Mineralogical and engineering characteristics of dry flue gas desulfurization products

Fifty-nine coal combustion products were collected from coal-fired power plants using various dry flue gas desulfurization (FGD) processes to remove SO₂. X-ray diffraction analyses revealed duct injection and spray dryer processes created products that primarily contained Ca(OH)₂ (portlandite) and CaSO₃·0.5H₂O (hannebachite). Most samples from the lime injection multistage burners process contained significant amounts of CaO (lime), CaSO₄ (anhydrite), and CaCO₃ (calcite). Bed ashes from the fluidized bed process were often dominated by CaSO₄ but also contained CaCO₃, CaO (lime), and MgO (periclase). Cyclone ashes were similar in composition to the bed ashes but contained more unspent sorbent and CaSO₄ and less MgO. Fly ash in all samples ranged from 10 to 79 wt%. Samples usually exhibited two distinct swelling episodes. One occurred immediately after water was applied due to hydration reactions, especially the conversion of CaO to Ca(OH)₂ and CaSO₄ to CaSO₄·2H₂O (gypsum). The second began between 10 and 50 d later and involved formation of the mineral ettringite (Ca₆[Al (OH)₆](SO₄)₃·26H₂O). The final pH after 112 d ranged from 10.0 to 12.1. If samples are incubated under ‘closed’ (i.e. incomplete recarbonation with atmospheric CO₂) and alkaline weathering conditions, gypsum and portlandite are initially formed followed by the conversion of the gypsum to ettringite. Closed, alkaline conditions typically can occur when FGD products are placed in confined settings such as a road embankment or buried as a discrete layer as occurs in some surface mine reclamation projects.     

Tricalcium aluminate hydration in the presence of calcium sulfite hemihydrate

This article reports on the study to evaluate the potential possibility of regulating the tricalcium aluminate (C₃A) hydration process by the addition of calcium sulfite hemihydrate. The kind and the form of hydration products were studied in the system: C₃A-CaSO₃·0.5H₂O-H₂O and C₃A-CaSO₃·0.5H₂O-Ca(OH)₂-H₂O by use of XRD, DTA and SEM/EDS methods as well as the kinetics of hydration along with chemical composition development of the liquid phase. The results thus obtained were compared to the hydration process of C₃A with the addition of natural gypsum. The results show that the reaction rate of C₃A with the addition of calcium sulfite hemihydrate differs from the analogous hydration process of C₃A in the presence of calcium sulfate dihydrate. Also, the kind of hydration products obtained in the presence of CaSO₃·0.5H₂O is different.     

Hydration of tricalcium aluminate in the presence of various amounts of calcium sulphite hemihydrate: Conductivity tests

Hydration of calcium aluminate C₃A (3CaO·Al₂O₃) in the presence of calcium sulphite hemihydrate (CaSO₃·0.5H₂O), with the molar ratio of substrates close to 1, produces the C₃A·CaSO₃·11H₂O calcium monosulphite aluminate phase. Small amounts of calcium sulphite added to calcium aluminate (the ratio of CaSO₃·0.5H₂O / C₃A equalling 0 : 1) change the rate of C₃A hydration and influence the whole reaction. Reaction processes for various ratios of the C₃A-CaSO₃·0.5H₂O mixture were examined in pure distilled water with a considerable amount of liquid W / S = 38-50 (constant W / C₃A). Processes in the liquid phase were monitored with conductivity equipment, and the XRD analysis was used to identify the phases precipitated during the examined reactions.     

Mineralogical characteristics of Ettringites synthesized from solutions and suspensions

Due to the wide technical importance of Ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) in hydrated cement based systems, four different synthesis methods were applied to get a better overview on how and under what favoring conditions Ettringite can be formed. In the first step different precipitation methods resulting in Ettringite were established. For all methods Al³⁺ was supplied by aqueous CO₂-free solution of Al₂(SO₄)₃·18H₂O. As a source of CaO-supply a clear solution and otherwise a Ca(OH)₂-suspension was utilized. Four synthesis routes with and without sucrose were employed.Variation of reaction parameter like temperature, time of reaction, pH-value of synthesis solution and influence of additional Gypsum were investigated. The qualitative phase analysis was performed by X-ray diffraction method (XRD). Morphological aspects of some synthesis products were studied by scanning electron microscopy (SEM). Finally lattice and structural parameter of the Ettringites were determined by refinement of XRD-pattern using the Rietveld-Method.     

