Raw materials recovery from WEEE : metal ions interaction with clay and carbon based sorbents

Waste Electrical and Electronic Equipment (WEEE) is constantly increasing in quantity and its disposal is of great interest, in particular because some precious and strategic metals are therein present. In this perspective WEEE cannot be considered only as a waste but it can be a resource. In particular electronic boards are among the electronic scraps the richest in precious metals. The most promising process to recover metals from electronic scraps is the dissolution with acids followed by a liquid-liquid extraction, but although this process is used in industrial scale the resulting efficiency is not very high since it comes from equilibrium reactions. Furthermore this kind of separation uses cost effective chemical agents that have great environmental impact, and so alternative treatment processes are nowadays being studied. One of these alternatives is the adsorption by solid sorbents that compared to the liquid-liquid extraction presents some advantages: a good recovery efficiency, easy separation of the two phases, short reaction times, low costs and low consumption of organic solvents. This thesis has the aim to investigate an emerging adsorption methodology to recover precious metals and Rare Earths, removing the ionic metals from water and subsequently releasing them with another reaction step in a new system. The solid matrices that have been analysed are: montmorillonitic clay and activated carbons. These solids have been tested in their original status but also after their modification with a commercial pentaethylenhexamine, hereafter named L6. Previous works have analysed these solid matrices in terms of uptake capability and efficiency testing different metals and different operating conditions. The results of these works have been considered as valuable starting point for this thesis. In the first part of the work a global overview of the state of art, the used materials and the adopted methodologies are reported, then the uptake of lanthanum on modified L6 clay is studied. The clay has been modified with a solution of polyamine 270 mM, since previous studies had analysed the same system but with the intercalation of polyamine performed at lower concentration (180 mM and lower). Afterwards the uptake behaviour towards copper has been studied on all the mentioned four different solid matrices (STx, modified L6 STx, AC and modified L6 AC). The copper was selected because it is one of the most abundant metals in WEEE but also because it is a very valuable material and has a strategic importance in the electronic devices. From the experiments performed on copper system the isotherms of all the solid matrices are derived and some speculations on their trend and on their behaviour have been advanced. Furthermore a more complex system is studied; indeed the real solution that can be generated during the WEEE treatment and recycling is generally composed by a mixture of ions, in which copper usually is the most abundant but not the only metal in solution. Knowing the uptake behaviour and efficiency of lanthanum and copper mono-ionic system, a bi-ionic solution composed by lanthanum and copper was chosen for the experiments. The aim of the bi-ionic experiments was to verify: a) if the presence of more than one element in solution generates interference in the uptake of the two elements b) if a different selectivity is present between the solid matrices c) if the uptake mechanisms are influenced by the bi-ionic system. In order to carry out this study the samples of each experiments have been analysed with different techniques to characterise both solid and liquid phases. The solid matrices have been analysed with X-ray diffraction (XRD) while the COD analysis, performed on the liquid phase before and after the reaction step, is used to determine the loading of the polyamine on the solid matrix. At the end the liquid phase of each experiment is analysed with ICP-OES technique in order to evaluate the metal concentrations in solution.