Preparation And Application Of Micro/Nano Hydrated Magnesium Carbonate

Micro/nano hydrated magnesium carbonate is a new kind of high value-added inorganic functional materials. Because of the restrictions of technology and product cost, the industrial preparation for hydrated magnesium carbonate has not achieved. So it is urgent to search for low cost raw materials and technology. This paper aims to study the preparation technology and growth mechanism of nesquehonite, magnesium carbonate pentahydrated and hydromagnesite, by using magnesite as raw material. The modification and application in polypropylene of nesquehonite whisker is also studied. Based on these, theoretical guidance and technical support can be provided for control growth, low-cost preparation and application of hydrated magnesium carbonate.Nesquehonite whiskers were prepared with the method of magnesite. The results showed that smooth rod-like MgCO3·3h2O whiskers were prepared at pyrogenation temperature of 50℃, pyrogenation time of 2.0h, milling speed of 500r/min, Mg(HCO3)2 concentration of 3.20g/L and in natural pH of Mg(HCO3)2 solution with the assistance of SDS of 5.0g/L. The obtained whiskers have an average length of 150μm with an aspect ratio of 50. The types such as inorganic salts, alcohol, organic alcohol, surfactants and amino acids and consumptions of additives had a profound influence on the composition and morphology of products. For example, SDS promoted oriented growth of nesquehonite with high aspect ratios. Flower-like magnesium carbonate pentahydrated was prepared with the assistance of KH2PO4. In the presence of pure alcohol and isopropyl alcohol, pyrogenation products could change from MgCO3·2H2O to 4MgCO3·Mg(OH)2·4H2O, as the conditions changed.The direct and indirect preparation techniques of hydromagnesite were studied systemically. The study showed that porous flower-like hydromagnesite microspheres with an average diameter and length of 15μm and 50μm were synthesized without additives. Rod-like hydromagnesite, the surface of which consisted of leafy structure, was prepared at low temperature (50℃) with the assistance of isopropyl alcohol for the first time. The acquired rods have an average diameter of 1.0μm and average length of 10μm. In addition, MgCO3·3H2O was severed as a precursor to prepare sheet hydromagnesite crystals with an average diameter and length of 10μum and 100μm. The pH of solution and agitation had a significant impact on its morphology.The formation and growth mechanism of micro/nano hydrated magnesium carbonate was analyzed. (1)The growth-units of MgCO3-3H2O were the MgO6 octahedron. The growth of MgCO3·3H2O whiskers was the result of extension of screw dislocation. The supersaturated Mg(HCO3)2 solution, negative value of △GV and the nucleated MgCO3 with a radius r≥ rc was the drive force of crystallization for MgCO3·3H2O. From the calculation results, induction time decreased, but nucleation rate and crystallization rate both increased with increasing concentration. The crystallization rate was influenced litter by stirring action when the supersaturation degree was low. While the supersaturation degree was higher, the stirring effect was more satisfactory. From the calculation, it was shown that Eα1 was less than Eα3 and Eα2 was the minimum. The induction time reduced, while nucleation rate, crystallization rate and reaction rate all increased with the pyrolysis temperature increasing. Furthermore, the phase transition process speeded up with increasing temperature. The calculated results accorded well with the experimental results. Additives lead positives or negatives infections to the nucleation and growth of nesquehonite through different adsorption ways. (2) H2PO4- and HPO42- ions in KH2PO4 could restrain the nucleation of MgCO3·5h2O by changing the solubility of the original solution or producing the adsorption on the surface. As a result, flower-like MgCO3·5H2O was formed. (3) The crystal structure of 4MgCO3·Mg(OH)2·4H2O consisted of two different configuration of the MgO6 octahedron. The two octahedrons were combined in a co-apex way. According to the linear fitting calculation result and comparative analysis, the reaction rate accelerated, and the activation energy of the crystallization at the assistance of isopropyl alcohol was less than that of no additives. The isopropyl alcohol could reduce significantly the barrier of two phases and accelerate the phase transformation.The modification of nesquehonite whiskers was studied by wet surface modification, as well as modification mechanism. Experimental results showed that better surfactant was stearic acid, and under the condition of surfactant dosage of 5wt.%, initial slurry concentration of 5wt.%, modification time of 60 min, modification temperature of 50℃, baking temperature of 70℃ and baking time of 3.0h, the modification effect of nesquehonite whiskers was best, and the activation index and contact angle of modified whiskers were 100% and 130°. (CH3(CH2)16COO)2Mg with good waterproof property was generated by chemical adsorption of stearic acid and Mg2+, Mg(OH)+, MgHCO3+ onto the surface of nesquehonite whiskers. As a result, nesquehonite whiskers possessed good hydrophobicity.The application of modified nesquehonite whiskers in polypropylene was also studied. Results revealed that the tensile strength increased by 10.8%, the tensile elastic modulus increased by 35%, the elongation at break reduced by 54.79%, and impact strength reached the biggest value 76J/m for composites at the mass ratio of 10% to nesquehonite whiskers.