Phosphorous Problem. AkMB Rashid Professor, Department of MME BUET, Dhaka

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09 Phosphorous Problem AkMB Rashid Professor, Department of MME BUET, Dhaka Today s Topics Behaviour of phosphorous in metal and slag Oxidation of phosphorous

1 09 Phosphorous Problem AkMB Rashid Professor, Department of MME BUET, Dhaka Today s Topics Behaviour of phosphorous in metal and slag Oxidation of phosphorous Effect of temperature Effect of metal and slag composition Conditions of dephosphorisation Conditions for simultaneous removal of carbon and phosphorous Rephosphorisation

2 Introduction Phosphorus can be dissolved in iron in substantial quantities. Dissolution of phosphorus proceeds with evolution of a certain quantity of heat: 1/2{P 2 } = [P]; DG = - 140, T Phosphorus is usually regarded as a harmful impurity in steel 1. widens the pasty zone and narrows the g-region cause segregation 2. produces structural inhomogeneities impairs the plastic properties of steel Sources of P in iron: Pig iron Scrap and ferro-alloys Classification of iron (based on P level): Class A iron (P < 0.15%) Class B iron (P up to 0.20%) Class C iron (P up to 0.30%) 3/23 Behaviour of Phosphorous in Metal and Slag At the steelmaking temperatures, the stable state of phosphorus is gaseous {P 2 } and oxide of phosphorus, P 2 O 5 is also a gas. Phosphorus has a very high solubility in liquid iron as well. Considerable interactions between iron and phosphorus exists in liquid alloys leading to strong deviations from ideality. Dissolution of P from its gaseous state: 1/2{P} = [P] 1wt% ; DG = -122, T J/mol Influence of other alloying elements on activity of phosphorus in iron solutions (determined using Wagner s approximation) e P O = e P S = e P Si = /23

3 o 3CaO + P 2 O 5 = Ca 3 (PO 4 ) 2 ; DH 298 = -677,840 J/mol Large negative heats of formation reflect the strength of the attraction between the PO 4 3 anions and the cations of the basic oxides This indicates that the activity coefficient of P 2 O 5 in basic slags is extremely small, reaching as low as in lime-rich slags. Basic slags thus make an excellent sink for phosphorus in steelmaking. Although Henrian behaviour is generally assumed for phosphorus both in slag and metal, the relation between activities of P in metal and slag solution is not linear. 5/23 Oxidation of Phosphorus Phosphorus removal from hot metal is regarded as the most important refining reaction. It can be effectively carried out only in primary steelmaking operations to achieve phosphorus contents of around to wt.% in steel. With increasing demand for ultra-low phosphorus steel (wt.% P < 0.005) for some special grades (like steel used in automobiles), removal of phosphorus has become of even greater significance. 6/23

4 Phosphorus has atomic number 15 and it can give up all 5 electrons from its outermost shell to become P 5+ or accept 3 electrons to become P 3 to attain stable configuration. This means that phosphorus can be removed both under oxidizing as well as reducing conditions. But removal of phosphorus under reducing conditions is not practical since its removal is highly hazardous. Thus, P removal is practised mostly under oxidizing conditions. 7/23 Oxidation of Phosphorus 2[P] 1wt% + 5[O] 1wt% = (P 2 O 5 ); DG = -740, T J/mol K P = log K P = a P2 O 5 h P 2 h O 5 G o RT = T At T > 1382 K, ΔG becomes positive, which results in decomposition of P 2 O 5 to P and O. Thus, removal of P requires that a P2 O 5 must be reduced. O = a FeO O sat N P2 O 5 P 2 = K P a FeO γ P2 O [O] sat To have a higher index of phosphorisation, γ P2 O 5 must be low in the slag Index of phosphorisation 8/23

5 K P = a P2 O 5 h P 2 h O 5 log K P = G o RT = T Assuming [h P ] = [P] and [h O ] = [O], at 1600 C and at equilibrium K P = a P2 O 5 P 2 O 5 = 4.0x10 8 Assuming wt% P = 0.01 and wt% O = 0.08% in a steel at turndown, a P2 O 5 = 1.57x10 17 If wt% P 2 O 5 = 2.0 wt%, then N P2 O 5 = 0.01 and γ P2 O 5 = 1.31x10 15 Such a low value of γ P2 O 5 is possible only in a highly basic slag since P 2 O 5 is an acidic oxide. 9/23 The different basic oxides have different ability to lower γ P2 O 5. log γ P2 O 5 = N CaO + 15N MgO + 13N MnO + 12N FeO 2N SiO T CaO is the most powerful dephosphorizer of all basic oxides. Na 2 O and BaO are more powerful than CaO but they cannot be used in steelmaking because of their tendency to attack and corrode the refractory lining of the furnace. The approximate dephosphorization power for some cations in slag : Ca 2+ Mg 2+ Mn 2+ Fe 2+ 30,000 : 1000 : 3 : 1 Effect of metal oxide on activity coefficient of P 2 O 5

6 Effect of Temperature (a) Oxidation of P with oxygen of the gaseous phase: 4/5[P] + {O 2 } = 2/5(P 2 O 5 ); DG = -618, T J/mol; (b) Oxidation of P with oxygen dissolved in the metal: 4/5[P] + 2[O] = 2/5(P 2 O 5 ); DG = -384, T J/mol; (c) Oxidation of P with oxygen present in iron oxides of the slag: 4/5[P] + 2(FeO) = 2/5(P 2 O 5 ) + 2[Fe]; DG = -142, T J/mol oxidation of dissolved phosphorus [P] is associated with heat evolution In some processes based on the conversion of high-phosphorus pig iron (such as the basic Bessemer process), phosphorus is the main fuel a decrease in temperature favours oxidation of phosphorus 11/23 Effect of Metal Composition The process of phosphorus removal is impeded when the metal has a high concentration of easily oxidizable impurities (such as Si, Mn or C) These elements interact with iron oxides and thus diminish the oxygen content of the slag. 12/23

7 Effect of Slag Composition A higher oxygen content and a high activity of iron oxides of the slag promotes phosphorus oxidation To remove phosphorus from metal and retain it in slag, the activity of P 2 O 5 in the slag must be decreased. This can be achieved by forming a basic slag by adding lime (or limestone). 13/23 CaO, reacts with P 2 O 5 to form stable compounds of (4CaO.P 2 O 5 ) or (3CaO.P 2 O 5 ). 2[P] + 5(FeO) + 4(CaO) = (4CaO.P 2 O 5 ) + 5[Fe] 2[P] + 5(FeO) + 3(CaO) = (3CaO.P 2 O 5 ) + 5[Fe] The extent of dephosphorization is often characterized by the index (4CaO.P 2 O 5 )/[P] 2. Simpler ratios such as (P 2 O 5 )/[P] 2, (P 2 O 5 )/[P], or (P)/[P] are also used. Dephosphorization increases with increase in (FeO) content of slag and becomes maximum in between 15-16% FeO. For (FeO) > 15-16%, dephosphorization decreases. The above behaviour can be observed at all basicities of slag. Dependence of (P 2 O 5 )/[P] ratio on FeO content of slag at different CaO/SiO 2 ratios in laboratory experiments at 1685 C

8 Why this dual behaviour of FeO? FeO is the source of oxygen for oxidation of P. So, for a given basicity of slag, (FeO) increases the oxidizing power of the slag. Beyond the optimum value of FeO in slag, FeO replaces CaO and may either combine with CaO or with P 2 O 5. Thus, higher is the wt% FeO, lower will be the wt% CaO, and this will adversely affect the dephosphorization ability of the slag. 15/23 The maximum dephosphorization ratio increases with the increase in the slag basicity. Higher basicity requires higher amount of CaO dissolved in slag. Any undissolved CaO will not be effective for dephosphorization. As the optimum value of FeO is more or less independent of slag basicity, control of FeO in slag is important for efficient dephosphorization. Dependence of (P 2 O 5 )/[P] ratio on FeO content of slag at different CaO/SiO 2 ratios in laboratory experiments at 1685 C

