Acid Mine Drainage (AMD), caused by the oxidation of iron sulphide minerals exposed during mining, is a source of water pollution worldwide and is one of the most significant environmental issues facing the mining industry. Acid mine drainage is characterised by low pH, high sulphate and dissolved metal concentrations, and high acidity. AMD impacts the chemical, physical, biological and ecological processes of the environment where it occurs. In seriously impacted water courses, all aquatic life disappears, pH decreases and the river bottom becomes covered with oxidised iron precipitates. AMD is so devastating to aquatic communities that community structure collapses and recovery is suppressed due to habitat elimination, niche reduction, substrate modification, sediment toxicity, and bioaccumulation of metals in flora and fauna.
Red mud is the fine-grained residue left after the NaOH-extraction of alumina from bauxite using the Bayer process. Composition varies with bauxite composition and process conditions, but the residues typically contain 11-54 wt% iron, 6-43 wt% aluminium, and 2-20 wt% silica. The iron is in the form of oxidised iron compounds, the alumina in boehmite and desilication product minerals, and the silica as un-reacted quartz and desilication product minerals. Bayer process red muds are highly alkaline with high sodium content and high pH. Post-disposal re-use requires neutralisation; typical neutralisation processes are by acid or gypsum addition. Seawater neutralised bauxite refinery residues as produced by Queensland Alumina Ltd. (QAL), Gladstone, have a high acid neutralisation and metal uptake capacity. QAL produces red mud in large quantities, 8000 dry tonnes a day. Disposal and management of refinery residues constitutes a major cost to the alumina industry. Using this waste in the treatment of AMD benefits in two ways. It controls AMD and its use off-site effectively subsidises waste storage for the refinery by eliminating the need for on-site management of red mud disposal sites. The thesis presented here studies the use of seawater neutralised bauxite refinery residues to treat acid mine drainage.
In order to fully characterise of the material, seawater neutralised red mud is analysed for composition, mineralogy, and morphology. Acid mine drainage waters, collected from several mine sites, are analysed chemically and then used in batch experiments. An application rate model is developed from batch experiments that establish the acid neutralising and metal uptake capacity of seawater neutralised red mud in the treatment of AMD. Treatment kinetics and the effect of temperature on the rate of acid neutralisation and metal uptake are also determined.
Mechanisms of metal capture are investigated through sequential extractions, titrations, aging studies, and heat of adsorption estimations. Equilibrium modelling compares the results of the titration experiments with theoretical solubility curves. Scanning electron microscopy creates a visual image of particle morphology. X-ray analysis describes compositional variations within the material, before and after treatment of AMD. The environmental fate of metals held by seawater neutralised red mud is simulated through a standard leaching test, a multiple exposure experiment, and by leaching under anoxic conditions.
Seawater neutralisation of bauxite refinery residues causes chemical changes and improves physical characteristics of the material. At the refinery, faster settling due to agglomerate consolidation and increased ease of handling after neutralisation are positive impacts for management. In post disposal reuse, seawater neutralisation results in an increased acid neutralisation capacity, improved soil properties, and increased phosphate adsorption capacity.
The primary mechanism for removal of metal cations from solution is established as precipitation of carbonate, hydroxide and hydroxycarbonate compounds on the surfaces provided by the red mud. Capacity and kinetics depend on the supply of alkaline anions. The readily soluble products of seawater neutralisation supply initial alkalinity with longer-term supply from the dissolution of less soluble minerals. The precipitation mechanism dictates the application and potential environmental impacts of the use of seawater neutralised red mud in the treatment of AMD. If the red mud is not fully saturated with respect to metal capacity, the metals are more difficult to leach, a finding that influences the choice of dosing rate. If the capacity is pushed to the limit to minimise material use and transport, the increased leach potential of metals from the spent material may limit disposal options at landfill. When the spent material is left on-site, capacity must not be exceeded. If neutral to alkaline conditions prevail after decommissioning, metal transport would be minimal.
Used within constraints, seawater neutralised red mud is a potentially powerful option for use in the treatment of acid mine drainage. Transport costs associated with the volumes of material required may constrain use to sites within shipping distance of refineries but refineries should be able to subsidise transport costs through reduced waste management costs.