A multi-stage magnetic separation closed-loop process for magnetite

Abstract

This invention discloses a magnetic separation process for low-grade magnetite, comprising the following steps: Step 1: Crushing the raw ore, wherein the raw ore is magnetite with a magnetic iron content of not more than 10%, and the raw ore is crushed to obtain raw ore particles with a particle size range of 1~2.5cm; Step 2: The raw ore particles are subjected to a first-stage strong magnetic separation to obtain middlings and tailings I, wherein the iron content of the middlings is 14%~22%, and the iron content of the tailings is 3%~5%; Step 3: The middlings are sequentially subjected to a first-stage grinding, a second-stage magnetic separation, a third-stage grinding, and a fourth-stage magnetic separation to obtain iron concentrate and tailings III; the tailings I are sequentially subjected to tailings degranulation and tailings magnetic separation to obtain tailings II and tailings III, and the tailings II are returned to the second-stage grinding. This invention optimizes the degranulation-re-grinding process for tailings I. Tailings I is dissociated a second time by a high-pressure roller mill or a dry crusher to release fine magnetic minerals. After magnetic separation, tailings II are returned to the original process for further degranulation and magnetic separation to obtain qualified iron concentrate products, thus avoiding resource waste.

Description

Technical Field

[0001] This invention application is a divisional application of the invention patent application filed on March 27, 2025, with application number 202510370059.8 and titled "A Magnetic Separation Process for Low-Grade Magnetite".

[0002] This invention relates to the field of mineral processing technology, and in particular to a magnetic separation process for low-grade magnetite, used to obtain high-grade iron concentrate from low-grade magnetite. Background Technology

[0003] In recent years, with the rapid development of China's economy and the rapid growth of steel production capacity, the over-exploitation of my country's iron ore resources has led to a gradual decrease in high-grade iron ore resources with good beneficiation properties, resulting in a decline in the sustainable supply capacity of iron ore resources and a shortage of reserve resources. Currently, my country possesses a large amount of low-grade magnetite, which generally meets the requirements for open-pit mining. However, the lower mining costs are usually accompanied by higher beneficiation ratios and lower recovery rates. Beneficiation technology for low-grade magnetite has always been a challenge for the efficient utilization of resources, especially when processing fine-grained disseminated ores, where low separation efficiency and high beneficiation costs are the main challenges. Therefore, how to efficiently and cost-effectively develop low-grade magnetite resources is particularly important and has become a key to ensuring the sustainable development of my country's steel industry.

[0004] Traditional processes commonly employ a combination of multi-stage magnetic separation and staged grinding to improve concentrate grade. However, effective solutions are lacking for recovering residual magnetic minerals from tailings, leading to resource waste. In recent years, research on tailings reprocessing has gradually become a hot topic, but existing technologies still suffer from problems such as complex processes, high energy consumption, or insufficient recovery rates.

[0005] A patented mineral processing technology for the strong magnetic separation and recovery of ultrafine-grained low-grade hematite (CN201610025736.3) proposes to perform ultrafine grinding (over 95% of the tailings after weak magnetic separation) and combine it with dispersants and flocculants to enhance magnetic separation and recovery. Although this process can significantly improve the recovery rate of ultrafine-grained iron minerals, it requires the addition of chemical agents such as sodium humate and anionic polyacrylamide, which not only increases costs but also may have potential negative environmental impacts.

[0006] Another invention patent, "Magnetic Ore Ultrafine Crushing-Classification Magnetic Separation Method" (ZL201610346096.6), uses a high-pressure roller mill to perform ultrafine crushing and classification magnetic separation of the raw ore, thereby recovering the liberated magnetite in advance and reducing the amount of subsequent grinding. Although this method optimizes the process energy consumption, it mainly targets the processing of the raw ore and does not address the secondary recovery of difficult-to-process minerals in the tailings, nor does it solve the problem of reusing residual magnetic minerals in the tailings.

