Understanding sulphuric acid leaching of uranium

Hydrometallurgy 155 (2015) 125–131

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Understanding sulphuric acid leaching of uranium from ore by means of 234 U/238U activity ratio as an indicator Bagdat Satybaldiyev a,⁎, Jukka Lehto b, Juhani Suksi b, Hanna Tuovinen b, Bolat Uralbekov a,c, Mukhambetkali Burkitbayev a a b c

Al-Farabi Kazakh National University, Department of General and Inorganic Chemistry, Almaty, Kazakhstan University of Helsinki, Department of Chemistry, Laboratory of Radiochemistry, Helsinki, Finland LLP “EcoRadSM”, Almaty, Kazakhstan

a r t i c l e

i n f o

Article history: Received 28 January 2015 Received in revised form 23 April 2015 Accepted 26 April 2015 Available online 29 April 2015 Keywords: Column leaching Uranium extraction Uranium redox state 234 U/238U activity ratio

a b s t r a c t Column leaching test was carried out for uranium ore samples obtained from a uranium in-situ leaching (ISL) mining site using 2.45 bed volumes (BVs) of 0.255 M sulphuric acid to study the evolution of uranium concentration and its isotopic 234U/238U activity ratio in the pregnant leach solution. The activity ratio of the two uranium isotopes in the pregnant leach solution was measured by alpha-spectrometry following radiochemical separation. Three phases of uranium leaching were identified based on the observed uranium concentration and the 234 U/238U activity ratio. The first phase, up to 0.8 BVs, was characterised by low uranium extraction at approximately 3% and a 234U/238U activity ratio in the range of 1.21–1.25, which can be attributed to hexavalent uranium leaching from mineral grain surfaces. The second phase, up to 1.8 BVs, with somewhat over 80% uranium extraction is related to the oxidation and dissolution of tetravalent uranium from uranium-bearing minerals, mostly uraninite, with a 234U/238U activity ratio of 0.92–1.00. Uranium leaching was very slow in the third phase (between 1.8 and 2.45 BVs), with only 0.5% leached and the 234U/238U activity ratio raised to 1.05–1.16. The total recovery of uranium at 2.45 BVs was 85%. © 2015 Elsevier B.V. All rights reserved.

1. Introduction As worldwide demand for energy continues to increase resulting from population growth and the emergence of new, fast-growing economies, nuclear power is likely to play a significant role in meeting future energy needs, reducing over-reliance on fossil fuels while achieving supply security and minimising carbon emissions. Kazakhstan is a major producer of uranium, with over a quarter of the worldwide production in 2012 mined in Kazakhstan. Uranium extraction by in-situ methods involving underground leaching using dilute sulphuric acid solutions has been conducted in Kazakhstan since the 1970s (Mudd, 2001). The in-situ leaching (ISL) method is based on the extraction of metals from the underground bedding layers by chemical solutions and on their recovery from the pregnant leach solution at the surface. Uranium extraction is conducted by injecting a leaching solution, typically sulphuric acid, into the roll front below the water table and through processes resulting in oxidation, dissolution and complexation of uranium from its minerals; the pregnant leach solution is directed through outlet wells to the surface and uranium is recovered from it (Benes et al., 2001).

⁎ Corresponding author. E-mail address: [email protected] (B. Satybaldiyev).

http://dx.doi.org/10.1016/j.hydromet.2015.04.017 0304-386X/© 2015 Elsevier B.V. All rights reserved.

Significant improvements have been made over the years concerning the leaching techniques employed to extract uranium from the ore bodies. Many physico-chemical studies on the effectiveness of uranium leaching using various complexing agents and oxidants have indeed been conducted as part of this optimisation process (Beletskii et al., 1997; Mamilov et al., 1980; Munoz et al., 1995). Problems related to low uranium extraction yields in certain rock types still remain in in-situ mining, despite great efforts to identify the mechanism involved in uranium leaching processes when using sulphuric acid as a complexing agent. A number of reasons have been proposed by experts to explain the low uranium recoveries, including lack of selectivity in the leaching solution employed (with a consequent need of introducing more effective oxidants to improve extraction), the immobilisation of layer mudding formation, incorrect borehole placement and formation of bacterial films in the borehole walls (Mamytbekov et al., 1998; Mudd, 2001). A need exists to achieve a better understanding of the interactions between ISL solutions and rock materials with a view for improving the uranium extraction yield in process solutions. Past physico-chemical studies on sulphuric acid leaching have not considered isotopic signatures and the disequilibrium between different nuclides pertaining to the uranium chain series (Beletskii et al., 1997; Brovin et al., 1997; Mamilov et al., 1980). Uranium isotopic disequilibrium techniques have been extensively used since the mid1950s to e.g., locate uranium deposits, estimate groundwater conditions

