JP6354074B2 - Raw material charging method to blast furnace - Google Patents

Description

  The present invention relates to a raw material charging method for a blast furnace. Specifically, in blast furnace operation in which a coke mixed ore layer and a coke layer are formed in layers in a blast furnace, the amount of coke mixed in the coke mixed ore layer is appropriately controlled according to the properties of the coke forming the coke layer. In addition, the present invention relates to a method for charging raw materials into a blast furnace to ensure air permeability inside the blast furnace.

In recent years, CO ₂ reduction has been demanded from the viewpoint of preventing global warming. In the steel industry, about 70% by weight of CO ₂ emissions are discharged from a blast furnace to produce pig iron, reduced CO ₂ emissions from the blast furnace is required. Reduction of CO ₂ in the blast furnace is possible by reducing reducing materials (coke, pulverized coal, natural gas, etc.) used in the blast furnace. In a blast furnace, iron ore that is a raw material (simply also referred to as “ore”) and coke that is a reducing material are usually charged from the top of the furnace so that they are alternately layered, and placed in the blast furnace. An ore layer and a coke layer are formed.

  On the other hand, when reducing the reducing material, particularly coke, the coke that secures the air permeability in the furnace is reduced, so that the ventilation resistance in the blast furnace furnace is increased. In a general blast furnace, when the iron ore charged from the top of the furnace reaches a temperature at which softening starts, the ore layer is deformed while filling the voids by the weight of the raw material existing in the upper part. Therefore, at the lower part of the blast furnace, the ventilation resistance of the ore layer is very large, and a fused layer (referred to as a “fusion zone”) in which almost no gas flows is formed. The air permeability of the cohesive zone has a great influence on the air permeability of the entire blast furnace, and the productivity in the blast furnace is limited.

  As means for improving the ventilation resistance of the cohesive zone, a raw material obtained by mixing ore and coke having a relatively small particle diameter (referred to as “mixed raw material”) and coke having a relatively large particle diameter in a blast furnace. It is known that it is effective to alternately charge and form a coke mixed ore layer made of a mixed raw material and a coke layer made of coke having a relatively large particle size in a layer shape. That is, it is known that mixing coke with an ore layer is effective, and many techniques for forming a coke mixed ore layer have been proposed.

  For example, in Patent Document 1, in a bell-less blast furnace, coke is charged into a downstream hopper among ore hoppers, and coke is deposited on the ore on a conveyor, and then charged into a furnace bunker. A technique for charging ore and coke into a blast furnace through a turning chute has been proposed.

  In Patent Document 2, in a bell-less blast furnace, when charging coke or ore stored in a plurality of furnace top bunkers from the center of the furnace toward the furnace wall in the radial direction of the furnace, Ore stored in another furnace bunker from the time when the discharge amount of the coke stored in one from the furnace bunker reaches a predetermined amount between 5 to 50% by mass of the amount of coke charged for one batch Has been proposed, and technology for simultaneously charging coke and ore is proposed. Thereby, it is said that three kinds of batches, ie, a normal charging batch for coke, a central charging batch for coke, and a batch for mixing charging can be performed simultaneously.

  In Patent Document 3, all ore and all coke are completely removed in order to prevent instability of the cohesive zone shape in blast furnace operation and decrease in gas utilization rate near the center, and to improve stable operation and thermal efficiency. A raw material charging technique for charging into a blast furnace after mixing has been proposed.

  In addition, in Patent Document 4, as a means of enjoying the reactivity improvement effect by mixed coke, by mixing highly reactive coke and ore with low JIS reducibility, low reactive ore is reacted with high efficiency, Technologies for improving the reactivity of the blast furnace have been proposed.

  On the other hand, since the amount of coke charged into the furnace (also referred to as “coke ratio”) is substantially constant, when coke is mixed with ore, the thickness of the coke layer relatively decreases. It is empirically known that when the thickness of the coke layer is reduced in the blast furnace, the ventilation resistance in the cohesive zone where the ore softens and melts increases, thereby impeding stable operation.

  Several proposals have been made to prevent an increase in the airflow resistance due to such a reduction in the thickness of the coke layer. For example, in Patent Document 5, in order to prevent local reduction of the coke layer thickness, the charging range of the coke at the furnace port portion is set to an area of 40% or more from the furnace wall side in the furnace radial direction. A technique has been proposed in which the average thickness of one layer of coke at the furnace port is 60 cm or more. Patent Document 6 proposes a technique for adjusting the amount of coke charged at the top of the furnace so that the average thickness of the coke layer at the furnace belly is 250 mm or more.

