A CONCEPTUAL CIRCUIT DEVELOPMENT FOR GOLD PROCESSING BASED ON BROMINE/BROMIDE LIXIVIANT

13 Pages·00.664 MB·English

by  Mariam Melashvili, Chris Fleming, Mykolas Gladkovas, Mike Dry, Mani Manimaran

COM 2015 | THE CONFERENCE OF METALLURGISTS
hosting AMCAA | America's Conference on Aluminum Alloys ISBN: 978-1-926872-32-2
A CONCEPTUAL CIRCUIT DEVELOPMENT FOR GOLD PROCESSING BASED ON
BROMINE/BROMIDE LIXIVIANT

*Mariam Melashvili1, Chris Fleming1, Mykolas Gladkovas1, Mike Dry2, Mani Manimaran3

1SGS Canada Inc.
185 Concession Street
Lakefield, Ontario K0L 2H0
(*Corresponding author: [email protected])

2 Arithmetek
1331 Hetherington Drive
Peterborough, Ontario K9L1X4

3Albemarle Corporation
451 Florida Street
Baton Rouge, LA 70821

ABSTRACT
Development of a new hydrometallurgical flowsheet requires quantitative description and
optimisation of all chemical processes in each step in the process, as well as mass and energy balances
across a fully integrated circuit incorporating all the unit operations. This paper covers steps leading to
early modelling of a gold processing plant based on a bromine/bromide lixiviant. Research conducted
at SGS Lakefield in collaboration with Albemarle provided the data that were used in the development
of this model. A previous investigation in this series described the leaching behaviour of various
mineral types in Stabilized Bromine reagent. This paper is concerned with the gold recovery operations
associated with a conceptual bromine leaching process. The recovery of gold by different types of resin
and activated carbon was studied, along with various methods of stripping the gold from the extractant.
The circuit is closed by electrolysing the spent leach liquor to oxidize some of the contained bromide
back to bromine that would allow recycling the regenerated leach solution back to the leach step in the
process.

KEYWORDS
gold bromide adsorption, elution, bromine regeneration, electrolysis

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INTRODUCTION

Worldwide gold mining and extraction is primarily based on the more than century old gold
cyanidation process. Due to its highly poisonous nature, the process has been banned in a number of
countries. Additionally, there are refractory ores that are not amenable to cyanide leaching. As a result,
there has been considerable past research work on the development of alternatives to cyanide leaching
systems. One of the promising gold lixiviant systems is based on bromine/bromide chemistry but in
view of some shortcomings, this system has yet to gain popularity among other alternatives.
It is well known that bromide stabilizes the gold by forming a stable complex in acidic to
neutral pH conditions. The oxidizing agent normally used in this process is the bromine. This is,
however, a corrosive liquid with high vapour pressure, which causes difficulty in handling. Albemarle
developed a reagent, “Stabilized Bromine” which has a considerably lower vapour pressure than liquid
bromine. Gold leaching from various ore mineralogies by this novel bromine reagent has been shown
to be successful, and details have been given in a previous paper of this series (Melashvili, M., et al
2012). The current paper focuses on gold recovery methods such as activated carbon and resin
adsorption techniques.
Unlike cyanidation, the recovery of gold bromide is expected to involve gold reduction by
activated carbon (Wan 1993; Freiberg 1993) which would make the conventional elution methods
ineffective. However, the reductive precipitation by carbon seems to be the function of free bromine
concentration according to Wan (1993) to explain the results (gold bromide adsorption) obtained by
Pesic and Storhok (1992). With respect to ion-exchange resins, the anticipation is that resins will load
the gold bromide (Mensah-Biney 1991, Reid and Mensah-Biney 1988) but the resin’s functional
groups might degrade in the presence of bromine due to high oxidation potential it imparts to the leach
solution.
Various types of ion-exchange resins as well as activated carbon were studied to recover the
gold bromide and findings are presented in this paper. Furthermore, this paper is concerned with
investigating the possibility of re-oxidizing free bromine from bromide for recycling in the process
which is vital for the economic viability of this process. Finally, the paper refers the process model
constructed based on this study.

