The use of cyanide for the recovery of gold has become an increasingly contentious issue following, on the one hand, a number of high-profile and negative cyanide-related environmental incidents in recent years, and, on the other, increasingly vocal and visible non-governmental organisations (NGOs), such as Greenpeace and Oxfam, calling for the banning of cyanide use.
Cyanide is used around the world to recover low concentrations of gold that could otherwise not be extracted, from its host rock through a leaching step. (See box on the Understanding the Valley Leach Facility at CC&V). Unlike the frequent public contention that the mining industry is a major user of cyanide, globally this industry accounts for only 20% of consumption a year. Approximately 1.4 million tonnes of hydrogen cyanide are produced annually worldwide. The remaining 80% is used in industrial applications including the production of plastics, adhesives, fire retardants, cosmetics, pharmaceuticals, food processing and as an anti-caking additive for table and road salts. (Source:
AngloGold Ashanti is acutely aware of the potential impact of cyanide on both people and the environment and the importance of the correct management of cyanide. The company was actively involved in the development of the International Cyanide Management Code. AngloGold Ashanti was one of the first signatories to the code in 2005. By being involved in code activities, AngloGold Ashanti maintains its awareness of best industry practice. The Cresson Project will be reviewed by a third-party expert to determine certification under the code within the next three years (as allowed in terms of the code).
Yet the use of cyanide remains an issue of high-profile debate, particularly in North America. The responsible use of cyanide is critical to the viability of many North American operations, CC&V included.
The cyanide issue came to the fore in North America in 1992, when major problems were discovered at the Summitville Mine in Colorado, the same state in which CC&V is located. The Summitville Mine was a surface mining operation for gold and silver that used heap leaching with cyanide ore-processing reagents. The site had been permitted (or licensed) in the 1980s in an area of south-western Colorado at a high altitude known for deep snow accumulation. A liner for the leach pad ripped during construction and was never repaired, allowing water containing dilute cyanide and certain metals to leak from the heap-leach pad into an adjoining stream. In response, the company installed a water treatment plant to be used to neutralise the cyanide in the heap and related ponds. In 1992, the Colorado Department of Public Health and Environment (CDPHE) imposed new discharge limits on the operation. The inability to meet these lower standards, in conjunction with other factors led to the abandonment of the site in December 1992. Owing to the magnitude of the problems that remained, and the potential for additional issues related to uncontrolled flows from the site, the State of Colorado requested assistance from the US Environmental Protection Agency (EPA). The adjoining streams historically had naturally occurring low pH and high metal content as evidenced by such names as Alum Creek and Bitter Creek. While the initial cyanide contamination garnered the headlines, it was the release of certain metals through acid rock drainage that posed the biggest long-term issue. The EPA declared the mine a Superfund site, allowing use of the Superfund itself for site clean-up.
In 1993, the state, environmental NGOs, and the mining industry co-operated through the development and later enactment of new legislation and regulations designed to significantly strengthen the mining and reclamation requirements with the goal of avoiding a reoccurrence of Summitville. The revised laws are amongst the strongest mining and reclamation laws in the United States. Shortly after enactment, CC&V voluntarily sought re-licensing of those portions of the operations associated with the Valley Leach Facility (VLF) under the new, more stringent requirements and its Cresson Mine became the first heap-leach gold mine licenced in Colorado under the new law.
CC&V remains committed to the responsible use of cyanide and uses the latest containment technologies to assure that cyanide solution does not escape and the zero discharge status of the gold recovery operations is maintained. The VLF and Adsorption, Desorption and Recovery (ADR) facility are the primary zero discharge facilities involving complete containment of dilute cyanide solutions. The VLF features a double- and, where solution is collected in the internal ponding structures, a triple-lined design. Solution collection and detection systems are also included in the design. The ADR was similarly designed with solution collection and detection systems.
In addition to these sophisticated design features, a quality control and quality assurance (QA/QC) programme was instituted to achieve compliance with the stringent 1993 legal requirements. A third party expert consultant was used to rigorously monitor and test materials during construction of the VLF and other solution containing facilities such as the ADR. Prior to activation, all of the monitoring and testing results were compiled and submitted with a certification statement by a registered professional engineer to the Colorado Division of Minerals and Geology (DMG) for review and approval. DMG also conducted frequent inspections throughout the construction process. Only upon acceptance by DMG of the certification reports, could dilute cyanide containing solution be used.
Intensive monitoring conducted since the facilitys activation has verified the zero discharge status of the VLF and other facilities. Various prescribed monitoring of critical elements of the VLF and associated facilities as well as down-gradient water quality sampling are conducted at regular intervals. In addition, ground water monitoring wells have been installed around the Cresson Project and are routinely monitored. VLF and ground water monitoring results are submitted to the DMG. Surface water monitoring results are provided to CDPHE monthly. Based upon the results from this monitoring, there is no evidence of leakage or loss of containment from the VLF or associated facilities.
Another potential concern related to facilities such as the VLF is the atmospheric emission of hydrogen cyanide (HCN). CC&V minimises the HCN releases by maintaining the pH of the solution at high levels and the burial of the drip lines used to wet the ore. To address these concerns and others raised by local residents, the Agency for Toxic Substances and Disease Registry (ATSDR) completed a Public Health Assessment of the Cresson Project in 2000. This agency is affiliated with the Centers for Disease Control or CDC. The summary of the completed study states:
The Agency for Toxic Substances and Disease Registry has concluded that the ambient air emissions associated with Cripple Creek and Victor Mining Company do not pose a threat to human health. The cancer incidence and birth defects in the Victor and Cripple Creek area are not elevated when
to those in similar areas. The local drinking water has not been impacted by mining activities. The dust and metals levels in Victor and Cripple Creek do not [represent] a threat to public health. Current hydrogen cyanide levels on-site and in nearby residential areas are not at levels of health concern.
CC&V believes that its record shows that the Cresson mine represents the state-of-the-art in cyanide solution facility design, construction quality assurance and operation. We continue to strive to improve operations, where possible.
Understanding the Valley Leach Facility at CC&V
At CC&Vs Cresson mine, gold is removed from the crushed ore through the same basic process used all over the world. Naturally occurring minerals, including gold and silver, that are exposed on the broken faces of the crushed ore, are dissolved using a dilute sodium cyanide solution. This is called the leaching process.
At the Cresson mine, leaching is undertaken out of doors in a valley leach facility (VLF), a valley area with clay and plastic liners upon which the crushed ore is placed for the removal process. The bottom and sides of the VLF which are made up of impermeable double and triple liner systems. The crushed ore is placed in layers of about 11 metres and a dilute solution of sodium cyanide (about 100 parts per million) is applied to each successive layer using agricultural-type drip irrigation tubes. As the solution soaks through the ore, it dissolves the gold and silver on the surface of the ore. The so-called pregnant solution is then captured in specifically designed internal ponding structures at the lowest point of the VLF, and pumped into the recovery facility.
There are no external ponds where the solution is held prior to processing; rather the solution is kept in the facility within the pore space of the ore, much as ground water is held in porous bedrock. The VLF therefore has a large excess capacity to cater for significant precipitation events such as rain or snow. A sophisticated and comprehensive water monitoring system is in place to maintain the water balance as the VLF is a zero discharge facility.
Once it leaves the VLF the pregnant solution enters the Adsorption, Desorption and Recovery (ADR) facility where the solution is pumped through steel tanks containing activated carbon (roasted coconut shell) granules which attract (or adsorb) the dissolved gold-cyanide complex. The barren process solution from which the gold has been removed is recirculated to the VLF to start the leaching cycle all over again. Meanwhile the gold is removed from the carbon, before refining.
Through this process about 70% of the gold is removed. This is because the cyanide process dissolves only that gold or silver which remains on the surface of the rock. To recover all the metals the rock would need to be crushed so finely that the process solution would not flow through it.
The ore on the VLF is not removed once the gold has been recovered from its surface. Rather, new ore is stacked on top of the leached rock.