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During the leaching process of ionic rare earth ore (ICREO), ion-exchangeable phase calcium (IEP-Ca) and ion-exchangeable phase aluminum (IEP-Al) are leached along with rare earth, which causes many problems in the enrichment process, such as increasing the precipitant agent consumption and rare earth loss, etc. The agitation leaching kinetics and the column leaching mass transfer process of IEP-Ca and IEP-Al were studied to understand the leaching behavior of impurity in ICREO, which provides guides for the adjustment of the leaching process and to limit the co-leaching of impurities. IEP-Ca and IEP-Al were leached by ion exchange, with the leaching agent cations and the leaching kinetics described by an internal diffusion-controlled shrinking core model with an apparent activation energy of 8.97 kJ/mol and 10.48 kJ/mol, respectively. In addition, a significant reduction in the leaching efficiency of aluminum was caused by the hydrolysis reaction reinforced by the increase in MgSO4 concentration and temperature. The leaching kinetic data of IEP-Ca and IEP-Al was verified by the column leaching mass transfer process. There was a synchronous increase in the peak concentration of the outflow curve and leaching efficiency of calcium with the concentration of MgSO4 since IEP-Ca was easily leached. Therefore, as the leaching efficiency of calcium was already very high in the 0.20 mol/L MgSO4 leaching process, the leaching rate of calcium was limited by the leaching temperature and injection rate of MgSO4. For aluminum, the hydrolysis of Al3+ was promoted by increasing the MgSO4 concentration and the leaching temperature, thereby effectively reducing the content of aluminum in the leachate.

The magnesium sulfate leaching technology for the ion-absorbed rare earths ore can solve the ammonia pollution problem existing in ammonium sulfate leaching process. However, the leaching capacity of magnesium sulfate is slightly weaker than that of ammonium sulfate, resulting in a bigger consumption of magnesium sulfate. In this paper, the MgSO4-ascorbic acid compound leaching agent had been demonstrated to deal with the ion-absorbed rare earths ore. The ascorbic acid could form a stable coordination with rare earth ions, so that it can strengthen the leaching of ion-exchangeable phase. Moreover, ascorbic acid has a strong reductive property, it can leach the colloidal sediment phase rare earth as well. The present study investigates the effect of the initial pH and the composition of leaching agent on the rare earth leaching. It is determined that the rare earth leaching efficiency is 107.5% under the condition of pH 2.00, 0.15 mol/L magnesium sulfate and 1.0 g/L ascorbic acid in leaching agent. In this case, the content of the ion-exchangeable phase and colloidal sediment phase rare earth in the leaching residues are both only 0.02‰. The leaching efficiency of colloid sediment phase rare earth can be 85.7%, so that the Ce partition in the leaching liquor increases to be 5.77%. The magnesium-ascorbic acid compound leaching agent is proposed to be a promising choice to deal with the ion-absorbed rare earths ore, which can realize the efficient leaching, low consumption of MgSO4 and environmentally friendly leaching.

To examine the activation of organic acids on the leaching process of ion-adsorption type rare earth ore (IRE-ore), the leaching behavior of rare earth (RE) and zeta potential of IRE-ore were investigated in the absence and presence of carboxylic acids. The results show that all the tested organic acids (acetic acid, malonic acid, citric acid, tartaric acid, succinic acid, and malic acid) can promote RE extraction. At relatively high concentrations of organic acids, the activation efficiency of organic acids on RE extraction is generally consistent with their complexation ability; whereas at their low concentrations, the change of zeta potential on the IRE-ore surface with organic acid concentration and pH has a close association with RE extraction, which indicates that organic acids can impact the surface electrical property of IRE-ore via their adsorption/desorption, and thereby increase/decrease the affinity of RE ions to IRE-ore. Therefore the influence of organic acids on the IRE-ore surface electrical property also plays an important role in RE extraction in addition to their complexation with RE ions.

