Offsetting treatment costs with metals recovery
Wastewater treatment is a necessary cost of doing business in the mining industry. Mining involves numerous processes to extract and recover minerals or metals from the earth to produce items we use on a daily basis. Common to all ore extraction methods is the generation of large volumes of wastewater streams containing elevated concentrations of dissolved metals. Due to the lasting liability these waste streams pose to habitats, wildlife and people, they are fully regulated in many jurisdictions and require costly treatment before being reused or discharged to the environment.
According to Global Water Intelligence’s 2011 report, “Water for Mining,” total global expenditures for water infrastructure in the mining industry were estimated at $7.7 billion for the year. This amount is projected to grow to $13.6 billion by 2014. Driving this demand is a booming industry looking to treat wastewater to meet increasingly strict regulations and stronger commitments to corporate social responsibility. Mining companies can no longer leave behind a legacy of wastewater problems.
A cost center line item, wastewater treatment also can have a significant impact on the life-cycle costs of the operation. This is an industry in which remediation can continue to be required after the mine has been shut down. Technology innovations, however, are making it possible for these waste streams to generate revenue through metals recovery while being treated to comply with water quality discharge limits. This revenue can be applied toward the cost of waste- water treatment and improve the bottom line.
Traditional Treatment Limitations
Lime neutralization is the most common method used to reduce the concentration of dissolved metals in mining effluents. But the changing nature of the industry is limited by this method. The process works by adding lime or limestone to neutralize the acidity of the wastewater and precipitate the dissolved metals as a waste sludge. Because of the non-selective nature of the lime process, all metals are removed together as a hydroxide sludge that cannot be processed further to recover the metal value. Rather, the sludge must be stored in an impoundment designed for long-term storage.
The amount of sludge produced from this process can be voluminous. In Canada alone, it has been estimated that as much as 6.7 million cu meters of sludge is produced annually from mining. While the use of lime removes dissolved metals from wastewater, it essentially only transfers the metals from one waste medium to another. Preventing the metals in the sludge from resolubilizing and re-entering the environment can represent an ongoing environmental and financial liability, long after the mine has ceased operations.
It is apparent that new approaches for treating dis- solved metals in mining effluents are required. Among the options are sulfide precipitation processes such as BioSulphide and ChemSulphide to treat metal-laden wastewater to comply with water quality regulations and, in the process, reduce or eliminate sludge production and generate revenue from concurrent metal recovery. These technologies use either biological or chemical sulfide sources to selectively recover dissolved metals from wastewater, producing a high-grade saleable metal product and treated water.
Sulfide precipitation works by introducing a sulfide re- agent to wastewater in a contactor tank where conditions are adjusted to precipitate individual metals. From there, the precipitated metals and treated water are pumped to a clarifier. The treated water is separated from the metal solids for reuse or discharge and the metal solids are filtered to remove excess water to produce a saleable metal product. If more than one metal is to be recovered, multiple circuits can be placed in series.
Both technologies have been successfully applied at sites around the world to clean heavy metals from mining effluent and recapture the metal value in these streams. In doing so, sulfide precipitation processes have improved process efficiency and as a result, reduced the cost of environmental remediation and improved the life-cycle costs of the operation.
One project that has benefited from sulfide precipitation technology is the Dexing Mine, an active copper mine in China. Prompted by a government-wide initiative to improve the country’s environmental performance for the 2008 Olympics, Jiangxi Copper Co., the largest copper producer in China, led the effort by targeting the site for a new wastewater treatment plant.
The Dexing Mine produces 120,000 tons of copper annually. During production, low-grade ore waste is dumped into stockpiles that generate copper-laden acid mine drainage. To treat the wastewater, a 1,000-cu-meter per-hour ChemSulphide plant was commissioned in the spring of 2008. Not only does the plant successfully treat the effluent to reduce copper concentration levels, it also generates revenue by recovering the copper from the waste stream. Average feed levels are 149 mg/L and average effluent levels are less than 3 mg/L.
Since operations began, plant performance has exceeded expectations. On average, the plant treats more than 6.3 million cu meters of wastewater annually while recovering close to 1.8 million lb of copper per year from waste streams. The treated water is recycled on site and the recovered copper is sold to a copper smelter to generate revenue that offsets the cost of wastewater treatment. On this particular project, return on investment for the plant was achieved in less than three years, therefore, the plant now operates at a profit.
Removing and recovering the copper also includes the benefit of reducing the volume of sludge produced. Based on the amount of copper the plant recovers, it has been estimated that sludge reduction for the Dexing site measures approximately 4,650 cu meters per year. This is copper that will not potentially mobilize and will not require long-term management and storage. Sludge reduction also delivers operational savings, which can only improve project life-cycle costs.
Awareness of water’s value is growing. Rising public concern about water quality and the availability of clean water supplies are driving new regulations, which in turn have contributed to the increased costs of doing business for industries with intensive water use and large wastewater production. At the same time, technological innovation is making it possible to recover value from mining wastewater. Therefore, it is possible to treat mine waste streams for environmental compliance and economical benefit. A more sustainable future can be achieved.