Effect of Ammonium (NH₄⁺) Impurity on the Crystallization of Cobalt Sulfate Hexahydrate from Aqueous Solutions Using Cooling Method

Abstract

This research examines the influence of ammonium (NH₄⁺) impurities on the kinetic behavior, activation energy, crystal structure, morphology, and purity of cobalt sulfate hexahydrate using the cooling crystallization method. Characterization results indicate that ammonium at all concentrations affects the crystallization process, with a minimum concentration required to alter crystal characteristics. At ammonium concentrations up to 3.75 g/L, the crystal growth rate decreases, while activation energy increases. Furthermore, the crystal structure does not change, and crystal purity decreases by approximately 0.2%. This decline is insignificant and tends to stagnate, suggesting a maximum adsorption limit of impurities onto the crystal. At ammonium concentrations of 5 g/L, the crystal growth rate increases, and activation energy decreases. This shift in behavior is caused by the formation of Tutton’s salt, (NH₄)₂Co(SO₄)₂·6H₂O, which significantly reduces crystal purity by 1%. Additionally, the presence of ammonium does not alter the crystal shape.

1. Introduction

Cobalt sulfate hydrate is an important chemical compound nowadays because it is widely used in various industrial applications, including battery manufacturing, electroplating, and as a precursor in the production of catalysts [1,2,3]. Its significance has grown even further with the increasing global demand for sustainable energy solutions and the rapid transition toward electric mobility, which heavily relies on lithium-ion batteries (LiBs) [4]. In 2022, the market price of cobalt sulfate hydrate rose by 23% due to high demand in the battery industry. However, production did not increase accordingly, creating a supply–demand imbalance. This shortage has raised concerns about resource availability and price stability, emphasizing the need for improved production while maintaining quality [5].

Cobalt sulfate hexahydrate (CoSO₄·6H2O) is a hydrated form that is easy to synthesize and commonly found in nature. It can be produced at relatively low temperatures and moderate humidity levels, making it a practical choice for industrial applications [6]. The production of cobalt sulfate hexahydrate is typically achieved through crystallization. Among the various crystallization methods, evaporative crystallization is one of the most commonly used techniques. However, it has certain limitations, such as requiring significant heat input to promote evaporation, which can lead to energy inefficiency [7,8]. Due to these drawbacks, cooling crystallization has become a promising alternative method to produce cobalt sulfate hydrate [9]. It offers the advantage of low energy consumption and the ability to achieve high purity, making it a more efficient and effective choice for producing cobalt sulfate hexahydrate [4,10].

In the hydrometallurgical process, crystallization is carried out using a purified solution obtained through various methods, including solvent extraction. The resulting cobalt sulfate solution varies in purity depending on the efficiency of the purification process [11]. In industry, certain impurities often remain in the solution, with ammonium being one of the most common. This impurity persists in the solution because ammonia is used as a leaching agent and a saponification agent during the extraction process [12,13,14]. The presence of ammonium in the solution can negatively affect the crystallization process by reducing the crystal quality. Impurities may also influence key aspects of crystallization including kinetic behavior, crystal structure, morphology, and purity [15]. Therefore, a comprehensive study of the effects of ammonium impurities on cobalt sulfate crystallization is important.

The aim of the present work is to investigate the effect of ammonium impurity on the crystallization behavior of cobalt sulfate hexahydrate. This study conducted a kinetic analysis to determine the crystallization growth rate and activation energy, as well as the crystal structure, morphology, and purity. By exploring these factors, this research sought to provide insights into how ammonium ions influence the crystallization process, crystal quality, and purity, which are important for optimizing the cobalt sulfate industry.

2. Materials and Methods

2.1. Chemical Reagents

The materials used in this research were cobalt sulfate heptahydrate (CoSO₄·7H₂O, ≥99.8%), ammonium sulfate ((NH₄)₂·SO₄, ≥99%), and distilled water. Cobalt sulfate heptahydrate was battery-grade, provided by GEM. Co., Ltd., Jingmen, China, and ammonium sulfate was analytical-grade, purchased from Tianjin Huasheng. Co., Ltd., Tianjin, China. Cobalt sulfate heptahydrate was used as the main material and crystal seed. The crystal seed was prepared by grinding and sieving to obtain a uniform size of 0.154 mm.

