Thorium in high-titania slag - J. NELL
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NELL, J. Thorium in high-titania slag. The 6th International Heavy Minerals Conference ‘Back to Basics’, The Southern African Institute of Mining and
Metallurgy, 2007.
Thorium in high-titania slag
J. NELL
Mintek (now at Hatch)
Introduction range of compositions were selected for bulk chemical
Heavy mineral deposits often contain relatively high levels analysis, scanning electron microscopy and micro-analysis.
of radioactive elements (thorium and uranium in particular). The samples were crushed to a maximum particle size of
It is difficult to obtain clean separation of ilmenite and 1 mm and from each sample a 20-kg batch was riffled out
monazite (the main impurity mineral containing radioactive for analysis.
elements), and physical intergrowths of the two minerals
are, in fact, not uncommon. As a result, ilmenite Bulk chemical composition
concentrates obtained from such deposits often contain high Chemical compositions of the samples selected for test
levels of thorium and uranium. Because of the large work are given in Table I. TiO2, Al2O3, SiO2, MgO, MnO,
negative free energies of formation of ThO2 and U3O8 both FeO and V2O5 were analysed by ICP-OES, Na2O, CaO and
thorium and uranium report to the slag during smelting and Cr2O3 by atomic absorption spectroscopy and U and Th by
if an ilmenite concentrate contaminated with monazite is XRF (the XRF detection limit for U and Th was around
used to make high titania slag, the concentrations of these 4 ppm). The ‘B’ series has a higher silica content than the
elements in the slag end up about 40% higher than in the others as a result of small silica additions that were made
ilmenite feed. during smelting. All samples have elevated alumina levels
Previous work, patented by RGC Mineral Sands Limited, as a result of furnace refractory contamination during
and subsequently confirmed by Mintek, showed that smelting.
radioactivity may be reduced by roasting an ilmenite As expected, there is an inverse relationship between the
concentrate with borax at 1000°C to 1100°C and leaching it concentrations of TiO2 and FeO in the samples (Figure 1).
with hydrochloric acid1. It is evident that the cost of an The TiO2 and FeO levels are similar to the levels found in
ilmenite roast/leach process will be high and there may be industrial slags2. Extreme conditions of reduction under
economic benefits in removing the radioactive elements which reduction of more ‘refractory’ oxides (e.g., chromia,
from the slag instead (the volume of slag to be treated is magnesia, silica, alumina and even urania and thoria) might
smaller than the volume of ilmenite feed). take place were not explored in the study. Note that BB1
To remove radioactive elements from the slag it is and BBB1 contain more than 7% combined silica and
necessary to know how these elements occur in the slag. In alumina (SiO2 as a flux addition during smelting, and Al2O3
other words, the distribution of uranium and thorium from refractory contamination) and they do not fall on the
between the various oxide, silicate and metallic phases in general trend defined by the other samples.
high titania slag must be determined. Once the deportment The U and Th concentrations of sample E2 is noteworthy.
of thorium and uranium between the various phases has In the preparation of slag E2, monazite concentrate was
been determined, an appropriate process for slag added to the ilmenite to increase the concentrations of
purification can be developed. radioactive elements in the slag. This was done primarily to
facilitate the detection of Th and U during the phase
Test samples chemical characterization of the slag.
High-titania slag was prepared in a pilot-scale DC furnace
from ilmenite concentrates with different levels of uranium Radioactivity measurements
and thorium. Subsequently, ten slag samples representing a Gamma ray emissions from the samples were measured
Table I
Bulk chemical compositions of the slag samples selected for test purposes
THORIUM IN HIGH-TITANIA SLAG 7solid-solution series (in high-titanium slags M may be Ti4+,
Ti 3+ , Fe 2+ , Mg 2+ and Al 3+ , appropriately combined to
maintain charge balance in the structure). All the slags
contain TiO2 (rutile) in trace amounts.
The shape of the background at small diffraction angles
indicates the presence of small amounts of silicate glass in
the slags. This feature was particularly prominent in the ‘B’
series of samples and is consistent with the addition, during
smelting, of SiO2 to these samples.
