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Efforts directed to reduction of greenhouse gas emissions have led to the introduction of new firing/co-firing technologies and alternative fuels in the coal-based power industry. This has resulted in the formation of combustion products with new properties that can affect the reuse of these wastes and/or pose a hazard to the environment. One of the power-plant fly ash (FA) reuse options is its application as a solidifying dense mixture with water for backfilling mine workings or in engineering constructions. In this comparative study, geotechnical properties of three groups of FA were evaluated: (i) weathered and freshly generated ash from hard coal combustion in conventional pulverized coal boilers without (C-PCA, C-PFA) and with selective non-catalytic reduction installations for NO x reduction (NC-PFA); (ii) FA from hard coal co-firing with alternative fuels: off gases (GC-PFA) or biomass (BC-PFA) in pulverized coal boilers; (iii) FA from coal (C-FFA) or biomass combustion (B-FFA) in fluidized-bed boilers. The transportability, bonding and solidification properties, uniaxial compression strength, and rewetting of dense mixtures were evaluated by measurements of volumetric density, fluidity, water retention capacity, bonding time, solidification time, uniaxial compression test, and slakeability at the background of the FA chemical composition. Calcareous C-FFA > B-FFA displayed the best geotechnical properties. Other materials showed poorer geotechnical properties than FFA and could be aligned in the order BC-PFA > GC-PFA > NC-PFA > C-PFA > C-PCA.

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This paper presents the results of the technological and economic analysis of mine water treatment systems before their discharge into the environment. The following analysis enabled us to determine the profitability of the investment, taking into account the TSS (total suspended solids) concentration in mine water. The simulation results showed that it is economically profitable to apply a water treatment system if natural sedimentation carried out in underground mine water passages, or in sedimentation tanks located on the ground, is ineffective for TSS removal. Economic and financial parameters allow us to conclude that all analyzed variants of the application of a pre-treatment system are characterized by high economic effectiveness. This mainly results from the high profitability of an analyzed investment, comparatively low capital expenditure, and present low market percentage rates. The most profitable variant (TSS concentration is 1000 mg/dm3) brings significant economic indicators, i.e., high NPV–Net Present Value (100 319 270.28 PLN), a high NPVR–Net Present Value Ratio (8.96 PLN/PLN), and a short discount payback period (1 year 236.6 days). A high internal rate of return (157.8%) for this variant reduces the risk of losing profitability in a situation of growing capital costs in the monetary market.

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