The development of KR hot metal desulfurization process, the principle of desulfurization and its research

Molten iron pretreatment has become a modern steelmaking production process: molten iron pretreatment – double blowing converter – refining outside the furnace – full continuous casting and hot charging and hot feeding. Nowadays, users have increasingly stringent requirements for steel quality, generally requiring The sulfur content is controlled below 0.015%, and some even require it to reach the “double zero” ultra-low sulfur level. In addition, taking into account the goals of reducing the smelting tasks of the converter and reducing converter consumption, the tasks of each smelting equipment are more unified and specialized. To give full play to their respective expertise, in recent years, new domestic converter steel plants have been equipped with hot metal desulfurization equipment, and old plants have been transformed and equipped with desulfurization equipment. As a mainstream desulfurization process, the mixing method
It is well used in many domestic steel plants.

1 Mixing desulfurization process Introduction

1.1 Development of mixing desulfurization process in China

The KR mixing method is a hot metal outside-furnace desulfurization technology used in industrial production by Nippon Steel Guangzhou Steel Works in 1965 [1]. As early as 1976, Wuhan Iron and Steel No. 2 Steelmaking Co., Ltd. introduced the first domestic machine from Nippon Steel Co., Ltd. mix
Method desulfurization equipment, with a single tank processing capacity of 70-80t, and a processing cycle of about 85 minutes. CaC2-based was selected as the desulfurizer. Because the consumption indicators and operating costs of this set of equipment were high at that time, and the processing cycle was long, it was not used domestically. It has been widely promoted. With the passage of time, the mixing desulfurization process has developed into a mature and stable desulfurization process after nearly two decades of development. The consumption indicators, operating costs and processing cycles have been greatly reduced. In 2000, Wuhan Iron and Steel Co., Ltd. On the basis of digesting the first set of mixing desulfurization technology, steelmaking cooperated with the former Wuhan Iron and Steel Research and Planning Institute to independently design and create the second set of mixing desulfurization equipment. In 2001, Baosteel Group No. 1 Steel Co., Ltd. introduced two sets of mixing desulfurization equipment from Japan’s Kawasaki Heavy Industries 150t mixing desulfurization equipment. In 2002, the former Wuhan Iron and Steel Research and Planning Institute built two sets of 55t mixing desulfurization equipment in Kunming Iron and Steel Co., Ltd. In 2003, the former Shanghai Metallurgical Planning and Research Institute built two sets of 40t mixing desulfurization equipment in the Third Steel Plant of Baosteel Group. Desulfurization equipment. In 2007, two sets of 200t mixing desulfurization equipment were built in Wuhan Iron and Steel Co., Ltd.’s No. 2 Steelmaking Co., Ltd., and two sets of 300t mixing desulfurization equipment were built in Maanshan Iron and Steel Co., Ltd. No. 4 Steelmaking Co., Ltd. Shaogang Steel Co., Ltd. is building two sets of 130t mixing and desulfurization equipment in the No.1 Steelmaking Project. 300t, 200t, 150t, 130t, 80t, 55t, 40t large, medium and small series of mixing desulfurization.

1. 2 Principle of mixing desulfurization process

The so-called mixing desulfurization process is to insert a cross-shaped mixing head of cast refractory materials and baked into a molten iron tank with a certain amount of molten iron to rotate, so that the molten iron forms a vortex, and then the weighed desulfurization agent is oscillated to the Material (or rotary feeder) is added to the rotating molten iron. After the desulfurizer enters the molten iron tank, it is quickly drawn into the molten iron by the vortex. During the continuous stirring process, it fully reacts with the sulfur in the molten iron, thereby desulfurizing. Affects desulfurization There are two main factors for speed, one is the type of desulfurizer, and the other is the kinetic conditions. Research has proved that the influence of kinetic conditions is greater than the influence of the type of desulfurizer. The mixing speed is as high as 120r/min, and the molten iron is fully rotated, achieving excellent metallurgical results. Under the dynamic conditions, the input desulfurizer can fully react, so the desulfurization efficiency is as high as more than 95%.
The current mixing desulfurization process uses lime as the desulfurization agent, and then adds a small amount of fluorite and aluminum slag as flux. When the silicon content in the molten iron is above 0.05!, the desulfurization reaction is:
The CO gas generated by the reaction stirs the molten iron and accelerates the desulfurization reaction. Since the silicon content in the blast furnace molten iron is generally greater than 0.05%, the desulfurization reaction is formula (1). In the reaction formula (1) ), the Ca 2 SiO 4 layer generated in the layer surrounds the lime particles. This layer has a tight texture and a high melting point, which prevents the sulfur in the molten iron from dispersing to depth through it, slowing down the desulfurization speed, and the generated fine layer surrounds the lime particles. The newly added lime increases the consumption of lime, so fluxes such as fluorite are added to the desulfurizer to generate low-melting-point substances, so that the sulfur in the molten iron further reacts with the lime, which can increase the desulfurization efficiency by about 20%[2] . Since lowering the oxygen potential can improve desulfurization efficiency, some steel plants add aluminum slag to the molten iron to reduce the oxygen potential through aluminum deoxidation[3].

