Advances in Ma terials Physics and Che mist ry, 2012, 2, 21-24
doi:10.4236/ampc.2012.24B006 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Fluoride Process ing of Titaniu m-Containing Minerals
N. M. Laptash , I. G. Maslennikova
Institute of Chemistry, Far Eastern Branch of RAS, Vladivostok, Russi a
Received 2012
ABSTRACT
Fluoride processing of natural ilmenite with the use of ammonium hydrogen difluoride (NH4HF2) as an effecti ve flu or in atin g agent is
suggested. Chemistry, composition, structure, thermal and hydrolytic properties of fluorination products were investigated. Ammo-
nium fluoro- and oxofluorotitanates are suitable for preparing of titanium dioxide as pigmentary product or as doped by nitrogen and
fluorine.
Keywords: Ilmenite; Fluorination Reactions; Ammonium Hydrogen Difluoride; Ammonium Fluoro- and Oxofluorometallates;
Thermal Behavior; Hydrolysis; N-F-TiO2.
1. Introduction
Titanium dioxide (TiO2) has long been at the center of photo-
catalyst research due to its catalytic efficiency coupled with
wide availability, biocompatibility, chemical stability, low cost,
and safety toward both humans and the environment. It is much
more effective as photocatalyst in the form of nanoparticles
modified by doping with cations and anions [1,2]. The nitrogen
and fluorine-doped titanium dioxide (N–F–TiO2) nan omaterials
exhibit high photocatalytic activity for water-splitting and pho-
todegradation of organic pollutants [3–8]. It was shown that
co-doping with nitrogen and fluorine is advantageous for the
reduction of defect formation and lowers the energy cost for the
incorporation of nitrogen owing to the charge compensation
effect between the donor (F) and acceptor (N) [9].
Multifunctional properties and vast applications of nano–
TiO2 require its production in a mass scale. The large-quantity
production of rutile nanorods from ilmenite sands was recently
suggested [10,11]. Ilmenite (FeTiO3) is abundant feedstock for
industrial production of TiO2. At pr esent, ilmenite is commonly
used in industry for making white pigment via a sulfate or chlo-
rine route having serious disadvantages, such as the treatments
of byproducts in the former and the lack of raw rutile minerals
in the latter. Fluoride processing of titanium-bearing minerals
can serve as an alternative. Ammonium hydrogen difluoride
(NH4HF2, solid, melting point is 126oC, boiling point is 240oC)
was recogni zed as versati le fluorinating agent for recovering of
titanium-containing raw materials [12,13]. It should be noted
that foundation of ilmenite processing with ammonium hydro-
gen difluoride was created by Svendsen as early as the thirties
[14,15]. The suggested methods comprised fluorination with
molten NH4HF2 followed by sublimation of fluoride titanium
compound but the detailed chemistry was not completely un-
derstood. Since the fluorination products are ammonium fluoro-
or oxoflluorotitanates, it is reasonable to consider them as pre-
cursors for the N–F–TiO2 obtaining.
Indeed, the N–F–TiO 2 nanoparticles of anatase crystalline
structure were recently prepared by a facile method of
(NH4)2TiF6 pyrolysis [16]. The synthesis of N–F-codoped TiO2
powders with a homogenous anatase structure via a thermal
decomposition of different ammonium oxofluorotitanate pre-
cursors at 550oC was reported [17]. Uniform ammonium oxof-
luorotitanate (NH4TiOF3) mesocrystals and their conversion to
mesocr ystals of TiO2 were descr ibed [1 8-20]. Titanium oxyflu-
oride TiOF2 was synthesized for obtaining of thermally stable
TiO2 of high photocatalytic activity [21] and for its use as
anode material for lithium-ion battery [22]. The synthesis of the
above precursors from natural ilmenite and investigation of
their physicochemical properties is the aim of presen t paper .
2. Fluorination of Ilmenite with NH4HF2
Interaction of ilmenite with NH4HF2 proceeds exothermally at
room temperature under grinding the initial components [23].
Similar reactions when two solids interact under mechanical
grinding with the formation of a new compound were being
studied by Indian authors since 1982 [24]. One should concen-
trate attention on nonstoichiometric composition of fluorinating
products due to some OH– groups substituting for fluorine since
water mol ecules are formed during fluor ination:
FeTiO3 + (5 –0.5y)NH4HF2
= NH4H2xFeOxF3 + (NH4)3Ti(OH)yF7-y + (1–0.5y)NH3
+ (3–x–y)H2O (x ≤ 0.3, y ≤ 0.4). (1)
The main titanium fluoride product is a double salt isostruc-
tural with (NH4)3TiF7 = (NH4)2TiF6∙NH4F which was isolated
in a single crystal form from fluoride aqueous solution. Its
crystal structure was determined. The parameters of tetragonal
unit cell were changed under X-rays, the stable phase is cha-
racterized by the following parameters: sp. gr.P4nc; a = 11.97,
b = 11.68 Å; z = 8. One of the three independent Ti octahedra is
diso rd ered so a ph ase tr ansi tion (P T) at abou t 280 K takes place.
