Weathered Crust Elution-deposited Rare Earth Research Articles

Influence of ammonium sulfate leaching agent on engineering properties of weathered crust elution-deposited rare earth ore

Influence of ammonium sulfate leaching agent on engineering properties of weathered crust elution-deposited rare earth ore

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.

Occurrence and vertical distribution of aluminum and rare earths in weathered crust elution-deposited rare earth ore: Effect of soil solution pH and clay minerals

Weathered crust elution-deposited rare earth ore is crucial source of medium and heavy rare earths, with in-situ leaching being the most common mining method. The high contents of impurity of aluminum in the leach solution are a significant challenge for the subsequent enrichment process of rare earths. A comprehensive understanding of the occurrences and vertical distribution of aluminum and rare earths within typical vertical profiles can provide valuable insights into entire design of the in-situ leaching. This paper improves a five-step sequential extraction method to analyze the occurrence and vertical distribution of rare earths and aluminum in vertical profiles from Chongzuo and Longyan. Experimental results demonstrate that soil solution pH is the main factor affecting the vertical distribution of ion-exchangeable rare earths. Both samples have distinct areas of enrichment for ion-exchangeable rare earths or aluminum. Ion-exchangeable rare earths are primary concentrated in the middle and lower parts of the ore layer (4–13 m in Chongzuo, 14–22 m in Longyan), while the ion-exchangeable aluminum is mainly enriched in the upper part of the ore layer (1–5 m in Chongzuo, and 2–14 m in Longyan). The vertical distribution of inorganic hydroxy aluminum is likely influenced by the micromorphology and particle size of the clay minerals. The inorganic hydroxy aluminum concentration in Chongzuo samples decreases continuously from 415.65 to 120.95 mg/kg with increasing sampling depth, whereas the concentration in Longyan samples (110.55–171.27 mg/kg) is almost independence with sampling depth. These results provide direct guidance for the entire design of the injection well depth and the leaching parameters, thereby inhibiting the leaching of impurity of aluminum and lower the consumption of leaching agent.

Effects of cations on rare earth adsorption and desorption in binding sites of montmorillonite

The exchangeability of rare earth (RE) in weathered crust elution-deposited rare earth ores largely depends on its interaction with clay minerals, which may be significantly influenced by various cations. Therefore, the effects of K<sup>+</sup>, Ca<sup>2+</sup> and Al<sup>3+</sup> on RE<sup>3+</sup> adsorption and desorption in binding sites of montmorillonite (MMT) were investigated. Through the pre-saturation, the interlayer ions of MMT had been replaced by K<sup>+</sup>, Ca<sup>2+</sup> or Al<sup>3+</sup>. RE<sup>3+</sup> can adsorb on the interlayer sites of Ca-MMT and K-MMT, but nearly not Al-MMT. The basal spacing of Ca-MMT is larger than K-MMT, which provides a smaller hinder effect of interlayer collapse for the interlayer diffusion of RE<sup>3+</sup>. The adsorption capacity followed the order: Ca-MMT＞K-MMT＞Al-MMT and La<sup>3+</sup>＞Y<sup>3+</sup>＞Eu<sup>3+</sup>. It can predict that the grade of the exchangeable RE in ores abundant in Ca<sup>2+</sup> is the most, followed by the ore rich in K<sup>+</sup> and Al<sup>3+</sup> the least. Clay minerals tend to adsorb light RE and hard to adsorb middle and heavy RE. The reversibility of RE adsorbed in interlayers, especially in collapsed interlayers, is far worse than that on externals. The desorption rates of RE were in the order of RE-Al-MMT＞RE-K-MMT＞RE-Ca-MMT and Eu<sup>3+</sup>＞Y<sup>3+</sup>＞La<sup>3+</sup>. For the desorption of interlayer RE<sup>3+</sup>, NH<sup>4+</sup> is better than Mg<sup>2+</sup> because the larger change of the basal spacings (Δd) provides more minor activation energy barriers (ΔE) for NH<sup>4+</sup> diffusion within interlayers. It can enrich the metallogeny theory of weathered crust elution-deposited rare earth ores and provide a certain theoretical basis for its efficient exploitation.

