Weathered Crust Elution-deposited Rare Earth Ore Research Articles

Role of Calcium Chloride on the Eluting Process of Residual Ammonium from Weathered Crust Elution-Deposited Rare Earth Ore Tailings

A large amount of ammonium salt leaching agent will remain in the leaching site of weathered crust elution-deposited rare earth ore (WREOs). The release of residual ammonium (RA) will seriously affect the water system ecology of the mining area, and it is urgent to control it. In this paper, column eluting was used to simulate the eluting process of RA in rare earth (RE) ore tailings, and the effects of calcium chloride concentration, eluting temperature, liquid-solid ratio, eluent pH and eluent flow rate on the eluting process of RA in rare earth ore tailings were discussed. It was found that calcium chloride could effectively elute the RA from ore tailings. Eluting agent pH almost had no effect on the eluting process of RA in the pH range of 4–6, and a greater impact on it at pH 8. The flow rate could effectively enhance the elution efficiency. The optimum conditions were calcium ion concentration of 0.1 mol/L, liquid-solid ratio of 2:1, pH 4–6, flow rate of 0.6 mL/min and elution at room temperature. At this time, the elution efficiency of RA was 91.85%. The eluting process of RA in ore tailings was controlled by the inner particle diffusing according to the kinetic analysis. The reaction order was 0.368, and the activation energy of the reaction is 12.450 kJ/mol. This will provide a theoretical basis and technical support for the efficient eluting process of residual ammonium in the leaching site of WREOs.

Enhancement of weathered crust elution-deposited rare earth ores leaching process with the addition of PQ-10

To solve the problems of the long development period, low mass transfer efficiency and high impurity content in the in-situ leaching process of weathered crust elution-deposited rare earth ores (WCE-DREO), cationic hydroxyethyl cellulose (PQ-10) was composited with conventional leaching agent ammonium sulfate ((NH4)2SO4) to form a novel composite leaching agent. The effects of PQ-10 concentration, leaching temperature and leaching flow rate of the composite leaching agent on the leaching kinetics and mass transfer processes of rare earth (RE) and aluminum (Al) were investigated. Compared to the single leaching agent (NH4)2SO4, the composite leaching agent (2 wt% (NH4)2SO4+0.02 wt% PQ-10) can reduce the RE leaching equilibrium time from 465 to 130 min and increase the RE leaching efficiency and decrease the Al leaching efficiency. It also facilitates the leaching process of WCE-DREO by increasing the peak concentrations of RE and Al, reducing the theoretical tower plate height (HETP) and improving the leaching mass transfer efficiency. It is indicated that PQ-10 can promote the leaching of WCE-DREO. The leaching process of the composite leaching system conforms to the diffusion kinetic control model. When the PQ-10 concentration is in the range of 0.005 wt%–0.020 wt%, the reaction orders of RE and Al are 0.73 and 0.54, respectively, which shows a positive effect on the leaching velocity; when the PQ-10 concentration is in the range of 0.030 wt%–0.060 wt%, the reaction orders of RE and Al are –1.16 and –0.75, respectively, which show a negative effect on the leaching velocity. In the range of 10–50 °C, the apparent activation energies of RE and Al are 15.02 and 17.31 kJ/mol, respectively, and the higher the leaching temperature, the smaller the HETP and the higher the leaching velocity and mass transfer efficiency. The increase in leaching flow rate contributes to the increase in the longitudinal diffusion velocity of the leaching agent within WCE-DREO, causing a shorter time for RE and Al to reach leaching equilibrium. In addition, the flow rate and HETP are consistent with the Van Deemter equation. At a flow rate of 0.8 mL/min, HETP was minimized and the optimal mass transfer efficiencies is achieved for RE and Al.

Promotion Mechanism of Ammonium Formate in Ammonium Salt Leaching Process for Weathered Crust Elution-Deposited Rare Earth Ores

Weathered crust elution-deposited rare earth ores (WREOs) are significant strategic mineral resources. In industry, in situ leaching technology is usually applied with ammonium chloride and ammonium sulfate as the leaching solution. However, the slow seepage velocity of the leaching solution and low rare earth leaching efficiency still need to be improved. Ammonium formate can effectively improve the WREO leaching process. In order to further explore the effects of ammonium formate on the ammonium salt leaching process for WREOs, ammonium chloride and ammonium sulfate compounded with ammonium formate were used as leaching agents to determine their effects on leaching efficiency, seepage velocity and swelling. The results show that in the presence of ammonium formate, the rare earth leaching efficiencies with ammonium chloride and ammonium sulfate are both slightly increased, the seepage velocity of ammonium chloride and ammonium sulfate is increased by 1.67 × 10−4 cm·s−1 and 1.18 × 10−4 cm·s−1, and the swelling percentage falls by 0.14% and 0.37%, respectively. The thickness of the adsorbed water layer and thermogravimetric and XRD results confirm that ammonium formate can inhibit surface hydration and thus improve the WREO leaching process.

Influence of surfactant on clay swelling inhibition of weathered crust elution-deposited rare earth ores

Clay minerals are the main components of the weathered crust elution-deposited rare earth ores (WREOs), and their swelling properties have a significant effect on the safe and green in-situ leaching of WREOs. Few researches have been reported on the inhibition of clay swelling in WREOs and our teammates used combination of ammonium salts to suppress the clay swelling in previous work. Surfactants combined with (NH4)2SO4 were utilized as inhibitor in this study to reduce the amount of ammonium salts and thus decrease the ammonium pollution. Effects of different surfactants on inhibition of clay swelling were investigated, and the swelling inhibition mechanism was further discussed by DLVO theory. CTAB exhibited best performance among five surfactants. The swelling efficiency of kaolinite, halloysite and illite in DI water is 2.52%, 1.65% and 1.32% respectively, and these values can be reduced to 0.45%, 0.65% and 0.82% respectively in presence of single (NH4)2SO4. As (NH4)2SO4 was compounded with 0.04% CTAB, the swelling efficiency of kaolinite, halloysite and illite was further reduced to 0.25%, 0.45% and 0.31% respectively. The XRD and SEM analysis indicated that no change of the morphology and crystal structure of minerals after soaking with surfactants were presented. It was further clarified by DLVO theory that the London-van der Waals energy between clay particles increased, while both of the repulsion electrostatic energy and the total potential energy decreased in the presence of CTAB, and this was beneficial to the clay swelling inhibition.

Leaching kinetics and permeability of polyethyleneimine added ammonium sulfate on weathered crust elution-deposited rare earth ores

At present, in-situ leaching of weathered crust elution-deposited rare earth ores (WCE-DREOs) encounter with problems such as long leaching cycles, slow infiltration rate and low product purity. In order to solve the above problems, the conventional leaching agent ammonium sulfate ((NH4)2SO4) was compounded with polymeric surfactant polyethyleneimine (PEI) to form a composite leaching agent. The effects of leaching temperature, PEI concentration, flow rate and pH on leaching kinetics and permeability of rare earths (RE) and aluminum (Al) in orebody were studied. It is found that with temperature increasing, the time required to reach leaching equilibrium for both RE and Al is shortened, the apparent activation energies of RE and Al are 14.79 and 13.45 kJ/mol, respectively, and the leaching processes are in accordance with the outer diffusion control. When the concentrations of (NH4)2SO4 and PEI in the composite leaching agent are 2.0 wt% and 0.4 wt%, the time required to reach leaching equilibrium for RE and Al is about 50% shorter than that using (NH4)2SO4 alone, and the leaching efficiencies of RE are slightly higher than that of Al. Properly increasing the temperature and flow rate of the composite leaching agent can improve the leaching efficiencies of RE and Al, but pH has neglected effects on the leaching efficiencies of RE. At PEI concentrations below 0.4 wt%, the addition of PEI promotes the leaching of RE and Al. In column leaching studies of the WCE-DREO, the addition of 0.4 wt% PEI to the traditional leaching agent (NH4)2SO4 has no impact on the leaching efficiencies of RE. However, it can significantly increase the infiltration rate of WCE-DREO, shortening the leaching time per 10 mL effluent from about 30 min for (NH4)2SO4 leaching system to 20 min for the composite leaching system. The leaching time is shortened by one-third, and the leaching cost is reduced, which can provide theoretical guidance for the development and commercial implementation of novel composite leaching agent for WCE-DREO.

