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Abstract
The study investigates the high-temperature chlorination of rhodium (Rh) and its oxides using alkali-metal and alkaline-earth-metal chlorides. Thermodynamic analyses reveal the potential of magnesium chloride (MgCl2) as an effective chlorinating agent in an oxygen-containing atmosphere. Experimental results confirm the efficient chlorination of Rh and Rh2O3 using MgCl2, highlighting its practical advantages over toxic Cl2 gas. The findings suggest a safer and more feasible method for the separation, recovery, and purification of Rh from various raw materials.
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Abstract
This study was aimed at investigating an effective chlorination method for Rh and its oxide to develop an efficient Rh extraction process. The feasibility of using alkali-metal and alkaline-earth-metal chlorides (NaCl, KCl, MgCl2, and CaCl2) as chlorinating agents was evaluated from the perspective of thermodynamics; the prediction results revealed MgCl2 as a suitable agent for chlorinating metallic Rh and Rh2O3 in an oxygen-containing atmosphere. The thermodynamic analysis results were then experimentally validated. Metallic Rh converted to RhCl3 when mixed with MgCl2 and heated at 973 K (700 °C) in an O2 atmosphere. The Rh2O3 powder was efficiently chlorinated when reacted with liquid MgCl2 at 1073 K (800 °C) in an Ar–1 pct O2 atmosphere. Therefore, the chlorination of Rh using MgCl2 is feasible; its use has potential to make the extraction and recovery of Rh from various raw materials more efficient.
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Introduction
Rhodium (Rh) belongs to the platinum group elements (PGEs) and is present in low abundance in the Earth’s crust. Furthermore, Rh is expensive and experiences significant price volatility. Therefore, high Rh yields and short processing times are required when extracting and recovering Rh from various raw materials.
The final separation and purification of PGEs are typically achieved through a combination of various hydrometallurgical techniques, such as solvent extraction, precipitation, and ion exchange.[1‐4] Therefore, the extraction and recovery of PGEs commonly include their acid dissolution step. By contrast, metallic PGEs and their oxides are poorly soluble in acids. In particular, the metallic Rh and its oxides are difficult to dissolve even in aqua regia or Cl2/HCl solutions.[1,5] Consequently, the dissolution (or leaching) of Rh is often an important challenge in extraction and recovery processes.
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For efficient dissolution and extraction of PGEs (including Rh), the extraction medium parameters, such as solution composition (including oxidizing agent, complexing agent, and acid concentration), must be optimized. Furthermore, converting the PGEs in the raw material into a state that facilitates dissolution and extraction through pretreatment, such as concentration, micropulverization, and alloying, is beneficial.[6‐11] For example, when recovering PGEs from spent catalysts, smelting with collector metal is often performed for concentrating and alloying the PGEs prior to their acid dissolution.[2,3]
Chlorination with high-temperature Cl2 gas has been widely used as a pretreatment for solubilizing PGEs.[1,12‐17] For example, when purifying Rh metal containing impurities, it is often mixed with NaCl and heat-treated in a Cl2 atmosphere at ~973 K (700 °C).[1,13] Because the resulting chlorides are soluble in acid, Rh can be efficiently purified using hydrometallurgical techniques. Reactions with Cl2 gas in molten salts, such as a NaCl-CsCl melt,[17] are also available for chlorinating Rh and other PGE metals.
Additionally, chlorination treatments with Cl2/CO gas (and phosgene generated by reacting Cl2(g) and CO(g)) have been investigated for the separation and recovery of PGEs from spent catalysts.[18‐22] For example, Kim et al.[19] investigated the effects of temperature, time, gas flow rate, and gas composition on the effectiveness of the treatment with Cl2/CO gas. After the treatment under optimized conditions (823 K (550 °C), CO:Cl2 = 4:6), the Pt and Rh contained in the catalyst sample were distributed mainly in the non-volatilized residue and could be effectively dissolved and extracted using 1 M HCl(aq) (the extraction rates of Pt and Rh were 95.9 and 92.9 pct, respectively).
