Abstract
Rare earth elements (REEs) are valuable raw materials which are in great demand in modern high technology industries. Developing methods to produce/recover REEs from waste is significant to the national security of any developed country. This study was focused on investigating the use of supercritical CO2 (sCO2) to extract REEs from anthracite acid mine drainage (AMD). Four different mine drainage water source locations at Blaschak Coal Corp. in Pennsylvania, USA were selected for sample collection. An extraction process was developed and demonstrated for two of those water sources containing the highest concentration of REEs. A method involving metal ion coagulation, their dissolution from the sludge into a concentrated aqueous HNO3 solution, complexation with organic ligands and sCO2 extraction was developed to recover REEs from AMD. Specifically, sodium aluminate (NaAlO2) was used as the coagulant to concentrate REEs from the AMD into a solid precipitate. Consequently, over 99% of the REEs in AMD is concentrated in the remaining sludge. During the coagulation process, the effects of pH and NaAlO2 concentration on REE precipitation were investigated. Fuming nitric acid (HNO3) was used to digest the pre-concentrated sludge and tributyl phosphate (TBP) was used to form REE/TBP/HNO3, a non-polar complex with selected REEs, specifically, cerium (Ce), lanthanum (La) and neodymium (Nd). HNO3 concentration and organic/aqueous phase ratio were considered as the variables to improve complexation efficiency. Dynamic extraction experiments using sCO2 and REE/TBP/HNO3 solutions were then conducted at optimal conditions of 60 °C and 20 MPa. The overall REE extraction efficiencies are found to increase with the atomic number of the REE. As a result, the average overall REE extraction efficiencies of 41.8%, 40.1% and 58.2% for Ce, La and Nd, respectively, are obtained. The potential improvements in the overall extraction efficiency are also discussed.
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