Question 3 of 6: Acid Rock Drainage

Question 3: Acid Rock Drainage (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) Acid-base accounting (ABA)

Given. A representative drill-core or waste-rock sample is assayed for total and sulphide sulphur content (from which the Acid Potential is derived) and for its Neutralization Potential (from a Sobek fizz test followed by acid digestion and back-titration). Find. A single screening ratio – the Neutralization Potential Ratio – that classifies the sample's long-term acid-generating risk.

Acid-base accounting is a static geochemical test that compares a sample's capacity to generate acid against its capacity to neutralize that acid. The Acid Potential (AP, in kg CaCO₃-equivalent per tonne) is calculated from the sulphide-sulphur assay using the stoichiometric factor $AP = 31.25 \times \%S$, which assumes all sulphide sulphur oxidizes fully to sulphuric acid via the pyrite-oxidation pathway. The Neutralization Potential (NP, same units) is measured directly by the Sobek method: the sample is reacted with excess standardized HCl, any unreacted acid is back-titrated with NaOH, and the acid consumed by carbonate and other neutralizing minerals is reported as NP. The two are combined into the Neutralization Potential Ratio, $NPR = NP/AP$, and screened against the criteria in the table below (Price, MEND 1.20.1, 1997/2009): $NPR < 1$ is classified likely acid generating, $1 \le NPR \le 4$ is possible/uncertain (kinetic testing recommended to resolve the classification), and $NPR > 4$ is non-acid generating. For example, a sample assaying 1.5% sulphide sulphur has $AP = 1.5 \times 31.25 = 46.9$ kg CaCO₃-eq/t; if its measured NP is 95 kg CaCO₃-eq/t, $NPR = 95/46.9 \approx 2.0$, placing it in the possible/uncertain band and flagging it for follow-up humidity-cell (kinetic) testing rather than a final call from the static test alone.

b) Conventional active water treatment plant

A conventional active ARD treatment plant is a chemical lime-neutralization process operated continuously on the impacted water stream. Raw, acidic (typically pH 2–4), metal-laden influent is first dosed with hydrated lime, Ca(OH)₂, in an agitated reaction tank, which neutralizes the free acidity ($H_2SO_4 + Ca(OH)_2 \rightarrow CaSO_4 + 2H_2O$) and raises the pH toward 9–10. As pH rises, dissolved metal ions precipitate as insoluble hydroxides ($M^{2+} + 2OH^- \rightarrow M(OH)_2\!\downarrow$), which is the actual removal mechanism for most regulated metals. The water then passes to an aeration/oxidation stage, where dissolved oxygen or forced air oxidizes ferrous iron to ferric iron ($4Fe^{2+} + O_2 + 10H_2O \rightarrow 4Fe(OH)_3\!\downarrow + 8H^+$), producing a denser, faster-settling floc and consuming additional lime to neutralize the acidity this reaction itself generates. Flocculant is typically dosed ahead of a clarifier or settling pond, where the metal-hydroxide/gypsum floc settles out as sludge while clarified water overflows to discharge; the settled sludge is thickened, dewatered where practical, and disposed of to the tailings facility or a dedicated sludge cell, and the treated effluent is continuously monitored against MDMER limits at the compliance point before release. Benefits of this approach are that it is a proven, well-understood technology capable of handling variable influent flow and chemistry, reliably achieves regulatory compliance across a wide range of water qualities, and can be commissioned relatively quickly. The principal disadvantages are a high ongoing operating cost – reagent (lime), energy, labour and sludge disposal must be sustained for as long as ARD is generated, which is very often in perpetuity after closure – the large sludge volumes it produces, and the fact that it treats the symptom (contaminated water) rather than the source (sulphide oxidation), so treatment cannot simply stop once discharge ends. Relative to the source-control and passive-treatment methods in part (c), active treatment is the most effective and most certain method of achieving compliance on demand, but its lifecycle cost – especially the present value of a perpetual post-closure operating obligation – is by far the highest of the available options, which is exactly why prevention (source control) is preferred wherever it is achievable.

c) Five ARD prevention methods (active and passive)