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Development of ramming material for ferroalloy acidic slag electric furnace bottom

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Table of Contents

The development process of ferroalloy acidic slag electric furnace bottom ramming material is introduced. Tests have shown that replacing the furnace bottom with carbon bricks can facilitate masonry construction, repair the furnace bottom, increase the life of the furnace bottom, and reduce the unit consumption of refractory materials.

Keywords: ferroalloy; Al₂O₃-C-SiC series furnace bottom; ramming material

Preface

The ferroalloy electric furnace lining is subject to high-temperature heat load, as well as erosion and erosion of the charge, high-temperature furnace gas, molten iron, and slag liquid, resulting in a short service life of the furnace lining and high refractory material consumption. The selection of refractory materials for the furnace lining and the methods used to repair and build the furnace can reduce consumption and shorten the time for repairing and building the furnace. Improving the service life of the furnace lining is an important issue in ferroalloy production.

It is used to develop furnace bottom ramming materials for acidic slag iron alloy electric furnaces instead of carbon bricks to build the furnace bottom. The overall unit consumption of refractory materials is better than that of carbon bricks. Moreover, the construction of this kind of furnace bottom material can use tools such as cranes, packaging, and vibrating rammers to carry out cold repairs or hot repairs on the furnace bottom. It is easy to operate and highly efficient. It can greatly reduce the furnace repair time and thereby increase ferroalloy production.

Ramming material material

To withstand the harsh conditions of high temperature and acidic slag, furnace bottom ramming materials should use acidic or neutral refractory materials. The preferred materials are carbon materials, corundum, silicon carbide, etc. At present, the MgO-CaO-Fe₂O₃ series electric furnace bottom ramming materials produced by Haicheng Huayu Group and other units have been widely used in ferroalloy and electric furnace steelmaking plants at home and abroad. This kind of alkaline bottom ramming material has produced great economic and social benefits in the production of alkaline slag steelmaking and alkaline slag smelting ferroalloys (such as chromium series alloys). However, the linings of acidic slag electric furnaces for smelting manganese-silicon alloys and high-carbon ferromanganese-less flux methods are still mainly constructed with carbon bricks. The process of building the furnace bottom with carbon bricks has the following shortcomings: (1) The production process of carbon bricks is complex and the cost is high; (2) Workers use carbon bricks to build the furnace bottom with high labor intensity, low efficiency and poor operating environment. (3) The repairability of the carbon brick furnace bottom is poor, especially when the furnace bottom temperature is high, hot repair is more difficult.

Carbon materials

Carbon materials refer to solid raw materials mainly composed of graphite. Graphite has high thermal conductivity (840~2100W/m·℃) and low thermal expansion coefficient (1.4×10-6/℃ at 1000℃). Very high melting point and chemical stability. Graphite is inert in non-oxidizing media. Except for strong acids and strongly oxidizing media, graphite is not corroded by other acids, alkalis, and salts, and does not react with any organic compounds. It is difficult to infiltrate molten metals and metal oxides and has good corrosion resistance; graphite (melting point 3500°C) is also a high-temperature resistant material and has no eutectic relationship with Al₂O₃, SiC, SiO₂, etc. The strength of general materials gradually decreases at high temperatures, while graphite doubles in strength above 2000°C and has good thermal shock resistance. But the weaknesses of carbon are poor oxidation resistance and low low-temperature strength.

Corundum

Because ionic bonds dominate the corundum crystal structure. Therefore, it has high hardness, high strength (Mohs hardness level 9), high melting point (2050°C), medium thermal expansion coefficient, good thermal conductivity, stable chemical properties, and strong corrosion resistance to Na₂CO₃ and molten iron. Therefore, corundum refractory products (i.e. high-temperature ceramics) have become excellent refractory products with high temperature resistance, corrosion resistance, high strength and other properties.

