Showing papers on "Calcium aluminates published in 2011"

Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part I. Background, Experimental Techniques and Analysis Methods

Abstract: In aluminum-killed steels, modification of solid alumina inclusions is often carried out by calcium treatment, converting the alumina to liquid calcium aluminates. When calcium treatment is performed, calcium can either react with sulfur in the melt or with solid alumina. Calcium sulfide inclusions are solid at steel casting temperatures and thus would be detrimental to castability if they remained in the steel after calcium treatment. The aim was to study the transient evolution of inclusions after calcium treatment, testing the hypothesis that calcium sulfide may form as an intermediate reaction product, which can subsequently react with alumina to form modified calcium aluminates. The first part gives the project background and describes the experimental and quantification techniques adopted, including the effect of sampler size in laboratory melts. Results of the formation of intermediate calcium reaction products in laboratory and industrial heats are presented in the second part.

Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part II. Results and Discussion

Abstract: The formation of intermediate reaction products after calcium addition to aluminum-killed steel was studied. Steel samples were taken from laboratory and industrial heats before and at various times after calcium treatment. Inclusions were characterized by automated and manual scanning electron microscopy and X-ray microanalysis of polished cross sections and inclusions extracted by dissolution of the steel. Industrial and laboratory melts containing more than 40 parts per million (by mass) of dissolved sulfur showed calcium sulfide as the main reaction product after calcium injection, with calcium aluminates appearing later. It is proposed that the calcium aluminates are formed by reaction between the calcium sulfide and the alumina. A laboratory heat containing 7 parts per million of sulfur showed calcium oxide as the main initial calcium reaction product. A simple mechanism is proposed for the modification of alumina inclusions by calcium, considering transient CaO and CaS formation.

The structure of liquid calcium aluminates as investigated using neutron and high energy x-ray diffraction in combination with molecular dynamics simulation methods.

Abstract: The O–O and Al–O weighted partial structure factors were incorrectly labelled in both figures 4 and 5 of the original version of the paper; see the PDF for the correct figures.

Elaboration and characterization of alumina - fluorapatite composites

Abstract: Alumina and fluorapatite powder were mixed in a wet medium in order to elaborate biphasic ceramics composites. The effect of fluorapatite addition (26.5 wt%) in the densification and the mechanical properties of the alumina matrix were measured. The phase developments have been systematically analysed by scanning electronic microscopy, X-ray diffraction, Infrared spectroscopy and 31P and 27Al magic angle scanning nuclear magnetic resonance. The Brazilian test was used to measure the mechanical resistance of alumina - 26.5 wt% fluorapatite composites. The densification and strength rupture of composites increase versus sintering temperature and holding time. At 1600°C, the composites densities reached 85% and the rupture strength was about 22 MPa. Also, the composites sintering at 1500°C for 5 hours provides samples with similar density and having higher mechanical resistance, above 26 MPa. For longer holding times, the mechanical properties were hindered by the exaggerated grain growth and the formation of intragranular porosity. From 1400°C, the characterization of the alumina - 26.5 wt% fluorapatite composites indicates the formation of calcium aluminates.

The Effect of Sulfur Concentration in Liquid Iron on Mineral Layer Formation During Coke Dissolution

Abstract: The effects of sulfur concentration in liquid iron on mineral layer development between coke and iron as coke dissolves in a 2 mass pct carbon-iron liquid have been investigated at 1773 K (1500 °C). The initial sulfur in iron concentrations used ranged from 0.006 to 0.049 mass pct. Key findings include that the two-stage dissolution behavior exhibited in the carbon transfer from coke to iron, as reported in a previous study by the authors, at low initial sulfur in iron contents, was also apparent at the higher values used in this study. This two-stage behavior was attributed to a change in the mineral layer density as a result of changes in mineral morphology at the interface. In addition to confirming the two-stage behavior of the carbon-transfer kinetics at the higher sulfur concentration in iron levels, after a period of time, a solid calcium sulfide layer formed on the mineral layer. The sulfide layer formed after approximately 40 minutes, and the proportion of sulfide in the mineral layer increased with increased experimental time and initial sulfur concentration in iron. It was usually found at the iron side of the mineral layer and was associated with calcium-enriched calcium aluminates. Thermodynamic analysis of this layer confirmed that the sulfide is stabilized as the mineral layer is enriched by calcium.

Thermodynamics of Reactions Among Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes

Abstract: The thermodynamic of the chemical reactions among Al2O3, CaO, SiO2 and Fe2O3 in the roasting processes was investigated in this chapter. The chemical reactions are classified into SiO2-Al2O3 system, Fe2O3-Al2O3 system, SiO2-Fe2O3 system, CaO-Al2O3 system, SiO2-CaO system, SiO2-calcium aluminates system, CaO-Fe2O3 system, Al2O3-calcium ferrites system and Al2O3-CaO-SiO2-Fe2O3 system. When the roasting temperature is over 1100K, 3Al2O3·2SiO2 is preferentially formed in SiO2-Al2O3 system; FeO·Al2O3 can be formed in Fe2O3-Al2O3 system; ferric oxide and SiO2 could not generate iron silicate; 12CaO·7Al2O3 is preferentially formed in CaO-Al2O3 system when one mole Al2O3 reacts with CaO; 2CaO·SiO2 is preferentially formed in SiO2-CaO system; except for CaO·2Al2O3 and CaO·Al2O3, the other calcium aluminates can transform into calcium silicate by reacting with SiO2 in SiO2-calcium aluminates system; 2CaO·Fe2O3 is preferentially formed in CaOFe2O3 system; alumina is unable to form 3CaO·Al2O3 with calcium ferrites(2CaO·Fe2O3 and CaO·Fe2O3), but able to form 12CaO·7Al2O3 with 2CaO·Fe2O3; when CaO, Fe2O3, Al2O3,SiO2 coexist, they are more likely to form ternary compound 2CaO·Al2O3·SiO2 and 4CaO·Al2O3·Fe2O3.

The use of thermal analysis in the study of Ca3Al2O6 formation by the polymeric precursor method

Abstract: Single-phase Ca3Al2O6 was prepared via polymeric precursor method. The influence of the reactants nature in the Ca3Al2O6 synthesis was investigated. For this purpose, citric acid and soluble salts of calcium (nitrate, chloride, carbonate) and aluminium (nitrate, chloride, acetate) were used as starting materials, in the presence and, respectively, in the absence of ethylene glycol. Ca3Al2O6 resulted as single-phase after annealing at 1050 °C for 1 h only starting from calcium nitrate or carbonate and aluminium nitrate or acetate as salts precursor for Ca2+ and Al3+ cations. The formation of Ca3Al2O6 is not conditioned by the ethylene glycol presence in these mixtures. Using calcium and aluminium chlorides, the phases present at 1050 °C are Ca12Al14O33 and unreacted CaO.

Cementitious properties of steelmaking slags

Abstract: The present study is directed towards the use of steelmaking slags as raw material for sulphoaluminate belite cement (SAB). Another important objective was to characterise the cementitious properties of phases in ladle furnace slag (LFS) specifically the calcium aluminates. Mayenite (C12A7) is considered one of the most important calcium aluminate in LFS, and since comparatively limited data on the kinetic properties of this phase are available, it was decided to study C12A7 more closely with regard to both particle size and temperature sensitivity. The behaviour of high-temperature reactions of tested SAB mixtures was investigated using thermogravimetric analysis coupled with a quadrupole mass spectrometer. Mineralogical observations were carried out with x-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results proved that steelmaking slags have the potential to work as raw material, since sulphoaluminate (C4A3 S ) along with polymorphs of dicalcium silicate (C2S) and ferrite phase (C4AF) were detected after firing at 1200oC in an air atmosphere. The hydration properties of the specimens were analysed through conduction calorimetry, and compressive strength of specimens hydrated for 2 and 28 days. The compressive strength was in accordance with that suggested in the literature for slow hardening SAB cement. Both mixtures tested behaved the same with regard to heat development as well as the amount of ettringite (AFt) formed during the first 24 hours of the hydration. The formation of AFt was characterised with both differential scanning calorimeter (DSC) and XRD. The crystallographic distribution in LFS samples was quantified using Rietveldanalysis. Calorimetric studies were performed at 20, 25 and 30°C in order to calculate the activation energy of hydration and thereby to suggest a kinetic model for tested compositions within this temperature interval. In addition to heat of hydration, compressive strength tests were completed on mortar prisms of LFS, and LFS in a blend with ground granulated blast furnace slag (GGBFS) which hydrated for 2, 7 and 28 days. Both compositions reached acceptable early strengths, whereas, after 28 days hydration, the blend was superior to neat LFS. Related activation energy was according to the Avrami-Erofeev model determined to 58 kJ/mol for the LFS and 63 kJ/mol for the blend. Corresponding calorimetric studies at the same temperatures were performed on a fine and coarse size fraction (Fraction A and Fraction B) of a synthesised C12A7. The purity was confirmed by XRD, and the hydraulic behaviour was investigated in excess water with respect to the dissolution. The apparent activation energy was calculated to 33 and 79 kJ/mol, respectively, for Fractions A-B using the Avrami-Erofeev model. From the model, it was also concluded that the acceleration period can be ascribed to a phase-boundary controlled mechanism. The principal calcium aluminate hydrates obtained were C2AH8 and C2AH7.5, and it was further observed that C12A7 is accompanied by an anomalous…

Solid state combustion synthesis of nano to macroscale portland cement and other high value nano particles

Abstract: A method of making Portland cement, white cement, calcium aluminates, calcium aluminum silicates and similar oxides using solid state combustion synthesis is described. The method uses less energy and produces lower CO 2 emissions than conventional processes. The method uses green fuels like biomass and lignin and eliminates most of the coal used in traditional cement production. A batch reactor and a semi-continuous reactor that can be used for the combustion synthesis are also described.

