Method for detecting melt height

Abstract

A method for detecting the melt height of a smelting furnace having a discharge hole for discharging the melt at the bottom of the furnace, wherein the discharge distance of the melt discharged from the discharge hole is measured, the discharge speed of the melt discharged from the discharge hole is calculated using the discharge distance, the height of the discharge hole and the discharge angle of the melt, and the discharge speed is used to detect the melt height.

Description

Technical Field

[0001] The present invention relates to a method for detecting the height of a molten material in a smelting furnace. Background Art

[0002] One type of smelting furnace whose interior cannot be directly observed is the blast furnace. A blast furnace is a smelting furnace where iron is smelted by charging ore and coke, which contain a high iron content, from the top of the furnace and blowing a mixed gas such as air or pure oxygen from the bottom. Molten pig iron slag, produced, accumulates at the bottom of the blast furnace and is discharged from a hole called a taphole before reaching the blast furnace's auxiliary equipment. When implementing this production method, stable and trouble-free operation is required to achieve the planned production volume.

[0003] However, due to factors such as malfunctions in taphole equipment and poor fluidity of the molten slag, excessive molten slag can accumulate at the bottom of the blast furnace. This can lead to poor air permeability due to a narrowing of the gas flow area, clogging of the tuyere by the molten slag, and in the worst case, even halting production. Therefore, for stable operation of the blast furnace, it is necessary to monitor the melt height.

[0004] Technologies exist for determining the height of the melt in a smelting furnace. For example, Patent Document 1 discloses capturing two or more images of the surface of the melt discharged from the discharge port of a smelting furnace over a short period of time. The melt discharge rate is calculated based on the changes in the images, and the melt height can be calculated based on the discharge rate and the pressure within the furnace.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-160498

[0008] Non-patent literature

[0009] Non-Patent Document 1: Takashi Sugiyama and three others, "Analysis of Liquid Flow in the Blast Furnace Drip Zone," Iron and Steel, 1987, Vol. 15, pp. 2044-2051

[0010] Non-patent literature 2: Masaki Sawamoto, "Flow Dynamics (Flow Dynamics)", Kyoritsu Publishing, 2005 edition, P58-59 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] However, the melt flowing inside the discharge hole encounters resistance from the hole's inner wall. Therefore, the melt flowing in contact with the inner wall has a slower velocity than the melt flowing in the center of the hole. Consequently, the melt on the outside of the discharge hole, which contacts the hole's inner wall, flows slower than the melt on the inside of the hole, which flows in the hole's center. Furthermore, the melt discharged from the discharge hole is captured on the outside, so the melt tapping speed calculated from the capture is slower than the actual discharge speed. Thus, the method disclosed in Patent Document 1 cannot accurately determine the melt discharge speed, resulting in reduced accuracy in calculating the melt height based on the discharge speed. The present invention was developed in response to these prior art issues and aims to provide a melt height detection method that can accurately measure the melt discharge speed and accurately detect the melt height.

[0013] Methods used to solve problems

[0014] A method for solving the above-mentioned problem is as follows.

[0015] (1) A method for detecting the height of a melt, which is a method for detecting the height of a melt in a smelting furnace having a discharge hole at the bottom of the furnace for discharging the melt, wherein the discharge distance of the melt discharged from the discharge hole is measured, the discharge speed of the melt discharged from the discharge hole is calculated using the discharge distance, the height of the discharge hole and the discharge angle of the melt, and the melt height is detected using the discharge speed.

[0016] (2) The method for detecting the height of the melt according to (1), wherein the height of the discharge hole is measured.

[0017] (3) The method for detecting the melt height according to (1) or (2), wherein the melt height is detected using the resistance to which the melt is subjected from the discharge hole until the melt is discharged from the discharge hole and the discharge speed.

[0018] Effects of the Invention

[0019] The melt height detection method of the present invention calculates the melt discharge velocity using the melt discharge distance, the height of the discharge hole, and the melt discharge angle. This allows for more accurate calculation of the melt discharge velocity compared to methods that determine the melt discharge velocity by photographing the melt. By using the melt discharge velocity to detect the melt height, high-precision melt height detection is possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a partial cross-sectional view of the lower portion of the blast furnace 10 .

[0021] Figure 2 It is a schematic diagram showing a state in which the melt 12 is discharged from the tap hole 14 .

