Method for preparing rare earth steel by refining prefabricated slag of CaO-Al2O3-REO-MgO slag system

Abstract

The invention relates to a method for preparing rare earth steel by refining prefabricated slag of a CaO-Al2O3-REO-MgO slag system, belongs to the technical field of ferrous metallurgy, and solves the technical problem of how to prepare the rare earth steel by refining the prefabricated slag of the CaO-Al2O3-REO-MgO slag system so as to improve the rare earth yield. The invention discloses a method for refining and preparing rare earth steel by using prefabricated slag of a CaO-Al2O3-REO-MgO slag system. The method comprises the following steps: S1, paving a deoxidizing agent, the prefabricated slag and lime at the bottom of a steel ladle before tapping, and tapping; s2, slag adjustment operation of the LF furnace; step S3, alloying is carried out; s4, treating white slag for more than or equal to 20 minutes, and performing calcium treatment; s5, argon blowing is carried out, and an LF furnace station is left; s6, RH treatment is carried out; s7, adding a rare earth iron alloy; s8, vacuum breaking is carried out after vacuum circulation, and the robot leaves the RH station; s9, a tundish covering agent is scattered on the surface of the molten steel; and S10, the surface of the molten steel is protected through continuous casting crystallizer casting powder. The yield of the rare earth elements is 40% or above.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a method for preparing rare earth steel by refining with a prefabricated slag using a CaO-Al2O3-REO-MgO slag system. Background Art

[0002] Rare earth elements have the functions of refining grains, improving properties, inhibiting harmful elements, deoxidizing, desulfurizing and modifying in steel. However, rare earth elements have strong chemical activity, resulting in not only low and uncontrollable recovery rates during smelting, but also the fact that rare earths are not easily present in the form of metal bonds in steel and tend to exist in the form of ionic bonds, thus unable to fully, effectively and controllably play the role of rare earth elements. Therefore, the smelting process of rare earth-treated steel has attracted increasing attention.

[0003] Due to the above characteristics of rare earths, during the preparation of rare earth steel, the molten steel produced by converters and electric arc furnaces needs to be further processed, which is called secondary refining, such as LF furnace refining, VOD refining, VAD refining, AOD refining, RH refining, etc. To achieve refining purposes, such as degassing, deoxidizing, dephosphorizing, desulfurizing, removing inclusions and fine-tuning the composition, a slag system composed of different oxide components needs to participate in the metallurgical reaction, such as slag composed of metal oxides such as CaO, SiO2, Al2O3, MgO, FeO, MnO, etc., that is, refining slag.

[0004] Existing research shows that refining slag containing rare earth oxides can not only improve the recovery rate of rare earth elements in rare earth steel refining and improve the existing form of rare earths in steel, but also rare earth oxides can improve the deoxidation ability of deoxidizing elements in the refining slag and absorb non-metallic inclusions in the molten steel, thereby improving the cleanliness of the molten steel. In addition, rare earth oxides also affect the physical properties of the refining slag such as melting point, viscosity and basicity. By reasonably controlling the composition and physical properties of the refining slag, the cleanliness of the molten metal can be improved. Further, due to the above advantages of the refining slag containing rare earth oxides, during the continuous casting process after refining, protective slag containing rare earth oxides is also used to protect the molten steel being cast, such as tundish covering agent, continuous casting mold powder, etc.

[0005] In summary, in the refining and protective casting processes, using refining slag added with a certain proportion of rare earth oxides or rare earth oxide slag systems is an effective means to improve the quality of the refining process and the recovery rate of rare earths. However, due to the scarcity and activity of rare earths, it is difficult to obtain rare earth raw materials with suitable compositions and good economy. Generally, waste steel slag obtained during the smelting of rare earth steel or tailings after the beneficiation and smelting of rare earth-containing ores are used, not only the composition is difficult to control, but also the effect is uncertain.

[0006] Therefore, how to find rare earth oxides with good economy and apply a refined slag containing rare earth oxides with appropriate composition of the rare earth oxides and other slag-making materials to the refining process of rare earth steel has become a technical problem faced by those skilled in the art. Summary of the Invention

[0007] In view of the above technical problems, the present invention provides a method for refining and preparing rare earth steel using a prefabricated slag of CaO-Al2O3-REO-MgO slag system, which can at least solve one of the technical problems in the existing steelmaking process, namely, how to use the prefabricated slag of CaO-Al2O3-REO-MgO slag system to refine and prepare rare earth steel to improve the rare earth recovery rate, and how to achieve low-cost and efficient recycling of rare earth polishing powder waste.

[0008] The object of the present invention is mainly achieved by the following technical solutions:

[0009] The present invention provides a method for refining and preparing rare earth steel using a prefabricated slag of CaO-Al2O3-REO-MgO slag system. The rare earth steel contains 0.002-0.02% cerium and / or lanthanum. The method includes the following steps:

[0010] Step S1: In converter or electric furnace steelmaking, before tapping, spread deoxidizer, prefabricated slag of CaO-Al2O3-REO-MgO slag system and lime at the bottom of the ladle, then tap the steel, and perform slag blocking operation at the end of tapping;

[0011] Step S2: After the ladle is transferred to the LF furnace station, blow argon for stirring, heat up to above 1610 °C, and then perform slag adjustment operation;

[0012] Step S3: Keep above 1610 °C and blow argon for stirring, and perform alloying operation;

[0013] Step S4: Keep above 1610 °C and blow argon for stirring, the white slag time ≥ 20 minutes, and then perform calcium treatment;

[0014] Step S5: Keep above 1610 °C, blow argon for stirring for 10-15 min, and then the ladle leaves the LF furnace station;

[0015] Step S6: After the ladle is transferred to the RH station, under the condition of vacuum degree ≤ 100 Pa, keep for ≥ 20 min, and control the molten steel [H] ≤ 4.0 ppm;

[0016] Step S7: Determine the oxygen content. When the oxygen content in the steel is less than 4 ppm, add rare earth ferroalloy;

[0017] Step S8: After adding rare earth ferroalloy, perform vacuum circulation for 3-5 min and then break the vacuum and leave the RH station;

[0018] Step S9: The ladle is transferred to the tundish station of continuous casting, and the tundish covering agent is spread on the molten steel surface.