Micro-analysis
The slags, after tapping, consist mostly of iron-titanium
oxides (M3O5 solid solutions where M = Ti4+, Ti3+, Fe2+,
Mg2+, Mn2+ and Al3+) and silicate phases. The aim of this
part of the characterization is to determine how the
radioactive elements are distributed between these phases.
This information is critical to develop methods for the
Figure 1. Relationship between the TiO2 and FeO removal of impurities from the slag.
concentrations in the selected slags
Proton induced X-ray emission spectroscopy (PIXE)
Measurements on slag F 3, which was doped with
Table II monazite, were performed by iThemba Laboratories. The
Gamma ray emissions from samples selected for test work.
Results are expressed as (counts.second-1.gram-1), standard
advantage offered by PIXE is a relatively low detection
deviations are given in parentheses limit for elements with high atomic mass, such as thorium
and uranium. However, there are also draw backs to the
technique:
• The inability to analyse elements with an atomic
number below about 20 (Ca) when these are contained
in phases with a high mean atomic number
• The semi-quantitative nature of the analyses
(measurements are standardless, although matrix
corrections are performed).
It is therefore not possible to perform comprehensive,
quantitative chemical analyses of the phases present in the
slag using this technique.
However, the instrument can be used to generate
concentration maps for areas of slag measuring almost
with a gamma ray detector. Measurements were performed 0.5 mm2. This provides a powerful visual image of the
on 50-g samples, counting for 400 seconds. Repeated distribution of trace elements between the coexisting phases
measurements (between 3 and 6) were performed on each over a relatively large area of slag. Such maps for a grain of
sample to calculate mean values and standard deviations slag consisting of silicate and oxide phases are shown in
(Table II). There is good agreement between gamma ray Figure 3 and Figure 4. In Figure 3 the silicate phases are
emissions and the combined thorium and uranium characterized by high concentrations of iron and manganese
concentrations of the samples (Figure 2). A regression of and a relatively low titanium content. The observations may
the data gives: be summarized as follows:
[1]
where NC is the normalized gamma-ray emission in units
of counts.gram -1.seconds -1. It is important to note that
Equation [1] is strictly valid for the slag samples that were
analysed and that it cannot be applied to other materials.
Reconciliation of the radioactivity of the slags with the
concentrations of radioactive elements was not undertaken.
There can be no doubt that the radioactive decay series in
the slags are not in equilibrium owing to factors such as the
volatilization of some of the daughter elements during
smelting.
Phase-chemical characterization of the slag
X-ray diffractometry
Powder X-ray diffraction was used to identify the phases
present in the samples. In all the samples, the most Figure 2. Gamma ray emissions from the samples as a function of
abundant phase belongs to the M 3O 5 multi-component the combined uranium and thorium concentrations
8 HEAVY MINERALS 2007900000 300 Sr 1500
Ti Tal
840000 280 1400
780000 260 1300
720000 240 1200
650000 220 1100
600000 200 1000
540000 180 900
480000 160 800
420000 140 700
360000 120 600
300000 100 500
240000 400
180000 80
60 300
100 μm 120000 200
60000 100 μm 40 100 μm
20 100
0 0
0
Fe 180000 Mn 900000 1500 9000
168000 Y Nb
840000 1400 8400
156000 780000
144000 1300 7800
720000 1200 7200
132000 650000 1100 6600
120000 600000
108000 1000 6000
540000 900 5400
96000 480000
84000 800 4900
420000
72000 360000 700 4200
60000 300000 600 3600
48000 240000 500 3000
36000 180000 400 2400
100 μm 24000
100 μm 120000 300 1800
12000 60000 100 μm 200 100 μm 1200
0 0 100 800
0 0
Figure 3. Backscattered electron image (a) and distribution maps
of titanium (b), iron (c) and manganese (d), to illustrate the Figure 4. continued. Elemental maps of tantalum (a), strontium
location of silicate and oxide phases in a fragment of slag from (b), yttrium (c) and niobium (d) in silicate- and titanium oxide-
sample F3. In the backscattered electron image the silicate phases phases in the grains shown in Figure 3
are black in colour and the titanium oxides are grey. Elemental
maps were composed using the PIXE facility at iThemba
Laboratories 6000 3000
Ce La
5600 2800
5200 2600
4800 2400
4400 2200
4000 2000
3600
Thi 600 60 1800
Ui 3200 1600
560 56 2800
520 1400
52 2400 1200
480 48 2000 1000
440 44 1600
400 800
40 1200
360 600
36 100 μm 800 400
320 32 400 100 μm
200
280 28 0
240 0
24
200 20
160 16
120 12
100 μm 80 100 μm 8 Figure 4. (continued). Elemental maps of cerium (a) and
40 4
0 0 lanthanum (b) in silicate and titanium oxide phases in
the grains shown in Figure 3
6000 Ba 1200
Zr
5600 1120
5200 1040
960
4800
880
while others such as U, La and Ba are concentrated in
4400
4000 800 localized regions within silicate grains.
