LEACHING AND RECOVERY OF RARE EARTH ELEMENTS FROM ALTERED ALKALINE GRANITE ROCK FROM NUSAB EL-BALGUM AREA, SOUTH WESTERN DESERT, EGYPT

Taysser A. Lashen 1 , Saeyda A. Mohamed 2 , Mohamed F. Cheira* 1 , Doaa I. Zaki 1 and Eman M. Allam 1 . 1. Nuclear Materials Authority, P.O. Box 530 El Maadi, Cairo, Egypt. 2. Chemistry Department, Faculty of Science, Menoufiya University, Egypt. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

788 solution as soluble sulfates or chlorides 15 . Ammonium chloride and ammonium nitrate were used as a mixed leachate to treat weathered deposited rare earth ores 16 . Other researchers suggested magnesium sulfate as leaching agent for ion-adsorption type rare earth ore to reduce or even eliminate ammonia-nitrogen emissions 17,18 . On the other hand releasing of rare earth ions from a mineral concentrate containing minerals require severe conditions to destroy their structure. Xue et al., reported that the hydrochloric acid leaching process of the activated bastnaesite occurred at 90 ℃ leaching temperature and 20:1 liquid to solid ratio 19 . In fact, leaching ores by HCl are environmentally beneficial for those containing bastnaesite.
Three-liquid-phase system (TLPS) is a process for simultaneous extraction and separation of multiple target components even with slight differences in physicochemical properties. In a well-designed TLPS, multiphase extraction and selective separation of different components respectively into three different liquid phases can be achieved by only one-step of extraction 37 . TLPE is composed of an organic solvent-rich top phase, a polymer-rich middle phase and a salt-rich bottom phase. According to the strategy developed by Sui et al., it is possible to separate REEs in groups and also from undesirable ions by this approach 38 . They concluded that most of the rare earths in their solutions remained in salt-rich bottom phase due to the formation of hydrophilic complexes between them and complexing agents. On the basis of this data, it could be separate and recover ∑REEs in relatively pure form by controlling variant parameters whereas the obtained leach liquor containing about 90% light rare earth ions.
The current work aimed to study the leaching and recovery parameters of total REEs from Nusab EI Balgum altered alkaline granite rock sample. The leaching parameters included the type of leaching agents, the concentration of hydrochloric acid, agitation time, solid/liquid ratio, stirring rate, grain particle size and leaching temperature are studied. The leaching performance of total iron which is the primary impurity ion in the leachate is also investigated. In addition, thermodynamic parameters of the leaching process are also determined. Recovery of total REEs from the obtained leach liquor is carried out using the three-liquid-phase system. The effect of acidity, concentration of ammonium sulfate and the suitable amount of polyethylene glycol are determined.

Characteristic of working sample:-
The studied sample is obtained from Nusab EI Balgum area, in the central southwestern desert of Egypt. The area is bounded by latitudes 23° 15 / to 23° 20 / N and longitudes 29° 15 / to 29° 20 / E. It covers about 40 km 2 and forms the northern part of Bir Safsaf area. It consists mainly of sandstone, volcanic rocks, altered granitic. The working sample is firstly subjected to preparation through crushing, grinding and sieving followed by proper quartering, then analyzed quantitatively after complete dissolution using the suitable techniques.

Reagents:-
The chemicals and reagents used in this work are of the analytical grade in all experiments, double-distilled water was used for aqueous solutions preparation and dilution. The main chemicals used in this work are hydrochloric acid 37% obtained from Adwic, ascorbic acid from Sigma-Aldrich, formic acid from Prolabo and Arsenazo III from BDH, England. Bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272, purity of 85 wt%) is supplied by Sigma, polyethylene glycol (PEG) with average molecular weight of 2000 is purchased from Loba Chemie. The stock solution of (NH 4 ) 2 SO 4 salt is prepared by dissolving an appropriate amount of (NH 4 ) 2 SO 4 in double distilled water, ethylene diamine tetraacetic acid (EDTA) is supplied from Loba Chemie and 1,10-phenanthroline (phen) is obtained from Sigma-Aldrich.

