El Tratado General de Paz (CPA) y la dimensión posconflicto

What is the effect of ore layer thickness on the process? If thicker ore layers are charged, less ore layers are present in the operating furnace and less coke slits are available to distribute the gas. But, especially in conveyor belt fed furnaces, the thicker the ore layer, the more charging capacity is available.

For reduction and melting two effects must be considered, those being the reduction in the granular zone of the furnace and the melting of the layers in the cohesive zone.

7.7.1 Reduction in granular zone

The reduction capacity of gas entering thicker ore layers will be depleted faster and as a consequence, the reduction of ore burden in the granular zone will be poorer.

7.7.2 Softening and Melting

As soon as an ore layer starts to soften and melt, it becomes impermeable for gas. This means that ore layers are heated up at the contact surface between the coke and ore layer. The thicker the ore layer, the longer it will take to melt down completely. Moreover, the melting of the ore layer slows down because there is more oxygen in the ore layer, because of lower rate of pre–reduction (see preceding section). So the thicker the ore layer, the more difficult the melting of the layer (Figure 7.20, next page).

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100% Thickness

Area in thicker layer heats slowly and has poor gas reduction

Figure 7.20 Melting of thin and thick ore layers compared

7.7.3 Optimizing ore and coke layer thickness

So, the blast furnace operator wants good permeable coke layers (i.e. thick layers) and good melting ore layers i.e. thin layers. As is often the case in BF operation the best operational results can only be reached with a compromise between these two factors. Generally speaking, from operational observation, the ore layers should not exceed 70–80 cm in the throat of a blast furnace and coke layers should not be smaller than 32 cm. The operational optimization depends on local situations.

Experience has shown that:

– Permeable ore layers can be maintained even when the layers have become quite thick, provided a permeable ore burden is used. For pellet burdens this would require screening of the pellets, and for sinter it would have to be sized to a relatively large diameter (more than 5 mm).

– The minimum coke layer thickness experienced was 14 cm metallurgical coke in the belly.

Conveyor belt fed furnaces tend to work with thicker ore layers. This is caused by the fact that in a conveyor fed furnace the charging capacity increases with increasing layer thickness. In skip–fed furnaces the optimum charging capacity is reached with full skips of coke. In the past the volume of coke was normally the determining factor, so furnaces tended to work with full skips of coke. At high coal injection rates the skip weight is normally the determining factor and thus furnaces now work with full skips of ore.

Another aspect of the optimization of the coke layer thickness has to do with the gas permeability of the coke layer. The coarser the coke is screened in the blast furnace stockhouse, the more permeable the layer is. There are, however, two drawbacks of the coarse (35 mm or more) screening of coke.

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Consequence 1: The coarser the coke is screened, the more nut coke or small coke is produced. The nut coke is added to the ore burden layer, increasing the thickness of the ore burden layer and decreasing the size of the coke layer.

Consequence 2: The coarser the coke is screened at the stockhouse, the thicker the formation of a mixed layer at the coke–burden interface.

Optimization depends on local conditions, but high productivity has been reached with a coke screen size in the stockhouse of 25 mm and a nut coke quantity of 25 kg/tHM.

7.7.4 “Ideal” burden distribution

The ideal burden distribution for high productivity and high PCI rates is—

according to the authors—as follows:

– An ore free centre,

– Nearly horizontal layers of coke and ore burden, – Some nut coke in the ore burden in the wall area and – Coarse coke in the centre.

Figure 7.21 “Ideal” burden distribution Ore free centre

The ore free centre allows the gas to distribute itself through the coke layers from inside to outside. We can consider the coke layers as layers with equal pressure. If the total internal pressure difference in 1.2 bar, the pressure difference over each of the 40 ore layers is about 0.03 bar. The ore free centre typically has a diameter of 1.5 to 2 metres. The ore free centre can be made in a furnace with a bell–less top by discharging 10–15 % of the coke on a very inward chute position. In furnace with a double bell top, formation of an ore free centre is more difficult.

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Nearly horizontal layers

Using nearly horizontal layers of coke and ore minimizes the effect of natural deviations of parameters important for the formation of the layers. E.g. wet pellets have a different angle of repose as compared with dry pellets. This does not affect burden distribution if nearly horizontal layers are used. Care should be taken, that there is no inversion of the profile, i.e. a pile in the centre of the furnace. This can be monitored with e.g. a profilemeter.

Nut coke

The gas in the wall area is cooled by the heat losses to the wall. Moreover, in the wall area a relatively large percentage of fine ore burden materials is located and reduction disintegration is strongest (because of slower heating and reduction).

For these reactions, reduction and melting of the ore burden in the wall area is most difficult. Nut coke in the wall area helps to reduce reduction gas and heat requirements in the wall area. The nut coke has a lower heat capacity than the ore burden. Moreover, when the ore burden in the wall area starts melting, the nut coke is immediately available for direct reduction. In doing so, it prevents the direct reduction attack on the metallurgical coke.

Coarse coke in the centre

The coke charged in the centre is the least attacked by the solution loss reaction and has the smallest chance to be burnt in front of the tuyeres. Therefore, it is thought that the coke charged in the centre finally constitutes the coke in the hearth. Good permeability of the hearth helps to improve casting and prevents preferential flow of iron along the wall, thus increasing hearth campaign length.