MASTER'S THESIS. Improvement of the Desulphurisation Process by Slag Composition Control in the Ladle Furnace

Size: px
Start display at page:

Download "MASTER'S THESIS. Improvement of the Desulphurisation Process by Slag Composition Control in the Ladle Furnace"

Transcription

1 MASTER'S THESIS 2009:065 Improvement of the Desulphurisation Process by Slag Composition Control in the Ladle Furnace Stephen Famurewa Mayowa Luleå University of Technology Master Thesis, Continuation Courses Minerals and Metallurgical Engineering Department of Chemical Engineering and Geosciences Division of Process Metallurgy 2009:065 - ISSN: ISRN: LTU-PB-EX--09/065--SE

2 IMPROVEMENT OF THE DESULPHURISATION PROCESS BY SLAG COMPOSITION CONTROL IN THE LADLE FURNACE Famurewa Mayowa Stephen Supervisors Professor Bo Björkman(LTU) Sven-Olof Ericsson(OVAKO) Luleå University of Technology Master Thesis in Minerals and Metallurgical Engineering Department of Chemical Engineering and Geosciences Division of Process Metallurgy

3 ABSTRACT The cleanliness of steel with respect to non-metallic inclusions and the precise alloy compositions in the steel products have always been of great concern in steel making technology. The development of steel making process to meet the compositional requirements for specific mechanical properties such as ductility, toughness, fatigue and machinability requires dynamic and continuous investigations. The refining of molten steel in the ladle furnace to meet the required compositional range requires the optimisation of the process parameters. For sulphur removal control, parameters such as argon gas flow rate through the porous plugs, inductive stirring effect, vacuum pressure of the tank degasser, amount & composition of the top slag should be optimised. In this thesis project an investigation was carried out on the factors that influence the top slag composition before vacuum treatment and also to optimise the top slag composition for precise sulphur removal. 12 heats were followed during the project; slag samples, steel samples, temperature and oxygen activities were taken at eight different process stages at Ovako steel mill. A relatively large variation was observed for all the oxide components of the slag phase before vacuum treatment in all the heats followed. A PLS analysis made shows that topslag composition before degassing is influenced by the amount of slag former added, oxygen potential at tapping, the yield of Al and Si deoxidants into the steel at tapping. The model has a poor predictability because some important parameters such as ladle glaze condition, amount of EAF slag tapped and refractory wear could not be measured. An alternative solution of extra slag practice was suggested instead of modelling the composition and mass of carry over slag left after slag removal. The extra slag practice involves the addition of lime during tapping so as to aid the removal of all the slag before ladle refining and thus optimisation of the new synthetic slag for precise sulphur removal could be easily achieved. Finally the investigation of the desulphurisation process shows that degassing time, argon gas flow rate through the porous plugs are as well important as the slag mass and composition in order to achieve a precise sulphur removal. ii

4 ACKNOWLEDGEMENT I am eternally grateful to my creator and my saviour whose mercy and love has been without limit in my life. To him who gave this opportunity, His mercy endures forever I want to appreciate Swedish institute (S.I), who has granted me the scholarship to study in LTU. My profound gratitude goes to my supervisor at Ovako Steel AB, Sven-Olof Ericsson for accepting me to carry out this research work under his supervision and also for sharing his rich experience with me during the course of the work. I appreciate my supervisor in LTU Professor Bo Björkman for his contribution in this project work and his pedagogic style of knowledge transfer in the classroom. I also like to appreciate the technical support of Jan-Eric Andersson, Robert Eriksson, Patrik Undvall, Sölve Hagman, Lars-Erik Borgström, Ove Grelsson, Rolf Nilsson and all the team members working at the EAF and Ladle furnace at Ovako Steel AB Hofors. I also appreciate the moral support of the members of Pingst Kyrkan, Hofors during my stay and all my friends in Luleå. This will be incomplete if i don t appreciate my dearly beloved Abiola, who has been a good companion for me. My parents Mr and Mrs I. B Famurewa you are part of what I am today. This could not have been, without the support of my wonderful brothers, Sunday and Festus. I am grateful unto you all. Famurewa Mayowa Stephen July 2009, Hofors iii

5 TABLE OF CONTENTS ABSTRACT... ii ACKNOWLEDGEMENT... iii TABLE OF CONTENTS... iv 1.0 INTRODUCTION Background Historical Background of Ovako Process Description at Ovako Steel AB Hofors Effects of Sulphur on Steel Aim of the Project LITERATUTRE REVIEW General Steelmaking Electric Arc Furnace Ladle Furnace Refining Refining Processes Deoxidation Alloying Stirring Desulphurisation Thermodynamic Theory Slag Properties Composition Sulphide capacity Oxides Activities Sulphur Distribution Ratio Temperature Kinetic Theory Argon Gas flow rate iv

6 Viscosity Dilution Slag MATERIAL AND METHOD Material Method Experimental Procedure Analysis Procedures and Techniques: RESULTS AND DISCUSSIONS Synthetic Slag Composition Top slag compositional changes Mass Balance Regression Analysis for the Top Slag Variation Oxygen Activities Equilibrium sulphur Distribution Regression Analysis for the Desulphurisation Process Equilibrium Condition during Vacuum Treatment Equilibrium Sulphur Content in the Bulk steel Optimisation of the top slag composition CONCLUSION AND RECOMMENDATIONS Conclusion Recommendations REFERENCES APPENDICES v

