672
Metallurgical Effects of Milling Media and Water Composition in
the Flotation of a Mixed-Mineral Ore
S.N. Nyoni, K.C. Corin, and C.T. O’Connor
Centre for Minerals Research, University of Cape Town, Cape Town, South Africa
ABSTRACT: Chemical reactions occur during milling and in flotation, mostly driven by interactions between
minerals, reagents, milling media, and water chemistry. Furthermore, the valuable minerals may become more
difficult to float in the case of polymetallic ores and closed-water circuits as a result of these interactions. This
study uses a mixed mineral (pyrite and galena) synthetic ore to investigate how metallurgical performance is
affected by using different milling media, pHs in the mill, and process water ionic strengths. Mixed potential,
DO, and pH were closely monitored in situ during milling. Using pyrite as one of the ores allows for the effect
of Fe in the mineral to be decoupled from that arising from the mill shell and Fe-bearing milling media.
Keywords: Milling media, water quality, water recycling, ionic strength, mixed ore
INTRODUCTION
The milling environment plays a crucial preparatory role for
flotation, enabling the liberation of desirable minerals from
host rocks and facilitating primary chemical or physico-
chemical interactions of the ore. These interactions extend
to those resulting from the milling media and water quality,
which may vary depending on the season, water internal
reuse within the concentrator, external water reuse, and the
presence of multiple sources of raw or freshwater [1], [2].
When targeting sulfide minerals, knowledge of the electro-
chemical interactions which may occur between such min-
erals during the milling stage is of particular importance,
as these interactions may affect electron transfer with other
components of the milling or flotation system. These inter-
actions include those of dissolved ions emanating from the
milling media or recycled water, suspended solids, reagent
additions, and any dissolved gas, which can result in reac-
tions such as surface oxidation, collector adsorption, and
precipitate formation on the mineral or gangue surfaces [3].
Collector adsorption is often studied as a collective
effect of the other reactions mentioned above, as they affect
the potential for the collector to attach to the mineral
surface, and thus its modification to a more hydrophobic
character and subsequent mineral-gas bubble attachment
during separation by flotation [4]. Reactions resulting from
one or a combination of these interactions may significantly
alter the floatability of the ore, by shifting the electrochemi-
cal or redox potential (Eh) of the slurry phase unfavourably
or changing the hydrophobicity of the targeted mineral or
the associated gangue.
The presence of cationic species introduced from the
milling media, reagent addition, or oxidation of sulfides,
which interact with minerals present, targeted or gangue, in
a manner that affects their role/identity as reductants or oxi-
dants in the system, compounds the latter case. Lead (Pb)
ions, for example, have been identified as contributing to
the activation of pyrite (Py), often considered an unwanted
and problematic gangue mineral, resulting in its enhanced
floatability and increased affinity for the collector [5].

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Extracted Text (may have errors)

672
Metallurgical Effects of Milling Media and Water Composition in
the Flotation of a Mixed-Mineral Ore
S.N. Nyoni, K.C. Corin, and C.T. O’Connor
Centre for Minerals Research, University of Cape Town, Cape Town, South Africa
ABSTRACT: Chemical reactions occur during milling and in flotation, mostly driven by interactions between
minerals, reagents, milling media, and water chemistry. Furthermore, the valuable minerals may become more
difficult to float in the case of polymetallic ores and closed-water circuits as a result of these interactions. This
study uses a mixed mineral (pyrite and galena) synthetic ore to investigate how metallurgical performance is
affected by using different milling media, pHs in the mill, and process water ionic strengths. Mixed potential,
DO, and pH were closely monitored in situ during milling. Using pyrite as one of the ores allows for the effect
of Fe in the mineral to be decoupled from that arising from the mill shell and Fe-bearing milling media.
Keywords: Milling media, water quality, water recycling, ionic strength, mixed ore
INTRODUCTION
The milling environment plays a crucial preparatory role for
flotation, enabling the liberation of desirable minerals from
host rocks and facilitating primary chemical or physico-
chemical interactions of the ore. These interactions extend
to those resulting from the milling media and water quality,
which may vary depending on the season, water internal
reuse within the concentrator, external water reuse, and the
presence of multiple sources of raw or freshwater [1], [2].
