Resourceful treatment method for magnesium electrolytic slag

Abstract

The invention relates to the technical field of industrial waste residue resourceful utilization, in particular to a magnesium electrolysis residue resourceful treatment method which comprises the following steps: S1, mixing magnesium electrolysis residues and water according to the volume mass ratio of (4-6): 1, and collecting leachate and leaching residues; s2, adding magnesium hydroxide powder into the leachate, adjusting the pH value of the solution to 3.5-4.5, adding a flocculating agent into the leachate, and separating and precipitating to obtain filtrate; s3, adding a proper amount of oxalic acid solution into the filtrate, and removing precipitates; s4, evaporating and concentrating the filtrate obtained in S3 to 1 / 3-1 / 2 of the original volume, and separating the separated crystal from the concentrated solution; s5, separating out potassium chloride crystals and sodium chloride crystals from the concentrated solution. The process route is clear, operation is easy to implement, the treatment effect is stable, and environmental benefits and economic benefits are achieved, so that the method has good industrialization potential and wide application and popularization prospects in the field of resourceful treatment of the magnesium electrolysis slag.

Description

Technical Field

[0001] This invention relates to the field of industrial waste slag resource utilization technology, and in particular to a method for the resource utilization of magnesium electrolytic slag. Background Technology

[0002] Magnesium electrolysis production generates a large amount of complex magnesium electrolysis slag, mainly containing high-value components such as potassium chloride, sodium chloride, and magnesium chloride, as well as various impurities such as iron, aluminum, and calcium. Currently, this waste slag is primarily disposed of through stockpiling or landfilling, which not only occupies land resources but also easily leads to serious environmental problems such as soil salinization and groundwater pollution due to the soluble chlorides in it being leached by rainwater. Simultaneously, a significant amount of valuable metal resources are not recovered, resulting in substantial waste. Therefore, developing a resource-based treatment technology for magnesium electrolysis slag that can achieve clean conversion and efficient recovery is both an urgent environmental need and of significant economic value.

[0003] In the prior art, Chinese patent CN104495876A discloses a method for stepwise crystallization recovery of KCl, NaCl, and MgCl2 by adjusting the pH using a water immersion combined with an alkali solution (such as NaOH). However, this type of method has several key and deep-seated technical defects in practical applications and industrial promotion. First, its impurity removal process is usually relatively crude, and it is not suitable for Al 3+ Fe 3+ The removal mechanism for impurity ions lacks precise control. While simple alkali precipitation can form hydroxides from iron and aluminum, the wide pH control range easily leads to finely fragmented precipitates that are difficult to settle and filter, or co-precipitate with magnesium ions, resulting in the loss of valuable metals. More importantly, this method typically fails to systematically address the issue of Ca... 2+ The problem lies in the deep removal of calcium impurities, as the presence of calcium impurities can seriously affect the purity and application performance of subsequent products such as magnesium chloride.

[0004] Furthermore, existing technologies primarily focus on the separation of the main components, neglecting the efficiency of solid-liquid separation after impurity removal, the subsequent treatment of precipitates, and the potential introduction of new ions (such as excess Na). + The lack of comprehensive consideration of issues such as pollution leads to "breakpoints" or hidden costs in the process flow, thus restricting the robustness of the entire process and the stability of the product. In contrast, the method proposed in this invention has been specifically optimized and refined in its technical approach: after water immersion, magnesium hydroxide (which can itself be derived from intermediate products in the process) is used as a pH adjuster to precisely control the system within the range of 3.5-4.5. Under these conditions, Fe... 3+ And Al 3+ It can almost completely precipitate into an easily filterable hydroxide while minimizing magnesium loss. Subsequently, oxalic acid is introduced into the filtrate to selectively precipitate Ca. 2+This technology achieves efficient and deep removal of the key impurity, calcium. The process chain is complete, with each step specifically addressing particular impurities. This not only significantly improves the purity of the final recovered salts but also makes the operation simpler and more controllable, providing a more feasible and competitive solution for the high-value-added resource utilization of magnesium electrolytic slag.

[0005] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventors studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0006] This invention relates to the field of industrial waste slag resource utilization technology, and in particular to a method for the resource utilization of magnesium electrolytic slag.

