Case Studies Chapter 7

From GARDGuide
1. The Argo Tunnel - Pulsed Limestone Bed Treatment
2. Bisbee No. 7 stockpile – BioSulphide process
3. Equity Silver – High Density Sludge Treatment Plant
4. Keystone Mine – Constructed Wetlands
5. Brukunga Pyrite Mine Site - High Density Sludge Lime Neutralization

1. The Argo Tunnel - Pulsed Limestone Bed Treatment



The Argo Tunnel is located in Idaho Springs, Clear Creek County, Colorado, approximately 30 miles west of Denver. The tunnel was constructed to provide drainage and transportation for several connected gold mines. The tunnel continues to drain acidic mine water at an average rate of 280 gallons per minute. The environmental media affected are surface water and, to a much lesser extent, groundwater.

Treatment Applied

A conventional lime water treatment plant was constructed in 1998 and has been operating continuously. Primary contaminants include acidity and a host of heavy metals, including aluminum, copper, iron, manganese and zinc.

A pilot treatment system was operated and studied periodically from 2004 through 2007 by the United States Geological Survey (USGS) Leetown Science Center utilizing a pulsed limestone bed treatment system at 230 L/min.



Metals removal for iron and aluminum was >98%. Copper had removals of 50 to >99%, while zinc had removals from 5 to 65%. Manganese concentrations were generally unaffected. The effluent of the limestone reactor required post-treatment with lime to raise the pH high enough to remove zinc and manganese to dischargeable levels. The sludge from the limestone/lime treatment scheme had settled volumes that were 60% of the lime treatment alone.


Sibrell, P. L., T. R. Wildeman, M. Frienmuth, M. Chambers, and D. Bless. 2005. “Demonstration of a Pulsed Limestone Bed Process for the Treatment of Acid Mine Drainage at the Argo Tunnel Site.” Abstract.

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2. Bisbee No. 7 stockpile – BioSulphide process


The Copper Queen Mine closed in the 1970s after nearly one hundred years of mining. One of the major issues at this site was drainage from a large ore stockpile (No. 7). This drainage was optimal for BioSulphide treatment due to its flow rate and copper concentration. The plant was commissioned in 2004.

Treatment Applied

BioteQ and Phelps Dodge have a Joint Venture to use the process to recover copper at Bisbee, Arizona. The fully commissioned BioSulphide® plant recovers copper from dump drainage. The resulting concentrate (50% Cu) reports to the Miami smelter for profitable water treatment. The plant has a design capacity of 3.6 tonnes Cu/day




The feed water to the plant contains 0.5 to 2 g/L iron and 340 mg/L copper at a pH of 2.2. After treatment, the effluent contains less than 1 mg/L copper and iron. The plant is currently recovering more than 2 tonnes Cu per day.


Nelson L. Ashe, Ian McLean, and Max Nodwell. 2008. “Review of Operations of the Biosulphide® Process Plant at the Copper Queen Mine, Bisbee, Arizona”. In Hydrometallurgy 2008 - 6th International Symposium - Honoring Robert Shoemaker. Editors Courtney A. Young, Patrick R. Taylor, Corby G. Anderson - 2008 - Technology & Engineering - 1186 pages

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3. Equity Silver – High Density Sludge Treatment Plant


The Equity Silver mine is a former open pit and underground mine, located 35 kilometres southeast of Houston in north central British Columbia. The Equity Silver mine operated from 1980 to 1994 and then closed due to depletion of the economic resource. The mining occurred from three open pits and a small underground mine. Copper, silver and gold were extracted through a conventional mill flotation circuit plus a cyanide leach circuit.

Shortly after the mine opened, acidic drainage was found to be occurring from the oxidization of sulphide minerals contained in the mined rock. Equity Silver’s original low density sludge process was unable to handle the usually large runoff events so a new high density sludge (HDS) plant was commissioned to treat acidic drainage post closure.

Treatment Applied

Installation of a conventional high density planted was completed in 2004 at a cost of $10M. The 600 m3/h water treatment plant started up in December 2004, with placement of the treatment sludge in an abandoned pit. The plant was designed for full automation and remote control.