[0019] Step S10: The molten steel enters the continuous casting mold, and the continuous casting mold powder is used to protect the molten steel surface.

[0020] Further, in step S1, the deoxidizer includes ferromanganese, ferrosilicon, pure aluminum or aluminum-iron alloy; by mass, the aluminum content in the added pure aluminum or aluminum-iron alloy is 2.5 - 3.5 times the estimated oxygen content in the steel, and the estimated oxygen content in the steel is determined according to the predetermined carbon-oxygen product and the carbon content at the end of smelting.

[0021] Further, the prefabricated slag composition of the CaO - Al2O3 - REO - MgO slag system in step S1 includes rare earth oxides, Al2O3, CaO, and MgO. By mass percentage, the rare earth oxide content is 10 - 20%, the Al2O3 content is 30 - 50%, the CaO content is 30 - 50%, and the MgO content is 5 - 10%.

[0022] Further, when smelting rare earth steel containing cerium, in the rare earth oxides, by mass percentage, the cerium oxide accounts for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of lanthanum oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing lanthanum, in the rare earth oxides, by mass percentage, the lanthanum oxide accounts for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of cerium oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing cerium and lanthanum, in the rare earth oxides, by mass percentage, the cerium and lanthanum oxides account for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of neodymium oxide and praseodymium oxide.

[0023] Further, in step S1, the total addition amount of the prefabricated slag of the CaO - Al2O3 - REO - MgO slag system and lime is 0.5 - 1.0% of the tapping weight, and the addition amount of lime is calculated such that the total mass of CaO in the prefabricated slag and lime is 1.5 - 2.5 times the mass of Al2O3 in the prefabricated slag.

[0024] Further, in step S2, the slag adjustment is achieved by adding lime to keep the CaO mass in the slag at 1.5 - 2.5 times the Al2O3 mass, and at the same time, keep CaO / SiO2 in the range of 5 - 8.

[0025] Further, in step S4, the calcium treatment is such that the calcium content in the steel is in the range of 0.0010 - 0.0020%.

[0026] Further, the tundish covering flux in step S9 is a tundish covering flux mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system; the continuous casting mold powder in step S10 is a continuous casting mold powder mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0027] Further, the tundish covering flux is obtained by mixing a prefabricated slag of the CaO-Al2O3-REO-MgO slag system, lime, and magnesite in a mass ratio of (95 - 105):(40 - 60):(10 - 20), and then making spherical particles with a diameter ≤ 1.0 mm.

[0028] Further, the preparation of the continuous casting mold powder is to mix 55 - 65% of the prefabricated slag of the CaO-Al2O3-REO-MgO slag system, 10 - 25% of SiO2, 3 - 10% of graphite or carbon black, 2 - 5% of sodium sulfate, and 5 - 10% of CaCO3 by mass percentage using a ball mill or high-speed stirring equipment, and then make a particle size of 60 - 100 mesh.

[0029] Compared with the prior art, the present invention can achieve at least one of the following technical effects:

[0030] (1) The prefabricated slag of the CaO-Al2O3-REO-MgO slag system used in the present invention for refining rare earth steel enables the recovery rate of rare earth elements in rare earth steel to be above 40%, and the rare earth recovery rate at the end of refining is more than 10% higher than that of the high-aluminum synthetic refining slag commonly used.

[0031] (2) When continuously casting rare earth steel in the present invention, using a tundish covering flux mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system, the loss rate of rare earth elements from the end of refining to before tapping from the tundish does not exceed 18%.

[0032] (3) When continuously casting rare earth steel in the present invention, using a continuous casting mold powder mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system, the difference between the rare earth recovery rate of the casting blank and the rare earth recovery rate after tapping from the tundish does not exceed 18%.

[0033] (4) The prefabricated slag of the CaO-Al2O3-REO-MgO slag system mainly composed of cerium oxide used in the present invention for refining rare earth steel uses rare earth polishing powder waste as a raw material, has a simple preparation process, and has the advantages of low cost, stable and controllable composition. Specific embodiments

[0034] The following further describes in detail a method for refining and preparing rare earth steel using a prefabricated slag of CaO-Al2O3-REO-MgO slag system in combination with specific embodiments. These embodiments are only for the purpose of comparison and explanation, and the present invention is not limited to these embodiments.

[0035] The present invention provides a method for refining and preparing rare earth steel using a prefabricated slag of CaO-Al2O3-REO-MgO slag system. The rare earth steel is rare earth steel containing 0.002-0.02% cerium and / or lanthanum. The method includes the following steps:

[0036] Step S1: In converter or electric furnace steelmaking, before tapping, a deoxidizer, a prefabricated slag of CaO-Al2O3-REO-MgO slag system, and lime are spread at the bottom of the ladle, and then tapping is carried out. A slag blocking operation is carried out at the end of tapping.

[0037] Step S2: After the ladle is transferred to the LF furnace station, argon stirring is carried out, and the temperature is heated up to above 1610 °C, and then a slag adjusting operation is carried out.

[0038] Step S3: Keeping above 1610 °C and argon stirring, an alloying operation is carried out.

[0039] Step S4: Keeping above 1610 °C and argon stirring, the white slag time is ≥ 20 minutes, and then calcium treatment is carried out.

[0040] Step S5: Keeping above 1610 °C, after argon stirring for 10-15 min, the ladle leaves the LF furnace station.

[0041] Step S6: After the ladle is transferred to the RH station, under the condition of a vacuum degree ≤ 100 Pa, the holding time is ≥ 20 min, and the molten steel [H] is controlled ≤ 4.0 ppm.

[0042] Step S7: Oxygen is determined. When the oxygen content in the steel is less than 4 ppm, rare earth ferroalloy is added.

[0043] Step S8: After adding rare earth ferroalloy, vacuum circulation is carried out for 3-5 min and then the vacuum is broken, and the ladle leaves the RH station.

[0044] Step S9: The ladle is transferred to the continuous casting tundish station, and a tundish covering agent is used to spread on the surface of the molten steel.

[0045] Step S10: The molten steel enters the continuous casting mold, and a continuous casting mold powder is used to protect the surface of the molten steel.

