Hot Blast Stoves in Steel Making

A blast furnace is a type of metallurgical furnace used for smelting to produce metals, typically iron and general steel, http://www.smallcapinvestor.com/smallcapinsights/china/2008-06-19-check_on_china_general_steel_holdings_inc. In a blast furnace, fuel and ore are continuously introduced and combined through the top of the furnace, while air (sometimes with oxygen enrichment) is forced into the bottom of the chamber, so that the chemical reactions occur throughout the furnace as the material flows downward. The end products are normally molten metal and slag phases tapped from the bottom, and flue gases leaving from the top of the furnace.

The Scotch tuyere (A tuyere is a tube, nozzle or pipe through which air is blown into a furnace or hearth.) is of cast iron, with a wrought-iron coil of pipe, for the cooling water, cast in the walls. The Lancashire tuyere is a hollow truncated cone with double walls, the cooling water flowing between them. An open spray tuyere is somewhat similar to the above, but the rear end is open (open tuyere), and the walls are kept cool by jets of water from perforated pipes inside. In the spreader tuyere, which is of a somewhat similar type, the water is distributed over a sheet to cool the upper part of the shell, a jet of water cooling the nose, and the water running back over the bottom to the outlet.

A vacuum or exhaust tuyere is one from which the water is sucked out, instead of being forced through, to lessen the leakage in the furnace if the tuyere breaks. The blast from the stoves is led into a large pipe (bustle pipe) surrounding the furnace and above the tuyeres with which it is connected. At a point near the top of the hearth is a hole for tap- ping the slag (slag notch, cinder notch, flushing hole, monkey), and lower down and to one side is the iron notch or tapping hole for tapping out the molten iron.

In modern furnaces the hearth is entirely within the furnace, and is called a closed hearth, the contents being tapped through a hole in the wall. This arrangement was first introduced by Lurmann, and was called a closed front (Lurmann front). “It was formerly the custom to have an open fore part (open front). In front of the furnace there was an arched-over opening extending from the furnace bottom to a little above the level of the tuyeres; the sides and roof of this opening formed a cavity known as the fore hearth.

A section of one of the fire-brick, called the dam, was Duquesne blast furnaces carried to the tuyere level; it formed the back of the fore hearth, and was supported by a stone (dam stone) or cast-iron dam plate; in the dam plate was a vertical slit which was stopped with loam, and which allowed of a tapping hole being made at any convenient level, while the excess of cinder ran off through a semicircular cinder notch. The arch above the dam was called the tymp; it was kept in position by a tymp plate (tymp stone) of cast iron, and generally cooled by running water.

In Lurmann's closed front arrangement the slag was tapped through a water-cooled tuyere (scoria block) situated in a water-cooled cast-iron plate (scoria plate). At the top of the furnace is the charging arrangement consisting usually of two bells, a large one below, and a smaller one above, each fitting in a hopper (bell and hopper, cup and cone), and so arranged that only one is lowered at a time, thus preventing the escape of gas. Various forms of ore distributing devices have been designed to insure the even disposition of the charge within the furnace.

Charging - The ore is kept in huge piles (stock pile) in a yard, and is handled by large traveling cranes (Ore Bridge, gantry crane). It is loaded into bins from which it is run into small hopper-bottom cars provided with scales for weighing, and from these is dumped into the buckets or boxes (skip) which are hoisted to the top of the furnace up an inclined track (skip bridge), and dumped automatically on the upper bell.

The limestone is similarly handled; the coke is measured by volume (occasionally weighed), the contents of the skip representing a definite weight. This method is known as skip charging. The older method, now seldom employed for new furnaces, is to weigh the materials in wheelbarrows which are raised to the top of the furnace by an elevator (hoist, lift), and dumped around the bell by hand (barrow charging). One complete unit or charge of ore, coke, and limestone is called a round.

Modern blast furnaces may have more than 30 tuyeres, through which the hot blast is forced into the furnace. They are usually constructed from copper and cooled with water to withstand the very high, extreme temperatures necessary to construct general steel and other products.