Autoclave breakdown of zircon with ammonium fluorides

Article abstract of DOI:10.1134/S1070427206110036

Autoclave breakdown of zircon with ammonium fluorides was studied.

Full text of DOI:10.1134/S1070427206110036

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 11, pp. 1757 1760. Pleiades Publishing, Inc., 2006.
Original Russian Text V.V. Guzeev, A.N. D’yachenko, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 11, pp. 1777 1780.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Autoclave Breakdown of Zircon with Ammonium Fluorides
V. V. Guzeev and A. N. D’yachenko
Tomsk Polytechnic University, Tomsk, Russia
Received March 30, 2006; in final form, June 2006
Abstract Autoclave breakdown of zircon with ammonium fluorides was studied.
DOI: 10.1134/S1070427206110036
Zircon is one of the main mineral sources of zir-
conium materials, but the technology of its breakdown
and processing is still labor-consuming, and much
efforts are made to improve it.
nating agents consists in their convenient physico-
chemical properties allowing their regeneration and
recycling.
The experiments were carried out with zircon sam-
ples from Tugan deposit of Tomsk oblast.
The classical industrial methods of breaking down
zirconium concentrates are sintering with fluosilicates
and chlorination. Numerous studies summarized in [1]
were devoted to zircon fluorination; however, it did
not gain industrial application. The fluoride technol-
ogy allows the scheme of the zirconium product proc-
essing to be reduced considerably, but high cost and
corrosive behavior of HF and F , and also SiF liber-
The reactions of zircon with ammonium fluoride
and hydrofluoride were studied under isochoric condi-
3
tions in a 100 cm stainless steel autoclave. A weighed
portion of zircon with a grain size of 0.074 mm was
placed in the autoclave together with a weighed por-
tion of ammonium fluoride taken in a 20% excess in
relation to the stoichiometric amount calculated for
the reaction
2
4
ation restrain industrial application of these reagents.
In this study we examined the autoclave breakdown
of zircon with ammonium fluorides.
ZrSiO4 + 13NH4F = (NH4)3ZrF7 + (NH4)2SiF6 + 8NH3
+ 4H2O.
Unlike fluorine and hydrogen fluoride, ammonium
fluorides are very convenient to use, as these are solid
crystalline substances under normal conditions. Am-
monium fluorides can be obtained from metal fluor-
ides by the action of ammonia water, which is the
basis for conversion and recycling of ammonium
fluorides.
The autoclave was heated to 150, 200, 250, 300,
350, and 400 C and kept at these temperatures for 0.5,
1, 2, and 4 h.
After opening the autoclave, its content was trans-
ferred into a crucible and calcined for 30 min at
Ammonium fluoride is known to react slowly with
zircon [2, 3]. At temperatures below 240 C, the reac-
tion rate is negligible and the degree of breakdown is
unsatisfactory for the industrial use, whereas at higher
temperatures ammonium fluoride evaporates with
decomposition.
400 C to remove by sublimation unchanged NH F
4
and formed (NH ) SiF . The residue in the crucible
4 2
6
was a mixture of unchanged ZrSiO and NH ZrF , to
4
4
5
which 5 ml of concentrated sulfuric acid was added,
and the mixture was kept until H SO vapor disap-
2
4
peared. Zirconium in the form of ZrOSO solution
4
was leached with water from the formed cake, con-
verted to zirconium dioxide, and weighed to deter-
mine the degree of conversion.
Here we report new data on the zircon breakdown
with ammonium fluoride and hydrofluoride.
The essence of the method consists in the reaction
of zircon with ammonium fluoride or hydrofluoride
under isochoric conditions (in an autoclave), after
which the reaction products are separated by sublima-
tion to give zirconium tetrafluoride and dioxide. The
advantage of ammonium fluorides over other fluori-
The same procedure was used to study the reaction
of ammonium hydrofluoride with zircon
2ZrSiO4 + 13NH4F HF = 2(NH4)3ZrF7 + 2(NH4)2SiF6
+ 3NH3 + 8H2O.
1757

1758
GUZEEV, D’YACHENKO
Scheme 1.
The data on the kinetics of zircon breakdown with
The rate equation for the range 300 400 C is
ammonium fluoride are given in Fig. 1, which con-
tains two series of curves: the first corresponds to
temperatures below ammonium fluoride boiling point
and the second, to elevated temperatures.
1/3
1
1/3 (1
)
= exp( 5500/RT)t.
1
The activation energy E = 55 kJ mol , which
corresponds to the kinetic control, the preexponential
factor k = 1 s , and the gas constant R = 8.31.
Material streams and material balance were calculated
in accordance with Scheme 1.
A
The standard mathematical treatment of the kinetic
data showed that the curves for 150, 200, and 250 C
are described with an adequate accuracy by Gray
Weddington’s, and the curves for 300, 350 and 400 C,
by Crank Ginstling Brounshtein’s equation.
1
0
Scheme 1 clearly illustrates the breakdown of zir-
con with ammonium fluoride and the formation of
complexes followed by their hydrolysis and ammoni-
um fluoride regeneration. Uniqueness of ammonium
fluorides consists in the fact that their melt is the most
powerful fluorinating agent reacting with zircon to
form zirconium and silicon complex fluorides. Zirco-
nium tetrafluoride and ammonium hexafluorosilicate
are easy to separate by sublimation. After separation,
the fluorides are subjected to hydrolysis with am-
monia. Ammonium fluoride is regenerated, and oxides
are precipitated from the solution. Unlike ammonium
fluoride melt, its solution is inert and does not react
with oxides.
The rate equation for the range 150 250 C is
= 1
[1
10 3 exp( 17000/RT)t]3.
1
The activation energy E is 17 kJ mol , which
A
corresponds to the diffusion control, the preexponen-
tial factor k = 10 s , and the gas constant R =
3
1
0
8.31.
, %
The experimental dependences of the conversion
on time and temperature for the breakdown of zircon
with ammonium hydrofluoride are given in Fig. 2.
The kinetic curves are linearized most accurately by
Yander’s equation
= 1
{1
[6.6exp( 54000/RT) ]1/3}3.
, h
Fig. 1. Kinetic curves for the reaction of ammonium fluor-
ide with zircon under isochoric conditions. ( ) Degree of
conversion degree and ( ) time; the same for Fig. 2. Tem-
perature, C: (1) 150, (2) 200, (3) 250, (4) 300, (5) 350, and
(6) 400.
1
The preexponential factor is k = 6.6 s , and the
0
activation energy calculated from the Arrhenius equa-
tion is E = 54 kJ mol , which indicates that the
process is kinetically controlled and temperature
1
A
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 11 2006

AUTOCLAVE BREAKDOWN OF ZIRCON WITH AMMONIUM FLUORIDES
1759
Scheme 2.
exerts a decisive effect on its rate. Scheme 2 of ma-
terial streams in the breakdown of zircon with ammo-
nium hydrofluoride is given below.
Apparently, the use of ammonium fluoride at
high temperatures is not appropriate because high
pressures arise and hence more sophisticated equip-
ment is required; therefore, the use of ammonium
hydrofluoride is the most efficient. As follows from
Fig. 3, the vapor pressure of ammonium hydrofluoride
at 300 C does not exceed 45 atm, which allows its use
in the technology of the zircon breakdown with am-
monium hydrofluoride. A considerable experience of
using high-pressure devices in the industrial synthesis
of ammonia and in the transportation of natural gas
has been accumulated.
The above-given experimental data allow us to
suggest the optimal process parameters: with NH F
4
HF, the conversion higher than 95% is reached at
300 C within 1.5 h and at 400 C, within less than
0.5 h [4].
When NH F was used, an elevated pressure was
4
developed in the autoclave. The heating of ammonium
fluoride is known to result in its thermal dissociation
into ammonia and hydrogen fluoride, which increases
the total pressure in the system [5].
The temperature of 300 C is optimal for imple-
mentation of the process, as relatively low tempera-
ture will not cause strong corrosion of the instrumen-
tation even at a long reaction time (4 h).
The pressure of the mixture of HF and NH F
4
vapors calculated by the Mendeleev Clapeyron equa-
tion considerably exceeds the actual pressure owing to
the condensation of the mixture of hydrogen fluoride
and ammonia at a temperature below 240 C and also
owing to considerable deviations of the behavior
of this mixture from that of an ideal gas to which
theoretical calculations are applicable.
A distinctive feature of the zircon breakdown with
ammonium hydrofluoride is the complete regeneration
, %
To determine the real pressure which is developed
under the considered conditions, we have carried out
special experiments.
The temperature dependence of the vapor pressure
was studied for two substances: ammonium fluoride
3
and hydrofluoride. In a 100 cm autoclave, 10 g of salt
was placed, and the system was sealed and gradually
heated to 350 C with continuous monitoring of the
pressure with an MTP-1M manometer. The plots of
the vapor pressure of ammonium fluoride and hydro-
fluoride are given in Fig. 3.
, h
Fig. 2. Kinetic curves for the reaction of ammonium hydro-
fluoride with zircon. Temperature, C: (1) 250, (2) 300,
(3) 350, and (4) 400.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 11 2006

1760
P, atm
GUZEEV, D’YACHENKO
In the suggested scheme, the number of operations
is minimized and complete recycling of the reagents is
provided. Among disadvantages of the scheme is
certain complexity of the instrumentation of the main
process, ammonium fluoride breakdown of zircon,
which is caused by the buildup of high pressure under
isochoric conditions. The corrosive action of ammoni-
um fluoride on stainless steel can be reduced by add-
ing sodium fluoride [7]; however, the use of sodium
fluoride will pose a problem of its regeneration and
removal from the products.
CONCLUSIONS
(1) Optimal parameters of zircon breakdown with
ammonium fluoride and hydrofluoride in an autoclave
under isochoric conditions were found. In the case
of ammonium hydrofluoride, the 95% decomposition
at 300 C is reached within 1.5 h, and a pressure of
45 atm is developed in the system.
T, C
Fig. 3. Dependence of vapor pressure P of (1) ammonium
fluoride and (2) ammonium hydrofluoride on temperature T
(2) The overall scheme of the cycle of zircon
breakdown with the regeneration of ammonium fluor-
ide and production of zirconyl hydroxide was sug-
gested.
3
under isochoric conditions in 100 cm of air.
of ammonium fluoride, which makes the process
economically attractive and environmentally safe.
The formed ammonium fluosilicate sublimes at
temperatures above 320 C and is removed from the
mixture [6].
REFERENCES
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regiona (Zircon Ilmenite Gravel Deposits as a Potential
Source of the Development of the Western Siberia
Region), Kemerovo: Sars, 2001.
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Ovsyannikova, A.A., Zh. Prikl. Khim., 1994, vol. 67,
no. 5, pp. 737 741.
Heating to 300 C removes unchanged ammonium
fluoride from the mixture. As the temperature is in-
creased, (NH ) ZrF decomposes to zirconium tetra-
4 2
6
fluoride, and ammonia and hydrogen fluoride are
liberated.
The overall process of ammonium fluoride break-
down of zircon is shown in Scheme 3.
3. Mel’nichenko, E.I., Ftoridnaya pererabotka redko-
metall’nykh rud Dal’nego Vostoka (Fluoride Processing
of Rare-Metal Ores of the Far East), Vladivostok:
Dal’nauka, 2002.
Scheme 3.
Autoclave
breakdown
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purification
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