Title：Novel Method for Stirring BOF Melts in Conjunction With Slag Splashing
Keyword：BOF Nozzle, porous plug, BOF Melts
This paper discusses a newly developed breathable brick that eliminates the problems of porous plug while maintaining the operational advantages of splashing slag to protect the furnace. The new breathable brick achieves this goal by providing operational flexibility, using advanced controls, the slag splash guard keeps the converter running for long periods of time, and maintaining the agitation effect of the breathable brick on the molten steel.
Basic Oxygen Converter (BOF) blows oxygen into the hot molten iron in the furnace to decarburize the molten iron and remove harmful impurities, and convert the molten iron into molten steel. In the converter, blowing oxygen into the molten iron can reduce the carbon content in the molten steel. The process also uses chemical fluxes such as calcined limestone or dolomite to adjust the composition of the slag, facilitate the removal of impurities, and protect the refractory lining of the converter wall. The use of furnace bottom stirring can improve the metallurgical effect and efficiency of the top-blown oxygen converter. When the converter bottom blows and stirs, the gas is introduced from the bottom or side of the converter to stir the molten steel. The advantages of combined blowing with oxygen blowing on the top of the converter and stirring on the bottom of the furnace are well documented in various literatures. This compound blowing improves the reaction kinetic conditions, makes the temperature of the molten pool more uniform, and can better control the carbon-to-oxygen ratio, thereby improving the yield and dephosphorization effect, and improving the yield of manganese. Reduced consumption of alloy materials.
Economic benefits of compound blowing
Table 1 summarizes the economic benefits of combining bottom-blown agitation with top-blown flue gas. Savings of $3.9 per ton of hot metal are expected due to improved yields, a reduction in slag-forming material, an increase in residual manganese in the molten steel, and other benefits. The cost of equipment installation and gas agitation is estimated at $1.12 per ton of molten iron. This estimated savings of approximately $2.78 per ton of hot metal using combined blowing are well known in the converter steelmaking community.
The current converter process has no bottom stirring
At United Steel Corporation (eg in the US). The converter steelmaking process has four stages. The different steps are shown in Figure 1. Hot metal adding stage (step 1), oxygen blowing smelting stage (starting oxygen blowing step 2, oxygen blowing ending step 3), tapping stage (step 4), and slag splash protection stage (step 5). In this cycle, after step 5 is completed, step 1 of the next smelting cycle is entered.
In step 1 (hot metal charging), scrap steel and hot molten iron (metal material) are charged or poured into the converter hearth through the upper opening of the converter. In step 2 (starting oxygen blowing), a large flow of oxygen is impinged into the molten iron bath by inserting a furnace top oxygen lance. During this blowing process, slag is formed on the upper surface of the molten pool. In step 3 (the end of oxygen blowing), the oxygen supply is stopped, and the oxygen lance is drawn out from the furnace top. In step 4 (tapping), the converter is tilted, the molten steel is poured from the tapping hole on the side of the furnace into the ladle, and the slag is left in the furnace. In step 5 (slag splash guard furnace), the converter is returned to the vertical position and the oxygen lance is inserted. The oxygen lance sprays supersonic nitrogen at this time, similar to the supersonic oxygen blowing oxygen, and the splashed slag sticks to the inner wall of the converter, resulting in a coating of slag on the furnace wall refractory. It replaces part of the refractory material consumed or eroded in the converter smelting process to a certain extent, and forms a protective layer on the refractory material.
However, if slag splashing is carried out in a converter with bottom blowing agitated permeable bricks, it often leads to partial or complete blockage of the permeable bricks at the bottom of the converter. The blockage of the permeable bricks basically prevents or restricts the bottom blowing gas from entering the converter through the permeable bricks to stir molten steel, and eventually leads to the complete loss of the bottom blowing stirring function after multiple slag splashing operations.
Therefore, one of the main challenges of using converter bottom blowing agitated porous plug is. Over time, as the slag or molten steel cools in the gas slits in the permeable bricks, they may experience partial or complete blockage of the gas passages, and the gas may partially pass through these permeable brick elements. But the agitation effect is diminished or completely lost, especially if there is still agitation gas flowing through the bottom vent bricks, and these blockages are difficult to detect.
