A method for dephosphorizing in a vacuum induction furnace by recycling dephosphorization slag

Abstract

This invention discloses a method for dephosphorizing in a vacuum induction furnace by recycling dephosphorized slag. The steps are as follows: (1) Add steel material into the furnace and add dephosphorizing agent into the silo; the dephosphorizing agent is a mixture of pre-mixed dephosphorizing agent and reduced slag; (2) Install the slag scraper onto the tamping rod, then cover the furnace and start heating until the steel material melts completely; (3) After the steel material has completely melted, add the dephosphorizing agent in two batches and keep the temperature of the molten steel at 1450-1480℃, and then carry out a dephosphorization reaction for 30-35 minutes. Then, use the slag scraper to scrape the slag into the slag hopper in the furnace. (4) Take out the slag, cool and crush it, and then mix the slag with coke and heat it to reduce it. The reduced slag can be reused. This invention can control the phosphorus content in the molten steel to below 0.015%. Since it uses reduced slag, it has the advantages of low cost, fast slag formation in the early stage, simple operation, and the slag is pollution-free and reusable.

Description

Technical Field

[0001] This invention belongs to the field of vacuum smelting technology, and in particular to a method for dephosphorizing in a vacuum induction furnace by recycling dephosphorizing slag. Background Technology

[0002] Vacuum induction furnace smelting utilizes the electromagnetic induction changes of a power source to generate an induced current within the furnace charge, thereby melting the charge. Because slag has poor conductivity and cannot be melted by induction heating, it is typically not added in induction furnace smelting. Furthermore, the poor fluidity of slag would affect safety during tapping. However, controlling the phosphorus content in induction furnace smelting is quite difficult. Constrained by factors such as smelting conditions, furnace lining composition, and overall smelting composition, precise control of the phosphorus content within the furnace is challenging, necessitating the use of pure iron with low phosphorus content. This further increases smelting costs for enterprises.

[0003] Phosphorus in steel originates from phosphorus in iron ore, which is reduced and then enters the molten iron, participating in the steelmaking process. The principle of oxidative dephosphorization is to introduce the dephosphorization products as phosphates into the slag. In actual converter oxygen blowing smelting, the oxidation of phosphorus in the molten steel occurs at the slag-steel interface, where FeO is oxidized to P₂O₅, which combines with CaO to form stable phosphates. Fe continues to be oxidized to FeO, participating in subsequent dephosphorization processes. The dephosphorization principle is shown in the following reaction formula (A):

[0004]

[0005] As shown in reaction formula (A) above, dephosphorization is an exothermic process, therefore low temperature is favorable for the dephosphorization reaction. One difficulty limiting slag recycling is that the recycling of phosphorus in the slag may pollute the molten steel. If a method is used to separate phosphorus from the slag, the remaining slag can be reused. Since phosphorus in the slag forms phosphates in the form of oxides, a reduction method can be used to reduce the phosphorus in the slag to phosphorus gas, which can then be discharged from the slag. The basic principle is shown in reaction formula (B) below:

[0006] 4CaO·P2O 5(s) +5C=4CaO+P2+5CO(B)

[0007] As can be seen from the reaction formula (B), after reduction, the phosphorus in the slag will escape in the form of gas along with CO, thereby reducing the phosphorus content in the slag so as to achieve the purpose of recycling.

[0008] Patent application CN201110157597.7 discloses a method for dual dephosphorization using an intermediate frequency furnace and a vacuum induction furnace. The method involves first performing dephosphorization in the intermediate frequency furnace, followed by transferring the dephosphorized molten steel to a vacuum induction furnace for refining. While this method offers good dephosphorization efficiency, the operation is cumbersome and requires dangerous handling of the high-temperature molten steel. During the transfer process, some elements in the steel readily absorb oxygen, leading to the formation of inclusions that are difficult to remove within the molten steel.

[0009] Patent application number CN201810527391.0 discloses a desulfurization and dephosphorization process for a vacuum induction furnace. Its shortcomings are: (1) the slag used contains F, which can easily affect the sealing of the furnace body; (2) the slag contains F, which is harmful to the environment and cannot be recycled; (3) after reducing phosphorus and sulfur to the required level, it is impossible to fine-tune the alloy composition.

[0010] In summary, although existing technologies can achieve dephosphorization, there are few reports on dephosphorization in a vacuum furnace and the recycling of dephosphorized slag. Therefore, developing a method for dephosphorization in a vacuum induction furnace and recycling of dephosphorized slag is of great practical significance. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to provide a method for dephosphorizing in a vacuum induction furnace by recycling dephosphorizing slag, so as to achieve dephosphorization and slag recycling.

[0012] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0013] (1) Add steel material into the furnace and add dephosphorizing agent into the silo; the dephosphorizing agent includes pre-mixed dephosphorizing agent and reduced slag;

[0014] (2) Install the slag scraper onto the tamping rod, then cover the furnace with the lid and start powering on to heat up until the steel material is melted and clear;

[0015] (3) After all the steel material has melted, add the dephosphorizing agent in two batches and keep the temperature of the molten steel at 1450-1480℃; then carry out the dephosphorizing reaction for 30-35 minutes, and then use the slag removal equipment to remove the slag into the slag hopper in the furnace.

[0016] (4) After the steel tapping is completed, the slag is taken out and crushed. Then the slag is thoroughly mixed with coke and heated to reduce. The reduced slag can be used as the dephosphorizing agent in the subsequent furnace step (1).

[0017] Furthermore, the main components of the pre-prepared dephosphorizing agent in step (1) are in the following weight ratios: FeO 15%–22%, CaO 40%–50%, SiO2 15%–25%, MgO 9%–11%, and MnO 2%–3%.

[0018] Furthermore, in step (1), in addition to the dephosphorizing agent, Fe2O3 is also added, and the amount of Fe2O3 added is 8% to 12% of the total weight of the dephosphorizing agent.

[0019] Furthermore, in step (5), the heating reduction temperature is 1400-1430℃, the amount of coke added accounts for 10%-15% of the total weight of the slag, and the reduction time is controlled at 30-40 minutes.

[0020] The beneficial effects of adopting the above technical solution are as follows: This invention utilizes favorable dephosphorization conditions by adding dephosphorizing slag and using argon gas stirring to achieve highly efficient dephosphorization in a vacuum induction furnace, thereby effectively improving smelting quality. Furthermore, the process is low-cost, environmentally friendly, simple to operate, and has significant economic and environmental benefits. This invention can control the phosphorus content in molten steel to below 0.015%, and has advantages such as low cost, simple operation, pollution-free slag, and reusability. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to specific embodiments.

[0022] The method for dephosphorizing in a vacuum induction furnace by recycling dephosphorization slag adopts the following process steps:

[0023] (1) Add steel and alloy materials into the furnace and add dephosphorizing agent into the silo; the dephosphorizing agent is composed of pre-mixed dephosphorizing agent and reduced slag; the weight ratio of the pre-mixed dephosphorizing agent is: FeO 15%~22%, CaO 40%~50%, SiO2 15%~25%, MgO 9%~11%, MnO 2%~3%, with the balance being impurities mainly Al2O3 and S; the pre-mixed dephosphorizing agent accounts for ≥80% of the total weight of the dephosphorizing agent; the amount of dephosphorizing agent added is 4%~5% of the total weight of steel and alloy materials.

[0024] In addition to the dephosphorizing agent, a portion of Fe2O3 is added together with it, meaning Fe2O3 is also added to the silo. The amount of Fe2O3 added is 8% to 12% of the total weight of the dephosphorizing agent. The purpose of adding Fe2O3 is as follows: during the steelmaking process, phosphorus (P) is oxidized at the slag-steel interface by [FeO], and Fe will continue to be oxidized to FeO. Since there is no large amount of oxygen blown in during the operation of the vacuum induction furnace as in the actual converter smelting process, a certain amount of Fe2O3 is added to the dephosphorizing agent to simulate oxygen supply.

[0025] (2) Install the slag scraper onto the tamping rod, then cover the furnace with the lid and start powering on to heat up until the steel material is melted.

[0026] (3) After all the steel material has been melted, the materials are added to the silo in two batches, namely dephosphorizing agent and Fe2O3, and the temperature of the molten steel is kept at 1450-1480℃. After adding the first batch of materials to the silo, the bottom blowing argon gas is turned on for stirring. After 5-10 minutes, the second batch of materials to the silo is added, and the dephosphorization reaction is carried out for 30-35 minutes. Then, the slag is removed into the slag hopper in the furnace using the slag removal equipment. After the slag removal is completed, 0.03-0.05 MPa of argon gas is introduced into the furnace before alloying.

[0027] (4) After the steel tapping is completed, the slag is taken out and crushed to 0.1-0.2 mm. The slag is then thoroughly mixed with coke and heated for reduction. The amount of coke added accounts for 10%-15% of the total weight of the slag, and the effective carbon content in the coke is ≥80wt%. The heating and reduction temperature is 1400-1430℃ and the heating and reduction time is 30-40 min. The reduced slag can then be used as the dephosphorizing agent in step (1) of the subsequent furnace.

[0028] The reduction process utilizes the strong reducing properties of coke to reduce phosphorus pentoxide in the slag, generating phosphorus vapor that volatilizes, thus allowing the slag to retain its dephosphorization capacity for further dephosphorization. The principle of coke reducing slag is shown in the following reaction equations (C) and (D):

[0029] C(s)+FeO(l)=CO(g)+Fe(l), ΔGθ=141660-139.45T(C);

[0030] 5C(s)+P2O5(l)=5CO(g)+P2(g), ΔGθ=603609-648.94T(D).

[0031] The composition of the slag before and after reduction is shown in Table 1:

[0032] Table 1: Changes in major components of slag before and after reduction (wt)

[0033] Element TFe CaO <![CDATA[SiO2]]> <![CDATA[P2O5]]> Before reduction / % 15～20 40～50 15～20 2.5～3.05 After reduction / % 12～16 42～52 16～22 0.95～1.35

[0034] As shown in Table 1, the P2O5 content decreased significantly after reduction.

[0035] Examples 1-6: The following specific process is used for the method of dephosphorizing in a vacuum induction furnace by recycling dephosphorization slag.

[0036] (1) Steelmaking in a vacuum induction furnace with a nominal capacity of 50-500 kg is used for dephosphorization treatment. Steel and alloy materials are added to the crucible. Before adding, the surface of the steel is polished and then dried at high temperature. Reduced slag, pre-mixed dephosphorizing agent and Fe2O3 are added to the silo. The reduced slag used in each embodiment is the result of the reduction of the previous furnace slag. The amount of material added in each embodiment is shown in Table 2, and the composition content of the pre-mixed dephosphorizing agent in each embodiment is shown in Table 3.

[0037] Table 2: Feeding Amount for Each Embodiment

[0038]

[0039] In Table 2, the ratio of Fe2O3 is the ratio of its weight to the total weight of the dephosphorizing agent.

[0040] Table 3: Component content (wt) of the pre-formulated dephosphorizing agent described in each embodiment

[0041]

[0042]

[0043] In Table 3, the remaining amount of the pre-mixed dephosphorizing agent is considered as impurities.

[0044] (2) Install the slag skimmer onto the tamping rod, then cover the furnace and start heating until the steel is completely melted. After the steel is completely melted, add the materials in two batches, namely the dephosphorizing agent and Fe2O3, and maintain the temperature of the molten steel at 1450-1480℃. After adding the first batch of materials, turn on the bottom blowing argon gas at 0.2-0.3MPa and stir for 5-10 minutes. Then add the second batch of materials and carry out the dephosphorization reaction for 30-35 minutes. Then use the slag skimmer to skim the slag into the slag hopper in the furnace. After skimming, before alloying, charge the furnace with 0.03-0.05MPa of argon gas. Take samples for testing, fine-tune according to the target composition, and alloy. After alloying, tap the steel. The process parameters of each embodiment are shown in Table 4.

[0045] Table 4: Process parameters for each embodiment

[0046]

[0047] In Table 4, the argon stirring refers to the pressure and time of bottom-blowing argon stirring between two batches of materials.

[0048] (3) After the steel tapping is completed, the slag is taken out, cooled and crushed to 0.1-0.2 mm. Then the slag is thoroughly mixed with coke and heated for reduction. The amount of coke added accounts for 10%-15% of the total weight of the slag, the effective carbon content in the coke is ≥80wt%, the heating and reduction temperature is 1400-1430℃, and the heating and reduction time is 30-40 min. The slag reduction parameters of each embodiment are shown in Table 5.

[0049] Table 5: Slag Reduction Parameters for Each Example

[0050]

[0051]

[0052] The reduced slag can be used for the preparation of the dephosphorizing agent in step (1) of the next furnace.

[0053] (4) In Example 1, the P content in the final molten steel decreased from 0.06 wt% to 0.012 wt%; in Example 2, the P content in the final molten steel decreased from 0.05% to 0.0099%; in Example 3, the P content in the final molten steel decreased from 0.065% to 0.011%; in Example 4, the P content in the final molten steel decreased from 0.064% to 0.010%; and in Example 5, the P content in the final molten steel decreased from 0.057% to 0.011%. It is evident that this method effectively reduces the phosphorus content in the molten steel.

Claims

1. A method for recycling dephosphorized slag for dephosphorization in a vacuum induction furnace, characterized by, The method comprises the following steps: (1) adding steel materials into a vacuum induction furnace, and adding dephosphorizing agents into a material bin; the dephosphorizing agents are composed of pre-prepared dephosphorizing agents mixed with reduced furnace slag; the pre-prepared dephosphorizing agents are composed of the following components in percentage by weight: FeO 15% to 22%, CaO 40% to 50%, SiO2 15% to 25%, MgO 9% to 11%, MnO 2% to 3%, and the balance being impurities mainly including Al2O3 and S; and 8% to 12% of Fe2O3 in total weight of the dephosphorizing agents is also added into the material bin; (2) installing a slag scooper on a material stirring rod, then covering a furnace cover, starting to send power and heat until the steel materials are completely melted; (3) after the steel materials are completely melted, the dephosphorizing agents and Fe2O3 are added in two batches, and the temperature of the molten steel is controlled at 1450 to 1480 ℃; after the first batch is added, bottom argon blowing is started, and after stirring for 5 to 10 minutes, the second batch is added, and the dephosphorization reaction is carried out for 30 to 35 minutes, and the furnace slag is scooped out into a slag hopper in the furnace by using the slag scooper; (4) after tapping is completed, the scooped-out furnace slag is taken out and crushed to a particle size of 0.1 to 0.2 mm, and then is fully mixed with coke, wherein the amount of coke added accounts for 10% to 15% of the total weight of the furnace slag; the mixture is heated and reduced at 1400 to 1430 ℃ for 30 to 40 minutes, and the reduced furnace slag is used to prepare the dephosphorizing agents in step (1) of the next furnace.

2. The method according to claim 1, characterized in that: In step (3), the pressure of the bottom argon blowing is 0.2 to 0.3 MPa.

3. The method according to claim 1 or 2, characterized in that: In step (4), the effective carbon content in the coke is not less than 80 wt%. In step (4), the effective carbon content in the coke is not less than 80 wt%.

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