2.2. Experimental Methods

The batch cooling crystallization method was used to synthesize cobalt sulfate hexahydrate under the influence of NH₄⁺ ion impurity. The experiment commenced by dissolving 220 g of cobalt sulfate heptahydrate crystal into 200 mL of distilled water to make a cobalt solution. The influence of NH₄⁺ ion impurity on the crystallization of cobalt sulfate hexahydrate was investigated through the addition of ammonium sulfate into the cobalt solution. The amounts of NH₄⁺ ion in the solution were 0 g/L, 1.25 g/L, 2.5 g/L, 3.75 g/L, and 5 g/L. These values were chosen based on the potential impurity content present in the cobalt solution. The solution was then mixed at 60 °C and stirred at 300 rpm for 30 min to ensure homogenous solution. Once the solution was homogenous, the solution was cooled down to the initial temperature. The initial temperatures used in this research were 25 °C, 30 °C, 35 °C, and 40 °C. A relatively low initial temperature value was chosen because of the constraints on temperature control during the filtration process. A rapid temperature change during the filtration process could promote spontaneous crystallization, which affects impurity absorption, kinetic behavior, and crystal properties. The selected concentration at the initial temperature equated to supersaturation values (S) = 1.26, 1.17, 1.07, and 1, respectively. The supersaturation value was determined by comparing the solubility of cobalt sulfate heptahydrate crystals at 40 °C with the initial temperature. A low supersaturation value was selected to prevent spontaneous crystallization when the temperature was lowered to the initial temperature. Figure 1 shows the experimental results from this study for the solubility of CoSO₄·7H₂O in 200 mL of water at various temperatures and without impurities.

After the solution was cooled to the initial temperature, seeds were added to the solution (0.5% seed ratio) to assist the crystallization process. The crystallization occurred spontaneously when seeds were added to the solution. The crystallization ran for 10, 20, 30, and 40 min. Variations in crystallization time and initial temperature were used to determine the isothermal crystallization’s kinetic behavior. The accumulated crystals and solution were further separated by filtering the solution. During the filtration process, the crystals were periodically washed with ethanol to minimize the possibility of additional crystallization and to minimize the attachment of impurities from the solution to the crystal surface. Ethanol was chosen to wash the crystals because the crystals do not dissolve in ethanol. The filtered crystals were then placed in a laboratory incubator at 50 °C. The laboratory-scale instrumented crystallizer used in this study is illustrated in Figure 2.

2.3. Characterization and Analysis Method

Several characterizations were carried out to determine the effect of ammonium impurity on the crystallization kinetic behavior, crystal structure, functional group, morphology, and crystal purity of cobalt sulfate hexahydrate. The isothermal crystallization kinetic behavior was determined by assessing the crystal output yield (x) value in accordance with Equation (1):

$$Output yield \left(x\right)={\displaystyle \frac{m_{out}}{m_{in}}}={\displaystyle \frac{m_{out}}{m_{sol}+m_{seed}}}$$

(1)

where x is the crystal output yield that ranges from 0 to 1, m_out is the weight of crystal output (g), m_in is the weight of crystal input (g), m_sol is the weight of crystal in the solution, and m_seed is the weight of crystal seed. The crystal output yield (x) value was then analyzed using the Johnson–Mehl–Avrami (JMA) theory. The JMA theory was selected because no crystallization process occurs before the seeds are added to the solution. The crystallization occurs spontaneously when seeds are added to the solution. The JMA theory is described with the following equation:

$$x \left(t\right)=1-exp (-{kt}^n)$$

(2)

The equation is then derived as follows:

$$1-x \left(t\right)=exp (-{kt}^n)$$

(3)

$$ln (1-x \left(t\right))=-{kt}^n$$

(4)

$$\mathit{ln}\left[-\mathit{ln}\left(1-x \left(t\right)\right)\right]=\mathit{ln}\left(-{kt}^n\right)$$

