XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 767
• The rate of free NaCN production is no longer the
same for synthetic and actual leach solution
• Cell voltage is no longer the same for synthetic and
actual leach solution
• Cell voltage continues to drop without reaching a
steady state
Figure 4 (b) shows that the rate of rise in free NaCN con-
centration (slope) was 31% greater/steeper with the syn-
thetic solution compared to the actual leach solution, which
corresponds to a current efficiency of only 58% for the
actual leach solution. This drop in the current efficiency at
high current density was expected because higher cell cur-
rent promotes parasitic anodic reactions (5) through (8).
This was confirmed by cyanate (OCN) assays in the final
solution after the completion of the EO tests, where the
concentration of cyanate in the actual leach solution pro-
cessed through the EO was 2.5 times higher at high current
density compared to low current density (given in Table 2).
This table further shows that cyanate production was only
1.3 times different for the synthetic solution, indicating
that solution chemistry becomes more important for the
EO process operating at high current density. In particu-
lar, reactions involving chloride and iron cyanides become
more important.
Sensitivity of Cyanide Recovery to Solution
Composition
The three main constituents that may have an outsized
influence on the EO process performance include: thiocya-
nate, free cyanide, and chloride. The concentration of these
constituents affects the EO process as follows:
• SCN is the primary reactant and the mass transfer of
SCN to the electrode surface maybe important for
achieving high current efficiency
• Free CN generated on the anode is the main product
of the EO process, but it can also be oxidized to cya-
nate via reaction (7)
• Chloride can be involved in the generation of chlo-
rine on the anode. If generated, chlorine would then
react with free CN and reduce the overall yield of the
EO process
For all these reasons, tests were conducted using synthetic
solutions with variable concentrations of SCN, free CN,
and Cl, to gain insight into the sensitivity of the EO process
to leach solution composition.
Effect of Initial Thiocyanate Concentration. Figure 5
(a), (b) and (c) give the thiocyanate concentration drop,
resulting cyanide increase, as well as voltage throughout the
testwork, respectively. Although various synthetic solutions
were created (Solutions B – E, only the boundary condi-
tion concentrations are shown). The lower cell operating
voltage observed with Solution E compared to Solution B
was due to the much higher electric conductivity of the
solution, which is based purely on the increased TDS con-
tent (Solution B with 1 g/L SCN was 16 mS/cm, while
Solution E at 15 g/L SCN was 38.6 mS/cm). The portion
Figure 4. High Current Density Testing (943 A/m2). Variation of (a) thiocyanate concentration (b) cyanide concentration
Table 2. Cyanate analysis at the end of the EO test
Current Density,
A/m2 Solution [OCN], mg/L
283 Actual leach solution 71
Synthetic Solution A 121
943 Actual leach solution 176
Synthetic Solution A 163
• The rate of free NaCN production is no longer the
same for synthetic and actual leach solution
• Cell voltage is no longer the same for synthetic and
actual leach solution
• Cell voltage continues to drop without reaching a
steady state
Figure 4 (b) shows that the rate of rise in free NaCN con-
centration (slope) was 31% greater/steeper with the syn-
thetic solution compared to the actual leach solution, which
corresponds to a current efficiency of only 58% for the
actual leach solution. This drop in the current efficiency at
high current density was expected because higher cell cur-
rent promotes parasitic anodic reactions (5) through (8).
This was confirmed by cyanate (OCN) assays in the final
solution after the completion of the EO tests, where the
concentration of cyanate in the actual leach solution pro-
cessed through the EO was 2.5 times higher at high current
density compared to low current density (given in Table 2).
This table further shows that cyanate production was only
1.3 times different for the synthetic solution, indicating
that solution chemistry becomes more important for the
EO process operating at high current density. In particu-
lar, reactions involving chloride and iron cyanides become
more important.
Sensitivity of Cyanide Recovery to Solution
Composition
The three main constituents that may have an outsized
influence on the EO process performance include: thiocya-
nate, free cyanide, and chloride. The concentration of these
constituents affects the EO process as follows:
• SCN is the primary reactant and the mass transfer of
SCN to the electrode surface maybe important for
achieving high current efficiency
• Free CN generated on the anode is the main product
of the EO process, but it can also be oxidized to cya-
nate via reaction (7)
• Chloride can be involved in the generation of chlo-
rine on the anode. If generated, chlorine would then
react with free CN and reduce the overall yield of the
EO process
For all these reasons, tests were conducted using synthetic
solutions with variable concentrations of SCN, free CN,
and Cl, to gain insight into the sensitivity of the EO process
to leach solution composition.
Effect of Initial Thiocyanate Concentration. Figure 5
(a), (b) and (c) give the thiocyanate concentration drop,
resulting cyanide increase, as well as voltage throughout the
testwork, respectively. Although various synthetic solutions
were created (Solutions B – E, only the boundary condi-
tion concentrations are shown). The lower cell operating
voltage observed with Solution E compared to Solution B
was due to the much higher electric conductivity of the
solution, which is based purely on the increased TDS con-
tent (Solution B with 1 g/L SCN was 16 mS/cm, while
Solution E at 15 g/L SCN was 38.6 mS/cm). The portion
Figure 4. High Current Density Testing (943 A/m2). Variation of (a) thiocyanate concentration (b) cyanide concentration
Table 2. Cyanate analysis at the end of the EO test
Current Density,
A/m2 Solution [OCN], mg/L
283 Actual leach solution 71
Synthetic Solution A 121
943 Actual leach solution 176
Synthetic Solution A 163