Evaluating the efficiency of a metal recycling process by means of life cycle assessment and exergy analyses

[摘要] ENGLISH ABSTRACT: Various recycling technologies for metal production from electronic waste (e- waste) have been proposed in literature. Traditionally, pyrometallurgical processes are employed to recover non-ferrous metals as well as precious metals from e-waste. Hydrometallurgical processes offer an alternative to metal recovery and present some advantages over pyrometallurgical processes such as lower capital cost, and being more predictable and easier to control. The aim of this project was to investigate the overall environmental impact and thermodynamic inefficiencies of a hydrometallurgical process proposed for copper and gold recovery. Exergy and life cycle assessment (LCA) analyses were used to determine the thermodynamic inefficiencies and the environmental impact of the overall process.The process consists of a mechanical pre-treatment step for size reduction, chemical removal of solder in the form a nitric acid leaching system, and copper recovery via an (Leaching- Solvent Extraction and Electrowinning (L-SX-EW) process configuration with sulphuric acid as the lixiviant in the presence of hydrogen peroxide as the oxidising agent. The gold recovery process followed the traditional cyanidation and carbon-in-pulp (CIP) process route, with the final recovery with an EW-unit. A nitric acid washing circuit to remove the remaining base metals prior to the gold recovery processes were also considered. The overall copper and gold recovery is 81% and 97%, respectively.The exergy analysis showed that the copper and gold recovery processes are the most inefficient processes in the overall process chain. This was established by determining the exergy destruction of each process unit as well as its relative contribution to the overall exergy destruction. Copper and gold recovery processes contributed 39% and 37% respectively to the overall exergy destruction. Unit process analysis showed that the copper leaching process and the gold elution process were the most inefficient process units within these two sections. Therefore, optimisation of these unit processes should be prioritised. The overall exergetic efficiency of the process was determined to be 0.85%. The sensitivity analysis showed that even though the process's exergy destruction values do vary with a change in electricity requirement and metal concentration, the relative contributions of the various sections remained the same.The life cycle assessment (LCA) revealed that the gold elution, nitric acid washing stage and copper leach are the major contributors to the overall environmental impact. Their contribution respectively ranges from a minimum of 2.62% to 74.2%, 0.44% to 77.9% and 3.17% to 29.8% across the different impact categories. The main causes of impact are due to the use of sodium hydroxide and coal-based electricity. Therefore, the reduction of these process inputs within the plant will reduce the overall environmental burden. The latter was confirmed by replacing sodium hydroxide with hydrated lime in the copper leach and reducing the electricity and sodium hydroxide requirement. Both LCA and exergy analysis highlighted the roles of gold elution and copper leach as the foremost contributors to the overall process inefficiencies.

[发布机构] Stellenbosch University