In iron and steelmaking processes, engineering design and the lining materials chosen are critical factors in controlling the efficiency and energy usage of the production equipment. As a result, it is critical that industrial designers understand the advantages and disadvantages of the materials they choose. For example, it is especially important to select insulating firebricks (IFBs) that minimize energy losses. Recent studies conducted on IFBs produced using the three most common manufacturing methods – cast, slinger, and extrusion – show that the cast process offers the lowest thermal conductivity and provides the greatest energy savings.

IFBs are highly versatile, and used variously in iron and steel production, including blast furnace stoves, ductwork in direct reduction units and reheat furnaces, back-up insulation in coke ovens, and in tundishes and ladles. They also are used to form sidewalls, roofs and hearths for heat-treating and coating lines.

IFBs are manufactured in various ways too, the most common of which are casting, slinger, and extrusion. The casting process uses gypsum plaster as a rapid setting medium for a high water-content clay mix, containing additional burnout additives. The slinger process is a form of low-pressure extrusion of a wet clay mix containing high levels of burnout additives, with an additional processing step in which the semi-extruded material gets ‘slung’ onto a continuous belt to generate additional porosity, before drying and firing. The extrusion process forces a damp clay mixture containing burnout additives through an extrusion nozzle, and the extruded material is cut into bricks, dried and fired.

The brick chemistries and microstructures produced may vary widely among these methods, leading to different thermal conductivities within products of the same temperature rating. This variation in turn has an effect on the ability of different IFB types to control energy loss.

To understand the effect of the three main IFB manufacturing methods on thermal conductivity and energy loss behavior, researchers conducted a study to quantify the differences in energy usage that can be achieved within Class 23 and Class 26 IFBs.

Two graphs show the thermal conductivity of the IFBs tested, a critical property because IFBs are primarily used for their insulating abilities. In each class of IFB, cast brick has the lowest thermal conductivity, followed by the slinger-produced brick, with the extruded brick displaying the highest conductivity.

Researchers designed two identical electrically heated laboratory muffle kilns and conducted energy usage studies comparing among the IFB bricks. They lined the first kiln with Class 23 cast IFBs and this formed the benchmark, since they had the lowest thermal conductivity in the class. Test results are shown in Table 1.

The thermograph image shown above shows the kilns during the 1,000°C firing test; the cast IFB lined kiln is on the left. It shows how much heat is wasted through the body of the kiln lined with the higher thermal conductivity IFB and how the surface temperature of the kiln becomes overheated. This shows both the effect of wasting energy costs and health and safety issues caused by hazardous working temperatures.

Significantly less energy was needed to run the test kiln through a 1,000°C firing cycle with the cast IFB compared to the extrusion IFB (37% less for Class 23 and 38.5% less for Class 26.) These energy usage differences are due to the differing thermal conductivities of the IFBs. In materials of similar chemistry, thermal conductivity is controlled by the structure of the material. The different manufacturing methods of the IFBs studied produce materials with inherently different macro- and microstructures and it is these that control the thermal behavior of the products. The texture of the IFBs is finest for the cast product and is coarsest for the extruded product.