Method for removing aluminum from a nickel sulfate solution by fluoride-free phosphate gradient precipitation

Abstract

The application discloses a method for removing aluminum from a nickel sulfate solution by gradient precipitation of non-fluoride phosphate, and belongs to the technical field of hydrometallurgy. The method comprises the following steps: adjusting the pH of an aluminum-containing nickel sulfate solution to be less than or equal to 2.5 to perform pre-acidification treatment and form a homogeneous solution; slowly adjusting the pH of the homogeneous solution to be between 3.3 and 3.7, adding phosphate and a dispersant to perform dispersion, slowly adjusting the pH to be between 4.0 and 4.2, adding a crystal form directing agent to induce AlPO4 crystallization, and obtaining a crystallization solution; and sequentially performing aging, cooling and solid-liquid separation on the obtained crystallization solution to obtain a purified nickel sulfate solution. The method can realize high-selective precipitation and crystallization of Al 3+ in the nickel sulfate solution, avoid the co-crystallization loss of Ni 2+ , and the AlPO4 crystal formed has a compact structure and a large particle size, and is easy to be separated by a conventional method. The method also has the characteristics of simple operation, mild conditions and low cost, and can completely avoid the fluorine pollution and reduce the nickel loss caused by the traditional method.

Description

Technical Field

[0001] This invention relates to a method for removing impurities from nickel sulfate solution, and more particularly to a method for removing aluminum from nickel sulfate solution by phosphate gradient precipitation with no fluorine and low nickel loss, belonging to the field of hydrometallurgical technology. Background Technology

[0002] Nickel sulfate is a core raw material in fields such as positive electrode materials for power batteries (e.g., ternary materials, lithium iron phosphate), electroplating, and electroless plating. Its purity directly determines the performance of the final product. During nickel smelting (e.g., wet leaching, laterite nickel ore smelting) and nickel sulfate preparation, aluminum impurities readily exist as colloidal Al(OH)3 or ionic Al. 3+ If aluminum impurities are mixed into the solution and not effectively removed, they can lead to lattice defects in the cathode material, degradation of electrochemical performance, and problems such as scaling in production equipment and clogging of filtration systems. Therefore, the deep purification of aluminum impurities in nickel sulfate solution is a key technical challenge in the industry.

[0003] Existing aluminum removal technologies mainly include fluoride precipitation, hydroxide precipitation, and phosphate precipitation. Fluoride precipitation utilizes fluoride ions to form stable AlF3 with aluminum, exhibiting high selectivity. However, it generates highly corrosive fluoride-containing hazardous waste, exacerbating equipment wear and causing significant nickel co-precipitation losses (typically >0.8%), and is currently being gradually restricted by industry policies. Hydroxide precipitation generates Al(OH)3 by adjusting the pH, but its selectivity is poor; when the pH is close to neutral, Ni... 2+ Ni loss also begins to occur, exceeding 1%. Al(OH)3, being in a colloidal state, easily clogs the filter cloth, making it difficult to meet the requirements of deep purification processes. Traditional phosphate precipitation methods directly add phosphate to generate AlPO4 precipitate, but this easily forms amorphous colloidal precipitates, leading to problems such as incomplete aluminum removal, precipitate agglomeration, and excessive phosphorus residue. Furthermore, it does not address the technical challenges of nickel co-precipitation. Clearly, existing fluorine-free aluminum removal technologies still have significant technical deficiencies in selectivity, solid-liquid separation efficiency, and nickel loss control, necessitating the development of green, economical, and scalable new deep aluminum removal processes. Summary of the Invention

