Quality of Lime for Steelmaking in Converter
Quality of Lime for Steelmaking in Converter
Lime is a white crystalline solid with a melting point of 2572 deg C. It is a basic oxide and is used to react with the acidic oxides (e.g. silica). It is calcium oxide (CaO) produced on heating (calcination) of limestone (CaCO3) to a temperature of 900 deg C and above (usually 1100 deg C).
CaCO3(s) + heat = CaO(s) + CO2 (g)
This reaction is reversible. Calcium oxide reacts with carbon dioxide to form calcium carbonate. The reaction is driven to the right by flushing of carbon dioxide from the mixture as it is released.
Hydrated lime Ca(OH)2 is formed by reaction of lime with water (slaking). Hydrated lime is also known as slaked lime.
CaO + H2O = Ca(OH)2 + heat
Lime as a basic flux in steel production and it plays an important role in the sequence of metallurgical reactions taking place in a converter. Steel is produced from hot metal by oxidizing sulphur (S), phosphorus (P), carbon (C), silicon (Si), manganese (Mn), and other impurities so that they can enter the slag or gas phases, thus separating from the metal phase. Lime in steelmaking is mainly used to produce slag for the removal of these harmful elements in liquid bath and optimize the quality of liquid steel.
The basic oxygen process oxidizes impurities in an oxygen converter also known as basic oxygen furnace (BOF) where the hot metal comes in contact with oxygen. Oxidized impurities of the hot metal are absorbed in a slag, which is formed with the help of calcined lime.
Metallurgical lime in the fifties consisted of a mixture of particles of all sizes from very coarse to very fine, with additional components such as silicon dioxide and sulphur concentrated in the fine particles. The lime quality was obviously less important at that time because the open hearth process for making steel had a smelting period of 6 to 8 hours. With the introduction of the basic oxygen converter (BOF), the process of steelmaking got speeded up with oxygen blowing periods of 18 to 20 minutes. The speeds of reactions taking place in the BOF are very fast. Since a large amount of calcined lime is charged into the converter within a short period of time, careful selection of the lime quality is important to improve its dissolution in the slag and to cope up with the speed of reactions. It is very important to pay attention to the chemical and physical properties of lime.
Chemical properties of lime
Great importance is attached to the CaO content of the lime, which is to be as high as possible but in no case less than 90 %. The content of carbon dioxide is to be as low as possible, which is important for the heat balance and for quiet oxygen blowing conditions in the converter. CO2 depends on the extent of calcination while producing lime and the upper limit for soft burnt converter grade lime is normally 1.5 % of CO2.
Silica (SiO2) content of the lime is very important. SiO2 is an acidic oxide and has a big corrosive influence on the basic magnesia-carbon refractory lining of the converter. One of the main purposes of lime addition in the converter is to neutralize SiO2 produced by the oxidation of Si present in the hot metal and scrap. Any introduction of SiO2 through lime in the converter is counterproductive. Hence SiO2 content of the lime is to be as low as feasible. Ideally it is to be less than 1.0 %.
Magnesium oxide (MgO) content of lime is of increasing importance. There is a favourable influence of lime containing MgO on the refractory lining life of the converter. Some metallurgical limes, used in the converter process have MgO content of around 2 %.
The sulphur content of the lime, which limits the desulphurisation in the converter, is important. In some conditions sulphur content of lime may result in an increasing re-sulphurisation of the steel. The sulphur content is of great importance for soft and low-manganese steel. Lower sulphur content aids in controlling the sulphur content in the steel. The sulphur content in lime depends on the kind of fuel used for lime calcining, the quality of limestone and the calcining conditions. Normally sulphur in the calcined lime is to be restricted to 0.05 %. Typical analysis of lime shows 0.025 % sulphur.
Properly calcined lime aids in the dephosphorization of liquid bath in the converter. Introduction of phosphorus through lime in the converter is counterproductive. Hence P content of the lime is to be as low as feasible. Normally it is to be less than 0.03 %.
Physical properties of calcined lime
The size of the calcined lime for the BOF process of steelmaking is very important. Normally lime in the size fractions between 8 mm to 40 mm is used in the converter since it helps in its dissolution in the converter bath and aids in the slag formation. If the lime size is less than 8 mm then the majority of the finer fraction of lime (less than 8 mm) is blown off with the converter exhaust gases and creates a favourable condition for the formation of accretions in the hood. The allowable limit for the finer fraction of lime charged in the converter is normally around 5 %.
The second physical property of calcined lime which has got a major influence on the slag formation during the BOF process is the lime reactivity. Lime reactivity is the property of lime which determines its faster dissolution and higher reaction ability for the removal of the harmful elements from the converter bath. Since it is not possible to determine the reactivity of lime in the liquid steel, it is usually determined in water using a standardized test procedure so that reproducible test results can be achieved.
Calcined lime gets its reactivity property because of good porosity which results into a large specific surface. The large specific surface has a favourable effect on the metallurgical reactions.
If the lime gets over-calcined during its production in the kiln then the pores get closed and there is reduction in the specific surface area in the lime and hence loss of reactivity. Similarly in case of under-calcined lime, the core of lime is not calcined. Such lime has residual carbon di oxide (CO2) and hence has low reactivity. Under-calcined lime generates more fines during its handling. The reactivity of lime is dependent on its homogeneity, the degree of thermal decomposition, and its specific surface area.
Because of high porosity, calcined lime is highly hygroscopic in nature. The surface of the lime lumps picks up moisture from the air and gets hydrated. During handling of lime the hydrated surface falls off as powder and exposes fresh surface of lime to moisture for getting hydrated. With the absorption of moisture, the lime loses its reactivity. The hydrated lime does not take part in the reactions in the converter which take place at high speeds. Hence it is essential that the calcined lime is consumed in the converter within shortest period of time after its production.
The common method for the determination of the reactivity of lime is by titration with 4N hydrochloric acid (HCl). In this test 12.5 g sample of lime in the size range of 1 mm to 3 mm is taken and a saturated solution is prepared with 100 ml of water at 60 deg C. Two drops of methyl orange reagent is added to this solution. Then HCl acid of 4N (mole) concentration is added to it drop by drop till it is fully neutralized. The end point is the known by the colour change from orange to pink. The volume of HCl used for neutralization is then multiplied by 4 and the value is generally referred as the reactivity value. The higher is the volume of HCl needed for the neutralization, higher is the reactivity of lime.
Physical characteristics of good calcined lime
The main physical characteristics of the lime (Fig 1) which has been properly calcined in the lime kiln are given below.
- It has low bulk density with value varying in the range of 1.5 tons/cum to 1.7 tons/cum.
- Its porosity is high normally and is in the range of around 50 %.
- Its specific surface area is larger than the surface area. It is in the range of 1.5 sq m/kg to 2.0 sq m/kg.
- Its mineral crystals are small.
- Its reactivity is to be very high. Normally the reactivity value is more than 300 ml of 4N HCl.
- Its residual CO2 content is low. The value is generally the less than 2 %.
- Its fines content is low. Usually it is to be less than 5 %.
Fig 1 Characteristics of lime for use in converter
Lime and steelmaking slag
Lime is critical to steelmaking. Near the start of the blow calcined lime and calcined dolomite are added, although sometimes these are added gradually throughout the heat. The CaO of calcined lime and CaO + MgO of calcined dolomite react with the oxidized impurities and iron oxide to form a slag. The slag is thus composed of complex calcium and magnesium silicates, alumino-silicates, ferrites, and phosphates etc. The formation of a good basic slag is essential to ensure the steel quality. Basicity (CaO/SiO2) levels are normally kept in the range 2.8 and 3.5.
The term slag basicity is a means by which it is determined to how acidic or how basic a slag is in relationship to the basic refractory utilized in converter. If the slag is more acidic then it will erode the bricks, if it is more basic it will protect the bricks. The basicity of the slag is normally fixed by the amount of lime and calcined dolomite added. The lower value of the ratio indicates that the slag is acidic and as the ratio increases the slag becomes more and more basic
A properly calcined lime with good basicity favours dephosphorization and desulphurization, and facilitates the steelmaking process with reduced spitting. The sulphur distribution between bath and slag is better with proper calcined lime than with the lime which is over calcined even with slags of the same basicity. The same applies to a lesser degree for the dephosphorization. Also in this case, especially with low iron oxide contents in the slag, the phosphorus contents in the steel are reduced by the use of the properly calcined lime.
With the use of a properly calcined lime, it is possible to reduce the lime additions in the converter and thus to have better control of slag and temperatures in the converter. It also helps in increasing the yield of the liquid steel in the converter.