Carbonate-rich limestone quarry waste as a doping agent to implement AMD buffering potential of feldspar mine waste

Buffering Acid Mine Drainage (AMD) is an essential part of the remediation procedures aimed to reduce the environmental impact of high-sulfur waste and tailing dumps. As solubility of most of the heavy metals dramatically decreases at increasing pH, buffering also results in a strong reduction of potentially toxic elements load of waters.

Adding carbonate-based materials, like lime, can neutralize acid mine drainage by raising pH of water and triggering precipitation of metals as hydroxides (Grieco et al., 2021), but the treatment processes are often expensive in terms of capital and operating costs. The use of mining waste as buffering agent, on the other hand, could positively contribute to a circular economy strategy for the secondary recovery of metals from abandoned mine wastes and tailings. The addition of the buffering agent to reworked tailings can result in secondary dumps with highly reduced or even absent acid mine drainage potential.

The aim of the present research is to evaluate the buffering potential and the leaching waters heavy metal load reduction of feldspar mining wastes and the effect of doping it with a minor fraction of carbonate-rich limestone quarrying wastes. The frame of the research is the evaluation of potential metal recovery from the dumps of the many abandoned sulfide mines of Sardinia as the island also hosts major feldspar mines that produce a high amount of carbonate-bearing waste.

Selected samples were collected from sulfide-rich lead-zinc Campo Pisano and gold Furtei abandoned mines. The former is part of a giant and high-grade MVT and SEDEX Zn-Pb-Fe district while the latter exploited an epithermal deposit.

Mineralogy and chemistry of the samples were determined by XRD and XRF and the acidification potential by standard Lapakko modified Acid Base Accounting tests. Leaching tests simulated the contaminant release from tailings following the Synthetic Precipitation Leaching Procedure. Leachates were analyzed for major and trace elements by ICP-MS and were then buffered with material from feldspar wastes, provided by Minerali Industriali. The Potential Toxic Element contents of buffered materials was also determined by ICP-MS.

All samples show positive Net Acid Potential Production, with higher values at Campo Pisano. Leaching tests show pH around 3 and high metal contents at Furtei. Campo Pisano leachates are only slightly acidic and the metal load is much lower due to the initial internal buffering by the carbonate gangue.

Buffering with feldspar waste was efficient on highly acidic Furtei samples, even though the amount of buffered material required is high. Doping feldspar waste with carbonate-rich limestone waste grants the same buffering capacity at a highly reduced amount of buffering agent added.

The results show that limestone waste-doped feldspar waste can be an effective acid buffering agent both for mixing with secondary tailings and for treatment of polluted acidic waters.

References

Grieco, G., Sinojmeri, A., Bussolesi, M., Cocomazzi, G. and Cavallo, A. (2021). Environmental impact variability of copper tailing dumps in Fushe Arrez (Northern Albania): The role of pyrite separation during flotation. Sustainability, 13(17), 9643.