Research article
Mineralogical constraints on the
determination of neutralization
potential and prediction of acid
mine drainage
A.D. Paktunc
Abstract Acid-base accounting tests, commonly
used as a screening tool in acid mine drainage
(AMD) predictions, have limitations in (1) measuring with confidence the amount of neutralizers
present in samples and (2) affording an interpretation of what the test results mean in terms of predicting the occurrence of acid mine drainage. Aside
from the analytical difficulties inherent to the conventional methods, a potential source of error in
neutralization potential (NP) measurements is the
contribution from the dissolution of non-carbonate
minerals. Non-carbonate alkalinity measured during static tests may or may not be available to neutralize acidity produced in the field. In order to assess the value-added of extending the NP with the
knowledge of mineralogical composition and evaluate potential sources of errors in NP measurements,
a suite of samples were examined and characterized
in terms of their mineralogical and chemical compositions. The results indicate that although the
acid-base accounting tests work well for simple
compositions, the tests may result in overestimation or underestimation of NP values for field samples. Mineralogical constraint diagrams relating NP
determinations to Ca, Mg and CO2 concentrations
were developed with the purpose to serve as supplementary guides to conventional static tests in
identifying possible NP contributions from noncarbonate minerals and checking the quality of the
chemical testing results. Mineralogical NP makes it
possible to interpret the meaning of NP results and
to assess the behaviour of samples over time by
predicting the onset of AMD and calculating NP
values for individual size fractions.
Introduction
Acid-base accounting (ABA) tests, involving the determination of acid generating potential (AP) and neutralization potential (NP) of samples, are commonly used as a
screening tool to predict the occurrence of acid mine
drainage (AMD). These tests are designed to examine the
balance between the acid-producing and acid-consuming
components of wastes (Coastec Research 1991). The tests
are static in nature and as such they do not predict drainage quality from the wastes. Instead, the tests allow simple, rapid and low-cost screening of samples to make a
preliminary prediction for acceptable or unacceptable water quality (Coastec Research 1991). The ABA tests have
limitations in terms of providing reliable measurements
of NP. Secondly, the meaning of measured NP in terms
of predicting the occurrence of AMD is often not clear.
Uncertainties or errors in NP measurements can arise
due to any or a combination of mineralogical composition, particle size, procedural limitations and human error. Among these, uncertainties due to particle size, procedural limitations and human error are inherent to the
techniques and as such they can be minimized by revising the procedures and by improving laboratory quality
control/assurance standards. Readers should refer to Norecol Environmental Consultants (1991), Lawrence and
Wang (1997), and Lapakko (1994) for an evaluation and
review of the conventional NP determinations.
Measured NP values do not necessarily indicate alkalinity
that is readily available to neutralize acidity produced by
the oxidation of sulfide minerals in mine wastes. One
limitation as noted by Coastec Research (1991) which involves the inability of static tests in making a distinction
between the various acid consuming minerals and their
neutralization capacity, can lead to significant over or
Key words Acid mine drainage 7 Acid rock
underestimation of NP. Norecol Environmental Consuldrainage 7 Neutralization 7 Acid-base accounting 7
tants (1991) recommended that routine ABA tests include
Mineralogy 7 Tailings 7 Waste rock
a supporting mineralogical description to provide confidence in the chemical results. In addition, Price and others (1997) pointed out that existing static tests do not
identify minerals that contribute to effective field NP. In
cases where measured NP values reflect non-carbonate alReceived: 1 June 1998 7 Accepted: 6 October 1998
kalinity, effective field NP is likely to be overestimated.
A.D. Paktunc
CANMET, Mining and Mineral Sciences Laboratories, 555 Booth In summary, as stated by Lawrence and Wang (1997), reliable and confident determination of the practical NP
St, Ottawa, Ontario, K1A0G1, Canada
value of a waste cannot be achieved by a single static
e-mail:dpaktunc6NRCan.gc.ca
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
103
Research article
test. It is clear that there is a need to make the static test
results more meaningful from the point of view of their
ability to better screen samples and to establish mineralogical parameters prior to designing and conducting kinetic geochemical tests. Mineralogical composition is a
critical factor as it indicates the sources of NP and AP.
Provided that other conditions that cause and/or influence AMD are met, a static testing technique, based on
the premise that bulk dissolution is complete and incorporating mineralogical composition, should provide a
reasonable estimate of the acid generating potential of
rock and tailings samples (Paktunc 1999a). Recognizing
these limitations, Lawrence and Scheske (1997) proposed
a new method to calculate NP based on mineralogy. This
method involves the calculation of NP values for individual minerals based on their mineral abundances determined through the use of CIPW norm calculations and
relative reactivity factors. As this approach can predict
the presence of “neutralizing” minerals, not actually present in the sample, its usefulness and applicability for the
intended purpose is questionable (Paktunc 1999b). Consequently, the objectives of this study were to present mineralogical constraints as a guide to interpret the NP
measurements and discuss the mineralogical NP method.
cast in terms of kg CaCO3 equivalent per tonne, the following consideration must be taken into consideration.
