Gold is one of the few metals that dissolves in the presence of cyanide and oxygen ions. The soluble gold species is dicyanoaurate. From which it can be recovered by adsorption on activated carbon. A process called “cyanidation”, or cyanide leaching, has been the dominant gold mining technology since the 1970s.
In this process, sodium cyanide is used, in a dilute solution of between 100 ppm and 500 ppm or cyanide of 0.01% to 0.05%, to selectively dissolve gold from the mineral. The two most common processes that use cyanide for gold recovery are heap leaching and milling, also known as carbon leaching (CIL). Although important scientific research has been carried out for many years, no other chemical reagent has been found that comes close to exhibiting the superior economic and environmental qualities of cyanide in the recovery of precious metals. As part of their best practices, mines use as little cyanide as possible for environmental, safety and economic reasons.
The cyanide leaching process is often carried out following other physical processes such as crushing and crushing. Once the gold dissolves, the solutions are further processed to recover the gold, which is then melted into gold ingots. Waste, sludge and solutions containing leached material, water and waste cyanide are treated with different chemical and physical methods to reduce or eliminate cyanide left over from the gold dissolution process before it is discharged into a tailings facility. In addition, different technologies are used to recover and reuse residual cyanide in the above-mentioned processing circuits.
The solutions that accumulate in a waste facility generally have cyanide concentrations that are not harmful to people, birds, or animals. While a cyanide concentration lower than 50 ppm is required to comply with the International Cyanide Management Code to protect wildlife, many mining operations achieve cyanide concentrations lower than 10 ppm in their tailings facilities. After discharge, the residual cyanide is rapidly diluted and destroyed by natural processes, such as oxidation and ultraviolet catalysis (by sunlight). Local, state, and national regulations limit the amount and concentration of cyanide that can be discharged at a tailings facility.
The dissolution and dissociation (or ionization) of molecular or ionic cyanide in aqueous solutions creates free cyanides. The results also revealed that the complete electrodegradation of cyanide can be achieved with a current efficiency of 100%. According to the ICMC, the amount of cyanide used in commercial gold operations usually ranges from 0.01% to 0.05% cyanide (100 to 500 parts per million), and the code's website states that mining industries in Australia and North America have not documented accidental human deaths from cyanide poisoning for more than 100 years. These complexes can dissociate and release free cyanide in the presence of UV radiation and strong acids.
The purpose of this chapter is to provide basic information on these cyanide treatment processes, including the basic uses of chemistry and reagents, common areas of application, and treatment performance that can be expected on a large scale. After several high-profile leaks and spills, the use of cyanide in mining operations faces growing public opposition, and some countries completely prohibit it. While cyanide is not used in the recently opened Kensington mine, it would be used in the proposed Donlin Creek mine and, most likely, also in the proposed pebble mine. At an optimal gold mining pH of 10.5 or higher, most of the free cyanide in the solution is in the form of a cyanide anion (CNâ), where cyanide loss through volatilization is limited.
Liquid or gaseous hydrogen cyanide and alkaline cyanide salts can enter the body by inhalation, ingestion, or absorption through the eyes and skin. It was also discovered that 100% of cyanide can be degraded from real samples obtained from a Mexican mining tailings dam. Christine Blackmore, ICMI chief cyanide auditor and associate director of Wardell Armstrong, says mining companies need to adopt greater education and transparency when it comes to cyanide and local communities. Again, local, state and national regulations limit the amount and concentration of cyanide that may be present in these solutions.
The bones and seeds of common fruits, such as apricots, apples and peaches, can naturally contain up to 700 parts per million (ppm) of cyanide. . .