A Mathematical Model for Reactions During Top-Blowing in the AOD Process: Validation and Results
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THE argon oxygen decarburization (AOD) process is the most common process for refining stainless steel.[1] The gas-blowing mixture is diluted with argon or nitrogen in order to decrease the partial pressure of the carbon monoxide, thus promoting the decarburization reaction and reducing the chromium losses to slag.[2] Due to the lesser oxidation of metallic species, the consumption of reducing agents is likewise reduced.[3] In the original concept, the blowing mixture was delivered through tuye`res located on the sidewall of the vessel, but modern designs also feature a top lance.[1,4] Figure 1 shows an example of a blowing practice, which consists of a combined-blowing decarburization stage, multiple side-blowing decarburization stages, and a reduction stage with desulphurization and alloying.[5] The primary interest of this work is the combined topand size-blowing decarburization stage, which aims to VILLE-VALTTERI VISURI, AKI KA¨RNA¨, PETRI SULASALMI, EETU-PEKKA HEIKKINEN, and TIMO FABRITIUS are with the Process Metallurgy Research Unit, University of Oulu, PO Box 4300, 90014 Oulu, Finland. Contact e-mail: ville-valtteri.visuri@oulu.fi MIKA JA¨RVINEN is with the Department of Mechanical Engineering, Aalto University, PO Box 14440, 00076 Aalto, Finland. PENTTI KUPARI is with the Outokumpu Stainless Oy, 95490 Tornio, Finland. Manuscript submitted June 11, 2015. Article published online March 27, 2017. 1868—VOLUME 48B, JUNE 2017
maximize the decarburization rate by employing a high oxygen supply rate. Typical blowing rates at this stage are approximately 1.0 to 1.6 Nm3/(t min) through the top lance and 0.80 to 1.25 Nm3/(t min) through the tuye`res.[6] Both subsonic and supersonic lances can be employed in the AOD process.[1,4] While supersonic lances are water-cooled and positioned one to four meters from the surface of the metal bath, subsonic lances are placed closer to the bath and are not necessarily water-cooled.[1] The supersonic gas jet is obtained with a de Laval nozzle, which can feature one or more exit ports. The main advantages of supersonic lances over subsonic lances are the increased oxygen delivery rate and improved penetrability of the gas jet.[1] On the other hand, subsonic lances enable more post-combustion, which in turn improves the energy efficiency of the process.[1] It has been suggested that approximately 30 to 100 pct of the blown oxygen reacts with the metal bath depending on the design and position of the lance.[1] In comparison to decarburization during side-blowing, modeling of reactions during top-blowing in the AOD process has received relatively little attention. Watanabe and Tohge[7] conducted a study on the mechanisms of decarburization under reduced pressure and proposed a simple reaction scheme for top-blowing with O2, an Ar-O2 mixture, and CO2. Later, Tohge et al.[8] proposed a model for combined-blowing. In their
METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
MATERIALS AND METHODS
In contemporary practice, both tuye`res and a top lance are employed for delivering the req
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