Regeneration of cobalt-contaminated activated carbon by supercritical carbon dioxide extraction

Abstract

A mixture of Di-(2-ethylhexyl) phosphoric acid (D2EPHA) and n-hexane or methanol is used as the extractant solution for extracting cobalt from activated carbon using supercritical carbon dioxide extraction technology. In this work, a semi-continuous pilot unit of SFE is employed to conduct the extraction. In order to feed the viscous extractant by HPLC pump, n-hexane or methanol is added as a diluent to reduce the viscosity of the extractant. The amount of cobalt removed along the time course of the extraction is recorded and plotted as an extraction curve. A kinetic model is also established and fit to the extraction curve, and the obtained parameters of the model are used to explain the regeneration mechanism. The effects of temperature ranged from 40 to 80 °C and the effect of the concentration of diluents on the extraction are investigated and discussed based on the established model. It is also found that the removal of cobalt ions reaches a maximum; this varies with the operational conditions and is known as maximum removal efficiency. It is presumed that the maximum removal efficiency is affected by the adsorption kinetics of the extractant and the rate of ion exchange between the extractant and metal ions on the surface of the activated carbon. After increasing the extraction temperature from 60 to 80 °C it is observed that the maximum removal efficiency is greatly increased, presumably resulting from the diminishing competitive adsorption between the extractant and diluents. The established model can help to reveal the extraction mechanism and to promote maximum removal efficiency for regenerating activated carbon without secondary pollution.