Silicon carbide

Silicon carbide is resistant to high temperatures, has a high melting point (>2200℃), and has high strength at low and high temperatures (the elastic modulus at 25℃ is 4.76×10-⁶kg/cm², and at 1500℃, the elastic modulus is 2.8×10kg/cm²). High wear resistance (hardness value up to 2500kg/cm²), small thermal expansion coefficient (25~1000℃, 5.0×10-6/℃). Thermal conductivity is high (38456J/m·h·℃ at 1000℃), so it has good thermal shock resistance. The contact angle between SiC and slag is greater than 90°, it hardly reacts with low-alkalinity slag, and will not be wetted by slag. It can prevent slag penetration and inhibit reaction with slag, so it has strong corrosion resistance. SiC remains stable in strong reducing atmosphere up to 2454°C. The temperature at which SiC and C can coexist is as high as (2545±40)°C, indicating that SiC-C refractory materials are very stable refractory materials at high temperatures.

Ingredients for furnace bottom ramming material

In order to resist the erosion of the furnace bottom by acidic slag and molten iron, it is planned to develop a high-carbon binder using corundum, graphite and silicon carbide as the main raw materials. This high-carbon binder enables the furnace bottom material to be formed using corundum and silicon carbide as aggregates and graphite as the matrix. The furnace bottom material can be dry stored, transported and constructed with the help of cranes, vibrating rammers and other tools. After the furnace bottom construction is completed, smelting can be carried out without baking. The adhesion of high carbon binder is used to evenly surround the graphite on the surface of corundum and silicon carbide particles. Utilizing the thermoplasticity of high-carbon binders, materials can form structures with higher density under the action of external forces. As the temperature increases, the high-carbon binder will produce bonding, bridging, condensation and coking, turning the material system into a composite of carbon bonding and ceramic bonding.

Tests

According to the above design, the proportion screening test process was carried out in the laboratory as follows:

Ingredients

Special grade bauxite clinker Al₂O₃≥88%, Fe₂O₃≤1.5%, K₂O+Na₂O≤0.4%; volume density ≥3.1g/cm³; water absorption ≤4%; particle composition is 8~1 mm, adding amount 50%~60 %.

Silicon carbide SiC≥90%; particle composition is 3~0mm, adding amount is 15%~25%.

Fused corundum Al₂O₃≥98%; particle size ≤0.074mm, adding amount 10%~20%.

Flake graphite C≥97%, adding amount 12%~14%.

Additives include 10% to 15% of binding agents, antioxidants, expansion agents, explosion-proof agents, etc.

Physical and chemical properties of test products (see Table 1)

Table 1 Physical and chemical properties of test products

Chemical composition/% Volume density/(g/cm³)Compressive strength/MPaApparent porosity/%Linear change rate
Al₂O₃CSiC160 ℃×16 h1550×3 h
56.0~63.314~18.712~13.42.35~2.456~98~10-1.05~-1.24

Slag resistance test

Use this product to make a crucible, add manganese-silicon alloy acidic slag, and conduct a slag resistance test in a high-temperature furnace. When the furnace temperature reaches 1500°C, keep it warm for 3 hours. Then cool to room temperature, cut the crucible open, and observe the slag invasion of the crucible. It can be seen from the cut section that the interface between the slag and the crucible is very obvious, indicating that there is basically no reaction between the slag and the crucible, that is, the slag has minimal corrosion on the crucible, which proves that the furnace bottom material is feasible for acidic slag furnace lining. See Figure 1 for details.

Figure 1 Micrographs of samples 1° and 2° in the slag resistance test

It can be seen from Figure 1 that under the high temperature load of the furnace bottom ramming material sample, the molten slag and the ramming material basically have no reaction. There is a clear boundary between the two, and the slag erosion layer and transition layer are not obvious. It shows that the ramming material has strong resistance to slag erosion.

Conclusion

1. It is feasible to use carbon-bonded Al₂O₃-C-SiC furnace bottom material as acidic slag for smelting electric furnace bottoms and can be promoted and applied.

2. The overall cost of Al₂O₃-C-SiC furnace bottom ramming material is lower than that of carbon bricks. The furnace repair construction is simple, the operating environment is good, the construction efficiency is high, and the furnace shutdown and furnace repair time are reduced. The repairability is good, which creates conditions for increasing the life of the furnace bottom and reducing the unit consumption of refractory materials.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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