Refractories mullite

Mullite Refractories. MuUite refractories are classified under ASTM C467. This brick must have an Al O content between 56 and 79% and contain less than 5% impurities. Impurities are considered metal oxides other than those of aluminum and siUcon. The hot-load subsidence is 5% max is 1593°C. 

The first type of these refractories were mullite products melted from a mixture of bauxite or AI2O3 with clay. Besides mullite crystals, the product also contained less corundum and glass. It has been proved, however, that a still superior corrosion resistance is exhibited by similar corundum and corundum-baddeleyitic materials and the manufacture of mullite refractories has been abandoned. 

The work by Boyer [50] was concerned with mullite refractories utilized in blast-furnace applications. The major reaction appeared to be K2C03 fluxing of the mullite-glass samples to form minor kaliophilite. Boyer described the reaction as a soft black hygroscopic crust confined to the... 

Mullite refractories Figure 10 shows the Na20-muIlite vertical section. The phases formed are corundum, /3-Al203, and nepheline. In both cases, the mullite phase decreases in amount as Na20 reaction increases. The... 

Fig. 17. Photograph of mullite refractory before and after reaction with soda vapor at 1350°C/12 h.

It has already been noted that the primary use of the sillimanite minerals is for the production of mullite refractories. These refractories are used extensively in the steel and glass industries. Because of the association of the sillimanite minerals with higher alumina content minerals in their deposits, the range of alumina contents for the aluminosilicate refractories that they produce is approximately 50-80 wt% alumina. 

A mullite bonded refractory was made by mixing colloidal silica and basic aluminum chloride in proportions to form mullite and using this as a binder for mullite powder to form a mullite refractory body at 1300 C (489). Refractory bodies of silli-manite have been bonded with a mixture of colloidal silica and basic aluminum chloride acting as a binder for sillimanite powder, fired at 1300-I400 C (488). 

The values of thermal shock cycles temperature - water for fireclay and mullite refractories are within 5-7, and up to 30-50 cycles for silicon carbide refractories. The values of thermal shock cycles temperature - air for fireclay and mullite refractories are within 25-30 cycles, and up to 50-100 cycles for silicrHi carbide refractories. 

Crouch AG, Jolliffe KH. The effect of stress rate on the rupture strength of alumina and mullite refractories. Proc Brit Cer Soc 1970 15 37-46. 

High Alumina Refractories. The desired alumina content, from 100% to just above 45%, is obtained by adding bauxites, synthetic aluminosihcates, and synthetic aluminas to clay and other bonding agents. These refractories are used in kilns, ladles, and furnaces that operate at temperatures or under conditions for which fireclay refractories are not suited. Phosphate-bonded alumina bricks have exceptionally high strength at low to intermediate temperatures and are employed in aluminum furnaces. High alumina and mullite are used in furnace roofs and petrochemical apphcations. 

Sihca and aluminosihcate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobaUte (1). Cristobahte is a form of crystalline siUca that can cause siUcosis, a form of pneumoconiosis. lARC has deterrnined that cristobaUte should be classified as 2A, a probable carcinogen. The amount of cristobahte formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobahte. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Other types of refractory that find use are forsterite, zirconia, and zircon. Acid-resisting bricks consisting of a dense body like stoneware are used for lining tanks and conduits in the chemical industry. Carbon blocks are used as linings for the crucibles of blast furnaces, very extensively in a number of countries and to a limited extent in the United States. Fusion-cast bricks of mullite or alumina are largely used to line glass tanks. 

Note that these three minerals, which are common in the Earth s crust, are not stable at ambient pressure at high temperatures. At ambient pressure, mullite (3Al203-2Si02), is usually found in refractory materials based on these minerals. 

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

Tests at 1000°C. After 1 hour of pumping the apparent leak rate of a mullite double-walled vessel is 1.7 X 10 8 l.-mm. of mercury per second or 2 X 10-8 cc. (N.T.P.) per second. In terms of permeability rates this value is equivalent to 3 X 10-10 cc./sq. cm./second/mm./cm. of mercury. Roeser (28) has studied a number of refractory procelain tubes from several manufacturers. His permeability values vary from 8.3 X 10 10 to 5 X 10-8 cc./sq. cm./second/mm./cm. of mercury. We have studied two double-walled vessels. These two tubes give nearly identical apparent leak rates although one is constructed of mullite and the other from zircon. It may be possible that more sensitive tests would show up differences in apparent leak rates. 

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