Preparation and Characterization of Ceramic Membranes
2.1 Experimental
2.1.1 Raw materials
The selection of suitable raw materials and their relative compositions is the most important aspect for the fabrication of ceramic membrane. Different functional properties of the membrane depend upon the raw materials and hence the choice of raw materials and their composition eventually influence membrane morphological parameters. In this work, two types of membranes were prepared by two different compositions of inorganic precursors namely composition A and composition B. Composition A constitutes six common inorganic
Table 2.1: Raw materials composition (wet and dry basis) of ceramic membranes.
Composition A (wt %) Composition B (wt %) Material
wet basis dry basis wet basis dry basis
Kaolin 29.63 40 37.04 50
Quartz 11.11 15 11.11 15
Calcium carbonate 18.52 25 0 0
Feldspar 0 0 11.11 15
Sodium carbonate 7.40 10 7.41 10
Boric acid 3.71 5 3.7 5
Sodium metasilicate 3.71 5 3.7 5
Water 25.92 0 25.93 0
raw materials such as kaolin, quartz, calcium carbonate, sodium carbonate, boric acid and sodium metasilicate. Composition B was prepared using kaolin, quartz, feldspar, sodium carbonate, boric acid and sodium metasilicate. The formulations for composition A and composition B are summarized in Table 2.1. These formulations were determined from trial and error fabrication procedures through experimentation. It is further interesting to note that to date lower submicron range symmetric membranes prepared from low cost precursors was not addressed and this work uniquely contributed to the same.
Kaolin and sodium metasilicate were obtained from CDH, India. Feldspar was obtained from National Chemicals Ltd, India, Quartz was collected from Research Lab Fine Chem Industry, India. All other inorganic precursors such as calcium carbonate, boric acid, sodium carbonate were obtained from Merck, India. All these raw materials used for inorganic membrane preparation were graded at least 99.5 % pure and were used without further purification.
Various raw materials used in this work for the fabrication of inorganic membrane serve for different functional attributes. Kaolin and feldspar provide low plasticity and high refractory
properties to the membrane. Quartz contributes to the mechanical and thermal stability of the membrane. The regulation of porous texture in the ceramic was achieved by calcium carbonate (CaCO3) and sodium carbonate (Na2CO3). These materials under sintering conditions would dissociate into calcium oxide (CaO) and sodium oxide (Na2O), respectively and release CO2 gas. The path taken by the released carbon dioxide (CO2) gas thereby created the porous texture of the inorganic membrane and contributed to the membrane porosity during the sintering process. On the other hand, sodium carbonate and boric acid act as a colloidal agent and improved dispersion properties of the inorganic precursors thereby addressing homogeneity in the membrane structure. Boric acid also increased membrane mechanical strength by the formation of metallic metaborates at sintering temperatures. In a similar way, sodium metasilicate acts as a binder by creating silicate bonds among the elements to induce higher mechanical strength in the ceramic membrane [79]. The final formulation reported in this work has been deduced from a trial and error based fabrication approach using various ratios of inorganic precursors. The suggested precursor formulations provided crack free membranes with good structural integrity and desired submicron range pore diameters. The main difference between composition A and composition B was in the amount of pore forming materials (sodium carbonate, calcium carbonate) and clay materials (kaolin, quartz and feldspar). Composition A contains about 35 % total pore forming materials and 55 % clay materials. On the other hand, composition B constitutes only 10 % pore forming materials and 80 % clay materials. These alternations in compositions were identified to yield membranes with diverse structural morphologies.
Table 2.2 compared the identified compositions in this work with those presented in the literature. From the table, it can be observed that Potdar et al., [80] have provided optimal
Table 2.2: Comparison of identified membrane compositions with those presented in literatures.
Literature Materials Composition
(dry basis wt %)
Sintering temperature Kaolin 12.7
Ball clay 16.1
Quartz 23.6 Feldspar 5.1 Calcium carbonate 28.1
Potdar et al., [80]
Pyrophallite 14.3
900 oC
Clay 21 Kaolin 35 Feldspar 20 Belouatek et al., [32]
Sand 24
1100 oC
Kaolin 40 Quartz 15 Calcium carbonate 25
Sodium carbonate 10
Boric acid 5
Nandi et al., [34]
Composition A
Sodium metasilicate 5
850-1000 oC (This work)
Kaolin 50 Quartz 15 Feldspar 15 Sodium carbonate 10
Nandi et al., [35]
Composition B
Boric acid 5
800-950 oC (This work)
Sodium metasilicate 5
inorganic formulations (based on dry basis) using kaolin (12.7 wt %), ball clay (16.1 wt %), quartz (23.6 wt %), feldspar (5.1 wt %), CaCO3 (28.1 wt %) and pyrophallite (14.3 wt %) for the fabrication of MF range inorganic membranes. Amongst these precursors, quartz, feldspar
and pyrophallite could be regarded as expensive materials when compared to kaolin, ball clay and calcium carbonate. The total contribution of expensive materials to the composition was 43.3 %. In a similar approach, Belouatek et al., [32] have reported optimal inorganic formulations using clay (21 wt %), kaolin (35 wt %), feldspar (20 wt %) and sand (24 wt %) to prepare inorganic supports applicable for liquid waste treatment. Of these ingredients reported by the authors, only feldspar can be regarded as an expensive raw material when compared to clay, kaolin and sand. In this case, the total contribution of expensive precursors in the formulation was 20 %. Incidentally, the sintering temperature for the membrane preparation was about 1100 oC. In this context, costly materials used in this work contributed only 15 % in composition A and 30 % in composition B. Further, the sintering temperature was kept below 1000 oC. Therefore, the identified inorganic precursor formulations (composition A and composition B) for membrane fabrication tend to be competent with those of presented in the literature in the context of materials cost. In addition, it can be also inferred that given the challenging task of ceramic membranes in the lower submicron range pore size, the higher contribution of expensive precursors (30 %) was justified.