The material used for testing was fly ash from pulverized coal boilers, taken from a power plant located in southern Poland. The power plant utilizes a blend of energy coal, classified according to ISO 11760 (2005) as subbituminous coals. The coal contained 20.7% ash, and its calorific value was 21300 kJ/kg. The ash sample was collected and prepared in accordance with EN 14899 (2005).
The chemical and phase composition of the fly ash was determined on the basis of tests carried out using:
X-ray fluorescence spectrometer (XRF),
X-ray diffractometer (XRD),
scanning electron microscope (SEM / EDS).
The main chemical components of the ash sample were determined using the XRF X-ray fluorescence spectrometry using a Philips PW 2400 spectrometer and SuperQ software. The results are expressed as a percentage of the main oxides of the elements.
X-ray tests have been carried out using X-ray powder diffractometry with the help of a PANalytical Empyrean diffractometer with Co Kα radiation. The quantitative content of individual phase components was determined using the Rietveld method.
The ash characteristics, determined using the SEM scanning electron microscope, included the determination of the morphology and size of grains and the elemental composition based on the observation of grain surface and X-ray microanalysis. The SEM / EDS analysis was performed with the help of the Hitachi SEM SU3500 variable pressure scanning electron microscope, with the use of an X-ray spectrometer with energy dispersion of the EDD UltraDry from Thermo Scientific NORAN System 7. The BSE (Backscattered Electron) detector was used for analysis because of its ability to illustrate contrast in the composition of multiphase samples.
The size distribution of ash was determined using a wet sieving method according to ISO 1953 (1994). The measure of the amount of unburned coal was loss on ignition (LOI). The amount of unburned coal (LOI) was determined in accordance with the procedure described in EN 196-2 (1994) at 900oC.
The float and sink analysis was performed according methodology of standard ISO 7936 (1992). Organic liquids with densities of 1.4 to 2.0 g/cm3, every 0.1 g/cm3 were used for the tests.
Ash samples with a grain size above 0.1 mm were used to study the recovery of unburned coal from ash. The distribution of the unburned coal content, in relation to the grain size in the total ash sample (Fig. 1), and the technical requirements of some separators, was the basis for this selection.
The unburned coal extraction with the wet gravitational method was carried out in two stages. In the first stage, cenospheres were separated which float on the surface of the suspension. In the second stage, the technique of separation in a rising current of water was used. The methodology of the above technique was described by Bialecka et al. (2020).
Two different gravitational separation techniques were also applied using the dry method. One of them was the traditional fluid bed method, the other was a fluid bed method with vibrations and a classifier.
Separation fluid bed method was carried out in a laboratory fluid bed separator, which was a properly instrumented quartz tube with a diameter of 100 mm and a height of 500 mm. Quartz sand with a grain size of 0.3 - 0.385 mm was used as a fluid-forming agent. The separation ash was gradually dosed in portions to ensure the required pressure drops over the deposit. After the end of the ash dosing and the setting of the pressure over the deposit at a stable level, the air flow velocity was reduced in order to arrange the grains in characteristic layers. In the upper layer of the deposit, unburned coal grains formed, and in the lower layer, ash grains and a layer of quartz sand grains formed.
Fluid bed separation method with vibrations was carried out in a device consisting of a separator with a flat nozzle bottom and a classifier with vertical air flow. This is a combination of the classification method with vertical air flow and elutriation. Furthermore, vibrations were used to loosen the grains at the fluid deposit forming stage.
Electrostatic enrichment tests were carried out using the Boxmag-Rapid Limited device from England. The main variable was the voltage between the electrodes regulated within 10-25 kV. Increasing the voltage between the electrodes to above 25 kV caused a spark and prevented separation.
The separation products of individual tests were dried (in the case of wet methods), the losses on ignition (LOI) were determined as a measure of unburned coal content, and a mass balance was prepared. The recovery of unburned coal in the separation process products were calculated from the dependence:
Where: ε - unburned coal recovery ,
β - loss on ignition value in the product
α - loss on ignition value in the enrichment feed,
γ - product weight yield.