Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 693

FIGURE 17.3
Typical Acid Generation Profi le

Quantity
of Acid
Generated

Time

17.2 EVALUATING THE OCCURRENCE OR RISK OF ARD

Identifying Areas of Active ARD

Active oxidation of sulphide minerals, the precursor to ARD, may be identii ed by visual
observation of its common effects such as staining by hydrated oxides of iron and manganese,
or by the presence of efl orescent sulphate salts, coni rmed by tasting. Other, less common,
indicators are emissions of steam and or oxides of sulphur with their characteristic acrid
odour, and of course the presence of acidic solutions, identii ed by routine pH tests. Visual
observations may similarly be used to identify actively oxidizing intervals in drill cores,
particularly where the cores have been stored for several years in a humid environment. In
some cases, acid produced by oxidation of drill cores may result in rapid corrosion and col-
lapse of metal core trays, thereby providing an early indication of the potential for ARD.
The simplest tests used to delineate areas of developing ARD in the i eld, are the paste
pH and electrical conductivity tests. For these tests, samples of less than 50 g are crushed
to less than 1 mm and mixed with distilled water at a solid to liquid ratio of 1:2 (w/w) to
form a paste. After a period of equilibration, usually overnight, the pH and conductivity
are measured. A pH of less than 4 indicates an acidic sample. An electrical conductivity of
more than 2 deci-siemens per metre (ds/m) indicates a high level of soluble salts, a result of
oxidation with possible neutralization.
Similar tests can be conducted on drill cores. However, usually such cores are subjected
to static geochemical tests, as subsequently described.

The simplest tests used to
delineate areas of developing
ARD in the fi eld, are the paste
pH and electrical conductivity
tests.

Assessing the Potential for ARD

Theoretically, there is the potential for ARD from rocks containing a signii cant compo-
nent of sulphide minerals. However, whether or not ARD develops depends on numerous
factors including the particular sulphide minerals present; the size, shape, and texture of
the sulphide grains; the permeability of the substances surrounding the sulphide grains;
the oxygen concentration; pH, the presence of bacteria; and the presence of acid-neutral-
ising substances. Generally, the presence of sulphide minerals at concentrations of 1% or
more in the waste rock, tailings or pit walls, indicates the need for further investigation.
The steps involved in detailed assessment are shown in Figure 17.4 .

The presence of sulphide
minerals at concentrations of
1% or more in the waste rock,
tailings or pit walls, indicates the
need for further investigation.