Mining Waste & Water Management Flashcards
Outline the main components required for the formation of mine water.
- Pyrite
- Air
- Water
- Bacteria (acts as catalyst)
- Creates metal laden acidic water
What determines minewater quality?
- Pyrite / sulphide minerals create minewater
- Alkaline minerals neutralize / inhibit minewater formation
- Local composition and form of minerals dictate minewater quality (e.g. fine crystal pyrite surrounded by inert material may not contact water)
An abandoned sulphide mine has a substantial amount of residual minerals exposed in its underground workings in a host rock with limited available alkalinity throughout. Pumping was necessary during operation and since closure water levels have recovered to the level of an adit just below the water table. Mine water is decanting into the adit and flowing into the local river system.
Explain how the iron content of the mine water could be expected to change over the years from the time of closure, using a schematic graph and diagrams to illustrate your answer.
- Peak = “first flush” quality water that is discharged (“first flush” quality is determined by mineral composition and form of larger contact area below surface).
- Resulting in Red River
- Until all Fe3+ is consumed, reactions will continue to take place.
- Flooded workings eventually become inert
- Can pump water out so that groundwater levels held below decant level (that of adit)
- Treatment to avoid a “Red River”
- Stop pumping when Fe conc. curve flattens out
- Sulphides above pumping level give “long term” quality (from seeping rainwater)
Outline active treatment
- Minewater abstraction = pumping
- Metal ppt to get contaminants out of solution
- Expect few particulates in liquid, but..
- Polishing removes remaining particulates
Outline lime dosing / recirculating sludge
- Ca(OH)2 adjusts pH
Outline the biochemical sulphidisation process
- Sulphate to sulphide
- Sludge of sulphide
Outline the Unipure HDS process
- Flocculant promotes settling
- Recirculated sludge acheives onion ring growth - get better settling quality and t.f. better final effluent quality
An abandoned copper mine has an historic waste rock dump which is causing contamination of the local water courses. A small stream with an average flow of approximately 5 l/s has been identified containing variable concentrations of iron, aluminium and copper and a pH of 3.5. The authorities have requested that a treatment scheme be installed. They have stated their preference for a passive scheme based on pre-treatment in a limestone bed and then a combination of aerobic reed beds for the removal of iron and anaerobic cells containing compost and sawdust for the removal of copper.
Provide a critique of this proposal by noting the limitations associated with the installation of this passive treatment concept for these local circumstances, with consideration given to location, cost, maintenance and design requirements.
// Why was passive treatment not adopted at WJ?
- Land area requirement
- Need for flat area (terracing costly if can’t find flat area)
- Need for large area (for WJ - 5 l/s = 2 ha, 440 l/s = 0.05 ha)
- Capital (plus maintenance) cost
- High capital cost for passive treatment construction compared with active treatment
- Maintenance required where aluminium present (Al(OH)3 ppt in water - blocks l.stone drain t.f. not passive anymore).
- Effluent quality
- Seasonal variability (some)
- Unable to meet required discharge consent @ WJ due to low pH and high metal conc. (Fe, Zn, Mn).
Summarise the Passive Treatment at Wheal Jane (WJ)
- Limestone drains can remove acidity (H+)
- Present in raw minewater
- Also H+ generated by iron hydrolysis reaction
- Aerobic cells
- 63-74% iron removal in the aerobic cells
- Arsenic removal to below detection limits
- Anaerobic cells
- 45-86% zinc removal
- Copper and cadmium removal to below detection limits (at reletively low pH)
- Rock filters
- 97% manganese removal in the ALD System durng summer 1996
- Decrease in pH in aerobic cells affects downstream treatment stages (hence potential benefit from intermediary limestone drains)
Give a pilot Passive Treatment plant layout
- Lime dosing gets pH from 2.5 up to 3-4 (increase alkilinity).
