99-136

8/12/2019 99-136

1/8

Copyright 1999 by SME1

SME Annual Meeting

March 1-3, 1999, Denver, Colorado

Preprint 99-1 36COLUMN STUDIES OF THE ADSORPTION OF GOLD CYANIDE COMPLEX BY GOLDSTRIKE

ORE CARBONACEOUS MATERIAL

P. A. Schmitz

S. Duyvesteyn

W. P. Johnson

Univ. of Utah

Salt Lake City, UT

L. Enloe

J. McMullen

Barrick Gold Corp. Inc.Toronto, ON, Canada

ABSTRACT

The characteristics of gold leaching from carbonaceous

matter by sodium cyanide, and sorption of gold cyanide to

carbonaceous matter in autoclaved and non-autoclaved gold

ore from Barrick Goldstrike Mines Inc. (BGMI), Nevada, was

investigated. Much greater gold cyanide complex sorption

was observed for the carbonaceous matter in high

preg-robbing ores relative to the low preg-robbing ores. This

observation corroborates previous reports that have attributed

observed preg-robbing behavior to the carbon in the ore. Gold

cyanide complex sorption was found to be reversible for

carbonaceous matter from low preg-robbing ore, but was

irreversible for carbonaceous matter from high preg-robbing

ores. Autoclaving the ore appeared to enhance the sorptive

behavior of the carbonaceous matter from both high and low

preg-robbing ores.

INTRODUCTION

Barrick Goldstrike Mines, located within the Carlin trend in

Nevada, ore can be subject to two forms of refractoriness. It

has been estimated that more than 50% of the gold inGoldstrike ore is originally encapsulated within a sulfide

phase (Chryssoulis et al., 1996). encapsulation of gold by

sulfides requires oxidation to make the gold amenable to

cyanide leaching. In the processing of the ore, oxidation of

the sulfides is achieved through autoclaving, which releases

elemental particles of gold that are sub-micron in size.

Additionally, naturally occurring organic carbon is found in

many of the gold deposits located within the Carlin trend in

northeastern Nevada. The natural organic carbon has been

implicated in a phenomenon known as preg-robbing

(Hausen and Bucknam, 1985). Preg-robbing was firs

described as the active adsorption of gold from pregnan

cyanide solutions by Smith (196 8).

Smith (1968) recognized that two types of carbonaceous ore

occurred in the Carlin deposits: ores that would adsorb

additional gold from the cyanidation procedure and ores that

would not. Physical and chemical characteristics of the

carbonaceous matter measured from X-ray diffractometry

Raman spectroscopy, FTIR spectroscopy, and LECO

oxygen/carbon analyses, show that the carbonaceous matter

present in the ore is similar to that of commercial activated

carbon (Nelson et al., 1986; Sibrell et al., 1990; Stenebrten

and Johnson, 1998). This is significant because commercia

activated carbon is used in the carbon-in-leach process used

to recover the gold from these ores. It has been shown tha

there is a rough correlation between the preg-robbing

behavior of Goldstrike ore and the microcrystallite

dimensions of the demineralized carbonaceous matter from

non-autoclaved ore (Stenebrten and Johnson, 1998). This

correlation suggests that the micro-crystallite dimension ofthe ore is related to its tendency to adsorb gold.

The motivation for this study was to determine to what extent

autoclaving during processing changes the physical and

chemical characteristics of the carbonaceous matter and its

Au(CN)2- sorbing behavior. Carbonaceous matter is the end

result of a demineralization process used to remove the

quartz, carbonate and some sulfide components of the ore

8/12/2019 99-136

2/8


Two batch adsorption tests were performed on carbonaceous

matter from autoclaved ores classified according to

preg-robbing behavior, e.g. high preg-robbing (HPR) or low

pregrobbing (LPR). The preg-robbing test is described in

Stenebrten and Johnson (1998). The carbonaceous matter

was contacted with a 0.5g.L NaCN solution containing 5ppm

of gold. It was found, by measuring gold in solution after 24hrs, that while the HPR carbonaceous matter sorbed nearly all

of the gold in solution, the solution exposed to the LPR

carbonaceous matter resulted in an equilibrium gold

concentration over 11 ppm. Presumably, gold that was either

associated with phases destroyed during autoclaving (e.g.

sulfides), or possibly originally associated with the

carbonaceous matter, was leached by the cyanide ions in

solution. The gold cyanide ion was then taken up strongly by

the HPR carbonaceous matter but negligibly by the LPR

carbonaceous matter.

Alternatively, it must also be considered whether the CN -

extractable gold content after autoclaving was equal for bothLPR and HPR ores, since an inequality in the amount of gold

present would also result in apparent differences in sorption

behavior. Although the LPR and HPR ores had similar opt

values, variation in the sulfide content of the ore may have

accounted for differences in gold amenable to leaching. This

study was therefore designed to relate the gold adsorption

behaviors of LPR and HPR carbonaceous matter, both

autoclaved and nonautoclaved. This is a problematic task

since the presence of any gold in the carbon interferes with a

straightforward batch adsorption test.

EXPERIMENTAL PROCEDURE

Gold leaching with NaCN and elution with a 1: 1 mixture of

NaCN and NaOH, followed by the adsorption of Au(CN)2-,

and the subsequent desorption Au(CN)2- by NaCN, were

studied sequentially in the carbonaceous matter of both

autoclaved and non-autoclaved LPR and HPR ore. The initial

task was performed to determine how much gold was

available to cyanide leaching followed by sodium hydroxide

elution from the carbonaceous matter of both autoclaved and

non-autoclaved demineralized Goldstrike ore. This was

performed with the recognition that gold associated with the

carbonaceous matter may not be completely released into

solution, but may instead be sorbed by the carbon, especiallyin the case of the HPR carbon. Exposure of the carbonaceous

matter to a gold cyanide solution was then performed to study

the tendency of the carbon to uptake Au(CN)2-. However, any

gold not released during the initial leaching phase may have

influenced the sorption of Au(CN)2- by the carbon. Lastly,

NaCN and a mixture of NaCN and NaOH was used in an

attempt to remove the sorbed gold from the carbonaceous

matter.

The ore was demineralized according to the procedure of

Stenebrten and Johnson (1998). In all ores, HCI and HF

were used to remove the carbonate and silicate minerals

respectively.

Approximately 80-90% of the sulfides were removed form theautoclaved ores during the oxidation process according to

typical values determined by LECO analysis performed by the

BGMI metallurgical services laboratory. An attempt to

remove the sulfides from the nonautoclaved ore was made by

heavy medium separation of the ore with sodium poly-

tungstate. However, electron microprobe analysis of the

demineralized non-autoclaved ores showed significant sulfide

as inclusions in the carbonaceous matter. This demineralized

fraction of carbon including variable concentrations of

sulfides (depending on the success of heavy medium

separation in nonautoclaved ores and the extent of oxidation

in oxidized ores) is hereafter referred to as carbonaceous

matter (CM). The carbon content of the non-autoclaved HPRCM used in this study was 52.5%, whereas the carbon content

of the non-autoclaved LPR CM was 13.3%. The carbon

content of the autoclaved CM was not known at the time of

the study, but was estimated to be 75%, LECO carbon and

sulfur analyses are currently being performed.

Stock solutions of NaCN, Au(CN)2-, and a mixture of

NaCN/NaOH were prepared as follows. The NaCN solution

was prepared at a concentration of 0.5g/L, adjusted to a pH o

10. 5 with 0. 1 M NaOH. The stock gold solution of 5ppm Au

was prepared by adding gold standard to a solution of NaCN

(0.5g/L) adjusted to a pH of 10. 5. A 1: 1 elute mixture of the

NaCN solution and 0. 1 M NaOH was also prepared. The

solutions were pulled through a mini-column prepared by

packing 60 mg of CM in Teflon tubing (1/8"id). A glass woo

plug was used to prevent the passage of the fine particulate

of the CM. A 10 ml syringe was fastened to the top of the

tubing to provide a fluid reservoir. Negative pressure to drive

fluid flow was provided by a Digi-Staltic peristaltic pump

placed on the effluent side of the column.

