Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
`" 1047744
The present invention relates to a process for
the production of ceramic products and particularly of wall-tiles
or floor-tiles.
The conventional process used for the production of
tiles is a double-firing process wherein the body of the tile,
after having been formed by pressing the raw material, or in any
other suitable way, is fired as such is subsequently glazed on ~--
one of its faces, the obtained glaze then being fired.
This conventional process, which is used for a wide
range of ceramic products, provides finished products of the
desired quality but is obviously time consumming and expensive.
Attempts have been made to replace it with faster and cheaper
processes, particularly by so-called single-firing processes in ---
which the ceramic body is coated with glaze, thereafter eventually
dried, and the glazed body is subsequently fired in a single-
phase. -
This single-phase firing processes are more critical and
difficult than the conventional process and have been carried out
more or less successfully according to the raw materials used
and the shape and nature of the requested finished product.
However, these do not always afford the desired savings, for
instance in cases in which the single-firing requires a considera-
ble length of time and produces a poor quality product or a high
rate of rejects.
A relative fast single-firing is feasible, as well
known in the art, by using certain raw materials, especially
batches with a high content, for example 70%, of talcum and ~ -
wollastonite.
However, such a fast single-firing is economically
advantageous only in some specific areas, like the United States,
but not is most areas like Europe, South America and Asia, where,
for economic reasons and according to the availability of
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materials, there is required the use of natural clay bodies or -
depending on the areas - of bodies firing with clay, kaolin, cal-
cium or magnesium carbonates, silica and feldspathic materials and
other natural components as well as pre-treated or synthetic
materials.
The present invention relates to the production of ti-
les made from materials or bodies which will be referred to in
the following specification with the term "clay materials" it
being understood that materials containing high percentages (50%
or more) of talcum and wollastonite, as well as materials contain-
ing addition of clay as binder, are excluded from the above term.
The "clay materials" may contain raw clays and/or
pre-fired clays, a certain percentage, at least about 40% by
weigth of raw clay, being necessary to confer to the material the
desired mechanical properties, and they must not contain, as
mentioned above, substantial amounts of talcum and wollastonite.
As far as the rest is concerned, they may contain various mate-
rials, such as siliceous or feldspathic sand and alkaline-earth
metals, iron compounds and others.
The composition should be preferably such that the sum
of magnesium and calcium oxides does not exceed much more than
20% by weight (a percentage of 30%, no doubt, should be excessi-
ve) and such that the percentage of aluminium oxide contained
be within the same percentage. Even more preferably the composi-
tion may contain an amount of iron oxide of a few units per cent
by weight.
The present invention proposes single-firing process
for the production of perfect tiles in an exception short time,
as was never achieved heretofore and consequently with substantial
savings. The tiles are made of "clay materials", as specified
before, thus resulting in a final
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product of quality, of regular geometric shape and free of defects.
The process according to this invention comprises
the steps of preparing a raw tile consisting of a dry, unfired
tile body with a dried glaze coating on one face of the body,
and with the body consisting predominantly of clay comprising
at least 40% by weight of raw clay, an aggregate calcium oxide
and magnesium oxide content of less than 30% by weight and alu-
minum oxide content of less than 30% by weight, and an iron oxide
content of less than 10% by weight; advancing the raw tile at a
speed of the order of 1.5 - 2 meters per minute through a treat-
ment chamber; in a heating step having a duration of 4 - 8 minu-
tes, contacting both faces of the tile body concurrently with
flowing hot gases as the tile body advances through the treat-
ment chamber to raise both faces concurrently and, by substan-
tially only direct convection heating, substantially to firing
surface temperature; in subsequent firing step having a duration -
of 11 - 20 minutes, contacting both faces of the tile body
concurrently with the flowing hot gases as the tile body continues -
to be advances through the treatment chamber to maintain both
surfaces substantially at the firing surface temperature concur-
rently, by substantially only direct convection heating, to fire
the tile body and the glaze of the raw tile; and, following
completion of the firing step, finally cooling the fired tile in
a cooling step having a duration of less than 12 minutes; whereby
the sum of the durations of the heating and firing steps is not
in excess of about 28 minutes and the overall heating, firing and
cooling time is not in excess of 40 minutes.
