Mining Waste and Management – Waste Rock: 24-MMP-A6 Mining and the Environment, May 2017, Q3

Question 3: Mining Waste and Management – Waste Rock (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. General waste rock knowledge (8 marks)

i. Definition (1 mark). Waste rock is the barren or sub-economic rock excavated to access and mine the ore body (overburden and interburden stripped from an open pit, or development rock from underground headings) that does not meet the cut-off grade for processing and is instead stockpiled or placed in an engineered waste rock storage facility.

ii. Two types of contaminant testing (2 marks). Static testing (acid-base accounting, ABA) measures a rock sample's Acid Potential and Neutralization Potential in a single laboratory analysis to give a snapshot classification (via the NPR ratio) of whether the rock is likely, possibly, or unlikely to generate acid – fast and inexpensive but blind to reaction RATE. Kinetic testing (humidity cell or column tests) instead subjects a sample to repeated wet/dry cycles over weeks to months, measuring how leachate chemistry (pH, sulphate, metals) actually evolves over time – slower and more costly, but the only way to determine reaction rate, lag time before acid onset, and realistic field-scale drainage chemistry.

iii. Top-down vs. bottom-up storage (2 marks). Bottom-up (end-dumping) construction builds the waste dump progressively upward from the base, with each new lift dumped on top of the previous one from the crest – simple and low-cost but tends to segregate coarse material to the toe and can trap air within the pile, promoting oxygen ingress and acid generation if sulphidic material is present. Top-down (paddock/free-dump-then-push) construction instead places and compacts material in horizontal lifts, working from the surface downward/outward in engineered layers, which allows better control of material placement (e.g. encapsulating PAG material under NAG cover as it is placed) and can reduce internal air/oxygen ingress compared to bottom-up dumping, at the cost of more handling and higher construction cost.

iv. A common sulfide mineral (1 mark). Pyrite (FeS₂) is the most common acid-generating sulphide mineral found in waste rock; pyrrhotite (Fe_(1-x)S) is another frequently-cited example, notable for oxidizing faster than pyrite.

v. AMD vs. ARD (1 mark). "Acid Mine Drainage" (AMD) refers specifically to acidic, metal-laden drainage originating from anthropogenic mining disturbance (waste rock, tailings, exposed pit walls, underground workings). "Acid Rock Drainage" (ARD) is the broader term covering the same oxidation chemistry occurring from ANY exposed sulphidic rock, including natural outcrops with no mining disturbance at all – AMD is therefore a subset of ARD specific to mining-caused exposure.

vi. Reaction type and electron flow (1 mark). Acid mine drainage is an oxidation-reduction (redox) reaction. Electrons flow from the sulphide sulphur and ferrous iron in pyrite (which are OXIDIZED, losing electrons, as S²⁻ is converted toward SO₄²⁻ and Fe²⁺ toward Fe³⁺) to dissolved oxygen (which is REDUCED, gaining electrons, forming H₂O) – a transfer that, once ferric iron accumulates, becomes bacterially catalyzed and self-sustaining (Part B below).

B. The four acid mine drainage reactions (5 marks)

The four AMD reaction steps and what each represents
Reaction	What is occurring
(1) $FeS_2 + O_2 + H_2O \leftrightarrow Fe^{2+} + SO_4^{2-} + 2H^+$	Primary pyrite oxidation. Dissolved oxygen directly oxidizes pyrite in the presence of water, releasing ferrous iron, sulphate, and hydrogen ions (acidity) into solution – this is the initiating step that begins acid generation whenever a sulphidic rock surface is freshly exposed to air and moisture.
(2) $Fe^{2+} + O_2 + H^+ \rightarrow Fe^{3+} + H_2O$	Ferrous-to-ferric oxidation. The ferrous iron produced in Reaction 1 is oxidized to ferric iron. This step is comparatively slow abiotically but is dramatically accelerated (by several orders of magnitude) by acidophilic iron-oxidizing bacteria such as Acidithiobacillus ferrooxidans once pH drops low enough for them to thrive – making it widely regarded as the rate-limiting step that governs how quickly the overall reaction chain accelerates.
(3) $Fe^{3+} + H_2O \leftrightarrow Fe(OH)_2^+ + 3H^+$	Ferric iron hydrolysis and precipitation. Ferric iron reacts with water to precipitate as an iron hydroxide/oxyhydroxide solid (the characteristic orange "yellow boy" staining seen at AMD-affected sites), releasing additional hydrogen ions (further acidity) as a by-product of the hydrolysis.
(4, balanced) $FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+$	Ferric iron attack on pyrite (the propagation/autocatalytic step). Ferric iron generated in Reactions 2-3 itself acts as an oxidant, attacking fresh pyrite surfaces directly (independent of dissolved oxygen availability) and regenerating more ferrous iron that can cycle back through Reaction 2 – this is what makes AMD self-sustaining and able to continue even where oxygen ingress is limited, as long as ferric iron and fresh pyrite surface remain available.