One important thing to consider is the fact that one or
two moles of CaCO3 are required to neutralize one mole
of sulfuric acid. This depends upon which one of the following reactions takes precedence during neutralization.
CaCO3cH2SO4
2 CaCO3cH2SO4
*Ca 2ccCO2cH2OcSO 2P
4
2P
*2 Ca 2cc2 HCO P
3 cSO 4
(2)
(3)
NP can be recast in terms kg CaCO3 equivalent per tonne
by multiplying the Eq. 1 by 1.02 in the case of the reaction in Eq. 2 and 2.04 in the case of the reaction in Eq. 3.
For pyrite, ni is 2 when the reaction 2 is applicable and 4
in the case of the the reaction in Eq. 3. For pyrrhotite, ni
is 1 when the reaction in Eq. 2 is considered and 2 for
the reaction in Eq. 3. ns is 2 for pyrite and 1 for pyrrhotite. Other parameters needed in the above NP equations
are given in Table 1 for common carbonate species. NP
values for calcite can be written as:
NPp9.8!Xi
kg H2SO4 eq/t for reaction in Eq. 2
(4)
NPp4.9!Xi
kg H2SO4 eq/t for reaction in Eq. 3
(5)
NPp10!Xi
kg CaCO3 eq/t
(6)
Although equal moles of calcite and dolomite are needed
to neutralize sulfuric acid, dolomite’s NP is 1.1 times
Mineralogical NP and AP
greater than that of calcite. Siderite is initially a neutralizer; however, with continued dissolution, it produces
NP which is a measure of the amount of neutralizing
acidity as ferrous iron is oxidized to ferric iron and ferric
bases present in a sample, is determined by various
hydroxide precipitates. Thus, net contribution of siderite
chemical methods as outlined by Sobek and others
to neutralization is considered to be zero. It is assumed
(1978), Lawrence and others (1989) and Coastec Research that Mn is not oxidizable; therefore, Mn in carbonates
(1991). Recognizing the difficulties involved in the meas- contributes to NP. If Mn is deemed to be oxidizable, then
urement and interpretation of NP values, Paktunc (1999a) the parameters for Mn-bearing carbonate species listed in
proposed a new approach to determining NP values
Table 1 must be adjusted for the amount of non-oxidizabased on mineralogical composition. In this approach,
ble cations.
NP of each mineral is calculated based on its abundance In conventional ABA tests, AP is calculated based on
in the sample and the stoichiometry of neutralization
bulk sulfur concentrations. Although this is straightforreactions. NP is expressed in terms of sulfuric acid equivalent or the conventional CaCO3 equivalent. In order to
take into account the presence of more than one type of
Table 1
neutralizing mineral, a bulk NP is calculated
Parameters used in the calculation of mineralogical NP.
98!10!Xi!ci!ns
NPp A
ni!vi
ip1
k
(1)
where NP is the mineralogical NP in kg sulfuric acid
equivalent per tonne, 98 is the molecular weight of
H2SO4, 10 is the conversion factor for recasting in kg.t –1,
Xi is the amount of mineral i in wt%, ci is the number of
non-oxidizable cations in one formula unit of neutralizing mineral i, ns is the moles of sulfuric acid formed by
the oxidation of one mole of sulfide mineral s, ni is the
moles of mineral required to consume ns moles of sulfuric acid produced by the oxidation of one mole of sulfide
mineral s, vi is the molecular weight of neutralizing mineral i (g.mol –1) and k is the number of neutralizing minerals in the sample. If the mineralogical NP needs to be
104
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
w molecular weight; c number of non-oxidizable cations in one
formula unit
Mineral
Formula
w
c
Calcite
Siderite
Magnesite
Dolomite
Ankerite
Ankerite
Magnesian siderite
Magnesian siderite
Magnesian siderite
Magnesian siderite
Rhodochrosite
Kutnohorite
CaCO3
FeCO3
MgCO3
Mg0.5Ca0.5CO3
Ca0.5Fe0.3Mg0.2CO3
Ca0.5Fe0.1Mg0.4CO3
Fe0.8Mg0.2CO3
Fe0.6Mg0.4CO3
Fe0.4Mg0.6CO3
Fe0.2Mg0.8CO3
MnCO3
Ca0.5Mn0.5CO3
100.0
116.0
84.3
92.0
101.7
95.3
109.5
103.2
96.9
90.6
114.9
107.5
1.0
0.0
1.0
1.0
0.7
0.9
0.2
0.4
0.6
0.8
1.0
1.0
Research article
ns!98!Xs!10
APp A
ws
sp1
m
(7)
250
20
200
NP (kg CaCO 3 eq./t)
ward, ABA tests crudely assume that all sulfur is present
as pyrite. If the sample that is being tested contains pyrrhotite as well, overestimation of AP values by up to 1.5
times may occur. In order to take into consideration the
presence of more than one type of sulfide mineral in
samples, AP should be calculated in the following manner:
APp16.33!Xs
for pyrite
APp11.15!Xs
for pyrrhotite
calcite
Fe0.2Mg0.8CO3
Ca0.5Fe0.3Mg0.2CO3
150
10
Fe0.4Mg0.6CO3
100
5
50
1
where AP in kg sulfuric acid equivalent per tonne, ns is
the number of moles of H2SO4 formed by the oxidation
of one mole of sulfide mineral s as already defined for
NP equation above, 98 is the molecular weight of H2SO4,
10 is the conversion factor for recasting in kg.t –1, Xs is
the amount of sulfide mineral s in wt%, vs is the molecular weight of sulfide mineral s (g.mol –1) and m is the
number of sulfide minerals in the sample. Molecular
weight of pyrite is 120 whereas that of pyrrhotite is 87.9.
