Materials Transactions, Vol. 55, No. 7 (2014) pp. 1083 to 1090
© 2014 The Japan Institute of Metals and Materials
Dissolution Behavior of Platinum in Na2O­SiO2-Based Slags
Chompunoot Wiraseranee1,+1, Takeshi Yoshikawa2,+2, Toru H. Okabe2 and Kazuki Morita1,2
1
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan
2
With the aim of minimizing the loss of platinum into slags by controlling the slag composition during the high-temperature recycling
process, the effects of representative slag components, namely, Al2O3, MgO, Fe2O3, and CuOx, on the dissolution behavior of platinum into
Na2O­SiO2-based slags were investigated. The solubility of platinum in the slags was measured by equilibrating the Na2O­SiO2-based slags
with pure solid platinum at 1473 K in air. The dissolution of platinum in the slags was found to be suppressed by the addition of Al2O3, MgO,
and Fe2O3. Al2O3 and Fe2O3 behaved as acidic oxides, whereas MgO behaved as a diluent and decreased the solubility of platinum in the slags.
CuOx behaved as a weakly basic oxide and slightly enhanced the dissolution of platinum into the slags. The correlation between the platinate
capacity of the slag, which is a parameter proposed in this paper, and the optical basicity enabled the content of platinum in slags at 1473 K to be
estimated from the slag composition. [doi:10.2320/matertrans.M2014042]
(Received February 18, 2014; Accepted April 25, 2014; Published June 6, 2014)
Keywords: platinum, molten slags, solubility, thermodynamics, recycling
1.
Introduction
Platinum is indispensable for a number of specialized
applications, for example, as a catalyst for decomposition of
toxic gases in automobiles, as a highly corrosion-resistant
container for molten glass, and as an electrical contact
material in electronic devices. Thus, the worldwide scarcity
and high price of platinum make the recycling of platinum
essential nowadays.1)
The use of liquid copper or liquid iron as a collector metal
has been one of the major methods for the recovery of
platinum from scraps.2­4) In this process, the platinum in the
scraps is collected by the collector metal after the scraps are
melted with fluxes and the collector metal in a reducing
atmosphere. In order to obtain a platinum-rich alloy as a
product, the collector metal is partly removed into slags by
oxidation. Theoretically, platinum is stable as a metal at high
temperature. However, the loss of platinum into slags during
oxidation in a highly oxidizing atmosphere is a critical
problem. Therefore, the process conditions must be controlled to minimize the loss of platinum into slags.
In the recycling process, the composition of the slags
depends on the compositions of the scraps, fluxes, and the
oxidized collector metal. Spent automobile catalysts have
been the main materials recycled for platinum, so MgO,
Al2O3, and SiO2 from the honeycomb structure (i.e.,
cordierite, 2MgO·Al2O3·5SiO2) in the catalysts are often the
major components in the slags. Moreover, because of the
oxidation of the liquid collector metal (i.e., iron or copper),
the content of Fe2O3 or copper oxides (CuOx) in the slags can
be relatively high in an oxidizing atmosphere.
Previously, Nakamura and Sano5) reported that the
solubility of platinum in BaO­SiO2, Na2O­SiO2, and CaO­
SiO2 binary slags under an air atmosphere increased with
increasing slag basicity. Baba and Yamaguchi6) suggested
+1
Graduate Student, The University of Tokyo. Present address: Central
Research Institute, Mitsubishi Materials Corporation, Iwaki 971-8101,
Japan
+2
Corresponding author, E-mail: [email protected]
that when copper­platinum liquid alloys are in equilibrium
with FeOx­SiO2 slag at 1573 K at oxygen partial pressures
( pO2 ) ranging from 10¹9 to 10¹6 atm, the solubility of
platinum increases with increasing copper content in the
slags. Furthermore, in our previous study, it was found that
the solubility of rhodium, one of the platinum-group metals,
in Na2O­SiO2-based slags at 1473 K and at a pO2 of 0.21 atm
was decreased by the addition of Al2O3 and MgO into the
slags, but instead remained constant when the content of
CuOx was increased.7) According to these previous investigations, the dissolution of platinum into slags is apparently
influenced by the slag composition, therefore minimization
of the platinum loss into slags by controlling the slag
composition is feasible.
In the present study, in order to design a slag composition
for the recovery of platinum in an oxidizing atmosphere, the
effects of the addition of Al2O3, MgO, Fe2O3, and CuOx on the
dissolution behavior of platinum in the slags were examined.
2.
