Kinetics Leaching Process of Uranium Ions from El ...

International Journal of Nuclear Energy Science and Engineering, Volume 6, 2016 doi: 10.14355/ijnese.2016.06.004

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Kinetics Leaching Process of Uranium Ions from El-Erediya Rock by Sulfuric Acid Solution Y. M. Khawassek2, A.A. Eliwa E.2, A. Haggag2, Saad A. Mohamed1, S. A. Omar2 Faculty of Science - Ain Shams University

1

Nuclear Materials Authority, P.O. Box 530, El Maadi, Cairo, Egypt

2

Abstract The leaching process of uranium (VI) from El-Erediya rock sample in sulfuric acid solution using hydrogen peroxide as an oxidant was investigated. The leaching conditions temperature, hydrogen peroxide concentration, sulfuric acid concentration; contact time, particle size, solid-liquid ratio and agitation rate were studied. The optimum process operating parameters were ore particle size range (100-63 µ m), sulfuric acid concentration (1.5M), contact time (120 min), the solid-liquid ratio (1:3), H2O2 concentration (1 M) and agitation rate (600 rpm) at room temperature. The leaching efficiency of uranium(VI) was about 95.2%. The experimental kinetic leaching data were well interpreted with a shrinking core model with diffusion control through a porous product layer. The leaching process followed the kinetic model: 1-3 (1-X) 2/3 + 2 (1-X) = k1t with an apparent activation energy of 25.02kJ/mole. Keywords Leaching; Uranium; Shrinking Core Model; Kinetics; Diffusion

Introduction Uranium mineralization was studied on the El Erediya area which is located in the Egyptian Eastern Desert, and it is one of the most important uranium occurrences in Egypt that it is bounded by latitudes 26º18`30`` - 26º23`30``N and longitudes 33º27`23``- 33º29`45``E. This Mineralization is structurally controlled and associated with asteroid veins that are hosted by a granitic pluton. This granite exhibits extensive alteration, including silicification, argillization, sericitization, chloritization, carbonatization, and hematization. The primary uranium mineral is pitchblende, while the uranpyrochlore, uranophane, and kasolite are the most abundant secondary uranium minerals [1-5]. Recovery of metal values from the ores includes three main processes, namely the physical upgrading, leaching and finally the metals recovery then purification with a lot of chemical treatments through the extraction of metal ions from the obtained solution. Leaching (or solid extraction) is defined as the hydrometallurgical process which is used to dissolve valuable matter from its mixture with an insoluble solid by means of a suitable reagent. In the case of uranium ore material, uranium exists mainly in the hexavalent or tetravalent states. The hexavalent uranium oxide (UO3) is the main constituent in secondary uranium minerals and may be considered as the amphoteric uranyl oxide [(UO2) O] which is capable of forming salts with both acid and alkaline reagents [6]. Simple oxides and the other primary or all of the secondary uranium ores are amenable to both acid and alkaline leaching but not easily so in either dilute acids or alkalis. On the other hand, tetravalent uranium oxide (UO2) is the basic constituent in primary uranium minerals and is soluble in strong acids but not easily so in either dilute acids or alkaline generally refractory requiring strong acid concentrations or rather severe conditions for their dissolution [7]. Kinetics of leaching for different metals ions from different ores were recently discussed such as batch leaching of uranium ore in Canada [8], oxidative leaching of molybdenum-uranium ore in wadiSikait, Egypt [9], dissolution of nickel from lateritic nickel ore in Eskisehir region of Turkey [10], column leaching of lanthanides from Abu Tartur

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International Journal of Nuclear Energy Science and Engineering, Volume 6, 2016

phosphate ore in Egypt [11], leaching process of El-sella uranium ore in Egypt [12], dissolution of total gold from Ijero-ekiti (Nigeria) gold ore deposit [13], leaching of TiO2 from Egyptian ilmenite [14], leaching of nickel and cobalt from Chinese laterite ore [15]. The leaching of cobalt and copper have been leached successfully by HCl from Co-Cu ores in the Democratic Republic of Congo [16] and phosphorus leaching from high phosphorus iron ores in China [17]. The aim of this work is to investigate a simple leaching process of uranium ore material from El-Erediya area, Eastern Desert, Egypt using H2SO4 acid in the presence of H2O2 as an oxidant. This paper considers the kinetic aspects of uranium leaching, the examination of the effects of the main system variables on the leaching rate, as well as the determination of the kinetic model and the apparent activation energy. Experimental Characterization of the Uranium Working Sample Samples were collected from El-Erediya area, Eastern Desert, Egypt. The mineralization is considered mainly as uranium ore materials which are also associated with other economic minerals such as REE and other elements. A series of experiments was carried out upon 5g sample portion of ore. After each experiment, the leach slurry was filtered, washed thoroughly with hot distilled water and then both filtration and washing were made up to 50 ml. Uranium leaching efficiency was calculated according to the following equation: ?Leaching efficiency of Uranium , % 

Leached Metal ion Conc . x 100 Original Metal ion Conc .

