Influence of copper recovery on the limnology and geochemistry of the Berkeley Pit lake, Butte, Montana

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1 Influence of copper recovery on the limnology and geochemistry of the Berkeley Pit lake, Butte, Montana Nick Tucci Montana Bureau of Mines & Geology Copper recovery return water into Berkeley Pit Chris Gammons, Montana Tech Butte, MT Duaime, T.E., Tucci, N.J. (2011) Butte Mine Flooding Operable Unit Water-Level Monitoring and Water-Quality Sampling Consent Decree Update Butte, MT, MBMG Open File Report #599

2 Critical Level 5410 Berkeley Pit, 1982 Background Porphyry copper mine Dewatering pumps turned off in 1982 Pit lake began forming in 1983 Main water inputs Groundwater from surrounding underground mines Surface water diversions Precipitation < Evaporation Critical lake level = 5410 Expected to reach C.L. > 2023 Remedy in perpituity HDS lime treatment

3 Continental Fault Continental Pit (active) Tailings Pond Cementation plant & Horseshoe Bend Berkeley Pit lake

4 Background

5 Water inputs: Groundwater (~ 3 million gal/day) Sludge Discharge: (211,000 gal/day) Horseshoe Bend drainage ( million gal/day) Precipitation (~ 12 /yr) Storm runoff Water output: Evaporation (~ 24 /yr) North Sludge Discharge P E Pre-mining water table South minor Evapo-concentration evapoconcentration GW Seepage Berkeley Pit Lake

7 Monitoring the Berkeley Pit lake Monitored by MBMG Over 700 ft deep Over 40 Billion Gallons No outlet (terminal lake) Poor water quality 12 g/l TDS ph 2.5 to 2.7 Fe: 500 to 1000 mg/l Zn: 600 mg/l Cu: 70 to 150 mg/l

8 Berkeley Pit (not drawn to scale) Black and Tan Ale (unfortunately, not to scale either) Cu 2+ Cd 2+ Cu 2+ Fe 3+ Mg 2 + Chemocline

9 Berkeley Pit lake: Conceptual Model landslides photochemical reactions? O 2 diffusion evaporation rain, snow surface inputs Horseshoe Bend Spring lime treatment sludge Cu recovery return flow storm runoff Cu cementation water table leaching of soluble salts from weathered bedrock adsorption, subaqueous pyrite oxidation Fe 2+ Fe 3+ schwertmannite, jarosite Fe-oxidizing seasonal bacteria overturn H +, Fe 2+ gravitational settling deep groundwater influx Fe 3+ Cu recovery intake epilimnion hypolimnion monimolimnion pit sediment Gammons & Duaime, 2006 Flooded underground mine workings (warm water)

10 Copper Recovery Process Process developed in Butte, MT: W. Ledford (1890) Cu 2+ + scrap Fe Fe 2+ + Cu Cu plated steel rails in underground mine water Lexington Tunnel, Butte

11 Resource Recovery: Copper Cementation ~ 150 ppm Cu ~ 30 ppm Cu Berkeley Pit-lake Cu 2+ + scrap Fe Fe 2+ + Cu Over 40 million lbs Cu dissolved in B-Pit prior to recovery Process is 75-90% efficient

13 Lake Volume, Gal = times when Horseshoe Bend drainage was diverted to lake Meromictic (?) Holomictic Meromictic Holomictic 8.0E E E E E E+10 Landslide Volume 3.2E E E E E Critical Water Level (5410 feet) Elevation Lake Elevation, Ft

14 Effect of Cu recovery on limnology Over 31 billion gallons cycled in 6 years Mixolimnion Fe III >Fe II [Cu] = 60mg/L Cu-depleted Fe Rich water returning to lake Pit Bottom

16 Effect of Cu recovery on Cu concentrations Depth, Ft Cu, ug/l 0 50, , , , ,000 10/02 5/05 10/06 11/07 Replenishable source of Cu = chalcocite and other secondary Cu minerals? chalcocite /09 05/10 06/08 pyrite Reflected light photomicrograph

17 Effect of Cu recovery on Fe concentrations Depth, Ft Fe, mg/l ,000 1,200 10/02 5/05 10/06 11/ / ~ 4 months solids collection /10 11/ ~ 150 million lbs of Fe precipitated in 6 years!

18 Effect of Cu recovery on Fe Speciation Fe II (Pit bottom) Fe III (Surface) Fe III (Pit Bottom) Fe II (Surface)

19 Cu 2+ + Fe Fe 2+ + Cu Cu cementation O 2 diffusion schwertmannite Fe 3+ Fe 2+ epilimnion gravitational settling Cu recovery intake hypolimnion pit sediment 8Fe H 2 O + SO O 2 = Fe 8 O 8 (OH) 6 SO 4 (s) + 14H +

20 Scavenging of PO 4 3, AsO 4 3 by Fe III oxy hydroxides 2002 IDL, 0.5 mg/l P Depth, ft. m 11/09 time 2009 IDL, mg/l P time 6/08 10/ P, mg/l As, g/l

21 Sorption onto secondary Fe(III) minerals at ph 2.5, expect sorption of anions Cu cementation O 2 diffusion schwertmannite Fe 3+ Fe 2+ epilimnion H 2 AsO 4 - H 2 PO 4 - SO 4-2 hypolimnion pit sediment Cu recovery intake

22 Haven t seen a drop in ph ph buffered by secondary Fe minerals? 3Fe 8 O 8 (OH) 6 SO 4 + 6H 2 O + 18H SO K + schwertmannite 8KFe 3 (OH) 6 (SO 4 ) 2 K-jarosite Pit waters close to equilibrium with schwertmannite Slightly supersaturated with K jarosite Schwertmannite transforms to K jarosite in pit sediment (Twidwell et al. 2006)

23 ph buffering by aqueous sulfate? SO H + HSO 4 - ~ 8000 mg/l SO 4 (> 80 mmol/l) At ph 2.5, about 5 to 10% of SO 4 is protonated Fast reaction

24 Effect of Cu recovery on total acidity of lake Total acidity = [H + + HSO 4- ] + 2 [Fe 2+ + Cu 2+ + Zn 2+ + Mn 2+ ] + 3 [Fe 3+ + Al 3+ ] [ ] = mmol/l concentrations Precipitation of schwertmannite 8Fe O 2 + SO H 2 O Fe 8 O 8 (OH) 6 SO H + 16 units of acidity 14 units of acidity 2/16, or a 12.5% decrease in total acidity

25 Same idea, with jarosite 3Fe /4O 2 + 2SO K + + 9/2H 2 O KFe 3 (OH) 6 (SO 4 ) 2 + 3H + 6 units of acidity 3 units of acidity 3/6, or a 50% decrease in total acidity!

26 jarosite

27 In other words Acidity is being transferred from aqueous solution to solid precipitates, which then settle to the bottom of the lake Could be a significant savings in lime when treatment begins New acidity titrations of Berkeley Pit water are needed to quantify this!

28 Treatment plant Sludge Treatment of Berkeley Pit water begins ~ February 2023

29 Summary 6 years of Cu recovery have eliminated vertical stratification in the Berkeley Pit lake Dissolved Cu and Fe concentrations have been cut in half Dissolved P and As concentrations have decreased about an order of magnitude Total acidity has decreased

30 Questions

Neutralization of Acid Mine Drainage Using Stabilized Flue Gas Desulfurization Material

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