7 1.0 INTRODUCTION 1.1 Background Steel and its products are undoubtedly the pillar and anchor of material developments through the ages. It is a substantial part of material science and a key material in product development in modern technological advancement. It is the base material for over 2500 different grades of products (1). The potential ability to modify its structures, crystal arrangements, chemical compositions and several other material properties leads to its wide areas of present use and continuous possibility of future developments (1). The world production of crude steel as reported by world steel association is to a great extent more than any other metal product, this also proves its wide versatility in material consumption. Its world productions in million metric tons are 1251, 1251 and 1329 in the year 2006, 2007 and 2008 respectively (2). Figure 1 shows the production of steel in the world in There was a decrease in the crude steel produced in the world as well as in Europe and Sweden in 2008 compared to The production of steel in Sweden has been between 5.2 and 6 million metric tons in the past 6 years with minimum of 5.2 in 2008 (2). Figure 1: World Steel production (2) 1

8 The production of steel could be classified into two, based on the raw materials; ore based and scrap based raw materials. Steel from iron ore (hematite or magnetite) are mainly produced in integrated steel mills while steel from scrap based materials are produced in EAF operated mills. Steel products could also be classified into three based on the composition of alloy additives; low alloy steel, medium alloy and high alloy steel (1). 1.2 Historical Background of Ovako Ovako is a leading European long special steel producer whose production covers low alloy steels and carbon steels in the form of bar, wire, rod, tubes and rings. The primary operation areas include, heavy vehicle, automotive and engineering industries. It has 15 production sites in Sweden Finland, Italy, France and Netherlands with several sales companies in Europe and the USA with a total annual production of about 2million tones of steel of "right quality" (3). The origin of Ovako could be traced to strong Nordic steel production technology and the forerunners to the company were founded for over 300 years ago. Present day Ovako was established in 2005 by a merger of 3 re-known steel companies, Ovako Steel, Fundia Steel and Imatra Steel. Due to strategic and technical reasons, the new company decided to continue its operation in a specific steel product (3). Ovako consists of four product divisions namely; Bar, Wire, Bright Bar and Tube&Ring. Figure 2 below shows the four product divisions with their respective production sites. Figure 2: The Group Structure of Ovako showing the products and their production sites (3) 2

9 1.3 Process Description at Ovako Steel AB Hofors At Ovako Steel two scrap baskets with a total weight of about 110ton are charged into the oval bottom tapped (OBT) electric arc furnace (EAF). The electrical melting with graphite electrode and combustion from the oxy-fuel burners proceed after the first scrap charge with 1.9ton of slag former (lime) addition. The second scrap charge into the furnace is followed by the addition of 1.6ton slag former (lime or dolomite) and then by carbon and oxygen injection for slag foaming. Dust and off gas produced during the melt down are collected by off gas evacuation system. Sampling is carried out during the melting to check the temperature in the furnace and also elemental compositional of the molten scrap. The desired phosphorous refining and heat condition is achieved after about 48 minutes of power-on. The steel is tapped into the ladle where it is deoxidized with aluminum and silicon (FeSi). Sample of the steel is taken after tapping and deoxidation in advance for further refining. The ladle is transported further by crane to the ASEA-SKF unit and the ladle glaze from the previous heat, tap hole sand, EAF slag and part of deoxidation products (Al 2 O 3 and SiO 2 ) which have floated to the top of the steel and other impurities are removed at the mechanical deslagging process. The steel in the ladle is then transported further in a ladle wagon to the heating unit where it is heated using electric energy through three graphite electrodes. Alloying is done through lumpy alloys and wire feeder. Also slag formers are added. The ladle proceeds to the vacuum degassing where desulpurisation is done as well as gas and inclusion removal. A schematic description of the entire steel making process at Ovako Steel AB is shown in Figure 3. Slag Removal Melting Scrap Charging Ingot To Rolling Mill Stripping Ingot Teeming Vacuum Ladle Furnace Figure 3: Steel making Process at Ovako Steel AB 3

10 The steel temperature is finally adjusted to casting temperature and the composition is checked to be in agreement with the aimed composition. The 100ton refined molten steel is finally teemed into 24 ingots each of 4.2ton weight using up-hill teeming. The ingots are stripped and then transported to the pit furnaces for heat treatment prior to rolling or forging. The final products after processing in the rolling mill and tube& ring mill are in the form of bar, tube and ring. The production of steel grades used for the manufacturing of ball bearing requires very low oxygen content in other to reduce the possibility of formation of non metallic oxide inclusions such as Al 2 O 3, and etc, which have deleterious effect on the final products (fatigue life, crack initiation point) (4). A lot of research work has been done on the reduction of total oxygen content of steel; a successful result achieved was a further reduction from about 20ppm to 5ppm. This success led to the increase in the effect of sulphur in the steel, these effects become somewhat more intense than earlier noticed (4). 1.4 Effects of Sulphur on Steel Sulphur has a positive effect on steel when good machinability is desired of the steel product. In some other steel products sulphur content is refined to its minimum due to its negative effect on the mechanical properties. The following effects of sulphur become more significant when the oxygen content is successfully reduced. i. Formation of undesirable sulphides which promotes granular weaknesses and cracks in steel during solidification. ii. It lowers the melting point and intergranular strength and cohesion of steel iii. Sulphur contributes to the brittleness of steel and when it exists in sulphide phase it acts as stress raiser in steel products. (4,5) 1.5 Aim of the Project The above mentioned effects of sulphur are highly undesirable in the production of some special steel products, for example ball bearing steel grades; since the operating condition of such steel grades requires high fatigue strength and other similar mechanical properties. The present state of production in Ovako at the commencement of this project was able to meet 4