- Lime manually fed in
- Reduces (area) for reed beds
- Improve performance
- Anoxic cell
- Anoxic Limestone drain increases pH to 6
- Enclosed system - pdfe layer to keep air out
- Having smaller anaerobic cells in LD and ALD systems increases pH and alkilinity
- Aerobic cells of LF system = reed beds
- active removal of arsenate
- Anaerobic cells of LF system have sulphidising conditions
- generate reducing environment
- some Fe ppt
- Rock filter generate algae that remove manganese
- high pH (in theory)
- water trickle to remove sulphide
What are the 3 options for changing alkilinity in passive treatment?
- Lime-dosed system
- ALD system
- Lime-free system
What are the sources and types of waste associated with mining?
- Mining - waste rock
- Processing - tailings
- Mining/processing - marginal ore stockpile?
- Various other sources
- Discarded packaging
- Oil and grease
- Laboratory waste
- Etc
- Usually regulated under standard industrial legislation
Describe waste rock as produced from mining
- Exposed pit face
- Big boulders to mm particles
- Non-impounded disposal
Describe tailings
- From mm to microns range
- Wet processing
- Usually impounded disposal
Describe marginal ore stockpile
- Non-impounded
What are the two types of factors that must be considered regarding the issue of ARD?
What about the factors need to be considered?
- Physical
- Weather (water balance)
- Site conditions
- Disposal method
- Particle size distribution
- Geochemical
- Mineralogy (encapsulated in silicate or no?)
- Type of sulphide (ARD/ML)
- Alkalinity releasing minerals
- State of physical development of facility
In waste characterization, what are the key issues?
- Will it generate acidity?
- If so, what intensity?
- How long?
- Leachate chemistry - what metals are expected to go into solution?
- If not, metal leachability issues?
- Conc. of elements of environmental concern (establish from elemental analysis)
- Temporal dimension
Define acidity and alkilinity.
Give the units.
Acidity and alkalinity are capacity factors that represent the acid- and base-neutralizing capacities of an aqueous system.
Units: mg CaCO3/L
Acidity and alkalinity are measures of what?
Acidity and alkalinity are measures of the potential impact of waste water on the environment
In the context of solid waste, what does AP stand for?
Acid-generating Potential of solid waste,
in kg CaCO3/t
In the context of solid waste, what does NP stand for?
Neutralization Potential of solid waste,
in kg CaCO3/t
In the context of solid waste, what does NPR stand for?
Neutralization Potential Ratio of solid waste,
NPR = NP / AP (-)
Name the reactive sulphides
- Pyrite (FeS2)
- Pyrrhotite (Fe1-XS)
- Marcasite (FeS2)
- Other sulphides
- ZnS
- CuS
- CuFeS2
- FeAsS
- CdS
- PbS
Pyrite (FeS2), Pyrrhotite (Fe1-XS) and Marcasite (FeS2) are examples of what?
Sulphides
Give examples of neutralizing minerals
- Carbonates (dissolving)
- Ca-fsp, olivine (fast reaction)
- Pyroxenes, amphiboles (intermediate)
- Sorosilicates, phyllosilicates (slow)
- Plagioclase fsp (v. slow)
- (Qtz, inert)
What are the controls on metal solubility?
- Different metals have different solubility (in addition, metal oxides or hydroxides can be amphoteric, and are able to react both as a base and as an acid).
- Mineralogy - critical (exposed from grinding?)
- pH has a very strong influence - both acid and alkaline (parabolic shaped curve)
- Reducing conditions tend to ppt metals, but not all metals (i.e. As; As removed if high amount of Fe)
- Bacteria (catalyst in ppt and adsorption)
What is the ARD testing approach?
How would one calculate AP
S- x 31.25 (kg CaCO3 / t)
How would one calculate NP?
How does the relationship between AP and NP tell of the likelyhood of ARD?
B.C. Screening Criteria
During testing for ARD and metal leachability, why is mineral characterization critical?
- Sulphides
- Types
- Grain size
- Grain to grain relationships
- Trace elements
- Neutralizing minerals
- Carbonate types
- Grain relationships
What technique can be used to determine the types and quantities of carbonates and sulphides?
Reitveld XRD
What is Reitveld XRD?