Materials and Methods
The gold containing bromide/bromine solution was prepared by dissolving pure gold foil in a
Stabilized Bromine reagent. This synthetic gold bromide solution was used for testing various
adsorbents for their gold bromide adsorption capacity. This was accomplished by contacting a
predetermined volume of this solution with the various adsorbents in a conical flask, on a shaker table.
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The elution tests were conducted in a water-jacketed glass column. The adsorbent was placed
in the column and deionised water passed through the column to remove air pockets in the adsorbent
bed. Hot water, heated to a specific temperature, was pumped through the outer jacket of the column
when needed. A volume of 300-400 mL of eluant solution was prepared and pumped at a specified
flow rate (bed volume per hour) into the bottom of the column and out the top. At the end of the test,
DI (wash) water was pumped to collect residual aqueous gold. Some tests were run on a shaker table
by placing a loaded adsorbent into known composition of eluant for a pre-determined reaction time.
Most of the bromide to bromine electrolytic oxidation tests were performed in a regular
electrolytic cell. The total volume of this cell was 1000 mL. The electrochemical working area of the
anode (mixed metal oxide coated titanium) was 100 cm2 and for the cathode (stainless steel SS316)
was 120 cm2. Direct current power was supplied to the cell electrodes. The cell solution was pumped
to a reservoir and recycled back to the cell using peristaltic pumps. The cell was filled with about 700
mL solution and about 1200 mL of solution was circulated between the cell and the reservoir. The
circulation flow rate was 1000 mL/minute. Some of the tests were conducted using a two-compartment
cell with a membrane to minimize reduction of bromine to bromide at the cathode. The bromide and
bromate were analysed by ion chromatography. The bromine was measured by titration after acidifying
the sample with sulphuric acid. The latter provides total bromine concentration. The equivalent
concentration of active bromine was calculated from bromine (direct) and bromate assay results. The
efficiency of current utilization for bromide oxidation was calculated based on amount of active
bromine (equivalent bromine) produced in the cell during electrolysis.
The gold leaching experiments in Stabilized Bromine were conducted by leaching a gold
bearing ore in a suitable size reactor equipped with an overhead stirrer. The gold analyses were
performed using the Fire Assay method.

RESULTS AND DISCUSSION

Search for the best adsorbents
Identification of the best adsorbent was accomplished by comparing the gold loadings on
different adsorbents from synthetic gold bromine solution of similar composition. The list of
adsorbents included: strong base resin , A-500; weak base resins, Aurix 100 and Reilex 425; non-
anionic resin, XAD-4; weak ion-exchange fiber, Smopex 205; and activated carbon. Additionally, in
order to simulate the performance of recycled resins, the resins (A-500 and Aurix 100) were pretreated
with bromine solution prior to contacting with gold bromide medium. The loading data for these
adsorbents are presented in Tables 1 through 4.
Table 1: Adsorption of gold bromide on strong-base resin, A-500
Adsorbent Solution, mL Adsorbent, g Au, g/t Au, mg/L % Au recovery
A 500 (fresh) 1000 1.10 18829 1.99 91.2
A 500 (spent) 1000 1.12 18289 1.75 92.1

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Table 2: Adsorption of gold bromide on weak-base resin, Aurix 100
Adsorbent Solution, mL Adsorbent, g Au, g/t Au, mg/L % Au recovery
Aurix 100 (fresh) 1000 0.92 18754 4.53 79.2
Aurix 100 (spent) 1000 1.03 17557 4.60 79.7

Table 3: Adsorption of gold bromide on activated carbon
Adsorbent Solution, mL Adsorbent, g Au, g/t Au, mg/L % Au recovery
Activated carbon 1000 1.32 10683 3.51 80.1

Table 4: Adsorption of gold bromide on ion-exchange resins and fiber
Adsorbent Solution, mL Adsorbent, g Au, g/t Au, mg/L % Au recovery
XAD-4 1000 0.86 12733 9.52 53.4
Reillex 425 850 0.69 24790 3.93 83.7
Smopex 205 850 1.13 18375 0.18 99.3

