Wellington-Oro mine water treatment plant

Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi

Introduction

Description of the site

The French Gulch/Wellington-Oro mine site is located in the town of Breckenridge, in Summit County, Colorado, USA. The mine is situated in Rocky Mountains in a valley hosting several abandoned mine sites of which the Wellington-Oro mining complex is the largest one. The mine is 3.5 km upstream of the confluence of alpine stream French Gulch and the Blue River which is a famous trout stream. The main water source to French Gulch is snowmelt runoff, the highest flow rates occurring in May and June. In some parts of the French Gulch a major part of the flow goes below the surface, especially during summer. (Kimball et al. 1999, USEPA 2013a)

The overall climate in the region is considered high-alpine with the average maximum daytime temperatures around 21°C in July-August and around -1°C in December-February. The yearly precipitation is ca. 500 mm. (Western Regional Climate Center 2014)

The local geology comprises bedrock ore with several faults cutting through the site area. The water surface in the mine area’s western limit is above the French Gulch water level which causes mine water to discharge to the valley via bedrock faults and fractures, shallow groundwater flow and from springs discharging mine water all year round. (USEPA 2013a)

The main mining activities at the site took place between 1880’s and 1930’s and the mining ceased completely in 1972. Pb, Zn, Cu, Ag and Au ores were extracted, ca. 50% of which below the groundwater table. Ore was concentrated in a gravity mill (1908-1929, capacity 100 t/d) and iron and sulphur were removed from the Zn ore in a roaster and magnetic separation plant (1912-1927, capacity 50 t/d). Flotation mill was commissioned in 1927. The abandoned mine contains over 20 km of tunnels, adits, drifts and stopes most of which are flooded today. (International mining 2009, Berger et al. 2012, USEPA 2013a)

Need for water treatment and other remedial actions

The investigations on the environmental contamination at the Wellington-Oro mine site begun in the late 1980’s. In 1989 the site was identified as a potential Superfund site by US Environmental Protection Agency (USEPA). Superfund is an USEPA programme and a related fund which addresses abandoned hazardous waste sites and allows USEPA to clean up those sites and to compel responsible parties to carry out decontamination. In the Superfund process the site is assessed, placed on the National Priorities list and appropriate, often long-term clean-up plans are put together and implemented. In the Wellington-Oro site a specific community-based approach was applied and a comprehensive stakeholder group was formed in 1995 to develop plans to address mining related environmental impacts within French Gulch area. As an implication of the Superfund process, the town of Breckenridge and Summit County purchased the land to be developed as a public recreational area in 2005. (International mining 2009, Berger et al. 2012, USEPA 2013a, USEPA 2013b)

A series of Engineering Evaluation/Cost Analyses (EE/CA) were undertaken with different focuses in order to evaluate the remedial activities to be carried out at the Wellington-Oro site. As for solid wastes at the site, an administrative order was given to remove the mine wastes (roaster fines, mill tailings and waste rock) from the site in order to reduce potential for human contact. The materials were capped with impermeable clay and clean gravel and drainage system was installed to reduce water infiltration. The work was finalized in 1999. (USEPA 2013a, USEPA 2014)

Flooded underground workings were identified as the largest source of metals (primarily Zn and Cd) loading to groundwater and surface water. Mine water, primarily its high zinc content, was severely affecting important downstream fish populations in French Gulch, a tributary of the Blue River. In the investigations in the late 1980’s, Wellington-Oro Mine pool was identified as the major contributor of zinc and cadmium loading from French Gulch into the Blue River. A natural seep of groundwater flowing year round, FG-6C, was the major route of mine pool water into French Gulch. Thus, seep FG-6C was defined as the target for water treatment activities. (USEPA 2013a, USEPA 2014)

Mine water quality and treatment objectives

According to discharge monitoring data 2008-2012, the average concentration of Zn in the FG-6C was ca. 154 mg/l at flow rate ca. 180 l/min (peak flows during spring runoff up to 1900 l/min). The aim of the remedy activities at Wellington-Oro mine is to improve the water quality in the Blue River by limiting the concentration of dissolved Cd and Zn in river water to 4.0 μg/l and 225 μg/l, respectively. These values were also set as the discharge standards of the treatment plant. (USEPA 2013a, USEPA 2014)