When targeting sulfide minerals, knowledge of the electro-
chemical interactions which may occur between such min-
erals during the milling stage is of particular importance,
as these interactions may affect electron transfer with other
components of the milling or flotation system. These inter-
actions include those of dissolved ions emanating from the
milling media or recycled water, suspended solids, reagent
additions, and any dissolved gas, which can result in reac-
tions such as surface oxidation, collector adsorption, and
precipitate formation on the mineral or gangue surfaces [3].
Collector adsorption is often studied as a collective
effect of the other reactions mentioned above, as they affect
the potential for the collector to attach to the mineral
surface, and thus its modification to a more hydrophobic
character and subsequent mineral-gas bubble attachment
during separation by flotation [4]. Reactions resulting from
one or a combination of these interactions may significantly
alter the floatability of the ore, by shifting the electrochemi-
cal or redox potential (Eh) of the slurry phase unfavourably
or changing the hydrophobicity of the targeted mineral or
the associated gangue.
The presence of cationic species introduced from the
milling media, reagent addition, or oxidation of sulfides,
which interact with minerals present, targeted or gangue, in
a manner that affects their role/identity as reductants or oxi-
dants in the system, compounds the latter case. Lead (Pb)
ions, for example, have been identified as contributing to
the activation of pyrite (Py), often considered an unwanted
and problematic gangue mineral, resulting in its enhanced
floatability and increased affinity for the collector [5].

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XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 671
Zanin, M., Ametov, I., Grano, S., Zhou, L., &Skinner, W.
2009. A study of mechanisms affecting molybdenite
recovery in a bulk copper/molybdenum flotation cir-
cuit. International Journal of Mineral Processing, 93(3–
4), 256–266. doi: 10.1016/j.minpro.2009.10.001.
Zhang, N., Liu, W., Liu, W., &Chen, X. 2022. Flotation
separation of molybdenite from chalcopyrite using
mechanically degraded polyacrylamide as a novel
depressant. Colloids and Surfaces A: Physicochemical
and Engineering Aspects, 652, 129897. doi: 10.1016
/j.colsurfa.2022.129897.
Zhu, H., Li, Y., Lartey, C., Li, W., &Qian, G. 2020.
Flotation kinetics of molybdenite in common sulfate
salt solutions. Minerals Engineering, 148, 106182. doi:
10.1016/j.mineng.2020.106182

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Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 673
In some studies, pH, and dissolved oxygen (DO) con-
trol have been used, to varying extents, in adjusting the
electrochemistry of the slurry phase, as measured by the
Eh [6], [7], [8]. The measured Eh in a mineral slurry phase
is the net result of the oxidizing and reducing species pres-
ent in the system. Compared to other minerals often found
associated with sulfidic ores, Py often has the highest rest
potential, implying that it is often the mineral least suscep-
tible to oxidation [9]. Thus, knowledge of the difference
between the rest potential of Py and that of the targeted
mineral, for example, galena (Gn) is important in adjust-
ing the processing protocol to cater for any anticipated oxi-
dation of the targeted mineral surface before flotation. In
related studies, using deionised water (DI) at a pH of ~6,
the rest potentials of Py and Gn have been measured to be
+660 mV, and +400 mV respectively, and the Py-Gn mixed
potential in the range 351–401 mV [10].
Another important challenge arising from the pres-
ence of Py in mineral processing and extraction circuits is
its contribution to acid mine drainage, which is deleterious
to the environment [11], [12], [13].
This paper aims to determine the effects of pH, water
quality, and milling media used in the milling process on
the flotation of a synthetic mixed ore containing the sulfide
minerals Py and Gn, with talc and quartz (Qz) added as
gangue components. The effect on the slurry electrochem-
istry, as measured by Eh, was monitored during the milling
operation and studied in relation to the flotation response
of the ore, particularly with respect to the ultimate effect
on the ease of separation of Gn from Py. Additionally, the
impact of milling media on the presence of dissolved Fe in
the system, and the presence of various dissolved salts typi-
cal of many flotation operations that recycle process water,
were investigated, and compared with a control condition
which used inert ceramic (Cer) media and deionized water,
respectively. The findings of this study could have signifi-
cant implications for the optimisation of the grinding and
flotation processes for sulfide minerals, which can help
improve the efficiency of mineral processing operations.
