|OTP appearance||grey solid|
Iron is one of the seven metals of antiquity. Because of its relatively high melting point, iron is produced by subtractive smelting. All non-iron metals are removed from the ore by smelting processes, leaving just the iron behind.
|Wrought Iron||≥97||0.04-0.08||≤2 Si||Carbothermic reduction of ore|
|Cast Iron||≥94||2.1-4||≤3 Si|
- Elemental iron does not occur naturally
- The nickel alloy occurs rarely in meteorites
- -oxides occur naturally in the minerals hematite and magnetite
- -hydroxides occur naturally in the mineral goethite
- -sulfides occur naturally in the mineral iron pyrite
Subtractive smelting requires the metal amalgam (called the bloom) to be kept at high temperature (bright yellow heat) for several hours to throw off the silicon dioxide and aluminum oxide slag/gangue, and release other metallic impurities as liquids. Since the temperature is so high and the duration so long, if the forge is made of more primitive materials it is essentially single-use. It's normal to create a forge for a single smelt, and tear it completely down following the smelt.
A second unusual characteristic of smelting iron is that typically no container is used. The "stack" of the forge is repeatedly covered with a layer of charcoal and a layer of iron ore. When the fire at the bottom of the stack burns down some of the charcoal, the layers move down and more room is made at the top of the stack for more layers of charcoal and iron. This process is repeated until either the iron bloom grows large enough to block the air vents or ash removal openings, or all the iron ore has been consumed. In the latter case, a final "filling" of just charcoal is put into the forge to allow the last of the iron ore to merge with the existing bloom.
The spongy bloom is moved directly from the reducing forge to a nearby hard surface and hammered into a form as compact as possible. This bursts any embedded pockets of gasses or metal impurities. This first hammering must be done as quickly as possible (less than 1 minute) after the bloom's removal from the forge , or it will lose too much heat to keep the impurities liquid and the bloom soft. If it cools off too much, it must be placed in a reducing forge once again to raise its temperature back to bright yellow, and the hammering process repeated. After this first hammering, the spongy bloom which contained pockets of gasses and liquid metal impurities has been made into a more compact bar containing only solid impurities such as grains of sand (silicon dioxide, aluminum oxide, quartz, etc) which did not melt in the reducing forge.
Decarburizing cast iron
Cast iron is more readily produced in large quantities than pure/wrought iron.
This process is as simple as melting pig iron in an oxidizing environment: usually by forming a large pool of cast iron which is exposed to air. This is a difficult task, since the heat required is great but intimate contact with the burning fuel will result in a reducing, rather than oxidizing, environment. One tool suited to this process is the reverberatory furnace. With air circulating in at the access port, an oxidizing environment can be achieved. Tools inserted to agitate the pool of iron increase the overall surface (reaction) area, resulting in quicker and more complete oxidation.
Introducing sodium (not potassium) nitrate to the bottom of a cylinder of molten cast iron removes phosphorus, sulfur, and carbon. Sufficiently accuracy can produce steel directly from cast iron this way. Several reactions present themselves, including:
- 4 NaNO3 + 5 C → 3 CO2 + 2 Na2CO3 + 2 N2
- 2 NaNO3 + 4 C → 3 CO + Na2CO3 + N2
Any resulting potassium carbonate would liquify and float atop the iron while the gasses escaped. Any CO produced might serve to reduce small particles of iron oxides entrained in the cast iron.
Refining takes place in a second, lower temperature (red heat) forge. The iron bar undergoes a repeated heat-draw-fold process. The idea is that after each drawing-out, any pocket of impurities has a chance of being on the "outside" of the bar. When on the outside, they will be removed either by exposure to oxygen, exposure to heat, or physically separated from the iron by the hammering. After some number of draws, the iron will be sufficiently pure for use.
Specifically, we reheat the bar in a refining forge to a medium-red heat and hammer it into a strip (usually a few cm wide, less than a cm thick, and as much as a meter long), then fold it in half lengthwise twice, to produce something more like the dimentions of the original bar. This bar is placed back in the refining forge, heated to bright red heat, and the process repeated.
Scientific American April
link courtesy Scientific American.
Van Nostrand's Eclectic Engineering Magazine 1; pp75-79.