A review of sources and sinks for nitrate in the mining environment

Christopher H. Gammons, Ph.D.Professor, Dept. of Geological Engineering

Montana TechButte, Montana

cgammons@mtech.edu

Acknowledgements

This research was supported by Goldcorp Inc. Thanks to mines and agencies who supplied data and photographs

Sources of nitrate in mine settings

• Explosives• Cyanide breakdown• Sewage

• Atmospheric deposition• Geological background

Mine-related

Non-mine-related

Explosives• ANFO

– 96% ammonium-nitrate, 4% fuel oil– Both ammonium and nitrate are highly soluble in water– Very little residual nitrate if explosives are handled

carefully and combustion is complete• Emulsions, gels

– Also contain ammonium-nitrate, but mixture is contained in a gel or slurry that minimizes contact with water

– Leach slower, but over time will also release nitrate and ammonium

Nitrate concentrations in mine waters from blasting range: < 1 to > 10 mg/L (NO3-N)

Depends on many factors, including:- Blasting efficiency (% detonation)- How much precipitation on site- Contact time of water with mine waste- Evapoconcentration effects (ponds and lakes)- Presence/absence of algae/aquatic plants

Ammonium concentrations can also be high, but over time ammonium will oxidize to nitrate

Breakdown of cyanide

CN-

OCN-

SCN-

NH4+ + HCO3

-

NO3-

cyanide

cyanate

thiocyanate

ammonium

nitrateNO2-

SO42-

HCN

NOx(g) in air

VOLATILIZATION

OXIDATIONpH < 9

Breakdown of cyanide

CN-

OCN-

SCN-

NH4+ + HCO3

-

NO3-

cyanide

cyanate

thiocyanate

ammonium

nitrateNO2-

SO42-

HCN

NOx(g) in air

VOLATILIZATION

OXIDATIONpH < 9

Breakdown of cyanateOCN- + 2H2O → NH3 + HCO3

-

SCN- + 2H2O + 2O2 → SO42- + CO2 + NH4

+

Breakdown of thiocyanate

Both pathways generate ammonium

pH total CN WAD CN SCN- OCN- NO3-N NO2-N NH3+NH4PINBC S97 8.55 3.56 2.68 1.25 - 110 6.3 6.3PINAC S97 8.22 2.47 1.3 1.33 - 110 6.33 5PINBAR S97 9.32 70.9 66.3 0.71 - 79 4.5 6.3PINMBC S97 11.43 5.96 6.63 2.73 - 1 0.2 1.3PINMAC S97 11.43 5.91 6.63 2.95 - 1 0.18 1.3PINRCLSM S97 10.18 18.1 18 8.29 - 2 0.38 1.3LTFL2 S97 3.27 3.78 1.24 6.76 - 110 2.73 21.2LTFL3 S97 2.68 5.08 5.73 18.03 - 86 0.15 21.2LTFL7 S97 8.04 0.24 <.15 - 36 4.33 1.5LTBC S97 7.51 2.25 1.07 2.49 - 97 8.1 2LTAC S97 8.82 2.22 1.05 2.86 - 95 8.05 13.7LTBAR S97 9.66 63.9 63.9 2.28 - 99 8.05 27.5LTP1/2 S97 7.23 1.46 0.88 2.46 - 130 10.4 13.7LTLG S97 8.93 14.8 7.33 15.5 - 81 10.2 37.5LTDP S97 8.26 31.2 27.6 53.8 - 23 0.95 48.7LTFL2 O97 3.22 60.4 68.8 13.3 - 110 4.58 25LTFL3 O97 2.93 24.8 13.7 16.6 - 99 2.15 30LTFL7 O97 8.95 4.31 2.2 5.2 - 57 6.08 8LTBC J99 - - - - 15 140 13 23LTAC J99 - - - - 18 130 13 23LTBAR J99 - - - - 27 100 11 24

Concentrations (mg/L) of cyanide and cyanide breakdown products from gold mines in Nevada (from Johnson et al. , 2000).

Removal (sinks) of nutrients from mine water

• Many technologies exist for removal of nutrients from treated sewage

• In theory, these same technologies can be used for mine waters

PROBLEMS• Massive volumes of water

– e.g., tailings ponds, pit lakes• Low organic carbon in source • Remote settings, extreme climate• Cost

Treatment Relative cost

Time to desired result

Comments

Natural attenuation

Very low Very long (decades?)

May be a reasonable approach if there is no likelihood of discharge to receiving surface water and groundwater. For pit lakes, may need artificial mixing to accelerate oxidative processes. Example: Colomac Mine.

Enhanced natural attenuation

Low Long (several years)

Addition of limiting nutrient (usually P) to a water body may stimulate oxidative or reductive processes that remove N-compounds. Example: Colomac Mine.

Land application

Low-medium

n/a Need to monitor carefully for contaminated groundwater. Some compounds may be toxic to plants (thiocyanate?). Example: Beal Mine.

Constructedwetlands: aerobic

Medium months Large, shallow ponds remove nutrients through assimilation. May need to harvest plant mass. Evaporation. Possible impacts to aquatic life. Example: Warm Springs Ponds, Montana.

