Effect of the Occurrence State of Dodecylamine on the Adsorption Behavior of Calcium Sulfate Dihydrate and Silica

Abstract

In this work, the effects of dodecylamine storage state on the adsorption behavior of calcium sulfate dihydrate and silica were systematically investigated by using Raman detection, solution equilibrium calculation, and calculation based on density functional theory. The results show that the selective adsorption behavior of dodecylamine with calcium sulfate dihydrate and silica is closely related to its occurrence state. The adsorption of dodecylamine in the ionic state with calcium sulfate dihydrate and silica is dominated by the strong electrostatic adsorption between the H-O atoms under acidic conditions, while that of dodecylamine in the molecular state is dominated by the weak electrostatic adsorption between the Ca-N or Si-N atoms under alkaline conditions. Finally, by comparing the distribution coefficients and adsorption energies of the ionic/molecular states of dodecylamine with the change in pH, the reason why dodecylamine adsorbs calcium sulfate dihydrate more readily under acidic conditions was explained at the atomic level.

1. Introduction

Phosphogypsum is a large waste byproduct of the phosphochemical industry. For every ton of phosphoric acid produced, about 5 tons of phosphogypsum are produced [1]. The global inventory of phosphogypsum has exceeded 6 billion tons [2,3], and the cumulative storage of phosphogypsum in China has exceeded 830 million tons, with an annual utilization rate of about 45% [4]. Long term storage not only occupies land, but also easily causes comprehensive pollution of water, soil, and air [5]. The main component of phosphogypsum is calcium sulfate dihydrate (CaSO₄·2H₂O, 80%–90%), and the main impurity is silicon dioxide (SiO₂, 4%–20%) [4]. The industrial application of phosphogypsum is greatly restricted by coexisting impurities [6,7,8,9,10,11,12,13]. Selective separation of calcium sulfate dihydrate and silicon dioxide is the key to impurity removal.

The methods for removing impurities from phosphogypsum include water washing (to remove water-soluble fluorine/phosphorus, etc.) [14], acid leaching (to remove impurities such as sulfuric acid/hydrochloric acid, etc.) [15], and organic amine flotation (to remove insoluble impurities such as silicon) [16,17]. The organic amine flotation method is becoming a research hotspot due to its ability to effectively remove silicon and prepare high-purity calcium sulfate dihydrate, as shown in

Table 1. Previous research on the flotation process to remove silica from phosphogypsum.

Collector	Frother	Depressant	pH	Gypsum (%)	Silica (%)	Yield (%)	Ref.
DH	FX	None	2.0	96.33	9.08→0.62	69.85	[18]
DDBA	MIBC	None	2.3–2.5	94.88	7.28→1.91	——	[19]
2HEAC-12	MIBC	None	2.3–2.5	93.47	7.28→2.71	——	[19]
DDA	Pine Oil	Sodium silicate	2.5	80.65→95.63	——	——	[20]
ODA	Pine Oil	Sodium silicate	2.5	80.65→92.80	——	——	[20]
DTAC	Pine Oil	Sodium silicate	2.5	80.65→93.86	——	——	[20]
Mixed Amine	Pine Oil	Sodium silicate	2.5	80.65→95.89	——	79.93	[20]
DDA	None	None	2.0	89.0→97.5	12.03→1.17	98.58	[21]

. The reason why amine collectors are effective is that they have different adsorption properties on the surfaces of calcium sulfate dihydrate and silica, which changes the hydrophobicity of calcium sulfate dihydrate and silica in aqueous solution and promotes the separation of silica and gypsum. As shown by Qi et al. [18], before and after the addition of the dodecylamine hydrochloride (DH) collector, gypsum exhibits strong N-H bond stretching vibration peaks in FTIR, while quartz only shows very weak N-H bond stretching vibration peaks. At the same time, the zeta potential of gypsum and quartz changes with pH. At pH 2.0, the contact angle of gypsum increases from 34.70° to 62.97°, and the contact angle of quartz increases from 34.41° to 42.44°. After interacting with the collector, gypsum becomes more hydrophobic and floats in water by binding with bubbles through hydrophobic interactions, while quartz still maintains hydrophilicity and cannot stably bind with bubbles, thus achieving selective separation of gypsum and quartz.

The organic amine collectors used in the experimental study of silicon removal by phosphogypsum flotation include dodecyl dimethyl ethylbenzyl ammonium chloride (DDEA), dodecylbis(2-hydroxyethyl) methylammonium chloride (2HEAC-12), dodecylamine hydrochloride (DH), dodecylamine (DDA), octadecylamine (ODA), dodecyltrimethylammonium chloride (DTAC), mixed amine, etc. Among them, the direct flotation of phosphogypsum by dodecylamine has the most significant effect and the widest source. Zhang et al. used dodecylamine as a collector to obtain a concentrate product with a gypsum purity of 97.50% and a silica content of 1.17%, which meets the first-grade requirements of the Chinese national standard.

Previous work has shown that pH significantly affects the flotation efficiency of dodecylamine. For example, Qi et al. treated phosphogypsum with dodecylamine (30 mg/L) at pH 2.0, which increased the purity of gypsum from 84.44% to 96.33% and reduced the silica content from 9.08% to 0.62% [22]. Guo et al. found that at pH 3–11, the purity of gypsum concentrate decreased from 90.03% to 82.55%, and the silica content increased from 2.60% to 6.50% [23]. The acid flotation effect of dodecylamine on phosphogypsum was significantly better than that of alkaline flotation. Current research has mostly focused on describing experimental phenomena and lacks in-depth analysis at the atomic level of the correlation between pH and the existence state (ionic/molecular) of dodecylamine and its adsorption mechanism.

This study investigated the changes in the occurrence state of dodecylamine at different pH values and its effects on the adsorption mechanism of calcium sulfate dihydrate and silica from the perspective of interaction by combining Raman detection, solution equilibrium calculation, and density functional calculation. The reason why dodecylamine is more prone to adsorb calcium sulfate dihydrate under acidic conditions was identified at the atomic level.

2. Materials and Methods

All chemical reagents (dodecylamine, calcium sulfate dihydrate, silica, and deionized water) were commercially purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China), analytically pure, and used without further purification. A certain amount of analytical pure dodecylamine liquid was measured and dissolved in deionized water to prepare the dodecylamine solution with the concentration of 2% wt. The pH of the solution was adjusted with sulfuric acid or sodium hydroxide solution to obtain dodecylamine solutions at different pH values as Raman samples. X-ray powder diffraction (XRD, D8 Advance, Bruker Instruments, Karlsruhe, Germany) was used to confirm the crystalline structures of the samples. The Raman analysis was performed using the LabRAM HR Evolution Raman spectrometer from HORIBA (Montpellier, France).

Density functional theory (DFT) calculations were performed using the DMol [3]program with the spin unrestricted approach [24,25]. The generalized gradient approximation functional externalized using the Perdew–Burke–Ernzerhof [26] and DFT semi-core pseudopots were employed in all calculations. The cutoff energy was set as 500.0 eV. Reciprocal space integration was performed with a Monkhorst–Pack grid of 2 × 2 × 1. The unit cell parameters of calcium sulfate dihydrate are a = 6.284 Å, b = 15.200 Å, c = 6.523 Å; α = 90.000°, β = 127.414°, γ = 90.000° [27]; the unit cell parameters of silica are a = 4.913 Å, b = 4.913 Å, c = 5.405 Å; α = 90.000°, β = 90.000°, γ = 120.000°. The thickness of the vacuum layer was set to 30 Å.

The adsorption energy (E_abs) of dodecylamine adsorbed on the surface can be calculated as:

$$E_{abs}=E_{surface+dodecylamine}-E_{surface}-E_{dodecylamine}$$

where E_{surface+dodecylamine} is the total energy of the surface with a dodecylamine molecule or ion adsorbed, E_surface is the energy of the blank surface (calcium sulfate dihydrate or silica), and E_dodecylamine is the energy of a dodecylamine molecule or ion.

