Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 698

NAG test is a variation of this test, used for samples containing relatively high sulphur con-
tents or high ANC which may not react completely during a single stage NAG test. In this
multi-stage test, the same sample is tested repeatedly until no further reaction occurs.

Kinetic Geochemical Testing
Many geochemists consider that NAPP and NAG tests are not sufi ciently conclusive to
provide the sole basis for the management of mine wastes, and that selection of ARD con-
trol measures should be based on more dei nitive kinetic tests. This may be so in some cases,
but there are also many situations where a combination of static testing and local experience
enable coni dent predictions to be made which provide the basis for effective management.
The simplest kinetic test is a variation of the single stage NAG test - the Kinetic NAG
test - in which temperature, pH, and electrical conductivity are recorded during the test,
thereby providing information about reaction rates that can be related to i eld tests or site
observations.
The most common kinetic tests are laboratory column leach tests which are used to con-
i rm predictions of acid generation and leaching from the static testing programme, and to
evaluate:
●

Rates of acid generation over time;

●

Neutralization, and

●

Leachate water quality, including concentrations of metals over time.

As these tests are time consuming and comparatively expensive, relatively few are con-
ducted compared to numbers of samples subjected to static tests. One approach is to
undertake a column leach test on a single, representative, usually blended sample of each
identii ed waste rock unit or tailings type. An alternative approach is to use column leach
tests for those units where the static tests yielded inconclusive or contradictory results.
While laboratory leach tests attempt to simulate site conditions to the extent that is prac-
ticable, actual i eld conditions will be different. If such differences are considered criti-
cal, small-scale leach pad trials may be carried out to assess acid generating and leaching
behaviour of mine wastes at the mine site. In particular, such trials enable mine wastes to
be placed as in operating waste dumps or tailings storages. They also enable the effective-
ness of various treatment measures to be assessed. In the simplest cases, samples of leachate
are collected and analyzed periodically. In more sophisticated cases the leach pads are also
instrumented, with probes to measure moisture content, temperature and oxygen levels.

While laboratory leach tests
attempt to simulate site
conditions to the extent that is
practicable, actual fi eld conditions
will be different.

Characterization and Classifi cation
Various classii cation schemes have been used to characterize material types in terms of their
acid-forming potential. A system adapted by EGi (Miller and Jeffery 1995), and commonly
applied to both waste rock and tailings, classii es materials into four categories: barren; non-
acid forming (NAF); potentially acid forming (PAF); and uncertain (UC). The criteria used
for this classii cation are shown in Table 17.1 . Figure 17.6 illustrates this classii cation scheme.
Further subdivision of the PAF category is commonly carried out with, for exam-
ple, low and high or low, medium and high sub-categories being recognized. Another
approach, used by BHP-Billiton at its Cannington silver mine in Queensland, Australia,
uses a classii cation that combines acid-forming potential with salinity, based on manage-
ment requirements. This classii cation system is shown in Table 17.2 .
In general, simple classii cations are preferred in most situations, as geochemical complex-
ity makes it difi cult to distinguish and separate materials of differing ARD potential with-
out i rst conducting large numbers of tests. As discussed later, however, the measurement of
relative acid-generating potential may be required for certain ARD control strategies.

Simple classifi cations are
preferred in most situations, as
geochemical complexity makes
it diffi cult to distinguish and
separate materials of differing
ARD potential.