Analysis on the Material Type, Mode, Method and Economic Benefit of Adding Scrap Steel to Blast Furnace

In recent ten years, the process operation of the blast furnace has changed greatly, especially the promotion of the coal injection process, but there is almost no change in the charge. Although there are some new methods to control the burden distribution, the actual burden is still composed of lump ore, sinter, pellets, coke and a small amount of flux. This article will explore the idea of using scrap steel cut into pieces as furnace charge.

There are many possible advantages of adding scrap blocks to blast furnace burden. As the scrap is a fully reduced metal, it only needs energy to heat and melt into molten iron. Therefore, if scrap is added in the charging process of the blast furnace, the productivity of the blast furnace can be improved and the fuel ratio can be reduced. By adjusting the particle size of the scrap, the porosity of the charge bed, that is, the permeability, can also be adjusted, which can offset the reduction of the porosity of the blast furnace at a high coal injection ratio.

Considering the overall process flow, the blast furnace smelting process is a relatively complete process compared with the electric furnace, so the blast furnace has lower requirements on the composition and grade of scrap steel.

To sum up, we simply divide the blast furnace scrap adding methods into two types: pre iron adding and post iron adding. The pre iron adding method is mainly to add scrap before the blast furnace charging system for smelting, and the post iron adding method is mainly to add scrap at the casting house.

1. Add scrap before iron

Before exploring the way of adding scrap before iron, we should first analyze the resistance of blast furnace smelting to scrap raw materials, the most important of which is the way of distribution in the furnace. Under the preset experimental conditions, for example, it is determined that the temperature at which scrap begins to melt is the eutectic melting point of iron 1147 ℃, the coke ratio is 450kg/tFe, the coal ratio is 57kg/m3, and the average diameter of scrap is about 10mm.

There are three distribution methods: central scrap distribution, uniform scrap distribution and furnace wall scrap distribution. Through a large number of experiments and calculations, it is found that the temperature of the raceway decreases significantly with the increase of scrap ratio under the central scrap distribution state, but the other two scrap charging schemes have only a small impact on the temperature of the raceway. These results can be explained as follows: in order to maintain the temperature of molten iron, when the blast speed is constant, the fuel ratio in unit time decreases, which makes the endothermic direct reduction reaction and the melting loss speed decrease.

The energy previously used for these reactions is now used for the melting of scrap. As the energy required for scrap melting in unit molten iron is less than that required for direct reduction reaction and melting, the productivity of molten iron has been improved in general. For the three distribution methods, after a large number of experiments and data analysis, in the case of central distribution, the shaft part becomes cool due to the need of scrap, resulting in unexpected deterioration of gas distribution in the blast furnace.

For furnace wall distribution, the stability of the blast furnace is not reduced. Because the permeability of scrap steel is less than that of coke but more than that of ore, the permeability will increase, thus increasing the reaction speed. Other conditions of the blast furnace are very similar to the reference period. Therefore, the conclusion is that furnace wall distribution is the best of the three schemes.

The influence of scrap diameter on furnace condition. Three kinds of scrap with diameters of 5mm, 10mm and 25mm are selected. Under the standard state of furnace wall distribution mode, through test and calculation, the output, fuel ratio and furnace top gas efficiency are almost not affected by the scrap diameter. When the scrap diameter decreases, the permeability of the bed near the furnace wall decreases with the reduction of the scrap diameter, and the permeability of the axial bed does not change. This increases the gas flow in the axis area, making the average temperature of the top gas rise. With the increase of scrap diameter, the heat loss of furnace wall will increase and the temperature of molten iron will decrease due to the increase of the permeability of the charge bed. Therefore, considering all kinds of factors, it is generally the most appropriate to select scrap with a particle size of about 10mm in the industry.

In general, the most appropriate distribution scheme is to distribute scrap steel to only more than half of the outside of the blast furnace radius, so that the internal state of the blast furnace has the smallest change compared with the operation without scrap steel, and obviously shows the increase of output and the decrease of fuel ratio. The diameter of scrap also has a little influence on the state of blast furnace, and the most significant changes are gas flow and pressure drop. This is caused by the change of the average permeability of the bed.

Method of adding scrap before iron

Scrap steel before iron is mainly added under the blast furnace trough and sintered. Two kinds of raw materials are mainly used for sintering addition, namely iron oxide scale with low cost and low grade, and high-quality steel sand and steel grain raw materials. The high grade sinter is produced by the sinter machine when the material is mixed and used for the blast furnace. The influence of this addition method on the change of blast furnace condition is almost zero. However, there are restrictions on the supply of raw materials. In addition, due to the haze problem in North China, there is a great pressure on environmental protection, and the sintering process is the primary process for dust control in the steelmaking process.

Another way is to add under the blast furnace trough. The distribution mode and particle size of scrap added under the blast furnace trough have been analyzed and tested in the previous article, and will not be detailed here. The adding position is divided into the feeding belt conveyor tail on both sides of the blast furnace and a small weighing hopper. Among them, the method of feeding at the end of the belt conveyor does not use the weighing of scrap steel. In this way, if it is necessary to weigh, additional belt weighers are needed for measurement, which increases the cost and is more complex.

