Volume 2 Number 7 Dennis R. Dinger 1 May 2004
Short Courses to be offered again in June 2004
If you are planning to attend or to send someone to one or more of the courses, NOW is the time to send in a registration form. The courses that will be offered are:
Course 1 -- 21-23 June 2004
Fine Particle Processing Using Predictive Process Control
(PPC) Principles
$1295.00 for the 3-day
course
Course 2 -- 24 June 2004
Performing Particle Calculations in
MS Excel®
Using the DRD Add-In Functions
$ 595.00 for the 1-day course
Course 3 -- 25 June 2004
Rheology for
Ceramists
$ 595.00 for the 1-day course
The complete short course announcement is available at: Short Course Announcement. The registration form and hotel information form are available at the web site through direct links on the announcement.
Some of you may be wondering who should take these courses and where they have been offered in the past. Professor Jim Funk and I offered a longer (5-day) version of Course 1 throughout the 1990s. A version of that course was taught at Clemson University to seniors and grad students during that same time period.
Many engineers and managers have told me over the years that they have had trouble finding ceramic and materials engineers who know anything about processing. For those who have made this statement, this is your opportunity to send your engineers and technicians to receive a short, but thorough, review of the fundamentals and application of fine particle technology to ceramic and materials processing. Send them to the 3-day Course #1. For those of you who feel the need to learn and/or review the fundamentals of these fine particle processing phenomena and/or the application and benefits of Predictive Process Control techniques to ceramic and materials processing systems, this course is for you as well. Over the years when this course was offered, we have had many engineers, technicians, and managers participate and benefit from this course. All who need to learn, use, and/or apply these techniques are encouraged to attend.
For those of you who are using (or plan to use) the DRD Add-In Functions in MS Excel®, the 1-day Course 2 explains what the functions do, and demonstrates (with sample spreadsheets) how to use them. The set of solved, sample spreadsheets that are distributed at this course can be used as the basis for calculations when participants return to their plants. The course text, which fully explains and demonstrates the use of all functions, is also a handy reference.
Course 3 is an in-depth study of the rheological properties of particulate/fluid suspensions. It covers all rheologies of importance to ceramic processing. Each type of rheology is defined and explained. The presentation includes explanations of gelation behaviors and shear-thinning, dilatant, and yield-dilatant rheologies -- all of which are important to ceramic processing. The emphasis of this course is not on understanding the mathematics of non-Newtonian rheologies, but on understanding how and why the different rheologies occur, on understanding what consequences can be expected when the rheologies do occur, and on knowing what can and should be done when and if they do occur.
If you plan to attend, please register early.
An Update
If this is the first issue of this 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 .
The two books, Rheology for Ceramists and Particle Calculations for Ceramists, can be purchased at the Books and Downloads page of the web-site. Quantity discounts are available on the paperback books. If interested, please contact me for details. Downloadable versions of each book are also available at the web-site.
As with all other issues of the E-zine, please forward this issue to any ceramists or materials engineers who might be interested. Or simply point friends and associates to the Dinger Ceramics web site.
The following topic is the result of another excellent suggestion from a reader.
Impurities Resulting from Forming Methods and Plant Procedures
The questioner requested my thoughts concerning discoloration during firing that results from trace impurities applied during certain forming methods (mechanical pressing, isopressing, extrusion, etc.) and/or from plant procedures (handling, fixtures, and/or other unknown sources.)
Impurities during Processing
Obviously, any time raw materials, suspensions, and bodies are in contact with production equipment, contamination can occur. Contamination can occur during mining, beneficiation, transport, and/or storage of raw materials, suspensions, and bodies, and then of course, contamination can occur during any of the traditional processing steps. We will look at each of these briefly. This won't be an all-inclusive list because there are an infinite number of possibilities. Some of the infinite number of possibilities cannot be controlled by local production engineers, but they may nevertheless apply.