Data suggest that the strong-base resin, A-500, recovered 91-92% gold regardless of whether the resin
was fresh or pre-soaked into a bromine solution before contacting the gold bromide. The weak-base
resin, Aurix-100, recovered close to 79-80% gold. Similarly, this resin was not affected by the bromine
that was used to pre-soak prior to gold loading. The performance of activated carbon was similar to the
activity of weak-base resin Aurix 100, which extracted about 80% of the gold in solution. However,
the loading capacity of Aurix 100 (~19 kg/t gold) was almost twice as high as that of carbon (~11
kg/t). The best adsorption capacity (99.3% gold recovered) was detected by an ion-exchange fiber,
Smopex 205. However, this option would probably only be selected if the intention was to burn the
loaded adsorbent to recover gold. The gold recovery by non-ionic resin XAD-4 and Reilex 425 were
53% and 84%, respectively.
Search for the best eluants
Based on gold bromide loading data the best adsorbent was identified as the strong base resin
although the final choice will also depend on the successful elution of concentrated gold bromide.
Therefore, the experimental research was continued for other two candidates (weak-base resin and
activated carbon) as well. Various compositions of reagents were tested to elute the gold that was
adsorbed from synthetic gold bromide solution but the most interesting results were obtained using
cyanide, thiourea and thiosulphate systems. These are presented below.
Caustic cyanide elution of gold bromide from weak base resin and activated carbon was
conducted in a water jacketed glass column. The elution conditions and results are presented in Table 5
and profiles are shown in Figure 1.
Table 5: Cyanide elution summary
Adsorbent NaCN NaOH T Time Rate Au Eluted Au Residual Au Initial calc.
Type g M M oC h BV/h % g/t g/t
Aurix 100 9.9 0.1 2 60 5 2 85.5 1126 7951
Act. carbon 10.0 0.2 1 ~90 6 4 77.1 1783 8071

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100 100
90 90
80 80
ve 70 70
ulati60 60 ed
m ut
cu50 50 el
ed Au
ut40 40 %
el
Au 30 30
%
20 20
10 10
0 0
0 4 8 12 16 20 24 28
Bed Volumes
Aurix 100(CN) Act.Carbon(CN)
Figure 1: Caustic cyanide Elution Profiles
From Table 5 it is obvious that the caustic cyanide solubilised 86% and 77% gold of gold bromide
from weak base resin and activated carbon, respectively. The curves in Figure 1 suggest elution of gold
bromide from weak base resin with caustic cyanide is fast and efficient compared to activated carbon,
which is slow and incomplete even after double the volume of eluant. Industrial practice has
demonstrated that gold can be eluted effectively from carbon with caustic cyanide solution, but that the
kinetics are slow unless elution is conducted in a pressure reactor at 124 to 140OC. Presumably the
same will apply to carbon that has been loaded from gold bromide leach solution. With further
optimization, it is expected that all the gold could be eluted from the adsorbents, since these are both
commercial elution processes.
In a similar manner as for cyanide, the acidic thiourea was passed through the elution column
to collect the aqueous gold. This eluant was tested on strong-base resin (A-500) and activated carbon.
The data are presented in Table 6 and profiles are shown in Figure 2.
Table 6: Thiourea elution summary
Adsorbent Thiourea H SO T Time Rate Au Eluted Au Residual Au Initial calc.
2 4
Type g M g/L oC h BV/h % g/t g/t
A 500 8.9 1.0 100 22 5 2 97.5 225 8970
Act. carbon 6.7 1.0 100 22 6 4 2.9 2675 2754
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100 100
90 90
80 80
e 70 70
v
ati
ul 60 60
m d
e
cu 50 50 ut
uted 40 40 Au el
el %
u 30 30
A
%
20 20
10 10
0 0
0 4 8 12 16 20
Bed Volumes
A-500(Tu) Act. Carbon(Tu)

