Volume 6  Number 9                            Dennis R. Dinger                                1 July 2008

Updates

The E-zine

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"... for Ceramists" Series Books

          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.

          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.

This topic was suggested by a reader.

Raw Materials' Effects on Ceramic Processing:
Part III -- Ions and Solid Impurities in the Raw Materials & in the Interparticle Fluid

Introduction

As raw materials and body properties change, casting rates and cast structures change.  One of the problems we have had regarding casting, cast rates, and hard/soft casts has to do with impurities in the raw materials.  As materials are added into production batches, soluble impurities and impurities of all particle sizes appear in the batch.  Some soluble impurities transfer from powder surfaces to interparticle fluid.  Colloidal particles contribute their enormous surface areas to the batch.  Etc.

Both types of impurities cause casting and cast problems.  Both must be dealt with in some way.  And these two problems work together --- the particle impurities provide the surface areas to carry the chemical impurities.  

Particle Impurities

Most companies control their particle size distributions (PSDs) so this should not be a problem.  But when a company attempts to use local raw materials which are less expensive than the 'good' materials shipped from afar, one must be careful of impurities that arrive with the raw materials.  

          Beneficiation

All raw materials have impurities.   The question is:  Which species form the impurities and how many of them are present?  You can count on all raw materials having impurities.  The 'good' materials mentioned above tend to be well-washed and well-cleaned by the supplier before shipment and processing.  If you are doing the cleaning and beneficiation yourselves, it then becomes your responsibility to achieve 'good' materials.

Here's a question for you to ponder:  If you are preparing a raw material and you can use between 1 to 10 gallons of water per pound to clean the material, how much water will you use?  ... and why?  There is typically one answer but for two reasons.  The answer:  You will use as little water as possible because water is a valuable and expensive ingredient in batches.  Why?  The reasons:  (1)  Cost.  1 gallon is less expensive than 10 gallons, so you use as little as possible.  (2)  The customer (which may be your own company) cannot tell the difference between material washed with 3 gallons of water and material washed with 10 gallons.  If there is no apparent difference, you choose the lower amount for Reason #1.

IF a company does not have the capabilities to measure all impurity species, to err on the safe side, they should use the larger amount of wash water.  Some companies prefer the least expensive forms of raw materials.  Just remember --- 'least expensive' frequently means 'least well processed.'

          Particle Size vs Surface Area

Typical non-plastics, such as calcined aluminas, have surface areas in the single digits range (e.g., 2.0 m²/g).  But if you want to use aluminas for catalysis, aluminas are available that have greater than 100 m²/g.  Calcined aluminas can have median particle sizes in the 1-10 m²/g range.  The catalyst aluminas are tiny by comparison.  These examples of aluminas are typical of a wide variety of other powder species. 

Some companies have the particle size analysis and other analytical instruments that can measure coarse as well as colloidal size ranges and analyze for mineral species.  Those that do can try to separate impurity particles, measure their sizes, and analyze their mineral species.  

Also note that for some raw materials, the least expensive way to buy 4 micron powders is as 100+ micron agglomerates.  The suppliers can mill such powders to free up the individual particles (for a price.)  Or the customer can do the milling.  When you choose the least expensive material, you may need to do further processing such as milling, prior to using the materials in production batches.  This applies to a wide variety of materials.

The smaller the particle size, the higher the specific surface area (SSA).  Consider a 1cm³ cube which has 6cm² of surface area.  If you cut this first cube into eight ½ cm cubes and then again calculate the total surface area, you now have 12 cm² of surface area.  As you continue to halve these smaller and smaller cubes, the total surface area per 1cm³ of material increases by a factor of 2 each time.  

One cm³ of 10 micron diameter spherical particles will define a SSA of 0.597 m²/g.  One cm³ of 0.01 micron diameter spherical particles will define a SSA of 597 m²/g.  

Surface areas become enormous as particle sizes decrease into the colloidal range.  Two factors are important to consider regarding colloidal impurities:  (1)  Chemical impurities (ions) usually attach to particle surfaces;  and (2)  Colloidal particles usually cling tightly to larger particles and/or to each other.

