Introduction

Uranium is one of the most important heavy metals because of its strategic significance in the energy field. The leaching of uranium is a term to describe the process of extracting uranium from its ore which considered as an essential step in the uranium processing. Uranium leaching is carried out via conventional or non-conventional techniques depending on the type of uranium mineral, uranium ore grade, reagents availability, economics of the process and environmental impacts (Cheira et al. 2014; Azimi and Azari 2017; Gado 2018).

Uranium could actually be leached from its ore materials by either acid or alkaline leaching. The choice of the best reagent depends on various technical and economic aspects, namely type of uranium mineral, gangues, availability and reagents costs, oxidant requirements and construction materials of the equipment. In uranium leaching, several variables must be assessed to determine the optimum leaching conditions such as grain size, slurry density, degree of agitation composition of leach liquor, reagent conc., oxidation potential, temperature, pressure and leaching time (kinetics and leaching mechanism) (International Atomic Energy Agency 1990).

The kinetics and practical facts of the uranium leaching, HSS system (H2O2–Na2SO4–H2SO4) were applied on the typical New Mexico ore material at temperature ranged between 30 and 80 °C and constant pH 4.6. The effect of Na2SO4 and H2O2 concentration, pH, temperature and Fe2+ addition on the rate of uranium extraction was determined (Eligwe et al. 1982; Yildiz et al. 2017a; Sen et al. 2017a, b, c, d). Studying the oxidation of uranium species coming from some Romanian uranium ores in the presence of some oxidizing agents such as KMnO4 or H2O2 with H2SO4 was carried out. The leachability of some natural and man-made radionuclide from soils and sediments was subject to attack by various acid mixtures such as 6 M HCl, 8 M HNO3 and/or 8 M aqua regia at a temperature range 20–75 °C for the time intervals from 2 to 24 h. The data indicated that the use of H2SO4 resulted in 25% overall increase in the minerals dissolution (Benedik et al. 1999).

Recently, (Smirov et al. 2009) the kinetic features of underground uranium leaching from Russian ores of hydrogenous uranium deposits were explained. A dilute aqueous acidic ferric nitrate solution (0.002 M) was used to extract 97% of uranium (IV) from ores occurring in the Elliot Lake area of Canada at optimum leaching conditions which are 0.1 g/ml S/L phase ratio, at 75 °C for 24 h.

A comparative batch and counter-current acid leaching were conducted on low-grade Quirke Mine and high-grade complex Midwest Lake, Canada uranium ores with various oxidants. The leaching results indicate that uranium extraction is least affected by the kind of oxidant and type of leaching but counter-current leaching generally provides higher uranium extraction (Haque and Laliberte 1987). The leaching of uranium from raw phosphorite was studied using sodium carbonate leachate at high selective temperature (Ketzinel et al. 1984). An oxidative leaching of uranium from SIMFUEL using Na2CO3–H2O2 leachate was also proposed by Chung et al. (2010).

Several kinetic studies of UO22+ dissolution using different leaching agents with different oxidants were carried out (Amme et al. 2007; Sharma et al. 1996; Goff et al. 2008; Peper et al. 2004; Lee et al. 2009). The oxidation of Fe2+ in uranium leaching solutions with a gaseous system mixture of SO2 and air was studied (Umanskii and Klyushnikov 2011). The kinetic of leaching process of uranium from EL-Missikat shear zone eastern desert and El-Erediya rock, Sella rock (Khawassek et al. 2016a, b) and El-Hammamat sediment in Egypt were applied by sulfuric acid solution.

The mineral monazite is a naturally occurring rare earth phosphate in which uranium and thorium commonly substitute for the REES in the cation sub-lattice. A modified alkaline dissolution of Egyptian monazite was applied and it was found that uranium could be selectively leached by a mixture of sodium carbonate, sodium hydroxide and hydrogen peroxide leaving thorium and rare earth elements as an insoluble hydrous oxide (El-Nadi et al. 2005). The leaching of uranium from monazite specimens with carbonate–bicarbonate leachate was explained (Eyal and Olander 1990a, b, c).

