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Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid
Accepted Manuscript Title: Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid Author: V. Madakkaruppan Anitha Pius T. Sreenivas Nitai Giri Chanchal Sarbajna PII: DOI: Reference:
S0304-3894(16)30272-2 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.03.050 HAZMAT 17562
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-10-2015 12-3-2016 19-3-2016
Please cite this article as: V.Madakkaruppan, Anitha Pius, T.Sreenivas, Nitai Giri, Chanchal Sarbajna, Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.03.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid
V.Madakkaruppana, Anitha Piusb*, T.Sreenivasc, Nitai Giric and Chanchal Sarbajnaa
a
Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad-500016
b c
Department of Chemistry, Gandhigram Rural Institute, Gandhigram, Dindigul district, Tamil Nadu-624302
Mineral Processing Division, Bhabha Atomic Research Center, AMD Complex, Begumpet, Hyderabad-500016
*
Corresponding author.
Anitha Pius Department of Chemistry, Gandhigram Rural Institute, Gandhigram, Dindigul district, Tamil Nadu 624 302, India Telephone/Fax +91-0451-2452371/+91-9443108504 E-mail: [email protected]
1
Highlights
U Leaching from a low-grade Si-rich ore studied in H2SO4 medium with (MW) irradiation.
MW heating is more efficient in terms of U recovery, kinetics and purity of liquor.
U leachability of 84% obtained in 90 min. at 95°C with 0.38 M H2SO4 at 450mV.
Conventional conductive heating gave about 74% leachability with less purity liquor.
U leaching was found to be product layer diffusion as controlling mechanism.
Abstract This paper describes a study on microwave assisted leaching of uranium from a low-grade ore of Indian origin. The host rock for uranium mineralization is chlorite-biotite-muscovitequartzo-feldspathic schist. The dominant presence of siliceous minerals determined leaching of uranium values in sulfuric acid medium under oxidizing conditions. Process parametric studies like the effect of sulfuric acid concentration (0.12-0.50M), redox potential (400-500 mV), particle size (600-300 µm) and temperature (35°-95°C) indicated that microwave assisted leaching is more efficient in terms of overall uranium dissolution, kinetics and provide relatively less impurities (Si, Al, Mg and Fe) in the leach liquor compared to conventional conductive leaching. The kinetics of leaching followed shrinking core model with product layer diffusion as controlling mechanism. Keywords: Uranium, Microwaves, Acid Leaching, Leach Kinetics.
1. Introduction Uranium is one of important strategic metal where large supply demand gap is predicted in coming decades [1]. The interest in uranium metal stems from its widespread use in commercial nuclear power reactors (Light Water or Pressurized Heavy Water variety) as a main fuel source [2]. Nuclear energy is attracting renewed world-wide attention, 2
particularly in developing nations like China and India, as a large-scale energy generating source, mainly because it is bestowed with the advantage of very-low or negligible green house gas emissions (GHG) during the process [3]. The main source of uranium is conventional ore deposits. Uranium is recovered from any land based resource like ores or tailing dumps etc. by hydrometallurgical process, consisting of size reduction, leaching, solid-liquid separation, purification and precipitation of dissolved uranium as yellow cake [46]. Leaching is a critical unit operation in the process flowsheet and temperature of reaction is an important controlling parameter for this unit operation. The complexity in processing resources of above-cited variety arises predominantly due to the chemical reactivity of gangue or unwanted minerals along with minerals of interest or valuable minerals. During chemical leaching, which is the stage where the release of various ionic species from solid mineral phase/s to aqueous medium occurs, presence of unwanted ionic species in solution phase increases the rigorousness of purification operations like ion exchange or solvent extraction, besides aggravating the effluents treatment protocol [5]. Therefore, research is pursued in various laboratories on methods of achieving selectivity during the leaching stage itself to the extent possible.
The chief methods pursued in this endeavor include
manipulation of chemical conditions [6] or/and application of microwaves which is having the attribute of selective heating of minerals [7]. The application of microwave technology in minerals processing was being studied by isolated groups world-wide for last two decades [7-20]. Considering its potential, several reviews on principles and application of microwaves in different ore processing unit operations like size reduction, roasting, flotation, leaching etc. was reported [7, 8]. However, its relevance for low-grade or other complex resources is better felt now than ever before due to concomitant developments in scaling-up of the microwave technology [21]. With regard to leaching operation microwave has distinct advantage over conventional conductive heating
3
method. Microwave heating is rapid and selective in nature, distributes uniformly in the cavity, high efficiency, fast switch on and off as well as flexible and available in modular design.
Microwave heating is a kind of electro-heat process such as induction, radio
frequency, direct resistance, and infrared heating that utilize specific parts of electromagnetic energy [17, 18]. Microwave energy is non-ionized electromagnetic radiation which has frequencies changing in the range of between 300 MHz to 300 GHz. Materials that couple to microwave energy are called dielectric and contain dipoles. The dipoles align themselves in an electric field and flip around in an alternating electric field. Stored internal energy is lost through friction thus heating the minerals. Based on rate of heating (with microwaves) the minerals were classified into hyperactive, active, difficult-to-heat and inactive. From the large number of minerals and inorganic compounds tested by several laboratories it was noted that most silicates, carbonates and sulfates, and some oxides and sulfides are transparent to microwave energy, while most sulfides, arsenides, sulfosalts and sulfarsenides, and some oxides, heat well when subjected to microwave irradiation [7]. For instance it was reported that pure UO2 attained a temperature of 1100° C in 0.1 min of irradiation with MW of 2450 MHz and heating rate noticed was 200° C per sec [7]. In comparison to this the SiO2 and CaCO3 which are the common host rocks for uranium minerals heats at the rate of 2 – 5 and 5 °C/sec [7]. In view of these positive factors like improved extraction efficiency and attractive leaching kinetics investigations on application of microwaves for leaching of uranium values from a low-grade under oxidizing conditions using sulfuric acid as lixiviant are carried out and results discussed. Though studies were carried out earlier on MW assisted dissolution of spent nuclear fuel and thorium [22, 23] it is for the first time an attempt is being on evaluating its role on leaching of uranium from its ore. Though the application of MW for treating the ores is highly energy intensive the enormous energy packed in uranium nucleus
4
would definitely give net positive energy balance and this factor is the driving force behind these investigations [24].
2. Leaching chemistry Leaching of oxides of uranium in sulfuric acid medium takes place by two steps, (i) oxidation of insoluble U (IV) to U (VI) and (ii) complexation of oxidized U (VI) by sulphate anions to form uranyl sulphates. The uranyl sulfates remain as stable aqueous complexes under specified redox and pH conditions [4-6]. The leaching process is reported to follow electrochemical mechanism depicted in the equation 1 to 3 [25]. Sulfuric acid leaching of uranium minerals requires the presence of Fe (III) regardless of the reagent used as an oxidant [26, 27]. Fe (III) oxidizes U (IV) as given in Eq. 1, while the oxidant reagent oxidizes the Fe (II) to Fe (III) as given in Eq. 2; thus, the Fe (II) – Fe (III) couple serves as an electron transfer catalyst between the oxidant and UO2. 2
→
2
2 4
→2
(1) 2
(2)
The chemical interaction of soluble UO22+ species with sulfate anions yields uranyl sulphate complexes like UO2 (SO4)n2-2n as illustrated in Eq. 3. →
(3)
Satisfactory leaching of uranium values occurs by maintaining a redox potential (Eh) of 400 500 mV vs saturated calomel electrode (SCE.) [26, 27].
3. Experimental 3.1. Characterization studies The uranium ore sample used in the present studies was collected from Narwapahar mine, Singhbhum district, Jharkhand, India. The ore was crushed and ground to 600 µm, 425
5
µm and 300 µm size using laboratory jaw crusher and rod mill. All the chemicals used in the test work were of AR grade from standard chemical suppliers like BDH, Loba and Merck. Identification of various phases in the ore sample was made using Siemens D500 Xray diffractometer (XRD) with CuKα radiation (1.5418Ȧ) as source.
The accelerating
voltage and amperage was 35 kV and 22 mA. A 1° divergence slit was used to analyze between the 2θ range of 5 - 90° with a step size of 0.02° and 2s/step on a 30s delay. Phase identification was made with the aid of International Centre for Diffraction database (ICDD). Quantitative mineralogical composition of the Narwapahar uranium ore was estimated using optical microscopy. Micro-morphological study on the leach residue was performed using Philips XL-20 Scanning Electron Microscope attached with Energy Dispersive analyzer (SEM-EDS). Uranium content in the ore and various product streams were determined using pellet fluorimetry on Model FL6224 fluorimeter (M/s. Electronic Corporation of India Limited, India). Powdered rock sample weighing 1 g accurately was mixed with 3 ml HF and 5 ml HNO3 and the contents were evaporated to dryness on hot-water bath. This was carried out for two more times. 5 ml of HNO3 was further added to the contents and re-evaporated to dryness. This was also repeated for two times. Finally, 10 ml of HNO3 was added to the residue in the container and after about 30 minutes of digestion the mixture was filtered. The filtrate was collected in 100 ml volumetric flask and the balance volume adjusted with 10 % HNO3. Uranium was extracted with ethyl acetate using aluminum nitrate as the salting-out agent. The aliquot from the organic layer was dried under an infrared lamp, combined with a flux (Na2CO3: NaF = 4:1) in a platinum planchet, and fused at 850° C. After cooling, the fused pellet was irradiated using Hg-vapor lamp at 365 nm wavelength. The characteristic uranium fluorescence emanating from the sample pellet at 555 nm was measured by PMT detector.
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Silica content in the ore was determined by UV-visible spectrophotometer (Unicam) by molybdenum-blue method. The procedure involves decomposing the ore samples (0.20g) with NaOH in nickel crucibles, followed by water leaching and acidification with HCl. The clear solution was used for estimations in UV-Vs spectrophotometer. Na and K were determined by Flame photometer (Flame photometer-128, M/s Systronics, India). 0.04g of sample was weighed in a platinum crucible; 2 ml of hydrofluoricsulphuric acid mixture was added followed by heating the crucible gently on a hot plat until white fumes of sulphuric acid evolved. The contents were cooled and 5-10 ml of warm deionized water added to bring the solid residue into solution. The contents were transferred to a 50 ml volumetric flask and diluted to desired volume with de-ionized water followed by aspiration. All other elements (Al, Fe, Mg, Mn, Ca & P) were determined using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (ULTIMA-2, M/s Horiba Jobin Yvon, France). The sample preparation for ICP analysis involves weighing of 0.5 g of powdered material in a platinum dish and incinerating in a furnace at 650° C for one hour for removal of organic matter. The contents of the Pt dish were treated with 5 ml of 40% HF and 3 ml. of concentrated HNO3. This mixture was evaporated to dryness on a boiling water bath. The same process was repeated three times. Further, the sample was again treated with 3 ml HNO3 for three more times and the residue of the last stage was re-dissolved in HNO3 (3ml) and the contents made up to 100 ml volume such that the final solution has 3% acidity. The moisture content in the samples was determined by accurately weighing 1.0 ± 0.05 g of ore in an analytical balance using cleaned silica crucible. The crucible contents were maintained at 110°C in an oven for 1 hour. The weight loss was recorded as moisture content of the ore.