In the initial leaching phase, a total of 30ml of the NaCN

solution was introduced to the column (0.3ml/min) and

collected sequentially in 5ml aliquots. Following the NaCN

NaCN/NaOH eluent was passed through the column(0.3ml/min) and collected in 5ml aliquots. Both the

autoclaved and non autoclaved HPR CM were exposed to 15

ml of the NaCN/NaOH eluent, however both of the LPR CMs

were contacted with only 10 ml of the NaCN/NaOH eluent. A

third step in the leaching process involved contacting the CM

with I pore volume of stock NaCN solution for 24hrs. The

CM was eluted with additional NaCN solution to a total

volume of 5ml. In the subsequent adsorption experiment, a

total of 30ml of gold cyanide stock solution was introduced to

8/12/2019 99-136

3/8


the column (0.3ml/min). The column effluent was collected in

5ml aliquots. This was followed by the equilibration of 1 pore

volume of gold cyanide stock solution with the CM for a 24hr

period. The pore volume was washed with additional gold

cyanide to a total volume of 5ml. The column was the rinsed

with 5 ml of deionized water. The desorption stage of the

experiment was performed by equilibrating the CM with onepore volume of the NaCN solution for 24hrs. The pore

volume was rinsed with additional NaCN to a total volume of

5ml. All solutions were analyzed for gold using ICP-AES

against 1 ppm and 5ppm gold standard solutions. The

solutions were compared at the 242.795nm wavelength using

a Perkin-Elmer Plasma 4000 Emission Spec ICP. Gold

leaching and desorption into NaCN was measured directly,

while gold adsorption was measured by calculating the

amount of gold lost from the initial 5ppm solution.

EXPERIMENTAL RESULTS

The results of the leaching and elution of gold from thevarious CMs are shown if Figures 1-4. The first 5 ml of

leachate run through the autoclaved LPR CM showed almost

10 ppm of gold (Figure 1). Subsequent volumes of NaCN

eluent showed exponentially decreasing gold concentrations.

A final aliquot of NaCN contacted with the CM for 24 hrs

leached greater gold concentrations into solution, indicating

kinetic limitations in leaching under these conditions. Figure

2 shows almost an identical curve for the non-autoclaved

LPR, the major difference is that the overall concentration of

gold leached into solution is lower. This difference is

presumably due to differing amounts of gold amenable to

leaching between these samples. Removal of the sulfides in

the second sample during heavy medium separation reduced

the gold content of that sample. Furthermore, the remaining

gold was largely non-amenable to leaching due to

encapsulation in the sulfides. In contrast, autoclaving of the

first sample released gold from sulfides, and it appears that

the released gold remained with the carbonaceous matter

during demineralization.

Figure 3 shows there was a significant amount of gold

leached from the autoclaved HPR CM during the first 5 ml of

NaCN exposure. However in subsequent aliquots, the

concentration of gold in solution was near zero. Although the

NaCN/NaOH eluent did not show any measurable gold insolution, there was a significant amount of gold released into

NaCN after 24 hrs contact time. In figure 4, the non-

autoclaved HPR shows there was significant gold leaching

into the first 5 ml aliquot of NaCN and no elution of gold by

the NaCN/NaOH. However, unlike the autoclaved HPR, the

non-autoclaved HPR showed a gradual decrease in gold

concentration during the NaCN leach. Also, the leachate from

the non-autoclaved HPR differed from the autoclaved HPR in

that no gold was measured in the NaCN leachate after 24 hr

contact time.

The amount of gold leached and eluted from the CMs was

calculated by multiplying the concentration of gold by the

volume of each aliquot. In the autoclaved HPR, 55% of the

total amount of gold leached and eluted from the CM wasreleased during the 24 hr contact with NaCN, indicating a

significant kinetic limitation. Leaching of gold from the LPR

CMs also showed kinetic limitation with 5% gold recovery

during 24 hr leaching from the autoclaved LPR, and 15%

gold recovery during 24 hr leaching from the non-autoclaved

LPR. In both the autoclaved and non-autoclaved LPR CMs

elution of gold by the NaCN/NaOH resulted in detectable, but

insignificant amounts (less than 1%) relative to the tota

amount of gold obtained from the CM.