[0046] Specifically, in step S1, the tapping temperature is controlled between 1620-1650 °C; the deoxidizer includes ferromanganese, ferrosilicon, pure aluminum or aluminum ferroalloy. Among them, the addition amounts of ferromanganese and ferrosilicon are controlled according to the midline of the manganese and silicon component ranges of the rare earth steel.

[0047] By mass percentage, the aluminum content in pure aluminum is more than 95%, and the aluminum content in aluminum-iron is 40 - 95%. Calculated by mass, the addition amount of pure aluminum or aluminum-iron alloy is determined according to the estimated oxygen content in the steel, so that the aluminum content in the added pure aluminum or aluminum-iron alloy is 2.5 - 3.5 times the estimated oxygen content in the steel. The estimated oxygen content in the steel is determined according to the predetermined carbon-oxygen product and the carbon content at the end of smelting. The value range of the predetermined carbon-oxygen product is 0.0015 - 0.0035%. 2 , and the specific value of the predetermined carbon-oxygen product is determined according to the specific conditions of converter or electric furnace smelting, such as tapping temperature, furnace age, carbon content at the end point, etc.

[0048] The prefabricated slag of the CaO - Al2O3 - REO - MgO slag system, where REO is the abbreviation of Rare Earth Oxides, that is, rare earth oxides. The composition of this prefabricated slag includes rare earth oxides, Al2O3, CaO, and MgO. By mass percentage, the rare earth oxide content is 10 - 20%, the Al2O3 content is 30 - 50%, the CaO content is 30 - 50%, and the MgO content is 5 - 10%. Further, in the prefabricated slag, the mass ratio of CaO:Al2O3 is 0.9 - 1.3. The prefabricated slag is granular, with a particle size of 20 - 50 mm and a melting point of 1380 - 1420 °C.

[0049] It should be noted that when smelting rare earth steel containing cerium, among the rare earth oxides, by mass percentage, the cerium oxide accounts for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of lanthanum oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing lanthanum, among the rare earth oxides, by mass percentage, the lanthanum oxide accounts for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of cerium oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing cerium and lanthanum, among the rare earth oxides, by mass percentage, the cerium and lanthanum oxides account for 50 - 100% of the total rare earth oxides, and the rest is a combination of one or more of neodymium oxide and praseodymium oxide. Among them, cerium oxide is a compound formed by the combination of cerium and oxygen, such as Ce2O3, CeO2, etc.; lanthanum oxide is a compound formed by the combination of lanthanum and oxygen, such as La2O3, LaO, LaO2, etc.; neodymium oxide is a compound formed by the combination of neodymium and oxygen, such as Nd2O3, etc.; praseodymium oxide is a compound formed by the combination of praseodymium and oxygen, such as Pr2O3, Pr6O 11 etc.

[0050] Specifically, for the prefabricated slag of the CaO-Al2O3-REO-MgO slag system where the cerium oxide accounts for 50-100% of the total rare earth oxides, the present invention uses rare earth polishing powder waste to prepare it. The rare earth polishing powder waste is a solid waste with a mass percentage of rare earth oxides above 40%, so it has a high recycling value. The mass percentage of rare earth oxides in the rare earth polishing powder is above 40%, and the remaining components are SiO2, Al2O3, CaO, ZrO2, etc.; the main component of the rare earth oxides is CeO2, and the rest consists of La2O3, Nd2O3, Pr6O 11 and so on.

[0051] The preparation process of the prefabricated slag of the CaO-Al2O3-REO-MgO slag system where the cerium oxide accounts for 50-100% of the total rare earth oxides is to use the rare earth oxides in the rare earth polishing powder waste as the rare earth oxide source material, mix it with the calcium oxide source material, aluminum oxide source material, and magnesium oxide source material in proportion, then sinter and crush and screen to obtain the prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0052] Specifically, in step S1, the total addition amount of the CaO-Al2O3-REO-MgO slag system prefabricated slag and lime is 0.5-1.0% of the tapping amount, and the addition amount of lime is calculated according to the total mass of CaO in the prefabricated slag and CaO in the lime being 1.5-2.5 times the mass of Al2O3 in the prefabricated slag. The lime particle size is 20-50mm, and the CaO content is ≥90%.

[0053] It should be noted that there is heat loss during the tapping process and the ladle transfer process in step S1. In addition, during the tapping process, part of the converter slag enters the ladle, and there are aluminum deoxidation and slag-steel reactions, and some elements, such as Al, Si, Mn, are oxidized and enter the slag. Therefore, in step S2, the molten steel at the LF furnace station needs to be heated and raised in temperature first, such as raising the temperature to the range of 1610-1630°C, and then adjusting the slag. The specific requirements for adjusting the slag are to add lime to make the mass of CaO in the slag maintain 1.5-2.5 times the mass of Al2O3, and at the same time, the basicity R of the refining slag is maintained in the range of 5-8, where R = CaO / SiO2.

[0054] In step S3, the alloying operation is to add other necessary elements for rare earth steel to the steel in the form of adding ferroalloys or pure metals, such as Ti, Nb, V, Ni, etc.

[0055] In step S4, the calcium treatment is carried out by feeding pure calcium wire to make the calcium content in the steel within the range of 0.0010-0.0020%.

[0056] It should be noted that for the argon blowing and stirring in steps S2 and S3, it is appropriate when the molten steel is exposed and there is no large-scale turnover; for the argon blowing and stirring in steps S4 and S5, it is appropriate when the molten steel is not exposed and there is a slight fluctuation on the slag surface.

[0057] Specifically, in step S7, the rare earth ferroalloy is a commercial rare earth ferroalloy containing rare earth cerium and / or lanthanum, or a high-purity rare earth ferroalloy prepared by the reduction smelting method or the vacuum reduction smelting method. By mass percentage, the iron content is 60-90%, the balance is cerium and / or lanthanum, the total impurity content is <0.1%, the oxygen content is <0.005%, and the sulfur content is <0.002%.

[0058] The high-purity rare earth ferroalloy is prepared by the reduction smelting method or the vacuum reduction smelting method.

