Preparation And Performance Investigation Of Novel Low-cost Porous Ceramic Separation Membrane

As a type of novel separation media,inorganic ceramic separation membranes exhibit more merits than their high polymer counterparts,such as high separation efficiency,good high temperature resistance,good structural stability,low energy consumption and easy clean-regeneration.As a result,they are increasingly applied in many industrial separation fields,including food industry,medical industry, metallurgy,bio-technology.With the strong environmental demands for ceramic membranes nowadays,such as massive liquid waste treatment,strong acidic or alkali media separation and high temperature thermal shock applications,commercialized porous ceramic separation membranes could not be used on a large scale in industry due to their several drawbacks,such as high cost,rare membrane materials and narrow application range.Accordingly,in my Ph.D dissertation work,the efforts were focused on the investigation of preparation and performance characterization of low cost porous ceramic membranes based on the needs of lowering cost and meeting the specific environmental applications(high temperature,strong acidic or alkali applications).Several cost-effective novel porous separation membranes with applied value have been developed.And the application performances were also studied from the viewpoint of extending application fields,for example,separation of strong acidic or alkali liquids and the removal or recovery of solids from powder-containing hot gas. Some phasic achievements have been obtained.The work in this dissertation can be described briefly as follows.1.Preparation and performance investigation of tubular porous cordierite ceramic membrane supportsAs a classic ceramic material,cordierite(2MgO₂·Al₂O₃·5SiO₂)is usually applied in the fields of refractory products and automobile exhaust catalyst carriers due to its many advantages,especially low thermal expansion co-efficient.It is said that in Janpan' scientific and technical community cordierite has been used in the preparation of porous ceramics.However,specific preparation process has not been reported in the relevant literatures.In this work,the preparation process of cordierite ceramic membrane tubes was studied.The corrosion performances and the corresponding mechanism were also studied in detail.Low cost commercial grade coarse cordierite powders were used as the starting materials,while the "extrusion-sintering" process was adopted.The effects of sintering temperature on pore size and its distribution,porosity,mechanical strength,permeation flux and thermal expansion coefficient were studied in detail. The main properties(porosity,pore size and permeation flux)of these two supports prepared from different powders were compared.Under the optimized process system, two tubular ceramic membrane tubes with average pore sizes of 8.66-9.49μm and 17.03-19.24μm were prepared respectively.Furthermore,these porous tubes could be used as the supports for the preparation of micro-filtration or ultra-filtration membranes.To verify the practicability in the specific environmental applications,the acidic and alkali corrosion resistance tests were carried out for the fabricated cordierite tubes C-1(average pore size:8.66μm)and the commercial alumina supports(average pore size:2.96μm).In particular,their corrosion mechanisms were analyzed.The results indicated that,after 10 wt%NaOH solution corrosion for 4 h,the alkali-resistant cordierite ceramic membrane tubes exhibited a slight change in pore-structure,and lower strength loss ration(8.16%)than the alumina supports(73.6%).This has not been reported in international research community.On the contrary,after the acid corrosion(for example,20 wt.%H₂SO₄ solution for 4 h),the strength loss ratio of the cordierite ceramic membrane tubes was 54.87%,while that of acid-resistant alumina ceramic tubes is only 3.56%.Therefore,the commercial alumina supports are suitable for the treatment of strong acidic liquids.By comparison,due to cheap and abundant raw materials and low sintering temperature,the fabricated alkali-resistant cordierite supports with low expansion co-efficient are expected to have potential large-scale applications in the pre-treatment of strong alkali media and high temperature solid-gas or liquid-solid separation.This study is expected to fill the research blank of the environmental application of porous ceramic membrane in the separation of alkali caustic media and high temperature hot gas.2.Preparation investigation of porous mullite ceramic membrane supportsMullite(3Al₂O₃·2SiO₂)is the only stable existing phase for the Si-Al system.As an excellent structural ceramic,it shows many unique advantages.However,the use of mullite for the porous ceramic membrane preparation is rarely reported in the literatures.In the first part,industrial coarse mullite was used as the aggregate materials while the mixture of kaolin and basic carbonate magnesium as additive.The influences of sintering temperature,pore-forming agent,additives on the properties of the porous mullite were studied,such as sintering characteristics,phase composition and pore structure.At the optimized sintering temperature of 1500℃,low-cost porous mullite ceramics with average pore size of 26.21μm and thermal expansion co-efficient of 6.24×10^-6℃^-1were prepared.In the second part,for the first time,the porous mullite ceramic membrane supports with controllable micro-structure were prepared from the mullite precursors(fly ash core and aluminum hydroxide shell)by the reaction sintering technique.The phase transition mechanism and sintering behavior were studied in detail during the course of heat treatment.Although there is a slight increase in sintering temperature for porous mullite,some advantages could be still achieved as follows:a.extension of sintering temperature range;b.easier control of micro-structure of the sintered bodies; c.consumption of harmful phase cristobalite;d.increase in the phase concentration of mullite.This study solved the scientific difficulties for the sintering of fly ash porous