Why is this important?  A little bit of the wrong impurity in a material can define an enormous SSA and carry with it a large quantity of chemical impurities.  Or said from a different point of view, a little bit of tiny particles can provide the carrier surfaces for chemical impurities that can literally kill the process.

Removal of such colloids is difficult, although possible.  It is best accomplished by a supplier who can make use of both the coarse and fine fractions after separation.

         The Nature of the Impurity Particles

In one plant where local kaolins are used, they wash them on site before use.  A different variety of local kaolin was substituted for this first one, but it also needed to be washed on site prior to use.  

The original reasoning suggested that the fine impurities travelling with the kaolins were no different than ball clays which were also added to the batches.  But something was different.  To my knowledge, it was not determined what species these fine impurities represented.  Sand (silica) tends to be a major impurity in ball clays and kaolins and there was a lot of sand removed from the kaolin by this washing.  It is possible, therefore, that the colloidal particles removed by washing were tiny silica particles which will behave very differently in body slips from similarly sized clays or kaolins.  Pure silica is a NON-plastic material, while ball clays and kaolins are plastic materials.  If silica forms a substantial portion of the fines in a body, that body will behave very differently from one which has only plastic fines in those same size ranges.

It is not necessary to actually identify the species of each impurity powder.  It would be valuable information if possible to determine.  But knowing that they must be removed and actually removing them prior to batching are the important matters.  In the case of this example, they were removed by washing prior to batching to improve the casting process.

Once again, this same line of reasoning is applicable to all types of ceramic processing --- whether traditional ceramics or advanced ceramics.

Chemical Impurities

The surfaces of particles will attract and carry many chemical impurities which can be detrimental to forming processes.  The question to be asked here deals with whether or not the materials have ever been exposed to such impurities.  

          Two Coal Examples

I'm sure, if you have been reading these e-zine articles that you have heard this one before, but I'll tell it again anyway.  (What good is getting old if you can't repeat your stories??)

To make our coal/water slurry (in the late 1970s and early 1980s), we were milling coal + deflocculants to achieve high solids coal/water slurries with very low viscosities.  One particular coal made great slurry.  We picked up our original truckload of the coal at the mine in West Virginia during the summer.  When we ran out the coal during the winter, we were going to drive to the mine again, but they told us we could get the same coal at a distribution site in Buffalo, NY (which was much closer).  So we picked some up there.  To make a long story short, we could NOT make slurry with that coal.  It just didn't work.

We concluded that something was different, but no one admitted to doing anything differently.  Finally, after many, many, many phone calls, someone commented that it was winter and the rail cars (in the winter, but not in the summer) are automatically sprayed with calcium chloride solution to prevent the coal from freezing during tranport into gigantic rail car sized and shaped particles that would lock themselves into the rail cars (and could not be removed) until they thawed in the spring.

Calcium chloride is a powerful flocculant, so to start with coal covered with a powerful flocculant made it impossible to successfully make slurry.  Hours were spent washing this coal to remove the calcium chloride.  At the same time, we sent someone to West Virginia to pick up some more coal that had not been sprayed.

At the same time, we were testing coals from around the world to determine whether they could be used to make good slurry.  Some of the British coals were deep mined from sites below the ocean, where they had been subjected to sea water and its many, many ions.  These coals too carried surface chemical impurities that prevented us from making good slurry from them.

These two examples are reminders of the power of chemical surface impurities that travel with raw materials.

          Mixing Within a Production Plant

Another way to vary the distribution of chemicals in slips and suspensions regards the level of mixedness of each batch.  Poor mixing, which typically occurs in holding tanks, and/or the hurried addition of chemical additives, can achieve suspensions with high concentrations of additives in some regions of a batch, and low concentrations of the same additives in other regions of the same batch.  In some cases, additives (when added in too high concentrations) form rocks within the suspension which settle out.  This can easily occur in storage tanks which are not designed to do mixing.  All you need is a person who is in a hurry to quickly empty a liter of additive into a holding tank to tune the suspension.  In high concentrations, flocculants can deflocculate and deflocculants can flocculate.  Without sufficient mixing energy, concentrated regions of deflocculants can flocculate and settle out before they are homogeneously distributed throughout the batch.  "Did you add the deflocculant?"  "Yes!"  "Well, add some more -- it isn't working!"

