Furnace lining is a protective and insulating layer of heat resistant material attached to the inside of the shell, hearth, and tap holes of a furnace. This layer serves to protect the furnace parts from the extreme heat developed during smelting operations. It also prevents excessive heat loss from the external furnace surfaces making the process more efficient. Also known as refractory materials, furnace lining materials are typically ceramics or combination metal/ceramics. These protective layers may be made up of individual bricks, poured fluid aggregates, or semi-moist aggregates which are rammed into place.
Metal smelting requires extremely high furnaces temperatures with averages running between 1,500° and 2,000° Celsius (2,730° and 3,630° Fahrenheit). To prevent the destruction of the furnace shell by these high temperatures, the assembly is equipped with a furnace lining. This lining is composed of materials that can withstand these extreme heat without degrading. In addition to protecting the furnace construction, the lining also insulates the furnace to prevent unnecessary heat loss. This lowers the amount of external heating required to maintain smelting temperatures and makes the furnace more efficient.
The furnace lining is typically applied to all interior surfaces. This includes the dome, shell, hearth, and tap holes used to pour or drain off the molten metal. Furnace linings may be of composite construction with separate materials being used in different parts of the furnace. This composite construction depends on the types of metal being melted, the capacity and configuration of the furnace. This is an important consideration as different thermal, chemical and mechanical loads are placed on separate parts of the furnace. Typically the rim, hearth, and tap holes are lined with different materials than those used on the sides of the furnace shell.
Furnace lining materials are chosen mainly for their ability to withstand protracted exposure to extreme temperatures. Other desirable characteristics include resistance to mechanical shock, abrasion, and chemical reactions within the molten metal. The most commonly used furnace lining materials are ceramic compounds and metal/ceramic combinations. Ceramic lining materials are made from a variety of raw materials each with its own particular strengths. Common ceramic furnace lining materials include aluminum oxide, magnesite, silicon carbide, and dolomite.
Metal and ceramic combinations, also known as cermets, blend the high heat resistance of ceramics with the desirable physical characteristics of steel. Ceramics used in these cermet combinations include tungsten carbide, zirconium bromide, and aluminum oxide. The steel components of cermets include iron, cobalt, chromium, and nickel.
Furnace linings are attached to the interior of furnaces in a number of different ways. These include pouring or compacting liquid or moistened aggregates into forms on the inside of the furnace. Another common form of furnace lining is the installation of layers of individual fire proof bricks which are attached with a suitable heat resistant mortar.