[0007] To address the aforementioned technical problems, one objective of this invention is to provide a method for the resource utilization of magnesium electrolytic slag, comprising the following steps: S1 magnesium electrolytic slag with a volume-to-mass ratio of 4-6:1 is mixed with water and stirred at 150-200 r / min for 1-3 h. Then, solid-liquid separation is performed, and the leachate and leaching residue are collected. S2 Add magnesium hydroxide powder to the leachate, adjust the pH of the solution to 3.5-4.5, add flocculant to the leachate, stir and react for 30-60 min, let it stand to precipitate for 1-2 h, separate the precipitate, and obtain the filtrate; Add an appropriate amount of oxalic acid solution to the S3 filtrate to remove the precipitate. The concentration of oxalic acid ions in the mixture is 2-5 times that of calcium ions. S4 The filtrate obtained from S3 is evaporated and concentrated to 1 / 3-1 / 2 of its original volume, and the precipitated crystals are separated from the concentrate. The S5 concentrate was precipitated sequentially as potassium chloride and sodium chloride crystals using an evaporation-crystallization-cold precipitation method.

[0008] According to a preferred embodiment, the leaching residue is washed with water and then dried to recover TiO2, Ca, and SiO2.

[0009] According to a preferred embodiment, in S1, the particle size of the magnesium electrolytic slag is no greater than 8 cm.

[0010] According to a preferred embodiment, in S1, the water immersion temperature is 25-40°C.

[0011] According to a preferred embodiment, in S1, solid-liquid separation is performed after stirring at 30°C and 180 r / min for 2 h.

[0012] According to a preferred embodiment, in S1, solid-liquid separation is performed by pressure filtration.

[0013] According to a preferred embodiment, in S2, the flocculant is polyaluminum chloride (PAC) with a mass fraction of 0.1-0.3%.

[0014] According to a preferred embodiment, in step S2, the precipitate is separated by centrifugal filtration. Preferably, the centrifugal speed is 2500-3000 r / min.

[0015] According to a preferred embodiment, in step S3, oxalic acid solution is added and then stirred for 1 hour.

[0016] According to a preferred embodiment, in S4, the evaporation and concentration conditions are: vacuum degree 0.06-0.08 MPa and temperature 70-80℃.

[0017] According to a preferred embodiment, the conditions for the evaporation crystallization-cold precipitation method are as follows: vacuum degree 0.06-0.08 MPa, temperature 80-100℃ until a large number of crystals precipitate from the mother liquor, and then cooling to 10-15℃. At this time, potassium chloride crystals are separated and then washed with 1-3 times the amount of saturated potassium chloride solution to remove impurities and obtain a potassium chloride product with high purity. The remaining mother liquor is subjected to the above operation, and is evaporated, concentrated, cooled, and washed with saturated sodium chloride to obtain pure sodium chloride crystals. One of the objectives of this invention is to provide a magnesium electrolytic slag resource utilization system, comprising a crushing unit, a reaction tank unit, and a crystal reaction unit.

[0018] The crushing unit is used to crush and screen magnesium electrolytic slag. The crushing unit is, for example, an integrated crushing and screening machine.

[0019] The reaction tank unit contains a set of reaction tanks arranged in sequence according to the process flow and with specific functions. It is equipped with a first reaction tank for water immersion treatment, a second reaction tank for iron and aluminum treatment, and a third reaction tank for calcium and magnesium treatment.

[0020] Specifically, the main structure of the first reaction tank (leaching reaction tank) can be a corrosion-resistant tank equipped with a high-strength stirring system. The tank body is typically equipped with a jacket or built-in coils, allowing the introduction of steam or hot water to maintain the optimal temperature required for the leaching process and promote the rapid dissolution of soluble salts. The tank bottom is designed with a slight taper or has special flow channels to facilitate the discharge and emptying of high-solids content slurry. The feed inlet is connected to the magnesium electrolytic slag and process water pipelines and is equipped with a metering device; an exhaust port is located at the top to balance the operating pressure. The core structural feature of the first reaction tank is to achieve high-intensity, uniform mixing and efficient mass transfer between the solid and liquid phases.

[0021] The structural design of the second reaction tank (iron-aluminum sedimentation tank) focuses on achieving precise pH control and efficient pre-conditioning for solid-liquid separation. The tank is typically divided into sections: the upper section is a mixing reaction zone equipped with a variable frequency speed-controlled stirrer to ensure rapid and uniform mixing of the magnesium hydroxide regulator and the leachate; a high-precision online pH monitoring probe is integrated into the tank wall for real-time monitoring and feedback control of the reaction environment. The lower section transitions into a flocculation and sedimentation zone, which can be designed with a gradually expanding structure or multiple inclined plates to significantly increase the effective sedimentation area and promote the growth, aggregation, and initial sedimentation of ferric hydroxide and aluminum hydroxide flocs. The bottom of the tank is conical, collecting the precipitate and connecting to the sludge discharge valve. The purpose of the second reaction tank is to provide a suitable environment for sedimentation and pre-enrichment.

[0022] The third reaction tank (calcium precipitation tank) is designed with precise oxalic acid dosing, a gentle reaction, and protection of the formed calcium oxalate crystals in mind. This tank typically employs a larger aspect ratio to provide a smooth fluid flow. The oxalic acid solution is injected through a multi-point, micro-dosing system and mixed online with the inlet pipe to prevent localized over-concentration. The tank's agitator is designed with a low-speed paddle to ensure uniform reaction while minimizing shearing and breakage of the already formed fine crystals. The lower half of the tank serves as a crystal maturation zone, allowing sufficient residence time for crystal growth, facilitating subsequent filtration. The tank bottom is also designed for easy solid collection.

[0023] The crystal reaction unit is configured as a sealed, corrosion-resistant evaporative crystallizer. The crystallizer body is typically designed as a jacketed vessel or a structure with built-in heating tubes to allow the introduction of steam or a high-temperature heat transfer medium, providing a uniform and controllable heat source for solution evaporation. The liquid level and evaporation area within the vessel are precisely calculated to optimize the high-temperature heating state while ensuring processing capacity and preventing product loss and contamination caused by mist entrainment. The crystallizer also integrates a cold precipitation function, such as a cold precipitation vessel with a cooling jacket. The hot saturated liquid (i.e., the filtrate evaporated and concentrated to 1 / 3-1 / 2 of its original volume in the above process) is transported to one or more cold precipitation vessels with cooling jackets. The cold precipitation vessels are typically equipped with slow stirring, and the solution temperature decreases slowly and uniformly through the circulation of the cooling medium within the jacket. The final separation of the crystals is completed in an integrated or adjacent solid-liquid separation unit. The crystal reaction unit typically transports the concentrated slurry containing the target crystals to a centrifuge or vacuum filter through a bottom crystal slurry discharge port, achieving a high-yield recovery of high-purity crystals.

[0024] The beneficial effects of this technical solution are: This invention provides a method and system for the resource-based treatment of magnesium electrolytic slag, offering a practical and feasible technical approach for the large-scale and high-value utilization of magnesium electrolytic slag. Compared to conventional treatment processes, this method features targeted optimizations and refinements in its technical approach: after extracting valuable components through water leaching, it innovatively uses magnesium hydroxide (which can be derived from intermediate products in the process) as a pH adjuster to precisely and stably control the system within the range of 3.5-4.5. Under these conditions, Fe... 3+ And Al 3+ It can efficiently precipitate into hydroxides that are easy to settle and filter, thus effectively removing them while minimizing magnesium loss. Subsequently, oxalic acid is introduced into the filtrate after iron and aluminum removal, which can selectively precipitate Ca. 2+ This process achieves deep purification of calcium impurities. The entire process chain is tightly integrated, with each step highly targeted. While effectively separating the main impurities, it significantly improves the purity of the recovered salt products. Moreover, the process conditions are mild, and the operation is simple and controllable, providing a more feasible and competitive solution for the resource utilization of magnesium electrolytic slag.

[0025] According to the national mandatory standard "Standard for Pollution Control of General Industrial Solid Waste Storage and Landfill" (GB 18599-2020), since July 2021, the soluble salt content in general industrial solid waste disposed of by landfill must be less than 2%. However, the soluble salt content in magnesium electrolytic slag is as high as about 95%. Traditional stockpiling or landfilling methods no longer meet environmental regulations, making resource utilization an inevitable choice. The purpose of this invention is to provide a method and system that can efficiently treat magnesium electrolytic slag and achieve comprehensive recovery and utilization of its valuable components, thereby completely solving its environmental pollution risks and improving resource utilization.

[0026] The process route of this invention is clear, easy to operate, and has stable treatment effect, combining environmental and economic benefits. Therefore, this invention has good industrialization potential and broad application prospects in the field of resource utilization of magnesium electrolytic slag. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0028] In the description of this invention, terminology is used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly defined.

[0029] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the materials, reagents or instruments used, unless otherwise specified by the manufacturer, are all commercially available reagents and materials; the conditions not specified in the examples are all carried out according to conventional conditions or conditions recommended by the manufacturer. At the same time, the present invention does not limit the source of the raw materials used. Unless otherwise specified, the raw materials used in the present invention are all commercially available products in this technical field.

[0030] like Figure 1 As shown, the process of the present invention includes the following 7 steps.

[0031] (1) Pretreatment of magnesium electrolytic slag: The magnesium electrolytic slag is crushed to make the particle size less than 8 cm, and then large impurities are removed by screening. Crushing and screening can increase the contact area between the slag and the subsequent leaching agent, thereby improving the leaching efficiency.

[0032] (2) Water leaching treatment: The pretreated magnesium electrolytic slag is mixed with water at a liquid-to-solid ratio of 4-6:1 (volume-to-mass ratio), and leached for 1-3 h at a stirring rate of 150-200 r / min and a temperature of 25-40℃. During this process, soluble chlorides such as NaCl, FeCl3, AlCl3, MgCl2, and KCl dissolve into the solution to form the leachate, while insoluble components such as TiO2, Ca, and SiO2 remain in the leaching residue. After leaching, solid-liquid separation is performed using plate and frame filtration, and the leachate and leaching residue are collected separately.

[0033] (3) Leachate purification - iron and aluminum removal: Slowly add magnesium hydroxide powder to the leachate to adjust the pH of the solution to 3.5-4.5. At this time, Fe 3+ Al 3+ Ferrous hydroxide and aluminum hydroxide precipitates will form. To improve the precipitation separation effect, add 0.1-0.3% (mass fraction) of polyaluminum chloride (PAC) to the solution as a flocculant, stir the reaction for 30-60 minutes, let it stand to precipitate for 1-2 hours, and then separate the precipitate by centrifugation filtration (speed 2500-3000 r / min). The precipitate can be recovered as an industrial by-product.

[0034] (4) Leachate purification - calcium removal: Add an appropriate amount of oxalic acid solution to the filtrate after removing iron and aluminum, so that the calcium... 2+ The calcium oxalate precipitate is formed by reacting with oxalate ions. The concentration of oxalate ions in the solution is controlled to be 2-5 times that of calcium ions. After stirring for 1 hour, the calcium oxalate precipitate is removed by filtration.

[0035] (5) Separation and recovery of valuable components - recovery of magnesium: The filtrate after calcium removal is sent to an evaporation and concentration device and concentrated to 1 / 3-1 / 2 of the original volume under the conditions of vacuum degree of 0.06-0.08 MPa and temperature of 70-80℃. Then it is cooled to room temperature to allow magnesium chloride to crystallize out. Magnesium chloride hexahydrate crystals are obtained by centrifugation. After drying, the crystals can be sold as a product.

[0036] (6) Sodium and potassium separation and recovery: The mother liquor after separating magnesium chloride mainly contains NaCl and KCl, which are separated by evaporation crystallization-cold precipitation method. The mother liquor is further evaporated and concentrated until a large amount of crystals precipitate, and then cooled to 10-15℃. At this time, KCl crystallizes preferentially because its solubility is greatly affected by temperature. Potassium chloride crystals are obtained by centrifugation. The remaining mother liquor is evaporated, concentrated and cooled again to obtain sodium chloride crystals.

[0037] (7) Treatment of leaching residue: The leaching residue mainly contains TiO2, Ca, SiO2 and other components. After washing with water and drying, it can be used as an additive for building materials or further processed to recover elements such as titanium.

[0038] Example 1 Analysis revealed the following composition of the magnesium electrolytic slag, as shown in Table 1. The raw materials—magnesium hydroxide powder, oxalic acid, potassium chloride, sodium chloride, and magnesium chloride—were all analytical grade reagents.

[0039] The specific operating steps are as follows: Take 100 kg of magnesium electrolytic slag, crush it to a particle size of less than 5 cm using a jaw crusher, and then remove impurities by screening with a vibrating screen.

[0040] Water leaching treatment: The crushed and screened residue is placed in a leaching tank, 500 L of water is added, and leaching is carried out for 2 hours at a stirring rate of 180 r / min and a temperature of 30℃. After leaching, solid-liquid separation is performed using a plate and frame filter press to obtain leachate and leaching residue.

[0041] Leachate purification: Add magnesium hydroxide powder to the leachate, adjust the pH to 4.0, then add 0.5% polyaluminum chloride, stir and react for 60 minutes, let stand for 2 hours, and centrifuge and filter (3000 r / min) to remove iron and aluminum precipitates.

[0042] Add oxalic acid solution (oxalate ion concentration is 2.5 times that of calcium ion concentration) to the filtrate, stir and react for 1 hour, then filter to remove calcium oxalate precipitate.

[0043] Separation and recovery of valuable components: The filtrate was sent to an evaporation and concentration device and concentrated to 1 / 3 of the original volume under a vacuum of 0.07 MPa and a temperature of 75°C. After cooling to room temperature, it was centrifuged to obtain crude magnesium chloride hexahydrate crystals. Then, it was washed with a saturated magnesium chloride solution to remove impurities, centrifuged again, dried, and weighed to 31.5 kg with a purity of 99.21%.

[0044] The mother liquor after magnesium chloride separation was further evaporated and concentrated until a large amount of crystals precipitated. It was then cooled to 12°C and centrifuged to obtain crude potassium chloride crystals. These crystals were then washed with a saturated potassium chloride solution to remove impurities, and centrifuged and dried to obtain 41.23 kg of potassium chloride crystals with a purity of 99.32%. The remaining mother liquor was evaporated, concentrated, and cooled again to obtain crude sodium chloride crystals. These crystals were then washed with a saturated sodium chloride solution to remove impurities, and centrifuged and dried to obtain 16.64 kg of sodium chloride crystals with a purity of 99.53%.

[0045] Treatment of leaching residue: The leaching residue was washed three times with clean water and then dried at 105℃ to obtain approximately 8.3 kg of residue, which can be used as an additive for building materials.

[0046] By employing the core impurity removal pathways of "precise precipitation of iron and aluminum with magnesium hydroxide under precise pH control" and "selective deep calcium removal with oxalic acid," the key impurity issue related to product purity has been resolved. Experimental verification shows that the purity of the precipitated sodium chloride and potassium chloride crystals is consistently higher than 99%, and the product quality can directly meet the industrial-grade and even higher standards of use in multiple fields such as chemical, metallurgical, and agricultural industries.

[0047] Table 1. Composition data of magnesium electrolytic slag (wt%)

[0048] It should be noted that the specific embodiments described above are exemplary, and those skilled in the art can devise various solutions inspired by the disclosure of this invention. These solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents.

Claims

1. A method for resource utilization of magnesium electrolysis slag, characterized in that, Comprising the following steps: S1: Magnesium electrolysis slag with a volume-mass ratio of 4-6:1 is mixed with water, stirred at 150-200 r / min for 1-3 h, and then solid-liquid separation is performed, the leaching solution and leaching residue are collected; S2: Magnesium hydroxide powder is added to the leaching solution, the pH value of the solution is adjusted to 3.5-4.5, a flocculating agent is added to the leaching solution, and after stirring and reaction for 30-60 min, standing and precipitation for 1-2 h, the precipitate is separated, and the filtrate is obtained; S3: An appropriate amount of oxalic acid solution is added to the filtrate, and the precipitate is removed, wherein the concentration of oxalic acid ions in the mixed solution is 2-5 times that of calcium ions; S4: The filtrate obtained from S3 is evaporated and concentrated to 1 / 3-1 / 2 of the original volume, and the crystals and concentrated solution are separated; S5: The concentrated solution is sequentially precipitated with potassium chloride crystals and sodium chloride crystals by using the evaporation crystallization-cold separation method.

2. The magnesium electrolytic slag valorization process of claim 1, wherein, In S1, the particle size of the magnesium electrolysis slag particles is not greater than 8 cm.

3. The magnesium electrolytic slag valorization process of claim 1, wherein, In S1, the water immersion temperature is 25-40℃.

4. The magnesium electrolytic slag valorization process of claim 1, wherein, In S1, the stirring is performed at 30℃ and 180 r / min for 2 h, and then solid-liquid separation is performed.

5. The magnesium electrolytic slag valorization process of claim 1, wherein, In S2, the flocculating agent is polyaluminum chloride with a mass fraction of 0.1-0.3%.

6. The magnesium electrolytic slag valorization process of claim 1, wherein, In S2, the separation of the precipitate is performed by centrifugal filtration separation.

7. The magnesium electrolytic slag valorization process according to claim 6, characterized in that, The centrifugal speed is 2500-3000 r / min.

8. The magnesium electrolytic slag valorization process of claim 1, wherein, In S4, the evaporation and concentration conditions are: vacuum degree 0.06-0.08 MPa, temperature 70-80℃.

9. The magnesium electrolytic slag valorization process of claim 1, wherein, The leaching residue is dried after water washing, and TiO2, Ca and SiO2 are recovered.

10. A magnesium electrolysis slag resourceful treatment system for the magnesium electrolysis slag resourceful treatment method according to any one of claims 1 to 9, characterized in that, Comprising a magnesium electrolysis slag crushing unit for crushing and screening, a reaction pool unit, and a crystal reaction unit for crystal precipitation of the filtrate in the reaction pool, wherein, The reaction pool unit comprises a set of reaction pools arranged in sequence according to the process flow, which are provided with a first reaction pool for water immersion treatment, a second reaction pool for iron and aluminum treatment, and a third reaction pool for calcium and magnesium treatment.

Images