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Technical Paper


system is higher, which indicates higher refractoriness. Furthermore, the liquid content of the calcia-free system at 1600 and 1700°C is clearly lower than in the system with calcia, which would indicate better anti-corrosion characteristics. In general, the system of Al2 better than the system Al2


O3 O3 oxide slag attack.


3.3 Review analysis on used steel ladle purging plug A used purging plug based on corundum/spinel low cement castable was taken as an example for review analysis. The original height of the purging plug was 420 mm, with a top diameter of 120 mm and bottom diameter of 180 mm. There were two rows of slits in the purging plug, with 10 in the inner area and 20 in the outer area. After 25 heats (including Ladle Furnace and Vacuum Degassing treatment) of, in total, about 2000 min, the residual height of the purging plug was reduced to 250 mm, with a concave shape in the hot face area. A sample of the hot face of the purging plug was taken for SEM analysis.


Figure 10 shows the transition zone of infiltrated area and non-infiltrated area. The bright phase areas are infiltrated steel. The lighter grey areas beside the infiltrated steel were verified as spinel solid solution. The EDS results indicate FeO rich spinel ss with minor content of MgO. Due to the high amount of liquid steel and iron-rich slag infiltrated into the refractory material and the high temperatures during oxygen lancing, the iron oxide can easily react with magnesia aluminate spinel and corundum to form the spinel solid solution. According to Figure 4, such iron rich spinel solid solution is thermodynamically much more stable than any other phase in the system. The darker grey phase beside spinel ss and infiltrated steel is CA6


, and does not show a solid solutioning with Fe2 EDS results.


4. Results and discussion 4.1 Mechanical properties


Data of the test castables pre-fired at 1650°C are given in Table 3. More data for these properties and a discussion of the differences at different temperatures are given in a previous paper [6]


. At 1650°C sintering shrinkage


occurs for all tested castables regardless of the binder. The open porosity of the hydratable alumina castable is about 25 % higher when compared to the cement bonded castables. This can be explained by a higher mixing water demand (4.7 % of the hydratable alumina vs. 3.9 to 4.2 % for the cement bonded mixes) due to the very high specific surface area, 250 – 300 m²/g, of the Alphabond 300 which absorbs water.


The CMoR and CCS results are given in Figures 11 and 12. The strength levels vary depending upon temperature. The curing strength at 20°C is the lowest. During “drying” at 110°C further hydration takes place and the


O3 according to the -MgO can be considered -MgO-CaO regarding resistance against iron Castable


Bulk density (g/cm³) Open porosity (%)


Permanent linear change (%)


C2S26 3.20 12.1


-0.42


www.ireng.org


C5S26 3.16 12.3


-0.38


C5S0 3.20 11.8


-0.65


A4S26 3.13 16.3


-0.34


Table 3: Open porosity, bulk density and permanent linear change of cement bonded castables after firing at 1650°C/5 h


strength increases accordingly, because this temperature is too low to remove all pore water from these dense and low permeability castables. Tempering at 350°C, where most of the water is removed, does not change the strength level in general. The strength level of the ultra-low cement castable C2S26 and hydratable alumina castable A4S26 is lower when compared to the higher cement content of C5S26 and C5S0 (LCC). It is interesting to see that the hydratable alumina mix has slightly higher strength after drying and tempering than the ULCC. At 1000°C, the hydrates formed by the cement have decomposed and weak ceramic sintering started. Also for the hydratable alumina binder the hydrates formed have decomposed. However, here a clear difference shows as the strength level of the hydratable alumina mix is clearly lower at 1000°C. Obviously higher temperature is needed for enabling a sintering in this system, e.g. 1250°C as shown in Figures 11 and 12. High firing temperatures such as 1500 and 1650°C lead to clear strength increases. The strength level of the cement bonded mixes is about double of the hydratable alumina bonded mix, regardless if the cement content is 2 or 5 %.


For industrial application the strength level of the Alphabond 300 castable fired at 1250°C or higher is considered sufficient. When comparing different cement contents it appears that the cement content does not influence the sintered strength at high temperatures. However, when comparing the ULCC with the no-cement castables it becomes apparent, that even a small amount of cement has a clear effect on the sintered strength at high temperatures, although all castables have the same amount of reactive alumina in the matrix.


During the production of a purging plug, sufficient curing strength is needed in order to provide adequate mechanical strength for the de- molding, handling, and transportation of the green purging plug. The drying strength at 350-400°C is also important, because most purging plugs in Europe are tempered in that temperature range before delivery to the steel works. In China, purging plugs are generally already pre-fired at high temperature prior to application in the steel ladle. Strength at 1000°C might be irrelevant for those purging plugs which are pre-fired at high temperatures, for example 1600°C, as the ceramic bond has already formed. However it would be relevant to those tempered at lower temperatures of around 400°C, because each purging plug goes through a temperature of around 800~1000°C during pre-heating or operation, and the purging plug


Figure 10: SEM and EDS analysis on used purging plug: spinel solid solution (light grey phase, #1, EDS analysis left side); 2 indicates CA6 analysis right side; bright phase is infiltrated steel


20 ENGINEER THE REFRACTORIES


(dark grey phase, #2, EDS


July 2018 Issue


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