Mineral processing – Milling
Practical Action
Suitability of different mills to different operations
Some of the characteristics and requirements to be considered when selecting a mill are given
below. The mill manufacturer can usually be consulted concerning the application of a
particular mill or for sourcing a mill which is suitable for a particular application.
• Mineral properties. The choice of mill type is primarily dependent upon the properties
of the material it will be used process. It is vitally important to match the mill and
material characteristics properly.
• Capacity. The scale of the operation will determine the size of the mill which is
required. Throughput or capacity is often given in tonnes per hour (or kg per hour for
small mills). Always check capacities with as many sources as possible as sales
information can often be biased to encourage sales.
• Reduction ratio and final size requirement. This parameter will dictate whether a
single mill will be sufficient for final product requirements or if a multi-stage plant
will be needed. Generally speaking, the greater the reduction ratio, the larger the
likelihood of a multi-stage process being required.
• Power requirements and type of power supply. Access to a power supply of suitable
capacity is essential. Types of power supply for remote applications are discussed
later in this section. The power requirement for a particular mill will be given in the
mill specification document provided by the manufacturer. Specific power
consumption (e.g. kilowatt hours per tonne) is often quoted and is a good comparative
guide.
• Wet or dry product. Products which can be accepted in a wet state, such as slurries,
can be milled wet which will often save power and reduce dust related problems. As a
general rule, only tumbling mills are used for wet grinding, although other mills can be
used for wet grinding in certain circumstances.
• Continuous or batch operation. Some mills can be designed in such a way as to enable
continuous milling. This is important where the throughput is high, as well as making
loading and emptying easier within the process. Some mills will only accept batch
loads.
• Portable or stationary equipment required. Depending on the nature of the operation,
the equipment can be sited permanently or can be portable. Portable equipment is
useful for operations which move frequently due to the dispersed location of the raw
material or where a mobile milling service is offered.
• Classification. When considering a mill for a particular application, one needs to
consider the classification mechanism that will be required for the process and
whether this will have to be purchased separately or if it will be an integral part of the
mill.
• Cost. Obviously cost is an important factor. It is important to consider all the costs
beforehand. For an accurate analysis of the economic viability of a mill to be carried
out the following costs need to be considered:
• capital costs of mill (and capital depreciation against the useful life of the
mill)
• capital costs of peripherals, such as feeding and classification equipment,
power supply, dust suppression hoods or sleeves, etc.
• transport costs
• running costs of fuel or electricity, labour, etc.
• maintenance costs
Local availability of mills rather than import
In developing countries it is usually preferable to purchase a mill in-country. This helps reduce
transport costs and helps to support the local economy and engineering capacity (where the
mills are manufactured locally). Quality should be checked carefully in such situations as the
manufacturing ability and standards in developing countries are often lower than those of
developed countries. As mentioned previously, there are a variety of locally produced
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