Improvement of Synergistic Extraction of Neodymium Ions via Robust Model Predictive Control

Abstract

Abstract A neodymium, a high value rare earth element (REE), is widely used as a rare earth magnet. Neodymium magnets (also known as NdFeB) are permanent magnets made of neodymium, iron and boron alloy. These magnets used in several products require low magnet mass or strong magnetic fields such as microphones, in-ear headphones, and computer hard disks. As neodymium is a precious metal, high purity separation of neodymium from low concentrate is required. A hollow fiber supported liquid membrane (HFSLM) is a viable technique for metal extraction at very low concentration because it has a larger mass transfer per unit surface area, high selectivity property, and low energy consumption. Moreover, the HFSLM consumes lower amount of extractant solution than conventional solvent extraction methods. This research is aimed at studying the extraction of neodymium ions Nd(III) through the HFSLM by mean of synergistic extraction systems to increase in extraction efficiency. The separation of Nd(III) in nitric solution is carried out by 0.5 M of di-2-ethylhexyl phosphoric acid (D2EHPA) and 0.5 M of trioctyl phoshine oxide (TOPO) in the liquid membrane, and 1 M of sulfuric acid in the stripping solution. To achieve good separation of the Nd(III) throughout the continuous process, a Model Predictive Control (MPC) technique is applied to control the concentration of extracted neodymium at a desired set point by manipulating the flow rate of feed solution and its control performance is compared with that of a Proportional Integral Derivative (PID) controller. Simulation study has shown that the optimum extraction of 94.5 % can be achieved and the MPC and PID controllers can control the concentration of neodymium ions at the desired set point. However, the MPC provides better control responses than the PID controller does in both nominal and parameter mismatch cases.