Volume 2 Number 10 Dennis R. Dinger 1 August 2004
An Update
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.
Once again, please send suggestions for topics you'd like to see addressed in future issues of this E-zine. The list, which at one point was fairly long, is short once again.
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Don't forget -- Dr. Dinger is available for on-site consulting for a daily fee plus traveling expenses. A variety of ceramic processing short courses can also be taught on site. If interested, please inquire.
The topic of the main article in this issue was mentioned briefly in the previous issue. This is an important processing consideration which deserves its own article.
Errata
In recent issues, I referred to our particle size distribution equation, but I reversed the 'L' and 's' subscripts (a little bit of dyslexia there). The correct equation is:
CPFT/100 = (Dn - DSn) / (DLn - DSn)
Mixing Using an Extruder
As a college professor, I would be the first to harp on the ideas that "mixers are for mixing," and "extruders are for extruding." I have frequently told my students, "Use devices for the purposes for which they were designed!" Many pieces of equipment are used for functions which they happen to perform, but which they were not designed or optimized to perform.
For example, many ball mills are used for mixing. They don't do as good a job of mixing as mixers, but there can be no denying that mixing occurs in every ball mill. Mixers are not designed to break particles and change particle size distributions (PSDs), but PSDs typically change when mixing occurs in ball mills. Is this always considered when ball mills are used as mixers? At best, mixing in ball mills is of the relatively low shear variety and it can't compete with good high shear mixers. But it's done every day. When a mixing tank is needed, but one isn't available, and a ball mill IS available -- using the ball mill may be a good solution. It should be considered to be a temporary solution, but it is frequently a standard, permanent procedure in many plants.
Having said this, there is at least one exception to the rule: Most extruders improve the mixedness of bodies as they pass through the extrusion dies.
High Shear Rate/Low Shear Stress Mixing
Typical mixers perform in the high shear rate/low shear stress mixing regime, which means they produce high shear rates using relatively low shear stresses. High Intensity Dispersion (HID) systems, as well as many common, less intense mixers, operate in this regime. When solids contents in suspensions are too low and particles are not crowded, HID conditions do not efficiently perform HID mixing. The reason is that good HID requires particles to be batted about by impellor blades -- especially, it requires that they be batted into other particles. Intense particle/particle collisions are a primary reason why HID is successful.
Good HID deagglomerates and it frees all entities in the suspension to travel as individuals. But all mixers (even HID mixers) do not break particles. They can break the bonds that hold weak agglomerates together, but mixing intensities are simply not sufficient to break particles. This is the reason why specific surface areas of powders do not increase during normal or HID mixing. Normal mixing intensities may break some agglomerates. HID mixing intensities can break many agglomerates. Especially strong agglomerates, however, will not usually break apart even under good HID conditions.
Solids contents must be relatively high to facilitate good HID results. Most suspensions to be spray dried do not respond well to HID because they are usually mixed at relatively low solids contents. Most other suspended bodies do respond well to HID because particles are generally crowded at the solids contents used in forming bodies. When solids contents in forming bodies are not high, they can usually be increased until after HID, at which point the extra water can be added to reduce solids contents to the appropriate forming levels.
Low Shear Rate, High Shear Stress Mixing
What does one do with bodies at the other extreme? What does one do when only enough carrier fluid is added to bodies to allow them to be processed? This regime requires high shear stresses to produce the mixing action, but it necessarily must be performed at relatively low shear rates. Let's consider this, briefly.
Why Add Water?
This is an important question when dealing with high solids plastic forming bodies: Why add lots of water to mix and suspend a body when it must only be removed again before processing? Removing water is an expensive process. Even if one believes that filter pressing is relatively inexpensive (which it is), it still represents an additional production step. Raising solids contents higher than those possible using filter pressing requires drying -- and drying is expensive. Why add water to suspend a body knowing full well that it will require drying to remove it again?
The answer to these questions is that the addition of excess water allows bodies to be mixed well -- as suspensions. Levels of homogeneity can be achieved by mixing suspensions of powders that are difficult (impossible??) to achieve any other way.