Tungsten-rhenium mixed metal matrix cathodes

Abstract: In this paper, studies are carried out on W-Re mixed matrix cathodes developed in house, will be discussed. It has been observed that a proper choice of rhenium particle size is essential for obtaining good emission density. The cathodes have been prepared by impregnating W-Re (50%–50% by weight) mixed metal matrices with 5:3:2 barium calcium aluminates. A current density of 20 A/cm2 has been achieved at 1360K. Life testing of these cathodes has been carried out in closely spaced diode configuration. Variation of work function and emission current density after 35000 Hrs of life will be presented

Powder-like composition for boriding steel items

Abstract: FIELD: metallurgy. ^ SUBSTANCE: powder-like composition for boriding steel items contains following components, wt %: boron carbide 45-65, sodium fluoride 5-8, high-aluminous corrosion resistant cement on base of calcium aluminates HACRC-75-0.5 30-35, wood coal carburizer 8-12. ^ EFFECT: strengthened layer on steel items at repeated usage of saturating mixture. ^ 2 tbl

Thermodynamic Calculation and Application of Non-Metallic Inclusions for Ultra-Low-Oxygen Gear Steel

Abstract: Interrelations between oxygen content of gear steel and non-metallic inclusions,and transformations conditions for non-metallic inclusions were calculated using the thermodynamic calculation software Factsage.It is found that the components of non-metallic inclusions with higher plasticity in ultra-low-oxygen gear steel should be composed by SiO2 0-10%,Al2O3 22%-55%,CaO 42%-60%,and MgO 5%-10%,aluminum content would be about 0.020%,calcium content would be more than 0.7×10-6,and oxygen content would be about 0.0005% in molten steel when the reaction between the inclusion and liquid steel arrived in equilibrium.If the compositions of fining slag with w(CaO)40%,w(Al2O3)≤37%,w(MgO)=10%,(w(CaO)+w(MgO))/w(SiO2)=10,and SiO2 content as low as possible were selected,Al2O3 and MgO·Al2O3 in molten steel can transform into calcium aluminates with low melting point.

Formation of the phase composition of unsintered refractory concretes in the operating temperature range of glassmaking equipment

Abstract: Comprehensive studies of the phase composition of refractory concrete compositions heat-treated in different temperature intervals, including the operating range of glassmaking equipment, have been performed. The characteristic changes of the phase composition of unsintered concretes during the transformative heat-treatment have been established; a new phase appears after sintering at 1000°C — calcium aluminosilicate 2CaO - Al2O3 - SiO2, which attests to the interaction in the sample of calcium aluminates with free silica — microsilica or aluminosilicates.

Process for technogenic waste utilization

Abstract: The invention relates to ecology, particularly to complex utilization of high-silica alumina-containing technogenic waste of coal-mining, mineral resource, metallurgical industry, fuel and energy complex, and also overburden of dumps of open cast mines, containing as basic components alumina and silica A process for technogenic waste utilization comprises preparation of furnace charge from raw material based on non-alkali waste, containing alumina, silica and iron (III) oxide, and calcium carbonate, sintering the charge, cooling, desalination of alumina from the sinter by soda solution, separation of soda solution with alumina from slurry, separation of alumina from the solution and use of slurry for the preparation of cement clinker; at that as raw material mixture of argillite and kaolin of fraction less than is used, taken in weight fraction providing in mixture the content of iron (III) oxide, equal 18-28 wt %, in the course of charge preparation water is added therein which contains 01-03 wt % of surfactants, in quantity of 30-40 wt % from charge weight, sintering the charge is carried out under the temperature of 1260-1290 °C, after separation of soda solution with alumina from slurry, then remaining in slurry soda solution with alumina is displaced with water being delivered in amount equal to difference between the amount of soda solution taken to desalination and amount of separated soda solution The slurry for the preparation of cement clinker is used after its mixing with argillite and calcium carbonate, taken in ratio, providing such a content of CaO, SiO, AlOand FeOas in the prepared cement clinker The use of the proposed process provides the possibility of more complete calcium aluminates formation under decreased temperatures at prevention of conditions for forming a vitreous phase which leads to increasing alumina extraction degree at decreasing energy consumption and improvement of ecological situation

Effects of sintering atmosphere controlled using carbon pellet on translucency of AlN ceramics

Abstract: Translucent AlN ceramics were sintered with a carbon pellet in order to control the CO gas surrounding the sample during sintering. The total transmittance of the ceramics increased by sintering with the carbon pellet because of the decrease of light scattering. One of causes of the light scattering was the presence of calcium aluminates at grain boundaries. The light scattering can be controlled by the CO gas using the carbon pellet.

Effect of Slag-Metal Reaction on Transformation of Al2O3 inclusions in Pipeline Steel

Abstract: For high grade pipeline steel, elimination of Al2O3 inclusions can improve the susceptibility of hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC). In the present work, the transformation of Al2O3 inclusions with slag-metal reaction was studied by controlling ladle slag and relative technologies in LF-RH refining process. It is found that Al2O3 inclusions decrease with increasing refining time, and no pure Al2O3 inclusions exist in molten steel at the end of secondary refining; the remained inclusions in molten steel are liquid calcium aluminates and semi-liquid CaO-MgO-Al2O3 complex inclusions. The analysis of thermodynamic equilibrium proves that CaO and MgO in slag can be reduced by aluminum in molten steel, which is the source of calcium and magnesium in molten steel.