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       This study investigates the effects of NH4+ impurities and seed ratio on the growth mechanism and performance of nickel sulfate hexahydrate under discontinuous cooling crystallization, and examines the effects of NH4+ impurities on the growth mechanism, thermal properties, and functional groups of nickel sulfate hexahydrate. At low impurity concentrations, Ni2+ and NH4+ ions compete with SO42− for binding, resulting in decreased crystal yield and growth rate and increased crystallization activation energy. At high impurity concentrations, NH4+ ions are incorporated into the crystal structure to form a complex salt (NH4)2Ni(SO4)2 6H2O. The formation of the complex salt results in increased crystal yield and growth rate and decreased crystallization activation energy. The presence of both high and low NH4+ ion concentrations causes lattice distortion, and the crystals are thermally stable at temperatures up to 80 °C. In addition, the influence of NH4+ impurities on the crystal growth mechanism is greater than that of the seed ratio. When the impurity concentration is low, the impurity is easy to attach to the crystal; when the concentration is high, the impurity is easy to incorporate into the crystal. The seed ratio can greatly increase the crystal yield and slightly improve the crystal purity.
       Nickel sulfate hexahydrate (NiSO4 6H2O) is now a critical material used in a variety of industries, including battery manufacturing, electroplating, catalysts, and even in the production of food, oil, and perfume. 1,2,3 Its importance is growing with the rapid development of electric vehicles, which rely heavily on nickel-based lithium-ion (LiB) batteries. The use of high-nickel alloys such as NCM 811 is expected to dominate by 2030, further increasing the demand for nickel sulfate hexahydrate. However, due to resource constraints, production may not keep up with the growing demand, creating a gap between supply and demand. This shortage has raised concerns about resource availability and price stability, highlighting the need for efficient production of high-purity, stable battery-grade nickel sulfate. 1,4
       The production of nickel sulfate hexahydrate is generally achieved by crystallization. Among the various methods, the cooling method is a widely used method, which has the advantages of low energy consumption and the ability to produce high-purity materials. 5,6 Research on the crystallization of nickel sulfate hexahydrate using discontinuous cooling crystallization has made significant progress. At present, most research focuses on improving the crystallization process by optimizing parameters such as temperature, cooling rate, seed size and pH. 7,8,9 The goal is to increase the crystal yield and purity of the obtained crystals. However, despite the comprehensive study of these parameters, there is still a large gap in the attention paid to the influence of impurities, especially ammonium (NH4+), on the crystallization results.
       Ammonium impurities are likely to be present in the nickel solution used for nickel crystallization due to the presence of ammonium impurities during the extraction process. Ammonia is commonly used as a saponifying agent, which leaves trace amounts of NH4+ in the nickel solution. 10,11,12 Despite the ubiquity of ammonium impurities, their effects on crystal properties such as crystal structure, growth mechanism, thermal properties, purity, etc. remain poorly understood. The limited research on their effects is important because impurities can hinder or alter crystal growth and, in some cases, act as inhibitors, affecting the transition between metastable and stable crystalline forms. 13,14 Understanding these effects is therefore critical from an industrial perspective because impurities can compromise product quality.
       Based on a specific question, this study aimed to investigate the effect of ammonium impurities on the properties of nickel crystals. By understanding the effect of impurities, new methods can be developed to control and minimize their negative effects. This study also investigated the correlation between impurity concentration and changes in seed ratio. Since seed is widely used in the production process, seed parameters were used in this study, and it is essential to understand the relationship between these two factors. 15 The effects of these two parameters were used to study the crystal yield, crystal growth mechanism, crystal structure, morphology, and purity. In addition, the kinetic behavior, thermal properties, and functional groups of crystals under the influence of NH4+ impurities alone were further investigated.
       The materials used in this study were nickel sulfate hexahydrate (NiSO 6H2O, ≥ 99.8%) provided by GEM; ammonium sulfate ((NH)SO, ≥ 99%) purchased from Tianjin Huasheng Co., Ltd.; distilled water. The seed crystal used was NiSO 6H2O, crushed and sieved to obtain a uniform particle size of 0.154 mm. The characteristics of NiSO 6H2O are shown in Table 1 and Figure 1.
       The effect of NH4+ impurities and seed ratio on the crystallization of nickel sulfate hexahydrate was investigated using intermittent cooling. All experiments were conducted at an initial temperature of 25 °C. 25 °C was chosen as the crystallization temperature considering the limitations of temperature control during filtration. Crystallization can be induced by sudden temperature fluctuations during filtration of hot solutions using a low-temperature Buchner funnel. This process can significantly affect the kinetics, impurity uptake, and various crystal properties.
       The nickel solution was first prepared by dissolving 224 g NiSO4 6H2O in 200 ml distilled water. The chosen concentration corresponds to a supersaturation (S) = 1.109. The supersaturation was determined by comparing the solubility of dissolved nickel sulfate crystals with the solubility of nickel sulfate hexahydrate at 25 °C. The lower supersaturation was chosen to prevent spontaneous crystallization when the temperature was lowered to the initial one.
       The effect of NH4+ ion concentration on the crystallization process was investigated by adding (NH4)2SO4 to a nickel solution. The NH4+ ion concentrations used in this study were 0, 1.25, 2.5, 3.75, and 5 g/L. The solution was heated at 60 °C for 30 min while stirring at 300 rpm to ensure uniform mixing. The solution was then cooled to the desired reaction temperature. When the temperature reached 25 °C, different amounts of seed crystals (seed ratios of 0.5%, 1%, 1.5%, and 2%) were added to the solution. The seed ratio was determined by comparing the weight of the seed with the weight of NiSO4 6H2O in the solution.
       After adding the seed crystals to the solution, the crystallization process occurred naturally. The crystallization process lasted for 30 minutes. The solution was filtered using a filter press to further separate the accumulated crystals from the solution. During the filtration process, the crystals were regularly washed with ethanol to minimize the possibility of recrystallization and minimize the adhesion of impurities in the solution to the surface of the crystals. Ethanol was chosen to wash the crystals because the crystals are insoluble in ethanol. The filtered crystals were placed in a laboratory incubator at 50 °C. The detailed experimental parameters used in this study are shown in Table 2.
       The crystal structure was determined using an XRD instrument (SmartLab SE—HyPix-400) and the presence of NH4+ compounds was detected. SEM characterization (Apreo 2 HiVac) was performed to analyze the crystal morphology. Thermal properties of the crystals were determined using a TGA instrument (TG-209-F1 Libra). The functional groups were analyzed by FTIR (JASCO-FT/IR-4X). The purity of the sample was determined using an ICP-MS instrument (Prodigy DC Arc). The sample was prepared by dissolving 0.5 g of crystals in 100 mL of distilled water. The crystallization yield (x) was calculated by dividing the mass of the output crystal by the mass of the input crystal according to formula (1).
       where x is the crystal yield, varying from 0 to 1, mout is the weight of the output crystals (g), min is the weight of the input crystals (g), msol is the weight of the crystals in solution, and mseed is the weight of the seed crystals.
       The crystallization yield was further investigated to determine the crystal growth kinetics and estimate the activation energy value. This study was performed with a seeding ratio of 2% and the same experimental procedure as before. The isothermal crystallization kinetics parameters were determined by evaluating the crystal yield at different crystallization times (10, 20, 30, and 40 min) and initial temperatures (25, 30, 35, and 40 °C). The selected concentrations at the initial temperature corresponded to supersaturation (S) values ​​of 1.109, 1.052, 1, and 0.953, respectively. The supersaturation value was determined by comparing the solubility of dissolved nickel sulfate crystals with the solubility of nickel sulfate hexahydrate at the initial temperature. In this study, the solubility of NiSO4 6H2O in 200 mL of water at different temperatures without impurities is shown in Figure 2.
