Evaluation
Of Techniques for Preventing Acidic Rock Drainage Final Report
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report 2.35.2b
January 1997
EXECUTIVE
SUMMARY
The management
of waste rock produced from mining of sulphitic ores poses a challenge
to the mining industry. Acid generation occurs when sulphide minerals
(principally pyrite and pyrrhotite) contained in the rock are exposed
to air and water. In the absence of sufficient alkaline or buffering
minerals, the resulting leach water becomes acidic, and is characterized
by high sulphate, iron and, metal concentrations. This water, sometimes
called acid rock drainage (ARD), can contaminate surface water and
ground water courses, damaging the health of plants, wild life,
fish and, possibly, humans.
A study was
initiated by Noranda Technology Centre (NTC) and the Centre de recherches
minérales (CRW to evaluate the relative effectiveness of
various techniques for controlling ARD in waste mine rock. This
study was undertaken at NTC as part of the MEND (Mine Environment
Neutral Drainage) Program. The techniques investigated were water
cover, soil cover, wood bark cover, and addition of limestone and
phosphate rock (apatite).
Potentially
acid-generating waste rock samples used in the investigation were
obtained from the Stratmat site, located on the Heath Steele Mine
property, near Newcastle, New Brunswick, and from Les Mines Selbaie,
located near Joutel, Qu6bec. Both types of rock samples were crushed
to particle sizes between 25 and 50 mm. The investigation involved
outdoor lysimeter tests and indoor or laboratory column experiments.
Cover techniques investigated were a I m water cover, a soil cover
consisting of a 150 mm thick water-saturated clay layer sandwiched
between two 75 mm thick sand layers, and a 150 mm thick wood bark
layer. Limestone and phosphate were added at 1 and 3% dosages. Control
experiments, using waste rock without cover and additive, were also
installed for comparison. The outdoor tests were subjected to natural
weather conditions (rain, freeze-thaw and evaporation). The laboratory
or indoor tests were run at an average temperature of 20'C and subjected
to a cycle of 8 weeks of dry conditions and 8 weeks of wet conditions
(water addition). Water was added to simulate the average annual
precipitation for a nearby municipality, Dorval, Québec.
All tests were conducted in triplicate.
Monitoring
of the effluent water quality to three years (154 weeks) indicated
the control waste rock started producing acid very early in the
tests (about the 5th week). The rate of acid production was quantified
(mg of CaCO3 per day per kilogram of rock) and found to be higher
in the laboratory than outside. Higher laboratory temperatures are
most probably responsible for the higher rate. The Stratmat rock
generated acid at a higher rate than the Selbaie rock, although
the latter has a higher pyrite content. A detailed post-testing
investigation of the two rock types was conducted at The university
of Western Ontario, using mercury intrusion porosimetry, surface
analytical techniques and X-ray fluorescence and diffraction methods.
The results indicate the fresh, unoxidized Stratmat and Selbaie
rocks have a similar pore structure, but different gangue mineralogy.
The Stratmat rock consists of pyrite and minor amounts of metal
sulphides held in a matrix of silicate minerals including illite
and feldspar. The Selbaie rock, on the other hand, contains mainly
pyrite and quartz. Trace amounts of metal sulphides appear to be
in solid solution with the pyrite.
Results of
accelerated leaching tests clearly showed that the gangue composition
of the Stratmat rock has a major influence on its acid generation
ability and would explain the difference in acid production between
the Stratmat and Selbaie rocks. Water cover was found to be the
most effective control technique during the three years of indoor
testing, followed by 3% and 1% limestone, soil cover and, finally,
3% and 1% phosphate. The effectiveness of the various techniques
observed in the outdoor tests were as follows: water cover, 99%;
1% limestone, 93%; soil cover, 70%; and 1% phosphate, 9%. 10-15%
increase in effectiveness was observed (from 83 to 98%) when the
amount of limestone added to the rock was increased from 1 to 3%.
A similar increase in the amount of phosphate yielded higher effectiveness,
from 10 to 70%. All the techniques, with the exception of the water
cover, were found to be slightly more effective in the laboratory
than outside. The water cover maintained the same effectiveness
(> 99%) in both laboratory and outside tests. The soil cover
was more effective in the laboratory (98%) than outside (70%). The
difference may be explained by the effects of adverse natural climatic
conditions (for example, freezing and thawing) which were not resent
in the laboratory. It is also believed that oxygen and water enter
the soil covered waste rock mostly by the side walls of the lysimeters.
The phosphate was found to contain some carbonate mineral (calcite)
which probably delayed acid production (at the 3% dosage) for some
time. An increase in acidity and a decrease in pH were observed
in both the Stratmat and Selbaie rocks when all the calcite was
presumably consumed. It should be noted that the relative effectiveness
of the different techniques is likely to change with time, due to
depletion of alkalinity or phosphate materials.
The wood bark
accelerated acid production by about 60% in the laboratory and 500%
outside. The role of the iron oxidizing bacteria (Thiobacillus ferrooxidans)
was invoked to explain this acceleration. This was confirmed when
a bactericide (0.02% thymol solution), added to the wood bark cover,
reduced acid generation considerably. The iron oxidizing bacteria
are mostly autotrophs (that is, they require inorganic carbon for
their metabolism), and would become more active by using CO, produced
from fungal decomposition of the wood bark. Other heterotrophic
iron oxidizing bacteria would use organic carbon from the wood bark
for metabolism. Thus, a wood bark cover is not considered a good
technique for reducing acid generation in sulphide-bearing waste
mine rock. The water covered waste rock began to release low concentrations
of metals (zinc, iron and lead) after two and a half years of operation.
The delay in metal release may be attributed to the presence of
trace amounts of alkaline minerals which were probably depleted
after the initial two and a half years. The results of the study
thus far indicate that, although. a water cover may not completely
prevent oxidation, it will reduce acid generation considerably.
In fact, when considering both feasibility and efficiency, it is
the most promising ARD control technology known to the industry.
The rate of oxidation is decreased in two important ways: first,
the oxidation will begin much later if fresh rock is covered (two
and a half years in this case), and second, the oxidation will continue
at a considerably reduced rate, due to the oxygen diffusion barrier
the water presents. The delay before oxidation begins is probably
proportional to the neutralization potential of the rock. If oxidized
waste rock is covered with a layer of water, it is likely that the
alkaline materials will be depleted and that the oxidation will
begin immediately.
The effectiveness
of the water cover may be enhanced by increasing the depth of the
water or applying an organic layer on top of the waste. With an
organic layer, the oxygen may be consumed by biodegradation before
it can reach the sulphides. The practical implementation of a water
cover scheme presents some other questions (for example, maintaining
the required depth of water and long-term stability of holding structures)
which still have to be addressed through hydrological and engineering
studies. Laboratory studies such as this one are also necessary
to address initial uncertainties prior to implementation.
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