RECYCLING OF WASTE FLY ASH FORMED DURING COMBUSTION OF HARD COAL FOR USE IN MOULDING MIXTURES MANUFACTURE

The article is a compilation of the most important results obtained during execution of own research project regarding the possibility of using fly ash formed during combustion of hard coal as a base granular material in moulding mixtures and as a reinforcing phase in cast aluminium alloy-based composite materials (ALFA composites). The chemical composition of the domestic fly ash, characterised by the greatest stability of composition (the content of the main chemical compounds is the following - wt. %: SiO₂ = 81,66%, Al₂O₃ = 6,69%, Fe₂O₃ = 4,37%, CaO = 4,29%, MgO = 1,57%), was determined together with the composition of respective phases (quartz, mullite, larnite) and some typical phase transformation-related values of temperature (melting point = 1435°C, flow temperature = 1494°C, dimensional stability up to 1230°C). The effect of washing process and magnetic separation of fly ash on changes in the value of its helium density, specific surface, and chemical and phase composition was discussed. The process of washing decreases significantly the content of sodium (by about 20 wt. %), calcium (by about 14 wt. %) and magnesium (by about 11 wt. %), removed mainly from the glassy phase. Applied effectively, this process also causes a decrease in the value of the fly ash true density and specific surface. Magnetic separation of fly ash removes the magnetic particles in an amount of about 26% in convertion to Fe₂O₃. A decrease in the content of aluminium, sodium, potassium and calcium usually accompanies this effect. The removal of magnetic phase is accompanied by a drop in the value of true density and specific surface. The article presents the results of investigations regarding the fly ash agglomeration behaviour when conducting the process in a disc granulator and in a pressure granulator. It has been stated that neither of these two methods of agglomeration enables obtaining a truly compact and solid material. The obtained agglomerates are characterised by a porous structure, the agglomerates produced in disc granulators having definitely larger pores. The examined fly ash was used as a base granular material for the manufacture of moulding mixtures used in the Shaw Process, in the process of thermo-setting mixtures (with sodium silicate as a binder), and in mixtures for the CO₂ process. The basic technological properties of the manufactured moulding mixtures were determined along with their thermal resistance and residual strength. The applicability of these mixtures in production of foundry moulds and cores for elements cast from metal alloys at temperatures of up to about 1000°C was stated. The effect of the fly ash-based moulding mixture on the chemical composition and morphology of a phase boundary in aluminium alloy casting was analysed. The fly ash used as a base material in foundry mixture was observed to have no effect on changes in casting microstructure and morphology. Investigations of the possibility to use fly ash as a reinforcing phase in ALFA composites were preceded by investigations of the physico-chemical properties of fly ash used as a base material in foundry mixtures after pouring of these mixtures with molten metal. In spite of high reactivity of molten aluminium and its alloys in contact with the examined material, no interaction has been observed to take place. After a heat treatment of the ready, fly ash-containing, composite, where the said fly ash was acting as a reinforcing phase and was introduced in an amount of 5 wt. %, as well as a heat treatment of the base material, i.e. aluminium alloy samples, it was observed that the yield strength R_P02 and tensile strength R_m of the composite increased by about 10%, compared to analogical values obtained for the base, fly ash-free, aluminium alloy. The value of the composite elongation A₅ decreased by about 60%, while the value of HBW 5/250 raised by about 55%, compared to analogical values obtained for the base material.

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