Refractories lime bond

The coal had to have an adequate ash fusion temperature, preferably over 1200 °C and an acceptably low ash content. A high ash fusion temperature reduced the risk of crotching, caused by the ash fusing and bonding lumps of lime to each other and to the refractories. It also ensured that most of the ash did not adhere to the lime and could be removed by screening at (say) 6 mm. The ash-contaminated fine lime could be sold as a low-grade product. 

Refractory silica bricks, containing 96 to 98 % of Si02, are bonded using 1 to 3 % of CaO added as a milk of lime or as hydrate, together with a small quantity of a finely divided sodium-iron-silicate flux [32.3]. 

Lime is also used as a bonding and stabilising agent in the production of silicon carbide and zirconia refractories [32.4]. 

No-Cement Castable. Alumina and aluminosilicate castable refractories which do not contain hydraulic setting cement and in which the bonding agents contribute no significant amount of lime (CaO). The product might contain up to 0-2% total lime (CaO) on a calcined basis as contributed by the aggregate. (ASTM C401-91). 

Sandstone. A sedimentary rock which may be of several types, e.g. siliceous, calcareous, argillaceous, depending on whether the grains of silica in the rock are bonded with secondary silica, with lime or with clay. Siliceous and argillaceous sandstones both find some use in the refractories industry crushed sandstone is used as a source of silica in glass manufacture. 

In the 1990s, trials of doloma-based refractories in the continuous casting of steel were started. It was felt that the high lime content (57%) of the doloma refractory would make it very effective in the prevention of alumina buildup during the continuous casting of aluminum killed carbon and stainless steels. Doloma graphite SENs and ceramic bonded doloma tundish nozzles have both been developed. 

As an illustration. Table 10.7 shows the numerous reduetion reactions which take place in carbon magnesia refractories at high temperatures. These reactions bring into play carbon (originating from the bond and the graphite), periclase grains, lime siUcates which constitute the bonds between the magnesia crystals and the metal additions (which act as carbon antioxidants). 

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