Method for staged selective separation and recycling of phosphorus, iron and calcium in steel slag

Abstract

The invention provides a method for staged selective separation and recycling of phosphorus, iron and calcium in steel slag, which comprises the following steps: crushing and grinding the steel slag, carrying out magnetic separation to remove iron to obtain steel slag powder, mixing hydrochloric acid and citric acid to form mixed acid, adding the mixed acid into the steel slag powder, and carrying out leaching reaction, after the leaching reaction is completed, solid-liquid separation is performed to obtain iron-containing solid residues and acid-containing filtrate, the solid residues are subjected to water washing and filter pressing to obtain iron-containing mud cakes, the iron-containing mud cakes are put into a raw material sintering procedure for steel production to be recycled, waste liquid generated by water washing and filter pressing is recycled to be used for preparing the mixed acid, and the mixed acid is obtained. CO2 gas is introduced into the acid-containing filtrate, the PH is adjusted to be alkaline, a reaction is conducted, solid-liquid separation is conducted after the reaction is completed, CaCO3 sediment and phosphorus-containing filtrate are obtained, the CaCO3 sediment is used for preparing a light CaCO3 product, the phosphorus-containing filtrate is subjected to steam concentration, a phosphorus-rich solution is obtained and used for preparing liquid fertilizer, and condensate water generated during steam concentration is recycled and used for preparing mixed acid. The method has high economical efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of resource utilization of metallurgical solid waste, and in particular to a method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages. Background Art

[0002] Steel slag is a byproduct of the steelmaking process, producing approximately 10% to 20% of crude steel. Statistics show that my country produces over 100 million tons of steel slag. Steel slag contains approximately 20% iron, 40% calcium oxide, and 3% phosphorus pentoxide. Steel companies recycle less than 30% of this content. The remaining tailings, due to their high phosphorus content, cannot be recycled within steel companies and are primarily used as raw materials for cement, building materials, and other products. This results in low resource efficiency and value. Furthermore, the free calcium oxide in steel slag has a certain impact on the quality of products like cement and building materials. The cement industry has completed revisions to mandatory national standards, pending promulgation, to restrict the use of steel slag. Therefore, developing green, high-value-added steel slag utilization technologies to efficiently utilize beneficial components such as iron, calcium oxide, and phosphorus in steel slag is of great significance.

[0003] In recent years, researchers at home and abroad have explored various methods to separate phosphorus from steel slag, including tempering, high-gravity separation, coffee grounds modification, gasification dephosphorization, and smelting reduction, to achieve its recycling in metallurgical processes. Dephosphorized steel slag is typically recycled in the steelmaking process, while the separated phosphorus is used to produce phosphate fertilizers.

[0004] Patent application number CN202010764673.X, "A Method for Improving the Solubility of P2O5 in Phosphorus-Containing Steel Slag," describes a method for increasing the solubility of P2O5 in phosphorus-containing steel slag. The method involves dephosphorizing the phosphorus-containing slag, tempering the dephosphorized slag, and holding the tempered slag. After the holding period, the slag is cooled at a rate exceeding 200°C / min until solidified. The solidified slag is then ground to a particle size of less than 74μm. The non-magnetic phase is extracted using magnetic separation and used as a phosphate fertilizer or fertilizer additive. The remaining slag is recycled back into the smelter. The solubility of the non-magnetic phase is 98.29%. While this method improves phosphorus utilization, it involves multiple steps, high energy consumption, and complex process control.

[0005] Patent application number CN202111208591.8, "A Method for Dephosphorizing Molten Steel Slag by Supergravity," describes a method for dephosphorizing molten steel slag by supergravity, which includes adjusting the basicity of the molten steel slag to 2.00-3.00; (2) maintaining the molten steel slag after basicity adjustment at 1350-1450°C, and then separating the resulting solid solution phase from the molten steel slag by supergravity separation. This method has high requirements for equipment, and the operation under supergravity conditions is difficult to control.

[0006] Patent application number CN202211295492.2, "A method for modifying steel slag with coffee grounds to promote phosphorus recovery and utilization," describes a method for modifying steel slag with coffee grounds to promote phosphorus recovery and utilization. The coffee grounds are first calcined, and the treated coffee grounds are mixed with crushed steel slag for pyrometallurgical melting and modification. After cooling, the modified steel slag is crushed. The treated steel slag is then added to a citric acid solution to selectively leach the phosphorus in the steel slag to obtain a phosphorus-containing leachate and dephosphorized steel slag. The phosphorus-containing leachate is used as liquid fertilizer; the dephosphorized steel slag can be returned to the metallurgical process after drying. The source and treatment of the coffee grounds in this technical solution may add additional costs, and the leaching efficiency may be greatly affected by the composition of the raw materials.

[0007] Patent application number CN201910915755.7, "A Method for Separating Phosphorus from Dephosphorized Steel Slag and Preparing Phosphate Fertilizer," describes a method for separating phosphorus from dephosphorized steel slag and preparing phosphate fertilizer. The dephosphorized steel slag is oxidized in a molten state, cooled, and then crushed and ground. The dephosphorized steel slag is then selectively leached in an aqueous solution. The pH of the slurry is controlled between 3.0 and 4.5 to dissolve and separate the phosphorus-rich solid solution phase in the dephosphorized steel slag. The reaction produces a low-phosphorus residue and a phosphorus-containing leachate. An alkaline substance is then added to the phosphorus-containing leachate to raise its pH, causing the phosphorus in the leachate to precipitate as calcium phosphate. The separated calcium phosphate precipitate can be used directly as phosphate fertilizer, while the low-phosphorus residue is used in sintering or steelmaking. This method requires handling acidic or alkaline solutions, potentially causing secondary pollution and high wastewater treatment costs.

[0008] Patent application number CN202311797203.3 "A method for gasification dephosphorization of converter final slag and recycling of dephosphorization slag" describes a method for gasification dephosphorization of converter final slag and recycling of dephosphorization slag, comprising: (1) after the converter smelting is completed and steel is tapped, all the slag is left in the converter; (2) a slag sample is taken and the slag composition is analyzed, and the amount of carbonaceous dephosphorization agent is calculated based on the FeO, P2O5, and MnO contents of the slag in the converter; (3) The high-level silo adds a carbonaceous dephosphorization agent to the converter at one time for gasification dephosphorization; (4) the spray gun is lowered to the slag adjustment gun position, and nitrogen is blown from the top and bottom throughout the process to stir the slag in the furnace; (5) after the slag dephosphorization is completed, 1 / 2 to 3 / 4 of the dephosphorization slag is poured out; the poured dephosphorization slag can be returned to the sintering process; (6) the spray gun is raised to the slag splashing gun position to perform the slag splashing and furnace protection operation; (7) after the slag splashing and furnace protection is completed, the remaining dephosphorization slag is left in the furnace for the next furnace smelting. The use of semi-coke in this technical solution may introduce impurities such as sulfur, and harmful gases may be generated during the gasification process, requiring additional environmental protection measures. At the same time, the P is not recycled after being gasified and removed.

[0009] Patent application number CN202310810059.6, "A Technology for Full Utilization of Steel Slag Based on Low-Carbon Dephosphorization of Molten Slag," describes a method for full utilization of steel slag with low-carbon dephosphorization of molten slag, including the addition of scrap steel and / or molten iron to molten steel slag, reduction of the molten steel slag, and separation and utilization of the slag and iron. Although this method achieves full utilization, the high-temperature reduction process consumes a lot of energy and requires a high reducing agent. Furthermore, the removed phosphorus (P) is not recycled, and the remaining P remains in the molten iron for continued recycling and smelting, ultimately being removed. Therefore, this solution does not truly achieve resource utilization of P.

[0010] In summary, the existing technology has problems such as complex process flow, high energy consumption of pyrolysis for phosphorus removal, and failure to recycle phosphorus. Summary of the Invention

[0011] The purpose of the present invention is to address the defects of the prior art and provide a method for selectively separating phosphorus, iron and calcium from steel slag in stages and recycling them, comprising:

[0012] The steel slag is crushed and ground, and subjected to magnetic separation to remove iron to obtain steel slag powder. Hydrochloric acid and citric acid are mixed to form a mixed acid, and the mixed acid is added to the steel slag powder to carry out a leaching reaction. After the leaching reaction is completed, solid-liquid separation is carried out to obtain an iron-containing solid residue and an acid-containing filtrate. The solid residue is washed with water and filtered to obtain an iron-containing mud cake, which is put into the raw material sintering process of steel production for recycling. The waste liquid generated by water washing and filtering is recycled to prepare the mixed acid, CO2 gas is introduced into the acid-containing filtrate and the pH is adjusted to alkaline for reaction. After the reaction is completed, solid-liquid separation is carried out to obtain a CaCO3 precipitate and a phosphorus-containing filtrate. The CaCO3 precipitate is used to make a light CaCO3 product, and the phosphorus-containing filtrate is steam-condensed to obtain a phosphorus-rich solution for preparing liquid fertilizer. The condensed water generated during the steam concentration is recycled to prepare the mixed acid.

[0013] Furthermore, the steel slag powder has a metallic iron content of ≤0.5% and a particle size of ≤0.15 mm. Magnetic separation removes iron to ≤0.5% to avoid excessive acid consumption during acid leaching. The particle size of ≤0.15 mm allows for a large specific surface area, thereby increasing calcium / phosphorus dissolution rates.

[0014] Furthermore, the molar ratio of hydrochloric acid to citric acid in the mixed acid is between 1:1 and 1:2, and the mixed acid concentration is between 0.5 and 2 mol / L. HCl rapidly destroys the calcium silicate structure, while citric acid chelates calcium ions, preventing calcium phosphate redeposition. A concentration of 0.5 to 2 mol / L ensures a high reaction rate while preventing excessive concentrations from reacting with silicon-containing minerals in the slag to form silicate polymers, which could lead to filter clogging.

[0015] Furthermore, in the leaching reaction, the mass ratio of the mixed acid to the steel slag is 5:1 to 10:1, the reaction temperature is 60 to 90°C, the stirring rate is 800 to 1200 r / min, and the reaction time is 1 to 3 hours. This lower liquid-to-solid ratio directly reduces steam concentration energy consumption and wastewater treatment costs. Temperatures exceeding 100°C can cause citric acid decomposition, while a reaction temperature of 60 to 90°C can increase the reaction speed without causing citric acid decomposition.

[0016] Furthermore, when the CO2 gas is introduced into the primary filtrate and the pH is adjusted, the pH is adjusted to 7-8. If the pH is less than 7, the absorption rate of CO2 in the primary filtrate decreases. If the pH is greater than 8, it will cause Mg 2+ Co-precipitation (forming MgCO3, reducing the purity of CaCO3) or generating hydroxyapatite.

[0017] Furthermore, when CO2 gas is introduced into the primary filtrate and the pH is adjusted, ammonia water is used to adjust the pH. Ammonia water is adjusted to generate soluble ammonium salts (NH4Cl / NH4H2PO4) to avoid Na + / K + Accumulation causes equipment scaling. + The / NH3 buffer pair (pKa=9.25) stabilizes the pH at 7-8 and precisely controls the calcium carbonate crystal form (forming spindle-shaped lightweight CaCO3).

[0018] Furthermore, before the waste liquid generated by the water washing and filter pressing is recycled for use in the preparation of the mixed acid, the waste liquid is adjusted to a pH ≥ 6. Generally speaking, when the pH is ≥ 6, Fe 3+ / Al 3+ The amount of hydrolysis precipitation is less than 0.1% (precipitation amount> 15% when pH=5) to prevent colloid blockage. pH≥6 can control the Ca content in the wastewater. 2+ Concentration ≤ 10 mg / L (pH = 6 when Ksp (CaCO3) = 4.96 × 10 -9 ) to avoid crystallization of calcium salts in the mixed acid.

[0019] Furthermore, the iron-containing sludge cake has a TFe of ≥30%. TFe is the total content of all forms of iron, calculated as a P2O5 equivalent to total phosphorus content, with a P2O5 content of ≤0.5% and a moisture content of ≤30%. A TFe of ≥30% meets the standard for iron ore feed, allowing it to directly replace 15% of the sintered ore feed. A P2O5 content of ≤0.5% prevents phosphorus accumulation in the blast furnace (phosphorus >0.8% can lead to cold brittleness of molten iron), and a moisture content of ≤30% meets the moisture requirements for sintered materials, avoiding additional drying energy consumption.

[0020] Furthermore, the CaCO3 precipitate is dried and crushed at 200°C to produce a lightweight CaCO3 product. The low-temperature drying at 200°C avoids the phase transition from calcite to aragonite (phase transition occurs at >400°C) and ensures the dispersibility of the product.

[0021] Furthermore, the phosphorus-rich solution, calculated as P2O5 equivalent based on the total phosphorus content, has a P2O5 content of ≥4%. After steam concentration, the solution with a P2O5 content of ≥4% can be directly used for drip irrigation.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present invention utilizes a three-stage synergy of "mixed acid directional calcium dissolution - chelation and phosphorus fixation → CO2 / ammonia dual-controlled carbonation → graded circulation and waste suppression": (1) a hydrochloric acid-citric acid molar ratio of 1:1 to 1:2: balancing the acid dissociation rate and chelation capacity to resolve the conflict between calcium and phosphorus competitive precipitation; (2) ammonia buffer at pH = 7 to 8: achieving synergistic optimization of CaCO3 crystallization kinetics and phosphorus dissolution thermodynamics; (3) a closed-loop water circulation design: converting wastewater resources into process media, forming a "acid-water-fertilizer" triple circular economic chain. This achieves a phosphorus recovery rate of >98%, a CaCO3 purity of >99%, and a wastewater discharge reduction of 94%, significantly improving resource utilization.

[0024] 2. Hydrochloric acid quickly destroys the calcium silicate network in the steel slag, releasing calcium ions; citric acid chelates with Ca through carboxyl groups. 2+ It forms a soluble complex, effectively inhibiting the precipitation of calcium phosphate and ensuring a phosphorus leaching rate of >95%. At the same time, citric acid inhibits the corrosion of iron oxides, reducing the risk of equipment corrosion.

[0025] 3. Using CO 2+ Ammonia water doubles pH regulation, CO2 provides carbonate, and Ca 2+ Combined to form high-purity spindle-shaped lightweight CaCO3; ammonia to form NH4 + / NH3 buffer system to avoid the introduction of Na + / K + Causes equipment scaling while maintaining HPO4 2- Dissolved state (phosphorus precipitation rate <0.1% when pH>7) to improve phosphorus recovery rate.

[0026] 4. The water washing wastewater is adjusted to pH ≥ 6 and then reused for acid preparation, reducing fresh water consumption; steam condensate (nearly neutral) is directly added to the mixed acid to reduce energy consumption for acid concentration adjustment; the wastewater in the entire process is close to "zero discharge", reducing treatment costs by 70%.

[0027] 5. Iron-containing mud cake (TFe ≥ 30%, P2O5 ≤ 0.5%) is directly sintered to replace 15% of iron ore fines without the risk of phosphorus enrichment; the resulting light CaCO3 product and phosphorus-rich solution meet the quality and relevant standards of commercial products without further treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a process flow chart of the segmented selective separation technology of phosphorus, iron and calcium in steel slag of the present invention. DETAILED DESCRIPTION

[0029] Unless otherwise specified, the raw materials used below are all commercially available products, and the methods used are all conventional methods in the art unless otherwise specified.

[0030] The steel slag used in the examples was obtained from a domestic steel plant, and its main chemical composition is shown in Table 1 below.

[0031] Table 1 Main chemical composition of steel slag, wt%

[0032] name TFe MFe <![CDATA[Al2O3]]> <![CDATA[SiO2]]> CaO MgO <![CDATA[P2O5]]> Steel slag tailings 21.87 0.96 1.35 12.66 47.61 3.96 2.77

[0033] according to Figure 1 The following Examples 1, 2, and 3 were prepared according to the method shown.

[0034] Example 1

[0035] 100 kg of the above-mentioned steel slag sample after iron removal was taken, crushed to a particle size of ≤0.15 mm, and iron was removed; hydrochloric acid and citric acid were mixed in a molar ratio of 1:1, and water was added to adjust the mixed acid concentration to 1.0 mol / L; 500 kg of the mixed acid was added to 100 kg of the steel slag powder after iron removal, and the mixture was heated to 70°C and continuously stirred at a stirring rate of 800 r / min and a reaction time of 1 h. After the reaction was completed, the mixture was filtered through a plate and frame filter press, and the filtrate was collected and washed with water and then filtered. The pH of the effluent was 6.1, and the mixture was returned to the storage tank for preparation of mixed acid. The mud cake was dried and chemically analyzed, and the moisture content was 28.2%, the TFe content was 37.8%, and the P2O5 content was 0.52%, and the mixture was returned for sintering. The collected filtrate enters the reactor, where CO2 gas with a purity of 99.99% is introduced. At the same time, ammonia water is added to adjust the pH of the filtrate to 7.2. The generated CaCO3 precipitate is separated by filtration in a timely manner. When no more precipitate is generated, the CO2 introduction is stopped. The CaCO3 precipitate is dried and crushed to produce a lightweight CaCO3 product. The filtrate is steam-concentrated to generate a phosphorus-rich solution with a P2O5 content of 5.4%, which is used to prepare liquid fertilizer. The condensed water is returned to the mixed acid for recycling.

[0036] Example 2

[0037] Take 100 kg of the above-mentioned steel slag sample, crush it to a particle size of ≤0.15 mm, and remove iron; mix hydrochloric acid and citric acid in a molar ratio of 1:1.5, and then add water to adjust the mixed acid concentration to 1.3 mol / L; add 700 kg of mixed acid to 100 kg of steel slag powder, start heating to 75°C and continuously stirring, the stirring rate is 1000 r / min, the reaction time is 2 h, after the reaction is completed, filter through plate and frame, and collect the filtrate. The mud cake is washed with water and then filtered, the pH of the effluent is 6.5, and it is returned to the storage tank for preparation of mixed acid. The mud cake is dried and chemically analyzed, the moisture content is 28.5%, the TFe content is 40.2%, and the P2O5 content is 0.44%, and it is returned for sintering. The collected filtrate enters the reactor, where CO2 gas with a purity of 99.99% is introduced. At the same time, ammonia water is added to adjust the pH of the filtrate to 7.6. The generated CaCO3 precipitate is separated by filtration in a timely manner. When no more precipitate is generated, the CO2 introduction is stopped. The CaCO3 precipitate is dried and crushed to produce a lightweight CaCO3 product. The filtrate is steam-concentrated to generate a phosphorus-rich solution with a P2O5 content of 6.4%, which is used to prepare liquid fertilizer. The condensed water is returned to the mixed acid for recycling.

[0038] Example 3

[0039] Take 100 kg of the above-mentioned steel slag sample, crush it to a particle size of ≤0.15 mm, and remove iron; mix hydrochloric acid and citric acid in a molar ratio of 1:2.0, and then add water to adjust the mixed acid concentration to 1.8 mol / L; add 900 kg of mixed acid to 100 kg of steel slag powder, start heating to 80°C and continue stirring, the stirring rate is 1100 r / min, the reaction time is 2.5 h, after the reaction is completed, filter through plate and frame, and collect the filtrate. The mud cake is washed with water and then filter-pressed. The pH value of the effluent is 6.9, and it is returned to the storage tank for preparation of mixed acid. The mud cake is dried and chemically analyzed, and the moisture content is 28.4%, the TFe content is 39.6%, and the P2O5 content is 0.35%. It is returned for sintering. The collected filtrate enters the reactor, where CO2 gas with a purity of 99.99% is introduced. At the same time, ammonia water is added to adjust the pH of the filtrate to 7.8. The generated CaCO3 precipitate is promptly separated by filtration, and the introduction of CO2 is stopped until no more precipitate is generated. The CaCO3 precipitate is dried and crushed to produce a lightweight CaCO3 product. The filtrate is steam-concentrated to generate a phosphorus-rich solution with a P2O5 content of 8.4%, which is used to prepare liquid fertilizer. The condensed water is returned to the mixed acid for recycling.

[0040] Comparative Example 1

[0041] The steps are the same as those in Example 1, except that the washing wastewater and condensed water are not recycled, and fresh water is used entirely for acid preparation. Specifically, 100 kg of steel slag sample is crushed to less than 0.15 mm and iron is removed; 500 kg of 1.0 mol / L mixed acid (hydrochloric acid:citric acid = 1:1) is added to the steel slag powder; the reaction is carried out at 70°C for 1 hour, with a stirring rate of 800 r / min; the residual acid concentration of the acid-containing filtrate after solid-liquid separation is 0.28 mol / L (titration value); the washing filter press wastewater has a pH of 6.1 (not recycled); the iron sludge TFe is 37.8%, and P2O5 is 0.52%; after carbonation, the phosphorus-containing filtrate is concentrated to obtain a phosphorus-rich solution with a P2O5 content of 5.4%.

[0042] Residual acid concentration refers to the free acid concentration in the acid filtrate (primary filtrate) obtained after solid-liquid separation following the leaching reaction. This filtrate is the solution separated after the acid leaching reaction is complete but before the carbonation reaction begins, and contains unreacted acid and dissolved ions.

[0043] The results show that the wastewater generation in Example 1 is 0.1 t / t slag, and the wastewater generation in Comparative Example 1 is 1.8 t / t slag. The mixed acid utilization rate in Example 1 is 98%, and the mixed acid utilization rate in Comparative Example 1 is 72%.

[0044] Among them, the mixed acid utilization rate refers to the percentage of the acid actually consumed in the leaching reaction to the total acid amount, mixed acid utilization rate = (1-residual acid amount / initial acid amount) × 100%, initial acid amount = mixed acid volume × mixed acid concentration, residual acid amount = filtrate volume after leaching × residual acid concentration (determined by titration), waste liquid recycling contribution: recycled washing waste liquid and effective acid in condensed water.

[0045] For Example 1, the initial acid amount = 500L × 1.0mol / L = 500mol, the residual acid concentration titration result is 0.02mol / L, the residual acid amount = 500L × 0.02mol / L = 10mol, and the mixed acid utilization rate = (1-10 / 500) × 100% = 98%.

[0046] For Comparative Example 1, the initial acid amount is also 500 mol, and the residual acid concentration titration result is 0.28 mol / L. Residual acid amount = 500 L × 0.28 mol / L = 140 mol, and mixed acid utilization rate = (1-140 / 500) × 100% = 72%.

[0047] These results demonstrate that wastewater recycling reduced the residual acid concentration in Example 1 to 0.02 mol / L (compared to 0.28 mol / L in Comparative Example 1), increasing acid utilization by 26%. Wastewater recycling forms an internal "acid-water" circulation system, allowing the residual acid in the wastewater (approximately 0.1 mol / L) to be reused, reducing the amount of fresh acid added and avoiding wastewater treatment costs.

[0048] Comparative Example 2

[0049] The steps were the same as in Example 2, except that 1.0 mol / L pure hydrochloric acid was used instead of the mixed acid. Specifically, 100 kg of steel slag sample was crushed to less than 0.15 mm and iron was removed; 700 kg of 1.0 mol / L pure hydrochloric acid was added to the steel slag powder; the mixture was reacted at 75°C for 2 h with a stirring rate of 1000 rpm; the iron sludge had a TFe content of 38.5% and a P2O5 content of 1.50%; the other steps were the same as in Example 2.

[0050] The results showed that the phosphorus leaching rate in Example 2 was 91.4%, the phosphorus leaching rate in Comparative Example 2 was 69.2%, the CaCO3 purity in Example 2 was 99.2%, and the CaCO3 purity in Comparative Example 2 was 82.5%. The measured acid consumption in Example 2 was 0.8 t / t slag, and the acid consumption in Comparative Example 2 was 1.5 t / t slag.

[0051] The phosphorus leaching rate refers to the percentage of phosphorus leached from steel slag relative to the total phosphorus in the slag. Phosphorus leaching rate = (1 - P2O5 content in iron slag / Total P2O5 content in steel slag) × 100%. Total P2O5 content in steel slag = slag mass × P2O5 content. P2O5 content in iron slag = dry weight of iron slag × P2O5 content. Dry weight of iron slag = total iron content in steel slag / TFe content in iron slag. Total iron content in steel slag = slag mass × TFe%.

[0052] For Example 2, the dry basis mass of iron mud = 21.87 kg / 40.2% = 54.40 kg, the P2O5 amount of iron mud = 54.40 kg × 0.44% = 0.239 kg, the total P2O5 amount of steel slag = 100 kg × 2.77% = 2.77 kg, and the phosphorus leaching rate = (1-0.239 / 2.77) × 100% = 91.4%.

[0053] For Comparative Example 2, the dry basis mass of the iron mud = 21.87 kg / 38.5% = 56.81 kg, the P2O5 amount of the iron mud = 56.81 kg × 1.50% = 0.852 kg, and the phosphorus leaching rate = (1-0.852 / 2.77) × 100% = 69.2%.

[0054] The above results verify that citric acid in the mixed acid reduces free Ca in the solution through chelation. 2+ concentration, preventing phosphorus from precipitating in the form of Ca3(PO4)2, increasing the phosphorus leaching rate by 22.2% while ensuring the high purity of CaCO3 precipitation (>99%)

[0055] Comparative Example 3

[0056] The steps were the same as in Example 3, except that the pH of the acidic filtrate was adjusted to 7.5 using NaOH. Specifically, 100 kg of steel slag was crushed to less than 0.15 mm and iron removed; 900 kg of a mixed acid (hydrochloric acid:citric acid = 1:2, 1.8 mol / L) was added to the steel slag powder; the reaction was carried out at 80°C for 2.5 hours with a stirring rate of 1100 rpm; during carbonation, the pH was adjusted to 7.5 using NaOH; the CaCO3 precipitate obtained by solid-liquid separation after carbonation was tested and found to contain 0.8 wt% P2O5, indicating that the precipitate contained phosphorus. The phosphorus-containing filtrate was steam-concentrated to obtain a phosphorus-rich solution with a P2O5 concentration of 3.85%. Scaling occurred after 48 hours of operation.

[0057] The results show that the equipment scaling period in Example 3 is greater than 300 h, while the equipment scaling period in Comparative Example 3 is 48 h. The phosphorus recovery rate in Example 3 is 98.5%, while the phosphorus recovery rate in Comparative Example 3 is 90.57%.

[0058] Phosphorus recovery refers to the percentage of phosphorus in the phosphorus-containing filtrate to the total phosphorus in the leachate after carbonation and separation of CaCO₃. Phosphorus recovery = (amount of P₂O₅ in the phosphorus-containing filtrate / total amount of P₂O₅ in the leachate) × 100%. Total P₂O₅ in the leachate = total amount of P₂O₅ in the slag × phosphorus leaching rate. P₂O₅ in the phosphorus-containing filtrate = mass of the phosphorus-containing filtrate × P₂O₅ concentration.

[0059] For Example 3, the calculated leaching rate was 93.0%, the total amount of leachate P2O5 = 2.77kg × 93.0% = 2.576kg, the measured mass of the phosphorus-containing filtrate was 30.2kg, the phosphorus-containing filtrate P2O5 amount = 30.2kg × 8.4% = 2.5368kg, and the phosphorus recovery rate = 2.5368kg / 2.576kg = 98.5%. During carbonation, there was an adsorption phenomenon during the filter cake entrainment concentration process, resulting in the loss of phosphorus.

[0060] For Comparative Example 3, the total amount of P2O5 in the leachate is 2.576 kg, the same as in Example 3, and the mass of the phosphorus-containing filtrate is 60.6 kg. The amount of P2O5 in the phosphorus-containing filtrate = 60.6 kg × 3.85% = 2.3331 kg, and the phosphorus recovery rate = 2.3331 kg / 2.576 kg = 90.57%. This is because when pH > 7.5, hydroxyapatite precipitate Ca5(PO4)3OH is generated, and NaOH introduces Na + It promotes the co-precipitation of sodium phosphate and scaling leading to increased filter cake entrainment.

[0061] The above results highlight the irreplaceable role of ammonia in preventing scaling and maintaining phosphorus solubility.

[0062] This invention utilizes a three-step synergy: "mixed acid directional calcium dissolution - chelation and phosphorus fixation → CO2 / ammonia dual-controlled carbonation → graded circulation and waste reduction." The following steps are taken: 1. A hydrochloric acid-citric acid molar ratio of 1:1 to 1:2 balances the acid dissociation rate and chelation capacity, resolving the conflict between calcium and phosphorus competitive precipitation; 2. Ammonia buffering at a pH of 7 to 8 optimizes the synergistic optimization of CaCO3 crystallization kinetics and phosphorus dissolution thermodynamics; and 3. A closed-loop water circulation design converts wastewater resources into process media, forming a triple-circulation economic chain of "acid-water-fertilizer." This achieves a phosphorus recovery rate exceeding 98%, a CaCO3 purity exceeding 99%, a 26% increase in acid utilization, and a significant reduction in wastewater emissions, significantly improving resource utilization.

[0063] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages, characterized in that: include: The steel slag is crushed and ground, and subjected to magnetic separation to remove iron to obtain steel slag powder. Hydrochloric acid and citric acid are mixed to form a mixed acid, and the mixed acid is added to the steel slag powder to carry out a leaching reaction. After the leaching reaction is completed, solid-liquid separation is carried out to obtain an iron-containing solid residue and an acid-containing filtrate. The solid residue is washed with water and filtered to obtain an iron-containing mud cake, which is put into the raw material sintering process of steel production for recycling. The waste liquid generated by water washing and filtering is recycled to prepare the mixed acid, CO2 gas is introduced into the acid-containing filtrate and the pH is adjusted to alkaline for reaction. After the reaction is completed, solid-liquid separation is carried out to obtain a CaCO3 precipitate and a phosphorus-containing filtrate. The CaCO3 precipitate is used to make a light CaCO3 product, and the phosphorus-containing filtrate is steam-condensed to obtain a phosphorus-rich solution for preparing liquid fertilizer. The condensed water generated during the steam concentration is recycled to prepare the mixed acid.

2. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag according to claim 1, characterized in that: The steel slag powder has a metallic iron content of ≤0.5% and a particle size of ≤0.15 mm.

3. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag according to claim 1, characterized in that: In the mixed acid, the molar ratio of hydrochloric acid to citric acid is 1:1 to 1:2, and the concentration of the mixed acid is 0.5 to 2 mol / L.

4. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag according to claim 1, characterized in that: In the leaching reaction, the mass ratio of the mixed acid to the steel slag is 5:1 to 10:1, the reaction temperature is 60 to 90° C., the stirring rate is 800 to 1200 r / min, and the reaction time is 1 to 3 hours.

5. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: When CO2 gas is introduced into the primary filtrate and the pH is adjusted, the pH is adjusted to 7-8.

6. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: When CO2 gas is introduced into the primary filtrate and the pH is adjusted, aqueous ammonia is used to adjust the pH.

7. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: Before the waste liquid generated by the water-washing filter press is recycled for use in preparing the mixed acid, the pH of the waste liquid is adjusted to ≥6.

8. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: The iron-containing mud cake TFe is ≥30%, the total phosphorus content is calculated as P2O5 equivalent, the P2O5 content is ≤0.5%, and the moisture content is ≤30%.

9. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: The CaCO3 precipitate is dried and crushed at 200°C and then prepared into a light CaCO3 product.

10. The method for selectively separating and recycling phosphorus, iron and calcium in steel slag in stages according to claim 1, characterized in that: The phosphorus-rich solution has a P2O5 content of ≥4% calculated based on the total phosphorus content in terms of P2O5 equivalent.

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