Preparation of TiO₂ composite microfiltration membranes by the wet powder spraying method

Abstract

A composite TiO₂ membrane supported by a planar or tubular porous metallic substrate has been developed using the wet powder spraying (WPS) method. Intermediate layers made of the same material as that of the support were found to be necessary to prevent the penetration of the powder suspension into the pores of the support during the spraying process. The morphology of the membrane was examined by LM, SEM and XRD analysis, and the characterization was performed by bubble-point, flow rate and stability tests. By using the ultrasonic wave and grinding methods it was possible to distinctly decrease the imperfections in the membranes. The TiO₂ membrane itself has an average pore size of 0.11–0.12 μm, a thickness of 20–30 μm and an air flux up to 1.9 × 10⁵ L/h m² bar.

Introduction

Microfiltration (MF) is becoming increasingly popular as the preferred mode for a large variety of filtration applications involving the separation and concentration of particulate suspensions or solutions, the recovery of low molecular weight substances and in some instances, the recovery of macromolecules such as proteins. Generally, the pore diameters of the MF membranes range from 0.05 to 10 μm [1], [2], [3].

MF membranes can be divided into two main groups—inorganic membranes and organic membranes. Inorganic membranes can be subdivided further into ceramic and metallic membranes, while commonly used organic membranes are made of polymeric compounds. The development of inorganic microfiltration membranes for industrial applications enables the realization of high thermal capability, high chemical stability and good cleaning ability by means of high pressure or backflushing, which cannot be implemented by polymer membranes. Metallic membranes additionally offer good thermal shock resistance, high mechanical strength and capability for welding or brazing, but their application is limited since pore sizes <1 μm cannot be easily realized.

In most cases, MF membranes consist of several layers with a gradual decrease in pore size towards the feed side of the membrane [4]. The main support of an asymmetric MF membrane consists of a packing of rather coarse-grained material, which is produced in a classical way by uniaxial or cold isostatic pressing of a dry powder, by co-extrusion of a powder paste with additions of binders and plasticizers or by slip casting [5], [6]. After burning away the organic compounds, the so-called “green” compact is sintered. In order to obtain defect-free membranes, thin top layers on the support system must fulfill more stringent requirements than those utilized in the manufacture of commercially available porous materials. Pore size distribution and roughness must be smaller than usual. The quality of the support is especially critical if the formation of the top layer is mainly determined by capillary action of the support. Then, besides a narrow pore size distribution, the wettability of the support system plays also an important role [4], [7]. In cases where the precursor particles of the membrane layer are very small in size compared to the pore size of the bulk of the support, the membrane particles can significantly penetrate the support pores and the resulting permeability of the membrane/support composite will deteriorate. A practical solution is to add one or more intermediate layers, whose pore size and thickness are between those of the bulk support and membrane layer. The intermediate layer can be used to improve the quality of the support system. Due to the above-mentioned advantages, metal powders are interesting candidates for the support and the intermediate layers, while the MF membrane itself is preferrentially made of ceramic powders. Commonly used materials are therefore ZrO₂, Al₂O₃, TiO₂ and SiO₂. Up to now, slip casting [8], [9] and sol–gel [10], [11], [12] methods have normally been used to apply these materials.

In the present work, a composite MF membrane consisting of a ceramic layer on a porous steel substrate was developed, which combines the properties of metallic and ceramic filters advantageously. The wet powder spraying (WPS) technique was used due to its widespread possibilities of spraying metallic as well as ceramic powders on tubular and planar substrates without changing the equipment [13], [14], [15]. The potential of the WPS method has been already shown in former investigations carried out on graded metallic filters [16] and is already established in industrial production of graded metallic filters [17]. Ethanol based suspensions are preferred for metallic powders due to their fast drying speed. The problem of handling ethanol in an industrial production process is solved by an effective exhaust equipment.

TiO₂ pigment powder was used for the functional ceramic layer to be applied as a coating to the substrate made of stainless steel 316L. The WPS technique was also adopted for the manufacturing of the functional layer, because spraying of TiO₂ pigments is less time consuming than slip casting or sol–gel an has a high automation potential. Intermediate layers made of gas-atomized 316L stainless steel powders were found to be a prerequisite for coating the functional ceramic layer without obvious pin holes. The contribution describes each step of the production route in detail. The microstructure of the membranes was examined by light microscopy, SEM and XRD analysis and the properties were characterized by measuring the bubble point, flow rate and stability.