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TitleModern Blast Furnace Ironmaking an Introduction (hoogovens)
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Page 87

74 Chapter VII

be formed and the gas is distributed via the coke slits from the centre towards
outside radius of the furnace. Te melting zone gets an inverted or even
shape. Te central coke column not only serves as a gas distributor, but as well
as a t ype of pressure valve: it functions to stabilise the blast pressure.

Figure 7.10 Central working furnace

It depends on the type of burden distribution equipment how the coke can be
brought to the centre. ith a bell–less top the most inward positions of the
chute can be used. ith a double bell system the coke has to be brought to the
centre by coke push (see below) and by choosing the right ore layer thickness
in order to prevent the flooding of the centre with ore burden materials. In
the central working fu rnace there is a relatively small amount of hot gas at the
furnace wal l: hence low heat losses. As a result the melting of the burden in the
wall area takes place close to the tuyeres, so the root of the melting zone is low
in the furnace. Te risk of this type of process is that ore burden is not melted
completely before it passes the tuyeres. Tis could lead to the observation of
lumps of softened ore burden through the tuyere peep sites. Tis can lead from
slight chilling of t he furnace (by increased direct reduction ) and irregu lar hot
metal quality to severe chills and damage of the t uyeres.

Limiting the risk of a low melting zone root can be done with gas and burden
distribution. Operational measures include the following.

– Maintain a sufficiently high coke pe rcentage at the wall. sing nut coke in the

wall area can also do this. Note that an ore layer of 55 cm at the throat needsabout 20 to 22 cm of coke for the carburisation and direct reduction. So if the
coke percentage at the wall is under 27 %, a continuous ore burden column can
be made at the wall.

– Ensure a minimum gas flow alo ng the wall in bosh and belly , which can be
monitored from heat loss measurements and/or temperature readings. If the gas
flow along the wall becomes too small, it can be increased by means of burden
distribution (more coke to the wall or less central gas flow) or by increasing the
gas volume per tonne hot metal (by decreasing oxygen).

Page 88

The Process: Burden Descent and Gas Flow Control 75

– Control the central gas flow . Note that the gas flow through the centre leav es
the furnace at a high percentage of CO and H₂ and a high temperature. Te
energy content of the central gas is not efficiently used in the process and thus
the central gas flow should be kept within limits.

Te central working furnace can give very good, stable pr ocess results with
respect to productivity, hot metal quality and reductant rate. It also leads to
long campaign length for the furnace above the tuyeres. However, the process is

very sensitive for deviations in burden materials, especially the size distribution. WaWall wll wororkiking ng fufurnrnacacee

In Figure 7.11 the wall working furnace is presented. Melting ore burden blocks
the centre of the furnace and the gas flow is directed towards the wall area.

Figure 7.11 all working furnace

Te gas flow causes high heat losses in the area of the furnace where a gap can
be formed between burden and wall i.e. in lower and middle shaft. Te melting
zone gets a shape or even the shape of a disk. In this situation the root of the
melting zone is higher above the tuyeres, which makes the process less sensitive
for inconsistencies. Te process can be rather efficient. However, due to the high
heat losses the wear of the refractory in the shaf t is much more pronounced
than with the central working furnace. Te ga s passing along the wall can

also cool down rapidly and in doing so loses its reduction capabilities. As aconsequence, the fuel rate is high. Moreover the fluctuations in the pressure
difference over the burden are more pronounced, which leads to limitations in

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Page 175

164 Index

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