Extraction and recovery of metals from Waste Electrical and Electronic Equipment (WEEE) using both traditional and designer solvents

Abstract

Waste Electrical and Electronic Equipment (WEEE) are electrical and electronic gadgets known to have reached their end-of-life. They are made of printed circuit boards (PCBs) containing valuable metals like copper, gold, iron, lead nickel, silver, tin and zinc which can leach into the environment when landfilled. Current extraction methods for the recovery and recycling of metals from WEEE uses various solvents which are not selective, releases toxic gases and recover metals in a mixed form which requires further purification steps to recover individual metals. The main aim of this project is to design and optimise a recycling process which can be regarded as the best available technique (BAT) for the extraction and recovery of metals from WEEE. Two metal extraction methods namely hotplate and microwave were used in this research. The extraction solvents were divided into three categories namely i) inorganic, ii) organic and iii) designer solvents. For clarity, all experiments conducted using hotplate extraction method were labelled as 1-3 and microwave extraction method as 4-6. The extraction process optimisation was conducted by investigating the effects of various parameters on the extraction of five metals (Cu, Zn, Fe Ni and Pb) from WEEE under identical experimental conditions. The parameters were effect of dose of WEEE used, particle-size, H2O2, as an oxidising agent, temperature, extraction time and concentration of solvents. In total, fifteen solvents were used for the extraction of metals from WEEE, four inorganic solvents (aqua-regia, nitric acid, hydrochloric acid, sulfuric acid), three organic solvents (citric acid, malic acid, tartaric acid), seven ionic liquids as designer solvents and water. Three of these designer solvents were purchased (1-butyl pyridinium bromide, 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium bromide) while the remaining four designer solvents were synthesized in our laboratory (1-(1-cyanoethyl)-3-methylimidazolium bromide, 1-(3-cyanopropyl)-3-methylimidazolium bromide), 1-[2-(2-hydroxyethoxy)]-3-methylimidazolium chloride and 1-(2-cyanoethyl)-3-methylimidazolium bromide). Flame atomic absorption spectrometer (FAAS) was used for the analysis of metal concentrations. The limit of detection (LD) and limit of quantitation (LQ) were calculated for each metal using the calibration design method. The composition of metals in the WEEE sample was determined using aqua-regia at 70 0C for 6 hours and the composition of the metals were Cu (43.9 %), Zn (22.4 %), Fe (7.3 %), Ni (1.3 %), Pb (1.7 %) and other materials (23.4 %). The results revealed that inorganic solvents were more aggressive than the organic and designer solvents for the extraction of Cu, Zn, Fe, Ni and Pb from WEEE. The optimum conditions for the extraction of metals were 700C leaching temperature, 120 minutes leaching time, 0.5 g dose, 2 mol/L concentration for inorganic solvents, 25 g/L for organic solvents, 100 g/L for designer solvents and 4.0 mm particle-size. For the data quality assessment (DQA), principal component analysis (PCA) was used as this is the most widely used multivariate data analysis technique. The highest PCA contributors for the effect of dose, particle-sizes, H2O2, temperature and time were aqua-regia, nitric acid and hydrochloric acid. For concentration effect, it was aqua-regia and nitric acid. Apart from aqua-regia, the best leaching solvents for each of the metals are as follows: Cu (nitric acid), Zn (all solvents), Fe (hydrochloric acid), Ni (nitric acid) and Pb (nitric acid). The best parameter used was the effect of H2O2 which increased the extraction of the metals for most solvents. The extraction efficiency based on the solvents was in the order inorganic > organic > designer solvents. Aqua-regia was the best performing solvent followed by nitric acid and sulfuric acid. The organic acids and designer solvents were generally considered to have similar performance. The metals were recovered using electrochemical recovery method. Approximately 82 % of Cu and 30 % of Zn were recovered in 17 hours.