In the United States, the slag splashing method is commonly used to protect the furnace. Due to the poor reliability of the bottom blowing stirring element and the difficulty of maintenance, the bottom blowing stirring technology is not used. In equipments around the world that use bottom blowing stirring and slag splashing to protect the furnace, the service life of the existing ventilation bricks of the furnace bottom stirring element is much lower than that of the converter refractory material. For example, bottom-blown agitated porous plug rarely last more than 3000 – 5000 furnaces, beyond which they cannot be used, while converter refractories can last for thousands of furnaces. Therefore, at least half of the smelting time, and in some cases up to 85% of the smelting time, cannot be subjected to bottom blowing stirring, which affects the quality of the steel being smelted and produced, resulting in bottom blowing stirring or no bottom blowing stirring” double approach”. Therefore, these equipments must be rebuilt and replaced with refractory materials, so that the advantages of furnace bottom stirring can be maintained frequently.
New devices and processes
It is an object of the present invention to provide a new type of breathable brick that helps to eliminate the disadvantages discussed above, while maintaining the advantages of a (bottom or side) gas agitation operation submerged in molten steel at the bottom of the converter. The current design provides two different flexible modes of operation on the breathable brick: agitation mode and burner mode, which can be selected by using a control mechanism.
In the stirring mode, the bottom blowing gas helps the molten pool above the furnace bottom to perform proper stirring and mixing, and the bottom blowing nozzle works in the spraying state. The phenomena of air bubbling and jet flow state prove that the full expansion Mach number should be greater than 1.25 for the jet to be in a stable jet state. Bottom blowing jets help to: (a) prevent impact erosion of the bottom refractory and (b) enable more efficient agitation.
Suggestion to improve converter process
Figure 2 illustrates the operating strategy of the converter bottom-blown agitated gas-permeable brick nozzle. The proposed patented process (US Pat. No. 10,781,499) is described, which differs from the standard process of converter steelmaking in steps 1 to 3 (hot iron injection stage and oxygen blowing smelting stage) where the permeable bricks are in a state of outgassing agitation. In steps 4 to 5 (tapping stage and slag splashing protection stage), the ventilating brick mouth is in the burner state.
n step 1 (injection of molten iron charging), before starting to pour molten iron into the furnace, the inert gas is passed through the gas channel of the gas permeable brick (or continuously and uninterruptedly blown into the gas). Keeping the inert gas flowing while pouring molten iron and adding scrap prevents overheating and/or clogging of the bottom stirring elements. In step 2 (starting smelting with oxygen blowing), the inert gas continues to flow through the gas channel of the permeable brick at the same or different flow rates to achieve agitation of the molten pool. In step 3 (finishing oxygen blowing smelting), the inert gas blowing and stirring are continued after step 2.
In step 4 (tapping), when the converter is tilted to pour out molten steel, the airflow through the ventilation brick channels, one channel is converted into fuel, and the other channel is converted into oxidant, resulting in a flame that can produce combustion. In step 5 (slag splashing to protect the furnace), the burning flame prevents blockage of the ventilation brick passages and prevents any form of blocking bridges from being formed at the outlet of the ventilation bricks. Therefore, in steps 4 and 5, the fuel and oxidant are passed through the bottom blowing stirring element. It is recommended to use burner mode in splash guard furnaces where it is not necessary for all heats.
In the following sections, laboratory experiments and results using vented brick nozzles are discussed in detail.
Development of a New Type of Furnace Bottom Stirring Nozzle
Figure 3 outlines the complete breathable tile development and testing methodology. In order to achieve the new process method for bottom blowing agitation operation, the first step was to develop a new breathable brick design. As discussed earlier, this new design will allow the breather block to operate in two different modes: agitation mode and burner mode. In the stirring mode, the porous plug work in the spray state to effectively mix the molten pool of molten steel and reduce the number of blows to the refractory material. In the burner mode, the vented brick outlet can form a stable combustion flame.
The next step in the development phase is to test the vented brick under simulated environmental conditions to confirm the flow regime and the robustness of the vented brick to any liquid backflow. The experiments were carried out in a water tank, using water and oil as fluid media. After successfully demonstrating the jet flow regime of the breathable brick, the breathable brick was tested in flame mode using converter slag to understand the interaction of the flame with the converter slag.
The slag experiments were carried out in crucibles, the idea of which was to simulate a single gas-permeable brick section at the bottom of a converter. In the mixing mode, the interaction between the different porous plug is crucial to optimize the melt pool mixing time. In flame mode, however, each breathable brick acts around a small volume at its exit. The distance between the two porous plug is large enough that there is minimal interaction between any two flames. Each breathable brick must provide enough heat on its own to melt solid slag or solidified metal at the outlet.