Design Feed

Permit Limits



6.5 to 9.5



Al (mg/L)



Cu (mg/L)



Fe (mg/L)



Zn (mg/L)



SO4 (mg/L)


Cd (mg/L)



As (mg/L)




The effluent discharge consistently meets regulatory compliance.

Treatment Statistics






Drainage treated (m3)



Average acidity (mg/L)



Sludge produced (m3)



Water discharges (m3)




Goldcorp. 2008. Sustainability Report.

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4. Keystone Mine – Constructed Wetlands


The Keystone Mine was owned by Silver King Mines Inc., of Salt Lake City, Utah. The mine produced mainly copper from 1923 to 1925 and includes two adits and 2,000 ft of drifts and crosscuts in an area of 15 acres. The extraction of large quantities of ore from the mine resulted in extensive development of the underground workings. These workings discharge two miles upstream of the confluence with Lake Shasta, with copper, cadmium, and zinc the constituents of concern.

Treatment Applied

The typical metal concentrations and ranges of discharge are shown below.



5 gpm discharge

10 gpm discharge






2–13 mg/L




3–21 mg/L




0.02–0.12 mg/L




9–140 mg/L



In 1989, a constructed wetlands treatment system was commissioned. It consists of a vertical flow of water treated using anaerobic conditions at the base to precipitate heavy metals as sulphides. The technology is designed to treat the drainage in perpetuity. The system uses a ditch design with a topsoil substrate. The retention time through the 4200 square meter (1 m deep) system is 0.3 days at a flow rate of 8600 L/minute. The treatment system was constructed for $2M US. Operation and maintenance (O&M) are estimated at $10,000 per year indefinitely.


The constructed wetlands system has an efficiency of 90%. Details of its performance are presented below.



Quantity removed in
a 5 gpm discharge

Quantity removed in
a 10 gpm discharge


















Robert S. Hedin, Robert W. Nairn, and Robert L. P. Kleinmann. . Passive Treatment of Coal Mine Drainage. U.S. Dept. of the Interior, Bureau of Mines, 1994 - Technology & Engineering - 35 pages.

ITRC website - retrieved on May 5, 2012 from -

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5. Brukunga Pyrite Mine Site - High Density Sludge Lime Neutralization

Improved ARD Treatment With A High Density Sludge (HDS) Lime Neutralization System At The Brukunga Pyrite Mine Site, South Australia

An ARD Treatment case study prepared by Earth Systems Pty. Ltd.

Site history

The Brukunga pyrite mine is located approximately 40 km east of Adelaide in the Mount Lofty Range of South Australia. Pyrite and pyrrhotite were mined from 1955 to 1972 to supply feedstock for sulfuric acid production for the South Australian fertiliser industry. In August 1977 the South Australian State Government accepted responsibility for rehabilitation of Brukunga, with the Department of Manufacturing, Innovation, Trade, Resources and Energy (DMITRE) currently tasked with the management and remediation of the site.

Acid Rock Drainage (ARD) has been a significant issue at the Brukunga mine as a result of the oxidation of pyrite and pyrrhotite within the waste rock piles, tailings and pit highwall. This process continues to generate acidic water, characterised by pH values from of 2.5 to 2.9 and elevated acidities ranging from 2,500 to 12,000 mg/L CaCO3. ARD contains sulphate concentrations ranging between 6,000 and 10,000 mg/L, and elevated soluble Fe, Al and Mn concentrations, which comprise the key constituents of the metal acidity. An average of approximately 2.0 tonnes of H2SO4 acidity is generated by the site each day. It has been conservatively estimated that acidity generation will continue for hundreds of years, and unless a comprehensive remediation strategy is implemented, water treatment in perpetuity is the only option for environmental protection.

Since 1980, the ARD affected seepage from the tailings storage facility and mine site have been directed to collection ponds, and then pumped to a central water treatment facility. The original water treatment plant commissioned by the State Government in 1980 was a low-density sludge (LDS) lime neutralization system, designed to treat a maximum of 20kL/h of ARD affected water.

