Complex Processing of Pulverized Fly Ash by Dry Separation Methods

Transcription

1 Journal of Environental Protection, 2010, 1, doi: /jep Published Online Septeber 2010 ( 293 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods Vladiir Vasilievich Zyryanov 1, Ditry Vladiirovich Zyryanov 2 1 Institute of Solid State Cheistry and Mechanocheistry, Russian Acadey of Sciences, Siberian Branch, Novosibirsk, Russia; 2 Nanopowder Technology Ltd, Novosibirsk, Russia. Eail: vladinetta@gail.co, info@nanopowder-technology.co Received May 18 th, 2010; revised June 16 th, 2010; accepted June 19 th, ABSTRACT Pulverized fly ash (PFA) is produced about 500 billions tons every year in the world in a result of coals cobustion. Most of the fly ash collected in power plants is disposed by deposition in landfills, situated as a rule near big cities with well developed infrastructure and high cost of land. Moreover, the pollution of environental by fine solid wastes is inevitable and takes place in area of residing of a basic part of the population. The only solution is a coplex processing of fine wastes with a production of value added aterials. New conception of coplex processing of PFA is proposed on the base of facilities of Electro-ass-classifier (EMC) and other techniues. The characterization of separated fractions was carried out by SEM and optic icroscopy, XRD, laser diffraction, Mössbauer spectroscopy and other ethods. A fine fraction of glass icrospheres presents the ain interest as filler in various aterials. Keywords: Pulverized Fly Ash (PFA), Processing, Solid Wastes, Utilization, Dry Separation, Fly Ash Coponents, Glass Microspheres, Magnetospheres, Fillers, Electro-Mass-Classifier 1. Introduction Coal fly ash is the particulate atter reaining after cobustion of the carbonaceous coponent of coal. The residual, accessory inerals, predoinantly clay, siliceous and iron inerals generally coprise the bulk of the ash. Fly ash was considered to be the sixth ost abandoned ineral in the USA [1]. Usually less than 20% is reused coercially all over the world, predoinantly in ceent or as a fill aterial in construction [2]. The ain cause of low utilization of PFA is a polydisperse inhoogeneous ix containing renants of unburnt carbon coke or char. Other proble for processing of PFA is related to high fineness of this waste coplicating the separation of carbon by dry physical ethods. Such proble ay be solved by the use a green engineering Electro-ass-classifier (EMC) techniue operating with charged aerosols in closed volue in a wide range of particle sizes. Acid fly ash of type F according to ASTM prevails aong different coal cobustion products. In addition, coercial potential of this PFA after separation is axial [2]. So, the perspectives of PFA coplex processing technology based on the dry separation ethods for environental protection and profitable production of value added aterials are better to show in such class of waste. The possible solutions for utilization with profit for other types of PFA fly ash C according to ASTM classification and ash fro low teperature boilers with fluidized bed were found as well. Based on the echanical activation in EMC, these solutions are presented in our book [2]. 2. Materials and Methods Various speciens of PFA were used for investigations produced in the largest power plants with high teperature boilers of the forer USSR Reftinskaya GRES (specien No 1, coal fro Ekibastuz deposit), Troitzkaya TEC (specien No 2, coals fro Ekibastuz and Kuznetsk deposits, both plants fro Ural region), Novosibirskaya TEC 5 (specien No 3, coal fro Kuznetsk deposit, Siberian region). The fineness of PFA speciens was close one to another due to use of siilar electrostatic precipitators. Ash particles were ranged fro 200 n to 500 with ean particles size about 40. Different techniues were used for dry physical separation of PFA including Electro-ass-classifier (EMC) [3,4], agnetic separator with peranent agnet fro