Analytical Procedures and Instrumentation The El-Erediyaore materialwas analyzed for its major and minor elements using the reported methods and the results are shown in the table 1. TABLE 1 CHEMICAL ANALYSIS OF EL-EREDIYAORE MATERIAL

M. Oxides

%

T. Element

Ppm

SiO2

74

U

1250

Al2O3

14.34

∑REEs

1500

Fe2O3

2

Zn

400

P2O5

0.78

Pb

870

CaO

0.56

Cd

92

MgO

0.4

Cu

100

Na2O

1.87

Ni

100

K2O

2.54

Nb

340

MnO

0.05

Sr

194

TiO2

0.96

Zr

420

L.O.I**

1.7

Th

38

Total

99.29

Ba

400

L.O.I**Total loss in ignition



Generally, the samples used in this work were weighed using an analytical balance produced by Shimadzu (AY 220).

 

Hot plate magnetic stirrer model Fisher Scientific. The hydrogen ion concentration of the different solutions was measured accurately using the pH- meter model (HAANA pH-mV-temp). The quantitative analysis of uranium was carried out by UV-spectrophotometer “single beam multi-cellspositions model SP-8001”, MetretechInc., version 1.02 using Arsenazo III indicator (Sigma-Aldrich) [18]and confirmed by an oxidimetric titration against ammonium metavanadate using N-phenyl anthranilic acid indicator (Sigma-Aldrich) [19, 20].



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Results and Discussion Leaching Results 1)

Effect of Acid Type

All mineral acids can dissolve uranium under reasonable conditions with different efficiency. The effect of the acid type upon uranium dissolution efficiency from the ore material was tested using 1.5 M of H2SO4, HNO3and HClindividually under the following conditions: 1/3 solid/liquid ratio, 149-100 µ m ore particle size range, stirring rate 700 rpm at room temperature and agitated for 240 min. Under these mentioned conditions, the uranium leaching efficiency was52%, 40%and 46% of H2SO4, HNO3and HCl, respectively.From the obtained results indicated in table 2, it is clear that all the mineral acids can dissolve U from the ore material with different efficiencies. The use of H2SO4 attained higher dissolution efficiency of 52 % for uranium than the other two acids. Therefore, the cheapest acid which is the sulfuric acid is the preferred choice for other experiments of El-Erediyaore material leaching process to follow the behavior of other parameters. TABLE 2 EFFECT OF ACID TYPE ON LEACHING EFFICIENCY OF U (VI)

2)

Acid type

U. Leaching Efficiency, %

H2SO4

52

HNO3

40

HCl

46

Effect of H2SO4 Concentration

A series of leaching experiments was carried out using different H2SO4 concentrations (0.25 to 2.5 M). The other leaching conditions were kept constantat the ore particle size of 149-100 µ m, thesolid/liquid ratio of 1:3, the contact time of 240 min and the stirring rate of 700 rpm at 25°C. The leaching efficiencyis shown in Figure (1), it is clear that the H2SO4 concentration increased from 0.25M to 2.5M, the uranium dissolution efficiency increased from 24% to 56%. However, 1.5M H2SO4which was the concentration choice applied in thesubsequent leaching experiments achievd the highest dissolution efficiency.

Leaching Efficiency, %

60

40

20

0 0

0.5

1

1.5

2

2.5

3

H2SO4 concentration, M FIG. 1 EFFECT OF H2SO4 CONCENTRATION ON THE LEACHING EFFICIENCY OF URANIUM (ORE PARTICLE SIZE 149-100 µ M, SOLID/LIQUID RATIO 1:3, STIRRING SPEED 700 RPM, CONTACT TIME 240 MIN AT 25°C)

3)

Effect of Contact Time

In these experiments, different leaching times (15 to 300 min) weretested. The other leaching conditions were kept fixed, namely; 149-100 µ more grain size, 1.5M H2SO4, 700 rpm stirring speed, the solid/ liquid ratio of 1:3 at 250C. For a leaching time of 120 min, the leaching efficiency of uranium reached 57.6%. As the leaching time was extended, the leaching efficiency of uranium did not increase. Therefore, it can be concluded that 120 min contact time represents the preferred condition for the subsequent uranium ions dissolution experiments. The result is shown in Figure (2).

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Leaching Efficiency, %

70

50

30

10 0

100

200

300

400

Contact time, min. FIG. 2 EFFECT OF CONTACT TIME ON THE LEACHING EFFICIENCY OF URANIUM (ORE PARTICLE SIZE 149-100 µ M, 1.5M H2SO4, SOLID/LIQUID RATIO 1:3, STIRRING RATE 700 RPMAT 25°C)

4)

Effect of H2O2 Concentration

The uranium minerals presented in the working sample need to be oxidized before dissolving because they are in oxidative states that cannot be dissolved under normal conditions. For this reason, H2O2 was chosen as a strong oxidizing agent [21].It has been shown that hydrogen peroxide under acidic conditions can oxidize the low valence states of uranium to high valence states which are readily soluble in the leaching solution. In order to evaluate the effect of H2O2, a series of leaching experiments wascarried out using 1.5M H2SO4. These experiments were performed in the absence and the presence of different concentrations of H2O2 varying from 0.1M to 1M. The other leaching conditions were fixed at: a solid/liquid ratio of 1:3, contact time of 120 min, stirring rate of 600 rpm, 149-100 µ m ore grain size at 25°C. The result is shown in Figure (3). About 57.6% of uranium dissolved in the absence of H2O2. As the milliliters H2O2 added increased the leaching efficiency of uranium increased to 95.2%. Therefore, 1M H2O2ore represents the preferred condition for dissolution experiments. Leaching Efficiency, %

100

80

60

40 0

0.2

0.4

0.6

0.8

1

1.2

Hydrogen peroxide Concentration, Molar

FIG. 3 EFFECT OF THE H2O2 CONCENTRATION ON THE LEACHING EFFICIENCY OF URANIUM (PARTICLE SIZE 149-100, 1.5M H2SO4, SOLID/LIQUID RATIO 1:3, STIRRING RATE 700 RPM, CONTACT TIME 120 MIN AT 25°C)

5)

Effect of Particle Size

The effect of particle size on the leaching of uranium was studied using six different size fractions, namely,400μm, 300-149 μm,149-100 μm, 100-63 μm,63-32μm and 32μm. It was found thatthe leaching efficiency of uranium increased as the particle size of the working solid sample decreased. When a particle size of 400 μm was used, the leaching efficiency of uranium sharply decreased to 52%. As a result, the fraction of the smallest particle size 100-63 μm gave the highest dissolution result. This is due to the highest surface area of the smallest particle size fraction; the conversion rates are inversely correlated with an average initial diameter of the particles. The results are shown in Figure (4).

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FIG. 4 EFFECT OF PARTICLE SIZE ON THE LEACHING EFFICIENCY OF URANIUM (1.5M H2SO4, 1M H2O2, CONTACT TIME 120 MIN, SOLID/LIQUID 1:3, STIRRING RATE 700 RPM AT 25°C)

6)

Effect of Temperature

Leaching experiments were carried out at a room temperature, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C and 100°C using the same conditions. The resulting leaching efficiency is shown in Figure (5) and indicates that the temperature did not play a critical role in the leaching of uranium. For example, when working at room temperature, the obtained leaching efficiency for uranium was 95.2%. By increasing the temperature from 30°C to 50°C, the uranium leaching efficiency gradually increased from 96% to 97.6%. A further increase in temperature to 100°C, gave uranium leaching efficiency of 98.4%. Therefore, it can be concluded that higher leaching temperatures result in higher leaching efficiency. On the other hand, increasing the temperature enhances the solubility of the undesirable impurities such as sulfates, arsenide, silicates, chlorites, clays and phosphates [22]; moreover, the cost of the leaching step increases.

Leaching Efficiency,%

100

95

90 20

40

60

80

100

120

Temperature, °C FIG. 5 EFFECT OF TEMPERATUREON THE LEACHING EFFICIENCY OF URANIUM (1.5M H2SO4, 1M H2O2, CONTACT TIME 120 MIN, SOLID/LIQUID 1:3, THE PARTICLE SIZE100-63 ΜM, STIRRING RATE 700 RPM).

