Oxidation of Iron, Silicon and Manganese

Transcription

1 08 Oxidation of Iron, Silicon and Manganese AkMB Rashid Professor, Department of MME BUET, Dhaka Today s Topics Oxidation of iron Oxidation and reduction of silicon Oxidation and reduction of manganese

2 Oxidation of Iron Oxidation of iron according to the reaction [Fe] + [O] wt% = (FeO) is the most important since it controls FeO content of slag and oxygen content of steel Oxidation potential of slag Loss of iron in slag and hence affects productivity In addition to the above, FeO also helps in dissolution of lime in slag 3/21 [Fe] + [O] wt% = (FeO) a FeO a Fe x h O = a FeO a Fe x f O x [O] log 6150 T Since Fe in steel is almost pure, a Fe 1. For Fe-O system, the oxygen activity shows a small deviation from ideality: log f O = 0.17 [O] O x [O] = a FeO a FeO /T log [O] sat = T T, C [O] sat Increase in T increases oxygen dissolved in molten iron [O] in contact with pure FeO. Control of temperature is important to limit the dissolution of oxygen in molten iron 4/21

3 In steelmaking, FeO is present along with CaO, MgO, SiO 2, MnO activity of FeO is influenced by other solute oxides a FeO = γ FeO x (FeO) γ FeO = activity coefficient of FeO, which depends on slag composition. In CaO-SiO 2 -FeO system as CaO/SiO 2 ratio increases, a FeO increases physically it means that CaO replaces FeO from FeO.SiO 2. The following expression is used to express a FeO : γ FeO = (FeO) Consider a slag with (FeO) = 0.5 a FeO = 0.31; [O] = saturation concentrations of oxygen is about half of the saturation value calculated from the [O] relation 5/21 Oxidation and Reduction of Silicon Silicon can be melted with iron in any proportions with a noticeable quantity of heat. Solutions of silicon in iron showed a pronounced negative deviation from Raoult's law bonds existing between silicon and iron silicon is present in iron alloys with a composition close to FeSi The dissolution reaction: [Si] = [Si] 1wt% Activities in liquid Fe Si alloys at 1600 C DG = T log γ 0 Si = T 6/21

4 Silicon is present in all steelmaking slags as SiO 2. Equilibrium between silicon and oxygen in the steelmaking process : [Si] 1wt% + 2[O] 1wt% = (SiO 2 ) a SiO2 h Si x h O 2 If Henrian behaviour is assumed for silicon and oxygen in iron, then for silica saturated slags (a SiO2 = 1) : Si x O 2 = log T 1 K 11.4 The product of [Si] and [O] is constant up to 15% Si Lowering activity of oxygen by silicon is compensated by an increase in the activity of silicon by oxygen Influence of silicon on oxygen content of iron in equilibrium with solid silica [Si] 1wt% + 2[O] 1wt% = (SiO 2 ) Conditions favourable for silicon oxidation : Low temperature Low a SiO2 in slag a basic slag favours silicon oxidation. a SiO2 h Si x h O 2 log T 11.4 For a typical BOF slag, a SiO2 5x10 4. Calculations at 1600 C yields Si x O 2 = 5x x Assuming [O] = 0.08% at turndown, [Si] at equilibrium = 1.6x10 6 % (extremely low!!) virtually all Si present in iron should get oxidised and goes to slag in primary steelmaking 8/21

5 Effect of Temperature (a) (b) (c) Oxidation of Si by dissolved oxygen in the metal: [Si] + 2[O] = (SiO 2 ); DG = 542, T; Oxidation of Si by oxygen in the gaseous phase: [Si] + (O 2 ) = (SiO 2 ); DG = 775, T; Oxidation of Si by iron oxides in the slag: [Si] + 2(FeO) = 2[Fe] + (SiO 2 ); DG = 29, T All these reactions occur with evolution of much heat. Oxidation of Si occurs intensively in the presence of an oxidant in the whole range of steelmaking temperatures 9/21 Effect of Slag Composition In basic slags, silica formed reacts with basic oxides activity of SiO 2 in basic slags is negligibly low the reaction of silicon oxidation occurs practically to the end In acid slags slags are saturated with silica, activity of silica in acid slags is close to unity, a SiO2 = 1. 10/21

6 Reduction of silica If there is no intensive supply of oxidants (oxygen, air, iron ore) to a melt under an acid slag, Si reduction takes place: (SiO 2 ) + 2[Mn] = 2(MnO) + [Si]; DG = 32, T (SiO 2 ) + 2[C] = 2{CO} + [Si]; DG = 611, T These reactions occur with heat absorption. Si reduction is favoured by a high temperature. Equilibrium curves of reaction SiO 2 + 2C = Si + 2CO in iron-carbon melts The process of Si reduction is influenced by the compositions of the metal and slag. (SiO 2 ) + 2[Fe] = 2(FeO) + [Si] a 2 FeO x a Si a SiO2 a Si = K 2 a FeO (For SiO 2 saturated slag) SiO 2 reduction increases for slags with low a FeO. C, Mn lower [O] (thus lower a FeO ) in slag and favours SiO 2 reduction In basic slag, CaO breaks iron silicates, frees FeO, and favours SiO 2 reduction 11/21 Silicon as a Deoxidising Agent Silicon has a very high affinity for oxygen silicon can be used as a deoxidizing agent [Si] 1wt% + 2[O] 1wt% = (SiO 2 ) O = a SiO2 K x 1500 C, , and using a SiO2 = 1 O = x10 5 Si addition of Si reduces [O] content of steel Compared to Mn, deoxidation with Si is one order magnitude better In practice, better deoxidation resulted when both Si and Mn are used Silicon, wt% Manganese, wt% Oxygen, wt% /21