[0059] The method for preparing a high-purity rare earth ferroalloy by reduction smelting is to add rare earth oxides and metallic calcium to the molten iron in a smelting furnace for an oxidation-reduction reaction, then add a prefabricated slag of the CaO-Al2O3-REO-MgO slag system to the top of the molten pool for slag treatment, and finally perform vacuum treatment to obtain the high-purity rare earth ferroalloy.

[0060] Specifically, it includes the following steps:

[0061] Step a: Add pure iron and pure aluminum to a smelting furnace for melting;

[0062] Step b: Heat the molten iron to 1600-1650 °C and keep it, remove the scale on the surface of the molten iron, add rare earth oxides and metallic calcium, and perform inert gas stirring by bottom blowing in the molten pool while adding. The stirring intensity is 0.15-0.25 Nm 3 / t·min, and the treatment time is 5-10 min; the rare earth oxides are oxides of cerium and / or lanthanum;

[0063] Step c: After the treatment, keep the molten iron at 1600-1650 °C, add a prefabricated slag of the CaO-Al2O3-REO-MgO slag system to the top of the molten pool, with a slag thickness of 300-500 mm, and perform inert gas stirring by bottom blowing in the molten pool. The stirring intensity is 0.05-0.15 Nm 3 / t·min, and the treatment time is 20-30 min;

[0064] Step d: After the treatment, pump the molten iron under the slag layer into a vacuum environment for vacuum treatment. After cooling, the high-purity rare earth ferroalloy is obtained;

[0065] It should be noted that the prefabricated slag of the CaO-Al2O3-REO-MgO slag system in step c is the same as the prefabricated slag of the CaO-Al2O3-REO-MgO slag system added in step S1.

[0066] A method for preparing high-purity rare earth ferroalloy by vacuum reduction melting is to add cerium chloride and / or lanthanum chloride, calcium or barium into the molten iron in a vacuum melting furnace for reaction to obtain high-purity rare earth ferroalloy.

[0067] Specifically, it includes the following steps:

[0068] Step I: Add pure iron into the melting chamber of the vacuum melting furnace, add rare earth chloride and reducing agent into the feeder of the vacuum melting furnace. When the vacuum degree in the vacuum melting furnace reaches 67 - 150 Pa and is maintained, start powering on to increase the temperature; the rare earth chloride is cerium chloride and / or lanthanum chloride; the reducing agent is calcium or barium;

[0069] Step II: Heat the molten iron to 1600 - 1650 °C and maintain it, add the rare earth chloride and reducing agent in the feeder into the molten iron, and process for 20 - 30 min. During the process, maintain the vacuum degree in the vacuum melting furnace at 67 - 150 Pa;

[0070] Step III: After the treatment, cast at 1600 - 1650 °C, and then keep warm for 5 - 15 min. During the process, maintain the vacuum degree in the vacuum melting furnace at 67 - 150 Pa;

[0071] Step IV: Cool with the furnace to obtain high-purity rare earth ferroalloy. During the process, maintain the vacuum degree in the vacuum melting furnace at 67 - 150 Pa;

[0072] Specifically, in step S9, the tundish covering agent is preferably a tundish covering agent mainly composed of prefabricated slag of CaO - Al2O3 - REO - MgO slag system. When preparing the prefabricated slag of CaO - Al2O3 - REO - MgO slag system as the main component of the tundish covering agent, first mix the prefabricated slag of CaO - Al2O3 - REO - MgO slag system, lime, and magnesite according to the mass ratio of (95 - 105):(40 - 60):(10 - 20), and then make it into spherical particles with a diameter ≤ 1.0 mm, that is, obtain the tundish covering agent mainly composed of the prefabricated slag of CaO - Al2O3 - REO - MgO slag system; among them, by mass percentage, the CaO content in lime is ≥ 90%, and the MgO content in magnesite is ≥ 90%. After the molten steel is poured into the tundish, use an iron shovel or an automatic spreader to evenly spread the tundish covering agent on the surface of the molten steel, and the dosage of the tundish covering agent is 1.5 - 3.0 kg / m 2 On the surface of the molten steel, maintain the thickness of the slag layer between 20 - 40 mm.

[0073] Specifically, in step S10, the continuous casting mold powder preferably uses a continuous casting mold powder mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system. When preparing a continuous casting mold powder mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system, according to the mass percentage content, 55-65% of the prefabricated slag of the CaO-Al2O3-REO-MgO slag system, 10-25% of SiO2, 3-10% of graphite or carbon black, 2-5% of mirabilite, and 5-10% of CaCO3 are mixed evenly using a ball mill or a high-speed stirring device, and the particle size is maintained at 60-100 mesh, thus obtaining a continuous casting mold powder mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system. SiO2 in the mold powder can reduce the melting point and improve fluidity; graphite or carbon black is used for heat preservation and preventing crust formation; the main components after the high-temperature decomposition of mirabilite are Na2O or K2O, which can increase the initial melting speed; CaCO3 is used to increase the CaO content in the mold powder.

[0074] During specific operation, before the molten steel enters the mold, a layer of mold powder is first spread at the bottom of the mold to form an initial covering layer. In the initial stage, the slag is manually spread, and the amount of slag spreading is controlled at 0.5-1.5 kg / t of steel. Subsequently, an automatic device is used to add materials regularly and quantitatively, and the addition amount is controlled at 0.2-0.5 kg / min according to the drawing speed, and a stable slag layer thickness is maintained at about 15-30 mm.

[0075] It should be noted that the prefabricated slag of the CaO-Al2O3-REO-MgO slag system in step S9 and step S10 is the same as the prefabricated slag of the CaO-Al2O3-REO-MgO slag system added in step S1.

[0076] Since the prefabricated slag of the CaO-Al2O3-REO-MgO slag system mainly composed of cerium oxide used in the present invention for refining rare earth steel uses rare earth polishing powder waste as a raw material, the preparation process is simple, and it has the advantages of low cost and stable and controllable composition.

[0077] The prefabricated slag of the CaO-Al2O3-REO-MgO slag system used in the present invention for refining rare earth steel can improve the recovery rate of rare earth elements, improve the existing form of rare earth in steel, and increase the rare earth recovery rate at the end of refining by 10%.

[0078] When continuously casting rare earth steel in the present invention, a tundish covering agent mainly composed of a prefabricated slag of the CaO-Al2O3-REO-MgO slag system is used, so that the loss rate of rare earth elements from the end of refining to before tapping from the tundish does not exceed 18%.

[0079] When continuously casting rare earth steel, the present invention uses a continuous casting mold powder mainly composed of a prefabricated slag of a CaO-Al2O3-REO-MgO slag system, so that the difference between the rare earth recovery rate of the cast slab and the rare earth recovery rate after tapping from the tundish does not exceed 18%.

[0080] When the high-purity rare earth ferroalloy used for refining rare earth steel in the present invention is prepared by the reduction smelting method, a prefabricated slag of a CaO-Al2O3-REO-MgO slag system is used, which not only expands the application range of the prefabricated slag, but also provides a new method for obtaining high-purity rare earth ferroalloy.

[0081] Comparative example

[0082] This comparative example is used to illustrate the effect of smelting cerium-containing rare earth steel under the condition of not using the prefabricated slag of the CaO-Al2O3-REO-MgO slag system. This process uses traditional high-aluminum synthetic refining slag and conventional rare earth ferroalloy addition methods for operation.

[0083] The smelting target is rare earth steel with a cerium content of 0.010%, the total amount of molten steel is 120 tons, the estimated oxygen content is 0.034%, the rare earth content in the rare earth ferroalloy is 20%, and the rare earth recovery rate is estimated to be about 40% according to experience.

[0084] Step S1: Before tapping from the converter, conventional deoxidizers and traditional high-aluminum pre-melted slag are sequentially laid at the bottom of the ladle. The deoxidizers are ferrosilicon, ferromanganese, and pure aluminum; the addition amount of pure aluminum is calculated based on the estimated oxygen content of 0.034%. According to 95% of the aluminum content in pure aluminum, about 128.8 kg of pure aluminum needs to be added to meet the requirement that the aluminum content is 3 times the oxygen content in the steel; the ratio of the high-aluminum pre-melted slag is Al2O3 55%, CaO 25%, MgO 8%, SiO2 10%, and the rest MnO+FeO 2%. The lime addition amount is 400 kg, and the CaO content is 90%. The tapping temperature is controlled at 1620 °C, and slag blocking operation is performed at the end.

[0085] Step S2: After the ladle is transferred to the LF furnace, it is heated to 1625 °C, argon is blown for stirring and lime is added additionally to adjust the mass ratio of CaO / Al2O3 to about 1.49, and the basicity R (CaO / SiO2) is adjusted to 5.3. The composition of the refining slag is shown in Table 1:

[0086] Table 1 Composition of the refining slag in the comparative example of the present invention (wt%)

[0087]

[0088] Step S3: Maintain at 1620-1630 °C and argon stirring, and carry out alloying operations to add Cr, Ni, and Cu elements.

[0089] Step S4: Maintain the temperature at 1620 - 1630°C with argon blowing and stirring. Keep the white slag time at 15 minutes, and conduct calcium treatment by feeding pure calcium wire to make the Ca content in the steel reach 0.0012%.

[0090] Step S5: Maintain the temperature at 1620 - 1630°C. After argon blowing and stirring for 10 minutes, the ladle leaves the LF furnace.

[0091] Step S6: Conduct RH vacuum treatment at below 100 Pa for 20 minutes to control the [H] in the steel at around 4.5 ppm.

[0092] Step S7: After the oxygen determination detects 3.5 ppm, directly add ferrosilicon cerium alloy with a commercial cerium content of 20%. To achieve a final cerium content of 0.010% in the steel, 30 kg of rare earth cerium needs to be added according to a 40% recovery rate, corresponding to an addition amount of 150 kg of rare earth ferrosilicon alloy.

[0093] Step S8: After adding the commercial ferrosilicon cerium alloy, continue RH circulation treatment for 3 minutes and then break the vacuum and leave the station.

[0094] Step S9: After the ladle is transferred to the continuous casting tundish station, use a calcium-aluminum-based tundish covering agent. The composition of the tundish covering agent is shown in Table 2:

[0095] Table 2 Composition of the tundish covering agent in the comparative example of the present invention (wt%)

[0096]

[0097] Step S10: Pour the molten steel into the mold, and use a protective slag to protect the surface of the molten steel in the mold. The composition of the continuous casting protective slag is by mass percentage: SiO2 38%, CaO 25%, Al2O3 15%, C 5%, Na2O 3%, and others 14%.

[0098] After detection, the content of rare earth cerium in the finished product is 63 ppm, and the recovery rate of rare earth cerium is 31.5%. Among them, the recovery rate of rare earth at the refining station is 70%, the loss rate of rare earth elements from the refined steel to before the tundish tapping is 19%, and the loss rate of rare earth elements in continuous casting is 19.5%. The chemical composition of the corrosion-resistant steel containing rare earth cerium obtained is shown in Table 3 by mass percentage.

[0099] Table 3 Chemical composition of the finished product rare earth steel in the comparative example of the present invention (wt%)

[0100] C Si Mn P S Als Ca Cr Ni Cu Ce 0.05 0.087 1.35 0.011 0.002 0.032 0.0005 0.73 0.31 0.42 0.0079

[0101] Example 1

[0102] This example is used to illustrate the method of smelting a rare earth corrosion-resistant steel with a target cerium content of 0.010% using a prefabricated slag of the CaO-Al2O3-CeO2-MgO slag system. The total amount of molten steel is 120 tons, the rare earth content in the rare earth ferroalloy is 20%, and the rare earth recovery rate is calculated at 50%.