Analytical methods:-
The mineralogical analyses of working sample are assayed by X-ray Diffraction analysis (XRD), which is used to determine bulk mineral assemblages based on the diffraction of X-rays interacting with a rock sample. The quantitative analysis of the studied sample after its digestion is then carried out for the determination of the major oxides and trace elements. The major oxides SiO 2 , Al 2 O 3 and TiO 2 have been spectrophotometrically analyzed using Unicam UV2-100 UV/Vis spectrometer according to standard methods of analysis while Na and K oxides have 789 determined by the flame photometric technique. The Fe 2 O 3 , MgO and CaO have however been titrimetrically determined 39 . In the meantime, the concentration of the trace elements in the working sample has been determined through the inductively coupled plasma optical emission spectrometry (ICP-OES). The ∑REEs concentration in the leachate solution is spectrophotometrically determined by Arsenazo III at 650 nm 40 . All analysis is repeated 3 times for each sample.

Characterization of working sample:-
The representative sample is subjected to complete chemical analysis using the mentioned procedures and the obtained results are shown in Table ( Table (2) that refer to an enrichment of light REEs than heavy REEs. These results agree with X-ray powder diffraction (XRD) identification ( Figure 1) whereas bastnaesite mineral incorporated REEs. Bastnaesite found as solid solutions adsorbed on quartz and iron oxides and not found as separated mineral 41 . Besides, the silicon minerals seem as quartz. The total REE concentration is around 0.34 wt%.

Effect of acid type:-
The effect of the acid type upon the REEs leaching efficiency from the powdered sample is tested using 3M of H 2 SO 4 , HNO 3 and HCl under the 1/3 solid/liquid ratio, 80 mesh particle size, 200 rpm stirring rate for 60 min agitation time at room temperature. Under these mentioned conditions, the ∑REEs leaching efficiencies are 24.70, 28.22 and 66.00% while the iron dissolution efficiencies are 9.30, 10.78 and 13.63% for H 2 SO 4 , HNO 3 and HCl respectively. The latter results are graphically represented in Figure (2), as can be seen the use of HCl attains the higher dissolution efficiency (66.00%) than the other two acids. Moreover, processing of ore bearing bastnaesite is more favorites with HCl than H 2 SO 4 to avoid fluoride emission, it causes series air pollution. Also, it is interesting to mention here that the choice of HCl as leachant is very useful for further procedure suggested for recovery of total REEs. Therefore, it is the preferred choice for the next experiments of leaching process. 791

Effect of stirring rate:-
The effect of stirring rate on the ∑REEs leaching efficiency is investigated from 100 rpm to 350 rpm under the previous recommended conditions. The obtained data indicates that the leachability of REEs is increased from 73.31 to 92.24% as the stirring speed increased from 100 to 200 rpm but it remains almost constant above 200 rpm ( Figure  6). Increasing the stirring rate more than the latter value leads to more iron releasing in the leach liquor. Therefore, the chosen stirring rate is 200 rpm for the subsequent tests.

Effect of the grain particle size:-
The effect of changing grain particle size is studied from 40 to 200 mesh sizes under the previous optimum conditions. The obtained results for total REEs and total iron leaching efficiencies are illustrated in Figure (7). It is found that the ∑REEs leaching efficiency increases by decreasing the particle size from 40 to 80 mesh. On this regard, the leaching efficiency is increased from 44.25 to 92.4%, while the leaching efficiency of iron is increased from 3.75 to 15.5%. It is worthy to mention that more grinding would increase the dissolution of the most amounts of gangue constituents as well as increase the complication of the filtration and the washing steps, in addition to the costs of the grinding step itself. Thus, the particle size of 80 mesh is considered the preferred particle size to perform the leaching process.   (Table 3), it is found that the leaching efficiency of ∑REEs is slightly increased from 92.4 to 95.5% with increasing the leaching temperature from 25 to 80 °C respectively. Besides, the iron leaching efficiency is increased with increasing the temperature. Therefore, the leaching temperature of 25 °C is chosen in actual production to avoid the rare earth ions hydrolysis and economize the operating cost.

