agglomeration of ferroalloy fines for use in bulk steelmaking processes

Vol 7 No 3 APRIL 2013

Gokarn et al Agglomeration of Ferro Alloy Fines for Use in Bulk Steel Making Process of 10



Agglomeration of Ferro Alloy Fines for Use in Bulk Steel Making Process

Authors Prabhash Gokarn*, Veerendra Singh, A Kumar, B D Nanda & A Bhattcharjee, Tata Steel Ltd., India.

(*corresponding author - [email protected])

Abstract Ferroalloys are added as deoxidizing agents and additives to increase strength, elasticity and abrasion & corrosion resistance of steel. The preferred size of ferroalloy lumps for steel making is 10mm – 80 mm to optimize the operational efficiency. Ferroalloy lumps are produced by manual breaking of casted alloy cakes which generates 5-10% fines which cannot be used in bulk steel making process (like the commonly used LD process) because of handing and operational difficulties. Therefore, we at Tata Steel developed an agglomeration process for ferroalloy fines and used the briquettes thus produced for making steel. The developed process described in the paper is an economic, environment friendly and efficient way to utilize the ferroalloy fines in steel making.

1. Introduction

Ferroalloys are used in Steel Making as deoxidizing agents and additives to increase mechanical properties (strength, toughness, wear resistance, springiness), high temperature properties(creep strength, hardness), electrical properties or corrosion resistance.

Most bulk ferroalloys - like FeMn, SiMn, FeSi and FeCr manufactured by carbo-thermic reduction of ores in submerged arc furnaces. Noble ferroalloys like FeMo, FeV, FeTi etc. manufactured through the Alumino-Thermic process. In both cases, the ferroalloy is produced in form of liquid metal.

The liquid metal is cast into cakes and crushed into ~10mm to ~60mm size lumps, with co-generation of fines during sizing.

The fines generated during the sizing of metal cake cannot be used in the bulk steel making processes like the BOF(LD) process, as these fines get oxidized quickly and this reduces the overall recovery during steel making [1-2]. Though, ferroalloy fines in the size range of 3 to 20mm have better dissolution characteristics, the higher surface area (due to small size) also transports undesirable gases and moisture into the furnace. Small alloy size also increases dust losses and leads to handling difficulties [3-6].

Agglomeration into lumps is the best method to utilize these fines. Binder composition and physical strength of the agglomerate are two main constraints to develop a cost effective method. Various attempts have been made in the past to agglomerate these fines using conventional binders like molasses, tar, resin, etc [6-10], which failed due to a variety of reasons and could not be adopted commercially.

A briquetting process has been developed in this study to utilize ferroalloy fines of manganese alloys (ferro-manganese and silico-manganese) in the steel making process. The briquettes produced by the patented process developed was tested in the laboratory as well as in commercially in the LD shops of Tata Steel. 2. Lab Scale Studies 2.1. Characterization of Fines: Samples of ferroalloy fines were collected from Ferro Alloy Plants being operated by Tata Steel (viz FeMn at Joda, FeCr at Cuttack and SiMn under tolling at Durgapur). These were classified into three different size ranges (>10mm; -10+3mm; -3mm). Five important constituents (Mn/Cr, Si C, S, and P) were analyzed using ICP-OES (Spectro-Analytical Instruments; Ciros) to find the chemical composition of the prepared agglomerate. Particle shape



and surface characteristics were also analyzed using scanning by electron microscope to study the agglomeration behavior of fines. 2.2 Briquetting of Fines: Selection of binder for alloy fines determines the strength of briquettes and thus is most important. The binders should not add any unwanted ingredient like sulphur, phosphorus, nitrogen etc. in the steel, and it should be cost effective. Molasses and other conventional organic binders were rejected because these binders contain sulphur and phosphorus. Sodium silicate, Bentonite, Acrylic resins and Phenolic resins were tried as binders and tested, and the results are given in Table-1. The experimental work plan is described in Table 1 and Fig. 1. Ferroalloy fines were mixed thoroughly with the binder in a muller mixer. Sixty to seventy grams of the mixture was compacted in a cylindrical die of diameter 3cm at different loads and the green compact was cured at different temperatures (100˚ and 150˚ C) for one hour. Briquette density, compressive strength, tumbling index, abrasion index, shatter index and dissolution characteristics were studied.

Binder % Load (ton) Curing Condition

Sodium Silicate 5, 7.5 & 10 1 & 5 100 C, 1 hour

Sodium Silicate+ Bentonite 5+2, 7.5+2 & 10+2 1 100 C, 1 hour

Acrylic Resin 5, 8 & 10 1 & 3 100 C, 1 hour

Phenol formaldehyde Resin 5, 8 & 10 1 & 5 100 & 150 C, 1 hour

Table-1: Briquetting conditions

Figure-1: Process Methodology for Binder Selection 2.3. Smelting of Briquettes: Twenty kilograms of steel scrap was melted in a 25 kg induction furnace and 5 kg of ferro manganese (FeMn) lumps were added. Experiments were repeated for FeMn fines and FeMn briquettes under the same test conditions for comparison. The mixing behavior of the materials was observed. Slag and metal samples were collected and the manganese recovery was calculated. Figure-2 shows the lab scale setup to test the dissolution behavior of lumps, fines and briquettes. Similar trials were conducted for SiMn and FeCr fines, for reasons of space and clarity trials with FeMn fines have been described in detail in this paper.

