Slag formation in top blown convertersSlag formation in the converter is a a very important process issue. Steel

refining is slag related and most important technical and economical features are seriously affected by it (P removal, slag volume, hot metal/scarp ratio, refractory wear, iron and metallic yield etc…). To have an acceptable understanding of the converter process and slag formation few things have to be made clear on separate bases. It is important to understand the main features and process related of oxygen jet, its consequences and our way of controlling it. It also important to understand the thermodynamics of oxidation processes and those related to potential liquid phase reactions among formed oxides, activities in the liquid slag, kinetic aspects of lime and magnesia dissolution as well as the saturation related aspects. To have a shorter story independent chapters have been prepared for each of the above mentioned issues.

1. Supersonic jet decay.This chapter is mainly dedicated to generate a sustainable oxygen jet related

mathematics infrastructure for the jet impact conditions on slag formation.1.1 Calculation of Laval nozzle

Please refer to the sketch bellow

The presumption that the mass is preserved in any section of the pipe allow the follow- ing equation:

W=u*A=constant 1where is the gas density, u is the gas velocity and A is the cross section of the

tube

This means that its differential equals zero ddu/u+dA/A=0 2

For un ideal gas, considering also an adiabatic process the following relations are truep=R*T 3p/=k (constant) 4

Where R is the gas constant, T is the absolute gas temperature and Cp/Cv. I have to remind that for oxygen , and that following general thermodynamic relation is also valid :

Cp-Cv =R 5If the differential of equation 4 is considered, then after calculation the following relation can be written:

Po,

Pe,

De

Dt

Pt,

dp/dP=*R*T 6Speed sound in the gas is pressure-density related. The equation describing the sound speed velocity in gas is:

s= (p/0.5 = (*R*T)0.5 7Bernoulli law, for a frictionless fluid in one direction is:

u*du+1/*dp=0 8By substituting equation 6 in 8 following form is obtained:

u*du+s2*(d Substituting the d value from 9 into 2 the following form is obtained:

(1-u2/s2)*du/u+dA/A=0 10Mach number is defined by ratio of actual speed to sound speed in gas

Ma=u/s 11

The final form which is of real interest is:

dA/A=(Ma2 –1)*du/u 12This equation shows that if Ma<1, than dA/A is negative if a positive figure is considered for du/u ( an increase in gas velocity). However, when Ma>1, the exit area of the nozzle should be larger if one is willing to get an increase in exit speed of gas. This is the base of selecting a Laval type nozzle for the oxygen lance of the converter (higher exit speed, with higher kinetic energy at the impact area and improved process related consequences)Calculations based on energy conservation of the gas in various conditions (as the ones presented by the above sketch allow the following equations to be developed:

Po(Pe

(y-1)/y *(1+(ae2/2) 13

Toe *(1+(ae2/2) 14

o(e

(y-1) *(1+(ae2/2) 15

If we consider point “e” as being in fact the throat (“t”) similar equations with 13-15 can be written, in this case the symbol “e” being substituted with “t”. This means, that we can calculate the throat conditions for oxygen as follows (considering that at throat the Ma=1:

Pt= 0.5283*P0 16

Tt= 0.823*T0 17t= 0.6339*0 18

The exit speed of the gas can be calculated either using the previous relation (considering Mae as the independent variable or using equation 19 (derived from above equation 13)

Mae2 =2/(Po/Pe)

The exit area of the throat At and the exit area Ae are realated by the following equation:

At/Ae= [(Pe/P0)2/Pe/P0)( 20

The maximum flow through the throat can be calculated with equation 21:

m= At*[P0**(2/(kg/sec) 21

Example of calculation

Design a nozzle for 400 Nm3/min which operates at Mach 2. The exit pressure should slightly positive (1.05 bar).

From equation 19, by substituting the known data (including a P0 =8.28 bar is obtained. The mass flow equivalent to 400 Nm3 /min of oxygen is 9.524 kg/sec. Oxygen density at P0 can be calculated from the equation:

0=P0*MO2/Ru/T 22

(where MO2 is the molecular mass of oxygen, Ru is the gas constant-8314 KJ/Kmol/K and T is the absolute gas temperature -K).

