Certain extruders have been redesigned internally to delaminate (or in some cases deagglomerate) raw materials.  Delaminators are most applicable to kaolinitic- and clay-based ceramic processing bodies.  They can sometimes be successfully applied to other bodies that contain agglomerated, chemically prepared (and sometimes calcined) raw materials.  These will be discussed in the next two sections. 

As the name implies, their purpose of such devices is to delaminate (or deagglomerate -- as the case may be.)

Delamination of Kaolinitic Minerals

Kaolinitic minerals frequently contain kaolin platelets in so-called book stacks.  That is, they are neatly stacked and tightly packed like the pages of a book.  In this form, the majority of the beneficial properties contributed by the kaolin minerals are withheld from the batch because the kaolinite platelets are tightly stacked, rather than being uniformly dispersed throughout the body. 

Why are the kaolinitic minerals (including kaolinitic ball clays) present in the body in the first place?  In the majority of cases, kaolinitic minerals are present to provide plastic forming properties.  For this reason, it is important that kaolinitic minerals are delaminated and optimally dispersed to maximize the plastic properties they provide to the overall body.  Delamination of any existing book stacks and uniform dispersion of the individual platelets throughout the bodies are paramount. 

When we move from one house to another, newspaper is frequently used for packing the delicate dishes and collectibles into cartons.  If the paper is going to perform this cushioning function, it is never used in the neat stack in which it arrives.  Sheet by sheet, each piece is crumpled and stuffed into place to provide the desired level of cushioning between the delicate objects.  Similarly, a book stack of kaolinite platelets will only provide plastic properties associated with the two outer surfaces (front cover and back cover) and the edges.  This is the equivalent of one platelet plus all of the edges.  After a book stack is delaminated so all platelets report as individuals, the plastic properties provided will be proportional to all of the surface areas of all of the platelets plus all of the edges.

The exposed surface area contained in a book, or in a stack of newspaper packing paper, is not very great, but the total exposed surface areas of all of the individual pages of a book, or the total volume and cushioning effects of all of the crumpled newspaper sheets, can both be enormous.  Similarly, the exposed surface area of a book stack of kaolin platelets is not very great, but the total exposed surface area of the delaminated stack (when all platelets report as individuals) can be enormous.

The fact that kaolinite and clay minerals may be less expensive than other minerals has no effect on this line of reasoning.  When ingredients are selected for inclusion in bodies, you expect that they will each be in their optimal forms and will be appropriately distributed to fulfill their intended purposes.  Forming bodies which contain remnant book stacks of kaolinite platelets will be much less plastic than they could be if the platelets were separated and well-dispersed throughout the body.

Deagglomeration of Agglomerates

Similarly, some raw materials, especially those that have been chemically prepared and precipitated, and even more particularly, those that have been chemically prepared, precipitated, AND calcined, need to be properly deagglomerated and dispersed to fulfill their intended functions in ceramic bodies.  It is possible that the relatively pure, highly processed, expensive forms of materials used in laboratory test preparations will be properly deagglomerated so they can be dispersed properly.  On the other hand, it is also highly possible that the larger bulk shipments of these same materials, although similarly formed and processed may not be as well deagglomerated.  If this is the case, lab samples and results will differ markedly from the results of plant trials.

Finely precipitated powders have great tendencies to agglomerate as they are dewatered and processed.  This is especially true when crystallite sizes are in the colloidal size range.  Agglomeration of colloidal particles occurs quickly and is quite common.  If these materials are not properly deagglomerated during body preparation, these particles may not be as well dispersed in production bodies as in their corresponding lab samples. 

Consider an agglomerate of chemically prepared, precipitated, and calcined raw materials.  We have estimated that commercially available agglomerates of 4 micrometer diameter particles can contain tens of thousands of agglomerated particles.  The effective (outer) surface area of the single agglomerate will be orders of magnitude smaller than the effective total surface area when all particles are free to report individually. 

If you are paying a premium price to add pure, chemically prepared powders to your forming body, which case would you want to have?  (1) agglomerated particles which don't provide the desired properties to the forming body?  ... or (2) deagglomerated particles where each particle is reporting individually and where each particle is contributing its special properties to the body?

The Mechanism of Delamination/Deagglomeration

The delaminator of this discussion outwardly resembles a production extruder.  Inwardly, instead of having one long continuous auger and no stators, it has many split augers and stators located alternately along the barrel (e.g, auger, stator, auger, stator, etc.)  The delaminator does not need a vacuum chamber for de-airing because the purpose of the device is to delaminate, not to produce de-aired extrusion blanks for the process.  Not to worry -- once bodies are delaminated, they won't relaminate -- which means delaminated product can then be fed into the current production extruder.   

Delaminators' sizes need to be physically larger than the production extruders they supply.  Depending upon the number of delamination sections (1 split auger plus 1 stator = 1 section) in the device, one or more delaminators may be needed to feed the final vacuum extruder.

To properly delaminate, the process must intentionally be slow.  For example, when testing a lab extruder that had been redesigned and refitted to function as a delaminator, we passed the test batch through the delaminator multiple times to achieve desired levels of plasticity.

Multiple passes through short delaminators or a single pass through longer barrel lengths plus slower throughputs can each provide the desired body properties.  The reason the process is (read: must be) slow is that the body must be repeatedly cut by the split augers and sheared under great pressure against the stators.  As a result, the throughput from a properly designed delaminator will usually be much less than the throughput of a comparably sized extruder.

The goal of a delaminator is not high throughput, but delamination.  For this reason, repeated shear of the body between the split augers and their associated stators and slow throughput rates achieves the work of delamination.

Results

A delaminator of this design provides the same results in high solids extrusion bodies as high intensity dispersion (HID) does in high solids production suspensions.  In the case of HID mixing, particles are batted about by the high velocity blades so they collide with each other.  In this way, book stacks and agglomerates are broken apart by high shear and high collision energies imparted by the high speed HID impellor blade in lower solids content suspensions.  In some cases, such as for calcined, agglomerated raw materials, HID does not provide sufficient energy to break agglomerates and free all constituent particles.  HID is usually sufficient to delaminate suspended book stacks of kaolin platelets but it may be insufficient to deagglomerate calcined materials.

When HID cannot adequately perform delamination or deagglomeration, the solution is to raise the solids contents even higher into the range of extrusion bodies.  The delaminator can then complete the task.  The goal is to apply low shear at low speeds but under extremely high pressures to cause delamination and deagglomeration.

The two options are to use the high shear/low solids content and normal pressure conditions found in HID mixers, or to use the low shear/high solids content and extremely high pressure conditions found in delaminators.

When successfully delaminated, kaolin- and clay-based forming bodies will be noticeably more plastic to the touch.

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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Copyright © 2006  Dennis R Dinger

103 Augusta Rd, Clemson, SC 29631   (864) 654-5731

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