[0022] Figure 3 It is a schematic cross-sectional view of the tap hole 14 through which the melt 12 passes.

[0023] Figure 4 These are images showing the discharge flows of Invention Examples 1 and 2.

[0024] Figure 5 Graphs showing the results of detecting the melt height H in Inventive Examples 1 and 2.

[0025] Figure 6 This is a graph showing the temporal change of the molten pig iron-slag equilibrium. DETAILED DESCRIPTION

[0026] The present invention is described below using embodiments thereof. In this embodiment, a blast furnace is used as a smelting furnace, and an embodiment of a method for detecting the melt level in the blast furnace is described. However, the smelting furnace's molten material level detection method is not limited to blast furnaces; it can be applied to any smelting furnace having a discharge port in its lower portion for discharging the molten material to the outside of the furnace.

[0027] In the method for detecting the melt height of this embodiment, the discharge speed of the melt discharged from the discharge hole to the outside of the furnace is calculated, and the discharge speed is used to detect the melt height accumulated in the lower part of the blast furnace. Figures 1 to 3 A method for calculating the discharge rate of the melt discharged from the discharge hole to the outside of the furnace will be described.

[0028] Figure 1 This is a partial cross-sectional view of the lower portion of a blast furnace 10. The lower portion of the blast furnace 10 is provided with multiple tapholes 14 for discharging the melt 12 accumulated therein. The melt 12 is molten pig iron and slag, or a mixture thereof, present in liquid form within the furnace. The melt 12 is discharged from the tapholes 14 to the outside of the furnace, forming a parabolic discharge stream 16 and falling onto the melt 12 liquid level 20 in a ditch 18. In this embodiment, the tapholes 14 are an example of a discharge hole for discharging the melt 12 accumulated in the lower portion of the smelting furnace to the outside of the furnace.

[0029] Figure 2 Schematic diagram showing the state of the melt 12 being discharged from the tap hole 14. In the method for detecting the melt height of this embodiment, the melt height is detected by using Figure 2 (a) The discharge distance L of the melt 12 tap , the height h of the tap hole 14 tap and the discharge angle θ of the melt 12 tapThe discharge speed of the melt 12 is calculated. The discharge distance L of the melt 12 tap It is the distance from when the molten material 12 is discharged from the tap hole 14 to when it falls to the liquid surface of the molten material 12. tap For example, the measurement can be performed using an image obtained by capturing the discharge flow 16 together with an object serving as a length reference.

[0030] Height h of the tap hole 14 tap It is the height from the center of the taphole 14 to the liquid level 20 where the molten material 12 falls. The height from the center of the taphole 14 to the liquid level 20 where the molten material 12 falls can be measured using an image obtained by photographing the discharge flow 16 together with a subject serving as a length reference. Alternatively, the design value for the taphole 14 can be used. It should be noted that the center position of the taphole 14 changes due to the dissolution of the refractory material as the molten material 12 is discharged from the taphole 14. In addition, the height of the liquid level 20 sometimes also changes. Therefore, the height from the center of the taphole 14 to the liquid level 20 where the molten material 12 falls is preferably measured using an image obtained by photographing the discharge flow 16.

[0031] The discharge angle θ of the melt 12 tap It is the angle between the discharge flow 16 and the horizontal plane when the molten material 12 is discharged from the tap hole 14. tap The height h of the tap hole 14 can be measured by using an image obtained by photographing the discharge flow 16. Alternatively, the height h can be the same as the designed value of the inclination angle of the central axis of the tap hole 14 relative to the horizontal plane. tap and the discharge angle θ of the melt 12 tap No measurement is necessary.

[0032] Figure 2 (b) is a diagram showing a state where an obliquely projected object 30 moves along a parabolic trajectory. Figure 2 As shown in (b), for example, if an object 30 is projected obliquely from a height h at a projection angle of θ and just flies a flight distance L and lands on the ground, the only force acting on the object 30 is gravity. Therefore, if the left and right directions are set as x and the up and down directions are set as z, the motion equations of the object 30 are the following equations (1) and (2).

[0033]

[0034]

[0035] v in the above formula (1) x is the initial velocity in the x direction (m / s), v in (2) zis the initial velocity in the z direction (m / s). In addition, m is the mass of the object 30 (kg), g is the acceleration due to gravity (9.8 m / s 2 ).

