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Application practice of magnesite in converter steelmaking

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Table of Contents

In the steelmaking process, the cost of auxiliary materials used in the converter is one of the structural costs of the steelmaking process. In order to reduce the cost of the steelmaking process, an attempt is made to use magnesite instead of light-burned dolomite for slag making, which can achieve the normal furnace protection effect while reducing the cost of converter steelmaking auxiliary materials. This article mainly studies and summarizes the practical process of adding magnesite directly to the converter for slag making, and draws relevant conclusions.

Keywords: magnesite; converter; steelmaking cost

In order to further reduce the cost of the steelmaking process, the steel industry is exploring effective ways to reduce costs and increase efficiency. Directly adding magnesite to the converter to make slag is a feasible method to reduce costs and increase efficiency. This article analyzes and discusses the conclusions drawn from our company’s practice of directly adding magnesite to the converter for slag making.

1 Principle of slagging reaction when magnesite is directly added to converter

Magnesite is my country’s dominant mineral resource and one of the important export mineral products. Its molecular formula is MgCO3. It belongs to the trigonal crystal system and is a carbonate mineral of magnesium. Its theoretical composition is MgO accounting for 47.81% and CO2 accounting for 52.19%. It is a manufacturing The main raw material of alkaline magnesia refractory materials. Magnesite begins to decompose into magnesium oxide and carbon dioxide when heated above 640°C. When calcined at 100°C to 1000°C, the carbon dioxide does not completely escape. The product is a powdery substance that becomes light burnt magnesium, which has high chemical activity. When calcined at 1400℃~1800℃, carbon dioxide completely escapes. Due to the occurrence of recrystallization and sintering, magnesium oxide forms a dense block of periclase, which is called dead burned magnesium. This dead burned magnesium has high refractoriness. The decomposition temperature of magnesite provides favorable conditions for adding to the converter. When the converter temperature reaches 1250°C, magnesite can decompose quickly after adding magnesite, and the decomposition product is light burned magnesium, which has high chemical activity and can quickly form slag. The main reaction principles of magnesite thermal decomposition:

MgCO3=MgO+CO2

The decomposition reaction of magnesite is an endothermic reaction. Adding magnesite will reduce the temperature of the molten pool. According to the decomposition temperature of magnesite and the slag-making mechanism of converter steelmaking, magnesite should be added in the early stage of blowing. On the one hand, it can quickly decompose magnesite to generate MgO, increase the MgO content in the slag, reduce the melting temperature and initial flow temperature of the slag, slow down the formation of a dense C2S shell on the surface of lime, increase the melting speed of lime, and facilitate early slag melting. On the other hand, the CO2 gas generated by the decomposition of magnesite increases the stirring of the molten pool and promotes rapid slag formation.

2 Analysis of MgO content using magnesite slag

2.1 Chemical and physical indicators of magnesite

Table 1 Chemical and physical indicators of magnesite

Test itemsMgO%Loss on ignition%SiO₂%CaO%   Fe₂O₃%Al₂O₃%Particle size mmMoisture %
Index quantity45.3550.271.771.231.280.1020-50≤1

2.2 Test data

See Table 2.

Furnace number Amount of auxiliary materials addedConverter slag sample (%)Converter slag sample (ton)Theoretical MgO content/%
Active lime (kg) Limestone (kg)Magnesite (kg) )Dolomite (kg) Sinter (kg)CaOSiO2MgOR
8401810370028002000  54.0719.337.212.649.289.77
8401812400030002300 60048.5121.087.782.5012.978.04
8401814430045002000 100049.6820.458.282.4210.608.56
8401815490040002000 80043.5021.069.562.3516.955.35
8401816480040002100 50046.2820.269.642.2811.148.55
8401818500037002000 80047.6820.769.652.294.4520.40
840183642001300200050060048.8617.229.532.728.3310.88
8401837420020001500 150049.4518.999.972.548.138.37
8401838420020001500 210049.2620.118.992.468.228.28
8401839420020001500 250048.9220.6110.382.3710.356.57

2.3 Test conclusion

According to theoretical calculations, if 2000kg magnesite is added to each furnace of steel, the final slag MgO can reach 8.7%. From the actual MgO content of the slag, the average is 9.0%, which meets the process requirements.

2.4 Magnesite cooling effect

Through analysis and calculation of test data: the temperature drop is 39.34℃ for every 1000Kg of magnesite added.

2.5 Analysis of slag alkalinity

The average slag R of the test heat is 2.51, and the slag R of the normal heat is 2.8. The difference in slag R mainly reflects the CaO content of light-burned dolomite, dolomite and magnesite. The CaO content of light-burned dolomite is 52%, the CaO content of dolomite is 30%, and the CaO content of magnesite is 1.23%.

To achieve normal slag R, using 2000Kg of magnesite requires adding lime:

=2.14*0.29*0.0065*130*1000/0.89/1.66=354Kg.

2.6 Analysis of the operation process of adding magnesite to the converter

(1) There is no obvious change in the converting process when magnesite is added to the converter. There is no abnormal splashing or slag overflow in the converter, which is the same as in normal furnaces.

(2) Observing the state of the slag at the end of the converter shows that the fluidity of the slag is good.

(3) Converter furnace type and furnace conditions during operation: The furnace director confirmed the furnace conditions during blowing with magnesite converter to the end point and tapping process, and no abnormality occurred.

3 Summary of test results

(1) Use magnesite to replace light-burned dolomite and dolomite to form slag and protect the furnace. The MgO content in the slag is theoretically calculated by adding the amount and compared with the actual final slag test MgO. Theoretical MgO: 8.7%, actual MgO: 9.0%. achieve experimental results.

(2) During the converter smelting process, no abnormality occurred in the process operation after adding magnesite, and the test results were achieved.

(3) After using magnesite, the converter furnace director confirmed the converter furnace conditions and furnace type, and no obvious changes in furnace conditions were found.

(4) When adding 1000Kg of magnesite during the smelting process, 177kg of white ash will be required.

(5) This test has achieved the expected test results. Since the dosage is small, it is recommended to expand the test in the future and use it rationally with other materials.

4 Application effect analysis

During the test, magnesite was used to completely replace light-burned dolomite and dolomite, and there was no significant change in the converter lining. Through experimental data, it is tested that magnesite can be used to completely replace light-burned dolomite and dolomite for slagging. The comprehensive economic benefits are as follows.

The analysis cost is as follows based on the MgO content of converter slag reaching 8%.

4.1 Use 1000kg of light burnt dolomite, 2000kg of dolomite and 400kg of magnesium balls to make slag.

Unit price including tax: light burnt dolomite 441.12 yuan/t, dolomite 135.59 yuan/t, magnesium ball 1050 yuan/t, magnesite 580 yuan/t.

Cost: (1000*441.12/1000+2000*135.59/1000+400*1050/1000)/130=8.7 yuan/t.

4.2 Use 1700kg magnesite to make slag

Cost: 1700*580/1000*130=7.58 yuan/t.

Conclusion: Using magnesite for slag making alone can reduce the cost by 8.7-7.58=1.12 yuan/t.

Due to the limitation of scrap steel resources, thermal balance cannot be achieved by adjusting the structure of scrap steel entering the furnace. Therefore, using magnesite to completely replace light-burned dolomite, dolomite and magnesium ball slag making does not meet the on-site production process, and some dolomite needs to be used to balance heat.

Therefore, under the current conditions, light-burned dolomite, dolomite, and magnesite are used together. The magnesite consumption after equivalent conversion of magnesite and magnesite balls is: 400*1080/580=744kg.

Using 700kg magnesite, 2000kg dolomite and 500kg light-burned dolomite will meet the requirement of 8% MgO content in converter slag.

Cost of slagging using light burnt dolomite, dolomite and magnesite:

(700*580/100 0+2000*135.59/1000+500*441.12/1000)/130=6.9 yuan/t. Compared with the use of light-burned dolomite, dolomite and magnesium balls, the slagging cost is reduced by 8.7-6.9=1.8 yuan/t.

5 Conclusion

In the converter smelting process, it is feasible to directly add magnesite to the converter to slag and reduce the cost of the steelmaking process.

From the experience summarized in the production test, it can be seen that adding magnesite to the converter for slag making can not only reduce costs, but also use magnesite to completely replace dolomite for slag making on the premise that scrap steel resources meet the requirements.

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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