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Fig. 1. Cross section of an aquifer with a redox front.

and establish the mixing degree of different groundwater sources (Osmond and Ivanovich, 1992; Suksi et al., 2001; Uralbekov et al., 2014). Our study reports changes in total uranium concentration with 234 U/238U activity ratio during leaching in a laboratory column test and provides interpretation to explain variations of different elements, particularly uranium, in the pregnant leach solution. The main aim is to demonstrate the use of uranium isotopic disequilibrium as a tool for better interpreting uranium extraction processes and monitoring its migration from the ore minerals. 2. 234U/238U disequilibria in uranium roll bedding deposits The 234U/238U activity ratio in geological materials can vary due to physical and chemical processes. Studies on developing the conceptual model of oxidative isotope fractionation of 234U have shown that 234 Th, the daughter of 238U, is due to alpha-recoil pushed into areas of rock matrix where oxygen atoms accumulate around it (Adloff and Roessler, 1991; Regil et al., 1989). Extra energy generated during decay may increase the oxidation potential at the end of the 234Th recoil trajectory. As a result 234U may mobilise at the more mobile hexavalent oxidation state while 238U mostly remains at the less mobile tetravalent state. The behaviour of uranium isotopes and their activity ratio are essential information in monitoring the genesis of uranium at different geological sites such as ore deposits. One of the most interesting sites for observing uranium isotope disequilibria are uranium roll bedding deposits. The uranium roll front moves slowly downflow, in the direction of the fluid flow. It recycles continuously by dissolving from the oxidising zone and moving to the reducing zone at the front where it precipitates again. As the roll front advances the precipitated uranium becomes part of the oxidising zone again. During the motion of the 238 U downflow, its daughter nuclide 234U moves ahead to some extent because of recoil mobilisation (Osmond and Ivanovich, 1992). Fig. 1 presents the 234U/238U activity ratio changes in an aquifer solid during its downflow motion through a redox front. Information on uranium disequilibrium in roll bedding deposits will be used in this study to identify the roll front zone from which ore samples were collected and also to interpret the features of uranium mineral leaching. 3. Materials and methods 3.1. The ore sample and its characterisation A uranium ore sample originating from a deposit in South Kazakhstan was obtained as drill chips, sampled by drilling plants from three newly drilled wells. This ore deposit is related to infiltration type and is located in the south-eastern part of Shu-Sharasu uranium province, where uranium moralization is associated with regional