7
The Loss on Ignition (LOI) was estimated by heating the sample in a furnace in a stepped schedule for 2 hour to 950° C.
The fired sample was cooled down to room
temperature in desiccators and then weighed. The weight loss was recorded as LOI. pH and Eh was measured on respective instruments (ELICO, India) both during start and end of the experiment. The values reported were those obtained after the completion of the experiment. The redox potential values reported are those taken with saturated calomel electrode (SCE) as reference.
3.2. Leaching studies Microwave assisted leaching experiments were carried out on a microprocessor controlled ETHOS-One microwave digester (Milestone Srl, Italy) [28]. The digester consists of 2 x 800 W magnetron operating on a 1-s duty cycle at a frequency of 2450 MHz. The delivered microwave power is 1500 W.
The unit include a 360° reversing turn-table
operating at 3 revolutions/min and a 10 position carousel. A variable-speed exhaust fan is present in the cavity for removal of corrosive fumes.
The equipment has facility for
monitoring and controlling of both external (infrared) and internal temperature (ATC400/ATCFO) of the reaction vessels in real time during the run. Similarly it also has an automatic pressure control for direct sensing of reaction pressure. Temperature and pressure sensors were connected to microwave-transparent and chemical resistant PTFE vessels in which the sample is placed. The unit is designed for maximum temperature of 300° C and pressure upto 55 bars. The entire unit is operated through a touch-screen control terminal operating on software for full GLP documentation with PID algorithms to exactly duplicate the required temperature curve. 5 g of uranium ore sample was weighed directly into the PTFE leaching vessel to which other reagents like sulfuric acid (leachant) and manganese dioxide (oxidant) were
8
added in measured quantities such that 1:3 solid/liquid ratio (weight-wise) is achieved. The vessels were sealed and positioned in the microwave carousel followed by mounting of temperature and pressure sensors. The microwave irradiation of the vessel contents was carried out in isothermal or fixed set-temperature mode without stirring.
5 minutes of
ramping time was given for attaining the predefined temperature and the reaction was subsequently carried out for 90 minutes. After the reaction period was over the contents were automatically cooled to ambient temperature and taken out for solid-liquid separation. Slurry was filtered on a vacuum filter (60 kPa) using Whatman filter paper (No.42). The leach residue was washed twice using acidified water in all the experiments. The pH and Eh of the solution was recorded and samples were chemically analyzed for various desired analytes. Data analysis was made on the basis of chemical assays of metal ions in the feed and reaction products and leachability (R) is calculated using the relation given in Eq. 4.
(4)
where R is the leach recovery in percent, vl is the volume of the leach liquor plus wash, gl is the U3O8 assay of the liquor, ws is the weight of the leach residue and gs is the U3O8 assay of the leach residue.
4. Results and discussion 4.1. Characterization of ore The optical microscopic studies indicate that the host rock for uranium mineralization in Narwapahar ore is chlorite-biotite-muscovite-quartzo-feldspathic schist.
XRD studies
indicate uraninite as principal uranium bearing phase (Fig. 1) and the major gangue minerals are quartz, chlorite, biotite, epidote and magnetite with minor quantity of apatite, albite, pyrite, chalcopyrite, zircon and molybdenite. The quantitative mineralogical composition of
9
the Narwapahar ore (Table 1) indicates presence of siliceous minerals (quartz, chlorite and biotite mica) to the extent of about 95% by weight. CN film auto-radiography for a period of 72 hours gave high density alpha tracks corresponding to uraninite mineral with a radioactive halo, occurring in close association with chlorite (Fig. 2 a & b). The U3O8 content of the ore is 0.025% (Table 2). The SiO2, Al2O3 and FeO contents are 55.4, 14.8 and 8.7% respectively confirming overwhelming presence of siliceous and micaceous minerals besides iron oxides (Table 1).
4.2. Microwave leaching The main objective in the present studies is to utilize MW irradiation as source of heat during leaching. The other process variables like the particle size of ground ore, oxidant and sulfuric acid dosage were fixed (Table 3) on the basis of results of conventional leaching studies carried out in our laboratory on identical system (Narwapahar ore) earlier [29].
4.2.1 Effect of microwave power Microwave power is a factor greatly influencing the rate of microwave heating. The dependence between the applied microwave power and the increase of temperature of the irradiated system is described by the Eq. 5 [30]: ∆
(5)
where: ΔT – the increase of the mean temperature of the heated body, P – microwave power used for heating, V volume, cp heat capacity, ρ density and t – time of heating. In order to determine the effect of MW power on the leachability of uranium minerals, the ore slurry was irradiated at two different MW power levels namely, 300 and 600 W. However, the slurry temperature was kept constant at 75° C using an inbuilt feedback system of the MW unit. This was done to maintain the same range of reaction temperatures as used in conventional 10
leaching experiments (Table 3). The PID controller in MW unit regulates the power output of magnetron to keep the solution/slurry temperature at pre-defined level (within ± 1° C). The other process conditions maintained constant in the experiment are: H2SO4: 0.38M, Eh 450 mV (using MnO2 dosage of 5kg/t), particle size: 300 µm and solid/liquid ratio (weight-wise) 1:3 and irradiation time 90 minutes. The progress of the leaching reaction was monitored kinetically. Results of the experiment are given in Fig. 3 and the MW settings and average energy received for maintaining the set temperature is given in Table 4. The slurry irradiated at input MW power of 300 W gave uranium leachability of about 28% during the initial 15 minutes of reaction period and reached about 53% in 90 minutes. The corresponding results with MW power level of 600 W, was about 46% and 77% respectively. Two notable aspects apparent from the MW experimental details (Table 4) at the two input power levels are (i) higher average MW power (or energy) received during the ramping period at 600 W in comparison to 300 W and (ii) almost identical average power (or energy) taken at both the set power levels after the slurry attained the set temperature. It was also observed that in the case of experiments carried out with MW power setting of 300 W, the duty cycles of energy bursts (on – off cycle of magnetron) are closely spaced or like a continuum and of lower energy when compared to that irradiated with 600 W power setting. In the latter case, the duty cycles are well spread out but the maximum energy delivered in each pulse is almost double than that given with 300 W power setting. Therefore, the higher energy burst delivered per pulse at 600 W power setting during the ramping period as well as that given during intermittent duty cycles resulted in upward shift of leachability by about 18 – 24% over the lower power setting of 300 W. MW assisted heating of the uranium ore slurry gave about 5% higher leachability over that attained under conventional leaching conditions for similar process conditions adopted [29]. Similar trends of enhancement were reported earlier by Zhao and Chen [22] in MW assisted leaching of pure uranium oxide in
11
nitric acid medium and by other on ores of Cu, Ni, Au and Zn [14-15, 19, 20, 22]. The results were no different in the studies carried out Xia and Pickles [13] for recovery of valuable metals from electric arc furnace dust. Al-Harahsheh et al [31] made detailed studies on the role of thermal and non-thermal effects in the MW assisted processes for aqueous dissolution of metals/minerals. Based on analysis of published information from different systems Al-Harahsheh et al [31] attributes the observed enhancement in systems irradiated with MW over the conventional methods of heating to predominantly thermal effects. The shallow penetration depth of microwave energy in conducting solutions (like leach solutions) leads to dissipation of microwave energy within the first few millimeter distance of the reactor walls. This implies that the temperature at the walls of the vessel is higher than the temperature in the bulk or in the middle where temperature sensor exists. Under such arrangement though the sensor displays only 75° C, the temperature of the leach solution and the solid particles close to the walls of the vessel is significantly high, particularly when the minerals in the ore are hyperactive or active to MW (like UO2, iron oxides and sulfides) [7]. This phenomenon becomes all the more significant when the contents of the slurry are unstirred. Dissolution studies of pure uraninite and pitchblende by conventional heating mode showed that the processes are chemically controlled and the reaction rate is sensitive to temperature changes [6]. This means that a small change in the reaction temperature results in significant change in the leachability of uranium oxide minerals. This explains the reason for enhanced leachability of uranium values from their ores when heated with microwaves.
4.2.2. Effect of acid concentration Effect of sulfuric acid concentration on the leachability of uranium was studied in the concentration range of 0.12 to 0.50 M at 75°C using input MW power of 600W. The other process conditions kept constant are: Eh 450 mV (using MnO2 dosage of 5kg/t), particle size:
12
300 µm and solid/liquid ratio (weight-wise) 1:3, irradiation time 90 minutes with no stirring of the reactor contents. Fig. 4 gives the results of the experiments. Maximum leachability, about 77% was noticed upon using sulfuric acid concentration of about 0.38 M in MW assisted process as against 72% achieved under conventional conditions. Further increase in the acidity did not show any enhancement of the dissolved uranium values.
The
improvements noticed with varying leachant dosages in MW treated slurry are also due to differential breakage of ore particles under the effect of MW [32]. Since quartz and mica, the two dominant minerals in the ore (Table 1) are transparent and low loss minerals, it is assumed that microwave energy passed through those minerals without any loss and thereby did not cause any change nor induced any stress within its matrix. However, composite particles like U in silica matrix or U in mica matrix (Figure 2) or U – Fe – silica matrix absorbs microwave energy and transform it into heat causing differential expansion and induces fractures within the composite particles facilitating enhanced accessibility of uranium phases to the leachant. Such an opportunity does not exist in conventional leaching under quiescent agitation conditions [32].
4.2.3. Effect of oxidant dosage The leaching of uranium is known to proceed through electrochemical mechanism as discussed in the Section on process chemistry of leaching therefore, the redox conditions of the slurry control the leaching efficiency critically. Effect of oxidant dosage on the leaching of uranium values from Narwapahar ore was studied by varying the redox potential from 400 to 500 mV (versus saturated calomel electrode) by addition of different dosages of manganese dioxide. The other process conditions followed in the tests are H2SO4 0.38M, temperature 75°C, particle size 300µm, MW power 600 W and reaction time 90 minutes. Higher leachability of uranium values – 70 to 81% was obtained with MW heating (Figure 5)
13
compared to 46 to 74% dissolution observed in conventional leaching –at all redox conditions namely 400 mV to 475 mV (vs SCE) investigated. The leachability reached near constancy (80%) for Eh value 500 mV.
