Refractories are ceramic materials designed to withstand the very high temperatures (in excess of 1,000°F [538°C]) encountered in modern manufacturing. More heat-resistant than metals, they are used to line the hot surfaces found inside many industrial processes.
In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories can withstand physical wear and corrosion caused by chemical agents. Thus, they are essential to the manufacture of petrochemical products and the refining of gasoline.
Refractory products generally fall into one of two broad categories: preformed shapes or unformed compositions, often called specialty or monolithic refractories. Then, there are refractory ceramic fibers, which resemble residential insulation, but insulate at much higher temperatures. Bricks and shapes are the more traditional form of refractories and historically have accounted for the majority of refractory production.
Refractories come in all shapes and sizes. They can be pressed or molded for use in floors and walls, produced in interlocking shapes and wedges, or curved to fit the insides of boilers and ladles. Some refractory parts are small and possess a complex and delicate geometry; others, in the form of precast or fusion-cast blocks, are massive and may weigh several tons.
Benefits from material property enhancement
Regardless of how complex or sophisticated the refractory castable is that is selected for an application, the physical properties of the material can be drastically reduced if care is not taken during the mixing, pouring, and curing processes. Particularly with the use of more complex refractory castables to solve specific wear issues, installation variables become even more critical to the performance of a lining. Unfortunately, lining quality is often compromised by field conditions during material placement. Project schedules, crew skill levels, equipment availability, job cost pressures, or other demands can sometimes influence proper refractory installation. Improper water addition, mix time variations, over- or under-vibration, and improper curing can drastically affect material quality. With precast shapes, cast in a controlled shop environment, the physical properties of a castable can be more fully optimized.
Initial drying and firing of a refractory castable is a critical installation variable that can influence lining performance. Precast shapes are typically fired in a digitally-controlled furnace prior to shipment, ensuring that the refractory manufacturer & recommended bake-out schedule is closely followed. Since the shapes are fired slowly from all sides, the moisture is removed through the entire thickness of the shape in a controlled manner. Depending on the temperature to which the shape is fired, this can optimize the physical properties of the material through the entire thickness of the shape, not just the hot face surface. This results in a truly homogeneous lining. Micro-cracks within the shape, which are often introduced during field bake-out but may go unnoticed, may also be reduced since the initial firing is more controlled.
In service, linings composed of precast shapes often see less stress and cracking, due to the independent, & floating nature of the lining. The performance of the lining can also be more predictable, resulting in better opportunities to plan for maintenance and repairs.