Leaching characteristics of fly ash

Transcription

1 Leaching characteristics of fly ash Aysenur Ugurlu Abstract The disposal of fly ash as a byproduct of thermic power stations, results in significant environmental problems. The leaching of coal fly ash during disposal is of concern for possible contamination, especially for the aquatic environment when ash is in contact with water. The aim of this study was to investigate the leaching behaviour of fly ashes currently disposed in Kemerkoy Power Plant (Turkey) fly-ash-holding pond. The studies were conducted with fly ashes from the electrostatic precipitators (fresh fly ash) and from the fly ash pond (pre-leached fly ash). The fly ashes has alkaline in nature and ph ranges between 11.9 to The pre-leached fly ash exhibited lower EC values (7,400 ls) than the fresh fly ash (10,300 ls). In contrast to Fe and Pb, the elements such as Cr, Cd, Cu and Co did not leach from the fly ash. The Ca and Mn concentrations decreased with increasing temperature whereas, Na and K concentrations increased. The results showed that the most important effects of fly ash leaching were ph, Na, Ca, K, Fe, Mg, Mn and Pb. Keywords Ash pond Æ Coal solid waste Æ Fly ash Æ Leaching Introduction Coal is the most abundant and widely spread fossil energy resource in the world (Benito and others 2001). Fly ash is a byproduct of incineration of coal. More than 150 million Received: 8 July 2003 / Accepted: 7 May 2004 Published online: 9 June 2004 ª Springer-Verlag 2004 A. Ugurlu Environmental Engineering Department, Hacettepe University, Beytepe, Ankara, Turkey ugurlu@hacettepe.edu.tr Tel.: Fax: tons of fly ash are produced annually worldwide from the combustion of coal in power plants. Fly ash is utilized in cement and construction industry. However, the rate of production is greater than the consumption. The unused fly ash is disposed into the holding ponds, lagoons, landfills and slag heaps. Disposal of huge amounts of fly ash in landfills and surface impoundments or its re-use in construction materials is of environmental concern (Piekos and Paslawska 1998). Fly ash is classified as a hazardous residue. Coal contains significant quantities of various trace elements and, during combustion of coal, trace elements are enriched as a result of carbon loss as carbon dioxide and the trace elements are associated with the surface of the ash particle due to evaporation and condensation. The characteristics of the coal used and the type of the installation employed in generating the solid combustion wastes (fly ash) have a direct influence on chemical and mineralogical composition of fly ash (Benito and others 2001). The disposal of fly ash is considered a potential source of contamination due to the enrichment and surface association of trace elements in the ash particles (Choi and others 2002). The elements Mn, Ba, V, Co, Cr, Ni, Ln, Ga, Nd, As, Sb, Sn, Br, Zn, Se, Pb, Hg and S in the coal are volatile to a significant extent in the combustion process. However, the elements Mg, Na, K, Mo, Ce, Rb, Cs and Nb appear to have smaller fraction volatilized during combustion. Whereas, Si, Fe, Ca, Sr, La, Sm, Eu, Tb, Py, Yb, Y, Se, Zr, Ta, Na, Ag, and Zn are either not volatilized or only show minor trends related to geochemistry of mineral matter (Iyer 2002). During transport, disposal and storage phases, the residues from coal combustion are subjected to leaching effects of rain and part of the undesirable components in the ashes may pollute both ground and surface waters (Benito and others 2001). These solid residues (fly ash) can be leached in higher concentrations than drinking water standards and can cause contamination in drinking water sources. Therefore, it is important to predict the leaching behaviour of residues to prevent the environmental effects, especially for the aquatic environment when ash is in contact with water. The toxic elements leached from fly ash can contaminate soil, ground water and surface water. Therefore, effective water management plans are required for fly ash disposal. Although chemical composition of coal waste can 890 Environmental Geology (2004) 46: DOI /s

2 give us an idea about the pollutants passing through water, in order to quantify these phenomena it is necessary to carry out leaching tests. Lau and Wong (2001) found that different elements have different leaching behaviours because of differences in elemental properties and ph of the solution and leaching time, which strongly influence the leaching behaviour. Seferinoglu and others (2003) reported that trace-element leaching from bottom ash is slower and often requires that the entire bulk matrix be dissolved. The aim of this study is to investigate the leaching behaviour of fly ashes disposed in the Kemerkoy Power Plant (Turkey) ash pond and to investigate the potential influence from the ash disposal on ground water quality. The study includes batches