Figure 2: Thiourea Elution Summary
Thiourea and sulphuric acid eluted the gold bromide successfully off the strong base resin. However,
thiourea was ineffective as an eluant to solubilise the gold bromide adsorbed on carbon. This outcome
is not surprising since gold forms a positively charged complex with thiourea, which would tend to
stay in the phase and not re-adsorb onto resin due to its charge difference. Elution of gold cyanide
from strong base resin with acidic thiourea solution is in fact a successful industrial process. On the
contrary, the activated carbon has a strong affinity for gold thiourea complexes, which would re-adsorb
even if the ligand exchange between bromide and thiourea occurred.
A different method, a shaker table, was used to elute the gold bromide from resin and carbon
with thiosulphate solution. The testing conditions and results are presented in Table 7.
Table 7: Thiosulphate elution summary
Adsorbent Na S O NaOH Cu pH T Au Eluted Au Residual Au Initial calc.
2 2 3
Type g M M g/L oC % g/t g/t
A 500 1.9 1.0 2 0 - 50 0.61 7514 7560
Act. carbon 1.1 1.0 0 0.2 10 22 80.0 414 2071
The data in Table 7 suggest that thiosulphate was unable to elute gold bromide from A-500, strong-ion
exchange resin but was successful to solubilise about 80% of gold bromide off the carbon. This option
should be further explored in search for non-cyanide eluant for gold bromide loaded carbon.
Bromine regeneration by electrolysis
Bromine regeneration by electrolysis was carried out in a synthetic sodium bromide solution.
Two variables were tested: current density and initial bromide concentration. These tests were run in a
regular one compartment electrolytic cell.
Various current densities in the range of 500 to 2000 A/m² were investigated to establish its
effect on the bromide oxidation reaction. The initial bromide concentration was fixed at 60 g/L. The
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amount of current passed through the cell was 10 ampere-hours for all the tests of this series. The test
results for current density variation tests are summarized in Table 8.
Table 8: Summary for current density experiments
Curr/density Average Final BrO g/L Br ,g/Lby titration Br , g/L equiv. Curr/Efficiency
3, 2 2
A/m2 V pH EH,mV (direct) (direct) (acidified) ( calc) %
500 6.8 10.7 955 4.6 0.32 14.75 17.56 57.2
1000 8.8 11.5 855 4.28 0.49 16.04 16.53 62.6
1500 10.5 11.1 881 4.47 0.30 13.52 17.06 51.9
2000 13.2 10.8 797 5.75 0.41 17.35 18.21 67.7

The results show that the variation of current density between 500 and 1500 A/m2 had a minimal
impact on current efficiency, which varied between 60 and 70%.
A current density of 1000 A/m2 was selected to investigate the effect of initial bromide
concentration on bromine production efficiency. The amount of current passed through the cells was
10 ampere-hours for each test. The summary of test results with solutions containing different sodium
bromide concentrations is provided in Table 9.
Table 9: Summary for solution composition experiments
NaBr Average Final BrO g/L Br ,g/Lby titration Br , g/L equiv. Curr/Efficiency
3, 2 2
mg/L V pH E ,mV (direct) (direct) (acidified) (calc) %
H
30 12.1 11.0 924 2.13 0.44 12.39 8.42 49.1
60 8.8 11.5 855 4.28 0.49 16.04 16.53 62.6

These tests results indicated that the current efficiency as well as concentration of bromine that was
produced significantly increased with higher initial bromide in electrolyte.
A test was run using a two-compartment electrolysis cell equipped with an ion exchange
membrane to prevent migration of bromine to the cathode. The anode compartment of the cell was
filled with sodium bromide solution (60 g/L) and the cathode compartment with sodium hydroxide
solution (12 g/L). The applied current was 10 A in all these tests. The pH in the anode compartment
was maintained at ~8 with 50% NaOH solution. The tests results are presented in Table 10.
Table 10: Summary for membrane electrolysis
Test Average Final BrO g/L Br ,g/Lby titration Br , g/L equiv. Curr/Efficiency
3, 2 2
V pH E ,mV (direct) (direct) (acidified) (calc) %
H
EL-2 (60 min) 9.8 9.5 1264 - - 35.92 - 82.5

It is obvious that membrane electrolysis is much more efficient because most of the current was
consumed by bromine formation reaction. The current efficiency value in EL-2 tests that was run for
60 minutes was ~20% higher values compared to the tests in undivided cells.