The practice of in-situ leaching of the ion-adsorption type rare earths ore with ammonium sulfate could only leach most of rare earth in ion-exchangeable phase, but not the colloidal sediment phase. Therefore, the reduction leaching of rare earth from the ion-adsorption type rare earths ore with ferrous sulfate was innovatively put forward. The soak leaching process and the column leaching process were investigated in the present study. It was determined that ion-exchangeable phase could be released, and part of colloidal sediment phase rare earth could be reduction leached by the cations with reduction properties. The mechanism of reduction leaching was discussed with the Eh-pH diagram of cerium. Moreover, the stronger reduction of reductive ions, the greater acidity of leaching agent solution, and the higher reductive ion concentration, could result in the higher rare earth efficiency and the bigger cerium partition in the leaching liquor. In the ferrous sulfate column leaching process, the rare earth leaching rate and the rare earth efficiency were a little higher than with (NH4)2SO4 agent, and the rare earth efficiency and the partitioning of cerium in leaching liquor could be about 102% and 5.31%, respectively. However, the ferrous sulfate leaching process revealed some problems, so compound leaching with magnesium sulfate and a small amount of ferrous sulfate was proposed to an excellent alternative leaching agent for further studies, which may realize efficiency extraction and be environment-friendly.

The adsorption behaviors of rare earth elements on clay minerals would have great influence on the mineralization process and the leaching process of the ion-adsorption type rare earths ore. In this work, the adsorption thermodynamics of REEs on kaolin were investigated thoroughly and systematically. The experimental results showed that the adsorption characteristics of La, Nd, Y on kaolin did fit well with the Langmuir isotherm model and their saturated adsorption capacities were 1.731, 1.587 and 0.971 mg/g, respectively. The free energy change (ΔG) values were −16.91 kJ/mol (La), −16.05 kJ/mol (Nd) and −15.58 kJ/mol (Y), respectively. The negative values of ΔG demonstrated that the adsorption of rare earth on kaolin was a spontaneously physisorption process. The deposit characteristic of the volcanic ion-adsorption type rare earths ore and the behavior of the rare earth in the column leaching process were also developed here. With the increase of the ore body depth, the distribution of the LREEs decreased and the HREEs increased. And the slight differences in the adsorption ability of REEs on clay minerals led to the fractionation effect in the column leaching process. These developed more evidences and better understanding of metallogenic regularity, and provided a theoretical basis and scientific approach to separation of the HREEs and LREEs in the leaching process.

Magnesium sulfate was proposed to be leaching agent to deal with the ion-adsorption type rare earths ore to reduce or even eliminate ammonia–nitrogen emissions. The effects of temperature, particle size and stirring speed on rare earth leaching process and the leaching behaviors of the single rare earth element were investigated in order to reveal the rare earth leaching characteristics. Besides, the comparison of leaching effects between magnesium sulfate and ammonium sulfate was also studied. The results showed that the rare earth leaching process could be well described with inner diffusion control model and the apparent activation energy was 9.48 kJ/mol. The leaching behaviors of the single rare earth element were brought into correspondence with rare earths. Moreover, when the concentration of leaching agent was 0.20 mol/L, the rare earth leaching efficiency could all reach above 95% and the leaching efficiency of aluminum impurities could be restrained by 10% using magnesium sulfate compared with ammonium sulfate.

Based on the excellent properties and potential applications of nano-ceria with special morphology, the synthesis of nano-ceria has attracted more and more attention. In this paper, the carbon dioxide carbonization method is introduced into the preparation of nano rare earth oxides. C e 2 ( C O 3 ) 3 Δ 6 H 2 O nanosheets with a uniform morphology and a thickness of about 180 ​nm were synthesized by carbonization precipitation under the pH ​= ​5.0 and cerium chloride concentration of 0.3 ​mol/L. It is shown that Oriented attachment (OA) and Ostwald ripening (OR) mechanism coexist during the growth of cerium carbonate nanosheets. And highly uniform ceria nanosheets with cubic phase fluorite structure which composed of 50–100 ​nm crystal grains and has a particle size distribution of 0.59 can be obtained by calcining cerium carbonate at 400 ​°C for 2 ​h. The preparation of ceria nanosheets by carbon dioxide carbonization has the advantages of simple operation, mild conditions and low cost, which would provide a effective reference for preparing other kinds of nano rare earth compounds.