[0004] To address the technical shortcomings of existing nickel sulfate solution aluminum removal processes, such as fluoride pollution, incomplete aluminum removal, significant nickel loss, difficulty in precipitation separation, and high environmental pressure, this invention aims to provide a fluoride-free phosphate gradient precipitation method for aluminum removal from nickel sulfate solutions. This method, through a coupling technology of "gradient pH control - phosphate selective precipitation," can achieve highly selective precipitation and crystallization of aluminum ions in nickel sulfate solutions, avoiding co-crystallization loss of nickel ions. The resulting AlPO4 crystals have a dense structure and large particle size, making them easy to separate efficiently using conventional solid-liquid separation methods. This method also features simple operation, mild conditions, and low cost, and can completely avoid the technical problems of fluoride pollution and reduced nickel loss associated with traditional methods.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a method for removing aluminum from nickel sulfate solution by gradient precipitation of fluorophosphate, which includes the following steps: 1) The pH of the aluminum-containing nickel sulfate solution was adjusted to below 2.5 for pre-acidification treatment to form a homogeneous solution; 2) Slowly adjust the pH of the homogenized solution to 3.3-3.7, and add phosphate and dispersant to disperse it, thus obtaining a dispersion solution; 3) Slowly adjust the pH of the dispersion solution to 4.0~4.2, and add a crystal-directing agent to induce AlPO4 crystallization to obtain a crystallized solution; 4) The crystallization solution is subjected to aging, cooling and solid-liquid separation in sequence to obtain nickel sulfate purified solution.

[0006] The key to this invention's method of removing aluminum from nickel sulfate solution via gradient precipitation with phosphate lies in the coupling mechanism of "gradient pH control - selective precipitation with phosphate," combined with the synergistic effect of composite agents (phosphate, dispersant, and crystal-directing agent). This achieves highly efficient and selective removal of aluminum ions while simultaneously obtaining AlPO4 crystals with high crystallinity and large particle size, which can be efficiently separated using simple solid-liquid separation methods. First, a reaction environment preferentially precipitating aluminum ions is constructed by precisely controlling the pH value in stages. In the pre-acidification stage, the pH is controlled to ≤2.5 to dissolve colloidal Al(OH)3, allowing aluminum to be completely released as free Al. 3+ The system exists in the form of aluminum phosphate crystals. The first pH adjustment stage controls the pH within the range of 3.3 to 3.7. Within this pH range, the dispersant maintains good complexing ability, ensuring sufficient dispersion of aluminum ions and complete dissolution of phosphate in the solution system. This avoids local supersaturation and the formation of amorphous aluminum phosphate precipitates, promoting the formation of uniform aluminum phosphate crystal nuclei. The second pH adjustment stage controls the pH within the range of 4.0 to 4.2, providing the optimal pH environment for AlPO4 crystallization and inducing Al… 3+ With PO4 3-Directed formation of dense crystalline precipitates. Secondly, the combined use of phosphates, dispersants, and crystal-directing agents improves nucleation and crystallization effects. Among these, phosphates act as the primary precipitant, providing PO4. 3- With Al 3+ The reaction produces AlPO4 precipitate, utilizing Al 3+ With PO4 3- Its solubility product in a specific pH range (4.0~4.2) is much lower than that of Ni. 2+ With PO4 3- The solubility product property of Al 3+ Selective precipitation of Ni reduces 2+ The risk of co-precipitation exists, and dispersants, such as polyepoxysuccinic acid (PESA), are introduced during the precipitation of aluminum ions to complex Al. 3+ Formation of soluble complexes controls Al 3+ The release rate of Al 3+ Initially, uniform AlPO4 crystal nuclei are formed, and their carboxylic acid groups (-COOH) can be adsorbed onto the surface of the AlPO4 crystal nuclei through electrostatic adsorption, reducing the surface energy. This not only promotes the dispersion of AlPO4 crystal nuclei and avoids agglomeration, but also promotes the formation of dense α-type AlPO4 crystals. Simultaneously, the polyepoxysuccinic acid enhances the formation of AlPO4 crystals. 3+ The weak complexing effect (logK≈6.5) regulates Al 3+ The release rate can avoid the formation of amorphous precipitates and reduce nickel co-precipitation. The introduction of crystal-directing agents mainly improves the growth process of AlPO4 crystal nuclei. For example, carboxymethyl cellulose (CMC) is used as a crystal-directing agent. The growth rate of AlPO4 crystal faces is controlled by hydroxyl-directed adsorption, which promotes the preferential growth of crystals along the (001) direction to form plate-like crystals (length-to-diameter ratio > 5) and improves the air permeability of the filter cake.

