Closure, Remediation & Management of Precious Metals Heap Leach Facilitiesedited by Dorothy Kosich and Glenn Miller. January 14-15, 1999

CYANIDE BEHA VIOR IN HEAP LEACH CIRCillTS: A NEWPERSPECTIVE FROM STABLE CARBON- AND NITROGEN-ISOTOPE

DATA

Craig A. Johnson, David J. Grimes, and Robert O. RyeU.S. Geological Survey, Box 25046, MS 963, Denver, CO 80225

ABSTRACT

The stable isotopic composition of process solution cyanide (CN-) was analyzed at three heap leach operations inNevada in order to detennine cyanide loss pathways and compare the losses along specific pathways. Pure cyanidefrom the suppliers and cyanide in barren solutions has cSlsN and cSl3C values of -3:2%0 (relative to air) and -36:t:2%0(relative to PDB). respectively. These compositions reflect the nitrogen and carbon sources used by themanufacturers. commercial ammonia and natural gas methane. Cyanide in pregnant solutions returning from theheaps displays downward isotopic shifts of up to 7%0 in cSlsN and up to 4%0 in cS13C. The shifts mimic the isotopicshifts observed in laboratory experiments in which cyanide was progressively precipitated as a cyanometalliccompound. and are opposite in sign and much smaller in magnitude than the isotopic shifts observed in experimentsin which HCN was offgassed. We infer that at the time of our sampling. offgassing of HCN was a minor cyanideloss pathway and retention within the heaps by adsorption or precipitation was the major loss pathway. Theimplications of these findings are that (1) any changes made to reduce offgassing. such as improving control ofsolution pH. may not significantly reduce cyanide consumption. and (2) cyanide rinsing from heaps at the time ofclosure may be controlled by dissolution or desorption kinetics as well as by heap hydraulic properties.

INTRODUCTION

Dilute solutions of cyanide (CN") are used at a large number of heap leach operations to extract gold from ores. Thebasic process is well established and has been used on a commercial scale for more than a century (von Michaelis,1985). Heap leaching consumes cyanide, and it is standard procedure to maintain cyanide concentrations at thedesired level by continuously adding reagent to recycled process solutions before they are applied to the ore heaps.The chemical reactions and mechanisms that are responsible for cyanide consumption have been studied intensivelyover the past 20 years (Smith and Mudder, 1991), but cyanide chemistry is complex and the relative importance ofcompeting chemical reactions remains difficult to detennine.

An understanding of the fate of cyanide in process solutions is important for evaluating the environmental impact ofany process solution releases and for the best possible control of the substance, either for the purpose of conservingit to minimize reagent costs during active leaching, or for the purpose of destroying it in any discharge or at closure.Infonnation on the fate of cyanide will also assist in completing Toxic Chemical Release Inventory reports tocomply with Section 313 of the Emergency Planning and Community Right-to-Know Act and Section 6607 of thePollution Prevention Act.

The purpose of this paper is to review highlights from an ongoing U.S. Geological Survey project in which stableisotopic measurements are being used to examine cyanide behavior in heap leach operations. Data and findings thatare summarized here are discussed in greater detail in Johnson et aI. (1998).

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THE CYANIDE CYCLE IN HEAP LEACH OPERATIONS

Figure 1 is an illustration of the cyanide cycle in a typical heap leach operation. The central box represents totalcyanide in the process solution which includes free cyanide (CN", HCNaq) and all cyanide comp1exed with metals.State of Nevada mine effluent regulations consider only weak acid-dissociable cyanide (Smith and Struhsacker,1988), which includes free cyanide and cyanide contained in weak complexes such as Cd(CN)3-1 and Zn(CN)4-2.Strong complexes, such as Fe(CN)6-4, Fe(CN)6"3, CO(CN)6-4, and AU(CN)2-1, are not subject to regulation in Nevada,but can serve as a sink for free cyanide where they form, or a source of cyanide where they dissociate.

To maintain free cyanide concentrations at a level that promotes efficient leaching of gold. reagent cyanide is addedto recycled solutions before they are applied to the heaps. This cyanide supply is easily quantified by examiningpurchasing records.

