Full resolution (JPEG) - On this page / på denna sida - Second part - IX. Mining Industry and Metal Production - 2. The Iron and Steel Industry. By the late Prof. J. G. Wiborgh - Wrought iron
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730- ix. mining industry and metal production op sweden.
Bloom-Hammer.
In the degree that the charcoal is consumed by the blast, the pig iron naturally
sinks towards the bottom of the hearth, and in order to prevent this, a bar is
inserted under the pig iron pieces, by which they are again lifted above the tuyeres.
The pig iron melts gradually and is collected on the top of the slag, which is on
the bottom of the hearth, where it solidifies on account of the cooling off of the
bottom. As soon as all the iron is melted and collected on the bottom, the work
of breaking loose is begun, which consists in loosening the iron from the bottom
of the hearth with the bar and lifting it up towards the tuyere in order to melt
it again, and thus the work continues uninterruptedly until the silicon, manganese,
and finally the carbon are oxidized and the pig iron converted into »fårskon
(pieces of refined iron) of soft iron, which are at last melted and welded together
into a large ball, which is taken out of the hearth and placed under the bloom
hammer (fig. above), under which it is compressed and cut into several blooms.
The heating of these blooms to be forged into bar-iron is sometimes done in the
same kind of hearths (half Walloon forging), but usually in reverberatory furnaces,
which are then called heating furnaces.
The above described Lancashire hearth is the most common one, but somewhat
deviating constructions also exist. Thus, there are similar hearths with only one
tuyere, and others with three, the third being placed at the back of the hearth.
There are also quite often somewhat larger hearths in use with two tuyeres and two
working ports placed opposite each other; these are called double Lancashire hearths.
Pig iron suitable for Lancashire forging ought to be white to half-white and
have a low percentage of silicon, 0 3 to 0-5 %, wherefore it is usually produced
with a hot blast of only 200’ to 300° C. The percentage of manganese ought
also to be low, preferably only a few tenths of one per cent, because the refining
process is retarded by manganese.
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