Synthetic Slag for Secondary Steelmaking
Synthetic Slag for Secondary Steelmaking
Synthetic slag consists of prepared mixture of several individual oxides which is used during secondary steelmaking to assist the steel treatment in the ladle from the viewpoint of effective refinement. Synthetic slag practice is normally used to obtain clean steels and also for the desulphurization of the liquid steel.
Secondary steelmaking is a critical quality control step between the primary steelmaking and the continuous casting of the liquid steel. A key feature for success with the secondary steelmaking processes is the slag control. Use of synthetic slag which is specifically designed to have the required chemical composition and physical properties helps in the slag control.
The desirable properties of the synthetic slag include (i) slag is to have high sulphide capacity, (ii) it is to be basic in nature, (iii) it is to be fluid to obtain faster reaction rates, and (iv) it is not to cause excessive refractory wear.
The secondary steelmaking slag is in liquid form in the ladle and floats on the surface of liquid steel which is usually at temperature of 1,600 deg or higher. It acts like a sponge to absorb the impurities consisting mainly of sulphur and non-metallic inclusions. The design of the slag is a critical step impacting the efficiency of the steel refining processes during the secondary steelmaking.
Slag regime in secondary steelmaking significantly influences the final quality of the produced steel, particularly with respect to the achieved desulphurization of steel. One of the possibilities for influencing the slag regime is the application of synthetic slags to the ladle slag, formed from slag-making additions during the liquid steel tapping.
Synthetic slag practice during secondary steelmaking maximizes the efficiency of the steel refining process by (i) improving steel quality, (ii) improving productivity, (iii) reducing costs, and (iv) protecting the environment (since it removes the need of addition of fluorspar also called fluorite). The other objectives of use of the synthetic slag during secondary steelmaking are as follows.
- To cover the liquid steel with an insulating layer to reduce heat losses which otherwise takes place due to heat radiation.
- To remove the possibility of reoxidation of steel from atmospheric oxygen.
- To absorb inclusions present in the liquid steel.
- To desulphurize liquid steel. Using synthetic slag of desired basicity and sulphide capacity, deoxidized steel can be desulphurized to as low as 0.005 % of sulphur.
- The use of the synthetic slag practice is to be attractive especially when there is low capital cost of equipment associated with the application of this practice.
Synthetic slag is used during the secondary steelmaking because of the following.
- Synthetic slag greatly influences deoxidation and desulphurization of the liquid steel.
- It greatly shortens the steelmaking time, efficiently remove impurities and improve the steel quality.
- It is a required additive for production of special steels such as bearing quality steel.
- It reduces dust float pollution effectively.
- It lowers melting temperature of slag.
- It helps in increasing the slag fluidity.
- It helps in absorbing inclusions and impurities, thus producing cleaner steel.
- Synthetic slag is stable and nearly dust-free and hence easy to handle and there is no pollution during its handling.
The basic task of the synthetic slags, which are normally added to ladle slags, is to increase the quality of liquid steel by forming an active slag for an improvement of the kinetic conditions in the ladle for the refining processes. Added synthetic slags influence the properties of the ladle slag not only with its chemical and phase compositions, but also with the manner of their preparation and the granularity of the used raw materials. The aim of applying synthetic slags is to create a fluid, sufficiently basic, and liquid slag with a low melting temperature, which contributes to an acceleration of the physical and chemical processes at the slag-metal interface, thus influencing the efficiency of the secondary steelmaking.
Synthetic slag modifies the composition of ladle slag and physically, thermally and chemically protects the liquid steel during ladle refinement and casting. Synthetic slag products are specifically formulated to specific conditions existing in a steel melting shop for appropriate desulphurization and non-metallic inclusion absorption without adversely affecting ladle refractory.
Treating the liquid steel with synthetic slag in the ladle is an efficient and relatively cheap method of reducing the non-metal inclusion content by reducing the sulphur and oxygen content. The principle of the process of using synthetic slag consists in making a contact on a large surface between the liquid steel and a slag having a composition selected to ensure an advanced steel deoxidation and desulphurization.
Design of synthetic slag
Slag design in secondary metallurgy is crucial for the performance in steelmaking. It guarantees the quality and cleanliness of the steel and the consistency of its performance. It is critical since it impacts the efficiency of the steel refining process during the secondary steelmaking. The slag is to have necessary physical and chemical properties. It is to have low melting temperature, low viscosity of liquid slag, and optimum fluidity.
Main components of synthetic slag are CaO, Al2O3 and a small amount of SiO2. Synthetic slag having these components is also known as calcium aluminate (CA) flux.
When the ladles are lined with magnesia carbon or dolomite refractories then MgO forms an important component of the synthetic slag. This synthetic slag is also called calcium magnesium aluminate (CMA) flux. CMA slag allows a quick formation of a homogeneous and liquid slag with a high capacity to absorb sulphur and oxide inclusions from the steel bath, very similar to CA slag. CMA synthetic slag brings the MgO-content in the slag, right after tapping, close to the saturation concentration and minimizes the dissolution of magnesia carbon or dolomite refractories in the slag.
Earlier CaF2 was also used to be a component of the synthetic slag since fluorite was added during the production of synthetic slag. CaF2 helps in increasing the slag fluidity as well as sulphide capacity of slag but it attacks the refractories and has environmental issues because of formation of gaseous compound SiF4 due to interaction of CaF2 with SiO2 in the slag. Use of fluorite in the preparation of synthetic slag is generally no more done these days.
Sometimes aluminum (Al) is added in the synthetic slag to deoxidize the liquid steel since transfer of sulphur from liuid steel to slag is followed by transfer of oxygen from slag to steel. Therefore deoxidation of steel is essential for efficient desulphurization.
The synthetic slag is to be designed for each application. The CaO and Al2O3 contents in calcium aluminates is varied in large ranges mainly to suit local conditions of secondary steelmaking in the steel melting shop of the plant. Synthetic slag has CaO content usually in the ranges from 25 % to over 55 %, Al2O3 content in the ranges from 30 % to over 55 %. The slag is normally low in SiO2, Fe2O3, and TiO2. MgO percentage in CMA synthetic slag ranges from 3 % to 6 %. In case aluminum is added in synthetic slag then usually itis in the range of 5 % to 16 %.
The melting point of the slag is generally in the range of 1350 deg C to 1400 deg C. Bulk density of the synthetic slag usually ranges from 2.85 tons/cum to 3.0 tons/cum. Product size of the synthetic slag is to facilitate its quick melting in the ladle as well its easy handling. Size distribution is normally tailored as per requirement. Usually synthetic slags are supplied in three size ranges consisting of 1 mm to 50 mm, 3 mm to 30 mm, and 10 mm to 100 mm.
Generally synthetic slag is basic in nature. However, special synthetic slag can be designed for a specific purpose. For removal of oxide inclusions, a neutral slag with CaO/SiO2=1 or 1.2 can be used especially when no desulphurization is needed.
Calcium aluminate system
Synthetic slag basically consists of calcium aluminates which are the salts of aluminic acid and calcium hydroxide. The composition of various minerals is obtained by heating calcium oxide (lime) and aluminum oxide (bauxite) together at high temperatures (around 1600 deg C). The phase diagram of calcium aluminate system is given at Fig 1.
Fig 1 Phase diagram of calcium aluminate system
The phase diagram of the CaO-Al2O3 system shows that there are several calcium aluminates appear in the system out of which few are stable. Calcium aluminate for synthetic slag is made of select raw materials of calcium and aluminum, blended at a proper proportion, ground to powder, pelletized after churning, sintered or melted in kiln. The mineral composition of calcium aluminate for synthetic slag is to be CA, CA2, C3A and C12A7. The phase diagram shows that there are several calcium aluminates appear in the system out of which few are stable at atmospheric pressure under an atmosphere of normal humidity. The stable phases shown in the phase diagram (formed at atmospheric pressure under an atmosphere of normal humidity) are as follows.
- Tricalcium aluminate, 3CaO·Al2O3 (C3A) – It is the most basic of the calcium aluminates. Melting point is 1,542 deg C and density of 3.064 g/cc.
- Dodeca calcium hepta-aluminate, 12CaO·7Al2O3 (C12A7) (Mayenite) -Its composition and properties have been the subject of much debate, because of variations in composition that can arise during its high-temperature formation. Melting point is 1400 deg C and density of 2.68 g/cc.
- Monocalcium aluminate, CaO·Al2O3 (CA) – It is formed when the appropriate proportions of calcium carbonate and aluminum oxide are heated together until the mixture melts. It melts incongruently at 1390 deg C. Density is 2.945 g/cc.
- Monocalcium di aluminate, CaO·2Al2O3 (CA2).
- Monocalcium hexa-aluminate, CaO·6Al2O3 (CA6).
In the steel industry during secondary steelmaking, a number of synthetic slags based on Al2O3 are used which are produced from natural or secondary raw materials in different forms. However, the use of synthetic slags is influenced by the choice of manufacturing process being used.
Based on the manufacturing process, there are two types of calcium aluminates which are used. These are sintered calcium aluminate and fused calcium aluminate. The first type is sintered in rotary kiln and the second type is fused in electric arc furnace. Sintered calcium aluminate is produced by sintering of high purity alumina and low silica limestone. Fused calcium aluminate is produced by melting of high grade bauxite and low silica limestone and then solidifying and crushing the molten material. In both the process of manufacturing selection of raw materials is an important aspect. The objective during manufacture of calcium aluminate flux is to have the mineral composition of CA, CA2, C3A and C12A7. Since bauxite is a material which is mined, it is always associated with certain impurities and hence the composition wise, fused calcium aluminate flux is inferior to the sintered calcium aluminate flux provided pure alumina is used for sintering.
The specific consumption of calcium aluminate varies from 6 kg/ton to 12 kg/ton depending on the process condition, steel’s initial and final chemistry, and composition of the calcium aluminate flux. It is advisable to add 1/3rd to 1/2 at the ladle bottom and balance on the top for better performance of the calcium aluminate flux.
The advantages associated with the use of secondary slag during the secondary steelmaking include the following.
- Protection of the ladle and tundish refractories.
- Savings of energy at the ladle furnaces.
- Consistent chemistry of the secondary steelmaking slag.
- Faster steel refining times during the secondary steelmaking.
- Sulphur and inclusion removal from the steel making the steel cleaner.
- Greater yields during the continuous casting through improved castability.
- A good cover slag for the prevention of gaseous pick-up from the atmosphere.
- An effective fluid sink that absorbs inclusions from steel.
- Synthetic slag generally contains no fluorine compounds, thus avoids refractory attack and atmospheric pollution.
Synthetic slag practice during secondary steelmaking is quite simple and not much capital investment is needed for it. Certain issues are associated with this practice are given below.
- Desulphurization need may vary from one heat to other heat, if slag carry- over from primary steelmaking furnace is not controlled. Oxygen content of steel from primary steelmaking furnace may also vary. If these two aspects are not controlled then consistent results from synthetic slag practice may not be achieved.
- CaO is one of the major components of the synthetic slag. It is hygroscopic and leads to hydrogen pick up.
- Argon rinsing is necessary with the use of synthetic slag to stir the bath.
- The slag attacks the ladle refractory. In case there is Ca F2 in the synthetic slag then the refractory wear is higher.
- Heat is necessary for melting the synthetic slag.