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Technology Overview as part of a Web-based Technical and Regulatory Guidance

Chemical Precipitation

1. Introduction
Click Here to view case study table at the end of this document.
Chemical precipitation is a conventional technology used to treat mining-influenced water (MIW), including acid mine drainage, neutral drainage, and pit lake water. Chemical precipitation processes involve the addition of chemical reagents, followed by the separation of the precipitated solids from the cleaned water. Typically, the separation occurs in a clarifier, although separation by filtration or with ceramic or other membranes is also possible. Chemical precipitation can also be used in pit lakes or other water bodies, in which case the precipitated solids can simply remain in the bottom of the pool.

Precipitation can be induced by the addition of an alkali, sulfide, coagulant, or other reagent that will bond with dissolved metal ions. Alkali sources include caustic sodium hydroxide (NaOH), hydrated lime (Ca(OH)2), quick lime (CaO), limestone (CaCO3), and magnesium hydroxide (Mg(OH)2). Sulfide reagents used to cause precipitation of contaminants include iron sulfide (FeS), sodium hydrosulfide (NaHS) (Wellington-Oro Water Treatment Plant), sodium sulfide (Na2S), calcium sulfide (CaS), and biogenic sulfide generated in situ by sulfate reduction. Coagulants can include alum KAl(SO4)2, iron hydroxide (Fe(OH)3), or ferric chloride (FeCl3). Carbonates can also be used in chemical precipitation, including sodium carbonate (Na2CO3), calcium carbonate (CaCO3), or CO2 under pressure (Toby Creek Mine).

Additional methods reported are neutralization using the Rotating Cylinder Treatment System (RCTS) (Leviathan Mine, Sunshine Mine, Cement Creek, American Tunnel, Inactive Copper Mine in Vermont, Zortman Landusky), FeCl3 for arsenic removal (Lava Cap Mine), use of CO2 under pressure (Toby Creek Mine), and a limestone/steel slag system (Ohio Mines). Advantages of steel slag are low cost and less degradation over time than limestone alone. The steel slag produces extreme alkalinity and can precipitate manganese and other trace metals. Some metals may be removed by co-precipitation with iron or aluminum oxyhydroxide species.

The technologies discussed in this guidance are based on representative case studies. Additional information about this technology and other acid drainage treatments can be found in the GARD Guide (INAP 2009).

2. Applicability
Chemical precipitation technology is applicable to the following situations:

Chemical precipitation is a flexible permanent technology that can address metal contamination in MIW at mine sites. This technology can be used in conjunction with other treatments or by itself, depending on site conditions. The treatment system can be designed to deal with a variety of site conditions. The optimal process and its efficiency depend on several factors, including flow rate or volume, contaminants and their concentrations, other water parameters, discharge criteria, site access, and sludge disposal options.

Chemical precipitation is a standard treatment technique used across the United States and around the world. At least 12 case studies were received, and all show that it was successful. Additional lab-scale technology studies were also reported.

3. Advantages
Chemical precipitation has the following advantages:

Chemical precipitation offers many advantages as a treatment alternative. It is able to meet stringent discharge criteria. It has been used effectively for many years. The design of the treatment process can be customized and thus can be used in a variety of situations. Chemical precipitation is a long-term remedy that can address both acute and chronic risks to human and ecological receptors. It provides a relatively rapid effect in the reduction of contamination in downgradient surface water bodies. Advances in remote monitoring have increased the ability for chemical precipitation to be used in locations previously prohibitive.

4. Limitations

Disadvantages or limitations of chemical precipitation include the traditionally active nature of the process. Chemical reagents need to be procured, energy inputs and manual oversight are required, and a waste stream is generated. These can equate to a relatively high cost for treatment.

5. Performance
Chemical precipitation is a proven, large-scale technology that offers permanent results. It is applicable for many mine drainage sites and can be used solo or in conjunction with other treatment technologies. Chemical precipitation has demonstrated achievement of stringent water quality limits in acid mine discharges and has reduced/eliminated migration of metal contaminants to downgradient water bodies, wetlands, and watersheds. The performance and results are specific to initial water quality and site limitations. Each site must be fully characterized physically and chemically to ensure that the right technology(s) are being used and state regulations are met.

6. Costs
Chemical precipitation is generally considered a high-cost treatment. Impacts to cost result from variables, including flow rate, contaminants being treated, quantity and characteristic of sludge generated, variability of contaminant concentrations, reagents used, labor demand, etc.

Reagent cost will greatly impact O&M costs of treatment. In September 2000, the National Lime Association reported a comparison of costs. Sodium hydroxide was reported to cost $228 to neutralize 1 ton of sulfuric acid. Magnesium hydroxide was $179. Calcium hydroxide cost $66, while calcium oxide was the lowest of the reagents compared at $37.

The Wellington-Oro Treatment Plant operates at a low of 50 gpm to a high of 150 gpm, treating for zinc and cadmium at a pH of 6.4. The capitol cost associated with this plant is approximately $4.3 million. It has not been in operation long enough to report O&M costs.

7. Regulatory Considerations
A National Pollution Discharge Elimination System (NPDES) permit may be required under the Clean Water Act. An active and ongoing treatment process may require oversight of a certified operator. Additionally, the quantity of chemicals and sludge contained on site may trigger regulation under the Resource Conservation and Recovery Act (RCRA). Disposal of the waste sludge must comply with applicable regulations. Additional treatment mechanisms should be considered when the primary treatment system cannot achieve regulatory standards or state regulators will not agree to alternative abatement standards for individual cases.

8. Stakeholder Considerations
Chemical precipitation is a well-accepted technology which offers permanent results for metal contaminants removal and achieves stringent discharge limits that are protective of public health and the environment.

The Wellington-Oro site was identified a potential Superfund site in 1989, which eventually galvanized a community-based team to review treatment options at the site. This resulted in a unique settlement agreement that enabled the land to be purchased for public open space, while providing for treatment of contaminated water emanating from the site.

9. Lessons Learned
In some cases, an effective industry/regulatory working group with regular meetings, intervening conference calls, and general correspondence greatly assist with constructive feedback and generate public cooperation towards design/implementation of the remediation project. Once projects are approved, technologies can be improved upon and enhanced for other applications.

10. Case Studies

Table 10-1. Case studies using chemical precipitation (full scale)

11. References
INAP (International Network for Acid Prevention). 2009. The Global Acid Rock Drainage (GARD) Guide.

USEPA (United States Environmental Protection Agency) Region 8. n.d. “Superfund Program: French Gulch.”

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