ENHANCEMENT OF LIFE OF BLAST FURNACE RUNNER THROUGH DESIGN OF RUNNER CASTABLE

Dr. A. K. Samanta, Anindya Roy, Dr. S. Adak & Dr. A. K.Chattopadhyay

Yoshihiro Fujitake*, Yasuhiro Eguchi* & Yukihiro Suekawa* TRL Krosaki Refractories Limited, Belpahar – 768 218, Odisha (INDIA) *Krosaki Harima Corporation, Yahatanishiku, Kitakyusyu City 806-8586

(JAPAN) 1.0 Abstract With the recent development of iron making technology, the productivity of blast furnace has increased phenomenally. In order to deal with the increased productivity of blast furnace, the increase in cast-house availability also has become a necessity. In recent days a lot of developments have been made in the trough castable to achieve higher campaign through-put with minimum down time. The major factors determining the campaign life of main trough are: i) Corrosion at the slag line ii) Erosion and corrosion at the metal line and iii) Mechanical abrasion caused by the stream at the impact area. A detail study has been done on the erosion and corrosion mechanism inside the trough and a suitable castable has been developed to enhance the campaign through-put to almost double. The present paper deals with the development of new generation thixotropic trough castable with the design of advanced recipe by strengthening the matrix phase. The different physico-chemical and thermo-mechanical properties of the castables have been evaluated. The mechanism of better erosion and corrosion resistance of the castable have been studied in the laboratory as well as the same is monitored during the real campaign. The suitable application procedure to ensure enhancement of trough campaign life have also been studied. 2.0 Introduction: 2.1 Difficulties to Achieve Higher Performance The primary aim for any blast furnace is to minimize the loss in production by reducing the down-time. One of the attributing factors for higher down-time of furnace is the frequent maintenance of trough refractory working lining. The frequent maintenance is basically the effect of high corrosion and erosion of working lining castable. The corrosion of working lining castable occurs mainly due to the chemical reaction which happens at the slag-castable interface.

The excessive corrosion (chemical) and erosion (mechanical) of the trough working lining can be controlled to a great extent with the strengthening of the castable matrix and fluid motion control, respectively. 2.2 Design Aspects of Main Trough based on Blast Furnace Operation Parameters Main trough contributes more that 50% of the total specific consumption value in any trough management. The corrosion/erosion in the main trough is caused by the turbulent mix of metal and slag. The rate of corrosion and/or erosion of the trough becomes higher with reduced residence time of slag and metal in the main trough (Fig.1). A minimum residence time of 7 minutes is required to have a complete separation of hot metal and slag.

Fig.1: Hot Metal Residence Time vs. Castable

Wear Rate Considering the hot metal discharge rate of 4.5THM/min and hot metal/slag minimum separation time of 7mins, the main trough should have a hot metal holding capacity of 32THM. Considering the specific gravity of hot metal as 6.9T/m3, the minimum volume of hot metal that should reside inside the main trough before Skimmer Block is 4.64m3.

Fig.2: Overview of Main Trough

Considering a length of 14m up to Skimmer Block of main trough (Fig.2 & Fig.3); the cross sectional average area (A-A’) up to the slag-metal interface should be 0.33m2.

Fig.3: Cross-Section of Main Trough at A-A’

Mathematical Calculations: ⇒ 0.33*106 = ½*(X+Y)*375mm ⇒ ½*(X+Y) = 880mm Considering the vertical gradient as 10⁰; ⇒ tanФ = tan10⁰ = 0.1763 ⇒ Y = X + 2*375* tan10⁰ ⇒ ½*(X+X+2*375* tan10⁰) = 880mm ⇒ X = (1760-132)/2 ≈ 815mm (bottom width) ⇒ Z = X + 2*965* tan10⁰ ⇒ Z = 815 + 340 ≈ 1150mm (top width) After determining the inner profile of the main trough, it is very much important to design the refractory lining ensuring high degree of safety. Hot metal freezes around 1150⁰C. Even after sufficient erosion of the working lining, if the 1150⁰C isotherm line (freeze plane) can be placed within the working lining itself then, the chances of trough failures can be minimized. In a forced cooled trough, if the maximum amount of heat can be extracted from the refractory lining, the freeze plane shift towards the hot face. This phenomenon has been demonstrated in the below graphs (Fig.4(a) to

Fig.4(d)) where two different lining patterns (Fig.5 and Fig.6) are considered.

