XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 149
carbon strongly preference gold glycinate adsorption over
copper glycinate adsorption and that the presence of
increasing free glycine increases the adsorption of gold in
glycine-cyanide mixtures.
Cementation of Precious Metals onto Zinc Dust
Unpublished research has been internally reported on
zinc cementation from glycinate leachates and glycine-
starved-cyanide leachates. In both cases, gold and silver
showed strong cementation-precipitation onto zinc dust in
a de-aerated environment similar to cyanide only. Various
parameters were evaluated, and effective cementation could
be observed down to very low and zero levels of cyanide,
which is useful in the retrofitting of existing cementation
circuits where they are still in use.
REAGENT RECOVERY,
RECONCENTRATION, RECYCLING AND
REUSE AND WATER REUSE
The chemical stability of glycine, and its inherent regenera-
tion after NaHS precipitation, IX and SX, makes it valu-
able to recover if the concentration used (typically in base
metal operations) justifies recovery and recycle. To main-
tain an appropriate water balance in the plant, dilute wash-
effluents need to be reconcentrated so that glycine and
metal glycinates can be reused and water can be reused for
washing of filter cakes or for residue washing in counter-
current decantation (CCD) circuits. Various options exist
but depends on the nature of the spectator ions in the sys-
tem. During gold leaching for example, mere solid-liquid
separation and recycling of the barren leachate after carbon
adsorption may suffice. However, the oxidative leaching of
base metal sulfide minerals may need the rejection of sul-
fur from the circuit. Depending on the system this may be
achieved through crystallization or gypsum precipitation.
Glycine remains in solution while the sulfate is rejected.
The glycine, and most of the pH modifier is retained and
can be reused. Economics would dictate the extent to which
recovery and reuse is worthwhile. Nano-filtration has also
shown much promise to reconcentrate glycine and metal
glycinates to the retentate, while allowing monovalent and
smaller molecules to partition to the permeate. Evaporation,
while energy intensive, may also be considered for effluent
reconcentration. The alkalinity (pH9.5) and presence of
metal ions mitigates against algae and bacterial growth in
recycle streams. Glycine (in alkaline media as the glycinate
anion) has not been found to have any deleterious effect
of carbon activity and behaves like cyanide in this respect.
However, the impact of glycine in recycled process water on
flotation circuits performance has not been studied in great
detail and is an area for further research, particularly where
gold is associated with pyrite or arsenopyrite or the impact
on differential floatability in base metal sulfide circuits.
Evaporation
As glycine is non-volatile, water recovery through evapora-
tion is possible, although energy requirements and evapo-
rator costs will determine the feasibility of evaporation as a
water recovery method. It may be considered in conjunc-
tion with crystallization where sodium glycinate may for
instance be crystallized while evaporating water, thereby
delivering the salt that can be reused in leaching. However,
it would be preferable to evaporate enough water without
crystallization.
Nano Filtration
Nano filtration which utilizes ion-selective membranes has
shown much promise, particularly to reconcentrate the
effluent derived from washing the solid residues after solid-
liquid separation. Nano-filtration recovers large molecules
and polyvalent ions to the retentate while monovalent ions
typically pass through to the permeate, depending on the
permeate to retentate ratio, pH, operating pressure, charge
balance and species present. We have demonstrated that
we can feasibly recover metal glycinates from wash efflu-
ents while some glycine deports to the permeate to balance
sodium cations that co-deports.
Crystallization
Crystallization of sodium, potassium and ammonium sul-
fates is possible as a method to reject sulfur (which derived
from oxidizing mineral sulfides) from leachates to maintain
a steady sulfur balance. We have shown that sodium gly-
cinate can be crystallized in some cases, whereas sulfates
can be differentially crystallized in others, depending on if
cooling or evaporative crystallization is used, and whether
vacuum is used or not.
Gypsum Precipitation
In some cases, the precipitation of sulfate sulfur by lime
addition to form gypsum may be feasible. The choice of
sulfur rejection is highly dependent if preoxidation is per-
formed on concentrates, or the impact of residual sulfur in
the circuit when returned to the leach. Gypsum precipita-
tion is a cheap method to reject sulfur, but it depends how
the resulting solution will be reintegrated into the leach
circuit.
carbon strongly preference gold glycinate adsorption over
copper glycinate adsorption and that the presence of
increasing free glycine increases the adsorption of gold in
glycine-cyanide mixtures.
