Recent developments have indicated high oxygen consumption rates of about 35 g-mole/m³-min during oxidative pressure leaching. At such high oxygen consumption rates the mass transfer of dissolved oxygen at the gas-liquid interface may become rate-limiting. The objective of this study was to obtain an understanding of the gas-liquid mass transfer processes that take place in mechanically agitated pressure leaching systems. The classical reaction between sodium sulphite and dissolved oxygen to form sulphate at atmospheric pressure was used to determine the oxygen mass transfer rates in a 200-liter asymmetrical plastic tank, modelled after the shape of the first compartment of the zinc pressure leach. The effect of this asymmetry was compared with the work of Swiniarski who used a cylindrical symmetrical tank of similar volume. A number of process variables such as the impeller type and size, the impeller speed, the impeller immersion depth and the effect of full baffles that affect mixing were investigated. Also, the volumetric power consumption associated with the mass transfer rates were measured. The results indicate that the asymmetrical tank is at least 3.6 times more efficient in mass transfer than the symmetrical tank. There is a critical speed below which the mass transfer parameter, K[formula omitted], is almost zero and above which K[formula omitted] increases almost linearly with impeller tip speed. A simple energy balance model for bubble creation can predict the critical tip speed. It is shown that K[formula omitted] is enhanced at shallow depths, with a corresponding high mass transfer to energy ratio. The relative effectiveness of impeller types and sizes with regard to the use of power for gas-liquid mass transfer was established. Full baffles degrade the mass transfer rate at increased depth of impeller immersion. The results also add substantial support to the findings provided by DeGraaf [5] that: (i) The dimensionless correlations used in liquid mixing systems do not accurately predict dispersion rates by agitators. (ii) The optimum conditions for gas dispersion and the consequent generation of gas-liquid interfacial area are different from fluid mixing. (iii) The classical mixing power equations for impellers markedly overestimate power requirements during impeller gas dispersion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Hydrometallurgical processing of sulphide concentrates is an attractive method for the selective extraction of valuable metals. The dissolution of minerals in a leaching process involves several electrochemical parameters that need to be investigated• to ensure the development and growth of the base metal industry in South Africa. A study has been carried out to elucidate the leaching mechanism of a nickel-coppercobalt sulphide concentrate in an oxidative pressure-acid medium. The sulphide concentrate studied in this research, comprises mainly of the minerals pyrrhotite, (Fe1_xS) with x = 0 to 0.2, pentlandite, (Ni,Fe)9S8 and chalcopyrite, (CuFeS2). The leaching behaviour of these minerals was successfully studied by means of Atomic Absorption (AA) measurements, Scanning '•Electron Microscopy (SEM) and Moss bauer spectroscopy, after leaching took place in an oxidative pressure-acid medium. The dissolution of the valuable metals was achieved effectively with recoveries of well over 90% for nickel, copper and cobalt under the specific conditions studied. Mechanical activation by means of ultra fine milling improved metal extraction with an average of approximately 40%, after a leaching period of 150 minutes. The most suitable conditions for the oxidative pressure-acid leaching of the mechanically treated nickel-copper-cobalt sulphide concentrate in a dilute sulphuric acid medium were found to be: particle size 80% - 10J.Lm; temperature l10°C; oxygen partial pressure 10 bar; sulphuric acid concentration 30 kg/ton; solids content 15% by mass and an impeller agitation rate of 800 r/min. The values of the apparent activation energies of nickel, copper and cobalt, extracted from the sulphide concentrate, were found to be 20.6 (± 4.4) kJ/mol K, 33.6 (± 4.2) kJ/mol K and 17.4 (± 3.5) kJ/mol K respectively. / Thesis (MIng (Chemical Engineering))--Potchefstroom University for Christian Higher Education, 2001

Hydrometallurgical processing of sulphide concentrates is an attractive method for the selective extraction of valuable metals. The dissolution of minerals in a leaching process involves several electrochemical parameters that need to be investigated• to ensure the development and growth of the base metal industry in South Africa. A study has been carried out to elucidate the leaching mechanism of a nickel-coppercobalt sulphide concentrate in an oxidative pressure-acid medium. The sulphide concentrate studied in this research, comprises mainly of the minerals pyrrhotite, (Fe1_xS) with x = 0 to 0.2, pentlandite, (Ni,Fe)9S8 and chalcopyrite, (CuFeS2). The leaching behaviour of these minerals was successfully studied by means of Atomic Absorption (AA) measurements, Scanning '•Electron Microscopy (SEM) and Moss bauer spectroscopy, after leaching took place in an oxidative pressure-acid medium. The dissolution of the valuable metals was achieved effectively with recoveries of well over 90% for nickel, copper and cobalt under the specific conditions studied. Mechanical activation by means of ultra fine milling improved metal extraction with an average of approximately 40%, after a leaching period of 150 minutes. The most suitable conditions for the oxidative pressure-acid leaching of the mechanically treated nickel-copper-cobalt sulphide concentrate in a dilute sulphuric acid medium were found to be: particle size 80% - 10J.Lm; temperature l10°C; oxygen partial pressure 10 bar; sulphuric acid concentration 30 kg/ton; solids content 15% by mass and an impeller agitation rate of 800 r/min. The values of the apparent activation energies of nickel, copper and cobalt, extracted from the sulphide concentrate, were found to be 20.6 (± 4.4) kJ/mol K, 33.6 (± 4.2) kJ/mol K and 17.4 (± 3.5) kJ/mol K respectively. / Thesis (MIng (Chemical Engineering))--Potchefstroom University for Christian Higher Education, 2001