Sponge iron
As a sponge iron ( Eisenluppe , sponge iron , often also DRI for "direct reduced iron") one understands today mainly a product of the direct reduction of iron ore . The reduction of the iron ore results in a "pasty" spongy-pored product with a content of iron of 92-95%. Sponge iron is not an alloy created by a melting process ; no liquid pig iron is produced during production .
Due to its porosity (hence the name "... sponge" ), the sponge iron has to be compacted for further processing or - more recently - melted, since it contains undesirable impurities (especially slag ).
Basically, forged iron - for physical reasons - could only be produced as sponge iron until the invention of the raft furnace , the predecessor of the blast furnaces known from the 18th century .
Historical background and early technology
Sponge iron has been extracted by the Hittites in Asia Minor for at least 3800 years and since the beginning of the early Iron Age in Central Europe around 2800 years ago by smelting ferrous ore, mostly bog iron ore . In the German-speaking countries, the result was called Luppen . Based on this early term, glowing steel pieces that have already been cleaned and intended for further processing are still called lobes in industry.
In early processes, the ore was mostly mixed with charcoal or peat ( peat coal ) and annealed on open racing fires . Temperatures of around 700 to 900 ° C were created, which was sufficient for reduction.
Hence the developed smelting furnace , even racing stove , with a low shaft (usually made of clay ), the chimney effect it possible to reach temperatures of more than 1000 ° C. In terms of kiln technology, the racing kiln was an early form of the low- shaft kiln , often built using the slope technique in order to use the natural updraft. In order to achieve higher temperatures and more efficient iron extraction, however, it required the addition of blower technology to ensure the necessary supply of combustion air. Operated by hand with bellows very early on , until they understood - as an innovation - to use more powerful ones with water power as a drive.
The finished product was in good furnace operation in both cases, the so-called. Renneisen (from the speech importance of gutters - gutters removal of the slag after the opening of tapping).
The iron produced in this way contained both carbon and a very large proportion of slag and was therefore extremely brittle . In order to obtain a usable material (so-called wrought iron ) from this, the blacksmiths had to compress the sponge iron below the melting temperature of the iron and above the solidus temperature of the slag it contained by means of muscle power using a sledgehammer (sintering it together ). For thousands of years this was the method to "drive out" (press out) the still liquid slag . The iron compacted in this way was forged out and repeatedly folded over for further cleaning and then fire-welded in order to connect the iron piece obtained after the folding with one another. This process was also called fermentation .
With the Renneisen an initially carbon-rich iron was present (C 1%), which was according to the further process steps to a soft, malleable and very good weldable iron. Through the forging process, the treated iron was further decarburized and the structure was largely homogeneous . With the annealing process associated with forging , the precipitation of cementite and perlite was further reduced . In today's terminology, the product would be referred to as soft iron , in practice also pure iron . For further processing into usable weapons or tools , such as. B. chisel , further steps were required, such as carburizing in the forge and finally tempering such as hardening and tempering .
At the beginning of the late Middle Ages , larger sponge-like lobes (also called pieces or wolves ) weighing up to 100 kg could be produced in the further developed piece furnace , the yield of cleaned wrought iron barely exceeding 10–15 kg. As a forerunner of the blast furnace , the piece furnace consisted of a brick, square shaft and still had a temporary access made of clay, from which the rag or sponge iron was removed. The shafts, which were very strong in terms of cross-section and material, were - in some cases - up to 10 meters high at the beginning of Classicism . (At the same time, the “classic” clay kiln continued to be used in other regions.)
Due to the resulting higher process temperatures which has melting point of iron (1539 ° C) and there was already unintentionally, liquid pig iron, that of the former hut people as mud flow or Saueisen (engl .: pig iron was designated), as it malleable for further processing to Iron had to be laboriously refurbished in the forge . A process based on this is now known as the fresh- hearth process .
The sponge iron, compacted manually on the anvil, was forged in hammer forges , also known as iron hammers or horizontal hammers , with water-powered tail hammers (drop hammers) from the late Middle Ages onwards , even after the industrial revolution into the 20th century. At the same time, instead of mechanical hammers , the work of fermenting the rag was also carried out by hand by the blacksmiths' guild, which varies from region to region .
The further processing into high-quality steel ( fermentation steel , the early form of stainless steel ) - for example into stabbing weapons like epee blades - was effected by so-called refining hammers or by the blacksmith on site because he still had traditional knowledge. Basically, this technique is still used today in traditional Japanese blacksmiths' workshops to forge particularly high-quality swords. (More on this: see below )
The modern techniques of iron and steel production go back in principle to these original experiences of the time when today's blast furnace technology with a continuous supply of heated air- oxygen through blowers was not feasible with the given means.
