# Sustainability in Italian Ceramic Tile Production: Evaluation of the Environmental Impact
## 2 Materials and Methods  
### Materials and Production Process  
The main constituents of ceramic materials are clays, quartz and fluxing agents. Clays are a mixture of hydrous aluminum phyllosilicates forming compact grains characterized by particles of micrometric dimensions. Quartz is usually under the form of sand, while calcium carbonate and feldspars are used as fluxing agents [21; 22; 23]. The choice of the fluxing agent has a direct effect on firing temperature and open porosity of final products (e.g., feldspars allow porcelain tiles characterized by water absorption lower than 0.5% to be obtained). The ceramic tile production process can be defined as a powder technology, since all the applied raw materials are in the form of powders. For porcelain stoneware tiles, representing the top quality and most commercialized product in Europe, the production process is schematically described in Figure 2 with an indication of the main pollutants generated in each step. In particular, raw materials are extracted from quarries and transported to production sites by land or sea. Here, they are grinded and wet milled to form a ceramic slurry, in which pigments can be also added to give colored bodies. Then, there is the spray-drying phase, where the slurry is treated to form rounded ceramic granules ready to be efficiently pressed. After pressing, the residual humidity is lowered through a drying stage and then, the pressed body is ready for decoration. Figure 1: Materials, waste and emissions flows in ceramic tiles production plant. Figure 2: Scheme of the production stages for porcelain stoneware tiles and indication of the main pollutants generated in each step (PM: particulate matter; F: fluorine, CO\({}_{2}\): carbon dioxide; NO\({}_{X}\); nitrogen dioxides; VOCs: volatile organic compounds; SO\({}_{X}\): sulfur dioxide). In particular, ceramic tiles can be traditionally glazed and/or decorated by digital ink-jet printing. Traditional glazes contain silica, frits, pigments, and other compounds with the function of opacifiers or melting additives [24]. On the other hand, the ink-jet decoration, which has been introduced during the last decades as a process innovation, has recently gained an increasing importance and application. This technology involves the use of fine-grained pigments dispersed in organic media, especially alcohols and glycols. Recent studies are focusing on the use of more sustainable aqueous media [25; 26]. After the decoration step, the tiles are fired at temperatures up to 1150-1200 \({}^{\circ}\)C. In the final stages, the product is mechanically treated in order to reach its final format and/or enhance its aesthetic appearance and it is lastly packed and ready to be sold. The principal emission source of a ceramic tile production site is represented by the release of particulate matter (PM) during nearly all the stages of the production process. This is well expected because the ceramic production process is based on powder technology. In particular, the release of ceramic particles is mostly associated with the handling of raw materials and their physical transformations, above all the spray-drying and pressing stages. The released powder is taken up by aspirating systems at each of the working operations and driven through filtering treatments, before being discharged from the emission channels, or chimneys. With the exception of a limited number of plants employing a wet abatement, the most widely adopted filtering devices in the Italian ceramic district are fabric filter type [27]. Therefore, the PM separation is based on a physical treatment. The firing of ceramic tiles is the most sensitive stage of the whole process from the emission point of view. The high temperatures reached during firing (1000-1250 \({}^{\circ}\)C as a function of selected raw materials) promote the sintering process, thus allowing raw material transformations. At this stage, several pollutants eventually present as impurities in the raw materials can be released.