  Patent Document 5 and Patent Document 6 are operating conditions when a large amount of coke is not mixed in the ore. If a large amount of coke is mixed in the ore, the air permeability of the coke mixed ore layer (ore layer) is improved. Therefore, it is considered that the lower limit value of the coke layer thickness can be relaxed.

JP-A-3-211210 JP 2004-107794 A JP-A-53-152800 JP-A 64-36710 Japanese Patent Laid-Open No. 7-18310 JP-A-11-506393

  By the way, the coke forming the coke layer is used after being sieved with a sieve having a predetermined opening size, and coke having a size smaller than the opening size of the sieve (hereinafter referred to as “powdered coke”). Are not screened out. Therefore, in normal blast furnace operation, the properties of the coke forming the coke layer vary, and the content of the powdery coke in the coke layer varies depending on the properties of the coke.

In the coke layer, if the powdered coke content increases, the air permeability deteriorates.In this case, increase the thickness of the coke layer to ensure the air permeability of the coke layer, that is, the air permeability of the blast furnace. Is required. However, when the thickness of the coke layer is increased, it is necessary to reduce the amount of coke mixed with the mixed raw material. This is because when the amount of coke mixed with the mixed raw material is not reduced, the reducing material becomes excessive, and not only the CO ₂ emission amount increases but also the manufacturing cost increases.

  That is, in order to carry out stable blast furnace operation, the content of the powdery coke in the coke forming the coke layer is detected in advance, and the thickness of the coke layer is determined according to the detected content of the powdery coke. At the same time as increasing or decreasing, it is necessary to decrease or increase the amount of coke mixed with the mixed raw material. However, Patent Documents 1 to 6 do not consider this point at all.

  The present invention has been made in view of the above circumstances, and its object is to form a coke layer in a blast furnace raw material charging method in which a coke mixed ore layer and a coke layer are formed in layers in a blast furnace. The coke layer thickness is increased or decreased according to the content of powdered coke in the coke to be used, and the coke mixing amount of the coke mixed ore layer is appropriately controlled, thereby ensuring the air permeability inside the blast furnace. It is to provide a raw material charging method to a blast furnace.

The gist of the present invention for solving the above problems is as follows.
[1] Charging the mixed raw material and coke mixed with iron ore and coke alternately from the top of the blast furnace
A raw material charging method to a blast furnace in which a coke mixed ore layer made of the mixed raw material and a coke layer made of the coke are formed in a layer in a furnace,
A particle size measurement sensor installed above a transport facility for transporting coke for forming the coke layer to a blast furnace, wherein the short diameter of particles contained in the coke transported by the transport facility is in the range of 5 mm to 35 mm Measure the ratio of coke particles that are less than the minor axis of
When the coke for forming the coke layer is defined as a standard condition in which coke particles having a minor axis of 35 mm or less are not included, based on the measured ratio, the coke mixing amount in the mixed raw material is defined as a standard. Set less than the amount of coke in the mixed raw material
A method of charging a raw material into a blast furnace, in which a coke having a difference between a coke mixing amount under a reference condition and a coke mixing amount set based on the ratio is allocated as coke forming a coke layer.
[2] The coke mixing amount in the mixed raw material is set to be equal to or less than an upper limit value of the coke mixing amount calculated by substituting the measured ratio into the following equation (1). Raw material charging method to blast furnace.
Mix = [(9/10) × α−69 / 2] × β + 200 (1)
Here, in the formula (1), Mix is the upper limit value of the amount of coke in the mixed raw material (kg / molten iron-ton), α is the short diameter of the coke particles, and is an arbitrary value in the range of 5 mm to 35 mm. , Β is the ratio (% by mass) of coke particles having a minor axis of α mm or less.

