1193
The Canadian Mineralogist
Vol. 43, pp. 1193-1203 (2005)

CALCULATED ACID–BASE BALANCE FOR H2O2 OXIDATIONOF CARBONATE-POOR PYRITIC MINE-ROCK
PAUL A. WEBER§, JOAN E. THOMAS, WILLIAM M. SKINNER AND ROGER ST.C. SMART¶
Ian Wark Research Institute, University of South Australia, Adelaide 5095, Australia

Abst r act
An assessment of the net acid–base account following H2O2 oxidation of mine rock was established by chemical calculations based on the molar quantity of neutralizing cations (Na, K, Mg, Ca) and acid-generating S released during the Net Acid
Generation (NAG) test. These neutralizing cations are derived from both silicates and carbonates; in carbonate-poor samples(ANCcarb < 5 kg H2SO4/t), however, the acid-neutralization capacity of silicate may be more important in neutralizing acidity
than carbonate ANC. Testing is not recommended for samples that have significant concentrations of non-acid-forming S, nonneutralizing cations or non-acid-forming sulfide minerals, such as galena.
Keywords: acid rock-drainage, acid-neutralization capacity, net acid generation (NAG) test, H2O2, acid mine-drainage, pyrite
oxidation, acid–base accounting.

SOMMAIRE
Un bilan global entre acidité et basicité de roches exploitées dans les mines suite à leur oxydation par traitement au H2O2 aété établi par calculs fondés sur la quantité molaire des cations neutralisateurs (Na, K, Mg, Ca) et de S apte à générer lʼacidité
en utilisant le test NAG (génération globale dʼacidité). Ces cations neutralisateurs seraient dérivés à la fois de silicates et decarbonates; dans les échantillons à faible teneur en carbonate (ANCcarb < 5 kg H2SO4/t), il est clair que la capacité des silicates à
neutraliser lʼacide pourrait être plus importante que celle des carbonates. Un tel test nʼest pas recommandé pour les échantillons
qui possèdent une proportion importante de soufre non apte à former un acide, de cations non aptes à neutraliser, ou de minéraux
sulfurés non aptes à générer de lʼacidité, par exemple la galène.
(Traduit par la Rédaction)
Mots-clés: drainage minier acide, capacité à neutraliser lʼacide, test NAG, évaluation de la génération dʼacide, H2O2, oxydationde la pyrite, bilan acide–base.

INTRODUCTION
A comprehensive set of tools for predicting thegeochemical characteristics of discrete samples ofrock and their potential acid-forming (PAF) or nonacid-forming (NAF) behavior is available (e.g., Sobek
et al. 1978, Morin & Hutt 2001, Smart et al. 2002). A
common predictive test in cases of acid rock-drainage(ARD) used to characterize samples as to their overall
acid-production capacity is the net acid-generation(NAG) test (Miller et al. 1997, Smart et al. 2002). This
test employs H2O2 to oxidize sulfide minerals such as
pyrite, thereby generating acid, which then reacts with§¶neutralising minerals to produce a final NAG pH thatreflects the net acid-generation capacity of the sample.
Back-titration of the NAG liquor to pH 7.0 provides
an assessment of the NAG value (expressed in kg
H2SO4/t). Net acid-generation in conjunction with the
net acid-production potential (NAPP) can be used tofurther refine the classification of a sample in regard toits acid-generation potential (e.g., Smart et al. 2002).
In this paper, we investigate the acid-neutralization
reactions occurring in the NAG tests (and sequential
NAG test) using calculations based on neutralizing ions(Na, K, Mg, and Ca) released to solution during the test.
This is the first time that silicate acid-neutralization

Current address: Rock & Earth Ltd, Castle Hill Village, RMB55037, Christchurch, New Zealand. E-mail address:
rockearth@xtra.co.nz.
Current address: Applied Centre for Structural and Synchrotron Studies, University of South Australia, Adelaide 5095,
Australia.

1194

THE CANADIAN MINERALOGIST

capacity (SiANC) has been addressed by these methods
in the NAG test. A similar process has been described
previously for carbonate dissolution in the ANC test
(Weber et al. 2004a, 2005), and its dissolution in the
NAG test (Stewart et al. 2003a). In combination with
acid-generating reactions (determined from S release),
it is possible to obtain an acid–base account for the
NAG test, which provides an explanation of the final
pH values in the NAG test.

BACKGROUND INFORMATIONThe NAG test and variations thereof (e.g., Miller
et al. 1997, Smart et al. 2002) evaluate the net acidgeneration potential of a sample without specific separate estimation of the maximum potential acidity (MPA)or acid-neutralization capacity (ANC) (Miller et al.
1997), and is based on earlier similar procedures (e.g.,
Sobek et al. 1978, Finkelman & Giffin 1986, OʼShay
et al. 1990). Acid generated during the test by sulfide
oxidation reacts with carbonate and silicate minerals
present in the sample and may result in their dissolution.
Rapid decrease in pH during the NAG test indicates thepresence of reactive sulfide minerals or acidic sulfate
salts (Weber et al. 2004b), whereas a high final NAG
pH (above 4.5) indicates the presence of carbonates andsilicates with acid-neutralizing capacities in excess ofacid produced.
Oxidation reactions in the NAG test can generate
false readings of pyrite-induced acidity in organicmatter-rich samples (e.g., coal measures) owing to
the formation of organic acids under H2O2 oxidizing
conditions, leading to overestimation of the net acidgeneration capacity (derived from sulfide oxidation) of
the sample during back-titration (OʼShay et al. 1990,
Stewart et al. 2003a, Ahern et al. 2004). In samples withsignificant carbonaceous organic matter, these authors
showed that a better indication of the acid generation isto determine the acidity in the NAG test due to pyrite,
monitored by S release (Stewart et al. 2003a, Ahern etal. 2004). They reported that the total S in the NAG
digestion liquor may be representative of the pyrite
oxidized, and that this can be used to determine thepyrite-induced acidity generated in the NAG test (e.g.,
AcidityNAG; Equation 1). To determine the net acidgeneration capacity of a sample requires considerationof neutralizing reactions as well as acid-generating reactions in the test. Stewart et al. (2003a) suggested that theneutralization of acidity occurring in the NAG test canbe determined by Mg and Ca concentrations measured
in the NAG solution as an indication of carbonate dissolution (e.g., ANCNAG(Mg+Ca); Equation 2). However, incontrast to Stewart et al. (2003a), we suggest that Equation 2 also includes neutralization derived from Mg- andCa-bearing silicate minerals. These two equations canbe used to determine the calculated NAG value (CNV)
expressed in kg H2SO4/t (Equation 3).

AcidityNAG = [S  2]  49/wt(in kg H2SO4/t)(1)

ANCNAG(Mg+Ca) = [(Mg + Ca)  2]  49/wt(in kg H2SO4/t)(2)

CNV = AcidityNAG – ANCNAG(Mg+Ca)(in kg H2SO4/t)(3)

where S, Mg and Ca are in mg/L, wt is the sample
weight in g. Conversion of S in NAG solution toequivalent H2SO4/t assumes that all S is derived from
pyrite oxidation and 1 mole of H2SO4/t is generated
for every mole of S in solution, and all Mg and Ca
are derived from acid dissolution of carbonates andsilicates. However, Na- and K-bearing silicate minerals
also can neutralize acidity. This has been demonstrated
in the modified Sobek ANC test (Weber et al. 2005),
and can similarly be determined for the NAG test, e.g.,
Equation 4.
ANCNAG(Na+K) = [(Na + K)]  49/wt(in kg H2SO4/t)(4)where Na and K are in mg/L, and wt is the sample
weight in g. All Na and K are assumed to be from silicate neutralization. Thus this source of neutralization
should be considered when assessing the net acid–basebalance.
The assumption that all S released to solution during
the NAG test (or in other H2O2 methods) is derived from
pyrite dissolution and thus equates to the release of 2 H+
ions is erroneous. For instance, Jennings et al. (2000)
showed that arsenopyrite, pyrite, marcasite, pyrrhotite, and chalcopyrite generate acidity, and (low-Fe)
sphalerite, chalcocite, and galena do not. This finding
is similar to that of Stewart et al. (2003b), who demonstrated that there are significant differences between the
acid-forming potentials of different sulfide minerals in
the NAG test and that, of the sulfide minerals examined,
only pyrite, pyrrhotite, arsenopyrite and chalcopyrite
are acid-producing, whereas sphalerite, galena, bornite,chalcocite and covellite are not. Results also showedthat for samples with up to 5 wt% S (present as sulfides),a single-step NAG test is sufficient to oxidize themajority of sulfides present as pyrite, pyrrhotite, arsenopyrite, and sphalerite. However, this finding is at odds
with calculations based on stoichiometry, which suggestthat >5 wt% S should be oxidized. It can be determinedthat 15 vol.% H2O2 (6.35 mol/L) or 1.6 mol H2O2 isavailable in the NAG test (250 mL) to react with any
pyrite present. According to Equation 5, this amount issufficient to oxidize up to 0.21 mole of pyrite. If the 2.5g NAG sample was entirely pyrite (0.02 mole of pyrite),then theoretically there is ample H2O2 is for oxidation
purposes. However, owing to H2O2 loss associated with
its catalytic decomposition, this is not the case:

