| Summary: | The objective of this research was to explore an efficient process to recover cyanide
and copper from barren gold cyanide solution. The research work described here concerns an
investigation into fundamental and practical aspects o f two options for electrowinning copper
from cyanide solution. These two options are: (a) the use o f an alternative anode reaction to
limit the electro-oxidation o f cyanide in concentrated cyanide solutions and (b) the use of a
graphite fibre cathode to electrowin copper from dilute cyanide solution.
(1) A critical literature survey was conducted to examine the stability constants of
copper cyanide species. The distributions and the equilibrium redox potentials of copper
cyanide species were calculated using the most reliable stability constants. They are
dependent on the mole ratio of cyanide to copper, total cyanide concentration, pH and
temperature. Potential measurements have confirmed the validity of the calculated results.
The pH-potential diagram was drawn using the Gibbs free energy data derived by selecting
the most reliable stability constants.
(2) Direct copper electrowinning from dilute cyanide solutions was conducted in a
membrane cell. The accumulation o f deposited copper on the graphite felt as the plating
proceeds significantly improves the conductivity o f the graphite felt, increases the specific
surface area and benefits copper deposition. Copper can be deposited on the graphite felt
from low concentration solutions (1-2 gL-1 Cu and CN:Cu mole ratio = 3-4) with 50-80 %
current efficiency, the removal of around 40% Cu and an energy consumption of 1-2 kWh/kg
Cu in the superficial current density range 30 - 100 Am-2 at 40 °C.
(3) Copper electrowinning from concentrated copper cyanide solution (70 g L-1 Cu)
was conducted using four sacrificial species (sulphite, methanol, thiocyanate and ammonia)
at 40 to 60 °C. Only sulphite can decrease the anodic current efficiency of cyanide oxidation
from ~ 100 to 10-20 % over the current density range of 250-500 Am-2. With increasing
CN:Cu mole ratio from 3 to 4.5, the anodic current efficiency of cyanide oxidation increased
and the copper deposition current efficiency decreased. As regards the recovery of copper
from barren gold cyanide solution, it has been shown that using sulphite oxidation as an
alternative anode reaction, copper can be electrowon from a cyanide electrolyte containing
about 70 gL-1 Cu (CN:Cu = about 3) and 0.5 M Na2SO3 at a cathode current efficiency of
about 95% with a energy consumption of about 0.8 kWh/kg Cu at 250 Am-2.
(4) In alkaline solutions, sulphite is oxidized to sulphate on the graphite anode in a
two-electron reaction. The reaction order with respect to sulphite ions is below 1 at low
potentials(< 0.4 V vs. SCE) and 1 at high potentials. The reaction order for hydroxide ions is
close to zero. Two Tafel slopes were observed, 0.060 - 0.64 V decade-1 at low potentials and
0.19-0.20 V decade-1 at high potentials in the temperature range 40 - 60 °C . Sulphite
oxidation in alkaline solution appears to undergo an electron-radical mechanism.
(5) The anodic oxidation of copper cyanide has been studied using a graphite rotating
disk with reference to cyanide concentration (0.05-4 M) , CN:Cu mole ratio (3-12),
temperature (25-60 °C) and hydroxide concentration (0.01-0.25 M) . Copper had a significant
catalytic effect on cyanide oxidation. In the low polarization region (< about 0.4 V vs. SCE) ,
cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate.
At a CN:Cu ratio of 3 and [OH-] = 0.25 M, the Tafel slope was about 0.12 V decade-1.
Cu(CN)32-
was discharged on the electrode surface. With increasing CN:Cu mole ratio and
decreasing pH, the dominant discharged species shifted to Cu(CN)43-. In the high polarization
region (about 0.4 -0.6 V vs. SCE), cuprous cyanide complexes were oxidized to copper oxide
and cyanate. When the concentration of cyanide was high and the pH low, cyanogen was
formed, but no copper oxide.
(6) Sulphite oxidation is enhanced by the presence of copper cyanide. The effect of
sulphite on limiting the oxidation of copper cyanide decreases with increasing mole ratio of
cyanide to copper. This is related to the shift in the discharged species from Cu(CN)32- to
Cu(CN)43- with increasing mole ratio of cyanide to copper. Sulphite is oxidized to sulphate.
At [Cu] = around 1 M, CN:Cu = 3 -3.2, [OH-] = 0.05-0.25 M, [SO32-] = 0.4-0.6 M and the
temperature = 50 - 60 °C, the anode current efficiency of sulphite oxidation reached 80-90%)
as the anodic current efficiency of cyanide fell to 20 to 10 %. [Scientific formulae used in this abstract could not be reproduced.]
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