3600 720
3200 640 The partitioning of Th, U, Y, Zr, Nb, Ta, Ba and Ce into
2800 560
2400 480 silicate phases is noteworthy. These elements have large
400
2000
320
atomic masses and large ionic radii. This is also a
1600
1200 240 characteristic of the radioactive daughter elements
100 μm 800 100 μm 160
400 90 generated by the decay series of thorium and uranium. By
0 0 implication these elements should also concentrate in the
silicate phases, rather than in the oxide phases present in the
slag.
Figure 4. Elemental maps of thorium (a), uranium (b), zirconium The PIXE data for chromium are more ambiguous.
(c) and barium (d) in silicate and titanium oxide phases in the
Several grains of slag containing silicate and oxide phases
grains shown in Figure 3. Maps were composed using the PIXE
facility at iThemba Laboratories
were mapped; in one instance chromium showed a slight
preference for silicate phases but in other cases the
distribution was more uniform. Microprobe data (see
below) are in agreement with the latter scenario, indicating
• All the trace-elements detected by the instrument (Th, a slight preference of chromium for M3O5 oxide phases.
U, Y, Zr, Nb, Ta, Ba and Ce) partition preferentially The thorium content of the silicate phases was found to
into silicate phases (Figure 4) be approximately 4 500 ppm. This means that the thorium
• Some of the elements, such as Th, Zr, Nb and Ta, are content of the titanium oxide phases must be below
uniformly distributed throughout the silicate phases 220 ppm (the bulk thorium content of the sample). This
THORIUM IN HIGH-TITANIA SLAG 9constrains the distribution coefficient for thorium between per analysis; this was considered to be counter
Silicate–oxide
silicate and oxide phases (D Th = wt % Th in productive in terms of the objectives of the work
silicate/wt % Th in oxide) to be greater than 20. The • The volatiliszation of sodium during analysis
approximate uranium content of the silicate phases is about • Complicated interelement correction procedures due to
100 ppm but the distribution coefficient could unfortunately the high thorium and zirconium concentrations in the
not be estimated because the uranium content of the bulk phases.
sample was below the detection limit of the measurement In most slags more than one type of silicate could be
technique. identified on the basis of the concentrations of SiO2, MnO
and FeO. The silicate phases are characterized by complex
Electron microprobe analysis chemical compositions and a lack of crystalline textures,
Electron microprobe analysis using wavelength dispersive i.e., they have the chemical and morphological
X-ray spectroscopy (WDS) was performed to quantify the characteristics of glass. The presence of silicate glass in the
thorium content of the silicate phases present in each of the samples is consistent with the presence of amorphous
samples (the uranium concentration in the silicate phases material detected by XRD. The glass represents silicate
was similar to the detection limit and it was not measured). melt that coexists with the titanium oxide melt at furnace
The detection limit for thorium is about 500 ppm (for a operating temperatures.
counting time of 300 seconds); well below the Silica-poor glass is characterized by high concentrations
concentration level in the silicate phases, but above the of FeO and MnO. There is also a positive correlation
concentration in M3O5 titanium-oxide phases. The thorium between the concentrations of MnO and FeO in the silica-
content of the latter phases could therefore not be poor glass. Texturally, silica-rich glass occurrs as rounded
measured. droplets within the silica-poor glass (Figure 5), suggesting
With rare exceptions, between 5 and 10 points of each immiscibility of two silicate melts at high temperatures.
