Recovery of Rare Earth Elements from Solid Residue of El-Sela Ore, South Eastern Desert, Egypt

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1 Recovery of Rare Earth Elements from Solid Residue of El-Sela Ore, South Eastern Desert, Egypt A.A. Salman (1), C.M. Sharaby (1), W.A. Elnagar (2), Y.M. Khawassek (2) and SH.M. Abdo (2) (1) Chemistry Department, Faculty of Science, AL-Azhar University, Egypt. (2) Nuclear Materials Authority, P.O. Box 530 El Maadi, Cairo, Egypt Received: 6/2/2014 Accepted: 10/3/2015 ABSTRACT The study area of Gabal El Sela at Halaib environ is located at about 20 km west of Abu Ramad City, Egypt. An uraniferous ore material associated with REE was subjected to sulphuric acid leaching for extraction of uranium mainly followed by solid liquid separation through filtration then washing. Physical upgrading was performed upon the dry residue. Chemical treatment by 50% NaOH was carried out where about 250 g residue ground at -200 mesh were agitated at solid / liquid ratio of 1/2 for one hour. The cake was filtered then dried at 100 º C. The dried cake was subjected to dissolution by conc. HCl at 80 º C at a solid /liquid ratio 1:1 for one hour. More than 98% of REE was leached out, and then the leach liquor was subjected to selective precipitation by HF and oxalic acid then calcinations of REE oxalate. Keywords: REE/ Alkali treatment/ precipitation/ oxalic acid. INTRODUCTION The study area of Gabal El-Sela at Halaib environ is located at about 20 km west of Abu Ramad City covering an area of about 128 km 2. It is roughly bound by longitudes 36 o 8` - 36 o 17` E and latitudes 22 o 13` - 23 o 20` N (1), in the extreme south eastern part of Egypt, where a new uranium mineralization has lately been discovered. This discovery adds to the economic importance of this region (2) as it was very famous for its Mn-oxide ore of Gabal Elba, (Mn-W) minerals of Gabal Qash Amir and lately the newly discovered U- REE mineralization of Gabal El Sela (3). Microscopically, Gabal El Sela is covered by biotite-muscovite monzogranites (two -mica granites), which are medium to coarse-grained, pink in color, sheared and highly weathered. The modal composition is mainly of quartz (36%), potash feldspars (32%), (28%), biotite and muscovite (2.5%) (3). The U-mineralization occurs in the NW-part of El-Sela highly wreathed granites associated with veins in the ENE-WSW and N-S shear zones as well as the related NW-SE gash open fractures. The mineralized veins comprise microgranites, red and black silica veins and basic lamprophyre dykes (4,5). The mineralogic investigations revealed the presence of pitchblende, coffinite, and thorianite as primary accessory minerals especially in the microgranites. These radioactive minerals are associated with autunite and uranophane as secondary uranyl minerals (6). Other minerals observed that may be of economic importance are: thorite, monazite, apatite, rutile, columbite, fluorite and samarskite along with the commen accessories such as ilmenite, hematite, sphalerite, pyrite, epidote, garnet and jarosite (6). Corresponding author shimo.shosho86@yahoo.com 26