3. Results

3.1. The Effect of pH on the Occurrence State of Dodecylamine

Figure 1 shows the effect of pH on the Raman spectra of a 2% solution of dodecylamine. Spectral lines in the 400–600 cm⁻¹ region represent the vibrational peaks of dodecylamine carbon skeleton deformation; peaks in the 800–1200 cm⁻¹ region are the rocking vibration peaks of dodecylamine CH₂ and CH₃, as well as the stretching vibration peaks of CH₂-CH₂ and CH₂-CH₃; peaks in the 1300–1450 cm⁻¹ region consist of the CH₂ shear vibration peak and the CH₃ asymmetric deformation vibration peak of dodecylamine; and the shear peaks near 1600 cm⁻¹ represent NH₂ or NH₃⁺ [28]. Among them, the 1589 cm⁻¹ peak corresponds to the NH₃⁺ shear vibration peak, the 1587 cm⁻¹ peak corresponds to the NH₃⁺ and NH₂ shear vibration peaks, and the 1577 cm⁻¹ peak corresponds to the NH₂ shear vibration peak. The peak in the range of 400–1450 cm⁻¹ did not show a significant shift when the pH of the solution decreased from 12.0 to 2.0, but the NH₂ group gradually protonated to form NH₃⁺, causing the 1577 cm⁻¹ shear vibration peak to shift towards the higher wave number (1589 cm⁻¹ peak). The Raman spectroscopy detection results in Figure 1 indicate that under acidic conditions, dodecylamine in the solution is mainly in the ionic state of NH₃⁺. Under alkaline conditions, the dodecylamine in solution is mostly in the molecular state NH₂.

3.2. Calculation

Figure 2 shows the XRD patterns of pure calcium sulfate dihydrate and silica, with diffraction peaks located at 11.75°, 23.50°, and 29.21°, which correspond well to the (020), (040), and (041) crystal planes of CaSO₄·2H₂O (PCPDF 33-0311). The peaks located at 20.83°, 26.61°, and 50.11° correspond well to the (100), (101), and (112) crystal planes of SiO₂ (PCPDF 46-1045). The (020) and (101) crystal planes are the strongest peaks in XRD, so the (020) surface of calcium sulfate dihydrate and the (101) surface of silicon dioxide were selected to conduct further calculations.

Figure 3a–d show the stable adsorption configuration of CaSO₄·2H₂O (020) to C₁₂H₂₅NH₃⁺, CaSO₄·2H₂O (020) to C₁₂H₂₅NH₂, SiO₂ (101) to C₁₂H₂₅NH₃⁺, and SiO₂ (101) to C₁₂H₂₅NH₂, respectively. Calculation results show that the adsorption energies are E₁ = −20.105 eV, E₂ = −1.998 eV, E₃ = −13.036 eV, and E₄ = −3.169 eV, respectively. The isoelectric point of calcium sulfate dihydrate and silicon dioxide is approximately pH = 2.0 [29,30,31]. When pH > 2.0, both calcium sulfate dihydrate (020) and silicon dioxide (101) crystal surfaces are negatively charged. The amino group head of the dodecylamine ion (C₁₂H₂₅NH₃⁺) is an electron deficient group with a positive charge, while the amino group head of the dodecylamine molecule (C₁₂H₂₅NH₂) is an electron rich group with a negative charge. Dodecylamine adsorbs onto the crystal faces of calcium sulfate dihydrate (020) and silicon dioxide (101) through the amino group head.

Figure 4 shows the key atomic interaction distance of dodecylamine in ion/molecular state adsorbed with calcium sulfate dihydrate and silica, as well as the charge density difference. The blue area in Figure 4 represents the region where the electron density increases, while the yellow area represents the region where the electron density decreases. The isovalue is 0.01 e/ų. The atoms exposed on the (020) surface of calcium sulfate dihydrate are Ca atoms and O atoms of sulfate ions, while the atoms exposed on the (101) surface of silicon dioxide are Si atoms and O atoms.

Figure 4a,b show that there is a significant electronic interaction between two H-O atoms at the CaSO₄·2H₂O (020)-C₁₂H₂₅NH₃⁺ adsorption interface, with distances of 2.018 Å and 2.708 Å, respectively. The length of the H-O bond in the O-H…O hydrogen bond is approximately 1.200–1.215 Å [32], and the length of the H-O bond in the N-H…O hydrogen bond is approximately 1.95 Å [33], indicating electrostatic interaction between H-O atoms. Figure 4c,d show that there is a significant electronic interaction between one Ca-N atom and one H-O atom at the CaSO₄·2H₂O (020)-C₁₂H₂₅NH₂ adsorption interface. The distances between Ca-N atoms and H-O atoms are 2.140 Å and 2.501 Å, respectively. The bond length of Ca-N is approximately 2.31–2.51 Å [34], indicating the formation of coordination bonds between Ca-N atoms. Figure 4e,f show that there is a significant electronic interaction between one H-O atom and one H-Si atom at the SiO₂(101)-C₁₂H₂₅NH₃⁺ adsorption interface. The distances between H-O atoms and H-Si atoms are 2.707 Å and 2.987 Å, respectively, both of which are electrostatic interactions. Figure 4g,h show that there is a significant electronic interaction between one Si-N atom and two H-O atoms at the SiO₂ (101)-C₁₂H₂₅NH₂ adsorption interface. The distances between Si-N atoms and H-O atoms are 1.885 Å, 2.423 Å, and 2.637 Å, respectively. The Si-N bond length is approximately 1.70–1.78 Å [35], indicating that there is electrostatic interaction between Si-N atoms.

For the CaSO₄·2H₂O (020) surface, the smaller the distance between H-O atoms, the greater the electrostatic attraction and adsorption energy. For the SiO₂ (101) surface, the nitrogen atom at the head of the dodecylamine group has one lone pair electron, which coordinates or electrostatically attracts positively charged Ca and Si atoms on the crystal plane. The smaller the distance between the atoms, the greater the attraction and the higher the adsorption energy. The electronegativity of oxygen (O) atoms is 3.44, and the electronegativity of nitrogen (N) atoms is 3.04 [36]. Since the electronegativity of O atoms is greater than that of N atoms, the electrostatic attraction between H-O atoms is greater than that between Si-N atoms and Ca-N atoms. Therefore, the order of the adsorption forces on the surface is: CaSO₄·2H₂O (020)-C₁₂H₂₅NH₃⁺ > SiO₂(101)-C₁₂H₂₅NH₃⁺ > SiO₂ (101)-C₁₂H₂₅NH₂ > CaSO₄·2H₂O (020)-C₁₂H₂₅NH₂. This is in good agreement with the calculated results of the head charge and adsorption energy of dodecylamine, where the more positive the charge, the greater the adsorption energy; The more negative the charge, the smaller the adsorption energy, indicating that the ionic dodecylamine has a stronger adsorption effect on calcium sulfate dihydrate and silica, and is more easily adsorbed with calcium sulfate dihydrate. The adsorption energy, atomic distance, and interaction types of dodecylamine of ion/molecular states on the surface of calcium sulfate dihydrate and silica are summarized in

Table 2. The summary of adsorption properties of dodecylamine in ionic/molecular states on the surface of calcium sulfate dihydrate and silica.

Adsorption System	Adsorption Energy (eV)	Key Atomic Distances (Å)	Interaction Type	Ref.
CaSO4·2H2O (020)-C12H25NH3+	−20.105	H-O: 2.018, 2.708	Strong electrostatic adsorption	[32,33,36]
CaSO4·2H2O (020)-C12H25NH2	−1.998	H-O: 2.140; Ca-N: 2.501	Weak coordination	[34,36]
SiO2(101)-C12H25NH3+	−13.036	H-O: 2.707; H-Si: 2.987	Electrostatic adsorption	[35,36]
SiO2 (101)-C12H25NH2	−3.169	H-O: 2.423, 2.637; Si-N: 1.885	Coordination/Electrostatic	[35,36]

. The data in Table 2 indicate that CaSO₄·2H₂O (020)-C₁₂H₂₅NH₃⁺ is the dominant form of adsorption in competitive adsorption.

