This report documents the results of the first part of a research
project jointly funded by Noranda Inc. and the Mine Environment
Neutral Drainage (MEND) program. The second part is covered in a
separate report entitled Hydrology and Solute Transport in Oxidised
Waste Rock from Stratmat Site, N.B., published concurrently with
this report (MEND 2.36.2b). The overall objective of the project was
to understand the geochemical and hydrological interactions
between the partially oxidised waste rock and water and to improve
our capabilities and techniques in the prediction of acidic drainage
from waste rock piles.
Partially oxidised waste rock was sampled from the Stratmat pile at
Heath Steele Division of Noranda Inc. by grabbing, trenching, and
bulk excavation techniques. The samples were physically and
geochemically characterised in the laboratory whereas the bulk
density was measured in the field. The trenched samples were
used in column dissolution tests in which 25-kg composite
sub-samples were subjected to repeated washing with water to
observe the water quality evolution over time. The resulting data
were used to predict water quality for a hypothetical scenario where
the waste rock were backfilled in the Stratmat pit. In addition, water
quality profiles were measured in the Stratmat pit.
Results of the column dissolution tests suggest the following mass
balance for the Stratmat pile: Approximately 7% of the original
sulphide sulphur has been oxidised since deposition, releasing a
total acidity of 11 800 t CaCO3 equivalent, of which 56% has been
neutralised in situ. Currently, the acidity inventory is approximately
5200 t CaCO3 equivalent whereas the inventory of soluble zinc is
about 1660 t. The mass balance appears to support preferential
oxidation of sphalerite over pyrite.
The column dissolution experiments further indicate that dumping
the waste rock into the flooded Stratmat pit will cause significant
release of stored metals and sulphate. The long-term pore water
quality in the absence of ground water movement is predicted as
follows: pH 3.32, acidity 12 500 mg CaCO3/L, SO42- 19 500 mg/L,
Zn 4500 mg/L, Cu 180 mg/L and Pb 2.4 mg/L. In the presence of
uncontaminated ground water movement, the water quality would
gradually improve as the initial pore water is displaced or diluted. It
would take nine pore volumes of flushing to reduce the
concentrations of most metals (except Pb) to below 0.1 mg/L. For
Pb, this would take many more pore volumes.
Geochemical modelling suggests various concentration control
mechanisms. Concentrations of Pb and Fe in the pore water are
likely controlled by equilibria of the leachate solution with anglesite
and ferric hydroxide, respectively. On the other hand,
concentrations of SO4, Zn, Ca, Mg, Mn, and Al in the pore water
seem to be limited in the short term by dissolution/diffusion rate
controls. The presence of gypsum is found to inhibit the dissolution
of anglesite. As a result the anglesite stored in the waste rock
would not dissolve appreciably until gypsum storage is exhausted
by dissolution. This implies that decommissioning of the waste rock
by a backfilling-flushing-treatment process would last a long time
before the pore water in the backfilled waste rock becomes
acceptable for discharge to the receiving groundwater.