Study on the Leaching Kinetics of Weathered Crust Elution-Deposited Rare Earth Ores by Hydroxypropyl Methyl Cellulose

In the process of the in situ leaching of weathered crust elution-deposited rare earth ores (WCE-DREOs), there are many problems in the conventional leaching agent, such as a slow leaching rate, low leaching yield and long leaching period. In order to solve the above problems, 2.0 wt% ammonium sulfate was mixed with hydroxypropyl methyl cellulose (HPMC). The effects of the HPMC concentration, temperature, pH and flow rate on the leaching kinetics of rare earth (RE) and aluminum (Al) were investigated. The results showed that when the concentration of HPMC was 0.05 wt%, the leaching equilibrium time of RE and Al was about 60% shorter than that of single ammonium sulfate. With an increase in the leaching temperature, the leaching equilibrium time of RE and Al decreased, and the apparent activation energy of RE and Al was 23.13 kJ/mol and 17.31 kJ/mol, respectively. The leaching process was in line with the internal diffusion kinetic control model. When the pH of the leaching agent was 4.02~8.01, the leaching yield of RE and Al was basically the same, but the leaching yield of Al was greatly increased at pH 2.0 due to a large amount of adsorbed hydroxy-Al in the RE ore eluded. The leaching yield reached the maximum when the flow rate was 0.7 mL/min. The leaching time and the leaching cost of RE can be saved by the composite leaching agent. The results provide theoretical guidance for the development and industrial application of the new composite leaching agent.

Effect of surfactant addition on leaching process of weathered crust elution-deposited rare earth ores with magnesium sulfate

Surfactants were proposed to be added into magnesium sulfate solution to improve the leaching process of weathered crust elution-deposited rare earth ores (WREOs). Effects of surfactants and their concentration on the seepage of leaching solutions and the leaching efficiency of rare earth (RE) and aluminum (Al) were investigated, and the leaching kinetics, the mass transfer process, the adhesion work and the adhesion work reduction factor were analyzed to reveal its strengthening leaching mechanism. The results show that cetyltrimethylammonium bromide (CTAB) has a better strengthening effect on the leaching process than dodecyl trimethyl ammonium bromide (DTAB), sodium dodecyl sulfate (SDS), sodium oleate and oleic acid. In the presence of 0.04% CTAB in 0.2 mol/L solution, the permeability coefficient of WREOs increases from 0.945×10−5 to 1.640×10−5 cm·s−1, and the leaching efficiency of RE increases from 80% to 90%, confirming the promotion of surfactants on the leaching process of WREOs. Kinetic analysis shows that the leaching process conforms to the inner diffusion control model, and the leaching kinetics equations of RE and Al related to CTAB content are obtained. Mass transfer discussion shows a smaller height equivalent to theoretical plate (HETP) of RE and Al at CTAB content of 0.04%, suggesting the higher mass transfer efficiency here. According to the interfacial properties of leaching solutions, the calculated adhesion work and the adhesion work reduction factor further demonstrate the strengthening leaching effect of CTAB on the leaching process of WREOs.

Mechanisms of swelling inhibition and seepage promotion of weathered crust elution-deposited rare earth ore by hydroxypropyl methyl cellulose