H2O2 OXIDATION OF CARBONATE-POOR PYRITIC MINE-ROCKFeS2 + 15/2 H2O2 → Fe(OH)3+ 2 SO42– + 4 H2O(5)

Samples tested by the NAG test will commonly experience a temperature excursion during the test procedure,
which is associated with the catalytic exothermic
decomposition of the H2O2 (Stewart et al. 2003b) (e.g.,
Fig. 1). This temperature excursion has previously been
incorrectly attributed to the exothermic reactions ofminerals with H2O2. The cessation of effervescence wastaken to indicate that the reaction had run to completion (Jennings et al. 2000). Catalytic decomposition of
the H2O2 is triggered by metals such as Pb, Cu, Mn,
and Fe (Cotton & Wilkinson 1988) that are typicallyreleased during oxidation of sulfide minerals in the
NAG test. Following decomposition of the H2O2, nofurther significant oxidation of pyrite occurs.
Since total oxidation of sulfide may not occur insamples with high sulfide-bound S (owing to decomposition of the H2O2), the final NAG pH may be circumneutral, if there is sufficient acid-neutralization capacity
in the sample to neutralize the acid released, when infact, the acid-production potential (e.g., MPA) is far
greater than the ANC. To determine the acid-production potential in such samples requires a sequential

1195

NAG test (Smart et al. 2002), which involves repeatedadditions of H2O2 to the sample residue (after removalof the previous NAG solution by filtering) to determine
the true acid-generating potential of the sample. Some
problems with this method occur if the sample has ahigh clay content that can clog the filter paper. In thisinstance, the liquor must be decanted off, which leaves
a portion of solution and dissolved ions in the sample
residue. Unfortunately, these ions can then be calculated as being released again from the next sequentialstage.
LECO determinations of inorganic C are a common
and useful way to determine the carbonate content,
and hence, the calculated ANC derived from carbonates (ANCcarb), provided Fe carbonate is not an issue(e.g., Paktunc 1999, White et al. 1999, Weber et al.
2004a). The difference between the ANCcarb and the
ANCNAG(Mg+Ca) is therefore due to silicate ANC. The
difference between AcidityNAG and the ANCNAG should
provide an overall indication of the net acid–baseaccount of the NAG test (ABANAG). In this paper, weinvestigate these acid–base balances.

METHODS AND MATERIALSRock samples
Eight samples of rock were characterized as partof the Australian Minerals Industry Research Association (AMIRA) P387A project, which involved
X-ray diffraction (XRD), X-ray fluorescence (XRF),LECO total S, LECO organic and inorganic C, opticalmineralogy and scanning electron microscopy (SEM).
Mineral assemblages were determined by XRD–XRF
mineralogical reconciliation (similar characterization
was reported previously by Weber et al. (2004b). Two
samples were deemed unsuitable for ABANAG calculations because of significant non-acid-forming sulfides or
non-acid-forming sulfate salts. Thus six of the originaleight samples were assessed. Mineralogical data for

FIG. 1. Kinetic NAG test pH and temperature profiles forsample PAF–H. The rapid decrease in pH from 90 to 120
minutes is due to the inability of the pH-measuring system
to compensate for the sudden increase in temperature.