[0007] As a preferred embodiment, sulfuric acid is used as a pH adjuster during the pre-acidification process. The addition rate of the pH adjuster is controlled to decrease the pH by ≤0.5 pH / min, with the final pH controlled at 2.2~2.4. Using sulfuric acid as a pH adjuster avoids the introduction of new anionic impurities, and controlling the pH within the preferred range ensures sufficient dissolution of colloidal aluminum while reducing the consumption of subsequent alkaline adjusters.

[0008] As a preferred embodiment, the pre-acidification process employs a stirring-assisted method, with a stirring rate of 100-200 rpm and a time of 5-10 min. Stirring promotes the dissolution of colloidal Al(OH)3 under acidic conditions, forming aluminum ions, which is beneficial for obtaining a homogeneous solution.

[0009] As a preferred approach, in step 2), the pH increase rate is controlled at 0.2~0.5 pH / min. If the pH increase rate is too fast, it will lead to excessively high local alkali concentration, making it easy for amorphous AlPO4 precipitates to form. If the pH increase rate is too slow, it will reduce efficiency and increase the difficulty of control.

[0010] As a preferred embodiment, the phosphate includes sodium phosphate. Phosphates primarily provide phosphate ions, and common water-soluble phosphates generally meet the application requirements. However, to avoid the introduction of impurity ions, sodium phosphate, which has a smaller impact on nickel sulfate recovery, is preferred.

[0011] As a preferred embodiment, the dispersant comprises polyepoxysuccinic acid. Compared to conventional dispersants, the preferred polyepoxysuccinic acid has a large number of carboxyl groups, which can complex Al... 3+ Achieving dispersion facilitates the formation of uniform AlPO4 crystal nuclei with phosphate ions, while avoiding the physical aggregation of AlPO4 crystal nuclei. Especially during AlPO4 crystal growth, polyepoxysuccinic acid can be adsorbed onto the surface of AlPO4 crystal nuclei through electrostatic interactions, reducing surface energy and promoting the formation of dense α-type crystals (particle size 20~50μm), rather than colloidal amorphous precipitates, thereby improving the sedimentation rate and filtration performance. Aluminum phosphate precipitate formation process: Al 3+ +PO4 3+ → AlPO4↓ (main reaction); PESA + Al 3+ →[Al-PESA] + (Competitive complexation, regulating the crystallization process), the weak complexing ability of PESA (logK≈6.5) enables Al... 3+ The release is gradual during precipitation, ensuring orderly crystallization of AlPO4 rather than instantaneous nucleation. Traditional dispersants (such as polyacrylic acid (PAA) and sodium hexametaphosphate) suffer from poor temperature resistance (PAA fails above 70℃), low dispersion efficiency, and easy phosphorus contamination. In contrast, polyepoxysuccinic acid (PESA) possesses multi-carboxyl complexation, steric hindrance, and excellent temperature resistance, enabling directional adsorption of Al in the early stages of nucleation. 3+ It may adsorb onto the surface of primary crystal nuclei, inhibiting the formation of amorphous precipitates and preventing the aggregation of AlPO4 crystal nuclei.

[0012] As a preferred embodiment, the dispersant is added at a rate of 0.1~0.5 g per liter of homogeneous solution. The dispersant primarily plays an optimal dispersing role in the early stages of crystal nucleus formation, ensuring uniform dispersion of the formed AlPO4 crystal nuclei and preventing crystal agglomeration. If the concentration of the dispersant is <0.1 g / L, it will result in poor dispersion of aluminum ions, which is not conducive to the formation of uniform crystal nuclei. If the concentration of the dispersant is >0.5 g / L, it will lead to excessive complexation of aluminum ions, which is not conducive to the generation and growth of crystal nuclei. The amount of dispersant added is further preferably 0.1~0.3 g / L.

[0013] As a preferred embodiment, the amount of phosphate added is measured according to a molar ratio of phosphate to aluminum ions of 1.2 to 1.5:1. This is achieved by controlling the PO4 content. 3- / Al 3+ The molar ratio can balance the aluminum removal efficiency and the residual amount of phosphate. The optimal amount of phosphate added can maximize the conversion of aluminum ions into AlPO4. However, if the amount of phosphate added is too high, it will lead to a large residual amount of phosphate and also cause the loss of nickel ions. Therefore, the amount of phosphate added needs to be controlled within an appropriate range.

