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• operated at any one time. Tailings have been deposited across the east (upper pond) and west <br />(lower pond, main embankment) ends of the disposal azea in such a manner th~t the free water <br />pools are maintained adjacent to the splitter embankment and away from the main embankment. <br />Tailings solids have settled from the slurry to form beach azeas at the east and west ends of the <br />disposal area. The beach areas slope away from the spigots towards the free vyater pool at a 1 <br />to 2% grade. The spigots aze opened and closed to deposit a thin layer of tailings evenly across <br />the beach area. Thin layer deposition increases the desiccation and densificatign of the tailings <br />to produce a relatively high density, low permeability material within a relatively short period <br />of time. <br />2.1.2 Acidification-Volatization-Recovery Circuit <br />The San Luis mill facility and tailings disposal area began operations in April 1991. The mill <br />was designed with anacidification-volatization-recovery (AVR) cyanide detoxiltication circuit. <br />• An AVR circuit uses some type of acid (in this case sulfuric acid) to lower tailings pH levels <br />to below 9.0. Below a pH of approximately 9.0, cyanide in an aqueous form is not stable and <br />will volatilize as a cyanide gas. The gas can then be recovered, typically through the use of <br />scrubbers, and then recycled for use in the mill processing. <br />The AVR system at the San Luis mill facility was utilized from the start of opertions through <br />May of 1992. Tailings deposition during the period of time that the AVR system was in use <br />occurred in both the upper and lower impoundment areas. During the first !few months of <br />tailings deposition (April-October 1991), the operating spigots were sequenced across the main <br />embankment and the east end of the upper pond to cover and stabilize the liner cover material. <br />From October 1991 through January 1992, spigots in each of the upper and lot~+er ponds were <br />simultaneously operated to avoid freezing problems along the distribution line. In early <br />February 1992, tailings deposition was transferred entirely to the lower pond. <br />• The AVR cyanide detoxification circuit, acidification to a pH slightly below 9.0, Uid not produce <br />a <br />