7 Oxidation and Reduction of Manganese Blast furnace iron usually contains between 0.5 and 2.5 wt% Mn. Manganese is soluble in iron in any proportions, and forms almost ideal solutions in iron. In pure Fe-Mn alloys, the activity of Mn varies almost precisely by Raoult's law. [Mn] = [Mn] 1wt% DG = 9.11T In practice, Mn present as a ternary solution Fe-C-Mn. Carbon lowers the activity of Mn and gives a negative deviation from Raoult's law Variation of the activity coefficient of Mn with C content in liquid Fe-C-Mn alloys at 1540 C 13/21 Manganese oxide has extensive solubility in slags. In pure Fe-Mn system, simple deoxidation by manganese results FeO-MnO inclusions. In acid steelmaking, the slag is basically a FeO-MnO-SiO 2 ternary system. In these liquid slags, the maximum solubility of silica is ~ 50 wt% at 1600 C and the activity coefficient of MnO has a constant value of mol%. In basic steelmaking slags, MnO is present only as a minor constituent (<10 wt% of the slag) during the refining period. 14/21

8 (FeO) + [Mn] 1wt% = (MnO) + [Fe] a MnO x a Fe a FeO x h Mn γ MnO x N MnO γ FeO x N FeO x f Mn Mn Mn oxidizes readily to form the following oxides: MnO 2, Mn 2 O 3, Mn 3 O 4, and MnO. Only MnO is stable at high temperatures K γ FeO x f Mn γ MnO = (Mn) N FeO [Mn] = K Using (N MnO ) = (N Mn ) (Mn) K * is an equilibrium quotient and it depends on composition of the slag Distribution of Mn between slag and metal can then be written as (Mn) [Mn] = K N FeO log K = 7940 T 3.17 Factors favouring oxidation of manganese: a high activity of FeO in slag (which means an oxidizing slag) a high K* value (which is obtained at low temperatures) 15/21 Variation of manganese, silicon and oxygen contents of iron in equilibrium with silica saturated FeO-MnO-SiO 2 slags 16/21

9 Effect of Temperature Oxidation with the oxygen dissolved in metal: [Mn] + [O] = (MnO); DG = 244, T by direct interaction with the oxygen in the gaseous phase: [Mn] + 1/2{O 2 } = (MnO); DG = 361, T or by reaction with iron oxides in slag: [Mn] + (FeO) = (MnO) + [Fe]; DG = 123, T all reactions of manganese oxidation occur with heat evolution manganese oxidation is favoured by a decrease in temperature 17/21 Reduction of MnO in slag transfers Mn from slag to metal and increases the concentration of Mn. Under favourable conditions, MgO of the slag can be reduced by Fe, C or Si. (FeO) + [Mn] = (MnO) + [Fe] ; a MnO a FeO x a Mn a MnO a FeO x Mn (as Mn forms an almost an ideal solution in iron) Conditions for the reduction of MnO in slag: Low activity of FeO in slag (which means a reducing slag) High temperature (which decreases K*) 18/21

10 Effect of slag composition a MnO a FeO x Mn Mn = 1 K x a MnO a FeO [Mn] in metal is determined by the ratio of the activities of MnO and FeO in the slag In acid slag a MnO decreases due to silicate slag formation and, thus, Mn oxidation proceeds more deeply in a Bessemer converter, for instance, manganese is oxidized practically to its traces in the metal Effect of Mn content of metal on (MnO)/(FeO) ratio 19/21 In basic slags containing MnO, the content of Mn in the metal is determined by the temperature and (a FeO ). Addition of iron ore to the bath causes an increase of (a FeO ), which results in a decrease of manganese content in the metal on the contrary, a vigorous course of the reaction (FeO) + [C] = {CO} + [Fe] causes a slight decrease of (a FeO ), and therefore, increases [Mn]. Addition of strong deoxidants decreases oxygen content of the slag, which decreases (a FeO ), and raises [Mn]. Addition of manganese ores (containing higher manganese oxides e.g., MnO 2 ) Releases [O] as it reduced to more stable (MnO), raises (a FeO ) and decreases [Mn]. Addition of metallic manganese (in the form of ferro-manganese) partially oxidized and the remaining part is left as [Mn] 20/21

11 Example 3.1 The activity coefficient of MnO in slag having a slag of basicity 1.8 is 1.6. The mole fraction of FeO and MnO in slag is 0.25 and 0.05 respectively. Determine the equilibrium content of Mn and O in steel at 1873 K. Given that, at 1873 K, [O] sat = 0.233%, and [Mn] + (FeO) = (MnO) + [Fe]; ΔG = T J/mol a FeO = (N FeO ) a MnO h Mn x a FeO γ MnO x N MnO Mn x a FeO ln G0 RT = ln γ MnO x N MnO Mn x a FeO ln[mn] = G0 RT + ln γ MnO x N MnO a ΔG = 6880 J/mol For (N FeO ) = 0.25, (a FeO ) = 0.36 [Mn] = 0.048%. [O] = [O] sat a FeO = = 0.084% 21/21