Waste streams have increased due to advancements in technology and the increase in the global population, requiring innovative strategies to recover value from them whilst reducing their negative environmental impact and human health hazards. Thus, the increase in waste has led to research focused on circular economies. E-waste is the fastest growing waste stream in the world containing valuable metals that exceed those rich in ore from mines. In Africa, e-waste metal recycling remains largely informal and small scale resulting in inefficient metal recovery, increased negative environmental impact and human health hazards. E-waste metal recycling using pyrometallurgy is limited to secondary smelter feed and there are limited industrial plants dedicated solely for this purpose. While in hydrometallurgy, research in e-waste metal recycling has been largely focused on metal extraction whilst downstream metal recovery processing studies are limited. The strategy often employed in e-waste metal recovery via hydrometallurgy is base metal (BM) extraction before precious metal (PM) recovery due to the high concentration of these metals in e-waste. This results in the production of base metal-rich-leachate solutions. The heterogeneity of these leachate solutions and the high cost of downstream processing requires a multi-disciplinary approach that considers metal recovery selectivity and associated costs. Natural sorbents, chitin and chitosan found in large quantities in industrial food waste and precipitation with sulphides have received much attention due to their high metal recovery efficiencies, metal selectivity, scalable operation and low costs. Chitin and chitosan are mainly sourced from crustacean shell waste and there are limited techno-economic studies on the extraction and production methodology of these polymers. Chitin and chitosan from Black Soldier Fly (BSF) larvae shells, a waste product from BSF farming, is thought to have high adsorptive properties due to their low crystalline index. However, studies on metal adsorption onto chitin and chitosan sourced from BSF larvae and their potential combined application with sulphide precipitation to recover metals from e-waste leachate solutions remains limited. Therefore, the dissertation aimed to develop a cost-effective method of extracting chitin and chitosan from BSF larvae shell waste and investigated the techno-economic feasibility of the application of these polymers in combination with sulphide precipitation for the recovery of base metals from e-waste leachate solutions. The potential application of chitin/chitosan from BSF larvae in e-waste metal recovery may result in a circular economy where solid waste is utilized to produce BSF larvae. While the BSF larvae shell waste generated from BSF larvae production can be used to remediate electronic waste, recovering value from these waste streams while reducing their environmental impact. The cost-effective method for the extraction of chitin and production of chitosan from BSF larvae was investigated by a study into the effects of demineralisation, deproteination, decolourisation, de-acetylation processes on the chitin and chitosan character, metal adsorption performance and techno-economics. Chitin and chitosan were extracted and produced from BSF larvae (Hermetia illucens) using a combination of the processes stated prior. Adsorption studies with the produced chitin and chitosan were conducted on base metals ferrous, ferric, copper and aluminium ions in single and bimetal solutions. The adsorbed metals were then eluted using 0.1 M H2SO4. Precipitation studies were also conducted with various concentrations of copper in a ferrous, copper and aluminium solution. The techno-economic feasibility of the application of the chitin and chitosan and sulphide precipitation with NaHS in PCB leachate solutions was investigated by the development of a model based on the ascertained individual metal recovery performance in the adsorption and precipitation studies. Extracted chitin from BSF larvae was found to be in the alpha form. 4-hour Deproteination of the BSF larvae after liberation with 4 wt % NaOH and de-acetylation of the deproteinated chitin with 40 wt% NaOH was found to produce chitin and chitosan with the highest metal sorption capacities and lowest cost of production. The maximum adsorption capacity for ferrous, ferric, copper and aluminium ions onto chitin from BSF larvae was 2.29 ± 0.0001 mmol/g, 2.07 ± 0.0001 mmol/g, 1.69 ± 0.0001 mmol/g and 1.82± 0.0001 mmol/g respectively. While for chitosan, the maximum adsorption capacity for ferric, copper and aluminium ions was 0.951 ± 0.0012 mmol/g, 1.16 ± 0.0016 mmol/g and 0.961± 0.0013 mmol/g respectively. The order of metal adsorption selectivity for ferrous, ferric, copper and aluminium on chitin from BSF larvae was determined to be Fe2+>Fe3+>Al3+>Cu 2+. While for chitosan it was determined to be Cu2+>Fe3+>Al3+ and at a low pH (below pH of 2) it was observed to be Cu2+>Al3+>Fe 3+. Ferrous ion oxidation to ferric ions was observed during the adsorption of ferrous ions onto the chitin and chitosan. Adsorption of the metals onto chitin and chitosan were best modelled by the Freundlich isotherm and Pseudo 2nd order kinetic model. The adsorption on both polymers was found to be spontaneous, favourable, chemisorption and predominantly surface complexation. Sulphide precipitation with NaHS was observed to be selective towards copper precipitation however co-precipitation with aluminium occurred. The application of chitin and chitosan on the multi-metal synthetic PCB leachate solution resulted in the production of two refined streams respectively. The application of NaHS precipitation seems to be more feasible on the refined streams produced by the application of chitin. The combined application of NaHS and chitin from BSF larvae on the multi-metal synthetic PCB leachate solution showed economic feasibility. The recovery costs were $ 116 per kg metal recovered and an overall gross profit of $ 933/ kg metal recovered. However further economic studies which include consideration of capital costs need to be conducted to conclusively determine the economic feasibility of this downstream metal recovery process. This study shows the potential of chitin and chitosan extracted from BSF larvae to upgrade PCB metal leachate solutions for further downstream processing
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/35814 |
| Date | 22 February 2022 |
| Creators | Mabuka, Thabo |
| Contributors | Govender-Opitz, Elaine, Harrison Susan T L |
| Publisher | Faculty of Engineering and the Built Environment, Department of Chemical Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
Page generated in 0.0034 seconds