Synthesis of Fe-Ti(O,C,N) Nanosized Composite via Reduction of Ilmenite by
Methane in Nitrogen Thermal Plasma Flow
A.V.Samokhin1, N.V.Alexeev1, M.A.Sinaiskiy1, A.A.Fadeev1, Y.V.Tsvetkov1 , A.V.Kolesnikov2
1
Institute of Metallurgy and Material Science, Moscow, Russia
2
Tshwane University of Technology, Pretoria, South Africa
Abstract: Thermodynamic calculations revealed the equilibrium compositions in the FeTiO3 + а
CH4 + b N2 mixture. Ilmenite concentrate was reduced by methane in nitrogen thermal plasma,
generated by 25 kW torch. The experiments proved the possibility to produce Fe (Fe3C) –
Ti(O,C,N) nanopowder. The physical and chemical characteristics of the nanopowders were
studied. The influence of the process operational parameters on the nano-sized fraction properties
were investigated.
Keywords: synthesis, reactor, DC arc plasma torch, nanopowders, titanium carbonitride, ilmenite.
1 Introduction
Fe-TiC composite is increasingly used for
development of new wear-resistant materials and
coatings. The composite can be produced by hightemperature carbothermal reduction of natural mineral
ilmenite FeTiO3. Process intensification can be achieved
by FeTiO3 heating in plasma flow. Nano-sized composite
Fe – TiC can be used to create nano-structured materials
and coatings. Also composite can serve as minor
alloying treatment for refining grain size and metal
alloys modification. Current contribution reports the
results of experimental and theoretical investigations
aimed at production of nanocomposite Fe - Ti (C, N) by
the reduction of particulate ilmenite in a reactor with
confined nitrogen DC plasma flow using methane as
reductive agent.
2 Thermodynamic analysis
Thermodynamic equilibrium modelling software
ТERRA [1] was used to calculate the equilibrium
compositions and thermodynamic characteristics in the
following reactions:
FeTiO3 evaporation in neutral atmosphere, (1)
interaction FeTiO3 + а CH4 + b N2
(2)
Coefficients a and b in reaction (2) were varied as a
= 2-3.5, b = 30 and 100, which corresponded to the real
experimental conditions.
The calculations were carried out in the temperature
range T=3160–3800 K, step 20 K (reaction (1)) and in
the temperature range T=400–4000 K, step 50 K
(reaction (2)). Total pressure was equal to P=0.1 MPa.
Titanium compounds TiO, TiC and TiN have
similar crystal structure with NaCl type lattice. They can
form indefinite sequence of ideal solutions TiO – TiC –
TiN (titanium oxycarbonitride), where non-metal atoms
substitute each other. Due to absence of thermodynamic
properties of such solid solutions in literature and in the
database supplied with TERRA, the titanium
oxycarbonitride was approximated as ideal solid solution
TiO – TiC – TiN.
Calculations of ilmenite evaporation in the neutral
gas medium have shown that the evaporation takes place
at the temperature, exceeding 3220 K. Up to the
temperature 3360 K the gas phase composition forms
due to evaporation of FeO ( Fig. 1). Titanium dioxide
TiO2 dissociate at the temperature close to 3220 K
forming oxygen according to the overall reaction
4 TiO2 ---> Ti4O7 + 0.5 O2, (3)
At the temperature T= 3400 K titanium oxide Ti4O7
is transformed into Ti3O5
3 Ti4O7 ---> 4 Ti3O5 + 0.5 O2 (4)
Fig. 1. Ilmenite components conversion to gas
phase as function of temperature, 1 – Fe, 2 – Ti.
The significant formation of the gaseous
components during evaporation occurs at the
temperatures above 3350 K. The complete ilmenite
evaporation is reached at the temperature 3780 K.
The calculations of equilibrium in the reaction
between ilmenite, methane and nitrogen (reaction 2)
demonstrated that TiOxCyNz yield equal to 100 % is
obtained at а > 2.5 for b = 30 and 100. The expansion
of temperature interval where titanium oxycarbonitride
yield is equal to 100% is observed with methane
concentration increase.
The produced powders deposited on the inner watercooled surface of the plasma reactor as fluffy layer. The
layer was periodically removed from the surface into the
powder collector installed at the lower end of reactor.
Some powder was transferred to the filter.
