Furnace Tapping Tools

Liquid iron (hot metal) and slag produced during the continuous operations of blast furnaces are capable of interfering with furnace operation when appropriate measures are not put in place to prevent them from reaching the level at which they do so.

Hot metal and slag must be removed at regular intervals to ensure efficiency in blast furnace processes. For a slag and hot metal to be prevented from interfering with blast furnace operation, a tap hole is needed. The tap hole located slightly above the floor of the hearth of the furnace is used for tapping slag and hot metal from the furnace.

Furnace tapping is the process of removing slag and hot metal from furnace hearth. The tapping process is the most important factor that determines the in-furnace gas pressure and residual amounts of iron and slag in the hearth. Inefficiency in hearth drainage results in unstable furnace operation which leads to marked losses in furnace productivity and furnace longevity. When hot metals and slag levels approaches the tuyere level, the reducing gas flow in the bosh is severely affected. This results in irregularities in burden and descent of the furnace.

The tapping cycle starts when the tap hole is drilled open and ends by plugging the tap hole with tap hole mass when the furnace mast burst out. At the end of the tapping cycle, the gas-slag interface tilts down towards the tap hole and a considerable amount of slag remains above the tap hole level.

The most critical and primary requirement of the tapping process is to obtain and sufficiently secure the required rate of furnace products. In large furnaces, tapping rates of 7 ton/min, and liquid tapping velocities of 5m/sec, in tap holes of 70mm diameter and 3.5 long, are typically encountered. The condition of tap holes and their length typically influences the tapping rate.

The performance of a furnace and how long it lasts strongly depends on how effective the tap hole performs. In this regard, the tap hole plays a very critical role in the existence of a blast furnace. This is because it serves as the heart of the blast furnace upon which it relies for survival and continuous existence.

The tap holes as essential as they are not only ensures the longevity of blast furnaces, but also ensures continuous production and efficiency in operation. Should a blast furnace fail to function due to failure of its tap holes, operations will come to a stand still. This will serve a severe blow on productivity and significant loss of revenue.

Some steel manufacturing companies use very large furnaces that have about 2 to 4 tap holes which facilitates effectiveness in the continuous drainage of hot metal and slag. Small to medium sized furnaces however usually have only one tap hole. For the large furnace with multiple tap holes, one or two tap holes are opened for hot metal and slag evacuation for a period of time after which the other one or two overlap to also ensure continuous drainage. This is to ensure that the tap holes do not get congested and choked or blocked, as this would affect performance. When a tap hole is switched to another, the previous one is relieved off the slag and hot metal so that it can be made ready for another tapping process. Furnaces with single tap holes have tapping variation intervals of between 20 to 90 minutes.

When the time comes for a furnace to be tapped, time must not be wasted. This is to guarantee maximum production, efficiency and longevity of the furnace life.

The production of hot metal by Blast furnace is continuous, but tapping of the furnace occurs only periodically. Controlling the slag and hot metal level in the hearth is as such very important to prevent the catastrophic build-up of liquid iron in the hearth. The liquid iron level is estimated using a simple model to calculate the production rate of the furnace. Given the volume of the hearth, including the voids between the solid coke particles, an estimate is made of the level inside the hearth. This results in a target tapping interval, usually around 20 minutes at a normal production rate. The model must take into account the drill-bit diameter, tap-hole length, and hydrostatic pressure of liquids when calculating the tapping rate. This, however, cannot be accurate if the operator does not ensure the tap-hole is drilled thoroughly and properly opened.

The key components of the blast furnace complex include equipment or entire technological chains that are directly involved in the process and the failure of which can lead to an accident or a blast furnace process stop. The blast furnace complex can be divided into the following vital subsystems:

Central Unit
It includes the blast furnace proper, which in turn consists of the following parts:
A shell;
Cooling staves;
A refractory lining;
A cooling system;
A charging device (a bell-type top charger or a bell-less top charger, (usually a chute-type one), a hot blast feeding system (a straight and bustle main and a tuyere stock).
All the blast furnace process's main redox reactions occur in the central unit.

Cast House
It includes runners responsible for separating hot metal from slag, as well as their transportation to the discharge from the cast house into hot-metal ladle cars and slag cars. The cast house equipment includes taphole opening and ramming machines, runner cover manipulators, and other equipment.

Hot Blast Stoves
They are designed for heating combustion air and consist of a casing, lining, hot-blast stove checkerwork and a sub-checkerwork, a burner, valves, and a bustling main for feeding hot blasts into the furnace.

BFG Gas Cleaning Plant
It is designed for gas purification for further use when heating hot blast stoves and other consumers, such as factory thermal power plants, heating furnaces, etc. The gas cleaning plant is also used to maintain the required top pressure. The gas cleaning plant usually includes a dust catcher or cyclone, a scrubber, a Venturi tube, a demister and a throttle group (for a wet-type gas purification system) or bag filters, a heat exchanger, and a TRT (for a dry-type gas purification system).

BF Charging System
This system is responsible for preparing and supplying material of the required quality and at the required ratio into the blast furnace. It usually consists of storage bins, screens, chutes, feeders, conveyors, or a skip-hoist system for materials.

Role of Auxiliary Equipment
Auxiliary systems of the blast furnace complex include systems that are not directly related to the technological process, but with which the operation of a modern blast furnace seems possible.

Stockhouse and Cast House Dedusting Systems
They are designed to maintain standard indicators of air quality in workplaces and common indicators of emissions into the atmosphere. They include a gas duct system, shelter, draft equipment, and proper filters (bag, electric, or combined). Currently, bag filters are more often used due to environmental standards tightening.

Pumping Stations and Water Supply Systems for Blast Furnace Cooling
They are designed to supply water to the blast furnace cooling system, which maintains necessary thermal conditions in the cooling staves, preventing their burnouts and furnace shell damage. It comprises a pipeline system, fittings, pumping units, and heat exchange equipment.

Utility Networks
They include systems of supply of energy carriers (nitrogen, oxygen, natural gas, compressed air, electricity, etc.), heating, ventilation and air conditioning systems, compressor stations, etc.

Main Parts of Blast Furnaces
The furnace shell, cooling staves, and lining are the main components of the central unit, where the cooling staves play almost a decisive role in the blast furnace campaign duration. Adequately designed and selected for required zones cooling staves are the basis for the reliable operation of a blast furnace.
Cooling staves can be divided into three main types based on their material: cast iron, steel, and copper.

Cast Iron Cooling Staves
This equipment the most commonly used; their main advantage is wear resistance, which is very important in the upper part of the furnace stack and the blast furnace top, where the descending material still needs to be softened and is highly abrasive. Their main disadvantage is relatively low thermal conductivity compared to other types of cooling staves. A cast iron cooling stave is a cast iron plate cast in steel tubes (coils). The coils can be arranged either in one row or in two rows. The surface of the cooling staves facing the fireside of the furnace can be either flat (such staves are used in the lower part of the furnace: hearth, bottom, and tuyere zone) or have teeth, including those having a "dovetail” shape for the possibility of inserting refractory bricks. Such cooling staves are used in a blast furnace's bosh, belly, stack, and top. When embedding a steel tube into a cast iron cooling stave, there is a gap S = 0.3÷0.8 mm between the outer wall of the tube and the stave body, which appears during the casting process when the marshalite coating of the cooling stave pipes burns out. The multilayer structure of the cooling stave (cast-iron body, steel tube with a wall subjected to the process of carburization during the stave manufacturing) leads to the formation of cracks both in the cooling tubes and in the cast-iron body of the stave under sudden impact of peak thermal loads. This design also reduces the overall thermal conductivity of the stave itself.

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