METHODOLOGY
The present study aimed to investigate the impact of media
type, pH, and water chemistry on the grinding and flo-
tation of sulfide minerals, namely Pyrite (Py) and Galena
(Gn). To ensure the absence of other sulfide minerals
and their purity, the minerals, which were obtained from
Mineral World, Cape Town, South Africa, were analysed
by X-ray diffraction (XRD). The purity of Py and Gn was
found to be 95.1 wt.% and 78 wt.% (+11.8 wt.% angle-
site, PbSO4 and 10.2 wt.% cerrusite respectively. Parallel
experiments were carried out using forged steel (FS), and
Cer media in a Magotteaux Mill [14]. The mill was oper-
ated at 55 rpm with a targeted grind size of 60% passing
75 µm. The solids mill feed comprised 2.5 wt.% each for
Py and Gn, 2 wt.% talc, and 93 wt.% Qz, and was milled
with 2 l of either DI or synthetic plant water (SPW) with
an ionic strength of 0.1205 M, which was labelled 5SPW
[15], [16]. The milling pH was varied between a natural
pH (~6–8) and controlled pHs of 4 and 10, achieved by the
automated addition of 2 vol.% HCl and 1 wt.% Ca(OH)2
(lime), respectively. Eh, pH, and DO measurements were
recorded automatically online at 10-second intervals for the
duration of the milling process. After milling, the slurry was
transferred to a 4.5 l bottom-driven Magotteaux flotation
cell, operating at an impeller speed of 1100 rpm. Reagents
added to the cell were 100 g/t sodium isobutyl xanthate
(SIBX) collector and 60 g/t DOW 200 frother. Aeration
was initiated at 12 l/min, and concentrates were collected
after 2, 6, 12, and 20 min, with the systematic scraping of
loaded froth at 15-second intervals. Fe/Pb in the dried and
weighed feed concentrates and tailings solids was measured
by X-ray Fluorescence Spectroscopy (XRF). The standard
deviation error values associated with the experiments were
determined and presented accordingly as error bars.
RESULTS AND DISCUSSION
Mill Slurry Electrochemical Analysis
In the present study the electrochemical parameters such
as Eh, pH, and DO were measured. Notably, the pH was
observed to remain relatively constant with little varia-
tion, remaining within the range of 6–8 during the natu-
ral pH experiments, while it was maintained at pH 4 and
10 throughout the other experiments. The absence of gas
purging and the ingress of atmospheric air in the milling
system also resulted in minimal fluctuations in DO read-
ings, remaining stable at ±8 mg/l. Consequently, the focus
of this section’s discussion will be on the variations observed
in Eh, as depicted in Figure 1. This figure shows the varia-
tion in Eh during the milling process for different milling
media compositions, water types, and pHs.
Electrochemical potential (Eh) is a crucial measure in
understanding the redox balance in a system. In the context
of xanthate usage, the goal is often to generate dixantho-
gen, a highly hydrophobic species that imparts hydropho-
bicity to the desired mineral [17]. To achieve this, an Eh
of approximately +200 mV is often typically targeted, as it
facilitates the oxidation of xanthogen to dixanthogen [2].
In this study, the measured Eh values are compared to this
region.

Previous Page Next Page

Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 673
In some studies, pH, and dissolved oxygen (DO) con-
trol have been used, to varying extents, in adjusting the
electrochemistry of the slurry phase, as measured by the
Eh [6], [7], [8]. The measured Eh in a mineral slurry phase
is the net result of the oxidizing and reducing species pres-
ent in the system. Compared to other minerals often found
associated with sulfidic ores, Py often has the highest rest
potential, implying that it is often the mineral least suscep-
tible to oxidation [9]. Thus, knowledge of the difference
between the rest potential of Py and that of the targeted
mineral, for example, galena (Gn) is important in adjust-
ing the processing protocol to cater for any anticipated oxi-
dation of the targeted mineral surface before flotation. In
related studies, using deionised water (DI) at a pH of ~6,
the rest potentials of Py and Gn have been measured to be
+660 mV, and +400 mV respectively, and the Py-Gn mixed
potential in the range 351–401 mV [10].
Another important challenge arising from the pres-
ence of Py in mineral processing and extraction circuits is
its contribution to acid mine drainage, which is deleterious
to the environment [11], [12], [13].
This paper aims to determine the effects of pH, water
quality, and milling media used in the milling process on
the flotation of a synthetic mixed ore containing the sulfide
minerals Py and Gn, with talc and quartz (Qz) added as
gangue components. The effect on the slurry electrochem-
istry, as measured by Eh, was monitored during the milling
operation and studied in relation to the flotation response
of the ore, particularly with respect to the ultimate effect
on the ease of separation of Gn from Py. Additionally, the
impact of milling media on the presence of dissolved Fe in
the system, and the presence of various dissolved salts typi-
cal of many flotation operations that recycle process water,
were investigated, and compared with a control condition
which used inert ceramic (Cer) media and deionized water,
respectively. The findings of this study could have signifi-
cant implications for the optimisation of the grinding and
flotation processes for sulfide minerals, which can help
improve the efficiency of mineral processing operations.
METHODOLOGY
The present study aimed to investigate the impact of media
type, pH, and water chemistry on the grinding and flo-
tation of sulfide minerals, namely Pyrite (Py) and Galena
(Gn). To ensure the absence of other sulfide minerals
and their purity, the minerals, which were obtained from
Mineral World, Cape Town, South Africa, were analysed
by X-ray diffraction (XRD). The purity of Py and Gn was
found to be 95.1 wt.% and 78 wt.% (+11.8 wt.% angle-
site, PbSO4 and 10.2 wt.% cerrusite respectively. Parallel
experiments were carried out using forged steel (FS), and
Cer media in a Magotteaux Mill [14]. The mill was oper-
ated at 55 rpm with a targeted grind size of 60% passing
75 µm. The solids mill feed comprised 2.5 wt.% each for
Py and Gn, 2 wt.% talc, and 93 wt.% Qz, and was milled
with 2 l of either DI or synthetic plant water (SPW) with
an ionic strength of 0.1205 M, which was labelled 5SPW
[15], [16]. The milling pH was varied between a natural
pH (~6–8) and controlled pHs of 4 and 10, achieved by the
automated addition of 2 vol.% HCl and 1 wt.% Ca(OH)2
(lime), respectively. Eh, pH, and DO measurements were
recorded automatically online at 10-second intervals for the
duration of the milling process. After milling, the slurry was
transferred to a 4.5 l bottom-driven Magotteaux flotation
cell, operating at an impeller speed of 1100 rpm. Reagents
added to the cell were 100 g/t sodium isobutyl xanthate
(SIBX) collector and 60 g/t DOW 200 frother. Aeration
was initiated at 12 l/min, and concentrates were collected
after 2, 6, 12, and 20 min, with the systematic scraping of
loaded froth at 15-second intervals. Fe/Pb in the dried and
weighed feed concentrates and tailings solids was measured
by X-ray Fluorescence Spectroscopy (XRF). The standard
deviation error values associated with the experiments were
determined and presented accordingly as error bars.
RESULTS AND DISCUSSION
Mill Slurry Electrochemical Analysis
In the present study the electrochemical parameters such
as Eh, pH, and DO were measured. Notably, the pH was
observed to remain relatively constant with little varia-
tion, remaining within the range of 6–8 during the natu-
ral pH experiments, while it was maintained at pH 4 and
10 throughout the other experiments. The absence of gas
purging and the ingress of atmospheric air in the milling
system also resulted in minimal fluctuations in DO read-
ings, remaining stable at ±8 mg/l. Consequently, the focus
of this section’s discussion will be on the variations observed
in Eh, as depicted in Figure 1. This figure shows the varia-
tion in Eh during the milling process for different milling
media compositions, water types, and pHs.
Electrochemical potential (Eh) is a crucial measure in
understanding the redox balance in a system. In the context
of xanthate usage, the goal is often to generate dixantho-
gen, a highly hydrophobic species that imparts hydropho-
bicity to the desired mineral [17]. To achieve this, an Eh
of approximately +200 mV is often typically targeted, as it
facilitates the oxidation of xanthogen to dixanthogen [2].
In this study, the measured Eh values are compared to this
region.