[0036] First, consider the distance traveled in the z direction. If the time from when object 30 is thrown until it lands is Δt seconds, then object 30 descends exactly the same height h in Δt seconds. Therefore, for the distance traveled in the z direction, the following equation (3) holds true.

[0037]

[0038] v in the above formula (3) ini is the initial velocity of the object 30 (m / s), θ is the projection angle (rad), and h is the height (m). The second term on the right side of equation (3) is the initial velocity in the z direction multiplied by Δt.

[0039] Next, consider the distance traveled in the x-direction. Since no force is applied in the x-direction during oblique projection, the initial velocity in the x-direction is maintained until impact. Therefore, if the time from when object 30 is projected until impact is Δt, then object 30 travels the flight distance L in Δt seconds. Therefore, for the distance traveled in the x-direction, the following equation (4) holds true.

[0040] L=v ini ×cosθ×Δt…(4)

[0041] L in the above formula (4) is the flying distance (m) of the object 30 in the x direction. ini is the initial velocity of the object 30 (m / s), and θ is the projection angle (rad).

[0042] If the initial velocity v of the object 30 is calculated using the above equations (3) and (4), ini After arranging the above equations, the following formula (5) is derived. It should be noted that in the following formula (5), the reference for the height h is the landing position of the object 30 .

[0043]

[0044] If the above formula (5) is applied to the discharge velocity V of the melt 12 tap Calculation, then V ini is the discharge velocity V of the melt 12 tap (m / s), L is the discharge distance of the melt 12 tap (m), h is the height of the tap hole 14 tap (m), θ is the discharge angle θ of the melt 12 tap (rad). Therefore, by using the above formula (5), the discharge distance L of the melt 12 tap, the height h of the tap hole 14 tap and the discharge angle θ of the melt 12 tap , the discharge velocity of the melt 12 can be calculated. Specifically, this method estimates the melt velocity by capturing the shape of the discharge flow and combining it with an estimation formula based on mechanics. This method reduces the error in velocity measurement caused by the resistance of the inner wall of the taphole and accurately calculates the melt discharge velocity.

[0045] Next, according to the discharge speed V tap The method of detecting the height of the melt 12 is described below. In the method of detecting the height of the melt of this embodiment, the discharge speed V of the melt 12 is used. tap The height of the melt 12 is calculated using the following formula (6).

[0046]

[0047] ρ in the above formula (6) l is the density of the melt 12 (kg / m 3 ), v tap is the discharge speed of the melt 12 (m / s), P1 is the gas pressure in the furnace (Pa), g is the acceleration due to gravity (9.8m / s 2 ), H is the height of the melt 12 accumulated in the lower part of the furnace (m), P2 is the gas pressure at the outlet of the tap hole 14 (Pa), h tap is the height of the taphole 14 (m).

[0048] Density ρ of the melt 12 l In addition, when the melt 12 contains molten pig iron and slag, the density of the melt 12 can be calculated by dividing the density of the molten pig iron and the density of the slag in proportion to the mixing ratio. l The discharge velocity v of the melt 12 tap is v calculated by the above formula (5) ini The gas pressure P1 in the furnace is obtained from the measured value of the pressure gauge installed at the bottom of the blast furnace 10. The gas pressure P2 at the outlet of the tap hole 14 can be atmospheric pressure. In addition, the height H of the melt 12 and the height h of the tap hole 14 in the above formula (6) are tap The reference is the bottom position of the blast furnace 10.

[0049] The first term on the left side of the above equation (6) represents the mechanical energy of the melt 12 accumulated in the lower portion of the blast furnace 10. The second term on the left side of the above equation (6) represents the mechanical energy of the melt 12 immediately after being discharged from the taphole 14. It should be noted that the term representing the kinetic energy of the melt 12 accumulated in the lower portion of the furnace is sufficiently small compared to the other terms and can therefore be ignored. The above equation (6) is a formula in which these mechanical energies are equal (the difference between the first and second terms is 0). By using this equation (6), the height H of the melt 12 accumulated in the lower portion of the furnace can be detected.

