FR2721602A1 - Ceramic materials with low heavy metal content - Google Patents

Abstract

Process for prodn. of ceramics such as porcelain, glazed ceramics, vitreous enamel, using an oven in which the following successive stages are carried out; (i) a first baking to form a biscuit or unglazed body, (ii) dipping the biscuit or unglazed body in the glaze, and (iii) a second baking of the dipped biscuit or unglazed body whilst directing steam onto the biscuit or unglazed body, the water having the effect of bonding to the heavy metals on the surface during the baking so as to facilitate their elimination. Decoration may be applied to the biscuit after dipping in glaze and baking, and the decorated body then baked for a third time while applying steam to the surface; or decoration may be applied to the unglazed biscuit after the first baking and before dipping in the glaze.

Description

 The present invention relates to a process for the manufacture of ceramics, that is to say of any product of mineral origin, the manufacture of which is due to at least one firing at high temperature.

 These ceramics generally consist of porcelain, earthenware, stoneware, vitreous, glass, etc. They can be used with or without ornamental decoration.

 Whatever the embodiment of the ceramics, very often there are found, in their composition, heavy metals which can come from the ceramic paste used, from the vitrifying enamel of the ceramics and / or from the decorations which adorn said ceramics.

 Up to now, all the bright colors of decoration (red, blue, yellow ...) are the result of oxides, carbides, nitrides, borides, silicides etc. There are also metals such as lead, cadmium, the use of which is hardly compatible with health standards governing the metal content of enamel and decorations used in articles in contact with food.

 Nevertheless, the limit lead release rates are becoming more and more severe. This legislation, which came into force in the 1960s, was hardened in 1971 with a limit rate of 7 micrograms of lead per milliliter (often expressed in parts per million: ppm) for all types of ceramic products.

 In 1980, following additional medical studies and recommendations from the World Health Organization, this limitation was reduced to a range of values between 0.5 and 3 ppm depending on the category of products (pitchers, cups and mugs, large hollow products, small hollow products, flat products).

 However, currently in the United States, a Californian law must still bring down very significantly the levels of lead and other metals in ceramics. The new range is between 0.023 and 0.226 ppm. These new standards are incompatible with current manufactured products.

The state of the art can be defined by the document EP-A0,569,734 which relates to a process for the preparation of decorations with colors resistant to the firing of porcelain and to a pigment composition useful for this. Most pigments do not have sufficiently stable colors when firing porcelain. There is a need to increase the color stability of the pigments. Inclusion pigments based on sulphides and heavy metal selenides in an oxidized or silicate envelope, in particular
ZrSiO4, give decorations with colors resistant to the firing of porcelain, when are added, in the decoration or enamel layer, additives, as color stabilizers, chosen from compounds of Zn, in particular of ZnO or sintered glass plates containing zinc, or Cer (III) or Cer (IV) compounds, in particular CeO2. The amount introduced into the decoration layer is, for ZnO or CeO2, generally between 5 and 50% by weight relative to the inclusion pigment. The pigment composition according to the invention contains, in addition to the inclusion pigment, at least one usual color stabilization additive.

 This document shows the very important use which is made of heavy metals in ceramics.

 This process therefore does not make it possible to solve the hygiene problems due to the presence of metals.

 A solution has been found with the document EP-A-0.452.065 which proposes a composition of varnish of Chinese porcelains which is essentially free of lead and cadmium and which includes many substitute products, such as SíOX: 35 to 60% , Bu203: 5 to 45%, A1203: 5 to 20%, B203: 1 to 15%, etc. There are certain other optional ingredients, and the substance BaO is not in an amount greater than 2%.

 However, this invention relates only to the composition of the varnish, which limits its use all the more, which remains impossible with, for example, unglazed ceramics, ceramics with small fire decorations.

 The method according to the invention makes it possible to very considerably reduce the level of metals.

 Whatever the process of manufacturing ceramics, the baking to obtain a cookie can be replaced, if it is heated less, by obtaining a quartered.