Effects of Ammonium Salts on Rare Earth Leaching Process of Weathered Crust Elution-Deposited Rare Earth Ores

In order to reveal the influence of ammonium salts on the rare earth leaching process of weathered crust elution-deposited rare earth ores, ammonium acetate, ammonium chloride, and ammonium sulfate were used as leaching agents. The effects of the leaching agent on the rare earth leaching efficiency and the expansion, dissolution, and transformation behavior of clay minerals in the rare earth leaching process were studied. The results showed that rare earth leaching efficiency followed the order ammonium acetate > ammonium chloride > ammonium sulfate, with values of 90.60%, 85.96%, and 84.12%, respectively. The swelling ratio of clay mineral followed the order ammonium acetate < ammonium chloride < ammonium sulfate; the clay mineral swelling ratio was 2.09% when ammonium acetate was the leaching agent. Thermogravimetric analysis showed that the interlayer water content was the lowest when ammonium acetate was used as the leaching agent. Under the conditions of different leaching agents, the clay mineral contents changed from illite and halloysite to smectite and kaolinite. When ammonium acetate was used as the leaching agent, the relative conversion of illite was 1.49%, and that of smectite was only 0.17%. SEM analysis showed that the clay minerals expanded and dissolved obviously when ammonium chloride and ammonium sulfate were used as the leaching agents. Meanwhile, the clay mineral layered structure was relatively complete when ammonium acetate was used as the leaching agent. Therefore, when ammonium acetate was used as the leaching agent, it had the least effect on the swelling, dissolution, and transformation of clay minerals. This can provide a theoretical basis for the safe production of weathered crust elution-deposited rare earth ore, and for the screening of green and efficient leaching agents.

Elution of the residual water-soluble ammonium from the weathered crust elution-deposited rare earth tailings: Kinetics and mechanism

The water-soluble ammonium (WS-A) in the weathered crust elution-deposited rare earth tailings is adsorbed on the clay mineral surface through weak forces (van der Waals forces and hydrogen bonds), and it is easy to migrate with rainwater, causing serious environmental pollution. Therefore, the study of the elution process of water-soluble ammonium is critical for the remedy of rare earth tailings leached with ammonium. This work aims to investigate the elution process and mechanism of the WS-A from the tailings and determine the optimum elution conditions. Effects of the factors on the removal efficiencies of WS-A were studied under batch and dynamic conditions. Results illustrated that the elution of WS-A is particle size, liquid–solid ratio, pH, ion strength, and temperature dependent, and it was easy to be eluted at weak acid conditions, high liquid–solid ratio, and high temperature from the tailings with small particle size. Moreover, WS-A elution is a fast process controlled by both external and internal diffusion, and a high elution efficiency of 99.98% could be reached under the optimum dynamic conditions. FTIR and XPS showed that nitrogen content was decreased obviously after elution by water, and the concentration gradient diffusion was the main driving force for the elution of WS-A absorbed on illite, halloysite, and kaolinite at different interaction sites. This study had great significance in reducing the environmental contamination caused by residual water-soluble ammonium after the in-situ elution of weathered crust elution-deposited rare earth ore.

Rare Earth Occurrence States of Weathered Crust Elution-Deposited Rare Earth Ores in Southern Yunnan

To reveal the regularity of variation in the rare earth occurrence states of weathered crust elution-deposited rare earth ores, ore samples from different weathering crust layers were obtained by performing the sequential extraction procedure. The order of rare earth contents firmly obeyed the following sequence: the weathered layer > humic layer > partly weathered layer. The occurrence states of rare earth elements were mainly the ion exchange state, carbonate bound state, iron–manganese oxide state, organic binding state and residual state. The proportions of rare earth elements found in the rare earth ion exchange state of the weathered layer, humic layer and partly weathered layer were 78.55%, 73.53% and 53.88%, respectively. The light rare earth elements (LREEs) found in the rare earth ion exchange state were enriched in the upper part of the weathering crust, while the heavy rare earth elements (HREEs) were enriched in the lower part. There were also obvious negative anomalies in the content of cerium in the ion exchange state. The content of rare earth elements found in the carbonate bound state was small, and the rare earth partition pattern was basically consistent with that of the ion exchange state, which had little effect on the differentiation of the rare earth elements. The iron–manganese oxide state was mainly enriched with cerium, and the content of cerium increased with the depth of the weathering crust. The iron–manganese oxide state was the main factor causing the phenomenon of the anomaly in the cerium content. Meanwhile, the iron oxides in the iron–manganese oxide state were mainly hematite and goethite. The organic binding state mainly beneficiated yttrium and cerium by complexation and certain adsorption. The content of elements found in the rare earth residual state was related to the degree of weathering and reflected the release sequence of rare earth elements in the mineralization process. Clarifying the rare earth occurrence states is conducive to better revealing the metallogenic regularity of weathered crust elution-deposited rare earth ores. In addition, the results can provide a valuable reference for expanding the available rare earth resources and the efficient comprehensive utilization of rare earth ore.

