The choice of raw materials and their relative composition is very important to achieve defect free membranes. The structural and morphological properties of a membrane are largely dependent upon the raw materials used and hence selection of raw materials and their composition is very important. In this work, several locally available low cost raw materials were selected to fabricate a tubular shaped ceramic membrane. Contemplation on reducing the price of the membrane and to assess our natural resources, the local clay materials: kaolin, quartz, ball clay, pyrophyllite, feldspar and calcium carbonate are utilized as raw materials.
These clay materials utilized for elaboration of the membrane was of mineral grade and obtained in the vicinity (Kanpur, India). The composition of clay materials used for the
fabrication of membrane along with its significance is given in Table 2.1. The composition reported in this work was obtained with a trial and error approach using various ratios of raw materials. The reported raw material compositions in Table 2.1 enabled the fabrication of defect free membrane with good permeation, pore size distribution and porosity characteristics.
Table 2.1: Summary of identified raw material compositions for tubular membrane fabrication
2.1.2 Elaboration of tubular ceramic membrane
An extrusion process was adopted to form a tubular shaped membrane. The table top mechanized extruder (M/s. VB Ceramic Consultants, Chennai, India) made of stainless steel was used for obtaining tubular ceramic membrane. It essentially consists of a feed chamber, extruder screw shafts, cone and die assemblies. The procedure for the fabrication of membrane is diagrammatically presented in Figure.2.1. Tubular ceramic membrane was fabricated with length, inner and outer diameters of 100, 5.5, and 11.5 mm, respectively. Clay powders were
Raw Materials Composition (wt.%) Significance
Kaolin
(Al2.SiO5.(OH)4)
15 Low plasticity and high refractory property
Quartz (SiO2)
28 Increases mechanical and thermal strength
Calcium Carbonate (CaCO3)
18 Pore forming agent as well as sintering aid
Ball clay (3 SiO2.Al2.O3)
18 Provides plasticity and strength to the green support
Phyrophyllite (Al2. (SiO5)2. (OH)2
15 To reduce crazing
Feldspar
(Na, Ca AlSi3 O8)
6 Acts as a flux to form glassy phase at low temperature
accurately weighed according to the composition and mixed with Millipore water (Elix-3 Milli- Q, USA) to make the paste for extrusion without the addition of any organic chemicals for plasticity. The obtained paste was fed into the extrusion cylinder. Then, evacuation piston forced the paste through a die in a tabletop extruder to form a tubular shape membrane. The tubular membrane was extruded in the horizontal direction with the forwarding velocity of ~0.7 cm/s at room temperature. When the tube reached the length of around 120 mm, the process extruder stopped and the tube was cut with sharp blades. Then, the obtained tubular membrane was subjected to natural drying at room temperature for 12 h. After which, the membrane was dried at 100°C for 12 h and 200 °C for 12 h in a hot air oven. Subsequently, the membrane was taken to the sintering process with a heating rate of 2 °C/min and sintered at 950 °C for 6 h in a box furnace.
Figure 2.1: Schematic representation of tubular ceramic membrane fabrication Millipore water
Mixed raw materials
Extrusion
Natural drying for 12 h
Oven drying at 100 & 200 °C for 12 h
Polishing & Sizing
Ultrasonication
Membrane drying at 100 °C
Characterization Sintering at 950 °C for 6 h
These restrained thermal treatment steps were followed to avoid the formation of micro cracks and bends in the membrane. The obtained rigid and porous sintered tubular membrane was finally polished and sized using silicon carbide abrasive paper (No. C - 220) to obtain a uniform smooth surface. Then the ceramic membrane was cleaned with Millipore water in an ultrasonic bath for 30 min to remove the loose particles created during polishing and sizing. Finally, the elaborated membrane was dried at 100 °C for further characterization. The table top mechanized extruder used for the fabrication of the tubular membrane and fabricated tubular membrane at different views are presented in Figure 2.2.
Figure 2.2: (a) Table top mechanized extruder and (b & c) fabricated membrane at different views.
2.1.3 Characterization techniques
The particle size distribution (PSD) of raw materials were analyzed in Malvern Mastersizer 2000 (APA5005® model, hydro MU) instrument in wet dispersion mode by circulating the heterogeneous feed at constant flow rate (pump speed = 2700 rpm) with an ultrasound to avoid the agglomeration of clay powders during analysis. Scanning electron microscopy (SEM)
(a)
(b) (c)
investigations were conducted in a varying pressure digital scanning electron microscopy (LEO1430VP®) combined with an energy dispersive X-ray spectroscope. The X-ray diffraction (XRD) profiles recorded with Cu Kα (λ=1.5406 Å) radiation working at 40 kV and 40 mA in Bruker AXS machine in 2θ value ranging between 5 and 75° with a scan rate of 0.05°/s.
Thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis were performed using Mettler Toledo (TGA/SDTA 851®) thermogravimetric instrument (NETZSCH TG 209F1 Libra) to characterize the thermal decomposition activities of the individual and mixed clay powders in air atmosphere with a heating rate of 10 °C/min from 25 to 970 °C in a 150µL platinum container. Field emission scanning electron microscope (FESEM, JEOL JSM-5600LV) was used to analyze the presence of possible defects on the surfaces. A small size of the membrane sample was fixed on top of the stub and layered with gold using an auto fine coating instrument (JEOL JFC-1300) preceding to morphology assessment. The porosity of the membrane was measured by utilizing water as a soaking agent. The mechanical strength was measured using a standard three-point bending test by Computerized Universal Tester (DUTT- 101, India). Four tubular ceramic membranes (length of 100 mm) were tested with the bending strength instrument. The instrument support span adjusted to 80 mm, before putting the sample on the support beam. Then the load was applied at a constant load rate of 10 Ns-1 until failure in the breaking force value was observed. The mechanical strength of the membrane was calculated from arithmetic mean of all values obtained. The corrosion resistance of the membrane was evaluated by means of loss of mass after treating in aggressive environments.
The acid and alkali solutions were prepared with extreme conditions (HCl solution with pH 1 and NaOH solution with pH 14) and soaked the membrane into the solutions for one week. The corrosion resistance of the recovered membrane was evaluated by weight decrement of the membrane.