4.2.4. Effect of particle size Effect of particle size on the rate of leaching of uranium was studied at three grind sizes 600, 425 and 300 µm by maintaining the other conditions constant like H2SO4 0.38M, Eh 450mV, temperature 75°C, MW power input 600 W, irradiation time 90 minutes and solid/liquid ratio 1:3. The results are illustrated in Fig. 6. About 63% of uranium values could be leached from the coarsely ground (600 µm) material in 90 minutes contact time under MW effect. This enhanced to 77% when the ore was reduced to finer than 300 µm size.
The phenomenon of improved dissolution with finer grind could be due to the
combination of the thermal effects and consequent micro-cracks generation as well as increase in the surface area to volume ratio which paves way for better reaction between the leaching solution and the uranium mineral phases in the ore [32].
4.2.5. Effect of temperature Effect of temperature on the leaching of uranium minerals in the ore was studied by varying reaction temperature from 35° to 95° C using input MW power of 600W and keeping the other experimental conditions constant as follows: H2SO4 0.38M, Eh 450mV, MW irradiation time 90 minutes and solid/liquid ratio 1:3. The average MW power taken by the slurry for maintaining constant temperature of 35°, 55°, 75° and 95° C during the reaction period was 25, 45, 60 and 71 W respectively. As indicated from the results in Fig. 7 the uranium dissolution increased from about 48% obtained at 35o C to a maximum of 84% at 95° C. The maximum leachability of uranium resulted under identical process conditions in
14
conventional leaching was only 74% (Table 3) [29]. The increased energy deposited in the MW absorbing minerals with increase in set temperature gives differential heating rates in heterogeneous systems creating temperature gradients between the solid and liquid phases. The temperature gradient creates large thermal convection currents in the slurry resulting in agitation and sweeping away of the reaction products. Constant exposure of new surface to leachant solution due to this action results in increased leachability [31].
4.3. Leaching kinetics Leaching is a rate phenomenon. Kinetics of leaching of a low-grade uranium ore is influenced by several operating factors like: nature of uranium mineralization in the ore and in specific the liberation size, the ratio of U(IV) to U(VI), reaction temperature, redox potential, ionic composition of solution, etc. The un-reacted shrinking-core model is the most commonly used mathematical relation to describe the heterogeneous reactions like leaching of mineral from ores. The model is well-known and requires only limited description [33, 34]. In a binary reaction system in which only solids (ore particles) and solution phase exists, the rate of leaching is controlled by (i) solid or product layer diffusion and (ii) chemical reaction. The integrated rate expressions for the product layer diffusion and surface chemical reaction models are given in Eq. 6 and 7 respectively [33]. 1
3 1
1
1
2 1
(6)
(7)
where, ‘x’ is the conversion fraction of solid particle, ‘kd’ is the apparent rate constant (min-1) for diffusion through the product layer, ‘kc’ is the apparent rate constant (min-1) for the surface chemical reaction and ‘t’ is the reaction time. 15
The degree of linearity of the plot of left hand side of Eq. 6 and 7 against the reaction time (t), gives insights into the mechanism of reactions. It is further corroborated from the activation energy value computed using the Arrhenius relation given in Eq. 8. (8)
where ‘k’ is a reaction rate constant, ‘A’ is the frequency factor, ‘Ea’ is the apparent activation energy, ‘R’ is the universal gas constant and ‘T’ is the absolute temperature. The activation energy for diffusion-controlled reactions is generally below 20 kJ/mol while it is above 40 kJ/mol for chemical reaction controlled process [33, 34]. In order to understand the type of leaching mechanism prevalent for the Narwapahar uranium ore in the MW assisted leaching, the leaching kinetic data obtained at different reaction temperatures (Fig. 7) was analysed using the kinetic rate equations given in Eq. 6 and 7 and Arrhenius activation energy from Eq. 8. The kd and kc values computed from Eq. 6 and 7 are given in Table 5 & 6. The R2 values in the plots enable in understanding the closeness of the experimental data and that predicted with models (Eq. 6 and 7). The kd values given in Table 5 vary in the rage of 0.0011 to 0.0043 min-1 while the kc were between 0.0019 and 0.0042 min-1. The R2 value for kd was 0.98 while for kc it was in the range of 0.87–0.90.
It can therefore be inferred that the predominant dissolution mechanism of
uranium from the Narwapahar ore is diffusion controlled only. The kd values obtained in conventional leaching of uranium values from Narwapahar ore were in the range of 0.0018 to 0.0035 min-1 [29], being relatively lower than that obtained with MW irradiation. The mechanism of leaching was also confirmed from the apparent activation energy (Ea) value of 9.66 kJ/mol, computed using the data given in Fig. 8 and Eq. 8. The Ea value obtained from similar exercise in conventional leaching was 7.32 kJ/mole [29].
The
insignificant difference in activation energy for both the modes of heating was reported in the
16
case of dissolution of ceramic UO2 in nitric acid medium by Zhao and Chen [22] and for chalcopyrite and sphalerite system by Al Harahsheh et al [9, 31]. The closeness of Ea values implies that the MW irradiation has not altered the intrinsic leaching mechanism generally observed in conventional leaching. Studies by Al Harahsheh et al [31] on chalcopyrite and sphalerite minerals in ferric chloride medium also attribute proximity of Ea values to the limitation of penetration of microwaves when passing through a high loss leaching solution which cause significant temperature difference between the outer shell of the leaching solution or the zone close to walls of the reaction vessel where the MW directly impinge and the bulk position where the temperature probe exists. This behaviour is more pronounced for unstirred slurry system. The general equation relating the important operational parameters with the rate constant ‘kd’ is given in Eq. 9 [34].
exp
(9)
where ‘ko’ is the frequency factor or pre-exponential factor (min-1), ‘C’ is concentration of reagent/s or leachants in solution (moles/liter), ‘ro’ is the particle size, ‘p’ and ‘q’ are constants indicating the order of the reaction. The order of the reaction was determined from the slope of the lines generated by plotting the natural logarithm of apparent rate constants or ‘ln kd’ vs the natural logarithm of the respective variable. In the case of leaching of Narwapahar uranium ore it was noted that reaction temperature, concentration of sulphuric acid, manganese dioxide and the particle size of the ground ore were critical variables. Therefore the kinetic plots of these variables given in Table 7 were taken for determining the ‘kd’ values (Table 6) which were used for estimating the order of the reaction depicted in Eq. 10. Eq. 11 gives the relation developed for leaching of uranium from Narwapahar ore by conventional method [29], for comparison.
17
1
3 1
/
2 1
.
1
3 1
/
2 1
.
.
.
.
.
.
(10) .
(11)
The order of the reaction with respect to the particle size is relatively lower in MW irradiated system when compared to that obtained with conventional leaching.
5. Characterization of leaching products 5.1. Characterization of leach liquor The leach liquors obtained from the experiments carried out under optimized conditions (H2SO4 0.38M, Eh 450mV, MW power 600 W, temperature 75°C, irradiation time 90 minutes and solid/liquid ratio 1:3) were analysed for Si, Al, Fe and Mg to understand the gangue minerals (quartz, mica minerals, iron oxides) reactivity and compare the selectivity of MW assisted leaching versus the conventional leaching. The results are shown in Fig.9. Leaching of gangue elements was comparatively less under microwave irradiation conditions than the conventional heating. The concentration of various ions in the leach liquor obtained with MW irradiation was Si 6.62g/l, Al 4.54g/l, total Fe 9.50g/l and Mg 3.97 g/l while under conventional conditions the assays were Si 7.27g/l, Al 5.39 g/l, total Fe 11.20 g/l and Mg 4.19 g/l. 5.2. Characterization of leached residue The solid leach residues obtained under optimized conditions of leaching in microwave and conventional heating were characterized SEM-EDS technique. The morphology of leach residue (Fig. 10a) obtained under conventional conditions has clear surface, regular size and shape while that obtained with MW irradiation (Fig. 10b) was irregular and needle shaped with dominant presence of broken particles.
18
6. Conclusions Microwaves assisted leaching of uraninite from a low grade ore of Indian origin was investigated. The host rock for uranium mineralization is chlorite-biotite-muscovite-quartzofeldspathic schist. Leaching was carried out under oxidizing conditions using sulfuric acid as leachant and manganese dioxide as oxidant. Process parametric variation studies investigated include the MW irradiation power, dosages of leachant and oxidant (for Eh control), particle size and temperature of reaction. Kinetics of the reaction was also monitored for elucidating the mechanism of leaching. The results of the studies indicate that about 84% of uranium values can be leached in 90 minutes of irradiation with MW (average power 71 W) by maintaining other conditions like H2SO4 0.38 M, Eh 450 mV and temperature 95 °C. The uranium lechability in conventional conductive heating was about 74% with similar process conditions. The enhancement is mainly due to high microwave absorption ability of uranium oxide minerals over the silicate minerals matrix leading to rapid rise in localized temperature resulting improved dissolution. The kinetics of uranium leaching in MW assisted heating satisfied the shrinking core kinetic model with product layer diffusion process as principle mechanism. The Arrhenius activation energy was about 9.66 kJ/mole. MW assisted heating also gave leach liquors with relatively higher purity over that obtained in conventional leaching.
Acknowledgements Authors are very much thankful to Dr. A.K.Rai, Director, Dr. A.K.Chaturvedi, Additional Director (R & D) and Dr.Yamuna Singh, Head, MPG Group, Atomic Minerals Directorate for Exploration and Research for kind permission for publishing this research article. The authors are also thankful to MPG and Chemistry Group, AMD, Hyderabad for analytical support.
19
References: [1] Uranium 2014: Resources, Production and Demand, A Joint Report by the OECD Nuclear Energy Agency and the International Atomic Energy Agency (Vienna), NEA No. 7209, OECD (2014) 17 – 131 [2] Nuclear Power Reactors in the World, IAEA-RDS-2/35 ISBN 978–92–0–104915–5, ISSN 1011–2642, International Atomic Energy Agency Vienna (2015) 79. [3]http://www.world-nuclear.org/Nuclear-Basics/Greenhouse-gas-emissions-avoided (accessed in Jan 2016) [4] R.C. Merritt, The Extractive Metallurgy of Uranium, Colorado School of Mines research Institute, Colorado, (1971) 576 [5] C. Edwards, A. Oliver, Uranium processing: a review of current methods and technology, JOM (2000) 52 (9) 12–20. [6] S.K. Bhargava, Rahul Ram, Mark Pownceby, Stephen Groncott, Bob Ring, James Tardio, Lathe Jones, A review of acid leaching of uraninite, Hydrometallurgy (2015) 151 10-24. [7] K.E. Haque, Microwave energy for mineral treatment processes – a brief review, Int. J.Miner. Process (1999) 57 1-24. [8]
M.
