Otras herramientas informáticas utilizadas en este trabajo

The infiltration of liquid slag into the porous refractory lining has a significant effect on the degradation and, thus, on the lifetime of the reactor. By infiltration the reaction surface between the liquid slag and the refractory phases increases, leading to faster chemical corrosion of the lining. The reactions in the interior of the lining can also lead to new degradation mechanisms, limiting the lining’s lifetime. For example, the formation of new phases can lead to a volume expansion and the internal stresses in turn can lead to spalling of a part of the lining [23]. Due to the detrimental effect of slag infiltration on the lifetime of the refractory lining several methods have been studied trying to stop the infiltration. An overview is given below.

Brick production

Already during the production process the infiltration behavior of the slag can be controlled. From equation 3-4 it is clear that the infiltration behavior depends on the size of the open pores and the contact angle between the slag and the refractory phases. Both can be controlled during the production process. By selecting the size distribution of the grains before firing, the open pore size can be controlled, leading to a limited infiltration behavior. The contact angle between the slag and the refractory phases differs for the chromite and periclase phases. During the production process, the amount of both phases can be modified to increase the amount of the phase limiting the infiltration of the liquid.

Even after the firing of the brick the pore size can still be reduced. Deng et al. [24] used vacuum impregnation of the open pores in magnesia-chromite bricks using Cr2O3 precursor sol and MgCr2O4 spinel precursor sol to reduce

the median pore size diameter from 22.03 micron (untreated brick) to 14.75 micron (Cr2O3 precursor sol) and 13.31 micron (MgCr2O4 spinel

precursor sol). The effect of the reduced pore size on infiltration of a copper convertor (fayalite type) slag was studied; the results are shown in Figure 3.10. The reduction in pore size clearly leads to a reduction in the slag penetration depth.

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Figure 3.10: Effect of the impregnation of the open pores of a magnesia- chromite brick with Cr2O3 precursor sol (single sol) and MgCr2O4 spinel

precursor sol (mixed sol) on the penetration by and chemical corrosion caused by a fayalite slag from a copper convertor after 3 h at 1600 °C. Taken from [24].

Pretreatment of the brick

The infiltration of the slag is only possible due to the open pores in the refractory bricks. Several authors [25, 26] have therefore tried to create a completely dense surface layer on the bricks, thereby keeping the beneficial properties of a porous refractory lining, while at the same time preventing the infiltration and chemical degradation in the refractory lining. In order to get this dense layer, the surface of the porous brick is melted using a laser, which, after solidification leads to the formation of dense layer. The large temperature difference between the top layer and the rest of the brick can, however, result in internal stresses and cracks [27, 28], leading to slag infiltration or spalling of the dense layer. In order to mitigate this effect Bradley et al. [29] have used the setup shown in Figure 3.11 which heats the surrounding of the melted part of the lining to prevent internal stresses resulting into a complete dense layer without cracks.

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Figure 3.11: Schematic representation of the setup used to pretreat refractory bricks by melting the surface. Based on [29].

Reactions during the lifetime of the lining

The reaction between the slag and the refractory itself can limit or even stop the infiltration. As mentioned in section 3.4.3, the reaction during infiltration changes the slag composition inside the sample. This can lead to a change in slag properties like the viscosity or the liquidus temperature when specific components are removed from the infiltrating liquid. The system studied by Mukai et al. [18], for example, resulted in an increase in the SiO2 content of the slag with infiltration depth. The resulting increase in

viscosity slowed down any further infiltration while the increase of the liquidus temperature of the slag caused the slag to eventually freeze in the sample.

The formation of new phases can seal off the open pores and thus stop the infiltration of the liquid. A well-known mechanism in the steel industry [30], leading to the sealing of the open pores, occurs when MgO-C bricks are heated to the typical working temperature for the steel industry, usually 1700 °C. At this temperature the carbon reduces the MgO to form Mg and CO, both are gases at the working temperature. When this gas mixture exits the porous lining it can come into contact with a locally higher pO2, either in

the gas phase or in the slag. The reaction with oxygen again creates solid MgO, forming a layer at the hot face of the lining. When this layer seals the

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interior of the brick from the surroundings, the reaction between MgO and C inside the lining reaches an equilibrium state, preventing further oxidation of carbon. If part of the layer fails, the gas will start to flow through it again, reforming the sealing layer at all holes, thus resulting in a self-healing mechanism.

The closing of the open pores can also be caused by the formation of new phases due to the reaction between slag and refractory, preventing further slag infiltration into the brick. Kaneko et al. [31] for example showed that the infiltration of a synthetic Al2O3-SiO2-FeO-CaO-MgO-Na2O-K2O slag was

hindered by the formation of a sealing FeCr2O4 layer on top of a Cr2O3-Al2O3

refractory brick formed by reaction between the refractory brick and the FeO from the liquid slag.