Volume 5  Number 8                            Dennis R. Dinger                                1 June 2007

Updates

"... for Ceramists" Series 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.  Order your 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.

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For this month's article (and the next few articles as well), we will discuss PPC, its applications, and mind set.

Predictive Process Control (PPC)

Introduction

Throughout these articles, I have given lots of pointers and suggestions for process systems.  It is now time to discuss PPC -- Why is it important?   How does it differ from normal process controls?  and Why should it be implemented?

What is PPC?

When Jim Funk and I wrote our textbook, we gave it the title "Predictive Process Control".  Why?  From the early days of the coal/water slurry project at Alfred University, which starting in 1978, we decided we would do as much with computers as was possible.  I was the computer geek on the project. 

During those days, we learned that it was possible to quickly characterize many important properties of individual raw materials.  With those results in hand, we could then calculate and predict the corresponding properties of production bodies made from those characterized ingredients.  This is even more the case and more applicable today because most of today's characterization instruments are computer-automated and compatible with the popular spreadsheet programs.  It is even easier now, therefore, to characterize the important raw materials properties, import all of the results into MS Excel® (or some other such program), and then calculate predicted results for a variety of final production body compositions.

Our experiences with many production companies suggested that they were not taking advantage of the new characterization tools nor the high speed capabilities of computers.  Note that in those days (and this was only 30 years ago [seems like yesterday]), PCs were not yet readily available and simple lab computers cost $30,000 to $40,000 dollars each.  If you wanted to automate an instrument, you had to do it yourself.  It took a lot of effort and time to automate instruments, but it was worth the effort.  Over the period of that project, we went through more than 10,000 files containing particle size distribution data and we had similarly large numbers of several other types of files as well.  My point:  We took and used an enormous amount of data during those years -- much more than we would have taken or used had it all still been manual operations.  (The tendency for manually collected data was to file it in a file cabinet where it would be seldom, if ever, used thereafter.)

If we took that much data during the early 1980s with instruments we automated ourselves, just imagine how much data is being generated today when almost every instrument available is computer-automated!!!

Back to the topic -- PPC is "Predictive" Process Control.   PPC makes greatest use of today's characterization instruments and today's high speed computers.  Using the word "Predictive" with the words "Process Control" suggests the analysis of raw materials in advance of their use to predict the properties of the bodies in which they are later used.  By using PPC techniques, process bodies can actually be "controlled" to achieve the exact, desired properties.  Computers can perform such calculations very quickly.

Current (????) Industrial Practice 

At the time of the coal slurry project, current industrial practice assumed all raw materials properties to be constant.  Actually, in those days, they didn't have much choice.  It was difficult and time-consuming to obtain most important, useful results.  Therefore, mixing constant compositions of a batch formula day after day to achieve desired body and body properties was common practice.  It appears that this common practice may still be occuring in many companies today.  Even today with our modern instruments, it still takes a lot of time and effort to measure the full range of properties of interest (which represents orders of magnitude greater volumes of better data), bring them together into a single spreadsheet or database, and then decide which ingredients and compositions to use for any given batch.  Additionally, the person-in-charge needs to understand in great detail how the different ingredients and different fundamental properties affect body properties.  It remains easier and less mentally demanding to simply add all of the standard ingredients to the ball mill, mill for an hour, and be done with it.  The batch is finished.  Voilá!  (But have the process properties been controlled?)

I remember describing the PPC process to one engineer.  He listened to the whole explanation, and then he responded, "We don't do it that way."  I knew that, but if they were not doing it the new way, wasn't it about time they considered changing their methodologies?

Graph after graph of raw material properties variations are available to demonstrate that raw materials properties always vary.  It is unnecessary to show such graphs in this article because you can all look through your file cabinets to see how each of your properties varies from shipment to shipment.  Do not think that I am being critical of raw materials suppliers.  I am not.  They are great at controlling raw materials properties.  But it is a fact that all properties still vary.  The raw materials suppliers keep variations to a minimum, but production companies which are mixing raw materials from several suppliers must minimize property variations to much finer, more precise levels (and such controls cannot be performed by individual raw materials suppliers).

A New MindSet

If PPC requires a new way of thinking -- then so be it!  That is good!  All companies brag about their process control methods.  That is good!  They should brag!  But we have seen many companies whose "controls" are really "checks" --- they are not "controls".  Are your controls really controls?  Or are your controls really checks?  If your recipe calls for 3 payloader buckets-full of clay, 1 of sand, and 2 of feldspar, and your control is to measure the properties after each batch is mixed, ball milled, and ready for production --- are you really controlling the body?

Maybe some bodies and ceramic products don't need such precise controls as PPC affords -- and the "controls" just described are sufficient.  If that is the case, fine!  That is good!  You each need to decide how precise your controls must be.  But many of today's bodies might benefit from more precise controls.  If you have gremlins and other process problems that come and go according to the phase of the moon and the direction the wind is blowing --- you probably are not monitoring or controlling some vital property that you aren't aware is a "vital" property.  Most companies have such problems routinely.  If you don't --- then you are doing a wonderful job and you don't need PPC.