3.3. The Effect of pH on Selective Adsorption of Dodecylamine

As dodecylamine exists in the form of both ion and molecule in the aqueous solution, and the ion/molecule proportion is closely related to pH, the concept of mixed adsorption energy is proposed to investigate the effect of pH on selective adsorption of dodecylamine. The process is as follows.

In the aqueous solution of dodecylamine, dodecylamine will dissolve in water:

$$C_{12}{H}_{25}N{H}_2+H_{2}O\rightleftharpoons C_{12}{H}_{25}N{H}_3^{+}+O{H}^{-}$$

with $K_b=\left[C_{12}{H}_{25}N{H}_3^{+}\right]\left[O{H}^{-}\right]/\left[C_{12}{H}_{25}N{H}_2\right]$ and $p{K}_b=3.37$ [37].

Combined with $K_w=\left[H^{+}\right]\left[O{H}^{-}\right]={10}^{-14}$, the distribution coefficient of ionic dodecylamine

$$\delta \left[C_{12}{H}_{25}N{H}_3^{+}\right]={\displaystyle \frac{1}{1+\frac{K_w}{K_b{10}^{-pH}}}}$$

and the distribution coefficient of molecular dodecylamine

$$\delta \left[C_{12}{H}_{25}N{H}_2\right]={\displaystyle \frac{1}{1+\frac{K_b{10}^{-pH}}{K_w}}}$$

could be derived.

The mixed adsorption energy of dodecylamine on calcium sulfate dihydrate under different pH conditions is defined as

$$E_{mix1}=E_1\delta \left[C_{12}{H}_{25}N{H}_3^{+}\right]+E_2\delta \left[C_{12}{H}_{25}N{H}_2\right],$$

the mixed adsorption energy of dodecylamine on silica under different pH conditions as

$$E_{mix2}=E_3\delta \left[C_{12}{H}_{25}N{H}_3^{+}\right]+E_4\delta \left[C_{12}{H}_{25}N{H}_2\right],$$

and the difference in adsorption energy of calcium sulfate dihydrate and silica mixture by dodecylamine as

$$\Delta E=E_{mix1}-E_{mix2}$$

The distribution coefficient of ion/molecular state dodecylamine varies with pH as shown in Figure 5a. When pH < 10.63, dodecylamine mainly exists in an ionic state. When pH > 10.63, dodecylamine mainly exists in molecular form. At pH 10.63, the distribution coefficients of dodecylamine in both molecular and ionic states are equal, both being 0.5.

Figure 5b shows the variation in E_mix1, E_mix2, and ΔE with pH under different pH conditions. When pH < 10.63, the negative value of ∆E is large, indicating that dodecylamine has a much higher adsorption energy for calcium sulfate dihydrate than for silica, indicating that dodecylamine is more likely to adsorb calcium sulfate dihydrate at this time. After pH > 10.63, the negative value of ∆E sharply decreases, indicating that the difference in adsorption energy between dodecylamine and dodecylamine becomes smaller, and the selective adsorption becomes worse.

4. Conclusions

This work investigates the effect of the occurrence state of dodecylamine on the adsorption properties of calcium sulfate dihydrate and silica by combining Raman analysis, solution equilibrium calculation, and DFT calculation. At pH > 2.0, the calcium sulfate dihydrate (020) surface and silica (101) surface are negatively charged and adsorb with the head of the dodecylamine group. Computational results show that under acidic conditions, dodecylamine is mainly in an ionic state, and the head of the amine group is positively charged. The electrostatic adsorption between the H-O atoms of calcium sulfate dihydrate and silica is stronger, making it easier to adsorb calcium sulfate dihydrate. Under alkaline conditions, dodecylamine is mainly in a molecular state, with a negative charge on the head of the amine group. The adsorption between Ca-N and Si-N atoms of dihydrate calcium sulfate and silica is weak and the difference is not significant. By comparing the distribution coefficient of ion/molecular state dodecylamine and the variation in adsorption energy with pH, the reason why dodecylamine is more prone to adsorb dihydrate calcium sulfate under acidic conditions was identified at the atomic level, providing theoretical guidance for phosphogypsum flotation and process optimization.