Project Topics
Get Complete Project Material File(s) Now! »
The chemistry of the process
The oxygen blow is often divided into three periods, which coincide with the slag formation. The first period lasts for 4-5 minutes, and is called the slag forming period. The second period is where most of the decarburization happens and it lasts for 10-11 minutes. The third period is the final stage of the blow, it is a critical period for the yield loses of iron and it lasts for 2-3 minutes [4].
The first blow period
The converter is charged with about 160 ton pig iron and 45 ton scrap. Shortly after the blow starts, the converter is also charged with burnt lime (CaO) and dolomite lime (CaO and MgO).
The oxygen lance is lowered on top of the melt at a height controlled by a lance program. The lance is at a high position at the beginning of the blow. When the oxygen strikes the pig iron at supersonic speed (Mach 2,7[3]), it creates a crater which becomes a reaction zone (also called the hot spot), see Figure 3. The main combustion reaction at the crater is the oxidation reaction of iron, which is followed by the oxidation of silicon and manganese. A reaction index explanation is shown in Table 1.
¾ {Fe} + ½ O2 (g) → (FeO) (1).
¾ [Si] + O2 (g) → (SiO2) (2).
¾ [Mn] + ½ O2 (g) → (MnO) (3).
The products of these reactions join the slag. FeO and MnO acts as a flux aids which help to dissolve the added CaO. The dissolution of CaO can also be improved by adding additional flux aids and the use of bottom stirring. At the end of the first period, 30-40 % of the added CaO has dissolved and started forming slag[4]. Infusible dicalcium silicates begin to form in the slag. At this point the composition of the slag has gone from point 1 to point 2 in Figure 4.
The second blow period
At this point, most of the silicon is gone. See Figure 5. The lance is lowered. The temperature of the melt rises and the decarburization process starts along with the oxidation of phosphorous.
¾ [C] + ½ O2 (g) → CO (g) (4).
¾ 2[P] + 5/2 O2 (g) → (P2O5) (5).
Carbon exits the converter as CO gas. CO gas among other emissions is taken care of to protect the environment. Almost all CO is burned to CO2[4].
¾ CO (g) + ½ O2 (g) → CO2 (g) (6).
As the oxygen jet strikes the melt, droplets are thrown up in the slag. The droplets which contain carbon and other dissolved elements help reduce the FeO in the slag. The contact surface between the droplets and the slag becomes the second reaction zone, see Figure 3. There the following reactions take place:
¾ (FeO) + [C] (in the droplet) → {Fe} + CO (g) (7).
¾ 2(FeO) + [Si] → 2{Fe} + (SiO2) (8).
¾ (FeO) + [Mn] → {Fe} + (MnO) (9).
¾ 5(FeO) + 2[P] → 5{Fe} + (P2O5) (10).
The bottom stirring system
Some of the advantages of the LD-LBE process compared to other processes found in literature are [4]:
¾ Lower oxygen content in the steel.
¾ Higher Manganese content in the steel.
¾ Less lining wear.
¾ Better mass and heat transport in the melt.
The LD-LBE converter body is made of steel with a ceramic lining (magnesite of different qualities, MgCO3). The stirring plugs are made of fused magnesia (very high MgO content), which can withstand the high temperatures of the process. The LD-LBE converter in Oxelösund has 8 stirring plugs, which are represented as dark dots at the bottom of the converter in Figure 7, and in Figure 8. Each plug has an individually applied flow. The used gas, volume and flow are controlled by the stirring programs in the LD process computer. The bypass is always used when no flow is applied to the plugs by the stirring programs; it consists of a constant counter pressure, which prevents steel from getting into the plugs and cause clogging. The stirring plugs can also be turned off individually, in which case they are applied a counter pressure. The previous thesis with the bottom stirring in Oxelösund was performed using only every other stirring plug[2]. In this thesis, all stirring plugs were used.
Nitrogen in LD
The pig iron charged in to the LD-LBE converter has a nitrogen content of about 60 ppm[19]. Nitrogen is also found in the atmosphere (about 78%), and as bottom stirring gas in the LD-LBE converter.
Nitrogen is used as a bottom stirring gas when the converter is uncharged to prevent plug clogging and to save on Argon costs. Nitrogen may also be used during the blow, depending of the type of steel (e.g. nitrogen alloyed steels).
A thesis work performed at SSAB Europe steel plant in Luleå charted the whole nitrogen absorption/desorption course in the LD-LBE process[19]. Several samples were taken in different steps of the LD-process during the thesis work in Luleå. Pig iron samples taken from a charged converter before the blow showed that the atmospheric exposure during the charging raised the nitrogen contents in the pig iron with about 5 ppm. It was found that Nitrogen’s transition from gas to melt occurred in the following steps, also seen in Figure 10:
¾ Transport within the gas phase by diffusion.
¾ Gas-melt interface reaction.
¾ Transport within the melt by diffusion.
Table of contents :
1 Introduction
1.1 Background
1.2 Aim and scope
2 Process description
2.1 The chemistry of the process
2.1.1 The first blow period
2.1.2 The second blow period
2.1.3 The third blow period
2.2 The bottom stirring system
2.2.1 Stirring programs
2.2.2 Limitations
3 Theory
3.1 Previous work
3.2 The dephosphorisation process
3.2.1 The formation of dicalcium silicates (phosphorous carriers)
3.3 Nitrogen in LD
3.4 Slopping
4 Method of work
4.1 Test programs
4.2 The bottom stirring condition
4.3 Sampling
5 Results and discussion
5.1 General results
5.3 Dephosphorisation results
5.4 Nitrogen results
5.5 Slopping control
6 Conclusions
6.1 Recommendations
7 Further work
8 Acknowledgments
9 References
