What Are The Causes Of Refractory Failures

Refractories are heat-resistant materials that constitute the linings for high-temperature furnaces and reactors and other processing units. In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also withstand physical wear and corrosion by chemical agents. Any failure of refractory could result in a great loss of production time.

The refractory material failure may caused by many different factors, such as chemical reaction and corrosion, spalling, material selection, plant operations, material storage, mixing, installation, curing, and drying. Only by understanding all aspects pertaining to the design and installation of the refractory material can one find the cause of the failure and help eliminate future failures.

The most common cause for failure of refractory is chemical reaction with the environment in which it is operating and chemical corrosion from molten slag and hot gas/molten salt. Chemical corrosion of a refractory is caused by slag attack at the refractory surface. The material selected must match the chemical environment that exists. For example, an acidic refractory should not be used in furnaces using basic fluxes, slag, etc. and vice-versa.

The porosity of refractory plays an important role in the chemical reaction. The more porous it is, the greater will be the depth to which the slag will penetrate and destroy the refractory. As the temperature increases, the rate of chemical reaction gradually increases. Sometimes, rise in temperature beyond the safe limit quickly brings about the destruction of the refractory. These chemical aspects are complementary to the engineering plant aspects and must be taken into consideration for a successful realization of the process.

Another important cause is spalling. It may be thermal, mechanical or structural. Thermal spalling may be due to unequal expansion or contraction caused by the difference in temperature at different parts. Mechanical spalling is mostly due to carelessness in loading the furnace or in the removal of materials from furnace, thereby damaging the refractory. Structural spalling takes place due to change in composition of the refractory because of reaction with slags, flux, etc. as a result its coefficient of expansion changes. Thus, different parts expand and contract to a different extent.

Improper material storage, mixing, installation, curing and drying will also cause refractory failure. Refractory material should always be stored in dry, well-ventilated conditions. Use fresh refractory materials and follow proper storage procedures to ensure that the refractory will not lose strength. Use potable water (suitable for drinking) for mixing. The use of the wrong type of water will hinder the ability of the refractory material to reach its proper strength. Using the right type of mixer, following proper mixing procedures, and staying within recommended pot life are other important installation factors. Using the wrong mixer or pneumatic gun could also affect the strength of the refractory material. Almost all refractory materials (except those that are phosphate bonded) must be cured prior to the drying process. Failure to properly cure a cement-bonded refractory material is the number one contributor to refractory failure and lack of longevity.

Problems with the quality of the refractory material itself is also an important reason for the failure of refracoties. A selection of the right refractories for a specific application is important.