Mineral processing – Milling
Practical Action
Pulverised coal for power generation
Glass, sand, lead oxide, potash and arsenic
for glass making
Salt
Charcoal for briquetting
Table 1: Some applications of the milling process
In this brief we will concentrate on mineral grinding, rather than the grinding of grain and
other foodstuffs, because a comprehensive publication already exists in the area of grain
milling (see final section on resources).
Material Characteristics
When a material is to be milled there are certain characteristics which have to be taken into
account. These include the following
• Hardness
• Brittleness
• Toughness
• Abrasiveness
• Stickiness
• Softening and melting temperature
• Structure (e.g. close grained or cellular)
• Specific gravity
• Free moisture content
• Chemical stability
• Homogeneity
• Purity
The hardness of a material is probably the most important characteristic to consider when
deciding on what type of mill to choose. Trying to grind a material which is too hard, such as
sand in most types of beater mill, will result, either in costly damage to the mill or an
expensive maintenance requirement. Most types of readily available hammer mills for
agricultural grinding are not suitable for grinding most types of minerals.
Hardness of minerals is expressed on Mohs scale - a numerical index ranging from 1 for talc
(the softest mineral) to 10 for diamond (the hardest known material). Table 2 below shows
Mohs’ scale of hardness.
Hardness No.
Mineral (example)
1 Talc or graphite
2 Rock salt or gypsum
3 Calcite
4 Fluorspar
5 Apatite
6 Felspar
7 Quartz
8 Topaz
9 Sapphire
10 Diamond
Table 2: Mohs’ scale of hardness
Common practical test
Marks paper - like a pencil
-
Can be marked with fingernail
Can mark a copper coin
-
Can mark window glass
Can mark a knife blade
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-
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2