Using additives to lower the firing temperature in the heavy clay ceramic industry

1 Introduction Energy-saving in relation to the production process in the ceramic industry is increasingly becoming a topic of discussion. In the first place, there is increasing political pressure to reduce the consumption of energy, in particular energy derived from fossil fuels. In the second place, the energy used for production is becoming an increasingly large cost item. Generally speaking, most of the energy used is needed to ensure evaporation of water from the products before they can be fired. In addition, a great deal of energy is also ­needed for firing the products in the kilns. There are various ways of trying to further reduce the consumption of energy within a company by modifying the ingredients/recipe used. One possible way is to reduce the amount of water in the wet-shaped product. This can be reached by using specified additives or by the addition of opening agents. Another method is the use of an additive that causes the clay to sinter effectively at a lower top temperature. Additives that lower the sintering temperature are natural or synthetic compounds containing chemical elements that already exhibit a flow-type or sintering type of behaviour at a lower temperature. The sintering behaviour of a clay body is determined by the presence of basic oxides which are able, at higher temperatures, to break into the rigid and stable network structures of silicon (and aluminium) oxides, thereby causing the formation of amorphous and glass-like structures. The extent to which this occurs or has an effect depends not only on the chemical composition but also on the size of the individual particles containing these elements and on the temperature. For the production of technical ceramics and, in some cases, whitewares, it is possible to adjust the fineness of the material used providing the body is ground. However, for the production of heavy clay ceramics, this does not play a role. Chemical elements that have the potential to lower the top temperature include alkaline and alkaline earth compounds such as sodium, potassium, calcium, barium and magnesium. Various metal oxides, such as the oxides of manganese, lead and divalent iron as well as borates, can also lead to a decrease in the required maximum temperature. Some of these elements are bound to clay minerals or present in clay in some other form. On the basis of practical experience, it is also known that the higher the concen­tration of very fine particles present in a clay (i.e. the higher the concentration of clay minerals present), the lower the required top temperature will be. When fired at this lower top temperature, the fired colour of the iron present in the clay becomes more orange-red. In order to achieve the desired quality and at the same time retain this colour, a clay recipe with a higher concentration of very fine particles must be used. However, due to the extra water retention in a clay with a higher concentration of very fine particles, the energy consumption in the dryer will increase. As a result, the addition of a clay with a higher concentration of very fine particles to the recipe is not a realistic option when it comes to reducing the consumption of energy. If one does not want the energy consumption in the dryer to increase, then the elements mentioned above must be added in a non-water-binding form. The following aspects also deserve further attention: › If the additive used to lower the sintering temperature contains water-soluble components, consideration must be given to the risk that these components will collect on the outer surface of the product during the first drying phase and result in a discoloration of the surface after the firing process or else cause the products to melt together during the firing process › The additive must be dosed early enough in the clay preparation process so that it becomes completely homogenized. A lack of homogeneity will result in undesirable quality variations › If the additive is present in the form of particles, the particles should be fine enough to ensure that no melting spots occur › Many additives contribute their own colour › A lower top temperature may result in a different product colour › The addition of additives to lower the sintering temperature can lead to greater differences in density and therefore differences in size and colour, if the firing process in a particular installation is subject to a certain degree of temperature inhomogeneity. In addition, products may more readily become subject to deformation and packages in the kiln may become unstable › The price of an additive and the quantity that needs to be dosed should be such that the energy-saving achieved is not overshadowed by the increase in cost of the ingredients needed for the recipe   2 Project The use of additives in ceramic bodies in order to lower the firing temperature was investigated. In the course of the investigation, the (theoretical) effect of such additives was described. The effect of two selected additives, namely glazing waste and ground monitor screen glass in a dosage of up to 4%, was determined empirically. In particular, the influence was investigated of such additives on the thermal behaviour of the clay bodies and on the physical and mechanical characteristics of the fired product as well as the leaching behaviour of the product at various temperature intervals.   3 Investigation The investigation into the addition of additives to lower the sintering temperature was conducted using recipes based on three different types of basic clays: › a red-firing Maas clay › a yellow-firing, calcium-rich “loess” loam and › a white-firing Westerwald clay These recipes are typical and representative of the naturally occurring raw materials and the properties of the clays used in the Netherlands and Belgium. With regard to the additives, the following two sub­stances were chosen › ground monitor screen glass supplied by the company Sims/Mirec (particle size < 100 µm); › glazing waste from the whitewares industry For each clay ingredient, three different recipes were tried out per additive: a control (i.e. with no additive) and two dosage levels of the additive concerned. The dosage levels were chosen on the basis of the experience already acquired by TCKI. The low dosage level used involved a concentration of 1 mass % and the higher dosage was 4 mass %. This was done in order to get a good idea of the effects of the additive. The chemical composition was determined for each component as well as the three basic clays used. For every recipe, the particle size distribution and the specific surface area were determined. For the recipe with the highest dosage of an additive and for each control, the thermal behaviour