Uranium recovery from sandstone-type uranium deposit by acid in-situ leaching - an example from the Kujieertai

Journal Pre-proof Uranium recovery from sandstone-type uranium deposit by acid in-situ leaching - an example from the Kujieertai

Yipeng Zhou, Guangrong Li, Lingling Xu, Jinhui Liu, Zhanxue Sun, Weijun Shi PII:

S0304-386X(18)30668-6

DOI:

https://doi.org/10.1016/j.hydromet.2019.105209

Reference:

HYDROM 105209

To appear in:

Hydrometallurgy

Received date:

13 September 2018

Revised date:

5 November 2019

Accepted date:

9 November 2019

Please cite this article as: Y. Zhou, G. Li, L. Xu, et al., Uranium recovery from sandstonetype uranium deposit by acid in-situ leaching - an example from the Kujieertai, Hydrometallurgy(2019), https://doi.org/10.1016/j.hydromet.2019.105209

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Uranium Recovery from Sandstone-Type Uranium Deposit by Acid In-Situ Leaching - an example from the Kujieertai Yipeng Zhou, Guangrong Li, Lingling Xu, Jinhui Liu, Zhanxue Sun, Weijun Shi State Key Laboratory of Nuclear Resources and Environment, East China

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University of Technology，Nanchang, Jiangxi, 330013, China

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Abstract: The factors influencing uranium recovery in water-rock systems

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during acid in-situ leaching (ISL) were studied at the Kujieertai uranium

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deposit in Xinjiang. Using an ISL unit, a field leach trial (FLT) had been carried out to test the sequential effects of a leaching solution without oxidant

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(H2SO4 solution 4–8 g/L) and a leaching solution with oxidant (H2SO4 3–7 g/L,

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and Fe (Ⅲ) 2–6 g/L). The observation of the leaching process revealed clearly

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defined stages of uranium release from the solid mineral to solution. Uranium mobilization from solid mineral into solution can be described in four stages. At the beginning of the acid ISL process, there was no oxidant to be added to the leaching solution and the desorption of hexavalent uranyl ions in the open pores, as well as dissolution of

hexavalent uranium minerals, led to a short-

term peak in the pregnant solution, which happened while pH decreased from about 5.3 to 2.62. Following the depletion of the adsorbed hexavalent uranium and a decline in uranium dissolution intensity, the addition of Fe(III) facilitated 

Corresponding author: [email protected]

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the oxidation of tetravalent uranium, which enabled intensive uranium mobilization again. During this process, the dissolution of uranium had a strong positive correlation with the reduction of Fe(III) and Eh in the leach solution. Beside hydrochemical factors, the deportment of uranium was also an important factor affecting uranium recovery. Uranium located in the open

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pores can be completely exposed to the solution and the mobilization intensity

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was significantly affected by hydrogeochemical conditions; but the uranium

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present in microfissures and in the ore matrix could not be fully exposed to the

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solution, so, their dissolution intensity was primarily controlled by corrosion

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and permeability of the ore. In general, the hydrogeochemical conditions and the deportment of uranium were the external and internal factors that

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significantly affected the dissolution and recovery of uranium in the early and

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middle stages of the FLT. However, in the latest stages, due to uranium

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depletion, enhancing the chemical potential of the leaching solution, specifically acidity and/or the amount of oxidant, had little improvement on uranium recovery.

Keywords: Sandstone-type uranium deposit; acid in-situ leaching; water-rock interaction; uranium recovery; uranium deportment in the ore 1. Introduction

The factors that affect the recovery of natural uranium in water include the mineral form of the uranium, the physical and chemical properties of the rock

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and water, and the hydrodynamic conditions. (Langmuir, 1978; Shi, 1990). Under natural conditions, uranium migrates slowly, but for recovery purposes, the addition of sulfuric acid can accelerate this process. ISL (in-situ leaching) is a technology used in sandstone-type uranium deposits, which causes rapid uranium dissolution. The process involves the injection of an oxidant and a

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leaching solution (such as sulfuric acid or carbonic acid) into mineralized

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strata. Through this process, uranium is oxidized, resulting in its rapid

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dissolution (Bilietsiki, et al., 2000).

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Over the past decades, research on uranium mobilization has focused primarily on the migration and mineralization of uranium under natural conditions, as

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well as the impact of uranium migration on the environment (Evсееvа et al.,

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1965; Langmuir et al., 1980; Dаnnеv et al.,1982; Morrison et al., 1995; Fuller

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et al., 2002; Noubactep et al., 2006; Yang et al., 2006; Yanase et al., 2008; Yue et al., 2011; Alam et al., 2014; Golubev et al., 2016; Tsarev et al., 2017; Lammers et al., 2017; Post et al., 2017). Because of the increasing demand to develop low-grade uranium resources, there have been many studies on various uranium mobilization mechanisms in sandstone-type deposits related to ISL. These studies have considered factors such as the geochemical properties of uranium, the hydrochemistry of the environment, and the environmental hydrodynamic conditions. The studies mostly focused on hydrochemical conditions including pH, redox potential (ORP) and ionic

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components in the water. Using synthetic UO2, Ram et al. (2011) studied the relationship between the uranium dissolution and Fe (FeⅢ, FeⅡ), and found that while the redox potential (ORP) was 460~565mV, the dissolution of uranium had a strong linear positive correlation with Fe. This correlation happens in a wide range of Fe concentration (Fe(Ⅲ): 1.1×10-4-4.2×10-3M; Fe(

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Ⅱ): 3.2×10-4-1.8×10-2 M, Fe(Total): 4.2×10-4-2.4×10-2M). The correlation was

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strongest while ORP is of 420~460mV and it became weaker while ORP is

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below 420mV. There was a significant linear positive correlation between

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uranium dissolution rate and Fe( Ⅲ ) concentration when the Fe( Ⅱ )

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concentration kept constant. Whereas Laxen (1973) found that uranium dissolution rate varied with 1/[Fe(Ⅲ)]2. Munoz et al. (1995) studied the effect

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of Fe concentration on uranium dissolution in microbial natural uranium ore

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leaching and found that the increase of ferrous iron concentration in the range

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of 2-10g/L had no obvious effect on the dissolution rate. This may possibly be due to the precipitation of jarosite which may block release of uranium from the mineral lattice. Zhou et al. (2014; 2016) also conducted microbial leaching tests of natural sandstone-type uranium ores and found that uranium dissolution is positively correlated with Fe(Ⅲ) concentration in the range of 02g/L, but while Fe(Ⅲ) concentration exceeds 2g/L, further increase of Fe(Ⅲ) concentration does not enhance the dissolution of uranium. As to the influence of anions on uranium mobilization,

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Ram et al. (2013) conducted leaching experiments using synthetic UO2, and found that increasing the concentration of SO 42- from 1.46×10-2M to 5.36×101

M lead to a significant decrease in the dissolution of UO 2, it is because

excessive SO42- causes the decrease of ORP and hinders the formation of effective Fe( Ⅲ ) complexes. Regarding the influences of pH and Eh on

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uranium dissolution-precipitation process in aqueous solution, Oddo et al.

