## 1 Introduction
It is a non-extractable salt containing the same anion as the extractable components, the addition of which increases the total content of anions and reduces the activity coefficient of water. This results in a shift of the equilibrium to the formation of the organic soluble complex, i.e., enhances the extraction. Nitrate media is more preferable during ore processing due to high solubility of REE nitrates in aqueous solutions and lower corrosion activity towards materials of the separation equipment (e.g., in comparison with chlorides). The choice of a salting-out agent is largely determined by its availability. Previously, we have shown that calcium nitrate can be used as an effective salting-out agent [18] for the REE separation in the system \"100% TBP-nitric acid solutions of REE(NO\({}_{3}\))\({}_{3}\)\". Despite the large volume of experimental results existing at the moment, very often there are no data in literature on specific conditions of separation (concentrations of extractants and other components of extraction systems, temperature, stream flows, etc. ), especially in the case of macro-quantities (grams per liter) of REEs. The existing data are insufficient to calculate parameters of the process for separating a pair of elements with a certain composition. In particular, such an object can be a mixture of praseodymium and neodymium with a Pr/Nd ratio = 70:30, which can be formed in technological schemes during the isolation of neodymium [26]. The main systems for the separation of a Pr-Nd pair described in literature are Aliquat 336 in the presence of NH\({}_{4}\)NO\({}_{3}\) as a salting-out agent [27; 28]; organophosphorus acids (di-(2-ethylhexyl) phosphoric acid, HDEHP, and 2-ethyl-hexylphonic acid mono-2-ethylhexyl ester, EHEHPA) in chloride media [29] or synergetic systems of EHEHPA + 8-hydroxyquinoline [30]. TBP was also used to separate of the \"light\" group of REEs in the presence of nitric acid as a salting-out agent [31] and to model the extraction of Nd(NO\({}_{3}\))\({}_{3}\) and Pr(NO\({}_{3}\))\({}_{3}\) in the presence of NH\({}_{4}\)NO\({}_{3}\) as a salting-out agent [32]. There are also some examples of the application of phosphine oxides for extraction of REEs from nitric acid media [33]. Nevertheless, even with using these systems, separation of the Pr-Nd pair is still a difficult task, and the growing consumption of these metals requires the development of new effective and cheap extraction systems. Therefore, the aim of this work was to apply existing commercially available extractants as a basis for new extraction systems for Pr and Nd separation (in both static and dynamic conditions). We have chosen the following extractants: tri-butyl-phosphate (TBP), di-(2-ethylhexyl) phosphoric acid (HDEHP) and Aliquat 336 (a mixture of quaternary ammonium bases). The choice of these three extractants is based on their availability and their fundamentally different extraction mechanisms: solvation (TBP), cationic exchange (HDEHP) and anionic exchange (Aliquat 336) [15]. As already mentioned, a large amount of experimental data have been obtained for extraction systems based on these extractants, but in this work, it was important to examine the extraction of a specific mixture with a Pr/Nd ratio of 70:30 and assess how the main affecting factors can influence extraction and separation performance. As the main factors, we have chosen concentration of Ca(NO3)2 as a salting-out agent in the case of TBP and Aliquat 336, concentration of REEs and nitric acid in the case of HDEHP and presence of a water-soluble complexing agents in the case of Aliquat 336.