The high anode overvoltage exhibited by fluorine cells has been variously attributed to bubble overvoltage and to inhibition of the electron transfer process by a layer of insulating carbon fluoride on the surface of the electrode. This thesis presents the results of various experiments which attempt to assess the contribution of the carbon fluoride film to the anode overvoltage in the absence of bubble overvoltage. Fluorine evolution from the molten salt KF.2HF at 85°C was studied at vertical carbon anodes. The techniques employed to study the kinetics and mechanism of this reaction included cyclic voltammetry, steady state potentiostatic polarisation, A.C. impedance, potential pulse and constant current electrolysis. Such measurements indicated that several types of carbon fluoride can be produced on the anode evolving fluorine, depending on the potential of electrolysis. These measurements also show the difficulty in obtaining kinetic parameters for the fluorine evolution reaction. The nature of the fluoride film produced on the carbon anode was investigated using XPS and SIMS analysis of electrodes polarised in the KF.2HF melt at 5V and 9V. These measurements indicated that the film is not one single carbon fluoride but more of a graded nature becoming more heavily fluorinated at higher potentials. The inhibiting effect of the solid film was studied by transfering carbon electrodes, following fluorine evolution in the KF.2HF melt under well' defined conditions, to other electrochemical systems using aqueous or organic electrolytes at room temperature, where electrode kinetic studies were made of redox reactions. Kinetic data were compared with those obtained with unfluorinated carbon surfaces. Observations obtained by the above methods have been related to estimates of the thickness of the carbon fluoride film as determined by measurements of the double layer capacitance in the KF.2HF melt and in other solvent systems. Both capacitance measurements and kinetic studies of redox reactions indicate that the film became progressively thicker as the potential of the fluorine anode was raised. Rates of redox reactions fell by at least two orders of magnitude in the presence of a film formed by polarising the carbon at 6.0V in the KF.2HF melt. Attempts were made to overcome the inhibiting effect of the fluoride films by incorporating transition metals in the carbon anodes. The last section of this thesis explains the manufacture of doped carbon samples and presents results obtained in the KF.2HF melt using such anode materials.

MSc dissertation - Faculty of Engineering and the Built Environment / Waste electrolyte from fluorine cells is a major waste problem for the fluorine chemical industry. Processes have to be developed to reduce, recycle and re-use the spent electrolyte that has up to now been stockpiled. This dissertation is a compilation of the research work that has been done to derive a process to treat waste electrolyte for re-use. Different conversion processes were investigated to develop a Waste Management plan for the fluorine generating facility in respect of the electrolyte. Gravity settling, centrifuging, filtration, the addition of KF.HF to the to the electrolyte to decrease the HF concentration in the electrolyte and consequently decrease the solubility of Fe, Cu and Ni and addition of NaOH to the electrolyte to convert soluble Fe to the insoluble triple salt were tested. Gravity settling and centrifuging were shown to produce the best solution. However, significant sedimentation of the insoluble metal impurities in the electrolyte is timeously. The implementation of sedimentation as an industrial separation process to purify waste electrolyte of excess metal impurities is therefore impractical. The results indicated that sparging molten electrolyte with N2 gas to remove HF (thus precipitating soluble Fe, Cu and Ni, and removing moisture to reduce corrosion of metal components), followed by sediment centrifuging, appears to be a practical basis for an industrial waste electrolyte treatment process. During an assessment carried out by the Economics Trends Research Group (ETRG) (3) at the University of Cape Town a strong argument was made for the need to direct companies in South Africa to address environmental concerns with high priority. In South Africa there is very little awareness of the concept of Clean Technology. Not only must the level of contamination be reduced before waste is released into the environment, but natural resources like water must be conserved, and energy consumption must be reduced. Public concern over degradation of the environment can no longer be ignored. Globally, the chemical industries are considered to be the main culprits in the degradation of the environment. The assessment carried out by the ETRG showed that the chemical industries are classed among the top 5 generators of toxic and hazardous waste in every country. The metallurgical sector (mining) is in most cases classed as the top waste generator. Development and implementation of technologies that are more efficient are not a matter of choice any more. Each new facility that is developed should meet the challenge of generating as little waste as possible. Unfortunately, many old industries and facilities did not focus on increasing efficiency and minimising waste. These old facilities experience a challenge now to develop technology to make them part of this Cleaner Production and Technology era. Cleaner Production implies generating less effluent or waste and recycling waste to be used as raw material in the same or another facility. Cleaner Production also concentrates on the increase of efficiency but this is often limited by the chemical properties of substances. This research was based on the ideas for implementation of Cleaner Production in the fluorine generation facility at Necsa. Waste reduction almost always implies investment in equipment and development of new technologies. However there is ample evidence to show that the cost of rehabilitation of contaminated environment is exceedingly high in comparison with the precautionary steps taken to prevent contamination. Waste/Effluent Management have become new buzz words in the industrial environment.