The manufacturing process for ceramic tiles
The principal phases are as follows: preparing the raw materials, pressing, glazing, firing, sorting and finally packaging
Draw of the process
The manufacturing process starts with the selection of the raw materials that make up the ceramic tile, essentially clays, feldspars, sand, carbonates and kaolins. As the raw materials are normally used exactly as they are extracted from the mines or quarries, it is essential to begin by ensuring that they are homogenised, in order to guarantee that their characteristics are consistent.
They are therefore ground in order to disperse and reduce the particles. This grinding process may be wet or dry. Wet grinding is the most common method, whereby the clays are dissolved in water in a ball mill. The resulting suspension is known as ‘slip’. Spray-drying is used to reduce its water content for the later compacting and drying processes.
The slip is pumped, sprayed and dried by a stream of hot gases inside the spray-dryer. These gases come from a conventional air-gas burner, or alternatively the exhaust gases from a cogeneration turbine may be used, thereby cutting the cost of the drying process. A spray-dried powder is produced, made up of spherical granules that are uniform in shape and hollow inside, allowing for a fine loose powder which makes the mould-filling and pressing processes easier.
Dry pressing is the most common moulding method. Hydraulic presses are used, guaranteeing maximum compaction, high productivity rates and optimum consistency in the press cycle time. The tiles produced have a moisture content of between 5 and 7 per cent, which means they must be dried to reduce it to between 0.2 and 0.5 per cent in order to ensure successful firing, and, where appropriate, glazing. An excessively high water content would cause the tiles to crack or break.
In the case of unglazed products, after drying, the next phase is firing. If the tiles are to be glazed, then they must first be covered with one or more layers of glaze. This gives the tiles a series of technical and aesthetic features including colour, gloss and surface texture. Glazed tiles are also impermeable and easy to clean, and are chemically and mechanically resistant. Ceramic tiles are line-glazed and the most common techniques are waterfall glazing, spraying, dry glazing or decorating.
Firing is one of the most important stages in the manufacturing of ceramic tiles. During this process, a series of reactions take place that change the microstructure of the tile, creating the required final properties such as mechanical strength, size stability, resistance to chemical agents and fire and easy cleaning. During the firing stage, the key variables in the thermal cycle are the firing time and temperature and the kiln atmosphere, which depend on the composition of the raw materials and the type of product required.
Today, fast firing is the most common method used in the production of ceramic tiles. Single layer roller kilns are now used, which have drastically reduced the firing cycle times to less than 40 minutes (thanks to the improved heat-transfer coefficients, as well as their uniform nature and flexibility). In these single layer kilns, the tiles travel over the rollers and the heat needed to fire them is supplied by natural gas-air burners located in the kiln walls. During this process, the heat is basically transmitted by convection and radiation.
In order to guarantee maximum quality at each stage of the process, the next stage is to sort the tiles and detect any faulty pieces. Although sorting was originally done by hand, today this process is carried out automatically using recognition software based on a series of pre-determined parameters. This enables tiles that are irregularly shaped or with faulty colouring to be removed. Packing is the final stage in the ceramic tile manufacturing process.