[0050] Figure 3 : is a schematic cross-sectional view of the taphole 14 through which the molten material 12 passes. When the molten material 12 accumulated in the lower portion of the blast furnace 10 is discharged from the furnace, the molten material 12 encounters resistance at the taphole 14. Therefore, it is preferable to calculate the height H of the molten material 12 using the following equation (7), which takes into account the influence of this resistance in the above equation (6).

[0051]

[0052] f in the above formula (7) tap is the friction coefficient of the tap hole 14 (-), W tap is the length of the tap hole 14 (m), D tap is the inner diameter of the taphole 14 (m), and Ke is the pipe inlet loss coefficient (-). It should be noted that (-) is dimensionless.

[0053] The length W of the tap hole 14 tap The inner diameter D of the tap hole 14 is calculated based on the penetration length of the drill bit when setting the tap hole 14. tap It is calculated based on the diameter of the drill bit used when setting the tap hole 14. In addition, the pipe friction coefficient f of the tap hole 14 is tap It can be calculated using the Swamee-Jain formula shown in the following formula (8).

[0054]

[0055] e in the above formula (8) tap is the taphole roughness (m), D tap is the inner diameter of the tap hole 14 (m), and Re is the Reynolds number of the fluid flowing through the tap hole 14 (-).

[0056] Tap hole roughness e tap The value varies depending on the drilling method, tapping clay, and the time elapsed from the start of tapping, but it has been confirmed from operational analysis that a value within the range of 0.0001 to 0.01 m is appropriate. The Reynolds number Re can be calculated using the following formula (9).

[0057]

[0058] In equation (9), μ represents the viscosity of the melt 12 (Pa·s). The viscosity μ of the melt 12 is calculated by dividing the viscosities of the molten pig iron and slag in proportion to the mixing ratio. Past actual values ​​can be used for the viscosities of the molten pig iron and slag. Alternatively, the viscosity of the slag can be estimated using the method described in Non-Patent Document 1, which estimates the viscosity based on the concentrations of components such as CaO, MgO, Al₂O₃, SiO₂, and FeO and the temperature.

[0059] The tube inlet loss coefficient Ke can be calculated using the following formula (10) described in Non-Patent Document 2.

[0060] Ke=0.5+0.3×sin(θ tap )+0.2×sin 2 (θ tap )…(10)

[0061] In the above formula (10), θ tap is the discharge angle of the melt 12 (rad).

[0062] Thus, in the method for detecting the melt height of the present embodiment, the discharge distance L of the melt 12 discharged from the tap hole is used. tap , the height h of the tap hole 14 tap and the discharge angle θ of the melt 12 tap To calculate the discharge velocity v of the melt 12 discharged from the tap hole 14 to the outside of the furnace tap Therefore, the discharge speed v of the melt 12 can be calculated more accurately than the method of obtaining the discharge speed of the melt 12 by photographing the shape of the melt 12. tap .

[0063] Then, the calculated discharge velocity v of the melt 12 is used. tap The height H of the melt 12 is calculated by the above formula (6), which states that the mechanical energy of the melt 12 accumulated in the lower part of the furnace is equal to the mechanical energy of the melt 12 immediately after being discharged from the tap hole 14 to the outside of the furnace. In the melt height detection method of this embodiment, the discharge speed v of the melt 12 is calculated more accurately than the method of obtaining the discharge speed of the melt 12 by photographing the shape of the melt 12. tap The height H of the melt 12 is calculated, and therefore the detection accuracy of the height H of the melt 12 is also improved.

[0064] Furthermore, when detecting the height H of the melt 12, it is preferable to use the above equation (7) instead of the above equation (6) in consideration of the resistance the melt 12 encounters from the tap hole 14. This further improves the detection accuracy of the height H of the melt 12.

[0065] Example

[0066] Next, the capacity is about 5000m 3 The validity of the method for detecting the height of the melt of this embodiment is confirmed by an example in which a large blast furnace is used. In this example, the discharge flow of the melt immediately after the hole is opened is photographed, and the discharge distance L is measured based on the image generated by the photographing. tap and the height h of the taphole tap The discharge flow is captured using a camera that has a guaranteed operating temperature of 70°C and can capture image data of 2064×1544 pixels. In this embodiment, the discharge flow is captured twice using a camera with varying timings. The discharge velocity v is detected based on the first captured image. tap The embodiment of the melt height H is set as Invention Example 1, and the discharge speed v is detected based on the second captured image. tap and melt height H are referred to as Invention Example 2.

