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24-MMP-A6 Mining and the Environment · December 2018

Question 4 of 6: Mining Waste and Management – Tailings

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A6 Mining and the Environment, 2018-Dec. 3 hours duration, open book (any non-communicating calculator permitted). SIX questions are printed on the paper; FIVE questions constitute a complete exam paper, and only the first five questions as they appear in the answer book are marked. Most questions require a concise, point-form-acceptable answer rather than a full essay. Every question is solved in full below (including all six, not just the five a candidate would normally submit) so this set also serves as complete study material.

Reference texts: Government of Canada, Fisheries Act and the Metal and Diamond Mining Effluent Regulations (MDMER, the current name for the exam's "Metal Mining Effluent Regulations"); Government of Canada, Canadian Environmental Protection Act, 1999 (CEPA); Government of Canada, Impact Assessment Act (successor to the 2012 Canadian Environmental Assessment Act named in the exam); Government of Canada, Species at Risk Act; International Network for Acid Prevention (INAP), Global Acid Rock Drainage (GARD) Guide; Price, W.A., MEND Report 1.20.1, Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials (1997/2009); Canadian Dam Association (CDA), Dam Safety Guidelines (2013/2019); Global Industry Standard on Tailings Management (GISTM, 2020); Vick, S.G., Planning, Design, and Analysis of Tailings Dams (1990); ATSDR, Toxicological Profiles for mercury; BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia.

Question 4: Mining Waste and Management – Tailings (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 (2 marks)

i. Mine tailings are the fine-grained waste material remaining after valuable minerals have been extracted from ore during processing/milling – typically discharged as a slurry of finely ground rock and process water. As-deposited conventional tailings slurry is typically about 30–50% solids by weight (thickened or paste tailings can be dewatered well above this range before deposition).

ii. True. Tailings water chemistry depends entirely on ore mineralogy and processing reagents: cyanide-leach gold tailings water is typically basic/alkaline (lime is added to suppress HCN generation), sulfide-ore tailings can generate acidic water through the same ARD mechanism as waste rock (Question 3), and tailings from arid or evaporite-affected sites (or where process water is recycled and concentrates dissolved salts) can be saline.

B. Tailings storage/disposal practices and site-specific considerations (6 marks)

i) Four common practices:

ii) Four site-specific environmental considerations: proximity/hydraulic connectivity to fish-bearing water bodies (to avoid or, if unavoidable, require an MDMER Schedule 2 listing); seismic activity and foundation geology (liquefaction potential, competent bearing foundation); site climate/water balance (precipitation surplus versus evaporation, freeze-thaw behaviour); and downstream population and consequence-of-failure classification (dam-breach inundation risk to communities, infrastructure and sensitive habitat).

C. Comparison table and cross-sectional sketches (12 marks)

Raised tailings embankment methods: upstream, downstream, centerline
AttributeUpstreamDownstreamCenterline
Description/Details (1 mark each)Each raise is built progressively toward the pond, founded mostly on the settled/cycloned tailings beach of the prior raise; minimal imported fill.Each raise steps entirely away from the pond; the full cross-section at every stage is imported engineered fill, never resting on tailings.Each raise's crest stays vertically above the starter dyke centerline; the downstream face is engineered fill while the upstream side is partly tailings-supported.
Advantage (0.5 marks each)Lowest cost and simplest construction – minimal imported fill and equipment.Best seismic resistance and water-storage capability – can be engineered like a conventional dam.Intermediate cost with materially better seismic performance and raise-rate flexibility than upstream.
Disadvantage (0.5 marks each)Poorest seismic resistance (liquefaction-prone loose sand fill); raise rate limited by tailings consolidation.Highest cost – full engineered fill volume at every raise for the same overall height.Still partly reliant on tailings for the upstream zone, so not as strong as full downstream construction.
Sketch (2 marks each)See the three cross-sectional sketches below.
Original groundSuccessive raises step UPSTREAM (toward pond)Cycloned/spigotted tailings beach(coarse fraction) supports each new raiseSupernatant pond (fines)Phreatic surfaceUpstream Method
Fig. 4C-1 – Upstream method: successive raises step upstream (toward the pond), founded largely on the settled/cycloned tailings beach of the prior raise.
Original groundCrest steps DOWNSTREAM (away from pond) --widest structural footprint of the 3 methodsBeachDownstream Method
Fig. 4C-2 – Downstream method: each raise steps entirely downstream (away from the pond); the whole cross-section at every stage is engineered fill, never resting on tailings.
Original groundCrest stays vertically overstarter dyke centerlinePondDownstream face: engineered fill / rockfill massCenterline Method
Fig. 4C-3 – Centerline method: each raise's crest remains vertically above the starter dyke's centerline; the downstream face is engineered fill, the upstream face is partly tailings-supported.