of leaching tests with fly ash before disposal and with fly ash slurry collected from the fly ash pond. Materials and methods Chemical composition The main method of disposal of fly ash from the power plant is mixing with water. The resultant slurry is transferred to an ash-disposal pond. Random samples of freshly generated fly ash have been collected (5 kg) from the electrostatic precipitators in the Kemerkoy Thermoelectric Power Plant and then mixed. The pre-leached fly ash was collected at various parts of the fly ash pond located 800 m to the Plant at 5 cm below the surface and then mixed. The samples were taken from the part where the fly ash had been stored for 1 week to 3 months. The coal and fly ash samples were analysed by X-ray diffraction (XRD). The physico-chemical properties of the elements in the fly ash is related to their chemical form in the coal, coal combustion process and mechanisms of emission-control devices. The fly ash was produced through the combustion of lignite coal, which is transported from the nearby area (Yatagan). The composition of the lignite coal and some physical properties are shown in Table 1. The ash content of the coal was about 33%. The lignite coal burnt in the power plant produced fly ash with a high lime content ( %) as well as Fe 2 O 3 ( %), SiO 2 ( %) and Al 2 O 3 ( %; Baba 2000). The fly ash is a fine powdery residue with particle sizes in the range of 63 to 125 lm. Data on chemical composition of fly ash taken from the incinerator (FA-1) and from the fly ash pond Table 1 Chemical and physical characteristics coal Parameter Coal (mean ± SD) Moisture (%) 31.6±5 Ash (%) 33.4±10 Sulfur (%) 0.82±0.25 Gross calorific value (kcal/kg) 1,805±288 Density (g/cm 3 ) 1.14±0.06 (FA-2) are shown in Table 2. The fly ash contains high concentrations of silica together with oxides of Ca, Al and Fe. The fly ash has very high organic content that was not lost during combustion (12.7%). Mg, Na, Ti, K and P content account for only 3.5%. Trace elements make up the rest of the fly ash. As can be seen from the Table 2, there are slight differences between the chemical composition of FA-1 and FA-2. The wet disposal of the fly ash into the ash pond caused leaching of some constituents from the fly ash due to weathering. Some constituents showed an increase due to leaching of some constituents from the fly ash particles. Batch leaching tests A series of batch leaching tests were conducted in the laboratory. In order to better simulate the natural conditions and susceptibility to release a lower-liquid-to-solid (L/S) ratio was used. Therefore, 5-g fly ash samples were mixed with 25 ml of de-ionized water, giving a liquid/solid ratio of 5. Three subsequent extractions with the same volume were applied and gentle stirring was continued during the extraction (2 h). The experiments were carried out at room temperature (23 C), and at 40 and 50 C. The results were calculated using the mean value of duplicates. The fly ash samples were tested for characteristic properties related to leaching behaviour. The tests were conducted in order to distinguish the easily leachable and less readily released loads of soluble components. The fly ash samples were tested for characteristic properties related to leaching behaviour. In these studies, the leaching of Ca, Mg, Na, K, SO 4, Mn, Pb, Cd, Cu, Co, Fe, ph and conductivity (EC) from fly ash were investigated in order to predict potential pollution. The results were calculated using the mean value of duplicates of each extraction. Table 2 Chemical analysis of fly ash. FA-1 fly ash from Incinerator; FA-2 fly ash from pond; dolomitic limestone; cherty limestone Fly-ash-1 Fly-ash-2 Major elements (wt%) SiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O Ti 2 O P 2 O Trace elements (ppm) Nb Zr Y Sr Rb Ga <0.01 <0.01 Ni Co <0.01 <0.01 Cr V Ba < Environmental Geology (2004) 46:

3 In addition to the leaching tests using de-ionized water, adsorption studies were also conducted by using the typical ground profile beneath the ash pond. The fly ash was stored in a valley located on a karstic and fractured dolomitic and cherty limestone basin. The ash level was 3 m at the time of sampling (during plant start-up studies). It was expected that the ash level in the pond would increase to 130 m. Limestone is the most extensive rock unit in this area and it is reported that it is densely fractured, porous and partly karstified (Baba 2000). These basins are present beneath an alluvial layer (1 m). The leaching studies with these rock types were also conducted in order to determine the background concentrations, the possible elemental contamination from fly ash disposal and possible adsorption on these rock types. These rock samples were collected nearby the ash-disposal site from drilling studies (1 50 m), which did not come into contact with leachates. In these studies, the leaching behaviour of FA-1 and