[0067] Figure 4 These are images showing the discharge flows of Invention Examples 1 and 2. Figure 4 (a) is an image of the discharge flow of Invention Example 1, Figure 4 (b) is an image of the discharge flow of Invention Example 2. Figure 4 As shown in (a) and (b), the surface portion of the exhaust flow, that is, the boundary surface between the exhaust flow and the external gas, is not a smooth surface, but an irregularly undulating shape. This indicates that the surface portion of the exhaust flow is subject to strong viscous resistance from the inner wall of the exhaust hole and the external gas, and therefore, the exhaust flow becomes turbulent only near the surface of the exhaust flow. At this time, the flow velocity of the exhaust flow near the surface fluctuates irregularly due to the turbulence, and therefore, it is confirmed that, for example, the calculation method based on the present invention is more accurate than the previous method of estimating the flow velocity based on the position change of the image. In Invention Examples 1 and 2, the exhaust distance L is measured based on the image. tap and the height h of the taphole tap , use them to calculate the discharge velocity v tap . The discharge distance L tap , height of the taphole h tap The measurement results and the inclination angle θ of the taphole tap and discharge velocity v tap These are shown in Table 1 below.

[0068] [Table 1]

[0069]

[0070] Furthermore, using the above-mentioned discharge speed v tap, the melt height H in Invention Example 1 and Invention Example 2 was calculated. The parameters used in calculating the melt height H are shown in Table 2 below.

[0071] [Table 2]

[0072]

[0073] Figure 5 This graph shows the results of melt height H detection for Invention Examples 1 and 2. The melt height H detected in both Invention Examples 1 and 2 falls between the taphole height and the tuyere height. Furthermore, in Invention Example 2, where the discharge speed is faster than in Invention Example 1, the melt height H is even higher.

[0074] Next, using the molten pig iron slag balance as blast furnace operating data, we confirmed Figure 5 The validity of the calculation result of the melt height H shown. The molten pig iron slag balance refers to the value obtained by subtracting the amount of ironmaking slag from the amount of tapping slag calculated in volume conversion. A molten pig iron slag balance of 0 means that the amount of tapping slag is equal to the amount of ironmaking slag. When the molten pig iron slag balance is greater than 0, it means that the amount of tapping slag is greater than the amount of ironmaking slag, and the amount of melt accumulated in the lower part of the furnace decreases, so the melt height H decreases. On the other hand, when the molten pig iron slag balance is less than 0, it means that the amount of tapping slag is less than the amount of ironmaking slag, and the amount of melt accumulated in the lower part of the furnace increases, so the melt height H increases.

[0075] The amount of slag is calculated based on the oxygen balance of the blast furnace per unit time, the ratio of oxygen contained in the iron oxide in the raw materials charged from the furnace top, and the ratio of gangue components in the charged raw materials. Specifically, the amount of iron produced is calculated using the following formula (11), and the amount of slag is calculated using the following formula (12). These are then added together to calculate the amount of slag.

[0076] W p =(N top -N tuy )×(R Fe / o )×M Fe ÷R FeHM …(11)

[0077] W s =W p ×R G / Fe …(12)

[0078] In the above formulas (11) and (12), W p is the amount of iron produced (t / hour), W s is the slag production (t / hour). In addition, N top is the amount of oxygen atoms per unit time in the top gas (mol / hour), N tuyis the mass of oxygen atoms blown in from the tuyere per unit time (mol / hour), R Fe / O is the average number of iron atoms per oxygen atom in the iron oxide in the raw materials charged from the furnace top (-), M Fe is the molar mass of iron (t / mol), R FeHM is the mass fraction of iron in the molten pig iron (-), R G / Fe It is the mass of the charged gangue component per 1 t of molten pig iron (t / molten pig iron).

[0079] N top Calculated by analyzing the composition of the furnace top gas. tuy It is determined by analyzing the components of the air blown in from the tuyere. Fe / O and R G / Fe It is determined by analyzing the components of the raw materials charged from the furnace top. FeHM It is determined by analyzing the components of the molten pig iron tapped from the tap hole.