[0103] Step S1: Before tapping from the converter, a deoxidizer, CaO-Al2O3-CeO2-MgO prefabricated slag, and lime are sequentially laid at the bottom of the ladle. The deoxidizer is ferrosilicon, ferromanganese, and pure aluminum; the addition amount of pure aluminum is based on the estimated oxygen content of 0.034%, and calculated according to 95% of the aluminum content in pure aluminum. Approximately 128.8 kg of pure aluminum needs to be added to meet the requirement that the aluminum content is 3 times the oxygen content in the steel; by mass percentage, CeO2 in the CaO-Al2O3-CeO2-MgO prefabricated slag is 10%, the Al2O3 content is 34%, the CaO content is 50%, and the MgO content is 6%; the CaO content in the lime is 90%; the particle size of the prefabricated slag and lime is 20 - 50 mm; the total addition amount of the prefabricated slag and lime is 1.0% of the tapping amount, including 1000 kg of prefabricated slag and 200 kg of lime, and the total mass of CaO is twice that of Al2O3 in the prefabricated slag; the tapping temperature is controlled at 1630 °C, and slag blocking operation is carried out at the end;

[0104] Step S2: After the ladle is transferred to the LF furnace station, argon blowing and stirring are started and the temperature is heated up to 1625 °C. Subsequently, slag adjustment operation is carried out. By adding lime to adjust the mass ratio of CaO / Al2O3 in the slag to 1.8, and at the same time adjusting the basicity R (CaO / SiO2) of the refining slag to maintain at 6.3. The composition of the refining slag is shown in Table 4:

[0105] Table 4 Composition of the refining slag in Example 1 of the present invention (wt%)

[0106]

[0107] Step S3: Maintain at 1620 - 1630 °C and argon blowing and stirring, and carry out alloying operation by adding Cr, Ni, and Cu elements;

[0108] Step S4: Maintain at 1620 - 1630 °C and argon blowing and stirring, control the white slag time at 25 minutes, and carry out calcium treatment by feeding pure calcium wire to make the Ca content in the steel reach 0.0015%;

[0109] Step S5: Maintain at 1620 - 1630 °C, after argon blowing and stirring for 10 minutes, the ladle leaves the LF furnace;

[0110] Step S6: RH furnace vacuum treatment, control the vacuum degree at ≤100 Pa, treat for 25 minutes to ensure [H] ≤ 4.0 ppm;

[0111] Step S7: After oxygen determination, the oxygen content in the steel is measured to be 3.2 ppm. Subsequently, high-purity ferrocerium alloy is added. By mass percentage, the iron content in the high-purity ferrocerium alloy is 80%, the balance is cerium, the total impurity content is 0.09%, the oxygen content is 0.004%, and the sulfur content is 0.001%. It is prepared by the reduction melting method; to achieve a final cerium content of 0.010% in the steel, 24 kg of rare earth cerium needs to be added according to a 50% recovery rate, and the corresponding addition amount of rare earth ferroalloy is 120 kg;

[0112] Step S8: After adding the ferrocerium alloy, continue the RH circulation treatment for 3 minutes and then break the vacuum and leave the station;

[0113] Step S9: After the ladle is transferred to the tundish station of the continuous casting, use a tundish covering agent made of CaO - Al2O3 - CeO2 - MgO prefabricated slag, lime, and magnesite in a mass ratio of 100:50:15. The CaO content in the lime is 90%, the MgO content in the magnesite is 90%, the particle size of the tundish covering agent is ≤1 mm, and it is manually spread, with the controlled dosage of 2.5 kg / m 2 on the molten steel surface to maintain a slag layer thickness of about 30 mm; The composition of the tundish covering agent is shown in Table 5:

[0114] Table 5 Composition of the tundish covering agent in Example 1 of the present invention (wt%)

[0115]

[0116] S10: Lay 0.5 kg / t of initial protective slag before the molten steel is poured into the mold. After the molten steel enters the mold, add slag automatically at a rate of 0.3 kg / min to maintain a slag layer thickness of 20 mm; The ratio of the protective slag is: prefabricated slag 55%, SiO2 25%, graphite 10%, mirabilite 5%, CaCO3 5%. The composition of the protective slag is shown in Table 6:

[0117] Table 6 Composition of the protective slag in Example 1 of the present invention (wt%)

[0118]

[0119] After detection, the content of rare earth cerium in the finished product is 89 ppm, and the recovery rate of rare earth cerium is 44.5%. Among them, the recovery rate of rare earth at the end of refining is 80%, the loss rate of rare earth elements from the refined molten steel to before the molten steel is poured out of the tundish is 17.5%, and the loss rate of rare earth elements in continuous casting is 18%. The chemical composition of the corrosion-resistant steel containing rare earth cerium obtained is shown in Table 7 by mass percentage:

[0120] Table 7 Chemical composition of the finished product rare earth steel in Example 1 of the present invention (wt%)

[0121] C Si Mn P S Als Ca Cr Ni Cu Ce 0.05 0.090 1.34 0.010 0.002 0.035 0.00085 0.73 0.31 0.42 0.0089

[0122] Example 2

[0123] Steps S1 - S8 of this embodiment are exactly the same as those of Step S1 - S8 of Embodiment 1, and Steps S9 and S10 of this embodiment are exactly the same as those of Steps S9 and S10 of the comparative example.

[0124] After detection, the content of rare earth cerium in the finished product is 83 ppm, and the recovery rate of rare earth cerium is 41.5%. Among them, the rare earth recovery rate at the end of refining is 80%, the loss rate of rare earth elements from the end of refining to before tapping from the tundish is 19%, and the loss rate of rare earth elements in continuous casting is 19.5%. The chemical composition of the corrosion - resistant steel containing rare earth cerium obtained is shown in Table 8 by mass percentage.

[0125] Table 8 Chemical composition of the rare earth steel in the finished product of Embodiment 2 of the present invention (wt%)

[0126] C Si Mn P S Als Ca Cr Ni Cu Ce 0.05 0.087 1.35 0.011 0.002 0.032 0.0005 0.73 0.31 0.42 0.0083

[0127] Embodiment 3

[0128] Steps S1 - S9 of this embodiment are exactly the same as those of Step S1 - S9 of Embodiment 1, and Step S10 of this embodiment is exactly the same as that of Step S10 of the comparative example.

[0129] After detection, the content of rare earth cerium in the finished product is 86 ppm, and the recovery rate of rare earth cerium is 43%. Among them, the rare earth recovery rate at the end of refining is 80%, the loss rate of rare earth elements from the end of refining to before tapping from the tundish is 17.5%, and the loss rate of rare earth elements in continuous casting is 19.5%. The chemical composition of the corrosion - resistant steel containing rare earth cerium obtained is shown in Table 9 by mass percentage:

[0130] Table 9 Chemical composition of the rare earth steel in the finished product of Embodiment 3 of the present invention (wt%)

[0131]

[0132]

[0133] Embodiment 4

[0134] This embodiment is used to illustrate a method for smelting a furnace of rare earth wear - resistant steel with a target lanthanum content of 0.010% using a pre - formed slag of CaO - Al2O3 - La2O3 - MgO slag system. The total amount of molten steel in this furnace is 280 tons, the rare earth content in the ferrolanthanum alloy is 30%, and the rare earth recovery rate is calculated at 50%.

