(25a) Reutilization of Coal Fly Ash for the Production of Highly Beneficial Products

Coal fly ash (CFA), generated during the combustion of coal in power generation plant, is an industrial waste that continues to increase as a result of the growth in power demand. This large amount of industrial waste can cause environmental and human health problems if it is not treated properly. Research on the potential reuse of CFA has therefore been undertaken intensively worldwide. From those perspectives, this work aims to reduce the problems related to the disposal of CFA by converting them into beneficial products, including zeolite, geopolymer and fertilizer. 

Zeolites: Hydrothermal alkaline reaction was employed for converting CFA to zeolites. Two different types of alkali (i.e. NaOH and KOH) were used as activating agents. Different Si/Al ratio, time and temperature used for the hydrothermal reaction were investigated for their effects on the type and amount of zeolites synthesised. It was found that, by using NaOH as activating agent, a mixture of Na-A and hydroxy-sodalite was mostly formed. On the other hand, a different mixture of merlinoite and perlialite was formed when KOH was used as the activating agent. The obtained zeolites were then employed for dye removal from wastewater and it was found that zeolite obtained from NaOH has the highest adsorption capability of 16.26 mg/g.

Geopolymer: In order to produce CFA-based geopolymer, CFA was firstly treated with the aqueous solution of Na₂SiO₃ and NaOH. Concentration of Na₂SiO₃, SiO₂/Na₂O mole ratio, ash/alkaline solution mass ratio, temperature and curing time were varied to study their effects on heat resistance and compressive strength of geopolymers. It was found that the compressive strength of geopolymer increases with Na₂SiO₃ concentration, temperature and curing time, whereas heat resistance of geopolymer is insignificantly affected by varying those parameters (dry weight loss between 2.5 and 4.9%). Moreover, it was found that geopolymers produced have higher compressive strength (1115 MPa) and better heat resistance as compared to the hardened Ordinary Portland Cement (OPC) (400 MPa).

Fertilizer: CFA consists of all the elements present in soil except organic carbon and nitrogen, making it becomes a valuable raw material for producing fertilizer. In order to produce fertilizer, CFA was mixed with another two components: (i) wood compost, which contains significant amounts of basic nutrients for the plants i.e. carbon, hydrogen oxygen nitrogen and sulfur; and (ii) biochar, which can provide a physical domain for bacteria and fungi, including beneficial mycorrhizal species and also enhance the water-holding capacity of the soil. From the ICP analysis, the fertilizer produced contains significant amounts of useful nutrients for plants, including phosphorus and potassium as primary macronutrients; calcium and magnesium as secondary macronutrients; cobalt, copper, iron, manganese, sodium, and zinc as micronutrients. Moreover, the final product also contains a significant amount of silica (~18.9 wt. %). Last but not least, it was found that the fertilizer obtained can significantly enhance the quality of soil.