1

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Accounting for cyanide and its degradation products at three Nevada gold mines:

Constraints from stable C- and N-isotopes

by

Craig A. Johnson1, David J. Grimes1, and Robert O. Rye1

Open-File Report 98-753

This report is preliminary and has not been reviewed for conformity with U.S.

Geological Survey editorial standards or with the North American Stratigraphic

Code. Any use of trade, product, or firm names is for descriptive purposes only

and does not imply endorsement by the U.S. Government.

1

Denver, Colorado

1998
2

Accounting for cyanide and its degradation products at three Nevada

gold mines: Constraints from stable C- and N-isotopes*

By Craig A. Johnson, David J. Grimes, and Robert O. Rye

ABSTRACT

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 δ15N and δ13C 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 δ15N and δ13C 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 δ13C air

CO2 has been important in many barren and pregnant solutions. However, DIC in reclaim pond

waters at all three mines has low δ

13C 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.

INTRODUCTION

Dilute solutions of cyanide (CN-) are used at a large number of mines to leach gold from

ores. The basic process is well known, having been used on a commercial scale for more than a

century (von Michaelis, 1985).

It is important to understand the fate of cyanide in process waters because of its toxicity and

because of increasing environmental concerns. Cyanide levels are regulated in mine discharge

(Smith and Struhsacker, 1988), and cyanide use at U.S. mines will soon be subject to Toxic

Chemical Release Inventory (TRI) reporting as a consequence of the extension to metal mining

facilities of Section 313 of the Emergency Planning and Community Right-to-Know Act and

Section 6607 of the Pollution Prevention Act. Mines will need to give an annual accounting for the

 

*

 This report contains the text of a talk that was delivered at the Geological Society of America

annual meeting in Toronto, Ontario on October 26, 1998. An abstract of the talk was published

previously (Johnson et al., 1998).
3

cyanide imported on-site including quantities treated for destruction, quantities accumulated, and

quantities released to the environment. A second reason to understand the fate of cyanide in

process waters is that this will identify steps to conserve it and minimize mining costs (Botz and

others, 1995).

Cyanide chemistry is complex, and there are several pathways by which it can be lost or

degraded in process waters (Smith and Mudder, 1991). The operative loss pathways can be

difficult to identify, however, so accurate TRI reporting is going to be a formidable task at many

mines. We have been investigating the utility of stable isotope methods for identifying specific

loss pathways and for quantifying the amounts of cyanide lost. The purpose of this report is to

provide an overview of case studies that were carried out at active cyanidation operations at the

Lone Tree, Pinson, and Getchell mines in north-central Nevada. We are grateful to Newmont

Gold Company, Pinson Mining Company, and the Getchell Gold Corporation for access and

assistance in carrying out the work.

DESCRIPTIONS OF THE HEAP LEACH CIRCUITS, SAMPLES TAKEN, AND

PROBABLE CYANIDE LOSS MECHANISMS

At the three mines, barren cyanide solutions with pH about 10.5 are sprayed onto the ore

heaps. The solutions percolate through the crushed ores and dissolve gold as the cyanide complex,

Au(CN)2

-1. They then hit an impermeable pad beneath the heaps, and are channelled outward and

through a series of tanks containing activated carbon. The carbon adsorbs the gold, and the barren

solutions then flow out to a holding pond where they are amended with fresh cyanide and base for

another pass through the heaps. The activated carbon is removed from the tanks periodically for

gold stripping.

At each mine, water samples were taken at several different points in the heap leach circuit

over a time period of an hour or two, so the samples represent snapshots of the circuits. The heap

leach operations were the primary focus of the study, but for comparison purposes mill reclaim

pond waters were also sampled at each of the mines as were mill process waters at Pinson.

