Deadly chemicals can be a gold mine
Nov. 20 2020
Even the word cyanide conjures up fear and images of toxic chemical seeps for the majority of us who fortunately have never even come in contact with it. It derives its fearsome reputation from the fact that it is a rapidly acting, and potentially deadly chemical.
In the mining industry it has been an important component of the gold (and precious metal) extraction process since the late 1800s. Sodium cyanide (NaCN) is used in the milling of high grade ores and also for extracting gold from low-grade ore by converting the gold to a water-soluble complex. Production of reagents for the mining sector accounts for approximately 13% of cyanide consumption globally.
Gold will dissolve in an aqueous solution of cyanide, allowing it to be separated from the ore it is found in. Once in solution, the gold can then be recovered from the cyanide solution through a process that typically involves activated carbon. The gold cyanide complex is adsorbed onto the carbon creating a particle large enough that it can be separated using wire mesh.
Once the gold is removed, the cyanide that remains in the tailings from the milling process, or any seepage from heap leach facilities, is potentially hazardous. The majority of mining companies globally have adopted the International Cyanide Management Code which defines strict protocols for how cyanide is used and managed, including the detoxification of cyanide in waste streams. This step lowers the concentrations of cyanide compounds, using a process that oxidizes cyanide to cyanate. As a result of the various oxidation and reduction processes employed by gold mines and their use of cyanide, it is important that as a premier analytical services provider to this sector, that we can provide a broad range of testing.
The cyanide molecule (CN) forms complexes with certain inorganics and trace metals. The form in which cyanide may be found in a particular environment is governed by equilibria which are dependent on temperature and pH. Due to the potential for simultaneous presence of multiple forms of cyanide, analysis can be challenging. As such, various methods have been developed to quantify each particular form. Since the classification of different cyanide species is very strongly dependent on pH, analytical methods have been correlated accordingly.
Free cyanide species include free ions (CN–) and hydrogen cyanide (HCN). These forms are highly bioavailable and pose the greatest risk of toxicity. From an analytical perspective, free cyanide is the sum of HCN and CN- in solution. These species are separated from other forms of cyanide by diffusion through a membrane using a solution at a pH between 6 and 7.
Weak acid dissociable (WAD) cyanide species include simple complexes, such as those formed with alkali metals, as well as some heavy metals that dissociate at a pH between 4 and 6. As shown in Figure 1, WAD cyanide includes simple cyanide compounds, as well as weak metal cyanide and moderately strong metal-cyanide complexes.
Total cyanide represents the total of all cyanide species, including free, WAD and strong metal cyanide complexes. This analysis is performed under strong acid conditions (pH < 2) in order to dissociate the strong metal cyanide species.
The relative stability of cyanide compounds is also illustrated in Figure 1 below. As the species becomes more stable, a progressively lower pH and increased heat and UV radiation are required for dissociation to the CN–/HCN form.
Figure 1: Least to Most Stable Forms of Cyanide