Volume 6 Number 10 Dennis R. Dinger 1 August 2008
Updates
"... 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.
The E-zine
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Chemical Additives: When, Where, Why, & How They Are Added
Introduction
In the last e-zine article, we discussed in some detail the nature of additives for ceramic slips and bodies. We will continue that discussion in this article. Without the help of chemical additives, most ceramic bodies could not be successfully formed. In this article, we will look at several common additives and categories of additives and consider details surrounding their use.
Hydroxyl Ions
Why?
This first ion represents the simple addition of a base to the water or slip. Hydroxyl ions are generally added for two reasons: (1) Hydroxyl ions raise the pH of the suspension; and (2) Many raw materials partially deflocculate in a basic solution.
The first reason simply says that if the original batch water is too acidic, pH can be raised by adding hydroxyl ions. The second reason is related to the first. Many ceramic raw materials tend to partially flocculate when the water or interparticle fluid is too acidic. For this reason, it is easier and almost always less expensive to partially deflocculate the body using hydroxyl ions first than to use the commonly available, more expensive deflocculants to do the whole deflocculation job. Following the original addition of hydroxyl ions, other deflocculants can then be used. The overall result is a less expensive chemical deflocculation package.
How?
The most common and least expensive source of hydroxyl ions is sodium hydroxide. This works well, but as discussed in the previous article, not only does it raise the pH by putting more OH- ions into solution, but it also adds Na+ ions which raises the interparticle fluid's conductivity. Another source of hydroxyl ions is ammonium hydroxide or other soluble hydroxides. One must consider cost and the effects of the ammonium ions (or other cations) to the behavior of the body before one uses that chemical.
When?
The simple answer is to add bases whenever they are needed. This could be before, during, or after blunging. If sodium hydroxide or some other chemical is used in a body, it can be added to pretreat the water by raising its pH, or to adjust the pH of the slip or body at any in the production process.
Make sure that you are not adding this, or any additive, too soon to prepare for a coming event --- that would have been self-correcting in the body had you waited long enough. Remember that you are usually interested in one of the two ions that make up a soluble salt. The ion (cation or anion) in which you are interested will be available to perform its intended function. The other ion (anion or cation) will typically remain in solution to muddy up the interparticle fluid and cause the conductivity of that fluid to increase.
Where?
The first ingredient into a production blunger is typically water. If preadjustment of pH or pretreatment of the water is deemed necessary, the base additions can go directly into the water. If pH needs to be adjusted anywhere later in the process, bases can be added as needed. When doing so, they should be added slowly and carefully to minimize large regions of highly concentrated basic slip. Whenever regions of the slip or body have been dosed with high concentrations of additive, deleterious results can occur.
Inorganic Flocculants --- Ca++ and Mg++
Why?
Inorganic flocculants such as Ca++ and Mg++ are usually added to raise viscosities and gelation rates. They work because of electrostatic attractions between the multivalent positive charges and the typically negative particle surface charges. These additions cause flocculation, gel structure enhancement, and higher viscosities. When added in conjuction with dilution water, they can also change the rheological nature of the slip from dilatant to shear-thinning, or from mildly shear-thinning to stronger shear-thinning.
How?
These ions must be added as soluble salts. They can be added as soluble chlorides or sulfates or other species, but the first two are sulfates and chlorides are most commonly used. They should not be added as partially soluble salts, because then a time effect occurs and slip properties will change with time. Calcite and dolomite are examples of partially soluble salts that should be avoided, if possible.
If too many such ions are in the bodies, they can be removed as insoluble silicates by adding solutions of sodium silicate. This means that the Ca++ and Mg++ ion concentrations can be adjusted (both increasing and decreasing) to achieve a particular value. If these are added as sulfates, the sulfate contents can be adjusted as well (see the sulfate discussion below), but if these are added as chlorides, the chlorides cannot be removed except by dewatering in a filter press. Once added, chlorides usually remain in the interparticle fluid.
When?
These are usually added after batching is complete to adjust suspension viscosities. These additives can also be added in holding tanks, but that is not advisable due to the poor mixing present in holding tanks.
