Cadmium separation from phosphoric acid using the emulsion liquid membrane
An emulsion liquid membrane (ELM) containing Cyanex 302 has been successfully
used for the recovery of cadmium from synthetic phosphoric acid solutions containing
30% wt. P205.
The ELM formulation was optimised for high extraction rate and stability. Cyanex
302, and a mixture of NaC1 (3M) and HC1 (1M), were selected from a set of possible
reagents and stripping phases, following equilibrium and kinetic experiments in two-phase
systems. Under chemical control the rate of Cd2 extraction was pseudo first
order with respect to Cd2 and Cyanex 302 concentration. Arlacel C was found to be
the most suitable surfactant for emulsion stability.
The dependence of the ELM rate of Cd 2 extraction on the concentrations of
cadmium, Cyanex 302, and surfactant, as well as on phase and treat ratio, and
homogeniser and stirrer speed, was investigated. Cadmium extraction levels were
found to be above 90% for most conditions. The influence of operating variables on
emulsion swelling and rupture was also evaluated as a function of time. Swelling of
the internal phase of the emulsion reached values greater than 50%, whilst emulsion
rupture was below 2%. Emulsion globule size distributions in the stirred tank were
measured for different impeller speeds and diameters, reagent concentrations, and
emulsion hold-up, using an endoscope attached to a high-speed video camera. A
correlation for globule size, accounting for swelling and rupture phenomena, changes
in viscosity and hold-up, was proposed and found to be in good agreement with
experimental results. This allowed the effect of the stirring speed on the specific
interfacial area and the mass transfer coefficient to be decoupled, and led to the
conclusion that the overall rate of Cd 2 recovery was controlled by diffusional
resistances in the continuous phase.
A time- dependent model, and a quasi-steady state model were proposed for the
extraction of cadmium with the ELM. Simulations with both models were found to be
in good agreement with experimental results.