XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2453
nitrogen. If the pulp potential was to be varied, the pulp
potential was adjusted to the required value with either
oxidizing or reducing reagents. For pulp potentials above
the air-set potential, the pulp potential was adjusted with
sodium hypochlorite (NaOCl) and the flotation gas was
air. For pulp potentials below the air-set potential, the
pulp potential was lowered during grinding by flushing the
steel grinding mill with nitrogen prior to the addition of
the minerals. The pulp potential was then raised above the
nitrogen-set potential with NaOCl and the flotation gas
was nitrogen. In this way, the pulp potential was always
adjusted from a region of lower pulp potential to a region of
higher pulp potential which eliminates any possible hyster-
esis effects. Distilled water was used in all flotation mixture
tests. In tests below the air-set potential (both grinding and
flotation stages), the distilled water was deoxygenated by
bubbling nitrogen through the water for at least 20 minutes
prior to use.
Grinding
For all but one of the flotation tests, 50 g of the nickel sul-
phide and 450 g of quartz were mixed with distilled water
and ground for 20 minutes in a ball mill at the natural pH
and 67% solids by weight. Two mills were used: a stainless-
steel mill with stainless steel media, and a mild steel mill
with steel grinding media. The ball mill and charge were
cleaned prior to testing by grinding with a small quantity
of quartz and water for 10 minutes. In one test, a small
stainless steel rod mill with 5 stainless steel rods was used to
grind 50 g of the nickel sulphide with 50 ml distilled water
for 5 minutes. The ratio of sulphide mineral to quartz dilu-
ent (50 g to 450 g) was selected to enable a flotation feed
grade comparable to a typical nickel sulphide ore.
Flotation
Equipment
In all but one test, the ground pulp was floated in a modi-
fied 3 dm3 Denver style cell in which the impeller was
driven from below to allow the whole surface of the froth to
be scraped with a paddle at constant depth and rate. In one
test, a 1 dm3 cell was used. The pH and Eh were monitored
continuously during testing. The pH was measured with a
glass/calomel electrode calibrated using standard pH 4, pH
7 and pH 10 buffer solutions before each test. Pulp poten-
tials were measured with a high-impedance differential vol-
tameter using a polished platinum flag electrode and silver/
silver chloride reference electrode pair. The performance of
the electrode system was checked using a standard ferric-
ferrous ion solution (Light, 1972). Measured values were
converted to the standard hydrogen electrode (SHE) scale
by the addition of 0.2 V.
Procedure
The flotation procedures used for the mineral-quartz mix-
tures followed a standard procedure developed by CSIRO.
The ground pulp was transferred to the flotation cell and
the pulp level adjusted with water. The pH was adjusted to
pH 9, and the pulp aerated for 5 min. Collector was then
added, and the pulp conditioned for a further 5 minutes.
Flotation concentrates were collected at 0.5, 1, 2, 4, and 8
minutes by hand scraping at a constant depth and rate.
For tests where the Eh was controlled during flota-
tion, there were some minor differences in test procedures
for tests conducted below and above the air-set potential.
These modifications were necessary to ensure the reducing
or oxidising conditions required during flotation were not
compromised during any prior steps, such as slurry transfer
from mill to cell, when setting the pulp Eh or pH, or when
conditioning the pulp. For tests below the air-set potential,
the ground slurry was transferred from the mill to the cell
under nitrogen and a blanket of nitrogen was maintained
above the pulp until flotation concentrates were taken.
This helped minimise exposure of the ground pulp to air
before flotation commenced. In these tests, nitrogen was
the flotation gas and the makeup water used during flo-
tation was deoxygenated distilled water. The pH and Eh
were set to their test values and there was no aeration stage.
Collector was added, and the pulp conditioned for 5 min-
utes. Flotation concentrates were then collected.
For tests above the air-set potential, the flotation feed
was prepared in the same way as for tests below the air-set
potential except that none of the above precautions to avoid
exposure of the pulp to oxygen were taken, and the flota-
tion gas was synthetic air. The order of addition of reagents
and the conditioning times were unchanged.