Experimental Procedures
2.1 Equilibrium experiments
Slag samples were prepared by melting mixtures of
reagent-grade Na2CO3, SiO2, Al2O3, MgO, Fe2O3, and
Cu2O in a pure platinum crucible at 1573 K prior to the
experiments. Measurements of the solubility of platinum in
the slags were carried out by equilibrating 4 g of liquid
Na2O­SiO2­Al2O3, Na2O­SiO2­MgO, Na2O­SiO2­Fe2O3, or
Na2O­SiO2­CuOx slags with a fixed Na2O/SiO2 molar ratio
of 0.97 using a pure platinum crucible in an electric furnace
in air ( pO2 ¼ 0:21 atm) at 1473 K. A schematic diagram of
the experimental apparatus is shown in Fig. 1.
The oxygen partial pressure in the system was controlled
by flowing dried air through the system at a rate of 200
mL/min. CO2 was removed from the gas stream by passing it
through soda-lime. Moisture was removed from the gas by
passing it through silica gel and magnesium perchlorate. The
temperature was controlled with an accuracy of «1 K using a
proportional-integral-derivative (PID) controller with a Pt/
6%Pt­Pt/30%Pt thermocouple.
1084
C. Wiraseranee, T. Yoshikawa, T. H. Okabe and K. Morita
Fig. 1
Schematic diagram of the experimental apparatus.
The dissolution of iron and copper from the Na2O­SiO2­
Fe2O3 and Na2O­SiO2­CuOx slags into solid platinum was
observed. In order to determine the iron and copper contents
in the solid platinum in equilibrium with the slags, a piece of
platinum wire (diameter 100 µm, purity ² 99.9%) was added
into the slags. After equilibration, the samples were withdrawn from the furnace and quenched to room temperature
by flushing them with Ar gas.
Solubility of Pt/ mass ppm
150
2.2 Chemical analysis
The platinum, sodium, aluminum, magnesium, iron, and
copper contents in all slag samples and the iron and copper
contents in the platinum wire were determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES).
The Fe2+ content in the Na2O­SiO2­Fe2O3 slags was
determined by titration with potassium dichromate. The
silicon content in the slags was determined by the SiO2
gravimetric method.
2.3 Raman spectroscopy
Raman spectra of the quenched slag samples were
obtained at room temperature by a laser Raman spectrometer
(Horiba T-64000) using the macro analysis method. All
silicate slag samples were excited by an Ar ion laser
(514.5 nm) operated at 300 mW.
3.
Results and Discussion
The experimental results obtained on the dissolution of
platinum in slags after equilibration are summarized in
Table 1. The iron and copper contents in the solid platinum
equilibrated with the Na2O­SiO2­Fe2O3 and Na2O­SiO2­
CuOx slags are summarized in Table 2.
3.1 Equilibration time
The equilibration time required for the dissolution of
120
Na2O-SiO2-21.5(mass pct)CuOx
Na2O-SiO2-29.4(mass pct)Fe2O3
Na2O-SiO2-7.9(mass pct)MgO
Na2O-SiO2-20.4 (mass pct)Al2O3
90
60
30
0
0 12 24 36 48 60 72 84 96
Time, t /h
Fig. 2 Dependence of the solubility of platinum in the Na2O·SiO2 based
slags at 1473 K in air ( pO2 ¼ 0:21 atm) on the equilibration time.
platinum into slags was determined first. The solubility of
platinum in each slag system increased with time and became
constant after 18 h (Fig. 2). In the systems with the Na2O­
SiO2­Fe2O3 and Na2O­SiO2­CuOx slags, the iron and copper
contents in the solid platinum also became steady after 18 h
(Fig. 3). Therefore, the equilibration times of all slag systems
were assigned to be 18 h in this study.
3.2
Solubility of platinum in Al2O3-, MgO-, Fe2O3-, and
CuOx-bearing slags
The solubility of platinum in slags with varying contents of
Al2O3, MgO, Fe2O3, and CuOx is shown in Fig. 4. It was
found that the solubility of platinum decreased when the
Dissolution Behavior of Platinum in Na2O­SiO2-Based Slags
Experimental results on the dissolution of platinum in slags at 1473 K and pO2 ¼ 0:21 atm.
Table 1
Slag composition,
/mass%
Sample
No.
