2580 cubic blast furnace hearth pouring and repair

2580 cubic blast furnace hearth pouring and repair

The integral pouring of the blast furnace hearth is a new technology that uses amorphous refractory materials to repair the hearth lining. Compared with the traditional hearth carbon brick and ceramic cup brick masonry process, it has the advantages of short construction period and saving maintenance costs. The seventh generation of the No. 7 blast furnace (2580m³) of the Ironmaking Plant of Anshan Iron and Steel Co., Ltd. began on September 11, 2004. In November 2011, the local ring carbon temperature of the hearth began to rise continuously, and the ring carbon temperature of the two layers on the north side of the west iron mouth exceeded 520℃. In February 2012, measures to control the strength of the blast furnace were taken, and the blast furnace utilization factor was reduced from 2.09t/(m³·d) to 1.88t/(m³·d), and the erosion rate of the hearth refractory material was slowed down. In February 2017, the temperature of the newly added ring carbon below the west iron mouth rose to 328℃, and the remaining thickness of the carbon brick was calculated to be 369mm. Titanium ore was used for long-term furnace protection, but the refractory temperature was still high. The above phenomenon shows that the No. 7 blast furnace has entered the end of its service life, which does not meet Anshan Iron and Steel's guidelines of "safety, longevity, stable operation, and optimized indicators". In order to ensure the safe production of the blast furnace and improve its production efficiency, the No. 7 blast furnace was shut down on August 13, 2020, and the furnace was repaired for the first time using the integral casting technology.

The structure of the hearth of Anshan Iron and Steel No. 7 blast furnace

The hearth of No. 7 blast furnace adopts a hearth and bottom structure that combines microporous carbon bricks with ceramic cups. The bottom of the furnace is fully covered with 4 layers of 350mm thick domestic carbon bricks, of which the 1st, 2nd and 3rd layers from bottom to top are Wu Peng semi-graphite carbon bricks, and the 4th layer is Wu Peng microporous carbon bricks; the 5th layer of carbon bricks is microporous carbon bricks (7RDN) produced by SGL Company, with a thickness of 600mm. The 1st to 6th layers of ring-laid carbon bricks in the hearth are also 7RDN; the ring-laid carbon bricks on the upper part of the hearth are domestic molded small-block carbon bricks; the working surface of the hearth adopts a domestic ceramic cup structure; the cup bottom is composed of 2 layers of corundum-mollite combination bricks; the cup wall is composed of 1 layer of corundum combination bricks, of which the upper part is composite brown corundum and the lower part is corundum-mollite; the upper surface of the ring-laid carbon bricks and the ceramic cup adopts large-block cover bricks. The tuyere area adopts gray corundum large-block combination bricks, and each tuyere combination brick consists of two bricks, upper and lower.

Investigation and Analysis of Causes of Blast Furnace Damage

Hearth cleaning

On August 13, 2020, the blast furnace lowering line was shut down and the residual iron discharge operation began, with a total of about 600t of residual iron. On August 16, 2020, the protective cleaning of the hearth began, that is, removing the bulk material and residual iron, retaining the residual carbon bricks that have not been corroded, and blowing the surface of the residual carbon bricks clean, so as to facilitate the hearth pouring operation and make the pouring material and carbon bricks better bonded together. The thickness of the residual iron edge is 0.4~0.5m, and the center thickness is 1.0~1.4m. The hearth cleaning was completed on August 31, 2020, with a total of about 200t of bulk material and about 400t of residual iron.

The characteristics of furnace erosion are as follows:

(1) The furnace bottom is severely eroded, with a pot-bottom-shaped erosion. There are 2 layers of full-covered carbon bricks in the middle of the furnace bottom, with a thickness of 700mm, and 3~4 layers of full-covered carbon bricks on the edge, with a thickness of about 700~1400mm.

(2) The 1~2 layers of ring carbon erosion in the foot area of ​​the furnace are relatively light, with the thinnest remaining thickness of 410mm, located below the east iron mouth. The thickness of the rest of the foot area is greater than 500mm.

(3) The most serious erosion of the furnace side wall is on both sides of the iron mouth and in the area 0.5m below the iron mouth. The thinnest part is on both sides of the east iron mouth, with a thickness of 300mm.

Analysis of the causes of furnace erosion

(1) Molten iron penetration. Carbon bricks are in direct contact with molten iron in the furnace for a long time. Since the carbon in the molten iron is in an unsaturated state, the molten iron will penetrate into the carbon bricks along the pores and cracks of the carbon bricks. The molten iron that penetrates the carbon bricks will stay in the pores and cracks of the carbon bricks. After long-term immersion and chemical erosion, when the iron content in the carbon bricks reaches 5%, the volume of the carbon bricks will expand by more than 3 times, causing the structure of the carbon bricks to be destroyed.