By the term "direct convection heating" it is meant
that the tile is heated by the moving hot gases, with exclusion
or wit~out determining presence of other types of heat transmis-
sion (such as the conduction from the supporting means on which
the tile rests and/or radiation from the walls of the treatment
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chamber. Obviously, this is accomplished when the hot gases
forms the only source of heat within the chamber and therefore
heat the tiles as well as the supporting means and the walls
(eventually other high temperature bodies being placed into
such a position as not to be able to directly contribute to
the heating of the tiles).
Usually, the advancing of the tiles with both faces
(which means the front glazed face and the rear face without tak-
ing into account the edges which are of relatively negligible
surface) substantially exposed to the direct transmission of
heat contact with underlying transporting supports, preferably
having
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a rotating surface and fixed axis, with each of which they come
into contact ideally along a generatrix
The feeding takes place at high speed, considerably
higher than the ~peeds adopted heretofore for materials of this
type, at the rate of 1.5 - 2 meters/minute (these figures not
being considered binding), such speeds being optimum for wall-
tiles and therefore 4 - 5 mm. thick, but may be slightly reduced
for thicker tiles.
The heat treatment chamber is temperature-controlled a~d
usually comprises a tunnel-kiln where in successive areas of the
same the tile~ go through the successive steps of heatiDg a~d
keeping at the firing temperature (or briefly firing, although
proper firing actually starts during the heating step) and subse-
quently cooled outside or eventually partially cooled iD the
treatment chamber.
The drying of the ceramic body is a common operation iD
the manufacturing of ceramic products and may be usually carried
out at a speed differing from that of the above mentioned opera-
tion or at the same speed when the dryer i~ an integral part
of the kiln. The same applies to drying of the glazing. In the
following description reference is made to a process wherein ~ -
dryi~g of the tile body i9 performed previously and separately
whilst drying of the glazing is carried out immediately prior to
heating and at the same speed; in this case it lasts a few
minutes, for example from 5 to i minutes, at suitable temperatures,
for example up to 200C, a~d under a~ adequate and intense gas
circulation.
When drying of the glazing is completed and the actual
heat treatme~t starts, the temperature of the tiles, according
3 to the present invention, is quickly raised up to the firing
temperature. The firing temperature, measured rear the tiles - as
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all the temperatures stated in this specification - may vary
quite substantially according to the compositions, but it is
generally comprised around 1000C, as for example between 800C
and 1200C, but more commonl~, between 1000C and 1100C, for
example around 1050C and 1060C.
~ he heating of the tiles is carried out, as referred to
above, in an environment where heat tranSmiSSioD takes place
by CoDVeCtion with gas flowing in counter-current to the tiles,
and besides heating the tiles, it heats also the walls of the
heat treatment chamber which are normally made of refractory
material and at times may become incaDdescent turning into a
light-red colour. I~ these conditions the temperature differeDce
between walls and tiles is always ~ept to such an amount that
heat exchanges by radiation do Dot have a determining effect.
It is important to dose heating in the heating zone, so
as to produce a quick rising of the temperature of the tile, the
o.nly upper limitatlon regarding the rapidity of heating, since
the material may be damaged by a too quick evolution of volatile
products by uDcontrolled thermal co~traction and expansion iD the
still raw tile and such a limit being set case by case. General-
ly, a heating.time of 4 - 6 minutes is considered adequate and
at this point it may be assumed that the tile has reached - at
least OD the surface - a temperature a few degrees lower than
chamber temperature.
During the actual firing step, the tile remains at a
practically constant surface temperature (even if not exactly
constant due to the fact that heat-exchanges st~1l continue an~
the temperature of the chamber is Dot exactly co~stant) and
conti.nues to advaDce in the firing zone which is at an almost
e~eD temperature tending to be lower at the start and at the
end due to closeness of to the heating and cooli~g Z0De
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respectively, and in gas counter-current. ~his step lasting
slightly over 10 minutes, for example between 11 and 15 minutes.
