The Mechanism of Selective Separation of Antimony from Chloride Leachate of Copper Anode Slime

The mechanism of selective separation of antimony from copper anode slime leachate is studied. Density functional theory is used to calculate the unit cell energy, band structure, density of states, number of charge distributions and overlap populations of the SbCl3 hydrolysates SbOCl, Sb3O4Cl and Sb4O5Cl2, and these are studied by infrared spectroscopy. The pathways for formation of SbOCl, Sb3O4Cl and Sb4O5Cl2 in water, ethanol and ethylene glycol systems are clarified. The results show that the energy of the SbOCl unit cell is the lowest, and the structure is more stable. SbOCl exhibits strong electronic localization, weak orbital expansion, and more stability; SbOCl has a wider pseudoenergy gap, and its covalent bonds are stronger than those of Sb3O4Cl and Sb4O5Cl2. Hydrolysis of SbCl3 leads to preferential generation of more stable structures. The charge distribution, the sizes of overlap populations, and the infrared spectra indicate that SbCl3 has been hydrolysed. There are two basic paths for the process of alcoholysis: a wateror alcohol-ionized hydroxyl replaces one chlorine atom in SbCl3 to form an [Sb(OH)Cl2] monomer, and as the degree of hydrolysis increases, the H and Cl atoms are removed to form SbOCl. Conversely, Sb3O4Cl, Sb4O5Cl2 or Sb atoms may be formed an [Sb-OH] monomer generated directly by wateror alcohol-ionised hydroxyl groups, and Cl atoms replace the H atom in the hydroxyl group of the [Sb-OH] monomer to form SbOCl, Sb3O4Cl and Sb4O5Cl2.


Introduction
Copper anode slime is an important by product of the copper electrolytic refining process, and its yield accounts for approximately 0.2%-0.8% of the output of refined copper [1]. In 2019, China's copper anode slime output reached 78,000 t. Copper anode slime is rich in many rare and precious metals, such as gold, silver, selenium, tellurium, antimony, bismuth, and metals of the platinum group [1,2]. At present, there are many processes for treating copper anode slime to recover these valuable metals, such as the traditional pyrometallurgical process [3][4][5], a combined process of beneficiation and metallurgy [3,5,6], the Kaldor furnace process [3,5], and a hydrometallurgical process [3,5,7]. Nevertheless, many problems still arise in these processes during industrial production, such as high energy consumption for pyrometallurgical processes, heavy pollution for combined processes, poor raw material applicability for Kaldor furnace processes, and difficult subsequent metal separation for hydrometallurgical processes. To recover valuable metals from anode slime in a more economical, efficient and environmentally-sound fashion, an innovative process was proposed and industrialized at Jiangxi Copper Co., Ltd. The process mainly includes "mixed acid leaching of bismuth and antimonyseparation of Cu by sulfated roasting followed by water leaching -sodium hydroxide leaching of tellurium -recovery and separation of gold and silver". In this process the following metals in the indicated proportions are effectively separated from copper anode slime: > 9-% Bi, 95% Sb, 90% Se, 95% Cu, 95% Au and 96% Ag. This shows that the process allows comprehensive recovery of valuable metals from copper anode slime. However, there are few reports on the mechanism of selective separation of valuable metals from copper anode slime. This makes it difficult to accurately control the metal separation process, which is not conducive to the stable operation of smelters.

