Solubility of Transition Metal Halides in Chloroaluminate Melts

Low melting temperatures of KCl-AlCl₃ binary mixtures make chloroaluminate melts attractive as working media for transition metals’ electrowinning and electrorefining. These electrolytes can also be effectively used as heat transfer agents in the second loop of molten salt nuclear reactors. However the implementation of new electrochemical and nuclear technologies is limited by the lack of some essential information about physical and chemical properties of these ionic liquids; in particular there is no data on solubility of transition metal halides in chloroaluminate melts under different conditions.

In the present study solubility of nickel, chromium, molybdenum and iron chlorides was determined in КCl-AlCl₃ based melts with varying K : Al mole ratio and working temperature. Potassium-to aluminum mole ratio and the transition metal content in the melt was determined by analyzing samples of chloroaluminate melts saturated with the required transition metal chloride at desired temperature and then rapidly quenched. Melt samples were analyzed by atomic emission spectroscopy combined with ICP.

The relation between the solubility of nickel chloride and melt composition (different K : Al mole ratios) is presented in Fig. 1a. Solubility curve of NiCl₂ is V-shaped. The solubility minimum (<0.02 wt.% NiCl₂) is reached at the mole ratios of K : Al of 0.92 at 350 ^оС and 0.97 at 500 ^оС. Addition of potassium chloride added to the КCl-AlCl₃ equimolar mixture resulted in increase of nickel chloride solubility presumably due to formation of nickel-containing chloride complexes, NiCl₄^2-. Increasing nickel chloride solubility upon lowering K : Al mole ratio (from unity down) can be explained by the formation of six-coordinated complexes of nickel, NiCl₆^4-, existing in the form of Ni(Al₂Cl₇)₂ compound. Increasing temperature does not alter the profile and behavior of the solubility curve of nickel chloride in КCl-AlCl₃ based melts. NiCl₂ solubility increases with temperature at potassium-to-aluminum mole ratios below unity and noticeable decreases in the melts with potassium chloride content exceeding the equimolar КCl-AlCl₃ composition.

The results of molybdenum(III) chloride solubility determination at different melt compositions are presented in Fig. 1b. Molybdenum chloride solubility at K : Al molar ratios below 0,95 is small (equals to 0,015–0,03 wt.% MoCl₃) and does not depend on melt composition and temperature. When KCl content in the melt increases the molybdenum(III) chloride solubility becomes higher and at molar ratios of K : Al around 1.1–1.4 reaches 2.1 wt.% at 350 ^оС and 1.7 wt.% at 500 ^оС. The shape of the solubility curve indicates that molybdenum complex ions, MoCl₆^3-, are formed during dissolution of molybdenum chloride in the presence of excess of potassium chloride.

Solubility of iron(II) chloride in КCl-AlCl₃ based melts depends on solubility of double chlorides KFeCl₃ and K₂FeCl₄, that have relatively high melting temperatures (399-406 and 374-380 ^оС, respectively). The obtained data (Fig. 1c) show that iron chloride solubility is relatively high and its dependence on the melt composition has a maximum at K : Al mole ratios between 1.05–1.15.

Solubility behavior of chromium(II) chloride in chloroaluminate melts is more complex (Fig. 1d). At the molar ratio of K : Al<0.9 the CrCl₂ solubility is small (less than 0.01 wt.% CrCl₂) and does not depend on the melt composition. The increase of KCl concentration in the melt results in increasing chromium chloride solubility. It reaches maximum when K : Al mole ratio is close to 1.0. At further increasing potassium chloride content solubility of CrCl₂ shows a tendency to decreasing. This tendency is more pronounced at higher temperatures (500 ^оС). The maximum solubility of chromium(II) chloride in the melt is however not that high and does not exceed 0.15 wt.% CrCl₂.

The performed experiments thus indicate that the mole ratio of potassium and aluminum chlorides in the melt is the major factor controlling solubility of transition metal chlorides in the chloroaluminate melts.

In a special series of experiments solubility of transition metal chlorides was investigated in chloroaluminate melts contacted with Hastelloy C-276 corrosion-resistant alloy. It was found that the presence of nickel-chromium-molybdenum-iron alloy in the contact with studied electrolyte does not lead to noticeable changes of solubility of transition metal chlorides in КCl-AlCl₃ based melts.