A method for preparing iron oxide and silica gel using neodymium iron boron waste

Abstract

This invention relates to the field of rare earth industrial waste recycling technology, and more particularly to a method for preparing iron oxide and silica gel from NdFeB waste. The method involves reducing and roasting the NdFeB waste with biochar, followed by magnetic separation with oxalic acid to obtain iron-rich and silicon-rich components. The obtained iron-rich component is then subjected to acid leaching, alkali precipitation, and roasting to obtain iron oxide. The obtained silicon-rich component is then acid-leached with the iron-rich component, and the resulting acid leaching residue is roasted with sodium carbonate, leached in water, and then treated with carbon dioxide to obtain silica gel. The advantages of this invention are that it first pre-treats the NdFeB waste, separating it into iron-rich and silicon-rich components, then chemically refines the iron-rich component to obtain iron oxide, and prepares the silicon-rich component into silica gel, achieving the synergistic recovery and value-added utilization of iron and silicon elements in NdFeB waste.

Description

Technical Field

[0001] This invention relates to the field of rare earth industrial waste recycling technology, and in particular to a method for preparing iron oxide and silica gel using neodymium iron boron waste. Background Technology

[0002] Neodymium iron boron permanent magnet materials are composed of neodymium, iron, and boron (Nd2Fe). 14 B) Tetragonal crystals possess advantages such as light weight, low cost, good magnetic energy product and coercivity, and high energy density. They are currently the best-performing magnetic materials in terms of overall magnetic properties and have been widely used in national defense, aerospace, medical devices, electronic information, and metallurgical engineering. In 2021, my country's output of sintered NdFeB blanks reached 207,100 tons, a year-on-year increase of 16%; other NdFeB output was 9,380 tons, a year-on-year increase of 27.2%. The rapid growth of NdFeB permanent magnet materials has accelerated the generation of NdFeB waste. Approximately 30% of the waste is also generated in the alloy smelting and sintering cutting processes during sintering.

[0003] Currently, the recycling of NdFeB permanent magnet materials mainly focuses on extracting high-value rare earth elements (such as Nd, Pr, and Dy) through hydrometallurgical techniques. However, this rare earth-centric recycling strategy has created a new problem: generating a large amount of secondary waste, namely, leaching residue rich in iron and silicon elements after acid leaching (NdFeB Leaching Residue, hereinafter referred to as NdFeB waste or NFB). NFB is the largest solid waste in the primary waste recycling process of NdFeB, mainly composed of iron oxides, while also enriched with silicon (mainly in the form of SiO2) and trace amounts of residual rare earth elements (approximately 0.5%). Since the iron in the residue is mostly in the form of insoluble Fe2O3 and is easily encapsulated by SiO2 to form a dense structure, it is usually stockpiled at low cost or directly used in metallurgy, resulting in the waste of key elements such as iron and silicon and potential environmental risks. In the context of the circular economy and zero-pollution goals, improving the value and utilization rate of NFB is becoming increasingly important.

[0004] Patent application CN118561330 A discloses a method for preparing iron tetroxide and nepheline using neodymium iron boron waste, comprising the following steps: premixing → reduction roasting → magnetic separation → targeted melting → acid leaching separation. 1) Select NdFeB waste containing 0.31%-0.76% total rare earth oxides and mix it with biochar in a certain proportion and then ball mill it to obtain a premix; 2) Place the obtained premix in a furnace and calcine it in an oxygen-free environment at 700-900℃ for 1-3 hours. After reduction calcination, obtain a magnetic solid powder; 3) Place the obtained solid powder in ethanol for magnetic separation. Collect the magnetic black powder and the solid powder remaining after magnetic separation. Perform continuous magnetic separation N≥2 times. Wash the obtained magnetic black powder with water and dry it to obtain iron(III) oxide; 4) Mix the obtained solid powder remaining after magnetic separation in carbonate and target it for smelting at 900-1100℃ for 2-4 hours to obtain clinker; 5) After acid leaching of the obtained clinker, the rare earth elements enter the solution. After solid-liquid separation, wash and dry the filter cake to obtain nepheline. The iron tetroxide and nepheline prepared by this method have low purity and low selling price, which cannot meet the requirements for high-value utilization of iron and silicon components in NdFeB waste. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing iron oxide and silica gel using NdFeB waste. The method first pre-treats the NdFeB waste to separate it into iron-rich and silicon-rich components. Then, the iron-rich component is chemically refined to obtain iron oxide, and the silicon-rich component is prepared into silica gel, thereby realizing the synergistic recovery and value-added utilization of iron and silicon elements in NdFeB waste.

[0006] The present invention is achieved through the following technical solution: On the one hand, a method for preparing iron oxide and silica gel using neodymium iron boron waste is provided, wherein the neodymium iron boron waste is reduced and roasted with biochar and then magnetically separated with oxalic acid to obtain iron-rich components and silicon-rich components.

[0007] The obtained iron-rich component was subjected to acid leaching, alkali precipitation, and calcination to obtain iron oxide. The obtained silica gel is prepared by acid leaching the silicon-rich component and the iron-rich component together with the acid leaching residue, followed by calcination with sodium carbonate, water leaching, and carbon dioxide purging.

[0008] Furthermore, the reduction calcination is carried out in an inert atmosphere at a temperature of 540–680°C for a time of 60–180 min.

[0009] Furthermore, the amount of biochar added is 6% to 9% of the mass of NdFeB waste; the concentration of oxalic acid is 0.08 to 0.1 mol / L, and the magnetic separation liquid-solid ratio is 3 to 7:1.

[0010] Furthermore, the acid leaching uses sulfuric acid with a concentration of 4-6 mol / L, a liquid-to-solid ratio of 3-7:1, a leaching temperature of 50-80℃, and a leaching time of 120-240 min.

[0011] Furthermore, the alkaline precipitation is carried out by adjusting the pH to 3.5-4.5 with sodium hydroxide solution, and the precipitation reaction temperature is 50-70℃; the calcination temperature is 700℃ and the time is 2h.

[0012] Furthermore, the amount of sodium carbonate added is 0.7 to 1.0 times the mass of the mixture obtained after mixing the silicon-rich component and the acid leaching residue; the calcination temperature is 700 to 900°C, and the calcination time is 60 to 120 minutes.

[0013] Furthermore, the pH is controlled at 8 and the reaction temperature is 50–70°C when carbon dioxide is introduced.

[0014] Furthermore, the reduction calcination process involves reactions as shown in formulas (1) and (2): 3Fe2O3+ C → 2Fe3O4+ CO(1); 3Fe2O3+ CO → 2Fe3O4+ CO2(2).

[0015] Furthermore, the acid leaching process involves reactions as shown in formulas (3) to (8): Fe3O4+ 4H2SO4→ FeSO4+ Fe2(SO4)3+ 4H2O (3); Fe2O3+ 3H2SO4→ Fe2(SO4)3+ 3H2O(4); FeO + H2SO4→ FeSO4+ H2O (5); Al2O3+ 3H2SO4→ Al2(SO4)3+ 3H2O(6); Nd2O3+ 3H2SO4→ Nd2(SO4)3+ 3H2O(7); Pr2O3+ 3H2SO4→ Pr2(SO4)3+ 3H2O (8).

[0016] Furthermore, during the sodium carbonate addition and calcination process, the reaction shown in formula (9) occurs: SiO2+ Na2CO3→ Na2SiO3+ CO2(9).

[0017] Beneficial effects Iron in NdFeB waste mainly exists in the form of Fe2O3, which has weak magnetism and is difficult to distinguish effectively from the weakly magnetic silicon-containing phase using conventional magnetic separation. This invention utilizes the reduction roasting with added biochar to convert Fe2O3 into Fe3O4, significantly enhancing its magnetic properties. Simultaneously, the introduction of oxalic acid has a dual effect: firstly, it acts as a dispersant to prevent the agglomeration of magnetic particles; secondly, oxalate ions can coordinate with surface iron ions, weakening the van der Waals forces and magnetic dipole interactions between particles, thereby improving magnetic separation selectivity. Experimental data shows that oxalic acid-assisted magnetic separation increases the iron recovery rate in iron-rich components by approximately 10%, reduces the number of magnetic separation steps by two, and shortens the separation time by 30%. Traditional processes directly use acid leaching to dissolve iron, resulting in high acid consumption and the generation of large amounts of acidic wastewater. This invention employs the synergistic effect of reduction roasting and oxalic acid-assisted magnetic separation to achieve highly efficient pre-separation of iron and silicon components, overcoming the dual challenges of low efficiency in traditional physical separation and high cost in chemical separation.

[0018] After the iron-rich components are leached with sulfuric acid, Fe, Al, and rare earth elements simultaneously enter the solution. 3+ The initial precipitation occurs at a pH of approximately 2.0–2.5, and complete precipitation occurs at a pH of 3.5–4.0; while Al 3+ The initial pH of precipitation is approximately 4.0–4.5, with rare earth ions (Nd) present. 3+ Pr 3+ The initial pH of the precipitation is greater than 6.0. This invention precisely controls the pH at 4.0 ± 0.2, achieving selective precipitation of iron while retaining Al and rare earth elements in the solution. Subsequent calcination at 700℃ completely converts ferric hydroxide into α-Fe₂O₃, resulting in regular crystals and uniform particle size. In other words, this invention, through selective acid leaching and precise pH control of precipitation, overcomes the technical bottleneck of co-precipitation of iron with aluminum and rare earth elements, obtaining iron oxide with a purity higher than 96%, meeting the requirements for high-value-added applications such as pigments, magnetic materials, and catalyst supports.

[0019] After the silicon-rich component is combined with the acid leaching residue, the SiO2 (amorphous or crystalline) undergoes a solid-phase reaction in the presence of sodium carbonate: Na2CO3 + SiO2 → Na2SiO3 + CO2↑. This reaction is carried out at 700–900℃. Na2SiO3 is soluble in water, thus achieving the separation of silicon from residual metallic impurities.

[0020] The sodium silicate solution obtained from water leaching undergoes hydrolysis and polymerization upon the introduction of CO2: Na2SiO3 + CO2 + H2O → H2SiO3↓ + Na2CO3. By controlling the pH to 8 and the temperature to 50–70℃, the specific surface area and pore structure of the silica gel can be adjusted. The product performance is superior to commercially available ordinary silica gel. The byproduct sodium carbonate can be concentrated, crystallized, and returned to the calcination process, achieving recycling and significantly reducing reagent consumption. Compared to traditional NdFeB waste recycling processes, where silicon-rich residue is typically landfilled as solid waste, generating approximately 0.15–0.2 tons of silicon slag per ton of waste, this invention uses an alkali fusion transformation-carbonation method to prepare silica gel, achieving a leap from silicon waste to high-value products. Simultaneously, it establishes a closed-loop recycling pathway for sodium carbonate. The silica gel product can be applied in fields such as desiccants, chromatographic packing materials, and catalyst supports.

[0021] This invention uses an aqueous solution of oxalic acid, which is safe, inexpensive, and easy to recycle; the roasting process emits no toxic or harmful gases, CO2 is recycled, and the wastewater can meet emission standards or be reused after neutralization.

[0022] This invention transforms the recycling of NdFeB waste from a rare earth-oriented approach to a synergistic utilization of all components, including iron, silicon, and rare earths, significantly enhancing the comprehensive value of the waste, reducing solid waste landfill, and extending to the resource utilization of iron- and silicon-containing industrial solid wastes such as red mud, fly ash, and molybdenum tailings. Attached Figure Description

[0023] Figure 1 This is the XRD pattern of the neodymium iron boron waste of this invention; Figure 2 This is the XRD pattern of the iron-rich component obtained by magnetic separation in this invention; Figure 3 This is the XRD pattern of the silicon-rich component obtained by magnetic separation in this invention; Figure 4 The appearance morphology of the raw materials and products of this invention (a. NFB waste; b. iron oxide; c. silica gel); Figure 5 This is the XRD pattern of iron oxide obtained by this invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise stated, all percentages, ratios, proportions, or parts are by weight.

[0026] Unless otherwise specified, the reagents and raw materials used in the embodiments and comparative examples of this invention are commercially available. The main components of the NdFeB waste used in the following embodiments are shown in Table 1 below.

[0027] Table 1 Chemical composition of NdFeB waste

[0028] Example 1 A method for extracting iron and silicon elements from NdFeB waste includes the following steps: Step S1: Preprocessing; Neodymium iron boron waste was mixed with 6% biochar and ground to obtain mixture A. Mixture A was placed in a furnace and calcined in a nitrogen atmosphere at 660℃ for 60 min to obtain magnetic solid powder. The obtained solid powder was then mixed with 0.12 mol / L oxalic acid at a liquid-to-solid ratio of 3:1 and placed in a magnetic separator for magnetic separation to obtain iron-rich and silicon-rich components. Step S2: Acid leaching to remove silicon; The iron-rich component obtained in step S1 was leached with 4 mol / L sulfuric acid at a liquid-to-solid ratio of 7:1 and a leaching temperature of 80°C. Solid-liquid separation was then performed to obtain an acid leaching solution (mainly composed of ferric sulfate, aluminum sulfate, and rare earth sulfate) and an acid leaching residue (mainly composed of silica). The acid leaching residue was washed until neutral and dried for later use. Testing and calculations showed that the Fe leaching rate was 91.85%. Step S3: Prepare iron oxide; Add 0.08 mol / L sodium hydroxide solution to the leachate obtained in step S2 to adjust the pH to 4. The reaction produces ferric hydroxide precipitate. The ferric hydroxide precipitate is calcined at 700℃ for 2 hours to obtain ferric oxide with an iron content of 62.38%, which meets the requirements of GBT25953-2010 Grade II product.

[0029] Step S4, roasting transformation; The silicon-rich component obtained in step S1 is mixed evenly with the leaching residue obtained in step S2 to obtain mixture B. Mixture B is mixed and ground with 0.7 times the amount of sodium carbonate, and then calcined at 900℃ for 60 minutes to obtain clinker. The clinker is soaked in water, and solid-liquid separation is performed to obtain a solution containing sodium silicate and water-leached residue. The calcination tail gas is collected, dust is removed, and then sent to process S5 for reaction gas.

[0030] Step S5: Prepare silica gel; The sodium silicate solution obtained in step S4 is passed through CO2 gas, and the temperature is controlled at 50℃ and the pH is 8. A white gelatinous precipitate is generated by reaction. The solid and liquid are separated to obtain a white precipitate and a filtrate. The white precipitate is dried to obtain silica gel, and the filtrate is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is returned to step S4 for recycling.

[0031] Comparative Example 1 The amount of oxalic acid added in step S1 was changed; instead of adding oxalic acid, deionized water was used instead. Other operations were the same as in Example 1.

[0032] Compared with Example 1, adding oxalic acid increases iron recovery by about 10%, reduces the number of magnetic separations by 2, and shortens the magnetic separation time by about 30%.

[0033] Example 2 A method for extracting iron and silicon elements from NdFeB waste includes the following steps: Step S1: Preprocessing; Neodymium iron boron waste was mixed with 9% biochar and ground to obtain mixture A. Mixture A was placed in a furnace and calcined in a nitrogen atmosphere at 540℃ for 180 min to obtain magnetic solid powder. The obtained solid powder was then mixed with 0.11 mol / L oxalic acid at a liquid-to-solid ratio of 4:1 and placed in a magnetic separator for magnetic separation to obtain iron-rich and silicon-rich components. Step S2: Acid leaching to remove silicon; The iron-rich component obtained in step S1 was leached with 4.5 mol / L sulfuric acid at a liquid-to-solid ratio of 6:1 and a leaching temperature of 70°C. Solid-liquid separation was then performed to obtain an acid leaching solution (mainly composed of ferric sulfate, aluminum sulfate, and rare earth sulfate) and an acid leaching residue (mainly composed of silica). The acid leaching residue was washed until neutral and dried for later use. Testing and calculation showed that the Fe leaching rate was 92.03%. Step S3: Prepare iron oxide; Add 0.09 mol / L sodium hydroxide solution to the leachate obtained in step S2 to adjust the pH to 4. The reaction produces ferric hydroxide precipitate. The ferric hydroxide precipitate is calcined at 700℃ for 2 hours to obtain ferric oxide with an iron content of 63.32%, which meets the requirements of GBT25953-2010 Grade II product.

[0034] Step S4, roasting transformation; The silicon-rich component obtained in step S1 is mixed evenly with the leaching residue obtained in step S2 to obtain mixture B. Mixture B is mixed and ground with 0.8 times the amount of sodium carbonate, and then calcined at 850°C for 80 minutes to obtain clinker. The clinker is then soaked in water, and solid-liquid separation is performed to obtain a solution containing sodium silicate and water-leached residue. The calcination tail gas is collected, dust is removed, and then sent to process S5 for use as reaction gas.

[0035] Step S5: Prepare silica gel; The sodium silicate solution obtained in step S4 is passed through CO2 gas, and the temperature is controlled at 50℃ and the pH is 8. A white gelatinous precipitate is generated by reaction. The solid and liquid are separated to obtain a white precipitate and a filtrate. The white precipitate is dried to obtain silica gel, and the filtrate is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is returned to step S4 for recycling.

[0036] Comparative Example 2 The acid leaching temperature in step S2 was changed from 70°C to 40°C, and other operations were the same as in Example 2.

[0037] Compared with Example 2, it can be seen that when the acid leaching temperature is too low, the leaching is incomplete, and the Fe leaching rate drops significantly, from 92.03% to 78.24% (Table 2).

[0038] Comparative Example 3 The liquid-to-solid ratio in step S2 was changed from 6:1 to 2:1, and other operations were the same as in Example 2.

[0039] Compared with Example 2, it can be seen that the acid leaching solution-solid ratio was too low, the leaching was incomplete, and the Fe leaching rate decreased significantly, from 84.24% to 40.02% (Table 2).

[0040] Example 3 A method for extracting iron and silicon elements from NdFeB waste includes the following steps: Step S1: Preprocessing; Neodymium iron boron waste was mixed with 7.5% biochar and ground to obtain mixture A. Mixture A was placed in a furnace and calcined in a nitrogen atmosphere at 620℃ for 90 minutes to obtain a magnetic solid powder. The obtained solid powder was then mixed with 0.1 mol / L oxalic acid at a liquid-to-solid ratio of 5:1 and placed in a magnetic separator for magnetic separation to obtain an iron-rich component and a silicon-rich component. Step S2: Acid leaching to remove silicon; The iron-rich component obtained in step S1 was leached with 5 mol / L sulfuric acid at a liquid-to-solid ratio of 5:1 and a leaching temperature of 60°C. Solid-liquid separation was then performed to obtain an acid leaching solution (mainly composed of ferric sulfate, aluminum sulfate, and rare earth sulfate) and an acid leaching residue (mainly composed of silica). The acid leaching residue was washed until neutral and dried for later use. Testing and calculations showed that the Fe leaching rate was 90.64%. Step S3: Prepare iron oxide; Add 1 mol / L sodium hydroxide solution to the leachate obtained in step S2 to adjust the pH to 4. The reaction produces ferric hydroxide precipitate. The ferric hydroxide precipitate is calcined at 700℃ for 2 hours to obtain ferric oxide with an iron content of 63.62%, which meets the requirements of GBT 25953-2010 Grade II product.

[0041] Step S4, roasting transformation; The silicon-rich component obtained in step S1 is mixed evenly with the leaching residue obtained in step S2 to obtain mixture B. Mixture B is mixed and ground with 0.9 times the amount of sodium carbonate, and then calcined at 800℃ for 100 min to obtain clinker. The clinker is soaked in water, and solid-liquid separation is performed to obtain a sodium silicate-containing solution and water-leached residue. The calcination tail gas is collected, dust is removed, and then sent to process S5 for reaction gas.

[0042] Step S5: Prepare silica gel; The sodium silicate solution obtained in step S4 is passed through CO2 gas, and the temperature is controlled at 60℃ and the pH is 8. A white gelatinous precipitate is generated by the reaction. The solid and liquid are separated to obtain a white precipitate and a filtrate. The white precipitate is dried to obtain silica gel, and the filtrate is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is returned to step S4 for recycling.

[0043] Comparative Example 4 The calcination temperature in step S1 was changed from 620°C to 460°C, and the other operations were the same as in Example 3.

[0044] Compared with Example 3, it can be seen that when the calcination temperature is too low, the reduction is incomplete and the Fe recovery rate decreases significantly, from 95.61% to 78.42% (Table 2).

[0045] Comparative Example 5 The proportion of biochar added in step S1 was changed from 7.5% to 2.5%, and other operations were the same as in Example 3.

[0046] Compared with Example 3, it can be seen that the addition of biochar was too low, the reduction was incomplete, and the Fe recovery rate decreased significantly, from 94.53% to 78.22% (Table 2). Moreover, unreacted ferric oxide remained in the roasted clinker.

[0047] Example 4 A method for extracting iron and silicon elements from NdFeB waste includes the following steps: Step S1: Preprocessing; Neodymium iron boron waste was mixed with 8% biochar and ground to obtain mixture A. Mixture A was placed in a furnace and calcined in a nitrogen atmosphere at 580℃ for 100 min to obtain magnetic solid powder. The obtained solid powder was then mixed with 0.09 mol / L oxalic acid at a liquid-to-solid ratio of 6:1 and placed in a magnetic separator for magnetic separation to obtain iron-rich and silicon-rich components. Step S2: Acid leaching to remove silicon; The iron-rich component obtained in step S1 was leached with 5.5 mol / L sulfuric acid at a liquid-to-solid ratio of 4:1 and a leaching temperature of 60°C. Solid-liquid separation was then performed to obtain an acid leaching solution (mainly composed of ferric sulfate, aluminum sulfate, and rare earth sulfate) and an acid leaching residue (mainly composed of silicon dioxide). The acid leaching residue was washed until neutral and dried for later use. Testing and calculation showed that the Fe leaching rate was 89.71%. Step S3: Prepare iron oxide; Add 1.1 mol / L sodium hydroxide solution to the leachate obtained in step S2 to adjust the pH to 4. The reaction produces ferric hydroxide precipitate. The ferric hydroxide precipitate is calcined at 700℃ for 2 hours to obtain ferric oxide with an iron content of 62.66%, which meets the requirements of GBT25953-2010 Grade II product.

[0048] Step S4, roasting transformation; The silicon-rich component obtained in step S1 is mixed evenly with the leaching residue obtained in step S2 to obtain mixture B. Mixture B is mixed and ground with 1.0 times the amount of sodium carbonate, and then calcined at 700℃ for 120 minutes to obtain clinker. The clinker is soaked in water, and solid-liquid separation is performed to obtain a sodium silicate-containing solution and water-leached residue. The calcination tail gas is collected, dust is removed, and then sent to process S5 for reaction gas.

[0049] Step S5: Prepare silica gel; The sodium silicate solution obtained in step S4 is passed through CO2 gas, and the temperature is controlled at 65℃ and the pH is 8. A white gelatinous precipitate is generated by the reaction. The solid and liquid are separated to obtain a white precipitate and a filtrate. The white precipitate is dried to obtain silica gel, and the filtrate is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is returned to step S4 for recycling.

[0050] Comparative Example 6 The amount of sodium carbonate added in step S4 was changed from 1.0 times to 0.6 times, and other operations were the same as in Example 4.

[0051] Compared with Example 4, it can be seen that the addition of sodium carbonate was insufficient, the reaction to generate nepheline was incomplete, and the silicon reaction rate decreased significantly, from 85.80% (1.0 times) to 65.43% (0.6 times) (Table 2).

[0052] Example 5 A method for extracting iron and silicon elements from NdFeB waste includes the following steps: Step S1: Preprocessing; Neodymium iron boron waste was mixed with 7% biochar and ground to obtain mixture A. Mixture A was placed in a furnace and calcined in a nitrogen atmosphere at 640℃ for 80 minutes to obtain a magnetic solid powder. The obtained solid powder was then mixed with 0.08 mol / L oxalic acid at a liquid-to-solid ratio of 7:1 and placed in a magnetic separator for magnetic separation to obtain an iron-rich component and a silicon-rich component. Step S2: Acid leaching to remove silicon; The iron-rich component obtained in step S1 was leached with 6 mol / L sulfuric acid at a liquid-to-solid ratio of 3:1 and a leaching temperature of 50°C. Solid-liquid separation was then performed to obtain an acid leaching solution (mainly composed of ferric sulfate, aluminum sulfate, and rare earth sulfate) and an acid leaching residue (mainly composed of silica). The acid leaching residue was washed until neutral and dried for later use. Testing and calculation showed that the Fe leaching rate was 85.38%. Step S3: Prepare iron oxide; Add 1.2 mol / L sodium hydroxide solution to the leachate obtained in step S2 to adjust the pH to 4. The reaction produces ferric hydroxide precipitate. The ferric hydroxide precipitate is calcined at 700℃ for 2 hours to obtain ferric oxide with an iron content of 63.48%, which meets the requirements of GBT25953-2010 Grade II product.

[0053] Step S4, roasting transformation; The silicon-rich component obtained in step S1 is mixed evenly with the leaching residue obtained in step S2 to obtain mixture B. Mixture B is mixed and ground with 0.9 times the amount of sodium carbonate, and then calcined at 750°C for 100 min to obtain clinker. The clinker is then soaked in water, and solid-liquid separation is performed to obtain a sodium silicate-containing solution and water-leached residue. The calcination tail gas is collected, dust is removed, and then sent to process S5 for reaction gas.

[0054] Step S5: Prepare silica gel; The sodium silicate solution obtained in step S4 is passed through CO2 gas, and the temperature is controlled at 70℃ and the pH is 8. A white gelatinous precipitate is generated by the reaction. The solid and liquid are separated to obtain a white precipitate and a filtrate. The white precipitate is dried to obtain silica gel, and the filtrate is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is returned to step S4 for recycling.

[0055] Effect Example The above Examples 1-5 and Comparative Examples 1-6 were tested under the following conditions: The Fe and Si content in each material was determined by inductively coupled plasma optical emission spectrometry (ICP-OES); the percentage values ​​were obtained through material balance and elemental mass conservation calculations. The determination of total iron content was performed according to GB / T6730.5, the determination of silicon dioxide content according to GB / T6730.9, and the determination of alumina content according to GB / T6730.11 or GB / T6730.56.

[0056] Fe recovery rate (%): The percentage of iron in the final iron oxide product relative to the total iron in the original NdFeB waste after the entire process is completed.

[0057] Fe leaching rate (%): The percentage of the mass of iron element entering the acid leaching solution in step S2 relative to the total mass of iron element in the iron-rich component treated in this step.

[0058] Silicon element reaction rate (%): The percentage of the mass of silicon element entering the sodium silicate solution after roasting and water immersion in step S4 relative to the total mass of silicon element in the mixture B processed in this step.

[0059] The specific test results are shown in Table 2 below: Table 2 Comparison of effects between comparative examples and implementation examples

[0060] In summary, examples 1-5 all achieved good results with addition amounts ranging from 6% to 9%. Comparative Example 2, lacking biochar, resulted in a 49.7% decrease in iron recovery, demonstrating the necessity of reduction roasting. Comparative Example 3 had an excessively low biochar addition amount, specifically below 6%, leading to a 9.6% decrease in iron recovery, and XRD analysis showed residual Fe2O3, indicating incomplete reduction. In Examples 1-5, sulfuric acid concentrations of 4-6 mol / L, liquid-to-solid ratios of 3-7:1, and leaching times of 120-240 min all achieved highly efficient leaching of iron, aluminum, and rare earth elements, with leaching rates >95%, and the silica purity in the acid leaching residue met the requirements for subsequent silica gel production. Therefore, this invention utilizes the iron oxide content of approximately 70wt%~80wt% in dry NdFeB waste, and converts the iron oxide into magnetic Fe3O4 by adding a small amount of biochar. Then, the iron element is separated by magnetic separation. The operation is simple and low-cost, overcoming the technical difficulties of high acid / alkali consumption, high cost, and low product purity in existing acid / alkali recovery processes.

[0061] This invention transforms the recycling of NdFeB waste from a rare earth-oriented approach to a synergistic utilization of all components, including iron, silicon, and rare earths, significantly enhancing the comprehensive value of the waste, reducing solid waste landfill, and extending to the resource utilization of iron- and silicon-containing industrial solid wastes such as red mud and fly ash.

[0062] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing iron oxide and silica gel using neodymium iron boron waste, characterized in that, Iron-rich and silicon-rich components were obtained by reducing and roasting neodymium iron boron waste with biochar followed by magnetic separation with oxalic acid. The obtained iron-rich component was subjected to acid leaching, alkali precipitation, and calcination to obtain iron oxide. The obtained silica gel is prepared by acid leaching the silicon-rich component and the iron-rich component together with the acid leaching residue, followed by calcination with sodium carbonate, water leaching, and carbon dioxide purging.

2. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The reduction calcination is carried out in an inert atmosphere at a temperature of 540–680°C for 60–180 min.

3. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The amount of biochar added is 6% to 9% of the mass of NdFeB waste; the concentration of oxalic acid is 0.08 to 0.1 mol / L, and the liquid-to-solid ratio of magnetic separation is 3 to 7:

1.

4. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The acid leaching uses sulfuric acid with a concentration of 4-6 mol / L, a liquid-to-solid ratio of 3-7:1, a leaching temperature of 50-80℃, and a leaching time of 120-240 min.

5. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The alkaline precipitation is carried out by adjusting the pH to 3.5-4.5 with sodium hydroxide solution, and the precipitation reaction temperature is 50-70℃; the calcination temperature is 700℃ and the time is 2h.

6. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The amount of sodium carbonate added is 0.7 to 1.0 times the mass of the mixture obtained after mixing the silicon-rich component with the acid leaching residue; the calcination temperature is 700 to 900°C and the calcination time is 60 to 120 minutes.

7. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, When carbon dioxide is introduced, the pH is controlled at 8 and the reaction temperature is 50-70℃.

8. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The reduction roasting process involves reactions as shown in formulas (1) and (2): 3Fe2O3+ C → 2Fe3O4+ CO(1); 3Fe2O3+ CO → 2Fe3O4+ CO2(2).

9. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The acid leaching process involves reactions as shown in formulas (3) to (8): Fe3O4+ 4H2SO4→ FeSO4+ Fe2(SO4)3+ 4H2O (3); Fe2O3+ 3H2SO4→ Fe2(SO4)3+ 3H2O(4); FeO + H2SO4→ FeSO4+ H2O (5); Al2O3+ 3H2SO4→ Al2(SO4)3+ 3H2O(6); Nd2O3+ 3H2SO4→ Nd2(SO4)3+ 3H2O(7); Pr2O3+ 3H2SO4→ Pr2(SO4)3+ 3H2O (8).

10. The method for preparing iron oxide and silica gel using NdFeB waste according to claim 1, characterized in that, The reaction shown in formula (9) occurs during the sodium carbonate addition and calcination process: SiO2+ Na2CO3→ Na2SiO3+ CO2(9).

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