770 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
• Non-selectivity
Due to the non-selectivity of the biological system,
the system needs to be sized appropriately to oxidize
the other contaminants along with the SCN. This
additional oxygen demand increases the plant capital
costing as well as operating costs (additional reagents,
additional power for equipment, additional heating).
• Variation in hydraulic and mass loading
Biological systems are inherently averse to changes.
As this site is in the Abitibi region, the water flows
follow a distinct yearly trend, where the flows are very
low in the winter due to freezing, and then very high
immediately upon the spring thaw (freshet). This
large change in hydraulics, and the associated change
in mass removal duty is a difficult endeavor for a bio-
logical system, particularly in the colder climates.
The proposed electro-oxidation process design would either
replace the MBBR or be placed upstream of it to both miti-
gate the downsides of the MBBR while adding the electro-
oxidation circuit’s benefits to the site’s treatment process.
Description of EO Process Integration into Existing
Metallurgical Flowsheet
Due to the prevalence of deposits within the Abitibi Gold
Belt, it is not uncommon for a mill to accept ore from
mines throughout the area. In particular, the metallurgical
process studied within this second case study uses ore from
two mines. The main source is a sulphidic base metal ore,
which is treated prior to cyanidation in a conventional flo-
tation circuit. This produces copper and zinc concentrates
with the flotation tailings sent to the cyanide leach, where
the second ore is added as well. The cyanide leach is con-
ventional with leach tanks followed by CIP. The tailings are
treated in a cyanide detoxification circuit before a thick-
ener separates the solids and sends this to paste backfill.
The overflow solution is sent to the tailings storage facilities
(TSFs). While there is a significant amount of mill process
solution reuse from the TSFs, the site has a positive water
balance and must discharge excess solution. As the detox
effluent does not meet metal and toxicity limits, the water
must be treated in two discrete water treatment plants the
initial treats the water with lime, peroxide and ferrous sul-
phate to remove metals and residual cyanide complexes,
while the second uses an MBBR to oxidize ammonia and
thiocyanate. Figure 8 shows the block flow diagram for the
site’s metallurgical process.
Described above, the proposed IX-EO circuit would
be placed upstream of the existing MBBR, as shown in
Figure 9.
The columns are filled with ion exchange (IX) resin
which selectively replaces thiocyanate with sulphate to pro-
duce clean effluent. Once the IX resin reaches saturation,
it’s regenerated via sulfuric acid. The regenerant is directed
to the EO, which is the same as the process described for
the Mexican project (i.e., thiocyanate is oxidized to cya-
nide, with the HCN captured in a wet scrubber).
Testwork Results and Discussion
Samples of mine effluent collected after lime and before
biological treatment (MBBR) were shipped from site to
BQE Water’s laboratory in Vancouver, BC, Canada, with
the goal to prove out the ion exchange process to ensure
effluent quality met site objectives, and to validate the EO
process on the regenerant.
Table 4 shows the dissolved concentrations of the main
constituents in the solution received from site.
Ion Exchange Test Results
Prior to loading on the IX columns, the as-received solu-
tion was bulk filtered to remove suspended solids. The same
bed of ion exchange resin was then used for five (5) back-
to-back cycles, to show the reproducibility of the process,
and the gauge the extent of capacity changes. Figure 10
provides the breakthrough curves for the initial and final
loading cycles.
Table 3. Summary of Electro Oxidation results for Mexican Minesite
Test Parameter Solution Tested
Slope,
mg CN·L–1·min–1
Slope @100% CE,
mg CN·L–1·min–1
Current
Efficiency
Low current Density
(283 A/m2)
Actual leach solution 8.00 9.89 81%
Synthetic solution 7.92 80%
High Current Density
(943 A/m2)
Actual leach solution 19.04 32.97 58%
Synthetic Solution 24.89 75%
SCN concentration
Effect on CE
1 g/L SCN 8.45 32.97 26%
15 g/L SCN 23.95 73%
• Non-selectivity
Due to the non-selectivity of the biological system,
the system needs to be sized appropriately to oxidize
the other contaminants along with the SCN. This
additional oxygen demand increases the plant capital
costing as well as operating costs (additional reagents,
additional power for equipment, additional heating).
