Potentiality of caro’s acid in leaching of uranium and radioactive measurements from Abu-Rusheid mylonitic gneiss rocks, South Eastern Desert, Egypt


 Abu Rusheid area is located at the Southern Eastern Desert of Egypt and composed of Mylonitic gneiss rocks (mineralized rock), Serpentinite rocks, Ophiolitic metagabbro, Ophiolitic mélange, Monzogranites, post- granitic dykes (lamprophyre and dolerite), veins and recent alluvial deposits. This paper is concerned with the study of potentiality of sulphuric and caro’s acid in uranium dissolution from Abu Rusheid mineralized rocks. For this purpose, many batch dissolution experiments were conducted. The obtained results showed that 91.5% and 52% uranium leachability for Caro’s acid and dilute sulfuric acid respectively. The reaction mechanism was described using shrinking core models.

Batch leaching tests on Ranger, Nabarlek, Koongarra, and Jabiluka ores showed that uranium extraction was unaffected by choice of oxidant, but that Caro's acid reduced acid and lime requirements by 15-25% and 20-30% respectively [16]. To gain a better understanding of the leaching process and its operation, kinetic studies should be concerned. Since the leaching kinetics interprets the complex behavior of leaching process occur on grain particles [17]. Thus, this study includes investigation and discussion of the leaching kinetics and dissolution of uranium on Caro's acid solution.
For leaching uranium from Abu-Rusheid uranium mineralization, phosphoric acid was examined. Results obtained encourage recommending phosphoric acid as an alternative to sulphuric acid leaching agent for uranium [18]. It is demonstrated by the laboratory experiment that heap leaching is suited to process Abu-Rusheid uranium mineralization; D263B strong base anion resin can effectively concentrate and purify the obtained pregnant solutions to produce a quality products, namely yellow cake [19].
The present paper concerned with study the leachability of Abu Rusheid uranium minerals ores situated at the Southern Eastern Desert of Egypt. The potentiality of Caro`s acid was established in batch experiments followed by kinetics investigation. Geologic Setting

Regional geology
Abu Rusheid is a small part of the Arabian-Nubian Shield that outcrops across an area of around 73.5 km 2 in Egypt's South Eastern Desert (ANS). The shield, which covers more than 6 x 10 6 km 2 [20,21], is one of the world's greatest Neoproterozoic crustal development episodes, having been exposed by uplift and erosion during the Oligocene and younger periods [22,23]. The Arabian-Nubian Shield (ANS) is made up of Precambrian rocks that can be found in western Arabia and northeastern Africa on both sides of the Red Sea ( Fig. 1a; [24]). The majority of the ANS is made up of juvenile continental crust [25][26][27], which crust is formed by mantle-derived melts. When the Mozambique ocean closed due to arc terrane accretion, it formed between 900 and 550 Ma [22]. Abu Rusheid area is a part of the Arabian-Nubian Shield and can be considered a key domain in that shield, beside its very complex structures. In addition, this area is considered as the southeastern extension of the Migif-Hafa t metamorphic complex [28-30], which is highly tectonized and characterized by presence of several types of mineralization and alteration processes along time spans. Abu Rusheid-Sikait area is bordering to the major shear zone recognized by many authors as the Nugrus thrust fault [31] or the Nugrus strike-slip fault [32] and or Shait-Nugrus shear zone [33]. Geology of the study area The main rock units encountered in this area are grouped from older to younger as follows; -(a) Mylonitic gneiss rocks, (b) Serpentinite rocks, (c) Ophiolitic metagabbro, (d) Ophiolitic mélange, (e) Monzogranite rocks, (f) Post-granitic dykes and veins (Fig. 1b). The mylonitic gneiss rocks (2.0 km 2 ) represent the oldest rocks exposed in W. Abu-Rusheid area. These mylonitic gneisses were originally identi ed as psammitic gneisses [34][35][36][37], occur in the eld down thrusted the ophiolitic mélange and foliated in ENE-WSW direction (Fig. 2a). The mylonitic gneiss rocks of Abu Rusheid area is characterized by development of mylonitic fabric close to the shear and contact zones, low to moderate topography and highly sheared as well as show a well-developed planer banding, gneissosity and folding. They are highly metasomatized and re ect high radioactive anomalies that contain abundant crystals of thorite, uranothorite, zircon, uorite and Nb-bearing minerals (samarskite, pyrochlore, beta te) [38]. They are cross cut by three main shear zones; the rst two shear zones are parallel to each other (NNW-SSE) and perpendicular to the third one (ENE-WSW), the NNW-SSE shear zones were emplaced by lamprophyre dykes which act as a good barrier (physical and chemical trap) capturing, adsorbing and protecting the uranium minerals that introduced to the gneiss with moving the uranium-rich hydrothermal solution ( Fig. 2b-d). They have general NNW trend and dips steeply (80º-85º) toward SW. The serpentinite is only found in a small part in the NW corner of the mapped area. Abu-Rusheid -Sikait granitic pluton is an elongated body extending NW-SE for about 12 km with width about 3 km. The granitic rocks occupy the major part of the mapped area surrounding the mylonitic gneisses ( Fig. 1b). They are represented from the NW direction by porphyritic biotite monzogranite followed by deformed biotite monzogranite and two mica monzogranite, whereas the muscovite granites occupy the SE part of the pluton [39].
The descending (in ltrational) mineralization-bearing hydrothermal solutions are mainly derived from meteoric water, migrating under gravity from high relief peripheral hot uraniferous muscovite-biotite granite to low central part of the basin (mylonitic gneiss rocks) and redeposited along banding planes and fractures in mylonitic gneiss .
After the emplacement of lamprophyre dykes (mantled-derived with high temperature and volatiles, as well as, CO 2 ), the ascending (ex ltrational) hydrothermal solutions are dominantly derived from groundwater, and are mainly derived by high gas pressures penetrating into the basin, along banding planes and fractures in the host rocks with low temperatures and containing F1 − and CO 3 2

Results And Discussion
Radioactivity of the Abu Rusheid mylonitic gneiss samples, The radioactivity of the studied mylonitic gneiss samples, Abu Rusheid area, SED, Egypt were measured then related to the standards for U and Th provided by International Atomic Energy Agency (IAEA). The measurements are given in Table. 1. The table shows that the eU shows very high level from 69 and 1106 ppm with an average of 364.84 ppm, while the eTh content ranges between 45 and 402 ppm with an average of 147.59 ppm. The eTh/eU ratios range from 0.04-5.43 with an average of about 0.86. These values indicated that; there is uranium leach in. On the eU-eTh plot Fig. (4a); eU shows very high level and show the relationships among eU and eTh for the mineralized mylonitic gneiss, where the relation of eU and eTh is negative due to the slight enrichment of uranium. The increase of eTh content in some samples is regard due to the presence of uranothorite. Also, the eTh/eU ratio versus eU (Fig. 4b) show negative correlation, in which eTh/eU ratio increases with a decrease of eU in the most of representive samples re ect the presence of enrichment uranium mineralization.   Where A is the sulfuric acid concentration (g/L) [44].
The presence of MnO (0.1%) in mineralized ore could be useful as oxidant of iron as in the following Eq. (2) 2Fe 2+ + MnO 2 +4H + 2Fe 3+ + Mn 2+ +2H 2 O (2) The above equation shows that the reaction requires large amounts of hydrogen ions to take place, which consequently consume the acid leach.
The acid required to achieve the pervious equivalent oxidation of ferrous ion could be reduced by 50% if using another oxidant such as sodium chlorate or Caro`s acid as evident in the following equation As shown in Fig. 5, the mineralized ore need large amounts of sulfuric acid to achieve almost uranium leaching .Since the leachability record low percent 9 to19% at free acidity 0.25M to 1M with redox potential 215 to 275 mv then increase periodically as in Fig. 6. Which; owing to the studying mineralized ore containing a lot of acid consumer minerals such as uorite mica.
Introducing the oxidant as H 2 O 2 mixed with sulfuric acid as the leach liquor improve the leaching performance and increase the leachability from 19 to 41% and redox potential from 275 to 425mv. Since increase the oxidation rate of iron consequently increase the uranium dissolution. In other hand, reduce consuming of sulfuric acid by gangues metals associated uranium and oxidation. 1M sulfuric acid was used as the maximum concentration of ore leaching to decrease the dissolution of gangues metals such as silica and alumina which exist as the major oxides in ore. High free acidity, access to the uranium inside individual particles.

Effect of contact time
By varying the parameter in the range of 3-7 hr with keeping other variables as follows: 1M free acidity, temperature 40 o C, particle size 300µm and solid/liquid ratio 1:2.
The results are shown in Fig. 7, about 31.5, 17 % of uranium values were leached in 3 hr using 1M H 2 SO 4 and Caro's acid respectively .Then, gradually increased to about 41.8, and 19.5% in 4-7 hr reaction time, respectively. The less increase in leachability of uranium is result of simultaneous increase of acid consumption, during leaching in case of using only sulfuric acid since.

Effect of grain size
The leachability as function of grain size was established by varying the grain size from 425 to 75 µm and constant the other parameters and illustrated in Fig. 8. As shown in Fig. 7, the leachability increase gradually with grain size decrease could be attributed to increase in the area of reaction consequently increase dissolution of the contained uranium and oxidant (MnO) on solution.
Effect of temperature Temperature studied range were from room temperature to lower 85 o C to avoid dissolution of associated minerals like monazite and rare earth silicate [40]. Consequently, temperature effect was investigated by varying reaction temperature from 30 to 70 o C and xing the other parameters to avoid dissolution of associated minerals like monazite and rare earth elements silicate .Consequently, temperature effect was investigated by varying reaction temperature. Figure 9, shows that, gradual increase in uranium leachability with temperature increase, as increase from 19.2 to 36% at temperature from 30 to 70 o C in case of diluted H 2 SO 4 .On using Caro`s acid ,there is slightly increase in leachability from 41 to 44 at temperature 30-50 o C, then tend to decrease with temperature increase from 50 to 70 o C.
From above, we can concluded that the high temperature accelerate the uranium dissolution when using diluted H 2 SO 4 .On other hand, accelerate rstly then decrease after that on using Caro`s acid which can be attributed to the dissociation of H 2 SO 5 into H 2 SO 4 and water at temperature above 50 o C.

