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

i. Waste rock is uneconomic, sub-cut-off-grade rock and overburden that must be removed to access ore, and which is not sent to the mill for processing; it is typically stored in surface piles or dumps.

ii. Static vs. kinetic testing. Static testing (acid-base accounting) is a single-point laboratory measurement comparing a sample's acid potential (from total/sulphide sulphur) against its neutralization potential to give a snapshot ratio (NPR); it is fast and cheap but says nothing about reaction rate. Kinetic testing (humidity cells, column tests) instead cycles a sample through repeated wet/dry leaching over weeks to months, measuring the actual rate and lag time before acid generation begins – essential for samples that static testing flags as "possible/uncertain."

iii. Top-Down vs. Bottom-Up storage. Top-down (end-dump) construction places waste rock by tipping from the crest of an advancing dump face, letting coarser material roll and segregate to the toe; it is fast and low-cost but offers little control over material placement or compaction. Bottom-up (paddock-dump) construction places and compacts waste rock in engineered horizontal lifts from the base upward, which is slower and costlier but is used where selective handling – e.g. keeping potentially acid-generating (PAG) material encapsulated away from the surface and water table – is required.

iv. A common sulfide mineral in waste rock is pyrite ($FeS_2$).

v. AMD vs. ARD. Acid Rock Drainage (ARD) is the broad term for acidic, metal-laden drainage from the oxidation of sulphide minerals in any exposed rock, including natural, unmined outcrops. Acid Mine Drainage (AMD) is the narrower term specifically for such drainage originating from mine wastes – waste rock, tailings, pit walls and underground workings – i.e. AMD is a subset of ARD caused directly by mining activity.

vi. Electrochemical mechanism. Pyrite oxidation proceeds as an electrochemical cell reaction on the mineral grain surface: an anodic site oxidizes Fe and S (releasing electrons, Fe$^{2+}\rightarrow$Fe$^{3+}$ and S$\rightarrow$SO$_4^{2-}$), while a spatially separate cathodic site on the same (semi-conductive) sulphide grain consumes those electrons by reducing dissolved oxygen; electrons flow slowly through the mineral lattice from anode to cathode, which is why the overall reaction proceeds gradually rather than instantaneously once sulphides are exposed to oxygen and water.

B. The four acid-generation equations, and balancing Equation 4

$$FeS_2 + \tfrac{7}{2}O_2 + H_2O \rightarrow Fe^{2+} + 2SO_4^{2-} + 2H^+ \quad (1)$$

Equation (1) is the primary oxidation of pyrite by dissolved oxygen, releasing ferrous iron, sulphate and acidity (H$^+$) directly – this is the initiating step of acid generation.

$$Fe^{2+} + \tfrac{1}{4}O_2 + H^+ \rightarrow Fe^{3+} + \tfrac{1}{2}H_2O \quad (2)$$

Equation (2) oxidizes the ferrous iron produced in (1) to ferric iron; at pH below about 4 this step is very slow abiotically and becomes the rate-limiting step of the overall process, but is strongly catalyzed biologically by iron-oxidizing bacteria such as Acidithiobacillus ferrooxidans.

$$Fe^{3+} + 3H_2O \rightarrow Fe(OH)_3 + 3H^+ \quad (3)$$

Equation (3) is the hydrolysis and precipitation of ferric iron as ferric hydroxide (the orange "yellow boy" precipitate seen in AMD-impacted streams), which itself releases further acidity.

Equation (4) as printed on the exam ($FeS_2 + Fe^{3+} + H_2O \leftrightarrow Fe^{2+} + SO_4^{2-} + H^+$) is the self-propagating step: ferric iron generated in (2) now acts as the oxidant attacking fresh pyrite directly (faster than dissolved oxygen at low pH), which is what makes AMD a self-sustaining cycle once initiated. Balancing it by conserving Fe, S, O, H and charge gives:

$$FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+ \quad \boxed{(4,\ \text{balanced})}$$

Check: Fe, $1+14=15$ both sides; S, $2=2\times1$; O, $8\times1=2\times4$; H, $8\times2=16\times1$; charge, $14(+3)=+42$ on the left equals $15(+2)+2(-2)+16(+1)=+42$ on the right.

C. Negative effects of AMD on the environment

• Acute and chronic toxicity to aquatic life – low pH and elevated dissolved metals (Fe, Al, Cu, Zn, Cd) damage fish gills and are lethal to sensitive invertebrates, reducing or eliminating aquatic populations downstream.
• Physical habitat smothering – precipitation of ferric hydroxide ("yellow boy") and other metal precipitates as pH recovers downstream coats streambed gravels and reduces light penetration, degrading spawning habitat and primary production.

D. Prevention/treatment methods

• Active chemical treatment (lime or hydrated-lime dosing with aeration and clarification to precipitate and settle metal hydroxides) – active: requires continuous reagent supply, mechanical mixing/aeration and pumping (significant energy and operator input), and generates a metal-hydroxide sludge requiring ongoing disposal.
• Passive treatment (constructed wetlands, anoxic limestone drains, or sulphate-reducing bioreactors) – passive: relies on natural biological and chemical processes and gravity flow with little ongoing reagent or energy input, at the cost of lower throughput and periodic (not continuous) maintenance.
• Source-control prevention via encapsulation/covers (soil, till, or engineered synthetic covers, or subaqueous/water covers) that exclude oxygen and/or water from contacting sulphide material – a largely passive approach once constructed, since it functions by physically limiting reactant supply rather than by ongoing operator intervention.

E. Northern Canada considerations

• Permafrost and deep seasonal frost affect cover-system performance (freeze-thaw cycling can crack low-permeability covers and disrupt water covers that freeze to the bottom in shallow ponds), so cover design must specifically account for the local frost regime.
• Passive biological treatment (wetlands, bioreactors) slows dramatically or stops at winter temperatures, and short open-water seasons limit when treated effluent can be discharged, so northern sites typically need larger equalization storage and/or a greater reliance on active treatment through the winter months, along with higher-cost reagent and equipment logistics due to limited road access.