| Neodymium carbonate is widely used in lighting,catalysis and electronic information industries.It is also the most important synthetic precursor of neodymium oxide and its crystallization process determines the properties of the subsequent calcined neodymium oxide,such as size,morphology and specific surface area.At present,most of the industrial preparation of neodymium carbonate adopts the precipitation method,the precipitation process is simple and extensive,and the obtained rare earth carbonate primary products have uneven particle size and complex morphology.Aiming at the problems of unclear control mechanism of particle size and morphology of neodymium carbonate and low product quality during the production process of neodymium carbonate,this paper analyzes the basic properties of the amorphous neodymium carbonate(ANC)produced at the initial stage of crystallization,and studies the crystallization activity(crystallization activity refers to the degree of difficulty of crystallization,short crystallization time indicates high activity,and vice versa)and influencing factors of neodymium carbonate.We focus on analyzing the mechanism and evolution law of the transformation process from amorphous to crystal.Through kinetic calculation of neodymium carbonate crystallization process,grasp the relationship between mass transfer process,surface reaction and nucleation growth,optimize temperature,feed ratio and stirring and other important operating conditions to initially control the quality of crystals,and use additives and external field enhancement on this basis to futher realize the control of particle size and morphology.We successfully prepare qualified products with ultra-fine and uniform morphology.The main results are as follows:(1)Use X-ray diffraction,Fourier infrared spectroscopy,Raman spectroscopy,thermogravimetry,scanning electron microscopy,high-resolution transmission electron microscopy,inductively coupled plasma atomic emission spectrometry to study the structure,composition,morphology and solubility of the amorphous phase at the initial stage of precipitation dissolution characteristics.We analyzed that it is an amorphous hydrated carbonate with a molecular formula of Nd2(CO3)3·1.87H2O.The microscopic morphology of ANC was compared at different temperatures,indicating that they do not change with the increase of reaction temperature,and they are all spherical particles with a size of 50 nm.The equilibrium method is used to study the dissolution behavior of ANC in different temperature and different solutions.The results show that the solubility of ANC in ammonium chloride solution is greater than that in pure water,and the solubility is 10 times that of neodymium carbonate normal salt.An order of magnitude,increasing the temperature can also increase the amount of amorphous substance dissolved.(2)The stability of the ANC in the dry state and the precipitation mother liquor was evaluated,and the ion potential was used to judge the cry stallization activity of the amorphous carbonate.The temperature can significantly improve the crystallization activity of amorphous neodymium carbonate.When the temperature is increased from 20℃ to 80℃,the crystallization time decreases from 9 h to 1 h.During the crystallization of the amorphous material,the morphology evolution,Nd3+concentration,pH value and crystal diffraction peak intensity changes of the particles were studied,and the analysis showed that the crystallization of neodymium carbonate involves a process of dissolution-recrystallization.(3)The kinetics of the crystallization process of neodymium carbonate was studied and the process parameters were optimized.It is determined that the macroscopic crystallization rate of neodymium carbonate conforms to the zero-order equation,and the crystallization rate increases with the increase of temperature.The primary nucleation activation energy is calculated to be 36.98 kJ·mol-1,indicating that it is controlled by the mass transfer process and the surface reaction.The secondary nucleation rate decreases first and then increases as the temperature rises.The effects of important parameters in the crystallization process such as temperature,feed ratio,feed rate and stirring on crystallization were studied.Temperature significantly affects the morphology of crystals.At lower temperatures(20-40℃),amorphous materials mainly undergo one-dimensional growth and eventually form needle-like crystals.When the temperature rises,it changes from one-dimensional growth to two-dimensional lamella growth.Continue to increase the temperature,the morphology of the crystal tends to develop toward the shape of a three-dimensional polyhedron.When[HC03-/Nd3+]=2,the concentration of residual[Nd3+]in the mother liquor is 230 mg·L-1,and the yield is 96.81%.When[HCO3-/Nd3+]≥3,the content of[Nd3+]in the mother liquor is below 1 mg·L-1,and the yield of rare earths exceeds 99.99%.The feed ratio also affects the morphology of the final crystal.When[HCO3-/Nd3+]=2-3,the products are all needle-like crystals.When[HCO3-/Nd3+]=4-5,a sticky flake is formed.When the feed rate is 10-30 ml·min-1,the obtained crystals are all needle-shaped and the maximum size is almost the same.When the feed rate was increased to 40 ml·min-1,the crystals evolved from needle-like to point-connected spines,and the size also increased from the original 1-2 μm to 20 μm.Stirring can shorten the crystallization time of the rare earth carbonate produced by the positive and reverse precipitation methods,but it reduces the crystallization rate under the concurrent precipitation conditions.(4)Based on the optimization of the previous process conditions,the effects of dispersants and ultrasound on reducing particle size were explored,and the effects of several precipitating agents and surfactants on the morphology of the formed neodymium carbonate crystals were studied.After adding polyethylene glycol,the effect of particle size reduction is most obvious.Isopropanol has little effect on reducing the particle size,but the addition of n-butanol and ethylene glycol increases the particle size.50 W ultrasound can reduce the particle dso by 50.88%,the ultrasonic power can be increased to 100 W,and the dso can be reduced by 55.39%(compared with the dso of particles without ultrasound).Ultrasound can not only alleviate the agglomeration between particles,but also inhibit local supersaturation and affect crystal growth.When NaHCO3 and NH4HCO3 are used for precipitation,the different types of cations(Na’and NH4+)will not bring much change to the crystal morphology.The difference in the morphology of the crystals formed by sodium carbonate and sodium bicarbonate(ammonium bicarbonate)is caused by the difference in pH during the feeding process.In a high pH solution,flaky particles with a larger particle size are obtained.The addition of SDS(sodium dodecyl sulfate)makes the amorphous precipitate no longer nanospheres,and accelerates the formation of finer needle-like particles.After CTAB(cetyl trimethyl ammonium bromide)is added,the initially formed precipitate is agglomerated porous block particles,and after 6 hours,it gradually grows into a network structure.Betaine under the condition of 4 g·L-1 makes the neodymium carbonate crystals become clusters of superimposed flowers. |