11 the low sulphur requirement ranges of the different steel grades but with a low level of accuracy. These involve extra sulphur addition when the sulphur removal is too high or further refining when the removal is too low. However the focus of this project work is to improve the desulphurisation process during vacuum degassing at the ladle furnace, by slag composition control. It is focussed on increasing the level of accuracy of the process to meet the desired sulphur content of the steel product and also to shorten the degassing period. This involves an extensive study of the thermodynamics of the process and kinetics. The ladle refining of different steel grades and different slag practice were followed daily. Slag properties (especially composition), steel compositions, temperature and some other factors were analysed for their sulphur removal potential using some empirical models and later compared with actual measurements in the plants. The compositional variation of the slag formed after deoxidation was studied with respect to the dissolved oxygen content of the steel at tapping. The mass of slag remaining after deslagging and its influence on the final slag composition were also investigated. Optimum synthetic top slag practice with improved sulphide capacity, for accurate desulphurization for different steel grades was to be estimated and the effects of the different kinetic parameters were to be investigated. 5

12 2.0 LITERATUTRE REVIEW 2.1 General Steelmaking Electric Arc Furnace The Electric Arc Furnace is a Steel making technology which is employed for about twentyfive percent of the world steel production (5). External high current electric arc heating with a better thermal control than the basic oxygen Process is used to melt steel scrap and converts it into liquid steel. The cycle of operation for the production of steel in the EAF involves; charging of scrap (direct reduced iron is included in some charges), melting down, refining, sampling (composition and temperature) and tapping. The scarp charged into the furnace could be home scrap (scraps within the steel mill), process scrap (scraps from the manufacturing of steel products) or obsolete (scraps from the end of life of used equipments), and the choice depends on type of steel products (1). All the mentioned scrap types are used in Ovako steel production process. Metallurgically, preheating the scrap is beneficial, as it reduces the energy requirement for melting the scrap which further reduces tap to tap time and the overall productivity. It also decreases the hydrogen contents in the steel as dry charge are fed into the furnace but the extent of preheating is limited to avoid evolution of undesired dioxin. The furnace is mainly eccentric bottom tapped vessel (though oval bottom tapped vessel, equipped with spout also exist), made of heavy steel plates with a dish-shaped refractory hearth and three vertical graphite electrodes extending downward from a domeshaped removable roof. The furnace could be tilted backward for slag removal and forward for about 10-18º for tapping. The furnace is also often equipped with oxy fuel burners for energy efficiency reason. Fluxing agents (lime and dolomite) are added as slag formers to remove impurities. Oxygen and carbon are also injected into the furnace for slag foaming. Slag Foaming This is a common praxis in the EAF. Carbon or coke is injected into the furnace to increase the melt down efficiency by supplying additional energy from combustion with injected oxygen and also to cause carbon boil which promotes stirring to achieve a good slag/metal mixing. Another important function of this praxis is to cause a foaming of the slag provided 6

13 the viscosity of the slag is not too low. The slag foam decreases the energy loss, decreases refractory wear and protect the water cold panel at the top of the vessel (1, 7). The injection of oxygen performs some refining operation in the EAF, especially phosphorous removal although manganese, silicon, chromium and iron are also oxidized. The oxygen content of molten steel is often extrapolated using the carbon content in an online production process. In theory dissolved oxygen and carbon content of steel will react to form carbon monoxide until equilibrium is reached C + O = CO(g) Gº = T (1) % C X % O = X CO pressure (2) Reaction 1 will reach equilibrium when the relationship in equation 2 is attained (5) Ladle Furnace Refining The secondary stage of steelmaking process is done in an open topped cylindrical container lined with refractory called ladle. The primary step is done either in the converter or EAF and crude steel is produced (7). The unit metallurgical processes in the ladle include; Electrical heating, deslagging, wire feeding, stirring with gas or electromagnetic fields and vacuum treatment (5, 7). The units of operation mentioned above enables the following refining and adjustment operations; i. Deoxidation ii. Alloying iii. Stirring to achieve temperature and composition homogeneity and improved refining iv. Desulphurisation v. Degassing to remove hydrogen, nitrogen and other gaseous inclusions vi. Removal and modification of inclusion vii. Adjustment of temperature to optimum range before casting (7). 7

14 2.2 Refining Processes Deoxidation The oxygen content of the steel tapped into the ladle after melting in the EAF is often high, as oxygen is injected into the EAF for refining, slag foaming, and other process control measures. Final steel products require a very low content of oxygen and also further refining and alloying are most desirable at the minimum oxygen content, for this reasons there is a need to kill or deoxidize the crude molten steel. The addition of strong deoxidizers such as aluminum and ferrosilicon is done during tapping, they could either be placed in the preheated ladle before tapping or run into the tapping stream so as to utilize the mixing effect of the tapping stream to achieve thorough deoxidation (5,8). The reaction is shown in equation (3) and (4), and the oxides nucleate to diffuse to the ladle wall or absorbed into EAF slag. 2Al + 3O = Al 2 O 3(slag) Gº = T (3) Si + 2O = SiO 2 (slag) Gº = T (4) The deoxidation reactions shown in equations (3) and (4) are exothermic and thus the temperature of the liquid steel is increased, however the steel also loses heat by radiation from the top surface, heating of ladle lining and by flux through the lining and shell (5). The rate of heat loss is reduced in most ladle operation by preheating the ladle before tapping Alloying The adjustment of the final composition of the molten metal is done at the heating and wire feeding position of ladle station. A wire feeder runs wire of alloying elements at controlled speed into the steel (5,7). Most of the alloying elements are lumpy ferroalloy since they are cheaper to produce and available in different grades to suit the final steel compositional requirements. The addition of alloying elements results into temperature drop of the molten steel and the calculation to meet the final composition is often done by computer programmes. 8

