Monolithic refractories

Monolithic refractories are suitable for walls that must be gas tight. The weight of the furnace itself is sustained by supports that help the monolithic material adhere to the shell and prevent gas leakage. 

Monolithic refractories have lower thermal expansion than most refractory bricks. Whatever small expansion does occur can usually be absorbed by the supports. Therefore, unlike refractory bricks, monolithic refractory walls do not require clearances for thermal expansion. Clearances required for brick construction may allow passage for furnace gas leaks out or air into a furnace. The superior sealing capability and reduced expansion of monolithic refractories make them suitable for higher furnace 

Castable refractories consist of course and fine grains with suitable bonding cement. After mixing with water, these are poured in place using molds or pouring forms. 

Trowelable refractories are a kind of castable refractory mortar with a consistency that makes it easy to trowel into place—very useful for patching and for shaping complex surfaces. 

Plastic refractories contain a binder material, and are tempered with water so that they have suitable plasticity for pounding or ramming into place. 

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A. Nishikawa, Technology of Monolithic Refractories, Pbbrico Japan Company, Tokyo, Japan, 1984. 

Castable Monolithic Refractories Standard portland cement is made of calcium hydroxide. In exposures above 427°C (800°F) the hydroxyl ion is removed from portland (water removed) below 427°C (800°F), water is added. This cyclic exposure results in spalling. Castables are made of calcium aluminate (rather than portland) without the hydroxide they are not subject to that cychc spaJhng failure. 

Monolithic refractory coatings have been applied to metallic components in furnaces for fuel ash corrosion control. Results have been less than satisfactory because of the large thermal expansion mismatch between the metal and refractory. Failure usually occurs upon thermal cycling which causes cracking, eventual spalling of the refractory, and direct exposure of the metal to the effects of the fuel ash. 

Baueijee, S. Monolithic Refractories. World Scientific Piiblislniig Company. Iuc,... 

A common trend seen today in this industry is plant integration through the merger and alliance of refractories companies. In general, the global refractory production has increased, due in part to process optimization (better usage of refractories) or to improved refractory materials with better performance. Monolithic refractories consumption, however, has markedly increased and represents today a large portion of the total. For instance, it is about 60% of the total production in Japan [63],... 

Technology of Monolithic Refractories, Chap. 1 Revised edition, Plibrico Japan Co., Ltd., Tokyo Insho Kan Printing Co., Ltd., 1999. 

S. Banerjee, Monolithic Refractories, Chap. 2, The American Ceramic Society/World... 

R.E. Moore, J.D. Smith, and T.P. Sander, Dewatering monolithic refractory castables experimental and practical experience, Proc. UNITECR 97, 1997, pp. 573-583. 

K. Murakami, T. Yamato, Y. Ushijima, and K. Asano, The trend of monolithic refractory technology in Japan, Refract. Appl. News, 8(5) 12-16 (2003). 

ASTM C 417 Standard Test Method for Thermal Conductivity of Unfired Monolithic Refractories, December 1993, Revised 1998. 

Dead-burned dolomite is used in the production of refractory bricks, shaped refractories and for monolithic refractories. High purity, low iron dolomite for brickmaking is generally sintered at temperatures of 1800 °C or higher (see section 16.9). A lower purity product (which is often pre-blended with 5 to 10 % of iron oxide to assist sintering) is used for fettling purposes. It is sintered at 1400 to 1600 °C. 

Monolithic refractories creep under compressive stress. At stresses much less than the crush strength, the creep rate diminishes with time and approaches zero. Creep occurs under nominally constant stress. When strain instead of stress is held constant, the stress relaxes by the same mechanism that causes creep. Creep rate increases at lower temperatures and drops off with temperature. 

Wygant, J. F., and Crowley, M. S, Designing Monolithic Refractory Vessel Linings, Ceramic Bulletin, Volume 43, No. 3, 1964. 

Continuing improvements in monolithic refractories, particularly in bonding, have resulted in their steadily increasing usage— now substantially over 60% monolithic. 

Firebrick originally provided load bearing walls, heat resistance, and containment. As steel framing and casing became more common, and as monolithic refractories were improved, furnaces were built with externally suspended roof and walls. 

The basic components of most refractories are oxides of various origins. Tables 9.1 and 9.2 list properties of some monolithic refractory materials. 

Furnace construction with monolithic refractories is determined by the method(s) to be used in installing the furnace lining, which may be dictated by furnace configuration, time limitations, or other local site conditions. The furnace designer must determine the minimum refractory thickness required. (See table 9.3.) Thicker-than-minimum linings are usually mandated by fundamental economic considerations such as fuel conservation (less heat loss), extended lining life, and reduced maintenance. Additional lining thickness also may be required because of workplace environmental considerations (e.g., external shell temperature or interal atmosphere). 

Although these two terms are sometimes used interchangeably, anchors are ceramic or high-temperature metal alloy shapes embedded in a monolithic refractory whereas hangers are usually the metal holders for the anchors. The hangers and anchors not only support the refractory wall or roof but do so while allowing shght expansion and... 

Figure 9.8 shows some more typical monolithic refractory supports. 

Fig. 9.8. Monolithic refractories in roof (arch) construction and in nose construction, using supports consisting of ceramic anchors held by alloy hangers.

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