Possible cyanide sinks are (1) loss of HCN gas to the atmosphere, (2) leakage of process solutions to theenvironment, (3) retention within the heaps by adsorption or precipitation, and (4) oxidation to fonn dissolvedinorganic carbon and nitrogen species (Fig. 1). Offgassed HCN is removed from the process solution by diffusiveand advective dispersion into the atmosphere. Offgassing has been proposed to be the major cyanide lossmechanism in process solutions (Smith and Mudder, 1991) on the basis of theoretical considerations and laboratoryexperiments (e.g., Simovic et al., 1985). At heap leach operations, offgassing fluxes have not been measureddirectly because of the difficulty of obtaining accurate results.

Leakage of process solutions from active operations usually leads to cyanide losses that are small relative to thelosses from other pathways. Where leaks are recognized, the cyanide flux can be estimated from analyses of groundwater samples obtained from monitoring or extraction wells.

Retention of cyanide within the heaps can occur by adsorption onto secondary minerals, precipitation ofcyanometallic compounds, or coprecipitation with iron oxides. Retained cyanide can return to solution if solutionchemistry changes, as during heap rinsing prior to closure. The stability of cyanide-containing solids and thekinetics of dissolution and desorption are important controls on the release of this cyanide (c.f., Meeussen et al.,1992; Theis et al., 1994), and these factors may be important to consider in planning the rinsing and closure ofinactive heaps. At the present time, the importance and long-term stability of retained cyanide is poorlycharacterized and represents a part of the cyanide cycle in critical need of research (Heriba, 1991).

The final loss pathway shown in Figure 1 is oxidation, which includes a number of possible chemical pathwaysinvolving hydrolysis and abiotic or bacterial oxidation reactions (Chatwin, 1989). Fonnate (COOH-) and cyanate(CNOl can appear as intermediate species. The end products of carbon oxidation are the dissolved carbonatespecies, H2CO3, HCO3", or CO32". The relative abundance of these species is a function of solution pH which, for theprocess solutions samples discussed below, ranges from 10 to as low as 3. The nitrogen species produced are NH3or NH4+ which can further oxidize to nitrite and nitrate. An important feature of the oxidation pathways is that theyare irreversible (Barbosa-Filho and Monhemius, 1994), and thus provide for pennanent destruction of the cyanide.A related pathway is the formation of thiocyanate (SCN1 by reaction of the cyanide anion with intennediate-valencesulfur species, and the subsequent oxidation of the thiocyanate. Thiocyanate fonnation may not permanentlydestroy the cyanide anion, however, because oxidation can give rise to sulfate plus the CN- anion (Barbosa-Filhoand Monhemius, 1994).

Estimates of the flux of cyanide along the oxidation pathways can be made by monitoring increases in dissolvedinorganic carbon and nitrogen species as the solutions flow through the ore heaps. However, estimates made by thismethod are of questionable value because the inorganic carbon and nitrogen species are removed from solution by avariety of mechanisms, and may have non-cyanide sources. For dissolved inorganic nitrogen, ammonia offgassingor nitrate denitrification would reduce the concentrations, and dissolution of explosive residues may increase them.For dissolved inorganic carbon, exchange of carbon dioxide with the atmosphere is a complicating factor.

Among the cyanide fluxes shown in Figure 1, only the addition of reagent cyanide and the loss via process solutionleakage are straightforward to quantify. Cyanide fluxes related to offgassing, retention, and oxidation are notamenable to direct measurement, and they are generally poorly known.

STABLE ISOTOPE BACKGROUND

Stable isotope techniques have been used routinely over the past several decades to study the cycles of elements innatural systems (see Faure, 1986 and Hoefs, 1987 for overviews). The value of stable isotope measurements is thatthey can reveal the sources and sinks for chemical species, they can be used in mass balance calculations, and theycan indicate the relative rates of chemical reactions.

Isotope techniques can be used in cyanide studies because nitrogen and carbon each has two naturally occurringstable isotopes. The more abundant, or major, isotopes are 14N and 12C, and the minor isotopes are lsN and 13C.Overall, ISN comprises about 0.37% of terrestrial nitrogen, and 13C comprises about 1.1 % of terrestrial carbon(Hoefs, 1987). There are slight variations in the abundances of lsN and 13C in natural and synthetic compounds thatcan be understood using thermodynamic and kinetic theory.