Materials and methods
Using first-principles density functional theory (DFT) combined with the plane pseudopotential method, the CASTEP [20,21] module of MS software was used to optimize the unit cell structure and calculate the unit cell system energy and band structure of antimony compounds, as well as the total density of states and sub-state density, number of charge layouts, and number of overlapping clusters. The method established unit cell models for SbCl3, SbOCl, Sb3O4Cl, and Sb4O5Cl2 according to the relevant parameters in Table 1; the generalized gradient approximation (GGA) was used to correct the exchange effect from interactions between electrons in different unit cells; The Perdew-Burke-Ernzerh (PBE) in Generalized Gradient Approximation (GGA) was used to deal with interactions between electrons. The Monkhorst-Pack scheme was used to select the grid points in k-space and calculate the total energy and charge density of the system in the Brillouin zone. Brillouin zone k vectors were selected as 1×2×2, 2×2×3, 2×4×3 and 3×4×1; plane wave truncation energies were set to 244.9 eV, 489.8 eV, 489.8 eV and 489.8 eV, the self-consistent accuracy was set to 2.0×10-6 eV/atom, and the force acting on each atom did not exceed 0.05 eV/nm. Solid SbCl3 and a certain concentration of SbCl3 solution were used to carry out hydrolysis and alcoholysis reactions in different systems. SbCl3 solid powder was dissolved in hydrochloric acid to prepare a solution with concentrations of 0.05 mol/L Sb 3+ and 1 mol/L Cl -, and this was added to equal volumes of deionized water, ethanol, and ethylene glycol. The reactions were stirred at room temperature (25℃) for 2 h, the pH values of the solutions were adjusted to 2 with aqueous ammonia, they were placed in an ultrasonic reactor for 1 h, and the dry product was obtained by centrifugal filtration at 60℃ for 6h. It can be seen from Table 2 that the SbOCl unit cell has the lowest energy. A preliminary analysis of the hydrolysis of SbCl3 suggests that priority will be given to the generation of SbOCl, due to its low energy and stable structure.

Energy band analysis
The wider the energy band is, the greater the fluctuations, indicating that the effective mass of the electrons in the energy band is smaller, the extent of delocalization is greater, and the atomic orbitals that make up this energy band are more extended and more accessible for reactions. A narrow energy band width indicates that the eigenstate of the energy band is mainly composed of atomic orbitals at a certain lattice point in the local area. The electrons in this energy band are strongly localized, exhibiting limited orbital expansion and contributing to greater stability [25,26].
The calculated energy band diagram for each substance is shown in Figure 1. This shows that the energy band of SbCl3 extends from -28.5 eV to 15.9 eV, so the bandwidth is 44.4 eV and the band gap is 4.152 eV; the energy band of SbOCl extends from -28.6 eV to 16.4 eV, so the bandwidth is 45.0 eV and the band gap width is 3.101 eV; the energy band of Sb3O4Cl extends from -28.6 eV to 17.8 eV, so the bandwidth is 46.4 eV and the band gap is 2.737 eV; the energy band of Sb4O5Cl2 extends from -28.8 eV to 17.1 eV, so the bandwidth is 45.9 eV and the band gap is 2.730 eV. By comparison, it can be seen that SbOCl has the narrowest energy band width and smallest fluctuations, indicating that electronic localization in the SbOCl energy band is very strong, orbital expansion is limited, the properties are relatively stable, the energy of the unit cell is consistent, and SbOCl more easily forms a stable structure.

Density analysis
The size of the pseudoenergy gap in the density of states map (the peaks and valleys of the lowenergy bonding state and the high-energy antibonding region produce the pseudoenergy gap) reflects the strength of covalent bonding. The wider the pseudoenergy gap is, the greater the covalent character. The larger the abscissa of the peak density of states, the easier it is for extranuclear electrons to be distributed in the high-energy region, and the easier it is to remove electrons. In contrast, it is also easier to accept electrons [27,28].
The calculated density of states diagram for each substance is shown in Figure 2. It can be seen that the pseudo-energy gap of SbCl3 extends from -0.386 to 4.290 eV and the pseudo-energy gap is 4.676 eV, which is mainly contributed by the electrons in Cl 2p and Sb 6s and 6p orbitals; the pseudo-energy gap of SbOCl extends from -1.400 to 3.930 eV and the pseudo-energy gap is 5.330 eV; the pseudoenergy gap of Sb3O4Cl extends from -0.393 to 3.610 eV and the pseudo-energy gap is 4.003 eV; and the pseudo-energy gap of Sb4O5Cl2 extends from -0.524 to 3.500 eV and the pseudo-energy gap is 4.024 eV. Antimony oxychlorides are mainly contributed by electrons on the electron orbitals of Cl2p, O2p and Sb6s6p. By comparison, it can be seen that the pseudoenergy gap of SbOCl is the largest, and the extent of covalent bonding among its atoms is the greatest. Therefore, the hydrolysis of SbCl3 more easily generates stable SbOCl.