[0014] As a preferred embodiment, the phosphate is added in the form of a dilute solution with a mass percentage concentration of 8% to 12%, and the addition is carried out in multiple batches. Adding the phosphate in multiple batches ensures its thorough and uniform dispersion, which is beneficial for the formation of uniform crystal nuclei and avoids the risk of nickel ion co-precipitation due to excessively high local phosphate concentrations. The preferred addition method is to add phosphate in 3 to 5 batches. For example, when adding phosphate in 3 batches, the addition ratio of the three batches is (25% to 35%) + (35% to 45%) + (25% to 35%), with an interval of approximately 5 minutes between each addition.

[0015] As a preferred approach, in step 3), the pH adjustment rate is controlled at 0.1~0.2 pH / min. A slower pH adjustment rate is beneficial for precise pH control, but if the pH increase is too slow, it will lead to insufficient crystallization kinetics. Conversely, if the pH increase rate is too fast, AlPO4 will precipitate too quickly, forming amorphous AlPO4 precipitates that are prone to co-precipitation with nickel ions. Slowly increasing the pH to the range of 4.0~4.2 is the optimal range for aluminophosphate crystallization, maximizing precipitation selectivity and crystal density.

[0016] As a preferred embodiment, the crystal-directing agent comprises carboxymethyl cellulose. The preferred carboxymethyl cellulose can be directionally adsorbed and coated onto a specific surface of AlPO4, which not only regulates the growth rate of AlPO4 crystal faces but also promotes preferential crystal growth along the (001) direction, forming plate-like crystals (aspect ratio > 5), thus improving the air permeability of the filter cake.

[0017] As a preferred embodiment, the amount of the crystal-directing agent added is 0.03~0.08g per liter of dispersion solution.

[0018] As a preferred embodiment, the aging conditions are: temperature of 60~70℃ and time of 30~40min; the preferred temperature range can promote the rapid growth of aluminum phosphate crystals and avoid nickel ion co-precipitation, and the preferred aging time ensures sufficient crystal growth and improves crystal density and particle size.

[0019] As a preferred embodiment, the cooling conditions are as follows: first, cool to 45-55°C at a cooling rate of 1-3°C / min, hold at that temperature for 5-10 minutes, and then cool to room temperature. Stepwise cooling can enhance crystallization stability and reduce the difficulty of subsequent crystal filtration and separation.

[0020] In this invention, the reaction process is promoted by stirring throughout the precipitation and aluminum removal process, with the stirring rate maintained at 150~200 rpm.

[0021] The precipitate obtained from solid-liquid separation in step 4) of this invention is mainly composed of AlPO4 (containing 32-35% P2O5), which can be converted into slow-release phosphate fertilizer after calcination at 550°C for 2 hours.

[0022] Ni in the nickel sulfate solution of the present invention 2+ Concentration of 100~120g / L, Al 3+ The concentration is 80~300ppm.

[0023] Compared with existing technologies, the beneficial technical effects of the present invention are as follows: 1) The phosphate precipitation system replaces the traditional fluorination method. The aluminum impurities are removed by selective precipitation of aluminophosphate, which completely avoids the problems of equipment corrosion, hazardous waste treatment and environmental pressure caused by fluorides. The precipitate is aluminophosphate, which is free of fluorine pollution and can be recycled. The cost of treating 1 ton of solution is reduced by about 30-50% compared with the traditional fluorination method, which meets the requirements of green production.

[0024] 2) An innovative "gradient pH control-phosphate selective precipitation" coupling technology is adopted. By precisely increasing the pH in stages, the system sequentially achieves aluminum colloid depolymerization, uniform diffusion of phosphate ions, and directional formation of dense crystalline AlPO4. Utilizing the preferential reaction of aluminum with phosphate ions within a specific pH range, aluminum is deeply precipitated while nickel remains unprecipitated. This technology enables highly efficient removal of aluminum (Al2PO4). 3+ <5ppm, while nickel loss rate <0.3%.