In recent times, the determination of U(VI) and U(IV) in phosphate ores (El-Sebayia, Qatrani and Abu Tatur) through HCl selective leaching was suggested by Fouad (2010). The uranium recovery from a uranium-bearing phosphatic sandstone (phosphate reserves present in Qatrany area, south west of Cairo with a uranium assay of 0.048% U) was proposed using a heap leaching technique using Fe2(SO4)3/H2SO4 leachate (Shakir et al. 1992a, b).

Moreover, the bioleaching is an interesting branch of hydrometallurgy used for metal extraction from their ores using microorganisms. The microbial technique offers an economic alternative for the mining industry at a time the high-grade mineral resources are being depleted (Devasia and Natarajan 2004). The bioleaching treatment of Abu Zeneima uraniferous Gibbsite ore for uranium recovery was applied (Ibrahim and El-Sheikh 2011).

Furthermore, the dissolution of alkali metal urinates in carbonate melts under the influence of CO2 and O2 partial pressure was performed (Volkovich et al. 2000). Billard et al. (2007) studied the dissolution of UO2 and UO3 in the ionic liquid, 1-methyl-3-butyl imidazolium bis-triflimide (BumimTf2N) with the help of small amounts of HNO3 leading to the formation of UO2(NO3) −3 . The imidazolium-based Fe-containing ionic liquids (ILS) were directly dissolved UO22+ in the presence of their corresponding imidazolium chlorides without additional oxidants (Yao and Chu 2013). A comparative study on the dissolution rate of sintered (Th–U) O2 pellets in nitric acid by microwave and conventional heating was also reported (Singh et al. 2011). The basic features of the leachability of depleted uranium (DU) projectiles in soil were surveyed (Schimmack et al. 2005; Pulhani et al. 2007).

This study deals with the current state of uranium leaching from Abu Zeneima ferruginous siltstone, Southwestern Sinai, Egypt, using H2SO4 acid. The kinetic aspects of uranium leaching and the examination of the effects of the main system variables are investigated on the leaching rate, as well as the apparent activation energy for uranium (VI) and iron (III).

Experimental

Materials and methods

All reagents used in the experiments were prepared from analytical grade chemicals (Fluka, POCH S.A. Poland and Scharlau Chemie S.A. Spain).

Abu Zeneima ferruginous siltstone sample was obtained from Abu Zeneima area, Southwestern Sinai, Egypt. The rock was crushed, grinded and sieved with American Society for Testing and Materials (ASTM) standard sieves into four size fraction; 0.297, 0.149, 0.105 and 0.074 mm mesh size.

The chemical composition of Abu Zeneima ferruginous siltstone sample has been determined as major oxides by colorimetry technique and the traces by ICP-OES technique (Prodigy High Dispersion ICP, TExxLEDYNE-Leeman Labs USA). The quantitative analysis of uranium was carried out by a single beam spectrophotometer, Meterch Inc (SP-8001) using Arsenazo III indicator (Janberty et al. 2013) and confirmed by an oxidometric titration against ammonium metavanadate using N-phenyl anthranilic acid indicator by computerized titrator, SCOTT instrument, GmbH, Germany (Mathew et al. 2009). The mineralogical composition of the shale sample was examined using XRD, PHILIPS PW 3710/31 diffractometer, Scintillation counter, Cu-target tube and Ni filter at 40 kV and 30 mA. EDAX with scanning electron microscope (SEM) pictures is used to indicate the mineralogical state of the Abu Zeneima ferruginous siltstone ore and its solid residue after dissolution.

Experimental dissolution procedures

Experiments were carried out by agitation leaching using a covered 500-ml Pyrex flask and mechanically stirred with a magnetic stirring bar at 0–400 rpm. Typically and for each run, 50 ml of H2SO4 solution of a definite molarity was charged into the glass reactor and heated to the required temperatures. Thereafter, 5 g of the shale sample was added and the contents were well agitated. The concentration of H2SO4 which gave the maximum dissolution of uranium and iron was subsequently used for the optimization of the other dissolution parameters. The fraction of uranium and iron dissolved (X) was calculated from the initial difference in weight of the amounts dissolved and undissolved at various time intervals up to 60 min (Ayanda and Adekola 2012). After oven drying at 50 °C, The post-dissolution residual was scanned and analyzed by both EDAX and ICP-OES techniques.

Results and discussion

Characterization of Abu Zeneima ferruginous siltstone

The chemical composition of Abu Zeneima ferruginous siltstone ore and its post-dissolution solid residue were analyzed as major oxides and trace elements by ICP-OES (Table 1) and EDAX, SEM images (Fig. 1a, b). From the obtained data, it is clear that the concentration of Fe2O3 decreases from 18.03 to 0.55% (about 96.94% leachability) and uranium decreases from 498 to 90 mg/kg (about 81.92% leachability) using H2SO4 as leaching agent. The measurements were calculated as triplicates with relative standard deviation.

Table 1 Chemical composition of Abu Zeneima ferruginous siltstone ore sample and its post-dissolution solid residue
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Fig. 1
figure 1

SEM images and EDAX pattern of a Abu Zeneima ferruginous siltstone ore, b post-dissolution solid residue

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The EDAX pattern of the ore reveals high uranium and iron concentrations before dissolution, but after dissolution low concentration was found. Moreover, the micro-structure of the ore and its post-dissolution solid residue were determined by scanning electron microscope (SEM) which reveals a clear difference in surface morphology with coarse grains before dissolution (Fig. 1a), while a smooth grains and fine surface after dissolution (Fig. 1b) due to penetration effect of H2SO4 leaching agent.

On the other hand, the X-ray diffraction data reveal that the ore is dominated by Quartz (SiO2), Hematite (Fe2O3), Kaolinite (Al2Si2O5(OH)4), Gypsum (CaSO4.2H2O) and Dolomite (CaMg (CO3)2) minerals and there is no a definite uranium mineralization. Uranium was suggested to exist as a deposit altered and replaced by other elements. After dissolution with H2SO4, uranium almost vanished from the undissolved residue under optimized dissolution conditions. Also, the concentration of some trace elements in the dissolute ferruginous siltstone increased by the penetration action of H2SO4. Furthermore, hematite mineralization is responsible for the presence of iron in the ore which easily dissolved by sulfuric acid.

Uranium and iron dissolution studies

Temperature, H2SO4 concentration, particle size, stirring speed and solid to liquid phase ratio (S/L) are influential parameters affecting the dissolution of uranium and iron dissolution.

Effect of dissolution temperature

Temperature is a significant parameter especially in studying dissolution kinetics of uranium and iron from Abu Zeneima ferruginous siltstone ore. The fraction dissolved of uranium and iron was studied against dissolution time ranged between 15 and 60 min at different temperatures ranged between 25 and 90 °C. From Fig. 2a, b, it was observed that the fraction dissolved (X) of both uranium and iron increased with increasing both dissolution time and temperature. At 90 °C about 81.92% of uranium and 96.94% of iron were dissolved through 60 min. Hence, 60 min and 90 °C were used as optimum parameters for uranium and iron dissolution.

Fig. 2
figure 2

Effect of temperature on fraction dissolved of a uranium b iron conditions: 5 g ore, 50 ml of 0.8 M H2SO4, 0.074 mm size fraction, 400 rpm

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Effect of H2SO4 concentration

Sulfuric acid was chosen for special considerations of economic feasibility calculations, since it is cheap and available, but it has since proved to be non-selective as it dissolves uranium, iron and many impurities. Therefore, attention will then be given to studying the use of selective leaching agent or designing of extraction units to separate the two elements from each other.

The effect of H2SO4 concentration on uranium and iron dissolution rate from Abu Zeneima ore is explored in concentrations ranged between 0.2 and 0.8 M. In this study, other parameters were kept constant. From Fig. 3a, b, it was concluded that increasing H2SO4 concentration enhancing the uranium and iron fraction dissolves (X) attaining 0.8192 and 0.9694, respectively. Therefore, 0.8 M H2SO4 was chosen for optimum dissolution kinetics.

Fig. 3
figure 3

Effect of H2SO4 concentration on fraction dissolved of a uranium b iron conditions: 5 g ore, 90 °C, 0.074 mm size fraction, 400 rpm

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Effect of particle diameter

Particle diameter is an important factor which affects uranium–iron dissolution kinetics. Different particle diameter was examined in the range between 0.297, 0.149, 0.105 and 0.074 mm. The dissolution experiments were performed while other factors were kept constant. From Fig. 4a, b, it was found that the fraction dissolved (X) for both uranium and iron increased with decreasing the particle diameter due to the high surface area exposed to H2SO4 attack. Hence, 0.074 mm particle diameter was, therefore, chosen for further dissolution kinetics giving efficiency of 81.92% and 96.96% for uranium and iron, respectively.

Fig. 4
figure 4

Effect of particle diameter on fraction dissolved of a uranium b iron conditions: 5 g ore, 50 ml of 0.8 M H2SO4, 90 °C, 400 rpm

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Effect of solid to liquid phase ratio (S/L)

The effect of solid to liquid phase ratio (S/L) on the dissolution kinetics of uranium–iron was studied using ratios of 5/50, 6/50, 7/50 and 8/50 g/ml at constant optimized factors. As shown in Fig. 5a, b, it was found that increasing S/L ratio leads to decrease in uranium–iron dissolution rate as there is no enough H2SO4 concentration to complete the dissolution process. Hence 5/50 g/ml ratio was selected as optimum factor which gives the best uranium–iron dissolution rate.

Fig. 5
figure 5

Effect of S/L ratio on fraction dissolved of a uranium b iron conditions: 0.074 mm size fraction, 0.8 M H2SO4, 90 °C, 400 rpm

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Effect of stirring speed

The effect of stirring speed on uranium–iron dissolution kinetics using 0.8 M H2SO4 was studied at stirring speeds ranged between 100 and 400 rpm. Particle size of 0.074 mm, S/L ratio of 5/50 g/ml, and temperature at 90 °C were kept constant. According to the experimental data shown in Fig. 6a, b, it was observed that uranium–iron dissolution rate increases with increasing stirring speed. Hence, 400 rpm was chosen to give the best uranium and iron fractions dissolved.

Fig. 6
figure 6

Effect of stirring speed on fraction dissolved of a uranium b iron conditions: 0.074 mm size fraction, 0.8 M H2SO4, 90 °C, 5 g ore, 50 ml of 0.8 M H2SO4

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Dissolution kinetic analysis

In a fluid–solid reaction system, the reaction rate is generally controlled by one of the following steps: diffusion through the fluid film, diffusion through the ash (or solid product) layer on the particle surface or the chemical reaction at the surface of the core of reacted particles. There are three controlling models for the rate of reaction: chemical reaction at the particle surface, diffusion through the product layer and a combination of both. The rate of the process is controlled by the slowest of these sequential steps (Karatepo et al. 2016; Yildiz et al. 2017b).

The most important model suggested for derivation of the expression of the fluid–solid reaction is the shrinking core model (SCM) which is thought that the reaction takes place in the outer surface of the solid and this surface shrinks towards the center of the solid as the reaction proceeds leaving behind an inert solid layer, called ash layer, around the unreacted shrinking core (Wei et al. 2010; Dasdelen et al. 2017; Yildiz et al. 2016; Erken et al. 2015, 2016; Aday et al. 2016).

Considering that, a solid particle M is immersed and reacts with a fluid N as the following equation:

$$ N \, \left( {\text{fluid}} \right) \, + \, bM \, \left( {\text{solid}} \right) \to {\text{products}}. $$
(1)

If the reaction rate of the particle is controlled by diffusion of the fluid N through the ash layer, the time t required for a spherical solid to react can be calculated by the following equation:

$$ 1 - 3(1 - x)^{2/3} + 2(1 - x) = \frac{{6bDC_{ \circ } }}{{C_{B} r_{ \circ }^{2} }}t = K_{1} t, $$
(2)

where X is the uranium or iron fraction dissolved, t is the dissolution time (min), D is the diffusivity of uranium or iron ions through the ash layer (m2/s), \(C_{ \circ }\) is the concentration of the fluid outside the particle (mol/L), CB is the apparent concentration of the solid reactant (mol/L), \(r_{ \circ }\) is the initial outside radius of the particle (m), and K1 and K2 are the apparent rate constants.