Fe(II) forms a cubic fluoroperovskite type structure and easily
oxidized in air and in aqueous solution, and Fe(III) fluoride
compound crystallizes in cubic fluoroelpasolite structure. Its
octahedral single crystals of the (NH4)xFe(OН)3-xF2x (х =
2.70–2.85) composition were grown. Usually, natural ilmenite

N. M. LAPTASH, I. G. M ASLENNIKOVA
22
contains some Fe(III). We dealt with the real composition of
0.8Fe TiO3·0.1Fe2O3 and investigated carefully its fluorinated
process. The corresponding thermal curves are shown in Figure
1. An exoeffect at 125oC is evident.
The corresponding equation can be expressed as follows:
0.8Fe TiO3·0.1Fe2O3 + 4.4NH4HF2
= 0.8 NH4H0.4FeO0.2F3 + 0.2(NH4)2.8Fe(OН)0.2F5.6
+ 0.8(NH4)3Ti(OН)0.4F6.6 + 0.64NH3 + 2.18H2O. (2)
The endoeffect at 280oC corresponds to thermal decomposi-
tion of titanium double salt. Further effects are connected with
thermal behavior of (NH4)2Ti(OН)0.4F5.6 and ammonium fluoro-
ferrat es :
(NH4)3Ti(O Н)0.4F6.6 = (NH4)2Ti(OН)0.4F5.6 + NH3 + HF (3)
(NH4)2.8Fe(OН)0.2F5.6 = FeF2 + 0.17N2 + 2. 4 7N H 3
+ 3.6HF+ 0.2H2O (4)
NH4Н0.4FeO0.2F3 = FeF2 + NH3 + HF + 0.2H2O (5)
(NH4)2Ti(O Н)0.4F5.6 = NH4TiO 0.4F4.2 + NH3 + 1.4HF (6 )
NH4TiO0.4F4.2 = NH4TiO 0.4F4.2↑ (7)
FeF2 + NH4TiO0.4F4.2 = FeTiF6 + NH3 + 0.2HF + 0.4H2O (8)
FeTiF6 = FeF2 + TiF4↑. (9)
One should mention the evolution of volatile titanium fluo-
ride compound NH4TiO0.4F4.2 which sublimes incongruently
with the formation, probably, of the titanium adduct with NH3.
We succeeded in obtaining of single crystal of this volatile
compound and determined its chain structure (Figure 2). Infi-
nite chains of cis-con nected [TiF6]-octahedra are joint via NH4
groups by N–H···F hydrogen bonds with the average N···F
distance of 2.85–2.98 Å [25]. It is necessary to take into ac-
count that some Ti4+ is reduced to Ti3+by NH3 evolved, so we
used simple aqueous leaching of the cake to separate titanium
from iron. One can expect the formation of ammonium oxoflu-
orotitanate (NH4)3TiOF5 at this stage at pH = 7–8 [26]. This
compound is isostructural with iron fluoroelpasolite, their crys-
tal structures were d etermined [27].
Figure 1. Thermal curves of the mixture of ilmenite with NH4HF2.
3. Dynamic Orientational Disorder in Crystals of
Iron and Titanium Fluoroelpasolites
Classical cubic structure of A2BMX6 (A>B) elpasolite (Fm3m,
Z = 4) comprises a central atom M to be in the 4a position,
ligands in 24e, larger cations in 8c, and smaller cations in 4b.
NH4 in the latter position is accepted to be disordered on two
orientations. We advanced in the refining of this structure and
published recently the paper on this subject [28]. In fact, the
ligand atoms are distributed on mixed 24e + 96j positions, and
ammonium group in the 4b position is distributed on the 32f
position taking 8 equivalent orientations. Ammonium groups in
the 8c position are tetrahedrally shifted into the 32f position. In
the Ti oxofluoroelpasolite, a central atom is disordered on 6
orientations. The Ti atom is shifted towards the O atom with
the formation of short triple Ti–O bond that allows to determine
the real geometry of TiOF5 octahedron. Figure 3 presents dis-
ordered structure of the discussed elpasolites.
The observed disorder has a dynamic nature that the NMR
data support (Figure 4). Two phase transitions at lower tem-
perature are evident. The M2 jump at 265 К coincides with the
temperature o f phase tran sition (PT) detect ed by the di fferential
scanning microcalorimetry method (DSM). The rather large
value of entropy change ∆S at this PT (18.1 J mol-1 K-1 or Rln9)
characterizes this first order PT as of order-disorder type [29].
