Synthetic Coal Slags

Name: Synthetic Coal Slags
Diagram No.: 1140
Type of diagram: TTT
Chemical composition in weight %: See the table
Group: Slags
Note: TTT diagrams of the sample slags with different CaO ratios.
Isothermal experiments were carried out under temperatures ranging from 1000 to 1350 C for each slag. After melting, the slag was rapidly quenched to the desired temperature, which was maintained throughout the crystallization process. Crystals did not form immediately, but began to appear after a period of time. The time before crystal growth began, known as the incubation time, is the time required to initiate nucleation at a fixed temperature and is an important parameter for characterizing the crystallization property. Figure 3 shows the TTT diagram. The first on the left is the starting point when crystals began to appear and the second represents when no more apparent crystal growth was observed. To make a convenient comparison, both the x and y coordinate axes were set in the same range, except for the sample with 45% CaO, because it had a high crystallization temperature. Each TTT curve has an extremum (nose) that represents the least time required to achieve a given degree of crystallinity. In the nose region, the overall crystallization process (including both nucleation and crystal growth) is much faster than at other temperatures.29 As a whole, the TTT curves of the samples with CaO between 25 and 40% demonstrated a shift toward a lower temperature region compared to the slags with low CaO contents. When the content of CaO was increased from 10 to 40%, the nose temperature decreased from 1150 to 1000 C and the incubation time around the nose was shortened.
Figure shows a comparison of the final changeless images of the crystals after the end time of TTT at 1100 C. When there was a small amount of CaO (5 or 10%), the crystals appeared as a thin layer on the surface of the slags. At successively higher CaO concentrations, more crystals occurred and gathered together. The resulting grains made it easier to distinguish from the liquid. When the percentage of CaO increased to 40%, however, the crystals were not obvious on the surface from the image. However, the exothermic peak of the DSC signal for the 35% CaO sample showed significant crystallization. Therefore, it is assumed another new mineral generated but is difficult to distinguish in the image because of the similar color to the liquid.
Crystals formed on the slag surface were observed to dissolve into the bulk molten slag when the content of CaO exceeded 40%. The time of dissolution was marked with a red point in the TTT diagrams. At first, the crystals occurred dendritic in a specific area, but after several seconds (149 s), the crystals started to dissolve and spread into a thin layer on the surface of the slag that is not easily distinguished. This phenomenon was observed at particular isothermal temperatures between 1100 and 1200 C. It is an indication that some crystal formed at first may change as the slag flows along the wall in the gasifier. Therefore, the viscosity can also be highly affected.
Reference: Not shown in this demo version.

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