KR desulfurization method

Abstract

The invention relates to the field of metallurgy, and discloses a KR desulfurization method, in a molten iron pretreatment process, under the conditions that a KR stirring head is inserted into molten iron in a ladle and the KR stirring head rotates, a first desulfurization agent and a desulfurization auxiliary agent are added to the surface of the molten iron at the same time, and then a second desulfurization agent is added to the surface of the molten iron; and the molten iron in the ladle is desulfurized. The KR desulfurization method disclosed by the invention is beneficial to shortening the desulfurization time and improving the production efficiency while meeting the desulfurization effect, and is suitable for popularization and application in iron and steel enterprises.

Description

Technical Field

[0001] The invention relates to the field of metallurgy, and in particular to a KR desulfurization method. Background Art

[0002] At present, the following methods are widely used for hot metal desulfurization: mechanical stirring method, injection method, injection method, KR method, and magnesium-based desulfurization method. The mechanical stirring method is to immerse the stirrer in the hot metal and rotate it at high speed to generate strong vortex and convection motion, so that the added desulfurizer is fully mixed with the hot metal. The desulfurizer reacts chemically with the sulfur in the hot metal, and the generated sulfide enters the slag phase, thereby reducing the sulfur content of the hot metal. The equipment structure of the mechanical stirring method is complex, and the maintenance and replacement cost of the stirrer is high. A certain amount of air may also be brought in during the stirring process, increasing the oxidation of the hot metal. The injection method uses an inert gas as a carrier to spray the desulfurizer into the hot metal through a spray gun. The gas carries the desulfurizer to form a high-speed jet, so that the desulfurizer is evenly dispersed in the hot metal. The desulfurizer reacts with sulfur to generate sulfides that enter the slag phase or gas phase to achieve desulfurization. In the injection method, the desulfurizer is not in sufficient contact with the sulfur in the hot metal, the desulfurization efficiency is low, and the desulfurizer consumption is large. The injection method is to inject the desulfurizer directly into the surface of the molten iron. The desulfurizer melts and mixes with the molten iron under the heat of the molten iron, and reacts chemically with sulfur. The sulfide produced by the reaction enters the slag phase, and then the sulfur-containing slag is removed by slagging. In the injection method, it takes a certain amount of time for the desulfurizer to melt and mix on the surface of the molten iron, and it is easy to cause local overreaction and surface slagging, the desulfurization efficiency is low, and the utilization rate of the desulfurizer is not high. The KR method is a special form of mechanical stirring method, which is similar to the general mechanical stirring method, but the stirrer is a special cross or T-shaped, which can produce a stronger stirring effect when rotating at high speed. At the same time, the KR method has good adaptability to desulfurizers and can use a variety of desulfurizers. The KR method has a good desulfurization effect and can reduce the sulfur content of molten iron to a very low level, but the equipment is complex, the maintenance cost of the stirrer is high, and the technical requirements for operators are also high. The magnesium-based desulfurization method uses magnesium or magnesium-based alloy as a desulfurizer. Magnesium vaporizes rapidly in molten iron, and the generated magnesium vapor reacts with sulfur in the molten iron to form magnesium sulfide. The reaction speed is fast and can quickly reduce the sulfur content of molten iron. The magnesium-based desulfurizer can be added to the molten iron by spraying or pouring. The magnesium-based desulfurization method has extremely high desulfurization efficiency and fast reaction speed; however, the cost of magnesium is relatively high, and there are certain safety risks during the operation, which requires strict control of the operating environment and conditions.

