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This Internet case study is copyrighted (©) 1998 by Kevin A. Morin and Nora M. Hutt, and is adapted from Chapter 6 of Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.
Where the chemistry of minesite drainage is unacceptable for release to the environment, there must be some type of control prior to release in order to minimize damage to the environment. There are two basic approaches to drainage-chemistry control: reactive ("we'll handle it when it happens") and proactive ("a stitch in time saves nine"). While proactive controls sounds best, it can actually be more expensive, riskier to the environment, and requires detailed predictive work to control the timing, extent, and severity of the potential problem. If the predictive work is wrong, proactive work may turn out to be fruitless.
Reactive control allows a minesite component to generate its drainage chemistry, the drainage is then collected and treated, and the waste products from treatment like metal precipitants ("sludge") are disposed. Long-term stable disposal of treatment waste is a major problem with reactive control because the treated contaminants are more concentrated.
Proactive control adjusts physical, chemical, and/or biological conditions within a component to create lower concentrations in the drainage as it leaves the component. Because proactive control of most drainages is rarely successful to the degree needed for discharge to the environment, some reactive control is frequently included or planned as a contingency. Because of (1) costs for this backup reactive control, (2) the lag time before proactive measures become effective, and (3) the risk of failure of proactive controls, the cost for reactive control alone may be lowest (see below).
To give you some idea on how overwhelming drainage control can be, imagine a minesite with a perimeter of 2 km by 2 km. If net precipitation onto the site is 0.5 m/yr (20 inches/yr), then the minesite will have to handle and control 2 billion (2x109) liters of water annually if the chemistry is unacceptable. Even if control costs US$0.001/L, then the total cost would be US$2,000,000 a year! Some minesites are actually spending this much for control annually.
There have been some studies done on unit costs for proactive and reactive treatment. For example, Meek (1994) compared the economics of proactive prevention techniques to reactive collection and treatment for acidic drainage. This comparison is based on field-scale tests over seven years and includes reagent costs. The most economical approach on a hectare basis was simply collection and reactive treatment -- all proactive techniques were more expensive.
Geocon (1995) also summarized costs for reactive and various proactive control techniques (Table 2), based on various assumptions and scenarios. Active collection and treatment were again the least expensive for waste rock and were among the least expensive for tailings.
There has been strong opposition to reactive drainage-chemistry control is some countries and correspondingly strong pressure for proactive control. In wetter climates, the most popular proactive option is water covers (see Mining and Oceans) which significantly reduce the reaction rates of some minerals. As a result, the minerals virtually do not react and thus persist for millennia.
For example, if acid-generating pyrite is submerged behind a water-retaining dam, then this pyrite generates very little acidity underwater. However, the dam must stand and retain water forever or the acid will eventually be generated when the pyrite is exposed to air. Thus, in some cases, this proactive control could simply be a postponement of the inevitable.
From the technical perspective of environmental risk (probability and consequence), the storage of pyrite underwater does not lower risk, only preserves and maintains the risk. For this and other reasons, the preferred underwater placement is in natural lakes and oceans. However, placing mined materials into natural waterbodies, which typically have various forms of life, causes environmental degradation at least over the depositional period. In this case, does a proactive stitch in time really save nine? There are no simple answers here.
|Technique||Cost (US$/ha)||Technique||Cost (US$/ha)|
|Collect and treat||$26,900||Quicklime addition||$56,800|
|Lime addition||$32,900||Phosphate/apatite addition||$38,500|
|Clay cover and alkaline trench||$50,200||Controlled layering of acid-generating material||$31,400|
|Control Technique||Initial Capital Cost1 (1994CDN$/t)||Final Total Cost1 (1994CDN$/t)|
|Collect and treat (C&T)||0.03-0.16||0.26-0.64|
|C&T with simple soil cover||0.16-0.55||0.34-0.85|
|Multilayer soil cover||0.89-1.12||1.07-1.31|
|Plastic cover (200-yr lifespan)||1.38||1.59|
|Control Technique||Initial Capital Cost1 (1994CDN$/ha)||Final Total Cost1 (1994CDN$/ha)|
|Collect and treat (C&T)||62,000-97,000||214,000-238,000|
|C&T with simple soil cover||91,000-182,000||200,000-264,000|
|C&T with partial water cover||93000||225000|
|Multilayer soil cover||231,000-307,000||291,000-415,000|
|Multilayer soil cover with partial water cover||223000||303000|
|Self-sustained water cover||13,000-254,000||71,000-349,000|
|Maintained water cover||11000||83000|
|1 Initial Capital Cost includes implementation costs incurred at the time of minesite closure; Final Total Cost includes Initial Capital Cost plus the Net Present Value of costs incurred after minesite closure, like long-term collection and treatment, inspection and maintenance, and plastic-cover replacement.|
© 1998 Kevin A. Morin and Nora M. Hutt
Geocon. 1995. Economic Evaluation of Acid Mine Drainage Technologies. Canadian Mine Environment Neutral Drainage (MEND) Report 5.8.1.
Meek, F.A., Jr. 1994. Evaluation of acid prevention techniques used in surface mining. IN: International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, USA, April 24-29, Volume 2, p. 41-48. U.S. Bureau of Mines Special Publication SP 06A-94.
For more details and case studies, see Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.
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