Smith and Mudder (1991) have reviewed a large number of laboratory and case studies that

were carried out to determine the chemical pathways by which cyanide can be lost in heap leach

circuits. The results of these studies indicate that the loss pathways likely to be important at Lone

Tree, Pinson, and Getchell are:

1) offgassing as HCN,

2) hydrolysis, which is a term that we will use in this report for a family of oxidative pathways,

some mediated by bacteria, that break the C-N bond to form dissolved inorganic carbon and

nitrogen species,

3) sulfidation to thiocyanate (SCN-),

4) precipitation of insoluble cyanometallic compounds, and

5) adsorption or coprecipitation on solid phases.

ISOTOPIC ANALYSES OF CYANIDE AND IMPLICATIONS FOR CYANIDE

LOSS PATHWAYS

Figure 1 shows carbon and nitrogen isotopic analyses of dissolved cyanide from the three

mines. Cyanide was separated from the samples by reflux distillation to an alkaline trap following

standard techniques (USEPA, 1986). The cyanide in the trap was then precipitated and analyzed

isotopically by a method described by Johnson (1996). The analyses are of total cyanide which

includes the CN- anion and aqueous HCN (the relative proportion of which changes with pH) and

cyanide complexed with metals.

The isotopic compositions are expressed in δ-notation relative to internationally recognized

standards where δ (in ‰) = (RSA/RST - 1) x 1000. For nitrogen, RSA and RST refer to the ratio

15N/14N in the sample and air N2, respectively; for carbon, RSA and RST refer to the ratio 13C/12C in
4

the sample and Vienna Pee Dee Belemnite (VPDB), respectively. The reproducibility of the

measurements is typically within ±0.5 ‰.

Barren solutions and fresh cyanide as delivered to the mines define a triangular field with

average values of -4 ‰ for nitrogen and -36 ‰ for carbon. Overall, these compositions reflect the

sources used in the common manufacturing processes, air for nitrogen and natural gas methane for

carbon (Kroschwitz, 1993). The small shift downward from the 0 ‰ air δ15N value indicates that

the manufacturing process fractionates the isotopes of nitrogen slightly. Also shown in Figure 1

are analyses of cyanide from the mill operation at Pinson and the mill reclaim pond. The δ13C

values for these samples are -49 to -53 ‰, much lower than in the heap leach operations. This

discrepancy illustrates the fact that the natural gas methane used to manufacture cyanide can span a

very broad δ

13C range, from -20 to -65 ‰, depending on its point of origin (Schoell, 1988). The

Nevada mines periodically change cyanide suppliers and the low δ13C values in the pond likely

reflect the δ

13C of an earlier batch of cyanide from a different supplier.

The pregnant solutions returning from the heaps show wider isotopic ranges than the

barren solutions. For some samples, the isotopic compositions are indistinguishable from the

starting barren solutions, but for others there was a shift to lower δ15N and δ13C values along a

slope of about unity. This shift is a fingerprint of the cyanide loss pathways in the ore heaps.

Figure 2 shows the variation in cyanide δ

15N values with the fraction of cyanide remaining

(relative to the barren solution concentrations at a given mine). The curve is a least squares fit to the

Lone Tree mine data using a Rayleigh distillation model. At the time of our sampling, cyanide

losses were high at all three mines. The losses produced negative (downward) isotopic shifts

meaning that the heavier isotope 15N was preferentially lost as the solutions travel through the ore

heaps. This is the opposite of chemical reactions that are kinetically controlled. The implications

of this behavior will be discussed below. 

The cyanide behaved differently depending on whether the solutions encountered oxidized

ores or sulfide ores in the heaps. Figure 3 shows a correlation between cyanide concentration, or

cyanide δ

15N, and thiocyanate (SCN-

). Thiocyanate forms from intermediate valence sulfur species

during the oxidation of sulfide minerals, and its concentration can be taken as an index of the

amount of sulfide ore that a solution encounters in the heaps. Where thiocyanate is low (where the

solutions encountered predominantly oxidized ores) the cyanide losses were greatest and the δ15N

values were shifted downward. Where thiocyanate is high (where the solutions encountered

sulfide ores) the cyanide losses were less and δ15N was unchanged from the original values in the

barren solutions. As a point of reference, the highest thiocyanate levels observed in the solutions

would account for about 10 % of the cyanide loss in one pass through the ore heaps. Unless

thiocyanate has a very fast removal rate, the results suggest that sulfidation is a minor cyanide loss

mechanism at these mines.