With these values the throat area is 0.00461 m2 and the At/Ae calculated is 0.5925 which allows the calculation of the exit area of the Laval Nozzle.

1.2 Calculation of jet decay.

The supersonic jet speed drops after leaving the Laval nozzle due to friction with surrounding atmosphere , jet free expansion as well as gas entrainment from surrounding environment.

The distance from the nozzle at which the oxygen speed drops to 1 Mach can be calculated with the following equation:

X= De/(2*c0.5) 23

The “c” value, named nozzle constant can be calculated using the equation 24

c= 0.25* (de/x)2*(Pde/Pdx) 24

(where x is the distance from nozzle tip, Pde is the dynamic pressure at exit and Pdx is the dynamic pressure at distance x from nozzle tip ). Obviously, jet expansion studies are required to define properly the “c” value. Such experiments carried out at Hoogovens showed that for Mach2 nozzles, a good approximation of the relation between the variable is offered by following relation-25

Pdx/P0=230*(x/dt)-2.4 25

This is generally leading to “c” values ranging from 3.89*10^-3 to 8*10^-3.

To appreciate better the jet decay condition and its physical relation with actual operating conditions figure 1 is considered useful. As known, the oxygen speed in

the axis of the jet is higher then at its periphery (due to friction with the environment). If the speed ratio at the periphery is reported to axial speed along the jet travel toward steel surface, then a jet expansion picture becomes clear. It has been found more suitable however to plot the ratio of diameter of jet having at periphery half of the gas speed in the axis to throat diameter against the lace height measured in throat diameters. Figure 1 offer such a plot (data valid for nozzle exit speed of Mach 2)

Figure 1

1.3 Evaluation of jet entrainment ratio

The mass stream at the exit of the nozzle consist of pure oxygen. Free jet expansion is taking place outside the nozzle, the jet decay is taking place also and the entrapment of surrounding gases is taking place. The ratio of mass stream at nozzle exit to mass stream at distance x from the nozzle denoted as Rg is an important jet parameter. For a Mach nozzle of about 2 (2.1 in reality) the equation used for Rg calculation is:

Rg=0.132*[(Px/Pe)*(T/Tx)*(Pe/P0)(Pe/P0)]0.5*(x/dt)1.2 26

Gas recirculation inside the converter is directly proportional with Rg.

1.4 Specific reaction surface of metal droplets.

Metal droplets are generated as a result of jet impact. Formed droplets size distribution obeys the Rosin-Rammer-Sperling distribution function (RRS):

RRS=100*exp[-(d/f)n] % 27

where d is droplet diameter and f is a measure of fineness of particles its- reciprocal describes the drop break-up, while n is a measure of homogeneity having a typical value of 1.26.

The droplets size distribution , RF (in fractions) over a wide range of blowing conditions is given by relation 28:

Dependence of logarithm of d/dt on lance height measured in nozzle throat units.

y = 1.1286x - 1.0987R2 = 0.9998

0

0.2

0.4

0.6

0.8

1

1.2

1 1.3 1.6 1.9log (h/dt)

log(

d/dt

)

Log(RF)= -3*(d/dl)1.26 28

(where dl is the droplet diameter corresponding to RRS=0.1%. For non coalescent jet nozzles dl

can be calculated (in mm) with relation 29:

dl= 5.513*10-3*{106*(dt/x2)*Pe*[1.27*(P0/Pe)-1]*cos

where dt is the throat diameter in m, x is the height of the nozzle above metallic bath in m, is the angle of divergent part of the nozzle, while the pressure parameters expressed in atm have the already known meaning). Generally speaking, the droplets diameters are ranging from about 0.5mm up to 5-6 mm. Based on equation 28, and 29 the fraction of droplets having the diameter d is described by equation 30:

f(d)=8.7308*RF*d0.26/dl1.26 30

Cumulating the area of droplets of various diameters of each kg of liquid steel transformed into droplets we can calculate the SRS-specific reaction surface. The total reaction surface will depend however on the amount of metallic droplets ejected per minute –Mg - due to jet impingement.

