Mineral processing – Milling
Practical Action
(or other gas). The particles collide violently and this causes comminution to take place.
Various designs of fluid energy mill exist, the most common being the microniser. This mill
has a shallow circular grinding chamber and a series of peripheral jets set tangentially to a
common circle. The turbulence causes bombardment which effects a rapid reduction in
particle size. A centrifugal classification system keeps larger particles within the chamber
while allowing fine particles to leave. In a well designed fluid energy mill there will be almost
no contact between the charge and the mill lining.
Characteristics:
Suitable for hard or soft materials to be reduced to 0.02mm, or less, this method of milling
tends to be energy intensive and slow but is suitable where the product is highly sensitive to
heat or contamination from grinding media.
Mineral
Product size
Production
(Kg / hr)
Alumina
100% - 0.0075mm
50% - 0.003mm
5500
Feldspar
90% - 0.0075mm
3600
Graphite
90% - 0.01mm
25
Mica
95% - 0.075mm
725
Table 12: Typical jet mill performance
Fluid use
2850 kg/hr steam at 7000 kPa and 400oC
85 m3 /min air at 7000 kPa and 20oC
2m3 / min air at 7000kPa and 20oC
20m3 / min air at 7000kPa and 425oC
Other mills and reduction processes
Attrition mills
Attrition mills are the most common type of mill found in developing countries. The
traditional grain mills of many regions of the world are based on attrition grinding between two
circular stones, one rotating and the other stationary. Plate mills use a similar principle but
are constructed of steel or ceramic plates and used more commonly in the vertical plane.
Output from such a mill is low and only of practical use for small scale milling.
Cutting machines
Many ductile or fibrous materials such as plastics, rubbers and miscellaneous chemicals
cannot be milled using conventional milling equipment. Instead they are often cut or
shredded. High speed rotating knife blades set in cutting mills will reduce such materials to a
usable size. In certain cases reduction can be down to 0.25mm.
Cryogenic comminution
Ductile materials such as steel, plastics and rubber, which cannot be milled easily, can be
embrittled by lowering the temperature of the material. Once embrittled the material will lend
itself more easily to comminution by conventional methods, usually with the use of a hammer
mill. Liquid nitrogen is a gas used for this purpose. The process is expensive due to the cost
of the gas but is used for some specialised applications.
Traditional mills in developing countries
As mentioned in the previous section there are a number of traditional mills in use throughout
the world. Some of these mills date back thousands of years and have changed little in design.
Many are precursors to modern mills. They are usually constructed from materials found
locally by indigenous craftsmen. Often the quality of the product varies considerably and the
throughput for such a mill is low, but in many circumstances, where the fineness is not critical
and the quantity to be milled is low, choosing a traditional mill can be the best option. They
are often simple and cheap to construct and can be powered by one of a wide variety of power
sources. Some examples are given below.
10