[0003] CN 109609715 A discloses a desulfurization method of a desulfurization accelerator composition and molten iron and its application. The method comprises contacting the desulfurization accelerator composition and a desulfurizer with molten iron, wherein the desulfurization accelerator composition comprises a desulfurization accelerator and a desulfurization aid; wherein the desulfurization accelerator contains aluminum and aluminum oxide, and the desulfurization aid comprises carbonate and / or calcium aluminate; and the desulfurization aid is selected from calcium carbonate, sodium carbonate, 12CaO· 7Al2O3; the dosage ratio of the desulfurization accelerator to the desulfurization auxiliary agent is 1:0.02-0.2; the aluminum content in the desulfurization accelerator is 10-20% by weight, and the aluminum oxide content is more than 50% by weight; the particle size of the desulfurization accelerator is less than 2mm; the desulfurization agent contains calcium oxide; relative to 100 parts by weight of the desulfurizer, the dosage of the desulfurization accelerator is 10-30 parts by weight; relative to 100 parts by weight of the desulfurizer and the desulfurization accelerator, the dosage of the desulfurization auxiliary agent is 0.2-2 parts by weight; relative to 1 ton of molten iron, the dosage of the desulfurizer is 5-7kg, and the dosage of the desulfurization accelerator composition is 0.5-3kg. The prior art desulfurization accelerator composition and desulfurization method do not use fluorite, can make the fluorine content in the wastewater and waste residue meet the environmental benchmark value, and have a good desulfurization effect. However, the alumina in the desulfurization accelerator of the prior art may react with other elements in the molten iron to affect the quality of the molten iron, and the desulfurization agent quicklime reacts with other components in the slag at high temperature to corrode the furnace lining and increase the equipment maintenance cost.

[0004] CN 112877505 B discloses a fluorine-free KR desulfurizer and a desulfurization method thereof. The desulfurizer comprises the following components in weight percentage: active lime: 75%-85%, aluminum ash: 6%-9%, sodium borate calcite: 6%-12%, and calcium carbide: 3%-4%; the weight percentage content of B2O3 in sodium borate calcite is ≥45%, and the weight percentage content of Na2O is ≥5%; the weight percentage content of CaO in lime is ≥90%, and the weight percentage content of SiO2 is ≤2%. The prior art realizes the fluorine-free desulfurizer, reduces the pollution to the environment, and can also effectively promote the dissolution of limestone, improve the use efficiency of the desulfurizer, and thus improve the desulfurization effect. In addition, it also effectively reduces the agglomeration phenomenon of the desulfurizer, and improves the utilization rate and desulfurization efficiency of the desulfurizer. However, the aluminum ash, the slag formed by the sodium borate calcite, and the products generated by the calcium carbide in the prior art have a corrosive effect on the refractory material and increase the equipment maintenance cost.

[0005] CN 111254257 B discloses a method for mechanically stirring desulfurization of molten iron based on bottom-blowing powder agent. After the rotation speed is increased to the working rotation speed, the desulfurizer is sprayed into the molten iron from the bottom of the molten iron tank; the particle size of the desulfurizer is less than 0.1 mm, and the desulfurizer is sprayed into the molten iron through a permeable brick installed at the bottom of the molten iron tank; the permeable brick adopts a slit-type powder-spraying permeable brick, and the installation position is at a distance D from the center of the tank bottom, 0≤D<L / 2, wherein L is the rotating diameter length of the stirrer; the mass percentages of the desulfurizer components are: CaO: 58-60%, Al2O3: 12-14%, SiO2: 3-5%, MgO: 10-11%, CaF2: 2-3%, soda ash: 3-4%, K2O: 3-5%, Na2O: 3-4%. This prior art improves the thermodynamic conditions of the desulfurization reaction, increases the effective length of the agitator blades, and effectively reduces the serious slag sticking between the agitator blades caused by the rigid rotor, which in turn leads to reduced stirring performance and short agitator life. However, the CaF2, K2O and Na2O in this prior art have an erosive effect on refractory materials and increase equipment maintenance costs. At the same time, the air-permeable bricks are easily blocked, affecting production.

[0006] Based on this, there is room for improvement in the desulfurization agent for KR desulfurization. Summary of the invention

[0007] In view of this, the present invention provides a KR desulfurization method to solve the technical problems of low production efficiency and large consumption of desulfurizer in the existing KR desulfurization process.

[0008] The present invention provides a KR desulfurization method, comprising: in the process of molten iron pretreatment, under the condition that the KR stirring head is inserted into the molten iron in the ladle and the KR stirring head is rotated, by simultaneously adding a first desulfurizer and a desulfurization auxiliary agent to the surface of the molten iron respectively, and then adding a second desulfurizer to the surface of the molten iron, so as to desulfurize the molten iron in the ladle.

