MAGNESIUM REDUCTION OF TITANIUM TETRACHLORIDE
491
In assessing the rate of the reaction between TiCl4 face coverages, the density of active sites is almost con-
stant. Under these assumptions, the reaction is first-
order, and its rate is proportional to the TiCl4 concentra-
vapor and a clean surface of molten magnesium, we
relied on the theory of rate processes (steady-state the-
ory) [2]. According to this theory, for a chemical reac- tion in the gas phase.
tion to occur, not only the reactant molecules should
It follows from our calculations for typical condi-
approach one another closely enough but also the
parameters of the molecules in the gas phase should
match in a certain way those of the condensed phase.
tions of industrial processes that, if the only reaction
were the one between TiCl4 molecules and a clean sur-
face of liquid magnesium, it would take 2450 h for the
TiCl4 pressure in the reactor to decrease from 105 to
This theory makes it possible to assess the probabil-
ity and effectiveness of the collisions of gaseous mole-
cules against the surface of the condensed phase and to
evaluate the barrier heights and reaction rates involved
using data on the geometry and properties of the reac-
tant particles (TiCl4 molecules and Mg surface).
0.9 × 105 Pa. Because of the decrease in the density of
active sites on the magnesium surface upon the forma-
tion of Mg–Cl bonds, the rate of the reaction between
TiCl4 and the surface of molten Mg should be even
lower. Taking into account the other types of collision
(the surface impacted by a tetrahedron edge or vertex)
reduces this period of time by about one order of mag-
nitude, but still the initial reaction rate is much faster
than the actual one.
Consider the interaction between titanium tetrachlo-
ride molecules and the surface of liquid magnesium.
The TiCl4 molecule has the shape of a tetrahedron, with
the Ti atom residing in the center position and the Cl
atoms sitting in the vertices. The Ti–Cl distance in the
TiCl4 tetrahedron is 2.18 Å, and the angle at vertex is
109°, so that the Cl–Cl distance is 3.6 Å. Crystalline
magnesium has a hexagonal structure, with an Mg–Mg
bond distance of 3.2 Å. In molten magnesium, the inter-
atomic distance is 15–20% larger.
In the initial stages of the process, a more important
part is played by the gas-phase reactions between tita-
nium chlorides and Mg vapor. The predominance of
gas-phase reactions is evidenced by the fact that, in the
initial stages of the exothermic reduction process, the
temperature is maximal some distance away from the
magnesium surface [3].
The full structural match (necessary for chemical
interaction) between the TiCl4 molecule and the surface
of liquid magnesium is only achieved if one of the faces
of the TiCl4 tetrahedron (equilateral triangle with a side
length of 3.6 Å) is parallel to the liquid surface and the
three Cl ions are situated directly over three Mg atoms.
Since the probability of triple collisions is low, the
gas-phase reactions yield lower titanium chlorides,
magnesium chloride, and magnesium subchloride. The
rate of reaction between TiCl4 molecules and Mg vapor
was calculated for the processes
It is only in this configuration that a collision of a
TiCl4 molecule against the Mg surface may result in the
formation of three bonds, leading to strong chemisorp-
tion and a reaction between titanium tetrachloride and
magnesium.
TiCl4 + Mg = TiCl3 + MgCl,
TiCl4 + Mg = TiCl2 + MgCl2.
The activation energies and rates of these reactions
are difficult to assess theoretically [4]. Semiempirical
estimates led us to conclude that their rates are rather
high. At 1000 K, the rate-limiting process is magne-
sium vaporization. The formation of stable Ti nuclei as
a result of gas-phase reactions appears unlikely because
there is no uncombined titanium in the gas phase,
despite the high supersaturation with Ti.
The probability of such a collision is extremely low.
All the other types of collision (with a vertex or edge
coming in contact with the surface) are more likely but
do not ensure the formation of three bonds and are,
hence, less effective. The low probability of effective
collisions and the low effectiveness of the other colli-
sions account for the slow rate of the reaction between
TiCl4 and the Mg surface.
The formation of a liquid magnesium chloride film
on the Mg surface impedes Mg vaporization, thereby
reducing the rate of gas-phase reactions, as evidenced
by the fact that the maximum in temperature shifts to
the Mg surface.
The calculation results lead us to conclude that a
clean surface of liquid magnesium is essentially nonre-
active with titanium tetrachloride. Therefore, to explain
the low initial reaction rate, there is no need to assume
that the Mg surface is covered with a film preventing
chemical interaction, as was done in many studies.
The reactions occurring in the liquid magnesium
chloride film by the vapor–liquid–solid mechanism
seem to play a key role in the deposition of spongy tita-
nium on the reactor wall.
In assessing the rate of the reaction between tita-
nium tetrachloride and a clean surface of liquid magne-
sium in an industrial reactor, the following assumptions
were made: All the sites on the magnesium surface are
active (unoccupied); if an incident TiCl4 molecule has
That a chloride film on the metal surface may not
only retard but also substantially accelerate interaction
one of its faces parallel to the surface (full structural between the metal and gas phase was shown by measur-
match), the collision effectiveness is 100%. At low sur- ing the temperature-dependent rate of reaction between
INORGANIC MATERIALS Vol. 38 No. 5 2002
Evdokimov