In-situ leaching process of weathered crust elution-deposited rare earth ore (WCE-DREO) often faces problems such as poor permeability and occurrence of landslides. The main reasons are the swelling of clay minerals by water absorption and blockage of seepage channels by fine particles. This study proposed using hydroxypropyl methyl cellulose (HPMC) as a novel green swelling inhibitor with the combination of ammonium sulfate ((NH4)2SO4) as an efficient composite leaching agent. The swelling inhibition performance of HPMC on swelling inhibition was evaluated through linear swelling tests and mud ball immersion tests. The mechanisms of HPMC on swelling inhibition and seepage promotion were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM), Surface area analysis (BET), X-ray photoelectron spectroscopy (XPS), Thermogravimetric analysis (TGA), and the contact angle, zeta potential and particle size distribution were characterized. The results indicated that HPMC showed a good performance on swelling inhibition, which improved with the increase of HPMC concentration. HPMC expels the water from the interlayer. Moreover, HPMC adsorbs onto the surface of clay minerals by hydrogen bonding to form a hydrophobic film, which can prevent the clay minerals from hydration swelling. Due to the electric neutrality property of HPMC, the electrostatic repulsion of clay minerals can be reduced. Therefore, the fine particles of the clay minerals can be bridged to form larger particles by the long carbon chain of HPMC which leads to expansion of the seepage channels as well as the improvement of the permeability of the ores. According to the column leaching tests, the novel leaching agent composed of 0.05 wt% HPMC and 2 wt% (NH4)2SO4 not only showed a good leaching capacity of rare earth, but also inhibited the swelling of clay and enhanced the seepage when comparing with traditional leaching agent ((NH4)2SO4). Developing a novel leaching agent benefits the green, safe and high-efficiency exploitation of WCE-DREO.

Application of surfactant for improving leaching process of weathered crust elution-deposited rare earth ores

Weathered crust elution-deposited rare earth ores (WREOs) in China are the main source of medium and heavy rare earths in the world. In order to improve the seepage and the mass transfer of traditional ammonium salt leaching process of WREOs, surfactants were added in the ammonium sulfate leaching solution and the ammonium chloride leaching solution. The leaching kinetics, the mass transfer process, and the adhesion work reduction factor calculated from the interfacial properties were studied to reveal the strengthening mechanism of surfactant. In the presence of the tested five surfactants, cetyl trimethyl ammonium bromide (CTAB), dodecyl trimethyl ammonium bromide (DTAB), sodium dodecyl sulfate (SDS), sodium oleate and oleic acid, the permeability of WREOs is improved, the rare earth (RE) leaching efficiency increase and the impurity aluminum (Al) leaching efficiency decrease, indicating its promotional effect on the leaching process of WREOs. Furthermore, CTAB shows a better leaching enhancement, and the optimal addition dosage is 0.4 g/L for the two ammonium salt leaching agent systems. The kinetics analysis shows the internal diffusion controls model of RE and Al leaching process, and the leaching kinetics equations of RE and Al related to CTAB concentration were obtained for the two ammonium salt leaching systems. According to the chromatographic plate theory, the mass transfer efficiency of RE increases with the CTAB concentration increasing until 0.4 g/L, which confirms that the optimal CTAB addition is 0.4 g/L. Smaller adhesion work and adhesion work reduction factor indicate more favorable permeation as well as the leaching process.

Anti-swelling mechanism of DMDACC on weathered crust elution-deposited rare earth ore

In order to inhibit the swelling of the clay minerals in the in-situ leaching process of weathered crust elution-deposited rare earth ores (WCE-DREO), diallyl dimethyl ammonium chloride (DMDACC) was introduced as an anti-swelling agent and combined with (NH4)2SO4 as a novel composite leaching agent. It can be found that the novel composite leaching agent exhibits a good anti-swelling performance and leaching capacity of rare earth, and has great potential on the actual exploitation of WCE-DREO. The anti-swelling mechanism of DMDACC was studied by characterization analysis. The results show that DMDACC with positive charges can be adsorbed on the clay particles by the electrostatic attraction and hydrogen bonds, and neutralize the negative charge of the clay particles. The double electrical layers are suppressed and the repulsion force between clay sheets decreases. It causes the clay particles prone to aggregate. Moreover, DMDACC can enter the interlayer and expel out the water molecules in interlayer. The interlayer spacing is decreased and the hydration swelling of the clay particles is weakened. It can provide a theoretical basis for the development of novel anti-swelling agents.

Sorption/desorption of Eu(III) on halloysite and kaolinite

Weathered crust elution-deposited rare earth (WED-RE) ores are important rare earth element (REE) resources around the world. In these ores, the REE are believed to mainly exist as exchangeable REE(III) ions on the surfaces of clay minerals, such as tubular halloysite and platy kaolinite. However, the mechanisms underlying the sorption and desorption of REE(III) ions on clay minerals in the environment of weathered crusts remain ambiguous. In this study, the sorption/desorption characteristics of Eu(III) ions onto halloysite and kaolinite were investigated. The results revealed that Eu(III) was adsorbed onto both clay minerals through inner-sphere complexation and outer-sphere complexation, where the former was the primary sorption mechanism. Halloysite displayed a higher sorption capacity for Eu(III) than kaolinite in a weakly acidic environment (pH 4–6). The desorption efficiency increased with the increasing concentration of NH4+ ions. When the NH4+ concentration was 0.002 mol/L, which is similar to the typical cation concentration of groundwater/rainwater, the amount and rate of Eu(III) desorption from halloysite were lower than those from kaolinite. The desorption of Eu(III) ions from the two clay minerals is proposed to be governed by the attraction between the outer-sphere complexes and clay minerals when the NH4+ concentration is 0.002 mol/L. Therefore, halloysite has a higher sorption capacity and stronger retention capacity for Eu(III) than kaolinite, which suggests that halloysite may play a greater role in REE(III) enrichment in WED-RE ores than kaolinite owing to the similar chemical properties of REE(III) ions. The fundamental results obtained in this study afford new insights into the migration of REE and the role of clay minerals in REE enrichment during ore formation.

Vertical distribution and occurrence state of the residual leaching agent (ammonium sulfate) in the weathered crust elution-deposited rare earth ore

Weathered crust elution-deposited rare earth ore (WCE-DREO) are rich in middle and heavy rare earth, and ammonium sulfate ((NH4)2SO4) was often used as leaching agent to leach rare earths by in-situ leaching method. However, much of (NH4)2SO4 would remained in the ore body during the leaching process, and release of it would cause seriously environmental pollution after the mine closure. To efficiently remove it, the rare earth ore properties and vertical distribution and occurrence state of the residual leaching agent at mine roof (GP1), mine waist (GP2), and mine foot (GP3) with different depth were investigated and efficient elution method was proposed in this study. Results showed that the rare earth ore mainly consist of quartz, clay minerals (halloysite, illite, and kaolinite) and rock-forming minerals, and pH and moisture contents of them were ranged from 4.0 to 5.0 and 10–20%, respectively. Residual agent was mainly enriched in the middle and deep layer of the ore body with the main form of ammonium nitrogen (NH4+-N), and content of it at the three sites followed the order of GP1>GP3>GP2, which was related to the content of the clay minerals and the moisture. Occurrence state experimental results illustrated that about 95% of the NH4+-N existed as water-soluble ammonium (WS-AN) and adsorbable ammonium (AS-AN), and 5% of it existed as fixed ammonium (FX-AN), and concentration ratio of them was in order: WS-AN > AS-AN ≫ FX-AN. Based on the results above, MgCl2 solution was used as an eluent to remove the leaching agent from the ore, and results showed that higher than 90% of residual ammonium could be removed from the ore by it. This study provided a valuable guidance for the residual leaching agent removal from the WCE-DREO body.

Partial rehydration of tubular halloysite (7 Å) immersed in La(NO3)3 solution for 3 years and its implication for understanding REE occurrence in weathered crust elution-deposited rare earth ores

Clay minerals are the main carriers of rare earth elements (REE) in weathered crust elution-deposited rare earth (WED-RE) ores. The clay mineral-water interface reaction is among the most important interactions controlling the aggregation and transportation of REE(III) ions. Yet long-term experiments of interactions between REE(III) ions and clay minerals are scarce in the literature. Here, two typical clay minerals that commonly occur in WED-RE ores, namely tubular halloysite (7 Å) and platy kaolinite, were immersed in a La solution for 3 consecutive years. The specimens' characteristics were systematically determined using X-ray diffraction (XRD), transmission electron microscope (TEM), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS). The partial rehydration of tubular halloysite (7 Å) was observed, with the resulting rehydrated halloysite characterized by lattice fringes having d(001)-values of ~1.000 and 0.830 nm, whose distribution features demonstrated that the partial rehydration happened initially at the outermost layers of the halloysite (7 Å). By contrast, both the morphology and structure of kaolinite remained intact after its 3-year-long immersion in the La solution. For both immersed halloysite and kaolinite, the La existed as inner-sphere complexes, outer-sphere complexes, and hydroxide particles adsorbed on the surface of minerals. However, La(III) ions partially penetrated the interlayer of halloysite but did not enter that of kaolinite. Accordingly, the intercalation of La(III) in halloysite may facilitate the rehydration of halloysite (7 Å). These experimental results provide novel insight into the conditions for rehydrating tubular halloysite (7 Å), and indicate a new possible state of REE(III) in WED-RE ores, both of which help us to better understand the geochemical behavior of REE and clay minerals in WED-RE ores.