Sample Preparation

(0-3mm) FeMn fine)

Mixing

Pressing

(1-5ton)

Curing

(100 & 150° C, 60 minutes)

Compressive strength Test

Binder



(a) (b) (c) Figure-2: Lab Scale trials in Induction furnace (a) Induction furnace (25kg) (b) Melting of scrap (c) Sample

collection before and after the addition 3. Results and Discussions 3.1 Characterization Studies: Chemical analysis of various size fractions is given in Table-2. Despite the slightly lower percentage of silicon (Si) and manganese (Mn) in fines compared to lumps, fines are suitable for use in steel making. Size analysis of the samples of ferro manganese fines (0-10mm) was carried out and it was found that ~70 % fines are of 0 to 3mm size (fines) and 30 % are of 3 to 10mm size (chips). Particle size and shape analysis is shown in Figure-3 and 4. Finer particle sizes are preferred for briquetting, but presence of significant amount of very angular particles makes the agglomeration process more challenging. Very angular particles enhance the mechanical interlocking but require high pressure compaction.

Size Range % C Mn S P Si

>10mm 93 6 >68 0.01 0.193 0.54

-10, +3mm 2 6.75 66.30 0.01 0.175 1.72

<3mm' 5 6.7 65.90 0.01 0.188 1.33

Table-2: Size and Size wise chemical analysis of Ferromanganese fines

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

20

40

60

80

100

Comm.Pass

%, P

asse

d

Particle Size (mm) Figure-3: Particle Size Analysis



Figure-4: Particle Shape Analysis

SEM analysis shown in Figure 5 reveals that these fines are not oxidized. Some small slag inclusions were also seen in the briquetted samples.

Figure-5: SEM micrograph of Lumps (Pt1-High carbon Phase, Pt2-Low Carbon Phase) and Briquettes (Pt1-High carbon Phase, Pt2- Slag particle)

Pt-1

Pt-2

Pt-1

Pt-2



3.2 Briquetting Studies: Metallic fines show a different binding behavior compared to conventional ore particles. Figure 6 shows surface of manganese ore and ferro manganese metal particles. The ore particles usually contain small cracks and cleavages which play important role in binder absorption and binding of the particles. Figure-6: Surface Roughness of Mn ore and FeMn Metal Particle Three different combination of sodium silicate were tried and it was found that the prepared agglomerate does not attain the suitable compressive strength and it varies between 90 and 240 kgf/sample. The strength achieved by machine compaction was 700-1150 kgf/sample. The strength of the briquettes is not suitable for handling and presence of alkalis and silicon are a concern which prevents its use in the steel making process. Acrylic resins and phenol based resins were then used and it was found that acrylic resins produce an agglomerate of strength of 650-1050 kgf/sample and 720 to 1100 kgf/sample at 1 ton and 3 ton loads, respectively. Thermosetting resin produces the best agglomerate with minimum compressive strength of 1050 kgf/sample. Agglomerate strength varies between 1600 and 2000 kgf by machine compaction with a 15 MPa load. This binder produces good strength with manual compaction also and strength varies between 1050 to 1440 kgf/ sample for 5 and 10 % binder content, respectively. A comparative analysis of maximum cold compressive strength achieved using different binders is given in Figure-7 and it shows that phenolic resin based agglomerate achieves maximum strength. Handling properties of these briquettes were tested and shown in Table 3 for the briquettes produced with the most suitable binder combination. The physical characteristics of briquettes are acceptable to existing LD steel making process.

Properties Briquette

Size & Shape Diameter : 30mm, L : 20mm

Apparent Density 5200 kg/m3

Compressive Strength 55Mpa

Tensile Strength (Load Applied in radial direction) 15Mpa

Tumbler Index (Wt 15kg, rpm 200@25) 95% (>6.3mm)

Abrasion Index (Wt: 15kg, rpm 200@25) 3%( <0.5mm)

Shatter Index (Wt : 10 kg, No of Drops : 4, Height : 2m) 98%(<5mm)

Table-3: Properties of briquettes

(a) Mn Ore Particle (b) FeMn Particle



Figure-7: Maximum Cold Compressive Strength of Briquettes Achieved using Different Binders

3.3 Smelting Studies: Initially these briquettes were tested in laboratory and subsequently larger trials (0.5, 10 & 100 ton) were conducted at the plant. Mixing and other operational performance parameters were observed during the lab scale induction furnace operations. It was observed that fines do not mix properly in the liquid steel but get trapped in the foam on top of the liquid steel. It also generates a significant amount of slag. The slag generation was lowest for lumps and highest for fines. Mn recovery was lowest for the fines but it was similar for lumps and briquettes. A comparison is given in Figure 8. Mn recovery was also observed for different types of briquettes tested for tumbling test. The best recovery was observed for the briquettes of 30mm diameter and 20mm thickness (Weight: 65gm) and same were used for the plant trial.