Where?
Inorganic flocculants should be added after batching is complete, but while the batches are still being mixed in the main blungers. Since we are talking about flocculants, all flocculants should be added in the presence of reasonably high shear mixing. Because these additions raise viscosities, flocculant additives are more difficult to mix into a body. To accomplish this, they are best distributed in the presence of high shear mixing.
To add flocculants to slips in holding tanks is to invite inhomogeneously mixed body suspensions.
Inorganic Deflocculants --- Sodium Silicate
Why?
Sodium silicate is a traditional deflocculating chemical for slips. It works by removing the flocculating cations as insoluble silicates. Note: Sodium silicate deflocculates by removing flocculating cations. Other deflocculants work differently.
How?
Sodium silicate is the only commonly available soluble silicate. Most other silicates are insoluble, so they cannot be used to perform this function. When sodium silicate is added to a slip, the sodium and silicate ions go their separate ways. As soon as a silicate ion comes in contact with a multivalent cation, they combine, and an insoluble silicate is precipitated. Sodium doesn't combine with anything, but remains in the interparticle fluid. If the suspension is filter pressed, sodium will come out with the filtrate. Otherwise, sodium ions remain in the interparticle fluids where they, too, raise the interparticle fluid conductivities.
When?
Sodium silicate can be added at any time during the process. If partially soluble salts are present, i.e., those which add multivalent cations slowly into the body slip, sodium silicate can be added whenever necessary to remove the free, multivalent cations. Some desire to add this deflocculant all at once so that when multivalent cations dissolve and enter the interparticle fluid, they can be immediately removed. The only problem here is whether or not the silicate ions will find some other ions in the slip with which they can combine before the partially soluble cations appear. Our experience says that tuning chemicals should be added when and where they are needed. To add them in advance so they can wait around until needed is to reduce their effectiveness because there are almost always other ions available with which additives can combine when their primary targets are not yet available.
One should not add this to pretreat the water in preparation for the blunging operation (in expectation that it will sit around and be available when the powders are added) because any calcium or magnesium or other multivalent cations that are present in the batch water will be removed by the silicate ions. Since this is sodium silicate, any additions of this deflocculant will add sodium ions to the batch. Any time it is added to a body, the silicate ions will usually be tied up and precipitated by multivalent cations, but the sodium ions will remain in the interparticle solution.
On the other hand, if you are not using deionized water for batches, you may want to add sodium silicate to the blunger water specifically to remove the multivalent cations. If your goal is to keep a reasonably clean interparticle fluid throughout the batching operation, it is better to use deionized water than to use sodium salts to remove cations in the blunger.
Where?
As a deflocculant, sodium silicate will mix easily and well. For this reason, sodium silicate can be added at any point in a process --- wherever it is needed. It must still be added slowly and then immediately be mixed well, regardless of the point of addition. This is especially true when the point of addition is in the holding tanks.
Organic Deflocculants --- Organic Poly-Electrolytes
Why?
Organic deflocculants form the broad category of organic polyelectrolytes. These function by covering particle surfaces, surface charges, and adsorbed ions and by showing their own negative charges to the interparticle environment. With such additives, the powders exhibit the predominant surface charges of the deflocculants.
Slips containing organic polyelectrolytes often produce softer casts and these additives are easier on polymer molds.
Note, however, that the organic deflocculants function differently from sodium silicate. Sodium silicate removes flocculating cations by precipitating them as insoluble silicates. Removal is not a reversible process. Once a calcium ion is precipitated as insoluble calcium silicate, it will not redissolve.
Organic deflocculants cover and mask flocculating cations --- like paint covers a wall. Unlike paint, there is no indication that these additives every "dry" so they cannot be easily removed. It appears that they are removable during high shear conditions, whereafter, they can readsorb onto the particles in new equilibrium positions. Flocculating cations are not permanently removed from bodies by organic deflocculants, but only temporarily tied up, covered, or rendered invisible and ineffective. Depending on the magnitude of shear to which the suspensions are later subjected, it is possible that multivalent flocculating cations may or may not ever be allowed to surface again.