  In the present invention, in a raw material charging method for forming a coke mixed ore layer and a coke layer into a blast furnace, the particle size distribution of the coke for forming the coke layer is measured by a coke conveyance facility to the blast furnace. . And based on the measurement result of this particle size distribution, the coke mixing amount to the coke mixed ore layer is set, and the coke mixing amount of the set coke mixing and the coke particles having a short diameter of 35 mm or less, which is the standard condition, are set. The coke amount that is the difference between the coke mixing amount when the ratio in the coke to be formed is zero is allocated as the coke that forms the coke layer. As a result, in the case of coke with a large amount of fine coke, the thickness of the coke layer increases, and as a result, the air permeability of the coke layer, that is, the air permeability inside the blast furnace is ensured, and the blast furnace operation is stable. Is realized.

FIG. 1 is a diagram for defining the minor axis of coke particles. FIG. 2 is a schematic view of a test apparatus used for measuring the relationship between the thickness of the coke layer and the cohesive zone ventilation resistance. FIG. 3 is a diagram showing the relationship between the ratio of coke particles having a minor axis of 5 mm or less and the upper limit value of the coke mixing amount in the mixed raw material. FIG. 4 is a diagram showing the relationship between the ratio of coke particles having a minor axis of 35 mm or less and the upper limit value of the coke mixing amount in the mixed raw material. FIG. 5 is a diagram showing the relationship between the ratio β when the upper limit value Mix is 50 kg / molten iron-ton and the minor axis α of the coke particles.

  Hereinafter, the present invention will be described in detail.

  In blast furnace operation in which a coke mixed ore layer composed of a mixed raw material in which iron ore and coke are mixed in a blast furnace and a coke layer are formed in layers, powdered coke in the coke forming a coke layer (sieving If the content of coke having a size smaller than the opening size of the distributor is increased, the air permeability of the coke layer, that is, the air permeability inside the blast furnace is deteriorated. Therefore, in this case, the amount of coke mixed into the coke mixed ore layer is reduced, and the amount corresponding to this reduced amount is charged as a coke layer, increasing the thickness of the coke layer, thereby increasing the air permeability inside the blast furnace. It is necessary to secure.

  The present inventors, for the purpose of stably ensuring the air permeability inside the blast furnace even if the coke content of the coke forming the coke layer changes, the ventilation resistance of the cohesive zone inside the blast furnace. The test was performed using a test apparatus that can simulate the above.

  Usually, the coke forming the coke layer is sieved with a sieving machine having an opening size of 35 mm, then conveyed to a blast furnace, and charged into the blast furnace. It is a well-known matter to those skilled in the art that if the coke has a mesh size of 35 mm and does not pass through a sieve, the air permeability of the cohesive zone inside the blast furnace can be secured. However, coke particles having a size that should be sieved by a sieve having an opening size of 35 mm are also mixed in the coke after being sieved by a sieve having an opening size of 35 mm. In addition, coke is pulverized by a drop impact during transportation to a blast furnace.

  In the present specification, coke having a size to be sieved by a sieving machine having an opening size of 35 mm and contained in the coke forming the coke layer is referred to as “coke having a minor axis of 35 mm or less”. Similarly, coke having a size that should be sieved by a sieving machine having an opening size of α mm is referred to as “coke having a minor axis of α mm or less”. Here, “the minor axis of coke particles” means that, as shown in FIG. 1, the distance between the intersections of the straight line passing through the center of gravity of the coke particles and the intersection of the circumference of the projection surface is the shortest on the projection surface of the coke particles. It is defined by the distance between intersections.

  In the test, focusing on the content of coke particles having a minor axis of 35 mm or less in the coke forming the coke layer, the condition that the minor axis is in the range of 5 mm to 35 mm as a condition for ensuring the air permeability of the cohesive zone inside the blast furnace. The relationship between the content ratio of coke particles having an arbitrary minor axis or less and the amount of coke mixed in the coke mixed ore layer was investigated.

  FIG. 2 shows a schematic diagram of a test apparatus used for measuring the relationship between the thickness of the coke layer and the cohesive zone ventilation resistance. In the figure, reference numeral 1 denotes a sample heating furnace, and the sample heating furnace 1 includes a sample filling container 2 and a heating device 3 therein. In addition, a sample filling layer 6 in which the coke layer 4 and the coke mixed ore layer 5 are filled in layers is formed inside the sample filling container 2. The temperature of the sample packed layer 6 is controlled by the heating device 3. Reference numeral 7 denotes a gas heating furnace, and the gas heating furnace 7 also includes a heating device 8 therein. Reference numeral 9 is a gas mixer, 10 is a gas distribution pipe, 11 is a pressure gauge, 12 is a thermocouple, 13 is a holding plate, 14 is a pedestal, 15 is a connecting rod, and this connecting rod 15 is made of graphite or metal. It is preferable to make it. Reference numeral 16 denotes a load means. In this example of the test apparatus, a weight 16 is used as the load means. The weight 16 applies a load simulating the inside of the blast furnace to the sample packed layer 6.