1.5 Mg calculation.

The specific blowing impact is defined by equation 31:

Ix=104*Atot*(0.155*Foxy-9.81)/x2 31(where Atot stands for the cumulated exit area of the lance nozzles-m2, x is the lance height and Foxy is the oxygen flow rate (m3/min)Mg value can be then calculated based on following linear correlation 32:

Mg=-223.067+76.632*log(Ix) (tons/min) 32

The numeric values have to be tuned for each specific converter and lance. Therefor a more convenient relation is based on Weber number-33

We=2*Pd,x/(e*g*e)0.5 33(where e is liquid steel surface tension-1.6 N/m, e is the oxygen density close to the impact area of the stream-1.38 kg/m3 and g is the gravity-9.81 m/sec2). Pd,x is the dynamic pressure at impact. It can be calculated either from the kinetic energy of the oxygen jet or from the amount of depression generated at impact –34:

Pd,x= (e*ux2)/2=(l*g*x) 34

(where ux is the gas speed at impact, l is the liquid steel density-7000 kg/m3 and x is the depression generated by oxygen jet at impact.

Mg dependence on Weber number is illustrated by figure 2.

Figure 2.

It is clear that when the We is below 18 ( equivalent with 50 m/sec impact speed) the amount of entrained droplets is zero and therefore lance height has to be selected to have an acceptably good droplets generation.The calculation of the depression depth is made at Hoogovens based on following equations (35,36):

ln(Pd/h)=-2.3654*ln(h/dt)+5.4402 35h=Pd/(l*g) 36

with the data properly expressed. The data have a better fit for a larger size converter than for small ones. Another set of equations which seem to offer a better approach for smaller size converters from a different author (Lange K.W) is presented by equations 37 and 38.

Mh= 0.7854*dt2*Pe*[1.27*(P0/Pe)-1]*cosl*g*h3) 37

h/h= 4.469*Mh0.66 38.

In general terms, higher is the We number (lower lance position for same oxygen supply pressure) higher the Mg value will be. The exposed reaction surface is higher and therefore more oxygen from the same available amount will be consumed in the reaction on droplets surface, availability for other oxidation purposes, post-combustion of CO for example diminishing.

2. Oxidation processes

The oxidation process of iron accompanying chemical elements is taking place during oxygen blowing. The process is having, as consequence the oxides generation which is accompanied by corresponding thermal effects. All the oxidation reactions ( oxide formation reactions) are generating heat ( heat of formation). Liberated heat will change the actual thermal status of the system driving it toward higher temperature. The actual temperature of the system is averaging its heat balance for selected unit of time,

Mg dependence (kg.m3) on Weber number

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140 160

Weber number

Mg

/FO

2-

kg/m

3

and doesn’t represent the reaction site temperature. While the reaction site identification, its local temperature and reactants supply to this site are typically micro-system related parameters, the liquid steel evolution in the converter is a macro-system parameter. As can be understood the oxidation conditions (macro and micro) are changing permanently. The reactions free energy are usually considered as thermodynamic approach for the order of various oxidation reactions. Hot metal refining imposes various oxidation reactions:

[Si]+ {O2} (SiO2) 12[Mn]+{O2}2(MnO) 20.8[P]+ {O2}0.4(P2O5) 3[Ti]+{O2}(TiO2) 42[C]+ {O2}2{CO} 52[Fe]+ {O2}2[FeO] 62{CO}+{O2}2{CO2} 7{CO2}+[C]2{CO} 8*

The heat of reaction , for the reaction equations as written above should consider the dissolution heat, the required heat to lift the temperature of reactants and reaction products to the actual reaction temperature as well as the dissolution heat of various oxides into the slag. Post-combustion reaction (7) as well as the C oxidation reaction are playing a certain role into actual heat liberated by the process. While reaction 8 (which is endothermic) can be neglected, the CO post-combustion is important bothe for the thermal balance and actual oxygen requirements. Its share changes with the actual lance position, the Si content of metal and CO/CO2 temperature dependent stability. Table 1 id offering the net reaction heat value calculated for 1600oC for the above reactions.