The produced powders were analysed by the
following methods:
X-ray diffraction (XRD) analysis was done using
difractometer
RIGAKU
Ultima
– 4 with
monochromatic CuK
radiation and high-speed
detector D/teX, PDXL software and PDF-2 database;
Specific Surface Area (SSA) measurements were done
using Micromeritics TriStar 3000 porosity analyser
particle size distribution (both raw powder and
synthesized powder) were measured using laser
difractometer Mastersizer 2000
particle morphology was studied using Helios 650
NanoLab (SEM + EDX) with Apollo X SDD analyser.
Final carbon content in the produced powder was
measured by LECO RC-412 analyser.
Final nitrogen content in the produced powder was
measured by LECO ТС-600 analyser.
Fig. 2. TiOxCyNz yield and composition as function of
temperature at a = 3, b = 100.
At the maximal methane content ( а = 3.5) the
titanium carbonitride TiOxCyNz shows 100% yield in the
temperature ranges 1350 – 2500 К (for b = 30) and 1250
– 2500 К (for b = 100). When the temperature increases
from lower to the higher values in the abovementioned
ranges, the composition of TiOxCyNz changes from
predominantly titanium nitride ( z = 0.95 – 0.98) to
titanium carbonitride ( y = 0.4 – 0.45, z = 0.6 – 0.55).
Oxygen content is relatively low and is equal to x = 0.01
– 0.02.
In the temperature range corresponding to the
TiOxCyNz presence in the system, iron is completely
reduced to metallic Fe and carbide Fe3C. With increase
of initial methane content the amount of Fe3C grows and
at а = 3.5 (maximum methane excess) the concentration
of Fe3C is dominant.
3 Experimental setup, synthesis conditions and
product characterisation methods
The ilmenite carbothermal reduction was carried out
in the plasma reactor based on a DC arc plasma torch
with the input power up to 25 kW (see Figure 3).
Ilmenite concentrate powder had the following
chemical composition (mass. %): TiO2 – 65.6 %, SiO2 1.5%, Al2O3 – 2.5 %, FexOy – remaining. Concentrate
particle size was less than 63 microns and less than 5
microns. Also microgranules (granule size less than 63
micron) were used as another type of raw material. Each
microgranule consisted of concentrate particles with
average size 1 micron.
Fig. 3. Experimental plasma setup.
4 Results and discussion
The experimental conditions were as follows:
Process parameter
Range
1 Power in plasma jet, kW
5.5 – 10
2
Plasma gas flow rate N2,
m3(STP)/h
1.3 – 1.8
3
Plasma jet full enthalpy, kWh/m3
3. 3 – 7.2
4
Raw ilmenite flowrate, g/min
1.1 – 9.2
5
Carrier gas composition
N2, N2 + СH4
(0 – 73 vol.%)
6
Carrier gas flowrate, m3(STP)/h
0.21 – 0.4
7
Metahe/ilmenite flow rates ratio,
m3(STP)/kg
0.2 – 0.96
The products of ilmenite reduction in plasma
reactor have polydispersive composition, including
nanoparticles and micron-sized particles as well (see
Figure 4).
Particles of micron-sized fraction have complex
structure and are represented by hollow spheres. (Fig. 4).
Inside hollow spheres the smaller size spherical particles
could be observed. Besides spherical particles, the
irregular shaped particles were observed. The irregular
shapes were formed by sintering of the submicron
particles. The formation of the hollow spheres could have
been caused by the sequential evaporation of iron oxides,
followed by evaporation of titanium oxides (as follows
from thermodynamic calculations).
According to XRD analysis, the micron-sized
particles consist of titanium oxide phases, and complex
iron-titanium oxides. EDS results have shown that iron
content in complex oxides is significantly lower. The
mass fraction of micron-sized particles in the products
varied from 45 to 17 mass. % depending on the dispersity
of the raw ilmenite concentrate.
The yield of micron-sized fraction can be minimized
by using granulated ilmenite concentrate, containing
particles with average diameter of 1 micron. The worst
results (high percentage of micron-sized fraction in
products) were obtained using larger ilmenite concentrate
particles with average size below 5 microns. Most
probably, some aggregation and formation of
agglomerates could have taken place in the two-phase
flow inside the pipeline transporting the raw material.
The following melting of agglomerates probably took
place.
Nano-sized fraction in the produced powder
consists mainly of the spherical particles (Fig. 4). The
fraction morphology stays almost the same for all
investigated experimental conditions.
Nano-sized fraction consists of particles with
various shapes - equiaxial (including spheres), plate-like
shapes and needle-type shapes. The variety of shapes
could exist due to various chemical composition of the
particles. Iron and titanium oxycarbonitride with cubic
NaCl type lattice are the main phases in the nano-sized
fraction. The nano-sized particles were formed as result
of condensation from the gas phase. The specific surface
area of nanosized powders varied in the range 12-22
m2/kg when methan/ilmenite flow rates ratio was
changed in the range 0.27 – 0.96 m3/kg. As XRD analysis
results indicate, the variation of the methan/ilmenite
flow rates ratio from 0.27 to 0.49 m3/kg leads to
formation of TiN with cubic lattice.