Characterisation of the surface thermodynamic properties of cement components by inverse gas chromatography at infinite dilution

The surface thermodynamic properties of three main inorganic compounds formed during hydration of Portland cement: calcium hydroxide (Ca(OH)₂), ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) and calcium-silicate-hydrates (C-S-H), respectively, and one mineral filler: calcium carbonate (CaCO₃), have been characterised by inverse gas chromatography at infinite dilution (IGC-ID) at 35 °C. The thermodynamic properties have been investigated using a wide range of non-polar (n-alkane series), Lewis acidic (CH₂Cl₂ and CHCl₃), Lewis basic (diethyl ether) and aromatic (benzene) and n-alkene series molecular probes, respectively. The tested samples are fairly high surface energy materials as judged by the high dispersive contribution to the total surface energy (the dispersive components γ_s^d range from 45.6 up to 236.2 mJ m^− 2 at 35 °C) and exhibit amphoteric properties, with a predominant acidic character. In the case of hydrated components (i.e. ettringite and C-S-H), the surface thermodynamic properties have been determined at various temperatures (from 35 up to 120 °C) in order to examine the influence of the water content. The changes of both dispersive and specific components clearly demonstrate that the material surface properties are activated with temperature. The changes in the acid-base properties are correlated with the extent of the overall water loss induced by the thermal treatment as demonstrated by thermogravimetric analysis (TGA). The elemental surface composition of these compounds has been determined by X-ray photoelectron spectroscopy (XPS).     

Investigation of the structure of ettringite by time-of-flight neutron powder diffraction techniques

The crystalline structure of ettringite, Ca₆[Al(OH)₆]₂(SO₄)₃·∼26H₂O, was investigated using high-resolution time-of-flight neutron powder diffraction techniques. The powder diffraction data were subjected to Rietveld crystal structure refinement. The resultant ettringite crystal structure confirmed the positions of Ca, Al, and S atoms while permitting a more precise determination of the locations of O and H atomic positions than in previous X-ray and neutron diffraction studies. A discussion of the ettringite hydrogen bonding network is presented, illustrating the role of hydrogen bonding in the stabilization of the ettringite structure.     

Amorphisation mechanism of a flint aggregate during the alkali-silica reaction: X-ray diffraction and X-ray absorption XANES contributions

Flint samples at different stages of the Alkali-Silica Reaction were prepared and analyzed by X-ray diffraction (XRD) and silicon K-edge X-ray absorption near edge structure techniques (XANES). The results are compared to those of measurements performed on alpha quartz c-SiO₂ and rough flint aggregate. The molar fraction of Q₃ sites is determined as a function of the time of reaction. Up to 14 h of attack, the effect of the reaction seems of little importance. From 30 to 168 h, we showed an acceleration of the effect of the reaction on the crystal structure of the aggregate resulting in an amorphisation of the crystal. During this period, the amorphous fraction increases linearly with the number of Q₃ sites. The results of the XANES confirm the amorphisation of the aggregate during the reaction and show the presence of silicon in a tetrahedral environment of oxygen whatever the time of attack.     

Anion water in gypsum (CaSO4·2H2O) and hemihydrate (CaSO4·1/2H2O)

The bonding nature of water in gypsum, CaSO₄·2H₂O, and hemihydrate, CaSO₄·1/2H₂O, was suggested by characteristic absorption bands of water and sulphate ions. The infrared spectral data indicate the presence of anion water in gypsum and hemihydrate through hydrogen bonding. Negative shifting of bending vibration of SO₄ ion and lowering and broadening of the O-H stretching vibration at around 3600 cm⁻¹ indicate the presence of both anion water and hydrogen bonding in gypsum and hemihydrate     

X-ray investigation of sintered cadmium doped hydroxyapatites

Pure and cadmium (Cd) doped hydroxyapatites (HA, Ca₁₀(PO₄)₆(OH)₂) were synthesized by a precipitation method from aqueous solutions of Ca(NO₃)₂4·H₂O for the former and Cd(NO₃)₂4·H₂O for the latter, by using (NH₄)₂HPO₄ as the phosphate source, while pH was kept in the range of 11–12. The effect of incorporation of Cd²⁺ ions into the structure of HA was investigated after the air sintering at 1100 °C for 1 h. The results indicate that Cd²⁺ addition into HA yields nearly fully densified products with respect to pure stoichiometric HA. The XRD patterns showed that Cd doping increases the crystallinity of HA. The 2, 4.4, and 8.8 mol% Cd doped HAs had calcium oxide (CaO) impurity phase in their lattice. The CaO phase in the HA structure gradually disappeared with increasing Cd amount, and was replaced with cadmium oxide (CdO) in the CdHA doped with 11 mol% Cd. Cd²⁺ ion incorporation decreased the a- and c-axis lattice constants and unit cell volume of HA.     