phase were analysed per sample. Mean values and standard The ThO2 and ZrO2 concentrations in the different silicate
deviations for silicate and oxide phases are reported in phases in a particular slag are inversely proportional to the
Table III and Table IV respectively. Several factors may silica content, i.e., thorium and zirconium partition
contribute to the low analytical totals obtained for some of preferentially to the silica-poor glass. These relationships
the analyses: are illustrated by the data for samples E2 and H4 in
• The presence of sulphur (up to about 4% by weight in Table IIII (owing to the poor optical imaging on the
extreme cases), potassium (maximum of about 1% by instrument used for the WDS analysis it was not possible to
weight), and other trace elements such as niobium in distinguish between different silicate phases in the slag.
the silicate phases. To include these elements would Consequently, no analyses of the silica-poor phase were
have extended counting times well beyond 5 minutes obtained for B4, F3 and G3).
Table III
Electron microprobe analyses of silicate phases in the slag samples. (standard deviations are given in parentheses)
*Analyses of the silica-poor phase in E2 are incomplete due to instrumental difficulties at the time of analysis
10 HEAVY MINERALS 2007Table IV
Electron microprobe analyses of iron-titanium oxide phases in the slag samples. (standard deviations are given in parentheses)
The oxide phases contain MgO in quantities similar to
those present in the coexisting silicate phase but virtually
no CaO. To the extent that this signifies a tendency for
alkali earth metals with large ionic radii to partition into
silicate phases, this is highly significant (such a tendency is
in agreement with the PIXE results that show preferential
partitioning of barium to the silicate phases). Radium, an
Fe-Ti-oxide alkali earth metal considered to be an important source of
radioactivity in slag, has an ionic radius of 1.40 Å, much
Silica-rich phase larger than the radii of Ba2+ and Ca2+ (1.35 Å and 0.99 Å
respectively). The observed partitioning behaviour of MgO,
Metal CaO and BaO suggests that RaO also will partition to the
silicate phases present in the slag, but even more so than the
Silica-poor phase lighter alkali earth metals.
Distribution of elements between silicate and
oxide phases
The distribution of elements between silicate and oxide-
phases is a reflection of the thermodynamic behaviour of
Figure 5. Backscattered electron image illustrating textural the system. The distribution coefficient for element i, Di, is
relationships between different silicate phases in slag H4. defined as:
Droplets of a silica-rich phase are surrounded by a [2]
silica-poor phase
The value of D i , depends on variables such as
temperature, oxygen partial pressure (i.e., slag-grade) and
The high thorium content of the silicate phases is one of melt composition. If there is a sufficient dependence on
the most important observations of this study. It is melt composition it would, in principle, be possible to
important to note that the concentration levels of thorium in manipulate the behaviour of impurities during smelting. In
silica-poor glasses are still well below the values at which the context of this study it would be highly desirable to
silicate melts (glasses) become saturated with thorium. calculate D Th for each of the slags. Due to the low
High alkali-borosilicate glasses quenched from 1250°C concentration of thorium in the oxide phases it was
contain between 2% and 5% Th (2.3%–5.7% ThO 2 by unfortunately not possible to measure DTh directly, although
weight) in solution3; data cited by Farges 4 suggest similar minimum values were estimated from mass-balance
solubility levels in alumino-silicate glasses quenched from constraints.
the same temperature. He also noted an increase in the Compositional data for silicate- and oxide-phases were
solubility of thorium with increasing temperature. The used to calculate DMn, DMg, DAl, DFe, DTi and DCr for each
silicate glasses in the high-titanium slags are therefore not of the slags (Table V). Di values for Ca and Si were not
expected to be saturated in thorium (the reason for the calculated since the concentrations of these elements in the
difference in thorium content of the different silicate M3O5 solid solutions were not accurately measured. Data
glasses will be discussed in a later section). for silicate phases with different compositions in the same
The distribution of thorium between silicate and oxide- slag were used to calculate average values for the D i.
phases will be reported later; as an initial observation it is Where data for silica-poor glass were not available,
sufficient to note that thorium has a strong preference for inequalities were used to indicate the bias (samples B4, E2,
silicate phases. F3 and G3).