Al-Harahsheh,
S.W.
Kingman,
Microwave-assisted
leaching-a
review,
Hydrometallurgy (2004) 73 (3-4) 189-203. [9] M. Al-Harahsheh, S.W. Kinghman, N. Hnkins, C. Somerfield, Bradshaw, W. Louw, The Influence of microwaves on the leaching kinetics of chalcopyrite, Miner.Eng. (2005) 18 1259-1268. [10] N.A. Rowson, N.M. Rice, Magnetic enhancement of pyrite by caustic microwave treatment, Miner.Eng. (1990) 3 (3-4), 363-368. [11] S. Marland, B. Han, A. Merchant, N. Rawson, The effect of microwave radiation on coal grindability, Fuel (2000) 79 (11) 1283-1288.
20
[12] K.E. Haque, Microwave irradiation pre-treatment of refractory gold concentrate, In: R.S. Salter, D.M. Wysouzil, G.W. McDonald (Eds.), Proceedings of the International Symposium on Gold, Winnipeg, Canada (1987) 327-339. [13] D.K. Xia, C.A. Pickles, Microwave caustic leaching of electric arc furnace dust, Miner.Eng. (2000) 13 79-94. [14] P.A. Olubambi, J.H. Potgieter, J.Y. Hwang, S. Ndlovu. Influence of microwave heating on the processing and dissolution behaviour of low-grade complex sulphide ores, Hydrometallurgy 89 (2007) 127–135. [15] Y. Hua, Z. Lin, Z. Yan, Application of microwave irradiation to quick leach of zinc silicate ore, Miner.Eng. 15 (2002) 451–456. [16] M. Lova, I. Murova, A. Mockovciakova,N. Rowson, S. Jakabsky, Intensification of magnetic separation and leaching of Cu-ores by microwave radiation, Separation and Purification Technology 31 (2003) 291-299. [17] C.A. Pickles, Microwaves in extractive metallurgy: Part 1 – Review of fundamentals, Miner.Eng. (2009) 22 1102–1111. [18] C.A. Pickles, Microwaves in extractive metallurgy: Part 2 – A review of applications, Miner.Eng. (2009) 22 1112–1118. [19] C.A. Pickles, Microwave heating behaviour of nickeliferous limonitic laterite ores, Miner.Eng. (2004) 17 775–784. [20] B. Nanthakumar, C.A. Pickles, S. Kelebek,
Microwave pretreatment of a double
refractory gold ore, Miner.Eng. (2007) 20 (11) 1109–1119. [21] F. Bergamelli, M. Iannelli, J.A. Marafie, J.D. Moseley, A Commercial Continuous Flow Microwave Reactor Evaluated for Scale-up, Organic Process Research & Development (2010) 14 (4) 926-930.
21
[22] Zhao YunGeng, Chen Jing, Kinetics study on the dissolution of UO2 particles by microwave and conventional heating in 4 mol/l nitric acid, Sci China Ser B-Chem (2008) 517 700-704. [23] G.K. Mallik, Microwave Processing in Thorium Fuel Cycle, BARC News letter (http://www.barc.gov.in) (2013) 331 26-30. [24] Physics of Uranium and Nuclear Energy, http://www.world-nuclear.org/info/NuclearFuel- Cycle/Introduction/Physics-of-Nuclear-Energy. [25] A.R. Burkin, Chemical Hydrometallurgy : Theory and Principles, Imperial College Press (London) (2001) 424. [26] R.J. Ring, Ferric sulphate leaching of some Australian uranium ores, Hydrometalurgy (1980) 1-2 89–101. [27] M.J. Maley, S. Burling, R. Ring, The effect of oxidation reduction potential and ferric iron concentration on leaching of uranium ores. In: Lam EK, Rowson JW, Ozberk E (eds) Proceedings of the 3rd international conference on uranium. 40th annual hydrometallurgy meeting, Saskatoon, Saskatchewan (2010) (1) 563–574. [28] Ethos One-Operator Manual MA133, Milestone Srl (www.milestonesrl.com). [29] V. Madakkaruppan, Anitha Pius, T. Sreenivas, K. Shiv Kumar, Leaching kinetics of uranium from a quartz-chlorite-biotite rich low grade Indian ore, J Radioanal Nucl Chem (2015) 303 (3) 1793-1801. [30] H.M. Kingston, L.B. Jessie, Introduction To Microwave Sample Preparation Theory And Practice, American Chemical Society, Washington. D.C. (1988). [31] M. Al-Harahsheh, S. Kingman, S. Bradshaw, The reality of non-thermal effects in microwave assisted leaching systems? Hydrometallurgy (2006) 84 1-13.
22
[32] P. Hartlieb, M. Toifl, F. Kuchar, R. Meisels, T. Antretter, Thermo-physical properties of selected
hard
rocks
and
their
relation
to
microwave-assisted
comminution,
http://dx.doi.org/10.1016/j.mining.2015.11.08. [33] F.K. Crundwell, The dissolution and leaching of minerals: Mechanism, myths and misunderstandings, Hydrometallurgy (2013) 139 132-140. [34] O. Levenspiel, Chemical reaction engineering, Wiley, New York (1999).
23
Figure captions Fig. 1. XRD pattern of Narwapahar uranium ore Fig. 2. Microscopic photograph of ground feed from Narwapahar ore. a) Liberated and composite grain of uraninite and b) uraninite in association with chlorite (inset alpha track). Fig. 3. Effect of microwave irradiation on leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, particle size: 300 µm, temperature: 75°C). Fig. 4. Effect of acid concentration on microwave leaching of uranium as a function of time (MnO2: 5 kg/t, particle size: 300 μm, temperature: 75°C, input microwave power: 600W) Fig. 5. Effect of redox potential on microwave leaching of uranium as a function of time (H2SO4: 0.38M, particle size: 300 μm, temperature: 75°C, input microwave power: 600W). Fig. 6. Effect of particle size on microwave leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, temperature: 75°C, input microwave power: 600W) Fig. 7. Effect of temperature on microwave leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, particle size: 300 μm, input microwave power: 600W) Fig. 8. Microwave leaching of uranium from a low grade ore – Arrhenius plot. Fig. 9. Dissolution pattern of Silica, Alumina, iron and magnesia under microwave and conventional conditions. Fig. 10. SEM-EDS of leach residue in a) conventional, b) microwave.
24
Ura
Fig. 1.
Q, B
210
: Uraninite : Pyrite : Chalcopyrite : Molybdenite : Epidote : Chlorite : Biotite : Quartz
Ura
Py
Ura
Py
Py
Py
Cp Ep
Ch
Ura
Cp
Py
Q
60
B
Mol
Ch
110
Ura
160
B
Intensity (cps)
Ura Py CP Mol Epi Ch B Q
Py
260
60
50
2 theta
40
30
20
10
10
25
Fig. 2 a & b.
a
b
26
Fig. 3.
Fig. 4.
27
Fig. 5.
Fig. 6.
28
Fig. 7.
Fig. 8.
29
Fig. 9
30
Fig. 10a & b.
a
31
b
32
Table 1 Mineralogical composition of Narwapahar ore Minerals
Weight (%)
Quartz
45.00
Chlorite/biotite
50.00
Magnetite
3.00
Pyrite and chalcopyrite
0.67
Apatite
1.00
Tourmaline
0.30
Radioactive minerals
0.03
(uraninite)
Table 2. Whole rock composition of Narwapahar ore Elements (Oxides)
Weight (%)
SiO2
55.43
TiO2
0.52
Al2O3
14.83
Fe2O3
3.71
FeO
8.66
MnO
0.08
MgO
4.13
CaO
2.78
Na2O
1.13
K2O
2.45
P2O5
1.60
U3O8
0.025
H2O
0.13
LOI
3.80
33
Table 3 Sulfuric acid leaching of uranium from Narwapahar ore: Optimum parameter in conventional leaching experiments [29] Parameters
Range
Optimum parameter
Temperature (°C)
35°-95° C
75° C
Acid
concentration 0.12-0.50
% U3O8 leachability
0.38
(Moles) Oxidant dosage/redox 3-10 kg MnO2/t potential (MnO2 kg/t)
(400-500 mV)
Particle size (µm)
300-600 µm
72%
5 kg/t (450 mV) 300 µm
Table 4 Microwave power settings used for irradiation of ore slurry
Set Temp. (0C)
Average MW power (Watts)/Energy (kJ) received during ramping period of 5 min.
Average MW power (Watts)/ Energy (kJ) received after attaining set temperature (Total reaction time 90 min.)
300
75
65 (19.5)
49 (264.6)
600
75
97 (29.1)
55 (297.0)
Input MW Power (Watts)
Table-5 Kinetics of MW leaching: Apparent rate constant at different temperature
Temperature (°C)
Apparent rate constant (min-1)
Coefficient determination (R2)
Reaction control
Reaction control
Diffusion control
(kc)
Diffusion control (kd)
35
0.0019
0.0011
0.9094
0.9817
55
0.0027
0.0020
0.8736
0.9844
75
0.0038
0.0036
0.8708
0.9811
95
0.0042
0.0043
0.8804
0.9874
34
Table 6 Kinetics of MW leaching: Apparent rate constant and coefficient determination by variation of acid concentration, oxidant dosage and particle size H2SO4 (Mole)
Apparent Rate constant
Coefficient determination ( R2)
MnO2
Apparent Rate (Mole) constant
(min-1)
Apparen t Rate constant
(min-1)
Coeffi Size cient (µm) determ ination ( R2 )
Coefficient determinati on ( R2)
(min-1)
0.12
0.0030
0.9969
0.11
0.0028
0.9839
600
0.0020
0.9901
0.24
0.0034
0.9920
0.19
0.0036
0.9814
425
0.0026
0.9874
0.38
0.0039
0.9900
0.27
0.0038
0.9841
300
0.0036
0.9811
0.50
0.0042
0.9794
0.38
0.0042
0.9942
Table 7 Kinetics of MW leaching: Order of reaction by variation of acid concentration, oxidant dosages and particle size Parameters
Order of reaction
Coefficient of determination (R2)
Acid concentration
02372
0.9853
Oxidant dosage
0.3193
0.9600
Particle size
-0.8481
0.9965
35
S0304-3894(16)30272-2 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.03.050 HAZMAT 17562
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-10-2015 12-3-2016 19-3-2016
Please cite this article as: V.Madakkaruppan, Anitha Pius, T.Sreenivas, Nitai Giri, Chanchal Sarbajna, Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.03.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of microwaves on the leaching kinetics of uraninite from a low grade ore in dilute sulfuric acid
V.Madakkaruppana, Anitha Piusb*, T.Sreenivasc, Nitai Giric and Chanchal Sarbajnaa
a
Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad-500016
b c
Department of Chemistry, Gandhigram Rural Institute, Gandhigram, Dindigul district, Tamil Nadu-624302
Mineral Processing Division, Bhabha Atomic Research Center, AMD Complex, Begumpet, Hyderabad-500016
*
Corresponding author.
