JP7513895B2 - How to calm foaming - Google Patents

Description

The present invention relates to a method for calming foaming during blowing.

In the converter process, oxygen is blown into the furnace to remove impurities from the molten metal while raising the temperature, and the component concentrations and temperature of the molten metal after blowing are controlled to fall within specified ranges. During this process, the oxygen blown from the top blowing lance reacts with the carbon in the molten iron to generate CO gas, which causes the slag to foam (hereafter referred to as "foaming"). If the amount of expansion of the slag due to foaming is large, the slag will overflow from the furnace (hereafter referred to as "slopping"). Sloping not only reduces the iron yield, but also poses a danger to operators and can lead to operational interruptions, so methods to calm or suppress slag foaming are being investigated.

A common method for calming the foaming of slag is to add a calming agent to the foamed slag. For example, Patent Document 1 discloses a method for suppressing slag foaming, in which carbonaceous material is added within three minutes of starting oxygen supply from the top blowing lance.

In addition, methods of calming foaming other than the method of adding calming material are also known. Patent Document 2 discloses a method of calming foaming in a reaction vessel other than a converter-type refining furnace, in which a rod is inserted and removed from the top of the slag to form a gas venting opening in the slag. Patent Document 3 discloses a method of calming foaming in which a jet stream is collided with the slag surface, and the collision force generated at that time forms a gas bubble vent hole in the surface layer of the slag to vent the gas bubbles remaining in the slag. Furthermore, Patent Document 4 discloses a method of calming foaming in a standby state where no blowing process is being performed, in which gas is blown from a top blowing lance when foaming occurs.

Patent No. 3888313 Japanese Patent Application Laid-Open No. 10-183217 Patent No. 6221705 JP 2019-94522 A

Toshihiko Shagochi: Jet Engineering, Morikita Publishing (2004). P.34

Compared to foaming when not blowing, refining reactions are actively progressing during blowing, and foaming is also intense. Therefore, in order to calm the foaming during blowing, a large amount of silencing material is required if silencing material is added, and even if the foaming is calmed, the benefit of silencing may not be sufficient from a cost perspective.

The method described in Patent Document 1 is a method for preventing slag foaming before it occurs, so it cannot deal with foaming if it actually occurs. The method described in Patent Document 2 does not provide a sufficient calming effect when foaming is severe, as the gas venting openings disappear quickly.

In addition, in the method described in Patent Document 3, a top blowing lance capable of blowing gas at high pressure must be installed in addition to the existing top blowing lance in order to calm the foaming. Therefore, even if the foaming is calmed, it is disadvantageous in terms of cost. Furthermore, the method described in Patent Document 4 is a method used in a standby state where no blowing process is being performed, so it cannot deal with slag foaming that occurs during blowing.

In consideration of the above problems, the present invention aims to provide a method for stabilizing foaming during blowing at low cost.

When oxygen gas is blown from a top lance in a converter-type refining process, the oxygen jet penetrates the slag and collides with the molten iron. The inventors noticed that blowing gas from the top lance has a certain degree of foaming suppression effect because slag is entrained from the side of the jet flow and the bubbles in the entrained slag are physically destroyed by the oxygen jet. In addition, the flow characteristics of the oxygen jet can be divided into two regions: an initial region near the nozzle where there is a potential core region (hereinafter referred to as the core region) where the velocity does not decay, and a development region downstream of that. The inventors further noticed that it is the development region where the oxygen jet spreads and entrains the surrounding fluid, and that the initial region where the core region exists does not contribute significantly to entraining the fluid.

On the other hand, if the height of the slag surface rises suddenly due to foaming and becomes greater than the lance height, the slag present above the lance tip from which the oxygen jet is ejected will not be caught in the jet flow and will remain there, making it more susceptible to slopping. This invention was therefore based on the assumption that the foaming suppression effect can be increased by controlling the lance height so that no slag is present above the lance tip and the amount of slag caught in the jet flow is maximized.

The present invention is as follows.
(1)
A method for suppressing foaming during blowing in a refining furnace having a top blowing lance, comprising:
A method for calming foaming, comprising adjusting the height of the top blowing lance to a range that satisfies formula (1) before the rate of rise of the height of the slag surface due to foaming during blowing exceeds 14 cm/s, and maintaining the height of the top blowing lance adjusted to a range that satisfies formula (1) at least until the rate of rise of the height of the slag surface becomes 0 cm/s or less.
H _core ≦LH-(MH-L ₀ )≦5H _core ... (1)
Here, _Hcore in the formula represents the length (mm) of the core region of the jet from the top lance where the jet velocity does not attenuate near the nozzle. Also, LH represents the distance (mm) from the stationary bath surface of the molten iron to the nozzle tip of the top lance. Furthermore, MH represents the average height (mm) from the hearth to the slag surface, and _L0 represents the bath depth (mm).
(2)
The method for calming foaming described above in (1) is characterized in that, when the rate of increase in the height of the slag surface is 6 cm/s or more, the height of the top blowing lance is adjusted to a range that satisfies the formula (1).

The present invention provides a method for stabilizing foaming during blowing that can be done inexpensively.

FIG. 2 is a view for explaining a state in which the height of the slag surface during blowing is measured.

A method for calming slag foaming during blowing according to one embodiment of the present invention will be described below with reference to Fig. 1. The blowing process may be any one of desiliconization blowing, dephosphorization blowing, and decarburization blowing, and also includes a blowing process in which these are performed in series.
As shown in FIG. 1, the refining furnace 1 has a furnace body 11 and a top-blowing lance 2. The furnace body 11 is a refining vessel with an opening 12 formed at the top, and the inside is covered with refractory material. In addition, a plurality of bottom-blowing tuyeres (not shown) are provided at the bottom of the furnace body 11, and agitation gas is blown into the furnace body 11 from the bottom-blowing tuyeres. The top-blowing lance 2 is a lance that can be raised and lowered in the vertical direction (up and down direction in FIG. 1). Oxygen gas is supplied to the top-blowing lance 2 from a gas supply path (not shown) connected to the upper end side, and a jet 3 of oxygen gas is sprayed from at least one nozzle hole formed at the lower end. There is no particular provision regarding the lance specifications in this embodiment, and a conventionally used lance can be used as it is.

As mentioned above, the jet 3 consists of a core region where the velocity does not decay and a development region downstream of that, and it is known that if the radius of the nozzle hole is _r0 (mm), the length Hcore of the _core region is _10r0 (see non-patent document 1).

In order to calm or suppress foaming, it is necessary to understand the foaming behavior inside the furnace. Therefore, in order to understand the foaming behavior inside the furnace online, there is a method that uses a level meter that uses microwaves that propagate in a straight line even in an environment where dust, flames, etc. are present. In this method, after molten iron and scrap or alloys are charged into the converter, microwaves are transmitted into the furnace from a fixed or movable microwave transmitting/receiving antenna installed in the sublance hole of the converter's upper hood, and the height of the slag surface is measured from the received signal.

In this embodiment, in order to grasp the foaming behavior inside the furnace, the height of the slag 4 in the vertical direction (the up and down direction in FIG. 1) is measured using a microwave level gauge that can measure continuously. Below, we will explain the implementation situation when measuring the height of the slag surface during converter blowing using microwaves.

As shown in FIG. 1, a sublance hole 6 with a sliding or openable lid (not shown) is provided at the top of the furnace body 11. The microwave level measuring device 7 is, for example, a mobile device installed above the sublance hole 6, and is connected to a transmitting/receiving antenna 9 via a waveguide 8. Measurements of the bath surface and the like in the converter are performed through the sublance hole 6. The microwave level measuring device 7 may be installed above a dedicated measurement hole rather than above the sublance hole 6. The microwave level measuring device 7 may also be fixed rather than mobile.

By measuring the height of the slag surface of the slag 4 using the above method, the rate of rise of the slag surface height during blowing is calculated. During blowing, the slag 4 and molten iron 5 below the oxygen gas jet 3 are pushed aside and become thinner, but as shown in Fig. 1, the average slag surface height MH and bath depth _L0 in the furnace are measured. When calculating the rate of rise of the slag surface height during blowing, the rate of rise of the slag surface height t seconds after the start of blowing is obtained from the gradient of the slag surface height in the time interval (10 to 30 seconds) immediately before t seconds.

Next, a specific method for calming the foaming of the slag in this embodiment will be described. In this embodiment, the lance height is adjusted based on the measurement results of the change over time in the height of the slag surface using the microwave level measurement device 7.

At the end of blowing (mainly at the end of dephosphorization blowing when desiliconization blowing, dephosphorization blowing, and decarburization blowing are performed in succession), foaming is in a severe state, and the rate at which the slag surface rises is also high. Therefore, before the rate at which the slag surface rises exceeds 14 cm/s, the lance height is adjusted to a range that satisfies the following formula (1).
H _core ≦LH-(MH-L ₀ ) ≦5H _core ... (1)

Here, _Hcore in formula (1) represents the length (mm) of the core region of the oxygen gas jet where the jet velocity does not decrease near the nozzle. Also, LH represents the distance (mm) from the stationary bath surface of the molten iron to the nozzle tip of the top lance. Furthermore, MH represents the average height (mm) from the hearth to the slag surface, and _L0 represents the bath depth (mm).

If you try to adjust the lance height to a range that satisfies formula (1) after the rate of rise of the slag surface height exceeds 14 cm/s, the momentum of the foaming cannot be stopped, making slopping more likely to occur and disrupting operations. Therefore, adjust the lance height to a range that satisfies formula (1) below before the rate of rise of the slag surface height exceeds 14 cm/s.

Furthermore, when adjusting the lance height, if the value of {LH-(MH-L ₀ )} is adjusted to be smaller than H _core , the slag fluid will be entrained in the core region, or the fluid near the slag surface will not be entrained by the jet, resulting in insufficient fluid entrainment and insufficient foaming suppression. On the other hand, if the lance height is adjusted to be larger than 5H _core , the distance from the nozzle to the slag surface will _be too large, reducing the effect of entraining the slag fluid by the jet, resulting in insufficient foaming suppression. Therefore, the lance height is adjusted to satisfy formula (1).

By adjusting the lance height in this way, slag is drawn in from the sides of the jet flow, and the bubbles in the drawn-in slag are physically destroyed by the oxygen gas jet, decreasing the rate at which the slag surface rises. Therefore, the lance height is adjusted to a range that satisfies formula (1) at least until the rate at which the slag surface rises falls to 0 cm/s or less. When the slag surface is lowered, foaming has subsided, and it is not necessary to satisfy formula (1). On the other hand, frequent changes to the lance height may cause the refining effect of the blowing process to become unstable, so it is preferable to fix the lance height even after the rate at which the slag surface rises to 0 cm/s or less.

In addition, by adjusting the lance height before the rate of rise of the slag surface exceeds 14 cm/s, the foaming can be suppressed, but adjusting the lance height to the range of formula (1) when foaming is not yet so intense may affect the refining behavior by blowing. On the other hand, if the top blowing lance is made higher than the conditions suitable for refining from the early stage of blowing with the sole purpose of increasing the amount of slag entrained in the jet, the strength of the jet on the molten iron becomes weak, resulting in a soft blow, and the amount of FeO in the slag increases. In other words, in a soft blow, the amount of slag increases, foaming becomes intense, and the yield also decreases. Therefore, it is preferable to adjust the lance height to the range that satisfies formula (1) when foaming rises sharply at the end of blowing, specifically, when the rate of rise of the slag surface is 6 cm/s or more. However, depending on the blowing conditions, the rate of rise of the slag surface may rise slowly or suddenly, so it is preferable to change the timing of adjusting the top blowing lance height depending on the situation.

In this embodiment, an example of injecting a jet of oxygen gas from the top lance has been described, but in dephosphorization refining, the same can be applied to the case where a dephosphorization material is injected together with the oxygen gas from the top lance.

Next, an embodiment of the present invention will be described. However, the conditions are merely examples of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this embodiment. The present invention can be implemented in various ways that achieve the object of the present invention without departing from the gist of the present invention.

In the experiment, a refining furnace with a maximum capacity of 280 tons of molten iron was used as shown in FIG. 1. A total of 250 tons of molten iron and scrap were charged into the refining furnace, and dephosphorization material was added. Oxygen gas was blown from the top lance at a flow rate of 40,000 Nm ³ /h toward the molten iron to perform dephosphorization blowing. The top lance had a nozzle tip radius of 27.5 mm or 22.5 mm, and the lance height LH ₀ from the stationary bath surface of the molten iron to the nozzle tip was unified at 3,000 mm as an initial condition. During blowing, the vertical height of the slag was measured using a microwave level measuring device, and the rising speed of the slag surface height was monitored. When the rising speed of the slag surface height reached the values shown in Table 1, the lance height was adjusted to the value of {LH-(MH-L ₀ )} shown in Table 1, and the lance height was fixed as it was. In addition, the foaming calming effect was evaluated by measuring the increase in the height of the slag surface for 5 seconds after adjusting the lance height.

In Examples 1 to 9, the lance height was adjusted to satisfy formula (1) before the rate of rise of the slag surface exceeded 14 cm/s, so the slag surface height subsequently decreased, and the foaming calming effect was confirmed. On the other hand, in Comparative Example 1, the top-blowing lance was set too high when adjusting the lance height, so the effect of the jet entraining the slag fluid was small, and foaming could not be sufficiently calmed. In Comparative Example 2, the lance height was adjusted to a state where the slag was buried within a range of 100 mm from the tip of the top-blowing lance when adjusting the lance height, so the slag was not entrained in the jet flow near the slag surface, and foaming could not be sufficiently calmed. Furthermore, in Comparative Example 3, the lance height was adjusted at a stage when the rate of rise of the slag surface exceeded 14 cm/s, so the momentum of foaming could not be stopped, and the slag surface height did not decrease.

Reference Signs List 1 Refining furnace 2 Top blowing lance 3 Jet 4 Slag 5 Molten iron 6 Sublance 7 Microwave level measuring device 8 Waveguide 9 Transmitting/receiving antenna 11 Furnace body 12 Opening

Claims (2)

A method for suppressing foaming during blowing in a refining furnace having a top blowing lance, comprising:
A method for calming foaming, comprising adjusting the height of the top blowing lance to a range that satisfies formula (1) before the rate of rise of the height of the slag surface due to foaming during blowing exceeds 14 cm/s, and maintaining the height of the top blowing lance adjusted to a range that satisfies formula (1) at least until the rate of rise of the height of the slag surface becomes 0 cm/s or less.
H _core ≦LH-(MH-L ₀ )≦5H _core ... (1)
Here, _Hcore in the formula represents the length (mm) of the core region of the jet from the top lance where the jet velocity does not attenuate near the nozzle. Also, LH represents the distance (mm) from the stationary bath surface of the molten iron to the nozzle tip of the top lance. Furthermore, MH represents the average height (mm) from the hearth to the slag surface, and _L0 represents the bath depth (mm). The foaming calming method described in claim 1, characterized in that the height of the top blowing lance is adjusted to a range that satisfies formula (1) when the rate of rise of the height of the slag surface is 6 cm/s or more.

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