Indirect Methods of Determination of K:Al Mole Ratio in Molten Chloroaluminates

Chloroalumninate melts are prospective media for the second loop of molten salt nuclear reactors and low-temperature electrowinning and electrorefining processes for metals that can not be obtained from aqueous solutions. Application of chloroaluminates in such technologies is limited by the problem of corrosion resistance of construction materials. The alloys and steels corrosion resistance strongly depends on KCl-AlCl₃ melt composition (K : Al mole ratio). When the mole ratio of potassium-to-aluminum is high (basic melts) precipitation of insoluble potassium chloride can cause the abrasion of the material. From the other side increasing AlCl₃ content in the melt leads to increase of the red-ox potential due to formation of Al₂Cl₇^- species (acidic melts). Thus the importance of knowledge and ability to control K : Al mole ratios is undoubted for the industrial application of chloroaluminate melts.

Chemical analysis based methods allow to determine the general content of potassium and aluminium in the melt. However chloroaluminate melts are very sensitive to moisture that can cause AlCl₃ hydrolysis and result in decreasing «actual» aluminum chloride concentration in the melt. At increasing K : Al mole ratio (for example, due to evaporation of AlCl₃) precipitation of solid potassium chloride can take place and this may lead to erroneous results of hemical analysis. Therefore indirect methods for determining K : Al mole ratio in molten chloroaluminates are needed to obtain correct data and perform possible required adjustment of the acid-base properties of KCl-AlCl₃ electrolytes.

In the present work potentiometry (with aluminum working electrode), cyclic voltammetry (with tungsten working electrode) and X-Ray diffraction analysis were suggested for indirect control of K : Al mole ratio. The electrochemical measurements were conducted using an Autolab PGSTAT 302N potentiostat/galvanostat. The electrode potentials were measured vs. aluminum reference electrode (ARE) consisting of aluminum wire lowered into KCl-AlCl₃ melt containing solid KCl and separated from the working electrolyte by a diaphragm. X-ray diffraction patterns were recorded on an X’PERT PRO MPD X-Ray spectrometer with high speed 3D detector PIXCEL and an inert chamber Anton Paar HTK 1200N.

Calibration curves of aluminum working electrode vs. ARE in studied chloroaluminates were obtained in temperature range from 300 to 500 ^oC. They strongly depend on acid-base properties of KCl-AlCl₃ electrolytes and thus can be used for K/Al molar ratio determination. Reproducibility of the obtained values with temperature and salt composition was demonstrated.

Cyclic voltammetry can be effectively used for determining acid-base properties of molten КСl-AlCl₃ systems. In case of basic melts the cyclic voltamogramm consists of two edge peaks at 2.2-2.3 V and -0.3-0 V that correspond to dissolution of tungsten and deposition of aluminum. The presence of acidic Al₂Cl₇^- species leads to the appearance of additional maxima on the voltammograms. The influence of AlCl₃ concentration on the position of these peaks and their intensity was investigated.

The use of modern X-ray diffraction methods allows to perform high-precision qualitative and quantitative analysis. However the determination of low concentrations of «free» potassium and aluminum chlorides in quenched chloroaluminate electrolytes is complicated due to overlapping of weak lines of KCl and AlCl₃ phases (present in small amounts) with the lines of major KAlCl₄ monoclinic phase. High inclination of salt crystals to preferential orientation during sample preparation constitutes another difficulty for determining K : Al mole ratio by X-ray diffraction. In the present work we suggest using full profile Rietvelds analysis that allows to separate the contribution of diffraction patterns of different crystal phases even in the case of their overlapping. The results of X-ray diffraction determination of the composition of potassium chloroaluminate melts agree well with the electrochemical data.