Effect of thermal processing on the characteristics of incineration fly ash

Figures & data

Table 1. Chemical compositions of fly ash and its products for the main oxides, total carbon, total sulfur, and total halides (wt%)

Analyte	Fly Ash	Glass-Ceramics from Bulk Glass 900 °C	Glass-Ceramics from Powdered Glass 900 °C	Sintered Fly Ash 1050 °C
SiO2	21.06	31.27	30.56	24.99
Al2O3	7.49	13.46	13.57	8.96
Fe2O3	2.72	4.55	4.75	3.31
MgO	2.68	3.91	3.93	3.25
CaO	22.96	32.32	32.02	27.54
Na2O	7.04	2.48	2.25	4.12
K2O	6.26	0.61	0.51	2.87
TiO2	1.96	2.95	3.61	2.36
P2O5	1.59	2.31	2.24	1.85
MnO	0.15	0.23	0.24	0.18
Cr2O3	0.194	0.283	0.436	0.243
Total halides	16.86	1.4	1.5	12.35
Total carbon	1.11	0.04	0.06	0.11
Total sulfur	3.20	0.13	0.10	4.84

Table 2. Contents of selected minor elements in fly ash and its products, dry basis (mg/kg)

		Glass-Ceramics from Bulk Glass	Glass-Ceramics from Powdered Glass	Sintered Fly Ash
Element	Fly Ash	900 °C	900 °C	1050 °C
Ba	1993	1856	1811	2202
Cs	9.7	0.2	0.2	0.4
Ga	12.2	20.9	20.6	14.8
Rb	115.0	7.0	5.5	32.1
Sn	1377	2156	2156	1656
Sr	350.0	442.8	432.7	429.2
Zr	140.6	207.8	204.3	161.6
Eu	9.02	14.73	14.04	10.07
Gd	2.15	3.52	3.55	2.79
Tb	10.38	16.61	15.96	12.28
Mo	30.4	12.6	57.7	31.5
Cu	1376.9	433.6	434.7	299.0
Pb	5977.4	2056.3	2120.5	4235
Zn	>10000	>10000	>10000	>10000
Ni	85.2	165.1	580.8	48.7
As	22.1	13.1	16.6	34.4
Cd	466.9	86.2	74.0	20.3
Sb	583.5	192.4	303.1	337.2
Bi	29.8	12.1	9.5	10.0
Hg	0.64	<0.01	0.01	<0.01
Se	9.4	1.6	0.8	<0.5

Figure 1. Backscattered electron image of the surface of a fly ash particle.

Table 3. The phases in samples as determined using XRD and EDS

Sample	Phase Composition
Fly ash	Halite NaCl, silvite KCl, α-quartz (α-SiO2), CaSO4, PbSO4, CaO and Fe2O3
Bulk glass	Amorphous matrix, small fractions of spinel (MgO·(Al, Cr, Fe)2O3), ZnAl2O4)
Glass-ceramics from bulk glass (900 °C)	Mg-gehlenite Ca2(Mg0.25Al0.75)Si1.25Al0.75O7, ackermanite Ca2(Mg0.75Al0.25)(Si1.75Al0.25)O7, spinel (MgO·(Al, Cr, Fe)2O3), α-Ca silicate α-Ca2(SiO4), Ca-Al oxide Ca3(AlO3)2, anortite Ca(Al2Si2O8)
Glass-ceramics from powdered glass (900 °C)	Ackermanite (Ca2(Mg1-xAlx)Si2-xAlxO7, calcium aluminium oxide CaAl2O4 natrium calcium aluminium silicate NaCaAlSi2O7, magnesium aluminium oxide MgAl2O4, spinel (MgO Al, Cr, Fe)2O3)
Sintered fly ash (1050 °C)	Halite (NaCl), silvite (KCl), α-quartz (α-SiO2), calcium sulfate (Ca(SO4)), Mg-gehlenite Ca2(Mg0.25Al0.75)Si1.25Al0.75O7)

Figure 2. XRD traces of fly ash and fly-ash products obtained using different processing methods.

Figure 3. Results of thermogravimetry (TG) and differential scanning calorimetry (DSC) for fly ash.

