Paper 14_Breuer:text

The implementation of more stringent regulations and the recent development of the International Cyanide Management Code (ICMC) by the industry is aimed at avoiding future environment incidents with cyanidation plants. To meet the ICMC discharge to tailings requirements and/or regulations, many operations now adopt cyanide recovery or destruction processes. In Australia, there is also the reporting requirement of the National Pollution Inventory (NPI), whereby all cyanide used in the gold recovery process must be accounted for. Thus, there is a need to clearly understand the cyanide deportment in gold processing operations, particularly in Australia. The recovery of gold from sulphide ores and concentrates is becoming more widespread as the supply of oxide orebodies diminishes. Many sulphide minerals are problematic in the cyanidation of these ores and concentrates, as they are known to consume oxygen and cyanide (Hedley and Tabachnick, 1968; Nicol, Fleming and Paul, 1987; Deschenes, 2005; Dai and Jeffrey, 2006; May et al., 2005). Most metal sulphides decompose in aerated, alkaline cyanide solution to form metal ions, metal oxides or metal cyanide complexes and various sulphur-containing species including thiocyanate, sulphide and thiosulphate ions, with polythionates and polysulphides having also been detected (Marsden and House, 2006). Gold processing plants today typically have the ability to measure ‘free’ cyanide and weak acid dissociable (WAD) cyanide on site and in some operations online analyses are used to control reagent addition. However, these analyses provide little information on the deportment of cyanide within the processing plant and thus samples must be sent out to analysis laboratories to attain this information. This typically requires the application of several analysis techniques to quantify the metal cyanides, thiocyanate and cyanate; atomic adsorption spectroscopy or ion coupled plasma for elemental analysis and wet chemistry techniques for other species. Ion chromatography has also been used to analyse for the various cyanide species and metal cyanides (Fagan, 1995; Huang, Paull and Haddad, 1996; Pohlandt, 1984; Fagan et al., 1998; Muir, 1994; Miura and Kawaoi, 2000; Fagan et al., 1997; Otu, Byerley and Robinson, 1996; Haddad, 1988; Hilton, 1986; Zvinowanda, Okonkwo and Gurira, 2008; Sumiyoshi, Yagi and Nakamura, 1995). Sulphur dioxide (or sulphite salt) / air or Caro’s acid (sulphuric acid + hydrogen peroxide) are commonly used in the destruction of cyanide. However, the understanding and mechanisms of such reactions are unclear or have not been published in the open literature. Thus, there is a clear need to be able to measure the deportment of sulphur species, in addition to cyanide, to better understand the chemical processes taking place and the reaction products generated within a gold processing plant. This information is important in evaluating and monitoring the environmental risk of an operation. This paper describes the methodology developed utilizing ion chromatography to analyse various cyanide and sulphur species present during cyanidation of complex gold ores and cyanide destruction processes. The application of this method to enhance the understanding of the chemistry related to the deportment of cyanide in gold processing is demonstrated.