15 2.2.3 Stirring Generally, ladle furnace technology is equipped with either one or both of the two stirring facilities; electromagnetic accessory for inductive stirring and permeable refractory block at the bottom of the ladle called porous plugs for gas stirring. These two stirring means are important for good metal/slag interaction to achieve an effective ladle refining. It enhances homogenous temperature and composition of the steel. It also aids continuous slag metal reaction with the aim of sulphur, hydrogen, nitrogen and inclusion removal (6). 2.3 Desulphurisation Desulphurisation is an essential practice in the production of clean steel products such as bearing steels with high fatigue strength which function under high impact operational requirement. Ovako Steel AB specializes in the production of bearing steel products, desulphurisation becomes an important subject to be continuously investigated for highly clean products which can withstand market competition and satisfy customer's demand. Based on the production route and the type of steel product, desulphurisation could be done at different points in the steelmaking process and with different reagents; however it is mainly carried out in a reducing conditions when the oxygen activity is low (8). At Ovako Steel AB, desulphurisation is done during the vacuum degassing in the ladle furnace using lime saturated multicomponent slag. The parameters which influence the desulphurisation process are either thermodynamic or kinetic parameters; the main parameters are discussed later in the report while the theories related to this project work are explained below Thermodynamic Theory When studying the thermodynamics of slag, metal and gas interactions in the ladle refining with consideration to sulphur removal, the reactions below are important. 2 2 [ S] + ( O ) = [ O] + ( S ) S( g ) + ( O ) = 1 2O( g ) + ( S ) (5) (6) [ S] + 1 2O( ) + = [ O] S( ) (7) g g The equilibrium constant for the reaction in equation (6) is expressed as; 9

16 K K 6 6 = = a a f 2 S 2 O 2 S (% S) a P 2 O 2 O P 2 S slag P 2 O P 2 S (8) Also sulphide capacity can be written as C C s S K6 a = f = 2 S (% S) 2 O slag (9) P 2 O P 2 S Where [S] and [O] are dissolved sulphur and oxygen in the steel respectively while (S 2- ) and (O 2- ) are sulphide and oxide (with free oxygen ion) in the slag respectively. a S2- and a O2- are the activities of sulphide and oxide in the slag. K 6 is the equilibrium constant for gas-slag reaction in equation (6) (4, 8, 9). Since oxides activities and the partial pressure of gaseous phases are not readily available as process parameters, sulphide capacity is often expressed in terms of temperature and composition as process control tool (5, 13'). Sosinky and Sommerville derived an expression to correlate optical basicity with sulphide capacity at temperature range between 1400 andd 1700 C (4, 14) Λ Log CS = Λ (10) T X1n1Λ th1 + X 2n2Λth2 + X 3n3Λ th where OpticalBasicity Λ = (11) X n + X n + X n X is the mole fraction of the oxides in the slag system, n is the number of oxygen atom in a molecule of each oxide and Λ th represents the optical basicity of each oxide (14). Young et al showed that equation (11) only applied to range where Λ = 0.8, and therefore reported correlations for ranges with Λ < 0.8, Log C s (% SiO ) (% Al O ) (12) 2 1 = Λ 23.82Λ 11710T

17 Andersson et al (4) studied the distribution of sulphur and the extent of sulphur removal using equation (7) K 7 a = a Also, Log K L S = Log (% S) [% S] L S O S P P 7 S2 O2 (% S) ao = f C [% S] 935 = T 935 = Log C T S S S + Log f S Log a O (13) To calculate the activity coefficient of sulphur in the metal, Wagner's expression can be used Log f S j = e [% i]. (14) i a O and a S are the activities of oxygen and sulphur in the molten steel respectively. j e S, is the interaction parameter between sulphur and other elements j in the steel. K 7 is the equilibrium constant for the gas-metal reaction in equation (7). The activity of oxygen in the steel could be calculated assuming equilibrium between the dissolved oxygen and aluminum in the steel and alumina in the slag or alumina inclusion in the steel bulk. Al + 3O = Al O K 15 G = exp = RT 2 3 a G = T 2 3 [ a ] 2 [ a ] 3 Al Al O O ( KJ / mol) (15) Where a Al2O3 is the activity of alumina in the slag or as inclusion, and a O and a Al are the activities of oxygen and aluminum respectively. The activity of Al can be calculated using equations (14) and (16) while the alumina activity can be calculated using Ohto and and Suito empirical expression in equation (17). a Al2O3 is taken to be 1, if alumina inclusion is considered in the equilibrium. K 17 is the equilibrium constant for the deoxidation reaction shown in equation (15) (4, 15). Log a a Al Al2O3 = = f Al [ % Al] [ 0.275( % CaO) ( % MgO) ] % SiO (16) (% Al O ) (17) The oxide composition is in weight percent and the expression has been proven to be valid for temperature ranges close to 1600ºC (4). Also the expression is suggested for CaO;

18 60wt%,SiO 2 ; 10-50wt%, Al 2 O 3 ;0-50wt% and MgO: 0-30wt% which is a fairly good range for the slag studied in this project work Slag Properties The process of desulphurisation depends to a great extent on the properties of the slag phase. The ability to extensively describe the thermodynamic and thermophysical properties of the notable phases as a function of the composition and temperature of the slag is a strong control tool for desulphurisation (16). It has also been established that high amount of slag is favourable for sulphur removal (17) Composition The different phases in a multi-component slag system play significant roles in ladle refining with focus on desulphurisation. CaO + [S] = CaS + [O] (18) The general equation for desulphurisation is written in equation (18) above, one of the important conditions to enhance the reaction is the activity of CaO. An optimum slag composition should be saturated with CaO, in other words the activity of CaO in the slag should be close to unity to facilitate the exchange of the dissolved sulphur in steel with oxygen ion (6,18). The ternary phase diagrams of CaO-SiO 2 -Al 2 O 3 can be used to establish the optimum slag composition at a particular temperature. Figure 2 shows an isothermal section of CaO-SiO 2 -Al 2 O 3 slag system, the highlighted portion shows the homogeneous liquid region at temperature 1600ºC, while the remaining sections show undesired solid regions. Any composition within the homogeneous liquid region with an activity of CaO close to unity is good for desulphurisation process. For the system considered in Figure 4, liquidus line 'ab' represents a perfect unity of CaO activity. 12