[0007] In summary, existing technologies have low tailings utilization rates. Traditional processes for tailings reprocessing are mostly limited to simple magnetic separation, without combining degranulation and regrinding technologies. This makes it difficult to release the fine magnetic minerals encapsulated in the gangue. At the same time, some processes rely on chemical agents to improve separation efficiency, which increases costs and is not environmentally friendly. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing technologies by providing a magnetic separation process for low-grade magnetite. This process can re-granulate tailings, then grind and magnetically separate them before returning them to the original beneficiation process to produce a qualified product with a grade of 60.3%, thus achieving efficient utilization of tailings.

[0009] This application provides a magnetic separation process for low-grade magnetite, including the following steps:

[0010] Step 1: Raw ore crushing process. The raw ore is magnetite with a magnetic iron content of not more than 10%. After crushing, the raw ore is used to obtain raw ore particles with a particle size range of 1~2.5cm.

[0011] Step 2: The raw ore particles undergo primary high-intensity magnetic separation to obtain middlings and tailings I. The middlings have an iron content of 14%~22%, and the tailings have an iron content of 3%~5%.

[0012] Step 3: The middlings are sequentially subjected to primary grinding, secondary magnetic separation, secondary grinding, and tertiary magnetic separation to obtain iron concentrate and tailings III;

[0013] Tailings I is successively subjected to tailings granulation and tailings magnetic separation to obtain tailings II and tailings III. Tailings II is then returned to the secondary grinding process.

[0014] Tailings III enter the tailings dam;

[0015] The particle size ratio of the ore particles obtained after the first-stage grinding, the ore particles obtained after the second-stage grinding, and the ore particles obtained after the tailings degranulation is 4:5:3.

[0016] The magnetic separation intensities of the secondary magnetic separation and the tailings magnetic separation are the same;

[0017] The ratio of magnetic separation intensity between the secondary magnetic separation and the tertiary magnetic separation is 6:5;

[0018] After the secondary magnetic separation, the iron content is 55%.

[0019] The iron content of tailings II obtained after magnetic separation of the tailings is 18%~22%;

[0020] After the three-stage magnetic separation, the iron content is 60%~63%.

[0021] Furthermore, the raw ore crushing process adopts a three-stage crushing method, including a first stage crushing, a second stage crushing, and a third stage crushing;

[0022] The first stage of crushing breaks the raw ore to 5-8cm, the second stage of crushing breaks the raw ore after the first stage of crushing to 3-5cm, and the third stage of crushing breaks the raw ore after the second stage of crushing to 1-2.5cm.

[0023] Furthermore, the first and second stages of crushing employ jaw crushers, while the third stage of crushing employs cone crushers.

[0024] Furthermore, the primary high-intensity magnetic separation adopts a dry high-intensity magnetic separator.

[0025] Furthermore, the magnetic separation intensity of the first-stage high-intensity magnetic separation is 2600 Gs.

[0026] Furthermore, the particle size of the ore particles obtained after the first-stage grinding is 160 mesh;

[0027] The ore particles obtained after the two-stage grinding process have a particle size of 200 mesh.

[0028] The tailings were granulated to obtain mineral particles with a particle size of 120 mesh.

[0029] Furthermore, the primary and secondary grinding processes employ ball mills; the secondary, tertiary, and tailings magnetic separation processes employ countercurrent magnetic separators.

[0030] Furthermore, the tailings are further disintegrated by a high-pressure roller mill or a dry crusher to perform secondary disintegration of tailings I.

[0031] Furthermore, a hammer crusher is used to perform secondary separation of tailings I.

[0032] Furthermore, the magnetic separation intensity of the secondary magnetic separation and tailings magnetic separation is 1800 Gs;

[0033] The magnetic separation intensity of the three-stage magnetic separation is 1500 Gs.

[0034] Furthermore, the three-stage magnetic separation employs two countercurrent magnetic separators connected in series. The raw material after two-stage grinding first passes through the first countercurrent magnetic separator to obtain a product with an iron content of 50% to 60%. The product then enters the second countercurrent magnetic separator to obtain an iron concentrate with an iron content of 60% to 63%.

[0035] The beneficial effects of this invention are as follows:

[0036] 1. For tailings I produced by a single magnetic separation, which has a high iron content, in order to achieve deep utilization of the tailings and improve the yield, this scheme optimizes the degranulation-re-grinding of tailings I. Tailings I is dissociated twice by a high-pressure roller mill or a dry crusher to release fine magnetic minerals. After magnetic separation, tailings II are returned to the original process for further degranulation and magnetic separation to obtain qualified iron concentrate products, thus avoiding resource waste.

[0037] 2. This scheme adopts reagent-free magnetic separation. Through a multi-stage magnetic separation design with different gradient magnetic field strengths, magnetite is crushed and magnetically separated multiple times to obtain qualified products, reducing the dependence on chemical reagents.

[0038] 3. The closed-loop recycling system returns the reprocessed tailings to the upstream process, forming a closed-loop system for efficient resource reuse and reducing overall energy consumption. This process is significantly superior to existing technologies in terms of tailings reuse rate, environmental protection, and economy, providing a new approach for the efficient development of low-grade magnetite. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0040] Figure 1 This is a flow chart of a low-grade magnetite magnetic separation process according to an embodiment of the present invention.

[0041] Figure 2 This is a flow chart of a low-grade magnetite magnetic separation process according to another embodiment of the present invention. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0043] For low-grade ores, the goal is to refine raw ore with a magnetic iron (MFE) content of only about 9% into an iron concentrate with a content of 60.3% to meet market demands. Due to the special characteristics of the raw ore and the primitive and outdated beneficiation process, the recovery rate is low, costs are high, and enterprises suffer significant losses. Therefore, this embodiment provides a magnetic separation process for low-grade magnetite, the process flow diagram of which is shown below. Figure 1 As shown, it includes the following steps:

[0044] Step 1: Raw ore crushing. The raw ore is magnetite with a magnetic iron content of no more than 10%. After crushing, the raw ore is obtained as raw ore particles with a particle size range of 1~2.5cm.

[0045] Step 2: The raw ore particles undergo primary high-intensity magnetic separation to obtain middlings and tailings I. The iron content of the middlings is 14%~22%, and the iron content of the tailings is 3%~5%.

[0046] Step 3: The middlings are successively subjected to primary grinding, secondary magnetic separation, secondary grinding, and tertiary magnetic separation to obtain iron concentrate and tailings III;

[0047] Tailings I is successively processed by tailings granulation and tailings magnetic separation to obtain tailings II and tailings III. Tailings II is then returned to the secondary grinding process.

[0048] Tailings III enter the tailings dam;

[0049] The particle size ratio of the ore particles obtained after primary grinding, the ore particles obtained after secondary grinding, and the ore particles obtained after tailings degranulation is 4:5:3.

[0050] The magnetic separation intensity is the same for both the secondary magnetic separation and the tailings magnetic separation;

[0051] The ratio of magnetic separation intensity between the secondary and tertiary magnetic separation stages is 6:5;

[0052] After two-stage magnetic separation, the iron content is 55%.

[0053] The iron content of tailings II obtained after tailings magnetic separation is 18%~22%;

[0054] After three-stage magnetic separation, the iron content is 60%~63%.

[0055] In this embodiment, the raw ore particles are first strictly controlled within the range of 1~2.5cm through crushing. Then, the middlings and tailings I are initially separated by strong magnetic separation. After tailings I is granulated to 120 mesh, tailings II containing 18%~22% iron are obtained by magnetic separation of tailings I and returned to the secondary grinding process. The recovery of tailings II (containing 18%~22% iron) increases the total metal recovery rate by 8%~12%, optimizes resource utilization, and reduces waste.

[0056] The middlings are ground into ore particles in the first stage, and the tailings I are granulated into ore particles with a particle size ratio of 4:3. Since the particle sizes of the middlings after grinding and the tailings I after granulation are similar, experimental analysis shows that using the same magnetic separation intensity can ensure that magnetic minerals are captured efficiently, while avoiding a decrease in recovery rate or energy waste due to intensity differences. Therefore, the second-stage magnetic separation and the tailings magnetic separation use the same magnetic separation intensity. In the third-stage magnetic separation process, in order to avoid the adsorption of non-magnetic impurities or intergrowths, the magnetic separation intensity of the third-stage magnetic separation must not be higher than that of the second-stage magnetic separation. Therefore, in this embodiment, the magnetic separation intensity of the second-stage magnetic separation and the tailings magnetic separation is the same, and the magnetic separation intensity ratio of the second-stage magnetic separation and the third-stage magnetic separation is 6:5.

[0057] Meanwhile, multi-stage grinding and graded magnetic separation are combined for different stages. By using the synergistic parameters of grinding and magnetic separation, the grade of primary grinding to 160 mesh → secondary magnetic separation increases to 55%, secondary grinding to 200 mesh → tertiary magnetic separation finally reaches 60%~63%. This solution solves the problems of low recovery rate, high energy consumption and resource waste of low-grade magnetite by precisely controlling the crushing particle size, recycling tailings, and synergistic use of multi-stage grinding and magnetic separation parameters.

[0058] Traditional magnetite crushing processes often employ a two-stage crushing method (such as jaw crusher + cone crusher), directly crushing the raw ore (particle size ≤ 50 cm) to 1-5 cm. However, for low-grade magnetite (containing ≤ 10% iron), the mineral intercalation is fine and the gangue inclusions are complex, resulting in a wide particle size distribution in existing technologies: undissociated coarse particles (> 3 cm) and over-crushed fine particles (< 1 cm) coexist within the 1-5 cm range, leading to a 15%-20% increase in subsequent grinding energy consumption. To achieve strict control over the particle size of the raw ore, a multi-stage crushing scheme is adopted. Specifically, the raw ore crushing process employs three stages: a first stage crushing, a second stage crushing, and a third stage crushing. The first stage crushes the raw ore to 5-8 cm, the second stage crushes it to 3-5 cm, and the third stage crushes it to 1-2.5 cm.

[0059] like Figure 2 As shown, this embodiment adopts a three-stage closed-circuit crushing process to construct a gradient crushing system of 5~8cm→3~5cm→1~2.5cm. The first and second stages of crushing use jaw crushers, and the third stage of crushing uses cone crushers.

[0060] First stage crushing: A 600×900 jaw crusher is selected;

[0061] Second stage crushing: A 250×1200 jaw crusher is selected;

[0062] Third stage crushing: Equipped with a 900 cone crusher.

[0063] This embodiment uses three-stage crushing to gradually reduce the particle size range, so that the particle size concentration of the final product is ≥95%; the secondary jaw crusher performs medium crushing and the cone crusher performs fine crushing, with the single-stage crushing ratio decreasing step by step, reducing over-crushing and giving full play to the energy efficiency advantages of each piece of equipment.

[0064] Highly coordinated with downstream processes, the 1~2.5cm particle size matches the subsequent primary high-intensity magnetic separation (magnetic field strength 1.2T), and the uniform particle size reduces the loss of coarse iron particles in the magnetic separation tailings, and improves the adsorption efficiency of magnetic minerals by 20%.

[0065] In this embodiment, according to the requirements of magnetic separation, such as Figure 2 As shown, a dry-type strong magnetic separator is used in the first-stage strong magnetic separation.

[0066] After comparing the magnetic separation intensity:

[0067] The iron content of the middlings obtained after magnetic separation at a strength of 2200 Gs is 15%~22%, and the iron content of tailings I is 5%~6.5%.

[0068] The iron content of the middlings obtained after magnetic separation at a strength of 2600 Gs is 14%~22%, and the iron content of tailings I is 3%~5%.

[0069] The iron content of the middlings obtained after magnetic separation at a strength of 3000 Gs is 13%~19%, and the iron content of tailings I is 3%~5%.

[0070] Through data comparison, in another embodiment of the present invention, the magnetic separation intensity of the first-stage strong magnetic separation is 2600Gs. The advantage is that it can improve the iron content of the middlings to ensure the unit output, and the iron content of tailings I is moderate.

[0071] To improve magnetic separation efficiency, in this embodiment, the particle size of the ore particles obtained after primary grinding is 160 mesh; the particle size of the ore particles obtained after secondary grinding is 200 mesh; and the particle size of the ore particles obtained after tailings degranulation is 120 mesh.

[0072] The raw ore is subjected to a first-stage strong magnetic separation to obtain middlings and tailings I. The middlings are then subjected to a first-stage grinding, a second-stage magnetic separation, a third-stage grinding, and a fourth-stage magnetic separation to obtain iron concentrate and tailings III.

[0073] Primary grinding: The particle size of the ore particles obtained from primary grinding is 160 mesh;

[0074] Secondary magnetic separation: The iron content after secondary magnetic separation is 55%;

[0075] Secondary grinding: The particle size of the ore particles obtained from secondary grinding is 200 mesh;

[0076] Three-stage magnetic separation: The iron content after three-stage magnetic separation is 60%~63%.

[0077] To achieve the above technical effects, such as Figure 2 As shown, in this embodiment, ball mills are used for the first and second stages of grinding; countercurrent magnetic separators are used for the second and third stages of magnetic separation.

[0078] Specifically:

[0079] Primary grinding: Using a Φ1.5×3m ball mill, the middlings are ground to -160 mesh (96μm) to account for 85%, and the slurry concentration is 60%~65%;

[0080] Secondary magnetic separation: A countercurrent magnetic separator is used to separate the rough concentrate and tailings III with an iron content of 55%;

[0081] Secondary grinding: A Φ1.5×3m ball mill is used to grind the rough concentrate to -200 mesh (74μm) with 90% content, and the slurry concentration is 50%~55%;

[0082] Third-stage magnetic separation: A counter-current magnetic separator is used to separate iron concentrate and tailings with an iron content of 60%~63%.

[0083] In this embodiment, the first-stage grinding to 160 mesh (96μm) preferentially dissociates coarse intergrowths, avoiding mud formation caused by direct ultrafine grinding (mud formation rate reduced by 40%); the second-stage grinding to 200 mesh (74μm) specifically dissociates remaining inclusions. The total grinding energy consumption is comparable to that of single-stage grinding, but the concentrate grade is improved; on the other hand, a countercurrent magnetic separator is used to improve separation efficiency. The second-stage magnetic separation and countercurrent design extend the magnetic particle capture time, improving the recovery rate of coarse concentrate; the third-stage magnetic separation enhances the magnetic field strength to suit fine-grained minerals (-200 mesh), and the concentrate grade exceeds 60%.

[0084] For the tailings I produced by the first magnetic separation, which has a high iron content, in order to achieve deep utilization of the tailings and improve the yield, this scheme optimizes the degranulation-re-grinding of tailings I. The tailings I are then dissociated a second time by a high-pressure roller mill or a dry crusher to release fine magnetic minerals. After magnetic separation, tailings II are returned to the original process for further degranulation and magnetic separation to obtain qualified iron concentrate products, thus avoiding resource waste.

[0085] The tailings granulation process yields ore particles with a size of 120 mesh; the tailings II obtained after tailings magnetic separation has an iron content of 18%~22%. In this embodiment, tailings I is subjected to secondary dissociation by a high-pressure roller mill or a dry crusher.

[0086] Specifically, for the recycling process of magnetite tailings I (containing 3%~5% iron), the tailings I is degranulated-magnetically separated-returned through a process design. The tailings I are degranulated and magnetically separated to obtain tailings II (containing 18%~22% iron), which is then returned to the secondary grinding system, thus realizing resource recycling and reducing tailings emissions.

[0087] Specifically:

[0088] Tailings shredding: A hammer crusher is used to perform secondary shredding of tailings I, shredding tailings I to -120 mesh (125μm) accounting for 85%;

[0089] Tailings magnetic separation: A counter-current magnetic separator is used to separate the following:

[0090] Tailings II: Contains 18%~22% iron, returned to secondary grinding;

[0091] Tailings III: Contains 2%~3% iron, and is discharged into the tailings dam.

[0092] After grinding, the middlings are processed into iron concentrate through secondary and tertiary magnetic separation. Tailings I are returned to the middlings processing flow after grinding and tailings magnetic separation. They are then mixed with the product after secondary magnetic separation and subjected to secondary grinding and tertiary magnetic separation to obtain iron concentrate.

[0093] The middlings are ground to a particle size of 160 mesh, and the tailings I are shredded to a particle size of 120 mesh. Since the particle sizes of the middlings after grinding and the tailings I after shredding are similar, experimental analysis shows that using the same magnetic separation intensity can ensure that magnetic minerals are captured efficiently, while avoiding a decrease in recovery rate or energy waste due to intensity differences. Therefore, the same magnetic separation intensity is used for the secondary magnetic separation and the tailings magnetic separation.

[0094] When the secondary magnetic separation intensity is 1800 Gs, the iron content of the middlings after magnetic separation is 55%, and the iron content of tailings III is below 4%. When the secondary magnetic separation intensity is 1500 Gs, the iron content of the middlings after magnetic separation is 57%, and the iron content of tailings III is about 4% to 5%. By comparing the two, choosing 1800 Gs for the secondary magnetic separation intensity can reduce the resource loss of iron ore, and at the same time, qualified products can be selected through subsequent beneficiation processes.

[0095] Prioritizing middlings processing is crucial for ensuring efficiency. A secondary magnetic separation intensity of 1800 Gs was chosen to minimize iron ore resource loss. After secondary magnetic separation, the minerals undergo further grinding and tertiary grinding to obtain iron concentrate. Secondary grinding efficiently recovers most of the magnetite. The 1800 Gs high-intensity magnetic field used in secondary grinding rapidly adsorbs strongly magnetic minerals, reducing the loss of medium- and high-value minerals. Tertiary grinding further purifies the middlings and tailings (II), requiring the treatment of weakly magnetic minerals not fully separated in the previous stage. To avoid adsorbing non-magnetic impurities or intergrowths during tertiary magnetic separation, the magnetic separation intensity must not exceed that of the secondary magnetic separation.

[0096] In another embodiment, the magnetic separation intensity of the secondary magnetic separation and tailings magnetic separation is 1800 Gs; the magnetic separation intensity of the tertiary magnetic separation is 1500 Gs.

[0097] When the three-stage magnetic separation process is carried out by a 1500Gs countercurrent magnetic separator, the iron content of the product is 50%~60%, which does not meet the market demand for iron concentrate.

[0098] To improve the efficiency of the three-stage magnetic separation, this embodiment uses two counter-current magnetic separators connected in series, such as... Figure 2 As shown, the raw material after two-stage grinding first passes through the first countercurrent magnetic separator to obtain a product with an iron content of 50% to 60%, and the product then enters the second countercurrent magnetic separator to obtain an iron concentrate with an iron content of 60% to 63%.

[0099] In this embodiment, two countercurrent magnetic separators are used in series. The slurry flow direction of the countercurrent magnetic separator is opposite to the movement direction of the magnetic product, which prolongs the residence time of magnetic particles in the magnetic field and increases the capture probability. Using two devices in series allows for two "scavenging" processes of magnetic minerals, reducing the loss of iron minerals in the tailings.

[0100] The iron concentrate is purified in stages to gradually improve its grade. The raw material, after two-stage grinding, first enters a counter-current magnetic separator to initially separate products with higher iron content (50%–60%), removing most gangue and weakly magnetic impurities. The key to this stage is rapid enrichment of iron minerals, reducing the load on subsequent processing. The initially enriched product then enters a second counter-current magnetic separator. Through finer separation conditions (such as adjusting magnetic field strength and slurry flow rate), residual non-magnetic impurities are further removed, significantly increasing the iron concentrate grade to 60%–63%. Through the synergistic effect of the two stages, the first magnetic separator focuses on "roughing," recovering most of the iron minerals; the second focuses on "fine cleaning," emphasizing grade improvement. This staged purification avoids the mixing interference of high-grade and low-grade materials from a single magnetic separation, improving the final concentrate quality.

[0101] The above embodiments are preferred implementations of the present invention. Any obvious substitutions are within the protection scope of the present invention without departing from the inventive concept of the present invention.

Claims

1. A multi-stage magnetic separation closed-loop process for magnetite, characterized in that, Includes the following steps: Step 1: Raw ore crushing process. The raw ore is magnetite with a magnetic iron content of not more than 10%. After crushing, the raw ore is used to obtain raw ore particles with a particle size range of 1~2.5cm. Step 2: The raw ore particles undergo primary high-intensity magnetic separation to obtain middlings and tailings I. The middlings have an iron content of 14%~22%, and the tailings have an iron content of 3%~5%. Step 3: The middlings are sequentially subjected to primary grinding, secondary magnetic separation, secondary grinding, and tertiary magnetic separation to obtain iron concentrate and tailings III; Tailings I are successively subjected to tailings granulation and tailings magnetic separation to obtain tailings II and tailings III. Tailings II are then returned to the secondary grinding process. Tailings III enter the tailings dam; The particle size ratio of the ore particles obtained after the first-stage grinding, the ore particles obtained after the second-stage grinding, and the ore particles obtained after the tailings degranulation is 4:5:

3. The primary grinding process uses a ball mill to grind the middlings to -160 mesh (85%), with a slurry concentration of 60% to 65%. The secondary grinding process uses a ball mill to grind the coarse concentrate to -200 mesh (90%) and the slurry concentration to 50%~55%. The tailings granulation process involves secondary separation of tailings I using a high-pressure roller mill or a dry crusher, resulting in mineral particles with a particle size of 120 mesh. The magnetic separation intensities of the secondary magnetic separation and the tailings magnetic separation are the same; The ratio of magnetic separation intensity between the secondary magnetic separation and the tertiary magnetic separation is 6:5; The secondary magnetic separation, tertiary magnetic separation, and tailings magnetic separation are performed using countercurrent magnetic separators. The magnetic separation intensity of the secondary magnetic separation and tailings magnetic separation is 1800 Gs; The magnetic separation intensity of the three-stage magnetic separation is 1500 Gs; The three-stage magnetic separation uses two countercurrent magnetic separators connected in series. The raw material after the two-stage grinding first passes through the first countercurrent magnetic separator to obtain a product with an iron content of 50% to 60%. The product then enters the second countercurrent magnetic separator to obtain an iron concentrate with an iron content of 60% to 63%. After the secondary magnetic separation, the iron content is 55%. The iron content of tailings II obtained after magnetic separation of the tailings is 18%~22%; After the three-stage magnetic separation, the iron content is 60%~63%; The raw ore crushing process adopts a three-stage crushing method, including a first stage crushing, a second stage crushing, and a third stage crushing; The first stage of crushing breaks the raw ore to 5-8cm, the second stage of crushing breaks the raw ore after the first stage of crushing to 3-5cm, and the third stage of crushing breaks the raw ore after the second stage of crushing to 1-2.5cm. The first stage uses a 600×900 jaw crusher, the second stage uses a 250×1200 jaw crusher, and the third stage uses a 900 cone crusher. Through three stages of crushing, the particle size range is gradually reduced so that the particle size concentration of the final product is ≥95%. The secondary jaw crusher is used for medium crushing, followed by the cone crusher for fine crushing. The single-stage crushing ratio decreases step by step.

2. The multi-stage magnetic separation closed-loop process for magnetite according to claim 1, characterized in that, The primary high-intensity magnetic separation uses a dry high-intensity magnetic separator, and the magnetic separation intensity of the primary high-intensity magnetic separation is 2600 Gs.

3. The multi-stage magnetic separation closed-loop process for magnetite according to claim 1, characterized in that, A hammer crusher was used to perform secondary separation of tailings I.

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