Cohesion and expansion in polycrystalline solids formed by hydration reactions — The case of gypsum plasters

When powdered plaster (CaSO₄·1/2H₂O) is mixed with sufficient water to give a plastic paste, hydration occurs rapidly, forming a hardened mass of gypsum (CaSO₄·2H₂O), usually with slight bulk expansion. The addition of certain compounds can greatly increase the expansion, which may lead to destructive pressures. Here I show that this effect increases with the size of the alkyl group in an homologous series of simple water-soluble calcium carboxylate salts: Ca(HCOO)₂; Ca(CH₃COO)₂; Ca(CH₃CH₂COO)₂. The latter two, when used at aqueous concentrations of 10% or more, cause large expansions. The results can be explained by a delicate balance between crystal growth pressures and cohesive interactions at the interfaces between crystallites, on the assumption that only two principal classes of interface exist in the hardened structure: “bridging” and “non-bridging.” This hypothesis allows us to make some useful conjectures about the performance of mineral-based hydraulic binders in general, with potential implications for their durability.     

Metaettringite, a decomposition product of ettringite

Dehydroxylation of ettringite in an atmosphere of constant partial water vapour pressure (30-400 Torr) and controlled temperature (55-95 °C) yields an X-ray amorphous product containing 11-13 H₂O per ettringite formula unit. The X-ray amorphous product does, however, give electron diffraction patterns similar to those of ettringite but with a considerably reduced from ∼1.123 nm (ettringite) to 0.85 nm in the partially dehydroxylated product, termed metaettringite. The structure of metaettringite is closely related to that of fleischerite, Pb₃Ge[(OH)₆](SO₄)₂·3H₂O and the isostructural despujolsite, Ca₃Mn^IV(SO₄)₂(OH)₆·3H₂O. These minerals contain columnar units. The columnar structure is like that of ettringite but with closer packing of columns in the a direction, resembling metaettringite.The mechanism of collapse of ettringite to metaettringite, involving loss of water and motion of columns, cannot be achieved without scissoring and possibly rotation of individual columns; this and other defect-producing mechanisms result in loss of crystallinity of the metaettringite product.     

Stability and reactivity of thaumasite at different pH levels

Thaumasite (CaCO₃·CaSO₄·CaSiO₃·15H₂O) has been reported to form at low temperatures (below 15 °C) during sulfate attack. Reactions between calcium silicate hydrate (C-S-H) and Ca⁺², CO₃⁻², SO₄⁻², CO₂ and water, or between ettringite and C-S-H, CO₃⁻² and/or CO₂ and water, result generally in the formation of thaumasite. In some instances, thaumasite may be affected by the presence of other chemicals in the surrounding environment (i.e., phosphates and ammonia in agricultural soil). There are insufficient data regarding the stability of thaumasite at different pH levels in the presence of other chemical ions. Understanding this issue might help in the detection of the thaumasite form of sulfate attack, and, therefore, in one's choice of the appropriate protection technique. This work reports the reactivity of thaumasite with phosphate, carbonate and bicarbonate ions at different pH levels ranging from 6.00 to 12.00, as well as the stability of thaumasite at high pH levels (greater than 12.00). Thaumasite was found to react with these ions at pH levels at and below 12.00; however, thaumasite was stable with minimal reactivity at pH levels greater than 12.00.     

The hydration products of Portland cement in the presence of tin(II) chloride

The hydration products of Portland cement pastes cured using water containing tin(II) chloride have been compared with those using distilled water. In the latter case, the expected products—portlandite, ettringite and calcite—were observed. The X-ray diffraction patterns of the cement pastes cured in the presence of tin(II) chloride showed several additional peaks that have been attributed to the formation of calcium hydroxo-stannate, CaSn(OH)₆, and Friedel's salt (tetracalcium aluminate dichloride-10-hydrate), Ca₃Al₂O₆·CaCl₂·10H₂O. The amount of portlandite formed was reduced in the presence of tin(II) chloride. Calcium hydroxo-stannate contains tin in the +IV oxidation state and equations are presented to account for the oxidation of Sn(II) to Sn(IV) preceding the formation of CaSn(OH)₆ and Friedel's salt.     