In the meantime the heat penetrates more deeply in the
tile body firing the inner layers of the tile. Finally, the
cooliDg step takes place in a shorter time slightly shorter than
the firing time for example slightly shorter than 12 minutes,
say 8 - 10 minutes and may be determined by a partly direct and
partly indirect cooli~g and occurs partially inside and partially
or entirely outside the chamber.
The times specified above by way of example are close to
optimum for wall-tiles, 4 - 5 mm. thick, and may be increased
with a variatio~ ratio generally not linear, for thicker tiles,
such as those used for flooring, which may even be 10 mm thick
and for which the length of time for heating may be for example
up to 6 - 8 minutes and for firing 17-20 minutes.
It has been found, according to the present inventioD
that the best compromise between the necessity of suitably
supporting the tile OD the generatrices of the fixed-axis rota-
ting supports for feeding, and the necessity of not introducing
an excessive difference in the thermal conditions of the two
faces of the tile, is obtained when the tile is kept in constant
~ contact with two or three of that geDeratrices.
i, This is obtained in the more common case of the support
rotating rollers, when the distance between the centers of the
roller is e~ual to half thelength of the tile itself. In such
a caæe it is ensured that the product obtalned is of regular
geometric shape and free from defects. However, it is possible
to modify to a reasonable extent this condition when in practice
it i8 necessary to produce with the same equipment, tiles of
~ varying dimensions.
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According to the present invention, the tile body is
formed of one of the bodies termed as "clay materials", as speci-
fied above. ~ypical materials form~ng said bodies are for
example: plastic t~pe clays; no fat or semi-fat clays; kaolins;
feldspars; dolomite and calcium carbonate; siliceous sand. The'
formulation of the glazing materials varies according to the
required product and to the type of support used and may be raw,
semi-raw or with fritted glazes.
I~ the carrying out of the present invention it must be
considered some physical properties of the material'as speoified
in the following. A temperature/weight diagram of the material
indicates a loss of weight with temperature increase according
to the development of volatile substances.
With the increase in temperature and with the tile body
kept at that temperature for some time, there appears first an
increase and next, iD the phase of specific interest, a loss
oi permeability of the tile body itself, which logically starts
on the outer surface exposed to a quicker increase of temperature.
~he ceramic material used must have a total weight'loss during
the eDtire process, which, although not ~ery low when the above
materials are dealt with, is however not excessive, for example,
not more than 15%. 'In addition, it must be able to keep substan-
tially its surface permeability at least up to a temperature
of 700 - 800~ and until there is no substantial weight loss,
as shown i~ the temperature/weight diagram, obviously under the
conditions of the process according to this invention, particu-
larly with the thermo-gravimetry gradients contemplated by the
invention. ~urther, the thermo-dilatometric behaviour of the
material, i.e. variation of linear dimensio~s with increase
in temperature, must be such as not to produce excessive diffe-
rence in dimensions between the two faces of the tile, which due
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to the unavoidable unevenness in heating, may not in fact be at
the same temperature at least during part of the process. Such
behaviour of the material may be illustrated by plotting a
thermo-dilatometric diagram.
By operating according to the invention, particularly
under the preferred and optimum conditions, much better results
are obtained, on an industrial scale with respect to those
obtained heretofore and actually beyond expectations. In the
ceramic art the attempts made heretofore to introduce single-
firing of tiles made of common materials which required a
firing time of a few hours and resulted in the production of
mostly defective tiles, led to believe that such results were
impossible. The speed increase, the lack of supports for the tile
and the drastic reduction of operating times are all factors which,
based on past technical experience, were supposed to be negative,
so that it was assumed tihat manufacturing of excellent products
from materials of the above mentioned type required operation un-
der gentler, more gradual and mechanically easier conditions than
those previously adopted and not the contrary. ~-
The fact that by making all operating conditions
simultaneously more severe, i.e. both speed and rapidity and
mechanical feeding conditions of the tile, the result obtained
was never achieved before even in conditions remotely approaching
those of the invention, represents in fact a remarkable techni-
cal achievement.