- Quantitative x-ray diffraction
- Provides an estimate (wt. %) of the mineralogical composition of a sample
- Useful for determining the types and quantities of carbonates and sulphides
Short term leachability is an important step during testing for ARD and metal leachability.
What are the kinds of short term leaching?
- Synthetic Precipitation Leaching Procedure (SPLP, US EPA)
- Meteoric Water Mobility Procedure (Nevada)
- BC Special Waste Extraction Procedure (SWEMP)
- EN 12457 - X (EU Compliance Test)
- CEN/TS 14429 (EU Basic Characterization Test)
Describe the process that takes place in Humidity Cells during Kinetic Testing.
- 1kg of tailings, ground down to <2mm
- 3 days humid air
- 3 days dry air
- 1 day flushing / sampling for later analysis (flush with synthetic, pH 6 rainwater)
- Repeat for 6-12 moths, 2 yrs in rare occasions
- This is to simulate what happens in a waste rock dump. Moist and dry air to simulate for all envs.
- Finest material most reactive
What is the purpose of using Columns in Kinetic Testing?
- Used to simulate conditions in saturated or partially saturated tailings
- Leachates can be used as an estimate of drainage quality
- Water cover is to examine leachability from tailings material
- Anoxic conditions
What potential sources of water might be considered to provide process water supply to an open pit metalliferous mining operation?
(For each, briefly discuss the advantages and disadvantages that would prioritise one type of source over another.)
- Decant pond from Tailings Management Facility (TMF)
- Water Storage Dam (WSD)
- Fresh river water
- Non-contaminated groundwater resource (pumped) (particularly in extreme dry climates, where lack of water restricts process - water from distant groundwater borefield required)
- Dewatering arising from open pit
Illustrate the relationship between pulp density and moisture content
How would one determine the possibility of ARD generation from waste materials using the graph below?
During the screen testing phase, and given a value for AP and NP, how might one determine the likelyhood of ARD?
Using the ratio of NP : AP
i.e. the NPR (=NP/AP)
Discuss how the cumulative expenditure associated with a project changes as it progresses with time, input and the importance of making informed choises regarding treatment routes at the early stages of a project.
Make use of the diagram below showing the projected phases.
- High impact on “bottom line” at the beginning bc details are inaccurate (+/- 30%).
- As time passes, technical details become more clear and the impact on the “bottom line” (or where the project is going) decreases.
Why would it be better to dewater an open pit mine with peripheral boreholes rather than just a sump system in the pit?
- To prevent saturated walls from occuring peripheral dewatering boreholes can be set up to depress the water table
- No longer have saturated walls (have dry walls instead) and inflows of water into sump - less required to pump from slump;
- Inflows now only from drainage, runoff etc that still needs to be pumped out).
- Low inflows may evaporate.
- ARD, ochre on surface w/ flooded sump
Discuss the factors influencing the management and location of waste rock dump(s) and the tailings dam for the following plan:
ARD classification:
T1 Low
WR1 No
WR2 Low
WR3 Possible
WR4 Likely
WR5 No ARD/High alkilinity
WR6 No ARD/Neutral
WR7 No ARD/High alkilinity
- Need to consider haulage distance / haulage costs so waste rock dump (WRD) shouldn’t be too far from excavation.
- Don’t want to put WRD anywhere you might find more ore, so consider area for extension aorund the mine.
- Don’t want to put the WRDs near the river - unless completely inert (don’t want to contaminate the river water).
- WR5 and WR7 have high alkilinity and no ARD t.f. can be used as a cover for tailings and mine closure if not leaching (test leachability) - t.f. they should be kept close by to the TMF.
- WR6 is inert and can be used for tailings dam embankment (save on geomaterials cost) bc it will not react and retain its strength.
- WRs with ARD problems need to be tested (select appropriate kinetic testing) and treated and put in WRDs furthest from river.
- Use knowledge and common sense!
Explain how the mineralogy within a prospect mine is in the situation of good minewater quality pre-mining.
- Oxidation of sulphides near surface only (leached out often) forming oxide cap.
