Investigation of various methods for recovery of rare, precious and critical metals from industrial wastewater and sludges

Industrial wastewaters and sludges contain toxic metals that harm aquatic ecosystems and drinking water sources. Recovering metals not only prevents pollution but also contributes to the conservation of natural resources. Additionally, it is more economical compared to new metal production and reduces dependence on raw materials. In this study, the recovery of rare, valuable, and critical metals from industrial wastewater and sludge was investigated. Alongside literature reviews, practical studies were conducted on two automotive and two mining industry wastewaters, as well as three different treatment sludges. The wastewaters were concentrated and dewatered using a vacuum evaporator, and the resulting sludge and raw samples were analyzed. Six different hydrometallurgical methods were applied to three treatment sludges and four evaporation sludges. The studies were divided into preliminary and main experiments. In the preliminary tests, sulfuric acid, oxalic acid, and hydrochloric acid leaching were followed by pH adjustments and evaporation. In the main experiments, NaOH was used instead of oxalic acid, pH ranges were modified, and a second-stage precipitation step was added. The methods differed in leach solution type, solution normality, pH levels for precipitation, and solid/liquid ratios. In Sample-7, sodium sulfide was additionally used for silver recovery. In the analyses, 35 metals were evaluated in the preliminary tests and 38 in the main experiments. After the applied methods, the final solid precipitation product was assessed for recovery potential. HCl leaching was found to be effective in increasing the purity of metals such as Ag, Cu, and Mn, while reducing unwanted impurities. With 1N H₂SO₄ leaching, more controlled precipitation was achieved, while higher normalities resulted in loss of selectivity. NaOH leaching offered process stability but did not yield high recovery in every sample. Oxalic acid was found insufficient for complex samples. Zn generally showed an increasing trend and was identified as the main impurity. A solid/liquid ratio of 1/7.5 yielded higher metal concentrations; however, its disadvantage was the potential dehydration during the leaching stage due to the high solid content. The oil content in raw samples was also found to cause uncertainty in the experiments. The results obtained provide guiding data for scaling successful laboratory methods to pilot plant applications.