Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore

Journal Pre-proof Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore

Wenrui Nie, Rong Zhang, Zhengyan He, Jie Zhou, Ming Wu, Zhigao Xu, Ruan Chi, Huifang Yang PII:

S0304-386X(19)31035-7

DOI:

https://doi.org/10.1016/j.hydromet.2020.105295

Reference:

HYDROM 105295

To appear in:

Hydrometallurgy

Received date:

15 November 2019

Revised date:

18 January 2020

Accepted date:

15 February 2020

Please cite this article as: W. Nie, R. Zhang, Z. He, et al., Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore, Hydrometallurgy(2020), https://doi.org/10.1016/j.hydromet.2020.105295

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Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore Wenrui Nie a, Rong Zhanga , Zhengyan He a, Jie Zhoua, Ming Wua , Zhigao Xua* , Ruan Chib* , Huifang Yanga,b

a

Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education &

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Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for

Technology, Wuhan, 430073, China.

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*Corresponding authors: Zhigao Xu

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Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of

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b

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Nationalities, Wuhan 430074, China;

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Tel: +86 027 67842157; Fax: +86 027 67843918; E-mail address: [email protected]

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Address: Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan 430074, China.

*Corresponding authors: Ruan Chi

Tel: +86 027 87195682; Fax: +86 027 87195682; E-mail address: [email protected]

Address: Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan, 430073, China.

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Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore Abstract Rare earth (RE) with specific application value has become the relatively important strategic resource for all countries. Weathered crust elution-deposited rare earth ore (WCED-REO) are rich in middle-heavy RE, and its exploitation restrict the global supply of

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the middle-heavy RE. The leaching technology directly determines the leaching efficiency of RE in WCED-REO. The selection and use of the leaching agents are the most important parts

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of the leaching technology. The leaching mechanism is at the core of the leaching technology

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and also affects the effective utilization of the leaching agents. The research and development

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of leaching technologies, leaching agents, and leaching mechanisms of WCED-REO are discussed in this article. The development processes of exiting leaching technologies are

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introduced. The advantages and disadvantages of barrel leaching, pool leaching, heap

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leaching, in-situ leaching, and other leaching technologies are compared. And the new and

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practical leaching technologies are put forward. The research progresses of ammonium salt leaching agents, composite ammonium salt leaching agents, and novel non-ammonium leaching agents in recent years are reviewed. The leaching effects of different leaching agents are compared comprehensively to provide guidance for industrial application and research direction of the best leaching agent in the later period. The leaching basic theory, as well as the mass transfer process and leaching kinetics of rare earth are introduced. The mass transfer processes in leaching mechanism are explained from the macroscopic and microscopic perspectives. The further research directions of the leaching technologies, leaching agents, and leaching mechanism of WCED-REO are put forward, which provide the theoretical 2

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guidance and technical supports for the green and efficient exploitation of WCED-REO.

Keywords Rare earth; Weathered crust; Leaching technology; Leaching agent; Leaching mechanism

1. Introduction

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Rare earth (RE) is an industrial vitamin (Meng, 2011) with high application value that

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can significantly improve the performance of materials. RE elements include 15 lanthanides,

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scandium, and yttrium. The light RE elements refer to La, Ce, Pr, and Nd. The middle RE

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elements refer to Sm, Eu, and Gd. The heavy RE elements refer to Tb, Dy, Ho, Er, Tm, Yb,

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Lu, and Y (Xu, 2002). Especially, the middle-heavy RE elements are widely used in the manufacture of permanent magnet materials, luminescent, laser materials, superconducting

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materials, and electronic materials (Moldoveanu and Papangelakis, 2016; Gupta and

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Krishnamurthy, 2015). With the continuous development of science and technology, RE has

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become the relatively important strategic resource for all countries due to its specific value and low reserves (Chen, 2011).

Rare earth ores (REOs) generally have two types: the mineral type rare earth ores (MT-REOs) and the weathered type rare earth ores (WT-REOs). The MT-REOs are rich in light RE and mostly represented by of bastnaesite and monazite. This type REOs are mainly distributed in China, America, Australia, Canada, and South Africa (Ober, 2016). The RE in MT-REOs can be enriched by the conventional physical separation methods such as flotation, magnetic separation, and gravity separation (Moldoveanu and Papangelakis, 2012).

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Unlike MT-REOs, the WT-REOs are mainly rich in middle-heavy RE, mostly represented by the weathered crust elution-deposited rare earth ore (WCED-REO, also known as ion adsorption rare earth ore). WT-REOs are mainly distributed in the southern provinces of China, such as Jiangxi, Guangdong, Fujian, Guangxi, Hainan, Hunan, and Yunnan (Chi et al., 2014b). With the continuous deepening of mine exploration degree, the WT-REOs have

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been also discovered in Laos (Sanematsu et al., 2009), Thailand (Sanematsu et al., 2013,

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2015), Indonesia (Maulana et al., 2014), Madagascar (Berger et al., 2014), Philippines

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(Padrones et al., 2017), America (Bern et al., 2017), and other countries in recent years. The

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WT-REOs are mainly composed of clay minerals, quartz sand, and rock-forming mineral

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feldspar. The contents of clay minerals range from 40 to 70% and its main mineral compositions are kaolinite, halloysite, illite, and montmor illonite (Chi et al., 2014b). Rare

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earth ions (REEs) adsorbed on these clay minerals are mainly in the forms of hydrated or

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hydroxyl hydrated ions (Chi et al., 2014b; Tian et al., 2010a). According to the specific

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occurrence states of RE in WCED-REO, the RE can be easily recovered through ion exchange. That is to say, the REEs in the RE raw ore can be exchanged into the solution by electrolytes leaching. The accompanying impurity ions are also being exchanged into the solution, the impurities mainly contain iron and aluminum, but also calcium and barium.

The recovery of RE from the WCED-REO is highly dependent on the leaching processes. Fig. 1 displays the numbers of published papers on the topics of "Weathered crust elution-deposited rare earth & Ion adsorption rare earth" and "Weathered crust elution-deposited rare earth leaching & Ion adsorption rare earth leaching" from 2010 to 2019. The number of published papers on RE leaching has grown rapidly in the past decades, which 4

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indicates that this scientific area has become a research hot spot and has attracted more researchers in this filed.

Recently, the review articles of RE leaching for WCED-REO are limited. Moreover, the existing review publications are mainly focus on introducing the mining and leaching processes of REOs. These papers do not summarize the latest leaching technologies, and

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rarely discuss the development and application trend of leaching agents, nor do they explain

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the leaching mechanism and research methods of REOs.

The leaching technology directly determines the leaching efficiency of RE in

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WCED-REO. Incorrect leaching technology or improper leaching process operation (such as

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liquid injection and liquid collection) may lead to serious groundwater pollution or geological

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disasters such as landslides while failing to achieve ideal RE leaching efficiency. In this

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article, the development of the leaching technologies from barrel leaching and pool leaching

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(Chi et al., 2014b), heap leaching technology to in-situ leaching and other leaching technologies are introduced. And the new and practical leaching technologies are introduced. The advantages & disadvantages for different leaching technologies are discussed, in order to guide the selection of a suitable leaching technology for specific ore bodies or mines.

The leaching agent directly relates to the exchange efficiency of REEs in WCED-REO, the content of impurities in leachate. Whether the leaching agent meets the environmental requirements is also the hotspot for the leaching process of WCED-REO. On the basis of sodium salt and ammonium salt leaching agents, the RE leaching agents develop into composite ammonium salt and green non-ammonium leaching agents (Chi et al., 2019). It is 5

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believed that ammonium salt leaching agent is still difficult to be completely replaced in a short period of time. In this article, the performance of different leaching agents are reviewed , which can provide guidance for using and developing green RE leaching agent with high efficiency.

With the existing leaching technology and leaching agent, the leaching efficiency of RE

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has not reached the theoretical maximum. It is still impossible to leach the RE from the WCED-REO completely. Because the leaching mechanism has not been thoroughly studied.

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In this article, the main methods and achievements in the study of leaching mechanism for

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WCED-REO are reviewed, such as the basic ion exchange theory of leaching, the mass

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transfer process explained from macro and micro perspective, and the leaching kinetics. These

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the leaching efficiency of RE.

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are aims to guide the readers to further study the leaching mechanism, and to further enhance

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2. Leaching technology of weathered crust elution-deposited rare earth ore

The leaching technology of WCED-REO has gone through three stages: barrel leaching and pool leaching technology, heap leaching technology, and in-situ leaching technology (Chi et al., 2019; Huang et al., 2005).

2.1. Barrel leaching and pool leaching technology

The barrel leaching technology was developed in the early 1970s. The screened rare earth ore was placed inside wooden barrels, and REEs were leached by sodium chloride solution. Due to the small production scale, low output, and high cost, the barrel leaching 6

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technology was rapidly replaced by the pool leaching technology in the mid-1970s.

During the pool leaching, the screened REOs were firstly placed in the cement leaching pool with a certain angle tilted bottom, which facilitate the outflow of the RE leachate. The pool leaching technology, as well as the leaching facilities, is simple and lack of further protections, thereby has caused serious environmental pollution. For instance, the vegetation

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restoration on polluted sites is difficult to achieve. Moreover, the low recovery efficiency of RE also restricted its further applications. As a consequence, sodium chloride was replaced by

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ammonium sulfate in the early 1980s. In contrast, ammonium sulfate leaching has the lower

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dosage, higher RE leaching efficiency, and brings purer RE products (Chi and Tian, 2008).

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In order to further achieve higher exploitation efficiency of WCED-REO, new leaching

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technology.

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technologies were developed, such as the heap leaching technology and in-situ leaching

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2.2. Heap leaching technology

For heap leaching, a clean heap leaching field is required. Junction canals surrounded with flood protection ditches are designed at the lower side of the heap, and the leakproof layer are covered. Meanwhile, sludge settling tanks are set to link the junction canals, so that the leachate can be clarified. Then the REOs are placed on the heap leaching field. The leaching agent solution is injected to the top of the heap, then naturally penetrate downward, and therefore leach REEs (Deng et al, 2016). The flowsheet of the heap leaching technology is shown in Fig. 2. Compared with the pool leaching technology, heap leaching technology can effectively control the solid-liquid ratio of the leaching process by changing the height of 7

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the ore heap (Li, 2014). The leaching agents can be used more effectively, and the technology has higher recovery efficiency so that result in lower costs.

The RE leaching efficiency is affected by the permeation of leaching solution in ore (Zhou et al., 2019). High permeability is beneficial to the leaching efficiency of RE. However, orebody of some WCED-REO are weak in permeability. In this case, Wei et al. (2016) found

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that adopting the heap leaching technology could have a shorter leaching period and higher RE recovery efficiency. At the same time, it could mitigate the anti-adsorption phenomenon

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of RE as well as the influence of the climatic conditions. Mei et al. (2017) effectively

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improved the permeability of solution in ore and the leaching efficiency of RE by optimizing

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the conditions and parameters of heap site constructions. These optimum proposals include a set bottom pad inclination angle in the heap site, a reduced heap height, adopted heap

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construction in sections and layers, and added permeators within the ore heap.

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Most of the main impurity aluminum in RE leachate came from the humus layer. In order to reduce the content of impurities in RE leachate in heap leaching technology, Chi et al. (2014a) stripped the humus layer and only piled the REOs of the completely weathered layer and partly weathered layer for heap leaching. Use this method to leach could help improve the purity of RE leachate, so as to obtain purer RE products, and reduce the cost of subsequent recovery.

2.3. In-situ leaching technology

In-situ leaching technology include mine prospecting, injection system, collection system, and some follow-up work. For in-situ leaching, the mine should be prospected first, to 8

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make sure the orebody has false floor, non-fissure and high permeability (Li, 2014). If these geological conditions are lacked, a large amount of RE enrichment fluid will be lost , or the leaching efficiency will be very low. And even worse, the underground water will be seriously polluted by the leakage of leaching agent. At this point, it is necessary to replace in-situ leaching technology with heap leaching technology.

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After geological prospecting, the leaching agents are allowed to be vertically injected into the orebody through injection holes drilled at the top of the RE mine. RE leachate is

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collected from deflector holes and collection ditches at the foot of the mountain, anti-seepage

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measures were applied in the collection ditches and drainage ditches to avoid the loss of RE

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enrichment fluid (Deng et al, 2016). The flowsheet of the in-situ leaching technology is shown in Fig. 3. Compared with pool leaching and heap leaching technologies, in-situ

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leaching technology does not dig topsoil or ore. There are no damages to the mountains and

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vegetation, little destruction of mine surroundings, less workload, and lower production cost

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in the in-situ leaching technology. These obvious advantages make in-situ leaching technology widely used.

