Sub-Saharan Africa’s Afrox, has revealed that ozone might be a viable solution for cyanide destruction in the gold mining sector, following trials at a gold mine currently in operation in South Africa.

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.

Due to this hazardous nature, cyanide management is at the forefront of safety concerns at mines. Now however, cyanide destruction via ozonation has been given the green light by Afrox as an ideal solution to destroy or eliminate harmful cyanide species in gold processing solutions.

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 that will significantly improve the efficiency of converting oxygen to ozone, thereby reducing the operating costs of using ozone for cyanide destruction.

Trials
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 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.

Results were 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.

Management
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, has been implemented by the world’s major gold producers. 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.

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.