Drying step in a method of producing ceramic articles

A method of drying green ceramic articles with a hot air stream in a drier, wherein the green articles, immediately or soon after entering the drier, are heated to 100.degree. C. while the moisture content thereof is maintained equal to that of the green articles at the beginning of heating step.

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement to the drying step utilized 
in a method for producing ceramic articles, earthenware, bricks or the 
like, wherein the drying is achieved by means of a stream of hot air in a 
continuous tunnel drier or in a static drier. 
The term "hot air" used in the rest of this description and in the claims 
refers to the medium for drying the ceramic articles. The medium can be 
preheated air, hot gases, hot combustion vapours or the like. 
A method for producing ceramic articles, such as earthenware, bricks or the 
like, substantially comprises the following steps in succession: moulding 
the clay material, drying the moulded articles and firing the articles 
after drying. 
The clay is moulded in conventional devices, e.g. presses or extruders, by 
a "wet" method using vapour if required, or by a dry method wherein clay 
is moulded using the moisture in the material itself, combined with 
adequate moulding pressure. The ceramic articles, irrespective of the 
manner in which they have been moulded, must be dried for a suitable 
period in order to eliminate most of the water mixed therewith or 
hygroscopically absorbed therein. This operation is necessary to ensure 
that the green articles become sufficiently compact and strong to 
withstand the subsequent manipulation and loading into the furnaces 
without being deformed and without cracking, shrinking or breaking, which 
they would tend to do if the water was removed too quickly from the green 
ceramic articles, i.e. if they were placed in the furnaces immediately 
after being moulded. 
The ceramic articles can be dried naturally or artificially. Natural 
drying, brought about simply by the atmospheric air around the ceramic 
articles, is not desirable due to the high cost of storing and taking out 
the articles and the fact that the production of finished articles is 
dependent on the weather. Artificial drying is brought about in static 
chamber driers or, more commonly, in tunnel driers. The medium used for 
drying the ceramic articles in the desired manner is a stream of hot air, 
generally circulated in counter-current relationship with the articles. 
The prior-art drying of ceramic articles has technical and economic 
disadvantages which have not heretofore been overcome. In order to produce 
the desired heat-exchange conditions and remove the boundary layer of 
vapour surrounding the ceramic articles, a considerable amount of air has 
to be conveyed inside the drier, and the hot drying air has to move at 
high speed. This results in considerable energy consumption required in 
driving the fans outside or inside the drier, in order to ensure the 
required flow rate of the hot drying air. 
Furthermore, the temperature difference between the hot drying air and the 
ceramic articles to be dried is still rather small. The reasons are as 
follows: 
(1) The temperature of the ceramic articles entering the drier is 
relatively low. 
(2) The heat exchange between hot air and the articles to be dried occurs 
in counter-current relationship, and 
(3) At the end of the drying process the articles can withstand only a 
small amount of surface heating, since they are in the phase during which 
water is diffusing in vapour form into a porous material and the volume of 
vapour is about 10.sup.3 times the volume of liquid water. Consequently, 
the ceramic articles will burst if they are surface-heated above a 
predetermined maximum value. 
However, drying is mainly due to heat exchange by forced convection, the 
maximum exchange being dependent on the speed and amount of hot air being 
circulated. 
Another technical disadvantage is that, at the temperature of the ceramic 
entering the drier (25.degree.-40.degree. C.), the water in the form of 
moisture in the green ceramic articles is very viscous and there is a high 
bonding force between the water and the clay constituting the ceramic 
articles. As a result, the surface of the articles is dried and shrinks, 
whereas the interior remains moist. The difference between the interior 
and the shrinking exterior of each ceramic article produces high tensions 
in the clay and may result in permanent deformation, microscropic cracking 
or breakage of the ceramic articles. The danger of micro-cracking is 
increased by the phenomenon known as thermo-osmosis, consisting mainly of 
migration of liquid water from the hot surface to an internal, colder 
region of each ceramic article being dried. 
The invention is based on the problem of providing a method of producing 
ceramic articles, earthenware, bricks and the like, wherein the step of 
drying the articles is carried out so as to simultaneously to obviate all 
of the disadvantages of the prior art, so that the ceramic articles can be 
dried in much shorter times than those at presently required in 
corresponding known methods. 
SUMMARY OF THE INVENTION 
To this end, according to the present invention, the ceramic articles, 
immediately or soon after entering the drier, are heated to temperatures 
which can reach also 100.degree. C. and, during the heating, the moisture 
content of the ceramic articles remains substantially equal to the 
moisture content of the articles at the beginning of the heating process. 
