Abstract
The effective and selective leaching of rare earth elements (REEs) from various sources is frequently possible in practice by adopting a carefully coordinated strategy incorporating a selective precipitation of these elements from undesired ones in solution. In this study, the behavior of chemical precipitation of REEs with commonly used precipitants such as sulfate, carbonate, fluoride, phosphate, and oxalate was examined using thermodynamic principles and calculations. It was found that the pH of the system has a profound effect on determining particular chemical species of precipitants, which are subsequently responsible for the precipitation of REEs. The role of various anions such as Cl−, NO3−, and SO42− derived from the acid used in the leaching process on the precipitation behavior of REEs was examined. These anions form complexes with REEs and display a very positive effect on the precipitation behavior. The nitrate environment exhibits most conducive to precipitation followed by sulfate and then chloride.
Highlights
Rare earth elements (REEs) are becoming very important in modern society
The precipitation behavior of REEs with carbonate, fluoride, phosphate, sulfate, and oxalate was considered in this study, since these precipitants are most widely used in precipitation of REEs in practice
It is interesting to note that the precipitation of nitrate and sulfate complexes is significantly better than free REE ions
Summary
Rare earth elements (REEs) are becoming very important in modern society. REEs are essential ingredients in the transition to green technology and there have been numerous recent articles describing their extraction processes from various sources [1,2]. Most rare earth bearing minerals are refractory in nature and a pretreatment or strong acid treatment of ores bearing these minerals is usually necessary [3,4,5,6] before leaching is affected Such pretreated ores are subjected to the leaching process of REEs. The leach liquor is frequently subjected to the removal of impurities such as iron, calcium, uranium, and thorium by going through stage-wise precipitation using various compounds such as hydroxide, carbonate, sulfate, and oxalate [7,8,9,10,11,12,13]. The concentration of the free REEs at equilibrium can be estimated if the concentration of all species involved in Equation (1) is known Such calculated values are far from real values since the chemicals produced from the leaching process undergo hydrolysis and complexation [14,15]. The precipitation behavior of REEs with carbonate, fluoride, phosphate, sulfate, and oxalate was considered in this study, since these precipitants are most widely used in precipitation of REEs in practice
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