Biological oxidation of cyanide: A viable treatment option for the minerals processing industry?

Abstract

For over 100 years, cyanide has been the leach reagent of choice for the extraction of precious metals. Cyanide is used as a lixiviant in milling leach circuits, as well as in heap leach operations. Increasingly stringent environmental regulations have created much interest in a wide variety of methods to destroy residual cyanide. Traditional means of cyanide destruction include alkaline chlorination, hydrogen peroxide, and the INCOIS0 2 air process. Biological degradation of cyanide has often been offered as a potentially inexpensive, environmentally friendly alternative to conventional processes. In response to the costs of cyanide destruction, cyanide use has become increasingly efficient, and recovery and recycle processes are being employed in many operations. Because of this, and because the pilot studies necessary to develop a biological treatment process can be expensive and time consuming, biological treatment has not seen widespread use for the detoxification of cyanide. Recently, however, biological degradation of cyanide has received a great deal of consideration as a method for cyanide detoxification for heap leach closures. In the decommissioning of heap leaches, residual cyanide must be detoxified before the heap can be closed. Smaller heaps have been successfully decommissioned and larger heaps are in the process of decommissioning. During closure, residual concentrations in the heap must be reduced so that the leachate meets discharge requirements. This paper examines the various alternative methods of heap closure, and compares closing costs for 1.2 and 25 million ton heaps.