In one large ceramic industry (you can guess which one), I have seen many batch houses that contain long rows of large ball mills which are each used to mix and mill bodies.  All raw ingredients go into the mills.  Completed bodies come out of the mills.  Then, each body is checked to determine if it is acceptable or not.  (That last "check" is their "control".)

I know of a tile company which needed nine different warehouses to store their tiles.  They routinely produced tiles that were the right size, plus some larger and some smaller sizes.  Of all these tiles, some had the right color of glaze, while some others were darker and yet others were lighter.  They had to sort their products and send them to the appropriate warehouses.  They had nine warehouses because three sizes and three shades of each color produced nine possible combinations.  This meant they had to match each customer with a particular warehouse.  This company, by the way, implemented PPC and eliminated the need for the extra eight warehouses.  Their new controls produce only the single desired size with the single desired color. 

The PPC mindset requires a major change of thinking for many.  A typical PPC batch formulation may look very different than a traditional batch formulation.  For example, a PPC formulation may target a specific PSD (using several size categories and amounts), along with target percentages of sodium oxide, potassium oxide, silica, ball clay, specific surface area (SSA), etc.  This compares with the traditional formulation that calls for fixed percentages of each raw material ingredient (kaolin, ball clay, feldspar, and quartz).

To understand the PPC approach, consider this example:  Let's say that your body formulation calls for 20% kaolin, 30% ball clay, 20% quartz, and 30% feldspar.  What would you do if you realized that your 'ball clay' contained 50% quartz impurities instead of the normal 0% quartz impurities?  If you do not measure quartz content in the 'ball clay' and you simply add the usual 30% ball clay and 20% quartz, the body will actually be receiving only 15% ball clay and it will contain an excess of quartz (35%).  Obviously, the new batch will not perform anywhere near that of the planned body.  Too little ball clay with too much quartz will behave very differently during forming, as well as very differently during firing.  Final body properties will be very different as well.  If we know in advance of batching that the 'ball clay' contains lots of quartz impurities, we would change the batch composition to accommodate the new 'ball clay'.  We would start by adding 60% 'ball clay' to achieve the 30% called for by the original recipe.  Then we would realize that without adding any 'quartz' at all, we will have an excess of quartz in the body.  This will force us to look for a different ball clay which contains much lower quartz impurities to achieve the desired levels of both ball clay and quartz.  Knowing the exact composition of the 'ball clay' will set a whole series of adjustments into motion to allow the daily batch to actually produce the desired recipe.

This very problem occurred in some plants in South America which were purchasing clays from small, local suppliers.  They were simply adding the appropriate amounts of their 'ball clay' (whatever it happened to be) to their bodies without paying any attention to how much quartz and ball clay were actually present in the patio labeled 'ball clay.'  Some of their yields dropped precipitously to zero (as one would expect).

So if you knew that an ingredient's properties changed in a major way from one shipment to the next, you would attempt to accommodate that change.  Right?  That is what PPC does with all important properties.  PPC calls for monitoring each important property in each candidate ingredient.  Then, one decides BEFORE mixing the ingredients into the batch how much of each should be used.  

This is "predictive" process control.  Once body raw materials are mixed, it is generally too late to control anything but the solids content and the chemical additive contents.  Controls need to be applied predictively (in advance) rather than trying to make corrections after the fact. 

Why Is PPC Important?

PPC is important because it provides a methodology for engineers to closely control the properties of their process bodies.  It is easier to control body properties by mixing appropriate amounts of ingredients in advance than to try to adjust body properties after the whole body has been mixed.  The predictive approach allows close control.  The after-the-fact-adjustment-process does not.

Why Should PPC Be Implemented?

PPC should be implemented because it makes best use of today's analytical tools and computers.  It requires engineers who really understand their processes in great detail but it allows precise control of all important body properties.  PPC is the process control methodology that best matches today's plant process methods with today's computer-automated capabilities.  Very few plants and processes, if any, should be relying on the old, seat-of-the-pants methods in today's industrial world.

PPC Implementation

Current instrumentation allows quick analysis of many (all?) important ingredient properties.  Analysis times are generally short.  Data can be made available quickly.  Many particle physics and chemical properties are additive (PSD, SSA, elemental and mineral analyses, etc.)  Therefore, one can easily calculate the percentage of each candidate ingredient required to achieve desired batch properties.

To implement PPC, it is necessary to have two or three different sources of each candidate raw material available for use at any time.  All such raw materials must be well characterized.  In a traditional ceramic industry, this may require having several patios or silos containing each different type of raw material.  Each of these needs to be carefully characterized for all important properties.  Then, the engineer-in-charge can calculate on his or her desktop PC how much of each material needs to be added to achieve target properties.  After this is accomplished, the daily batch formulation can be printed and sent out to the production staff for implementation.

This is a different way of thinking than the traditional way, but the example above with quartz impurities in the ball clay shows that this is actually a normal way of thinking to handle a known problem that is about to occur.  The only new information required to implement PPC is to know (measure) the current raw materials properties.  If we know what properties are coming in our next raw materials, we can deal with them.  This is PPC.