Analysis
Chemical Analysis
Flotation products were weighed both wet and dry (to allow
calculation of water recoveries) and were prepared for anal-
ysis in a standard manner. Products were assayed for nickel,
iron and sulphur by Inductively Coupled Plasma-Optical
Emission Spectroscopy (ICP-OES). As both millerite and
pentlandite contain nickel, the pentlandite content of a flo-
tation product was estimated by assuming all the iron was
associated with the pentlandite. Energy Dispersive X-ray
(EDX) spot analysis of the pentlandite grains had shown
that the pentlandite contained on average 45.8% Ni,
nitrogen. If the pulp potential was to be varied, the pulp
potential was adjusted to the required value with either
oxidizing or reducing reagents. For pulp potentials above
the air-set potential, the pulp potential was adjusted with
sodium hypochlorite (NaOCl) and the flotation gas was
air. For pulp potentials below the air-set potential, the
pulp potential was lowered during grinding by flushing the
steel grinding mill with nitrogen prior to the addition of
the minerals. The pulp potential was then raised above the
nitrogen-set potential with NaOCl and the flotation gas
was nitrogen. In this way, the pulp potential was always
adjusted from a region of lower pulp potential to a region of
higher pulp potential which eliminates any possible hyster-
esis effects. Distilled water was used in all flotation mixture
tests. In tests below the air-set potential (both grinding and
flotation stages), the distilled water was deoxygenated by
bubbling nitrogen through the water for at least 20 minutes
prior to use.
Grinding
For all but one of the flotation tests, 50 g of the nickel sul-
phide and 450 g of quartz were mixed with distilled water
and ground for 20 minutes in a ball mill at the natural pH
and 67% solids by weight. Two mills were used: a stainless-
steel mill with stainless steel media, and a mild steel mill
with steel grinding media. The ball mill and charge were
cleaned prior to testing by grinding with a small quantity
of quartz and water for 10 minutes. In one test, a small
stainless steel rod mill with 5 stainless steel rods was used to
grind 50 g of the nickel sulphide with 50 ml distilled water
for 5 minutes. The ratio of sulphide mineral to quartz dilu-
ent (50 g to 450 g) was selected to enable a flotation feed
grade comparable to a typical nickel sulphide ore.
Flotation
Equipment
In all but one test, the ground pulp was floated in a modi-
fied 3 dm3 Denver style cell in which the impeller was
driven from below to allow the whole surface of the froth to
be scraped with a paddle at constant depth and rate. In one
test, a 1 dm3 cell was used. The pH and Eh were monitored
continuously during testing. The pH was measured with a
glass/calomel electrode calibrated using standard pH 4, pH
7 and pH 10 buffer solutions before each test. Pulp poten-
tials were measured with a high-impedance differential vol-
tameter using a polished platinum flag electrode and silver/
silver chloride reference electrode pair. The performance of
the electrode system was checked using a standard ferric-
ferrous ion solution (Light, 1972). Measured values were
converted to the standard hydrogen electrode (SHE) scale
by the addition of 0.2 V.
Procedure
The flotation procedures used for the mineral-quartz mix-
tures followed a standard procedure developed by CSIRO.
The ground pulp was transferred to the flotation cell and
the pulp level adjusted with water. The pH was adjusted to
pH 9, and the pulp aerated for 5 min. Collector was then
added, and the pulp conditioned for a further 5 minutes.
Flotation concentrates were collected at 0.5, 1, 2, 4, and 8
minutes by hand scraping at a constant depth and rate.
For tests where the Eh was controlled during flota-
tion, there were some minor differences in test procedures
for tests conducted below and above the air-set potential.
These modifications were necessary to ensure the reducing
or oxidising conditions required during flotation were not
compromised during any prior steps, such as slurry transfer
from mill to cell, when setting the pulp Eh or pH, or when
conditioning the pulp. For tests below the air-set potential,
the ground slurry was transferred from the mill to the cell
under nitrogen and a blanket of nitrogen was maintained
above the pulp until flotation concentrates were taken.
This helped minimise exposure of the ground pulp to air
before flotation commenced. In these tests, nitrogen was
the flotation gas and the makeup water used during flo-
tation was deoxygenated distilled water. The pH and Eh
were set to their test values and there was no aeration stage.
Collector was added, and the pulp conditioned for 5 min-
utes. Flotation concentrates were then collected.
For tests above the air-set potential, the flotation feed
was prepared in the same way as for tests below the air-set
potential except that none of the above precautions to avoid
exposure of the pulp to oxygen were taken, and the flota-
tion gas was synthetic air. The order of addition of reagents
and the conditioning times were unchanged.
Analysis
Chemical Analysis
Flotation products were weighed both wet and dry (to allow
calculation of water recoveries) and were prepared for anal-
ysis in a standard manner. Products were assayed for nickel,
iron and sulphur by Inductively Coupled Plasma-Optical
Emission Spectroscopy (ICP-OES). As both millerite and
pentlandite contain nickel, the pentlandite content of a flo-
tation product was estimated by assuming all the iron was
associated with the pentlandite. Energy Dispersive X-ray
(EDX) spot analysis of the pentlandite grains had shown
that the pentlandite contained on average 45.8% Ni,