Carbide refractory

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

Refractories. Its low coefficient of expansion, high thermal conductivity, and general chemical and physical stabihty make sihcon carbide a valuable material for refractory use. Suitable apphcations for sihcon carbide refractory shapes include boiler furnace walls, checker bricks, mufflers, kiln furniture, furnace skid rails, trays for zinc purification plants, etc (see Refractories). 

Modem installations often impose furnace conditions so severe that refractories other than lire-clay are needed. High aluminum and silicon carbide refractories are typical of tlie.se. The heal conductivities of the super-refractories are larger than those of lire-clay brick, and such construction should be backed up with high temperature insulation. Silicon carbide blocks are the most refractory and have the quality of resisting clinker adhesion heitcr than ordinary fire-brick. Their fusion temperature is about 4000 F (2204 C)... 

The largest quantity of Si3N4 by weight is applied in nitride bonded silicon carbide refractories which are produced in amounts of 20,000 t/year [14]. 

Bond systems are generally the key in assessing how any refractory, including silicon carbides, will perform in contact with a corrodent. Permeable silicon carbide refractories can be classified as ... 

Silicon carbide refractories are employed in considerable quantities in kiln furniture and in muffles in the ceramic industry, where use is made of their thermal conductivity. The life depends on conditions it is impaired by access of air and water vapour. Oxidation results in volume expansion, embrittlement and cracking of the product. 

Refrax [Carborundum.]. TM for silicon nitride-bonded silicon carbide refractories. Available in brick and precision-formed shapes and parts. 

Decomposition of batch materials can produce extremely large quantities of gases such as CO2, SO3, NOx, H2O, etc. Reactions with metals in contact with the melt can generate oxygen, carbon dioxide, or hydrogen by electrolytic reactions. Corrosion of refractories can open previously closed pores to the melt, releasing the gas contained in those pores into the melt. Residual carbon in refractories, or carbide refractories such as SiC, can react with oxide melts to form CO2 or CO. The products of all of these reactions can agglomerate to form bubbles. 

Reddy N. K., lYoperties of Silicate-Bonded Silicon Carbide Refractories, Materiak Lettern 47 (2001) 305-307. 

ASTM C-863 Evaluating oxidation resistance of silicon carbide refractories at elevated temperatures... 

Continuous Kiln. A kiln in which the full firing temperature is continuously maintained in one or other zone of the kiln. There are two types annular KILN (q.v.) and tunnel kiln (q.v.). Continuous Vertical Retort. A type of gas retort, built of silica or siliceous refractories. Coal is charged into the top of the retort, coke is extracted from the bottom, and town gas is drawn off, the whole operation being continuous (cf. HORIZONTAL retort). Continuous vertical retorts are also used in the zinc industry, in which case they are built of silicon carbide refractories. 

Silicon carbide refractory 15-120 Floor quarry 30-50 Cement-Lime Mortar 1 2 8-9... 

One characteristic of a refractory is the mineral composition. In the Al industry, the most common are alumina silica refractories, carbon, and silicon carbide refractories. 

In general, the porosity of traditional fireclay refractories (Table 1.2) is 22-25 %, while modern alumina silica materials reach 13-15 %, silicon carbide refractories have porosity 12-18 %, cathode carbon refractories have porosity 15-22 %, while almost all the pores are open and permeable. The porosity of low-cement and ultralow-cement castables is 12-15 %, while the ratio of permeable pores is very low (the permeability is 0.1-0.4 pm ). 

Hardness testing of the refractories is not an everyday procedure, yet it is very useful to remember that the hardness of self-bonded silicon carbide ceramics is in the range of 20-25 GPa, the hardness of nitride-bonded silicon carbide refractories is 16-20 GPa, and the hardness of alumina ceramics is 12-16 GPa, which is three to four times higher than with dense alumina silica dense refractories. 

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. 

Yurkov A, Danilova O, Dovgal A. Nitride bonded silicon carbide refractories structure variations and corrosion resistance, 13th Biennial Worldwide Congress on Refractories UNITECR 2013, the Unified International Conference on Refractories, 10-13 Sept. 2013, Victoria, p. 991-6. 

Etzion R, Metson J, Depree N. Wear mechanism of silicon nitride bonded silicon carbide refractory materials. Light Met. 2008 137 955-9. 

Etzion R, Metson JB. Factors affecting corrosion resistance of silicon nitride bonded silicon carbide refractory blocks. J Am Ceram Soc. 2012 95 410-5. 

As we mentioned earlier in this chapter, fireclay, muUite, and alumina refractories have been used for the layer that is in contact with molten Al. There were trials of magnesia, zirconium, aluminium titanate, and silicmi carbide refractories. The results of these trials cannot be called tmsuccessful. Most likely, these trials were sustainable for the entire range of actions necessary at implementation of a new material. In a limited amount, these materials are used today (crucibles for secondary Al for low-pressure dye casting, runners between melting and holding furnaces for special alloys, burner blocks, tap hole blocks). Yet the existing technical decision is to use alumina silica refractories with a relatively sophisticated structure and antiwetting additives. 

Stroms, E. K. (1997). The Refractory Carbides, Refractory Materials. London Academic Press. 

---