1196

THE CANADIAN MINERALOGIST

these rock samples are presented in Table 1. Maximum
potential acidity (MPA) is based on total wt% S, as
determined by LECO total S analysis, multiplied by thestoichiometric factor 30.6 to determine the MPA in kg
H2SO4/t; the ANC is determined by the Modified Sobek
ANC test, although the ANCActual (Weber et al. 2004b)
was determined on samples that were affected by acidgeneration during storage and the testing process itself(derived from acidic sulfate salts and reactive sulfides).
The NAPP is based on the difference between MPA
and ANC (using ANCActual where available; e.g., see
Weber et al. 2005).
The Modified Sobek ANC test
The Modified Sobek ANC test is based on themethodology of Sobek et al. (1978) as adapted byEnvironmental Geochemistry international (Smart et al.
2002). The test involves adding the prescribed quantity
and molarity of acid to 2 g dry pulverized sample (<75m) in a 250 mL flask. Twenty mL of deionized water
is used to flush any sample adhering to the flask walls
and the bottom. The flask covered by a watch-glass isheated to 80–90°C on a hot plate for 2 h and then cooledat room temperature for 1 h. A back-titration using theprescribed quantity and molarity of NaOH (see Smartet al. 2002, Weber et al. 2004a) is then made. This is
stopped at pH 4.5 to add 2 drops of 30% H2O2 to oxidize
the remaining Fe2+ in solution. The titration is continued
to pH 7.0, and the volume of NaOH recorded. The pHof the solution was measured with a Meterlab PHM201
pH – T meter calibrated with standard buffers at pH 4.0
and 7.0. Blanks were also run to quality-check results
and to improve the ANC calculation process (Weberet al. 2005).
Previously, it has been shown that reactive sulfides
and acidic sulfate salts can generate acidity in the ANC
test (Weber et al. 2004b), which needs to be addressed
to determine the actual ANC (ANCActual) that is notbiased by acid formation masking ANC. This sulfide
and sulfate acidity (SAmax) was determined by assessing
the Modified Sobek ANC digestion liquor for dissolved
S, which can be used to determine the ANCActual of thesample. Further details of this method are provided
elsewhere (Weber et al. 2004b, 2005).
Net Acid-Generation (NAG) test
The NAG test involves the addition of 250 mL 15
vol.% (unstabilized) H2O2 to 2.5 g of pulverized sample(<75 m), to encourage the rapid oxidation of reactive
sulfides. Unstabilized H2O2 with a pH of 4.5 is usedrather than stabilized H2O2 with a pH of 3.5 to preventthe sample being classified as acid-generating, when
it may simply be a function of the pH 3.5 stabilized
H2O2. After digestion, the final pH is recorded (NAG
pH), and the sample is back-titrated with either 0.1 M
or 0.5 M NaOH first to pH 4.5 and then to pH 7.0, andthe net acid-generation (NAG) capacity is calculated.
The sample is back-titrated initially to pH 4.5, as thisis the starting pH of the solution. Titration to pH 7.0
accounts for acidity derived from metals such as Cu
and Zn (Smart et al. 2002). To obtain information on
the acid-generation kinetics of a sample, the pH andtemperature of the NAG liquor are monitored during thesingle-addition NAG test to produce a pH–temperature
profile for the duration of the test. This is known as the
kinetic NAG test (Fig. 1).
To determine the acid-production potential insamples that experience H2O2 decomposition prior to
total oxidation of sulfide minerals requires a sequential
NAG test, which involves repeated additions of H2O2
to the sample residue (after removal of the previous
NAG solution by filtration) to determine the true acidgenerating potential of the sample. The NAG liquor isback-titrated to determine the net generation of acidfor that stage. The test is usually repeated until there
is no further reaction, and the pH remains above 4.5.
Further details of the NAG test and sequential NAG
tests are provided in the ARD Test Handbook (Smartet al. 2002).
NAG acid-base account (ABA) calculation
The acid-neutralization capacity of a sample
subjected to the NAG test (ANCNAG) can be determinedby calculation (Equation 6) and includes ions derived
from both carbonates and silicates:
ANCNAG = [(Na + K) + (Mg + Ca)  2]
 49/wt (in kg H2SO4/t)(6)

where Na, K, Mg, and Ca are the mmoles of eachcation released in the test, 49 is the conversion factor toconvert to kg H2SO4/t, and wt is the weight of sample
used in the test (2.5 g). The NAG acid–base account(ABANAG) can therefore be determined by:
ABANAG = ([(S  2)] – [(Na + K)
+ (Mg + Ca)  2])  49/wt (in kg H2SO4/t) (7)and should be comparable to the NAG value determined by back-titration to pH 4.5, the starting pH of
the initial NAG solution. This calculation assumes
that there are no significant sulfide minerals other thanpyrite, that no undetermined non-acid-forming sulfate
salts or non-acid-neutralizing cations are present, andthat there is no significant precipitation of secondary
minerals (excluding Fe and Al) during the test. Where
the ANC NAG is greater than Acidity NAG, negative
ABA NAG results may occur. This typically occurs
where the final NAG pH is greater than, or close to,the starting pH of the H2O2 solution (pH 4.5), andindicates that neutralizing reactions have dominatedover acid-generating reactions in the NAG test. In thisinstance, the neutralization capacity in the NAG test

H2O2 OXIDATION OF CARBONATE-POOR PYRITIC MINE-ROCKcan be determined by titration with HCl down to pH
4.5 to determine a negative NAG value that matches
the ABANAG value. Further details and examples areprovided by Weber (2003).