Anitha Pius Department of Chemistry, Gandhigram Rural Institute, Gandhigram, Dindigul district, Tamil Nadu 624 302, India Telephone/Fax +91-0451-2452371/+91-9443108504 E-mail: [email protected]
1
Highlights
U Leaching from a low-grade Si-rich ore studied in H2SO4 medium with (MW) irradiation.
MW heating is more efficient in terms of U recovery, kinetics and purity of liquor.
U leachability of 84% obtained in 90 min. at 95°C with 0.38 M H2SO4 at 450mV.
Conventional conductive heating gave about 74% leachability with less purity liquor.
U leaching was found to be product layer diffusion as controlling mechanism.
Abstract This paper describes a study on microwave assisted leaching of uranium from a low-grade ore of Indian origin. The host rock for uranium mineralization is chlorite-biotite-muscovitequartzo-feldspathic schist. The dominant presence of siliceous minerals determined leaching of uranium values in sulfuric acid medium under oxidizing conditions. Process parametric studies like the effect of sulfuric acid concentration (0.12-0.50M), redox potential (400-500 mV), particle size (600-300 µm) and temperature (35°-95°C) indicated that microwave assisted leaching is more efficient in terms of overall uranium dissolution, kinetics and provide relatively less impurities (Si, Al, Mg and Fe) in the leach liquor compared to conventional conductive leaching. The kinetics of leaching followed shrinking core model with product layer diffusion as controlling mechanism. Keywords: Uranium, Microwaves, Acid Leaching, Leach Kinetics.
1. Introduction Uranium is one of important strategic metal where large supply demand gap is predicted in coming decades [1]. The interest in uranium metal stems from its widespread use in commercial nuclear power reactors (Light Water or Pressurized Heavy Water variety) as a main fuel source [2]. Nuclear energy is attracting renewed world-wide attention, 2
particularly in developing nations like China and India, as a large-scale energy generating source, mainly because it is bestowed with the advantage of very-low or negligible green house gas emissions (GHG) during the process [3]. The main source of uranium is conventional ore deposits. Uranium is recovered from any land based resource like ores or tailing dumps etc. by hydrometallurgical process, consisting of size reduction, leaching, solid-liquid separation, purification and precipitation of dissolved uranium as yellow cake [46]. Leaching is a critical unit operation in the process flowsheet and temperature of reaction is an important controlling parameter for this unit operation. The complexity in processing resources of above-cited variety arises predominantly due to the chemical reactivity of gangue or unwanted minerals along with minerals of interest or valuable minerals. During chemical leaching, which is the stage where the release of various ionic species from solid mineral phase/s to aqueous medium occurs, presence of unwanted ionic species in solution phase increases the rigorousness of purification operations like ion exchange or solvent extraction, besides aggravating the effluents treatment protocol [5]. Therefore, research is pursued in various laboratories on methods of achieving selectivity during the leaching stage itself to the extent possible.
The chief methods pursued in this endeavor include
manipulation of chemical conditions [6] or/and application of microwaves which is having the attribute of selective heating of minerals [7]. The application of microwave technology in minerals processing was being studied by isolated groups world-wide for last two decades [7-20]. Considering its potential, several reviews on principles and application of microwaves in different ore processing unit operations like size reduction, roasting, flotation, leaching etc. was reported [7, 8]. However, its relevance for low-grade or other complex resources is better felt now than ever before due to concomitant developments in scaling-up of the microwave technology [21]. With regard to leaching operation microwave has distinct advantage over conventional conductive heating
3
method. Microwave heating is rapid and selective in nature, distributes uniformly in the cavity, high efficiency, fast switch on and off as well as flexible and available in modular design.
Microwave heating is a kind of electro-heat process such as induction, radio
frequency, direct resistance, and infrared heating that utilize specific parts of electromagnetic energy [17, 18]. Microwave energy is non-ionized electromagnetic radiation which has frequencies changing in the range of between 300 MHz to 300 GHz. Materials that couple to microwave energy are called dielectric and contain dipoles. The dipoles align themselves in an electric field and flip around in an alternating electric field. Stored internal energy is lost through friction thus heating the minerals. Based on rate of heating (with microwaves) the minerals were classified into hyperactive, active, difficult-to-heat and inactive. From the large number of minerals and inorganic compounds tested by several laboratories it was noted that most silicates, carbonates and sulfates, and some oxides and sulfides are transparent to microwave energy, while most sulfides, arsenides, sulfosalts and sulfarsenides, and some oxides, heat well when subjected to microwave irradiation [7]. For instance it was reported that pure UO2 attained a temperature of 1100° C in 0.1 min of irradiation with MW of 2450 MHz and heating rate noticed was 200° C per sec [7]. In comparison to this the SiO2 and CaCO3 which are the common host rocks for uranium minerals heats at the rate of 2 – 5 and 5 °C/sec [7]. In view of these positive factors like improved extraction efficiency and attractive leaching kinetics investigations on application of microwaves for leaching of uranium values from a low-grade under oxidizing conditions using sulfuric acid as lixiviant are carried out and results discussed. Though studies were carried out earlier on MW assisted dissolution of spent nuclear fuel and thorium [22, 23] it is for the first time an attempt is being on evaluating its role on leaching of uranium from its ore. Though the application of MW for treating the ores is highly energy intensive the enormous energy packed in uranium nucleus
4
would definitely give net positive energy balance and this factor is the driving force behind these investigations [24].
2. Leaching chemistry Leaching of oxides of uranium in sulfuric acid medium takes place by two steps, (i) oxidation of insoluble U (IV) to U (VI) and (ii) complexation of oxidized U (VI) by sulphate anions to form uranyl sulphates. The uranyl sulfates remain as stable aqueous complexes under specified redox and pH conditions [4-6]. The leaching process is reported to follow electrochemical mechanism depicted in the equation 1 to 3 [25]. Sulfuric acid leaching of uranium minerals requires the presence of Fe (III) regardless of the reagent used as an oxidant [26, 27]. Fe (III) oxidizes U (IV) as given in Eq. 1, while the oxidant reagent oxidizes the Fe (II) to Fe (III) as given in Eq. 2; thus, the Fe (II) – Fe (III) couple serves as an electron transfer catalyst between the oxidant and UO2. 2
→
2
2 4
→2
(1) 2
(2)
The chemical interaction of soluble UO22+ species with sulfate anions yields uranyl sulphate complexes like UO2 (SO4)n2-2n as illustrated in Eq. 3. →
(3)
Satisfactory leaching of uranium values occurs by maintaining a redox potential (Eh) of 400 500 mV vs saturated calomel electrode (SCE.) [26, 27].
3. Experimental 3.1. Characterization studies The uranium ore sample used in the present studies was collected from Narwapahar mine, Singhbhum district, Jharkhand, India. The ore was crushed and ground to 600 µm, 425
5
µm and 300 µm size using laboratory jaw crusher and rod mill. All the chemicals used in the test work were of AR grade from standard chemical suppliers like BDH, Loba and Merck. Identification of various phases in the ore sample was made using Siemens D500 Xray diffractometer (XRD) with CuKα radiation (1.5418Ȧ) as source.
The accelerating
voltage and amperage was 35 kV and 22 mA. A 1° divergence slit was used to analyze between the 2θ range of 5 - 90° with a step size of 0.02° and 2s/step on a 30s delay. Phase identification was made with the aid of International Centre for Diffraction database (ICDD). Quantitative mineralogical composition of the Narwapahar uranium ore was estimated using optical microscopy. Micro-morphological study on the leach residue was performed using Philips XL-20 Scanning Electron Microscope attached with Energy Dispersive analyzer (SEM-EDS). Uranium content in the ore and various product streams were determined using pellet fluorimetry on Model FL6224 fluorimeter (M/s. Electronic Corporation of India Limited, India). Powdered rock sample weighing 1 g accurately was mixed with 3 ml HF and 5 ml HNO3 and the contents were evaporated to dryness on hot-water bath. This was carried out for two more times. 5 ml of HNO3 was further added to the contents and re-evaporated to dryness. This was also repeated for two times. Finally, 10 ml of HNO3 was added to the residue in the container and after about 30 minutes of digestion the mixture was filtered. The filtrate was collected in 100 ml volumetric flask and the balance volume adjusted with 10 % HNO3. Uranium was extracted with ethyl acetate using aluminum nitrate as the salting-out agent. The aliquot from the organic layer was dried under an infrared lamp, combined with a flux (Na2CO3: NaF = 4:1) in a platinum planchet, and fused at 850° C. After cooling, the fused pellet was irradiated using Hg-vapor lamp at 365 nm wavelength. The characteristic uranium fluorescence emanating from the sample pellet at 555 nm was measured by PMT detector.
6
Silica content in the ore was determined by UV-visible spectrophotometer (Unicam) by molybdenum-blue method. The procedure involves decomposing the ore samples (0.20g) with NaOH in nickel crucibles, followed by water leaching and acidification with HCl. The clear solution was used for estimations in UV-Vs spectrophotometer. Na and K were determined by Flame photometer (Flame photometer-128, M/s Systronics, India). 0.04g of sample was weighed in a platinum crucible; 2 ml of hydrofluoricsulphuric acid mixture was added followed by heating the crucible gently on a hot plat until white fumes of sulphuric acid evolved. The contents were cooled and 5-10 ml of warm deionized water added to bring the solid residue into solution. The contents were transferred to a 50 ml volumetric flask and diluted to desired volume with de-ionized water followed by aspiration. All other elements (Al, Fe, Mg, Mn, Ca & P) were determined using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (ULTIMA-2, M/s Horiba Jobin Yvon, France). The sample preparation for ICP analysis involves weighing of 0.5 g of powdered material in a platinum dish and incinerating in a furnace at 650° C for one hour for removal of organic matter. The contents of the Pt dish were treated with 5 ml of 40% HF and 3 ml. of concentrated HNO3. This mixture was evaporated to dryness on a boiling water bath. The same process was repeated three times. Further, the sample was again treated with 3 ml HNO3 for three more times and the residue of the last stage was re-dissolved in HNO3 (3ml) and the contents made up to 100 ml volume such that the final solution has 3% acidity. The moisture content in the samples was determined by accurately weighing 1.0 ± 0.05 g of ore in an analytical balance using cleaned silica crucible. The crucible contents were maintained at 110°C in an oven for 1 hour. The weight loss was recorded as moisture content of the ore.
