Mine-rock piles are complex hydrogeologic systems. As a result, the current knowledge of their physical and chemical hydrogeology is too limited to permit accurate predictions of water chemistry through time based on detailed simulations of their internal processes. However, a simplistic empirical model based on general knowledge and available data can be used to obtain rough estimates of seepage chemistry through time. This empirical model is based on five factors: (1) the production rates of metals, nonmetals, acidity, and alkalinity under acid and pHneutral conditions, (2) the volume rate of flow through the rock pile based on infiltration of precipitation, (3) the elapsed time between infiltration events, (4) the residence time of the water within the rock pile, and (5) the percentage of mine rock in the pile flushed by the flowing water. The last factor is most difficult to define at many minesites, but can be assumed due to its apparently frequent narrow range of 5 to 20%. A hypothetical example illustrates how to use the model and highlights other complications like secondary-mineral precipitation that may also have to be considered. A field example based on data from an actual minesite demonstrates the accuracy of the model as compared to measured concentrations. Again, this model ignores many complexities of mine-rock piles and is thus only useful for rough estimates of future chemistry.
As the number of completed kinetic tests grows, an international database is forming on rates of acid generation, acid neutralization, and metal leaching. As the results become available and are compiled, valuable lessons are learned on the prediction of acidic drainage and on the variability of reaction rates caused by site-specific factors.One of the more important lessons from the kinetic-test database pertains to acid-base accounting (ABA). A great deal of effort is expended on the development of universal ABA criteria to predict if a sample may eventually become net acidic. Based on the amount of carbonate or feldspar minerals (Neutralization Potential or NP) and sulfide minerals (Acid Potential or AP), the criteria are usually expressed as some form of Net Neutralization Potential (=NP-AP) or NP/AP ratio. Widely reported criteria for NP/AP state that a value above 2.5 to 3.0 can be considered non-net-acid-generating through time, but there is still some argument on the appropriate values. The Canadian database shows that one site required NP/AP>4.0 for the prediction of consistently neutral pH after 40 weeks of testing.This paper presents some of the information in the Canadian database. In particular, the rates of NP and AP depletion are presented for several kinetic tests, showing that the amount of NP often has to exceed AP by a factor of 1.3 to 4.0 for the predicted maintenance of near-neutral pH into the distant future. This is in partial agreement with theoretical relationships that show the factor should often be between 1.0 and 2.0 for carbonate-based neutralization. In effect, the kinetic database shows that attempts to identify universal ABA criteria can be either too cautious or faulty for a particular minesite. Instead, appropriate ABA criteria should be determined on a sitespecific basis, reflecting reaction rates of the acid-generating and acid-neutralizing minerals at the particular site.
This paper provides an approach for roughly estimating the capital and annual operating costs, and the longevities (time to substantial replacement or upgrade), of alkali-based water-treatment plants for ARD. Under most combinations of flow and acidities, these plants will be the only cost-effective option for long-term consistent adherence to water-quality restrictions. Median and mean capital costs were roughly US$3.8 and $4.5 million, and there was a correlation with average annual flow rate. There were no strong correlations among total annual operating costs, flows, and acidity concentrations due to highly variable individual contributions from factors like power and reagents. Nevertheless, an average unit-volume treatment cost was US$0.27/m 3 for acidities less than 800 mg/L, and was US$2.24/m 3 for acidities above 4000 mg/L. The reported longevity of these water-treatment plants is around twenty years, attributable to factors such as increasing chemical loadings, increasing flows, and improved technology. A replacement period of 20 years can have a significant, but not dominant, effect on net present value over 100 years. Passive treatment systems and soil covers do not usually attenuate concentrations consistently to non-toxic discharge levels, so additional treatment can be required. If this involves a water-treatment plant, an interplay of combined costs, longevities, durations, and risks leads to a myriad of waste-management scenarios. For example, a soil cover that lessens annual acidity loadings can extend the number of years a treatment plant operates; thereby increasing (1) the length of time the mining company must maintain a site presence and thus (2) the risk posed by a greater probability of intense storm events or other problems leading to an accidental release of contaminated water. Long-term costs for passive treatment systems, like wetlands, and soil covers are not as well defined. This can give the false impression that water-treatment plants are more expensive in the long term.
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