zones of formation oxidation. The uranium content of the drilled wells is close to the average of the entire site. The b 2-mm particle size fraction was separated for the column test, after the air-dry sandy material was thoroughly stirred. This treatment removed the inclusions of pebbles, gravel and rock fragments, which may cause problems in modelling (Benes et al., 2001). The ore sample was examined by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA) to investigate the texture and mineralogical composition, and to identify uranium-bearing minerals. XRD patterns were measured with a Philips PW3020 goniometer equipped with curved graphite crystal monochromator, and an X'Pert PW3710 MPD control unit. A long fine-focus Cu X-ray tube was run at 40 kV and 40 mA power settings from a PW1830 generator. The 2Θ range of 5–75° was measured in continuous mode applying a scanning rate of 0.015°/s (total time 1 h 17 min 46 s). Electron probe micro-analysis was performed by wavelengthdispersive spectroscopy using a Cameca SX100 instrument at a 15-kV acceleration voltage and a 10-nA beam current with 1–5 μm. Natural minerals were used as standards and the results were processed using the PAP on-line correction programme. Scanning electron microscopy analysis was performed using a Hitachi S-4800 FESEM instrument at a 20-kV acceleration voltage and a 15–20-μA beam current after carbon coating of the primary ore grains. The total organic carbon (TOC) of the ore was determined by first treating the dried ore sample with hydrochloric acid to remove inorganic carbon and then by the dry-oxidation of organic carbon in a furnace followed by the collection and determination of evolved CO2 (Tiessen and Moir, 1993). Inorganic carbon and carbonates were determined by treating an ore sample with concentrated phosphoric acid followed by trapping the evolved CO2 in KOH and the titration of CO2 with hydrochloric acid. Sulphur in sulphide form was determined by subtracting the value of sulphate sulphur from the total sulphur. Total sulphur was determined by dissolving all sulphur species by digestion of the sample using sodium peroxide and carbonate followed by determining the sulphate ions gravimetrically. The sulphate fraction was determined by treating the ore sample with a sodium carbonate solution followed by gravimetrically determining the sulphate ions in solution. Total and ferrous iron was determined by spectrophotometry, using orthophenanthroline as the coloured complex forming reagent using the method described in Herrera et al. (1989). Effective porosity was determined by kerosene saturation under vacuum of dried sample (Beletskii et al., 1997), while porosity volume is a product of effective porosity and bed volume. 3.2. Column ore leaching test A 1-m-long PVC column (ID 3 cm) was filled with a total of 1.3 kg of ore. A 2-cm layer of inert quartz gravel and a glass fibre were added to both ends of the column to support the ore bed (Benes et al., 2001). The column was filled with the ore sand batchwisely by pressing and wetting it with formation water to reach a density of 1.69 g/cm3. Column parameters are presented in Table 1. The column leaching test was conducted in a horizontal mode at room temperature under atmospheric pressure in conditions with a constant pressure gradient (Fig. 2). The ore bed was first treated with 7.8 bed volumes (BVs) of formation water to wash out the watersoluble uranium portion possibly oxidised by air when stored, as well as for controlling the permeability. Formation water was original frontal zone water sampled from the uranium roll front from the same wells as

Table 1 Column parameters used in uranium leaching from ore. Ore sample mass, g

Cross sectional area, cm2

Bed length, cm

Bulk density, g cm−3

Porosity volume, cm3

Effective porosity, %

Bed volume cm3

1328

8.2

96.0

1.69

262

33.29

787


127

3.3. Analysis of metals, uranium isotopes and oxidation states Metal concentrations, including uranium, were analysed using inductively coupled optical emission spectrometry (Optima 8000). Uranium isotope activities were determined by alpha spectrometry (Alpha analyst) following radiochemical separation of uranium by solvent extraction techniques as described in León Vintró and Mitchell (2000). The tetra- and hexa-valent oxidation states of uranium in the ore material were chemically separated by DOWEX anion exchange resin after dissolution in 6 M HCl solution using argon bubbling according to the procedure described in Pidchenko et al. (2013). The isotopic composition of both oxidation states was determined using alpha spectrometry after radiochemical separation as described above. Fig. 2. Scheme of uranium ore column leaching. 1 — Marriot vessel for leaching agent; 2 — capillary; 3 — leaching column; 4 — Erlenmeyer flask for pregnant solution.

4. Results and discussion 4.1. Characteristics of the ore sample

the ore sand. Major dissolved ions in formation water (Table 2) were analysed by flame photometry (Na, K), by titration with standardised − 2− solutions (Ca, Mg, Cl−, NO− 3 , NO2 ) and by gravimetric analysis (SO4 ). After removal of air-oxidised uranium, the leaching by formation water, made 0.255 M with respect to sulphuric acid (25 g L−1), was started. The experiment was continued for 27 days. Pregnant leach solution samples (30–40 mL) were taken every 12 h and the chemical and radiochemical analyses of solutions, pH and Eh values were measured, as well as the acid concentration in it. All redox potentials in this study are quoted with respect to the standard hydrogen electrode (SHE). A total of 1927 mL of solution was applied to the column. The number of BVs used for leaching was thus 2.45 and the leaching rate was 0.074–0.090 BVs day−1. The leaching rate, presented as a filtration coefficient calculated according to Benes et al. (2001), was equal to 0.01 m day−1 at the start and decreased to 0.08 m day−1 by the end of formation water leaching.