A density functional theory (DFT) on the leaching process of weathered crust elution-deposited rare earth ore with lixiviants

The properties of the solution are closely related to their structures. For efficient exploitation of the weathered crust elution-deposited rare earth ores, the properties of RE ions of the solution have been studied in the leaching process of weathered crust elution-deposited rare earth ores. The structures of 10 hydrated RE clusters were optimized at the PBE0/6-311+G(d, p) level by density functional theory (DFT). The electrostatic potential distribution, the interaction, and the binding energy of hydrated RE clusters with different anions were analyzed. The effect of the anion of the lixiviant in the leaching process was studied at the molecular level. The results show that thermodynamics dominates the static leaching of rare earth (RE) in the leaching process. In the leaching of REs from weathered crust elution-deposited rare earth ore, the stability of the hydrated clusters can affect the leaching rate. The anions that form more stable hydrated clusters with RE ions are more conducive to rare earth leaching. The interaction between inorganic anion and water can also reduce the energy of the whole cluster and make the structure more stable. Besides, the binding energy of the clusters indicates that hydrated clusters with heavy REs have lower binding energy than those with light REs. So the heavy REs are more likely to migrate with the solution. This conclusion also fits with the fractionation effect of rare earth partition.

Modified tailings of weathered crust elution-deposited rare earth ores as adsorbents for recovery of rare earth ions from solutions: Kinetics and thermodynamics studies

Weathered crust elution-deposited rare earth ores tailings are mainly composed of clay minerals, which can be used as efficient adsorption materials. In order to explore the possibility of recovering rare earth ions using tailings from mine waste liquid, the weathered crust elution-deposited rare earth ores tailings were modified to adsorb La3+ and Y3+ with respect to initial concentrations of La3+ and Y3+, temperatures and impurity ions, and the adsorption kinetics and thermodynamics were further discussed to analyze the adsorption behavior. The results show a higher adsorption capacity of heavy rare earth Y3+ than that of light rare earth La3+. The presence of impurity ions does not adversely affect adsorption of rare earth ions by the modified tailings. The adsorption performance is well fitted with the Langmuir adsorption isotherm model rather than the Freundlich adsorption isotherm model, and the maximum adsorption capacity is 0.0226 mmol/g and 0.0197 mmol/g for Y3+ and La3+, respectively. Pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich model were applied to fit the adsorption results. The results show that the adsorption process of modified tailings for La3+ and Y3+ is more in line with the pseudo-second-order kinetic model, suggesting the dominated chemical adsorption. The adsorption activation energies of La3+ and Y3+ on modified tailings are 11.60 KJ/mol and 7.60 KJ/mol, respectively. Thermodynamic studies show that the adsorption process of modified tailings for La3+ and Y3+ is a spontaneous endothermic reaction in the experimental temperature range. The recovery of rare earth ions form the modified tailings can be carried out by the leaching solution of weathered crust elution-deposited rare earth ores, 0.2 mol/L (NH4)2SO4 solution. In addition, the modified tailings are reversible and keep functionality to reuse in several cycles after elution process.

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.

Isolation and Nitrogen Removal Efficiency of the Heterotrophic Nitrifying-Aerobic Denitrifying Strain K17 From a Rare Earth Element Leaching Site.

K17, an indigenous and heterotrophic nitrifying-aerobic denitrifying bacterium, was isolated from the soil of a weathered crust elution-deposited rare earth ore leaching site in Longnan County, China. Strain K17 was identified as Pseudomonas mosselii. In this study, the morphological characteristics of strain K17 were observed and the optimal ammonia nitrogen removal conditions for the strain were studied using a single-factor experiment. Key enzyme activities were determined, and we also explored the ammonia nitrogen removal process of strain K17 on simulated leaching liquor of the rare earth element leaching site. Based on the determination of ammonia nitrogen removal and enzyme activity, it was found that strain K17 has both heterotrophic nitrifying and aerobic denitrifying activities. In addition, single-factor experiments revealed that the most appropriate carbon source for strain K17 was sodium citrate with a C/N ratio of 10 and an initial NH4+-N concentration of 100 mg/l. Furthermore, the optimal initial pH and rotation speed were 7 and 165 r/min, respectively. Under optimal conditions, the ammonia nitrogen removal efficiency of strain K17 was greater than 95%. As an indigenous bacterium, strain K17 has great potential for treating residual ammonium leaching solutions from rare earth element leaching sites.

Removal of ammonia nitrogen from residual ammonium leaching solution by heterotrophic nitrification-aerobic denitrification process

