1989-11-22_REVISION - M1988112

• Permit No: M1988112
• IBM Index Class Name: Revision
• Doc Date: 11/22/1989
• Doc Name: TAILINGS COLUMN TESTING PROCEDURES BATTLE MTNS SAN LUIS PROJECT
• Type & Sequence: AM1
• Media Type: D
• Archive: No

introduction
This paper deals primarily with the
acid mine drainage problem associated with
the bituminous coal fields of northern
Appalachia. Past research dealt with the
identification of the geologic factors
affecting the occurrence of acid mine drain-
age on a regional scale. More recently,
efforts are being directed toward the iden-
tification of potentially acid or alkaline
producing material. This latter effort,
when coached in a multifactored conceptual
model that predicts mine drainage quality,
has Led to the formulation of a field
analytical technique that rapidly and ac-
curatelp identifies acid prod uci ny strata
(which are to be selectively handled and
isolated from the surface environment to
minimize the environmental impact), alkaline
producing strata (which are ideally suited
for soil-vegetational substrate) and
chemically inert material (which lends it-
self to stable backfill material).
Because of the interrelationship of
the regional and site specific facets of
mine drainage characterization, this paper
is presented in two parts; the first is
an overview of the acid mine drainage prob-
lem and the geologic factors responsible for
its occurrence, while the second part is a
discussion of a study that led to the iden-
tification of two parameters that can be
used to assess the rock's potential to pro-
duce alkaline oz acid leachates.
Background of the Acid Mine Drainage Problem
During earth moving operations, such
as in strip mining of coal, large volumes
of rock overlying the coal (the overburden)
are removed to expose the seam. This land
disturbance may impact the environment with
mine drainages that are laden with suspended
sediment, dissolved solids, heavy and minor
metals and, in certain cases, acidic mine
waters. Within a particular strip mine,
numerous strata of varying mine ra logics are
disrupted by the mining process and the
quality of the drainage emanating from the
mine site is a blend of water chemistries
produced by each rock type, In some mines
the combination of water chemistries yields
acidic drainages.
Acid mine drainage, as the name im-
plies, is an extremely acidic, iron and
sulfate rich drainage that forms under
natural conditions when certain coal seams
are mined and the strata are exposed to
accelerated oxidizing environments. During
this process, a variety of iron sulfides
(Fe52), occurring commonly as marcasite and
pyrite, oxidize in the presence of oxygen
and water to form soluble hydrous iron sul-
fates. These compounds appear as the white
and yellow-green salt crusts on the surface
of weathered rock faces. Natural waters
flwinq over the weathered surfaces readily
dissolve these compounds which hydroli ze
and form acidic high-sulfate, high-iron
drainages. The first step of the race [ion
releases ferrous iron, which can be oxidized
to the ferric state, and forms oxyhydroxides
that impart the red and yellow color that
is characteristic of acid mine drainage.
These iron hydroxide precipitates are the
'yellow boy that is often observed in the
streams and drainages of some coal mine
areas.
The oxidation of ferrous iron to Eerric
iron usually proceeds slowly under normal
conditions. Hoveve r, certain iron bacteria
act as catalysts and accele r.a to the chemical
reaction rate with an increase in acidity.
These iron oxidizing bacteria are indigenous
to aqueous environments with pH values that
range from 2.8 io },2 and are generally
absent or inactive in hydcogrologic ragimes
wi t5 pH values greater than around 5.5.
Factors Affecting Mine Drainage Quality
Aciaitr. It is generally recognized that
the oxidation of the iron disulfides (here-
after called pyrite) and the subsequent
hydrolysis of the oxidation products is the
cause of acidity. Because of this relation-
ship, it is often assumed that the amount of
acidity produced is related +ro the amount of
pyrite present, i.e., the hiyher the pyrite
(or sulfur) content the greater the poten-
tial of the sample to produces acid. How-
ever, Caruccio showed that rcpcks with a
paucity of carbonate produced acidities
that varied more directly with the particle
size of the pyrite than with the amount of
pyrite present. 2,} It was shown that small
particles of pyrite (less than 0.25 microns)
rapidly decomposed when exposed to the atmo-
sphere while coarse grained (pyrite particles
(greater than 50 microns) remained relative-
ly stable. In terms of a sample's poten-
tial to produce acidity, all other things
being equal, the samples with a preponder-
ance of fine grained (framboi~dal) pyrite
generate orders of magnitude more acid than
samples having coarse grained (secondary,
plant replacement, cleat coats, etc.)
pyrite.
Alkalinity. Previous work by Caruccio2 and
Caruccio and reide l4 has shown that the de-
gree of acidity is also a function of the
calcareous material content crf the strata,
which has the potential to produce alka-
linity and a highly buffered and potentially
neutralizing drainage. In addition, the
calcareous material maintains the pH of the
ground water regime in the alkaline range
which effectively surpresses the iron bac-
teria and reduces the catalye+is of the oxi-
dation of ferrous iron.
However, unlike the ac icl-formi nq reac-
tions (which have feu rate limiting steps
due to the high solubility of the oxidation
products) the geochemistry of alkaline pro-
duction from calcareous material is con-
strained by the low solubility of the spe-
cific carbonate mineral in water (sideri to
excluded).5 As a result, alkalinity con-
centrations are fixed by [he partial pres-
sure of carbon dioxide, the tt ime of water
contact and the solubility of the specific
mineral or rock, e.q., calcine, dolomite,
limestone.
438