[0009] In some embodiments, the first desulfurization agent is passivated active lime, the desulfurization auxiliary agent is liquid aluminum, and the second desulfurization agent is ultrafine calcium carbonate powder.

[0010] In some embodiments, the KR desulfurization method comprises the following steps: Step 1, inserting a KR stirring head into molten iron in a ladle and rotating the KR stirring head at a first rotation speed; Step 2, adding a first desulfurizing agent in a first amount to the surface of the molten iron in the ladle, and simultaneously adding a desulfurizing auxiliary agent in a second amount to the surface of the molten iron; Step 3, after the first desulfurizing agent and the desulfurizing auxiliary agent are added, the rotation speed of the KR stirring head is increased to the second rotation speed and maintained for the first time; Step 4: adding a second desulfurizing agent in a third amount to the surface of the molten iron in the ladle.

[0011] In some embodiments, in step 1, the KR stirring head is inserted into the molten iron in the ladle at a position 300 to 500 mm from the bottom of the ladle, and the first rotation speed is 90 to 110 rpm.

[0012] In some embodiments, in step 2, the first added amount is 3-4 kg / t molten iron.

[0013] In some embodiments, in step 2, the second added amount is 0.2-0.4 kg / t molten iron.

[0014] In some embodiments, the particle size of the first desulfurizing agent is less than 2 mm.

[0015] In some embodiments, in step 3, the second rotation speed is 110-130 rpm, and the first time is 4-5 minutes.

[0016] In some embodiments, in step 4, the third added amount is 0.6-1.2 kg / t molten iron.

[0017] In some embodiments, the particle size of the second desulfurizing agent is less than 30 μm.

[0018] Beneficial effects of the present invention: The KR desulfurization method of the present invention adds desulfurizing agent passivated active lime and ultrafine calcium carbonate powder to molten iron in a suitable adding mode according to the properties, and selects a suitable addition amount, so that the desulfurization effect of the passivated active lime is optimized, and ultrafine active calcium oxide produced by decomposition of calcium carbonate is used for deep desulfurization, so that the molten iron is effectively desulfurized, so that the sulfur content in the desulfurized molten iron is less than 20ppm, and the synergistic effect of the desulfurizer shortens the desulfurization time by more than 2 minutes. The KR desulfurization method of the present invention not only meets the desulfurization effect, but also helps to shorten the desulfurization time and improve production efficiency, and is suitable for promotion and application by steel enterprises, and shortening the desulfurization time has certain economic benefits. DETAILED DESCRIPTION

[0019] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments.

[0020] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. The subsequent embodiments will not explain this one by one.

[0021] The KR desulfurization method of the present invention comprises: during the molten iron pretreatment process, under the condition that the KR stirring head is inserted into the molten iron in the ladle and the KR stirring head is rotated, the molten iron in the ladle is desulfurized by adding a first desulfurizer and a desulfurization auxiliary agent to the surface of the molten iron at the same time and then adding a second desulfurizer to the surface of the molten iron.

[0022] In some embodiments, the first desulfurization agent is passivated active lime, the desulfurization auxiliary agent is liquid aluminum, and the second desulfurization agent is ultrafine calcium carbonate powder.

[0023] Passivated active lime desulfurization is mainly the reaction of calcium oxide (CaO) with sulfur (S) in molten iron to generate calcium sulfide (CaS) and oxygen (O). Under the condition of molten iron temperature of about 1300-1400℃, when the initial sulfur content is high, the desulfurization reaction is easy to proceed, thereby achieving the purpose of desulfurization. As the reaction proceeds, the oxygen content in the system increases, which is not conducive to the desulfurization reaction. Adding aluminum liquid to deoxidize reduces the oxygen potential, which is conducive to the desulfurization reaction, thereby improving the desulfurization efficiency. Passivated active lime is a lime product after active lime has been passivated. The reaction activity has been properly controlled. During the desulfurization process of molten iron, it can stably react with sulfur in molten iron, and will not cause safety accidents such as molten iron splashing due to excessive reaction, making the desulfurization process more stable and safe, which is conducive to improving the controllability and desulfurization efficiency of the desulfurization operation. The method of adding passivated active lime to the surface of molten iron can adopt the commonly used method in the field. For example, a lime silo with a controllable opening and closing outlet at the bottom above the molten iron ladle can be used to drop the passivated active lime onto the surface of the molten iron by gravity. The passivating active lime powder can be sprayed onto the surface of the molten iron using a special spray gun device using a gas such as nitrogen as a carrier.

