The tests that eventually lead to the conception of argon-oxygen decarburisation were conducted at Union Carbide from 1954 to 1955 [17]. Subsequently, a patent application was submitted by William A. Krivsky in 1956 [46]. Initially, two different tuyère injection designs were tested: the first design combined injection of both oxygen and argon through tuyères, while the second design employed the separate injection of oxygen through a top lance and used tuyères for delivering the argon [47]. The experiments pointed out that the combined injection design was a simpler and more reliable design, and thus it was selected as a basis for the first 15 t commercial installation at the Joslyn Fort Wayne plant in 1968 [17, 47].
Following the success of the first commercial installation, the AOD process was adopted throughout the world in the 1970s [5, 18, 19, 21, 48] and became the dominating refining process already in the early 1980s [5]. Nowadays, the AOD process accounts for approximately three quarters of the total world production of stainless steel [9, 49].
The advent of the AOD process marked a breakthrough in converter technology: a high chromium yield could be achieved with a significantly lower consumption of reductants [5, 10, 17, 47, 50]. The carbon content could be reduced to below 50 ppm without vacuum treatments [51]. Additional advantages over the preceding processes include:
– high cleanness of steel [48, 52],
– relatively low sulphur content of steel [52], – good predictability of the process [48], and – the ability to use inexpensive charge materials [21].
In addition to refining, part of the alloying of stainless steel is conducted in the AOD process. Typical alloying additions include, but are not limited to, high carbon ferrochrome, stainless steel scrap, carbon steel scrap, nickel, iron, high carbon ferroman-ganese, and molybdenum oxide [5]. Minor alloying elements such as molybdenum, vanadium, and tungsten can be added either as ferroalloys or as oxides [53]. The total weight of the additions varies in the range of 5–30% of the tap weight [5]. Alloying additions can also be used for cooling the metal bath [54]. The alloying additions during the reduction stage primarily serve this aim [55].
2.1 Equipment
A schematic illustration of a typical geometry is shown in Fig. 5. The vessel is attached to its foundations via a supporting structure which allows the tilting of the vessel for charging and tapping. Owing to the high blowing rates, the AOD process provides highly efficient mixing characteristics [56–58]. One of the drawbacks of high gas injection rates is that the resulting fluid flows can bring the whole vessel into an oscillating motion [59–61], causing wear of the support bearings, structural steelwork, and foundations [62–64]. The oscillation behaviour is dominated by blowing procedure and is most intense during the reduction stage or at the end of the decarburisation stage [62, 65–67].2
Tuyères Top lance
Gas plume Metal bath Slag
Gas jets
Fig 5. Schematic illustration of an AOD vessel. Reproduced from Article I by permission of Springer Nature.
The AOD vessel is lined with refractory materials in order to protect the steel shell from the high temperatures prevailing in the process. Magnesia-chrome and dolomite
2Dampening systems have been proposed as a solution for reducing the vessel oscillation [64].
have been the most common refractory materials since the first AOD furnace, although rebonded and semi-bonded bricks have replaced direct-bonded bricks [68]. In general, magnesia-chrome stones have a better durability with acid slags, while dolomite is better suited for basic slag practice [69]. In terms of temperature, the chemical potential of reactions and fluid flows, the most wearing circumstances are found at the tuyère wall and the trunnion area [70–72]. Dolomite refractories typically last for approximately 100–150 operating hours [69].
The off-gas consists mainly of carbon monoxide and inert gases [55]. Its com-position changes during the processing so that the CO content is proportional to the decarburisation rate [55]. The main mechanisms of dust formation are the ejection of metal droplets, ejection of slag droplets, entrainment of solids, and vaporisation of metal [73]. Experimental evidence suggests that the ejection of metal and slag droplets are the most important of the aforementioned mechanisms [73]. The solid oxide particles found in the off-gas amount to 5–8 kg per ton of metal on average [55], and consist mainly of FeO, Cr2O3, MnO, and CaO [55, 74].
Because the attachment of the flue structure to the converter mouth is not air-tight, the oxygen contained in the leakage air may react with CO to form CO2, thus heating up the flue gas [75]. Numerical calculations suggest that during simple side-blowing post-combustion takes place primarily in the AOD flue and is sensitive to outlet fan gauge pressure [75, 76]. If the pressure in the ventilation hood rises, most of the gas exits to the atmosphere through the gap between the vessel and the hood [77]. However, the intrusion of cold air into the vessel appears to be impossible under normal operating conditions [78].
