Method for making glass silicate tiles

A method of making glass-silicate tiles includes pouring an input raw material containing glass granulate into a heat-proof mold, wetting the input raw material and making an initial blank thereby, heat treating of the blank by gradual heating and by gradual cooling by stages with holding period between the stages wherein a first heating stage is performed predominantly by heating a bottom side of the blank with higher speed of heating of a lowering layer than of an upper layer of the blank to accelerate gases to release the blank through the upper layer up to reach of the temperature of beginning of glass granulate sintering (T.sub.f) in the lower layer, and the temperature not exceeding a glass granulate transformation temperature (T.sub.g) in the upper layer, a first holding period at these condition to expel generated gases, and heating the upper layer with higher speed than the lower layer until a Littleton temperature is reached in the lower layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and an apparatus for making glass 
silicate tiles. 
One of such methods is disclosed in patent application PV-2751-93 of Czech 
Republic. In this reference flat glass silicate tiles are produced from 
sand and glass wastes by heating a work piece located in a refractory mode 
to a temperature which is equal to an average value between the 
temperature of the beginning of sintering and the Littleton temperature. 
Thereafter the workpiece is subjected to a thermal shock, and successive 
cooling in several stages. The process is performed in a multi-chamber 
furnace with electrical heating, and the temperature in the successive 
chambers is maintained in accordance with the properties of the glass 
granulate used in the workpiece. 
This method has the disadvantages that the heating of the workpiece is 
performed from the surface, and during the process of thermal shock gas 
inclusions remain in the lower layers. They reduce strength of the tiles 
on the one hand, and can migrate to the surface on the other hand and 
distort the decorative layer by forming crates on its surface. It is 
therefore believed that it is advisable to improve the above mentioned 
existing method. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of present invention to provide a method and 
an apparatus for making glass-silicate tile which avoids the disadvantages 
of the prior art. 
In keeping with these objects and with others which will become apparent 
hereinafter, one feature of the present invention resides, briefly stated 
in a method of making glass silicate tiles which has the following steps 
pouring an input raw material containing glass granulate into a heat-proof 
mold, wetting the input raw material and making an initial blank thereby, 
heat treating of the blank by gradual heating and by gradual cooling by 
stages with holding period between the stages wherein a first heating 
stage is performed predominantly by heating a bottom side of the blank 
with higher speed of heating of a lowering layer than of an upper layer of 
the blank to accelerate gases to release the blank through the upper layer 
up to reach of the temperature of beginning of glass granulate sintering 
(T.sub.f) in the lower layer, and the temperature not exceeding a glass 
granulate transformation temperature (T.sub.g) in the upper layer, a first 
holding period at these condition to expel generated gases, and heating 
the upper layer with higher speed than the lower layer until a Littleton 
temperature is reached in the lower layer. 
In accordance with another embodiment of the inventive method, the method 
includes the following steps creating of an initial blank by pouring an 
initial raw material containing glass granulate into a heat-proof mold and 
its consolidation; gradual heating the blank with holding periods between 
stages comprising following stages: a first stage of heating, 
predominantly from bottom of the blank with lower layer heating 
temperature higher than the temperature of upper layer to accelerate gas 
removal through the upper surface layer until a glass granulate sintering 
temperature is reached in the lower layer, a first holding period at these 
sintering temperature is reached in the lower layer, a first holding 
period at these conditions to expel generated gases, heating the upper 
layer with higher speed than the lower layer until a Littleton temperature 
is reached in the lower layer and sintering temperature is reached in the 
upper layer and having a second holding period with these conditions, 
during the second holding period pressing the blank by a gas permeable 
press, after which additionally heating the blank until the upper layer 
reaches a temperature T.sub.4, 5 corresponding to the glass granulate 
viscosity logarithm 4.5 Pa.s, under which the third holding period is 
realized until the lower layer reaches a temperature T.sub.5,5 on which a 
glass granulate viscosity logarithm is equal to 5.5 Pa.s; after the third 
holding period the blank is cooled with holding periods between the 
following stages; a first stage of accelerated cooling is realized until 
the blank surface reaches a temperature T.sub.L, which is followed by a 
holding period for the period of time sufficient to achieve the Littleton 
temperature in the lower layer; after the first holding period a second 
cooling stage is performed until the annealing temperature is reached, 
after which a second holding period ensures a product annealing; after the 
first annealing a third cooling stage is realized until the surface 
reaches a temperature T13,5 on which the melt viscosity logarithm is equal 
to 13.5 Pa.s, after which the product is annealed, and after the second 
annealing, cooled down to the room temperature. 
