| Pulverized coal(PC) furnace or boiler and circulating fluidized bed(CFB) boiler ashes are two of the most common and important types of fly ash in China currently. Due to the significant differences in combustion conditions(particularly the burning temperature), dust collecting and pipeing methods, desulphurization technology and other aspects, there are dramatical differences of material composition and physico-chemical properties between PC fly ash and CFB fly ash, which would affect their utilization orientation and prospects.1. This research focuses on Chongqing power plant PC ashes and Anwen power plant CFB desulfurization ashes. With these mineralogy, petrology, geochemistry element methods, such as X-ray fluorescence spectrometry(XRF), inductively coupled plasma mass spectrometry(ICP-MS), spectrometer field emission scanning electron microscope(SEM-EDS) and Siroquant TM mineral quantitative techniques, comparative studies of elements and mineral composition and rock characteristics between Luohuang and Anwen power plant coal combustion products(fly ash, bottom slag, gyps) have been done. Through the discussion of differentiation and enrichment mechanism of trace elements in coal combustion products and the examination of the occurrence modes of rare metal elements in fly ash, the results turn out:(1) Trace elements As, Se, Mo, Sb, Hg, Pb and Bi are significantly enriched in fly ashes compared to a loss in the bottom ash, Nb, Ta and Ti are relatively enriched in the bottom ash; the differentiation of trace elements among different coal combustion products is caused by element volatile.In fly ashes of Anwen power plant, Fe(10.26%) and rare metal elements Ga(47.83?g/g), Nb(120.5 ?g/g), REE(719.4 ?g/g) are highly enriched(ash-based content). With a higher potential for recycling, the contents of these elements exceed the minimum standards of corresponding industrial use. In bottom ashes, the contents of Ga(48.92 ?g / g), Nb(136.8 ?g/g) and Ti(2.13%) are higher than those in fly ash, and they also have some potential as resource utilization. Additionally, harmful elements F(1286 ?g / g) and Hg(1407 ng/g) are abnormally enriched in fly ashes of Anwen power plant, whose environmental hazard is of concern.The temperature of Anwen(CFB) boiler furnace is 900?. The decomposition and melting degree of minerals in coal is low. The average content of amorphous sialic vitreum is 46.6%, far below that in the luohuang PC ash. The use of limestone powder to adsorb sulfur during coal combustion leads to a higher content of anhydrite(10.8%). The mineral composition of fly ash in Anwen CFB is relatively complex: the average content of hematite is 15.2%, the content of quartz is 10.5%, the mixed-layer minerals of illite and smectite account for 8.0% of the total, in addition, there are a small amountof anatase(2.3%), lime(1.2%) and sodium feldspar(1.5%). Other major mineral phase is mullite(11.4%), hematite(1.6%) and a small amount of anhydrite(0.8%).The fly ashes of CFB desulfurization are fine-grained, particle size of which below 45 ?m account for 88%. Hematite monomer with high dissociation is conducive to magnetic seperation. There are REE sulfate and phosphate monomers in fly ashes. However, independent minerals of Ga and Nb were not detected by XRD and SEM-EDS. 1%-3% Ti were detected in clay minerals and amorphous vitreum, from which it can be inferred that carrier minerals of Ti include at least two- anatase and clay minerals. According to preliminary findings, Ga may be present in clay minerals or the product or the amorphous sialic vitreum(Ga can isomorphously replace Al). The EDS detection shows that a part of Ti are present in clay minerals. It has been proved that Ti can isomorphously replace Si, Al and enter into the clay mineral lattice. Nb, the nature of which is similar to Ti, can replace Ti isomorphously and enter into anatase or rutile(Nb was undetected in anatase of Anwen fly ashes). Therefore, it can be speculated that Nb isomorphously enters into clay minerals like Ti does. And the occurrence modes of Ga and Nb need further research.(2) Differentiations of trace elements among coal combustion products in Luohuang power plant are more significant than that in Anwen power plant. Zn, Ga, Ge, As, Se, Mo, Cd, In, Sn, Sb, F, Hg, Pb, Tl and Bi, these volatile elements are enriched in fly ash, however, Ti, Zr, Nb, Hf, Ta and W tend to be enriched in the bottom ash by gravity. Rare metal elements Ga(43.15 ?g/g), Nb(70.35 ?g/g), REE(546.0 ?g/g) are highly enriched in Luohuang PC fly ashes. With a potential for recycling, the contents of these elements get close to or exceed the minimum standards of corresponding industrial use. The contents of Ga(35.42 ?g/g), Nb(71.99 ?g/g), REE(604.5 ?g/g) in gypsum samples on dry ash-free basis were close to those in fly ashes. The potential of its resource utilization is also of concern. Harmful elements F(1213 ?g/g) and Hg(846.5 ng/g) in gypsum are abnormally enriched, because of which much more attention should be paid to the environmental and health hazards during the process of resource utilization of gypsum.Pulverized coal(PC) furnace was used in Luohuang power plant, central temperature of which is 1400?. Mineral decomposition and melting degree of raw coal was relatively higher, which led to higher contents(74.6% on average) of amorphous silicon-aluminum glass in fly ashes; the content of Fe2O3 in fly ashes is 9.5%, in which the content of the maghemite with strong magnetism(?-Fe2O3) is 5.8%, providing favorable conditions to recycle the iron-rich material in fly ash; other mineral phases are mainly mullite(11.4%), hematite(1.6%) and a small amount of anhydrous gypsum(0.8%).The major particles in the fly ash are different types of microspheres. Only a few irregular detrital minerals and unburned carbon particles were observed in SEM images.Microspheres in the Luohuang fly ash show an elemental change through their cross-section: the inner layer consists of pure Si-O mass; the chemical component of the middle layer is Si-Al-Fe-Ti-Ca-O; and Fe-O mass composes the outer layer. Iron-rich microspheres surrounded by maghemite(?