Table 1

Element Value (kcal/kg) CommentsSi 6600

Mn 1320P 6250C 2390 Post-combustion level =0.12Fe 750 Fe2+/Fe3+=0.23

The oxidation of Si, Mn and C are taking part practical simultaneously. Mass law however, despite its heat of formation which is low, imposes a very significant share of oxygen to be consumed on iron oxidation (reaction 6) which can be converted in an oxygen carrier to the reaction site where the following reactions can also take place

x[ M]+y(FeO) (MxOy) 9

Evolution curves for actual bath chemistry during blowing and slag chemistry during blowing can found in many publications (no need to reproduce them). They show that Si is practically completely oxidized during the first quarter of the blowing time, Mn is strongly oxidized during the same period but, during the main C removal period (30% to 80% of the blowing time) a slow Mn recovery is taking place, while in the last part of the blow its oxidation accompanies the C removal. Similar behavior with the Mn has the P content, but in this case P recovery is less

significant and the reasons of its behavior are different that the one driving the Mn oxidation process.

Iron oxidation is very important, and its behavior during blowing is similar to some extent with the Mn behavior. High FeO contents are found in the slag during silicon oxidation process. The iron oxides content in the slag drops until the critical carbon is reached and is increased again toward the end of blow. The reasons for such an FeO behavior during the blow are different for each moment of the blow and they deserve attention as slag formation is seriously related to FeO evolution. As a result, both bath chemistry evolution calculation and slag chemistry evolution calculations should be related to specific conditions along the process route followed.

3. Slag chemistry Liquid slag is formed from the start of oxygen blowing. The oxidation

process previously mentioned is generating the oxides (FeO,MnO,SiO2) which are facing the process environment completed by the lime, dolomite lime and refractory presence. A permanent lime, dolomitic lime and partly refractory dissolution process is taking place during the blow. The ideal aim is to end up the process with a completely liquid slag, which performed its duty ( for which the desired chemistry has to be achieved) required by some other metallurgical processes (P and S removal for example). Achievement of these targets should be made possible under a minimum dissolution of refractories.

Few statements concerning the slag chemistry and process realities have to be made:

Slag formation process in most of cases doesn’t end up with a completely liquid and homogeneous slag

Undisolved lime or dolomite, precipitated Calcium disilicate or magnesio-wustite are part of slag realities (desired obviously in some specific cases) . Slag have to be treated as heterogeneous system

Analysis of slag chemistry should be carefully considered as they represent in fact the total content of one component with little chance of separating the part of it in solution and the other one situated in precipitated stage

Slag properties (like viscosity, foaming index etc.. for example) are related to the liquid phase properties and are affected by precipitates presence

Chemical and mineralogical analysis of the slag may allow the separation of liquid phase and precipitates chemistries and is required for understanding of slag formation and slag behavior.

Optimization of slag chemistry is not an easy task. Too small lime and dolomite lime additions may end up with unnecessary high refractory consumption and low refining capability while too large additions may unnecessary increase the heat consumption for slag formation, “dead “slag” volume –not fully utilized refining capability, not to mention the yield and mechanical wear of refractories.

3.1 Slag formation

As mentioned the oxides originally generated by initiation of oxygen injection are forming a liquid slag of ( FeO+MnO)-SiO2 type. Its low volume (which can increased by carried over slag high in silica from the charging ladle) is low compared with the amount of heat generated by chemical reactions and its presence at the reaction site end up with liquid slag temperature far higher than the actual bath temperature; its behavior should be considered in connection with it. Overheating in excess of 300 oK of initially formed slag have been reported. The level of overheating drops during the process due to various reasons ( drop of post-combustion, foaming and increased contact area with converter lining, improved heat transfer as the result of an increased iron conversion value (Mg), transfer of reaction site from originally impact area to impact area and steel droplets surface in the liquid slag, limitation of chemical reactions generating heat to C removal and light iron oxidation, expanding heat consumption heating and dissolution of fluxes etc…) toward the end and the typical slag overheating at the blow end is around 50 oK.

