Automation of membrane based solvent extraction for Zr and Hf separation
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In recent years, research into metal ion extraction using hollow fibre membranes has grown, focussing on the extraction of a wide range of metals including base, transition, and rare earth metals. This non dispersive solvent extraction technique does not have many of the disadvantages such as emulsion formation, foaming, and flooding, which are associated with the more traditional dispersive solvent extraction techniques. Current studies using hollow fibre membranes for the separation of Zr and Hf generally use the same experimental setup consisting of a manually controlled system monitored by analogue gauges. Although the manual system is sufficient for initial proof of concept studies, it does lack the accuracy and repeatability to advance this technology to the next step of commercialisation. The aim of this study was therefore to design, simulate and construct an automated membrane based solvent extraction (AMBSX) system for use in Zr and Hf extraction research. Boundary conditions entailed that the system should be chemically resistant to acids (up to 9 mol/L) as well as to a variety of organic solvents commonly used in solvent extraction. In addition, the apparatus was designed to be able to function over a range of pressures (0-200 kPa), flow rates (100 - 900 mL/min) and flow directions (co- and counter-current) up to a temperature of 35 °C. The objective was to attain independent automated control of both flow rate and pressure in the system, while improving the accuracy and repeatability of the results. The automation of the experimental setup was done using National Instruments hardware and a central controller programmed using LabVIEW™. The system was first simulated using LabVIEW’s built in simulation functionality as a proof of concept for the control of the system. Flow rate and pressure were controlled using proportional derivative and integral (PID) control algorithms and optimised using the Cohen-Coon tuning method. It was shown that proportional integral (PI) control was preferential for the AMBSX system. After the design, construction and commissioning of the AMBSX, the system was optimised using the method obtained from the simulation of the AMBSX. After optimisation, a case study for the extraction of Zr and Hf, using Cyanex 301® as extractant, was conducted on the AMBSX using pressures of 100 kPa and 70 kPa and flow rates of 450 mL/min and 350 mL/min for the aqueous and organic phases, respectively. Five separate runs of 120 minutes each were done to determine the control and repeatability obtainable with the AMBSX. It was shown that the automated system was able to accurately control the flow rate and pressure to desired set points. This improvement of accuracy led to highly reproducible extraction results with the standard deviations between the five extractions varying by less than 1.2%. From this it can be concluded that the design, simulation and construction of an automated system was successfully implemented with independent control of the flow rate and pressure.