|
Chemical Analyses: Solids: Carbonate
Carbonate minerals, such as calcite (CaCO3), aragonite, and dolomite
(CaCO3�MgCO3), are considered to be the main sources of neutralizing
potential (NP) in soils, tailings and waste rock. By determining
the carbonate content, the amount of total neutralization potential
attributable to carbonate minerals can be determined. The remaining
neutralization potential is attributable to non-carbonate minerals
(i.e. silicate minerals, natural oxides, hydroxides, and phosphates).
In addition, carbonate, in various forms, is often added to soils,
tailings, and waste rock to increase the neutralization potential
of these potentially acid producing materials.
The carbonate content of soils, tailings, and waste rock is important
for predicting the neutralization potential of the material when
doing an acid-base account (ABA). See the technologies
for prediction section for more information on ABA. Neutralization
potential information is used in prediction to determine the potential
for acid mine drainage formation. Since most carbonate minerals
react quickly to neutralize acid, their contribution to mine reclamation
is of major significance. By predicting the amount of acid that
will form and the amount of neutralization potential natural occurring
in the material, the amount of additional carbonate mineral that
should be added to soil, tailings, and waste rock to completely
neutralize all acid formation can be determined.
Often times, various forms of carbonate minerals are added to soil
and water to neutralize acid. Carbonate addition can be a major
component of the reclamation process since it can neutralize the
acid generation of waste material and acid mine drainage. Reclamation
activities often involve one or more of the following practices
for mitigating acid generating waste material: (i) excavation
and treatment, (ii) in-situ
treatment, (iii) excavation and transport to a containment
structure. Often times, an analysis of reclamation alternatives
will determine whether the waste material should be contained or
treated. If treatment is selected as the reclamation alternative,
often times, treatment involves incorporation of carbonate minerals.
Carbonate minerals are also used to mitigate acid mine drainage
in some cases. Water is either treated by active chemical treatment
(i.e. carbonate addition), passive treatment, or a combination of
the two. For more information on water treatment and carbonate addition
for mitigation of acid mine drainage, see the water
treatment section. Similar to mitigation of waste material,
an analysis of reclamation alternatives is often conducted to determine
the best method for treating acid mine drainage. To look at an actual
plan that accesses different reclamation alternatives, click here
(link to High Ore).
Carbonate content is determined indirectly by reacting a finely
ground sample with a mineral acid to liberate CO2:
CaCO3 + 2H+ –-> Ca2+ + CO2 + H2O
The CO2 is then measured and expressed in equivalent kg CaCO3/tonne
of waste material. The evolved CO2 can be determined by volumetric
or gravimetric methods (MEND,
2001). Scroll down for a brief description of each of these
methods.
Volumetric Determination
The volumetric method involves leaching a 0.20 g pulverized sample
with dilute hydrochloric acid. Evolved carbon dioxide is carried
into a measuring burette by a stream of oxygen. The gases are then
passed through a potassium hydroxide solution that dissolves carbon
dioxide. The oxygen is returned to the burette and the volume is
again measured. The difference in volume, corrected for temperature
and atmospheric pressure, is proportional to the percentage of inorganic
carbon in the sample. The detection limit is 0.2% carbonate (Jeffrey,
1981).
Gravimetric Determination
The gravimetric method involves decomposition of inorganic carbonates
by treating the soil sample with 2N H2SO4 containing FeSO4 as an
antioxidant to prevent release of CO2 from organic matter. The air
stream containing evolved CO2 is purified by passing it through
a series of traps to remove water and extraneous constituents, after
which the evolved CO2 is absorbed in a Nesbitt absorption bulb.
The CO2 is determined by weighing the bulb before and after absorption
(Page
et al., 1982).
Chemical
Analysis | Physical
Properties |
