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The blast furnace is the primary process for converting haematite ore (iron oxide) to pig iron. It is a large, steel vessel lined with refractory brick, where iron ore, coke and limestone are charged into the top, and preheated air is blown into the bottom. The burden takes several hours to descend to the bottom of the furnace where they are reduced to molten slag and iron.
There is a more detailed description of the process here, and the BBC host an animation of an early (water powered) blast furnace here. (Start the animation by clicking the start button).
This British Pathe film about blast furnaces post dates Carnforth but the principles are exactly the same.
The above illustration was featured on the cover of a document of the Iron and Steel Institute in Barrow-in-Furness in September 1874.
It is the base of a blast furnace (Cast House) and while the detail is lacking, it suggests stacks of cold pigs in the foreground, a rail mounted hand crane and one, possibly two horses at work.
Right is an 1874 engraving of the cast house floor of a blast furnace at Barrow in Furness.
The furnaceman is using a gallows crane to lift a slag bogie off one of two tracks that deliver empty slag bogies to the slag notch. Early blast furnaces produced a relatively low temperature, viscous slag that was poured into these rectangular, lidded boxes. In West Cumberland "Cornthwaite" bogies made at Lowca were in widespread use. The bogies were hauled to the tip by horse and later by steam locomotive. The bogie was positioned under a three legged crane, a chain running round a pulley at the top then back to the locomotive. The loco pulled on the chain and lifted the lid off the bogie then the poor employee would have to lever the lid off the still hot, solidified block of slag with a pinch bar. There are records in Carlisle CRO of an employee claiming against his employer when he was injured by a still partly molten block of slag.
The furnace top is open but there is no attempt to depict any method of charging the furnace.
Pig iron gets its name from the open topped patterns cast in sand in the pig beds - all set out by hand in advance of each cast. The molten iron ran down the main channel (the Sow) and fed hundreds of moulded depressions in the sand (the Pigs).
Slag floats on top of molten iron so the process of tapping the furnace starts with raking off the slag through the slag notch then opening up the lower tap hole to allow the molten iron to run down to the pig beds. The crew would use simple spade-like gates to control the direction of the iron.
Simple hand tools were used to prepare the pig beds.
The pigs would be allowed to cool and then broken up manually (back breaking work) and stacked - possibly for transport by horse drawn bogie. Typically the furnace would be tapped say every 4-8 hours so the iron had to be broken up, stacked, shipped out and sand in the pig beds relaid continuously. Lowca Engineering Company near Whitehaven patented a machine to break combs of pigs.
Slag from the original furnaces was dumped at Keer Marsh to the North of the plant (and North of the F&M Joint Railway line) for a period up to about 1880. Thereafter slag was transported to Warton Sands, to the West of the Keer. Philip Grosse describes the change from rectangular to circular tapered slag boxes at this time.
Blast furnaces at Middlesbrough circa 1880. Molten iron running in to the pig beds and in the foreground is a circular, tapered side slag box on a dumb buffered four-wheeled flat top bogie.
In the background are the hydraulic lifts ( between the furnaces) that elevated barrows of coke, iron ore, and limestone to the charging gantry at the top of the furnace.
Carnforth's lifts were enclosed by brick and iron plate structures.
Below the bunkers at Workington, and right, charging the furnace. The counter weighted small bell would be loaded through four of these side opening doors with the iron ore, coke and limestone evenly distributed. At intervals the bell momentarily opened to release the charge. This was an unhealthy and risky job. The furnace top crews worked continuously for their entire shift. You can work out for yourself how they coped with nature's needs.
Safety chains prevent the bell falling in to the furnace should the main link fail.
This is a cross section of one of the blast furnaces from the West Cumberland Iron and Steel company of Workington.
a - Cup: The cup combines with the bell to support the charge before it is released forming a gas tight seal.
b - Cone: Usually termed the bell.
c- Gas Box: Off-takes, specially shaped to minimise dust leaving the burden. this design has one, but it created uneven passage of gas through the burden, so they evolved to four per stack.
d - Gas downcomer: Guiding the dust laden gas to the Dust box below. The bore of the downcomer was large to minimise gas velocity.
e - Dust box: Simple, early dust catcher.
f - Furnace shaft: The tapered body of the furnace.
g - Bosh: The widest part of the furnace.
h - Crucible: More commonly called the hearth.
j - Twyer pipes : Water cooled nozzle through which the hot air blast was directed in to the furnace. Five Tuyeres (rhymes with fears) on this furnace.
k - Gas culvert : The underground chamber for cleaned gas.
l - Twyer holes: Holes in the steel shell and refractory lining that accommodate the Tuyeres.
m - Circular main: Bustle Pipe surrounding the bosh/hearth area. Hot blast from the stoves was blown to the Bustle Pipe via the Hot Blast Main. The Bustle Pipe connected to the Tuyeres via a short pipe called a Gooseneck.
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