was determined with the help of a dilatometer curve. For each recipe, trial bricks were formed by hand (»1). These trial bricks were first dried in the laboratory area and then in a drying oven set to 40° C. The products made from Maas clay and from loess loam (four bricks per recipe) were fired at a top temperature of 1 070° C. The products made from Westerwald clay were fired at a top temperature of 1 200° C. In addition, products from all the recipes were fired at lower top temperatures in order to determine the lowest temperature at which it was still possible to produce products with identical product characteristics. The lower top temperatures used were 1 050°  C and 1 030° C for the Maas clay and loess loam recipes, and 1 170° C and 1 140° C for the recipes based on Westerwald clay. The fired products (three bricks per recipe) were analyzed for the following: › net dry density › initial rate of water absorption and free water absorption › compressive strength › leaching behaviour (selected samples) for critical components in reference with the Dutch Building Materials ­Decree (SO4, F, As, Mo, Cr and V) as well as components specifically present in the additives (Ba, Pb and Zn)   4 Results The chemical composition of the basic clays used and the additives are presented in »Table 1. The differences in chemical composition between the various clay types are clear. The “loess” loam has a low concentration of aluminium (contains comparatively few clay minerals) but is rich in calcium. The Westerwald clay has a higher concentration of aluminium (contains more kaolin clay minerals). The Maas clay clearly contains iron. In particular, the glazing waste has high concentrations of sodium, zinc and zircon. The monitor screen glass has high concentrations of sodium, potassium, lead and barium. In both additives used, substances were identified which appear suitable for lowering the sintering temperature. The additives used do not significantly affect the specific surface area. »2, »3 and »4 show the dilatometer curves. It is clear that addition of the two additives leads to a certain degree of firing shrinkage at a lower temperature and that the sintering curves for the different recipes are very different. If sintering takes place at a lower temperature, the interval during which organic material is burned out may be shortened. If the sintering curve slopes steeply downwards, it may be an indication of an increased risk of thermal deformation of the products. The photographs of the products fired at the various top temperatures are presented in »5. »Tables 2 through 5 provide an overview of the physical and mechanical characteristics of the various products.   5 Conclusions For certain clay types, it is feasible to use additives to lower the sintering temperature and thereby realize energy savings. For the production of masonry bricks from Maas clay, the addition of 4% (100 µm finely ground) monitor screen glass or of 4% glazing waste can reduce the top temperature required from 1 070° C to 1 030° C without a significant deterioration in product colour and/or physical/mechanical characteristics. For the production of masonry bricks from a white-firing Westerwald clay, a reduction in top temperature was realized from 1 200° C to 1 140° C, whereby the same effects were realized as for Maas clay. For the production of yellow-firing masonry bricks (from calcium-rich clay such as the “loess” loam used in this investigation), the issues involved are more complex. Here also it is possible to use the additives mentioned to reduce the top temperature from 1 070° C to 1 030° C while at the same time maintaining the physical and mechanical product characteristics. However, the problem is that at this lower top temperature, it is not possible to maintain the yellow colour of the product. As a result, the use of additives to lower the sintering temperature for yellow-firing products based on calcium would not seem to be a realistic option. This might also be true for bronze-firing products and product colours other than the red and white products investigated so far in this study. For products based on calcium-rich clay, the lower top temperature also leads to a large increase in the quantity of leachable chrome and sulphate. The latter would increase the risk of efflorescence. No extra leaching was detected in this investigation with regard to the components specifically added to the recipes in the form of additives used (e.g. barium, zinc and lead). However, the risk of extra flue gas emissions for these components may increase. In addition to lowering the top temperature, the addition of glazing waste in particular leads to a reduction in the quantity of leachable arsenic, molybdenum and vanadium. This may be related to a change in the oxidation state of these components, which could reduce their mobility. With regard to the production process in practice (at lower firing temperatures as well), the following aspects will also have to be taken into account: increased thermal instability of the products and a more limited timeframe for the combustion of organic compounds due to the fact that the products will already become dense at a lower temperature. The components must also be ground finely enough and homogeneously distributed throughout the entire clay body in order to prevent excessive stickiness and melting spots. The energy savings realized by lowering the top temperature is estimated at between 1.2 and 5.6 m³ of natural gas gross per tonne fired product, depending on the top temperature and the reduction achieved. However, due to the lower energy content of the products cooling down, there will be less heat available for the drying process. As a result, the net energy savings are estimated at from 0.7 to 3.4 m3 of natural gas per tonne fired product. As energy is also needed for grinding monitor screen glass down to a fraction < 100 µm, the actual energy savings realized by adding this material will be somewhat lower. The amount of energy needed for grinding the material has not been quantified.   6 Résumé It was concluded that, in particular for the red-firing Maas clay and the white-firing Westerwald clay, for soft mud hand-moulded facing bricks there are real possibilities for realizing a significant lowering of the sintering temperature (by 40° C to 60° C) and for achieving energy savings. The possible energy-saving was estimated at between 0.7 and 3.4 m3 of natural gas per tonne product. For yellow-firing products based on a calcium-rich clay, it would be difficult to implement these steps, as it is not possible to retain the yellow product colour if the top temperature is lowered. For products based on Maas clay, a possible additional benefit is that the leaching of arsenic can be reduced in this fashion.