The AP formula (7) can be simplified as:
dolomite
0
Fe0.6Mg0.4CO3
Fe0.8Mg0.2CO3
2
20
10
5
1 2
0
2
4
6
CO 2 (wt%)
8
10
Fig. 1
Mineralogical NP and CO2 relations for various carbonate
minerals. Mineral quantities corresponding to 1, 2, 5, 10 and 20
wt % are indicated by symbols on each carbonate line. Calcite
and dolomite lines define the maximum NP values for a given
CO2 concentration. Dolomite has slightly higher NP than
(8) calcite, but the difference is too small to show up at this scale
of illustration
(9)
Similar to NP, AP can be recast in terms kg CaCO3 equivalent per tonne by multiplying Eq. 7 by 1.02 in the case
of the reaction in Eq. 2 and 2.04 in the case of the reaction in Eq. 3.
Alternately, AP can be calculated from bulk sulfur analysis. This is done by substituting mineral quantities (Xs) in
the above equations by the following:
Xsp1.85!C
for pyrite
Xsp2.7!C
for pyrrhotite
where C is the sulfur concentration in wt%.
As the dolomite and calcite lines define the maximum
NP values for common carbonate species, no NP value
should plot in the region above the calcite line. Thus,
points above the calcite/dolomite line are either due to
NP contributions from non-carbonate minerals or overestimated NP values. In addition, mineral quantity values
indicated on each mineral line provide an additional con(10) straint for interpreting NP values.
(11) Theoretical NP variations with CacMg concentrations
are shown on Fig. 2. Calcite and magnesian siderite lines
on this plot define the upper and lower limits of NP and
CacMg values due to carbonate minerals. Four magne-
Mineralogical constraints
200
NP (kg CaCO 3 eq./t)
Theoretical or mineralogical NP of common carbonate
minerals, calculated for a range of mineral quantities by
the above technique, are illustrated in Figs. 1 and 2 as a
function of CO2 and CacMg concentrations. These plots
can serve as the quality checks for NP measurements by
defining theoretical limits or constraints placed upon NP
by the carbonate minerals. Lines for calcite, dolomite, ankerite and four magnesian siderite compositions are
shown on Fig. 1. Magnesian siderite compositions,
Fe0.8Mg0.2CO3, Fe0.6Mg0.4CO3, Fe0.4Mg0.6CO3 and
Fe0.2Mg0.8CO3 are for illustrative purposes to represent
complete solid solution between siderite and magnesite.
On the NP vs. CO2 plot, calcite and dolomite lines overlap and determine maximum NP values due to carbonate
minerals for a given CO2 concentration. On this plot, the
lowest NP values for a given CO2 concentration are from
the magnesian siderite example with the highest Fe content (i.e. Fe0.8Mg0.2CO3). Ankerite (Ca0.5Fe0.3Mg0.2CO3)
line is between the Fe0.2Mg0.8CO3 and Fe0.4Mg0.6CO3 lines.
250
20
150
100
10
50
5
0
1
0
2
1
2
3
4
5
Ca+ Mg (wt%)
Calcite
Dolomite
Ca0.5Fe0.3Mg0.2CO3
Fe0.2Mg0.8CO3
Fe0.4CO3
Fe0.6Mg0.4CO3
Fe0.8Mg0.2CO3
Plagiodase
Hornblende
6
7
8
9
Fig. 2
Mineralogical NP and CacMg relations for various carbonate
species shown on Fig. 1. Magnesian siderite and calcite lines
define the upper and lower limits for carbonate minerals.
Mn-bearing carbonates may plot above the magnesian siderite
line. Also shown for comparison are plagioclase (CaAl2Si2O8)
and hornblende (Ca1.7Mg3.5Fe1.3Al1.3Si7O22(OH)22) lines
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
105
Research article
sian siderite compositions overlap and define the uppermost limit of NP for a given CacMg concentration.