Na2O
1085
SiO2
Al2O3
Mole fraction
of Al2O3,
XAl2 O3
Time,
t/h
Solubility of Pt,
/mass ppm
Activity
coefficient of PtO,
£ PtO
Platinate
capacity,
CPtO2 2 103
00
50.1
49.9
®
®
18
115
330
29
01
38.3
41.3
20.4
0.13
18
12
2900
3.0
02
38.3
41.3
20.4
0.13
42
15
2330
3.8
03
45.5
45.3
9.2
0.057
18
42
870
11
Time,
t/h
Solubility of Pt,
/mass ppm
Activity
coefficient of PtO,
£ PtO
Platinate
capacity,
CPtO2 2 103
Slag composition,
/mass%
Na2O
SiO2
MgO
Mole fraction
of MgO,
XMgO
04
44.3
47.8
7.90
0.12
18
65
610
29
05
44.3
47.8
7.90
0.12
36
60
660
23
06
48.2
46.0
5.7
0.084
18
91
430
17
Time,
t/h
Solubility of Pt,
/mass ppm
Activity
coefficient of PtO,
£ PtO
Platinate
capacity,
CPtO2 2 103
Sample
No.
Slag composition,
/mass%
Na2O
SiO2
Fe2O3
Mole fraction
of Fe2O3,
XFe2 O3
07
08
33.6
33.6
37.0
37.0
29.4
29.4
0.14
0.14
18
48
65
62
480
500
14
14
09
41.1
44.6
14.3
0.060
18
78
440
17
10
32.8
37.6
29.6
0.14
18
62
®
®
11
29.8
28.9
41.3
0.21
18
50
560
11
Time,
t/h
Solubility of Pt,
/mass ppm
Activity
coefficient of PtO,
£ PtO
Platinate
capacity,
CPtO2 2 103
Sample
No.
Slag composition,
/mass%
Na2O
SiO2
CuOx
Mole fraction
of CuOx,
XCuOx
12
39.0
39.5
21.5
0.18
18
77
350
19
13
39.0
39.5
21.5
0.18
36
70
410
21
14
39.0
39.5
21.5
0.18
48
78
360
19
15
16
39.0
47.0
39.5
47.1
21.5
5.9
0.18
0.049
92
18
77
100
330
®
29
®
17
44.1
44.1
11.8
0.10
18
83
400
20
18
37.0
38.3
24.7
0.20
18
70
400
21
19
32.5
32.8
34.7
0.31
18
66
380
19
Sample
No.
Table 2 Iron and copper contents in solid platinum in equilibrium with
the Na2O­SiO2­Fe2O3 and Na2O­SiO2­CuOx slags at 1473 K and
pO2 ¼ 0:21 atm.
Sample No.
XFe
in alloy
XPt
in alloy
07
0.0014
0.9986
08
0.0019
0.9981
09
11
0.0004
0.0025
0.9996
0.9975
Sample No.
XCu
in alloy
XPt
in alloy
12
0.22
0.78
13
0.18
0.82
14
0.19
0.81
15
0.24
0.76
17
0.10
0.90
18
0.19
0.81
19
0.24
0.76
Al2O3, MgO, Fe2O3, and CuOx contents in the slags
increased. The iron and copper contents in the solid platinum
in equilibrium with the Fe2O3- and CuOx-bearing slags
increased with increasing Fe2O3 and CuOx contents in the
slags (Fig. 5).
3.3
Activity coefficient of PtO in Al2O3-, MgO-, Fe2O3-,
and CuOx-bearing slags
The dissolution of iron and copper from the slags into solid
platinum causes a decrease in the activity of solid platinum
from unity. Thus, instead of discussing the effects of the
addition of oxides on the solubility of platinum in slags, its
effect on the activity coefficient of platinum in the slags was
investigated as follows.
Since platinum dissolves into slag as PtO5) according to
eq. (1), the activity coefficient of PtO in slags can be
determined from eq. (2).
1
PtðsÞ þ O2 ðgÞ ¼ PtOðsÞ
ð1Þ
2
Gf,PtOðsÞ
¼ 62;800 þ 73T ðJ=molÞ8Þ
£ XPtO
Gf,PtOðsÞ
¼ RT ln PtO 1=2
ð2Þ
aPt pO2
Here, G and pO2 are the standard Gibbs energy for the
reaction and oxygen partial. £i, Xi and ai are the activity
coefficient at the Raoultian standard, mole fraction and
activity of component i, respectively.