RESULTS
Sample PAF–H
Sample PAF–H is derived from the PT Freeport Grasberg mine, Papua Province, Indonesia. It is described asa potentially acid-forming sample with a high capacity
to generate ARD and low acid-neutralization capacity.
A classification system for this site has been previously
described (Neale et al. 2003). For sample PAF–H, the
ABANAG calculation provided a good indication of the
NAG value determined by titration (Table 2). AcidityNAG is invariably greater than the NAG value, whichis expected if neutralization reactions are occurring. In
this instance, the neutralization is essentially derived
from silicate minerals, as only 1 kg H2SO4/t ANC is due
to carbonates (e.g., ANCcarb; Table 1). The majority ofthis neutralization is derived from Na- and K-bearing
silicates (e.g., ANCNAG(Na+K)). After an initial surge inneutralization during Stage 1, which is predominantly
derived from Na and K minerals, the ANCNAG remains
stable at an average of 8.4 kg H2SO4/t for stages 2–4. In
Figure 2A, we summarize the individual AcidityNAG and
ANCNAG released during the PAF–H sequential NAG
test as compared to final NAG pH. Acid-generating
reactions dominated over acid-neutralization reactions
during the first three stages, as indicated by the low
pH and the AcidityNAG being greater than ANCNAG(Fig. 2A). Most acidity is released during the first twostages. By Stage 4, neutralization reactions (ANCNAG)
dominated over acid generation, as shown, and the
pH remains at 4.5. Results indicate that neutralization(ANCNAG) derived over four sequential NAG stages
was 36.7 kg H2SO4/t, which is significantly greater thanthe ANCActual of 11.1 kg H2SO4/t.
Sample PAF–L
Sample PAF–L is also derived from the PT Freeport Grasberg mine, Papua Province, Indonesia. It is
described as a potentially acid-forming sample with alow capacity to generate ARD and low acid-neutralization capacity (Neale et al. 2003). ABANAG provided
a good indication of the NAG value determined forsample PAF–L (Table 3). Again, AcidityNAG is greaterthan the NAG value, indicating significant silicateinduced neutralization reactions are occurring during
the test, as only 1 kg H2SO4/t ANC is derived fromcarbonates (Table 1). Figure 2B summarizes the individual AcidityNAG and ANCNAG released during thesequential NAG test as compared to final NAG pH.
Acid-generation reactions dominated over acid-neutralization reactions during the first two stages, although

1197

ANCNAG is also significant in proportion to the total
acid load. For Stage 4, the final NAG pH was 7.4(Table 3), which indicates that neutralization reactions
dominate; no analysis of the solution was undertaken
for this stage. During Stage 3, ANCNAG is greater than
AcidityNAG, which indicates that although acidity has
been generated (as evident by the pH drop from 4.5
to 3.2), the ABANAG calculation does not indicate that
acid-generating reactions are dominating over acidneutralization reactions. This could either be related
to precipitation of sulfate compounds such as jarosite,or non-neutralizing cations that have been released
from soluble salts such as gypsum. This will affect thechemical calculation process by either overestimating
the acid-generation or the acid-neutralization reactions.
During Stage 2, the difference between AcidityNAG and
ANCNAG is greatest, and this is reflected by the lowest
pH recorded. Results indicate that the ANCNAG derived
over three stages is 28.9 kg H2SO4/t, which is greaterthan the ANCActual of 21.3 kg H2SO4/t.
Sample C
Sample C is derived from the Newcrest Miningʼs
Cadia mine in Australia. It is a rock-chip sample derived
from blast hole drilling and is classified as potentially
acid-forming, with a low capacity to generate acid andwith a moderate ANC. Sequential NAG testing indicatedthat strongly acid conditions developed during stages 2
and 3 (Table 4) after exhaustion of the readily available
ANC. This was determined to be 11 kg H2SO4/t, asmeasured in the Modified Sobek ANC test. Analysis of
the solution data shows that the initial ANC surge inthe sequential NAG test is derived from approximately
equal proportions of ANCNAG(Mg+Ca) and ANCNAG(Na+K).
By Stage 2, the ANCNAG(Mg+Ca) is only 30% of the initialvalue, and ANCNAG(Na+K) is only 10%. Although the
ANCNAG(Na+K) is low, its contribution to ANCNAG is
significant. Figure 2C shows that AcidityNAG dominates
over acid-neutralization reactions during the first four
NAG stages, which is reflected in final NAG pH values
being less than 4.5. The lowest values of pH occur at

1198

THE CANADIAN MINERALOGISTStage 2, when the difference between AcidityNAG and
ANCNAG is the greatest for any stage. For the final
three stages, ANCNAG dominates over AcidityNAG,
which is reflected in gradually increasing values ofpH. By Stage 7, ANCNAG dominates over AcidityNAG,
and the final NAG pH remains at 4.5. In this instance,
the total ANCNAG (13.0 kg H2SO4/t) is similar to the
ANC determined by the modified Sobek ANC test, 11kg H2SO4/t.

