Modification of fly ash produced in the process of burning fossil fuels - granulation method

The low grain size of fly ashes from the combustion of fossil fuels creates a number of ecological problems both during storage and transport, and often makes their re-use more difficult. One way to avoid this problem is to compact these materials in the granulation process. The granulation process generally requires the use of different binders, which are divided into matrix, film, and chemical type binders. Depending on the chemical composition of the fly ash and the type of the binder used, the obtained aggregate forms (granulates) find numerous applications, e.g. in the production of adhesives, zeolites and/or concrete, in manufacture of building materials, and in aggregates for road works or for ground leveling. In many applications, especially in road engineering works or in ground leveling, the basic requirements, next to the compressive strength of the granules obtained, are the leachability of sulfate ions and the pH-value of the eluates. The paper presents a research study investigating the granulation process of fly ashes produced by the combustion of hard and brown coals burned in both pulverized and fluidized furnaces, with the application of the following binders: water, calcium sulfate hemihydrate, and mixture of calcium sulfate hemihydrate and calcium hydroxide. For the resulting granulates, tests were conducted to determine compressive strength, SO₂ sorption capacity, the pH-value, and the content of sulfate ions in the eluate before and after sorption tests. The obtained results show that the granulation process mostly decreases leachability of sulfate ions and the pH-value of the eluates while simultaneously providing adequate compressive strength of the obtained compacted forms (granulates).