In accordance with the present invention also an apparatus is proposed 
which has a transport system for location and transportation of heat-proof 
molds having bottom parts; the heat-proof molds are arranged on the 
transport system with a gap between the transport system and bottom part 
of the mold and means for heat treating glass-silicate tiles comprising 
following modules being lined up in the process flow; a module for input 
raw material filling into the heat-proof molds for formation blanks 
therein, a module of gradual heating of the blanks with holding periods 
between stages having a pre-heating chamber and heat stress chamber; a 
module of gradual cooling blanks including modules of inter-stage holding 
and annealing; wherein each of the heating and cooling modules is equipped 
with insulation walls and a roof wherein gas burners are installed in the 
side walls of the pre-heating chamber, while the burner height from the 
floor corresponds to the height of the gap between the upper surface of 
the transport system and bottom part of the heat-proof molds, and gas 
flame cone of the gas burners is located in the gap; gas exhaust channels 
are located in the side walls and roof of the pre-heating chamber; the 
heat-proof molds with the blanks located in them are moving continuously 
with transport unit through the above modules ensuring gradual heat 
treatment of input product and production of the glass-silicate tiles. 
The novel features which are considered as characteristic for the present 
invention are set forth in particular in the appended claims. The 
invention itself, however, both as to its construction and its method of 
operation, together with additional objects and advantages thereof, will 
be best understood from the following description of specific embodiments 
when read in connection with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference to the drawings, FIG. 1 shows a line of a thermal treatment 
which includes modules for loading of raw material 12, for preliminary 
lower heating 13, for accelerated heating of a blank surface 14, for 
thermal shock 15, for adjustable heating and annealing 16, and for a final 
cooling 17. The modules of the preliminary heating and final cooling are 
connected by a gas pipe 9, wherein I is an inlet for hot gasses from 
chambers of the preliminary heating, and II is an exit of gasses after 
their passage in a ceiling of the chamber for the final cooling. The mold 
with the initial blanks 2 is transported along the line on movable 
trolleys 6. 
FIG. 2 shows a chamber for the preliminary heating. The initial material in 
heat resistant molds 1 is arranged on a special support 2'. A flame 3 from 
burners 4 is located under the bottom of the mold. The support with the 
mold is arranged on the movable trolley 6 inside a heat insulating casing 
5, whose ceiling has passages for withdrawal of gasses 7. The mold on the 
support is held by spacer projections 8. 
FIG. 3 shows the support for molds with initial material. The support 
includes a bottom 2 and projections 8 which are shaped in correspondence 
with the sizes of the used molds. The heat resistant mold is schematically 
shown in FIG. 3a together with a three-layer initial blank. The mold in 
FIG. 3b is illustrated in a moment when in the upper layer a temperature 
of transformation T.sub.g is reached, while in the lower layer a 
temperature of the beginning of sintering T.sub.f is reached, and pressing 
is performed by a gas-tight press which is provided with micro passages 
for withdrawal of gasses from the lower layers of the blank as shown by 
arrows. 
FIG. 3c shows the stage of thermal treatment when in the lower layer the 
Littleton temperature of T.sub.n is reached. Melting of the layer occurs 
and the remaining gasses are expelled through loose upper layer. FIG. 3d 
shows a stage when on the surface a maximum temperature of thermal 
treatment is obtained, the surface is melted, and a structure of the final 
product is formed. FIG. 3e illustrates a stage of cooling of the product. 
The temperature of the upper and lower layers T.sub.s is lower than the 
temperature of the middle layer T.sub.m, and the thermal expansion 
coefficient of the upper layer is minimal. During this stage thermal 
stresses of compression (-.delta.) are generated, to provide its strength. 
FIG. 4 schematically shows a mode of thermal treatment of the lower 18 and 
upper 19 layers of the blank during the time (.tau..sub.0 
.div..tau..sub.13) and when in these layers certain temperatures T are 
achieved during the subsequent stages. FIG. 5 finally schematically shows 
the values of the temperature gradient (.DELTA.T) through a thickness of 
the blank which is thermally treated, as well as values of temperature 
stresses (.delta.) in the upper layer during all stages of thermal 
treatment (.tau..sub.0 .div..tau..sub.13). 
The apparatus operates in the following manner. In the heat resistant mold 
the initial material is poured in one or several layers, and then the mold 
is supplied on the immovable trolleys into chambers of preliminary 
heating, where the blank is heated through the bottom of the mold first 
until the temperature gradient (T.sub.f -T.sub.g) is reached so that an 
additional pressing of the blank can be performed, and then until the 
temperature gradient (T.sub.l -T.sub.f) is reached and in the lower layer 
of the blank melting of the raw material occurs. After this, the molds on 
the trolleys are supplied into the chamber for predominant heating of the 
surface 15, where an accelerated heating of the upper layer is performed. 
