Understanding effect of process variables on extraction of silica (SiO2) from Coal Fly Ash (CFA) and Pulp Waste Ash (PWA)

Abstract

The disposal of coal fly ash (CFA) and pulp waste (PW) is an environmental problem due to the large amount of it generated. Silica (SiO2) is contained in CFA and PW is 57.9% and 33.04%, respectively. SiO2 is used in the manufacturing of various silica products such as; silica bricks, silica glasses, water extender (Na2SiO3), etc. The commercial production of SiO2 is highly energy-intensive, manufactured by smelting quartz at a temperature of 1400 ºC. While several studies have been done on SiO2 extraction from CFA, none have been done on SiO2 extraction from pulp waste ash (PWA). Thus, there are no comparative studies of SiO2 extraction between CFA and PWA. Although some studies studied the optimization of SiO2 extraction from CFA (Matlob et al., 2012), none studied the optimization of the extraction of silica by studying the four variables, namely; the NaOH concentration, extraction time, NaOH solution to waste ratio, and extraction time. Thus, there is a need of studying the optimization of coal fly ash from coal fly ash. In this study the extraction of SiO2 using sol-gel extraction of silica from CFA and PWA is investigated, as well as the optimization of SiO2 extraction from CFA. To extract SiO2, PW was first calcinated at 650 °C to form PWA. The two wastes materials (CFA and PWA) were then characterized using; X-Ray Fluorescence (XRF), Scanning Electron Microscopy (SEM) and Thermogravimetric Analysis (TGA). SiO2 was extracted from CFA and PWA by treating them with sodium hydroxide (NaOH) and hydrochloric acid (HCl). The extracted SiO2 was analyzed using; TGA and SEM. The extraction of SiO2 from CFA was then optimized by using the response surface methodology (RSM) approach. Furthermore, to determine whether or not the extraction of SiO2 from CFA can be explained with a kinetic model, a kinetic study was done. To perform a kinetic study of SiO2 extraction, the optimized extraction conditions were used. The results showed that when SiO2 is extracted at 2 M NaOH, 3.5 mL/g NaOH solution to waste ratio, for 45 min at 70 ºC in both wastes materials, a silica yield of 10.98% and 37.59% in CFA and PWA is obtained, respectively. The model describing SiO2 extraction from CFA was very satisfactory as it gave a p and R2 value of 0.0001 and 0.8804, respectively. A p value < 0.05 indicates that there is less than 5% chance that the model the values represent could be due to noise. An R2 value close to 1 shows that there is good correlation between the observed and predicted values for SiO2 extraction from CFA. About 88.03% of the data can be explained and analyzed by the model. In addition, results of the cross-validation of the model showed an error of about 1%, emphasizing further the validity of the proposed model for SiO2 extraction from CFA. The results also showed that the optimum iii conditions found for SiO2 extraction from CFA using numerical optimization by desirability function were; 86.69 ºC, 3 M NaOH, 5 mL/g NaOH solution to waste material ratio and 30 min for SiO2 yield of 31.40%. The actual SiO2 yield for CFA was 31.06%. The kinetic study results showed that the process of the extraction of SiO2 from CFA followed a first-order reaction kinetics. Thus, the encouraging results obtained from this thesis could provide a platform for the beneficiation of these waste material. This would not only lower the amount of PW and CFA that goes into landfills but would subsequently reduce the costs that go into managing the landfills and the energy that is used to manufacture quartz.