The capacity of the treatment plant was often exceeded, by up to 25KL/h, due to high flows in the mine creek during high intensity rainfall events. In 2003, to assist with improved mine water management, a river diversion system was installed to divert unpolluted water from above the mine away from the sulfidic waste materials. Improvements in the treatment plant were also designed and implemented.

Treatment plant upgrade

In 2004 the South Australian Government commissioned an upgrade of the LDS plant. The key objectives of the WTP upgrade included:

  • Improve the hydraulic capacity of WTP to better manage heavy rainfall events.
  • Improve the neutralization and precipitate settling capacity of WTP to effectively treat the higher acidity loads reporting to the plant following construction of the clean water diversion system around the mine.
  • Minimise sludge volumes produced at the plant in order to reduce pumping, storage, dewatering and handling costs.
  • Improve the quality of discharge water by enhancing the oxidation of soluble Fe2+ and Mn2+ within the plant.
  • Optimise water treatment to minimise operating costs.

The objectives of the WTP upgrade were met by transforming the existing LDS plant to a High Density Sludge (HDS) system, and installing additional hydraulic capacity with HDS capability. The HDS conversion strategy involved recirculating treatment precipitates (sludge) back into untreated ARD prior hydrated lime addition.

The advantages of using the HDS approach include:

  • Ability to deal with mine waters characterised by high acidity.
  • A substantial reduction in sludge volume resulting from increases in sludge density from 2-6 wt.% solids up to 40 wt.% solids.
  • Reduced sludge management costs.
  • Slightly reduced reagent costs.
  • Ability of HDS precipitates to settle faster minimises sizing requirements for thickeners/clarifiers.

Detailed chemical testwork was conducted on site to identify the optimum operating parameters for the HDS upgrade.

HDS plant operational parameters

Results from the testwork and a site water balance identified the following process variables for the new HDS system:

  • Total HDS treatment capacity: 50kL/h.
  • Sludge recycle rate: 10-15kL/h.
  • Recycled sludge and raw ARD residence time in Reactor 1: 10-15 minutes.
  • Overflow pH of ARD/sludge mix from Reactor 1 = 4-7 pH units.
  • Target pH in Reactor 2 using hydrated lime slurry addition is ~10.
  • Recycled sludge, raw ARD and lime slurry residence time in Reactor 2: 15 minutes.
  • Aeration rate in Reactor 2 =110 m3/h.
  • Recycled sludge, raw ARD and hydrated lime residence time in Reactor 3: 15 minutes.
  • Aeration rate in Reactor 3 =110 m3/h.
  • Anionic polymer flocculant addition to Reactor 3 overflow prior to thickener.
  • Target pH = 9.2-9.5 in supernatant overflow from thickener.
  • Sludge density: 25-40 wt% at base of thickener unit; and >50 wt% after dewatering in the drying ponds.

The HDS plant at Brukunga has been consistently meeting its design objectives both in terms of treated water quality and sludge density for the past 7 years.

The future

While the total volume of sludge produced at Brukunga has decreased substantially, the total tonnage remains unchanged and hence issues associated with its management and safe storage continue. Approximately 3.0-4.0 tonnes (dry basis) of gypsum-rich sludge are produced for every tonne of hydrated lime (dry basis) added in the plant. Hence, an estimated 2,000-3,000 tonnes of dry sludge are generated annually and disposed against the highwall at the mine site (Figure 1). The mine site currently hosts close to 60,000 tonnes of dry sludge material. Each additional 100 years of water treatment can be expected to add a further 200,000-300,000 tonnes of relatively soluble gypsum-rich sludge to the mine site.

While treatment in perpetuity can often represent the lowest cost option for dealing with ARD on a Net Present Value basis, it sustains residual risk for the South Australian Government associated with the obligation for unfailing treatment in perpetuity and a growing stockpile of unstable metalliferous sludge.

To better manage these risks and community expectations, DMITRE has engaged in a process to devise a comprehensive remediation strategy for the site. A key aim of rehabilitation is to devise and implement a strategy that supersedes the need for perpetual treatment.

Figure 1: Dry treatment sludge stored against the highwall at the Brukunga mine site.

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