       Johnson-Mail-Avrami (JMA theory) is used to analyze isothermal crystallization behavior. JMA theory is chosen because the crystallization process does not occur until seed crystals are added to the solution. JMA theory is described as follows:
       Where x(t) represents the transition at time t, k represents the transition rate constant, t represents the transition time, and n represents the Avrami index. Formula 3 is derived from formula (2). The activation energy of crystallization is determined using the Arrhenius equation:
       Where kg is the reaction rate constant, k0 is a constant, Eg is the activation energy of crystal growth, R is the molar gas constant (R=8.314 J/mol K), and T is the isothermal crystallization temperature (K).
       Figure 3a shows that the seeding ratio and the dopant concentration have an effect on the yield of nickel crystals. When the dopant concentration in the solution increased to 2.5 g/L, the crystal yield decreased from 7.77% to 6.48% (seed ratio of 0.5%) and from 10.89% to 10.32% (seed ratio of 2%). Further increase in the dopant concentration led to a corresponding increase in the crystal yield. The highest yield reached 17.98% when the seeding ratio was 2% and the dopant concentration was 5 g/L. The changes in the crystal yield pattern with the increase of the dopant concentration may be related to the changes in the crystal growth mechanism. When the dopant concentration is low, Ni2+ and NH4+ ions compete for binding with SO42−, which leads to an increase in the solubility of nickel in the solution and a decrease in the crystal yield. 14 When the impurity concentration is high, the competition process still occurs, but some NH4+ ions coordinate with nickel and sulfate ions to form a double salt of nickel ammonium sulfate. 16 The formation of double salt leads to a decrease in the solubility of the solute, thereby increasing the crystal yield. Increasing the seeding ratio can continuously improve the crystal yield. Seeds can initiate the nucleation process and spontaneous crystal growth by providing an initial surface area for solute ions to organize and form crystals. As the seeding ratio increases, the initial surface area for ions to organize increases, so more crystals can be formed. Therefore, increasing the seeding ratio has a direct effect on the crystal growth rate and crystal yield. 17
       Parameters of NiSO4 6H2O: (a) crystal yield and (b) pH of nickel solution before and after inoculation.
       Figure 3b shows that the seed ratio and dopant concentration affect the pH of the nickel solution before and after seed addition. The purpose of monitoring the pH of the solution is to understand the changes in chemical equilibrium in the solution. Before adding the seed crystals, the pH of the solution tends to decrease due to the presence of NH4+ ions that release H+ protons. Increasing the dopant concentration results in more H+ protons being released, thereby decreasing the pH of the solution. After adding the seed crystals, the pH of all solutions increases. The pH trend is positively correlated with the crystal yield trend. The lowest pH value was obtained at a dopant concentration of 2.5 g/L and a seed ratio of 0.5%. As the dopant concentration increases to 5 g/L, the pH of the solution increases. This phenomenon is quite understandable, since the availability of NH4+ ions in solution decreases either due to absorption, or due to inclusion, or due to absorption and inclusion of NH4+ ions by crystals.
       Crystal yield experiments and analysis were further conducted to determine the kinetic behavior of crystal growth and calculate the activation energy of crystal growth. The parameters of isothermal crystallization kinetics were explained in the Methods section. Figure 4 shows the Johnson-Mehl-Avrami (JMA) plot which shows the kinetic behavior of nickel sulfate crystal growth. The plot was generated by plotting the ln[− ln(1− x(t))] value against the ln t value (Equation 3). The gradient values ​​obtained from the plot correspond to the JMA index (n) values ​​which indicate the dimensions of the growing crystal and the growth mechanism. While the cutoff value indicates the growth rate which is represented by the constant ln k. The JMA index (n) values ​​range from 0.35 to 0.75. This n value indicates that the crystals have one-dimensional growth and follow a diffusion-controlled growth mechanism; 0 < n < 1 indicates one-dimensional growth, while n < 1 indicates a diffusion-controlled growth mechanism. 18 The growth rate of the constant k decreases with increasing temperature, indicating that the crystallization process occurs faster at lower temperatures. This is related to the increase in supersaturation of the solution at lower temperatures.