XFe in solid Pt
0.010
0.008
0.006
0.004
0.002
0.4
0.3
0.2
0.1
0.0
0.004
0.003
0.002
0.001
0
Na2O-SiO2-29.4(mass pct)Fe2O3
0.000
0.4
XCu in solid Pt
XCu in solid Pt
C. Wiraseranee, T. Yoshikawa, T. H. Okabe and K. Morita
XFe in solid Pt
1086
0.3
0.2
0
0.1
Na2O-SiO2-21.5(mass pct)CuOx
0.0
0 12 24 36 48 60 72 84 96
0.1
0.2
0.3
XCuOx, XFe2O3 in slag
0.4
Fig. 5 Dependence of the iron and copper contents in solid platinum on the
Fe2O3 content in the Na2O­SiO2­Fe2O3 slags and the CuOx content in
Na2O­SiO2­CuOx slags in air (pO2 ¼ 0:21 atm) at 1473 K.
Time, t/ h
Fig. 3 Dependence of the iron and copper contents in solid platinum in
equilibrium with the Na2O­SiO2­29.4(mass%)Fe2O3 and Na2O­SiO2­
21.5(mass%)CuOx slags at 1473 K in air (pO2 ¼ 0:21 atm) on equilibration time.
150
Na2O-SiO2-Al2O3
100
Solubility of Pt/ mass ppm
50
0
150
Na2O-SiO2-MgO
100
50
activity of platinum in the Pt­Fe solid alloys in equilibrium
with the Na2O­SiO2­Fe2O3 slags was assumed to obey
Raoult’s law. However, the content of copper dissolved into
the solid platinum was high (³25 mol%). Therefore, the
activity of platinum in the Pt­Cu solid alloys was calculated
from the activity-composition relations of the solid Pt­Cu
alloys at 1473 K.9) The dependence of the activity coefficient
of PtO on the Al2O3, MgO, Fe2O3, and CuOx contents in the
slags is shown in Fig. 6.
The activity coefficient of PtO sharply increased with
increasing Al2O3 content, slightly increased with increasing
MgO and Fe2O3 content, and remained constant with
increasing CuOx content in the slags.
3.4
50
Effects of addition of Al2O3, MgO, Fe2O3, and CuOx
on the dissolution of platinum into slags
In order to understand the role of each oxide in the
dissolution of platinum into slags, the effects of the addition
of each oxide are discussed individually in this section.
3.4.1 Effects of Al2O3 addition
It is generally known that Al2O3 is amphoteric. It has
been suggested that in the Na2O­SiO2­Al2O3 melts, Na2O
modifies the silicate network (eq. (3)) and Al2O3 tends to be
incorporated as a tetrahedral unit in the composition range of
Al/Na <1 (eq. (4)).10)
0
Na2 O þ Si­O­Si ¼ 2Si­O þ 2Naþ
0
150
Na2O-SiO2-Fe2O3
100
50
0
150
Na2O-SiO2-CuOx
100
0
0.1
0.2
0.3
Xoxide additive in slag
0.4
Fig. 4 Dependence of the solubility of platinum on the Al2O3, MgO,
Fe2O3, and CuOx contents in the Na2O·SiO2 slags at 1473 K in air
(pO2 ¼ 0:21 atm).
At pO2 ¼ 0:21 atm and 1473 K, no dissolution of any
components in the Na2O­SiO2­Al2O3 and Na2O­SiO2­MgO
slags into solid platinum was observed. Therefore, the
activity of solid platinum (aPt) in eq. (2) was assigned to
unity. Meanwhile, because the amount of iron dissolved
into the solid platinum was insignificant (<0.25 mol%), the
Na2 O þ Al2 O3 ¼ 2½AlO2 Na
þ
ð3Þ
ð4Þ
The Al2O3 formed in the melts consumes Na2O to form
NaAlO4 units in the network, and the remaining Na2O is
consumed to modify Si­O­Si and Si­O­Al bonds. In
previous research, the changes in silicate structures when
the Al2O3 content in Na2O­SiO2­Al2O3 ternary slags
increased were determined by Raman spectroscopy.11) These
results suggested that the Al3+ ions in the melts were
incorporated into the SiO44¹ tetrahedral structure. Figure 7
shows the Raman spectra of the quenched slag samples
obtained in this study. The slags were melted at 1573 K in
air with the addition of 8 mass% MgO, 20 mass% Al2O3,
20 mass% Fe2O3, and 20 mass% Cu. The increases in Al2O3
Dissolution Behavior of Platinum in Na2O­SiO2-Based Slags
4.0
3.5
3.0
2.5
2.0
500
Na2O-SiO2-Al2O3
3.5
Na2O-SiO2-MgO
3.0
2.5
2.0
4.0
Na2O-SiO2-Fe2O3
3.5
log aNa2O in slag
Solubility in slag/ mass ppm
-6.4
4.0
log γ °PtO in slag
1087
400
300
(8a)
-6.6
-6.8
-7.0
-7.2
0
200
5 10 15 20 25
Al2O3 in slag/ mol pct
100
0
3.0
-7.0
2.5
-6.9
-6.8
-6.7
-6.6
-6.5
log a Na2O in slag
2.0
Fig. 8 Dependence of the solubility of platinum on the activity of Na2O in
the Na2O­SiO2­Al2O3 slags at 1473 K.
4.0
Na2O-SiO2-CuOx
3.5
3.0
2.5
2.0
0.0
0.1
0.2
0.3
Xoxide additives
0.4
Fig. 6 Dependence of the activity coefficient of PtO on the Al2O3, MgO,
Fe2O3, and CuOx contents in Na2O·SiO2 slags at 1473 K in air ( pO2 ¼
0:21 atm).
Fig. 7 Raman spectra of Na2O­SiO2, Na2O­SiO2­MgO, Na2O­SiO2­
Al2O3, Na2O­SiO2­Fe2O3, and Na2O­SiO2­CuOx slags (Na2O/SiO2
molar ratio ³ 0.97).
content in the sodium metasilicate slags caused decreases in
the intensities of the 600 cm¹1, 850 cm¹1 (NBO/Si = 4), and
950 cm¹1 bands (NBO/Si = 2) but caused increases in the
intensity of the 1050 cm¹1 (NBO/Si = 1) band. It was
claimed that these changes in the silicate structures due to the
addition of Al2O3 were caused by decreases in the amount of
the network modifier and the construction of three-dimensional network structures, which appear as the band at
550 cm¹1. It is thus deduced that the Al2O3 in the sodium
metasilicate slags behaved as a network former.
In addition, the activity of Na2O in the Na2O­SiO2­Al2O3
slags (Na2O/SiO2 ratio = 1) at 1400 K decreased when the
Al2O3 content in the slags increased.10) Figure 8(a) shows the
dependence of the activity of Na2O in the Na2O­SiO2­Al2O3
slags on the Al2O3 content, where the standard state of Na2O
was adjusted from that in the 40(mol%)Na2O­60SiO2 melt,
which was reported in the previous study, to pure liquid
Na2O. The behavior of the platinum in the slags as an acidic
oxide is indicated by the decrease in solubility of platinum in
the slags with decreasing Na2O activity in the Na2O­SiO2­
Al2O3 slags (Fig. 8).
Accordingly, since the platinum dissolved into the slags as
an acidic oxide anion, the consumption of the basic oxide by
Al2O3, which behaves in the sodium metasilicate melts as a
network former, suppresses the dissolution of platinum into
the slags.
3.4.2 Effects of MgO addition
MgO is commonly considered to be a weakly basic oxide,
especially when compared to Na2O and CaO. For example,
the optical basicity ($) of Na2O is higher than that of MgO.
Table 3 shows the optical basicity of various oxides.12­14)
The electrical conductivity of a more basic slag is higher
than that of a less basic slag. For instance, the electrical
conductivity of the Na2O·SiO2 slag ($ = 0.70) was
3 ³¹1 cm¹1 at 1773 K, whereas that of the MgO·SiO2 slag
($ = 0.58) was 0.4 ³¹1 cm¹1 at 1873 K.13) The role of MgO
as a basic oxide in the CaO­SiO2­MgO ternary slags was
reported previously. The electrical conductivity in the CaO­
SiO2­MgO melts increased when the MgO content in
the slags increased at a fixed CaO/SiO2 ratio. However,
the electrical conductivity of the Na2O­SiO2­MgO slags
1088
C. Wiraseranee, T. Yoshikawa, T. H. Okabe and K. Morita
Table 3
Optical basicity of various oxides.
Pure oxides
Optical basicity, $
Ref. No.
