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Ironmaking – What is the slag iron stagnation layer in blast furnace hearth?

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The decisive factor for the first-generation blast furnace operation is the hearth bottom. In the design of the blast furnace body, the bottom of the furnace is generally made of multi-layer refractory materials, from top to bottom are clay bricks (which are eroded and washed away in a short time after the furnace is opened), ceramic pads (ceramic materials, low thermal conductivity, resistant to scouring), large carbon bricks (microporous carbon bricks, ultra-microporous carbon bricks, graphite/semi-graphite carbon bricks, etc.), and the lower part of the furnace bottom is the bottom cooling water pipe. It is rare for modern blast furnace bottoms to burn through or to be seriously eroded, but hearth burn-through and “high-risk” conditions occur frequently. What is the reason? Today, let’s discuss a key to the erosion of blast furnace hearth – slag-iron stagnation layer.

blast furnace refractory

I. Overview

The slag-iron stagnation layer is located on the inner surface of the hearth bottom and has a certain thickness and flow rate of solid-liquid mixture.

Under the normal production condition of blast furnace, a layer of slag iron shell with self-protection effect is formed at the bottom of the hearth, and the form of the slag iron shell is solid, and a semi-liquid and semi-solid material form is still formed between the slag iron and the liquid slag. Compared with liquid slag iron, the slag iron stagnation layer has high viscosity, low flow rate, low thermal conductivity (large thermal resistance), and thin thickness (from the actual anatomy of the hearth bottom of the blast furnace, it is generally between 50-100mm, and the The flow rate, viscosity and tapping of slag and iron in the hearth are all related). The existence of slag-iron stagnation layer is the key to realize the self-protection of blast furnace hearth bottom.

2 The discovery of the anatomy of the blast furnace hearth

During the actual anatomy of the bottom of the blast furnace hearth, it was found that there are few slag iron layers in some blast furnace hearth areas. The actual structure from the inside to the outside is: dead iron layer→adhesive→graphite carbon→ring crack→carbon brick→ The original protective structure (such as filler, cooling wall, etc.).

In the actual production process, there is a slag-iron stagnation protective layer on the sidewall of the hearth. The anatomical process of the hearth bottom of the blast furnace was not found, because the blast furnace mainly used the operation of reducing the material surface during the shutdown process. If the blast furnace adopts nitrogen quenching, sufficient water and other measures, the deposition layer in the side wall area of the hearth is obvious. .

Image of hearth erosion survey.

There is a slag iron deposition layer in the furnace bottom area, and the thickness of the titanium carbonitride protective layer on the bottom of the blast furnace with titanium protection furnace can reach more than 200mm.
3. Slag-iron stagnation layer and slag-iron shell
The slag-iron stagnation layer is a protective layer similar to a “film” formed between the hearth bottom and the molten iron during normal production.
Hearth bottom slag iron shell is a solid slag-iron mixture formed on the inner wall due to the reduction of the thickness of the refractory material on the inner wall of the hearth and hearth due to changes in the erosion shape and production conditions during the normal production process.
In fact, the interface between the slag-iron protective layer and the slag-iron stagnation layer is not obvious, and is related to the slag-iron composition, flow, tapping, and dead coke state. Under normal circumstances, we set the 1150°C isotherm as the inner type of the hearth erosion, which is based on many industrial tests, and the sudden change of temperature such as thermal conductivity and fluidity is used as the point of performance distinction.
4. The stability of different blast furnace slag iron shell and stagnation layer
The slag-iron stagnation layer and the slag-iron shell are the key to realize the self-protection of the blast furnace hearth, and the long-term stability of the two is the key to realize the long-term stability of the blast furnace hearth.
In the blast furnace production process, there are many unstable factors, such as water leakage from cooling stave and tuyere, tapping splash, increased circulation, large changes in slag iron basicity and composition, etc., which will aggravate the instability of stagnation layer and slag iron shell, resulting in The stagnation layer thins and disappears, and the slag iron shell thins, cracks or even falls off. Judging from the long-term tracking of domestic large, medium and small blast furnaces, the temperature of the hearth thermocouple of blast furnaces with a furnace capacity of less than 1500 m³ changes the most, mainly due to unstable raw materials, water leakage from tuyere and cooling walls, and furnace conditions. The production of molten iron changes drastically, and the employees operate in reverse.
The stability of the thermocouple in the blast furnace hearth of the 1500-3000m³ blast furnace has been improved, but the premise is still the level of production, equipment and operation.
The state stability of the blast furnace hearth of a blast furnace with a furnace capacity of more than 3000 m³ is better, mainly because the level of large blast furnaces in terms of coke quality, molten iron production, slag iron composition, and equipment maintenance is significantly higher than that of small and medium-sized blast furnaces. According to the analysis of the whole furnace operation before Baosteel’s No. 3 blast furnace overhaul, its thermocouple fluctuation period is more than 3 months. From Shougang Qiangang No. 1, 2 and 3 blast furnaces (2650m³, 2650m³, 4000m³), Baotou Steel No. 4 blast furnace (3200m³), Xiwang Special Steel No. 1 blast furnace, Qinggang No. 1 blast furnace and other blast furnaces with different furnace capacities and production conditions From the point of view of the changing conditions of the hearth thermocouple from the start of the furnace to the stable production period, it takes more than 400 days for the slag iron shell and stagnation layer of the newly opened blast furnace to reach stability at the longest, and it still takes about 300 days for the shortest period. Opening a blast furnace requires exploration in terms of cooling, equipment, raw materials, and production. It takes a period of time to achieve a relatively long-term balance between the erosion rate of the blast furnace lining and the cooling intensity of the hearth bottom.
The hearth bottom erosion situation in the middle and late stage of the furnace can be reflected by the change of thermocouple. Through the calculation of the inner shape of the hearth bottom erosion of the blast furnace, combined with the trend analysis, a comparison can be made for the hearth bottom erosion state of the large and medium-sized blast furnaces. accurate predictions.

5. Problems

Through the above analysis, in fact, there are still many problems and disputes in the research process of blast furnace hearth bottom erosion.

(1) The slag-iron stagnation layer cannot be directly observed, how to determine the existence and thickness of the stagnation layer?

(2) What is the difference between the slag-iron shell and the slag-iron stagnation layer, in addition to the difference in state, in terms of composition?

(3) The slag-iron stagnation layer should not only exist in the hearth bottom, but also exist in the lower part of the blast furnace shaft, the waist, and the bollard. The composition and performance of the stagnation layer and protective layer at these positions?

(4) According to the position of the stabilized slag iron shell and the stagnant layer, when designing the hearth bottom of the blast furnace, it can be directly designed to such a size, and through enhanced cooling, hearth pouring, etc., the opening of the furnace to the hearth bottom can be reduced. Consumption of refractory materials and loss of production during this period of time when the state is stable?

(5) Can a seamless hearth bottom overall pouring method be used to reduce and avoid traditional carbon brick joints, thereby reducing alkali metal damage, stress concentration, etc., so as to make the slag iron shell and stagnation layer more stable?

The above questions are for everyone to think about, and those who are interested can leave a message for discussion.

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