Today's industrial extraction and use of sponge iron
The " Krupp racing process " developed around 1930 made it possible for the first time to process "poor" iron ores with a low percentage composition, but also iron-containing dust from blast furnaces contained in the blast furnace gas together with inexpensive coke abrasion or fine coal without using a blast furnace throughput. This process is currently not economical.
Newer direct reduction processes such as B. the Midrex process , which processes iron-rich ores with oxidizing additives in a shaft furnace or a rotary kiln and wins sponge iron in the final stage of the process. The sponge iron obtained in this way is usually "briquetted" before further processing, i.e. smashed, segmented and sintered . These briquettes or pellets are then Gattierungsbestandteil for cast iron from the cupola furnace or for the production of steel in electric arc furnaces . The porosity that still exists after pelleting accelerates the chemical reaction in the furnace. The initially existing mechanical strength under thermal stress prevents caking, so that the reducing gases can rise unhindered through the raw material.
The newer direct reduction processes for steel production usually melt the briquetted pellets from sponge iron together with oxide-rich scrap , thus further reducing the carbon content and thus skipping the pig iron stage of the blast furnace process.
Overview of the industrial production of sponge iron
There are currently four direct reduction processes in use for the production of sponge iron:
- Midrex process , currently the most economically important process with 59.7% (as of 2010) of the worldwide production of directly reduced iron (DRI). The iron ore is reduced using a hydrogen-rich gas using the countercurrent principle. As a rule, natural gas is used for this . The end product sponge iron is very pure (C content between 0.5 and 3%) and can be used directly for steel production instead of pig iron.
- HYL III, a method comparable to Midrex. The difference between Midrex and HYL III is how the reducing gas is obtained from the natural gas. At Midrex, the carbon dioxide content of the furnace gas is used for this , while at HYL the natural gas is split up in a steam reformer .
- Corex process, a newer method of smelting reduction, combines the process of direct reduction (pre-reduction of iron to sponge iron) with a smelting process in the melter gasifier and carburization (final reduction). The aim is to produce liquid iron with a C content between 3.5 and 4%, the quality of which corresponds to that of blast furnace pig iron. Uncoked coal, which is processed in the coal gasification process, is used as an energy source . The large amounts of exhaust gas produced during the process, such as B. Carbon monoxide and hydrogen are used as reducing gas; Surpluses have to be sold in order to make the operation economical.
- Finex process, a process similar to Corex. Here, fine ore is dried in a fluidized bed dryer and processed into sponge iron in a multi-stage fluidized bed reactor . After compaction (Hot Compacted Iron, HRI) it is melted into liquid pig iron by means of a melter gasifier.
Around 75 million tons of sponge iron are produced annually (as of 2012). Based on DRI plants under construction, plants operating according to the Midrex process estimate that DRI production could increase to 200 Mt / year by 2025/2030.
Various steel groups in Europe are currently making initial efforts to make the energy-intensive production of steel more cost-effective through direct reduction using the controversial fracking process . Here, the cheaper so-called shale gas (a natural gas stored in clay stones ) is to be used in the Midrex process to reduce the iron ore / iron oxide mixture - instead of the classic natural gas. The procedure, which is already practiced in the USA, has not been approved in Europe since 2011, despite ongoing applications, due to unexplained environmental risks - primarily due to contamination of groundwater and surface water. In May 2013, the Advisory Council on Environmental Issues spoke out against fracking in Germany, as it would neither lower gas prices in Germany nor increase security of supply.
Use of sponge iron in crafts
In the handicraft sector, the time-consuming and therefore cost-intensive self-production of sponge iron has almost died out in western cultures due to the industrial supply of forgeable, low-carbon semi - finished products .
Racing fire symposia for the production of sponge iron
Because of this, there were numerous attempts in Germany, France and Switzerland from the end of the 1990s in the form of racing fire symposia to rediscover the knowledge of historical iron production using racing furnaces , which was only lost from around 1900 . On the initiative of individual art blacksmiths or blacksmiths associations, numerous public racing fire experiments to investigate the early Iron Age iron smelting took place - often parallel to blacksmiths' meetings in conjunction with exhibitions of contemporary metal sculpture . At the same time, attention should also be drawn to the great achievements of earlier generations of blacksmiths.
To make it clear: up to the late Middle Ages, rarely more than 20 kg of sponge iron was produced per kiln cycle, which lasted two to three days. After the hard work of compacting and refining with the corresponding burn-off (scale), a fist-sized lump (approx. 3 kg) of malleable iron was created. After further processing, at best two small sword blades or pieces of armor could be made from this quantity . This required about 250 kg of charcoal and 60 kg of ore. The clay kiln had to be rebuilt for each kiln.