The total amount of gold leached from the CM (Table 1) can

be related to the amount of gold that was originally present in

the bulk ore prior to demineralization. The amount of goldleached and eluted from the CM was divided by the carbon

content of the CM, and multiplied by the % carbon of the

bulk ore to back-calculate the gold content in ounces per ton

In the autoclaved LPR CM, 99.7 g of gold was leached. This

accounts for 57.8% of the gold originally present in the ore as

determined by fire assay (Table 1). In contrast, 2.2% of the

gold, based on original opt values, was leached from the

non-autoclaved LPR. However, when the total amount of gold

leached by the nonautoclaved LPR CM in subsequent phases

(adsorption and desorption) was included, the non-autoclaved

LPR CM released 6.8% of the gold content of the ore (by fire

assay). Approximately 5% of the gold of the original opt

value was leached from both the autoclaved and

non-autoclaved HPR CM.

The results of the adsorption of gold on the LPR CM are

shown in Figure 5. The autoclaved LPR CM samples showed

sorption in the initial aliquot of gold-cyanide solution

However, successive aliquots indicated varying amounts of

sorption and desorption. The autoclaved LPR CM showed

significant additional adsorption of gold onto the carbon after

subsequent equilibration with gold cyanide solution for 24

hrs. The total sorption of gold cyanide on the autoclaved LPR

CM was 0.021 g Au/mg CM, which corresponds to a value

of 0.028 g Au/mg C (after division by carbon content of theCM) (Table 2). The non-autoclaved LPR CM showed initial

sorption of gold onto the CM, followed by significan

leaching of gold into the gold cyanide solution. The net resul

of the adsorption process on the non-autoclaved LPR CM

was the leaching of 0.083 g Au/mg CM. Figure 6 shows

gold cyanide sorption onto the HPR CM. Both autoclaved and

non-autoclaved samples showed continued sorption of gold

throughout the experiment with significant additiona

8/12/2019 99-136

4/8


sorption during the subsequent 24 hr contact with gold

cyanide solution. There was slightly less total sorption of gold

cyanide per gram of carbon (1. 175 g Au/mg CM vs. 1.240

g Au/mg CM) onto the autoclaved HPR CM relative to the

non-autoclaved HPR CM (Table 2).

The removal of the sorbed gold by desorption into NaCN(Table 3) showed that both the autoclaved and non-autoclaved

LPR CM desorbed significant amounts of gold back into

solution. The autoclaved LPR CM desorbed an amount of

gold equivalent to what was adsorbed during the step where

the CM was contacted with gold cyanide. In contrast, gold

was leached from the non-autoclaved LPR during the

adsorption stage of the experiment, and an additional 2.6

g of gold (almost 30% of the total amount of gold leached

from the nonautoclaved LPR throughout all stages of the

experiment) was leached during the desorption stage. As

expected, both the nonautoclaved and autoclaved HPR CMs

showed limited desorption of sorbed gold; however, the

non-autoclaved HPR showed slightly higher recovery (3.2%)relative to the autoclaved HPR (2.1%).

ANALYSIS AND DISCUSSION

Autoclaved LPR CM released significantly more gold than

the other CMs in the initial NaCN leaching phase of the

experiment. Since the low preg-robbing behavior of LPR ore

has been theorized to be associated with the low sorbing

behavior of the CM, and since autoclaving releases gold from

the sulfides, high concentrations of gold released into solution

would be expected from the autoclaved LPR. If it is assumed

that autoclaving the HPR ore released similar amounts of gold

from the sulfide phase as in the LPR, the extremely low

concentrations of gold released by the HPR ore corroborates

the theory that the sorbing behavior of the CM is responsible

for preg-robbing; that is, the gold solubilized by NaCN was

sorbed by the HPR CM and therefore not released into

solution. The percentage of gold released during the initial

leaching, relative to initial opt values determined by fire

assay, was 17% and 10.4% for the non-autoclaved LPR and

HPR, respectively (Table 1). Assuming that the HPR and LPR

samples initially had similar NaCN extractable gold contents,

the greater percentage of gold released from the LPR CM

relative to the HPR CM supports the theory that pregrobbing

is due to sorption of gold onto the CM upon initial contactwith NaCN.

