Ceramic Technology and Processing, King

Mixing Coarse Grained Materials 133

Check List Uniformity

• Segregation

Particle packing

Amount of coarse/ medium/ fines Microstructure

Pelletize with binder Impeller speed

• Firing Shrinkage

Increase intermediate sizes Microstructure

REFERENCES

1.Alan G. King and Santosh T. Keswani, "Colloidal Mills: Theory and Experiment," J. Am. Ceram. Soc., 77[3] 769-77 (1994).

2.Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion. New York: Reinhold, 1970.

3.Z. Tadmor and C.G. Gogos, Principles of Polymer Processing. New York: John Wileyn & Sons, 1979.

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drying (as compared to pan drying) when the particle size is small. However, one is still dealing with the same problem except on a smaller scale.

Selecting an appropriate method is usually governed by the requirements of the press mix, which can be either damp or dry from using a binder and/or plasticizer. Each drying method will be addressed.

A slip is usually dried to make a press mix, with the drying method selected based on the desired properties of the pressed part. Drying is much more critical for fine-grained, technical ceramics than it is for coarsegrained slips. Coarse materials are permeable with an abundance of fracture sources due to the coarse size. Fine slips form agglomerates that act as fracture sources, degrading the strength. Therefore, drying is an important issue for fine-grained slips. Several drying methods will be discussed.

Pan Drying

This drying method is by far the most widely used in ceramic laboratories since it is so easy to do. One can use stainless pans or glass pans. Place the pan in an oven that has some exchange of air. Since the slip contains binders and other constituents, it is not a good idea to overheat as you could cook the organic components. Sixty degrees Celsius is a reasonable drying temperature. During drying, the coarser particles will sink to the bottom and the slimes, which are very fine materials, will migrate to the free surface. In addition, the dissolved materials will also migrate to the free surface, creating a crust. When broken up, the crust will form a distribution of moderately hard agglomerates that upon sintering will shrink more than the rest of the body and create voids. This is not good. Sometimes, the choices are limited and pan drying might have to be the method of choice. High speed dry milling will help to break up these agglomerates, but this is not perfect. Spray drying, stir drying, rotary drying, and freeze drying methods eliminate segregation of the coarse particles and slimes.

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The dryer has several principle parts: a slip pump, a two-fluid spray nozzle, a stainless drying chamber, a conical base, a connecting stainless tubing, a cyclone, and a glass collection chamber. Room air is introduced into the drying chamber by a blower through a heater. Controls include drying air temperature, air flow rate, and a slip feeding rate. On a different dryer than that shown, the following modifications were made: the diameter of the collection chamber was increased to reduce swirling of the granules, and a 12-inch HEPA filter was put on the air intake. The thought was that all of the dust in the air gets into the mix. Clean up is an appreciable task. Thankfully some driers have a body that tips upwards for better access.

To clean, dismantle the dryer by lifting the top and sliding out the nozzle. The glass drying chamber lifts upward and releases the conical base, which is on casters. One can roll out the base for cleaning purposes. The cyclone and collecting chamber are fastened with clamps that are unsnapped and lifted off. This is a lot of work for just one press mix, but what is the alternative? Other lab-sized, spray driers are now available and one should consider them. Cleaning is a very important criterion. Small bench-sized spray driers, because of their size, do not furnish enough time to dry the slip droplets. Therefore, the powder may not have the large agglomerate size needed for flowability and die fill. Buchi (and others) makes a well designed bench spray drier with all of the necessary features. However, to dry the droplets they use a very fine atomizer. This results in a very finely divided press mix that is not especially free flowing.

A nozzle cleaning mechanism is important since there is a tendency for the dry slip to cause clogging problems. There can be two parts: an air jet that blows across the tip, and a needle - coaxial with the jet - that can be forced through the orifice to push out dried slip.

Much of the dried slip adheres to the chamber walls, conical base, and in the tubing. This material is not spherical and most of it can be screened out. Recovery is not very good at 50%; however, one at least gets some good press mix. Figure 6.2 shows the press mix recovered from screening. Particles are semi-spherical with some of them having internal holes. Holes are typical of spray-dried particles and while it is better not to have them, they can be tolerated.

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Figure 6.2: Spray Dried Press Mix. Particles are fairly round and show internal cavities. Scale bar 100 μm.

The spray drying process consists of drying the droplets while they are in free fall. They are at least dry enough to avoid sticking to the walls and to each other. To achieve this, the slip should have flow characteristics suitable for atomization. Lab dryers use either two-fluid or vibratory (ultrasonic) atomizers. Commercial spray driers also can use a rotating disc, but lab equipment is too small to adapt to a rotor. Anyway, the slip should be pseudoplastic with a viscosity of about a thousand cps. Droplets must have sufficient residence time to dry. Air temperature, air flow velocity, slip flow rate, volume % solids, the binder system, and the length of the free fall path are important parameters. High air temperature, low air velocity, a low slip flow rate, high solids content, a non-skinning binder system, and a long free fall path are adjustments that will enhance drying. The following are restraints on drying: high air temperature that can volatilize or cook the binder, slow air velocity that lowers the production rate, a low slip flow rate that also cuts down the production rate, a high

Forming 139

solids content slip that can cause clogging problems in the nozzle, a binder that forms a skin on the droplet surface, and the free fall path restricted and fixed in a small lab dryer. There will be some experimentation at first to find the optimum drying conditions, which can be different for each material. A good place to start is in the middle of the drying parameters. One can increase the drying process until the dryer's capacity is exceeded or the press mix is not spherical. After achieving this, one can then, back off just a little. For lab applications, there is less interest in the production rate than in obtaining a good press mix, which makes it easier to back off.

After obtaining a spray-dried material, observation will show that there is a range of sizes and shapes in the mix. This mix will not be free flowing. With screening the spray-dried mix, one can remove both the fines, which are the result of satellite droplets and tumbling, and the coarse, which are the result of dried material on the walls and coalescent droplets. With this technique, one can typically get a 50% yield of press mix. Evaluation of the spray-dried mix is by how well it fills the die cavity and how well the granules coalesce during pressing. This is determined by the density of the fired part and by microscopic examination.

Stir Drying

This is done in a mixer that has a heater. As the material loses water, it is continuously mixed to keep the constituent's distribution uniform. This is a very common technique in the chemical industry. Steam jacketing is a common practice. Commercial equipment is oversized for the lab, but one can make a lab-sized, stir drier. For instance, one can fabricate a heating mantle for a Hobart mixer. This works well except that one has to periodically scrape down material that sticks to the sides of the bowl. Vendors of heated driers will run tests on their equipment. The slip can form a dilatant mass after it has lost some water. (This can overload the stirring motor and bend the stirring mechanism into a pretzel.

One thing to remember is that evaporation of the water is endothermic and the mix will be cooled during drying. When open to the air, condensation will occur in the mix unless it is heated. The worst case is a mix with a high vapor pressure solvent such as methylene chloride. The

140 Ceramic Technology and Processing

mixer will become so cold that a thick layer of frost will form on the bowl. One can imagine the amount of water that is condensing in the mix.

One needs to scrape down the bowl and blade regularly during drying. This hardened material will form agglomerates in the mix, but will have the same composition as the powder.

The blade (as in the Hobart) will not conform closely to the sides of the bowl creating a dead space. Rubber scrapers can be fastened to the sides of the blade and act as squeegees. It will still be necessary to scrape down the parts.

Solvent Drying

As the mix loses the liquid, menisci draw the particles together by surface tension, agglomerating the mix. Interfacial tension with liquids other than water is lower and this effect is reduced. In solvent drying, the starting liquid is probably water that has to be replaced with the solvent. The first step is to recover the solids. Methods to do this include: sedimentation, filtering, centrifuging, or just pouring the slip out onto a flat slab of plaster, which is an old potter's trick. Then, the material is reblunged with a solvent such as acetone or methanol. As water is miscible with these solvents, it is diluted and is decanted along with the solvent. A repetition of this process will further reduce the water content. The powder can now be dried. Alternatively, it can be reblunged with a nonpolar solvent with even a lower interfacial tension, such as a low molecular weight hydrocarbon often containing a wetting agent.

Again, these solvents are flammable and some are toxic, so safety should be considered. An oven can be converted to be explosion proof by removing the thermostat relay to a remote position and plugging the oven into a variable transformer to reduce the element temperature to as low as is practical. One should still have a good air flow to keep the solvent vapor concentration low. Be sure to check the voltage each time to make sure that somebody has not turned up the dial. When there is a substantial amount of solvent drying, the oven should have a hood and an explosion panel directing the explosion away from people. Alternatively, one can let the

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solvent air dry. When the amount is small, this can be done on a table top in a ventilated room. As before, there is the problem of water condensation.

Rotary Drying

In this process, the wet material is fed into a pitched tube that is slowly rotating. Heaters raise the temperature to where the water is evaporated, at which point the dry mix falls out of the other end. Finegrained materials stick in the tube and coat its surface. In some production instances, chains are hung in the tube to break up the cake. Agglomeration occurs as one would expect. One can place a high-intensity, dry mill at the exit end of the tube to mechanically break up the larger agglomerates. One does not apply this process to powders containing binders or other additives. Ordinarily but not always true, rotary driers are used in the plant rather than in the lab. A major problem is adherence of the cake to the walls of the tube.

Freeze Drying

This process has the potential of being very useful as the water is removed by sublimation and this does not result in agglomerates, at least from the drying process.

Steps in the process include pouring the slip into shallow trays, placing them in a freezer, and subliming the water. The temperature of the tray has to be higher than that of the cooling coil to transfer the water to the coil. Trays are heated but have to be below freezing or the slip will melt. To control the temperature differential, the slip has to be in a thin layer and all of the temperatures have to be below the freezing point of water.

Other freeze drying techniques involve spraying the slip into liquid nitrogen, alcohol cooling with dry ice, or acetone cooling with dry ice. The spherical particles are then recovered, kept cold, and freeze dried. One will need to screen the mix as spray nozzles produce a wide range of sizes.

The industrial process for commercial freeze drying is done at a

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huge facility in the Midwest. The problem is the restriction of the thin layer. This cuts into productivity, unless the market is large and the facility is automated. Instant coffee is sometimes freeze dried.

Freeze drying is a valid laboratory procedure for drying a slip and producing a powder. Laboratory-sized, freeze driers are available. One should anticipate a scale up to production. One should also inform manufacturing if the lab is successful as the plant will invest in freezedrying equipment.

Check List, Drying

•Pan drying is the most commonly used technique, but it leads to segregation and agglomeration.

•Spray drying, at present, is the preferred technique for fine particles despite all of its faults.

•Spherical particles are a notable advantage in mold fill.

•Segregation is a serious problem in drying. Mix while drying.

•Agglomeration is a serious problem in drying. Use volatile solvents.

•Coarse-grained mixes can be pan dried as segregation is not as much of a problem, provided that the mix is only damp.

•Freeze drying is a good lab method to prevent drying agglomerates.

•Controlling the uniformity of the powder is not easy, and much of this results from the drying process.

3.0GRANULATION

For die cavity filling, the press mix should be uniform and free flowing. When it is a fine stiff cake or formless mass coming out of the mixer, granulate the material. One type of granulator is a U-shaped screen with an arm that oscillates back and forth to press the mix through the