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MDAG.com Internet Case Study 25

A Case Study of Important Aluminosilicate Neutralization

by K.A. Morin and N.M. Hutt

© 2007&2008 Kevin A. Morin and Nora M. Hutt

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Abstract

            At a molybdenum-related site with more than 2 km of underground workings and 60,000 tonnes of waste rock, acid-base accounts say the drainage waters should be acidic, but several decades of monitoring shows no ARD. We looked more closely at the Neutralization Potential (NP), particularly the aluminosilicate-based (“silicate”) NP, based on (1) the mathematical conversion of aluminosilicate minerals to NP, (2) the dissolution-precipitation of minerals to explain the water chemistry at the portal and in humidity cells, and (3) the dissolution (neutralization) rates of aluminosilicate minerals based on measured pH values.

            Silicate NP values were calculated from solid-phase levels of aluminosilicate minerals in the five humidity-cell samples. As a result, all except the already acidic metasedimentary Cell 1 sample would be net acid neutralizing, as long as a substantial portion of this silicate NP successfully neutralized pH to above 6. However, this was not true for the Cell 2, because it was already acidic, so most of its silicate NP derived from biotite, magnetite, and epidote cannot neutralize fully. The remaining three cell samples were still near neutral (as of Week 105), contained minor carbonate, and derived most of their silicate NPs from plagioclase. Their mineralogy-based Sulphide Net Potential Ratios indicate they will not become acidic, if much of their silicate NP can neutralize fully.

            The U.S. Geological Survey SpreadBal-2002 software estimated the amounts of silicate minerals dissolving and precipitating to explain water chemistries seen at the portal and in recent weeks of the five humidity cells. Epidote, plagioclase, calcite, and biotite were often calculated as major dissolving components to create the observed water chemistries, with substantial precipitation of quartz, ferric oxides, and aluminum oxides also needed to explain the chemistries. Subsequent conversions to neutralization potentials and alkalinities showed that silicate neutralization played a major role in the partial (below pH 6) to full neutralization in these humidity cells. This work also showed that calcite was not needed to explain the water chemistry, with atmospheric CO2 potentially supplying the carbon for alkalinity. However, slightly better agreement was obtained when calcite was included.

            Literature-derived, pH-dependent rates of neutralization by aluminosilicate minerals in the near-neutral humidity cells were similar to, or substantially greater than, the measured neutralization rates in the three near-neutral cells. The substantially larger measured rates apparently reflect (1) the additional contribution of calcite and/or (2) micro-scale acidic conditions (pH < 6) around the aluminosilicate mineral grains which would cause them to react faster. However, such a small-scale pH cannot be estimated from available information.

1. Introduction

            At some minesites, standard interpretations of acid-base accounting (ABA) can predict widespread ARD when none has been observed for decades (Morin et al., 2001; Eary and Williamson, 2006). This contradiction is especially seen at some molybdenum-related sites.

            The contradiction can be traced to two factors associated with the surrounding rock, which is usually similar to granodiorite. First, sulphide minerals at some minesites react relatively slowly, producing acidity at a low rate. Second, slow-neutralizing aluminosilicate minerals, which are not detected in the hours-long Neutralization Potential (NP) test, can successfully neutralize the low acidity rate.

            For one site with more than 2 km of underground workings and approximately 60,000 t of waste rock (not discussed by Morin et al., 2001), we looked more closely at three aspects of rock’s capacity to neutralize acidity:

1) The amount of NP represented by the aluminosilicate minerals in the rock, which cannot be detected in the standard, short, hours-long acid bath (Section 2). In this study, this is called “silicate NP”, and will provide a better estimate of NP than ABA for this site.

2) The amounts of aluminosilicate minerals and carbonate minerals that must dissolve or precipitate to explain the water chemistry from the humidity cells and the underground drainage (Section 3).

3) The rate at which the aluminosilicate minerals dissolve and neutralize aqueous acidity and pH (Section 4), which in this study is labelled “rate of silicate neutralization”. This may indicate to what rate the sulphide oxidation would have to increase before the silicate minerals could no longer keep pH above 6.

            Ideally, the silicate NP and the rate of silicate neutralization should come from site-specific testwork. For this site, the testwork included expanded acid-base accounts, total-element contents, mineralogy including types and amounts of silicate minerals, and long-term kinetic tests for bulk reaction rates including small-scale laboratory tests and large-scale monitoring of the existing minesite components.

2. Calculation of Silicate, Slow-Reacting, and Effective Neutralization Potential

            As part of acid-base accounting (ABA), a small amount of pulverized sample is placed in an acid bath and heated until the reaction (visible bubbling, usually created by fast-reacting carbonate minerals) ceases. A subsequent titration with hydroxide then shows how much of the original acid had been neutralized by minerals in the sample. This is known as bulk or “Sobek” Neutralization Potential (NP; Sobek et al., 1978). It can also be considered fast-reacting “short-term NP”, often representing calcium-magnesium-bearing carbonate minerals.

            It is important to note that, despite the abundance of aluminosilicate minerals, fast-reacting carbonate minerals can still be found in rock types like granodiorite. White et al. (2005) frequently identified calcite, in granitoid rocks around the world, at levels of 0.028 to 18.8 kg/tonne, with a mean of 2.52 kg/t. Jambor et al. (2006) also detected carbonate minerals in rocks ranging from granitic to ultramafic.

            Bulk NP often represents the actual amount of potential neutralization to a pH greater than 6.0, which is important for in situ control of ARD. However, theoretical cases (Eary and Williamson, 2006) and actual cases (Morin et al., 2001) show measured short-term NP can substantially underestimate total neutralizing capacity (called “Effective NP” here to distinguish it from measured NP). This is due to the presence of aluminosilicate minerals that can neutralize water to pH above 6.0 under certain conditions.

