Blast Furnace

Blast Furnace, the furnace used for the smelting of iron, i.e. the extraction of the metal from its naturally occurring compounds or ores. In shape, size, and proportions, blast furnaces vary considerably according to the nature of the fuel, the character of the ores, etc., employed. The general shape may be described as of two truncated cones, united at their bases, the angular junction being rounded off, forming the boshes. The furnace is built of firebricks; outside this, and separated by a space filled with sand, etc., is another layer of firebricks, and surrounding all are wrought iron plates united by rivets. The part of the furnace above the boshes is known as the stack; and the top portion of the stack forms the throat, which is generally capable of closure, to admit of the collection of the gaseous products. The bottom of the furnace constitutes the hearth, around which are openings through which the twyers, or pipes from the blowing engines, deliver the blast. In most cases the hot gases passing off from the furnace are utilised for the purpose of heating the blast. The front of the hearth is continued forward beneath an arch of the walls - the tymp arch - to form a cavity known as the fore-hearth. In front this is dammed by a block of firebrick supported by a metal damplate. On the top of the dam is a groove known as the cinder notch, through which, when the furnace is working, the slag runs into trucks placed to receive it. In the dam also is the tapping hole, which, except when open for the purpose of allowing the molten metal to flow out, is closed by a tightly rammed plug of clay. The height of such furnaces is about 70 feet. When starting the furnace, wood and coke are introduced, then layers of limestone and coke with small quantities of the ore, till the furnace is about one-third full. The wood is then ignited, ore, fuel, and limestone (the flux) being added lightly, and the blast slowly increased, the normal condition not being reached for some days. It is then kept continuously working or "in blast," by filling in from the top the mixture of ore, fuel and flux. The slag runs off as before stated, and the iron is tapped when necessary. The furnace itself remains in blast frequently for years without intermission. For the chemistry of the process see Iron.