Dry Mixing
Dry mixing with the goal of attaining homogeneity is difficult to achieve. Even if it were possible to achieve homogeneity during dry mixing, maintaining the 'homogeneity' of loose, dry particles, after mixing as dry powders, but before forming (as dry powders), is nigh unto impossible. A glass process engineer once asked, "How can I maintain the level of homogeneity achieved by dry mixing until the powders are fed into my glass tanks?" It his case, he was dry mixing powders in one building, and sending the powders by conveyor belt to another building before feeding them into a glass tank. In other cases, mixed dry powders are stored before further processing. How does one prevent dry particles from segregating by size as one handles them? The answer I gave to the engineer was, "You can't!" Dry powders will always unmix (i.e., they will always separate by particle size and density as they are moved about and handled.)
If you don't believe this, mix a tablespoon of coffee creamer to a tablespoon of Folger's instant coffee in a cup. Mix them with the spoon to the best, most homogeneous consistency you can achieve. It's fairly easy to see the level of homogeneity in this experiment because the creamer is fine white powder and the Folger's is coarse dark brown powder. When it's as homogeneous as you can make it, tap the cup against the table a few times and watch the powders unmix.
The solution in some plants is to dry mix, and then spray the dry powders with water, or NaOH solutions, before further processing. In the case of glass plants, this means dry mix, add water, and then feed the powders into glass tanks where the water is almost immediately burnt off.
This example was about dry mixing. What happens when a little water and other additives are added before mixing?
High Solids Mixing
What happens when mixing is to occur at the solids content used in forming operations? This applies to ram pressing, roller forming, extruding, injection molding, and other similar forming processes. A variety of mixers are available to perform the mixing in such cases. Many labs use Kitchen Aid mixers for these bodies. Many plants use different types of muller mixers.
When particles are free to move around in relatively stress-free environments, it is difficult to mix such bodies to achieve homogeneity. This is especially the case when binders and other additives are to be mixed into such bodies. The additives will stick to the first available surfaces, and will not easily be spread further throughout the body. Higher stresses are required to achieve homogeneity.
When such bodies are to be extruded (and even when this is not the case), extruding bodies through a pelletizing die will help achieve homogeneity. The high shear stress and low shear rate conditions in a typical extrusion die will help to force binders, additives, and other wet ingredients to spread more evenly throughout the body. Homogeneity should not be expected to happen in only one pass through an extruder. It will typically require several passes.
Quality mixing won't occur within the auger chamber because the body is not sheared much at all in that part of an extruder. It won't happen within the piston chamber in a standard piston extruder for the same reason. But quality mixing will happen as particles in the body are sheared, under pressure, as they are forced to flow through a die.
This translates into several extra production steps, but if mixing problems are apparent in extruded products, or in other plastic forming products, one should consider extrusion through a pelletizing die as a potential solution to the problem.
When we tried this type of mixing in the lab, the first pass of the body through the die did not produce a quality extrusion, but after several passes, the extrusions flowed smoothly with high quality surfaces which are signs of a well-mixed body.
A Mixer that Uses This Phenomenon
There is a company which is selling a cream whipping device -- it looks like a large bicycle pump with a clear plastic cylinder and a piston (with holes, like a pelletizing die) attached to the handle. One puts whipping cream into the cylinder, and forces it quickly back and forth through the holes in the piston by pumping the piston back and forth.
I believe (although I haven't been able to find it) that there is a production mixer available that works similarly. It's a single barrel/double piston arrangement with a pelletizing die at the center. The body is put into one side of the cylinder and forced back and forth through the die by the pistons at each end of the cylinder.
Summary
An extruder can work well as a mixer because of the high pressure/low shear environment it creates. High pressure/high shear causes dilatancy. Low pressure/low shear is typical in standard low shear mixers. Low pressure/high shear is typical in HID mixers. High pressure/low shear occurs in all extruders.
I suggest that 'extruders used as mixers' should be considered as possible solutions to many high solids forming body mixing problems. It won't apply to all extrusion bodies because homogeneity of mixing is not a prime requirement in all bodies. But many relatively pure 'high tech' bodies require homogeneity of mixing.
Not only does homogeneity of mixing improve using this technique, but well mixed bodies typically have improved forming properties (i.e., rheological properties.)
Miscellany
If anyone knows of a double piston mixer like the one that I described above, please send me the particulars.
Please continue to send your ideas or questions for future topics. Thanks.
Until next time ...
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