Observed Preferential Depletion of Neutralization Potential over Sulfide Minerals in Kinetic Tests: Site-Specific Criteria for Safe NP/AP Ratios

Abstract

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.

Long-term evaluation of coal fly ash and mine tailings co-placement: A site-specific study

“…The acid-base accounting evaluates the balance between acid generation processes (oxidation of sulfide minerals) and acid neutralizing processes (dissolution of alkaline carbonates, displacement of exchangeable bases, and weathering of silicates) (Sobek et al, 1978;Ritcey, 1989). Static tests are fast and cost-effective, but are considered qualitative and have a relatively large uncertainty zone for which it is impossible to state on the long-term acid generation potential (Morin and Hutt, 1994).…”

Revue de littérature détaillée sur les tests statiques et les essais cinétiques comme outils de prédiction du drainage minier acide

“…Cependant, un rejet ayant un PNN>20 kg CaCO 3 /t est considéré comme non générateur d'acidité, alors que si son PNN <20 kg CaCO 3 /t, il est considéré comme générateur d'acidité. Morin et Hutt (1994), Adam et al (1997) et Price et al (1997) suggèrent des valeurs de classifications des rejets en se basant sur le rapport NPR. La classification basée sur ce rapport comprend aussi une zone d'incertitude dont les limites sont assez variables selon les auteurs (Figure 1).…”

Acid/Base Account and Minesoils: A Review

“…is equal to NP (neutralization potential, carbonate or feldspar minerals) minus AP (acid potential = sulfide minerals). Morin and Hutt (1994) found that a NP/AP ratio greater than 4 was needed to prevent acid drainage from a site in Canada. They mention that NP/ AP values between 2.5 and 3.0 have been reported to maintain a near-neutral pH.…”

“…Other research reports that samples with an ABA less than 33 tons/1,000 tons should be considered as potential acid producers and be evaluated using kinetic tests (Sullivan and Essington;. Morin and Hutt (1994) mention that predictions of acidity are more valid when kinetic tests (i.e. : humidity cells) are conducted along with the standard ABA (a static test).…”

“…Procedures used for ABA are reasonably accurate and reproducible, however, major errors in interpretation may occur if there are interfering substances in the sample. Additionally, the ABA method is a static test; it characterizes the sample at that point in time -there is no information on what may actually happen with time, since there are variations in the kinetics between acid aoo alkaline production (Harvey and Dollhopf, 1985;Morin and Hutt, 1994). Kinetic tests, involving simulated weathering methods, have been developed (Caruccio, 1968;Geidel, 1979;Smith et al, 1974b) in an attempt to obtain more practical information.…”