Manganese carbonates such as kutnohorite
(Ca0.5Mn0.5CO3) and rhodochrosite (MnCO3) plot above
and to the left of the magnesian siderite line. Similarly,
Ca and Mg bearing silicates with high neutralization potentials such as plagioclase and hornblende (Paktunc
1999a) also plot above the magnesian siderite lines. An
exception to this would be olivine which plots along the
magnesian siderite lines. In conclusion, any data plotting
outside the carbonate array should be considered suspect.
One cautionary note concerning plagioclase, hornblende
and olivine lines on the NP vs. CacMg plot, is that maximum NP values were considered as the calculation assumed congruent dissolution of the silicates (Paktunc
1999a).
Evaluation of the mineralogical
approach
characterized by the formulas: CaCO3 and Ca0.5Mg0.5CO3
(Table 3). Siderite contains a significant proportion of
Mg; thus, it is referred to as magnesian siderite with the
formula Fe0.6Mg0.4CO3. Minor amounts of Mn and Ca are
present in the magnesian siderite sample. Magnesite contains minor levels of Fe and Mn.
Replicate measurements and analysis of the ABA reference material NBM-1 from CANMET ranged from 31 to
44 kg CaCO3 equivalent per tonne with an average of
40.6B3.7 kg per tonne. Comparing these results to the
recommended value of 42 kg per tonne for NBM-1 demonstrates that the method gave reproducible and accurate results; therefore, the NP results for the samples can
be accepted with confidence.
Measured NP values of the carbonate mineral and quartz
mixtures are compared against their theoretical counterparts. As illustrated in Fig. 3, the measured NP values
correlate well with the mineralogical NP values for calcite, dolomite and magnesian siderite with correlation
coefficients of 0.999, 0.993 and 0.985, respectively. The
situation is different for the magnesite samples, however.
It appears that the measured NP values of magnesite
mixtures are consistently underestimated. Deviation from
the mineralogical NP values increase gradually with an
increase in the amount of magnesite in the sample. The
NP values measured at pH 8.3 and 4.3 are similar; therefore, the discrepancy appears to be related to incomplete
dissolution of magnesite. Magnesite is not fully soluble in
cold HCl (Mason and Berry 1968); therefore, the modified
NP measurement technique does not appear to be suitable for magnesite-bearing samples.
The mineralogical NP values of the Kemess South samples correlate with the measured NP values by defining
the following relationship with a correlation coefficient of
0.91 (Fig. 4):
Measured NP (kg CaCO 3 eq./t)
In order to assess the value-added of extending the NP
with the knowledge of mineralogical composition, a suite
of samples were examined and characterized in terms of
their mineralogical and chemical compositions. The sample set represents a wide range of varying amounts of
carbonate minerals including calcite, dolomite, ankerite
and magnesian siderite as well as some reactive rockforming silicates with neutralizing capacity. The set includes rock samples from the Kemess South deposit in
British Columbia and tailings samples from the Louvicourt mine in Quebec. In addition, four sets of samples
were prepared by mixing 1, 2, 4, 6, 8, 10, 20 and 40 wt %
of calcite, dolomite, siderite and magnesite with quartz.
Whole rock chemical compositions were determined by a
NPp14.74c0.91!mNP
(12)
combination of atomic absorption spectrometry, inductively-coupled plasma atomic emission spectrometry at
the Analytical Services Group of CANMET. CO2 determinations were made by LECO furnace method. In addition
500
to bulk analyses, Ca and Mg were analyzed following seCalcite
lective dissolution of the samples by aqua regia. With the
Dolomite
400
exception of the Kemess South samples, NP values were
Magnesian siderite
determined by the modified acid base accounting test
1
Magnesite
1:
procedure (Lawrence and others 1989) and the measure300
ments were made at pH 8.3 and 4.3 (Table 2). NP values
of the Kemess South samples, based on the standard ABA
200
technique of Sobek and others (1978), were determined at
Chemex Labs Ltd. in North Vancouver. Mineralogical
compositions were determined by a combination of opti100
cal microscopy, scanning electron microscopy, X-ray diffraction and electron microprobe techniques. Mineral
0
quantities were determined by a mathematical technique
based on linear mixing calculations of mineral chemistry
100
200
300
500
0
400
and bulk compositions (Paktunc 1998). Partial analytical
Mineralogical NP (kg CaCO 3 eq./t)
results and mineralogical compositions are listed in Table 4.
Fig. 3
Electron probe microanalysis of carbonate minerals indi- Comparison of mineralogical NP against the measured NP
cate that calcite and dolomite compositions are ideal,
values for the synthetic mixtures of carbonate minerals
106
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
Research article
Table 2
Neutralization potentials (NP) of carbonate samples.