The small weighing hopper feeding method is relatively simple and easy. One of the many small weighing hoppers under the blast furnace slot is selected to be used for waste steel weighing and feeding. A short waste steel feeding belt is built near the bottom of the slot. The tail of the machine directly feeds into the small weighing hopper. The upper part of the belt head is a vibrating feeder and a waste steel silo. The action of the vibrating feeder is interlocked with the small weighing hopper. After the weight of waste steel in the hopper reaches the set weight, The feeder stops automatically. Scrap steel silo is directly fed by forklift. This method can realize scrap feeding without many changes, and can be integrated into the automatic feeding control process to realize automatic feeding after setting the material batch.

2. Add scrap after iron

The scrap added after iron is mainly divided into hot metal tank and iron trench. Among them, the efficiency of adding scrap steel to the hot metal trench is low, and it is most appropriate to use the steel sand with smaller particle size, which increases the working strength of the furnace workers greatly, so the use of scrap steel is relatively small. Here, we mainly discuss the way of adding scrap steel to hot metal cans.

The biggest problem is that adding scrap to the hot metal tank will cause the temperature of hot metal to drop, and may also cause the bottom of the ladle to settle.

Therefore, it can be concluded that in the absence of a ladle roaster to preheat the scrap inside the hot metal tank, the scrap material should be mainly light and thin materials, and the volume should not exceed 4% of the volume of the hot metal tank. Experience has proved that the inside of the hot metal tank can be filled with light and thin materials, without obvious bottom effect.

It is inevitable that the consumption of molten iron in steelmaking will be slow. Equipment maintenance, failure and other reasons will lead to slow consumption. Long residence time of molten iron ladle will lead to crusting of molten iron ladle, which will cause losses. Therefore, it is meaningful to study the residence time of molten iron ladle. Compare the steel without scrap with the steel with scrap to see if there is any influence on the length of residence time. According to the experience of fault treatment, the maximum residence time of molten iron is generally within 4h, so 4h is selected as the standard when doing research. Without preheating, scrap is light and thin electrical steel, and the percentage of scrap added to the capacity of molten iron ladle is 3%.

The results show that within four hours, with or without scrap, there is little effect on the presence or absence of crusts in molten iron. In fact, molten iron crusting is a problem of molten iron fluidity. There are only two reasons that affect the fluidity, one is the physical heat of molten iron, and the other is the chemical composition of molten iron. If the molten iron is placed in the ladle for too long, it will cause crusting. If it is serious, it will be sent to the cast iron machine, which will affect the output of steel. The data shows that there is no crust, indicating that the crust cycle has not yet arrived.

3. Economic benefit analysis

Taking the 800m3 blast furnace as an example, under the normal production without adding scrap, the daily output is about 3200 tons. After selecting the most direct benefit of adding scrap steel in hot metal tank and under the blast furnace trough, the daily output has soared to 4500 tons, and the daily output of hot metal has increased by 1300 tons.

In the way of adding scrap steel to the hot metal tank, the 800m3 blast furnace will cast iron 18 times a day, with 3 cans each time. Without preheating by using the ladle roaster, 3 tons of light and thin materials will be added to each can, at 1600 yuan per ton. The cost of the original ton of iron is 2250 yuan, which can directly calculate the economic benefits.

Yield increase benefit of adding scrap steel to hot metal ladle every day=18 * 3 * 3 * (2250-1600)=105300 yuan

Under the mode of adding scrap steel under the blast furnace slot, the material type mainly includes punch, iron pin pressing cake with a diameter of 100mm and a thickness of 50mm, and rebar head with a diameter greater than 10mm and a length less than 10cm. The cost per ton is about 2100 yuan based on various material types. The 800m3 blast furnace is charged with 9 batches per hour and 216 batches per day. Each batch is charged with 5.3 tons of scrap steel. The daily increase of the blast furnace is 216 * 5.3=1144.8 tons.

The effect of adding scrap steel under the tank every day=(2250-2100) * 1184=177600 yuan.

The daily efficiency increase of the two methods=105300+177600=282900 yuan, the annual efficiency increase is 8.48 million yuan per month, and the annual efficiency increase is nearly 100 million yuan.

The benefit calculation is only the direct economic benefit of the ironmaking blast furnace smelting process, and does not calculate the energy saving, consumption reduction and efficiency, and the subsequent steelmaking process to increase production and efficiency. Therefore, the benefits of adding scrap to blast furnace for steel enterprises are immeasurable.

4. About scrap processing and production process in steel enterprises

To put it simply, if the steel enterprise sets up its own production line for deep processing of scrap steel, take the processing of steel bar heads as an example. For example, 1700 to 1800/tons of steel bars with a diameter of more than 10 mm are demolished, and 1800 to 2000/tons of construction steel bar waste are processed into steel bar heads with a diameter of less than 10 cm. The acquisition value is about 2300/tons. Each ton of waste steel bar head can save about 150 yuan. The daily direct efficiency increase is 150000 yuan, and the annual efficiency increase is 50 million yuan. In theory, the cost per ton of iron can be reduced to about 2000 yuan.

Since this year, we have been constantly exploring ways and means of adding scrap in the steel smelting process. The various ways summarized in this paper can reduce environmental pollution while increasing the scrap ratio, improve the utilization rate of scrap resources, and bring significant cost reduction benefits.