Chemistry of Steelmaking by Basic Oxygen Furnace
Chemistry of Steelmaking by Basic Oxygen Furnace
During the steelmaking process by basic oxygen furnace (BOF), impurities in hot metal such as carbon (C), Silicon (Si), manganese (Mn), phosphorus (P) etc. are removed by oxidation for the production of liquid steel. Oxidation is carried out with high purity oxygen gas which is blown in the BOF. The oxidation reactions result into the formation of CO, CO2, SiO2, MnO, and iron oxides. While CO and CO2 are in gaseous form and removed from BOF top as converter gas, other oxides are dissolved with the fluxes added to the BOF, to form liquid slag that is able to remove sulphur (S) and phosphorus (P) from the metal.
BOF process has two characteristics. First, the process is autogenous that meaning that no external heat source is needed. The oxidation reactions during the oxygen blow provide the energy needed to melt fluxes and scrap and to achieve the desired temperature of liquid steel. Second, the process refines hot metal at high production rates for the production of liquid steel. The fast reaction rates are due to the large surface area available for reactions. Large amount of gas is evolved when oxygen is injected into the bath of metal. This gas forms an emulsion with the liquid slag and metal droplets sheared from the bath surface by the impingement of the oxygen jet. The large surface area generated by gas- metal- slag emulsion increase the rates of the refining reactions.
Since the impurities are dissolved in the molten metal, reactions between impurities and oxygen occurs with dissolved oxygen. Further since oxidation of carbon takes place at higher temperature, carbon oxidation to CO is highly probable and hence majority of C is removed as CO. The oxidation reactions which take place during BOF steelmaking are given below. All the reactions are exothermic.
Fe + O = FeO
Mn + O = MnO
Si + 2O = SiO2
C + O = CO
2P + 5O = P2O5
Decarburization of the carbon available in the bath is the most extensive and the important reaction during oxygen steelmaking. The change in the carbon content and the bath composition of the bath with the progress of the oxygen blow is shown in the Fig.1. There are three distinct stages during this decarburization reaction. In the first stage which occurs during first few minutes of the blow, decarburization takes place at a slow rate, since most of the oxygen supplied reacts with the silicon of the bath. During the second stage, which occurs at high carbon content of the bath, decarburization takes place at a higher rate and is controlled by the rate of supplied oxygen. The third stage occurs when the carbon content of the bath reaches around 0.3 %. At this stage, the decarburization rate drops since lesser carbon is available to react with all the oxygen supplied. At this stage, the rate is controlled by mass transfer of carbon, and the oxygen will mostly react with iron to form iron oxide. At this stage, since the rate of CO generation drops, the flame at the BOF mouth becomes less luminous and practically disappears when the carbon drops to a level of around 0.1 %
Fig 1 Change of composition with the progress of the blow
Conditions favorable for silicon oxidation are (i) low temperature, and (ii) low amount of silica in the slag. A basic slag favours silicon oxidation. In basic, silicon oxidation occurs practically to a very low value since SiO2 reacts with CaO and decreases activity of silica in the slag.
Almost all the Si gets oxidized and removed early in the blow because of a strong affinity of oxygen for Si. The silicon of the hot metal is oxidized to a very low level (<0.005 wt. %) in the first 3 to5 minutes of the blow. The oxidation of Si to silica (SiO2) is exothermic and it produces significant amount of heat which raises the bath temperature. It also forms a silicate slag that reacts with the added lime and calcined dolomite to form a basic slag. Since the oxidation of silicon is the main heat source, it amount in hot metal determines the amount of the cold charge (scrap, pig iron etc.) that can be added to the converter. It also determines the volume of slag and hence affects the yield and dephosphorization of the bath. As per rule, more slag will result into lower phosphorus but also lesser yield.
Oxidation of iron is the most important since it controls (i) FeO content of the slag and oxygen content in the steel, (ii) loss of iron in the slag and hence affects the productivity of the steelmaking process, (iii) oxidation potential of the slag, and (iv) FeO helps in the dissolution of lime in the slag.
Manganese oxidation reaction in oxygen steelmaking is rather complex. In a top blown converter, Mn is oxidized to MnO in the earlier stages of the blow and after most of the silicon is oxidized then Mn reverts back into the bath metal. Finally during the blow end when more oxygen is available for the oxidation then Mn gets reduced in the bath metal. In case of the bottom blowing or combined blowing in the converter, the oxidation of Mn has got a similar pattern but the residual Mn content of the liquid steel in the converter bath is higher than the top blown converter.
The oxidizing conditions in the converter favour the dephosphorization of bath metal. The reaction of dephosphorization takes place due to the interaction of metal and slag in the bath. Parameters such as lower bath temperatures, higher slag basicity (CaO/SiO2 ratio), higher content of FeO in the slag, higher slag fluidity and good stirring of the bath favour the dephosphorization reaction. The removal of phosphorus follows the pattern as shown in Fig 1. The phosphorus content of the bath metal reduces in the beginning of the blow, then during the main decarburization period when the FeO is reduced, P reverts into the bath metal and finally it reduces again at the blow end. Bath stirring improves the mixing of metal and slag and helps in the rate of dephosphorization. Good stirring with the addition of the fluxing agents such as flour spar etc., also improves phosphorus removal by increasing the dissolution of lime, resulting in a highly basic and fluid liquid slag.
Sulphur removal is not very effective in the BOF steelmaking process due to the highly oxidizing conditions. Sulphur distribution ratio (% S in slag / % S in the metal) is around 4 to 8 which is much lower than in the steel ladle (around 300 to 500) during the secondary steel refining. During the BOF process, approximately 10 % to 20 % of sulphur in the bath reacts with oxygen directly to form SO2. The remaining sulphur is removed by the following slag – metal reaction.
S + CaO = CaS + FeO
Removal of S by the slag is assisted by high basicity and low Fe content of the slag. S content of the liquid steel is greatly influenced by the S contained in the hot metal and scrap which is charged in the BOF.
Formation of slag
Fluxes (lime and calcined dolomite) that are charged early in the blow dissolve with the developing oxides to form a liquid slag. The rate of dissolution of these fluxes strongly affects the slag metal reactions occuring during the blow. At the beginning of the blow, the lance height above the bath is more which causes an iniyial slag rich in SiO2 and FeO. During this period large amount of fluxes are charged in the BOF. The lance is then lowered and the slag starts to foam at around one third of the blow due to reduction of FeO in the slag in conjunction with CO formation. The drop in the FeO content in the slag is shown in Fig 2. As the blow progresses, the CaO dissolves in the slag, and the active slag weight increases. After the blow has progressed around three fourth of the time, the FeO content in the slag increases because of a decrease in the rate of decrburization. During the blow, the temperature of the liquid steel gradually increases from about 1350 deg C to 1650 deg C at turndown, and the slag temperature is about 50 deg C higher than that of the liuid steel. The slag at turndown may contain regions of undissolved lime mixed with the liquid slag, since the dissolution of lime is limited by the presence of dicalcium silicate (2CaO.SiO2) coating, which is solid at steelmaking temperatures and prevents rapid dissolution. The presence of MgO in the lime weakens the coating. Thus, earlier charging of MgO speeds up slag formation due to quicker solution of lime.
Fig 2 Formation of slag components in BOF