By convention. the abundances of stable isotopes are expressed in o-notation as ratios of the minor isotope to themajor isotope relative to a reference standard where:

( (ISN/14N)SamPle-l ) XlOOO SISN(in pennil, %0) = (IS N/14 N)standard (1)

and

(13C/12C)sample -11 x 1000

(13C/12C)standard )(2)813C(in pennil, %0) = I

The reference standard for nitrogen is air N2, and the reference standard for carbon is calcite from a marine fossilwhich is referred to as PDB for Pee Dee Belemnite (Hoefs, 1987).

In chemical reactions that are irreversible -such as cyanide offgassing and cyanide oxidation in the heap leachenvironment -the stable isotope partitioning can be predicted using a Rayleigh distillation model. Takingoffgassing as an example, the chemical reaction is:

(3)HCNaq => HCN,.

By analogy with the evaporation of water (Dansgaard, 1964), the lighter isotopic species (H12CI4N) will offgas fasterthan the heavier isotopic species (HI3CI4N, HI2C1sN, etc.), and the liquid will become enriched in the heavierisotopic species. The degree of isotopic enrichment per amount of HCN offgassed will depend on the ratio of theoffgassing rates for the different isotopic species, which is termed the a.-factor. The isotopic change for the residualHCNaq and the HCN, is shown in Figure 2 for a hypothetical a.-factor of 1.005 (the offgassing rate constants for theisotopic species of HCN are not known, so the a-factor cannot be calculated from theory). The o-value of theaqueous cyanide increases as offgassing proceeds. The O-value of the offgassed cyanide is lower than that of theaqueous cyanide, and it also increases progressively.

RESULTS AND DISCUSSION

To investigate the utility of stable isotope methods for tracing cyanide behavior, analyses were perfonned onsolutions prepared in laboratory experiments and on process solutions collected at active heap leach operations at thePinson. Lone Tree. and Getchell mines in northern Nevada. Pinson Mining Company. Newmont Gold Company.and Getchell Gold Corporation personnel assisted with planning and with collection of samples, and providedinfonnation that was helpful in interpreting the analytical results. Methods used for the isotopic analysis weredescribed previously (Johnson. 1996). Reproducibility was :t:l%o (one-sigma).

57

Figure 3 shows the O13C and olsN values of total cyanide in process solutions from the three operations. Barrensolutions and liquid cyanide delivered by the suppliers define a triangular field with olsN of -3:t2%0 and Ol3C of-36:t2%0. The data reflect the isotopic compositions of the materials used in the common cyanide manufactUringprocesses, commercial ammonia for nitrogen and natural gas methane for carbon (Kroschwitz, 1993). Methanespans a wide range of O13C, from -20 to -65%0, depending on the geologic setting of the source (Schoell, 1988), sosome variation in cyanide Ol3C is expected from one supplier to another. In fact cyanide O13C values of -55 to -500/00were observed in another ore processing circuit (unpublished data) which indicates that the large isotopic variabilityof natural gas methane is represented in the cyanide that is marketed today in Nevada. Commercial ammonia issynthesized from air N2 and typically has a olsN value within a few permil of the 0%0 air value (Bohlke and Coplen,1995). Thus commercial cyanide is unlikely to show significant olSN variation from one manufacturer to another.

Pregnant solutions returning from the heaps in some cases show isotopic shifts to lower olsN and Ol3C values (Fig.3). Figure 4 illustrates that the nitrogen isotopic data are compatible, within error, with a Rayleigh distillationmodel.

In Figure 5, the Lone Tree nitrogen isotopic data are compared with data from laboratory experiments. Theexperimental data show the nitrogen isotopic shifts that occurred in residual dissolved cyanide during progressiveoffgassing of HCN and during progressive complexation and precipitation as copper(II)ferrocyanide. Theexperimental results confirm that the behavior of the isotopes during cyanide offgassing is analogous to the behaviorof the isotopes during the evaporation of water in that the lighter species are preferentially lost and the O-values ofthe residual cyanide progressively increase. In contrast, the complexation and precipitation experimental resultsindicate small negative isotopic shifts. The shift to lower O-values indicates that the complexation/precipitationprocess was not strictly unidirectional, and that dissolution and reprecipitation occurred. No experiments werecarried out to investigate cyanide loss by oxidation but. because the oxidation reactions are irreversible, isotopicfractionations will be controlled by kinetics and any shifts in O-values of the residual cyanide would be positive(Hoefs, 1987).