Where sulfide minerals are present in the heaps, solution pH usually drops and metals are

taken into solution. Figure 4 shows the variation in dissolved iron with thiocyanate concentration.

There is a large increase moving into the sulfide ores (note that iron is plotted on a log scale). The

horizontal line in Figure 4 is the iron concentration sufficient to accomodate all the cyanide in the

starting barren solutions as an iron complex. The solutions that encountered sulfide ores clearly

have the capacity to complex large amounts of cyanide.

Figure 4 also shows the concentrations of several other metals (Mn, Zn, Ni, Co, Cd, and

Ag) that are known to combine with the ferri- or ferrocyanide complexes to form insoluble

compounds. These metals also increase as more sulfidic ores are encountered. The data in Figure

4 don’t prove that the cyanide in these heap leach operations is precipitating as insoluble

cyanometallic compounds, but they are at least permissive evidence that precipitation is an

important cyanide loss pathway in portions of the heaps that contain sulfide ores.

Another point to be made with respect to Figure 4 is that the iron concentrations are very

low in the solutions that encountered predominantly oxidized ores (the low thiocyanate solutions).

The reason for this is that large amounts of ferric oxy/hydroxides are precipitated. Discrete

cyanometallic compounds may not have precipitated from these solutions , but coprecipitation of

cyanide with ferric oxides, or adsorption onto ferric oxides, may have been important.
5

To determine whether the isotopic shifts observed at the three mines (Figures 1 and 2) are

compatible with the cyanide precipitation or coprecipitation hypothesis, we carried out experiments

in the laboratory in which we precipitated copper(II)ferrocyanide from simple solutions and then

measured the isotopic shifts in the residual free cyanide. The results are shown in Figure 5, and

they match reasonably well the small, non-kinetic isotope fractionations that were observed at the

mines. It is important to remember though that the samples with the greatest cyanide losses and the

lowest δ15N values have encountered mainly oxidized ores and they are low in metals. The fact

that these samples follow the same trend on Figure 5 as the samples with higher metal contents

(and correspondingly greater cyanide complexing capacity) suggests that coprecipitation of cyanide

with ferric oxides may produce similar small, non-kinetic fractionations. This is a question that

needs further investigation.

Another cyanide loss pathway favored by lower pH is offgassing of HCN. One of our

original hypotheses was that offgassing would be analogous to the evaporation of water which is

kinetically controlled and in which the lighter isotopes are preferentially lost (Dansgaard, 1964).

We confirmed this by conducting offgassing experiments in the laboratory using simple solutions,

and the results are illustrated in Figure 5. Cyanide concentrations declined, and δ15N (and δ13C)

values of the residual cyanide progressively increased. Note that the isotopic shifts are quite large.

The absence of similar increases in δ

15N at the mines indicates that HCN offgassing is not

an important mechanism at Lone Tree, Pinson, and Getchell. It is possible that some offgassing

did take place, and that the resulting positive δ15N shifts were offset by negative shifts caused by

precipitation or coprecipitation reactions. Still though, offgassing could only have been a minor

pathway because the largest downward δ

15N shifts that we have observed, some 4 to 6 ‰, would

be offset by offgassing of only 10 or 20 % by analogy with our experiments.