(5)

$$\mathit{ln}\left[-\mathit{ln}\left(1-x \left(t\right)\right)\right]=ln\left(k\right)+n ln\left(t\right)$$

(6)

where x (t) is the transformation at time t, k represents the transformation rate constant, t represents the transformation time, and n represents the Avrami exponent. The kinetic behavior calculation results are then analyzed further to determine the activation energy for crystallization using the Arrhenius equation:

$$k_g=k_0\exp {\displaystyle \frac{E_g}{RT}}$$

(7)

where k_g represents the reaction rate constant, k₀ denotes a constant, E_g signifies the activation energy for crystallization growth, R is the molar gas constant (R = 8.314 J/mol·K), and T indicates the isothermal crystallization temperature (K).

Several samples synthesized at an initial temperature of 25 °C were then characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Inductively Coupled Plasma–Mass Spectroscopy (ICP-MS) to determine the crystal structure, functional group, morphology and crystal purity. Samples with an initial temperature of 25 °C were chosen because the filtration process was carried out at room temperature so that the crystal characteristics were not affected during the filtration process. The XRD (SmartLab SE-HyPix-400) instrument was utilized to determine the crystal structure and to detect the presence of NH₄⁺ compounds. The instrument use Cu-Kα radiation with a wavelength of 1.5418 Å and a scanning rate of 0.3°/min. FTIR (JASCO-FT/IR-4X) was utilized to analyze the functional group. The analysis was conducted in the wavenumber range of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹. All XRD and FTIR analyses were conducted within a maximum of 48 h after synthesis to prevent any alterations in the crystal characteristics. The SEM (Apreo 2 HiVac) instrument was utilized to analyze the crystal’s morphology. The ICP-MS (Prodigy DC Arc) instrument was employed to identify the purity of the sample with a detection limit of up to 0.1 mg/L. The sample for ICP-MS characterization was prepared by dissolving 0.5 g of the crystal in 100 mL of distilled water. ICP-MS testing was carried out three times for each sample to determine the error bar value. The purity of the cobalt sulfate hexahydrate crystal was determined with following equation:

$$Crystal purity=100\%-\left({\displaystyle \frac{M_a}{M_o}}\times 100\%\right)$$

(8)

$$M_a=C_a\times V$$

(9)

where M_a represents the mass of impurities in the crystal (mg), M_o represents the mass of the crystal (mg), C_a is the concentration of impurities in the solution (mg/L), and V is the volume of the solution (L).

3. Results and Discussions

The kinetic behavior of the cobalt sulfate hexahydrate crystals is illustrated in Figure 3. The graphs are plotted with the value of ln [−ln (1 − x (t))] in relation to the value of ln t, which corresponds to Equation (6). The equation of each plot contains the JMA exponent value (n) and the crystal growth rate. The JMA exponent value (n) is represented by the gradient value, while the nucleation and crystal growth rate are represented by the intercept value (constant ln k). The JMA exponent value (n) spans from 0.28 to 0.58, indicating a similar dimensionality of growth across varying temperatures [16]. The nucleation and crystal growth rate value spans from −1.88 to −6.07, which indicates a low crystal output yield. The low crystal output yield in this study is correlated with a low supersaturation value that was intentionally selected to prevent spontaneous nucleation and control the crystal growth process. However, these low values do not reflect an industrial-scale crystallization process, as industrial operations typically use higher supersaturation levels to maximize crystal output yield. The nucleation and crystal growth rate value also shows an increase as the initial temperature decreases, which indicates that the crystallization process occurs faster at lower temperatures. An increase in the crystal growth rate at a low initial temperature is related to an increase in the supersaturation value of the solution at a lower temperature, so that the crystallization process can occur more easily.