9 Conditions for Dephosphorization 2[P] + 5(FeO) + 3(CaO) = (3CaO.P 2 O 5 ) K = a 3CaO.P2 O 5 [h P ] 2 a FeO 5 a CaO 3 a 3CaO.P2 O 5 [h P ] 2 P P = K a FeO 5 a CaO 3 The conditions for an efficient dephosphorization are: a CaO in slag should be high. This means slag should have free dissolved lime. A highly basic, lime-rich slag is required. The slag should contain a lowest possible amount of undissolved free lime. a FeO in slag should be high i.e., slag should be oxidizing. For efficient dephosphorization the FeO content of slag should be in between 15 to 16%. Low temperature favours high K 17/23 Principal conditions essential for dephosphorization : 1. formation of an oxidizing furnace atmosphere and oxidizing slag with a high activity of iron oxides in it (the slag must have a (FeO) 15 wt%); 2. high basicity of the slag (high in CaO and low in SiO 2 ) and high activity of CaO in it; 3. quick formation of ferruginous-limy slag; 4. a relatively low temperature (especially at moderate and high concentrations of carbon); 5. low activity (concentration) of phosphorus in the slag. 18/23

10 In practice, these conditions are provided by the following techniques: (a) addition of iron oxides to the melt (in the form of iron ore or scale); (b) addition of CaO (lumpy or powdered lime or limestone); (c) blowing the bath with oxygen (or air); (d) removing most phosphorus at an early stage of the heat when the temperature of the metal is still not high; (e) forming an active free-running slag as early as possible, which can be achieved by bath stirring, adding certain fluidizers to the slag, etc. Sometimes, the fluid final slag from the previous heat, containing high CaO and FeO and little phosphorus, is left in the furnace; (f) slag renewal (i.e. by slagging-off and the formation of new slag free from P). 19/23 Conditions for Simultaneous Removal of C and P C + O = CO ; K C = p CO C O = 1305 T P + 5 O = P 2 O 5 ; K P = N P 2 O 5 γ P2 O 5 [P] 2 [O] 5 = T Henrian activity is assumed equal to (wt %) Both reactions require oxidising conditions, but the P reaction requires a slag, which is basic in nature in addition to oxygen. Thus, if C and P are to be removed simultaneously, an important requirement is the availability of slag which acts as a sink for (P 2 O 5 ). Thermodynamically, a slag is required in which activity coefficient of P 2 O 5 is very low. The question is how low activity of P 2 O 5 should be? 20/23

11 K C = p CO C O = 1305 T K P = N P 2 O 5 γ P2 O 5 [P] 2 [O] 5 = T Consider molten metal contains 2% C and 0.15% P and the mole fraction of P 2 O 5 in slag is 0.1. K P [P] 2 γ P2 O 5 = 5 K C [C] 5 N P2 O 5 (using p CO = 1 atm) T (K) γ P2 O x x x Both decarburisation and dephosphorization are possible simultaneously in presence of slag in which γ P2 O 5 has extremely low value. At low temperature, γ P2 O 5 in slag is lower. Thus, low temperature is favourable. 21/23 Rephosphorization At the end of a heat if the metal temperature is too high, and deoxidants are added to the metal (part of them can pass to slag (but not to metal) and lower the activity of oxygen in metal and that of iron oxides in slag) phosphorus that has been oxidized and passed to slag can be reduced again and returned to the metal (rephosphorization). 22/23

12 In the final melt, slag basicity gradually decreases as slag interacts with the ladle lining (which consists of SiO 2 and Al 2 O 3 ) SiO 2 content increases due to the addition of FeSi as deoxidant All these factors (high temperature and the decrease in a (FeO) and CaO/SiO 2 ) form favourable conditions for the reverse passage of phosphorus to the metal. As a result, the last portions of steel poured from the ladle during teeming may turn out to contain noticeably more phosphorus than the first ones. Preventive measures against rephosphorisation Add CaO at the later stage to increase slag basicity Tap steel containing much lower P level than the required value 23/23

Oxidation of Iron, Silicon and Manganese

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