2 Recovery of the valued metals from the ores includes three main processes which are physical upgrading, leaching and finally the metals recovery and purification with a great deal of chemical treatments through the extraction of metals from the obtained solution. The physical treatments involve minerals beneficiation to liberate and separate heavy minerals from ores or residues either by gravitational differences or slight difference in magnetic susceptibility (7). The aim of this study is the recovery of REE after chemical treatment to achieve the goal of dissolving most of REE constituents. The chemical treatment experiments involve two ways; acid treatment and digestion with sodium hydroxide. The acid treatment experiments were conducted upon heavy fraction of El-Sela mineralization, in order to study the factors affecting the leaching efficiency. The optimum conditions of acid treatment did not achieve the desirable results thus another method called treatment with alkali (NaOH) followed by aci d leaching (HC l) was performed. Finally, the recovery of REE from the filtrate of the HCl dissolution would be carried out. EXPERIMENTAL The experimental work was performed upon a solid residue remaining after sulfuric acid leaching of the ground uraniferous material from El-Sela ore. The mineralization is considered mainly as uranium ore materials which is also associated with other economic elements such as REE and other minerals. Several experiments were carried out upon the residue to specify some of these economic elements (REE) such as physical upgrading and chemical treatment. The procedure of chemical treatment includes two ways acid treatment and treatment with alkali to achieve the goal of dissolving most of REE constituents. Instrumentation: 1- Generally, the samples used in this work were weighed using an analytical balance (Shimadzu, AY 220), having a maximum sensitivity of 10-4 g and an accuracy of ± 0.01 %. 2- The hydrogen ion concentration of the different solutions was measured. The ph-meter used was HAANA ph-mv-temp, provided with H11270 combination electrode and thermometer sensor made from stainless steel. 3-The samples were analyzed using conventional wet chemical techniques (8), for major elements. The SiO 2, Al 2O 3, TiO 2and P 2O 5 were determined using a spectrophotometric methods. The contents of Na and K were determined by a flame photometric technique. Total Fe as Fe 2O 3, MgO and CaO were determined by titration methods. The loss on ignition (L.O.I) was determined gravimetrically. The estimated error for major constituents is not more than ±1%. 4- Total REE was determined by arsenazo III where the absorbance of its complex was measured at the wavelength 650nm (9) by using UV-spectrophotometer single beam multi-cells-positions model SP-8001, Metretech Inc., version 1.02, with glass cell of 10 mm length. 5- (Prodigy Axial high dispersion ICP-OES USA) inductively Coupled Plasma Optical Emission Spectrometer used for determination individual REE, 6- An atomic absorption model G.B.C.A.A, was used for measuring trace elements. 7- Uranium was determined by titration against ammonium metavanadate (10). Sample preparation: A gross sample of El-Sela ore was firstly subjected to uranium leaching at Inchass pilot unit using suitable conditions for high uranium leaching and associated REE with minimum dissolution of gangues (represented as iron). These conditions include grinding the ore to -60 mesh size with 100g/l sulfuric acid at acid : ore ratio of 2 for 6 hours at room temperature. The residue was then subjected to filtration and drying. In this work about 60 kg of solid residue from El-Sela ore was first treated by shaking tables to obtain heavy mineral concentrate (11). 27

3 Chemical Treatment by NaOH: A series of experiments was carried out upon 50 g samples of heavy concentrate, ground to -200 mesh size which was mixed with NaOH at different ratios. The alkali and the sample were mixed in 250 ml conical flask using a hot plate magnetic stirrer under the conditions given in table (1). Table (1): The studied factors affecting the alkali treatment of the solid residue from El-Sela ore. Factor Variable Values NaOH, % Fixed conditions (A/S) Temp ( o C) Time (min) 1- NaOH, % conc. 30,40, 50, 70 varied 2 2- Alkali/Solid (A/S) ratio 2, 2.5, 3 varied Temperature ( o C) 100, 120, 140, varied 4- Digestion Time (min) 30,45, 60, varied The temperature was controlled by adding a small amount of distilled water to the mixture. After one hour, the mixture was diluted with distilled water at 105 C and agitated for one hour. After each experiment, the leach slurry was filtered using filter paper, washed thoroughly with hot distilled water then dried at 100 C. The dried cake was subjected to dissolution by conc. HCl at 80 C at solid /liquid ratio 1:1 for one hour. Selective precipitation of the REE from the acid solution (filtrate from hydrochloric acid dissolution) was precipitated with HF and oxalic acid and finally calcinations of REE oxalate. RESULTS AND DISCUSSION A gross sample of El-Sela ore containing 0.042%U and 0.045% REE was firstly subjected to uranium, REE leaching at Inchass pilot unit using the suitable conditions that verify high uranium leaching and associated REE analysis of the solid residue after leaching indicated that it contains some valuable elements such as REE that represents the aim of this study (11). Physical up Grading of the Leached Solid Residue: The solid residue under study after the filtration step and good washing was left to dry before the physical upgrading. The dry solid residue was reground to -60 mesh using a roll mill. The weight of solid residue after dryness was about 60 kg which was subjected to separate its constituents of silicate and other light minerals fraction using the shaking tables. The sample obtained from this heavy fraction (weighed about 3 kg). The heavy concentrate was subjected to magnetic separation using hand magnet. The separated magnetite represented about 34 g of the heavy fraction. The remaining heavy concentrate samples after magnetic separation is considered the starting material for the experimental work of this study. The sample obtained after magnetic separation (weighed about kg). Characterization of the Heavy Fraction Produced from Physical up Grading Step: The heavy fraction produced from Physical upgrading step was studied for its mineralogical and chemical composition. Mineralogical Characterization: The mineralogical composition of El-Sela heavy fraction of the shaking table is investigated. Photomicrographs of this heavy minerals fraction under microscope refer to presence the light minerals such as feldspare and quartz. This is most probably due to the presence of mixed grains of the 28