The infinite number of possibilities brings to mind the First Law of Computer Programming. (I may have already mentioned this law in an earlier issue of this E-zine.) In the early days of computing and computer programming, before all programs were designed to be "user friendly," computer programs were supposed to be "idiot-proof." You know -- the computer programs prompts the user to enter the value of a mass (in grams) and the person enters a percentage instead, or they enter a word, or they make a typographical error. Then, if the programmer had not anticipated any of those possibilities, the program would either bomb, produce ridiculous answers, or lock up the computer. In the early days, when computing errors occurred, computers frequently locked up and had to be restarted, which took relatively long periods of recovery time -- not only to restart the computer, but to re-start the program, re-enter data, and get back to the prompt that caused the problem so you could try entering another value (and maybe cause it to bomb again.)
The First Law of Computer Programming states, "No computer program will ever be idiot-proof because idiots are so ingenious." This First Law is appropriate to many non-computer topics today. For example -- Who could predict that somebody would set a hot cup of McD's coffee on the car seat between their legs? ... and then spill it, and scald themself? You get the picture. To make a computer program totally fool-proof, one has to code each program to handle any and all possible inputs -- regardless how crazy those inputs may be. Today, generic error handling routines can be used which, for example, re-ask the question that caused the problem (if and when a problem occurs.) Such error handling routines were not always available. One had to predict all the possible answers one could give to a prompt for information, and then handle all of those possibilities. That's a rather difficult assignment, especially when an infinite number of responses are possible.
An equally difficult assignment is to predict all possible sources of contamination in ceramic bodies. It's somewhat easier to find the source of a contamination problem after it surfaces, than to predict and prevent (in advance) all possible contamination problems (many of which are not even within an engineer's or a production company's control.) When searching for all possible sources of contamination, one is dealing with an infinite number of possibilities, and it's nigh unto impossible to predict and prevent them all.
Having said that, let's consider some possibilities.
Scraping and Impact Phenomena
Before looking at individual possibilities, two generic phenomena need to be considered. Ceramic raw materials are strong in compression, so when they scrape against other materials, or they impact other materials, they can easily gouge or otherwise abrade those materials and introduce impurities. This is a most important consideration when trying to find an impurity source: Did the powder, suspension, or body scrape, impact, or abrade a contamination source?
Certainly, any time two dissimilar materials come into contact, contamination can occur. But when two dissimilar materials scrape against each other or impact each other, abrasion and contamination are almost certain to occur. Such phenomena occur frequently in certain types of process equipment -- less frequently in others. Consideration of these two phenomena will be mentioned as appropriate in the sections below.
Mining
We are accustomed to receiving pure materials from suppliers, but impurities can enter raw materials during all processing steps during mining operations. I am reminded of the large, strong magnet suspended over a conveyor belt at a raw materials processing plant. The surface of the magnet held wrenches, screw drivers, pipes, bolts, etc., that had been lost among the raw materials and had been recovered at this point in the process.
I mentioned in a previous another article that one company found copper impurities in their product that were traced back to the wires used to connect explosives during mining operations. The spool of aluminum wire had been emptied, and a worker in the field substituted copper wire until the aluminum spool could be replaced.
The question of concern is this: Is a particular contaminant important enough to any process to include it in the materials specifications? In the example above, no one expected or predicted copper impurities, so there was no specification made for copper contents in the raw material. The copper impurity content was small and difficult to trace. And it was only visible in some of the final products.
Beneficiation and/or Chemical Preparation
Depending on the raw material and the types of beneficiation processes used, surface impurities may remain on powders. This type of impurities also will be found on chemically prepared powders. If these are organic impurities, they will usually burn off during firing. But they may affect processing properties. Don't think that you are free from contamination problems because you are using all chemically prepared materials. All processes are susceptible to contamination problems!
Transport
Contamination can easily occur during transportation of materials. For example, it is more expensive to reserve hopper and tank cars exclusively for a particular material. That requires a full car shipped in one direction, and an empty car returned to the supplier. But regardless of the extra price, that is an excellent way to prevent contamination of one mineral with another. When a transportation company decides to fill the empty car with a different material for the return trip, contamination can then occur. The question is one of cleaning: How well has the car been cleaned of the second material, prior to refilling with the original material? If one doesn't realize the car has been used for a second material, cleaning prior to refilling might be minimal. The metals from which the cars are built and any surface coatings can also each contaminate raw materials. Minerals shipped in the holds of ships are also susceptible to this type of problem.