The purpose of the water treatment plant is to ensure the compliance of discharged water to discharge standards. Another, secondary target is to recover Zn and Cd. (International Mining 2009)

Description of the technology

In order to find the best available technology for water treatment at the site, an international call for proposals was put in place in 2005. A number of proposals were assessed with regard to e.g. their performance and costs. Sulphide precipitation was evaluated as most suitable as it reduces metals to very low concentrations sufficient to meet the effluent quality requirements. Furthermore, the produced sludge can be utilized and thus requires no costly disposal. According to BioteQ (Bratty et al. 2008), inorganic sulphide source is often more feasible in smaller scale installations of sulphide precipitation process such as Wellington-Oro mine compared to biogenic sulphide as the additional capital costs of a bioreactor would in many cases be too high. (International mining 2009, BioteQ 2013)

The project was started with the operation of one of BioteQ’s mobile plants at the site in order to confirm design criteria and the quality of the sludge formed. The full scale ChemSulphide® plant at Wellington-Oro mine was commissioned in 2008. Process design, commissioning, training and engineering operational support were provided by BioteQ. The operation of the plant including all collection and conveyance systems and site maintenance is the responsibility of the Breckenridge town’s water division. The operation is funded jointly by Summit County and the town. EPA and Colorado Department of Health and Environment oversee the treatment plant operation. The produced sludge is shipped to a smelter and the treated water is discharged to the environment. (International mining 2009, BioteQ 2011, BioteQ 2013, USEPA 2013a)

The basic process configuration is rather simple consisting of feed buffer tank, sulphide precipitation reactors, flocculation tank, sludge conditioning tank, clarifier, filter press and granular media pressure filters as a polishing step. Figure 1 shows a schematic flow chart of the process. The reagents include NaHS as the precipitation agent, Na₂CO₃ (soda ash) for pH control, flocculent and FeCl₃. NaHS arrives to the plant as 35-44% solution whereas soda ash and flocculent are prepared on site from dry materials. Reagent dosing is carried out by automated system. Sulphide dosing is controlled so that zinc and cadmium are removed to discharge limit but no excess H₂S gas is formed and not too much iron is precipitated. pH is adjusted to the optimal range for sulphide precipitation by soda ash addition. (ITRC 2010, Hall 2012, USEPA 2013a)

The volume of contactors (main reaction vessels) is 7 m³ each and they are connected in series. In contactors the influent metals react with sulphide to form metal sulphide precipitates, mainly ZnS, CdS and PbS. The aim is to avoid Fe precipitation as the smelter prefers to use iron poor material in its process. The dosing of NaHS is controlled by the redox potential and dosing of soda ash by the measured pH level. After precipitation flocculent is added and precipitated solids are separated in a clarifier (diameter 4.9 m, depth 2.4 m). Part of the sludge is recycled back to the sludge conditioning tank, where additional NaHS is dosed, and further to the first contactor. Overflow from the clarifier flows to the filter feed tank which also has a possibility for pH adjustment. (USEPA 2013a)

Four granular mixed media (containing anthracite and sand) filters operate in parallel. The area of filters is 0.3 m² each. Each filter is backflushed daily and meanwhile three other filters remain operational. The backflush water is provided by the three filters in service. In case of filter malfunction the plant operates in recycle mode where the effluent is pumped back to the mine pool. The building is compact due to local zoning limitations. (USEPA 2013a)

Figure 1. Schematic flow chart of the Wellington-Oro water treatment plant (modified from USEPA 2013a)

Performance

Capacity

The design capacity of the water treatment plant is 816 m³/d (150 gpm, maximum pumping rate) and it is designed to operate 24 h/d, 7 d/week. At spring peaks the flow rate of seep FG-6C is expected to exceed the plant capacity; the overflow then bypasses the plant and is directed to the Blue River.  In 2012, total 45,000 m³ mine water was treated and ca. 10.5 t of mixed Zn/Cd concentrate was recovered. (BioteQ 2013) Thus, in that year the plant utilized only ca. 15% of the full designed volumetric treatment capacity. The plant typically operates at less than 50% capacity. Therefore, the plant could receive and treat additional mine water if necessary. (USEPA 2013a, USEPA 2014)

Treatment results

BioteQ (2013) reports the performance of the treatment plant as presented in table 1.