- Steady state reached over geological time.
Explain how the mineralogy within a mine can lead to the situation of poor minewater quality during early shallow mining.
- Exposing minerals initiates minewater formation.
- Contact between sulphides, water and air.
- Red river produced (mine water flows down adit).
During the final years of a mining operation the quality of mine water pumped from the deep levels of the mine continues to meet the permit for discharge to the local river system. After closure, when pumping ceases in 1992, the water level in the mine recovers and the mine water quality deteriorates as shown by the steep rise in dissolved metal concentration as it approaches ground level.
Outline how the mineralogy within a mine can lead to the situation of good minewater quality before closure.
- Groundwater drawn down significantly by pumping (rainwater purcolates down).
- But process of mining exposes further sulphides to oxidation (down the mine; purcolating water also brings down air)
- Leading to minewater with potenital for contamination.
- Potential dependent on balance of acid generating sulphides to alkaline minerals
During the final years of a mining operation the quality of mine water pumped from the deep levels of the mine continues to meet the permit for discharge to the local river system. After closure, when pumping ceases in 1992, the water level in the mine recovers and the mine water quality deteriorates as shown by the steep rise in dissolved metal concentration as it approaches ground level.
Outline how the mineralogy within a mine can lead to the situation of poor minewater quality as water levels recover.
- Pumping ceases post-mining.
- Groundwater levels (aerated water) start to recover, and the larger contact area produced from mining gives “first flush” quality.
- Quality determined by mineral composition and form.
- Groundwater levels then recover to decant level (w/ flooded workings) and “first flush” quality water discharges to surface water, resulting in a Red River.
- (Note: Flooded workings will eventually become inert after all Fe3+ used up)
During the final years of a mining operation the quality of mine water pumped from the deep levels of the mine continues to meet the permit for discharge to the local river system. After closure, when pumping ceases in 1992, the water level in the mine recovers and the mine water quality deteriorates as shown by the steep rise in dissolved metal concentration as it approaches ground level.
Once the water level recovers to ground level, the water quality slowly improves with time, following a saw-tooth pattern with fluctuations in metal concentration. Explain why this occurs, using sketch diagrams to support your answer.
- Groundwater levels held below decant level by pumping.
- Pumped water is treated to avoid Red River.
- Sulphides (mineral make-up) above pumping level give “long term” quality.
- Re: seasonal changes give saw-tooth pattern (see image for how)
An abandoned sulphide mine has a substantial amount of residual minerals exposed in its underground workings in a host rock with limited available alkalinity throughout. Pumping was necessary during operation and since closure water levels have recovered to the level of an adit just below the water table. Mine water is decanting into the adit and flowing into the local river system.
At the point the adit reaches the local river system, the mine water has a pH of 2.5. All of the dissolved iron present in the mine water has been oxidised to iron (III) and some ochre precipitation is evident along the adit. Use the graph below to determine the maximum dissolved iron concentration in the mine water before it flows into the river, expressing your answer as mg/l.
(Note that the atomic weight of iron is 55.85 g /mol).
M = mol/L
mol/L x g/mol = g/L
g/L x 1000 = mg/L
M = 10-3.3 mol/L
Mr Fe = 55.85 g/mol
10-3.3 x 55.85 = 0.280 g/L
0.280 x 1000 = 28.0 mg/L
The mine water has a pH of 2.5. All of the dissolved iron present in the mine water is in its reduced form, iron (II).
Use the graph below to determine the maximum dissolved iron and sulphide concentration in the mine water, expressing your answer in mg/l.
(Note that the atomic weight of iron is 55.85 g/mol, and sulphur is 32.07 g/mol)
M = mol/L
mol/L x g/mol = g/L
g/L x 1000 = mg/L
M Fe2+ = 10-1.5
M S2- = 10-15
Mr Fe = 55.85 g/mol
Mr S = 32.07 g/mol
10-1.5 x 55.85 = 1.76 g/L → 1766 mg/L Fe2+
10-15 x 32.07 = 3.207e-14 g/L → 3.207e-11 mg/L S2-
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