Reaction (4) as printed is not stoichiometrically balanced; balancing it requires 14 mol of ferric iron and 8 mol of water per mole of pyrite to produce 15 mol of ferrous iron, 2 mol of sulphate, and 16 mol of hydrogen ions: $$FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+$$

C, D, E. Effects, prevention/treatment, and northern-climate considerations (7 marks combined)

C. Negative environmental effects of AMD (2 marks).

Acute and chronic aquatic toxicity: low pH and elevated dissolved metal concentrations (iron, aluminum, copper, zinc, and others mobilized under acidic conditions) are directly toxic to fish and aquatic invertebrates, and can eliminate sensitive species from affected reaches entirely.
Long-term degradation of downstream water quality and habitat: iron oxyhydroxide precipitation ("yellow boy") coats stream substrate, smothering benthic habitat and spawning gravels, while the contamination can persist and require treatment for decades to centuries after the source rock is no longer actively being disturbed.

D. Three prevention/treatment methods, active or passive (3 marks).

Chemical neutralization (lime/limestone treatment plant) – active. Continuously doses collected drainage with a neutralizing reagent (hydrated lime, limestone) to raise pH and precipitate metals as a sludge; requires ongoing reagent supply, energy, operator attention, and sludge management for as long as drainage is generated.
Waste segregation and encapsulation (PAG material isolated under a low-permeability/oxygen-excluding cover, often combined with NAG blending) – passive (prevention). Prevents acid generation at the source by excluding oxygen and/or water from reactive material rather than treating drainage after the fact; once constructed it requires comparatively little ongoing energy input, though periodic cover integrity inspection is still needed.
Constructed wetlands / anoxic limestone drains – passive (treatment). Routes drainage through a engineered wetland or limestone-lined channel where biogeochemical processes (sulphate reduction, alkalinity generation, metal precipitation via plant/microbial action) neutralize and remove metals with minimal external energy or reagent input, at the cost of a larger land footprint and reduced performance in cold conditions (Part E).

E. Considerations for Canada's northern environment (2 marks).

Passive treatment systems (constructed wetlands, biological reactors) rely on biological and chemical reaction rates that slow substantially in cold temperatures and can effectively stop under winter ice cover, so a northern site often needs a larger system (to bank capacity for the low-performance season), supplemental heating, or a hybrid active/passive design to maintain year-round performance.
Remoteness and short shipping seasons increase the logistics cost and risk of maintaining an active treatment plant's ongoing reagent (lime) supply, favouring passive or semi-passive approaches where feasible; conversely, permafrost and freeze-thaw cycling can crack or degrade cover systems intended to exclude oxygen/water from encapsulated waste, requiring cover designs specifically engineered for northern climate durability.

Topic. The bacterially-catalyzed pyrite oxidation reaction chain that produces acid mine drainage, and the static/kinetic testing and active/passive management methods used to predict, prevent and treat it.

Key relations. Reactions 1-4 above; the balanced ferric-attack step $FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+$ is what makes the overall process autocatalytic once initiated.

Why this works. Reaction 2 (ferrous-to-ferric oxidation) is the kinetic bottleneck for the WHOLE chain because Reaction 4 cannot proceed without the ferric iron that Reaction 2 supplies – this is why bacterial catalysis of Reaction 2 (once pH is low enough for acidophiles to establish) causes such a dramatic acceleration in observed field drainage chemistry, and why early intervention (before bacterial populations establish) is far more effective than treatment after AMD is fully self-sustaining.

Common pitfall. Treating Reaction 4 as a simple 1:1:1 stoichiometric substitution (as printed, unbalanced) rather than recognizing it needs 14 mol of ferric oxidant per mole of pyrite – a small coefficient error here misrepresents just how much ferric iron (and hence how much prior Reaction 2 activity) is required to sustain the autocatalytic cycle.

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

Question 3 of 6: Mining Waste and Management – Waste Rock

Question 3: Mining Waste and Management – Waste Rock (20 marks)

A. General waste rock knowledge (8 marks)

B. The four acid mine drainage reactions (5 marks)

C, D, E. Effects, prevention/treatment, and northern-climate considerations (7 marks combined)