It can be assumed that the total amount of gold released from

the non-autoclaved LPR represents gold initially associated

with the carbon or left behind from the demineralization

process. With this assumption, it can be presumed that the

much greater amount of gold released into solution from the

autoclaved LPR represents release of gold from sulfides

during autoclaving. The difference in gold leached between

the non-autoclaved and autoclaved LPR indicates over 50% of

the gold in the ore is associated with the sulfides

corroborating the work of Chryssoulis et al. (1996).

Because there was additional gold present in the autoclaved

CM due the release of encapsulated gold during the oxidationof the sulfides, it can be presumed that the autoclaved HPR

CM was exposed to a higher Au(CN)2-concentration relative

to the non-autoclaved HPR during the initial NaCN leaching

Given this, it is strange that the non-autoclaved HPR CM

actually released more gold (0.033 g Au/mg C) relative to

the autoclaved HPR (0.020 g Au/mg CM). It is hypothesized

that autoclaving increased the sorption of Au(CN)2- by HPR

CM, possibly due to a change in functional group conten

during oxidation. This has been tentatively corroborated by

the appearance of additional peaks (1720 cm-1) in the FTIR

spectra data of HPR CM after autoclaving (manuscript in

preparation). This peak was not found in the FTIR spectra

data of non-autoclaved CM (Stenebrten and Johnson, 1998)This may also explain why autoclaved HPR CM showed

similar sorption of gold relative to non-autoclaved HPR CM

during the adsorption stop despite the probable higher

gold-cyanide ion concentrations experienced by the

autoclaved HPR CM relative to nonautoclaved HPR CM

Presuming a higher gold-cyanide loading on autoclaved

relative to nonautoclaved HPR, one would expect a lesser

tendency of the autoclaved CM to sorb Au(CN)2-during the

adsorption step. Since this is not observed, one can infer an

increased sorptivity of the carbon due to autoclaving. The

same effect may also be observed in the LPR CM, where the

autoclaved LPR CM shows slight uptake of Au(CN)2-whereas

nonautoclaved LPR CM shows release of gold into solution

even during the adsorption stage of this study.

During the leaching stage, the LPR CM released much more

gold (99.7 g autoclaved, 3.8 g non-autoclaved) than the

HPR CM (0.88 g autoclaved, 1.0 g non-autoclaved)

During the adsorption stage, LPR CM sorbed minor

amounts (1.2 g autoclaved, -4.8 g nonautoclaved) o

gold-cyanide complex relative to HPR CM (51.7 g

autoclaved, 37.4 g nonautoclaved). This indicates HPR CM

had much higher gold loading than LPR CM prior to the

desorption phase, yet the HPR CM showed only 2-3%

recovery of gold, whereas the autoclaved LPR CM gave 100%recovery. Desorption from non-autoclaved LPR CM could no

be evaluated due to lack of gold sorption. The above result

indicate that gold cyanide complex sorption onto LPR CM is

readily reversible, whereas gold cyanide complex sorption

onto HPR CM is irreversible under the conditions of the

experiment. For both the LPR and HPR CMs, desorption of

gold cyanide complex differed between the autoclaved and

non-autoclaved CMs. Non-autoclaved CMs showed higher

8/12/2019 99-136

5/8


recovery of gold relative to the autoclaved CMs indicating

that autoclaving the CM is deleterious to gold recovery.

Sorption of gold to the non-autoclaved HPR CM was almost 2

orders of magnitude greater than sorption to the

non-autoclaved LPR CM, assuming that both of these CMs

had similar original gold content. If sorption is related to thephysical characteristics of the CM, this difference may help to

explain the correlation observed between the physical

characteristics of the non-autoclaved carbon and the %

pregrobbing behavior of the whole ore (Stenebrten and

Johnson, 1998). Furthermore, although autoclaving appears

to increase the sorption tendency of both the LPR and HPR

CMs, the autoclaved HPR CM remains much more sorptive

than the autoclaved LPR CM. This may explain why physical

properties of the non-autoclaved CM also correlated with %

recovery (Stenebrten and Johnson, 1998), despite the ore

being autoclaved during the recovery test.

