Prefabricated slag of CaO-Al2O3-REO-MgO slag system as well as preparation and application of prefabricated slag

Abstract

The invention relates to prefabricated slag of a CaO-Al2O3-REO-MgO slag system as well as preparation and application of the prefabricated slag, belongs to the technical field of ferrous metallurgy, and solves the technical problems of how to prepare low-melting-point refining slag containing rare earth oxide at low cost and how to realize environment-friendly, economic and efficient recovery of rare earth polishing powder waste in the existing steelmaking process. The prefabricated slag of the CaO-Al2O3-REO-MgO slag system is prepared from the following components in percentage by mass: 10 to 20 percent of rare earth oxide, 30 to 50 percent of Al2O3, 30 to 50 percent of CaO and 5 to 10 percent of MgO. The invention further discloses a preparation method of the prefabricated slag of the CaO-Al2O3-REO-MgO slag system. The prefabricated slag adopts the rare earth polishing powder waste as the raw material, the preparation process is simple, the prefabricated slag has the advantages of low cost and stable and controllable components, the recycling process of the rare earth polishing powder waste is simple, efficient, environment-friendly and economical, waste is turned into wealth, and the prefabricated slag has an extremely strong demonstration effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a prefabricated slag of a CaO-Al2O3-REO-MgO slag system, its preparation and application. Background Art

[0002] At present, the main steelmaking methods are converter steelmaking method and electric arc furnace steelmaking method. The above two steelmaking processes can only meet the requirements of general users for the quality of steel. In order to obtain high-quality and more varieties of high-grade 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. For some special purposes, when special high-quality steel is required and secondary refining still cannot meet the requirements, special steelmaking methods need to be used. Such as electroslag remelting, etc.

[0003] In order to achieve refining purposes, such as degassing, deoxidation, dephosphorization, desulfurization, removing inclusions and fine-tuning of components, etc., 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. The important physical and chemical properties of slag are: basicity, viscosity, melting temperature, oxidizing property and reducing property, etc. These properties play a decisive role in the smooth progress of the refining process and the quality of the final molten steel. Existing research shows that 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; rare earth oxides will affect the physical properties such as melting point, viscosity and basicity of the refining slag. By reasonably controlling the composition and physical properties of the refining slag, the cleanliness of the molten metal can be improved. Further, 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.

[0004] In addition, rare earth elements have the effects of refining grains, improving properties, inhibiting harmful elements, deoxidizing and desulfurizing, and modification in steel. However, rare earth elements have strong chemical activity. Not only is the recovery rate low and difficult to control during smelting, but also rare earth is not easy to exist in the form of metal bonds in steel and is more inclined to exist in the form of ionic bonds, so that the role of rare earth elements cannot be fully, effectively and controllably exerted. Therefore, the smelting process of rare earth-treated steel has attracted more and more attention. Relevant research shows that the refining slag containing rare earth oxides can not only improve the recovery rate of rare earth elements, but also improve the existence form of rare earth in steel. In addition, the process practice of electroslag remelted steel shows that the rare earth slag system reduction method has good desulfurization and deoxidation effects and can effectively avoid the oxidation and burning loss of rare earth, which is a relatively good method for adding rare earth elements to steel.

[0005] In summary, in the refining and protective casting processes, using a 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. However, rare earth oxides need to be combined with other slag-making materials to exert their unique functions. Additionally, due to the scarcity and reactivity of rare earth elements, how to prepare an economical and appropriately composed rare earth oxide-containing refining slag has become a technical problem to be solved. Summary of the Invention

[0006] To address the above technical problems, the present invention provides a prefabricated slag of the CaO-Al2O3-REO-MgO slag system, as well as its preparation and application, which can at least solve one of the technical problems in the existing steelmaking process, namely how to prepare a low-melting-point refining slag containing rare earth oxides at low cost, and how to achieve environmentally friendly, economical, and efficient recycling of rare earth polishing powder waste.

[0007] The object of the present invention is mainly achieved through the following technical solutions:

[0008] On the one hand, the present invention provides a prefabricated slag of the CaO-Al2O3-REO-MgO slag system. The prefabricated slag includes rare earth oxides, Al2O3, CaO, and MgO. In terms of mass percentage content, 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%.

[0009] Further, the mass ratio of CaO to Al2O3 is 0.9-1.3.

[0010] Further, in the rare earth oxides, in terms of mass percentage content, 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.

[0011] Further, the particle size of the prefabricated slag is 20-50 mm, and the melting point is 1380-1420 °C.

[0012] The present invention also provides a method for preparing the above prefabricated slag, including the following steps:

[0013] Step S1: Clean the rare earth polishing powder waste.

[0014] Step S2: Conduct component detection on the waste rare earth polishing powder after cleaning treatment. Classify the waste rare earth polishing powder into type A and / or type B according to the content of rare earth oxides in the waste rare earth polishing powder. Among them, the mass percentage content of rare earth oxides in type A waste rare earth polishing powder is greater than 90%, which is used as the rare earth oxide source material; the mass percentage content of rare earth oxides in type B waste rare earth polishing powder is 40 - 90%. Subject type B waste rare earth polishing powder to acetic acid treatment to obtain a solid compound containing rare earth, aluminum, and calcium, which is used as the rare earth oxide source material.

[0015] Step S3: Mix the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material in proportion, then sinter them, and then crush and screen to obtain a prefabricated slag of the CaO - Al2O3 - REO - MgO slag system.

[0016] Furthermore, the acetic acid treatment in step S2 includes the following steps:

[0017] Step a: Pour type B waste rare earth polishing powder into an acetic acid aqueous solution with a concentration of 0.8 - 1.3 mol / L at a ratio of 45 - 55 g / L, and stir at a temperature of 70 - 100 °C for more than 24 h;

[0018] Step b: Filter the precipitate to obtain a solution;

[0019] Step c: Heat, evaporate, and dry the obtained solution to obtain a solid compound containing rare earth, aluminum, and calcium, which is used as the rare earth oxide source material.

[0020] Furthermore, in step S3, the calcium oxide source material is a solid with a CaO mass percentage content of more than 80%; the aluminum oxide source material is a solid with an Al2O3 mass percentage content of more than 80%; the magnesium oxide source material is a solid with a MgO mass percentage content of more than 80%. The particle sizes of the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material are controlled below 100 mesh.

[0021] Furthermore, in step S3, the sintering is carried out in a natural atmosphere, and the sintering equipment is any one of a rotary kiln, a resistance sintering furnace, a gas - flow sintering furnace, a continuous sintering furnace, and an intermittent sintering furnace.

