Acid Rock Drainage – Characterization and Formation: 24-MMP-A6 Mining and the Environment, December 2014, Q2

Question 2: Acid Rock Drainage – Characterization and Formation (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Humidity cell test and column test (both kinetic methods described)

The humidity cell test is a kinetic (as opposed to static) geochemical characterization method: a crushed, sieved sample (typically 1–2 kg) is loaded into a laboratory column and subjected to a repeating weekly cycle – commonly three days of dry air, three days of humidified air, then a flush with a fixed volume of deionized water on the seventh day – run continuously for 20 weeks or longer (often 40–52+ weeks for a definitive result). Each week's leachate is analyzed for pH, conductivity, acidity/alkalinity, sulphate and dissolved metals, producing a time series rather than the single static number an acid-base accounting (ABA) test gives. The results are used to determine the RATE of sulphide oxidation and acid generation, to identify the lag time before net-acidic drainage begins as fast-reacting neutralizing minerals are consumed, and to track which metals mobilize first as pH drops – all inputs that size the closure cover, water-treatment plant and monitoring program. Its main advantage is that it captures behaviour ABA cannot, since two samples with an identical NPR can have very different lag times and peak loading rates depending on mineral reactivity and grain size; its main limitation is time and cost, since a defensible result takes many months to a year of continuous lab operation, and the small, crushed sample may not fully represent full-scale particle-size and oxygen-diffusion effects in an actual waste rock pile.

The column test is the field-scale complement: a larger, less-crushed (often run-of-mine or coarse-crushed) sample is packed into a much larger column or barrel, typically metres rather than centimetres tall, and leached by percolating water downward through the packed bed – either continuously or on a periodic irrigation schedule – to simulate infiltration through an actual waste rock pile or heap rather than the tightly cycled wet/dry regime of a humidity cell. Effluent collected at the base is analyzed on the same schedule (pH, acidity, sulphate, metals) over a comparable multi-month-to-year duration. Because the sample retains a more realistic particle-size distribution and pore structure, a column test better represents field-scale oxygen diffusion, preferential flow paths and the physical armouring effects that can slow oxidation in a real pile, which is its main advantage over the humidity cell for predicting full-scale pile behaviour. Its limitations are a larger footprint and sample volume requirement, a still-imperfect scale-up to a pile that may be tens of metres high, and (like the humidity cell) a long test duration before a defensible rate is obtained.

b) Sulphide oxidation process

Acid rock drainage from pyrite (FeS₂) oxidation proceeds through three coupled reactions. The process begins with direct oxidation of pyrite by dissolved oxygen, which releases ferrous iron, sulphate and acidity into solution:

\text{FeS}_2 + \tfrac{7}{2}\text{O}_2 + \text{H}_2\text{O} \;\longrightarrow\; \text{Fe}^{2+} + 2\,\text{SO}_4^{2-} + 2\text{H}^{+}\qquad\text{(Reaction 1)}

As pH falls, the second reaction – oxidation of ferrous iron to ferric iron – becomes the rate-limiting step; below about pH 4.5 it is strongly catalyzed by acidophilic bacteria such as Acidithiobacillus ferrooxidans, which can accelerate it by several orders of magnitude:

\text{Fe}^{2+} + \tfrac{1}{4}\text{O}_2 + \text{H}^{+} \;\longrightarrow\; \text{Fe}^{3+} + \tfrac{1}{2}\text{H}_2\text{O}\qquad\text{(Reaction 2)}

The ferric iron produced is itself a strong oxidant and attacks fresh pyrite directly, without needing dissolved oxygen – this third reaction is what makes the cycle self-sustaining (autocatalytic) once started, since it regenerates the Fe²⁺ consumed in Reaction 2 while releasing far more acidity per mole of pyrite than Reaction 1 alone:

\text{FeS}_2 + 14\text{Fe}^{3+} + 8\text{H}_2\text{O} \;\longrightarrow\; 15\text{Fe}^{2+} + 2\,\text{SO}_4^{2-} + 16\text{H}^{+}\qquad\text{(Reaction 3)}