Our experience on the coal slurry project with coals shipped by rail was less than wonderful. This problem appears to apply only to coal shipments and only to the coal slurrying process, but it makes a good example for discussions of contamination sources. During the winter months, all coal cars are sprayed with calcium chloride solution to prevent the coal chunks from freezing together. Without the calcium chloride solution, the coal in each car freezes into one giant mass which cannot easily be released from and emptied from the car. With the spray, coal cars can be emptied. As I suggested, this ONLY affected our process. We were processing the coal in an unusual way -- and using it differently than the tons and tons of coal that went straight into combustion chambers. The calcium chloride solution was not a problem for any of the coal shipped directly to power plants or for other combustion uses. In fact, the calcium chloride solved a major problem for all coal going to combustion facilities. But we happened to be making the coal into deflocculated slurries, and the calcium chloride not only caused flocculation, but it ruined milling, rheological, and processing properties. No one in the coal industry knew it would be a poison to our process. In fact, the calcium chloride spray was such a commonly used technique that hardly anyone paid any attention to it. Most didn't even remember that it was a standard procedure that was used in the winter months as the coal cars left the yard. So when we asked, "What's different about this coal, compared to the first shipment we received?" brought the standard answer, "Nothing! It's exactly the same process and exactly the same coal!" It took many phone calls until we found a person who told us about this spray. Many!
Storage
Some storage yards at processing plants are neat, clean, and under cover. Some storage yards are disorganized. That is, they allow different minerals to contaminate one another. In some storage yards, many of the minerals are exposed to the weather, and runoff can them be a problem. Each piece of equipment used to move materials around storage yards can provide contamination. Payloader buckets, carts, wagons, conveyors, etc., can all contaminate the materials.
Batching and Mixing
What types of materials are used to make batch tanks and/or mixers? What materials are used to line batch tanks and/or mixers? What equipment is used to load powders into tanks and mixers? Are all of these devices well-maintained?
What materials are used for impellor blades? Sharp tungsten carbide teeth on high intensity dispersion (HID) impellor blades hold up fairly well when mixing traditional ceramic slips and slurries. Sharp stainless steel HID blades, however, turn into smooth, rounded pillow-shaped beads in relatively short order -- even when only mixing clay slurries. WC, other carbides, and stainless steels will all contaminate slurries and slips, but stainless steel wears much faster than carbide.
Ball Milling and Crushing
What types of balls and liners are used in ball mills? All milling media wear and provide contamination to batches. Are silica or alumina or steel or zirconia media used? Are rubber liners used in the mills? What surfaces are exposed to the raw materials in crushing and milling equipment? Are the production bodies sensitive to silica, alumina, steel, or zirconia contamination? Are any of these contaminants inert to the body? Can any such contaminants be removed from bodies prior to forming and firing?
Kiln Setting
Are kiln cars cleaned properly to minimize contamination? Are high pressure air hoses used to clean wares, kiln cars, and kiln furniture, or are vacuum systems used? High pressure air throws dust and particles all over everywhere. Vacuums minimize the spread of such contamination. One technique is somewhat 'easier' to use than the other. Is the better procedure used?
What types of burners are used in the kilns? High velocity burners can entrain dust and spread it around settings. Low velocity burners do not have this problem. So if kiln cars, kiln furniture, and wares are coated with dust when they enter the kiln, certain types of burners can entrain and spread dust around inside the kiln.
Firing
Are tunnel kilns or periodic kilns used? Kiln atmospheres and combustion gases in tunnel kilns flow from the firing zone towards the ware entrance. Combustion products move from the high temperature zones, in which they are released, toward the cooler zones, the cooler ware, and the flue. Below the temperatures at which organics begin to burn (~300-350degC), impurities in the flue gas stream can condense on cool ware and cause glaze and surface contamination.
Impurities during Forming
The scraping and impacting issues mentioned earlier apply here. When and where do body powders scrape against or impact the surfaces of process equipment? Comminution equipment produces impacts between raw materials, media, and liner materials. Extruders, dry presses, and injection molding equipment produce lots of scraping and abrasion.
