Process for preparing battery-grade nickel sulfate by stripping crystallization

Abstract

The application belongs to the technical field of lithium battery recycling, and particularly relates to a process for preparing battery-grade nickel sulfate by back extraction and crystallization, which comprises four steps of extraction, washing and impurity removal, back extraction and standing and aging. In the step of extraction, a nickel-containing loaded organic phase is obtained. In the step of washing and impurity removal, the loaded organic phase is washed. In the step of back extraction, the washed loaded organic phase is back extracted by using a saturated nickel sulfate solution in an S1 tank. Fine crystals are directly precipitated. The S1 tank comprises a crystallization tank, a spiral agitator is arranged in the crystallization tank, a coating is arranged on the inner wall of the crystallization tank, a phase-controlled array ultrasonic probe for on-line detection of crystal particle size distribution is arranged on the outer wall of the crystallization tank, and the rotation speed of the spiral agitator and the discharge frequency of the crystallization tank are controlled according to the average particle size of the crystals in the crystallization tank. After the fine crystals are grown by standing in the step of standing and aging, solid-liquid separation is performed, and type I battery-grade nickel sulfate is obtained. The process of the application omits the step of evaporation and concentration, and greatly reduces the equipment cost and energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery recycling technology, specifically relating to a process for preparing battery-grade nickel sulfate by back-extraction and crystallization. Background Technology

[0002] With the explosive growth of the new energy vehicle industry, higher requirements are being placed on the purity and preparation efficiency of battery-grade nickel sulfate during the recycling of ternary lithium batteries. Hydrometallurgy, as the mainstream recycling process, typically requires multiple steps to prepare nickel sulfate after leaching, impurity removal, and extraction, including back-extraction, evaporation and concentration, and crystallization. However, the nickel sulfate solution obtained from back-extraction has a low concentration and fluctuates greatly in impurity composition. Subsequent evaporation and recrystallization are necessary to reach battery-grade standards. This results in a lengthy process, high energy consumption, difficulty in controlling sodium ion introduction, and problems such as uneven particle size and impurity encapsulation in the crystallized product, affecting product consistency and the performance of subsequent cathode materials.

[0003] While existing nickel sulfate preparation processes each have their own characteristics, they all have certain limitations. The traditional multi-step purification-evaporation crystallization method is mature, but it is lengthy, costly, and prone to introducing sodium ion impurities. Chinese invention publication CN121759713A discloses a method for separating nickel and cobalt from a nickel-containing solution and preparing nickel salts, employing an extraction-concentration crystallization method. This method achieves good separation of nickel and cobalt, but the chelating agent and extractant are expensive, and a separate chelation and impurity removal unit is still required, meaning the process is not completely simplified. Chinese invention publication CN108441649A discloses a method for extracting nickel from nickel sulfide materials through chemical precipitation. This method treats nickel sulfide raw materials through chemical precipitation, offering strong raw material adaptability, but it consumes a large amount of precipitant and requires multiple subsequent purification and evaporation crystallization steps, resulting in high overall energy consumption. Chinese invention patent CN111411228A discloses a method for extracting and separating nickel, cobalt, and magnesium from a mixed solution of nickel, cobalt, and magnesium, achieving efficient separation of these components. However, its extraction system is complex, requiring large amounts of extractant and diluent, resulting in a low solution concentration after back-extraction. It still necessitates evaporation, concentration, and crystallization steps, making the process lengthy. Furthermore, while resin adsorption and solvent displacement crystallization methods are effective in specific scenarios, they face limitations in adsorption capacity and solvent recovery, respectively, making it difficult to balance economic efficiency with scalability requirements. Therefore, it is necessary to develop a simpler process for preparing battery-grade nickel sulfate with lower recovery costs. Summary of the Invention

[0004] The purpose of this invention is to provide a process for preparing battery-grade nickel sulfate by back-extraction crystallization, so as to solve the problems of lengthy process flow, excessive sodium content and high cost of traditional process.

[0005] To achieve the above objectives, the present invention provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization, comprising an extraction step, a washing and impurity removal step, a back-extraction step, and a settling and aging step. In the extraction step, the leachate after removing impurities and extracting cobalt from waste lithium battery cathode materials is used as raw material, and nickel is extracted by extracting the organic phase to obtain the supported organic phase; In the washing and impurity removal step, the supported organic phase is washed with acid. In the back-extraction step, a saturated nickel sulfate solution is used in tank S1 to back-extract the washed loaded organic phase, directly precipitating fine crystals. Tank S1 includes an extraction tank and a crystallization tank, with the aqueous phase outlet of the crystallization tank directly connected to the extraction tank. The crystallization tank is inverted conical in shape, and its inner wall is coated with a nano-ceramic particle-modified Teflon coating or a PTFE-nano SiO2 composite coating. A spiral stirrer is installed inside the crystallization tank. The side of the spiral stirrer facing the extraction tank is a hydrophilic-hydrophobic contrast surface, while the side facing away from the extraction tank is a hydrophobic surface. The hydrophilic-hydrophobic contrast surface is composed of alternating hydrophilic nano-titanium dioxide coatings and hydrophobic fluorosilane coatings, while the hydrophobic surface is composed of hydrophobic fluorosilane coatings. A phased array ultrasonic probe for online detection of crystal particle size distribution is installed on the outer wall of the crystallization tank. When the average crystal particle size in the crystallization tank is <150 μm, the speed of the spiral stirrer is increased to 200 rpm; when the average crystal particle size in the crystallization tank is ≥220 μm, the speed of the spiral stirrer is decreased to 150 rpm. rpm; when the average crystal size in the crystallization tank is stable at 150-220μm, the spiral stirrer maintains the current speed. In the static aging process, the fine crystals undergo solid-liquid separation after static crystal growth to obtain Type I battery-grade nickel sulfate.