7
The Loss on Ignition (LOI) was estimated by heating the sample in a furnace in a stepped schedule for 2 hour to 950° C.
The fired sample was cooled down to room
temperature in desiccators and then weighed. The weight loss was recorded as LOI. pH and Eh was measured on respective instruments (ELICO, India) both during start and end of the experiment. The values reported were those obtained after the completion of the experiment. The redox potential values reported are those taken with saturated calomel electrode (SCE) as reference.
3.2. Leaching studies Microwave assisted leaching experiments were carried out on a microprocessor controlled ETHOS-One microwave digester (Milestone Srl, Italy) [28]. The digester consists of 2 x 800 W magnetron operating on a 1-s duty cycle at a frequency of 2450 MHz. The delivered microwave power is 1500 W.
The unit include a 360° reversing turn-table
operating at 3 revolutions/min and a 10 position carousel. A variable-speed exhaust fan is present in the cavity for removal of corrosive fumes.
The equipment has facility for
monitoring and controlling of both external (infrared) and internal temperature (ATC400/ATCFO) of the reaction vessels in real time during the run. Similarly it also has an automatic pressure control for direct sensing of reaction pressure. Temperature and pressure sensors were connected to microwave-transparent and chemical resistant PTFE vessels in which the sample is placed. The unit is designed for maximum temperature of 300° C and pressure upto 55 bars. The entire unit is operated through a touch-screen control terminal operating on software for full GLP documentation with PID algorithms to exactly duplicate the required temperature curve. 5 g of uranium ore sample was weighed directly into the PTFE leaching vessel to which other reagents like sulfuric acid (leachant) and manganese dioxide (oxidant) were
8
added in measured quantities such that 1:3 solid/liquid ratio (weight-wise) is achieved. The vessels were sealed and positioned in the microwave carousel followed by mounting of temperature and pressure sensors. The microwave irradiation of the vessel contents was carried out in isothermal or fixed set-temperature mode without stirring.
5 minutes of
ramping time was given for attaining the predefined temperature and the reaction was subsequently carried out for 90 minutes. After the reaction period was over the contents were automatically cooled to ambient temperature and taken out for solid-liquid separation. Slurry was filtered on a vacuum filter (60 kPa) using Whatman filter paper (No.42). The leach residue was washed twice using acidified water in all the experiments. The pH and Eh of the solution was recorded and samples were chemically analyzed for various desired analytes. Data analysis was made on the basis of chemical assays of metal ions in the feed and reaction products and leachability (R) is calculated using the relation given in Eq. 4.
(4)
where R is the leach recovery in percent, vl is the volume of the leach liquor plus wash, gl is the U3O8 assay of the liquor, ws is the weight of the leach residue and gs is the U3O8 assay of the leach residue.
4. Results and discussion 4.1. Characterization of ore The optical microscopic studies indicate that the host rock for uranium mineralization in Narwapahar ore is chlorite-biotite-muscovite-quartzo-feldspathic schist.
XRD studies
indicate uraninite as principal uranium bearing phase (Fig. 1) and the major gangue minerals are quartz, chlorite, biotite, epidote and magnetite with minor quantity of apatite, albite, pyrite, chalcopyrite, zircon and molybdenite. The quantitative mineralogical composition of
9
the Narwapahar ore (Table 1) indicates presence of siliceous minerals (quartz, chlorite and biotite mica) to the extent of about 95% by weight. CN film auto-radiography for a period of 72 hours gave high density alpha tracks corresponding to uraninite mineral with a radioactive halo, occurring in close association with chlorite (Fig. 2 a & b). The U3O8 content of the ore is 0.025% (Table 2). The SiO2, Al2O3 and FeO contents are 55.4, 14.8 and 8.7% respectively confirming overwhelming presence of siliceous and micaceous minerals besides iron oxides (Table 1).
4.2. Microwave leaching The main objective in the present studies is to utilize MW irradiation as source of heat during leaching. The other process variables like the particle size of ground ore, oxidant and sulfuric acid dosage were fixed (Table 3) on the basis of results of conventional leaching studies carried out in our laboratory on identical system (Narwapahar ore) earlier [29].
4.2.1 Effect of microwave power Microwave power is a factor greatly influencing the rate of microwave heating. The dependence between the applied microwave power and the increase of temperature of the irradiated system is described by the Eq. 5 [30]: ∆
(5)
where: ΔT – the increase of the mean temperature of the heated body, P – microwave power used for heating, V volume, cp heat capacity, ρ density and t – time of heating. In order to determine the effect of MW power on the leachability of uranium minerals, the ore slurry was irradiated at two different MW power levels namely, 300 and 600 W. However, the slurry temperature was kept constant at 75° C using an inbuilt feedback system of the MW unit. This was done to maintain the same range of reaction temperatures as used in conventional 10
leaching experiments (Table 3). The PID controller in MW unit regulates the power output of magnetron to keep the solution/slurry temperature at pre-defined level (within ± 1° C). The other process conditions maintained constant in the experiment are: H2SO4: 0.38M, Eh 450 mV (using MnO2 dosage of 5kg/t), particle size: 300 µm and solid/liquid ratio (weight-wise) 1:3 and irradiation time 90 minutes. The progress of the leaching reaction was monitored kinetically. Results of the experiment are given in Fig. 3 and the MW settings and average energy received for maintaining the set temperature is given in Table 4. The slurry irradiated at input MW power of 300 W gave uranium leachability of about 28% during the initial 15 minutes of reaction period and reached about 53% in 90 minutes. The corresponding results with MW power level of 600 W, was about 46% and 77% respectively. Two notable aspects apparent from the MW experimental details (Table 4) at the two input power levels are (i) higher average MW power (or energy) received during the ramping period at 600 W in comparison to 300 W and (ii) almost identical average power (or energy) taken at both the set power levels after the slurry attained the set temperature. It was also observed that in the case of experiments carried out with MW power setting of 300 W, the duty cycles of energy bursts (on – off cycle of magnetron) are closely spaced or like a continuum and of lower energy when compared to that irradiated with 600 W power setting. In the latter case, the duty cycles are well spread out but the maximum energy delivered in each pulse is almost double than that given with 300 W power setting. Therefore, the higher energy burst delivered per pulse at 600 W power setting during the ramping period as well as that given during intermittent duty cycles resulted in upward shift of leachability by about 18 – 24% over the lower power setting of 300 W. MW assisted heating of the uranium ore slurry gave about 5% higher leachability over that attained under conventional leaching conditions for similar process conditions adopted [29]. Similar trends of enhancement were reported earlier by Zhao and Chen [22] in MW assisted leaching of pure uranium oxide in
11
nitric acid medium and by other on ores of Cu, Ni, Au and Zn [14-15, 19, 20, 22]. The results were no different in the studies carried out Xia and Pickles [13] for recovery of valuable metals from electric arc furnace dust. Al-Harahsheh et al [31] made detailed studies on the role of thermal and non-thermal effects in the MW assisted processes for aqueous dissolution of metals/minerals. Based on analysis of published information from different systems Al-Harahsheh et al [31] attributes the observed enhancement in systems irradiated with MW over the conventional methods of heating to predominantly thermal effects. The shallow penetration depth of microwave energy in conducting solutions (like leach solutions) leads to dissipation of microwave energy within the first few millimeter distance of the reactor walls. This implies that the temperature at the walls of the vessel is higher than the temperature in the bulk or in the middle where temperature sensor exists. Under such arrangement though the sensor displays only 75° C, the temperature of the leach solution and the solid particles close to the walls of the vessel is significantly high, particularly when the minerals in the ore are hyperactive or active to MW (like UO2, iron oxides and sulfides) [7]. This phenomenon becomes all the more significant when the contents of the slurry are unstirred. Dissolution studies of pure uraninite and pitchblende by conventional heating mode showed that the processes are chemically controlled and the reaction rate is sensitive to temperature changes [6]. This means that a small change in the reaction temperature results in significant change in the leachability of uranium oxide minerals. This explains the reason for enhanced leachability of uranium values from their ores when heated with microwaves.
4.2.2. Effect of acid concentration Effect of sulfuric acid concentration on the leachability of uranium was studied in the concentration range of 0.12 to 0.50 M at 75°C using input MW power of 600W. The other process conditions kept constant are: Eh 450 mV (using MnO2 dosage of 5kg/t), particle size:
12
300 µm and solid/liquid ratio (weight-wise) 1:3, irradiation time 90 minutes with no stirring of the reactor contents. Fig. 4 gives the results of the experiments. Maximum leachability, about 77% was noticed upon using sulfuric acid concentration of about 0.38 M in MW assisted process as against 72% achieved under conventional conditions. Further increase in the acidity did not show any enhancement of the dissolved uranium values.
The
improvements noticed with varying leachant dosages in MW treated slurry are also due to differential breakage of ore particles under the effect of MW [32]. Since quartz and mica, the two dominant minerals in the ore (Table 1) are transparent and low loss minerals, it is assumed that microwave energy passed through those minerals without any loss and thereby did not cause any change nor induced any stress within its matrix. However, composite particles like U in silica matrix or U in mica matrix (Figure 2) or U – Fe – silica matrix absorbs microwave energy and transform it into heat causing differential expansion and induces fractures within the composite particles facilitating enhanced accessibility of uranium phases to the leachant. Such an opportunity does not exist in conventional leaching under quiescent agitation conditions [32].
4.2.3. Effect of oxidant dosage The leaching of uranium is known to proceed through electrochemical mechanism as discussed in the Section on process chemistry of leaching therefore, the redox conditions of the slurry control the leaching efficiency critically. Effect of oxidant dosage on the leaching of uranium values from Narwapahar ore was studied by varying the redox potential from 400 to 500 mV (versus saturated calomel electrode) by addition of different dosages of manganese dioxide. The other process conditions followed in the tests are H2SO4 0.38M, temperature 75°C, particle size 300µm, MW power 600 W and reaction time 90 minutes. Higher leachability of uranium values – 70 to 81% was obtained with MW heating (Figure 5)
13
compared to 46 to 74% dissolution observed in conventional leaching –at all redox conditions namely 400 mV to 475 mV (vs SCE) investigated. The leachability reached near constancy (80%) for Eh value 500 mV.
4.2.4. Effect of particle size Effect of particle size on the rate of leaching of uranium was studied at three grind sizes 600, 425 and 300 µm by maintaining the other conditions constant like H2SO4 0.38M, Eh 450mV, temperature 75°C, MW power input 600 W, irradiation time 90 minutes and solid/liquid ratio 1:3. The results are illustrated in Fig. 6. About 63% of uranium values could be leached from the coarsely ground (600 µm) material in 90 minutes contact time under MW effect. This enhanced to 77% when the ore was reduced to finer than 300 µm size.
The phenomenon of improved dissolution with finer grind could be due to the
combination of the thermal effects and consequent micro-cracks generation as well as increase in the surface area to volume ratio which paves way for better reaction between the leaching solution and the uranium mineral phases in the ore [32].
4.2.5. Effect of temperature Effect of temperature on the leaching of uranium minerals in the ore was studied by varying reaction temperature from 35° to 95° C using input MW power of 600W and keeping the other experimental conditions constant as follows: H2SO4 0.38M, Eh 450mV, MW irradiation time 90 minutes and solid/liquid ratio 1:3. The average MW power taken by the slurry for maintaining constant temperature of 35°, 55°, 75° and 95° C during the reaction period was 25, 45, 60 and 71 W respectively. As indicated from the results in Fig. 7 the uranium dissolution increased from about 48% obtained at 35o C to a maximum of 84% at 95° C. The maximum leachability of uranium resulted under identical process conditions in
14
conventional leaching was only 74% (Table 3) [29]. The increased energy deposited in the MW absorbing minerals with increase in set temperature gives differential heating rates in heterogeneous systems creating temperature gradients between the solid and liquid phases. The temperature gradient creates large thermal convection currents in the slurry resulting in agitation and sweeping away of the reaction products. Constant exposure of new surface to leachant solution due to this action results in increased leachability [31].
4.3. Leaching kinetics Leaching is a rate phenomenon. Kinetics of leaching of a low-grade uranium ore is influenced by several operating factors like: nature of uranium mineralization in the ore and in specific the liberation size, the ratio of U(IV) to U(VI), reaction temperature, redox potential, ionic composition of solution, etc. The un-reacted shrinking-core model is the most commonly used mathematical relation to describe the heterogeneous reactions like leaching of mineral from ores. The model is well-known and requires only limited description [33, 34]. In a binary reaction system in which only solids (ore particles) and solution phase exists, the rate of leaching is controlled by (i) solid or product layer diffusion and (ii) chemical reaction. The integrated rate expressions for the product layer diffusion and surface chemical reaction models are given in Eq. 6 and 7 respectively [33]. 1
3 1
1
1
2 1
(6)
(7)
where, ‘x’ is the conversion fraction of solid particle, ‘kd’ is the apparent rate constant (min-1) for diffusion through the product layer, ‘kc’ is the apparent rate constant (min-1) for the surface chemical reaction and ‘t’ is the reaction time. 15
The degree of linearity of the plot of left hand side of Eq. 6 and 7 against the reaction time (t), gives insights into the mechanism of reactions. It is further corroborated from the activation energy value computed using the Arrhenius relation given in Eq. 8. (8)
where ‘k’ is a reaction rate constant, ‘A’ is the frequency factor, ‘Ea’ is the apparent activation energy, ‘R’ is the universal gas constant and ‘T’ is the absolute temperature. The activation energy for diffusion-controlled reactions is generally below 20 kJ/mol while it is above 40 kJ/mol for chemical reaction controlled process [33, 34]. In order to understand the type of leaching mechanism prevalent for the Narwapahar uranium ore in the MW assisted leaching, the leaching kinetic data obtained at different reaction temperatures (Fig. 7) was analysed using the kinetic rate equations given in Eq. 6 and 7 and Arrhenius activation energy from Eq. 8. The kd and kc values computed from Eq. 6 and 7 are given in Table 5 & 6. The R2 values in the plots enable in understanding the closeness of the experimental data and that predicted with models (Eq. 6 and 7). The kd values given in Table 5 vary in the rage of 0.0011 to 0.0043 min-1 while the kc were between 0.0019 and 0.0042 min-1. The R2 value for kd was 0.98 while for kc it was in the range of 0.87–0.90.
It can therefore be inferred that the predominant dissolution mechanism of
uranium from the Narwapahar ore is diffusion controlled only. The kd values obtained in conventional leaching of uranium values from Narwapahar ore were in the range of 0.0018 to 0.0035 min-1 [29], being relatively lower than that obtained with MW irradiation. The mechanism of leaching was also confirmed from the apparent activation energy (Ea) value of 9.66 kJ/mol, computed using the data given in Fig. 8 and Eq. 8. The Ea value obtained from similar exercise in conventional leaching was 7.32 kJ/mole [29].
The
insignificant difference in activation energy for both the modes of heating was reported in the
16
case of dissolution of ceramic UO2 in nitric acid medium by Zhao and Chen [22] and for chalcopyrite and sphalerite system by Al Harahsheh et al [9, 31]. The closeness of Ea values implies that the MW irradiation has not altered the intrinsic leaching mechanism generally observed in conventional leaching. Studies by Al Harahsheh et al [31] on chalcopyrite and sphalerite minerals in ferric chloride medium also attribute proximity of Ea values to the limitation of penetration of microwaves when passing through a high loss leaching solution which cause significant temperature difference between the outer shell of the leaching solution or the zone close to walls of the reaction vessel where the MW directly impinge and the bulk position where the temperature probe exists. This behaviour is more pronounced for unstirred slurry system. The general equation relating the important operational parameters with the rate constant ‘kd’ is given in Eq. 9 [34].
exp
(9)
where ‘ko’ is the frequency factor or pre-exponential factor (min-1), ‘C’ is concentration of reagent/s or leachants in solution (moles/liter), ‘ro’ is the particle size, ‘p’ and ‘q’ are constants indicating the order of the reaction. The order of the reaction was determined from the slope of the lines generated by plotting the natural logarithm of apparent rate constants or ‘ln kd’ vs the natural logarithm of the respective variable. In the case of leaching of Narwapahar uranium ore it was noted that reaction temperature, concentration of sulphuric acid, manganese dioxide and the particle size of the ground ore were critical variables. Therefore the kinetic plots of these variables given in Table 7 were taken for determining the ‘kd’ values (Table 6) which were used for estimating the order of the reaction depicted in Eq. 10. Eq. 11 gives the relation developed for leaching of uranium from Narwapahar ore by conventional method [29], for comparison.
17
1
3 1
/
2 1
.
1
3 1
/
2 1
.
.
.
.
.
.
(10) .
(11)
The order of the reaction with respect to the particle size is relatively lower in MW irradiated system when compared to that obtained with conventional leaching.
5. Characterization of leaching products 5.1. Characterization of leach liquor The leach liquors obtained from the experiments carried out under optimized conditions (H2SO4 0.38M, Eh 450mV, MW power 600 W, temperature 75°C, irradiation time 90 minutes and solid/liquid ratio 1:3) were analysed for Si, Al, Fe and Mg to understand the gangue minerals (quartz, mica minerals, iron oxides) reactivity and compare the selectivity of MW assisted leaching versus the conventional leaching. The results are shown in Fig.9. Leaching of gangue elements was comparatively less under microwave irradiation conditions than the conventional heating. The concentration of various ions in the leach liquor obtained with MW irradiation was Si 6.62g/l, Al 4.54g/l, total Fe 9.50g/l and Mg 3.97 g/l while under conventional conditions the assays were Si 7.27g/l, Al 5.39 g/l, total Fe 11.20 g/l and Mg 4.19 g/l. 5.2. Characterization of leached residue The solid leach residues obtained under optimized conditions of leaching in microwave and conventional heating were characterized SEM-EDS technique. The morphology of leach residue (Fig. 10a) obtained under conventional conditions has clear surface, regular size and shape while that obtained with MW irradiation (Fig. 10b) was irregular and needle shaped with dominant presence of broken particles.
18
6. Conclusions Microwaves assisted leaching of uraninite from a low grade ore of Indian origin was investigated. The host rock for uranium mineralization is chlorite-biotite-muscovite-quartzofeldspathic schist. Leaching was carried out under oxidizing conditions using sulfuric acid as leachant and manganese dioxide as oxidant. Process parametric variation studies investigated include the MW irradiation power, dosages of leachant and oxidant (for Eh control), particle size and temperature of reaction. Kinetics of the reaction was also monitored for elucidating the mechanism of leaching. The results of the studies indicate that about 84% of uranium values can be leached in 90 minutes of irradiation with MW (average power 71 W) by maintaining other conditions like H2SO4 0.38 M, Eh 450 mV and temperature 95 °C. The uranium lechability in conventional conductive heating was about 74% with similar process conditions. The enhancement is mainly due to high microwave absorption ability of uranium oxide minerals over the silicate minerals matrix leading to rapid rise in localized temperature resulting improved dissolution. The kinetics of uranium leaching in MW assisted heating satisfied the shrinking core kinetic model with product layer diffusion process as principle mechanism. The Arrhenius activation energy was about 9.66 kJ/mole. MW assisted heating also gave leach liquors with relatively higher purity over that obtained in conventional leaching.
Acknowledgements Authors are very much thankful to Dr. A.K.Rai, Director, Dr. A.K.Chaturvedi, Additional Director (R & D) and Dr.Yamuna Singh, Head, MPG Group, Atomic Minerals Directorate for Exploration and Research for kind permission for publishing this research article. The authors are also thankful to MPG and Chemistry Group, AMD, Hyderabad for analytical support.