  As shown in the figure, this test apparatus has the greatest feature in that the sample heating furnace 1 and the gas heating furnace 7 are arranged in series. Thus, the series heating is performed in the gas heating furnace 7. The gas thus made enters the sample heating furnace 1 from the lateral direction.

  In this test apparatus, coke in which the ratio of particles having a minor axis of 5 mm or less was adjusted to a range of 0 to 5.0% by mass, and coke in which the ratio of particles having a minor axis of 35 mm or less was adjusted to 0 to 50% by mass. The coke layer 4 was formed using, the coke mixing amount of the coke mixed ore layer 5 was variously changed, and the air permeability was investigated. In the present specification, the condition that the coke for forming the coke layer does not contain coke particles having a minor axis of 35 mm or less is defined as the reference condition.

  Specifically, when the ratio of coke particles having a minor axis of 35 mm or less in the coke for forming the coke layer is zero, that is, the coke mixing amount of the coke mixed ore layer 5 under the standard condition is 200 kg / molten iron. -Tons. In addition, since the air permeability deteriorates as the content of the powdery coke in the coke for forming the coke layer 4 is increased, the coke mixed ore layer 5 is mixed in order to secure it. The coke was allocated to the coke layer 4 under various conditions, and the air permeability was investigated. And the coke mixing rate (kg / molten iron-ton) in the coke mixed ore layer 5 in the test in which the pressure loss in the test and the pressure loss under the standard conditions (conditions not containing coke particles having a minor axis of 35 mm or less) are equal. ) Was determined according to the ratio of particles having a minor axis of 5 mm or less and the ratio of particles having a minor axis of 35 mm or less.

  The results obtained in the test are shown in FIGS. FIG. 3 shows the relationship between the ratio of coke particles having a minor axis of 5 mm or less and the upper limit of the amount of coke mixed in the mixed raw material in a test using coke in which the ratio of particles having a minor axis of 5 mm or less is changed. FIG. FIG. 4 shows the relationship between the ratio of coke particles having a minor axis of 35 mm or less and the upper limit of the amount of coke mixed in the mixed raw material in a test using coke in which the ratio of particles having a minor axis of 35 mm or less is changed. FIG. 3 and 4, the coke mixing ratio (kg / molten iron-ton) at which the pressure loss is equal to the pressure loss of the reference condition is displayed as the upper limit value.

  As shown in FIGS. 3 and 4, the ratio of particles having a minor axis of 5 mm or less or 35 mm or less shown on the horizontal axis and the upper limit value of the coke mixing amount in the mixed raw material shown on the vertical axis are linearly related. It was. From this relationship, it is understood that the upper limit value of the coke mixing amount in the mixed raw material and the ratio of coke particles having a minor axis of 5 mm or less or 35 mm or less are expressed by a linear expression. Moreover, the influence of the ratio of the coke particles on the upper limit value of the coke mixing amount differs depending on whether the minor axis is 5 mm or less and the minor axis is 35 mm or less.

  Therefore, the upper limit value of the amount of coke in the mixed raw material is set to Mix (kg / molten iron-ton), the short diameter of the coke particles is set to α (mm), and the short diameter is 5 mm or less or 35 mm or less. When the particle ratio is β (mass%), these factors are expressed by the following formula (2). In the equation (2), 200 is the coke mixing amount (kg / molten iron-ton) under the reference conditions, and A and B are coefficients.

Mix = (A × α + B) × β + 200 (2)
The condition that Mix is 50 kg / molten-ton when the ratio β of coke particles having a minor axis of 5 mm or less in FIG. 3 is 5 mass%, and the ratio β of coke particles having a minor axis of 35 mm or less in FIG. 4 is 50 mass%. Substituting the condition in which Mix is 50 kg / molten-ton into Equation (2) to obtain the coefficients A and B, A = 9/10 and B = −69 / 2 are obtained. That is, the expression (2) is expressed by the following expression (1).

