The aim of the work was to draw attention to the usefulness of the alkaline thermal activation process with sodium hydroxide in the process of rare earth metal leaching (REE), from fly ash with hydrochloric acid and nitric acid(V). The work is a part of the authors’ own research aimed at optimizing the REE recovery process coming from fly ash from hard coal combustion.

The article contains an assessment of the possibility of leaching rare earth metals (REE) from fly ash originating from the combustion of hard coal in one of the Polish power plants. The process was carried out for various samples consisting of fly ash and sodium hydroxide and for different temperatures and reaction times. The process was carried out for samples consisting of fly ash and sodium hydroxide containing respectively 10, 20 and 30% on NaOH by weight in relation to the weight of fly ash. Homogenization of these mixtures was carried out wet, and then they were baked at 408K, 433K and 473K, for a period of three hours. The mixture thus obtained was ground to a particle size of less than 0.1 mm and washed with hot water to remove excessive NaOH. The solid post-reaction residue was digested in concentrated HCl at 373K for 1 hour at a weight ratio fs/fc of 1:10. The results of chemical analysis and scanning microscopic analysis along with EDS analysis and X-ray analysis were used to characterize the physicochemical properties of the tested material.

The results indicated that REE recovery from fly ash strictly depends on heat treatment temperature with NaOH, and an increase in REE recovery from alkaline-activated fly ash along with increasing the amount of NaOH in relation to fly ash mass.

This paper presents the results of a cost-effectiveness analysis and a cost-benefit analysis for the production of X-type zeolites from fly ash.
Positive results of the laboratory tests on the quality of zeolites derived from fly ash initiated a cost analysis on the production of this materials on an industrial scale. The cost-effectiveness analysis was conducted using the dynamic generation cost indicator (DGC). The calculated DGC expresses the technical manufacturing cost of 1 Mg of synthetic zeolites. Whereas the cost-benefit analysis (CBA) was completed using the economic net present value (ENP V) and the economic internal rate of return (EIRR ) indicators.
The calculated unit technical cost of producing 1 Mg of zeolites using an installation consisting of five reactors with a capacity of 25 m3 each is 211 EUR and is lower than the current market price of this product, including transportation costs. This proves the financial viability of the investment. The calculations of the economic efficiency of the installation (CBA method) show that it is fully economically viable to operate and use the products from a social point of view.