762
Management of Thiocyanate and Thiosalts in
Gold Extraction Project
Bret Baker, Kresimir Ljubetic, Guillermo Armendariz
BQE Water
ABSTRACT: As ore bodies and metallurgical flowsheets for gold extraction become increasingly complex,
managing by-products generated during mineral processing requires growing attention. One group of
constituents that can cause significant environmental and economic impacts for gold projects are thiospecies
containing reduced or partially oxidized sulphur atoms such as thiocyanate and thiosalts. This paper discusses
the implications of thiocyanate and thiosalts and reviews management systems to avoid and mitigate their
impacts through holistic water management in gold processing. Findings from case studies of active gold mines
will be provided, including life cycle cost-benefit analyses and assessments of system constraints and limitations
that need to be carefully considered for each project.
INTRODUCTION
While Thiocyanate and thiosalts are present in many indus-
trial processes, this paper focuses on their significance in
base and precious metals mining. Although thiocyanate is
less toxic than cyanide, its impact on the receiving envi-
ronment can be significant. In general, thiosalts can lead
to toxicity in the environment through oxidation down-
stream which contributes acid and can remobilize metals or
drop pH to a toxic level. Further, the oxidation by-products
can increase total dissolved metals (TDS) of the down-
stream solution, also affecting the health of the receiving
environment.
In Canada, Metal and Diamond Mining Effluent
Regulations (MDMER) impose limits for all mining oper-
ations to ensure any effluent does not cause acute lethal-
ity to rainbow trout and Daphnia magna. Thiocyanate has
an LC50 value of 150 ppm for rainbow trout in 96-hour
period, which is incorporated into Ontario operating mines
being held to a discharge limit of 150 mg/L SCN (Douglas
Gould 2012). Given that thiocyanate can be toxic to aquatic
life at relatively low concentrations, many mines are facing
a need to develop new strategies for the removal of thiosalts
and thiocyanate before the effluent can be considered safe
for discharge to the receiving environment.
Physical separation by membrane (reverse osmosis and
nanofiltration), biological oxidation in aerobic bioreactors,
and chemical oxidation in advanced oxidation processes
are current removal methods of thiosalts and thiocyanate.
Biological and chemical oxidation completely oxidize these
thio species into sulphate, and for thiocyanate nitrate and
carbonate, per reaction 1:
x SCN O2 yH2O SO CO NO3
4
2-
2 "++++-(1)
Despite its potential efficacy, biological treatment of thio-
cyanate contains weaknesses, including:
ii. Risk of acute and chronic toxicity by increasing
metal bioavailability
iii. Reaction rates are sensitive to temperature
fluctuations

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Extracted Text (may have errors)

762
Management of Thiocyanate and Thiosalts in
Gold Extraction Project
Bret Baker, Kresimir Ljubetic, Guillermo Armendariz
BQE Water
ABSTRACT: As ore bodies and metallurgical flowsheets for gold extraction become increasingly complex,
managing by-products generated during mineral processing requires growing attention. One group of
constituents that can cause significant environmental and economic impacts for gold projects are thiospecies
containing reduced or partially oxidized sulphur atoms such as thiocyanate and thiosalts. This paper discusses
the implications of thiocyanate and thiosalts and reviews management systems to avoid and mitigate their
impacts through holistic water management in gold processing. Findings from case studies of active gold mines
will be provided, including life cycle cost-benefit analyses and assessments of system constraints and limitations
that need to be carefully considered for each project.
INTRODUCTION
While Thiocyanate and thiosalts are present in many indus-
trial processes, this paper focuses on their significance in
base and precious metals mining. Although thiocyanate is
less toxic than cyanide, its impact on the receiving envi-
ronment can be significant. In general, thiosalts can lead
to toxicity in the environment through oxidation down-
stream which contributes acid and can remobilize metals or
drop pH to a toxic level. Further, the oxidation by-products
can increase total dissolved metals (TDS) of the down-
stream solution, also affecting the health of the receiving
environment.
In Canada, Metal and Diamond Mining Effluent
Regulations (MDMER) impose limits for all mining oper-
ations to ensure any effluent does not cause acute lethal-
ity to rainbow trout and Daphnia magna. Thiocyanate has
an LC50 value of 150 ppm for rainbow trout in 96-hour
period, which is incorporated into Ontario operating mines
being held to a discharge limit of 150 mg/L SCN (Douglas
Gould 2012). Given that thiocyanate can be toxic to aquatic
life at relatively low concentrations, many mines are facing
a need to develop new strategies for the removal of thiosalts
and thiocyanate before the effluent can be considered safe
for discharge to the receiving environment.
Physical separation by membrane (reverse osmosis and
nanofiltration), biological oxidation in aerobic bioreactors,
and chemical oxidation in advanced oxidation processes
are current removal methods of thiosalts and thiocyanate.
Biological and chemical oxidation completely oxidize these
thio species into sulphate, and for thiocyanate nitrate and
carbonate, per reaction 1:
x SCN O2 yH2O SO CO NO3
4
2-
2 "++++-(1)
Despite its potential efficacy, biological treatment of thio-
cyanate contains weaknesses, including:
ii. Risk of acute and chronic toxicity by increasing
metal bioavailability
iii. Reaction rates are sensitive to temperature
fluctuations

