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Manufacturing of ceramic membranes for oil-in-water emulsification and oil recovery from wastewater

Subject Area Glass, Ceramics and Derived Composites
Synthesis and Properties of Functional Materials
Production Automation and Assembly Technology
Term from 2017 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 327896934
 
Final Report Year 2022

Final Report Abstract

The aim of this German-Brazilian cooperation project is to develop ceramic membranes with a property profile adapted to the production of oil-in-water emulsions or the separation of oily wastewaters. The production and processing of emulsions is an important process in the food or cosmetics industry, which is conventionally energy-intensive and thus cost-intensive. Membrane processes - in particular using chemically and thermally stable ceramic membranes - could make these processes more efficient and also lead to more homogeneous droplet distributions. The project focus is on the development of new or improved manufacturing routes for both oxide ceramic and polymer-derived ceramic (PDC) SiOC membranes. For this purpose, symmetrically constructed ceramic flat membranes were produced by tape-casting and also asymmetric membranes with a thin separation layer and a broad support structure were developed by combining two layers produced by (i) tape-casting or freeze casting, by (ii) direct freeze-tape-casting or by (iii) phase inversion tape-casting. The latter was applied to PDC materials for the first time, opening up new, versatile possibilities for the production of tailored PDC membranes, whose surface characteristics could also be made more hydrophilic or hydrophobic by composition, but especially by pyrolysis temperature. The membranes prepared show narrow pore size distribution (0.1-3 µm) and medium porosities (25-75%), which qualify them for use as microfiltration membranes and were investigated with respect to their water permeability, oil-in-water emulsion generation, and separation of oil-containing emulsions. High water permeabilities were obtained for particularly hydrophilic membranes prepared by incorporating TiO2 fillers (SiOC membran) or subsequent surface functionalization with silanes or a TiO2-coating. A further increase in water permeability could be observed for the asymmetric membrane geometries with very thin separation layer. A photocatalytic activity of the TiO2-containing membranes and thus the possibility to decompose organic substances on the membrane surface by UV irradiation could be demonstrated for TiO2-coated Al2O3 membranes for smaller organic molecules such as methylene blue (MB), but oil contaminants could not be decomposed, since they apparently deactivate the catalyst in different ways. For TiO2-containing SiOC membranes, only very low photocatalytic activities could be detected, since fewer TiO2 particles are located at the membrane surface. Interestingly, however, very high MB adsorptions were observed, making these membranes also suitable for the purification of dye-contaminated wastewater. To increase the antifouling properties of TiO2 coated membranes further, the decomposing effect can be enhanced by a conductive membrane material. Such catalytic membrane processes can combine separation and catalytic degradation of oil, minimizing fouling and extending membrane life. A conductive SiOC/C membrane has already been developed as a first step in the project, but initially tested very successfully as a gas diffusion layer. Future work will investigate the antifouling properties of these membranes in more detail. In oil separation experiments, hydrophobic SiOC membranes could not be measured at pressures that the membrane could still withstand due to the already very low water permeabilities. Nevertheless, good oil separation performances up to 96% oil removal could be demonstrated especially with asymmetric, hydrophilic Al2O3 and SiOC membranes. Emulsification tests starting from a premix (o/w emulsion) showed that the membranes act in a very complex way on the droplet reduction, where not only the pore size but also the thickness of the membrane can play a decisive role. In general, droplet sizes of 2-5 µm could be achieved with 1-2 membrane passes. While asymmetric membranes have the advantage of achieving higher membrane flux, a symmetric membrane with a smaller average pore size across the membrane thickness can still result in smaller droplet sizes due to the longer transport path and associated interactions. Overall, the manufacturing processes developed for membranes and the resulting membranes with a wide variety of pore structures form an excellent basis for investigating the suitability of different material/structure combinations on the production of emulsions or their separation in greater detail in the future, and thus to provide membranes for a wide variety of separation or emulsification processes. This work is currently being continued in the Advanced Ceramics Group.

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