[0080] Figure 6 This is a graph showing the temporal change of the molten pig iron-slag equilibrium. Figure 6 The time of Invention Example 1 shown is the time when the discharge flow was photographed as Invention Example 1. The time of Invention Example 2 is the time when the discharge flow was photographed as Invention Example 2.

[0081] The temporal change in the equilibrium of the molten pig iron and slag indicates that the amount of melt accumulated in the lower portion of the furnace increases from the time of Invention Example 1 to the time of Invention Example 2. Therefore, the height H of the melt accumulated in the lower portion of the blast furnace is higher in the case of Invention Example 2 than in the case of Invention Example 1. Figure 5 As shown, the melt height H detected by the melt height detection method of the present embodiment is also higher in Invention Example 2 than in Invention Example 1, and the trends are consistent.

[0082] Next, the melt is highly balanced Figure 5 The results of the confirmation of the detection results of the melt height H shown are explained. The melt height balance is calculated based on the molten pig iron balance and the slag balance. Here, the molten pig iron balance is the value obtained by subtracting the iron making iron amount (t) from the tapping iron amount (t) in the tapping as the object, and the slag balance is the value obtained by subtracting the slag making amount (t) from the tapping slag amount (t). If the values ​​of these balances are negative, it means that the amount of molten pig iron slag in the furnace has increased, and if the values ​​of these balances are positive, it means that the amount of molten pig iron slag in the furnace has decreased. These balances are weight (t), so by dividing these balance values ​​by the density and the cross-sectional area of ​​the hearth, it can be converted into the melt height balance. Specifically, the melt height balance is calculated using the molten pig iron balance, the slag balance and the following formula (13).

[0083] Melt height balance = [molten pig iron balance (t) / molten pig iron density (kg / m 3 )+slag balance (t) / slag density (kg / m 3 )] / Effective cross-sectional area of ​​hearth (m 2 )…(13)

[0084] Will Figure 6 The molten pig iron balance, slag balance, and melt height balance calculated using these at the time of Invention Example 1 and Invention Example 2 are shown in Table 3 below. It should be noted that the effective cross-sectional area (m2) of the hearth in the above formula (13) was calculated based on past performance by setting the void ratio of the lower furnace to 0.35 and multiplying the cross-sectional area of ​​the lower furnace by this void ratio.

[0085] [Table 3]

[0086]

[0087] As shown in Table 3, the difference in melt height between Invention Example 1 and Invention Example 2 calculated based on melt height balance is 1.63 m (-0.89-(-2.52)=1.63). Figure 5 The difference in melt height between Invention Example 1 and Invention Example 2 is also substantially the same. This result confirms that the melt height in a blast furnace can be detected with high accuracy using the melt height detection method of this embodiment.

[0088] Explanation of symbols

[0089] 10 Blast Furnace

[0090] 12 Melt

[0091] 14 taphole

[0092] 16 Discharge Flow

[0093] 18 ditch

[0094] 20 Liquid level

[0095] 30 objects

Claims

1. A method for detecting the height of a melt, which is a method for detecting the height of a melt in a smelting furnace having a discharge hole at the bottom of the furnace for discharging the melt, wherein: measuring the discharge distance of the melt discharged from the discharge hole, Measuring the gas pressure inside the furnace and the gas pressure at the outlet of the discharge port of the smelting furnace, The discharge speed of the melt discharged from the discharge hole is calculated using the discharge distance, the height of the discharge hole, and the discharge angle of the melt. The melt height is detected using the discharge speed, the gas pressure in the furnace, the gas pressure at the discharge hole outlet, and the following formula (6): ρ in the above formula (6) l is the density of the melt, its unit is kg / m 3 ;v tap is the discharge speed of the melt, its unit is m / s; P1 is the gas pressure in the furnace, its unit is Pa; g is the acceleration due to gravity, which is 9.8m / s 2 ; H is the height of the melt accumulated in the lower part of the furnace, its unit is m; P2 is the gas pressure at the outlet of the discharge hole, its unit is Pa; h tap is the height of the discharge hole, in m.

2. The method for detecting the melt height according to claim 1, wherein: The height of the discharge hole is measured.

3. The method for detecting the melt height according to claim 1 or claim 2, wherein: The melt height is detected using the resistance encountered by the melt from the discharge hole and the discharge speed until the melt is discharged from the discharge hole.

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