Behavior and mechanism of different fraction lead leach with several typical sulfate lixiviants in the weathered crust elution-deposited rare earth ore.

Recently, some new leaching agents without ammonium, such as magnesium sulfate (MgSO4) and aluminum sulfate [Al2(SO4)3], have been developed to eliminate ammonia nitrogen pollution in in situ mining process of the weathered crust elution-deposited rare earth ore (WCED-REO), but they might cause heavy metal contamination. In this study, characteristics and mechanisms of different fractions of lead (Pb) released by (NH4)2SO4, MgSO4 and Al2(SO4)3 leaching agents were investigated using batch experiments and column leaching tests. The experimental results showed that the amounts of Pb released by the different leaching agents followed the trend of Al2(SO4)3 > (NH4)2SO4 > MgSO4 under the same total cationic charge, and both the acid extractable and reducible fractions of Pb were released. The release of acid extractable fraction Pb was related to the cation hydration radius of NH4+, Mg2+, and Al3+, whereas the release of reducible fraction Pb was mainly influenced by the concentration of H+, especially at pH < 4.0. Furthermore, column leaching tests indicated that pH has little effect on the Pb contents of different fractions released by (NH4)2SO4 and MgSO4 in leaching the WCED-REO. Although Al2(SO4)3 released the largest contents of rare earth and Pb in leachate, the content of residual acid extractable fraction Pb in soil was the most after water injection (simulating the cleaning process after mining). This work can provide a scientific method and theoretical basis for comprehensively assessing the environmental impact of new leaching agents on WCED-REO mining.

Adsorption and desorption performances of ammonium on the weathered crust elution-deposited rare earth ore

Release of the remaining ammonium salts from the weathered crust elution-deposited rare earth ore (WCE-DREO) would pose a serious threat to the environment. Thus, in this study, the adsorption and desorption performances of the WCE-DREO and four minerals that composed it were investigated to select an efficient eluent for the remained ammonium removal. Results showed that the ammonium ion adsorbed on the WCE-DREO and the clay minerals as a monolayer, and the calculated adsorption capacities were as follows: montmorillonite > WCE-DREO > halloysite > illite > kaolinite due to their different structure. pH experiment showed that it had great effects on ammonium adsorption on both the WCE-DREO and the investigated clays. Kinetic experiments showed that ammonium sorption and desorption process were both a fast process. Co-ions experiments showed that Mg2+ showed the highest inhibition effect on the ammonium adsorption on the WCE-DREO due to its ionic character. Based on this data, magnesium salt was chosen as the eluent, and results showed that more than 93 % of the remaining ammonium ion could be removed from the WCE-DREO in the batch and column experiments.

Preparation of crystalline rare earth carbonates with large particle size from the lixivium of weathered crust elution-deposited rare earth ores

Crystalline rare-earth (RE) carbonates having large particle size were prepared from the lixivium of weathered crust elution-deposited rare-earth ores using the precipitation method with ammonium bicarbonate as the precipitant. Their chemical composition was studied using elemental and thermogravimetric analyses (TGA), and their structure and morphology were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results demonstrate that the crystalline rare-earth carbonate is a hydrated basic carbonate or oxycarbonate and not astable intermediate carbonate in the process of thermal decomposition. The particle size of crystalline rare-earth carbonates with large particle size is in the range of 50–200 µm. With an RE2O3 content of up to 95wt%, the quality of crystalline rare-earth carbonates is higher compared to the Chinese National Standard (GB/T 28882-2012). The quality of the product is superior to the Chinese National Standard.

