GEOLOGICAL, MINERALOGICAL, CERAMIC APPLICATIONS 



Figs. 14 and 15. Sintered iron ore with 3% addition of coke and 20% flue dust sho^\'ing extensive 're- 

 crystallization of iron oxides around the periphery of original ore grains alone with primary dendrites 

 of Fe304 in slag pool. Note the characteristic "shaded" cracking which is found to precede final 

 break-up and recrystallization of ore grains. X93. Microradiograph —Cu radiation. 



the structures difficult due to falling out of 

 parts of the surface. 



The structure of a magnesite refractor}^ 

 uncontaminated by iron is clearly shown by 

 radiography in Fig. 16. Here a large piece of 

 "fused cast" magnesia is embedded in finer- 

 grained magnesia. The whole structure is 

 bonded by silicate which appears white on 

 the radiograph due to the very low absorp- 

 tion of magnesium compared to the calcium 

 associated with the silicate. The "fused 

 cast" magnesia demonstrates the three di- 

 mensional aspect of radiography. 



It is in the field of chrome/magnesite re- 

 fractories, however, that microradiography 

 has proved of even greater value by giving 

 an understanding of the structures formed 

 in service, in particular, in open-hearth fur- 

 nace roofs. Figs. 17, 18 and 19 show typical 

 radiographs on sections near the working 

 face of such bricks. All the sections are com- 

 pletely opaque except for some tiny pools of 

 silicate. The magnesia in Fig. 17 which has 

 recrystallized into crystals elongated per- 

 pendicular to the working face is consider- 

 ably lighter than in Fig. 16. This is due to 

 the high iron content of the MgO which 



^ff?a' 



Fig. 16. Microradiograph of part of used mag- 

 nesite brick with a large piece of "fused cast" mag- 

 nesia embedded in a finer magnesia, the whole 

 being bonded with silicate (white, because of Ca 

 content). Note the three dimensional effect vis- 

 ible in the large grains. X25 Ni radiation. 



actually contains a fine precipitate of MgO- 

 Fe203 — usually unresolved on the radio- 

 graph. In Fig. 18 a coarser precipitate is 

 clearly resolved. 



Both Figs. 17 and 18 show the intimate 

 bonding between the chrome grains and the 

 magnesia which often nucleates on the front 

 of the grain during recrystallization. An iron 

 rich spinel phase is found on the other side 

 away from the furnace and grows as co- 



579 