Consider fluid flow and extrusion. According to fluid mechanics, when a simple fluid is flowing in a pipe, the layer of fluid nearest the pipe wall remains stationary relative to the wall. When a particle/fluid suspension is flowing, however, the layer of particles nearest the wall will scrape along the wall and not remain stationary relative to the wall. Professor Jim Funk often said that if the layer of suspension at the wall actually remained stationary with the wall, extrusion dies would rust. But they don't rust -- they are polished. This means that when suspensions flow in pipes, and plastic forming bodies flow in extruders and extruder dies, materials abraded from pipes, extrusion augers, extrusion barrels, and extrusion dies will all be added to the body. Auger contamination may remain internal within the extrusions as they exit the dies, but barrels and dies can contaminate the surfaces of the extrusions. Whether or not surface contamination is a problem depends upon whether raw extrusions are formed in the final shape, or whether extrusion blanks are further processed, shaped, and surface contaminations are removed.
Consider dry pressing systems. The process of pressing is only a simple contact phenomenon between the dies and the powder. But the die ejection process is a scraping phenomenon. Consider the formation of a round pellet in a die. As the pellet is pressed from top and bottom, it expands laterally and locks itself into the die cylinder. As pellets are forcibly ejected from the die, they will scrape along the die surface and the pellets' surfaces will be contaminated with die material. If the die cavity is a multipart cavity that can be opened up to release the pressed pieces, without requiring surfaces to scrape against the die for removal, contamination can be minimized.
Isostatic pressing operations are different, however, because such wares are usually formed by pressing against rubber, latex, polyurethane, or other polymer materials. In such systems, particles usually do not have the opportunity to scrape against hard die materials. Surface properties of iso-pressed pieces, however, may not be as smooth as those pressed against hard metal surfaces. There is always a trade-off to be made when selecting die materials and coatings.
We must also consider filler systems. In all of the processes just mentioned, scraping and abrasion can occur as dry powders are fed to the process equipment. The faster the flow of particles, the more abrasion they will produce. Dry pneumatic flow, suspension flow, and plastic body flow all produce scraping and abrasion. All such processes can and will contaminate bodies.
Impurities from Plant Procedures
Handling
Contamination of wares can occur any time the wares are handled by workers. It is more likely, however, that wares will be damaged by rough handling, than that they will be severely contaminated by handling. But it can happen.
Fixtures
Again, the materials from which support fixtures are made can cause contamination. The issue once again should be whether or not scraping or impacts occur when using the fixtures. Rough handling can cause sharp corners to be broken, or glazes to be scratched. The question then becomes: Are wares located so broken pieces and dust from other wares cannot fall on them and further contaminate their surfaces?
Miscellaneous
The search for sources of contamination requires good detective skills and common sense. Some plants are quite dusty, and the dust settles up in the rafters over process equipment. Murphy's Law obviously applies to this problem. Whenever it's absolutely necessary that no contamination occurs, that's when it will occur. Sometimes contamination occurs due to temporary construction projects. Anything that causes major vibrations and jarring can cause dust contamination. A drill rig may be thumping along, drilling a well outside a building. A backhoe may be breaking blacktop or concrete to dig a hole. A car with well-amplified woofers may drive by. A forklift driver may bump a building support. All such phenomena can produce contamination. The only advantage to some of these types of unusual occurrences is that they aren't routine, continuous sources of vibration or jarring.
Some sources of contamination are continuous. We must constantly be on the lookout for all contamination sources.
Summary
Surface contamination will occur whenever powders, suspensions, and ceramic forming bodies scrape against or impact with the surfaces of process equipment. Look for it.
Other sources of contamination can be totally elusive. When contamination occurs, one has to analyze all possible local sources and all possible sources back to the original deposit. Do not presume, however, to point the finger at your suppliers' procedures. Don't be afraid to ask questions of your suppliers, but don't automatically assume "We didn't do it! -- therefore, you must have."
If you're very lucky, no contamination problems will ever occur. (Do you know what fat chance means?) If you're not so lucky, such problems will come and go without ever having been solved. If you're lucky, contamination problems will occur and remain long enough for you to identify and eliminate them.
Miscellany
Please continue to send your ideas or questions for future topics. Thanks.
Until next time ...
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Copyright © 2004 Dennis R Dinger
103 Augusta Rd, Clemson, SC 29631 (864) 654-3155
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