FA-2 samples (5 g) were studied when they were in contact with dolomitic () or cherty () limestone samples (5 g) and 25 ml of distilled water. The ash samples were dried at 110 C prior to batch analysis. Analytical methods All the analyses were carried out according to standard methods (APHA 1989). The Ca, Mg, Na, K and trace elements were analysed by an atomic adsorption spectrophotometer (Perkin Elmer 2280). ph was measured by an EDT instruments BA 350 ph meter. All the samples were filtered through a 0.45-lm micropore membrane filter prior to analyses. Results and discussion Leaching studies are important in predicting the environmental impacts associated with the disposal into fly ash ponds. In these studies, the leachability of elements from fresh fly ash (FA-1) and weathered fly ash samples taken from the ash pond (FA-2) in the Kemerkoy Power Plant were compared. During these studies, leaching of calcium, sodium, magnesium, sulphates, potassium and various heavy metals from fly ash was determined. The fly ash samples were extracted three times with each extraction lasting 2 h. The effect of number of extractions on the leachability of EC and ph was studied. It is suggested that the leachability of toxic trace elements from fly ash particles is affected by the number of extractions. The EC values decreased with the number of extractions at all temperatures applied and the highest EC values were observed in the first extraction (see Table 3). Therefore, cation concentrations decreased after each extraction. The EC from FA-1 reduced from 10,300 to 5,400 ls after the third extraction at room temperature. The leachate of the pre-leached fly ash (FA-2) exhibited lower EC values than the fresh fly ash (FA-1). The difference was significant. This is probably related to the higher ion dissolution that occurred from the FA-1. The lower EC values of the FA-2 can be explained by leaching to some extent in the ash pond. The ECs of the rock samples were low and average values were about 70 ls for both rock types. The studies with the underlying rock samples showed that a decrease in EC values of the fly ash samples was not significant for both rock types and, therefore, it can be expected that high EC values are released to the ground water. The pre-leached fly ash reaction with both rock samples resulted in lower EC values than fresh fly ash. In these studies, FA-1 with dolomitic limestone exhibited slightly lower EC values than that of cherty limestone. There are significant differences among the conductivity of the leachates at room temperature and at 40 and 50 C. The first extraction of FA-2 at room temperature was 7,420 ls. It decreased to 4,170 ls at 40 C and increased to 7,000 ls at 50 C. The conductivity change of leachate from FA-1, with respect to temperature, was different than that of FA-2. The EC decreased with increasing temperature. However, higher conductivity cannot be attributed to the higher dissolution rates of K and Na for both fly ashes in the heated leaching experiments with respect to ambient extractions. Table 3 EC values from the batch leaching tests (ls). FA-1 fly ash from incinerator; FA-2 fly ash from ash pond; FA-1+ FA-1 mixed with dolomitic limestone; FA-1+ FA-1 mixed with cherty limestone; FA-2+ FA-2 mixed with dolomitic limestone; FA-2+ FA-2 mixed with cherty limestone Batch tests leaching T FA-1 FA-2 FA-1 + ( C) FA-1 + Test 1 1st extraction 23 10,300 7, ,840 8,540 3,260 4,830 2nd extraction 23 5,730 4, ,630 6,470 2,530 3,680 3rd extraction 23 5,400 3, ,370 5,630 2,820 2,960 Test 2 1st extraction 40 7,860 4, ,450 8,880 1,980 2,700 2nd extraction 40 6,140 4, ,130 5,620 1,718 2,750 3rd extraction 40 5,990 4, ,860 5,700 1,388 2,430 Test 3 1st extraction 50 7,500 7, ,160 5,800 4,160 3,480 2nd extraction 50 3,400 4, ,610 4,170 2,900 2,480 3rd extraction 50 2,350 3, ,950 3,750 1,906 2, Environmental Geology (2004) 46:

4 Batch leaching tests showed that the ph values of the weathered and fresh fly-ash samples were both alkaline in nature and the ph of the leachate from FA-1 and FA-2 ranged from and , respectively (Table 4). This was probably due to the high Ca content of the fly-ash samples. Similar results have been observed in previous studies (Choi and others 2002). The change in ph with the number of extractions was insignificant. The effect of temperature on leaching of fly ash was studied in the range of C using distilled water. The effect of temperature on ph-leaching tests showed that for both fly ash samples ph values decrease when the temperature was increased to 40 C and increase when the temperature was further increased to 50 C. The leaching tests were also conducted for rock samples. Both rock types have alkaline properties and the ph of the dolomitic limestone () ranged from 8.31 to 8.65 and the cherty limestone () ranged between 8.36 to When the ash samples were mixed with or samples, the ph of the leachate was slightly