在超重力系統中製備氫氧化鎂與氧化鎂奈米粉體

Abstract

超重力技術已發展多年，其包含了高重力旋轉填充床與旋轉盤反應器，近年來已經成功應用於蒸餾、吸附等程序。於結晶應用上，其有助於得到微奈米級的超細粉體，對於現今之奈米技術有極大的提升。 氫氧化鎂為耐火材料中常使用的粉體，其用途廣泛，包含醫藥與化妝品工業；其煅燒後的產物為氧化鎂，是最為常見的陶瓷材料之一，常用於防火磚、坩堝等。製備奈米級氫氧化鎂的方法日新月異，許多方法如水熱法、溶膠凝膠法都可使粒子大小達到奈米尺寸，然而此類方法多半高成本、高耗能，發展於工業上有其困難之處；而氧化鎂多半因缺乏粒徑小之前驅物，往往於煅燒後粒子無法達到奈米級的要求。 本研究以超重力系統製備奈米粒子，係利用超重力系統合成奈米級氫氧化鎂，並以之作為氧化鎂的前驅物。實驗方法為在超重力系統中，以氯化鎂與氫氧化鈉進行液-液相反應，於連續式操作下合成氫氧化鎂，並將之煅燒成為氧化鎂，探討系統中各變數包含轉速、濃度、流量、填料等對氫氧化鎂粒子粒徑的影響，以及煅燒溫度對氧化鎂之影響，目標為使兩種粉體粒徑縮小至最小尺寸。 本研究結果發現，在轉速達2000rpm、氯化鎂濃度高達0.83M、氫氧化鈉濃度為1.66M時，於旋轉填充床與旋轉盤反應器中皆可得到長約50-100nm、厚度小於10nm的片狀氫氧化鎂粉體。於旋轉盤反應器中，將兩反應溶液之流量由0.75L/min降至0.28L/min時，以動態光散射儀測得之數目平均粒徑會由143.6nm降至47.5nm，後者之BET比表面積高達77m2/g。使用不鏽鋼網填料之旋轉填充床，發現流量由0.32L/min上升至0.75L/min時，所得之數目平均粒徑會由61.9nm下降至51.8nm；使用馬鞍狀填料時，粒徑與流量無太大關係。 將平均粒徑為47.5nm之片狀氫氧化鎂粉體煅燒至600℃後，可得到立方晶相之氧化鎂，以SEM照片觀察為約50nm之多面體，動態光散射儀測得的平均粒徑約150nm，BET比表面積為32m2/g。 本實驗可得到大小約50nm的氫氧化鎂奈米粒子，而且為片狀結晶，這些粒徑小且具阻氣難燃特性的粒子應用於抗火材料上有很大的優勢；氧化鎂粒子約150nm，作為觸媒或催化劑能有優良的效果。本研究展現了超重力技術適於製備奈米材料的特性，又因為此結晶技術低耗能且易於放大，於工業發展上應有廣大的潛力。

The high-gravity system has been developed for many years. It consists of a high-gravity rotating packed-bed reactor and a spinning disk reactor. In recent years, it has been successfully applied in distillation and adsorption process. In the field of crystal engineering, it can be used to produce nanoparticles. Magnesium hydroxide is usually used in flame retardant materials, and it can be also used in medicines and cosmetics. It can be calcined to produce magnesium oxide, which is one of the most useful ceramic materials and is usually used in fire-resisting bricks or crucibles. Many methods for synthesis magnesium hydroxide nanoparticles, including hydrothermal method and sol-gel method, have been developed, but they were always high-cost and hard to scale up. Size of magnesium oxide cannot be developed to nano-order because of no suitable nano-sized precursors. The purpose of this research was to synthesize nanoparticles in a high-gravity system. Nanoparticles of magnesium hydroxide were synthesized in a high-gravity system, and used as precursors for magnesium oxide. First, magnesium hydroxide was prepared in the high-gravity system by a continuous, MgCl2-NaOH liquid-liquid phase reaction. Then the product was calcined to produce magnesium oxide. In this research, the effects of operating variables, including rotating speed, reactant concentration, liquid flow rate, and packing material on particle size and shape of magnesium hydroxide nanoparticles was studied. Besides, the effect of calcination temperature on particle size of magnesium oxide was also investigated. The aim of this project was to explore operation conditions for producing Mg(OH)2 and MgO particles as small as possible. In this study, lamellar-like magnesium hydroxide nanoparticles, which is 50-100nm in length, less than 10nm in thickness, can be obtained either in a spinning disk reactor or in a rotating packed-bed reactor, with rotating speed being 2000rpm, MgCl2 concentration being 0.83M, and NaOH concentration being 1.66M. Using a spinning disk reactor, the number mean size of Mg(OH)2 particles measured by a dynamic light scattering analyzer decreases from 143.6nm to 47.5nm when liquid flow rates of MgCl2 and NaOH solutions both decrease from 0.75L/min to 0.28L/min. The BET surface area of 47.5nm Mg(OH)2 particles is 77m2/g. On the other hand, using a rotating packed-bed reactor filled with stainless-steel meshes, Mg(OH)2 particle size decreases from 61.9nm to 51.8nm when liquid flow rates of reactants solutions both increase from 0.32L/min to 0.75L/min. When a rotating packed-bed reactor is filled with ceramic Intalox saddles, liquid flow rate shows no significant effect on the particle size of Mg(OH)2. Polyhedral nanoparticles of magnesium oxide, which is about 50nm observed by a scanning electron microscope, can be obtained by calcination of 47.5nm Mg(OH)2 powder in lamellar shape, using a programmed heating up to 600oC. The number mean size of these MgO particles is about 150nm and the BET surface area of that is 32m2/g. The Mg(OH)2 nanoparticles of lamellar-like and 50nm in equivalent diameter obtained in this research would present great advantages when applied to flame-retardant materials. MgO particles produced in this research are about 150nm for number mean size, which can be used as fine catalyst. This research shows that the high-gravity system is a powerful tool to synthesize nanoparticles. This technology has a great potential in commercialization because of its low energy consumption and its simplicity in scale-up.