Method for recovering high-purity magnesium product from magnesium-containing circulating water

Abstract

The invention discloses a method for recovering a high-purity magnesium product from magnesium-containing circulating water, which comprises the following steps: pretreating the magnesium-containing circulating water, filtering to remove impurities, and adjusting the water quality, so that the removal rate of heavy metal ions in the magnesium-containing circulating water is not less than 95% to obtain a pretreatment solution; sequentially adding a first precipitant and a second precipitant into the pretreatment liquid, stirring and reacting, separating out calcium-containing precipitate, and performing solid-liquid separation to obtain decalcified magnesium-containing liquid; carrying out reduced pressure evaporation on the decalcified magnesium-containing liquid until the magnesium ion concentration is 30-40g / L, and purifying to obtain a purified liquid with the total impurity ion concentration of less than or equal to 100ppm; and separating out MgSO4. 7H2O crystals from the purified liquid by adopting a gradient cooling method, and then purifying and storing the MgSO4. 7H2O crystals. The magnesium-containing circulating water is subjected to selective calcium removal, magnesium enrichment and purification and magnesium crystal separation, the magnesium ion recovery rate is larger than or equal to 98%, and the magnesium crystal with the purity larger than or equal to 99.9% is finally obtained.

Description

Technical Field

[0001] This application relates to the field of power plant circulating water treatment technology, and more specifically to a method for recovering high-purity magnesium products from magnesium-containing circulating water. Background Technology

[0002] With the deepening of industrial water conservation and emission reduction goals, the water-saving, energy-saving, and resource recovery potential of industrial circulating cooling water systems, which are major water consumers in power plants, is receiving increasing attention. After long-term circulation and concentration, power plant circulating water has extremely high concentrations of calcium and magnesium ions, and is accompanied by various impurities such as suspended solids (e.g., silt, corrosion products), carbonates, sulfates, and silicates. The harmful mechanisms and removal conditions of different impurities vary.

[0003] The discharge of such high-calcium and magnesium wastewater not only burdens the environment but also leads to a serious waste of magnesium resources. However, current mainstream treatment processes (such as chemical precipitation softening) suffer from large land area requirements and high sludge treatment loads, severely restricting their application and development. Currently, the magnesium ion concentration in concentrated power plant circulating water can reach as high as 1100 mg / L. In addition, large amounts of magnesium-containing wastewater are also generated from brine, mother liquor from lithium extraction in salt lakes, fertilizer plant wastewater, and rare earth industrial wastewater. The discharge of these wastewaters not only burdens the environment but also signifies a serious waste of magnesium resources. Magnesium and its compounds are important industrial raw materials, widely used in building materials, chemicals, pharmaceuticals, agriculture, and other fields. Therefore, how to economically and efficiently recover and produce high-purity, high-value-added magnesium compound crystals from such wastewater with magnesium ion concentrations as high as 1100 mg / L has become a key technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] This application aims to at least partially address one of the technical problems in related technologies. This application proposes a method for recovering high-purity magnesium products from magnesium-containing circulating water. The method involves selectively removing calcium from the magnesium-containing circulating water, then enriching and purifying magnesium, and finally separating the magnesium crystals. This achieves a magnesium ion recovery rate of ≥98% and ultimately obtains magnesium crystals with a purity of ≥99.9%. Therefore, embodiments of this application propose a method for recovering high-purity magnesium products from magnesium-containing circulating water, comprising the following steps: Magnesium-containing circulating water is pretreated by filtration to remove impurities and then the water quality is adjusted so that the removal rate of heavy metal ions in the magnesium-containing circulating water is not less than 95% to obtain the pretreated solution. After adding the first precipitant and the second precipitant to the pretreatment solution and stirring to react, calcium-containing precipitate is separated, and after solid-liquid separation, decalcified magnesium-containing solution is obtained. The total concentration of impurity ions in the purified solution obtained by evaporating the decalcified magnesium-containing solution under reduced pressure to a magnesium ion concentration of 30-40 g / L is ≤100 ppm. The purified solution was subjected to gradient cooling to precipitate MgSO4·7H2O crystals, which were then purified and preserved.