[0135] Step S1: Before tapping the converter steel, deoxidizer, CaO-Al2O3-La2O3-MgO prefabricated slag and lime are sequentially laid at the bottom of the ladle. The deoxidizer is ferrosilicon, ferromanganese and pure aluminum. The addition amount of pure aluminum is based on the estimated oxygen content of 0.029%. Calculated according to 95% of the aluminum content in pure aluminum, about 256.4 kg of pure aluminum needs to be added to meet the requirement that the aluminum content is 3 times the oxygen content in the steel. By mass percentage, La2O3 in the CaO-Al2O3-La2O3-MgO prefabricated slag is 15%, the Al2O3 content is 40%, the CaO content is 40%, and the MgO content is 5%; the CaO content in lime is 90%; the particle size of the prefabricated slag and lime is 20 - 50 mm; the total addition amount of the prefabricated slag and lime is 0.8% of the tapping amount, including 1550 kg of prefabricated slag and 690 kg of lime. The total mass of CaO is twice that of Al2O3 in the prefabricated slag; the tapping temperature is controlled at 1620 °C, and slag blocking operation is carried out at the end.

[0136] Step S2: After the ladle is transferred to the LF furnace, argon is blown for stirring and the temperature is raised to 1620 °C. Lime is added to adjust the slag system composition to make the mass ratio of CaO / Al2O3 1.8 and the slag basicity R(CaO / SiO2) 7. The composition of the refining slag is shown in Table 10:

[0137] Table 10 Composition of the refining slag in Example 4 of the present invention (wt%)

[0138]

[0139] Step S3: Maintain at 1615 - 1625 °C and blow argon for stirring, and carry out alloying operations to add Cr, Mo, Ni, Nb, Ti, V elements;

[0140] Step S4: Maintain at 1615 - 1625 °C and blow argon for stirring. After maintaining the white slag state for 20 minutes, calcium treatment is carried out, and pure calcium wire is fed to make the Ca content in the steel reach 0.0016%;

[0141] Step S5: Maintain at 1615 - 1625 °C. After blowing argon for 12 minutes, the ladle leaves the LF furnace;

[0142] Step S6: Vacuum treatment in the RH furnace. The vacuum degree of the RH furnace is ≤100 Pa, and the treatment is carried out for 25 minutes to control [H] ≤ 4.0 ppm.

[0143] After oxygen determination, the measured oxygen content is 3.0 ppm. Subsequently, high-purity lanthanum iron alloy is added. By mass percentage, the iron content in the high-purity lanthanum iron alloy is 70%, the balance is lanthanum, the total impurity content is 0.05%, the oxygen content is 0.003%, and the sulfur content is 0.0015%. It is prepared by the vacuum reduction melting method; to achieve a final lanthanum content of 0.010% in the steel, calculated according to a 50% recovery rate, 56 kg of rare earth lanthanum needs to be added, and the corresponding rare earth iron alloy addition amount is 187 kg;

[0144] Step S8: After adding the lanthanum-iron alloy, continue RH treatment for 4 minutes and then break the vacuum and leave the station;

[0145] Step S9: After the ladle is transferred to the tundish station of continuous casting, use a tundish covering agent made of CaO - Al2O3 - La2O3 - MgO prefabricated slag, lime, and magnesia in a mass ratio of 100:50:15. The CaO content in the lime is 90%, the MgO content in the magnesia is 90%, the particle size of the tundish covering agent is ≤1 mm, and it is automatically spread by machine, with the controlled dosage of 2.0 kg / m 2 On the molten steel surface, maintain the slag layer thickness at about 25 mm; the composition of the tundish covering agent is shown in Table 11:

[0146] Table 11 Composition of the tundish covering agent in Example 4 of the present invention (wt%)

[0147]

[0148] Step S10: Before the molten steel is poured into the mold, initially lay 0.5 kg / t of the mold powder manually. After the molten steel enters the mold, the automatic slag addition rate is 0.35 kg / min, and maintain the slag layer thickness at 25 mm; the ratio of the mold powder is: prefabricated slag 65%, SiO2 10%, graphite 10%, mirabilite 5%, CaCO3 10%. The composition of the mold powder is shown in Table 10:

[0149] Table 12 Composition of the mold powder in Example 4 of the present invention (wt%)

[0150]

[0151] The final finished product has a lanthanum content of 96 ppm and a rare earth recovery rate of 48%. Among them, the rare earth recovery rate at the end of refining is 82%, the loss rate of rare earth elements from the end of refining to before tapping from the tundish is 17%, and the loss rate of rare earth elements in continuous casting is 17%. The chemical composition of the wear-resistant steel containing rare earth lanthanum obtained is shown in Table 13 by mass percentage:

[0152] Table 13 Chemical composition of the finished rare earth steel in Example 4 of the present invention (wt%)

[0153]

[0154] Example 5

[0155] Steps S1 - S8 of this example are exactly the same as Steps S1 - S8 of Example 4, and Steps S9 and S10 of this example are exactly the same as Steps S9 and S10 of the comparative example.

[0156] The lanthanum content of the final product is 88 ppm, and the rare earth recovery rate is 44%. Among them, the rare earth recovery rate at the end of refining is 82%, the loss rate of rare earth elements from the end of refining to before tapping from the tundish is 19%, and the loss rate of rare earth elements in continuous casting is 19%. The chemical composition of the wear-resistant steel containing rare earth lanthanum obtained is shown in Table 14 by mass percentage:

[0157] Table 14 Chemical composition of the finished rare earth steel in Example 5 of the present invention (wt%)

[0158]

[0159] Example 6

[0160] Steps S1 - S9 of this example are exactly the same as steps S1 - S9 of Example 1, and step S10 of this example is exactly the same as step S10 of the comparative example.

