## 2 Materials and Methods
### Aspects Inherent to the Process That may Deteriorate the Refractory
#### 2.1.1 Hard Pushes (Mechanical Stress and Abrasion)  
For the extraction of coke, it is necessary that the coal cake undergoes a contraction, which separates it moderately from the walls during the time it remains in the oven [16]. In this way, the evacuation of the coke cake from the oven will exert less resistance in its friction with the oven walls (see Figure 1) [8]. The way to quantify the difficulty of pushing is by measuring the amperage in the electrical motor of the extraction device (usually measured in amperes) [8]. It is possible to make this measurement through the design of the pusher ram [17]. Hard pushes lead to inadequate stress on the walls of the oven during coke removal [18]. This mechanical stress can seriously damage the refractory. At the same time, there is an increased friction of the coke against the refractory walls, increasing the abrasion of the silica material. In addition, if the area of the wall where excessive pressure is produced is a previously repaired or defective zone, its resistance decreases with respect to the rest of the zones of the oven [18]. Figure 2a shows damage on the oven wall caused by hard pushes while Figure 2b shows damage from abrasion or thermal shock. Figure 1: Friction of the coke cake on the oven walls. Figure 2: (**a**) Hole produced by hard pushes; (**b**) damage in the refractory produced by thermal shock. It would be possible to analyze the stress generated by hard pushes using the finite element method, which would permit to calculate their tolerable limit [19]. Figure 3 shows how the force exerted by hard pushes can displace the oven wall refractory. The mortar joints of refractory pieces (tongue and grooves) are particularly sensitive to this mechanical stress [20]. These are the most common causes of coke extraction difficulty:  
* Inadequate loading of coal mixture. If the charge is higher than necessary, not all the coal will be transformed into coke and consequently the needed shrinkage will not occur and greater resistance in the pushing of the oven will be generated [4]. * Incorrect charge density. Coal mixture must contain between 8 and 10 per cent of H\({}_{2}\)O. A deviation from this rate will affect the charge density and the thermal energy required to transform the coal into coke. Again, the shrinkage of the coke cake may be compromised and lead to a hard pushing [4]. * Low temperature. Incomplete transformation of the coal mixture due to poor heating is one of the most frequent causes of hard pushes. If there is a deficit of thermal energy supplied by the combustion chambers to the coking chambers, the transformation of the mixture of coals into coke will not be carried out completely and, therefore, the reduction and separation of the oven walls will not take place. * Hight temperature. Excessive energy input into the coke mixture will accelerate the process of transformation into coke, provoking a reduction of coke size in the final stage and making it mechanically less consistent and more difficult to extract. Damage to the refractory. Deformation or damage to the refractory, which prevents the coke from sliding through the oven, can lead to \"hard\" pushes, which can also generate damage in other areas of the ovens. * Carbon deposits on the oven walls. The gas obtained during coking can crack, producing carbon deposits (graphite) on the walls. This graphite turns into an obstacle during the pushing of the oven [4]. * Deviation in the coking time. For a given battery production schedule, there must be a selection of time for the coal to remain in the oven. This time is set precisely, according to the calories provided during combustion, which in turn depends on the average temperature target of the battery. The coking time must be the same for all the ovens. That is, for a given amount of coal, a given oven geometry and a fixed calorific input, the residence time should be equal. This concept is called global coking time (GCT). Positive deviation occurs when the charge remains in the oven for a period of time longer than scheduled.