Environmental Engineering Reference
In-Depth Information
FIGURE 4.16
Truck Teams with Shovel operate in
Most Open Pit Applications
4.6 ORE DRESSING AND THICKENING
At most modern mining operations, whether surface or underground, the ores are not of
sufi ciently high grade to ship long distances to smelters, and they are subjected to mill-
ing, mineral dressing, or benei ciation, usually at or near the mine site. All of these terms
are sometimes referred to as ore dressing. Ore dressing is the mechanical separation of the
grains of ore minerals from the worthless gangue. The resulting concentrate contains most
of the ore minerals, and the waste is called tailings. The separation from valuable minerals
and worthless gangue requires the following two steps, size reduction (milling and grind-
ing) and separation of ore minerals from gangue materials (concentrating).
The separation from valuable
minerals and worthless gangue
requires the following two
steps, size reduction (milling and
grinding) and separation of ore
minerals from gangue materials
(concentrating).
Milling and Grinding
The reduction of ores to small particles is generally the most expensive phase of mineral
benei ciation. The i rst ore breakage occurs during blasting. Subsequently, the crushing plant
reduces the ore to i st-sized rocks. Two stages of crushing using different equipment may
be required. Crushed ore is then ground into i ne particles in ball mills or rod-mills which
are large, rotating, cylindrical machines ( Figure 4.17 ). The mining, crushing, and grinding
portions of the processing are extremely energy intensive since the rock must be reduced
essentially to i ne sand and silt-sized particles in order to liberate the valuable minerals for
subsequent extraction. The manipulation of particle size by crushing and grinding the ore,
combined with particle size classii cation is also termed comminution ( Table 4.5 ) .
It is advantageous to remove ores from the crushing and grinding circuit as soon as the
desired grain size is achieved. Further crushing or grinding is energy intensive, and over-
grinding may in fact hinder subsequent metal recovery. Apart from operational safety and
health issues, and potential dust emissions at i rst stage crushing, milling and grinding
have no signii cant direct environmental impacts. Indirect impacts are associated with the
high energy consumption.
The mining, crushing, and grinding
portions of the processing are
extremely energy intensive.
Concentrating
Milling aims to liberate commercially valuable minerals from gangue minerals.
Concentrating refers to the subsequent separation and removal of the valuable constituents
 
 
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