Mining Waste and Management – Tailings: 24-MMP-A6 Mining and the Environment, May 2017, Q4

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. Tailings basics (2 marks)

i. What are mine tailings, and % solids (1 mark). Mine tailings are the finely-ground process residue left after the economically valuable minerals have been extracted from ore during milling/beneficiation – typically discharged from the processing plant as a slurry of ground rock particles suspended in process water. As-discharged tailings slurry is commonly about 30-45% solids by weight (conventional slurry disposal), though thickened or paste tailings can be produced at substantially higher solids content (upward of 60-70%) as part of a dry-stack or reduced-water-footprint disposal strategy.

ii. True or False (1 mark). True – tailings water chemistry depends entirely on the ore mineralogy and process reagents used, and can be acidic (sulphide-bearing ore with ARD potential, as in Question 3), basic (e.g. lime-dosed circuits, or ore with abundant carbonate gangue), or saline (process water reuse/recycling that concentrates dissolved salts, or ore bodies with naturally saline groundwater).

B. Tailings storage/disposal practices and site selection (6 marks)

i. Four common tailings storage/disposal practices (4 marks).

Conventional slurry impoundment. Tailings slurry (30-45% solids) is pumped and discharged behind an engineered raised embankment (Part C), where solids settle and supernatant water is reclaimed from a decant pond – the most common method historically, but requiring the largest water inventory and dam footprint.
Thickened/paste tailings. Tailings are dewatered (thickened, or further to non-segregating paste consistency) before discharge, substantially reducing the water retained in the facility, the decant pond size, and in many cases the required containment structure height, at the cost of additional thickening/pumping infrastructure and energy.
Dry-stack (filtered) tailings. Tailings are mechanically filtered to a low-moisture, unsaturated "cake" that is trucked or conveyed and compacted in engineered lifts like a conventional earthfill, eliminating the need for a water-retaining dam altogether – the highest capital/operating cost per tonne but the lowest long-term dam-failure risk profile.
Underground (paste) backfill. A portion of the tailings stream is mixed with cement binder and pumped back underground to fill mined-out stopes, simultaneously providing ground support and reducing the surface tailings volume requiring above-ground storage – only applicable where an underground mining method and suitable stope geometry exist.

ii. Four site-specific environmental considerations for tailings facility siting (2 marks).

Seismicity of the region, since embankment stability and liquefaction risk (Question 6A) are directly governed by expected ground motion at the site.
Proximity to and connectivity with fish-frequented or otherwise sensitive water bodies, since siting near or requiring diversion of such a water body triggers substantially more stringent regulatory review (Question 2B).
Local hydrology and climate (precipitation/evaporation balance, watershed size draining to the site), which govern how much water must be managed and the facility's flood/spillway design requirements.
Foundation geotechnical conditions (bearing capacity, presence of weak layers such as glaciolacustrine clay), since an inadequately characterized foundation is a documented root cause of catastrophic embankment failure (Question 6A).

C. Tailings impoundment construction: upstream, centerline, downstream (12 marks)

Raised tailings embankment methods: upstream, centerline, downstream
Deliverable	Upstream	Centerline	Downstream
Description/Details	Each raise is built mostly from cycloned/spigotted tailings sand itself atop a small starter dyke, with the crest progressively stepping toward the pond (upstream) as the facility is raised.	Each raise's crest remains vertically above the starter dyke centerline; the downstream face of every raise is engineered fill while the upstream side rests partly on the settled tailings beach.	Each raise steps entirely away from the pond (downstream); the full cross-section at every raise is engineered fill, none of the structural mass ever rests on tailings.
Advantage	Lowest cost and simplest construction – minimal imported fill, uses the tailings stream itself as structural material.	Balances cost against performance – better seismic/water-storage capability than upstream without the full fill cost of downstream.	Best seismic resistance and water-storage capability – can be engineered to the same standard as a conventional earth/rockfill dam.
Disadvantage	Poor seismic resistance and highest liquefaction susceptibility; raise rate limited by tailings consolidation; unsuitable for significant water storage.	Still carries some tailings-supported (upstream-side) weak zone, so not the top performer for either water storage or seismic resistance.	Highest cost and largest fill volume/footprint of the three methods for a given height.
Sketch	See the three cross-sectional sketches below.

Fig. 4C-1 – Upstream method: successive raises step toward the pond, founded largely on the settled/cycloned tailings beach of the prior raise.

Fig. 4C-2 – Centerline method: crest remains vertically above the starter dyke; downstream face is engineered fill, upstream face partly tailings-supported.

Fig. 4C-3 – Downstream method: each raise steps entirely away from the pond; the full cross-section at every stage is engineered fill.

Topic. Tailings disposal practice options and the three raised-embankment construction methods that dominate conventional slurry tailings storage.

Key relations. None quantitative; the governing design trade-off across all three raised-embankment methods is how much of the structural mass relies on tailings (cheap, weak) versus imported engineered fill (expensive, strong).

Why this works. Site-specific factors (Part B(ii)) directly determine which disposal practice and embankment method are appropriate – a seismically active or high-precipitation site pushes the design toward downstream/centerline construction or a dry-stack alternative, while a benign, low-seismicity, arid site can more defensibly support the lower-cost upstream method.

Common pitfall. Selecting the cheapest disposal method (conventional slurry impoundment, upstream construction) purely on capital cost without weighting site-specific seismic, water-balance and foundation risk factors – several of the industry's best-documented catastrophic failures involved exactly this trade-off (Question 6A).

Source. Vick, S.G., Planning, Design, and Analysis of Tailings Dams (1990); Canadian Dam Association, Dam Safety Guidelines.

Question 4 of 6: Mining Waste and Management – Tailings

Question 4: Mining Waste and Management – Tailings (20 marks)

A. Tailings basics (2 marks)

B. Tailings storage/disposal practices and site selection (6 marks)

C. Tailings impoundment construction: upstream, centerline, downstream (12 marks)