19 1600ºC Figure 4: Isothermal section of the phase diagram of the system, CaO-SiO 2 -Al 2 O 3 at 1600 C. The shaded area indicates the anticipated homogeneous liquid region at 1600 C. C 3 S=3CaO SiO 2, C 2 S=2CaO SiO 2, (18) CA=CaO Al 2 O 3, CA 2 =CaO 2Al 2 O 3, CA 6 =CaO 6Al2O 3, A 3 S 2 =3Al 2 O 3 SiO 2. Figure 5, 6 and 7 show the activities of CaO, SiO 2 and Al 2 O 3 respectively at different points in the homogeneous liquid region of the slag system at the 1600C. An optimum slag composition for good desulphurisation can be carefully chosen by exploring these diagrams to have the highest possibilities of CaO activity, basicity and efficient fluidity. It should be noted that composition affects viscosity of the slag, if the CaO of the multicomponent slag system is higher than 60% it has a negative effect on the sulphide capacity of the slag as it becomes heterogeneous and more viscous, thus the kinetic will be negatively influenced (6). Figure 5: Activities of CaO in CaO-SiO 2 -Al 2 O 3 multicomponent system. Standard State of pure AlO1.5 with the relations 2AlO 1.5 = Al 2 O 3 and (a AlO1.5 ) 2 = a Al2O3 at 1600 C. (14) 13

20 Figure 6: Activities of SiO 2 in CaO-SiO 2 -Al 2 O 3 multicomponent system at 1600 C (14) Figure 7: Activities of Al 2 O 3 in CaO-SiO 2 -Al 2 O 3 multicomponent system. Standard State of pure AlO1.5 with the relations 2AlO 1.5 = Al 2 O 3 and (a AlO1.5 ) 2 = a Al2O3 at 1600 C ). (14) 14

21 Sulphide capacity An important property of slags which plays a vital role in the investigation and control of desulphurization process is sulphide capacity. It is the potential ability of a completely homogeneous molten slag to remove sulphur during slag metal interaction (4, 19). This potential ability is used to estimate the amount of sulphur that a slag of a given composition will retain under a specified condition of oxygen and sulphur pressures (19). It is often used to establish the sulphur distribution ratio between slag and metal at equilibrium. Its ability to compare the desulphurization characteristics of different slags has led to the creation of several models for its measurement (6, 10, 17, 16, 14). Figure 8 shows the sulphide capacities of a CaO-Al 2 O 3 -SiO 2 system at different compositions. For an optimal slag composition aimed to achieve an effective desulphurisation, compositions close to line ab (-Log C s = 1) should be ensured. Compositions close to line ab also have CaO activity close to unity and high basicity which are important for desulphurisation (14). a b Figure 8: Isothermal section of the system, CaO-SiO2-Al2O3 at 1650 C showing the Log of Sulphide capacity with composition in mass % (14) 15

22 Basicity is defined in its simplest form as the ratio of %CaO / %SiO 2, it is known that desulphurization is improved with slags of higher basicity. Basic slags have high content of basic oxides which are network breakers with ability to release its oxygen ion (O 2- ) in exchange for the dissolved sulphur in steel. Significant correlations have been made between sulphide capacity and basicity. Figure 9 shows three different sulphide capacity models; Sosinky & Sommerville, Young et al and KTH models. The first two models were calculated from optical basicity while the third is a model developed in the division of process Metallurgy in KTH (4). The three models in figure 9 show that sulphide capacity is improved with increased basicity. Figure 9: Sulphide Capacity values as functions of Basicity (4) In this project work optical basicity, a measure of the electron donor power of slag will be used in the estimation of the sulphide capacity, because its model have been proven to have a fair agreement with empirical data and the parameters can be accessed Oxides Activities The activities of oxides in the molten slag and alloying elements in the molten metal as well as the temperature of the process determine the equilibrium oxygen potential in the system. The measurements of the activities of oxides in the slag and dissolved oxygen in the steel are important for control of desulphurization process (10). Oxides activities in the slag affect the equilibrium activity of oxygen in the steel and also the basicity of the slag. Figure 10 shows that at high basicity, the activity of alumina is low and the oxygen activity will also be low provided the Al content of the steel is high at this condition. This is a necessary requirement 16

23 for desulphurization. Also Turkdogan E. established that low SiO 2 content of the slag is favorable for improved sulphur removal, due to its equilibrium with Al and Si content of the steel. (20) Sulphur Distribution Ratio Figure 10: Alumina activities for typical ladle furnace slag using the KTH model and Ohta and Suito Equation (16). It is an estimation of the sulphur reduction in a desulphurization process. It is the ratio of the sulphur content in the slag and metal phase at the end of vacuum treatment. A good estimation of this parameter indicates a good control of the process. It is a function of temperature, sulphide capacity of the slag, oxygen activity and sulphur content of the molten steel (16). An optimized slag which can be used to control desulphurisation can be obtained by exploring the ternary diagram shown in figure 11 Figure 11; Isothermal section of the system, CaO- SiO 2 - Al 2 O 3 -MgO with 5%MgO at 1600 C showing the sulphur distribution between Metal and Slag at equilibrium (14) 17

