Lixiviação convencional e lixiviação assistida por ultrassom de elementos terras raras a partir de fosfogesso

Abstract

Rare earth elements (REE) are a group of elements with very similar physical and chemical properties. They include the 15 elements of the lanthanide series (La–Lu), Y, and Sc. The need for these elements in different sectors of modern technology and the dependence on China has encouraged the search for secondary resources to obtain them. In this context, phosphogypsum, a waste generated in large amounts in phosphoric acid production, can be considered a promising alternative for obtaining REE. Therefore, this work aimed to evaluate the possibility of using phosphogypsum waste to obtain REE through conventional acid leaching and the process intensification through ultrasound-assisted leaching. Initially, the phosphogypsum waste was characterized regarding the chemical and nanomineralogical composition. The presence of several toxic elements in ultrafine particles and nanominerals was revealed, demonstrating the dangerous potential of this waste. In addition, REE at a concentration of about 5000 mg kg–1 was found, suggesting the high potential of phosphogypsum as a secondary source of these elements. From this, the recovery of REE was initially studied by conventional acid leaching. A leaching efficiency of 90% was achieved using a sulfuric acid concentration of 2.9 mol L−1 , solid/liquid ratio of 1.7/20 g mL−1 , and temperature of 55 °C. Under these conditions, the kinetic study showed that the equilibrium of the conventional leaching was reached in about 20 minutes. In sequence, the ultrasoundassisted leaching was investigated using an ultrasonic probe system. The optimization showed that a leaching efficiency of 84% was achieved using a sulfuric acid concentration of 0.6 mol L −1 , amplitude of 77%, and pulse of 93.6%, keeping set the solid/liquid ratio of 1.7/20 g mL−1 and temperature of about 40 °C. Under these same conditions, the leaching efficiency at silent condition was only 68%. The kinetic study demonstrated that the equilibrium of the ultrasound-assisted leaching was reached in about 20 minutes, while for the leaching at silent condition, the equilibrium was not verified within 60 minutes. These results demonstrated that the ultrasound could contribute to the REE leaching from phosphogypsum, leading to an increase of about 20% in the leaching efficiency and a considerable reduction in the time required to reach the equilibrium. According to the characterization results, the ultrasound effects on the REE leaching were mainly due to a large reduction in the particle size of the phosphogypsum. In summary, ultrasound was an effective method for the REE leaching from phosphogypsum, allowing obtaining a leaching efficiency comparable to that obtained in the conventional leaching, but using a sulfuric acid concentration about 5 times lower, thus representing a great advantage from the environmental point of view.