Carbon Dioxide-Based Neutralization of High-Density Sludge Effluents as a Sustainable Climate and Water Quality Alternative to the Use of Strong Mineral Acids

The neutralization of high-density sludge (HDS) effluent is a required process involved in the treatment of acid mine drainage (AMD). In their last treatment stage, effluents with high pH values are acidified to reach legal standards before being released to the environment using hydrochloric or sulfuric acid. In this investigation, CO 2 was tested as an alternative way to decrease the pH of the HDS effluent, together with an economic analysis comparing the results with the use of strong mineral acids, considering a full-scale 300 m 3 /h plant. HDS samples were collected from a PAN American Silver operation in Cajamarca, northern Peru. Four acidification tests were carried out on 20 L containers, with a subsequent evaluation of reaction time and CO 2 consumption to regulate the final pH of the treated solution. The results suggest that by adding CO 2 (0.5 L/min) to the solution, the pH was successfully decreased from 10–10.5 to 6.5–7.5 (which falls within the legal limits) in a matter of minutes. An average of 130 g of CO 2 was sequestrated per m 3 of solution to decrease the pH within legal limits, representing around USD 0.031/m 3 in terms of treatment cost for a full-scale plant. While this is more expensive than using other acids, with a CO 2 credit of USD 100/ton, sequestrated CO 2 neutralization is 12% cheaper and only 6% more expensive than using H 2 SO 4 and HCl, respectively. Moreover, in terms of the costs per ton of avoided CO 2 of USD 133 and USD 262 for replacing hydrochloric and sulfuric acid, respectively, it is markedly lower than the cost of other CO 2 abatement technologies, like, for instance, solar photovoltaic panels (PV) that can cost between USD 368 and USD 684/ton of avoided CO 2 in Peru and require substantial capital investments. Moreover, the use of CO 2 implicates a series of additional safety, operational, and environmental advantages that should be considered. Therefore, the use of CO 2 to decrease HDS effluent’s pH should be further explored in Peru and elsewhere as a sustainable alternative.