The JMA plot shows that ammonium ions can change the crystallization growth rate pattern of cobalt sulfate hexahydrate. The addition of ammonium up to 3.75 g/L can reduce the growth rate value. While the addition of 5 g/L of ammonium can increase the growth rate, as shown by the higher values of the purple dots and lines compared to the green ones in Figure 3, this pattern is shown at initial temperatures of 25 °C and 30 °C. At initial temperatures of 35 °C and 40 °C, the crystal growth rates with the addition of 3.75 g/L and 5 g/L ammonium tend to be similar. This phenomenon is likely due to the lower supersaturation at higher temperatures, which results in a lower concentration of cobalt ions, thereby reducing the interaction between ammonium and cobalt ions. As a result, the influence of ammonium on the crystal growth rate becomes less significant. On the other hand, at higher supersaturation levels, the interaction between ions is more intense and leads to a greater impact of ammonium on the crystallization process [17].

The sudden change in the crystal growth rate pattern with the addition of impurities indicates that the interaction of ammonium ions during the crystallization process might change. In the absence of impurities, cobalt ions (Co²⁺) undergo solvation during their dissolution and interact with water molecules to form a cobalt aquo complex [Co(6H₂O)]²⁺. The cobalt aquo complex then binds with SO₄²⁻ ions without any obstacle to produce the Co(SO₄)₂·6H₂O nucleus after the seed is added or attached to the seed and grows into a cobalt sulfate hexahydrate crystal. When a low-impurity concentration is added to the solution, there is a competing process between the cobalt aquo complex and ammonium ions to bind with SO₄²⁻ ions, which results in increasing the solubility of the solute and reducing the crystal growth rate. Furthermore, when a high-impurity concentration is added to the solution, there is a possibility that ammonium ions (NH₄⁺) will interact with the cobalt aquo complex because the availability of sulfate ions is not sufficient to interact with both ions. The interaction between ammonium ions with the cobalt aquo complex causes the production of Tutton’s salt. The formation of this salt results in decreasing the solubility of the solute and increasing the crystal growth rate [18]. Figure 4 illustrates a schematic of the crystallization process of cobalt sulfate hexahydrate with the addition of NH₄⁺ impurities.

The formation of Tutton’s salt is proven by the XRD results shown in Figure 5. The XRD data are matched with PDF #16-0304, which indicates CoSO₄·6H₂O (moorhouseite). The crystal has a monoclinic system with the space group C2/c, and parameter cell a = 10.04 Å, b = 7.234 Å, c = 24.3 Å, α = γ = 90°, and β = 98.34, with a volume of 1746.2 ų [19]. The incorporation of ammonium ions into the crystal structure leads to the formation of ammonium cobalt sulfate hydrate (NH₄)₂Co(SO₄)₂·6H₂O (as the expected Tutton’s salt, under a high concentration of ammonium). The data are matched with PDF #18-0086. The crystal has a monoclinic system with space group P21/a, and parameter cell a = 9.23 Å, b = 12.49 Å, c = 6.23 Å, α = γ = 90°, and β = 106.9°, with a volume of 687.2 ų [20,21]. Figure 5b is an enlarged image of (a). Blue arrows 1–3 point to the peaks of Tutton’s salt.

The JMA plot functions were further analyzed to determine the crystallization activation energy. The activation energy indicates the energy barrier that must be overcome in order for the crystallization process to take place. Higher activation energy could decrease the crystal growth rate, while lower activation energy could increase the crystal growth rate. The activation energy was calculated according to the Arrhenius equation described in the characterization and analysis method of Equation (7). Figure 6a demonstrates the relationship between the ln (k_g) value and the 1/T value. The slope of the plot was then used to calculate the activation energy. The crystallization activation energy of cobalt sulfate hexahydrate under the influence of ammonium ions is shown in Figure 6b. The crystallization activation energy of cobalt sulfate hexahydrate without impurity is 150.93 kJ/mol. When a low-ammonium concentration is added, the activation energy increases. The highest increase in the activation energy value was shown by adding 3.75 g/L ammonium ions with an increase of 5.56 % to 159.33 kJ/mol. The crystallization activation energy then decreases by 2.24% when the concentration of added ammonium was 5 g/L. This pattern explains why there is a decrease in the crystal growth rate with the addition of ammonium up to a certain concentration, and an increase thereafter.