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(1981) emphasized the importance of Eh-pH conditions in ISL. According to

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pH and concentration of ligands (including HCO3-, CO32-, PO43- and SO42-)

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which control the distribution of uranium in water, Shi et al. (1986) calculated

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the speciations of uranyl complexes in natural water of supergene origin under normal temperature and atmospheric pressure using a computer program that

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they wrote. They pointed out that UO2CO3 is the main form of uranium in

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weakly acidic water. In acidic water of the sulfide oxidation zone, only when

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SO42- concentration is higher than 400mg/L dissolved uranium can be dominated by UO2(SO4)n2-2n.

The researches mentioned above are not focused on the domain of in-situ leaching of sandstone-type uranium, but the fundamentals can still be applied to it. Many experimental studies focused on sandstone type uranium leaching were conducted. The results of a column leaching experiment by Gao et al. (2003) showed that hexavalent uranium dissolves while pH is 2.0-4.6 and Eh 420-650mV, tetravalent uranium dissolves while pH < 2.0 and Eh > 650mV,

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and dissolved uranium precipitates while pH>4.6 and Eh < 420mV. Shi et al. (2004) emphasized that pH=2.0 and Eh=700mV are the most important and critical parameters that affect the dissolution and migration of tetravalent and hexavalent uranium. Briganti et al. (2017) conducted a study using chemical leaching and adsorption tests and a simple modelling using PHREEQC. They

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found that the geochemical behavior of uranium during ignimbrite–water

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interaction was controlled mainly by temperature, pH, and solution chemistry

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(esp., alkalinity).

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In general, current studies of uranium ISL at field scale focus mainly on the process and application, whereas studies that consider uranium mobilization

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are rare, contributing to uncertainty and debate on this topic. In this study, a

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field leach trial (FLT) of ISL was carried out at the Kujieertai deposit, which is

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the earliest and largest acid ISL sandstone-type uranium deposit in China. Uranium mobilization, and the relationship to the physicochemical conditions of water-rock systems were analyzed.

2. Mineralogy, materials and Methods

2.1 Geological and hydrological profile of the ore deposit

The Kujieertai Deposit is located within the Yili Basin in northwest China and

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is a typical interlayer oxidation zone variant of the sandstone-type uranium deposit. The ore body is mainly found in the monoclinal strata of the mid to lower Jurassic Shuixigou Group, at a burial depth of 170–230 m. The strata gently dip to the north at angles of between 5–8° in the Kujieertai Deposit. The layers above and below the mineral-bearing strata are impermeable and retain

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confined pore water at a groundwater depth of between 43–81 m. The water

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flows to the northeast at about 14° with a hydraulic gradient of 0.03. The water

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has a pH between 7–8, and a mineralization level of 0.24–0.63 g/L, consisting

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mainly of HCO3, SO4, Ca, Na, and Mg (Wang et al., 1997; Liu et al., 2002).

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That is consistent with the chemical composition of groundwater at the FLT

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site (Table. 1).

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The ore body is situated 100 m ahead of the interlayer oxidation front and 200

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m behind the oxidation front, where yellow sandstone contacts grey sand, indicating oxidized zone and reduced zone, respectively. The ore body is tilted upward and exhibits a complicated roll form (Fig. 1).

The FLT was carried out at the upper limb of the roll-front. Five wells for insitu leaching were constructed forming an independent "four injection wells and one production well" leaching unit. Among them, well C1 is for production, while well Z1~Z4 are for injection. The distance between the production well and the injection well is 14.2m. The well distribution and the

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lithologic characteristics of ore-bearing aquifers are shown in figure 2.

The mineralized aquifer was approximately 20m thick and consisted of medium and coarse-grained grey and yellow sand. The ore-bearing strata consists of coarse-sandstone at the bottom grading to fine sandstone at the top.

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It is loose and highly permeable. The upper rock surrounding the ore body is

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medium grained well sorted sandstone. The lower surrounding rock has

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relatively complex lithology, including loose medium-coarse sand and coarse

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sandstone, siltstone and silty mudstone, as well as silty cemented fine

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sandstone. The permeability coefficient of the upper surrounding rock aquifer

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0.77m/ d.

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is 0.62m/d, the ore layer is 0.61m/d, and the lower surrounding rock aquifer is

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The upper aquitard of the ore-bearing aquifer consists of mudstone, silty mudstone and carbonaceous mudstone. These are dense and have poor permeability. The bottom aquitard consists of argillaceous siltstone which is compacted and poor in permeability. The detailed results of porosity and mudcontent analysis of core samples are shown in Table 2.

Study of the lithology of the leaching wells shows that the ore body is located in the upper part of the roll front and is spatially discontinuous. The ore body

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thickens eastward and thins to the north and south. It pinches out between wells C-1 and Z-3. Mineralization in the ore body shown in each well is listed in table3.

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Major minerals in the ore bearing acquifer are:

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(1) Quartz, accounting for 30 to 50%. The quartz is angular suggesting a

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nearby source and is 0.1 to 0.5 cm size. (2) Clay minerals account for 40 to

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60%, mainly sericite, illite, and kaolinite, along with some chlorite. (3) Mostly

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weathered feldspars with a few polycrystalline plagioclase aggregates. (4)

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material from 0 to 5%.

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Rock debris, less than 5%, mostly granite and mudstone. (5) Carbonaceous

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The minor minerals include calcite, pyrite, zircon, rare earth minerals, apatite, ilmenite, rutile, and uranium ore. The distribution of calcite is very uneven. It can be localized as calcareous cemented masses or calcareous cemented thin layers. The content of calcium carbonate in non-calcareous cemented core samples is of 0.23%-0.97%. The content of pyrite is between 0.038% and 1.2%, which mainly occurs in three textures: lumps, star shaped masses and colloids (strawberry shape). Most of the pyrite occurs in fractures in quartz grains and between clay minerals. A small amount of pyrite is associated with

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uranium minerals. Uranium minerals are mainly uraninite, brannerite, autunite and coffinite (Fig. 3).

Chemical analyses of 19 core samples from the orebody and 11 core samples

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from surrounding rock near the orebody, are shown in Table 4.

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2.2 In-situ leaching (ISL) process

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2.2.1 Process flow

The flow dynamics of the ISL process are outlined in Fig. 4. First, the leaching

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solution was injected into the ore-bearing strata where the pregnant solution

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with uranium was pumped through the production well to the surface.

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Afterward, the solution was delivered to the processing plant for recovery of the uranium（Uranium in the solution was adsorbed by anion exchange resin in the fixed-bed adsorption tower. The saturated resin was eluted by the NH4NO3 solution to recover the uranium. The tailings solution that was depleted of uranium was then delivered to the solution settling pond where it was recycled to be used as a leaching solution.