Fig. 5 XRD of nano-sized fraction.
Fig. 4 Produced powder: micron-sized fraction and nanosized fraction.
The
nano-sized
fraction
produced
when
methan/ilmenite ratio was equal to 0.49 m3/kg consists of
TiN, iron and unconverted ilmenite. Chemical analysis
results show that the nano-sized fraction consists of 39
mass. % Ti, 30 mass. % Fe, 3.2 mass. % C and 4.2 mass.
% N. The products were treated by acid solutions in
order to remove iron and iron compounds. After acid
treatment the XRD analysis showed presence of TiN,
Ti3O5 and FeTiO3 (Figure 5).
The presence of titanium oxides in the product after
acid treatment can be explained by incomplete
evaporation of raw material particles in plasma stream.
Conclusion
The goal of current work was to investigate both
theoretically and experimentally a possibility to produce
the nanosized composite Fe – Ti(O,C,N) by reduction of
disperse ilmenite with methane in the thermal plasma
reactor with confined nitrogen flow. The nitrogen
plasma flow was generated by the 25 kW DC arc plasma
torch .
Equilibrium
compositions
and
thermodynamic
characteristics of reactions products were calculated.
Two reactions were considered: evaporation of FeTiO3 in
neutral gas media (Ar) and interaction FeTiO3 + a CH4 +
b N2. The temperature range was taken as 400-4000 K,
and total pressure was equal to 0.1 MPa. Titanium
oxycarbonitride Ti(O,C,N) was modeled as ideal solid
solution TiO – TiC – TiN. The dependence of TiOxCyNz
equilibrium yield and (x+y+z=1) composition upon
temperature and initial ratio CH4/FeTiO3 were
determined. Also thermodynamic calculations revealed
the amount of reduced metallic iron content in the
system as function of system temperature and ratio of
raw reagents. It was found that 100 % TiOxCyNz yield
can be achieved when coefficient a in the reaction (2) is
greater or equal to 2.5. The increasing methane content in
the raw reagents mixture gives broadening up of the
temperature range where titanium oxycarbonitride has
100 % yield. When temperature increases from lower end
to the higher end of the abovementioned 100% yield
range, the chemical composition TiOxCyNz varies from
essentially titanium nitride ( z = 0.95 – 0.98) to titanium
carbonitride ( y = 0.4 – 0.45, z = 0.6 – 0.55). At the
same time the oxygen content is quite small and it is
equal to x = 0.01 – 0.02 .
The experimental investigations of dispersive
ilmenite concentrate reduction by methane in nitrogen
thermal plasma jet were carried out. In different
experiments the used ilmenite particles have average size
less than 50 microns or less than 5 microns. Also
ilmenite/carbamide granules with granule size less than
63 microns were used. The ratio methane/ilmenite was
varied in the experiments. The obtained products are
represented by polydispersive powders. The mass
fraction of sub-micron particles and micron-sized
particles in the product has reached 85 %, and specific
surface was equal to 12 – 22 m2/g. The fraction
containing sub-micron and micron-sized particles was
separated from the produced powder by gravitational
sedimentation in liquid.
The relative amount of the fraction containing submicron and micron-sized particles increases with
decrease of the raw material particle size. The submicron fraction consists of separate particles with
different shapes. According to XRD results, main phases
are iron and titanium oxycarbonitride with NaCl type
lattice. These particles were formed as result of
condensation process in gas phase.
Micron-sized fraction consists mainly of true
spherical particles. Many hollow spheres were observed
amongst these particles. The formation of hollow spheres
could occur as result of consecutive evaporation of iron
oxides and titanium oxides from raw ilmenite particles.
This conclusion is based on the findings of
thermodynamic calculations.
The achieved experimental and theoretical
results prove possibility to produce nanopowders having
chemical composition Fe – Ti(O,C,N) by reduction of
disperse ilmenite with methane in thermal plasma
reactor.
The work was supported by Russian Fund for
Fundamental Research (Grant 13-03-00733) and by
President of Russian Federation Grant Council for
support of leading scientific schools (Grant NSH854.20.12.3).
References
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Symposium "Combustion and plasmachemistry", 24 – 26
August 2005. Almaty, Kazakhstan.
Kazakh State
University, pp. 52 – 57, 2005.