CHARGING OF SOLID MATERIALS FROM THE TOP

In Chapter 1, Section 1.1.3, the evolution of ironmaking in Europe was briefly discussed. In order to clean the top gas for its utilisation as fuel, it was necessary to have a top charging device by which solids could be charged into the furnace without significant leakage of gas. This led to the invention of the two-bell charging system (one small bell and one large bell), which was a standard feature of all blast furnaces till about 1970. Even now, many smaller furnaces have this arrangement. Figure 2.5 shows the arrangement, and the procedure for its operation is also described therein.

It may be noted that gas flows through uptakes continuously while the charging cycle goes on. The solid burdens are not uniform. Coke is lighter than iron ore (about 3.5–4 times lighter).

Again the prepared iron burdens—sinter and pellets—have a range of densities. The sizes of burdens are also not uniform and have a range.

Chapter 7 will deal with internal zones and gas flow in blast furnaces. Figure 2.1 shows the approximate temperature levels at various heights of the furnace. It is an ideal situation and is very much desirable. However, in reality there is considerable variation in temperature, gas composition and gas velocity at any horizontal section of a blast furnace. These are the consequences of non-uniform size of burden, since a finer particle offers more resistance to gas flow.

Chapter 10 has discussed the importance of uniform gas flow for smooth operation, as well as efficient productivity of a blast furnace. Efforts towards this goal are three-fold, viz.

∑ Screening of solid charges before charging into the furnace to eliminate fines below a certain size

∑ Agglomeration of fines by sintering, pelletising

∑ Proper top charging device to make burden size distribution as uniform as possible on horizontal section.

Gas uptake Skip

Skip bucket

Small bell

Large bell

(a) Small bell and large bell both closed; skip bucket tipped to dump charge in hopper above small bell. Gas flowing from top of furnace through uptakes located in dome (top cone).

(b) Large bell remains closed while small bell opens to admit charge to large bell hopper.

(c) Small bell closed to prevent escape of gas to atmosphere and large bell open to admit charge to the furnace.

(d) Both bells closed, ready to repeat charging cycle.

Note that the rod supporting the large bell passes through a hollow rod supporting the small bell, permitting independent operation of bells.

Small bell rod Large bell rod

Receiving hopper Revolving hopper

Gas seal Large bell hopper

Top cone

Lip ring

Figure 2.5 Stages in two-bell charging cycle.

There have been subsequent modifications for improvement of charge distribution of the bell-type charging, as follows.

∑ Bell-type with movable throat armour

∑ Four-bell charging device.

With the advent of the movable throat armour systems in which a movable deflector is introduced into the stream of material falling from the big bell, a much wider cross section of the stockline can be controlled. Separate charging of iron-bearing materials and coke at different deflector settings as well as the proper distribution of fines has improved furnace performance and reduced the coke rate.

However, the most modern device is the so-called ‘Bell-less Top’ charging equipment, invented by Paul Wurth in Luxembourg in 1972. All modern furnaces are now having this system. Figure 2.7 shows this system. This new system comprises a combination of a hopper and gate. The material is fed on to a rotating chute at variable angles, through a system of seal valves and flow control gates. Mathematical models and instrumentation systems are available to predict the stockline profile and to measure the effect of change in the distribution pattern. All modern furnaces incorporate a high top pressure operation, where the exit gas pressure is above 1.5–2 atm gauge. This leads to more leakage as well as rapid erosion of bells. Bell-less top performs much better. In addition to this, the control of charge distribution is much superior.

The distribution monitoring system includes the following:

∑ Heat flux monitoring equipment to measure the heat flow in different zones (both above and under the burden)

∑ Profile meters for the measurement of surface profiles

∑ Thermocouples in the throat, stack and bosh regions to measure temperature

In order to improve the uniformity of charge distribution in horizontal section of the furnace, a revolving chute is an added feature. Material is introduced onto the small bell through two openings, each equipped with a seal valve, making sealing more effective. The revolving chute distributes the materials evenly on the small bell, and this assists in realising a more uniform distribution. Figure 2.6 shows two types of stockline profiles obtained by this device. It may be noted that there is some segregation of small and large particles. This is due to their different trajectories when they fall into the furnace upon opening of the big bell.

Ore Coke Ore Coke

2

1 1

2 2

1

2

1

V profile M profile

Figure 2.6 Typical burden profiles showing ore and coke layers, qualitative size segregation in ore layer (1 large and 2 small particles); iron ore includes sinter and pellets.

∑ Stack pressure monitoring and pressure drop measurement along the furnace height

∑ Special instruments such as infrared probes to monitor the burden surface temperature, devices in the stack region to measure individual layer thicknesses and local descent rate, and tuyere probes to sample materials at the tuyeres level

∑ Mathematical models for charge distribution control, overall heat and mass balance and interpretation of probe data.

The advantages accruing from improved distribution control can be summarised as follows:

∑ Increased productivity, decreased coke rate, improved furnace life

∑ Reduced refractory erosion

∑ Improved wind acceptance and reduced hanging as well as slips

∑ Improved efficiency of gas utilisation and its indirect reduction

∑ Lower silicon content in hot metal and consistency in the hot metal quality

∑ Reduced tuyere losses and minimisation of scaffold formation

∑ Lower dust emission owing to uniform distribution of fines.

All these advantages have improved the overall efficiency, thereby making the process more competitive.

CL Receiving hopper

Equalising valve Relief valve Hopper positioning

mechanism

Upper sealing valve (open)

Material bin 2

Material flow regulating gate valve (closed) Lower sealing valve (closed)

Clean gas

Rotating distributing chute Feeder spout chute drive

Lower sealing valve (open) Material flow regulating gate valve (open) Material bin 1 Upper sealing valve (closed)

Figure 2.7 Bell-less top with pressurised hoppers and a rotating distributing chute.