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

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 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 (or, if net-neutralizing, the rate of alkalinity release), to identify the lag time before net-acidic drainage begins as fast-reacting carbonate neutralization potential is 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: 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 – a defensible result takes many months to a year of continuous lab operation – and the small (kg-scale), crushed sample may not fully represent full-scale particle-size and oxygen-diffusion effects in an actual waste rock pile.

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 acid generating	$NPR < 1$	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 (humidity cell, 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 NAG cover/encapsulation stock (Question 3b) or placed without special acid-management handling, subject to confirmation on a representative subset by kinetic testing.

Check

This exam's own printed NPR boundary between "possible/uncertain" and "non-acid generating" is 2. Some agencies (e.g. Price, MEND 1.20.1) instead use a wider uncertain band up to $NPR = 3$–$4$ before calling a sample confidently non-acid-generating – this answer uses the paper's own stated criteria, as instructed.

Worked example. A waste rock sample assays 0.80% sulphide sulphur, giving Acid Potential $AP = 31.25 \times \%S = 31.25 \times 0.80 = 25.0$ 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 $NP = 70.0$ kg CaCO₃-eq/t, then $NPR = NP/AP = 70.0/25.0 = \boxed{2.8}$, which under this exam's own criteria classifies the sample as non-acid generating.

Topic. Static (ABA) versus kinetic (humidity cell) 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 answers the much more expensive but decision-critical question "how fast, and starting when?" – a mine plan needs both, using ABA to triage thousands of samples and reserving kinetic testing for the uncertain/possible 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

b) Sulphide oxidation process

c) Acid-base accounting: NPR screening table