[0161] The lanthanum content of the final product is 88 ppm, and the rare earth recovery rate is 46%. Among them, the rare earth recovery rate at the end of refining is 82%, the loss rate of rare earth elements from the end of refining to before tapping from the tundish is 17%, and the loss rate of rare earth elements in continuous casting is 19%. The chemical composition of the wear-resistant steel containing rare earth lanthanum obtained is shown in Table 15 by mass percentage:

[0162] Table 15 Chemical composition of the finished rare earth steel in Example 6 of the present invention (wt%)

[0163]

[0164] As can be seen from the above comparative examples and examples, the refining slag prepared by using the CaO - Al2O3 - REO - MgO slag system prefabricated slag in the present invention enables the recovery rate of rare earth elements in steel to be above 40%. Among them, the rare earth recovery rate at the end of refining can be increased by more than 10% compared with the conventional high - alumina synthetic refining slag, and the loss rate of rare earth elements from the end of refining to before tapping from the tundish and the loss rate of rare earth elements in continuous casting are both below 18%.

[0165] The above - mentioned is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing rare earth steel by refining prefabricated slag of CaO-Al2O3-REO-MgO slag system, characterized in that: The rare earth steel is a rare earth steel containing 0.002-0.02% cerium and / or lanthanum; the method comprises the following steps: Step S1, steelmaking in a converter or electric furnace, before tapping, a deoxidizer, prefabricated slag of CaO-Al2O3-REO-MgO slag system and lime are spread on the bottom of the ladle, and then the steel is tapped, and a slag blocking operation is performed at the end of tapping; Step S2, after the ladle is transferred to the LF furnace station, argon is blown for stirring, the temperature is raised to above 1610°C, and then slag adjustment is performed; Step S3, maintaining the temperature above 1610° C. and stirring with argon blowing to perform alloying operation; Step S4, maintaining above 1610°C and stirring with argon blowing, the white slag time is ≥ 20 minutes, and then calcium treatment is performed; Step S5, maintaining the temperature above 1610°C, blowing argon and stirring for 10-15 minutes, after which the ladle leaves the LF furnace station; Step S6, after the ladle is transferred to the RH station, the vacuum degree is ≤100 Pa, and the holding time is ≥20 min, and the molten steel [H] is controlled to be ≤4.0 ppm; Step S7, oxygen determination, when the oxygen content in the steel is less than 4ppm, adding rare earth iron alloy; Step S8, after adding rare earth iron alloy, vacuum cycle for 3-5 minutes and then break the vacuum and leave the RH station; Step S9, the ladle is transferred to the continuous casting tundish station, and a tundish covering agent is spread on the surface of the molten steel; Step S10: The molten steel enters the continuous casting mold, and the surface of the molten steel is protected by continuous casting mold protection slag.

2. The method according to claim 1, characterized in that: In step S1, the deoxidizer includes ferromanganese, ferrosilicon, pure aluminum or aluminum-iron alloy; in terms of mass, the aluminum content in the added pure aluminum or aluminum-iron alloy is 2.5-3.5 times the estimated oxygen content in the steel, and the estimated oxygen content in the steel is determined based on the predetermined carbon-oxygen product and the carbon content at the smelting endpoint.

3. The method according to claim 1, characterized in that: The prefabricated slag components of the CaO-Al2O3-REO-MgO slag system in step S1 include rare earth oxides, Al2O3, CaO, and MgO. Calculated by mass percentage, the rare earth oxide content is 10-20%, the Al2O3 content is 30-50%, the CaO content is 30-50%, and the MgO content is 5-10%.

4. The method according to claim 3, characterized in that: When smelting rare earth steel containing cerium, among the rare earth oxides, calculated by mass percentage, cerium oxide accounts for 50-100% of the total amount of rare earth oxides, and the rest is one or a combination of lanthanum oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing lanthanum, among the rare earth oxides, calculated by mass percentage, lanthanum oxide accounts for 50-100% of the total amount of rare earth oxides, and the rest is one or a combination of cerium oxide, neodymium oxide, and praseodymium oxide; when smelting rare earth steel containing cerium and lanthanum, among the rare earth oxides, calculated by mass percentage, cerium and lanthanum oxides account for 50-100% of the total amount of rare earth oxides, and the rest is one or a combination of neodymium oxide and praseodymium oxide.

5. The method according to claim 1, characterized in that: In the step S1, the total amount of precast slag and lime added in the CaO-Al2O3-REO-MgO slag system is 0.5-1.0% of the steel output, wherein the amount of lime added is calculated based on the total mass of CaO in the precast slag and CaO in the lime being 1.5-2.5 times the mass of Al2O3 in the precast slag.

6. The method according to claim 1, characterized in that: In step S2, the slag is adjusted by adding lime so that the mass of CaO in the slag is maintained at 1.5-2.5 times the mass of Al2O3, and the CaO / SiO2 is maintained in the range of 5-8.

7. The method according to claim 1, characterized in that: In the step S4, the calcium treatment is performed to reduce the calcium content in the steel to a range of 0.0010 to 0.0020%.

8. The method according to claim 1, characterized in that: The tundish covering agent in the step S9 is a tundish covering agent with the precast slag of the CaO-Al2O3-REO-MgO slag system as the main component; the continuous casting crystallizer protective slag in the step S10 is a continuous casting crystallizer protective slag with the precast slag of the CaO-Al2O3-REO-MgO slag system as the main component.

9. The method according to claim 8, characterized in that: The tundish covering agent is obtained by mixing the prefabricated slag of the CaO-Al2O3-REO-MgO slag system with lime and magnesia in a mass ratio of (95-105):(40-60):(10-20), and then making it into spherical particles with a diameter of ≤1.0mm.

10. The method according to claim 8, characterized in that: The preparation of the continuous casting crystallizer protective slag is to mix 55-65% of the prefabricated slag of the CaO-Al2O3-REO-MgO slag system, 10-25% of SiO2, 3-10% of graphite or carbon black, 2-5% of Glauber's salt, and 5-10% of CaCO3 according to the mass percentage, using a ball mill or high-speed stirring equipment to evenly mix them, and then make them into a particle size of 60-100 mesh.

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