NP values in kg CaCO3 equivalent per tonne; Mod. NP modified NP; mNP Mineralogical NP
Sample
Mineral
Carbonate
wt%
Mod. NP
@pH 8.3
Mod. NP
@pH 4.3
mNP
cc-4
cc-5
cc-6
cc-7
cc-8
cc-9
cc-10
cc-11
dol-4
dol-5
dol-6
dol-7
dol-8
dol-9
dol-10
dol-11
mag-4
mag-5
mag-6
mag-7
mag-8
mag-9
mag-10
mag-11
sid-4
sid-5
sid-6
sid-7
sid-8
sid-9
sid-10
sid-11
Calcite
Calcite
Calcite
Calcite
Calcite
Calcite
Calcite
Calcite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Dolomite
Magnesite
Magnesite
Magnesite
Magnesite
Magnesite
Magnesite
Magnesite
Magnesite
Mg siderite
Mg siderite
Mg siderite
Mg siderite
Mg siderite
Mg siderite
Mg siderite
Mg siderite
40
20
10
8
6
4
2
1
40
20
10
8
6
4
2
1
40
20
10
8
6
4
2
1
40
20
10
8
6
4
2
1
409
204
102
85
74
57
35
19
434
205
87
102
92
70
38
15
70
37
20
19
13
8
4
3
146
89
22
16
13
7
3
4
410
206
105
88
77
60
38
23
437
208
89
105
95
73
41
16
72
39
22
20
15
10
6
5
336
188
51
38
28
16
7
6
400
200
100
80
60
40
20
10
435
217
109
87
65
43
22
11
474
237
119
95
71
47
24
12
155
78
39
31
23
16
8
4
Table 3
Electron microprobe analysis (wt%) of carbonate minerals.
CO2 calculated based on stoichiometry; avg average; st. dev. standard deviation; n number of analysis
Sample
Mineral
N
067994
Dolomite
15
PC-18
Calcite
12
PC-25
Siderite
25
13189
Magnesite
15
avg
st. dev
avg
st. dev
avg
st. dev
avg
st. dev
CaO
MgO
FeO
MnO
CO2
Total
56.40
~0.04
~0.09
~0.08
44.32
100.72
0.32
30.14
22.49
~0.08
~0.08
48.24
100.87
0.39
0.37
0.30
14.46
42.14
1.06
42.49
100.45
0.12
1.19
1.44
0.30
~0.04
47.64
0.40
0.62
52.66
101.32
0.58
0.52
0.29
Formula
CaCO3
Ca0.5Mg0.5CO3
where NP is the measured NP and mNP is the mineralogical NP. This relationship indicates that the majority of
the measured NP values are slightly overestimated in
comparison with the theoretical values. This is also apparent on the NP vs. CO2 plot where majority of the Ke-
Fe0.6Mg0.4CO3
MgCO3
mess South samples plot above the calcite or dolomite
lines (Fig. 5). The Kemess South samples contain calcite
as the dominant carbonate mineral. Since calcite or dolomite line on this plot defines the maximum values of
carbonate NP, data points plotting above the calcite or
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
107
Research article
Table 4
Partial bulk chemical composition, mineral quantities, and estimates of acid generation and neutralization potentials.
KS Kemess South; Lv Louvicourt; FeO total iron; Caaq Ca aqua regia dissolution; Mgaq Mg aqua regia dissolution; AP, NP
(measured), NNP, mNP (calculated based on mineralogy) in kg CaCO3 eq/t; mNP1 in kg H2SO4 eq/t; NP values for the Kemess
South samples are based on Sobek and others (1978) whereas the Louvicourt samples modified NP. A complete list of samples
used in this study and analytical results are available upon request from the author
Sample
Location
26138 26284
KS
KS
27219 27226 27245 27607 27911
KS
KS
KS
KS
KS
Partial bulk chemical analyses (wt%)
FeO
7.88
4.68 3.06
MgO
1.90
1.96 0.80
CaO
2.50
2.77 1.70
1.71
1.40 1.21
Caaq
0.84
0.83 0.31
Mgaq
CO2
0.33
1.17 0.11
S
1.22
2.25 0.07
Partial mineral quantities (wt%)
Plagioclase 11.9
25.8
K feldspar 20.4
7.4
Muscovite 16.9
24.1
9.1
Biotite
Chlorite
13.8
6.4
Amphibole
Calcite
3.3
3.2
1.2
Ankerite
Mg siderite
Pyrite
2.3
4.0
0.1
Acid-base accounting
AP
38
70
NP
40
31
NNP
2
P39
NP/AP
1
0
mNP
33
32
32
32
mNP1
Measured NP (kg CaCO 3 eq./t)
250
2
42
40
18
12
12
9.73
0.70
2.50
1.69
0.35
0.05
0.68
8.41
3.70
1.60
1.16
1.93
0.05
0.08
8.54
2.90
5.50
3.07
0.90
2.24
0.00
17.1
38.3
7.2
19.3
5.6
22.5
0.5
2.0
4.9
1.3
0.2
2.8
21
35
14
2
5
5
3
39
36
15
20
19
0
56
56
49
48
1:1
150
100
50
0
0
150
200
100
50
Mineralogical NP (kg CaCO 3 eq./t)
250
Fig. 4
Comparison of mineralogical NP against the measured NP
values
108
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
6.20
1.50
2.00
1.29
0.48
0.95
0.03
4.86
1.81
2.99
0.61
0.75
2.05
2.69
3.8
16.3
28.4
34.6
35.1
19.2
11.3
5.3
17.6
3.2
3.8
1
120
119
154
176
173
Kemess South
Louvicourt
200
6.23
2.60
8.40
7.19
0.42
6.18
0.03
28115 28190
KS
KS
1
29
28
37
32
32
84
45
P39
1
38
37
28205
KS
28298 28579
KS
KS
14.20
5.20
5.10
2.41
2.66
1.50
2.50
7.31
1.90
2.70
2.19
0.93
1.58
0.30
37.5
1.7
8.9
11.77
0.80
0.40
0.28
0.08
0.05
4.63
28594
KS
19.11
3.60
3.20
2.24
1.70
1.83
6.60
29706
KS
9.01
3.40
1.60
1.14
1.73
0.18
0.20
5.9
27.2
14.2
13.8
30.5
23.6
41.2
4.7
4.7
18.4
4.7
0.4
5.4
2.1
4.0
0.5
7.7
11.1
0.4
78
49
P29
1
47
46
9
59
50
6
47
46
145
4
P141
0
4
4
206
53
P153
0
54
53
6
32
26
5
21
20
dolomite lines are indicative of overestimated NP values.
