Modelling of heavy metals adsorption to a chelating resin in a fluidized bed reactor

Adsorption to a chelating resin is a method for recovering heavy metals from wastewater containing very light quantities of heavy metals (<0.3 mol m−3, which approximately corresponds to 20 ppm). A thermodynamical study in a closed vessel showed that equilibrium is well represented by a Langmuir isotherm. Adsorption kinetics in a continuous stirred tank reactor were also conducted. Experiments were simulated by a global kinetics model comprising mass transfer in a liquid film around the resin particles, with diffusion through the pores and reaction on the adsorption sites. Kinetics has been found to be limited by film mass transfer for all metals studied (Cu2+, Ni2+, Co2+ and Zn2+). The mass transfer coefficient kL was found to be around 10−4 m s−1. Adsorption of heavy metals was then carried out at mini-pilot scale. Problems due to a decrease in particles mean radius during the adsorption prompted us to use a fluidized bed. It is also possible with this reactor to treat solutions containing suspended solids which would clog fixed beds. The decrease in particle radius (or increase in apparent density of the resin) produces a contraction of the fluidized bed: unloaded particles remain at the top of the bed and a density gradient appears throughout the column, leading to a stabilization of the fluidized bed. The measured phenomenon is well described by a liquid plug flow model with immobile resin in the column. As for the adsorption in the continuous stirred tank, the Langmuir model and the kinetics limited by film mass transfer were considered. The mass transfer coefficient kL was adjusted to a value around 10−5 m s−1.