The Effect of Neutralisation Method and Reagent on the Rate of Cu and Zn Release from Acid Rock Drainage Treatment Sludges

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 using a relatively oxidised synthetic ARD (containing Fe[~70% Fe], Al, Cu and Zn) with a variety of reagents and processes, and two reduced synthetic ARD compositions (Fe, Al, Cu and Zn; Fe, Cu and Zn) treated with hydrated lime. The sludges were all 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 the oxidised ARD by standard ARD neutralisation procedures and reagents (lime, limestone, Bauxsol, KB-1 and HDS lime) 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. The Cu and Zn are adsorbed onto the surface of the amorphous ferrihydrite present. 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. Additional