The line fitted to the Kemess South samples is parallel to
the calcite or dolomite line, suggesting a systematic
measurement error associated with the NP measurements. When the Sobek NP values are converted to modified NP values by the use of the relationship defined by
Lawrence and Wang (1997), most of the Kemess South
data points shift to lower NP values below the calcite line
(Fig. 5). Similarly, when the Kemess South samples are
evaluated on the NP vs. CacMg plot (Fig. 6), it becomes
apparent that some samples plot outside the carbonate
array. This indicates Ca and/or Mg contributions from
non-carbonate minerals with neutralization potentials
that are less than those of carbonate minerals. Such incidents will cause an overestimation of NP values based on
Ca and Mg concentrations by aqua regia dissolution;
therefore, CaaqcMgaq concentrations as a surrogate to
NP determinations can only be used with caution.
Measured NP values are consistently lower than the mineralogical NP values for the Louvicourt samples (Fig. 4).
The samples contain ankerite and magnesian siderite as
the dominant carbonate minerals (Paktunc and Wilson
1998); therefore, it is expected that the bulk NP values
are controlled by ankerite and magnesian siderite
Research article
Table 4
Continued
Sample
Location
29784
KS
29791
KS
29874
KS
Partial bulk chemical analyses (wt%)
FeO
8.18
9.52
7.34
MgO
1.10
1.60
1.60
CaO
2.30
2.20
1.80
1.55
1.46
1.03
Caaq
Mgaq
0.28
0.63
0.69
1.61
1.80
0.56
CO2
S
0.02
0.02
0.62
Partial mineral quantities (wt%)
Plagioclase
K feldspar
Muscovite
31.4
30.6
Biotite
Chlorite
7.5
Amphibole
Calcite
4.0
Ankerite
5.3
Mg siderite
Pyrite
Acid-base accounting
AP
0
NP
46
NNP
46
NP/AP
98
mNP
40
mNP1
39
NP (kg CaCO3 eq./t)
250
1
47
46
68
40
39
31709
KS
31988
KS
32047
KS
5.42
0.80
2.60
1.85
0.42
1.39
0.45
6.65
3.20
3.70
5.00
0.88
2.14
0.06
8.45
1.30
11.90
9.65
0.71
7.40
3.60
20.1
16.9
11.4
3.2
15.8
10.3
2.5
21.6
4.2
6.8
1.1
6.4
0.8
0.1
19
27
8
1
25
25
113
254
142
2
216
212
3.2
25.7
7.3
14.4
14
63
49
5
42
42
2
84
82
46
68
66
Louvicourt
Kemess (Sobek)
Kemess (MNP)
200
150
Carbonate field
100
Fe0.6Mg0.4CO3
50
0
Fe0.8Mg0.2CO3
0
1
2
3
4
5
6
CO 2 (wt%)
7
8
9
10
Fig. 5
NP vs. CO2 of Kemess South and Louvicourt samples.