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XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 761
Lin, Xiaoqing, Qianlin Huang, Gaoxiang Qi, Silan Shi,
Lian Xiong, Chao Huang, Xuefang Chen, Hailong
Li, and Xinde Chen. 2017. “Estimation of Fixed-Bed
Column Parameters and Mathematical Modeling of
Breakthrough Behaviors for Adsorption of Levulinic
Acid from Aqueous Solution Using SY-01 Resin.”
Separation and Purification Technology 174:222–31.
doi: 10.1016/j.seppur.2016.10.016.
Liu, Haibo, Tianhu Chen, and Ray L. Frost. 2014. “An
Overview of the Role of Goethite Surfaces in the
Environment.” Chemosphere 103:1–11. doi: 10.1016
/j.chemosphere.2013.11.065.
Maji, Sanjoy Kumar, Anjali Pal, Tarasankar Pal, and Asok
Adak. 2007. “Modeling and Fixed Bed Column
Adsorption of As(III) on Laterite Soil.” Separation and
Purification Technology 56(3):284–90. doi: 10.1016
/j.seppur.2007.02.011.
Malik, Davendra Singh, Chakresh Kumar Jain, and Anuj
Kumar Yadav. 2018. “Heavy Metal Removal by Fixed-
Bed Column—A Review.” ChemBioEng Reviews
5(3):173–79.
McBride, Diane, James Gebhardt, Nick Croft, and
Mark Cross. 2018. “Heap Leaching: Modelling and
Forecasting Using CFD Technology.” Minerals 2018,
Vol. 8, Page 9 8(1):9. doi: 10.3390/MIN8010009.
Rajendran, Saravanan, A. K. Priya, P. Senthil Kumar, Tuan
K. A. Hoang, Karthikeyan Sekar, Kar Yeen Chong,
Kuan Shiong Khoo, Hui Suan Ng, and Pau Loke
Show. 2022. “A Critical and Recent Developments on
Adsorption Technique for Removal of Heavy Metals
from Wastewater-A Review.” Chemosphere 303:135146.
doi: 10.1016/j.chemosphere.2022.135146.

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Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 763
iv. Slow response to changes in flow or contaminant
mass loading
v. The long-term stability and storage of biomass
increases the process fluctuations, not economically
viable for some mine sites.
vi. Addition to the total dissolved solids (TDS) of the
effluent solution. In many of the industries where
thiosalts present themselves, the TDS of either the
recycled process solution, or the effluent being dis-
charged to the environment can be a key driver for
process selection, where increased TDS could lead to
reduced process efficiency or increased toxicity.
To reduce the impact of thiosalt removal on associated pro-
cesses, physical processes, in particular ion exchange and
electro oxidation, are gaining interest due to their ability to:
1. completely remove SCN from process solutions
without otherwise impacting the effluent, and
2. control the extent of oxidation. For thiosalts, the
preference is for complete oxidation to sulphate,
but for thiocyanate, the oxidation can be con-
trolled such that cyanide can be recovered and
recycled back to the cyanidation process, reducing
a mines operating cost and long-term environment
liabilities.
BQE Water (BQE) has developed an electro-oxidation
(EO) process for controlled thiosalt oxidation. This paper
presents results from two lab studies performed on actual
mine waters at conditions mimicking site conditions:
3. Reducing SCN buildup in a recirculating process
solution from a gold mine in Guerrero, Mexico,
with high TDS and elevated temperatures.
4. Addressing an existing environmental discharge
issue at a site in the Abitibi gold belt, Canada,
using ion-exchange (IX) and EO processes on a
low TDS solution at low temperatures.
Electro-oxidation process description
The EO process used for thio species oxidation utilizes
commercial electrocells. The main anodic and cathodic
reactions involved in the EO process for SCN are shown
in reactions (2) and (3) with the overall reaction shown as
reaction (4). In addition to the target reactions, there are
other parasitic reactions (5) through (11) that can occur
depending on solution chemistry, cell design, and operat-
ing conditions.
EO of SCN -Process Reactions
e 6
:4
8H
Anode SCN H O
CN SO
2
4
2-
"+
+++
-
-+-(2)
6e 6 6OH Cathode: H2 O H2 "++--(3)
4
3 2H
Overall: SCN H O
CN SO H
2
4
2-
2
"+
+++
-
-+(4)
Possible Parasitic Reactions
Anode
4e 2 4 H O O H
2 2 "+++-(5)
2Cl Cl2 "-(6)
e 2 2 CN H O OCN H
2 "+++--+-(7)
e Fe^CNh Fe^CNh
6
4-
6
3- "+-(8)
Cathode
2e 2 H H2 "++-(9)
e 3CN Cu CNh32- Cu "++--^(10)
e Fe^CNh63- Fe^CNh64- "+-(11)
Reactions (6) and (7) are especially undesirable as they
reduce free NaCN concentration. Reaction (6) generates
chlorine that can destroy cyanide, while Reaction (7) oxi-
dizes cyanide to cyanate. Finally, reactions involving strong-
acid dissociable iron-cyanide complexes, reactions (8) and
(11), can lower EO process efficiency.
CASE STUDY 1: GOLD MINE IN
GUERRORO STATE, MEXICO
Description of Issue, and Potential for Improvement
The ores at this mine in Guerrero state, Mexico contain
a significant amount of pyrite and pyrrhotite, which is an
increasing trend for gold ores found throughout the world.
When treated through the cyanidation process, these ores
result in significant concentrations of thiocyanate, which
correspond to an associated loss of cyanide and represents a
significant operating cost to the project.
Further, while the site can have significant rainfall,
due to the mountainous terrain, there is little available
fresh water. As such, the metallurgical circuit has a closed
water balance, leading to increasing process solution TDS.
As thiocyanate is generated during the leach, there is con-
tinual production of this contaminant contributing to TDS
increase, which is significant for two reasons:

Previous Page Next Page

Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 763
iv. Slow response to changes in flow or contaminant
mass loading
v. The long-term stability and storage of biomass
increases the process fluctuations, not economically
viable for some mine sites.
vi. Addition to the total dissolved solids (TDS) of the
effluent solution. In many of the industries where
thiosalts present themselves, the TDS of either the
recycled process solution, or the effluent being dis-
charged to the environment can be a key driver for
process selection, where increased TDS could lead to
reduced process efficiency or increased toxicity.
To reduce the impact of thiosalt removal on associated pro-
cesses, physical processes, in particular ion exchange and
electro oxidation, are gaining interest due to their ability to:
1. completely remove SCN from process solutions
without otherwise impacting the effluent, and
2. control the extent of oxidation. For thiosalts, the
preference is for complete oxidation to sulphate,
but for thiocyanate, the oxidation can be con-
trolled such that cyanide can be recovered and
recycled back to the cyanidation process, reducing
a mines operating cost and long-term environment
liabilities.
BQE Water (BQE) has developed an electro-oxidation
(EO) process for controlled thiosalt oxidation. This paper
presents results from two lab studies performed on actual
mine waters at conditions mimicking site conditions:
3. Reducing SCN buildup in a recirculating process
solution from a gold mine in Guerrero, Mexico,
with high TDS and elevated temperatures.
4. Addressing an existing environmental discharge
issue at a site in the Abitibi gold belt, Canada,
using ion-exchange (IX) and EO processes on a
low TDS solution at low temperatures.
Electro-oxidation process description
The EO process used for thio species oxidation utilizes
commercial electrocells. The main anodic and cathodic
reactions involved in the EO process for SCN are shown
in reactions (2) and (3) with the overall reaction shown as
reaction (4). In addition to the target reactions, there are
other parasitic reactions (5) through (11) that can occur
depending on solution chemistry, cell design, and operat-
ing conditions.
EO of SCN -Process Reactions
e 6
:4
8H
Anode SCN H O
CN SO
2
4
2-
"+
+++
-
-+-(2)
6e 6 6OH Cathode: H2 O H2 "++--(3)
4
3 2H
Overall: SCN H O
CN SO H
2
4
2-
2
"+
+++
-
-+(4)
Possible Parasitic Reactions
Anode
4e 2 4 H O O H
2 2 "+++-(5)
2Cl Cl2 "-(6)
e 2 2 CN H O OCN H
2 "+++--+-(7)
e Fe^CNh Fe^CNh
6
4-
6
3- "+-(8)
Cathode
2e 2 H H2 "++-(9)
e 3CN Cu CNh32- Cu "++--^(10)
e Fe^CNh63- Fe^CNh64- "+-(11)
Reactions (6) and (7) are especially undesirable as they
reduce free NaCN concentration. Reaction (6) generates
chlorine that can destroy cyanide, while Reaction (7) oxi-
dizes cyanide to cyanate. Finally, reactions involving strong-
acid dissociable iron-cyanide complexes, reactions (8) and
(11), can lower EO process efficiency.
CASE STUDY 1: GOLD MINE IN
GUERRORO STATE, MEXICO
Description of Issue, and Potential for Improvement
The ores at this mine in Guerrero state, Mexico contain
a significant amount of pyrite and pyrrhotite, which is an
increasing trend for gold ores found throughout the world.
When treated through the cyanidation process, these ores
result in significant concentrations of thiocyanate, which
correspond to an associated loss of cyanide and represents a
significant operating cost to the project.
Further, while the site can have significant rainfall,
due to the mountainous terrain, there is little available
fresh water. As such, the metallurgical circuit has a closed
water balance, leading to increasing process solution TDS.
As thiocyanate is generated during the leach, there is con-
tinual production of this contaminant contributing to TDS
increase, which is significant for two reasons: