Accounting for cyanide and its degradation products at three Nevada gold mines; constraints from stable C- and N-isotopes

An understanding of the fate of cyanide (CN-) in mine process waters is important for
addressing environmental concerns and for taking steps to minimize reagent costs. The utility of
stable isotope methods in identifying cyanide loss pathways has been investigated in case studies at
three Nevada gold mines.
Freshly prepared barren solutions at the mines have cyanide d15N and d13C values
averaging -4 ? and -36 ?, respectively, reflecting the nitrogen and carbon sources used by
commercial manufacturers, air and natural gas methane. Pregnant solutions returning from ore
heaps display small isotopic shifts to lower d15N and d13C values. The shifts are similar to those
observed in laboratory experiments where cyanide was progressively precipitated as a
cyanometallic compound, and are opposite in sign and much smaller in magnitude than the shifts
observed in experiments where HCN was offgassed. Offgassing is inferred to be a minor cyanide
loss mechanism in the heap leach operations at the three mines, and precipitation as cyanometallic
compounds, and possibly coprecipitation with ferric oxides, is inferred to be an important loss
mechanism.
Isotopic analysis of dissolved inorganic carbon (DIC) shows that uptake of high d13C air
CO2 has been important in many barren and pregnant solutions. However, DIC in reclaim pond
waters at all three mines has low d13C values of -28 to -34 ? indicating cyanide breakdown either
by hydrolysis or by other chemical pathways that break the C-N bond. Isotope mass balance
calculations indicate that about 40 % of the DIC load in the ponds, at a minimum, was derived
from cyanide breakdown. This level of cyanide hydrolysis accounts for 14-100 % of the dissolved
inorganic nitrogen species present in the ponds. Overall, isotope data provide quantitative evidence
that only minor amounts of cyanide are lost via offgassing and that significant amounts are
destroyed via hydrolysis and related pathways. The data also highlight the possibility that
significant cyanide may be either retained in the ore heaps or destroyed via other chemical
pathways.