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

De Waal, Alicia (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

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.

AFRIKAANSE OPSOMMING: Verskeie herwinningstegnologieë vir metaalproduksie van elektroniese afval (e-afval) is in die literatuur voorgestel. Tradisioneel word pirometallurgiese prosesse aangewend om nie-ysterhoudende metale sowel as edelmetale van e-afval te herwin. Hidrometallurgiese prosesse is 'n alternatief vir metaalherwinning en bied 'n paar voordele bo pirometallurgiese prosesse, soos laer kapitaalkoste en die feit dat dit meer voorspelbaar en makliker is om te beheer. Die doelwit van hierdie projek was om ondersoek in te stel oor die omgewingsimpak en termodinamiese ondoeltreffendheid van die hidrometallurgiese proses wat voorgestel was om koper en goud te herwin. Eksergie- en lewensiklus assessering (LSA) ontledings tegnieke was gebruik om die termodinamiese ondoeltreffendheid en die omgewingsimpak van die algehele proses te bepaal. Die proses fokus op die produksie van koper en goud. Die proses maak gebruik van meganiese skeidingstegnieke, chemiese verwydering van die soldeer-werke in die vorm van 'n salpetersuurloging stap, koperherwinning deurmiddel van ‘n loging-oplosmiddel ekstraksie en elektroherwinning (L-SE-EW) proses konfigurasie waar swaelsuur as die logingsmiddel in die teenwoordigheid van waterstofperoksied as die oksideermiddel dien. Die goudherwinningsproses volg ‘n tradisionele sianied loging stap gevolg deur geaktiveerde koolstof adsorpsie en desorpsie prosesroete waarvan elektroherwinning as finale stap dien. Die goud seksie maak ook gebruik van ‘n salpetersuur-was stap om die oorblywende basismetale te verwyder voordat die sianied loging stap uitgevoer word. Die ontwerp en prosesmodellering het algeheel 81% koper en 97% goud herwin. Die eksergie analise het getoon dat die koper- en goudherwinningsprosesse die mees ondoeltreffende prosesse is. Dit is bepaal deur die eksergie vernietiging van die algehele proses te analiseer en die relatiewe bydrae van die algehele prosesaanlegte te analiseer. Die koper herwinning afdeling het 39% tot die eksergie vernietiging bygedra waarvan die bydra van die goud herwinnings prosesse 37% is. Eenheidsproses-analise het getoon dat die koper-logingsproses en die goud-elueringsproses die oneffektiefste proseseenhede is in die twee afdelings. Optimisering van hierdie seksies moet dus geprioritiseer word as enige prosesverbetering aangebring word. Die algehele eksergie doeltreffendheid van die proses is 0.85%. Sensitiwiteitsanalise het getoon dat alhoewel die proses se eksergie vernietiging wissel met elektrisiteit en metaalkonsentrasie, het die relatiewe bydraes van die verskillende afdelings stabiel gebly. Die lewensiklus assessering het getoon dat die goud afstropings proses, salpetersuur-was en koper-loging die grootse bydraes tot die algehele omgewingsimpak het. Die afsonderlike bydrae wissel onderskeidelik van ‘n minimum 2,62% tot 74,2%, 0.44% tot 77,9% en 3,17% tot 29,8%. Die hoofoorsaak van impak kan toegeskryf word aan die gebruik van natriumhidroksied en steenkool-gebaseerde elektrisiteit. Daarom sal die vermindering van hierdie kommoditeite binne die aanleg die omgewingslas verminder. Die laasgenoemde is bevestig deur natriumhidroksied met gehidreerde kalk in die koper logingsproses te vervang en die behoefte aan elektrisiteit en natriumhidroksied te verminder. Beide LSA- en eksergie-analise het die rolle van die goud afstropings proses en koper logings proses geïdentifiseer as die hoof bydraers tot die algehele proses-ondoeltreffendheid.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106003
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