Structural Study of Copper Oxide Supported on a Ceria-Modified Titania Catalyst System

A structural study of CuO supported on a CeO₂–TiO₂ system was undertaken using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. The results of XRD revealed the presence of only two phases, TiO₂ anatase and CeO₂ cerianite. A trend towards smaller TiO₂ crystallites was observed when cerium content increased. When the amount of cerium increased, Ti K-edge XANES analysis showed an increasing distortion of Ti sites. The results of Ce L_III-edge EXAFS showed that Ce atoms are coordinated by eight oxygen atoms at 2.32 Å. For the sample containing a small amount of cerium, the EXAFS analysis indicated that the local structure around Ce atoms was highly distorted. The catalysts presented quite different Cu K-edge XANES spectra compared to the spectra of the CuO and Cu₂O reference compounds. The Cu–O mean bond length was close to that of the CuO and the Cu atoms in the catalysts are surrounded by approximately four oxygen atoms in their first shell. Copper supported on the ceria-modified titania support catalysts displayed a better performance in the methanol dehydrogenation when compared to copper supported only on titania or on ceria.     

Effects of water content and temperature on the crystallization behavior of FGD scrubber sludge

Massive quantities of flue gas desulfurization scrubber sludges, both sulfate- and sulfite-rich, are generated by coal burning power plants. Environmentally friendly but economical disposal of this sludge is of great importance to the coal industry. To achieve the goal of effective utilization of the scrubber sludges, we have begun to explore developing value-added materials from these sludges. In this paper, we report how water-to-scrubber sludge (w/s) ratio and the fabrication temperature affected the crystal growth behavior of the sludge under pressure. This was accomplished by conducting scanning electron microscopy, X-ray diffraction, Fourier transform infrared, differential scanning calorimetry, and thermomechanical analyzer measurements on the samples formed at 24.2 MPa. Our results suggest that the hydration of hemihydrate, generated from the sludge, resulted in the formation of two phases of calcium sulfate in our materials formed at four different temperatures using two different w/s ratio settings. At T<130 °C, parallelogram-shaped (columnar) gypsum (CaSO₄·2H₂O) and rectangular-shaped (acicular) β-hemihydrate (CaSO₄·0.5H₂O) crystals dominated our materials for w/s=0.2. However, for w/s=0.6, needle-shaped but orientated crystallites formed in the samples. At T>130 °C, the samples mainly exhibited hemihydrate phase in the form of fibrous growths formed by the splitting of gypsum crystallites. The samples fabricated at 90°C exhibited a higher degree of interlocking of the crystallites, which imparted better mechanical property to our materials as depicted by the fracture strength.     

Removal of hydrogen sulfide using red mud at ambient conditions

Red mud (RM) is a caustic waste product of alumina industry. A laboratory study was conducted to investigate the removal of hydrogen sulfide (H₂S) gas using RM at ambient conditions. Red mud was characterized before reaction and after reaction by using XRD, BET, SEM/EDX, TG-DSC, FT-IR, and CHNS. The XRD and EDX data confirmed that the H₂S was removed in the form of FeS₂, FeS, CaSO₄·2H₂O, sulfur, sulfide, and bisulfide of Na. During the sulfidation reaction, the color of some RM changes from red to black, indicating that iron oxide was converted to iron sulfide. The black color in this case was an indicator of mineral sulfide compounds which indicated that some H₂S sorption and removal occurred. The removal capacity of sulfidic filtrate was higher as compared to the sulfidic red mud as revealed by thermogravimetric and EDX analysis.     