24 Temperature An essential parameter in desulphurization is the temperature at which the process is carried out. It influences the viscosity (favourable kinetic condition) and sulphide capacity of the slag and also sulphur distribution in the metal and slag. Most models that have been developed to evaluate the sulphide capacity of slag were mainly functions of temperature and composition (10,11,12,17). Figure 12 shows that sulphide capacity is improved at higher temperature. The calculation was done at constant MgO and SiO 2 contents of the slag and also at constant %Al and %C content of the steel (11). It is also reported that desulphurisation is slower at the later period of vacuum degassing due to reduced sulphur content of the steel and temperature drop during the process which is unfavorable for the sulphide capacity (4). Figure 12: Sulphide capacity as a function of temperature and Al 2 O 3 in the topslag (11) Kinetic Theory The transfer of sulphur atoms from the metal phase to the slag phase and the transfer of oxygen ions from the slag phase to the metal phase during sulphur refining process is controlled by mass transfer through diffusion (12, 19). Fick s law of diffusion could be applied to the process as below, 18

25 Φ J = D x V m = D A t δ [ % S] Kt ρm A = t M [% S] [% S ] [% Si] Where J- Diffusion flux D- Diffusion constant Φ - Concentration gradient x [% S ] [ % S ] ( 19) [% ] Initial Concentration of Sulphur in the melt %wt S m m [% S e ]- Concentration of Sulphur in the slag/metal interface at equilibrium %wt δ - Boundary Layer K t - Total mass transfer coefficient M- Mass of steel A - Slag-Metal interface area V- Volume of steel ρ - Density of steel m [ S] % t e - Sulphur removal rate and [%S] is the instantaneous sulphur concentration in steel As earlier mentioned sulphur removal depends on the stirring rate and viscosity, both properties affect the slag metal interface area and also mass transfer coefficients of the process. It should be noted that these constant δ and K t are difficult to measure in a real process and the modeled values are specific for particular stirring conditions (12). The conventional assumption of a flat and horizontal slag metal interface area 'A' has been proven to be an under estimation as the slag is dispersed in the steel and the interaction area is more than supposed (12) Argon Gas flow rate The manipulation of the inductive and gas stirring during the Ladle refining is a very important factor in the desulphurization process control. An investigation of the influence of argon gas flow rate on desulphurization during vacuum treatment is shown in Figure 13. With respect to desulphurization, the optimum condition for vacuum treatment in the figure is at the argon gas flow rate of 1.8m 3 Ar/min, thus a better desulphurisation is achieved optimum slag/metal mixing (6). 19

26 Figure 13: Steel Desulphurisation during Vacuum treatment at various Ar stirring rates (13) The position of the inductive stirrer and the rate of flow of argon gas through the porous plugs were studied by Hallberg et al (17) in the creation of a process model for sulphur refining at Ovako Steel AB. It could be deduced from figure 14 that argon gas flow rate through the porous plug has an influence on the fluid flow in the ladle, a low flow rate at the first porous plug which is closer to the inductive stirrer and high flow rate at the second porous plug which is at the opposite side is necessary for a good desulphurisation (17). Figure 14: Influence of different argon gas flow rates on sulphur removal for combined gas and inductive stirring (17) 20

27 Most of the top slag is concentrated above the second porous slag where they are hit by the gas plumes from the plug and thus a greater contact area between slag and steel is created. This is substantiated by the authors, using computational fluid flow dynamic predictions. It should be noted that the flow rates through the porous plugs are not fixed throughout a heat at Ovako Steel AB, they are changed by the operators based on reactions observed in the camera view. Some previous projects done at Ovako Steel AB have also investigated the effects of argon gas stirring before and after vacuum treatment on desulphurisation (6). The control of argon gas flow rate was difficult at the heating station of ladle due to poor flow of gas through the porous plug. The only trial that was done was unsuccessful with a poor sulphur removal and poor vacuum treatment. Another trial with inductive stirring and calm argon gas stirring at the final heating unit process after the vacuum treatment shows no influence on the final sulphur content of the steel though it had a positive impact on inclusion removal (6) Viscosity It is a thermo-physical property that influences the kinetics of the ladle metallurgy (16). The viscosity of both the steel and slag affects the mass transfer during the ladle refining. At low viscosity of the slag, mass transfer rate of sulphur is improved due to easy dispersion in the steel and the slag/metal interfacial area is increased (13). A low melting point CaO rich slag can be synthesized by adding a correct proportion of Al 2 O 3, and the viscosity is adjusted in some steel plants by the addition of CaF (6,13) 2.. Viscosity values for steels are reasonably well established at steel making conditions but the viscosity of slags are not, they are rather extrapolation of temperature and composition in a multicomponent slag system (16). 2.4 Dilution Slag A major quest in this research is to identify the source, composition and mass of the slag which remains after mechanical slag removal in steel making process line at Ovako. This poses a problem to the optimization of the slag mass and composition for a precise sulphur 21

28 removal. A preliminary study of the process shows the following possible sources of the dilution slag; 1. EAF Slag: Hot heel is a common praxis at Ovako steel, about 110tons of steel scrap is charged into the EAF and less than 105tons is tapped into the Ladle leaving some steel behind in the furnace. Despite the hot heel practice, it is unavoidable to have a small mass of slag entrained in the tapped steel. 2. Tap hole sand: After each tapping, the tap hole is blinded with Olivine sand. It is of course certain that this sand will be lost into the steel during tapping, as the hole is opened before steel falls into the ladle. The tap hole sand is rich in MgO and SiO 2 and its quantity in the tapped stream depends on the age of the tap hole, this also considered as a probable source of dilution slag during ladle refining. 3. Ladle glaze: Ladle glaze is formed when draining the Ladle into the mould, as top slag comes in contact with refractory (21). As teeming proceeds the temperature of the system drops and fluidity of the slag reduces, thereby forming non metallic particles after reacting with refractory, this particles hang on the wall of the ladle as glaze and are flushed off when liquid steel is poured into the Ladle during subsequent heat. The condition, composition and the amount of the glaze depends on the steel type produced at a particular heat. 4. Deoxidation Products: To enhance further refining after melting of scrap in the EAF, reduction of oxygen contents of the steel is important. The oxygen contents which is often between 100 and 1000ppm before tapping depending on the extent of refining and the heat condition in the EAF, is reduced by the addition of aluminium metal and Ferrosilicon alloy. The reaction products, Al 2 O 3 and SiO 2 indigenous inclusions are major sources of inclusion in steel making and as well increase the amount of the EAF slag as they are absorbed after nucleation and separation from the steel bulk. With consideration to all the theories of sulphur removal as well as thermodynamic and kinetic properties discussed earlier, this project work will advance sulphur removal process at Ovako steel AB Hofors using slag composition control. It will investigate the slag mass and composition during different process stages and attempt to optimize the slag with the aim of controlling sulphur refining process. 22

