Environmental Engineering Reference
In-Depth Information
19
Approaches to Waste Rock
Disposal
Issues and Risks
Waste rock, also known as spoil or mullock, is produced in most mining operations
as a necessary consequence of accessing an ore body. Various terms are used for
the waste rock, depending on its distribution in relation to the ore. For example,
the term overburden is used for waste material overlying an ore body. Similarly
in a stratifi ed situation such as a sedimentary sequence involving multiple seams
of coal, the waste rock between coal seams is known as interburden. The ratio of
waste rock to ore is known as the stripping ratio. A stripping ratio of 3 means that
3 tonnes of waste is excavated for each tonne of ore.
In general, underground mining operations produce much smaller quantities of waste rock
compared to surface mines. In an open Pit mine, the overall stripping ratio is determined by the
dimensions and geometry of the ore body, the value of the ore and the shape of the pit, which
itself may be determined by slope stability considerations. Large, low grade ore bodies, low
value commodities such as limestone, or low rank coal deposits such as lignite, are usually une-
conomic to mine unless stripping ratios are low - from less than 1 to 3. High rank coal seams
and high grade ore bodies, particularly for valuable commodities such as precious metals, may
have stripping ratios of 10 or more.
Figure 19.1
shows a cross-section through a typical open pit
mine with a high stripping ratio. In this case, the ore body is a relatively narrow, steeply dipping
vein, similar to many gold mining situations. The ore body continues below the base of the pit
but to mine deeper would be uneconomic because the cost of removal of additional waste rock
would exceed the value of additional ore. In such a situation, an increase in the price of the
commodity may justify deepening the Pit, resulting in an increased stripping ratio.
Waste rock is produced in most
mining operations as a necessary
consequence of accessing an ore
body.
19.1 NATURE AND CHARACTERISTICS OF WASTE ROCK
Waste rock from mining operations exhibits a wide range of chemical and physi-
cal properties, the understanding of which can be of critical importance in determining
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