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
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
Continental Fault Continental Pit (active) Tailings Pond Cementation plant & Horseshoe Bend Berkeley Pit lake
Background
Water inputs: Groundwater (~ 3 million gal/day) Sludge Discharge: (211,000 gal/day) Horseshoe Bend drainage (2000-2003 2-3 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
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
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
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)
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
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
Lake Volume, Gal = times when Horseshoe Bend drainage was diverted to lake Meromictic (?) Holomictic Meromictic Holomictic 8.0E+10 7.2E+10 6.4E+10 5.6E+10 4.8E+10 4.0E+10 Landslide Volume 3.2E+10 2.4E+10 1.6E+10 8.0E+09 0.0E+00 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1985 1984 1983 5460 5260 5060 4860 4660 4460 4260 Critical Water Level (5410 feet) Elevation Lake Elevation, Ft
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
Effect of Cu recovery on Cu concentrations Depth, Ft 0 100 200 300 400 500 600 Cu, ug/l 0 50,000 100,000 150,000 200,000 250,000 10/02 5/05 10/06 11/07 Replenishable source of Cu = chalcocite and other secondary Cu minerals? chalcocite 700 800 900 11/09 05/10 06/08 pyrite Reflected light photomicrograph
Effect of Cu recovery on Fe concentrations Depth, Ft 0 100 200 300 400 500 Fe, mg/l 0 200 400 600 800 1,000 1,200 10/02 5/05 10/06 11/07 600 06/08 700 ~ 4 months solids collection 800 05/10 11/09 900 ~ 150 million lbs of Fe precipitated in 6 years!
Effect of Cu recovery on Fe Speciation Fe II (Pit bottom) Fe III (Surface) Fe III (Pit Bottom) Fe II (Surface)
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 2+ + 10H 2 O + SO 4 2- + 2O 2 = Fe 8 O 8 (OH) 6 SO 4 (s) + 14H +
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, 0.032 mg/l P time 6/08 10/02 0.00 0.25 0.50 0.75 1.00 P, mg/l 0 200 400 600 800 As, g/l
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
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 + + 13SO 4 2- + 8K + 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)
ph buffering by aqueous sulfate? SO 4 2- + 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
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 2+ + 2O 2 + SO 4 2- + 10H 2 O Fe 8 O 8 (OH) 6 SO 4 + 14H + 16 units of acidity 14 units of acidity 2/16, or a 12.5% decrease in total acidity
Same idea, with jarosite 3Fe 2+ + 3/4O 2 + 2SO 4 2- + 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!
jarosite
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!
Treatment plant Sludge Treatment of Berkeley Pit water begins ~ February 2023
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
Questions