2 294 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods FeNdB-alloy (MS), triboadhesion separator (TAS), electrostatic separator with electric field 6 kv/c (ES) and their cobinations. All these devices are able to operate in discrete and continuous odes that are iportant for laboratory studies and industrial processing respectively. Moreover, all these installations are copatible and can operate in closed volue without aerosol containation of air atosphere. Flow sheet for processing of PFA ay be realized for one run in production odule with capacity TPH. Operating in the range of particles size ~50 n-2, EMC techniue allows to use with high efficiency other ore selective separators after reoving fro PFA of the dust with particles size < 20. Principle of operation of ultifunctional green EMC engineering is based on the generation of charged aerosols and their separation to unliited nuber of fractions under action of centrifugal and other forth s [3,4]. The cobination of instruental and cheical approaches was used for characterization of separated ash fractions: XRD (DRON-3M, Russia, Bruker D8 Advance, Gerany, Cu K radiation), SEM + EDS (Hitachi TM-1000, JSM-6700F), optical icroscopy Neophot 21, Gerany, particle size analyzer (Laska, Russia), based on a laser diffraction ethod, Mössbauer spectroscopy (NZ-640, Hungary). Losses on ignition (LOI) were deterined at 850, 1 hour. Cheical copositions were deterined in spectroeter ARL-9900XP, Swiss. 3. Separation In a result of investigation of dozen various PFA speciens of F type, the optial schee for coplex processing was found, Figure 1 [2]. After reoving of fine and partly ediu fractions in EMC, the rest of PFA can be divided by custo separators. Additionally four fractions can be obtained [2]. Fro practical point of view, fractions were called as potential products: Super-alpha (fine fraction No 1 in Figure 1), Alpha (ediu fraction, No 2 + No 3), Delta (large fraction, No 4), coarse byproduct (No 5), agnetic byproduct (agnetic fraction, No 6). Particle size distributions for obtained products Figure 1. Flow sheet for coplex dry processing of F type PFA.

3 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods 295 are shown in Figure 2 for specien No 1 (No 2 and 3 are very siilar). Magnetic fraction has a particles size distribution close to Delta. After refineent of agnetic byproduct, agnetospheres with proising catalytic properties in ethane conversion process can be received [5]. Magnetospheres present spherical particles of nanocoposites consisted fro spinel and heatite in iron-rich silica glass atrix [5,6]. Coarse byproduct is a concentrate of relatively expensive aterials coke and cenospheres (hollow glass icrospheres with density < 1 g/c 3 ). Dry physical ethods are not efficient for ore deep separation of coarse fraction. So, the yield of this byproduct ust be iniized to decrease the role of wet technologies in coplete processing of PFA [2]. Fractions No 2 and 3 have close fineness and properties. So, for siplification of flow sheet these fractions are reasonable for uniting in a product Alpha. The yields of products and their LOI are presented in Table 1. For enrichent of coarse fraction by coke, relatively low electric field with E = 6 kv/c was applied in ES. The carbon content in fine fraction hardly is higher than in initial PFA due to illing of porous char particles (Figure 3) during treatent in EMC in discrete ode of operation. The decrease of velocity of rotor rotation in EMC results to lowering of LOI in fine fraction. In EMC with continuous ode of operation, the life tie of ash particles in caera of charged aerosol generation sharply drops resulting to a little illing of coke and lowered contents of carbon in fine and ediu fractions Superalpha and Alpha respectively. The ratio of illed carbon particles and capacity of EMC installations in separation process are linked paraeters. Fro the other side, the pozzolana properties of Super-alpha product don t depend on the LOI. Moreover, after coke cobustion at 850, activity of Superalpha even decreases a little. There is a siple explanation of such effects. Fine carbon particles with a size ~n after illing get in pore space of ceent stone with a little influence on the strength. In uenched ash glass particles echanical strains results to high cheical activity in basic solution in ortar (so called echanocheical activity). After theral annealing, cheical activity decreases due to relaxation of echanical strains in glass particles. Weak effects of carbon and strain relaxation on the strength of ortar with additives of Super-alpha show counterly signs and copensate each other. 4. Cheical Coposition Cheical copositions of obtained products are presented in Table 2. These results are very siilar to other known data [2]. The iron content in Delta is lowered 12 % in particle size, Super-alpha Alpha Delta Coarse byproduct Initial PFA Figure 2. Particle size distributions for obtained products fro PFA No 1. Figure 3. Porous coke particle fro PFA No 1. relatively Alpha due to agnetic separation of large fraction only. Nevertheless, in EMC there is a facility to reove agnetic particles fro fine fractions as well, if this operation has practical sense for better coercialization. 5. Phase Coposition Phase copositions of all studied PFA speciens are very siilar sall crystallites of ullite () and uartz () in silica glass, Figure 4. The content of ullite depends on the particle size. Fine fractions are enriched by uartz, and coarse fractions by ullite that correlates with cheical coposition. Cenospheres are usually enriched by silica and Na/K, so the ullite content is lowered relatively ain part of icrospheres with density > 1 g/c 3. Phase copositions of agnetic fractions differ slightly as well, Figure 5. The ain crystal phase is fer-

4 296 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods Table 1. Yields and LOI (%) of products obtained in flow sheet in Figure 1. Products PFA, No LOI Super-alpha Alpha Delta Coarse byproduct Magnetic byproduct Y LOI Y LOI Y LOI Y LOI Y LOI Table 2. Cheical coposition of obtained products (for agnetospheres after refineent). PFA Product SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO TiO 2 K 2 O Na 2 O P 2 O 5 MnO S-alpha Alpha Delta Magnetospheres S-alpha Alpha Delta Magnetospheres S-alpha Alpha Delta Magnetospheres h No 1 No theta Figure 4. XRD pattern for PFA No 1. rospinel, and inor phases are heatite (h) and uartz (). The content of ferrospinel depends on the agnetosphere size with axiu content in ediu fractions theta Figure 5. XRD patterns of agnetospheres. 6. Morphology Morphology of the ost ash particles presents hollow

5 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods 297 glass icrospheres, Figures 6-8 (arks in ). In large fractions soe glass particles have irregular fors. In coarse fraction ost particles are presented by aggloerates, including plerospheres and coposites, Figures For any applications, spherical for of glass particles is very attracting, especially as filler in ceent, concrete ixtures, polyer coposites, and so on. So, the reoving fro PFA of coarse byproduct, consist of coke and nonspherical glass particles with irregular for and porous icrostructure sharply enhances the uality of obtained products as fillers. Part of carbon is in aggregates with silica glass icrospheres, Figure 10. Because of this, the efficient reoving of coke fro PFA is liited by individual coke particles only. EDX analysis of glass particles in Figure 9 displays a different cheical coposition: for particle 1 - Figure 7. SEM iage of Super-alpha No 1. Figure 8. Optical iage of Delta No 2. Figure 9. SEM iage of coarse byproduct No 1. Figure 6. Product Alpha. optical iage (PFA No 2); SEM iage (PFA No 1). Si/Al = 1.1, for particle 2 - Si/Al= 2.1, dark particles coke aggregates. Magnetospheres display various orphological types in every kinds of PFA, Figures Magnetospheres fro different power plants were studied by cobination

6 298 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods Figure 10. Aggregate fro coke and glass icrospheres in coarse byproduct No 1. Figure 12. Optical and SEM iages of agnetospheres No 2. Figure 11. Optical and SEM iages of agnetospheres No 1. of structural ethods XRD with Rietveld analysis and Mössbauer spectroscopy [6]. Structural forulas of ferrospinels were deterined that allows to search a correlations with catalytic properties of agnetospheres with different origin [5]. Such correlations help to select optial agnetospheres and odify the for proising applications in catalysis. Cenospheres separated fro coarse byproducts by wet technology are shown in Figure 14. Cenospheres are well known product with a relatively high cost and large area for applications [2]. The content of cenospheres in coarse byproduct is about 2-4% (up to 30% of total content in PFA). The iron contents in nonagnetic fractions of glass icrospheres and cenospheres in studied speciens of PFA are in the range 3-5%. As a result, the color of glass icrospheres usually grey, that coplicates their applications in white and colored polyer copositions. According to studies of glasses by Mössbauer spectroscopy

7 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods 299 Figure 13. Optical and SEM iages of agnetospheres No 3. [2], Fe 2+ state have coordination nuber 4.5 and Fe (usual octahedral position). Paraeters of iron in glass are presented in Table 3. After theral treatent in air, the content of Fe 2+ decreases with narrowing of lines that corresponds to annealing of echanical strains in uenched glass. The color of powders acuires poorly red shade. Other valuable product which can be separated fro coarse byproduct is a coke with LOI > 90%. The content of extracted coke in coarse byproducts was about 2-4%. A coke represents a practical interest not only as a clean solid fuel, but ostly a source of rare volatile eleents V, In, Co, U (Figure 16), and as absorbent [2]. The obtaining of cenospheres and coke akes possible a profitable wet cheical processing of coarse byproduct. In such a case the waste after separation coke and cenospheres presents the source of rare earth eleents, secondary products are silica, aluina and raw for zeolite synthesis [2]. Coarse byproduct ay be used without (c) Figure 14. Optical iages of cenospheres divided fro PFA No 1, 2 and 3 (c). processing in fabrication of low dense building ceraics. However, the capacity of region arket usually is less than uantity of produced coarse byproduct. Diversification of value added products allows to realize with profit a coplete utilization of produced PFA.