Usually, an in-situ leaching mining area consists of dozens of in-situ leaching units, and each unit consists of five or seven wells, a production well in the

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center and several surrounding injection wells, forming a "five-spot" or "seven-spot" pattern well field. In order to avoid the influence of in-situ leaching production of the mine on the FLT, an independent unit of “five-spot” pattern leaching unit away from production zone of the mine was developed for the FLT. As mentioned above, four injection wells (Z1~Z4) and one

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pumping well (C1) form a "five-spot" pattern well field without monitoring

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wells around. The length of the screen of well C1 is 6.1m, and that of well Z1,

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Z2, Z3 and Z4 is 2.47m, 3.96m, 4.27m and 4.05m, respectively.

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2.2.2 Leaching solution

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In the in situ leaching system, the solution flows in a closed cycle which does

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not discharge solution or introduce foreign water. So, the residual acid and Fe

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ions in the tail liquid (chemical composition shown in Table 3) can be reused. This recycle mining is not only environmentally friendly but also economical. According to the requirements of the FLT, the leaching solution of this study was prepared by adding sulfuric acid and ferrous sulfate to the tailings solution that was depleted of uranium (<1mg/L). Before injecting them into underground ore-bearing aquifers, the Fe(Ⅱ) in the solution was almost totally oxidized into Fe(Ⅲ) by thiobacillus ferrooxidans which was cultured in a facility composed of 4 contact oxidation pools of the same size, 8 meters long, 2 meters wide, and 3 meters high. The FLT was divided into four stages with

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different treatments on the leaching solution: in the first stage, only the planned amount of sulfuric acid was added into the leaching solution; in the second, third and fourth stages, a scheduled amount of ferrous sulfate was added into the tail solution and ferrous oxidization followed. Then an appropriate amount of sulfuric acid was added into the oxidized leaching solution (ferric

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containing solution) before it was injected underground. When the tail liquid

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could not meet the required injection amount, a small balance amount of the

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tail liquid from the industrial leaching system of the mine was introduced (the

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composition of the industrial mining tail fluid is shown in Table 5), and no

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3. Results and discussion

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other reagents were introduced into FLT system.

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The flow rate of FLT was not stable as shown in Fig. 5 and Table 6. In the first stage both production and injection flow rate decreased. Well washing was carried out at the end of this stage so that the production flow in the second stage was obviously increased to a relatively high level. In the third stage and the early of fourth stage, the production flow rate was relatively low because of technical problems. It was increased again when a new extraction pump was employed in the middle of the fourth stage. In general, during the whole FLT period, the production flow rate changed remarkably, while the injection flow rate was relatively stable with the injection pressure remaining between

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0.2~0.3MPa.

As mentioned above, the injection solution was prepared from the tail solution formed by the uranium depleted pregnant solution, so, except for the control of

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acidity (throughout the FLT) and the concentrations of ΣFe and Fe (Ⅲ) (in the

injection solution came from the pregnant solution.

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the first stage in the

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second, third, fourth stages), the Ca2+, Mg2+ and NO3- in all stages and ΣFe in

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Therefore, the variation of the concentration of these ions is mainly the result

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of water-rock interaction. Changes in acidity, Eh and some ions concentration

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are shown in figure 6.

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3.1 Results of uranium leaching

Figure 7 shows the variation in the concentrations of uranium in the pregnant solution over time.

In stage 1, the overall uranium concentration was low. The concentration peaked on day 13 at 40.13 mg/L, and rapidly dropped to 14 mg/L on day 22. Afterwards, concentration remained stable for the rest of the stage, between 10–15 mg/L. In stage 2, the leaching solution containing Fe(Ⅲ) was

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continuously injected into the ore-bearing strata, and the concentration of uranium in the solution rose quickly. In stage 3, the amount of Fe(Ⅲ) was decreased, and the acidity was increased. At the beginning of stage 3, the increase in uranium concentration observed in stage 2 was sustained and remained at a level of 53–55 mg/L through the rest of this stage. The uranium

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concentration peaked at 67.98 mg/L in early stage 4, before beginning to

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decline at a decreasing rate over time. The uranium concentration at the end of

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the experiment was 23 mg/L.

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3.2 Discussion

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The factors affecting uranium mobilization in water-rock systems during acid

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ISL are complicated. This study focused on hydrochemical and uranium

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deportment. Uranium found in aqueous solution occurs mainly as complex compounds that are controlled by the chemical properties of the water medium (Shi, 1990). In the water-rock system undergoing acid ISL, the pH, Eh, complex ligand, and Fe(Ⅲ) are important chemical conditions that affect the release of uranium from ore to the leaching solution (Suzuki et al., 1990; Gao et al., 2003; Shi et al., 2004).

3.2.1 Relationship between pH and uranium mobilization

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At the beginning of the FLT, after the addition of sulfuric acid, the dissolved uranium concentration in pregnant solution got a short-term rapid increase, and reached a peak at the 13th day, then dropped down and stayed at a low level until the oxidization process of adding Fe(Ⅲ) was conducted. The question is, what caused this short-term peak of uranium concentration to occur? It is well

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known that sandstone-type uranium deposits form mainly due to the

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precipitation of uranium from water, and the adsorption of uranium by peat,

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clay, and other minerals. Therefore, there is usually adsorbed hexavalent

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uranium (in the form of uranyl ions and their complexes) in sandstone-type

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uranium deposits This kind of uranium is often amorphous in the form of fine particles located on mineral surfaces. That makes it vulnerable to the changes

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in hydrogeochemical conditions. The pH value of water has a notable effect on

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the adsorption of uranium ions. In addition, hexavalent uranium could also

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exist in the original ore in form of hexavalent uranium minerals that undergo acid-consuming dissolution in the first phase of FLT, which would contribute to the short-term peak. It is noteworthy that this short-term peak of uranium concentration could also be partially caused by oxygen introduced to the ore formation during drilling and well construction (thus resulting in a preoxidation of tetravalent uranium minerals). As the pH value decreases, the adsorbed UO22+ can be replaced by protons. Regarding the first 40 days of the FLT, figure 8 shows the curves of uranium concentration and pH against time. It shows that the increase of uranium concentration was closely accompanied

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by pH decrease, typically, when pH dropped from about 5 to 4, the uranium concentration increased significantly from lower than 2 mg/L to over 24 mg/L. Besides pH value, the ions in the water also affect the adsorption of uranium. Anions can affect the composition of uranium species in water, which in turn affects uranium adsorption. The cations, especially Ca 2+ and Mg2+, can hinder

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the adsorption process of uranium (alkaline-earth metal ions can be adsorbed

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competitively with uranium by peat, clay and other media). Therefore, with the

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increase of salinity of water, the adsorption of uranium decreases (Shi, 1990).

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As we can see in figure 6, due to the effect of sulfuric acid, ions like Ca 2+,

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Mg2+, and SO42-, kept rising during early of the FLT. All these changes,

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including pH decrease, made uranium species in solution different.

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Regarding the first 40 days of the FLT (pregnant solution compositions are

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shown in Table 7), the molarities of 52 possible uranium species in pregnant solution were calculated using PHREEQCI (version 3.4.0). Results show that UO2(HPO4)22-, UO2SO4, UO22+, and UO2(SO4)22-, are the dominant uranium species in the pregnant solutions during the whole first 40 days of the FLT, and they together make more than 99.5% of total uranium species. The percentage of these 4 species against time are shown in Figure 9. Figure 9 demonstrates that almost all dissolved uranium was in form of UO 2(HPO4)22- when the pH value of solution was about 7 at the beginning of the FLT because there was 915mg/L ΣPO4 in solution. The complex stability constant of UO 2 (HPO4)

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2

is

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large (next only to that of UO2(CO3)34+), so, in weak acid-neutral-alkaline natural water, when ΣPO4 > 0.01mg/L, UO2 (HPO4)

2-

2

can be the major

uranium species (Shi, 1990). As pH decreased from about 5.3 to 2.6, UO2(HPO4)22- decreased while the other 3 species (UO2SO4, UO22+, and UO2(SO4)22-) increased. Meanwhile, the major species of uranium shifted from

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UO2(HPO4)22- to UO2SO4 in the pH region of 3.7 to 2.6. Eventually, when pH

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got lower than 2.0, UO2SO4 was up to 66.4%, UO2(SO4)22- up to 13.6%, UO22+

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to 19.6% (the peak with 20.9% at pH of 2.54 and then went down slightly as

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pH fell), and UO2(HPO4)22- down to a negligible level of 0.23%. From the

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above discussion, it can be inferred that the desorption of uranium, was driven by the ion exchanges, e.g. adsorbed UO 22+ was replaced by protons, and

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complexing of UO2SO4 making uranium less absorbable. This is the main

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solution

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reason for the first short-term peak of uranium concentration in the pregnant

As mentioned above, after shortly reaching a peak in the 13th days of the field trial, the uranium concentration kept at a low level until the ferric solution was injected. It indicates that oxidation is necessary for uranium dissolution after adsorbed uranium becomes depleted.

3.2.2 Relationship between uranium mobilization and Fe(Ⅲ) - Eh

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Fe(Ⅲ) is an important natural oxidant for converting tetravalent to hexavalent uranium (Shen et al., 1960; Sani et al., 2005), and the relationship between Fe(Ⅲ) concentration and uranium leaching in acid ISL has been the focus of previous studies (Munoz et al., 1995; Que et al., 1999; Bilietsiki et al., 2000; Yu et al., 2003; Abhilash et al., 2013; Charalambous et al., 2014; Zhou et al.,

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2016).

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As discussed above, Fe was not added to the leaching solution in stage 1 of the

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FLT, and Fe ions in the solution originated from water-rock interactions. The

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initial concentration of ΣFe was approximately 0.05 g/L with no Fe(Ⅲ). At the end of stage 1, ΣFe increased to 0.8 g/L with Fe(Ⅲ) 0.2 g/L. In stages 2, 3, and

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4, high Fe(Ⅲ) solution was injected into the uranium-bearing strata. Thirteen

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days later, after the addition of Fe(Ⅲ), ΣFe and Fe(Ⅲ) began to increase

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synchronously in the pregnant solution. And 70 days later, ΣFe and Fe(Ⅲ) peaked at 2.31 g/L and 1.24 g/L respectively, then dropped slowly due to the decrease of them in the injection solution, as shown in Fig. 10.

Figure 10 shows that uranium concentrations, Fe(Ⅲ), and ΣFe are positively correlated and highly synchronized. Uranium concentration peaked two days later than Fe(Ⅲ) and ΣFe, and decreased following the decreasing of Fe(Ⅲ) and ΣFe. From the 2th to 4th stage, Fe(Ⅲ) was reduced to Fe(Ⅱ) and the Fe(Ⅲ) in ΣFe dropped from an average of 90 % to around 45 %, representing a

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decrease of approximately half (shown in Fig. 11).

There are many reducing substances that can contribute to the Fe(Ⅲ) oxidation-reduction reaction in the water-rock interaction of acid ISL, including pyrite and organic carbon, among others. Tetravalent uranium takes

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only small account of the total reducing substances in this FLT, and most of the

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Fe(Ⅲ) reduction was not caused by interactions with uranium minerals.

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Nevertheless, the FLT revealed that the kinetic rate of uranium oxidation and

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dissolution is positively correlated with the kinetic rate of Fe(Ⅲ) reduction. As

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shown in Figure 12, the curves of uranium dissolution kinetic rate is consistent with that of Fe(Ⅲ) reduction (Fig. 12a), with the ratios of uranium dissolution

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to Fe(Ⅲ) reduction between 0.041 and 0.091 (Fig. 12b).

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The Eh of the solution is undoubtedly the key factor affecting the dissolution of tetravalent uranium during the acid leaching of uranium, which is related to the composition and content of the variable valence ions in the solution. The Eh in solution is controlled by an electrical potential difference between the same elements of two different valence states (EPDS) with a relatively high ion concentration and a fast reaction rate (Shi et al., 2005). In the acid leaching solution, the EPDS of Fe(Ⅱ)/Fe (Ⅲ) is usually the key factor controlling Eh because of its higher concentration and faster reaction kinetic rate. As shown in Figure 13, solution Eh and Fe(Ⅱ)/Fe (Ⅲ) shows a strong positive correlation in

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both the injection and the pregnant solution, confirming this conclusion.

The relationship between pregnant uranium concentration and Eh as function of time (Fig. 14) shows that the rapid rises of uranium concentration, which occurred at early stage 1 and from stage 2 to early stage 4 respectively, are

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accompanied by a significant Eh increase. The peak of the uranium

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concentration in stage 4 is also due to the high Eh injection in stage 2 and stage

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3. It takes time for solution to flow from injection wells to the extraction well,

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so the change in the uranium concentration in the pregnant solution is time

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lagged. It is noteworthy that, in pregnant solution, the uranium concentration peak occurred synchronously with that of high Eh. The rapid fall after the peak

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in stage 4 is obviously due to the decrease of Eh at end of the stage.

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It should be noted here that the second and third stage injections are not only high Eh (high Fe(Ⅲ)/Fe(Ⅱ) ratio), but also high Fe(Ⅲ) concentration (3~6g/L), and the large amount of ferric ions reacting with uranium minerals caused a rapid increase of uranium concentration. If a solution has low Fe(Ⅲ) (typically lower than 0.5g/L) but with high Fe(Ⅲ)/Fe(Ⅱ) ratio, its Eh could also be high as that of stage 2 and stage 3. When this kind of solution is used for injection, Fe(Ⅲ) would be consumed quickly, and as a result, the uranium concentration and Eh would not be as high as shown in figure 14.

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As the uranium minerals that can readily react with the solution gradually dissolved, the positive correlation between uranium mobilization and Fe(Ⅲ) concentration and Eh also weakened, as shown in Fig. 12. Although both Fe(Ⅲ) concentrations and Eh were maintained at a high level and even greatly increased in the leaching solution at the end of stage 4, uranium concentrations

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kept declining mainly due to the depletion of uranium in the leaching zone.

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C3.2.3 The relationship between uranium mobilization and uranium

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deportment in the ore

The relationship between the intensity of uranium mobilization (dissolved

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uranium concentration) and the residual uranium content in the ore in this FLT

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is presented in Figure 15. Uranium concentration fluctuates, and the whole

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process can be divided into section AB, BC, CD, and DE (Fig. 15). Besides the factors mentioned above and the well-known effect of the depletion of uranium in ore, the staged difference of uranium mobilization intensity was also related to the differing deportment of uranium in the ore. According to previous researches (Cheng et al., 1995; Qin et al., 1998; Wang et al., 2009) and the backscatter photograph in figure 3, uranium in ore can be generally categorized into five types according to its deportment: (1) surface uranium in the open pores, (2) subsurface uranium in open pores, (3) surface uranium in microfissures, (4) subsurface uranium in microfissures, and (5) uranium in

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cemented materials. Combined with the characteristics of the uranium minerals shown in Fig. 3, the deportment of the five types of uranium speciation in the porous medium is represented in Fig. 16.

In ISL systems, the contact reaction between leaching agent and uranium

f

mineral can be divided into two categories: One is called external diffusion.

oo

That is, in relatively large connected open pores (the A-type pores in Fig. 16

pr

where water flows through by gravity), driven by convection and dispersion,

e-

the leaching agents can easily move to the surface of ore rock, then diffuse

Pr

through water film on the rock surface and reacts with the uranium mineral. Also, by molecular diffusion, the dissolved uranium enters the solution through

al

the water film. Another is called internal diffusion. The leaching agents enter

rn

the microfissures (B-type pores in Fig. 16, where there is thin film water and

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capillary water which cannot be driven by gravity) via molecular diffusion from the open pores and diffuses through the water film to react with the uranium mineral. In the same way, the dissolved uranium migrates from mineral to the microfissure and to the open pores. During ISL process, the release of uranium is related to uranium occurrence and the manner of solute transportation and corresponds to the four stages of uranium mobilization shown in Figure 15.

In section AB, dissolved uranium was dominated by hexavalent uranium

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(mainly adsorbed uranium mentioned before) that occurred on the mineral surfaces exposed fully to leaching solution in the open pores (hexavalent uranium of part Ⅰ shown in Fig. 16). Under natural hydrogeochemical conditions, uranium primarily dissolves as uranyl carbonate, but at very low intensity due to the low concentrations of CO 32- and HCO3- in the water. This

f

type of uranium dissolved quickly when the acid ISL solution was injected into

e-

pr

the concentration of uranyl complex ligands.

oo

the strata, and the intensity of mobilization was controlled by the pH level and

Pr

In section BC, the dissolution involved surface tetravalent uranium and subsurface hexavalent uranium in open pores (part Ⅰ and Ⅱ shown in Fig. 16,

al

respectively), and surface uranium and subsurface uranium in microfissures

rn

(part Ⅲ and Ⅳ shown in Fig. 16, respectively). The tetravalent uranium of part

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Ⅰ was dissolved via weak oxidation by the sulfuric acid solution. Due to ore corrosion the subsurface hexavalent uranium of part Ⅱ was exposed to the reagent and dissolved. The sulfuric acid solution diffused into microfissures and reacted with uranium of part Ⅲ and part Ⅳ. The dissolved uranium was transported from microfissures to open pores by diffusion.

The factors that control uranium mobilization in this stage are the oxidation of tetravalent uranium, the corrosion of the ore and the diffusion of solutes. Because all these three processes proceed slowly, uranium dissolution is

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relatively weak, and the dissolved uranium concentration is very low during BC stage. It is noteworthy that if oxidant had not been added into the system in later stages, uranium concentration in the pregnant solution would have been kept at low level, about 10mg/L, or even lower.

f

Dissolution in section CD was dominated by tetravalent uranium on the

oo

mineral surfaces (part Ⅰ shown in Fig. 16). Here, strong oxidation caused by

pr

Fe(Ⅲ) enabled tetravalent uranium on the mineral surfaces to be oxidized and

Pr

oxidation caused by the Fe(Ⅲ).

e-

dissolved rapidly. Therefore, leaching was controlled by the kinetic rate of the

al

In section DE, the dissolution mainly took place of the uranium occurring in

rn

the subsurface of open pores (part Ⅱ shown in Fig. 16) and in the

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microfissures (part Ⅲ and Ⅳ shown in Fig. 16, respectively). It was driven by gradual corrosion of the ore surfaces by sulfuric acid and the following diffusion and permeation of the Fe(Ⅲ)-rich solution into the microfissures. During this period, the intensity of the uranium mobilization was mainly controlled by the diffusion and permeation of the solution into the subsurface of ore.

Most of deep cemented uranium in the ore (part Ⅴ shown in Fig. 16) could not contact with the leaching solution and could not be released into the solution.

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The four leaching levels described above progressed successively. When the conditions for adequate acidity and oxidation are met, the surface uranium in the open pores will easily dissolve. However, because surface uranium is only a small portion of total uranium in the undisturbed ore underground, more than

f

80% of the uranium dissolution took place in the final stage (after point D in

e-

pr

oo

Fig. 15).

Pr

4. Conclusions

al

Water-rock acid ISL of a sandstone-type uranium deposit was observed in this

rn

study and displayed four prominent stages of uranium mobilization from solid

Jo u

mineral to solution. The hydrogeochemical conditions and the deportment of uranium in the deposit are the main causes of such staged characteristics of uranium mobilization.

At the very beginning of the acid ISL process, a short-term peak of uranium concentration in the pregnant solution, which happened while pH value decreased from about 5.3 to 2.62 without oxidation, was mainly due to the desorption of uranyl ions and dissolution of hexavalent uranium minerals in the open pores. Ion exchanges, (e.g. adsorbed UO22+ was replaced by protons,

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and complexing of UO2SO4,) maybe the main mechanisms making uranium less adsorbable and led the rapid and short dissolution of uranium. But as the adsorbed hexavalent uranium became depleted, uranium dissolution intensity would decline rapidly and an oxidation process, such as injecting high Fe(III) concentration solution, was necessary.

Addition of Fe(III) enabled intensive

f

uranium mobilization. In this process the dissolution of uranium had a strong

oo

positive correlation with the reduction of Fe (III) and Eh of the leach solution.

pr

A strong process of uranium dissolution (typically tetravalent uranium) during

Pr

enough concentration of ferric ions.

e-

the oxidation process needs not only high Eh (high Fe(Ⅲ)/Fe(Ⅱ) ratio) but also

al

Furthermore, the dissolution of uranium in the microfissures was controlled by

rn

the corrosion and permeation of the leaching solution towards the uranium-

Jo u

bearing minerals. As the uranium depleted from the ore, the intensity of mobilization also gradually decreased. In the final stage of the ISL, the increasing acidity and/or concentration of oxidant had no significant impact on the dissolution of uranium.

Acknowledgements: This study was financially supported by the National Natural Science Foundation of China (Nos. 41572231, 41772266), the National Basic Research Program (973) (No. 2015CB453002), and the Jiangxi Province Key Research and Development Program (No. 20161BBH80051).

