Oxidative Leaching of metals from electronic waste with solutions based on quaternary ammonium salts

摘要

The treatment of electric and electronic waste (WEEE) is a problem which receives evermore attention. An inadequate treatment results in harmful products ending up in theenvironment.This project intends to investigate the possibilities of an alternative route for recycling ofmetals from printed circuit boards (PCBs) obtained from rejected computers. The processis based on aqueous solutions composed of an etchant, either 0.2 M CuCl2.2H2O or 0.2 MFeCl3.6H2O, and a quaternary ammonium salt (quat) such as choline chloride orchlormequat. These solutions are reminiscent of deep eutectic solvents (DES) based onquats. DES are quite similar to ionic liquids (ILs) and are used as well as alternativesolvents with a great diversity of physical properties, making them attractive forreplacement of hazardous, volatile solvents (e.g. VOCs).A remarkable difference between genuine DES and ILs with the solutions used in thisproject is the addition of rather large quantities of water. It is shown the presence of waterhas a lot of advantages on the leaching of metals, while the properties typical for DES stillremain.The oxidizing capacities of Cu(II) stem from the existence of a stable Cu(I) component inquat based DES and thus the leaching stems from the activity of the Cu(II)/Cu(I) redoxcouple. The advantage of Fe(III) in combination with DES is the fact that the Fe(III)/Fe(II)redox couple becomes reversible, which is not true in pure water. This opens perspectivesfor regeneration of the etching solution.In this project the leaching of copper was studied as a function of gradual increasing watercontent from 0 - 100w% with the same concentration of copper chloride or iron(III) chlorideat room temperature and 80ºC. The solutions were also tested on real PCBs. At roomtemperature a maximum leaching effect for copper was obtained with 30w% cholinechloride with 0.2 M CuCl2.2H2O. The leaching effect is still stronger at 80°C, b ut of coursethese solutions are more energy consuming. For aluminium, tin, zinc and lead, theleaching was faster at 80ºC. Iron and nickel dissolved easily at room temperature. Thesolutions were not able to dissolve gold, silver, rhodium and platinum.