Mechanisms of bastnasite formation: replacement of calcite by rare earth carbonates.

<div> <p>The interaction of rare earth bearing (La, Nd, Dy) aqueous solutions with calcite crystals at was studied at ambient and hydrothermal conditions (25-220 °C) and resulted in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of calcite by rare earth carbonates. Calcite grains were replaced from their periphery inwards, and the newly formed REE-bearing carbonates follow the crystallisation sequence lanthanite [REE<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>·8H<sub>2</sub>O] → kozoite [orthorhombic REECO<sub>3</sub>(OH)] → hydroxylbastnasite [hexagonal REECO<sub>3</sub>(OH)]. The specific rare earth involved in these processes and the temperature have a significant role in the polymorph selection, crystallisation pathways and kinetics of mineral replacement. La- and Nd-bearing kozoite, grows oriented onto the calcite surface, forming an epitaxy, due to their structural similarities. This phase forms elongated crystals on [100], with the {011} and {0-11} as major forms. The epitaxial relationship is (104) [010]<sub>cc </sub>║(001) [100]<sub>koz</sub> and is strongly dependent on the ionic radius of the rare earth in the structure of kozoite. These results have strong implications for the understanding of mineralisation reactions occurring in REE-bearing carbonatite deposits, the most important resources of rare earths in the world.</p> </div>

Bio-inspired mineral fluorescent hydrogels cross-linked by amorphous rare earth carbonates

Rare earth carbonates act as cross-linkers to fabricate mineral plastic hydrogels based on polyacrylic acid and provide fluorescence properties.

New Insights in the Hydrothermal Synthesis of Rare-Earth Carbonates

The rare-earth carbonates represent a class of materials with great research interest owing to their intrinsic properties and because they can be used as template materials for the formation of other rare earth phases, particularly of rare-earth oxides. However, most of the literature is focused on the synthesis and characterization of hydroxycarbonates. Conversely, in the present study we have synthesized both rare-earth carbonates—with the chemical formula RE2(CO3)3·2-3H2O, in which RE represents a generic rare-earth element, and a tengerite-type structure with a peculiar morphology—and rare-earth hydroxycarbonates with the chemical formula RECO3OH, by hydrothermal treatment at low temperature (120 °C), using metal nitrates and ammonium carbonates as raw materials, and without using any additive or template. We found that the nature of the rare-earth used plays a crucial role in relation to the formed phases, as predicted by the contraction law of lanthanides. In particular, the hydrothermal synthesis of rare-earth carbonates with a tengerite-type structure was obtained for the lanthanides from neodymium to erbium. A possible explanation of the different behaviors of lighter and heavier rare-earths is given.

Application of response surface methodology for optimization of parameters for microwave heating of rare earth carbonates

Microwave heating has been applied in the field of drying rare earth carbonates to improve drying efficiency and reduce energy consumption. The effects of power density, material thickness and drying time on the weight reduction (WR) are studied using response surface methodology (RSM). The results show that RSM is feasible to describe the relationship between the independent variables and weight reduction. Based on the analysis of variance (ANOVA), the model is in accordance with the experimental data. The optimum experiment conditions are power density 6 w/g, material thickness 15 mm and drying time 15 min, resulting in an experimental weight reduction of 73%. Comparative experiments show that microwave drying has the advantages of rapid dehydration and energy conservation. Particle analysis shows that the size distribution of rare earth carbonates after microwave drying is more even than those in an oven. Based on these findings, microwave heating technology has an important meaning to energy-saving and improvement of production efficiency for rare earth smelting enterprises and is a green heating process.