Volume 6  Number 7                            Dennis R. Dinger                                1 May 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   .

If you have friends, business associates, etc., who are ceramists, materials engineers, or any other type of engineer or technician, and they are interested in receiving this e-zine, please forward this issue to them and encourage them to sign up.  Or simply point them to the Dinger Ceramics web site.  Also -- whether you are a new or continuing reader -- please send suggestions for topics you'd like to see addressed in future issues of this E-zine.

"... 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.

This topic was suggested by a reader.  Part II next month will cover process and product properties

Raw Materials' Effects on Ceramic Processing:
Part I -- Slip and Pressure Casting Suspensions

Introduction

As raw materials and properties change, casting rates and cast structures change.  On one job site, we struggled with low casting rates for a long time and the solution came down to levels of chemical additives, the order in which the chemicals were added, and particle size distributions (and specific surface areas.)  In this article, we will briefly discuss some pertinent points regarding slip preparations that may be helpful to you.

Plastic vs Non-Plastic Raw Materials

The first major distinction between raw materials is whether they are considered to be 'plastic' or 'non-plastic' raw materials.  The very titles of each type points to the main distinction between the two:  the 'plastic' raw materials build gel structures which produce plastic properties, while the 'non-plastics' do not.  Since slip and pressure casting bodies are both supposed to build structures, they both need lots of 'plastic' raw materials to do so.  When one is trying to cast using non-clay, 'non-plastic' systems, one must substitute the proper chemical additives to help build the gel structures in the absence of the clay raw materials.

Four major categories of consideration are particle size distributions (specific surface areas -- SSA), rheological properties, chemical additives, and mixing.

Particle Size Distributions

In the days when we were presenting the results of our research to identify systems that could provide dense-packing of particles, many engineers told us, "but we don't want dense packing in our bodies."  Dense packing requires the right particle size distribution and severe levels of deflocculation to achieve.  Few if any processes require such bodies.  But everyone wants to achieve excellent forming properties and shear-thinning behavior.  Our research and experience showed that bodies that can pack densely (correct PSD with correctly adjusted additive chemicals) generally have the best particle size distributions with which to achieve the most desirable rheological properties and processing behaviors.  Even if you don't need dense production bodies, we believe you still need the particle size distributions that produce dense packing, but you must tune them to be partially flocculated.  Similar PSDs, different chemical tuning!

For this reason, everyone should be wanting to achieve a particle size distribution that 'can' pack densely.  For most process bodies, when dense-packing particle size distributions are achieved, but the bodies are diluted and flocculated properly, they WILL NOT pack densely, but they WILL contian the best possible particle size distribution to produce excellent rheological and forming properties.  Partially flocculated clay-based bodies WILL NOT form dense particle compacts.

          Dense Packing Distributions

Readers are referred to several earlier e-zine articles which discussed dense-packing distributions.  To summarize briefly, however:  to achieve dense-packing distributions, one wants complete, smooth particle size distributions from finest to coarsest particles.  All size classes should contain particles and the whole distribution, when plotted as a log-log histogram, should produce a relatively straight line with a slope of ~0.37 or less (for starters, we recommend a slope of 0.2.)

The reason dense packing distributions work well is because by design, the particles in dense-packing distributions interfere only minimally with each under shear.  When clay-based bodies are partially flocculated, gel structures form, which opens the structures as they tie all particles into the structure.  In slip casting systems especially, it is important that colloids be immobilized.  This means that all colloidal particles must be pulled into the forming gel structure and flocculation does this.  When shear is applied to such structures, collisions and particle interferences are minimized. Under shear, such structures can easily be broken down so particles flow as individuals.  When shear is removed, the gel structures will reform similar to those torn down by the shear.   

By way of contrast, poorest packing and poorest rheological and forming properties will be produced when all particles in a body are the same, exact size.  Such a PSD is known as a monodispersion.  Although monodispersions are really NOT distributions, the worst possible 'distribution' to have in a process is a monodispersion.  To have all particles in a single size class is BAD.  To have some particles in each size class is GOOD. 