High anionic and cationic mobility is reflected in thermal beha-
vior of this complex. The easy transfer of hydrogen from ammo-
nium group to the O atom emerges in the IR spectrum as the
appearance of strong hydrogen bond of the O–H···F type at
700–800 cm-1. As a result, only NH3 and H2O, but no HF
evolve during the thermal decomposition of the compound.
Figure 2. Crystal structure of NH4TiOxF5-2x (x = 0.15): sp. gr.
P21/n, a = 14.683, b = 6.392, c = 20.821 Å; α, γ = 90 o, β = 110.538o,
Z = 16.
Figure 3. Disordered crystal structure of (NH4)3Fe F6 or (NH4)3
TiOF5.

N. M. LAPTASH, I. G. M ASLENNIKOVA
23
4. Thermal and Hydrolytic Properties of
Ammon iu m Fluo rometallates
Thermal behavior of ammonium oxofluorotitanates were ex-
amined [30]. Thermal curves of the (NH4)3TiOF5 decomposi-
tion are presented in Figure 5. The corresponding reactions can
be expressed as follows:
2(NH4)3TiO F5 = (NH4)2Ti F6 + (N H 4)2TiOF 4 + 2NH3 +H2O (10)
3(NH4)2TiO F4 = (NH4)2Ti F6 + 2NH4TiOF3 + 2NH3+ H2O (11)
4NH4TiO F3 = (NH4)2Ti F6 + 3( NH 4)0.3TiOF2 + 0.15N2
+ 0.8NH3 +H2O. (12)
The process is accompanied by sublimation of volatile tita-
nium compound and by the formation of hexagonal ammo-
nium-containing TiOF2.
Hydrolysis process of volatile ammonium fluorotitanate,
NH4TiOxF5-2x, is p ractically important. Its aqueous solution has
an acid reaction meaning the strong hydrolysis [31]. According
to 19F, 17O, and 49Ti NMR data, dimers with bridging OH or
even trimers ( cyclic or lin ear) are formed:
NH4TiO0.2F4.6 + 0.5H2O = 0.25(NH4)2[TiF6]
+ 0.1(NH4)3[Ti2(OH)F10] + 0.15(NH4)1.3H1.7[Ti3(OH)3F12]
+ 0.025H2[Ti4( OH) 6F12], (13)
Oligomerization (polymerization) is the main feature of py-
rohydrolisis of ammonium fluorotitanates and fluoroferrates.
Kinetic curves show that it takes about 40 min to convert am-
monium fluorometallates to oxides [32,33]:
Figure 4. Temperature dependence of the second moment (M2) of
the 19F NMR spectrum of (NH4)3TiOF 5.
Figure 5. Thermal curves of (NH4)3TiOF5.
(NH4)2Ti(OH)xF6-x → NH4TiOF3 → (NH4)0.8TiOF2.8 →
(NH4)0.3TiOF2.3 → TiO2; (14)
(NH4)xFe(OH)3-xF2x → (NH4)1-хF e(OH)yF4-х-y →
(NH4)yFe(OH)xF3-x+y → Fe2О3. (15)
Using the data obtained we suggested py-
ro-hydro-metallurgical method of ilmenite processing [34]
which comprises the fluorination of ilmenite with NH4HF2 at
20–200 oC followed by a simple aqueous leaching of the cake.
Combination of hydrolysis and pyrohydrolysis processes gives
pigmentary Ti and Fe oxides. We tried to burn volatile ammo-
nium fluorotitanate in an oxygen atmosphere at 1000oC and
obtained N–F-doped TiO2 with rutile structure. Crystals with
splendid color like sapphire were grown. They are good UV-
and visible light absorbers. We suspect th at sel ectin g condit ion s
of hydrolysis (pyrohydrolysis) or/and pyrolysis of ammonium
fluorotitanates, it will be possible to design nanosized
N–F-doped TiO2.
Thus, to obtain useful product from natural raw materials
using NH4HF2 we should to take into account that thermody-
namically possible fluorination reactions proceed spontaneous-
ly (exothermally) with the formation of high symmetry phases
of ammonium fluoro- and oxofluorometallates. The essence of
high symmetry is dynamic orientation disorder of both ammo-
nium groups and anionic polyhedra.
Under dynamic disorder, it is possible to identify O and F
atoms on local scale by common X-ray diffraction and to find
the real geometry of oxofluoride polyhedron. Orientational
dynamic disorder is responsible for PTs at lower temperatures
which p roceed with rather large ∆S and are characterized as PT
of order-disorder type.
Oligomerization is the main feature of thermal and hydrolyt-
ical decomposition of ammonium fluoro- and oxofluorometal-
lates which can be used for designing perspective functional
materials doped by F and N atoms.
5. Acknowledgements
We thank Dr. A.A. Udovenko, Prof. I.N. Flerov, Prof. V.Ya.
Kavun, Prof. S.P. Gabuda and Prof. V.K. Goncharuk for their
help and useful discussion.
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