Science, Technology and Engineering

Environmental Geoscience

Postgraduate Projects

Chemical stability of Acid Rock Drainage (ARD) treatment sludges

Danny McDonald

Supervisor: Dr John Webb

Neutralisation of Acid Rock Drainage (ARD) by adding an alkaline reagent produces sludges generally dominated by amorphous ferrihydrite, often with high levels of adsorbed heavy metals. The effect of neutralisation method and reagent on the long-term chemical stability of these treatment sludges was investigated experimentally in this thesis.

Synthetic ARD was treated with hydrated lime (batch and high density sludge process), limestone and 2 proprietary reagents (KB-1 and Bauxsol). The amorphous metal hydroxide sludge produced was leached using deionised water, US EPA methods (TCLP, SPLP), and the new Strong Acid Leach Test (SALT), simulating acid disposal conditions. Sludges precipitated by all reagents had very similar leachability except for KB-1 and Bauxsol which released more aluminium. SALT showed that the leachability of ARD treatment sludges is primarily determined by the final pH of the leachate, rather than by the strength of chemical bonds. A higher neutralisation potential (e.g. a greater content of unreacted neutralising agent) makes sludges inherently more chemically stable; a larger volume of acid leachate can be neutralized before the pH drops to levels where metals in the sludge are mobilised.

The sludges were also subjected to kinetic testing to determine the rate of Cu and Zn release at pH 4, simulating disposal in a moderately acidic pit lake. All sludges precipitated from an oxidised ARD (i.e. Fe³⁺) by standard ARD neutralisation procedures and reagents showed similar chemical stability, with an initial rapid rate of release of Cu and Zn (~50% of leaching in the first 5 minutes) that reduced over time, apparently exponentially. Sludges initially precipitated as ferrous hydroxide and then oxidised may contain a finely crystalline goethite component; if the ARD was Al-free the crystalline content is greater and coarser-grained and may consist of magnetite as well as goethite. Leaching of zinc from these crystalline sludges can be as much as an order of magnitude less than from sludges precipitated as ferric hydroxide, due to the incorporation of Zn within the mineral structure. Copper is more readily released from the sludges with a higher goethite/magnetite content, because it is adsorbed on the crystallite surfaces. However, it appears that Al-rich goethite can incorporate Cu, reducing its leachability.

Thus modifications to the ARD treatment procedure, in particular control of oxidation state and Al levels, can substantially increase the crystallinity and improve the chemical stability of the sludge precipitated, and have a much greater influence on sludge leachability than the neutralisation agent used. These modifications are probably applicable to any water treatment sludges formed by pH adjustment (e.g pickle liquor, landfill leachate).