[0024] Ultrafine calcium carbonate (CaCO3) powder will decompose in the environment of high-temperature molten iron to generate calcium oxide and carbon dioxide. The calcium oxide desulfurization is as described above. Because ultrafine calcium carbonate powder has a large specific surface area, it can decompose quickly at high temperature, and the calcium oxide produced by decomposition is more active and can react quickly with the sulfur in the molten iron, achieving efficient desulfurization in a short time. Ultrafine calcium carbonate powder can be better dispersed in the molten iron, increasing the contact area with the molten iron, which can make the desulfurization reaction more sufficient, thereby improving the desulfurization efficiency. Ultrafine calcium carbonate powder is relatively pure and will not introduce too many impurity elements during the desulfurization process, which is very beneficial for ensuring the purity and quality of molten iron. The method of adding ultrafine calcium carbonate powder to the surface of molten iron can adopt the commonly used method in the field. For example, the ultrafine calcium carbonate powder can be discharged to the surface of molten iron by cooperating with a vibrating feeder and a chute. The ultrafine calcium carbonate powder can be discharged to the surface of molten iron by a screw feeder and a conveying pipeline.

[0025] The present invention uses aluminum liquid to reduce temperature drop and shorten melting time on the one hand; on the other hand, it can quickly deoxidize and create thermodynamic conditions with low oxygen potential for desulfurization. The aluminum liquid can be added to the surface of the molten iron in a manner commonly used in the art. For example, the aluminum liquid can be placed above the ladle and the container can be slowly tilted to flow along a specific chute into the surface of the molten iron or directly into the surface of the molten iron. The aluminum liquid can be sprayed into the molten iron by a special spray gun.

[0026] The KR desulfurization method of the present invention comprises the following steps: Step 1, inserting a KR stirring head into molten iron in a ladle and rotating the KR stirring head at a first rotation speed; Step 2, adding a first desulfurizing agent in a first amount to the surface of the molten iron in the ladle, and simultaneously adding a desulfurizing auxiliary agent in a second amount to the surface of the molten iron; Step 3, after the first desulfurizing agent and the desulfurizing auxiliary agent are added, the rotation speed of the KR stirring head is increased to the second rotation speed and maintained for the first time; Step 4: adding a second desulfurizing agent in a third amount to the surface of the molten iron in the ladle.

[0027] In some embodiments, in step 1, the KR stirring head is inserted into the molten iron in the ladle at a position 300-500 mm from the bottom of the ladle, and the first rotation speed is 90-110 rpm (revolutions per minute).

[0028] In some embodiments, in step 2, the first added amount is 3-4 kg / t molten iron.

[0029] In some embodiments, in step 2, the second added amount is 0.2-0.4 kg / t molten iron.

[0030] In some embodiments, the particle size of the first desulfurizing agent is less than 2 mm.

[0031] In some embodiments, in step 3, the second rotation speed is 110-130 rpm, and the first time is 4-5 minutes.

[0032] In some embodiments, in step 4, the third added amount is 0.6-1.2 kg / t molten iron.

[0033] In some embodiments, the particle size of the second desulfurizing agent is less than 30 μm.

[0034] In some embodiments, the first desulfurizer, the auxiliary desulfurizer, and the second desulfurizer flow from the center of the molten iron in the ladle toward the surface of the molten iron.

[0035] The KR desulfurization method of the present invention achieves deep desulfurization of molten iron and shortens the desulfurization treatment time by matching the type of desulfurizer and the method of adding the desulfurizer to molten iron and selecting a suitable addition amount of the desulfurizer.