Table 2 Chemical composition of formation water used in the column leaching of uranium ore. Units in mg L−1, except pH. Aqueous component pH Na K Ca Mg Fe U

Concentration

Aqueous component

Concentration

7.85 1164 12 140 70 b0.05 b1

CO2− 3 HCO− 3 2− SO4 −

b1.5 160 590 1820 b2 0.002

Cl NO− 3 NO− 2

4.1.1. Mineralogy of the ore sample Quartz was overwhelmingly the most common mineral component in the ore sample (N 90%), accompanied by lower contents of potassium feldspar (specifically microcline, ~ 4%), and sodium-rich plagioclase feldspar (albite–oligoclase in composition). The X-ray diffraction pattern also showed observable minor peaks of sheet silicates, muscovite, kaolinite and illite. Their mineral contents were, however, so low that their fractions in the ore could not be reliably determined from the Xray diffraction pattern. SEM analysis along with EPMA results (Cameca SX100 instrument) showed that the main part of uranium was present as small particles (≈ 1 μm) of uraninite (Fig. 3). The EPMA results further showed that uranium minerals were found as inclusions in mixed grains of quartz, potassium feldspar and other silicates. Uranium was largely present in a form of uraninite (A) and with minor amounts as coffinite (B), Tirich brannerite (C) and monazite (D), which were present as bigger formations with a size variation of ≈20–100 μm (Fig. 4). Particle size distribution was determined using a sieving method. Most of the ore mass (51%) lies in the 0.1–0.5 mm particle size range (Table 3). A substantial mass proportion (29%) in the particle size fraction b0.1 mm was also found. This high fraction of small particles affects the permeability of the ore body as it clogs the pores during leaching agent flow. Ore permeability characterised by the filtration coefficient decreased from 0.1 m day−1 at the start to 0.08 m day−1 at the end during the formation water leaching stage. One main reason for this may be the mechanical colmatation resulting from the clogged pores in the ore body. The filtration coefficient increased back to its original value of 0.1 m day−1 after acid leaching was switched on, possibly due to the dissolution of small particles clogging the pores .

b a

Fig. 3. SEM image (a) and EDX semiquantitative elemental analysis spectrum (b) of ore sample grains.

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a

b

c

d

Fig. 4. EPMA images of ore grains: uraninite (A), coffinite (B), Ti-rich brannerite (C) and monazite (D).

Table 3 Particle size distribution of the uranium ore sample (%). 2.0–1.0

1.0–0.5

0.5–0.25

0.25–0.1

0.1–0.05

0.05–0.01

0.01–0.005

0.005–0.001

b0.001

Total

mm

mm

mm

mm

mm

mm

mm

mm

mm

%

10.2

5.8

26.9

28.3

6.5

7.8

3.0

6.5

5.2

100

4.1.2. Chemical characteristics of the ore sample The ore sample contained low contents of organic material (0.043%) and sulphides (b 0.28%). The uranium concentration in the ore was 0.225%. CaCO3 was present at a maximum level of 0.25%, which is clearly below the recommended maximum value of 5% that may cause chemical colmatation of the ore bedding (Benes et al., 2001). Sulphuric acid is therefore a suitable leaching agent for this ore. The ore material contains relatively high concentrations of ferrous (0.5%) and ferric (0.7%) iron. According to Ding et al. (2013) ferric ions are the main oxidisers of tetravalent uranium during ILS (reactions (1) and (2)). Due to low contents of reducing components, such as organic matter and sulphides, the contents of ferric iron ions sufficiently oxidise all tetravalent uranium in the column. 4Fe2þ þ O2 þ 4Hþ →4Fe3þ þ 2H2 O

ð1Þ

UO2 þ 2Fe3þ →UO2 2þ þ 2Fe2þ

ð2Þ

4.2. 234U/238U activity ratio in different uranium oxidation states in the ore sample The isotopic activity ratio 234U/238U in the ore was close to unity at 0.99 ± 0.02. We may thus conclude that the sample was obtained from approximately the middle section of the bedding front, where the 234U leaching and precipitation processes are in equilibrium. The hexavalent uranium fraction in the ore sample was found to have a predominance of 234U isotope and the 234U/238U isotopic activity ratio was 1.20 ± 0.03. During sampling and pretreatment the ore sample was wet and interacted with air oxygen. In these conditions part of the tetravalent uranium may have been oxidised by ferric ion according to reactions (1) and (2). Oxidation most probably occurs on the uranium mineral (uraninite) grain surfaces covered by a layer with elevated 234 U content due to the alpha-recoil process. The ore sample was airdried during pretreatment. It is possible that the oxidised and mobilised uranium is adsorbed onto the uranium mineral surfaces as UO2+ 2 cations from the formation water during the pretreatment phase. The UO22 + cations may precipitate as UO2(OH)2 when drying. The 234U/238U isotopic activity ratio in the tetravalent uranium fraction was found to be somewhat below unity at 0.95 ± 0.05. The tetravalent uranium fraction in the studied ore sample represented uraninite and coffinite minerals. The low 234U/238U activity ratio in the tetravalent

