Life cycle assessment of process strategies for value recovery and environmental mitigation of mining waste

Thesis (MEng)--Stellenbosch University, 2020.ENGLISH ABSTRACT: By 2031, South African primary ore gold grades are forecasted to decline to the gold grades expected in mine tailings resources. Furthermore, the reprocessing of mine tailings does not require the costly excavation and size reduction unit operations necessary for primary ore resources. Mine tailings can therefore be viewed as a secondary gold resource. Hazardous pollutant sand acid mine drainage (AMD) emanating from Witwatersrand stockpiled tailings dams affect human and ecosystem health. Potential exists to valorise mine tailings to the circular economy as construction raw materials and mine backfill. Sequestration of toxic compounds from mine tailings is however necessary to avert the promotion of environmental impacts up the consumer value chain. Process flowsheets proposed in literature for gold recovery from mine tailings have neglected the evaluation of life cycle impacts of technologies that need to be evaluated before they can be presumed to be “green” alternatives. Life cycle assessment (LCA) was identified as an environmental impact assessment tool for assessment of ecological impacts of gold recovery process flowsheets.Outotec® HSC chemistry and Thinkstep’s GaBi® was identified as software solutions to conduct LCA with the ReCiPe® 2016 life cycle impact assessment (LCIA) methodology.LCAfor gold recovery from mine tailings successfully identified environmental hotspots in process flowsheets. The LCA model predicted that the thiosulphate flowsheet reduced environmental impacts for 18 out of 19 impact categories compared to the cyanide flowsheet, apart from freshwater consumption impact category. Electricity consumption during cyanide destruction and emissions from the conventional cyanide flowsheet were recognized as reasons for the increased environmental impacts compared to the thiosulphate flowsheet. Suggestions to further reduce environmental impact based on LCA results were made. Cyanide and thiosulphate leaching process flowsheet solutions were identified for recovery of gold and ammonium diuranate (yellow-cake uranium)from a hypothetical scenario of West Rand, Witwatersrand mine tailingsresource.Environmental impact of mine tailings was reduced. Sulphides and uranium in mine tailings were reduced from 0.18% to 0.03%(82% reduction)and from54.9g/tonneto 13.0g/tonne respectively(75% reduction),thereby decreasing environmental impacts related to acid mine drainage and uranium radionuclide emissions.The processes proposed reduce heavy metal emissions and are not compliant with National Environmental Management: Waste act 2008 (Act No. 59 of 2008)(NEMA)for the protection of water resources. The exceptions are manganese and copper emissions for thethiosulphate flowsheet and manganese, copper and lead emissions for the cyanide flowsheet. Subsequent mine tailings remediation strategies are necessary to render mine tailings inert according to NEMA. LCA provides a basis for environmental impact assessment, but social-and economic-assessments are necessary to determine the viability of process flowsheets proposed and ensure sustainable development in the mineral processing industry.AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsommingMaster