CAKING PROCESS OPTIMIZATION PRINCIPLES
L.Leontiev, S.Shavrin1, V.Gorbachev, G.Maizel 2
Ural’s Department of Russian Academy of Sciences, S.Kovalevskoi st. 4, 620219,
Ekaterinburg, Russia.
2
Scientific-industrial-inculcation enterprise TOREX, Studencheskaya st., 16, 620219,
Ekaterinburg Russia.
1
ABSTRACT
The model of caking processes activation is suggested. At its basis lies the caking
particles surface statement composite tense statement and their influence on the
diffusion processes in the system analysis.
In it is shown the possibility of the passing conditions of phase transformations, ions
with large radius microadditions on the strengthening as well as the weakening of the
cake. On these positions the way to solve the task of “useful” for the composite
structure and properties processes stimulation and braking of the undesirable ones is
formulated and suggested. The oxide systems caking on the example of iron oxides is
considered.
The main requirements to the optimal mode of caking process is the achievement of
the process maximal speed, the product shape preservation, receiving of the required
structure and service properties, physical-chemical processes passing completeness
etc. These factors, determining the caking quality, are mostly often interrelated.
However the unite optimization model building for each material has required the
consideration of caking technology separate aspects. For the shape preservation or the
product warping absence it is necessary to provide such heating and cooling mode in
which the acting tensions do not exceed the given material yield limit. That is why at
the foundation of the model lies the calculation of all the acting tensions (thermal,
structural and residual) during all the process and the comparison of their values with
the material mechanical properties temperature relationship, mostly with the yield
limit, proportionality and temporary tensile strength. If there is no such data for the
concrete material its caking optimization will require the experimental determining of
the mechanical properties temperature relationships.
One of the means to increase the caking speed is its activation. The “activity” notion
is not formally determined, and, though its mechanism is studied and is the mutual
particles motion, it is understood under it simply more speedy pressing thickening. In
this connection there were no approaches to the directed search for the caking
activation methods till the recent times. The caking activation principles have been
formulated by us. At their basis lies the directed influence both on the particles
surface structure and on the driving forces of their mutual motion. The first action is
expressed both in the powder receiving way and in special surface amorphous
rendering that may be carried out by choosing the conforming gas phase. The target
experiments, carried out on Fe3O4 powder pressings, have shown that small additions
of oxygen in gas phase (5-7%) increase the caking speed 4-5 times what conforms
with the model calculated conclusions. The important factors in the influence on
particles motion driving forces is the tense statement of the specimen. The energetic
analysis of both the first type tensions and micro tensions (the second type) allowed to
make the conclusion about the possibility of considerable increase of the caking
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speed. The particular interest is represented by the additions to powders which do not
influence the cake quality and properties, though the appearing due to them micro
tensions ( as a consequence of the big difference of heat expansion ratios) may
increase the shrinkage speed several times in the initial stages of caking. The next
caking activation principle is the choice of the conforming liquid phase (or eutectics).
The caking with liquid phase participation kinetics and mechanisms are studied and
fully enough described in literature, so, we will not stop at them, though they are
taken into account in the frame of generalized caking model. One more important
factor is the presence of phase transformations in heating and roasting process. In this
case not only tense statement(tens and hundreds times exceeding thermal tensions in
value) appears but radically changes the contacting particles division border structure.
Role of each of these factors is assessed individually for each concrete case. The
peculiarity of the phase transformations passing is crystal-chemical constants tensor
view. It is shown that activation is possible only in the case of symmetric tensor. If it
is asymmetric, the phase transformation presence may lead not to the caking
activation but to the weakening or even destruction of the caked specimen. So, at the
basis of active caking model of powder compositions lies the quantitative assessment
of the following interrelations:
Caking speed transformations
Liquid phase forming
Thermal tensions in heating
Thermal tensions of the second type in
multicomponental cakes
Polymorphemic
Phase transformations accompanied
by mass exchange processes
Particles shape and powder receiving methods
Selfactivation
The next requirement for caking quality increase is the optimal structure receiving.
For this purpose the mathematical model of forming of not only pore structure but
also different phases separation forms in the main substance matrix is developed. At
its basis lie the concepts of symmetry elements in crystal-chemical transformations.
At their foundation lies the definition of oriental-size conformity and symmetry
elements of the crystals undergoing the phase transformations. This allowed to
develop the views system and mathematical model for calculation of the deformation
value and character. Using the free energy minimization principles the relationships
allowing to determine the new phase separation form and to tie it with the process
passing conditions are received. Also the possibility of phase transformations passing
conditions influence both on strengthening and weakening of the whole cake. One of
the applications of this direction is the problem of dispersed strengthening of
compositions and alloys. Choosing the type and shape of particles of dispersed phase
it is possible to considerably increase the matrix service properties. At the basis of
such strengthening mechanism lies the grain borders and dislocations fixing in the
main phase. It is shown that the optimal particles shapes are: sphere – for metals, disk
– for carbides and needle (stylus) – for nitrides. On the basis of the model approbation
in practice the ways were shown and algorithms of disperse-strengthened
compositions structure optimization were developed to receive the required service
properties.
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From the already stated positions the caking conditions optimization required to solve
one more task – stimulating of “useful” to composition structure processes and
braking of undesirable ones. For example, the reduction process in the caked ferric
oxide may pass in two ways:
1.Fe2O3  Fe3O4
2. .Fe2O3  Fe2O3 Fe3O4
In the first case the sudden weakening and even destruction of the cake takes place
while in the second case the process is accompanied by insignificant durability
decrease. That is why the choice of temperature-time conditions of caking is rather
relevant also from these positions. Let us illustrate the abilities of the developed
model on the example of quartz ceramics, the basic difficulty in receiving of which is
the phase transformation of  -cristobalite into  -cristobalite in low (160-270C)
temperatures. Such a transition is accompanied by the volume asymmetric change of
about 5% what leads to the specimen weakening and even destruction. The developed
model allowed to optimize the heat treatment process of cheap quartz glass with the
help of heating speeds and time lag combination on intermediate stages of roasting.
Another application of the model is the directed search of micro additions allowing to
decrease, and in the range of cases “to eliminate”, the crystallization process. In
particular, it is shown that the most braking action is possessed by trivalent ions of
large sizes (for example, Ba3+ ).
The development of the model as such and its software is the task of the further
research.
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