THORIUM IN HIGH-TITANIA SLAG 11Table V
Average distribution coefficients (silicate oxide) for Mn, Mg, Al, Fe, Ti and Cr for each of the slags
(the range over which Di varies is reflected as an uncertainty)
Distribution coefficients for B4, E2, F3 and G3 are not well constrained due to a lack of analytical data. Symbols are used to indicate the bias resulting from
the lack of data.
Table VI
Calculated mass-percentage of silicate and oxide phases in each slag and the calculated thorium distribution
‘Bulk Th’ is the bulk Th content of the slag (from Table I)
‘Th concentration in silicates’ is the average Th concentration in silicate phases determined by microprobe
‘Th in slag contained in silicates’ is the amount of thorium contained in the silicate phases in the slag
‘Th in slag contained in oxides’ is the amount of thorium contained in the oxide phases in the slag
‘Th conc. in oxides’ indicates the maximum Th concentration in oxide phases calculated from the Th distribution
DTh is the minimum value of the silicate-oxide distribution coefficient for Th
The Th distribution in E2 and H4 are subject to large uncertainties due to the large difference in the Th content of the silicate phases
Calculations are based on semi-quantitative PIXE data
Elements may be classified according to their partition The chemical behaviour of thorium in
coefficients: high-titania slag
• Silicon, calcium (and thorium) have large partition
coefficients which means that they are strongly Distribution of thorium between silicate and oxide-
concentrated in the silicate phases. Manganese also phases
prefers silicate phases but its partition coefficient is Although the distribution coefficient could not be measured
considerably smaller (about 10) directly, it is possible to constrain the distribution of
• Iron, magnesium, chromium and aluminium have thorium through calculation.
distribution coefficients that are around unity, which First, the weight fractions of silicate and oxide phases in
means that they are evenly distributed between silicate each slag are calculated. The average concentrations of
and oxide phases MnO, SiO2, TiO2 and CaO in silicate and oxide phases are
• Titanium has a very small distribution coefficient used for this purpose because of the magnitudes of the
(~ 1/100), which means that it is strongly concentrated silicate-oxide distribution coefficients of these elements.
in oxide phases. Next, the amount of thorium reporting to silicate phases (in
μg/g slag) is calculated from the weight percentage silicate
The distribution coefficients indicate how the removal of
phases in the slag and the average thorium concentration in
silicate phases from the slag will affect the composition of the silicate phases. Standard error-propagation techniques
the oxide residue. Elements that preferentially partition to are used to calculate the uncertainties in these quantities.
silicate phases (Di > 1) will be depleted in the residue (i.e., Within the uncertainty of the data, the amount of thorium
the composition of the residual oxide slag will be upgraded reporting to the silicate phases is enough to account for the
through the removal of these elements), while elements for mean total thorium content of the slags and, as a result, only
which Di < 1 will be enriched in the residue (providing a maximum value for the amount of thorium reporting to
insoluble oxides do not precipitate during leaching). oxide phases can be calculated (this being the difference
Leaching of silicate phases should therefore result in a between the total thorium content of the slag and the
decrease in the concentrations of SiO2, CaO and Th (to a amount of thorium present in silicate phases—taking the
lesser extent also MnO), and an increase in the uncertainties in the data into account). Following the
concentration of TiO 2 in the oxide residues. The calculation of a maximum value for the amount of thorium
concentrations of FeO, MgO, Al2O3 and Cr2O3 in the oxide present in oxide phase, a minimum value for the thorium
residues will not be significantly affected by the removal of distribution coefficient can be calculated. Results of these
silicate phases. calculations are reported in Table VI.
12 HEAVY MINERALS 2007The calculation is complicated by the large difference in Using thermodynamic data of Barin6, the equilibrium
thorium content of the silica-rich and silica-poor glasses. In oxygen partial pressure for reaction 1 at 1900 K is 10-9.3 bar
the case of slags B4 and G3, calculation of the distribution (this is about 1.3 log10-units more reducing than the Fe-FeO
coefficient was not attempted because of a lack of data for buffer). The oxidation of solid Ti4O7 to TiO2 (rutile) at
the thorium-rich, silica-poor, glass. Although WDS data are 1900 K takes place at a very similar oxygen partial pressure
not available for F3 either, semi-quantitative PIXE analyses (10-9.2 bar), indicating that solid Ti3+-bearing oxide phases
for this sample are consistent with a value for the thorium are stabilized at oxygen partial pressures approximately 7
distribution coefficient of at least 20. Consistently large log 10 -units more oxidizing than the Ti-TiO 2 buffer.
distribution coefficients for thorium lead to an important Assuming similar behaviour in the thorium-oxygen system,
conclusion: the formation of Th 3+-bearing oxide phases at 1900 K
• The removal of silicate phases will also remove the might be expected at oxygen partial pressures of about 10-17
bulk of the thorium and cause a significant decrease in bar. This is about 8 log10-units more reducing than the
the thorium content of the oxide residue. minimum oxygen partial pressure likely to be attained
during smelting (10-9.3 bar). Although these calculations are
It is informative to consider the significance of a large
not directly applicable to the oxidation state of multi valent
distribution coefficient for thorium between silicate and
cations in melt phases, the data suggest that the ratio of
oxide phases. Consider a slag containing 5 wt% silicate
Th3+ to Th4+ is likely to be very low, even under the most
phases and a bulk Th content of 200 ppm. Assuming
Silicate–oxide reducing conditions expected during smelting.
DTh = 100, it follows that the Th concentration in
The structural environment around Th4+ in silicate glasses
the silicate and oxide phases will be 3360 ppm and 33.6
containing between 1% and 3% (by weight) Th 4+ was
ppm respectively. Therefore, if the silicate phases could be
investigated by Farges4 using extended X-ray absorption
fully leached from the slag, it should be possible to reduce
fine structure (EXAFS) spectroscopy. Several conclusions
the thorium content by at least 84%.
from the work by Farges4 are directly applicable to current
A large value for D Th is consistent with the general
problem:
incompatible behaviour of thorium during the
crystallization of magma with different compositions5 (an • Th4+ is mainly six co-ordinated by oxygen with a mean
element is said to be ‘incompatible’ if it is concentrated in bond length, ⟨ VI Th-O.⟩ of about 2.3 Å. Eight-co-
the residual melt once different oxide and silicate minerals ordinated Th4+ (⟨VIIITh-O⟩ ~ 2.4 Å) is present at high
start to crystallize during cooling). Unfortunately, there are thorium concentration levels (3%) and/or when
no data for D Th between silicate melt and M 3O 5 oxide polymerization increases. Mean metal-oxygen bond
phases in the geological literature that could be used for lengths for the two six-co-ordinated cation sites (M1
comparison with the results of this study. and M2) in orthorhombic Ti3O5-rich solid solutions
⟨VIM1-O⟩ and ⟨VIM2-O⟩ are approximately 2.04 Å and
Discussion 2.01 Å, respectively4. The much shorter metal-oxygen
bond lengths in Ti3O5-rich solid solutions mean that
The silicate phases in the slags were previously identified thorium is unlikely to partition into oxide phases
as quenched silicate melt (i.e., glass). It is therefore relevant crystallizing from the slag during cooling
to consider the chemistry of thorium in silicate melts as it • The amount of six-co-ordinated Th4+ (VITh4+) increases
may suggest options for optimizing the amount of thorium in the presence of major quantities of alkali cations
reporting to the silicate melt phases during smelting. The such as Na+ and K+. Should the addition of such fluxes
most stable oxidation state for thorium is +4; with respect be practical, it will facilitate the collection of thorium
to thorium metal, it is also one of the most stable oxides. by silicate melt phases during smelting
The stability of ThO2 relative to Th plus oxygen is evident • The bond strength around six-co-ordinated Th 4+ in
from the Gibbs free energy of formation of ThO 2 ; silicate melt or glass is high relative to that in Th-
approximately -870 KJ.mol -1 at 1900 K (1627°C). By bearing minerals such as ThO 2 and ThSiO 4 . This
comparison, the Gibbs free energy of formation of TiO2 explains in part why these phases are not observed in
(relative to Ti and O2) at 1900 K is about -603 KJ.mol-1. In the slags, even at high bulk thorium concentration
terms of oxygen partial pressure (PO2) it means that ThO2 levels (> 300 ppm)
will form when the PO2 exceeds 10-23.8 bar, while TiO2 will • Increasing polymerization of the silicate glass or melt
(form from Ti and O2) when the PO2 exceeds 10–16.5 bar. A (high concentrations of SiO2), will favour higher co-
practical implication of this result is that metallic thorium ordinations around Th4+ due to a lack of non-bridging
will not form during smelting and the Th content of the pig oxygens (NBOs) to which thorium preferentially
iron will be negligible. bonds. This explains why the silica-rich glass phase
It is not known what the effective oxygen partial pressure contains less thorium than the co existing, low silica,
during smelting is; it will also be different for the different glass phase in the slags.