3.11 Slag liquidus temperature is an important issue. At the process start, melting point is governed by the melting of above mentioned system (for FeO-SiO2 system the fayalitte has melting point of 1208 oC while the lowest melting point of the binary system MnO-SiO2 is slightly higher-1252 oC) which should be expected to be situated below 1200 oC. The slag overheating drops dramatically the viscosity and surface tension of the this slag. Its lime pores penetration capability increase and the dissolution of lime and dolomite lime commences. Higher the available liquid slag volume is higher the dissolution rate of the lime is (expressed in kg of lime dissolved out of total lime added to that specific moment). Of course, delayed lime addition may end up in serious refractory wear. For this reason some plants are adding lime before charging the scrap (it may partly lower the charging side erosion softening the scarp mechanical wear when charged) but its extra heat accumulated to the blow start should not be neglected. Too large volume of lime added under these circumstances is risky also as the generated liquid slag to available lime for dissolution is too low and a dry slag formation may be the result, not to mention the difficulties of process ignition.

The lime and dolomite dissolution reactions are described by following equations:

(2FeO.SiO2 )+4<CaO> 2(CaO.FeO)+(2CaO.SiO2) 10(2FeO.SiO2 )+ 2<CaO.MgO>)(2CaO.SiO2)+2(MgO.FeO) 11

The (CaO.FeO) –iron reach calciowustite is liquid and it is helping the enrichment of originally formed slag in lime ( dissolved). 2CaO.SiO2 calcium disilicate is a solid compound (melting temperature in excess of 2010 oC) and it is formed on the reaction surface having a blocking effect on further lime dissolution. For the soft burnt lime, it is reported to be formed with some delay after the calciowustite formation, allowing the evacuation of liquid reaction product before blocking the reaction surface. Hard burnt lime however forms calcium disilicate on reaction initiation spots followed by calciowustite formation which ends up with a sintering conditions for the reactions products. As a result, the net transfer of calciowustite to liquid slag pool is much less and CaO content of the forming liquid slag increases much slowly . The magnesiowustite has a high melting point and is obviously solid at this stage of process. As a result, the chances of having a significant MgO transfer to the liquid slag pool is less than the one

for CaO from reaction 10 which explains partly the lower dissolution rate of dolomite lime than that of soft burnt lime.

The result of slag enrichment in lime and MgO (at a lower pace) is the permanent rise of slag liquidus temperature. Figure 3 shows schematically the expected liquidus temperature evolution for lime addition and dolomite lime addition. As is known the MgO is lowering the liquidus temperature to certain extent and to limited contents.

Figure 3.

The above calculated liquidus temperature values are obtained from a Kapoor-Frohberg type computer model which is also used to calculate the activity of various slag components. Obviously, liquid slag chemistry are difficult to be obtained (and not total slag chemistry) and this is explaining the large variations achieved in reality. The graph is underlining also the fact that in the first third of the blow the slag formation is governed by different rules than during the main decarburisation period (up to 75 % of the blown oxygen amount) or finishing stage of the heat. Liquid slag volume is also evolving on a curve similar with the above curves, indicating an accelerated increase toward the blow end. It is thus clear that until the end of Si oxidation (meaning large amounts of oxygen utilized for Si, Mn and much less available for C removal) the liquid slag chemistry is controlled by above mechanism, with a significant amount of Calciumdidilicate present in the liquid as a precipitate. Magesiowustite which is solid dissolves also to small extent. As the quantity of SiO2 is dependent on the Si content of hot metal and MnO is also depending on Mn content of hot metal, their concentration into the formed liquid slag (which includes also the dissolved lime according the above described process) can vary with the available hot metal chemistry. Slag formation path will therefore be different. Primary generated liquid slag volume, by the oxidation process of Si and Mn will also be affected.