[0036] Example 1 During the pretreatment of molten iron, the sulfur content in the molten iron in the ladle before desulfurization is 0.06%. The KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 90rpm. The position where the KR stirring head is inserted into the molten iron in the ladle is 500mm from the bottom of the ladle. Then, in the ladle, passivation active lime is added to the surface of the molten iron in an amount of 3kg / t molten iron, and the particle size of the passivation active lime is less than 2mm, and aluminum liquid is added to the surface of the molten iron at the same time in an amount of 0.2kg / t molten iron. After the passivation active lime and aluminum liquid are added, the rotation speed of the KR stirring head is increased to a rotation speed of 110rpm and maintained for 4 minutes. Then, in the ladle, calcium carbonate ultrafine powder is added to the surface of the molten iron in an amount of 0.6kg / t molten iron, and the particle size of the calcium carbonate ultrafine powder is less than 30μm. After the molten iron is desulfurized, the sulfur content in the molten iron is reduced to 15ppm, and the desulfurization treatment time is 18 minutes.

[0037] Example 2 During the pretreatment of molten iron, the sulfur content in the molten iron in the ladle before desulfurization is 0.08%, the KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 100rpm, and the position where the KR stirring head is inserted into the molten iron in the ladle is 400mm from the bottom of the ladle. Then, in the ladle, passivation active lime is added to the surface of the molten iron in an amount of 3.5kg / t molten iron, and the particle size of the passivation active lime is less than 2mm, and aluminum liquid is added to the surface of the molten iron at the same time in an amount of 0.3kg / t molten iron. After the passivation active lime and aluminum liquid are added, the rotation speed of the KR stirring head is increased to a rotation speed of 120rpm and maintained for 4.5 minutes. Then, in the ladle, calcium carbonate ultrafine powder is added to the surface of the molten iron in an amount of 0.9kg / t molten iron, and the particle size of the calcium carbonate ultrafine powder is less than 30μm. After desulfurization of the molten iron, the sulfur content in the molten iron was reduced to 17 ppm, and the desulfurization treatment time was 19 minutes.

[0038] Example 3 During the pretreatment of molten iron, the sulfur content in the molten iron in the ladle before desulfurization is 0.09%. The KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 110rpm. The position where the KR stirring head is inserted into the molten iron in the ladle is 300mm away from the bottom of the ladle. Then, in the ladle, passivation active lime is added to the surface of the molten iron in an amount of 4kg / t molten iron, and the particle size of the passivation active lime is less than 2mm, and aluminum liquid is added to the surface of the molten iron at the same time in an amount of 0.4kg / t molten iron. After the passivation active lime and aluminum liquid are added, the rotation speed of the KR stirring head is increased to a rotation speed of 130rpm and maintained for 5 minutes. Then, in the ladle, calcium carbonate ultrafine powder is added to the surface of the molten iron in an amount of 1.2kg / t molten iron, and the particle size of the calcium carbonate ultrafine powder is less than 30μm. After the molten iron is desulfurized, the sulfur content in the molten iron is reduced to 19ppm, and the desulfurization treatment time is 20 minutes.

[0039] Comparative Example 1 During the molten iron pretreatment process, the sulfur content in the molten iron in the ladle before desulfurization is 0.06%. The KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 100 rpm. The position where the KR stirring head is inserted into the molten iron in the ladle is 500 mm from the bottom of the ladle. Then, passivated active lime is added to the surface of the molten iron in the ladle at an addition amount of 5 kg / t of molten iron. After the molten iron is desulfurized, the sulfur content in the molten iron is reduced to 25 ppm, and the desulfurization treatment time is 21 minutes.

[0040] Comparative Example 2 During the molten iron pretreatment process, the sulfur content in the molten iron in the ladle before desulfurization is 0.08%. The KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 110rpm. The position where the KR stirring head is inserted into the molten iron in the ladle is 400mm from the bottom of the ladle. Then, passivated active lime is added to the surface of the molten iron in the ladle at an addition amount of 6kg / t of molten iron. After the molten iron is desulfurized, the sulfur content in the molten iron is reduced to 27ppm, and the desulfurization treatment time is 23 minutes.