- Surrounding environment: water, oil medium experiment
The flow characteristics of the new type of breathable brick at normal temperature and pressure were tested in a simulated box. The size of the box is a typical converter container size ratio of 1:6, the ventilation brick is located in the center of the furnace bottom, and the experiment is simulated by two media, water and oil. Water was chosen because its viscosity is similar to that of molten steel. The viscosity of converter slag is simulated by the selection of oil. The main purpose of this simulation test is to replicate the operation of the vent block in the splash hoist mode: to ensure that no liquid penetrates back into the vent block gas channel in either mode of agitation or burner. Experiments in this vessel replicated “the area around a single breathable brick” in a converter vessel. This enables testing using an actual prototype breather brick. The electric furnace is used to control the electric furnace to ensure that there is no water infiltration in the ventilating block under the two operating modes of the ventilating block.
- Hot metal and slag experiments
Simulation experiments of liquid slag and molten steel were carried out in the crucible. The breathable brick is located at the bottom of the crucible, and the crucible is filled with converter solid slag, iron, or a mixture of solid slag and iron mixture. A pile of solid materials is used as a test device to simulate the worst case where the outlet of the breathable brick is partially or completely blocked by solid slag or solidified molten steel. The heat from the bottom flame is used to create a molten pool of metal or slag.
Laboratory test results
- Water and oil medium: converter box simulation experiment
Prototype vent bricks were tested in the laboratory to verify the functionality of the device and the operation according to the design calculations, Figure 4 shows the theoretical and laboratory determined flow pressure characteristics of one of the prototype vent bricks. This graph also shows the expansion Mach number of the prototype vent tiles. The left y-axis is the fluid supply pressure and the right y-axis is the expansion Mach number. The graph shows that the expansion Mach number is above 1.25 at supply pressures above 80 psia. This is the critical expansion Mach number, above which the breathable brick operates under the spraying condition. Figure 5a shows the operation of the nozzle under the spraying condition. The liquid level is three feet above the vent outlet.
Furthermore, Figure 4 shows that the pressure required for the jet flow regime can be achieved using a standard liquid gas supply tank without the use of a compression device. In addition, the flow pressure characteristics measured in the laboratory are within 10% of the theoretically determined pressure flow characteristics of the permeable brick.
Prototype venting bricks were tested with an aqueous medium in a container, and the results showed that as long as there was gas passing through the venting bricks, no water would penetrate into the interior of the venting bricks. In addition, the permeable brick operation was tested using oil with a viscosity similar to that of converter slag as the fluid medium in the simulated converter. Air has the same momentum as the fuel and oxidant, and in the oil-medium test, air is used to pass through the vented block, and the oil level in the tank is maintained at about 18 inches from the vented block outlet. As can be seen in Figure 5b, the momentum of the fluid flowing out of the breathable brick is large enough to occupy the oil column above it.
Experiments under ambient conditions help to establish the credibility of the breathable brick design, and the next section discusses experiments using converter slag.
Converter Liquid Slag Experiment
Burner mode operation tests were carried out in crucibles using converter slag. Table 2 summarizes the main components in the converter slag used in the experiments. Figure 6 is a top view of the crucible testing the vented brick as burner mode operation. Load converter solid slag and 5% iron mass into crucible. After the tapping operation (step 4 in Figure 2), iron is added to replicate the traces of iron remaining in the converter slag. Initially, the slag and iron in the crucible were cooled by a stream of nitrogen gas from a vented brick at the bottom.
Figure 6b shows the initial stage when the permeable brick starts the burner mode, when the temperature in the crucible is higher than the autoignition temperature of the fuel, a flame is generated. The energy released by the bottom flame melts the slag, forming a slag pool, and the combustion gas causes the slag to splash, as shown in Fig. 6d. The slag from the bottom burn formed solidified slag spots on the viewing window approximately 6 feet above the crucible. These “black spots” can be seen in Figures 6d-6h. The slag splashed from the bottom burning indicates that the bottom burning clears the silt at the outlet of the ventilating brick, and the slag is sprayed with sufficient strength.
