Freshen up

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Refining is the name of various processes that primarily serve to reduce the carbon content, but also the content of various other undesirable constituents of pig iron such as phosphorus and sulfur with the help of oxygen . Depending on the type of process, scrap can be processed in addition to pig iron and different types of steel or cast iron can be produced.

The most widely used process is the Linz-Donawitz process (LD process). This means that 72% of world crude steel production is currently produced (as of 2013).

Late medieval blacksmiths freshening the high-carbon rags on a fresh fire . Source: Agricola : De re metallica libri XII. (1556)

Depending on the temperature of the finished iron or steel, a distinction is made between three basic Frisch processes:

  • Liquid pig iron is processed in the welding steel process. In the course of the fresh treatment, however, the reduction in the carbon content increases the melting point of the iron more than its temperature, so that the resulting steel begins to crystallize and becomes doughy.
    There are four variants of the welding steel process:
    • The fresh hearth process emerged from the Rennofen process , in which sponge iron ( lobes ) was produced. The further development of the racing furnace for piece oven and later raft oven were due to higher temperatures - undesirable - already liquid carbon-rich and not malleable pig iron produced, the so-called Saueisen (. English pig iron ), which are refined consuming after solidification had. Today as then, introduced charcoal ensures the supply of heat and at the same time has an oxidizing effect to reduce the unwanted iron components. The refined iron must then be freed of slag components by forging . Historically under multiple fire welding on the anvil .
    • In the puddling process , also known as flame furnace freshening , the pig iron is heated in a pan until it is liquid, and hot air is blown over it with constant stirring, which ensures that the iron components are burned.
    • The packaging process mainly works with welded steel scrap, which is bundled in a solid state, i.e. in the form of a package (name!). In the welding furnace, these packets are heated to approx. 1400 ° C and the welding sand, which acts as a hearth, combines with the iron oxide produced on the surface to form slag, which then flows off.
    • The Aston process is used to manufacture welded steel and was developed in the USA. Already prepared, about 1500 to 1550 ° C hot, liquid steel is poured onto a slag that is also liquid, but about 250 ° C colder. As a result of the quenching process and the strong movement, the steel loses some of the dissolved gases and foreign additions. As a result of the cooling, the temperature also drops below the melting point and solidifies into a pellet weighing several tons .
  • In the mild steel process , the feedstock is processed in either the solid or liquid state. However, the temperature increases so much during the refining process that the finished steel is always liquid. The mild steel process has five different subspecies.
    • The crucible steel process is one of the heated fresh processes and originally served to free welding steel by remelting it from impurities and to homogenize it. As the demands on steel increased, the process was also used to produce steel with properties tailored to the respective purpose.
    • The stove freshening process is one of the heated fresh processes in which the pig iron is placed in a flat stove. The oxygen required for freshening is brought into the bath by oxygen carriers (e.g. ore, mill scale, etc.). Siemens-Martin furnaces and electric arc furnaces are among the stove-refreshing processes.
    • The wind freshening process is one of the unheated fresh processes, since so much heat is generated during the process that an additional heat source is unnecessary and even some scrap can be processed. The oxygen is either blown through small holes in the crucible or with a lance onto the molten pig iron from above. The wind-freshening processes with oxygen blown in from below include the OBM process (Oxygen-Bottom-Maxhütte or Oxygen-Bottom-Blow-Metallurgy Process) and the Thomas process . Air or oxygen is blown from above into the Bessemer pear and the Linz-Donawitz process (LD process).
    • The electric steel process is one of the heated fresh processes in which the required heat is generated by electrical energy and transferred into the feed material either with electrodes as in an electric arc furnace or by induced alternating magnetic fields as in an induction furnace .
    • In the Perrin process , developed by René Perrin in 1932, liquid pig iron is poured from a height of several meters onto a slag bath, which is also liquid, thereby intensively mixing the two. Due to the large contact area between the two phases, the fresh process is completed within a few minutes.

Another process that counts for freshening is vacuum oxygen decarburization (VOD), with which stainless steel is decarburized under vacuum.

literature

  • Robert Durrer : Basics of iron extraction . Verlag Francke AG, Bern 1947, p. 149-199 .

Individual evidence

  1. ↑ stahl-online.de
  2. ^ A b Robert Durrer : Basics of iron production . Verlag Francke AG, Bern 1947, p.  164 .