Thermodynamic leaching studies:-
The acidic leaching of REEs and total iron from the working sample are thermodynamically studied to explore the feasibility and spontaneity of the leaching process through the determination of thermodynamic parameters. The thermodynamic parameters are calculated from the variation of the thermodynamic distribution coefficient K d with a change in temperature according to the following equation: where C L and C S are the amounts of metal ions in leaching solution (mg) and the solid sample (mg) at equilibrium (mg metal/kg solid sample), respectively. The thermodynamic parameters including the Gibbs free energy ΔG (kJ/mol), the enthalpy ΔH (kJ/mol) and the entropy ΔS (J/mol.K) are calculated for this system using the following Van ʼ t Hoff equations 42 where R is the universal gas constant (8.314 J/mol.K) and T is temperature (K). Accordingly, the values of both the enthalpy ΔH and the entropy ΔS are calculated from the slope (-ΔH/R) and intercept (ΔS/R) of the Log K d versus 1/T plot (Figure 8). From the obtained data, the values of R 2 of thermodynamic model are near to unity, indicating a good linearity. The values of ΔH, ΔS and ΔG for leaching process are obtained and reported in Table (

Preparation of leach liquor:-
A leach liquor is prepared by treating 1 kg of 80 mesh size properly ground sample with 6 liters of 3 M hydrochloric acid solution for 180 min and agitated at room temperature. The insoluble gangue residue is then filtered and the obtained leach liquor is found to assay 532 mg/L of total REEs indicating a leaching efficiency of 92.4%. The complete chemical analysis of leach liquor using the mentioned procedures is shown in Table (5).

Three-liquid-phase extraction studies:-
In this study, the three-liquid-phase extraction approach composed of Cyanex 272/PEG 2000/ (NH 4 ) 2 SO 4 -H 2 O which is used to remove impurities from the working leach solution which contains total REEs by controlling partitioning and selective enrichment of those impurities and rare earths in the three-liquid-phase system.

Effect of pH:-
The effect of partition of ∑REEs in salt-rich bottom phase of TLPS versus aqueous pH value is carried out within the pH range from 1 to 3 to avoid Fe (III) and Al (III) hydrolysis. The obtained result plotted in Figure (   797

Effect of PEG 2000 amount:-
The effect of partition of total REEs with the added amount of PEG 2000 is illustrated in Figure (11). From the obtained results, it is clearly evident that the partition of total ∑REEs in the bottom salt-rich phase of TLPS varied with increase of PEG 2000 up to 3 g, more increase than this value has no effect on the partition of ∑REEs in the latter phase. The increase of PEG 2000 could lead to the transfer of H 2 O molecules from salt-rich phase to PEG-rich phase, which in favor of the transferring of hydrophilic complexes making discordancy in the partition of ions in three phases.

Effect of (NH 4 ) 2 SO 4 Concentration:-
The effect of (NH 4 ) 2 SO 4 concentration on the partition of ∑REEs in the bottom salt rich layer is presented in Figure  (12).The obtained data showed an enrichment of ∑REEs in this layer. It increases with increasing the (NH 4 ) 2 SO 4 concentration up to 1.6 M. Over the later value of (NH 4 ) 2 SO 4 concentration has, it is no effect on ∑REEs extraction in bottom phase. However, the increase of salt concentration in aqueous phase could result in stripping of the hydration shell of the polymer molecular chain, and thereby has a significant influence on the partitioning of rareearth ions. Precipitation of ∑REEs by oxalic acid:-Precipitation of the collected rare earth elements separated from the salt bottom layer is carried out using oxalic acid and then roasted to produce rare earth oxides. A common observation in REEs precipitation of REEs by oxalic acid is conducted where the 20% oxalic acid is gradually added with continuous stirring at pH1 43 . The obtained precipitate has been identified using the environmental scanning electron microscope (ESEM) ( Figure 13) and then analysed quantitatively using ICP-OES technique to identify its chemical constituents and individual REEs distribution in oxalate and oxides forms (Tables 6, 7). The obtained data seemed that the rare earth oxides are assaying 44.37 and 98.60% in oxalate and oxide concentrates respectively. Finally, a schematic flow sheet of leaching and recovery of REEs is constructed and presented in Figure (14).

Conclusion:-
The leaching procedures of REEs using HCl acid form the Nusab El-Balgum altered alkaline granite rock sample have indicated that REEs dissolution is strongly dependent on the HCl acid concentration. The maximum leaching efficiency ( 92.4 %) is obtained at 3 M hydrochloric acid concentration, 200 rpm stirring speed and 1/6 S/L ratio for 180 min contact time at room temperature. The thermodynamic results of negative ΔG value confirm the feasibility and the spontaneous nature of the studied leaching process of ∑REEs while the positive ΔH value reflects the endothermic nature of dissolution. On the contrary, the positive ΔS value suggests an increase in randomness at the S/L interface during dissolution. It is also indicated that the rare earth ions are most probably transported from the solid phase into the bulk of solution to improve the leaching efficiency.