Mix = [(9/10) × α−69 / 2] × β + 200 (1)
Here, in the formula (1), Mix is the upper limit value of the amount of coke in the mixed raw material (kg / molten iron-ton), α is the short diameter of the coke particles, and is an arbitrary value in the range of 5 mm to 35 mm. , Β is the ratio (% by mass) of coke particles having a minor axis of α mm or less.

  In addition, the formula (1) indicates that the amount of coke mixed in the coke mixed ore layer when the ratio of coke particles having a minor axis of 35 mm or less in the coke for forming the coke layer is zero (standard condition) is 200 kg / In this case, it is not necessary to limit the amount of coke mixed under the standard condition to 200 kg / molten-ton.

  In blast furnace operation, the coke ratio (kg / molten iron-ton) required for the iron ore reduction reaction and the temperature rise of the molten iron produced is generally about 300 kg / molten iron-ton, but it depends on the operational status of each blast furnace. Will change accordingly. The coke ratio is the total amount of coke charged (kg / molten iron-ton) blended in both the coke mixed ore layer and the coke layer. That is, if the coke ratio is CR (kg / molten iron-ton), the amount of coke blended in the coke mixed ore layer under the reference condition is an amount obtained by multiplying the coke ratio CR by a certain blend ratio γ (−). (CR × γ) can be displayed.

  The present invention has been made based on the above test results, and the raw material charging method to the blast furnace according to the present invention is that the mixed raw material mixed with iron ore and coke and coke are alternately charged from the top of the blast furnace. And a raw material charging method for forming a coke layer in the blast furnace, in which a coke mixed ore layer made of the mixed raw material and a coke layer made of the coke are formed in a layer in a furnace. With the particle size measurement sensor installed above the transport facility (such as a belt conveyor) for transporting, the minor axis of the particles contained in the coke transported by the transport facility is not more than an arbitrary minor axis in the range of 5 mm to 35 mm. Coke ratio is measured, and based on the measured ratio, the amount of coke mixed in the mixed raw material is the reference condition, the coke layer of the coke particles having a minor axis of 35 mm or less. The amount of coke that is less than the coke mixing amount when the ratio in the coke to be formed is zero, and the difference between the coke mixing amount under the reference condition and the coke mixing amount set based on the ratio is determined as coke. Allocate as coke to form a layer. That is, in blast furnace operation with a constant coke ratio (kg / molten iron-ton), it was set based on the coke mixing amount under the standard conditions and the content of powdered coke in the coke for forming the coke layer 4. The difference in coke amount from the coke mixing amount is blended as coke forming the coke layer.

  In addition, it is preferable to transfer the difference coke amount from the coke mixed layer to the coke layer because the air permeability can be ensured without increase / decrease in the reducing material ratio, but it is preferable to have ± 5 kg / molten iron-ton in the transfer amount. Is possible.

  When the coke mixed in the coke mixed layer is transferred to the coke layer, the amount of coke mixed in the mixed raw material is calculated by substituting the measured ratio into the above formula (1). It is preferable to set it below the upper limit.

  That is, in the present invention, when fine grains (minor axis of 35 mm or less) included in the coke forming the coke layer increase, the air permeability of the coke layer deteriorates. Allocate the mixed coke to the coke layer to ensure air permeability inside the blast furnace. By adjusting the amount of coke in this way, the coke ratio (kg / molten iron-ton) is maintained at a certain predetermined value.

  As a particle size measurement sensor for measuring the particle size distribution of coke, for example, disclosed in publication 1 (publication 1; Japanese Patent Application Laid-Open No. 2003-83868), “a measurement object is imaged by an imaging device and imaged. Obtaining a blurred image obtained by performing a blurring process on the original image from the original image, and binarizing the blurred image, thereby measuring the particle size distribution of the measurement object having a predetermined particle size or more, and By performing a binarization process on the difference image formed by the difference between the captured original image and the blurred image, the distribution of the particle size of the measurement object smaller than the predetermined particle size is measured, and these two types of particle size measurement are performed. What is necessary is just to use the measuring apparatus etc. which utilized the particle size distribution measuring method "which measures the whole particle size distribution based on the measurement result of distribution. Specifically, a particle size measuring sensor capable of detecting the distance between intersections shown in FIG. 1 by image processing is used.

