Blast Furnace Construction
BLAST FURNACE
The blast furnace is a counter-current gas/solids reactor in which the descending column of burden materials [coke, iron ore and fluxes/additives] reacts with the ascending hot gases. The process is continuous with raw materials being regularly charged to the top of the furnace and molten iron and slag being tapped from the bottom of the furnace at regular intervals.
Key steps of the process are as follows:
- upper part of the furnace – free moisture is driven off from the burden materials and hydrates and carbonates are disassociated.
- lower part of the blast furnace shaft – indirect reduction of the iron oxides by carbon monoxide and hydrogen occurs at 700-1,000°C.
- Bosh area of the furnace where the burden starts to soften and melt – direct reduction of the iron [and other] oxides and carbonization by the coke occurs at 1,000-1,600°C. Molten iron and slag start to drip through to the bottom of the furnace [the hearth].
Between the bosh and the hearth are the tuyeres [water cooled copper nozzles] through which the blast – combustion air, preheated to 900-1,300°C, often enriched with oxygen – is blown into the furnace. Immediately in front of the tuyeres is the combustion zone, the hottest part of the furnace, 1,850-2,200°C, where coke reacts with the oxygen and steam in the blast to form carbon monoxide and hydrogen [as well as heat] and the iron and slag melt completely.
Molten iron and slag collect in the furnace hearth. Being less dense, the slag floats on top of the iron. Slag and iron are tapped at regular intervals through separate tap holes. For merchant pig iron production, the iron is cast into ingots; in integrated steel mills, the molten iron or hot metal is transferred in torpedo ladle cars to the steel converters. Slag is transferred to slag pits for further processing into usable materials, for example raw material for cement production, road construction, etc.
Source : metalurgica engineering