Description:
Field of Invention
The present invention relates to enhancing slag fluidity in steel plants. Particularly, the present invention relates to an improved method for enhancing fluidity of slag using Al-killed ladle furnace slags (AKLFS).

Background and Prior art of the invention
In the steelmaking process, a substantial amount of slag is generated, posing a significant waste management challenge and contributing to environmental pollution. Recognizing the adverse impact of this waste, there is a growing imperative to explore sustainable solutions, and recycling of these materials emerges as an effective strategy to mitigate these concerns.

Traditionally, fluorspar (CaF2) played a crucial role in enhancing slag fluidity during secondary steel refining. However, due to environmental regulations and issues such as refractory erosions in fluorine-bearing slag, the use of CaF2 has been phased out in many steelworks. Instead, calcium aluminates based synthetic slags have gained popularity, offering improved slag fluidity, enhanced lime dissolution, and desulphurization capabilities.

The steelmaking industry faces constant pressure to deliver high-quality and high-performance steel products. To meet these demands, continuous improvements in the quality and service properties of steel are imperative.

A pivotal aspect of secondary metallurgy is optimizing the slag regime, which can significantly enhance the quality and performance of steel. One effective approach to achieve this optimization is through the utilization of synthetic slags. Tailored to the refining process's specific requirements, synthetic slags are custom-designed compounds aimed at creating an effective refining slag.

The adoption of synthetic slags empowers steelmakers to exercise precise control over the refining process, resulting in superior quality steel products. These slags can be formulated to eliminate impurities, fine-tune the chemical composition of the melt, and regulate the refining process's temperature. Additionally, they contribute to reducing the consumption of costly alloying elements, translating into substantial cost savings for the steelmaking industry. However, synthetic slags, mostly low melting calcium aluminates based, are costly.

The high cost of synthetic slags has prompted a search for alternative materials to serve as flux and enhance slag fluidity. One promising alternative is the use of Al-killed ladle furnace slags (AKLFS) as a substitute for synthetic calcium aluminates. Al-killed ladle furnace slag (AKLFS) closely resembles synthetic calcium aluminates in terms of chemistry and exhibits a low melting point.

Behera, N., Raddadi, A., Ahmad, S., Tewari, N., Zeghaibi, O. (2016). Use of Al-Killed Ladle Furnace Slag in Si-Killed Steel Process to Reduce Lime Consumption, Improve Slag Fluidity. In: Reddy, R.G., Chaubal, P., Pistorius, P.C., Pal, U. (eds) Advances in Molten Slags, Fluxes, and Salts: Proceedings of the 10th International Conference on Molten Slags, Fluxes and Salts 2016. Springer, Cham. https://doi.org/10.1007/978-3-319-48769-4_110 disclose that Al-killed ladle slags coming from its Flat Product Ladle Furnace process could be used in place of Calcium Fluoride/ Bauxite/Calcium Aluminate fluxes for the production of Si- killed steel grades, thus reducing Lime consumption, reducing waste and improving desulphurization levels.

Slags produced during Al-killing in secondary refining have phases closely resembling synthetic slag and can be used as a slag modifier. However, these slags, owing to high basicity, disintegrate into fine powder, making their handling difficult, and in their raw form, they are treated as slag waste.
Accordingly, a challenge associated with the utilization of Al-Killed Ladle Furnace slag (AKLFS) is its tendency to disintegrate into fines upon cooling. Therefore, there is a need to address this issue and harness the benefits of Al-killed Ladle Furnace slag as a slag modifier in silicon-killed steel.

Object of the present invention
It is an object of the invention to develop techniques to consolidate disintegrated Al-Killed Ladle Furnace slags (AKLFS) into larger agglomerates, simplifying handling, facilitating bunker addition, and exploring effective methods to use AKLFS as a slag modifier in secondary refining.

It is also an object of the invention to provide a cost saving and a more sustainable steel production process.

It is a further object of the invention to enhance the quality and performance of steel in secondary refining processes.

It is yet another object of the present invention to reduce calcined lime consumption, leading to cost savings and improved operational efficiency.

Summary of the present invention
In an aspect of the present invention there is provided a method for fluidizing ladle top slag in Si killed/Si-Al killed steel with a slag basicity (CaO/SiO2) in the range of 1.5-3.5, the method comprising adding1.5-2.0 kg/tcs of air-cooled Al-killed ladle furnace slag (AKLFS) after the addition of 6-10 kg/tcs of calcined lime during the tapping ladle operation, resulting in a fluid ladle top slag.