 A first process for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven in which the following successive steps are carried out - a first firing to obtain a cookie, - a quenching of the cookie in the enamel, and - a second enamel baking of the cookie, the process is characterized in that steam is sent to the soaked cookie during the second baking, the water having the function of binding to the heavy metals, located at the surface during this cooking, in order to facilitate their elimination.

 According to a second mode of use, the process for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven in which the following successive steps are carried out: - a first firing to obtain a cookie, - a soaking the cookie in the enamel, - a second firing of enameling the cookie, - a setting of decorations on the enameled cookie, and - a third firing, or more, depending on the colors of the decorations, obtaining ceramic definitive, said method is characterized in that water vapor is sent to the enameled cookie receiving the decorations during the third baking, the water having the function of binding to the heavy metals, located on the surface during this cooking, in order to facilitate their elimination.

According to a third mode of use, the process for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven in which the following successive steps are carried out
- a first baking to obtain a cookie or quartered,
- a setting of decorations on the cookie or stretched,
- quenching of the biscuit or stretched decorated in the enamel, and
a second enamel baking of the soaked and decorated biscuit or debris, said process is characterized in that steam is sent to the soaked or debris biscuit during the second baking, the water having the function of bind to heavy metals, located on the surface during this cooking, to facilitate their elimination.

 In order to improve the second and third processes, water vapor is sent to the cookie soaked during the second baking, the water having the function of binding to heavy metals, located on the surface during this baking , to facilitate their elimination.

 According to a fourth initialization mode, the method of manufacturing objects such as glass, of the type using an oven in which the objects coated with decorations are baked, is characterized in that steam is sent to objects decorated during cooking, water having the function of binding to heavy metals, located on the surface during this cooking, in order to facilitate their elimination.

 Whichever method is used, water vapor is applied homogeneously in the form of a water vapor curtain. The steam curtain is applied horizontally.

 The water vapor comes from pure water treated at around 200 degrees Celsius (C) and under a pressure of 1 bar.

 Before each injection of water vapor, an accelerator injector makes it possible to aerate and oxygenate the ceramics, or any other object coated with cooked decorations, at a temperature of approximately 500 C.

 The injector-accelerator produces a curtain of air applied uniformly and vertically from bottom to top.

 The oven used is an electric oven, that is to say without combustion product.

 The process can be used for enamelled sheets.

 The attached drawings are given as indicative and non-limiting examples. They represent a preferred embodiment according to the invention. They will allow the invention to be easily understood.

 FIG. 1 represents a side view in section along C-C of FIG. 2 of an oven allowing the implementation of the method according to the invention.

 Figure 2 shows a top view of the oven according to the invention.

 3 shows a cross section along A-A of Figure 1 made at the injector-accelerator.

Finally, Figure 4 shows a cross section along
BB of Figure 1 made at the level of the steam injector.

 The present invention relates to a method of manufacturing ceramics. Any type of ceramic can be manufactured such as porcelain, earthenware, vitreous, stoneware.

 This process uses an oven 1 which is well represented in FIGS. 1 and 2.

 In these figures, it is a tunnel oven 1 allowing the continuous production of ceramics, due to the use of a conveyor belt 4 mounted movably, according to F1, on a set of rollers 16, 17 and 18 The current of cooking air is a flow which moves along F2, against the movement along Fl of the conveyor belt 4. So that this flow of cooking air is always present, a set of elements, such as that extractor hoods 7, turbines 6 and fans 3, are present.

 However, any other type of oven could be used for continuous or discontinuous operation of the manufacturing process.

 Whatever the embodiment of the furnace 1, whatever the method of producing the furnace 1 and whatever the manufacturing method used, the essential object of the invention is to provide a process for removing heavy metals, the presence in ceramics is unfit for human consumption.

 Thus ceramics form all of the pieces of the dressers, pitchers of water, glasses, etc. who are in contact with food.

 The excessive presence of heavy metals, such as lead, cadmium, does not correspond to the latest standards laid down by law or to the current sensitivity of the public, which is inclined towards ecology.

