Phosphate fertilizer is produced using phosphoric acid as an important chemical component. This essential ingredient is usually extracted from phosphate rock by wet process phosphoric acid (WPPA). However, WPPA contains impurities, including heavy metals such as cadmium (Cd), zinc (Zn), and copper (Cu) (HMS), as well as unwanted elements such as iron, and other valuable elements, especially rare earth elements (rare-earth elements) such as yttrium (Y), holmium (Ho), and ytterbium (Yb). The current study aims to develop a new process for simultaneous and selective recovery of HMS and REES from WPPA via solvent extraction. Single-stage extraction, tandem extraction, and collaborative extraction were explored to gain insight into the mechanisms and determine the most efficient processes. D2EHPA was used to extract 90%, 80% and 99% of Y, Ho and Yb, respectively. Cyanex301 extracted 89% of Zn, 99% of Cu and Cd. On the other hand, 60% of Yb74% of Y and 78% of Ho were extracted using H2SO4, while 98% of cd and 99% of Zn were extracted using HCI. The organic phase can be successfully reused and has effective extraction and desorption. Oxalic acid (C2H2O4) is used to precipitate rare earth elements, but sodium carbonate is used to precipitate heavy metal elements. Finally, solvent extraction of WPPA solution was carried out to ensure extraction performance under industrial conditions

Six different organic extractants were used to evaluate extraction efficiency. The concentration of organic solutions varies from 0.02 to 2 M, as reported in the literature. Figure 1a illustrates the functional relationship between REE extraction efficiency and different extractants. The results showed that the efficiency of all extractants was lower than that of D2EHPA. Even TBP, which is used to extract rare earth elements through solvation effects, only extracts 19%, 9%, and 24% of Y, Ho, and Yb, and its effectiveness is still low due to the greater complexing capacity of phosphoric acid. Figure 1b shows the efficiency of extracting HMs in the study using the same series of extractants. The study found that Cyanex301 showed excellent extraction performance in the removal of Zn, Cu and Cd, reaching 89% Zn yield and 100% Cu and Cd yield, making it the most effective HMs extraction agent.

From a functional group perspective, Cyanex572 is similar to D2EHPA, and the observed difference in extraction efficiency is due to differences in pKa values (pKa of 1.35 for D2EHPA and 6.03 for Cyanex572). For other extractants, their complexation capacity is significantly weaker than phosphoric acid. The results show that D2EHPA has the highest extraction efficiency for the three rare earth elements, and the co-extraction rates are 90%, 80% and 99%, respectively. The order of base extraction efficiency of the solvent is as follows :D2EHPA > TBP > Fentamine AT0810 > Cyanex572 > Cyanex923 > Cyanex301. This can be attributed to the chemical structure of D2EHPA, which enables it to act as a cation exchanger and form coordination compounds with dilute ions

The influence of extractant concentration was studied in the concentration range of 0.05-2 M. At low concentrations (about 0.05M), D2EHPA has limited ability to extract rare earth metals. However, as the concentration increases: its effect in metal extraction becomes better. At 0.3M, only Yb can be extracted, but at 0.5M, the extraction efficiency of Yb is the highest, reaching 98%. Cyanex 301 demonstrated significant extraction capabilities, achieving a zinc extraction rate of 87%, as well as full extraction of Cu and Cd.

The first stage of tandem extraction (path 1) is shown, including treatment using D2EHPA in the initial channel. It demonstrates REEs' similar extraction efficiency while capturing small amounts (~5%) of HM, especially Zn and Cu. In contrast, in the second channel, Cyanex301 shows a complete preference for HM, excluding the REES take. The results of the second stage extraction (Path 2) are shown. In the initial channel, Cyanex301 showed a strong preference for copper and cadmium, while the extraction efficiency of zinc was slightly reduced to about 60%, due to the small amount of rare earth elements extracted in this channel. In contrast, in the second channel, the extraction efficiency of D2EHPA decreased slightly due to competition with zinc extraction. The results show that the extraction of zinc has a 12% preference.

In particular, this was demonstrated by a significant increase in the amount of Zn extracted to 95%. Since each extractant has a specific affinity for a particular ingredient, their combination collaboratively improves the extraction process, leading to higher yields and overall performance enhancements in this case, Cyanex301 acts as a more effective modifier rather than a base extractor for Zn2+. At the same time, D2EHPA plays a key role in the extraction of Zn2+ as the main extraction agent.

In order to investigate the use of H2SO4 as an eluent for the extraction of rare earth elements from the supported D2EHPA solution, several experiments were conducted, varying the acid solution concentration in the range of 2 to 12 M. The data obtained in Figure 6a show that the optimal concentration of H2S04 is 7M and the overall eluate efficiency is 70%, which is suitable for all rare earth elements, 60%, 74% and 78% of Yb, y and Ho respectively. This effectiveness can be attributed to the strong dependence of the equilibrium constant on the concentration of hydrogen ions. As shown in Figure 6b, the extraction efficiency of HM varies with the acid strength. Because Cu* is strongly complexed by Cyanex301 at low pH, extreme conditions are required to shift the PH-dependent extraction equilibrium in the opposite direction.

The regeneration of organic solvents has both economic benefit and environmental impact. Because it contributes to the sustainability of resources and the efficiency of the extraction process. The extractant regenerated during the extraction stage is reused during the extraction process (Figure 7).

The focus of this study is on a challenging industrial problem involving the purification of industrial phosphoric acid versus the opportunity to recover valuable metals. Using D2EHPA and Cyanex 301 as solvent extractants, the closed-loop extraction process was designed and optimized. In fact, the process can make the extraction of HMs and ree very efficient. The results show that the separation rate of rare earth elements from HMs is high due to the high selectivity of extractant. The stripping stage enables the regeneration of the extractant to be reused during the extraction stage. More importantly, the precipitation of rare earth and HMs was successfully achieved, resulting in the production of a mixed rare earth concentrate and the regeneration of the stripping agent for further recycling. In addition, the process can be effectively applied to industrial phosphoric acid to produce purified PA for the manufacture of fertilizers. In terms of environmental impact, reducing the HMs content in fertilizers, especially the Cd content, can reduce pollution of soil and aquatic ecosystems.