Effect of S/L ratio
The effect of S/L ratio was investigated by varying through the S/L ratio range between 1:2 to 1:5, while xing the other parameters. The obtained results (Fig. 10) show that the uranium leachability increased gradually as the ratio decrease from 1:2 to 1:5 in case of H 2 SO 4 .On other hand, multiplied as ratio decrease from 1/2 to 1/3 then increase gradually to reach 96.5% from 1/3 to 1/5 ratio. Which proved that the Caro's acid save in sulfuric acid consuming during leaching and give concentrated uranium leach liquor.

Leaching kinetics
In order to gain more understood of leaching behavior ,the uranium leachability % values were determined along each experiment performed with time .The obtained results would be formulated in core shrinking models which the best model can describe the kinetics and reaction mechanism.
Shrinking core model derived equations as the following [46]. 1-3 (1-X) 2/3 + 2 (1-X) = K d t, (5) when the leaching rate controlled by diffusion through product layer 1 -(1 -X) 1/3 = K c t, (6) when the leaching rate controlled by chemical reaction on surface layer X = K f , (7) when leaching rate controlled by lm diffusion While K d , Kc and K f are the apparent rate constants (min − 1 ), t is the leaching time, and X is the metal fraction.

Effect of free acidity
To elucidate the mechanism of the Caro's acid concentration on uranium dissolution, the shrinking core models as described by Eqs. (5) and (6) are operated and graphed as in Fig. (11 a,b).
(a) diffusion reaction control; (b) Chemical reaction control As shown in Fig. 11a,b ,there are a linearity relation of slopes > 0.9 between the uranium fraction mole dissolved at varied times and Caro's acid concentration. There are very little changes between two reaction models, so that another relationship will be plotted between logarithm apparent rate constant against logarithm M H 2 SO 4 as in Fig. 12, to differentiate between them. Figure 12, shows that the linearity in diffusion model than other chemical one, which proved that the leaching rate is controlled by diffusion process. The slope of 0.986 indicates strong dependence of the rate on sulfuric acid concentration.

Effect of Temperature
Measuring uranium adsorbed each time interval during dissolution experiments with varying reaction temperature and xes other conditions. The obtained resulted used in construction of two shrinking core models to be examined at different temperatures as shown Fig. 13a,b. As observed, the two models have good linear ts and their R 2 values are close to each other. Therefore, it is di cult to distinguish between these reaction models.
Activation energy is the important parameter that can be used to justify the rate-determining step in hydrometallurgical process. It was calculated based on the Arrhenius equation [47]: K=A e −Ea/RT ; LnK=LnA-Ea/RT (8) Where K is the rate constant, Ea is the activation energy, R is the ideal-gas constant (8.314 J/Kmol), T is the temperature in K and A the frequency or pre-exponential factor.
As shown in Fig. 14, Arrhenius equation was plotted as Ln(K) versus (1/T ) for each temperature, and the activation energies were calculated from the slopes of straight lines where the slope is -Ea/R. From above Figure the calculated activation energy Ea are − 11.03 and − 8.48 kJ/mole for chemical reaction control and diffusion reaction control respectively. These indicate that the reaction rate is controlled by chemical reaction on the particle surface. Consequently, temperature have pronounced effect on reaction rate since, its increase lead to dissociate the Caro's acid into H 2 O and H 2 SO 4 on the particle surface, then increase the diffusion of ions dissociated through the liquid lm and particle layer.

Conclusion And Future Applications
Batch experiments show the superiority of Caro's acid in uranium dissolution with low concentration than dilute sulfuric acid. Leachability of 91.5% and 52% uranium for Caro's acid and dilute sulfuric acid respectively was obtained. Shrinking core models used to determines the reaction mechanism, which shows that the uranium dissolution in Caro's acid controlled by diffusion reaction model. Arrhenius plot were constructed to calculate the activation energy, which is found ranging from − 11.03 to -8.48kj/mole.
In future, we can applied a two-stage leaching for achieving maximum uranium solubilisation with minimum dissolution of gangue elements like chlorite and biotite in sulfuric acid leaching of uranium ore. Firstly, leaching with Caro's with optimized concentration of less than 100g/l H 2 SO 4 until Eh and pH of leach liquor reached 450 and 1.6 respectively followed by leaching with diluted H 2 SO 4 that keep the Eh and pH of leach liquor at the same values.       Effect of grain size on uranium leachability %using H2SO4 and Caro's acid Figure 9 Effect of Temperature on uranium leachability %using H2SO4 and Caro's acid Effect of S/L ratio on uranium leachability % using H2SO4 and Caro's acid Figure 11 The kinetic curve of uranium leaching at different concentration of Caro's acid based on different models.