lower than the ph values observed for FA-1 and FA-2 leachates. This may indicate that the high ph values will probably reach the groundwater. The main components of the fly ash are Ca, Na, K and Mg and they were leached in high amounts from the fly ash samples. Calcium amounts leached from FA-1 and FA-2 showed significant differences. The leaching studies at room temperature showed that about 50% lower Ca was extracted from FA-2 than FA-1. The Ca concentration leached from FA-1 was 880 mg/l whereas it was 460 mg/l from FA-2 at this temperature. This can be explained by the leaching of Ca in some extent in the ash pond. It probably combined with CO 2 present in the rock and/or in the air, and resulted in the formation of low solubility calcium carbonate (CaCO 3 ). The concentration of Ca decreased when temperature was increased. A similar trend was also observed by Khanra and others (1998). It is suggested that Ca can be precipitated as CaSO 4 and/or CaCO 3. The Ca concentrations leached at 50 C were similar for both fly ash samples (about 600 mg/l). However, Na and K concentrations increased with temperature. The coal solid waste is disposed of in an ash pond lying on a dolomitic and cherty limestone basin. The leaching studies of dolomitic and cherty limestone showed very low Ca concentrations. When the rock samples were in contact with fly ash samples, this resulted in Ca concentrations being increased. The FA-2 reaction with rock samples exhibited lower Ca values than FA-1. In these studies both fly ash samples, in contact with, showed lower Ca values than that of. However, most of the Ca was not retained in the rock. Higher concentrations of Mg were extracted from rock samples than that of both fly ash samples. Magnesium was found as a less readily soluble component of the fly ash and this suggests that it was incorporated within the interior of the fly ash. Slightly lower Mg values were measured from the pre-leached fly ash. Potassium and sodium were also leached in higher amounts from FA-2 than FA-1. They were probably present in the interior part of the fly ash and were leached for longer periods. Significantly higher K concentrations were leached from FA-2 samples (255 mg/l) than FA-1 (49 mg/l). It was seen that when both of the fly ash samples were in contact with rock samples, the K concentration did not interact with any rock samples. The potassium concentration leached from FA-2 was slightly reduced when it was in contact with. Therefore, it can be expected that K concentrations will also be transported to the ground water. High Na concentrations will also be transported to the ground water. The leachability of Cu, Cr, Pb, Mn, Co and Fe from fly ash was also studied. Chromium did not leach from the fly ash samples. The leached Cu concentrations were close to the detection limit. This is probably because Cr and Cu are precipitated as their insoluble hydroxides. The lead concentration of FA-1 and FA-2 samples did not show differences. Higher Mn concentrations ( mg/l) were leached out from both rock types than any fly ash samples at any temperature applied. In the present studies, Ca, SO 4, Pb, Mn and Co leached in higher amounts from fresh fly ash than the pre-leached fly ash. However, Mg, Na, K, Cu and Fe are higher in the pre-leached fly ash. The effect of temperature was insignificant for some ions. Mn and Cu concentrations did not change with temperature. Iron oxides have a lower solubility in distilled water. Iron leached from fly ash is probably precipitated as hydroxides Table 4 ph values from the batch leaching tests. FA-1 fly ash from Incinerator; FA-2 fly ash from ash pond; FA-1+D FA-1 mixed with dolomitic limestone; FA-1+C FA-1 mixed with cherty limestone; FA-2+D FA-2 mixed with dolomitic limestone; FA-2+C FA-2 mixed with cherty limestone Batch tests leaching T FA-1 FA-2 FA-1 + ( ) FA-1 + Test 1 1st extraction nd extraction rd extraction Test 2 1st extraction nd extraction rd extraction Test 3 1st extraction nd extraction rd extraction Environmental Geology (2004) 46:

5 due to the alkaline nature of fly ash. The concentration of Pb, Fe and Mn in leachates depends on the test conditions. During the leaching studies, the resulting high ph in the leachates led to a limited removal of Pb and Fe from fly ash, which was even less removal than the other metals (Cu, Co, Mn). The leachability of trace elements will depend more or less on the leachability of iron (Khanra and others, 1998). Leaching of the metals decreased generally with increasing temperature. In most cases, a direct link between extraction and leachability could not be found. It is suggested that the degree of extraction does not exclusively determine the leachability of all metals, rather, it is temperature. However, the applied leaching conditions did not lead to the complete removal of soluble compounds due to a low L/S ratio of only 5. Choi and others (2002) have