Refractories Review: Refractory Fallacies
Myth #1: A Brick is a Brick is a Brick.Refractories of any given type—e.g., super duty fireclay, 70% alumina, 96% MgO, SiC—made by different manufacturers are identical and don’t need to be compared for suitability in the intended use application. Reality: While stainless steel alloy components purchased from any of several manufacturers will have essentially identical composition and properties, this is not always true for refractories. Refractories of any type from different manufacturers can vary significantly, so the selection and purchase must be carefully considered.
Myth #2: Refractories are Forever. Refractories can be expected to exhibit good, unlimited service life in any application, no matter how they are used and abused. Reality: Most refractories are consumable materials that are going to wear out, hopefully at an acceptable rate to allow the desired optimum performance to be achieved. But there is a great disparity in the optimum life of refractories, depending on the application, ranging from <10 minutes to more than 25 years. The careful consideration of multiple factors is very important for the optimization of any refractory lining life.
Myth #3: Better Products through Chemistry. The use of fluxing additives (e.g., fluorspar [CaF2], B2O3, NaOH, etc.) in a process will have no effect on the life of the refractory lining. Reality: Intentional and unintentional additions can have a major detrimental effect on refractories. The common result will be accelerated refractory wear/melting and significantly reduced lining life.
Myth #4: Bricks are Better than Castables. Castable refractories can’t possibly have properties and service life as good as pressed and fired bricks. Reality: In the past, when more than 15 wt% calcium aluminate cement binder was common in castables, they were not as good as bricks. But since the mid-80s, low-, ultralow- and no-cement castables have been developed with carefully controlled particle size grading, and ultra-fine and other additions. These new generation castables do have properties and service life that are better than equivalent bricks. So based on material and installation improvements, castables and other monolithic refractories have surpassed the usage of pressed and fired shapes in many countries.
Myth #5: More is Better.It is OK to add whatever amount of water is needed to make the installation of a castable quicker and easier for the workers. Reality: Castables actually have an optimum range of water addition, and using more or less than the optimum amount will result in property deterioration and reduced service life.
Myth #6: H2O is H2O. It is OK to use muddy water from a river or canal for mixing a castable. Reality: Suspended solids (e.g., clay) in the water will modify the castable composition, causing various detrimental effects that will reduce the life of the castable, including possible premature catastrophic collapse/failure. A general guideline for the water used to mix a castable is that it should be “drinkable.”
Myth #7: Thermal Shock Happens. The heatup of a refractory lining should be controlled, but the cooldown can be as fast as possible—even by spraying water on the hot lining to accelerate the cooling. Reality: The heatup and cooldown of a refractory lining should both be controlled, because damage can be induced in either case. A heating/cooling rate of less than 100°F/hour is commonly recommended.
Myth #8: Who Cares about Alberta Clippers? The mixing, installation and storage of castable refractories by a contractor will not be affected by cold temperatures (below 50°F). Reality: Cool/cold temperatures have a significant effect on the phase and bond development of castables, and thus a direct effect on the integrity and performance of the installed lining. All necessary precautions must be taken to insure that the detrimental effects of cool/cold temperatures are avoided.
Many important topics like raw materials, microstructure, inverse property relationships, quality control, shelf life and others could also have been mentioned here. Suffice it to say that proper knowledge, experience and careful attention are necessary to realize the desired optimum cost effective performance of refractories. But it is not uncommon that the corporate refractory engineer(s) is overworked, and maybe inexperienced, causing refractory issues to be sometimes overlooked. The results can range from insignificant to catastrophic, with small to huge cost and time effects. The best approach is to thoroughly evaluate and minimize the risks that can accompany refractory-related problems.