Preparation Of Fused Silica Membrane For High-temperature Gas-solid Separation

In China,coal accounts for more than 60%of primary energy consumption.The treatment of high-temperature dust is vital for environmental protection.Though there are many methods for high-temperature dust removal,existing technologies?e.g.cyclone dust removal,electrostatic precipitation and filter dust separator?have certain disadvantages.The porous ceramics have been considered as one of the most promising material in high-temperature flue gas filtration,which have high-temperature stability,chemical corrosion resistance stability,excellent mechanical strength,and also can be loaded with desulfurization and denitration catalysts.Fused silica ceramic have a low thermal expansion coefficient(0.54×10^-6/K,25⁸00oC),and also exhibit high thermal shock resistance and excellent chemical stability.The aim of this study was to fabricate ceramic membranes using fused silica particles as the main raw material and to characterize and discuss the performance.This study primarily consists of two parts,namely ceramic support preparation and membrane preparation.1.The fused silica particles was used as the main raw materials,hexagonal boron nitride?h-BN?as the sintering aid and crystallization inhibitor,starch as the pore-former,and polyvinyl alcohol?PVA?as the molding binder to prepare the fused silica ceramic supports.Because of the low expansion coefficient of fused silica,they are low-cost compared with silicon carbide materials for high temperature dust filtration.In this study,porous fused silica ceramic supports were fabricated with h-BN as a sintering aid.The results shown that h-BN could inhibit the crystallization of fused silica ceramic particles at high temperature and act as a sintering addictive to reduce firing temperature.The effects of starch content,firing temperature and h-BN content on the properties of the support were primarily analyzed here.The obtained supports exhibits an average pore diameter of 72?m,an open porosity of 42%,a bending strength of 16.5 MPa,a Weber modulus of 8.8,as well as a gas permeation flux of4.23×10⁵ m³/?m² h bar?.After 30 times of thermal shock resistance cycle test,the bending strength of the ceramic support remained 16 MPa,and the value only decreased by 3%.According to the results,the support displayed high thermal shock resistance.The acid corrosion test also verified that fused silica ceramics has excellent acid corrosion resistance.2.Fused silica membranes was prepared using fused silica particles as the main raw materials,h-BN as the sintering aid and crystallization inhibitor,starch or graphite as the pore-former,PVA as the binder,and silane coupling agent as the dispersant.It was primarily studied using four methods,namely dip-coating method,precursor film firing method,sealing and spray method,and one step co-firing method.The dip-coating method is a membrane forming process simple in operation,whereas the preparation of a macroporous fused silica membrane is considered a huge challenge?10?m?.The main reason is that the average pore size of the support is 72?m,and the membrane formation particle size is d₅₀=40?m.In order to avoid the penetration of the membrane-forming particles into the pores of the support to increase the filtration resistance,the experiment avoids this phenomenon by preparing a high viscosity slurry,but at the same time causes an increase in the suction resistance of the support.In addition,the capillary force of the large pore size support is relatively small.During the dip-coating process,the high viscosity membrane-forming tends to pull off the slurry from the surface of the support,which makes it difficult to form a complete membrane layer of a proper thickness.According to the preparation of fused silica membranes by precursor film firing method,the surface roughness of the supports seriously affected the structural integrity of the membranes and the bonding strength between the membrane layer and the support.Owing to the large pore size of the support and the preparation of the aggregate particles(d₅₀=288?m),the surface roughness of the support was high,and the contact area was narrowed between the membrane-forming particles and the support surface,thereby reducing the bending strength of the membrane and the support.However,this process may be of great value if used to make small pore size ceramic membranes.To address particle penetration and bonding strength between the membrane and the support,the fused silica membranes was prepared using sealing and spray coating.A layer of PVA film was pre-filling on the surface of the support,and the pre-sealing hole could prevent the membrnae-forming slurry from being sprayed onto the inner channel of the support.After the PVA film was dried,the slurry was sprayed on the surface of the support.Subsequently,the effects of heating rate,graphite content and spray slurry volume on the structure of fused silica membrane were investigated experimentally.The fused silica membranes with a thickness of 100±5?m and an average pore size of 9.85?m had been prepared after sprayed3 cycles and sintered at 1130°C for 1 h.The membranes exhibited a gas permeability value of8.2×10⁴ m³/?m² h bar?and a dust removal efficiency of 98.6%.After 32 times of thermal shock resistance cycle test,the membrane showed good thermal shock resistance.Overall,the fused silica membranes displayed outstanding permeability,high thermal shock resistance,good removal efficiency,and renewability.The fused silica membranes was also prepared by spraying membrane-forming slurry on the surface of the green body following the one step co-firing process.The graphite content,h-BN and firing temperature were adjusted to fit the shrinkage ratio between the green body and the green membrane.Besides,the bonding strength between the membrane layer and the support was 0.6 MPa.