[0005] In some embodiments, the pretreatment process of the magnesium-containing circulating water includes solid-liquid separation and water quality equalization adjustment; the solid-liquid separation is used to remove particulate impurities; the water quality adjustment process includes adding an impurity removal agent to precipitate sulfates.

[0006] In some embodiments, the impurity removal agent includes sulfuric acid or sulfate and calcium chloride; the ratio of the total molar amount of the sulfuric acid and the sulfate to the total molar amount of barium and strontium salts in the magnesium-containing circulating water is 1.1-1.5:1; and the calcium chloride is 0.05-0.1% of the mass of the magnesium-containing circulating water.

[0007] In some embodiments, after the impurity removal agent is added, the reaction is carried out at 200-300 r / min for 30-60 min, and then plate and frame filtration is performed at a pressure of 0.8-1.0 MPa to obtain the pretreated liquid.

[0008] In some embodiments, the first precipitant is sodium sulfate, the reaction temperature is 50-60°C, the residence time of the sodium sulfate is 1-2 hours, and the stirring rate is 150-200 r / min. And / or, the sodium sulfate is added in batches, with the total amount added being 40%-90% of the theoretical reaction amount of sulfate.

[0009] In some embodiments, the second precipitant is oxalic acid or ammonium oxalate; the reaction temperature is 50-60℃, the time is 1-2h, and the stirring rate is 50-100r / min.

[0010] In some embodiments, the decalcified magnesium-containing liquid is evaporated under reduced pressure at a vacuum of -0.08 to -0.09 MPa and a temperature of 60-80°C.

[0011] In some embodiments, gradient cooling includes first lowering the purified solution to 40°C and holding it at that temperature for 30-40 minutes; then lowering it to 20°C and holding it at that temperature for 120-180 minutes.

[0012] In some embodiments, the purified solution is subjected to gradient cooling to precipitate MgSO4·7H2O and separate solid and liquid to obtain a first crystal; then the first crystal is washed and centrifuged and filtered to obtain a second crystal; the second crystal is vacuum dried to obtain MgSO4·7H2O crystals, and the water content of the MgSO4·7H2O crystals is ≤0.5%.

[0013] In some embodiments, the first crystal is washed 2-3 times with ice water at a temperature of 0-5°C; And / or, the second crystal is first dried at 40-50℃ and vacuum degree -0.08MPa for 2-3 hours; then heated to 60-70℃ and dried for 4-6 hours.

[0014] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0015] Figure 1 This application presents a method for recovering high-purity magnesium products from magnesium-containing circulating water. Detailed Implementation

[0016] The embodiments of this application are described in detail below, with examples of these embodiments illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0017] Therefore, embodiments of this application propose a method for recovering high-purity magnesium products from magnesium-containing circulating water, such as... Figure 1 This includes the following steps: S1: After pretreatment and filtration to remove impurities, the magnesium-containing circulating water is adjusted to ensure that the removal rate of heavy metal ions in the magnesium-containing circulating water is not less than 95% to obtain the pretreated solution. S2: Add the first precipitant and the second precipitant to the pretreatment solution in sequence, stir and react to separate the calcium-containing precipitate, and obtain the decalcified magnesium-containing solution after solid-liquid separation; S3: The total concentration of impurity ions in the purified solution obtained after evaporating the decalcified magnesium-containing liquid under reduced pressure to a magnesium ion concentration of 30-40 g / L is ≤100 ppm. S4: The purified solution was subjected to gradient cooling to precipitate MgSO4·7H2O crystals, which were then purified and preserved.

[0018] In step S1, the pretreatment process for magnesium-containing circulating water includes solid-liquid separation and water quality equalization. Solid-liquid separation is used to remove particulate impurities, ensuring that the removal rate of heavy metal ions (barium ions, strontium ions) in the magnesium-containing circulating water is not less than 95%, resulting in a pretreated solution. For example, magnesium-containing circulating water uses a grid assembly for impurity filtration. The initial magnesium-containing circulating water contains large particulate impurities such as hair and silt, large suspended solids, gravel, as well as suspended solids such as grease, scum, and some colloids. Before selective treatment of calcium and magnesium ions in the magnesium-containing circulating water, pretreatment is required to remove macroscopic impurities, creating optimal conditions for the subsequent efficient, economical, and stable ion recovery unit.

[0019] In this embodiment, the bar grid assembly includes a first grid, a second grid, and a multi-media filter layer arranged sequentially upstream and downstream; wherein the spacing between the first grid, the second grid, and the multi-media filter layer decreases sequentially. In other words, the bar grid assembly of this application includes a first grid, a second grid, and a multi-media filter layer, and the spacing between the first grid is greater than that between the second grid, and the spacing between the second grid is greater than that between the multi-media filter layer. According to the flow direction of the magnesium-containing circulating water, it passes through the first grid, the second grid, and the multi-media filter layer sequentially. As can be seen from the above, the magnesium-containing circulating water first passes through the first grid with a larger spacing to intercept large particulate impurities such as hair and silt; after being filtered by the first grid, the magnesium-containing circulating water then passes through the second grid, which further filters and retains particulate impurities, suspended particulate matter, scum, and some colloids. After being filtered by the second grid, the magnesium-containing circulating water then passes through the multi-media filter layer with the smallest spacing, which further removes the smallest particulate impurities, thereby achieving distributed and phased impurity removal of the magnesium-containing circulating water.

[0020] For example, the spacing of the first grid is 5-20 mm and the spacing of the second grid is 1-10 mm. In some embodiments, the multi-media filter layer includes sand particles with a particle size of 0.5-1.2 mm, coal slag with a particle size of 2.0-5.0 mm and stones with a particle size of 1-2 mm, wherein the magnesium-containing circulating water passes through the coal slag, sand particles and stones in sequence.

[0021] In particular, the multi-media filter layer of this application is composed of cinders, stones, and sand, wherein the cinders, stones, and sand are arranged sequentially as a cinder layer, a stone layer, and a sand layer. The particle size of the cinders is 2.0-5.0 mm, the particle size of the stones is 1-2 mm, and the particle size of the sand is 0.5-1.2 mm. The density of the cinders is less than that of the sand, and the density of the sand is less than that of the stones. For example, the density of the cinders is 0.6-1.0 g / cm³, the density of the sand is 2.0-2.5 g / cm³, and the density of the stones is not less than 2.65 g / cm³. According to the flow direction of the magnesium-containing circulating water, the magnesium-containing circulating water passes sequentially through the cinder layer, the sand layer, and the stone layer.

[0022] The cinder layer has the lowest density and the coarsest particles. As the first line of defense for magnesium-containing circulating water as it flows through the multi-media filtration layer, it can intercept most of the larger suspended solids in the magnesium-containing circulating water. Because of the coarse particles and large pores of the cinder, contaminants in the magnesium-containing circulating water can penetrate to deeper areas, resulting in high dirt-holding capacity and preventing rapid surface clogging. The sand layer, located downstream of the cinder layer, is used to intercept medium-sized suspended solids. Since the cinder layer has already removed large particles, the flow velocity of wastewater is more stable when it reaches this layer, and the smaller pores provided by the sand layer allow for further fine filtration. The stone layer, as the final barrier, ensures that its high density allows it to remain stably at the bottom after backwashing.

[0023] The thickness ratio of the cinder, stone, and sand layers in the example is (10-20):(5-10):(15-20), for example, (10, 12, 13, 15, 16, 18, 20):(5, 6, 8, 9 or 10):(15, 16, 18, 19, 20). Magnesium-containing circulating water passes sequentially through the cinder and sand layers. As the magnesium-containing circulating water flows from top to bottom, the pore size of the filter media decreases, allowing suspended solids to be trapped layer by layer throughout the multi-media filter layer according to their size, rather than simply accumulating on the surface. This greatly improves the dirt-holding capacity of the entire multi-media filter layer and extends the filtration cycle.

[0024] Secondly, due to the significant density differences among the three materials—coal slag, sand, and stone—the filter residue is completely flushed up and fluidized during backwashing (water or combined air-water flushing). After backwashing, the densest stones settle the fastest and fall to the bottom layer; the medium-density sand falls to the middle layer; and the lightest coal slag falls to the top layer last, making it easier to remove and clean the contaminants, resulting in a better backwashing effect. Finally, deep filtration through a multi-media filter layer slows down the increase in head loss, achieving deep filtration and efficient interception of contaminants, as well as a highly efficient, stable, and easy-to-maintain filtration effect. Then, a 5μm filter membrane is used for precision filtration to prevent impurities from accumulating and settling in subsequent reactions, thus avoiding affecting the calcium-magnesium separation effect. The turbidity of the magnesium-containing circulating water after filtration is ≤5 NTU.

[0025] The water quality adjustment process involves adding a purifying agent to the filtered magnesium-containing circulating water. This purifying agent includes sulfuric acid or sulfates and calcium chloride. In other words, the purifying agent includes sulfuric acid and calcium chloride, or sulfates and calcium chloride; the sulfates include sodium sulfate, etc. The ratio of the total molar amount of sulfate ions in the sulfuric acid and sulfates to the total molar amount of barium and strontium ions in the magnesium-containing circulating water is 1.1-1.5:1; the calcium chloride content is 0.05-0.1% of the mass of the magnesium-containing circulating water. In other words, the molar ratio of sulfuric acid or sulfates to barium and strontium salts in the water is 1.1-1.5:1; the calcium chloride content is 0.05-0.1% of the mass of the magnesium-containing circulating water. After the purifying agent is added, the reaction is carried out at room temperature for 30-60 minutes at 200-300 rpm. At this time, the Ba... 2+ 、Sr 2+ Heavy metal ions form insoluble sulfate precipitates, and suspended matter aggregates and settles under the action of impurity removal agents.

[0026] In this embodiment, the reaction system with the added impurity removal agent is subjected to plate and frame filter press filtration at a pressure of 0.8-1.0 MPa to obtain a pretreated liquid. That is, after the impurity removal agent reaction is completed, the liquid is filtered through a plate and frame filter press at a pressure of 0.8-1.0 MPa to obtain a clear pretreated liquid with impurities removed. The filter residue is collected and disposed of separately. The removal rate of heavy metal ions in magnesium-containing circulating water is not less than 95%.

[0027] In step S2, the first precipitant and the second precipitant are added sequentially to the pretreatment liquid obtained in step S1, and the mixture is stirred and reacted to separate the calcium-containing precipitate. After solid-liquid separation, a decalcified magnesium-containing liquid is obtained. In other words, the pretreatment liquid obtained in step S1 is fed into the first-stage reactor of a multi-stage series reaction device. A bottom liquid is added to each stage of the reactor. The bottom liquid is one or more of water, subsequent process treatment liquid, or circulating water, and the amount added is 80% of the reactor volume. At the same time, the pretreatment liquid and the first precipitant, such as sodium sulfate slurry, are added to the first-stage reactor in parallel flow. The sodium sulfate addition ratio in the first-stage reactor is controlled to be 40%-90% of the theoretical reaction amount of sulfate ions to carry out the preliminary decalcification reaction.

[0028] The slurry after the initial decalcification reaction enters the 2~(n-1) stage reactors (n is the total number of stages, 2≤n≤10) through the overflow port. Sodium sulfate is added to this stage reactor (the addition ratio is 0-60% of the theoretical amount of sulfate, and the total amount of sodium sulfate added to each stage is 60%-100% of the theoretical amount) to continue the reaction and fully convert calcium ions into calcium sulfate precipitate. The reaction temperature of each stage reactor is controlled at 50-60℃, the sodium sulfate residence time is 1-2 hours, and the stirring rate is 150-200 r / min.

[0029] The slurry after initial decalcification enters the nth stage reactor, where a second precipitant, oxalic acid or ammonium oxalate, is added. The reaction temperature is 50-60℃, the reaction time is 1-2 hours, and the stirring rate is 50-100 r / min. After stirring and reaction, calcium-containing precipitates are separated. At this point, residual calcium ions combine with oxalate ions to form insoluble calcium oxalate precipitates. Magnesium ions hardly participate in the reaction, achieving efficient separation of calcium and magnesium. After solid-liquid separation of calcium sulfate and calcium oxalate precipitates by a sieving device or hydrocyclone, a magnesium-rich decalcified magnesium-containing solution is obtained. The calcium content in the decalcified magnesium-containing solution can be reduced to no more than 10 ppm, meeting the requirements for the preparation of high-purity magnesium crystals.

[0030] In step S3, the magnesium-rich decalcified magnesium-containing solution obtained in step S2 needs further concentration and purification to ensure the purity of the final magnesium crystals. Therefore, the decalcified magnesium-containing solution is transferred to a vacuum evaporator and evaporated under reduced pressure at a vacuum of -0.08 to -0.09 MPa and a temperature of 60-80°C; example temperatures are 60°C, 65°C, 70°C, 75°C, or 80°C. High temperatures are avoided to prevent magnesium ion hydrolysis, which would reduce the purity of the later crystals. Evaporation is stopped when the magnesium ion concentration reaches 30-40 g / L, i.e., near saturation, and then purification is performed. In this embodiment, reduced pressure evaporation can shorten the concentration time, reduce energy consumption, and prevent impurities from volatilizing into the concentrate.

[0031] In some embodiments, a concentrated solution with a magnesium ion concentration of 30-40 g / L is purified sequentially by passing it through a cation exchange resin column and an anion exchange resin column. Specifically, the concentrated solution with a magnesium ion concentration of 30-40 g / L is passed through a cation exchange resin column, such as a strongly acidic cation exchange resin (e.g., type 001×7), to remove trace amounts of cationic impurities such as calcium and iron ions. Then, it is passed through an anion exchange resin column, for example, a strongly basic anion exchange resin (e.g., type 201×7), to remove trace amounts of anionic impurities such as sulfate and fluoride ions. After this purification process, the total concentration of impurity ions in the purified solution is ≤100 ppm, and the purity of magnesium ions is further improved.

[0032] The purified solution has a magnesium ion concentration of 30-40 g / L and a total impurity ion concentration of ≤100 ppm. A gradient cooling method is used to precipitate MgSO4·7H2O crystals. The gradient cooling process includes first transferring the purified solution into a cooling crystallizer, then lowering the temperature of the purified solution to 40℃ and holding it at that temperature for 30-40 min; then lowering it to 20℃ and holding it at that temperature for 120-180 min. This promotes the slow precipitation of MgSO4·7H2O crystals. The slow cooling can reduce the inclusion of impurities in the crystals and improve the purity.

[0033] For example, the temperature of the purification solution is first lowered from 60℃ to 40℃ and held for 30 min, 31 min, 32 min, 33 min, 34 min, 35 min, 36 min, 37 min, 38 min, 39 min, or 40 min; then lowered to 20℃ and held for 120 min, 130 min, 140 min, 150 min, 160 min, 170 min, or 180 min. This promotes the slow precipitation of MgSO4·7H2O crystals. Slow cooling allows for slow and controllable crystal growth, enabling solute molecules / atoms to arrange themselves more orderly in the crystal lattice, reducing the probability of mother liquor or impurities being encapsulated in the crystal, and improving purity. Furthermore, simple rapid cooling can easily lead to excessive instantaneous nucleation, a sudden drop in system supersaturation, and subsequent slow and uncontrollable growth. The gradient cooling method of this application makes nucleation events more gentle and predictable, avoiding explosive nucleation, thereby achieving separation and optimized control of the two competing processes of crystal nucleation and growth, ultimately obtaining a product that meets specific size, morphology, crystal form, and purity requirements.

[0034] In some embodiments, the purification solution is subjected to gradient cooling to precipitate MgSO4·7H2O and separate solid and liquid to obtain the first crystal; then the first crystal is washed and centrifuged and filtered to obtain the second crystal; the second crystal is vacuum dried to obtain MgSO4·7H2O crystal, and the water content of the MgSO4·7H2O crystal is ≤0.5%.

[0035] In this embodiment, a gradient cooling method is used to precipitate MgSO4·7H2O and separate the solid and liquid to obtain the first crystal. Centrifugal filtration is then employed at a speed of 3000-4000 r / min for 10-15 min. The specific speeds are 3000 r / min, 3100 r / min, 3200 r / min, 3300 r / min, 3400 r / min, 3500 r / min, 3600 r / min, 3700 r / min, 3800 r / min, or 4000 r / min; the specific times are 10 min, 11 min, 12 min, 13 min, or 15 min. This application utilizes centrifugal force of 3000-4000 r / min to quickly and thoroughly remove the fine MgSO4·7H2O crystals from the mother liquor. This method is also very effective for crystals with slow settling rates, as described in this embodiment. The powerful centrifugal force not only separates the crystals but also significantly removes the mother liquor trapped between the crystal particles. The filter cake obtained after centrifugation is typically drier, with a significantly lower moisture content (or solvent residue) than that obtained by atmospheric pressure filtration or vacuum filtration. This process is highly advantageous for subsequent drying steps, saving considerable drying time and energy. Furthermore, compared to gravity filtration or vacuum filtration, which requires hours or even longer, this application utilizes centrifugal filtration to reduce separation time from "hours" to "minutes," greatly improving the separation efficiency of the first crystal.

[0036] For example, the obtained first crystal is washed, such as by washing the first crystal 2-3 times with ice water at a temperature of 0-5°C. Example: The first crystal is washed 2-3 times with ice water at temperatures of 0°C, 1°C, 2°C, 3°C, 4°C, or 5°C to remove impurities adsorbed on the surface of the first crystal. During the brief contact during washing, ice water at 0-5°C minimizes the solubility of the first crystal, thus minimizing the loss of target product due to dissolution by the ice water, which improves the yield of the first crystal. Furthermore, during washing, fine crystals have higher solubility than large crystals. Using room temperature or hot water would accelerate the dissolution of small crystals and recrystallization on the surface of large crystals, leading to changes in crystal size distribution. Therefore, this application maintains the crystal form of the first crystal, preventing transformation to other crystal forms during washing. Finally, ice water has a slightly higher viscosity than warm water, maintaining a lower viscosity and better permeability, making it easier to enter the gaps in the first crystal to remove impurities and maintain the chemical stability of the first crystal. In some embodiments, the ice water used to wash the first crystal is recycled to improve the recovery rate of magnesium ions.

[0037] In this embodiment, after washing the first crystal, centrifugation is performed to obtain the second crystal at a speed of 3000-4000 r / min for 10-15 min. This application utilizes centrifugal force of 3000-4000 r / min to quickly and thoroughly separate the fine MgSO4·7H2O crystals from the mother liquor. This is also very effective for crystals with slow settling rates, as described in this embodiment. The powerful centrifugal force not only separates the crystals but also significantly removes mother liquor trapped between the crystal particles. This application uses centrifugal filtration to reduce the separation time from "hours" to "minutes," greatly improving the washing efficiency of the second crystal and enabling the recycling of the mother liquor for reuse as much as possible.

[0038] Finally, the second crystal is vacuum dried by transferring it to a vacuum drying oven and using segmented drying. For example, the second crystal is first dried at 40-50℃ and a vacuum of -0.08MPa for 2-3 hours; then the temperature is raised to 60-70℃ and dried for 4-6 hours. The second crystal is dried in segments: first, it is dried at 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, or 50℃ under a vacuum of -0.08MPa for 2 or 3 hours to remove surface moisture; then the temperature is raised to 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, or 70℃ and dried for 4-6 hours. In some embodiments, excessively high drying temperatures are avoided to prevent the MgSO4·7H2O crystals from losing their water of crystallization.

[0039] This application employs EDTA complexometric titration to determine the purity of magnesium ions in magnesium crystals and atomic absorption spectrophotometry to determine the content of calcium ions and other heavy metal impurities. The magnesium ion recovery rate is calculated as (mass of magnesium ions in the actual obtained magnesium crystals / mass of initial magnesium ions in the wastewater) × 100%, with a target recovery rate ≥ 98% and crystal purity ≥ 99.9%. This application is environmentally friendly: it utilizes low-energy-consumption processes such as reduced-pressure evaporation and cooling crystallization, avoiding harmful gas emissions, and ensuring proper treatment of filter residue, thus contributing to environmental friendliness.

[0040] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0041] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between components; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0043] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0044] In this application, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0045] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for recovering high-purity magnesium products from magnesium-containing circulating water, characterized in that, Includes the following steps: Magnesium-containing circulating water is pretreated by filtration to remove impurities and then the water quality is adjusted so that the removal rate of heavy metal ions in the magnesium-containing circulating water is not less than 95% to obtain the pretreated solution. After adding the first precipitant and the second precipitant to the pretreatment solution and stirring to react, calcium-containing precipitate is separated, and after solid-liquid separation, decalcified magnesium-containing solution is obtained. The total concentration of impurity ions in the purified solution obtained by evaporating the decalcified magnesium-containing solution under reduced pressure to a magnesium ion concentration of 30-40 g / L is ≤100 ppm. The purified solution was subjected to gradient cooling to precipitate MgSO4·7H2O crystals, which were then purified and preserved.

2. The method according to claim 1, characterized in that, The pretreatment process of the magnesium-containing circulating water includes solid-liquid separation and water quality equalization adjustment; the solid-liquid separation is used to remove particulate impurities; the water quality adjustment process includes adding an impurity removal agent to precipitate sulfate.

3. The method according to claim 2, characterized in that, The impurity removal agent includes sulfuric acid or sulfate and calcium chloride; the ratio of the total molar amount of sulfuric acid and sulfate to the total molar amount of barium and strontium salts in the magnesium-containing circulating water is 1.1-1.5:1; the calcium chloride is 0.05-0.1% of the mass of the magnesium-containing circulating water.

4. The method according to claim 2 or 3, characterized in that, After the impurity removal agent is added, the mixture is reacted at 200-300 r / min for 30-60 min, and then plate and frame filtration is performed at a pressure of 0.8-1.0 MPa to obtain the pretreated liquid.

5. The method according to claim 4, characterized in that, The first precipitant is sodium sulfate, the reaction temperature is 50-60℃, the residence time of the sodium sulfate is 1-2h, and the stirring rate is 150-200r / min; And / or, the sodium sulfate is added in batches, with the total amount added being 40%-90% of the theoretical reaction amount of sulfate.

6. The method according to claim 4, characterized in that, The second precipitant is oxalic acid or ammonium oxalate; the reaction temperature is 50-60℃, the time is 1-2h, and the stirring rate is 50-100r / min.

7. The method according to claim 6, characterized in that, The decalcified magnesium-containing liquid is evaporated under reduced pressure at a vacuum of -0.08 to -0.09 MPa and a temperature of 60-80℃.

8. The method according to claim 4, characterized in that, Gradient cooling involves first lowering the purified solution to 40°C and holding it at that temperature for 30-40 minutes; then lowering it to 20°C and holding it at that temperature for 120-180 minutes.

9. The method according to claim 8, characterized in that, The purified solution is subjected to gradient cooling to precipitate MgSO4·7H2O and separate solid and liquid to obtain the first crystal; then the first crystal is washed and centrifuged and filtered to obtain the second crystal; the second crystal is vacuum dried to obtain MgSO4·7H2O crystal, and the water content of the MgSO4·7H2O crystal is ≤0.5%.

10. The method according to claim 9, characterized in that, The first crystal is washed 2-3 times with ice water at a temperature of 0-5℃; And / or, the second crystal is first dried at 40-50°C and under a vacuum of -0.08MPa for 2-3 hours; Then raise the temperature to 60-70℃ and dry for 4-6 hours.

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