The charge distribution number analysis
The charge distribution number can reflect the gain or loss of electrons in an atom. A positive charge indicates a loss of electrons, and a negative charge indicates a gain of electrons. The gain and loss of electrons can strengthen the bonding interactions between atoms. The greater the number of electrons transferred, the stronger the bond interaction between atoms, and vice versa [29].
The calculated formal charge for each substance is shown in Table 3. It can be seen from Table 3

Overlap population analysis
The overlap population can be used to reflect the interaction between atoms, analyse the bonding properties between atoms and reflect the strength of bonds between atoms. The numbers for overlap populations are positive, which means that the atoms are bonded by covalent bonds. The larger the value is, the stronger the covalent bond and the more stable the unit cell structure; if the value for an overlap population is negative, it means that atoms are combined in the form of anti-bonds. The smaller the value, the stronger the repulsion between atoms and the weaker the stability; when the overlap population is 0, there are ionic bonds between atoms [30][31][32]. The calculated overlap populations for each substance are shown in Table 4. We can see that the overlap populations of SbCl3 are both positive (0.29 and 0.31), indicating that Cl and Sb atoms are connected by covalent bonds. The sums of the overlapping populations of Sb-Cl bonds are 1.16 and 2.48, respectively and the total is 3.64; the greater the population, the stronger the stability of the covalent bond; therefore, each [SbCl3] unit contains one Sb-Cl bond with an overlap population of 0.29 and two Sb-Cl bonds with overlap populations of 0.31. Therefore, since 0.31>0.29, when SbCl3 is hydrolysed, the OH-produced by water ionization replaces a Cl atom from one of the Sb-Cl bonds with an overlap population of 0. 29  By comparing the overlap populations of SbOCl, Sb3O4Cl and Sb4O5Cl2, it can be seen that the sum of the overlap populations in SbOCl covalent bonds is 9.92, the sum of the overlap populations for antibonding is -1.06, the sum of overlap populations for covalent bonds and overlap populations of antibonding orbitals for the aggregate population is 8.86; therefore, the covalent bonding effect is 9.36 times that of the anti-bonding effect. The greater the covalent bonding contribution is, the more stable the structure [33,34]; the sum of the aggregated overlap populations for Sb3O4Cl covalent bonds is 4.56, the sum of the overlap populations for antibonding interactions is -0.72, the overall sum of the overlap populations of both covalent bonding and antibonding is 3.84, so the covalent bonding effect is 6.33 times the effect of antibonding. For Sb4O5Cl2, the sum of the overlap populations for covalent bonds is 6.52, the sum of the overlap populations for antibonding is -0.78, the overall sum is 5.74, and the effect of covalent bonding is greater than that of anti-bonding. The covalent bond of SbOCl is the strongest, the force between atoms is the largest, and the stability of the unit cell is the highest. Therefore, the Sb(OH)Cl2 generated during the hydrolysis of SbCl3 more easily continues on to eliminate HCl and generate SbOCl; that is, SbOCl is more stable than Sb4O5Cl2 or Sb3O4Cl.
In summary, compared with Sb3O4Cl and Sb4O5Cl2, SbOCl has the lowest unit cell energy, the narrowest energy bandwidth and the widest pseudoenergy gap, i.e., the strongest covalent bonds between Sb, O and Cl atoms. During the hydrolysis of SbCl3, it is easier to generate stable SbOCl; from the formal charges, it can be seen that SbCl3 preferentially breaks the [Sb-Cl] bond with the weaker bond energy and forms Sb(OH)Cl2; among the group SbOCl, Sb3O4Cl and Sb4O5Cl2, the [Sb-O] bond is stronger than the [Sb-Cl] bond, and it is easy to remove the Cl atom in a later stage of the process to obtain Sb2O3 or Sb powder. By comparing the overlap populations for antibonding interactions and covalent bonds, it is seen that SbOCl is more stable than Sb4O5Cl2 and Sb3O4Cl; that is, Sb(OH)Cl2 produced during the hydrolysis of SbCl3 can continue on to eliminate HCl and generate SbOCl.
To further clarify whether the rupture and formation of covalent bonds during the hydrolysis of SbCl3 is consistent with the results of the calculation, infrared spectroscopy was used to analyse the hydrolysis of SbCl3 and the mechanism for formation of chlorine-oxygen covalent bonds.

