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How the tower of the first Blast Furnace was developed may never be known but the associated process of iron making increase the volume of iron that could be smelted while also reducing the price. The first record of a Blast Furnace in the U.K. is in 1496.
Early furnaces were best located on sloping ground, close to a reliable stream. Water was used to drive the early bellow to create the drought, while the slope helped to provide a near level roadway onto the top of the furnace.
The key to the process is the removal of the oxygen from the iron ore at the same time as separating as many of the other impurities as possible.
A blast furnace works on a continuous process lasting weeks, months, or in modern times, years and it will be assumed that the furnace is in the middle of a campaign and so the lighting the furnace (blowing in) can be ignored.
Clean carbon (Charcoal or later Coke), Iron Ore and Limestone are added to the top of the furnace. Little and often is best as it has the least affect on the burning of the furnace. Also important is that the charge material is alternated (e.g.. Iron Ore, then Coke, then Limestone, and then more Iron Ore).
At the top of the furnace the charge is heated and dried by the hot gases being blown through the furnace. Lower down, the iron ore melts as the carbon starts to burn and from just below the middle of the furnace, molten iron drips down through the remaining carbon fuel onto the hearth at the very bottom of the furnace.
As there is insufficient oxygen in the air blast to properly burn the carbon fuel oxygen is captured from the iron ore, however, in spite of this, the majority of the gas produced is still Carbon Monoxide.
In the lower part of the furnace, the limestone acts as a flux and draws together many impurities together into a layer of slag that floated onto of the molten iron.
The molten iron and slag is drawn off periodically.
The air blast is introduced a little way above the hearth and must be strong enough to stop the burning contents of the furnace stack dropping into the hearth, but must also not be so strong as to blow the contents out of the top.
Until the introduction of the Blast Furnace cast iron had not existed and iron had never been seen as a liquid in any great volume.
Since the start of the 18th Century the Blast Furnace has developed in a number of ways. Firstly Coke was introduced as a fuel in place of charcoal, allowing the size of furnace to be increase (charcoal would collapse under the extra weight from a large furnace). This was famously pioneered by Abraham Darby at Coalbrookdale in 1709 and was almost universal within 100 years, however a few charcoal furnace carried on until as late as 1921! Secondly the blast air was heated using heat recovered from the exhaust gases (energy conservation is not that new). Lastly, the Coke and Iron Ore are now mixed and heated, producing sinter, before they are charged into the furnace. Interestingly, you can tell from the texture and colour of the slag whether or not a furnace has had a hot or cold blast.
Modern Blast Furnaces can be 35m (120ft) high, 14m (45ft) diameter and can produce 10,000 tons per day.
The Iron produced by a Blast Furnace is always call 'Pig Iron'. The title of 'Cast Iron' is only generally used after the iron has been cast into a finished product.
Early furnaces producing small quantities of iron could be used to cast products directly and some furnaces, such as Rockley Furnace, had casting pits for large items such as Cannon. With larger furnaces, all iron was cast into pigs and was remelted but from the 1850s molten iron was charged into other types of furnace, mixer or converter. Little if any iron is now cast into pigs in the U.K., as steel making plants are incorporated into the same works as the Blast Furnaces.
|Property||Slag Type and Value|
|Specific Gravity||2.0 - 2.5||—||—|
|Compacted Unit Weight,
|1120 - 1360
(70 - 85)
|(800 - 1040)
(50 - 65)
|Absorption (%)||1 - 6||—||—|
|Calcium Oxide (CaO)||41||34-48||41||31-47||39||32-44||39||34-43|
|Silicon Dioxide (SiO2)||36||31-45||36||31-44||36||32-40||36||27-38|
|Aluminum Oxide (Al2O3)||13||10-17||13||8-18||12||8-20||10||7-12|
|Magnesium Oxide (MgO)||7||1-15||7||2-16||11||2-19||12||7-15|
(FeO or Fe2O3)
|a. Data source is the National Slag Association data: 1949 (22 sources); 1957 (29 sources); 1968 (30 sources) and 1985 (18 sources).|
|Los Angeles Abrasion (ASTM C131)||35 - 45%|
|Sodium Sulfate Soundness Loss (ASTM C88)||12%|
|Angle of Internal Friction||40 - 45|
|Hardness (measured by Moh's scale of mineral hardness)*||5 - 6|
|California Bearing Ratio (CBR), top size 19 mm (3/4 in)**||up to 250%|
|*Hardness of dolomite measured on same scale is 3 to 4
**Typical CBR value for crushed limestone is 100%
The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal".
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