4 present large particle sizes and so it would be preferred to reduce the particle size for better liberation before using the shaking table. The main valuable and economic minerals present in this heavy fraction include monazite, zircon, colorless fluorite, violet fluorite, muscovite, rutile, goethite, Mn minerals, epidote, hematite. Chemical Composition of Solid Residue Preparation of the residue under study for chemical analysis was carried out as follows: Each 0.5 gram finely ground sample portion from the residue was heated with 12 ml mixture of HNO 3:HCl in a ratio 1:3 in 100 ml teflon beaker till complete evaporation, then 25 ml HF was added and heated to dryness. Finally, 50 ml of 1:1 HCl was used to dissolve the residue and the solution was filtered (8).The remaining solid was fused with 1.0 gram of NaOH and dissolved in 1:1 HCl solution both the filtrate and the dissolved residue were combined and completed to 250 ml (12) volume in volumetric flask. The major and trace elements were analyzed as shown in table (2), and the chemical analysis of REE and (Sc+Y) from the solid residue by ICP-OES is tabulated in table (3). Table (2): Chemical analysis of the solid residue after physical upgrading from El Sela ore. Major oxides Constituent, % Trace Element SiO Cd Al 2O Cu Fe 2O Nb MnO 0.23 Ni MgO 2.42 Pb CaO 2.25 Cr Na 2O 1.24 V K 2O 1.49 Th TiO U P 2O Zn LOI 0.42 Zr REE Total ppm Table (3): Chemical analysis of REE from solid residue after physical upgrading process by (ICP OES). Element Concentration, ppm Element Concentration, ppm Sc Gd Y Tb 40.9 La Dy Ce 1035 Ho Pr Er Nd Yb 2.1 Sm Lu 2.04 Eu 4.23 Total