How?
These are generally added to body slips as sodium and ammonium polyelectrolytes. Once again, however, one must pay attention to the cations that are added to the bodies as the organics (which do the deflocculating) are added. There are numerous species that fall into this "polyelectrolyte" category. When selecting an organic deflocculant, the goal is to find the particular organic specie that best matches and fits the surface structures of the powders that are to be deflocculated.
Another less reported consideration is that these are polymers, and as polymers, they can have different chain lengths. The longer chain lengths should provide more binding power in addition to their deflocculating capabilities. If you find a particular specie that appears to work well with your body, you can then ask the supplier if other chain length versions of that same chemical are also available. If you do not ask this question, you will be sold the traditionally best acting chain length of the particular specie.
Another point of consideration: Although organic additives can perform their tasks very well, some of them are very expensive. Sometimes, the concentrations required to function properly must also be uncharacteristically high (as additives go). This forces an economic decision to be made.
When?
Organic deflocculants should be added to the body when they are needed.
If these additives are attracted to more than one mineral specie in the batch, you many want to add these deflocculants to the particular specie onto which you want them to adsorb, before you add the other powders to the batch. This might be tricky to achieve, but a necessary requirement so they will function properly. If these additives can be removed, it will be under HID (high intensity dispersion) conditions. So in this example, the additions should be made after HID but before powders are batched.
Although these are deflocculants, they should nevertheless be added into a properly sheared environment. If not, they may adsorb onto some powders at higher concentrations than planned.
Where?
They should be added in the main blunger, but they are frequently added to holding tanks. Additions to holding tanks requires careful, slow additions to achieve uniform distribution throughout the body.
Sulfate Ions
Why?
This is the ion that stumps me. Sulfate ions affect casting, but I have never seen any paper which describes how and why sulfate ions work. I do know that the sulfate ion is an ion that must be present in very specific concentrations for reasons of tradition. Sulfate ions appear to be valuable ions to be present in casting systems, but the only reason I've ever heard for using them, and for everything else about them, has been tradition.
I have seen a forming problem solved by using sulfate concentrations that were considered by tradition to be "taboo" --- sulfate concentrations were too high and in the taboo range for their use. In that case, the forming problem disappeared in the presence of higher sulfate concentrations.
I have also seen that same problem brought back when the traditionalists ordered the corrections to be removed and the body to be prepared by the original method with sulfate concentrations in the traditionally accepted range. Their words were, "Everyone knows this correction cannot work," even though the fix in the taboo concentration range was actually working.
One of my students ran a test where he filter pressed a casting slip, added distilled water to reconstitute it, filter pressed that slip again, and repeated the process several more times. He measured the sulfate contents of all the filtrates, and sulfate was always present. How, therefore, can one know with certainty the sulfate concentration in a slip without knowing the concentration of sulfate that was traveling with the dry powders? In that series of experiments, there was no rhyme nor reason to explain the sulfate concentrations we were seeing in the filtrates. My question: How does one define the taboo range of sulfate concentrations? Are we talking a taboo range of sulfate additions? ... or a taboo range of total sulfates? If so, how does one measure the total concentration of sulfates? I have never seen any good answers to such questions.
Obviously, sulfate ions work well and actually help in many slip casting operations. So we use them because they work! But, most everything we know about and do with sulfates is answered by one word: tradition! We need to learn more about why and how sulfates work in slip casting systems. Knowing wny and how they work will advance our understanding of sulfate additions in slip casting systems.
Until such time, we continue to use sulfates because they work!
How?
Sulfates are usually added as calcium and magnesium salts. The calcium and/or magnesium cations flocculate bodies. The sulfate ions enter the suspensions also and do 'something'. Some of the sulfates appear to be tied up in the gel structures within casting slips. Some of the sulfates appear to be free to wander within the interparticle fluids.
It is possible to remove excess sulfate ions from casting slips. Some will exit the body during filter pressing and casting operations by travelling with the interparticle fluid. Sulfate ions can also be removed by adding barium ions and precipitating barium sulfate.