Hydrolysis and related pathways may be more important where solutions encounter sulfide

than where they encounter oxidized ores. Figure 6 shows the relationship between thiocyanate and

ammonium, and there is an increase toward sulfide ores. Ammonium concentrations depend not

only on production from cyanide but also on any non-cyanide sources and on ammonium loss

pathways. At these mines though we believe that the increase is related to production from cyanide

because i) other possible sources, such as explosive residues, would be uniformly distributed in

the heaps and while they might contribute ammonium they would not lead to higher concentrations

in sulfide ores than in oxidized ores, ii) offgassing of ammonia would likely be uniformly low

because the pregnant solutions all have pH well below the ammonia-ammonium pKa, and iii)

oxidation of ammonia would be expected to proceed at a uniform rate independent of the ore types

encountered. As a point of reference, the highest ammonium levels observed in pregnant solutions

are equivalent to the amount of cyanide lost in one cycle through the heaps.

ISOTOPIC ANALYSES OF DISSOLVED CO2 AND NITRATE AND

IMPLICATIONS FOR CYANIDE LOSS PATHWAYS

Another way to monitor cyanide loss by hydrolysis and related pathways is to measure the

isotopic composition of the degradation products. Figure 7 shows the δ13C value of dissolved

inorganic carbon, or dissolved CO2, in barren solutions, pregnant solutions, and reclaim pond

waters. Barren solution holding ponds range from -19 to -28 ‰, and the pregnant solutions range

from -9 to -27 ‰. Reclaim ponds at all three mines are low in δ13C. If cyanide was the only

source of dissolved CO2 in these solutions, then one could measure the CO2 contents and calculate

the amount of cyanide degraded. There are other sources though, and their δ13C values are given

in Figure 7. They are atmospheric CO2, carbonaceous compounds which are indigenous to the

ores at all three of the mines, carbonate minerals, the activated carbon used for gold adsorption,

and cyanide, through several different pathways. Offgassing to the atmosphere and bacterial

reduction are sinks.

Looking just at CO2 production, the full equation for carbon isotope mass balance would

involve the five sources shown in Figure 7, their isotopic compositions, and any isotopic

fractionations. This is far too many unknowns to determine the contribution from cyanide.
6

However, cyanide is distinctive its low δ

13C value (Figure 7), so one can look at a limiting case of

binary mixing between cyanide carbon and the next lowest δ13C source, either activated carbon or

carbonaceous matter depending on the mine. This case will give a minimum amount of CO2 from

cyanide. The calculation isn’t useful for many barren or pregnant solutions because the dissolved

CO2 is dominated by air CO2, which is high in δ13C. The spraying procedure used to administer

the solutions to the ore heaps undoubtedly enhances air uptake. But the reclaim pond waters have

not exchanged as thoroughly with air because they sit static, and a low δ13C cyanide component is

required to explain their isotopic compositions (Figure 7). The results of the binary mixing

calculations (Table 1) indicate that about 40 %, at a minimum, of the dissolved CO2 in the ponds is

cyanide-derived at all three of the mines. 

For each carbon atom produced by cyanide breakdown, there must have been a nitrogen

atom produced. So one can calculate an equivalent amount of dissolved inorganic nitrogen (NO3

-

+NO2

-

+NH4

+

) from the CO2 data, and the results are shown in Table 1. Cyanide breakdown

accounts for all, 20 % and 14 % of the inorganic nitrogen inventory of the ponds. As with the

dissolved CO2 results, these are minimum values.

One can do a parallel isotopic analysis of nitrate, and we have just begun doing that. The

few data that we have at this time are shown in Figure 8. It would be premature to give an

interpretation of the results, but it is clear that nitrate is important in constraining the cyanide budget

because there is enough to account for large quantities of cyanide, some three times the amount that

is lost in one pass through the heaps. The critical question is whether it is all cyanide-derived.

