1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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1 1 d. Results from Acculab have very recently shown levels of 80-90 ppm and in the re6ent past have been as high as 170 ppm. ' Thus, there are two major problems with free CN at these levels; one is potential bird kill and the other is not conforming to state permits. 1 1 1 (2) At the San Luis site the levels of free CN have been debated among us. There seems to be a consensus that free CN levels range from 20-70 ppm and probably average 30 ppm. This presumption of 30 ppm is based upon a computer interpretation of a silver-nitrate filtration. BEI believes that 30 ppm is low and the real level is probably closer to 100 ppm for the following reasons: a. Typically, tailings contain 90-130 ppm free cyanide. b. The AVR system (to be discussed later) is inefficient to the degree that it can only be removing perhaps 10-20% of the free CN. c. From tests conducted by BEI on San Luis tailings (to be discussed later) the levels of free CN are typically 90 ppm. b. WAD CN (1) Two problems are associated with WAD cyanide. One problem relates to their instability iln that pH or chemical changes will often cause the relgase of CN and, thereby, the problems of CN toxicity. This is especially a problem after mine closure as the environmental pH changes and the WAD's that remain can then be influenced by this pH change. WAO's are also of concern in the case of pond leakage or spillage for the same reason. The second problem with WAD is that their breakdown can cause operations problems by consuming CN normally used for Au leach. In addition, deposition of the free metal may occur in the carbon circuit with its associated problems. (2) Everyone agrees that WAD cyanide is about 140/170 ppm at San Luis and all analyses support this. Most recently Acculab determined WAD to be 180/200 ppm. The particular WAD is copper. BE/