Permeability of Refractories

Diffusion through refractories is of technical importance. According to the fineness of the pore structure and the pressure the fiow should conform predominantly to the equations for Poiseuille flow or Knudsen flow. In general the numerical value of the permeability depends on a variety of factors(26) which include  

As a rule the physical constitution produces greater changes in the permeability than the chemical nature of the refractory. Thus one finds a very great range in permeability in different samples of the same chemical nature, as is illustrated in the following data (Table 3) due to Kanz(27), In agreement with 

Type of substance Permeability range (c.c./sec./cm.Vcm./sec./cm. of water difference in pressure) 

Fireclay product Silica product Magnesite product Chromite product Insulating product 0 00278-0-476 0 0158 -0132 0 0276 -0 265 0147 -1 42 0 00597-0 539 

One may look for interesting results by considering the influence of temperature upon the rate of permeation of refractories. The effects of temperature help to sort out the different types of flow, Poiseuille, Knudsen, and activated diffusion. In a refractory should occur large pores and tubes, small pores, and pores of molecular or sub-molecular dimensions. In the large pores one would anticipate Poiseuille flow the permeability Pq is proportional to = viscosity)  

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ASTM C-577-99. Standard test method for the permeability of refractories. 

C577-99 Standard Test Method for Permeability of Refractories. Annual Book of ASTM Standards Vol. 15.01. West Conshohocken, PA American Society for Testing and Materials, 2001 99 102. 

Bell DA. Cold cmshing strength A waste of time Refract Eng 2000 March 22. Innocentini MDM, Silva MG, Menegazzo BA, Pandolfelli VC. Permeability of refractory castables at high temperatures. J Am Ceram Soc 2001 84(3) 645-647. Innocentini MDM, Pardo ARF, Pandolfelh VC. Modified pressure decay technique for evaluating the permeability of highly dense refractories. J Am Ceram Soc 2000 83 220-222. 

Refractories are materials that resist the action of hot environments by containing heat energy and hot or molten materials (1). There is no well-established line of demarcation between those materials that are and those that are not refractory. The ability to withstand temperatures above 1100°C without softening has, however, been cited as a practical requirement of industrial refractory materials (see CERAMICS). The type of refractories used in any particular application depends on the critical requirements of the process. For example, processes that demand resistance to gaseous or liquid corrosion require low permeability, high physical strength, and abrasion resistance. Conditions that demand low thermal conductivity may require entirely different refractories. Combinations of several refractories are generally employed. 

Another chondrite component is an optically opaque (in thin section) assortment of very fine-grained minerals that fills the spaces between the larger chondrules, refractory inclusions, and metal grains. This material is called matrix. Characterization of matrix minerals is hampered by their tiny particle sizes (as small as 50-100 nm). Moreover, the fine grain sizes, high porosity, and permeability of matrix make it especially susceptible to alteration during later heating or exposure to aqueous fluids. 

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. 

It should be emphasized that world-wide the treatment industry is based on comparatively few moderately permeable timbers. Problems can arise when there is commercial interest in using a timber that is somewhat less than ideal, perhaps because it is the main plantation species of that country (for example in the use of eucalypts and spruee). Although treatment of refractory species is not ideal, by drying to a low moisture eontent and with a high preservative loading in the surface layer, adequate serviee life may be achievable for certain end uses. 

Permeability is the material property that most influences the drying process of refractory castables [43—45]. The permeability of compressible fluids flowing through rigid and homogeneous porous media is described by the Forchheimer equation, which includes a quadratic term for the flow rate q. For small changes in pressure, the Forchheimer s equation leads to Darcy s law ... 

For further examples of flow through porous plates one may study the permeability of various refractories, the properties of which, in relation to the diffusion problem, are given in the next section. 

Labyrinth Factor. A correction to be applied in calculating the gas permeability of a porous refractory, to allow for the diminished air flow because pores are neither circular nor straight. The labyrinth factor is the ratio of that part of the total porosity which contributes to the air-flow, to the total porosity. 

Permeability. The rate of flow of a fluid (usually air) through a porous ceramic material per unit area and unit pressure gradient. This property gives some idea of the size of the pores in a body -whereas the measurement of porosity (q.v.) evaluates only the total pore volume. From the permeability of a compacted powder the specific surface (q.v.) of the powder can be deduced. B.S. 1902 Pt. 3.9 describes a gas permeability test applicable to refractory materials, as does ASTM C577. The ASTM C866 test for the filtration rate of whiteware clays depends on their water permeability. See blaine test carman equation lea and nurse... 

In industrial practice, the structure of refractories is characterized by the apparent density and open porosity, and, usually, the real density and closed porosity and not used. The exception is the quality of carbon cathode blocks, where the real (true) density is taken into account not only in research, but in industrial practice as well (because it characterizes the ratio of graphitation). Water absorption is not used frequently. In research, the real density and closed porosity are important. In several cases, it is important to know the gas permeability of the material and the... 

Permeability is the abiUty of the refractory material to transmit the gas or liquid. The permeable porosity is a part of the capillary porosity. The permeability of the material may aid in understanding the corrosion resistance, the resistance to infiltration of metal, slag, or electrolyte, although there is no direct consequence. 

The specific inner surface of refractories can be calculated from the open porosity and gas permeability [31]. It can be determined by the low-temperature... 

The corrosion includes the process of chemical corrosion itself, but it is also necessary to take into accoimt the penetration of the aggressive liquid in open permeable pores, the diffusion of the aggressive melt (or constituents of the aggressive melts) into the structure of refractory materials, and others. 

The Influence of Porosity and Permeability on Corrosion Resistance of Refractories... 

The book by Prof. Yanko [1] discusses a mechanism of degradation of the flue walls, stating that the main corrosion agents are aluminium fluoride and sodium fluoride. The temperature of evaporation of aluminium fluoride is - 950 °C, while the temperature of evaporation of sodium fluoride is - 1,050 °C. Volatile sodium and aluminium fluoride penetrate the permeable pores of refractory [1] and interact with silica, giving volatile silicon tetrafluoride, while sodium oxide is released in the pores ... 

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