Combining the environmental advantage of magnesium salt compound leaching agent, calcium oxide was used as a precipitant to enrich rare earth elements from rare earth leaching liquor in a magnesium salt system. The use of magnesium salt compound leaching agent and calcium oxide precipitant can achieve the effective circulation of Ca and Mg, and finally solve the ammonia-nitrogen pollution problem in the whole extraction process of the weathered crust elution-deposited rare earth ore. The study suggested that alkaline rare earth sulfates and rare earth hydroxides could generate in the precipitation process, which was proved by the Hazen model calculation and the calcium oxide addition experiments. What's more, it was showed that, under the optimum precipitation conditions, the precipitation efficiency of rare earth was 99.72% and the purity of rare earth concentrate is 82.21%, and the SO3 content in the rare earth concentrate was as high as 13.44% because of the formation of alkaline rare earth sulfates during the precipitation process. In order to reduce the SO3 content, the stirring washing of the precipitation enrichment with an sulfate removal agent was proposed in this paper. The anion of the sulfate removal agent could compete with sulfate ions in the stirring washing process. As a result, sodium hydroxide could be the first choice for further detailed study. When the precipitation enrichment was washed with sodium hydroxide, the purity of the rare earth concentrate was 88.03% and the SO3 content was 7.51% respectively. Hence, a feasible operation would be provided for the non-ammonia enrichment of rare earth from leaching liquor in a magnesium salt system. The research in this paper could have great significance for environmentally-friendly extraction of the weathered crust elution-deposited rare earth ore and the improvement of resource utilization.

The differences in the leaching and mass transfer characteristics of ion-adsorption type rare earth ore between rare earth and non-rare earth impurities were investigated. It is determined that the leaching kinetics of rare earths (REs), Mg and Ca could be described by the shrinking-core model. The leaching rate is controlled by inner diffusion and is in the order of Mg > Ca > RE. The apparent activation energy is 8.48, 6.79 and 6.32 kJ·mol−1, respectively. In leaching system of Al, the leaching efficiency decreases with the increase in temperature because of hydrolysis reactions. The leaching efficiency of aluminum increases firstly and then decreases with the passage of time when the temperature is higher than 35 °C. Besides, there is a good consistency in leaching characteristics and mass transfer, and the order of the time-to-peak is Mg < Ca < RE < Al. Therefore, rare earth and non-rare earth may be partly separated by expanding the separation coefficient. The research could provide a theoretic basis for the separation of rare earth and non-rare earth impurities. Furthermore, compound leaching with magnesium salt and calcium salt is innovatively proposed to solve the problem of calcium and magnesium nutrient loss in the soil as well as eliminate ammonia-nitrogen emissions from the source.

Two-dimensional metal oxide nanostructures, such as nanoplates, nanowalls, and nanosheets, have received further attentions in recent years, due to their outstanding properties resulted from the small thickness and quantum size effects. In this work, lanthanum oxide nanosheets with the thickness of 56 nm had been successfully prepared by an originally simple method of carbon dioxide carbonation; the preparation process was green without using surfactants and detergent. The X-ray diffraction, transmission electron microscopy, and scanning electron microscopy were used to characterize the products. Thermogravimetric-differential thermal analysis and FT-IR were introduced to prove that the lanthanum oxide precursor with lamellar morphology was La2(CO3)3·3H2O. The carbon dioxide carbonation method would provide significant benefits containing mild reaction conditions, high efficiency, low cost, and easy to realize large-scale production.

Magnesium bicarbonate, prepared by the carbonation of magnesium hydroxide slurry, was used as a novel saponifier to eliminate the ammonia nitrogen pollution in the rare earth extraction separation process. The effect of impurity ions introduced by system on the carbonation reaction of magnesium hydroxide was studied in the work. The results showed that the presence of Ca2+ could lead to side reactions so as to reduce the conversion rate of magnesium hydroxide, and a small number of rare earth ions would have great influence on the carbonation reaction. What's more, there was no influence on carbonation reaction with the low concentration of Na+ or Mg2+, the conversion rate of magnesium hydroxide could reach above 96%. This paper showed a practical theory which could provide scientific guidance for the preparation of novel saponifier in rare earth extraction separation process.

The purpose of this study was to overcome the disadvantages of ammonia-nitrogen wastewater pollution and high cost of sodium saponification in rare earth separation process. The study focused on the non-saponification extraction technology with magnesia. The influences of the content and particle size of magnesia, reaction time, reaction temperature, and O/A on cerium extraction rate were also discussed. The results showed that the hydrogen ions of extractant were exchanged by rare earth ions when organic extractant and rare earth solution were mixed with magnesia powder, and then the exchanged hydrogen dissolved magnesia to make the acidity of the system stable. The magnesium ions were not participated in the extraction reaction. Non-saponification extraction process of rare earth had been realized. The cerium extraction rate could reach up to 99% in single stage within the optimal reaction conditions.