suggested that the elements in the ash particles were mainly associated with the surface and these surface-associated fractions might dominate the leachate chemistry at the early stages of fly-ash disposal in contact with water. Some elements showed a rapid release. However, the elements incorporated within the interior of the fly ash dissolved in a slower mode compared with the readily leachable surface-associated elements. This may explain why EC, Ca, Na, K and Fe leaching was different for un-leached (FA-1) and pre-leached (FA-2) samples. The fly ash from the ash pond was pre-leached and exhibited lower EC and Ca values than FA-1. However, the calcium that is leached from the pre-leached fly ash (FA-2) is probably retained in the interior. This supports the theory that these elements were surface associated and they leached in the ash pond where they were weathered. In contrast, FA-2 exhibited higher K, Na and Fe values than FA-1, which are thought to be associated within the interior of the fly ash. Although both fly-ash samples showed no increase in conductivity, the dissolution of K and Na increased with temperature. It is suggested that leaching with respect to the ambient extraction principally affected the aluminium silicate fraction of the fly ash, mainly K and Na. However, a major impurity such as Ca and many trace elements, such as Cd, Co and Cu, did not increase considerably. Sulfates were leached in high amounts from the fly ash (Table 5). The difference between the leached SO 4 concentration of the FA-1 and FA-2 was not significant. The interaction of both fly ash samples with dolomitic limestone was slightly stronger than cherty limestone. It can be expected that the leachable fraction of this ion can be washed out completely into the ground water. The leached concentrations of some ions decreased when the fly ash samples were in contact with typical rock samples from beneath the ash pond. In these studies, FA-1 mixed with cherty limestone samples exhibited lower leaching of any constituents than FA-1 with dolomitic limestone. In contrast, FA-2 mixed with dolomitic limestone showed lower concentrations of constituents than FA-2 with cherty limestone. Therefore, the elements that do not or are that are weakly interact with rock will probably be transported to deeper layers and ultimately to the ground water. Some elements, such as Cd, Cu, Co and Table 5 Concentrations of major and minor elements in leachates. FA-1+ FA-1 mixed with dolomitic limestone; FA-1+ FA-1 mixed with cherty limestone; FA-2+ FA-2 mixed with dolomitic limestone; FA-2+ FA-2 mixed with cherty limestone Element T FA-1 FA-2 FA-1 ( ) + FA-1 + FA-2 + Ca Mg Na K SO Pb Mn Co Cu Fe Environmental Geology (2004) 46:

6 Cr, are not expected to leach from fly ash. In contrast, elements such as Na, K, Mg, Pb and Mn will be transported to the ground water. Conclusions A major part of the water soluble compounds was removed in the fly ash pond. The results of batch tests, especially with pre-leached fly ash, showed that the release of elements into the solution continues over long periods. The EC values decreased after each extraction. In the both flyash samples Ca, Na, K, Mn, Fe, S and Pb showed maximum leachability, whereas, Cd, Mg, Cu, Cr, Zn and Co showed minimum leachability. The leaching of heavy metals was low for the studied fly ash. The low metal leaching due to high ph resulted in low contamination of the leachate. The leached concentration of Mg, Pb and Mn decreased when they were in contact with typical rock samples from beneath the ash pond. However, the elements (Na, K, Mg, Pb, Mn, SO 4 ) that do not or that weakly interacted with the underlying rock types will probably be transported to the ground water. References APHA (1989) Standard methods for the examination of water and wastewater. American Public Health Association, Washington, DC Baba A (2000) Leaching characteristics of wastes from Kemerkoy (Mugla-Turkey) Power Plant. Global Nets: Int J 2:51 57 Benito Y, Ruiz M, Cosmen P, Merino JL (2001) Study of leachates obtained from the disposal of fly ash from PFBC and AFBC processes. Chem Eng J 84: Choi SK, Lee S, Song YK, Moon HS (2002) Leaching characteristics of selected Korean fly ashes and its implications for the groundwater composition near the ash mound. Fuel 81: Iyer R (2002) The surface chemistry of leaching coal fly ash. J Hazard Material B93: Khanra S, Mallick D, Dutta SN, Chaudhuri SK (1998) Studies on the phase mineralogy and leaching characteristics of coal fly ash. Water Air Soil Pollut 107: Lau SSS, Wong WC (2001) Toxicity evaluation of weathered coal fly ash: amended manure compost. Water Air Soil Pollut 128: Piekos R, Paslawska S (1998) Leaching characteristics of fluoride from coal fly ash. Fluoride 31: Seferinoglu M, Paul M, Sandstrom A, Koker A, Toprak S, Paul J (2003) Acid leaching of coal and coal-ashes. Fuel 82: Environmental Geology (2004) 46: