Removal of Oxide Impurities from Alkali Haloaluminate Melts Using Carbon Tetrachloride

Abstract

Small amounts of oxide impurities in alkali chloroaluminate and fluorochloroaluminate melts can complicate markedly the electrochemical and spectroscopic behavior of other solute species in these melts. A simple method for the removal of oxides from these melts has been developed in our laboratory. This method is based on the reaction of carbon tetrachloride with oxides to convert them to chlorides. Spectroscopic techniques (UV-visible and IR spectroscopy) have shown that addition of carbon tetrachloride results in the complete conversion of oxides to chlorides.Oxide impurities in molten chloroaluminates ~ may have pronounced effects on the behavior of other solute species in these media; 2-4 these impurities are difficult to avoid. Recently we reported on the removal of oxide impurities from a sodium chloroaluminate melt saturated with NaC1 with phosgene (COC12). 8 Prior studies on the determination and removal of oxide species from chloroaluminate melts are summarized briefly in that paper. 5Phosgene is a very poisonous gas and must be handled with extreme caution. In addition, we have found that the removal of oxide impurities from acidic sodium chloroaluminate melts (A1C1JNaC1 mole ratio > 1) using COC12 is not complete. 5We report here a method for the removal of oxides from both acidic and basic alkali chloroaluminate melts, as well as fluorochloroaluminate melts. This method is based on the reaction of carbon tetrachloride with oxide species to convert these species to the corresponding chloride complexes. Using CC14 as a chlorinating reagent is advantageous compared .to the COC12-treatment in that CC14 is much easier to handle. FTS-7 Fourier transform infrared (FTIR) spectrophotometer which was controlled by a microcomputer system. The in situ infrared spectra of the molten salts were obtained using a cell similar to that described by Flowers and Mamantov. 8'~ The cell utilized silicon windows which were torch-sealed to the Pyrex body of the cell. AceThred adapters (Ace Glass Inc.) on the top of the cell provided access for loading and sample addition, and produced an airtight seal when closed. One AceThred adapter on the cell was covered with a septum. An appropriate amount of CC14 was added by injecting it through the septum using an airtight microsyringe (Baxter Diagnostics, Inc.). A furnace with diametrically opposed holes, which was constructed in house, allowed heating of the melt inside the sample chamber of the FTIR instrument. Results and Discussion AICl3-NaCls~ melt at 200~1 shows infrared absorption spectra in the region from 640 to 880 cm 1 for Experimental Aluminum chloride (Fluka, >99.0%) was purified by subliming it twice under vacuum in a sealed Pyrex tube. Sodium chloride (Mallinckrodt, reagent grade) was dried under vacuum (<50 mTorr) at 450~ for at least 48 h. High purity sodium fluoride (AE SAR, puratronic, 99.995 %), niobium pentachloride (AESAR, puratronic, 99.99%), and tungsten oxychloride (WOC14, Aldrich) were used without further purification. Carbon tetrachloride (water 0.001%) w...

Re‐establishing the paradigm for evaluating halide salt compatibility to study commercial chloride salts at 600°C–800°C

“…The commercial 20.1%K–12.9%Mg–1.6%Na–65%Cl salt contained impurities including 5% H 2 O and lesser amounts of Br, Fe, B, Mn, and SO 4 based on data from the manufacturer Table ). The salt was purified using a two‐step process: first the carnallite method mixing the salt with NH 4 Cl (2:1) followed by heating to 450°C for 2 hr and 750°C for 1 hr; the next step was sparging at 850°C (a) with CCl 4 for 35‐40 hr, (b) with UHP Ar for 0.5 hr, (c) with Ar‐4%H 2 for 12–15 hr, and (d) with UHP Ar for 0.5 hr. The commercial salt purified using only the first process was designated as salt #2 in Table and the two‐stage purified salt was designated salt #1.…”

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…Very similar behaviors to produce hydroxychloride during salt dehydration had been found in CaCl2 salt as well [16,17,[31][32][33][34][35]. To mitigate the risks of the corrosive hydroxychloride species, multiple methods for salt dehydration and purification have been proposed, such as thermal treatment by controlled heating [26,29,43,31,[36][37][38][39][40][41][42], reduction by elemental Mg [19,22,23,25], dehydration with ammonium chloride [14,29,42,44,45], and carbochlorination with CO, Cl2, and/or CCl4 to remove oxide species [13,14,46,47]. Because the targeting CSP industry needs a significant amount of salt inventory (>10,000 Mt) for each power plant, it is our intent to investigate the most cost-effective and scalable salt purification process.…”

“…Figures 18 and 19 show the DSC heating and cooling traces for baseline salts with additional MgCl2 and NaCl. 46 Table 10 summarizes the measured phase-transition temperatures during heating of six salt compositions off the baseline salt composition. 45 Research partners at the University of South Australia independently verified our results with DSC after performing the prescribed salt purification using the same raw materials.…”

Progression to Compatibility Evaluations in Flowing Molten Salts

“…The water content of the anhydrous MgCl2 is reported as ≤0.5%. For purification, carbochlorination via carbon tetrachloride (CCl4) is used to remove oxides based on the work of Young and Mamantov [Young 1990, Chen 1993. The KCl was melted and equilibrated at 850°C followed by sparging the molten salt with carbon tetrachloride (CCl4) for 3 h, argon for 0.5 h, argon with 4% hydrogen in argon for 1 h, and argon again for 0.5 h. All gases were Ultra-high Purity (UHP) grade from Airgas and further purified to remove trace moisture with phosphorus pentoxide (P2O5) layered with Drierite™.…”