PSI - Issue 37

Alexandre Fragoso et al. / Procedia Structural Integrity 37 (2022) 533–539 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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After the material is crushed, pre-homogenization is carried out, which consists on the deposition in several layers of limestone, marls, alternative raw materials, and corrective materials for their composition (Harrisson, A.M., 2019). The homogenized material, called raw, is then transported by conveyors to milling, where it is subjected to a drying operation and new fragmentation, becoming a material called flour (Huntzinger & Eatmon 2009). A mill failure during the comminution phase often occurs (Barata et al. 2017). The next step is the production of clinker. Initially, the flour is subjected to preheating in the preheating cyclone tower (Fig. 1a). At this stage, the kiln exhaust gases are used to heat the flour as it descends the different levels of the cyclone tower until it reaches the kiln (Fig. 1b), where it is transformed into clinker. Clinker is then obtained through a series of chemical reactions at a temperature of 1450°C (Mian et al. 2013) (Rackley 2017). When leaving the kiln, the clinker is suddenly cooled and goes to a new storage area. Finally, the last step is cement grinding. In this stage, the dosage and grinding of the clinker and additions, namely plaster, ashes, steel slag, limestone, among others, are carried out to produce different types of cement (Habert 2014) (Kääntee et al. 2004), which are subsequently packaged and shipped.

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Fig. 1. (a) Preheating cyclone tower; (b) Flow of flour as it descends the cyclone tower; (c) External view of the cyclone under study. The working principle of cyclones consists of the tangential injection of the gas loaded with flour into the conical chamber (Fig. 1b), creating a vortex that concentrates the denser particles close to the wall by centrifugal force. Consecutively, the relatively clean gas goes to the cyclone level above, and the heated mixture precipitates, passing through a pendulum valve (one-way) to the cyclone level below. The number of levels in a preheat tower varies between one and six. The more steps used, the more efficient the pre-heater will be. However, at each level, the pressure drops through which the exhaust fan work must increases. Thus, for each production system, it is necessary to calculate the best configuration of the cyclone tower and kiln to obtain the best possible efficiency. With a suitable cyclone configuration, it is possible to subject the flour to preheating, which results in a final kiln inlet temperature of between 750 to 900°C, and a 90% decarbonization percentage, using 60% of the total of thermal energy. Hence, cyclones are exposed to temperatures that can reach 900°C; for this reason, their metallic structure is coated with an insulating material and concrete to prevent the metal from suffering corrosion by a chemical attack and fatigue and/or creep due to extreme temperature conditions (Fig. 2a) (Infante et al. 2003) (Martins & Branco 2004). However, it was identified that concrete cracks, giving rise to chemical attack, exposure to temperature, and consequent fall of the concrete, as shown in Fig. 2b. This problem forces the production line to be stopped to carry out a corrective maintenance action to repair the equipment, which consists of the complete removal and application of a new layer of concrete at the cyclone ceiling.

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Fig. 2. (a) Thermal insulation scheme of the cyclone; (b) Failure of the concrete layer at the cyclone ceiling.