Hi everyone!
After I completed the sintering reaction to reduce fly ash at 1500oC, the XRD pattern shows the SiO2 (quartz) phase in the raw material remains unchanged and did not transform into cristobalite, rather the SiO2 component of the mullite. However, according to the SiO2 phase diagram, quartz is supposed to change at a temperature ~> 573oC at least to another polymorph.
Could someone offer reasons/explanations for this?
Thank you
Dear Joshua Olatunji Ayeni
Maybe your furnace has not reached the target temperature or your XRD readings maybe have to be re-noted again, we can not make a phase diagram different from the typical phase diagram in textbooks for more ... can you please tell me the type of silica (ore) and the type of furnace you used it in sintering SiO2 after you check if the furnace work regularly ... I will see this case deeply ...best regards
Dear Prof Raghd Muhi Al-Deen Jassim , thank you for your response.
I used Fly Ash with 42% SiO2 composition as the raw material, and C as the reducing agent
The furnace description:
Name: ZY 1700oC, Luoyang Branch Furnace Ind. Co. Ltd. China
Tube: Corundum
Model KJ-60-17
Heating Element: MoSi2
Thank you
The process of mixing the alumina- and silica-containing reactants determines the mechanism of mullite formation. It's also related to the temperature at which the mullite-forming reaction occurs (mullitisation temperature). Temperatures for mullitisation have been stated to vary by hundreds of degrees Celsius depending on the synthesis process used.
Fly ash is an aluminosilicate and Mullite (3Al2O3 ·2SiO2 ) is the only stable phase in the Al2O3 -SiO2 system and is used in numerous applications owing to its high-temperature chemical and mechanical stabilities. As a result, fly ash may be a cost-effective way to make mullite.
The main oxide components, based on composition, are Al2O3 and SiO2, which result from the fact that clay and sand are the primary mineral phases of coal. Although SiO2 is the most abundant component of glassy spherical particles, undissolved -quartz (SiO2) is the most common crystalline phase.
Mullite (3Al2O32SiO2) is another important crystalline species that forms during coal combustion, with lower levels of magnetite (Fe3O4) and/or hematite (Fe2O3). The presence of mullite in fly ash means that these fly ashes could be used as sources of mullite-based materials.
Mullite's high melting point of 1850 C, as seen in the Al2O3-SiO2 phase diagram, suggests high-temperature applications. Mullite also has the advantage of being stable in both oxidizing and reducing environments.
Raghd Muhi Al-Deen Jassim Prof, Please I don't get your explanation. The question is why does SiO2 still exist after carbothermal reduction of fly ash at 1500C for 5h?