Table 1. Wellington Oro Mine Water Chemistry (BioteQ 2013)

At filter down time the treatment result for Zn is ca. 1 mg/l and thus the limit value for Zn is not met. In these cases, the discharge is diverted to the mine pool. (USEPA 2013a)

The efficiency of the plant to improve the status of the Blue River could not be evaluated by USEPA review (2013a) as the continuous operational period of the plant was still rather short. Furthermore, metals loading from other sources are not known. The flow of FG-6C represents ca. 2% of the total flow of the French Gulch.

Waste generation and management

The sulphide sludge (10.5 t in 2012) is removed from the clarifier and dewatered in a filter press to 35-50% DS. According to the Toxicity Characterization Leaching Procedure (TCLP) the sludge is classified as non-hazardous waste and therefore it may be disposed of in a municipal landfill. Another disposal option is the abandoned mine workings. However, the quality of the sludge allows its utilization in a smelter for zinc recovery. (ITRC 2010, USEPA 2013a, USEPA 2014)

The filter cake contains 50-57% zinc and ca. 38% of sulphur on a dry weight basis. The dried sludge is stored in plastic lined sacks and has been shipped to a smelter Nyrstar in Clarksville, Tennessee which is the primary zinc producer in the US. Nyrstar’s specifications for the material include the zinc concentration >50% of the dry weight and low levels of other metals. Thus the produced sludge is acceptable to the Nyrstar smelter and the process does not generate any residuals requiring disposal. (Whysner et al. 2012, USEPA 2013a)

The smelter has covered the shipping costs (0.31 USD/kg Zn content) and paid additional 0.33 USD/kg Zn content for purchasing the sludge. The total potential of zinc recovery (based on the design flow rate and influent Zn concentration) of the plant would be ca. 40 t/a, corresponding 13,200 USD/a on the price level mentioned above. (Smith et al. 2013, USEPA 2013a)

Advantages and disadvantages

Advantages of the sulphide precipitation technology include (Bratty et al. 2008, Lopez et al. 2009):

• The generated sludge can be utilized to recover metals; no need for sludge disposal → lower operating costs
• Metal concentrations of discharged effluent are very low; water reuse possible
• Lower capital costs compared to lime treatment due to faster process kinetics, better settling rate and smaller plant footprint
• Recovered metals may partly offset the treatment costs

General disadvantages of the sulphide precipitation technology include (e.g. INAP 2009, Lewis 2010, Mokone et al. 2012)

• Problems related to potential excess sulphide: toxicity of gases, odours, corrosion; residual sulphide in the effluent
• Dosing of sulphide and process control might be challenging due to the sensitivity of the process (very low solubility of sulphides)

Treatment costs

The capital costs of the plant were approximately ca. 4.3 million USD (ITRC 2010). USEPA (2013a) reported the approximate annual operating costs of the plant at 260,000 USD/a (ca.  0.6 USD/m³ water treated in 2012). The largest cost items were project management and labour (40%) and maintenance/subcontractors (31%). Chemical costs accounted ca. 16% of the total operating costs. The recovered zinc could only offset a minor share of the treatment costs in 2012.

Plant reliability and maintenance needs

According to the USEPA review (2013a) the maintenance costs of the plant are high relative to the size of the plant. This indicates that the plant experiences continuous, non-routine operating problems. These problems are mainly related to mechanical functioning of the plant. There are also issues on the building that could be improved in order to enhance plant performance and working conditions of the staff, according to USEPA (2013a). These include e.g. ventilation, insulation and accessibility of the equipment.