When the crucible slag began to splash, the burner mode was turned off, and nitrogen flow was introduced from the bottom to solidify the slag. The solidified slag can be seen, marked as “center slag hole” in Figure 6e. After the slag has solidified, change the ventilation brick operation to burner mode. Figures 6f and 6g show that the flame is formed in the center of the crucible where the breathable brick is located. After it was determined that a clear hole could be created above the vent outlet, the vent operation was switched back to nitrogen flow. Figure 6i shows the crucible at the end of the experiment. The results show that the solidified slag layer above the outlet of the breathable brick has obvious slag holes.
Experiments show that the transition process from stirring mode to burner mode and then to stirring mode is smooth, and the slag has no backflow phenomenon in the air-permeable brick. The flow slip and control system worked well, showing the practicality of switching between the two operating modes of the bottom breather brick, as shown in Figure 2 for the converter process recommendations. This experiment simulates the worst conditions faced by permeable bricks, namely the formation of solid slag shells on and around the permeable bricks.
- Detecting liquid entering the breathable brick device
In the laboratory of “Air Products”, an experimental study was carried out on the detection of the blockage of the ventilation brick and the control mechanism of the feedback control valve of the ventilation brick. In this prototype design, thermocouples and flow measurement devices were used as active sensor elements to test and validate the control mechanism. Thermocouples are installed at key positions A, B and D on the bottom wall of the refractory crucible and inside the breathable brick.
Create a molten pool of molten steel in the refractory crucible above the outlet of the breathable brick. Since the viscosity of molten steel is lower than that of liquid slag, molten molten steel is used in this experiment. Therefore, the penetration of molten steel into the breathable brick will cause the worst impact. In order to simulate the situation of molten steel entering the permeable brick, the bottom blowing gas flow was gradually reduced to zero. Figure 7 shows temperature data obtained by installing thermocouples in refractory crucibles and prototype porous plug. Temperature and time are represented on the y-axis and x-axis, respectively. After 236.5 minutes of operation, the gas flow dropped to zero. As can be seen from Figure 7, when the bottom blowing gas flow begins to decrease, molten steel permeates into the gas channel of the permeable brick, resulting in an increase in the temperature readings of thermocouples A, B, and d. During this operation, the bottom wall temperature of the crucible was approximately 1775°F (968°C). Thermocouple A and Thermocouple B read close to 725°F (385°C) per minute, and the controller feedback signal can be used to initiate gas entry into the permeable brick again to avoid further penetration of molten steel or liquid slag into the permeable brick. The thermocouple reading D shows the temperature rise in the tube due to the loss of cooling effect of the breathable brick. Since the molten steel solidifies before reaching the position of thermocouple D, the temperature of thermocouple D is lower than that of thermocouple A and thermocouple B.
The molten steel penetrates into the breathable brick to a depth of approximately 2 inches. This molten steel penetration represents laboratory data and does not fully reflect the actual blockage of the converter ventilation bricks. In the next stage of the experiment, the key is to determine how to use the temperature sensor signal of the permeable brick to feedback control the bottom blowing gas to reduce the penetration of molten steel into the permeable brick.
- Advantages of the new type of breathable brick
The new type of ventilation brick developed in this project is that a single ventilation brick can realize two operation modes: stirring mode, the bottom blowing and stirring of the ventilation brick acts on it; burner mode, to avoid any blockage of the ventilation brick at the outlet. Compared to standard conventional porous plug, the features of this self-cleaning breathable brick help maintain bottom-blown agitation efficiency in converter operations and have a long life.
The ventilation bricks are easy to install and can be installed without waiting for the bottom of the converter to be replaced. Compared with the conventional bottom-blown agitated and ventilated bricks (usually installed by replacing the bottom of the furnace), this installation method is more convenient. Additionally, the patented breathable brick works at existing supply pressures, using similar gas flow rates. These gases come from standard high pressure liquid storage vessels or air separation units and do not require additional compressors, reducing the capital and operating costs of the system without the use of compressors. Additionally, temperature and flow sensors mounted on the vent tiles help detect any blockages on the vent tiles and take corrective action with active feedback.
In the North American market, the newly developed porous plug can be used in combination with bottom-blowing agitation of converters and slag splashing to protect the furnace. This can significantly improve economic efficiency and molten steel quality. In the global market, this new breathable brick can help maintain the efficiency of bottom-blown agitation and a longer life cycle of converter refractories, which can be extended from 3,000 to 5,000 furnace ages to tens of thousands of furnace ages.