-Fe2O3) reveal strong magnetic which facilitates the separation of mag-microspheres. Independent minerals of other trace elements(e.g., REE, Nb, Ti, Zr and Ga, et al.) were not detected by neither SEM-EDS nor XRD. It is inferred that these rare metal elements evenly disperse in the Si-Al-Fe-Ti-Ca-O layer of microspheres.2. Reutilization is the the most effective approch to solve the environmental problems caused by fly ash. In order to improve the value-added and the utilization rate of fly ash, experiments including magnetic separation, flotation(float-sink test), friction electric selected, particle size classification and joint chemical extraction of rare metal elements(e.g. REE, Ga and Nb et al.) were carried out, on basis of a comprehensive understanding of the compositon and mineral and the mineralogical and petrological characteristics of the Luohuag and Anwen fly ash. Eventally, two preliminary comprehensive utilization procedures were built for the Luohuang PC fly ash and the Anwen CFB desulfurization fly ash.(1) The extravagant contents of SO3(6.96%) and the LOI(Loss On Ignation, 8.31%) obstructed digestion of the CFB(Circulating Fluidized Bed) sulfurization fly ash generated from the Anwen Plant in the production of cment and cencret. Hence, a synchronous reducing roast was employed to decresse the percentage of SO3 and LOI.Meanwhile, the weak magnetic hematite(?-Fe2O3) was magnetize to magnetite(?-Fe2O3) which has a relative higher magnetism in the reducing atmosphere. The optimizational conditions for the synchronous reducing roast were demarcated as follows: with the unburned carbon in fly used as the reducing agents, hermetically roast the CFB sulfurization fly ash under a temperature of 900? for 15 to 30 minites, followed by a rapid water quenching to avoid the re-oxidation of magnetite; A wet low-intensity magnetic separator was used for the separation of iron from the fly ash roast-quench thick liquid, with a magnetic field intensity of 160 m T. Contents of LOI, Fe and SO3 in the treated CFB desulfurization fly ash decline to 5.3%, 5% and 1.32%, respectively. The concreted iron powder separated from from the fly ash contains Total iron(TFe) of 52.9%, with an iron recovery rate of around 50%. The iron powder obtained through the above pocedrue can be used as the ironmaking matearial or as the heavy medium allplied in the coal preraration. The comprehensive economic benefits would be significant.Experiments on an joint chemical extraction of tare metal elements including Ga?Nb?REE from the treated Anwen CFB fly ash were proceed. The procedure and technique parameter is shown as follow:A). The treated fly ash was firstly sintered with Na CO3 under a temperature of 860? for 30 minites,aimming to release the elements fixed in mineral to water-soluble or acid-soluble compound.B). Water leaching for Gallium extraction:Leaching the alkaline sintered fly ash with pure water under 90? for 2 hours, obtained a Ga leaching rate of 84.7%. Also 15.4% of the alluminium was leached out synchronously;C). Acid leaching for the extraction of rare earth elements(REE) and titanium(Ti):The water leaching residual was then soaked with hydrochloric acid(6 mol/L) under 60? for 4 hours. As a result, 80.1% of REE and 76.1% of Ti were extracted. Addationally, 53.2% of alliminium was released from the residual in this step. A total of 68.6% of aluminium was extracted till now. Leaching leaching rate of Niobium(Nb) in the above two steps was extremely limited; Nb was clined to enrich in the acid leaching residue.D). Resin adsorption separation of Gallium:The D201 rare was used to adsorpt the Gallium ion from the water leaching extraction liquid under a temperatue of 40? with an adsorption rate of 28.0%. Pure water was used to desorpt Ga ionfrom the resin on temperature 40 ?, too. The desorption rate was 35.2%.Through the above two step comprehensive utilization process, valuable elements including iron, REE and gallium can be valuable from the Anwen CFB desulfurization fly ash, with a considerable value added. Accordingly, the stock pressure and environmental pollution problems cause by fly ash would be alleviate effectively. Technologies for preconcentration of Nb and separation of REY, Ti from the acid leaching solution are the focus of our subsequent work.The Luohuang fly ash is enriched of different kind of microspheres. Cenospheres with diametres 150-500 ?m is separated by means of the water-medium sink-float separation firstly. Cenospheres account for 0.15 wt.% of the whole fly ash. And then magnetic microspheres(Fe2O3 of 44.65%) were available through the wet low intensity magnetic(160 m T) separation; finally an air classification of the remaining nonmagnetic microspheres was applied to obtain microspheres with six different size fraction, 89.6 ?m, 40.2 ?m, 21.3 ?m, 12.7 ?m, 8.1 ?m and 4.9 ?m respctively. Compared with the similar products of 3MTM, the above six size level glass microspheres have a narrower particle size distribution. Thus, these glass microspheres should be available for many application area, e.g. industrial paint, resin, rubber and so on.According to the above systematic and fine separation procedure, a variety of fly ash sphere products(cenosphere, magnetic microsphere, superfine glass microsphere)adapting to special application fields can be acquired, making both the utilization rate and economic value added of the Luohuang fly ash maximum. Moreover, the environmental pressure caused by the fly ash will be relieved effectively. The relevant separation or classification technologies are already mature, and the existent rough grading device in the Luohuang Plant can continul to work in order to reduce investment. |
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