In-situ leaching technology is not perfect. Collapse of the injection hole or leakage of the leachate is usually appeared during the leaching process, which is easy to cause landslides, environmental damages and reduce the recovery efficiency of RE. To a certain extent, these problems can be improved by optimizing injection and collection technology.

2.3.1. Injection technologies

Injection technology is the first step of in-situ leaching technology. In the traditional 9

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injection technology, the leaching agent is injected directly into the ore body through the injection holes, the drilled holes enlarge contact areas so that enhance the ore leaching. But there are some problems, such as the poor stability of injection holes, the overtopping of leaching liquid in injection holes, and the increase of landslides probability. In order to overcome these issues, the injection technology has been optimized. Zhang et al. (2016)

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proposed a new injection technology. In this technology, the circular injection holes with a

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diameter of 0.16m were drilled vertically downward along the mountain surface. The

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downcomers were placed into the holes, the space between the hole and the downcomers were

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filled with fillings, which could withstand the mountain surface pressure. The marker poles

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were placed into the downcomer to monitor liquid level heights. This method not only guarantees the effective utilization rate of injection holes, but also reduces the occurrence of

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safety accidents and improved the recovery efficiency of RE. Wang et al. (2017) designed a

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device to promote the permeability of the ore, this was also a new type of injection tube. It

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was comprised of the inner tube, the outer tube with seepage holes, and cavity with the sealed bottom. The cavity was filled with porous anticorrosive medium. This device can reduce the depth and quantity of the injection holes, therefore enhance the stability of the injection holes. Zhang et al. (2016a) controlled the height of the liquid level in the injection holes by adjusting the flow rates of liquid injection, so that the leaching agent only contacted with the completely and partly weathered layer with more ionic phase rare earths and less impurities, and avoided contacting with humus layer with few ionic phase rare earths and more impurities. This method can not only decrease the content of the impurity ions in the leachate from the source, but also reduce the geological disasters such as the landslide caused by the 10

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water swelling of clay minerals in the humus layer. Gui et al. (2017a, 2017b) calculated the total amount of liquid injection and steady seepage flow of the single-hole injection during in-situ leaching by determining relationship between the leaching efficiency of RE and the saturation of the hole bottom. Based on these, a design method of network hole parameters for in-situ leaching was proposed to reduce the no-control rates of leaching ore and landslide

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incidence.

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2.3.2. Collection technologies

The collection technology is the key to ensure the recovery efficiency of RE and prevent

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pollution diffusion. Traditional collection technology not only has low RE recovery efficiency,

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but also has the hidden danger of leaching liquid leakage. In the meanwhile, the collection

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ditch built by cement can also pollute the mountain environment. For these reasons, new

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collection technologies have been put forward one after another. Hu et al. (2015) proposed a

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new method to improve RE leachate recovery efficiency. In this technology, the deflector roadways in mine were opened between the completely weathered layer and partly weathered layer. The deflector holes had been increased from 1 to 2 rows, and were located at the side of the partly weathered layer, near the slight weathered layer. The horizontal inclination angle, distance of deflection holes, and row spacing of deflection holes were improved. This technology can avoid the loss of RE resources in the completely weathered layer and the loss of RE enrichment fluid in leachate by means of opening the combination of deflector roadways and deflector holes. Aimed at the WCED-REO with complex orebody, Yuan et al. (2017) proposed a new interception and collection technology to improve RE leachate

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recovery efficiency. The technology included four processes: collecting roadways, deflector holes, cutting grooves and anti-seepage treatment. The most important was that the deflector hole and the groove could form a manual interception plate, which could intercept more than 90% of the leaching fluid flowing through the area. In order to effectively reduce the leakage of the leachate, Chi et al. (2018) designed multi-stage intercept pools according to the high

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terrain to low terrain, which were built in the catchment water at the foot of the mine.

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According to the content of RE in the interception pool, the dosage of leaching agent could be

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controlled precisely to reduce the extra consumption of leaching agent, thus reduced the costs

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and the corresponding pollution. Zou et al. (2016) invented an aquifer water collection device.

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The devices were connected with the collection ditches and were placed surround REOs. This device effectively improves the collection efficiency of leachate, therefore avoided the loss of

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RE enrichment solution and reduced the recovery costs.

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2.4. Other leaching technologies

In the leaching process of WCED-REO, some external conditions can be applied, such as heating (Moldoveanu and Papangelakis, 2015), electric field (Zou et al., 2007), magnetic field (Qiu et al., 2008), and ultrasound (Yin et al., 2018). The techniques mentioned above can change the physicochemical properties of the surface of mineral particles, enhance the diffusion rate of the ions between the solid-liquid interface, therefore improve the leaching efficiency of RE (Wang et al., 2018). However, these auxiliary conditions are difficult to be applied in the practical industrial exploitation, and only keep the laboratory exploration stage.

2.5. Comparison of different leaching technologies 12

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In order to comprehensively evaluate the above-mentioned leaching technologies, the advantages and disadvantages of which are illustrated in Table 1 (Li, 2014). It can be seen that in-situ leaching technology is the best and most promising technology at present, but it is easy to cause geological problems due to improper technical treatment or management. Therefore, more attention should pay for the ore geology prospecting before using in-situ

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leaching technology, and improving the injection technologies. More attention should also be

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payed to the improvement of the collection technologies, because which is not only affect the

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leaching efficiency of RE, but also affect the pollution degree of surface water and

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groundwater. It is important to point out that, for the orebody with weak permeability, fissure,

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and non-false floor or the flat land, the heap leaching technology is particularly recommend. It can realize the high recovery efficiency of RE and avoid the leakage of the RE leachate to

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pollute the water body (Chi et al., 2019). Also, vegetation removed during heap leaching also

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needs to be handled properly. Other novel leaching techniques need to take the actual

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situation into account, otherwise it is an armchair strategist.