fraction can be explained by the alpha-recoil effect. Due to alpha-recoil and further mobilisation and dissolution of 234U as UO2+ 2 , the tetravalent uranium fraction contains less 234U. The isotopic ratio is, however, dependent on the location in the bedding roll front; the fresh ore precipitated in the reduction zone may show higher 234U/238U isotopic activity ratios than unity. 4.3. Leaching with formation water When leaching the ore column with formation water, the pH slightly increased from 7.90 at the beginning of leaching to 8.37 at the end, while Eh increased from 175 mV to 220 mV. Leaching with formation water showed that ore permeability was low based on calculated filtration coefficients ranging from 0.085 to 0.147 m day−1; the filtration coefficient remained approximately constant at 0.08 m day−1 after 5.5 bed volumes of formation water. Furthermore, calcium, iron and magnesium concentrations decreased after a formation water BV of 1.5 and reached the values of metals in the initial formation water solution (Fig. 5). The aluminium concentration increased during formation water leaching and the maximum concentration in the effluent at the end of the water leaching stage was 0.015 mg L−1. The slow dissolution of kaolinite and/or other clay minerals might explain the increase in aluminium concentration. Total uranium extraction during leaching with formation water was ca. 20% and this value was not taken into consideration in further uranium extraction calculations when leached with sulphuric acid.

Fig. 5. Ca, Mg, U, Al and total Fe concentrations in the effluent of formation water leaching stage.


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Fig. 6 shows the 234U/238U activity ratio in the effluent at the formation water leaching stage aiming to wash out the soluble forms of uranium. The ratio remained constant with an average value of 1.20 ± 0.05. As discussed above, hexavalent uranium is characterised with high 234 U/238U activity ratios due to the processes followed by the alpharecoil of daughter nuclides. At the formation water leaching stage the 234 U/238U activity ratios in effluent were close to the value observed on the hexavalent uranium in the untreated ore, suggesting that this is the source of uranium at this stage. 4.4. Leaching with sulphuric acid The pH and Eh of the pregnant leach solution when leaching with dilute sulphuric acid (0.225 M) are presented in Fig. 7. Eh values decreased from ca. 250 mV at the start of the experiment and remained at ca. 130 mV before rising to a value of ca. 400 mV after 1.6 BVs of sulphuric acid solution. The pH value in the pregnant leach solution remained at ca. 8 during the first 0.5 BVs and rapidly decreased thereafter. The pH constancy at the beginning was a reason of remaining formation water in the column; the loaded column pore volume is 262 cm3 (see Table 1), which is equal to 0.33 BVs. Another 0.17 BVs presented with a stable pH is a consequence of the neutralisation of the first portions of sulphuric acid by interaction with calcium and magnesium minerals, possibly calcite and dolomite. Increasing calcium and magnesium concentrations were observed at corresponding BVs as shown in Fig. 8. The iron and acid concentrations are significant factors influencing uranium ore leaching. Ding et al. (2013) showed that ca. 50% of the iron in the solution is present in the ferric state at a redox potential equal to 400 mV. The oxidation power of the solution is mainly generated by the ferric and ferrous iron redox pair. As shown by the chemical characteristics of the ore sample, the ferric and ferrous iron concentrations in the ore are higher than the tetravalent uranium concentration. As the reductive component contents are low, e.g., sulphides and organic matter, it is reasonable to conclude that the oxidation power of a solution in the column is enough to oxidise all U(IV) in the ore and therefore it is not necessary to use an additional oxidiser for this ore material. The evolution of Fe, Ca, Mg and Al concentrations in the pregnant leach solution is presented in Fig. 8. Concentrations of all metals at the start of acid leaching were at approximately similar levels as in the formation water. The dissolution of calcium- and somewhat later of magnesium- and iron-containing minerals probably took place at ca. 0.3 BVs, after the formation water was removed from the column. These minerals were most probably iron sulphides and calcite and dolomite, which neutralise the first portions of sulphuric acid in the column. The increasing concentration of iron in the column was followed by