[0006] Optionally, in the back-extraction step, the outlet of the crystallization tank is equipped with a shut-off valve for controlling the discharge. When the average particle size of the crystals in the crystallization tank is ≥220 μm, the speed of the spiral stirrer is reduced to 150 rpm while the discharge frequency of the crystallization tank is increased.

[0007] Optionally, in the back-extraction step, the width of the hydrophilic nano-titanium dioxide coating on the hydrophilic-hydrophobic contrast surface is 1.5–2.0 cm, and the width of the hydrophobic fluorosilane coating is 0.6–0.8 cm.

[0008] Optionally, in the back-extraction step, the top radius of the crystallization tank is H, the horizontal distance between the top edge of the stirring blade of the spiral stirrer and the inner wall of the crystallization tank is L, and the radius difference between adjacent blades of the spiral stirrer is h. The constraints of H, L and h are as follows: 0 < L ≤ 10 cm, 0 < h ≤ (H / 10).

[0009] Optionally, in the extraction step, 3 to 5 stages of countercurrent extraction are used, the extraction equilibrium pH is 5.5 to 6, and the extraction temperature is 20 to 30°C.

[0010] Optionally, in the washing and impurity removal step, a 2-3 stage countercurrent washing is adopted, and the acid solution is a nickel sulfate solution containing sulfuric acid, wherein the concentration of nickel sulfate in the nickel sulfate solution is 0.1-0.5 mol / L and the concentration of sulfuric acid is 3-10 g / L.

[0011] Optionally, in the back-extraction step, the saturated nickel sulfate solution contains 2–6 mol / L sulfuric acid, and the back-extraction temperature is 20–30°C.

[0012] Optionally, the aging time in the static aging step is 5 to 12 hours.

[0013] Optionally, in the back-extraction step, the loaded organic phase after back-extraction is acid-washed and water-washed and then recycled back to the extraction step.

[0014] Optionally, in the extraction step, the extractable organic phase consists of P507 and a diluent, with the volume concentration of P407 being 30–40%.

[0015] Compared with the prior art, the present invention has the following beneficial effects: Compared to the traditional back-extraction-evaporation-crystallization process, this invention eliminates the evaporation and concentration steps, significantly reducing equipment costs and energy consumption. Furthermore, it directly prepares high-purity nickel sulfate by combining back-extraction kinetics and crystallization kinetics during the back-extraction process. Simultaneously, it controls crystal size using a spiral stirrer to improve crystal purity, resulting in Type I battery-grade nickel sulfate. This achieves low-cost nickel recovery with zero overall lithium loss and less than 0.01% nickel loss, demonstrating promising prospects for industrial applications. Attached Figure Description

[0016] Figure 1 This is a flowchart of a process for preparing battery-grade nickel sulfate by back-extraction crystallization in an embodiment of the present invention; Figure 2 This is a schematic diagram of the S1 tank involved in a process for preparing battery-grade nickel sulfate by back-extraction crystallization in an embodiment of the present invention; Figure 3 This is a top view of the crystallization tank in an embodiment of the present invention. Detailed Implementation

[0017] The present invention will be further described in detail below through specific examples to enable those skilled in the art to implement it based on the description. The following description is intended to disclose the present invention so that those skilled in the art can implement it. The preferred embodiments described below are merely examples, and those skilled in the art can conceive of other obvious modifications. The basic principles of the present invention defined in the following description can be applied to other embodiments, modifications, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the present invention. In the present invention, unless otherwise specified, the raw materials and equipment used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0018] The markings in the accompanying drawings of the instruction manual include: extraction tank 1, crystallization tank 2, motor 3, spiral stirrer 4, hydrophilic-hydrophobic contrast surface 5, hydrophobic surface 6, phased array ultrasonic probe 7, temperature sensor 8, insulation jacket 9, and shut-off valve 10.

