Alumina-silica brick

Fireclay bricks (alumina silica bricks, alumina calcium oxide silica bricks, and other silicate bricks) are not optimal barrier materials for A1 reductirMi cells. As we have mentioned, cryolite-based electrolyte for A1 reduction is a substance that dissolves alumina better than anything else. Certainly, it will dissolve all alumina-based refractory compositions and almost all other oxides similar in chemical structure to alumina. From a chemical point of view, the effective refractory barriers against the penetration of cryolite might be tin oxide, nickel oxide, compounds of nickel oxide, iron oxide, or zinc oxide (such as spinel Fe NiOs). These oxides almost do not react with NaF and aluminium fluoride [175]. Yet the cost of these materials, which is 50-100 times higher than that of firebrick, provides the impetus to find less costly variants of alumina silica materials. 

The working layer of the furnaces above the level of molten metal is made from conventional alumina silica bricks and castables. The temperatures are in the range of 1,000 °C, and even under the roof it is below 1,200 °C, so the requirements for refractories are not strict. 

V. CASE STUDIES OF CORROSION A. Alumina-Silica Brick in Ferrous Foundry Applications... 

Figure 7 Representation of corrosion of alumina-silica brick in foundry/steel-making slag.

Table 1 Classes of Alumina-Silica Brick Explained in Terms of the Al203-Si02 Phase Equilibrium Diagram...

Table 2 Typical Raw Materials Used in Alumina-Silica Brick...

Refractory producers may have a unique or standard compliment of raw materials in their inventory to produce all of their alumina-silica brick products. Any one producer may have a special material that is unavailable to other producers. In North America, this was the case for Missouri diaspore until its depletion in the 1960s. As another example, one refractory producer had special bauxitic kaolin in Alabama used to produce 70% AI2O3 brick, and the deposit was subsequently depleted in the 1980s. In both cases cited, the producer with the special raw material had a competitive advantage during the life of its particular mineral deposit. 

The sillimanite family members, used to produce 50% and 60% alumina class brick, all exhibit characteristic permanent expansions on heating due to decomposition of the mineral and the formation of muUite. This is usually of interest in alumina-silica brick on heating the brick for the first time (during manufacture). The decomposition temperatures and expansions are given in Table 3. 

Alumina-silica bricks are manufactured from a blend of sized raw material aggregates and clays by mixing, forming, drying, and firing. These are traditional processes in manufacturing any ceramic product. Since refractories are used at... 

The best alumina-silica bricks are made in plants with common attributes. If an inspection trip is made to a refractory plant, here are the attributes worth looking for ... 

Table 5 Alkali Phases Formed at the Hot Face of Alumina-Silica Brick Resulting in Localized Melting...

Pure cordierite is an expensive material, and it is known that less pure materials containing some cordierite also exhibit low thermal expansion coefficients. For this reason, semicordierite materials are frequently used as kiln furniture and car deck block in ceramic kilns. In some cases, alumina-silica brick compositions are also used for the same product apphcations. Properties of some products used in kiln furniture/deck block applications are shown in Table 7. The maximum use temperature of these products is typically 1200°C or higher. 

One of the weaknesses of alumina-silica brick is their potential for reaction with basic slag or other corrosives to form melted phases or liquid at relatively low temperatures. The reaction potential can be illustrated by considering the... 

It is interesting that alumina-silica bricks are attacked by acids in chemical applications at room temperature. For example, 70% AI2O3 bricks have been known to exhibit service of only several weeks in hot hydrochloric and sulfiulc acids. Acid-proof bricks are commonly used in chemical service, and these bricks last longest if they exhibit extremely low apparent porosities and if then-composition exhibits extremely low alkalis. Alkali phases form glasses in manufacturing that are more readily soluble in acid media, so the bond phase in acid-proof brick must contain higher alumina content and lower alkali content. 

VI. HEALTH AND SAFETY CONSIDERATIONS WITH ALUMINA-SILICA BRICK... 

Alumina-silica brick of 60% AI2O3 class or lower (including fireclay) can contain a small amount of free or uncombined crystalline silica. Not all refractory bricks contain free silica, as its presence complicates manufacture of the brick, and certain silica phases—like cristobalite—contribute to accelerated wear. 

It is well known that free silica, as quartz or cristobalite, is a respiratory hazard. Since cristobalite is the expected phase in fired alumina-silica brick, it is of the most potential concern. It is inexpensive to determine if a brick contains quartz or cristobalite. The issue is sawing of the brick during masonry construction. Wet sawing eliminates dust exposure, and it should be used if there are concerns about silica exposure. 

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