 In a first embodiment, a first baking is carried out in the oven 1, making it possible to obtain a biscuit or quartered. Depending on the nature of the ceramic (porcelain, earthenware, vitreous, etc.), the firing temperature varies. The cookie is then dipped in the enamel to make its surface as uniform as possible. We then proceed to a second firing allowing the enameling of the biscuit and thus the production of the final ceramic. The invention essentially consists in sending steam onto the cookie soaked during the second baking. The function of water vapor is to bind to the heavy metals which are located on the surface during cooking.

This bond will allow the elimination of heavy metals in the form of oxides or others.

 This first process therefore allows the elimination of heavy metals present in the original material making it possible to produce ceramics. The product thus obtained contains heavy metals within it but which will not be in contact with food.

The surface part, for its part, is characterized by the absence of these heavy metals, which will make the ceramic suitable for use as a supply support.

 However, and this is often the case for porcelain makers, the enameled biscuit is generally partially covered with colors in order to brighten up its presentation and attract the eye, it is the second process. Two essential types of decor are possible. In a first case, these are decorations on the enamel known as "small fire" decorations, the fixing of which on the ceramics is done simply by heating at a low temperature, this allows the decoration to be encrusted. In the second case, these are decorations in the enamel called "big fire" decorations for which the cooking is more important. The enamel will therefore cover the decorations which will no longer be in contact with the outside.

 With regard to "small fire" decorations, the ceramics thus manufactured must be treated with the process in order to remove heavy metals, on the one hand, coming from enamel and / or ceramics, and on the other hand , "small fire" decorations present on the surface.

 With regard to the "grand feu" decorations, the process will allow the elimination of heavy metals from enamel and / or ceramics. Heavy metals from "big fire" decorations are inaccessible or not depending on the thickness of the enamel layer.

 Whether the decoration is a "small fire" or large fire "decoration, the manufacturing process is almost identical to the process of manufacturing ceramics without decoration, with the only exception that the second enamel firing of the biscuit is followed by a laying decoration on the enameled biscuit, and finally, a third firing to obtain the ceramic, this third firing as a function of the temperature which will be applied to the ceramic creating "small fire" or "large fire" decorations.

 In this case, the water vapor is sent to the enameled cookie receiving the decorations during the third baking, the water again having the function of binding to the heavy metals located on the surface during this baking, which will facilitate the elimination of said heavy metals.

 The third method is on the same principle as the two previous methods. However, it consists of dipping the cookie in the enamel, once the decoration has been affixed to said cookie. There are therefore two successive firings which frame the setting of the decor.

 It is during the second baking that the steam is sent to the decorated cookie to do its job.

 Of course, in these last two processes, the injection of water vapor could be combined at the second and third firing.

 A fourth process is possible, the application of which is specific to any decorated object from which the heavy metals which are present in the decorations are to be removed. This is the case for all glass objects.

 It is during the cooking of the decorated object that the water vapor is sent to evacuate said heavy metals.

 In order for this removal of heavy metals to be uniform, the water vapor should be applied homogeneously in the form of a curtain of water vapor. As can be seen in FIG. 4, the water vapor is conducted inside the furnace 1 by means of two supply lines 10 which eject horizontally, on either side of the partitions Il of the furnace 1, said vapor through openings 19 and according to arrow F4. The water vapor curtain is therefore complex since a "shear" effect is applied to it. This effect creates a cloud of water vapor.

For the process to be most effective, it is necessary to use pure water which will be treated at about 200 degrees
Celsius (C) and under a pressure of 1 Bar. This water vapor is produced by a water vapor injector 5 which has means for heating the pure water to be treated. In fact, and as can be seen in FIG. 2, there is a supply 8 which supplies water to a steam generator 9, the assembly forming the injector 5.

 Before each injection of steam, an accelerator injector 2 aerates and oxygenates the ceramics at a temperature of approximately 500 C.

 This injector-accelerator 2 produces a curtain of air applied in a homogeneous manner in order to obtain final ceramic products all similar in quality. This curtain of hot air is produced by the combined action of a fan 12 and a heating resistor 13. The air coming from the fan 12 along F5 is heated in contact with the resistor 13, then a hot air supply pipe 14 leads to openings 15, where the air is expelled vertically upwards, according to F3, to create the curtain of hot air.