19
References: [1] Uranium 2014: Resources, Production and Demand, A Joint Report by the OECD Nuclear Energy Agency and the International Atomic Energy Agency (Vienna), NEA No. 7209, OECD (2014) 17 – 131 [2] Nuclear Power Reactors in the World, IAEA-RDS-2/35 ISBN 978–92–0–104915–5, ISSN 1011–2642, International Atomic Energy Agency Vienna (2015) 79. [3]http://www.world-nuclear.org/Nuclear-Basics/Greenhouse-gas-emissions-avoided (accessed in Jan 2016) [4] R.C. Merritt, The Extractive Metallurgy of Uranium, Colorado School of Mines research Institute, Colorado, (1971) 576 [5] C. Edwards, A. Oliver, Uranium processing: a review of current methods and technology, JOM (2000) 52 (9) 12–20. [6] S.K. Bhargava, Rahul Ram, Mark Pownceby, Stephen Groncott, Bob Ring, James Tardio, Lathe Jones, A review of acid leaching of uraninite, Hydrometallurgy (2015) 151 10-24. [7] K.E. Haque, Microwave energy for mineral treatment processes – a brief review, Int. J.Miner. Process (1999) 57 1-24. [8]
M.
Al-Harahsheh,
S.W.
Kingman,
Microwave-assisted
leaching-a
review,
Hydrometallurgy (2004) 73 (3-4) 189-203. [9] M. Al-Harahsheh, S.W. Kinghman, N. Hnkins, C. Somerfield, Bradshaw, W. Louw, The Influence of microwaves on the leaching kinetics of chalcopyrite, Miner.Eng. (2005) 18 1259-1268. [10] N.A. Rowson, N.M. Rice, Magnetic enhancement of pyrite by caustic microwave treatment, Miner.Eng. (1990) 3 (3-4), 363-368. [11] S. Marland, B. Han, A. Merchant, N. Rawson, The effect of microwave radiation on coal grindability, Fuel (2000) 79 (11) 1283-1288.
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[12] K.E. Haque, Microwave irradiation pre-treatment of refractory gold concentrate, In: R.S. Salter, D.M. Wysouzil, G.W. McDonald (Eds.), Proceedings of the International Symposium on Gold, Winnipeg, Canada (1987) 327-339. [13] D.K. Xia, C.A. Pickles, Microwave caustic leaching of electric arc furnace dust, Miner.Eng. (2000) 13 79-94. [14] P.A. Olubambi, J.H. Potgieter, J.Y. Hwang, S. Ndlovu. Influence of microwave heating on the processing and dissolution behaviour of low-grade complex sulphide ores, Hydrometallurgy 89 (2007) 127–135. [15] Y. Hua, Z. Lin, Z. Yan, Application of microwave irradiation to quick leach of zinc silicate ore, Miner.Eng. 15 (2002) 451–456. [16] M. Lova, I. Murova, A. Mockovciakova,N. Rowson, S. Jakabsky, Intensification of magnetic separation and leaching of Cu-ores by microwave radiation, Separation and Purification Technology 31 (2003) 291-299. [17] C.A. Pickles, Microwaves in extractive metallurgy: Part 1 – Review of fundamentals, Miner.Eng. (2009) 22 1102–1111. [18] C.A. Pickles, Microwaves in extractive metallurgy: Part 2 – A review of applications, Miner.Eng. (2009) 22 1112–1118. [19] C.A. Pickles, Microwave heating behaviour of nickeliferous limonitic laterite ores, Miner.Eng. (2004) 17 775–784. [20] B. Nanthakumar, C.A. Pickles, S. Kelebek,
Microwave pretreatment of a double
refractory gold ore, Miner.Eng. (2007) 20 (11) 1109–1119. [21] F. Bergamelli, M. Iannelli, J.A. Marafie, J.D. Moseley, A Commercial Continuous Flow Microwave Reactor Evaluated for Scale-up, Organic Process Research & Development (2010) 14 (4) 926-930.
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[22] Zhao YunGeng, Chen Jing, Kinetics study on the dissolution of UO2 particles by microwave and conventional heating in 4 mol/l nitric acid, Sci China Ser B-Chem (2008) 517 700-704. [23] G.K. Mallik, Microwave Processing in Thorium Fuel Cycle, BARC News letter (http://www.barc.gov.in) (2013) 331 26-30. [24] Physics of Uranium and Nuclear Energy, http://www.world-nuclear.org/info/NuclearFuel- Cycle/Introduction/Physics-of-Nuclear-Energy. [25] A.R. Burkin, Chemical Hydrometallurgy : Theory and Principles, Imperial College Press (London) (2001) 424. [26] R.J. Ring, Ferric sulphate leaching of some Australian uranium ores, Hydrometalurgy (1980) 1-2 89–101. [27] M.J. Maley, S. Burling, R. Ring, The effect of oxidation reduction potential and ferric iron concentration on leaching of uranium ores. In: Lam EK, Rowson JW, Ozberk E (eds) Proceedings of the 3rd international conference on uranium. 40th annual hydrometallurgy meeting, Saskatoon, Saskatchewan (2010) (1) 563–574. [28] Ethos One-Operator Manual MA133, Milestone Srl (www.milestonesrl.com). [29] V. Madakkaruppan, Anitha Pius, T. Sreenivas, K. Shiv Kumar, Leaching kinetics of uranium from a quartz-chlorite-biotite rich low grade Indian ore, J Radioanal Nucl Chem (2015) 303 (3) 1793-1801. [30] H.M. Kingston, L.B. Jessie, Introduction To Microwave Sample Preparation Theory And Practice, American Chemical Society, Washington. D.C. (1988). [31] M. Al-Harahsheh, S. Kingman, S. Bradshaw, The reality of non-thermal effects in microwave assisted leaching systems? Hydrometallurgy (2006) 84 1-13.
22
[32] P. Hartlieb, M. Toifl, F. Kuchar, R. Meisels, T. Antretter, Thermo-physical properties of selected
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Figure captions Fig. 1. XRD pattern of Narwapahar uranium ore Fig. 2. Microscopic photograph of ground feed from Narwapahar ore. a) Liberated and composite grain of uraninite and b) uraninite in association with chlorite (inset alpha track). Fig. 3. Effect of microwave irradiation on leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, particle size: 300 µm, temperature: 75°C). Fig. 4. Effect of acid concentration on microwave leaching of uranium as a function of time (MnO2: 5 kg/t, particle size: 300 μm, temperature: 75°C, input microwave power: 600W) Fig. 5. Effect of redox potential on microwave leaching of uranium as a function of time (H2SO4: 0.38M, particle size: 300 μm, temperature: 75°C, input microwave power: 600W). Fig. 6. Effect of particle size on microwave leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, temperature: 75°C, input microwave power: 600W) Fig. 7. Effect of temperature on microwave leaching of uranium as a function of time (H2SO4: 0.38M, MnO2: 5 kg/t, particle size: 300 μm, input microwave power: 600W) Fig. 8. Microwave leaching of uranium from a low grade ore – Arrhenius plot. Fig. 9. Dissolution pattern of Silica, Alumina, iron and magnesia under microwave and conventional conditions. Fig. 10. SEM-EDS of leach residue in a) conventional, b) microwave.
24
Ura
Fig. 1.
Q, B
210
: Uraninite : Pyrite : Chalcopyrite : Molybdenite : Epidote : Chlorite : Biotite : Quartz
Ura
Py
Ura
Py
Py
Py
Cp Ep
Ch
Ura
Cp
Py
Q
60
B
Mol
Ch
110
Ura
160
B
Intensity (cps)
Ura Py CP Mol Epi Ch B Q
Py
260
60
50
2 theta
40
30
20
10
10
25
Fig. 2 a & b.
a
b
26
Fig. 3.
Fig. 4.
27
Fig. 5.
Fig. 6.
28
Fig. 7.
Fig. 8.
29
Fig. 9
30
Fig. 10a & b.
a
31
b
32
Table 1 Mineralogical composition of Narwapahar ore Minerals
Weight (%)
Quartz
45.00
Chlorite/biotite
50.00
Magnetite
3.00
Pyrite and chalcopyrite
0.67
Apatite
1.00
Tourmaline
0.30
Radioactive minerals
0.03
(uraninite)
Table 2. Whole rock composition of Narwapahar ore Elements (Oxides)
Weight (%)
SiO2
55.43
TiO2
0.52
Al2O3
14.83
Fe2O3
3.71
FeO
8.66
MnO
0.08
MgO
4.13
CaO
2.78
Na2O
1.13
K2O
2.45
P2O5
1.60
U3O8
0.025
H2O
0.13
LOI
3.80
33
Table 3 Sulfuric acid leaching of uranium from Narwapahar ore: Optimum parameter in conventional leaching experiments [29] Parameters
Range
Optimum parameter
Temperature (°C)
35°-95° C
75° C
Acid
concentration 0.12-0.50
% U3O8 leachability
0.38
(Moles) Oxidant dosage/redox 3-10 kg MnO2/t potential (MnO2 kg/t)
(400-500 mV)
Particle size (µm)
300-600 µm
72%
5 kg/t (450 mV) 300 µm
Table 4 Microwave power settings used for irradiation of ore slurry
Set Temp. (0C)
Average MW power (Watts)/Energy (kJ) received during ramping period of 5 min.
Average MW power (Watts)/ Energy (kJ) received after attaining set temperature (Total reaction time 90 min.)
300
75
65 (19.5)
49 (264.6)
600
75
97 (29.1)
55 (297.0)
Input MW Power (Watts)
Table-5 Kinetics of MW leaching: Apparent rate constant at different temperature
Temperature (°C)
Apparent rate constant (min-1)
Coefficient determination (R2)
Reaction control
Reaction control
Diffusion control
(kc)
Diffusion control (kd)
35
0.0019
0.0011
0.9094
0.9817
55
0.0027
0.0020
0.8736
0.9844
75
0.0038
0.0036
0.8708
0.9811
95
0.0042
0.0043
0.8804
0.9874
34
Table 6 Kinetics of MW leaching: Apparent rate constant and coefficient determination by variation of acid concentration, oxidant dosage and particle size H2SO4 (Mole)
Apparent Rate constant
Coefficient determination ( R2)
MnO2
Apparent Rate (Mole) constant
(min-1)
Apparen t Rate constant
(min-1)
Coeffi Size cient (µm) determ ination ( R2 )
Coefficient determinati on ( R2)
(min-1)
0.12
0.0030
0.9969
0.11
0.0028
0.9839
600
0.0020
0.9901
0.24
0.0034
0.9920
0.19
0.0036
0.9814
425
0.0026
0.9874
0.38
0.0039
0.9900
0.27
0.0038
0.9841
300
0.0036
0.9811
0.50
0.0042
0.9794
0.38
0.0042
0.9942
Table 7 Kinetics of MW leaching: Order of reaction by variation of acid concentration, oxidant dosages and particle size Parameters
Order of reaction
Coefficient of determination (R2)
Acid concentration
02372
0.9853
Oxidant dosage
0.3193
0.9600
Particle size
-0.8481
0.9965
35