1994-02-28_REVISION - M1988112 (2)

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INTRODUCTION Several static tests have been developed to estimate quickly and inexpensively if a mine waste will produce acidic drainage. The objective of the standard Acid-Base Accounting (ABA, Sobek et al., 1978), Modified Acid-Base Accounting (Lawrence, 1990), and B.C. Research Initial Tests (Duncan and Bruynesteyn, 1979) is to determine the maximum potentials for acid production (acid production potential or APP) and acid neutralization (neutraliration potential or NP). The APP is based on sulphur analysis and the NP is determined by the amount of acid neutralized under when the sample is in contact with a solution in the approximake pH range of 1 to 3.5. Whether or not a mine waste will produce acidic drainage is determined by the difference (Net NP) or ratio (NP/APP) of these values. The Net Acid Production Test (NAP, Coastech Research Inc., 1989) is a static test which was developed due to a perceived need for both an alternative laboratory technique and a simple field procedure to allow rapid on-site waste management/placementdacisions. The test has the advantage over other static methods that sulphur analyses, to allow calculation of the theoretical APP of a sample, are not required. This feature makes the test particularly suitable for field use. The NAP test uses hydrogen peroxide to accelerate the oxidation of sulphide minerals present in a sample of mine waste, and the acid generated then reacts with other host rock minerals. In the referenced method, five grams of mine waste is subjected to oxidation by 100 mL of 154 hydrogen peroxide. One hour after the reaction is apparently complete, the pH of the mixture of solids and liquid is determined. (It was indicated that the results were comparable when only five minutes were allowed after the apparent completion of the reaction). The solution is then titrated to pH 7.0, yielding a value which allows the Net Acid Producing Potential to be measured directly. In the Coastech Research Inc. (1989) study, this technique correctly identified all nine samples which produced AMD in the field, and two of the three samples which did not produce acid in the field. In general, however, this techhnique yielded higher equivalent Net NP values than the other static tests for the samples examined. A test, perhaps based on the NAP test, referred to as the Net Acid Generation test, or NAG, was presented by Miller et al. (1990, 1991), and has been used at mine sites in Australia, New Zealand and the U.S. The authors stated that the NAG typically yielded lower potential acid generation than indicated by the standard ABA method. They correlated the NAG results for one mine site with the Net Acid Production Potential (NAPP) of the samples (NAPP = -Net NP). The NAG values were roughly two-thirds of the standard ABA values. In 8ddition, these workers indicate that simply recording the pulp pH following peroxide digestion could provide a classification of a sample as potentially acid producing or not. It is further suggested that the lag period before acid generation starts in a waste material could be predicted, al least in some qualitative sense, by monitoring rates of temperature and/or pH change during digestion. In any application, however, it is indicated that the methods were site specific. In particular, preliminary testing was required to determine methods of sample preparation, the ratio of mine waste sample to hydrogen peroxide solution, hydrogen peroxide strength, and reaction time. 146