However, the leaching step can be conducted at ambient temperature economically through the heat of dilution provided by the addition of the required amount of the concentrated sulfuric acid directly to the slurry mixture of the material and the water. 7)

Effect of Solid/Liquid Ratio

Working with fixed concentrations of 1.5 M H2SO4, the effect of six other solid/liquid ratios (1:1, 1:1.5, 1:2, 1:2.5, 1:3 and 1:3.5) were tested under the same leaching conditions used for the particle size of 100-63 μm. The results are shown in Figure (6). The leaching efficiency of uranium increased at solid/liquid ratio of 1:1to 1:3.5. 8)

Effect of Stirring Rate

The effect of the stirring rate was studied using conditions 100-63 μmparticle size, 1.5M H2SO4, 1 M H2O2, 1:3 solid/ liquid ratio at 25°C for 120 min. Stirring rate of 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm and 700 rpm were examined.

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FIG. 6 EFFECT OF SOLID/LIQUID RATIO ON THE LEACHING EFFICIENCY OF URANIUM (THE PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, CONTACT TIME 120 MIN, STIRRING RATE 700 AT 25°C)

Leaching Efficiency,%

100 90 80 70 60 50 40 0

200

400

600

800

rpm FIG. 7 EFFECT STIRRING RATE ON THE LEACHING EFFICIENCY OF URANIUM (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 120 MIN AT 25°C)

The leaching rates for uranium increased as the stirring rate increased to 600 rpm reaching 98.4% leaching efficiency and then remained almost constantlyabove 600 rpm. Therefore, the preferred speed was 600 rpm which was used for all the subsequent tests. The results are shown in Figure (7). 3.2 Leaching kinetics of Uranium 1)

Effect of Temperature

Figure (8) presents the effect of the reaction temperature on the uranium leaching rate in the range of 25°C– 60°C under conditions of 100-63 μm particles, 1.5M H2SO4, 1 M H2O2 with a 1:3 solid/liquid ratio. The results show that the leaching rate of uranium increases as the temperature increases. In order to obtain the kinetic equation and the apparent activation energy for the dissolution of uranium in the presence of H2O2, the experimental data in Figure (8) were correlated to various kinetic models for solid-liquid reactions. Several equations were studied including [22, 23]: 1-3 (1-X)2/3 + 2 (1-X) = k1t,

(1)

1 - (1 - X)1/3= k2t,

(2)

X = k3t,

(3)

wherek1, k2and k3 are the apparent reaction rate constants (min–1) for each case respectively and t is the leaching time (min) and X is the fraction reacted expressed as X = % Leaching/100

(4)

By plotting the above equations, it was noticed that the data did not fit Eqs. (2) or (3). The best fit with the data from 0 min to 60 min was for Eq. (1), which can be expressed asdiffusion controlled kinetic equation. The relationships between 1-3 (1-X) 2/3 + 2 (1-X) and the leaching time for uranium at various temperature are plotted in Figure (9).

40

Fraction of U dissolved (X)


1.2

Film Diffusionat 25°C Film Diffusionat 40°C Film Diffusionat 60°C

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Film Diffusionat 30°C Film Diffusion at 50°C

0.8

0.4

0 0

20

40 Time, min

60

80

FIG. 8 EFFECT OF DIFFERENT TEMPERATURES ON THE URANIUM LEACHING EFFICIENCY (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN) Particle Diffusion at 25 °C Particle Diffusion 40 °C Particle Diffusion 60 °C

Particle Diffusion at 30 °C Particle Diffusion at 50 °C y = 0.0137x R² = 0.9962

1-3(1-X)2/3+2(-X)

1 0.8

y = 0.0110x R² = 0.9954

0.6

y = 0.0083x R² = 0.9936

0.4

y = 0.0060x R² = 0.9949 y = 0.0046x R² = 0.9835

0.2 0 0

20

40 Time, min

60

80

FIG. 9 RELATIONSHIP BETWEEN 1-3 (1-X) 2/3 + 2 (1-X) AND LEACHING TIME FOR URANIUM AT VARIOUS TEMPERATURES (ORE PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN)

TheR-squared values for all the lines are greater than 0.9. This indicates that the linear relationship is significant and suggests that the leaching rate of uranium is diffusion controlled. The apparent activation energy was determined from the Arrhenius equation [24]: ln k = ln A– Ea/RT

(5)

wherek is the reaction rate constant, A is the frequency factor, Ea is the apparent activation energy and R is the ideal gas constant. The data for the fivetemperatures are plotted and given in Figure (10), and the regression analysis for these plots also shows that the linear relationship is significant. The apparent activation energy (Ea) was determined to be25.02 KJ/mol, whichsuggests a diffusion controlled process for El-Erediyalow-grade uranium ore material. 0 y = -3.0096x + 4.7885 R² = 0.99 ln(Kd)

-2

-4

-6 2.9

3

3.1

1000/T

3.2

3.3

3.4

FIG. 10 ARRHENIUS PLOT FOR URANIUM LEACHING (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN)

The different values of the apparent rate constants k1 and k2 at different temperatures and their corresponding correlation coefficient rate are summarized in table 3).

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TABLE 3 THE VALUE OF THE APPARENT RATE CONSTANTS, K1 ANDK2, MIN-1 WITH THE CORRELATION COEFFICIENTAT DIFFERENT TEMPERATURE RANGE

Apparent K2, min-1 0.000058 0.000069 0.000091 0.000108 0.000127

T, °C 25 30 40 50 60

2)

R2 0.757 0.702 0.694 0.725 0. 759

Apparent K1, min-1 0.0046 0.0060 0.0083 0.0110 0.0137

R2 0.983 0.994 0.993 0.995 0.996

Effect of H2SO4 Concentration

The effect of the H2SO4 concentration was studied from 0.25 to 1.5 M on the uranium leaching rate in the presence of H2O2 with a 1:3 solid/liquid ratio at 25°C. There is a general increase in the leaching rate as the H2SO4 concentration increases. The corresponding plots of 1-3 (1-X)2/3 + 2 (1-X) versus time at various concentrations are graphed in Figure (11). It can be seen that an initial H2SO4 concentration of 1.5 M is necessary to obtain a high dissolution rate of uranium. 0.25 molar

0.5 molar

0.75 molar

1 molar

1-3(1-X)2/3+2(1-X)

0.0025

1.5 molar y = 7E-05x R²= 0.9969

0.002

y = 6E-05x R²= 0.9967

0.0015

y = 4E-05x R²= 0.9909

0.001 y = 3E-05x R²= 0.998

0.0005

y = 2E-05x R²= 0.998

0 0

10

20 Time,min

30

40

FIG.11 RELATIONSHIP BETWEEN 1-3 (1-X) 2/3 + 2 (1-X) AND LEACHING TIME FOR URANIUM LEACHING AT VARIOUS H2SO4 CONCENTRATION (PARTICLE SIZE 100-63 ΜM, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 30 MIN AT 25 °C)

In order to obtain the reaction order for the total H2SO4 concentration, log-log plots of the rate constants versus, the total H2SO4 concentration are plotted and given in Figure (12). The slope of the line or the reaction order of the total H2SO4 concentration is 0.8. Hence, the leaching rate of uranium strongly depends on the acid concentration. -3

y = 0.8001x - 4.2702 R²= 0.9844

-3.5

log K

-4 -4.5 -5 -5.5 -6 -0.8

-0.6

-0.4

-0.2

0

0.2

0.4

logMH2SO4 FIG. 12 LOG-LOG PLOT OF THE RATE CONSTANT VERSUSH2SO4 CONCENTRATION (PARTICLE SIZE 100-63 ΜM, 1M H2O2, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 30 MIN AT 25 °C)

3)

Effect of H2O2 Concentration

The effect of the H2O2 concentration was studied from 0.2 to 1M on the uranium leaching rate with 1:3 solid/liquid ratio at 25 °C. There is a general increase in the leaching rate as the H2O2 concentration increases.

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The corresponding plots of 1-3 (1-X)2/3 + 2 (1-X) versus time at various concentrations are graphed in Figure (13). It can be seen that an initial H2O2 concentration of 1 M is necessary to obtain a high dissolution rate of uranium.

1-3(1-X)2/3+2(1-X)

0.2 molar

0.4 molar

0.6 molar

0.8 molar

1 molar

0.0025

y = 3.30E-05x R²= 0.9933

0.002

y = 2.77E-05x R²= 0.9958

0.0015

y = 2.411E-05x R²= 0.9968

0.001

y = 1.44E-05x R²= 0.9953

0.0005

y = 8.7E-06x R²= 0.9953

0

0

20

40 Time,min

60

80

FIG.13 RELATIONSHIP BETWEEN1-3 (1-X) 2/3 + 2 (1-X) AND LEACHING TIME FOR URANIUM LEACHING AT VARIOUS H2O2 CONCENTRATION (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN AT 25°C)

In order to obtain the reaction order for the total H2O2 concentration, log-log plots of the rate constants versus the total H2O2 concentration are plotted and given in Figure (14). The slope of the line, or the reaction order of the total H2O2 concentration, is 0.85. Hence, the leaching rate of uranium strongly depends on the H2O2 concentration. -3 y = 0.8474x - 4.4704 R²= 0.9873

-3.5

log K

-4 -4.5 -5 -5.5 -6 -0.8

-0.6

-0.4 -0.2 logMH2O2

0

0.2

FIG. 14 LOG-LOG PLOT OF THE RATE CONSTANT VERSUS H2O2 CONCENTRATION (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN AT 25°C)