  As described above, according to the present invention, the coke transport facility to the blast furnace measures the particle size distribution of the coke forming the coke layer, and the coke to the coke mixed ore layer based on the measurement result of the particle size distribution. Since the mixing amount and the amount of coke added to the coke layer are controlled, air permeability in the blast furnace is ensured, and stabilization of the blast furnace operation is realized.

  In the actual blast furnace, when the raw material charging is performed by applying the present invention under the same coke ratio and the same output ratio, and when the raw material charging is performed outside the scope of the present invention, the gas utilization rate And the result of investigating the pressure loss in the packed bed is shown in comparison. As a particle size measuring sensor for measuring the particle size distribution of coke, a measuring device using the particle size distribution measuring method disclosed in publication 1 is used, and this measuring device conveys coke for forming a coke layer to a blast furnace. Installed above the belt conveyor.

  In the coke for forming the coke layer, the ratio of coke particles having a minor axis of 5 mm or less was measured by a particle size measuring sensor. The coke mixing amount in the mixed raw material (kg / molten iron-ton), the upper limit value Mix (kg) of the coke mixing amount calculated by substituting the measured value of the ratio of coke particles having a minor axis of 5 mm or less into the equation (1) / Hot iron-ton) (Invention examples 1 and 2) and when the coke mixing amount (kg / hot metal-ton) in the mixed raw material is set to a value exceeding the upper limit according to the formula (1) (comparison) Examples 1 and 2) are compared and shown in Table 1.

  In the coke for forming the coke layer, the ratio of coke particles having a minor axis of 35 mm or less was measured by a particle size measuring sensor. The coke mixing amount (kg / molten-ton) in the mixed raw material, and the coke mixing amount upper limit value Mix (kg) calculated by substituting the measured value of the ratio of coke particles having a minor axis of 35 mm or less into the equation (1) / Cold iron-ton) (Invention Examples 3 and 4) and when the amount of coke mixed in the mixed raw material (kg / hot metal-ton) exceeds the upper limit according to the formula (1) (comparison) Examples 3 and 4) are compared and shown in Table 2.

  As shown in Tables 1 and 2, it was confirmed that when the present invention was applied, the gas utilization rate was improved and the pressure loss in the packed bed was reduced. That is, it has been confirmed that the application of the present invention enables stable blast furnace operation.

DESCRIPTION OF SYMBOLS 1 Sample heating furnace 2 Sample filling container 3 Heating device 4 Coke layer 5 Coke mixed ore layer 6 Sample packed layer 7 Gas heating furnace 8 Heating device 9 Gas mixer 10 Gas distribution pipe 11 Pressure gauge 12 Thermocouple 13 Press plate 14 Base 15 connecting rod 16 weight

Claims (2)

The mixed raw material mixed with iron ore and coke and coke are alternately charged from the top of the blast furnace,
A raw material charging method to a blast furnace in which a coke mixed ore layer made of the mixed raw material and a coke layer made of the coke are formed in a layer in a furnace,
Coke particles having a particle diameter measuring sensor installed above a transport facility for transporting coke for forming the coke layer to a blast furnace, wherein the minor axis of the particles contained in the coke transported by the transport facility is 5 mm or less. the ratio of the measured,
When the coke for forming the coke layer is defined as a standard condition in which coke particles having a minor axis of 35 mm or less are not included, the standard condition is based on the minor axis of the coke particle and the measured ratio. Calculating a coke mixing amount equal to the pressure loss of the coke, and setting the coke mixing amount in the mixed raw material to be equal to or less than the calculated coke mixing amount ,
The difference in the amount of coke and coke mixing amount before Symbol set coke mixed amount of the reference condition and allocation as coke that forms a coke layer, the raw material charging process of the blast furnace. The raw material to the blast furnace according to claim 1, wherein the coke mixing amount in the mixed raw material is set to be equal to or less than an upper limit value of the coke mixing amount calculated by substituting the measured ratio into the following equation (1). The charging method.
Mix = [(9/10) × α−69 / 2] × β + 200 (1)
Here, in the equation (1),
Mix is the upper limit of the amount of coke in the mixed raw material (kg / molten iron-ton),
α is 5 mm, which is the short diameter of the coke particles,
β is a ratio (mass%) of coke particles having a minor axis of 5 mm or less.

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