The net effect of the coupled cycle is that a small initial charge of dissolved oxygen can drive a very large amount of continued pyrite dissolution through the Fe³⁺/Fe²⁺ shuttle, which is why ARD prevention strategies (Question 3) focus on excluding either oxygen or water – removing either reactant stalls the whole cycle even though Reaction 3 itself needs no free O₂.

c) Acid-base accounting: NPR screening table

NPR screening criteria (as printed on the exam paper)
Potential for ARD	Initial screening criteria	Interpretation
Likely	$NPR < 1$	Neutralization Potential is insufficient to offset the sample's Acid Potential; acid drainage is expected once the limited available NP is exhausted (or promptly, for a fast-reacting, low-NP sample) – treat as acid-generating for waste-management planning without further testing.
Possible/uncertain	$1 < NPR < 2$	NP and AP are close in magnitude; the static test alone cannot resolve long-term behaviour because it does not capture the relative REACTION RATES of acid-generating sulphides versus neutralizing carbonates. Kinetic testing (part a) is recommended before this material is classified as safe for unrestricted disposal.
Non-acid generating	$NPR > 2$	Neutralizing capacity comfortably exceeds acid-generating capacity under the static test; the material can generally be used as neutralizing cover/encapsulation stock (Question 3) or placed without special acid-management handling, subject to confirmation on a representative subset by kinetic testing.

Check

The paper prints NPR as "acid potential / neutralization potential", but that is inverted: the neutralization potential ratio is conventionally $NPR = NP/AP$, and only that definition makes its own screening bands consistent ($NPR<1$ = acid-generating means NP falls short of AP). This answer uses $NPR = NP/AP$ with the paper's printed breakpoints of 1 and 2, which match MEND 1.20.1 (Price, 2009); some older guidance (e.g. BC's 1998 Price & Errington guidelines) carried a wider uncertain band up to $NPR \approx 4$.

Worked example. A waste rock sample assays 1.60% sulphide sulphur, giving Acid Potential $AP = 31.25 \times \%S = 31.25 \times 1.60 = 50.0$ kg CaCO₃-eq/t (the standard Sobek/MEND stoichiometric factor, assuming full oxidation of sulphide-S to sulphuric acid). If its titrated Neutralization Potential is $NP = 60.0$ kg CaCO₃-eq/t, then $NPR = NP/AP = 60.0/50.0 = \boxed{1.2}$, which under this exam's own criteria classifies the sample as possible/uncertain – a candidate for kinetic testing before it is committed to a NAG cover or an unrestricted waste dump.

Topic. Static (ABA) versus kinetic (humidity cell/column) ARD characterization, and the pyrite-oxidation reaction chain that generates acidity.

Key relations. $AP = 31.25 \times \%S$; $NPR = NP/AP$; the coupled cycle FeS₂ $\xrightarrow{\text{O}_2}$ Fe²⁺ $\xrightarrow{\text{O}_2/\text{bacteria}}$ Fe³⁺ $\xrightarrow{\text{attacks FeS}_2}$ more Fe²⁺.

Why this works. ABA is a cheap, fast screening tool that answers "could this ever generate net acid?" from bulk chemistry alone; the humidity cell and column tests answer the much more expensive but decision-critical question "how fast, and starting when, at what scale?" – a mine plan needs both, using ABA to triage thousands of samples and reserving kinetic testing for the uncertain band and for representative samples that will inform closure design.

Common pitfall. Treating Reaction 1 (direct O₂ oxidation) as the only reaction that matters and concluding that excluding oxygen alone always stops ARD – Reaction 3 shows that once enough Fe³⁺ has accumulated, the cycle can continue attacking pyrite with little or no dissolved oxygen present, which is why a water cover (excludes O₂ diffusion far more effectively than a soil cover) is generally the most robust single prevention measure.

Source. Price, W.A., MEND Report 1.20.1 (1997/2009); INAP, Global Acid Rock Drainage (GARD) Guide.

Question 2 of 6: Acid Rock Drainage – Characterization and Formation

Question 2: Acid Rock Drainage – Characterization and Formation (20 marks)

a) Humidity cell test and column test (both kinetic methods described)

b) Sulphide oxidation process

c) Acid-base accounting: NPR screening table