slags (hence the differences in Fe/Ti ratio of the slags).
Phase-chemically, the composition of the M3O5 phases that To summarize: data on the chemical behaviour of
crystallize from iron-titanium oxide slag may effectively be thorium in silicate melts and glasses support the mass-
described in terms of the FeTi 2O 5-Ti 3O 5 solid solution balance calculations according to which the amount of
series. The activity of Ti3O 5 in the slag increases with thorium in silicate glass phases is sufficient to account for
increasing grade, and when all the Fe 2+ is reduced to the total thorium content of the slags.
metallic iron (Fe0) the activity of Ti3O5 is almost unity. An
indication of the minimum oxygen partial pressure during Summary
smelting may therefore be obtained from the equilibrium Elements with a large ionic radius preferentially partition to
between solid Ti3O5 and TiO2 (rutile): the immiscible silicate melt that forms during smelting.
[3] These elements include thorium (which is present in such
large quantities that its concentration in silicate phases
THORIUM IN HIGH-TITANIA SLAG 13could be measured by electron microprobe), zirconium, References
barium, lanthanum, niobium, cerium (the distribution of
these elements was determined by PIXE), and possibly also 1. RGC Mineral Sands Ltd., S.A. Patent no. 93/5474, 29
the radioactive daughter elements of the thorium- and July 1993.
uranium-decay series. 2. BESSINGER, D., DU PLOOY, H., PISTORIUS,
Mass balance calculations suggest that the thorium P.C., and VISSER, C. Characteristics of some high
content of the silicate phases is high enough to account for titania slags. Heavy Minerals. R.E. Robinson. (ed.)
the total thorium content of the slags. This is also reflected Johannesburg, South African Institute of Mining and
by the silicate-oxide distribution coefficients for Th that are Metallurgy, 1997. pp. 151–156.
overwhelmingly in favour of the silicate phases. Depending
on the total silica content, silicate phases constitute 3. ELLER, P.G., JARVINEN, G.D., PURSON, R.A.,
approximately 4 to 8 weight per cent of the slags that were PENNEMAN, R.R., LYTLE, F.W., and GREEGOR,
selected for test work. R.B. Actinide abundances in borosilicate glass.
Spectroscopic data in the literature are consistent with the Radiochimica Acta, vol. 39, 1985. pp. 17–22
partitioning of thorium into depolymerized (low-silica) 4. FARGES, F. Structural environment around Th4+ in
glasses and melts relative to crystalline phases and more silicate glasses: Implications for the geochemistry of
polymerized glasses and melts. These results are in incompatible M 4+ elements. Geochimica et
agreement with the observations of this study and confirm Cosmochimica Acta, vol. 55, 1991. pp. 3303–3319.
that silicate phases contain most of the thorium present in
5. MAHOOD, G.A. and HILDRETH, W. Large partition
the slag.
In principle, the radioactivity levels of high titania slag coefficients for trace elements in high silica rhyolites.
can be lowered by the removal (physical or chemical) of Geochimica et Cosmochimica Acta, vol. 47, 1983.
silicate phases from the slag. Disposal of the radioactive, pp. 11–30.
silicate fraction will be onerous and the economics of such 6. BARIN, I. Thermochemical Data of Pure Substances,
a step are questionable. Parts I and II. VCH Verlagsgesellschaft. 1989.
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