The evaluation of the mechanisms involved in the extraction of nickel from low concentration effluents by means of supported liquid membrane

Abstract

From an economic point of view, the use of membranes at the present time is intermediate between the development of first generation membrane processes and second generation processes such as supported liquid membranes. The objective of this research was to investigate the mechanisms involved in the extraction of nickel from low concentration effluents by means of supported liquid membranes (SLM). A custom-made reactor/extractor was used for experimentation, based upon a process flow diagram that closely emulated the flow diagram of an industrial application and was used to determine the scientific and technical feasibility of the SLM process. The extraction equilibrium of the nickel/di-(2-ethylhexyl) phosphoric acid (Ni/D2EHPA) system is an integral part of the extraction process and was determined with extraction experiments. The results of the extraction experiments showed that there is no significant influence of temperature on the equilibrium for the temperature range of 30 /C to 70 /C. It was assumed that the nickel-organic complex exists in two forms, one in which the nickel coordinates with two D2EHPA molecules and another one in which the nickel coordinates with six D2EHPA molecules. It was found that the experimental data supported this assumption. Another augmentation of the equilibrium model was the incorporation of the activity of the aqueous species, as well as the effect of aqueous speciation of the nickel species. The SLM-process was modelled by solving a system of equations that describe all six steps involved in the extraction process and a special computer program was written to solve the system of equations. The process model showed that the nickel flux through the SLM is determined by the diffusion of the nickel through the feed boundary layer as well as the diffusion of the organo-metallic species through the membrane and although temperature does not have an effect on the extraction equilibrium, it does have a beneficial effect on both of these transfer steps. It was found that, as long as a sufficiently low pH (pH < 2.0) was kept in the strip solution, the strip side will not be rate limiting. The process model showed that there exists an extractant concentration at which the nickel flux is an optimum and that this optimum is dependent on temperature. The effect of all the variables involved in the extraction process are interdependent and the model is capable of predicting the effect of this interdependence. The research reported in this thesis leads to a better understanding of the SLM process and suitable recommendations are made towards a possible industrial application of this technology.