In a preferred embodiment of the invention, the ceramic articles are heated 
to a temperature between 40.degree. and 100.degree. C. (the dew point) in 
an environment at a high partial water pressure. 
More particularly, the ceramic articles are heated to a temperature between 
40.degree. and 100.degree. C. (the dew point) by a stream of hot air at a 
temperature of 120.degree. to 400.degree. C., having a moisture content 
between 48 and 850 g per kg dry air. 
In the case where the ceramic articles are dried in a tunnel drier, 
according to another feature of the present invention, the articles are 
heated within the aforementioned temperature range by the aforementioned 
stream of hot air, which is supplied to the drier in co-current 
relationship with the ceramic articles. 
When the ceramic articles are heated in an environment at a high partial 
pressure of water vapour, the result, due to the known cold-wall effect, 
is that the water vapour condenses on the surface of earth article, thus 
giving up heat of condensation which uniformly heats the ceramic article 
over its entire surface and inside, before the water begins to evaporate 
from the surface of the article. The reason is that, during the first step 
of the method, the flow of heat entering the article is greater than the 
flow of heat leaving it, and this increases the temperature of the 
article. This is due to the high partial pressure of water and the 
consequent high coefficient of heat exchange at the place where heating 
occurs. The exchange coefficient is 10 to 100 times greater than in 
conventional hot-air driers. 
During the process of drying the ceramic articles, control of the partial 
water pressure and temperature control are fundamental factors in 
balancing the flow of heat entering each ceramic article with the flow of 
water or vapour leaving it, which are dependent upon the various drying 
steps or on the characteristics of the ceramic article, e.g. its porosity, 
shape, thickness or mechanical strength. According to the present 
invention, the aforementioned control is brought about not only by varying 
the amount of air or fuel burnt inside or outside the drier and/or by 
recycling the exhaust vapours from the drier but more particularly by 
injecting vapour and/or water, atomized if necessary, into the drier in 
order to obtain the aforementioned desired temperature and humidity 
conditions of the drying air. 
In static driers, the aforementioned controls are brought about by 
injecting vapour and/or finely atomized water into the combustion products 
in an appropriate fuel burner. In continuous tunnerl driers, the vapour or 
atomized water is injected only during the starting phase of the drier, 
whereas during normal operation some of the hot air or vapours discharged 
from the drier may advantageously be recycled, after reheating if 
necessary, thus greatly reducing the energy consumption. 
According to another embodiment of the present invention, the ceramic 
articles are heated to between 40.degree. and 100.degree. C. by conveying 
the articles through a bath of water maintained at the desired 
temperature. 
Because of the aforementioned heating, the ceramic articles during the 
actual drying step are in conditions such that there is: 
A considerable reduction in the bonding forces between the water and the 
clay, 
a decrease in the viscosity of the water, and 
an increase in and balancing of the diffusivity of water through the 
ceramic article. 
The results, during drying, are: 
More uniform shrinkage of each ceramic article until all the water has been 
lost, and 
a considerable reduction in tension inside each ceramic article, so that 
the tension can be reduced below the limit where breaking or permanent 
deformation occurs, thus eliminating the risk of deformation, microscopic 
cracks, fracture and the like. 
Another advantage is that ceramic articles under the aforementioned 
conditions are dried in a hot air or gas environment at a controlled 
temperature and a controlled humidity, i.e. a controlled partial pressure 
of water. In the case where a tunnel drier is used, the air is driven in 
co-current relationship with the ceramic articles. Due to these associated 
conditions, drying can be brought about by a heat exchange based mainly on 
the temperature difference between the ceramic articles and the hot air, 
instead of heat exchange through forced convection. The temperature 
difference can be adequately increased, particularly at the beginning of 
the drying process, since the ceramic articles are heated up to 
100.degree. C. and can undergo greater surface heating than the very small 
amount utilized in known drying processes, without any risk of exploding 
as a result of vapour spreading inside each ceramic article. The reason is 
that water flows in liquid form to the surface of the article, where it 
evaporates. 
Another result of the aforementioned associated conditions of each ceramic 
article, which is heated to a temperature up to 100.degree. C., and of the 
co-current flow of the hot, moist drying air and the control of the 
temperature and humidity of the air, is that the ceramic articles can be 
completely dried in much shorter times (reduced by a factor of 10 to 100) 
than the times required by the known methods.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The following are some non-limitative examples of drying ceramic articles 
according to the present invention. 
EXAMPLE 1 
100 ceramic articles were inserted in a tunnel drier, after being piled in 
conventional manner on a trolley which was guided through the drier. The 
ceramic articles had been moulded by the dry moulding method, and had been 
taken out of a warehouse. 
The ceramic articles were made of clay having substantially the following 
composition: 
SiO.sub.2 :45%; Al.sub.2 O.sub.3 :40%; Fe.sub.2 O.sub.3 :2%; MO:1%; 
CaO:10%; Na.sub.2 O:2%. 
The average water content of each ceramic article was 8% when it was taken 
out of the warehouse. 
Immediately after entering the drier, the ceramic material travelled 
through a region at a temperature of 200.degree. C., supplied with hot, 
moist air containing 800 g water per kg dry air. 
The hot air was supplied in co-current relationship with the ceramic 
articles at a flow rate of 10 m.sup.3 /min. 
The ceramic material was completely dried in 400 seconds. 
The drying air was discharged from the drier at a temperature of 
110.degree. C. Some was discharged, whereas some was reheated to 
200.degree. C. and sent into the drier co-current with additional ceramic 
articles. Each ceramic article coming out of the drier had an average 
temperature of 80.degree. C. and a residual water content of 0.8%. Under 
these conditions, it was directly supplied to a conventional furnace. Out 
of 100 ceramic articles subjected to the aforementioned drying step, no 
defects were found due to cracking, breaking or deformation. 
EXAMPLE 2 
100 ceramic articles were introduced into a continuous tunnel drier in the 
manner described in Example 1. The ceramic articles had been moulded by 
the conventional wet method, e.g. using an extruder and vapour. The 
ceramic articles were made of clay having substantially the same 
composition as the clay in Example 1. The average water content of each 
ceramic article was 25% when taken from a warehouse. 
Immediately after entering the drier, the ceramic material travelled 
through the first heated region in a stream of hot moist air at 
250.degree. C. and containing 800 g water per kg dry air. The air was 
supplied in co-current with the ceramic articles. 
On leaving the heated region, each ceramic article was at 80.degree. C. and 
had substantially the same moisture content (25%) as when it entered the 
heated region. Under the aforementioned temperature and moisture 
conditions, the ceramic articles travelled through the tunnel drier in a 
stream of hot, moist air supplied in co-current at a flow rate of 10 
m.sup.3 /min and at a temperature slightly below 250.degree. C. 
The ceramic articles were completely dry in 900 seconds. 
Some of the air discharged from the drier at 110.degree. C. was heated to 
250.degree. C. and recycled to the drier inlet. The ceramic articles 
coming from the tunnel drier had an average temperature of about 
90.degree. C. and a residual water content of 0.8%. In this state, they 
were directly supplied to a conventional furnace. 
Out of 100 ceramic articles processed in the aforementioned manner, no 
defects were found due to cracking, breaking or deformation. 
EXAMPLE 3 
100 ceramic articles were supplied to a continuous tunnel drier in the 
manner described in Example 2. The average water content of each ceramic 
article was 25%. 
Immediately after entering the drier, the ceramic articles travelled 
through a water bath kept at boiling-point. On coming out of the bath, 
each ceramic article was at a temperature slightly below 100.degree. C. 
and had substantially the same moisture content (25%) as before entering 
the bath. Under the aforementioned temperature and moisture conditions, 
the ceramic articles made contact with a stream of hot air containing 400 
g H.sub.2 O per kg dry air supplied in co-current with the articles at a 
temperature of 350.degree. C. and at a flow rate of 10 m.sup.3 /minute. 
The ceramic material was completely dry in 900 seconds. 
Some of the hot, moist air discharged from the drier at approx. 150.degree. 
C. was reheated to 350.degree. C. and recycled to the drier inlet. The 
ceramic articles leaving the drier had an average temperature of about 
90.degree. C. and a residual water content of 0.8.degree.. In this state, 
they were directly supplied to a conventional furnace. 
At the end of this drying operation likewise, 100 processed ceramic 
articles did not show any defects due to cracking, breaking or 
deformation. 
EXAMPLE 4 
100 ceramic articles moulded by the wet method were placed in a chamber 
drier (i.e. a static drier). The articles had a moisture content of about 
25%. The chamber drier was closed, after which vapour at 200.degree. C. 
was introduced therein, the vapour containing 800 g water per kg dry 
vapour. 
The flow rate of vapour was 5 m.sup.3 /minute. The vapour was obtained by 
injecting finely atomized water into the combustion products of a burner 
until the aforementioned moisture content was obtained. 
The ceramic articles were completely dry in 900 seconds. 
The exhaust vapours discharged from the drier were at 120.degree. C. 
On being discharged from the drier, the ceramic articles were at 80.degree. 
C. and had a residual water content of 0.8%. In this state, they were 
directly supplied to a conventional furnace. 
100 ceramic articles were processed but did not show any defects due to 
micro-cracking, breaking or deformation. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.