[0019] Example 1 This embodiment provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization, and the process flow is as follows: Figure 1 As shown, the specific steps include: S1, Extraction: The leachate from waste lithium-ion battery cathode materials after impurity removal and cobalt extraction was used as the extraction solution. This leachate contained 2.64 g / L lithium, 15.48 g / L nickel, 0.2 mg / L copper, 0.1 mg / L iron, 1.3 mg / L aluminum, 1.12 mg / L calcium, 0.8 mg / L manganese, and 52 mg / L cobalt. The leachate was pumped into extraction tank E1 at a flow rate of 60 mL / min, and the organic phase was pumped into extraction tank E5 at a flow rate of 90 mL / min. The extraction equilibrium pH was adjusted to 6.0 using 30 wt% sodium hydroxide solution. Five stages of countercurrent extraction were performed (from tank E1 to tank E5) at room temperature (25℃), with each stage lasting 5 min. A settling time of 15 min was then allowed for phase separation to occur, yielding a nickel-containing loaded organic phase. The organic phase for extraction consists of 30 vt% P507 and 70 vt% kerosene; the extraction equilibrium pH (6.0) is the pH of the aqueous phase after each extraction stage.

[0020] S2, Washing and removing impurities: The loaded organic phase overflowed into tank W1 and was washed in a two-stage countercurrent process using a 0.5 mol / L nickel sulfate solution containing 5 g / L sulfuric acid (in both tanks W1 and W2). The flow rate of the nickel sulfate solution was 60 mL / min, the washing temperature was room temperature (25℃), the washing time was 5 min, and the settling time for phase separation was 15 min. The nickel sulfate solution, after enriching lithium ions and other impurities, was partially returned to the extraction solution, and then intermittently replenished by water washing solution, sulfuric acid washing solution, and saturated nickel sulfate solution.

[0021] S3, Back Extraction: The washed loaded organic phase overflowed into tank S1, where a first-stage back-extraction was performed using a saturated nickel sulfate solution containing 4 mol / L sulfuric acid. The back-extraction temperature was room temperature (25℃), the back-extraction time was 5 min, and the phase separation crystallization time was 15 min, directly precipitating fine crystals to obtain a crystal slurry. Figure 2 As shown, the S1 tank includes an extraction tank 1 and a crystallization tank 2. The crystallization tank 2 is directly connected to the aqueous phase outlet of the extraction tank 1. The crystallization tank 2 is inverted conical in shape. Figure 2 (R=35°). The inner wall of the crystallization tank 2 is coated with a Teflon coating modified with nano-ceramic particles with a thickness of 80 μm (water contact angle 125°). A spiral stirrer 4 driven by a motor 3 is installed inside the crystallization tank 2. The side of the spiral stirrer 4 facing the extraction tank 1 (front) is a hydrophilic-hydrophobic contrast surface 5, and the side of the spiral stirrer 4 away from the extraction tank 1 (back) is a hydrophobic surface 6. The hydrophilic-hydrophobic contrast surface 5 is composed of alternating layers of hydrophilic nano-titanium dioxide coating (2 cm wide) and hydrophobic fluorosilane coating (0.8 cm wide), thus creating a significant surface energy difference on the front side of the spiral stirrer 4, achieving efficient oil-water separation; the hydrophobic surface is composed of a hydrophobic fluorosilane coating. Figure 3 As shown, the top radius of the crystallization tank 2 is H, the horizontal distance between the top edge of the stirring blade of the spiral stirrer 4 and the inner wall of the crystallization tank is L, and the radius difference between adjacent blades of the spiral stirrer 4 is h. The constraints of H, L and h are as follows: 0 < L ≤ 10 cm, 0 < h ≤ (H / 10).

[0022] A phased array ultrasonic probe 7 for online detection of crystal particle size distribution is installed on the outer wall of the crystallization tank 2. A temperature sensor 8 for monitoring the temperature inside the tank is installed on the side wall of the crystallization tank 2. An insulation jacket 9 is installed outside the crystallization tank 2 to allow constant temperature water to flow in and stabilize the temperature inside the crystallization tank 2 at 25±1℃. A shut-off valve 10 for controlling the discharge is installed at the bottom outlet of the crystallization tank 2. When the average particle size of the crystals in the crystallization tank 2 is <150 μm, the speed of the spiral stirrer 4 is increased to 200 rpm; when the average particle size of the crystals in the crystallization tank 2 is ≥220 μm, the speed of the spiral stirrer 4 is reduced to 150 rpm, and the discharge frequency of the crystallization tank 2 is increased (from 30 min / time to 15 min / time); when the average particle size of the crystals in the crystallization tank 2 is stable between 150 and 220 μm, the spiral stirrer 4 maintains the current speed and the current discharge frequency (discharge occurs when the shut-off valve 10 is open and stops when it is closed).

[0023] After back-extraction, the nickel sulfate solution is recycled, with a portion intermittently discharged into the pickling solution, i.e., the W1-W2 system. The loaded organic phase overflows into the S2 tank after back-extraction and undergoes primary washing with a 1.0 mol / L sulfuric acid solution at a flow rate of 40 mL / min, a washing temperature of room temperature (25℃), a washing time of 5 min, and a settling time of 15 min for phase separation. After the sulfuric acid washing solution is recycled and enriched, a portion is intermittently discharged into the pickling solution, i.e., the W1-W2 system.