4)

Effect of Particle Size Particle Diffusion at 400 µm Particle Diffusion at 149-100 µm Particle Diffusion at 32 µm

Particle Diffusion at 300-149 µm Particle Diffusion at 100-63 µm

y = 0.0112x R² = 0.9942 y = 0.00667x R² = 0.9926 y = 0.004448x R² = 0.9854 y = 0.00247x R² = 0.9937 y = 0.00111x R² = 0.9902

1-3(1-X)2/3+2(1-X)

1

0 0

20

40 Time, min

60

80

FIG. 15 RELATIONSHIP BETWEEN 1-3 (1-X) 2/3 + 2 (1-X) AND LEACHING TIME FOR REE LEACHING WITH DIFFERENT PARTICLE SIZES(1.5M H2SO4, 1M H2O2 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN AT 25°C)

The effect of particle size 400 μm, 300-149 μm, 149-100 μm, 100-63 μm, 63-32 and 32μmon the rate of the uranium leaching reaction in the presence of 1.5 M H2SO4, 1 M H2O2 and a 1:3 solid/liquid ratio is illustrated in Figure (6). There is a general increase in the leaching rate as the particle size decreases. One reason for this is that the smaller particle sizes have the largest surface area. There is an increase in the reaction surface area

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which enhances the mass transfer process of leaching. Another reason is that the solid particles were activated during grinding. The plots of 1-3 (1-X)2/3 + 2 (1-X) against time for the various particle sizes are graphed in Figure 15. The apparent rate constant was determined and plotted versus the log inverse of the initial average particle diameter (d) and the results are shown in Figure (16) which indicates that the ash layer diffusion reaction on the particle surface is the rate determining step of the dissolution process. As seen from Figure (16), the order of the reaction was found inversely proportional to power 1.0484 of particle size ([d0]-1.048). 0

y = 1.0484x - 0.3197 R²= 0.9851

-0.5

log(k)

-1 -1.5 -2 -2.5 -3 -3.5 -3

-2.5

-2 -1.5 log(d -1 ), μm-1

-1

FIG. 16 PLOT OF THE LOG RATE CONSTANT VERSUS THE LOG INVERSE OF THE PARTICLE DIAMETER (1.5M H2SO4, 1M H2O2 1:3 SOLID/ LIQUID RATIO, CONTACT TIME 60 MIN AT 25°C)

5)

Effect of Solid/Liquid Ratio

1-3(1-X)2/3+2(1-X)

Particle Diffusion at 1:1 g/ml Particle Diffusion at 1:2 g/ml Particle Diffusion at 1:3 g/ml

Particle Diffusion at 1:1.5 g/ml Particle Diffusion at 1:2.5 g/ml y = 0.0109x R²= 0.9977

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

y = 0.0073x R²= 0.9959 y = 0.0042x R²= 0.9923 y = 0.0022x R²= 0.9932 y = 0.0008x R²= 0.9812

0

20

40 Time,min

60

80

FIG.17 RELATIONSHIP BETWEEN1-3 (1-X) 2/3 + 2 (1-X) AND LEACHING TIME FOR URANIUM LEACHING AT VARIOUS THE SOLID/LIQUID RATIO (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, CONTACT TIME 60 MIN AT 25°C) y = -2.2927x - 3.053 R²= 0.9997

0

log(k)

-1 -2 -3 -4 -0.7

-0.5

-0.3 log(S/L )

-0.1

0.1

FIG. 18 LOG-LOG PLOT OF THE RATE CONSTANT VERSUS THE SOLID/LIQUID RATIO (PARTICLE SIZE 100-63 ΜM, 1.5M H2SO4, 1M H2O2, CONTACT TIME 60 MIN AT 25°C)

The effect of the solid/liquid ratio was studied from (1:1, 1:1.5, 1:2, 1:2.5 and 1:3) on the uranium leaching rate with 1.5 M H2SO4, 1 M H2O2, the particle size of 100-63 μmat 25°C. There is a general increase in the leaching

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rate with the solid/liquid ratio increases. The corresponding plots of 1-3 (1-X)2/3 + 2 (1-X) versus time at various solid/liquid ratio are graphed in Figure (17). It can be seen that the solid/liquid ratio of 1:3 is required to obtain a high dissolution rate of uranium. Figure (18) represents the relation the log-log plots of the rate constants and the solid/liquid ratio. From this figure, it is clear that a straight line was obtained with slope of 2.29, correlation coefficient of 0.99 and intercept of -3.035. Determination of Empirical Kinetic Model Equation Of Uranium 1)