When?
Soluble sulfates should be added as needed after a batch has been mixed, but while it is still in the production blunger. If the sulfates are added too early in the process, both cations and anions can adsorb to alternate sites where they were not intended to be.
Where?
As flocculants, sulfates should be added with sufficient shear to achieve uniform distribution throughout the body. These should not be added, if at all possible, to slip in holding tanks. A good place to add them is just ahead of a stirred ball mill or continuous HID mixer which is placed in a recirculation loop from the blunger to the SBM or mixer and back to the blunger. By doing this, the sulfates can be well dispersed within the slip in the SBM or mixer, which makes it easier for that slip to be well distributed throughout the whole batch when it returns to the main blunger.
Barium Ions
Why?
Barium ions are added to reduce sulfate ion contents as insoluble barium sulfate.
How?
Barium ions are typically added in hydroxide or carbonate form. Barium compounds are hazardous, so care should be taken when they are used. If one makes every attempt throughout processing to hold sulfate ion concentrations in the acceptable range, barium ions may not be needed.
When?
These should only be added when it is deemed absolutely necessary.
Where?
Additions should take place in high shear environments because these additions are usually in relatively small quantities, which makes it more difficult to achieve uniformity of distribution. A recirculation loop from the blunger or holding tank, through an SBM or high shear mixer, and returning to the blunger or holding tank (as described above for sulfates) would be a good place to add barium ions. This would be especially good if the recirculation loop came from a holding tank because holding tank impellors are notoriously poor mixing impellors.
Binders
Why?
We add binders to increase green and dry strengths of wares. CMC (carboxy methyl cellulose) and PVA (poly vinyl alcohol) are frequently used. By their very natures, binders raise slip and body viscosities.
How?
These are usually mixed in solution by themselves at elevated temperature and the binder solutions are then added to the bodies.
When & Where?
Binders should be added last --- otherwise, all process steps following binder additions must be performed at high viscosities --- and this is not advisable. Binder solutions are nasty thick so they must be added to slips and mixed in high shear environments or the bodies will not be uniform.
Compatibility?
Compatibility applies to all additives discussed above, but especially to binders.
Are the particular binders used in a process compatible with the rest of the chemical treatment package?
In one plant, they were adding binders to raise dry strengths. Since binder additions raise viscosities of body slips, they added their binder which raised the slip viscosity and they added their deflocculant to reduce slip viscosity back into the desired process range. After the first binder and deflocculant additions, the dry strength had not increased sufficiently, so they added more binder and deflocculant. This was no good either, so they repeated the process several more times.
It ultimately turned out that the binder and deflocculant were incompatible. When the two pure chemicals were mixed together, the binder pulled together into a ball in the center of the beaker, surrounded by the deflocculant. That same phenomenon was also happening in the body. Rather than attaching to the particles, the binder was balling up in the pores between particles --- where it was ineffective.
Make sure you test the compatibility of all additives in a chemical treatment package before adding them to a production slip or body.
Dopants
Why?
Many advanced ceramics bodies require small percentages of dopants to produce desired body properties.
How?
When a dopant must be added, most ceramists immediately think of powder additions. But remember that many dopants are simply cations or anions. For this reason, one should also consider soluble forms of the desired dopant ions. With particularly low concentrations of dopants required, consider adding the soluble dopant ions at pHs where the desired cations (or anions) are the same charges as the powder surfaces.
For example, if you want to add a small percentage of a highly charged cation to a powder, make sure the powder surface charges are not negative. If the dopant and surface are oppositely charged, the dopants will latch onto the first available powder sites and uniformity of mixing will be poor. If, however, you know that the powder surfaces are positively charged below pH=2 and negatively charged above pH=2, the dopant ions should be added to highly acidic suspensions. Positive ions will repel positive surface charges, so uniformity of mixing will be excellent. When the desired uniformity of mixing is achieved, then one can titrate the suspension to values higher than pH=2 and the dopant ions will latch onto the first available sites. Since the dopants are already uniformly distributed throughout the suspension, this should produce an excellently uniform distribution.