       Johnson-Mehl-Avrami (JMA) plots of nickel sulfate hexahydrate at different crystallization temperatures: (a) 25 °C, (b) 30 °C, (c) 35 °C and (d) 40 °C.
       The addition of dopants showed the same pattern of growth rate at all temperatures. When the dopant concentration was 2.5 g/L, the crystal growth rate decreased, and when the dopant concentration was higher than 2.5 g/L, the crystal growth rate increased. As mentioned earlier, the change in the pattern of crystal growth rate is due to the change in the mechanism of interaction between ions in the solution. When the dopant concentration is low, the competition process between ions in the solution increases the solubility of the solute, thereby decreasing the crystal growth rate. 14 Furthermore, the addition of high concentrations of dopants causes the growth process to change significantly. When the dopant concentration exceeds 3.75 g/L, additional new crystal nuclei are formed, which leads to a decrease in the solubility of the solute, thereby increasing the crystal growth rate. The formation of new crystal nuclei can be demonstrated by the formation of the double salt (NH4)2Ni(SO4)2 6H2O. 16 When discussing the crystal growth mechanism, X-ray diffraction results confirm the formation of a double salt.
       The JMA plot function was further evaluated to determine the activation energy of crystallization. The activation energy was calculated using the Arrhenius equation (shown in Equation (4)). Figure 5a shows the relationship between the ln(kg) value and the 1/T value. Then, the activation energy was calculated using the gradient value obtained from the plot. Figure 5b shows the activation energy values ​​of crystallization under different impurity concentrations. The results show that the changes in the impurity concentration affect the activation energy. The activation energy of crystallization of nickel sulfate crystals without impurities is 215.79 kJ/mol. When the impurity concentration reaches 2.5 g/L, the activation energy increases by 3.99% to 224.42 kJ/mol. The increase in activation energy indicates that the energy barrier of the crystallization process increases, which will lead to a decrease in the crystal growth rate and crystal yield. When the impurity concentration is more than 2.5 g/L, the activation energy of crystallization decreases significantly. At an impurity concentration of 5 g/l, the activation energy is 205.85 kJ/mol, which is 8.27% lower than the activation energy at an impurity concentration of 2.5 g/l. A decrease in the activation energy indicates that the crystallization process is facilitated, which leads to an increase in the crystal growth rate and crystal yield.
       (a) Fitting of the plot of ln(kg) versus 1/T and (b) activation energy Eg of crystallization at different impurity concentrations.
       The crystal growth mechanism was investigated by XRD and FTIR spectroscopy, and the crystal growth kinetics and activation energy were analyzed. Figure 6 shows the XRD results. The data are consistent with PDF #08–0470, which indicates that it is α-NiSO4 6H2O (red silica). The crystal belongs to the tetragonal system, the space group is P41212, the unit cell parameters are a = b = 6.782 Å, c = 18.28 Å, α = β = γ = 90°, and the volume is 840.8 Å3. These results are consistent with the results previously published by Manomenova et al. 19 The introduction of NH4+ ions also leads to the formation of (NH4)2Ni(SO4)2 6H2O. The data belong to PDF No. 31–0062. The crystal belongs to the monoclinic system, space group P21/a, the unit cell parameters are a = 9.186 Å, b = 12.468 Å, c = 6.242 Å, α = γ = 90°, β = 106.93°, and the volume is 684 Å3. These results are consistent with the previous study reported by Su et al.20.
       X-ray diffraction patterns of nickel sulfate crystals: (a–b) 0.5%, (c–d) 1%, (e–f) 1.5%, and (g–h) 2% seed ratio. The right image is an enlarged view of the left image.