Figure 4 is a sketch of actual slag formation path. On the figure, the Calcium disilicate precipitation curve has been overimposed to allow some comments on slag path formation. Added lime concentration correspond to point A while the calculated end lime concentration of the final slag should be inside or close the precipitation boundary of calcium disilicate area. Theoretically we are aiming to achieve same end slag chemistry, but the real liquid slag chemistry may depend on some other factors (like achieved end point C, addition of ore for temperature control reasons, variable amount of

Slag liquidus temperature changes during combined blowing

1200

1300

1400

1500

1600

0 12.5 25 37.5 50 62.5 75 87.5 100

Percentage of oxygen blown, %

Liq

uid

us

tem

pe

ratu

re,

C

Lime

dolomite

carried over slag, left over slag in the converter, impact of slag deposit from N splashing or gunning, etc…)

Figure 4As can be observed, high Si content is leading to a higher starting SiO2

content in the slag. The expected path should follow the dashed line linking point A with the starting point, ending somewhere on the line depending on lime amount. The solid line is showing a different slag path, broken into three sections equivalent for each one third of blown oxygen. Data are based on total slag analysis. While the liquid slag chemistry will be situated at the boundary of calcium disilicate area, the lime to the balance of added amount will be precipitated as calcium disilicate with a chemistry equivalent to a point located inside precipitation area. The calcium disilicate precipitation line is achieved rather late in the process in case of high Si content while at lower Si content it may be achieved earlier. The amount of calcium disilicate precipitated and liquid slag can be calculated using a typical metallurgical calculation. All the selected examples are underlining the fact, that an accelerated lime dissolution (including an enlargement of the liquid slag volume) will take place when the calcium disilicate envelope of exposed lime pores collapses and new lime surfaces are exposed to slag penetration again. The rate of lime dissolution in the absence of dolomite lime addition is considered to be higher than when dolomite is being added (despite some contrary statements found in the published literature). The rate of dolomite dissolution is obviously smaller than the lime dissolution, dissolution speed practically achieved for dolomite lime being half of that achieved with metallurgical lime under similar process conditions.

The presence of other oxides in the lime dissolution process deserve some comments also.

3.1.2 MnO presence.As stated previously for a constant oxidation potential (generally measured as

FeO content ) the MnO in the slag is Mn activity in the bath and temperature related. Higher Mn content in the hot metal is leading to higher MnO content in the initially formed slag. There are few important consequences of its presence:

The available liquid slag volume at the beginning of slag formation increases, avoiding the formation of dry slag in the lime dissolution process in its early stages. It is good to mention, that its presence to

CaO

SiO2

FeO

0.8% Si

0.6% SI

0.4 % Si

C2S

C3S

A

reasonable levels (3.8-4.5 %) is making the slag formation more forgiving (especially for lance position and oxygen flow relation) reducing significantly the lance head skulling and diminishing the clogging of the converter cone zone. It imposes a Mn content in the hot metal in excess of 0.5%. This condition is more important for the shops operating with a rather high P content in hot metal (in excess of 0.1%) which are forced to operate on high slag volume if no hot metal dephosphorisation in the torpedo ladle is available. However a much higher Mn content in nthe hot metal is not desirable because its effect on metallic yield becomes significant and Mn cost in the hot metal can’t be covered by its positive impact in the steelmaking operation.

The liquid slag fluidity is increased for two separate reasons-higher MnO means lower melting point, the second important reasons being related with the basic behavior of MnO which interrupts the silicate chains, leading thus at macrocosmos level to smaller liquid pfase structural units and higher mobility (lower activation energy also)

MnO-SiO2 compound formed due high SiO2 content is more stable than the FeO-SiO2 silicate bound compound. These increases the FeO activity in the liquid slag allowing a higher calcium ferrite formation during the dissolution process

Due to its basic behavior, its presence increases the S distribution ratio ( (S)/[S] ratio) and has no detectable effect on P distribution ratio. As the amount of formed calcium disilicate is expected to drop (and keeping in mind that is has rather high P2O5 solubility-close to 2.7 %) the expectation would be to see lower P distribution ratio. The practice so far could not provide sustainable results.

