Sintering Lignite Fly and Bottom Ashes via Two-step versus Conventional Process

Lignite combustion class-C (highly-calcareous) fly ash and bottom ash were sintered employing two-step sintering (TSS) and compared to conventionally sintered ones. TSS sintering is a new promising approach mainly used to obtain effectively densified ceramics. This alternative process is generally characterized by the absence of the final stage of grain growth occurring upon conventional sintering and by the development of nanograin microstructure. The ceramic microstructures successfully obtained in the present research were characterized by means of X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray analysis as well as by density measurements. The effectiveness of the solidification process was thoroughly studied, and the specific microstructural features attained are compared between each other and evaluated in relation to the sintering method applied. The results show that the valorization of lignite calcareous ashes into ceramic materials is feasible through different sintering techniques.


Introduction
Approximately 500 million tons of coal fly ash and 100 million tons of lignite fly ash are produced annually worldwide, and the particular amount is predicted to increase in the near future [1,2].Ash is formed by combustion of coal and especially of low calorific value lignite, in the respective power stations, as a waste product, specifically: a) fly ash, about 80% of the total ash amount, entrained in the gas flow and collected by electrostatic precipitators; b) bottom ash, the remaining, a dark gray, granular, and porous material.Nowadays, special emphasis should be placed on resources optimization, to minimize uncontrolled waste disposal [3,4].Especially, management of fly and bottom ashes is of great environmental concern, as only a limited amounts of ashes are currently used, while the rest is landfilled, a situation that will possibly cause severe long-term environmental effects [5][6][7][8].Predicting the fate of the pollutants and the risk they pose to human and ecological receptors has been attempted through extensive study of the physicochemical processes and interrelationships of environmental contaminants including heavy metals present in the soil and subsurface [9,10].Nevertheless, the chemical, mineralogical and morphological properties of these by-products render their valorization as secondary raw materials into value-added products, a challenge with important technological, environmental and economic aspects [11][12][13][14][15].
In particular, the utilization of lignite combustion ashes for ceramics production represents a significant research area.Conventional solid-state sintering is a generally established manufacturing technique for industrial ceramics, tiles, etc.For the development of fired ceramics, lignite ashes appear secondary attractive candidate materials, where silica and alumina are among their main components.However, the incorporation of fly ash in the clayey raw materials for brick production has rarely exceeded a 30 wt.%, mainly due to a certain incompatibility of the ash with the clay mixtures, especially with regard to a reduced plasticity at high levels of ash leading to extrusion difficulties [16].Recently, research has been undertaken on the development of ceramics starting from 100% various Class-F ashes through conventional sintering procedures [17,18].
In the last few years, a new two-step method (TSS) was proposed for sintering powdery materials to produce dense and fine ceramic microstructures without detrimental final-stage grain growth, thus leading to improved mechanical properties [19].This sintering method uses two steps in the heating schedule: the sample is first heated to a higher temperature to achieve an intermediate but sufficiently high starting density, then cooled down and held at a lower temperature to approach full densities or even just for the pore size control.The feasibility of densification without grain growth relies on the suppression of grain-boundary migration while keeping grain-boundary diffusion active.The two-step sintering procedure appears an important milestone for modern technical ceramics, and its feasibility has been verified in various ceramic systems [20][21][22][23][24][25][26][27][28][29].The efficiency of this method is considered more pronounced in ceramics with crystalline phases of higher symmetry, whereas its applicability is questioned only when the activation energy for consolidation is higher than that for grain growth.This new process provides sufficient motivation to investigate its potential for the treatment and valorization of industrial by-products in ceramics development, as an efficient alternative to currently employed traditional heating procedures.
In the present research, compacts prepared from lignite combustion class-C fly ash and bottom ash mixtures originating from Region of Western Macedonia (Northern Greece) were heated applying the alternative TSS sintering procedure and are compared to similar specimens that were conventionally sintered.The valorization of such combustion byproducts as the raw materials for the development of sintered ceramics would not only provide environmental but also economic advantages from to the low cost of these industrial by-products and from possible energy savings during the ash mixture firing due to the noticeable carbon content of fly ash and especially of bottom ash.Moreover, the rich-in-Ca ash composition can possibly be expected to yield an interesting mineralogy in the sintered materials, while the Ca-bearing phases may also act as a flux enabling melting to begin at lower temperatures, thus using less energy.Furthermore, the low thermal conductivity of the ashes, as they are mainly consisted of hollow sphere-shaped particles (cenospheres), should also influence the sintering result.In previous study, ashes of similar composition have been tested by the authors with microwave sintering [30].By heating the lignite ashes that were under investigation along with employing the two-step method, interesting solidification processes, microstructure and properties can be attained.The sintering results are evaluated as a function of the two-step sintering program employed and the ash specimen composition.