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Miles Silbeman from University of Texas is specially appreciated for correcting English grammar and spelling, and professional advice on geology.

References

f

Abhilash, Pandey, B.D., 2013. Role of ferric ions in bioleaching of uranium

oo

from low tenor Indian ore [J]. Canadian Metallurgical Quarterly, 50(2),

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102-112;

e-

Alam, M.S., Cheng, T., 2014. Uranium release from sediment to groundwater:

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Influence of water chemistry and insights into release mechanisms [J]. Journal of Contaminant Hydrology,164(4), 72-87.

al

Briganti, A., Armiento, G., Nardi, E., Proposito, M Tuccimei, P., 2017.

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Understanding uranium behaviour in a natural rock–water system: leaching

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and adsorption tests on the Tufo Rosso a Scorie Nere ignimbrite (Viterbo area, central Italy) [J]. Environmental Earth Sciences, 76(20), 680. Charalambous, F.A, Ram, R., Mcmaster, S,, Pownceby, M.I., Tardio, J., Bhargava, S.K., 2014. Leaching behaviour of natural and heat-treated brannerite-containing uranium ores in sulphate solutions with iron(Ⅲ) [J]. Minerals Engineering, 57(2), 25-35. Cheng M.G., Jan X.F., 1995. Geological characteristics and prospective assessment of in-situ leaching sandstone type uranium deposit [J]. Uranium Geology, (1), 11-18.

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Fuller, C.C., Bargar, J.R., Davis, J.A, Piana, M.J., 2002. Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation [J]. Environmental Science & Technology, 36(2), 158-65. Gao, B., Shi, W.J., Wang, G.H., 2003. Study of migration characteristics of solute (uranium) during in-situ leach process [J]. Uranium Geology, 19(2),

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100-105.

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Golubev, V.N., Dubinina, E,O., Chernyshev, I.V., et al., 2016. Behavior of

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isotope (18O/16O, 234 U/238U) systems during the formation of uranium

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deposits of the “sandstone” type [J]. Doklady Earth Sciences,466(1), 28-

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Lammers L.N., Rasmussen, H., Adilman, D., Delemos, J.L., Zeeb, P., Larson,

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D.G., 2017. Groundwater uranium stabilization by a metastable

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hydroxyapatite [J]. Applied Geochemistry, 84, 105-113.

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Langmuir, D., 1978. Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits [J]. Geochimica Et Cosmochimica Acta, 42 (6), 547~569. Langmuir, D., Chatham, J.R., 1980. Groundwater prospecting for sandstonetype uranium deposits: a preliminary comparison of the merits of mineralsolution equilibria, and single-element tracer methods [J]. Journal of Geochemical Exploration, 13(2–3), 201-219. Laxen, P.A., 1973. A Fundamental Study of the Dissolution in Acid Solutions of Uranium Minerals from South African Ores [D], PhD Thesis, University

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of Witwatersrand, Johannesburg. Liu J.H, Sun Z.X., Chen G.X. 2002. Hydrogeochemical Characteristics of 512 Deposit in Yili Basin, Xinjiang [J]. Journal of East China University of Technology (Natural Science), 25(4), 271-274. Morrison, S.J., Tripathi, V.S., Spangler, R.R., 1995. Coupled reaction/transport

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modeling of a chemical barrier for controlling uranium (VI) contamination

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in groundwater [J]. Journal of Contaminant Hydrology, 17(4), 347–363.

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Munoz, J.A., Ballester, A., Gonzalez, F. & Blazquez, M.L., 1995. A study of

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the bioleaching of a Spanish uranium ore. Part II: orbital shaker

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experiments [J]. Hydrometallurgy, 38 (1), 59–78. Noubactep C, Schöner A, 2006. Meinrath G. Mechanism of uranium removal

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from the aqueous solution by elemental iron [J]. Journal of Hazardous

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Materials, 132(2–3), 202-212.

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Oddo, J.E, Adams, J.A.S., Thompson, M.B., 1981. Novel Approach to Eh-pH Diagrams and Their Relation to Uranium In-Situ Leaching: Abstract [J]. Amer Assoc Petroleum Geologist Bulletin, 966. Post, V., Vassolo, S.I., Tiberghien, C., Baranyikwa, D., Miburo, D., 2017. Weathering and evaporation controls on dissolved uranium concentrations in groundwater - A case study from northern Burundi [J]. Science of the Total Environment, s, 607–608, 281-293. Qin M.K., Wang Z.B., 1998. Zhao Ruiquan, et al. Characterisitics of clay minerals and their relationship with uranium mineralization in uranium

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deposit No.512, Yili basin [J]. Earth Science, 23(5), 508-512. Que W.M, Yao Y.X., Wang X.W.,1999. Factors influencing the reaction rates of in -situ leaching. Uranium Mining and Metallurgy,18(3), 156-163. Ram, R., Charalambous, F., Tardio, J., Bhargava, S., 2011. An investigation on the effects of Fe (FeIII, FeII) and oxidation reduction potential on the

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dissolution of synthetic uraninite (UO2) [J]. Hydrometallurgy, 109 (1), 125

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–130

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, Ram, R., Charalambous, F.A., McMaster, S., Pownceby M.I., Tardio J.,

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Bhargava S.K., 2013. Chemical and micro-structural characterisation

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studies on natural uraninite and associated gangue minerals [J], Minerals Engineering, 45(complete), 159-169

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Sani, R.K., Peyton, B.M., Dohnalkova, A., Amonette, J.E., 2005. Reoxidation

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of reduced uranium with iron(Ⅲ) (hydr)oxides under sulfate-reducing

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conditions [J]. Environmental Science & Technology, 39(7), 2059-66. Shen Z.L., Wang H.C., 1960. Radioactive hydrogeology [M]. Beijing: Geological Publishing House, 41. Shi W.J., 1986. Probe into the main existing form of uranium in natural water of hypogene zone [J]. Journal of East China University of Technology (Natural Science), (2), 69-83. Shi W.J., 1990. Principles of uranium geochemistry [M]. Beijing. Atomic Energy Press, 26. Shi W.J., Gao B., Wang G.H., 2004. Mechanism of Solute Migration During

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In-situ Leaching l Sandstone type Uranium Deposit [J]. Journal of East China Institute of Technology, 27(1), 24-32. Shi W.J., Sun Z.X., Applied hydrogeochemistry [M]. Beijing: Atomic Energy Press, 93-94 Suzuki S., Hirono S., Awakura Y., Majima H., 1990. Solubility of uranous

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sulfate in aqueous sulfuric acid solution [J]. Metallurgical Transactions B,

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21(5), 839~844.

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Tsarev, S., Collins, R., Ilton, E.S., Fahy, A., Waite, T.D., 2017. The short-term

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reduction of uranium by nanoscale zero-valent iron (nZVI): role of oxide

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shell, reduction mechanism and the formation of U(V)-carbonate phases [J]. Environmental Science: Nano, 4.,1304-1313.

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Wang B.Q., 1997. In-situ leaching condition and techicaleconomical appraisal

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of uranium deposit NO.512 [J]. Uranium Geology, (3), 147-153.

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Wang J., Geng S.F., 2009. Characteristics of the interlayer oxidation zone and the Kujieertai uranium deposit in Yili Basin [J]. Geology in China, 36(3), 705-713.

Yang X.Y., Ling M.X., Sun W., Liu C.Y., 2006. Study on the ore-forming condition and occurrence of uranium minerals in sandstone-type uranium deposits from Ordos basin, Northwest China[J]. Geochimica Et Cosmochimica Acta, 70(18), A720. Yu R.L., Liu S.J., Cao Y., 2003. Study on improving the efficiency of oxidation for in-situ leaching process [J]. Uranium Mining and Metallurgy, 21(3),

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122-125. Yue S., Wang G., 2011. Relationship between the hydrogeochemical environment and sandstone-type uranium mineralization in the Ili basin, China[J]. Applied Geochemistry, 26(1), 133-139. Zhou Y.P., Ji H.B., Sun Z.X., 2016. Uranium Migration Kinetics in Acid

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Solution Containing containing Ferric Iron [J]. Acta Geologica Sinica,

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90(12), 3554-3562.

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Zhou Y.P, Shen Z.L., He J.T., Liu J.H., Shi W.J., 2014. Bioleaching of uranium

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Metlallurgy), (10), 54-56.

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ore from a sand-type uranium deposit [J]. Nonferrous Metals (Extractive

Bilietsiki, B.Y., Bogatcolf L.К., Wolkolf N.Y. 2000. Handbook of in-situ

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industry, 21.

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leaching uranium mining [M].Hengyang: Sixth Institute of nuclear

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Dаnnеv, F.Y., Strеlianov N.P., Yao Z.K.. 1982. Some geochemical characteristics of uranium in the external ore forming process [J]. Earth and Environment, (6), 45-47. Evсееvа, L.S., Fомinа N. L. 1965. Oxidation-reduction character sedimentary uranium-bearing rock [M]. Beijing: Atomic Energy press, 5-12.

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Table 1. Chemical composition of groundwater at the FLT site Eh

F-

HCO3-

Cl-

SO42-

NO3-

Fe2+

Fe3+

K+

Na+

Ca2+

Mg2+

mV

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

7.23

389

3.77

53.12

34.75

68.93

15.13

＜0.01

＜0.01

1.53

18.08

53.26

10.06

7.09

383

3.89

71.36

32.71

78.33

15.05

0.71

＜0.01

2.86

21.42

55.14

6.87

Z3

7.16

391

9.60

68.14

43.35

64.68

27.71

＜0.01

＜0.01

1.65

27.96

61.32

5.62

Z4

7.08

395

2.41

75.23

23.75

60.64

10.80

0.95

＜0.01

3.32

21.42

54.60

4.81

C1

7.16

378

3.11

63.28

30.81

56.91

16.36

1.76

＜0.01

4.55

20.68

37.83

5.02

sample

pH

Z1 Z2

Note: Eh was measured with platinum electrode-calomel electrode. Groundwater temperature is 17 ℃, and saturated KCl

oo

f

electrode potential (for NHE) is 249 MV. Measurement method for Eh is the same in follows.

Table 2. The porosity and mud-content of the core samples from FLT site

porosity

fine sand

0.31～0.35

0.19～0.33

17.85～23.45

17.26～26.58

0.19～0.28 30.56～37.24

silt 0.14～0.19 40.13～42.31

clay 0.03~0.08 87.11~93.18

Pr

mud-content（%）

medium sand

pr

coarse sand

e-

analyzed item

Grade (μg/g) 658 982 527 432 604

rn

Z-1 Z-2 Z-3 Z-4 C-1

mineralization character Thickness (m) Mass of uranium per square meter (kg/m2) 1.2 1.42 3.1 5.48 1.9 1.8 0.7 0.54 2.3 2.39

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Well number

al

Table 3. The mineralization of the orebody revealed by each well

Note: Mass of uranium per square meter means total uranium mass contained in a column with cross-sectional area of 1 square meter throughout the ore-bearing aquifer.

Table 4. The average chemical composition of ore body and surrounding rock （%） rock type

U(T)

U(VI)

SiO2

TiO2

Al2O3

Fe2O3

FeO

CaO

K2O

Na2O

surrounding rock

0.016

0.014

68.42

0.25

13.84

1.21

0.86

0.67

4.85

2.84

ore

0.054

0.031

71.23

0.25

13.46

1.64

0.8

0.56

4.14

2.66

rock type

MgO

MnO

P2O5

CO2

S2-

S6+

CaCO3

FeS2

LOI

Total

surrounding rock

0.25

0.17

1.98

0.24

0.33

0.11

0.54

0.53

4.75

101.87

ore

0.21

0.11

1.67

0.19

0.28

0.22

0.43

0.52

3.33

101.79

Note: LOI represents loss on ignition.

Table 5. The composition of the industrial mining tail liquid

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pH 1.92

Eh

F-

Cl-

SO42-

NO3-

Fe2+

Fe3+

K+

Na+

Ca2+

Mg2+

Al3+

U

mV

mg/L

mg/L

g/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

606

186.1

334.47

11.9

582.1

431.8

205.5

63.9

276.1

69.1

166.8

627.1

＜1

Table 6. The average flow rate in each stage Production flow rate（ m3/d） C1 45.95 74.88 59.40 66.64

Injection flow rate（m3/d）

stage Z1 12.98 15.40 17.31 18.96

Z2 13.68 16.51 16.87 19.985

Z3 14.13 12.89 12.52 18.25

Z4 12.94 12.43 12.95 17.855

oo

f

stage 1 stage 2 stage 3 stage 4

Table 7. Chemical constituents of the pregnant solutions during the first 40 days

Mg2+ NO33.8 15 16.1 31 50.3 56 113.2 96 150.8 56 183.6 212 208.8 254 301.6 353 243.6 320 266.8 270 232 306 278.4 413 290 273 296 135 301.9 118

SO4269 155 752 2419 3686 4471 5256 5494 4634 5090 5606 5168 6348 6184 6458

Na+ 18.1 23.1 31.4 43.2 59.4 67.6 76.3 80.0 75.1 78.3 87.4 90.0 87.9 93.4 73.0

e-

Ca2+ 37 62 163 274 323 318 323 304 323 380 380 380 475 566 576

Pr

Fe2+ 10 10 47 47 89 96 142 132 123 137 126 152 170 180 210

al

Fe3+ 0 0 0 0 7 27 36 77 82 104 112 116 170 150 150

rn

U 0.29 0.29 0.82 1.51 24.40 26.17 40.13 23.31 18.81 13.87 12.39 11.80 11.21 11.21 11.21

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pH 7.06 7.03 6.89 5.32 4.07 3.77 2.62 2.62 2.54 2.34 2.18 2.28 2.14 2.05 1.99

pr

(concentration unit: mg/L) K+ 1.5 1.8 2.9 6.6 20.3 26.3 39.2 42.1 39.2 53.2 58.8 51.3 60.0 63.1 41.5

F3.8 3.3 3.9 4.5 3.9 5.3 5.0 5.8 4.1 4.8 5.8 6.2 6.8 6.4 8.3

Cl34.7 31.5 32.7 26.3 19.4 25.4 22.2 29.1 24.2 19.7 31.3 23.1 27.4 29.8 12.6

PO4314.9 11.2 13.1 9.4 12.1 15.5 10.8 13.3 12.0 13.1 15.5 11.2 13.5 14.9 11.2

Al3+ HCO34.5 73 3.2 78 40.8 61 265.9 3 405.7 0 584.1 0 679.7 0 692.8 0 548.0 0 534.2 0 652.8 0.00 582.5 0.00 642.5 0.00 495.9 0.00 517.8 0.00

Note: The content of Al3+ was not analyzed in the experiment, the Al 3+ value in the table was calculated using the anioncation balance, in which the initial concentration is almost consistent with the background in the deposit’s underground water.