[0025] 3) Construct a composite reagent synergistic system of "precipitant-dispersant-crystal guide agent". Phosphate, as the main precipitant, provides phosphate ions. The dispersant plays a regulatory role in the early stage of crystal nucleus formation. The crystal guide agent directionally guides crystal growth. The synergistic effect of the three promotes the formation of dense, large-particle-size crystals of aluminum phosphate, which solves the technical pain points of precipitation agglomeration and difficult filtration in the traditional phosphate precipitation method. 4) Through constant temperature aging and step-cooling crystallization enhancement treatment, the density and stability of aluminophosphate crystals are further optimized, crystal dissolution is reduced, and solid-liquid separation efficiency is improved. At the same time, by precisely controlling the dosage of reagents and reaction conditions, the aluminum removal efficiency and phosphorus residue risk are balanced to ensure that the purified liquid meets production requirements. Detailed Implementation

[0026] To illustrate the technical content, objectives, and effects of the present invention in detail, the following embodiments are provided for further explanation. Example 1

[0027] Measure 1 L of nickel sulfate solution into the reactor, where Ni 2+ The concentration is 110 g / L, Al 3+ The concentration was 200 ppm. The pH of the solution was adjusted to 2.2 using 10% (w / w) dilute sulfuric acid at a rate of approximately 0.4 pH / min, with continuous stirring for 15 min for pre-acidification. Then, a second pH adjustment was performed using 20% ​​(w / w) sodium hydroxide solution, raising the pH to 3.5 at a rate of 0.3 pH / min. Simultaneously, 10% Na₂PO₄ solution (according to PO₄) was added. 3- / Al 3+ The total amount was calculated using a molar ratio of 1.3:1, and PESA solution (final concentration 0.2 g / L) was added. Na3PO4 solution was added in three portions (30% + 40% + 30%), with each addition 5 minutes apart. The pH was then adjusted twice using 20% ​​sodium hydroxide solution at a slow rate of 0.1 pH / min until it reached a precise pH of 4.1. CMC (final concentration 0.05 g / L) was then added. The system temperature was raised to and maintained at 65°C for 35 minutes. After the reaction was complete, the temperature was first lowered to 50°C at a program speed of 2°C / min, held for 10 minutes, and then allowed to cool naturally. Solid-liquid separation was then performed, and the composition of the filtrate was analyzed. Analysis showed that Al... 3+ The residue was 3.8 ppm, Ni 2+ The loss rate was 0.22%, the average particle size of the precipitate was D50≈25 μm, the turbidity of the filtrate (NTU) was 3, and the filtration time was 20 min.

[0028] Comparative Example 1

[0029] The operating procedures were the same as in Example 1, except that the rate of the secondary pH adjustment (from 3.5 to 4.1) was changed to 0.05 pH / min. The results showed that Al 3+ The residue was 5.1 ppm, Ni 2+ The loss rate was 0.24%, the average particle size of the precipitate was D50≈15 μm, the turbidity of the filtrate (NTU) was 8, and the filtration time was 28 min. 3+ The residual amount was higher than that in Example 1, indicating that slow adjustment may lead to insufficient reaction driving force and incomplete crystallization.

[0030] Example 2 The operating steps are the same as in Example 1, except that the secondary pH adjustment rate is changed to 0.2 pH / min. Results show that Al 3+ The residue was 2.9 ppm, Ni 2+ The loss rate was 0.23%, the average precipitate particle size D50 ≈ 28 μm, the filtrate turbidity (NTU) = 2, and the filtration time was 18 min. The nickel loss rate was comparable to that of Example 1, but the precipitate crystallization density was higher. Combining Examples 1 and 2, it is shown that the secondary adjustment rate range of 0.1–0.2 pH / min is the preferred choice.

[0031] Comparative Example 2

[0032] The operating steps are the same as in Example 1, except that PO4 is used. 3- / Al 3+ The molar ratio was changed to 1.0:1.0. The results showed that Al... 3+ The residue was 12.5 ppm, Ni 2+ The loss rate was 0.18%, the average particle size of the precipitate was D50≈18 μm, the turbidity of the filtrate (NTU) was 12, and the filtration time was 25 min. Aluminum removal was incomplete, but the nickel content in the precipitate was low (0.15%), indicating that insufficient precipitant was the limiting factor.