29 3.0 MATERIAL AND METHOD 3.1 Material The raw materials used at Ovako Steel AB are steel scraps of different grades, including home, process and obsolete scraps. The selection of the scrap materials is based on the size and grade of the scrap and also on the cleanliness or type of the steel to be produced. The additives used in the process includes anthracite, coal, oxygen, slag formers (Lime, dolomite Alumina and pure Alumina), deoxidants (Aluminium metal and Ferrosilicon) and other alloys. The chemical composition of the slag formers used at the ladle furnace refining is given in table 1. Table 1: Chemical Composition of slag formers SiO 2 MnO S TiO 2 CaO Al 2 O 3 MgO FeO Lime 2,91 0,17 0,065 0,050 92,30 0,93 0,80 0,430 Alumina 2,78 0,08 0,017 0,198 17,58 63,78 12,13 0,679 Pure Alumina 2,22 0,02 0,006 0,020 4,22 91,90 0,45 Measurement is in wt-% 3.2 Method Experimental Procedure A number of heats were followed to observe the compositional changes in the steel and slag at different process stages, right from melt down in the EAF to the end of casting. The aim was to observe the consequential effects on desulphurisation. Different steel grades were followed to investigate the variation in the studied parameter. The studied parameters include temperature, slag composition, steel composition, oxygen activity, mass of additives and also consideration was taken of the vacuum pressure and argon gas flow rate at degassing. 23

30 Eight sampling points were observed at different subunits in the integrated process line for thorough follow up of the equilibrium conditions and changes in the system. The schematic diagram of the sampling points is shown in Figure 15. Figure 15: Sampling points for plant trials B - Bulk steel sample, S - Slag Sample, O - Oxygen activity, T - Temperature 1. B 1, S 1, O 1, T 1 : Steel sample, Slag sample, Oxygen activity and Temperature in the EAF just before tapping respectively 2. B 2, S 2 : Steel sample and Slag sample at the end of tapping; 3. S 3 : Slag sample before slag removal 4. B 4, O 4, T 4 : Steel sample, Oxygen activity and Temperature at arrival at the ASEA- SKF Ladle furnace station before alloying 5. B 5, S 5, O 5, T 5 : Steel sample, Slag sample, Oxygen activity and Temperature before degassing 6. B 6, S 6, O 6, T 6: Steel sample, Slag sample, Oxygen activity and Temperature after degassing. 7. B 7, S 7, O 7, T 7 : Steel sample, Slag sample, Oxygen activity and Temperature after extra alloying (if there is any). 8. B 8 : Steel sample during casting. 24

31 3.2.2Analysis Procedures and Techniques: Temperature: The temperature of the molten steel was measured in the EAF with the aid of Robot and at the Ladle furnace using the automatic sampling lance. In some occasions for the purpose of this thesis work the temperature was measured using the electro nite Celox R7 oxygen activity measuring equipment. Chemical Composition Steel Samples: Automatic lances, similar to the temperature lances said earlier were used to take steel samples at each point mentioned above, the samples were sent to the operation laboratory were immediate analysis was made. The samples were analysed by optical emission spectroscopy (Bausch & Lomb, ARL OES 4460) for the concentrations of Al, Cr, Mn, Si, P, Mo, V, Ca, Ti, Mg and other minor elements. The relative analysis accuracy of these elements depends on their concentrations. Sulphur and carbon in the samples were analysed with LECO CS-444 using the melting and combustion method. Chemical Composition of Slag Samples: The slag samples were taken at each designated process stage using the slag spoon, the samples were saved for further analysis. The samples were prepared before analysis; they were ground into powder in a ring mill, sieved to collect fine particles less than 10µm which are void of metallic iron. Chemical reagents were added according to standard and then thoroughly mixed together. The prepared samples were then heated in a laboratory furnace for about 8 minutes before they were arranged in the PAN analytical Axios equipment to analyse the oxide composition using the wavelength dispersive X-ray fluorescence technique. The concentration in weight percent of CaO, Al 2 O 3, SiO 2, MgO, MnO, TiO 2, Cr 2 O 3 and other oxides in minor concentration were measured. A portion of the samples taken after grinding to fine size was also analysed for sulphur and carbon content. This analysis was done in LECO-CS 200 equipment using the melting and combustion method. 25

32 Oxygen content The dissolved oxygen content in the steel bulk is a key parameter in refining during steel making process. In the project work the activity of oxygen in the steel melt was measured using the Celox sensor designed by Heraeus Electro-Nite. The sensor contains ZrO 2 elctrolyte with a molybdenum wire in Cr/Cr 2 O 3 as a reference electrode while the bulk steel is the second electrode. An electromotive force (e.m.f) is built up when different oxygen activities are sensed by the two electrodes, the value of which is displayed on the screen of the equipment. The thermocouple attached to the sensor measures the temperature of the system. A calculation is generated automatically using the measured temperature and e.m.f to obtain the oxygen activity which is then displayed on the screen of the equipment. 26