8 osivstrmpaalpha300 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods Table 3. Paraeters of Mössbauer spectra of nonagnetic cenospheres separated fro PFA No 3. paraeters content, % content, % Dublet Fe 3+ Dublet Fe 2+ initial Т = width of lines, - cheical shift, - uadruplet splitting. C preseength,s3 Spha28 uper-al0,0 0,2 0,4 0,6 0,8 1,0 ubsitutionofceentfigure 15. Typical SEM iages of cenospheres obtained fro PFA No ,R = 0.95Upp[U] = [C] Ccontent,%Figure 16. Correlation between U and carbon contents in fractions of PFA No Pozzolana Properties The utilization of PFA products is ostly as fillers in ceent and concrete. The profitable processing of PFA is deterined by the capacity of construction arket. The uality of PFA products deterines the possible volue of utilization. The pozzolana properties are shown in Figure 17 for the fine PFA products. Figure 17. Copressive strength for ortars ceent:sand = 1:3 with substitution of ceent by products fro PFA No 1 after 3, 7 and 28 days of curing in wet atosphere. Partial substitution of ceent by Super-alpha drastically increases the strength of ortars even after short tie of curing. This enhanceent of building aterial is related to increasing of density of concrete due to filling of pores between ceent particles and absorption of excessive Ca(OH) 2. The substitution of 30% ceent by Alpha results to the sae strength properties of concrete. The fineness of Alpha and custo ceent are very siilar, so the effect of substitution is related to pozzolana properties. A possible use of Delta in ortars or ready ix is due to partial substitution of sand. In this case a workability of ortars becoes better thanks to spherical for of particles, but the copressive strength displays a weak dependence on the degree of substitution. However, the density of concrete decreases, so noralized on the density effect becoes significant, Table 4 [2]. The possible volue of PFA utilization in construction industry becoes uch ore significant after processing. Relatively low uality of ortars with initial PFA is related to renants of coke and porous coarse ash particles with irregular for. In the case of specially designed coal ix with inorganic additives, prepared with optial orphology, the cost of PFA after cobustion becoes ore significant than produced electric and heat energy [7]. In other words, electric and heat energy ust becoe byproduct

9 Coplex Processing of Pulverized Fly Ash by Dry Separation Methods 301 Table 4. Paraeters of 28 day concretes with products separated fro PFA No 1, which are eual on the volue to ix ceent:sand = 1:2 (weight ratio). No Coposition Density ρ, g/c 3 Copressive strength σ, MPa Noralized strength, σ/ρ 1 Ceent: 2sand (3) Ceent: sand + Delta (3) Ceent: 2Delta (2) Ceent: 2agnetospheres (1.5) (2/3ceent + 1/3Alpha): 2sand (1.5) (2/3ceent + 1/3Alpha): sand + Delta (3) (2/3ceent + 1/3PFA): sand + PFA (1) 11.5 only in cheical reactors for the production of high teperature aterials glass icrospheres and other fillers, binding aterials, and so on. 8. Conclusions PFA fro coal power plants consist of coponents with a nuber of proising applications. One run dry physical separation in green EMC-based installation of PFA to 3 products and 2 byproducts provides wide possibilities for utilization with profit of solid wastes. Refineent of agnetic byproduct and wet processing of coarse byproduct can provide coplete processing of solid wastes to value added products. In such a case, the effect of eliination environental pollution is accopanied by econoy of natural resources and lowering of CO 2 eission. Obtained results prootes to new eco-friendly conception to produce a power as a byproduct in relatively sall cheical reactors which can provide better econoic perspectives due to local green production of distributed energy fro carbonaceous sources. 9. Acknowledgeents The first author gratefully acknowledges the support of this research in part through grant Russian Found for Basic Researches REFERENCES [1] G. L. Fisher, Bioedically Relevant Cheical and Physical Properties of Coal Cobustion Products, Environental Health Perspectives, Vol. 47, 1983, pp [2] V. V. Zyryanov and D. V. Zyryanov, Fly Ash Technogenic Raw, Maska Ltd, Moscow, 2009, p [3] V. V. Zyryanov, Electro-Mass-Classifier, USSR Invention Certificate No , [4] V. V. Zyryanov, D. V. Zyryanov and V. A. Sadykov, Creation of Coatings by Charged Aerosol Deposition Method, Russian Nanotechnologies, Vol. 3, No. 5-6, 2008, pp [5] A. G. Anshits, E. V. Kondratenko, E. V. Foenko, et al., Novel Glass Crystal Catalysts for the Processes of Methane Oxidation, Catalysis Today, Vol. 64, No. 1-2, 2001, pp [6] V. V. Zyryanov, S. P. Petrov and A. A. Matvienko, Morphology and Structure of Magnetic Spheres Based on Heatite or Spinel and Glass, Inorganic Materials, Vol. 46, No. 6, 2010, pp [7] V. V. Zyryanov and D. V. Zyryanov, Cheical Reactors as Alternative to Coal Burning Boilers in Power Plants, Proceedings of 7th All-Russian Conference with International Participation Cobustion of solid fuel, Vol. 3, Noveber 2009, Novosibirsk, pp