Page 308 - Biosystems Engineering

Bioseparation Pr ocesses 285

mills are classified as grinders, they do not do much actual grinding.
Their value lies in ensuring a breakdown of agglomerates or, in the
case of emulsions, shearing the fluid phases to produce droplets of
fine size, approximately 1 μm.
Colloid mills, which are used for dispersion or emulsification, fall
into four main groups: hammer or turbine, smooth–surface disk,
rough-surface type, and valve orifice device. The principle of opera-
tion lies in creating a high-velocity fluid stream with very great shear
forces existing within the fluid, which acts to disrupt the particles.
Chemical aid in the form of dispersing agents is sometimes used.
Because of heat generation resulting from friction, a cooling water
jacket is provided in many systems.
A pan crusher is used not only for crushing but also for mixing
liquids and solids. A pan crusher consists of one or more grinding
wheels or mullers revolving in a pan; the pan may be driven, whereas
the mullers revolve by friction. Mullers are made of tough alloys such
as hard nickel, iron scrapers, or plows. The material is properly fed
under the mullers. Chambers of dry pan use air cylinders to regulate
the grinding pressure under each of the muller tires from 7500 to
20,000 lb. The bottom of the pan rotates and has a central solid-crushing
ring as well as an outer ring of screen plates with openings from 0.16
to 1.3 cm. In some instances, a solid pan bottom is used in place of a
perforated screen bottom, and the ground material is discharged
through a slot in the rim (known as a rim-discharge grinder). It will
give greater throughput with wetter materials but will require a
greater screening area and a higher circulating load.
A dry pan is useful for crushing medium-hard or soft materials
such as clays, shales, cinders, and soft minerals (e.g., barite). The feed
size should be 7.5 cm or less and should be a product of a particular
feed size. Finer products can be obtained by operating the pan in a
closed circuit (a circulating load of 75 percent is common) with a
vibrating screen. A high reduction ratio with low power and low
maintenance are some of the features of pan crushers (Fig. 9.2).
Typical dimensions of a dry pan are as follows:
Pan diameter: 1.8 to 3 m
Muller diameter: 0.71 to 1.6 m
Muller width: 13 to 46 cm
Power: 15 to 100 hp or 1 to 5 hp/ton of product
The production rate varies from 1 to 6 tons/h depending on the
pan size and hardness of the material as well as fineness of the feed
product. A wet pan is used for developing plasticity or molding qual-
ities in ceramic feed materials. The abrasive and kneading action of
the mullers blend finer particles with coarser ones during the crush-
ing operation.