Acidic heap leaching behavior of uranium from El-Sela area, South Eastern Desert, Egypt

The percolation technique was used to examine the leachability of uranium from El-Sela uranium mineralization. The agglomeration effect, particle size, flow rates, free acidity, solid/liquid ratio, and the ratio of cobblestone effect were all studied in the kinetics of uranium leaching. At a height of 3.0 m within the column, the optimal ratio of cobblestone to ore was found to be 1:4, and the irrigation rate was reduced from 0.15 to 0.1L/m2/min, increasing the leachability of uranium from 47.5 to 79%. Finally, column leaching is confirmed by fitting data from an application first-order kinetic law with data collected.


Introduction
Heap leaching is an industrial mining method that extracts precious metals by slowly percolating a leaching solution down into an ore heap. Temperature, flow rates, particle size distribution, and pores distribution, as well as ore mineralogy, all have an impact on this process. In order to achieve effective uranium heap leaching, the permeability of the stacked ore must be considered. Fine particles and clays are often the cause of clogging in heaps. This causes variable permeability, which can reduce the efficiency and homogeneity of the leaching process [1].
After leaching, however, freshly formed fine particles flow downward through the pore space among coarse particles and are deposited at local areas due to the combined action of multiple factors such as hydraulic energy, gravity, and chemical reactions, and the solution transports a specific amount of fine particles out of the reaction [2,3]. The permeability of the leach and even percolation of the lixiviant solution in the ore are the most critical aspects of heap leaching. Excessive fines formed during the leaching process clog natural flow channels and form impermeable layers within the heap, limiting lixiviant percolation and reducing permeability, which could have a negative effect on percolation, particularly over long periods of heap leaching. It can also regularly block the flow of solution through the heaps. This results in a weak contact between the ore and the leach solution, resulting in low metal recoveries or failure to recover any metal at all.
Fine mineral particle agglomeration is usually would not minimize or eliminate poor permeability issues caused by the fine particles which tend to enhance heap leaching performance [4][5][6]. This process allows adjusting the particle size distribution by gathering fine particles, using mostly capillary forces, cohesion forces between particles and adhesion forces (e.g. hydrogen bonds, electrostatic and Van der Waals forces). This process may be thanks to improving the heap permeability and to reducing fine particle migration during leaching [7][8][9].
Acid agitation and column leaching with sulphuric acid were used in many chemical experiments on El-Sela mineralization, followed by recovery using IRA-400 and D236B anion exchange resin [10,11]. Using a shrinking core model with diffusion control through a porous product layer, the experimental data from the leaching of uranium from El-Sella uranium ore with hydrochloric acid solution were well interpreted [12]. Column leaching experiments will be investigated in this study first to see if they can be used as a heap leaching technique.

Geology and mineralogy of El-Sela uranium mineralization
El-Sela region is situated in the southern part of Egypt's Eastern Desert, near the Sudan border, 22 kms Southwest of Abu-Ramad city as shown in Fig. 1. Many authors studied the area geologically, mineralogically, geochemically, and radiometrically as part of their study of the Eastern Desert's uranium potentiality in granites, concluding that El-Sela granite is a highly fractionated high-K calc-alkaline (HKCA) magma containing primary muscovite and occurring as a medium-sized granite pluton affected by high alteration [13]. They also discovered that uranium mineralization such as uranothorite, high U-monazite, and high U-zircon.
The geochemical data will determine the characteristics of the study uranium fertile granites. The uranium fertility depends essentially on the typology of their host granites. Peraluminous, high-K and low Na and Ca contents are the main indicators of the U-fertility of granites. Under certain conditions of alteration, the high-K calc alkaline (HKCA) granites can form uranium ore with less tonnage [15]. In mineral exploration, the alteration halos around ore bodies in altered zones are used as primary guides. Depending on the type of U-deposit, important alteration types accompanying uranium mineralization develop alteration minerals such as chlorite, albite, silicate minerals, sericite, clay, and illite [16]. On the other hand, the data of XRD pattern (Fig. 2) showing the peaks of goethite, cryptomelane and quartz as constituents of the uraniferous manganese oxide. According to geochemistry and mineralogy of El-Sela granite presented above, it's obvious that, the weakness of ore bonds and joints lead to be destroyed and moldered during crushing process.

Sampling
The rate of leaching is influenced by the particle size. To prepare the samples for agitation and column leaching tests, they are crushed into a different particle size. This particle size meets the requirement that the real particle diameter does not exceed 1/10 of the inside diameter of the column [17]. The size distributions of a representative sample are shown in Table 1.

Ore agglomeration procedure
Water and sulfuric acid were used to agglomerate the uranium mineralization at El-Sela. For the agglomeration experiment, an El-Sela uranium mineralization feed charge of 20.0 kg dry ore with a known amount of H 2 SO 4 binder was used. This loading is approximately 10-15% of the total effective drum size. Before being moved into the drum granulator, the powder was pre-mixed with a predetermined volume of acid in an acid resistance glass ware for 10 min. A cumulative batch time of 30 min was used for the majority of the agglomeration samples.

Agitation leaching experiment
Crushed to a grain size of approximately "-100 mesh" was a representative sample from the El-Sela field (− 0.15 mm). Because of the natural of chemical composition of the study sample, the acid agitation leaching technique was chosen for this study. Except for the study element, all of the leaching experiments were carried out under the same general conditions: 250 gm sample, 50 gm/l acid concentration, 1/2 S/L ratio, room temperature (25 °C), and a 6 h contact time. The uranium content of the final agitation leaching residue was also determined.

Column experiments
To determine the optimum leaching parameters for the treatment of the El-Sela samples, a series of column tests were conducted. For the first part of these experiments, small PVC columns (100 mm internal diameter and 1500 mm height) were used (agglomeration and non-agglomeration testes). In the column leaching apparatus mentioned above,  silicate sand is put at the bottom of column as filter layer. According to the required weight and grain size, test samples are prepared and blended well, then put into the column. The column was flushed with fresh water before the acid leaching to remove any soluble phases that may have developed during sampling and storage. Sulfuric acid solution is used as lixiviant in the experiment and leaching solution was delivered to each column using plastic container and a peristaltic pump. Columns are run for 16 days or until very low uranium concentrations appear in the pregnant solution, after which experiments are run to see what impact the following parameters on uranium leaching, as ore particle size, application flow rate, lixiviant acid concentrations and consumption.
The second part of this work to study the effect of cobblestone addition in which, cobblestone of the same size of the ore sample was distributed along four columns (A, B, C, and D) as sheets to allow the resulted fines to agglomerate on it, where the ratio of 1/3, 1/4, 1/5, and ore without cobblestone were tested. Three different flow rates were testing (one for every column) 0.15, 0.10, and 0.05 L/m 2 /min. Samples from pregnant leach liquor ware taken, and their uranium content, pH, and other impurities are analyzed to determine if the effluent should be recycled as intermediated leaching solutions (ILS) or transferred to recovery circles pregnant leaching solution (PLS). Leaching can be steeped if the value of the extracted uranium is less than the cost of operation. Of course, if a technical indicator like leaching efficiency is the only thing that matters, then leaching can continue.

Analytical method
Under different conditions, uranium was analysed in the corresponding solutions using Arsenazo III reagent [18]. A Lambada UV/VIS spectrophotometer (Perkin-Elmer, USA) was used for this purpose. Prior to titration, uranium was analysed using an oxidimetric titration process against ammonium meta vanadate in the presence of diphenylamine sulfonate indicator; proper uranium reduction was accomplished using ferrous sulphate [19].

Chemical composition of El-Sela uranium mineralization
The chemical composition of studied uranium mineralization sample was found sufficiently simple assay is low in CaO (0.6%) and MgO (1.1%) as shown in Table 2. In addition, the silicic gangue minerals (feldspars and quartz) equivalents to 71.6% SiO 2 are frequently predominating and which are practically inert to acid attack. The other major oxides; namely Al 2 O 3 , Na 2 O and K 2 O assaying 15.15, 0.07 and 1.08% respectively are mainly present in the feldspar minerals. The presences of Fe 2 O 3 and MnO in relatively high percent (5.37 and 0.75 respectively) enhance the uranium dissolution, since act as an oxidizing agent during the leaching process. In other hand, the presence of P 2 O 5 have harmful effect during leaching if pH exceed 2.0.

Agitation leaching tests
El-Sela mineralized samples with 700 ppm U were subjected to uranium agitation leaching in the lab under a variety of conditions to determine the optimum conditions that lead to high uranium leaching results. The following conditions were used to test cost savings as well as high kinetics: 50 g/l sulfuric acid concentration, 6.0 h as agitation time, ore size of − 0.15 mm, solid/liquid ratio of 1/2, and room temperature [11].

