150 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
FINAL EFFLUENT DETOXIFICATION,
REAGENT DESTRUCTION AND
NEUTRALISATION NEEDS
Where possible and financially feasible, it would be
advantageous to perform solid-liquid separation of the bar-
ren /leached pulp and recycle the reagents back to the start
of the leach circuit. This is particularly important where
lixiviant concentrations are high, e.g., for concentrate
leaching. Where a leachate effluent is directed to tailings,
detoxification is either eliminated (for some of the cyanide-
free glycine options), or significantly minimized (in the case
of GlyCat, or glycine with cyanide at starvation levels and
no free cyanide present). Where strong oxidants are used
to destroy any residual cyanide, they will also affect glycine
degradation.
REAGENT GENERATION AND THE
INTEGRATION OF RENEWABLE ENERGY
The use of caustic soda and hydrochloric acid as key pH
modifiers can be achieved through chlor-alkali production
or via bipolar membrane electrodialysis. In the presence
of renewable or decarbonized energy, this is a strong con-
tender to lime-based processes, considering the large CO2
footprint associated with the calcination of limestone to
produce burnt lime or quicklime. Where saline brines are
readily available, this may be a suitable source of some of
the reagents during alkaline glycine leaching. Glycine pro-
duction itself does not differ significantly from a carbon
footprint perspective over cyanide, given that natural gas is
used as primary raw material for both reagents. However,
the strong capability of glycine to be recovered, recycled,
and reused can reduce the overall carbon footprint by
significantly lowering the need for new reagent addition.
Under dilute conditions and in an open circuit configura-
tion when tailings water is not reclaimed and reused, glycine
is inevitable, like cyanide, lost to tailings and represent a net
loss to the circuit. In the GlyCat ™ approach (with cyanide
catalyst) overall cyanide consumption is often reduced with
80%. However glycine addition may be equal or slightly
higher than cyanide, even though glycine unit costs may
be less. In the case of base metals, or where cyanide-free
GlyLeach ™ is used the concentrations of glycine required
requires solid-liquid separation and recycling of the leach-
ate after the targeted metals are recovered. Dosage rates are
ore dependent, as for any lixiviant.
PROCESS STABILITY, MEASUREMENT
AND CONTROL
New process chemistries should, as far as possible, simplify
process chemistry (therefore the number of unwanted side
reactions should be at a minimum). Assaying and monitor-
ing of chemical species concentrations should be feasible,
practical within short time frames and allow sufficient time
for process rectification and control. Total metal assays are
easily established through AAS, ICP-MS or ICP OES.
Species concentrations are however harder to determine,
particularly in complex multicomponent chemical matri-
ces. For example, the development of an acceptable and reli-
able measurement method to quantitatively determine free
cyanide, ferrocyanide, cyanate and thiocyanate species, and
copper cyanide and WAD cyanide species took a long time.
It has been shown that copper glycinate has a very distinct
UV-Visible fingerprint, which is also the case for nickel and
cobalt glycinates (Tanda, Oraby and Eksteen, 2017). Free
glycine can be reliably determined by HPLC. In simple sys-
tems titration techniques may suffice. The benign, stable,
and non-hygroscopic nature of glycine makes the control
of addition straightforward. Unlike cyanide leach circuits
for which on-line analyzers have been developed, glycine
circuits, being at cusp on industrial implementation, do
not have the equivalent on-line analyzers yet. This is an area
where further development would be beneficial.
TRANSPORT AND STORAGE
Some chemical reagents, such as cyanide, are classified as
chemical warfare agents and some jurisdictions do not
allow transport and storage of such chemicals on and to
site. The European REACH (Registration, Evaluation,
Authorization and Restriction of Chemicals) seeks to mini-
mize and restrict the transport and handling of dangerous
goods. Similar initiatives are in place elsewhere. Having
reagents that are not highly reactive, oxidizing, strong acids
or bases, non-volatile, non-corrosive, non-toxic, non-explo-
sive and is acceptable in human foods and animal feed cer-
tainly alleviates much of the risks associated with transport
and storage. The non-volatile, non-noxious, non-irritant,
non-hygroscopic and free-flowing nature of glycine there
significantly improved handleability and storability.
LIMITATIONS AND BEST CANDIDATES
TO DEPLOY GLYCINE LEACH PROCESSES
Glycine shares many of the application opportunities of
cyanide leach processes, where the pH, %solids, tempera-
tures are similar allowing retrofitting into existing circuits.
