**1. Introduction**

Molybdenum (Mo) is a strategic metal that has an extensive demand in different branches of the industry. Rhenium (Re) is also a strategic metal, although less common with wide applications in the oil industry (e.g., production of reforming catalyst) and in heat-resistant alloys (e.g., aerospace). Typically, in Mo sulphide concentrates (molybdenite, MoS2), Re coexisting with varying concentrations

ranging from 0.001% to 0.1% is identified [1]. Upon roasting, molybdenite transforms into technical-grade Mo oxide, while Re content escapes the reactor in the form of Re2O7, which partly deposits in the filters alongside the flue dust. Scrubbing the flue gases and leaching flue dusts are effective methods to recover Re values. Such solutions typically have 5–10 g/L Mo and 0.4–0.9 g/L Re. Due to similar chemical properties, Mo–Re separation is a challenge, and a number of studies have been devoted to introducing ad hoc technologies in this regard. Ion exchange and solvent extraction are the most used separation methods [2]. However, the separation of Mo and Re from liquors obtained from leaching molybdenite roasting flue dust is an essential step in order to produce high-purity final products, such as ammonium perrhenate and ammonium (para) molybdate. The following hydrolysis and acidic reactions illustrate the reactants/products involved in the leaching of flue dust [3,4]:

$$\text{MoO}\_3\text{ (s)} + \text{H}\_2\text{O (l)} \implies \text{H}\_2\text{MoO}\_4 \text{ (aq)} \quad \Delta\_\text{r}G^\circ = -31.9 \text{ kJ/mol} \tag{1}$$

$$\text{H}\_2\text{MoO}\_4 \text{ (aq) } + 2\text{H}^+ \text{ (aq) } \rightleftharpoons \text{MoO}\_2^{2+} \text{ (aq) } + 2\text{H}\_2\text{O (l) } \quad \Delta\_\text{F}\text{O}^\circ = -2.79 \text{ kJ/mol} \tag{2}$$

$$2\text{ Re}\Omega\text{\textquotedbl{}(s)} \; + \; \text{H}\_2\text{O}\; (\text{l}) \; \rightleftharpoons \; 2\text{Re}\text{O}\_4^-\; (\text{aq}) \; + \; 2\text{H}^+\; (\text{aq}) \; \quad \Lambda\_\text{F}\text{\textquotedbl{}} = -64.4\,\text{kJ/mol} \tag{3}$$

$$\text{HCO}\_4^-\text{ (aq) }+\text{ H}^+\text{ (aq) }\rightleftharpoons \text{ HReO}\_4\text{ (aq) }\quad\Delta\_\text{r}\text{O}^\circ=-1465.8\,\text{kJ/mol}\tag{4}$$

At high acid concentration, the anionic oxyspecies of rhenium (i.e., perrhenate ion (ReO− <sup>4</sup> )) is highly stable, and other possible short-life species readily hydrolyse to it. On the other hand, molybdenum forms cationic oxyspecies (MoO2<sup>+</sup> <sup>2</sup> ) at high acid concentration, evolving into neutral and anionic species with increasing pH [5].

Many attempts have been made on Mo and Re separation. Solvating extractant TBP (tributyl phosphate) is used for Re removal in near-zero pH, followed by Mo removal employing commercial extractant LIX984N [6]. Stepwise removal of Mo with TBP was conducted at pH near 2, and afterwards, Re was removed at pH lower than 0 [7]. Similarly, the coextraction of molybdenum and rhenium by N235 (tri-octyl amine) and their separation from stripping solution by using D201 ion-exchange resin (containing quaternary ammonium group [N-(CH3)2C2H4OH]) has been reported [8]. Selective extraction of rhenium over molybdenum from alkaline solutions has also been studied by employing an organic phase composed of 20% N235 and 30% TBP diluted in kerosene [9]. An organic phase composed of 5 vol% N235 (in kerosene) was found to selectively recover Re over Mo at equilibrium pH 0.0 [10]. Table 1 lists recent attempts to separate Re and Mo from their aqueous mother liquor.

The use of appropriate solvents is a challenging problem. The commercially available extractant Cyanex 923 has been shown to be an appropriate choice for the purpose of rhenium recovery [11]. The lower solubility of Cyanex 923 in water compared with that of TBP (0.05 to 0.4 g/L at 25 ◦C, respectively) and its complete miscibility with diluents at low temperature are mentioned as some of its advantages over other solvents such as TOPO (trioctylphosphine oxide) and Aliquat 336. Pathak et al. [12] studied the extraction behaviour of molybdenum from acidic radioactive wastes using PC88A. They found that by increasing HNO3 concentration in the aqueous phase, Mo extraction decreases, while increasing the organic concentration until 0.15 M causes an increase in metal extraction.

In the present work, the application of PC88A (2(ethylhexyl)phosphonic acid mono-2(ethylhexyl)-ester) extractant was studied on Mo–Re separation in solutions obtained from leaching molybdenite flue dust under various conditions of organic concentration and aqueous solution acidity. Next, the performance of PC88A was compared with that of D2EHPA using both experimentation and density functional theory simulations.


**Table 1.** Literature on Mo and Re solvent extraction.

N235 = tri-octyl amine, TBP = tributyl phosphate, Cyanex 923 = trialkylphosphine oxide, LIX 63 = 5,8-diethyl-7-hydroxy-6-dodecanone oxime, D2EHPA = di-(2-ethylhexyl)phosphoric acid, Alamine 304-1 = tri-n-dodecyl amine, PC88A = 2(ethylhexyl)phosphonic acid mono-2(ethylhexyl)-ester.

### **2. Experimentation**