Despite their effectiveness, Cl2, Cl2/CO, and phosgene gases are highly toxic. Therefore, researchers have examined safer and more facile alternatives, such as chlorination treatment with metal chlorides which are solid at room temperature and less toxic.[23‐30] For example, Bronshtein et al.[25] reported that when a sample of spent catalysts was mixed with CaCl2 hydrates and Ca(ClO)2 and then heated at 1273 K (1000 °C) in air, Pt could be efficiently chlorinated and volatilized, while Pd could not. They also reported that adding silica alongside CaCl2 hydrates promoted the chlorination volatilization of Pd; however, they did not investigate the reaction behavior of Rh. Mpinga et al.[26] investigated chlorination treatment of PGE-containing ore (high-chromite content ore) with alkaline-earth-metal chlorides. After mixing MgCl2 and CaCl2 hydrates and heat-treating at 923 K (650 °C), nearly all the Pt and Pd could be extracted using hot 6 M HCl(aq). Horike et al. reported that Mg–Pt alloys could be chlorinated at 773 K (500 °C) using CuCl2 as the chlorinating agent.[24] Yoshimura et al. reported that metallic PGEs could be chlorinated at ~623 K (~350 °C) through immersion in a FeCl3-containing molten salt.[27,28]
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Although various studies on the high-temperature chlorination of PGEs have been reported to date, the feasibility and reaction mechanisms of the chlorination of Rh and its compounds using metal chlorides remain unclear. To address this knowledge gap, in this study, thermodynamic analyses were conducted on the chlorination reaction of metallic Rh and its oxide using alkali-metal and alkaline-earth-metal chlorides. Furthermore, the analysis results were validated through experiments.
Thermodynamic Considerations
Thermodynamic Data of Rh Compounds
Figure 1 shows the temperature dependence of the standard Gibbs energy of formation (ΔGf°) for RhCl3 and Rh2O3, and Table I lists the ΔGf° of RhCl3 at 1073 K (800 °C).[31‐34] The data of RhCl3 from various sources indicate notable differences in values, suggesting significant uncertainty. For example, Bell et al.[31] measured the dissociation pressure of RhCl3 at 995 K−1236 K (722 °C–963 °C). According to the outcomes, the ΔGf° of RhCl3 at 1073 K (800 °C) is −39.3 (±8.4) kJ/mol, and the logarithm of the partial pressure of chlorine (\(\log p_{{{\text{Cl}}_{{2}} }}\) (atm)) under the Rh(s)/RhCl3(s) equilibrium at 1073 K (800 °C) is −1.3 (±0.3). Meanwhile, according to a previously reported database,[32] the ΔGf° of RhCl3 and the \(\log p_{{{\text{Cl}}_{{2}} }}\) under the Rh(s)/RhCl3(s) equilibrium are −29.9 kJ/mol and −1.0, respectively, at 1073 K (800 °C). Furthermore, according to another database,[33,34] the ΔGf° of RhCl3 and the \(\log p_{{{\text{Cl}}_{{2}} }}\) under the Rh(s)/RhCl3(s) equilibrium are −59.6 kJ/mol and −1.9, respectively, at the same temperature. Owing to these differences in the thermodynamic data of RhCl3, further investigations are required to determine their true values. In this study, the parameters reported by Bell et al.[31] were used, because their experimental method and thermodynamic assessment are reasonable.
Fig. 1
Standard Gibbs energy of formation of RhCl3 and Rh2O3, derived from Refs. [31‐34]
In addition to Rh2O3, RhO2 has also been reported as a Rh oxide.[35,36] However, in our preliminary experiments, in which Rh and Rh2O3 powders were heated at 873 K−1273 K (600 °C–1000 °C) in air for more than 24 h, diffraction peaks corresponding to RhO2 could not be identified in X-ray diffraction (XRD) patterns. Even if RhO2 is formed on the outermost surfaces of the powder particles, its amount is negligible. Therefore, in the following analyses, only Rh2O3 was considered for rhodium oxide.
High-Temperature Chlorination of Metallic Rh and Rh2O3
Figure 2 shows the Ellingham diagram for chlorides,[31,34] presenting the standard Gibbs energy of the reaction, ΔGr°, for the chlorination per mole Cl2(g) of various metals, hydrogen, and carbon in the temperature range 800 K−1200 K (527 °C–927 °C). The ΔGr° values for the chlorination of Rh with Cl2(g) are negative, indicating that metallic Rh can be converted to RhCl3 through a reaction with pure Cl2(g). Figure 2 also shows the high stability of alkali-metal and alkaline-earth-metal chlorides (RClx, where R: Na, K, Mg, and Ca).
Fig. 2
Standard Gibbs energy of chlorination reactions of certain metals with Cl2(g). Data for the reactions between H2(g) and Cl2(g) and between C(s) and Cl2(g) are also shown. Thermodynamic data were obtained from Refs. [31, 34]
Figure 3 shows the temperature dependence of ΔGr° for the chlorination of Rh with various chlorides (RClx, HCl(g), and CCl4(g)),[31,34] considering metallic R, hydrogen, or carbon as the by-product. The ΔGr° for the chlorination of Rh with RClx or HCl(g) has large positive values at 800 K−1200 K (527 °C–927 °C).
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ 2 NaCl}}\left( s \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ 2 Na}}\left( l \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {1} \right)}} = { 598}.{\text{8 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(1)
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ 2 KCl}}\left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ 2 K}}\left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {2} \right)}} = { 64}0.0{\text{ kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(2)
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ MgCl}}_{{2}} \left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ Mg}}\left( l \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {3} \right)}} = { 448}.{\text{3 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(3)
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ CaCl}}_{{2}} \left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ Ca}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {4} \right)}} = { 6}0{5}.{\text{7 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(4)
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ 2 HCl}}\left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ H}}_{{2}} \left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {5} \right)}} = { 176}.{\text{4 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(5)
These values suggest that metallic Rh cannot be chlorinated by RClx and HCl(g) unless the activity of the reaction product (RhCl3) and/or by-product is kept at a significantly low level. For the reaction with CCl4(g), ΔGr° is negative, which indicates that the chlorination of Rh by CCl4(g) can proceed.
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + { 1}/{\text{2 CCl}}_{{4}} \left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + { 1}/{\text{2 C}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {6} \right)}} = - {49}.{\text{1 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(6)
Fig. 3
Standard Gibbs energy of chlorination reactions of Rh metal with metal chlorides. Data for the reactions of Rh(s) with HCl(g) or CCl4(g) are also shown. Thermodynamic data were obtained from Refs. [31, 34]
Figure 4 shows the temperature dependence of ΔGr° for the chlorination of metal oxides (Rh2O3 and R2/xO), H2O(g), and CO2(g) with Cl2(g).[31,34] We consider the reactions per mole of Cl2(g), with the by-product as O2(g). Below ~1073 K (~800 °C), the ΔGr° for the chlorination of Rh2O3 is negative (Figure 4(b)), indicating that pure Cl2(g) can chlorinate Rh2O3 at low temperatures:
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ Cl}}_{{2}} \left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + { 1}/{\text{2 O}}_{{2}} \left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {7} \right)}} = - {13}.{\text{7 kJ at 873 K }}({6}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(7)
Furthermore, Figure 4 indicates that the difference in stability between the oxide and chloride varies significantly depending on the metal element. Among the metal elements considered in this study, Mg is relatively stable in oxide form compared with the chloride form.
Fig. 4
(a) Standard Gibbs energy of chlorination reactions of metal oxides with Cl2(g). Data for the reaction between H2O(g) and Cl2(g) are also shown. Thermodynamic data were obtained from Refs. [31, 34]. (b) Enlarged view
Figure 5 plots the ΔGr° values for the chlorination of Rh2O3 with RClx as functions of temperature, with the by-products set as R2/xO.[31,34] Below ~1200 K (~927 °C), the ΔGr° for the chlorination of Rh2O3 with MgCl2 is negative (Figure 5(b)). Thus, Rh2O3 is expected to be chlorinated by MgCl2, as follows:
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ MgCl}}_{{2}} \left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ MgO}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {8} \right)}} = - {9}.{\text{5 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}). \hfill \\ \end{aligned} $$
(8)
Moreover, the ΔGr° for the chlorination of Rh2O3 with NaCl, KCl, and CaCl2 has large positive values at 800 K−1200 K (527 °C–927 °C):
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ 2 NaCl}}\left( s \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ Na}}_{{2}} {\text{O}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {9} \right)}} = { 354}.{\text{8 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(9)
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ 2 KCl}}\left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ K}}_{{2}} {\text{O}}\left( l \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{1}0} \right)}} = { 455}.{\text{6 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(10)
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ CaCl}}_{{2}} \left( l \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ CaO}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{11}} \right)}} = { 11}0.0{\text{ kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}). \hfill \\ \end{aligned} $$
(11)
For these chlorides to act as chlorinating agents, the activity of the reaction products (RhCl3 and/or R2/xO) must be maintained at a significantly low level.
Fig. 5
(a) Standard Gibbs energy of chlorination reactions of Rh oxide with metal chlorides. Data for the reactions between of Rh2O3(s) with HCl(g) or CCl4(g) are also shown. Thermodynamic data were obtained from Refs. [31, 34]. (b) Enlarged view
In Figure 5, the ΔGr° values for the chlorination of Rh2O3 with HCl(g) are also plotted. The ΔGr° for chlorination with HCl(g) increases with temperature and becomes positive at ~973 K (~700 °C); the reaction temperature suitable for chlorination by HCl(g) is expected to be sufficiently lower than this temperature.
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ 2 HCl}}\left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ H}}_{{2}} {\text{O}}\left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{12}} \right)}} = - {13}.{\text{3 kJ at 873 K }}({6}00\, \, ^\circ {\text{C}}) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{12}} \right)}} = { 14}.{\text{7 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(12)
As a source of HCl(g), the thermal decomposition reaction of metal chloride hydrates (e.g., 2 MgCl2·2H2O → MgCl2·H2O + MgOHCl + 2 H2O + HCl[37]) is also expected to be available.
Additionally, the ΔGr° for the chlorination of Rh2O3 with CCl4(g) is large negative at 800 K−1200 K (527 °C–927 °C), as shown in Figure 5. Thus, CCl4(g) is expected to chlorinate Rh2O3.
$$ \begin{aligned} {1}/{\text{3 Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + { 1}/{\text{2 CCl}}_{{4}} \left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + { 1}/{\text{2 CO}}_{{2}} \left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{13}} \right)}} = - {22}0.{\text{2 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(13)
The chemical potential of oxygen in the system strongly influences the chlorination reaction of metals and their oxides.[38‐40] Therefore, the chemical potential diagram of the Rh-O-Cl system was considered to further discuss the chlorination reactions. Figure 6 shows the corresponding diagram at 1073 K (800 °C), using the logarithms of \(p_{{{\text{Cl}}_{{2}} }}\) and the partial pressure of oxygen (\(p_{{{\text{O}}_{{2}} }}\)) as the abscissa and ordinate, respectively.[31,34] The dotted lines in the figure represent the \(p_{{{\text{O}}_{{2}} }}\) and \(p_{{{\text{Cl}}_{{2}} }}\) determined by the R2/xO/RClx and H2O(g)/HCl(g) equilibria, \(p_{{{\text{O}}_{{2}} }}\) determined by the C(s)/CO(g) and CO(g)/CO2(g) equilibria, and \(p_{{{\text{O}}_{{2}} }}\) with a relatively high value of 0.01 atm (e.g., \(p_{{{\text{O}}_{{2}} }}\) in an Ar−1 pct O2 atmosphere).
Fig. 6
(a) Chemical potential diagram of the Rh-O-Cl system at 1073 K. The thermodynamic data reported in Refs. [31, 34] were used. (b) Enlarged view
Figure 6 indicates that the effectiveness of MgCl2 as a chlorinating agent for Rh and Rh2O3 depends on the \(p_{{{\text{O}}_{{2}} }}\) of the reaction system. When a sufficient amount of MgCl2 is present in the reaction system, \(p_{{{\text{Cl}}_{{2}} }}\) and \(p_{{{\text{O}}_{{2}} }}\) can be controlled by the MgO/MgCl2 equilibrium. Furthermore, when carbon coexists in the reaction system, \(p_{{{\text{O}}_{{2}} }}\) can be controlled by the C(s)/CO(g) equilibrium. Therefore, when using MgCl2 in the presence of carbon, the chemical potential of the reaction system can be maintained near the potential point A* in Figure 6. Because metallic Rh is thermodynamically stable near this chemical potential, Rh and Rh2O3 cannot be chlorinated. For Rh2O3, the following reduction reaction will proceed even in the presence of MgCl2.
$$ \begin{aligned} {\text{Rh}}_{{2}} {\text{O}}_{{3}} \left( s \right) \, + {\text{ 3 C}}\left( s \right) \, = {\text{ 2 Rh}}\left( s \right) \, + {\text{ 3 CO}}\left( g \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{14}} \right)}} = - {\text{539 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}) \hfill \\ \end{aligned} $$
(14)
Meanwhile, chlorination with MgCl2 becomes feasible when the oxygen chemical potential is sufficiently high. For example, when the \(p_{{{\text{O}}_{{2}} }}\) of the reaction system is 0.01 atm and MgCl2 is present in sufficient quantity, the chemical potential of the reaction system can be maintained near point A in Figure 6. At this chemical potential, the RhCl3 phase is thermodynamically stable, enabling the chlorination of Rh2O3 through reaction [8]. In the case of metallic Rh, chlorination can proceed through a reaction with both MgCl2 and O2(g):
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ MgCl}}_{{2}} \left( l \right) \, + { 1}/{\text{2 O}}_{{2}} \left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ MgO}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{15}} \right)}} = - {36}.{\text{5 kJ at 1}}0{\text{73 K }}({8}00\, \, ^\circ {\text{C}}). \hfill \\ \end{aligned} $$
(15)
Under a CaO/CaCl2 equilibrium, even at a high \(p_{{{\text{O}}_{{2}} }}\) of 0.01 atm, the chemical potential of the reaction system is maintained near point B in Figure 6. Near this chemical potential, the Rh2O3 phase is thermodynamically stable. Therefore, even when CaCl2 is used as a chlorinating agent in a high \(p_{{{\text{O}}_{{2}} }}\) atmosphere, chlorination of Rh does not occur unless the activity of the reaction products is maintained at a sufficiently low level. The same principle applies to the utilization of Na2O/NaCl and K2O/NaCl equilibria under a high \(p_{{{\text{O}}_{{2}} }}\) (represented by points D and C in Figure 6, respectively). In such cases, NaCl and KCl are not suitable as chlorinating agents for Rh and Rh2O3.
Additionally, the \(p_{{{\text{Cl}}_{{2}} }}\) and \(p_{{{\text{O}}_{{2}} }}\) of the reaction system can be maintained near the potential points E and E* in Figure 6 when using Cl2−1 pct O2 gas and pure Cl2 gas in the presence of carbon, respectively. In both cases, the RhCl3 phase is stable under the controlled environment, enabling the chlorination of Rh and Rh2O3. Point F in Figure 6 corresponds to the atmosphere under H2O/HCl equilibrium and a \(p_{{{\text{O}}_{{2}} }}\) of 0.01 atm. Although the Rh2O3 phase is thermodynamically stable at point F, the corresponding chemical potential is located near the phase boundary between Rh2O3 and RhCl3. As indicated in Table I, the thermodynamic data of RhCl3 have large uncertainties. Furthermore, the partial pressure of H2O on Rh or Rh2O3 surfaces can be maintained at a low level by flowing dry gas. Flowing dry HCl−1 pct O2 gas over Rh and Rh2O3 may allow their chlorination, because the H2O(g) generated as the by-product can be continuously removed from their surfaces.
Thermodynamic analyses predicted that metallic Rh and Rh2O3 can be chlorinated using MgCl2 under an oxygen-containing atmosphere. Figure 7 shows the temperature dependence of the vapor pressure of the metal chlorides.[31,34] At 1073 K (800 °C), the saturated vapor pressures of MgCl2 and RhCl3 are below 10−3 atm. Therefore, when the chlorination treatment is conducted below 1073 K (800 °C) under atmospheric pressure without a strong gas flow, the loss of MgCl2 and RhCl3 from the reaction system due to volatilization can be prevented.
Fig. 7
Temperature dependence of vapor pressure of RhCl3 and other metal chlorides. Thermodynamic data were obtained from Refs. [31, 34]
Metallic Rh and its oxide (Rh2O3) were reacted with metal chlorides in oxygen-containing atmospheres. Table II lists the Rh samples and metal chloride (RClx) reagents used in this study. Chloride reagents were stored and handled inside a glove box (UN-800L, UNICO) under a high-purity Ar atmosphere to prevent moisture absorption.
Table II
Materials Used in this Study
Form
Purity
(Mass Pct)
Supplier
Rh
powder
(micrometer-sized, gray)
> 99.9
Tanaka Kikinzoku Kogyo K.K.
Rh2O3
powder
(micrometer-sized, black)
99.8
Mitsuwa Chemical Co., Ltd.
MgCl2
powder
98.8
Nacalai Tesque, Inc.
CaCl2
powder
99.0
Kojundo Chemical Laboratory Co., Ltd.
NaCl
powder
99.8
FUJIFILM Wako Pure Chemical Corp.
KCl
powder
> 99.5
FUJIFILM Wako Pure Chemical Corp.
Table III presents the conditions of chlorination tests. Mixed powders of Rh and RClx and those of Rh2O3 and RClx were maintained at 1073 K (800 °C) in an Ar–1 pct O2 atmosphere. Mixtures subjected to heat treatment were prepared using an agate mortar. In addition, a Rh/MgCl2 mixture was heated at 973 K (700 °C) in a pure O2 atmosphere. In all tests, RClx was mixed in excess to allow the complete chlorination of the Rh sample. For example, to completely chlorinate metallic Rh with MgCl2, the molar amount of MgCl2 should be at least 1.5 times greater than that of Rh, as shown in Eq. [15]. In the test, the molar amount of MgCl2 was set at three times that of Rh.
Table III
Experimental Conditions of Chlorination Tests
Exp. no.
Rhodium Sample
Chlorinating Agent
Molar Ration of Rh Sample and Chlorinating Agent
Heat Treatment
Name
Mass,
ms/g
Name
Mass,
mc/g
Temp.,
T/K (°C)
Time, t/min
Atmosphere
CT1-231115
Rh
0.063
MgCl2
0.175
Rh : MgCl2 = 1 : 3
1073 (800)
40
Ar−1 pct O2
Rh2O3
0.051
MgCl2
0.117
Rh2O3 : MgCl2 = 1 : 6
CT2-231219
Rh
0.065
MgCl2
0.193
Rh : MgCl2 = 1 : 3
973 (700)
40
O2
CT3-231115
Rh
0.060
CaCl2
0.198
Rh : CaCl2 = 1 : 3
1073 (800)
40
Ar−1 pct O2
Rh2O3
0.035
CaCl2
0.110
Rh2O3 : CaCl2 = 1 : 7
CT4-231128
Rh2O3
0.051
NaCl
0.144
Rh2O3 : NaCl = 1 : 12
1073 (800)
40
Ar−1 pct O2
Rh2O3
0.050
KCl
0.174
Rh2O3 : KCl = 1 : 12
Figure 8 shows the experimental apparatus. The mixture of Rh sample and metal chloride was placed in a quartz crucible, which was installed inside a quartz vessel with a lid (Figure 8(a)). All the operations were performed inside the glove box. The vessel was then transferred to a gas-tight quartz tube (Figures 8(b,c)). The inside of the quartz tube was purged with Ar−1 pct O2 or pure O2 gas. Subsequently, the quartz tube was placed in a preheated electric furnace. Figure 9 shows the temperature profile of the quartz vessel containing the samples. The holding time was ~40 min at both 973 and 1073 K (700 and 800 °C). During the heat treatment, the filling gas was supplied at a flow rate of 20 mL/min to maintain a pressure of 1 atm in the quartz tube.
Fig. 8
Schematic of (a) quartz vessel with lid containing the sample and (b) gas-tight quartz tube used for chlorination tests. (c) Photograph of the lower part of the quartz tube in which the reaction vessel is placed (Exp. CT2-231219)
After the heat treatment, the mixture samples were transferred inside the glove box, and then pulverized and homogenized. Subsequently, the crystalline phases were analyzed by XRD using a SmartLab (Rigaku, Cu-Kα radiation). To prevent reactions with moisture in the ambient air, the samples were covered with a polyimide film in the glove box before the XRD analysis. Furthermore, the obtained samples were analyzed through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), using a JCM-7000 (JEOL) system.
Results and Discussion
Figure 10(a) shows the XRD pattern of the Rh/MgCl2 sample heated at 1073 K (800 °C) in an Ar−1 pct O2 atmosphere (Exp. CT1-231115). The major diffraction peaks correspond to Rh and MgCl2, and there is no peak corresponding to RhCl3. The MgO formed in the sample was attributed to the reaction between a portion of MgCl2 and O2 in the gas phase. Although thermodynamic analyses implied that the metallic Rh would be chlorinated upon its reaction with MgCl2(l) and O2(g) (reaction [15]), the chlorination reaction hardly proceeded in the experiment. This was probably due to an insufficient oxygen concentration in the gas phase. Additionally, the heat treatment temperature in this experiment was higher than the melting point of MgCl2. This resulted in the metallic Rh particles becoming immersed in liquid MgCl2 during heat treatment, preventing them from making direct contact with the gas phase; this could be another reason for the insufficient chlorination of Rh.
Fig. 10
XRD patterns of the Rh/MgCl2 mixture sample (a) after heating at 1073 K for 40 min in an Ar−1 pct O2 atmosphere (Exp. CT1-231115) and (b) after heating at 973 K for 40 min in an O2 atmosphere (Exp. CT2-231219)
Figure 10(b) shows the XRD pattern of the Rh/MgCl2 sample heated at 973 K (700 °C; below the melting point of MgCl2) in an O2 atmosphere (Exp. CT2-231219). After 40 min of reaction, the peaks of metallic Rh were replaced by those of RhCl3, indicating the progress of the chlorination of Rh. Accordingly, the sample appeared reddish brown color after heat treatment. The overall reaction for Rh chlorination at this experimental temperature could be expressed as:
$$ \begin{aligned} {2}/{\text{3 Rh}}\left( s \right) \, + {\text{ MgCl}}_{{2}} \left( s \right) \, + { 1}/{\text{2 O}}_{{2}} \left( g \right) \, = { 2}/{\text{3 RhCl}}_{{3}} \left( s \right) \, + {\text{ MgO}}\left( s \right) \hfill \\ \Delta G^\circ_{{{\text{r}},\left( {{16}} \right)}} = - {51.4}\,{\text{kJ at 973 K }}({7}00\, \, ^\circ {\text{C}}). \hfill \\ \end{aligned} $$
(16)
The area of the three-phase interface of Rh(s)/MgCl2(s)/O2(g) should be limited. Therefore, we can conclude that metallic Rh was chlorinated mainly owing to its contact with Cl2(g), which is generated by the following reaction: MgCl2(s) + O2(g) → MgO(s) + Cl2(g).