In the event that the concentrated volume of deflocculant hasn't totally been removed, it will reappear in the body as soon as it sees sufficient mixing energy.  That could be in a pump, in the piping, etc.

The reason this has been included in the section "Chemical Impurities" is that the concentration of chemical additives present in a slip may be totally different than expected, and therefore, it will appear similar to the cases when you are dealing with impurities.  It is also important from the point of view of having the proper amount of chemical additives in a batch (which will be discussed below).

Impurities in Water

The other important raw material in most ceramic plants is water.  How pure is your water?  Should a plant go through the process of deionizing all batch water, or can the water be used directly without further processing?  The answer is, "It depends."

          Non-Industrial Examples

The easiest example to show this deals with the coffee maker in my kitchen.  When we lived in Alfred, NY, where they have naturally hard water, I had to clean my coffee maker weekly for water to flow through it properly.  Wash it --- you know --- fill it with vinegar and run it to clean out the calcium deposits in the tubing.  Had I not done this, it would have clogged completely within a month --- and I would have thrown it away and bought a new one.

We also ran a humidifier during the winter in our lab at Alfred.  We were in a lab in a new building where the air in each lab was changed several times per hour.  In the winter time, the air coming in was essentially dry (i.e., very low humidity).  Static electricity was a major problem.  Huge sparks frequently jumped from my fingers to my keyboard.  One of these sparks ruined one of the terminal ports in the computer.  So we bought and used a humidifier.  It was essentially a large tank of water within which the mechanism dunked a large fiber belt.  When the belt came out of the water, a large fan blew air through it, which deposited the moisture in the room.  We had the same problem with this belt as with the coffee maker.  Every week or two, the belt crusted up and solidified from the calcium deposits and had to be cleaned so it would work properly.  In this case, we could break up the deposits by crushing the belt with our hands.  Then, the deposits loosened and they could more easily be removed.

Back to the coffee maker.  We have been in Clemson, SC, for 20 years now, and I have never had to run vinegar through the coffee maker.  The water here comes from the SC lakes systems and the water is naturally soft.  I'm sure it has impurities in it, but not the calcium impurities present in Alfred's hard water.

          Industrial Considerations

It is almost a necessity to deionize production water when the local water is hard, like it is in upstate New York.  Calcium ions are flocculants, so an abundance of calcium ions is not the best impurity to expose dry powders to as batching takes place.  My initial tendency would be to assume that a deionization system would be absolutely necessary in upstate New York, and because I have had no problems with my coffee maker here in SC, it would not be necessary to deionize local water to build a ceramic production plant here.  But, the decision to deionize, or not, should be made after testing the local water.  Chloride and fluoride ions (which are present in the SC water) may also be detrimental to ceramic production processes, even though they do not form deposits typical of the calcium ions.

My advice is to err on the safe side --- if it is a concern that local water may contain detrimental impurities, clean up the water and eliminate the uncertainties.  This does not mean using giant water softeners, either, because they exchange sodium ions for the magnesium and calcium ions that they replace.  Excess sodium ions also can cause equally bad problems in production slips and bodies as calcium ions.  At least excess calcium ions can be removed from suspension.  Excess sodium ions cannot.

Several plants around the world use deionizers.  Is it an expensive process?  Yes.  Is it a necessary process?  Yes.

Proper Amounts of Chemical Additives

          How Many Impurity Ions or Additives Should We Use?

If asked the question, "How much of each additive chemical should be used in a process?", the answer should always be "the least amount necessary to achieve the desired result."

What control over chemical species contents do we have?  We can control the amount of additives that we use.  We have little control over the impurities that are present in the water and/or in the powders used in a batch.  The best we can do is try to remove all impurities in the water and use powders with no impurities.  The impurities can be fairly completely removed from batch water, but we must either live with impurities in the powders, or resort to further powder cleaning operations at the production site.

          What Problems Can Occur?

The biggest problem that can occur when too many impurity or additive ions are present is that the conductivity of the interparticle fluid increases and increases to the point where electrostatic interparticle forces no longer function.  When this happens, the ability to flocculate or deflocculate is no longer possible.

                    Interparticle Fluid Conductivity Too High

If you alternate between flocculation and deflocculation, over and over again, eventually the conductivity rises to the point where electrostatics no longer function.  I always bumped up against this in the labs at Clemson University because I'd add too much deflocculant to the 5 gallon batch of slip and the viscosity would decrease too far.  Then, I'd add flocculant and being impatient, I'd add too much until the viscosity was too high.  This process would be repeated until eventually, the viscosity was within the target range.  

Consider:  We typically used calcium or magnesium chloride (or sulfate) as the flocculant, and sodium silicate as the deflocculant.  The silicate ion removes excess flocculating cations.  Let's use calcium chloride as the flocculant and sodium silicate as the deflocculant in this example.  Adding too much flocculant adds calcium ions and chloride ions.  The addition of sodium silicate removes calcium ions because sodium silicate is soluble, but calcium silicate is insoluble.  So the silicate bonds with calcium ions to precipitate calcium silicate.  Adding more calcium chloride will flocculate again, and adding more sodium silicate will remove the flocculating cations (calcium) and deflocculate again.  But notice --- nothing happens to the chloride ions that are added with the calcium chloride and nothing happens to the sodium ions that are added with the sodium silicate.  

Repeated use of these two chemicals allows the addition and removal of calcium ions.  But every addition adds more and more chloride or sodium ions to the interparticle fluid.  Over time, repeated use of these two chemicals adds higher and higher concentrations of NaCl to the interparticle fluid.  If you want to ruin a slip or production body, dump NaCl into it.  That is what you are doing as you repeatedly flocculate and deflocculate.  

Put this phenomenon together with the unknown levels of impurity ions in the water and raw materials and you have a real problem on your hands.  At some point, when fluid conductivity is too high, all further additions of any ionic species will FLOCCULATE.  When fluid conductivity is too high, deflocculants don't work.

This point of concentration is difficult to pin down, especially when impurity ion concentrations fluctuate from batch to batch.

Sodium hydroxide additions can also be used to set pH, but notice that NaOH contains SODIUM ions, which contributes to this problem.  Do you want to be adding more and more chemical additives when one of the ions (the cation or the anion) is typically useful, and the other ion (the anion or the cation) is typically unremovable and simply adds to the conductivity of the interparticle fluid?

                    Can the Impurity Ions Be Removed?

Many additives are sodium salts because sodium salts tend to be soluble.  For example, sodium silicate is soluble.  Most other silicates are not.  Even with organic additives, many of them are sodium additives as well.  For example, sodium polyacrylate.

To my knowledge, it is essentially impossible to remove sodium and chloride ions once they are present in the interparticle fluid.  The question then follows:  Can you achieve desired properties without adding more and more of these unremovable ions?

Which ions can be removed?  Calcium, magnesium, and other soluble +2 and +3 ions can be removed by silicate ions (and vice versa).  Sulfate ions can be removed by additions of barium ions (and vice versa).  Removal of ions requires chelating agents for each particular ionic species.

Many ions are not removable.  Sodium, chloride, hydroxyl, and most polyelectrolyte ions cannot be removed.  Once you add them to the body, they will remain.  Some of these ions can be removed with the filtrate water during dewatering operations, but they cannot be removed by additions of other chemicals.

The sum of these considerations is this:  Do not add any more chemicals (impurities or additives) to a body than absolutely necessary!

Order of Chemical Additions & Point of Addition

Two other considerations are that (1) the order of chemical additions is important, as well as (2) the particular point at which the addition enters the process slip is also important.  

