Blast Furnace Tap Hole and Tapping of the Furnace
The operation of a blast furnace is a continuous process. The blast furnace continues to produce liquid iron (hot metal) and slag as long as it is in operation. The hot metal and slag accumulate in the hearth of the furnace, but since there is a limit to the amount that can be accumulated before it interferes with the furnace operation, hot metal and slag must be removed from the furnace at regular intervals. The tap hole also known as iron notch, is used for tapping the hot metal from the furnace. It is located slightly above the floor of the hearth.
Regardless of the specific tap hole configuration or operating philosophy, due to the addition of dynamic (often periodic) and more intense process conditions (exposure to higher temperatures leading to accelerated corrosion, greater turbulence, and elevated rates of mass and heat transfer), and higher concurrent thermo-mechanical forces (from thermal or flow shear stresses), the performance and longevity of the blast furnace is intimately linked to the performance of the tap hole. Hence tap hole is very critical to the blast furnace. It is the heart and the lifeline of the blast furnace since without a tap hole a blast furnace cannot exist. The criticality and relevance of tap hole continues even in the modern automated blast furnaces.
Tap hole is an essential part of a blast furnace. Large furnaces usually have 2 to 4 tap holes and the drainage of hot metal and slag is practically continuous by periodically drilling and plugging the tap holes with one of the tap holes is always open and two alternate tapings usually overlap for some period of time. Medium or small sized blast furnaces have normally one tap hole and the time interval between two tappings generally varies from 30 min to 90 min. Some blast furnaces are equipped with a slag (cinder) notch (generally referred to as the monkey) for removing slag from the blast furnace, and it is located in a plane typically 1 m to2 m above the tap hole.
In earlier days when the burden of the blast furnace was not improved to present standards, the weight of the slag produced in the blast furnace was more than half the weight of the hot metal. The lower density of the slag caused it to fill up the space in the hearth above the metal, and it would interfere with the penetration of the blast air and the combustion process at the tuyeres long before the accumulation of hot metal had reached the desired amount for tapping. Hence it was necessary to remove the excess slag through the slag notch once or twice between two tappings. However presently because of better prepared burdens, the slag volumes are at around 250–320 kg/ton level. Therefore the monkey is seldom used and the slag is typically removed only through the tap hole during the blast furnace tapping.
Tapping, also referred to as casting or drainage, is a process that removes hot metal and slag from the furnace hearth. The tapping process critically determines the in-furnace gas pressure and residual amounts of iron and slag in the hearth. Poor hearth drainage usually leads to unstable furnace operation which is generally connected to marked losses in furnace productivity and campaign life. An inefficient tapping also gives rise to excessive accumulation of liquids and thus high liquid levels in the hearth. If the liquid slag approaches the tuyeres level, the reducing gas flow in the bosh is severely disturbed, often resulting in irregular burden descent.
A tapping cycle begins as the tap hole is drilled open and is terminated by plugging the tap hole with the tap hole mass when the furnace gas bursts out. At the end of the tapping, the gas-slag interface tilts down towards the tap hole and a considerable amount of slag remains above the tap hole level. The iron phase can be drained from levels below the tap hole because of the large pressure gradient that develops near the tap hole in the viscous slag phase. The average slag-iron interface is therefore lower than the tap hole level. Depending on a number of factors, such as liquid production rates, hearth volume and tapping strategies, the initial stage of a tapping cycle varies and can be categorized as follows.