Sample U
Sample U is from the Placer Pacific Osbourne
mine. It is a composite tailings sample characterized as
potentially acid-forming, with a moderate ANC and alag period prior to acid rock-drainage. Acid generation
dominates over acid-neutralization reactions during thefirst two stages of the sequential NAG test, as evidenced
by the low pH. However, by Stage 3, neutralization
reactions dominated over acid-generating reactions,and the pH is greater than the starting pH, 4.5 (Table 5).

FIG. 2. Comparison of ANCNAG, AcidityNAG and final NAG pH for six waste-rock samples. A. Sample PAF–H. B. Sample
PAF–L. C. Sample C. D. Sample U. E. Sample T. F. Sample PF.

H2O2 OXIDATION OF CARBONATE-POOR PYRITIC MINE-ROCK

This trend continues in Stage 4, with an even higher
final NAG pH value and a greater disparity between
AcidityNAG and ANCNAG. On average, the ANCNAG is
9.2 kg H2SO4/t after Stage 1, The total ANCNAG overfour sequential NAG stages for this sample is 44.4 kg
H2SO4/t, which is half of the total AcidityNAG (Table 5).
At all stages, the AcidityNAG was greater than the NAG
value, suggesting significant silicate-induced neutralization of acid as the carbonate ANC (ANCcarb) was only
3 kg H2SO4/t. AcidityNAG and ANCNAG released during
the sequential NAG test as compared to final NAG pH
are shown in Figure 2D. The trend of increasing final
NAG pH subsequent to sulfide exhaustion in stages 2–3
is clearly visible.
Sample T
Sample T is from the Newcrest Telfer mine inWestern Australia. It is a rock-chip sample consideredpotentially acid-forming, with a short lag prior to acidrock-drainage. Sequential NAG geochemical data
for this sample are shown in Table 6. Most ANC isderived from Stage 1 of the sequential NAG test (15kg H2SO4/t), and this is predominantly derived from

1199

ANCNAG(Na+K), which indicates acid neutralization
by Na- and K-bearing silicate minerals. AcidityNAG
is greater than the final NAG value, suggesting thatsignificant neutralization by silicate occurred in Stage
1 as the carbonate ANC (ANCcarb) is only 2 kg H2SO4/t (Table 1). Predominantly, this acid-neutralization
capacity of silicates is related to potassium release at
Stage 1. Figure 2E summarizes the AcidityNAG and
ANCNAG results as compared to final NAG pH values.
ANCNAG results indicate that there is significant neutralization occurring during Stage 1, and this is almost
equal to the acidity generated in that stage. However,following its exhaustion, the ANCNAG in subsequent
sequential NAG steps is much lower. With the removalof this inherent ANC and continued formation ofacid, Stage 2 generates the lowest pH values recorded
during the sequential NAG test, which agrees with thegreatest disparity between AcidityNAG and ANCNAG. In
subsequent stages, ANCNAG reactions dominated overAcidityNAG, which is reflected in the final NAG pH
being greater than pH 4.5.
Sample PF
Sample PF is from the Rio Tinto Kaltim Primacoal mine, Kalimantan, Indonesia. It is a potentially

1200

THE CANADIAN MINERALOGIST

FIG. 3. ABANAG and AcidityNAG versus NAG (titration) in kg H2SO4/t. A. Total ABANAG and AcidityNAG versus total NAG forsix samples. Total NAG for sample PF corrected to 97 kg H2SO4/t. B. ABANAG and AcidityNAG versus NAG for individual
sequential NAG stages for six samples. NAG for sample PF (Stage 3) is not included in the correlation, but is shown toindicate significant variance due to organic acids.