CONCLUSIONS

To conclude, where cyanide solutions encounter oxidized ores, the lack of δ15N increases

indicates that offgassing of HCN is not an important loss pathway. We hypothesize that

coprecipitation with ferric oxides is a major loss pathway. Where cyanide solutions encounter

sulfide ores, the dissolved metal contents suggest that precipitation of cyanometallic compounds

may be important. The δ15N values do not show downward shifts though, and although it is

possible that negative shifts were offset by positive shifts caused by HCN offgassing, the amount

of offgassing could not have been more than 10 or 20 %.

Cyanide hydrolysis and sulfidation are more important loss pathways in sulfide ores than in

oxidized ores. Where solutions sit ponded, isotopic analyses of dissolved CO2 place lower bounds

on the amount of hydrolysis occurring, and it is considerable. Isotopic analysis of dissolved

nitrate may allow better quantification of hydrolysis and related pathways, and this is an area that

we are currently pursuing.

REFERENCES CITED

Botz, M.M., Stevenson, J.A., Wilder, A.L., Richins, R.T., Mudder, T.L., and Burdett, B.,

1995, Cyanide recovery and destruction: Engineering and Mining Journal, v. 196, p. 44-

47.

Dansgaard, W., 1964, Stable isotopes in precipitation: Tellus, v. 16, p. 436-468.

Johnson, C.A., 1996, Determination of 15N/14N and 13C/12C in solid and aqueous cyanides:

Analytical Chemistry, v. 68, p. 1429-1431.

Johnson, C.A., Grimes, D.J., and Rye, R.O., 1998, Accounting for cyanide and its degradation

products at three Nevada gold mines: Constraints from stable C- and N-isotopes [abs.]:

Geological Society of America Abstracts with Program, v. 30. p. A-79.

Kroschwitz, J.I., ed., 1993, Kirk-Othmer encyclopedia of chemical technology, 4th ed.: New

York, John Wiley & Sons.

Schoell, M., 1988, Multiple origins of methane in the Earth: Chemical Geology, v. 71, p. 1-10.

Smith, A., and Mudder, T., 1991, The chemistry and treatment of cyanidation wastes: London,

Mining Journal Books Limited, 345 p.
7

Smith, A., and Struhsacker, D.W., 1988, Cyanide geochemistry and detoxification regulations, in

van Zyl, D.J.A., Hutchison, I.P.G., and Kiel, K.E., eds., Introduction to evaluation,

design and operation of precious metal heap leaching projects: Littleton, Colorado, Society

of Mining Engineers, Inc., p. 275-292.

U.S. Environmental Protection Agency, 1986, Test methods for evaluating solid wastes, 3rd ed.,

SW-846, Washington, D.C., U.S. Government Printing Office, vol. 1C, p. 9010A-1-

9010A-15.

von Michaelis, H., 1985, Role of cyanide in gold and silver recovery, in van Zyl, D., ed.,

Cyanide and the environment: Fort Collins, Colorado State University, p. 51-64.
opic analyses of total cyanide 

opic analyses of total cyanide

-12-10

-8-6-4-2

0

-55 -50 -45 -40 -35 -30

δ13C of total cyanide

Barren

solutions

Pregnantsolutions

Pinson mill solutions 

& reclaim pond

gure 1. Carbon and nitrogen isotopic analyses of total cyanide in process water samples from

the Pinson (blue symbols), Lone Tree (red symbols), and Getchell (green symbols) mines.
nide loss fractionates isotopes 

nide loss fractionates isotopes

-12-10

-8-6-4-2

0

1 0.8 0.6 0.4 0.2

0

Fraction of cyanide remaining

α=1.0018

ure 2. Variation in δ15N of total cyanide with the fraction of cyanide remaining. The curve is a

least squares fit to the data from the Lone Tree mine using a Rayleigh distillation model.Data from Pinson are shown in blue, Lone Tree in red, and Getchell in green.
dized 

dized vs sulfide ores sulfide ores

0

0000

-12-10-8-6-4-20

0 5 10 15 20

δ15N of total cyanide

SCN (mg/L)

δ15N

Total cyanide

Oxidized ores Sulfide ores

Figure 3. Variation in total cyanide concentrations and δ15N values with thiocyanate showing that

there is a systematic change in the behavior of cyanide depending on the amount of sulfideore that a solution encounters in the ore heaps. Data from Pinson are shown in blue, LoneTree in red, and Getchell in green.
ipitation & coprecipitation 

ipitation & coprecipitation

1

1

0

0

0

0

4

01020304050

0 5 10 15 20

Mn+Zn+Ni+Co+Cd+Ag (mg/L)

SCN (mg/L)

100% Fe(CN)

6 equivalent

gure 4. Variation in iron and other metals with thiocyanate. As they encounter increasing

amounts of sulfide ores, the solutions show an increasing capacity to carry metals. Thehorizontal line is the iron concentration sufficient to accomodate all the cyanide in the barrensolutions as the ferri- or ferrocyanide complex. Iron concentrations are plotted as filled

circles, combined minor metals are plotted as open circles. Data from Pinson are shown in

blue, Lone Tree in red, and Getchell in green.
parison with lab experiments 

mparison with lab experiments

-12

-8-4

04

1 0.8 0.6 0.4 0.2

0

Fraction of cyanide remaining

HCN offgassing

expts 

α=0.979

Complexation & precipitation

expts 

α=1.0005

Lone Tree

Mine

e 5. Comparison of the isotopic shifts observed in the Lone Tree mine pregnant solutions

with those observed in laboratory experiments where cyanide was offgassed or precipitatedas a cyanometallic complex. The Lone Tree data are compatible with cyanide loss by

precipitation, but not with cyanide loss by offgassing. Experimental results are plotted in

black, data from Lone Tree are plotted in red.
rolysis and related pathways 

rolysis and related pathways

0

1020304050

0 5 10 15 20

SCN (mg/L)

Figure 6. Variation in ammonium with thiocyanate. Hydrolysis and other pathways which break

the C-N bond appear to be more important where solutions encounter sulfide ores than

where they encounter oxidized ores, but the possibility of non-cyanide sources and a poorunderstanding of ammonium sinks makes this conclusion tentative. Data from Pinson areshown in blue, Lone Tree in red, and Getchell in green.
olved CO 

olved CO

2

-50-40-30-20-10

0

0 20 40 60 80 100

dissolved CO

2 (mg/L C)

Carbonate minerals

Air CO

2 Carbonaceous matter 

(G)

Activated carbonCarbonaceous matter 

(P)

Cyanide

Cyanide

Pregnant solutions

Reclaim ponds

Barren ponds

Carbon isotopic compositions of dissolved CO2 in barren solution holding ponds,

gnant solutions, and reclaim pond waters. The isotopic compositions of the potential

2 sources were taken from the literature or from our own unpublished measurements.

laim pond waters have low δ13C values, which is a fingerprint of cyanide breakdown.

a from Pinson are shown in blue, Lone Tree in red, and Getchell in green.
ate 

ate

-10

-5

05

0 100 200 300 400 500

NO3 (mg/L)

Cyanide

Reclaim

pond

Pregnant

solution

Figure 8. Nitrogen isotopic compositions of nitrate in a pregnant heap leach solution and in

reclaim pond water from the Getchell mine. The reclaim pond isotopic composition

resembles the composition of the cyanide in use at the Getchell mine, but the pregnant

solution has a substantially higher δ-value.
16

Table 1.-Minimum percentages of dissolved inorganic carbon (CO2+HCO3

-

+H2CO3) and dissolved

inorganic nitrogen (NH4

+

+NO2

-

+NO3

-

) derived from cyanide in reclaim ponds

---------------------------------------------------------------------------------------------------------------------

Mine Dissolved inorganic carbon Dissolved inorganic nitrogen

---------------------------------------------------------------------------------------------------------------------

Pinson 40 100

Lone Tree 43 20

Getchell 38 14

---------------------------------------------------------------------------------------------------------------------