Brief description of accompanying drawings
Figure 1 illustrates a XRD pattern of LF slag collected from the flat product plant’s slag dump yard
Figure 2 illustrates a picture AKLFS slag dump in the flat product plant
Figure 3 illustrates Fluid ladle top slag after AKLFS addition
Figure 4 illustrates Pellets made out of the -0.5 mm fraction of AKLFS
Figure 5 illustrates Briquettes made from LF slag powder

Detailed description of invention
The present invention addresses the drawbacks associated with traditional slag modifiers such as fluorspar and synthetic slags, introducing Al-Killed Ladle Furnace Slag (AKLFS) as a promising substitute.

In Silicon-killed steels, primary deoxidation relies on Silicon-based ferro-alloys such as Silico-Manganese and Ferro-silicon, lowering oxygen levels to 20-50 ppm. Silicon-killed secondary refining slags often exhibit a thick and crusty nature, requiring modifiers like Fluorspar (Calcium Fluoride) or synthetic Calcium Aluminate to fluidize the slag and aid in refining. For Al-killed steels, aluminum serves as the major deoxidant, reducing oxygen levels to less than 5 ppm. This results in a slag with a significant amount of fused Calcium Aluminate phase.

An alternative flux material, with a composition similar to synthetic slags, is generated as slag waste from Al-killed deoxidation practice during secondary refining. The Al-killed ladle furnace slags have a similar low melting point and mineralogical phases of calcium aluminate-based synthetic slags. Since this material is non-corrosive and environmentally less detrimental and is treated as slag waste, it can be used as a replacement for calcium aluminate-based synthetic slag.

The di-calcium-silicate (2CaO·SiO2 or C2S) phase, formed due to excess lime additions during secondary steelmaking, undergoes a phase transition during cooling from β-C2S (monoclinic) to γ-C2S (orthorhombic) accompanied by a volume increase of around 12%. This causes the disintegration of the slag into a fine powder.

The present inventors have overcome the challenge tied to the utilization of Al-Killed LF slag pertaining to its tendency to disintegrate into fines upon cooling, by further refinement in its implementation.

A long products steel making plant specializes in Si-killing deoxidation practice. However, the generated slag often adheres to the ladle bottom, complicating slag dumping. To address this, the plant adds calcined lime and calcined bauxite during tapping to enhance slag basicity (CaO/SiO2 ratio) to over 1.5, improving fluidity and desulfurization efficiency.

Another steel plant making flat products, utilizes Al-killing of steel in its two steel melting shops. The AKLFS generated during secondary refining has high basicity (Basicity > 7) and alumina content (Al2O3 > 30%). This AKLFS can serve as an effective slag modifier for the long product plant or any plant using Si-killing of steel. Adding AKLFS improves basicity, fluidity, and reduces ladle bottom sticking during dumping.

The most dominant mineralogical phases in Al-Killed LF slag from flat product plant are gehlenite (2CaO·Al2O3 ·SiO2, Ca2Al2SiO7), di-calcium silicates of larnite (β-2CaO·SiO2, β-Ca2SiO4), and shannonite (γ-2CaO·SiO2, γ-Ca2SiO4), and tricalcium aluminate (3CaO·Al2O3, Ca3Al2O6, and C3A).

The present invention is illustrated with specific examples from steel plants such as Durgapur Steel Plant (DSP) and Bokaro Steel Plant (BSL). The use of Al-killed ladle slag as a slag modifier at DSP, a Si-killing deoxidation practice facility, holds the potential to reduce calcined lime consumption, leading to cost savings and improved operational efficiency. Similarly, at BSL, the AKLFS generated during secondary refining is suggested as an effective slag modifier for facilities using Si-killing of steel, providing benefits in terms of basicity, fluidity, and reduced ladle bottom sticking during dumping.

In conclusion, the invention offers a practical solution to the challenges associated with the disintegration of Al-killed ladle slag, making it a valuable slag modifier in silicon-killed steel. The potential benefits include cost savings, environmental friendliness, and enhanced operational efficiency, making it a significant contribution to the steelmaking industry's ongoing quest for sustainable and high-performance solutions.

The study found that Al-killed Ladle Furnace Slag (AKLFS) can effectively replace synthetic flux or calcined bauxite in refining silicon-killed steel. Adding AKLFS improved the fluidity of ladle top slag without negatively affecting desulphurisation. The slag's composition also led to easier ladle bottom cleaning, reducing failures in subsequent heats. The developed process can be adopted by any plant making Si/Si-Al killed varieties of steel to get a fluid slag for efficient secondary metallurgical operations.

The agglomeration techniques developed for making pellets and briquettes from the fines of Al-killed ladle furnace slag can also be adopted easily to make aggregates from the fines of AKLFS for the sustainability of the process.

The present invention thus provides a method for fluidizing ladle top slag in Si-Killed/Si-Al Killed steel without the addition of traditional fluxes like fluorspar, calcined bauxite, and calcium aluminate-based synthetic slag. The method employs waste AKLFS, eliminating the need for costly fluxes and synthetic slags, providing a cost-effective and sustainable approach.

In an embodiment, there is provided a method for fluidizing ladle top slag in Si killed/Si-Al killed steel with a slag basicity (CaO/SiO2) in the range of 1.5-3.5, the method comprising adding 1.5-2.0 kg/tcs of air-cooled Al-killed ladle furnace slag (AKLFS) after the addition of 6-10 kg/tcs of calcined lime during the tapping ladle operation, resulting in a fluid ladle top slag.

As used herein, 1.5-2.0 kg/tcs of air-cooled Al-killed ladle furnace slag refers to the amount (kg) of AKLFS added per tonne of crude steel(tcs).

The AKLFS added in the method is selected from briquetted AKLFS, and pelletized AKLFS.

In an embodiment, when the AKLFS added in the method is briquetted AKLFS the method further comprises briquetting of AKLFS fines before addition during the tapping ladle operation comprising the steps of:
a. dry mixing of the AKLFS fines followed by wet mixing of the fines by adding water up to 10% by weight to form nodule shaped briquettes of 15 mm sizes;
b. pressing the nodule-shaped briquettes of 15mm size in the briquetting machine;
c. curing and drying the briquettes obtained in step (b) to obtain the briquetted AKLFS.

The briquetted AKLFS is formed from AKLFS fines with a particle size ranging from 0.5 mm to 5.0 mm.

In an embodiment, when the AKLFS added in the method is pelletized AKLFS the method further comprises pelletizing of AKLFS fines before addition during the tapping ladle operation comprising the steps of:
a. mixing the AKLFS fines with water; and
b. pelletizing the mix in a pelletizer to obtain pellets with a size of 10-15mm.

The pelletized AKLFS is formed from AKLFS fines with a particle size of less than 0.5 mm.

The invention will now be illustrated with a non-limiting example.

Example
Plant trials were conducted at the Si-killed secondary refining facility to test the effectiveness of adding AKLFS (Al-killed slag from a flat-products plant) to the secondary refining process. The AKLFS generated at the flat product plant is rich in pre-fused Calcium-Aluminate phases and has a low melting point.

Figure 1, which is the XRD pattern of AKLFS sample from the flat product plant, too reveals that the slag primarily consists of metal oxides, silicates, and aluminate forms. The most dominant mineralogical phases detected in the slag sample are gehlenite (2CaO·Al2O3 ·SiO2, Ca2Al2SiO7), di-calcium silicates of larnite (β-2CaO·SiO2, β-Ca2SiO4), and shannonite (γ-2CaO·SiO2 , γ-Ca2 SiO 4 ), and tricalcium aluminate (3CaO·Al 2 O3 , Ca3Al2O6 , and C3A). The chemical composition, phase diagram analysis, dominant mineralogical phases present all suggest that the AKLFS from the flat product plant can be used as a synthetic flux for refining of Si-killed steels.

The LF slag dump at the flat product steel plant contained a significant amount of disintegrated LF slag and a small number of slag lumps. Nine tons of as-dumped AKLFS, without any screening and containing fine powders, were packed into 20kg bags and transported to the steel melting shop of the long product plant (Figure 2).

In the trial heats, steel deoxidation and alloying additions were carried out as per grade requirements during steel tapping from the BOF to the ladle. All additions were made following established practices, except for the omission of calcined bauxite addition. After tapping, a prescribed amount of AKLFS was manually added to the top of the liquid steel in the ladle by dropping packed bags. The liquid steel was then vigorously gas-stirred to mix the material and transported to the LF for subsequent refining. The objective was to determine if the AKLFS material could effectively replace calcined bauxite. AKLFS was added in a number of heats over a period wherein the tap sulfur in the liquid steel ranged from 0.052 to 0.027%. During the trial heats, the addition of AKLFS ranged from 160 to 240 kg. After each heat, the slag formed was visually inspected for fluidity. The addition of 160 kg of AKLFS resulted in inadequate fluidity, while the addition of 240 kg created very thin slag, which caused difficulties during arcing and necessitated more lime addition in secondary refining. To achieve good slag consistency, the AKLFS addition amount was adjusted to 200 kg. During trial heats, it was observed that slags with approximately 200 kg of AKLFS added to them were fluid in nature. During secondary refining in the trial heats, the slag was readily mixed with the added flux and created a smooth and fluid slag cover (Figure 3). All the slag was very easily dumped off after the completion of casting. A thin layer of slag glaze coating formed on the sides of the ladles lining, giving it a protective coating. In normal heats, without AKLFS addition, the slag that forms is thick, viscous, and sticky. After casting, during slag dumping, this sticky slag remains attached to the bottom and is challenging to separate easily.

Manual addition is a costly and labor-intensive process, and the AKLFS contains a significant amount of fine powder. If the material can be agglomerated into larger sizes that do not disintegrate easily, it can be added during tapping through bunkers, which would be more economically viable.

Pelletization
Pelletization technology was developed for AKLFS fines. Pellets of various sizes were made (Figure 4) from the -0.5 mm-sized fraction of AKLFS by adding water to the fine powder and pelletizing the mix in a pelletizer. The free lime present in AKLFS provided binding properties, making the material self-binding in the presence of water. These pellets, with a size of 10-15mm, have a crushing strength varying between 40-60 kg.

Briquetting
Additionally, a briquetting method was developed for other size fractions of AKLFS. The +3mm, -3mm, and -0.5mm screened fractions for a batch weight of 10kg were first dry-mixed in a mixer in different proportions. Gradually, water up to 10% by weight was added during the mixer operation for wet mixing. The free lime present in the material provided the binding properties. Nodule-shaped briquettes of 15mm size were pressed in the briquetting machine. The green briquettes were then allowed to cure and dry in the shade in open air. Once the briquettes were cured and dry, they were tested for crushing strength. The cured briquettes had a crushing strength varying between 80-100 kg. Figure 5 shows images of 15mm briquettes made from the powder.

The results of the plant trials and agglomeration studies indicate that the Al-killed Ladle Furnace Slag (AKLFS) can be used as a substitute for synthetic flux or calcined bauxite addition in the secondary refining of silicon-killed steel. The addition of AKLFS did not have any significant adverse impact on the desulphurization efficiency in ladle refining. On the contrary, it made the slag more fluid, ensuring a good ladle top slag cover. Agglomeration techniques were developed by hydrating the AKLFS mixture and then pelletizing or briquetting the mix.

Implementing AKLFS at the long product plant has the potential to reduce calcined lime consumption, replacing calcined bauxite, leading to cost savings. Additionally, AKLFS enhances desulfurization efficiency, a critical aspect of steelmaking, resulting in increased operational efficiency and cost savings for Si-killing deoxidation practice. Therefore, it is a viable option for slag modification at any long product steel making facilities.

The invention addresses a crucial issue in the steelmaking industry, particularly in secondary refining processes, where the effective utilization of slag plays a pivotal role in enhancing the quality and performance of steel. The background of the invention highlights the challenges associated with the traditional use of fluorspar and synthetic slags, prompting the search for alternative materials.

Moreover, the invention aligns with the steel industry's constant pursuit of high-quality and high-performance steel products. The optimization of the slag regime, achieved through the use of synthetic slags or innovative substitutes like Al-killed ladle slag, empowers steelmakers to exercise precise control over the refining process. This results in superior quality steel products, reduction of impurities, fine-tuning of chemical composition, and regulation of temperature during refining.
, Claims:
1. A method for fluidizing ladle top slag in Si killed/Si-Al killed steel with a slag basicity (CaO/SiO2) in the range of 1.5-3.5, the method comprising adding1.5-2.0 kg/tcs of air-cooled Al-killed ladle furnace slag (AKLFS) after the addition of 6-10 kg/tcs of calcined lime during the tapping ladle operation, resulting in a fluid ladle top slag.

2. The method as claimed in claim 1, wherein the AKLFS is selected from briquetted AKLFS, and pelletized AKLFS.

3. The method as claimed in claim 1, wherein the method further comprises briquetting of AKLFS fines before addition during the tapping ladle operation comprising the steps of:
a. dry mixing of the AKLFS fines followed by wet mixing of the fines by adding water up to 10% by weight to form nodule shaped briquettes of 15 mm sizes;
b. pressing the nodule-shaped briquettes of 15mm size in the briquetting machine;
c. curing and drying the briquettes obtained in step (b) to obtain the briquetted AKLFS.

4. The method as claimed in claim 3, wherein the briquetted AKLFS is formed from AKLFS fines with a particle size ranging from 0.5 mm to 5.0 mm.

5. The method as claimed in claim 1, wherein the method further comprises pelletizing of AKLFS fines before addition during the tapping ladle operation comprising the steps of:
a. mixing the AKLFS fines with water; and

b. pelletizing the mix in a pelletizer to obtain pellets with a size of 10-15mm.

6. The method as claimed in claim 5, wherein the pelletized AKLFS is formed from AKLFS fines with a particle size of less than 0.5 mm.