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

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 (8 marks)

i. Waste rock is mined rock that does not contain ore of sufficient grade to be processed economically; it is segregated from ore at the point of mining and stored separately in engineered piles or dumps.

ii. Static testing (e.g. acid-base accounting, ABA) measures the acid-generating potential (AP) and neutralizing potential (NP) of a sample at a single point in time, giving a quick screening classification. Kinetic testing (e.g. humidity cell tests) exposes a sample to repeated wet/dry weathering cycles over weeks to months and measures the actual rate of acid generation and metal leaching over time, confirming (or refuting) the static-test prediction for material classified as uncertain.

iii. Top-down storage places waste rock by end-dumping from the crest of the pile, letting each truckload roll and self-segregate down the face (coarse material to the toe, fines near the crest); it is simple and low-cost but creates coarse, high-permeability zones that promote oxygen and water ingress. Bottom-up storage builds the pile in successive horizontal lifts, each placed and often compacted from the base upward before the next lift begins; it gives much better control over material blending, compaction and oxygen-ingress pathways, at higher construction cost and slower placement rate.

iv. Pyrite (FeS₂) is the most common acid-generating sulfide mineral (pyrrhotite is another common example).

v. Acid Rock Drainage (ARD) is the general term for acidic, metal-laden drainage generated by the oxidation of sulfide minerals in any rock exposure, including natural, undisturbed outcrops. Acid Mine Drainage (AMD) is the subset of ARD specifically caused by mining activity (waste rock, tailings, pit walls, underground workings) that exposes sulfide minerals to accelerated oxidation.

vi. AMD generation is an oxidation–reduction (redox) reaction. Electrons flow from the sulfide sulfur in pyrite (oxidized from S⁻ toward S⁶⁺ in sulfate) to dissolved oxygen (reduced to water), with dissolved iron acting as an electron-transfer catalyst – ferrous iron (Fe²⁺) is oxidized by O₂ to ferric iron (Fe³⁺), which then oxidizes further pyrite, regenerating Fe²⁺ and closing a self-sustaining cycle.

B. The four acid-generating reactions (5 marks)

1. Equation 1 – primary pyrite oxidation by oxygen. $FeS_2 + O_2 + H_2O \leftrightarrow Fe^{2+} + SO_4^{2-} + 2H^+$. Direct oxidation of pyrite by dissolved/atmospheric oxygen releases ferrous iron, sulfate and acidity (H⁺) – this is the initiating step of acid generation whenever a fresh sulfide surface is exposed to air and water.
2. Equation 2 – oxidation of ferrous to ferric iron. $Fe^{2+} + O_2 + H^+ \rightarrow Fe^{3+} + H_2O$. Oxygen oxidizes the ferrous iron produced in Equation 1 to ferric iron. This step is slow abiotically at low pH and is strongly catalyzed biologically (by iron-oxidizing bacteria such as Acidithiobacillus ferrooxidans), making it the practical rate-limiting step of the whole AMD cycle.
3. Equation 3 – ferric iron hydrolysis and precipitation. $Fe^{3+} + H_2O \leftrightarrow Fe(OH)_3 + 3H^+$. Ferric iron hydrolyzes in water to precipitate as ferric hydroxide (the orange "yellow boy" staining seen at AMD seeps), releasing additional acidity; this reaction proceeds preferentially once pH rises above about 3.5, buffering pH but adding more H⁺ to the system.
4. Equation 4 – ferric iron attack on further pyrite (balanced). Ferric iron generated in Equation 2 can itself oxidize additional pyrite even without direct oxygen contact, propagating acid generation deeper into a waste rock pile once sufficient Fe³⁺ has built up – this pathway dominates once pH is low and Fe³⁺ is abundant. Balancing for atoms and charge:
   $$\boxed{FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+}$$
   Fe balances (1 + 14 = 15), S balances (2 = 2), O balances (8 = 8), H balances (16 = 16), and charge balances (+42 on each side).

C. Effects, prevention/treatment, and northern considerations (7 marks)

i) Negative environmental effects (2 marks):

• Acute and chronic toxicity to aquatic life from low pH and elevated dissolved metals (aluminum, iron, copper, zinc), which can extirpate fish and benthic invertebrate populations downstream of a discharge.
• Long-term degradation of surface and groundwater quality, including precipitation of iron/aluminum hydroxide coatings that smother stream substrate and habitat, potentially affecting downstream drinking water sources for decades.

ii) Three prevention/treatment methods (3 marks):

• Active water treatment (e.g. a lime neutralization/high-density-sludge plant) – adds alkalinity to raise pH and precipitate metals as hydroxide sludge; active, requiring continuous reagent supply, energy, staffing and sludge disposal.
• Passive treatment (e.g. constructed wetlands, anoxic limestone drains, sulfate-reducing bioreactors) – relies on natural biological/geochemical processes (bacterial sulfate reduction, limestone dissolution) to raise pH and remove metals with minimal ongoing input; passive, suited to lower flows/concentrations and post-closure walk-away sites.
• Source control by encapsulation (low-permeability soil or synthetic covers, or a permanent water cover) – excludes oxygen and/or water from reactive sulfide material to prevent generation at the source rather than treating it after the fact; a prevention approach that is largely passive once constructed.

iii) Canada's northern environment (2 marks):

• Extreme seasonal temperature swings and long winters limit conventional active treatment plant operation (chemical reactions slow at low temperature, pipes/reagents can freeze), requiring heated/insulated infrastructure or seasonal water storage and batch treatment.
• Permafrost can be used deliberately as a "cold storage" strategy – sulfide material kept perennially frozen reacts negligibly – but ground disturbance risks thawing permafrost, causing subsidence that can crack or destabilize an encapsulation cover designed on the assumption of a stable frozen substrate.