Preparation Of Nano-Mg（OH）₂ And High Activity Nano-MgO From Microcrystalline Magnesite And Its Adsorption Properties For Cr（Ⅵ）

Magnesite（Mg CO₃）is the dominant mineral in China,and its reserves account for more than one fourth of the total magnesite reserves in the world.It is characterized by rich mineral resources,low impurity content and simple post-treatment.It is a high-quality mineral source for the preparation of magnesium containing compounds.Magnesite can be divided into crystalline magnesite and microcrystalline magnesite according to the different crystal state of the ore.At present,the magnesite resources in China are mainly crystalline magnesite,but the output of microcrystalline magnesite is not much.Therefore,the previous research on magnesite mostly uses crystalline magnesite as raw material,and the research on microcrystalline magnesite as raw material is not much.How to make good use of China’s Magnesite resources,improve its economic benefits,and prepare high-quality magnesium compounds,is the main research direction of magnesite resources.Nano magnesium hydroxide and nano magnesium oxide have attracted more and more attention due to their excellent properties and have been widely used in various fields.Therefore,in this study,microcrystalline magnesite was used as raw material to prepare nano magnesium hydroxide and nano magnesium oxide with high activity,which were applied to the purification of Cr（Ⅵ）containing wastewater.This study is of great significance for the comprehensive utilization of microcrystalline magnesite.In this paper,microcrystalline magnesite was used as raw material and light burned（calcined at 1000℃）to obtain light burned magnesia with good activity（CAA value of 24.12 s）.Nano magnesium hydroxide was prepared by hydration and hydrothermal process.The effects of hydration temperature,hydration time,solid-liquid ratio,dispersant（PVP、PEG6000）and other conditions on the morphology of magnesium hydroxide were investigated,and the morphology differences of magnesium hydroxide under different preparation routes were explored.The results show that:hydration temperature 80℃,hydration time 90 min,hydration solid-liquid ratio 1:10,as the best hydration conditions,the hydration rate can reach 92.75%,can prepare"card room"shape nano flake magnesium hydroxide.In the hydrothermal process,the amount of light burned magnesia is 20%～30%,and 4%PVP is used as dispersant.Under this condition,the dispersibility of the product can be improved and the morphology tends to be regular,and the slightly thick flake nano magnesium hydroxide can be prepared.Five routes to prepare nano magnesium hydroxide were proposed.Among them,the light burned magnesium hydroxide prepared by hydration calcination hydrothermal method is hexagonal thick sheet with the most regular morphology and the best dispersion.Nano magnesium oxide was prepared by calcining the"card room"shape nano flake magnesium hydroxide obtained by hydration.The effects of calcination temperature and holding time on the activity of nano magnesia were investigated,and the optimal calcination conditions were determined.In addition,the effect of precursor magnesium hydroxide with different morphology on the morphology and activity of nano-sized magnesium oxide was investigated.The results show that the calcination temperature is 400℃and the holding time is 0.5 h.as the best calcination conditions,the nano Mg O with high specific surface area(205.5164 m²·g^-1)and high activity（CAA<10 s,hydration rate>95%）can be prepared.It is confirmed that the morphology and activity of nano magnesium oxide are closely related to the morphology and activity of precursor nano magnesium hydroxide.Due to the morphology memory effect,the morphology of nano magnesium oxide inherits the main morphology characteristics of precursor magnesium hydroxide.The results show that the morphology and activity of nano magnesia prepared by light calcined magnesia hydration calcination hydrothermal calcination route are the most regular,and the activity of nano magnesia prepared by light calcined magnesia hydration calcination route is the highest.Finally,the high activity nano magnesium oxide was used as adsorbent to explore its adsorption performance for Cr（Ⅵ）.The optimal adsorption conditions were determined as Mg O dosage of 2.000 g·L^-1,adsorption time of 20 min,solution p H of 4and adsorption temperature of room temperature.The removal rate of Cr（Ⅵ）in water by Mg O could reach 98.12%.The regeneration and recycling properties of the prepared nano Mg O adsorbent were investigated.After five"adsorption desorption"tests,the removal rate of Cr（Ⅵ）in water can still reach more than 90%.The adsorption of Cr（Ⅵ）on Mg O was studied by kinetic and isotherm models.It is more consistent with pseudo second-order kinetic model（R²=0.9992）and Langmuir isotherm model（R²=0.9987）respectively,indicating that the adsorption is mainly chemical adsorption.Cr（Ⅵ）is adsorbed on Mg O surface in the form of monolayer,and the maximum adsorption capacity is 460.8 mg·g^-1.In addition,the adsorption performance of high activity nano magnesium oxide for other metal ions was tested.The results show that it not only has good adsorption performance for Cr（Ⅵ）,but also has good adsorption capacity for other metal ions.It can be seen from the above results that the adsorbent has the characteristics of high efficiency,renewable and strong functionality,and it is a kind of adsorbent with practical application value.