Blending
and Layering Waste Rock to Delay, Mitigate or Prevent Acid Rock
Drainage and Metal Leaching: A Case Study Review
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.37.1
April 1998
SUMMARY
As a long term
means of preventing the onset of acidic drainage, potentially acid
generating and acid consuming waste rock can theoretically be blended
or layered to produce a geochemically benign composite. This report
presents the general theory behind blending and layering for prevention
and control of acid rock drainage (ARD), and presents selected case
studies on the use of this technique at mines in Canada, United
States and other parts of the world.
The focus of
this report is the blending, mixing and layering of waste rock,
and, to some extent, the addition of limestone and other alkaline
materials. The addition of alkaline materials was initially considered
beyond the scope of this MEND sponsored project (Project2.37.1).
However, the scope was expanded as very few case studies were found
which exclusively examined blended waste rock, especially in the
coal fields. Both coal and metal mines are covered in the review.
Coal
Mines
Most of the
reviewed coal case studies were located in the Appalachian region
of the eastern United States with pyrite sulphur values in the range
of 0. 1 to 1 %. At these sites, the effect of waste rock blending
was often confused or masked by additional management measures such
as lime and flyash addition, and/or the presence of acidic drainage
from adjacent historic sites. Comprehensive reports, providing compilations
and assessments of acid-base accounting (ABA) data and kinetic tests,
predictions of long term effluent quality, pre and post mining monitoring
of rock characteristics, and pre and post mining monitoring of drainage
quality, were not found in the course of this project. However,
numerous papers in the published literature reviewed specific and
general management practices at individual sites.
Researchers
in Appalachia have compiled estimates of volume-weighted ABA data
derived from drill core analyses, and compared these values to effluent
quality at associated seepages. These compilations (dipretoro and
Rauch, 1988; Ericksen and Hedin, 1988; Brady et aL, 1994)
have resulted in the development of general criteria for overall
volume-weighted waste rock blends considered likely to produce net
alkaline drainage but not necessarily prevent metal leaching. The
commonly cited criteria in the Appalachia (P. Ziemkiewicz, pers.
comm., 1997) are:
Net Neutralization
Potential (NNP) > 10 kg CaCO3 equivalent/tonne;
Neutralization
Potential (NP) > 15 kg CaCO3 equivalent/tonne; and,
Ratio of Neutralization
Potential to Acid Potential (NP/AP) ratio > 2.
The reviewed
studies on coal wastes tended to examine the addition of alkaline
materials rather than the deliberate blending of acid generating
and acid consuming waste rock types. These studies indicated that..
a) Fine alkaline
material distributed uniformly throughout a waste pile were effective
in delaying the onset of acidic effluent,
b) Layering
of limestone within test piles was effective in reducing the amount
of net acidity produced, and in delaying the onset of acidic effluent,
but did not effectively prevent acid conditions from developing
within the potentially acid generating (PAG) portions of the piles,.
c) Lime kiln
dust was more effective in reducing the amount of net acidity produced
and in preventing the onset of acidic effluent than limestone, possibly
due to the higher surface area and the greater reactivity of CaO;
d) The order
in which overburden was placed in relation to acid drainage generating
strata had a significant effect on leachate quality, and,
e) Placement
of overburden as soon as possible after excavation was beneficial
in reducing ARD. The critical ratio of alkaline addition to acid
potential which would permanently prevent the onset of acidic drainage
at specific sites was generally not predicted.
Metal
Mines
Few metal mine
sites were identified that had deliberately examined or applied
blending and/or layering as a prevention method. Typically, older
sites that provided long term water quality data to support the
success or failure of this technique Jacked sufficient documentation
on materials in the waste piles and the level of blending employed
to allow conclusions to be drawn. Newer sites that characterized
their materials and operational procedures have had limited time
in which to demonstrate success or failure.
Site data and
laboratory test data for metal mines tended to indicate that:
a) The majority
of cases evaluated blends with NP/AP ratios less than 2;
b)
Blending did not reduce sulphide oxidation rates in the potentially
acid generating material unless highly reactive neutralizing material
(limestone) was applied and the blending was near ideal. Perfect
mixing was generally only possible in column or humidity cell tests.
Such ideal blending was not considered feasible at field scale;
c) Layering
thicknesses down to 1 0 cm did not reduce sulphide oxidation rates
in potentially acid generating material;
d) Blending
and layering were effective in delaying the onset of acidic effluent,
and reducing the metal and acidity loadings exiting from the combined
material. The presence of alkaline drainage from neutralizing materials
appeared to temper the locally produced acidic leachate, resulting
in less acidic drainage and lower metals levels due to decreased
solubility of hydroxide and carbonate phases at the elevated pH.
e) Prevention
of acidic effluent from the blended or layered materials did not
necessarily prevent dissolved metal levels from being problematic.
The reviewed
metal mine case studies did not identify safe waste rock blends
which would prevent ARD and metal leaching at those sites.
From the literature
and case studies reviewed, it appears that blending and layering
of acid generating and acid consuming waste rock may be a legitimate
method of neutralizing acidic drainage with a consequent reduction
in metal leaching through precipitation of metals within the pile.
However, there remains considerable uncertainty, and further investigation
into the application and practice of this method is required. Specifically,
more information on large scale, controlled field studies is needed.
The identification and detailed documentation of additional sites
should also be encouraged, in association with predictive laboratory
tests. At issue is what portion of alkalinity is available to neutralize
the acidity produced by sulphide oxidation, and the extent that
this is influenced by the thoroughness with which alkaline and acid
producing waste rock are blended at a given site.
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