Preparation Of Silicon Nitride Based Porous Ceramic Membranes And Their Applications In Membrane Separation

With the rapid growth of global population and development of industrialization,the demand for fresh water is rising continuously year by year,making the fresh water resources of equal importance with petroleum energy.Because of the inhomogeneous distribution,global climate change,industrial pollution and domestic pollution,the infrequent fresh water resources gradually reducing,especially in Africa and the Middle East.This situation is not optimistic not only in the middle and west China,but also in some eastern coastal cities of China,including Beijing,Tianjin,Dalian and Qingdao.Lack of fresh water resources is becoming a big problem that perplexes China and the whole world.To solve this problem,it is necessary to start with two things,i.e.asking the ocean for fresh water by desalination and reclamation of waste water.Sea water desalination came into the human's minds as early as the 16th century,and got a great development after World War II.The oil economy along with the population of Middle East is booming from then on,leading to the demand for fresh water increase rapidly and then sea water desalination industry grow fast.More than 70%fresh water in Israel and Saudi Arabia comes from sea water desalination.To protect the fresh water resources,the Euramerican developed countries are also concentrating on developing desalination industry.In China,an annual output of exceeding ten million tons of fresh water has been obtained by sea water desalination.Scaled desalination plants accompanied by new methods of desalination,among which the membrane method reverse osmosis?RO?,thermal methods multistage flash evaporation?MSF?,low temperature multi-effect distillation?MED?and the thermal membrane coupling process have been adopted in desalination widely with industrialization scale,affording fresh water around the world continuously.With the rapid development of membrane technology,the membrane distillation?MD?has been studied widely as a competitive desalination technology.But the lack of a stable and efficient membrane material make the MD unable to be applied in large scale.Large amount of fresh water is consumed in the industries like petroleum smelting,pharmaceutical,metallurgy and chemical fertilizers,along with the production of oily wastewater,polluting fresh water area.The low concentration of hundreds to thousands ppm of oil in the suspension and the small size of the droplets make it difficult to be separated from the waste water.Ultrasonic separation,air flotation,chemical demulsification,gravity static and biological method have been adopted in the oil and water separation process.High cost and low separation efficiency are insurmountable disadvantages of some of these methods,making the membrane separation process that developed in the past several years an alternative.The aim of our work is to explore several new porous non-oxide ceramic membrane materials that have potential application in the membrane distillation and oil-water separation processes.In the fabrication of porous planar ceramic membranes with asymmetric structures,the simple craft that combines phase-invasion tape casting and reaction sintering makes the preparation cost lower.The behavior of prepared membrane in oil-water separation is evaluated and the MD experiment is done for the as-synthesized membrane after modified to hydrophobic by inorganic SiCNO according to the previous research results of our laboratory.The details are as below.Chapter 1 firstly introduces the status of the fresh water resources and the pollution of oily waste water.Secondly,the present situation of the development of sea water desalination industry as well as the advantages and disadvantages of the technologies that applied in sea water desalination is discussed.The oil-water separation process is similarly introduced like desalination.Finally,the theory and device of MD and oil-water separation,especially the requirements for separation membranes in these two processes are introduced.In view of these requirements,the research ideas of this paper are determined.Chapter 2 simply introduces the raw materials,characterization instruments,the technology and equipment of phase-invasion tape casting method that applied in the synthesis of asymmetric porous membrane.Chapter 3 prepares the a-Si₃N₄ porous ceramic membrane and apply it into the MD experiments.First of all,a homogeneously and stably dispersed nonaqueous silicon slurry is prepared.To increase the fluidity of the slurry,the original silicon powder is heat treated at 600 °C for 5 hours.The suitable dispersing agent is selected through settling experiment,and the best content of dispersant is determined,then the solid content is adjusted to get uniform and stable silica slurry with suitable viscosity and rheological property.Secondly,the a-Si₃N₄ ceramic membrane with rich nanowire structure is obtained by selecting the suitable catalyst and determining the reaction atmosphere and sintering temperature.Thirdly,the membrane surface was modified to hydrophobic by inorganic SiCNO that derives from pyrolysis of chlorosilane.The obtained super hydrophobic membranes are used for membrane distillation experiments and the operation can run stably for a long time.In addition,the membranes operate stably in strong acid and strong alkali solution.In chapter 4,reaction sintering bonded porous ?-Si₃N₄?SRBSN?ceramics with asymmetric structure were prepared by phase inversion casting and sintering.Compared with a-Si₃N₄,?-Si₃N₄ has higher strength due to the interlacing structure of rod like crystal.In the preparation of SRBSN,the starting material of silicon powder reduce the cost greatly comparing with the fine silicon nitride powder.Due to the natural hydrophilicity and high porosity of silicon nitride,it can be used for the separation of oil emulsion in water,resulting in large yield and high oil drop rejection rate.The membranes are applied to membrane distillation after hydrophobic modification,and ideal results are obtained.In the fifth chapter,silica/oxynitride silicon composite ceramic membranes with petal like nanostructure was prepared and modified.Because of the low surface energy and petal like microstructure on the modified ceramic membrane surface,super hydrophobicity with a water contact angle of above 160° is obtained,and it has strong thermal stability and chemical stability.It also has excellent performance in MD experiments.Chapter 6 summarizes the whole work,and discusses the development prospects and shortcomings of our work.