Comprehensive Utilization Of Hydromagnesite

Hydromagnesite?4MgCO₃·Mg?OH?₂·4H₂O?is a rich trona carbonate mineral.At present,the proven content of hydromagnesite resources in China is over 100 million tons,which is a rare large deposit,mainly distributed in the northern Tibet salt lake area and some salt lake areas in Qinghai.Hydromagnesite has pure texture,white color and low impurity content,and is a high-quality mineral raw material for preparing magnesium products such as flame retardant,active magnesium oxide,heavy basic magnesium carbonate,nano magnesium hydroxide and the like.However,due to geographical,transportation,technology and other factors,it has limited its development and utilization,so far there has been little research on it in China.Therefore,how to develop and utilize hydromagnesite to prepare a series of high-quality magnesium compounds is currently the main research work.Nano-magnesium oxide and nano-magnesium hydroxide have unique properties,are important inorganic chemical raw materials,and are widely used in many fields.Therefore,this study is devoted to the development of environmentally friendly,low-cost and suitable industrial production methods of high-quality nano-magnesium oxide and nano-magnesium hydroxide using hydromagnesite as raw materials and explores its application in the treatment of heavy metal wastewater.In this paper,magnesium oxide nanoparticles with different morphologies were prepared from hydromagnesite by a simple process of"calcination-hydration-calcination"without adding any other reagents.The effects of calcination conditions on the morphology,crystallinity,grain size and specific surface area of the samples were investigated experimentally.The results show that the increase of temperature or the extension of calcination time are helpful to improve the crystallinity of magnesium oxide,promote crystal growth and reduce specific surface area.The results show that the mesoporous magnesia with a specific surface area of 188.3 m²/g can be obtained by calcining at 650?for 1 h.After calcination at 850?for 4 h,rod-shaped and dumbbell-shaped MgO nanoparticles can be obtained.The experiment of"hydration+calcination"is repeated for the mesoporous network magnesium oxide with high specific surface area.The results show that repeating the hydration step can improve the crystallinity of magnesium hydroxide sample and promote grain growth.The calcined magnesium oxide can basically maintain the structure of the precursor.Finally,the experiment shows that the hexagonal flake nano magnesium hydroxide can be obtained by secondary hydration,and the hexagonal flake nano magnesium oxide can be obtained by calcining the hexagonal flake nano magnesium hydroxide at 650?for 1 h;Calcination at 850?for 1 h can obtain lamellar mesoporous nano magnesium oxide.Then,taking hydromagnesite as raw material,through comparison of various methods,the process route of"calcination-hydration-calcination-hydrothermal"is determined to prepare nano magnesium hydroxide with regular hexagonal plates and high dispersion.The experiment explored hydrothermal conditions.The results show that controlling hydrothermal conditions can change the growth direction of crystal,increasing the exposure of crystal face 001 with weak polarity and decreasing the exposure of crystal face 101 with large polarity.The experiment finally determined that the optimum hydrothermal conditions were magnesium oxide dosage:25%,hydrothermal temperature:150?,hydrothermal time:3h.The experiment explored the modification conditions.The results showed that under the condition of 4%PVP,the growth direction of magnesium hydroxide tended to be consistent,and the dispersibility was effectively improved.Finally,nano magnesium hydroxide with an average diameter of 300 nm to 400 nm,hexagonal plate thickness of 40 nm to 60 nm and good dispersibility was obtained.Finally,the removal ability of high specific surface area mesoporous network magnesium oxide prepared from hydromagnesite to heavy metal ions was investigated.Taking the adsorption of Pb???as an example,the effects of adsorbent dosage,adsorption time,temperature and pH on its adsorption performance were investigated experimentally.The results showed that the optimal adsorption conditions were as follows:adsorbent dosage of 0.05 g/L,adsorption time of 45min,solution pH of 6-8,adsorption temperature of room temperature,and the removal rate of lead ions could reach more than 99.8%.The adsorption kinetics and isotherm data of the magnesium oxide adsorbent for Pb???are highly consistent with the pseudo-second-order model and Langmuir model,which indicates that the single-layer chemisorption of heavy metal ions has a maximum adsorption capacity of 7431.5 mg/g,which is the highest value in relevant reports based on MgO adsorbent at present.An adsorption mechanism involving hydroxyl functional groups and ion exchange between Mg???and heavy metal ions on MgO surface is proposed.In addition,the magnesium oxide can adsorb various ions simultaneously,and has excellent adsorption performance on Cd,Cr,Ni,As,Co,P,Se,Be,Bi,Cu,Fe,Mn,V,Zn and Al ions.The above results show that the MgO adsorbent has the advantages of high efficiency,low cost,easy preparation and easy popularization,is harmless to the environment,and is expected to become an excellent adsorbent for quickly removing heavy metal ions in wastewater.