          Where Are the Plastics and Non-plastics in the Distribution?

The particles that will gel most easily, and therefore provide the most help building floc structures, are the plastic colloidal materials.  For this reason, one wants at least some of the clay minerals to be colloidal in size.  Generally, ball clays fill this role.  Non-plastics will not provide the gelation properties.  Non-plastics can be finely milled, but when their surface areas are too high, they will adversely affect gelation and forming properties. 

It is frequently tricky to achieve smooth PSDs with several different raw materials, but we always tried to fill in holes in the histogram distributions with the various PSDs from both plastic and non-plastic materials.  The cost of finer and finer non-plastic materials typically goes up, up, up --- which helps prevent the use of non-plastics that are too fine.  But we want some fineness because we wanted to maximize the distribution of those particles throughout the body.  The finer the particles, the greater the number of those particles for distribution throughout the body.  

Typical non-plastics are the feldspars, which help to decrease firing temperatures, and the various types of silica, which help adjust the overall chemical content of the body.  It is usually a balancing act however to achieve and maintain excellent casting and rheological property behaviors while also achieving and controlling the firing behaviors of the bodies.  

We know, for example, that by purchasing (or milling) ultrafine feldspars, we can decrease firing temperatures by up to 100^oC (or more.)  In some cases, we may want to use the finest feldspars to minimize firing temperatures.  But too many ultrafine feldspars (which are non-plastics) can cause forming and gelation problems.  It is always a balancing act, and lately, more and more engineers want to produce bodies that break all of the rules.  They want excellent gelation behavior with 100% colloidal non-plastics.  Such bodies can be tricky (??impossible??) to produce and to control.

Regarding immobilized colloids, colloidal clays (plastics) when partially flocculated can pull properly treated non-plastics into the forming gel structures --- so all colloidal particles are immobilized.  Why immobilize colloids?  During casting, whether at normal pressures in plaster molds or at high pressures in polymer molds, colloids that are not immobilized will typically travel with the carrier fluids.  The first water into the mold will typically travel with the greatest velocities.  So if the colloids are not immobilized, they will travel with this first water and form the first layer of cast at the surface of the mold.  This layer will be poorly packed (monodispersions pack poorly and colloidal non-plastics will behave like monodispersions), but since the first layer is formed from the finest particles, its porosity will be high and its pore size distribution will be particularly fine.  As a result, the pores within the bulk slip may be large (providing for easy flow of water), but the pores in this first, thin layer of fine particles will be especially small (providing for poor flow of water.)  Unless colloids are immobilized so the casting process successfully compacts the gel structure within the slip (retaining its relatively large pore passages), the first cast layer of fine non-immobilized colloids can contain especially fine pores which prevent further successful casting.  In pressure casting systems with non-immobilized colloids, one solution to a slow casting rate might be to increase casting pressure.  This only makes the problem worse.  One must immobilize colloids by tying them into the slip gel structure so casting can proceed properly and quickly.

Rheological Properties

Partially flocculated suspensions generally lead to shear-thinning rheologies, good cast properties, and excellent gel structures.  Over-flocculated suspensions generally lead to extreme shear thinning behaviors, especially soft casts, and syneresis.  Deflocculated and over-deflocculated suspensions both tend to produce poor gel structures, hard casts, and dilatant rheologies. 

Keep in mind that the PSD and solids content of the body generally define the type of additive needed to achieve casting viscosities.  In many plants, solids content is considered sacred, so the only available adjustment is chemical additive concentration.  A poor PSD (resembling a monodispersion) will generally require deflocculation to achieve casting viscosities and this will maximize dilatancy.  Good PSDs generally require flocculation to achieve casting viscosities which will produce shear-thinning properties.  

Solids content can and should also be used as a variable, especially in pressure casting systems.  Many engineers believe the more water present in a slip, the longer it will take to do the casting.  But this is not correct.  The more flocculated a body, the larger the pores will be within the gel structure.  This means that the more flocculated a body, the faster the casting can be performed.

When a casting shop switchs over from plaster casting to pressure casting, some of the traditional slip characteristics (such as specific gravities) are frequently carried over.  In pressure casting systems, however, the amount of water to be removed is not a problem (like it can be in plaster casting when the molds become saturated.)  The tendency of some to use slips of identical specific gravities for plaster and pressure casting, is a logical transition.  But pressure casting can be performed quickly at lower specific gravities than could be used in plaster casting systems.

The logic for this follows:  lower specific gravity means more water and lower viscosity;  but lower viscosity requires more flocculation which generally produces larger pores in the gel structure and corresponding cast structure;  and larger pores in the cast structure allow faster water flow through the cast and shorter casting times.  In addition to pressure casting, this also applies to filter pressing operations. 

When gelation rates are insufficient, adding more water (reducing specific gravity) and more flocculant can help.  A gelation curve that proceeds slowly can sometimes be improved by these two adjustments.  Sometimes gelation rates are slow due to the types of raw materials that have been used.  For example, somewhat dirty kaolins with lots of fines and other impurities work differently from the ball clays.  This is NOT a problem with kaolins purchased from the major suppliers.  But in some countries, smaller, local suppliers may offer lower priced kaolins which have seen less in-house beneficiation before shipment.  With such kaolins, we found that gelation rates and firmer casts can sometimes be achieved by washing them before use at the production site.  We believe washing removes the fine impurities which cause SSA, gelation, and chemical additive problems.  

Chemical Additives

One always needs to 'play' around with the many available chemical additives to determine which works best with any particular body.  There is no one 'best' or no one 'correct' additive to use.  Here again, this usually requires a trade-off.  Some additives work well, but are expensive, while others don't work as well, but are within the desired price range.  Each production site needs to test the available additives to determine which works best for their body.

          Inorganic vs Organic Additives

Frequently, +2 cations of magnesium and calcium are used as inorganic flocculants in slips.  Inorganic deflocculants such as sodium silicate work by tying up and retiring the flocculating cations just mentioned.  Other organic deflocculants such as the polyacrylic acid salts work by covering the flocculation cations (like paint covers a wall.)  Flocculating cations, once removed by sodium silicates, are no longer available to the slips.  Extensive, high intensity mixing cannot bring them back into solution because magnesium and calcium silicates are insoluble.  Extensive, high intensity mixing MAY bring those cations back into solution which were covered by organic deflocculants.  Unless organic deflocculants are 'chelating agents,' it is possible that the cations could return.  This suggests that the order of addition of the additives and corresponding levels of mixing can be important to achieving desired casting viscosities and rheologies.

          Repeated Additions of Flocculating and Deflocculating Ions

Note also that when inorganic additives are used, repeated additions of flocculants, then deflocculants, then flocculants, etc. -- back and forth, will deposit the anions attached to the flocculant cations and sodium (attached to the silicate ions) into the interparticle fluids.  At some point after repeated back-and-forth dosages, all further chemical additions will cause flocculation.  We used to have our grad students repeatedly flocculate, then deflocculate, then flocculate, etc., a slip while monitoring the viscosity after each addition.  When the slip becomes saturated with sodium ions and possibly chloride ions (if the flocculant is MgCl₂ and the deflocculant is sodium silicate), all further chemical additions of either flocculant or deflocculant will cause increased flocculation.  The reason for this is that the inorganic additives work by changing the electrostatic charging on the surfaces of suspended particles.  When the conductivity of the interparticle fluid is too high because it contains high concentrations of Na⁺ and Cl^- ions, electrostatic interactions are effectively shorted out so they no longer function.

          Impurity Ions or Pre-Adjustments  

In some cases, NaOH is added early in the slip batching process to achieve a somewhat deflocculated chemical environment for the first raw material additives.  This increases the sodium ion concentration in the water and it can reduce the total amount of additive possible before the inorganic additives no longer work (as discussed in the previous paragraph.)

          Order and Place of Additions

One must also consider the order in which additives are introduced to the slips, the concentrations used, impurities in the raw materials and water, and the places in the process at which the additives can best be introduced.

Where should certain additives be added to a slip system.  If the choice is to add chemicals to the main production blunger, or to add them in the holding tank, one only needs to ask oneself which tank is better suited for mixing and distributing chemical additives.  Blunger -- Yes!  Holding tank -- No!  Even so, many processes traditionally do the majority of their chemical adjusting in the holding tanks.  Sure -- given enough time, it may work.  But a properly controlled holding tank impellor is designed to prevent settling -- not to do extensive mixing.  

In one of our systems, we pumped slip from the main production blunger through a continuous stirred ball mill, or through a continuous high intensity disperser, and then returning it to the production blunger.  Given this set up, we would add the additives to the slip at the entrance to the stirred ball mill or at the entrance to the high intensity disperser.  Our work with the coal slurry showed that the best place to add chemicals was right at the point where the chemicals were needed.  It also showed that we had to use proper mixing intensities to achieve proper distribution of the chemicals.  

We also had to be certain that we did not over-flocculate or over-deflocculate the suspension.  Sometimes we had to wash raw materials because they had been previously dosed with an additive that prevented proper control of suspension rheology.  For example, we could not produce coal slurry that had been dosed with calcium chloride at the mine.  It took us quite a few phone calls to find out what was being done differently in the winter than in the summer.  We learned that in the winter, coal railroad cars were routinely and automatically sprayed with calcium chloride as they left the mine site to prevent the coal in each car from freezing into one gigantic mass as the coal was transported during winter months.  These calcium ions poisoned our slurrying process.  Coal that was treated with sea water behaved similarly.  

Similar types of poisoning can happen to other raw materials, too.  At one of our seminars, an engineer who used barium titanate told us that at that time, there were only three major suppliers of barium titanate in the US and he could identify each supplier's materials by analyzing them for impurities.  Each supplier used a different process to make their barium titanate, so each had different impurities.  If a particular impurity does not work with your process, it would certainly be nice to know --- sooner rather than later.

Some companies deionize all water before use in slip casting operations.  This is good!  Some companies add chemicals to the water supply in the blunger tank before any raw materials are added to the blunger.  This can be good, and it could be bad.

          Removing Ions vs Masking Ions

As mentioned above, some cations can be added and completely removed again from the process.  Magnesium and calcium ions can be added as a variety of salts.  They can also be removed with the addition of sodium silicate.  Sodium ions and chloride ions --- the associated anions, however, once added to the slip, remain in the slip in the interparticle fluids.  Sulfate ions, which are traditionally important for slip casting bodies, can be removed by additions of barium ions.  

Regarding sulfate ions, it is not clear how and why they are important to slip casting systems --- but by tradition --- they are --- and they must be present at very specific levels of concentration.  This is another one of those sacred traditions that dare not change.  We once tried a test where we filter pressed a slip, added distilled water to reconstitute it, then filter pressed it again, etc. --- over and over again.  After every filter pressing, we measured the content of sulfate ions in the filtrate, and it was always present.  The overall concentration in the slip may have been reduced a little by the filter pressing, but there was always more present when the system was reconstituted with distilled water.  This suggested that some sulfate ions adhere to particles (so they are stuck somehow in the gel structure), and others are free to move into the interparticle fluid and be removed with the filtrate water.  This experiment raised more questions than it provided answers. 

This all shows that we know and understand how some ions behave in slip systems, and we don't know nor understand how others behave.  All is not clear yet!

          Partial vs Full Flocculation or Deflocculation

NEVER tune to states of full flocculation or deflocculation.  Full flocculation produces syneresis (a densifying of the gel and body structures with time.)  This is a major problem in some bodies --- especially when a soluble chemical (a cation) is present in the raw materials of the body and it enters the interparticle fluid slowly during holding.  