Figure-8: Comparative Analysis of Mn Recovery from Lumps, Briquettes and Fines

4. Product Validation First phase plant trials were carried out using 500 kg of FeMn briquettes. The Plant adds 150 to 600 kg of ferro manganese in ladles of heat size of 155 tons to produce different grades of steel. 200 kg and 300 kg ferro manganese briquettes were added in two heats. It was found that the Mn recovery was 5 to 10% higher when using briquettes (over lumps) compensating the lower Mn content of fines. The improved dissolution characteristic is the likely reason for improved Mn recovery. Nitrogen level did not show any unexpected variation (and was within ~13ppm). In second phase of plant trials, 10 ton of ferro manganese briquettes were prepared and added manually in 20 different heats of different grades of steel in varied quantities. These trials too were found satisfactory and in further trials 100 tons of FeMn briquettes were filled in the working chute and added through the actual plant feeding system. These results, presented in figure 9, confirm the results of the previous trials.

After successful implementation at the plant scale, a vendor was identified and developed for supply of 200 tpm of ferro manganese briquettes. Later after successful lab and plant scale trials with briquetting of silico-manganese fines, the capacity at the vendor was increased and briquetting of silico manganese fines for use at the LD Shops(BOF Steelmaking) was commercially undertaken. Lab scale trials have also successfully been completed using FeCr fines.



Figure-9: Slag-Metal Analysis During Plant Trials

5. Conclusions Ferroalloy fines cannot be used in bulk steel making processes like BOF(LD) as the small size increases losses, reduces recovery and can act as carrier for moisture and gasses. High quality briquettes can be produced by mixing the resin binder, compaction and by curing at 150˚ C temperature. The process flow sheet developed for briquetting is shown in Figure-10. The developed product was tested in the lab and commercialized after successful plant trials.

Apart from financial benefits of using ferroalloy fines, use of briquettes is environment friendly and it can significantly reduce the amount of metallic dust and fines generated during handling and use of ferroalloy fines in smaller furnaces.

The method of briquetting developed by Tata Steel for bulk ferroalloys (FeMn, SiMn and FeCr) can be extended to noble ferro alloy fines and fines of manganese metal which will further reduce costs of steel making and increase competitiveness.

Figure-10: Process flow Sheet to Agglomerate the FeMn Fines

5. Acknowledgements The authors express their sincere thanks to Dr. D. Bhattacharjee, Director, RD&T, TATA Steel, Mr. Rajeev Singhal, EIC, FAMD and Mr. Debashis Das Chief LD#1, Tata Steel for their keen interest and guidance during the development of the process and its commercialization.

Metal Analysis Slag Analysis



6. References [1] K.D. Peaslee, D.S. Webber, S. Lekakh and B. Randall: 58th SFSA Technical and Operating

Conference, 4.1, (2005). [2] P.G. Sismanis and S. A. Argyropoulos: Proc. of the 69th Steelmaking Conf., 69(1986), 315. [3] Y. Lee, H Berg, B. Jensen, and J. Sandberg: Iron and Steel Society, 54(1996), 237. [4] M. Tanaka, M. Mazumdar and R.I. L. Guthrie: Metall. Trans. B, 24B, 4, (1993), 639. [5] H. Berg, H. Laux,, S. T. Johansen and O. S. Klevan: Ironmaking Steelmaking, 26,2, (1999),127. [6] V Singh, S M Rao, B D Nanda and D Srinivas: International Patent Application No. 2009-

PCT/IN2009/000532. [7] Vance, L Calbert. : United States Patent 1946-2405278. [8] Saunders, R. Earle, Pope, L. Richard: United States Patent 1960- 2935397. [9] L. Robert and Ranke: United States Patent 1975-3898076. [10] J.P.Beukes, J. Nell and S. D McCullough: South Africa Patent: 2001-4091. [11] A Ramu, P K Banerjee and B Roy Choudhury: Unpublished Report, R&D Tata Steel, India,

R&D-INV-011-96-1-13-97(1997). 7. Abbreviations

Mn Manganese Cr Chrome Fe Ferro / Iron Si Silicon C Carbon P Phosphorus kg Kilograms mm Millimeters kgf Kilogram-force

BOF/LD Basic Oxygen Furnace Processes like LD

agglomeration of ferroalloy fines for use in bulk steelmaking processes

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