Drying and Firing of Refractory Castables
Refractory monolithic linings are dried on site by one-side heating. During drying, rapid heating rates might lead to degradation of mechanical properties, and in extreme cases, to excessive buildup of pore pressure and even explosive spalling. A slow heating rate, on the other hand, is more energy and time consuming. Drying involves coupled heat and mass transfer in a porous solid undergoing microstructural changes (i. e., pore size and shape) and chemical changes (i. e., dehydration). Steam pore pressure is the main driving force for moisture transfer, as well as the force that could cause failure of the refractory concrete when it builds above its mechanical strength. Several material properties (such as permeability, thermal conductivity, and mechanical strength) are strongly affected by temperature and moisture content during the drying process. Dewatering is affected by the coupled and interactive influences of a number of variables, which include texture, mix constitution, permeability, strength, thermal conductivity, moisture content, casting and curing practice, binder level and type, dry-out schedule, and installation geometry. A common method for improving the spalling resistance of refractory concretes has been to add organic fibers to the mixes to increase permeability.
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:
where the pressure drop ЛР is the difference between the absolute fluid pressure at the entrance and at the exit of the sample, L is the sample thickness, and K is the coefficient of permeability, used in computer simulations [i. e., 46]. Very low permeabilities of refractory castables are measured using a vacuum decay approach [47, 48]. A vacuum decay curve is generated by monitoring the pressure change across a specimen-slab in a vacuum chamber, as a function of time.