AbstractSeafloor massive sulphide (SMS) mining is becoming an attractive solution to meet increasing demands for metal commodities, necessitating further research on the effects of mining on hydrothermal environments and ecosystems. This thesis is focused on material generated through SMS extraction, processing and return waste to the seafloor, specifically accelerated dissolution through galvanic interactions between sulphide mineral pairs. Galvanic cell formation is investigated by comparing sulphide mixtures, mechanical and naturally intergrown contacts.
Monomineralic and polymineralic sulphide oxidative dissolution semi batch experiments were run with artificial seawater, to assess the effect of mineral pairs on metal leachate using ICP-OES. Experiments were conducted with stock sulphides and SMS, 18-25°C, pH ~8.1, air equilibrated; representative of processing at ocean surface and/or return waste. 50°C experiments were representative of return waste deposited near active vents.
Surface area and temperature were dominant controls on dissolution. Increases in metal leachate in polymineralic experiments, attributed to galvanic interactions, were observed. Most significant stocksulphide pairings were: pyrite-galena, with a ~10% increase in Pb and pyrite-chalcopyrite-sulphidemix, with a ~33% increase in Cu; pyrite being cathodically protected in both. Logatchev
polymineralic (chalcopyrite-secondary-Cu-sulphide) saw sustained Cu and the absence of Fe leaching, with ~75% and ~63% reductions in Zn and Ni when compared to monomineralic Logatchev (chalcopyrite). 50°C experiments saw higher metal leachate than 18-25°C experiments, ~6-84x and ~2-4x for Fe and Pb respectively. Cu, Zn and Pb were identified as major toxicants released from sulphide pairs in this study. Cu and Zn were higher in natural SMS experiments, resulting in much higher concentrations than guidelines recommend for seawater systems (AZMECC/ARMCANZ,
2000), ~7.4x104x higher for Cu in the extreme case.
Accelerated metal leaching through galvanic cells could pose a threat to vent species through metal toxicity, worsening where mine waste settles near active vents. Natural vent samples were found to be more representative of leachate concentrations in these environments, favouring the use of natural samples when assessing the environmental impact of SMS mining.
|Date of Award||28 Sep 2021|
|Supervisor||Richard A Brooker (Supervisor) & Emily K Fallon (Supervisor)|