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f

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rn

al

Pr

e-

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Fig. 1 Cross-section view of the ore body

Fig. 2 Distribution of wells in the FLT and the characteristics of ore-bearing aquifers

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f

Fig.3 The backscatter photographs of the uranium ore

Jo u

rn

al

Pr

e-

pr

oo

Qz: quartz, Kf: feldspar, Kt: Kaolinite, ①brannerite, ②rare-earth mineral, ③pitchblende, ④coffinite

Fig. 4 flow dynamics of the in-situ leaching process

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Z1 Z2 Z3 Z4 C1

100 80 3

Flow rate (m /d)

④

② ③

①

60 40

oo

f

20 0 50

100

150

200

250

300

350

400

450

pr

0

Time (d)

e-

Fig.5 the profile of flow rate

al

Acidity

Ca

2+

Mg2+

Fetotal

Eh

-

800

NO3

10

-

rn

700 600

Jo u

Acidity, Fe (III), Fetotal (g/L)

12

900

Fe(III)

8

6

500 400 300

4 200 2

100

0 0

50

100

150

200

250

300

350

400

450

0 500

Time (d) Fig.6 the profile of acidity, Eh and ions concentration of the injection solution

2+

14

Eh (mV), Ca2+, Mg , NO3 (mg/L)

Pr

① Stage 1; ②Stage 2; ③Stage 3; ④Stage 4

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①

80

④

② ③

60 50 40 30 20

f

Uranium concentration (mg/L)

70

oo

10 0 50

100

150

200

250

300

350

400

450

pr

0

Time(d)

e-

Fig. 7 Change in uranium concentration in the leaching solution

Pr

①Stage 1; ②Stage 2; ③Stage 3; ④Stage 4

al

Uranium concentration pH

7

30 20

5

pH

6

rn

40

8

4

Jo u

Uranium concentration (mg/L)

50

3

10

2

0

1 0

10

20

30

40

Time (day) Fig. 8 Profiles of uranium concentration and pH in pregnant solution during the early stage of the FLT

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100

UO2(SO4)2

UO2

UO2SO4

80

Percentage (%)

2-

2+

UO2(HPO4)2

2-

60

40

20

0 10

20

30

f

0

oo

Time (day)

40

② ③

Pr

70

al

60

9 8 7 6

rn

50

10 Dissolved uranium Fe(III) in injection solution Total Fe in injection solution Fe(III) in pregnant solution Total Fe in pregnant solution

40

5

Jo u

Uranium concentration (mg/L)

④

30 20

4 3 2

10

1

0

0 0

50

100

150

200

250

300

350

400

Time (d)

Fig. 10 Uranium concentration, Fe(Ⅲ), and ΣFe concentration curves ①Stage 1; ②Stage 2; ③Stage 3; ④Stage 4

450

Fe(III)， Fetotal (g/L)

① 80

e-

pr

Fig. 9 Profiles of major uranium species in pregnant solution during the early stage of the FLT

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90 80 70 60 50 40 30

pregnant solution injection solution

20 10

st 1 stage

2

nd

to 4

th

stages

0 50

100

150

200

250

300

350

oo

0

f

Ratio of Fe(III) to Fetotal (%)

100

400

450

500

Time(d)

40 30 20 10

al

50

rn

60

4

8

70

6

4

2

Uranium dissolution rate (Kg/d)

Uranium dissolution rate

5

Jo u

Reduction rate of Fe (Ⅲ) (Kg/d)

80

10

Uranium dissolution rate (Kg/d)

Reduction rate of Fe (Ⅲ)

Pr

90

e-

pr

Fig. 11 Curves for the percentage of Fe(Ⅲ)in ΣFe in both the injection and leaching solutions

k2=0.091

3

2

k1=0.041 1

(b)

(a) 0 0 100 150 200 250 300 350 400 450 500

Time (d)

0 0

10

20

30

40

50

60

70

80

Reduction rate of Fe (Ⅲ) (Kg/d)

Fig. 12 Correction between speed of uranium oxidation-dissolution and the speed of Fe(Ⅲ) reduction

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pregnant solution

injection solution

800

670

780

660

760

650

Eh (mV)

Eh (mV)

820

740 720

640

700

630

680

620

660

610

640 600

10

100

0.1

oo

Fe (III) / Fe (II)

1

f

1

Fe (III) / Fe (II)

800

Pr

60

750 700

al

50

650

rn

40 30

600

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Uranium concentration (mg/L)

70

④

20 10

Eh (mV)

②③

e-

①

pr

Fig. 13 Bivariate plot showing strong positive correlation between solution Eh and Fe2+/Fe3+

550

Uranium concentration in pregnant solution Eh of the injection solution Eh of the pregnant solution

0

500 450

0

50

100

150

200

250

300

350

400

450

Time (d) Fig.14 Eh of injection and pregnant solution, pregnant uranium concentration as a function of time

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Surface U(Ⅵ) in open pores Surface U(Ⅳ)in open pores and U in microfissures Surface U(Ⅳ)in open pores Subsurface U in open pores and U in microfissures

60 50 40 D (80.95, 67.98)

30

10

oo

f

20

C (93.82, 15.03) B (98.57, 14.46) A (100, 0.29)

0 100

90

pr

Uranium concentration (mg/L)

70

80

70

60

E (40.3, 23.38)

50

40

Pr

e-

Residual uranium rate in the ore (%)

Fig. 15 Relationship between uranium concentration in pregnant solution and residual uranium rate in the

Jo u

rn

al

ore rock

Fig. 16 Schematic showing uranium speciation in the porous medium and related dissolvedleaching path

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rn

al

Pr

e-

pr

oo

f

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

Reliable and long range field data of uranium in-situ leaching was presented; PH, complex ligand ions and redox potential (mainly controlled by ratio of Fe(III)/Fe(II)) are the main hydrogeochemical factors affecting uranium dissolution and migration. Deportment of uranium determines the difficulty of uranium mineral contact with solution, and restricts the hydrochemical reactions, thus affecting the process of uranium migration from mineral to solution.

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 

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Declaration of interests XThe authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

Jo u

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XThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests: NONE.

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