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THE


REFRACTORIES ENGINEER


observed in the castable prisms, they showed no pop out or cracks (Fig. 1). Therefore, CAC 50 despite a wide difference in iron oxide content developed CO resistant castables (ASTM -Class A), when used with tabular alumina aggregates.


Technical Paper


a)


Table 3: Metallic iron content of regular castables and their CO resistance after 100h at 500°C


b)


c)


d)


Fig. 2: Castable test prisms after CO resistance test according to BS 1902 for 100h at 500°C Experiment 3:


Fig. 1: Castable prisms after CO resistance test according to ASTM C288 for 200h at 500°C in DIFK – a) & b) with CAC 50 L – Fe2


O3 Experiment 2:


A range of castable compositions (Table 2) was prepared with tabular alumina, 50% alumina-containing calcium aluminate aggregate (R50), CAC 50 and CAC 70. R50 and CAC 50 were produced from the same fused clinker. Despite having the same chemistry (Fe2


O3 -1.5%) R50


contained more metallic iron than CAC 50, caused by crushing of the former. CAC 70 was produced by sintering and had lower Fe2


O3 (0.2%)


and metallic iron than CAC 50. Each composition was cast with same amount of water (8.5%). The metallic iron content of each composition showed a clear relation with its CO resistance classification (Table 3, Fig. 2) [9]. Composition 2 and 7 showed Class A, although CAC 50 in composition 7 contributed higher Fe2


O3 O3 than CAC 70 in composition 2,


primarily because the overall limit for more sensitive metallic iron [10] had not crossed for composition 7. Iron compounds present in CAC 50 include Fe2


in solid solution with CA, C12


solution in C2 FeO and Fe2


O3 metallic Fe can react immediately. A7 and Perovskite, FeO in solid


AS and metallic iron generated during milling operation. are not readily available to catalyse the reaction, whereas


Table 4: Composition of regular castable with different grades of Andalusite for CO resistance test


1.38%, c) & d) with CAC 50 H - Fe2 O3 2.12%


The effect of the free iron and iron oxide content in aggregates can further be observed in the following experiment [11]. Andalusite grades with various chemistry was used to make a regular castable composition with 70% alumina-containing calcium aluminate cement as shown in Table 4. The castable prisms after curing and drying were prefired 5h at 540°C before exposing to CO- test as per ASTM 288 for 200h at 500°C. The results showed a clear relation with the total iron content (metallic iron + iron oxides, expressed as total iron oxide %) of andalusite grades by XRF. Purer grades of andalusite such as Durandal (D59 and D60) performed significantly better than others with higher iron content (Table 5, Fig. 3).


Table 2: Compositions of regular castables for CO resistance test


Table 5: Total iron content (metallic iron + iron oxides expressed as iron oxide %) of andalusite grades and CO resistance classification (ASTM) of respective castable compositions


September 2019 Issue


19


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