Characterization of raw materials .1 Thermogravimetric analysis

In the thermogravimetric analysis of raw material mixture with Na-CMC (Fig. 2.4), the first minimal weight loss occurred around 110-130 ˚C owing to the evaporation of physisorbed moisture present in the sample (Kumar et al., 2015b). The second peak in the DTG curve occurs at a temperature of 284.84 ˚C, which may be attributed to the thermal decomposition of sodium salt of carboxymethyl cellulose (Na-CMC), the binder used in the membrane fabrication process. At this temperature, the thermal oxidative degradation of Na-CMC causes different reactions to take place, thus changing the composition of the sample. During thermal degradation process, the fragments obtained from Na-CMC molecular chain undergo rearrangement that leads to the formation of some polyunsaturated chain products. These

materials may get oxidized at a higher temperature, leading to the generation of CO2 and minor weight loss (Tan et al., 2017). In the TGA (inset) and DTG graphs of the raw material mixture without additive, the absence of second peak at around 280 ˚C temperature also confirms that that particular peak corresponds to decomposition of Na-CMC. The similarity of both the graphs, except that particular peak, reveals the decomposition of Na-CMC at that particular temperature.

Fig. 2.4 (a) TGA and (b) DTG of raw material mixture with and without Na-CMC (K3

The significant weight loss observed around 710-730 ˚C is owing to the thermal decomposition of calcium carbonate (CaCO3), yielding calcium oxide (CaO) and carbon dioxide (CO2) as products, according to the reaction mentioned in equation (2.8).

2

CaCO3→CaO+CO (2.8)

The carbon dioxide formed as a result of this process leaves the sample and is considered responsible for the formation of pores in the membrane (Cheryan, 1998). The process of pore formation by CaCO3 decomposition and diffusion of CO2 through membrane matrix is depicted pictorially in Fig. 2.5.

It is worth mentioning that the mass loss for the mixture without Na-CMC is around 10% at 1100 ˚C, while it increased to around 24% for the mixture containing Na-CMC. It has been observed that weight loss after 1000 ˚C is minimal. Moreover, for fly ash-based membranes, the mechanical strength of membranes increases with increasing sintering temperature. Reports have mentioned that only 50 ˚C increase in temperature leads to an enormous increase in membrane strength (Černý and Tůmová; 2016).

Fig. 2.5 Formation of pores in the membrane matrix

2.2.1.2 X-ray Diffraction analysis

Fig. 2.6 XRD patterns of individual raw materials (Q: Quartz)

X-ray Diffraction analyses of individual raw materials used in the fabrication of membranes, i.e., fly ash, quartz and calcium carbonate, were successfully carried out and the results of the same are depicted in Fig. 2.6. Diffractograms show that peaks of calcium carbonate occurred at 2𝜃 values of 23.05, 26.2, 29.35, 35.95, 39.35, 43, 47.5 and 48.45˚; while the peaks of quartz appeared at 2𝜃 values of 20.8, 23.6, 26.6, 36.5, 39.4, 42.75, 50.1, 59.9 and 68.3˚ respectively.

Prior art regarding the X-ray analysis of calcium carbonate reveals the occurrence of peaks at similar positions and the peaks obtained for quartz too matched with standard JCPDS pattern number 46-1045 (JCPDS. 2000), thus verifying the authenticity of the instrument as well as raw materials (Rahman et al., 2013; Thriveni et al., 2014). Peaks corresponding to fly ash appeared at 2𝜃 values of 16.5, 20.95, 26.7 and 50.15˚, thus signifying that fly ash has quartz as its main component.

2.2.1.3 Laser particle size analysis

Fig. 2.7 Laser particle size analysis of raw materials used for membrane fabrication Particle size distribution is utmost important for getting a membrane with good physical and mechanical properties. Finer particles lead to the formation of a compact membrane with very

coarser particles are comparatively larger, which results in a membrane with lesser mechanical strength (Wang et al., 2007). Porosity is also less for membranes fabricated using smaller particle sizes (Norliza et al., 2001). In this work, laser particle size analysis revealed that the raw materials used for fabrication of ceramic membranes, namely fly ash, quartz and calcium carbonate, have d0.5 values of 37.733, 2.459 and 7.356 µm, respectively (Fig. 2.7). As evident from literature, membranes fabricated with raw materials having similar particle size have proven to offer excellent properties (Almandoz et al., 2004; Norliza et al., 2001). Therefore, the raw materials with aforementioned particle sizes were used further for membrane fabrication, looking at these prior arts.

2.2.1.4 X-Ray Fluorescence Spectrometer Analysis

The X-Ray Fluorescence Spectrometer analysis (XRF) of fly ash presented in Table 2.3 reveals that it contains silica as its main constituent along with some quantities of alumina and iron oxide and traces of oxides of manganese, magnesium, calcium, potassium, titanium and phosphorus. Similarly, the XRF analysis of quartz proves that the quartz used mostly comprises of silica, thus signifying the purity of raw materials used. It has been found that the procured data by carrying out XRF analysis of fly ash and quartz appreciably matches with those available in earlier literatures (Ibrahim et al., 2015; Singh and Subramaniam, 2018). Moreover, presence of any harmful heavy metals was not detected in the XRF analysis of fly ash samples.

It is worth mentioning that as the calcium carbonate used for membrane fabrication is having greater than 99% purity, XRF analysis of the same has not been carried out.

Table 2.3 XRF analysis of fly ash and quartz (All values are in wt.%)

Sample SiO2 Al2O3 Fe2O3 MnO MgO CaO K2O TiO2 P2O5

Fly ash 65.39 17.64 5.38 0.068 0.54 1.80 1.50 2.11 0.41 Quartz 86.97 7.59 0.49 0.022 0.03 0.18 0.20 0.02 0.005

2.2.1.5 Energy Dispersive X-ray Analysis

Energy Dispersive X-ray analysis of individual raw materials (Fig. 2.8) helped to get an idea about the various elements present in them. EDX analysis of fly ash revealed the presence of aluminium (Al), silicon (Si), oxygen (O2) as the main elements with little amounts of iron (Fe), magnesium (Mg) and potassium (K). Presence of similar elements in the aforementioned raw materials have already been reported in the previous literature (Längauer et al., 2021). The higher quantity of Si and O2 signifies that quartz is the major component of fly ash, which was also confirmed through XRF and XRD analysis (Ahmaruzzaman, 2010). Moreover, the EDX analysis also reveals that the fly ash used in this study does not contain any heavy metals such as nickel (Ni), cadmium (Cd), which are harmful to human beings. Quartz showed the presence of silicon (Si) and oxygen (O2) with 0.1 wt% Aluminium (Al), thus signifying its high purity with silica as the main constituent. EDX analysis of calcium carbonate (CaCO3) demonstrates the presence of calcium (Ca), carbon (C) and oxygen (O2), thus showing the purity of the raw material used.