Hydrolysis mechanism for antimony chloride systems
In the experiment, the filtrates from SbCl3 hydrolysis, ethanolysis, and glycolysis were analysed by infrared spectroscopy [35][36][37][38][39][40][41] to determine the reaction pathway for SbCl3 and the mechanism for replacement of chlorine by oxygen. The results are shown in Figure 3 and Figure 4. Characteristic infrared peaks are shown in Table 5.  Figure 3 shows the infrared spectrum of the hydrolysis product from solid SbCl3. From A and B in Figure 3, it can be seen that the peak at 3313.21 cm -1 was due to the O-H stretching vibration, and the peak at 1634.38 cm -1 was due to the O-H intermolecular bending vibration. After adding SbCl3, the hydrolysis of SbCl3 facilitated the ionization of hydroxyl groups in water, the number of free hydroxyl groups in solution increased, and the concentration of hydroxyl groups was relatively increased. The H atom of [H-OH] formed an [Sb-OH] monomer. For [Sb-OH], there is a hydrogen bond between the oxygen atom of the -OH in the monomer and a water molecule that is not easy to break. The electronegativity of chlorine is 3.16, the electronegativity of hydrogen in water is 2.1, and free Cl ions are produced in water. It is easy to replace the hydrogen atom in the hydroxyl group to form [Cl-O-Sb], that is, SbOCl, since the Sb nucleus is larger than the H nucleus, and it is easy to form Sb[OH]3 with the hydroxyl groups generated by ionisation of water. Because Sb[OH]3 is unstable, it continued to hydrolyse to form SbOCl, Sb3O4Cl and Sb4O5Cl2. Cl replaced the H atom in the water hydroxyl group so that the electron density increased, the force constant k increased, and an inductive effect was induced. The group frequency shifted to a higher wavenumber, the infrared spectrum of the water was redshift, and the electronegativity of the element increased. The stronger the inductive effect was, the more obvious the shift of the absorption peak to a higher wavenumber.