Essentially all the Kemess South samples plot within the
carbonate field when the Sobek NP values are converted to
MNP values. The exception are samples with nearly zero CO2
concentrations but indicating variable NP values of up to about
30 kg CaCO3 eq/t. These are believed to be due to contributions
from non-carbonate minerals
32320
KS
32371
KS
10.13
0.30
0.40
0.26
0.12
0.15
1.65
19.2
17.1
2.9
52
14
P38
0
0
0
11.73
5.60
1.70
2.39
1.01
0.11
0.28
32564
KS
8.75
3.40
5.40
3.54
1.44
2.87
0.01
6.2
19.5
17.9
17.0
39.8
25.6
1.8
9.8
32929 37005
KS
KS
9.97
3.30
5.40
2.96
1.32
0.08
0.02
53.5
17.4
0.5
9
32
23
4
18
18
0
79
79
281
98
96
1
22
21
29
0
0
3.96
1.80
7.40
2.73
1.86
1.76
1.30
37014
KS
6.44
1.90
3.30
1.18
3.35
0.07
0.07
11.1
51551
KS
13.62
3.60
5.90
3.10
1.75
2.46
3.70
34.5
9.3
11.4
9.3
6.2
7.1
16.9
34.9
4.0
6.8
2.2
0.1
6.0
41
136
95
3
71
70
2
63
61
28
40
40
116
75
P41
1
68
67
(Fe0.7Mg0.3CO3). Instead of plotting above the
Fe0.6Mg0.4CO3 line, the Louvicourt samples plot between
the Fe0.6Mg0.4CO3 and Fe0.8Mg0.2CO3 lines on the NP vs.
CO2 plot (Fig. 5). This indicates that the NP values of the
Louvicourt samples are underestimated by the use of the
modified NP technique. This appears to be due to high
pyrite content of the Louvicourt samples. According to
the modified acid base accounting procedure, titration is
completed at pH 8.3. Observations of the back-titration
curves of some samples suggested the presence of an inflection point at pH 4.3. During titration, hydrolysis of
Fe 3c and the precipitation of Fe(OH)3 at about pH 4
probably contributed to residual acidity and underestimation of NP. Thus, NP determinations made at pH 8.3 for
these samples are not indicative of the true neutralization
potentials of the samples. Instead, NP measurements
made at pH 4.3 are comparable with the calculated mineralogical NP values. Although, the NP measurements at
pH 8.3 may give more realistic values applicable to field
conditions for high sulfide bearing samples, the intend of
the NP measurement techniques is to determine the
amount of neutralizers present in the sample. In conclusion, NP values determined by conventional acid-base accounting techniques should be treated with caution for
samples containing high concentrations of sulfide. True
neutralization potentials of such samples may be higher,
probably closer to values determined at pH 4.3.
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
109
Research article
Table 4
Continued
Sample
Location
Cell 1A
Lv
Cell 1B
Lv
Partial bulk chemical analyses (wt%)
FeO
26.24
24.19
MgO
4.64
3.81
CaO
0.98
0.98
Caaq
Mgaq
CO2
3.00
3.54
S
15.80
15.70
Partial mineral quantities (wt%)
Plagioclase
K feldspar
Muscovite
11.3
16.4
Biotite
Chlorite
21.6
13.7
Amphibole
Calcite
Ankerite
1.9
3.0
Mg siderite
6.4
5.9
Pyrite
27.6
28.4
Acid-base accounting
AP
494
491
NP
19
22
NNP
P475
P469
NP/AP
0
0
mNP
47
54
46
53
mNP1
Cell 1C
Lv
Cell 1D
Lv
Cell 1E
Lv
Cell 2A
Lv
Cell 2B
Lv
Cell 2C
Lv
Cell 2D
Lv
Cell 2E
Lv
19.43
6.47
1.96
20.46
4.48
1.54
20.84
3.98
1.54
27.15
4.64
1.19
23.93
4.14
1.06
19.43
6.13
2.10
21.74
4.81
2.10
18.65
4.31
1.82
6.04
9.36
5.77
11.20
5.20
11.70
3.26
17.60
3.49
14.60
6.44
9.53
6.00
12.10
5.17
9.01
15.7
16.0
16.6
13.0
15.1
15.0
12.1
16.3
17.8
12.9
13.7
17.9
16.8
16.4
14.8
16.6
7.4
8.0
16.8
4.8
9.5
20.0
4.7
8.4
20.9
2.7
5.8
30.8
2.9
6.1
26.2
7.5
8.8
16.9
6.8
8.3
21.6
5.6
7.4
16.0
293
52
P241
0
100
98
350
40
P310
0
86
85
366
39
P327
0
80
79
550
18
P532
0
51
50
456
21
P435
0
54
53
298
55
P243
0
105
103
378
51
P327
0
97
95
282
42
P240
0
82
81
may serve as rough guides in the classification of geological materials for their potential for AMD.
In addition, the new approach can address some of the
shortcomings of the static tests. For instance, an assessAn assessment of the AMD potential of individual samment of the behaviour of a sample over time can be
ples can be made on sulfide vs. carbonate diagrams. Fig- made with the help of the mineralogical NP approach.
ure 7 illustrates the cases for the pyrite-calcite and pyrrSuch an assessment includes the prediction of onset of
hotite-calcite pairs. Similar plots can be derived for other AMD which is normally accomplished through the use of
acid producing and neutralizing minerals. Lines reprekinetic tests. The mineralogical NP also allows the calcusenting various NP/AP ratios are drawn to visually inter- lation of NP values for individual size fractions. Once the
pret the AMD potential of individual samples based on
required parameters are determined, NP and AP values
their mineral contents. Based on the British Columbia
are calculated for individual mineral and size fractions
ABA screening guidelines (Price and others 1997), samdissolving at particular time intervals. These are done
ples plotting above the NP/AP line of 1 are considered to through the use of a series of iterative calculations debe the likely producers of AMD. The NP/AP line of 1 cor- signed to simulate shrinking particles or shrinking core
responds to the calcite/pyrite ratio of 1.67 by weight and particle models (Paktunc 1999a). Such information will
3.1 by volume. Samples plotting below the NP/AP line of not only enhance the meaning of NP and AP values over
4 possess no potential for AMD according to British Co- time but also greatly aid the design and interpretation of
lumbia ABA guidelines. This value corresponds to the
kinetic tests. Furthermore, NP values of individual minercalcite/pyrite weight ratio of 6.67 (12.4 by volume). Simi- als are useful in the assessment of long-term behaviour
larly, NP/AP ratios of 1, 2 and 4 correspond to calcite/
or capacity of a waste rock pile or tailings. Contribution
pyrrhotite volume ratios of 1.9, 3.9 and 7.8. In other
of slowly dissolving minerals to the overall buffering cawords, in order for a waste material to be considered as
pacity in the long term can be significant which should
having no AMD potential, it must contain at least 12
be incorporated in the overall AMD evaluation.
times as much calcite as pyrite or approximately 8 times
as much calcite as pyrrhotite. These mineral proportions
Mineralogical NP and prediction
of AMD
110
Environmental Geology 39 (2) December 1999 7 Q Springer-Verlag
Research article
300
NP (kg CaCO 3 eq./t)
250
Carbonate
array
200
150
100
50
0
0
1
2
3
4
5
6
7
8
Ca aq +Mg aq (wt%)
9
10
11
12
Fig. 6
NP vs. CaaqcMgaq plot of the Kemess South samples. The
carbonate array is defined by the magnesian siderite and calcite
lines shown on Fig. 3
25
Likely
potential for AMD
NP
/A
P
=1
;p
o
30
Sulfide (vol%)
20
=
P
/A
o
NP
;p
=2
P
/A
NP
15
10
=
/AP
NP
y
2; p
4; po
AP =
/
P
N
= 4; py
NP/AP
5
0
py
1;
0
10
20
30
40
Calcite (vol%)
50
60
Fig. 7
Sulfide vs. calcite diagram to evaluate the potential for AMD.
Pyrite is represented by py and pyrrhotite by po. NP/AP values
of 1, 2 and 4 which represent boundary values between likely,
possibly, low and no potential for AMD according to BC
guidelines are shown for illustrative purposes
Conclusion
Presence of high concentrations of Fe-sulfide in samples
may result in undermeasurement of NP by the modified
acid-base accounting technique. Although, it appears that
such underestimated NP values may be more realistic for
high sulfide bearing samples in field conditions, the intend of the NP measurement techniques is to determine
the amount of neutralizers present in the sample. NP
measurements can be made with caution at lower pH values for samples with high concentration of sulfide.
Mineralogical constraint diagrams such as NP vs. CO2
and NP vs. CacMg graphs can be used to assess the
quality of NP measurements. Mineralogical NP provides a
greater degree of confidence to the chemical results.
Since individual contributions of minerals to the overall
NP values are determined, ambiguities on the interpretation of the results are greatly reduced.
Carbonate to sulfide ratios can be used to assess the
AMD potential of samples similar to NP/AP ratios. Calcite/pyrite and calcite/pyrrhotite ratios may serve as
rough guides to screen samples for the determination of
their potential to generate and neutralize acid.
The mineralogical NP can address an important shortcoming of the static tests. NP values of individual minerals which are useful in the assessment of long-term behaviour or capacity of mine wastes can be calculated individually. This allows an interpretation of the static testing results and evaluation of the NP and AP values within a kinetic framework. The approach also allows the use
of particle size variations in predictive calculations. These
features form the basis of a modelling approach that can
bridge the gap between the static and kinetic test results
by determining the contribution of slowly dissolving minerals to the overall buffering capacity in the long term.
Such information can be used to design and interpretation of kinetic tests.
Acknowledgements Samples along with some chemical data
from the Kemess South property were kindly provided by Silvia
M. Heinrich of El Condor Resources in Vancouver. Carbonate
minerals from the Geological Survey of Canada’s National Mineral Collection were obtained from Gary Ansell. Whole rock
chemical analysis and acid-base accounting tests were carried
out by the Analytical Services Group of CANMET. X-ray diffraction and electron microprobe analyses were carried out by
Paul Carriere and Gilles Laflamme. Mike Beaulne prepared the
samples used in the study. Discussions with Henry Steger and
John Graham of CANMET on NP determinations and other aspects of acid-base accounting techniques were helpful. Dr. Henry Steger and Carl Weatherell reviewed the manuscript and provided useful comments.
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