1994-02-28_REVISION - M1988112 (2)

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~~ Incomlt:e':e oxidation of the sulphide to a soluble product with ari oxidation state less than +4 could have: resulted in an underestimatiorrof acid production. Oxidation of sulphide to an ;xidetion state. less than +4 neutralizes acid. For example, oxidation o` one. mole of sulphide to elemental sulphur neutralizes two moles of acrid. However, when sulphide is oxidized to sulphate there is no net acid consumption or production. Incoml~let.e oxidation of ferrous iron released from iron sulphides would also lead to underestimation c)f the APP. If ferrous iron were not oxidized during; the test and subsequent titration, it would remain in solution and not ~;ontribute to the measured acid production. However, over a longer time frame, the ferrous iron would oxidize azui ferric. hydroxide would precipitate, wish a consequent generation of acid. Thus, ferrous iron in solution will produce acid which would not be measured by the NAP Test. This seems more probable than incomplete oxidation of sulphide since the rate of ferrous ircm oxidation would lie inhibited by the acidic conditions produced in the test. Excessive dissolution of mine waste components other than calcium carbonate and magnesium carbonate probably neutralized acid during the hydrogen pl;;roxide oxidation of some samples. Such acid neutralization would overestimate the actual NI' available to neutralize drainage and, consequently, result in an unrealisically high NAP N~:r. NP. The potential for such dissolution increases as the solution pH decreases and the content of minerals susceptible to such dissolution increases. Solution pH in turn decreases, as the difference between the acid- producing and acid-consuming mineral content (ntineralogic Net NP) increases. Excessive dissolution of silicate minerals most likely contribut~:d to elevated NAP Net NP values for TLS and TL6. These samples hacl the highest mineral~~gic Net NP (difference between acid-producing and acid-consuming minerals content) of the: samples examined and, therefore, produced solutions more acidic than other samples Burin;; the hydrogen peroxide oxidation. As a result, host rock components were subjected a highly acidic solution, in addition to elevated temperatures, for one to two hours during the NAP Test. Such conditions were conducive to dissolution of minerals other than c~abonates and consequent acid neutralization. TL-5 had the highest feldspar (mostly potassic) content of the samples and TL-6 had the highest pyroxene content ('T'able 2). The dissolution of these minerals, and the attendant acid neutralization, was probably accelerated under the test conditions. Excessive feldspaz dissolution may have also contributed to the lesser degree of P7et NP elevation for sample RK-2, which had a feldspaz (mostly calcic) content similar to that of TL-5 but a sulpln~r content of only 0.64 percent. Incomplete oxidation of ferrous iron released from iron carbonate minerals (siderite, ankerite) may have contributed excess acid neutralization by sample T:L-3 (mineralogic Net NP _ -44 kg CaCO3/t), which had the highest iron carbonate content of the: samples examined (8.7 weight percent). It is likely that some iron carbonate dissolved during the test, due in part to the attack by the acid generated, with an attendarn neutralization of :u;id. If the ferrous iron released were not oxidized during the hydrogen peroxide oxidation or the subsequent titration, the iron carbonate dissolution would neutralize acid. Under oxidizing conditions in the environment the ferrous iron released would oxidize to ferric iron which would precipitate as ferric hydroxide. Thus, the dissolution of iron carbonate and subs~Jquent reactions would provide no net acid neutralisation. 155