[0041] Comparative Example 3 During the molten iron pretreatment process, the sulfur content in the molten iron in the ladle before desulfurization is 0.09%. The KR stirring head is inserted into the molten iron in the ladle and the KR stirring head rotates at a rotation speed of 120rpm. The position where the KR stirring head is inserted into the molten iron in the ladle is 300mm from the bottom of the ladle. Then, passivated active lime is added to the surface of the molten iron in the ladle at an addition amount of 6.5kg / t molten iron. After the molten iron is desulfurized, the sulfur content in the molten iron is reduced to 30ppm, and the desulfurization treatment time is 25 minutes.

[0042] The sulfur content in the molten iron in the ladle before desulfurization is different, and the amount of desulfurizer and desulfurization time required to achieve a better desulfurization effect are different. Compared with the existing KR desulfurization method, the KR desulfurization method of the present invention shortens the desulfurization time, reduces the total amount of desulfurizer used, and improves the desulfurization effect accordingly, under the premise that the sulfur content in the molten iron in the ladle before desulfurization is the same.

[0043] The KR desulfurization method of the present invention adds desulfurizer passivated active lime and ultrafine calcium carbonate powder to molten iron in a suitable adding mode according to the properties, and selects a suitable addition amount to achieve the best desulfurization effect of the passivated active lime, and performs deep desulfurization by ultrafine active calcium oxide produced by decomposition of calcium carbonate, thereby achieving effective desulfurization of the molten iron, so that the sulfur content in the desulfurized molten iron is less than 20ppm, and the synergistic effect of the desulfurizer shortens the desulfurization time by more than 2 minutes. The KR desulfurization method of the present invention has a suitable amount of desulfurizer, and the impurities generated by the desulfurizer in the molten iron are very few, which will not affect the quality of the molten iron, and the erosion effect on the refractory material is weak or almost negligible, which will not affect the service life of the refractory material.

[0044] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0045] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A KR desulfurization method, characterized in that: include: During the molten iron pretreatment process, under the condition that the KR stirring head is inserted into the molten iron in the ladle and the KR stirring head is rotated, the molten iron in the ladle is desulfurized by adding the first desulfurizer and the desulfurization auxiliary agent to the surface of the molten iron at the same time and then adding the second desulfurizer to the surface of the molten iron.

2. The KR desulfurization method according to claim 1, characterized in that: The first desulfurization agent is passivated active lime, the desulfurization auxiliary agent is aluminum liquid, and the second desulfurization agent is ultrafine calcium carbonate powder.

3. The KR desulfurization method according to claim 2, characterized in that: The following steps are involved: Step 1, inserting the KR stirring head into the molten iron in the ladle and rotating the KR stirring head at a first rotation speed; Step 2, adding the first desulfurizing agent in a first amount to the surface of the molten iron in the ladle, and simultaneously adding the desulfurization auxiliary agent in a second amount to the surface of the molten iron; Step 3, after the first desulfurizing agent and the desulfurizing auxiliary agent are added, the rotation speed of the KR stirring head is increased to a second rotation speed and maintained for a first time; Step 4: adding the second desulfurizing agent in a third amount to the surface of the molten iron in the ladle.

4. The KR desulfurization method according to claim 3, characterized in that: In the step 1, the KR stirring head is inserted into the molten iron in the ladle at a position 300 to 500 mm away from the bottom of the ladle, and the first rotation speed is 90 to 110 rpm.

5. The KR desulfurization method according to claim 3, characterized in that: In step 2, the first added amount is 3-4 kg / t molten iron.

6. The KR desulfurization method according to claim 3, characterized in that: In step 2, the second added amount is 0.2-0.4 kg / t molten iron.

7. The KR desulfurization method according to claim 3, characterized in that: The particle size of the first desulfurizing agent is less than 2 mm.

8. The KR desulfurization method according to claim 3, characterized in that: In the step 3, the second rotation speed is 110-130 rpm, and the first time is 4-5 minutes.

9. The KR desulfurization method according to claim 3, characterized in that: In step 4, the third added amount is 0.6-1.2 kg / t molten iron.

10. The KR desulfurization method according to claim 3, characterized in that: The particle size of the second desulfurizing agent is less than 30 μm.