Since LPR CM showed good recovery of the adsorbed gold,this suggests that the Au(CN)2

-sorbing mechanism for LPR is

readily reversible. This correlates well with the high values of

gold released during the initial leaching of gold in the first

phase of the experiment. Although the LPR CM is unlike

commercial activated carbon used in the CIL process because

it shows little ability to sorb gold from solution, it is similar to

commercial activated carbon in terms of readily reversible

aurocyanide sorption. The difference in sorbing behavior

could be due to a significantly smaller surface area of the

naturally occurring LPR carbon compared to that of

commercial activated carbon, and future studies will attempt

to bear this out. It is evident that the aurocyanide sorption

reversibility of HPR CM is quite different from that of both

LPR CM and commercial activated carbon. Physical and

chemical analyses have indicated strong similarity between

commercial activated carbon and Goldstrike ore CM

(Stenebrten and Johnson, 1998). However, the relative

irreversibility of gold cyanide complex sorption onto HPR

CM indicates a fundamental difference between the

mechanism of gold cyanide complex sorption on HPR CM

and the mechanism of gold cyanide complex sorption onto

commercial activated carbon.

CONCLUSIONS

There was a significant difference in the amount of gold

cyanide complex leached from the carbonaceous matter from

HPR and LPR Goldstrike ore despite there being some kinetic

limitation on the leaching process. After initial gold leaching,

the HPR CM adsorbs an amount of gold cyanide complex

which is almost two orders of magnitude greater than the

amount of gold cyanide complex adsorbed by the LPR CM.

Additionally, whereas the gold cyanide complex adsorbed by

the LPR CM is readily reversible, the gold cyanide complex

adsorbed by the HPR CM is not reversible under the

conditions studied. Finally, autoclaving increases the

sorptivity of the CM: there was more gold sorbed by the

autoclaved CM from both the LPR and HPR ores, and the

recovery of sorbed gold was higher for the non-autoclaved

CM. It is suggested that there is a mechanistic difference in

the sorptive behavior of HPR and LPR CMs and that theoxidation of the CM due to autoclaving has an effect on the

chemical properties of the CM.

REFERENCES

Chryssoulis, S., Weisner, C. and Wong, C., 1996

Deportment of gold in composites HPR- and LPR-l o

Goldstrike, Barrick Goldstrike, unpublished company report

10 pp.

Hausen, D.M. and Bucknam, C.H., 1985, Study of

preg-robbing in the cyanidation of carbonaceous gold ores

from Carlin, Nevada, Applied Mineralogy, Proceeding ofthe Second International Congress on Applied Mineralogy

Park, W.C., Hausen, D.M., Hagni, R.D. eds., AIME

Warrendale, PA, pp. 833-856.

Sibrell, P.L., Wan, R.Y. and Miller, J.D., 1990

Spectroscopic analysis of passivation reactions for

carbonaceous matter from Carlin trend ores, Gold '90, SME

symposium, Salt Lake City, UT, pp. 355-363.

Smith, G.C., Feb. 20, 1968, Discussion of refractory ore,

Carlin Gold Mining Company unpublished report.

Stenebrten, J.F. and Johnson, W.P., 1998

Characterizations of Goldstrike ore carbonaceous material

1. Chemical Characteristics, submitted for publication, 33

pp.

Stenebrten, J.F. and Johnson, W.P., 1998

Characterizations of Goldstrike ore carbonaceous material

2. Physical characteristics, submitted publication, 47 pp.

8/12/2019 99-136

6/8


Table

3.DesorptionrecoveryofAdsorbedGol

din

HPR

andLPR

CM

T

able2.AuadsorptiondataforHPR

and

LPR

CM

8/12/2019 99-136

7/8


8/12/2019 99-136

8/8


Documents

99-136