• Variation in hydraulic and mass loading
Biological systems are inherently averse to changes.
As this site is in the Abitibi region, the water flows
follow a distinct yearly trend, where the flows are very
low in the winter due to freezing, and then very high
immediately upon the spring thaw (freshet). This
large change in hydraulics, and the associated change
in mass removal duty is a difficult endeavor for a bio-
logical system, particularly in the colder climates.
The proposed electro-oxidation process design would either
replace the MBBR or be placed upstream of it to both miti-
gate the downsides of the MBBR while adding the electro-
oxidation circuit’s benefits to the site’s treatment process.
Description of EO Process Integration into Existing
Metallurgical Flowsheet
Due to the prevalence of deposits within the Abitibi Gold
Belt, it is not uncommon for a mill to accept ore from
mines throughout the area. In particular, the metallurgical
process studied within this second case study uses ore from
two mines. The main source is a sulphidic base metal ore,
which is treated prior to cyanidation in a conventional flo-
tation circuit. This produces copper and zinc concentrates
with the flotation tailings sent to the cyanide leach, where
the second ore is added as well. The cyanide leach is con-
ventional with leach tanks followed by CIP. The tailings are
treated in a cyanide detoxification circuit before a thick-
ener separates the solids and sends this to paste backfill.
The overflow solution is sent to the tailings storage facilities
(TSFs). While there is a significant amount of mill process
solution reuse from the TSFs, the site has a positive water
balance and must discharge excess solution. As the detox
effluent does not meet metal and toxicity limits, the water
must be treated in two discrete water treatment plants the
initial treats the water with lime, peroxide and ferrous sul-
phate to remove metals and residual cyanide complexes,
while the second uses an MBBR to oxidize ammonia and
thiocyanate. Figure 8 shows the block flow diagram for the
site’s metallurgical process.
Described above, the proposed IX-EO circuit would
be placed upstream of the existing MBBR, as shown in
Figure 9.
The columns are filled with ion exchange (IX) resin
which selectively replaces thiocyanate with sulphate to pro-
duce clean effluent. Once the IX resin reaches saturation,
it’s regenerated via sulfuric acid. The regenerant is directed
to the EO, which is the same as the process described for
the Mexican project (i.e., thiocyanate is oxidized to cya-
nide, with the HCN captured in a wet scrubber).
Testwork Results and Discussion
Samples of mine effluent collected after lime and before
biological treatment (MBBR) were shipped from site to
BQE Water’s laboratory in Vancouver, BC, Canada, with
the goal to prove out the ion exchange process to ensure
effluent quality met site objectives, and to validate the EO
process on the regenerant.
Table 4 shows the dissolved concentrations of the main
constituents in the solution received from site.
Ion Exchange Test Results
Prior to loading on the IX columns, the as-received solu-
tion was bulk filtered to remove suspended solids. The same
bed of ion exchange resin was then used for five (5) back-
to-back cycles, to show the reproducibility of the process,
and the gauge the extent of capacity changes. Figure 10
provides the breakthrough curves for the initial and final
loading cycles.
Table 3. Summary of Electro Oxidation results for Mexican Minesite
Test Parameter Solution Tested
Slope,
mg CN·L–1·min–1
Slope @100% CE,
mg CN·L–1·min–1
Current
Efficiency
Low current Density
(283 A/m2)
Actual leach solution 8.00 9.89 81%
Synthetic solution 7.92 80%
High Current Density
(943 A/m2)
Actual leach solution 19.04 32.97 58%
Synthetic Solution 24.89 75%
SCN concentration
Effect on CE
1 g/L SCN 8.45 32.97 26%
15 g/L SCN 23.95 73%