Na2O
CaO
1.15
1.00
12
12
MgO
0.78
12
Al2O3
0.60
12
12
SiO2
0.48
Cu2O
0.97
13
CuO
0.90
14
decreased with increasing MgO content at a fixed Na2O/SiO2
ratio.15) The effect of MgO on the electrical conductivity in
the Na2O­SiO2­MgO slags can be explained as follows. The
parameters that influence the electrical conductivity of the
slags include the concentration of the mobilized species, the
charge, and the mobility of the ionic species. For the Na2O­
SiO2 binary slags, when the Na2O content in the slag
increased, that is, when the concentration and mobility of
ionic species (e.g., Na+) increased because of the looser
network due to the higher amount of the network modifier,
the electrical conductivity of the melts increased. In contrast,
in the Na2O­SiO2­MgO slags at a constant Na2O/SiO2 molar
ratio of 1, the electrical conductivity of the slags decreased
when the MgO content increased. Since MgO is less basic
than Na2O, in this study, MgO behaved as a diluent of slag
basicity in highly basic melts such as the Na2O·SiO2 slags
and suppressed the dissolution of platinum into the slags.
3.4.3 Effects of Fe2O3 addition
The stable oxidation states of iron in the slags are Fe2+ and
3+
Fe . Both species always coexist in steelmaking slags and
copper smelting slags.16) Chemical analysis has confirmed
that there is no Fe2+ in the Na2O­SiO2­Fe2O3 slags after
equilibration experiments carried out in air. Therefore, the
oxidation state of iron in the slags was considered to be Fe3+.
It has been suggested that Fe2O3 dissolves into basic slags
as the cation Fe3+ (eq. (5)) or the anion FeO2¹ (eq. (6)).16)
Fe2 O3 ¼ 2Fe3þ þ 3O2
2
ð5Þ
Fe2 O3 þ O ¼ 2FeO2
ð6Þ
As shown in Fig. 7, the addition of Fe2O3 causes changes
in the silicate structures in the sodium silicate melts. The
Raman spectra of the Fe2O3- and Al2O3-bearing slags are
similar to each other, except for some differences in the peak
shapes at the Raman shifts lower than 950 cm¹1. In short,
the addition of Fe2O3 caused some changes in the silicate
structures. Fe2O3 behaved like an acidic oxide, partly
consumed Na2O in the melts, and thus caused a decrease in
the solubility of platinum in the slags.
3.4.4 Effects of CuOx addition
Cu2O was added into the sodium metasilicate slags in
experiments. Because the critical oxygen partial pressure for
the oxidation of Cu+ to Cu2+ oxide at 1473 K is higher than
1 atm, as calculated from eqs. (7) and (9), both Cu+ and Cu2+
oxides tend to coexist in the slags (i.e., CuOx) in air.
CuðsÞ þ
1
O2 ðgÞ ¼ CuO0:5 ðsÞ
4
ð7Þ
¼ 85;000 þ 37T J=mol17Þ
Gf,CuO
0:5 ðsÞ
aCuO0:5
Gf,CuO
¼ RT ln
ð8Þ
0:5 ðsÞ
£ Cu XCu p1=4
O2
1
ð9Þ
CuðsÞ þ O2 ðgÞ ¼ CuOðsÞ
2
Gf,CuOðsÞ
¼ 153;200 þ 86T J=mol18Þ
aCuO
Gf,CuOðsÞ
¼ RT ln
ð10Þ
£ Cu XCu p1=2
O2
As for the behavior of the CuOx in the melts, the molar
ratio of Cu+/Cu2+ in the Na2O­SiO2­CuOx slags in air
increased with increasing Na2O content.19) The addition of
CuOx did not influence the silicate structure, indicating that
CuOx behaved as either a basic oxide or a diluent of slag
basicity. If one assumes that the platinum dissolved into slags
as a platinate anion,5) the CuOx should behave as a weakly
basic oxide in the sodium metasilicate melts and slightly
enhance the dissolution of platinum into the slags.
3.5 Activities of Fe2O3 and CuOx in the slags
In addition to the platinum dissolution behavior in the
slags, the activities of iron (aFe) and copper (aCu) in the
platinum alloys in equilibrium with Na2O­SiO2 slags were
measured. Consequently, the activities of Fe2O3 and Cu2O in
the respective slags could be determined.
The activity of Fe2O3 in slags is expressed by the
following equations:
3
2FeðsÞ þ O2 ðgÞ ¼ Fe2 O3 ðsÞ
ð11Þ
2
Gf,Fe
¼ 816;100 þ 255T J=mol20Þ
ð12Þ
2 O3 ðsÞ
a
Fe2 O3
¼ RT ln 2
Gf,Fe
2 O3 ðsÞ
£ Fe X2Fe p3=2
O2
Figure 9 shows the relationship between the Fe2O3 content
in slags and the Fe2O3 activity relative to that of the pure
solid Fe2O3 standard state. It was found that the Fe2O3
activity exhibited a large negative deviation from ideality.
From the activity-composition relation in the Cu­Pt system
at 1473 K,9) the activities of CuO0.5 and CuO relative to those of
pure solid CuO0.5 and pure solid CuO as the standard states
could be determined from eqs. (8) and (10). At a fixed Na2O/
SiO2 ratio, the molar ratio of Cu2+ to Cu+ in the Na2O­SiO2­
CuOx slags in air was reported to be constant at 0.9 regardless of
the CuOx content.19) Accordingly, in this study, it was assumed
that the molar ratio of CuO0.5 to CuO in the slags was 0.9.
The activity of CuOx in the slags is shown in Fig. 10, and
for comparison, the reported activity of CuO0.5 in the Na2O­
SiO2­Cu2O slags at 1573 K, which exhibited positive
deviation from ideality, is shown together.21) The activity of
CuO0.5 in the Na2O­SiO2­CuOx slags at 1473 K exhibits
almost Raoultian behavior. Meanwhile, the activity of CuO
in the Na2O­SiO2­CuOx slags exhibits a slight negative
deviation from ideality. Because of this difference in the
activity of CuO0.5 in the Na2O­SiO2 slags, it is hypothesized
that the increase in the CuO content in the slags results in a
decrease in the activity of CuO0.5 in the slags.
3.6
Platinate capacity of Al2O3-, MgO-, Fe2O3-, and
CuOx-bearing slags
For the recycling of a noble metal such as platinum, it will
Dissolution Behavior of Platinum in Na2O­SiO2-Based Slags
0.015
CPtO2 2 ¼
a Fe2O3 in slag
0.012
0.009
0.006
0.003
0.000
0.0
0.1
0.2
0.3
XFe2O3 in slag
Fig. 9 Dependence of the activity of Fe2O3 on the content of Fe2O3
in Na2O­SiO2­Fe2O3 slags at 1473 K (pO2 ¼ 0:21 atm) relative to the
activity of solid Fe2O3 as a standard state.
aCuO0.5 or aCuO in slag
1.0
0.8
0.6
ðmass pct PtO2 2 in slagÞ
a 2
¼ Kð13Þ O
1=2
fPtO2 2
aPt ðpO2 Þ
aCuO0.5
0.2
aCuO
Na2O-SiO2-Cu2O slag
(liquid Cu satd.)21)
Na2O-SiO2-CuOx slag
Na2O-SiO2-CuOx slag
0.0
0.0
0.2
0.4
0.6
0.8
XCuO0.5 or XCuO in slag
1.0
Fig. 10 Dependence of the activities of CuOx in the Na2O­SiO2­CuOx slag
at 1473 K (= 0.21 atm; in circles) and at 1573 K (in equilibrium with
liquid copper; in square)21) on the content of copper in slags at the Na2O/
SiO2 ratio of 0.97.
be advantageous if the amount of the platinum loss into slags
is predictable under controllable process conditions. It was
previously proposed that the platinum group metal (such as
ruthenium and rhodium) capacity of slags could be used to
calculate of the possible platinum group metal contents in
slags in which the thermodynamic properties of the metal
are unknown.22,23) In the present study, the platinate capacity
of the slags (CPtO2 2 ) is defined as the capacity of the slags
to hold platinum in the form of platinate ions (PtO22¹),
according to the following reaction for dissolution of
platinum into slags proposed by Nakamura and Sano (1997).
1
5Þ
PtðsÞ þ O2 ðgÞ þ O2 ðin slagÞ ¼ PtO2 2 ðin slagÞ ð13Þ
2
The platinate capacity of the slags is expressed in terms of the
measureable parameters by the following equation.
ð14Þ
Here K(13) is the equilibrium constant of eq. (13), aO2 is the
activity of oxide ion (O2¹) in slags, and fPtO2 2 is the activity
coefficient of the platinate ion in slags. In the same sense as
the activity coefficient of a metal in slags, the platinate
capacity is a specific property that indicates the behavior of
platinum in slags under an oxidizing atmosphere when the
standard state of the species of platinum in the slags is
arbitrary. From eq. (14), accordingly, the possible platinum
content in the slags at a particular temperature can be
estimated from the platinate capacity, provided that the
activity of platinum in the metal phase, slag composition, and
oxygen partial pressure in the system are known.
Optical basicity has been widely used as a generic measure
of slag basicity to correlate the capacities of slags with the
slag composition. In order to calculate the platinum content
in slags from a slag composition for which the thermodynamic properties of platinum are unknown, the calculated
optical basicity ($th) can be employed to correlate the slag
basicity with the platinate capacity of the slags.
Typically, the optical basicity of slags is calculated from
eq. (15), where $i is the optical basicity of the oxide i, n is
the mole number of oxygen in the oxide, and Xi is the mole
fraction of the oxide in slags.
th ¼
0.4
1089
a na Xa þ b nb Xb þ c nc Xc þ . . .
na Xa þ nb Xb þ nc Xc þ . . .
ð15Þ
Originally, the optical basicity represented the power of an
oxide to donate a negative charge. Therefore, for any slag
system, the optical basicity of the slag is affected by the
structural behavior of the slag, which includes the degree of
polymerization of the network former (e.g., silicate anions),
the nature of behavior of the network modifier (e.g., Na2+),
and the charge compensation behavior of the intermediate or
amphoteric oxides (e.g., Al2O3 and Fe2O3).24) Consequently,
in the presence of intermediate oxides, the calculation of the
optical basicity needs to take into account the fact that some
of cations (network modifiers) in the slags can be consumed
for charge compensation. Therefore, the “corrected optical
basicity” ($corr) is determined by subtracting the mole
fraction of the intermediate oxide (Al2O3 or Fe2O3) from
the mole fraction of the most basic oxide.
The relationship between the platinate capacity in the
Na2O­SiO2­Al2O3, Na2O­SiO2­MgO, Na2O­SiO2­Fe2O3,
and Na2O­SiO2­CuOx slags and the calculated optical
basicity is shown in Fig. 11. According to this figure, the
platinate capacity of the slags increased with increasing
optical basicity. The best fit to the correlation was found to be
the linear relationship in eq. (16).
log CPtO2 2 ¼ 8:4th 7:5
ð16Þ
For the recovery of platinum in an oxidizing atmosphere, it
is expected that the slags will contain various oxide systems
for which the thermodynamic properties of platinum are not
known. Thus, in order to estimate the possible platinum
content in the slags, we propose the use of the correlation
between the platinate capacity and the calculated optical
basicity.
1090
C. Wiraseranee, T. Yoshikawa, T. H. Okabe and K. Morita
0
to be the best-fit correlation between the platinate
capacity and the calculated optical basicity. This
correlation can be used to calculate the platinate
capacity in multicomponent recycling slags.
log CPtO22- in slag
-1
-2
Acknowledgement
-3
This research was supported by the New Energy and
Industrial Technology Development Organization (NEDO),
Japan. The authors would like to thank Mr. Yuzuru
Nakamura, Dowa Metals & Mining Co., Ltd., Japan, for
fruitful advice and discussions.
Na2O-SiO2 5)
CaO-SiO2 5)
Na2O-SiO2-CuOx
Na2O-SiO2-MgO
Na2O-SiO2-Al2O3
Na2O-SiO2-Fe2O3
-4
-5
-6
0.5
0.6
0.7
0.8
Λ th, corr
0.9
Fig. 11 Correlation between the platinate capacity and the calculated
optical basicity.
4.
Conclusions
With the aim of minimizing the platinum loss into slags
during the recovery of platinum in an oxidizing atmosphere,
the effects of Al2O3, MgO, Fe2O3, and CuOx addition on the
dissolution behavior of platinum in Na2O·SiO2-based slags
in air ( pO2 ¼ 0:21 atm) at 1473 K were investigated. The
following findings were obtained.
(1) The solubility of platinum sharply decreased with
increasing Al2O3, Fe2O3, and MgO content in the slags
but slightly decreased with increasing CuOx content.
Al2O3 and Fe2O3 behaved as acidic oxides in the slags
and drastically suppress the dissolution of platinum.
MgO is less basic than Na2O, and so it behaved as a
diluent of slag basicity, resulting in an increment in the
activity coefficient of PtO when the content of MgO
increased. The activity coefficient of PtO in the CuOxbearing slags slightly decreased with increasing CuOx
content, so it was deduced that copper oxides behaved
as basic substances and weakly enhanced the dissolution of platinum into the slags.
(2) The iron and copper in the Na2O­SiO2­Fe2O3 and
Na2O­SiO2­CuOx slags were found to dissolve into
solid platinum and to decrease the activity of solid
platinum in the system.
(3) The platinate capacity was proposed as the capacity of
the slags to hold platinum in the form of a platinate ions
(PtO22¹), and was found to increase with increasing
optical basicity of slags. A linear relationship was found
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