Free Paper Sample: Solvent Extraction Method for Separation of Rare Earths

Solvent extraction is a process that has been widely employed in the separation of REE. In some instances, the process is one of the many processes that is involved in the separation of REE. That is, the separation process can begin with one process, but the solvent extraction process is employed at some stage. Solvent extraction process plays a critical role during the production process of not only neodymium but also NdFeB magnets (Sprecher, Kleijn, & Kramer, 2014). The commercial recovery of neodymium begins with a mineral that is known as bastnasite (RECO3F) and in some instances monazite. The conventional method of open pit mining, as well as beneficiation techniques, is used during the initial stages. The RECO3F first undergo acid roasting where both carbonate and fluoride are removed. The product is a water-soluble compound known as rare earth sulfate (RE2 (SO4)3). Since RE2 (SO4)3 is soluble, it is possible to extract the RE from their ore. This can be achieved by mixing them with cold water and then allowing the water to pass through the heap of ore. However, the water leaving the heap of core still contains numerous impurities such as phosphates, thorium, and iron. To remove them, the pH of the leachate is increased which make the impurities to precipitate. This is followed by addition of caustic soda which leads to the precipitation of the rare earths. The final step of the leaching process involves the addition of HCl to convert the precipitates of rare earths to RECl3. Once a concentrated mixture of RECl3 has been obtained from the leaching process, the individual elements of the rare earths then need to be separated from each other. Separating the rare earths from each other demand the application of a technique that is known as a solvent extraction method. Solvent extraction technique utilizes the idea that the various types of rare earths differ in their levels of basicity although slightly.

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In solvent extraction technique, the liquid containing RECl3 is mixed with an organic solvent in which a specific type of RECl is soluble. Since variance the basicity among the various RECls is negligible, the process ought to be conducted several times until all the RECls are successfully separated. The ultimate product of this solvent extraction process is an oxide of rare earth with a purity of over 99.99% (Sprecher, Kleijn, & Kramer, 2014). The high purity of the rare earths achieved through this process has made it popular method.

New Ionic Liquids for Separation of Rare Earths

The recovery of REEs from permanent magnets may involve the use of hydrometallurgical paths. The hydrometallurgical paths involve the use of new solvent-ionic liquids. In this method, the alloys of magnets are dissolved in strong mineral acids. This is followed by selective precipitation of REEs either as fluorides, oxalates, or sulfates. However, this technique demands the use of a huge quantity of chemicals. Often, the technique involves the selective dissolution of the rare earths as well as boron which is also valuable but leaving behind the Fe. But one problem of selective leaching is that some of the unwanted elements end up in the solution. For example, the protective coatings of the magnets contain nickel and copper, and that also boron need to be separated from the rare earths. Consequently, the solution that is rich in REEs ought to be separated from elements that are not REE. This ought to be done before transferring the solution of REEs in an existing separation plant. The REE in the swarf can then be separated without additional investment. One of the major advantages of this method is that it results in the recovery of REEs of high purity solutions as compared to any other applications. However, the major disadvantage comes in the cost involved in pretreating the swarf by a process known as calcination which allows the step of selective dissolution as well as the generation of waste that ought to be discarded. Some of the processes involving leaching and precipitation lead to the recovery of over 99% of the rare earths (Binnemans et al., 2013). However, certain disadvantages have led to the development of other methods such as the new ionic liquids.

The new ionic liquids have been found to be potential candidates for enhancing the efficiency of solvent extraction techniques especially during the extraction of rare earth elements. Ionic liquids have been described as solvents that comprise entirely of ions. That is solvents that exist in ionic form. The ionic liquids possess unique characteristics that make them suitable for use during the solvent extraction process. In addition to possessing low vapor pressure, the ionic liquids have also been found to possess intrinsic electrical conductivity. Due to their promising use in solvent extraction processes, they can be used to replace the organic phases in processes that involve liquid-liquid extraction. Consequently, their applications help to enhance the safety of the systems since they are non-volatile. Previous experiments involving ionic liquids were performed in small laboratory scale, but it has been demonstrated that their use in large scale is highly feasible (Binnemans et al., 2013). Researchers have demonstrated that ionic liquids can be easily employed in the recovery of rare earth metals from leach liquors of scrap magnet that contain a huge quantity of iron impurities.

Drawbacks of Solvent Extraction Process

While solvent extraction process has gained a lot of popularity in the mining as well as the ore beneficiation industries over the past half a century, and especially in the rare earths sector, it has been found to be disadvantageous. Critics of the process argue that the use of solvents in an industrial process is going against the principles of green chemistry. Often, organic solvents in a chemical process result in huge wastes, and elimination of them result in environmental conservation. The solvent extraction processes such as the one used in the recovery of the rare earths often contain some conventional organic solvents that are corrosive, flammable, and toxic. It has been found that the volatility of the organic solvents used in the method has largely contributed to land, air, and global pollution (Izatt et al., 2016). Besides increasing risk due to worker exposure, the organic solvents have also been linked to serious accidents that not only injure employees but may also cause plant shut down for some time. Even in situations where there is a possibility of recovery as well as reuse of the solvents, the distillation processes involved are usually energy-intensive and may result in cross-contamination failure to dispose of the solvents carefully results in serious environmental and health consequences. The wider externalities arising from the use of solvents have a large negative impact to not only the workers but also the wider population. It has also been found that solvent extraction methods sometimes result in low rates of metal recovery as well as low metal selectivity. Consequently, achieving effective separation demands several stages (Xie, Zhang, Dreisinger, & Doyle, 2014). Such inefficiencies associated with solvent extraction leads to the heavy expenditures of labor, reagents, space, and time. Eventually, it results in large in-process inventories of metals which subsequently elongates the time needed to produce the final products.

References

Binnemans, K., Jones, P. T., Blanpain, B., Van Gerven, T., Yang, Y., Walton, A., & Buchert, M. (2013). Recycling of rare earths: a critical review. Journal of cleaner production, 51, 1-22.

Izatt, S. R., McKenzie, J. S., Izatt, N. E., Bruening, R. L., Krakowiak, K. E., & Izatt, R. M. (2016). Molecular recognition technology: a green chemistry process for separation of individual rare earth metals. White Paper on Separation of Rare Earth Elements, 1-13.

Sprecher, B., Kleijn, R., & Kramer, G. J. (2014). Recycling potential of neodymium: the case of computer hard disk drives. Environmental science & technology, 48(16), 9506-9513.

Xie, F., Zhang, T. A., Dreisinger, D., & Doyle, F. (2014). A critical review on solvent extraction of rare earths from aqueous solutions. Minerals Engineering, 56, 10-28.