Abstract
Since the inception of metal precipitation by hydrogen, it has been recognized that adding some organic compounds to the reduction solution accelerates the reaction, inhibits plating and helps control the resulting powder properties such as morphology, microstructure and particle size distribution, to name but a few. The powder properties are directly linked to the particulate processes taking place during reduction. Controlling these properties is crucial for downstream processes. For instance, in fuel cell technologies, the porosity of materials used to construct the electrodes is a critical parameter amongst others. However, in addition to their relatively high cost, additives’ function and mechanism of action are not well understood. Furthermore most of the additives are used on a trial and error basis. Since additives constitute one of the major operational costs, it becomes evident their use should be optimized. One of the most plausible options to achieve this is to develop an understanding of their mechanism of action.
This dissertation assesses the effect of calcium lignosulphonate and ethylene maleic anhydride on the particulate processes taking place during the reduction of nickel ammine sulphate solutions by hydrogen gas. Reactions were carried out in an autoclave operated at 28 bar and 180°C under stirring conditions of 850 rpm. Five densifications were performed for each additive at the dosage of 2, 5, 7 and 10 mg/L. Particulate processes were studied by analysing the particle size distribution and the corresponding normalized moments. These were further validated by scanning electron microscopy and nitrogen physisorption analyses. The powder phase identification and purity were determined by means of X-ray diffraction and X-ray fluorescence respectively.
Both calcium lignosulphonate and ethylene maleic anhydride acted as reduction catalysts under experimental conditions. Calcium lignosulphonate promoted growth and by extension agglomeration which was more pronounced at 10 mg/L. At lower concentrations of calcium lignosulphonate, the system was found to be dominated by breakage as validated by scanning electron micrographs. Although there was no significant difference in the nickel content of the powder obtained in the presence and absence of calcium lignosulphonate, the powder obtained at 10 mg/L additive dosage resulted in the highest sulphur content. Ethylene maleic anhydride on the other hand acted as a growth inhibitor and an anti-agglomerating agent, thus...
M.Tech. (Chemical Engineering)