Further characterization was carried out to examine the effect of ammonium presence on any disorder or structural change in the host lattice of the crystal. The FTIR characterization results are shown in Figure 7. The FTIR graph pattern for all samples mostly tends to be the same with noise arising from the absorption of atmospheric water vapor (H₂O) due to fluctuations during the measurement process. However, the noise does not affect the material’s characteristic peaks, ensuring the accuracy of the analysis. The notable change observed with the addition of ammonium up to a concentration of 3.75 g/L is a reduction in the transmittance value, while an increase in transmittance occurs when the ammonium concentration reaches 5 g/L. The addition of ammonium at a concentration of 5 g/L also generates an additional peak at wavenumber 1155 cm⁻¹. This behavior is associated with the formation of Tutton’s salt at higher ammonium concentrations. In general, all samples have four principal vibrational modes, υ₁, υ₂, υ₃, and υ₄, which indicate the stretching vibrations of SO₄²⁻. The four principal vibrational modes are shown at wavenumbers 983, 443, 1105, and 611 cm⁻¹, respectively. When a high concentration of ammonium is added, SO₄²⁻ ions will coordinate with the ammonium ion to form an unidentate complex. This coordination process causes the splitting of υ₃ into two bands, which can be seen by increasing the peak at a wavenumber 155 cm⁻¹ [22].

The SEM characterization results are displayed in Figure 8. The characterization shows that the addition of ammonium impurity does not affect the crystal shape of cobalt sulfate hexahydrate. As seen in the figure, the size of the crystals formed is also relatively the same. However, further characterization needs to be carried out to determine the effect of ammonium on the average size of the crystals produced.

The crystal purity of cobalt sulfate hexahydrate is shown in Figure 9. The test was carried out three times for each sample to determine the average and error bar value. The characterization results show that ammonium ions tend to have a threshold limit for incorporation into the cobalt sulfate hexahydrate crystal. This is demonstrated by the purity value, which remains nearly unchanged with the addition of ammonium up to 3.75 g/L. The crystal purity without any impurities is 99.97 ± 0.07%. The crystal purity values with the addition of ammonium up to 3.75 g/L are 99.85 ± 0.14%, 99.8 ± 0.2%, and 99.78 ± 0.16%, respectively. Although there was a slight decrease in the average purity, the reduction was not significant, and the error bar width remained almost within the same range. This suggests that the decrease in crystal purity was primarily due to the adsorption of ammonium onto the crystal, as illustrated in the impurity uptake mechanism in Figure 3. Furthermore, crystal purity began to decrease significantly after adding 5 g/L of ammonium with a purity value of 99.09 ± 0.25%. This observation is consistent with the earlier explanation and is further supported by the XRD and FTIR results, which show that ammonium started to incorporate into the crystal lattice to form Tutton’s salt of ammonium cobalt sulfate hexahydrate.

4. Conclusions

This research demonstrated that ammonium (NH₄⁺) ions influence the crystallization process and characteristics of cobalt sulfate hexahydrate crystals. The findings of this research are the following:

• Competitive interaction between ammonium ions and the cobalt aquo complex for binding with SO₄²⁻ ions reduces the crystal growth rate and promotes an elevation in the crystallization activation energy. The incorporation of ammonium ions into the crystal structure produces ammonium cobalt sulfate hexahydrate crystals (Tutton’s salt), hence increases the crystal growth rate and reduces the crystallization activation energy.
• The addition of ammonium up to a concentration of 3.75 g/L does not alter the resulting crystal structure, whereas the addition of 5 g/L ammonium leads to the formation of a new crystal structure (Tutton’s salt). The addition of ammonium at any concentration does not alter the crystal shape.
• The reduction in crystal purity upon adding a low concentration of ammonium was mainly caused by the adsorption of ammonium onto the crystal.