3. Leaching agent of weathered crust elution-deposited rare earth ore

The initial leaching agent of WCED-REO was NaCl solution (6-8 wt%). However, NaCl leaching agent gets low RE leaching efficiency and high impurity in leachate. What’s more, it would produce a large number of high-salt wastewater, improper treatment or disposal will lead to soil salinization and eventually damage the ecological environment. (NH4 )2 SO4 as the leaching agent, by contrast, can not only decrease the concentration and consumption volume of the leaching agent, but also improve the leaching efficiency of RE and reduce the leaching

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of impurities (eg. calcium and barium) by forming the insoluble sulfate to stagnate in ore body. Thus, (NH4 )2 SO4 (1-4 wt%) replaced NaCl as the main leaching agent in the exploitation of WCED-REO, and widely used at present.

In addition, (NH4 )2 SO4 leaching agent can also be injected in stages with different concentration. For the WCED-REO with high muddy, Kong (2018) suggested that the

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leaching process could be divided into two stages to improve the permeability coefficient of the muddy ores and shorten the production cycle. In the earlier stage, the leaching agent was 4%

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(NH4 )2 SO4 , and the concentration of (NH4 )2 SO4 was decreased to 2% in the later stage. High

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concentration of (NH4 )2 SO4 in the earlier stage had high acidity, which could dissolve alkaline

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minerals or colloids of micro particles in mineral soil and expand pores in mineral soil, the permeability of ores was improved. The dosage of leaching agent could be reduced by

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reducing the concentration in later stage.

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In order to further improve leaching effect (e.g. RE leaching efficiency, impurity removal efficiency, and environmental impact and so on.), many scholars tried to optimize leaching agents or develop novel leaching agents such as composite ammonium salt, magnesium salt, and aluminum salt leaching agents (Wang et al., 2018; Yang et al., 2019). Applying more advanced leaching agents could help realize a green, higher efficiency, low impurity, and low consumption exploitation of WCED-REO.

3.1. Composite ammonium salt leaching agents

The common ammonium salt leaching agents are (NH 4 )2 SO4 , NH4Cl, and NH4 NO3. The cation exchangeability and permeability of NH4 NO3 and NH4 Cl are better than (NH4 )2 SO4 14

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(He et al., 2017). NH 4 NO3 as a dual-nitrogen fertilizer provides more available nitrogen during leaching, which will aggravate ammonia nitrogen pollution in mining areas. Although NH4 Cl is more available and inexpensive, its poor selectivity performance lead to more remained impurity ions in the leachate. (NH4 )2 SO4 as the leaching agent can obtain the leachate with fewer impurity ions, but the leaching efficiency of RE is slightly lower and the

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anti-swelling effect is worse (He et al., 2016). Comprehensive evaluation shows that the

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leaching effect of (NH 4 )2 SO4 is better than that of NH 4Cl and NH4NO3. In order to optimize

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the performance of single ammonium salt leaching agent, some methods are proposed, like

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compounding ammonium salts or adding additives (e.g. inorganic or organic agents) into the

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leaching agents (Li, 2014).

Zhang et al. (2013) and Zhang et al. (2014) compounded NH 4 Cl, NH4 NO3 , and

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(NH4 )2 SO4 with a mass ratio of 4: 5: 6, NH4 Cl and NH4 NO3 were compounded with a mass

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ratio of 8: 2, respectively. Compared with single ammonium salt leaching agents, composite

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ammonium salt leaching agents could maintain high RE leaching efficiency. Besides, they could effectively reduce the linear expansion rate of clay minerals, the linear expansion rates were both lower than 2.8%. As an assistant leaching agent, the addition of some quaternary ammonium salt surfactants can also play an anti-swelling role (Wang et al., 2018). The geological disasters like landslides caused by the swelling of clay minerals after adsorption of water (Zhang et al., 2018) could be reduced in the leaching process.

Tian et al. (2013a) and Tang et al. (2013) found that when 0.03% sesbania gum (SG) was added with 3% (NH4 )2 SO4 , the leaching efficiency of RE could be increased by 7.12%. Also,

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the concentration of leaching agent decreased from 4% to 3%. Then Tian et al. (2013b) modified SG with carboxymethyl to obtain carboxymethyl sesbania gum (CSG), which was added into (NH4 )2 SO4 to further improve the leaching efficiency of RE, and reduced the concentration of (NH4 )2 SO4 to 2.5%.

Qiu et al. (2014a) found that the composite leaching agent of 0.1% of the impurity

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inhibitor LG-01 (organic agents containing hydroxyl and carboxyl groups) and 4% (NH 4 )2 SO4 could obtain a high RE leaching efficiency and up to 92% of the impurities removal efficiency.

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Qiu et al. (2014b) found that adding YZJ-01 (organic impurity inhibitor) to (NH4 )2 SO4

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leaching agent could make RE leaching efficiency up to 98% under the best conditions. At the

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same time, the leaching of impurities such as aluminum and iron are greatly inhibited. Subsequently many novel leaching agents with ammonium salts were also proposed, and the

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main leaching agents were listed in Table 2.

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3.2. Novel non-ammonium leaching agent

Composite ammonium salt leaching agents still contain NH4 + or NO3 -, which will produce ammonia-nitrogen wastewater. Thus, developing novel green non-ammonium leaching agents, to replace the ammonium salt leaching agents, have become a research hotspot.

Xiao et al. (2015a, 2015b) found that the use of 0.20 mol/L MgSO 4 could obtain a low leaching efficiency of impurity aluminum and above 93% of RE can be leached out. The leaching effect of MgSO4 was nearly the same as that of (NH4 )2 SO4 . It indicated that ammonia nitrogen pollution could be avoided by using the non-ammonium leaching agent. 16

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Due to its pollution-free and relatively high leaching efficiency, MgSO 4 has been added into the leaching agents and used in industry. In order to further improve the leaching performance and greenization of MgSO4 , MgSO4 was mixed with NH4 Cl and CaCl2 at a molar ratio of 15:25:60 to obtain a new composite leaching agent. The leaching efficiency of RE could be increased to above 94%, while that of aluminum could be reduced to 49.2% (Xiao et al.,

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2016a). This novel composite magnesium salt leaching agent makes the ratio of exchangeable

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calcium to magnesium in soil meet the requirement of soil nutrient ratio, and solves the

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problem of ammonia nitrogen pollution from the source, so it has been applied in industry.

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The mechanism of the leaching agents mentioned above is mainly leaching of ionic

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phase RE instead of the colloidal sedimentary phase RE. Xiao et al. (2016b) tried to leach the colloidal sedimentary phase RE (mainly CeO 2 /Ce(OH)4 ) using FeSO4 as leaching agent by the

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reductivity of Fe 2+. To some extent, the leaching efficiency of RE could reach about 102%,

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but the content of impurity ions in the leachate was also greatly increased, it was not

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conducive to the impurity removal. Whereupon, the dissolution of CeO 2 /Ce(OH)4 became easier by adding ascorbic acid with a strong reductivity to MgSO4 in the leaching process, then Ce3+ could be acquired (Xiao et al., 2017). This novel compound leaching agent could improve the leaching efficiency of colloid sediment phase RE, reduce greatly the leaching efficiency of impurities (Al and Fe) and decrease the dosage of MgSO 4 (Lai et al, 2018a, 2018b).

Beside the leaching agents of magnesium salts, Yang et al. (2018) put forward aluminum salt as the novel leaching agent. The leaching efficiency of RE for Al2 (SO4 )3 solution in

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0.128mol/L could reach 98.25%, higher than that of the ammonium salts and magnesium salts. Because of the low zeta potential of Al2 (SO4 )3 , the repulsion force between mineral particles during leaching is the least and the anti-swelling effect is the best, which can reduce the risk of landslide (Zhang et al., 2018; Yang et al., 2018). Thus, Yang et al. (2019) used Al2 (SO4 )3 in multi-stage leaching ore process. (NH 4 )2 SO4 was used as the leaching agent in the first stage,

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0.02 mol/L Al2 (SO4 )3 was used as the leaching agent in the second stage. Due to the addition

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of Al2 (SO4 )3 , the residual ammonium in tailing decreased from 11.2% to 0.6%, the absolute

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value of zeta potential of the tailings decreased and approached 1.5, the required amount of

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lime in subsequent treatment decreased, which led to the low cost and low environmental

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impact. However, the effect of impurity content in leachate was not considered when Al2 (SO4 )3 was used as the leaching agent, so it is only proposed in the experimental stage, and

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has not been applied in industry.

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3.3. Comparison of different leaching agents

Generally, the development of leaching agent has been gradually on the road of green and pollution-free development. Table 3 illustrates advantages and disadvantages of various leaching agents to comprehensively evaluate each leaching agent. The more plus signs (+), the better leaching effect would be gets. Obviously, the leaching agent containing ammonium salt is more efficient. It may be inevitable that there are leaching agents containing ammonium salt in the future. Thus, more attention should be payed to the development of additives to reduce ammonia nitrogen pollution. At present, the most promising alternative green leaching agent is the magnesium salt, proposed by academician Huang Xiaowei (Huang

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et al., 2016; Xiao et al., 2016a), which has been widely used in REOs.

Besides, the colloidal sedimentary phase of RE also accounts for a certain proportion of RE in ore. Although the currently used leaching agents can leach more RE in the colloidal sedimentary phase, the leaching efficiency has not reach theoretical maximum, which is what we should pay attention to develop new leaching agent and research the leaching mechanism.

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4. Leaching mechanism of weathered crust elution-deposited rare earth ore

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The leaching mechanism is benefit to enrich the theoretical basis of WCED-REO, and

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provide theoretical guidance for the development of novel leaching agents so that help

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optimize the leaching technologies. The leaching mechanism of WCED-REO is introduced from the following three aspects: leaching basic theory of rare earth, mass transfer process,

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and leaching kinetics.

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4.1. Leaching basic theory of rare earth

The main mineral constituent of WCED-REO is clay minerals, which are aluminosilicate and can be seen as a natural inorganic ion exchange agent. When rare earths adsorbed on the clay minerals meet the ammonium salt solution, the ion exchange reaction is shown as Eq. (1): [Al4 (𝑆𝑖4 𝑂10 )(𝑂𝐻)8 ]𝑚 ∙ 𝑛𝑅𝐸 3+ (𝑠) + 3𝑛𝑁𝐻4+(𝑎𝑞) ⇌ [Al4 (𝑆𝑖4 𝑂10 )(𝑂𝐻 )8 ]𝑚 ∙ 3𝑛[𝑁𝐻4+ (𝑠)] + 𝑛𝑅𝐸 3+(𝑎𝑞 )

(1)

Where: s is solid phase; aq is liquid phase.

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The ion exchange reaction is a reversible heterogeneous reaction, which is the theoretical basis of chemical leach of RE from WCED-REO. Therefore, the RE leaching basis theory is to inquire the adsorption sites, solution ion strength, solution acidity, ion complexation, and surface electrical properties of clay minerals in WCED-REO. It will be conducive to reveal the internal influencing factors of RE leaching efficiency.

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Xu et al. (2018) considered that except the surface of clay minerals, the REEs also adsorbed on the interlamination and marginal fracture structures of clay minerals and the

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colloidal particles of iron, manganese, and aluminum with strong adsorption to REEs. Among

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the adsorbed sites, the interlayer of clay minerals was the most difficult for the RE desorption.

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Sun et al. (2017) thought that the leaching process of WCED-REO by the low concentration of (NH4 )2 SO4 belonged to two-stage inter-diffusion process. This reflected that the adsorption

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sites of REEs on clay minerals are different, one is adsorbed on the surface, the other is

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adsorbed on the interlamination. In the first stage, the leaching efficiency of RE was fast and

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the ion exchange reaction main occurred on the surface of the clay minerals. In the second stage, the leaching efficiency of RE was slow and the ion exchange reaction main occurred among the interlamination of the clay minerals.

Moldoveanu and Papangelakis (2012) believed that under the condition of low ion strength and slightly acidic/neutral, it was easy for the cation exchange to be occurred in the permanent charge position inside clay particles. The ions complexed on the surface or edge of clay minerals were easily affected by the pH, so the appropriate increase of solution acidity was conducive to the leaching of RE. Alshameri et al. (2019) also found that at pH >5, the

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permanent chemical adsorption could occur on montmorillonite and muscovite white mica. Then the availability of ion-exchangeable between NH 4 + with RE3+ adsorbed on the clay minerals was reduced, when using (NH4 )2 SO4 as the leaching agent. Moreover, when the pH is higher than 5, the adsorption of rare earth ions at amphoteric edge surface sites c ould be reduced (Sinitsyn et al.,2000; Moldoveanu and Papangelakis, 2013). So the increase of pH

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value reduced the leaching efficiency of RE. Wang et al. (2017a) pointed out that the

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relatively high concentration of organic acids could promote the RE desorption and inhibit the

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RE re-adsorption, due to their complexation ability with RE in the solution (Li, 2014). The

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dissociation of the relatively low concentration of organic acids could change the surface

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charge properties of RE minerals with increasing H+ absorbed on REOs (Naidu and Harter, 1998; Chiang et al.,2011), decrease the affinity of RE minerals and REEs, thereby organic

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acids could promote the leaching of RE. Hu et al. (2018a) believed that aluminum ions on the

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surface of kaolin could inhibit the adsorption ability of kaolin with REEs, and the kaolin

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would release aluminum ions to affect the adsorption of REEs at a certain acidity.

4.2. Mass transfer process

The leaching mass transfer process of WCED-REO was once regarded as the chromatographic leaching process (MacDonald et al., 2004). The mass transfer of RE leaching could be highly expressed by the height equivalent to a theoretical plate (HETP). The mass transfer effect increased with the decrease of HETP (Tian et al. 1996). The relation between leaching flowrate and HETP can be described by Van Deemter equation to determine the optimum flowrate (Tian et al. 2010b). This theory can reveal the leaching mechanism of

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REO from a macro perspective, and guide RE leaching in industry. Meanwhile, ore grade and ore size can largely affect the leaching efficiency. In order to select and use the leaching agent more accurately, it is necessary to study the leaching mechanism from the micro perspective.

For WCED-REO, the REEs are mainly adsorbed on clay minerals in the form of hydrated or hydroxyl hydrated ions (Chi et al., 2014b; Tian et al., 2010a). Diffusion electric

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double layer theory can explain the interaction among the cations of clay mineral surface and interlayer, water molecules surrounded the cations and the solution (Gao, 2017). At a more

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microscopic level, the leaching mass transfer process of REO is actually the ion exchange

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process of REEs in the diffusion electric double layer, so the leaching mechanism of

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WCED-REO can be explained by the diffusion electric double layer model.

al

According to Poisson Boltzmann equation, Xiao et al. (2018) deduced the relationship

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between the potential φ(x) and the distance x in the electric double layer (EDL). And the

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potential φ(x) was related to the charge density, temperature, and anti-ion concentration. The higher these values are, the higher RE leaching efficiency will be obtained. The leaching process of WCED-REO was actually a process that a new stable EDL with cations in leaching agent established and the old EDL with REEs collapsed. When the potential φ(x) in the new EDL was smaller than that in the original EDL during the leaching process, the REEs in the diffusion layer will be replaced by the cations from the leaching agents, therefore new EDL clay minerals were formed and REEs were exchanged into the leachate. The smaller potential φ(x) in the new EDL is, the greater RE leaching ability is.

At present, the stern model (Gao, 2017) is widely used in the diffusion EDL theory, the 22

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model is shown in Fig. 4. In the stern model, the EDL is divided into stern layer and diffusion layer. The value of zeta potential is very close to the value of the stern layer potential, which reflects the adsorption capacity of clay mineral surface to metal ions (Sans et al., 2017). The greater the absolute value of zeta potential is, the stronger the adsorption capacity to metal ions will be. And the solute ions in the leaching agent exist in the form of hydration and enter

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into the stern layer. The leaching capacity of RE depends on the valence state and hydration

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ion radius. On this basis, Xu et al. (2019) and Zhang et al. (2018) found that with the same

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concentration of the leaching agent, the higher the cation valence state of the leaching agent

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was, the easier the cations enter the stern layer and be exchanged by REEs, and the stronger

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the leaching capacity of the leaching agent to RE was. Xu et al. (2019) believed that anion in water solution was in the form of a hydrate state as well, and their adsorption capacity was

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also determined by the ion potential or charge density. Compared with chloride ions and

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nitrate ions, hydrated sulfate ions have much higher ionic potential and are more likely to be

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attached onto the positive charged regions on the edge surface of clay mineral particles (Xu et al., 2018), thus making negatively charged clay mineral surface more negative. Due to the enhanced negative charged surface of clay mineral particles in stern layer, the cations from leaching agents are more likely to be adsorbed on the surface so that exchange out the REEs. Therefore, sulfate ions could improve the exchange rate of REEs. However, in view of this conclusion, some scholars hold opposite perspectives, Zhang et al. (2018) thought that since sulfate ions carry two negative charges, the force between sulfate ions and ammonium ions was stronger than that between monovalent anions (eg. Cl- or NO3 -) and ammonium ion. Therefore, compared with the leaching agent of NH4Cl and NH4 NO3 , ammonium ions and 23

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sulfate ions in the leach agent solution are more likely to form "positive and negative ion association", which reduced the chance for ammonium ions to enter the double electrode layer (Raymond et al., 2018; Ermakova et al., 2006). Obviously, the exchange of REEs will also be reduced. The zeta potential of clay minerals in (NH4 )2 SO4 is lower than that in NH 4Cl and NH4NO3, the anti-swelling effect is worse. Therefore, NH 4 Cl instead of (NH4 )2 SO4 was used

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as the leaching agent in actual mine leaching operation.

In terms of the hydration of hydrated ion in the EDL, Moldoveanu and Papangelakis

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(2012) thought when the hydrated monovalent cation (M+) arrived at the position adsorbed

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REEs (Ln3+), cations with low ΔHhyd (e.g. < 400 kJ/mole, such as M+) could dehydrate

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completely, and adsorbed on the clay minerals surfaces to form a strong electrostatic bond. But cations with high ΔHhyd (e.g. >3000 kJ/mole, such as Ln3+) retained at least some of the

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hydration water, and adsorbed on the clay minerals surfaces to form a weaker electrostatic

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cations with low ΔHhyd .

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bond. So, the REEs with high ΔHhyd were easy to be desorbed from the clay minerals by the

4.3. Leaching kinetics

In the leaching process of WCED-REO, cations in the leaching agents will exchange the REEs adsorbed on clay minerals, which is a typical liquid-solid heterogeneous ion exchange process. At present, the most typical leaching reaction model is "shrinking unreactive core model" (Chi and Tian, 2008), and the schematic diagram is shown in Fig. 5. The leaching process of WCED-REO can be divided into five stages: solute ions in the leaching agent diffuse from the solid-liquid interface membrane to the surface of mineral particles (I External 24

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diffusion), solute ions further diffuse in the mineral particles (II Internal diffusion), solute ions exchange with REEs on mineral particles (III Chemical reaction), the exchanged REEs then diffuse from the surface of the mineral particles to the solid-liquid membrane (IV Internal diffusion), the exchanged REEs continue to diffuse into the solution (V External diffusion). Among the five stages, internal diffusion is the key to control RE leaching efficiency of

f

WCED-REO (Tian et al., 2010b and 2010c). However, the REOs in this model are regarded

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as the spherical particles, while the shape of the ore particle in the actual leaching process is

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various, which is also the reason why the model is not rigorous.

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Except the shrinking unreactive core model, the Kerr model (Kerr, 1928) proposed by

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Kerr H W is also a widely used solid-liquid ion exchange model in edaphology, but the model is rarely used in the leaching process of WCED-REO. Hu et al. (2018) thought that using the

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(NH4 )2 SO4 as the leaching agent to leaching RE, the ion exchange process between

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ammonium ions in liquid phase and REEs in solid phase could be described accurately by the

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Kerr model. As shown in Eq. (2), the selection coefficient Kk of this model could reflect the adsorption capacity of clay minerals to NH 4 + and RE3+. The larger Kk was, the easier the RE3+ in solid phase were exchanged by NH4 + in liquid phase. The optimal concentration of (NH4 )2 SO4 could be determined by this model. Long et al. (Long et al., 2019) also used the Kerr model and the convection-dispersion equation to simulate the migration process of REEs during column leaching, and found that the optimal concentration of (NH 4 )2 SO4 leaching agent had a linear relationship with the RE grade.

𝐾𝑘 =

[𝑅𝐸 3+ ]𝑙𝑞[𝑁𝐻4 +]𝑠 [𝑅𝐸 3+]𝑠 [𝑁𝐻4 +]𝑙 25

3

3

(2)

Journal Pre-proof Where [RE3+]lq and [NH4 +]l are the molar concentrations of rare earth ions and ammonium ions in liquid phase, mol/L, respectively. [RE3+]s and [NH4 +]s are the molar concentrations of rare earth ions and ammonium ions in solid phase, mol/g, respectively.

Lattice Boltzmann (LBM) reveals the macroscopic transport regularity from the microscopic perspective using the numerical simulation method (Chen and Doolen,1998;

oo

f

McNamara and Zanetti, 1988). Hence, LBM can be used to simulate the transport process of the solute in porous media, namely the leaching process of WCED-REO. On this basis, Qiu et

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al. (2017) proposed the LBM model with coupled chemical reaction, and the D2Q9 model

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was used to simulate the exchange process of REEs. The model represented the discrete

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velocity of particles in two spatial dimensions and nine directions. The discrete velocity of particles and its weight ratio had some influence on the dynamic equilibrium of solvent and

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solute. The model could not only simulate the concentration distribution of leaching agents in

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each area, but also determine the rate control steps in the leaching process of RE by

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simulating the main parameters of the leaching kinetics of RE.

5. Conclusion and prospect

In the past 50 years, aimed at the WCED-REO, the leaching technology obtained a breakthrough. But the RE leaching efficiency and environmental pollution still need to be improved further. And most researches on the leaching mechanisms are in the macroscopic aspect, which needs to be explored in the microscopic aspect.

Through the summary of leaching technologies, leaching agents and leaching mechanisms of WCED-REO, it can be seen that the main research directions in the future are 26

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as the followings:

(1) Optimize the leaching technology of WCED-REO: Enhance the prospecting of geological environment information of ore body. Geological environment information such as fault position, plane extension direction, and section extension are obtained by means of remote sensing, GIS, and other digital measures, so as to take correct and effective

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measurements to reduce the occurrence of geological disasters and environmental pollution caused by leaching. It is necessary to further optimize the in-situ leaching technology,

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including injection technologies and collection technologies. Eventually realize the

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greenization, high efficiency, and intellectualization prospecting of WCED-REO.

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(2) Develop the leaching agents for WCED-REO: Additives such as impurity inhibitor,

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permeability promoter, and swelling inhibitor should be developed to make up for the

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deficiencies of single leaching agents. The green and non-pollution novel non-ammonium

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leaching agents with better RE leaching performance in ionic or colloidal sedimentary phase need more attention and investigation.

(3) Intensify the leaching mechanism of WCED-REO: The ion exchange mechanism between REEs and solute ions may be further revealed by referring to the solid-liquid ion exchange model commonly used in edaphology or digital simulation. The effects of surface charge properties of clay minerals on REEs and impurity ions should be investigated from the perspective of microscopic atoms. The adsorption site and water content of rare earth ions on clay minerals, and the effects of surface water and interlayer water in clay minerals on the adsorption and desorption of REEs should be studied with more characterizations. From the 27

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aspects of quantum chemistry, the exchange forms and modes of REEs/impurity ions need further research as well as the solvent cations and the effect of solvent anion on RE leaching.

Acknowledgments

The work is financially supported by grants from the National Natural Science Foundation of China (No. 51704339, No. 51734001 and No. 41472071) and "the

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Fundamental Research Founds for the Central Universities", South-Central University for

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Nationalities (No. CZY19033, No. CZP19002, No. YZZ16002 ,and No. CZQ19016).

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Author Statement The following is the individual contributions.

Author

Individual contribution Conceptualization

Wenrui Nie

Writing - Original Draft

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Writing - Review & Editing Investigation

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Rong Zhang

Writing - Review & Editing

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Zhengyan He

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Jie Zhou

Writing - Review & Editing

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Ming Wu

Investigation

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Zhigao Xu

Supervision Funding acquisition

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Supervision

Ruan Chi

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Project administration

Huifang Yang

Investigation

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

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We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Figures Fig. 1. The number of published papers with topics on "Weathered crust elution-deposited rare earth & Ion adsorption rare earth" and "Weathered crust elution-deposited rare earth leaching & Ion adsorption rare earth leaching", respectively (literature search in the database of Web of Science).

Fig. 2. Flowsheet of heap leaching technology of the weathered crust elution-deposited rare

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earth ore.

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Fig. 3. Flowsheet of in-situ leaching technology of the weathered crust elution-deposited rare

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earth ore.

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Fig. 4. A electric double layer schematic of clay minerals (Xiao et al., 2018).

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Fig. 5. Schematic diagram of leaching process.

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(a is the non-leached mineral particle; b is remainder of solid; c is diffusion region of solution)

43

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Fig. 1. The number of published papers with topics on "Weathered crust elution-deposited

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rare earth & Ion adsorption rare earth" and "Weathered crust elution-deposited rare earth

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of Web of Science).

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leaching & Ion adsorption rare earth leaching", respectively (literature search in the database

44

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Fig. 2. Flowsheet of heap leaching technology of the weathered crust elution-deposited rare earth ore.

45

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Fig. 3. Flowsheet of in-situ leaching technology of the weathered crust elution-deposited rare earth ore.

46

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al

Pr

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Fig. 4. A electric double layer schematic of clay minerals (Xiao et al., 2018).

Fig. 5. Schematic diagram of leaching process . (a is the non-leached mineral particle; b is remainder of solid; c is diffusion region of solution)

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Tables Table 1 Advantages and disadvantages of above-mentioned leaching technologies. Table 2 The main novel leaching agent with ammonium salts.

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Table 3 Advantages and disadvantages of various leaching agents.

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Table 1 Advantages and disadvantages of above-mentioned leaching technologies. Heap leaching

In-situ leaching

Other leaching

technology

technology

technologies

Popularized

Experimental

leaching

leaching

technology

technology

Industry technology

Less widely used

policy

leaching technology

permeability,

false floor or flat

fissure, and

land, non-fissure

non-false floor or

and high

the flat land.

permeability.

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Orebody needs

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Mine condition

Orebody with weak

vegetation

Suffer great damage

RE leaching efficiency

About 90%

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Comparison item

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field.

No damage 90%-95%

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No leaching in the

89.52%-99.62%

Journal Pre-proof Table 2 The main novel leaching agent with ammonium salts. Ammonium salts

Additives

Literature

Effects

2.0% of

sources

The leaching efficiency of RE

(NH4 )2 SO4 and

0.05%

exceeded 80%.

Yang et al.

NH4 NO3 with

CH3 COONH4

The removal efficiency of Al in

(2015)

molar ratio of 4:1

the leachate exceeded 80%. The leaching efficiency of RE increased by 8.38%.

f

0.1% C14 H12 O8

The concentration of (NH4 )2 SO4

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4% (NH4 )2 SO4

Luo et al. (2015)

was reduced by 25%.

1% of impurity

exceeded 92%.

Peng et al.

inhibitor 2#

The concentration of Al in the

(2016)

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3% (NH4 )2 SO4

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The leaching efficiency of RE

leachate was less than 1 mg /L.

2% QZX-02

increased to 97.58%.

Yan et al.

The concentration of Al in the

(2018)

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2% (NH4 )2 SO4

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The leaching efficiency of RE

0.35% C4 H6 O6

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3% (NH4 )2 SO4

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leachate was 0.016 g/L. The leaching efficiency of RE exceeded 98%. The removal efficiency of Al and Fe in the leachate exceeded

Fang et al. (2018)

90%. The leaching efficiency of RE

0.1 mol/L

0.032 mol/L

was 92.97%.

Feng et al.

(NH4 )2 SO4

HCOONH4

The leaching efficiency of Al

(2018)

was 37.79%.

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Journal Pre-proof Table 3 Advantages and disadvantages of various leaching agents.

Leaching Main leaching agents

Additives

efficiency of RE

Linear

Removal

expansion

efficiency of

rate of clay

impurity

minerals

Al3+/Fe3+

Impurity content in leachate

10g/L of NH4 Cl,

(NH4 )2 SO4 with a

no

++++

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+++

2.5% (NH4 )2 SO4

0.03% CSG

++++

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10g/L of NH4 Cl and no

A

p

A

++++

p

A

p

+

p

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ratio of 8:2

g

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0.03% SG

0.1% LG-01 0.018mol/L

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0.3mol/L (NH4 )2 SO4 2.0% of (NH4 )2 SO4 and NH4 NO3 with molar ratio of 4:1 4% (NH4 )2 SO4

0.20 mol/L MgSO4

YZJ-01 0.05%

CH3 COONH4 0.1% C14 H12 O8 no

g

++++

+++

++++

+++

+

+++

A

p

A

p

A

p

A

++++

+++

p

U

++

n

1% of 3% (NH4 )2 SO4

i

p

e-

3% (NH4 )2 SO4

4% (NH4 )2 SO4

in leachate

p

++

mass ratio of 4:5:6

NH4 NO3 with a mass

E

A

f

NH4 NO3 and

Ce partition

impurity

+++

++

inhibitor 2# 0.20 mol/L of MgSO4 , no

+++

+ 51

A

p

L

Journal Pre-proof NH4 Cl and CaCl2 with

a

molar ratio of

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15:25:60

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ascorbic acid

0.35%

3% (NH4 )2 SO4

0.032 mol/L

++++

++

+++

HCOONH4 1 g/L ascorbic

++

N

n

U

n

A

p

A

+++

p

A

+

p

+

Pr

acid

no

++

++++

++

U

n

N

n

t

g

rn

0.128mol/L Al2 (SO4 )3

+

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0.15 mol/L MgSO4

++

++++

C4 H6 O6

0.1 mol/L (NH4 )2 SO4

+

f

2% QZX-02

-

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2%(NH4 )2 SO4

0.5 g/L

++++

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0.20 mol/L MgSO4

no

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0.20 mol/L FeSO4

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Note: the degree of leaching efficiency of rare earth can be expressed as + + + + (>95%), + + + (90-95%), + + (85-90%), and + (<85%). The degree of linear expansion rate of clay minerals can be expressed as + + (<2.5%) and + (2.5-3%). The degree of removal efficiency of impurity can be expressed as + + + (>90%), + + (80-90%), and + (<80%). The degree of impurity content in leachate can be expressed as + + (<20mg/L), + (20-40mg/L) ,and - (>40mg/L) . The degree of Ce partition in leachate can be expressed as + + (>5.5%) and + (<5.5%).

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Highlights 

Discussion of the development of the leaching technologies.



Description of the current new and practical leaching technologies.



Compilation of leaching agents in literature.



Overview of advantages and disadvantages of various leaching agents.



Explanation of the leaching mechanism in terms of leaching basic theory of rare earth,

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mass transfer process, and leaching kinetics.

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