Its presence diminishes the solubility limit for MgO, allowing the achievement of good refractory protection with a lower amount of dolomite lime and reducing by this, at least partly the slag formation problems (an increase of MnO from 2.5 % to 4% decreases the MgO solubility limit with about 0.5 % for the same FeO content in slag and slag temperature).

3.1.3 Alumina presence.Alumina presence in the slag is related to three main sources: Blast furnace slag charged into the furnace with the hot metal (incomplete

slag removal in the charging ladle). From the charged fluxes (small percentage presence in the lime and

dolomite) Oxidation of aluminum from scrapAlumina behaves like acid oxide in the slag (despite its amphoter character). As a result the effective free lime amount diminishes to some extent affecting P distribution ratio (decreases). Its impact on liquidus temperature is positive but due to the limited content its real impact can be neglected.According to some studies related to lime dissolution speed, its presence in lime diminishes lime reactivity and lime dissolution speed. Limitations of Alumina below 0.8% or even more severe in the lime for converter process are found in many plants specifications for lime chemistry.

Its impact on Magnesia solubility is negligible for the concentrations limits of the converter slag (1-3%) but it becomes significant for secondary metallurgy slags.3.1.4 TiO2 presence. Ilmenite is a mineral rich in TiO2. Its presence in the liquid slag, at low concentrations seems to be positive from the liquidus temperature point of view and it has been used many years ago as fluorite substitute due to its capability to help the lime dissolution. In fact, TiO2 is acting as corrosive agent of grain boundaries of precipitated calcium disilicate . The collapse of precipitated calcium disilicate network occurs faster in TiO2 presence, accelerating thus the lime dissolution.In our days, its presence in the slag is related to the Ti content of hot metal (generally close to 0.1 %) leading to a TiO2 content in the slag of 1 to 1.5%.Its presence doesn’t affect P distribution ratio while S distribution seem to be very little negatively affected. MgO solubility in slag is not affected at this concentration level.

3.1.5 Carbon removal impact on slag formation.C removal process is the main aim of refining process in the converter. The ignition of process is followed by initiation of C removal process. As mentioned previously, Si and Mn oxidation is simultaneously initiated. Iron oxide is generated also. The available oxygen share for C removal is this stage of the process is controlled by the other parallel oxidation reactions. The heterogeneous oxidation reactions speed are controlled by the reactants transport to the reaction site and the specific reaction surface. The specific reaction surface is depending on actual Iron conversion value (Mg) and the droplets size distribution. High iron conversion value at the process start is risky as the amount of available liquid slag is too small, droplets presence in the slag may decrease significantly the slag fluidity diminishing its availability for lime dissolution and ending up in a very dry slag. Therefor Iron conversion level at the process is low and this can be achieved by lance position above the metallic bath. During the process start lance position is highest. It makes no sense to drop the oxygen flow rate during this period as the oxidation reactions speed are oxygen supply related. In general lance position and oxygen flow are connected by a adjustment relation like:

H= k(FO2)0.5 12Where k is constant which is considering the jet decay and lance construction (throat diameter, exit diameter and nozzle angle, inclination of the nozzle axis in relation the longitudinal vertical axis of the lance, number of nozzles, selected Mach number )As the Si oxidation and Mn oxidation speed drops ( strongly favored by the liquid pool relatively low temperature) mainly due to diminished Si content or because approaching equilibrium ( in Mn case) the C available for C removal reaction increases. Simultaneously, the droplets surface is carrying more FeO in a free condition ( lower SiO2 content) able to react with the C ( in the surface of droplets neighborhood ) sometimes ending up in breaking the droplet into smaller ones. This is reducing the FeO content transferred to the slag and FeO temporary balance in the slag becomes poor at its entry. The iron conversion can now be increased. This means that the lance has to be

