively large diameter and the flow velocity can be relatively low to minimize pressure drop.  Replace this by a suspension with similar viscosity and the design is all wrong.  To prevent the particles from settling, the flow velocity should be maintained relatively constant.  If the suspension is shear-thinning, it might be beneficial to have a smaller diameter pipe to maintain a higher level of shear within the pipe to prevent settling.  And the last thing one should do when pumping a suspension is to start and stop the pump frequently.  This will allow particles to settle and cause restart problems as well as helping to allow blockages to form.  A good design for a high viscosity fluid is simply not adequate for a high viscosity suspension. 

Mixtures of Dry Powders with Dry Powders

Even when mixing dry powders, shear is required to achieve well-mixed bodies.  Many dry mixers seem to have been designed under the assumption that all powders travel as individuals.  This is not an accurate assumption.  When powders are agglomerated, or when they are not totally dry, powders will travel in agglomerates or clumps, and only sufficient shear will cause the agglomerates and clumps to break up.  Many dry powder mixers (e.g., ribbon mixers or V-blenders) do not have the capacity to produce sufficiently high shear to do this.

Successful mixing of dry powders is one of the most difficult types of mixes to achieve.  To achieve homogeneity on a particle by particle basis requires that all particles travel as individuals (which they frequently don't).  If homogeneity can actually be achieved at the particle level, it is difficult to maintain because as soon as the mixing shear conditions are removed, dry particles will tend to segregate by size and surface area.  In many glass plants, feed powders are mixed dry and then they are sprayed with fluids to try to maintain the homogeneity achieved as they are transferred from the mixers to the glass tanks.  It is a strange process to dry mix, then wet the mixture immediately prior to feeding the wet mixture into the glass tank to be dried and melted into the glass.  It is a necessary process, however, to minimize the unmixing that would otherwise occur as the dry powders are transported from one process to the next.

The maintenance of homogeneity is easy to test.  A spoonful of Folger's coffee crystals and a spoonful of non-dairy coffee creamer in a cup will show the problem.  Stir them up until you believe they are homogeneously mixed.  Then tap the cup on the table while watching the mixture.  Tap!  Tap!  Tap!  That's all that's needed to unmix the powders.   The problem in most dry ceramic bodies is that you have white powders mixed with white powders.  By definition, they will always look mixed -- even when they are not.

Summary

Regardless of the fluids or powders that are being mixed, the first major requirement of a good mixer is capability to produce shear.  Sufficient shear!  The second requirement for good mixing is a good definition of homogeneity which is based on actual measurement of an appropriate powder property (other than an eyeball measurement.)   

In my opinion, high intensity mixing is the way to achieve homogeneity regardless of the materials being mixed.  High intensity mixing allows the engineer to control the mixing conditions rather than simply waiting on lower levels of shear and longer mixing durations to produce the same results.  Sometimes the same results are not possible with lower shear and longer durations.  

Mixing suspensions presents a series of unique problems that don't exist in fluids.  Underpowered motors, abrasion, and the high probability of segregation are problems unique to suspensions.  

Dry mixing also has a segregation problem -- one that is different from that in suspensions.  But this segregation problem is equally important to the achievement and maintenance of truly homogeneous mixtures.

The biggest problem with mixing is the achievement of sufficient shear to produce the desired results.  All too often, we rely on the eyeball method to define homogeneity.  It looks homogeneous -- therefore, mixing is deemed sufficient.  This is an inadequate argument.  The production of shear in a mixer costs money.  It has to be an appropriate shear for the materials being mixed and it has to be a high intensity shear to produce homogeneity at the particle and/or molecular level.  Short of this, mixing will be insufficient and properties of the mixed materials during storage periods will vary.

If a system is truly homogeneously mixed and the storage method maintains that homogeneity, later stages in the production process will face consistently homogeneous bodies.  This is the goal of production -- consistency of all body properties at each stage in the process.  Without consistency of mixing, all later stags in the production process will face continuously changing body properties and production bodies and ware will vary as a result of the inconsistencies.

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