EFFECT OF SOLID REAGENT DISPERSION
IN HETEROGENEOUS CHEMICAL REACTIONS
S. P. Bogdanov,
B. A. Lavrov,
Yu. P. Udalov,
and A. M. Germanskii
Translated from Novye Ogneupory, No. 2, pp. 42 – 49, February, 2016.
Original article submitted November 11, 2015.
A new type of heterogeneous process is considered, i.e., a solid – liquid type self-accelerating heterogeneous
reaction. For these processes there is a typical development stage of a progressive increase in reaction surface,
caused by solid phase dispersion. Steel corrosion in hydrochloric acid, reaction of boron nitride with molten
alkali, molten silicon phosphate with carbon reducing agent, hematite with molten silicate, and molten iron
with hematite ceramic, are studied. The effect of solid phase dispersion as an independent stage of chemical
reaction is confirmed by experiment. Possible process mechanisms are suggested.
Keywords: heterogeneous reaction, dispersion, reaction mechanism, reaction kinetics.
Heterogeneous reactions play an important role both in
chemical production, and in natural processes, for example
corrosion, in geological processes, etc. A feature of heteroge-
neous processes is the fact that the chemical reaction cannot
proceed at any point of a reaction space, and the act of reac-
tion occurs within a zone of small thickness at a phase inter
face. A phase interface is quite nonuniform with respect to
reaction capacity. It is well known that at the surface of solid
crystalline substances a plane with different crystallographic
orientation exhibits different activity. In turn, at crystal
planes with the greatest active centers there are surface de
fects, including dislocation outlets and block boundaries.
The whole heterogeneous process is multistage in nature.
In analyzing kinetics for these reactions there is separation of
outward diffusion, inward diffusion, and kinetic stages .
With an outward diffusion regime a diffusion layer,
through which liquid or gaseous reaction component enters,
is controlled by the action of a number of forces: temperature
and concentration gradients, gravitation, turbulent melt flow,
and molten liquid bubbling with gas bubbles. Reaction, lim
ited by outward diffusion processes with participation of
melts, concerns a number of hydrodynamic uncertainties .
Mass transfer is accomplished by vortex pulses of compara
tively small volumes of liquid or gas , and the role of a
contact surface for reacting phases in the resultant process
rate becomes secondary.
Inward diffusion and kinetic stages are connected with
surface chemical reaction. If all reaction surface is accessible
to liquid or gaseous reagent, then reaction rate V
, i.e., overall
amount of reagent converted in a unit of time, is proportional
to the reaction interface are S:
where k is proportionality coefficient.
Consequently, the rate of these processes depends mark
edly on reaction surface size and shape.
In the majority of theories it is assumed that reaction
commences at a whole surface and spreads by a continuous
front over the normal to a surface at a constant linear rate. A
change in reaction surface area in time for a heterogeneous
reaction is either not considered (model of constant surface
area (Fig. 1a )), or it is considered that the area of reaction
surface in reaction development decreases (model of a com
pressed sphere or figure of another shape (Fig. 1b )) . In
more detailed and complex models it is possible to consider
the difference in reaction rate at different particle (crystal)
faces, the change in time of proportionality coefficient k in
the equation above, and the polydispersed composition of a
It is well known that for kinetic curves of heterogeneous
processes a sigmoid form is typical. The reaction rate in the
Refractories and Industrial Ceramics Vol. 57, No. 1, May, 2016
1083-4877/16/05701-0085 © 2016 Springer Science+Business Media New York
FGBOU VPO St. Petersburg State Technological University
(Technical University), St. Petersburg, Russia.