The uranium extraction degree did not exceed 85% during the acid leaching stage. The uranium extraction degree during acid leaching is shown in Fig. 9. The maximum uranium concentration in the pregnant leach solution reached 5800 mg L−1. The results showed rapid dissolution of uranium after 0.9 BVs of acid solution as the pH dropped below 2. The 85% extraction was attained at 1.6 BVs, after which no significant change in recovery was observed. The evolution of uranium in the pregnant leach solution can be divided into three phases according to the measured uranium concentration and the 234U/238U activity ratio (Fig. 10). The first phase, characterised by low uranium extraction at ca. 3% and the 234U/238U activity ratio in the range of 1.21–1.25, represents the first 0.8 BVs of the solution. This fraction of extracted uranium is expected to be attributed to the leaching of hexavalent uranium resulting from the formation water elution from the column and can be considered as a continuation of leaching with formation water.

Fig. 6. Evolution of the uranium isotope activity ratio during the formation water leaching stage.

Fig. 8. Evolution of Fe, Mg, Ca and Al during ore leaching with dilute sulphuric acid.

Fig. 7. Evolution of Eh and pH during ore sample leaching with dilute sulphuric acid.

increasing Eh from ca. 130 mV to ca. 400 mV. Above 0.5 BVs the pH values decreased from 8 to ca. 1 at 1.2 BVs. The iron concentration reached a maximum at ca. pH 4 and slowly decreased towards the end of the acid leaching. Aluminium-containing clay minerals began dissolving from 0.6 BVs onwards, and reached a maximum at ca. pH 2.

4.5. Uranium concentrations and 234U/238U activity ratio in acid leaching solutions

130


was characterised by a slow rate and will not succeed uranium extraction in a high recovery. Established relationship between the activity ratio of uranium isotopes and uranium extraction can potentially be used in real leaching operations as an indicator for uranium sources during in-situ leaching. 5. Conclusions

Fig. 9. Uranium extracted from ore in a column with sulphuric acid.

The second phase in the evolution curve, after ca. 0.8 BVs of acid solution, begins dissolving the U(IV) minerals. This phase is characterised by elevated uranium concentrations and a lower 234U/238U activity ratio between 0.92–1.00. This phase represents the oxidation and dissolution of uraninite, the major uranium mineral in the ore. The same 234U/238U activity ratio values were recorded in the tetravalent uranium fraction of the untreated ore sample. At this uranium dissolution phase the Eh increased up to 400 mV, a sufficient value to transform U(IV) into U(VI). The total uranium fraction extracted during this phase was ca. 80%. The high oxidation power of the solution was provided by the dissolution of iron-bearing minerals following the generation of ferric ions oxidising the U(IV) species into acid-soluble U(VI) and simultaneously reducing into ferrous ions (Lottering et al., 2008). The reduced ferrous ions can then be oxidised into ferric ions by the oxygen from the air (see reactions (1) and (2)). Our experiment was conducted in an open system where the dissolution of oxygen from the air occurs and in principle U(IV) can also be oxidised directly by the oxygen. However, molecular oxygen is a relatively inefficient oxidant for uranium (Shakir et al., 1992). The third phase in the evolution curve was represented by the 234 U/238U activity ratio rising from about 1 to 1.05–1.16. During this phase less than 15% of the uranium was left in the ore sample. The extracted uranium was attributed to sparingly soluble uranium minerals at pH = 1 and an Eh of ca. 400 mV. We assume that the third phase mainly represents uranium leaching from low soluble uranium minerals such as monazite, brannerite and other minerals (Polinovskij, 2012). The activity ratio rising from 1 to 1.05–1.16 is explained by preferable leaching of the more mobile 234U when leaching U from sparingly soluble U minerals. It can be concluded that this final phase is not sufficient for extracting uranium using the ISL method, as the uranium leaching

Fig. 10. Evolution of uranium and 234U/238U activity ratio in column leaching of ore with sulphuric acid.