Nitrate: Treatment alternatives

Treatment Relative cost

Time to desired result

Comments

Constructed wetlands: anaerobic

Medium 1-2 year startup time

May be effective for low-cost removal of low-level COCs. Not feasible for high concentrations or high flows. Sensitive to climate extremes and changes to source water chemistry. May clog up.

Permeable reactive barrier

Medium 1-2 year startup time

Used to remove COC’s from groundwater in situ. Longevity uncertain. May clog up. Reactive material may become coated.

Bioreactors High 1-2 year startup time

Effective at removal of nitrate by denitrification. Sensitive to changes in source water chemistry. Example: Landusky Mine.

Ion Exchange

High immediate Robust technology, but expensive. Effective for some COCs but not others. Column regeneration creates waste.

Reverse osmosis

Very High immediate Robust technology, but expensive. Lowers concentration of multiple COCs simultaneously. Creates brine waste that must be disposed.

Nitrate: Treatment alternatives (cont.)

Some Examples1) Butte, Montana2) Landusky, Montana3) Stillwater, Montana

1. Butte

• Berkeley Pit lake• pH 2.6• Very high dissolved metals• No nitrate!

• Flooded underground workings• pH 4 to 7• Low to high dissolved metals• No nitrate!

Why no nitrate in Butte mine waters?

• All of these waters are anoxic• All of these waters are in contact with pyriteConclude: pyrite catalyzes denitrification

(but only in anoxic waters):5FeS2 + 14NO3

- + 4H+ → 7N2 + 5Fe2+ + 10SO42- + 2H2O

(Plenty of published literature on this)

2000: Note very large cyanide heap leach pad in upper left.

2005Slides courtesy of David Williams, Butte BLM

Example 2: Zortman-Landusky, Montana

Sept. 2005(mg/L)

L87 & L91 Pad influent

BR-3Outflow

Potential ARAR limit

Nitrate 177 - 241 < 1 10

Selenium 0.42 - 1.26 0.024 0.05

CN (wad) 0.041 - 0.223 0.159 na

CN (total) 0.200 - 0.547 0.231 0.0052

Landusky leach pad treatment system

three bioreactors

In series

Avg. flow ~ 285 L/min

Bioreactor performance• At first, great. Denitrifying bacteria NO3 → N2

• More recently, having problems with removal efficiency due to: – Changes in leach pad water chemistry

• Drop in pH from neutral to around 4• Increase in nitrate-N from 200 to > 300 mg/L

– Buildup of “organic sludge” in the bioreactors• State is exploring options

Example 3: Stillwater Mine, Montana

• Large underground platinum-palladium mine• High nitrate (20 to 40 mg/L as N) in mine

waters from blasting residues

Treatment scheme

• Need to treat 100-300 gallons/minute– 15 to 50 kg N per day

• Biological treatment – Moving Bed Bioreactors (MBBR) – Anaerobic/Aerobic – Add methanol, SRP

• Land Application– Irrigated pasture

Land Application site

Moving Bed Bioreactors (MBBR) at Stillwater

Empty Full

Photo courtesy Stillwater Mining

Stillwater Mine: Summary

• Performance has been very good for > 5 years• Optimal temperature ~ 15-20°C• Some problems in winter when water

temperature drops below 10°C– Need a heater to keep the MBBR cells warm

Summary

• N-contamination is a significant problem for the mining industry for which there is very little published information

• N- and CN-chemistry is complicated• There are multiple physical, chemical,

biological pathways • Some of these pathways can be used to a

mine’s advantage to minimize later N-impacts

Some references• Akcil, A., and Mudder, T. (2003) Microbial destruction of cyanide wastes in

gold mining: process review. Biotechnology Letters, 25(6), 445-450.• Chapman J.T., Coedy W., Schultz S., Rykaart M. (2007) Water treatment and

management during the closure of the Colomac Mine. Proc. Mine Closure 2007 Conference, Santiago, Chile.

• Ferguson, K.D., and Leask, S.M. (1988) The Export of Nutrients from Surface Coal Mines, Environment Canada Regional Program Report 87-12, March, 1988, 127 p.

• Forsyth B., Cameron A., Miller S. (1995) Explosives and water quality. Proc. of Sudbury 1995, Vol. 95, 795-803.

• Koren, D. W., Gould, W. D., and Bedard, P. (2000) Biological removal of ammonia and nitrate from simulated mine and mill effluents. Hydrometallurgy, 56(2), 127-144.

• Morin K. A. and Hutt N.M. (2009) Mine-water leaching of nitrogen species from explosive residues. Proc. GeoHalifax 2009.

• Revey, G.F. (1996) Practical methods to control explosives losses and reduce ammonia and nitrate levels in mine water. Mining Engineering, July, p. 61-64.

Questions?

Breakdown of cyanide

CN-OCN-

SCN-

NH4+ + HCO3

-

NO3-

cyanide

cyanate

thiocyanate

ammonium

nitrateNO2-

SO42-

HCN

NOx(g) in air

VOLATILIZATION

OXIDATIONpH < 9

ANFO

Stable isotope composition of different forms of nitrate