Experimental procedure 2.1 Materials
The fly ash (FA) utilized as a secondary raw material in the present research, a fine powder, was obtained by the electrostatic precipitation of dust-like particles from the flue gases of a lignite combustion power plant situated in Northern Greece (Region of Western Macedonia where the main lignite deposits of the country are located).The bottom ash (BA) that was used, a granular material much coarser than FA and also formed during lignite firing, was removed from the bottom of dry boilers of the same power plant.The characterization results for these ashes are given in Tabs.I and II.It can be seen that, particularly FA, is characterized by high % CaO content, similarly to other fly ashes from Northern Greece power units belonging to Class-C ashes (ASTM C 618).BA is less abundant in Ca, but contains higher residual (unburned) carbon (≈5%).SEM micrographs of FA and BA are provided in Fig. 1.

Preparation of ash compacts
FA/BA mixtures were prepared.Simple fabrication techniques were applied for the compacts preparation: the mixtures were uniaxially cold pressed using a stainless steel die using a hydraulic press (Specac, 15011) to form 13 mm diameter disc-shaped green specimens.The specimen green density and strength were evaluated and the compaction pressure was optimized, so that the pressed compacts had sufficient green density and strength to ensure safe handling and submission to heating.

Conventional sintering
Upon conventional heating, slow heating rates are normally selected to reduce abrupt thermal gradient that can possibly lead to process-induced stresses.In the present study, all specimens were sintered in a laboratory chamber programmable furnace (Thermoconcept, ΚL06/13) from room temperature up to 1150 o C with a heating rate of 10 o C/min, and then held at the maximum sintering temperature for 2h.Finally, they were gradually cooled to ambient temperature.The chosen sintering conditions were optimized on the basis of preliminary experimental trials.

Two-step sintering (TSS)
A temperature slightly lower than the melting point of the ashes was selected for the first sintering step (T1=1150 o C).As soon as T1 was attained, the samples were rapidly cooled down in the furnace, and held at a lower temperature, this of the second sintering step (T2=950 o C).Two different sintering programs where tested: TSS1, where the samples remained at T2 for 2h, and TSS1, where the samples remained at T2 for 4h (Fig. 2).In order to evaluate the intermediate sintering result at the end of the first sintering step, a series of specimens were sintered only up to 1150 o C and then taken out of the furnace.Finally, the specimens were gradually cooled to ambient temperature in the furnace.

Characterization of sintered ashes
Phase characterization of green and two-step sintered specimens was realized by Xray diffraction device (XRD) (Siemens, Diffractometer D-5000).The microstructures produced were studied using Scanning Electron Microscope (SEM -Jeol, JSM-6400).Shrinkage of the samples was calculated as the volume change (%) obtained upon sintering.Apparent density was measured according to the Archimedes principle by means of a specific apparatus (Shimadzu, SMK401-ΑUW220V).In order to determine water absorption capacity, sintered specimens were first oven dried to constant weight, cooled to room temperature, and weighed (W1).Then, the specimens were immersed in distilled water for 24h, and subsequently weighed again (W2), after the excess water removing from their surfaces by wiping with a damp cloth.The water absorption was calculated as the percentage increase in mass of oven-dried mass.Vickers microhardness was measured with a load of 200g and a dwell time of 15s (Shimadzu, HMV-2T).In order to enable reliable comparisons, mean microhardness values were calculated over five valid indentations per specimen.

