Blast furnace reaction zones

Thermochemistry. From an overall heat and mass balance point of view, the main chemical reactions of the blast furnace include oxidation of carbon in the zone in front of the tuyeres (raceway) to give CO plus heat. 

For the purposes of discussion, it is useful to consider the blast furnace as operating in four consecutive zones. At the fourtlr, bottom, zone the oxidation of coke at the tuyeres canres the temperamre to levels in excess of 2100 K. The next zone, which operates in the temperature range 1600-1900 K is where the liquid metal and slag are formed. The second zone, sometimes referred to as the thermal reserve zone, is where the CO2-C reaction to produce CO, the so-called solution reaction mainly occurs, and die reduction of h on is completed. In the first zone, at the top of the furnace, tire primaty reduction of... 

FIGURE 16.39 The reduction of iron ore takes place in a blast furnace containing a mixture of the ore with coke and limestone. Different reactions take place in different zones when the blast of air and oxygen is admitted. The ore, an oxide, is reduced to the metal by reduction with carbon monoxide produced in the furnace. 

The alkali-bearing minerals found in the blast furnace include kalio-philite-nepheline, leucite, plagioclase, and alkali carbonates [40-44], Kaliophilite was the dominant reaction product on the inside surface of the lining and in the joints between brick. The formation of this compound is accompanied by about a 45% volume increase. No kaliophilite, however, formed in the interior of the brick except in the carbon-disintegration zones. 

In Figure 6, a SEM picture is given which focusses on the reaction rim around a grain of blast furnace slag. An analysis by EDX reveals that, within this zone, goethite must have reacted with the blast furnace slag and the converter slag to form a complex metal oxide/hydroxide compound. 

Figure 6.5.13 shows schematically the different zones of a blast furnace. At the bottom, where blast air is injected, temperatures of up to 2000 °C are reached. At such high temperatures CO2 formed by coke combustion (Eq. (6.5.14)] is instantaneously gasified to CO via the Boudouard reaction, Eq. (6.5.15). Note that the primary product of coke combustion is at temperatures above about 1000 °C CO, but CO is quickly oxidized further as oxygen is present. 

Figure 6J.13 Reactions and reaction zones in a blast furnace.

At the tuyere zone of the furnace, carbon combustion takes place predominantly forming CO and generating most of the heat required to operate the furnace. Temperatures up to 1600 C are generated in the blast zone or tuyere raceways. As gases rise through the shaft, zinc and lead oxides are reduced by CO, and carbon is consumed by the CO2 so generated to reform CO. These reactions are endothermic and temperatures decline towards the top of the shaft. 

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