Infrared spectroscopy of antimony oxychloride prepared by hydrolysis
By comparing A and B in Figure 3, it can be seen that the absorption peak at 1634.38 cm -1 only shifted by 0.12 cm -1 , while the absorption peak at 3331.21 cm -1 shifted by 2.39 cm -1 ; this clearly illustrates that the redshift of the absorption peak at 3313.21 cm -1 was not only due to the increased concentration of hydroxyl groups in solution but was also due to the inductive effect of the chlorine that replaced the hydrogen in the hydroxyl group.  Figure 4 shows the infrared spectrum for the alcoholysis of SbCl3 solid in ethanol. It can be seen from line C in Figure 4 that the absorption peak at 3316.10 cm -1 was caused by an OH stretching vibration, the absorption peak at 2972.54 cm -1 was caused by a -CH asymmetric contraction vibration in -CH3, the peak at 2880.15 cm -1 was generated by the -CH symmetric contraction vibration in -CH3, the peak at 1379.29 cm -1 was generated by the -CH symmetric deformation vibration in -CH3, and the peak at 2880.15 cm -1 was generated by the symmetric stretch of -CH in -CH2. The -CH in -CH2 generated a peak at 1379.29 cm -1 due to shear vibration. The absorption peaks at 1087.46 cm -1 and 879.88 cm -1 were generated by C-C bond stretch vibrations. The peak at 1046.48 cm -1 was caused by a C-O stretching vibration, and the absorption peak at 636.44 cm -1 was caused by an -OH out-of-plane bending vibration. From the comparison between the D and C spectra in Figure 4, it can be seen that the absorption peak at 3316.10 cm -1 shifted by 1.58 cm -1 , and the absorption peak at 636.44 cm -1 shifted by 10.4 cm -1 to low frequency. After the H on -OH was replaced by Cl, the electron cloud density increased and the force constant k increased as a result of the inductive effect; therefore, the band at 3316.10 cm -1 shifted to a higher wavenumber, and the infrared spectrum of ethanol redshifted. The electronegativity of Cl (3.16) is greater than that of H (2.1). After Cl replaced the H on -OH, a [C-O-Cl] monomer was formed. Since the electronegativity difference for the bonded atoms is greater, the absorption peak was stronger, the polarity was stronger, and the absorption peak was redshift; the weaker the polarity, the more the absorption peak was blueshift. Therefore, the polarity of -OCl is less than that of -OH, so the absorption peak at 636.44 cm -1 shifted to lower frequency and a blueshift occurred, which proves that the H on -OH was replaced by Cl.

Infrared spectroscopy of antimony oxychloride prepared by alcoholysis in ethanol
Since the alcoholysis of SbCl3 in ethanol was slow, the reaction was promoted by adding ammonia. By comparing lines E and F in Figure 4, we can see that the peak at 1634.96 cm -1 was caused by the stretching vibration of the -NH bond in -NH3, and the absorption peak at 3358.72 cm -1 redshift by 0.19 cm -1 and at 562.79 cm -1 blueshift. by 22.67 cm -1 . That verifies the occurrence of an SbCl3 substitution reaction in ethanol.

Infrared spectroscopy of antimony oxychloride prepared by glycolysis
To further clarify the mechanism of the SbCl3 hydrolysis process, it was determined whether an alcoholysis reaction of SbCl3 would occur in ethylene glycol. The results show that the absorption peak at 3329.97 cm -1 in line G line was caused by an -OH stretching vibration, the absorption peaks at 2936.68 cm -1 and 2873.77 cm -1 were caused by -CH2 asymmetric and symmetric stretching, respectively, the 1408.20 cm -1 absorption peak was caused by a -CH2 bending vibration, the peak at 1204.06 cm -1 was caused by a C-C stretching vibration, the peaks at 1082.99 cm -1 and 1031.95 cm -1 were caused by C-O bond stretching vibrations, the peaks at 881.27 cm -1 and 860.18 cm -1 were produced by two -OH out-ofplane bending vibrations; since the vibrational frequency of the two [CH2OH] chemical bonds is the same, vibrational coupling occurred between them, causing the frequency of one of the peaks to be higher than the original frequency and the frequency of the other peak to be lower.
From a comparison of the H and G spectra in Figure 5, it can be seen that the band at 3329.97 cm -1 exhibited a shift of 9.99 cm -1 to lower frequency. This is because the electronegativity of Cl (3.16) is greater than that of H (2.1), and Cl replaces -OH. After H loss, the formation of the [C-O-Cl] monomer occurred; the absorption peak at 881.27 cm -1 was redshift by 0.19 cm -1, and the absorption peak at 860.18 cm -1 was blueshift by 0.01 cm -1 . One of the -OH groups in the alcohol was replaced by Cl, and the other was not, resulting in an increase in the electron density of the substituted -OH, an increase in the force constant k, and an inductive effect causing the absorption at 881.27 cm -1 to redshift, while the absorption peak at 860.18 cm -1 was blueshift.
The alcoholysis reaction of solid SbCl3 was slow in ethylene glycol, so the reaction was promoted by adding ammonia water. By comparing lines I and J in Figure 5, it can be seen that the peak at 1643.32 cm -1 was caused by the stretching vibration of the -NH bond in NH3. There are absorption peaks at 3284.52 cm -1 , 882.33 cm -1 and 860.99 cm -1 and the absorption peak at 3284.52 cm -1 was blueshift by 4.88 cm -1 , the peak at 882.33 cm -1 was blueshift by 0.33 cm -1 , and the peak at 860.99 cm -1 was redshift by 0.33 cm -1 , further verifying that the substitution of SbCl3 occurred in ethylene glycol.