2.1.1 Tuyères
Most AOD vessels feature two to nine horizontally aligned tuyères along the converter wall [5]. Fig. 6 illustrates a typical double-tube structure: the gas mixture is injected through a coaxial inner stainless steel tube, while inert gas (N2or Ar) is injected through the outer copper tube in order to cool the tuyère [62, 68, 79]. The total flow rates through the central pipe and the outer tube are typically 0.80–1.25 Nm3/(t·min) and 0.05–0.08 Nm3/(t·min), respectively [80].
Owing to limited measurements available from actual AOD vessels, the effectiveness of side-blowing has been subject to numerous physical and numerical modelling studies [58–63, 65, 67, 79, 81–104]. Physical modelling studies suggests that the penetration
Ar
Ar + O2
Fig 6. Schematic illustration of an AOD tuyère. Adapted from [68].
depth is independent of the bath height [92] and increases with the increasing momentum of the gas mixture [62, 67]. Some estimates on the penetration depth in an actual vessel have also been obtained throughcomputational fluid dynamics(CFD) modelling:
Odenthalet al.[62] reported that the penetration depths were in the range of 0.35 to 0.4 m in a 120-tonne vessel (7 tuyères; total flow rate of 120 Nm3/min). Some authors [62, 101, 105] have found that a reasonable agreement with CFD calculations can be obtained using the penetration depth correlation proposed by Hoefele and Brimacombe [106]:
L=10.7 Fr00.46dtuyère
ρG
ρL 0.35
, (1)
whereLis the penetration depth, Fr0is the modified Froude number,dtuyèreis the diameter of the tuyère,ρGis the density of the gas phase, andρLis the density of the liquid phase. It should be bore in mind that Eq. 1 does not account for the interaction of the gas jets [65].
2.1.2 Top lance
The experiences from refining of stainless steels in top-blowingbasic oxygen furnace (BOF) vessels indicated that although top-blowing enabled a high oxygen injection rate, the vessel was ill-suited for efficient mixing of slag during silicon reduction [107].
Relatively soon after the introduction of the AOD process, steel producers in Germany [108, 109] and Japan [35, 110–112] conducted tests on the use of combined top- and side-blowing for refining of stainless steels. Following the success of these attempts, it has become common to equip AOD vessels with a top lance, which is employed usually only in the early part of the decarburisation stage [5, 22, 80]. The gas flow rate through the top lance is typically 1.0–1.6 Nm3/min per ton of molten metal [80]. Similarly to side-blowing, the top-blowing gas mixture can be diluted with N2or Ar [55, 80], often
up to an oxygen-inert ratio of 1:1 in the final combined-blowing step [80].
Both sonic/subsonic and supersonic lances can be employed [5, 55]. Supersonic lances are always water-cooled and positioned one to four meters from the metal bath surface, while subsonic lances are placed closer to the bath and are not necessarily water-cooled [5, 55]. The advantages of supersonic lances in comparison to subsonic lances are the increased oxygen delivery rate and improved penetrability of the gas jet [5]. On the other hand, subsonic lances enable more post-combustion, which improves the energy efficiency of the process [5]. The supersonic gas jet is obtained with a de Laval nozzle [113, 114], which can feature one or more exit ports [114]. For a properly designed nozzle, the gas velocity at the nozzle exit can be calculated from [115]:
uexit=
whereγ is the isentropic expansion factor,ρ0is the density of the gas at supply condition (p0andT0), p0is the supply pressure, and pexit is the pressure at nozzle exit. After exiting the nozzle, the gas jet forms a potential core, which has a length of approximately six times the nozzle exit diameter [114]. In this region, ambient pressure and temperature have only a small effect on the spread and decay of the gas jet [114, 116, 117]. Upon entering the converter atmosphere, the gas jet becomes subject to ambient conditions, which have a considerable effect on its behaviour [114, 117–120].
The length of the supersonic region is affected greatly by the ambient temperature in the converter: if the ambient temperature increases from room temperature to 1923 K (1650◦C), the length of the supersonic region of an oxygen jet increases from 10–20 to 20–35 times the nozzle exit diameter [114]. The effect of ambient pressure (pamb) on the expansion phenomena of a supersonic gas jet can be summarised as follows [113, 114, 118, 119, 121]:
– If the exit pressure is lower than the ambient pressure,over-expansionoccurs and overlapping compression and expansion waves cause a diamond-shaped pattern, which occurs periodically until the jet becomes subsonic (Fig. 7a).
– If the exit pressure is equal to the ambient pressure, the expansion of the gas jet is optimal; this is the design point of the de Laval nozzles (Fig. 7b).
– If the exit pressure is higher than the ambient pressure,under-expansionoccurs:
expansion waves are formed after the nozzle exit and overlapping compression waves continue until the jet the end of the supersonic region (Fig. 7c).