            Morin and Hutt (1997 and 2001) provided the following equation for Effective NP:

            Effective NP = measured (short-term Sobek) NP + Slow-Reacting NP - Unavailable NP

This equation recognized that additional, unmeasured NP (SRNP) is not detected by the NP procedure, and that some measured NP (UNP) is “unavailable” for neutralization. SRNP in rock is the focus of this case study, with SRNP assumed to be the same as silicate NP. UNP, often around 5-15 kg/t, has been discussed elsewhere (Morin and Hutt, 1997 and 2001).

            Because the additional silicate NP (SRNP) provided by aluminosilicate minerals cannot be detected using standard ABA procedures, the NP represented by these minerals can be calculated. This is not a simple task, because several assumptions must be made about the minerals and about the water into which they are dissolving (Morin and Hutt, 2006). Nevertheless, a general estimate of silicate NP and SRNP can be obtained by setting the final neutralized pH to near-neutral values (around 7-8) so that simplistically all iron precipitates as Fe(OH)₃, all aluminum precipitates as Al(OH)₃, and all silicon forms aqueous H₄SiO₄⁰.

            Detailed mineralogy of rock in this study is available from five samples tested in humidity cells (Table 1, and Figures 1 to 3) and from hundreds of related historical thin sections. The five humidity-cell samples contained mixtures of minerals that can partially or fully neutralize acidity, including:

- carbonates mostly as calcite (calcium carbonate) detected at levels below 1% in the three cell samples that are currently near neutral (Cells 3, 4, and 5),

- plagioclase containing more sodium than calcium in the granodiorite,

- potassium feldspar (K-feldspar),

- muscovite (mostly in metasedimentary Cell 1 and lesser amounts in the granodiorite) and biotite (mostly in the volcanics), which in their ideal forms release potassium,

- biotite (mostly in the volcanics) and chlorite and clinochlore (both at low levels in four cell samples), which in their ideal form release magnesium, and

- clinozoisite and epidote (seen visually in both volcanics cell samples and one granodiorite cell sample), which in their ideal forms release calcium.

Figure 1. Weekly Effluent pH from the Five Humidity Cells.

Figure 2. Weekly Sulphate-Production Rate from the Five Humidity Cells.

Figure 3. Temporal Trends of Weekly Mole Ratios of Calcium, Magnesium, Sodium, and Potassium from the Five Humidity Cells.

            Based on more than two years of humidity-cell testing (Figures 1 to 3), aqueous calcium concentrations dominated over those of magnesium, sodium, and potassium in most humidity cells and in drainage from the full-scale underground portal. The major exception was acidic Cell 2, with elevated potassium. Thus, calcite, plagioclase, and clinozoisite-epidote appear to be the most important minerals.  However, ion exchange and secondary-mineral precipitation (like smectites) can remove potassium, sodium, and/or magnesium from solution and thus mask their contributions to neutralization.

            For the five humidity-cell samples (Table 1), petrographic-based values were considered more representative for most minerals, and any minerals at levels considered “trace” were ignored (Table 1). Also,

- all illite, sericite, and muscovite were combined as muscovite;

- gypsum, actinolite, and rutile were ignored as sources of neutralization;

- all sulphides were combined as pyrite although they might not all be acid generating;

- due to the significant discrepancies between petrographics and XRD for K-feldspar and plagioclase, the XRD proportions of K-feldspar and plagioclase which appear more reasonable for a granodiorite were applied to the petrographics values (e.g., for Cell 2, its petrographics-based 5% K-feldspar was adjusted to 0.5% K-feldspar and 4.5% plagioclase);

- plagioclase was assigned an Anorthite ratio of An20 (20% calcium and 80% sodium), which has slightly less neutralizing capacity than the rough average of An30 reported in the historical thin sections, and

- weight-percent was assumed equal to volume-percent, which may underestimate sulphide levels but is offset by assuming all sulphides were acid-generating pyrite.

            This resulted in mineralogy-based NP values of 54 to 230 kg CaCO₃ equivalent/ tonne based on mineralogy (Table 2), compared to measured Sobek NP values of 5-15 kg/t. Silicate NP represented most to all of the mineralogy-based NP values. The samples with the highest and lowest NPs were already acidic, so their mineralogy-based NPs derived mostly from muscovite and biotite were not applicable. The remaining three near-neutral cells drew most of their mineralogy-based NP from plagioclase.

            For the Cell 1 metasedimentary sample, its 40% illite-sericite (considered as muscovite) provided most of the silicate NP of 54 kg/t, with biotite supplying the remainder. This failed to raise its SNPR values above 1.0 and thus it is still considered net acid generating. More important, it became acidic quickly (Figure 5-1), so the silicate minerals provided virtually no NP to maintain near-neutral conditions.

            For the high-NP volcanics sample (Cell 2), roughly half its silicate NP of 230 kg/t was derived from biotite, with most of the remainder from epidote and magnetite. This raised its mineralogy-based SNPR above 2.0. Thus, it would be net acid neutralizing using standard SNPR criteria, but it became acidic relatively quickly during testing (Figure 1), so its actual effective NP was very low. Nevertheless, unlike the other four cells, weekly effluents show that potassium has been dominant, with calcium not far less on a mg/L basis. The elevated potassium would be consistent with dissolution of biotite and K-feldspar, but much lower magnesium would rule out biotite as the primary source of potassium. This is discussed further in Section 3 of this case study.