Figure 11 shows the XRD pattern of the Rh2O3/MgCl2 sample heated at 1073 K (800 °C) in an Ar−1 pct O2 atmosphere (Exp. CT1-231115). The XRD and SEM-EDS analyses indicated the disappearance of Rh2O3 and formation of RhCl3 and MgO. Reaction [8] predicted in the thermodynamic analysis proceeded efficiently. Owing to the formation of RhCl3, the sample appeared reddish brown. No migration of the formed RhCl3 out of the crucible due to volatilization was visually observed.
Fig. 11
XRD pattern of the Rh2O3/MgCl2 mixture sample heated at 1073 K for 40 min in an Ar−1 pct O2 atmosphere (Exp. CT1-231115)
Figure 12 shows the XRD patterns of the Rh/CaCl2 and Rh2O3/CaCl2 samples after heating at 1073 K (800 °C) in an Ar−1 pct O2 atmosphere (Exp. CT3-231115). Figure 13 shows the XRD patterns of the Rh2O3/NaCl and the Rh2O3/KCl samples after the same heat treatment (Exp. CT4-231128). In all cases, the formation of RhCl3 was not confirmed by XRD and SEM-EDS analyses, consistent with the thermodynamic prediction.
Fig. 12
XRD patterns of the (a) Rh/CaCl2 and (b) Rh2O3/CaCl2 mixture samples after heating at 1073 K for 40 min in an Ar−1 pct O2 atmosphere (Exp. CT3-231115)
These findings demonstrate that metallic Rh and Rh2O3 can be effectively chlorinated using MgCl2 as a chlorinating agent under an oxygen-containing atmosphere at a temperature below 1073 K (800 °C). Compared with Cl2(g), MgCl2 is less toxic and easier to handle because of its solid form at ambient temperature. Therefore, chlorination method using MgCl2 has potential to be utilized in the separation, recovery, and purification of Rh. The kinetics and dynamics of the MgCl2-based chlorination reactions are important topic for future study. The applicability of this chlorination method in the processing of different Rh-containing raw materials should also be investigated. In practical applications, technical issues related to the transport and use of anhydrous MgCl2 need to be addressed.
Conclusion
The feasibility of high-temperature chlorination of metallic Rh and Rh2O3 with alkali-metal and alkaline-earth-metal chlorides (NaCl, KCl, MgCl2, and CaCl2) was investigated thermodynamically and experimentally. Thermodynamic consideration indicated that MgCl2 can act as an effective chlorinating agent in an oxygen-containing atmosphere. When a mixture of Rh and MgCl2 powders was heated at 973 K (700 °C) in an O2 atmosphere, metallic Rh converted to RhCl3 within 40 minutes. A powder sample of Rh2O3 was efficiently chlorinated by reaction with liquid MgCl2 at 1073 K (800 °C) in an Ar–1 pct O2 atmosphere. In contrast, Rh2O3 was not chlorinated when NaCl, KCl, or CaCl2 were used under the same temperature and atmosphere. Compared with Cl2 gas, MgCl2 is less toxic and easier to handle owing to its solid form at ambient temperature. Therefore, the reaction with MgCl2 in an oxygen-containing atmosphere represents a feasible and effective technique for chlorinating Rh, and its use can potentially make the extraction and recovery of Rh from various raw materials more efficient.
Acknowledgments
The authors thank Mr. Osamu Hashino and Ms. Reina Takeda (Asahi Pretec Corporation) for their valuable comments. This research was financially supported by JSPS KAKENHI Grant Number 22K18906.
Conflict of interest
The authors declare that they have no conflict of interest.
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