Cementation of Precious Metals onto Zinc Dust
Unpublished research has been internally reported on
zinc cementation from glycinate leachates and glycine-
starved-cyanide leachates. In both cases, gold and silver
showed strong cementation-precipitation onto zinc dust in
a de-aerated environment similar to cyanide only. Various
parameters were evaluated, and effective cementation could
be observed down to very low and zero levels of cyanide,
which is useful in the retrofitting of existing cementation
circuits where they are still in use.
REAGENT RECOVERY,
RECONCENTRATION, RECYCLING AND
REUSE AND WATER REUSE
The chemical stability of glycine, and its inherent regenera-
tion after NaHS precipitation, IX and SX, makes it valu-
able to recover if the concentration used (typically in base
metal operations) justifies recovery and recycle. To main-
tain an appropriate water balance in the plant, dilute wash-
effluents need to be reconcentrated so that glycine and
metal glycinates can be reused and water can be reused for
washing of filter cakes or for residue washing in counter-
current decantation (CCD) circuits. Various options exist
but depends on the nature of the spectator ions in the sys-
tem. During gold leaching for example, mere solid-liquid
separation and recycling of the barren leachate after carbon
adsorption may suffice. However, the oxidative leaching of
base metal sulfide minerals may need the rejection of sul-
fur from the circuit. Depending on the system this may be
achieved through crystallization or gypsum precipitation.
Glycine remains in solution while the sulfate is rejected.
The glycine, and most of the pH modifier is retained and
can be reused. Economics would dictate the extent to which
recovery and reuse is worthwhile. Nano-filtration has also
shown much promise to reconcentrate glycine and metal
glycinates to the retentate, while allowing monovalent and
smaller molecules to partition to the permeate. Evaporation,
while energy intensive, may also be considered for effluent
reconcentration. The alkalinity (pH9.5) and presence of
metal ions mitigates against algae and bacterial growth in
recycle streams. Glycine (in alkaline media as the glycinate
anion) has not been found to have any deleterious effect
of carbon activity and behaves like cyanide in this respect.
However, the impact of glycine in recycled process water on
flotation circuits performance has not been studied in great
detail and is an area for further research, particularly where
gold is associated with pyrite or arsenopyrite or the impact
on differential floatability in base metal sulfide circuits.
Evaporation
As glycine is non-volatile, water recovery through evapora-
tion is possible, although energy requirements and evapo-
rator costs will determine the feasibility of evaporation as a
water recovery method. It may be considered in conjunc-
tion with crystallization where sodium glycinate may for
instance be crystallized while evaporating water, thereby
delivering the salt that can be reused in leaching. However,
it would be preferable to evaporate enough water without
crystallization.
Nano Filtration
Nano filtration which utilizes ion-selective membranes has
shown much promise, particularly to reconcentrate the
effluent derived from washing the solid residues after solid-
liquid separation. Nano-filtration recovers large molecules
and polyvalent ions to the retentate while monovalent ions
typically pass through to the permeate, depending on the
permeate to retentate ratio, pH, operating pressure, charge
balance and species present. We have demonstrated that
we can feasibly recover metal glycinates from wash efflu-
ents while some glycine deports to the permeate to balance
sodium cations that co-deports.
Crystallization
Crystallization of sodium, potassium and ammonium sul-
fates is possible as a method to reject sulfur (which derived
from oxidizing mineral sulfides) from leachates to maintain
a steady sulfur balance. We have shown that sodium gly-
cinate can be crystallized in some cases, whereas sulfates
can be differentially crystallized in others, depending on if
cooling or evaporative crystallization is used, and whether
vacuum is used or not.
Gypsum Precipitation
In some cases, the precipitation of sulfate sulfur by lime
addition to form gypsum may be feasible. The choice of
sulfur rejection is highly dependent if preoxidation is per-
formed on concentrates, or the impact of residual sulfur in
the circuit when returned to the leach. Gypsum precipita-
tion is a cheap method to reject sulfur, but it depends how
the resulting solution will be reintegrated into the leach
circuit.