Although cyanide has long been identified as a hazardous and controversial chemical, to date there is no viable replacement for processing gold ore, and it continues to be a key reagent in the extraction of gold. Following a series of accidents involving cyanide in gold mines, the most notable being a spill of cyanide tailings into the Danube river system in Romania in 2000, environmentalists and other stakeholders convened to develop the International Cyanide Management Code (ICMC). The Code was developed under the auspices of the United Nations Environmental Programme to improve cyanide management on mines, thereby reducing the frequency and severity of cyanide releases. The Code was completed in 2002 and although compliance was voluntary by 2004, it has been implemented by the worldâ€™s major gold producers including AngloGold Ashanti, Barrick Gold Corporation, Newmont Mining Corporation, Placer Dome Inc. and Rio Tinto. The fundamental objective of the code is to manage the use of cyanide in the gold process to limit the release of solutions containing cyanide into the environment from spillages and tailings disposal.
The Code stipulates that tailings-related accessible water must have a maximum value of 50 ppm CNWAD (Cyanide Weak Acid Dissociable). However, the discharge limit into a public water system is 0,022 ppm, as aquatic life tolerance to cyanide is 0,030 ppm. A recent survey found that most gold mines in South Africa regularly exceed the 50 ppm level by considerable margins. Cyanide destruction may be a viable means of voluntary adherence to the Code. Sites that are situated in ecologically sensitive areas may even consider reducing cyanide in their tailings to much lower levels than specified, making a cyanide destruction step essential.
There are a number of proven processes that destroy or eliminate harmful cyanide species in gold processing solutions, each with advantages and disadvantages, but none offering an ideal solution. Cyanide destruction through ozonation is a technically proven process and is environmentally friendly; however, it has not been used mainly due to a lack of demand as a result of the relatively high capital and operating (oxygen) cost compared to other processes.
Following the adoption of the Code, investigations into cyanide destruction by the gold mining industry began in earnest. With Afrox already supplying oxygen through the Goldox application to most of the mines, utilising ozone for cyanide destruction could become a very feasible process. In addition, Linde is developing technology which will significantly improve the efficiency of converting oxygen to ozone, thereby reducing the operating costs of using ozone for cyanide destruction.
Advantages of the ozonation process for cyanide destruction:
No adjustments are required to the tailings stream
The reaction is rapid, complete and irreversible
There are no undesirable by-products
Ozone can be generated on site
Iron cyanate in not destroyed
High capital cost for the ozonation unit
As a first step in commercialising the use of ozone in cyanide destruction, Afrox embarked on a trial at an operating gold mine, Transvaal Gold Mining Estate (TGME), which is currently employing the Afrox Goldox application. TGME (a subsidiary of Simmers), expressed interest in the trial as cyanide destruction in their tailings is a long-term environmental requirement for the mine. The trial was conducted on the gold plant tailings prior to deposition on the tailings dam. The ozone was produced using on-site gaseous oxygen and a portable generator. The ozone gas produced was injected using a sparger into the tailings to ensure optimal reaction efficiency before it was pumped into the tailings dam. Samples were taken at the pump and analysed for free cyanide to determine the effectiveness of the cyanide destruction. The trial indicated that cyanide oxidation occurred quickly, with free cyanide being reduced by in excess of 85%. The ozone consumption during the trial was approximately 2 g of ozone per gram of cyanide, similar to the theoretical value of 1.85 g of ozone per gram of cyanide. The results were very encouraging, suggesting that using ozone in cyanide destruction could prove to be a viable process route for gold plants with available oxygen wishing to reduce cyanide levels in their tailings.