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Surface Chemistry and Flotation Behavior of Monazite, Apatite, Ilmenite, Quartz, Rutile, and Zircon with Octanohydroxamic Acid.

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Abstract

Global increase in rare earth demand and consumption has led to a further understanding of their beneficiation and recovery. Monazite is the second most important rare earth mineral that can be further exploited. In this study, the surface chemistry of monazite in terms of zeta potential, adsorption density, and flotation response by microflotation using octanohydroxamic acid is determined. Apatite, ilmenite, quartz, rutile, and zircon are minerals that frequently occur with monazite among other minerals, hence were chosen as gangue minerals in this study. The Iso Electric Point (IEP) of monazite, apatite, ilmenite, quartz, rutile, and zircon are 5.3, 8.7, 3.8, 3.4, 6.3, and 50.1 respectively. Thermodynamic parameters of adsorption were evaluated. Ilmenite has highest driving force for adsorption. Adsorption density shows that hydroxamate adsorbs on to monazite and its gangue minerals. This observation was further confirmed by microflotation experiments. Increasing temperature to 80°C rases the adsorption and floatability of monazite and gangue minerals, which does not allow for separation. Appropriate use of depressant is recommended in order to achieve separation of monazite from its gangue.
THEMATIC SECTION: GREEN RARE EARTH ELEMENTS: INNOVATIONS IN ORE PROCESSING, HYDROMETALLURGY, AND ELECTROLYSIS
Surface Chemistry and Flotation Behaviors of Monazite–Apatite–
Ilmenite–Quartz–Rutile–Zircon with Octanohydroxamic Acid
J. Nduwa-Mushidi
1,
C. G. Anderson
1,
Published online: 26 January 2017
The Minerals, Metals & Materials Society (TMS) 2017
Abstract Global demand and consumption of rare earth
elements and compounds have led to increasing research to
further our understanding of their beneficiation and
recovery. Monazite is the second-most important rare
earth-bearing mineral that can be exploited. In this study,
the surface chemistry of monazite in terms of zeta poten-
tial, adsorption density, and flotation responses using
octanohydroxamic acid is determined. Apatite, ilmenite,
quartz, rutile, and zircon are the minerals that frequently
occur with monazite, and hence they were chosen as
gangue minerals in this study. The isoelectric points of
monazite, apatite, ilmenite, quartz, rutile, and zircon are
5.3, 8.7, 3.8, 3.4, 6.3, and 5.1, respectively. Thermody-
namic parameters of adsorption were evaluated. Ilmenite
has the highest driving force for adsorption. Adsorption
density value shows that octanohydroxamic acid adsorbs
onto monazite and its gangue minerals. This observation
was further confirmed by microflotation experiments.
Increasing the temperature to 80 C raises the adsorption
and floatability of monazite and gangue minerals, which
does not allow for separation. Monazite is best recovered at
a pH range of 7.5–10. Appropriate use of depressant is
recommended in order to enhance the separation of mon-
azite from its gangue.
Keywords Rare-earth Beneficiation Flotation
Monazite Heavy mineral sand
Introduction
Increase in technological advancement has led to an
enhancement in global demand for rare earth metals and
compounds. Despite their name implying so, rare earth
elements, however, are abundant in the earth’s crust;
however, their concentrations in most places are relatively
too low to allow for an efficient metallurgical extraction.
The main deposits of rare earth minerals are located in
China, the United States, Australia, Canada, Brazil, and
distributed across various countries in Central Africa.
Monazite is the second-most important source of rare
earths, after bastnaesite. Monazite is a rare earth phosphate
mineral that contains various amounts of thorium. Origi-
nally, monazite was the primary source of rare earth ele-
ments as byproducts of the production of thorium and
uranium. Thorium was viewed as a possible fuel for
nuclear reactors. The decreasing interest in thorium due to
environmental concerns shifted the importance of monazite
toward the extraction of rare earth elements by established
metallurgical processes. The most important sources of
monazite are placer deposits that are easy to mine. Mon-
azite often accumulates with other minerals such as ilme-
nite, rutile, zircon, quartz, magnetite, and sometimes gold.
Due to its relatively high specific gravity (4.9–5.5),
monazite is beneficiated along with other heavy minerals
by means of gravity separation. Monazite is further sepa-
rated from other heavy minerals by a series of magnetic
and electrostatic separation processes. However, these
physical methods of separation become less efficient when
the size of the liberated particle gets smaller. In the latter
case, efficient separation can be achieved by flotation.
The literature on the flotation of monazite is rather
scarce. The available literature focuses on the separation of
monazite from xenotime, bastnaesite, rutile, and zircon.
The contributing editor for this article was Bernd Friedrich.
&J. Nduwa-Mushidi
josuenduwa@gmail.com
1
Kroll Institute of Extractive Metallurgy, Critical Materials
Institute, Colorado School of Mines, Golden, CO 80401,
USA
123
J. Sustain. Metall. (2017) 3:62–72
DOI 10.1007/s40831-016-0114-0
The floatability behavior of monazite is very similar to that
of its associated minerals, and thus is a challenge to sep-
arate. This can be seen in the work done by Pavez and
Peres in a system of monazite–rutile–zircon using sodium
oleate and hydroxamates [1], by Ren in a system of mon-
azite–bastnaesite using benzoic acid [2], by McEwen in a
system of monazite–feldspar–ilmenite–rutile–garnet–zir-
con using amine acetate and sodium petroleum sulfonate
[3], and by Abeidu in a system of monazite–zircon using
oleic acid [4].
There is a wide range of reported isoelectric point (IEP) for
monazite. Cheng reported IEP values ranging from 1.1 to 9
[5]. However, most values reported in the literature fall within
the range from5.0 to 6.4. The differences in IEP values may be
attributed to impurities and differences in mineral composi-
tion and/or in experimental methods, and toradiation damage
of the lattice caused by the presence of thorium.
Very little is known about adsorption density values of
different collectors onto the surface of monazite. Only a
few studies have been conducted on adsorption density on
monazite with an oleate collector [68].
Hydroxamic acid, which may be considered as a
derivative of hydroxyl amine and carboxyl acid, adsorbs
onto mineral surfaces by a chelating reaction as shown in
Fig. 1[9].
In this study, apatite, ilmenite, quartz, rutile, and zircon
were chosen as associated minerals due to their occurrence
with monazite in mineral deposits. The separation by
flotation of monazite from these minerals relies on the
difference in the surface chemistry profile of the minerals.
Understanding the surface properties of these minerals is
the basis for establishing the concentration of monazite
and, ultimately, improving the production of rare earth
elements and compounds.
This study presents the surface chemistry profiles and
flotation behaviors of monazite and associated minerals in
terms of zeta potential measurements, adsorption density, and
microflotation experiments. Thermodynamic data, calculated
from adsorption studies, are also presented in this paper. The
experimental procedures used in this study are similar to that
of Pradip’s on bastnaesite, barite, and calcite [10].
Materials and Methods
Minerals and Reagents
A high-purity monazite mineral was obtained from Persson
Rare Minerals. The sample originated from New Mexico.
The composition and mineralogy of the sample were con-
firmed by Mineral Liberation Analysis (MLA) and X-Ray
Diffraction (XRD) analyses performed at Montana Tech
Camp. XRD results reported 100% purity for monazite,
while the MLA revealed 97.6% purity, with other minerals
such as 0.72% auerlite (Th
0.8
,Ca
0.2
)(VO
4
,SiO
4
,PO
4
).
High-purity apatite (fluorapatite), ilmenite, and quartz
samples were obtained from Van Waters and Rogers
(VWR). The purity of samples was determined by X-Ray
Fluorescent (XRF) and XRD. Rutile and zircon samples
were obtained from Excalibur Mineral Corp. The samples
were also confirmed by XRF and XRD.
Research grade octanohydroxamic acid (C
8
H
17
NO
2
) was
obtained from Tokyo Chemical Industry Co., Ltd (TCI).
Experimental Procedures
Zeta Potential
The zeta potential of each mineral was evaluated as a
function of pH by streaming potential method using a
Stabino Particle Charge Mapping from Microtrac. Pulver-
ized samples were added into aqueous suspensions in
50-mL polyethylene tubes, and conditioned on a shaking
table for 24 h. Suspensions were 0.05% solid. 0.1 N solu-
tions of both HCl and NaOH were used as pH modifiers
throughout zeta potential measurements.
Effects of cerium- and phosphate-determining ions on the
zeta potential of monazite were evaluated. A series of
experiments were also conducted to determine the effect of
octanohydroxamic acid on the zeta potential of each mineral.
Adsorption Density
The adsorption of octanohydroxamic acid on the surface of
each mineral was evaluated by the solution depletion
method; the concentrations of hydroxamic acid are mea-
sured before and after adsorption. The difference was
assumed to be due to adsorption onto the surface. The
adsorption density was calculated according to the fol-
lowing equation:
Cd¼DCV
mA ;
where DCis the change in molar concentration of the
solution before and after adsorption, Vis the volume of
hydroxamic acid solution in liters, mis the mass of solid
used in grams, and Ais the specific area in m
2
/g.
Predetermined amounts of solid minerals were mixed
with a known concentration of hydroxamic acid. Each
suspension was shaken at the temperature of experiment
until equilibrium was reached. The mixture was then cen-
trifuged to separate the solid from the liquid; the latter is
needed to determine the concentration.
The concentration of the liquid was evaluated by col-
orimetry using a UV–Visible spectrophotometer; hydrox-
amic acid forms a complex with ferric perchloride. The
J. Sustain. Metall. (2017) 3:62–72 63
123
ferric hydroxamate complex has a characteristic peak
detectable by the spectrophotometer at 500 nm.
The surface area of solid minerals was evaluated by
Brunauer-Emmett-Teller (BET) method using nitrogen gas.
Each mineral was grinded to -325 mesh.
Adsorption density experiments consisted of determin-
ing solid–liquid ratio, kinetics of adsorption, and adsorp-
tion isotherm. Each of these experiments was conducted for
all minerals at room temperature and at 80 C. In addition,
the effect of pH on adsorption was evaluated at room
temperature.
The free energy of adsorption was calculated at 25 and
80 C using the Stern–Grahame equation [11]. Given the
free energy at two different temperatures, the enthalpy and
entropy of adsorption could also be calculated.
Microflotation
The floatability of each mineral was evaluated by
microflotation experiment in a modified Hallimond tube.
Half a gram of pure mineral was conditioned in a beaker on
a stirring plate for 15 min in 55 mL of the collector solu-
tion at desired concentration and pH. The slurry was
transferred into the Hallimond tube and stirred throughout
the flotation experiment. Two-minute flotation tests were
conducted by passing air through the tube at 60 cc/min.
High-temperature flotation tests were conducted by
conditioning for 15 min at 80 C prior to flotation. Careful
measures were taken to prevent evaporation of the aqueous
solution during conditioning.
The concentrate and tailing were filtered, dried, and
weighed. The results were expressed on a weight basis
(floatability).
Results and Discussion
Zeta Potential
Results in Fig. 2show the zeta potential values of mon-
azite, apatite, ilmenite, quartz, rutile, and zircon. The IEP
values are 5.3, 8.7, 3.8, 3.4, 6.3, and 5.1, respectively. The
zeta potential of monazite is negative at higher pH, and
becomes more positive as the pH reaches lower values. The
IEP of monazite is similar to that reported in the literature
by Pavez and Peres [12], Cheng et al. [5], and Houlo et al.
[13].
The IEPs of the other minerals have been reported in the
literature: Apatite is reported at variant IEP values at pH
3.5, 5.5, 6.7 [14], pH 4.2 [15], and pH 7.5 [16]. The IEP of
ilmenite found in this study (3.8) is lower than the values
(4.2–6.25) found by Mehdilo [17]. The IEP of quartz is
Fig. 1 Formation of ferric
hydroxamate complexes [9]
64 J. Sustain. Metall. (2017) 3:62–72
123
higher than the value found by Zhou [18]. In another study,
Zhou reported that the surface potential of quartz remained
negative at all pH values. The literature value on the IEP of
rutile is 4.5 [1].
The differences in reported IEP values may be due to
impurities, differences in mineral composition, and in
experimental methods, or to lattice damage caused by
radiation due to the presence of thorium in the case of
monazite.
The effects of cerium potential- and phosphate potential-
determining ions are also investigated and presented in
Fig. 3. As expected, the zeta potential of monazite shifts
toward more positive values, and the IEP shifts to the right
when monazite is in equilibrium with a cerium nitrate
aqueous solution. This behavior results from the adsorption
of cerium cations or cerium hydroxyl cations onto the
surface of monazite, which renders the surface more pos-
itively charged. Similarly, the IEP shifts to the left with the
addition of phosphate ions, and zeta potential remains
negative in the range of pH of this experiment. Other rare
earth ions, such as La
3?
,Nd
3?
, etc., may also be potential-
determining ions. Their effects on the zeta potential of
monazite are expected to be similar to that of cerium ions.
Figures 4,5,6,7,8, and 9present the zeta potential of
monazite and its associated minerals as a function of pH in
10
-3
molar solutions of octanohydroxamic acid. For
comparison, the zeta potential in water is also plotted. With
the exception of quartz and ilmenite, the IEPs of minerals
shift to the left with the addition of octanohydroxamic acid.
Moreover, the octanohydroxamic acid has no effect on the
zeta potential of quartz, which can be explained by the
limited interaction of quartz and octanohydroxamic acid, as
reflected in the adsorption density results.
The change in the zeta potential is an indication that
octanohydroxamic acid collector adsorbs onto the surfaces
of minerals.
Adsorption occurs at pH lower than 9.5, which is the
pK
a
of hydroxamic acid [5]. In this range, octanohydrox-
amic acid molecules are predominant. This indicates that
the octanohydroxamic acid molecule is the adsorbing
-80
-40
0
40
80
2 4 6 8 10 12
Potenal [mV]
pH
Monazite Apatite
Ilmenite Quartz
Rutile Zircon
Fig. 2 Zeta potentials of monazite, apatite, ilmenite, quartz, rutile,
and zircon as a function of pH in water (Color figure online)
-80
-60
-40
-20
0
20
40
60
24681012
Zeta Potenal [mV]
pH
H2O
10-3 M K3PO4
10-3 M CeNO3
H2O
10-3 M K3PO4
10-3 M CeNO3
Fig. 3 Zeta potential of monazite as a function of pH in water,
10
-3
MK
3
PO
4
, and 10
-3
M CeNO
3
-60
-50
-40
-30
-20
-10
0
10
20
30
24681012
Zeta Potenal [mV]
pH
Monazite H2O
HXY
H2O
Fig. 4 Zeta potential of monazite in 10
-3
M octanohydroxamic acid
-40
-20
0
20
40
60
80
24681012
Zeta Potenal [mV]
pH
Apate
HXY
H2O
Fig. 5 Zeta potential of apatite in 10
-3
M octanohydroxamic acid
J. Sustain. Metall. (2017) 3:62–72 65
123
agent. At higher pH, however, octanohydroxamate anion is
responsible for adsorption.
Octanohydroxamic acid adsorbs on the surface of neg-
atively charged particles. This is an indication that
adsorption mechanism might be of chemical nature.
Adsorption Density
In adsorption kinetic experiments, the adsorption density
values were plotted as a function of time for monazite,
apatite, ilmenite, quartz, rutile, and zircon. The initial
concentration was 10
-3
M for all kinetic experiments. The
result is shown in Fig. 10. Twenty-four hours were found
sufficient to reach equilibrium for apatite, 48 h for mon-
azite, quartz, and rutile, and 96 h was sufficient for ilme-
nite and zircon. The following adsorption experiments at
room temperature, including adsorption isotherm and effect
of pH on adsorption, were carried using the resulted
equilibrium times.
Similar experiments were performed at 80 C. The
resulting equilibrium times, as seen in Fig. 11, are 24 h for
apatite and zircon, 60 h for ilmenite and quartz, and 90 h
for monazite and rutile. These equilibrium times were used
for subsequent adsorption experiments at 80 C.
Due to its relatively elevated and fast solubility in water
[19], apatite reaches equilibrium in much shorter time
period.
Minerals are abbreviated as follows:
Monazite: Mnz
Apatite: Ap
Ilmenite: Ilm
Quartz: Qtz
Rutile: Rt
Zircon: Zrn
The abbreviations were adapted from Siivalo and Sch-
miid-published minerals abbreviation list [20].
It is important to note that initial concentrations have
negligible effect on the equilibrium time in kinetics
experiments. Figure 12 illustrates the kinetics of adsorption
of monazite at initial concentrations of 10
-3
M and
2910
-3
M octanohydroxamic acid. In both cases, the
-70
-60
-50
-40
-30
-20
-10
0
10
24681012
Zeta Potenal [mV]
pH
Ilmenite
H2O
HXY
H2O
Fig. 6 Zeta potential of ilmenite in 10
-3
M octanohydroxamic acid
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
24681012
Zeta Potenal
pH
Quartz
H2O
HXY
H2O
Fig. 7 Zeta potential of quartz in 10
-3
M octanohydroxamic acid
-80
-60
-40
-20
0
20
40
60
2 4 6 8 10 12
Zeta Potenal [mV]
pH
Rule
H2O
HXY
H2O
Fig. 8 Zeta potential of rutile in 10
-3
M octanohydroxamic acid
-80
-60
-40
-20
0
20
40
24681012
Zeta Potenal [mV]
pH
Zircon H2O
HXY
H2O
Fig. 9 Zeta potential of zircon in 10
-3
M octanohydroxamic acid
66 J. Sustain. Metall. (2017) 3:62–72
123
equilibrium time is 24 h; however, higher degree of
adsorption is observed at higher initial concentration.
Adsorption isotherm experiments were conducted to
delineate the effect of equilibrium concentration and tem-
perature on the adsorption density. Figure 13 presents the
adsorption isotherm of monazite and associated minerals at
room temperature (25 C) and at 80 C.
Assuming the vertical and horizontal cross-sectional
areas of hydroxamate group to be 20.5 and 55 A
˚
2
,
respectively [10], the vertical and horizontal monolayer
adsorption density values were calculated to be 8.1 910
-6
and 3.02 910
-6
mol/m
2
, respectively. The vertical and
horizontal monolayers of adsorption are represented in
Fig. 13 by black and red horizontal dashed lines,
respectively.
Ilmenite, rutile, and zircon experience higher adsorption
density values at room temperature. The vertical mono-
layer coverage of octanohydroxamic acid on the surface of
these minerals occurs at relatively low equilibrium con-
centration (2 910
-3
–10
-3
M) in the bulk solution. This
indicates high driving forces of adsorption, and hence, high
free energies of adsorption. Apatite has a medium
adsorption level.
Apatite does not exhibit a plateau at either temperature.
This continuously increasing adsorption density recorded
for apatite may be due to bulk precipitation of octanohy-
droxamate with calcium ions in solution. Three types of
interactions between collector and minerals can happen in
chemisorption [21]: Chemisorption occurs during a
monolayer adsorption by interaction between the collector
and the surface without the movement of atoms from their
lattice sites. Surface reaction happens by interaction with
the movement of the lattice atoms, when multilayer
adsorption occurs. Bulk precipitation occurs by reaction of
the metal reagent and the collector away from the surface.
This happens when the rate of dissolution is faster than the
rate of reaction of collector and the lattice metal. In this
adsorption density study, bulk precipitation could not be
detected due to limitations of the solution depletion
method; therefore, it would be interpreted as adsorption
onto the surface.
The adsorption uptake of octanohydroxamic acid on
monazite shows lower adsorption density. The plateau
corresponding to monolayer adsorption occurs at
6±0.2 lmol/m
2
.
Quartz shows the lowest adsorption density among the
minerals under investigation in this study. The adsorption
on quartz does not reach vertical monolayer coverage,
which indicates that octanohydroxamic acid adsorbs onto
the surface of quartz in a horizontal configuration.
Temperature has remarkable effects on the adsorption
density, with the exception of quartz, which shows limited
temperature dependency on adsorption. As opposed to the
other minerals, increasing temperature results in a decrease
in adsorption density on quartz mineral.
Increasing the temperature from 25 to 80 C leads to an
increase in adsorption density on the surface of monazite
by a factor of 3.5. This temperature dependency indicates a
higher entropy of adsorption as it is observed from the
thermodynamics calculations. It is well known that the
0 50 100 150 200
Adsorpon [mol/m2]
Time [Hrs]
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 10 Kinetics of adsorption at 25 C
0 20 40 60 80 100 120
Adsorpon [mol/m2]
Time [Hrs]
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 11 Kinetics of adsorption at 80 C
0 1020304050607080
Adsorpon [mol/m2]
Time [Hrs]
0.001 M
0.002 M
Fig. 12 Kinetics of adsorption of octanohydroxamic acid onto
monazite at 25 C, at initial concentrations of 10
-3
and 2 910
-3
M
J. Sustain. Metall. (2017) 3:62–72 67
123
adsorption density generally increases with the increasing
temperature for chemical types of adsorption. This phe-
nomenon can be observed in Fig. 13 for the adsorption
density values of all minerals with the exception of quartz.
The free energy of adsorption at 25 and 80 C were
calculated using the Stern–Grahame equation:
Td¼2rCexp DG
ads=RT

;
where S
d
is the adsorption density in the stern plane, ris
the effective radius of the adsorbed ion, Cis the equilib-
rium concentration, DG
ads is the standard adsorption free
energy. Results are presented in Fig. 14.
Knowing the free energy at two different temperatures,
the enthalpy and entropy were calculated by means of the
following thermodynamic equations:
0 0.001 0.002 0.003 0.004 0.005
Adsorpon [mol/m2]
Eq. Concentraon [M]
Monazite
25 C
80 C
25°C
80°C
0 0.001 0.002 0.003 0.004 0.005
Adsorpon [mol/m2]
Eq. Concentraon [M]
Apate
25 C
80°C
25°C
0 0.001 0.002 0.003 0.004
Adsorpon [mol/m2]
Eq. Concentraon [M]
Ilmenite
25 C
80 C
25°C
80°C
0 0.002 0.004 0.006
Adsorpon [mol/m2]
Eq. Concentraon [M]
Quartz
25 C
80 C
25°C
80°C
0 0.001 0.002 0.003 0.004 0.005
Adsorpon [mol/m2]
Eq. Concentraon [M]
Rule 25 C
80 C
25°C
80°C
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
Adsorpon [mol/m2]
Eq. Concentraon [M]
Zircon
25 C
80 C
25°C
80°C
ab
cd
ef
Fig. 13 Adsorption isotherms of octanohydroxamic acid onto amonazite, bapatite, cilmenite, dquartz, erutile, and fzircon (Color
figure online)
68 J. Sustain. Metall. (2017) 3:62–72
123
DH
ads ¼DG
1=T1

DG
2=T2

1=T11=T2
ðÞ
DS
ads ¼DG
1DG
2

T2T1
ðÞ
Table 1summarizes the thermodynamic results obtained
from this study.
Thermodynamic values were calculated with the
assumption that DHand DSare independent of tempera-
ture, in the temperature range of 25 to 80 C. As a result,
DGappears to be linearly related to temperature. In actu-
ality, DGcould experience nonlinear relationship at tem-
peratures within this interval, which can generate different
DHand DSvalues for different temperature ranges.
Experiments were conducted to evaluate the effect of pH
on adsorption density. Figure 15 presents the adsorption
density as a function of initial pH. The initial concentra-
tion, in this case, is 10
-3
M of octanohydroxamic acid.
Ilmenite has the highest adsorption density over the
entire range of pH used in this experiment with the
exception of the interval from pH 8.5 to 10. In this interval,
apatite has the highest adsorption with a peak at pH 9. The
adsorption density of octanohydroxamic acid on the sur-
faces of monazite, rutile, zircon, and quartz shows a weak
dependency of pH, as opposed to ilmenite and apatite. The
adsorption density of ilmenite increases with the decreas-
ing pH. This may be due to the increase of ferric ions that
occurs in more acidic environment [22]. Higher adsorption
density of octanohydroxamic acid on ilmenite is due to
higher stability constant of ferric hydroxamate [23].
Table 2shows the stability constant of certain metal
10
14
18
22
26
30
20 30 40 50 60 70 80
ΔG [kJ/mol]
Temperature [°C]
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 14 Free energy of adsorption as a function of temperature
Table 1 Thermodynamic
parameters of adsorption Mineral DG
298
(kJ/mol) DG
353
(kJ/mol) DH(kJ/mol) DS(J/mol)
Monazite -14.87 -23.29 30.75 153.08
Apatite -17.43 -28.03 39.99 192.68
Ilmenite -20.48 -29.1 26.22 156.72
Quartz -13.09 -14.91 -2.9 34.19
Rutile -22.10 -29.29 20.25 142.12
Zircon -22.4 -27.5 5.67 92.97
Table 2 Stability constant for metal acetohydroxamate at 20 C[24]
Cation Log K
1
Log K
2
Log K
3
Ca
2?
2.4
Mn
2?
4.0 2.9
Cd
2?
4.5 3.3
Fe
2?
4.8 3.7
Co
2?
5.1 3.8
Ni
2?
5.3 4.0
Zn
2?
5.4 4.2
Pb
2?
6.7 4.0
Cu
2?
7.9
La
3?
5.16 4.17 2.55
Ce
3?
5.45 4.34 3.0
Sm
3?
5.96 4.77 3.68
Gd
3?
6.10 4.76 3.07
Dy
3?
6.52 5.39 4.04
Yb
3?
6.61 5.59 4.29
Al
3?
7.95 7.34 6.18
Fe
3?
11.42 9.68 7.23
24681012
Adsorpon density [mol/m2]
pH
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 15 Adsorption density as a function of pH
J. Sustain. Metall. (2017) 3:62–72 69
123
acetohydroxamate complexes at 20 C[24]. It can be seen
that trivalent complexes have higher stability constant, the
strongest being that of iron.
At a higher pH value (12), the adsorption density
decreases remarkably. This occurs above the pH value of
dissociation of octanohydroxamate (9.5). The higher pH
values yield octanohydroxamate anions. In addition, the
increasing pH results in the increasing negative value of
surface charge of minerals, as shown in the zeta potential
measurements. The occurrences of these two phenomena
together enhance the repulsive interaction between
octanohydroxamate ions and negatively charged particles.
It is important to note that the effect of pH on adsorption
was evaluated with 10
-3
M of octanohydroxamic acid.
Different concentrations could result in different behaviors
in terms of pH dependency on adsorption. The combined
effect of pH and concentration on adsorption was not
within the scope of this study.
Microflotation
The first series of experiment aimed to determine the
flotation response of each mineral as a function of collector
concentration at their natural pH. The results are illustrated
in Fig. 16.
The recovery rates of monazite, apatite, ilmenite, rutile,
and zircon increase sharply at a low concentration range
(10
-4
–3 910
-4
M). The recovery rate of quartz remains
relatively low, and maximum recovery of 60% was at
2.5 910
-3
M concentration of collector solution.
The effects of pH on the floatability were evaluated at
concentrations of 2 910
-4
and 10
-3
M of octanohy-
droxamic acid. The results are plotted in Figs. 17 and 18,
respectively.
At 0.001 M, the recovery rates of ilmenite, rutile, and
zircon show little dependency on pH in the range of
3.5–12. The recovery decreases sharply at higher pH val-
ues. Monazite shows maximum floatability at pH in the
range of 7.5–10. Apatite floatability increases with the
increasing pH; at 12.6, apatite has maximum floatability.
Quartz has the lowest floatability with a peak at pH 10.4.
At 0.0002, the floatability values of ilmenite, rutile, and
zircon decrease at acidic pH values. The floatability of
monazite remains unchanged with a peak at pH 7.6–9.8. In
this interval, monazite has the highest floatability. How-
ever, the difference is too negligible to allow for reasonable
separation.
The effect of temperature was evaluated by conditioning
at 80 C. The results are shown in Fig. 19.
Similar to the result observed at room temperature, the
floatability behaviors of monazite and associated minerals
have parallel response with respect to temperature and
collector concentration. Quartz, on the other hand, shows
the lowest floatability, which is consistent with results
obtained from adsorption density. One important point to
0%
20%
40%
60%
80%
100%
30-E00.140-E00.1
Floatability
Concentraon [M]
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 16 Effect of concentration on floatability at 25 C
0%
20%
40%
60%
80%
100%
135791113
Flotability
pH
Mnz
Ap
Ilm
Qtz
Rt
Zrn
Fig. 17 Effect of pH on floatability at initial concentration of
0.001 M octanohydroxamic acid
0%
20%
40%
60%
80%
100%
24681012
Floatability
pH
Mnz
Ilm
Qtz
Rt
Zrn
Fig. 18 Effect of pH on floatability at initial concentration of
0.0002 M octanohydroxamic acid
70 J. Sustain. Metall. (2017) 3:62–72
123
note is that the floatability at lower concentration (10
-4
M
of octanohydroxamic acid) increases when the temperature
is increased to 80 C.
Although ilmenite has the highest adsorption densities at
both temperatures, its floatability is lower than those of
monazite, apatite, rutile, and zircon. High adsorption may
result in nearly total depletion of octanohydroxamic acid in
the solution, which will decrease the froth, and thus
decrease recovery.
The results obtained in microflotation experiments
illustrate the flotation responses of minerals in different
environments using octanohydroxamate acid as a collector.
These results can be used as a basis of mineral separation
and concentration in a system containing monazite and
minerals investigated in this study. A bulk flotation test,
however, may be complicated by the presence of various
dissolved species in solution.
Conclusion
The IEP values of monazite, apatite, ilmenite, quartz,
rutile, and zircon are 5.3, 8.7, 3.8, 3.4, 6.3, and 5.1,
respectively. Addition of octanohydroxamic acid alters the
zeta potential of minerals, due to interaction of collector
molecules with lattice atoms. Octanohydroxamate acid
adsorbs on the negatively charged solid minerals, which
verifies that chemisorption takes place.
Adsorption density measurements show that octanohy-
droxamate acid adsorbs onto the surfaces of monazite and
gangue minerals as well. At higher pH environments,
adsorption decreases due to the electrostatic repulsion
between hydroxamate anions and negatively charged sur-
face. As seen in thermodynamic calculations, ilmenite has
higher affinity to hydroxamate due to higher stability
constant of ferric hydroxamate.
Microflotation experiments confirm that the flotation
response of monazite is very similar to that of its gangue
minerals when using octanohydroxamic acid as a collector.
Monazite is best recovered at a pH range of 7.5–10. A
variation of the collector concentration alone is not suffi-
cient to establish the separation of theses minerals.
The results obtained in this study provide a strong basis
on the flotations of monazite, apatite, ilmenite, quartz,
rutile, and zircon minerals with octanohydroxamic acid.
This can be used as a foundation for the separation and the
concentration of monazite from its associated minerals.
Acknowledgements This project was supported by the Department
of Energy through the Critical Materials Institute and the Ames
Laboratory. The authors extend their gratitude to the faculty and staff
of the Colorado School of Mines.
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