[0024] After acid washing, the loaded organic phase overflows into tank W3 and is washed with pure water at a flow rate of 40 mL / min. The washing temperature is room temperature (25℃) and the washing time is 5 min. The settling and phase separation time is 15 min. After the washing solution is circulated and enriched, a portion is intermittently discharged into the acid washing solution, i.e., the W1-W2 system. The washed organic phase is then returned to step S1 (extraction process) for recycling.

[0025] S4. Let it stand and age: The slurry flowing from the bottom outlet of crystallization tank 2 is separated into small nickel sulfate crystals (fine crystals) and a saturated nickel sulfate solution after solid-liquid separation. The small nickel sulfate crystals are then aged in the saturated nickel sulfate solution at room temperature for 12 hours, while the saturated nickel sulfate solution is returned to step S3 (back-extraction process). After aging, solid-liquid separation is performed to obtain nickel sulfate crystals and a saturated nickel sulfate solution. The nickel sulfate crystals are washed and dried to obtain type I battery-grade nickel sulfate, while the saturated nickel sulfate solution is returned to the aging process for recycling.

[0026] The overall loss rate of Li in the above process was calculated to be 0, and the overall recovery rate of Ni was 99.99%. After digestion, the nickel sulfate product was analyzed by ICP, and the specific gravity of each element was as follows: aluminum 0.0037%, calcium 0.0017%, cobalt 0.008%, copper not detected, iron 0.0005%, lithium not detected, manganese not detected, nickel 24.16%, magnesium 0.0024%, chromium not detected, cadmium not detected, lead not detected, sodium 0.03%; water-insoluble matter 0.005%, magnetic foreign matter 0.00001%, oil 0.0008%. The average crystal particle size was 185 μm, and the coefficient of variation (CV) was 11.2%, which fully meets the Type I battery-grade nickel sulfate standard in HG / T 5919-2021 "Battery-Grade Nickel Sulfate".

[0027] Example 2 This embodiment provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization, specifically including the following steps: S1, Extraction: The leachate from waste lithium-ion battery cathode materials after impurity removal and cobalt extraction was used as the extraction stock solution. This leachate contained 1.98 g / L lithium, 14.41 g / L nickel, undetectable copper, 0.2 mg / L iron, 0.9 mg / L aluminum, 1.15 mg / L calcium, 0.9 mg / L manganese, and 45 mg / L cobalt. The leachate was pumped into extraction tank E1 at a flow rate of 90 mL / min, and the organic phase was pumped into extraction tank E5 at a flow rate of 90 mL / min. The extraction equilibrium pH was adjusted to 5.5 using 30 wt% sodium hydroxide solution. Five stages of countercurrent extraction were performed (from tank E1 to E5) at room temperature (25℃), with each stage lasting 5 min and a settling time of 15 min, yielding a nickel-containing loaded organic phase. The organic phase consisted of 30 vt% P507 and 70 vt% kerosene; the extraction equilibrium pH (5.5) was the pH of the aqueous phase after each extraction stage.

[0028] S2, Washing and removing impurities: The loaded organic phase overflowed into tank W1 and was washed in a two-stage countercurrent process using a nickel sulfate solution containing 5 g / L sulfuric acid and a concentration of 0.5 mol / L (in both tanks W1 and W2). The flow rate of the nickel sulfate solution was 60 mL / min, the washing temperature was room temperature (25℃), the washing time was 5 min, and the settling time for phase separation was 15 min. The nickel sulfate solution, after enriching lithium ions and other impurities, was partially returned to the extraction solution, and then intermittently replenished by water washing solution, sulfuric acid washing solution, and saturated nickel sulfate solution.

[0029] S3, Back Extraction: The washed loaded organic phase overflows into the S1 tank, where a first-stage back-extraction is performed using a saturated nickel sulfate solution containing 4 mol / L sulfuric acid. The back-extraction temperature is room temperature (25°C), the back-extraction time is 5 min, and the phase separation crystallization time is 15 min, directly precipitating fine crystals to obtain a crystal slurry. The S1 tank in this step is the same as the S1 tank in Example 1, and the working process of the S1 tank is also the same.

[0030] After back-extraction, the nickel sulfate solution is recycled, with intermittent portions discharged into the pickling solution, i.e., the W1-W2 system. The loaded organic phase after back-extraction overflows into tank S2 and undergoes primary washing with a 1 mol / L sulfuric acid solution at a flow rate of 40 mL / min, a washing temperature of room temperature (25℃), a washing time of 5 min, and a settling time of 15 min for phase separation. The sulfuric acid washing solution is then recycled and enriched, with intermittent portions discharged into the pickling solution, i.e., the W1-W2 system. The washed organic phase is then returned to step S1 (extraction process) for recycling.

[0031] S4. Let it stand and age: The crystal slurry flowing from the bottom outlet of the crystallization tank undergoes solid-liquid separation to obtain small nickel sulfate crystals (fine crystals) and a saturated nickel sulfate solution. The small nickel sulfate crystals are then aged in the saturated nickel sulfate solution at room temperature for 7 hours, while the saturated nickel sulfate solution is returned to step S3 (back-extraction process). After aging, solid-liquid separation is performed to obtain nickel sulfate crystals and a saturated nickel sulfate solution. The nickel sulfate crystals are washed and dried to obtain Type I battery-grade nickel sulfate, while the saturated nickel sulfate solution is returned to the aging process for recycling.

[0032] The total loss rate of Li in the above process was calculated to be 0.0001%, and the total recovery rate of Ni was 98.55%. After digestion, the nickel sulfate product was analyzed by ICP, and the specific gravity of each element was as follows: aluminum 0.0007%, calcium 0.0024%, cobalt 0.0082%, copper not detected, iron not detected, lithium not detected, manganese not detected, nickel 23.31%, magnesium 0.0008%, chromium not detected, cadmium not detected, lead not detected, sodium 0.032%; water-insoluble matter 0.002%, magnetic foreign matter 0.00001%, oil 0.0009%. The coefficient of variation (CV) was 14.4%, which fully meets the Type I battery-grade nickel sulfate standard in HG / T5919-2021 "Battery-Grade Nickel Sulfate".

[0033] Example 3 This embodiment provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization, and the process flow is as follows: Figure 1 As shown, the specific steps include: S1, Extraction: The leachate from waste lithium-ion battery cathode materials after impurity removal and cobalt extraction was used as the extraction stock solution. This leachate contained 2.64 g / L lithium, 15.48 g / L nickel, 0.2 mg / L copper, undetectable iron, 0.6 mg / L aluminum, 1.1 mg / L calcium, 0.8 mg / L manganese, and 35 mg / L cobalt. The leachate was pumped into extraction tank E1 at a flow rate of 75 mL / min, and the organic phase was pumped into extraction tank E5 at a flow rate of 75 mL / min. The extraction equilibrium pH was adjusted to 6.0 using 30 wt% sodium hydroxide solution. Five stages of countercurrent extraction were performed (from tank E1 to tank E5) at room temperature (25℃), with each stage lasting 3 min and a settling time of 12 min, yielding a nickel-containing loaded organic phase. The organic phase consisted of 30 vt% P507 and 70 vt% kerosene; the extraction equilibrium pH (6.0) was the pH of the aqueous phase after each extraction stage.

[0034] S2, Washing and removing impurities: The loaded organic phase overflowed into tank W1 and was washed in a two-stage countercurrent process using a 0.5 mol / L nickel sulfate solution containing 10 g / L sulfuric acid (in both tanks W1 and W2). The flow rate of the nickel sulfate solution was 50 mL / min, the washing temperature was room temperature (25℃), the washing time was 3 min, and the settling time for phase separation was 12 min. The nickel sulfate solution, after enriching lithium ions and other impurities, was partially returned to the extraction solution, and then intermittently replenished by water washing solution, sulfuric acid washing solution, and saturated nickel sulfate solution.

[0035] S3, Back Extraction: The washed loaded organic phase overflows into the S1 tank, where a first-stage back-extraction is performed using a saturated nickel sulfate solution containing 4 mol / L sulfuric acid. The back-extraction temperature is room temperature (25°C), the back-extraction time is 3 min, and the phase separation crystallization time is 12 min, directly precipitating fine crystals to obtain a crystal slurry. The S1 tank in this step is the same as the S1 tank in Example 1, and the working process of the S1 tank is also the same.

[0036] After back-extraction, the nickel sulfate solution is recycled, with a portion intermittently discharged into the pickling solution, i.e., the W1-W2 system. The loaded organic phase overflows into tank S2 after back-extraction and undergoes primary washing with a 2 mol / L sulfuric acid solution at a flow rate of 40 mL / min, a washing temperature of room temperature (25℃), a washing time of 3 min, and a settling time of 12 min for phase separation. After the sulfuric acid washing solution is recycled and enriched, a portion is intermittently discharged into the pickling solution, i.e., the W1-W2 system.

[0037] After acid washing, the loaded organic phase overflows into tank W3 and undergoes a first-stage wash with pure water at a flow rate of 40 mL / min, a washing temperature of room temperature (25℃), and a washing time of 3 min. The settling and phase separation time is then 12 min. The washing solution is circulated and enriched, and a portion is intermittently discharged into the acid washing solution, i.e., the W1-W2 system. The washed organic phase is then returned to step S1 (extraction process) for recycling.

[0038] S4. Let it stand and age: The crystal slurry flowing from the bottom outlet of the crystallization tank is separated into small nickel sulfate crystals (fine crystals) and a saturated nickel sulfate solution after solid-liquid separation. The small nickel sulfate crystals are then aged in the saturated nickel sulfate solution at room temperature for 7 hours, while the saturated nickel sulfate solution is returned to step S3. After aging, solid-liquid separation is performed to obtain nickel sulfate crystals and a saturated nickel sulfate solution. The nickel sulfate crystals are washed and dried to obtain Type I battery-grade nickel sulfate, while the saturated nickel sulfate solution is returned to the aging process for recycling.

[0039] The total loss rate of Li in the above process was calculated to be 0, and the total recovery rate of Ni was 99.42%. After digestion, the nickel sulfate product was analyzed by ICP, and the specific gravity of each element was as follows: aluminum 0.0006%, calcium 0.0031%, cobalt 0.002%, copper not detected, iron not detected, lithium not detected, manganese not detected, nickel 23.45%, magnesium 0.0016%, chromium not detected, cadmium not detected, lead not detected, sodium 0.018%; water-insoluble matter 0.001%, magnetic foreign matter 0.000007%, oil 0.0007%. The coefficient of variation (CV) was 13.6%, which fully meets the Type I battery-grade nickel sulfate standard in HG / T 5919-2021 "Battery-Grade Nickel Sulfate".

[0040] Example 4 This embodiment provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The only difference from Example 1 is that in step S2 of this embodiment, the loaded organic phase is washed in a 3-stage countercurrent wash with a nickel sulfate solution containing 3 g / L sulfuric acid and a concentration of 0.5 mol / L. All other steps are the same as in Example 1.

[0041] The total loss rate of Li in the above process was calculated to be 0, and the total recovery rate of Ni was 99.9%. After digestion, the nickel sulfate product was analyzed by ICP, and the specific gravity of each element was as follows: aluminum 0.0007%, calcium 0.0015%, cobalt 0.0073%, copper not detected, iron not detected, lithium not detected, manganese not detected, nickel 23.86%, magnesium 0.002%, chromium not detected, cadmium not detected, lead not detected, sodium 0.01%; water-insoluble matter 0.003%, magnetic foreign matter 0.000006%, oil content 0.0007%, coefficient of variation (CV) 11.8%. It fully meets the Type I battery-grade nickel sulfate standard in HG / T 5919-2021 "Battery-Grade Nickel Sulfate".

[0042] Example 5 This embodiment provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. Compared with Embodiment 1, the difference is that in step S3 of this embodiment, the coating on the inner wall of the crystallization tank 2 is replaced with a PTFE-nano SiO2 composite coating with a thickness of 100 μm (water contact angle 135°) instead of an 80 μm thick nano-ceramic particle modified Teflon coating; the hydrophilic nano-titanium dioxide coating on the front of the spiral stirrer is 1.5 cm wide and the hydrophobic fluorosilane coating is 0.6 cm wide; the spiral stirrer speed is 200 rpm; part of the crystal slurry is returned to the crystallization tank inlet, and the reflux ratio is 0.3; the temperature is controlled at 25±0.5℃ by the insulation jacket 9. The rest is the same as in Embodiment 1 (such as controlling the discharge frequency according to the average crystal particle size).

[0043] In this embodiment, the device remained stable after 200 hours of continuous operation, requiring no cleaning, and the batch consistency of the product was improved to over 99.5%. The specific gravity of each element was as follows: aluminum 0.0028%, calcium 0.0014%, cobalt 0.005%, copper not detected, iron 0.00043%, lithium not detected, manganese not detected, nickel 24.18%, magnesium 0.0018%, chromium not detected, cadmium not detected, lead not detected, sodium 0.022%; water-insoluble matter 0.003%, magnetic foreign matter 0.00001%, oil 0.0005%.

[0044] Comparative Example 1 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference from Example 1 is that in step S1 of this comparative example, the leaching solution contains 2.64 g / L lithium, 15.48 g / L nickel, 0.2 mg / L copper, 0.1 mg / L iron, 1.3 mg / L aluminum, 1.12 mg / L calcium, 0.8 mg / L manganese, and 110 mg / L cobalt. All other components are the same as in Example 1.

[0045] The overall loss rate of Li in the above process was calculated to be 0, and the overall recovery rate of Ni was 99.99%. After digestion of the nickel sulfate product, ICP analysis showed the following elemental proportions: aluminum 0.0037%, calcium 0.0017%, cobalt 0.053%, copper not detected, iron 0.0005%, lithium not detected, manganese not detected, nickel 24.16%, magnesium 0.0024%, chromium not detected, cadmium not detected, lead not detected, sodium 0.01%; water-insoluble matter 0.005%, magnetic foreign matter 0.00001%, and oil 0.0008%.

[0046] Comparative Example 2 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference between this comparative example and Example 1 is that in step S1, the extraction equilibrium pH is 5.0, while the rest are the same as in Example 1.

[0047] The overall loss rate of Li in the above process was calculated to be 0, and the overall recovery rate of Ni was 95.45%. After digestion of the nickel sulfate product, ICP analysis showed the following elemental proportions: aluminum 0.0037%, calcium 0.0017%, cobalt 0.008%, copper not detected, iron 0.0005%, lithium not detected, manganese not detected, nickel 23.75%, magnesium 0.0024%, chromium not detected, cadmium not detected, lead not detected, sodium 0.03%; water-insoluble matter 0.005%, magnetic foreign matter 0.00001%, and oil 0.0008%.

[0048] Comparative Example 3 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference between this comparative example and Example 1 is that this comparative example does not have the static aging process of step S4, while the rest is the same as Example 1.

[0049] The overall loss rate of Li in the above process was calculated to be 0, and the overall recovery rate of Ni was 99.9%. After digestion of the nickel sulfate product, ICP analysis showed the following elemental proportions: aluminum 0.0037%, calcium 0.0034%, cobalt 0.05%, copper not detected, iron 0.0009%, lithium not detected, manganese not detected, nickel 23.16%, magnesium 0.0037%, chromium not detected, cadmium not detected, lead not detected, sodium 0.03%; water-insoluble matter 0.009%, magnetic foreign matter 0.00001%, and oil 0.0008%.

[0050] Comparative Example 4 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference from Example 5 is that in step S3 of this comparative example, a spiral stirrer is not installed in the crystallization tank, the phased array ultrasonic probe is not working, and the discharge frequency cannot be adjusted according to the average crystal particle size. All other aspects are the same as in Example 5.

[0051] In this comparative example, the crystallization rate was slow, and the crystals were large and uneven, easily clogging the bottom of the crystallization tank and the shut-off valve. The product contained 22.3% nickel, 0.058% sodium, 0.0008% magnetic foreign matter, 0.005% oil, and a particle size variation coefficient (CV) of 37%. After 48 hours of continuous operation, a small amount of crystals were still attached to the inner wall of the crystallization tank.

[0052] Comparative Example 5 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. Compared with Example 5, the difference is that in step S3 of this comparative example, the inner wall of the crystallization tank is not coated with a nano-SiO2 composite coating, the phased array ultrasonic probe is not working, and the discharge frequency cannot be adjusted according to the average crystal particle size; the discharge frequency is fixed (30 min / time). All other aspects are the same as in Example 5.

[0053] In this comparative example, after 24 hours of operation, a 2-5 mm thick crystal shell appeared on the inner wall of the crystallization tank, causing a 15% increase in stirring current, localized disordered supersaturation distribution, and the shedding of attached crystals to form coarse particles (>500 μm), which blocked the shut-off valve. The product contained 0.0003% oil, 0.00005% magnetic foreign matter, 0.047% sodium, only 23.1% nickel sulfate, and a particle size variation coefficient (CV) of 32%.

[0054] Comparative Example 6 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference from Example 5 is that in step S3 of this comparative example, no spiral stirrer is installed in the crystallization tank, the inner wall of the crystallization tank is not coated with a nano-SiO2 composite coating, a phased array ultrasonic probe is used, and the discharge frequency is adjusted according to the average crystal particle size. All other aspects are the same as in Example 5.

[0055] In this comparative example, the crystallization rate was slow, and the crystals were large and uneven, easily clogging the bottom of the crystallization tank and the shut-off valve. The product contained 22.4% nickel, 0.054% sodium, 0.0009% magnetic foreign matter, 0.005% oil, and a particle size variation coefficient (CV) of 38%. After 48 hours of continuous operation, a small amount of crystals were still attached to the surface.

[0056] Comparative Example 7 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. Compared with Example 5, the difference is that in step S3 of this comparative example, the phased array ultrasonic probe on the outer wall of the crystallization tank is not working, and the discharge frequency is a fixed value (30 min / time). All other aspects are the same as in Example 5.

[0057] In this comparative example, due to the lack of online particle size monitoring, the discharge frequency could not be adjusted in a timely manner when the feed concentration fluctuated. Some batches had excessively fine crystals (average 80 μm), while others had abnormally large particles (>500 μm), resulting in poor product consistency. The nickel content fluctuated between 22.3% and 23.5%, and the particle size variation coefficient (CV) fluctuated between 25% and 32%, failing to consistently meet the Type I battery grade standard.

[0058] Comparative Example 8 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference from Example 5 is that in step S3 of this comparative example, no spiral stirrer is installed in the crystallization tank; instead, a phased array ultrasonic probe on the outer wall of the crystallization tank operates, and the discharge frequency is adjusted according to the average crystal particle size. All other steps are the same as in Example 5.

[0059] In this comparative example, the crystallization rate was slow, the crystals were large and uneven, and the natural settling of the crystals easily clogged the bottom of the crystallization tank and the shut-off valve. The product contained 22.6% nickel, 0.056% sodium, 0.0009% magnetic foreign matter, 0.006% oil, and a particle size variation coefficient (CV) of 36%. After 24 hours of continuous operation, a small amount of crystals were still attached to the coating surface.

