Preparation Of Low-cost Hollow Fiber Membrane Accompanied With The Stabilization Of Hazardous Metal

Porous ceramic membranes have been receiving increasing attention in a wide range of separation/purification applications such as oily wastewater treatment, water desalination, drinking water purification, bacteria removal, gas separation and catalytic reactions. In order to enhance the usage efficiency, hollow fiber ceramic membranes(HFCMs) with high packing density and area/volume ratio(~ 3×104 m2·m-3) are concerned and thus introduced. Among all available HFCMs, those, fabricated by using a one-step phase-inversion technique, appear particularly attractive due to their unique asymmetric structure consisting in a thin sponge-like separation layer and a thick finger-like support.To date, the majority of ceramic membrane is made from high-purity alumina. However, both high-purity alumina powder and the high sintering temperature yield to high preparation cost, increasing amount of studies are devoted to prepare ceramic membranes by using natural mineral powders as well as other industrial solid wastes instead. To the best of our knowledge, the fabrication of low-cost hollow fiber membrane by entirely employing cheap and abundant bauxite mineral to replace pure alumina has not yet been reported.On the other hand, through crystalline phase transformation at high temperatures, hazardous heavy metals can be stabilized by spinel structure formation with aluminum-rich sources. Employment of bauxite mineral as major starting material is expected not only to stabilize hazardous metals as a low-cost alumina-source but also to fabricate a low-cost spinel-based composite ceramic membrane support.(1)With bauxite mineral as starting material, a low-cost alumina-mullite composite hollow fiber ceramic membrane(HFCM), featuring a finger-like support layer and a sponge-like separation layer, was prepared via the phase-inversion method followed by high temperature sintering. Factors including bore fluid flow rate, air-gap distance and suspension composition on structure properties of HFCM were systematically explored. Based on our experiment results, it was suggested that a low bore fluid flow rate would lead to the deformation of inner walls of HFCM as a result of insufficient solidification; while a large air-gap distance would induce the distortion of finger-like voids in the membrane inner surface. Effects of sintering on the microstructure, pore size distribution, nitrogen gas flux and mechanical property were investigated in details. Acid-base titration was first proposed to quantitatively determine concentration of surface active sites of membrane surface after sintering. Importantly, increasing sintering temperature would exponentially enhance Weibull strength but linearly reduce the concentration of active surface hydroxyl sites. Compared with its alumina counterpart, this low-cost alumina-mullite hollow fiber membrane exhibited a lower sintering temperature, comparable mechanical strength and active surface hydroxyl site concentration, which was promising for future economic separation applications.(2)In this study, a new idea to immobilize hazardous Cu O as simulated Cu-rich waste via the reutilization of abundant bauxite minerals was demonstrated by thermally converting it into the stable spinel structure to prepare functional porous ceramic membranes. With the addition of aluminum-rich bauxite mineral, over 90 % conversion of Cu Al2O4 spinel structure was achieved at the 1000- 1100 ℃ sintering window. Higher sintering temperatures would destabilize Cu Al2O4 structure, leading to the formation of minor precipitates such as Al2O3 and Cu O phases. Furthermore, addition of calcined bauxite would induce more minor precipitates than the nature bauxite did. This was likely attributed to the fact that the evaporation-condensation reaction of copper ions was significant on coarse calcined bauxite particles. Following leaching experiments clearly indicated that these copper ions were securely associated in the stable Cu Al2O4 spinel structure, and no significant amount of copper ions leaching was found even in acidic solutions.(3)The low-cost alumina-mullite composite membrane supports with Cu O as sintering-aid was prepared by reactive sintering mechanism. Factors including different addition of sintering-aid, open porosity, shrinkage, mechanical property, pore size distribution, gas permeation flux and microstructure on structure properties of membrane supports were systematically explored. Particularly, membrane support Cu Al8 shows appropriate sintering behavior. At high porosity, performance of membrane supports Cu Al8 is comparative with membrane supports Al, which is proved by the empirical strength-porosity relationships. This fact can be attributed to the formation of Cu-Al spinel and inter-locked mullite crystals, which leads to the uniform pore diameter compared with membrane supports Al. Finally, membrane supports Cu Al8 is applied on oily wastewaters treatment, and shows a slight lower permeation flux but a stable and higher reject rate compared with membrane support Al.(4)In order to reduce environment risk of zinc, a spinel-based porous membrane support was prepared by the high-temperature reaction of zinc and bauxite mineral. The phase evolution process, shrinkage, porosity, mechanical property, pore size distribution, gas permeation flux and microstructure were systematically studied. The XRD results, based on a Zn/Al stoichiometric composition of 1/2, show a formation of Zn Al2O4 structure starts from 1000 ℃ and then is accomplished at 1300 ℃. For spinel-based composite membrane, shrinkage and porosity are mainly influenced by a combination of an expansion induced by Zn Al2O4 and a general densification due to amorphous liquid Si O2. The highest porosity as high as 44 % is observed in Zn Al4 membrane support among all the investigated compositions. Compared with pure bauxite(Al), Zn Al4 composite membrane support is reinforced by Zn Al2O4 phase and inter-locked mullite crystals, which is proved by the empirical strength-porosity relationships. Also, an increase in average pore diameter and gas flux can be observed in Zn Al4. A prolonged leaching experiment reveals the zinc can be successfully incorporated into ceramic membrane support via formation of Zn Al2O4, which has substantially better resistance toward acidic attack.