Recovery of Samarium from Waste SmCo Magnets via Selective Precipitation with Ammonium Bicarbonate: Optimization of Process Efficiency

Abstract

SmCo magnets are a common material utilized in advanced technological applications. These magnets contain elevated concentrations of Sm and Co within their structural compositions. Given that both Sm and Co are classified as critical metals, the recycling of these magnets after their operational lifespan is of significant economic and environmental importance. Hydrometallurgical recycling processes represent an effective method for the recycling of these magnets. In this study, a pH-controlled selective precipitation method was developed using two HNO3 solutions with distinct oxidizing properties for the recovery of Sm and Co from end-of-life (EoL) SmCo magnets. In the initial stage of the process, the magnets were leached in a low-oxidizing 2M HNO3 solution, with a 1:30 (w/v) solid-to-liquid ratio at 20 °C. This step was undertaken to ensure the dissolution of Fe, thereby creating an environment conducive to its removal from the solution. The leaching experiments resulted in dissolution efficiencies of 95%, 96%, and 96% for Sm, Co, and Fe, respectively. In the second stage, a leaching experiment was performed using 3M HNO3 with a 1:10 solid-to-liquid ratio at 60 °C. Under these conditions, Sm and Co achieved dissolution efficiencies of 99%, while Fe remained undissolved in the solid phase due to hydrolysis at the high temperature, thus increasing the solution purity. In the precipitation process, the pH of both leachates was initially adjusted to 4 to precipitate impurities such as Fe ions. As a result of precipitation at pH 4 in the 2M HNO3 leachate, Fe ions were almost completely removed. This was followed by selective Sm precipitation in the pH range of 5–6.5 using NH4HCO3. The highest purity of Sm precipitation was achieved when the pH reached 6.5. An increase in Sm precipitation efficiency was observed with increasing pH, with an efficiency of 12.75% at pH 5, which rose to 82.37% at pH 6.5. Furthermore, although the precipitation efficiency of Co increased from 6.25% to 10% within this pH range, no significant difference in the extent of this increase was observed. In the case of the 3M HNO3 leachate, the Sm precipitation efficiency at pH 5 was 44.28%, while at pH 6.5, nearly all of the Sm ions were precipitated. The co-precipitation efficiency at pH 5 was 1.89%, increasing to 36.43% at pH 6.5. This increase in co-precipitation was attributed to the system’s Eh value, which reflects the enhanced oxidizing properties of the 3M HNO3 leach solution. The results of the study indicate that as the oxidizing strength of the solution increased, the co-precipitation rate also increased with rising pH.