Temperature

Equation 6 can describe the leaching of uranium in El-Erediyarock by diluted sulfuric acid. However, its applicability is limited by the specific values used for different reaction parameters. kUranium= 120.12e (–25.02 /RT(

(6)

On the other hand, the application for the other different reaction parameters (sulfuric acid concentration, hydrogen peroxide concentration, solid/liquid ratio and particle size fraction) could be achieved by considering the effect of these parameters on the rate constant of El-Erediyarock leaching reaction. Therefore, equation 7 could be used to describe the effect of the different reaction parameters on the empirical kinetic model which representsthe leaching process of uranium by diluted sulfuric acid [25, 26]. K Uranium= ЌU C1c1C2 c2PbDd e –Ea/RT

(7)

whereC1is the acid concentration, C2 is the hydrogen peroxide concentration,P is the particle size, D is the solid/liquid ratio,R is the universal gas constant (8.314 J K−1 mol−1) and T is the reaction temperature (in Kelvin). Eais the activation energy (J mol−1) for uranium, ЌU, c1, c2, b, and dare constants related to the different variable investigated. These constants are obtained in the next section. 2)

Sulfuric Acid Concentration

The effect sulfuric acid concentration on the leaching reaction kinetic model equation for uranium was analyzed to evaluate the constant c1in equation 7. According to the equation, the expressedrelation between the rate constant(k), obtained from Figure (11) for uraniumandthe sulfuric acid concentration would be expressed by the following relation: KU=k4C1c1

(8)

where k4 is a constant equal ЌU C2c2PbDde –Ea/RT. The relation between log KU values obtained at different sulfuric acid concentration against log MH2SO4for El-Erediya rock leaching reaction should give a straight line with a slope of (cforuranium) and intercept of logk4. Figure (12) represents the relation between the logarithmic values of leaching rate constant KU of the leaching process as a function of logarithmic values of sulfuric acid concentrationMH2SO4. From the results in figure (12), it is clear that a straight line was obtained with a slope of 0.8001, correlation coefficient of 0.984and intercept of -4.2702. According to equation 8 and figure (12), the constant c1 was found to be equal to 0.8001 and also k4 was found to equal 5.367 X 10-5. Therefore, equation 8 could be expressed as the following: KU=5.367X10-5C10.8001 3)

(9)

Hydrogen Peroxide Concentration

The effect hydrogen peroxide concentration on the leaching reaction kinetic model equation for uranium was analyzed to evaluate the constant c2in equation 7. According to the equation, the expressed relation between the rate constant(k), obtained from Figure (13) for uraniumand hydrogen peroxide concentrationscould be expressed by the following relation: KU=k5C2c2

(10)

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where k5 is a constant equal ЌU C1 c1PbDde –Ea/RT. The relation between log KU values obtained at different hydrogen peroxide concentration against log MH2O2for El-Erediya rock leaching reaction should give a straight line with a slope of (c2for uranium) and intercept of logk5. Figure (14) represents the relation between the logarithmic values of leaching rate constant (KU)for the leaching process as a function of logarithmic values of hydrogen peroxide concentration MH2O2. From the data in figure (14), it is clear that a straight line was obtained with a slope of 0.8474, correlation coefficient of 0.987and intercept of -4.4704. According to equation 10 and figure (14), the constant c2was found to be equal to 0.8474 and also k5 was found to be equal to 3.385 X 10-5. Therefore, equation 10 could be expressed as the following: KU=3.385X10-5C20.8474 4)

(11)

Ore Particle Size

The effect of ore particle size on the leaching reaction kinetic model equation for uranium was analyzed to evaluate the constants b in equation 7. Accordingly, the relation between the rate constant(k) obtained from Figure (15) for uranium at different particle size can be expressed by the following relation: KU = k6 Pb

(12)

where k6 is a constant equal ЌUC1c1C2c2 Dde –Ea/RT. Figure (16) represents the relation between the logarithmic values of leaching rate constant (log KU) of the leaching process as a function of logarithmic values of mean radius of particle size fraction (log P). From the obtained data in figure (16), it is clear that a straight line was obtained with a slope of 1.0484, correlation coefficient of 0.981 and intercept of -0.3197 for uranium. The slope of the straight lineof uranium indicates that the leaching process by diluted sulfuric acid is controlled by the layer diffusion model. According to equation 12 and figure (16), the constant b was found to be equal to 1.048and also k6was found to be equal to 0.4789. Therefore, equation 12 could be expressed as the following: KU= 0.4789P 1.048 5)

(13)

