Preparation Of In-situ Mg-Al Hydrotalcite And Its Research On Catalytic Hydrolysis Of Urea In Wastewater

Urea production capacity of global urea plants is up to 140 million tons per year, so the amount of wastewater from urea plants is 70 million tons, which not only contains carbon dioxide and ammonia, but also contains 0.5-2 wt% urea. It will result in not only the pollution of the environment, but also the waste of resources if the wastewater which cannot meet the emission standard is directly discharged. Thermal hydrolysis process, as a common treatment method in present urea plants, is carried out at an elevated temperature and pressure. In this method, the requirement for equipment materials is strict and a large amount of pressure steam is supplied to heat the wastewater fed into hydro lyser. Other methods, such as electrochemical, biological, enzymatic methods, those are still under investigation and currently have some drawbacks which hinder the industrialization. Therefore, looking for a new efficient approach which is low energy consumption and investment is an important and urgent task to treat urea wastewater.In this paper, MgAl-layered double hydroxides (MgAl-LDH) catalysts were introduced on the basis of thermal hydrolysis to catalyze urea hydrolysis in wastewater. A series of MgAl-LDH catalysts were prepared by different aluminum sources, synthesis temperatures, synthesis time and c(urea)/c(2+), then MgAl-LDH catalysts were modified by hydration and ethanol. X-ray diffraction, N2-BET, DSC and Hammett were applied to characterize the crystal structure and surface property of MgAl-LDH catalysts, further their structure-activity relationships were studied.1. The experimental results show that the particle size of η/-Al2O3-LDH is smallest, and the basicity and urea removal rate are highest and much higher than Al(NO3)3-LDH. Basic magnesium carbonate is generated with γ-AlOOH as aluminum source. The order of basicity and urea removal rate is as follows: η-Al2O3-LDH>γ-Al2O3-LDH>γ-AlOOH-LDH>Al(NO3)3-LDH. The basicity and urea removal rate are highest when synthesis temperature and time are respectively 130℃ and 4 h, and c(urea)/c(Mg2+) is 2. The urea concentration in wastewater is reduced to 7.46 mg/L at 165℃ after 120 min, which meets the emission standards of 10 mg/L. During repeated use of catalyst, the activity of MgAl-LDH firstly decreases and then trends to remain unchanged, the surface basicity and specific surface area have the same trend, which is caused by the increment of particle size.Magnesium aluminum oxide (LDO) generates when the calcination temperature reaches 450℃ and MgAl2O4 generates when the calcination temperature reaches 650℃, which cannot restore the layered structure of MgAl-LDH. The layered structure of MgAl-LDH recovers completely when hydration time is 4 h and particle size of hydrated MgAl-LDH (EMgAl-LDH) reduces, and the stability is good in the aqueous phase. Basicity, specific surface area and urea removal rate of RMgAl-LDH are higher than MgAl-LDH, and the urea concentration in wastewater is reduced to 5.03 mg/L. The increasing of particle size, the decreasing of specific surface area and basicity are caused by hydrothermal environment and have nothing with urea and its hydrolyzates in solutions.The modified MgAl-LDH (EMgAl-LDH) was quickly synthesized by adding ethanol to the solutions. Particle sizes decrease with the increment of the proportion of ethanol, and reach the minimum when the proportion of ethanol is 50%. Basicity, specific surface area and urea removal rate of EMgAl-LDH gradually increase with the increment of the proportion of ethanol, and reach the maximum when the proportion of ethanol is 50%.Two modified methods produce positive results on activity and stability of catalysts. The order of catalytic activities is as follows:RMgAl-LDH>EMgAl-LDH>MgAl-LDH. The order of stabilities is as follows:EMgAl-LDH>MgAl-LDH>RMgAl-LDH. EMgAl-LDH after continuous use possesses the highest hydrolysis activity.