       As shown in Figures 6b, d, f and h, 2.5 g/L is the highest limit of ammonium concentration in solution without forming additional salt. When the impurity concentration is 3.75 and 5 g/L, NH4+ ions are incorporated into the crystal structure to form the complex salt (NH4)2Ni(SO4)2 6H2O. According to the data, the peak intensity of the complex salt increases as the impurity concentration increases from 3.75 to 5 g/L, especially at 2θ 16.47° and 17.44°. The increase in the peak of the complex salt is solely due to the principle of chemical equilibrium. However, some abnormal peaks are observed at 2θ 16.47°, which can be attributed to the elastic deformation of the crystal. 21 The characterization results also show that a higher seeding ratio results in a decrease in the peak intensity of the complex salt. A higher seed ratio accelerates the crystallization process, which leads to a significant decrease in the solute. In this case, the crystal growth process is concentrated on the seed, and the formation of new phases is hampered by the reduced supersaturation of the solution. In contrast, when the seed ratio is low, the crystallization process is slow, and the supersaturation of the solution remains at a relatively high level. This situation increases the probability of nucleation of the less soluble double salt (NH4)2Ni(SO4)2 6H2O. The peak intensity data for the double salt are given in Table 3.
       FTIR characterization was performed to investigate any disorder or structural changes in the host lattice due to the presence of NH4+ ions. Samples with a constant seeding ratio of 2% were characterized. Figure 7 shows the FTIR characterization results. The broad peaks observed at 3444, 3257 and 1647 cm−1 are due to the O–H stretching modes of molecules. The peaks at 2370 and 2078 cm−1 represent the intermolecular hydrogen bonds between water molecules. The band at 412 cm−1 is attributed to the Ni–O stretching vibrations. In addition, the free SO4− ions exhibit four major vibration modes at 450 (υ2), 630 (υ4), 986 (υ1) and 1143 and 1100 cm−1 (υ3). The symbols υ1-υ4 represent the properties of the vibrational modes, where υ1 represents the non-degenerate mode (symmetric stretching), υ2 represents the doubly degenerate mode (symmetric bending), and υ3 and υ4 represent the triply degenerate modes (asymmetric stretching and asymmetric bending, respectively). 22,23,24 The characterization results show that the presence of ammonium impurities gives an additional peak at the wavenumber of 1143 cm-1 (marked with a red circle in the figure). The additional peak at 1143 cm-1 indicates that the presence of NH4+ ions, regardless of the concentration, causes a distortion of the lattice structure, which leads to a change in the vibration frequency of sulfate ion molecules inside the crystal.
       Based on the XRD and FTIR results related to the kinetic behavior of crystal growth and activation energy, Figure 8 shows the schematic of the crystallization process of nickel sulfate hexahydrate with the addition of NH4+ impurities. In the absence of impurities, Ni2+ ions will react with H2O to form nickel hydrate [Ni(6H2O)]2−. Then, the nickel hydrate spontaneously combines with SO42− ions to form Ni(SO4)2 6H2O nuclei and grows into nickel sulfate hexahydrate crystals. When a lower concentration of ammonium impurities (2.5 g/L or less) is added to the solution, [Ni(6H2O)]2− is difficult to completely combine with SO42− ions because [Ni(6H2O)]2− and NH4+ ions compete for combination with SO42− ions, although there are still enough sulfate ions to react with both ions. This situation leads to an increase in the activation energy of crystallization and a slowdown in crystal growth. 14,25 After the nickel sulfate hexahydrate nuclei are formed and grown into crystals, multiple NH4+ and (NH4)2SO4 ions are adsorbed on the crystal surface. This explains why the functional group of SO4− ion (wavenumber 1143 cm−1) in NSH-8 and NSH-12 samples remains formed without doping process. When the impurity concentration is high, NH4+ ions begin to be incorporated into the crystal structure, forming double salts. 16 This phenomenon occurs due to the lack of SO42− ions in the solution, and SO42− ions bind to nickel hydrates faster than to ammonium ions. This mechanism promotes the nucleation and growth of double salts. During the alloying process, Ni(SO4)2 6H2O and (NH4)2Ni(SO4)2 6H2O nuclei are simultaneously formed, which leads to an increase in the number of nuclei obtained. An increase in the number of nuclei promotes the acceleration of crystal growth and a decrease in the activation energy.