[0022] Furthermore, in step S3, the sintering process is as follows: First, preheat at 400 - 600 °C for 8 - 12 minutes, and then raise the temperature to 800 - 1200 °C and sinter for 25 - 35 minutes.

[0023] The present invention also provides an application of the above - mentioned prefabricated slag in slag for out - of - furnace refining, tundish covering agent, continuous casting mold powder, and slag for electroslag remelting.

[0024] Compared with the prior art, the present invention can achieve at least one of the following technical effects:

[0025] (1) The CaO and Al2O3 in the prefabricated slag of the present invention can form a low-melting-point calcium-aluminum compound, which provides a liquid environment for rapid fusion for the rare earth oxides and MgO to form a low-melting-point quaternary slag system. The addition of rare earth oxides and MgO further reduces the melting temperature, and the liquid CaO-Al2O3-REO-MgO slag system provides the prerequisite for the respective capabilities of rare earth oxides, Al2O3, CaO, and MgO in the slag-gold reaction.

[0026] (2) The CaO and Al2O3 components in the prefabricated slag of the CaO-Al2O3-REO-MgO slag system of the present invention meet the ratio requirements for forming a 12CaO·7Al2O3 compound, and the CaO content is relatively high, so that the slag has a low melting temperature, high basicity and high CaO activity, forming a rapid slagging and strong desulfurization ability.

[0027] (3) The Al2O3 in the prefabricated slag of the CaO-Al2O3-REO-MgO slag system of the present invention has a large capacity for containing the products of aluminum deoxidation and can absorb a large amount of deoxidation products.

[0028] (4) The rare earth oxides mainly composed of cerium in the prefabricated slag of the CaO-Al2O3-REO-MgO slag system of the present invention can further reduce the melting temperature of the CaO-Al2O3 system slag, and can improve the deoxidation ability of the deoxidizing elements, and absorb non-metallic inclusions in the molten steel. When smelting rare earth steel, it can increase the yield of rare earth elements and improve the existence form of rare earth in steel. It is particularly suitable for the electric slag remelting process of preparing rare earth steel using the rare earth slag reduction method.

[0029] (5) The CaO-Al2O3-REO-MgO slag system prefabricated slag of the present invention does not contain fluoride. The MgO in the slag not only reduces the erosion of the slag on the furnace lining, but also has a protective effect on the furnace lining, can increase the service life of the ladle and the purity of the molten steel, reduce the refining cost, but also can improve the fluidity of the slag, increase the desulfurization capacity of the slag, and improve the refining efficiency.

[0030] (6) The CaO-Al2O3-REO-MgO slag prefabricated slag of the present invention uses rare earth polishing powder waste as raw material, has a simple preparation process, and has the advantages of low cost and stable and controllable composition. It also makes the recycling process of rare earth polishing powder waste simple, efficient, environmentally friendly and economical, realizing the transformation of waste into treasure, and has a strong demonstration effect.

[0031] (7) The prefabricated slag of the CaO-Al2O3-REO-MgO slag system of the present invention has the characteristics of high alkalinity and good fluidity due to the presence of more CaO and a lower melting temperature. During actual use, other slag materials can be added according to needs on the basis of the prefabricated slag.

[0032] (8) The prefabricated slag of the CaO-Al2O3-REO-MgO slag system of the present invention has the characteristics of good dispersion of solid particles, low melting temperature, and fast slag melting. Therefore, it is not only suitable for the secondary refining and electroslag remelting processes of high-grade steel, but also can be used as an intermediate ladle covering agent and a continuous casting mold powder. Description of the Drawings

[0033] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference numerals represent the same components.

[0034] Figure 1 is the phase diagram of the CaO-Al2O3 system;

[0035] Figure 2 is the ternary phase diagram of the CaO-Al2O3-Ce2O3 system at 1500 °C. Detailed Description of the Invention

[0036] The following further describes in detail a prefabricated slag of a CaO-Al2O3-REO-MgO slag system and its preparation and application 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.

[0037] Refining slag is a technology in which raw materials are mixed in a certain proportion and particle size, heated below the melting point of the raw materials to sinter the raw materials together, and then broken into particle sizes for use. The prefabricated forms mainly include sintered type, pre-melted type, and mixed type. The main components of refining slag are CaO-CaF2 slag system, CaO-Al2O3-CaF2 slag system, and CaO-Al2O3-SiO2 slag system, etc. The slag system containing CaF2 has the advantages of low melting temperature, rapid slag formation, good fluidity, and good desulfurization ability. However, the large amount of CaF2 seriously erodes the refractory materials of the furnace lining, and the consumption of refractory materials significantly increases the cost of the secondary refining process. Secondly, the environmental pollution caused by fluorides cannot be ignored, and it belongs to the raw materials restricted for use in smelting; the CaO-Al2O3-SiO2 slag system has strong inclusion absorption ability, does not contain fluorine, and meets the environmental protection requirements, but has poor fluidity and relatively low slag alkalinity, and the desulfurization ability cannot be fully exerted.

[0038] In view of the above technical status, the present invention proposes a prefabricated slag of CaO-Al2O3-REO-MgO slag system, REO is the abbreviation of Rare Earth Oxides, i.e. rare earth oxides. The components of the prefabricated slag include rare earth oxides, Al2O3, CaO, and MgO. Calculated by mass percentage, the rare earth oxide content is 10-20%, such as 12%, 15%, and 18%; the Al2O3 content is 30-50%, such as 35%, 40%, and 45%; the CaO content is 30-50%, such as 35%, 40%, and 45%; and the MgO content is 5-10%, such as 8%.

[0039] It should be noted that CaO and Al2O3 in the prefabricated slag can form low-melting-point calcium-aluminum compounds, providing a rapid fusion liquid environment for the addition of rare earth oxides and MgO to form a low-melting-point quaternary slag system. The addition of rare earth oxides and MgO further reduces the melting temperature, and the liquid CaO-Al2O3-REO-MgO slag system provides the prerequisite for the respective capabilities of rare earth oxides, Al2O3, CaO, and MgO in the slag-gold reaction.

[0040] Furthermore, in the prefabricated slag, the mass ratio of CaO:Al2O3 is 0.9-1.3.

[0041] Specifically, in the rare earth oxides, cerium oxide accounts for 50-100% of the total rare earth oxides by mass percentage, and the rest is one or a combination of lanthanum oxide, 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 wait.

[0042] Furthermore, the prefabricated slag is granular, has a particle size of 20-50 mm, and a melting point of 1380-1420°C.

[0043] Taking into consideration the lower melting temperature, higher basicity and higher CaO activity of the prefabricated slag, the composition of the CaO-Al2O3 slag system is limited to a range in which the compound composition is mainly 12CaO·7Al2O3 and the CaO content is relatively high. For example, the mass percentage of Al2O3 in the CaO-Al2O3 system is 46-52%, that is, the mass ratio of CaO:Al2O3 is about 0.9-1.3. This ratio range allows the liquid phase to appear at 1450-1550°C.

[0044] Figure 1It is the CaO-Al2O3 system phase diagram. Based on the phase diagram analysis, CaO-Al2O3 system compounds that meet the above requirements are determined. Strongly basic CaO can form a series of complex compounds with amphoteric oxide Al2O3, such as 3CaO·Al2O3, 12CaO·7Al2O3, CaO·Al2O3, CaO·2Al2O3, CaO·6Al2O3. However, except for 12CaO·7Al2O3, other compounds have relatively high melting points. The melting point of 12CaO·7Al2O3 is 1455 °C, and the mass percentage of Al2O3 in it is about 52%. In addition, from Figure 1 it can be seen that to the right of 12CaO·7Al2O3 is the CaO·Al2O3 compound, and to the left is the 3CaO·Al2O3 compound. And CaO·Al2O3 is a congruent melting point compound, that is, a stable compound, while 3CaO·Al2O3 is an incongruent melting point compound, that is, an unstable compound.

[0045] It should be noted that the most stable valence state of Ce at room temperature is +4, so cerium oxides mainly exist in the form of CeO2 at room temperature. However, under the refining conditions of molten steel, in a high-temperature environment with a low oxygen partial pressure for a long time, the transformation of CeO2 to Ce2O3 will occur. Therefore, for the research on the Ce-containing CaO-Al2O3 slag system in the refining state, not only CeO2 but also Ce2O3 needs to be considered.

[0046] Considering the comprehensive balance of the activities and melting temperatures of various components in the slag, the cerium oxide in the prefabricated slag of the present invention is limited to the range of 5%-20%. Since the cerium oxide accounts for 50-100% of the total rare earth oxides, the lower limit of the mass percentage of rare earth oxides in the prefabricated slag is limited to 10%, and the upper limit is limited to 20%, such as 12%, 15%, 18%.

[0047] Figure 2 It is the ternary phase diagram of the CaO-Al2O3-Ce2O3 system at 1500 °C. Based on the phase diagram analysis, the content range of cerium oxide in the prefabricated slag that meets the above requirements is determined. From Figure 2As can be seen, the ternary phase of CaO-Al2O3-Ce2O3 is in a liquid phase at 1500 °C when the mass ratio of CaO:Al2O3 is 0.9 - 1.3 and the mass percentage of Ce2O3 is below 40%. In the ternary system of CaO-Al2O3-CeO2 (Ce2O3), as the content of CeO2 increases, the activities of CeO2 and Ce2O3 gradually increase, the activity of Al2O3 gradually decreases, and the activity of CaO first increases and then decreases. Near the mass percentage of CeO2 of 20%, the activity of CaO reaches its peak; as the content of CeO2 increases, the melting temperature shows a trend of first decreasing and then increasing. Near the mass percentage of CeO2 of 5%, the melting temperature is about 1360 °C. Near the mass percentage of CeO2 of 10%, the melting temperature drops to the lowest, about 1300 °C. Near the mass percentage of CeO2 of 20%, the melting temperature rises again to about 1360 °C.

[0048] Appropriate magnesium oxide in the slag can reduce the melting temperature of the slag, improve its fluidity, contribute to the separation of the slag from the molten metal, and improve the refining efficiency; magnesium oxide can be used as a desulfurizer to react with sulfur in the molten steel to form magnesium sulfide, thereby purifying the molten steel and improving the purity and corrosion resistance of the steel; in addition, magnesium oxide can also reduce the erosion of the furnace lining and extend the service life of the furnace. However, when the content of magnesium oxide in the slag is too high, it will lead to an increase in the viscosity of the slag, making the fluidity of the slag poor, which is not conducive to the floating and removal of impurities in the molten steel, thus affecting the purity of the molten steel. Therefore, the content of MgO in the prefabricated slag of the present invention is limited to 5 - 10%, such as 8%.

[0049] Secondly, the present invention also provides a preparation method for the above-mentioned CaO-Al2O3-REO-MgO slag system prefabricated slag, which includes the following steps:

[0050] Step S1, cleaning the rare earth polishing powder waste;

[0051] Step S2, detecting the composition of the cleaned rare earth polishing powder waste, and classifying the rare earth polishing powder waste into type A and / or type B according to the content of rare earth oxides in the rare earth polishing powder waste. Among them, the mass percentage of rare earth oxides in type A rare earth polishing powder waste is greater than 90%, which is used as the rare earth oxide source material; the mass percentage of rare earth oxides in type B rare earth polishing powder waste is 40 - 90%, and the type B rare earth polishing powder waste is treated with acetic acid to obtain a solid compound containing rare earth, aluminum and calcium, which is used as the rare earth oxide source material;

[0052] Step S3, mixing the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material in proportion, then sintering, and then crushing and screening to obtain the prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0053] It should be noted that rare earth polishing powder waste is a solid waste with a rare earth oxide mass percentage of more than 40%, so it has high recycling value. The current recycling methods mainly include ore dressing method, impurity chemical removal method, pyro-hydrometallurgical combined method, acid separation method, etc. Although the rare earth recovery rates of these methods are relatively high, the process flows are complex, the energy consumption is high, the cost is high, and the wastewater discharge is large. Industrialization still takes time.

[0054] Due to the scarcity and activity of rare earths, it is difficult to obtain rare earth raw materials with suitable composition and good economy from the existing actual smelting slag. Generally, the waste steel slag obtained during the smelting of rare earth steel or the tail slag after the beneficiation and smelting of rare earth-containing ores are used. Not only are the components difficult to control, but the effects are also uncertain. The present invention uses rare earth polishing powder waste to prepare precast slag, which not only solves the raw material problem of refining slag, but also obtains a more environmentally friendly, economical and efficient waste rare earth recycling method, which is of great significance for protecting the environment and alleviating the severe situation of tight rare earth resources.

[0055] Specifically, the mass percentage of rare earth oxides in rare earth polishing powder is more than 40%, and the rest are SiO2, Al2O3, CaO, ZrO2, etc.; the main component of rare earth oxides is CeO2, and the rest are composed of La2O3, Nd2O3, Pr6O 11 etc. According to the content ranges of rare earth oxides and CeO2, they are divided into two categories. The first category is that the mass percentage of rare earth oxides in rare earth polishing powder is 40-90%, and the mass percentage of CeO2 in rare earth polishing powder is 20-90%; the second category is that the mass percentage of rare earth oxides in rare earth polishing powder is greater than 90%, and the mass percentage of CeO2 in rare earth polishing powder is greater than 45%. The average particle size of rare earth polishing powder is below 10 microns.

[0056] Specifically, in step S1, the cleaning treatment of rare earth polishing powder waste includes: drying the rare earth polishing powder waste, grinding the lumps, picking out obvious impurities, and then sieving through a 100-mesh standard sieve.

[0057] Specifically, in step S2, the component detection of rare earth polishing powder waste includes the detection of rare earth oxides CeO2, La2O3, Nd2O3, Pr6O 11 , as well as the detection of non-rare earth oxides SiO2, Al2O3, CaO, ZrO2. The specific detection is carried out in accordance with GB / T20165 "Rare Earth Polishing Powder" and GB / T12690 "Chemical Analysis Methods for Non-rare Earth Inclusions in Rare Earth Metals and Their Oxides". The content of rare earth oxides in rare earth polishing powder waste is the sum of the mass percentages of rare earth oxides CeO2, La2O3, Nd2O3, Pr6O 11 in mass percentage.

[0058] Specifically, the acetic acid treatment in step S2 includes the following steps:

[0059] Step a: Pour the Class B rare earth polishing powder waste into the acetic acid aqueous solution at a ratio of 45 - 55 g / L, and stir for more than 24 h at a temperature of 70 - 100 °C;

[0060] Step b: Filter the precipitate to obtain a solution;

[0061] Step c: Heat and evaporate the obtained solution to dryness to obtain a solid compound containing rare earth, aluminum, and calcium as the rare earth oxide source material.

[0062] It should be noted that in step a, the concentration of the acetic acid aqueous solution is 0.8 - 1.3 mol / L, preferably 1.0 mol / L. By stirring, the Class B rare earth polishing powder waste is evenly dispersed in the acetic acid aqueous solution, increasing the contact interface between the solid and liquid reactants and improving the reaction efficiency.

[0063] By using acetic acid treatment, the rare earth oxides, aluminum oxide, and calcium oxide in the rare earth polishing powder waste are dissolved in the acetic acid aqueous solution, and the SiO2 and ZrO2 substances insoluble in the acetic acid aqueous solution are removed, thereby obtaining a solid compound containing rare earth, aluminum, and calcium as the rare earth oxide source material.

[0064] The reactions of CeO2, La2O3, Al2O3, and CaO in the rare earth polishing powder waste with the acetic acid aqueous solution are as follows:

[0065] CeO2 + 4CH3COOH → Ce(CH3COO)4 + 2H2O (1)

[0066] La2O3 + 6CH3COOH → 2La(CH3COO)3 + 3H2O (2)

[0067] Al2O3 + 6CH3COOH → 2Al(CH3COO)3 + 3H2O (3)

[0068] CaO + 2CH3COOH → Ca(CH3COO)2 + H2O (4)

[0069] According to reaction formulas (1) - (4), CeO2, La2O3, Al2O3, and CaO in the rare earth polishing powder waste are dissolved in the acetic acid aqueous solution. By filtration, impurities such as SiO2 and ZrO2 insoluble in the acetic acid aqueous solution are filtered out, and the remaining is an aqueous solution containing cerium acetate, lanthanum acetate, aluminum acetate, and calcium acetate. Then, after evaporation and drying, a solid compound containing cerium acetate, lanthanum acetate, aluminum acetate, and calcium acetate is obtained.

[0070] It should be noted that in step c, the solid compound containing rare earth, aluminum and calcium is subjected to component detection, such as the detection of rare earth components Ce, La, Nd, Pr, and non-rare earth components Al, Ca. The detection is carried out with reference to GB / T 20165 "Rare Earth Polishing Powder" and GB / T 12690 "Chemical Analysis Methods for Non-Rare Earth Inclusions in Rare Earth Metals and Their Oxides". The detection results are compared with the detection results in step S2 to obtain the conversion rates of rare earth components and Al, Ca after acetic acid treatment, such as a conversion rate of more than 90%.

[0071] Specifically, in step S3, the rare earth oxide source material includes the Class A rare earth polishing powder waste obtained in step S2 and the solid compound containing rare earth, aluminum and calcium; the calcium oxide source material is a solid with a CaO mass percentage content of more than 80%, such as limestone, etc.; the aluminum oxide source material is a solid with an Al2O3 mass percentage content of more than 80%, such as bauxite, etc.; the magnesium oxide source material is a solid with an MgO mass percentage content of more than 80%, such as light-burned magnesium oxide powder; the particle sizes of the rare earth oxide source material, calcium oxide source material, aluminum oxide source material and magnesium oxide source material are controlled below 100 mesh.

[0072] In step S3, the sintering is carried out in a natural atmosphere. The sintering equipment includes rotary kilns, resistance sintering furnaces, gas flow sintering furnaces, continuous sintering furnaces, intermittent sintering furnaces, etc., but is not limited to these equipment. The sintering process is as follows: First, preheat at 400 - 600 °C for 8 - 12 minutes, preferably preheat at 500 °C for 10 minutes; then raise the temperature to 800 - 1200 °C and sinter for 25 - 35 minutes, preferably raise the temperature to 1000 °C and sinter for 30 minutes; if the sintering temperature is too high and the time is too long, the furnace charge will agglomerate. During the high-temperature sintering in a natural atmosphere, the rare earth oxides in the rare earth oxide source material and the compounds containing rare earth, aluminum and calcium, such as CeO2, La2O3, Nd2O3, Pr6O 11 , as well as cerium acetate, lanthanum acetate, aluminum acetate and calcium acetate, etc., will undergo chemical changes, and correspondingly generate rare earth oxides, as well as Al2O3 and CaO.

[0073] After sintering, it is crushed and screened to obtain a CaO - Al2O3 - REO - MgO slag system prefabricated slag with a particle size of 20 - 50 mm. By mass percentage, the content of rare earth oxides in the prefabricated slag is 10 - 20%, the content of Al2O3 is 30 - 50%, the content of CaO is 30 - 50%, and the content of MgO is 5 - 10%. 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.

[0074] It should be noted that since the components in the prefabricated slag are already determined, the batching ratios of the rare earth oxide source material, calcium oxide source material, and alumina source material are determined by reverse deduction based on the finished product composition of the prefabricated slag.

[0075] The CaO-Al2O3-REO-MgO slag system prefabricated slag of the present invention uses waste rare earth polishing powder as a raw material, has a simple preparation process, and has the advantages of low cost, stable and controllable composition. Moreover, the recycling process of waste rare earth polishing powder is simple, efficient, environmentally friendly, and economical, realizing the transformation of waste into treasure and having a strong demonstration effect.

[0076] Finally, the present invention also provides an application of the above CaO-Al2O3-REO-MgO slag system in slag for secondary refining, tundish covering agent, continuous casting mold powder, and slag for electroslag remelting.

[0077] It should be noted that the CaO-Al2O3-REO-MgO slag system prefabricated slag of the present invention has the characteristics of high alkalinity and good fluidity due to containing more CaO and a lower melting temperature. In actual use, other slag materials can be added according to needs based on the prefabricated slag, such as adding lime, magnesium balls, dolomite, converter slag, electric furnace slag, mold powder, etc.

[0078] During secondary refining or electroslag remelting for steelmaking, CaO in the prefabricated slag mainly plays a role in desulfurization, and a higher content of CaO has a stronger desulfurization ability; a higher content of Al2O3 in the slag has a large capacity for accommodating the products of aluminum deoxidation and can adsorb a large amount of deoxidation products; the rare earth oxides in the slag can further reduce the melting temperature of the CaO-Al2O3-based slag, improve the deoxidation ability of deoxidizing elements, and absorb non-metallic inclusions in the molten steel. When smelting rare earth steel, it can improve the recovery rate of rare earth elements and improve the existing form of rare earth in steel, and is especially suitable for the electroslag remelting process of preparing rare earth steel by the rare earth slag system reduction method.

[0079] Due to the prefabricated slag of the present invention having a lower melting temperature, good fluidity, and strong inclusion adsorption ability, it can be combined with other slag materials to be used as a tundish covering agent or continuous casting mold powder, and can achieve the purpose of purifying the molten steel and reducing nozzle nodulation and blockage.

[0080] The prefabricated slag of the present invention contains MgO and does not contain fluorides, which not only reduces the erosion of the slag on the furnace lining, has a protective effect on the furnace lining, can improve the service life of the ladle and the purity of the molten steel, reduce the refining cost, but also can improve the fluidity of the slag, increase the desulfurization ability of the slag, and improve the refining efficiency.

[0081] Example 1

[0082] A prefabricated slag of the CaO-Al2O3-REO-MgO slag system, calculated by mass percentage, the content of rare earth oxide is 20%, the content of Al2O3 is 45%, the content of CaO is 30%, the content of MgO is 5%, the mass ratio of CaO:Al2O3 is 0.67, the oxide of cerium accounts for 100% of the total amount of rare earth oxides; the particle size is 20-50mm, and the melting point is 1420°C.

[0083] A preparation method of the above-mentioned prefabricated slag of the CaO-Al2O3-REO-MgO slag system includes the following steps:

[0084] Step S1: Clean the waste rare earth polishing powder; including drying the waste rare earth polishing powder, grinding the lumps, picking out the obvious impurities, and then sieving with a 100-mesh standard sieve.

[0085] Step S2: Conduct component detection on the cleaned waste rare earth polishing powder. The CeO2 in the polishing powder waste is 99% and the SiO2 is 1%. Classify the waste rare earth polishing powder as Class A waste rare earth polishing powder and use it as the source material of rare earth oxide.

[0086] Step S3: Mix the source material of rare earth oxide, the source material of calcium oxide, the source material of alumina and the source material of magnesia in proportion and then sinter, and then crush and screen to obtain the prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0087] The source material of calcium oxide is limestone with 80% CaO by mass percentage, the source material of alumina is bauxite with 80% Al2O3 by mass percentage, and the source material of magnesia is light-burned magnesia powder with 80% MgO by mass percentage; the particle sizes of the source material of rare earth oxide, the source material of calcium oxide, the source material of alumina and the source material of magnesia are controlled below 100 meshes, and the mass ratio of Class A waste rare earth polishing powder: bauxite: limestone: light-burned magnesia powder is 100:278:186:31.

[0088] The sintering is carried out in a natural atmosphere. The sintering equipment is a rotary kiln. The sintering process is as follows: first preheat at 400°C for 8 minutes; then raise the temperature to 800°C and sinter for 25 minutes; after sintering, crush and screen to obtain 2 tons of prefabricated slag of the CaO-Al2O3-REO-MgO slag system with a particle size of 20-50mm.

[0089] The application of the above-mentioned prefabricated slag of the CaO-Al2O3-REO-MgO slag system:

[0090] During the secondary refining of steel, according to the pre-determined slag-steel ratio of 2% and the composition of the refining slag, add the prefabricated slag and lime into the LF refining furnace containing 100 tons of molten steel as the slag for secondary refining. The slag formation speed during the refining process is 2 min, the sulfur content in the steel is 45 ppm, and the oxygen content is 25 ppm, which can meet the requirements.

[0091] During continuous casting, the tundish covering agent prepared from prefabricated slag and magnesia is covered on the molten steel surface in the tundish; the continuous casting mold powder prepared from prefabricated slag and graphite is covered on the molten steel surface in the continuous casting mold. They have good dispersibility on the molten steel surface, and the slag formation speed is 2.3 min, which can meet the requirements.

[0092] When electroslag remelting rare earth steel, according to the predetermined slag-steel ratio of 3% and slag composition, prefabricated slag and CaO are added into the electroslag remelting furnace and used as the slag for electroslag remelting. Using the rare earth slag system reduction method, the rare earth recovery rate is 45%, the sulfur content in the steel is 25 ppm, and the oxygen content is 22 ppm, which can meet the requirements.

[0093] Example 2

[0094] A prefabricated slag of CaO-Al2O3-REO-MgO slag system, calculated by mass percentage, the rare earth oxide content is 10%, the Al2O3 content is 30%, the CaO content is 50%, the MgO content is 10%, the mass ratio of CaO:Al2O3 is 1.67, the cerium oxide accounts for 50% of the total rare earth oxides, and the rest is a combination of lanthanum oxide, neodymium oxide, and praseodymium oxide; the particle size is 20 - 50 mm, and the melting point is 1410 °C.

[0095] A preparation method of the above-mentioned prefabricated slag of CaO-Al2O3-REO-MgO slag system includes the following steps:

[0096] Step S1, cleaning the rare earth polishing powder waste; including drying the rare earth polishing powder waste, grinding the lumps, picking out the obvious impurities, and then sieving with a 100-mesh standard sieve.

[0097] Step S2, detecting the components of the cleaned rare earth polishing powder waste, CeO2 30%, La2O3 15%, Pr6O 11 5%, Nd2O3 10%, SiO2 30%, Al2O3 5%, CaO 3%, ZrO2 2%. Classify the rare earth polishing powder waste as Class B rare earth polishing powder waste, and treat the Class B rare earth polishing powder waste with acetic acid to obtain a solid compound containing rare earth, aluminum, and calcium as the rare earth oxide source material;

[0098] The acetic acid treatment includes the following steps:

[0099] Step a, pour the Class B rare earth polishing powder waste into an acetic acid aqueous solution with a concentration of 0.8 mol / L at a ratio of 45 g / L, and stir at a temperature of 70 °C for 24 h;

[0100] Step b, filter the precipitate to obtain a solution;

[0101] Step c: Heat and evaporate the obtained solution, and then dry it to obtain a solid compound containing rare earth, aluminum, and calcium, which is used as the rare earth oxide source material; the conversion rates of the rare earth component and the acetic acid treatment of Al and Ca are both 90%.

[0102] Step S3: Mix the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material in proportion, then sinter them, and then crush and screen them to obtain a prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0103] The calcium oxide source material is limestone with 85% mass percentage of CaO, the aluminum oxide source material is bauxite with 85% mass percentage of Al2O3, and the magnesium oxide source material is light burned magnesium oxide powder with 85% mass percentage of MgO; the particle sizes of the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material are controlled below 100 mesh, and the mass ratio of the rare earth oxide source material: bauxite: limestone: magnesium oxide source material is 139.4:185.3:314.5:63.5.

[0104] The sintering is carried out in a natural atmosphere, the sintering equipment is a resistance sintering furnace, and the sintering process is as follows: first preheat at 600 °C for 12 minutes; then raise the temperature to 1200 °C and sinter for 35 minutes; after sintering, crush and screen to obtain 2 tons of prefabricated slag of the CaO-Al2O3-REO-MgO slag system with a particle size of 20 - 50 mm.

[0105] The application of the above-mentioned prefabricated slag of the CaO-Al2O3-REO-MgO slag system:

[0106] When refining rare earth steel outside the steel furnace, according to the predetermined slag-to-steel ratio of 2% and the composition of the refining slag, add the prefabricated slag and lime into the LF refining furnace containing 100 tons of molten steel as the slag for external refining. During the refining process, the slag formation speed is 1.8 min, the rare earth recovery rate is 83%, the sulfur content in the steel is 24 ppm, and the oxygen content is 24 ppm, which can meet the requirements.

[0107] During the continuous casting of rare earth steel, cover the molten steel surface in the tundish with a tundish covering agent prepared from the prefabricated slag, lime, and magnesite; cover the molten steel surface in the continuous casting mold with a continuous casting mold powder prepared from the prefabricated slag, carbon black, mirabilite, and CaCO3. They have good dispersibility on the molten steel surface, the slag formation speed is 2.1 min, and the rare earth loss rates in the tundish and continuous casting processes are 16% and 18% respectively, which can meet the requirements.

[0108] When electroslag remelting rare earth steel, according to the predetermined slag-to-steel ratio of 3% and the slag composition, add the prefabricated slag, CaO, and MgO into the electroslag remelting furnace for use as the slag for electroslag remelting. Using the rare earth slag system reduction method, the rare earth recovery rate is 48%, the sulfur content in the steel is 20 ppm, and the oxygen content is 20 ppm, which can meet the requirements.

[0109] Example 3

[0110] A prefabricated slag of CaO-Al2O3-REO-MgO slag system, calculated by mass percentage, the content of rare earth oxide is 10%, the content of Al2O3 is 50%, the content of CaO is 30%, the content of MgO is 10%, the mass ratio of CaO:Al2O3 is 0.6, the oxide of cerium accounts for 75% of the total amount of rare earth oxides, and the rest is the oxide of lanthanum; the particle size is 20-50mm and the melting point is 1400°C.

[0111] A preparation method of the above-mentioned prefabricated slag of CaO-Al2O3-REO-MgO slag system includes the following steps:

[0112] Step S1: Clean the rare earth polishing powder waste; including drying the rare earth polishing powder waste, grinding the lumps, picking out obvious impurities, and then sieving through a 100-mesh standard sieve.

[0113] Step S2: Conduct component detection on the cleaned rare earth polishing powder waste, CeO2 30%, La2O3 10%, SiO2 45%, Al2O3 5%, CaO 5%, ZrO2 5%. Classify the rare earth polishing powder waste as Class B rare earth polishing powder waste, and conduct acetic acid treatment on the Class B rare earth polishing powder waste to obtain a solid compound containing rare earth, aluminum and calcium as the rare earth oxide source material;

[0114] The acetic acid treatment includes the following steps:

[0115] Step a: Pour the Class B rare earth polishing powder waste into an acetic acid aqueous solution with a concentration of 1.3 mol / L at a ratio of 55 g / L, and stir at a temperature of 100°C for 25 h;

[0116] Step b: Filter the precipitate to obtain a solution;

[0117] Step c: Heat and evaporate and dry the obtained solution to obtain a solid compound containing rare earth, aluminum and calcium as the rare earth oxide source material; the conversion rates of Ce, La, Al, and Ca in the acetic acid treatment are 92%, 93%, 94%, and 91%.

[0118] Step S3: Mix the rare earth oxide source material, calcium oxide source material, alumina source material and magnesia source material in proportion, then sinter, and then crush and screen to obtain a prefabricated slag of CaO-Al2O3-REO-MgO slag system.

[0119] The calcium oxide source material is limestone with a CaO mass percentage of 90%, the aluminum oxide source material is bauxite with an Al2O3 mass percentage of 90%, the magnesium oxide source material is light-burned magnesium oxide powder with an MgO mass percentage of 90%, and the particle sizes of the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material are controlled below 100 mesh. The mass ratio of the rare earth oxide source material: bauxite: limestone: light-burned magnesium oxide powder is 114.9:200:117.9:41.

[0120] Sintering is carried out in a natural atmosphere. The sintering equipment is a gas-flow sintering furnace, and the sintering process is as follows: First, preheat at 500°C for 10 minutes; then raise the temperature to 1000°C and sinter for 30 minutes; after sintering, crush and screen to obtain 2 tons of CaO-Al2O3-REO-MgO slag system preformed slag with a particle size of 20 - 50 mm.

[0121] The application of the above-mentioned CaO-Al2O3-REO-MgO slag system preformed slag:

[0122] During the secondary refining of steel, according to the predetermined slag-to-steel ratio of 2% and the composition of the refining slag, the preformed slag and lime are added into the LF refining furnace containing 100 tons of molten steel as the slag for secondary refining. The slag formation rate during the refining process is 1.5 min, the sulfur content in the steel is 50 ppm, and the oxygen content is 30 ppm, which can meet the requirements.

[0123] During continuous casting, the tundish covering agent prepared from the preformed slag and lime is covered on the surface of the molten steel in the tundish; the continuous casting mold powder prepared from the preformed slag and mirabilite is covered on the surface of the molten steel in the continuous casting mold. They have good dispersibility on the surface of the molten steel, and the slag formation rate is 1.5 min, which can meet the requirements.

[0124] During the electroslag remelting of rare earth steel, according to the predetermined slag-to-steel ratio of 3% and the slag composition, the preformed slag and CaO are added into the electroslag remelting furnace as the slag for electroslag remelting. Using the rare earth slag system reduction method, the rare earth recovery rate is 42%, the sulfur content in the steel is 30 ppm, and the oxygen content is 30 ppm, which can meet the requirements.

[0125] Example 4

[0126] A preformed slag of the CaO-Al2O3-REO-MgO slag system, calculated by mass percentage, the rare earth oxide content is 12%, the Al2O3 content is 40%, the CaO content is 40%, the MgO content is 8%, the mass ratio of CaO:Al2O3 is 1.0, the cerium oxide accounts for 80% of the total rare earth oxides, and the rest is a combination of lanthanum oxide, neodymium oxide, and praseodymium oxide; the particle size is 20 - 50 mm, and the melting point is 1380°C.

[0127] A preparation method of the above-mentioned CaO-Al2O3-REO-MgO slag system prefabricated slag comprises the following steps:

[0128] Step S1: Clean the rare earth polishing powder waste; including drying the rare earth polishing powder waste, grinding the lumps, picking out the obvious impurities, and then sieving through a 100-mesh standard sieve.

[0129] Step S2: Conduct component detection on the cleaned rare earth polishing powder waste, with CeO2 74.4%, La2O3 10%, Pr6O 11 3.6%, Nd2O3 5%, SiO2 3%, Al2O3 2%, CaO 1%, ZrO2 1%. Classify the rare earth polishing powder waste as Class A rare earth polishing powder waste and use it as the rare earth oxide source material;

[0130] Step S3: Mix the rare earth oxide source material, calcium oxide source material, alumina source material, and magnesia source material in proportion, then sinter and crush and screen them to obtain the prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0131] The calcium oxide source material is limestone with 80% CaO by mass percentage, the alumina source material is bauxite with 80% Al2O3 by mass percentage, and the magnesia source material is light-burned magnesia powder with 80% MgO by mass percentage; the particle sizes of the rare earth oxide source material, calcium oxide source material, alumina source material, and magnesia source material are controlled below 100 mesh, and the mass ratio of Class A rare earth polishing powder waste: bauxite: limestone: light-burned magnesia powder is 100:385:386:77.5.

[0132] Sintering is carried out in a natural atmosphere, the sintering equipment is a continuous sintering furnace, and the sintering process is as follows: First, preheat at 550°C for 9 minutes; then raise the temperature to 900°C and sinter for 28 minutes; after sintering, crush and screen to obtain 1 ton of prefabricated slag of the CaO-Al2O3-REO-MgO slag system with a particle size of 20 - 50 mm.

[0133] Application of the above-mentioned CaO-Al2O3-REO-MgO slag system prefabricated slag:

[0134] When refining rare earth steel outside the steel furnace, according to the predetermined slag-steel ratio of 1.0% and the composition of the refining slag, add the prefabricated slag and lime into the AOD refining furnace containing 100 tons of molten steel as the slag for outside-furnace refining. During the refining process, the slag formation speed is 2.5 min, the rare earth recovery rate is 80%, the sulfur content in the steel is 28 ppm, and the oxygen content is 23 ppm, which can meet the requirements.

[0135] During the continuous casting of rare earth steel, the tundish covering agent prepared from prefabricated slag, lime, and magnesite is covered on the molten steel surface in the tundish; the continuous casting mold powder prepared from prefabricated slag, carbon black, mirabilite, and CaCO3 is covered on the molten steel surface in the continuous casting mold. They have good dispersibility on the molten steel surface, the slag formation speed is 2.0 min, and the rare earth loss rates in the tundish and continuous casting processes are 17% and 15% respectively, which can meet the requirements.

[0136] When electroslag remelting rare earth steel, according to the predetermined slag-to-steel ratio of 3% and slag composition, prefabricated slag and MgO are added into the electroslag remelting furnace and used as the slag for electroslag remelting. Using the rare earth slag system reduction method, the rare earth recovery rate is 43%, the sulfur content in the steel is 21 ppm, and the oxygen content is 27 ppm, which can meet the requirements.

[0137] Example 5

[0138] A prefabricated slag of the CaO-Al2O3-REO-MgO slag system, calculated by mass percentage, the rare earth oxide content is 18%, the Al2O3 content is 35%, the CaO content is 40%, the MgO content is 7%, the mass ratio of CaO:Al2O3 is 1.14, the cerium oxide accounts for 60% of the total rare earth oxides, and the rest is a combination of lanthanum oxide, neodymium oxide, and praseodymium oxide; the particle size is 20 - 50 mm, and the melting point is 1390 °C.

[0139] A preparation method of the above-mentioned prefabricated slag of the CaO-Al2O3-REO-MgO slag system includes the following steps:

[0140] Step S1: Clean the rare earth polishing powder waste; including drying the rare earth polishing powder waste, grinding the lumps, picking out the obvious impurities, and then sieving through a 100-mesh standard sieve.

[0141] Step S2: Detect the composition of the cleaned rare earth polishing powder waste, CeO2 48%, La2O3 20%, Pr6O 11 4%, Nd2O3 8%, SiO2 10%, Al2O3 6%, CaO 3%, ZrO2 1%. Classify the rare earth polishing powder waste as type B rare earth polishing powder waste, and perform acetic acid treatment on the type B rare earth polishing powder waste to obtain a solid compound containing rare earth, aluminum, and calcium as the rare earth oxide source material;

[0142] The acetic acid treatment includes the following steps:

[0143] Step a: Pour the type B rare earth polishing powder waste into an acetic acid aqueous solution with a concentration of 1.0 mol / L at a ratio of 50 g / L, and stir at a temperature of 85 °C for 28 h;

[0144] Step b: Filter the precipitate to obtain a solution;

[0145] Step c: Heat and evaporate the obtained solution, and then dry it to obtain a solid compound containing rare earth, aluminum, and calcium, which is used as the rare earth oxide source material; the conversion rates of Ce, La, Pr, Nd, Al, and Ca treated with acetic acid are 96%, 95%, 94%, 93%, 92%, and 91% respectively.

[0146] Step S3: Mix the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material in proportion, then sinter them, and then crush and screen them to obtain a prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

[0147] The calcium oxide source material is limestone with 80% mass percentage of CaO, the aluminum oxide source material is bauxite with 85% mass percentage of Al2O3, and the magnesium oxide source material is light burned magnesium oxide powder with 90% mass percentage of MgO; the particle sizes of the rare earth oxide source material, calcium oxide source material, aluminum oxide source material, and magnesium oxide source material are controlled below 100 mesh, and the mass ratio of the rare earth oxide source material: bauxite: limestone: magnesium oxide source material is 191.6:168:212:33.

[0148] The sintering is carried out in a natural atmosphere, the sintering equipment is an intermittent sintering furnace, and the sintering process is as follows: First, preheat at 450°C for 11 minutes; then raise the temperature to 1100°C and sinter for 32 minutes; after sintering, crush and screen to obtain 0.5 tons of prefabricated slag of the CaO-Al2O3-REO-MgO slag system with a particle size of 20 - 50 mm.

[0149] Application of the above-mentioned prefabricated slag of the CaO-Al2O3-REO-MgO slag system:

[0150] During the ladle furnace refining of steel, according to the predetermined slag-steel ratio of 1.0% and the composition of the refining slag, add the prefabricated slag, lime, and magnesite into the AOD refining furnace containing 100 tons of molten steel as the slag for ladle furnace refining. During the refining process, the slag formation rate is 2.2 min, the sulfur content in the steel is 48 ppm, and the oxygen content is 26 ppm, which can meet the requirements.

[0151] During the continuous casting process, cover the molten steel surface in the tundish with the tundish covering agent prepared from the prefabricated slag and MgO; cover the molten steel surface in the continuous casting mold with the continuous casting mold powder prepared from the prefabricated slag, MgO, and mirabilite. They have good dispersibility on the molten steel surface, and the slag formation rate is 1.6 min, which can meet the requirements.

[0152] During the electroslag remelting of rare earth steel, according to the predetermined slag-steel ratio of 3% and the slag composition, add the prefabricated slag and CaO into the electroslag remelting furnace for use as the slag for electroslag remelting. Using the rare earth slag system reduction method, the rare earth recovery rate is 41%, the sulfur content in the steel is 25 ppm, and the oxygen content is 21 ppm, which can meet the requirements.

[0153] As described above, it is only the preferred specific embodiment 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 prefabricated slag of CaO-Al2O3-REO-MgO slag system, characterized in that: The prefabricated slag comprises rare earth oxide, Al2O3, CaO and MgO. According to the 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%.

2. The prefabricated slag according to claim 1, characterized in that: The mass ratio of the CaO to the Al2O3 is 0.9-1.

3.

3. The prefabricated slag according to claim 1, characterized in that: In the rare earth oxides, cerium oxide accounts for 50-100% of the total rare earth oxides by mass percentage, and the rest is lanthanum oxide, neodymium oxide, praseodymium oxide or a combination of several thereof.

4. The prefabricated slag according to claim 1, characterized in that: The prefabricated slag has a particle size of 20-50 mm and a melting point of 1380-1420°C.

5. The method for preparing prefabricated slag according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: Step S1, cleaning the waste rare earth polishing powder; Step S2, testing the composition of the cleaned waste rare earth polishing powder, and classifying the waste rare earth polishing powder into Class A and / or Class B according to the rare earth oxide content in the waste rare earth polishing powder, wherein the mass percentage of rare earth oxide in the waste rare earth polishing powder of Class A is greater than 90%, and is used as a rare earth oxide source material; the mass percentage of rare earth oxide in the waste rare earth polishing powder of Class B is 40-90%, and the waste rare earth polishing powder of Class B is treated with acetic acid to obtain a solid compound containing rare earth, aluminum and calcium, which is used as a rare earth oxide source material; Step S3, mixing the rare earth oxide source material, the calcium oxide source material, the aluminum oxide source material and the magnesium oxide source material in proportion, sintering them, and then crushing and screening them to obtain the prefabricated slag of the CaO-Al2O3-REO-MgO slag system.

6. The method according to claim 5, characterized in that: The acetic acid treatment in step S2 comprises the following steps: Step a, pouring the Class B rare earth polishing powder waste into an acetic acid aqueous solution with a concentration of 0.8-1.3 mol / L at a ratio of 45-55 g / L, and stirring at a temperature of 70-100° C. for more than 24 hours; Step b, filtering the precipitate to obtain a solution; Step c: heating the obtained solution to evaporate and dry it to obtain the solid compound containing rare earth, aluminum and calcium as the rare earth oxide source material.

7. The method according to claim 5, characterized in that: In step S3, the calcium oxide source material is a solid material with a CaO mass percentage of more than 80%; the aluminum oxide source material is a solid material with an Al2O3 mass percentage of more than 80%; the magnesium oxide source material is a solid material with a MgO mass percentage of more than 80%, and the particle sizes of the rare earth oxide source material, the calcium oxide source material, the aluminum oxide source material and the magnesium oxide source material are controlled to be below 100 mesh.

8. The method according to claim 5, characterized in that: In the step S3, the sintering is carried out in a natural atmosphere, and the sintering equipment is any one of a rotary kiln, a resistance sintering furnace, an air flow sintering furnace, a continuous sintering furnace, and an intermittent sintering furnace.

9. The method according to claim 5 or 8, characterized in that: In step S3, the sintering process is: first preheating at 400-600° C. for 8-12 minutes, then heating to 800-1200° C. and sintering for 25-35 minutes.

10. Use of the prefabricated slag according to any one of claims 1 to 4 or the prefabricated slag obtained by the method according to any one of claims 5 to 9 in slag for external refining, tundish covering agent, continuous casting crystallizer protection slag, and slag for electroslag remelting.

Images