5 Chemical Treatment of the Heavy Concentrate by alkali(naoh): A Series of experiments were carried out upon 50 g samples of the heavy concentrate, ground to -200 mesh size which were mixed with NaOH. The digestion temperature was controlled by adding a small amount of distilled water to the mixture. After one hour, the mixture was diluted with distilled water at 105 C for one hour. In the resultant hot slurry, all the rare earths were present as a hydrous metal oxides cake and then this slurry was filtered out (13). The metal hydroxides cake was washed with hot water. The chemical treatment of heavy concentrate by NaOH involves some factors. These factors involve sodium hydroxide concentration, alkali/solid ratio, digestion temperature and digestion time, which were studied to determined the optimum conditions as will be discussed in the following paragraphs. Effect of Sodium Hydroxide Concentration: The effect of sodium hydroxide concentration on the content of REE in the hydroxide cake was studied in the range 30-70% NaOH under conditions of -200 mech ore particle size, alkali/solid ratio of 2 for 60 minute contact time at 140 C. The obtained results are represented in figure (1). The obtained results revealed that more than 85% of REE was extracted as hydroxide cake and increased to 99.5% as the alkali concentration increased from 30 to 50%, using an alkali concentration of 50% was considered the best concentration to achieve the highest REE content in the cake. 100 The content of REE in hydroxide cake, % The content of REE in hydroxide cake, % Sodium hydroxide concentration,% Fig (1): Effect of sodium hydroxide concentration on the content of REE in hydroxide cake from the solid residue of El-Sela ore at (-200 mesh, alkali/solid ratio 2, 1hour contact time at 140 C). Effect of Alkali/ Solid Ratio: The effect of A/S ratio ranging from 2 to 3 upon the content of REE in the hydroxide cake was studied while the other conditions were kept constant at -200 mesh ore size, 50% NaOH, at 140 C for 60 minute. The results shown in figure (2) indicated that the content of REE in hydroxide cake was slightly decreasing with increasing the A/S ratio. However, alkali/solid ratio of 2 gave the highest REE content in cake about 99.5% from the total REE after physical upgrading. 30

6 100 The content of REE in hydroxide cake,% The content of REE in hydroxide cake, % Alkali/ solid weight ratio Fig (2): Effect of alkali/ solid ratio on the content of REE in cake from the solid residue of El-Sela ore at (-200 mesh ore size, 50% NaOH one hour contact time at 140 C) Effect of Temperature: The effect of temperature upon the content of REE in the hydroxide cake was studied in the range 100 C to 160 C, while the other conditions were fixed at -200 mesh ore size, 50%NaOH, A/ S ratio of 2 for 1hour agitation time. The obtained results are represented in figure (3). From these results, it is clearly that the content of REE in the hydroxide cake increased from 100 C to 140 C. The highest REE content in the cake was about 99.5% is achieved at 140 C and remained constant at 160 C. The content of REE in hydroxide cake, % The content of REE in hydroxide cake, % Digestion temperature, C Fig (3): Effect of temperature on the content of REE in cake from the solid residue of El-Sela ore at (-200 mesh ore size, 50% NaOH, alkali/solid ratio 2and 1 hour contact time). 31

7 Effect of Agitation Time upon the REE Leaching Efficiency: The effect of varying time in periods ranging from 30 to 90 minutes upon the content of REE in hydroxide cake, while other conditions of -200 mesh ore size, 50% NaOH concentration, A/S ratio 2 were fixed as shown in fig.4. These results indicated that 90% of REE were extracted as hydroxide cake after 1/2 hour while the content of REE in cake increased till it reached about 99.5% after one hour. However, no further increase occurred above this time period. 100 The content of REE in hydroxide cake, % The content of REE in hydroxide cake, % Digestion time, min Fig (4): Effect of agitation time on the content of REE in hydroxide cake from the solid residue of El-Sela ore (-200 mesh ore size, 50% NaOH, alkali/solid ratio 2 at 140 C As a result, the optimum condition for digestion of solid residue with NaOH that achieve the highest REE content in the hydroxide cake were -200 mesh particle size with 50% NaOH concentration at solid / alkali ratio of 1/2 for 60 minutes digestion time, at 140 º C. The hydrous metal oxides cake contained 3405 ppm REE and dried in the drying oven at 100 C for one hour. The dried cake was transferred into a conical flask and dissolved in 37% hydrochloric acid at 80 C for one hour and acid /solid ratio 1:1. The reaction is presented in equation (1). RE (OH) 3 + 3HCl = RECl 3 + 3H 2O (1) The obtained solution was diluted with hot water for one hour. The rare earths and the other associated elements were dissolved in the acid solution where the extraction efficiency of REE reached 98% (3353ppm) REE while the undissolved impurities were left as residue. HF and oxalic acid were used to precipitate REE from the filtrate. The REEs were precipitated as REE fluorides using (1:1) HF acid (14). The oxalate precipitation is nearly the most specific precipitation method. This method has complete separation from iron, aluminum, nickel, chromium, manganese, zirconium, hafnium, and uranium. However, large amounts of ferric iron tend to inhibit precipitation of the rare earths because the oxalic acid was consumed in the reduction of the ferric to the ferrous iron (15). In this case, the rare earths should be separated by solvent extraction or by fluoride precipitation. On the other hand, ferrous iron if present in sufficient quantity, it would be precipitated as oxalate when standing overnight (16). 32