Figure 4. Electron backscattered micrographs of sintered fly ash sintered at (a) 850 °C and (b) and (c) 1050 °C.

Figure 5. Backscattered electron micrograph after melting at 1390 °C and casting.

Figure 6. DSC traces for melted fly ash: (a) cooling curve; (b) heating curve.

Figure 7. Backscattered electron micrographs of glass-ceramics. (a) GCPB; GCBG at (b) 850 °C; (c) 1050 °C.

Figure 8. Hardness of the fly-ash products formed after melting and heat treatment.

Table 4. The results of the compression tests

Sample	Average Compressive Strength (MPa)
Bulk glass	118
Glass-ceramics from bulk glass	124 (850 °C) 135 (900 °C) 137 (950 °C)
Glass-ceramics from powdered glass	134 (550 °C), 145 (750 °C), 169 (850 °C), 180 (900 °C), 187 (950 °C), 153 (1050 °C)
Sintered fly ash	109 (1050 °C)

Table 5. The results of leaching of fly ash and its products (dry basis, mg/kg)

		Glass	Glass-Ceramics from Bulk Glass			SFA	Glass-Ceramics from Powdered Glass
Element	Fly Ash	1390	850	900	950	1050	850	900	950	1050	Limiting value*
As	292	0.02	0.02	0.05	0.01	0.13	<0.01	0.01	0.02	<0.01	2
Ba	4760	0.66	1.32	2.73	8.23	0.76	1.01	8.40	7.70	8.60	100
Cd	7	0.08	0.01	0.01	1.66	0.03	0.01	0.12	0.05	1.65	1
Total Cr	26700	0.27	1.78	3.28	0.07	24.25	7.68	0.09	0.13	0.03	10
Cu	4270	0.74	0.05	0.02	0.18	0.02	0.01	0.09	0.09	1.44	50
Mo	13400	0.34	0.11	0.18	0.61	42.71	8.10	1.35	1.71	0.27	10
Ni	89	0.02	0.02	0.24	0.22	0.01	0.02	0.47	0.45	0.20	10
Pb	22400	0.01	0.05	0.03	0.03	0.01	<0.01	0.05	0.13	2.62	10
Se	1140	0.85	1.26	0.37	0.82	0.06	0.31	0.45	0.43	0.26	0.5
Zn	5550	3.34	0.40	0.17	4.60	0.25	0.16	1.81	1.63	5.57	50
Notes: *Limiting value for landfilling of stabilized and unreactive wastes. Bold numbers indicate conditions when the limiting values were exceeded.

Table 6. Advantages and disadvantages of the investigated processing routes

Product	Main Operations	Advantages/Disadvantages
Glass-ceramics from bulk glass	Fly ash - Melting (1290 °C/20 min) and cooling - Formation of bulk glass- Devitrification at 600 °C for 20 min and at 850–1050 °C for 2 hr	-High energy consumption when melting-Emissions of air pollutants during the vitrification stage (melting and/or annealing) -Products with good physical and chemical properties (higher hardness, better workability, and increased fracture toughness, strength) -Volume shrinkage of received products compared to fly ash-Expensive technology
Glass-ceramics from powdered glass	Fly ash	-High energy consumption when melting
	- Melting (1290 °C/20 min),	-Exhausts when melting and/or annealing
	- Quenching in water	-Water contamination when cooling
	- Drying	-Volume shrinkage compared to fly ash
	- Grinding	-Many operations
	- Pressing	-The most expensive technology
	- Sintering (850–1050 °C/2 hr)	-Products with good physical and chemical properties (high strength and hardness, high chemical stability)
		-Environmentally very stable products
Sintered fly ash	Fly ash- Pressing- Sintering (850–1050 °C/2 hr)	-Quality of end products is dependable of amount vitrifying objects in raw fly ash; pretreatment (washing and milling greatly improves quality of received product)
	-Requires preanalyzing of raw fly ash
	-Less volume shrinkage
		-Low energy consumption and less time for receiving end material
		-Less expensive technology
		-Lower strength and hardness and lower chemical stability