Results of column leaching (percolation)
Uranium dissolves in three stages during column leaching studies. The first release results when the column is flushed with water. During the acid leaching of the column, two further uranium releases occur: a rapid dissolution as soon as the pH decreases, followed by a slower dissolution at the end of the leaching process.

Effect of agglomeration
That process is the pre-mixing of the operated sample with lixivant solutions at atmosphere temperature as shown in Fig. 3. It improves the permeability of the ore [20]. In some times, agglomerates can be made stronger by adding a binder to the water [21]. Then agglomerates weathered for maturation a time period ranging from 14 to above 300 h [22]. Two kilograms of crushed ore have been agglomerated during 5.0 min with a L/S ratio of 1/10. In addition, a ratio of 20 kg/t has been used for sulfuric acid as binder. As shown, in Fig. 4, the agglomeration processes enhance the leaching kinetics and improve its dissolution performance at the first leaching period. The agglomerated sample gives the highest dissolution rate of uranium (45.5%) in a shorter time (11 days) than the non-agglomerated sample (40.2% uranium dissolution in 16 days). With time the leaching rate of agglomerated ore will resemble the behavior of non-agglomerated ore, which owing to the leaching resistance increases with time.

Effect of grain size
Three different particle sizes of El-Sela uranium mineralization have been studied; namely, − 40, − 20 and − 10 mm. As shown in Fig. 5, it's noticed that, the overlap between the leaching curves which owing to the alteration induced by acidic leach solution. Since, dissolve the oxides filling joints and fractures which lead to reducing the particle size of ore, in addition to the swelling of fine particles induced during acid leaching with time.

Effect of free acidity
The mineralogical and chemical composition of mineralized samples plays an essential role in the free acidity of leach solutions and its consumption. Acid concentrations range derived from batch experiment; about 30-50 g/L was tested in this experiment as shown in Fig. 6. As the acidity of lixiviant solutions increase, it's the redox state increase that leads to the solubility of uranium is increased indeed dissolves iron oxides, which enhances recovery efficiency. On another side, increasing acidity of the lixiviant solution (30-50 g/L of H 2 SO 4 ) leads to higher silicate dissolution which has a bad effect on leaching performance and acid consumption. Sulfuric acid concentration leads to greater acid consumption and volume of lixiviant to be handled, essentially due to stronger silicate dissolution and secondary precipitation.
Predicting chemical reactions and reagent use during the leaching process can be achieved using the pH values obtained from pregnant leach solutions. Figure 7 represents the transition in pH values of the pregnant leach solution over time. The sulfuric acid adsorbs on the mineralized sample for a short period of time, resulting in a pregnant leach solution with a high pH, which gradually decreases with time.
In general, the recycling of the intermediate pregnant solution (IPS) has little change in the acid consumption and the lixiviant volume. When the IPS is recycled after adjusting the pH, the uranium concentration was increased due to its higher Fe 3+ content which accelerate the uranium dissolution.

Effect of flow rate, L/m 2 /min
The flow rate of the solution also influences uranium recovery. As shown in Fig. 8, the flow rate shows little effect on uranium recovery due to the decrease in permeability induced with time, which come from the swelling of clay minerals which lead to decrease pores between grains. The maximum uranium dissolution after 15 days leaching at 0.15 L/m 2 /h flow rate was 48.5%.

Effect of clay minerals on the permeability of solution during heap leaching
There are some disadvantages to the leaching process due to the high percentage of clay minerals present, including the slowing down of the process, the appropriate porosity requirement of the leachate ore, and the weak attack of acidic solutions on refractory ores. Clay swelling has a detrimental impact on the porosity and permeability of the solution. Clay minerals can also adsorb positively charged metals onto their surfaces, as well as adsorb metals by interlayer substitutions.
The initial crack network in the particles increases during the leaching process, which splits every single particle into two or more new ore particles and produces a particle size distribution different from the initial reactor feed [19]. Excessive amounts of the fines generated during the leaching process and their mobilization would result in reduced permeability as the leaching process progresses, which could have a negative effect on percolation especially over long periods of heap leaching operation. It would also prevent a uniform flow of the solution through the heaps which leads to poor interaction between the ore and leach solution, producing inadequate metal recoveries, or the need to extend the leach time.