lowered and the amount of droplets for each m3 of oxygen entering the slag increases. The available oxygen for CO post-combustion drops. It is generally agreed that during this part of the process the C removal speed is increasing almost linearly with the blowing time until it reaches the maximum C removal speed which is entirely oxygen supply related. The increased level of Iron conversion (Mg) increases also the oxygen transfer toward the liquid pool ( as FeO which was not washed away by the slag or as dissolved oxygen[O]. This means that C removal reaction sites are now numerically increased reaction taking place at the droplet bath contact or deeply inside the bath if CO germination conditions exists. However the oxygen is not changed. Increased amount of CO from C removal is foaming the slag, as the result the lance is now working submerged and the entrainment of oxygen jets becomes more serious. Slag and gases are entrained into the oxygen jets increasing the amount of available stirring energy and leading to massive droplets oxidation including collapse of calcium disilicate network existing on lime particles surface and speeding up the lime dissolution. As the slag foaming is controlled by its viscosity, strongly affected by the precipitates presence its expansion may be sometimes higher than the available free converter volume and slag splashes are the consequences. As can be easily to understand, splashing is a lime and dolomite lime dissolution consequence and should be adjusted by controlling the lime dissolution process.When the C content of metallic bath drops bellow critical C, the C removal reaction speed is controlled by the C supply to the reaction site which becomes the limiting speed controlling factor. The turbulent diffusion to reaction site due to CO evolution drops and additional stirring energy is required to have a high share of blown oxygen consumed in the C removal reactions diminishing thus the Iron oxidation. To this a different level of C-O equilibrium is also contributing. This can be achieved by lowering the lance but, in case of combined blowing, lance position should be corrected for the bath level increase due to argon stirring.It is thus obvious that C removal is promoting due to its consequences (slag foaming, additional stirring energy, oxygen jet entrainment, temperature evolution ) slag formation but is also helps the C removal due to foaming properties and transferring part of the C removal reaction from the jet impact area on droplets surface into the slag.

3.1.6 Phosphorous removal.

As mentioned previously, P is oxidized simultaneously with Fe, Mn and Si from the start of blowing. The P2O5 formed as a result of oxidation process is an acid oxide. Free lime is required to reduce permanently its activity in the slag. On the other hand, its solubility in Calcium silicate allows the retention of relatively high amount of P in the slag from early slag formation stages helping thus the P removal from the liquid steel. The presence of various oxides in slag are influencing the P distribution ratio ( (P)/[P] ). Among the factors which a positively impacting the P distribution ratio should be mentioned:: High basicity level (in reality a high CaO activity ) of the slag. It has to be

mentioned that a basicity in excess of 3.5 doesn’t help the increase of P distribution ratio due to its negative impact on FeO activity (which drops

with further increase of slag basicity). It should be mentioned that for a fixed Si content of hot metal, a higher basicity means a higher slag volume and no matter that the P distribution ratio remains unchanged, the end P content of the liquid steel obviously drops.

High FeO content in the slag is strongly impacting the P distribution ratio. It has been practically established that an FeO content above 23 % doesn’t show a clear impact on the P distribution ratio ( data scatter becomes obvious). Under these circumstances, a higher FeO content helps only to increase the slag volume, controlling the end P level in the liquid steel via slag volume

Low temperature is positively influencing the P removal due to thermodynamic considerations.

There are some other factors which are negatively influencing the P distribution level achieved like :

Too low CxO product in the combined blowing process (especially for higher than 0.1 % P content in hot metal) due to negative impact on FeO content in the liquid slag especially toward the process end. It should be mentioned that this statement should be correctly interpreted. In combined blowing the P distribution ratio achieved a re slightly higher than when combined blowing is out of operation due to additional stirring energy which is driving the P removal closer to equilibrium. However the FeO impact remains clear.

Too high MgO content in the slag due to the precipitates presence in the liquid slag which increases the slag viscosity and is negatively affecting the mass transfer, not allowing the achievement of elevated P distribution ratio. These aspect is most obvious in the heats made after converter lining gunning which are seriously affected .