          Order of Addition

If three or four additives must be added to a suspension, they should not all be added at once.  We learned on the coal slurry project that the best place to add a chemical to the process is exactly at the point where it is needed and at the exact time when it is needed.  If the additives are all added at the same time, they will adsorb and combine in ways that are not wanted --- and frequently not expected.  They should be added one at a time in the order in which they are needed to do the particular task at hand.  Nor should they be added in excess, because in many cases, once the species has adsorbed onto a particle, it will not easily desorb so it can be repositioned.  

Order of additions need to be thoroughly tested in the lab.  It is not an easy series of tests, but it is a necessary series of tests.

The key thought in this section is to add only as much as necessary to do the task at hand.  Excess additives will typically be tied up in the wrong places.  You can't simply add any and all chemicals to a suspension and expect them to do what they are supposed to do while they wait around until the desired opportunities appear.  You must add them one at a time so they will perform the one function you want them to perform.  Then, you add the next one at the appropriate time to perform its desired function.  Etc.

          Point of Addition

Not only is it important to add chemicals in the order in which they are needed, but they must be added exactly when and where they are needed.  Additives are not little robots which you can program to do exactly what you want them to do.  Additives are simply chemical species that will adhere or adsorb on powders, or remain suspended in the fluid.  All of these phenomena will occur as the chemical species randomly come into contact with each other or with powders.  If the site at which you want a particular ion to adsorb is not ready, it is highly probable that that ion will adsorb somewhere else.

On the coal slurry project, we learned that if we could add deflocculants at the slurry entrance into a stirred ball mill, the deflocculants would adsorb onto the new surfaces created within the ball mill.  Adsorption isn't instantaneous, so adding the chemicals at the entrance to the stirred ball mill is close enough to where they will be needed --- so they have a high probability of reaching their desired adsorption sites --- which is good.  BUT, adding half of the additives at the entrance and half of the additives half way along the stirred ball mill is better (and this can be done on stirred ball mills).  BUT, adding 10% of the additives at the entrance and the remainder at 10% increments along the length of the stirred ball mill is best.   

Add each chemical when and where they are needed and you will optimize their desired performances.  Add them all at once to allow each of them to do their thing in the suspension --- will not work well.  Each additive will have alternative sites and alternative functions that they can perform when they enter a suspension.  When you do this, you are rolling the dice.  "Well ... I hope each chemical species knows what I want it to do in this suspension." 

Chemical additives don't work that way.  They will seek equilibrium locations as soon as possible --- and many such equilibrium locations will not be the locations you desire if you offer chemicals several different choices.  

Processing Consequences 

Too many impurity ions, or too high interparticle fluid conductivities, or too many additives adsorbed on the wrong sites cause soft casts.  The achievement of a high gelation rate, as shown in Figure B in Part II of this topoc, does not guarantee a firm cast.  Both the proper shape of gel curve AND a relatively clean interparticle fluid are needed.  The word "clean" suggests a minimal number of sodium and chloride ions (or other ionic species) and a low conductivity in the interparticle fluid.  

It should be relatively easy to achieve desired gel curve shapes using chemical additives, but it may not be possible to achieve the desirable firmness of casts until powder impurities are removed and chemical additives are minimized.  

Non-plastic colloidal particles and excess additive chemicals contribute to the softness of cast wares.   

Conclusions

In this article, we have considered many of the phenomena that apply to gelation, slip casting, additives, and cast properties.  This whole subject encompasses a very complex process.  It is NOT a simple process, but a very COMPLEX process.  Each production company needs to consider all of these phenomena and test their bodies with these phenomena in mind.

The subject of chemical additives is also very COMPLEX.  Everybody's production body is different:  different raw materials, different chemicals, different processes, different etc.  For this reason, there are no simple, all encompassing answers.  We can point out considerations, but it is up to each engineering group to test when and where these different phenomena impact their processes.

I wish it were simpler than this, but it is not. 

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...