8 Selective Precipitation by HF: A set of experiments were conducted using 50 ml of the acid solution assaying (3353ppm) REE, where portions of 0.5 to 3 ml 50 % HF were tested with 0.5 ml increment between each experiment. After each experiment, the slurry was left to settle out and the filtrate was analyzed against the remaining REE. The precipitation efficiency was calculated according to equation (2). The results reveal that complete precipitation of rare earths was achieved when 2.5 ml HF/50 ml solution is added. The effect of hydrofluoric acid addition on the precipitation efficiency of rare earth is indicated in figure (5). (Original Unprecipitated) REE conc. Precipitation efficiency, % = X 100 (2) Original REE conc. Fig (5): Effect of hydrofluoric acid addition on the precipitation efficiency of rare earth A sample of the rare earth fluorides (precipitated using 2.5 ml HF/50 ml solution) was subjected to filtration, good washing and drying in oven at 100 C then analyzed by ICP- OES including Sc and Y. the results are shown in table (4). Table (4): Chemical analysis of REE from fluoride precipitation by ICP OES Element Concentration, Concentration, Major Concentration, Element % % Elements % Sc 0.69 Gd 0.69 F 27.2 Y 5.02 Tb 0.02 Na 7.1 La 5.2 Dy 0.01 Al 9.8 Ce Ho 0.88 Si 5.3 Pr 2.11 Er 7.12 Ca 2,32 Nd 2.03 Yb 1.42 Sm 2.32 Lu 0.58 Eu Total

9 Selective Precipitation By Oxalic Acid: The chloride solution containing REE was firstly adjusted at different ph values (3, 4, 5) using 50% NaOH solution. On one hand, when the solution ph was 3 and 4 the REE was partially precipitated. On the other hand, when the solution ph was increased to 5, most of REE content was precipitated with other undesirable gangues especially, iron. Thus ph 5 was preferred. Different quantities of oxalic acid from (1-5%) was added to the precipitate to dissolve the undesirable gangues while the REE oxalate remained in the precipitate form. Then a complete precipitation was achieved at a final ph of 1.6 by adding 5% oxalic acid solution. The precipitation efficiency of the obtained REE was 98%. The rare earth oxalate precipitate was ignited at 650 C to convert the REE to oxide. Results of the quantitative analysis for the obtained precipitate using ICP OES were shown in table (5) including Sc and Y. Table (5): Chemical Analysis of REE from oxalate precipitation BY ICP OES. Element Concentration, Element Concentration, % Major Concentration, % Elements % Sc 0.52 Gd 2.35 Na 12.1 Y 8.5 Tb 0.92 Al 7.3 La 9.02 Dy 1.1 Si 6.8 Ce Ho 0.3 Cl 1.1 Pr 5.65 Er 4.05 K 1.2 Nd 8.3 Yb 0.16 Ca 2.7 Sm 2.82 Lu 0.05 Eu 0.49 Total Figure (6) represents a flow diagram for the recovery of the REE from the solid waste from El-Sela U-ore. CONCLUSION Based on the aforementioned results. Accordingly the solid residue obtained from El-Sela mineralization rock eastern desert, Egypt, was subjected to physical upgrading process using shaking tables. When the chemical treatment was performed on heavy concentrate by using acid treatment process, the leaching efficiency of REE did not achieve more than 60%. Thus another chemical treatment by digestion with NaOH upon 250g of residue was carried out. As a result, the suitable digestion of solid residue with NaOH that achieve the highest REE content in the cake were -200 mesh particle size with 50% NaOH concentration at solid/ alkali ratio of 1/2 for 60 minutes digestion time, at 140 º C. The hydrous metal oxide cake was subjected to drying at 100 o C in oven then the dried cake was subjected to dissolution in conc. HCl at 80 o C for one hour where acid /ore 1:1. As a result, more than 98% of REE was extracted. The leach liquor was subjected to selective precipitation of REE by HF and oxalic acid. The rare earth fluoride cake contained 47.3 % REE. By using oxalic acid in precipitation followed by calcination of REE oxalates at 650 o C, a final product of rare earth concentrate of about 68.5% was obtained. 34