To try to do this with solid dopants will produce less uniform distributions throughout the body. Remember: Tiny particles, no matter how small, contain bazillions of ions. If the choice is to distribute small particles containing bazillions of ions, or to distribute the bazillions of ions as free, soluble ions, the better distribution will result from the distribution of the free, soluble ions.
When & Where?
Dopants should be added when and where they can only deposit on the desired powder specie. If there are other powder ingredients in a body, do not wait until they are all present in hopes that the dopants will attach to the right powders. Do this in a suspension of the single specie. Then mix the rest of the powders into the batch.
The Generic "Where?"
Where should additives be mixed into suspensions? This consideration applies to all additives. There are two main choices:
Add Chemicals to the Main Blunger
This is the desirable answer in almost all cases because the impellors in blungers are designed to produce high shear, good mixing conditions. If it is possible to pull a stream off the main blunger and send it through a recirculation loop containing a stirred ball mill or some other even higher shear device, as discussed above, the best place to add the chemicals is at the entrance to the mill or high shear device. This should allow small additions of additive to be dispersed well within several liters of slip, and then the treated slip can then be dispersed fairly easily and well throughout the whole batch in the original tank.
If the recirculation loop is not possible, then slow, careful addition of the additives to the main blunger is recommended. If the additive can be dripped into or pumped into the blunger near the impellor where it will see the high shear conditions provided by the impellor, that is next best. The worst way to add chemicals to a tank is to hand someone a bottle of additive and tell them to "pour this into Tank 1."
At one plant, we put the additive chemicals into a bucket and used an IV tube, valve, and needle to add the chemicals from the bucket to the slip at the entrance of the high shear device in a recirculation loop (as discussed above). This process worked well.
There are many ways to add chemicals to a tank to tune the slip. In general, just remember to use common sense when choosing the location and method to add chemicals.
Add Chemicals to the Holding Tanks
The last place one should add chemicals is to slip in holding tanks. The reason: Holding tank impellors are not designed for mixing, but to prevent settling. They are low shear impellors and they function poorly as mixers.
I am familiar with one batch house foreman who decided to do the majority of his body prep mixing in the main blungers and only the really fine tuning later in the holding tanks. He cut his batching time approximately in half by doing this. Why? The original, traditional method he was using required him to transfer body slips to holding tanks ASAP after mixing. Then, the majority of the chemical tuning was done in the holding tanks. This tuning step in the holding tanks was the slow step which took all of the time. When he decided to perform most of the tuning to the main blungers, and only the last, final, fine tuning to the holding tanks, he eliminated most of the slow adjustment steps and replaced them with the faster mixing in the main blunger tank with the main blunger impellor (which was designed for mixing). The original method took him all day. With the new method, he was finished by lunch.
Why do we add chemicals to holding tanks? Some of the answer is by necessity. When it is necessary to tune in a holding tank, additions should be slow and careful to avoid over-dosing regions of slip in the tank.
A large part of the answer, however, is TRADITION. A batch that was prepared two days ago may reside in a holding tank, and it may need adjusting. In that case. it is necessary to do the adjusting in the holding tank. Today's batches, however, have no such reason. If we do only the coarse mixing in the main blunger and then quickly transfer the body to the holding tanks to do all of the fine tuning -- that is TRADITION -- especially if the main blunger sits empty for the rest of the day until the next morning when it will be used for the next batch.
One needs to use one's head and apply some common sense when designing and performing batching and tuning operations.
Conclusions
There are many possible additives that can be used in ceramic process systems. Each must be considered in great detail by the local production plant engineers. Can you use a sister plant's processing as a guide? Yes! Must two processes at different plants several hundred miles apart be identical? No.
Each plant will have its own batching process which will be designed for their specific raw materials, batch composition, and additive chemicals --- after having considered and test all of the points and considerations in this article.
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 ...
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103 Augusta Rd, Clemson, SC 29631 (864) 654-5731
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