Like in cyanide leaching, the gold (and other precious met-
als) cyanide complex adsorbs well onto activated carbon,
allowing CIP and CIL applications. However, like for cya-
nide processing, the presence of glycine during the leach
of gold-robbing ores, where the ore contains active natural
carbon, leaching results will be poor due to re-adsorption of
FINAL EFFLUENT DETOXIFICATION,
REAGENT DESTRUCTION AND
NEUTRALISATION NEEDS
Where possible and financially feasible, it would be
advantageous to perform solid-liquid separation of the bar-
ren /leached pulp and recycle the reagents back to the start
of the leach circuit. This is particularly important where
lixiviant concentrations are high, e.g., for concentrate
leaching. Where a leachate effluent is directed to tailings,
detoxification is either eliminated (for some of the cyanide-
free glycine options), or significantly minimized (in the case
of GlyCat, or glycine with cyanide at starvation levels and
no free cyanide present). Where strong oxidants are used
to destroy any residual cyanide, they will also affect glycine
degradation.
REAGENT GENERATION AND THE
INTEGRATION OF RENEWABLE ENERGY
The use of caustic soda and hydrochloric acid as key pH
modifiers can be achieved through chlor-alkali production
or via bipolar membrane electrodialysis. In the presence
of renewable or decarbonized energy, this is a strong con-
tender to lime-based processes, considering the large CO2
footprint associated with the calcination of limestone to
produce burnt lime or quicklime. Where saline brines are
readily available, this may be a suitable source of some of
the reagents during alkaline glycine leaching. Glycine pro-
duction itself does not differ significantly from a carbon
footprint perspective over cyanide, given that natural gas is
used as primary raw material for both reagents. However,
the strong capability of glycine to be recovered, recycled,
and reused can reduce the overall carbon footprint by
significantly lowering the need for new reagent addition.
Under dilute conditions and in an open circuit configura-
tion when tailings water is not reclaimed and reused, glycine
is inevitable, like cyanide, lost to tailings and represent a net
loss to the circuit. In the GlyCat ™ approach (with cyanide
catalyst) overall cyanide consumption is often reduced with
80%. However glycine addition may be equal or slightly
higher than cyanide, even though glycine unit costs may
be less. In the case of base metals, or where cyanide-free
GlyLeach ™ is used the concentrations of glycine required
requires solid-liquid separation and recycling of the leach-
ate after the targeted metals are recovered. Dosage rates are
ore dependent, as for any lixiviant.
PROCESS STABILITY, MEASUREMENT
AND CONTROL
New process chemistries should, as far as possible, simplify
process chemistry (therefore the number of unwanted side
reactions should be at a minimum). Assaying and monitor-
ing of chemical species concentrations should be feasible,
practical within short time frames and allow sufficient time
for process rectification and control. Total metal assays are
easily established through AAS, ICP-MS or ICP OES.
Species concentrations are however harder to determine,
particularly in complex multicomponent chemical matri-
ces. For example, the development of an acceptable and reli-
able measurement method to quantitatively determine free
cyanide, ferrocyanide, cyanate and thiocyanate species, and
copper cyanide and WAD cyanide species took a long time.
It has been shown that copper glycinate has a very distinct
UV-Visible fingerprint, which is also the case for nickel and
cobalt glycinates (Tanda, Oraby and Eksteen, 2017). Free
glycine can be reliably determined by HPLC. In simple sys-
tems titration techniques may suffice. The benign, stable,
and non-hygroscopic nature of glycine makes the control
of addition straightforward. Unlike cyanide leach circuits
for which on-line analyzers have been developed, glycine
circuits, being at cusp on industrial implementation, do
not have the equivalent on-line analyzers yet. This is an area
where further development would be beneficial.
TRANSPORT AND STORAGE
Some chemical reagents, such as cyanide, are classified as
chemical warfare agents and some jurisdictions do not
allow transport and storage of such chemicals on and to
site. The European REACH (Registration, Evaluation,
Authorization and Restriction of Chemicals) seeks to mini-
mize and restrict the transport and handling of dangerous
goods. Similar initiatives are in place elsewhere. Having
reagents that are not highly reactive, oxidizing, strong acids
or bases, non-volatile, non-corrosive, non-toxic, non-explo-
sive and is acceptable in human foods and animal feed cer-
tainly alleviates much of the risks associated with transport
and storage. The non-volatile, non-noxious, non-irritant,
non-hygroscopic and free-flowing nature of glycine there
significantly improved handleability and storability.
LIMITATIONS AND BEST CANDIDATES
TO DEPLOY GLYCINE LEACH PROCESSES
Glycine shares many of the application opportunities of
cyanide leach processes, where the pH, %solids, tempera-
tures are similar allowing retrofitting into existing circuits.
Like in cyanide leaching, the gold (and other precious met-
als) cyanide complex adsorbs well onto activated carbon,
allowing CIP and CIL applications. However, like for cya-
nide processing, the presence of glycine during the leach
of gold-robbing ores, where the ore contains active natural
carbon, leaching results will be poor due to re-adsorption of