acid-forming sample with a high capacity to generateacid rock-drainage. It has been described previously indetail (Weber et al. 2004b) and contains a significant
amount of organic matter. In sample PF, the majorityof the acid-neutralization capacity calculated from the
NAG test solution data is derived from silicate minerals.
Assuming that all Mg and Ca was derived from carbonates in this sample, one could incorrectly infer that
4.9 kg H2SO4/t of carbonate ANC was present (Stage 1).
However, only 0.4 kg H2SO4/t carbonate ANC is available (Table 1). The Mg and Ca are most likely derived
from clays, such as smectite-group minerals, present in
the sample. The AcidityNAG calculated from solution
analyses is in reasonable agreement with the NAG as
determined by back-titration. This is with the exceptionof Stage 3, where the NAG value for Stage 3 was muchhigher than expected owing to the release of organic
acids. Organic acids have previously been discussed
by Stewart et al. (2003a) for similar samples. This
stage was ignored to determine a correlation between
ABANAG and NAG (ABANAG = 1.19  NAG; R2 =0.98). The correlation between AcidityNAG and NAG
was determined to be AcidityNAG = 1.3 NAG (R2 =0.99). The Stage-3 NAG measured by back-titration
can be corrected using this correlation for NAG versus
AcidityNAG, which therefore indicated a NAG value of
7 kg H2SO4/t. Thus the total NAG value can be recalculated using a Stage-3 NAG of 7 kg H2SO4/t to give a
total NAG of 97 kg H2SO4/t, which is in agreement with
the ABANAG (corrected stage-3 NAG values are given
in brackets in Table 7). In Figure 2F, we summarize theindividual AcidityNAG and ANCNAG released during the
PF Sequential NAG test as compared to final NAG pH.

Results indicate that with depletion of pyrite within
the sample during the sequential NAG stages, there isa corresponding increase in final NAG pH. AlthoughStage 5, the final stage of the sequential testing, did
not reach pH 4.5, the general trend in pH increased asacidity was exhausted (Fig. 2F).

DISCUSSION
Neutralization by carbonate versus silicate
Stewart et al. (2003a) suggested that the neutralization of acidity occurring in the NAG test can be
determined by the molar concentration of Mg and CaH2O2 OXIDATION OF CARBONATE-POOR PYRITIC MINE-ROCKmeasured in the NAG solution (Equation 2). Furthermore, using the S release as a measure of acid generation, the calculated NAG value (Equation 3) could thenbe determined based on S, Mg, and Ca (Stewart et al.
2003). This is a suitable approach for rock samples withhigh carbonate content, such that this carbonate neutralization-source dominates. However, for carbonate-poor
samples, such as those assessed in this work, thisapproach is unsuitable, as significant neutralization
is derived from Na- and K-bearing silicate minerals.
For samples PAF–L, C, U, and PF, a better match to
the NAG associated with each sequential NAG stage
occurred if Na and K ions released to solution are
considered to be derived from neutralization processes.
For samples PAF–H and T, the addition of these ions
(Na and K) did not provide a convincing argument thatthey were important sources of acid neutralization.
There is no correlation between the carbonate ANC(ANCcarb) and ANCNAG(Mg+Ca) measured at Stage 1,even though final NAG pH ranged from 2.0 to 4.0,
which is sufficient for rapid dissolution of carbonate.
The assumption that Ca and Mg measured during the
NAG test (and subsequent sequential stages) are derived
entirely from carbonates is thus incorrect.
NAG acid–base accounting
In Figure 3A, we compare the total calculated
ABANAG and the AcidityNAG to the total NAG determined by back-titration to pH 4.5 for the samples
investigated (e.g., the sum of all sequential stages).
In the values calculated for ABANAG, we consider thenegative values (as determined by ABANAG calculation)
to be zero, as the NAG does not address the neutralization capacity if the final NAG pH is greater than 4.5.
The trend has the following correlation: ABANAG =1.004  NAG. For comparison, the 1:1 line is directly
over the ABANAG trend line, which represents the true
ABANAG : NAG ratio if for every mole of S released
to solution, there are two hydrogen ions neutralized bycations such as Na, K, Ca, and Mg. Total AcidityNAG
also is plotted to show that it is invariably greater thanthe NAG value, which indicates that acid neutralization
is occurring during the test.

1201

Figure 3b shows the correlation between ABANAG
and NAG values (as determined by titration) for eachsequential NAG stage (a total of 27 stages). Again, theresults show that AcidityNAG generally overestimates
the NAG value, and that the neutralization reactions
occurring during the test need to be addressed to provide
an accurate acid–base account that is comparable to
the NAG value determined by back-titration. Figure 4
is a close-up of the ABANAG versus NAG diagram foreach sequential stage (Fig. 3B) for values below 20 kg
H2SO4/t. Of particular interest is the sudden cessationof the ABANAG : NAG trend below the NAG value 0 kg
H2SO4/t. This occurred wherever the final NAG pH isgreater than or close to pH 4.5. The calculated ANCNAG
thus was in excess of any acidity (AcidityNAG) generated during the test. This could have been determinedby back-titrating the NAG solution with HCl down to
the starting pH of 4.5 to produce a negative ABANAG
value.
To determine the suitability of the derived ABANAG
value (Equation 7), which addresses the neutralization derived from Na, K, Mg, and Ca versus the CNV
value (Equation 3), which only addresses Mg and Ca
neutralization, the ABANAG and the CNV are plotted in
Figure 5A (total NAG values) and Figure 5B (individual
sequential NAG values). Results conclusively demonstrate that ABANAG provides a better indication of the
NAG (as determined by back-titration) than the CNV(in which the neutralizing effects of Na- and K-bearing
minerals are not considered) for these samples.
The ANCNAG values for Stage 1 and the average
ANCNAG values for Stage 2 through to the final sequential NAG Stage are shown in Table 8 for all samples
tested. Results conclusively demonstrate that subse-

FIG. 4. Close-up of the ABANAG values shown on Figure 3B.
This plot illustrates the effects of pH values that are greaterthan 4.5 on the ABANAG values. Results suggest that thetrend line should continue into negative numbers.

1202

THE CANADIAN MINERALOGIST

FIG. 5. ABANAG and CNV (calculated NAG value) versus NAG (as determined by titration). Values in kg H2SO4/t. A. Total
NAG (Total NAG for sample PF corrected to 98 kg H2SO4/t). B. Individual sequential NAG stages (NAG for sample PF notincluded in correlation owing to the presence of organic acids). Negative ABANAG values are assumed to be zero.

quent NAGANC values are lower than the initial surge
occurring in Stage 1 (carbonate dissolution and reactive
surface-sites and rapid cation–proton exchange on thesilicates), yet are still significant. In the case of thesilicates, the initial rapid-dissolution surge and cation
release, and the subsequent decrease in dissolution
rates with time, which generates a parabolic dissolution-trend, have been identified by many researchersinvestigating the dissolution kinetics of these minerals
in batch and flow-through kinetic tests (e.g., Busenberg& Clemency 1976, Chou & Wollast 1984, Amrhein &Suarez 1992, Suarez & Wood 1996). Further testing isrequired to determine if ANCNAG (Stage 1) is comparable to the ANC derived from the Modified Sobek
ANC test for potentially acid-forming samples with alow content of carbonate.

CONCLUSIONS
A satisfactory method was developed to determine the net acid-generation and net acid-neutralization occurring during the H2O2 NAG test based on
calculations using the molar quantity of neutralizing
cations and acid-forming S released to solution. This
provided an overall acid–base balance for the test,
which provided an explanation for NAG pH trends.
However, this method is unsuitable for samples that
contain significant salts and significant quantities of
non-acid-forming sulfide minerals.
Furthermore, results demonstrate that subsequent to
H2O2 oxidation of sulfidic samples, the ANC derived
from silicate minerals can be greater and more significant than carbonate ANC in carbonate-poor samples(e.g., ANCcarb < 5 kg H2SO4/t). In some instances, theANC derived from Na- and K-bearing silicate minerals
is more important than neutralization derived fromMg- and Ca-bearing minerals (carbonate and silicate).
We do not imply that the ANC measured by this test isapplicable to kinetic tests such as the column leach testor the field situation. Another approach is still required
to assess silicate ANC under those conditions.

ACKNOWLEDGEMENTS
Thanks to the AMIRA P387A sponsor companies,
which included Newcrest Australia Ltd, Northern
Territory Department of Mines and Energy, Placer
Pacific Ltd, PT Freeport Indonesia Ltd, and Rio Tinto
Ltd. Thanks to our research partner Environmental
Geochemistry international (EGi) for undertaking theModified Sobek ANC tests and the sequential NAG tests
on the AMIRA P387A waste-rock samples. Support wasprovided for PW by an Ian Wark Research InstitutePostgraduate Scholarship, and for WS, JT, and RS by anAustralian Research Council SPIRT grant in association
with Environmental Geochemistry international, Levay
and Co., and AMIRA International Ltd. Thanks to
Dogan Paktunc for early comments and in his capacity
as Associate Editor, two anonymous reviewers, andRobert F. Martin for constructive comments.

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Received November 26, 2003, revised manuscript accepted
May 1, 2005.