The main difficulty has been the operation of the filters and the short lifetime of the media due to cementing after relatively short periods of operation. The underlying reason for this might be the inadequate efficiency of filter backflushing. The problems have resulted in frequent and longish periods of downtime when the water from FG-6C has been directly or as partially treated pumped back to the mine pool. The filter problems have been the reason for the plant failing to meet the effluent standards. USEPA reviewers proposed the change of the filter type to bag filters which could solve many of the current problems associated with the mixed media filters. (USEPA 2013a)

Another major operation and maintenance problem has been the soda ash dosing system. The formation of solid Na₂CO₃ precipitates causes clogging of the pumps and pipelines even though soda ash is diluted to 8%. The deposits cause additional cleaning work and premature need for replacing system parts. Changing the pH adjustment chemical to NaOH might be an option to reduce these problems. (USEPA 2013a)

Monitoring and control needs

The control system of the plant is a proprietary system by BioteQ. (USEPA 2013a)

The environmental permit (National Pollutant Discharge Elimination System (NPDES)) of the site sets requirements for the monitoring program. In addition to the permit requirements, the operational staff collects extra samples on a monthly basis (marked green in Table 2).

Table 2. Wellington-Oro water treatment plant monitoring programme (USEPA 2013a)

The samples specified in Table 2 are analysed in a laboratory. Operational staff carries out additional on-site analysis daily on influent and influent Zn and Fe concentrations as well as pH. Other process locations are sampled periodically. (USEPA 2013a)

Environmental water samples are collected monthly at three surface water locations and analysed for Zn and Cd. (USEPA 2013a)

Conclusion

The Wellington Oro mine water treatment plant is rather a small scale unit treating mine water from an abandoned metal mine site. The plant was established as a result of EPA Superfund process. Funding for the project was gained from the local community, Town of Breckenridge and the Summit County. One important interest of the community for initiating reclamation process was the plan to develop the land area for housing and recreational activities. A part of the project was financially covered by the former owner of the site, B&B Mines. Thus, the project presents an example of a public-private-partnership for dealing with environmental liabilities of a mine site where active mining ceased already long ago.

The water treatment plant currently utilizes only a part of the designed volumetric capacity. Therefore, it could possibly be used to treat mine water flows from other local sources. Common mine water treatment facilities could be feasible in regions where several separate mines are/have been working and mine water with relatively similar characteristics is generated from multiple sources.

The basic treatment technology, sulphide precipitation, can be evaluated as potentially suitable for mine water treatment in northern conditions as the process is not subject to seasonal variations. The technical problems that the plant has experienced emphasize the need of thorough design and testing phase to evaluate the suitability of the technology for the case in question. In the start-up phase it is important to transfer all the critical information from the technology provider to the plant operators in order to keep the plant running smoothly and effectively.

While today the monetary value of the produced zinc containing sludge and its relevance in the total operating cost budget at Wellington-Oro treatment water plant is rather small, the production of a sealable commodity is an asset supporting the long-term plant operation. In general terms, the feasibility to utilize metal-rich sludge in a smelter depends on several factors, such as the location of the nearest smelter, transportation costs, produced sludge amounts and its quality.

References

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Lopez, O., Sanguinetti, D., Bratty, M. & Kratochvil, D. 2009. Green technologies for sulphate and metal removal in mining and metallurgical effluent Enviromine Conference, October 2009, Santiago, Chile

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Smith, K.S., Figueroa, L.A. & Plumlee, G.S. 2013. Can treatment and disposal costs be reduced through metal recovery? In: Brown, A.; Figueroa, L. & Wolkersdorfer, Ch.: Reliable Mine Water Technology (Vol I). – p. 729 – 735; Denver, Colorado, USA (Publication Printers). Annual International Mine Water Association Conference. Golden, Colorado, USA 2013

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Whysner, K., Figueroa, L., Petri, E. & Holmes, M. 2012. Economic recovery of zinc from Mining Influenced Water (MIW). U.S. EPA Hardrock Mining Conference, April 5, 2012. Available: http://www.clu-in.org/download/issues/mining/Hard_Rock/Thursday_April_5/Opportunities_for_Re-Use_and_Site_Management/01_Whysner.pdf