Oil fired crucible furnaces have been employed in gold mining plants and refineries for a long time. There are two basic types of furnaces, the stationary type and the tilting type. The first one needs the crucible to be installed in and out of the furnace. When a stationary furnace is recessed into the foundry floor brick, it is called pit type furnace. In a tilting furnace, a crucible with a special lip is used and the molten charge is poured from the melting crucible by tilting the furnace. The furnace consists of the following parts: shell, lining, brick block or pedestal and combustion equipment. The shell is heavy-gage steel. The lining is mainly a preformed, highly refractory unit that is cemented into place. The base block brick supports the crucible. The combustion unit is most the time of the premixing type, which mixes the fuel oil and air for proper combustion.
The best linings are preformed and fired bricks of highly refractory material such alumina clay or silicon carbide bricks set the furnace and cemented into place with a refractory cement. When the preformed linings are not available, a lining may be made by burning a suitable refractory material. A rammed lining may be prepared in an emergency from stiff mixture of crushed firebrick or gravel, 10-15% fire clay and water. It is important to mix well all the ingredients and the best results can be obtained is the mixture is made up a day ahead. The mixture is rammed into a solid slab and then cut off it with a shovel as it is required for ramming. The fired brick or silica must be refractory and of an appropriate size. When it is necessary to employ successive layers must be rammed into until the lining has been built up to the right desired thickness.
Linings must be rammed around a form. Id a form is not available and successive layers have to be rammed against the furnace shell, it is important to be sure that is layer is roughened before the next is applied. Otherwise, the layers may be separate later. If delays occur and the lining is allowed to dry out between layers, it must be thoroughly dampened before ramming is complete. Basically, the finished lining must be dried slowly and completely before it is used.
Cross of a stationary crucible furnace