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:
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: