Molienda

tabular alumina, and polycrystalline grogs (coarse granules) made from alumina, mullite, and zirconia. One way of making Si3N4 is to heat silicon metal in a nitrogen atmosphere. Carbo-thermal reactions at a high temperature are a common way to make a variety of carbides. Powdered raw materials usually come from chemical processes where the material in solution chemically reacts to form particles in suspension. An important class is Bayer alumina that starts as bauxite. The Hall Cell fuses the bauxite with the cryolite flux forming a sodium aluminate that is then leached with caustic and washed to produce a moderately pure alumina powder. The powder is agglomerated and then milled for ceramic formulations. Several varieties are available according to crystallite size and purity.

In other processes, the material is dissolved and is then precipitated chemically as a powder. High purity ceramic powders are sometimes made from an organic precursor decomposed to make the ceramic powder. Included in this class are some alumina and zirconias with or without the stabilizer. Much of this information is about ball mills used for processing ceramic slips. Emphasis is on fine-grained slips, with limited information on equipment for coarse materials.

Some materials are more difficult to mill than others. Generally, the order of difficulty from the most difficult to the least difficult is densefused materials, sintered materials, and precipitated powders. Although one might not expect this, glasses are very difficult to mill to micrometer sizes,

but they are easy to crush to granules.

2.0 DRY MILLING

Production milling is sometimes done dry as this avoids a separate drying step. Dry milling also avoids the formation of hard agglomerates as there is no liquid present. Dry milled ceramics are usually used in pressing operations to make a shape and to consolidate the particles. Crushing and

milling are sequential processes for particle size reduction. They will be discussed separately.

Crushing

Crushing helps reduce the particle size of hard materials to about 80 mesh using Tyler Sieves. After achieving this size reduction, other finer reduction techniques can be used in the lab. Two types of crushers are most commonly used: jaw crushers and roll crushers.

Roll Crushers

Roll crushers have two counter rotating steel rolls that are about 6inches in diameter with an adjustable gap through which the material is crushed. These crushers can crush materials to about 80 mesh. Particles that press onto the roll surfaces will contaminate future batches. Wire brushing the roll surfaces reduces contamination, but the only way to completely clean the roll surfaces is to have them machined though this is not at all practical. Roll crushers are dangerous as loose clothing such as sleeves, neckties, necklaces, or gloves can be caught between the rolls. One should avoid wearing or using anything that can be caught between the rolls.

Ball milling

Usually dry milling is done in a ball mill with a milling media. Ball mills and media are discussed later in this chapter. Portland cement is dry milled commercially using steel balls for efficiency. An Alumina media about three inches in diameter is used for milling white cement where the color has to be controlled. Hard ferrites are also dry milled in air swept ball mills. Bayer aluminas are dry milled by the producer.

In the lab, dry milling is not as common since the cost of drying is not a factor. However, when the product, which is originally wet milled in the lab, is transferred to production where it is dry milled, the properties of the powders will be different and will cause start-up problems. Dry milling is not effective in small diameter mills as it lacks enough energy to fracture the particles. Mills with about a 12-inch diameter are suitable. Higher energy mills such as planetary and stirred mills are also available.

A serious problem with dry milling is that the powder will cake onto the sides of the mill and will not receive further size reduction. Scraping the mill down periodically helps to reduce this caking problem. Another way to reduce the caking problem is to add a surface active agent. It is believed that Bayer alumina dry milling involves the use of ethylene glycol. Bone-dry

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