By analogy with the laboratory experiments, the o-value of process solution cyanide should be very high whereHCN offgassing is occurring and where cyanide losses are large. For the 77 to 98% cyanide losses calculated for thepregnant process solutions (Fig. 3), the olsN values of the residual cyanide can be predicted using a Rayleighdistillation model and the experimentally-detennined a-factor for offgassing (Fig. 5). The results are 28 to 83%0,clearly very different from the measured values of -10 to -3%0 (Fig. 3). In detail, the a-factors associated withHCN offgassing may vary with the rate of offgassing, but the rate in the experiments (IO%/hr) was sufficiently closeto the probable rate of cyanide loss at the heap leach operations (1 %/hr or greater) that large, positive olsN shifts arestill "to be expected if off gassing was important

Process solution 51SN shifts reflect the sum of the shifts caused by each of the cyanide loss mechanisms that operate.For this reason, isotopic effects of different loss mechanisms could offset or augment one another. However, it isdifficult to escape the conclusion that HCN offgassing was a minor loss mechanism at the operations examined inthis study. The reason for this is that the a-factor for offgassing is large enough that only a small offgassing losswould cause 51SN values in the residual cyanide to increase relative to their starting value. For example, for the a-factors determined in our experiments, an offgassing loss of only 1.5% of the starting cyanide would be sufficient tooffset the 51SN decrease caused by complexation/precipitation another 50% of the cyanide. Because cyanideoxidation is irreversible, oxidation would also offset 51SN decreases caused by complexation/precipitation, althoughpossibly less strongly than offgassing. Thus the negative 51SN shifts observed in the process solutions indicate thatcyanide loss was largely by reversible reactions, such as precipitation within the heaps, and that offgassing wasminor. The extent to which cyanide was oxidized cannot be determined from the cyanide isotope data withoutadditional information on oxidation a-factors. With regard to strategies for reducing cyanide consumption at theoperations examined in this study, we predict that steps taken to reduce offgassing, such as practicing more stringentpH control, would lead to only limited reductions in consumption. Our finding that cyanide retention within theheaps may be important means that cyanide elution during heap rinsing prior to closure may be controlled bydesorption and dissolution kinetics as well as by the hydraulic properties of heaps.

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Overall, stable isotope methods are useful in constraining cyanide losses via two pathways that are difficult toquantify using other methods. offgassing of HCN and retention within the heaps. Isotopic analyses at three heapleach operations in northern Nevada suggest that offgassing is a minor cyanide loss pathway, and retention withinthe heaps is a major loss pathway. With additional development of isotopic techniques, it may be possible toquantify cyanide offgassing fluxes and oxidation fluxes. Retention within the heaps could then be detennined bybalancing the overall cyanide cycle for an operation.

REFERENCES

Barbosa-Filho, 0., and Monhemius, A.J., 1994, Leaching of gold in thiocyanate solutions-Part 1: chemistry andthermodynamics: Transactions of the Institution of Mining and Metallurgy (Section C), v. 103, p. CI05-ClIO.

Bohlke, J.K., and Coplen, T.B., 1995, Interlaboratory comparison of reference materials for nitrogen-isotope-ratiomeasurements: International Atomic Energy Agency TECDOC-825, p. 51-66.

Chatwin, T .D., 1989, Cyanide attenuation/degradation in soil, final report: Resource Recovery & ConservationCompany, Salt Lake City, 60 p.

Dansgaard, W., 1964, Stable isotopes in precipitation: Tellus, v. 16, p. 436-468.Faure, G., 1986, Principles of isotope geology, 2nd ed.: New York. John Wiley & Sons, 589 p.Heriba, A., 1991, Factors in the development of a standard test for cyanide in neutralized, spent ore: Unpublished

Ph.D. dissertation, South Dakota School of Mines and Technology, Rapid City, 73 p.Hoefs, J., 1987, Stable isotope geochemistry, 3rd ed.: Berlin, Springer-Verlag, 241 p.Johnson, C.A., 1996, Determination of IsNf4N and 13C11~ in solid and aqueous cyanides: Analytical Chemistry, v.

68, p. 1429-1431.Johnson, C.A., Grimes, DJ., and Rye, R.O., 1998, Accounting for cyanide and its degradation products at three

Nevada gold mines: Constraints from stable C- and N-isotopes: U.S. Geological Survey Open-File Report98-753,16 p.

Kroschwitz. J.I.. ed., 1993, Kirk-Othmer encyclopedia of chemical technology, 4th ed.: New York. John Wiley &Sons.

Meeussen, J.C.L., Keizer, M.G., van Riemsdijk, W.H., and de Haan, F.A.M., 1992, Dissolution behavior of ironcyanide (Prussian blue) in contaminated soils: Environmental Science arid Technology, v. 26, p. 1832-1838.

Schoell, M., 1988, Multiple origins of methane in the Earth: Chemical Geology, v. 71, p. 1-10.Simovic, L., Snodgrass, WJ., Murphy, K.L., and Schmidt. J.W., 1985, Development of a model to describe the

natural degradation of cyanide in gold mill effluents, in van Zyl, D., ed., Cyanide and the environment:Fort Collins, Colorado State University, p. 413-430.

Smith, A., and Mudder, T., 1991, The chemistry and treatment of cyanidation wastes: London, Mining JournalBooks Ltd., 345 p.

Smith, A., and Struhsacker, D. W., 1988, Cyanide geochemistry and detoxification regulations, in van Zyl, DJ.A.,Hutchison, I.P.G., and Kiel, K.E., eds., Introduction to evaluation, design and operation of precious metalheap leaching projects: Littleton, Colorado, Society of Mining Engineers, Inc., p. 275-292.

Theis, T.L., Young, T.C., Huang, M., and Knutsen, K.C., 1994, Leachate characteristics and composition ofcyanide-bearing wastes from manufactured gas plants: Environmental Science and Technology, v. 28, p.99-106.

von Michaelis, H., 1985, Role of cyanide in gold and silver recovery, in van Zyl, D., ed., Cyanide and theenvironment: Fort Collins, Colorado State University, p. 51-64.

59

1Offgassingof

HCN

TotalCyanide

in Solution

Makeup CN-..ReagentCyanide

Oxidation~ Dissolved Inorganic

C- & N-species

~pump~ck~ ~~ sorption

Adsorption Dissolution

-Release Precipitation

Figure 1. The cyanide cycle in a hypothetical heap leach operation. Cyanide reservoirs are shown as boxes, andprocesses that transfer cyanide to or from the reservoirs are shown as arrows.

15

101Q...~0z.,~

5

0

OffgassedHCN

-5

10 ' I f I I I

1 0.8 0.6 0.4 0.2 0

Fraction of cyanide remaining

Figure 2. The change in 51sN of cyanide during off gassing predicted by analogy with the known isotopic behaviorof water undergoing evaporation. Calculated using a Rayleigh distillation model and a hypothetical a-factor of1.005.

m"CCco>-I;)

"in:§"0

.?"iQ

Figure 3. The measured olsN and Ol3C values of process solution cyanide. Open symbols are analyses of barrensolutions or liquid cyanide delivered to the mines by the suppliers. Filled symbols are analyses of pregnantsolutions. Circles are data from Pinson, diamonds from Lone Tree, and triangles from Getchell.

Figure 4. Nib"ogen isotopic compositions of process water cyanide. The curve is a least squares fit to the Lone Treedata (diamonds) using a Rayleigh distillation model. Circles are data from Pinson, and triangles are data fromGetchell.

61

4

! .

LoneTree!

m"CCCD>.0

"iEB'0~"CQ

.12

I I , , ,

1 0.8 0.6 0.4 02 0

Fraction of cyanide remaining

Figure 5. Nitrogen isotopic d.lta from offgassing experiments (open circles) and complexation/precipitationexperiments (open diamonds) compared with empirical data from Lone Tree process solutions (filled diamonds).

62

0

-4

-8