10 60 kg of El-Sela (ore) dry solid residue 170 ppm REE Grinding Shaking table 3kg Heavy minerals Magnetic separation 47 gm Magnetite NaOH Digestion 3420 ppm REE Filtration Filter cake HCl Dissolution REE Selective precipitation Oxalic acid calcinations 68.5 % REE Fig. (6): A flow diagram for the recovery of the REE from the solid waste from El-Sela U-ore. 35

11 REFERENCES (1) A. H. El-Afandy, F. S. Bakhit, A. A. Yonan, and G. M. Saleh; Egyptian Mineralogist; 14, 117(2002). (2) M. E. Ibrahim, A. A. Zalata, H. S. Assaf, I. H. Ibrahim, and M. A. Rashed, Egy. J. of Geol; 42/2, 690 (2003). (3) H. Abu Khoziem; Ph.D. Thesis, Faculty of Science, Cairo University, Egypt, (2006). (4) H. S. Assaf, M. Y. Attawiya, M. E. Ibrahim, S. E. Ammer and M. E. Shalaby; Egyptian Mineralogist; 11, (1999). (5) T. M. M. Ibrahim et al.; International symp. Uranium production and raw materials for the nuclear fuel cycle, IAEA, Vienna, Austria, 20-24, (2005) (6) T. M. M. Ibrahim et al.; internal report, NMA, (2011). (7) C.K. Gupta, and N. Krishnamurthy Extractive metallurgy of Rare earths, CRC press, Washington (2005). (8) L. Shapiro, and W.W. Brannock Rabid analysis of silicate, carbonate, and phosphate rocks, U.S. Geol. Survey Bull. 144-A(1962). (9) Z. Marczenko Spectrophotometric Determination of Elements, John Wiley and Sons Inc., New York (1986). (10) A. R. Eberle, M. W. Lerner, C. G. Cold beck, and C. J. Rodden ;New Brunswick Laboratory, USAEC (1964). (11) Y. M. Khawassek; Nucl. Sc. Sci. J., (in press) (2013). (12) E.R. Sholkovitz ; Chemical Geology, 88, 333(1990). (13) L. Th. Shwe, N. N. Soe and K. Th. Lwin ; World Academy of Science, Engineering and Technology, 24(2008). (14) J.H. Lee and R.H. Byrne ; Journal of solution chemistry, 22, 733(1993). (15) M. M. Woyski and R. E Harris The Rare Earths and Rare Earth Compounds, John Wiley and Sons Inc., New York, 8, 1(1963). (16) R. Chi and Z. Xu., ;Metallurgical and Materials Transactions B, 30, 2, 189(1999). 36

Certificate of Analysis

Certificate of Analysis First issued: June 2018 Version: June 2018 REE-3 Certified Reference Material for an Ore with Rare Earth Elements and Zirconium Table 1 REE-3 Certified Values note: Please refer