[0033] Example 3 The operating steps are the same as in Example 1, except that PO4 is used. 3- / Al 3+ The molar ratio was changed to 1.5:1. The results showed that Al... 3+ Residue was further reduced to 2.3 ppm, Ni 2+ The loss rate was 0.25%, the average particle size of the precipitate was D50≈30 μm, the turbidity of the filtrate (NTU) was 12, and the filtration time was 22 min. However, the PO4 content in the filtrate was high. 3- The residual concentration was 35 ppm, requiring further treatment. This indicates that increasing the ratio can enhance aluminum removal, but the issue of phosphate residue needs to be balanced, as excessive PO4... 3- Using Fe 3+ It is converted into FePO4 precipitate for removal.

[0034] Comparative Example 3

[0035] The operating procedures were the same as in Example 1, except that the PESA concentration was changed to 0.05 g / L. Results showed that the turbidity of the filtrate increased to 18, and the pressure filtration time increased by approximately 40% compared to Example 1. 3+ The residue was 4.5 ppm, Ni 2+ The loss rate was 0.26%, the average particle size of the precipitate was D50≈18 μm, the turbidity of the filtrate (NTU) was 18, and the filtration time was 28 min. This indicates that insufficient dispersant led to poor colloidal dispersion, which affected filtration.

[0036] Example 4 The operating steps were the same as in Example 1, except that the PESA concentration was changed to 0.4 g / L. Results showed that Al... 3+ The residue was 3.5 ppm, Ni 2+ The loss rate was 0.41%, the average particle size of the precipitate was D50≈22μm, the filtrate turbidity (NTU) was 5, and the filtration time was 24 min. Analysis suggests that excessive PESA significantly complexed with nickel, leading to the entrainment of a small amount of nickel ions by the precipitate. This also demonstrates that the optimal PESA concentration should be controlled within the range of 0.1~0.3 g / L.

[0037] Comparative Example 4

[0038] Aluminum removal was performed using the traditional sodium fluoride method. 1 L of the nickel sulfate solution from Example 1 was taken, and the pH was adjusted to 4.0 with 20% sodium hydroxide (by mass percentage). Sodium fluoride (according to F...) was then added. - / Al 3+ The reaction mixture was reacted at 60°C for 30 minutes (molar ratio 2:1) and then filtered. The Al content of the filtrate was measured. 3+ The residue was 4.2 ppm, Ni 2+ The loss rate was 0.85%, and the filter press time was 25 min. The precipitate is classified as fluorine-containing hazardous waste and requires special treatment.

[0039] Comparative Example 5 The operation steps are the same as in Example 1, but without adding PESA. The results show that Al 3+ The residue was as high as 32 ppm, the filtrate was severely turbid (NTU=65), and the filter cloth was quickly clogged by colloids during filtration, making it impossible to proceed normally.

[0040] Comparative Example 6

[0041] The operating steps were the same as in Example 1, but the gradient adjustment was omitted. The pH was directly and rapidly adjusted from 2.2 to 4.1 using 20% ​​sodium hydroxide (by mass percentage) (0.5 pH / min). Then, sodium phosphate (Na3PO4), polyepoxysuccinic acid (PESA), and carboxymethyl cellulose (CMC) were added, with the same amounts of all three agents as in Example 1. The results showed that Al 3+ The residual concentration was 15.7 ppm, the nickel content in the precipitate was 0.38%, an amorphous colloid was formed with a particle size D50 < 5 μm, the filtrate turbidity (NTU) was 25, and the filtration time was > 40 min. This demonstrates that rapid pH adjustment easily leads to local supersaturation, forming an amorphous precipitate that encapsulates nickel ions, thus reducing the selectivity for aluminum removal.

[0042] Comparative Example 7

[0043] The operating steps were the same as in Example 1, but without the addition of carboxymethyl cellulose (CMC). Results showed that Al 3+ The residue was 4.2 ppm, Ni 2+ The loss rate was 0.25%, the average particle size of the precipitate was D50≈18 μm, the turbidity of the filtrate (NTU) was 10, and the filtration time was 30 min.