Gold adsorption isotherm and regeneration of bromine from leach liquor
In order to demonstrate that the adsorbents perform the same way as in the case of synthetic
gold bromide solution, the actual leach products were produced and used to create adsorption
isotherms for both resin types and activated carbon. Furthermore, the filtrates of isotherm products
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were electrolysed to produce stabilized bromine under conditions established for synthetic sodium
bromide.
Leaching of gold bearing material achieved 89-93% gold extraction using an acidic (pH 2
with sulphuric acid) lixiviant with the composition of 47-48 g/L bromine and 10 g/L bromide. The
leach test results are presented in Table 11. The ICP scan data for major elements are shown in Table
12.
Table 11: Leach tests results
Test % Solids Br , g/L Br-, g/L % Au extraction Au, mg/L Au, g/t residual Au, g/t feed
2
Leach 1 32.5 46.9 10 88.6 13.9 4.31 36.7
Leach 2 32.4 48.1 10 92.7 16.8 2.53 36.5

Table 12: ICP data for Leach 1 and 2
Elements, mg/L Ag Al As Ba Be Bi Ca Cd
Leach 1 2.05 192 26 0.09 0.05 <1 1020 <0.5
Leach 2 2.51 235 99 0.08 0.06 <1 1180 <0.5
Elements, mg/L Co Cr Cu Fe K Li Mg Mn
Leach 1 5.2 1.1 860 881 137 <2 217 55
Leach 2 6 1.9 950 1880 127 <2 236 60
Elements, mg/L Mo Na Ni P Pb Sb Se Sn
Leach 1 <0.6 26300 6.3 <8 <2 <1 <3 <2
Leach 2 <0.6 29000 8 20 <2 <1 <3 <2
Elements, mg/L Sr Ti Tl U V W Y Zn
Leach 1 4.6 0.08 <3 <1 <0.2 <2 0.7 27
Leach 2 5.1 0.18 <3 <1 <0.6 <2 0.8 31

The ICP scan data is presented to indicate certain metals’ reactivity towards acidic bromine/bromide.
The acidic bromine leach was carried out to determine the dissolution of impurity elements and to
provide a competitive environment for gold loading onto the adsorbents.
To generate the adsorption isotherm, the leached pulp was contacted with predetermined
amounts of carbon, A-500, and Aurix100 in separate experiments. The solution to adsorbents ratio
ranged from ~30 to 2000 on a dry mass basis of adsorbent. Each test was run for 24 hours, after which
time the slurries were filtered and the solutions and adsorbents were analyzed for gold. The adsorption
profiles are depicted in Figure 3 .
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40000
35000
30000
g/t) 25000
(
n
o 20000
pti
or
ds 15000
A
u
A
10000
A-500
5000 Act. Carbon
Aurix 10
0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Au in solution (mg/L)

Figure 3: Equilibrium Isotherm

From Figure 3 it can be seen that the highest gold loading (36.6 kg/t) was achieved with the strong
base resin, A-500. The plots for Aurix 100 and carbon were similar, although higher loading was
achieved with activated carbon (~ 25 kg/t Au) than with Aurix 100 (22 kg/t). Hence the efficiency of
adsorption with respect to gold decreases in the following order: A-500 > Activated carbon > Aurix
100.
The filtrates from isotherm tests were combined and used to demonstrate that the bromine
consumed can be regenerated. The electrolysis was run in a regular undivided electrolysis cell for
duration of three hours. The current applied to the cell was 1000A/m2. The summary of test results is
provided in Table 13.
Table 13: Bromine regeneration from leach barren solution
Time Voltage pH E Analytical, g/L Br titration, g/L Curr. Effic.
H
min V mV Br- BrO (direct) (acidified) %
3
0 7.5 2.4 726 55 - 0.32 0.28 -
30 7.0 3.4 1052 - - - 9.4 74.3
60 7.2 5.6 1029 - - - 16.8 58.5
120 7.5 10.5 796 - - - 33.4 63.4
180 8.0 11.0 765 42.0 <0.5 21 49.3 58.3

