Mineral processing – Milling
Practical Action
traditional mills available in some countries which are suitable for certain milling applications.
Sometimes it is possible to find second-hand mills, especially in areas where there is intensive
mineral mining activity. Again great care should be taken when purchasing used plant, as it
could be counter-productive if the machinery then has to be shipped overseas for an expensive
overhaul or reconditioning.
After-sales service and spare parts is an important consideration in many developing countries
where it could be difficult to obtain either, or in remote locations where the time required for a
service engineer to arrive could mean a significant ‘down-time’ for the mill. It is worth looking
into this before buying a mill and having contingency plans in place in case of breakdowns. If
a mill is purchased locally there is more likelihood of finding spare parts and competent
technicians.
Milling Within an Operational Plant
Siting
The siting of the mill is dependent upon several factors. Firstly the type of operation will
determine whether the mill is placed at a centralised location to where minerals can be
transported from a number of sources or, if the operation is confined to one area, how close it
should be sited to the operations area at the mine or quarry. There should always be sufficient
space around the mill for easy loading, unloading and access and where necessary the mill can
be housed. Topographically the mill should be sited in such a way that the flow of minerals
can be aided by gravity thus reducing expensive handling costs. A convenient power supply
(see the second paragraph following) and a convenient water supply are also prerequisites for a
milling operation.
As mentioned earlier the milling process is often part of a larger process of a mining or
quarrying operation, and the specific siting of the mill within this process will obviously be
dependent upon the overall process and its various components.
Power supply
There are a variety of options when considering a power supply for a mill.
• Electricity - the grid. Commonly, where the mill is sufficiently near a grid electricity
supply, advantage will be taken of this facility. Most mills will be sold with an electric
drive motor fitted as standard and connection is merely a case of ‘plugging in’. Many
developing countries suffer problems with regular interruptions in the electricity
supply and insufficient capacity to deal with demand. This should be considered when
thinking about the power supply for a milling operation. If occasional unpredictable
stoppages can be tolerated then this is usually the cheapest option.
• Independent power supply. In situations where no grid connection is possible or where
the local supply is not sufficiently dependable, an independent or ‘stand-alone’ power
supply will need to be considered. There are various options available, the most
common being a diesel engine. This can be used as part of a diesel generator set to
provide an electricity supply or can be used to provide a direct shaft drive for the
plant. Other options include the construction of a dedicated power supply from a
renewable energy source, such as a small-scale hydro-power plant (either generating
electricity of providing direct shaft power). Careful analysis of the options and correct
selection of power supply can provide great savings in running costs.
• Animal power. In certain circumstances animal power can be harnessed to provide
power for a milling process. Animal traction is widely used in developing countries for
providing power for a range of applications, usually requiring rotary shaft power. Where
the milling process is in the small scale range, this is one possible solution to
providing a cheap renewable power supply.
• Human power. Where very small scale milling is required the power can be provided
by humans. Many small hand (or foot) operated mills have been developed in many
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