The data on uranium isotope activity ratios was used to interpret the uranium extraction processes in the column leaching of uranium ore. The evolution of uranium concentration in a column test was supported by measurements of 234U/238U activity ratios, with a view of identifying uranium sources during leaching with dilute sulphuric acid. We have shown that 234U/238U activity ratio measurements along with other parameters (pH, Eh and metal concentrations) in the pregnant leach solutions are a good tool for better understanding dynamic uranium leaching. Uranium ore leaching in a laboratory column showed three leaching phases based on observed uranium concentration and its 234 U/238U activity ratio. The first phase with a recovery of 3% (up to 0.8 BVs) was related to oxidised, hexavalent uranium with 234U/238U activity ratios of 1.21–1.25. The second phase with a recovery of 80% was mainly associated with tetravalent uranium minerals, particularly uraninite, the dissolution of which took place below 1.8 BVs of acid solution, as evidenced by our measured 234U/238U activity ratio in the range of 0.92–1.00. The third phase with a low recovery of 0.5%, related to the leaching of low soluble uranium minerals, was represented by the 234 U/238U activity ratio rising from 1 to 1.05–1.16. Acknowledgements This work has been carried out under the sub-project “Practical application of natural radionuclide isotope ratio in uranium hydrometallurgy and radioecology”, which is funded under the Technology Commercialization Project (contract No 538 from November 27, 2012), supported by the World Bank and the Government of the Republic of Kazakhstan. The authors would also like to thank Mr. Jussi Ikonen for his help with the SEM analysis and Mr. Kai Kaksonen for his help with uranium oxidation state determination. References Adloff, J.P., Roessler, K., 1991. Recoil and transmutation effects in the migration behaviour of actinides. Radiochim. Acta 52 (1), 269–274. Beletskii, V.I., Bogatkov, L.K., Volkov, N.I., 1997. Handbook on uranium geotechnology. Under the editorship of Skorovarov DI-Moscow: Energoatomizdat, p. 672. Benes, V., Boitsov, A.V., Fuzlullin, M., Hunter, J., Mays, W., Novak, J., Underhill, D.H., 2001. Manual of Acid in situ Leach Uranium Mining Technology. International Atomic Energy Agency, Vienna. Brovin, K.G., Grabovnikov, V.A., Shumilin, M.V., Yazikov, V.G., 1997. Forecast, Surveys, Exploration and Industrial Assessment of Uranium Deposits for Development of in Situ Leaching. Gylym, Almaty. Ding, D.X., Fu, H.Y., Ye, Y.J., Hu, N., Li, G.Y., Song, J.B., Wang, Y.D., 2013. A fractal kinetic model for heap leaching of uranium ore with fractal dimension of varied particle size distribution. Hydrometallurgy 136, 85–92. Herrera, L., Ruiz, P., Aguillon, J.C., Fehrmann, A., 1989. A new spectrophotometric method for the determination of ferrous iron in the presence of ferric iron. J. Chem. Technol. Biotechnol. 44 (3), 171–181. León Vintró, L., Mitchell, P.I., 2000. Determination of actinides and other alpha emitters. Encyclopedia of Analytical Chemistry: Instrumentation and Applications, Part II, Radio-chemical Analysis. John Wiley and Sons Ltd., Chichester, pp. 12848–12884. Lottering, M.J., Lorenzen, L., Phala, N.S., Smit, J.T., Schalkwyk, G.A.C., 2008. Mineralogy and uranium leaching response of low grade South African ores. Miner. Eng. 21 (1), 16–22. Mamilov, V.A., Petrov, R.P., Shushaniya, G.R., 1980. Production of Uranium by Underground Leaching Method. Atomizdat, Moscow. Mamytbekov, G.K., Raimkhanov, A.Ye., Smaylov, Ye.K., Kozhakhmetov, S.K., 1998. A new method for improve work well of geotechnological complex of underground leaching. Sci. Tech. Soc. KAKHAK 67 (In Russian). Mudd, G.M., 2001. Critical review of acid in situ leach uranium mining: 2. Soviet Block and Asia. Environ. Geol. 41 (3–4), 404–416. Munoz, J.A., Blazquez, M.L., Ballester, A., Gonzalez, F., 1995. A study of the bioleaching of a Spanish uranium ore. Part III: column experiments. Hydrometallurgy 38 (1), 79–97.

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