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BMRI -San Luis Pit Backfill: HCT Tests <br />3.0 KINETIC TESTS -HUMIDITY CELLS <br />Static acid-base accounting techniques are screening tests that separately evaluate the <br />acid-generating and acid-neutralizing potentials of mine wastes. In particular, the acid- <br />generating potential evaluation is based on an assumption that all of the sulfur will be <br />available for conversion to sulfuric acid. Kinetic tests are designed to evaluate the <br />relative reactivity of potentially acid-generating and acid-neutralizing phases in bulk <br />samples of rock (or other mining waste). Because the true field weathering will <br />develop over a period of years or decades, it is necessary to adopt an "accelerated life <br />testing" strategy for kinetic tests of waste rock. <br />Due to the amount of waste rock available at this time, Geochimica proposed in July, <br />1995 to use small-scale kinetic tests called humidity cell tests (HCT). In a humidity <br />cell, a sample of approximately 200 g of a single interval of waste rock is crushed and <br />treated in a closed cell (the "humidity cell") using cycles of dry air, humid air, and <br />rinsing with de-ionized water. The cycle is intended to simulate weathering conditions <br />in waste rock, in which periodic flushing by infiltration that exceeds field capacity is <br />followed by time periods in which air -sometimes moist and sometimes dry -circulates <br />through the waste rock pile. During the air cycling, reaction products build up on <br />solids, and these are flushed in the rinsing cycle. Each cycle lasts seven days (3 days <br />of moist air, three days of dry air, and one day of rinsing), and the tests run for periods <br />of 12-30 weeks, depending on the results. <br />The principal purpose of kinetic tests is to evaluate the net acid-generating behavior of <br />samples subjected to "weathering" cycles. At the end of each weekly cycle, the drain- <br />down effluent is tested for pH, alkalinity and acidity, sulfate, and iron, as these <br />parameters are expected to vary with the extent to which oxidation of sulfides and/or <br />reaction of acidic water with solid phases consumes (or releases) alkalinity (see <br />following section for geochemical discussion). Parameters are tracked as a tprtte series, <br />and graphs of these parameters allow rapid evaluation of the important chemical trends <br />in the on-going weathering reactions <br />Because the available samples of gneiss from San Luis remaining after other test work <br />were too small to perform standard HCTs, the samples have been composited to <br />produce sufficient sample (200 g) for testing. The samples were composited by their <br />total sulfur content, in order to minimize concerns about "dilution" of sulfur in the <br />compositing: <br />Geochimica, Inc 3 9502r/1-Feb-96 <br />