It is interesting to note that the bromine concentration in the electrolyte increases practically linearly
with electrolysis time. A total of 49.3 g of bromine was produced during electrolysis from the
electrolyte initially containing ~55 g/L bromide. The current efficiency for overall electrolysis of the
eluate was over 60%, which agrees well with current efficiency numbers for the tests using synthetic
sodium bromide solutions.
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The ICP scan analyses before (isotherm filtrate) and after electrolysis are presented in Table 14. It
shows the mobility of some base and alkali earth metals in the cell during electrolysis.
Table 14: ICP scan on isotherm filtrate and final electrolyte
Elements, mg/L Ag Al As Ba Be Bi Ca Cd
Isotherm filtrate 0.74 250 57 0.06 0.06 <1 1150 <0.3
After Electrolysis 0.23 7.6 <3 0.02 <0.02 <1 791 <0.09
Elements, mg/L Co Cr Cu Fe K Li Mg Mn
Isotherm filtrate 6 1.8 882 1260 103 <2 234 66
After Electrolysis <0.3 0.5 <0.1 3.9 100 <2 0.37 0.05
Elements, mg/L Mo Na Ni P Pb Sb Se Sn
Isotherm filtrate <0.6 28300 8.5 <8 <2 <1 <3 <2
After Electrolysis <0.6 28100 <0.6 <5 <2 <1 <3 <2
Elements, mg/L Sr Ti Tl U V W Y Zn
Isotherm filtrate 5.3 <0.1 <3 <1 <0.5 <2 0.84 31
After Electrolysis 4.27 <0.02 <3 <1 <0.2 <2 <0.02 <0.7

A further objective was to prove the feasibility of regenerating the bromine in a continuous
manner. To accomplish this task RIL and CIL experiments were conducted, providing some additional
data on gold recovery under the harsh conditions of leaching.
The CIL and RIL experiments were carried out on a similar feed material but using more
diluted bromine lixiviant compared to previous leaching tests. The gold recoveries were 82% and
81.1% for CIL and RIL experiments, respectively. The leaching data are presented in Table 15.
Table 15: Gold recovery summary in CIL and RIL
Test % Solids Br , g/L Br-, g/L % Au extraction Au, mg/L Au, g/t residual Au, g/t feed
2
CIL 23.6 9.4 10 82.0 0.05 8.97 50.0
RIL 23.2 9.3 10 81.1 0.03 8.22 43.5

The metal deficient solutions (CIL and RIL barren) were mixed and used for bromine
regeneration testing. The target concentration of bromine to be produced continuously during
electrolysis was 15 g/L. Initially, the electrolysis was run in a batch mode until the target bromine
concentration was reached. Afterwards, the feed solution was pumped to the cell and the electrolysis
was run in continuous mode. The feed flow rate was carefully set to ensure a constant bromine
concentration of ~15 g/L in the product solution emerging from the cell. The data are presented in
Table 16.
Table 16: Bromine regeneration from leach barren solution in continuous mode
Test Time Voltage pH E Analytical, g/L Br titration, g/L Curr. Effic.
H
mode min V mV Br- BrO (direct) (acidified) %
3
Batch 0 17.5 2.25 769 19.5 - - 0.08 56.5
50 11.5 11.2 749 - - - 7.20 57.5
60 11.5 11.3 749 - - - 15.4 57.0
Continuous 0 13.0 11.3 762 - - - - -
40 13.2 11.2 760 - - - 15.4 64.9
60 13.5 11.2 760 - - - 14.3 55.0
90 13.5 11.1 766 - - - 14.3 54.7
112 13.5 11.2 771 16.0 <0.5 6.3 16.4 48.9

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