Volume 5  Number 9                            Dennis R. Dinger                                1 July 2007

Updates

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"... 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.

This month's article is another in a series discussing PPC, its applications, and mind set.

Traditional vs PPC Batch Formulations

Introduction

One of the biggest differences between Predictive Process Control (PPC) techniques and traditional techniques are manifest in the way one specifies and formulates each batch.  Over the years, many engineers have given us horrified looks when we said. "The PPC batch formula can change on a day to day basis."  If we assume that all raw materials have constant properties day to day, shipment to shipment, and batch to batch, then we have no need to change batch formulas.  But when raw materials properties vary slightly from day to day, how do we accommodate the variations?  Can we accommodate the variations?  Or do we simply ignore the variations?

Certainly, if we know that materials properties are constantly varying, we should try to accommodate those variations.  Knowing that properties vary constantly is easy!  They do!  Knowing which ones vary, in which direction, and by how much --- is the problem.  PPC procedures require that we characterize each material IN ADVANCE OF USING THE MATERIAL so we know which properties are varying and by how much.  If we knew all these things, we would change batch formulas to accommodate the variations to produce batches with constant properties to the plant.  (See the example in the previous e-zine in which the quartz impurity content of a ball clay varied.)

On the other hand, if we are NOT characterizing ingredient materials in advance, but we are simply assuming that the suppliers are shipping us materials with constant properties, then we CANNOT make any adjustments to our batch formulations.  If we know the surface area of one ingredient has increased by 50% from the previous shipment, we certainly would try to correct/adjust for it in advance in the batch formulation.  If we know that the impurity level in one ingredient has jumped substantially from the previous shipment, we would certainly try to correct for it in advance (once again, see the previous e-zine article for this example.)  But if we don't know that any properties have changed, or which properties have changed and by how much they have changed, we CANNOT make any advance adjustments.  

If we don't know that anything has changed from the previous shipment, we can only continue to use our standard, traditional formula and we must wait until the batch has been mixed to learn its properties.  Then, if possible, we must change our forming procedures to accommodate the property variations that exist in the current batch.  This is a standard practice in some plants.  The production foreman arrives early to learn how the day's body is behaving so he can tell his production staff how they will need to treat the body to successfully make ware that day.  When this is the standard operating procedure in a plant, this must occur EVERY DAY!!  The fact that this process is in place in a plant tells us that the company KNOWS that their body properties vary from day to day --- and this is the way they compensate for it --- they change production procedures to accommodate the changing properties.  

I have heard many times over the years that casters in slip casting shops want slips with CONSTANT PROPERTIES from day to day.  They say they would gladly accept lousy properties --- as long as they could get those same lousy properties every day --- day after day.  They want CONSISTENCY!!!!  ... and even a consistently lousy body exhibits consistency.

The PPC formulation addresses this.

Traditional vs PPC Formulations

The following is a traditional formulation shown side-by-side with a PPC formulation:

                                  Traditional                                      PPC

                        20% Herman's Ball Clay                     8.5-9.0 SSA
                        30% Clyde's Kaolin                            4.0-4.3 MBI
                        20% ABC Quartz                               n = 0.3-0.32
                        30% XYZ Feldspar                            4.0 - 4.1 % alkalis
                                                                                  40 - 42% total SiO₂ 
                                                                                  etc.

(I created these names and recipes, so if there actually is a Herman's ball clay, a Clyde's kaolin, an ABC Quartz, or an XYZ Feldspar, I apologize.)

          The Traditional Recipe

Traditional recipes are based on percentages of common raw materials.  The assumption behind this type of  recipe is that each raw material has properties that remain constant from year to year.  I know that some suppliers have different named materials that they have tried to maintain constant over the years.  Again -- I have no beefs with raw material suppliers.  They are excellent at providing consistent materials from shipment to shipment and from year to year.  But they ARE NOT in positions to offer the precise controls required by each ceramic production companies.

Ceramic production companies must mix several raw materials, usually from several suppliers, and do this while providing consistent properties from production batch to production batch.  This is not the type of problem that can be pushed onto any one raw materials supplier (or all raw materials suppliers).  They each have their own problems maintaining consistency, but they have no idea which other raw materials (other than their own) go into each customer's body formulation.  For this reason, each production company MUST BE RESPONSIBLE for their own body's consistency.  It cannot be passed off onto the suppliers.  It is the production company's problem.

I have had this discussion with engineers who do not want to be responsible for their own body's properties.  Sorry -- but if you are a process engineer in a ceramic production company, body property consistency IS your problem.  You cannot push it off onto your suppliers.  If your body properties vary severely from batch to batch, it is your fault (the ceramic production company's) -- not the suppliers'.

By saying this, I am not absolving all raw materials suppliers of their duties to continue to produce and ship materials with constant properties.  If a single raw material's SSA increases by 50% from one shipment to the next --- that is a producer problem.  If the production company doesn't notice that a single raw material's SSA increased by 50% from the previous shipment --- that is a production company problem.  Everybody has their problems, and there's lots of blame to go around.  

The point is, however, that if your recipe calls for 20% of Herman's Ball Clay, and the bags are labeled "Herman's Ball Clay", the tendency is to assume that it is exactly the same "Herman's Ball Clay" you have been using for the last 50 years.  With that kind of thinking, the recipe is just exactly like the traditional recipe given above.  It has been that way for 50 years.  It will continue to be that way into the future.  End of story!