(2) Chemical erosion of zinc. The damage investigation found that the zinc content in the slag iron solidification layer at the front end of the furnace hearth carbon bricks reached 29.5%. Part of the zinc metal entering the furnace circulates and accumulates in the blast furnace, and the zinc vapor reacts chemically with the additives in the carbon bricks, causing the carbon bricks to expand in volume or even disintegrate, resulting in reduced strength of the carbon bricks and loose brick linings and peeling.

(3) Stress caused by molten iron circulation. Due to the existence of dead material columns in the furnace hearth, molten iron circulation is inevitable. The various stresses generated by the molten iron circulation intensify the molten iron penetration and the erosion of the carbon bricks by alkali metals, causing the carbon bricks to be delaminated and pulverized in stages.

Integral pouring repair of blast furnace hearth

The casting range of the furnace hearth includes the furnace bottom and the side walls of the furnace hearth, with a total casting material of 952t. The furnace bottom is cast in two layers, with the first and second layers being 500mm and 600mm thick respectively, and the total casting thickness being 1100mm. After casting, the dead iron layer is 3000mm deep, 800mm deeper than the original furnace type. The side walls are cast to the upper edge of the original tuyere combined bricks. The non-iron mouth area is cast according to the original furnace size. The area below the iron mouth is cast in a straight chord shape, and the iron mouth channel is cast and formed at one time, and the iron mouth depth is adjusted to 2800mm. The elephant foot area of ​​the furnace hearth is cast with an inclined mold to increase the thickness. All damaged refractory materials are removed from the tuyere belt, and no buffer layer is set, and it is cast as a whole with the side walls of the furnace hearth. See Figure 2 for a schematic diagram of the integral casting of the blast furnace hearth.

Furnace Cylinder Brick Lining Temperature Measurement Technology

A total of 318 thermocouples are installed in the furnace cylinder and furnace bottom. Among them, 32 are installed in the furnace bottom and 286 are installed in the furnace cylinder. The furnace bottom thermocouples are divided into two layers, the first layer of thermocouples is laid between the carbon bricks and the castables on the furnace bottom; the second layer of thermocouples is laid between the two layers of castables. The furnace bottom thermocouples all enter from the north side slag removal door and extend inward to the center and sub-center of the furnace bottom. Since the residual brick lining of the furnace is retained, the furnace thermocouple is installed in two ways. The non-iron mouth area is installed by drilling holes outside the furnace shell. Two thermocouples are installed in each thermocouple hole. The depth of the thermocouple entering the carbon brick is 150mm and 50mm respectively. The iron mouth area is blocked by the main iron groove outside the furnace shell, so the non-iron mouth area installation method cannot be used. Therefore, the thermocouple is installed in the four sections of the furnace and laid on the surface of the carbon brick to the iron mouth area. Flexible thermocouples are used. Two thermocouples are set at each temperature measurement point, one of which is buried on the contact surface between the carbon brick and the castable, and the other is bent 90°

Into the furnace and inserted into the castable. The arrangement of thermocouples for temperature measurement in the iron mouth area is shown in Figure 3.

Production status of blast furnace hearth after pouring repair

Strengthening of blast furnace production

The blast furnace overhaul lasted 37.5 days and was put into operation on September 19, 2020. The main economic and technical indicators before and after the blast furnace overhaul are shown in Table 1. It can be seen that after the hearth casting, the longevity safety hazard of the blast furnace hearth was eliminated, the blast furnace production was strengthened, the utilization coefficient increased from 1.727t/(m³·d) to 2.092t/(m³·d), the average daily output of the blast furnace increased by 941t, and the air volume and air pressure increased significantly.

Hearth and furnace bottom brick lining temperature

After the blast furnace is put into operation, the hearth and furnace bottom brick lining temperature rises to a certain height and then operates stably. The hearth ring carbon temperature and furnace bottom refractory material temperature are shown in Figures 4 and 5 respectively.

The thermocouple measuring point in the non-iron mouth area (the curve below 300°C in Figure 4) is inside the original carbon brick, the deep point is 150mm away from the cold surface of the carbon brick, and the temperature is less than 240°C. The thermocouple in the iron mouth area (the curve above 300°C in Figure 4) is installed on the contact surface between the carbon brick and the castable, the measuring point is 400-600mm away from the cold surface of the carbon brick, the temperature is less than 550°C, and the furnace monitoring temperature is within the normal production range.

Conclusion

At the end of the campaign of No. 7 blast furnace of Anshan Iron and Steel Co., Ltd., the hearth erosion was serious. Therefore, the damage of the hearth was investigated, the characteristics and causes of the hearth erosion were analyzed, and the hearth integral casting technology was promptly adopted during the blast furnace overhaul to repair the hearth refractory, eliminating the potential safety hazards of the hearth and ensuring the safe production of the blast furnace. After the furnace was put into operation, the blast furnace production was stable, the hearth monitoring temperature was within the normal production range, the utilization coefficient was increased from the original 1.727t/(m³·d) to 2.092t/(m³·d), and the blast furnace output was greatly increased.