Mineral processing – Milling
Practical Action
• Turbo mill
• Fluid energy mill
• Stirred media mill
There is also a section which looks at traditional mills used in developing countries and other
forms of size reduction other than milling:
• Attrition mills e.g. stone milling
• Cutting machines
• Cryogenic comminution
Glossary for the milling process
Milling circuit - open and closed. The milling circuit is the complete mill system from
beginning to end, including feed mechanism, mill, classifier, separator, product collector, etc.
In a closed mill circuit the oversize particles are returned from the post milling processes for
milling again (see figure below) whereas with an open circuit the process has no feedback
loop.
Air classification. Classification or sizing of particles using a mechanical air separator.
Batch mills. Mills which receive a discrete quantity of charge which is milled and then
discharged. The process is then repeated.
Continuous mills. A mill which can accept a continuous flow of feedstock and hence can
operate on a continuous basis. Both batch and continuous mills have their relative merits.
Peripheral and trunnion discharge. For cylindrical mills which are continuously fed, the
discharge of the final product can be either through the periphery of the mill (peripheral
discharge) or through the far end of the mill (trunnion discharge).
In this section we will now look in more detail at the mill types mentioned above.
Tumbling Mills
Autogenous mills
Description:
This type of mill consists of a large diameter, short length cylinder fitted with lifting bars. The
cylinder is fed with a coarse feedstock of up to 250mm in size and in rotating the feedstock is
lifted and then allowed to drop through a significant height. Three significant mechanisms
cause the breakdown of the mineral; impact due to the fall of the mineral onto the charge
below causes a reduction in the size of the feedstock; attrition of smaller particles between
larger grinding bodies; abrasion or rubbing off of particles from the larger bodies. Steel or
ceramic balls are often added to aid with the reduction process (the mill is then referred to as
a semi-autogenous mill). The process can be carried out wet or dry. Removal of the final
product can be carried out using air (where the process is dry) removing only the fines.
Rotational speed is usually fairly low, about 80% of critical speed (critical speed is the speed
at which the charge will be pinned to the rotating drum and does not drop) and the typical
drum diameter ranges from 2 to 10 metres. This type of mill is often used as a single stage
process, providing sufficient size reduction in a single process. Alternatively, it can be part of
a two stage process where further size reduction is required.
Characteristics:
This type of mill is only suited to certain kinds of mineral - one which has a fairly coarse nature
but once it is broken will disintegrate readily into a small size. In certain circumstances this type
of mill can deliver a product with a fineness of less than 0.1mm. Testing is required beforehand to
determine the suitability of a mineral for processing in an autogenous mill.
Suitable minerals such as copper or iron ore are listed in table 4. This type of mill has the
distinct advantage of accepting coarse feedstock and supplying a relatively fine finished
product, often sufficient as an end product. This can provide a reduction in plant costs if a
single mill is used as a substitute for two or more stages. There is little wear as the grinding is
often carried out by the mineral itself. Autogenous mills are most suited to large installations
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