Electrochemical behaviour of sulphur containing species in molten salts
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The behaviour of sulphur containing species has been investigated during the last few decades in various molten chloride and fluoride electrolytes but their effect on the performance of the cells producing aluminium still remains a subject of controversy. In the present work the electrochemical behaviour of sulphur containing species was studied in various molten chlorides and fluorides with the aim to contribute to a better understanding of the behaviour of sulphur impurities in the Hall-Héroult process. The sulphur electrochemistry and chemistry in the systems containing molten electrolytes are of interest for aluminium electrowinning because of the serious impact of the sulphur containing gases on the environment and the effect of sulphur compounds on the efficiency of the electrolysis. The complexity of the sulphur chemistry is briefly summarised in chapter 2.1. The literature review focused on the electrochemistry of sulphur species in molten salts is discussed in chapter 2 while chapter 3 is dealing with the description of the electrochemical and analytical methods used in this study. Preliminary experiments in electrolytes consisting of molten NaCl and mixtures of NaF-AlF3 revealed the necessity to find an inert electrode material for electrochemical studies. Platinum in the chloride melts and gold in the fluoride systems were selected as the most suitable materials for the working electrodes. The behaviour of the anions containing sulphur in various oxidation states was investigated by means of cyclic voltammetry and chronoamperometry and is described in chapter 4.3. The behaviour of sulphate anions was studied in a single NaCl melt and compared with the molten CaCl2-NaCl (10-90 mol%) mixture at 840 ºC. It was found that the sulphate reduction proceeds differently in these two electrolytes which was ascribed to the difference in the oxoacidity of the melts. The number of electrons transferred during sulphate (SVI) reduction in pure NaCl melt was found to be two with probable sulphite formation which possibly decomposed to sulphur oxide. In the molten CaCl2-NaCl mixture sulphate seemed to be transformed to SO3 which is further reduced in a one-electron exchange. The electrochemical signals recorded in the voltammograms also suggested that the character of the reduction products involved in those two melts is different. However, the “ec” mechanism, where a charge transfer is followed by a chemical reaction seemed to be the common feature in both electrolytes. The sulphate reduction was also studied in the eutectic LiCl-KCl mixture in the range of temperatures from 450 ºC to 840 ºC. In spite of the fact that the sulphate solubility seemed to rapidly decrease at temperatures below 700 ºC, the possible presence of a chemical reaction taking place after the reduction process of sulphate anions was suggested to be very similar to that found in molten NaCl.From the analysis of the data obtained by cyclic voltammetry followed that the reduction of sulphate anions to a lower oxidation state is probably diffusion controlled in all the three above mentioned systems at the sweep rates where the effect of the coupled chemical reaction can be neglected. Chapter 4 also includes an additional investigation of the behaviour of the sulphite and sulphide anions in the eutectic LiCl-KCl mixture. Chapter 5 is devoted to the investigation of the sulphate species in the NaF-AlF3 mixtures saturated by Al2O3 with varying cryolite ratio, i.e. CR = 3, 2, 1.5 and 1.2. It contains a description of sulphate modification at various experimental conditions and revealed its complicated and complex electrochemical behaviour which was monitored by cyclic voltammetry, chronoamperometry and square wave voltammetry. For the electroactive species in the electrolyte with CR equal to 3 two reduction steps were suggested involving three and two electrons respectively. The “ce” reaction mechanism was assumed where a chemical reaction seems to precede a charge transfer. The chemical step might be the partial decomposition of sulphate to SO2. The sulphate reduction in the electrolyte with CR = 2 appeared to be considerably different. Since the temperature difference in these two systems is only 40 ºC the reason was likely to be due to the higher concentration of AlF3 in the electrolyte. In this electrolyte only one cathodic process was observed and the reaction mechanism involving three electrons with a preceeding chemical reaction was suggested. Because the production of aluminium metal in the electrolysis cells operating with inert anodes is an attractive idea and have opened a question about the behaviour of sulphur species in very acidic electrolytes (high AlF3 content) a part of the present studies was devoted to the study of the electrochemical behaviour of the sulphate anions in the electrolytes with CR = 1.5 and 1.2. Cyclic voltammetry and square wave voltammetry revealed that the sulphate is then reduced in two cathodic steps. Simulations and the obtained square wave voltammograms indicated that the cathodic process probably involves more electrons in the very acidic NaF-AlF3 mixtures than in electrolytes with higher CR equal to 3 or 2. Observation of volatile species formed in the NaF-AlF3 mixtures with CR = 1.5 and 1.2 was revealed as another important difference. This could be due to the formation of sulphur which at high temperature is present in the gaseous form and thus likely to escape from the laboratory furnace. The difference in the electrochemical behaviour of the sulphate anions in the cryolitic based melts was related to the difference in the bath composition and experimental temperature. The present study shows the complexity and variability in the electrochemical behaviour of sulphur containing species as well as the difficulties and limitations resulting from the sulphur chemistry in the molten salts at high temperatures.