If the reaction rate is controlled by chemical reaction, the integrated rate equation is expressed by the following equation:

$$ 1 - (1 - x)^{1/3} = \frac{{bk_{\text{d}} C_{ \circ } }}{{C_{\text{B}} r_{ \circ } }}t = K_{2} t, $$
(3)

where Kd is the chemical reaction rate constants. To explore the effect of temp., particle size, S/L ratio, H2SO4 conc. and stirring speed on the reaction kinetics of uranium and iron, a plot of 1 − (1 − x)1/3 against dissolution time at different dissolution parameters is established and shown in Figs. 7, 8.

Fig. 7
figure 7

Plot of 1 − (1 − x)1/3 against dissolution time at different parameter for uranium: a temperature; b H2SO4 concentration; c particle diameter; d S/L ratio; e stirring speed

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Fig. 8
figure 8

Plot of 11 − (1 − x)1/3 against dissolution time at different parameter for iron: a temperature; b H2SO4 concentration; c particle diameter; d S/L ratio; e stirring speed

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From the two shrinking core models examined, only Eq. (3) has been found to give a perfect straight line with a good correlation coefficient. Hence, all resulted data were found to fit the shrinking core model with chemical reaction model as the rate determining step.

The apparent rate constants K1 and K2 of uranium and iron for the two shrinking core models examined at different temperature were calculated from the slopes of the straight lines obtained from Figs. 7, 8. The values of K1 and K2 and their corresponding correlation coefficients are summarized in both Tables 2, 3.

Table 2 Apparent rate constants (K1, K2) and their correlation coefficients for uranium dissolution
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Table 3 Apparent rate constants (K1, K2) and their correlation coefficients for iron dissolution
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The apparent rate constant values of K2 were used to obtain the activation energy of the dissolution reaction (Ea) from the Arrhenius equation:

$$ \ln K = \frac{{ - E_{\text{a}} }}{R}\left( {\frac{1}{T}} \right) + \ln A, $$
(4)

where K is the apparent rate constant, Ea is the activation energy in kJ/mol, R is the molar gas constant in J/mol K and A is the Arrhenius constant. The plot of ln K against 1/T in Fig. 9a, b gives a slope that represents the activation energies 31.59 kJ/mol for uranium and 26.02 kJ/mol for iron. Hence, all resulted data are found to fit the shrinking core model with chemical reaction as a rate determining step (Yuan 2018; Luo et al. 2010; Li et al. 2011; Ekinci et al. 1998; Tavakoli et al. 2017). Also, shrinking core model (SCM) with a mixed chemical and diffusion control mechanisms was suggested in the dissolution kinetics of many ores (Souza et al. 2007; Silva 2004).

Fig. 9
figure 9

Arrhenius plot of a uranium and b iron kinetic analysis

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Conclusions

Based on results of mineralogical, dissolution, kinetic and thermodynamic investigations, it can be concluded that both H2SO4 and temperature have a significant effect on increasing uranium–iron dissolution from ferruginous siltstone ore. The dissolution rate also increases by increasing stirring speed and decreases by both particle diameter and S/L phase ratio. With 0.8 M H2SO4 and temperature of 90 °C using 0.075 mm particle diameter with 400 rpm, about 81.92% of uranium and 96.96% of iron are dissolved.

Studying the dissolution kinetics for both uranium and iron is very important as the two elements can be separated from each other if there is a great difference in the dissolution rate and activation energy. The dissolution kinetics and thermodynamics indicate that shrinking core model is applicable. The reaction mechanism for uranium and iron dissolution is based on chemical reaction as a rate determining step with activation energy of 31.59 kJ/mol and 26.02 kJ/mol for uranium and iron, respectively.

Therefore, I give an important information through the dissolution kinetic studies to the decision-makers in the Nuclear Materials Authority in Egypt to decide how to deal with this ore in the case of the use of sulfuric acid as a leaching agent which proved to be non-selective and that the activation energy needed to dissolve uranium and iron close together. Some will thus find the two elements with each other. Therefore, either the search for a selective leaching agent to separate uranium from iron or the design of selective extraction units to separate the two elements using different separation techniques were taken into consideration according to economic feasibility calculations.