Mining Methods Vary Widely - Mining and the Environment: From Ore to Metal - page 237

grade sulphide and oxide rock dumped specii cally for leaching. Copper dump leaches are
typically large, with low-grade rock piled into heaps ranging from 10 m to over 30 m in
height. These may cover hundreds of hectares and contain millions of tons of waste rock
and low-grade ore (Biswas and Davenport 1976).
Dump leaching may involve the application and recovery of leach solutions as for heap
leaching, or it may involve recovery and processing of leachate from rainfall percolating
through the dump. Generally, the waste rock will not have been crushed, so that particle
sizes will be relatively large and metal recovery will be low compared to heap leach opera-
tions. Copper is the main metal recovered in this way, with sulphuric acid the main leach
solution. The principles that apply are the same as for heap leaching. Groundwater and
surface water contamination are the main potential impacts of concern.

Vat Leaching

Vat leaching, a hydro-metallurgical rather than a mining process which is further discussed
in Chapter Six, works on the same principles as the dump and heap leaching operations
except that it is a high-production-rate method conducted in a system of vats or tanks using
concentrated leaching solutions (solvent). Vat leaching is typically used to extract copper
from oxide ores by exposing the crushed ore to solvents (concentrated sulphuric acid) in
a series of large tanks or vats. Similarly, gold is extracted in vats, using cyanide solutions
as discussed elsewhere. The vats are usually designed in a series coni guration, which acts
to concentrate the copper content of the solutions as a function of ore-solvent contact time
(USEPA 1989). In contrast to heap and dump leaching, vat leaching operations may be con-
ducted under a number of environments, including slightly sub-atmospheric, atmospheric,
or super-atmospheric pressures, and under ambient or elevated temperatures (Weiss 1985).
Industrial leaching processes applying countercurrent stage-wise leaching can deliver the
highest possible concentration in the pregnant solution (also termed extract), but multi-
stage hydrometallurgical processes are often expensive, technically challenging, and only
applicable to large-scale mining operations. Agitation accelerates leaching, but due the high
energy consumption for ore preparation and leaching, the method is only warranted for
high-grade ore (see Figure 6.7 ).

In contrast to heap and dump
leaching, vat leaching operations
may be conducted under a
number of environments.

In situ Leaching and Solution Mining

In situ leaching (ISL) involves dissolving minerals from the ore without its removal from
the ground. In situ leaching includes the leaching of either disturbed or undisturbed
ore. In either case, in situ leaching allows only limited control of the solution compared
to a lined heap leach type operation. Theoretically, this approach to mineral extrac-
tion could be applied to a range of minerals. However, to the present time, its commer-
cial use has been coni ned mainly to various uranium minerals which are soluble in acid
or alkaline solutions. To a lesser extent ISL has been applied to recover copper from
low-grade oxidized ore. In each of these cases solution is pumped into the ore body
through a network of injection bores or wells and then pumped out together with salts
dissolved from the ore body through a series of extraction bores (production wells) as
shown in Figure 5.4 . Further processing of the pregnant solution is usually carried out at
the surface to precipitate the mineral product (Yellowcake -U 3 O 8 - in the case of uranium
ISL operations) from the solution which is subsequently re-used.