Nickel hydrometallurgical waste residue characterization, geochemical reactive transport modeling and disposal risk management

Steel, M. Abigail (2010) Nickel hydrometallurgical waste residue characterization, geochemical reactive transport modeling and disposal risk management. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

There has been limited experience in the use of hydrometallurgy to process nickel sulfide concentrate, thus the hydrometallurgical process residue is generally not well characterized in the open literature. This research will assist in ensuring the long-term stability of the waste and increase the understanding of its degradation and reactivity on disposal. The research pertains to the mobility of metals and sulfur compounds, their stability in minerals and phases associated with hydrometallurgical residues and the development of a risk-based methodology for selection of mine waste disposal designs. The research focuses on sulfur compounds and ore metals, such as nickel, cobalt and copper, which are target metals for the proposed hydrometallurgical processing facility in Long Harbour, Newfoundland. It will be particularly important to understand the effect of high sulfur waste material in Newfoundland's wet, temperate climate and generally slightly acidic surface water conditions. The research objectives are to characterize the mobility of metals from hydrometallurgical residue and assess residue reactivity/stability under different disposal conditions in order to determine the most favourable waste disposal procedures. Specific research objectives include: 1) characterization of the waste residues through mineralogical studies and elemental analysis; 2) assessment of acid and metal generating potential of the waste through static and kinetic tests and geochemical modeling; 3) assessment of decant water conditions in the residue impoundment through a calibrated numerical model; 4) evaluation of residue subsurface disposal conditions on a spatial and temporal basis through numerical modeling calibrated by in-situ field testing; and 5) prediction of the fate of heavy metals in the receiving environments. Finally, a risk-based, multi-criteria decision making approach is developed to assess various mine waste disposal options and applied through a case study. -- As there is very limited experience in the processing of nickel sulfide concentrate through hydrometallurgy the high sulfur, process residue is generally not well characterized. The mineralogical and sequential extraction work provided key residue mineral and microstructure information; suggested how target metals are present in the residue minerals and phases; and provided metal partitioning results which are important in understanding the residues metal leaching potential. The static and kinetic testing conducted further characterized the residues by assessing their acid generating and metal leaching capacity. Geochemical modeling of process residues is not widely reported in the literature due in part to the complexity of the mineralogical assemblage. This work, through calibrated models, was successfully able to model the residue that led to a greater understanding of factors impacting the chemistry of groundwater and surface water and enabled the prediction of longer term subsurface conditions in the residue impoundment. -- The design of a mine waste disposal site is waste and site specific and is complex. Using a risk-based decision-making to assess design options for a mine waste disposal project is novel and effective approach. This approach integrated the results from the mineralogical characterization and contaminant fate and transport modeling and included uncertainty in the human health and ecological risk analysis; then incorporated this risk analysis in a multi-criteria decision making analysis to evaluate the optimal mine waste disposal alternative.

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