Science, Technology and Engineering

Environmental Geoscience

Postgraduate Projects

The chemistry of Anoxic Limestone Drains (ALD's) for the remediation of acid mine drainage

Silvana Santomartino

Supervisor: Dr John Webb

Acid mine drainage (AMD) occurs when sulphide minerals (eg. pyrite and pyrrhotite) are exposed to the atmosphere during mining operations. The oxidation and hydrolysis of the sulphides results in the production of acid which is then leached into local waterways. Elevated concentrations of sulphate (SO42-), iron (Fe2+, Fe3+), aluminium (Al3+), manganese (Mn2+), lead (Pb), zinc (Zn), cadmium (Cd) and other metals are characteristic of ARD due to sulphide oxidation and the dissolution of oxides, carbonate and aluminosilicates by acidic water. Under the low pH conditions (pH less than 5), metals present in ARD remain in solution. If left untreated and transported off site, acid drainage will contact neutral waters downstream where the increase in pH will result in the precipitation of metals and the generation of further acidity.

The treatment of acid mine drainage involves the neutralisation of acidity with chemicals, particularly limestone. Anoxic Limestone Drains (ALD) were first described by Turner and McCoy (1990). They recognised the potential of buried beds of limestone to neutralise AMD passively. The system functions by promoting the contact of acid mine drainage with limestone gravel under anoxic conditions. The anoxic conditions limit the oxidation of ferrous iron, thereby minimising the armouring of limestone with ferric hydroxide. ALDs function is to raise the pH of the water to circumneutral levels (pH 6-7) and to introduce bicarbonate alkalinity. Upon exiting the ALD, the circumneutral pH level promotes metal precipitation and the bicarbonate alkalinity neutralises the acidity produced by metal hydrolysis. Studies of the effluent chemistry of ALDs have documented that many systems successfully generate alkalinity; however, large variation exists in the amount of alkalinity generated and in the effects that ALDs have on metal concentrations. The causes of variability in alkalinity generation and metal retention are unresolved. Also unclear is the contribution that CO2 makes to acid neutralisation processes, particularly upon its exsolution from effluent waters. Several authors hypothesise based on theoretically accepted chemical equations, that CO2 exsolution plays a significant role in ALD technology; however, there is currently no experimental evidence in support of this statement. Therefore, the initial aims of this study are to:

• Attempt to obtain reproducible results on effluent water quality using a small-scale laboratory set-up of an ALD
• Account for any differences in ionic concentrations between influent and effluent waters using mathematical modelling based on charge balanced chemical equations
• Assess the effect of CO2 exsolution on the pH, acidity and alkalinity levels in the effluent waters

Since the discovery of ALDs, they have been widely implemented but their effectiveness has often been less than anticipated. Iron hydroxide has been found to precipitate around limestone particles, reducing limestone dissolution and therefore the neutralising ability of the ALD. The decrease in the longevity of the ALD means that on-going maintenance to replenish the limestone is required. In order to improve the design of ALDs, greater knowledge is required on the conditions under which iron precipitates. Therefore, an additional aim of the PhD is to investigate the conditions under which iron precipitates by studying the location, distribution and chemical nature of Fe precipitates within an ALD and the mechanisms governing precipitation.

The formation of Fe coatings around limestone grains ultimately results in the failure of ALDs. To assist in predicting the lifetime of an ALD, this study is also investigating the time required for a certain mass of limestone to become ineffective at neutralising acid mine drainage. The method involves pumping AMD (pH 4, Fe = 100ppm) through a small column containing limestone. The limestone particles will be progressively coated in Fe precipitates and will be deemed ineffective at treating AMD when the effluent pH equals the influent pH. When this occurs, the limestone grains will be removed from the column and analysed using electron microprobing, XRD and Infra-red analysis. The composition, thickness and morphology of the Fe coating will be analysed as will be the strength of bonding of the Fe precipitate to the limestone surface.