RARE EARTHS RECOVERY BY SOLID-LIQUID EXTRACTION WITH NATURAL AND MODIFIED CLAYS

Nowadays there is an increasing need for Rare Earths (REs) due to their usage in numerous high-technology applications, so their recycling from Waste Electrical and Electronic Equipment (WEEE) can only be considered a valuable opportunity: this perspective is known as “urban mining” [1]. Among others, the use of solid-liquid extraction is obtaining more and more attention because of many advantages such as high recovery, short extraction time, high enrichment factor, low cost and no consumption of organic solvents [2]. The critical step in such a recovery process is the metal ions removal from the aqueous solutions. Recently, clays have been proposed as solid-phase being characterized by low cost, good mechanical properties, good acid tolerance, convenient solid-liquid separation and excellent reusability. Furthermore, clays show a natural adsorption behaviour towards REs [3]. On these bases, the aim of the present work is the study of REs recovery from WEEE via solid- liquid extraction. For this purpose different solid sorbents were considered, namely two natural smectite mineral clays (STx-1 and SWy- 2) and four modified organo-clays. Their effectiveness towards adsorption and release of lanthanum ions (selected as representing element of the REs family) model solutions was tested. Modified clays were synthesized by intercalating two different polymers having chelating properties, N-(methoxy-polyethylene glycol) ethylene diamine and N-(methoxy-polyethylene glycol) ethylene diamine trimethylene phosphonic acid. All the solid materials were characterized by FT-IR, thermo gravimetric analyses (TGA), X-ray diffraction (XRD) and XPS analyses, while the amounts of the intercalated polymer were estimated by COD (Chemical Oxygen Demand) analysis. Also the intercalation mechanism was investigated, varying the operating parameters, such as polymer concentration and pH in order to select the optimal intercalation conditions. The best organo-clay system was produced using 90 mM of initial polymer concentration and an intercalation pH of about 11, corresponding to a not-protonated condition of the chelating amino-groups. The adsorbed and released lanthanum ions were estimated by ICP-OES. A global efficiency (uptake and release) of lanthanum recovery around 30-35% was found for both pristine clays, while an improvement of the efficiency up to 43% was measured upon intercalation. Also the mechanisms and the kinetics of these reactions were evaluated and the results were mainly related to the amount and the nature of the intercalated polymer present in the clay.