Then the trolleys are moved into the chamber for holding, with a maximum 
temperature of thermal treatment 15, where the surface layer obtains a 
mirror smooth structure, and melting of the lower layers is performed, 
with lowering of the temperature to T.sub.l. Then the trolley with the 
molds are moved into the chamber of the first annealing 16 where the 
holding is performed at the upper temperature of annealing (T.sub.12), 
with linear lowering of the temperature to the lower temperature for 
annealing (T.sub.13,5), and the second holding at this temperature. After 
this the trolleys are moved into the chamber of final cooling, whose 
ceiling is heated to sudden thermal shock (lowering of the temperature) by 
heated gasses which are supplied from the chambers of the preliminary 
heating. After cooling to the temperature 150-200.degree. C. the trolleys 
with final product leave the line and are cooled to room temperature, and 
then they are removed from the molds. 
The present invention has advantages when compared with known methods and 
apparatus. In the present invention the realization of sintering and 
melting of initial material from the lower layer of the blank to the upper 
layer makes possible complete expelling of all gasses which are present in 
the initial condition as well as gasses which are formed during a thermal 
treatment as a result of chemical reaction. Therefore it is possible to 
obtain an article with a dense inner structure which is free of such 
defects as microbubbles, and has a bending strength not less than 30 MPa. 
This substantially exceeds the strength of such natural material as 
granite and marble. In addition, when the multi-layer tiles are produced, 
due to the corresponding selection of heat expansion coefficient of the 
lower and upper layers, in the upper layer thermal compression stresses 
are provided, which makes the article even more strong. The utilization 
during the thermal treatment of replacement of cations Na and K cations 
with L.sub.i cations permits to obtain articles with the bending strength 
more than 50-70 MPa, which makes possible the use of this tile for example 
for highly loaded floors of industrial spaces. 
The present invention is illustrated by the Examples presented hereinabove. 
EXAMPLE 1 
Crystal glass waste which contains 24% PbO in the form of scrap glass of 
together with silicate sand as used in glass making with particle size 0.3 
to 0.5 mm. The scrap glass is preliminary crushed to get granules of 
maximum particle size 2 to 3 mm. The input granulate made in the above way 
is distributed to hoppers (12). In one of the hoppers a mixture of the 
granulate and silicate sand is prepared to be poured as the lower layer 
with the ratio of 7 to 9 weight parts of granulate to 3 to 1 weight parts 
of silicate sand. In other bunker a granulate of different colors is mixed 
to be used for pouring the upper, decorative layer. The lower layer is 
poured and water is added to form a lower having a thickness of 8 to 10 mm 
into a heat-proof mold installed on a mobile trolley, which is followed by 
pouring the upper layer to the thickness of 3 to 4 mm. The moisture 
content does not exceed 4 to 5%. The mobile trolley with the blank 
prepared in this way i.e. poured into the heat-proof mold proceeds to a 
multi-chamber oven for heat treatment wherein it is moved in 20-minute 
steps through heating and cooling modules arranged in series. In the first 
chambers the blank is preliminary heated mostly by heating the bottom part 
of the mold by gas flame cone for 10 minutes to reach the temperature of 
approx. 600.degree. C. in the lower layer corresponding to the sintering 
temperature of scrap of glass i.e. to the viscosity logarithm approx. 9 
Pa.s, while the upper layer temperature reaches approx. 450.degree. C., 
which corresponds to the transformation temperature i.e. viscosity 
logarithm approx. 12 Pa.s. Then there is a 10-minute holding period 
(dwell) on the temperatures i.e. with the temperature gradient 150.degree. 
C. over the blank thickness. During the holding period the lower layer is 
consolidated because of its sintering, and gas impurities are released due 
to the rising lifting force (lower layers are hotter) from inter-granule 
space through the upper layer into the oven, and are removed through gas 
discharge channels in the chamber roof. In the next stage the lower layer 
temperature is increased to the Littleton temperature--in our case 
750.degree. C., and the upper layer temperature to 700.degree. C. in 10 
minutes, which is followed by a the second 10-minute holding period on 
heating. During the holding period the lower layer mixture at the 
Littleton temperature is consolidated, for the glass granulate is melted 
and, because of its own weight, evenly distributed in bottom part of the 
mold, forcing out the rest of the gases in the upper layer simultaneously. 
