An assessment of the efficacy of sodium carbonate for semi-passive treatment of circumneutral zinc-bearing mine waters

This study reports on the novel application of sodium carbonate dosing specifically for the semi-passive precipitation of Zn from circum-neutral mine water and demonstrates the use of Na2CO3 as a readily available, safe to handle reagent which can precipitate Zn at pH within environmental quality standards. >95 % Zn removal was achieved for each mine drainage investigated, requiring differing dosages of Na2CO3 with residual Zn concentrations <1 mgL-1. Geochemical modelling and precipitate characterisation using XRD, SEM-EDS, TGA and full chemical digest suggests that the precipitate predominantly comprised of hydrozincite. Laboratory trials demonstrated that (i) adding the precipitant dropwise could reduce the sludge volume by circa 50 %, and (ii) That following a laboratory protocol for simulating type II High Density Sludge (HDS) process resulted in substantial decreases in sludge volumes compared to single pass sludge, with 5.3 mL per litre mine water treated compared to 55.3 mL per litre for conventional single pass treatment after 25 cycles. Such reductions in sludge volume are a very advantageous outcome for practical applications. Particle size distribution and zeta potential determinations on the resultant HDS sludge suggest that increased heterogenous precipitation and decrease in zeta potential are important in the formation of HDS when using sodium carbonate to treat zinc-bearing waters. This work demonstrates that sodium carbonate is an efficacious reagent for semi-passive treatment of zincbearing mine waters and as such could find widescale application to this pressing global problem.

Automated summary

This study introduces the novel application of sodium carbonate dosing for the semi-passive precipitation of zinc from circum-neutral mine water. The research demonstrates that Na2CO3 can effectively precipitate zinc within environmental quality standards, achieving over 95% zinc removal for each mine drainage investigated. The use of sodium carbonate as a reagent for treating zinc-bearing mine waters could have widespread application and be advantageous due to reductions in sludge volume.