Solid/Liquid Ratio

The effect of solid/liquid ratio on the leaching reaction kinetic model equation for uranium was analyzed to evaluate the constant din equation 7. According to the equation, the expressedrelation between rate constant(k), obtained from Figure (17) for uraniumand different hydrogen peroxide concentration could be expressed by the following relation: KU=k7Dd

(14)

where k7 is constant equal ЌUC1c1C2c2Pbe –Ea/RT. Figure (18) represents the relation between the logarithmic values of leaching rate constant (log KU) of the leaching process as a function of logarithmic values of solid/liquid ratio (log D). From the data shown in figure (18), it is clear that a straight line was obtained with theslope of -2.2927, the correlation coefficient of 0.999and the intercept of -3.053. According to equation 14 and figure (18), the constant dwas found to be equal to-2.2927 and also k7was found to be equal to 3.385 X 10-5. Therefore, equation 14 could be expressed as the following: KU =8.851 X 10-4D-2.2927

(15)

From the analysis of the previous data for different factors affecting the leaching of uranium, equation 7 is given as the following; K Uranium = ЌU C10.8001 C2 0.8474 P1.0484 D-2.2927 e –25.02/RT

(16)

In El-Erediyaore material leaching process by diluted sulfuric acid, unless otherwise stated, the following conditions were taken in the leaching process. The ore leaching experiments were carried out from 5 g ore with

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a particle size of 100-63 μm, 1:3 solid/ liquid ratio, 1.5 M sulfuric acid, 1M hydrogen peroxideandthe temperature of 25o C. Under these conditions, ЌUcould be evaluated from equations 6, 9, 11,13 and 15 as the following: ЌU (0.0815) 1.0484(1.5)0.8001 (1)0.8474 (3)-2.2927= 120.12 So ЌU=14933.546

according to Eq. 6

ЌU(0.08151.0484(1)0.8474(3)-2.2927e(–25.02/8.314X298)=5.367X10-5 So ЌU=9.3X10-3 according Eq. 9 ЌU (0.0815) 1.0484(1.5)0.8001(3)-2.2927e (–25.02/8.314X298)=3.385 X 10-5 So ЌU=4.25X10-3 according Eq. 11 Ќ U(1.5)0.8001 (1)0.8474 (3)-2.2927e (–25.02/8.314X298)= 0.4789 So ЌU=4.341 according Eq. 13 ЌU (0.0815) 1.0484(1.5)0.8001 (1)0.8474 e (–25.02/8.314X298)=8.851 X 10-4 So ЌU=8.95X10-3according Eq. 15

Therefore, relative mean ЌU equals 2987.58. The empirical kinetic model equation for uraniumis evaluated as the following: K Uranium =2987.58C10.8001 C2 0.8474 P1.0484 D-2.2927 e –25.02/RT

Conclusions Uranium can be easily leached from uranium ions using H2SO4 acid in the presence of H2O2 as the oxidant. Using the optimum conditions of 1.5 M H2SO4; 1M H2O2; a stirring rate of 600 rpm, a solid/liquid ratio of 1:3, 120 min and a particle size of 100-63 μm, gave leaching efficiency of about 95.2% for uranium. The reaction order of the total H2SO4 concentration is 0.8001. Hence, the leaching rate of uranium strongly depends on the acid concentration. The linear relationship between the rate constant, k, and the inverse of the initial particle diameter indicates that the rate of uranium leaching is diffusion controlled.The order of the reaction was found inversely proportional to power 1.0484 of particle size ([d0]-1.0484). The reaction order of the total H2O2 concentration is 0.8474. Hence, the leaching rate of uranium strongly depends on the concentrations of acid and hydrogen peroxide. The leaching kinetics of uranium shows that the rate of uranium leaching using H2SO4 acid in the presence of H2O2 as an oxidant is diffusion controlled and follows the shrinking core model1-3 (1-X)2/3 + 2 (1-X) = k1t with an apparent activation energy of 25.02 kJ/mol. REFERENCES

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Amer, T.E. Ibrahim, T.M. & Omar, S.A. (2005): Micro-probe studies and some rare metals recovery from El Missikat mineralized shear zone. Eastern Desert, Egypt, vol. (2)p-p 225-238 Assiut-Egypt.

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Haggag, E. A; (2014): “Recovery of uranium and other economic elements from El-Missikat pluton, Central Eastern Desert, Egypt”. M.Sc. Thesis. Benha Univ., p 24.

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International Atomic Energy Agency (1980): Significance of Mineralogy in the Development of Flowsheets for Processing Uranium Ores, Technical Report Series, IAEA No. 196, Vienna.

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