[0060] Comparative Example 9 This comparative example provides a process for preparing battery-grade nickel sulfate by back-extraction crystallization. The difference from Example 5 is that in step S3 of this comparative example, the inner wall of the crystallization tank is not coated with a nano-SiO2 composite coating. All other aspects are the same as in Example 5.

[0061] In this comparative example, after 24 hours of operation, a 2-5 mm thick crystal shell appeared on the inner wall surface of the crystallization tank, causing a 15% increase in stirring current and localized disordered supersaturation distribution. Adhered crystals detached, forming coarse particles (>500 μm), which blocked the shut-off valve. The product contained 0.008% oil, 0.00005% magnetic foreign matter, 0.045% sodium, and 22.8% nickel sulfate. The average crystal size fluctuated significantly, with a coefficient of variation (CV) of 32%.

[0062] The above are merely embodiments of the present invention. The invention is not limited to the fields covered by these embodiments. Commonly known structures and characteristics are not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the application date or priority date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can improve and implement this solution based on the guidance provided in this application and their own capabilities. Some typical known structures or methods should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness or practicality of the invention. The scope of protection claimed in this application should be determined by the content of its claims. The specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A process for preparing battery-grade nickel sulfate by back-extraction crystallization, comprising an extraction step, a washing and impurity removal step, a back-extraction step, and a settling and aging step, characterized in that: In the extraction step, the leachate after removing impurities and extracting cobalt from waste lithium battery cathode materials is used as raw material, and nickel is extracted by extracting the organic phase to obtain the supported organic phase; In the washing and impurity removal step, the supported organic phase is washed with acid. In the back-extraction step, a saturated nickel sulfate solution is used in tank S1 to back-extract the washed loaded organic phase, directly precipitating fine crystals. Tank S1 includes an extraction tank and a crystallization tank, with the aqueous phase outlet of the crystallization tank directly connected to the extraction tank. The crystallization tank is inverted conical in shape, and its inner wall is coated with a nano-ceramic particle-modified Teflon coating or a PTFE-nano SiO2 composite coating. A spiral stirrer is installed inside the crystallization tank. The side of the spiral stirrer facing the extraction tank is a hydrophilic-hydrophobic contrast surface, while the side facing away from the extraction tank is a hydrophobic surface. The hydrophilic-hydrophobic contrast surface is composed of alternating hydrophilic nano-titanium dioxide coatings and hydrophobic fluorosilane coatings, while the hydrophobic surface is composed of hydrophobic fluorosilane coatings. A phased array ultrasonic probe for online detection of crystal particle size distribution is installed on the outer wall of the crystallization tank. When the average crystal particle size in the crystallization tank is <150 μm, the speed of the spiral stirrer is increased to 200 rpm; when the average crystal particle size in the crystallization tank is ≥220 μm, the speed of the spiral stirrer is decreased to 150 rpm. rpm; when the average crystal size in the crystallization tank is stable at 150-220μm, the spiral stirrer maintains the current speed. In the static aging process, the fine crystals undergo solid-liquid separation after static crystal growth to obtain Type I battery-grade nickel sulfate.

2. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the back-extraction step, the outlet of the crystallization tank is equipped with a shut-off valve for controlling the discharge. When the average particle size of the crystals in the crystallization tank is ≥220 μm, the speed of the spiral stirrer is reduced to 150 rpm while the discharge frequency of the crystallization tank is increased.

3. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the back-extraction step, the width of the hydrophilic nano-titanium dioxide coating on the hydrophilic-hydrophobic contrast surface is 1.5–2.0 cm, and the width of the hydrophobic fluorosilane coating is 0.6–0.8 cm.

4. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the back-extraction step, the top radius of the crystallization tank is H, the horizontal distance between the tip edge of the stirring blade of the spiral stirrer and the inner wall of the crystallization tank is L, and the radius difference between adjacent blades of the spiral stirrer is h. The constraints of H, L and h are as follows: 0 < L ≤ 10 cm, 0 < h ≤ (H / 10).

5. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the extraction step, 3 to 5 stages of countercurrent extraction are used, the extraction equilibrium pH is 5.5 to 6, and the extraction temperature is 20 to 30℃.

6. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the washing and impurity removal step, a 2-3 stage countercurrent washing is adopted. The acid solution is a nickel sulfate solution containing sulfuric acid, in which the concentration of nickel sulfate is 0.1-0.5 mol / L and the concentration of sulfuric acid is 3-10 g / L.

7. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the back-extraction step, the saturated nickel sulfate solution contains 2–6 mol / L sulfuric acid, and the back-extraction temperature is 20–30℃.

8. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the static aging process, the aging time is 5 to 12 hours.

9. The process for preparing battery-grade nickel sulfate by back-extraction crystallization according to claim 1, characterized in that: In the back-extraction step, the loaded organic phase after back-extraction is acid-washed and water-washed and then recycled back to the extraction step.

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