Refractories silica

Silica refractory bricks are manufactured from groimd ganister rock (quartzite) containing 98% Si02, to which 2% CaO is added as milk of lime. 

Mullite crystals in silica matrix formed by heating kaolinite (37,000x). (From W. D. Kingery et al.. Introduction to Ceramics, 2nd Edn., John Wiley, New York, 1976, pp. 783-812.) 

Quartz grain with cristobalite formed at surface (etched 20 min, 100°C, 50% NaOH, silica replica). 

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Acid Bessemer An alternative name for the original Bessemer steelmaking process in which the furnace is lined with a silica refractory. It is suitable only for ores relatively free from phosphorus. 

Acid Open Hearth The original version of the Open Hearth process for steelmaking in which the hearth is made of a silica refractory. The process does not remove phosphoms or sulfur, the acid impurities in the iron, so the raw materials must be relatively free from these. Pioneered by C. W. Siemens and F. M. E. and P. Martin at Sireuil, France, in 1864. 

Silica refractories, 27 516 Silica refractory brick, physical properties of, 27 495t... 

The standard ASTM PCE test is relative and used extensively only for alumina—silica refractories and raw materials (see Table 5). However, the upper service limit is generally several hundred degrees below the nominal PCE temperature because some load is generally applied to the refractory during service. In addition, chemical reactions may occur that alter the composition of the hot face and therefore the softening point. The relationship between PCE numbers and temperature is described in ASTM C24. 

Silica Refractories. This type consists mainly of silica in three crystalline forms cristobalite [1446446-1]> tridymite [1546-32-3]> and quartz [14808-60-7]. Quartzite sands and silica gravels are the main raw materials, although lime and iron oxides are added to increase the mineralization of the tridymite and cristobalite. Uses include roof linings, refractories for coke ovens, coreless induction foundry furnaces, and fused-silica technical ceramic products. Consumption of silica refractories has declined dramatically since the 1960s as a result of the changes in the steel industry. 

Cnthidia fasciculata - [SILICA - INTRODUCTION] (Vol21) - [CLAYS - SURVEY] (Vol 6) - [SILICA - SYNTHETIC QUARTZ CRYSTALS] (Vol 21) - [GLASS] (Vol 12) -from diatomite piATOMITE] (Vol 8) -OSHA specifications [ABRASIVES] (Vol 1) -m silica refractories [REFRACTORIES] (Vol 21) - [ANTIBIOTICS - ANSAMACROLIDES] (Vol 2)... 

Harvey F. A., McGee E. N., 1952, Factors affecting resistance of silica refractories to abrasion, J. Am. Ceram. Soc., 895. 

The annual world production of refractories is about 25 million tons. About 60% of this is consumed in metallurgy, the rest in the glass and cement industries, etc. The following shares of the individual types were reported for the late sixties 70% fireclay and high-alumina refractories, 17% basic refractories, 8% silica refractories and 5% other products. 

In technological practice, refractories are usually classified according to content of SLO2 and divalent oxides, as acid, neutral or basic types. The acid types comprise silica refractories and siliceous fireclay neutral refractories are alumina, mullite and chromite refractories magnesite, chrome-magnesite and dolomite refractories are... 

Figure 210 indicates that the character of the curves differs considerably with the individual materials. After an initial ascending branch, resulting from thermal expansion, there follows a bend which is gradual with fireclay and abrupt with silica refractories. The deformation interval for fireclay is 200 ""C and more, that of silica materials is only about 10 °C. 

Further complications occur when some of the phases undergo a phase transformation involving a change in volume. These effects are the cause of anomalous behaviour of the expansion and of a decrease of strength. A typical example is provided by inversions of SiO which is the main phase of silica refractories. The volume changes described in Section /.I arc projected into the expansion curve of silica refractories and also into that of high-silica fireclay. (Expansion curves of various refractories are shown in Fig. 212). 

In this way, it is possible to explain the low thermal expansion of silica refractories in the medium temperature range and a considerable expansion at low temperatures. This i.s also why silica differs considerably in thermal shock resistance in these temperature ranges. For this type of refractory, it is thus inadequate to specify the usual value of mean thermal expansion coefficient for a certain temperature range (e.g. 20—1000 C). The polymorphic inversion also results in an expansion anomaly of stabilized Z1O2 and of fusion-cast corundum-baddeleyite refractories around 1000 C (cf also Fig. 201). 

Silica refractories contain more than 93% Si02 and are made from natural quartzites comprising in excess of 96% Si02 which is converted to cristobalite and tridymite on firing while decreasing the density of the material. Besides the residual quartz, cristobalite and tridymite are the main crystalline phases of the product, which, in contrast with the materials described so far, undergoes volume expansion and even increase in porosity on firing. 

According to various data, the phase composition of silica refractories varies in the following ranges 60 — 80% cristobalite -1- tridymite, 5 — 30% quartz, 8 —15% glassy phase containing 70—80%SiO2-... 

The compressive strength decreases slightly with temperature in the case of magnesite and silica refractories. With silica refractories, indistinct extremes sometimes appear, which result from the structural inversions of Si02-... 

The principal properties of silica refractories are listed in Table 33. The porosity is relatively high, up to 30% with coke-oven silica. Attainment of values lower than those given in the table is rendered difficult by the volume changes involved in polymorphic inversions. Silica is typical by its very low content of closed pores, only 0.1 to 0.5%, whereas with fireclay their content amounts to 2 — 6%. There is no significant difference between the total porosities of standard fireclay and silica refractories however, substantially lower porosities can be attained with special grades of fireclay (less than 10%). 

The manufacture of silica refractories consists in the grinding of the previously crushed quartzite in a wet pan, the addition of the slaked lime and the molding of the prepared mixture by hand, in sanded steel molds. The pieces are then dried in tunnels and fired in down-draft kilns for about 9 days, carrying the temperature to the softening point of cones 16 to 18. The cooling requires from 5 to 7 days. The tunnel-car kiln is used in at least one plant, with a maximum heat treatment equivalent to cone 12 to 14. 

Properties of Silica Refractories.—The chemical composition of a typica material of this kind made from Pennsylvania quartzite is as follows ... 

Silica bricks withstand load conditions satisfactorily even at temperatures up to 1,500°C., under a pressure of 50 lb. per square inch. The specific heat of silica refractories varies from 0.2 to 0.266, between 0°C. and 1,200°. The thermal conductivity, in calories per second, through 1 cm. per degree Centigrade temperature difference, was found to be 0.0021, for the temperature range 0 to 100°C., and 0.0031, for 0 to 1,000°C. Dudley calculates the mean conductivity between any two temperatures, h and 2, from the relation K — 0.0020 -f 0.0000011 h + 2). Stockman and... 

Silica brick do not resist sudden temperature changes well but the harder they have been fired the better they behave in this respect. Refractories containing more tri-dymite should be expected to stand up better. As a class silica refractories show a far greater tendency to crack or spall upon chilling or rapid heating than clay materials. 

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