Introduction and Literature Review
CHAPTER 1 Introduction and Literature Review
After presenting a brief overview of fly ash materials and ceramic membranes in sections 1.1 and 1.2, respectively, this chapter addresses the available prior art in the fields of ceramic support fabrication, composite membrane fabrication, and ceramic membrane based oil-in- water emulsion separation studies and applications pertaining to response surface methodology for membrane separation processes in section 1.3. Thereby, possible scope for further research, objectives of the thesis and its organization are being presented in sections 1.4, 1.5 and 1.6, respectively.
1.1 Fly ash material
1.1.1 History
Electrical power generating produce millions of tons of fly ash and related by-products by burning coal. With increasing electricity demand and with abundance of coal in nature, fly ash generation is increasing enormously. According to Ahmaruzzaman (2010), only 16% of fly ash is utilized by existing techniques. Out of this, the cement industry utilizes about 30%
of fly ash and the remaining portion is disposed to landfill and ash ponds. Due to the existence of transition metal oxides, fly ash is known to provide adverse environmental effect. Among various viable alternatives for fly ash utilization, the fabrication of efficient ceramic membranes is an important alternative, owing to the utilization of fly ash to further address potential adverse environmental effects caused by discharged wastewater such as oil- in-water emulsions.
Listed as sixth largest in terms of electrical power generation, India produces significant amount of fly ash. While majority of fly ash is being used for cement manufacture, roadways
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and brick manufacture, large quantities of fly ash is stored in ponds. Currently, only 50% of the fly ash is used in the annual production capacity of 160 million tones in India.
Fly ash is abrasive, refractory and also alkaline. It consists of macronutrients (P, K, Ca, Mg) and micronutrients (Zn, Fe, Cu, Mn, B and Mo) and can be adopted for plant growth. Lime binding capacity of the fly ash enables it for cement manufacturing, concrete building materials and other relevant products. Chemical constitution of fly ash corresponds to significant proportion of silica (60-65%) followed with alumina (25-30%), magnetite and Fe2O3 (6-15%). Due to this, Iyer and Scott (2011) have opined that it can be used for the synthesis of zeolite, alum and precipitated silica. With appropriate physiochemical properties (bulk density, particle size, porosity, water holding capacity and active surface area), fly ash has been suggested to be adsorbent (Blissett and Rowson, 2012). Further, agricultural and engineering materials can be also developed from fly ash.
1.1.2 Physical Properties
Table 1.1 presents the physical properties of fly ash. Typically, fly ash contains spherical solid or hollow amorphous powder particles. Further, fly ash also contains angular shaped particles of carbonaceous materials. The particle size distribution is about 1-150µm (Ilic et al., 2003). Specific gravity and specific surface area vary from 2.1 – 3.0 and 170 – 1000 m2/kg, respectively. Depending upon unburned carbon content, the colour of fly ash varies from tan to black.
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Table 1.1: Physical properties of fly ash
Properties Range
Specific Gravity 2.1-3.0 Specific surface area (m2/kg) 170-1000
Color Grey-black
Particle size (µm) 1-150
1.1.3 Chemical Properties
Depending upon the type of coal burnt and handling/storage procedures, fly ash chemical properties vary significantly. Fundamentally, fly ash classification is based on the type of coal (anthracite, bituminous, sub-bituminous and lignite) used during combustion (Ahmaruzzaman, 2010; Iyer and Scott, 2011; Blissett and Rowson, 2012). While bituminous coal based fly ash consists of silica, alumina, iron oxide, calcium and carbon, the fly ash obtained from sub-bituminous coal consists of higher constitution of magnesium and calcium oxide and lower amounts of silica, iron oxide and carbon.
Table 1.2: Classification of fly ash by ASTM standards Class SiO2+Al2O3+Fe2O3
(%)
SO3
(%)
Ca (%)
Moisture (%)
Loss of Ignition (%)
C 50-70 <5 30-40 <3 <6
F >70 1-12 <12
The American Society for Testing Materials (ASTM) classification (C618) of fly ash refers to class F and C categories (Table 1.2). This is based on the constitution of alumina, silica, iron oxide and lime in fly ash. With low lime, class F fly ash possesses 70% overall weight percentage of alumina, silica, and iron oxide. However, for class C fly ash, the overall weight
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percentage of alumina, silica, and iron oxides is about 50 – 70% along with abundance of lime content. Generally, class C fly ash is obtained by low rank coals i.e. lignite and sub- bituminous coal with cementations properties. On the other hand, class F fly ash is obtained from the combustion of higher rank pozzolanic coals. The main differences between these ashes are in calcium, alumina, silica and iron content. While calcium content in class F varies from 1 – 12%, it varies from 30 – 40 % in class C. Another variation in the composition corresponds to alkali (sodium and potassium) and sulfate content. This is higher for class C fly ash but not class F fly ash. European standards based fly ash constitution is being presented in Table 1.3.
Table 1.3: Classification of fly ash based on European standards EN 450-1
Class SiO2+Al2O3+Fe2O3 (%) SO3 (%) Reactive Silica (%) LOI (%)
A >70 <3 >25 <5
B - - - 2-7
C - - - 4-9
The general classification of coal fly ash typically ignores the mineralogy aspects of the coal.
Vassilev and Vassileva (2007) indicated that the coal fly ash classification does not systematic scientific basis, despite being addressed from industrial coal utilization perspective. Thereby, European coal fly ash classification system has been developed in which the bulk oxides have been grouped together to classify fly ash (shown in Table 1.4).
Fly ash has been analyzed to have significant variations in available phases such as kaolinite, mullite, illite and siderate that includes calcite, pyrite and hematite. Low calcium ash has
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quartz and mullite as major constituents. On the other hand, fly ash with high calcium content consists quartz, Tricalcium Aluminate (C3A), Calcium Silicate (CS) and Tetracalcium Alumino Silicate (C4AS) (Bruce and Mathew, 2004). Fly ash has significant amount of glass, which is typically explored to develop road construction materials by facilitating reaction with lime (Pozzolanic reaction). Fly ash with its promising features of adhesivity, glass content, crystalline content and strength is eventually used in cement production and concrete admix.
Table 1.4: Classification of coal fly ash on chemical composition
Class SiO2 + Al2O3 + K2O + TiO2 + P2O5 (%)
CaO + MgO + SO3 +