Sulfuric acid breakdown of fluorite in the presence of silica

Article abstract of DOI:10.1134/S0036023609120055

We study the reaction of CaF₂ with concentrated sulfuric acid in the presence of silica at 120 and 140°C using fluorite mineral, fluorite concentrate, and mica-fluorite ore samples. When steam is fed to the reaction mixture, fluorosilicic acid

Full text of DOI:10.1134/S0036023609120055

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 12, pp. 1876–1879. © Pleiades Publishing, Inc., 2009.
Original Russian Text © G.F. Krysenko, P.S. Gordienko, D.G. Epov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 12, pp. 1958–1961.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Sulfuric Acid Breakdown of Fluorite in the Presence of Silica
G. F. Krysenko, P. S. Gordienko, and D. G. Epov
Institute of Chemistry, Far East Division, Russian Academy of Sciences,
pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia
Received June 17, 2008
Abstract—We study the reaction of CaF2 with concentrated sulfuric acid in the presence of silica at 120 and
140°C using fluorite mineral, fluorite concentrate, and mica–fluorite ore samples. When steam is fed to the reac-
tion mixture, fluorosilicic acid is formed and then distilled off, materially influencing the fluoride ion recovery.
Addition of quartz makes the reaction to proceed in the kinetic area and increases the fluoride ion recovery,
while the presence of mica-bound SiO2 displaces the reaction to the diffusion area and decreases the fluoride
ion recovery. We have found that rare alkali metals are concentrated as hexafluorosilicates in the insoluble res-
idue during the sulfuric acid breakdown of fluorite in the presence of SiO2.
DOI: 10.1134/S0036023609120055
Facilities for the production of fluoride compounds enhancing fluoride ion recovery, and to find ways for
are the main consumers of natural fluorspar. The largest concentrating rare alkaline metals in this processing of
fluorite concentrate producing facility is the Yaro- fluorite raw materials.
slavskii Mining and Concentration plant (Yaroslavskii
Settlement Primorskii region); its resources include not
only enormous fluorite ore resources but also a number
of strategic minerals, including lithium, rubidium, and
cesium compounds.
EXPERIMENTAL
The starting materials used were pulverized samples
of fluorite (F), a fluorite concentrate containing 92%
CaF2 (FC-92), a mica–fluorite ore containing 39.8%
CaF2 (MFO), muscovite mica with particle sizes of 20–
44 μm, crystalline α-quartz with particle sizes of 30–50 μm,
and chemically pure grade concentrated sulfuric acid.
The major component percentages and phase composi-
tions of the test samples are shown in Table 1.
Flotation is the most popular process for the concen-
tration of fluorite ores [1] and useful for processing
finely disseminated ores. The mineral resources of the
Primorskii region are distinguished by the complex
composition and fine-crystalline structure of ores,
which hampers the production of quality concentrates.
Chemical concentration is useful to improve the grade
of fluorite concentrates [2, 3]. The underlying idea of
this process is the digestion of calcite by solutions of
acids, ammonium hydrodifluoride, ammonium fluo-
ride, etc. When ammonium hydrodifluoride is used, the
concentrate is mixed with NH4HF2 and heated to 180–
200°ë. As a result, calcite transforms to calcium fluo-
ride and quartz to ammonium hexafluorosilicate, the
latter being removed upon heating above 300°ë. This
treatment increases the fluorite concentrate percentage
from 92 to 97% [4].
Experiments were carried out on a setup comprising
a Teflon reactor with an air-tight screw cover perforated
with two holes, one for feeding steam and the other for
removing gaseous reaction products; a steam generator;
and a cooler for condensing leaving gases ended with
an adapter in a conic flask filled with aqueous NaOH
andAlizarin Red indicator. The reactor was placed in an
electric furnace where temperature was adjusted using
a VRT-2 high-precision temperature regulator. Steam
was fed directly to the reaction mixture.
Sulfuric acid breakdown is now the main large-scale
process for the breakdown of fluorite concentrates [5].
The ores from the Yaroslavskii Mining and Concentra-
tion Plant contain up to 45% silica (SiO2) in the free
form as quartz and the bound form as mica, and it was
of interest to study the reaction of CaF2 contained in the
ore with concentrated sulfuric acid in the presence of
silica avoiding the stage of concentration.
Batch samples comprising CaF2 and SiO2 were
flooded with concentrated sulfuric acid and heated iso-
thermally at the set temperature with steam being con-
tinuously fed to the reaction mixture. Batch sizes were
0.7–0.5 g. The reaction course was monitored by pow-
der X-ray diffraction in the residues and measuring flu-
oride ion evolution. Kinetic experiments on the reaction
of fluorite with α-quartz and mica in concentrated sul-
furic acid used the same batch sizes in the isothermal
mode at 120 and 140°ë; the fluoride ion evolved during
This work was intended to study the reaction of
CaF2 with concentrated sulfuric acid in the presence of
silica, to determine the possibility and conditions for certain periods was determined by titration.
1876

SULFURIC ACID BREAKDOWN
1877
Table 1. Major component percentages and phase compositions of precursor samples
Weight percentages of
Sample*
Powder X-ray diffraction data
CaF2
CaCO3
SiO2
F
100
92
CaF2
CaF2
FC-92
MFO
1.9
4.3
2.45
18
39.8
CaF2, α-quartz, muscovite (KAl2.20(Si3Al)0.975O10(OH)1.72O0.28),
orthoclase (K4Al4Si12O32)
* All fluorite concentrate samples were purchased from the Yaroslavskii Mining and Concentration Plant.
Chemical analysis, X-ray powder diffraction, X-ray stoichiometric proportion corresponding to the equa-
radiometric analysis, and IR spectroscopy were used tion
for analyzing the residues.
3CaF2 + SiO2 + 3H2SO4
3CaSO4 + H2SiF6 + 2H2O, (1)
Fluorine was determined by distilling off H2SiF6 fol-
lowed by titration of the resulting solutions by thorium
nitrate. The fluoride ion amount eliminated during CaF2
hydrolysis was also determined via the titration of the
collected solution by thorium nitrate.
increases the fluorine amount evolved in the same reac-
tion time.
Figure 1 shows the fluoride ion recovery as a func-
tion of time for the sulfuric acid breakdown of the test
fluorite samples at 120°ë.
X-ray diffraction patterns were recorded on an
ADVANCE D-8 automated diffractometer with sample
spinning using CuKα radiation. X-ray phase analysis
was carried out using the search program EVA with the
PDF-2 database.
The rate curves in Fig. 1 show that addition of
α-quartz in the amount calculated for Eq. (1) for 2 h at
120°ë increases the fluoride ion recovery during the
sulfuric acid breakdown of fluorite by about 15%
(Cf. Fig. 1; curves ‡, b). In addition, the fluoride ion
recovery during the sulfuric acid breakdown of the flu-
orite concentrate containing 92% CaF2 (Fig. 1, curve c)
is higher than during the sulfuric acid breakdown of
neat fluorite mineral, evidently, because of 2.45% SiO2
contained in this concentrate, but when α-quartz is
added in the amount calculated from Eq. (1), it coin-
cides with the fluoride ion recovery achieved during the
Alkali metals were determined by X-ray radiometric
analysis using the 241Am radionuclide source and a
semiconductor detector with reference to proper
oxides.
IR spectra were recorded as Nujol mulls and KBr
disks on a Shimadzu spectrometer.
RESULTS AND DISCUSSION
α
The reaction of fluorite with sulfuric acid occurs in
two stages. First, the acid association species CaF2 ·
H2SO4 is formed during mixing, then decomposing at
70–130°ë to evolve one HF molecule; next, fluorite
finally decomposes at 130–270°ë with the evolution of
the second HF molecule. The first stage occurs in the
kinetic area; the second, in the diffusion one [6].
1.2
‡
b
c
d
e
1.0
0.8
0.6
0.4
0.2
In the presence of silica, the reaction of fluorite with
sulfuric acid results in the evolution of gaseous silicon
tetrafluoride as a result of the reaction SiO2 with the HF
evolved.
Thus, the admission of steam to the reaction mixture
was expected to improve the efficiency of silicon tet-
rafluoride evolution due to the formation of fluorosilicic
acid followed by the distillation off an HF + H2SiF6 +
H2O mixture, which would allow increasing the fluo-
ride ion recovery during the sulfuric acid breakdown of
fluorite.
0
20
40
60
80
100 120 140
τ, min
Fig. 1. Fluoride ion recovery α vs. time τ (min) during the
sulfuric acid breakdown of fluorite at 120°ë: (‡) fluorite
mineral, (b) fluorite mineral (or FC-92) in the presence of
α-quartz, (c) fluorite concentrate FC-92, (d) fluorite mineral
in the presence of 18% mica, and (e) mica-fluorite ore
(MFO).
Indeed, the experiments carried out on pure fluorite
mineral showed that addition of α-quartz taken in the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 12 2009

1878
1.0
KRYSENKO et al.
cessed using the generalized topochemical equation are
shown in Table 2.
0.5
0
The first process stage occurs in the kinetic area in
all three cases with small differences between the reac-
tion rates and the conversion being maximal (77%) in
the case of the fluorite + α-quartz mixture. At the sec-
ond stage, the sulfuric acid breakdown follows another
scenario: the digestion of neat fluorite and fluorite in the
presence of mica occurs in the diffusion area, and the
reaction occurs in the kinetic area, as previously, only
in the fluorite + α-quartz mixture. The reaction rate
drops in all three cases: by about 17, 4, and 30 times for
neat fluorite, fluorite in the presence of α-quartz, and
fluorite in the presence of mica, respectively.
A virtually complete fluoride ion recovery in 2 h of
the reaction at 120°ë is observed only in fluorite +
α-quartz mixtures. However, the character of the rate
curves (Fig. 1) implies that the fluoride ion recovery
will be enhanced by either longer experiment times or
temperature elevation. Indeed, when the process tem-
perature is 120°ë, complete fluoride ion recovery in 2 h
of sulfuric acid breakdown is observed even for the flu-
orite + 18% mica mixture. On the other hand, long pro-
cess times at 120°ë, e.g., as long as 3 h, increase the flu-
oride ion recovery only by 10 and 5% for neat fluorite
and the fluorite + 18% mica mixture, respectively.
X-ray radiometric analysis shows that rubidium and
cesium contained in the batch are concentrated in the
insoluble residues after water leaching of the reaction
mass (the weight of these residues does not exceed 15%
of the batch weight); IR spectroscopy identifies the
rubidium and cesium compounds in the residues as
hexafluorosilicates.
The results of our study are summarized as follows.
The presence of silica influences the sulfuric acid
breakdown of fluorite and, when steam is fed to the
reaction mixture, causes the formation and subsequent
distillation of fluorosilicic acid.
–0.5
–1.0
–1.5
–2.0
–2.5
‡
b
c
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
logτ [min]
Fig. 2. Log–log plot of fluoride ion recovery log[–log(1 –
α)] vs. time log τ during the sulfuric acid breakdown of flu-
orite at 120°ë: (‡) fluorite mineral, (b) fluorite mineral in
the presence of α-quartz, and (c) fluorite mineral in the
presence of 18% mica.
sulfuric acid breakdown of neat fluorite in the presence
of the stoichiometric silica amount.
Because micas frequently accompany fluorite ores,
we studied the influence of mica-bound silica on the
fluoride ion recovery during sulfuric acid breakdown.
Our experiments showed that the presence of mica
reduces fluoride ion recoveries, as shown by the rate
curve for the MFO sample (Fig. 1, curve e), which
results from the occurrence of a more complex process
in this case. When the fluorite mineral + 18% mica
model mixture was subjected to sulfuric acid break-
down, the fluoride ion recovery in 2 h of the reaction at
120°ë was reduced by about 7% (Fig. 1, curve d), as in
the MFO sample.
The experimentally determined fluoride ion recov-
eries at 120°ë were used to calculate the reaction order
n and reaction rate for the sulfuric acid breakdown of
the test samples under these conditions. The calcula-
tions used the Kolmogorov and Erofeev generalized
topochemical equation [7]. Figure 2 shows the log–log
plots of the fluoride ion recovery as a function of tem-
perature.
The sulfuric acid breakdown of fluorite at 120°C
first proceeds in the kinetic area and then in the diffu-
sion one. The presence of free silica shifts the fluorite
breakdown process to the kinetic area, increasing the
reaction rate and enhancing the fluoride ion recovery.
The presence of mica-bound SiO2 does not alter the
reaction character but decreases both the reaction rate
and the fluoride ion recovery. The complete fluoride ion
recovery from fluorite is reached by either elevating the
reaction temperature to 140°ë or adding free SiO2 as
The process, clearly, occurs in two stages with the
transition after 45–50 min of the reaction. The linear
trend of the log–log plot indicates the constancy of n
within the periods of time used. The kinetic data pro- quartz.
Table 2. Reaction orders (n) and rate constants (k) for the sulfuric acid breakdown of the test samples
Sample
α1max
n1
k1, min–1
α2max
n2
k2, min–1
F
0.65
0.77
0.49
2.65
2.50
2.14
0.06030
0.05920
0.05754
0.857
0.999
0.786
0.65
1.70
0.91
0.00363
0.01580
0.00195
F + α-quartz
F + 18% mica
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 12 2009

SULFURIC ACID BREAKDOWN
1879
4. E. I. Mel’nichenko, D. G. Epov, N. M. Laptash, and
S. A. Polishchuk, Proceedings of XV Mendeleev Meet-
ing “Chemical Topics of Ecology” (Minsk, 1993), Vol. 4,
p. 124 [in Russian].
During the sulfuric acid breakdown of fluorite in the
presence of silica, rare alkali metals are concentrated in
the form of hexafluorosilicates in insoluble residues
after water leaching of the reaction mass.
5. V. A. Zaitsev, A. A. Novikov, and V. I. Rodin, Production
of Fluoride Compounds by Processing Phosphate Raw
Materials (Khimiya, Moscow, 1982) [in Russian].
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