Reports on radio and television (e.g. broadcast with the mouse ) and the publication of the knowledge gained by Damascus blacksmith Heinz Denig aroused the interest of science. As a result, the knowledge gained was carefully examined and documented from an engineering point of view. In doing so, the scientists relied on the commitment and traditional knowledge of the art blacksmiths involved.
Traditionally cultivated blacksmithing in Japan
It is known that even today the traditional Japanese forging the necessary raw material ( Tamahagane = sponge iron) for the hand-forged swords (z. B. Katana ) in small, the (named above) bloomery similar herds even smelt . Instead of z. B. lawn iron ore or other iron ores iron sand used. The refining steel produced from it has better properties than industrially produced semi-finished products due to the special production method and the manual forging. The better properties manifest themselves mainly. in the manufacture of knives or bladed weapons , as the structure is more oriented in one direction due to the craftsmanship . It follows from this that the notched impact strength is higher in the transverse direction and lower in the longitudinal direction - which is particularly desirable here.
Historical swordsmithing in Europe
It can be assumed that in historical times in the European area of that time - and in the Middle East with Damascus as its famous center - weapons with similarly good properties in terms of notched impact strength, breaking strength and edge retention could be forged as in Japan today. Both in historical written sources and, above all, in the Germanic / German world of legends, myths about "miracle weapons" have been preserved, the core of which has been scientifically proven in relation to the extraordinary production. For example, the Wieland sword “Mimung” was previously filed for the blacksmith and given to the geese to eat, whereby the steel was nitrided and thus harder. The goose droppings were melted down again and a new sword was forged from it. Other examples are the legendary sword "Gram" or "Balmung" from the Nibelungen saga , the Eckesachs or the Durendal .
With the industrial revolution , this metallurgical knowledge was gradually lost. The reasons for this lay not only in the changed - more cost-effective - production conditions and the increasing use of semi-finished products , but also in the changed warfare, in which high-quality and durable blades were replaced by firearms. The general awareness of blade quality was changing; more optical effects came to the fore, such as B. Chromed sabers in the 20th century.
Inspired by the racing fire symposia and the following public discussion, knife and Damascus smiths are increasingly trying to make their own steel for high-quality knives from sponge iron. The German cutlery industry from Solingen has followed the general trend and has been ordering hand-forged Damascus steel from the well-known Balbach forge for the production of hunting knives since 2004 . In connection with the traditional manufacturing technique, the structure is also more aligned in one direction and thus, as a seal of quality, the notched impact strength is higher in the transverse direction. However, for reasons of cost, Damascus production from sponge iron is not used here.
See also
- Wheelwork
- Iron smelting by the Teutons , iron production in the Lahn-Dill area
- Oven sow
- Mining archeology
literature
- Ernst Brunhuber (founder): Foundry lexicon. 17th edition, completely revised and edited by Stephan Hasse. Schiele & Schön, Berlin 1997, ISBN 3-7949-0606-3 .
- AF Holleman , E. Wiberg : Textbook of Inorganic Chemistry . 24. & 25. Edition. De Gruyter, Berlin 1945, p. 488.
References and footnotes
- ^ Agricola : De re metallica libri XII. (1556)
- ↑ From iron to steel. Published by the Brandenburg an der Havel Industrial Museum , May 2002 (PDF; 168.3 kB).
- ↑ a b Matthias Zwissler, Roman Landes: Investigations into racing fire products . In: Racing fire symposium of the IGDF International Society for Damascus Steel Research eV in Polle at the blacksmith Georg Petau's, August 20, 2005. (PDF; 3.54 MB) .
- ↑ Pure Iron. Angele website (machine builder and forge supplier). Retrieved August 24, 2013.
- ↑ Ludwig Beck : Wrought iron preparation in fresh fires - Saueisen and fresh. S. 217 In: "The history of iron in technical and cultural-historical relationship", Vol. 2, (1895), 1332 S., ISBN 3253031926 , (Berlin State Library: HA 17 Jc 210-2), accessed on August 24 2013.
- ↑ For more information see professional literature on ironmaking
- ↑ see: direct reduction. In: Ernst Brunhuber (founder): Foundry Lexicon. 17th edition, completely revised and edited by Stephan Hasse. Schiele & Schön, Berlin 1997.
- ↑ Georg Küffner: The blast furnace is far from burned out. In: FAZ No. 135 of June 14, 2005. Retrieved on August 24, 2013.
- ↑ Werner Sturbeck: Fracking - The energy price gap. In: FAZ , August 3, 2013, No. 178, p. 13. Retrieved on August 24, 2013.
- ↑ Olaf Velte: The oven pig is reminiscent of old times. In: fr-online.de , June 4, 2010. Retrieved September 27, 2013.
- ↑ Heinz Denig: Explanation of the book "Old Blacksmithing Damascus Steel". Documentation of successful racing fire experiments. In: damaszenerschmiede.de. Retrieved September 27, 2013.