Treatment of Phosphoric Acid Sludge for Rare Earths Recovery I: Effect of Polymeric Flocculant Properties on Filtration and Recovery

Introduction

There is an ever-increasing demand for phosphorous as the world’s population expands. Since high-grade resources are depleting rapidly, researchers are motivated to design and develop new methodologies to extract phosphates from low-grade ores and other secondary resources. Tailings and wastewaters produced from the phosphate industry are the main secondary sources of phosphates (Alsafasfeh et al., 2017).

Phosphate is not only a necessary resource for plants growth and fertilizer industry, but also a significant secondary resource for many critical materials such as rare earth elements, uranium, magnesium and thorium (Zhang, 2014). These elements play an important role in green energy and high-tech industries. In order for the phosphoric acid to be suitable for high-grade phosphate fertilizer, such as di-ammonium phosphate (DAP), production, washing and floatation are generally required for upgrading P₂O₅ content and reducing gangue materials in the phosphate rock product extracted from phosphate ore. After desliming and sizing, the phosphate flotation feed is conditioned with fatty acids and fuel oil. Then, the apatite, which is one of the main minerals of phosphate rock, is floated from silica and carbonates in the rougher stage. The rougher concentrate is scrubbed with acid using a dilute sulfuric acid solution in order to remove the collector adsorbed on apatite particles. Then the rougher concentrate undergoes cleaner flotation with a cationic amine collector. In this step, silica is floated out leaving apatite in the sink fraction, which constitutes the final product. P₂O₅ in the rougher concentrate is 18.5-25% usually, and a cleaner concentrate contains around 30% P₂O₅ (Zarrinpour et al., 2009).

Mine tailing and industrial residues from phosphate processing streams are potential resources for recovering rare earth elements (Binnemans et al., 2009). There have been many studies for enhanced filtration of phosphoric acid sludge and recovery of rare earths. One of the methods, which has been reported, is using polymers as flocculant agents.

In the Wet Process phosphoric acid manufacturing process, the final phosphate rock product from the beneficiation plant is ground, followed by acidulation using sulfuric acid in the so-called attack tanks. This results in a slurry of calcium sulfate crystals, known as phosphogypsum in the industry, and phosphoric acid. The slurry is filtered, usually using a pan filter. The phosphogypsum is washed two to three times and pumped to the disposal site. The filter acid typically contains 28-30% P₂O₅. More often than not, the filter acid is further concentrated to about 54% P₂O₅ by evaporation. During the evaporation process, insoluble solids of aluminum, iron, magnesium and calcium, as well as other salts, precipitate out of solution gradually as P₂O₅ increases. The concentrated phosphoric acid slurry containing the solid precipitates is treated by either centrifugal separation or settling to achieve solid-liquid separation. The resulting thick material is called phosphoric acid sludge. Based on studies by the FIPR Institute and others, the solids in phosphoric acid sludge contains the highest REE concentration among the various phosphate mining and processing streams. As can be seen from Table 1, total REE content in solid fraction of the Florida phosphoric acid sludge is generally over 2000 ppm.