[0044] As can be seen from the above examples and comparative examples, when the pH gradient is strictly controlled and phosphate, PESA, and CMC are used in combination, the removal effect of aluminum ions in nickel sulfate solution by the present invention can achieve: Al 3+ <4ppm, Ni loss <0.25%.

[0045] Compared with the traditional sodium fluoride method for aluminum removal, the method of this invention reduces nickel loss by more than 65% and eliminates the risk of fluorine pollution. From an economic perspective: [This method is suitable for treating Al...] 3+ Based on an initial concentration of 200 ppm nickel sulfate solution, the reagent cost (sodium phosphate, PESA, CMC, and acid-base adjuster) per ton of nickel sulfate solution using this invention is approximately RMB 40-50. In contrast, the traditional fluorination method (costing RMB 60-90 per ton, including hazardous waste treatment fees) significantly reduces costs. Furthermore, because the nickel loss rate is reduced by more than 0.5%, the saved metal value is even more considerable for high-value nickel sulfate products. In addition, since the equipment does not require resistance to hydrofluoric acid corrosion, ordinary stainless steel can be selected, resulting in lower investment and maintenance costs.

[0046] The above description is a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitution or modification made by those skilled in the art within the scope of the technology disclosed in the present invention based on the technical solution and inventive concept of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for removing aluminum from nickel sulfate solution using a gradient precipitation of fluoride-free phosphate, characterized in that: Includes the following steps: 1) The pH of the aluminum-containing nickel sulfate solution was adjusted to below 2.5 for pre-acidification treatment to form a homogeneous solution; 2) Slowly adjust the pH of the homogenized solution to 3.3-3.7, and add phosphate and dispersant to disperse, obtaining a dispersion solution; wherein, the pH adjustment rate is controlled at 0.2-0.5 pH / min; the dispersant is polyepoxysuccinic acid; the amount of phosphate added is measured according to a molar ratio of phosphate to aluminum ions of 1.2-1.5:1; the amount of dispersant added is 0.1-0.5 g per liter of homogenized solution; 3) Slowly adjust the pH of the dispersion solution to 4.0~4.2, and add a crystal-directing agent to induce AlPO4 crystallization to obtain a crystallization solution; wherein, the pH adjustment rate is controlled at 0.1~0.2 pH / min; the crystal-directing agent is carboxymethyl cellulose; 4) The crystallization solution is subjected to aging, cooling and solid-liquid separation in sequence to obtain nickel sulfate purified solution; the aging conditions are: temperature of 60~70℃ and time of 30~40 min; the cooling conditions are: first cool down to 45~55℃ at a cooling rate of 1~3℃ / min, keep warm for 5~10 min, and then cool to room temperature.

2. The method for removing aluminum from nickel sulfate solution by gradient precipitation of fluorophosphate according to claim 1, characterized in that: In the pre-acidification process, sulfuric acid is used as a pH adjuster. The addition rate of the pH adjuster is controlled to reduce the pH by ≤0.5 pH / min, and the final pH is controlled to be 2.2~2.

4.

3. The method for removing aluminum from nickel sulfate solution by gradient precipitation of fluoride-free phosphate according to claim 2, characterized in that: The pre-acidification process is carried out using a stirring-assisted method, with a stirring rate of 100~200 rpm and a time of 5~10 min.

4. The method for removing aluminum from nickel sulfate solution by gradient precipitation of fluoride-free phosphate according to claim 1, characterized in that: The phosphate includes sodium phosphate.

5. The method for removing aluminum from nickel sulfate solution by gradient precipitation of fluoride-free phosphate according to claim 1, characterized in that: The phosphate is added in the form of a dilute solution with a mass percentage concentration of 8% to 12%, and the addition method is carried out in multiple batches.

6. The method for removing aluminum from nickel sulfate solution by gradient precipitation of fluoride-free phosphate according to claim 1, characterized in that: The amount of the crystal-directing agent added is 0.03~0.08g per liter of dispersion solution.