Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 258

TABLE 6.2
Characteristics of Copper Leaching - Hydrometallurgical methods include dump, heap, and vat leaching techniques, as well as underground (or in situ) leaching
methods. The leach cycle depends on ore type and applied leaching conditions. Each copper leaching method results in a pregnant leach solution (PLS). Copper is recovered from
the PLS through precipitation or by solvent extraction/electrowinning

Vat Leaching Heap Leaching Dump Leaching Underground and

in situ Leaching

Ore grade Moderate to high Moderate to high Low Low to high (dependent

upon mine conditions and
layout)

Types of ore Oxide, silicates, and some

sulphides

Oxides, silicates, and some
sulphides

Sulphides, silicates, and oxides Oxides, silicates, and some

sulphides

Ore preparation May be crushed to optimize

copper recovery

May be crushed to optimize
copper recovery

Blasting None

Container or pad Large impervious vat Impervious barrier of clay,

synthetic material, or both

None for existing dumps; new
dumps intended to be leached
would be graded, and covered
with an impermeable polyethylene
membrane, or bedrock, protected
by a layer of select fill

None

Solution Sulphuric acid for oxides;

acid cure and acid-ferric cure
provide oxidant needed for
mixed oxide/sulphide ores

Sulphuric acid for oxides;
acid cure and acid-ferric cure
provide oxidant needed for
mixed oxide/sulphide ores

Acid ferric-sulphate solutions with
good air circulation and bacterial
activity for sulphides

Sulphuric acid, acid cure,
acid-ferric cure, or acid
ferric-sulphate, depending
on the ore type

Length of leach cycle Days to months Days to months Months to years Months

Solution application
method

Spraying, flooding, and
circulation

Spraying or sprinkling Ponding/flooding, spraying, sprin-

kling, and trickle systems

Injection holes, recovery
holes

Metal recovery method SX/EW for oxides and mixed

oxide/sulphide ores; iron
precipitation for mixed ores

SX/EW for oxides and mixed
oxide/sulphide ores; iron
precipitation for mixed ores

SX/EW for oxides and mixed
oxide/sulphide ores; iron precipita-
tion for mixed ores

SX/EW for oxides and mixed
oxide/sulphide ores; iron
precipitation for mixed ores

Source:
US Congress, Offi ce of Technology Assessment 1988

The second route is removing copper from copper concentrate. This is done in tanks,
vessels or vats (VAT leaching). Considering the cost of ore grinding and concentrating,
VAT leaching of copper concentrates is only viable for high-grade ore when even a slight
increase in copper recovery adds substantial profit. VAT leaching is also applied to leach-
ing ores that are not amenable to simple atmospheric leaching as illustrated in a later sec-
tion using the Bayer Process to separate alumina from bauxite ore as an example.
In its simplest form, hydrometallurgical mineral processing requires low capital invest-
ment relative to smelting, and the process can be operated economically at a small scale.
Especially where applied to ore in situ rather than as concentrate the processing of low-grade
ores can be achieved at much lower cost. The net result is that metals can be produced from
low grade ore that in the past would have gone untouched or would have been discarded as
waste rock. As such, it is amenable to use by small 'mom and pop' operators. In China, for
example, there are about 40 to 50 small-scale leaching operations as of 2000 (Dresher 2001).

VAT leaching of copper
concentrates is only viable for
high-grade ore when even a
slight increase in copper recovery
adds substantial profi t.