ABSTRACT The aftermath of mining weathered crust elution-deposited rare earth ore produces a large amount of residual ammonium leaching solution, which causes ammonia and nitrogen pollution to the mine site. Recently, denitrification by heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria has attracted much attention. However, limited studies exist regarding the denitrification process of HN-AD bacteria. In this study, we combined four strains of HN-AD bacteria, Pseudomonas fulva K3, Pseudomonas mosselii K17, Klebsiella oxytoca A12, and Enterobacter hormaechei A16, obtained from rare earth element leaching sites, to select the best microbial consortium for ammonia nitrogen removal. We designed an ammonia removal process applicable to HN-AD bacteria to directly remove ammonia nitrogen from acidic leaching solutions. The experimental results demonstrated that the most efficient microbial consortium for ammonia nitrogen removal to be K3 + K17 + A16, with a removal efficiency of 89.68% for 8 h. In this process, considering the influencing factors of the ammonia removal process, the larger the influent flow rate and influent ammonia nitrogen concentration, the greater the ammonia nitrogen accumulation and pH decrease in the reactor. In consecutive multi-batch experiments, the ammonia removal process was used to remove ammonia nitrogen, at concentrations of 100–600 mg/L, from the simulated leaching solution at pH 4–7, whereby the effluent ammonia nitrogen concentration was lower than 15 mg/L. The results demonstrate that the ammonia removal process is highly feasible and stable. These findings will provide new ideas for the application of HN-AD bacteria and new methods for the removal of ammonia nitrogen from acidic leaching solutions.

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.

Dynamic elution of residual ammonium leaching agent from weathered crust elution-deposited rare earth tailings by magnesium chloride

The release of residual ammonium (RA) leaching agent from weathered crust elution-deposited rare earth tailings would cause serious environmental pollution, and it was necessary to efficiently remove it from the ore body before the mine closure. In this study, occurrence states of the RA were determined and dynamic elution of RA from rare earth tailings by using magnesium chloride as eluent was investigated. Effects of initial concentration, pH, flow rate, and particle size on the ammonium removal efficiency were investigated, and variations of ammonium occurrence states before and after elution were determined. Lastly, elution mechanism was discussed. Results showed that removal efficiency of RA by magnesium chloride was significantly higher than that by deionized water, and elution efficiency of RA could reach about 95.7% at the optimum laboratory experiment conditions. Energy dispersive spectrometer (EDS) analysis illustrated that the residual ammonium was replaced by Mg2+ during the elution process, and occurrence state experimental results showed that 94.0% of water-soluble and adsorbable ammonium was eluted. The empirical kinetic equation of eluting RA by magnesium chloride was established as 1–2/3α-(1-α)2/3= 0.02*C00.6t. This study provided a valuable method for reducing environmental pollution caused by the release of the residual ammonium from the rare earth tailings.

Dynamic Elution of Residual Ion-Exchangeable Ammonium from Weathered Crust Elution-Deposited Rare Earth Tailings

Dynamic Elution of Residual Ion-Exchangeable Ammonium from Weathered Crust Elution-Deposited Rare Earth Tailings

Inhibition on the swelling of clay minerals in the leaching process of weathered crust elution-deposited rare earth ores

Rare earths are mainly adsorbed on the clay minerals as ion-exchangeable phase which could be extracted by the ion-exchange method in weathered crust elution-deposited rare earth ore. The clay minerals would swell after adsorbing electrolyte solution during the rare earth leaching process, which would easily lead to the occurrence of landslides and other geological disasters. In this study, to alleviate the expansion of the clay minerals, swelling kinetics curves of the ore samples in the three anti-swelling agents: urea, NH4Cl and NH4NO3 were investigated. Synergy effects of these three anti-swelling agents in binary and ternary systems were also studied. Results showed that the lowest swelling ratio was obtained in the urea/ NH4Cl/NH4NO3 system: 3% wt NH4Cl and NH4NO3 with mass ratio: 7:3 and 6% wt urea at the optimum value of pH 5.0. The Swelling ratio of clay minerals increased with the increase of the temperature in the ternary system. Additionally, swelling kinetics equation δ = A•e-B/t of the clay minerals in different anti-swelling agents were also determined by mathematical regression analysis. The SEM, TG, FTIR and XRD analysis of rare earth tailing leached by ternary system verified the anti-swelling function of urea in the leaching process.

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.

Adsorption kinetics and thermodynamics of rare earth on Montmorillonite modified by sulfuric acid

Montmorillonite (Mt), the major clay mineral in the tailings of weathered crust elution-deposited rare earth ores, was modified to improve its adsorption capacity of rare earth ions (RE3+). The effect of initial RE3+ concentration, temperature and solution pH on the adsorption capacity of Mt to La3+ and Y3+ were investigated in this study, and further the adsorption kinetics and adsorption thermodynamics were discussed to analyze the adsorbing behavior of modified Mt to RE3+. The adsorption capacity of RE3+ on modified Mt is related to the initial RE3+concentration and temperature. The pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich models were applied to evaluate the adsorption kinetics. The results show that the adsorption process of RE3+ on modified Mt is more accurately represented by the pseudo-second-order model, and it is controlled by chemisorption rather than diffusion. The Arrhenius activation energy values of Y3+ and La3+ are 14.259 kJ/mol and 22.845 kJ/mol, respectively. The thermodynamics studies indicate that the adsorption process is a spontaneous endothermic process in the measured temperature range.

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.