Ammonia nitrogen sources and pollution along soil profiles in an in-situ leaching rare earth ore

The ammonium sulphate ((NH4)2SO4) in-situ leaching process is the most widely used extraction technology for weathered crust elution-deposited rare earth ores (WCED-REOs). Highly concentrated (NH4)2SO4, a representative leaching agent, is often used in the leaching process of WCED-REOs. However, this in-situ leaching process causes nitrogen pollution in the soil, surrounding surface and ground water due to the high concentrations of (NH4)2SO4 solutions used as a long term leaching agent. To date, the mechanism behind the variations in ammonia nitrogen (AN) in deep soil profiles is unclear. We conducted vertical and lateral soil sampling and analyzed the collected samples for soil moisture, pH, ammonia forms, and AN contents in soil profiles deeper than 500 cm in an in-situ leaching mining area of Ganzhou, Jiangxi Province, southern China. The results show that primary chemical pollutants in the soil are derived from residual leaching agents with high acidities and concentrations of AN. Twelve years after the mining process was completed, the mean pH values of the tailings in the mining area were 3.90 and 4.87 in its lower reaches. Due to the presence of chemical residues, the AN concentration was 12–40 times higher than that of the raw ore soil before it was mined. The percentages of different ammonium forms in the rare earth tailing soil were 65%, 30%, and 5% for the water-soluble, exchangeable, and fixed ammonium forms, respectively. The results of this study support effective prevention and remediation treatment of environmental problems caused by AN pollution of the soil in WCED-REOs.

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

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 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 of the leaching technology. The leaching mechanism is at the core of the leaching technology and also affects the effective utilization of the leaching agents. The research and development of leaching technologies, leaching agents, and leaching mechanisms of WCED-REO are discussed in this article. The development processes of exiting leaching technologies are introduced. The advantages and disadvantages of barrel leaching, pool leaching, heap leaching, in-situ leaching, and other leaching technologies are compared. And the new and 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 guidance and technical supports for the green and efficient exploitation of WCED-REO.

Leaching hydrodynamics of weathered elution-deposited rare earth ore with ammonium salts solution

In order to reveal the permeability rule of leaching agent solution and the effects of anions in the leaching process of weathered crust elution-deposited rare earth (RE) ores, the effects of ammonium concentration, temperature, particle size and porosity on the permeability were discussed in detail with (NH4)2SO4, NH4Cl and NH4NO3 as the leaching agent. It was found that the permeation velocity of ammonium salts increased linearly with the increase of hydraulic gradient. The seepage of ammonium salts solution in the RE ores followed Darcy's law and displayed a laminar flow. The properties of the leaching agent solution and RE ores were the main factors that affect the permeability of RE ores. With the decrease of ammonium concentration and increase of temperature, the viscosity of solution decreased and the permeability coefficients of RE ores increased. And the effects of temperature on the viscosity and permeability were larger than ammonium concentration. The permeability of RE ores became worse with the decrease of particle size and porosity, and the particle size played a more important role compared with porosity. The permeability coefficient of RE ores increased and the viscosity of ammonium salts solution decreased in the order of (NH4)2SO4, NH4Cl and NH4NO3, implying that the penetrating power of anions increased in the order of SO42–, Cl− and NO3−. The results may play a vital role in improving the permeability of weathered crust elution-deposited RE ores.

Column leaching process of rare earth and aluminum from weathered crust elution-deposited rare earth ore with ammonium salts

In order to better understand the leaching process of rare earth (RE) and aluminum (Al) from the weathered crust elution-deposited RE ore, the mass transfer of RE and Al in column leaching was investigated using the chromatographic plate theory. The results show that a higher initial ammonium concentration in a certain range can enhance the mass transfer process. pH of leaching agent in the range of 2 to 8 almost has no effect on the mass transfer efficiency of RE, but plays a positive role in the mass transfer efficiency of Al under strong acidic condition (pH<4). There is an optimum flow rate that makes the highest mass transfer efficiency. The optimum leaching condition of RE is the leaching agent pH of 4–8, ammonium concentration of 0.4 mol/L and flow rate of 0.5 mL/min. The mass transfer efficiencies of RE and Al both follow the order: (NH4)2SO4<NH4Cl<NH4NO3, implying the complexing ability of anion.