24-MMP-A6 Mining and the Environment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A6 Mining and the Environment, 2014-May. 3 hours duration, open book, no calculator permitted. Six questions of equal value (20 marks each) constitute the bank; a candidate answers any FIVE. Most questions require an essay-format answer.
Reference texts: BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia (current edition) (permitting, reclamation and closure planning requirements); Canadian Dam Association (CDA), Dam Safety Guidelines (2013/2019 update) (dam classification, embankment raising methods, design flood/PMF); Global Industry Standard on Tailings Management (GISTM, 2020) (tailings governance, upstream-construction restrictions); Price, W.A., MEND Report 1.20.1, Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials (1997/2009) (acid-base accounting, NPR screening criteria); International Network for Acid Prevention (INAP), Global Acid Rock Drainage (GARD) Guide (ARD prediction, prevention and treatment); Vick, S.G., Planning, Design, and Analysis of Tailings Dams, 2nd ed. (embankment construction methods, seepage control); Government of Canada, Metal and Diamond Mining Effluent Regulations (MDMER) under the Fisheries Act; Government of Canada, Impact Assessment Act (successor to the 2012 Canadian Environmental Assessment Act).
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 conventional active ARD treatment plant runs ARD-impacted mine water through neutralization, oxidation and solids separation in series before discharge. Table 1 sets out the process in the order the water passes through it.
| Stage | Purpose | Key reaction |
|---|---|---|
| 1. Lime (or hydrated lime slurry) neutralization | Raise pH from the ARD influent's typically acidic value toward 9–10, converting dissolved metal ions to insoluble hydroxides | $\text{H}_2\text{SO}_4 + \text{Ca(OH)}_2 \rightarrow \text{CaSO}_4 + 2\text{H}_2\text{O}$; $\text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe(OH)}_3\!\downarrow$ (and similarly for Al, Cu, Zn hydroxides at their own optimum pH) |
| 2. Aeration / oxidation | Oxidize remaining Fe2+ to Fe3+ so it precipitates as a faster-settling, more filterable floc | $4\text{Fe}^{2+} + \text{O}_2 + 10\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3\!\downarrow + 8\text{H}^{+}$ |
| 3. Clarification / settling | Separate the metal-hydroxide and gypsum (CaSO4) sludge from the clarified water | Physical settling, often with a flocculant aid |
The plant's main benefit is reliability: it is a mature, proven technology that handles variable influent flow and chemistry and consistently achieves high metal-removal efficiency (commonly >95% for Fe, Al and most base metals), which is why it is the default solution wherever MDMER compliance must be guaranteed. Its main disadvantages are the ones that matter most for closure planning: it requires continuous chemical (lime), energy and labour input essentially forever if the underlying rock continues to generate ARD, making it a perpetual-care liability rather than a one-time capital cost, and it produces a large volume of metal-hydroxide/gypsum sludge that itself needs permanent, secure disposal (commonly back into the tailings facility) since it can re-release metals if disturbed or if pH drops again. In terms of relative cost and effectiveness, active treatment is highly effective at meeting discharge limits but is generally the highest life-cycle-cost ARD management option of those covered in this exam, because chemical consumption scales directly with both flow rate and acidity and the plant must be staffed and maintained indefinitely; it is nonetheless frequently unavoidable where ARD flow and acidity are too high for a passive system (Practice Question 3b) to keep pace.
[Figure not reproduced: Generic waste rock encapsulation strategy (as printed on the exam paper, page 4, with page 7 an identical mark-up copy for submission): a PAG core is fully enclosed within a compacted NAG (or engineered) cover envelope placed over a compacted foundation, so the encapsulated PAG is isolated from both. See the official exam paper.]
The strategy shown is encapsulation: potentially acid-generating (PAG) waste rock is placed as the interior core of the pile and completely surrounded – on the flat top, the side slopes and the base – by potentially acid-neutralizing/non-acid-generating (NAG) material, so the PAG never has an exposed face in contact with the atmosphere. The five boxes are populated as follows: (1) the flat top surface nearest the crest is a vegetated growth medium / topsoil layer, providing erosion control and promoting evapotranspiration to shed infiltrating precipitation; (2) the remainder of the flat top is a compacted, low-permeability NAG till cover forming the primary oxygen/water barrier; (3) the side slopes carry the same low-permeability NAG cover material, maintaining an unbroken barrier down every exposed face of the pile; (4) the interior of the pile below the cover – both benches in the lower diagram – is the PAG waste rock core, kept fully isolated from oxygen and infiltrating water by the surrounding NAG envelope; (5) the foundation beneath the entire pile is compacted NAG or an engineered low-permeability liner, preventing oxygen ingress and uncontrolled seepage from below and directing any residual contact water to a collection ditch. The benched (lower) configuration adds progressive-cover and erosion-control benefits over the single continuous slope (upper) configuration, since each bench shortens the effective slope length exposed to surface runoff and allows the cover to be placed and vegetated progressively as the dump is built up, rather than only once the final slope is complete.