In the accompanying drawings:
Fig. 1 represents a tunnel-type furnace adapted for
carrying out the process according to the invention and
Fig. 2 represents an example of supporting and feeding
means for the tiles.
For actuation of the invention a tunnel-type furnace
running longitudinally on a single plane, as schematically
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illustrated in fig. 1, is best suited.
The detailed structure is not shown, as it is not
within the scope of the present invention, but generally
comprises, assuming that drying of the tile body is carried
e~e~ t~ly~
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a section 1 for drying of the glaze, a section 2 for the heating
step, a ~ection 3 for the firing step and a section 4 for the
cooling step.
The furnace is coDveniently heated by gas burners5 fitted
in the walls so as not to radiate on the tiles, as shown
schematically in fig. l, or by other suitable means, for example
electric heating, the furnace walls being adequately insulated.
In the case where gas burners are used, these will produce volumes
of combustio~ by-products which will be conviently moved iD
counter-current with respect to the tile, by means of suitable
suction devices (not shown) or other means so that gas masses
increase from the outlet to the i~let of the treatment chamber
and generally forthe ma~or portion of the chamber, usually in a
linear manner; and in the case of electric heating, air will have
to be introduced to provide such volumes.
The elimination of moisture in area 1 may re~uire a
separate circulation of hot gas, as showm schematically at 6
(gas inlet) and 7 (outlet) the burDers ~ot being provided iD this
area. Suitable means will be provided to convey hot gases in
counter-curreDt flow with respect to the tiles and suitable means
will thus be provided for loading and unloading of the tiles, as
showD schematically at 8, driving the rollers at the required
speeds and eventually for speed control, and so on.
Fig. 2 shows aD e~ample of a supporting and feeding means
for the tiles, two rollers 10 carried by shafts 11 supporting a
moving tile 12; the roller shafts being controlled by any suita-
ble means outside the furnace.
An example of carrying out the invention, illustrating
the production of a particul æ tile, will now be give~ by way
of example only, since the possibility of varying the composi-
tions based OD product~ available in nature for the industry is
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practically unlimited and those skilled in the art will be able
to actuate the inventior based on what has hereiDbefore been
described in the specification without departing from the true
scope of the invention, even with compositio.ns very differeDt
indeed.
The initial material is a mixture of "red beds" clays
having the following formulation: Ignit. lo~s O - 15%
2 ~0%~A1203 15 - 20%J Fe203 2- 8% CaO 1 - 10%
MgO 1 - 10%, ~2 1 - 6~o~ Na20 1 - 5%~ The percentages are by
weight.
The glaze used has the following composition: frits or
glazes made of alkaline-boro-silicates containiDg Pb, ~i, Ti,
Ba, Ca, Mg, Sr, Sn, Va, Zr; ceramic stains made of oxides or/
silicates or silico-aluminates of metals such as Fe, Co, Ni, Cr
III, Ti, Mn, Cu, Zn, Ba, 2r, Sn, etc.; various additives for
grinding such as kaolins aDd clays, Zr silicates; sodium-silicate,
-chloride and -carbonate.
The tiles are of two main types having dimensions of
150 ~ 150 x 4.5 ancl 200 x 200 x 9, respectively.
After pressing, drying and glazing the tiles are fed by
means of rollers, as shown in fig. 2, into an apparatus as
illustrated in fig. 1.
The feeding speed in the first case is 1.9 meters/
minute and in the second case 1.4 meters/minute. Drying of the
glaze lasts 5 and 6 minutes respectively, at a maximum temperature
of 200C. HeatiDg lasts 5 and 6 minute3 respectively. The time
required for firing is 13 and 17 minutes respectively and the
temperature measured in the chamber rear the tiles is 1060C
Cooling lasts 9 and 12 minutes respectively.
The tiles thus obtained are perfectly regular and meet
the required technical speclfications.
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