Aluminium silicate refractories

Acidic refractories (e.g. silica, fire, clay etc.) These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. Aluminium-silicate refractories These refractories vary widely in their physical, chemical and mineralogical characteristics depending on the nature and proportion of silica and alumina present on them. 

There are several fairly large niobium deposits around the world that belong to the refractory ore type. Some of these deposits can be found in Brazil, Africa and Greenland. Typically, these ores are heavily oxidized and mostly contain iron oxides and aluminium silicates. A typical example of such a deposit is the Mrima Hill deposit found in southeast Kenya, which was a case study in which new technology was examined. 

The chief source of beryllium oxide is the mineral beryl, or beryllium aluminium silicate, 3Be0.Al203.6Si02, which occurs in Argentina, Brazil and India. When pure, beryllium oxide has a specific gravity of 3.0, and is almost insoluble in water it is soluble in sulphuric acid and in fused alkalis. It has been used as a refractory (melting point over 2500°C) but its toxicity has restricted its use. 

Pyrophyllite. A natural hydrated aluminium silicate, AI2O3.4SiO2.H2O. It occurs in N. Carolina, Newfoundland, Japan, and S. Africa, and finds some use in whiteware bodies. The massive material can be turned on a lathe and then fired at 1000-1200 C with no appreciable change in dimensions. Pyroplastic Deformation. The irreversible deformation suffered by many ceramic materials when heavily stressed at high temperatures. The term has been applied more particularly to the slow deformation of fireclay refractories when loaded at high temperatures. Pyroscope. A device that, by a change in shape or size, indicates the temperature... 

This is by far the most frequently encountered interference in AAS. Basically, a chemical interference can be defined as anything that prevents or suppresses the formation of ground state atoms in the flame. A common example is the interference produced by aluminium, silicon and phosphorus in the determination of magnesium, calcium, strontium, barium and many other metals. This is due to the formation of aluminates, silicates and phosphates which, in many instances, are refractory in the analytical flame being used. 

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. 

Although in refractory practice there are hundreds of heat insulation materials, the list of heat insulation materials for the lining of reduction cells is rather limited. For one thing, economic considerations add some limitations, but for another, the heat insulation materials in reduction cells should withstand mechanical compression loads without deformation at temperatures up to 900 °C for a long time, and numerous inexpensive fiber heat insulation materials don t correspond to this requirement. In the Hall-Heroult reduction cell, the heat insulation materials should withstand the pressure of the layer of the electrolyte, the layer of molten aluminium, cathode carbon blocks (taking into account collector bars), and the refractory layer. Currently, only four or five heat insulation materials are used in the lining of reduction cells diatomaceous (moler) and perlite bricks, vermiculite and calcium silicate blocks (slabs), and sometimes lightweight fireclay bricks (but their thermal conductivity is relatively big, while the cost is not small) and fiber fireclay bricks. 

There are several publications [4—16] on admixtures of barium sulfate, aluminium fluoride, aluminium borate, aluminium titanate, calcium silicate (woUastonite), aluminium nitride, and silicon carbide and its combinations in alumina silica refractories for decreasing the wetting ability of Al in these refractories. 

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