Full deflocculation will produce dilatancy (extreme levels of collisions between particles and rising viscosities as shear rates rise.)  I have only ever met one person who suggested that he liked having a dilatant body slip.  I saw, first-hand, what the dilatancy was doing to his process --- and I don't think it was helping him at all.  

Trust me --- you don't what syneresis (a chemical problem) or dilatancy (a rheology problem) in any process.  Both of these cause major processing problems in ceramic systems.

Mixing

          Point of Addition / Adequate Mixing Intensity

Finally a word about mixing.  It was mentioned above that the point of addition of the chemicals must be as close as possible to the point at which they are needed.  This is not always possible.  The point of addition of the chemicals, however, should always be in a place where they will see adequate mixing immediately after their addition.

If the chemicals can be added just prior to the slip passing through a high shear mixer or a stirred ball mill --- by all means add the chemicals at that spot.  We used a bucket with an IV tube to add chemicals at such locations.  That gave excellent distribution within the slip that passed through the SBM or HID mixer and that mixture was then more easily and fully dispersed throughout the main batch in the blunger tank than had we added the chemicals directly to the blunger tank.

For similar reasons, adding chemicals to the main production blunger is much preferable to adding chemicals to holding tanks.  Why?  Blunger impellors are designed for mixing.  Holding tank impellors are designed to prevent settling --- not to do major mixing.  Blunger impellors produce reasonable intensities for mixing.  Holding tank impellors are low intensity by design.  

          Speed of Additions

If the batch formula calls for three 'glugs' of chemical additive, it is better to dilute this as much as possible and to add it slowly.  To take a bottle of concentrated sodium silicate (a deflocculant) to the side of a holding tank and pour it, "glug, glug, glug," into the blunger tank can produce a rock that will settle to the bottom of the tank without ever being mixed at all.  Common sense must prevail.

Remember also:  excess concentrations of deflocculants can flocculate, and excess concentrations of flocculants can deflocculate.  The explanation of this is another subject, but when not mixed properly, many additives can produce the opposite (and unwanted) effect.  This applies to the three 'glugs' problem above.  If three 'glugs' of deflocculant are not mixed adequately and quickly, they can cause the local particles to flocculate, which produces a rock, which sinks to the bottom of the tank.  When the boss asks if you deflocculated the tank, you can honestly answer, "Yes."  But if the additive is localized in a rock resting on the bottom of the tank, the tank will not have been effectively deflocculated.

          Were Ions Removed or Masked?

Finally, keep in mind that the inorganic deflocculant sodium silicate works by removing, and precipitating as insoluble silicates, soluble flocculating cations such as magnesium and calcium.  Also, keep in mind that some organic deflocculants such as the polyacrylic acid salts mask, but do not remove, magnesium and calcium ions.  These organic deflocculants work in a similar manner to paint on a wall.  The difference between such additives and paint, however, is that the organic deflocculants are not permanently attached so they can be dislodged under high shear conditions.  

Insoluble magnesium silicate and calcium silicate will not redissolve during high intensity dispersion.  A magnesium or calcium ion that is resting under an anionic polyelectrolyte could, under proper conditions, return as a free cation to the interparticle soup.

This means that since inorganic and organic additives work differently, the order of chemical additions, and the use (or not) of high intensity dispersion conditions can affect the outcome of the concentration of additives in the interparticle soup, and the state of equilibrium of the additives on all particle surfaces.  The goal of high intensity dispersion is to free up all individual particles and additive molecules or ions into the interparticle soup so that they can re-adsorb to equilibrium positions when the shear is removed.  Insoluble silicates cannot readsorb.  If organic additives can be stripped from surfaces and cations under them can be dislodged as well, the 'equilibrium' before HID and the 'equilibrium' after HID can be two different equilibriums.

Summary

This article has discussed some considerations regarding the preparation of slip and pressure casting bodies.  Part II of this topic will discuss product and process properties that result.  

If you have any specific questions about topics in this article or about the next part, please e-mail them to me by the end of May so they can be addressed in the June article.