 In a preferred embodiment, the furnace 1 used is an electric furnace, that is to say that no combustion product will disturb the course of the ceramic manufacturing process.

 The process can also be used for enamelled sheets or any other product such as glass undergoing baking, the characteristics of which contain heavy metals which it is desired to eliminate.

REFERENCES 1. Oven 2. Injector-accelerator 3. Fan 4. Tread or treadmill 5. Water vapor injector 6. Turbine 7. Extractor hoods 8. Water supply to injector 5 9. Steam generator water from injector 5 10. Steam supply lines 11. Oven partitions 12. Injector-accelerator fan 2 13. Injector-accelerator 2 heating resistance 14. Pipe hot air supply 15. Hot air expulsion holes 16, 17 and 18. Treadmill rollers 4 19. Water vapor expulsion port
F1. Direction of movement of ceramics
F2. Direction of movement of cooking heat
F3. Hot air movement
F4. Water vapor movement
F5. Air movement from the fan 12.

Claims (12)

 1. Method for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven (1) in which the following successive steps are carried out - a first firing to obtain a cookie or quartered, - a quenching of the cookie or of the redrawn in the enamel, and - a second enamel baking of the cookie or redrawn, the process is characterized by the fact  that steam is sent to the biscuit or soaked in water during the second baking, the function of the water being to bind to the heavy metals located on the surface during this baking, in order to facilitate their elimination.  2. Method for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven (1) in which the following successive steps are carried out - a first firing to obtain a cookie or quartered, - a quenching of the cookie or unblocked in the enamel, - a second enamel baking of the cookie or unblocked, - a setting of decorations on the biscuit or unglazed enamel, and - a third firing or more, depending on the colors of the decorations, obtaining the definitive ceramic, said process is characterized by the fact  that steam is sent to the enamelled biscuit or unglazed receiving the decorations during the third baking, the water having the function of binding to heavy metals, located on the surface during this baking, in order to facilitate their elimination.  3. Method for manufacturing ceramics such as porcelain, earthenware, vitreous, of the type using an oven (1) in which the following successive steps are carried out:  - a first baking to obtain a cookie or quartered,  - a setting of decorations on the cookie or stretched,  - quenching of the biscuit or stretched decorated in the enamel, and  - a second enamel baking of the biscuit or stretched soaked and decorated, the process is characterized by the fact  that steam is sent to the biscuit or stretched soaked and decorated during the second baking, the water having the function of binding to heavy metals, located on the surface during this baking, in order to facilitate their elimination .  4. Method according to any one of claims 2 or 3, characterized in that  that the water vapor is sent to the cookie or stretched during the second baking, the water having the function of binding to the heavy metals, located on the surface during this baking, in order to facilitate their elimination.  5. Method for manufacturing objects such as glass, of the type using an oven (1) in which the objects coated with decorations are fired, characterized by the fact  that water vapor is sent to the decorated objects during cooking, the water having the function of binding to heavy metals, located on the surface during this cooking, in order to facilitate their elimination.  6. Method according to any one of claims 1 to 5, characterized in that  that water vapor is applied evenly in the form of a water vapor curtain.  7. Method according to claim 6, characterized in that  that the water vapor curtain is applied horizontally.  8. Method according to any one of claims 1 to 7, characterized in that  that the water vapor comes from pure water treated at around 200 degrees Celsius (C) and under a pressure of 1 bar.  9. Method according to any one of claims 1 to 8, characterized in that  that before each injection of water vapor, an accelerator injector (2) makes it possible to aerate and oxygenate the ceramics, or any other object coated with cooked decorations, at a temperature of substantially 500 C.  10. Method according to claim 9, characterized in that  that the injector-accelerator (2) produces a curtain of air applied uniformly and vertically from bottom to top.  11. Method according to any one of claims 1, 2, 3 or 5, characterized in that  that the oven (1) used is an electric oven, that is to say without combustion products.  12. Method according to claim 5, characterized in that  that the process is used for enameled sheets.