Improvement of the leaching efficiency of uranium
Since the leaching effect splits the first agglomeration phase over time, we must solve the problem when leaching. Since fines that split during leaching can clog the heap again, we need to insert small sheets of strong cobblestones along the column to agglomerate on them and keep the kinetics stable during leaching, as well as choose the appropriate flow rate to spray over the ore, as well as the height of the ore within the column, which gives the highest percentage of extraction in the presence of fines.

Effect of cobblestone ratio addition on uranium extraction
The cobblestone was distributed as sheets along with four columns (A, B, C, and D) of the same size as the ore sample to allow the fines to agglomerate on it. The ratio of cobblestone to ore sample in leaching columns was as follows: A = none, B = 1/5, C = 1/4, and D = 1/3. As shown in Fig. 9, sand sheets have a positive effect on leaching efficiency; because dissolution increases as the sand ratio increases due to increased porosity until the ratio of 1/4 sand/ore ratio, leaching efficiency tends to decrease at column D (1/3) ratio due to the disproportion between irrigation flow rate and produced permeability. It's noticed that at the ratio of 1:4 sand (cobblestone) to ore the permeability increased uranium leachability from 48.5 to 73% in 16 day leaching period at flow rate of 0.15L/m 2 /min.

Effect of irrigation flow rate after cobblestone addition
As we explained previously, there is no effect of the flow rate on the rate of uranium extraction before adding sand, due to the decrease in permeability induced with time, come from the swelling of clay minerals which lead to decrease pores between grains. In this part, we are exposed to the effect of the flow rate on uranium extraction after adding sand by 1/4. Three different flow rates were testing (one for every column) 0.15, 0.10, and 0.05 L/m 2 /min. The results show that an increase in the irrigation rate leads to a decrease in the permeability and the uranium recovery reach to 70% in 17 days at an irrigation rate of (0.15 L/m 2 /min) compared to a recovery of 77.3% at an irrigation rate of (0.1L /m 2 /min) for the 20 day leaching period as shown in Fig. 10. This low rate prevents the accumulation of solutions on the heap surface and allows free air entry through the heap.

Effect of column height after cobblestone addition
In a previous study, the effect of the ore height in the column dissolution rate of uranium from El-Sella ores was studied, and the results indicated that the lowest uranium extraction was obtained at 3.0 m column height without treatment or addition of permeable materials [11]. In order to determine the effect of ore height using the above cobblestone addition and 0.1 L/m 2 / min scheme. Three column tests were conducted with (2.0, 3.0, and 4.0 m) ore height. Figure 11 demonstrates that uranium dissolution increased as the column height decreases. Therefore, the lowest uranium dissolution was achieved in the 4 m high column. While in the case of 2 and 3 m, the process of dissolving uranium is very close, which is about 79% for the 20 day leaching period.

Verification of kinetic model
The data of leaching experiments were discussed with two different approaches of the first-order kinetic model. As presented in the following equations; X t = X ∞ 1 − e kt) as a derivative of X t = X o e kt first order law where X is uranium leached fraction at Time t, X ∞ = maximum uranium leached fraction at t = ∞ X o is the initial uranium fraction at, k is rate constant and Time t, day [23,24].
The results of the test were evaluated with a general time dependent kinetic model and a modified kinetic model based on the ratio of the reagent consumption volume per mass of the initial valuable uranium.
As shown in Fig. 12, the column C (with sand cobblestone) gives linear relation between uranium leached fractions than column A (without sand cobblestone). From above Figure the rate constants for two columns has determined then used in calculation to predicted data on basis of above first order law.
As shown in Fig. 13 (A, C), there are the large deviation between experimental and calculated data in column A.On the contrary, in column C, there are a closeness between them with linear relationship between leached values. Thus, the column C with cobblestone show good leaching behavior in comparison with column A which appear bad behavior on column leaching.

Conclusion
Acid leaching of uranium from El-Sela low-grade uranium mineralization induces alteration by dissolving the oxides filling joints and fractures which lead to reducing the particle size of ore, in addition to the swelling of fine particles induced during acid leaching with time. Improved permeability with the addition of cobblestone would lead to increased permeability and leaching efficiency to 79%. Experiment data was fitted with a first-order kinetic model and also modeled based on the reagent consumption per mass of the initial valuable species. Both approaches of the first order kinetic model provide a good agreement with the column test data, which benefits the process planning of uranium heap leaching.