Hydrolysis mechanism of antimony in the Sb 3+ -HCl system
To verify whether the hydrolysis, alcoholysis, and glycolysis of solid SbCl3 were consistent with the Sb 3+ hydrolysis process occurring in aqueous solution, SbCl3 solutions with concentrations of 0.05 mol/L Sb 3+ and 1 mol/L Clwere prepared for studies of hydrolysis and alcoholysis. The reactions were carried out under the same conditions, and the products were analysed by infrared spectroscopy. The results are shown in Figure 6. By comparison, it was found that the infrared spectrum was completely consistent with that from the reaction of solid SbCl3, which confirms the mechanism of the SbCl3 hydrolysis process. There are two main ways to hydrolyse SbCl3 into oxychloride: 1) water and alcohol ionise hydroxyl groups and replace a Cl atom in SbCl3 to form [Sb(OH)Cl2] monomer; as the degree of hydrolysis progresses, the monomer loses H and Cl to form SbOCl, Sb3O4Cl, and Sb4O5Cl2; 2) Sb directly forms [Sb-OH] monomer with OH ionised by water and alcohol, and Cl replaces the H on the [Sb-OH] hydroxyl group in the monomer and directly forms SbOCl, Sb3O4Cl, and Sb4O5Cl2.

Conclusions
(1) The density functional method was used to calculate the electronic properties of SbCl3, SbOCl, Sb3O4Cl, and Sb4O5Cl2 unit cells using analyses of unit cell structure, unit cell energy, energy band, total density of states, partial density of states, overlap populations, formal charges, and covalent bond properties of SbCl3, SbOCl, Sb3O4Cl, and Sb4O5Cl2 in their respective unit cells. This was combined with information from analysis of infrared spectroscopic data for hydrolysis reactions leading to antimony oxychloride by atom transfer. There are two main ways to hydrolyse SbCl3 into oxychloride: the hydroxyl groups in water and alcohol are ionised to replace one Cl atom in SbCl3 and form a [Sb(OH)Cl2] monomer. During further hydrolysis, H and Cl atoms are removed. Alternatively, SbOCl, Sb3O4Cl, and Sb4O5Cl2 or Sb atoms are formed directly from [Sb-OH] monomers with OH ionised by water or alcohol. Cl atoms replace the H atoms on the hydroxyl groups in [Sb-OH] monomers to form SbOCl, Sb3O4Cl, and Sb4O5Cl2 directly.
(2) Infrared spectroscopy indicated that with SbCl3 in the water, ethanol, and ethylene glycol systems, the water hydroxyl group and alcohol hydroxyl group exhibited redshift and blueshift in the infrared spectra; the hydroxyl band at high frequency changed because the chlorine atom replaced the hydrogen atom, and the electron density increased. The force constant k increased, an inductive effect occurred, the group frequency shifted to higher wavenumber, the infrared spectrum was redshift; since the polarity of the -OCl bond is weaker than that of the -OH bond, the hydroxyl vibration shifted to lower frequency. A blueshift occurred, which indicates that the hydrogen atom on -OH was replaced by a chlorine atom; this is consistent with the calculated results on the strength of the covalent bond between chlorine and oxygen, which confirms the results of the density functional method.