Results and discussion
Photographs of ash mixtures (FA/BA:1/1) sintered through the two-step method, TSS1 (a) and TSS2 (b), are provided in Fig. 3.It can be seen that successfully consolidated integral and similar between them earth-yellowish specimens are obtained.
The main mineral phases present in the green specimens prepared of ash mixture (FA/BA:1/1), as well as in those conventionally sintered at 1150 o C and in the two-step sintered ones (TSS1 and TSS2), as determined by means of XRD, are shown in Fig. 4.  Obviously, the intensity of the peak associated with lime (CaO) predominates in the diffractogram of the green specimen (Fig. 4, A).This intense presence of lime in the ashes is mainly due to the high percentage of limestone (CaCO 3 ) in the feedcoal (lignite) of the power unit.Actually, an interesting ceramic microstructure and more complex than that of the raw materials is revealed, whose major crystalline phases are kyanite (Al 2 SiO 5 ), magnetite (Fe,Mg)(Al,Cr,Fe,Ti) 2 O 4 , quartz (SiO 2 ), gehlenite (2CaO .Al 2 O 3 .SiO 2 ), and calcite (CaCO 3 ).The result of the densification process in the sintered specimens can be evaluated upon microstructural observation using SEM analysis (Fig. 5).From Fig. 5, acceptable ceramic microstructures are obtained for all sintered specimens and the development of sintering necks that bind the powder particles can be observed.The efficient ash particle packing achieved in the green compacts, as it can be seen from Fig. 5a, should contribute to a successful sintering result.Higher densification degrees are attained after conventionally sintering (Fig. 5b) of the ash mixtures.Such microstructures would be potentially preferred for structural applications demanding reasonably dense ceramic materials combined with the substantial energy and production cost savings from the use of MW technology.Finer microstuctures are obtained when employing the TSS sintering (Fig. 5c).However, Fig. 5c also reveals an interconnected porosity in the TSS-sintered specimens, which exhibits no preferential orientation or shape.Therefore, longer holding times should probably be considered for the second sintering step of the TSS-heating program, in case that higher densification was demanded.On the other hand, such porous sintered-ceramic microstructures offer various advantages for specific applications, including a possible water-purifier performance due to bacteria immobilization properties in their pores, tailored insulation behavior as an alternative solution to reduce the energy consumption of buildings, and thermal shock resistance due to an improved expansion tolerance and a certain decrease in the modulus of elasticity.Moreover, from the economic point of view, a cost reduction is expected by producing objects of a reduced relative density.Shrinkage of the sintered ash specimens is lower than 5%.Density lies in the range of 2.3-2.4 g .cm -3 , whereas mean Vickers microhardness attains 120 HV.Water absorptivity varies up to 20 %, strongly depending on the existence of open and interconnected pores that can be verified from the SEM analyses.Moreover, rough pore wall surfaces are frequently observed in the micrographs, indicating a high specific surface area facilitating water adsorption on the pore wall (following the penetration and inhibition of water in the open pores).Precise preparation of porous ceramics having various pore size and enhanced porosity with two-step sintering program from Al 2 O 3 powder compact were also reported from other researchers [31].From a technological point view, such microstructures may be of interest for porous ceramic applications.

Conclusions
The results show that the valorization of lignite combustion by-products such as calcareous fly and bottom ash mixtures in synthesizing acceptable ceramic microstructures is feasible through different sintering techniques.The highly-calcareous nature of fly ash and the residual carbon of bottom ash influence the sintering results but do not hinder the synthesis of ceramics via the alternative methods used.
More pronounced crystallinity is obtained from lignite high-Ca fly ash and bottom ash employing two-step sintering processes.Also, finer microstuctures are obtained when employing the TSS sintering, however also exhibiting an interconnected porosity.In this case, the potential advantages of porous ceramic microstructures for specific applications combined with a cost reduction expected by producing objects of a reduced relative density should be taken into consideration.Moreover, sintering time is kept at lower temperatures upon TSS sintering, thus leading to possible energy consumption reductions.Further investigation of the heating conditions would enable a tailoring of the ceramic microstructures to meet the needs for specific applications.