At the same time, the glass granulate has not been fully melted in the 
upper layer; there have been pockets between individual granules through 
which gases are removed into the oven chamber, for the temperature 
gradient enabling gas release has still been maintained. The gases are 
then released through the gas discharge channels in the chamber roof. At 
the next 15-minute stage the blank surface is predominantly heated up to 
the temperature of 950.degree. C. corresponding to the viscosity logarithm 
of 4.5 of the selected sort glass, which is followed by a 5-minute holding 
period, by the end of which the lower layer temperature reaches 800 to 
820.degree. C. During the holding period the upper layer melts by its own 
weight, creating a mirrored glossy surface, and fuses with the lower 
consolidated and sintered layer forming a monolithic product. Then the 
upper layer of the blank is quickly cooled down to the Littleton 
temperature (750.degree. C.) or a bit lower, which is followed by a 15 to 
17-minute holding periods to equalize the temperature in the tile 
thickness, which is necessary to prevent its deformation due to uneven 
temperature field. Then, in 15 minutes the whole tile is rapidly cooled 
down to the temperature of 475.degree. C. that corresponds to the high 
annealing temperature, which is followed by 5-minute holding period on the 
temperature to equalize the temperature along the product thickness, to 
avoid a temperature could arise again and to remove temperature stress. 
After the holding period, in 30 minutes the temperature is decreased 
linearly to 435.degree. C. corresponding to the low annealing temperature, 
while the speed of temperature decrease is constant and equal to approx. 
1.3.degree. C. per minute. Then there is a 10-minute holding period, and 
then the product is finally cooled down with speed of 10 to 12.degree. C. 
per minute to room temperature. The final product is removed from the mold 
after it is cooled. 
EXAMPLE 2 
The example applies to making high-strength tiles with increased surface 
layer hardness. We are using the bottle glass scrap as a starting 
material. The starting material is prepared for pouring into the 
heat-proof mold in the way analogous to Example 1, with the difference 
that metal oxides are added to the glass granulate to provide the upper 
layer with decorative properties. In the given particular example, 0.5 to 
1% Cr.sub.2 O.sub.3 ; 1.5% Fe.sub.3 O.sub.4 and 8 to 10% Mn.sub.2 O.sub.3 
are added in the form of fine-dispersed powders to obtain green-black 
malachite imitating surface, and the granulate is mixed in such a way that 
the coloring agents are evenly distributed throughout the volume. Then the 
lower layer is formed by pouring the mixture of glass granulate and 
silicate sand to a thickness of 7 to 8 mm, which is followed by 3 to 4 mm 
upper layer consisting of not uniformly mixed mixture of glass granulate 
and oxides. The heat treatment step is set for 30 minutes in the oven. The 
initial heating is performed for 20 minutes to achieve the lower layer 
temperature of 755 to 760.degree. C. (log.congruent.8.5), and the upper 
layer temperature 580 to 590.degree. C. (log.congruent.12). After 
10-minute holding period on these temperatures the blank is heated for 25 
minutes to achieve the lower and upper layer temperature of 820.degree. C. 
and 760 to 770.degree. C. respectively. Then the second holding period is 
made for 5 minutes, during which the blank is pressed with a gas-permeable 
press having also the temperature of 760 to 770.degree. C. The pressing 
consolidates the lower layer, while a definite surface relief may be 
formed on the upper layer. Then, a thermal shock is applied by heating the 
surface up to a temperature of 970 to 1010.degree. C. in 3 to 5 minutes, 
while the lower layer temperature reaches the value of 900 to 950.degree. 
C. with maintaining the surface relief, which only becomes smoother. The 
thermal shock is followed by rapid cooling to a temperature of 820.degree. 
C. in 2 to 3 minutes with 15 to 25-minute holding period at this 
temperature to equalize the temperature along the blank thickness. Then a 
second cooling to a temperature of 580.degree. C. (high annealing 
temperature) in 15 minutes is applied. During the cooling the tile surface 
is treated with a reagent containing Li.sup.+ cations (e.g. Li.sub.2 
SO.sub.4), which results in replacement of Na.sup.+ and K.sup.+ cations 
with Li.sup.+ cation that has smaller radius and higher field intensity. 
The surface layer modified in such a way acquires a lower thermal 
expansion coefficient, and, on further cooling, compressive stresses--a 
hardened surface layer--are created in it. Further heat treatment is 
performed as in Example 1 in the following mode: 
5-minute holding period with a temperature of 580.degree. C.; linear 
cooling to a temperature 540.degree. C. in 40 minutes; 10-minute holding 
period with a temperature of 540.degree. C., and cooling to room 
temperature with a speed of 8 to 10.degree. C. a minute. After the 
cooling, the finished product is removed from the mold. The resulting 
product has a solid internal structure with a hardened surface layer. The 
bending strength of such a product is at least 35 to 40 MPa, and the 
surface layer hardeness is increased by 1 to 2 units (Moos) compared with 
the surface hardness of the starting glass.