ceramic,such as narrow sintering temperature range,difficult control of micro-structure,production of harmful phase and foam/deformation during high-temperature sintering.This not only has access to obtain the low-cost porous ceramic supports,but also provides a feasible practical way for high value-added utilization and environmental governance of industrial waste.3.Preparation investigation of tubular cordierite micro-filtration membranes with fly ash as the main starting materialsCommercialized porous ceramic membrane can not meet the demands for environmental applications,such as the large-scale waste liquid treatment, high-temperature gas-solid or liquid-solid separation.In the work,for the first time, low-cost asymmetric cordierite multiple-oxide porous membranes with controllable crystalline phases were in situ synthesized using fly ash as the main starting materials on the above-mentioned tubular supports(average pore size:18 um,open porosity: 41.40%).Asymmetric cordierite-based porous ceramic micro-filtration membranes were prepared using spherical industrial waste fly ash as the starting materials with basic magnesium carbonate as source of magnesium by the spray-coating/dip-coating process.Crack-forming mechanism of membrane layer and the corresponding dissolution method,phase transition mechanism and the control factors of micro-structure were studied during the preparation process.Also,the gas permeation performances and permeation mechanism were preliminarily studied.Optimization of the raw materials ratio was found to be necessary to avoid crack formation during sintering.The results also confirm thatα-cordierite was formed above 1100℃,and that the optimized sintering temperature range(1150-1200℃),cordierite-based porous membranes showed average pore size in the range of 3.6-6.4μm and nitrogen permeance of 7.52-9.82×10⁴ m³Â·m^-2·h^-1·MPa^-1under the average pressure of 0.15 MPa. Because the high nitrogen flux makes penetration resistance greatly reduced,also for low preparation cost,the fabricated membranes are expected to be applied in the massive pretreatment of dust-containing hot gas and waste liquids.4.Fabrication and characterization of tubular micro-filtration mineral membranes from natural zeoliteThe natural zeolite granulars are widely used in the fields of adsorption and ion-exchange because of its abundant active micro-pores.However,natural zeolite was applied to membrane preparation in our previous work.The narural zeolite-based porous mineral supports with coupling function of separation an ion-exchange have been successfully developed based on the combination of active micro-pores and the gaps between the accumulated particles.In order to develop low-cost mineral membranes with applied value,tri-layer mineral micro-filtration membranes have been fabricated on tubular porous mineral supports by dip-coating using cheap and abundant natural Heulandite (Ca[Al₂Si₇O₁₈]·6H₂O)mineral as the starting materials.The influence of the parameters during the preparation process,including the powder classification,the forming and sintering of membranes,was systematically studied.The XRD results reveals that the phase compositions of the membranes were related to the sintering temperature and the final major phases were almost quartz and albite.SEM studies subsequently indicate that solids in the membrane begun to sinter at about 850℃with alkali metal oxides as the aid fluxes.The inter-layers with average pore size in the range of 0.69-1.10μm were obtained at the optimum firing temperature(850- 950℃).Then,the top-layer membrane with average pore size of 0.54μm could be prepared on the above support.Nitrogen gas permeation flux and pure water permeation flux of the resulting tri-layer membranes are 1.96×10⁵l·m^-2·h^-1·bar^-1and 3.20×10³l·m^-2·h^-1·bar^-1,respectively,with the trans-membrane pressure of 0.1 MPa at room temperature.The prepared mineral ceramic membranes with low preparation cost are suitable for the large-scale separation of neutral medium.This not only added a new species for liquid separation ceramic membranes,but will lead to the technological innovations for the development of mineral materials.5.Synthesis and property characterization of low-cost ceramic membrane materials-mulliteIn a sense,the choice and synthesis of the ceramic support material are the basis for the inorganic ceramic membrane study.In the work,low cost mullite ceramics were for the first time prepared with natural bauxite as resource of alumina,which is a cheap and abundant substitute material for industrial grade alumina.The fired samples, including fly ash,bauxite and their mixture based on the composition of 3:2 mullite, were studied in terms of phase development mechanism and sintering characteristics. The results indicate that the secondary mullitization occurred by the solid state reaction of cristobalite and corundum below 1300℃,followed by the dissolution of corundum into transitory glassy phase at higher temperatures.The dilatometric results reveal that the formation of secondary mullite resulted in slight expansion despite of the shrinkage induced by sintering.In addition,the samples were fired at elevated temperatures and then characterized in terms of relative density,porosity, micro-structure and thermal and mechanical performances.At the optimized sintering temperature of 1600℃,the relative density and bending strength are 93.94%and 186.14 MPa,respectively.Only a small amount of nearly spherical closed pores were observed in the sintered body.The 1600℃sintered mullite samples exhibited better corrosion resistant performance in the strong acid or alkali solutions.The corrosion process can be divided into two stages:high-speed stage(0-5h)and low-speed stage (5-20 h),which was respectively corresponded with the surface corrosion and bulk corrosion process.The synthesized low-cost,high-performance mullite ceramic materials is suitable for the preparation of porous ceramic membranes,in particular the porous ceramic membrane supports after size reduction for classification.The study is of great significance and value for ceramic membrane preparation field.