Gold is mined at about 4,850 meters above sea level at the Veladero Gold Mine owned by the Barrick Gold Corporation. It is located about 374 km from San Juan, Argentina, in the Frontier district which is being intensively prospected. In 2017 it produced about 432,000 ounces of gold.
The Veladero mine uses traditional ore crushing and cyanide extraction techniques. The ore is crushed in two stages and carried overland by conveyor belt and is trucked to an area where it is treated in a leach pad. Cyanide is used in large quantities to extract the gold from the ore.
The Veladero management has started up a program to monitor every location around the mine where there is ground or surface water to detect cyanide contamination, as part of their drive to ensure the safe use, treatment, and containment of cyanide.
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The Barrick Gold Corporation has teamed up with OI Analytical to come up with a safe and responsible mine management approach. It has thus shown initiative and progressive thinking in anticipating and providing against social, economic, and environmental imbalances due to mine operations. The CNSolution 9310 Online Cyanide Analyzer from OI Analytical was the tool of choice for this important assignment.
The Instrument
The CNSolution 9310 Online Cyanide Analyzer is ideal for monitoring ground and surface water contamination with cyanide because it reports cyanide measurements with accuracy levels that are comparable to those obtained in a laboratory.
The analyzer is used on-site, near the point of sampling, providing analysis results within a few minutes. This prevents long holding times that make it difficult to accurately analyze substances for the concentration of cyanide in the laboratory.
Secondly, the immediate availability of the results means that plant operators are equipped to make quick decisions rather than being forced to delay their operations for hours or days. The ability to carry out continuous monitoring also means that the process allows the state of the environment, as well as the mining operations, to be visualized on a dynamic rather than a static, snapshot-like basis, as is the case when grab sampling is used.
The Veladero mine now monitors ground and surface water at 14 locations. The current site was chosen by the Barrick Corporation because it offered good shelter, power availability, proximity to the stream of ground water and to he data logging device.
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The Procedure
The CNSolution 9310 analyzer was set up in a site originally used to take shelter in during fierce thunderstorms. The stream could be sampled just 25 meters away from the shelter, a site which was also upstream of an existing water channel and monitoring location. The lines for retrieving samples and returning them to the stream were connected to the analyzer after setting them inside heavy rubber sleeves to protect them against environmental damage.
The data on cyanide concentration was transmitted via a 4-20 mA output from the analyzer to a Campbell Scientific data logger already functioning nearby, and then onwards to the Veladero database.
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The CNSolution 9310 had to be set up to determine the total cyanide concentration in the samples. In other applications, it is enough to determine the concentrations of free cyanide or cyanide obtained from compounds that undergo dissociation when in the presence of a weak acid.
The current situation required total cyanide detection because it was quite possible that cyanide would complex with many heavy metals such as gold or silver. This meant that an in-line UV digester had to be added to the system to complete the conversion of all such complexes formed by heavy metals with cyanide to hydrogen cyanide.
Cyanidation
Gold and silver are extracted from dilute or low-grade ores containing a high percentage of impurity by a hydro metallurgical process called cyanidation. In this process, cyanide is converted to a water-soluble complex of metal-cyanide, which is then precipitated using activated carbon, zinc dust, or ion-exchange resins.
The solutions used for leaching may have concentrations of cyanide between 50 ppm and 2,000 ppm. The International Cyanide Management Code requires that tailings discharges must have cyanide concentrations less than 50 mg/L.
The CNSolution 9310 Online Cyanide Analyzer is a gas-diffusion amperometry-based platform from OI Analytical which is used in USEPA testing methods OIA-1677 and ASTM D 6888-09. These are proven to be extremely reliable in cyanide determination, as they do not suffer interference from copper and metallic sulfides present in the solutions of the precious metals.
Gas Diffusion Amperometry
A simplified model of the basic principle of the CNSolution 9310 Online Cyanide Analyzer is shown below.
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