A thermogravimetric investigation of the synthesis of cobalt fluoride

Abstract

Fluorochemicals are valuable organic compounds manufactured by PELCHEM, the chemical manufacturing company of NECSA (The South African Nuclear Energy Corporation of South Africa). Fluorochemicals are manufactured through the fluorination of hydrocarbons to yield fluorocarbons. Fluorination is achieved using hydrogen fluoride and fluorine as the fluorination agent. The process in which fluorine is used as fluorinating agent is not viable owing to the high energy of reaction released with possible fragmentation of the hydrocarbon taking place. Cobalt trifluoride is known to be a mild fluorinating agent and yields less heat of reaction. Though the method has been used throughout the world, it has not been exploited within Necsa. It is therefore the aim of Necsa to establish another fluorination technology in which cobalt fluoride is utilized as a fluorinating agent. If successfully established, this technology will yield huge benefits for Necsa. Cobalt fluoride is quite expensive to obtain from the commercial market. It is therefore the aim of this study to investigate dry methods in which cobalt (III) fluoride is synthesised from the less expensive oxide form. This project will be undertaken in two phases, (i) the synthesis of cobalt fluoride and (ii) the usage of this compound in the fluorination of organic compounds. The current study focuses on the first phase, the investigation of synthesis of cobalt (III) fluoride from the mixed oxidation state oxide form, Co3O4. Cobalt (III) fluoride may be manufactured through direct fluorination using fluorine gas, but this process would result in high operational costs as fluorine is more expensive than hydrogen fluoride. As a result, our synthesis route involved sequential fluorination of cobalt oxide with hydrogen fluoride and then fluorine gas. Fluorine gas was used only for the fluorination of the cobalt (II) fluoride, resulting in lower amounts of fluorine used, thereby leading to significant savings in costs. Firstly, simulation of the reaction of Co3O4 with HF and F2 was carried out through thermodynamic equilibrium composition calculations using the HSC Chemistry software program. The thermodynamic data was then used as guideline to which the actual experimental reactions should be performed. Prior to the fluorination reactions, a study on the thermal analysis and spectroscopic characterisation of commercial Co3O4, CoO and CoF2 compounds used as starting materials, as well as CoF3 which was the desired product was performed. Spectroscopic characterisation techniques used included XRD, ATR-FTIR and Raman. The results of this study indicates that fluorination reactions should be carried out under dry conditions and at temperatures below 600 oC to limit decomposition and sublimation of CoF3. Four reactions were carried out to investigate the synthesis of CoF3: (i) Co3O4 with HF, (ii) Co3O4 with F2, (iii) CoF2 with F2, and (iv) a sequential reaction of Co3O4 with HF and then F2. These reactions were conducted on a thermogravimetric analyser. The degree of fluorination of the respective reactions was followed via the mass uptakes recorded at various isotherms. The ideal temperature condition for the synthesis of CoF2 through reaction of Co3O4 with HF was found to be 500 oC, whilst the ideal temperature for the synthesis of CoF3 through reaction of Co3O4 with F2 and CoF2 with F2 was found to be 300 oC and 400 oC respectively. These results were used to successfully synthesise CoF3 through a sequential reaction of Co3O4 with HF and F2 gas. XRD and ATR-FTIR analyses were instrumental in the characterization of the reaction products obtained. The next phase of the project will be the design and construction of a suitable laboratory scale reactor to produce sufficient cobalt trifluoride for the fluorination efforts.