Techno-economic analysis of integrated processes for the treatment and valorisation of neutral coal mine effluents

Abstract

The disposal of highly-concentrated neutral coal mine effluents into the environment constitutes a severe threat to the natural ecosystem. This work proposes and compares five novel treatment chains to purify the effluent and recover raw materials. The chains present different combinations of pre-treatment and concentration technologies. In all cases, the solution sent to the concentration step is concentrated up to saturation to recover water and sodium chloride. Concerning the technical performances, the treatment chains are compared in terms of total energy demand and salt recovery. Furthermore, the economic feasibility assessment is performed via a novel global parameter, i.e. the levelized cost of the produced NaCl crystals (Levelized Salt Cost, LSC). The energetic and economic analysis of the chains highlighted that the thermal energy demand of the concentration technology covers the highest share of the total demand and the relevant costs are among the highest expenditures. Also, the revenues given by Mg(OH)2 production were found to play a key role in offsetting the treatment costs. Among the treatment chains analysed, the one composed by two nanofiltration units and two crystallizers in the pre-treatment step and a multi-effect distillation unit in the concentration step showed the highest recovery of NaCl and turned out to be the most economically feasible. The relevant LSC (∼100 $/tonNaCl) was the lowest and it was comparable with the lower bound of the current range of price of high-purity NaCl crystals. In conclusion, the findings of this work contribute to improving the sustainability of the coal mine industrial sector, by proposing economically feasible solutions for the treatment and valorisation of its neutral effluent.