1. 3 Advantages and disadvantages of mixing desulfurization process

1. 3. 1 Advantages of mixing desulfurization post processing 

1) High and stable desulfurization efficiency

Due to its excellent kinetic conditions and reproducibility, the mixing method desulfurization process has high and stable desulfurization efficiency and less sulfur return. A domestic factory used the mixing method to treat 8 furnaces of molten iron in one shift, and the 7 furnaces reached 0. 001%. , one furnace is 0. 002%. However, it is difficult to achieve this level using the injection method of lime plus magnesium powder, and the sulfur return situation is serious [4].

2) Desulfurizer

The mixing method uses lime-based desulfurizer, and no special measures are required for transportation and storage. The magnesium powder used in the magnesium-based injection desulfurization process needs to be passivated, and protective measures are required for transportation and storage.

3) Operating costs

Whether it is the injection process or the mixing process, the main operating costs are desulfurizer and refractory materials. After years of improvement, the service life of the mixing head of the mixing equipment has been greatly improved. Now it is generally more than 250 furnaces, and in Wuhan Iron and Steel, it is as high as more than 500 furnaces. The service life of the injection method is generally more than 60 furnaces; the mixing equipment uses lime-based desulfurizer, which has a wide range of sources and low price, while the magnesium-based desulfurizer has a high price and is greatly affected by market fluctuations. Through the domestic Analysis of the production data of a certain factory shows that when the final sulfur of the molten iron is ≤ 0. 005%, the operating cost of the mixing method is lower than that of the injection method. When the final sulfur of the molten iron is > 0. 005%, the operating cost of the injection method is lower than that of the mixing method.

1. 3. 2 Defects of the mixing desulfurization process

1) The equipment is larger and takes up more area.
2) The one-time investment is large.
3) The temperature drop of molten iron is large.
4) The iron loss is large.
5) The processing cycle is long.

1. 4 Factors affecting the desulfurization efficiency of the mixing method

The main factors affecting the desulfurization effect of the mixing method are as follows.

1) Before mixing and desulfurization, there should not be too much slag on the molten iron surface, otherwise it will affect the full response of the desulfurizer. Therefore, it is necessary to remove slag before mixing and desulfurization, and it is appropriate to remove 70% of the slag. Perhaps the mature slag fishing technology should be used. The new slag fishing equipment from Yantai, Shandong was selected for the Shaoguan Iron and Steel KR desulfurization equipment.
2) The rotation speed of the mixing paddle cannot be too low, otherwise good dynamic conditions will not be achieved and the desulfurization efficiency will decrease. The normal rotation speed during general mixing operations is 100 ~ 120 r/s. With the wear of the mixing head, the mixing paddle can be increased appropriately. speed to ensure excellent dynamic conditions.
3) The desulfurizer must be a powder to increase the reaction area and fully contact the sulfur in the molten iron with the lime. If the desulfurizer particles are too large, the desulfurizer cannot fully react and the unit consumption is increased, which directly affects the desulfurization effect. General Desulfurizer is required <3 mm.
4) The main component of the desulfurizer is lime, so the quality of the lime has a great influence on the desulfurization effect, mainly the CaO content in the lime, the activity of the lime and the sulfur content in the lime.
5) The penetration depth of the mixing paddle must be appropriate. If the penetration depth is too deep or too shallow, it will directly affect the desulfurization effect. If it is too shallow, there will be serious splashing during mixing, and the stirring effect of the molten iron in the lower part of the molten iron tank will be poor; if it is too deep, the upper part will The molten iron is poorly stirred.

2 Comparison between mixing method and injection method

2.1 Comparison of desulfurization processes

A comparison of the two desulfurization processes is shown in Table 1.

2. 2 Comparison of desulfurization operation cost budgets

A comparison of desulfurization operation cost budgets is shown in Table 2.

2.3 Analysis and comments on two desulfurization methods
Through a comprehensive comparison of the desulfurization effects and operating costs of the two desulfurization processes, it can be seen that the mixing method has outstanding advantages in deep desulfurization and total cost. For large and medium-sized steel manufacturers, from a long-term perspective and combined with production practice, the KR mixing method for hot metal pre-desulfurization should be A choice with far-reaching value.

3 Conclusion

As an efficient and low-cost desulfurization process, the mixing desulfurization process has been widely promoted at home and abroad, and a series of products ranging from small to large have been formed in China. Although the one-time investment of the mixing desulfurization equipment is relatively large, the desulfurization effect is good , low operating cost, quick recovery of investment. Therefore, the mixing method desulfurization will become a mainstream desulfurization process in the future, be more widely promoted, and have a trend of evolving to the three desulfurization treatment process.