33 4.0 RESULTS AND DISCUSSIONS 4.1 Synthetic Slag Composition The synthetic compositions of the mixture of the different slag formers used at the steel mill during the trial sampling are given in Table 2. Synthetic slag 1 and 2 are mixtures of 65% Lime / 35%Alumina and 60%Lime / 40% Alumina respectively. The mass of the slag varies between 800 and 1000kg based on the degree of sulphur removal for the different steel grades. Synthetic slag 3 contains 73% Lime and 27% Pure Alumina, it has a higher mass than the previous slags (1300kg). Table 2: The Initial composition of three different synthetic slags Composition CaO Al 2 O 3 SiO 2 MgO S TiO 2 FeO MnO Synthetic slag 1 66,23 22,79 2,77 4,48 0,05 0,110 0,59 0,14 Synthetic slag 2 62,50 25,92 2,75 5,00 0,04 0,12 0,62 0,13 Synthetic slag 3 68,52 25,49 2,18 0,71 0,05 0,04 0,31 0,13 Synthetic slag 1: 65%Lime&35% Alumina, Synthetic slag 2: 60%Lime&40%Alumina, Synthetic slag 3: 73%Lime&27% Pure Alumina (Measurement is in wt-%) 4.2 Top slag compositional changes The analysis of the compositional variation of topslag before degassing for the 12 heats followed during the thesis work is given below. The usual slag practice for ladle refining is either synthetic slag blend 1 or 3(table 2), the compositional change of the top slag after heating and before vacuum treatment for the 12 heats is shown in figure 16. The change is quite high for some oxides while it is low for others. For SiO 2, MnO, MgO, FeO and Al 2 O 3, the average top slag composition before vacuum treatment for all the heats is higher than the synthetic slag blends added, while CaO is lower. It could be seen from table 3 that the range of wt%cao is between 49.8 %- 64.5%, and wt%al 2 O 3 is between 21.90% - 31,9% with relative deviation of 4.01% and 2,47% respectively. The range of SiO 2 is between % while MnO is %. 27

34 65 CaO % Al2O3 % SiO2 % 33 10, , ,0 MnO % FeO % MgO % Figure 16: Box Plot-Investigating the variation in the top slag composition before Degassing where represents mid spread of the data (50% of the heats), and represent the mean and median of the data set. Table 3: The composition of top slag before degassing for composition Charge %SiO2 %MnO %CaO %Al2O3 %MgO %FeO ao(ppm) Temp C 1 6,80 0,24 57,30 26,00 8,20 1,66 2, ,00 3,14 49,80 27,40 7,50 2,21 4, ,60 0,18 64,50 28,40 2,50 0,61 2, ,7 4 6,90 0,84 62,00 26,60 3,20 1, ,28 3,49 59,01 25,40 6,02 3,46 5, ,50 0,77 55,10 26,70 8,00 0,81 3, ,10 0,23 60,40 24,10 6,80 2,60 2, ,20 0,40 56,70 31,90 4,90 1, ,90 0,31 63,50 28,00 3,30 0,73 3, ,9 10 9,20 1,20 56,60 21,90 6,70 3,90 3, ,7 11 7,20 0,13 59,00 25,50 5,70 0,70 2, ,9 12 6,40 0,09 56,50 27,90 8,10 0,33 3, ,6 Mean 6,92 0,92 58,37 26,65 5,91 1,60 Std dev 1,43 1,17 4,01 2,47 2,03 1,18 Rel dev. 20,62 127,51 6,87 9,26 34,30 73,76 The composition is measured in wt-% The higher extreme values for the CaO in the box plots (though not considered as outliers for the distribution), are connected to high proportion of lime in the synthetic slag blend and also high mass of the synthetic slag (this is a common practice for high clean steel which requires extreme sulphur removal). The lower extreme values are peculiar for heats with high oxygen activities a o at tapping; which implies a low yield of Al and Si at tapping and also higher potential to retain some deoxidation products as inclusion in the steel bulk or on the ladle wall 28

35 until ladle refining stage when they are removed to the top slag. For %Al 2 O 3, the outlier is an uncommon blend of slag former (68% Lime and 32% pure Alumina) which contains very high %Al 2 O 3. On the contrary MnO, FeO and SiO 2 have high relative deviation with wide spread of the values and most of the values close to one extreme end. This is an indication that their compositions in the topslag are not controlled by the variation in the slag former blend. The upper extreme values for these oxides contents are results of high a o at tapping, Al/O/ Al 2 O 3 equilibrium before vacuum degassing and also the quantity of carry over slag remaining after the mechanical slag removal. The lower extreme values, especially for SiO 2, depict special heats with high purity requirements; they are refined with synthetic slag former of high mass and low initial SiO 2. The SiO 2 content of the slag of such heats is reduced further during vacuum treatment as the interaction between slag and steel degassing became improved. This reduction (equation 21) is aided by high %Al and low a o of such heats, and it in turn favours good desulphurisation. The trend for the variation of MgO from 2.5 to 8.2% is a little bit compounding, as it depends on the amount of EAF slag remaining after slag removal, age of the ladle in use, lime saturation of the top slag and it also depends on the slag former blend. If pure Alumina is used, then the MgO content of the top slag before vacuum treatment will be very low due to the low content of MgO in it. It can be clearly seen from figure 17 that the solubility of MgO refractory into the topslag varies with CaO content of the top slag; lime saturated topslag has a low solubility potential of the refractory and vice versa. It was observed that the ladle age is also an influencing factor for the MgO content of the topslag before vacuum treatment. Newly lined ladles have greater potential to wear than old ladles though they give better thermal resistance and heat conservation than the old ladles. 9 7 %MgO 5 Before Degassing After Degassing % CaO Figure 17: MgO pick up from the refractory into the top slag 29