       The chemical reaction of dissolving nickel sulfate hexahydrate in water, adding a small amount and a large amount of ammonium sulfate, and then carrying out the crystallization process can be expressed as follows:
       The SEM characterization results are shown in Figure 9. The characterization results indicate that the amount of ammonium salt added and the seeding ratio do not significantly affect the crystal shape. The size of the crystals formed remains relatively constant, although larger crystals appear at some points. However, further characterization is still needed to determine the effect of ammonium salt concentration and seeding ratio on the average size of the crystals formed.
       Crystal morphology of NiSO4 6H2O: (a–e) 0.5%, (f–j) 1%, (h–o) 1.5% and (p–u) 2% seed ratio showing the change of NH4+ concentration from top to bottom, which is 0, 1.25, 2.5, 3.75 and 5 g/L, respectively.
       Figure 10a shows the TGA curves of the crystals with different impurity concentrations. The TGA analysis was performed on the samples with a seeding ratio of 2%. The XRD analysis was also performed on the NSH-20 sample to determine the formed compounds. The XRD results shown in Figure 10b confirm the changes in the crystal structure. Thermogravimetric measurements show that all the synthesized crystals exhibit thermal stability up to 80°C. Subsequently, the crystal weight decreased by 35% when the temperature increased to 200°C. The weight loss of the crystals is due to the decomposition process, which involves the loss of 5 water molecules to form NiSO4 H2O. When the temperature increased to 300–400°C, the weight of the crystals decreased again. The weight loss of the crystals was about 6.5%, while the weight loss of the NSH-20 crystal sample was slightly higher, exactly 6.65%. The decomposition of NH4+ ions into NH3 gas in the NSH-20 sample resulted in slightly higher reducibility. As the temperature increased from 300 to 400°C, the weight of the crystals decreased, resulting in all crystals having the NiSO4 structure. Increasing the temperature from 700°C to 800°C caused the crystal structure to transform into NiO, causing the release of SO2 and O2 gases.25,26
       The purity of nickel sulfate hexahydrate crystals was determined by assessing the NH4+ concentration using a DC-Arc ICP-MS instrument. The purity of nickel sulfate crystals was determined using formula (5).
       Where Ma is the mass of impurities in the crystal (mg), Mo is the mass of the crystal (mg), Ca is the concentration of impurities in the solution (mg/l), V is the volume of the solution (l).
       Figure 11 shows the purity of nickel sulfate hexahydrate crystals. The purity value is the average value of 3 characteristics. The results show that the seeding ratio and impurity concentration directly affect the purity of the formed nickel sulfate crystals. The higher the impurity concentration, the greater the absorption of impurities, resulting in a lower purity of the formed crystals. However, the absorption pattern of impurities may change depending on the impurity concentration, and the result graph shows that the overall absorption of impurities by the crystals does not change significantly. In addition, these results also show that a higher seeding ratio can improve the purity of the crystals. This phenomenon is possible because when most of the formed crystal nuclei are concentrated on the nickel nuclei, the probability of nickel ions accumulating on the nickel is higher. 27
       The study showed that ammonium ions (NH4+) significantly affect the crystallization process and crystalline properties of nickel sulfate hexahydrate crystals, and also revealed the influence of the seed ratio on the crystallization process.
       At ammonium concentrations above 2.5 g/l, the crystal yield and crystal growth rate decrease. At ammonium concentrations above 2.5 g/l, the crystal yield and crystal growth rate increase.
       Addition of impurities to the nickel solution increases the competition between NH4+ and [Ni(6H2O)]2− ions for SO42−, which leads to an increase in the activation energy. The decrease in the activation energy after adding high concentrations of impurities is due to the entry of NH4+ ions into the crystal structure, thus forming the double salt (NH4)2Ni(SO4)2 6H2O.
       Using higher seeding ratio can improve the crystal yield, crystal growth rate and crystal purity of nickel sulfate hexahydrate.
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