Nickel aluminate reinforced porous ceramic hollow fibre membranes
thesisposted on 22.02.2017 by Fung, Yi-Lan Elaine
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Membranes are commonly used in industrial separation processes because of their absence of moving parts, simplicity of separation without phase change, potential of scaling and instantaneous response to system variations. In general, polymeric membranes are of low cost and could be easily prepared, yet they fail at processes that involved high temperatures, extreme pH, corrosive and organic chemicals. Ceramic membranes were therefore introduced to these harsh conditions because of their high thermal and chemical stability, while being insensitive to swelling and could be easily cleaned. Industries that make wide use of ceramic membranes include water treatment, chemical production, petrochemical, metal, automotive, textile, pulp and paper, tannery, biotechnology, cosmetic, pharmaceutical, food and beverage. Among all different membrane geometries, hollow fibre has the highest surface area to volume ratio and highest compactness. The major limitation of large scale production and application of ceramic hollow fibre membrane is the brittle nature of ceramic. Brittleness is a significant problem when the membrane is of a hollow fibre geometry which is small in size and has thin walls. Apart from cracking under pressure, brittle ceramic hollow fibre membranes are also difficult to be bundled up and to be sealed into equipment as orientated modules. This thesis presents the fabrication and characteristics of ceramic hollow fibre membranes with enhanced flexural strength, which involves nickel aluminate (NiAl2O4) as a reinforcement medium and as a pure membrane material. NiAl2O4 is a non-toxic ceramic material with high melting temperature and strong resistance to acids and bases. The ceramic nature and stability of the strengthened hollow fibre membrane was therefore maintained. NiAl2O4 was formed by the solid-solid reaction between nickel (II) oxide (NiO) and alumina (Al2O3) at high temperature of 1400 oC or above. All the ceramic hollow fibre membranes were prepared by the phase-inversion and sintering method. Apart from water the commonly used internal and external coagulant in a phase-inversion process, the use of ethanol-water mixture as internal coagulant was also attempted and its effect on the structure of membrane wall was investigated. Brittleness of the ceramic hollow fibre membranes, which was the main focus of this thesis, was indicated by flexural strength obtained by three-point bending test carried out on individual samples. On top of flexural strength, other characteristics of the ceramic hollow fibre membranes that determine their application, effectiveness and efficiency in separation processes were also studied. These include microstructure, porosity, active layer porosity and pore size, pure water flux and chemical stability. All the ceramic hollow fibre membranes prepared were porous, with their pore sizes in the microfiltration range, and are suitable for the separation of solid suspensions from liquid. The study of strength enhancement of porous ceramic hollow fibre membranes started with the reinforcement of porous ceramic materials by the inclusion of external element, which was discrete NiAl2O4 in the porous alumina matrix formed by the addition of NiO. X-ray diffraction (XRD) pattern proved the formation of NiAl2O4¬ without NiO remaining after the sintering process at 1500 oC or above for 5 hours. Energy dispersive X-ray spectroscopy (EDS) showed the even distribution of NiAl2O4 across the alumina matrix through ball-milling mixing of raw materials before sintering. Flexural strength of the porous NiAl2O4/Al2O4 increased with NiAl2O4 loading up to 146 MPa at 14.7 wt% of NiAl2O4, which was a result of closure force formed by thermal expansion mismatch between alumina and NiAl2O4 that deflects crack growth. Flexural strength decreased with further increase in NiAl2O4 content as densification and shrinkage were hindered by grain growth in random directions which formed agglomerates and loose particles. NiAl2O4/Al2O3 hollow fibre membranes were then fabricated through phase-inversion and sintering method with the same raw materials. A maximum flexural strength of 156 MPa was achieved by the NiAl2O4/Al2O3 hollow fibre membrane containing 16.4 wt% of NiAl2O4, which was 2.3 times of that of the pure alumina sample prepared under the same conditions. The 16.4 wt% NiAl2O4/Al2O3 hollow fibre membrane had a porosity of 46.8% which could give sufficiently low fluid resistance. It showed a pure water flux of 597 L/m2.h.bar under a feed pressure of 2 bar and a mean pore size of 330 nm on its active layer. In comparison with ceramic hollow fibre membranes reported in other literatures, NiAl2O4 reinforced alumina hollow fibre membrane achieved a higher flexural strength than other ceramic reinforcement methods such as the addition of Ni and NiO and a comparable flexural strength as yttria-stablized zirconia (YSZ) a stronger and higher cost ceramic than alumina and as within the porosity range of 47-55%. Followed by the reinforcement of alumina hollow fibre membrane by discrete NiAl2O4, nickel nanowire (Ni NW) which has a continuous structure was used as a raw material in the preparation of ceramic hollow fibre membrane precursor. Ni NW was oxidized into NiO in the sintering process in air, resulting in continuous NiAl2O4 chains in alumina hollow fibre membranes. On top of thermal expansion mismatch between NiAl2O4 and alumina, crack bridging effect of the NiAl2O4 chains also led to strengthening effect. Flexural strength was increased with NiAl2O4 content up to a loading of 2.00 wt% then decreased with further increase in NiAl2O4 like the case of discrete NiAl2O4. The maximum flexural strength achieved was 172 MPa and the corresponding porosity, pure water flux at a feed pressure of 2 bar and mean active layer pore size was 35%, 35 L/m2.h.bar and 220 nm respectively. Apart from acting as reinforcement medium in alumina hollow fibre membrane, pure NiAl2O4 hollow fibre membranes were prepared by mixing alumina and NiO powder in 1 : 1 molar ratio homogeneously, followed by sintering. A sintering time of 10 hours at 1600 oC was found to be sufficient for the solid state reaction to complete and sintering of NiAl2O4. Flexural strength of NiAl2O4 increased with sintering temperature. The flexural strength achieved at 1630 oC the highest sintering temperature attempted was 101 MPa with a porosity of 55% and pure water flux of 862 L/m2.h.bar at a testing pressure of 2 bar. Its active layer pore size ranged from 50 – 140 nm. In comparison with ceramic hollow fibre membranes with similar porosities, NiAl2O4 showed an appreciable flexural strength. This proved that NiAl2O4 was a mechanically strong ceramic and can potentially be used as industrial scale membrane with further studies. The ceramic hollow fibre membranes presented in this thesis have enhanced flexural strength, which indicates a decrease in their brittleness. The ceramic nature and hence high thermal and chemical stability of the hollow fibre membranes were maintained. These membranes could potentially undergo large scale production for industrial scale microfiltration processes. The strengthening mechanisms presented could also be potentially used in other ceramic materials which brittleness is a hindrance for their wide application.