(552f) Preparation of Economic and Robust Macroporous Membranes From Natural Kaolin Clays

Inorganic materials have been widely used in membrane separation technologies due to their attractive thermal and strength characteristics unmatched by the polymeric counterparts. Permeation properties have also been a focus of attention that led to many reported ceramic membrane configurations and preparation strategies. The high cost is one of the limitations in ceramic membranes stemmed by elevated costs of inorganic precursors and high energy requirements. A cost-effective ceramic membrane combining attractive transport and strength properties is highly desirable in this area. One way of addressing this challenge is to utilize natural clays. Jordan is abundant of many types of clays (e.g., Kaolin, bentonite, zeolite, quartz, etc) that can be used for manufacturing of composite membranes. Our ultimate objective is to prepare zeolite membrane supported on kaolin support from natural clays for use in ultra-filtration of brackish layer. This abstract summarizes our work in the preparation and testing of an effective kaolin membrane support.

We used three types of raw kaolin clays (K1, K2 and K3) mined from different regions in Jordan named: Assalat, Batnil-ghoul, and Wadil-hafira respectively. Kaolin and mineral contents were evaluated by XRD and XRF techniques. All clays were grinded into particles less than 20 micron and used without any pre-refining.  Membrane supports were prepared by pressing the wetted clay powder in a mold at 1000 psi followed by sintering at 1100C for four hours yielding three sets of supports: K1, K2 and K3 . The supports were tested by steady state helium gas permeation at room temperature for evaluation of flux permeance, and average pore size properties.

XRD patterns and XRF elemental analysis revealed that clays are composed of three major minerals (Kaolin, Quartz and Calcite). The proportions of minerals vary with the source with kaolin content increasing progressively from K1 to K3. This combination of minerals is very interesting because it contains the major constituents used in commercial supports. Kaolin (Al2O3.2SiO2.2H2O) is a plastic and moldable material, while quartz (SiO2) is refractory and can enhance the strength of the composite. During sintering, the quartz transforms into a glassy phase and holds the particles together. Calcite or limestone (CaCO3) may appear to be harmful because it yields CO2 upon heating which creates extra voids in the support. If these voids can develop into cracks, the calcite can be removed by an acid washing pretreatment. All supports were physically strong having a compressive strength exceeding 200 MPa. Attempts to purify the kaolin significantly decreased the strength to 78 MPa.

Gas permeation properties shows that from K1 to K3, the permeance progressively decreases two orders of magnitude from 1E-5 to 1E-7 mol/s.m2.Pa emphasizing the influence of minerals on the inner structure. These values are close to permeance of values of commercial alpha alumina supports. The supports are all macroporous as revealed by the average pore size evaluated from the fitted equations. The pore size decreases progressively from K1 to K3 in the same order of increasing kaolin content. It is interesting to note that gas transport in these membranes is a combination of viscous and Knudsen flows with percentage contributions ranging between 65/35 to 30/70 based on the pore size. The Knudsen mechanism becomes more dominant for smaller pore supports.