Characteristics and so2 capture capacities of sorbents prepared from products of spray-drying flue gas desulfurization

The characteristics and the SO₂ capture capacities of sorbents prepared from products of spray-drying flue gas desulfurization (FGD) have been studied. Sorbents were prepared by first slurrying Ca(OH)₂ and CaSO₃ and/or CaSO₄ with and without the addition of fly ash and then drying. Compared to the use of pure Ca(OH)₂, the SO₂ capture and Ca(OH)₂ utilization decreased for sorbents prepared without fly ash and increased for sorbents with fly ash. Flakelike ill-crystallized tobermorites were observed for all the sorbents containing fly ash. In addition, significant amounts of needle-shape Ca₄Al₂(OH)₁₂SO₄ . 6H₂O and Ca₆Al₂ (OH)₁₂(SO₄)₃ . 26H₂O (ettringite) were also observed for the sorbents containing CaSO₃ and/or CaSO₄. These newly formed compounds dissociated into CaSO₃ . 0.5 H₂O and inert precursors upon sulfation, and were responsible for the high SO₂ capture capacities and Ca(OH)₂ utilizations of the sorbents prepared with fly ash.     

Uptake of Se(IV/VI) oxyanions by hardened cement paste and cement minerals: An X-ray absorption spectroscopy study

The uptake of selenate (Se^VIO₄²⁻) or selenite (Se^IVO₃²⁻) by hardened cement paste (HCP) and important constituents of the cement matrix such as calcium silicate hydrate (C-S-H), portlandite (CH), ettringite (AFt) and monosulfate (AFm) was investigated using X-ray absorption spectroscopy (XAS). The XAS measurements were conducted on samples with Se loadings ranging between 1200 and 8800 ppm. X-ray absorption near edge structure (XANES) spectroscopy shows that redox reactions do not influence uptake processes in the cementitious systems. The EXAFS (extended X-ray absorption fine structure) spectra of Se(IV) and Se(VI) bound to CH, AFt, AFm and C-S-H are similar to those of SeO₄²⁻ and SeO₃²⁻ in solution, indicating a “solution-like” coordination environment upon uptake by the cement minerals. Similarly, the spectra of Se(IV)- and Se(VI)-treated HCP samples reveal the absence of backscattering atoms at short distances. These results suggest that the coordination sphere of the SeO₄²⁻ and SeO₃²⁻ entities is maintained upon immobilization by HCP and cement minerals and non-specific interactions dominate at the given Se loadings.     

Phase evolution during the low temperature formation of stoichiometric hydroxyapatite-gypsum composites

Both plaster of Paris (CaSO₄·(1/2)H₂O, POP) and bone-like synthetic calcium phosphates (CaP) have been used as bone-like cements. The current study investigated the formation of composites involving POP with each of three types of stoichiometric hydroxyapatites (abbreviated as SHAp, S-SH, and C-SH, each with a Ca/P molar ratio of 1.67). The kinetics, variations in solution chemistry during the formation of these composites and phase compositions of the formed products were investigated over a course of 24 h. Although the presence of gypsum precursors was shown to decrease the alkalinity of the medium involving SHAp formation from its precursors, a delay in the growth kinetics of gypsum was observed. The same behavior was observed in the presence of commercial apatite (C-SH), whereas in the presence of synthetic apatite (S-SH), no delay was observed. A possibility of formation of a calcium sulfate phosphate double salt, as an intermediate, was investigated and confirmed by XRD analysis.     

The high dispersion of CuCl2 in NaZSM-5 by using microwave technique

By using microwave technique, the high dispersion of CuCl₂ in NaZSM-5 zeolite (CuCl₂·2H₂O/NaZSM-5 ratio of 0–0.50 g/g) has been prepared. The mechanical mixture of CuCl₂-2H₂O and NaZSM-5 (CuCl₂·2H₂O/NaZSM-5 ratio of 0–0.50 g/g) shows characteristic XRD peaks of both NaZSM-5 and crystalline CuCl₂·2H₂O. Notably, after reaction of the above samples in a microwave oven for 10 min, the sample XRD patterns only exhibit the peaks assigned to NaZSM-5, while the peaks assigned to crystalline CuCl₂·2H₂O disappear completely, indicating that CuCl₂·2H₂O no longer exists in the crystalline state in the CuCl₂·2H₂O/NaZSM-5 samples. Additionally, in DTA curves, we cannot observe the melting point of CuCl₂ in a CuCl₂·2H₂O/NaZSM-5 sample (CuCl₂·2H₂O/NaZSM-5 ratio of 0–0.5 g/g) treated in a microwave oven, indicating that there is no CuCl₂ crystalline phase in the CuCl₂·2H₂O/NaZSM-5 samples (CuCl₂·2H₂O/NaZSM-5 ratio of 0–0.50 g/g).