Fig. 4. Thermal Profile Calculations in (a) Case-1: New Lining; (b) Case-1: 200mm Erosion; (c)

Case-2: New Lining & (d) Case-2: 200mm Erosion

(a)

(b)

(c)

(d)

Fig.5: Very High Thermal Conductive Lining

Fig.6: Insulation Lining in the Back-up

To substantiate the safe running of main trough, temperature at the back-up lining can be measured with the help of thermo-couples. Copper plates are placed in the back-up lining (Fig.7) throughout the length of main trough.

Fig.7: Temperature Measuring Arrangement of

Main Trough Back-up The tip of the K-type thermo-couples remain in contact with the copper plates and temperature against individual thermo-couple is displayed. These temperatures actually help to understand the residual thickness of working lining castable. 2.3 Design Aspect of Castable for Different Zones/Area of the Trough

The corrosion/erosion happens because of the following reasons:

Fig.8: Corrosion and Erosion Mechanism of Trough Castable

(i) Chemical reaction at the slag-castable interface: The FeO present in the slag reacts with the alumino-silicate fine particles present in the castable matrix and develop eutectic compound which get dissolved in the slag. (ii) Mechanical abrasion caused by liquids – Once the peripheral area becomes weak, the bigger grains get exposed and finally get dislodged from their position by the impact caused by liquid flow and been transported by the liquids (metal/slag). Table-1 describes the properties of trough castable with respect to specific areas of trough. Table 1: Requirement of Properties of Castable based on Specific Area of Applications

In order to achieve enhanced trough life, the working lining castable should have: a) Very good slag corrosion resistance; b) Excellent abrasion resistance; and c) Very good spalling resistance. Silicon Carbide has got all the desired properties but its best usage with respect to different area in a cast-house runner has been substantiated (Fig.9) by the help of following tests:

Copper Plate

SS Pipe

Brazing

a) Rotary Slag Corrosion test at 1600OC with BF Slag.

b) High Frequency Induction Furnace Corrosion Test at 1500OC for 30mins for 15 cycles with BF slag and pig iron.

Fig.9: Effect of Vaiable % of SiC on Wear Index of Castable

Based on the test results, different castables bearing different percentage of SiC have been developed for different application area as shown in the Table-2. Table-2: Development of Different Castables for

Blast Furnace Trough

2.4 Enhancement of Trough Performance with Improved Materials and Design The enhancement of the performance of the tough depends on number of factors including operational parameters, materials, maintenance practice, workmanship during installation etc. The improved performances of trough in three integrated Steel Plants in India are shown in the Fig.10(a) to Fig.10(c). In all cases, improved campaign life made a positive impact on the trough management with lower specific consumption of Refractories.

Fig.10(a): Performance in Plant-A with Criticality of Embedded Trough

Fig.10(b): Performance in Plant-B with Criticality of Bend Trough

Fig.10(c): Performance in Plant-C with Criticality of Slag Rate ≥ 500kg/THM

3.0 Value Addition from TRL Krosaki through Modern Installation Techniques TRL Krosaki is well equipped with state-of-the-art instruments for trough break out, maintenance and installation with expertise of dedicated service teams. In order to reduce the installation time, the following application gadgets are used by TRL Krosaki as shown in the Fig.11 (a) to Fig.11(c).

Fig.11(a): Photographs of Trough Breakers

Fig.11(b): Photographs of Mixer Machine and Casting Trolley

Fig.11(c): Photographs of Pumping and Gunning Machines

4.0 Conclusions Improved and consistent life of the trough is very important for uninterrupted operation of Blast Furnace with constant estimated production of hot metal. TRL Krosaki has developed new and innovative products for working lining of different areas of trough including back up lining with improved campaign life for different integrated steel plants in India. The newly developed materials show lower specific consumption of Refractories for the full campaign resulting a value proposition to the steel plants. The design capability of the trough including different state-of-the-art equipment and strong back up of Technical Services teams make TRL Krosaki a recognized player for Trough Management. 5.0 References

1) Lee W.E. & Chan C.F., Microstructures and Slag Resistance of Al2O3-SiO2-SiC, UNITECR 1997, pp 1593-1602.

2) Kanatani S., Improvement of Blast Furnace Trough Metal Line Castable, UNITECR 1997, pp 152-155.

3) Camelli S., Analysis of Wear Mechanism of Al2O3-SiC-C Blast Furnace Runner, UNITECR 1997, pp 156-159.

4) Golestani-Fard F., Ebrahim N. & Saberi A., Effect of Amount of SiC on the Physical, Mechanical and Corrosion Behaviour of Al2O3-SiC-C Castable, Proceedings of the International Colloquium on Refractories, 2006, Aachen, Germany, pp 143-146.