Design of hydrometallurgical stages for reprocessing artisanal mine tailings from Madre de Dios

Abstract

The objective of this work was to design hydrometallurgical stages to reprocess artisanal gravimetric table concentration tailings from Madre de Dios. These tailings are currently considered waste, even though they still contain gold and rare earths. The scope is limited to the design and technical-economic assessment of leaching stages of gold and rare earths, as well as their subsequent recovery and the neutralization of effluents. Leaching stages were experimentally evaluated in a laboratory scale through a three-step experimental design. Each stage evaluated one independent variable on two levels and its effect on reagent consumption and gold or rare earths extraction. The first step considered gold extraction through thiourea leaching with ferric sulfate as oxidizing agent in an equivalent molar proportion to thiourea. The second step evaluated the inclusion of a hydrochloric acid leaching stage at 80°C before gold leaching. Finally, the third experimental step included a pretreatment at 80°C with potassium hydroxide before the other two stages. According to experimental results, rare earth extraction was too low to consider it further, while thiourea leaching achieved 86.8% of gold extraction after 1 hour, using a thiourea concentration of 8 g/L on a 40% solids slurry. Based on these results, the designed batch process included the following stages: thiourea leaching, sedimentation of solids, cementation of gold using zinc powder, effluent neutralization, and zinc dissolution with sulfuric acid to recover zinc-free, high purity solid gold. Reagent and energy requirements were estimated for 40 kg batches of tailings. Approximately 0.5 g of gold are obtained per batch. An economic assessment indicated that for a five-year projection on which 500 batches are processed yearly, the net present value of the project is $2555.37, and the internal rate of return is 27.2%. A sensitivity analysis revealed that the project can remain profitable if the capital expenditure and the cost of reagents are modified within a ±20% range, while the price of gold and the number of yearly batches can only be reduced by 7.5% and 11.4%, respectively.