Characterization of the Cathodic Thermal Behavior of Molten CaCl₂ and Its Hygroscopic Chloride Mixture During Electrolysis

Abstract

A liquid metal electrochemically deposited in CaCl2 or its chloride melts serves as an effective reductant for active metal oxides. Although a very low oxygen concentration can be achieved at a considerably high electrolysis efficiency, the existence of small amount of water impurity in molten chlorides, which is very difficult to detect, causes low electrolysis efficiency. In this study, to clarify the morphological and thermal characteristics of a cathodic electrode in a slightly hygroscopic LiCl-KCl-CaCl2 melt, we simultaneously performed electrochemical measurements and thermal measurements using an ultrafine thermocouple inserted inside a Mo electrode (i.d. 1.57 mm). 2 Concomitantly, changes in the electrode interface were recorded at 500-μs intervals using a synchronized high-speed digital camera. Despite the small amount of water included in the system, the measured heat absorption was much smaller than thermodynamically predicted, which suggested that the generated hydrogen decreased the purity of the liquid alloy electrodeposited on the cathode surface possibly through hydride formation. By using the synchronized thermal measurement, it was possible to trace the change in the electrodeposition pattern of impurity water quickly and sensitively, which was difficult to determine in only the electrochemical potential-current response.

Recent Studies on Titanium Refining: 2017–2020

“…The morphological and thermal characteristics of a cathode in a slightly hygroscopic LiCl KClCaCl 2 melt were examined using a high-speed camera. 35) The measured heat absorption was considerably smaller than the thermodynamically predicted value, which suggested that the generated hydrogen formed a hydride by reacting with the liquid Ca that was electrodeposited on the cathode surface.…”

Steam-assisted electro-reduction of NiO: a sustainable alternative to conventional hydrogen reduction

“…29 Moreover, CaCl 2 is also known to possess hygroscopic characteristic, enabling the evolution of hydrogen. 30,31 Therefore, KCl and CaCl 2 were separately added into LiCl molten electrolyte and the reduction of NiO in such electrolytes was studied.…”

“…After that, the cathodic potential remains relatively stable at around −0.52 V vs. Mo-QRE, which is the smallest value in the stage I, in comparison with other electrolytes. In fact, the hygroscopic nature of CaCl 2 31 and its hydrolysis in the presence of H 2 O could support the formation of protons in the electrolyte. At the same time, the addition of 30 wt% CaCl 2 not only lowered the liquidus temperature, 35 elevating the superheat of the melt at 110 °C, but also could result in an accelerated hydrolysis reaction and enhancing the activity of generated H + , thus the cathodic hydrogen evolution reaction.…”