          The PPC Formulation

The major difference between Traditional recipes and a PPC Formulation is that the PPC Formulation is based on materials and body properties.  The assumption with traditional formulations is that the consistent properties provided by the consistent ingredients will produce consistent body properties.  PPC formulations simply target desired body properties instead of ingredient percentages that are supposed to produce those body properties.  With PPC, one does not assume that properties are always constant.  One assumes properties always vary, and therefore we measure those properties to learn exactly what they are and how they are varying.  Daily ingredient batch percentages are then adjusted to values that will produce desired body properties.  

If the traditional formulation is supposed to produce a consistent specific surface area (SSA) of 8.5-9.0 m²/g, then one sets an SSA target as part of the PPC formulation to 8.5-9.0 m²/g.  Using PPC, when the SSA of a particular raw material increases, its percentage in the body must decrease, or other raw materials percentages must be decreased to produce the target SSA body property.  We do this with ALL important body properties.

In this regard, the PPC formulation is a more fundamental description of the body that is supposed to be produced by the traditional formulation.  

Consider the 40-42% SiO₂ parameter in the PPC formulation above, for example.  Each of the raw materials listed in the traditional recipe contain silica.  Some of the silica is free quartz.  Some of it is bound into each of the minerals.  Total silica present in the fired body might be an important ware parameter.  If so --- then it should be specified as a target parameter in the PPC formulation.

Different types of alkalis are contained in different feldspars.  Maybe it is important that the total alkali content be between 4.0-4.1% as shown.  If so, that should be a parameter in the PPC formulation.  If it is necessary that 3.0-3.1% be sodium and 1.0-1.1% be potassium, then the specification should be changed to reflect those needs.

Notice the other parameters suggested in the PPC formulation above.  SSA is total surface area.  That is important because it relates to the efficiency of the additive chemicals that are used in each body and to the distribution of fluids within the body.  Methylene Blue Index (MBI) characterizes the plastic surface area contributed to the body by the ball clays and kaolins.  The traditional recipe assumes MBI (whatever it happens to be) is constant from batch to batch.  The PPC formulation shows this to be an important parameter --- important enough that it needs to be carefully specified and controlled.

The distribution modulus, n, relates to the particle size distribution of the body.  It characterizes the combined particle size distribution of all ingredients in the final body.  The distribution modulus, n, is just one way to characterize particle size distribution in the PPC formulation. It might be better to target percentages contained in each of several particle size classes than to simply calculate an overall distribution modulus.  It might be better to calculate the total number of particles, or to calculate an estimated packing factor --- each of which would also characterize PSD in the PPC formulation.  These are choices each plant must make regarding their own production bodies.  It is ABSOLUTELY NECESSARY, however, that one or more PPC parameters be directly related to particle size distribution.

The 4.0-4.1% alkali content takes into account any alkalis that are added to the body.  When sodium hydroxide, sodium silicate, and other sodium additives are routinely added to ceramic bodies, those sodium contents should be taken into account in the body formulation.  The PPC formulation allows one to do so.  The traditional recipe simply ignores the additives.

The total silica content in the body also takes into account the chemical variations in each raw material by targeting a total body percentage.  This parameter was chosen as an example.  Maybe it is more important that total alumina content should be targeted, or total iron oxide (impurity) content, or some other metal oxide, or all of them, be targeted.  If so, those percentages should be chosen, specified, and implemented as part of the PPC formulation.

Parameters in the PPC Formulation

Parameters chosen for the PPC formulation should take into account particle physics and chemical properties --- parameters that relate to forming properties as well as to final fired body properties.  Many particle physics parameters relate directly to forming and firing properties.  Chemical parameters tend to relate more to firing and fired ware properties.

The point is that the PPC formulation targets ALL important properties that define a ware:  both processing and ware properties.  If dry and fired shrinkages are important properties, then specific PSD parameters that define those properties should be included in the PPC formulation.  If certain chemical species are required in the final wares (and this includes impurity contents), then those parameters should be included in the PPC formulation.  

Figure out which properties of your ware are important, which define the final composition of the ware, which define the forming properties of the ware, etc., and use those parameters to define the PPC formulation.  Is viscosity and rheology important?  PSD parameters in the formulation are a must!  Is firing temperature important?  Alkali and other chemical contents should be included.  Etc.

Changing the Formula from Batch to Batch

When we say that the actual batch formula changes from day to day, some people seem to think we are suggesting that a whiteware body one day becomes a refractory body the next day.  This is NOT the case.  We are saying that if the MBI of one ball clay decreases in the current batch, and the plastic properties (MBI) are important for forming, then some percentage of another ball clay should be added to the current batch to maintain consistency of MBI from batch to batch.  Using the PPC methodology, this kind of adjustment happens simultaneously for all important properties.

The PPC formulation, even with all the formula changes, produces a much more constant body from day to day than the traditional recipe.  The other major point to note is that since PPC characterizes all of the raw materials in advance of the batching operation, the PPC formulation actually allows CONTROL of the body and its properties.  The traditional method simply mixes the same batch recipe day after day, and then tests are performed to determine IF the new batch behaves as desired.  This is not really a "control" --- it is a CHECK!

PPC Formulations -- State of the Art

The PPC methodology could not have been performed 30 years ago.  Characterization equipment was available 30 years ago, but most of it was not computerized.  Most of today's instruments are computerized, which means it is relatively easy to put characterization results into a spreadsheet program to calculate the percentages of each ingredient material needed to achieve PPC formulation targets.

In this regard, the PPC methodology and PPC formulations and procedures can be considered to be "state-of-the-art".  In this computerized world, are we going to ignore all of the latest advances, or will we take advantage of the current crop of incredibly capable computers to precisely control our production processes?

To follow traditional methods in this day and age is to ignore and cast aside the majority of today's computerized capabilities.